ABSTRACT Background Although antiretroviral therapies (ART) have substantially reduced HIV‐associated mortality, HIV infection is still linked to an unexplained earlier onset and increased incidence of aging‐associated conditions like sarcopenia that negatively impact healthspan. Complex syndromes, like sarcopenia, often arise from a combination of genetic and environmental factors, so in this study, we investigated effects of short‐term treatment with emtricitabine (2′,3′‐dideoxy‐5‐fluoro‐3′‐thiacytidine; FTC), an FDA‐approved ART, on skeletal muscle DNA methylation patterns and transcriptome‐wide responses in a male murine model of HIV phenotypic biology (Tg26 mice). Methods We treated 6‐month‐old male Tg26 (+/−) mice or wildtype (WT) littermates on a C57BL/6 genetic background with FTC in the drinking water for 1 month; control groups received drinking water vehicle alone (VEH). Muscle function and body composition were measured longitudinally. Skeletal muscle methylation patterns, transcriptional changes and histological features were quantified at sacrifice. Results Although neither gross structural nor functional muscle deficits were observed in this short‐term study with ART usage, relative decreases in muscle endurance measured by hang time over the study were twofold more severe in the Tg26 as compared to WT mice (p = 0.0453), and markers of myogenic cell maturation (Myf5, Myf6) were disrupted in the Tg26 HIV model as compared to WT littermates in a manner exacerbated by FTC treatment. Fat mass, measured by DXA, also tended (p = 0.085) to be uniquely increased by FTC treatment in the Tg26 mice over the study. Differential methylation patterns and pathway enrichment data suggested that the presence of an HIV phenotype and exposure to an FTC regimen altered the methylation status in skeletal muscle genes such as Camk2B, Pcolce2 and Lima1 in a manner consistent with promoting eventual functional impairment in muscle. Additionally, RNAseq revealed differential gene expression profiles and key regulatory pathways including cellular differentiation, regulation of lipid metabolism and neuroactive ligand‐receptor interactions. Lipodystrophy‐related genes including Lep and Adipoq involved in fat distribution and metabolism along with skeletal genes related to regulation of muscle strength were affected by the presence of an HIV phenotype and ART treatment. Conclusions The current study provides insights into mechanisms by which a clinically relevant ART may influence DNA methylation and transcriptome changes in skeletal muscle in the context of HIV biology. The differentially regulated pathways suggest novel targets for understanding and eventually abrogating the harmful effects of long‐term ART use in people living with HIV (PLWH) on skeletal muscle mass and function.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that integrates environmental, microbial, and metabolic signals to regulate gene expression across diverse tissues. Recent studies highlight AhR’s role in skeletal homeostasis and joint biology, particularly through its interactions with microbiome-derived tryptophan metabolites and the Wnt/β-catenin signaling pathway. This review synthesizes current knowledge on AhR signaling mechanisms, including canonical and non-canonical pathways, and explores the impact of exogenous toxicants and microbiome-derived compounds and endogenous host metabolites that activate AhR. We discuss emerging evidence linking AhR activity to cartilage development, inflammation, and osteoarthritis progression, and highlight the utility of gnotobiotic mouse models in dissecting microbiome-AhR interactions. Finally, we examine the therapeutic potential of AhR modulation in joint disease, emphasizing its relevance as a pharmaceutical target for osteoarthritis and age-related musculoskeletal decline.

