Metformin improves mitophagy-related pathways and mitochondrial-lysosomal homeostasis in chromosome 21 trisomic fibroblasts.

Mitochondrial dysfunction is a core cellular abnormality in Down syndrome (DS), yet the mechanisms underlying impaired mitochondrial quality control remain poorly defined. Here, we provide an integrated characterization of the autophagy-mitophagy-lysosome axis in human trisomic fetal fibroblasts (DS-HFFs). We show that trisomic cells display a paradoxical combination of elevated basal autophagy/mitophagy-related markers and defective mitochondrial clearance, indicative of a saturated and inefficient quality control system. DS-HFFs exhibit increased steady-state levels of PINK1, PARKIN, OPTN, NDP52, LC3-II, and p62, accumulation of autophagic vacuoles, reduced autophagic flux, and impaired delivery of mitochondria to lysosomes. We further demonstrate that metformin alleviates the saturation of the autophagy-mitophagy pathway by modulating multiple components of the mitochondrial quality control pathway, including attenuation of mTOR-associated signaling, enhancement of autophagic flux and normalization of mitophagy-related protein levels. Crucially, metformin increases mitochondria-lysosome association and rescues lysosomal degradative competence as shown by increased Dq-BSA activity, and increased maturation of Cathepsin D. Together, our findings support mitophagy inefficiency due to pathway saturation as a key pathogenetic mechanism in trisomic cells and identify metformin as a pharmacological tool capable of relieving the proteostatic overload intrinsic to trisomy 21. More broadly, these results align with a model in which aneuploidy imposes a chronic proteostatic burden that compromises organelle quality control.