Fibroblast-derived Hgf controls recruitment and expansion of muscle during morphogenesis of the mammalian diaphragm

Abstract

The diaphragm is a domed muscle between the thorax and abdomen essential for breathing in mammals. Diaphragm development requires the coordinated development of muscle, connective tissue, and nerve, which are derived from different embryonic sources. Defects in diaphragm development cause the common and often lethal birth defect, congenital diaphragmatic hernias (CDH). HGF/MET signaling is required for diaphragm muscularization, but the source of HGF and the specific functions of this pathway in muscle progenitors and effects on phrenic nerve have not been explicitly tested. Using conditional mutagenesis in mice and pharmacological inhibition of MET, we demonstrate that the pleuroperitoneal folds (PPFs), transient embryonic structures that give rise to the connective tissue in the diaphragm, are the source of HGF critical for diaphragm muscularization. PPF-derived HGF is directly required for recruitment of MET+ muscle progenitors to the diaphragm and indirectly (via its effect on muscle development) required for phrenic nerve primary branching. In addition, HGF is continuously required for maintenance and motility of the pool of progenitors to enable full muscularization. Localization of HGF at the diaphragm’s leading edges directs dorsal and ventral expansion of muscle and regulates its overall size and shape. Surprisingly, large muscleless regions in HGF and Met mutants do not lead to hernias. While these regions are likely more susceptible to CDH, muscle loss is not sufficient to cause CDH.

Distinct human stem cell subpopulations drive adipogenesis and fibrosis in musculoskeletal injury

“…VCAM1 marks activated FAPs in adult mice, while OSR1 and OSR2 mark embryologic FAP progenitor populations 13,19 . HGF is a ligand involved in the expansion of progenitor populations during muscle regeneration 20,21 . These findings suggest that the CD74+ population functions as activated progenitors.…”

“…The transcription factors RUNX1 and HIF1A have been shown to attenuate both fibroblast and adipocyte lineage specification 48–53 . FGF7 and HGF have known roles in regulating tissue regeneration via several downstream effects including activation of progenitor cells and immunomodulation 20,21,54–56 . These findings suggest that CD74+ cells upregulate genes that control stem/progenitor cell activation and lineage specification during injury.…”

Catenin signaling controls phrenic motor neuron development and function during a narrow temporal window

“…This indicates that cadherins have a much more predominant role in dendritic rather than axonal elaboration. While many signaling pathways contribute to phrenic axon growth and diaphragm innervation, including HGF/MET ( Sefton et al, 2022 ), Slit/Robo ( Charoy et al, 2017 ), and Col25a1 ( Tanaka et al, 2014 ), to our knowledge, cadherins are the first cell adhesion molecules to be implicated in phrenic MN dendritic development.…”