The impact of ischemic stroke on bone marrow microenvironment and extracellular vesicles: A study on inflammatory and molecular changes

An ischemic stroke (IS) is caused due to the lack of blood flow to cerebral tissue. Most of the studies have focused on how stroke affects the localized tissue, but it has been observed that a stroke can cause secondary complications in distant organs, such as Bone Marrow (BM). Our study focused on the effect of ischemic strokes on the bone marrow microenvironment. Bone marrow (BM) is a vital organ that maintains inflammatory homeostasis and aids in the repair of damaged tissue after injury/IS. We used the middle cerebral artery occlusion (MCAO) model of ischemic stroke on adult mice (6 months) and investigated the changes in the BM environment. BM cells were used for western blot and RT-PCR, and the BM supernatant was used for cytokine analysis and extracellular vesicle (EVs) isolation. We observed a significant increase in the total cell number within the BM and an increase in TNF-alpha and MCP-1, which are known for inducing a pro-inflammatory environment. Western blots analysis on the whole BM cell lysate demonstrated elevated levels of inflammatory factors (IL-6, TNF-alpha, and TLR-4) and senescence markers (p21 p16). EVs isolated from the BM supernatant showed no change in size or concentration; however, we found that the EVs carried increased miRNA-141-3p and miRNA-34a. Proteomic analysis on BM-derived EVs showed an alteration in the protein cargo of IS. We observed an increase in FgB, C3, Fn1, and Tra2b levels. The signaling pathway analysis showed mitochondrial function is most affected within the bone marrow. Our study demonstrated that IS induces changes in the BM environment and EVs secreted in the BM.

Chronic infusion of the tryptophan metabolite kynurenine increases mean arterial pressure in male Sprague-Dawley rats

Chronic kidney disease is the loss of renal function that can occur from aging or through a myriad of other disease states. Rising serum concentrations of kynurenine, a tryptophan metabolite, have been shown to correlate with increasing severity of chronic kidney disease. This study used chronic intravenous infusion in conscious male Sprague-Dawley rats to test the hypothesis that kynurenine can induce renal damage and promote alterations in blood pressure, heart rate, and decreased renal function. We found that kynurenine infusion increased mean arterial pressure, increased the maximum and minimum range of heart rate, decreased glomerular filtration rate, and induced kidney damage in a dose-dependent manner. This study shows that kynurenine infusion can promote kidney disease in healthy, young rats, implying that the increase in kynurenine levels associated with chronic kidney disease may establish a feed-forward mechanism that exacerbates the loss of renal function.NEW & NOTEWORTHY In humans, an elevated serum concentration of kynurenine has long been associated with negative outcomes in various disease states as well as in aging. However, it has been unknown whether these increased kynurenine levels are mediating the disorders or simply associated with them. This study shows that chronically infusing kynurenine can contribute to the development of hypertension and kidney impairment. The mechanism of this action remains to be determined in future studies.

The Role of Branched Chain Ketoacid Dehydrogenase Kinase (BCKDK) in Skeletal Muscle Biology and Pathogenesis

Muscle wasting can be caused by nutrition deficiency and inefficient metabolism of amino acids, including Branched Chain Amino Acids (BCAAs). Branched Chain Amino Acids are a major contributor to the metabolic needs of healthy muscle and account for over a tenth of lean muscle mass. Branched chain alpha-ketoacid dehydrogenase (BCKD) is the rate limiting enzyme of BCAA metabolism. Inhibition of BCKD is achieved through a reversible phosphorylation event by Branched Chain a-ketoacid dehydrogenase kinase (BCKDK). Our study set out to determine the importance of BCKDK in the maintenance of skeletal muscle. We used the Gene Expression Omnibus Database to understand the role of BCKDK in skeletal muscle pathogenesis, including aging, muscular disease, and interrupted muscle metabolism. We found BCKDK expression levels were consistently decreased in pathologic conditions. These results were most consistent when exploring muscular disease followed by aging. Based on our findings, we hypothesize that decreased BCKDK expression alters BCAA catabolism and impacts loss of normal muscle integrity and function. Further research could offer valuable insights into potential therapeutic strategies for addressing muscle-related disorders.

Is it time to reduce the length of postgraduate training for physician-scientists in internal medicine?

