Faculty Bibliography

Dietary protein is a key regulator of metabolic health in humans and rodents. Many of the benefits of protein restriction are mediated by reduced consumption of dietary branched-chain amino acids (BCAAs; leucine, valine and isoleucine), and restriction of the BCAAs is sufficient to extend healthspan and lifespan in mice. While the BCAAs have often been considered as a group, it has become apparent that they have distinct metabolic roles, and we recently found that restriction of isoleucine is sufficient to extend the healthspan and lifespan of male and female mice. Here, we test the effect of lifelong restriction of the BCAA valine on healthy aging. We find that valine restriction (Val-R) improves metabolic health in C57BL/6J mice, promoting leanness and glycemic control in both sexes. To investigate the molecular mechanisms engaged by Val-R with aging, we conducted multi-tissue transcriptional profiling and gene network analysis. While Val-R had a significantly greater molecular impact in the liver, muscle, and brown adipose tissue of female mice than males, there was a stronger gene enrichment with phenotypic traits in male mice. Further, we found that phenotypic changes are associated with a multi-tissue downregulation of the longevity associated PI3K-Akt signaling pathway. Val-R reduces frailty in both sexes and extends the lifespan of male by 23%, but does not extend female lifespan, corresponding with a male-specific downregulation of PI3K-Akt signaling. Our results demonstrate that Val-R improves multiple aspects of healthspan in mice of both sexes and extends lifespan in males, suggests that interventions that mimic Val-R may have translational potential for aging and age-related diseases.

Nonalcoholic fatty liver disease (NAFLD) and metabolic syndrome (MetS) have been linked to osteoporosis and osteoarthritis (OA), where the prevalence of all increase with age. Many individuals with NAFLD also exhibit MetS, a condition that is now termed metabolic dysfunction-associated steatotic liver disease (MASLD). MASLD spans from simple hepatic steatosis to hepatocyte ballooning and inflammation, termed metabolic dysfunction-associated steatohepatitis (MASH), which may occur with or without fibrosis. To delineate the contribution of liver injury to skeletal deterioration within the context of metabolic syndrome (MetS), we fed male mice a high-fat, cholesterol, and fructose (HFCF) diet that induces metabolic syndrome-associated steatohepatitis (MASH) and liver fibrosis, associated with moderate obesity but without profound insulin resistance. A nutrient-matched diet with high carbohydrates but low in fat, cholesterol, and fructose (LFCF), which induced steatosis in adult mice, served as the control. Micro-CT analysis of the femur of HFCF-fed mice that developed MASH with fibrosis revealed significant cortical thinning (reduced bone area and thickness), decreased trabecular thickness, and lower bone mineral density compared to LFCF-fed mice, with no liver fibrosis. In the knee joint, MASH with fibrosis was associated with subchondral bone loss, medial cartilage erosion, and elevated chondrocyte expression of iNOS, NLRP3, and β-galactosidase. Bulk RNA-seq of knee tissue identified 152 differentially expressed genes: interferon-related (Oas3, Nlrc5, Zbp1) and stress-response (Slc6a4, Alox12, Cirbp, Trpc6, Tap1, Ubash3, Nrg1) pathways were upregulated, while extracellular matrix organization pathways were downregulated. Our findings reveal a mechanistic connection between the degree of liver injury and deterioration of bone and joint integrity, pointing to a shared pathophysiological axis in MASLD and OA. Clinically, this raises the prospect that a single therapeutic, designed to modulate inflammation, or hepatic lipid metabolism, could be repurposed or developed to concurrently treat both steatohepatitis and osteoarthritic degeneration.

The effect of food intake patterns on growth remain largely unknown. In this issue of the JCI, Hornsby et al. provide compelling evidence that, in young males, confining food intake to three meals a day entrains preprandial ghrelin release, leading to postprandial growth hormone pulse release that is associated with an increase in epiphysial plate expansion - a measure indicative of increased bone growth. The positive effects of discrete meal intake, on bone, was dependent on an intact ghrelin signaling system. This Commentary posits that meal-entrained ghrelin release may enhance skeletal accrual, whether through direct action on bone cells, via stimulation of growth hormone secretion, or in concert with other nutrient-responsive hormones. Coordinating these hormonal cues with food intake could maximize bone acquisition and improve bone health throughout the lifespan.

