Molecular requirements for mammalian crinophagy highlight a key role for Ca2+-dependent Munc13-4

Secretory granules (SGs) in endocrine cells store and release peptide hormones with their turnover tightly controlled to maintain cellular hormone levels. We found that crinophagy, a specialized autophagy process, is the prevalent pathway that degrades older unused SGs in resting cells by SG-lysosome fusion. siRNA screening with a live cell assay for SG-lysosome merge identified SG components Rab27A, Munc13-4 and VAMP2 and lysosomal components PLEKHM1, HOPS subunits, and SNAREs STX7, STX8, and VTI1B required for docking SGs to and fusion with lysosomes. Munc13-4 is a central regulator of crinophagy that associates with many proteins that are functionally essential for the merge of SGs with lysosomes, and regulates the docking and fusion of SGs with lysosomes. SG-lysosome fusion was regulated by local or global calcium through binding and activation of Munc13-4. The findings reveal the critical docking/fusion machinery for mediating SG turnover in mammalian endocrine cells, and indicate how its dysregulation could impair hormonal and metabolic regulation.