Regulation of Osteoblast to Osteocyte Differentiation by Cyclin-Dependent Kinase-1

Osteocytes have recently been identified as a new regulator of bone remodeling, but the detailed mechanism of their differentiation from osteoblasts remains unclear. The purpose of this study is to identify cell cycle regulators involved in the differentiation of osteoblasts into osteocytes and determine their physiological significance. The study uses IDG-SW3 cells as a model for the differentiation from osteoblasts to osteocytes. Among the major cyclin-dependent kinases (Cdks), Cdk1 is most abundantly expressed in IDG-SW3 cells, and its expression is down-regulated during differentiation into osteocytes. Inhibition of CDK1 activity reduces IDG-SW3 cell proliferation and differentiation into osteocytes. Osteocyte and Osteoblast-specific Cdk1 knockout in mice (Dmp1-Cdk1KO ) results in trabecular bone loss. Pthlh expression increases during differentiation, but inhibiting CDK1 activity reduces Pthlh expression. Parathyroid hormone-related protein concentration is reduced in the bone marrow of Dmp1-Cdk1KO mice. Four weeks of Parathyroid hormone administration partially recovers the trabecular bone loss in Dmp1-Cdk1KO mice. These results demonstrate that Cdk1 plays an essential role in the differentiation from osteoblast to osteocyte and the acquisition and maintenance of bone mass. The findings contribute to a better understanding of the mechanisms of bone mass regulation and can help develop efficient therapeutic strategies for osteoporosis treatment.

FGF4 and FGF9 have synergistic effects on odontoblast differentiation

The purpose of this study was to investigate whether fibroblast growth factor 4 (FGF4) and FGF9 are active in dentin differentiation. Dentin matrix protein 1 (Dmp1) -2A-Cre transgenic mice, which express the Cre-recombinase in Dmp1-expressing cells, were crossed with CAG-tdTomato mice as reporter mouse. The cell proliferation and tdTomato expressions were observed. The mesenchymal cell separated from neonatal molar tooth germ were cultured with or without FGF4, FGF9, and with or without their inhibitors ferulic acid and infigratinib (BGJ398) for 21 days. Their phenotypes were evaluated by cell count, flow cytometry, and real-time PCR. Immunohistochemistry for FGFR1, 2, and 3 expression and the expression of DMP1 were performed. FGF4 treatment of mesenchymal cells obtained promoted the expression of all odontoblast markers. FGF9 failed to enhance dentin sialophosphoprotein (Dspp) expression levels. Runt-related transcription factor 2 (Runx2) was upregulated until day 14 but was downregulated on day 21. Compared to Dmp1-negative cells, Dmp1-positive cells expressed higher levels of all odontoblast markers, except for Runx2. Simultaneous treatment with FGF4 and FGF9 had a synergistic effect on odontoblast differentiation, suggesting that they may play a role in odontoblast maturation.

Involvement of Dmp1 in the Precise Regulation of Hair Bundle Formation in the Developing Cochlea

Dentin matrix protein 1 (Dmp1) is a highly phosphorylated, extracellular matrix protein that is extensively expressed in bone and teeth but also found in soft tissues, including brain and muscle. However, the functions of Dmp1 in the mice cochlea are unknown. Our study showed that Dmp1 was expressed in auditory hair cells (HCs), with the role of Dmp1 in those cells identified using Dmp1 cKD mice. Immunostaining and scanning electron microscopy of the cochlea at P1 revealed that Dmp1 deficiency in mice resulted in an abnormal stereociliary bundle morphology and the mispositioning of the kinocilium. The following experiments further demonstrated that the cell-intrinsic polarity of HCs was affected without apparent effect on the tissue planer polarity, based on the observation that the asymmetric distribution of Vangl2 was unchanged whereas the Gαi3 expression domain was enlarged and Par6b expression was slightly altered. Then, the possible molecular mechanisms of Dmp1 involvement in inner ear development were explored via RNA-seq analysis. The study suggested that the Fgf23-Klotho endocrine axis may play a novel role in the inner ear and Dmp1 may regulate the kinocilium-stereocilia interaction via Fgf23-Klotho signaling. Together, our results proved the critical role of Dmp1 in the precise regulation of hair bundle morphogenesis in the early development of HCs.

Generation of bicistronic Dmp1-Cre knock-in mice using a self-cleaving 2A peptide

INTRODUCTION

The conditional manipulation of genes using the Cre recombinase-locus of crossover in P1 (Cre/loxP) system is an important tool for revealing gene functions and cell lineages in vivo. The outcome of this method is dependent on the performance of Cre-driver mouse strains. In most cases, Cre knock-in mice show better specificity than randomly inserted Cre transgenic mice. However, following knock-in, the expression of the original gene replaced by Cre is lost.

