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BackgroundSingleton-Merten syndrome (SGMRT) is a rare immunogenetic disorder that variably features juvenile open-angle glaucoma (JOAG), psoriasiform skin rash, aortic calcifications and skeletal and dental dysplasia. Few families have been described and the genotypic and phenotypic spectrum is poorly defined, with variants in DDX58 (DExD/H-box helicase 58) being one of two identified causes, classified as SGMRT2.MethodsFamilies underwent deep systemic phenotyping and exome sequencing. Functional characterisation with in vitro luciferase assays and in vivo interferon signature using bulk and single cell RNA sequencing was performed.ResultsWe have identified a novel DDX58 variant c.1529A>T p.(Glu510Val) that segregates with disease in two families with SGMRT2. Patients in these families have widely variable phenotypic features and different ethnic background, with some being severely affected by systemic features and others solely with glaucoma. JOAG was present in all individuals affected with the syndrome. Furthermore, detailed evaluation of skin rash in one patient revealed sparse inflammatory infiltrates in a unique distribution. Functional analysis showed that the DDX58 variant is a dominant gain-of-function activator of interferon pathways in the absence of exogenous RNA ligands. Single cell RNA sequencing of patient lesional skin revealed a cellular activation of interferon-stimulated gene expression in keratinocytes and fibroblasts but not in neighbouring healthy skin.ConclusionsThese results expand the genotypic spectrum of DDX58-associated disease, provide the first detailed description of ocular and dermatological phenotypes, expand our understanding of the molecular pathogenesis of this condition and provide a platform for testing response to therapy.

Background Loeys-Dietz syndrome (LDS), an autosomal dominant rare connective tissue disorder, has multisystemic manifestations, characterised by vascular tortuosity, aneurysms and craniofacial manifestations. Based on the associated gene mutations along the transforming growth factor-beta (TGF-β) pathway, LDS is presently classified into six subtypes. Methods We present the oro-dental features of a cohort of 40 patients with LDS from five subtypes. Results The most common oro-dental manifestations were the presence of a high-arched and narrow palate, and enamel defects. Other common characteristics included bifid uvula, submucous cleft palate, malocclusion, dental crowding and delayed eruption of permanent teeth. Both deciduous and permanent teeth had enamel defects in some individuals. We established a grading system to measure the severity of enamel defects, and we determined that the severity of the enamel anomalies in LDS is subtype-dependent. In specific, patients with TGF-β receptor II mutations (LDS2) presented with the most severe enamel defects, followed by patients with TGF-β receptor I mutations (LDS1). LDS2 patients had higher frequency of oro-dental deformities in general. Across all five subtypes, as well as within each subtype, enamel defects exhibited incomplete penetrance and variable expression, which is not associated with the location of the gene mutations. Conclusion This study describes, in detail, the oro-dental manifestations in a cohort of LDS, and we conclude that LDS2 has the most severely affected phenotype. This extensive characterisation, as well as some identified distinguishing features can significantly aid dental and medical care providers in the diagnosis and clinical management of patients with this rare connective tissue disorder.

To test the hypothesis that ROSAH (retinal dystrophy, optic nerve oedema, splenomegaly, anhidrosis and headache) syndrome, caused by dominant mutation inThis cohort study systematically evaluated 27 patients with ROSAH syndrome for inflammatory features and investigated the effect ofThe majority of the cohort carried the p.Thr237Met mutation but we also identified a new ROSAH-associated mutation, p.Tyr254Cys.Nearly all patients exhibited at least one feature consistent with inflammation including recurrent fever, headaches with meningeal enhancement and premature basal ganglia/brainstem mineralisation on MRI, deforming arthritis and AA amyloidosis. However, there was significant phenotypic variation, even within families and some adults lacked functional visual deficits. While anti-TNF and anti-IL-1 therapies suppressed systemic inflammation and improved quality of life, anti-IL-6 (tocilizumab) was the only anticytokine therapy that improved intraocular inflammation (two of two patients).Patients' primary samples and in vitro assays with mutated ALPK1 constructs showed immune activation with increased NF-κB signalling, STAT1 phosphorylation and interferon gene expression signature. Knock-in mice with theROSAH syndrome is an autoinflammatory disease caused by gain-of-function mutations in

