Dentin sialophosphoprotein (DSPP) plays an essential role in the postnatal development and maintenance of mouse mandibular condylar cartilage

Long-term haplodeficency of DSPP causes temporomandibular joint osteoarthritis in mice

BACKGROUND

Extracellular matrix (ECM) protein malfunction or defect may lead to temporomandibular joint osteoarthritis (TMJ OA). Dentin sialophophoprotein (DSPP) is a mandibular condylar cartilage ECM protein, and its deletion impacted cell proliferation and other extracellular matrix alterations of postnatal condylar cartilage. However, it remains unclear if long-term loss of function of DSPP leads to TMJ OA. The study aimed to test the hypothesis that long-term haploinsufficiency of DSPP causes TMJ OA.

MATERIALS AND METHODS

To determine whether Dspp+/- mice exhibit TMJ OA but no severe tooth defects, mandibles of wild-type (WT), Dspp+/-, and Dspp homozygous (Dspp-/-) mice were analyzed by Micro-computed tomography (micro-CT). To characterize the progression and possible mechanisms of osteoarthritic degeneration over time in Dspp+/- mice over time, condyles of Dspp+/- and WT mice were analyzed radiologically, histologically, and immunohistochemically.

RESULTS

Micro-CT and histomorphometric analyses revealed that Dspp+/- and Dspp-/- mice had significantly lower subchondral bone mass, bone volume fraction, bone mineral density, and trabecular thickness compared to WT mice at 12 months. Interestingly, in contrast to Dspp-/- mice which exhibited tooth loss, Dspp+/- mice had minor tooth defects. RNA sequencing data showed that haplodeficency of DSPP affects the biological process of ossification and osteoclast differentiation. Additionally, histological analysis showed that Dspp+/- mice had condylar cartilage fissures, reduced cartilage thickness, decreased articular cell numbers and severe subchondral bone cavities, and with signs that were exaggerated with age. Radiographic data showed an increase in subchondral osteoporosis up to 18 months and osteophyte formation at 21 months. Moreover, Dspp+/- mice showed increased distribution of osteoclasts in the subchondral bone and increased expression of MMP2, IL-6, FN-1, and TLR4 in the mandibular condylar cartilage.

CONCLUSIONS

Dspp+/- mice exhibit TMJ OA in a time-dependent manner, with lesions in the mandibular condyle attributed to hypomineralization of subchondral bone and breakdown of the mandibular condylar cartilage, accompanied by upregulation of inflammatory markers.

Phenotype and molecular characterizations of a family with dentinogenesis imperfecta shields type II with a novel DSPP mutation

Background

Dentinogenesis imperfecta (DGI), Shields type-II is an autosomal dominant genetic disease which severely affects the function of the patients’ teeth. The dentin sialophosphoprotein (DSPP) gene is considered to be the pathogenic gene of DGI-II. In this study, a DGI-II family with a novel DSPP mutation were collected, functional characteristics of DGI cells and clinical features were analyzed to better understand the genotype-phenotype relationship of this disease.

Methods

Clinical data were collected, whole exome sequencing (WES) was conducted, and Sanger sequencing was used to verify the mutation sites. Physical characteristics of the patient’s teeth were examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The localization of green fluorescent protein (GFP)-fused wild-type (WT) dentin sialoprotein (DSP) and its variant were evaluated via an immunocytochemistry (ICC) assay. The behaviors of human dental pulp stem cells (hDPSCs) were investigated by flow cytometry, osteogenic differentiation, and quantitative real-time polymerase chain reaction (qRT-PCR).

Results

A novel heterozygous mutation c.53T > G (p. Val18Gly) in DSPP was found in this family. The SEM results showed that the participants’ teeth had reduced and irregular dentinal tubes. The EDS results showed that the Ca/P ratio of the patients’ teeth was significantly higher than that of the control group. The ICC assay showed that the mutant DSP was entrapped in the endoplasmic reticulum (ER), while the WT DSP located mainly in the Golgi apparatus. In comparison with normal cells, the patient’s cells exhibited significantly decreased mineralization ability and lower expression levels of DSPP and RUNX2.

Conclusions

The c.53T > G (p. Val18Gly) DSPP variant was shown to present with rare hypoplastic enamel defects. Functional analysis revealed that this novel variant disturbs dentinal characteristics and pulp cell behavior.

