Comparative Effects of Basic Fibroblast Growth Factor Delivery or Voluntary Exercise on Muscle Regeneration after Volumetric Muscle Loss

Volumetric muscle loss (VML) is associated with irreversibly impaired muscle function due to traumatic injury. Experimental approaches to treat VML include the delivery of basic fibroblast growth factor (bFGF) or rehabilitative exercise. The objective of this study was to compare the effects of spatially nanopatterned collagen scaffold implants with either bFGF delivery or in conjunction with voluntary exercise. Aligned nanofibrillar collagen scaffold bundles were adsorbed with bFGF, and the bioactivity of bFGF-laden scaffolds was examined by skeletal myoblast or endothelial cell proliferation. The therapeutic efficacy of scaffold implants with either bFGF release or exercise was examined in a murine VML model. Our results show an initial burst release of bFGF from the scaffolds, followed by a slower release over 21 days. The released bFGF induced myoblast and endothelial cell proliferation in vitro. After 3 weeks of implantation in a mouse VML model, twitch force generation was significantly higher in mice treated with bFGF-laden scaffolds compared to bFGF-laden scaffolds with exercise. However, myofiber density was not significantly improved with bFGF scaffolds or voluntary exercise. In contrast, the scaffold implant with exercise induced more re-innervation than all other groups. These results highlight the differential effects of bFGF and exercise on muscle regeneration.

Galectin-3 promotes the adipogenic differentiation of PDGFRα+ cells and ectopic fat formation in regenerating muscle

Worldwide prevalence of obesity is associated with the increase of lifestyle-related diseases. The accumulation of intermuscular adipose tissue (IMAT) is considered a major problem whereby obesity leads to sarcopenia and metabolic disorders and thus is a promising target for treating these pathological conditions. However, whereas obesity-associated IMAT is suggested to originate from PDGFRα+ mesenchymal progenitors, processes underlying their adipogenesis remain largely unexplored. Here, we comprehensively investigated intra- and extracellular changes associated with these processes using single-cell RNA sequencing (scRNA-Seq) and mass spectrometry. Our scRNA-Seq analysis identified a small PDGFRα+ cell population in obese mice directed strongly toward adipogenesis. Proteomic analysis showed that the appearance of this cell population is accompanied by an increase in galectin-3 in interstitial environments, which was found to activate adipogenic PPARγ signals in PDGFRα+ cells. Moreover, IMAT formation during muscle regeneration was significantly suppressed in galectin-3 KO mice. Our findings, together with these multi-omics datasets, could unravel microenvironmental networks during muscle regeneration highlighting possible therapeutic targets against IMAT formation in obesity.

NOX4 inhibition promotes the remodeling of dystrophic muscle

The muscular dystrophies (MDs) are genetic muscle diseases that result in progressive muscle degeneration followed by the fibrotic replacement of affected muscles as regenerative processes fail. Therapeutics that specifically address the fibrosis and failed regeneration associated with MDs represent a major unmet clinical need for MD patients, particularly those with advanced stage disease progression. The current study investigates targeting NAD(P)H oxidase (NOX) 4 as a potential strategy to reduce fibrosis and promote regeneration in disease-burdened muscle that models Duchenne muscular dystrophy (DMD). NOX4 is elevated in the muscles of dystrophic mice and DMD patients, localizing primarily to interstitial cells located between muscle fibers. Genetic and pharmacological targeting of NOX4 significantly reduces fibrosis in dystrophic respiratory and limb muscles. Mechanistically, NOX4 targeting decreases the number of fibrosis-depositing cells (myofibroblasts) and restores the number of muscle-specific stem cells (satellite cells) to their physiological niche, thereby, rejuvenating muscle regeneration. Furthermore, acute inhibition of NOX4 is sufficient to induce apoptotic clearing of myofibroblasts within dystrophic muscle. These data indicate that targeting NOX4 is an effective strategy to promote the beneficial remodeling of disease-burdened muscle representative of DMD and, potentially, other MDs and muscle pathologies.

Fasting Induces a Highly Resilient Deep Quiescent State in Muscle Stem Cells via Ketone Body Signaling

Short-term fasting is beneficial for the regeneration of multiple tissue types. However, the effects of fasting on muscle regeneration are largely unknown. Here we report that fasting slows muscle repair both immediately after the conclusion of fasting as well as after multiple days of refeeding. We show that ketosis, either endogenously produced during fasting or a ketogenic diet, or exogenously administered, promotes a deep quiescent state in muscle stem cells(MuSCs). Although deep quiescent MuSCs are less poised to activate, slowing muscle regeneration, they have markedly improved survival when facing sources of cellular stress. Further, we show that ketone bodies, specifically b hydroxybutyrate, directly promote MuSC deep quiescence via a non-metabolic mechanism. We show that b-hydroxybutyrate functions as an HDAC inhibitor within MuSCs leading to acetylation and activation of an HDAC1 target protein p53. Finally, we demonstrate that p53 activation contributes to the deep quiescence and enhanced resilience observed during fasting.