Department of Molecular Pharmacology, Tohoku University School of Medicine, Sendai 980-8575, Japan.
Department of Human Health and Nutrition, Shokei Gakuin University, Natori 981-1295, Japan.
Int J Mol Sci. 2022 Feb 14;23(4):2094. doi: 10.3390/ijms23042094.
The primary cilium is a hair-like immotile organelle with specific membrane receptors, including the receptor of Hedgehog signaling, smoothened. The cilium organized in preosteoblasts promotes differentiation of the cells into osteoblasts (osteoblast differentiation) by mediating Hedgehog signaling to achieve bone formation. Notably, 4.1G is a plasma membrane-associated cytoskeletal protein that plays essential roles in various tissues, including the peripheral nervous system, testis, and retina. However, its function in the bone remains unexplored. In this study, we identified 4.1G expression in the bone. We found that, in the 4.1G-knockout mice, calcium deposits and primary cilium formation were suppressed in the trabecular bone, which is preosteoblast-rich region of the newborn tibia, indicating that 4.1G is a prerequisite for osteoblast differentiation by organizing the primary cilia in preosteoblasts. Next, we found that the primary cilium was elongated in the differentiating mouse preosteoblast cell line MC3T3-E1, whereas the knockdown of 4.1G suppressed its elongation. Moreover, 4.1G-knockdown suppressed the induction of the cilia-mediated Hedgehog signaling and subsequent osteoblast differentiation. These results demonstrate a new regulatory mechanism of 4.1G in bone formation that promotes the primary ciliogenesis in the differentiating preosteoblasts and induction of cilia-mediated osteoblast differentiation, resulting in bone formation at the newborn stage.
初级纤毛是一种毛发状的不动细胞器，具有特定的膜受体，包括 Hedgehog 信号转导的受体 smoothened。初级纤毛在成骨前体细胞中组织，通过介导 Hedgehog 信号转导促进细胞分化为成骨细胞（成骨细胞分化），从而实现骨形成。值得注意的是，4.1G 是一种质膜相关细胞骨架蛋白，在包括外周神经系统、睾丸和视网膜在内的各种组织中发挥着重要作用。然而，其在骨骼中的功能尚未被探索。在本研究中，我们鉴定了骨骼中的 4.1G 表达。我们发现，在 4.1G 敲除小鼠中，钙沉积和初级纤毛形成在新生胫骨富含成骨前体细胞的小梁骨中受到抑制，这表明 4.1G 通过组织成骨前体细胞中的初级纤毛是成骨细胞分化的必要条件。接下来，我们发现初级纤毛在分化中的小鼠成骨前体细胞系 MC3T3-E1 中伸长，而 4.1G 的敲低抑制了其伸长。此外，4.1G 的敲低抑制了纤毛介导的 Hedgehog 信号转导的诱导及其随后的成骨细胞分化。这些结果表明 4.1G 在骨形成中的一个新的调节机制，促进分化中的成骨前体细胞中的初级纤毛发生和纤毛介导的成骨细胞分化，从而在新生阶段促进骨形成。
