Differential expression of syndecan isoforms during mouse incisor amelogenesis

An unexpected role of neurite outgrowth inhibitor A as regulator of tooth enamel formation

Neurite outgrowth inhibitor A (Nogo-A) is a major player in neural development and regeneration and the target of clinical trials aiming at promoting the regeneration of the central nervous system upon traumatic and ischemic injury. In this work, we investigated the functions of Nogo-A during tooth development to determine its role in dental physiology and pathology. Using immunohistochemistry and in situ hybridization techniques, we showed that Nogo-A is highly expressed in the developing mouse teeth and, most specifically, in the ameloblasts that are responsible for the formation of enamel. Using both Nogo-A knockout and K14-Cre;Nogo-A fl/fl transgenic mice, we showed that Nogo-A deletion in the dental epithelium leads to the formation of defective enamel. This phenotype is associated with overexpression of a set of specific genes involved in ameloblast differentiation and enamel matrix production, such as amelogenin, ameloblastin and enamelin. By characterising the interactome of Nogo-A in the dental epithelium of wild-type and mutant animals, we found that Nogo-A directly interacts with molecules important for regulating gene expression, and its deletion disturbs their cellular localisation. Furthermore, we demonstrated that inhibition of the intracellular, but not cell-surface, Nogo-A is responsible for gene expression modulation in ameloblasts. Taken together, these results reveal an unexpected function for Nogo-A in tooth enamel formation by regulating gene expression and cytodifferentiation events.

The significant role of glycosaminoglycans in tooth development

This review delves into the roles of glycosaminoglycans (GAGs), integral components of proteoglycans, in tooth development. Proteoglycans consist of a core protein linked to GAG chains, comprised of repeating disaccharide units. GAGs are classified into several types, such as hyaluronic acid, heparan sulfate, chondroitin sulfate, dermatan sulfate, and keratan sulfate. Functioning as critical macromolecular components within the dental basement membrane, these GAGs facilitate cell adhesion and aggregation, and play key roles in regulating cell proliferation and differentiation, thereby significantly influencing tooth morphogenesis. Notably, our recent research has identified the hyaluronan-degrading enzyme Transmembrane protein 2 (Tmem2) and we have conducted functional analyses using mouse models. These studies have unveiled the essential role of Tmem2-mediated hyaluronan degradation and its involvement in hyaluronan-mediated cell adhesion during tooth formation. This review provides a comprehensive summary of the current understanding of GAG functions in tooth development, integrating insights from recent research, and discusses future directions in this field.

Heparanase-Regulated Syndecan-1 Shedding Facilitates Herpes Simplex Virus 1 Egress

Herpes simplex virus 1 (HSV-1) can infect virtually all cell types in vitro An important reason lies in its ability to exploit heparan sulfate (HS) for attachment to cells. HS is a ubiquitous glycosaminoglycan located on the cell surface and tethered to proteoglycans such as syndecan-1. Previously, we have shown that heparanase (HPSE) facilitates the release of viral particles by cleaving HS. Here, we demonstrate that HPSE is a master regulator where, in addition to directly enabling viral release via HS removal, it also facilitates cleavage of HS-containing ectodomains of syndecan-1, thereby further enhancing HSV-1 egress from infected cells. Syndecan-1 cleavage is mediated by upregulation of matrix metalloproteases (MMPs) that accompanies higher HPSE expression in infected cells. By overexpressing HPSE, we have identified MMP-3 and MMP-7 as important sheddases of syndecan-1 shedding in corneal epithelial cells, which are natural targets of HSV-1 infection. MMP-3 and MMP-7 were also naturally upregulated during HSV-1 infection. Altogether, this paper shows a new connection between HSV-1 release and syndecan-1 shedding, a phenomenon that is regulated by HPSE and executed by the MMPs. Our results also identify new molecular markers for HSV-1 infection and new targets for future interventions.IMPORTANCE HSV-1 is a common cause of recurrent viral infections in humans. The virus can cause a range of mucosal pathologies. Efficient viral egress from infected cells is an important step for HSV-1 transmission and virus-associated pathologies. Host mechanisms that contribute to HSV-1 egress from infected cells are poorly understood. Syndecan-1 is a common heparan sulfate proteoglycan expressed by many natural target cells. Despite its known connection with heparanase, a recently identified mediator of HSV-1 release, syndecan-1 has not been previously investigated in HSV-1 release. In this study, we demonstrate that the shedding of syndecan-1 by MMP-3 and MMP-7 supports viral egress. We show that the mechanism behind the activation of these MMPs is mediated by heparanase, which is upregulated upon HSV-1 infection. Our study elucidates a new connection between HSV-1 egress, heparanase, and matrix metallopeptidases; identifies new molecular markers of infection; and provides potential new targets for therapeutic interventions.

