Functional Interaction of Auxiliary Subunits and Synaptic Proteins With CaV1.3 May Impart Hair Cell Ca2+Current Properties

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Abstract
We assessed the functional determinants of the properties of L-type Ca2+ currents in hair cells by co-expressing the pore-forming CaV1.3α1subunit with the auxiliary subunits β1A and/or α2δ. Because Ca2+channels in hair cells are poised to interact with synaptic proteins, we also co-expressed the CaV1.3α1 subunit with syntaxin, vesicle-associated membrane protein (VAMP), and synaptosome associated protein of 25 kDa (SNAP25). Expression of the CaV1.3α1 subunit in human embryonic kidney cells (HEK 293) produced a dihydropyridine (DHP)-sensitive Ca2+ current (peak current density −2.0 ± 0.2 pA/pF; n = 11). Co-expression with β1A and α2δsubunits enhanced the magnitude of the current (peak current density: CaV1.3α1 + β1A = −4.3 ± 0.8 pA/pF, n = 10; CaV1.3α1 + β1A + α2δ = −4.1 ± 0.6 pA/pF, n = 9) and produced a leftward shift of approximately 9 mV in the voltage-dependent activation of the currents. Furthermore, co-expression of CaV1.3α1 with syntaxin/VAMP/SNAP resulted in at least a twofold increase in the peak current density (−4.7 ± 0.2 pA/pF; n = 11) and reduced the extent of inactivation of the Ca2+currents. Botulinum toxin, an inhibitor of syntaxin, accelerated the inactivation profile of Ca2+ currents in hair cells. Immunocytochemical data also indicated that the Ca2+ channels and syntaxin are co-localized in hair cells, suggesting there is functional interaction of the CaV1.3α1 with auxiliary subunits and synaptic proteins, that may contribute to the distinct properties of the DHP-sensitive channels in hair cells.