An mTORC1-GRASP55 signaling axis controls unconventional secretion to reshape the extracellular proteome upon stress.
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ABSTRACT:
SUBMITTER: Nuchel J
PROVIDER: S-EPMC8382303 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:In cells, several cargoes are delivered to the cell surface or the extracellular space via unconventional secretion routes. GRASP55 is a Golgi protein that regulates unconventional secretion of distinct proteins and controls the assembly and membrane stacking of Golgi cisternae. Recent work suggested that the role of GRASP55 in unconventional secretion may involve its relocalization to other organelles. However, the stimuli that drive GRASP55-dependent unconventional secretion, the signaling events that regulate GRASP55 function, the subcellular locations where GRASP55 acts, and the cargoes that follow this route for secretion remain unclear. Here, we show that mTORC1 directly phosphorylates GRASP55 at multiple sites to maintain its Golgi localization. Cellular stresses or drugs that inhibit mTORC1 cause GRASP55 dephosphorylation and relocalization to autophagosomal / MVB structures. Using secretome and surfactome analyses in GRASP55-null cells, we identify for the first time numerous -previously unknown- cargoes that rely on this unconventional secretory pathway.
2021-09-09 | PXD020331 | Pride
Project description:Recent studies demonstrated that the Golgi reassembly stacking proteins (GRASPs), especially GRASP55, regulate Golgi-independent unconventional secretion of certain cytosolic and transmembrane cargoes; however, the underlying mechanism remains unknown. Here, we surveyed several neurodegenerative disease-related proteins, including mutant huntingtin (Htt-Q74), superoxide dismutase 1 (SOD1), tau, and TAR DNA-binding protein 43 (TDP-43), for unconventional secretion; our results show that Htt-Q74 is most robustly secreted in a GRASP55-dependent manner. Using Htt-Q74 as a model system, we demonstrate that unconventional secretion of Htt is GRASP55 and autophagy dependent and is enhanced under stress conditions such as starvation and endoplasmic reticulum stress. Mechanistically, we show that GRASP55 facilitates Htt secretion by tethering autophagosomes to lysosomes to promote autophagosome maturation and subsequent lysosome secretion and by stabilizing p23/TMED10, a channel for translocation of cytoplasmic proteins into the lumen of the endoplasmic reticulum-Golgi intermediate compartment. Moreover, we found that GRASP55 levels are upregulated by various stresses to facilitate unconventional secretion, whereas inhibition of Htt-Q74 secretion by GRASP55 KO enhances Htt aggregation and toxicity. Finally, comprehensive secretomic analysis identified novel cytosolic cargoes secreted by the same unconventional pathway, including transgelin (TAGLN), multifunctional protein ADE2 (PAICS), and peroxiredoxin-1 (PRDX1). In conclusion, this study defines the pathway of GRASP55-mediated unconventional protein secretion and provides important insights into the progression of Huntington's disease.
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Project description:Accumulation of misfolded proteins in the endoplasmic reticulum (ER), defined as ER stress, results in activation of the unfolded protein response (UPR). UPR activation is commonly observed in neurodegenerative diseases. ER stress can trigger unconventional secretion mediated by Golgi reassembly and stacking proteins (GRASP) relocalization in cell lines. Here we study the regulation of GRASP55 by the UPR upon pharmacological induction of ER stress in primary mouse neurons. We demonstrate that UPR activation induces mRNA and protein expression of GRASP55, but not GRASP65, in cortical neurons. UPR activation does not result in relocalization of GRASP55. UPR-induced GRASP55 expression is reduced by inhibition of the PERK pathway of the UPR and abolished by inhibition of the endonuclease activity of the UPR transducer IRE1. Expression of the IRE1 target XBP1s in the absence of ER stress is not sufficient to increase GRASP55 expression. Knockdown of GRASP55 affects neither induction nor recovery of the UPR. We conclude that the UPR regulates the unconventional secretion factor GRASP55 via a mechanism that requires the IRE1 and the PERK pathway of the UPR in neurons.
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