Emerging roles for estrogen in regulating skeletal muscle physiology

EditorialEmerging roles for estrogen in regulating skeletal muscle physiologyBridget Coyle-Asbil, Leslie M. Ogilvie, and Jeremy A. SimpsonBridget Coyle-AsbilDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, Canada, Leslie M. OgilvieDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, Canada, and Jeremy A. SimpsonDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, CanadaIMPART Investigator Team Canada, Saint John, New Brunswick, CanadaPublished Online:13 Feb 2023https://doi.org/10.1152/physiolgenomics.00158.2022This is the final version - click for previous versionMoreSectionsPDF (682 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations Skeletal muscle mass and strength decline with age, which contributes to impaired balance and reduced mobility, leading to falls. In fact, measures of muscle mass and strength are independent predictors of overall health in men and women (1). Throughout aging, there are sex-specific differences in skeletal muscle function, where declines in muscle strength occur at an earlier age in females than in males (2). Interestingly, this decline in muscle function in females is associated with reductions in circulating estrogen levels—a result of menopause (3–5). This observation is compelling and introduces sex hormones as a mechanism that directly regulates skeletal muscle physiology.Posttranslational modifications (PTMs) are chemical modifications to proteins that change the structure and subsequent function of proteins. There are over 200 different PTMs, which directly influence protein-protein interactions that subsequently drive the phenotype of the organ. One of the most common PTMs of muscle is phosphorylation, the addition of a phosphate group to either serine, or threonine, or tyrosine residues. Protein phosphorylation leads to the activation or inactivation of a protein and impacts homeostatic cell signaling. Modifications to the skeletal muscle phosphoproteome are involved in coordinating muscle growth, metabolism, repair, and contraction to generate force (6, 7). Throughout aging, there are modifications to the skeletal muscle phosphoproteome, indicating a connection between the muscle phosphoproteome and muscle function (8, 9). For example, phosphorylation levels of myosin light chain 2 and tropomyosin α, proteins in the contractile machinery, were increased in skeletal muscle of aged rats (8). Although several studies have investigated how protein phosphorylation impacts muscle function (10–13), these have been carried out largely in male subjects. Despite this, sex hormones are emerging as a novel mechanism regulating skeletal muscle function, which opens up new therapeutic avenues to target age-associated muscle dysfunction, enhance performance, and improve injury recovery. However, what remains to be discovered is the target protein and the specific amino acids that are directly phosphorylated by sex hormones (Fig. 1).Figure 1.Role of sex hormones in regulating skeletal muscle physiology in mice. Sex hormones (e.g., estrogen, testosterone, and progesterone) alter protein signaling through posttranslational modifications, isoform shifting, and de novo synthesis, which impacts skeletal muscle molecular signaling of sarcomeric proteins, transcription factors, calcium signaling, and metabolic proteins. However, the specific impact of each individual sex hormone on protein alterations and their functional impacts remains unknown. [Image created with BioRender.com and published with permission.]Download figureDownload PowerPointIn a recent issue of Physiological Genomics, Peyton and colleagues (14) performed a global phosphoproteomic analysis of tibialis anterior muscle in ovarian-hormone deficient mice to determine whether the decline of estrogen in females alters the skeletal muscle phosphoproteome. Using a mass spectrometric approach, they identified 22 proteins that are differentially phosphorylated in ovariectomized (OVX) versus sham mice, establishing that ovarian-derived sex hormones are involved in regulating the skeletal muscle phosphoproteome under resting conditions (i.e., noncontracting muscle). Pathway enrichment analysis, performed to gain further insight into the functional impact of the differentially regulated phosphopeptides in estrogen-deficient mice, showed an overrepresentation in proteins associated with calcium (e.g., calcium ion channel activity, myofilament calcium sensitivity, intracellular calcium release, and reuptake), metabolic signaling (e.g., insulin signaling, glycolysis, and gluconeogenesis), and cellular functions related to muscle maintenance and cytoskeletal integrity (e.g., sarcomeric