IP3R2 mediated inter-organelle Ca ²⁺ signaling orchestrates melanophagy

Organelle dynamics and crosstalk play a critical role in cellular functions thereby regulating physiological processes and pathological conditions. A variety of cellular processes are outcome of a balance between organelle biogenesis and degradation. Pigmentation is one such homeostatic state that is a result of melanosome biogenesis and melanosome degradation. Although melanosome biogenesis is partially understood, the melanosome degradation i.e. melanophagy remains largely unappreciated. Here, we reveal that Inositol 1,4,5-trisphosphate receptor 2 (IP ₃ R2) is a negative regulator of melanophagy. In this study, we developed two de novo ratio metric imaging probes to study melanophagy in live-cells. Using these probes, biochemical assays, ultrastructural studies, confocal microscopy, molecular analyses and calcium imaging; we demonstrate that IP ₃ R2, but not IP ₃ R1 or IP ₃ R3, keeps melanophagy in check. In vivo studies in zebrafish model system further substantiate IP ₃ R2’s functional relevance in pigmentation. Mechanistically, IP ₃ R2 silencing decreases mitochondrial Ca ²⁺ uptake, augments ADP/ATP ratio and thereby activates melanophagy. Simultaneously, IP ₃ R2 knockdown increases ER-lysosome proximity, enhances lysosomal Ca ²⁺ levels and decreases lysosomal pH. This in turn activates lysosomal TRPML1 channel and stimulates nuclear translocation of TFEB transcription factor, which facilitates transcription of key autophagy and two known melanophagy drivers. Taken together, we uncover that IP ₃ R2-mediated Ca ²⁺ signaling across organelles is a critical determinant of melanophagy and thereby skin pigmentation. Hence, this signaling cascade offers potential therapeutic prospects for the management of pigmentary disorders and skin malignancies.