For many years there has been a keen interest in developing regenerative treatment for temporomandibular joint-osteoarthritis (TMJ-OA). Currently, there is no consensus treatment due to the limited self-healing ability of articular cartilage and lack of understanding of the complex mechanisms regulating cartilage development in the TMJ. Endochondral ossification, the process of subchondral bone formation through chondrocyte differentiation, is critical for TMJ growth and development, and is tightly regulated by the composition of the extracellular matrix (ECM). Type VI collagen is a highly expressed ECM component in the TMJ cartilage, yet its specific functions are largely unknown. In this study, we investigated α2(VI)-deficient (

Fibrin gums up the works Plasmin is an abundant plasma protease that cleaves and deactivates the clot-associated protein fibrin. Human deficiencies in plasmin and its inactive proenzyme form, plasminogen (PLG), cause severe inflammation in mucosal tissues such as the mouth and eyes. Silva et al . report that, like humans, mice lacking plasminogen accumulate extravascular fibrin and develop an oral pathology that phenocopies human ligneous periodontitis (see the Perspective by Vicanolo and Hidalgo). The excess fibrin activates neutrophils through the αMβ2 (Mac-1) integrin receptor, which triggers the production of reactive oxygen species and neutrophil extracellular traps. Additionally, certain human polymorphisms in the PLG gene were found to be associated with increased likelihood of developing periodontitis, suggesting that fibrin–neutrophil interactions may be an attractive target for future treatments of this prevalent disease. —STS

In this case report, we focus on Muenke syndrome (MS), a disease caused by the p.Pro250Arg variant in fibroblast growth factor receptor 3 (FGFR3) and characterized by uni- or bilateral coronal suture synostosis, macrocephaly without craniosynostosis, dysmorphic craniofacial features, and dental malocclusion. The clinical findings of MS are further complicated by variable expression of phenotypic traits and incomplete penetrance. As such, unraveling the mechanisms behind MS will require a comprehensive and systematic way of phenotyping patients to precisely identify the impact of the mutation variant on craniofacial development. To establish this framework, we quantitatively delineated the craniofacial phenotype of an individual with MS and compared this to his unaffected parents using three-dimensional cephalometric analysis of cone beam computed tomography scans and geometric morphometric analysis, in addition to an extensive clinical evaluation. Secondly, given the utility of human induced pluripotent stem cells (hiPSCs) as a patient-specific investigative tool, we also generated the first hiPSCs derived from a family trio, the proband and his unaffected parents as controls, with detailed characterization of all cell lines. This report provides a starting point for evaluating the mechanistic underpinning of the craniofacial development in MS with the goal of linking specific clinical manifestations to molecular insights gained from hiPSC-based disease modeling.

Many studies have been conducted to elucidate the role of Type VI collagen in muscle and tendon, however, its role in oral tissues remains unclear. In this study, an α2(VI) deficient mouse (Col6α2-KO) model was used to examine the role of Type VI collagen in oral tissues. Tissue volume and mineral density were measured in oral tissues by µCT. Proteome analysis was performed using protein extracted from alveolar bone. In addition, alveolar bone was evaluated with a periodontitis induced model. µCT analysis showed the Col6α2-KO mice had less volume of alveolar bone, dentin and dental pulp, while the width of periodontal ligament (PDL) was greater than WT. The mineral density in alveolar bone and dentin were elevated in Col6α2-KO mice compared with WT. Our proteome analysis showed significant changes in proteins related to ECM organization and elevation of proteins associated with biomineralization in the Col6α2-KO mice. In induced periodontitis, Col6α2-KO mice had greater alveolar bone loss compared with WT. In conclusion, Type VI collagen has a role in controlling biomineralization in alveolar bone and that changes in the ECM of alveolar bone could be associated with greater bone loss due to periodontitis.

Tissue-specific cues are critical for homeostasis at mucosal barriers. Here, we document that the clotting factor fibrin is a critical regulator of neutrophil function at mucosal barriers. We demonstrate that fibrin engages neutrophils through the αMβ2 integrin receptor and activates effector functions, including the production of reactive oxygen species and NET formation. These immune-protective neutrophil functions become tissue damaging in the context of impaired plasmin-mediated fibrinolysis. Indeed, the accumulation of fibrin due to Mendelian genetic defects in plasmin leads to severe oral mucosal immunopathology in mice and humans. Concordantly, genetic polymorphisms in the human PLG gene, encoding plasminogen, are associated with common forms of the oral mucosal disease periodontitis. Our work uncovers fibrin as a critical regulator of neutrophil effector function within the mucosal tissue microenvironment and suggests fibrin-neutrophil engagement as a pathogenic instigator and therapeutic target in common mucosal disease.