Physician-scientists play a crucial role in advancing medical knowledge and patient care, yet the long periods of time required to complete training may impede expansion of this workforce. We examined the relationship between postgraduate training and time to receipt of NIH or Veterans Affairs career development awards (CDAs) for physician-scientists in internal medicine. Data from NIH RePORTER were analyzed for internal medicine residency graduates who received specific CDAs (K08, K23, K99, or IK2) in 2022. Additionally, information on degrees and training duration was collected. Internal medicine residency graduates constituted 19% of K awardees and 28% of IK2 awardees. Of MD-PhD internal medicine-trained graduates who received a K award, 92% received a K08 award; of MD-only graduates who received a K award, a majority received a K23 award. The median time from medical school graduation to CDA was 9.6 years for K awardees and 10.2 years for IK2 awardees. The time from medical school graduation to K or IK2 award was shorter for US MD-PhD graduates than US MD-only graduates. We propose that the time from medical school graduation to receipt of CDAs must be shortened to accelerate training and retention of physician-scientists.

The Role of Aryl Hydrocarbon Receptor in Bone Biology

Aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is crucial in maintaining the skeletal system. Our study focuses on encapsulating the role of AhR in bone biology and identifying novel signaling pathways in musculoskeletal pathologies using the GEO dataset. The GEO2R analysis identified 8 genes (CYP1C1, SULT6B1, CYB5A, EDN1, CXCR4B, CTGFA, TIPARP, and CXXC5A) involved in the AhR pathway, which play a pivotal role in bone remodeling. The AhR knockout in hematopoietic stem cells showed alteration in several novel bone-related transcriptomes (eg, Defb14, ZNF 51, and Chrm5). Gene Ontology Enrichment Analysis demonstrated 54 different biological processes associated with bone homeostasis. Mainly, these processes include bone morphogenesis, bone development, bone trabeculae formation, bone resorption, bone maturation, bone mineralization, and bone marrow development. Employing Functional Annotation and Clustering through DAVID, we further uncovered the involvement of the xenobiotic metabolic process, p450 pathway, oxidation-reduction, and nitric oxide biosynthesis process in the AhR signaling pathway. The conflicting evidence of current research of AhR signaling on bone (positive and negative effects) homeostasis may be due to variations in ligand binding affinity, binding sites, half-life, chemical structure, and other unknown factors. In summary, our study provides a comprehensive understanding of the underlying mechanisms of the AhR pathway in bone biology.

Alterations in Alzheimer's disease microglia transcriptome might be involved in bone pathophysiology

Aging is a major risk factor for multiple chronic disorders in the elderly population, including Alzheimer's disease (AD) and Osteoporosis. AD is a progressive neurodegenerative disease characterized by memory loss. In addition to dementia, several studies have shown that AD patients experience an increased rate of musculoskeletal co-morbidities, such as osteoporosis. Since tissue-specific macrophages contribute to both diseases, this study analyzed the microglia transcriptome of AD mice to determine a common gene signature involved in osteoclast biology. After comparing differentially regulated genes from GEO data sets (GSE93824 and GSE212277), there were 35 common upregulated genes and 89 common downregulated genes. Of these common genes, seven genes are known to play an important role in bone homeostasis. CSF1, SPP1, FAM20C, and Cst7 were upregulated and are associated with osteoclastogenesis and inflammation. Among the downregulated genes, LILRA6, MMP9, and COL18A1 are involved in bone formation and osteoclast regulation. We further validated some of these genes (CSF1, Cst7, and SPP1) in the cortex and the bone of AD mice models. The dysregulation of these microglial genes in AD might provide insights into the co-occurrence of AD and osteoporosis and offer potential therapeutic targets to combat disease progression.

Effects of Systemic Peroxisome Proliferator-Activated Receptor Gamma Inhibition on Bone and Immune Cells in Aged Female Mice

This study investigates the effects of peroxisome proliferator-activated receptor gamma (PPARγ) inhibition on bone and immune cell profiles in aged female mice, as well as in vitro stromal stem cell osteogenic differentiation and inflammation gene expression. The hypothesis was that inhibition of PPARγ would increase bone mass and alter immune and other cellular functions. Our results showed that treatment with PPARγ antagonist GW9662 for 6 weeks reduced bone volume and trabecular number and increased trabecular spacing. However, inhibition of PPARγ had no significant effect on marrow and spleen immune cell composition in aged female mice. In vitro experiments indicated that GW9662 treatment increased the expression of osteogenic genes but did not affect adipogenic genes. Additionally, GW9662 treatment decreased the expression of several inflammation-related genes. Overall, these findings suggest that PPARγ inhibition may have adverse effects on bone in aged female mice.

