A negative feedback regulation of MTORC1 activity by the lysosomal Ca²⁺ channel MCOLN1 (mucolipin 1) using a CALM (calmodulin)-dependent mechanism

Abstract

Macroautophagy/autophagy is an evolutionarily conserved pathway that is required for cellular homeostasis, growth and survival. The lysosome plays an essential role in autophagy regulation. For example, the activity of MTORC1, a master regulator of autophagy, is regulated by nutrients within the lysosome. Starvation inhibits MTORC1 causing autophagy induction. Given that MTORC1 is critical for protein synthesis and cellular homeostasis, a feedback regulatory mechanism must exist to restore MTORC1 during starvation. However, the molecular mechanism underlying this feedback regulation is unclear. In this study, we report that starvation activates the lysosomal Ca release channel MCOLN1 (mucolipin 1) by relieving MTORC1's inhibition of the channel. Activated MCOLN1 in turn facilitates MTORC1 activity that requires CALM (calmodulin). Moreover, both MCOLN1 and CALM are necessary for MTORC1 reactivation during prolonged starvation. Our data suggest that lysosomal Ca signaling is an essential component of the canonical MTORC1-dependent autophagy pathway and MCOLN1 provides a negative feedback regulation of MTORC1 to prevent excessive loss of MTORC1 function during starvation. The feedback regulation may be important for maintaining cellular homeostasis during starvation, as well as many other stressful or disease conditions.

Lysosome biology in autophagy

“…ALR begins with the reactivation of mTORC1 10,57 , initiated by lysosomal calcium-based negative feedback 59 as well as increased amino acid levels in the cytosol 60,61 and the lysosomes 62 . The link between mTORC1 reactivation and ALR initiation is not known but may be the phosphorylation of UVRAG by reactivated mTOR.…”

Amino acid-dependent control of mTORC1 signaling: a variety of regulatory modes

“…In addition to the ER, lysosomes also act as intracellular Ca 2+ stores. Lysosomal Ca 2+ can influence mTORC1 activity, especially under stressful conditions [152,153]. A lysosomal Ca 2+ channel, TRPML1 (also known as mucolipin 1), might represent the missing link between lysosomal Ca 2+ release and mTORC1 regulation in a context-dependent manner (Fig.…”

“…A lysosomal Ca 2+ channel, TRPML1 (also known as mucolipin 1), might represent the missing link between lysosomal Ca 2+ release and mTORC1 regulation in a context-dependent manner (Fig. 4d) [152][153][154][155], although it has also been suggested that TRPML1 does not affect mTORC1 activity [156,157]. Nevertheless, pharmacological or RNAi- mediated inhibition of TRPML1 resulted in reduced mTORC1 activity, whereas activation of TRPML1 by treatment with an agonist or TRPML1 overexpression retained to some extent mTORC1 activity during starvation [152].…”

“…Thus, TRPM L1-mediated lysosomal Ca 2+ release appears to affect mTORC1 activity through several different mechanisms. Moreover, it has been reported that transient nutrient starvation can induce TRPML1-mediated lysosomal Ca 2+ release [152,156]. Because mTORC1 has been reported to phosphorylate TRPML1 on Ser572 and Ser576, thus decreasing its Ca 2+ release activity [158], it is possible that inactivation of mTORC1 under nutrient starvation can relieve TRPML1 inhibition, leading to transient Ca 2+ release.…”

Energetic adaptations: Metabolic control of endocytic membrane traffic

“…As localized Ca 2+ signals control lysosomal fusion (Figure ), this suggests that mTORC1 may modulate lysosomal fusion to coordinate nutrient supply, although this phenomenon remains to be fully elucidated. Lysosomal Ca 2+ signals derived from MCOLN1 in turn also trigger mTORC1 activation leading to clathrin‐dependent lysosomal tubulation and fission, suggesting complex reciprocal regulation of nutrient sensing at the lysosome, mTORC1 and localized Ca 2+ signals, impacting lysosomal dynamics.…”