Mesenchymal stem cells (MSCs) and heterogeneous bone marrow stromal cells (BMSCs) are multipotent progenitors that can differentiate into osteoblasts and bone marrow adipocytes (BMAd). The role of BMAd in skeletal homeostasis is not yet fully understood, in part due to a lack of reproducible in vitro models that faithfully mimic the biology and molecular signatures of BMAds to study their behavior. Here, we report the in vitro generation of murine BMSC- and MSC-derived Osterix-expressing BMAd-like cells via a trans-differentiation model (TD-BMAd), the development of a semi-automated analysis platform for quantification of lipid-laden cells, and the use of these models to interrogate the role of the glucocorticoid receptor (GR) in bone marrow adipose tissue (BMAT) as a regulator of osteoclastic bone resorption. The TD-BMAd cells stored intracellular lipids and robustly expressed BMAd-associated genes, such as Sp7/Osx, Pparg, and Adipoq. The pro-osteoclastogenic gene Tnfsf11/Rankl was comparably expressed between TD-BMAd and osteoblasts. We previously reported that female mice with adult-onset conditional KO (CKO) of the GR in Osx-expressing cells exhibited a low cortical bone mass and high BMAT phenotype. Here, we demonstrate that this phenotype was associated with an increase in the abundance of cortical bone osteoclasts in female but not male GR-CKO mice, but that surprisingly, GR-deficient osteoblasts from these mice did not express higher levels of pro-osteoclastogenic Rankl. Instead, TD-BMAd from female (but not male) GR-CKO mice expressed significantly higher levels of Rankl as compared to cells from GR-WT mice, suggesting that GR-deficient BMAT may be the source of the sexually dimorphic osteoclastogenic phenotype seen in vivo. Although this methodology is not intricate in nature, this simple culture technique and subsequent quantification platform may have broader utility for further studies of BMAd biology and lipid-laden cells.

Tryptophan is an essential amino acid metabolized in the body primarily through enzymatic degradation involving indoleamine and tryptophan 2,3-dioxygenase (IDO and TDO), generating kynurenine as the initial step, and ultimately leads to multiple bioactive metabolites including NAD and quinolinic acid. We have previously shown that kynurenine accumulates with age and contributes to age-induced bone loss, and we hypothesized that knocking out IDO1 might be protective against bone loss in states such as aging by decreasing kynurenine levels in osteoprogenitor cells. To begin to characterize the target cell responsible for this effect, we generated conditional IDO1 knockout (KO) mouse model using floxed mice created by EUCOMM/IMPC and an osteoprogenitor-expressed (Osterix-promoter-driven) Cre possessing a Tet-off element. In an IACUC-approved protocol using male/female conditional IDO1 KO (cKO) C57BL/6 mice, we found sex-dependent differences in changes in bone mass. We report that bone mineral density (BMD) at the femoral (but not spinal) site was significantly higher in 21-month-old cKO male mice compared to wild-type (WT). In contrast, spinal (but not femoral) BMD was higher in 21-month-old female cKO mice versus WT. There were no significant differences in trabecular μCT parameters in female mice; however, male cKO mice showed increased trabecular number and decreased trabecular separation. In contrast, there were no differences between male mice in cortical bone measurements whereas female cKO mice had decreased cortical thickness. These data suggest that there are sex-dependent differences in the role of IDO1 in osteoprogenitor cells with aging.

Excessive mineralocorticoid receptor (MR) activation in the heart and vasculature leads to pathological effects such as extracellular matrix accumulation, oxidative stress, and sustained inflammation. While MR's role in cardiovascular and renal systems is well understood, MR signaling has also been implicated as a key driver of homeostasis and pathological changes in several other body systems including skeletal muscle and adipose tissue. The glucocorticoid receptor (GR) and MR are structurally and functionally linked, sharing 95% similarity in DNA binding domains and recognizing many of the same hormone response elements (HRE) as transcriptional regulators of target genes. The role of GR in bone has been defined through several mechanistic studies, whereas the role of MR in bone is understudied. Because mineralocorticoid signaling regulates renal sodium and calcium handling, chronic hyperaldosteronism may indirectly disrupt skeletal homeostasis through urinary calcium wasting and secondary alterations in parathyroid hormone signaling. Furthermore, MR inhibition through MR antagonists (MRAs) has been associated with beneficial skeletal effects, particularly in settings of hyperaldosteronism and 11β-HSD2 deficiency. In this review, we present historical and current scientific findings on the role of genomic MR signaling in bone and extra-skeletal tissues that may be involved in crosstalk with the skeletal system. Furthermore, we also highlight the availability of tools to study MR signaling in the context of the musculoskeletal system.

