Loss of Cbl-PI3K interaction modulates the periosteal response to fracture by enhancing osteogenic commitment and differentiation

Abstract

The periosteum contains multipotent skeletal progenitors that contribute to bone repair. The signaling pathways regulating the response of periosteal cells to fracture are largely unknown. Phosphatidylinositol-3 Kinase (PI3K), a prominent lipid kinase, is a major signaling protein downstream of several factors that regulate osteoblast differentiation. Cbl is an E3 ubiquitin ligase and a major adaptor protein that binds to the p85 regulatory subunit and modulates PI3K activity. Substitution of tyrosine 737 to phenylalanine (Y737F) in Cbl abolishes the interaction between Cbl and the p85 subunit without affecting the Cbl’s ubiquitin ligase function. Here, we investigated the role of PI3K signaling during the very early stages of fracture healing using OsterixRFP reporter mice. We found that the absence of PI3K regulation by Cbl resulted in robust periosteal thickening, with increased proliferation of periosteal cells. While the multipotent properties of periosteal progenitors to differentiate into chondrocytes and adipocytes did not change, osteogenic differentiation in the absence of Cbl-PI3K interaction was highly augmented. The increased stability and nuclear localization of Osterix observed in periosteal cells lacking Cbl-PI3K interaction may explain this enhanced osteogenic differentiation since the expression of Osterix transcriptional target genes including osteocalcin and BSP are increased in YF cells. Overall, our findings highlight a hitherto unexplored and novel role for Cbl and PI3K in modulating the osteogenic response of periosteal cells during the early stages of fracture repair.

PI3K activation increases SDF-1 production and number of osteoclast precursors, and enhances SDF-1-mediated osteoclast precursor migration

“…PI3K-AKT activity regulates activation of several transcription factors. We have previously reported that in YF mice enhanced AKT activity is responsible for upregulation of Sp7 activation, resulting in increased bone formation during fracture healing ( Scanlon et al, 2017 ). Sp1 transcriptional activity is also dependent on PI3K activation ( Chu, 2012 ; Zhang et al, 2006 ).…”

“…Our previous studies have shown that knock-in mice harboring the Y737F mutation (YF mice) have increased PI3K-AKT activation ( Adapala et al, 2010a , Adapala et al, 2010b , Adapala et al, 2014a ; Scanlon et al, 2017 ). YF mice increased bone volume due to a combination of increased non-resorptive osteoclasts and increased bone formation by osteoblasts ( Adapala et al, 2010a ; Brennan et al, 2011 ).…”

Tendon stem progenitor cells: Understanding the biology to inform therapeutic strategies for tendon repair

“…In several self‐renewing and regenerative tissues, such as muscle, intestine, and hair follicles, definitive markers have been established to enable identification and characterization of resident stem cell populations and their in vivo activity . Similarly, major advances have been made for bone marrow derived mesenchymal stem cells (BMSCs), adipose stem cells, and periosteal cells . In contrast, while stem/progenitor cells have been identified for tendons (frequently termed TSPCs), there remains a paucity of information regarding distinguishing markers, making it challenging to study these cells in vivo.…”

“…[2][3][4][5][6][7][8][9][10] Similarly, major advances have been made for bone marrow derived mesenchymal stem cells (BMSCs), adipose stem cells, and periosteal cells. [11][12][13][14][15][16][17][18][19] In contrast, while stem/progenitor cells have been identified for tendons (frequently termed TSPCs), there remains a paucity of information regarding distinguishing markers, making it challenging to study these cells in vivo. Given the molecular and structural similarities between tendons and ligaments, [20][21][22] we will focus this review on tendon and TSPCs, since the majority of the literature is concentrated on tendon biology.…”

A novel role for cathepsin K in periosteal osteoclast precursors during fracture repair

“…The periosteum is also known to be a reservoir of quiescent mesenchymal progenitor cells, which are activated upon injury. These cells differentiate to form cartilage and bone and thereby repair bone injury . The periosteum is a highly vascularized tissue and has been shown to contain myeloid cells of the hematopoietic lineage, such as tartrate‐resistant acid phosphatase (TRAP) cells .…”