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 CaV 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 CaV 1.3 channel affecting Ca2+ signaling and altered RPE function. Thus, we investigated the protein interaction between CaV 1.3 channels and bestrophin-1 by immunoprecipitation, CaV 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 CaV 1.3 channel activity, and changes in surface expression. Transfection of polarized RPE (porcine primary cells, iPSC-RPE) that endogenously express CaV 1.3 and wild-type bestrophin-1, with mutant bestrophin-1 confirmed reduction of CaV 1.3 surface expression. For the four selected BEST1 mutations, presence of mutant bestrophin-1 led to reduced CaV 1.3 activity by modulating pore-function or decreasing surface expression. Reduced CaV 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.

Anoctamin-4 is a bona fide Ca2+-dependent non-selective cation channel.

Changes in cell function occur by specific patterns of intracellular Ca2+, activating Ca2+-sensitive proteins. The anoctamin (TMEM16) protein family has Ca2+-dependent ion channel activity, which provides transmembrane ion transport, and/or Ca2+-dependent phosphatidyl-scramblase activity. Using amino acid sequence analysis combined with measurements of ion channel function, we clarified the so far unknown Ano4 function as Ca2+-dependent, non-selective monovalent cation channel; heterologous Ano4 expression in HEK293 cells elicits Ca2+ activated conductance with weak selectivity of K+ > Na+ > Li+. Endogenously expressed Ca2+-dependent cation channels in the retinal pigment epithelium were identified as Ano4 by KO mouse-derived primary RPE cells and siRNA against Ano4. Exchanging a negatively charged amino acid in the putative pore region (AA702-855) into a positive one (E775K) turns Ano4-elicited currents into Cl- currents evidencing its importance for ion selectivity. The molecular identification of Ano4 as a Ca2+-activated cation channel advances the understanding of its role in Ca2+ signaling.