organization and contractile function). As these signaling pathways are critical to maintaining normal muscle function, remodeling of the muscle phosphoproteome will have consequences on the protein’s function. These findings lay the molecular foundation linking sex hormones to muscle physiology and outline critical next steps to examining the functional impact of sex hormone depletion on skeletal muscle in females.Our current understanding of how sex hormones impact basic biological functions continues to expand. Sex hormones play important physiological roles that extend beyond their well-established reproductive functions. In fact, estradiol, the ovarian-derived form of estrogen, alters signaling pathways in the musculoskeletal, cardiovascular, and central nervous systems in both females and males (15–17). For example, in male rats, estrogen decreased eccentric cardiac hypertrophy following chronic volume overload, a model of cardiac stress (18). Thus, as estrogen levels are altered throughout aging, this has functional consequences on these biological systems (17, 19–22). As such, further investigation into how estrogen regulates pathways involved in skeletal muscle function may be of benefit to both sexes.The current issue follows their previous work, where Lai et al. (23) showed that OVX decreases phosphorylation of skeletal myosin regulatory light chain (RLC), which impairs muscle contractility in female mice. Furthermore, they demonstrated that muscle contractility and RLC protein phosphorylation are restored with the administration of exogenous estradiol, confirming that the effects of OVX on muscle function are estrogen-mediated rather than other ovarian-derived hormones (i.e., progesterone and testosterone). Although their previous work establishes that estrogen regulates muscle physiology at a sarcomeric level, a broader and unbiased investigation into the effects of estrogen on muscle proteins was lacking until now. Here, Lai et al. (23) present a comprehensive phosphoproteomic profiling of skeletal muscle and provide compelling evidence that sex hormones are involved in regulating signaling pathways such as energy metabolism, calcium signaling, and protein trafficking, which are critical for maintaining normal muscle function. Their results provide fundamental knowledge of female physiology and the impact of female sex hormones on skeletal muscle at a molecular level. These findings present a critical first step in identifying the molecular intermediates involved in driving specific muscle phenotypes. It is intriguing to consider the consequences of estrogen deficiency in older females and the impact this has on their strength, mobility, and quality of life. It will be of great interest to investigate the effects of sex hormones on other tissues such as cardiac and respiratory muscles.As estrogen is involved in regulating a variety of physiological mechanisms, it is imperative to investigate how the loss of ovarian-derived estrogen alters the regulation of normal physiology. Currently, there are two primary animal models used to evaluate the effects of estrogen loss in females. OVX, the surgical removal of one or both ovaries, has been the gold standard menopause model used to evaluate the effects of ovarian hormone loss, including estrogen, on various biological systems (24). However, when the ovaries are removed, the levels of other sex hormones (e.g., progesterone, testosterone, follicle-stimulating hormone, and luteinizing hormone) are also altered, which may influence the interpretation of how estrogen loss affects physiology. An emerging rodent model of menopause is produced by administering the chemical, 4-vinylcyclohexene diepoxide (VCD). VCD selectively targets and depletes the ovarian follicles, resulting in a hormone profile that is more similar to the natural menopause transition in women. However, VCD is toxic at high doses, which may present confounding effects on other organs if the appropriate administration doses are exceeded. Overall, the OVX and VCD models of menopause both provide the opportunity to study how ovarian-derived hormones influence biological systems. In both cases, it is important to consider the advantages of each model when designing research experiments and to understand the limitations to ensure that findings are interpreted in the appropriate biological context. Furthermore, how male sex hormones affect the phosphoproteome is an intriguing comparison that requires further consideration where orchiectomy, the surgical removal of the gonads and spermatic cord, may be used as a valuable model to examine changes in testosterone in humans with age.Throughout their manuscript, Peyton et al. (14) highlight