Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO

Abstract

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis. Ample evidence points to central role of NF-κB in such pathologic responses, yet the precise mechanisms underpinning specificity of these responses remain unclear. We propose that motifs of the scaffold protein IKKγ/NEMO partly facilitate such functions. As proof-of-principle, we used site-specific mutagenesis to examine the role of NEMO in mediating RANKL-induced signaling in mouse bone marrow macrophages, known as osteoclast precursors. We identified lysine (K)270 as a target regulating RANKL signaling as K270A substitution results in exuberant osteoclastogenesis in vitro and murine inflammatory osteolysis in vivo. Mechanistically, we discovered that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates inflammatory burden. Specifically, K270A directly or indirectly hinders binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets the modified proteins to autophagy-mediated lysosomal degradation. Taken together, our findings suggest that NEMO serves as a toolkit to fine-tune specific signals in physiologic and pathologic conditions.

Communications Between Bone Marrow Macrophages and Bone Cells in Bone Remodeling

“…Some studies have shown that TGF-β1 directly down-regulates NFATc1 activity by blocking the p65 in the receptor activator and then inhibits the generation and bone resorption of osteoclasts ( Figure 2 ) (Tokunaga et al, 2020 ). Recently, the role of the NEMO protein (a core component of the NF-kB signaling pathway) in mouse bone marrow macrophages was investigated that if the lysine-270 (NEMO-Lys270) in NEMO protein was mutated to Ala, the NF-kB signal in bone marrow macrophages would lose control, leading to the accelerated production of osteoclasts (Adapala et al, 2020 ). What's more, studies have found that the overexpression of cr6 interaction factor-1 (Crif1) in mouse BMSCs can increase the secretion of RANKL through the cAMP/PKA pathway, and then combine with RANK on macrophages to promote the formation of osteoclasts in vitro (Xiang et al, 2020 ).…”

Glutamine metabolism modulates chondrocyte inflammatory response

“…For example, intermittent fasting and rapamycin as autophagy promoting compounds have gained interest recently as modalities for driving protective autophagy ( Johnson et al, 2013 ; Mattson and de Cabo, 2020 ). Our prior work has also demonstrated that autophagy can also regulate inflammatory responses through modulation of NF-κB and other pathways ( Adapala et al, 2020 ), which may be involved in the anti-inflammatory effect of glutamine deprivation. Our future work will focus on understanding the role of ATF4 stress response system and autophagy in the modulation of chondrocyte inflammatory response and NF-κB activity.…”

Glutamine metabolism modulates chondrocyte inflammatory response

“…For example, intermittent fasting and rapamycin as autophagy promoting compounds have gained interest recently as modalities for driving protective-autophagy(Johnson, Rabinovitch, & Kaeberlein, 2013; Mattson & de Cabo, 2020). Our prior work has also displayed that autophagy can also regulate inflammatory responses through modulation of NF-κB and other pathways(Adapala et al, 2020), which may be involved in the anti-inflammatory effect of glutamine deprivation. Our future work will focus on understanding the role of ATF4 stress response system and autophagy in the modulation of chondrocyte inflammatory response and NF-κB activity.…”