Connexin 43 hemichannels protect bone loss during estrogen deficiency

Liang Ma,Hongyun Cheng,Yi Tian,Jean X Jiang,Daniel Brian Shropshire,Sumin Gu,Joseph J Pearson,Teja Guda,Rui Hua,Roberto Jose Fajardo

doi:10.1038/s41413-019-0050-2

Abstract

Estrogen deficiency in postmenopausal women is a major cause of bone loss, resulting in osteopenia, osteoporosis, and a high risk for bone fracture. Connexin 43 (Cx43) hemichannels (HCs) in osteocytes play an important role in osteocyte viability, bone formation, and remodeling. We showed here that estrogen deficiency reduced Cx43 expression and HC function. To determine if functional HCs protect osteocytes and bone loss during estrogen deficiency, we adopted an ovariectomy model in wild-type (WT) and two transgenic Cx43 mice: R76W (dominant-negative mutant inhibiting only gap junction channels) and Cx43 Δ130–136 (dominant-negative mutant compromising both gap junction channels and HCs). The bone mineral density (BMD), bone structure, and histomorphometric changes of cortical and trabecular bones after ovariectomy were investigated. Our results showed that the Δ130–136 transgenic cohort had greatly decreased vertebral trabecular bone mass compared to WT and R76W mice, associated with a significant increase in the number of apoptotic osteocyte and empty lacunae. Moreover, osteoclast surfaces in trabecular and cortical bones were increased after ovariectomy in the R76W and WT mice, respectively, but not in ∆130–136 mice. These data demonstrate that impairment of Cx43 HCs in osteocytes accelerates vertebral trabecular bone loss and increase in osteocyte apoptosis, and further suggest that Cx43 HCs in osteocytes protect trabecular bone against catabolic effects due to estrogen deficiency.

Highlights

Osteoporosis and osteopenia, diseases of low bone mineral density (BMD), are common among postmenopausal women and feature a high risk of fragility fracture.[1]
These data show that osteocytes in the cortical bone of Δ130–136 were more vulnerable to ovariectomy-induced cell death than those in WT and R76W mice, and further suggest that Connexin 43 (Cx43) HCs protect osteocytes and prevent bone loss caused by estrogen deficiency
In this study, we found that bone loss and defects in osteocytespecific Cx43 Δ130–136 OVX mice are more evident compared to R76W and WT mice, which includes increased vertebral trabecular bone loss, osteocyte apoptosis, compromised bone material quality, and increased oxidative stress (OS) levels

Summary

INTRODUCTION

Osteoporosis and osteopenia, diseases of low bone mineral density (BMD), are common among postmenopausal women and feature a high risk of fragility fracture.[1]. The importance of Cx43 in bone formation, remodeling, and responses to mechanical loading and parathyroid hormone has been illustrated using osteoblast- and osteocytespecific conditional Cx43 knockout (cKO) mouse models.[18,19,20,21,22,23] deficiency of Cx43 primarily in osteocytes shows increased apoptosis associated with empty lacunae.[24] these gene KO models could not elucidate the specific role of connexin channels given that Cx43 forms both gap junction channels and HCs. We generated two transgenic mouse models driven by DMP1 promoter with the expression of Cx43 mutants predominantly in osteocytes: R76W, with dominant-negative effects on gap junction channels; and Δ130–136, with dominantnegative effects on both gap junction channels and HCs.[25] We showed that impairment of Cx43 HCs negatively affected bone formation, remodeling, and osteocyte viability. The impairment of osteocytic HCs in OVX mice As reported previously,[27] at 4 weeks after surgery, the body weight of OVX mice was increased compared to sham-operated

RESULTS

DISCUSSION

Findings

MATERIALS AND METHODS