A PDGFRβ-PI3K signaling axis mediates periosteal cell activation during fracture healing

Abstract

Insufficient and delayed fracture healing remain significant public health problems with limited therapeutic options. Phosphoinositide 3-kinase (PI3K) signaling, a major pathway involved in regulation of fracture healing, promotes proliferation, migration, and differentiation of osteoprogenitors. We have recently reported that knock-in mice with a global increase in PI3K signaling (gCblYF) show enhanced femoral fracture healing characterized by an extraordinary periosteal response to injury. Interestingly, of all growth factor receptors involved in fracture healing, PI3K directly binds only to PDGFR. Given these findings, we hypothesized a PDGFR-PI3K interaction is necessary for mediating robust periosteal cell activation following fracture. In this study, we isolated primary periosteal cells from gCblYF mice to analyze cross-talk between the PDGFRβ and PI3K signaling pathways. We found PDGFRβ signaling contributes to robust Akt phosphorylation in periosteal cells in comparison with other growth factor signaling pathways. Additionally, we performed femoral fractures on gCblYF mice with a conditional removal of PDGFRβ in mesenchymal progenitors using inducible alpha smooth muscle actin (αSMA) CreERT2 mice. Our studies showed that depletion of PDGFRβ signaling within these progenitors in the early phase of fracture healing significantly abrogates PI3K-mediated periosteal activation and proliferation three days after fracture. Combined, these results suggest that PDGFRβ signaling through PI3K is necessary for robust periosteal activation in the earliest phases of fracture healing.

Regional specialization and fate specification of bone stromal cells in skeletal development

“…Blood vessels are known to be an important source of PDGF-B, which promotes the proliferation and recruitment of PDGFRb + pericytes and vascular smooth muscle cells (Hellströ m et al, ll OPEN ACCESS 1999). In bone, PDGFRb controls the behavior of skeletal stem and progenitor cells during regeneration (Bö hm et al, 2019;Doherty et al, 2019). In agreement with these results, we show that overexpression of PDGF-B enhances the number of OSX + OPCs, suppresses the emergence perilipin + adipocytes, and promotes type-H vessel and artery formation in the bone vasculature.…”

PDGFRα reporter activity identifies periosteal progenitor cells critical for bone formation and fracture repair

“… 20 A PDGFR-PI3K signaling axis mediates periosteal cell activation during healing. 20 , 48 In addition, PDGFRβ marks reparative skeletal stem cells in the periosteal, endosteal, and perivascular niches, which altogether give rise to osteoblastic, chondrogenic, and fibroblastic progeny in the callus. 11 The loss of PDGFRβ impairs callus formation.…”

An Improved Methodology to Evaluate Cell and Molecular Signals in the Reparative Callus During Fracture Healing

“…We performed IF labeling of the cryosections for the presence of two proteins that are highly expressed in stem/progenitor cells that give rise to osteoblasts, namely, smooth muscle actin (SMA) and Nestin. 15–18 We observed that SMA-positive cells are predominantly located at the periosteal regions of the day 7 fracture callus, but migrate to the central callus region by day 14 (Fig. 5).…”