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Stanje naponskih natrijevih kanala u depolariziranim potencijalima

Stanje naponskih natrijevih kanala u depolariziranim potencijalima


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Slučaj 1

To učimo na fiziologiji tijekom faze depolarizacije u membrani, više natrijevih kanala ima tendenciju otvaranja kako se membranski potencijal povećava tj. manje negativno. Ovo je vrsta pozitivne povratne sprege koja vodi do akcijskog potencijala i objašnjava prag depolarizacije koji je potreban za akcijski potencijal.

Dakle, ukratko, kako membranski potencijal postaje manje negativni, natrijevi kanali imaju tendenciju otvaranja.

Slučaj 2

S druge strane postoji poznati grafikon Bertrama Katzunga, Susan Masters, Anthonyja Trevora-Basica i Clinical Pharmacology, izdanje 12. koji pokazuje ovisnost funkcije natrijevog kanala na temelju membranskog potencijala koji prethodi podražaju.

Citirati,

Udio natrijevih kanala dostupnih za otvaranje kao odgovor na podražaj određen je membranskim potencijalom koji neposredno prethodi podražaju. Smanjenje udjela dostupnog kada je potencijal mirovanja depolariziran u odsutnosti lijeka (kontrolna krivulja) proizlazi iz naponski ovisnog zatvaranja h vrata u kanalima.

Oba ova slučaja mi izgledaju prilično kontraindicirana. Ali pokušao sam to objasniti s tri stupnja Na+ kanala.

Slučaj 1 - može se odnositi na prijelaz iz zatvorenih u otvorene kanale, tj. otvaranje vrata. (ovdje samo pretpostavljam pa me ispravite ako griješim)

Slučaj 2 - kao što je spomenuto u knjizi, govori o zatvaranju h vrata koja vodi do inaktiviranih vrata na višim membranskim potencijalima.

Da li se slučaj 1 i slučaj 2 događaju jedan za drugim? Jesu li rasponi napona za svaki različiti? Je li moje objašnjenje za slučaj 1 točno?


Fenestracije kontroliraju blok stanja mirovanja naponski upravljanog natrijevog kanala

Naponski natrijevi kanali pokreću električne signale u živčanom i srčanom mišiću, gdje njihova hiperaktivnost uzrokuje bol i srčanu aritmiju. Lokalni anestetici i antiaritmički lijekovi selektivno blokiraju natrijeve kanale u brzo aktiviranim živčanim i mišićnim stanicama kako bi ublažili ova stanja. Proučavali smo bakterijski natrijev kanal predaka kako bismo razjasnili strukturu mjesta vezanja lijeka i put ulaska lijeka do mjesta receptora. Otkrili smo da se mjesto vezanja lijeka nalazi u središtu transmembranske pore, kroz koju se kreću natrijevi ioni, a fenestracije tvore pristupni put za ulazak lijeka izravno iz stanične membrane. Ovi rezultati pokazuju kako ovi lijekovi koji se široko koriste blokiraju natrijeve kanale i imaju važne implikacije za strukturni dizajn lijekova sljedeće generacije.


Što je voltažni natrijev ionski kanal?

A napon-zatvoren natrijev ionski kanal je kanal to dopušta samo natrijevi ioni proći kroz i njihovu funkcija da li se otvaraju i zatvaraju kao odgovor na promjene u membrani.

Isto tako, što se događa ako su naponski natrijevi kanali blokirani? Na primjer, ako the naponski ograničen Na + kanal je blokiran, stanica se neće moći depolarizirati i akcijski potencijal se neće generirati. Jednostavnim dodavanjem 120 mM K+ u izvanstaničnu tekućinu, stanica bi se depolarizirala bez akcijskog potencijala.

Ljudi također pitaju, od čega su napravljeni naponski natrijevi kanali?

napon-zatvorene natrijeve kanale obično se sastoje od alfa podjedinice koja tvori pore ionske vodljivosti i jedne do dvije beta podjedinice koje imaju nekoliko funkcija uključujući modulaciju gating kanala. Sama ekspresija alfa podjedinice dovoljna je da se proizvede funkcionalna kanal.

Što uzrokuje otvaranje naponskih ionskih kanala?

napon-otvoreni kanali kada se transmembranski napon promjene oko njih. Aminokiseline u strukturi proteina osjetljive su na naboj i uzrok pora do otvorena na odabrane ion.


Genetika epilepsije

Ortrud K. Steinlein, u tijeku istraživanja mozga, 2014

1. Uvod

Naponski upravljani natrijevi kanali imaju ključnu ulogu s obzirom na funkciju neurona. Oni kontroliraju izmjenu natrija između izvanstaničnog i intracelularnog prostora, a bitni su za pokretanje i aktiviranje akcijskih potencijala (Hu i sur., 2009.). Njihova važna uloga u neuronskoj ekscitabilnosti čini ih glavnim kandidatima za epizodne neurološke poremećaje kao što je epilepsija. Stoga nije iznenađujuće da je utvrđeno da mutacije u različitim podtipovima natrijevih kanala s voltažnim naponom uzrokuju različite oblike epileptičkih poremećaja, te da su takve mutacije prepoznate kao jedan od najvažnijih uzroka genetske epilepsije (Mulley i sur., 2005.) . Fenotipovi napadaja uzrokovani mutacijama natrijevih kanala naponom su heterogeni i kreću se od benignih do teških, ako ne čak i razornih, što odražava važnost ove superfamilije ionskih kanala za regulaciju stanične ekscitabilnosti na nekoliko funkcionalnih razina (Tablica 1). Tipični primjeri za kliničke fenotipove uzrokovane mutacijama natrijevih kanala pod utjecajem napona su benigni obiteljski neonatalno-infantilni napadaji i teški i ponekad fatalni Dravetov sindrom (također poznat kao teška mioklonska epilepsija dojenčadi (SMEI)) (Baulac i sur., 1999. Escay i sur., 2000. Heron i sur., 2010. Kaplan i Lacey, 1983. Marini i sur., 2011. Meisler i Kearney, 2005. Reid i sur., 2009.). Ova dva epilepsijska sindroma predstavljaju krajnje krajeve spektra kliničke težine, dok se treći, genetska epilepsija s febrilnim napadajima plus (GEFS+), predstavlja s više srednjeg fenotipa koji može uključivati ​​i benigne i teške manifestacije (Baulac i sur. , 1999 Escayg i sur., 2000 Scheffer i Berković, 1997 Scheffer i sur., 2009) (Tablica 1).

Stol 1 . Fenotipovi epilepsije uzrokovani mutacijama natrijevih kanala ovisno o naponu

Klasa podjedinicaGenPodjedinica kanalaFenotipovi epilepsije a
α-Podjedinice
SCN1ANav1.1Febrilni napadaji
GEFS +
Dravetov sindrom
SMEB
West sindrom (infantilni grčevi)
Doose sindrom (mioklonska astatska epilepsija)
Neizlječiva dječja epilepsija s generaliziranim toničko-kloničkim napadajima (ICEGTC)
Rasmussensov encefalitis Lennox-Gastautov sindrom
SCN2ANav1.2Benigni obiteljski neonatalno-infantilni napadaji
Rana infantilna epileptička encefalopatija
Benigni obiteljski infantilni napadaj
SCN8ANav1.6Infantilna epileptička encefalopatija
β-podjedinice
SCN1BNavβ1GEFS +

Struktura VGSC

Naponski natrijevi kanali su heteromerni integralni membranski glikoproteini koji se mogu razlikovati po svojoj primarnoj strukturi, kinetici i relativnoj osjetljivosti na neurotoksin tetrodotoksin (TTX). Sastoje se od α-podjedinice od približno 260 kDa (� aminokiselina), koje su povezane s jednom ili više regulatornih β-podjedinica (㬡–𻉔 kDa od svake (𻉔) Catteral, 2000.). Detaljno ćemo opisati obje podjedinice (α i β) koje su u skladu s VGSC.