The role of heparan sulphate in development: the ectodermal story

Heparan sulphate (HS) is ubiquitously expressed and is formed of repeating glucosamine and glucuronic/iduronic acid units which are generally highly sulphated. HS is found in tissues bound to proteins forming HS proteoglycans (HSPGs) which are present on the cell membrane or in the extracellular matrix. HSPGs influence a variety of biological processes by interacting with physiologically important proteins, such as morphogens, creating storage pools, generating morphogen gradients and directly mediating signalling pathways, thereby playing vital roles during development. This review discusses the vital role HS plays in the development of tissues from the ectodermal lineage. The ectodermal layer differentiates to form the nervous system (including the spine, peripheral nerves and brain), eye, epidermis, skin appendages and tooth enamel.

Distribution of syndecan-1 protein in developing mouse teeth

Syndecan-1 is a cell surface proteoglycan involved in the regulation of various biological processes such as proliferation, migration, condensation and differentiation of cells, intercellular communication, and morphogenesis. The extracellular domain of syndecan-1 can bind to extracellular matrix components and signaling molecules, while its intracellular domain interacts with cytoskeletal proteins, thus allowing the transfer of information about extracellular environment changes into the cell that consequently affect cellular behavior. Although previous studies have shown syndecan-1 expression during precise stages of tooth development, there is no equivalent study regrouping the expression patterns of syndecan-1 during all stages of odontogenesis. Here we examined the distribution of syndecan-1 protein in embryonic and post-natal developing mouse molars and incisors. Syndecan-1 distribution in mesenchymal tissues such as dental papilla and dental follicle was correlated with proliferating events and its expression was often linked to stem cell niche territories. Syndecan-1 was also expressed in mesenchymal cells that will differentiate into the dentin producing odontoblasts, but not in differentiated functional odontoblasts. In the epithelium, syndecan-1 was detected in all cell layers, by the exception of differentiated ameloblasts that form the enamel. Furthermore, syndecan-1 was expressed in osteoblast precursors and osteoclasts of the alveolar bone that surrounds the developing tooth germs. Taken together these results show the dynamic nature of syndecan-1 expression during odontogenesis and suggest its implication in various processes of tooth development and homeostasis.

Structure and functions of syndecans in vertebrates

Syndecans constitute a family of transmembrane proteoglycans that perform multiple functions during development, damage repair, tumor growth, angiogenesis, and neurogenesis. Through mediating binding of a great number of extracellular ligands to their receptors, these proteoglycans trigger a cascade of reactions regulating, thereby, various processes in a cell: cytoskeleton formation, proliferation, differentiation, adhesion, and migration. In fibroblasts, syndecans are responsible for cell adhesion by modulating functions of integrins through interaction with fibronectin at the external side of a cell and with cytoskeleton and signaling molecules inside the cell. The extracellular domain of syndecans is subjected to periodic shedding from the cell membrane. This process may be stimulated in response to inflammation, tissue damage, and other pathological manifestations. Cleaved domain may act as either competitive inhibitor or activator of signaling cascades. This review summarizes and analyzes the available data regarding structure, main biochemical properties, and functions of syndecans in vertebrates.

Expression and roles of syndecan-4 in dental epithelial cell differentiation

Syndecan-4 (SDC4), a transmembrane heparan sulfate proteoglycan, acts as a signal transducer. It affects the growth and differentiation of a number of tissues and organs. However, the specific mechanisms through which SDC4 regulates the differentiation of dental epithelial cells (amelogenesis) and tooth development remains largely unknown. In the present study, to identify the SDC4-regulated processes in dental epithelial cells, the SDC4 expression pattern was examined in mouse molar and postnatal incisor tooth germs during the late bell stage of development. Small interfering RNA (siRNA) was designed for this study and used to downregulate SDC4 expression in the rat dental epithelial cell line, HAT-7. The results revealed that SDC4 was mainly present in the oral epithelium, the dental epithelial cells of enamel organs in the molars and the cervical loops in the incisors. When the inner enamel epithelial cells gave rise to ameloblasts, however, the loss of SDC4 expression was evident. SDC4 was also expressed in stratum intermedium (SI) cells in the incisors and in dental mesenchymal cells adjacent to the cervical loops in molars (E18) and postnatal incisors. Fibroblast growth factor 10 (FGF10) promoted proliferation and slightly decreased cell differentiation. The knockdown of SDC4 using specific siRNA led to a decrease in cell proliferation and a highly significant increase in amelogenin, ameloblastin, kallikrein 4 and matrix metalloproteinase 20 expression, molecules that are known to participate in the formation of enamel. These effects were attenuated by FGF10, which upregulated SDC4 expression. Taken together, these results suggest that SDC4 participates in amelogenesis, and FGF10 may modulate dental epithelial cell behaviors through the regulation of SDC4 expression.