Aging is associated with alterations in endogenous tryptophan (TRP) metabolism that contributes to musculoskeletal decline. In this study, we investigated the effects of the microbiota-derived TRP metabolite, indole-3-propionic acid (IPA), on musculoskeletal health in aged mice and lifespan in Drosophila melanogaster. Aged C57BL/6 mice received IPA (20 mg/kg, subcutaneous, three times per week for 12 weeks), while Drosophila were maintained on food supplemented with IPA (100 µM) throughout their lifespan. Our findings revealed that IPA-treated aged mice exhibited enhanced muscle function (grip strength and hang time). Histological and bone microCT analyses revealed no changes in muscle fiber size but enhanced bone microarchitecture. Furthermore, molecular studies have elucidated that IPA treatment prevents oxidative stress and reduces senescence, indicating improved cellular survival. Our Drosophila melanogaster longevity analysis revealed a significant extension of lifespan, but lifespan effects were genotype- and sex-specific. Collectively, our findings identify IPA as a promising microbiota-derived metabolite that improves musculoskeletal health and promotes longevity, highlighting its potential as a therapeutic intervention for age-related decline in function.

The aryl hydrocarbon receptor (AhR) is activated by kynurenine (Kyn), a tryptophan metabolite that accumulates with age, and this process drives osteoblast dysfunction. However, Kyn can be further metabolized, and the extent to which downstream metabolite molecules activate AhR in mesenchymal lineage cells and impact bone formation activity was unclear from previous studies. We hypothesized that Kyn metabolites activate AhR signaling and impair bone formation to drive bone loss. In the current study, tryptophan, Kyn, and 3-hydroxy-kynurenine (3HK) dose-dependently activated AhR in mesenchymal stem cell models, with 3HK being the most potent activator. Treating mesenchymal stem cells with 3HK and 3-hydroxyanthranilic acid (3HAA) dose-dependently induced DNA damage that at lower concentrations induced senescence and at higher concentrations promoted apoptotic cell death. This cell death was rescued upon scavenging reactive oxygen species with N-acetylcysteine, suggesting a mechanism of apoptosis related to increased oxidative stress. With regards to bone formation activity, the differentiation of primary bone marrow stromal cells into matrix-producing osteoblasts was blunted upon the introduction of Kyn, 3HK or 3HAA into osteogenic differentiation media, with 3HK and 3HAA inducing the greatest deficits in mineralized matrix production. In vivo administration of 3HAA to C57BL/6 mice was detrimental to whole-body bone mineral density and cortical bone mass, although trabecular bone was largely unaffected. Together, our results suggest that several intermediate metabolites in the tryptophan-Kyn pathway activate AhR and impede the differentiation of osteoblasts by inducing DNA damage, senescence and oxidative stress, which may have negative consequences for cortical bone in vivo.

HIV-associated mortality has been reduced by antiretroviral therapies (ART), but prolonged ART usage by people living with HIV (PLWH) is associated with frailty and poor healthspan. Mechanisms driving this phenomenon are not fully known, but clinical and preclinical studies suggest that HIV and ART may drive aberrant activation of the aryl hydrocarbon receptor (AhR) by kynurenine (KYN), an endogenous metabolite of tryptophan. Therefore, we investigated whether the combination of an HIV-like phenotype (Tg26 mice) and treatment with ART (emtricitabine; FTC) in female mice alters skeletal muscle homeostasis in an AhR-dependent manner to promote premature muscle aging phenotypes. Short-term FTC treatment increased serum KYN:tryptophan ratio and activated AhR signaling in skeletal muscle of Tg26 mice, although the study duration was not sufficient to induce significant FTC-related functional decline. FTC, alone or in combination with other ART (tenofovir alafenamide and tenofovir disproxil fumarate), activated AhR and induced senescence of female myoblasts in a manner comparable to KYN. Sequencing-based studies revealed targets and pathways related to the impacts of an HIV phenotype and ART in female skeletal muscle, including Gnas (encoding Gsα protein, critical for muscle glucose metabolism), inflammatory pathways, and lipid metabolism. Our studies suggest that the combined presence of HIV viral proteins and exposure to ART induced activation of AhR-mediated signaling in female muscle, as well as widespread changes across the skeletal muscle transcriptome and methylation landscape that may contribute to development of muscle dysfunction. This suggests AhR may represent a novel target for addressing persistent disparities in healthspan for PLWH.

