Smurf1 controls calcineurin complex and TFEB at a regulatory node for lysosomal biogenesis

Abstract

Abstract Autophagy is a homeostatic process in response to multiple signaling, such as lysosome-dependent physiological self-degradation process of cellular components. Lysosomal stress or starvation-induced mTOR inactivation and calcineurin activation were shown to promote the nuclear translocation of transcriptional factor EB (TFEB). However, the mechanisms via which signals from endomembrane damage are transmitted to activate calcineurin and orchestrate autophagic responses remain unknown. This study aimed to show that autophagy regulator Smurf1 controlled TFEB nuclear import for transcriptional activation of the lysosomal system. We showed that blocking Smurf1 affected the global transcriptional response to lysosomal damage by downregulating subsets of TFEB targets and lysosomal biogenesis. The findings revealed galectins recognized lysosomal damage and led to galectin-3 (Gal3) association with Smurf1 and the calcineurin apparatus on lysosomes. Smurf1 directly interacted with both Gal3 and calcineurin to form and stabilize the complex, thereby activating lysosomal biogenesis independent of mTOR. Smurf1 controlled the phosphorylation activity of the calcineurin A (CNA) subunit by promoting the dissociation of its autoinhibitory domain (AID) from its catalytic domain (CD) and ubiquitylated modification. In addition, the Gal3-Smurf1-calcineurin complex and TFEB feedback stabilized each other, and Smurf1 was required for calcineurin activity as a positive regulator of TFEB. The overexpression of Smurf1 showed similar effects as the overexpression of constitutive activation of PPP3CB. Surprisingly, Smurf1 also enhanced TFEB import by directly interacting with and ubiquitylating TFEB. Thus, Smurf1, which bridges environmental stresses with the core autophagosomal and autolysosomal machinery, interacted directly with TFEB and its phosphatase PPP3CB to control TFEB activation.