Several mouse lines with congenital growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis disruption have shown improved health and extended lifespan. The current study investigated how inactivating this axis, specifically during aging, impacts the healthspan. We used a tamoxifen-inducible global GH receptor (GHR) knockout mouse model starting at 12 months and followed the mice until 24 months of age (iGHRKO_12-24 mice). We found sex- and tissue-specific effects, with some being pro-aging and others anti-aging. Measuring an array of cytokines in serum revealed that inactivation of the GH/IGF-1 axis at 12 months did not affect systemic inflammation during aging. On the other hand, hypothalamic inflammation was significantly reduced in iGHRKO_12-24 mice, evidenced by GFAP⁺ (glial fibrillary acidic protein, a marker of astrocytes) and Iba-1⁺ (a marker for microglia). Liver RNAseq analysis indicated feminization of the male transcriptome, with significant changes in the expression of monooxygenase, sulfotransferase, and solute-carrier-transporter gene clusters. Finally, we found impaired bone morphology, more pronounced in male iGHRKO_12-24 mice and correlated with GH/IGF-1 inactivation onset age. We conclude that inhibiting the GH/IGF-1 axis during aging only partially preserves the beneficial healthspan effects observed with congenital GH deficiency.

Sodium-glucose transporter-2 inhibitor (SGLT2i) drugs are widely used for lowering blood glucose levels independent of insulin. Beyond this, these drugs induce various metabolic changes, including weight loss and impaired bone integrity. There is a significant gap in understanding SGLT2i-induced skeletal changes, as SGLT2 is not expressed in osteoblasts or osteocytes, which use glucose to remodel the bone matrix. We studied the impact of 1, 3, or 6 months of canagliflozin (CANA), an SGLT2i treatment, on the skeleton of 6-month-old genetically heterogeneous UM-HET3 mice. Significant metabolic adaptations to CANA were evident as early as 1.5 months post-treatment, specifically in male mice. CANA-treated male mice exhibited notable reductions in body weight and decreased proinflammatory and bone remodeling markers associated with reduced cortical bone remodeling indices. Bone tissue metabolome indicated enrichment in metabolites related to amino acid transport and tryptophan catabolism in CANA-treated male mice. In contrast, CANA-treated female mice showed increases in nucleic acid metabolism. An integrOmics approach of source-matched bone tissue metabolome and bone marrow RNAseq indicated a positive correlation between the two omics data sets in male mice. Three clusters of transcripts and metabolites involved in energy metabolism, oxidative stress response, and cellular proliferation and differentiation were reduced in CANA-treated male mice. In conclusion, CANA affects bone metabolism mainly via the 'glucose restriction state' it induces and impacts bone cell proliferation and differentiation. These findings underline the effects of SGLT2i on bone health and highlight the need to consider sex-specific responses when developing clinical treatments that alter substrate availability.

Absent in melanoma (AIM) 2, a gene induced by interferon, acts as a cytosolic sensor for double-stranded (ds) DNA. It forms the AIM2 inflammasome, producing interleukin (IL)-1β and IL-18. Our previous study demonstrated that mice lacking AIM2 exhibit spontaneous obesity, insulin resistance, and inflammation in adipose tissue. In this study, we aimed to explore the impact of AIM2 gene deletion on the bone marrow microenvironment and bone morphology in adult and aged mice. Utilizing micro-computed tomography (micro-CT), we discovered that female mice lacking AIM2 showed an increase in the total cross-sectional area at 5 months of age, accompanied by an increase in cortical thickness in the mid-diaphysis of the femur at both 5 and 15 months of age. At 15 months, the cortical bone mineral density (BMD) significantly decreased in AIM2 null females compared to wildtype (WT) mice. Trabecular bone volume and BMD at the distal metaphysis of the femur and the lumbar vertebra-4 were also significantly decreased in AIM2 null females. Histological examination of femurs from aged mice demonstrated increased bone marrow adiposity in AIM2 null mice, accompanied by a significant increase in CD45 - /CD31 - /Sca1 + /Pdgfa + adipogenic progenitor cells and a decrease in the ratio of CD45 - /CD31 - /Sca1 - /Pdgfa + osteogenic progenitor cells, as determined by flow cytometry of bone marrow cells. RNAseq analysis of the bone marrow revealed a significant increase in interferon-stimulated genes with Ifi202b as the top-upregulated gene in AIM2 null mice. Our findings suggest that AIM2 deficiency affects bone health by promoting adipogenesis in the bone marrow and inducing a pro-inflammatory environment, thereby contributing to decreased bone mineral density.