insightful similarities between cardiac and skeletal muscle physiology and discuss the implications of how alterations in the phosphoproteome are the basis for various pathologies in both muscle types. Indeed, phosphoproteomic modifications in cardiac muscle are key features of several cardiomyopathies. For example, cardiac troponin I (cTnI) and RLC are sarcomeric proteins that play important roles in calcium regulation and mediating actin-myosin interactions to generate force. In heart failure, both of these proteins are dephosphorylated, leading to severe impairments in cardiac muscle contraction and relaxation (25–27). This demonstrates the importance of regulating protein phosphorylation to maintain normal muscle function. Furthermore, estrogen acts directly on the heart through various estrogen receptors localized in different cardiac cells (e.g., cardiomyocytes, endothelial cells, and cardiac fibroblasts) suggesting that sex hormones are also involved in regulating cardiac function. Estrogen has a protective effect on the heart contributing to a lower incidence of heart disease in females premenopause compared with aged-matched males and postmenopausal females when estrogen levels decline (28). The identification of altered protein phosphorylation patterns and the role of sex hormones in muscle physiology may reveal targets for therapeutic intervention to improve muscle weakness and dysfunction in both cardiac and skeletal muscle pathologies.Here, Peyton and colleagues (14) have presented the first global phosphoproteomic analysis in skeletal muscle examining phosphorylation modifications following OVX in female mice. Their results show that with OVX, the phosphorylation of proteins associated with calcium signaling, metabolic regulation, and sarcomere organization is altered, all of which are important for regulating normal muscle function. With the identification of these phosphoproteins that are regulated by sex hormones, research into the receptors, signaling pathways, and kinases involved is required along with which sex hormones are driving each specific change. We are eager to learn the functional impact of these phosphoproteomic changes on skeletal muscle. These data provide fundamental insight into how female sex hormones regulate pathways important for maintaining normal muscle function. This information will ultimately provide novel therapeutic pathways for targeting muscle weakness and improving muscle function. Overall, given the importance of integrating sex and gender into research, a specific focus on improving our understanding of female physiology provides new insight into how estrogen deficiency impacts the skeletal muscle phosphoproteome and the consequences for muscle strength and function in aging females.GRANTSThis work was supported by the Canadian Institutes of Health Research (CIHR), Natural Sciences and Engineering Research Council of Canada (NSERC), and Heart and Stroke Foundation of Canada grants to J. A. Simpson. B. Coyle-Asbil was supported by a Canada Graduate Scholarship-Master’s NSERC and L. M. Ogilvie was supported by an Alexander Graham Bell Canada Graduate Scholarship-Doctoral NSERC.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the authors.AUTHOR CONTRIBUTIONSB.C.-A., L.M.O., and J.A.S. prepared figures; drafted manuscript; edited and revised manuscript; and approved final version of manuscript.REFERENCES1. Santilli V, Bernetti A, Mangone M, Paoloni M. Clinical definition of sarcopenia. Clin Cases Miner Bone Metab 11: 177–180, 2014. PubMed | Google Scholar2. Haynes EMK, Neubauer NA, Cornett KMD, O'Connor BP, Jones GR, Jakobi JM. Age and sex-related decline of muscle strength across the adult lifespan: a scoping review of aggregated data. Appl Physiol Nutr Metab 45: 1185–1196, 2020. doi:10.1139/apnm-2020-0081. Crossref | PubMed | ISI | Google Scholar3. Greising SM, Baltgalvis KA, Lowe DA, Warren GL. Hormone therapy and skeletal muscle strength: a meta-analysis. J Gerontol A Biol Sci Med Sci 64: 1071–1081, 2009. doi:10.1093/gerona/glp082. 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Download PDF Back to Top Next FiguresReferencesRelatedInformation Related ArticlesGlobal phosphoproteomic profiling of skeletal muscle in ovarian hormone-deficient mice 24 Oct 2022Physiological Genomics More from this issue > Volume 55Issue 2February 2023Pages 75-78 Crossmark Copyright & PermissionsCopyright © 2023 the American Physiological Society.https://doi.org/10.1152/physiolgenomics.00158.2022PubMed36622080History Received 8 November 2022 Accepted 5 January 2023 Published online 13 February 2023 Published in print 1 February 2023 Keywordsestrogenphosphoproteomesex hormonesskeletal muscle Metrics