Effects of DSPP Gene Mutations on Periodontal Tissues

Dentin sialophosphoprotein ( DSPP ) gene mutations cause autosomal dominantly inherited diseases. DSPP gene mutations lead to abnormal expression of DSPP, resulting in a series of histological, morphological, and clinical abnormalities. A large number of previous studies demonstrated that DSPP is a dentinal-specific protein, and DSPP gene mutations lead to dentin dysplasia and dentinogenesis imperfecta. Recent studies have found that DSPP is also expressed in bone, periodontal tissues, and salivary glands. DSPP is involved in the formation of the periodontium as well as tooth structures. DSPP deficient mice present furcation involvement, cementum, and alveolar bone defect. We speculate that similar periodontal damage may occur in patients with DSPP mutations. This article reviewed the effects of DSPP gene mutations on periodontal status. However, almost all of the research is about animal study, there is no evidence that DSPP mutations cause periodontium defects in patients yet. We need to conduct systematic clinical studies on DSPP mutation families in the future to elucidate the effect of DSPP gene on human periodontium.

Primary observation of the role of posttranslational modification of dentin sialophosphoprotein (DSPP) on postnatal development of mandibular condyle in mice

OBJECTIVES

We aimed to observe the posttranslational role of dentin sialophosphoprotein (DSPP) on postnatal development of mandibular condyle in mice.

METHODS

To explore the function of full-length DSPP, four groups of mice were employed: (1) wild type (WT) mice; (2)Dspp knockout (Dspp KO) mice; (3) mice expressing the normal DSPP transgene in the Dspp KO background (Dspp KO/normal Tg); (4) mice expressing the uncleavable full-length DSPP in the Dspp KO background (Dspp KO/D452A Tg). Firstly, Plain X-ray Radiography and Micro-computed Tomography were used to observe the condylar morphology changes of Dspp KO/D452A Tg mice in comparison with the other three groups. Then, Hematoxylin & eosin and toluidine blue staining were applied to uncover the histological changes of mandibular condylar cartilage (MCC) of Dspp KO/D452A Tg mice. To explore the function of the NH2-terminal fragments (i.e. DSP/DSP-PG), three groups of mice were employed: (1) WT mice; (2) Dspp KO mice; (3) mice expressing the NH2-terminal fragments of DSPP in the Dspp-null background (Dspp KO/DSP Tg). The former strategies were utilized to examine the differences of condylar morphology and histological structures changes within three groups of mice.

RESULTS

Transgenic full-length DSPP partially maintained mandibular condylar morphology and MCC thickness of Dspp KO mice. Transgenic DSP failed to do so, but led to smaller mandibular condyle and disordered cartilage structure.

CONCLUSIONS

Our observations provide insight into the role of posttranslational modification of DSPP in the postnatal development of healthy MCC and maintenance of condylar morphology.

Dentin Sialophosphoprotein Deletion Leads to Femoral Head Cartilage Attenuation and Subchondral Bone Ill-mineralization

Dentin sialophosphoprotein (DSPP), which expresses and synthesizes in odontoblasts of dental pulp, is a critical protein for normal teeth mineralization. Originally, DSPP was identified as a dentin-specific protein. In 2010, DSPP was also found in femoral head cartilage, and it is still unclear what roles DSPP play in femoral head cartilage formation, growth, and maintenance. To reveal biological functions of DSPP in the femoral head cartilage, we examined Dspp null mice compared with wild-type (WT) mice to observe DSPP expression as well as localization in WT mice and to uncover differences of femoral head cartilage, bone morphology, and structure between these two kinds of mice. Expression data demonstrated that DSPP had heterogeneous fragments, expressed in each layer of femoral head cartilage and subchondral bone of WT mice. Dspp null mice exhibited a significant reduction in the thickness of femoral head cartilage, with decreases in the amount of proliferating cartilage cells and increases in apoptotic cells. In addition, the subchondral bone mineralization decreased, and the expressions of vessel markers (vascular endothelial growth factor [VEGF] and CD31), osteoblast markers (Osterix and dentin matrix protein 1 [DMP1]), osteocyte marker (sclerostin [SOST]), and osteoclast marker (tartrate-resistant acid phosphatase [TRAP]) were remarkably altered. These indicate that DSPP deletion can affect the proliferation of cartilage cells in the femoral head cartilage and endochondral ossification in subchondral bone. Our data clearly demonstrate that DSPP plays essential roles in the femoral head cartilage growth and maintenance and subchondral biomineralization.