α-Podjedinice

Deset različitih izoforma α podjedinica sisavaca (NaV1.1–NaV1.9 i Nax) su okarakterizirane (Tablica 1) i najmanje sedam ih je izraženo u živčanom sustavu. NaV1.1, NaV1.2, NaV1.3 i NaV1.6 izoforme su uglavnom izražene u središnjem živčanom sustavu (CNS). Nasuprot tome, NaV1,7, NaV1.8 i NaV1.9 izoforme su pretežno locirane u perifernom živčanom sustavu (PNS Ogata i Ohishi, 2002.), poznato je da se akumuliraju u području ozljede perifernog živca i mogu biti važne kod kronične, neuropatske boli (Devor, 2006. Tablica 1). U nedavnim izvješćima SCN10A/NaV1.8 također je identificiran u ljudskim srcima (Facer i sur., 2011. Yang i sur., 2012.) i u intrakardijalnim neuronima (Verkerk et al., 2012.), gdje su genetske varijacije u SCN10A gena povezani su s promjenama u PR intervalu, trajanju QRS-a i ventrikularnoj provodljivosti (Chambers i sur., 2010. Sotoodehnia i sur., 2010.). Budući da ove izoforme (NaV1.1𠄱.3, NaV1.6𠄱.9) uglavnom su lokalizirani u živčanom tkivu, općenito se nazivaju “tip mozga” ili “neuronskog tipa” natrijevi kanali. NaV1.4 izoforma je uglavnom izražena u skeletnim mišićima, dok NaV1.5 je izoforma specifična za srce. Izoforma koja se naziva “Nax kanal” [također nazvan NaG/SCL11 (štakori), Nav2.3 (miševi) i/ili hNav2.1 (ljudi)] identificira podporodicu proteina sličnih natrijevim kanalima (George i sur., 1992.). Ovaj kanal ima značajne razlike u sekvenci aminokiselina u senzoru napona, inaktivacijskim vratima i području pora u usporedbi s ostatkom VGSC (George i sur., 1992. Goldin i sur., 2000.). Nax je normalno izražen u različitim organima uključujući srce, skeletni mišić, maternicu, ganglije dorzalnog korijena (DRG) i mozak [uglavnom u cirkumventrikularnim organima (CVO)]. Poteškoće u karakterizaciji biofizičkih svojstava ovog kanala uglavnom su posljedica nedostatka uspjeha u ekspresiji funkcionalnog proteina u heterolognim ekspresijskim sustavima. Hiyama i sur. (2002) generirali su model miša u kojem je Nax gen je nokautiran. Ova grupa je potvrdila da je Nax kanal je izražen u neuronima u CVO-ima koji imaju temeljnu ulogu u kontroli ravnoteže tjelesnih tekućina i iona. Ova skupina je izvijestila da u uvjetima žeđi miševi nemaju Nax pokazali su hiperaktivnost neurona u tim područjima i unosili prekomjernu sol, dok miševi divljeg tipa nisu. To je navelo istražitelje da predlože da Nax bio uključen u mehanizam koji osjeća razinu natrija u mozgu, gdje bi ovaj protein mogao osjetiti ekstracelularnu koncentraciju natrija (Hiyama i sur., 2002. Noda, 2006.).

stol 1. Sažetak različitih tipova VGSC i kanalopatija povezanih s mutacijama u genima koji kodiraju α podjedinice.

Svaka α-podjedinica je raspoređena u četiri homologne domene (DI𠄽IV) koje sadrže šest transmembranskih segmenata (S1–S6 Slika 1). Koristeći krioelektronsku mikroskopiju Sato i sur. (2001) su pokazali da su ove četiri domene raspoređene oko središnje pore kanala. Segment 4 svake domene sadrži visoku koncentraciju pozitivnih naboja (uglavnom arginina) i funkcionira kao jezgra senzora napona odgovornog za aktivaciju kanala ovisno o naponu. Segment 6 iz sve četiri domene čini unutarnju površinu pore. Petlja nalik na ukosnicu između segmenata 5 i 6 [S5–S6 P(ore)-petlja nalik na ukosnicu] dio je pora kanala i tvori uski (ionski selektivan) filter koji kontrolira ionsku selektivnost i permeaciju na ekstracelularnu stranu pore (Catterall, 2000 Yu i Catterall, 2003 George, 2005).

Payandeh i sur. (2011) nedavno su izvijestili o kristalnoj strukturi NaVAb, VGSC pronađen u bakteriji Arcobacter butzleri. NaVAb je dio obitelji kanala NachBac, koji je dobro uspostavljen model za proučavanje Na kralježnjakaV i CaV kanala (Ren i sur., 2001 Koishi i sur., 2004 Payandeh i sur., 2011). Payandeh i sur. (2011) uspjeli su uhvatiti ovaj kanal u zatvorenoj konfiguraciji kada je pora zatvorena s četiri aktivirana senzora napona na granici razlučivosti od 2,7 Å. Payandehov rad pruža prvi uvid u strukturnu osnovu za naponsko ovisnu ionsku selektivnost i blok lijeka u VGSC. Pora se sastoji od vanjskog cjevastog predvorja, selektivnog filtera, središnje šupljine (koja može smjestiti djelomično hidratizirane natrijeve ione) i unutarstanične aktivacijske kapije. Zavojnice koje čine pore postavljene su tako da stabiliziraju katione u središnjoj šupljini kroz heliko-dipol interakcije (Doyle i sur., 1998. Jogini i Roux, 2005.). Druga P2 spirala tvori izvanstanični lijevak i predstavlja visoko konzervirani element u natrijevim kanalima (Payandeh i sur., 2011.).

Payandeh i suradnici su to predložili u NaVAb, put ionske vodljivosti je elektronegativan i filter selektivnosti (uglavnom sastavljen od negativno nabijenih glutamatnih (Glu) bočnih lanaca) tvori najužu konstrikciju blizu izvanstanične strane membrane. Postoje 4 Glu 177 bočna lanca koji tvore skelu od 6,5-Å × 6,5-Å s otvorom širine približno 4,6 Å. Obilna mreža interakcija aminokiselinskih ostataka, uključujući vodikove veze između glutamina iz P-heliksa i karbonila Glu, stabilizira filtar selektivnosti. Radijus pora sugerira da hidratizirani ioni Na + mogu provoditi kroz kanal. Slobodna difuzija tada omogućuje hidratiziranom Na + da uđe u središnju šupljinu i krene kroz otvorena aktivacijska vrata prema citoplazmi (Payandeh i sur., 2011.). Ovaj put permeacije je u suprotnosti s filtrom selektivnosti u K + kanalima, koji je mnogo uži. U ovom slučaju manji radijus pore može provoditi samo dehidrirane ione K + kroz izravne interakcije s karbonilima okosnice kroz duge, uske pore (Morais-Cabral i sur., 2001 Ye et al., 2010).

Identifikacija primarne strukture VGSC dovela je do razvoja “klizna spirala” (Catterall, 1986b) i “spiralni vijak” (Guy i Seetharamulu, 1986.) modele (potvrđene studijama strukture i funkcije) kako bi bolje razumjeli kako radi senzor napona. Oba modela sugeriraju da pozitivno nabijeni ostaci u segmentu 4 unutar svake domene služe kao ulazni naboji koji se kreću prema van kroz membranu kao posljedica depolarizacije membrane, pokrećući proces aktivacije (Catterall, 1986a,b Guy i Seetharamulu, 1986, Catterall i sur., 2010.). Catteral i suradnici opširno su opisali ova dva modela. U osnovi, četiri do sedam pozitivno nabijenih ostataka unutar segmenta 4 uparilo bi negativno nabijene ostatke u segmentima 1, 2 i/ili 3. U ovoj konfiguraciji, pozitivno nabijeni ostaci u segmentu 4 povlače se prema unutra električnim poljem potencijala membrane u mirovanju koje je negativan. Kako depolarizacija napreduje, promjena polariteta membranskog potencijala oslobađa elektrostatičku silu i segmenti 4 se pomiču prema van, omogućujući svakoj pozitivno nabijenoj aminokiselini u segmentu 4 negativno nabijenu. Kao što je opisao Catterall (2010), ovo pomicanje ulaznih naboja prema van u segmentima 4 povlači poveznicu između segmenata 4 i 5, zakrivljuje segment 6 i pokreće otvaranje središnje pore kanala. Kretanje nabijenih čestica radi aktiviranja vodljivosti natrija (“gating pristojbe” ili “ulazna struja”) prvi su predvidjeli Hodgkin i Huxley (Hodgkin i Huxley, 1952, Catterall, 2010), ali su Armstrong i Bezanilla (1973) bili prvi koji su ga izmjerili 1973, kombinirajući tehnike unutarnje perfuzije, naponske kleme, i prosjek signala. Koristeći slične tehnike, Keynes i Rojas (1973.) su iste godine potvrdili postojanje izlazne struje. Armstrong i Bezanilla (1974.) izvijestili su o dodatnim svojstvima ove trenutne i jake dokaze koji ga povezuju s zatvaranjem natrijevih kanala sljedeće godine.

