Laminin gamma2 expression is developmentally regulated during murine tooth morphogenesis and is intense in ameloblasts

Phenotypic variability in LAMA3-associated amelogenesis imperfecta

OBJECTIVE

Amelogenesis imperfecta (AI) is defined as inherited enamel malformations. LAMA3 (laminin alpha-3) encodes a critical protein component of the basement membrane (laminin-332). Individuals carrying heterozygous LAMA3 mutations have previously been shown to have localized enamel defects. This study aimed to define clinical phenotypes and to discern the genetic etiology for four AI kindreds.

MATERIALS AND METHODS

Whole-exome analyses were conducted to search for sequence variants associated with the disorder, and micro-computed tomography (μCT) to characterize the enamel defects.

RESULTS

The predominant enamel phenotype was generalized thin enamel with defective pits and grooves. Horizonal bands of hypoplastic enamel with chalky-white discoloration and enamel hypomineralization were also observed and demonstrated by μCT analyses of affected teeth. Four disease-causing LAMA3 mutations (NM_198129.4:c.3712dup; c.5891dup; c.7367del; c.9400G > C) were identified. Compound heterozygous MMP20 mutations (NM_004771.4:c.539A > G; c.692C > T) were also found in one proband with more severe enamel defects, suggesting a mutational synergism on disease phenotypes. Further analyses of the AI-causing mutations suggested that both α3A (short) and α3B (long) isoforms of LAMA3 are essential for enamel formation.

CONCLUSIONS

Heterozygous LAMA3 mutations can cause generalized enamel defects (AI1A) with variable expressivity. Laminin-332 is critical not only for appositional growth but also enamel maturation.

ADAM10: Possible functions in enamel development

ADAM10 is A Disintegrin And Metalloproteinase (ADAM) family member that is membrane bound with its catalytic domain present on the cell surface. It is a sheddase that cleaves anchored cell surface proteins to shed them from the cell surface. ADAM10 can cleave at least a hundred different proteins and is expressed in most tissues of the body. ADAM10 is best characterized for its role in Notch signaling. Interestingly, ADAM10 is transported to specific sites on the cell surface by six different tetraspanins. Although the mechanism is not clear, tetraspanins can regulate ADAM10 substrate specificity, which likely contributes to the diversity of ADAM10 substrates. In developing mouse teeth, ADAM10 is expressed in the stem cell niche and subsequently in pre-ameloblasts and then secretory stage ameloblasts. However, once ameloblasts begin transitioning into the maturation stage, ADAM10 expression abruptly ceases. This is exactly when ameloblasts stop their movement that extends enamel crystallites and when the enamel layer reaches its full thickness. ADAM10 may play an important role in enamel development. ADAM10 can cleave cadherins and other cell-cell junctions at specific sites where the tetraspanins have transported it and this may promote cell movement. ADAM10 can also cleave the transmembrane proteins COL17A1 and RELT. When either COL17A1 or RELT are mutated, malformed enamel may occur in humans and mice. So, ADAM10 may also regulate these proteins that are necessary for proper enamel development. This mini review will highlight ADAM10 function, how that function is regulated by tetraspanins, and how ADAM10 may promote enamel formation.

A genetic model for the secretory stage of dental enamel formation

The revolution in genetics has rapidly increased our knowledge of human and mouse genes that are critical for the formation of dental enamel and helps us understand how enamel evolved. In this graphical review we focus on the roles of 41 genes that are essential for the secretory stage of amelogenesis when characteristic enamel mineral ribbons initiate on dentin and elongate to expand the enamel layer to the future surface of the tooth. Based upon ultrastructural analyses of genetically modified mice, we propose a molecular model explaining how a cell attachment apparatus including collagen 17, α6ß4 and αvß6 integrins, laminin 332, and secreted enamel proteins could attach to individual enamel mineral ribbons and mold their cross-sectional dimensions as they simultaneously elongate and orient them in the direction of the retrograde movement of the ameloblast membrane.

