PROJECT SUMMARY/ABSTRACT Preserving movement-related independence is a clinical and public health priority, as well as a major goal of the National Institute on Aging and NIH’s strategic plan for research on aging. Our work has demonstrated that low- functioning (LF) older adults have a more rapid functional decline than those who are high-functioning (HF). The biological mechanisms that lead to accelerated functional decline in LF older adults remain poorly understood, and few therapies are available to prevent its progression. As diverse as the etiologies of physical disability are, a growing body of evidence strongly implicates the mitochondria (Mt) as playing a key role in the initial onset and progression of functional decline in many individuals. Mitochondrial dysfunction has been directly linked to accelerate telomere attrition, genome instability, epigenetic alterations, stem cell exhaustion, cellular senescence, impaired proteostasis, and deregulated nutrient signaling, all key hallmarks of aging. What’s more, perturbations in cellular and mitochondrial iron transport and handling may contribute to increased mutations and deletions, which lead to a reduction in mtDNA copy numbers and thereby negatively affect mitochondrial (Mt) function. Cellular iron import and export are critical for optimal cellular function. Iron levels are modulated by the hormone hepcidin via binding and subsequent degradation of the iron export protein ferroportin (Fn). Iron acquisition (import) occurs through transferrin receptor and is highly responsive to intracellular iron levels. We also have documented increased skeletal muscle Mt iron stores which increased oxidative stress and the susceptibility of Mt permeability transition pore (PTP) opening (a measure of Mt resiliency) with age. Based on our pilot data our central hypothesis is that greater circulating hepcidin levels and muscle iron deregulation (↓Ferroportin → ↑ Cellular and Mt Iron Levels → ↓TfR-1) in LF older adults will lead to Mt dysfunction (↑Sensitivity to PTP, ↑Deletions/Damage, ↓Mitochondrial Respiration) and accelerated progression of functional decline in LF compared to HF older adults. To test our central hypothesis, we will conduct a prospective longitudinal study in which we will follow HF and LF research participants (70 to 80 years) for 3 years and obtain plasma and skeletal muscle biochemical measures of iron regulation and mitochondrial dysfunction at baseline and throughout the follow-up period. The Short Physical Performance Battery (SPPB) will be used to classify LF (SPPB ≤ 9) and HF (SPPB > 10) participants. We will annually assess the participants’ physical function through established measures (SPPB, 6-minute walk, and muscle strength). We will assess changes in health behaviors, including activity levels, dietary intake, and sleep at annual follow-up visits. For the proposed study, we will examine cross-sectional and longitudinal associations of dysfunctional iron regulation wi...