Mitochondrial Protein MAPL Drives Inflammatory Cell Death via Lysosomal mtDNA Release

Mitochondria are well-established regulators of cell death, but new research from McGill University published in Nature Cell Biology reveals a previously unknown inflammatory pathway with direct relevance to Parkinson's disease and immune dysregulation. The study centers on MAPL (Mitochondrial Anchored Protein Ligase, gene name MUL1), an outer mitochondrial membrane SUMO E3 ligase previously known to promote apoptosis. The authors used adenoviral overexpression of MAPL alongside a RING-domain deletion mutant (ΔRING) to show that MAPL's cell-killing activity requires its SUMOylation enzymatic activity — cells expressing ΔRING survived, while MAPL-expressing cells died across multiple human cell lines including U2OS, human fibroblasts, and HUH-7 hepatocytes (p<0.0001 for U2OS vs. rtTA control).

To systematically map the death pathway, the team conducted a genome-wide CRISPR knockout survival screen in HUH-7 cells transduced with Ad-MAPL. The screen's top protective hit was CXADR (coxsackievirus and adenovirus receptor), providing internal validation. Crucially, the second-most enriched hit was cGAS (MB21D1), the cytosolic DNA sensor, with STING also among protective knockouts. Gene set enrichment analysis of top hits pointed overwhelmingly to inflammatory signaling — IL-1 cascades, MyD88 signaling, and Toll-like receptor pathways — rather than classical apoptotic machinery. Protective knockouts also included NLRP3, ASC1, caspase-1, and the gasdermin effectors GSDMD and GSDME, firmly placing MAPL-induced death within the pyroptosis category.

Experimentally, MAPL overexpression caused plasma membrane rupture detected by SYTOX Green uptake (a cell-impermeant DNA dye), whereas the classical apoptosis inducer tBID did not breach the membrane. siRNA depletion of either GSDMD or GSDME significantly protected cells from MAPL-induced membrane rupture, indicating co-dependency of both gasdermins at distinct steps in the pathway. Unlike tBID-induced death, MAPL-induced death was fully preserved in BAX/BAK double-knockout cells, confirming this is a non-apoptotic mechanism. MAPL expression also drove RING-dependent upregulation of IL-6 mRNA and protein secretion, NLRP3 mRNA and protein, and cleavage of GSDMD and GSDME — all blocked by the pan-caspase inhibitor ZVAD.

A key mechanistic finding was that cells entirely lacking mtDNA (Rho0 cells derived from 143b osteosarcoma cells) were fully protected from MAPL-induced pyroptosis as measured by SYTOX Green uptake, establishing mtDNA as the essential cGAS-activating ligand. The authors demonstrated that MAPL promotes the biogenesis of mitochondrial-derived vesicles (MDVs) that carry mtDNA cargo to lysosomes. Gasdermin pores — particularly GSDMD — then permeabilize the lysosomal membrane, releasing mtDNA into the cytosol where it activates cGAS-STING signaling and completes the pyroptotic cascade. This identifies lysosomes as an unexpected intermediary in cytosolic mtDNA sensing.

Perhaps most clinically striking, multiple Parkinson's disease-associated genes — VPS35 (retromer complex) and LRRK2 (a kinase mutated in familial PD) — were found to regulate MAPL-induced pyroptosis. Depletion of MAPL, LRRK2, or VPS35 each inhibited inflammatory cell death in primary macrophages, demonstrating that this mitochondria-lysosome signaling axis operates in genuine innate immune cells and not just engineered cell lines. These findings place MAPL and the MDV-lysosome pathway at the intersection of mitochondrial biology, innate immunity, and neurodegenerative disease mechanisms.