Inhibiting MicroRNA-141-3p Improves Musculoskeletal Health in Aged Mice

Emerging evidence shows that the microRNA-141-3p is involved in various age-related pathologies. Previously, our group and others reported elevated levels of miR-141-3p in several tissues and organs with age. Here, we inhibited the expression of miR-141-3p using antagomir (Anti-miR-141-3p) in aged mice and explored its role in healthy aging. We analyzed serum (cytokine profiling), spleen (immune profiling), and overall musculoskeletal phenotype. We found decreased levels of pro-inflammatory cytokines (such as TNF-α, IL-1β, IFN-γ) in serum with Anti-miR-141-3p treatment. The flow-cytometry analysis on splenocytes revealed decreased M1 (pro-inflammatory) and increased M2 (anti-inflammatory) populations. We also found improved bone microstructure and muscle fiber size with Anti-miR-141-3p treatment. Molecular analysis revealed that miR-141-3p regulates the expression of AU-rich RNA-binding factor 1 (AUF1) and promotes senescence (p21, p16) and pro-inflammatory (TNF-α, IL-1β, IFN-γ) environment whereas inhibiting miR-141-3p prevents these effects. Furthermore, we demonstrated that the expression of FOXO-1 transcription factor was reduced with Anti-miR-141-3p and elevated with silencing of AUF1 (siRNA-AUF1), suggesting crosstalk between miR-141-3p and FOXO-1. Overall, our proof-of-concept study demonstrates that inhibiting miR-141-3p could be a potential strategy to improve immune, bone, and muscle health with age.

Hashimoto's thyroiditis and renal transplant rejection

PURPOSE

Hashimoto's thyroiditis (HT) is a common autoimmune thyroid disorder that can disrupt thyroid function and homeostasis. As HT results from a dysregulated immune system, we hypothesized that these patients might be more susceptible to transplant failure; however, literature on this association is limited. The purpose of this study is to examine the association of HT with the risk of renal transplant failure.

METHODS

We utilized the United States Renal Database System dataset collected from 2005 to 2014 and compared the time from first renal transplant to transplant failure in end-stage renal disease (ESRD) patients with a HT diagnosis to ESRD patients without a HT diagnosis that underwent renal transplant.

RESULTS

A total of 144 ESRD patients had International Classification of Disease-9 claim codes for HT prior to renal transplant, amongst a total cohort of 90,301 renal transplant patients aged 18-100 and meeting criteria. Patients with HT were significantly more likely to be female, white, and to have a diagnosis of cytomegalovirus compared to patients without. ESRD patients with a HT diagnosis that underwent renal transplant had a significantly increased risk of renal transplant failure compared to those ESRD renal transplant patients without an HT diagnosis. There was a significantly increased adjusted hazard ratio for graft failure in patients with a HT diagnosis compared to those without.

CONCLUSION

Thyroid health and HT may play a significant role in the development of the increased risk of renal transplant failure observed in this study. Additional studies are needed to investigate the underlying mechanisms for this association.

Orchiectomy sensitizes cortical bone in male mice to the harmful effects of kynurenine

Kynurenine (Kyn) is a tryptophan metabolite that increases with age and promotes musculoskeletal dysfunction. We previously found a sexually dimorphic pattern in how Kyn affects bone, with harmful effects more prevalent in females than males. This raises the possibility that male sex steroids might exert a protective effect that blunts the effects of Kyn in males. To test this, orchiectomy (ORX) or sham surgeries were performed on 6-month-old C57BL/6 mice, after which mice received Kyn (10 mg/kg) or vehicle via intraperitoneal injection, once daily, 5×/week, for four weeks. Bone histomorphometry, DXA, microCT, and serum marker analyses were performed after sacrifice. In vitro studies were performed to specifically test the effect of testosterone on activation of aryl hydrocarbon receptor (AhR)-mediated signaling by Kyn in mesenchymal-lineage cells. Kyn treatment reduced cortical bone mass in ORX- but not sham-operated mice. Trabecular bone was unaffected. Kyn's effects on cortical bone in ORX mice were attributed primarily to enhanced endosteal bone resorption activity. Bone marrow adipose tissue was increased in Kyn-treated ORX animals but was unchanged by Kyn in sham-operated mice. ORX surgery increased mRNA expression of the aryl hydrocarbon receptor (AhR) and its target gene Cyp1a1 in the bone, suggesting a priming and/or amplification of AhR signaling pathways. Mechanistic in vitro studies revealed that testosterone blunted Kyn-stimulated AhR transcriptional activity and Cyp1a1 expression in mesenchymal-linage cells. These data suggest a protective role for male sex steroids in blunting the harmful effects of Kyn in cortical bone. Therefore, testosterone may play an important role in regulating Kyn/AhR signaling in musculoskeletal tissues, suggesting crosstalk between male sex steroids and Kyn signaling may influence age-associated musculoskeletal frailty.

