The β 4 subunit of the voltage-gated calcium channel (Cacnb4) regulates the rate of cell proliferation in Chinese Hamster Ovary cells.

The β subunits of Voltage-Gated Calcium Channel (VGCC) are cytosolic proteins that interact with the VGCC pore -forming subunit and participate in the trafficking of the channel to the cell membrane and in ion influx regulation. β subunits also exert functions independently of their binding to VGCC by translocation to the cell nucleus including the control of gene expression. Mutations of the neuronal Cacnb4 (β4 ) subunit are linked to human neuropsychiatric disorders including epilepsy and intellectual disabilities. It is believed that the pathogenic phenotype induced by these mutations is associated with channel-independent functions of the β4 subunit. In this report, we investigated the role of β4 subunit in cell proliferation and cell cycle progression and examined whether these functions could be altered by a pathogenic mutation. To this end, stably transfected Chinese Hamster Ovary (CHO-K1) cells expressing either rat full-length β4 or the rat C-terminally truncated epileptic mutant variant β1-481 were generated. The subcellular localization of both proteins differed significantly. Full-length β4 localizes almost exclusively in the cell nucleus and concentrates into the nucleolar compartment, while the C-terminal-truncated β1-481 subunit was less concentrated within the nucleus and absent from the nucleoli. Cell proliferation was found to be reduced by the expression of β4 , while it was unaffected by the epileptic mutant. Also, full-length β4 interfered with cell cycle progression by presumably preventing cells from entering the S-phase via a mechanism that partially involves endogenous B56δ, a regulatory subunit of the phosphatase 2A (PP2A) that binds β4 but not β1-481 . Analysis of β4 subcellular distribution during the cell cycle revealed that the protein is highly expressed in the nucleus at the G1/S transition phase and that it is translocated out of the nucleus during chromatin condensation and cell division. These results suggest that nuclear accumulation of β4 at the G1/S transition phase affects the progression into S-phase resulting in a decrease in the rate of cell proliferation. Regulation of the cell cycle exit is a critical step in determining the number of neuronal precursors and neuronal differentiation suggesting that mutations of the β4 subunit could affect neural development and formation of the mature central nervous system.