Background: Stargazin (gamma2) and the closely related gamma3, and gamma4 transmembrane proteins are part of a family of proteins that may act as both neuronal voltage-dependent calcium channel (VDCC) gamma subunits and transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponinc (AMPA) receptor regulatory proteins (TARPs). In this investigation, we examined the distribution patterns of the stargazin-like proteins gamma2, gamma3, and gamma4 in the human central nervous system (CNS). In addition, we investigated whether human gamma2 or gamma4 could modulate the electrophysiological properties of a neuronal VDCC complex transiently expressed in Xenopus oocytes.
Results: The mRNA encoding human gamma2 is highly expressed in cerebellum, cerebral cortex, hippocampus and thalamus, whereas gamma3 is abundant in cerebral cortex and amygdala and gamma4 in the basal ganglia. Immunohistochemical analysis of the cerebellum determined that both gamma2 and gamma4 are present in the molecular layer, particularly in Purkinje cell bodies and dendrites, but have an inverse expression pattern to one another in the dentate cerebellar nucleus. They are also detected in the interneurons of the granule cell layer though only gamma2 is clearly detected in granule cells. The hippocampus stains for gamma2 and gamma4 throughout the layers of the every CA region and the dentate gyrus, whilst gamma3 appears to be localized particularly to the pyramidal and granule cell bodies. When co-expressed in Xenopus oocytes with a CaV2.1/beta4 VDCC complex, either in the absence or presence of an alpha2delta2 subunit, neither gamma2 nor gamma4 significantly modulated the VDCC peak current amplitude, voltage-dependence of activation or voltage-dependence of steady-state inactivation.
Conclusion: The human gamma2, gamma3 and gamma4 stargazin-like proteins are detected only in the CNS and display differential distributions among brain regions and several cell types in found in the cerebellum and hippocampus. These distribution patterns closely resemble those reported by other laboratories for the rodent orthologues of each protein. Whilst the fact that neither gamma2 nor gamma4 modulated the properties of a VDCC complex with which they could associate in vivo in Purkinje cells adds weight to the hypothesis that the principal role of these proteins is not as auxiliary subunits of VDCCs, it does not exclude the possibility that they play another role in VDCC function.