The G protein-coupled receptor ADGRG6 maintains mouse growth plate homeostasis through IHH Signaling.

ABSTRACT

The cartilage growth plate is essential for maintaining skeletal growth; however, the mechanisms governing postnatal growth plate homeostasis are still poorly understood. Using approaches of molecular mouse genetics and spatial transcriptomics applied to formalin-fixed, paraffin-embedded (FFPE) tissues, we show that ADGRG6/GPR126, a cartilage-enriched adhesion G protein-coupled receptor (GPCR), is essential for maintaining slow-cycling resting zone cells, appropriate chondrocyte proliferation and differentiation, and growth plate homeostasis in mice. Constitutive ablation of Adgrg6 in osteochondral progenitor cells with Col2a1Cre leads to a shortened resting zone, formation of cell clusters within the proliferative zone, and an elongated hypertrophic growth plate, marked by limited expression of PTHrP but increased IHH signaling throughout the growth plate. Attenuation of Smoothened (SMO)-dependent hedgehog signaling restored the Adgrg6 deficiency-induced expansion of hypertrophic chondrocytes, confirming that IHH signaling can promote chondrocyte hypertrophy in a PTHrP-independent manner. In contrast, postnatal ablation of Adgrg6 in mature chondrocytes with AcanCreERT2, induced after the formation of the resting zone, does not affect PTHrP expression but causes an overall reduction of growth plate thickness marked by increased cell death specifically in the resting zone cells and a general reduction of chondrocyte proliferation and differentiation. Spatial transcriptomics reveals that ADGRG6 is essential for maintaining chondrocyte homeostasis by regulating osteogenic and catabolic genes in all the zones of the postnatal growth plates, potentially through positive regulation of SOX9 expression. Our findings elucidate the essential role of a cartilage-enriched adhesion GPCR in regulating cell proliferation and hypertrophic differentiation by regulation of PTHrP/IHH signaling, maintenance of slow-cycle resting zone chondrocytes, and safeguarding chondrocyte homeostasis in postnatal mouse growth plates.

The cartilage growth plate is an essential structure for skeletal growth, however, the mechanisms that govern growth plate homeostasis are still poorly understood. In this study, we showed that an adhesion G protein-coupled receptor (GPCR) named ADGRG6 plays an essential role in maintaining the slow-cycling cells in the resting zone of the growth plate and directing appropriate proliferation and differentiation of the growth plate chondrocytes. Using a technique called spatial transcriptomics, we compared the gene expression profiles in control and Adgrg6 mutant growth plates and found that ADGRG6 prevents premature hypertrophic differentiation of the growth plate chondrocytes by negatively regulating Indian Hedgehog (IHH) signaling. In summary, our findings highlighted the essential role of a cartilage-enriched GPCR in maintaining growth plate homeostasis through IHH signaling.