PAC regulates endo-, and exocytosis, and lysosomal-mitochondrial stress signaling in human mast cells

In mast cells, endo- and exocytotic pathways are central to the (patho)physiological release of pro-inflammatory mediators, linking intracellular signaling with immune communication. Proton-activated chloride (PAC) channel mediates acid-sensitive outwardly rectifying anion channel (ASOR/PAORAC) currents, however, its physiological functions are poorly understood.Using electrophysiology, live-cell imaging, electron microscopy, and functional assays, we investigate the role of PAC in human mast cells.We detected ASOR/PAORAC in primary human mast cells and mast cell lines and demonstrated its essential role in vesicular signaling. PAC knockout reduced vesicular pH, increased endocytosis, decreased exocytosis, and disrupted endolysosomal homeostasis. Upon activation of exocytosis, the lack of PAC reduced CD107a (LAMP-1) surface expression. PAC-deficient cells also displayed increased colocalization of lysosomes and mitochondria, elevated ROS levels, and the appearance of C-shaped mitochondria, suggesting that PAC regulates inter-organelle stress signaling. Functionally, PAC knockout impaired mitochondrial respiration, linking ion channel activity to mast cell metabolic adaptation.These findings establish PAC as a key regulator of endo- and exocytosis-dependent signaling and lysosomal-mitochondrial stress response in human mast cells, highlighting its physiological relevance and potential as a therapeutic target in mast cell-associated disorders.