Sieck, G C

Abstract Age‐associated muscle weakness and atrophy of limb muscles, termed sarcopenia, is a major factor in the morbidity of the elderly. It is becoming increasingly recognized that a substantial contribution to the sarcopenic phenotype arises from motor neuron (MN) death in old age and subsequent muscle denervation. In human and rodent models, mitochondrial dysfunction is a leading culprit in both muscle and MN deterioration with age. In other motor pools within the Fischer 344 (F344) aging rat model, we showed that sarcopenia is selective to type IIx/b muscle fibers. We also showed a loss of larger MNs and denervation of the IIx/b fibers, which together comprise more fatigable fast (type FF) motor units. This selective vulnerability of larger MNs and type IIx/b muscle fibers to sarcopenia is further reflected by reductions in oxidative capacity of IIx/b fibers, as assayed by determining the maximum velocity of the succinate dehydrogenase reaction (SDHmax). Here, we hypothesize similar changes will occur in F344 tibialis anterior (TA) muscle from young (6‐months) and old (24‐months) groups. We also predict lumbar MN death in young and old F344 rats. We found selective atrophy of type IIx/b fibers in the TA from old rats. In old age, SDHmax was reduced but only in IIx/b TA fibers. These muscle observations were concomitant with the death of larger lumbar MNs in old age. This study indicates the remarkable selectivity of type FF TA motor units to age‐associated perturbations and sarcopenia.

Abstract Rationale Normal lung aging is characterized by progressive structural and functional decline that reduces respiratory efficiency. Aging is associated with excessive reactive oxygen species production and impaired homeostatic pathways that maintain mitochondrial quality control, involving mitophagy (removal of damaged mitochondria) and mitochondrial biogenesis (generation of new mitochondria). In human airway smooth muscle (hASM), reduced mitochondrial quality control can lead to the accumulation of damaged mitochondria, compromising ATP production and decreasing cellular viability. Maintaining healthy mitochondria involves a balance between mitophagy and mitochondrial biogenesis. A key regulator of this balance is PARIS, normally a target of the PINK1/pParkinS65 mitophagy pathway. PARIS transcriptionally inhibits PGC1α and NRF2 expression, key regulators of mitochondrial biogenesis and redox homeostasis. Furthermore, NRF2 can upregulate PINK1 expression, enhancing mitophagy. Therefore, we hypothesize that aging involves unmitigated mitochondrial dysfunction due to accumulation of PARIS and an imbalance between the removal of damaged mitochondria (mitophagy) and mitochondrial biogenesis. Methods hASM cells were dissociated from bronchiolar tissue samples from patients with no history of smoking or respiratory diseases and cultured (≤3 passages). Prior to experimentation, hASM cells from young (≤40 yrs) and old (≥70 yrs) patients (female and male) were serum deprived and phenotyped. In both age groups, targeted siRNA-mediated knockdown of PARIS was also performed. The extent of mitochondrial damage in hASM cells was determined by changes in mitochondrial membrane potential (Ψm) using TMRM (250 nM), and the extent of co-labeling of MitoTracker Red (200 nM) and CellLight GFP. Mitochondrial turnover was quantified by confocal imaging of hASM cells transfected with pMitoTimer. Mitophagy was determined by PINK1 recruitment to mitochondria, pParkinS65 and ubiquitin (pUbS65) phosphorylation. Total PARIS and pParkinS65-ubiquitinated PARIS (by immunoprecipitation) levels were determined. Mitochondrial biogenesis was determined by PGC1α and NRF2 mRNA and protein expression and mitochondrial DNA (mtDNA) copy number. Results In hASM cells from older patients, there was extensive mitochondrial damage, decreased PINK1 recruitment to mitochondria and decreased pParkinS65 and pUbS65 phosphorylation. Total PARIS accumulation increased due to decreased PARIS ubiquitination. Also, PGC1α and NRF2 expression, and mtDNA copy number decreased, likely due to PARIS-mediated inhibition. Knockdown of PARIS increased PGC1α and NRF2 expression, thereby promoting mitochondrial biogenesis and NRF2-mediated antioxidant responses and increasing PINK1 expression, thereby activating mitophagy. Conclusion Aging results in impaired mitochondrial quality control in hASM cells. The PARIS-NRF2 axis presents a promising target for therapeutic intervention to mitigate oxidative damage, maintain mitochondrial homeostasis and improve lung function with aging. This abstract is funded by: HL157984 (GCS) and AG44615 (GCS)