Expression and localization of amelotin, laminin γ2 and odontogenesis-associated phosphoprotein (ODAPH) on the basal lamina and junctional epithelium

At maturation stage of enamel development, a specialized basal lamina (sBL) was built between ameloblasts and enamel. After the teeth eruption, the ameloblasts transform into the inner cell layer of junctional epithelium. The inner cell layer forms the internal basal lamina of junctional epithelium. However, the composition of the sBL and internal basal lamina was not clarified. The objective of our study was to make a description of the localization of amelotin (AMTN), laminin γ2 (LAMC2) and Odontogenesis-associated phosphoprotein (ODAPH) on the sBL and internal basal lamina. In immunohistochemical study, AMTN, LAMC2 and ODAPH were detected on the sBL at maturation stage. AMTN was also detected in ameloblasts at maturation stage. The expression of AMTN decreased from early-to-late maturation stage. In contrast, the expression of LAMC2 and ODAPH was stable. Immunofluorescence double-staining showed the localization of AMTN was close to enamel surface. However, the localization of ODAPH was close to ameloblasts. LAMC2 and ODAPH were observed on internal basal lamina of junctional epithelium. In contrast, no expression of AMTN was detected on internal basal lamina of junctional epithelium. Our results suggested that ODAPH might participate in enamel maturation and periodontal health, which might provide a better understanding of enamel defects and periodontal disease in clinic.

Odontogenesis-associated phosphoprotein truncation blocks ameloblast transition into maturation in OdaphC41*/C41* mice

Mutations of Odontogenesis-Associated Phosphoprotein (ODAPH, OMIM *614829) cause autosomal recessive amelogenesis imperfecta, however, the function of ODAPH during amelogenesis is unknown. Here we characterized normal Odaph expression by in situ hybridization, generated Odaph truncation mice using CRISPR/Cas9 to replace the TGC codon encoding Cys41 into a TGA translation termination codon, and characterized and compared molar and incisor tooth formation in Odaph^+/+, Odaph^+/C41*, and Odaph^C41*/C41* mice. We also searched genomes to determine when Odaph first appeared phylogenetically. We determined that tooth development in Odaph^+/+ and Odaph^+/C41* mice was indistinguishable in all respects, so the condition in mice is inherited in a recessive pattern, as it is in humans. Odaph is specifically expressed by ameloblasts starting with the onset of post-secretory transition and continues until mid-maturation. Based upon histological and ultrastructural analyses, we determined that the secretory stage of amelogenesis is not affected in Odaph^C41*/C41* mice. The enamel layer achieves a normal shape and contour, normal thickness, and normal rod decussation. The fundamental problem in Odaph^C41*/C41* mice starts during post-secretory transition, which fails to generate maturation stage ameloblasts. At the onset of what should be enamel maturation, a cyst forms that separates flattened ameloblasts from the enamel surface. The maturation stage fails completely.

Epidermolysis bullosa: Molecular pathology of connective tissue components in the cutaneous basement membrane zone

Epidermolysis bullosa (EB), a group of heritable skin fragility disorders, is characterized by blistering, erosions and chronic ulcers in the skin and mucous membranes. In some forms, the blistering phenotype is associated with extensive mutilating scarring and development of aggressive squamous cell carcinomas. The skin findings can be associated with extracutaneous manifestations in the ocular as well as gastrointestinal and vesico-urinary tracts. The phenotypic heterogeneity reflects the presence of mutations in as many as 20 different genes expressed in the cutaneous basement membrane zone, and the types and combinations of the mutations and their consequences at the mRNA and protein levels contribute to the spectrum of severity encountered in different subtypes of EB. This overview highlights the molecular genetics of EB based on mutations in the genes encoding type VII and XVII collagens as well as laminin-332. The mutations identified in these protein components of the extracellular matrix attest to their critical importance in providing stability to the cutaneous basement membrane zone, with implications for heritable and acquired diseases.

Interactions of AMTN, ODAM and SCPPPQ1 proteins of a specialized basal lamina that attaches epithelial cells to tooth mineral

A specialized basal lamina (sBL) mediates adhesion of certain epithelial cells to the tooth. It is distinct because it does not contain collagens type IV and VII, is enriched in laminin-332, and includes three novel constituents called amelotin (AMTN), odontogenic ameloblast-associated (ODAM), and secretory calcium-binding phosphoprotein proline-glutamine rich 1 (SCPPPQ1). The objective of this study was to clarify the structural organization of the sBL. Fluorescence and immunogold labeling showed that the three proteins co-localize. Quantitative analysis of the relative position of gold particles on the sBL demonstrates that the distribution of ODAM is skewed towards the cell while that of AMTN and SCPPPQ1 tends towards the tooth surface. Bacterial two-hybrid analysis and co-immunoprecipitation, gel filtration of purified proteins and transmission electron and atomic force microscopies highlight the propensity of AMTN, ODAM, and SCPPPQ1 to interact with and among themselves and form supramolecular aggregates. These data suggest that AMTN, ODAM and SCPPPQ1 participate in structuring an extracellular matrix with the distinctive capacity of attaching epithelial cells to mineralized surfaces. This unique feature is particularly relevant for the adhesion of gingival epithelial cells to the tooth surface, which forms a protective seal that is the first line of defense against bacterial invasion.

