Osteoclast-Osteocyte Interaction's in Paget's Disease - Project Summary/Abstract: Paget’s Disease (PD) is usually diagnosed in patients >50 years of age, and is characterized by focal bone lesions (PDLs) that progress from an initial lytic to a mixed osteolytic-osteoblastic phase, due to rapid new bone formation. PD patients rarely develop new PDLs beyond those present at diagnosis. The primary cellular abnormality in PD resides in the osteoclast (OCL). However, the mechanisms responsible for the highly focal nature of PDLs, why PD is diagnosed in elderly patients, especially in patients harboring genetic mutations linked to PD, and the mechanisms driving the exuberant resorption/formation in PD, are unclear. We showed that OCLs from 70% of PD patients express measles virus nucleocapsid protein (MVNP), and transgenic mice with MVNP targeted to OCLs (MVNP mice) develop PDLs and the abnormal OCLs characteristic of PD. MVNP- expressing OCLs and OCLs from PD patients are hyper-responsive to RANKL and express high IL-6 levels, which induce high levels of IGF1 in OCLs (OCL-IGF1) to promote osteoblast (OB) differentiation in MVNP mice. Importantly, OCLs from PD patients but not normal donors also express increased IL-6 and IGF1. We confirmed OCL-IGF1’s role in PD and normal bone remodeling in vivo by generating WT and MVNP mice with targeted deletion of Igf1 in OCLs. We found that MVNP mice but not MVNP mice lacking OCL-Igf1 developed PDLs. Thus, high OCL-IGF1 levels in PD may be required to induce PDLs. However, these results do not explain why PDLs occur in aging patients or animal models and persist locally for long periods. These observations are most readily explained by stochastic changes in a long-lived bone cell population, most likely osteocytes (OCys). In this proposal we will test the Hypothesis: that pagetic OCL precursors, which are hyper-responsive to RANKL, home to specific skeletal sites containing senescent, pre-lesional OCys that produce or induce locally increased RANKL. As PD-OCLs numbers increase, they hyper-secrete sufficient IL-6 and IGF-1 to decrease Sost/sclerostin, increase expression of Wnts, block OCy differentiation and act directly on OB to enhance new bone formation. In preliminary studies, we found that OCys in PD (PD-OCys) express lower levels of OCy maturation markers, and display markedly reduced dendritic processes. Further, the percentage of sclerostin+ OCys adjacent to PDLs in MVNP mice was significantly lower compared to MVNP mice without PDLs or WT mice, and that IGF1 suppressed sclerostin expression in OCys. Importantly, similar morphologic changes occurred in OCys in bone samples of a PD patient, but not a normal donor. These results suggest that OCys may be key contributors to the pathogenesis of PD. To further test this hypothesis, we will: Aim 1. Characterize OCL-IGF1’s effects in PD and PD-OCys, identify the molecular mechanisms involved, and test if increased OCL-IGF1 is sufficient to induce PDLs and PD-OCys. Aim 2. Determine the role of senescent OCys in the location, development, progression and persistence of PDLs and PD. We will assess if prolonged exposure of PD-OCLs to normal OCys, or shorter exposure to OCys in older mice suffices for PDLs to develop.
