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Summary: Glioblastoma stem-like cells (GSCs) compose a tumor-initiating and -propagating population remarkably vulnerable to variation in the stability and integrity of the lysosomal compartment. Previous work has shown that the expression and activity of the paracaspase MALT1 control GSC viability via lysosome abundance. However, the underlying mechanisms remain elusive. By combining RNA sequencing (RNA-seq) with proteome-wide label-free quantification, we now report that MALT1 repression in patient-derived GSCs alters the homeostasis of cholesterol, which accumulates in late endosomes (LEs)-lysosomes. This failure in cholesterol supply culminates in cell death and autophagy defects, which can be partially reverted by providing exogenous membrane-permeable cholesterol to GSCs. From a molecular standpoint, a targeted lysosome proteome analysis unraveled that Niemann-Pick type C (NPC) lysosomal cholesterol transporters are diluted when MALT1 is impaired. Accordingly, we found that NPC1/2 inhibition and silencing partially mirror MALT1 loss-of-function phenotypes. This supports the notion that GSC fitness relies on lysosomal cholesterol homeostasis.

Summary: The tumor suppressor CYLD is a deubiquitinating enzyme that removes non-degradative ubiquitin linkages bound to a variety of signal transduction adaptors. CYLD participates in the formation of primary cilia, a microtubule-based structure that protrudes from the cell body to act as a “sensing antenna.” Yet, how exactly CYLD regulates ciliogenesis is not fully understood. Here, we conducted an unbiased proteomic screen of CYLD binding partners and identified components of the centriolar satellites. These small granular structures, tethered to the scaffold protein pericentriolar matrix protein 1 (PCM1), gravitate toward the centrosome and orchestrate ciliogenesis. CYLD knockdown promotes PCM1 degradation and the subsequent dismantling of the centriolar satellites. We found that CYLD marshals the centriolar satellites by deubiquitinating and preventing the E3 ligase Mindbomb 1 (MIB1) from marking PCM1 for proteasomal degradation. These results link CYLD to the regulation of centriolar satellites proteostasis and provide insight into how reversible ubiquitination finely tunes ciliogenesis. : Douanne et al. find that a subset of the deubiquitinating enzyme CYLD is part of the centriolar satellites and controls their proteostasis, therefore allowing the formation of primary cilia. They show that CYLD removes ubiquitin from the E3 ligase MIB1 and prevents MIB1-mediated dismantlement of centriolar satellites. Keywords: CYLD, centriolar satellites, PCM1, proteostasis, MIB1, ciliogenesis, ubiquitin