Osteoblasts express claudins and tight junction-associated proteins

Cingulin family: Structure, function and clinical significance

Cingulin and its paralog paracingulin are vital components of the apical junctional complex in vertebrate epithelial and endothelial cells. They are both found in tight junctions (TJ), and paracingulin is also detectable in adherens junctions (AJ) as TJ cytoplasmic plaque proteins. Cingulin and paracingulin interact with other proteins to perform functions. They interact with cytoskeletal proteins, modulate the activity of small GTPases, such as RhoA and Rac1, and regulate gene expression. In addition, cingulin and paracingulin regulate barrier function and many pathological processes, including inflammation and tumorigenesis. In this review, we summarize the discovery and structure, expression and subcellular distribution, and molecular interactions of cingulin family proteins and discuss their role in development, physiology, and pathological processes.

Targeted Overexpression of Claudin 11 in Osteoblasts Increases Trabecular Bone Mass by Stimulating Osteogenesis at the Expense of Adipogenesis in Mice

Mice lacking Claudin11 (Cldn11) manifest reduced trabecular bone mass. However, the impact of Cldn11 expression in osteoblasts in vivo remains understudied. Herein, we generated osteoblast-specific transgenic (Tg) mice expressing Cldn11 and characterized their skeletal phenotype. Micro-CT analyses of the distal metaphysis of the femur showed a 50% and a 38% increase in trabecular bone mass in Tg male and female mice, respectively, due to a significant increase in trabecular number and a reduction in trabecular separation. Histomorphometry and serum biomarker studies uncovered that increased trabecular bone mass in Cldn11 Tg mice was the consequence of enhanced bone formation. Accordingly, an abundance of bone formation (Alp, Bsp), but not bone resorption (Ctsk), markers were augmented in the femurs of Cldn11 Tg mice. Since the trabecular bone density is known to inversely correlate with the amount of marrow adipose tissue (MAT), we measured the MAT in osmium-tetroxide-labeled bones by micro-CT scanning. We found 86% less MAT in the proximal tibia of the Tg males. Consistently, the expression levels of the adipogenic markers, adiponectin and leptin, were 50% lower in the femurs of the Tg males. Our data are consistent with the possibility that claudin11 exerts anabolic effects in osteoblastic lineage cells that act via promoting the differentiation of marrow stem cells towards osteoblasts at the expense of adipocytes.

Near-ultraviolet irradiation to stimulate unmodified polyether ether ketone to achieve stable and sustainable antibacterial activity

OBJECTIVES

This study aims to investigate the cytotoxicity and sustainable antibacterial activity of unmodified PEEK under specific wavelength light treatment (365 nm), and its antibacterial mechanism was also preliminarily discussed.

METHODS

A near-ultraviolet source with a wavelength of 365 nm and a power of 5 W were selected. The irradiation time was 30 min, and the distance was 100 mm. A water contact angle tester was used to characterize the surface of the PEEK after 1-15 light treatments. MC3TC-E1 cells were used to evaluate the cytotoxicity of the materials under light treatment. Five kinds of common oral bacteria were detected in vitro, and antibacterial efficiency was determined by colony-forming unit (CFU) and scanning electron microscope (SEM). The antibacterial mechanism of PEEK under light was preliminarily discussed by spectrophotometry. The membrane rupture of Staphylococcus aureus and Escherichia coli was detected by lactate dehydrogenase. Staphylococcus aureus and Staphylococcus mutans were selected for the cyclic antibacterial test. Statistical analysis was performed by one-way analysis of variance and Tukey multiple range test. A significance level of 0.05 was considered (α = 0.05).

RESULTS

The results of the cell experiment showed that PEEK had no cytotoxicity (P > 0.05). CFU results showed that PEEK had an obvious antibacterial effect on Staphylococcus aureus, Staphylococcus mutans, Staphylococcus gordonii and Staphylococcus sanguis, but had no antibacterial effect on Escherichia coli (P < 0.05). The SEM results also verified the above antibacterial effect. The existence of singlet oxygen was confirmed by spectrophotometry. Meanwhile, the rupture of Staphylococcus aureus membrane was verified by lactate dehydrogenase assay. The water contact angle of the PEEK surface did not change significantly after 15 cycles of light treatment. Cyclic antibacterial experiments showed that the antibacterial effect was sustainable.

CONCLUSIONS

This study showed that PEEK has good cytocompatibility with stable and sustainable antibacterial properties under near-ultraviolet. It provides a new idea to solve the non-antibacterial property of PEEK, and also provides a theoretical basis for its further application in dentistry.

Comparison of Myosepta Development and Transcriptome Profiling between Blunt Snout Bream with and Tilapia without Intermuscular Bones

Simple Summary

The presence or absence of intermuscular bones (IBs) is directly related to the economic and edible value of fish. The specific regulatory mechanism of IB formation is not completely known yet. Here, we explored the molecular mechanisms that regulate the formation of IBs based on histological analysis, transcriptome profiling, and gene expression quantification using M. amblycephala (with IBs) and O. niloticus (without IBs) as models. As a result, we identified several bone-related genes and elucidated their regulatory roles in the development of IBs.

Abstract

Intermuscular bones (IBs) are small spicule-like bones located in the myosepta of basal teleosts, which negatively affect the edibleness and economic value of fish. Blunt snout bream (Megalobrama amblycephala, with epineural and epipleural IBs) and tilapia (Oreochromis niloticus, without epineural and epipleural IBs) are two major aquaculture species and ideal models for studying the formation mechanisms of fish IBs. Here, we compared myosepta development between M. amblycephala and O. niloticus, based on histological analysis, transcriptome profiling, and expression analysis of bone-related genes. The histological results showed that dye condensation began to appear in the myosepta 20 days post hatching (dph) in M. amblycephala, and IBs could be clearly observed 50 dph in the myosepta, based on different staining methods. However, in O. niloticus, dye condensation was not observed in the myosepta from 10 to 60 dph. Differentially expressed genes (DEGs) at different developmental stages were screened by comparing the transcriptomes of M. amblycephala and O. niloticus, and KEGG analysis demonstrated that these DEGs were enriched in many bone-related pathways, such as focal adhesion, calcium, and Wnt signaling pathways. Quantitative PCR was performed to further compare the expression levels of some bone-related genes (scxa, scxb, runx2a, runx2b, bgp, sp7, col1a2, entpd5a, entpd5b, phex, alpl, and fgf23). All the tested genes (except for alpl) exhibited higher expression levels in M. amblycephala than in O. niloticus. A comparison of the dorsal and abdominal muscle tissues between the two species also revealed significant expression differences for most of the tested genes. The results suggest that scxa, scxb, runx2a, runx2b, entpd5a, col1a2, and bgp may play important roles in IB development. Our findings provide some insights into the molecular mechanisms of IB formation, as well as clues for further functional analysis of the identified genes to better understand the development of IBs.

