Protocol for qualitative analysis of lysosome immunoprecipitation from patient-derived glioblastoma stem-like cells.

Lysosomes are critical for the sustenance of glioblastoma stem-like cells (GSCs) properties. We present a protocol to enrich and purify lysosomes from patient-derived GSCs in culture. We describe the steps required to stably express a tagged lysosomal protein in GSCs, mechanically lyse cells, magnetically immunopurify lysosomes, and qualitatively assess these organelles. We then detail the procedure for retrieving intact and purified lysosomes from GSCs. We also specify cell culture conditions, storage procedures, and sample preparation for immunoblotting. For complete details on the use and execution of this protocol, please refer to Maghe et al.1.

The paracaspase MALT1 controls cholesterol homeostasis in glioblastoma stem-like cells through lysosome proteome shaping.

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.

DNA damage repair kinase DNA-PK and cGAS synergize to induce cancer-related inflammation in glioblastoma.

Cytosolic DNA promotes inflammatory responses upon detection by the cyclic GMP-AMP (cGAMP) synthase (cGAS). It has been suggested that cGAS downregulation is an immune escape strategy harnessed by tumor cells. Here, we used glioblastoma cells that show undetectable cGAS levels to address if alternative DNA detection pathways can promote pro-inflammatory signaling. We show that the DNA-PK DNA repair complex (i) drives cGAS-independent IRF3-mediated type I Interferon responses and (ii) that its catalytic activity is required for cGAS-dependent cGAMP production and optimal downstream signaling. We further show that the cooperation between DNA-PK and cGAS favors the expression of chemokines that promote macrophage recruitment in the tumor microenvironment in a glioblastoma model, a process that impairs early tumorigenesis but correlates with poor outcome in glioblastoma patients. Thus, our study supports that cGAS-dependent signaling is acquired during tumorigenesis and that cGAS and DNA-PK activities should be analyzed concertedly to predict the impact of strategies aiming to boost tumor immunogenicity.

Inhibition of the pseudokinase MLKL alters extracellular vesicle release and reduces tumor growth in glioblastoma.

Extracellular vesicles (EVs) are lipid-based nanosized particles that convey biological material from donor to recipient cells. EVs play key roles in glioblastoma progression because glioblastoma stem-like cells (GSCs) release pro-oncogenic, pro-angiogenic, and pro-inflammatory EVs. However, the molecular basis of EV release remains poorly understood. Here, we report the identification of the pseudokinase MLKL, a crucial effector of cell death by necroptosis, as a regulator of the constitutive secretion of EVs in GSCs. We find that genetic, protein, and pharmacological targeting of MLKL alters intracellular trafficking and EV release, and reduces GSC expansion. Nevertheless, this function ascribed to MLKL appears independent of its role during necroptosis. In vivo, pharmacological inhibition of MLKL reduces the tumor burden and the level of plasmatic EVs. This work highlights the necroptosis-independent role of MLKL in vesicle release and suggests that interfering with EVs is a promising therapeutic option to sensitize glioblastoma cells.

The glycoprotein GP130 governs the surface presentation of the G protein-coupled receptor APLNR.

Glioblastoma is one of the most lethal forms of adult cancer, with a median survival of ∼15 mo. Targeting glioblastoma stem-like cells (GSCs) at the origin of tumor formation and relapse may prove beneficial. In situ, GSCs are nested within the vascular bed in tight interaction with brain endothelial cells, which positively control their expansion. Because GSCs are notably addicted to apelin (APLN), sourced from the surrounding endothelial stroma, the APLN/APLNR nexus has emerged as a druggable network. However, how this signaling axis operates in gliomagenesis remains underestimated. Here, we find that the glycoprotein GP130 interacts with APLNR at the plasma membrane of GSCs and arbitrates its availability at the surface via ELMOD1, which may further impact on ARF-mediated endovesicular trafficking. From a functional standpoint, interfering with GP130 thwarts APLNR-mediated self-renewal of GSCs ex vivo. Thus, GP130 emerges as an unexpected cicerone to the G protein-coupled APLN receptor, opening new therapeutic perspectives toward the targeting of cancer stem cells.

Lysosomes in glioblastoma: pump up the volume.

Lysosomes are acidic, dynamic organelles that supervise catabolism, integrate signaling cascades, and tune cellular trafficking. Moreover, the loss of their integrity may jeopardize cell viability. In cancer cells, lysosomes are qualitatively and quantitatively modified for the tumor's own benefit. For all these reasons, these organelles emerge as appealing intracellular targets to manipulate non-oncogene addiction. This is of particular interest for brain diseases, including neurodegenerative disorders and cancer, in which stem cells are exhausted and transformed, respectively. Recent publications had demonstrated that stem cells displayed disarmed lysosomes in terms of number and functions during aging and oncogenic progression. Likewise, our laboratory identified that the arginine protease MALT1, normally dedicated to the assembly of proper NF-kB activation and processing a number of substrates, arbitrates lysosome biogenesis and mTOR signaling in glioblastoma stem-like cells. Indeed, blocking either the expression or the activity of this enzyme leads to an aberrant increase of lysosomes, alongside of the down-regulation of the mTOR signaling. This surge of lysosomes eradicates glioblastoma stem-like cells. Targeting lysosomes might thus inspire the design of new strategies to face this devastating human cancer. Here, we provide an overview of the functions of the lysosome as well as its role as a cell death initiator, to highlight the potential of lysosomal drugs for glioblastoma therapy.

Paracaspase MALT1 regulates glioma cell survival by controlling endo-lysosome homeostasis.

Glioblastoma is one of the most lethal forms of adult cancer with a median survival of around 15 months. A potential treatment strategy involves targeting glioblastoma stem-like cells (GSC), which constitute a cell autonomous reservoir of aberrant cells able to initiate, maintain, and repopulate the tumor mass. Here, we report that the expression of the paracaspase mucosa-associated lymphoid tissue l (MALT1), a protease previously linked to antigen receptor-mediated NF-κB activation and B-cell lymphoma survival, inversely correlates with patient probability of survival. The knockdown of MALT1 largely impaired the expansion of patient-derived stem-like cells in vitro, and this could be recapitulated with pharmacological inhibitors, in vitro and in vivo. Blocking MALT1 protease activity increases the endo-lysosome abundance, impairs autophagic flux, and culminates in lysosomal-mediated cell death, concomitantly with mTOR inactivation and dispersion from endo-lysosomes. These findings place MALT1 as a new druggable target involved in glioblastoma and unveil ways to modulate the homeostasis of endo-lysosomes.