PGC-1α overexpression partially rescues impaired oxidative and contractile pathophysiology following volumetric muscle loss injury

Abstract

Volumetric muscle loss (VML) injury is characterized by a non-recoverable loss of muscle fibers due to ablative surgery or severe orthopaedic trauma, that results in chronic functional impairments of the soft tissue. Currently, the effects of VML on the oxidative capacity and adaptability of the remaining injured muscle are unclear. A better understanding of this pathophysiology could significantly shape how VML-injured patients and clinicians approach regenerative medicine and rehabilitation following injury. Herein, the data indicated that VML-injured muscle has diminished mitochondrial content and function (i.e. oxidative capacity), loss of mitochondrial network organization, and attenuated oxidative adaptations to exercise. However, forced PGC-1 overexpression rescued the deficits in oxidative capacity and muscle strength. This implicates physiological activation of PGC1-α as a limiting factor in VML-injured muscle adaptive capacity and provides a mechanistic target for regenerative rehabilitation approaches to address the skeletal muscle dysfunction.Oxidative capacity is a cornerstone of skeletal muscle health, and for the past 40 years, we have known that the most robust physiologic adaptation to regularly scheduled physical activity (i.e., exercise/overload training) is an increase in oxidative capacity (1,2). Improvements in muscle oxidative capacity are made possible with exercise training through adaptations affecting the density and function of the intramuscular mitochondrial network. The signaling pathways that initiate and coordinate mitochondrial improvements with exercise are complex, but advancements in molecular biology in the last two decades have revealed many of the key players involved (see for review (3)). Most notably, the transcription factor PGC-1 (peroxisome proliferator-activated receptor gamma, coactivator 1 alpha) is considered a critical molecular modulator of skeletal muscle oxidative plasticity because it regulates gene expression patterns for expansion of the mitochondrial network (i.e. mitochondrial biogenesis), angiogenesis, and motor neuron associated adaptations with exercise training(4-6). Expansion of the vasculature and mitochondrial network with exercise training enhances the functional capacity of the muscle (e.g., fatigue resistance), and in general, this physiologic type of acclimation is considered beneficial for human performance and health (7,8).Large-scale skeletal muscle trauma, such as volumetric muscle loss (VML) injury, is unique in that the muscle is not able to regenerate muscle fibers with endogenous repair systems and as a result, cannot fully recover strength. The loss of muscle function (i.e., contractility) can exceed the loss of tissue mass(9), and this permanent functional deficit leaves patients with lifelong disability(10) for which there is currently no corrective physical rehabilitation guidelines. Furthermore, the extent to which the remaining skeletal muscle can adapt to rehabilitation is unclear. Recent preclinical work has investigated various...

Mitochondrial dysfunction in skeletal muscle of fukutin‐deficient mice is resistant to exercise‐ and 5‐aminoimidazole‐4‐carboxamide ribonucleotide‐induced rescue

“…To assess the integrity of mitochondrial biogenesis signalling in Fktn KO muscle, eight additional untreated mice (4 LM; 4 Fktn KO, 100% female) underwent a 30 min in vivo electrical stimulation protocol to simulate an acute bout of exercise as previously described (Southern et al., 2019). Briefly, mice were anesthetized with isoflurane (1.5% to 2.0%) and platinum–iridium needle electrodes were placed around the sciatic nerve.…”

“…Mitochondrial respiratory function was assessed as previously described (Southern et al., 2019). Briefly, portions of the medial and lateral gastrocnemius muscles from uninjured limbs were dissected on a chilled aluminium block in 4°C buffer X (7.23 m m K 2 EGTA, 2.77 m m Ca‐K 2 EGTA, 20 m m imidazole, 20 m m taurine, 5.7 m m ATP, 14.3 m m PCr, 6.56 m m MgCl 2 ‐6H 2 O, 50 m m K‐MES) into thin muscle fibre bundles (<1 mg).…”

“…iQ SYBR Green Supermix (Bio‐Rad) and sequence‐specific primers were used to assess mRNA levels for the following genes: PGC‐1α (forward: 5′‐AGC CGT GAC CAC TGA CAA CGA G‐3′; reverse: 5′‐GCT GCA TGG TTC TGA GTG CTA AG‐3′) (Lin et al., 2004). Gene expression analysis was carried out by qRT‐PCR on a Bio‐Rad CFX instrument as previously described (Southern et al., 2019). Data were normalized to 18S (forward: 5′‐TTG ATT AAG TCC CTG CCC TTT GT‐3′; reverse: 5′‐CGA TCC GAG GGC CTA ACT A‐3′) (Jenkins, Witkowski, Spangenburg, & Hagberg, 2009) and relative gene expression was calculated using the method.…”

Musculoskeletal Regeneration, Rehabilitation, and Plasticity Following Traumatic Injury

“…It is currently unknown at this time what specific effect protracted in-flammation has on satellite cell viability and function within the remaining portion of the muscle; however, given the necessity of local satellite cells for muscle regeneration and their importance to plasticity, research investigating the quality of chronically injured muscle after VML, is highly needed. Other salient observations following VML injury include motor neuron axotomy, loss of neuromuscular junctions, heightened oxidative stress, devascularization, and mitochondrial dysfunction [83][84][85]; all of these may be deleterious to regenerative healing and the capacity to respond to tissue level physical therapies.…”

“…Clinical data of tissue level pathology and unresponsiveness to physical therapies is not currently available in this patient population. Available data from animal models of VML indicate that the remaining portion of muscle does not demonstrate a robust increase in oxidative capacity with endurance exercise type training due to inadequate activation of the necessary cellular signaling cascades (e. g. transcription factor PGC1α) [85]. Alarmingly, a recent systematic review, meta-analysis, and network meta-analysis of VML injury studies that included quantitative functional analyses [98] determined that in animal models, rehabilitation approaches for VML injury resulted in worse functional outcomes than if the injury was left to its natural sequela.…”