Sex-specific alteration in human muscle transcriptome with age

Sarcopenia is a medical condition that progressively develops with age and results in reduced skeletal muscle mass, alteration in muscle composition, and decreased muscle strength. Several clinical studies suggested that sarcopenia disproportionally affects males and females with age. Despite this knowledge, the molecular mechanism governing the pathophysiology is not well understood in a sex-specific manner. In this study, we utilized human gastrocnemius muscles from males and females to identify differentially regulated genes with age. We found 269 genes with at least a twofold expression difference in the aged muscle transcriptome. Among the female muscle samples, there were 239 differentially regulated genes, and the novel protein-coding genes include KIF20A, PIMREG, MTRNR2L6, TRPV6, EFNA2, RNF24, and SFN. In aged male skeletal muscle, there were 166 differentially regulated genes, and the novel-protein coding genes are CENPK, CDKN2A, BHLHA15, and EPHA. Gene Ontology (GO) enrichment revealed glucose catabolism, NAD metabolic processes, and muscle fiber transition pathways that are involved in aged female skeletal muscle, whereas replicative senescence, cytochrome C release, and muscle composition pathways are disrupted in aged male skeletal muscle. Targeting these novels, differentially regulated genes, and signaling pathways could serve as sex-specific therapeutic targets to combat the age-related onset of sarcopenia and promote healthy aging.

Update on the Role of Glucocorticoid Signaling in Osteoblasts and Bone Marrow Adipocytes During Aging

PURPOSE OF REVIEW

Bone marrow adipose tissue (BMAT) in the skeleton likely plays a variety of physiological and pathophysiological roles that are not yet fully understood. In elucidating the complex relationship between bone and BMAT, glucocorticoids (GCs) are positioned to play a key role, as they have been implicated in the differentiation of bone marrow mesenchymal stem cells (BMSCs) between osteogenic and adipogenic lineages. The purpose of this review is to illuminate aspects of both endogenous and exogenous GC signaling, including the influence of GC receptors, in mechanisms of bone aging including relationships to BMAT.

RECENT FINDINGS

Harmful effects of GCs on bone mass involve several cellular pathways and events that can include BMSC differentiation bias toward adipogenesis and the influence of mature BMAT on bone remodeling through crosstalk. Interestingly, BMAT involvement remains poorly explored in GC-induced osteoporosis and warrants further investigation. This review provides an update on the current understanding of the role of glucocorticoids in the biology of osteoblasts and bone marrow adipocytes (BMAds).

Sex Differences in Adipose Tissue Distribution Determine Susceptibility to Neuroinflammation in Mice With Dietary Obesity

Preferential energy storage in subcutaneous adipose tissue (SAT) confers protection against obesity-induced pathophysiology in females. Females also exhibit distinct immunological responses, relative to males. These differences are often attributed to sex hormones, but reciprocal interactions between metabolism, immunity, and gonadal steroids remain poorly understood. We systematically characterized adipose tissue hypertrophy, sex steroids, and inflammation in male and female mice after increasing durations of high-fat diet (HFD)-induced obesity. After observing that sex differences in adipose tissue distribution before HFD were correlated with lasting protection against inflammation in females, we hypothesized that a priori differences in the ratio of subcutaneous to visceral fat might mediate this relationship. To test this, male and female mice underwent SAT lipectomy (LPX) or sham surgery before HFD challenge, followed by analysis of glial reactivity, adipose tissue inflammation, and reproductive steroids. Because LPX eliminated female resistance to the proinflammatory effects of HFD without changing circulating sex hormones, we conclude that sexually dimorphic organization of subcutaneous and visceral fat determines susceptibility to inflammation in obesity.