Claudin-2 and claudin-12 form independent, complementary pores required to maintain calcium homeostasis

Calcium (Ca²⁺) homeostasis is maintained through coordination between intestinal absorption, renal reabsorption, and bone remodeling. Intestinal and renal (re)absorption occurs via transcellular and paracellular pathways. The latter contributes the bulk of (re)absorption under conditions of adequate intake. Epithelial paracellular permeability is conferred by tight-junction proteins called claudins. However, the molecular identity of the paracellular Ca²⁺ pore remains to be delineated. Claudins (Cldn)-2 and -12 confer Ca²⁺ permeability, but deletion of either claudin does not result in a negative Ca²⁺ balance or increased calciotropic hormone levels, suggesting the existence of additional transport pathways or parallel roles for the two claudins. To test this, we generated a Cldn2/12 double knockout mouse (DKO). These animals have reduced intestinal Ca²⁺ absorption. Colonic Ca²⁺ permeability is also reduced in DKO mice and significantly lower than single-null animals, while small intestine Ca²⁺ permeability is unaltered. The DKO mice display significantly greater urinary Ca²⁺ wasting than Cldn2 null animals. These perturbations lead to hypocalcemia and reduced bone mineral density, which was not observed in single-KO animals. Both claudins were localized to colonic epithelial crypts and renal proximal tubule cells, but they do not physically interact in vitro. Overexpression of either claudin increased Ca²⁺ permeability in cell models with endogenous expression of the other claudin. We find claudin-2 and claudin-12 form partially redundant, independent Ca²⁺ permeable pores in renal and colonic epithelia that enable paracellular Ca²⁺ (re)absorption in these segments, with either one sufficient to maintain Ca²⁺ balance.

Growth and mineralization of osteoblasts from mesenchymal stem cells on microporous membranes: Epithelial-like growth with transmembrane resistance and pH gradient

Osteoblasts in vivo form an epithelial-like layer with tight junctions between cells. Bone formation involves mineral transport into the matrix and acid transport to balance pH levels. To study the importance of the pH gradient in vitro, we used Transwell inserts composed of polyethylene terephthalate (PET) membranes with 0.4 μm pores at a density of (2 ± 0.4) x 106 pores per cm2. Mesenchymal stem cells (MSCs) prepared from murine bone marrow were used to investigate alternative conditions whereby osteoblast differentiation would better emulate in vivo bone development. MSCs were characterized by flow cytometry with more than 90% CD44 and 75% Sca-1 labeling. Mineralization was validated with paracellular alkaline phosphatase activity, collagen birefringence, and mineral deposition confirming MSCs identity. We demonstrate that MSCs cultured and differentiated on PET inserts form an epithelial-like layer while mineralizing. Measurement of the transepithelial resistance was ∼1400 Ω•cm2 at three weeks of differentiation. The pH value of the media above and under the cells were measured while cells were in proliferation and differentiation. In mineralizing cells, a difference of 0.145 pH unit was observed between the medium above and under the cells indicating a transepithelial gradient. A significant difference in pH units was observed between the medium above and below the cells in proliferation compared to differentiation. Data on pH below membranes were confirmed by pH-dependent SNARF1 fluorescence. Control cells in proliferative medium did not form an epithelial-like layer, displayed low transepithelial resistance, and there was no significant pH gradient. By transmission electron microscopy, membrane attached osteoblasts in vitro had abundant mitochondria consistent with active transport that occurs in vivo by surface osteoblasts. In keeping with osteoblastic differentiation, scanning electron microscopy identified deposition of extracellular collagen surrounded by hydroxyapatite. This in vitro model is a major advancement in modeling bone in vivo for understanding of osteoblast bone matrix production.

Probiotics Stimulate Bone Formation in Obese Mice via Histone Methylations

Rationale: Manipulation of the gut microbiome can prevent pathologic bone loss. However, the effects of probiotics on mitochondrial epigenetic remodeling and skeletal homeostasis in the high-fat diet (HFD)-linked obesity remains to be explored. Here, we examined the impact of probiotics supplementation on mitochondrial biogenesis and bone homeostasis through the histone methylation mechanism in HFD fed obese mice.

Methods: 16S rRNA gene sequencing was performed to study the microbiota composition in the gut and microbial dysbiosis in obese mouse model. High resolution (microPET/CT) imaging was performed to demonstrate the obese associated colonic inflammation. Obese-associated upregulation of target miRNA in osteoblast was investigated using a microRNA qPCR array. Osteoblastic mitochondrial mass was evaluated using confocal imaging. Overexpression of mitochondrial transcription factor (Tfam) was used to investigate the glycolysis and mitochondrial bioenergetic metabolism using Tfam-transgenic (Tg) mice fed on HFD. The bone formation and mechanical strength was evaluated by microCT analysis and three-point bending analysis.

Results: High-resolution imaging (µ-CT) and mechanical testing revealed that probiotics induced a significant increase of trabecular bone volume and bone mechanical strength respectively in obese mice. Probiotics or Indole-3-propionic acid (IPA) treatment directly to obese mice, prevents gut inflammation, and improved osteoblast mineralization. Mechanistically, probiotics treatment increases mitochondrial transcription factor A (Tfam) expression in osteoblasts by promoting Kdm6b/Jmjd3 histone demethylase, which inhibits H3K27me3 epigenetic methylation at the Tfam promoter. Furthermore, Tfam-transgenic (Tg) mice, fed with HFD, did not experience obesity-linked reduction of glucose uptake, mitochondrial biogenesis and mineralization in osteoblasts.

Conclusions: These results suggest that the probiotics mediated changes in the gut microbiome and its derived metabolite, IPA are potentially be a novel agent for regulating bone anabolism via the gut-bone axis.

Identifying the Potential Differentially Expressed miRNAs and mRNAs in Osteonecrosis of the Femoral Head Based on Integrated Analysis

Purpose

Osteonecrosis of the femoral head is a common disease of the hip that leads to severe pain or joint disability. We aimed to identify potential differentially expressed miRNAs and mRNAs in osteonecrosis of the femoral head.

Methods

The data of miRNA and mRNA were firstly downloaded from the database. Secondly, the regulatory network of miRNAs-mRNAs was constructed, followed by function annotation of mRNAs. Thirdly, an in vitro experiment was applied to validate the expression of miRNAs and targeted mRNAs. Finally, GSE123568 dataset was used for electronic validation and diagnostic analysis of targeted mRNAs.

Results

Several regulatory interaction pairs between miRNA and mRNAs were identified, such as hsa-miR-378c-WNT3A/DACT1/CSF1, hsa-let-7a-5p-RCAN2/IL9R, hsa-miR-28-5p-RELA, hsa-miR-3200-5p-RELN, and hsa-miR-532-5p-CLDN18/CLDN10. Interestingly, CLDN10, CLDN18, CSF1, DACT1, IL9R, RCAN2, RELN, and WNT3A had the diagnostic value for osteonecrosis of the femoral head. Wnt signaling pathway (involved WNT3A), chemokine signaling pathway (involved RELA), focal adhesion and ECM-receptor interaction (involved RELN), cell adhesion molecules (CAMs) (involved CLDN18 and CLDN10), cytokine-cytokine receptor interaction, and hematopoietic cell lineage (involved CSF1 and IL9R) were identified.

