The Role of Bestrophin-1 in Intracellular Ca²⁺ Signaling

Abstract

Mutations in the BEST1 gene lead to a variety of retinal degenerations, among them Best’s vitelliforme macular degeneration. To clarify the mechanism of the disease, the understanding of the function of BEST1 gene product, bestrophin-1, is mandatory. In overexpression studies bestrophin-1 appeared to function as a Ca²⁺-dependent Cl channel. On the other hand, bestrophin-1 is able to participate in intracellular Ca²⁺ signaling. Endogenously expressed bestrophin-1 largely localized to the cytosolic compartment close to the basolateral membrane of the retinal pigment epithelium (RPE) as it can be shown using differential centrifugation, immunohistochemistry, and transmission electron microscopy. To elucidate a cytosolic function of bestrophin-1, we explored the store-operated Ca²⁺ entry in short-time cultured porcine RPE cells. Depletion of cytosolic Ca²⁺stores by SERCA inhibition led to activation of Orai-1 Ca²⁺ channels. This resulted in an influx of extracellular Ca²⁺ into the cell which was reduced when bestrophin-1 expression was knocked down using siRNA techniques. Quantification of Ca²⁺ which can be released from cytosolic Ca²⁺ stores revealed that after reduction of bestrophin-1 expression less Ca²⁺ is stored in ER Ca²⁺ stores. Thus, bestrophin-1 functions as an intracellular Cl channel which helps to accumulate and to release Ca²⁺ from stores by conducting the counterion for Ca²⁺.