IRX3 and IRX5 Inhibit Adipogenic Differentiation of Hypertrophic Chondrocytes and Promote Osteogenesis

Abstract

Maintaining the correct proportions of different cell types in the bone marrow is critical for bone function. Hypertrophic chondrocytes (HCs) and osteoblasts are a lineage continuum with a minor contribution to adipocytes, but the regulatory network is unclear. Mutations in transcription factors, IRX3 and IRX5, result in skeletal patterning defects in humans and mice. We found coexpression of Irx3 and Irx5 in late‐stage HCs and osteoblasts in cortical and trabecular bone. Irx3 and Irx5 null mutants display severe bone deficiency in newborn and adult stages. Quantitative analyses of bone with different combinations of functional alleles of Irx3 and Irx5 suggest these two factors function in a dosage‐dependent manner. In Irx3 and Irx5 nulls, the amount of bone marrow adipocytes was increased. In Irx5 nulls, lineage tracing revealed that removal of Irx3 specifically in HCs exacerbated reduction of HC‐derived osteoblasts and increased the frequency of HC‐derived marrow adipocytes. β‐catenin loss of function and gain of function specifically in HCs affects the expression of Irx3 and Irx5, suggesting IRX3 and IRX5 function downstream of WNT signaling. Our study shows that IRX3 and IRX5 regulate fate decisions in the transition of HCs to osteoblasts and to marrow adipocytes, implicating their potential roles in human skeletal homeostasis and disorders.

Regulation and Role of Transcription Factors in Osteogenesis

“…Although apoptosis and replacement by bone have been widely considered as the terminal fate of hypertrophic chondrocytes, recent studies using different lineage tracing mouse models ( Col2a1 -CreERT, Sox9 -CreERT, Acan -CreERT, Osx -CreERT, Col10a1 -Cre, and Col10a1 -CreERT) have indicated that hypertrophic chondrocytes retain the plasticity of multi-lineage potential to differentiate into osteoblasts, adipocytes, pericytes, and stromal cells [ 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ]. The hypertrophic chondrocyte-derived osteoblasts play essential roles in maintaining bone homeostasis and fracture healing [ 35 , 39 ].…”

“…In mice, Irx3 and Irx5 compound deletion mutants die at mid-gestation due to cardiac defects and skeletal malformation [ 98 , 99 ], whereas conditional deletion of Irx3/5 using osteoblast-specific Osx -Cre in mice results in osteopenia in postnatal growth [ 100 ]. IRX3 and IRX5 regulate skeletal development in a dosage-dependent manner, with bone loss severity correlating with the number of functional alleles of Irx3/5 [ 40 ].…”

“…Loss of WNT signaling changes the fate commitment of osteoblasts to adipocytes from mesenchymal progenitors [ 51 ]. Notably, IRX3 and IRX5 act as downstream mediators of WNT signaling to determine the fate of progenitors, fine-tuning the divergence between osteogenesis and adipogenesis [ 40 ]. Clearly, the transcriptional network determining osteogenesis is complex, and further advances in molecular and informatic technologies will play an important role to further our understanding.…”

Only the Co-Transcriptional Activity of β-Catenin Is Required for the Local Regulatory Effects in Hypertrophic Chondrocytes on Developmental Bone Modeling

“…The two Irx genes have recently been shown to be positively regulated by Wnt/βcatenin signaling in hypertrophic chondrocytes. (30) To trace the fate of the hypertrophic cells expressing the signaling-defective Ctnnb1 allele, the Rosa26-flox-Stop-flox-YFP reporter line was introduced. This revealed a severe reduction in the percentage of chondrocyte-derived (OSX + YFP + ), as well as perichondrium-derived (OSX + YFP À ) osteoblasts (Fig.…”

Hypertrophic Chondrocytes Serve as a Reservoir for Marrow Associated Skeletal Stem and Progenitor Cells, Osteoblasts, and Adipocytes During Skeletal Development

“…6 Utilizing similar techniques, Tan et al also demonstrated the potential for mature adipocytes to arise from Col10a1-expressing hypertrophic chondrocytes. 9 Since the fates of mature osteoblasts, adipocytes, and pericytes are unlikely to arise from independent hypertrophic chondrocyte transdifferentiation events, we reasoned that the hypertrophic chondrocytes may undergo a dedifferentiation process to generate skeletal stem and progenitor cell (SSPC) population(s) that was not certified by peer review) is the author/funder. All rights reserved.…”