Bisphosphonates are primary agents in the current pharmacological arsenal against osteoclastmediated bone loss due to osteoporosis, Paget disease of bone, malignancies metastatic to bone, multiple myeloma, and hypercalcemia of malignancy. In addition to currently approved uses, bisphosphonates are commonly prescribed for prevention and treatment of a variety of other skeletal conditions, such as low bone density and osteogenesis imperfecta. However, the recent recognition that bisphosphonate use is associated with pathologic conditions including osteonecrosis of the jaw has sharpened the level of scrutiny of the current widespread use of bisphosphonate therapy. Using the key words bisphosphonate and clinical practice in a PubMed literature search from

Aging is associated with increased cellular senescence, which is hypothesized to drive the eventual development of multiple co-morbidities1. Here, we investigate a role for senescent cells in age-related bone loss by multiple approaches. In particular, we used either genetic (i.e., the INK-ATTAC “suicide” transgene encoding an inducible caspase 8 expressed specifically in senescent cells2–4) or pharmacological (i.e., “senolytic” compounds5,6) means to eliminate senescent cells. We also inhibited the production of the pro-inflammatory secretome of senescent cells using a JAK inhibitor (JAKi)3,7. In old (20–22-months) mice with established bone loss, activation of the INK-ATTAC caspase 8 in senescent cells or treatment with senolytics or the JAKi for 2–4 months resulted in higher bone mass and strength and better bone microarchitecture compared to vehicle-treated mice. The beneficial effects of targeting senescent cells were due to lower bone resorption with either maintained (trabecular bone) or higher (cortical bone) bone formation as compared to vehicle-treated mice. In vitro studies demonstrated that senescent cell-conditioned medium impaired osteoblast mineralization and enhanced osteoclast progenitor survival, leading to increased osteoclastogenesis. Collectively, these data establish a causal role for senescent cells in bone loss with aging and demonstrate that targeting these cells has both anti-resorptive and anabolic effects on bone. As eliminating senescent cells and/or inhibiting their pro-inflammatory secretome also improves cardiovascular function4, enhances insulin sensitivity3, and reduces frailty7, targeting this fundamental mechanism to prevent age-related bone loss suggests a novel treatment strategy not only for osteoporosis but also for multiple age-related co-morbidities.

Physiological effects of ␤ adrenergic receptor (␤2AR) stimulation have been classically shown to result from G s -dependent adenylyl cyclase activation. Here we demonstrate a novel signaling mechanism wherein ␤-arrestins mediate ␤2AR signaling to extracellularsignal regulated kinases 1/2 (ERK 1/2) independent of G protein activation. Activation of ERK1/2 by the ␤2AR expressed in HEK-293 cells was resolved into two components dependent, respectively, on G s -G i /protein kinase A (PKA) or ␤-arrestins. G proteindependent activity was rapid, peaking within 2-5 min, was quite transient, was blocked by pertussis toxin (G i inhibitor) and H-89 (PKA inhibitor), and was insensitive to depletion of endogenous ␤-arrestins by siRNA. ␤-Arrestin-dependent activation was slower in onset (peak 5-10 min), less robust, but more sustained and showed little decrement over 30 min. It was insensitive to pertussis toxin and H-89 and sensitive to depletion of either ␤-arrestin1 or -2 by small interfering RNA. In G s knock-out mouse embryonic fibroblasts, wild-type ␤2AR recruited ␤-arrestin2-green fluorescent protein and activated pertussis toxin-insensitive ERK1/2. Furthermore, a novel ␤2AR mutant (␤2AR T68F,Y132G,Y219A or ␤2AR TYY ), rationally designed based on Evolutionary Trace analysis, was incapable of G protein activation but could recruit ␤-arrestins, undergo ␤-arrestin-dependent internalization, and activate ␤-arrestin-dependent ERK. Interestingly, overexpression of GRK5 or -6 increased mutant receptor phosphorylation and ␤-arrestin recruitment, led to the formation of stable receptor-␤-arrestin complexes on endosomes, and increased agonist-stimulated phospho-ERK1/2. In contrast, GRK2, membrane translocation of which requires G␤␥ release upon G protein activation, was ineffective unless it was constitutively targeted to the plasma membrane by a prenylation signal (CAAX). These findings demonstrate that the ␤2AR can signal to ERK via a GRK5/6-␤-arrestin-dependent pathway, which is independent of G protein coupling.The ␤2-adrenergic receptor (␤2AR) 4 is a well studied member of the large and diverse group of seven transmembrane receptors (7TMRs), which have been shown classically to exert their intracellular effects through G protein activation (1-3). Agonist stimulation of the ␤2AR leads to G s -mediated activation of adenylyl cyclase, resulting in the production of cAMP and subsequent downstream signaling events. Moreover, additional studies both in cultured cell lines and in vitro have demonstrated that, in response to agonist, the ␤2AR can undergo PKAdependent phosphorylation leading to activation of G i (a process referred to as G protein "switching"), thereby effectively changing the signaling specificity of the receptor (4).Cessation of agonist-activated ␤2AR-G s -mediated signaling occurs via recruitment of modulatory proteins, ␤-arrestins, to the cytoplasmic surface of the receptor, a process that is enhanced by receptor phosphorylation by G protein-coupled receptor kinases (GRKs) (5). ␤-arrestin binding physically pre...