Laminin γ2 knockout mice rescued with the human protein exhibit enamel maturation defects

The epithelial ameloblasts are separated from the maturing enamel by an atypical basement membrane (BM) that is enriched in laminin 332 (LM-332). This heterotrimeric protein (α3, ß3 and γ2 chains) provides structural integrity to BMs and influences various epithelial cell processes including cell adhesion and differentiation. Mouse models that lack expression of individual LM-332 chains die shortly after birth. The lethal phenotype of laminin γ2 knockout mice can be rescued by human laminin γ2 (LAMC2) expressed using a doxycycline-inducible (Tet-on) cytokeratin 14 promoter-rtTA. These otherwise normal-looking rescued mice exhibit white spot lesions on incisors. We therefore investigated the effect of rescue with human LAMC2 on enamel maturation and structuring of the atypical BM. The maturation stage enamel organ in transgenic mice was severely altered as compared to wild type controls, a structured BM was no longer discernible, dystrophic matrix appeared in the maturing enamel layer, and there was residual enamel matrix late into the maturation stage. Microtomographic scans revealed excessive wear of occlusal surfaces on molars, chipping of enamel on incisor tips, and hypomineralization of the enamel layer. No structural alterations were observed at other epithelial sites, such as skin, palate and tongue. These results indicate that while this humanized mouse model is capable of rescue in various epithelial tissues, it is unable to sustain structuring of a proper BM at the interface between ameloblasts and maturing enamel. This failure may be related to the atypical composition of the BM in the maturation stage and reaffirms that the atypical BM is essential for enamel maturation.

Whole-exome sequencing, without prior linkage, identifies a mutation in LAMB3 as a cause of dominant hypoplastic amelogenesis imperfecta

The conventional approach to identifying the defective gene in a family with an inherited disease is to find the disease locus through family studies. However, the rapid development and decreasing cost of next generation sequencing facilitates a more direct approach. Here, we report the identification of a frameshift mutation in LAMB3 as a cause of dominant hypoplastic amelogenesis imperfecta (AI). Whole-exome sequencing of three affected family members and subsequent filtering of shared variants, without prior genetic linkage, sufficed to identify the pathogenic variant. Simultaneous analysis of multiple family members confirms segregation, enhancing the power to filter the genetic variation found and leading to rapid identification of the pathogenic variant. LAMB3 encodes a subunit of Laminin-5, one of a family of basement membrane proteins with essential functions in cell growth, movement and adhesion. Homozygous LAMB3 mutations cause junctional epidermolysis bullosa (JEB) and enamel defects are seen in JEB cases. However, to our knowledge, this is the first report of dominant AI due to a LAMB3 mutation in the absence of JEB.

Odontogenic ameloblast-associated and amelotin are novel basal lamina components

Odontogenic ameloblast-associated (ODAM) and amelotin (AMTN) are secreted by maturation stage ameloblasts and accumulate at the interface with enamel where an atypical basal lamina (BL) is present. This study aimed at determining and quantifying the ultrastructural distribution of ODAM and AMTN at the cell-tooth interface. Ultrathin sections of enamel organs from the early to mid- and late maturation stage of amelogenesis were processed for immunogold labeling with antibodies against ODAM, AMTN or with the lectins wheat germ agglutinin, Helix pomatia agglutinin (HPA) and Ricinus communis I agglutinin. Immunolabeling showed that both ODAM and AMTN localized to the BL. Quantitative analyses indicated that at the beginning of maturation there is a concentration of ODAM on the cell side of the BL while AMTN appears more concentrated on the enamel side. In the late maturation stage, such differential distribution is no longer apparent. All three lectins are bound to the BL. Competitive incubation with native lectins did not affect the binding efficiency of ODAM; however, AMTN binding was significantly reduced after incubation with HPA. In conclusion, ODAM and AMTN are bona fide components of the BL associated with maturation stage ameloblasts and they organize into different subdomains during the early maturation stage. The data also suggest that the BL is a dynamic structure that rearranges its organization as enamel maturation advances. Finally, the abrogation of AMTN antibody labeling by HPA supports the presence of O-linked sugars in the molecule and/or its close association with other O-glycosylated molecules.

Misnomers in oral pathology

BACKGROUND

Diseases which involve the oral cavity usually derive their names from either Greek or Latin. These terms are customarily based on etiology or description of the lesion. However, because of various reasons, some of these terms are misnomers.

OBJECTIVE

To review commonly encountered misnomers in oral pathology.

