This project arose from our unexpected preliminary data indicating that antagonism of 2AR signaling with the selective agent butaxamine (Butax) synergistically enhanced the response of bone to mechanical loading in aged (22 Mo) mice. Remarkably, the observed response exceeded what the same loading regimen would induce in a young adult (4 Mo) mouse by 50%. These data are in direct contrast with the minimal effects of propranolol (a non-selective blocker of 1AR and 2AR) on mechanically-induced bone formation in previous studies, and clearly indicate that although 1AR plays a predominant stimulatory role in SNS regulation of the bone mechanical response and in growth, the 2AR also plays a significant role in signaling SNS suppression of mechanically stimulated bone anabolism in the mature skeleton. The Wnt/-catenin pathway is a major mediator of the bone anabolic response to mechanical stimulation, and our subsequent in vitro studies demonstrated that activation of osteoblastic 2AR blocked both the mechanically stimulated increase in expression of Cox-2, a Wnt target gene, and the accumulation of nuclear -catenin protein, which is an essential aspect of Wnt/-catenin transcriptional activity. In combination, these results are consistent with a tonic 2AR mediated suppression of mechanotransduction through inhibition of Wnt/-catenin activity in osteocytes and osteoblasts. We have therefore hypothesized that antagonism or deletion of the beta 2 adrenergic receptor enhances the response of osteocytes and osteoblasts to mechanical stimuli. We will pursue this hypothesis through three closely related S.
Aims, each with a corresponding sub-hypothesis. The in vivo experiments of S.
Aim #1 are designed to demonstrate that pharmacological antagonism of 2AR with butaxamine synergistically enhances the periosteal anabolic response to mechanical stimulation of the tibia in young adult as well as aged mice and that the butaxamine enhanced mechanical response can be induced in both cortical and trabecular bone. S.
Aim #2, using experiments in knockout mice that lack the 2AR gene in all tissues or specifically in mature osteoblasts and osteocytes, or solely in osteocytes, is designed to prove that the mechanoresponsive synergy observed with butaxamine treatment in wild type mice is specifically due to antagonist effects on the 2AR, and that similar but reduced synergy can be achieved in loaded tibiae of mice that conditionally lack 2AR specifically in osteoblasts and osteocytes, or in osteocytes alone. In S.
Aim #3 we will implement a combined in vivo and in vitro approach to demonstrate that the cellular mechanism underlying this unexpected synergy involves 2AR antagonist enhancement of mechanically induced Wnt/-catenin pathway activity. If successful we believe the project would clearly demonstrate that reducing the activity of 2 adrenergic receptor will enhance Wnt mediated bone anabolism in response to mechanical stimulation and thereby define a novel interaction with this essential mechanotransduction pathway. Further, we believe the data will indicate that selective pharmacological suppression of 2AR activity holds potential as a novel adjuvant treatment that, in combination with modest exercise, could enhance skeletal strength even in the normally less responsive elderly skeleton.
This project seeks to understand how sympathetic nervous system activation of the osteoblastic beta 2 adrenergic receptor directly inhibits the bone formation response to mechanical stimulation. From a clinical perspective we believe that a demonstration of how inhibition of beta 2 adrenergic receptor signaling can enhance the bone mechanical response will directly enable more effective therapies to combat osteoporotic bone loss and the associated increase in fracture risk.