Role of iRhoms 1 and 2 in Endochondral Ossification.

Renpeng Fang,Gisela Weskamp,Tak W Mak,Miguel Otero,Carl P Blobel,Samantha Lessard,Coline Haxaire,David R Mcilwain,Stefan F Lichtenthaler

doi:10.3390/ijms21228732

Abstract

Growth of the axial and appendicular skeleton depends on endochondral ossification, which is controlled by tightly regulated cell–cell interactions in the developing growth plates. Previous studies have uncovered an important role of a disintegrin and metalloprotease 17 (ADAM17) in the normal development of the mineralized zone of hypertrophic chondrocytes during endochondral ossification. ADAM17 regulates EGF-receptor signaling by cleaving EGFR-ligands such as TGFα from their membrane-anchored precursor. The activity of ADAM17 is controlled by two regulatory binding partners, the inactive Rhomboids 1 and 2 (iRhom1, 2), raising questions about their role in endochondral ossification. To address this question, we generated mice lacking iRhom2 (iR2−/−) with floxed alleles of iRhom1 that were specifically deleted in chondrocytes by Col2a1-Cre (iR1∆Ch). The resulting iR2−/−iR1∆Ch mice had retarded bone growth compared to iR2−/− mice, caused by a significantly expanded zone of hypertrophic mineralizing chondrocytes in the growth plate. Primary iR2−/−iR1∆Ch chondrocytes had strongly reduced shedding of TGFα and other ADAM17-dependent EGFR-ligands. The enlarged zone of mineralized hypertrophic chondrocytes in iR2−/−iR1∆Ch mice closely resembled the abnormal growth plate in A17∆Ch mice and was similar to growth plates in Tgfα−/− mice or mice with EGFR mutations. These data support a model in which iRhom1 and 2 regulate bone growth by controlling the ADAM17/TGFα/EGFR signaling axis during endochondral ossification.

Highlights

The appendicular and axial skeleton develops through a process referred to as endochondral ossification, in which a highly-specialized cartilage intermediate represents an essential part of bone growth and maturation [1,2,3]
Previous studies have shown that simultaneous inactivation of iRhom1 and 2 in mice results in perinatal lethality and open eyes at birth as well as defects in heart valve and growth plate development, resembling the defects observed in mice lacking a disintegrin and metalloprotease 17 (ADAM17) [30]
Similar results were obtained in this study with primary chondrocytes, in which the stimulated shedding of the epidermal growth factor receptor (EGFR) ligands AREG, HB-EGF and EREG was significantly reduced in cells lacking iRhom2, whereas the stimulated shedding of TGFα, implicated in endochondral ossification [28], was not

Summary

Introduction

The appendicular and axial skeleton develops through a process referred to as endochondral ossification, in which a highly-specialized cartilage intermediate represents an essential part of bone growth and maturation [1,2,3]. Adam17−/− mice have an enlarged zone of mineralized hypertrophic cells in their growth plate, most likely caused by defects in TGFα signaling [15,16,17] These findings are consistent with the known role of the EGFR in regulating endochondral ossification and differentiation of hypertrophic chondrocytes [18,19,20]. Knock-in mice with the human EGFR have reduced EGFR signaling [21], resulting in a hypomorphic phenotype with an enlarged zone of hypertrophic cells in the growth plate This phenotype is seen in developing rats treated with the EGFR-inhibitor Gefitinib or in mice lacking the EGFR in chondrocytes [19], but not in mice lacking the EGFR in osteoclasts [22], suggesting that the EGFR signaling in chondrocytes is crucial for normal endochondral ossification [20]. Previous studies have demonstrated that chondrocyte proliferation and differentiation is regulated by the ADAM17/EGFR signaling pathway, with TGFα functioning as the best candidate EGFR-ligand responsible for regulating these processes [17,19]

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