BackgroundElevated transforming growth factor-beta (TGF-β) signalling has been implicated in the pathogenesis of Loeys-Dietz syndrome (LDS) and Shprintzen-Goldberg syndrome (SGS). In this study, we provide a qualitative and quantitative analysis of the craniofacial and functional features among the LDS subtypes and SGS.MethodsWe explore the variability within and across a cohort of 44 patients through deep clinical phenotyping, three-dimensional (3D) facial photo surface analysis, cephalometric and geometric morphometric analyses of cone-beam CT scans.ResultsThe most common craniofacial features detected in this cohort include mandibular retrognathism (84%), flat midface projection (84%), abnormal eye shape (73%), low-set ears (73%), abnormal nose (66%) and lip shape (64%), hypertelorism (41%) and a relatively high prevalence of nystagmus/strabismus (43%), temporomandibular joint disorders (38%) and obstructive sleep apnoea (23%). 3D cephalometric analysis demonstrated an increased cranial base angle with shortened anterior cranial base and underdevelopment of the maxilla and mandible, with evidence of a reduced pharyngeal airway in 55% of those analysed. Geometric morphometric analysis confirmed that the greatest craniofacial shape variation was among patients with LDS type 2, with distinct clustering of patients with SGS.ConclusionsThis comprehensive phenotypic approach identifies developmental abnormalities that segregate to mutation variants along the TGF-β signalling pathway, with a particularly severe phenotype associated with TGFBR2 and SKI mutations. Multimodality assessment of craniofacial anomalies objectively reveals the impact of mutations of the TGF-β pathway with perturbations associated with the cranium and cranial base with severe downstream effects on the orbit, maxilla and mandible with the resultant clinical phenotypes.

Small leucine rich proteoglycans (SLRPs), including Biglycan, have key roles in many organ and tissue systems. The goal of this article is to review the function of Biglycan and other related SLRPs in mineralizing tissues of the skeleton. The review is divided into sections that include Biglycan’s role in structural biology, signaling, craniofacial and long bone homeostasis, remodeled skeletal tissues, and in human genetics. While many cell types in the skeleton are now known to be affected by Biglycan, there are still unanswered questions about its mechanism of action(s).

Background Loeys-Dietz syndrome (LDS) is a rare connective tissue disorder whose oral manifestations and dental phenotypes have not been well-characterized. The aim of this study was to explore the influence of oral manifestations on oral health-related quality of life (OHRQoL) in LDS patients. Material and methods LDS subjects were assessed by the craniofacial team at the National Institutes of Health Clinical Center Dental Clinic between June 2015 and January 2018. Oral Health Impact Profile (OHIP-14) questionnaire, oral health self-care behavior questionnaire and a comprehensive dental examination were completed for each subject. OHRQoL was assessed using the OHIP-14 questionnaire with higher scores corresponding to worse OHRQoL. Regression models were used to determine the relationship between each oral manifestation and the OHIP-14 scores using a level of significance of p ≤ 0.05. Results A total of 33 LDS subjects (51.5% female) aged 3–57 years (19.6 ± 15.1 years) were included in the study. The OHIP-14 scores (n = 33) were significantly higher in LDS subjects (6.30 [SD 6.37]) when compared to unaffected family member subjects (1.50 [SD 2.28], p n = 32), the majority of LDS subjects reported receiving regular dental care (81%) and maintaining good-to-excellent daily oral hygiene (75%). Using a crude regression model, worse OHRQoL was found to be associated with dental hypersensitivity (β = 5.24; p p p p p p Conclusions The dental hypersensitivity, TMJ abnormalities, self-reported poor-to-fair oral health status and cumulation of four-or-more oral manifestations had significant influence on worse OHRQoL. Specific dental treatment guidelines are necessary to ensure optimal quality of life in patients diagnosed with LDS.