Conclusion

The identified differentially expressed miRNAs and mRNAs may be involved in the pathology of osteonecrosis of the femoral head.

Phylogeny and chemistry of biological mineral transport

Three physiologically mineralizing tissues - teeth, cartilage and bone - have critical common elements and important evolutionary relationships. Phylogenetically the most ancient densely mineralized tissue is teeth. In jawless fishes without skeletons, tooth formation included epithelial transport of phosphates, a process echoed later in bone physiology. Cartilage and mineralized cartilage are skeletal elements separate from bone, but with metabolic features common to bone. Cartilage mineralization is coordinated with high expression of tissue nonspecific alkaline phosphatase and PHOSPHO1 to harvest available phosphate esters and support mineralization of collagen secreted locally. Mineralization in true bone results from stochastic nucleation of hydroxyapatite crystals within the cross-linked collagen fibrils. Mineral accumulation in dense collagen is, at least in major part, mediated by amorphous aggregates - often called Posner clusters - of calcium and phosphate that are small enough to diffuse into collagen fibrils. Mineral accumulation in membrane vesicles is widely suggested, but does not correlate with a definitive stage of mineralization. Conversely mineral deposition at non-physiologic sites where calcium and phosphate are adequate has been shown to be regulated in large part by pyrophosphate. All of these elements are present in vertebrate bone metabolism. A key biological element of bone formation is an epithelial-like cellular organization which allows control of phosphate, calcium and pH during mineralization.

Responses of primary osteoblasts and osteoclasts from hemizygous β-globin knockout thalassemic mice with elevated plasma glucose to 1,25-dihydroxyvitamin D3

β-thalassemia is often associated with hyperglycemia, osteoporosis and increased fracture risk. However, the underlying mechanisms of the thalassemia-associated bone loss remain unclear. It might result from abnormal activities of osteoblasts and osteoclasts, and perhaps prolonged exposure to high extracellular glucose. Herein, we determined the rate of duodenal calcium transport in hemizygous β-globin knockout thalassemic (BKO) mice. Their bones were collected for primary osteoblast and osteoclast culture. We found that BKO mice had lower calcium absorption than their wild-type (WT) littermates. Osteoblasts from BKO mice showed aberrant expression of osteoblast-specific genes, e.g., Runx2, alkaline phosphatase and osteocalcin, which could be partially restored by 1,25(OH)₂D₃ treatment. However, the mRNA expression levels of RANK, calcitonin receptor (Calcr), c-Fos, NFATc1, cathepsin K and DMT1 were similar in both BKO and WT groups. Exposure to high extracellular glucose modestly but significantly affected the expression of osteoclast-specific markers in WT osteoclasts with no significant effect on osteoblast-specific genes in WT osteoblasts. Thus, high glucose alone was unable to convert WT bone cells to BKO-like bone cells. In conclusion, the impaired calcium absorption and mutation-related aberrant bone cell function rather than exposure to high blood glucose were likely to be the principal causes of thalassemic bone loss.

Cellular and extracellular matrix of bone, with principles of synthesis and dependency of mineral deposition on cell membrane transport

Bone differs from other connective tissues; it is isolated by a layer of osteoblasts that are connected by tight and gap junctions. This allows bone to create dense lamellar type I collagen, control pH, mineral deposition, and regulate water content forming a compact and strong structure. New woven bone formed after degradation of mineralized cartilage is rapidly degraded and resynthesized to impart structural order for local bone strength. Ossification is regulated by thickness of bone units and by patterning via bone morphogenetic receptors including activin, other bone morphogenetic protein receptors, transforming growth factor-β receptors, all part of a receptor superfamily. This superfamily interacts with receptors for additional signals in bone differentiation. Important features of the osteoblast environment were established using recent tools including osteoblast differentiation in vitro. Osteoblasts deposit matrix protein, over 90% type I collagen, in lamellae with orientation alternating parallel or orthogonal to the main stress axis of the bone. Into this organic matrix, mineral is deposited as hydroxyapatite. Mineral matrix matures from amorphous to crystalline hydroxyapatite. This process includes at least two-phase changes of the calcium-phosphate mineral as well as intermediates involving tropocollagen fibrils to form the bone composite. Beginning with initiation of mineral deposition, there is uncertainty regarding cardinal processes, but the driving force is not merely exceeding the calcium-phosphate solubility product. It occurs behind a epithelial-like layer of osteoblasts, which generate phosphate and remove protons liberated during calcium-phosphate salt deposition. The forming bone matrix is discontinuous from the general extracellular fluid. Required adjustment of ionic concentrations and water removal from bone matrix are important details remaining to be addressed.

MMP14 in Sarcoma: A Regulator of Tumor Microenvironment Communication in Connective Tissues

Sarcomas are deadly malignant tumors of mesenchymal origin occurring at all ages. The expression and function of the membrane-type matrix metalloproteinase MMP14 is closely related to the mesenchymal cell phenotype, and it is highly expressed in most sarcomas. MMP14 regulates the activity of multiple extracellular and plasma membrane proteins, influencing cell–cell and cell–extracellular matrix (ECM) communication. This regulation mediates processes such as ECM degradation and remodeling, cell invasion, and cancer metastasis. Thus, a comprehensive understanding of the biology of MMP14 in sarcomas will shed light on the mechanisms controlling the key processes in these diseases. Here, we provide an overview of the function and regulation of MMP14 and we discuss their relationship with clinical and pre-clinical MMP14 data in both adult and childhood sarcomas.

Calcium fluxes at the bone/plasma interface: Acute effects of parathyroid hormone (PTH) and targeted deletion of PTH/PTH-related peptide (PTHrP) receptor in the osteocytes

Calcium ion concentration ([Ca2+]) in the systemic extracellular fluid, ECF-[Ca2+], is maintained around a genetically predetermined set-point, which combines the operational level of the kidney and bone/ECF interfaces. The ECF-[Ca2+] is maintained within a narrow oscillation range by the regulatory action of Parathyroid Hormone (PTH), Calcitonin, FGF-23, and 1,25(OH)2D3. This model implies two correction mechanisms, i.e. tubular Ca2+ reabsorption and osteoclast Ca2+ resorption. Although their alterations have an effect on the ECF-[Ca2+] maintenance, they cannot fully account for rapid correction of the continuing perturbations of plasma [Ca2+], which occur daily in life. The existence of Ca2+ fluxes at quiescent bone surfaces fulfills the role of a short-term error correction mechanism in Ca2+ homeostasis. To explore the hypothesis that PTH regulates the cell system responsible for the fast Ca2+ fluxes at the bone/ECF interface, we have performed direct real-time measurements of Ca2+ fluxes at the surface of ex-vivo metatarsal bones maintained in physiological conditions mimicking ECF, and exposed to PTH. To further characterize whether the PTH receptor on osteocytes is a critical component of the minute-to-minute ECF-[Ca2+] regulation, metatarsal bones from mice lacking the PTH receptor in these cells were tested ex vivo for rapid Ca2+ exchange. We performed direct real-time measurements of Ca2+ fluxes and concentration gradients by a scanning ion-selective electrode technique (SIET). To validate ex vivo measurements, we also evaluated acute calcemic response to PTH in vivo in mice lacking PTH receptors in osteocytes vs littermate controls. Our data demonstrated that Ca2+ fluxes at the bone-ECF interface in excised bones as well as acute calcemic response in the short-term were unaffected by PTH exposure and its signaling through its receptor in osteocytes. Rapid minute-to-minute regulation of the ECF-[Ca2+] was found to be independent of PTH actions on osteocytes. Similarly, mice lacking PTH receptor in osteocytes, responded to PTH challenge with similar calcemic increases.

