Guide Cells Support Muscle Regeneration and Affect Neuro-Muscular Junction Organization.

Muscular regeneration is a complex biological process that occurs during acute injury and chronic degeneration, implicating several cell types. One of the earliest events of muscle regeneration is the inflammatory response, followed by the activation and differentiation of muscle progenitor cells. However, the process of novel neuromuscular junction formation during muscle regeneration is still largely unexplored. Here, we identify by single-cell RNA sequencing and isolate a subset of vessel-associated cells able to improve myogenic differentiation. We termed them 'guide' cells because of their remarkable ability to improve myogenesis without fusing with the newly formed fibers. In vitro, these cells showed a marked mobility and ability to contact the forming myotubes. We found that these cells are characterized by CD44 and CD34 surface markers and the expression of Ng2 and Ncam2. In addition, in a murine model of acute muscle injury and regeneration, injection of guide cells correlated with increased numbers of newly formed neuromuscular junctions. Thus, we propose that guide cells modulate de novo generation of neuromuscular junctions in regenerating myofibers. Further studies are necessary to investigate the origin of those cells and the extent to which they are required for terminal specification of regenerating myofibers.

Disruption of the gene encoding 3beta-hydroxysterol Delta-reductase (Tm7sf2) in mice does not impair cholesterol biosynthesis.

Tm7sf2 gene encodes 3beta-hydroxysterol Delta(14)-reductase (C14SR, DHCR14), an endoplasmic reticulum enzyme acting on Delta(14)-unsaturated sterol intermediates during the conversion of lanosterol to cholesterol. The C-terminal domain of lamin B receptor, a protein of the inner nuclear membrane mainly involved in heterochromatin organization, also possesses sterol Delta(14)-reductase activity. The subcellular localization suggests a primary role of C14SR in cholesterol biosynthesis. To investigate the role of C14SR and lamin B receptor as 3beta-hydroxysterol Delta(14)-reductases, Tm7sf2 knockout mice were generated and their biochemical characterization was performed. No Tm7sf2 mRNA was detected in the liver of knockout mice. Neither C14SR protein nor 3beta-hydroxysterol Delta(14)-reductase activity were detectable in liver microsomes of Tm7sf2((-/-)) mice, confirming the effectiveness of gene inactivation. C14SR protein and its enzymatic activity were about half of control levels in the liver of heterozygous mice. Normal cholesterol levels in liver membranes and in plasma indicated that, despite the lack of C14SR, Tm7sf2((-/-)) mice are able to perform cholesterol biosynthesis. Lamin B receptor 3beta-hydroxysterol Delta(14)-reductase activity determined in liver nuclei showed comparable values in wild-type and knockout mice. These results suggest that lamin B receptor, although residing in nuclear membranes, may contribute to cholesterol biosynthesis in Tm7sf2((-/-)) mice. Affymetrix microarray analysis of gene expression revealed that several genes involved in cell-cycle progression are downregulated in the liver of Tm7sf2((-/-)) mice, whereas genes involved in xenobiotic metabolism are upregulated.