Lineage tracing of cells expressing the ciliary gene IFT140 during bone development

Recent Progress on Genetically Modified Animal Models for Membrane Skeletal Proteins: The 4.1 and MPP Families

The protein 4.1 and membrane palmitoylated protein (MPP) families were originally found as components in the erythrocyte membrane skeletal protein complex, which helps maintain the stability of erythrocyte membranes by linking intramembranous proteins and meshwork structures composed of actin and spectrin under the membranes. Recently, it has been recognized that cells and tissues ubiquitously use this membrane skeletal system. Various intramembranous proteins, including adhesion molecules, ion channels, and receptors, have been shown to interact with the 4.1 and MPP families, regulating cellular and tissue dynamics by binding to intracellular signal transduction proteins. In this review, we focus on our previous studies regarding genetically modified animal models, especially on 4.1G, MPP6, and MPP2, to describe their functional roles in the peripheral nervous system, the central nervous system, the testis, and bone formation. As the membrane skeletal proteins are located at sites that receive signals from outside the cell and transduce signals inside the cell, it is necessary to elucidate their molecular interrelationships, which may broaden the understanding of cell and tissue functions.

Recent advances in primary cilia in bone metabolism

Primary cilia are microtubule-based organelles that are widespread on the cell surface and play a key role in tissue development and homeostasis by sensing and transducing various signaling pathways. The process of intraflagellar transport (IFT), which is propelled by kinesin and dynein motors, plays a crucial role in the formation and functionality of cilia. Abnormalities in the cilia or ciliary transport system often cause a range of clinical conditions collectively known as ciliopathies, which include polydactyly, short ribs, scoliosis, thoracic stenosis and many abnormalities in the bones and cartilage. In this review, we summarize recent findings on the role of primary cilia and ciliary transport systems in bone development, we describe the role of cilia in bone formation, cartilage development and bone resorption, and we summarize advances in the study of primary cilia in fracture healing. In addition, the recent discovery of crosstalk between integrins and primary cilia provides new insights into how primary cilia affect bone.

IFT140+/K14+ cells function as stem/progenitor cells in salivary glands

Stem/progenitor cells are important for salivary gland development, homeostasis maintenance, and regeneration following injury. Keratin-14⁺ (K14⁺) cells have been recognized as bona fide salivary gland stem/progenitor cells. However, K14 is also expressed in terminally differentiated myoepithelial cells; therefore, more accurate molecular markers for identifying salivary stem/progenitor cells are required. The intraflagellar transport (IFT) protein IFT140 is a core component of the IFT system that functions in signaling transduction through the primary cilia. It is reportedly expressed in mesenchymal stem cells and plays a role in bone formation. In this study, we demonstrated that IFT140 was intensively expressed in K14⁺ stem/progenitor cells during the developmental period and early regeneration stage following ligation-induced injuries in murine submandibular glands. In addition, we demonstrated that IFT140⁺/ K14⁺ could self-renew and differentiate into granular duct cells at the developmental stage in vivo. The conditional deletion of Ift140 from K14⁺ cells caused abnormal epithelial structure and function during salivary gland development and inhibited regeneration. IFT140 partly coordinated the function of K14⁺ stem/progenitor cells by modulating ciliary membrane trafficking. Our investigation identified a combined marker, IFT140⁺/K14⁺, for salivary gland stem/progenitor cells and elucidated the essential role of IFT140 and cilia in regulating salivary stem/progenitor cell differentiation and gland regeneration.

Role of primary cilia and Hedgehog signaling in craniofacial features of Ellis-van Creveld syndrome

Ellis-van Creveld syndrome (EvC) is an autosomal recessive genetic disorder involving pathogenic variants of EVC and EVC2 genes and classified as a ciliopathy. The syndrome is caused by mutations in the EVC gene on chromosome 4p16, and EVC2 gene, located close to the EVC gene, in a head-to-head configuration. Regardless of the affliction of EVC or EVC2, the clinical features of Ellis-van Creveld syndrome are similar. Both these genes are expressed in tissues such as, but not limited to, the heart, liver, skeletal muscle, and placenta, while the predominant expression in the craniofacial tissues is that of EVC2. Biallelic mutations of EVC and EVC2 affect Hedgehog signaling and thereby ciliary function, crucial factors in vertebrate development, culminating in the phenotypical features characteristic of EvC. The clinical features of Ellis-van Creveld syndrome are consistent with significant abnormalities in morphogenesis and differentiation of the affected tissues. The robust role of primary cilia in histodifferentiation and morphodifferentiation of oral, perioral, and craniofacial tissues is becoming more evident in the most recent literature. In this review, we give a summary of the mechanistic role of primary cilia in craniofacial development, taking Ellis-van Creveld syndrome as a representative example.

Cytoskeletal Protein 4.1G Is Essential for the Primary Ciliogenesis and Osteoblast Differentiation in Bone Formation

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.

CDC20 promotes bone formation via APC/C dependent ubiquitination and degradation of p65

The E3 ubiquitin ligase complex CDC20‐activated anaphase‐promoting complex/Cyclosome (APC/C^CDC20) plays a critical role in governing mitotic progression by targeting key cell cycle regulators for degradation. Cell division cycle protein 20 homolog (CDC20), the co‐activator of APC/C, is required for full ubiquitin ligase activity. In addition to its well‐known cell cycle‐related functions, we demonstrate that CDC20 plays an essential role in osteogenic commitment of bone marrow mesenchymal stromal/stem cells (BMSCs). Cdc20 conditional knockout mice exhibit decreased bone formation and impaired bone regeneration after injury. Mechanistically, we discovered a functional interaction between the WD40 domain of CDC20 and the DNA‐binding domain of p65. Moreover, CDC20 promotes the ubiquitination and degradation of p65 in an APC11‐dependent manner. More importantly, knockdown of p65 rescues the bone loss in Cdc20 conditional knockout mice. Our current work reveals a cell cycle‐independent function of CDC20, establishes APC11^CDC20 as a pivotal regulator for bone formation by governing the ubiquitination and degradation of p65, and may pave the way for treatment of bone‐related diseases.