Growth factor responsiveness: role of MyoD and myogenin

Chronic infection stunts macrophage heterogeneity and disrupts immune-mediated myogenesis

The robust regenerative potential of skeletal muscle is imperative for the maintenance of tissue function across a host of potential insults including exercise, infection, and trauma. The highly coordinated action of multiple immune populations, especially macrophages, plays an indispensable role in guiding this reparative program. However, it remains unclear how skeletal muscle repair proceeds in a chronically inflamed setting, such as infection, where an active immune response is already engaged. To address this question, we used a cardiotoxin injury model to challenge the reparative potential of chronically infected muscle. Compared with regenerating naive skeletal muscle, infected skeletal muscle exhibited multiple indicators of delayed muscle repair including a divergent morphologic response to injury and dysregulated expression of myogenic regulatory factors. Further, using both flow cytometric and single-cell RNA sequencing approaches, we show that reduced macrophage heterogeneity due to delayed emergence of restorative subsets underlies dysfunctional tissue repair during chronic infection. Our findings highlight how the preexisting inflammatory environment within tissue alters reparative immunity and ultimately the quality of tissue regeneration.

Regulatory T Cells Promote Myositis and Muscle Damage in Toxoplasma gondii Infection

The coordination of macrophage polarization is essential for the robust regenerative potential of skeletal muscle. Repair begins with a phase mediated by inflammatory monocytes (IM) and proinflammatory macrophages (M1), followed by polarization to a proregenerative macrophage (M2) phenotype. Recently, regulatory T cells (Tregs) were described as necessary for this M1 to M2 transition. We report that chronic infection with the protozoan parasite Toxoplasma gondii causes a nonresolving Th1 myositis with prolonged tissue damage associated with persistent M1 accumulation. Surprisingly, Treg ablation during chronic infection rescues macrophage homeostasis and skeletal muscle fiber regeneration, showing that Tregs can directly contribute to muscle damage. This study provides evidence that the tissue environment established by the parasite could lead to a paradoxical pathogenic role for Tregs. As such, these findings should be considered when tailoring therapies directed at Tregs in inflammatory settings.

Genomic organization and promoter analysis of the murine Id3 gene

Overexpression of the helix-loop-helix motif-containing transcription inhibitor Id3 has been shown to repress muscle-specific gene expression. Consistent with its putative negative regulatory role in the myogenic process, Id3 is highly expressed in proliferating myoblasts but down regulated when myoblasts are induced to differentiate. To investigate how Id3 expression may be transcriptionally regulated, we isolated a mouse Id3 genomic DNA fragment and characterized its organization and promoter activity. Comparison of the Id3 gene from human and mouse demonstrated a conserved exon-intron organization in which the first intron interrupts the C-terminal protein coding region and the second intron interrupts the 3' untranslated region at analogous positions in the two species. Sequence analysis of the 5'-flanking region revealed an unexpected mouse strain-specific genetic polymorphism due to a single base substitution. Deletion analysis revealed that as little as 180 base pairs of the mouse Id3 promoter upstream of the transcription start site is sufficient for a high level of gene expression in proliferating C2C12 myoblasts. In particular, the region between the nucleotide position -180 and -34 appeared to be crucial for maximal reporter gene activity and interacted specifically with C2C12 nuclear proteins. Finally, we showed that, despite the creation of a putative transcription factor-binding site, the genetic polymorphism observed did not affect Id3 promoter activity in proliferating C2C12 cells.

Evidence for the participation of nerve growth factor and its low-affinity receptor (p75NTR) in the regulation of the myogenic program

We have studied expression and function of neurotrophins and their receptors during myogenic differentiation of C2C12 cells, a clonal cell line derived from mouse muscle that is capable of in vitro differentiation. The genes coding for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and their common low-affinity receptor p75(neurotrophin receptor) (p75NTR) were shown to be expressed in C2C12 myoblasts and downregulated during myogenic differentiation and fusion into myotubes. Cocultures with dorsal root ganglia from day 8 chick embryos revealed neurite-promoting activities of C2C12 cells that ceased with myogenic differentiation. These data suggest a temporal and developmental window for the effect of myogenic cell-derived neurotrophins on neuronal as well as on myogenic cell populations. NGF was shown to increase DNA synthesis and cell growth of C2C12 myoblasts and to enhance myogenic differentiation in this cell line. We present evidence that NGF-mediated processes take place at stages preceding myogenic differentiation. Enhanced muscle differentiation was also seen in p75NTR-overexpressing C2C12 myoblasts which maintained high levels of receptors but ceased to produce NGF during differentiation. In contrast, when exogenous NGF was present at the onset of myogenic differentiation of receptor-overexpressing cells, muscle cell development was strongly repressed. This indicates that downregulation of p75NTR is necessary for guiding myogenic cells towards terminal differentiation. Since none of the trk high-affinity neurotrophin receptors could be demonstrated in C2C12 cells, we conclude that NGF mediates its nonneurotrophic effect via its low-affinity receptor in an autocrine fashion.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Loss of fibroblast growth factor receptors is necessary for terminal differentiation of embryonic limb muscle

Early in embryogenesis, precursors of the limb musculature are generated in the somite, migrate to the limb buds and undergo terminal differentiation. Although myogenic differentiation in culture is affected by several growth factors including fibroblast growth factor (FGF), it remains uncertain whether migration and differentiation of myogenic cells in vivo are directly regulated by such growth factors. To investigate the roles of FGF signaling in the regulation of myogenesis both in the somite and the limb bud, mosaic chicken embryos were generated that consist of somitic cells carrying transgenes expressing one of the following: FGF1, FGF4, the FGF receptor type-1 (FGFR1) or its dominant negative mutant (delta FGFR1). Cells infected with virus producing FGF ligand migrated into the somatopleure without differentiating into myotomal muscle, but differentiated into muscle fibers when they arrived in the limb bud. In contrast, cells overexpressing FGFR1 migrated into the limb muscle mass but remained as undifferentiated myoblasts. Cells infected with the delta FGFR1-producing virus failed to migrate to the somatopleure but were capable of differentiating into myotomal muscle within the somites. These results suggest that the FGFR-mediated FGF signaling (1) blocks terminal differentiation of myogenic cells within the somite and (2) sustains myoblast migration to limb buds from the somite, and that (3) down-regulation of FGFRs or FGFR signaling is involved in mechanisms triggering terminal differentiation of the limb muscle mass during avian embryogenesis.