Effects of familial hemiplegic migraine type 1 mutations on neuronal P/Q-type Ca ²⁺ channel activity and inhibitory synaptic transmission

Abstract

Inhibitory synapses play key roles in the modulatory circuitry that regulates pain signaling and generation of migraine headache. A rare, dominant form of this common disease, familial hemiplegic migraine type 1 (FHM1), arises from missense mutations in the pore-forming ␣1A subunit of P͞Q-type Ca 2؉ channels. These channels are normally vital for presynaptic Ca 2؉ entry and neurotransmitter release at many central synapses, raising questions about effects of FHM1 mutations on neuronal Ca 2؉ influx and inhibitory and excitatory neurotransmission. We have expressed the four original FHM1 mutant channels in hippocampal neurons from ␣1A knockout mice. Whole-cell recordings indicated that FHM1 mutant channels were less effective than wild-type channels in their ability to conduct P͞Q-type current, but not generally different from wild type in voltage-dependent channel gating. Ca 2؉ influx triggered by action potential waveforms was also diminished. In keeping with decreased channel activity, FHM1 mutant channels were correspondingly impaired in supporting the P͞Q-type component of inhibitory neurotransmission. When expressed in wild-type inhibitory neurons, FHM1 mutant channels reduced the contribution of P͞Q-type channels to GABAergic synaptic currents, consistent with a competition of mutant and endogenous channels for P͞Q-specific slots. In all cases, N-type channels took up the burden of supporting transmission and homeostatic mechanisms maintained overall synaptic strength. The shift to reliance on N-type channels greatly increased the susceptibility to G protein-coupled modulation of neurotransmission, studied with the GABA B agonist baclofen. Thus, mutant-expressing synapses might be weakened in a heightened state of neuromodulation like that provoked by triggers of migraine such as stress.action potential waveform ͉ inhibitory hippocampal neuron M igraine, the most common neurological disease (1, 2), can be associated with subclinical deficiencies in synaptic transmission in certain migraineurs (3). The hypothesis that migraine may be a synaptic disease has gained further credence through pioneering studies of familial hemiplegic migraine type 1 (FHM1), a rare hereditary form of migraine, and the identification of mutations in a neuronal Ca 2ϩ channel vital for neurotransmission (4). The affected CACNA1A gene encodes the pore-forming ␣ 1A subunit of the P͞Q-type channel (Ca V 2.1), a predominant mediator of voltage-gated Ca 2ϩ entry and transmitter release at many synapses in the central nervous system (5-9). Alterations in synaptic signaling are of likely importance for cross-talk between an ascending pain transmission pathway and inhibition by a powerful descending modulatory system, interactions that ultimately govern the generation of headache pain (10, 11). Despite extensive study of the role of P͞Q-type channels in pain circuits (12, 13) and the impact of FHM1 mutations on P͞Q-type channels (14-20), several important questions have not yet been addressed. What are the effects of FHM1 mutations on inhibi...

Familial Hemiplegic Migraine

“…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.…”

Different Relationship of N- and P/Q-Type Ca2+Channels to Channel-Interacting Slots in Controlling Neurotransmission at Cultured Hippocampal Synapses

“…Ca 2ϩ currents supported by P/Q-type channels are more apt to facilitate during a train of brief stimuli (Inchauspe et al, 2004(Inchauspe et al, , 2007Ishikawa et al, 2005). N-type channels are more susceptible to modulation by GPCRs for neurohormones (Bourinet et al, 1996;Currie and Fox, 1997;Zhou et al, 2003;Cao and Tsien, 2005). Because Ca V 2 family members differ in responsiveness to repetitive activity and neuromodulators, controlling their contribution to presynaptic release provides a powerful way to delimit the range of regulation.…”

Important Contribution of α-Neurexins to Ca2+-Triggered Exocytosis of Secretory Granules

“…An alternative explanation of how ␣-neurexins affect Ca 2ϩ channels could be that neurexins determine the correct spatial alignment between VDCCs and releasable vesicles, for example by anchoring VDCCs at their preferred slots within the active zone (Cao et al, 2004;Cao and Tsien, 2005). The microdomain hypothesis of Ca 2ϩ -dependent exocytosis proposes the importance of the "subsynaptic" spatial arrangement of fusioncompetent vesicles and the fusion machinery (Matthews, 1996;Klingauf and Neher, 1997;Neher, 1998).…”