Osteocytes translate fluid shear stress into biochemical signals critical for bone homeostasis. Here, we combined 3-dimensional (3D) osteocyte culture, microgravity simulation, fluid shear mimicking reloading after disuse, and real-time calcium signaling analysis to elucidate responses of osteocytes under different mechanical environments. Ocy454 cells were seeded onto 3D scaffolds and cultured under static (control) or simulated microgravity (disuse) conditions using a rotating wall vessel bioreactor. Elevated expression levels of Sost, Tnfsf11 (Rankl), and Dkk1 were detected following disuse, confirming efficacy of the microgravity model. Cell membrane integrity under mechanical challenge was evaluated by subjecting scaffold cultures to fluid shear in medium containing FITC-conjugated dextran (10 kDa). The proportion of dextran-retaining cells, indicative of transient membrane disruption and subsequent repair, was higher in microgravity-exposed osteocytes than controls, suggesting increased susceptibility to membrane damage upon reloading following disuse. Intracellular calcium signaling was assessed under a high but physiological fluid shear stress (30 dynes/cm2). Scaffolds cultured under disuse conditions demonstrated a larger sub-population of osteocytes with high calcium signaling intensity (F/Fo > 10 fold) during fluid shear. The maximum fold change in calcium signaling intensity over baseline and the duration of the peak calcium wave were greater for osteocytes cultured under disuse as compared to static controls, however the bioreactor-cultured osteocytes showed, on average, fewer calcium waves than those cultured under control conditions. Subsequent experiments demonstrated that the sub-population of osteocytes with high calcium signaling intensity following exposure to disuse were those that had experienced a transient membrane disruption event during reloading. Together, these results suggest that simulated microgravity enhances osteocyte susceptibility to formation of transient membrane damage and alters intracellular calcium signaling responses upon reloading. This integrated approach establishes a novel platform for mechanistic studies of osteocyte biology and could inform therapeutic strategies targeting skeletal disorders related to altered mechanical loading.

Background:Although antiretroviral therapies (ART) have substantially reduced HIV-associated mortality, the increased lifespan achieved by widespread ART deployment has revealed that HIV infection is linked to an unexplained earlier onset and increased incidence of aging-associated conditions like sarcopenia. Complex syndromes, like sarcopenia, often arise from a combination of genetic and environmental factors, so in this study, we investigated effects of short-term treatment with emtricitabine (2',3'-dideoxy-5-fluoro-3'-thiacytidine; FTC), an FDA approved ART, on skeletal muscle DNA methylation patterns and transcriptome-wide responses in a male murine model of HIV phenotypic biology (Tg26 mice). Methods:We treated 6 month old male Tg26 (+/-) mice or wildtype (WT) littermates on a C57BL/6 genetic background with FTC in the drinking water for one month; control groups received drinking water vehicle alone (VEH). Muscle function and body composition were measured longitudinally. Skeletal muscle methylation patterns, transcriptional changes, and histological features were quantified at sacrifice. Results:Although neither gross structural nor functional muscle deficits were observed in this short-term study with ART usage, relative decreases in muscle endurance measured by hang time over the study were 2-fold more severe in the Tg26 as compared to WT mice (p=0.0453), and markers of myogenic cell maturation ( Myf5, Myf6 ) were disrupted in the Tg26 HIV model as compared to WT littermates in a manner exacerbated by FTC treatment. Fat mass, measured by DXA, also tended (p=0.085) to be uniquely increased by FTC treatment in the Tg26 mice over the study. Differential methylation patterns and pathway enrichment data suggested that the presence of an HIV phenotype and exposure to an ART regimen altered the methylation status in skeletal muscle genes such as Camk2B, Pcolce2 and Lima1 in a manner consistent with promoting eventual functional impairment in muscle. Additionally, RNAseq revealed differential gene expression profiles and key regulatory pathways including cellular differentiation, regulation of lipid metabolism, and neuroactive ligand-receptor interactions. Lipodystrophy-related genes including LEP, ADIPOQ and PPARα involved in fat distribution and metabolism along with skeletal genes related to regulation of muscle strength were affected by the presence of an HIV phenotype and ART treatment. Conclusions:The current study provides insights into mechanisms by which a clinically relevant ART may influence DNA methylation and transcriptome changes in skeletal muscle in the context of HIV biology. The differentially regulated pathways suggest novel targets for understanding, and eventually abrogating, the harmful effects of long-term ART use in PLWH on skeletal muscle mass and function.

