A high-calcium diet failed to rescue an osteopenia phenotype in claudin-18 knockout mice

Novel Role for Claudin-11 in the Regulation of Osteoblasts via Modulation of ADAM10-Mediated Notch Signaling

The claudin (Cldn) family comprises 27 members of 20 to 34 kDa transmembrane tight junction proteins. In addition to Cldns' established canonical role as barriers controlling paracellular flow of molecules, a distinct noncanonical role for them as mediators of cell signaling is now emerging. In our studies evaluating Cldn family expression levels during osteoblast differentiation, Cldn-11 showed the largest increase (60-fold). Immunohistochemistry studies revealed high Cldn-11 expression in trabecular (Tb) bone lining cells. Micro-CT analysis of femurs and vertebrae of Cldn-11 knock-out (KO) mice at 12 weeks of age exhibited a 40% (p < 0.01) reduction in Tb bone volume adjusted for tissue volume compared with control mice, a change caused by significant reductions in Tb number and thickness and increase in Tb separation. Histomorphometry and serum biomarker studies revealed that reduced bone formation, not increased resorption, is the cause for reduced Tb bone volume in the Cldn-11 KO mice. Cldn-11 KO osteoblasts expressed reduced ALP and BSP, whereas Cldn-11 overexpression in MC3T3-E1 cells increased expression of ALP and BSP. Mechanistically, Cldn-11 interacted with tetraspanin (Tspan)3 in osteoblasts, and Tspan3 knockdown reduced osteoblast differentiation. Because members of the Tspan family regulate cell functions via Notch signaling, we evaluated whether Cldn-11/Tspan3 regulates Notch signaling in osteoblasts. Accordingly, Notch targets Hey1 and Hey2 were significantly upregulated in Cldn-11 overexpressing cultures but downregulated in both Cldn-11 KO and Tspan3 knockdown osteoblasts. Because ADAM10 has been shown to interact with Tspan family members to regulate Notch signaling, we evaluated whether Cldn-11 regulates ADAM10 expression. Cldn-11 overexpressing cells express more mature ADAM10, and an ADAM10 inhibitor blocked the Cldn-11 effect on osteoblast differentiation. Based on these data, we propose Cldn-11 as a novel component of an osteoblast cell surface protein complex, comprising Tspan3 and ADAM10, which regulates Notch signaling and cell differentiation. © 2019 American Society for Bone and Mineral Research.

Loss of Tight Junction Protein Claudin 18 Promotes Progressive Neoplasia Development in Mouse Stomach

BACKGROUND & AIMS

Loss of claudin 18 (CLDN18), a membrane-spanning tight junction protein, occurs during early stages of development of gastric cancer and associates with shorter survival times of patients. We investigated whether loss of CLDN18 occurs in mice that develop intraepithelial neoplasia with invasive glands due to infection with Helicobacter pylori, and whether loss is sufficient to promote the development of similar lesions in mice with or without H pylori infection.

METHODS

We performed immunohistochemical analyses in levels of CLDN18 in archived tissues from B6:129 mice infected with H pylori for 6 to 15 months. We analyzed gastric tissues from B6:129S5-Cldn18tm1Lex/Mmucd mice, in which the CLDN18 gene was disrupted in gastric tissues (CLDN18-knockout mice), or from control mice with a full-length CLDN18 gene (CLDN18+/+; B6:129S5/SvEvBrd) or heterozygous disruption of CLDN18 (CLDN18+/-; B6:129S5/SvEvBrd) that were infected with H pylori SS1 or PMSS1 at 6 weeks of age and tissues collected for analysis at 20 and 30 weeks after infection. Tissues from CLDN18-knockout mice and control mice with full-length CLDN18 gene expression were also analyzed without infection at 7 weeks and 2 years after birth. Tissues from control and CLDN18-knockout mice were analyzed by electron microscopy, stained by conventional methods and analyzed for histopathology, prepared by laser capture microdissection and analyzed by RNAseq, and immunostained for lineage markers, proliferation markers, and stem cell markers and analyzed by super-resolution or conventional confocal microscopy.

RESULTS

CLDN18 had a basolateral rather than apical tight junction localization in gastric epithelial cells. B6:129 mice infected with H pylori, which developed intraepithelial neoplasia with invasive glands, had increasing levels of CLDN18 loss over time compared with uninfected mice. In B6:129 mice infected with H pylori compared with uninfected mice, CLDN18 was first lost from most gastric glands followed by disrupted and reduced expression in the gastric neck and in surface cells. Gastric tissues from CLDN18-knockout mice had low levels of inflammation but increased cell proliferation, expressed markers of intestinalized proliferative spasmolytic polypeptide-expressing metaplasia, and had defects in signal transduction pathways including p53 and STAT signaling by 7 weeks after birth compared with full-length CLDN18 gene control mice. By 20 to 30 weeks after birth, gastric tissues from uninfected CLDN18-knockout mice developed intraepithelial neoplasia that invaded the submucosa; by 2 years, gastric tissues contained large and focally dysplastic polypoid tumors with invasive glands that invaded the serosa.

CONCLUSIONS

H pylori infection of B6:129 mice reduced the expression of CLDN18 early in gastric cancer progression, similar to previous observations from human gastric tissues. CLDN18 regulates cell lineage differentiation and cellular signaling in mouse stomach; CLDN18-knockout mice develop intraepithelial neoplasia and then large and focally dysplastic polypoid tumors in the absence of H pylori infection.

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.