Male sex hormones, aging, and inflammation

Adequate levels of androgens (eugonadism), and specifically testosterone, are vital compounds for male quality of life, longevity, and positive health outcomes. Testosterone exerts its effects by binding to the androgen receptor, which is expressed in numerous tissues throughout the body. Significant research has been conducted on the impact of this steroid hormone on skeletal, muscle and adipose tissues and on the cardiovascular, immune, and nervous systems. Testosterone levels have also been studied in relation to the impact of diseases, aging, nutrition and the environment on its circulating levels. Conversely, the impact of testosterone on health has also been evaluated with respect to its cardiac and vascular protective effects, body composition, autoimmunity and all-cause mortality. The male aging process results in decreasing testosterone levels over time. The exact mechanisms and impact of these changes in testosterone levels with age on health- and life-span are still not completely clear. Further research is needed to determine the optimal testosterone and androgen levels to protect from chronic age-related conditions such as frailty and osteoporosis.

Dietary interventions and molecular mechanisms for healthy musculoskeletal aging

Over the past decade, extensive efforts have focused on understanding age-associated diseases and how to prolong a healthy lifespan. The induction of dietary protocols such as caloric restriction (CR) and protein restriction (PR) has positively affected a healthy lifespan. These intervention ideas (nutritional protocols) have been the subject of human cohort studies and clinical trials to evaluate their effectiveness in alleviating age-related diseases (such as type II diabetes, cardiovascular disease, obesity, and musculoskeletal fragility) and promoting human longevity. This study summarizes the literature on the nutritional protocols, emphasizing their impacts on bone and muscle biology. In addition, we analyzed several CR studies using Gene Expression Omnibus (GEO) database and identified common transcriptome changes to understand the signaling pathway involved in musculoskeletal tissue. We identified nine novel common genes, out of which five were upregulated (Emc3, Fam134b, Fbxo30, Pip5k1a, and Retsat), and four were downregulated (Gstm2, Per2, Fam78a, and Sel1l3) with CR in muscles. Gene Ontology enrichment analysis revealed that CR regulates several signaling pathways (e.g., circadian gene regulation and rhythm, energy reserve metabolic process, thermogenesis) involved in energy metabolism. In conclusion, this study summarizes the beneficiary role of CR and identifies novel genes and signaling pathways involved in musculoskeletal biology.

Alterations in bone metabolites with age in C57BL/6 mice model

Understanding the pathophysiology behind age-related diseases is an urgent need as the elderly population continues to grow. With age, there is a high risk of musculoskeletal deterioration and associated morbidity and mortality. Although the exact mechanism behind age-related degeneration is unknown, it is well established that alteration in cellular metabolism is one of the important contributing factors. Alteration in signaling pathways with age leads to the accumulation or depletion of several metabolites that play a vital role in musculoskeletal pathophysiology. This study aimed to identify age-related changes in bone tissue metabolites in C57BL/6 mice. We then correlated the differentially expressed metabolites with their functions in bone biology. In both aged males and females, hydroxyproline, glutamine, and alpha-linolenic acid levels were decreased. In aged females, Ornithine (p value = 0.001), L-Proline (p value = 0.008), Uridine (p value = 0.001), Aspartic Acid (p value = 0.004) levels were significantly decreased, and glutamate (p value = 0.002) was elevated. In aged males, N-acetyl-D-glucosamine (pvalue = 0.010), Adrenic acid (pvalue = 0.0099), Arachidonic acid (p value = 0.029) and Allantoin (p value = 0.004) levels were decreased. Metabolic pathway analysis revealed that purine and D-glutamine and D-glutamate metabolism were significantly altered in both sexes, while arginine biosynthesis in females and lipid metabolism in males were highly affected. These differences in metabolic signaling might be one of the reasons for the discrepancy in musculoskeletal disease manifestation between the two sexes. Understanding the role of these metabolites play in the aging bone will allow for new sex-specific targeted therapies against the progression of musculoskeletal diseases.