Objectives The Bone Morphogenetic Proteins (BMPs) direct tooth development and still express in the adult tooth. We hypothesized that inhibition of BMP function would therefore disrupt dentinogenesis by differentiated odontoblasts. Methods We generated mice overexpressing the BMP-inhibitory protein Noggin in differentiated odontoblasts and osteocytes under control of a Dmp1 promoter-driven cre transgene. We compared the dentin phenotype in these mice with that in WT littermates and in mice with a Smad4 odontoblast/osteocyte knockout mediated by the same cre and therefore lacking all BMP and Tgfβ signaling in the same tissues. Results Three-month-old first molars from both Noggin-expressing and Smad4-deleted mice showed decreased dentin volume with enlarged pulp cavities, and both displayed less organized and mineralized dentinal tubules compared to WT. The Smad4-ablated phenotype was more severe. While dentin sialophosphoprotein (DSPP) and bone sialoprotein (BSP) were decreased in the dentin of both lines, dentin matrix protein 1 (DMP1) was sharply increased in Noggin-expressing teeth. Conclusions The phenotypes we observed in Noggin-overexpressing and Smad4-conditional knockout teeth resemble the phenotype of Dentinogenesis Imperfecta (DGI) type III. Our results show that BMPs regulate post-natal dentinogenesis and that BMP-inhibitory proteins like Noggin play a role in that regulation. The increased severity of the Smad4 phenotype indicates that Tgfβ ligands, in addition to BMPs, play a crucial role in post-developmental dentinogenesis.

Biglycan (Bgn) and Fibromodulin (Fmod) are small leucine rich proteoglycans (SLRPs) which are abundant in the extra-cellular matrix (ECM) of mineralized tissues. We have previously generated a Bgn/Fmod double knock-out (DKO) mouse model and found it has a 3-fold increase in osteoclastogenesis compared with Wild type (WT) controls, resulting in a markedly low bone mass (LBM) phenotype. To try and rescue/repair the LBM phenotype of Bgn/Fmod DKO mice by suppressing osteoclast formation and activity, 3- and 26-week-old Bgn/Fmod DKO mice and age/gender matched WT controls were treated with OPG-Fc for 6 weeks after which bone parameters were evaluated using DEXA, micro-computed tomography (μCT) and serum biomarkers analyses. In the appendicular skeleton, OPG-Fc treatment improved some morphometric and geometric parameters in both the trabecular and cortical compartments in Bgn/Fmod DKO female and male mice, especially in the repair module. For many of the skeletal parameters analyzed, the Bgn/Fmod DKO mice were more responsive to the treatment than their WT controls. In addition, we found that OPG-Fc treatment was not able to prevent or ameliorate the formation of ectopic ossification, which are common lesions seen in aged joints and are one of the phenotypical hallmarks of our Bgn/Fmod DKO model. Analysis of skull bones, specifically the occipital bone, showed the treatment recovered some parameters of LBM phenotype in the craniofacial skeleton, more so in the younger rescue module. Using OPG-Fc as treatment alleviated, yet did not completely restore, the severe osteopenia and mineralized tissue structural abnormalities that Bgn/Fmod DKO mice suffer from.

Additional file 1: Table S1. Distribution of OHIP-14 questionnaire. Table S2. Comparison of OHIP-14 health subdomains between three different age groups in the LDS cohort. Table S3. Comparison of OHIP-14 health subdomains between males and females in LDS cohort. Table S4. Comparison of OHIP-14 health subdomains between types of LDS mutations. Table S5. Frequency of oral manifestations for each age group in the LDS cohort. Table S6. Influence of Food Allergy on OHIP-14 (OHRQoL) in LDS Patients.

Fam20c is essential for the normal mineralization of dentin and bone. The generation of odontoblast and osteoblast cell lines carrying floxed Fam20c allele can offer valuable tools for the study of the roles of Fam20c in the mineralization of dentin and bone. The limited capability of the primary odontoblasts and osteoblasts to proliferate necessitates the development of odontoblast and osteoblast cell lines serving as substitutes for the study of differentiation and mineralization of the odontoblasts and osteoblasts. In this study, we established and characterized immortalized mouse floxed Fam20c dental papilla mesenchymal and osteoblast cell lines. The isolated primary mouse floxed Fam20c dental papilla mesenchymal cells and osteoblasts were immortalized by the infection of lentivirus containing Simian Virus 40 T-antigen (SV40 T-Ag). The immortalization of floxed Fam20c dental papilla mesenchymal cells and osteoblasts was verified by the long-term passages and genomic integration of SV40 T-Ag. The immortalized floxed Fam20c dental papilla mesenchymal and osteoblast cell lines not only proliferated at a high rate and retained the morphology of their primary counterparts, but also preserved the dentin and bone specific gene expression as the primary dental papilla mesenchymal cells and osteoblasts did. Consistently, the capability of the primary floxed Fam20c dental papilla mesenchymal cells and osteoblasts to mineralize was also inherited by the immortalized dental papilla mesenchymal and osteoblast cell lines. Thus, we have successfully generated the immortalized mouse floxed Fam20c dental papilla mesenchymal and osteoblast cell lines.

