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

Novel LRAP-binding partner revealing the plasminogen activation system as a regulator of cementoblast differentiation and mineral nodule formation in vitro

Amelogenin isoforms, including full-length amelogenin (AMEL) and leucine-rich amelogenin peptide (LRAP), are major components of the enamel matrix, and are considered as signaling molecules in epithelial-mesenchymal interactions regulating tooth development and periodontal regeneration. Nevertheless, the molecular mechanisms involved are still poorly understood. The aim of the present study was to identify novel binding partners for amelogenin isoforms in the cementoblast (OCCM-30), using an affinity purification assay (GST pull-down) followed by mass spectrometry and immunoblotting. Protein-protein interaction analysis for AMEL and LRAP evidenced the plasminogen activation system (PAS) as a potential player regulating OCCM-30 response to amelogenin isoforms. For functional assays, PAS was either activated (plasmin) or inhibited (ε-aminocaproic acid [aminocaproic]) in OCCM-30 cells and the cell morphology, mineral nodule formation, and gene expression were assessed. PAS inhibition (EACA 100 mM) dramatically decreased mineral nodule formation and expression of OCCM-30 differentiation markers, including osteocalcin (Bglap), bone sialoprotein (Ibsp), osteopontin (Spp1), tissue-nonspecific alkaline phosphatase (Alpl) and collagen type I (Col1a1), and had no effect on runt-related transcription factor 2 (Runx2) and Osterix (Osx) mRNA levels. PAS activation (plasmin 5 µg/µl) significantly increased Col1a1 and decreased Bglap mRNA levels (p < .05). Together, our findings shed new light on the potential role of plasminogen signaling pathway in the control of the amelogenin isoform-mediated response in cementoblasts and provide new insights into the development of targeted therapies.

Relevance of the plasminogen system in physiology, pathology, and regeneration of oral tissues - From the perspective of dental specialties

Plasmin is a proteolytic enzyme that is crucial in fibrinolysis. In oral tissues, the plasminogen system plays an essential role in physiological and pathological processes, which in addition to fibrinolysis include degradation of extracellular matrix, inflammation, immune response, angiogenesis, tissue remodeling, cell migration, and wound healing. Oral tissues reveal a change in the plasminogen system during pathological processes such as periodontitis, peri-implantitis, or pulpitis, as well as in response to mechanical load. The plasminogen system is also a key element in tissue regeneration. The number of studies investigating the plasminogen system in dentistry have grown continuously in recent years, highlighting its increasing relevance in dental medicine. In this review, we present the diverse functions of the plasminogen system in physiology and its importance for dental specialists in pathology and regeneration. We thus provide an overview of the current knowledge on the role of the plasminogen system in the different fields of dentistry, including endodontics, orthodontics, periodontics, and oral surgery.

MSX2 in ameloblast cell fate and activity

While many effectors have been identified in enamel matrix and cells via genetic studies, physiological networks underlying their expression levels and thus the natural spectrum of enamel thickness and degree of mineralization are now just emerging. Several transcription factors are candidates for enamel gene expression regulation and thus the control of enamel quality. Some of these factors, such as MSX2, are mainly confined to the dental epithelium. MSX2 homeoprotein controls several stages of the ameloblast life cycle. This chapter introduces MSX2 and its target genes in the ameloblast and provides an overview of knowledge regarding its effects in vivo in transgenic mouse models. Currently available in vitro data on the role of MSX2 as a transcription factor and its links to other players in ameloblast gene regulation are considered. MSX2 modulations are relevant to the interplay between developmental, hormonal and environmental pathways and in vivo investigations, notably in the rodent incisor, have provided insight into dental physiology. Indeed, in vivo models are particularly promising for investigating enamel formation and MSX2 function in ameloblast cell fate. MSX2 may be central to the temporal-spatial restriction of enamel protein production by the dental epithelium and thus regulation of enamel quality (thickness and mineralization level) under physiological and pathological conditions. Studies on MSX2 show that amelogenesis is not an isolated process but is part of the more general physiology of coordinated dental-bone complex growth.

