IgSF11 regulates osteoclast differentiation through association with the scaffold protein PSD-95

Abstract

Osteoclasts are multinucleated, giant cells derived from myeloid progenitors. While receptor activator of NF-κB ligand (RANKL) stimulation is the primary driver of osteoclast differentiation, additional signaling further contributes to osteoclast maturation. Here, we demonstrate that immunoglobulin superfamily member 11 (IgSF11), whose expression increases during osteoclast differentiation, regulates osteoclast differentiation through interaction with postsynaptic density protein 95 (PSD-95), a scaffold protein with multiple protein interaction domains. IgSF11 deficiency in vivo results in impaired osteoclast differentiation and bone resorption but no observed defect in bone formation. Consequently, IgSF11-deficient mice exhibit increased bone mass. Using in vitro osteoclast culture systems, we show that IgSF11 functions through homophilic interactions. Additionally, we demonstrate that impaired osteoclast differentiation in IgSF11-deficient cells is rescued by full-length IgSF11 and that the IgSF11-PSD-95 interaction requires the 75 C-terminal amino acids of IgSF11. Our findings reveal a critical role for IgSF11 during osteoclast differentiation and suggest a role for IgSF11 in a receptor-and signal transduction molecule-containing protein complex.Bone Research (2020) 8:5; https://doi.

CD44 Can Compensate for IgSF11 Deficiency by Associating with the Scaffold Protein PSD-95 during Osteoclast Differentiation

“…We then performed temporal osteoclast differentiation assays in low-cell-density and high-cell-density cultures (Supplemental Figure S1B). Consistent with our previous report [14], impaired osteoclast differentiation in IgSF11 −/− cultures was observed from day 2, and the differences became bigger on day 3 in low-cell-density cultures. On the other hand, IgSF11 +/+ and IgSF11 −/− cells cultures at a high cell density showed no differences in terms of osteoclast formation.…”

“…IgSF11 has an extracellular domain with a membrane-distal V-type domain and a membrane-proximal C2-type domain, a transmembrane domain, and a cytoplasmic domain with PDZ binding motif at C-terminal. We recently demonstrated a critical role for IgSF11 in osteoclast differentiation, with IgSF11 deficiency resulting in impaired osteoclast differentiation [14]. We found that IgSF11 functions through homophilic interactions.…”

“…Next, we examined the effect of IgSF11 stimulation on CD44-deficient (CD44 −/− ) cells. As we reported previously, IgSF11 engages in homophilic interactions, and adding recombinant soluble IgSF11-Fc protein that consists of the extracellular region of IgSF11 fused to the human IgG Fc region blocked osteoclast differentiation in a dose-dependent manner ( Figure S3) [14]. We therefore employed plate-bound recombinant IgSF11-Fc protein to instead stimulate IgSF11 (Figure 3).…”

“…We have previously reported that ablation of the gene IgSF11 in mice results in increased bone mass, not due to altered osteoblast activity, but rather because of impaired osteoclast differentiation [14]. Indeed, when bone marrow monocytes (BMMs) from wild-type (IgSF11 +/+ ) and IgSF11-deficient (IgSF11 −/− ) mice were cultured with M-CSF + RANKL to induce osteoclasts, IgSF11 −/− cultures showed decreased numbers of tartrate-resistant acidic phosphatase-positive (TRAP + ) multinuclear cells (i.e., mature osteoclasts), consistent with our previous findings ( Figure S1) [14]. However, given the importance of cell-cell interactions to osteoclast differentiation and maturation, we hypothesized that high cell culture density would increase the probability of cell-cell interactions that might be mediated by alternative cell surface molecules in the absence of IgSF11 homophilic interactions.…”

“…We sought to further investigate the molecular mechanism underlying the interrelationship between IgSF11 and CD44 in osteoclasts. Given the requirement for association between IgSF11 and PSD-95 during osteoclast differentiation [14] and the capacity of CD44 to compensate for IgSF11 deficiency, we hypothesized that CD44 may functionally associate with PSD-95 during osteoclast differentiation. To examine the association of CD44 with PSD-95, BMMs were cultured with M-CSF + RANKL for two days to induce pre-osteoclasts, and the lysates were used for coimmunoprecipitation.…”