Regulatory Mechanisms of Implant-Induced Osteolysis

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 DESCRIPTION (provided by applicant): Orthopedic implants are the most common approach to repair degenerative and failing joints as well as segmental bone defects. However, inflammation and ensuing osteolysis are frequent complications of orthopedic implants arising in response to implant-derived debris. Subsequent revision surgeries are more challenging and carry greater risk of failure, morbidity and mortality, especially in the aging population. Our lon-term research goal is to decipher the biologic and pathologic mechanisms underlying implant debris- mediated osteolysis. In previous work, we have shown orthopedic particles (PMMA, PE, etc.) target osteoclast (OC) progenitors and activate NF-B pathway, which is considered central for inflammatory responses and essential for OC differentiation. We further showed that PMMA particles enhance NF-B activation by targeting TAK1, NEMO, TRAF6, and IKK2. This process exacerbates osteoclastogenesis and leads to osteolysis. Recently, we identified a novel mechanism underlying regulation of TRAF6/NEMO/IKK2 signaling complex by the forkhead protein Foxp3 in regulatory T cells (Tregs), leading to NF-B activation, abundant production of growth factors, cytokines and chemokine. This hyper-cytokinemia led to hyperproliferation of hematopoietic stem cells (HSCs) and to expansion of early/primitive population of myeloid progenitors that possessed high OC potential. We now have evidence that PMMA particles induce this inflammatory pathway, in vivo. We also found that PMMA particles directly regulate Notch/RNBPJ signaling in myeloid progenitors leading to activation of OC-related transcription factors (NF-B and NFATc1). Hence, we posit that PMMA particles, and likely other implant materials, trigger inflammatory paracrinic and autocrinic signaling circuits in multiple cell typesin the bone marrow microenvironment including lymphocytes and myeloid progenitors. The signaling switches of these circuits include Foxp3, Notch/RBPJ, and NF-B transcription factors, driving myeloid cell expansion in response to bone marrow stress in paracrinic and autocrinic modes. Based on this information, we hypothesize that PMMA particles, by unidentified "stress" mechanism, modulate T cell activation by down-regulating Foxp3 function, resulting with T cell phenotypic switch into pathogenic T helper cells with enhanced NF-B activity. This step leads to production of osteotropic and pro-inflammatory factors that expand the myelo-progenitor population and enhance OC formation. We further hypothesize that PMMA particles contribute to activation of Notch signaling directly in myeloid progenitors leading to activation of RBPJ and NFATc1. Hence we propose to: 1) Investigate the paracrine mechanism(s) by which PMMA particles target lymphocytes to enhance myelo- proliferation and increase OC burden, 2) Investigate the autocrine mechanism underlying PMMA regulation of myelo-proliferation in OCPs by RBPJ and its contribution to osteoclastogenesis, and 3) Determine the role of Foxp3 and RBPJ in PMMA-induced tibial and calvarial osteolysis. Appropriate cellular, molecular, and genetic mouse models and tools will be used to test these Aims.