FAM20C mutations compromise the mineralization of skeleton and tooth in both human and mouse. Putatively, the mineralization disorder is attributed to the elevated fibroblast growth factor 23 (FGF23), which reduced the serum phosphorus by suppressing the reabsorption of phosphorus in kidney. Besides the regulation on systemic phosphorus homeostasis, FAM20C was also implicated to regulate cell behaviors and gene expression through a cell-autonomous manner. To identify the primary effects of Fam20c on dental mesenchymal cells, mouse Fam20c-deficient dental mesenchymal cells were generated by removing the floxed alleles from the immortalized mouse Fam20cf/f dental mesenchymal cells with Cre-expressing lentivirus. The removal of Fam20c exerted no impact on cell morphology, but suppressed the proliferation and mobility of the dental mesenchymal cells. Fam20c deficiency also significantly reduced the expression of Osterix, Runx2, type I Collagen a 1 (Col1a1), Alkaline phosphatase (Alpl) and the members of the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family, but increased Fgf23 expression. Consistently, the in vitro mineralization of Fam20c-deficient dental mesenchymal cells was severely disabled. However, supplements of the non-collagenous proteins from wild type rat dentin failed to rescue the compromised mineralization, suggesting that the roles of FAM20C in tooth mineralization are more than phosphorylating local matrices and regulating systemic phosphorus metabolism. Moreover, the down-regulated BMP signaling pathways in the Fam20c deficient dental mesenchymal cells revealed that the kinase activity of FAM20C might be required to maintain BMP signaling. In summary, our study discloses that Fam20c indeed regulates cell behaviors and cell signaling pathway in a cell-autonomous manner.

Objective The bone morphogenetic proteins (BMPs) play crucial roles in tooth development. However, several BMPs retain expression in the dentin of the fully patterned and differentiated tooth. We hypothesized that BMP signaling therefore plays a role in the function of the differentiated odontoblast, the job of which is to lay down and mineralize the dentin matrix. Design We generated mice deficient in Bmp2 and 4 using a dentin matrix protein 1 (Dmp1) promoter-driven cre recombinase that was expressed in differentiated odontoblasts. Results The first and second molars of these Bmp2 and Bmp4 double conditional knockout (DcKO) mice displayed reduced dentin and enlarged pulp chambers compared to cre-negative littermate controls. DcKO mouse dentin in first molars was characterized by small, disorganized dentinal fibers, a wider predentin layer, and reduced expression of dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), and bone sialoprotein (BSP). DcKO mouse odontoblasts demonstrated increased type I collagen mRNA production, indicating that the loss of BMP signaling altered the rate of collagen gene expression in these cells. Bmp2 and Bmp4 single Dmp1-cre knockout mice displayed no discernable dentin phenotype. Conclusions These data demonstrate that BMP signaling in differentiated odontoblasts is necessary for proper dentin production in mature teeth.

Objective Previous studies have shown that dentin sialophosphoprotein (DSPP) is not only essential to the formation and mineralization of dentin but also plays an important role in forming and maintaining a healthy periodontium. Under physiological conditions, DSPP is proteolytically processed into the NH2-terminal and COOH-terminal fragments, and these fragments are believed to perform different functions in the mineralized tissues. Previous studies in our group have demonstrated that the NH2-terminal fragment of DSPP inhibits the formation and mineralization of dentin, while the role of this fragment in periodontium is unclear.

Recent studies indicate that Family with sequence similarity 20 member C (FAM20C) catalyzes the phosphorylation of secreted proteins, and participates in a variety of biological processes, including cell proliferation, migration, mineralization, and phosphate homeostasis. To explore the local influences of FAM20C on osteoblast, Fam20c-deficient osteoblasts were generated by treating the immortalized Fam20cf/f osteoblasts with CMV-Cre-IRES-EGFP lentivirus. Compared with the normal Fam20cf/f osteoblasts, the expression of Bone sialoprotein (Bsp), Osteocalcin (Ocn), Fibroblast growth factor 23 (Fgf23), and transcription factors that promote osteoblast maturation were up-regulated in the Fam20c-deficient osteoblasts. In contrast, the expression of Dental matrix protein 1 (Dmp1), Dentin sialophosphoprotein (Dspp), Osteopontin (Opn), type I Collagen a 1 (Col1a1), and Alkine phosphatase (Alp) were down-regulated in the Fam20c-deficient cells. These alterations disclosed the primary regulation of Fam20c on gene expression. The Fam20c-deficient osteoblasts showed a remarkable reduction in the ability of forming mineralized nodules. However, supplements of extracellular matrix proteins extracted from the normal bone failed to rescue the reduced mineralization, suggesting that FAM20C may affect the biomineralization by the means more than local phosphorylation of extracellular matrix proteins and systemic phosphorus homeostasis. Moreover, although Fam20c deficiency had little impact on cell proliferation, it significantly reduced cell migration and lowered the levels of p-Smad1/5/8, p-Erk and p-p38, suggesting that the kinase activity of FAM20C might be essential to cell mobility and the activity of BMP ligands. In summary, these findings provide evidences that FAM20C may regulate osteoblast maturation, migration, mineralization, and BMP signaling pathways in a cell-autonomous manner.

