Loss of Gi G-Protein-Coupled Receptor Signaling in Osteoblasts Accelerates Bone Fracture Healing.
G-protein-coupled receptors (GPCRs) are key regulators of skeletal homeostasis and are likely important in fracture healing. Because GPCRs can activate multiple signaling pathways simultaneously, we used targeted disruption of G(i) -GPCR or activation of G(s) -GPCR pathways to test how each pathway functions in the skeleton. We previously demonstrated that blockade of G(i) signaling by pertussis toxin (PTX) transgene expression in maturing osteoblastic cells enhanced cortical and trabecular bone formation and prevented age-related bone loss in female mice. In addition, activation of G(s) signaling by expressing the G(s) -coupled engineered receptor Rs1 in maturing osteoblastic cells induced massive trabecular bone formation but cortical bone loss. Here, we test our hypothesis that the G(i) and G(s) pathways also have distinct functions in fracture repair. We applied closed, nonstabilized tibial fractures to mice in which endogenous G(i) signaling was inhibited by PTX, or to mice with activated G(s) signaling mediated by Rs1. Blockade of endogenous G(i) resulted in a smaller callus but increased bone formation in both young and old mice. PTX treatment decreased expression of Dkk1 and increased Lef1 mRNAs during fracture healing, suggesting a role for endogenous G(i) signaling in maintaining Dkk1 expression and suppressing Wnt signaling. In contrast, adult mice with activated Gs signaling showed a slight increase in the initial callus size with increased callus bone formation. These results show that G(i) blockade and G(s) activation of the same osteoblastic lineage cell can induce different biological responses during fracture healing. Our findings also show that manipulating the GPCR/cAMP signaling pathway by selective timing of G(s) and G(i) -GPCR activation may be important for optimizing fracture repair.
Endocrine Research Unit, VA Medical Center and Departments of Medicine and Physiology, University of California, San Francisco, CA.
Liping
L
Wang
Department of Medicine, the Program in Craniofacial Biology, and the Institute for Human Genetics, University of California, San Francisco, CA.
Edward C
EC
Hsiao
Department of Orthopedic Surgery, University of California, San Francisco General Hospital, Orthopaedic Trauma Institute, San Francisco, CA.
Shirley
S
Lieu
Department of Orthopedic Surgery, University of California, San Francisco General Hospital, Orthopaedic Trauma Institute, San Francisco, CA.
Mark
M
Scott
Endocrine Research Unit, VA Medical Center and Departments of Medicine and Physiology, University of California, San Francisco, CA.
Dylan
D
O'Carroll
Department of Medicine, the Program in Craniofacial Biology, and the Institute for Human Genetics, University of California, San Francisco, CA.
Ashley
A
Urrutia
Department of Medicine, the Program in Craniofacial Biology, and the Institute for Human Genetics, University of California, San Francisco, CA.|Gladstone Institute of Cardiovascular Disease, San Francisco, CA.|Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA.
Bruce R
BR
Conklin
Department of Orthopedic Surgery, University of California, San Francisco General Hospital, Orthopaedic Trauma Institute, San Francisco, CA.|Institut National de la Santé et de la Recherche Médicale (INSERM; National Institute of Health and Medical Research), Unités Mixtes de Recherche (UMR) 1163, Université Paris Descartes-Sorbonne Paris Cité, Institut Imagine, Paris, France.
Celine
C
Colnot
Endocrine Research Unit, VA Medical Center and Departments of Medicine and Physiology, University of California, San Francisco, CA.
Robert A
RA
Nissenson
D000818:Animals; D000242:Cyclic AMP / Q000235:genetics / Q000378:metabolism*; D005260:Female; D017102:Fracture Healing / Q000235:genetics*; D050723:Fractures, Bone / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000235:genetics / Q000378:metabolism; D036341:Intercellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D051821:Lymphoid Enhancer-Binding Factor 1 / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D010006:Osteoblasts / Q000378:metabolism* / Q000473:pathology; D043562:Receptors, G-Protein-Coupled / Q000235:genetics*; D015290:Second Messenger Systems
C501127:Dkk1 protein, mouse; D036341:Intercellular Signaling Peptides and Proteins; C497574:Lef1 protein, mouse; D051821:Lymphoid Enhancer-Binding Factor 1; D043562:Receptors, G-Protein-Coupled; D000242:Cyclic AMP; D019206:GTP-Binding Protein alpha Subunits, Gi-Go
FRACTURE REPAIR; G-PROTEIN-COUPLED RECEPTOR SIGNALING; OSTEOBLASTIC CELLS