Osteocalcin expressing cells from tendon sheaths in mice contribute to tendon repair by activating Hedgehog signaling

Abstract

Both extrinsic and intrinsic tissues contribute to tendon repair, but the origin and molecular functions of extrinsic tissues in tendon repair are not fully understood. Here we show that tendon sheath cells harbor stem/progenitor cell properties and contribute to tendon repair by activating Hedgehog signaling. We found that Osteocalcin (Bglap) can be used as an adult tendon-sheath-specific marker in mice. Lineage tracing experiments show that Bglap-expressing cells in adult sheath tissues possess clonogenic and multipotent properties comparable to those of stem/progenitor cells isolated from tendon fibers. Transplantation of sheath tissues improves tendon repair. Mechanistically, Hh signaling in sheath tissues is necessary and sufficient to promote the proliferation of Mkx-expressing cells in sheath tissues, and its action is mediated through TGFβ/Smad3 signaling. Furthermore, co-localization of GLI1+ and MKX+ cells is also found in human tendinopathy specimens. Our work reveals the molecular function of Hh signaling in extrinsic sheath tissues for tendon repair.

A Tppp3+Pdgfra+ tendon stem cell population contributes to regeneration and reveals a shared role for PDGF signalling in regeneration and fibrosis

“…We uncovered markers such as Lama4, Ly6a/e, and Plin2 for tdT + ScxGFP − stem cells ( Fig. 3b; Extended Data 3a), but a recently reported sheath marker, Osteocalcin 21 , was not detected, nor in any cell cluster from scRNAseq. We also found additional matrix biosynthesis and collagen genes in tdT − ScxGFP + tenocytes not previously reported 26,27 (Extended Data 3b).…”

“…To better the odds of uncovering adult tendon stem cells, here we chose to use the regeneration-competent Patellar tendon punch-injury model 11,18,19 . We took a logical approach to characterize cell types in the adult Patellar tendon, followed by making Cre-ER T2 drivers to define Tppp3 + paratenon sheath cells 20,21 as tendon stem cells. By exploiting heterogeneity within the Tppp3 + tendon stem cell population, we determined the cellular and molecular mechanisms that inextricably link tendon stem cell-mediated regeneration and fibrosis via shared usage of PDGF signalling.…”

Bringing tendon biology to heel: Leveraging mechanisms of tendon development, healing, and regeneration to advance therapeutic strategies

“…Commonly referred to as tendon stem progenitor cells, these cells can expand clonally and differentiate into multiple lineages in vitro, as well as generate tendon tissue upon subsequent transplantation in vivo. 125,[136][137][138][139][140] Yet to date, only a few studies have taken advantage of Cre-based lineage tracing methods in murine models to examine whether such a stem or progenitor cell exists in vivo and participates in natural tendon homeostasis and healing. Collectively, these findings have pointed to cells both in the surrounding peritenon and within the tendon proper as participating in tendon healing, but the functional significance of each distinct population remains unclear.…”

A robust method for RNA extraction and purification from a single adult mouse tendon

“…Several studies using large animal models such as sheep, rabbits, and rats have provided important information about tendon injury, biomechanics, surgical techniques, and bioengineering strategies for tendon repair ( Voleti, Buckley & Soslowsky, 2012 ). Other studies have used mouse genetics to gain an understanding of the molecular and cellular response of tendons to acute injuries, changing load environments, and in gene loss-of-function models ( Dunkman et al, 2014 ; Dyment et al, 2014 ; Howell et al, 2017 ; Mendias, Bakhurin & Faulkner, 2008 ; Wang et al, 2017 ). The mouse system offers unique advantages for implementing mechanistic studies of tendon biology as they permit genetic lineage tracing and conditional knockout strategies, and they can be housed simply and in large numbers to improve sample sizes for functional studies.…”