Knockdown of Foxg1 in supporting cells increases the trans-differentiation of supporting cells into hair cells in the neonatal mouse cochlea

Foxg1 is one of the forkhead box genes that are involved in morphogenesis, cell fate determination, and proliferation, and Foxg1 was previously reported to be required for morphogenesis of the mammalian inner ear. However, Foxg1 knock-out mice die at birth, and thus the role of Foxg1 in regulating hair cell (HC) regeneration after birth remains unclear. Here we used Sox2CreER/+ Foxg1loxp/loxp mice and Lgr5-EGFPCreER/+ Foxg1loxp/loxp mice to conditionally knock down Foxg1 specifically in Sox2+ SCs and Lgr5+ progenitors, respectively, in neonatal mice. We found that Foxg1 conditional knockdown (cKD) in Sox2+ SCs and Lgr5+ progenitors at postnatal day (P)1 both led to large numbers of extra HCs, especially extra inner HCs (IHCs) at P7, and these extra IHCs with normal hair bundles and synapses could survive at least to P30. The EdU assay failed to detect any EdU+ SCs, while the SC number was significantly decreased in Foxg1 cKD mice, and lineage tracing data showed that much more tdTomato+ HCs originated from Sox2+ SCs in Foxg1 cKD mice compared to the control mice. Moreover, the sphere-forming assay showed that Foxg1 cKD in Lgr5+ progenitors did not significantly change their sphere-forming ability. All these results suggest that Foxg1 cKD promotes HC regeneration and leads to large numbers of extra HCs probably by inducing direct trans-differentiation of SCs and progenitors to HCs. Real-time qPCR showed that cell cycle and Notch signaling pathways were significantly down-regulated in Foxg1 cKD mice cochlear SCs. Together, this study provides new evidence for the role of Foxg1 in regulating HC regeneration from SCs and progenitors in the neonatal mouse cochlea.

Mandible protraction alters Type I collagen, osteocalcin and osteonectin gene expression in adult mice condyle

Mandible condyle remodeling is a great challenge on craniofacial growth studies. The great majority of the reports deals with growing period. However, there is a great necessity to clarify the importance of functional stimulation on adult mandible condyle remodeling. By using an adult mouse model, we investigated the influence of mandible forwarding on condyle remodeling and gene expression by bone forming cells. Tomographic and scintigraphic evaluations showed sagittal growth and cell activity enhancement. RT-PCR showed that Type I collagen, osteocalcin and osteonectin expression level can be altered. We showed that functional stimulation is necessary to maintain the regular gene expression by condyle bone forming cells in adult mice. It opens new frame for further investigations aiming new clinical approaches to temporomandibular joint problems treatment, as well as mandible retrusion treatment.

Accelerated enamel mineralization in Dspp mutant mice

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.

Isolated dentinogenesis imperfecta and dentin dysplasia: revision of the classification

Dentinogenesis imperfecta is an autosomal dominant disease characterized by severe hypomineralization of dentin and altered dentin structure. Dentin extra cellular matrix is composed of 90% of collagen type I and 10% of non-collagenous proteins among which dentin sialoprotein (DSP), dentin glycoprotein (DGP) and dentin phosphoprotein (DPP) are crucial in dentinogenesis. These proteins are encoded by a single gene: dentin sialophosphoprotein (DSPP) and undergo several post-translational modifications such as glycosylation and phosphorylation to contribute and to control mineralization. Human mutations of this DSPP gene are responsible for three isolated dentinal diseases classified by Shield in 1973: type II and III dentinogenesis imperfecta and type II dentin dysplasia. Shield classification was based on clinical phenotypes observed in patient. Genetics results show now that these three diseases are a severity variation of the same pathology. So this review aims to revise and to propose a new classification of the isolated forms of DI to simplify diagnosis for practitioners.

Synthesis of bone-like nanocomposites using multiphosphorylated peptides

There is a great need for novel materials for mineralized tissue repair and regeneration. Two examples of such tissue, bone and dentin, are highly organized hierarchical nanocomposites in which mineral and organic phases interface at the molecular level. In contrast, current graft materials are either ceramic powders or physical blends of mineral and organic phases with mechanical properties far inferior to those of their target tissues. The objective of this study was to synthesize composite nanofibrils with highly integrated organic/inorganic phases inspired by the mineralized collagen fibrils of bone and dentin. Utilizing our understanding of bone and dentin biomineralization, we have first designed bioinspired peptides containing 3 Ser-Ser-Asp repeat motifs based on the highly phosphorylated protein, dentin phosphophoryn (DPP), found in dentin and alveolar bone. We demonstrate that up to 80% of serines in the peptide can be phosphorylated by casein kinases. We further tested the ability of these peptides to induce biomimetic calcium phosphate mineralization of collagen fibrils. Our mineralization studies have revealed that in the presence of these phosphorylated peptides, mineralized collagen fibrils structurally similar to the mineralized collagen fibrils of bone and dentin were formed. Our results demonstrate that using phosphorylated DPP-inspired peptides, we can successfully synthesize biomimetic composite nanofibrils with integrated organic and inorganic phases. These results provide the first step in the development of biomimetic nanostructured materials for mineralized tissue repair and regeneration using phosphopeptides.