CONCLUSIONS

Most of the misnomers encountered in oral pathology may arise from lack of understanding of underlying etiology, pathogenesis, histopathology, and/or concepts. Some misnomers are due to imprecise translations from word origins, etymological bungles, and/or factual errors. Clinical, histopathological, and/or etymological explanations are used to analyze and elucidate the nature of these misnomers. Alternative terms, where possible, have been suggested.

Bioactive nanofibers instruct cells to proliferate and differentiate during enamel regeneration

During tooth development, ectoderm-derived ameloblast cells create enamel by synthesizing a complex protein mixture serving to control cell to matrix interactions and the habit of hydroxyapatite crystallites. Using an in vitro cell and organ culture system, we studied the effect of artificial bioactive nanostructures on ameloblasts with the long-term goal of developing cell-based strategies for tooth regeneration. We used branched peptide amphiphile molecules containing the peptide motif Arg-Gly-Asp, or "RGD" (abbreviated BRGD-PA), known to self-assemble in physiologic environments into nanofibers that display on their surfaces high densities of this biological signal. Ameloblast-like cells (line LS8) and primary enamel organ epithelial (EOE) cells were cultured within PA hydrogels, and the PA was injected into the enamel organ epithelia of mouse embryonic incisors. The expression of amelogenin, ameloblastin, integrin alpha 5, and integrin alpha 6 was detected by quantitative real-time PCR and immunodetection techniques. We performed cell proliferation assay using BrdU labeling and a biomineralization assay using Alizarin red S staining with quantitative Ca(2+) measurements. In the cell culture model, ameloblast-like cells (LS8) and primary EOE cells responded to the BRGD-PA nanostructures with enhanced proliferation and greater amelogenin, ameloblastin, and integrin expression levels. At the site of injection of the BRGD-PA in the organ culture model, we observed EOE cell proliferation with differentiation into ameloblasts as evidenced by their expression of enamel specific proteins. Ultrastructural analysis showed the nanofibers within the forming extracellular matrix, in contact with the EOE cells engaged in enamel formation and regeneration. This study shows that BRGD-PA nanofibers present with enamel proteins participate in integrin-mediated cell binding to the matrix with delivery of instructive signals for enamel formation.

Junctional epidermolysis bullosa associated with hypoplastic enamel and pervasive failure of tooth eruption: Oral rehabilitation with use of an overdenture

This report describes a previously unreported case of generalized hypoplastic enamel and failure of eruption of the permanent maxillary teeth and only partial eruption of the permanent mandibular teeth in an 18-year-old male diagnosed with junctional epidermolysis bullosa. Similar anomalies were reported to have affected the deciduous dentition. Beginning at 4 years of age, oral rehabilitation has been conservatively managed with the fabrication of various maxillary complete overdentures. The use of this prosthesis has provided an economical, nonsurgical treatment option when oral soft tissue permits and with relative ease of construction.

Ultrastructure and composition of basement membrane separating mature ameloblasts from enamel

At a late stage of amelogenesis, a basement-membrane-like (BML) structure appears between mature ameloblasts and the enamel surface. Although this BML structure is known to contain certain basement membrane components, its detailed nature and role were not well defined. As such, this study examined the BML structure using high-resolution electron microscopy combined with immunohistochemical staining. Mandibular rat incisors were processed for the preparation of Epon sections for ultrastructural observations, and frozen sections were used for immunostaining laminin, heparan sulphate proteoglycan (HSPG) and type IV collagen. The BML structure was characterized by the presence of abundant ribbon-like 'double tracks', 4.5-5.0 nm wide; the form known to be taken by HSPG in basal laminae. The main ultrastructural component of basal laminae, known as 'cords', was replaced by fine filaments of type IV collagen. Immunohistochemical staining of the BML structure showed an intense reaction for HSPG, moderate staining for type IV collagen and negligible staining for laminin. These observations indicate that this structure is an atypical basement membrane in which the cord network is replaced by type IV collagen filaments. However, the BML structure was found to be unusually rich in HSPG, similar to kidney glomerular basement membrane. It is likely that this specialized basement membrane mediates firm attachment of mature ameloblasts to the enamel surface, and filters the influx and efflux of materials to and from enamel during maturation.

