Cdc42, a member of the Rho subfamily of small GTPases, is known to be a regulator of multiple cellular functions, including cytoskeletal organization, cell migration, proliferation, and apoptosis. However, its tissue-specific roles, especially in mammalian limb development, remain unclear. To investigate the physiological function of Cdc42 during limb development, we generated limb bud mesenchyme-specific inactivated Cdc42 (Cdc42(fl/fl); Prx1-Cre) mice. Cdc42(fl/fl); Prx1-Cre mice demonstrated short limbs and body, abnormal calcification of the cranium, cleft palate, disruption of the xiphoid process, and syndactyly. Severe defects were also found in long bone growth plate cartilage, characterized by loss of columnar organization of chondrocytes, and thickening and massive accumulation of hypertrophic chondrocytes, resulting in delayed endochondral bone formation associated with reduced bone growth. In situ hybridization analysis revealed that expressions of Col10 and Mmp13 were reduced in non-resorbed hypertrophic cartilage, indicating that deletion of Cdc42 inhibited their terminal differentiation. Syndactyly in Cdc42(fl/fl); Prx1-Cre mice was caused by fusion of metacarpals and a failure of interdigital programmed cell death (ID-PCD). Whole mount in situ hybridization analysis of limb buds showed that the expression patterns of Sox9 were ectopic, while those of Bmp2, Msx1, and Msx2, known to promote apoptosis in the interdigital mesenchyme, were down-regulated. These results demonstrate that Cdc42 is essential for chondrogenesis and ID-PCD during limb development.

In the 20th century, dentists largely classified periodontal diseases according to patient age at onset, rate of progression, and pathology. Accordingly, the main concepts of periodontitis were "adult," "early-onset," and "refractory"; however, these names did not correspond to the specific bacterial species identified, in such cases, using bacteriological approaches. Therefore, in 1999, the American Academy of Periodontology reorganized this tripartite classification into 2 broad groups: severe, rapid destruction of periodontal tissue not attributable to systemic diseases; and periodontitis as a manifestation of systemic diseases. 1 The former group is considered to be "aggressive periodontitis" (class III in the American Academy of Periodontology Periodontal Disease Classification System), while the latter can be grouped under periodontitis associated with hematological or genetic disorders (classes IVa and IVb in the American Academy of Periodontology Periodontal Disease Classification System), often presenting with comorbid immune (neutrophil) dysfunction. 1,2 However, researchers could not identify specific bacteria or pathologies unique to any of these classes. Subsequently, the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions led to a series of changes in periodontal disease taxonomy, transforming classification into a stage-and grade-based system. 3 | Periodontitis as a multifactorial diseaseClinical manifestations of incipient periodontitis vary considerably across individuals, depending on where the tissue damage originates and how it progresses; thus, it is impossible to explain the symptomatology in terms of bacterial infection alone. We now know that destruction of the periodontal tissue does not progress continuously and linearly, but is rather a result of repeated switches between dormant and active stages of resident bacteria. 4,5 Simplistically describing periodontitis as an infection with certain bacteria cannot completely explain the diverse patterns observed. The fact that periodontitis is a multifactorial disease lies at the heart of this complexity. The tissue damage is thought to be driven by a variety of factors, 6,7 which have started to be illuminated in recent years, and these risk factors have now been conceptually organized into 3 main groups: bacterial, host, and environmental. A host factor can include an underlying metabolic disease that affects the whole body, while smoking is an archetypal environmental factor. Changes in the biological properties of host cells can have an enormous influence on defense mechanisms in the periodontal tissue. In vitro analyses can be used to chronicle the detailed changes in immunocytes and resident cell populations; however, physiological samples are more useful for investigating systemic shifts in host defense mechanisms. Data from analyses of the gingival crevicular fluid have provided intriguing insights into how the protective state evolves and changes as humans grow and age. For example, by comp...

Cdc42 is a widely expressed protein that belongs to the family of Rho GTPases and controls a broad variety of signal transduction pathways in a variety of cell types. To investigate the physiological functions of Cdc42 during cartilage development, we generated chondrocyte-specific inactivated Cdc42 mutant mice (Cdc42(fl/fl); Col2-Cre). The gross morphology of mutant neonates showed shorter limbs and body as compared with the control mice (Cdc42(fl/fl)). Skeletal preparations stained with alcian blue and alizarin red also revealed that the body and the long bone length of the mutants were shorter than those of the control mice. Furthermore, severe defects were found in growth plate chondrocytes in the femur sections of mutant mice, characterized by a reduced proliferating zone height, wider hypertrophic zone, and loss of columnar organization in proliferating chondrocytes. The expression levels of chondrocyte marker genes, such as Col2, Col10, and Mmp13, in mutant mice were decreased as compared with the control mice. Mineralization of trabecular bones in the femur sections was also decreased in the mutants as compared with control mice, whereas osteoid volume was increased. Together these results suggested that chondrocyte proliferation and differentiation in growth plates in the present mutant mice were not normally organized, which contributed to abnormal bone formation. We concluded that Cdc42 is essential for cartilage development during endochondral bone formation.

Bone morphogenetic proteins (BMPs) control the expressions of many genes involved in bone formation. On the basis of our hypothesis that BMP2 stimulation-regulated gene expression plays a critical role in osteoblast differentiation, we performed genome-wide screening of messenger RNA from BMP2-treated and -untreated C2C12 cells using a DNA microarray technique. We found that the expressions of Gremlin1 and Gremlin2, which are known BMP antagonists, were bidirectionally regulated by BMP2. Gremlin1 was down-regulated by BMP2, while Gremlin2 was up-regulated in both time- and dose-dependent manners. Ablation of Gremlin1 or Gremlin2 enhanced osteoblast differentiation induced by BMP2. On the other hand, treatment with recombinant Gremlin1 inhibited BMP2-induced osteoblast differentiation. Furthermore, treatment with Smad4 siRNA and the p38 MAPK inhibitor SB203580 suppressed BMP2-induced Gremlin2 gene expression. The differential regulation of Gremlin1 and Gremlin2 gene expressions by BMP2 may explain the critical function of these genes during osteoblast differentiation.

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