Regulation of protein homeostasis by unconventional protein secretion in mammalian cells

Abstract

Secretion of proteins lacking leader sequence was deemed rare and unconventional, only accountable for the export of a limited number of clients by mechanisms that are poorly defined. However, recent studies have shown that many leaderless proteins misfolded in the cytoplasm can be selectively exported to extracellular milieu via an unconventional secretory path termed Misfolding-Associated Protein Secretion (MAPS). This process uses the surface of the endoplasmic reticulum (ER) as a platform to enrich abnormally folded polypeptides, and then transport them into the lumen of ER-associated late endosomes for subsequent secretion. Elimination of misfolded proteins via MAPS appears to serve a role in protein homeostasis maintenance, particularly for stressed cells bearing an excess of protein quality control (PQC) burden.

Keywords: DNAJC5/CSP; HSC70; Misfolding-associated protein secretion (MAPS); USP19; Unconventional protein secretion; cell-to-cell transmission; neurodegenerative diseases; proteasome; protein quality control; α-Synuclein.

Figure 1. Misfolding-associated protein secretion (MAPS)

Misfolded cytosolic proteins can be secreted by MAPS following these steps. In step 1, misfolded proteins are enriched on the surface of endoplasmic reticulum (ER) by binding to a receptor (e.g. USP19) that has a chaperone activity. For USP19-mediated secretion, USP19-associated chaperone HSC70 may assist USP19 in substrate recruiting. In step 2, misfolded proteins are transferred to DNAJC5, a HSC70 co-chaperone that is associated with the membrane of late endosomes (LE) (Note that Les are generally in close contact with the ER). Next, the complex of DNAJC5 and misfolded proteins are moved into the lumen of late endosome via a mechanism that is unclear. In step 4, late endosomes contact and fuse with the plasma membranes (PM) to release misfolded proteins.

Figure 2. Multiple pathways to generate secretory late endosome or lysosomes

Different unconventional protein secretion pathways may only differ in the initial substrate recruiting step. Cytosolic cargos can enter a secretory late endosomes (LE) via autophagosome (1) or through a Golgi independent vesicular trafficking route between the endoplasmic reticulum (ER) and LE (2), or in the case of MAPS (3), through direct translocation into the LE lumen. Invagination of LE membrane also generates intraluminal vesicles (green circles). The fusion of late endosomes with the plasma membrane (PM) results in the secretion of both exosome-bearing and exosome independent cargos.

Figure 3. Proposed models for intercellular transmission of misfolded proteins

i, Misfolded proteins or protein aggregates are packed in to late endosome or lysosomes. These vesicles are moved from cell to cell through intercellular connections formed by the so called nano tube. PM, Plasma membrane. ii, In misfolding-associated secretion (MAPS), misfolded proteins are moved into late endosomes. These vesicles fuse with the plasma membrane, resulting in the secretion of misfolded proteins. Secreted proteins can enter a recipient cell by endocytosis. The mechanism(s) by which misfolded proteins enter lysosomes or late endosomes in model i and ii is unclear. iii, In exosome-mediated secretion, a fraction of the cytosol is engulfed into multi-vesicular body (MVB) during endosome maturation. When late endosomes containing MVB fuse with the plasma membrane, the cytosolic contents are released into the extracellular space in small vesicles termed exosome. Exosomes can then enter a recipient cell either by endocytosis or by fusion with the plasma membrane.