Transcription factor scleraxis vitally contributes to progenitor lineage direction in wound healing of adult tendon in mice

Abstract

Tendon is a dense connective tissue that transmits high mechanical forces from skeletal muscle to bone. The transcription factor scleraxis (Scx) is a highly specific marker of both precursor and mature tendon cells (tenocytes). Mice lacking scx exhibit a specific and virtually complete loss of tendons during development. However, the functional contribution of Scx to wound healing in adult tendon has not yet been fully characterized. Here, using ScxGFP-tracking and loss-of-function systems, we show in an adult mouse model of Achilles tendon injury that paratenon cells, representing a stem cell antigen-1 (Sca-1)–positive and Scx-negative progenitor subpopulation, display Scx induction, migrate to the wound site, and produce extracellular matrix (ECM) to bridge the defect, whereas resident tenocytes exhibit a delayed response. Scx induction in the progenitors is initiated by transforming growth factor β (TGF-β) signaling. scx-deficient mice had migration of Sca-1–positive progenitor cell to the lesion site but impaired ECM assembly to bridge the defect. Mechanistically, scx-null progenitors displayed higher chondrogenic potential with up-regulation of SRY-box 9 (Sox9) coactivator PPAR-γ coactivator-1α (PGC-1α) in vitro, and knock-in analysis revealed that forced expression of full-length scx significantly inhibited Sox9 expression. Accordingly, scx-null wounds formed cartilage-like tissues that developed ectopic ossification. Our findings indicate a critical role of Scx in a progenitor-cell lineage in wound healing of adult mouse tendon. These progenitor cells could represent targets in strategies to facilitate tendon repair. We propose that this lineage-regulatory mechanism in tissue progenitors could apply to a broader set of tissues or biological systems in the body.

Scleraxis lineage cells contribute to organized bridging tissue during tendon healing and identify a subpopulation of resident tendon cells

“…A trace of adult Scx Lin revealed that not all tenocytes are Scx Lin during homeostasis, providing evidence that tenocytes are not a homogenous population. It has previously been shown that Scx-GFP mice (19) label most, but not all, tendon cells (8). In this work, we have shown that Scx-GFP mice label ;90% of intrinsic flexor tendon cells (Supplemental Fig.…”

“…Scale bars, 100 mm (B), 20 mm (C, D). *P , 0.05. that tendon cells lose Scx expression shortly after acute injury (8) and that Scx expression is mechanosensitive (17). Traces of Scx 022 and Scx 527 populations revealed differential Scx expression patterns during healing, with Scx 022 exhibiting fewer Scx Ai9 cells relative to Scx Lin and Scx 527 traces, and an absence of Scx Ai9 cells within the scar tissue.…”

“…Scx is a basic helix-loop-helix transcription factor that directs expression of extracellular matrix components and is necessary for the proper development of force-transmitting tendons (4)(5)(6). The localization of Scx + cells during healing differs greatly between specific tendons and injury models, with some studies showing Scx + cells localized within the defect site (7,8), whereas others exhibit a complete absence of Scx + cells within the scar (3). No studies have extensively characterized Scx lineage cell (Scx Lin ) localization in either a flexor tendon injury model or a model of complete transection and repair.…”

Scleraxis Lineage Cells Contribute to Organized Bridging Tissue During Tendon Healing, and Identifies Subpopulations of Resident Tendon Cells

“…Interestingly, a recent study by Sakabe et al . demonstrated that a partial Achilles tendon transection had Scx+ cells within the defect site by day 14 post-injury [8]. This suggests that keeping the cut ends of the tendon near one another and the maintenance of tensile force across the defect may be necessary for localization of Scx+ cells within the defect.…”

“…Scx is a basic helix-loop-helix transcription factor that directs expression of extracellular matrix components and is necessary for the proper development of force-transmitting tendons [4-6]. The localization of Scx+ cells during healing differs greatly between specific tendons and injury models, with some studies showing Scx+ cells localized at and bridging the defect site [7, 8] while others exhibit a complete absence of Scx+ cells within the scar [3]. No studies have extensively characterized Scx lineage cell localization in either a flexor tendon injury model or a model of complete transection and repair.…”