MATERIALS AND METHODS

We generated a new differentiated osteoblast- and osteocyte-specific Cre knock-in mouse line that carries the viral T2A sequence encoding a 2A self-cleaving peptide at the end of the coding region of the dentin matrix protein 1 (Dmp1) gene accompanied by the Cre gene.

RESULTS

We confirmed that Dmp1-T2A-Cre mice showed high Cre expression in osteoblasts, osteocytes, odontoblasts, and periodontal ligament cells and that the 2A self-cleaving peptide efficiently produced both Dmp1 and Cre proteins. Furthermore, unlike the Dmp1 knockout mice, homozygous Dmp1-T2A-Cre mice showed no skeletal abnormalities. Analysis using the Cre reporter strain confirmed differentiated osteoblast- and osteocyte-specific Cre-mediated recombination in the skeleton. Furthermore, recombination was also detected in some nuclei of skeletal muscle cells, spermatocytes, and intestinal cells.

CONCLUSION

2A-Cre functions effectively in vivo, and Dmp1-T2A-Cre knock-in mice are a useful tool for studying the functioning of various genes in hard tissues.

Off-target activity of the 8 kb Dmp1-Cre results in the deletion of Tsc1 gene in mouse intestinal mesenchyme

The Dmp1-Cre mouse, expressing Cre from an 8-kb DNA fragment of the mouse Dmp1 gene, is a common tool to study gene functions in osteocytes. Here we report that the deletion of Tsc1 (TSC complex subunit 1) by 8 kb Dmp1-Cre causes rectal prolapse in mice. Histological examination shows the presence of colon polyps in Tsc1-deficient mice in association with significantly larger colon and narrower lumen, which recapitulates the common polyps pathology in Tuberous Sclerosis, an autosomal dominant disorder caused by mutations in either TSC1 or TSC2. The intestine in Tsc1-deficient mice is also enlarged with the presence of taller villi. Using the Ai14 reporter mice that express a red fluorescence protein upon Cre recombination, we show that 8 kb Dmp1-Cre activity is evident in portion of the mesenchyme of the colon and small intestine. Lastly, our data show that Tsc1 deletion by Dmp1-Cre leads to an increased proliferation in the mesenchyme of colon, which at least partly contributes to the polyps pathology seen in this mouse model and is likely a contributing factor of the polyps in Tuberous Sclerosis.

Klf4 Promotes Dentinogenesis and Odontoblastic Differentiation via Modulation of TGF-β Signaling Pathway and Interaction With Histone Acetylation

Transcription factors bind to cell-specific cis-regulatory elements, such as enhancers and promoters, to initiate much of the gene expression program of different biological process. Odontoblast differentiation is a necessary step for tooth formation and is also governed by a complex gene regulatory network. Our previous in vitro experiments showed that Krüppel-like factor 4 (KLF4) can promote odontoblastic differentiation of both mouse dental papillary cells (mDPCs) and human dental pulp cells; however, its mechanism remains unclear. We first used Wnt1-Cre; KLF4^fx/fx (Klf4 cKO) mice to examine the role of KLF4 during odontoblast differentiation in vivo and demonstrated significantly impaired dentin mineralization and enlarged pulp/root canals. Additionally, combinatory analysis using RNA-seq and ATAC-seq revealed genomewide direct regulatory targets of KLF4 in mouse odontoblasts. We found that KLF4 can directly activate the TGF-β signaling pathway at the beginning of odontoblast differentiation with Runx2 as a cofactor. Furthermore, we found that KLF4 can directly upregulate the expression levels of Dmp1 and Sp7, which are markers of odontoblastic differentiation, through binding to their promoters. Interestingly, as a transcription factor, KLF4 can also recruit histone acetylase as a regulatory companion to the downstream target genes to positively or negatively regulate transcription. To further investigate other regulatory companions of KLF4, we chose histone acetylase HDAC3 and P300. Immunoprecipitation demonstrated that KLF4 interacted with P300 and HDAC3. Next, ChIP analysis detected P300 and HDAC3 enrichment on the promoter region of KLF4 target genes Dmp1 and Sp7. HDAC3 mainly interacted with KLF4 on day 0 of odontoblastic induction, whereas P300 interacted on day 7 of induction. These temporal-specific interactions regulated Dmp1 and Sp7 transcription, thus regulating dentinogenesis. Taken together, these results demonstrated that KLF4 regulates Dmp1 and Sp7 transcription via the modulation of histone acetylation and is vital to dentinogenesis. © 2019 American Society for Bone and Mineral Research.

