Loss of Gsα in the Postnatal Skeleton Leads to Low Bone Mass and a Blunted Response to Anabolic Parathyroid Hormone Therapy

Partha Sinha,Piia Aarnisalo,Rhiannon Chubb,Ingrid J Poulton,Jun Guo,Gregory Nachtrab,Takaharu Kimura,Srilatha Swami,Hamid Saeed,Min Chen,Lee S Weinstein,Ernestina Schipani,Natalie A Sims,Henry M Kronenberg,Joy Y Wu

doi:10.1074/jbc.m115.679753

Abstract

Parathyroid hormone (PTH) is an important regulator of osteoblast function and is the only anabolic therapy currently approved for treatment of osteoporosis. The PTH receptor (PTH1R) is a G protein-coupled receptor that signals via multiple G proteins including Gsα. Mice expressing a constitutively active mutant PTH1R exhibited a dramatic increase in trabecular bone that was dependent upon expression of Gsα in the osteoblast lineage. Postnatal removal of Gsα in the osteoblast lineage (P-Gsα(OsxKO) mice) yielded markedly reduced trabecular and cortical bone mass. Treatment with anabolic PTH(1-34) (80 μg/kg/day) for 4 weeks failed to increase trabecular bone volume or cortical thickness in male and female P-Gsα(OsxKO) mice. Surprisingly, in both male and female mice, PTH administration significantly increased osteoblast numbers and bone formation rate in both control and P-Gsα(OsxKO) mice. In mice that express a mutated PTH1R that activates adenylyl cyclase and protein kinase A (PKA) via Gsα but not phospholipase C via Gq/11 (D/D mice), PTH significantly enhanced bone formation, indicating that phospholipase C activation is not required for increased bone turnover in response to PTH. Therefore, although the anabolic effect of intermittent PTH treatment on trabecular bone volume is blunted by deletion of Gsα in osteoblasts, PTH can stimulate osteoblast differentiation and bone formation. Together these findings suggest that alternative signaling pathways beyond Gsα and Gq/11 act downstream of PTH on osteoblast differentiation.

Highlights

Parathyroid hormone (PTH) is an important regulator of osteoblast function and is the only anabolic therapy currently approved for treatment of osteoporosis
When doxycycline was administered in drinking water at 2 mg/ml to pregnant females from mating until 7 days after birth, constitutively active mutant form of PTH1R (caPTH1R); Gs␣OsxKO mice resembled caPTH1R mice with significantly greater trabecular bone mass, providing evidence that doxycycline prevented the deletion of Gs␣ (Fig. 1C)
These results demonstrate that the caPTH1R mutant receptor requires expression of Gs␣ in osteoblasts to increase trabecular bone mass and suggest that Gs␣ expression in Gs␣OsxKO mice can be regulated by doxycycline administration

Summary

Introduction

Parathyroid hormone (PTH) is an important regulator of osteoblast function and is the only anabolic therapy currently approved for treatment of osteoporosis. Mice expressing a constitutively active mutant PTH1R exhibited a dramatic increase in trabecular bone that was dependent upon expression of Gs␣ in the osteoblast lineage. Treatment with anabolic PTH[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34] (80 ␮g/kg/day) for 4 weeks failed to increase trabecular bone volume or cortical thickness in male and female P-Gs␣OsxKO mice.

Results

Conclusion