Synergistic Effects of Multiple Factors Involved in COVID-19-dependent Muscle Loss

The COVID-19 pandemic caused by the novel SARS-CoV-2 coronavirus is an ongoing pandemic causing severe health crisis worldwide. Recovered COVID-19 patients go through several long-term side effects such as fatigue, headaches, dizziness, weight loss, and muscle loss among others. Our study sought to determine the molecular mechanisms behind muscle loss in COVID-19 patients. We hypothesized that multiple factors such as cytokine storm and therapeutic drugs (glucocorticoid and antiviral drugs) might be involved in muscle loss. Using the Gene Expression Omnibus database, we identified several studies that performed RNA sequencing on skeletal muscles with the treatment of cytokine, glucocorticoid, and antiviral drugs. Our study identified cytokines, such as IL-1b, and IL-6, associated with altered regulation of several genes involved in the myogenic processes, including Ttn, Cxxc5, Malat1, and Foxo1. We also observed that glucocorticoid altered the expression of Foxo1, Lcn2, Slc39a14, and Cdkn1a. Finally, we found out that the antiviral (RNA-dependent RNA polymerase inhibitor) drug regulates the expression of some of the muscle-related genes (Txnip, Ccnd1, Hdac9, and Fbxo32). Based on our findings, we hypothesize that the cytokine storm, glucocorticoids, and antiviral drugs might be synergistically involved in COVID-19-dependent muscle loss.

Pearls of wisdom for aspiring physician-scientist residency applicants and program directors

Postgraduate physician-scientist training programs (PSTPs) enhance the experiences of physician-scientist trainees following medical school graduation. PSTPs usually span residency and fellowship training, but this varies widely by institution. Applicant competitiveness for these programs would be enhanced, and unnecessary trainee anxiety relieved, by a clear understanding of what factors define a successful PSTP matriculant. Such information would also be invaluable to PSTP directors and would allow benchmarking of their admissions processes with peer programs. We conducted a survey of PSTP directors across the US to understand the importance they placed on components of PSTP applications. Of 41 survey respondents, most were from internal medicine and pediatrics residency programs. Of all components in the application, two elements were considered very important by a majority of PSTP directors: (a) having one or more first-author publications and (b) the thesis advisor’s letter. Less weight was consistently placed on factors often considered more relevant for non-physician-scientist postgraduate applicants — such as US Medical Licensing Examination scores, awards, and leadership activities. The data presented here highlight important metrics for PSTP applicants and directors and suggest that indicators of scientific productivity and commitment to research outweigh traditional quantitative measures of medical school performance.

Loss of Indoleamine-2,3-Dioxygenase-1 (IDO1) in Knockout Mice Does Not Affect the Development of Skin Lesions in the Imiquimod-Induced Mouse Model of Psoriasis

Indoleamine-2,3-dioxygenase (IDO) degrades the essential amino acid tryptophan resulting in tryptophan depletion and the accumulation of catabolites such as kynurenine. The expression/activity of IDO in various cells, including macrophages and dendritic cells, results in an inhibition of T-cell responses in a number of situations, such as toward allogeneic fetuses and tissue grafts. Psoriasis is an immune-mediated skin disease involving T cells; kynureninase and its generation of catabolites downstream of IDO are reported to play an important role in this disease. We hypothesized that mice lacking the IDO1 gene would exhibit a hyperactive immune response and an exacerbation of skin lesions in the imiquimod-induced mouse model of psoriasis. Littermate wild-type and IDO1-knockout mice were treated with imiquimod for 5 days, and the severity of psoriasiform skin lesions assessed using the psoriasis area and severity index (PASI), ear edema measured using a digital caliper, and thickness of the epidermis determined by histology. Expression of pro-inflammatory mediators and tryptophan-metabolizing enzymes was monitored using quantitative RT-PCR. Imiquimod increased ear edema, PASI scores, and epidermal thickness in both WT and IDO1 knockout mice; however, there were no differences observed between the 2 genotypes. There were also no differences in imiquimod's induction of skin inflammatory mediators, indicating no effect of IDO1 gene loss in this psoriasis model. Although these data suggest a lack of involvement of IDO1 in psoriatic skin inflammation, other possible mechanisms, such as compensatory changes in other pathways and the involvement of the IDO2 isoform, must also be considered.