Slika 1. Shematski prikaz α- i β-podjedinica VGSC-a. Predstavljene su četiri homologne domene (I–IV) α-podjedinice S5 i S6 su segmenti koji oblažu pore, a S4 je jezgra senzora napona. U citoplazmatskom povezniku između domena III i IV naznačena je IFMT (izoleucin, fenilalanin, metionin i treonin) regija. Ovo je kritični dio “inaktivacijske čestice” (inaktivacijska vrata), a zamjena aminokiselina u ovoj regiji može poremetiti proces inaktivacije kanala. 𠇍ocking mjesto” sastoji se od više regija koje uključuju citoplazmatski poveznik između S4–S5 u domenama III i IV i citoplazmatski kraj segmenta S6 u domeni IV (*). Ovisno o podvrsti β-podjedinice koja se smatra, oni mogu komunicirati (kovalentno ili nekovalentno) s α-podjedinicom.

β-Podjedinice

To su također integralni proteini, sastavljeni od jedne izvanstanične domene (ECD, N-terminalna domena), jedne transmembranske domene i jedne intracelularne domene (C-terminalna domena). β-podjedinice su izražene u ekscitabilnim i ne-ekscitabilnim stanicama unutar živčanog sustava i srca, a postoje neki dokazi koji upućuju na to da se ti proteini mogu eksprimirati u stanicama čak i u odsutnosti α-podjedinice ( Patino i Isom, 2010. Tablica 2). Jedna ili više regulatornih β-podjedinica (㬡–㬤) može se povezati s jednom α-podjedinicom. Pojedinačna α-podjedinica može biti povezana s jednom nekovalentno (㬡 ili 㬣) i jednom kovalentno (㬢 ili 㬤) povezanom β-podjedinicom (Yu i Caterall20 et Caterall, al., 2005. Patino i Isom, 2010.). Ulogu β-podjedinica detaljno su pregledali Patino i Isom (2010). Autori primjećuju da su β-podjedinice regulatorni proteini koji mogu djelovati i kao molekule stanične adhezije (CAM) i modulirati ekspresiju VGSC na površini stanice, povećavajući gustoću natrijevih kanala i staničnu ekscitabilnost. Potonji može biti vrlo važan mehanizam koji regulira ekscitabilnost nociceptora in vivo (Lopez-Santiago i sur., 2011.). 㬡 povezanost s kontaktinom ili neurofascinom (NF)-186 također rezultira povećanom ekspresijom VGSC stanične površine (Kazarinova-Noyes i sur., 2001. McEwen i Isom, 2004.). Nadalje, 㬡 i 㬢 su proteini koji vežu ankirin. Miševi kojima nedostaje ankirin pokazuju smanjenu struju natrija (jaNa) gustoća i abnormalna jaNa kinetike (Chauhan et al., 2000.), što sugerira da β-podjedinice igraju važnu ulogu u kompleksu VGSC𠄺nkirina (Patino i Isom, 2010.). Interakcija između α- i β-podjedinica može biti posebno kritična na Ranvierovim čvorovima mijeliniziranih aksona, budući da miševi bez 㬡-podjedinice imaju smanjen broj čvorova, promjene u procesu mijelinizacije i drastično promijenjene kontakte između neurona i glijalnih stanica (Chen i sur., 2004.). Iako su proteini unutar nodalnih regija normalno lokalizirani kod ovih miševa, veza između VGSC i kontaktina je poremećena. Gubitak interakcija proteina ovisnih o 㬡-podjedinici–proteina može dovesti do promjena u strukturi Ranvierovih čvorova i poremećene slane provodljivosti (Chen i sur., 2004. Davis i sur., 2004.). Slično podjedinici 㬡-, 㬢 također može modulirati ekspresiju kanala na površini stanice i utjecati na jaNa gustoće (Isom i sur., 1995.). 㬢 (i 㬤) unutarstanične domene mogu se translocirati u jezgru i poboljšati SCN1A ekspresije, čime funkcioniraju kao transkripcijski regulatori VGSC α-podjedinice.

β-podjedinice su također kritične za staničnu migraciju. 㬡 i 㬢 posreduju migraciju fibroblasta (Xiao et al., 1999.) i stanica raka (Brackenbury i Isom, 2008.), adheziju i rast neurita (㬡 potiče i 㬡 promiče i 㬢, dok 㬢 ovaj proces #x003B23 i 4 nemaju učinka Davis i sur., 2004. McEwen i sur., 2009.). Učinci β-podjedinica na staničnu migraciju, adheziju i izrastanje neurita također ovise o događajima unutarstanične transdukcije kao što je aktivacija proto-onkogena tirozin-protein kinaze fyn od 㬡 za promicanje neurita (aksona) i/ili rasta dendrita (Brackenbury i sur., 2008.).

Tablica 2. Sažetak različitih tipova β podjedinica povezanih s različitim VGSC i povezanih kanalopatija povezanih s mutacijama u genima koji ih kodiraju (modificirano iz Patino i Isom, 2010.).


Promjene biofizičkih svojstava naponskih Na + struja tijekom sazrijevanja stanica okusa natrija u gljivičnim papilama štakora

Stanice okusa su heterogena populacija osjetilnih receptora koja prolazi kroz kontinuirani promet. Različite kemo-osjetljive stanične linije oslanjaju se na akcijske potencijale za oslobađanje neurotransmitera na živčane završetke. Električna ekscitabilnost je posljedica prisutnosti natrijeve struje osjetljive na tetrodotoksin, voltažom upravljane (jaNa) slično onom koji se nalazi u neuronima. Budući da su biofizička svojstva jaNa promjena tijekom razvoja neurona, pitali smo se da li se isto događa i u stanicama okusa. Ovdje smo za proučavanje koristili tehniku ​​snimanja patch-clamp jaNa u stanicama koje osjete natrij gljivičnih papila štakora. Identificirali smo ove stanice korištenjem poznatog blokirajućeg učinka amilorida na ENaC, natrijev (sol) receptor. Zatim, na temelju amplitude od jaNa i morfološkom analizom, podijelili smo natrijeve stanice u dvije široke razvojne skupine, odnosno nezrele i zrele stanice. Utvrdili smo da se: ovisnost o naponu aktivacije i inaktivacije promijenila u prijelazu iz nezrelog u zrelo stanje (depolarizacijski pomak) da se kapacitet membrane značajno smanjio u zrelim stanicama, povećavajući gustoću jaNa u zrelim stanicama pojavila se trajna natrijeva struja, odsutna u nezrelim stanicama. Analiza ekspresije mRNA α-podjedinica voltažiziranih natrijevih kanala u gljivičnim okusnim pupoljcima poduprla je elektrofiziološke podatke. U cjelini, naši nalazi pružaju dokaz za zamjetnu promjenu ekscitabilnosti membrane tijekom razvoja, što je u skladu s ključnom ulogom koju ima električna signalizacija u oslobađanju neurotransmitera od strane zrelih natrijevih stanica.

Sažetak ključnih točaka

Stanice okusa su senzorni receptori koji se stalno mijenjaju dok otkrivaju kemikalije u hrani i komuniciraju s aferentnim živčanim vlaknima.

Naponsko ograničena natrijeva struja (jaNa) je ključna ionska struja za generiranje akcijskih potencijala u potpuno diferenciranim i kemo-osjetljivim stanicama okusa, koje koriste električnu signalizaciju za oslobađanje neurotransmitera.

Ovdje izvještavamo da u stanicama okusa štakora koje su uključene u otkrivanje soli, svojstva jaNa, kao što je naponska ovisnost aktivacije i inaktivacije, prolaze kroz značajne promjene tijekom prijelaza iz nezrelog u zrelo stanje.