The aryl hydrocarbon receptor (AhR) is proposed to mediate the frailty-promoting effects of the tryptophan metabolite kynurenine, which increases with age in mice and humans. The goal of the current study was to test whether administration of pharmacological AhR inhibitors, BAY2416964 and CH-223191, could abrogate musculoskeletal decline in aging mice. Female C57BL/6 mice (18 months old) were treated with vehicle (VEH) or 30 mg/kg BAY2416964 (BAY) via daily oral gavage 5 days/week for 8 weeks. A second AhR antagonist, CH-223191, was administered to 16-month-old male and female C57BL/6 mice via intraperitoneal injections (3.3 mg/kg) 3 days/week for 12 weeks. While grip strength declined over time in VEH-treated mice, BAY preserved grip strength in part by improving integrity of neuromuscular junctions (NMJs), an effect replicated during in vitro studies with siRNA against AhR. Cortical bone mass was also greater in BAY- than VEH-treated mice. Similarly, CH-223191 treatment improved cortical bone and showed beneficial effects in skeletal muscle, including reducing oxidative stress as compared with VEH-treated animals. Transcriptomic and proteomic data from BAY-treated mice supported a positive impact of BAY on molecular targets that affect NMJ function. Taken together, these data support AhR as a therapeutic target for improving musculoskeletal health during aging.

Germline and osteoblast-directed deletion of G protein-gated inwardly rectifying K+ channel 3 (Girk3) was recently shown to increase bone mass after 18 wk of age in male mice. Here, we show that germline Girk3 deletion also increases trabecular and cortical bone mass and increases the mechanical strength of the femur in female mice after 18 wk of age. Unlike male mice, however, osteoblast-directed Girk3 deletion using 2.3 kb-Col1a1-Cre does not increase bone mass in adult female mice. To discover mechanisms underlying high bone mass in female Girk3-/- mice, bulk RNA-sequencing was performed on 2-d-old calvarial bone, revealing lower expression of proinflammatory cytokines such as IL-1β and IL-6 in Girk3-/- mice. Accordingly, cytokines and chemokines are largely suppressed in the circulation of adult Girk3-/- mice compared to WT littermates. The cytokines GM-CSF, IL-1β, IL-2, and IL-9 are reduced in the serum of both male and female Girk3-/- mice, while eotaxin, IFNγ, MIP-1α, and others are sexually dimorphic. Histomorphometry reveals that osteoclast activity is modestly reduced in Girk3-/- bone, which is supported by in vitro osteoclast resorption assays. However, deletion of Girk3 in myeloid-lineage cells with LysM-Cre is not sufficient to recapitulate high bone mass in either male or female mice. Moreover, female Girk3-/- mice are not protected from ovariectomy-induced bone loss. Finally, single-cell screening using cytometry by time-of-flight in the BM revealed no differences in immune cell abundances due to global Girk3 deletion. Taken together, while Girk3 regulates inflammatory cytokine expression in the bone and serum, deletion of Girk3 in myeloid-lineage cells does not affect bone mass.

