Cytoplasmic Cl couples membrane remodeling to epithelial morphogenesis.

Chloride is the major free anion in the extracellular space (>100 mM) and within the cytoplasm in eukaryotes (10 ∼ 20 mM). Cytoplasmic Cl level is dynamically regulated by Cl channels and transporters. It is well established that movement of Cl across the cell membrane is coupled with cell excitability through changes in membrane potential and with water secretion. However, whether cytoplasmic Cl plays additional roles in animal development and tissue homeostasis is unknown. Here we use genetics, cell biological and pharmacological tools to demonstrate that TMEM16A, an evolutionarily conserved calcium-activated chloride channel (CaCC), regulates cytoplasmic Cl homeostasis and promotes plasma membrane remodeling required for mammalian epithelial morphogenesis. We demonstrate that TMEM16A-mediated control of cytoplasmic Cl regulates the organization of the major phosphoinositide species PtdIns(4,5)P into microdomains on the plasma membrane, analogous to processes that cluster soluble and membrane proteins into phase-separated droplets. We further show that an adequate cytoplasmic Cl level is required for proper endocytic trafficking and membrane supply during early stages of ciliogenesis and adherens junction remodeling. Our study thus uncovers a critical function of CaCC-mediated cytoplasmic Cl homeostasis in controlling the organization of PtdIns(4,5)P microdomains and membrane remodeling. This newly defined role of cytoplasmic Cl may shed light on the mechanisms of intracellular Cl signaling events crucial for regulating tissue architecture and organelle biogenesis during animal development.