Ectopic bone formation after joint replacement or brain injury in humans is a serious complication that causes immobility of joints and severe pain. However, mechanisms underlying such ectopic bone formation are not fully understood. Bone morphogenetic protein (BMPs) are defined as inducers of ectopic bone formation, and they are regulated by several types of inhibitors. ANA is an antiproliferative molecule that belongs to Tob/BTG family, but its activity in bone metabolism has not been known. Here, we examined the role of ANA on ectopic bone formation activity of BMP. In ANA-deficient and wild-type mice, BMP2 was implanted to induce ectopic bone formation in muscle. ANA deficiency increased mass of newly formed bone in vivo compared with wild-type based on 3D-μCT analyses. ANA mRNA was expressed in bone in vivo as well as in osteoblastic cells in vitro. Such ANA mRNA levels were increased by BMP2 treatment in MC3T3-E1 osteoblastic cells. Overexpression of ANA suppressed BMP-induced expression of luciferase reporter gene linked to BMP response elements in these cells. Conversely, ANA mRNA knockdown by small interference RNA enhanced the BMP-dependent BMP response element reporter expression. It also enhanced BMP-induced osteoblastic differentiation in muscle-derived C2C12 cells. Immunoprecipitation assay indicated that ANA interacts with Smad8. Thus, ANA is a suppressor of ectopic bone formation induced by BMP, and this inhibitory ANA activity is a part of the negative feedback regulation of BMP function.

Most endoplasmic reticulum (ER)-retained proteins contain a carboxy-terminal signal sequence called the ER retention signal motif such as the Lys-Asp-Glu-Leu (KDEL) motif. Using this molecular mechanism, we developed a new dominant-negative assay, designated the KDEL-tag trap assay, to negatively regulate secretion of disulfide bond-dependent protein dimers, as typified by TGF-beta superfamily proteins. First, we tested this method on the Nodal protein Xnr5, which is a well-studied mesoderm inducer in vertebrates. Tagging of Xnr5 protein with KDEL at the carboxy-terminus effectively blocked the secretion of Xnr5, resulting in complete inhibition of mesoderm induction in Xenopus embryogenesis. Second, we examined the usefulness of the KDEL-tag trap assay on BMPs, which are well-known negative regulators of neural induction and ventralizing factors during early development, and demonstrated that the functions of the BMP family proteins BMP4 and ADMP were blocked by the KDEL-tag trap assay. Moreover, the technical feasibility of the KDEL-tag trap assay was confirmed in a cell culture system using mouse osteoblasts. Taken together, these results suggest that the KDEL-tag trap assay can be adapted to inhibit a variety of plasma membrane or secreted proteins of a multimeric nature.

Gorham–Stout disease (GSD), also called vanishing bone disease, is a rare osteolytic disease, frequently associated with lymphangiomatous tissue proliferation. The causative genetic background has not been noted except for a case with a somatic mutation in KRAS. However, in the present study, we encountered a case of GSD from a consanguineous family member. Whole‐exome sequencing (WES) analysis focusing on rare recessive variants with zero homozygotes in population databases identified a homozygous missense variant (c.823G > C, p.Asp275His) in gasdermin D (GSDMD) in the patient and heterozygous in his unaffected brother. Because this variant affects the Asp275 residue that is involved in proteolytic cleavage by caspase‐11 (as well as ‐4 and ‐5) to generate an activating p30 fragment required for pyroptotic cell death and proinflammation, we confirmed the absence of this cleavage product in peripheral monocytic fractions from the patient. A recent study indicated that a shorter p20 fragment, generated by further cleavage at Asp88, has a cell‐autonomous function to suppress the maturation of osteoclasts to resorb bone matrix. Thus, the present study suggests for the first time the existence of hereditary GSD cases or novel GSD‐like diseases caused by GSDMD deficiency. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.