Synaptic Gain-of-Function Effects of Mutant Ca_v2.1 Channels in a Mouse Model of Familial Hemiplegic Migraine Are Due to Increased Basal [Ca²⁺]_i

Abstract

Specific missense mutations in the CACNA1A gene, which encodes a subunit of voltage-gated Ca V 2.1 channels, are associated with familial hemiplegic migraine type 1 (FHM1), a rare monogenic subtype of common migraine with aura. We used transgenic knock-in (KI) mice harboring the human pathogenic FHM1 mutation S218L to study presynaptic Ca 2ϩ currents, EPSCs, and in vivo activity at the calyx of Held synapse. Whole-cell patch-clamp recordings of presynaptic terminals from S218L KI mice showed a strong shift of the calcium current I-V curve to more negative potentials, leading to an increase in basal [Ca 2ϩ ] i , increased levels of spontaneous transmitter release, faster recovery from synaptic depression, and enhanced synaptic strength despite smaller action-potential-elicited Ca 2ϩ currents. The gain-of-function of transmitter release of the S218L mutant was reproduced in vivo, including evidence for an increased release probability, demonstrating its relevance for glutamatergic transmission. This synaptic phenotype may explain the misbalance between excitation and inhibition in neuronal circuits resulting in a persistent hyperexcitability state and other migraine-relevant mechanisms such as an increased susceptibility to cortical spreading depression.

In vivo imaging reveals that pregabalin inhibits cortical spreading depression and propagation to subcortical brain structures

“…This notion fits with previous data demonstrating that cortical neurons from S218L mice display calcium currents with a distinct leftward shift in activation properties that is less pronounced in R192Q mice (16). As a result, S218L neurons are predicted to have the ability to conduct calcium at rest, endowing them with the ability to modulate neuronal excitability at a range of membrane potentials normally considered subthreshold (13).…”

“…FHM-1 mutations introduced into the orthologous Cacna1a gene produce transgenic mice with phenotypes that closely mimic both the milder (R192Q) and more severe (S218L) symptoms described in FHM-1 patients with these mutations (8,9). FHM-1 mutations have been shown to produce an overall gain-of-function increase in calcium conductance at physiological membrane potentials (10,11); given the wellestablished role of the channels in the calcium-mediated release of vesicular neurotransmitters, this increase can explain the increased synaptic activity observed in the mutant animals (12)(13)(14)(15)(16).…”

Abnormal synaptic Ca2+ homeostasis and morphology in cortical neurons of familial hemiplegic migraine type 1 mutant mice

“…Presynaptic [Ca 2+ ] i exhibits a power law relationship with the neurotransmitter release and synaptic transmission during depolarization 43,44 . A recent in-vitro study showed that elevated neuronal [Ca 2+ ] i at resting state in the S218L mutant increases spontaneous transmitter release and synaptic transmission strength in brainstem slices 45 . We found larger axonal boutons in FHM1 mutants at resting state that can also be explained by higher baseline [Ca 2+ ] i at presynaptic terminals of cortical and possibly other neurons 46 .…”

The Mechanisms and Functions of Synaptic Facilitation

“…At the calyx of Held, 1 ms voltage steps facilitated calcium current by 21% (Cuttle et al, 1998), but for action potentials there was only a 7% enhancement (Di Guilmi et al, 2014) (Figure 5B). 100 Hz trains of 1 ms steps enhanced calcium influx by 100%, whereas action potentials trains enhanced influx by 20% (Cuttle et al, 1998; Di Guilmi et al, 2014). This suggests that studies that used voltage steps of 5 ms and longer significantly overestimate the extent of enhancement that occurs under physiological conditions.…”