Dlx2 overexpression enhanced accumulation of type II collagen and aggrecan by inhibiting MMP13 expression in mice chondrocytes

Genetic studies revealed a crucial role of Distal-homebox (Dlx) genes in skeletal development, and our previous study demonstrated overexpressing Dlx2 in neural crest cells led to abnormal cartilage structure, including ectopic cartilage in the maxillary region and nasal bone in mice. The aim of this study was to investigate how Dlx2 overexpression affects chondrogenesis in mouse chondroblast cell line TMC23 and the underlying mechanism. We first demonstrated that Dlx2 expression was upregulated during chondrogenesis in TMC23 cells. Moreover, forced overexpression of Dlx2 in TMC23 cells led to increased accumulation of aggrecan and type II collagen, markers of early chondrocyte differentiation, but had little effect on mRNA and protein levels of Aggrecan and Col2α1, type II collagen gene. Importantly, Dlx2 overexpression decreased mRNA and protein levels of MMP13, a major collagenase degrading aggrecan and type II collagen during late stages of chondrogenesis. Luciferase-reporter and Chromatin-immunoprecipitation analysis demonstrated that MMP13 promoter contained two Dlx2-response elements, and Dlx2 inhibited MMP13 expression by directly binding to these two elements. Based on these observations, we propose that forced overexpression of Dlx2 enhances early chondrocyte differentiation by increasing accumulation of type II collagen and aggrecan, but interferes later stages of chondrocyte differentiation through inhibiting MMP13 expression.

Claudin expression during early postnatal development of the murine cochlea

BACKGROUND

Claudins are major components of tight junctions, which form the paracellular barrier between the cochlear luminal and abluminal fluid compartments that supports the large transepithelial voltage difference and the large concentration differences of K⁺, Na⁺ and Ca²⁺ needed for normal cochlear function. Claudins are a family of more than 20 subtypes, but our knowledge about expression and localization of each subtype in the cochlea is limited.

RESULTS

We examined by quantitative RT-PCR the expression of the mRNA of 24 claudin isoforms in mouse cochlea during postnatal development and localized the expression in separated fractions of the cochlea. Transcripts of 21 claudin isoforms were detected at all ages, while 3 isoforms (Cldn-16, - 17 and - 18) were not detected. Claudins that increased expression during development include Cldn-9, - 13, - 14, - 15, and -19v2, while Cldn-6 decreased. Those that do not change expression level during postnatal development include Cldn-1, - 2, - 3, - 4, - 5, - 7, - 8, -10v1, -10v2, - 11, - 12, -19v1, - 20, - 22, and - 23. Our investigation revealed unique localization of some claudins. In particular, Cldn-13 expression rapidly increases during early development and is mainly expressed in bone but only minimally in the lateral wall (including stria vascularis) and in the medial region (including the organ of Corti). No statistically significant changes in expression of Cldn-11, - 13, or - 14 were found in the cochlea of Slc26a4 ^-/- mice compared to Slc26a4 ^+/- mice.

CONCLUSIONS

We demonstrated developmental patterns of claudin isoform transcript expression in the murine cochlea. Most of the claudins were associated with stria vascularis and organ of Corti, tissue fractions rich in tight junctions. However, this study suggests a novel function of Cldn-13 in the cochlea, which may be linked to cochlear bone marrow maturation.

Treatment-related survival associations of claudin-2 expression in fibroblasts of colorectal cancer

Claudin-2 is a trans-membrane protein—component of tight junctions in epithelial cells. Elevated claudin-2 expression has been reported in colorectal cancer (CRC). The aim of this study was to investigate the expression patterns of claudin-2 in human CRC samples and analyze its association with clinical characteristics and treatment outcome. TMAs of primary tumors from two cohorts of metastatic CRC (mCRC) were used. Claudin-2 IHC staining was evaluated in a semi-quantitative manner in different regions and cell types. Claudin-2 expression was also analyzed by immunofluorescence in primary cultures of human CRC cancer-associated fibroblasts (CAFs). Initial analyses identified previously unrecognized expression patterns of claudin-2 in CAFs of human CRC. Claudin-2 expression in CAFs of the invasive margin was associated with shorter progression-free survival. Subgroup analyses demonstrated that the survival associations occurred among cases that received 5-FU+oxaliplatin combination treatment, but not in patients receiving 5-FU±irinotecan. The finding was validated by analyses of the independent cohort. In summary, previously unreported stromal expression of claudin-2 in CAFs of human CRC was detected together with significant association between high claudin-2 expression in CAFs and shorter survival in 5-FU+oxaliplatin-treated mCRC patients.

The critical role of the ZNF217 oncogene in promoting breast cancer metastasis to the bone

Bone metastasis affects >70% of patients with advanced breast cancer. However, the molecular mechanisms underlying this process remain unclear. On the basis of analysis of clinical datasets, and in vitro and in vivo experiments, we report that the ZNF217 oncogene is a crucial mediator and indicator of bone metastasis. Patients with high ZNF217 mRNA expression levels in primary breast tumours had a higher risk of developing bone metastases. MDA-MB-231 breast cancer cells stably transfected with ZNF217 (MDA-MB-231-ZNF217) showed the dysregulated expression of a set of genes with bone-homing and metastasis characteristics, which overlapped with two previously described 'osteolytic bone metastasis' gene signatures, while also highlighting the bone morphogenetic protein (BMP) pathway. The latter was activated in MDA-MB-231-ZNF217 cells, and its silencing by inhibitors (Noggin and LDN-193189) was sufficient to rescue ZNF217-dependent cell migration, invasion or chemotaxis towards the bone environment. Finally, by using non-invasive multimodal in vivo imaging, we found that ZNF217 increases the metastatic growth rate in the bone and accelerates the development of severe osteolytic lesions. Altogether, the findings of this study highlight ZNF217 as an indicator of the emergence of breast cancer bone metastasis; future therapies targeting ZNF217 and/or the BMP signalling pathway may be beneficial by preventing the development of bone metastases. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Osteoblast Differentiation and Bone Matrix Formation In Vivo and In Vitro