Naši rezultati pomažu razumjeti kako stanice okusa dobivaju električnu ekscitabilnost tijekom preokreta, svojstvo kritično za rad kao kemijski detektori koji prenose senzorne informacije u živčana vlakna.


Nasljedni poremećaji voltazijskih natrijevih kanala

Odjel za genetičku medicinu, Odjel za medicinu i farmakologiju, Sveučilište Vanderbilt, Nashville, Tennessee, SAD.

Adresa korespondencije: Alfred L. George Jr., Division of Genetic Medicine, 529 Light Hall, Sveučilište Vanderbilt, Nashville, Tennessee 37232-0275, SAD. Telefon: (615) 936-2660 Fax: (615) 936-2661 E-mail: [email protected]

Različiti nasljedni ljudski poremećaji koji utječu na kontrakciju skeletnih mišića, srčani ritam i funkciju živčanog sustava praćeni su mutacijama u genima koji kodiraju voltažom regulirane natrijeve kanale. Klinička težina ovih stanja kreće se od blage ili čak latentne bolesti do stanja opasnih po život ili onesposobljavanja. Natrijeve kanalopatije bile su među prvim priznatim bolestima ionskih kanala i nastavljaju privlačiti široki klinički i znanstveni interes. Proširujuća baza znanja značajno je unaprijedila naše razumijevanje odnosa strukture-funkcije i genotip-fenotip za voltažom upravljane natrijeve kanale i pružila nove uvide u patofiziološku osnovu uobičajenih bolesti kao što su srčane aritmije i epilepsija.

Naponski upravljani natrijevi kanali (NaVChs) važni su za generiranje i širenje signala u električno podražljivim tkivima poput mišića, srca i živaca. Aktivacija NaVChs u tim tkivima uzrokuje početni porast akcijskog potencijala spoja, što zauzvrat pokreće druge fiziološke događaje koji dovode do mišićne kontrakcije i aktiviranja neurona. NaVChs su također važne mete za lokalne anestetike, antikonvulzive i antiaritmičke lijekove.

Bitna priroda NaVChs je naglašen postojanjem nasljednih poremećaja (natrijevih "kanelopatija") uzrokovanih mutacijama u genima koji kodiraju ove vitalne proteine. Gotovo 20 poremećaja koji utječu na kontrakciju skeletnih mišića, srčani ritam ili funkciju neurona i u rasponu težine od blage ili latentne bolesti do stanja opasnih po život ili onesposobljavanja, povezano je s mutacijama u ljudskom NaVCh geni (tablica 1). Većina natrijevih kanalopatija je dominantno nasljedna, ali neke se prenose recesivnim nasljeđivanjem ili se pojavljuju sporadično. Dodatno, određeni farmakogenetski sindromi su praćeni varijantama NaVCh geni. Kliničke manifestacije ovih poremećaja prvenstveno ovise o obrascu ekspresije mutantnog gena na razini tkiva i biofizičkom karakteru Na.VCh disfunkcija na molekularnoj razini.

Nasljedni poremećaji NaVChs

Ovaj pregled će pokriti trenutno stanje znanja o ljudskim natrijevim kanalopatijama i ilustrirati važne veze između kliničkih, genetskih i patofizioloških značajki glavnih sindroma s odgovarajućim biofizičkim svojstvima mutanta Na.VChs. Početni kratki pregled strukture i funkcije NaVChs će pružiti bitne pozadinske informacije potrebne za razumijevanje nijansi ovih odnosa. Nakon toga slijedi pregled glavnih sindroma, organiziranih po zahvaćenom tkivu. Naglasak će biti stavljen na povezivanje kliničkih fenotipova s ​​obrascima disfunkcije kanala koji su u osnovi patofiziologije ovih stanja.

Natrijevi kanali su heteromultimerni, integralni membranski proteini koji pripadaju superfamiliji ionskih kanala koji se zatvaraju (otvaraju i zatvaraju) promjenama membranskog potencijala (1, 2). Proteini natrijevih kanala iz mozga, mišića i miokarda sisavaca sastoje se od jedne velike (približno 260 kDa) α podjedinice koja stvara pore i složena je s 1 ili 2 manje pomoćne β podjedinice (slika 1). Devet gena (SCN1A, SCN2Aitd.) koji kodiraju različite izoforme α podjedinice i 4 gena β podjedinice (SCN1B, SCN2Bitd.) identificirani su u ljudskom genomu. Mnoge izoforme eksprimiraju se u središnjem i perifernom živčanom sustavu (3), dok skeletni mišići i srčani mišić eksprimiraju ograničeniji NaVCh repertoari ( 4 – 9 ). α podjedinice su konstruirane s 4-strukom simetrijom koja se sastoji od strukturno homolognih domena (D1-D4) od kojih svaka sadrži 6 segmenata koji se protežu kroz membranu (S1-S6) i regiju (S5-S6 pora) koja kontrolira ionsku selektivnost i permeaciju (slika 1). Segment S4, koji funkcionira kao senzor napona (10, 11), je amfipatski s višestrukim bazičnim aminokiselinama (arginin ili lizin) na svakoj trećoj poziciji okružen hidrofobnim ostacima. Svaka domena nalikuje na cijelu podjedinicu kalijevog kanala s naponom, kao i na primitivnu bakterijsku NaVCh (12).

Struktura i genomski položaj ljudskog NaVChs. (A) Jednostavan model koji predstavlja transmembransku topologiju α i β NaVCh podjedinice. Označene su strukturne domene koje posreduju ključna funkcionalna svojstva. (B) Kromosomski položaj ljudskih gena koji kodiraju α (crvena) i β (plava) podjedinice u cijelom genomu. Zvjezdica uz naziv gena označava povezanost s nasljednom ljudskom bolešću. Dvostruka zvjezdica označava povezanost s fenotipovima miša.

NaVChs se prebacuje između 3 funkcionalna stanja ovisno o membranskom potencijalu (slika 2) (13). U ekscitabilnim membranama, iznenadna depolarizacija membrane uzrokuje brz porast lokalne propusnosti Na + zbog otvaranja (aktiviranje) od NaVChs iz njihovog zatvorenog stanja mirovanja. Da bi se to dogodilo, senzori napona (4 segmenta S4) unutar NaVCh protein mora se kretati u smjeru prema van, potaknut promjenom membranskog potencijala, a zatim prevesti ovu konformacijsku energiju u druge strukture (najvjerojatnije segmente S6) koje se maknu s puta dolaznih Na + iona. Ovo povećanje propusnosti Na + uzrokuje iznenadnu depolarizaciju membrane koja karakterizira početnu fazu akcijskog potencijala. Normalno, aktivacija NaVChs je prolazan zbog inaktivacija, još jedan proces ulaska posredovan strukturama smještenim na citoplazmatskoj strani proteina kanala (uglavnom D3-D4 linker). NaVChs se ne mogu ponovno otvoriti dok se membrana ne repolarizira i ne prođu oporavak od inaktivacije. Repolarizacija membrane postiže se brzom inaktivacijom NaVChs i povećava se aktivacijom naponski upravljanih kalijevih kanala. Tijekom oporavka od inaktivacije, NaVChs se može podvrgnuti deaktiviranje, prijelaz iz otvorenog u zatvoreno stanje (14). Aktivacija, inaktivacija i oporavak od inaktivacije događa se unutar nekoliko milisekundi. Osim ovih brzih prijelaza gajta, NaVChs su također osjetljivi na zatvaranje sporijim procesima inaktivacije (polagana inaktivacija) ako membrana dulje vrijeme ostane depolarizirana (15). Ovi sporiji događaji mogu pridonijeti određivanju dostupnosti aktivnih kanala u različitim fiziološkim uvjetima.

Funkcionalna svojstva NaVChs. (A) Shematski prikaz NaVCh prolazi kroz glavne prijelaze na ulazu. (B) Snimanje Naponske stezaljke NaVCh aktivnost kao odgovor na depolarizaciju membrane. Skretanje strujnog traga prema dolje (crveno) odgovara kretanju Na + prema unutra.

