PNPT1-induced mitochondrial dysfunction drives osteoclast activation via post-transcriptional Nrf2 suppression and lipid peroxidation signaling.

Background

Polynucleotide phosphorylase 1 (PNPT1) functions as a crucial mitochondrial enzyme; nevertheless, its potential genetic correlation with osteoporosis and its specific regulatory impact on osteoclastogenesis remain to be elucidated.

Methods

We executed a two-sample Mendelian randomization (MR) strategy to interrogate the causal link connecting PNPT1 expression to osteoporosis risk. For in vivo substantiation, we utilized both an ovariectomized (OVX) murine model and an adeno-associated virus (AAV)-driven overexpression system. Extensive in vitro assays employing RANKL-stimulated RAW264.7 macrophages were conducted to evaluate osteoclast differentiation, mitochondrial dynamics, autophagic flux, and intracellular oxidative stress through molecular and morphological analyses.

Results

MR evaluations pinpointed genetically predicted elevated PNPT1 expression as a potential genetic risk factor for osteoporosis. In vivo observations revealed a significant surge of PNPT1 within the osteoclast precursors of OVX subjects. In vitro, the ectopic overexpression of PNPT1 significantly enhanced osteoclastogenesis and bone degradation while simultaneously triggering severe mitochondrial depolarization alongside the accumulation of reactive oxygen species (ROS). On the contrary, targeted Pnpt1 silencing markedly suppressed osteoclast maturation. Mechanistic probes demonstrated that PNPT1 disrupted autophagic flux, marked by p62 accumulation. Notably, even with a compensatory transcriptional rise in Nrf2 mRNA, PNPT1 overexpression provoked a marked downregulation of Nrf2 and xCT proteins, suggesting a potent post-transcriptional suppression of the cellular antioxidant shield. This uncoupling invariably precipitated sub-lethal lipid peroxidation that amplifies osteoclastogenic signaling. Concordantly, AAV-mediated systemic PNPT1 amplification aggravated trabecular bone deterioration in vivo.

Conclusion

Guided by our MR findings and validated through our functional models, PNPT1 emerges as a potential genetic risk factor for osteoporosis. By inciting mitochondrial damage, provoking ROS buildup, and decoupling the protective autophagy-Nrf2/xCT axis, PNPT1 promotes osteoclastogenesis, thereby introducing a promising immunopharmacological target for restraining pathological bone resorption.