We review the characteristics of osteoblast differentiation and bone matrix synthesis. Bone in air breathing vertebrates is a specialized tissue that developmentally replaces simpler solid tissues, usually cartilage. Bone is a living organ bounded by a layer of osteoblasts that, because of transport and compartmentalization requirements, produce bone matrix exclusively as an organized tight epithelium. With matrix growth, osteoblasts are reorganized and incorporated into the matrix as living cells, osteocytes, which communicate with each other and surface epithelium by cell processes within canaliculi in the matrix. The osteoblasts secrete the organic matrix, which are dense collagen layers that alternate parallel and orthogonal to the axis of stress loading. Into this matrix is deposited extremely dense hydroxyapatite-based mineral driven by both active and passive transport and pH control. As the matrix matures, hydroxyapatite microcrystals are organized into a sophisticated composite in the collagen layer by nucleation in the protein lattice. Recent studies on differentiating osteoblast precursors revealed a sophisticated proton export network driving mineralization, a gene expression program organized with the compartmentalization of the osteoblast epithelium that produces the mature bone matrix composite, despite varying serum calcium and phosphate. Key issues not well defined include how new osteoblasts are incorporated in the epithelial layer, replacing those incorporated in the accumulating matrix. Development of bone in vitro is the subject of numerous projects using various matrices and mesenchymal stem cell-derived preparations in bioreactors. These preparations reflect the structure of bone to variable extents, and include cells at many different stages of differentiation. Major challenges are production of bone matrix approaching the in vivo density and support for trabecular bone formation. In vitro differentiation is limited by the organization and density of osteoblasts and by endogenous and exogenous inhibitors.

An Unbalanced Rearrangement of Chromosomes 4:20 is Associated with Childhood Osteoporosis and Reduced Caspase-3 Levels

The purpose of this study was to investigate the association of a chromosome 4:20 imbalance with osteoporosis in three related children. Bone biochemistry, bone turnover markers, and dual-energy X-ray absorptiometry (DXA) scanning were performed in all three cases and bone biopsy and histomorphometry in one. The chromosome imbalance was delineated by array comparative genomic hybridization (aCGH) and analyzed for candidate genes. A potential candidate gene within the deleted region is caspase-3, previously linked to low bone mineral density (BMD) in heterozygous mice thus caspase-3 activity was measured in cases and controls. Routine bone biochemistry and markers of bone turnover did not reveal any abnormality. DXA showed reduced total and lumbar spine bone mineral content. aCGH showed an 8 megabase (Mb) deletion of terminal chromosome 4q incorporating a region previously linked to low BMD and a 15 Mb duplication of terminal chromosome 20p. Bone biopsy showed a high bone turnover state, trabecularisation of cortical bone and numerous small osteoclasts coupled with normal bone formation. Basal serum caspase-3 activity was lower in cases compared with controls. We conclude that the early-onset osteoporosis with low basal levels of caspase-3 and abnormal osteoclasts is a feature of this chromosomal translocation. Further investigation of the role of the deleted and duplicated genes and especially caspase-3 is required.

A novel mutation in CLDN16 results in rare familial hypomagnesaemia with hypercalciuria and nephrocalcinosis in a Chinese family

BACKGROUND

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessively inherited disease characterized by excessive wasting of renal tubular magnesium and calcium. FHHNC is associated with various mutations in CLDN16 and CLDN19.

CASES

Two children from a consanguineous family of Chinese Han origin demonstrated manifestations of rickets, polyuria, polydipsia, hematuria and failure to thrive. Hypomagnesaemia (0.49-0.50mmol/L), hypercalciuria or a trend to hypercalciuria (24hour urine calcium: 3.8-5.1mg/kg/day), and secondary hyperparathyroidism (serum PTH level: 94.7-200pg/mL) were revealed upon laboratory examination. Using targeted next-generation sequencing and subsequent confirmation by Sanger sequencing, a novel homozygous mutation was identified in the CLDN16 gene of both FHHNC patients. This specific mutation, a 16bp deletion followed by a 23bp insertion in exon 3, led to the generation of a premature termination codon. The parents and an unaffected sister were all heterozygous carriers of this mutation.

CONCLUSIONS

We detected a novel mutation in CLDN16 for the first time. The clinical and genetic findings from this study will help to expand the understanding of this rare disease, FHHNC.

Mapping of Craniofacial Traits in Outbred Mice Identifies Major Developmental Genes Involved in Shape Determination

The vertebrate cranium is a prime example of the high evolvability of complex traits. While evidence of genes and developmental pathways underlying craniofacial shape determination is accumulating, we are still far from understanding how such variation at the genetic level is translated into craniofacial shape variation. Here we used 3D geometric morphometrics to map genes involved in shape determination in a population of outbred mice (Carworth Farms White, or CFW). We defined shape traits via principal component analysis of 3D skull and mandible measurements. We mapped genetic loci associated with shape traits at ~80,000 candidate single nucleotide polymorphisms in ~700 male mice. We found that craniofacial shape and size are highly heritable, polygenic traits. Despite the polygenic nature of the traits, we identified 17 loci that explain variation in skull shape, and 8 loci associated with variation in mandible shape. Together, the associated variants account for 11.4% of skull and 4.4% of mandible shape variation, however, the total additive genetic variance associated with phenotypic variation was estimated in ~45%. Candidate genes within the associated loci have known roles in craniofacial development; this includes 6 transcription factors and several regulators of bone developmental pathways. One gene, Mn1, has an unusually large effect on shape variation in our study. A knockout of this gene was previously shown to affect negatively the development of membranous bones of the cranial skeleton, and evolutionary analysis shows that the gene has arisen at the base of the bony vertebrates (Eutelostomi), where the ossified head first appeared. Therefore, Mn1 emerges as a key gene for both skull formation and within-population shape variation. Our study shows that it is possible to identify important developmental genes through genome-wide mapping of high-dimensional shape features in an outbred population.

Formation of the face, mandible, and skull is determined in part by genetic factors, but the relationship between genetic variation and craniofacial development is not well understood. We demonstrate how recent advances in mouse genomics and statistical methods can be used to identify genes involved in craniofacial development. We use outbred mice together with a dense panel of genetic markers to identify genetic loci affecting craniofacial shape. Some of the loci we identify are also known from past studies to contribute to craniofacial development and bone formation. For example, the top candidate gene identified in this study, Mn1, is a gene that appeared at a time when animals started to form bony skulls, suggesting that it may be a key gene in this evolutionary innovation. This further suggests that Mn1 and other genes involved in head formation are also responsible for more fine-grained regulation of its shape. Our results confirm that the outbred mouse population used in this study is suitable to identify single genetic factors even under conditions where many genes cooperate to generate a complex phenotype.

