Project Summary The interactions of osteoblasts (OB) and osteoclasts (OC) during bone remodeling are connected and highly coordinated. Paget’s Disease of Bone (PDB) is a focal disease in which this coordinated activity has been disrupted leading to exaggerated bone remodeling. As the second most common metabolic bone disease after osteoporosis, it affects 1-3% of the U.S. population after age 50. PDB is a genetically heterogeneous disease, however two strong associations have been linked to PDB: mutations in the Sequestosome 1 (SQSTM1) gene have been found in familial and somatic PDB and studies have linked PDB with the presence of the measles virus nucleocapsid protein (MVNP). Studies of the etiology of PDB has largely focused on osteoclast dysregulation, however, when we examined the somatic form of the disease, our data suggest a self-amplifying positive feedback loop signaling pathway between the pagetic OB and pagetic OC involving the chemokine signaling molecules CCL5, CXCL6 and their respective receptors CCR5 and CXCR1 that we hypothesize is responsible for the etiology of this disease. We found that these changes in chemokine signaling involved OBs carrying a SQSTM1mut and OCs expressing MVNP. We also found that MVNP-expressing OCs strongly upregulated a chemokine that attracts OB and pre-OC cells. Moreover, we found that not all the OB cells in the PDB lesion carried the SQSTM1 mutation suggesting that PDB lesions are composed of a complex mosaic of OB cell types, in which only a subset carry the SQSTM1 mutation validating the growth-by-recruitment hypothesis. To test this, we propose three specific aims. Aim 1 will compare genomic expression of pagetic OC from PDB with and without a SQSTM1 mutation and with and without MVNP expression. Aim 2 will test whether MVNP-expressing pre-OC cells preferentially migrate in response to signals from SQSTM1mut-expressing OB. Mixing experiments will use OB with or without a SQSTM1 mutation and pre-OC with or without MVNP expression to examine chemokine signals directing migration and attraction. Aim 3 will test for a self-amplifying positive feedback loop model involving MVNP-expressing OC cells amplifying the chemoattractant signals initiated by SQSTM1mut-carrying OB. Together, this paradigm-shifting research will foster new understanding of interactions between OB and OC during bone remodeling and disease and potentially open new approaches to PDB treatment that target only the mutant OB and OC cells.