in-vivo-measurement-of-knee-extensor-muscle-function-in-mice

In vivo Measurement of Knee Extensor Muscle Function in Mice

Skeletal muscle plasticity in response to countless conditions and stimuli mediates concurrent functional adaptation, both negative and positive. In the ...

University of Kentucky

Anterior cruciate ligament (ACL) tears result in significant quadriceps muscle atrophy that is resistant to recovery despite extensive rehabilitation. Recent work suggests an elevated fibrotic burden in the quadriceps muscle after the injury, which may limit recovery. Elucidating the mechanisms and cell types involved in the progression of fibrosis is critical for developing new treatment strategies.

Anterior Cruciate Ligament Tear Promotes Skeletal Muscle Myostatin Expression, Fibrogenic Cell Expansion, and a Decline in Muscle Quality.

Adequate muscle perfusion supports the transport of nutrients, oxygen and hormones into muscle fibers. Aging is associated with a substantial decrease in skeletal muscle capillarization, fiber size and oxidative capacity, which may be improved with regular physical activity. The aim of this study was to investigate the relationship between muscle capillarization and indices of muscle hypertrophy (i.e. lean mass; fiber cross sectional area (CSA)) in older adults before and after 12 weeks of progressive resistance exercise training (RET).

Low skeletal muscle capillarization limits muscle adaptation to resistance exercise training in older adults.

Aging induces physiological decline in human skeletal muscle function and morphology, including type II fiber atrophy and an increase in type I fiber frequency. Resistance exercise training (RET) is an effective strategy to overcome muscle mass loss and improve strength, with a stronger effect on type II fibers. In the present study, we sought to determine the effect of a 12-wk progressive RET program on the fiber type-specific skeletal muscle hypertrophic response in older adults. Nineteen subjects [10 men and 9 women (71.1 ± 4.3 yr)] were studied before and after the 12-wk program. Immunohistochemical analysis was used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area (CSA), satellite cell abundance, myonuclear content, and lipid droplet density. RET induced an increase in MyHC type II fiber frequency and a concomitant decrease in MyHC type I fiber frequency. Mean CSA increased significantly only in MyHC type II fibers (+23.3%,  < 0.05), but myonuclear content increased only in MyHC type I fibers ( < 0.05), with no change in MyHC type II fibers. Satellite cell content increased ~40% in both fiber types ( > 0.05). RET induced adaptations to the capillary supply to satellite cells, with the distance between satellite cells and the nearest capillary increasing in type I fibers and decreasing in type II fibers. Both fiber types showed similar decrements in intramuscular lipid density with training ( < 0.05). Our data provide intriguing evidence for a fiber type-specific response to RET in older adults and suggest flexibility in the myonuclear domain of type II fibers during a hypertrophic stimulus. In older adults, progressive resistance exercise training (RET) increased skeletal muscle fiber volume and cross-sectional area independently of myonuclear accretion, leading to an expansion of the myonuclear domain. Fiber type-specific analyses illuminated differential adaptation; type II fibers underwent hypertrophy and exhibited myonuclear domain plasticity, whereas myonuclear accretion occurred in type I fibers in the absence of a robust hypertrophic response. RET also augmented satellite cell-capillary interaction and reduced intramyocellular lipid density to improve muscle quality.

Resistance exercise training promotes fiber type-specific myonuclear adaptations in older adults.

Severe burn injury induces a myriad of deleterious effects to skeletal muscle, resulting in impaired function and delayed recovery. Following burn, catabolic signaling and myofiber atrophy are key fiber-intrinsic determinants of weakness; less well understood are alterations in the interstitial environment surrounding myofibers. Muscle quality, specifically alterations in the extracellular matrix (ECM), modulates force transmission and strength. We sought to determine the impact of severe thermal injury on adaptation to the muscle ECM and quantify muscle fibrotic burden. After a 30% total body surface area dorsal burn, spinotrapezius muscle was harvested from mice at 7 (7d,  = 5), 14 (14d,  = 4), and 21 days (21d,  = 4), and a sham control group was also examined (Sham,  = 4). Expression of transforming growth factor-β (TGFβ), myostatin, and downstream effectors and proteases involved in fibrosis and collagen remodeling were measured by immunoblotting, and immunohistochemical and biochemical analyses assessed fibrogenic cell abundance and collagen deposition. Myostatin signaling increased progressively through 21 days postburn alongside fibrogenic/adipogenic progenitor cell expansion, with abundance peaking at 14 days postburn. Postburn, elevated expression of tissue inhibitor of matrix metalloproteinase 1 supported collagen remodeling resulting in a net accumulation of muscle collagen content. Collagen accumulation peaked at 14 days postburn but remained elevated through 21 days postburn, demonstrating minimal resolution of burn-induced fibrosis. These findings highlight a progressive upregulation of fibrogenic processes following burn injury, eliciting a fibrotic muscle phenotype that hinders regenerative capacity and is not resolved with 21 days of recovery.

Thermal injury initiates pervasive fibrogenesis in skeletal muscle.

The inability of older adults to maintain independence is a consequence of sarcopenia and frailty. In order to identify the molecular mechanisms responsible for decreased physical function, it will be critical to utilize a small animal model. The main purpose of this study was to develop a composite comprehensive functional assessment battery (CFAB) of well-validated tests to determine physical function and exercise capacity in three age groups of male C57BL/6 mice (6 months old, n=29; 24-months, n=24; 28+ months, n=28). To measure physical function in mice, we used rotarod (overall motor function), grip meter (forelimb strength), treadmill (endurance), inverted cling (strength/endurance), voluntary wheel running (VWR, volitional exercise and activity rate), and muscle performance with in vivo contractile physiology (dorsiflexor torque). We hypothesized that CFAB would be a valid means to assess physical function of a given mouse across the lifespan. In addition, we proposed that CFAB could be used to determine relationships between different parameters associated with sarcopenia. We found that there was an overall age-related significant decline (p&0.05) in all measurements, and the CFAB score demonstrated that some individual mice (the upper quartile) retained the functional capacity of average mice one cohort younger. We conclude that the CFAB is a powerful, repeatable and non-invasive tool to assess and compare physical function and assess complex motor task ability in mice, which will enable researchers to easily track performance at the individual mouse level.

Article	Year
Anterior Cruciate Ligament Tear Promotes Skeletal Muscle Myostatin Expression, Fibrogenic Cell Expansion, and a Decline in Muscle Quality. The American journal of sports medicine\| PubMed ID: 30995070	2019
Low skeletal muscle capillarization limits muscle adaptation to resistance exercise training in older adults. Experimental gerontology\| PubMed ID: 31518665	2019
Resistance exercise training promotes fiber type-specific myonuclear adaptations in older adults. Journal of applied physiology (Bethesda, Md. : 1985)\| PubMed ID: 32134710	2020
Thermal injury initiates pervasive fibrogenesis in skeletal muscle. American journal of physiology. Cell physiology\| PubMed ID: 32432932	2020
Measuring Exercise Capacity and Physical Function in Adult and Older Mice. The journals of gerontology. Series A, Biological sciences and medical sciences\| PubMed ID: 32822475	2020

Camille R. Brightwell

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