A preliminary assessment of physical function and transciptome dynamics in aged mice with constitutive activation of the aryl hydrocarbon receptors in skeletal muscle

Background: Physical frailty is a major contributor to poor outcomes with aging, including a greater risk of death, underscoring the imperative for effective treatments. Recent investigations have uncovered a significant association between elevated levels of L-kynurenine (L-Kyn) and the onset of frailty, muscle weakness, and neuromuscular junction degeneration in both humans and murine models, though the causal mechanisms remain unverified. L-Kyn, a catabolic product of tryptophan metabolism, has been implicated in motor neuron degeneration, skeletal muscle atrophy, and mitochondrial dysfunction—phenotypic hallmarks of aging muscle tissue. Intriguingly, L-Kyn is an endogenous ligand for the aryl hydrocarbon receptor (AHR), a transcription factor primarily recognized for its role in the regulation of xenobiotic metabolism. While the AHR has been reported to exert both pro- and anti-aging effects in various contexts, chronic AHR activation is a feature of several pathological conditions that manifest with strikingly similar phenotypic profiles to those observed in the frail elderly. Objective: The objective of this study was to determine if constitutive activation of the AHR in skeletal muscle can exacerbate the decline of physical function with aging in mice. Methods: Aged (16-mo.) C57BL6 mice (male N=120, female N=120) from the NIA breeding colony were randomized to interventions employing muscle-specific adeno-associated virus (AAV) with the following three treatment groups: 1) a constitutively active AHR (CAAHR) mutant was constructed by deleting the ligand binding domain (amino acids 277-418); 2) a transcriptionally deficient AHR mutant (arginine to aspartate mutation at amino acid 39, R39D); and an empty vector control. Treadmill endurance, forelimb grip strength, and maximal walking speed were measured at baseline and at 26-mos. of age. RNAseq was also employed using untreated 16 mo. skeletal muscles (N=4) and 26 mo. gastrocnemius muscles from each of the AAV-treated groups (N=4/group). Results: In male mice, significant declines in performance were observed in most of the 26 mo. treatment groups compared to 16-mo., with the most pronounced declines seen in the constitutively active AHR treatment groups at 26 mo. Specifically, walking speed decreased 7.22% ( P =0.047), grip strength decreased 8.73% ( P =0.017), and treadmill endurance decreased 14.75% ( P =0.071) in 26 mo. constitutively active AHR treated groups compared to 16 mo. baseline. Though there were age dependent declines in physical function assessments in the female groups, this was not exacerbated in the CAAHR group compared to empty vector at 26 mo. RNAseq data revealed that CAAHR expression resulted in 1195 differentially expressed genes (511 up and 684 down) compared to empty vector control mice (only males?). Pathway analysis found that differentially expressed genes were enriched for processes such as cellular metabolism, energy production, and mitochondrial function and respiratory chain activity (were these down-regulated vs empty vector? Perhaps indicate a general direction after each process class above). Oxidative phosphorylation conductance was reduced 42% ( P <0.05) in 26 mo. control vector treated mice compared to 16 mo. baseline mice, with a further 18% ( P =0.07) decline seen in 26 mo. males with constitutively active AHR. Conclusions: These preliminary results in aging C57BL6 mice indicate that increased transcriptional activity of the AHR exacerbates the decline of physical function and mitochondrial oxidative phosphorylation conductance in aged mice in males only, which may suggest a role for AHR hyperactivation in frailty. Supported by grants from the National Institutes of Health R01AG076490 (Ryan & Hepple), NIA F32AG087637 amp; T32AG062728 (Wimberly). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.