Distinct Expression Patterns of Osteopontin and Dentin Matrix Protein 1 Genes in Pituitary Gonadotrophs

Cell-matrix interactions play important roles in pituitary development, physiology, and pathogenesis. In other tissues, a family of non-collagenous proteins, termed SIBLINGs, are known to contribute to cell-matrix interactions. Anterior pituitary gland expresses two SIBLING genes, Dmp1 (dentin matrix protein-1) and Spp1 (secreted phosphoprotein-1) encoding DMP1 and osteopontin proteins, respectively, but their expression pattern and roles in pituitary functions have not been clarified. Here we provide novel evidence supporting the conclusion that Spp1/osteopontin, like Dmp1/DMP1, are expressed in gonadotrophs in a sex- and age-specific manner. Other anterior pituitary cell types do not express these genes. In contrast to Dmp1, Spp1 expression is higher in males; in females, the expression reaches the peak during the diestrus phase of estrous cycle. In further contrast to Dmp1 and marker genes for gonadotrophs, the expression of Spp1 is not regulated by gonadotropin-releasing hormone in vivo and in vitro. However, Spp1 expression increases progressively after pituitary cell dispersion in both female and male cultures. We may speculate that gonadotrophs signal to other pituitary cell types about changes in the structure of pituitary cell-matrix network by osteopontin, a function consistent with the role of this secretory protein in postnatal tissue remodeling, extracellular matrix reorganization after injury, and tumorigenesis.

Role of osteocytes in mediating bone mineralization during hyperhomocysteinemia

Hyperhomocysteinemia (HHCY) is a risk factor for osteoporosis but whether HHCY affects bone mineralization or not is still ambiguous. Herein we evaluated whether homocysteine affects tissue mineral density (TMD) of cortical bone and if so the role of osteocytes. CD1 mice administered with homocysteine (5 mg/100 g body weight, i.p.) for 7, 15 and 30 days showed temporal changes in TMD and osteocyte lacunar density in femoral cortices. Short-term administration of homocysteine (day 7) increased osteocyte lacunar density and reduced TMD evidenced by microCT50 while prolonged administration of homocysteine (day 30) reinstated TMD and lacunar density to baseline values. Major differences were decreased number of nucleated osteocyte lacunae, increased number of empty lacunae and cleaved caspase 3-positive osteocyte lacunae in day 30 HHCY bone evidenced by H&E staining and immunohistochemistry. Other differences were induction in mineralization genes like Dmp1, Phex and Sost in cortical bone by real-time PCR and increased number of Dmp1- and Sost-positive osteocyte lacunae in day 30 HHCY bone evidenced by immunohistochemistry. Both HHCY day 7 and day 30 samples showed reduced Young's modulus demonstrating that biomechanical property of bone was lost during early HHCY itself, which did not improve with recovery of TMD. Our results thus demonstrate occurrence of two phases in cortical bone upon HHCY: the early phase that involved loss of TMD and increase in osteocyte numbers and a late phase that involved osteocyte reprogramming, apoptosis and mineralization, which reinstated TMD but compromised biomechanical property. To conclude, osteocytes have a potential role in arbitrating bone pathogenesis during HHCY.

Oncogenic and tumor-suppressive mouse models for breast cancer engaging HER2/neu

The human c-ErbB2 (HER2) gene is amplified in ∼20% of human breast cancers (BCs), but the protein is overexpressed in ∼30% of the cases indicating that multiple different mechanisms contribute to HER2 overexpression in tumors. It has long been used as a molecular marker of BC for subcategorization for the prediction of prognosis and determination of therapeutic strategies. In comparison to ER(+) BCs, HER2-positive BCs are more invasive, but the patients respond to monoclonal antibody therapy with trastuzumab or tyrosine kinase inhibitors at least at early stages. To understand the pathophysiology of HER2-driven carcinogenesis and test HER2-targeting therapeutic agents in vivo, numerous mouse models have been created that faithfully reproduce HER2(+) BCs in mice. They include MMTV-neu (active mutant or wild type, rat neu or HER2) models, neu promoter-driven neuNT-transgenic mice, neuNT-knock-in mice at the neu locus and doxycycline-inducible neuNT-transgenic models. HER2/neu activates the Phosphatidylinositol-3 kinase-AKT-NF-κB pathway to stimulate the mitogenic cyclin D1/Cdk4-Rb-E2F pathway. Of note, overexpression of HER2 also stimulates the cell autonomous Dmp1-Arf-p53 tumor suppressor pathway to quench oncogenic signals to prevent the emergence of cancer cells. Hence tumor development by MMTV-neu mice was dramatically accelerated in mice that lack Dmp1, Arf or p53 with invasion and metastasis. Expressions of neuNT under the endogenous promoter underwent gene amplification, closely recapitulating human HER2(+) BCs. MMTV-HER2 models have been shown to be useful to test humanized monoclonal antibodies to HER2. These mouse models will be useful for the screening of novel therapeutic agents against BCs with HER2 overexpression.

