Nonfunctional Na _V 1.1 familial hemiplegic migraine mutant transformed into gain of function by partial rescue of folding defects

Abstract

Familial hemiplegic migraine (FHM) is a rare subtype of migraine with aura. Mutations causing FHM type 3 have been identified in SCN1A, the gene encoding the Na v 1.1 Na + channel, which is also a major target of epileptogenic mutations and is particularly important for the excitability of GABAergic neurons. However, functional studies of Na V 1.1 FHM mutations have generated controversial results. In particular, it has been shown that the Na V 1.1-L1649Q mutant is nonfunctional when expressed in a human cell line because of impaired plasma membrane expression, similarly to Na V 1.1 mutants that cause severe epilepsy, but we have observed gain-offunction effects for other Na V 1.1 FHM mutants. Here we show that Na V 1.1-L1649Q is nonfunctional because of folding defects that are rescuable by incubation at lower temperatures or coexpression of interacting proteins, and that a partial rescue is sufficient for inducing an overall gain of function because of the modifications in gating properties. Strikingly, when expressed in neurons, the mutant was partially rescued and was a constitutive gain of function. A computational model showed that 35% rescue can be sufficient for inducing gain of function. Interestingly, previously described folding-defective epileptogenic Na V 1.1 mutants show loss of function also when rescued. Our results are consistent with gain of function as the functional effect of Na V 1.1 FHM mutations and hyperexcitability of GABAergic neurons as the pathomechanism of FHM type 3.spreading depression | Dravet syndrome | generalized epilepsy with febrile seizures plus | calmodulin | ankyrin

Pharmacology of the Na v 1.1 domain IV voltage sensor reveals coupling between inactivation gating processes

“…Strikingly similar phenotypes are exhibited by Na v 1.1 mutants linked to migraine (5)(6)(7)9). Of note, migraine mutations tend to cluster around Na v 1.1 VSDIV, lending further support to the utility of Hm1a for phenocopying FHM3.…”

“…Loss-of-function mutations in Na v 1.1 produce epilepsy syndromes, likely reflecting the critical importance of this Na v channel subtype in controlling the excitability of inhibitory interneurons in the brain (2)(3)(4). In contrast, gain-offunction mutations that diminish Na v 1.1 fast inactivation are linked to familial hemiplegic migraine type 3 (5)(6)(7)(8)(9). Therefore, Na v 1.1 has emerged as an important therapeutic target for brain disorders and, more recently, mechanical pain (10).…”

Three epilepsy-associated GABRG2 missense mutations at the γ+/β− interface disrupt GABAA receptor assembly and trafficking by similar mechanisms but to different extents

“…Trafficking-deficient mutant proteins have been shown to be rescued by lower temperature and molecular or pharmacological chaperones (Denning et al, 1992; Thomas et al, 2003; Varga et al, 2008; Guo et al, 2012; Saxena et al, 2012; Cestele et al, 2013). Here we found that decreased incubation temperature (30°C) increased surface and total levels of wildtype and mutant γ2L subunits.…”

Genetic Epilepsy Syndromes Without Structural Brain Abnormalities: Clinical Features and Experimental Models

“…This mechanism may generate phenotypic variability and also possibly be used in the development of therapeutic approaches. As recently shown for a missense SCN1A mutant identified in a family with pure familial hemiplegic migraine [43], rescue of folding defects may also transform nonfunctional loss-of-function mutants, an effect that is consistent with severe epilepsy, into gain-of-function ones, an effect that is consistent with familial hemiplegic migraine [44]. Animal models have confirmed that GEFS+ mutations cause loss of function of SCN1A and reduced excitability of GABAergic neurons [45].…”