The sympathetic nervous system (SNS) is an important regulator of bone, and may contribute to bone
pathology during aging. SNS activity is also heightened in post-menopausal women, causing reduced bone
formation by osteoblasts and increased bone resorption by osteoclasts, which leads to bone loss. Large meta-
analyses, osteoporosis cohort studies (preliminary data from co-I Dr. Lary) and new prospective trials,
consistently show that ß-adrenergic receptor (ßAR) antagonists (i.e. ß-blockers) are associated with reduced
fracture risk, increased bone mineral density (BMD), and reduced bone resorption. However, mechanistic
studies have focused largely on the osteoblast as the target of SNS activity. Humans and mice have three
ßARs: ß1AR, ß2AR and ß3AR. The gene encoding ß2AR (Adrb2) is highly expressed in bone, which also
expresses lower levels of Adrb1, but does not express Adrb3. In mice, Adrb2 deletion in osteoblasts improves
bone formation and prevents receptor activator of nuclear factor-kappa B ligand (RANKL)-mediated osteoclast
recruitment after stimulation with a ß-agonist. In humans, ß1-selective ß-blockers are used most often, but their
selectivity is not absolute, and many still bind ß2AR. Propranolol, a non-selective ß-blocker, increases BMD in
the majority of preclinical studies. Results from our laboratory show that propranolol can limit bone resorption
directly in vitro, and in vivo without changing RANKL levels. This is in contrast to the established dogma that
osteoblast expression of ß2AR regulates osteoclasts only indirectly via RANKL. Our proposed work will resolve
the outstanding mechanistic questions of how ß1AR and ß2AR directly influence osteoclast differentiation and
what their contributions are to age- and SNS-related bone loss. We propose a novel hypothesis that ßAR
signaling in osteoclasts promotes differentiation and resorption and contributes to bone loss in vivo. To fully
characterize ßAR receptor subtypes and novel signaling mechanisms in osteoclasts, and to determine the
contribution of ßAR subtypes to in vivo phenotypes of bone density and bone remodeling, we propose a
combination of genetic and pharmacologic in vivo and in vitro approaches in the following Specific Aims.
Specific Aim 1: We will identify signaling mechanisms activated by specific ßARs in osteoclasts. We expect
that we will identify novel and established target pathways to test in vivo for efficacy in modulating bone
resorption. Specific Aim 2: We will test the relative contributions of osteoclast ß1AR and ß2AR to SNS-
mediated and aging-related bone loss in vivo. The SNS has been implicated in osteoporosis, but the role of
ßARs in osteoclasts has never been investigated in this condition. We hypothesize that both deletion of ß1AR
and ß2AR, specifically in the osteoclasts, will attenuate ßAR agonist-induced and aging-related bone loss in
mice. Our work will provide a more complete understanding of the role of SNS signaling in age-related bone
loss in vivo, and will lead to studies targeting specific osteoclast ßARs and downstream signaling pathways for
treatment of bone diseases.
