Contribution of Ion Channels in Calcium Signaling Regulating Phagocytosis: MaxiK, Cav1.3 and Bestrophin-1

Abstract

Mutations in the BEST1 gene lead to a variety of retinal degenerations including Best's vitelliforme macular degeneration. The BEST1 gene product, bestrophin-1, is expressed in the retinal pigment epithelium (RPE). It is likely that mutant bestrophin-1 impairs functions of the RPE which support photoreceptor function and will thus lead to retinal degeneration. However, the RPE function which is influenced by bestrophin-1 is so far not identified. Previously we showed that bestrophin-1 interacts with L-type Ca²⁺ channels of the CaV1.3 subtype and that the endogenously expressed bestrophin-1 is required for intracellular Ca²⁺ regulation. A hallmark of Best's disease is the fast lipofuscin accumulation occurring already at young ages. Therefore, we addressed the hypothesis that bestrophin-1 might influence phagocytosis of photoreceptor outer segments (POS) by the RPE. Here, siRNA knock-down of bestrophin-1 expression as well as inhibition of L-type Ca²⁺ channel activity modulated the POS phagocytosis in vitro. In vivo CaV1.3 expression appeared to be diurnal regulated with a higher expression rate in the afternoon. Compared to wild-type littermates, Ca V 1.3 (-/-) mice showed a shift in the circadian POS phagocytosis with an increased activity in the afternoon. Thus we suggest that mutant bestrophin-1 leads to an impaired regulation of the POS phagocytosis by the RPE which would explain the fast lipofuscin accumulation in Best patients.

Robust lysosomal calcium signaling through channel TRPML1 is impaired by lysosomal lipid accumulation

“…Ca 2+ signaling makes a key contribution to several important functions of RPE cells, including ion and fluid transport, phagocytosis of photoreceptor outer segments, and secretion (56, 57). The TRPML1 channel has recently been implicated in multiple cellular functions, including phagocytosis (58), secretion (59), control of organelle motility (6), regulation of transcription factor TFEB (7), response to cellular starvation (8), and control of lysosome size (60).…”

Novel roles for voltage‐gated T‐type Ca2+ and ClC‐2 channels in phagocytosis and angiogenic factor balance identified in human iPSC‐derived RPE

“…30 We show here that specific inhibition of L-type Ca 2+ channels considerably decrease the rate of POS internalization, consistent with previous studies identifying L-type channels as regulators of RPE phagocytosis. [59][60][61] To date, a role for other types of Ca 2+ channels in this phagocytosis process was more speculative. 34 We demonstrate here that specific inhibition of T-type channels also considerably decreased the rate of POS internalization in hiPSC-derived RPE, thus, providing the proof that T-type Ca 2+ channels also regulate phagocytosis in human RPE.…”

Inhibition of Ca 2+ channel surface expression by mutant bestrophin‐1 in RPE cells

“…[24][25][26] Some of the interaction partners and data from cell and animal models indicate that bestrophin-1 multifunction contributes to intracellular Ca 2+ signaling in RPE cells by either modulating Ca 2+ conductance of the plasma membrane and/or the Ca 2+ content of cytosolic Ca 2+ stores. [24][25][26][27][28][29][30][31][32][33][34] Thus, mutation-dependent alterations in intracellular Ca 2+ signaling as a cause for RPE malfunction and subsequent disturbance of photoreceptor maintenance represent an attractive alternative hypothesis to that of the loss of Cl channel activity The observation that Best1 -/mice do not show reduction in RPE Ca 2+ -dependent Cl channel activity or a reduced light-peak amplitude of the DC ERG that is comparable with the human EOG supports this hypothesis. 29 Modulation of plasma membrane Ca 2+ conductance occurs by physical bestrophin-1 and L-type Ca 2+ channel interaction that is based on bestrophin-1 binding to Ca 2+ channel β-subunits via coupling of PxxP-motifs SH3 domain.…”

“…The PxxP motifs at the bestrophin‐1 C‐terminus permit potential interaction with all SH3 domain‐containing proteins . Some of the interaction partners and data from cell and animal models indicate that bestrophin‐1 multifunction contributes to intracellular Ca 2+ signaling in RPE cells by either modulating Ca 2+ conductance of the plasma membrane and/or the Ca 2+ content of cytosolic Ca 2+ stores . Thus, mutation‐dependent alterations in intracellular Ca 2+ signaling as a cause for RPE malfunction and subsequent disturbance of photoreceptor maintenance represent an attractive alternative hypothesis to that of the loss of Cl channel activity The observation that Best1 ‐/‐ mice do not show reduction in RPE Ca 2+ ‐dependent Cl channel activity or a reduced light‐peak amplitude of the DC ERG that is comparable with the human EOG supports this hypothesis …”