Poremećaji u funkciji mišićnog NaVChs može utjecati na sposobnost kontrakcije ili opuštanja skeletnih mišića. Dva simptoma su karakteristična za mišićnu membranu (sarkolemu) NaVCh disfunkcija, miotonija i periodična paraliza (16). Miotoniju karakterizira odgođena relaksacija mišića nakon iznenadne snažne kontrakcije i povezana je s repetitivnim stvaranjem akcijskog potencijala, manifestacijom sarkolemalne hiperekscitabilnosti. Suprotno tome, periodična paraliza predstavlja prolazno stanje hipoekscitabilnosti ili neuzbudljivosti u kojem se akcijski potencijali ne mogu generirati ili propagirati.

Periodična paraliza i miotonija. Periodična paraliza je karakterizirana epizodičnom slabošću ili paralizom voljnih mišića koja se javlja uz normalan neuromišićni prijenos i u odsutnosti bolesti motornih neurona. Bolesnici s obiteljskom periodičnom paralizom prisutnim tipično u djetinjstvu (17). Napadi slabosti često su povezani s promjenama u koncentraciji kalija u serumu (K+) kao rezultat nagle preraspodjele intracelularnog i izvanstaničnog K+. Ovaj klinički epifenomen čini osnovu za klasificiranje periodične paralize kao hipokalemične, hiperkalemične ili normokalemične. U paramyotonia congenita, dominantni simptom je hladnom izazvana ukočenost i slabost mišića (17, 18). Miotoniju pogoršanu kalijem karakterizira miotonija bez slabosti i pogoršanja simptoma nakon uzimanja K+ (19). Općenito, ovi poremećaji nisu povezani s onesposobljavajućim mišićnom distrofijom, iako se kronična slabost može razviti u nekih osoba s dugotrajnom hiperkalemijskom periodičnom paralizom (20).

Elektrofiziološke studije in vitro utvrdile su da su i miotonija i periodična paraliza povezane s abnormalnom vodljivošću natrijeve membrane mišićnih stanica (21), a ovi nalazi ukazuju na SCN4A kao najvjerojatniji gen kandidata. Studije genetskog povezivanja potvrdile su ovu hipotezu (22-24). Hiperkalemijska periodična paraliza, paramyotonia congenita i miotonija pogoršana kalijem povezani su s missense mutacijama u SCN4A. Postoje 2 prevladavajuće mutacije povezane s hiperkalemijskom periodičnom paralizom (T704M i M1592V), a one se javljaju neovisno u nepovezanim rodovima (20, 25). Alelna raznolikost je veća za paramyotonia congenita i miotoniju pogoršanu kalijem (26-32). In addition, approximately 15% of patients with genotype-defined hypokalemic periodic paralysis carry SCN4A mutations ( 33 ). Patients with SCN4A mutations may present rarely with life-threatening myotonic reactions upon exposure to succinylcholine resembling the syndrome of malignant hyperthermia susceptibility ( 34 , 35 ). In 1 report, congenital myasthenia has been linked to SCN4A mutations ( 36 ).

Characterization of SCN4A mutations and pathophysiology. Using heterologously expressed recombinant NaVChs, several laboratories have characterized the biophysical properties of many mutations associated with either periodic paralysis or various myotonic disorders. These studies demonstrated that variable defects in the rate or extent of inactivation occur in virtually all cases. Mutations associated with hyperkalemic periodic paralysis exhibit incomplete inactivation leading to a small level (1–2% of peak current) of persistent Na + current that is predicted to cause sustained muscle fiber depolarization (Figure 3) ( 37 , 38 ). Sustained depolarization will cause the majority of NaVChs (mutant and wild type) to become inactivated, and this explains conduction failure and electrical inexcitability observed in skeletal muscle during an attack of periodic paralysis ( 39 , 40 ). By this mechanism, mutant NaVChs exert an indirect dominant-negative effect on normal channels. In addition, some, but not all, mutations associated with hyperkalemic periodic paralysis have impaired slow inactivation ( 41 ), and this may contribute to sustaining the effect of persistent Na + current ( 42 ).

A common form of defective inactivation exhibited by mutant NaVChs associated with hyperkalemic periodic paralysis, long QT syndrome, and inherited epilepsy. The defect is caused by incomplete closure of the inactivation gate (left panel) resulting in an increased level of persistent current (right panel, red trace) as compared with NaVChs with normal inactivation (black trace).

SCN4A mutations in the myotonic disorders slow the rate of inactivation, speed recovery from inactivation, and slow deactivation ( 30 , 43 – 47 ). These biophysical defects are predicted to lengthen the duration of muscle action potentials ( 48 ). Prolongation of action potentials along T-tubule membranes will exaggerate the local rise in extracellular K + concentration by efflux through persistently activated potassium channels. Extracellular K + in T-tubules exerts a depolarizing effect on the resting membrane potential, increasing the probability of an aberrant afterdepolarization. A large afterdepolarization can trigger spontaneous action potentials in adjacent surface membranes, which in turn cause persistent muscle contraction and delayed relaxation, the physiological hallmarks of myotonia (Figure 4) ( 49 ).

Differences between normal and myotonic muscle action potentials. (A) Generation of action potential spikes during electrical stimulation (horizontal blue line and square wave) of a normal muscle fiber. Contraction occurs during action potential firing, followed by muscle relaxation when stimulation ceases. (B) Action potentials in myotonic muscle during and immediately after electrical stimulation. An afterdepolarization triggers spontaneous action potentials that fire after termination of the electrical stimulus (myotonic activity).

Treatment strategies for muscle sodium channelopathies. Pharmacological treatment for periodic paralysis with carbonic anhydrase inhibitors is often successful, but the mechanism of action is poorly understood ( 50 , 51 ). Certain local anesthetic/antiarrhythmic agents have antimyotonic activity and are sometimes useful treatments for nondystrophic myotonias ( 52 , 53 ). These drugs are effective because of their ability to interrupt rapidly conducted trains of action potentials through their use-dependent NaVCh-blocking action. Mexiletine is the most commonly used antimyotonic agent, and there have been in vitro studies demonstrating its effectiveness ( 54 ), but there have been no clinical trials comparing this agent with either placebo or other treatments. A more potent NaVCh blocker, flecainide, may also have utility in severe forms of myotonia that are resistant to mexiletine ( 55 ). The efficacy of flecainide for treating myotonia associated with certain SCN4A mutations may be greatest when there is a depolarizing shift of the steady-state fast inactivation curve for the mutant channel, whereas mutations that induce hyperpolarizing shifts in this curve are predicted to have greater sensitivity to mexiletine ( 56 ). Long-term treatment of myotonia with NaVCh blockers is often limited by drug side effects.

In the heart, NaVChs are essential for the orderly progression of action potentials from the sinoatrial node, through the atria, across the atrioventricular node, along the specialized conduction system of the ventricles (His-Purkinje system), and ultimately throughout the myocardium to stimulate rhythmic contraction. Mutacije u SCN5A, the gene encoding the principal NaVCh α subunit expressed in the human heart, cause inherited susceptibility to ventricular arrhythmia (congenital long QT syndrome, idiopathic ventricular fibrillation) ( 57 – 59 ), impaired cardiac conduction ( 60 ), or both ( 61 – 65 ). SCN5A mutations may also manifest as drug-induced arrhythmias ( 66 ), sudden infant death syndrome (SIDS) ( 67 , 68 ), and other forms of arrhythmia susceptibility ( 69 ).

Inherited arrhythmia syndromes: long QT and Brugada. Congenital long QT syndrome (LQTS), an inherited condition of abnormal myocardial repolarization, is characterized clinically by an increased risk of potentially fatal ventricular arrhythmias, especially torsade de pointes ( 70 , 71 ). The syndrome is transmitted most often in families as an autosomal dominant trait (Romano-Ward syndrome) and less commonly as an autosomal recessive disease combined with congenital deafness (Jervell and Lange-Nielsen syndrome). The syndrome derives its name from the characteristic prolongation of the QT interval on surface ECGs of affected individuals, a surrogate marker of an increased ventricular action potential duration and abnormal myocardial repolarization. Approximately 10% of LQTS cases are caused by SCN5A mutations, whereas the majority of Romano-Ward subjects harbor mutations in 2 cardiac potassium channel genes (KCNQ1 i HERG) ( 72 , 73 ). Triggering factors associated with arrhythmic events are different among genetic subsets of LQTS. SCN5A mutations often produce distinct clinical features including bradycardia, and a tendency for cardiac events to occur during sleep or rest ( 74 , 75 ).

