Muscle Energy Crisis is the Initial Driver of Locomotor Dysfunction in β-glucuronidase-Deficient Drosophila

Mucopolysaccharidosis type VII (MPS VII) is characterised by progressive locomotor decline, attributed to musculoskeletal and neurological defects. However, the severity varies according to the extent of β-glucuronidase (β-GUS) deficiency, with musculoskeletal deformities typically preceding neurological manifestations. To investigate the underlying cause of neuromuscular pathology, we employed β-GUS-deficient Drosophila models. In Drosophila , β-GUS is encoded by two genes, CG2135 and CG15117. Previously generated CG2135 ^-/- flies recapitulated several hallmark features of MPS VII, despite retaining approximately 30% residual β-GUS activity contributed by CG15117. To assess the consequences of complete loss of β-GUS, we now generated CG15117 ^-/- flies using CRISPR/Cas9 and subsequently established CG15117 ^-/- ;CG2135 ^-/- double knockout (DKO) flies. Phenotypic assessment revealed differences in susceptibility to starvation, lifespan, and locomotor function, with DKO flies exhibiting more severe impairments than either single knockout flies. Notably, CG15117 ^-/- did not show any significant defect in lifespan or locomotion, except under starvation conditions. Consistent with our earlier finding in CG2135 ^-/- fly brains, ATP depletion in DKO fly brains became evident only after 45 days of age, failing to explain the locomotory defects observed in earlier ages. Interestingly, analysis of muscles of 30-day-old CG2135 ^-/- and DKO flies revealed abnormal mitochondrial accumulation with autophagy defect and severe ATP depletion. Consequently, these defects led to locomotor impairment driven by apoptotic muscle degeneration. Collectively, this work provides the first evidence of tissue-specific vulnerability in MPS VII models, identifying muscle as an early pathological target and offering new insights into disease progression.