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Abstract Skeletal muscle regeneration by muscle satellite cells is a physiological mechanism activated upon muscle damage and regulated by Notch signaling. In a family with autosomal recessive limb‐girdle muscular dystrophy, we identified a missense mutation in POGLUT1 (protein O‐glucosyltransferase 1), an enzyme involved in Notch posttranslational modification and function. In vitro and in vivo experiments demonstrated that the mutation reduces O‐glucosyltransferase activity on Notch and impairs muscle development. Muscles from patients revealed decreased Notch signaling, dramatic reduction in satellite cell pool and a muscle‐specific α‐dystroglycan hypoglycosylation not present in patients' fibroblasts. Primary myoblasts from patients showed slow proliferation, facilitated differentiation, and a decreased pool of quiescent PAX7+ cells. A robust rescue of the myogenesis was demonstrated by increasing Notch signaling. None of these alterations were found in muscles from secondary dystroglycanopathy patients. These data suggest that a key pathomechanism for this novel form of muscular dystrophy is Notch‐dependent loss of satellite cells.

Summary: Background: The most common form of neuronal ceroid lipofuscinosis (NCL) is juvenile CLN3 disease (JNCL), a currently incurable neurodegenerative disorder caused by mutations in the CLN3 gene. Based on our previous work and on the premise that CLN3 affects the trafficking of the cation-independent mannose-6 phosphate receptor and its ligand NPC2, we hypothesised that dysfunction of CLN3 leads to the aberrant accumulation of cholesterol in the late endosomes/lysosomes (LE/Lys) of JNCL patients’ brains. Methods: An immunopurification strategy was used to isolate intact LE/Lys from frozen autopsy brain samples. LE/Lys isolated from samples of JNCL patients were compared with age-matched unaffected controls and Niemann–Pick Type C (NPC) disease patients. Indeed, mutations in NPC1 or NPC2 result in the accumulation of cholesterol in LE/Lys of NPC disease samples, thus providing a positive control. The lipid and protein content of LE/Lys was then analysed using lipidomics and proteomics, respectively. Findings: Lipid and protein profiles of LE/Lys isolated from JNCL patients were profoundly altered compared to controls. Importantly, cholesterol accumulated in LE/Lys of JNCL samples to a comparable extent than in NPC samples. Lipid profiles of LE/Lys were similar in JNCL and NPC patients, except for levels of bis(monoacylglycero)phosphate (BMP). Protein profiles detected in LE/Lys of JNCL and NPC patients appeared identical, except for levels of NPC1. Interpretation: Our results support that JNCL is a lysosomal cholesterol storage disorder. Our findings also support that JNCL and NPC disease share pathogenic pathways leading to aberrant lysosomal accumulation of lipids and proteins, and thus suggest that the treatments available for NPC disease may be beneficial to JNCL patients. This work opens new avenues for further mechanistic studies in model systems of JNCL and possible therapeutic interventions for this disorder. Funding: San Francisco Foundation.