Different α₂δ Accessory Subunits Regulate Distinctly Different Aspects of Calcium Channel Function in the Same Drosophila Neurons

Abstract

Voltage gated calcium channels (VGCCs) regulate neuronal excitability and translate activity into calcium dependent intracellular signaling. The pore forming α 1 subunit of high voltage activated (HVA) VGCCs operates not in isolation but associates with α 2 δ accessory subunits. α 2 δ subunits can affect calcium channel biophysical properties, surfacing, localization and transport, but their in vivo functions are incompletely understood. In vertebrates, it is largely unknown whether different combinations of the four α 2 δ and the 7 α 1 subunits mediate different or partially redundant functions or whether different α 1 /α 2 δ combinations regulate different aspects of VGCC function. This study capitalizes on the relatively simpler situation in the Drosophila genetic model that contains only two genes for HVA calcium channels, one Ca v 1 homolog and one Ca v 2 homolog, both with well-described functions in different compartments of identified motoneurons. We find that both dα 2 δ 1 and dα 2 δ 3 (stj) are broadly but differently expressed in the nervous system. Both are expressed in motoneurons, but with differential subcellular localization. Functional analysis reveals that dα 2 δ 3 is required for normal Ca v 1 and Ca v 2 current amplitudes and for correct Ca v 2 channel function in all neuronal compartments, axon terminal, axon, and somatodendritic domain. By contrast, dα 2 δ 1 does not affect Ca v 1 or Ca v 2 current amplitudes or presynaptic function, but it is required for correct Ca v 2 channel allocation to the axonal versus the dendritic domain. Therefore, different α 2 δ subunits are required in the same neurons to precisely regulate distinctly different functions of HVA calcium channels, which is in accord with specific α 2 δ mutations causing different brain diseases. Significance StatementCalcium influx through the pore forming α 1 -subunit of voltage gated calcium channels serves essential neuronal functions, such as synaptic vesicle release, control of action potential shape and frequencies, synaptic input computations, and transcriptional control. Localization and function of α 1 -calcium channel subunits depend on interactions with α 2 δ accessory subunits. Here we present in vivo analysis of Drosophila motoneurons revealing that different α 2 δ subunits independently regulate distinctly different aspects of calcium channel function in the same neuron, such as current amplitude and dendritic versus axonal channel localization. Our findings start unraveling how different α 1 /α 2 δ combinations regulate functional calcium channel diversity in different sub-neuronal compartments, and may provide an entry point toward understanding how mutations of different α 2 δ genes underlie brain diseases.