Biological function of laminin-5 and pathogenic impact of its deficiency

The basement membrane glycoprotein laminin-5 is a key component of the anchoring complex connecting keratinocytes to the underlying dermis. It is secreted by keratinocytes as a cross-shaped heterotrimer of alpha3, beta3 and gamma2 chains and serves as a ligand of various transmembrane receptors, thereby regulating keratinocyte adhesion, motility and proliferation. In intact skin, laminin-5 provides essential links to both the hemidesmosomal alpha6beta4 integrin and the collagen type VII molecules which form the anchoring fibrils inserting into the dermis. If the basement membrane is injured, laminin-5 production increases rapidly. It then serves as a scaffold for cell migration, initiates the formation of hemidesmosomes and accelerates basement membrane restoration at the dermal-epidermal junction. Mutations of the laminin-5 genes or auto-antibodies against one of the subunits of laminin-5 may lead to a significant lack of this molecule in the epidermal basement membrane zone. The major contributions of laminin-5 to the resistance of the epidermis against frictional stress but also for basement membrane regeneration and repair of damaged skin are reflected by the phenotype of Herlitz junctional epidermolysis bullosa, which is caused by an inherited absence of functional laminin-5. This lethal disease becomes manifest in widespread blistering of skin and mucous membranes, impaired wound healing and chronic erosions containing exuberant granulation tissue. Here, we discuss current understanding of the biological functions of laminin-5, the pathogenic impact of its deficiency and implications on molecular approaches towards a therapy of junctional epidermolysis bullosa.

Expression of basement membrane components in odontogenic tumors

OBJECTIVE

The objective was to characterize the expression of BMCs (laminins 1 and 5, collagen type IV, and fibronectin) in ameloblastomas, calcifying cystic odontogenic tumors (CCOTs), and adenomatoid odontogenic tumors (AOTs).

STUDY DESIGN

BMCs were analyzed in 14 ameloblastomas, 7 CCOTs, and 7 AOTs using immunohistochemistry.

RESULTS

In normal oral mucosa, linear deposits of these proteins were found at the epithelial-mesenchymal junction, but not in epithelial cytoplasm. In all tumors studied, linear deposits of all proteins were found at the epithelial-mesenchymal junction; laminin 1 was expressed in all tumor cells, regardless of cell types. For CCOTs, laminin 5 was found faintly in suprabasal cells, but expressed strongly in ghost cells. For AOTs, laminin 5 strongly decorated tumor cells adjacent to mineralization.

CONCLUSIONS

Laminin 1 may be a marker for odontogenic epithelium. Additionally, laminin 5 may be involved in ghost cell formation and initiation of calcification.

Rat wct mutation induces a hypo-mineralization form of amelogenesis imperfecta and cyst formation in molar teeth

Our previous findings have demonstrated that the rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induces enamel defects resembling those of human amelogenesis imperfecta (AI) in continuously growing incisor teeth. The present study clarifies the effect of the wct mutation on the morphogenesis and calcification of rat molar teeth. Formalin-fixed maxillae obtained from animals aged 4-30 days were examined by electron probe micro-analysis (EPMA) and by immunocytochemistry for amelogenin, ameloblastin, and enamelin. There were no distinct differences in the calcium and phosphorous contents and the amount of enamel between homozygous mutant and wild-type teeth during postnatal days 4-11. Although the mineral density in the enamel matrix considerably increased in the wild-type teeth until day 15, no changes occurred in mutant teeth during days 11-30. The immunoreactivity for enamel proteins in the secretory-stage ameloblasts in mutant teeth was similar to that in the wild-type teeth, and subsequently mutant maturation-stage ameloblasts became detached from the enamel surface, resulting in odontogenic cyst formation between the enamel organ and matrix until day 7 and the expansion of the cyst around the whole tooth crown on day 15. On day 30, the erupted mutant teeth presented morphological changes such as enamel destruction and tertiary dentin formation in addition to low mineral density in the enamel. Thus, the wct mutation prevents mineral transport without disturbing the synthesis of enamel proteins in molar teeth because of the absence of maturation-stage ameloblasts, in addition to the occurrence of odontogenic cysts.

Rat wct mutation prevents differentiation of maturation-stage ameloblasts resulting in hypo-mineralization in incisor teeth

A recent study provided genetic and morphological evidence that rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induced tooth enamel defects resembling those of human amelogenesis imperfecta (AI). The wct locus maps to a specific interval of rat chromosome 14 corresponding to human chromosome 4q21 where the ameloblastin and enamelin genes exist, although these genes are not included in the wct locus. The effect of the wct gene mutation on the enamel matrix synthesis and calcification remains to be elucidated. This study clarifies how the wct gene mutation influences the synthesis of enamel matrix and its calcification by immunocytochemistry for amelogenin, ameloblastin and enamelin, and by electron probe micro-analysis (EPMA). The immunoreactivity for enamel proteins such as amelogenin, ameloblastin, and enamelin in the ameloblasts in the homozygous teeth was the same as that in the heterozygous teeth from secretory to transitional stages, although the homozygous ameloblasts became detached from the enamel matrix in the transitional stage. The flattened ameloblasts in the maturation stage of the homozygous samples contained enamel proteins in their cytoplasm. Thus, the wct mutation was found to prevent the morphological transition of ameloblasts from secretory to maturation stages without disturbing the synthesis of enamel matrix proteins, resulting in the hypo-mineralization of incisor enamel and cyst formation between the enamel organ and matrix. This mutation also prevents the transfer of iron into the enamel.