Canagliflozin (CANA) is a sodium glucose cotransporter-2 inhibitor that reduces blood glucose levels. Sodium glucose cotransporter-2 is primarily expressed in the kidney, but not in any bone cells, therefore effects on the skeleton are likely to be non-cell autonomous. Originally developed to treat type II diabetes, CANA use has expanded to treat cardiovascular and renovascular disease. Clinical trials examining CANA in diabetic patients have produced contradictory reports on fracture risk, but there are limited data of CANA in nondiabetic conditions. In nondiabetic preclinical models, short-term treatment with CANA negatively affected trabecular bone whereas long-term treatment reduced cortical bone mineralization in male but not female mice. To investigate the skeletal effects of an intermediate period of CANA treatment, we treated male and female C57BL/6 J mice with CANA (180 ppm) for 6 months. Age at treatment initiation was also evaluated, with cohorts starting CANA prior to skeletal maturity (3-months-old) or in adulthood (6-months-old). Longitudinal assessments of bone mineral density revealed early benefits of CANA treatment in female mice. At euthanasia, both trabecular and cortical bone morphology were improved by CANA treatment in males and females. Bone formation was reduced at the endosteal surface. CANA decreased osteoblast number in male mice and bone marrow adiposity in females. Overall, more skeletal benefits were recorded in CANA-treated females than males. Urinary calcium output increased with CANA treatment, but parathyroid hormone was not changed. Despite reduced fasting blood glucose, body composition and whole-body metabolism were minimally changed by CANA treatment. For all outcome measures, limited differences were recorded based on age at treatment initiation. This study demonstrated that in nondiabetic C57BL/6 J mice, an intermediate period of CANA treatment improved bone morphology, but reduced osteoblast and bone marrow adipocyte number as well as serum procollagen type 1 N-terminal pro-peptide in a sex-specific manner.

BACKGROUND:Primary osteoarthritis (OA) occurs without identifiable underlying causes such as previous injuries or specific medical conditions. Age is a major contributing factor to OA, and as one ages, various joint tissues undergo gradual change, including degeneration of the articular cartilage, alterations in subchondral bone (SCB) morphology, and inflammation of the synovium. METHODS:We investigated the prevalence of primary OA in aged, genetically diverse UM-HET3 mice. Articular cartilage (AC) integrity and SCB morphology were assessed in 182 knee joints of 22-25 months old mice using the Osteoarthritis Research Society International (OARSI) scoring system and micro-CT, respectively. Additionally, we explored the effects of methylene blue (MB) and mitoquinone (MitoQ), two agents that affect mitochondrial function, on the prevalence and progression of OA during aging. RESULTS:Aged UM-HET3 mice showed a high prevalence of primary OA in both sexes. Significant positive correlations were found between cumulative AC (cAC) scores and synovitis in both sexes, and osteophyte formation in female mice. Ectopic chondrogenesis did not show significant correlations with cAC scores. Significant direct correlations were found between AC scores and inflammatory markers in chondrocytes, including matrix metalloproteinase-13, inducible nitric oxide synthase, and the NLR family pyrin domain containing-3 inflammasome in both sexes, indicating a link between OA severity and inflammation. Additionally, markers of cell cycle arrest, such as p16 and β-galactosidase, also correlated with AC scores. In male mice, no significant correlations were found between SCB morphology traits and cAC scores, while in female mice, significant correlations were found between cAC scores and tibial SCB plate bone mineral density. Notably, MB and MitoQ treatments influenced the disease's progression in a sex-specific manner. MB treatment significantly reduced cAC scores at the medial knee joint, while MitoQ treatment reduced cAC scores, but these did not reach significance. CONCLUSIONS:Our study provides comprehensive insights into the prevalence and progression of primary OA in aged UM-HET3 mice, highlighting the sex-specific effects of MB and MitoQ treatments. The correlations between AC scores and various pathological factors underscore the multifaceted nature of OA and its association with inflammation and subchondral bone changes.

Methylene blue (MB) is a well-established antioxidant that has been shown to improve mitochondrial function in both in vitro and in vivo settings. Mitoquinone (MitoQ) is a selective antioxidant that specifically targets mitochondria and effectively reduces the accumulation of reactive oxygen species. To investigate the effect of long-term administration of MB on skeletal morphology, we administered MB to aged (18 months old) female C57BL/J6 mice, as well as to adult male and female mice with a genetically diverse background (UM-HET3). Additionally, we used MitoQ as an alternative approach to target mitochondrial oxidative stress during aging in adult female and male UM-HET3 mice. Although we observed some beneficial effects of MB and MitoQ in vitro, the administration of these compounds in vivo did not alter the progression of age-induced bone loss. Specifically, treating 18-month-old female mice with MB for 6 or 12 months did not have an effect on age-related bone loss. Similarly, long-term treatment with MB from 7 to 22 months or with MitoQ from 4 to 22 months of age did not affect the morphology of cortical bone at the mid-diaphysis of the femur, trabecular bone at the distal-metaphysis of the femur, or trabecular bone at the lumbar vertebra-5 in UM-HET3 mice. Based on our findings, it appears that long-term treatment with MB or MitoQ alone, as a means to reduce skeletal oxidative stress, is insufficient to inhibit age-associated bone loss. This supports the notion that interventions solely with antioxidants may not provide adequate protection against skeletal aging.