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P/Q-type voltage-gated calcium channels are essential for Ca2+ influx and neurotransmitter release in hippocampal synaptic transmission. Through alternative splicing, the exclusion or inclusion of the NP splice variant determines the classification of the P-type or Q-type channels, respectively, which differ in their sensitivity to the peptide toxin ω-Agatoxin IVA (Aga-IVA). Familial hemiplegic migraine type-1 (FHM-1) is an autosomal dominant subtype of migraine caused by gain-of-function missense mutations in the CaV2.1 subunit of P/Q-type channels. The S218L FHM-1 mutation is associated with a particularly severe clinical syndrome which includes ataxia, generalized seizures and fatal cerebral edema, and causes a hyperpolarizing shift in channel activation resulting in an increased proportion of P/Q-type channels being open at the resting membrane potential and is predicted to increase glutamate release. Increased sensitivity of Aga-IVA has been observed in synaptic signaling in CA1 hippocampal neurons of a S218L FHM-1 mouse model although the underlying mechanism is not known. Here, performing subunit and splice-variant specific quantitative real-time PCR on mouse hippocampal regions, I demonstrate that CaV channel subunits and P/Q-type splice variants are not differentially expressed between WT and S218L mice. Using whole-cell patch-clamp electrophysiology on CA1 neurons in mouse brain slices, I further show that the contribution of P/Q-, N- and R-type channels to excitatory miniature release in WT and S218L mice is highly variable. Examining the contribution of P/Q-type channels to evoked release, I show that P/Q-type channels are an important contributor towards the rate of excitatory spontaneous action potential evoked release in WT neurons. Further, that WT CA1 neurons exhibit a large unitary EPSC response evoked by paired-pulse stimulation and was reduced when P/Q-type channels were blocked. In contrast, EPSC amplitude in S218L neurons tended to be smaller compared to EPSC amplitudes from WT although this effect was not consistent. Together, these data suggest that in CA1 neurons P/Q-type channels are predominant in evoked synaptic transmission in WT neurons and that the S218L mutation appears to cause decreased action potential evoked Ca2+ influx. Further investigation is required to determine whether other VGCCs act to compensate evoked release in S218L neurons.

Figure S1. Binding of SPTAN1 to Cav3.1-GFP ΔCT 1875–2377 mutant channels lacking a distal C-terminus region. Cav3.1-GFP ∆CT (1875–2377) and wild type channel immunoprecipitates from transfected tsA-201 cells probed with anti-Spectrin αII (SPTAN1) polyclonal antibody. Densitometry analysis of SPTAN1 bound to Cav3.1-GFP immunoprecipitates is shown. Figure S2. Disrupting Cav3.1 SPTAN1 interactions reduces cell surface expression of Cav3.1. Left: Surface biotinylation experiments on Cav3.1 channels transiently expressed in tsA-201 cells in the presence of a cell permeant Tat peptide corresponding to the putative spectrin interaction site (Tat-Cav3.1-CT) on the channel, or a scrambled peptide sequence. Right: Densitometry analysis of Cav3.1 surface pool normalized to the actin control. Note that the Tat- Cav3.1-CT peptide reduces the cell surface expression of the channel by ~ 40%. (DOC 118 kb)

Figure S1. Binding of SPTAN1 to Cav3.1-GFP ΔCT 1875–2377 mutant channels lacking a distal C-terminus region. Cav3.1-GFP ∆CT (1875–2377) and wild type channel immunoprecipitates from transfected tsA-201 cells probed with anti-Spectrin αII (SPTAN1) polyclonal antibody. Densitometry analysis of SPTAN1 bound to Cav3.1-GFP immunoprecipitates is shown. Figure S2. Disrupting Cav3.1 SPTAN1 interactions reduces cell surface expression of Cav3.1. Left: Surface biotinylation experiments on Cav3.1 channels transiently expressed in tsA-201 cells in the presence of a cell permeant Tat peptide corresponding to the putative spectrin interaction site (Tat-Cav3.1-CT) on the channel, or a scrambled peptide sequence. Right: Densitometry analysis of Cav3.1 surface pool normalized to the actin control. Note that the Tat- Cav3.1-CT peptide reduces the cell surface expression of the channel by ~ 40%. (DOC 118 kb)