Loss of calcium channels in the cerebellum of the ataxic and epileptic stargazer mutant mouse.

The stargazer mouse displays cerebellar ataxia and absence epilepsy as a result of a single, recessive mutation on chromosome 15 which silences the expression of the voltage-dependent calcium channel (VDCC) subunit gamma2, termed stargazin. Stargazin is the predominant gamma-subunit expressed in the cerebellum and is essential for correct assembly and trafficking of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-subtype of glutamate receptors (AMPARs) to postsynaptic membranes. As a functional association between AMPARs and VDCCs has been reported, and loss of stargazin results in a loss of AMPA receptors at cerebellar synapses, we investigated whether the loss of stargazin might also change the expression levels of calcium channels at cerebellar synapses. We present data showing that the stargazin mutation affects the expression of postsynaptic L-type Ca(v)1.2 (alpha(1C)-class) but not presynaptic P/Q-type Ca(v)2.1 (alpha(1A)-class) calcium channel proteins at cerebellar synapses. Both Western blot and immunogold analyses demonstrated a significant reduction in the levels of L-type calcium channel Ca(v)1.2 at stargazer cerebellar synapses compared to their non-ataxic littermates. This is in contrast to stargazer hippocampal synapses where no differences were detected in Ca(v)1.2 and 2.1 levels compared to controls, likely due to compensation by subunit gamma8. The loss of L-type calcium channel Ca(v)1.2 at stargazer cerebellar synapses suggests that stargazin mutation may contribute to the loss of VDCCs at postsynaptic sites. It is therefore possible that stargazin is involved in the trafficking of both AMPARs and VDCCs or in the formation of a functional AMPA receptor-calcium channel complex in the postsynaptic membrane.