Amelogenesis Imperfecta (AI) can be caused by the deficiencies of enamel matrix proteins, molecules responsible for the transportation and secretion of enamel matrix components and proteases processing enamel matrix proteins. In the present study, we discovered the double deletion of bone morphogenetic protein 2 (Bmp2) and bone morphogenetic protein 4 (Bmp4) in the dental epithelium by K14-cre resulted in hypoplastic enamel and reduced density in X-ray radiography as well as shortened enamel rods under scanning electron microscopy. Such enamel phenotype was consistent with the diagnosis of hypoplastic amelogenesis imperfecta. Histological and molecular analyses revealed that the removal of matrix proteins in the mutant enamel was drastically delayed, which was coincided with the greatly reduced expression of matrix metalloproteinase 20 (MMP20) and kallikrein 4 (KLK4). Although the expression of multiple enamel matrix proteins was down-regulated in the mutant ameloblasts, the cleavage of ameloblastin was drastically impaired. Therefore, we attributed the AI primarily to the reduction of MMP20 and KLK4. Further investigation found that BMP/Smad4 signaling pathway was down-regulated in the K14-cre;Bmp2f/f;Bmp4f/fameloblasts, suggesting that the reduced MMP20 and KLK4 expression may be due to the attenuated epithelial BMP/Smad4 signaling.

Dentin matrix protein 1 (DMP1) and dentin sialophosphoprotein (DSPP) belong to the Small Integrin-Binding Ligand N-linked Glycoprotein (SIBLING) family. In addition to the features common to all SIBLING members, DMP1 and DSPP share several unique similarities in chemical structure, proteolytic activation and tissue localization. Mutations in, or deletion of DMP1, cause autosomal recessive hypophosphatemic rickets along with dental defects; DSPP mutations or its ablation are associated with dentinogenesis imperfecta. While the roles and functional mechanisms of DMP1 in osteogenesis have been extensively studied, those of DSPP in long bones have been studied only to a limited extent. Previous studies by our group revealed that transgenic expression of Dspp completely rescued the dentin defects of Dmp1-null (Dmp1(-/-)) mice. In this investigation, we assessed the effects of transgenic Dspp on osteogenesis by analyzing the formation and mineralization of the long bones in Dmp1(-/-) mice that expresses a transgene encoding full-length DSPP driven by a 3.6-kb rat Col1a1 promoter (referred as "Dmp1(-/-);Dspp-Tg mice"). We characterized the long bones of the Dmp1(-/-);Dspp-Tg mice at different ages and compared them with those from Dmp1(-/-) and Dmp1(+/-) (normal control) mice. Our analyses showed that the long bones of Dmp1(-/-);Dspp-Tg mice had a significant increase in cortical bone thickness, bone volume and mineral density along with a remarkable restoration of trabecular thickness compared to those of the Dmp1(-/-) mice. The long bones of Dmp1(-/-);Dspp-Tg mice underwent a dramatic reduction in the amount of osteoid, significant improvement of the collagen fibrillar network, and better organization of the lacunocanalicular system, compared to the Dmp1(-/-) mice. The elevated levels of biglycan, bone sialoprotein and osteopontin in Dmp1(-/-) mice were also noticeably corrected by the transgenic expression of Dspp. These findings suggest that DSPP and DMP1 may function synergistically within the complex milieus of bone matrices.