Mutacije u SCN5A have also been associated with idiopathic ventricular fibrillation, including Brugada syndrome ( 59 , 76 ) and sudden unexplained death syndrome (SUDS) ( 77 , 78 ). Individuals with Brugada syndrome have an increased risk for potentially lethal ventricular arrhythmias (polymorphic ventricular tachycardia or fibrillation) without concomitant ischemia, electrolyte abnormalities, or structural heart disease. Individuals with the disease often exhibit a characteristic ECG pattern consisting of ST elevation in the right precordial leads, apparent right bundle branch block, but normal QT intervals ( 79 ). Administration of NaVCh-blocking agents (i.e., procainamide, flecainide, ajmaline) may expose this ECG pattern in latent cases ( 80 ). Inheritance is autosomal dominant with incomplete penetrance and a male predominance. A family history of unexplained sudden death is typical. SUDS is a very similar syndrome that causes sudden death, typically during sleep, in young and middle-aged males in Southeast Asian countries ( 81 – 83 ).

Disorders of cardiac conduction. Mutacije u SCN5A are also associated with heterogeneous familial disorders of cardiac conduction manifest as impaired atrioventricular conduction (heart block), slowed intramyocardial conduction velocity, or atrial inexcitability (atrial standstill) ( 60 , 62 , 84 , 85 ). The degree of impaired cardiac conduction may progress with advancing age and is generally not associated with prolongation of the QT interval or ECG changes consistent with Brugada syndrome. Heart block in these disorders is usually the result of conduction slowing in the His-Purkinje system. In most cases, inheritance of the phenotype is autosomal dominant. By contrast, atrial standstill has been reported to occur either as a recessive disorder of SCN5A (congenital sick sinus syndrome) ( 85 ) or by digenic inheritance of a heterozygous SCN5A mutation with a promoter variant in the connexin-40 gene ( 84 ).

Mutacije u SCN5A may also cause more complex phenotypes representing combinations of LQTS, Brugada syndrome, and conduction system disease. There have been documented examples of LQTS combined with Brugada syndrome ( 63 ) or congenital heart block ( 86 , 87 ), and cases of Brugada syndrome with impaired conduction ( 88 ). In 1 unique family, all 3 clinical phenotypes occur together ( 65 ). SCN5A mutations have also been discovered in families segregating impaired cardiac conduction, supraventricular arrhythmia, and dilated cardiomyopathy ( 64 , 89 ). Certain mutations may manifest different phenotypes in different families.

Characterization of SCN5A mutations and arrhythmogenesis. The clinical heterogeneity associated with SCN5A mutations is partly explained by corresponding differences in the degree and characteristics of channel dysfunction. In congenital LQTS, SCN5A mutations have a dominant gain-of-function phenotype at the molecular level. Specifically, most mutant cardiac NaVChs associated with LQTS exhibit a characteristic impairment of inactivation, leading to persistent inward Na + current during prolonged membrane depolarizations (Figure 3) ( 90 – 92 ). A general slowing of inactivation may be present in mutations associated with severe LQTS ( 93 ), while some mutations alter voltage-dependence of activation and inactivation but do not have measurable non-inactivating current ( 94 ). Persistent Na + current during the cardiac action potential explains abnormal myocardial repolarization in LQTS ( 95 ). By contrast with nerve and muscle, cardiac action potentials last several hundred milliseconds because of a prolonged depolarization phase (plateau), the result of opposing inward (mainly Na + and Ca 2+ ) and outward (K + ) ionic currents. Repolarization occurs when net outward current exceeds net inward current. Non-inactivating behavior of mutant cardiac NaVChs will shift this balance toward inward current and delay onset of repolarization, thus lengthening the action potential duration and the corresponding QT interval (Figure 5). Delayed repolarization predisposes to ventricular arrhythmias by exaggerating the dispersion of refractoriness throughout the myocardium and increasing the probability of early afterdepolarization, a phenomenon caused largely by reactivation of calcium channels during the action potential plateau ( 96 ). Both of these phenomena create conditions that allow electrical signals from depolarized regions of the heart to prematurely re-excite adjacent myocardium that has already repolarized, the basis for a reentrant arrhythmia. Additional proof of the role of cardiac NaVCh mutations in LQTS has come from studies of mice heterozygous for a prototypic LQTS SCN5A mutation (delKPQ). These mice have spontaneous life-threatening ventricular arrhythmias and a persistent Na + current in cardiac myocytes ( 97 ). SCN5A mutations associated with SIDS also exhibit this biophysical phenotype this suggests a pathophysiological relationship with LQTS ( 67 , 68 ).

Electrophysiological basis for LQTS. (A) Relationship of surface ECG (top) with a representative cardiac action potential (bottom). The QT interval approximates the action potential duration. Individual ionic currents responsible for different phases of the action potential are labeled. (B) Prolongation of the QT interval and corresponding abnormal cardiac action potential (blue) resulting from persistent sodium current. jaca, calcium current jaK1, inward rectifier current jaKr, rapid component of delayed rectifier current jaKs, slow component of delayed rectifier current jaNa, sodium current jaDO, transient outward current.

The proposed cellular basis of Brugada syndrome involves a primary reduction in myocardial sodium current that exaggerates differences in action potential duration between the inner (endocardium) and outer (epicardium) layers of ventricular muscle ( 96 , 98 , 99 ). These differences exist initially because of an unequal distribution of potassium channels responsible for the transient outward current (jaDO), a repolarizing current more prominent in the epicardial layer that contributes to the characteristic spike and dome shape of the cardiac action potential. Reduced myocardial Na + current will cause disproportionate shortening of epicardial action potentials because of unopposed jaDO, leading to an exaggerated transmural voltage gradient, dispersion of repolarization, and a substrate promoting reentrant arrhythmias (Figure 6). This hypothesis has been validated using animal models and computational methods. The theory helps explain the characteristic ECG pattern observed in Brugada syndrome and the effects of NaVCh-blocking agents to aggravate the phenotype.

Electrophysiological basis for Brugada syndrome. (A) Comparison of endocardial and epicardial action potentials in normal heart. The epicardial action potential is shorter because of large transient outward current. (B) Endocardial and epicardial action potentials in Brugada syndrome. Reduced sodium current causes disproportionate shortening of epicardial action potentials with resulting exaggeration of the transmural voltage gradient (horizontal double arrow).

Consistent with reduced sodium current as the primary pathophysiological event in Brugada syndrome, many SCN5A mutations associated with this disease cause frameshift errors, splice site defects, or premature stop codons ( 59 , 100 ) that are predicted to produce nonfunctional channels. Furthermore, some missense mutations have also been demonstrated to be nonfunctional because of either impaired protein trafficking to the cell membrane or presumed disruption of Na + conductance through the channel ( 101 – 104 ). However, other missense mutations associated with Brugada syndrome are functional but have biophysical defects predicted to reduce channel availability, such as altered voltage-dependence of activation, more rapid fast inactivation, and enhanced slow inactivation ( 105 – 107 ).

Pathophysiology of SCN5A dysfunction in cardiac conduction disorders. Defects in cardiac NaVCh function due to mutations associated with disorders of cardiac conduction exhibit more complex biophysical properties ( 61 , 62 ). Mutations causing isolated conduction defects have generally been observed to cause reduced NaVCh availability as a consequence of mixed gating disturbances. In the case of a Dutch family segregating a specific missense allele (G514C), the mutation causes unequal depolarizing shifts in the voltage-dependence of activation and inactivation such that a smaller number of channels are activated at typical threshold voltages ( 61 ). Computational modeling of these changes supports reduced conduction velocity, but the level of predicted NaVCh loss is insufficient to cause shortened epicardial action potentials, which explains why these individuals do not manifest Brugada syndrome. Two other SCN5A mutations causing isolated conduction disturbances (G298S and D1595N) are also predicted to reduce channel availability by enhancing the tendency of channels to undergo slow inactivation in combination with a complex mix of gain- and loss-of-function defects ( 62 ). However, other alleles exhibiting complete loss of function have also been associated with isolated cardiac conduction disease ( 108 , 109 ) without the Brugada syndrome. These observations suggest that additional host factors may contribute to determining whether a mutation will manifest as arrhythmia susceptibility or impaired conduction. This idea is supported by the observation that a single SCN5A mutation causes either Brugada syndrome or isolated conduction defects in different members of a large French family ( 88 ).