The Dmp1-SOST Transgene Interacts With and Downregulates the Dmp1-Cre Transgene and the Rosa(Notch) Allele

Activation of Notch1 in osteocytes of Rosa(Notch) mice, where a loxP-flanked STOP cassette and the Nicd coding sequence were targeted to the reverse orientation splice acceptor (Rosa)26 locus, causes osteopetrosis associated with suppressed Sost expression and enhanced Wnt signaling. To determine whether Sost downregulation mediates the effects of Notch activation in osteocytes, Rosa(Notch) mice were crossed with transgenics expressing Cre recombinase or SOST under the control of the dentin matrix protein (Dmp)1 promoter. Dmp1-SOST transgenics displayed vertebral osteopenia and a modest femoral cancellous and cortical bone phenotype, whereas hemizygous Dmp1-Cre transgenics heterozygous for the Rosa(Notch) allele (Dmp1-Cre;Rosa(Notch)) exhibited osteopetrosis. The phenotype of Notch activation in osteocytes was prevented in Dmp1-Cre;Rosa(Notch) mice hemizygous for the Dmp1-SOST transgene. The effect was associated with downregulated Notch signaling and suppressed Dmp1 and Rosa26 expression. To test whether SOST regulates Notch expression in osteocytes, cortical bone cultures from Dmp1-Cre;Rosa(Notch) mice or from Rosa(Notch) control littermates were exposed to recombinant human SOST. The addition of SOST had only modest effects on Notch target gene mRNA levels and suppressed Dmp1, but not Cre or Rosa26, expression. These findings suggest that prevention of the Dmp1-Cre;Rosa(Notch) skeletal phenotype by Dmp1-SOST is not secondary to SOST expression but to interactions among the Dmp1-SOST and Dmp1-Cre transgenes and the Rosa26 locus. In conclusion, the Dmp1-SOST transgene suppresses the expression of the Dmp1-Cre transgene and of Rosa26.

Klf10 regulates odontoblast differentiation and mineralization via promoting expression of dentin matrix protein 1 and dentin sialophosphoprotein genes

Klf10, a member of the Krüppel-like family of transcription factors, is critical for osteoblast differentiation, bone formation and mineralization. However, whether Klf10 is involved in odontoblastic differentiation and tooth development has not been determined. In this study, we investigate the expression patterns of Klf10 during murine tooth development in vivo and its role in odontoblastic differentiation in vitro. Klf10 protein was expressed in the enamel organ and the underlying mesenchyme, ameloblasts and odontoblasts at early and later stages of murine molar formation. Furthermore, the expression of Klf10, Dmp1, Dspp and Runx2 was significantly elevated during the process of mouse dental papilla mesenchymal differentiation and mineralization. The overexpression of Klf10 induced dental papilla mesenchymal cell differentiation and mineralization as detected by alkaline phosphatase staining and alizarin red S assay. Klf10 additionally up-regulated the expression of odontoblastic differentiation marker genes Dmp1, Dspp and Runx2 in mouse dental papilla mesenchymal cells. The molecular mechanism of Klf10 in controlling Dmp1 and Dspp expression is thus to activate their regulatory regions in a dosage-dependent manner. Our results suggest that Klf10 is involved in tooth development and promotes odontoblastic differentiation via the up-regulation of Dmp1 and Dspp transcription.

Reduced Euchromatin histone methyltransferase 1 causes developmental delay, hypotonia, and cranial abnormalities associated with increased bone gene expression in Kleefstra syndrome mice