Relationship between urokinase plasminogen activator receptor (uPAR) and the invasion of human prenatal hair follicle

During the morphogenesis of hair follicles, the invasive migration of basal keratinocytes resembles cell's dissemination of tissue remodeling. The urokinase-type plasminogen activator receptor (uPAR) appears to be a key molecule in the metastasis. In order to elucidate the relationship between uPAR and the invasion of the human hair follicle, immunohistochemistry, RT-PCR, plasmids transfection, and western blot were used. The results showed that uPAR was expressed in the outermost epithelial cells of the hair follicle and the basal keratinocytes of epidermis, and that the expression decreased with the development of the hair follicle. The cells of the outer root sheath (ORS) and interfollicle epidermis, which overexpressed uPAR, acquired increased invasiveness; however, they showed decreased invasion with overexpression of the urokinase-type plasminogen activator amino terminal fragment (uPA ATF), which inhibited the combination of uPAR and uPA competitively, and the cell invasive migration with overexpressed uPAR was required activated extracellular signal-regulated kinases (ERK). These results implied that overexpression of uPAR promote the invasive migration of hair follicle into the dermis in uPA-dependent and independent manner during human prenatal development.

Molecular genetics of ameloblast cell lineage

Late tooth morphogenesis is characterized by a series of events that determine crown morphogenesis and the histodifferentiation of epithelial cells into enamel-secreting ameloblasts and of mesenchymal cells into dentin-secreting odontoblasts. Functional ameloblasts are tall, columnar, polarized cells that synthesize and secrete a number of enamel-specific proteins. After depositing the full thickness of enamel matrix, ameloblasts shrink in size and regulate enamel maturation. Amelogenesis imperfecta (AI) is a heterogeneous group of inherited defects in enamel formation. Clinically, AI presents as a spectrum of enamel malformations that are categorized as hypoplastic, hypocalcified, or hypomaturation types, based upon the thickness and hardness of the enamel. The different types of AI are inherited, either as X-linked, autosomal-dominant, or autosomal-recessive traits. Recently, several gene mutations have been identified to cause the subtypes of AI. How these genes, however, coordinate their function to control amelogenesis is not understood. In this review, we discuss the role of genes that play definitive role on the determination of ameloblast cell fate and life cycle based on studies in transgenic animals.

Immunohistochemical detection of uPA, uPAR, PAI-1, and maspin in ameloblastic tumors

BACKGROUND

To evaluate the roles of extracellular matrix (ECM)-degrading serine proteinase in progression of odontogenic tumors, expression of urokinase-type plasminogen activator (uPA), uPA receptor (uPAR), plasminogen activator inhibitor-1 (PAI-1), and maspin was analyzed in ameloblastic tumors as well as in tooth germs.

METHODS

Tissue specimens of 10 tooth germs, 45 ameloblastomas, and 5 malignant ameloblastic tumors were examined immunohistochemically with the use of antibodies against uPA, uPAR, PAI-1, and maspin.

RESULTS

Immunohistochemical reactivity for uPA, uPAR, PAI-1, and maspin was detected in normal and neoplastic odontogenic tissues: uPA was recognized predominantly in mesenchymal cells, uPAR was evident in epithelial cells, PAI-1 was found in both epithelial and mesenchymal cells, and maspin was expressed only in epithelial cells. The levels of uPA and uPAR immunoreactivity in ameloblastic tumors were slightly higher than the levels in tooth germs, while PAI-1 reactivity in ameloblastomas tended to be lower than that in tooth germs. The level of maspin immunoreactivity in ameloblastomas was significantly higher than that in tooth germs, and ameloblastic carcinoma showed decreased maspin reactivity.

CONCLUSION

Expression of uPA, uPAR, PAI-1, and maspin in tooth germs and ameloblastic tumors suggests that interactions among these molecules contribute to ECM degradation and cell migration during tooth development and tumor progression. Altered expression of the serine proteinase and its associated molecules in ameloblastic tumors may be involved in oncogenesis of odontogenic epithelium.

Odontogenic keratocysts arise from quiescent epithelial rests and are associated with deregulated hedgehog signaling in mice and humans

Odontogenic keratocysts in humans are aggressive, noninflammatory jaw cysts that may harbor PTCH1 mutations, leading to constitutive activity of the embryonic Hedgehog (Hh) signaling pathway. We show here that epithelial expression of the Hh transcriptional effector Gli2 is sufficient for highly penetrant keratocyst development in transgenic mice. Mouse and human keratocysts expressed similar markers, leading to tooth misalignment, bone remodeling, and craniofacial abnormalities. We detected Hh target gene expression in epithelial cells lining keratocysts from both species, implicating deregulated Hh signaling in their development. Most mouse keratocysts arose from rests of Malassez--quiescent, residual embryonic epithelial cells that remain embedded in the periodontal ligament surrounding mature teeth. In Gli2-expressing mice, these rests were stimulated to proliferate, stratify, and form a differentiated squamous epithelium. The frequent development of keratocysts in Gli2-expressing mice supports the idea that GLI transcription factor activity mediates pathological responses to deregulated Hh signaling in humans. Moreover, Gli2-mediated reactivation of quiescent epithelial rests to form keratocysts indicates that these cells retain the capacity to function as progenitor cells on activation by an appropriate developmental signal.

