Perivascular Fibro-Adipogenic Progenitor Tracing during Post-Traumatic Osteoarthritis

Perivascular mural cells surround capillaries and microvessels and have diverse regenerative or fibrotic functions after tissue injury. Subsynovial fibrosis is a well-known pathologic feature of osteoarthritis, yet transgenic animals for use in visualizing perivascular cell contribution to fibrosis during arthritic changes have not been developed. Here, inducible Pdgfra-CreER^T2 reporter mice were subjected to joint-destabilization surgery to induce arthritic changes, and cell lineage was traced over an 8-week period with a focus on the joint-associated fat pad. Results showed that, at baseline, inducible Pdgfra reporter activity highlighted adventitial and, to a lesser extent, pericytic cells within the infrapatellar fat pad. Joint-destabilization surgery was associated with marked fibrosis of the infrapatellar fat pad, accompanied by an expansion of perivascular Pdgfra-expressing cellular descendants, many of which adopted α-smooth muscle actin expression. Gene expression analysis of microdissected infrapatellar fat pad confirmed enrichment in membrane-bound green fluorescent protein/Pdgfra-expressing cells, along with a gene signature that corresponded with injury-associated fibro-adipogenic progenitors. Our results highlight dynamic changes in joint-associated perivascular fibro-adipogenic progenitors during osteoarthritis.

Tissue cross talks governing limb muscle development and regeneration

For decades, limb development has been a paradigm of three-dimensional patterning. Moreover, as the limb muscles and the other tissues of the limb's musculoskeletal system arise from distinct developmental sources, it has been a prime example of integrative morphogenesis and cross-tissue communication. As the limbs grow, all components of the musculoskeletal system (muscles, tendons, connective tissue, nerves) coordinate their growth and differentiation, ultimately giving rise to a functional unit capable of executing elaborate movement. While the molecular mechanisms governing global three-dimensional patterning and formation of the skeletal structures of the limbs has been a matter of intense research, patterning of the soft tissues is less understood. Here, we review the development of limb muscles with an emphasis on their interaction with other tissue types and the instructive roles these tissues play. Furthermore, we discuss the role of adult correlates of these embryonic accessory tissues in muscle regeneration.

TGF-β-driven muscle degeneration and failed regeneration underlie disease onset in a DMD mouse model

Duchenne muscular dystrophy (DMD) is a chronic muscle disease characterized by poor myogenesis and replacement of muscle by extracellular matrix. Despite the shared genetic basis, severity of these deficits varies among patients. One source of these variations is the genetic modifier that leads to increased TGF-β activity. While anti-TGF-β therapies are being developed to target muscle fibrosis, their effect on the myogenic deficit is underexplored. Our analysis of in vivo myogenesis in mild (C57BL/10ScSn-mdx/J and C57BL/6J-mdxΔ52) and severe DBA/2J-mdx (D2-mdx) dystrophic models reveals no defects in developmental myogenesis in these mice. However, muscle damage at the onset of disease pathology, or by experimental injury, drives up TGF-β activity in the severe, but not in the mild, dystrophic models. Increased TGF-β activity is accompanied by increased accumulation of fibroadipogenic progenitors (FAPs) leading to fibro-calcification of muscle, together with failure of regenerative myogenesis. Inhibition of TGF-β signaling reduces muscle degeneration by blocking FAP accumulation without rescuing regenerative myogenesis. These findings provide in vivo evidence of early-stage deficit in regenerative myogenesis in D2-mdx mice and implicates TGF-β as a major component of a pathogenic positive feedback loop in this model, identifying this feedback loop as a therapeutic target.

Hic1 deletion unleashes quiescent connective tissue stem cells and impairs skeletal muscle regeneration

Skeletal muscle fibro-adipogenic progenitors (FAPs) are tissue-resident connective tissue cells and the main cellular source of pathological fibro-fatty scar associated with muscle disorders. Although our knowledge about skeletal muscle mesenchymal progenitor cells has exploded in the past decade, we still lack information about their origin, fate, gene regulation, function, and stemness. A recent study by Underhill and colleagues, published in Cell Stem Cell, described the last census of Hic1 mesenchymal progenitor/stem cells in skeletal muscle regeneration, providing valuable results and data to the ever-expanding community of scientists interested in tissue regeneration and fibrosis. This commentary contextualizes and summarizes these exciting new findings.

Single-cell revolution unveils the mysteries of the regenerative mammalian digit tip

The digit tip is an exciting model for studying regeneration in mammals, but the precise mechanisms and the populations of cells involved in the formation and remodeling of the blastema remain unknown. In an exciting new work, Storer et al. take advantage of single-cell RNAseq combined with Pdgfra+ ​lineage-tracing to open the way into the enigmatic world of mammalian tissue regeneration.

TGF-β-driven downregulation of the Wnt/β-Catenin transcription factor TCF7L2/TCF4 in PDGFRα+ fibroblasts

Mesenchymal stromal/stem cells (MSCs) are multipotent progenitors essential for organogenesis, tissue homeostasis, regeneration, and scar formation. Tissue injury upregulates TGF-β signaling, which modulates myofibroblast fate, extracellular matrix remodeling, and fibrosis. However, the molecular determinants of MSCs differentiation and survival remain poorly understood. The canonical Wnt Tcf/Lef transcription factors regulate development and stemness, but the mechanisms by which injury-induced cues modulate their expression remain underexplored. Here, we studied the cell-specific gene expression of Tcf/Lef and, more specifically, we investigated whether damage-induced TGF-β impairs the expression and function of TCF7L2, using several models of MSCs, including skeletal muscle fibro-adipogenic progenitors. We show that Tcf/Lefs are differentially expressed and that TGF-β reduces the expression of TCF7L2 in MSCs but not in myoblasts. We also found that the ubiquitin-proteasome system regulates TCF7L2 proteostasis and participates in TGF-β-mediated TCF7L2 protein downregulation. Finally, we show that TGF-β requires HDACs activity to repress the expression of TCF7L2. Thus, our work found a novel interplay between TGF-β and Wnt canonical signaling cascades in PDGFRα+ fibroblasts and suggests that this mechanism could be targeted in tissue repair and regeneration.

First person – Osvaldo Contreras

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Osvaldo Contreras is first author on ‘Cross-talk between TGF-β and PDGFRα signaling pathways regulates the fate of stromal fibro–adipogenic progenitors’, published in JCS. Osvaldo is a postdoctoral scientist in the laboratory of Enrique Brandan at Departamento de Biología Celular y Molecular and Center for Aging and Regeneration (CARE-ChileUC), Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile, investigating skeletal muscle tissue-resident mesenchymal progenitors in health and disease.