Overexpression of transforming growth factor-beta1 in teeth results in detachment of ameloblasts and enamel defects

Transforming growth factor-beta1 (TGF-beta1) is a key regulator of many cellular processes, including cell adhesion, the immune response and synthesis of extracellular matrix proteins. In the present study, we report the characterization of enamel defects in a transgenic mouse model overexpressing TGF-beta1 in odontoblasts and ameloblasts, its expression being driven by the promoter sequences of the dentin sialophosphoprotein gene. As reported earlier, these mice develop distinct dentin defects similar to those seen in human dentin dysplasia and dentinogenesis imperfecta. A further detailed examination of enamel in these mice revealed that from the early secretory stage, ameloblasts began to detach from dentin to form cyst-like structures. A soft X-ray analysis revealed that this cyst-like structure had a disorganized and partially mineralized matrix with an abnormal mineralization pattern and a globular appearance. In the molars, the enamel was not only pitted and hypoplastic, but enamel rods were completely lost. Thus, altered TGF-beta1 expression in the tooth seems to trigger detachment of ameloblasts and abnormal secretion and deposition of minerals in the cyst-like structures adjoining the dentin. We speculate that the altered expression of TGF-beta1 in teeth impacts the adhesion process of ameloblasts to dentin.

Expression of basement membrane components in odontogenic cysts

OBJECTIVE

To compare the expression of basement membrane components (BMCs), including laminins 1 and 5, collagen type IV, and fibronectin in odontogenic keratocysts (OKCs) with dentigerous cysts (DCs) and radicular cysts (RCs).

MATERIALS AND METHODS

Basement membrane components were analysed in 20 OKCs, 20 DCs and 20 RCs using an immunohistochemical technique.

RESULTS

Odontogenic keratocysts, DCs and RCs showed positive reaction to all BMCs studied, with different distributions and intensity. OKCs showed continuous linear deposits for laminins 1 and 5 but two staining patterns (continuous and discontinuous) for collagen type IV and fibronectin. DCs exhibited continuous linear deposits for laminins 1 and 5 and collagen type IV but a discontinuous linear deposit for fibronectin. RCs displayed similar results to DCs for laminin 1, collagen type IV and fibronectin. Laminin 5 in RCs had two staining patterns. Constant results in all cysts were strong intensity for laminin 1 and moderate intensity for laminin 5.

CONCLUSIONS

Substantial differences in the expression of BMCs among studied cysts were not observed, suggesting that the separation of the epithelial lining in OKCs is not associated with the existence of these proteins.

Urokinase-type Plasminogen Activator mRNA is Expressed in Normal Developing Teeth and Leads to Abnormal Incisor Enamel in αMUPA Transgenic Mice

The urokinase-type plasminogen activator (uPA) is a secreted, inducible serine protease implicated in extracellular proteolysis and tissue remodeling. Here we detected uPA mRNA through in situ hybridization in developing molar and incisor teeth of normal mice at multiple sites of the cap and bell developmental stages. The mRNA was confined to epithelial cells, however, was undetectable in ameloblasts or their progenitor preameloblasts and the inner enamel epithelium. Furthermore, mice of five lines of previously described alphaMUPA transgenic mice, carrying a transgene consisting of the uPA cDNA linked downstream from the alphaA-crystallin promoter, overexpressed uPA mRNA in the same epithelial sites. In addition, alphaMUPA mice showed remarkably high levels of uPA mRNA in ameloblasts, however, exclusively in two specific sites late in incisor development. First, at the late secretory stage, but only on sides of the ameloblast layer. Second, in a limited zone of ameloblasts near the incisal end, coinciding with a striking morphological change of the ameloblast layer and the enamel matrix. In adult alphaMUPA mice, the incisor teeth displayed discoloration and tip fragility, and reduction of the outer enamel as determined by scanning electron microscopy. These results suggest that balanced uPA activity could play a role in normal tooth development. The alphaMUPA tooth phenotype demonstrates a remarkable sensitivity to excessive extracellular proteolysis at the incisor maturation stage of amelogenesis.