Dentin sialophosphoprotein (DSPP) is one of the major non-collagenous proteins present in dentin, cementum and alveolar bone; it is also transiently expressed by ameloblasts. In humans many mutations have been found in DSPP and are associated with two autosomal-dominant genetic diseases - dentinogenesis imperfecta II (DGI-II) and dentin dysplasia (DD). Both disorders result in the development of hypomineralized and mechanically compromised teeth. The erupted mature molars of Dspp(-/-) mice have a severe hypomineralized dentin phenotype. Since dentin and enamel formations are interdependent, we decided to investigate the process of enamel onset mineralization in young Dspp(-/-) animals. We focused our analysis on the constantly erupting mouse incisor, to capture all of the stages of odontogenesis in one tooth, and the unerupted first molars. Using high-resolution microCT, we revealed that the onset of enamel matrix deposition occurs closer to the cervical loop and both secretion and maturation of enamel are accelerated in Dspp(-/-) incisors compared to the Dspp(+/-) control. Importantly, these differences did not translate into major phenotypic differences in mature enamel in terms of the structural organization, mineral density or hardness. The only observable difference was the reduction in thickness of the outer enamel layer, while the total enamel thickness remained unchanged. We also observed a compromised dentin-enamel junction, leading to delamination between the dentin and enamel layers. The odontoblast processes were widened and lacked branching near the DEJ. Finally, for the first time we demonstrate expression of Dspp mRNA in secretory ameloblasts. In summary, our data show that DSPP is important for normal mineralization of both dentin and enamel.

The Trps1 transcription factor is highly expressed in dental mesenchyme and preodontoblasts, while in mature, secretory odontoblasts, it is expressed at low levels. Previously, we have shown that high Trps1 levels in mature odontoblasts impair their function in vitro and in vivo. Col1a1-Trps1 transgenic (Trps1-Tg) mice demonstrate defective dentin secretion and mineralization, which are associated with significantly decreased Dspp expression due to direct repression of the Dspp gene by Trps1. Here, by crossing Trps1-Tg and Col1a1-Dspp transgenic (Dspp-Tg) mice, we generated Col1a1-Trps1;Col1a1-Dspp double transgenic (double-Tg) mice in which Dspp was restored in odontoblasts overexpressing Trps1. Comparative micro–computed tomography analyses revealed partial correction of the dentin volume and no improvement of dentin mineralization in double transgenic mice in comparison with Trps1-Tg and wild-type (WT) mice. In addition, dentin of double-Tg mice has an irregular mineralization pattern characteristic for dentin in hypophosphatemic rickets. Consistent with this phenotype, decreased levels of Phex, Vdr, and Fam20c proteins are detected in both Trps1-Tg and double-Tg odontoblasts in comparison with WT and Dspp-Tg odontoblasts. This suggests that the Dspp-independent dentin mineralization defects in Trps1-Tg mice are a result of downregulation of a group of proteins critical for mineral deposition within the dentin matrix. In summary, by demonstrating that Trps1 functions as a repressor of later stages of dentinogenesis, we provide functional significance of the dynamic Trps1 expression pattern during dentinogenesis.

Dentin sialophosphoprotein (DSPP), first discovered by cDNA cloning in 1997, plays a crucial role in dentinogenesis (1). It undergoes post-translational modification and is cleaved into the NH2-terminal fragments (existing in two forms: dentin sialoprotein (DSP) and the proteoglycan form termed DSP-PG) and COOH-terminal fragment (dentin phosphoprotein (DPP)) (1-3). Genetic studies have shown the association of DSPP mutations with dentinogenesis imperfecta (DGI) in humans (4-10). In animal models, Dspp-knock out (Dspp-KO) mice exhibit severe dentin hypomineralization defects with widened predentin zone resembling the dentin defects observed in human DGI (11). These findings strongly suggest the importance of DSPP in the formation and mineralization of dentin. Besides its established role in tooth formation, DSPP was also discovered in bone, cellular cementum and several non-mineralized tissues (12-20). Its expression in alveolar bone and cellular cementum is remarkably higher than in the long bone (14,19). Recent work from our laboratory showed that DSPP plays an essential role in these periodontal tissues (21). In Dspp deficient mice we observed severe alveolar bone loss with reduced cementum deposition and altered osteocyte morphology (21). The reduction in interdental alveolar bone caused apical migration of periodontal ligaments (PDL) leading to periodontal pockets (21). In vivo studies done by our group showed that proteolytic processing of DSPP is vital for its function in dentinogenesis (22). However, whether the same mechanism applies in periodontal tissues still remains to be tested. For this purpose, we systematically characterized the alveolar bone and cementum of the following groups of mice: 1) mice expressing the uncleavable D452A-DSPP in the Dspp-KO background (named as Dspp-KO/D452A-Tg mice); 2) Dspp-KO mice; 3) Wild-type (WT) mice and; 4) mice expressing the normal DSPP transgene in the Dspp–KO background (referred as Dspp-KO/normal-Tg mice). The results from this study showed that expression of normal DSPP was successful in rescuing the periodontal defects of Dspp-KO mice. However the transgenic expression of D452A-DSPP was unable to rescue the Dspp deficient defects. The findings from from this study provide in vivo evidence that the proteolytic processing of DSPP is necessary for maintaining the health of periodontal tissues.

