Presynaptic Ca2+ Channels Compete for Channel Type-Preferring Slots in Altered Neurotransmission Arising from Ca2+ Channelopathy

Abstract

Several human channelopathies result from mutations in alpha1A, the pore-forming subunit of P/Q-type Ca2+ channels, conduits of presynaptic Ca2+ entry for evoked neurotransmission. We found that wild-type human alpha1A subunits supported transmission between cultured mouse hippocampal neurons equally well as endogenous mouse alpha1A, whereas introduction of impermeant human alpha1A hampered the effect of endogenous subunits. Thus, presynaptic P/Q-type channels may compete for channel type-preferring "slots" that limit their synaptic effectiveness. The existence of slots generates predictions for how neurotransmission might be affected by changes in Ca2+ channel properties, which we tested by studying alpha1A mutations that are associated with familial hemiplegic migraine type 1 (FHM1). Mutant human P/Q-type channels were impaired in contributing to neurotransmission in precise accord with their deficiency in supporting whole-cell Ca2+ channel activity. Expression of mutant channels in wild-type neurons reduced the synaptic contribution of P/Q-type channels, suggesting that competition for type-preferring slots might support the dominant inheritance of FHM1.

Familial Hemiplegic Migraine

“…47 In contrast, synaptic transmission was unaltered and the contribution of P/Q Ca 2ϩ channels in controlling release was decreased at synapses between hippocampal neurons transfected with human R192Q Ca V 2.1␣1 subunits. 37 The functional studies just described lead to the important conclusion that FHM1 mutations produce gainof-function of human neuronal Ca V 2.1 channels. Mutant human Ca V 2.1 channels open at lower voltages and more readily than WT channels, and Ca 2ϩ influx through mutant channels can occur in response to small depolarizations insufficient to open WT channels.…”

“…In humans, mutations in the CACNA1A gene cause, in addition to FHM1, a few autosomal dominant neurological disorders characterized by cerebellar dysfunction, such as episodic ataxia type 2 (that may be associated with absence epilepsy in a few cases) and spinocerebellar ataxia type 6 9,26,27 (compare, Strupp 28 and Gomez 29 31,35,37,38 Recently, the generation of knockin mice carrying two different FHM1 mutations (R192Q and S218L) allowed the first analysis of mutant channels expressed at their endogenous level in neurons. [39][40][41] The studies in heterologous expression systems showed that the FHM1 mutations alter many biophysical properties of human Ca V 2.1 channels, in a complex way.…”

“…While either an increase or a decrease of the whole cell Ca V 2.1 current density elicited by strong depolarizations was measured in HEK293 cells depending on the FHM1 mutant analyzed 31,34,35 (and our unpublished observations), a consistent decrease was measured in neurons. 31,35,37,38 The R192Q mutation decreased the current density at high voltages in transfected neurons but increased it in transfected HEK293 cells. 31,34,37,38 Because the same mutation did not affect the whole-cell P/Q Ca 2ϩ current density elicited by strong depolarizations in neurons of knockin mice expressing Ca V 2.1 channels at their endogenous level, the changes in transfected cells likely reflect changes in the number of functional channels in the membrane that might be an artifact of overexpression.…”

“…31,35,37,38 The R192Q mutation decreased the current density at high voltages in transfected neurons but increased it in transfected HEK293 cells. 31,34,37,38 Because the same mutation did not affect the whole-cell P/Q Ca 2ϩ current density elicited by strong depolarizations in neurons of knockin mice expressing Ca V 2.1 channels at their endogenous level, the changes in transfected cells likely reflect changes in the number of functional channels in the membrane that might be an artifact of overexpression. One may therefore obtain misleading answers using transfected cells to study the effect of ion channel mutations on cellular functions that depend on the number of functional channels in the membrane.…”

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

“…Biophysical analysis of FHM1 Ca 2+ channel dysfunction in heterologus systems show that the FHM1 mutations alter many biophysical properties of human Cav2.1 channels in a complex way, which is rather debateable since both loss-of-function and gain-of-function phenotypes have been reported (Cao et al 2004, Barrett et al 2005, Tottene et al 2002, Kraus et al 2000, Hans et al 1999. The most consistent effect revealed by the analysis of single-channel properties of human Ca V 2.1 channels carrying FHM1 mutations was an increase in channel open probability and in single channel Ca 2+ influx over a broad voltage range, mainly due to a shift of channel activation to more negative voltages (Tottene et al 2002, Hans et al 1999).…”