Biophysical properties of mutant cardiac NaVChs associated with combined phenotypes are also more complex. An in-frame insertion mutation (1795insD) has been identified in a family segregating both LQTS and Brugada syndrome ( 63 ). This mutation causes an inactivation defect resulting in persistent Na + current characteristic of most other SCN5A mutations associated with LQTS, but it also confers enhanced slow inactivation with reduced channel availability that is more characteristic of Brugada syndrome ( 63 ). The 2 biophysical abnormalities are predicted to predispose to ventricular arrhythmia at extremes of heart rate by different mechanisms ( 110 ). Whereas persistent current will prolong the QT interval to a greater degree at slow heart rates, enhanced slow inactivation predisposes myocardial cells to activity-dependent loss of NaVCh availability at fast rates. In another unusual case, deletion of lysine-1500 in SCN5A was associated with the unique combination of LQTS, Brugada syndrome, and impaired conduction in the same family ( 65 ). The mutation impairs inactivation, resulting in a persistent Na + current, and reduces NaVCh availability by opposing shifts in voltage-dependence of inactivation and activation.

Unlike LQTS, Brugada syndrome, and isolated cardiac conduction disease, in which affected individuals are heterozygous for single NaVCh mutations, there are cases in which individuals with severe impairments in cardiac conduction have inherited mutations from both parents. Lupoglazoff et al. described a child homozygous for a missense SCN5A allele (V1777M) who exhibited LQTS with rate-dependent atrioventricular conduction block ( 86 ). In a separate report, probands from 3 families exhibited perinatal sinus bradycardia progressing to atrial standstill (congenital sick sinus syndrome) and were found to have compound heterozygosity for mutations in SCN5A ( 85 ). Compound heterozygosity in SCN5A has also been observed in 2 infants with neonatal wide complex tachycardia and a generalized cardiac conduction defect ( 111 ). In each case of compound heterozygosity, individuals inherited 1 nonfunctional or severely dysfunctional mutation from 1 parent and a second allele with mild biophysical defects from the other parent. Interestingly, the parents who were carriers of single mutations were asymptomatic, which suggests that they had subclinical disease or other host factors affording protection. These unusually severe examples of SCN5A-linked cardiac conduction disorders illustrate the clinical consequence of nearly complete loss of NaVCh function. Complete absence of the murine Scn5a locus results in embryonic lethality ( 112 ), and it is likely that homozygous deletion or inactivation of human SCN5A is also not compatible with life.

Treatment strategies for cardiac sodium channelopathies. Specific therapeutic options for SCN5A-linked disorders are limited. β-Adrenergic blockers remain the first line of therapy in LQTS albeit this treatment strategy may be less efficacious in the setting of SCN5A mutations ( 113 ). Clinical and in vitro evidence suggests that mexiletine may counteract the aberrant persistent Na + current and shorten the QT interval ( 114 , 115 ) in SCN5A mutation carriers, although there are no data indicating an improvement in mortality. Mexiletine has also been demonstrated to rescue trafficking defective SCN5A mutants in vitro ( 116 ). Flecainide has also been observed to shorten QT intervals in the setting of certain SCN5A mutations ( 117 , 118 ), but some have raised concern over the safety of this therapeutic strategy ( 119 ). Class III–type antiarrhythmic agents (quinidine, sotalol) may be beneficial in Brugada syndrome ( 120 , 121 ). Device therapy (implantable defibrillator for LQTS and Brugada syndrome pacemaker for conduction disorders) is also an important treatment option.

Neuronal NaVChs are critical for the generation and propagation of action potentials in the central and peripheral nervous system. Most of the 13 genes encoding NaVCh α or β subunits are expressed in the brain, peripheral nerves, or both ( 1 ). In addition to their critical physiological function, neuronal NaVChs serve as important pharmacological targets for anticonvulsants and local anesthetic agents ( 122 , 123 ). Their roles in genetic disorders including a variety of inherited epilepsy syndromes and a rare painful neuropathy have been revealed during the past 7 years.

Sodium channels and inherited epilepsies. Genetic defects in genes encoding 2 pore-forming α subunits (SCN1A i SCN2A) and the accessory β1 subunit (SCN1B) are responsible for a group of epilepsy syndromes with overlapping clinical characteristics but divergent clinical severity ( 124 – 129 ). Generalized epilepsy with febrile seizures plus (GEFS+) is usually a benignt disorder characterized by the frequent occurrence of febrile seizures in early childhood that persist beyond age 6 years, and epilepsy later in life associated with afebrile seizures with multiple clinical phenotypes (absence, myoclonic, atonic, myoclonic-astatic). Mutations in 3 neuronal NaVCh genes (SCN1A, SCN1B, i SCN2A) and a GABA receptor subunit (GABRG2) may independently cause GEFS+ or very similar disorders ( 130 , 131 ). Mutacije u SCN2A have also been associated with benign familial neonatal-infantile seizures (BFNIS), a seizure disorder of infancy that remits by age 12 months with no long-term neurological sequelae ( 129 , 132 ). Interestingly, despite expression of SCN1A i SCN1B in the heart ( 9 ), there are no apparent cardiac manifestations associated with these disorders.

By contrast, severe myoclonic epilepsy of infancy (SMEI) and related syndromes have severe neurological sequelae. The diagnosis of SMEI is based on several clinical features, including (a) appearance of seizures, typically generalized tonic-clonic, during the first year of life, (b) impaired psychomotor development following onset of seizures, (c) occurrence of myoclonic seizures, (d) ataxia, and (e) poor response to antiepileptic drugs ( 133 ). Two designations, borderline SMEI (SMEB) ( 133 , 134 ) and intractable childhood epilepsy with frequent generalized tonic-clonic seizures (ICEGTC) ( 128 ), have been assigned to patients with a condition resembling SMEI but in whom myoclonic seizures are absent and less severe psychomotor impairment is evident. SCN1A mutations have been identified in probands affected by all of these conditions.

More than 100 SCN1A mutations have been identified, with missense mutations being most common in GEFS+ ( 125 , 135 – 139 ) and more deleterious alleles (nonsense, frameshift) representing the majority of SMEI mutations ( 126 , 140 , 141 ). Only missense mutations in SCN1A have been reported for patients diagnosed with either ICEGTC or SMEB. There are rare reports of families segregating both GEFS+ and either SMEI or ICEGTC ( 128 ). The overlapping phenotypes and molecular genetic etiologies among the SCN1A-linked epilepsies suggest that they represent a continuum of clinical disorders ( 142 ).

Sodium channel dysfunction and epileptogenesis. The first human NaVCh mutation associated with an inherited epilepsy (GEFS+) was discovered in SCN1B encoding the β1 accessory subunit ( 124 ). However, mutations in this gene have very rarely been associated with inherited epilepsy. Only 2 SCN1B mutations have been described to date, including a missense allele (C121W) and a 5–amino acid deletion (del70–74) ( 124 , 143 ). Both mutations occur in an extracellular Ig-fold domain of the β1 subunit that is important for functional modulation of NaVCh α subunits ( 144 , 145 ) and mediates protein-protein interactions critical for NaVCh subcellular localization in neurons ( 146 ). The C121W mutation disrupts a conserved disulfide bridge in this domain, and functional expression studies demonstrated a failure of the mutant to normally modulate the functional properties of recombinant brain NaVChs ( 124 , 147 ). These findings and the observed seizure disorder in mice with targeted deletion of murine β1 subunit indicate that SCN1B loss of function explains the epilepsy phenotype ( 148 ). Functional characterization of the SCN1B deletion allele has not been reported.