Obesity is linked to increased fracture risk. Despite the negative effects of weight loss on the skeleton, patients with obesity are advised to lose weight via calorie restriction. Obesity and weight loss individually alter both whole-body and local metabolism. Little is known about changes to bone mass and metabolome following calorie restriction in obese preclinical models. We hypothesized that caloric restriction would reduce bone mass in obese mice and would alter the cortical bone metabolome. To induce obesity, 8-week-old male and female C57BL/6J mice received 60 % kCal high-fat diet for 12 weeks. From 20 to 30 weeks of age, mice either remained obese or lost weight through 30 % caloric restriction. Controls consumed 10 % kCal low-fat diet. Compared to obesity, calorie restriction elicited cortical bone loss and trabecular thinning. Weight loss also reduced bone formation. Both obesity and subsequent calorie restriction altered the cortical bone metabolome in a sex-dependent manner. Metabolic pathways altered with diet generally mapped to amino acid or fatty acid metabolism. In males, weight loss was associated with a downregulation of pathways related to tryptophan, tyrosine, ubiquinone, and fatty acids. In females, calorie restriction downregulated taurine and hypotaurine metabolism but upregulated pyrimidine metabolism, nicotinate and nicotinamide metabolism, and pantothenate and CoA biosynthesis. In summary, despite improvements in components of systemic metabolism, caloric restriction in obese preclinical models reduced bone mass and did not restore the cortical metabolome to control conditions.

Kynurenine (KYN), a tryptophan metabolite that increases with age, impairs osteoblast function. The aryl hydrocarbon receptor (AhR) has been proposed to mediate KYN's actions in bone. To test whether deletion of AhR in osteoblasts is beneficial for bone, we established an adult-onset AhR conditional knockout (CKO) model using Osx-Cre and examined the effects of AhR CKO at 4.5 and 6 mo of age (representing ~6 and 12 wk of CKO). While BMSC-derived osteoblasts from WT mice demonstrated reduced matrix formation from KYN treatment, AhR CKO osteoblasts were unaffected by KYN. Kynurenine's harmful effects were most pronounced in the middle of an osteoblastic differentiation time course, and these effects could be rescued via the AhR antagonist BAY2416964. In vivo, AhR deletion in Osx-expressing cells promoted sex- and compartment-specific skeletal phenotypes. Trabecular bone was increased in the distal femur of male and female AhR CKO mice at both 4.5 and 6 mo of age, potentially driven by a net decrease in the ratio of trabecular osteoclasts to osteoblasts despite a reduction in mineral apposition rate at 6 mo of age. In contrast, cortical bone phenotypes induced by AhR deletion depended on age and sex. In males, cortical bone volume fraction (Ct.BV/TV) was elevated in AhR CKO mice vs WT littermates at 4.5 mo of age, but differences resolved by 6 mo of age. In contrast, cortical bone was reduced in female AhR CKO as compared to WT littermates at 6 mo of age. These results underscore the complexity of AhR signaling in skeletal biology that must be considered while exploring AhR as a therapeutic target for conditions like osteoporosis and musculoskeletal frailty. Future studies will be needed to test the effects of osteoblastic AhR deletion at advanced ages, when the endogenous AhR ligand KYN is elevated in the circulation and skeletal niche.

Obesity and calorie restriction each negatively affect skeletal health. Despite the negative effects of weight loss on the skeleton, obese patients are advised to lose weight via calorie restriction. Additionally, obesity and weight loss individually alter both whole-body and local metabolism. Little is known about bone quality and changes to the cortical metabolome following calorie restriction in obese preclinical models. We hypothesized that caloric restriction would worsen bone quality in obese mice by shifting the cortical bone metabolome. To induce obesity, 8-week-old male and female C57BL6/J mice received 60% high-fat diet for 12 weeks. From 20 to 30 weeks of age, mice either remained obese or lost weight through 30% caloric restriction. Control animals received a 10% low-fat diet. Bodyweight and fat mass were increased by obesity and decreased with calorie restriction. Similarly, glucose and insulin tolerance were worsened with obesity but improved by weight loss. Compared to obesity, calorie restriction elicited more bone loss in both cortical and trabecular compartments. Weight loss also reduced bone formation. Both obesity and subsequent calorie restriction altered the cortical bone metabolome in a sex-dependent manner. Metabolic pathways altered with diet generally mapped to amino acid or fatty acid metabolism. In males, weight loss was associated with a downregulation of pathways related to tryptophan, tyrosine, ubiquinone, and fatty acids. In females, calorie restriction downregulated taurine and hypotaurine metabolism but upregulated pyrimidine metabolism, nicotinate and nicotinamide metabolism, and pantothenate and CoA biosynthesis. Our findings highlight the negative effects of obesity and subsequent caloric restriction on the skeleton. Despite improvements in components of systemic metabolism, caloric restriction in obese preclinical models did not restore bone morphology or the cortical metabolome to control conditions.

