Fibroblasts take the centre stage in human skeletal muscle regeneration

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137752/1/tjp12431_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137752/2/tjp12431.pd

RE: Talks BJ, Fernquest S, Palmer A, et al. 2019. No Evidence of Systemic Inflammation in Symptomatic Patients With Femoroacetabular Impingement

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152571/1/jor24427.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152571/2/jor24427_am.pd

Regulation of the Structure and Function of Skeletal Muscle and Tendon by Myostatin.

The ability of skeletal muscle and tendon to adapt to use and disuse has important consequences for determining the health, mobility and athletic performance of an individual. Muscle generates the forces for locomotion, and tendons link muscle to bone and can protect muscle fibers from contraction-induced injuries. Myostatin (GDF-8) is a member of the TGF-beta superfamily of cytokines. Inhibition of myostatin leads to a profound increase in muscle mass, and consequently there is much interest in the potential of myostatin inhibitors to treat injuries and diseases that lead to muscle atrophy. Using whole muscle and tendon mechanics experiments in the myostatin-deficient mouse model, along with cellular and molecular studies, we determined the impact of myostatin deficiency on the structure and function of skeletal muscle and tendon tissue. The deficiency of myostatin lead to muscle fiber hypertrophy and hyperplasia. Myostatin-deficient EDL muscles have a decrease in protein degradation due to a decrease in atrogin-1 expression. Myostatin deficiency decreased the type I collagen content of EDL muscles. The deficiency of myostatin increased the maximum isometric force of both EDL and soleus muscles, but decreased the specific force of EDL muscles. Furthermore, EDL muscles of myostatin-deficient mice are more susceptible to contraction-induced injury. Therefore, we determined if myostatin regulates the structure and function of tendons. Transcripts for myostatin and the myostatin receptors, ACVR2B and ACVRB, are present in tendons. Surprisingly, the tendons of myostatin-deficient mice are smaller, have a decrease in fibroblast density and type I collagen. Myostatin-deficient tendons also have a decrease in the expression of two genes that promote tendon fibroblast proliferation, scleraxis and tenomodulin. Treatment of tendon fibroblasts with myostatin activates the p38MAPK and Smad2/3 signaling cascades, increases cell proliferation and the expression of type I collagen, scleraxis and tenomodulin. Tendons from myostatin-deficient mice have a greater peak stress, lower peak strain and an increase in stiffness. We conclude that myostatin has a profound impact on the structure and function of skeletal muscle and tendon tissue. The partial inhibition of myostatin is likely to be beneficial in the treatment of many types of muscle injuries and diseases.Ph.D.Molecular and Integrative PhysiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/57707/2/cmendias_1.pd

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

Insulin‐like growth factor 1 signaling in tenocytes is required for adult tendon growth

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154662/1/fsb2fj201901503r.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154662/2/fsb2fj201901503r-sup-0001.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

Tissue‐engineered tendon constructs for rotator cuff repair in sheep

Current rotator cuff repair commonly involves the use of single or double row suture techniques, and despite successful outcomes, failure rates continue to range from 20 to 95%. Failure to regenerate native biomechanical properties at the enthesis is thought to contribute to failure rates. Thus, the need for technologies that improve structural healing of the enthesis after rotator cuff repair is imperative. To address this issue, our lab has previously demonstrated enthesis regeneration using a tissue‐engineered graft approach in a sheep anterior cruciate ligament (ACL) repair model. We hypothesized that our tissue‐engineered graft designed for ACL repair also will be effective in rotator cuff repair. The goal of this study was to test the efficacy of our Engineered Tissue Graft for Rotator Cuff (ETG‐RC) in a rotator cuff tear model in sheep and compare this novel graft technology to the commonly used double row suture repair technique. Following a 6‐month recovery, the grafted and contralateral shoulders were removed, imaged using X‐ray, and tested biomechanically. Additionally, the infraspinatus muscle, myotendinous junction, enthesis, and humeral head were preserved for histological analysis of muscle, tendon, and enthesis structure. Our results showed that our ETC‐RCs reached 31% of the native tendon tangent modulus, which was a modest, non‐significant, 11% increase over that of the suture‐only repairs. However, the histological analysis showed the regeneration of a native‐like enthesis in the ETG‐RC‐repaired animals. This advanced structural healing may improve over longer times and may diminish recurrence rates of rotator cuff tears and lead to better clinical outcomes. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:289–299, 2018.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142510/1/jor23642.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142510/2/jor23642_am.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

Inducible Depletion of Satellite Cells in Adult, Sedentary Mice Impairs Muscle Regenerative Capacity without Affecting Sarcopenia

A key determinant of geriatric frailty is sarcopenia, the age-associated loss of skeletal muscle mass and strength. Although the etiology of sarcopenia is unknown, the correlation during aging between the loss of activity of satellite cells, which are endogenous muscle stem cells, and impaired muscle regenerative capacity has led to the hypothesis that the loss of satellite cell activity is also a cause of sarcopenia. We tested this hypothesis in male sedentary mice by experimentally depleting satellite cells in young adult animals to a degree sufficient to impair regeneration throughout the rest of their lives. A detailed analysis of multiple muscles harvested at various time points during aging in different cohorts of these mice showed that the muscles were of normal size, despite low regenerative capacity, but did have increased fibrosis. These results suggest that lifelong reduction of satellite cells neither accelerated nor exacerbated sarcopenia and that satellite cells did not contribute to the maintenance of muscle size or fiber type composition during aging, but that their loss may contribute to age-related muscle fibrosis

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