Expression studies of α subunit mutations have demonstrated a wide range of functional disturbances. Early findings indicated that SCN1A mutations causing GEFS+ promote a gain of function, while mutations associated with SMEI are predicted to disable channel function. Two studies have demonstrated that increased persistent Na + current is caused by several GEFS+ mutations ( 149 , 150 ). This behavior is reminiscent of the channel dysfunction associated with 2 other human sodium channelopathies discussed above, hyperkalemic periodic paralysis and LQTS (Figure 3). Non-inactivating Na + current may facilitate neuronal hyperexcitability by reducing the threshold for action potential firing. However, not all GEFS+ mutations exhibit increased persistent current. For example, a shift in the voltage-dependence of inactivation to more depolarized potentials has been observed for 2 other GEFS+ mutations (T875M and D1866Y). This functional change is predicted to increase channel availability at voltages near the resting membrane potential and is sufficient to enhance excitability in a simple computational model of a neuronal action potential ( 150 ). This may be an oversimplification, as T875M also exhibits enhanced slow inactivation, which is predicted to decrease channel availability. For D1866Y, the changed voltage-dependence of inactivation was attributed to decreased modulation by the β1 subunit, a novel epilepsy-associated mechanism. Other GEFS+ mutations have been described that are nonfunctional (V1353L, A1685V) or exhibit depolarizing shifts in voltage-dependence of activation (I1656M, R1657C) predicted to reduce channel activity ( 151 ). These findings indicate that more than 1 biophysical mechanism accounts for seizure susceptibility in GEFS+.

Najviše SCN1A mutations associated with SMEI are predicted to produce nonfunctional channels by introducing premature termination or frameshifts into the coding sequence. This observation led to the notion that SMEI stems from SCN1A haploinsufficiency. Consistent with this idea was the finding that some missense mutations associated with SMEI are nonfunctional ( 151 , 152 ). However, a simple dichotomy of gain versus loss of function to explain clinical differences between GEFS+ and SMEI is not consistent with recent observations. As mentioned above, some GEFS+ mutations exhibit loss-of-function characteristics. More recently, 2 SMEI missense alleles (R1648C and F1661S) were demonstrated to encode functional channels that exhibit a mixed pattern of biophysical defects consistent with either gain (persistent Na + current) or loss (reduced channel density, altered voltage-dependence of activation and inactivation) of function ( 152 ). The precise cellular mechanism by which this constellation of biophysical disturbances leads to epilepsy is uncertain and motivates further experiments in animal models to help determine the impact of NaVCh mutations.

SCN9A and painful inherited neuropathy. Mutations in another neuronal NaVCh gene, SCN9A, encoding an α subunit isoform expressed in sensory and sympathetic neurons, have been discovered in patients with familial primary erythermalgia, a rare autosomal dominant disorder characterized by recurrent episodes of severe pain, redness, and warmth in the distal extremities. Two missense SCN9A mutations were recently identified in Chinese patients ( 153 ). Both mutations cause a hyperpolarizing shift in the voltage-dependence of channel activation and slow the rate of deactivation ( 154 ). This combination of biophysical defects is predicted to confer hyperexcitability on peripheral sensory and sympathetic neurons, accounting for the episodic pain and vasomotor symptoms characteristic of the disease. Consistent with overactive NaVChs are anecdotal reports of improved symptoms during treatment with local anesthetic agents (i.e., lidocaine, bupivacaine) or mexiletine ( 155 – 157 ).

NaVChs are important from many perspectives. Their recognized importance in the physiology and pharmacology of nerve, muscle, and heart is now further emphasized by their role in inherited disorders affecting these tissues. The sodium channelopathies provide outstanding illustrations of the delicate balances that maintain normal operation of critical physiological events such as muscle contraction and conduction of electrical signals.

Despite the extensive array of disorders listed in Table 1, it is likely that other inherited or pharmacogenetic disorders are caused by mutations or polymorphisms in NaVCh genes. Only 6 of the 13 known genes encoding NaVCh subunits have been linked to human disease. However, spontaneous or engineered disruption of 2 other genes (Scn8a i Scn2b) causes neurological phenotypes in mice ( 158 – 160 ), suggesting that other human sodium channelopathies might exist. Establishing new genotype-phenotype relationships, exploring pathophysiology, and developing new treatment strategies remain exciting challenges for the future.

The author is supported by grants from the NIH (NS32387 and HL68880) and is the recipient of a Javits Neuroscience Award from the National Institute of Neurological Disorders and Stroke.

Nonstandard abbreviations used: GEFS+, generalized epilepsy with febrile seizures plus ICEGTC, intractable childhood epilepsy with frequent generalized tonic-clonic seizures LQTS, long QT syndrome NaVCh, voltage-gated sodium channel SIDS, sudden infant death syndrome SMEB, borderline severe myoclonic epilepsy of infancy SMEI, severe myoclonic epilepsy of infancy SUDS, sudden unexplained death syndrome.

Sukob interesa: The author has declared that no conflict of interest exists.


Control of excitability can occur at the genomic level by the regulation of transcription of channel genes. The expression of Na + channels is developmentally regulated and tissue restricted. Patterns of electrical activity can also feed back upon and influence transcription: for example, seizures alter Na + channel gene expression in the brain. Denervation induces the expression of the cardiac isoform of the channel in skeletal muscle, while transiently suppressing expression of the mature skeletal muscle isoform ( Kallen, Sheng, Yang, Chen, Rogart & Barchi, 1990 Yang, Sladky, Kallen & Barchi, 1991 ). Chronic exposure to antiarrhythmic drugs which block Na + channels can increase the steady-state levels of Na + channel mRNA, in a manner that would tend to counteract the effects of channel blockade ( Duff, Offord, West & Catterall, 1992 ).

The mechanisms controlling Na + channel gene expression are only just beginning to be understood. Expression of the brain type II Na + channel is restricted to neurons by a transcription silencer known as REST ( Chong et al. 1995 Eggen & Mandel, 1997 Tapia-Ramirez, Eggen, Peral-Rubio, Toledo-Aral & Mandel, 1997 ). REST is a transcription factor with C2H2 zinc finger motifs homologous to the Drosophila repressor Krüppel that binds to a specific silencer element (RE-1) in the promoter of the brain II channel. REST is found in most tissues its absence in neurons is what permits expression of the brain II isoform.


The Absolute Refractory Period:

Just after the neuron has generated an action potential, it cannot generate another one. Many sodium channels are inactive and will not open, no matter what voltage is applied to the membrane. Most potassium channels are open. This period is called the absolute refractory period. The neuron cannot generate an action potential because sodium cannot move in through inactive channels and because potassium continues to move out through open voltage-gated channels. A neuron cannot generate an action potential during the absolute refractory period.


Summary of Process

Nerve impulses consist of action potentials fired in neurons. Understanding the process of firing and recovering from an action potential is necessary to understanding nerve signaling. This summary divides the action potential into stages, found below. Follow the links below to the Glossary for questions about unknown terms.

Before the action potential:

  • The neuron receives signals via its dendrites.
  • Signals that fail to raise the potential to the threshold potential cause no change.

The start of the action potential:

  • An incoming signal reaches threshold and opens the first set of voltage-gated sodium channels.
  • Positively-charged sodium pours into the cell, causing depolarization.

(“Voltage-gated channels,” 2016)

  • Depolarization in the first region causes the second set of sodium-channels to be depolarized to threshold.

The end of the action potential:

  • The action potential reaches the axon terminal.
  • This signals neurotransmitter to be launched into the synaptic cleft.
  • The neurotransmitter diffuses across the gap to bind to receptor proteins on the next cell.

Recovering from the action potential:

  • Once the neuron depolarizes to about +30 mV, the cell begins repolarization.
  • Sodium channels close and potassium channels open.
  • Positively-charged potassium leaves the cell, causing the potential to decrease.
  • Sodium channels are inactivated during repolarization so an action potential can’t re-fire.
  • The potassium exiting the cell will drive the membrane potential down to about -80 mV.

Returning to resting membrane potential:

  • Sodium channels can now be reopened, but it is more difficult.
  • A new signal must overcome the potential from threshold to resting plus the additional hyperpolarized potential.
    and natural diffusion bring the cell back to resting potential.

From the resting membrane potential, this process may repeat as the neuron receives new stimuli. To learn more about the process of beginning and transmitting a signal along a neuron, there is a video on the next tab and the Glossary of Terms contains more thorough definitions of the individual components described here. To test your knowledge of the nerve impulse, go to the Quiz tab.


Gledaj video: Depolarizing Neuromuscular Blockers for General Anesthesia during surgery (Lipanj 2022).


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