Precursor cells from Atlantic salmon (Salmo salar) visceral fat holds the plasticity to differentiate into the osteogenic lineage

In order to study the potential plasticity of Atlantic salmon (Salmo salar) precursor cells (aSPCs) from the adipogenic mesenchyme cell lineage to differentiate to the osteogenic lineage, aSPCs were isolated and cultivated under either osteogenic or adipogenic promoting conditions. The results strengthen the hypothesis that aSPCs most likely are predestined to the adipogenic lineage, but they also hold the flexibility to turn into other lineages given the right stimuli. This assumption is supported by the fact that the transcription factor pparγ , important for regulation of adiopogenesis, was silent in aSPCs grown in osteogenic media, while runx2, important for osteogenic differentiation, was not expressed in aSPCs cultivated in adipogenic media. After 2 weeks in osteogenic promoting conditions the cells started to deposit extracellular matrix and after 4 weeks, the cells started mineralizing secreted matrix. Microarray analyses revealed large-scale transcriptome responses to osteogenic medium after 2 days, changes remained stable at day 15 and decreased by magnitude at day 30. Induction was observed in many genes involved in osteogenic differentiation, growth factors, regulators of development, transporters and production of extracellular matrix. Transcriptome profile in differentiating adipocytes was markedly different from differentiating osteoblasts with far fewer genes changing activity. The number of regulated genes slowly increased at the mature stage, when adipocytes increased in size and accumulated lipids. This is the first report on in vitro differentiation of aSPCs from Atlantic salmon to mineralizing osteogenic cells. This cell model system provides a new valuable tool for studying osteoblastogenesis in fish.

Differential expression of claudin family members during osteoblast and osteoclast differentiation: Cldn-1 is a novel positive regulator of osteoblastogenesis

Claudins (Cldns), a family of 27 transmembrane proteins, represent major components of tight junctions. Aside from functioning as tight junctions, Cldns have emerging roles as regulators of cell proliferation and differentiation. While Cldns are known to be expressed and have important functions in various tissues, their expression and function in bone cells is ill-defined. In this study, the expression of Cldns was examined during osteoblast and osteoclast differentiation. The expression of Cldn-1, -7, -11, and -15 was downregulated during early stages of osteoclast differentiation, whereas Cldn-6 was upregulated. Moreover, the expression of several Cldns increased 3–7 fold in fully differentiated osteoclasts. As for osteoblasts, the expression of several Cldns was found to increase more than 10-fold during differentiation, with some peaking at early, and others at late stages. By contrast, only expression of Cldn-12, and -15 decreased during osteoblast differentiation. In subsequent studies, we focused on the role of Cldn-1 in osteoblasts as its expression was increased by more than 10 fold during osteoblast differentiation and was found to be regulated by multiple osteoregulatory agents including IGF-1 and Wnt3a. We evaluated the consequence of lentiviral shRNA-mediated knockdown of Cldn-1 on osteoblast proliferation and differentiation using MC3T3-E1 mouse osteoblasts. Cldn-1 knockdown caused a significant reduction in MC3T3-E1 cell proliferation and ALP activity. Accordingly, expression levels of cyclinD1 and ALP mRNA levels were reduced in Cldn-1 shRNA knockdown cells. We next determined if Cldn-1 regulates the expression of Runx-2 and osterix, master transcription factors of osteoblast differentiation, and found that their levels were reduced significantly as a consequence of Cldn-1 knockdown. Moreover, knocking down Cldn-1 reduced β-catenin level. In conclusion, the expression of Cldn family members during bone cell differentiation is complex and involves cell type and differentiation stage-dependent regulation. In addition, Cldn-1 is a positive regulator of osteoblast proliferation and differentiation.

High-throughput screening of mouse gene knockouts identifies established and novel skeletal phenotypes

Screening gene function in vivo is a powerful approach to discover novel drug targets. We present high-throughput screening (HTS) data for 3 762 distinct global gene knockout (KO) mouse lines with viable adult homozygous mice generated using either gene-trap or homologous recombination technologies. Bone mass was determined from DEXA scans of male and female mice at 14 weeks of age and by microCT analyses of bones from male mice at 16 weeks of age. Wild-type (WT) cagemates/littermates were examined for each gene KO. Lethality was observed in an additional 850 KO lines. Since primary HTS are susceptible to false positive findings, additional cohorts of mice from KO lines with intriguing HTS bone data were examined. Aging, ovariectomy, histomorphometry and bone strength studies were performed and possible non-skeletal phenotypes were explored. Together, these screens identified multiple genes affecting bone mass: 23 previously reported genes (Calcr, Cebpb, Crtap, Dcstamp, Dkk1, Duoxa2, Enpp1, Fgf23, Kiss1/Kiss1r, Kl (Klotho), Lrp5, Mstn, Neo1, Npr2, Ostm1, Postn, Sfrp4, Slc30a5, Slc39a13, Sost, Sumf1, Src, Wnt10b), five novel genes extensively characterized (Cldn18, Fam20c, Lrrk1, Sgpl1, Wnt16), five novel genes with preliminary characterization (Agpat2, Rassf5, Slc10a7, Slc26a7, Slc30a10) and three novel undisclosed genes coding for potential osteoporosis drug targets.

Emerging multifunctional roles of Claudin tight junction proteins in bone

The imbalance between bone formation and resorption during bone remodeling has been documented to be a major factor in the pathogenesis of osteoporosis. Recent evidence suggests a significant role for the tight junction proteins, Claudins (Cldns), in the regulation of bone remodeling processes. In terms of function, whereas Cldns act "canonically" as key determinants of paracellular permeability, there is considerable recent evidence to suggest that Cldns also participate in cell signaling, ie, a "noncanonical function". To this end, Cldns have been shown to regulate cell proliferation, differentiation, and gene expression in a variety of cell types. The present review will discuss Cldns' structure, their expression profile, regulation of expression, and their canonical and non- canonical functions in general with special emphasis on bone cells. In order to shed light on the noncanonical functions of Cldns in bone, we will highlight the role of Cldn-18 in regulating bone resorption and osteoclast differentiation. Collectively, we hope to provide a framework for guiding future research on understanding how Cldns modulate osteoblast and osteoclast function and overall bone homeostasis. Such studies should provide valuable insights into the pathogenesis of osteoporosis, and may highlight Cldns as novel targets for the diagnosis and therapeutic management of osteoporosis.

Gene expression of MC3T3-E1 osteoblastic cells on titanium and zirconia surface

PURPOSE

This study was performed to define attachment and growth behavior of osteoblast-like cells and evaluate the gene expression on zirconia compared to titanium.

MATERIALS AND METHODS

MC3T3-E1 cells were cultured on (1) titanium and (2) zirconia discs. The tetrazolium-based colorimetric assay (MTT test) was used for examining the attachment of cells. Cellular morphology was examined by scanning electron microscopy (SEM) and alkaline phosphatase (ALP) activity was measured to evaluate the cell differentiation rate. Mann-Whitney test was used to assess the significance level of the differences between the experimental groups. cDNA microarray was used for comparing the 20215 gene expressions on titanium and zirconia.

