Editors' ChoiceCellular Metabolism

Calcium signals for more lysosomes

Sci. Signal.  24 Mar 2015:
Vol. 8, Issue 369, pp. ec67
DOI: 10.1126/scisignal.aab1675

Under nutrient-depleted conditions, lysosomes reclaim nutrients by participating in the process of autophagy. The biogenesis of these organelles requires the transcription factor TFEB. Under nutrient-replete conditions, the transcriptional activity of TFEB is inhibited because phosphorylation mediated by mTOR causes TFEB to interact with 14-3-3 proteins, which retains the transcription factor in the cytoplasm. Medina et al. identified a signaling pathway that is initiated at lysosomes and activates TFEB. A siRNA screen revealed that knockdown of the calcium-activated phosphatase calcineurin (specifically, the calcineurin catalytic subunit isoform beta; PPP3CB) prevented the nuclear translocation of TFEB in starved cells, an effect that was mimicked by pharmacological inhibitors of calcineurin. Overexpression of a constitutively active form of calcineurin triggered the nuclear translocation of TFEB in cells grown in nutrient-replete conditions and in skeletal muscle cells in vivo. Exercise induces the activity of calcineurin and promoted the nuclear translocation of TFEB in skeletal muscle cells, a response that was attenuated by the electroporation of the endogenous calcineurin inhibitor CAIN. Various assays indicated that calcineurin dephosphorylated TFEB at Ser211 and Ser142, the phosphorylation state of which determines the subcellular localization of TFEB. The interaction between calcineurin and TFEB occurred in cells under nutrient-depleted and -replete conditions. Calcium chelation reduced the nuclear translocation of TFEB in starved cells or cells treated with an inhibitor of mTOR. Conversely, experimental manipulations that increased intracellular calcium concentrations, such as thapsigargin treatment, enhanced the nuclear translocation of TFEB in cells under nutrient-replete conditions, an effect that in thapsigargin-treated cells required calcineurin. Starvation did not increase global cytosolic calcium concentrations, but did increase the activity of a calcium reporter fused to the lysosomal calcium channel mucolipin 1 (MCOLN1; also known as TRPML1). The starvation-induced nuclear translocation of TFEB was reduced by knockdown of MCOLN1 or in fibroblasts from individuals with mucolipidosis type 4, a lysosomal storage disease caused by mutations in MCOLN1. Furthermore, electroporation of a shRNA targeting MCOLN1 reduced the exercise-induced nuclear translocation of TFEB in skeletal muscle. Knockdown of the regulatory and catalytic subunits of calcineurin or of MCOLN1 inhibited autophagy as measured by the abundance of various markers. Thus, starvation triggers a calcium signal at lysosomes through MCOLN1 that induces the dephosphorylation of TFEB by calcineurin and the activation of this transcription factor.

D. L. Medina, S. Di Paola, I. Peluso, A. Armani, D. De Stefani, R. Venditti, S. Montefusco, A. Scotto-Rosato, C. Prezioso, A. Forrester, C. Settembre, W. Wang, Q. Gao, H. Xu, M. Sandri, R. Rizzuto, M. A. De Matteis, A. Ballabio, Lysosomal calcium signalling regulates autophagy through calcineurin and TFEB. Nat. Cell Biol. 17, 288–299 (2015). [PubMed]

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