Tryptophan metabolism is a critical regulator of physiological and pathological processes, primarily through the kynurenine (KYN), serotonin and indole pathways. Dysregulation of indoleamine 2,3-dioxygenase 1 (IDO1) activity, serotonin and indole gut-microbial metabolism has been linked to a broad range of age-related chronic conditions, including cancer, cardiovascular disease, sarcopenia, and neurodegenerative disorders. Exercise emerges as a potent modulator of these pathways, redirecting tryptophan utilization to limit the accumulation of KYN metabolites while maintaining balanced indole and serotonin production. By regulating IDO1 activity and KYN flux, exercise alleviates inflammation, restores metabolic homeostasis, improved muscle integrity, neuroprotection, and overall systemic health. Mounting evidence supports the notion that lifestyle-based interventions targeting IDO1 and its downstream metabolites, particularly by physical activity, may offer a promising avenue for extending health span and mitigating the burden of chronic disease. This review synthesizes current advances in understanding the regulation of tryptophan metabolism (KYN, Serotonin and Indole) and highlights the unique capacity of exercise to remodel these pathways, underscoring their therapeutic potential in the context of healthy aging.

Musculoskeletal disorders (MSDs), notably sarcopenia and osteoporosis, profoundly affect aging individuals. This review explores lipid metabolism's role in age-related MSD pathophysiology, highlighting fatty acid uptake, lipid signaling, and lipotoxicity in muscle deterioration. It further addresses lipid-mediated regulation of osteoclasts, osteoblasts, and bone remodeling, emphasizing age-associated metabolic shifts exacerbating bone loss. Emerging therapeutic strategies targeting lipid pathways for MSD treatment are also discussed. This review integrates recent findings in muscle and bone lipid metabolism to deepen understanding of lipid dysregulation in musculoskeletal disorders and explore potential metabolic intervention strategies.

The glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) are ligand-activated transcription factors that regulate epidermal homeostasis, inflammation, and function. Prior studies using epidermal-specific conditional single and double knockout mice have shown their importance in skin physiology; however, clinically human disease is largely treated pharmacologically. Our objective was to examine how systemic MR/GR antagonism affects cutaneous gene expression and epidermal thickness in aged (18-month-old) C57BL/6J female mice. Mice were treated with selective GR (relacorilant), selective MR (eplerenone), or dual GR/MR (miricorilant) antagonists for 8 weeks. Quantitative RT-qPCR analysis of the skin showed that miricorilant significantly upregulated Sgk1, a GR/MR target. Miricorilant also increased the expression of keratinocyte differentiation markers and downregulated key inflammatory cytokines and Col3a1, a collagen subtype associated with tissue remodeling. Relacorilant suppressed Scnn1g, a subunit of the epithelial sodium channel. None of the antagonists significantly altered proliferation markers, epidermal thickness, or regulators of glucocorticoid activity. Our findings show that miricorilant downregulated inflammatory cytokines and increased differentiation marker expression without affecting epidermal thickness, suggesting its potential to treat inflammatory skin diseases. The results contrast with data from GR/MR knockout studies, highlighting the likely significance of receptor dynamics. Further studies of antagonist effects on receptor interactions with co-regulators appear warranted.