RESULTS

From the MTT assay, there was no significant difference between titanium and zirconia (P>.05). From the SEM image, after 4 hours of culture, cells on both discs were triangular or elongated in shape with formation of filopodia. After 24 hours of culture, cells on both discs were more flattened and well spread compared to 4 hours of culture. From the ALP activity assay, the optical density of E1 cells on titanium was slightly higher than that of E1 cells on zirconia but there was no significant difference (P>.05). Most of the genes related to cell adhesion showed similar expression level between titanium and zirconia.

CONCLUSION

Zirconia showed comparable biological responses of osteoblast-like cells to titanium for a short time during cell culture period. Most of the genes related to cell adhesion and signal showed similar expression level between titanium and zirconia.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Disruption of claudin-18 diminishes ovariectomy-induced bone loss in mice

Claudin-18 (Cldn-18), a member of the tight junction family of proteins, is a negative regulator of RANKL-induced osteoclast differentiation and bone resorption (BR) in vivo. Since estrogen deficiency decreases bone mass in part by a RANKL-mediated increase in BR, we evaluated whether estrogen regulates Cldn-18 expression in bone. We found that Cldn-18 expression was reduced in the bones of estrogen deficient mice, whereas it was increased by estrogen treatment in osteoblasts and osteoclasts in vitro. We next evaluated the role of Cldn-18 in mediating estrogen-induced bone loss. Cldn-18 knockout (KO) and littermate wild-type (WT) mice were ovariectomized (OVX) or sham operated at 6 wk of age, and the skeletal phenotype was evaluated at 14 wk of age. PIXImus revealed that total body, femur, and lumbar BMD were reduced 8-13% (P < 0.05) after 8 wk of OVX compared with sham in WT mice. As expected, total body, femur, and lumbar BMD were reduced 14-21% (P < 0.05) in Cldn-18 KO sham mice compared with sham WT mice. However, ovariectomy failed to induce significant changes in BMD of total body, femur, or vertebra in the Cldn-18 KO mice. μCT analysis of the distal femur revealed that trabecular (Tb) bone volume was decreased 50% in the OVX WT mice compared with sham that was caused by a 26% decrease in Tb number and a 30% increase in Tb separation (all P < 0.05). By contrast, none of the Tb parameters were significantly different in OVX Cldn-18 KO mice compared with sham KO mice. Histomorphometric analyses at the Tb site revealed that neither osteoclast surface nor osteoclast perimeter was increased significantly as a consequence of OVX in either genotype at the time point examined. Based on our findings, we conclude that the estrogen effects on osteoclasts may in part be mediated via regulation of Cldn-18 signaling.

Effects of a high-sodium diet on renal tubule Ca2+ transporter and claudin expression in Wistar-Kyoto rats

Background

Urinary Ca²⁺ excretion increases with dietary NaCl. NaCl-induced calciuria may be associated with hypertension, urinary stone formation and osteoporosis, but its mechanism and long-term effects are not fully understood. This study examined alterations in the expressions of renal Ca²⁺ transporters, channels and claudins upon salt loading to better understand the mechanism of salt-induced urinary Ca²⁺ loss.

Methods

Eight-week old Wistar-Kyoto rats were fed either 0.3% or 8% NaCl diet for 8 weeks. Renal cortical expressions of Na⁺/Ca²⁺ exchanger 1 (NCX1), Ca²⁺ pump (PCMA1b), Ca²⁺ channel (TRPV5), calbindin-D_28k, and claudins (CLDN-2, -7, -8, -16 and −19) were analyzed by quantitative PCR, western blot and/or immunohistochemistry.

Results

Fractional excretion of Ca²⁺ increased 6.0 fold with high-salt diet. Renal cortical claudin-2 protein decreased by approximately 20% with decreased immunological staining on tissue sections. Claudin-16 and −19 expressions were not altered. Renal cortical TRPV5, calbindin-D_28k and NCX1 expressions increased 1.6, 1.5 and 1.2 fold, respectively.

Conclusions

Chronic high-salt diet decreased claudin-2 protein and increased renal TRPV5, calbindin-D_28k, and NCX1. Salt loading is known to reduce the proximal tubular reabsorption of both Na⁺ and Ca²⁺. The reduction in claudin-2 protein expression may be partly responsible for the reduced Ca²⁺ reabsorption in this segment. The concerted upregulation of more distal Ca²⁺-transporting molecules may be a physiological response to curtail the loss of Ca²⁺, although the magnitude of compensation does not seem adequate to bring the urinary Ca²⁺ excretion down to that of the normal-diet group.

Claudin 18 is a novel negative regulator of bone resorption and osteoclast differentiation

Claudin 18 (Cldn-18) belongs to a large family of transmembrane proteins that are important components of tight junction strands. Although several claudin members are expressed in bone, the functional role for any claudin member in bone is unknown. Here we demonstrate that disruption of Cldn-18 in mice markedly decreased total body bone mineral density, trabecular bone volume, and cortical thickness in Cldn-18(-/-) mice. Histomorphometric studies revealed that bone resorption parameters were increased significantly in Cldn-18(-/-) mice without changes in bone formation. Serum levels of tartrate-resistant acid phosphatase 5b (TRAP5b) and mRNA expression levels of osteoclast specific markers and signaling molecules were also increased. Loss of Cldn-18 further exacerbated calcium deficiency induced bone loss by influencing bone resorption, thereby resulting in mechanically weaker bone. In vitro studies with bone marrow macrophages revealed Cldn-18 disruption markedly enhanced receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation but not macrophage colony-stimulating factor (MCSF)-induced bone marrow macrophage (BMM) proliferation. Consistent with a direct role for Cldn-18 in regulating osteoclast differentiation, overexpression of wild type but not PDZ binding motif deleted Cldn-18 inhibited RANKL-induced osteoclast differentiation. Furthermore, our findings indicate that Cldn-18 interacts with Zonula occludens 2 (ZO-2) to modulate RANKL signaling in osteoclasts. In conclusion, we demonstrate that Cldn-18 is a novel negative regulator of bone resorption and osteoclast differentiation.

Expression of renal distal tubule transporters TRPM6 and NCC in a rat model of cyclosporine nephrotoxicity and effect of EGF treatment

Renal magnesium (Mg2+) and sodium (Na+) loss are well-known side effects of cyclosporine (CsA) treatment in humans, but the underlying mechanisms still remain unclear. Recently, it was shown that epidermal growth factor (EGF) stimulates Mg2+ reabsorption in the distal convoluted tubule (DCT) via TRPM6 (Thébault S, Alexander RT, Tiel Groenestege WM, Hoenderop JG, Bindels RJ. J Am Soc Nephrol 20: 78–85, 2009). In the DCT, the final adjustment of renal sodium excretion is regulated by the thiazide-sensitive Na+-Cl− cotransporter (NCC), which is activated by the renin-angiotensin-aldosterone system (RAAS). The aim of this study was to gain more insight into the molecular mechanisms of CsA-induced hypomagnesemia and hyponatremia. Therefore, the renal expression of TRPM6, TRPM7, EGF, EGF receptor, claudin-16, claudin-19, and the NCC, and the effect of the RAAS on NCC expression, were analyzed in vivo in a rat model of CsA nephrotoxicity. Also, the effect of EGF administration on these parameters was studied. CsA significantly decreased the renal expression of TRPM6, TRPM7, NCC, and EGF, but not that of claudin-16 and claudin-19. Serum aldosterone was significantly lower in CsA-treated rats. In control rats treated with EGF, an increased renal expression of TRPM6 together with a decreased fractional excretion of Mg2+ (FE Mg2+) was demonstrated. EGF did not show this beneficial effect on TRPM6 and FE Mg2+ in CsA-treated rats. These data suggest that CsA treatment affects Mg2+ homeostasis via the downregulation of TRPM6 in the DCT. Furthermore, CsA downregulates the NCC in the DCT, associated with an inactivation of the RAAS, resulting in renal sodium loss.