The Glucocorticoid Receptor in Osterix-Expressing Cells Regulates Bone Mass, Bone Marrow Adipose Tissue, and Systemic Metabolism in Female Mice During Aging

Hallmarks of aging-associated osteoporosis include bone loss, bone marrow adipose tissue (BMAT) expansion, and impaired osteoblast function. Endogenous glucocorticoid levels increase with age, and elevated glucocorticoid signaling, associated with chronic stress and dysregulated metabolism, can have a deleterious effect on bone mass. Canonical glucocorticoid signaling through the glucocorticoid receptor (GR) was recently investigated as a mediator of osteoporosis during the stress of chronic caloric restriction. To address the role of the GR in an aging-associated osteoporotic phenotype, the current study utilized female GR conditional knockout (GR-CKO; GR^fl/fl :Osx-Cre+) mice and control littermates on the C57BL/6 background aged to 21 months and studied in comparison to young (3- and 6-month-old) mice. GR deficiency in Osx-expressing cells led to low bone mass and BMAT accumulation that persisted with aging. Surprisingly, however, GR-CKO mice also exhibited alterations in muscle mass (reduced % lean mass and soleus fiber size), accompanied by reduced voluntary physical activity, and also exhibited higher whole-body metabolic rate and elevated blood pressure. Moreover, increased lipid storage was observed in GR-CKO osteoblastic cultures in a glucocorticoid-dependent fashion despite genetic deletion of the GR, and could be reversed via pharmacological inhibition of the mineralocorticoid receptor (MR). These findings provide evidence of a role for the GR (and possibly the MR) in facilitating healthy bone maintenance with aging in females. The effects of GR-deficient bone on whole-body physiology also demonstrate the importance of bone as an endocrine organ and suggest evidence for compensatory mechanisms that facilitate glucocorticoid signaling in the absence of osteoblastic GR function; these represent new avenues of research that may improve understanding of glucocorticoid signaling in bone toward the development of novel osteogenic agents. © 2021 American Society for Bone and Mineral Research (ASBMR).

Long Non-coding RNA MALAT1 Is Depleted With Age in Skeletal Muscle in vivo and MALAT1 Silencing Increases Expression of TGF-β1 in vitro

Long non-coding RNAs (lncRNAs) are thought to function as “sponges” for microRNAs, but a role for such competing endogenous RNAs (ceRNAs) in muscle aging is not well understood. We therefore examined in skeletal muscles of young (4–6 months) and aged (22–24) male and female mice the expression of lncRNA MALAT1, which is predicted in silico to bind the senescence-associated microRNA miR-34a-5p. Results indicate a significant decrease in lncRNA MALAT1 expression in mouse skeletal muscle with age that coincides with an age-related increase in miR-34a-5p expression. In vitro studies using mouse C2C12 myoblasts demonstrate that MALAT1 silencing using siRNA increases miR-34a expression, consistent with a role for MALAT1 as an inhibitor of miR-34a-5p activity. Levels of reactive oxygen species (ROS) are known to increase in muscle with age, and so we treated C2C12 cells with hydrogen peroxide (10 and 100 μM) to examine changes in MALAT1 expression. MALAT1 expression decreased significantly with H2O2 treatment, but this effect was attenuated with p53 siRNA. Finally, miR-34a-5p is implicated in tissue fibrosis, and so we assessed the expression of TGF-β1 after MALAT1 silencing. MALAT1 siRNA significantly increased the expression of TGF-β1 in C2C12 cells. These findings suggest that age-related fibrosis and muscle atrophy mediated by ROS may result at least in part from an increase in miR-34a bioavailability resulting from a decline in miR-34a “sponging” due to ceRNA MALAT1 depletion. Crosstalk between MALAT1 and miR-34a may therefore represent a therapeutic target for improving muscle function with aging.

Expression of long noncoding RNA Xist is induced by glucocorticoids

Glucocorticoids (GCs) are potent anti-inflammatory and immunosuppressive agents. However, their clinical usage is limited by severe multisystemic side effects. Glucocorticoid induced osteoporosis results in significant morbidity and mortality but the cellular and molecular mechanisms underlying GC-induced bone loss are not clear. GC use results in decreased osteoblast differentiation with increased marrow adiposity through effects on bone marrow stem cells. GC effects are transduced through its receptor (GR). To identify novel GR regulated genes, we performed RNA sequencing (RNA-Seq) analysis comparing conditional GR knockout mouse made by crossing the floxed GR animal with the Col I promoter-Cre, versus normal floxed GR without Cre, and that testing was specific for Col I promoter active cells, such as bone marrow mesenchymal stem/osteoprogenitor cells (MSCs) and osteoblasts. Results showed 15 upregulated genes (3- to 10-fold) and 70 downregulated genes (-2.7- to -10-fold), with the long noncoding RNA X-inactive specific transcript (Xist) downregulated the most. The differential expression of genes measured by RNA-Seq was validated by qRT-PCR analysis of selected genes and the GC/GR signaling-dependent expression of Xist was further demonstrated by GC (dexamethasone) treatment of GR-deficient MSCs in vitro and by GC injection of C57BL/6 mice (wild-type males and females) in vivo. Our data revealed that the long noncoding RNA Xist is a GR regulated gene and its expression is induced by GC both in vitro and in vivo. To our knowledge, this is the first evidence showing that Xist is transcriptionally regulated by GC/GR signaling.