Expression of cytokeratin 8, vimentin, syndecan-1 and Ki-67 during human tooth development

Spatio-temporal immunolocalizations of cytokeratin 8 (CK8), vimentin, syndecan-1 and Ki-67 were analyzed in ten human incisors and canine tooth germs between the 7th and 20th developmental weeks. CK8 expression was mild to moderate in the epithelial tooth parts, while it shifted from absent or mild in its mesenchymal parts, but few cells, sparsely distributed throughout the tooth germ, strongly expressed CK8. As development progressed, CK8 expression increased to strong in preameloblasts, while expression of vimentin increased to moderate in the epithelial and mesenchymal tooth parts, particularly in the dental papilla and sac. Co-expression of CK8 and vimentin was observed in some parts of the tooth germ, and was increasing in the differentiating preameloblasts and preodontoblasts. Syndecan-1 showed characteristic shift of expression from epithelial to mesenchymal tooth parts, being particularly strong in dental papilla, sac and cervical loops, while co-expression of Ki-67/syndecan-1 was strong in the dental papilla. Our study demonstrated spatio-temporal expression and restricted co-expression of the investigated markers, indicating participation of CK8 and vimentin in cell proliferation and migration, and differentiation of preodontoblasts and preameloblasts. Our data also suggest involvement of syndecan-1 in morphogenesis of the developing tooth crown and cervical loops, and together with CK8 and vimentin in differentiation of preameloblasts and preodontoblasts.

Adenovirus gene transfer to amelogenesis imperfecta ameloblast-like cells

To explore gene therapy strategies for amelogenesis imperfecta (AI), a human ameloblast-like cell population was established from third molars of an AI-affected patient. These cells were characterized by expression of cytokeratin 14, major enamel proteins and alkaline phosphatase staining. Suboptimal transduction of the ameloblast-like cells by an adenovirus type 5 (Ad5) vector was consistent with lower levels of the coxsackie-and-adenovirus receptor (CAR) on those cells relative to CAR-positive A549 cells. To overcome CAR -deficiency, we evaluated capsid-modified Ad5 vectors with various genetic capsid modifications including “pK7” and/or “RGD” motif-containing short peptides incorporated in the capsid protein fiber as well as fiber chimera with the Ad serotype 3 (Ad3) fiber “knob” domain. All fiber modifications provided an augmented transduction of AI-ameloblasts, revealed following vector dose normalization in A549 cells with a superior effect (up to 404-fold) of pK7/RGD double modification. This robust infectivity enhancement occurred through vector binding to both α_vβ3/α_vβ5 integrins and heparan sulfate proteoglycans (HSPGs) highly expressed by AI-ameloblasts as revealed by gene transfer blocking experiments. This work thus not only pioneers establishment of human AI ameloblast-like cell population as a model for in vitro studies but also reveals an optimal infectivity-enhancement strategy for a potential Ad5 vector-mediated gene therapy for AI.

An important role for syndecan-1 in herpes simplex virus type-1 induced cell-to-cell fusion and virus spread

Herpes simplex virus type-1 (HSV-1) is a common human pathogen that relies heavily on cell-to-cell spread for establishing a lifelong latent infection. Molecular aspects of HSV-1 entry into host cells have been well studied; however, the molecular details of the spread of the virus from cell-to-cell remain poorly understood. In the past, the role of heparan sulfate proteoglycans (HSPG) during HSV-1 infection has focused solely on the role of HS chains as an attachment receptor for the virus, while the core protein has been assumed to perform a passive role of only carrying the HS chains. Likewise, very little is known about the involvement of any specific HSPGs in HSV-1 lifecycle. Here we demonstrate that a HSPG, syndecan-1, plays an important role in HSV-1 induced membrane fusion and cell-to-cell spread. Interestingly, the functions of syndecan-1 in fusion and spread are independent of the presence of HS on the core protein. Using a mutant CHO-K1 cell line that lacks all glycosaminoglycans (GAGs) on its surface (CHO-745) we demonstrate that the core protein of syndecan-1 possesses the ability to modulate membrane fusion and viral spread. Altogether, we identify a new role for syndecan-1 in HSV-1 pathogenesis and demonstrate HS-independent functions of its core protein in viral spread.

Syndecan-1 and syndecan-2 play key roles in herpes simplex virus type-1 infection

Herpes simplex virus type 1 (HSV-1) is an important human pathogen and a leading cause of infectious blindness in the developed world. HSV-1 exploits heparan sulfate proteoglycans (HSPG) for attachment to cells. While the significance of heparan sulphate (HS) moieties in HSV-1 infection is well established, the role of specific proteoglycan core proteins in the infection process remains poorly understood. The objective of this study was to assess the roles of syndecan-1 and syndecan-2 core proteins in HSV-1 infection, both of which are expressed by many HSV-1 target cell types. Our results demonstrate that syndecan-1 and syndecan-2 gene silencing by RNA interference reduces HSV-1 entry, plaque formation and facilitates cell survival. Furthermore, HSV-1 infection increases syndecan-1 and syndecan-2 protein synthesis and a resultant increase in cell surface expression of HS. Our observations suggest that changes in syndecan-1 and syndecan-2 expression levels may be related to active viral infection. Taken together, our findings provide new insights into HSPG functions during HSV-1 entry and spread.