Haploinsufficiency of Euchromatin histone methyltransferase 1 (EHMT1), a chromatin modifying enzyme, is the cause of Kleefstra syndrome (KS). KS is an intellectual disability (ID) syndrome, with general developmental delay, hypotonia, and craniofacial dysmorphisms as additional core features. Recent studies have been focused on the role of EHMT1 in learning and memory, linked to the ID phenotype of KS patients. In this study we used the Ehmt1(+/-) mouse model, and investigated whether the core features of KS were mimicked in these mice. When comparing Ehmt1(+/-) mice to wildtype littermates we observed delayed postnatal growth, eye opening, ear opening, and upper incisor eruption, indicating a delayed postnatal development. Furthermore, tests for muscular strength and motor coordination showed features of hypotonia in young Ehmt1(+/-) mice. Lastly, we found that Ehmt1(+/-) mice showed brachycephalic crania, a shorter or bent nose, and hypertelorism, reminiscent of the craniofacial dysmorphisms seen in KS. In addition, gene expression analysis revealed a significant upregulation of the mRNA levels of Runx2 and several other bone tissue related genes in P28 Ehmt1(+/-) mice. Runx2 immunostaining also appeared to be increased. The mRNA upregulation was associated with decreased histone H3 lysine 9 dimethylation (H3K9me2) levels, the epigenetic mark deposited by Ehmt1, in the promoter region of these genes. Together, Ehmt1(+/-) mice indeed recapitulate KS core features and can be used as an animal model for Kleefstra syndrome. The increased expression of bone developmental genes in the Ehmt1(+/-) mice likely contributes to their cranial dysmorphisms and might be explained by diminished Ehmt1-induced H3K9 dimethylation.

Depletion of ERK2 but not ERK1 abrogates oncogenic Ras-induced senescence

In response to oncogenic activation, cells initially undergo proliferation followed by an irreversible growth arrest called oncogene-induced senescence (OIS), an endogenous defense mechanism against tumorigenesis. Oncogenic activation of ERK1/2 is essential for both the initial phase of cellular proliferation as well as subsequent premature senescence, but little is known about the specific contribution of ERK1 versus 2 to OIS. Here we show that depletion of ERK2 but not ERK1 by shRNA knockdown in MEFs leads to continuous proliferation bypassing senescence even in the presence of oncogenic HRAS(V12). Upon depletion of ERK2, induction of both p19(Arf) and p16(Ink4a) was significantly compromised after oncogenic HRAS(V12) expression, attenuating activation of the key tumor suppressors p53 and pRb. Here we demonstrate that ERK2 but not ERK1 indirectly regulates p19(Arf) and p16(Ink4a) both at the transcriptional and translational level. Oncogenic Ras expression after ERK2 knockdown downregulates Fra-1 and c-Jun, components of the activator protein-1 (AP-1) heterodimer essential for transactivation of p19(Arf). Similarly we show a significant decrease in the activation of p38 MAPK and ETS family members which are involved in the induction of p16(Ink4a). The role of ERK2 in translational regulation is observed by the lack of tuberin (TSC2) and p70 ribosomal S6 kinase 1 (p70S6K1) phosphorylation, components of the mTOR pathway, which enhances p19(Arf) mRNA translation during oncogenic Ras-induced senescence. These observations suggest that ERK2 but not ERK1 contributes to upregulation of p19(Arf) and p16(Ink4a) in a transcription- and translation-dependent manner during oncogenic Ras-induced senescence. Taken together, our data indicate that ERK2 is the key ERK isoform mediating the senescence signaling pathway downstream of oncogenic Ras.

Signal transduction involving the dmp1 transcription factor and its alteration in human cancer

Dmp1 (cyclin D-interacting myb-like protein 1; also called Dmtf1) is a transcription factor that has been isolated in a yeast two-hybrid screen through its binding property to cyclin D2. Dmp1 directly binds to and activates the Arf promoter and induces Arf-p53-dependent cell cycle arrest in primary cells. D-type cyclins usually inhibit Dmp1-mediated transcription in a Cdk-independent fashion; however, Dmp1 shows synergistic effects with D-cyclins on the Arf promoter. Ras or Myc oncogene-induced tumor formation is accelerated in both Dmp1^+/− and Dmp1^−/− mice with no significant differences between Dmp1^+/− and Dmp1^−/−. Thus, Dmp1 is haplo-insufficient for tumor suppression. Tumors from Dmp1^−/− or Dmp1^+/− mice often retain wild-type Arf and p53, suggesting that Dmp1 is a physiological regulator of the Arf-p53 pathway. The Dmp1 promoter is activated by oncogenic Ras-Raf signaling, while it is repressed by physiological mitogenic stimuli, overexpression of E2F proteins, and genotoxic stimuli mediated by NF-κB. The human DMP1 gene (hDMP1) is located on chromosome 7q21 and is hemizygously deleted in approximately 40% of human lung cancers, especially those that retain normal INK4a/ARF and P53 loci. Thus, hDMP1 is clearly involved in human carcinogenesis, and tumors with hDMP1 deletion may constitute a discrete disease entity.