Cellular communications in bone homeostasis and repair

Cellular communication between the bone component cells osteoblasts, osteocytes and (pre-)osteoclasts is essential for bone remodeling which maintains bone integrity. As in the remodeling of other organs, cell death is a trigger for remodeling of bone. During the systematic process of bone remodeling, direct or indirect cell-cell communication is indispensable. Thus, osteoblasts induce migration and differentiation of preosteoclasts, which is followed by bone resorption (by mature multinuclear osteoclasts). After completion of bone resorption, apoptosis of mature osteoclasts and differentiation of osteoblasts are initiated. At this time, the osteoblasts do not support osteoclast differentiation but do support bone formation. Finally, osteoblasts differentiate to osteocytes in bone or to bone lining cells on bone surfaces. In this way, old bone areas are regenerated as new bone. In this review the role of cell-cell communication in bone remodeling is discussed.

Changes in the mRNA expression of osteoblast-related genes in response to beta(3)-adrenergic agonist in UMR106 cells

Activation of adrenergic receptors (AR) was demonstrated to result in either bone gain or bone loss depending on the activated AR subtypes and concentrations of agonists used. While beta(2)-AR agonist was extensively investigated as an osteopenic agent, effects of beta(3)-AR activation on osteoblasts were still elusive. Rat osteoblast-like UMR106 cells were herein found to express several AR subtypes, including beta(3)-AR. After exposure to a low-dose beta(3)-AR agonist BRL37344 (10 nmol L(-1)), UMR106 cells downregulated the mRNA expression of transcription factors Runx2 and Dlx5, which are important for initiation of osteoblast differentiation. Low-dose BRL37344 also decreased the expression ratio of receptor activator of nuclear factor kappaB ligand (RANKL) over osteoprotegerin (OPG), suggesting the protective effect of beta(3)-AR agonist against bone resorption. Alkaline phosphatase expression was markedly decreased, whereas expressions of osteocalcin and osteopontin were increased by 100 nmol L(-1) BRL37344, indicating that beta(3)-AR activation could accelerate the transition of matrix maturation stage to mineralization stage. In conclusion, beta(3)-AR activation in rat osteoblasts induced alteration in the expression of osteoblast-related transcription factor genes as well as genes required for bone formation and resorption. The present results also suggest that, besides beta(2)-AR, beta(3)-AR is another AR subtype responsible for the sympathetic nervous system-induced bone remodeling.

Claudin expression in the bone-lining cells of female rats exposed to long-standing acidemia

Besides enhancing osteoclast-mediated bone resorption, chronic metabolic acidosis (CMA) induces mineral efflux across the epithelial-like bone membrane formed by bone-lining cells (inactive osteoblasts), possibly via the paracellular pathway. However, there was a compensatory mechanism that restricted bone loss in the late phase of CMA, and changes in the expression of claudins, which are tight junction proteins known to regulate epithelial barrier function, were therefore anticipated in bone-lining cells. Herein, primary rat osteoblasts were found to express several transcripts of claudins, i.e., claudin-5, -11, -14, -15 and -16. Their protein expressions in bone-lining cells were demonstrated by immunohistochemistry in decalcified tibial sections. After exposure to CMA induced by oral administration of 1.5% NH(4)Cl for 21 days, expression of claudin-14, which normally seals the paracellular space and restricts ion movement, was increased, whereas that of claudin-15 and -16 which form pores for ion transport were decreased. Expressions of claudin-5 and -11 were not changed by CMA. In conclusion, the bone-lining cells of rats exposed to CMA for 21 days upregulated an ion-restrictive claudin (i.e., claudin-14), while downregulating ion-permeable claudins (i.e., claudin-15 and -16). These cellular responses might be parts of a compensatory mechanism accounting for deceleration of bone loss in late CMA.

State-of-the-art technologies, current opinions and developments, and novel findings: news from the field of histochemistry and cell biology

Investigations of cell and tissue structure and function using innovative methods and approaches have again yielded numerous exciting findings in recent months and have added important data to current knowledge, inspiring new ideas and hypotheses in various fields of modern life sciences. Topics and contents of comprehensive expert reviews covering different aspects in methodological advances, cell biology, tissue function and morphology, and novel findings reported in original papers are summarized in the present review.

IGF-I regulates tight-junction protein claudin-1 during differentiation of osteoblast-like MC3T3-E1 cells via a MAP-kinase pathway

Insulin-like growth factor I (IGF-I) is expressed in many tissues, including bone, and acts on the proliferation and differentiation of osteoblasts as an autocrine/paracrine regulator. Tight-junction proteins have been detected in osteoblasts, and direct cell-to-cell interactions may modulate osteoblast function with respect, for example, to gap junctions. In order to investigate the regulation of expression of tight-junction molecules and of function during bone differentiation, osteoblast-like MC3T3-E1 cells and osteocyte-like MLO-Y4 cells were treated with IGF-I. In both MC3T3-E1 cells and MLO-Y4 cells, the tight-junction molecules occludin, claudin-1, -2, and -6, and the gap-junction molecule connexin 43 (Cx43) were detected by reverse transcription with polymerase chain reaction. In MC3T3-E1 cells but not MLO-Y4 cells, mRNAs of claudin-1, -2, and -6, Cx43, and type I collagen, and proteins of claudin-1 and Cx43 were increased after treatment with IGF-I. Such treatment significantly decreased paracellular permeability in MC3T3-E1 cells. The expression of claudin-1 in MC3T3-E1 cells after IGF-I treatment was mainly upregulated via a mitogen-activated protein (MAP)-kinase pathway and, in part, modulated by a PI3-kinase pathway, whereas Cx43 expression and the mediated gap-junctional intercellular communication protein did not contribute to the upregulation. Furthermore, in MC3T3-E1 cells during wound healing, upregulation of claudin-1 was observed together with an increase of IGF-I and type I collagen. These findings suggest that the induction of tight-junction protein claudin-1 and paracellular permeability during the differentiation of osteoblast-like MC3T3-E1 cells after treatment with IGF-I is regulated via a MAP-kinase pathway, but not with respect to gap junctions.