Inhibition of Ca2+ channel surface expression by mutant bestrophin-1 in RPE cells

The BEST1 gene product bestrophin-1, a Ca2+-dependent anion channel, interacts with Ca(V)1.3 Ca2+ channels in the retinal pigment epithelium (RPE). BEST1 mutations lead to Best vitelliform macular dystrophy. A common functional defect of these mutations is reduced trafficking of bestrophin-1 into the plasma membrane. We hypothesized that this defect affects the interaction partner Ca(V)1.3 channel affecting Ca2+ signaling and altered RPE function. Thus, we investigated the protein interaction between Ca(V)1.3 channels and bestrophin-1 by immunoprecipitation, Ca(V)1.3 activity in the presence of mutant bestrophin-1 and intracellular trafficking of the interaction partners in confluent RPE monolayers. We selected four BEST1 mutations, each representing one mutational hotspot of the disease: T6P, F80L, R218C, and F305S. Heterologously expressed L-type channels and mutant bestrophin-1 showed reduced interaction, reduced Ca(V)1.3 channel activity, and changes in surface expression. Transfection of polarized RPE (porcine primary cells, iPSC-RPE) that endogenously express Ca(V)1.3 and wild-type bestrophin-1, with mutant bestrophin-1 confirmed reduction of Ca(V)1.3 surface expression. For the four selected BEST1 mutations, presence of mutant bestrophin-1 led to reduced Ca(V)1.3 activity by modulating pore-function or decreasing surface expression. Reduced Ca(V)1.3 activity might open new ways to understand symptoms of Best vitelliform macular dystrophy such as reduced electro-oculogram, lipofuscin accumulation, and vision impairment.