Genetic studies have shown that dentin sialophosphoprotein (DSPP) plays an essential role in dentinogenesis (1–3). For example, the dentin in Dspp knockout mice is hypomineralized and the predentin is widened, creating a phenotype similar to that of dentinogenesis imperfecta type III in humans (3). While studies have demonstrated the critical role of DSPP in dentin formation, the mechanism(s) by which DSPP function(s) in biomineralization remain largely unclear. DSPP, first identified by cDNA cloning (4), is a large protein that undergoes proteolytic processing to form the NH2-terminal fragment known as dentin sialoprotein (DSP), proteoglycan form of the NH2-terminal fragment referred to as DSP-PG and a COOH-terminal fragment named dentin phosphoprotein (DPP) (5–7). The sequence of the NH2-terminal fragment (DSP or DSP-PG) lies in the 5′-portion, while the DPP sequence is located in the 3′-region of the DSPP transcript (4, 8–10). Studies done in our laboratory and others established the presence of a proteoglycan form of DSP (known as DSP-PG) (5–7). DSP/DSP-PG and DPP are abundant in the dentin extracellular matrix (ECM) whereas the protein representing the entire sequence (DSPP) is scarcely present, which led to the belief that the processed fragments may be the functional forms of DSPP. Recent work in our laboratory has shown that blocking the proteolytic fragmentation of DSPP inactivates the function(s) of this molecule during dentinogenesis (11). This observation provides strong evidence that the proteolytic processing of DSPP is an activation event which releases the functional fragments that exert distinct molecular role(s) during dentin formation (11). DPP, discovered in 1967, is the most abundant non collagenous protein (NCP) in the dentin ECM (12, 13). The unusual feature of DPP is the presence of large amounts of aspartic acid (Asp) and phosphoserine (Pse), mostly found in the repeating sequences of (Asp-Pse-Pse)n and (Asp-Pse)n (14,15). Several in vitro mineralization studies have indicated that DPP is an important initiator and modulator for the formation and growth of hydroxyapatite (HA) crystals (16–18). It is believed that the DPP secreted by the odontoblasts is transported to the mineralization front where it binds to collagen fibrils promoting the formation of initial HA. As this mineralization process proceeds, and more predentin is converted to dentin, these mineral crystals grow in an oriented fashion under the modulation of DPP and other non-collagenous proteins (NCPs) that bind to the growing HA faces (19, 20). DSP, discovered in 1981, is much less abundant than DPP in dentin ECM (21). DSP [protein without the glycosaminoglycan (GAG) chains]does not have a significant effect on the in vitro mineral formation and growth (22). Since there is no in vivo data concerning the role of DSP in dentinogenesis, the functions of this fragment is presently undefined. The recently discovered proteoglycan form of DSP (DSP-PG) is a major component in the proteoglycan pool of the dentin extracellular matrix (ECM) and appears to be more abundant than DSP (5, 6). We believe that the NH2-terminal fragment of DSPP (DSP/DSP-PG) must play a role in biomineralization, distinct from that of the COOH-terminal fragment of DSPP (DPP). DSP-PG from the NH2-terminal sequence of DSPP was found in the dentin ECM at higher amounts than the core protein DSP, which led us to believe that DSP-PG may be the functional form of the NH2-terminal fragment. We hypothesize that the role of DSP-PG may be to prevent the predentin from being mineralized too rapidly and to serve as an antagonist of DPP in dentinogenesis. In order to examine the function of the NH2-terminal fragment of DSPP in dentinogenesis, we expressed the NH2-terminal fragment of DSPP in the Dspp-null background (referred as “Dspp KO/DSP Tg”) to analyze the in vivo effects of this fragment in a gain of function experimental approach. In this study, we systematically characterized the dentin of Dspp KO/DSP Tg mice and compared it to the dentin from Dspp-null (Dspp knockout or Dspp KO) mice and wild type (WT) mice. We observed that the dentin of the Dspp KO/DSP Tg mice appeared much worse compared to the Dspp-null mice. The results from this investigation support our hypothesis that the NH2-terminal fragment of DSPP has an inhibitory role in dentinogenesis.