Age-associated changes in microRNAs affect the differentiation potential of human mesenchymal stem cells: Novel role of miR-29b-1-5p expression

Age-associated osteoporosis is widely accepted as involving the disruption of osteogenic stem cell populations and their functioning. Maintenance of the local bone marrow (BM) microenvironment is critical for regulating proliferation and differentiation of the multipotent BM mesenchymal stromal/stem cell (BMSC) population with age. The potential role of microRNAs (miRNAs) in modulating BMSCs and the BM microenvironment has recently gained attention. However, miRNAs expressed in rapidly isolated BMSCs that are naïve to the non-physiologic standard tissue culture conditions and reflect a more accurate in vivo profile have not yet been reported. Here we directly isolated CD271 positive (+) BMSCs within hours from human surgical BM aspirates without culturing and performed microarray analysis to identify the age-associated changes in BMSC miRNA expression. One hundred and two miRNAs showed differential expression with aging. Target prediction and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that the up-regulated miRNAs targeting genes in bone development pathways were considerably enriched. Among the differentially up-regulated miRNAs the novel passenger strand miR-29b-1-5p was abundantly expressed as a mature functional miRNA with aging. This suggests a critical arm-switching mechanism regulates the expression of the miR-29b-1-5p/3p pair shifting the normally degraded arm, miR-29b-1-5p, to be the dominantly expressed miRNA of the pair in aging. The normal guide strand miR-29b-1-3p is known to act as a pro-osteogenic miRNA. On the other hand, overexpression of the passenger strand miR-29b-1-5p in culture-expanded CD271+ BMSCs significantly down-regulated the expression of stromal cell-derived factor 1 (CXCL12)/ C-X-C chemokine receptor type 4 (SDF-1(CXCL12)/CXCR4) axis and other osteogenic genes including bone morphogenetic protein-2 (BMP-2) and runt-related transcription factor 2 (RUNX2). In contrast, blocking of miR-29b-1-5p function using an antagomir inhibitor up-regulated expression of BMP-2 and RUNX2 genes. Functional assays confirmed that miR-29b-1-5p negatively regulates BMSC osteogenesis in vitro. These novel findings provide evidence of a pathogenic anti-osteogenic role for miR-29b-1-5p and other miRNAs in age-related defects in osteogenesis and bone regeneration.

Renal Contributions to Age-Related Changes in Mineral Metabolism

Aging results in a general decline in function in most systems. This is particularly true with respect to the skeleton and renal systems, impacting mineral homeostasis. Calcium and phosphate regulation requires tight coordination among the intestine, bone, parathyroid gland, and kidney. The role of the intestine is to absorb calcium and phosphate from the diet. The bone stores or releases calcium and phosphate depending on the body's needs. In response to low plasma ionized calcium concentration, the parathyroid gland produces parathyroid hormone, which modulates bone turnover. The kidney reabsorbs or excretes the minerals and serves as the final regulator of plasma concentration. Many hormones are involved in this process in addition to parathyroid hormone, including fibroblast growth factor 23 produced by the bone and calcitriol synthesized by the kidney. Sclerostin, calcitonin, osteoprotegerin, and receptor activator of nuclear factor‐κB ligand also contribute to tissue‐specific regulation. Changes in the function of organs due to aging or disease can perturb this balance. During aging, the intestine cannot absorb calcium efficiently due to decreased expression of key proteins. In the bone, the balance between bone formation and bone resorption tends toward the latter in older individuals. The kidney may not filter blood as efficiently in the later decades of life, and the expression of certain proteins necessary for mineral homeostasis declines with age. These changes often lead to dysregulation of organismal mineral homeostasis. This review will focus on how mineral homeostasis is impacted by aging with a particular emphasis on the kidney's role in this process. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.