The autophagic clearance of damaged lysosomes by lysophagy involves extensive modification of the organelle with ubiquitin, but the underlying ubiquitination machinery is still poorly characterized. Here, we use an siRNA screening approach and identify human UBE2QL1 as a major regulator of lysosomal ubiquitination, lysophagy, and cell survival after lysosomal damage. UBE2QL1 translocates to permeabilized lysosomes where it associates with damage sensors, ubiquitination targets, and lysophagy effectors. UBE2QL1 knockdown reduces ubiquitination and accumulation of the critical autophagy receptor p62 and abrogates recruitment of the AAA‐ATPase VCP/p97, which is essential for efficient lysophagy. Crucially, it affects association of LC3B with damaged lysosomes indicating that autophagosome formation was impaired. Already in unchallenged cells, depletion of UBE2QL1 leads to increased lysosomal damage, mTOR dissociation from lysosomes, and TFEB activation pointing to a role in lysosomal homeostasis. In line with this, mutation of the homologue ubc‐25 in Caenorhabditis elegans exacerbates lysosome permeability in worms lacking the lysosome stabilizing protein SCAV‐3/LIMP2. Thus, UBE2QL1 coordinates critical steps in the acute endolysosomal damage response and is essential for maintenance of lysosomal integrity.

Background: The ex vivo generation of human hematopoietic stem cells (HSCs) with long-term repopulating capacity and multi-lineage differentiation potential represents the holy grail of hematopoiesis research. In principle, human induced pluripotent stem cells (hiPSCs) provide the tool for both studying molecular mechanisms of hematopoietic development and the ex vivo production of ‘true' HSCs for transplantation purposes and lineage-specific cells, e.g. red blood cells, for transfusion purposes. CD43-expressing cells have been reported as the first hematopoietic cells during development, but whether or not these possess multilineage differentiation and long-term engraftment potential is incompletely understood. Methods: We performed ex vivo generation of hematopoietic cells from hiPSCs using an embryoid body(EB)-based, xeno-product-free differentiation protocol. We investigated the multilineage differentiation potential of different FACS-sorted CD43-expressing cell subsets by colony-forming assays in semisolid media. Further, erythroid differentiation was investigated in more detail using established protocols. Results: By using CD43, we were able to measure hematopoietic induction efficiency during hiPSC-derived EB differentiation. Further, we determined CD43+ cells as the cell population of origin for in vitro erythropoiesis. Furthermore, colony formation demonstrates that the multipotent hematopoietic stem and progenitor cell fraction is particulary enriched in the CD43^hi CD45+ population.

Significance Classic serine proteases are synthesized as inactive precursors that are proteolytically processed, resulting in irreversible activation. We report an alternative and reversible mechanism of activation that is executed by an inactive protease. This mechanism involves a protein complex between the serine protease HTRA1 and the cysteine protease calpain 2. Surprisingly, activation is restricted as it improves the proteolysis of soluble tau protein but not the dissociation and degradation of its amyloid fibrils, a task that free HTRA1 is efficiently performing. These data exemplify a challenge for protein quality control proteases in the clearing of pathogenic fibrils and suggest a potential for unexpected side effects of chemical modulators targeting PDZ or other domains located at a distance to the active site.