Laminins 2 (α2β1γ1, Lm-211) and 8 (α4β1γ1, Lm-411) are synthesized and secreted by tooth pulp fibroblasts and differentially promote neurite outgrowth from trigeminal ganglion sensory neurons

The tooth pulp innervation originates from the trigeminal ganglion (TG) and represents an illustrative example of tissue targeting by sensory nerves. Pulpal fibroblasts strongly promote neurite outgrowth from TG neurons in vitro. In the present study, we have investigated the possible participation of laminins (LNs), potent neuritogenic extracellular matrix components. Immunohistochemistry of human tooth pulp demonstrated expression of LN alpha1, alpha2, alpha4, alpha5, beta1 and gamma1, and laminin-binding integrin alpha3, alpha6, beta1 and beta4 chains in nerves. Though faintly stained for laminins in situ, pulpal fibroblasts reacted, once cultured and permeabilized, with antibodies to LN alpha2, alpha4, beta1 and gamma1 chains by flow cytometry. The cells also expressed the corresponding mRNAs and were able to assemble and secrete LN-2 (alpha2beta1gamma1, Lm-211) and LN-8 (alpha4beta1gamma1, Lm-411). LN-8 displayed a chondroitin sulphate (CS) modification in its alpha4 chain. In functional assays, mouse LN-1 (alpha1beta1gamma1, Lm-111) and recombinant human (rh) LN-8, but not native or rhLN-2, strongly promoted neurite outgrowth from TG neurons, mimicking the effect of cultured pulp fibroblast. Altogether, the results indicate that LN-2 and LN-8 are synthesized by tooth pulp fibroblasts and differentially promote neurite outgrowth from TG neurons. LN-8 may contribute to sensory innervation of teeth and other tissues during development and/or regeneration.

Animal models for skin blistering conditions: absence of laminin 5 causes hereditary junctional mechanobullous disease in the Belgian horse

Recent achievements in the genetic correction of keratinocytes isolated from patients with junctional epidermolysis bullosa have paved the way to a gene therapy approach for the disease. Because gene therapy protocols require preclinical validation in animals, we have characterized spontaneous animal models of junctional epidermolysis bullosa. In this study we have elucidated the genetic basis of the hereditary junctional mechanobullous disease in the Belgian horse, a condition characterized by blistering of the skin and mouth epithelia, and exungulation (loss of the hoof). Immunofluorescence analysis associated the condition to the absent expression of the gamma2 chain of laminin 5 and designated Lamc2 as the candidate gene. Comparative analysis of the nucleotide sequence of the full-length gamma2 cDNA isolated by reverse transcription polymerase chain reaction amplification of total RNA purified from the epithelium of a junctional epidermolysis bullosa foal and a healthy control disclosed a homozygous basepair insertion (1368insC) in the affected animal. Mutation 1368insC results in a downstream premature termination codon and is predicted to cause absent expression of the laminin gamma2 polypeptide. Our results also show that: (i) the horse junctional epidermolysis bullosa genetically corresponds to the severe Herlitz form of junctional epidermolysis bullosa in man; (ii) the amino acid sequence and structure of the horse laminin gamma2 chain are virtually identical to the human counterpart; (iii) the moderate eruption of skin blisters in the affected animals with respect to the human Herlitz junctional epidermolysis bullosa patients correlates with the protection provided by hair. Our observations suggest that the affected foals are a convenient source of epithelial cells from tissues that cannot be obtained from human junctional epidermolysis bullosa patients, and imply that hairless strains of animals with recessive skin disorders would be the best models for in vivo gene therapy approaches to skin blistering diseases.

A disease-associated glycine substitution in BP180 (type XVII collagen) leads to a local destabilization of the major collagen triple helix

BP180 is a homotrimeric transmembrane protein with a carboxy-terminal ectodomain that forms an interrupted collagen triple helix. Null type mutations in the BP180 gene produce a recessive subepidermal blistering disease, non-Herlitz junctional epidermolysis bullosa. Like the null mutations, a glycine substitution (G627V) within the longest BP180 collagenous domain (COL15) is also associated with the recessive skin disease; however, unlike the null mutations, this glycine substitution appears to act in a dominant fashion to give rise to a novel form of random pitting dental enamel hypoplasia. The dominant effects of this mutation were thought to be due to alterations in the assembly and/or stability of this BP180 collagenous region. To further investigate this issue, a structural analysis was performed on recombinant forms of the wild type and G627V mutant BP180 ectodomain. Both proteins were found to form collagen-like triple helices with very similar Stokes radii and melting temperatures and exhibited very similar rates of synthesis, secretion and turn-over. Tryptic digestion analysis revealed that the mutant G627V-sec180e contains an additional highly sensitive proteolytic site that maps within the region of the mutation. Thus, the disease-associated G627V mutation in BP180 does not grossly alter protein structure, but causes a local destabilization of the triple-helix that exposes sensitive residues to the in vitro effects of trypsin and possibly affects its structure-function in vivo.

