Tristetraprolin Regulation of MyoD mRNA Stability Commits Quiescent Adult Muscle Stem Cells to Myogenesis

In animals, tissue maintenance, plasticity and repair rely on adult stem cells which have been identified in nearly all tissues. Many adult stem cells are typically quiescent and only activate when required for maintenance and repair of adult tissues. Within hours of activation skeletal muscle stem cells called satellite cells begin to express MyoD, a muscle-specific transcription factor that functions as a master regulator, committing satellite cells to myogenesis. The earliest detectable event in satellite cells following muscle injury is phosphorylation of p38α/β MAPKs, which is required for MyoD induction and cell-cycle entry. Loss of Syndecan-4, a component of the satellite cell niche disrupts p38α/β MAPK activation and severely delays MyoD induction. We performed a microarray gene chip experiment to identify genes expressed during satellite cell activation. We identified differentially expressed genes by subtracting genes changing in Sdc4-/- satellite cells from those changing in WT satellite cells following 12h of muscle injury. Unexpectedly, we observed that 70% of RNA-binding proteins (RNA-BPs) decreased in activated satellite cells. Expression levels of the Tristetraprolin (TTP) family of RNA-BPs declined dramatically as satellite cells activated. The TTP family is known to direct mRNA decay and we identified the 3‘UTR of MyoD as a direct TTP target. Furthermore, p38α/β MAPK signaling inhibits TTP-mediated mRNA decay in satellite cells. HuR, an RNA-BP that is induced during satellite cell activation is known to stabilize MyoD mRNA. The coordinate inhibition of TTP and induction of HuR may together function as a feed-forward loop to commit satellite cells to myogenesis by rapid induction of MyoD. A similar feed-forward circuit could operate in other stem cell systems, implicating that post-transcriptional regulation of mRNA could play a major role in regulating adult stem cells to maintain and repair adult tissues

Post-transcriptional regulation of satellite cell quiescence by TTP-mediated mRNA decay.

Skeletal muscle satellite cells in their niche are quiescent and upon muscle injury, exit quiescence, proliferate to repair muscle tissue, and self-renew to replenish the satellite cell population. To understand the mechanisms involved in maintaining satellite cell quiescence, we identified gene transcripts that were differentially expressed during satellite cell activation following muscle injury. Transcripts encoding RNA binding proteins were among the most significantly changed and included the mRNA decay factor Tristetraprolin. Tristetraprolin promotes the decay of MyoD mRNA, which encodes a transcriptional regulator of myogenic commitment, via binding to the MyoD mRNA 3' untranslated region. Upon satellite cell activation, p38α/β MAPK phosphorylates MAPKAP2 and inactivates Tristetraprolin, stabilizing MyoD mRNA. Satellite cell specific knockdown of Tristetraprolin precociously activates satellite cells in vivo, enabling MyoD accumulation, differentiation and cell fusion into myofibers. Regulation of mRNAs by Tristetraprolin appears to function as one of several critical post-transcriptional regulatory mechanisms controlling satellite cell homeostasis

The novel immunomodulatory biologic LMWF5A for pharmacological attenuation of the “cytokine storm” in COVID-19 patients: a hypothesis

BACKGROUND: A common complication of viral pulmonary infections, such as in the ongoing COVID-19 pandemic, is a phenomenon described as a “cytokine storm”. While poorly defined, this hyperinflammatory response results in diffuse alveolar damage. The low molecular weight fraction of commercial human serum albumin (LMWF5A), a novel biologic in development for osteoarthritis, demonstrates beneficial in vitro immunomodulatory effects complimentary to addressing inflammation, thus, we hypothesize that LMWF5A could improve the clinical outcomes of COVID-19 by attenuating hyperinflammation and the potential development of a cytokine storm. PRESENTATION AND HYPOTHESIS: A variety of human in vitro immune models indicate that LMWF5A reduces the production of pro-inflammatory cytokines implicated in cytokine storm associated with COVID-19. Furthermore, evidence suggests LMWF5A also promotes the production of mediators required for resolving inflammation and enhances the barrier function of endothelial cultures. TESTING THE HYPOTHESIS: A randomized controlled trial, to evaluate the safety and efficacy of nebulized LMWF5A in adults with Acute Respiratory Distress Syndrome (ARDS) secondary to COVID-19 infection, was developed and is currently under review by the Food and Drug Administration. IMPLICATIONS OF HYPOTHESIS: If successful, this therapy may attenuate the cytokine storm observed in these patients and potentially reduce mortality, increase ventilation free days, improve oxygenation parameters and consequently lessen the burden on patients and the intensive care unit. CONCLUSIONS: In conclusion, in vitro findings suggest that the immunomodulatory effects of LMWF5A make it a viable candidate for treating cytokine storm and restoring homeostasis to the immune response in COVID-19

A role for RNA post-transcriptional regulation in satellite cell activation

Abstract Background Satellite cells are resident skeletal muscle stem cells responsible for muscle maintenance and repair. In resting muscle, satellite cells are maintained in a quiescent state. Satellite cell activation induces the myogenic commitment factor, MyoD, and cell cycle entry to facilitate transition to a population of proliferating myoblasts that eventually exit the cycle and regenerate muscle tissue. The molecular mechanism involved in the transition of a quiescent satellite cell to a transit-amplifying myoblast is poorly understood. Methods Satellite cells isolated by FACS from uninjured skeletal muscle and 12 h post-muscle injury from wild type and Syndecan-4 null mice were probed using Affymetrix 430v2 gene chips and analyzed by Spotfiretm and Ingenuity Pathway analysis to identify gene expression changes and networks associated with satellite cell activation, respectively. Additional analyses of target genes identify miRNAs exhibiting dynamic changes in expression during satellite cell activation. The function of the miRNAs was assessed using miRIDIAN hairpin inhibitors. Results An unbiased gene expression screen identified over 4,000 genes differentially expressed in satellite cells in vivo within 12 h following muscle damage and more than 50% of these decrease dramatically. RNA binding proteins and genes involved in post-transcriptional regulation were significantly over-represented whereas splicing factors were preferentially downregulated and mRNA stability genes preferentially upregulated. Furthermore, six computationally identified miRNAs demonstrated novel expression through muscle regeneration and in satellite cells. Three of the six miRNAs were found to regulate satellite cell fate. Conclusions The quiescent satellite cell is actively maintained in a state poised to activate in response to external signals. Satellite cell activation appears to be regulated by post-transcriptional gene regulation