Sex differences in tendon structure and function

Tendons play a critical role in the transmission of forces between muscles and bones, and chronic tendon injuries and diseases are among the leading causes of musculoskeletal disability. Little is known about sex‐based differences in tendon structure and function. Our objective was to evaluate the mechanical properties, biochemical composition, transcriptome, and cellular activity of plantarflexor tendons from 4 month old male and female C57BL/6 mice using in vitro biomechanics, mass spectrometry‐based proteomics, genome‐wide expression profiling, and cell culture techniques. While the Achilles tendons of male mice were approximately 6% larger than female mice (p  0.05) of plantaris tendons were observed. Mass spectrometry proteomics analysis revealed no significant difference between sexes in the abundance of major extracellular matrix (ECM) proteins such as collagen types I (p = 0.30) and III (p = 0.68), but female mice had approximately twofold elevations (p < 0.05) in less abundant ECM proteins such as fibronectin, periostin, and tenascin C. The transcriptome of male and female tendons differed by only 1%. In vitro, neither the sex of the serum that fibroblasts were cultured in, nor the sex of the ECM in which they were embedded, had profound effects on the expression of collagen and cell proliferation genes. Our results indicate that while male mice expectedly had larger tendons, male and female tendons have very similar mechanical properties and biochemical composition, with small increases in some ECM proteins and proteoglycans evident in female tendons. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2117–2126, 2017.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138868/1/jor23516-sup-0001-SuppTab-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138868/2/jor23516_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138868/3/jor23516-sup-0002-SuppTab-S2.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138868/4/jor23516.pd

Haploinsufficiency of myostatin protects against aging‐related declines in muscle function and enhances the longevity of mice

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112228/1/acel12339-sup-0003-TableS1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112228/2/acel12339.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112228/3/acel12339-sup-0004-TableS2.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112228/4/acel12339-sup-0002-FigureS1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112228/5/acel12339-sup-0001-DataS1.pd

Changes in skeletal muscle and tendon structure and function following genetic inactivation of myostatin in rats

Myostatin is a negative regulator of skeletal muscle and tendon mass. Myostatin deficiency has been well studied in mice, but limited data are available on how myostatin regulates the structure and function of muscles and tendons of larger animals. We hypothesized that, in comparison to wild‐type (MSTN+/+) rats, rats in which zinc finger nucleases were used to genetically inactivate myostatin (MSTNΔ/Δ) would exhibit an increase in muscle mass and total force production, a reduction in specific force, an accumulation of type II fibres and a decrease and stiffening of connective tissue. Overall, the muscle and tendon phenotype of myostatin‐deficient rats was markedly different from that of myostatin‐deficient mice, which have impaired contractility and pathological changes to fibres and their extracellular matrix. Extensor digitorum longus and soleus muscles of MSTNΔ/Δ rats demonstrated 20–33% increases in mass, 35–45% increases in fibre number, 20–57% increases in isometric force and no differences in specific force. The insulin‐like growth factor‐1 pathway was activated to a greater extent in MSTNΔ/Δ muscles, but no substantial differences in atrophy‐related genes were observed. Tendons of MSTNΔ/Δ rats had a 20% reduction in peak strain, with no differences in mass, peak stress or stiffness. The general morphology and gene expression patterns were similar between tendons of both genotypes. This large rodent model of myostatin deficiency did not have the negative consequences to muscle fibres and extracellular matrix observed in mouse models, and suggests that the greatest impact of myostatin in the regulation of muscle mass may not be to induce atrophy directly, but rather to block hypertrophy signalling.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111244/1/tjp6572.pd

Physiological loading of tendons induces scleraxis expression in epitenon fibroblasts

Scleraxis is a basic helix–loop–helix transcription factor that plays a central role in promoting fibroblast proliferation and matrix synthesis during the embryonic development of tendons. Mice with a targeted inactivation of scleraxis ( Scx −/− ) fail to properly form limb tendons, but the role that scleraxis has in regulating the growth and adaptation of tendons of adult organisms is unknown. To determine if scleraxis expression changes in response to a physiological growth stimulus to tendons, we subjected adult mice that express green fluorescent protein (GFP) under the control of the scleraxis promoter ( ScxGFP ) to a 6‐week‐treadmill training program designed to induce adaptive growth in Achilles tendons. Age matched sedentary ScxGFP mice were used as controls. Scleraxis expression was sparsely observed in the epitenon region of sedentary mice, but in response to treadmill training, scleraxis was robustly expressed in fibroblasts that appeared to be emerging from the epitenon and migrating into the superficial regions of tendon fascicles. Treadmill training also led to an increase in scleraxis, tenomodulin, and type I collagen gene expression as measured by qPCR. These results suggest that in addition to regulating the embryonic formation of limb tendons, scleraxis also appears to play an important role in the adaptation of adult tendons to physiological loading. © 2011 Orthopaedic Research Society. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:606–612, 2012Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90076/1/21550_ftp.pd

Scleraxis is required for the growth of adult tendons in response to mechanical loading.

Scleraxis is a basic helix-loop-helix transcription factor that plays a central role in promoting tenocyte proliferation and matrix synthesis during embryonic tendon development. However, the role of scleraxis in the growth and adaptation of adult tendons is not known. We hypothesized that scleraxis is required for tendon growth in response to mechanical loading, and that scleraxis promotes the specification of progenitor cells into tenocytes. We conditionally deleted scleraxis in adult mice using a tamoxifen-inducible Cre-recombinase expressed from the Rosa26 locus (ScxΔ), and then induced tendon growth in Scx+ and ScxΔ adult mice via plantaris tendon mechanical overload. Compared to the wild type Scx+ group, ScxΔ mice demonstrated blunted tendon growth. Transcriptional and proteomic analyses revealed significant reductions in cell proliferation, protein synthesis, and extracellular matrix genes and proteins. Our results indicate that scleraxis is required for mechanically-stimulated adult tendon growth by causing the commitment of CD146+ pericytes into the tenogenic lineage, and by promoting the initial expansion of newly committed tenocytes and the production of extracellular matrix proteins

Haploinsufficiency of myostatin protects against aging-related declines in muscle function and enhances the longevity of mice

Summary The molecular mechanisms behind aging-related declines in muscle function are not well understood, but the growth factor myostatin (MSTN) appears to play an important role in this process. Additionally, epidemiological studies have identified a positive correlation between skeletal muscle mass and longevity. Given the role of myostatin in regulating muscle size, and the correlation between muscle mass and longevity, we tested the hypotheses that the deficiency of myostatin would protect oldest-old mice (28-30 months old) from an aging-related loss in muscle size and contractility, and would extend the maximum lifespan of mice. We found that MSTN +/À and MSTN À/À mice were protected from aging-related declines in muscle mass and contractility. While no differences were detected between MSTN +/+ and MSTN À/À mice, MSTN +/À mice had an approximately 15% increase in maximal lifespan. These results suggest that targeting myostatin may protect against aging-related changes in skeletal muscle and contribute to enhanced longevity