Effects of fetal exposure to nicotine on dental development of the laboratory rat

Nicotine is one of the most widely used toxins in the world today. Most addiction research relating to nicotine in particular, as well as opioids and alcohol, has concentrated on the cellular and molecular biology of the mammalian brain and on features of organ structure and physiology associated with substance abuse. Thus, while numerous studies have been conducted to examine nicotine's detrimental physiological effects in a variety of soft tissues, this investigation attempts to examine further the gross morphological consequences of this drug on a hard tissue, the first molar crown of the laboratory rat. It is hypothesised that by providing nicotine to rats during and after the fetal cycle, changes in dental structure will occur, owing to perturbations of development induced by this toxin. The dentitions of Fisher rats exposed to nicotine during and after the fetal cycle, and those of their non-treated controls, were examined. By carefully measuring the length, width and occlusal (chewing) areas of the first maxillary molars, it was possible to identify any gross morphological effects of nicotine on dental development. It was found that dental asymmetries (calculated as a size difference between a tooth and its antimere) were significantly increased while occlusal areas were significantly decreased in nicotine-exposed rats compared to control rats. In addition, significant differences were detected within the experimental group, females tending to exhibit the deleterious effects of nicotine more so than males. These results are in accordance with the predicted outcome; in similar studies of physiological systems and soft tissues, dental development is affected by the presence of nicotine.

Expression of laminin-5 in ameloblastomas and human fetal teeth

Extracellular matrix proteins have been shown to play important roles in the cell migration and differentiation in both normal and pathological conditions. In the present study, we used immunohistochemistry and in situ hybridization to determine the distribution of laminin-5 in ameloblastomas and developing human teeth. In ameloblastomas, the immunoreaction for the laminin-5 gamma2 chain was confined to the tumor cells of the peripheral area. The staining reaction was variable, being mostly weak and fragmented in the basement membrane structures surrounding the neoplastic islands. Some peripheral epithelial cells and some invading small ameloblastoma cell islands showed intense intracellular staining for the gamma2 chain. Tumor cells in the proliferating areas of ameloblastomas expressed gamma2 chain mRNA. The laminin-5 gamma2 chain was located beneath the dental lamina and in the outer, but not in the inner, enamel epithelium of the developing teeth. During the early hard tissue apposition stage, intense staining for the gamma2 chain was confined to ameloblasts, which also gave a strong signal for gamma2 chain mRNA. These results suggest that laminin-5 may contribute to the infiltrative and progressive growing potential of ameloblastomas. During human tooth development, however, laminin-5 may participate in the terminal differentiation of ameloblasts and in enamel matrix formation.

Extracellular proteolysis alters tooth development in transgenic mice expressing urokinase-type plasminogen activator in the enamel organ

By catalyzing plasmin formation, the urokinase-type plasminogen activator (uPA) can generate widespread extracellular proteolysis and thereby play an important role in physiological and pathological processes. Dysregulated expression of uPA during organogenesis may be a cause of developmental defects. Targeted epithelial expression of a uPA-encoding transgene under the control of the keratin type-5 promoter resulted in enzyme production by the enamel epithelium, which does not normally express uPA, and altered tooth development. The incisors of transgenic mice were fragile, chalky-white and, by scanning electron microscopy, their labial surface appeared granular. This phenotype was attributed to a defect in enamel formation during incisor development, resulting from structural and functional alterations of the ameloblasts that differentiate from the labial enamel epithelium. Immunofluorescence revealed that disorganization of the ameloblast layer was associated with a loss of laminin-5, an extracellular matrix molecule mediating epithelial anchorage. Amelogenin, a key protein in enamel formation, was markedly decreased at the enamel-dentin junction in transgenics, presumably because of an apparent alteration in the polarity of its secretion. In addition, increased levels of active transforming growth factor-beta could be demonstrated in mandibles of transgenic mice. Since the alterations detected could be attributed to uPA catalytic activity, this model provides evidence as to how dysregulated proteolysis, involving uPA or other extracellular proteases, may have developmental consequences such as those leading to enamel defects.