A perilysosomal feedforward mechanism regulates starvation‐induced calcium signaling

Nutrient depletion triggers a starvation‐induced calcium (Ca 2+ ) signal (SICS) that promotes Ca 2+ ‐dependent responses. However, the components and regulations of SICS are unclear. Here, we explored SICS components and their regulation by the Ca 2+ sensor calmodulin (CaM). Overexpression of the stromal interaction molecule 1 (STIM1), a key switcher of store‐operated Ca 2+ entry (SOCE), enhances SICS by fourfold. This effect is abolished by the truncation of the Ca 2+ ‐binding loop within STIM1. Consistently, SOCE inhibition strongly suppresses SICS. Nutrient removal or stimulation of the transient receptor potential mucolipin‐1 (TRPML1, encoded by the Mcoln1 gene) triggers intracellular Ca 2+ release that is prevented by pre‐emptying of endoplasmic reticulum (ER) Ca 2+ . In the presence of extracellular Ca 2+ , inhibition and silencing of Mcoln1 reduces SICS by 35–40%. We identified a CaM‐binding site in the second cytoplasmic loop of TRPML1 that interacts with Ca 2+ ‐bound CaM. Mcoln1 overexpression enhances the upstroke and peak of SICS, effects that are absent with a mutated CaM‐binding domain. Further, we generated a genetically encoded biosensor for TRPML1 (BS‐ML1). BS‐ML1 produces a robust signal upon nutrient deprivation, which is substantially reduced with the Mcoln1 mutant. The response of BS‐ML1 to nutrient depletion is equally reduced by extracellular Ca 2+ removal or SOCE inhibition. Reduced CaM availability significantly prolongs SICS, consistent with an ~40% reduction in cytoplasmic Ca 2+ removal rate. Our data thus indicate that (1) SICS comprises multiple components, including linked Ca 2+ release from the lysosome and ER and subsequent SOCE; and (2) CaM regulates the kinetics and magnitude of SICS by controlling cytoplasmic Ca 2+ removal and a perilysosomal feedforward mechanism that promotes TRPML1 activity. The dynamics of this feedforward mechanism likely regulate subsequent tissue responses to nutrient starvation.