A mammalian taste bud is a collection of approximately 50-100 cells present in the lingual papillae, i.e., fungiform, foliate, and circumvallate papillae, and palatal epithelium. The most important function of taste buds is the detection of taste. Many animals can detect aqueous chemical stimuli immediately after birth, indicating that some taste buds are morphologically and functionally mature at birth. Developmental studies have revealed that the morphological maturation of taste buds in lingual papillae occurs postnatally, while those in the soft palate mature prenatally. Interestingly, some isolated cells in the gustatory epithelium display immunoreactivity for α-gustducin, a taste-specific G protein, during the development of taste buds, indicating the existence of another gustatory system during the neonatal stage in mammals.

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Deficiency of osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of NF-κB ligand (RANKL), in mice induces osteoporosis caused by enhanced bone resorption but also accelerates bone formation. We examined if bone formation is coupled with bone resorption in OPG-deficient (OPG^-/-) mice, using risedronate, an inhibitor of bone resorption. Histomorphometric analysis showed that bone formation-related parameters in OPG^-/-mice sharply decreased with the suppression of bone resorption by the daily injection of risedronate for 30 days. OPG^-/-mice exhibited high serum alkaline phosphatase activity and osteocalcin concentrations, both of which were decreased to the levels of wild-type mice by risedronate injection. The ectopic bone formation induced by bone morphogenetic protein-2 implantation into OPG^-/-mice was not accelerated. These results suggest that bone formation is coupled with bone resorption at local sites in OPG^-/-mice. Myeloid differentiation factor 88 (MyD88) plays essential roles in the signaling of the Toll/interleukin-1 (IL-1) receptor family. Toll-IL-1 receptor domain-containing adapter inducing interferon-β (TRIF)-mediated signals are involved in lipopolysaccharide (LPS)-induced MyD88-independent pathways. Using MyD88-deficient (MyD88^-/-) mice and TRIF-deficient (TRIF^-/-) mice, we examined the roles of MyD88 and TRIF in osteoclast differentiation and function. LPS and IL-1α stimulated osteoclastogenesis in co-cultures of osteoblasts and hemopoietic cells obtained from TRIF^-/-mice but not MyD88^-/-mice. Bone histomorphometry showed that MyD88^-/-mice exhibited osteopenia with reduced bone resorption and formation. These results suggest that the MyD88-mediated signal is essential for the osteoclastogenesis and function induced by IL-1 and LPS, and that MyD88 is physiologically involved in bone turnover.

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The synovial membrane in the temporomandibular joint (TMJ) consists of a lining and connective tissue sublining layers. Its synovial lining layer contains two types of synovial lining cells-macrophage-like type A and fibroblast-like type B cells. Ultrastructurally, the type A cell is characterized by numerous vacuoles, lysosomes, nuclei with rich heterochromatin, and cell surface filopodia while the type B cell possesses a well-developed rough endoplasmic reticulum, a nucleus with less chromatin, cytoplasmic projections, and cell surface caveolae. However, they are hardly distinguishable under the light microscope without specific cell markers. Immunocytochemistry and in situ hybridization histochemistry have revealed that fibroblast-like type B cells in the murine TMJ show an intense reaction for 25 kDa-heat shock protein (Hsp25), indicating that this protein is a useful cell marker for the type B cells. On the other hand, the macrophage-like type A cells react with ED1, which recognizes the macrophage/monocyte lineage because the type A cells are derived from monocyte. Double immunostaining for Hsp25 and ED1 demonstrated regional differences in the population of the lining cells among the synovial membrane of rat TMJ-the type A cell is predominant in the anterior portion while the type B cell is abundant in the posterior synovial folds. Hsp25 protein is the major substrate of mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK-2), which mediates a stress-activated signaling pathway. The activation of Hsp25 by MAPKAPK-2 has been considered to regulate cytoskeletal actin organization, resulting in cellular morphological changes. MAPKAPK-2 and Hsp25 are co-localized in the cytoplasm of the fibroblast-like type B cells in the rat TMJ, implying that these proteins possibly participate in the induction of morphological changes of the type B cells.

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Osteoporosis is a bone disease in which a loss of bone volume and bone strength leads to fragility factures, which is also suggested to be one of the risk factors of periodontitis. This disease is caused by both an increase in bone resorption and relative negative balance between such bone resorption and accelerated bone formation. Tob (transducer of erbB2) is a member of the antiproliferative family of proteins and acts as a negative regulator of bone morphogenic protein (BMP) signaling. Recently, we found that bone volume in ovariectomised Tob-deficient mice was comparable to sham-operated wild-type mice, and that Tob could be a target in the development of new therapeutic measures for osteoporosis. This review addresses (a) the concept and therapy of osteoporosis, (b) the prospect of Tob, and (c) the mechanism whereby Tob-deficiency protects against ovariectomy-induced bone loss.

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Cathepsin E is an intracellular aspartic proteinase that is predominantly localized in the endosomal compartments of antigen presenting cells including macrophages. Here, we have investigated the expression of cathepsin E in mouse macrophages treated with interferon (IFN)-γ, lipopolysaccharide (LPS), interleukin (IL)-10, and IL-4. The mRNA levels of cathepsin E in macrophages stimulated with IFN-γ and LPS were increased, but conversely, that with IL-10 was decreased. However, upon stimulation with IFN-γ or LPS, the activity levels of cathepsin E in the activated cells were markedly decreased, but those in the culture media were increased. Immunoblot analysis revealed that procathepsin E in the cells was largely converted to the mature form in the cells upon stimulation with IFN-γ. In addition, the extracellular enzyme was reacted with antibodies to mature cathepsin E but not with antibodies for procathepsin E. By contrast, when macrophages were treated with IL-4, cathepsin E production was significantly decreased in both the cell and the medium. These results indicate that, upon stimulation with IFN-γ and LPS, cathepsin E is up-regulated in macrophages, which is accompanied by the enhanced maturation and secretion of the enzyme. Conversely, cathepsin E is down-regulated by treatment with IL-4 and IL-10.

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