Oxygen‑sensing histone demethylase KDM6A modulates chondrocyte‑to‑osteoblast transdifferentiation by activating the Wnt/β‑catenin pathway.
KDM6A is critical for chondrocyte-to-osteoblast transdifferentiation and fracture healing, but its clinical relevance in humans requires further investigation.
Where it sits
this study against the rest of the abaloparatide (tymlos) corpusSummary and findings
The study investigates the role of KDM6A, an oxygen-sensitive histone demethylase, in chondrocyte-to-osteoblast transdifferentiation during fracture healing using a Kdm6a-knockout mouse model. Single-cell RNA sequencing identified chondrocyte-derived osteoprogenitors as key players in this process, with the Wnt/β-catenin pathway implicated in their differentiation. KDM6A deficiency was associated with impaired osteogenic differentiation and delayed fracture healing.
Abstract
Fracture healing is a complex biological process involving chondrocyte (CH) differentiation and endochondral ossification. A subset of CHs may transdifferentiate into osteoblasts, enhancing bone regeneration. The oxygen‑sensing histone demethylase lysine demethylase 6A (KDM6A) and local oxygen microenvironment are hypothesized to serve pivotal roles in modulating this transition; however, the precise regulatory mechanisms remain unclear. To assess the role of KDM6A, an oxygen‑sensitive histone demethylase, in endochondral ossification, an inducible cartilage‑specific Kdm6a‑knockout mouse model was generated. Single‑cell RNA sequencing (scRNA‑seq) analysis was performed in a mouse tibial fracture model to characterize CH subpopulations and their fate transitions during bone repair. scRNA‑seq identified distinct CH subpopulations, including chondrocyte‑derived osteoprogenitors (CDOPs), which acted as osteoblast precursors during endochondral ossification. Pseudotime trajectory analysis revealed a bifurcated differentiation pathway, with CDOPs exhibiting rapid osteoblast conversion. Functional enrichment analyses implicated the Wnt/β‑catenin pathway in this transition. In vitro, CHs isolated from bone callus of KDM6A‑knockout and control mice were induced to undergo transdifferentiation into osteoblasts under varying oxygen tensions. The expression levels of chondrogenic markers, osteogenic differentiation‑related indicators and canonical Wnt signaling molecules, as well as the levels of histone dimethylation of H3K27 (H3K27me2) and trimethylation of H3K27 (H3K27me3) at their promoter regions, were assessed. In vivo, the molecular and functional consequences of KDM6A deficiency were characterized through histopathological evaluation and bone microarchitecture analysis. <i>In vitro</i>, CHs cultured under normoxic conditions exhibited greater osteogenic differentiation than those cultured under hypoxic conditions. Conversely, loss of KDM6A impaired the pro‑osteogenic effect of normoxia on CH‑to‑osteoblast transdifferentiation, indicating the importance of KDM6A in oxygen‑mediated CH‑to‑osteoblast transdifferentiation. Mechanistically, chromatin immunoprecipitation analysis revealed that under normoxic conditions, KDM6A‑knockout CHs exhibited higher levels of the repressive histone marks H3K27me2 and H3K27me3 at the Wnt3a promoter region, as well as increased H3K27me3 levels at the Runt‑related transcription factor 2 (RUNX2) promoter region, compared with control cells. These findings indicated that KDM6A catalyzed the removal of H3K27 methylation at the promoters of Wnt3a and RUNX2, thereby relieving their transcriptional repression. <i>In vivo</i>, KDM6A‑knockout mice exhibited osteogenic defects and delayed fracture healing compared with control mice. KDM6A serves as a pivotal oxygen sensor that drives CH‑to‑osteoblast transdifferentiation and enhances fracture healing through Wnt/β‑catenin pathway activation. The KDM6A‑mediated oxygen response mechanism is a potential target for enhancing bone regeneration during fracture repair.
Background
Fracture healing involves complex biological processes, including the differentiation of chondrocytes and endochondral ossification. A subset of chondrocytes may transdifferentiate into osteoblasts, contributing to bone regeneration. The study explores the role of KDM6A, an oxygen-sensing histone demethylase, in this transdifferentiation process, which could have implications for enhancing fracture healing.
Methods
An inducible cartilage-specific Kdm6a-knockout mouse model was used to study the role of KDM6A in endochondral ossification. Single-cell RNA sequencing was performed on a mouse tibial fracture model to identify chondrocyte subpopulations and their fate transitions. In vitro experiments involved culturing chondrocytes under varying oxygen tensions to assess osteogenic differentiation, while in vivo analysis included histopathological evaluation and bone microarchitecture analysis.
Results
KDM6A-knockout mice showed osteogenic defects and delayed fracture healing compared to controls. Single-cell RNA sequencing identified distinct chondrocyte subpopulations, including chondrocyte-derived osteoprogenitors, which rapidly converted to osteoblasts. In vitro, normoxic conditions promoted osteogenic differentiation, but KDM6A deficiency impaired this effect. Chromatin immunoprecipitation analysis revealed increased repressive histone marks at the Wnt3a and RUNX2 promoters in KDM6A-knockout cells.
Interpretation
The study suggests that KDM6A plays a crucial role in chondrocyte-to-osteoblast transdifferentiation by modulating histone methylation and activating the Wnt/β-catenin pathway. While the findings provide mechanistic insights, the clinical significance remains uncertain due to the use of a mouse model. Further research is needed to determine if these mechanisms can be targeted to enhance fracture healing in humans.
Key findings
- KDM6A-knockout mice exhibited osteogenic defects and delayed fracture healing.
- scRNA-seq identified chondrocyte-derived osteoprogenitors as osteoblast precursors.
- Normoxic conditions enhanced osteogenic differentiation in vitro.
- Loss of KDM6A impaired the pro-osteogenic effect of normoxia.
- KDM6A catalyzed removal of H3K27 methylation at Wnt3a and RUNX2 promoters.
Limitations
- Mouse model may not fully replicate human biology.
- Mechanistic focus limits direct clinical translation.
- Oxygen tension effects studied only in vitro.