Mechanisms of Physiologic and Pathologic Osteoclastogenesis

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ABSTRACT: The transcription factor NF-kB is expressed ubiquitously in all cell types and is readily activated by numerous factors and cytokines. Baseline NF-kB activity is essential for skeletal development and physiologic cellular functions. In contrast, its exacerbated and often uncontrolled activity during inflammation leads to undesired harmful effects with major dysfunctional consequences including osteolysis. Hence, therapies targeting NF-kB have been highly pursued to combat most inflammatory diseases. Unfortunately, most available therapies are inefficient owing to lack of selectivity in such complex and ubiquitous signaling pathway wherein the essential beneficial functions of NF-kB are blocked along side the harmful effects leading to detrimental outcomes. Therefore, there is an unmet need to decode NF-kB signaling to identify specific targets that assign signal specificity and distinguish between physiologic and pathologic functions. To address this critical knowledge gap, we focused on RANKL-induced osteoclastogenesis as a proof of concept and set out to decipher the NF-kB molecular machinery and identify the signal-specific molecular signature that controls this response in osteoclast progenitors and maintains skeletal homeostasis. We hypothesize that the IKK scaffold IKKγ/NEMO serves as a platform that site-specifically assembles unique signal activating or suppressing protein complexes in cell and stimulus specific manners. This hypothesis is based on recent advances implicating NEMO as a scaffold that integrates signaling molecules in response to a wide range of stimuli at lysine (K) specific sites (refer to Fig 2). These modifications include, lysine poly-ubiquitination, SUMOylation, and according to our novel finding, ISGylation; a process of attaching the ubiquitin-like protein, ISG15 (IFN-stimulated gene) to target proteins. We conduced comprehensive NEMO lysine mutational analysis and identified the NEMO K270 residue as a crucial RANKL-regulation target. Specifically, NEMO harboring K270A mutation (NEMOK270A) elicits exacerbated osteoclastogenesis. More importantly, myeloid knock-in mice of the NEMOK270A that we generated displayed severe osteopenia and osteolysis. Mechanistically, autophagy is significantly decreased in NEMOK270A BMMs. Furthermore, proteomic screen identified interferon-stimulated gene-15 (ISG15) as a potential regulator of osteoclastogenesis and autophagy. Thus, our overarching hypothesis is: RANKL-induced binding of ISG15 to NEMO at K270 is essential to restrain osteoclastogenesis by assembling a negative-feedback response. We further posit that mutating K270 hinders this regulatory process leading to reduced autophagy and uncontrolled osteoclastogenesis. Our aims are: Aim 1: Determine the mechanism by which NEMO, through its K270 site, maintains physiologic and restrains pathologic/exacerbated osteoclastogenesis. Aim 2: Determine the role of RANKL-induced ISG15 as the ubiquitin-like protein that facilitates NEMO- K270-mediated autophagy and control of physiologic osteoclastogenesis.