Localization of perlecan and heparanase in Hertwig's epithelial root sheath during root formation in mouse molars

During cementogenesis, dental follicular cells penetrate the ruptured Hertwig's epithelial root sheath (HERS) and differentiate into cementoblasts. Mechanisms involved in basement membrane degradation during this process have not been clarified. Perlecan, a heparan sulfate (HS) proteoglycan, is a component of all basement membranes. Degradation of HS of perlecan by heparanase cleavage affects a variety of biological processes. We elucidated immunolocalization of perlecan and heparanase in developing murine molars to clarify their roles in cementoblast differentiation. At the initial stage of root formation, perlecan immunoreactivity was detected on the basement membrane of HERS. Weak heparanase immunoreactivity was detected in HERS cells. HERS showed intense staining for heparanase as root formation progressed. In contrast, labeling for perlecan disappeared from the basement membrane facing the dental follicle, and weak immunoreactivity for perlecan was detected on the inner side of the basement membrane of HERS. These findings suggest that perlecan removal is an important step for root and periodontal tissue formation. Heparanase secreted by the cells of HERS may contribute to root formation by degrading perlecan in the dental basement membrane.

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.

periostin null mice exhibit dwarfism, incisor enamel defects, and an early-onset periodontal disease-like phenotype

Periostin was originally identified as an osteoblast-specific factor and is highly expressed in the embryonic periosteum, cardiac valves, placenta, and periodontal ligament as well as in many adult cancerous tissues. To investigate its role during development, we generated mice that lack the periostin gene and replaced the translation start site and first exon with a lacZ reporter gene. Surprisingly, although periostin is widely expressed in many developing organs, periostin-deficient (peri(lacZ)) embryos are grossly normal. Postnatally, however, approximately 14% of the nulls die before weaning and all of the remaining peri(lacZ) nulls are severely growth retarded. Skeletal analysis revealed that trabecular bone in adult homozygous skeletons was sparse, but overall bone growth was unaffected. Furthermore, by 3 months, the nulls develop an early-onset periodontal disease-like phenotype. Unexpectedly, these mice also show a severe incisor enamel defect, although there is no apparent change in ameloblast differentiation. Significantly, placing the peri(lacZ) nulls on a soft diet that alleviated mechanical strain on the periodontal ligament resulted in a partial rescue of both the enamel and periodontal disease-like phenotypes. Combined, these data suggest that a healthy periodontal ligament is required for normal amelogenesis and that periostin is critically required for maintenance of the integrity of the periodontal ligament in response to mechanical stresses.

Msx2 controls ameloblast terminal differentiation

Late tooth morphogenesis is characterized by a series of events that determine cusp morphogenesis and the histodifferentiation of epithelial cells into enamel-secreting ameloblasts. Mice lacking the homeobox gene Msx2 exhibit defects in cusp morphogenesis and in the process of amelogenesis. To better understand the basis of the Msx2 mutant tooth defects, we have investigated the function of Msx2 during late stages of tooth morphogenesis. Cusp formation is thought to be under the control of the enamel knot, which has been proposed to act as an organizing center during this process (Vaahtokari et al. [ 1996] Mech. Dev. 54:39-43). Bone morphogenetic protein-4 (BMP4) has been suggested to mediate termination of enamel knot signaling by means of regulation of programmed cell death (Jernvall et al. [ 1998] Development 125:161-169). Here, we show that Bmp4 expression in the enamel knot is Msx2-dependent. We further show that during amelogenesis Msx2 is required for the expression of the extracellular matrix gene Laminin 5 alpha 3, which is known to play an essential role during ameloblast differentiation. This result thus provides a paradigm for understanding how transcription factors and extracellular matrix can be integrated into a developmental pathway controlling cell differentiation.

Plasminogen mediates the pathological effects of urokinase-type plasminogen activator overexpression

Increased expression of urokinase-type plasminogen activator (uPA) and its receptor (uPAR) is associated with different pathological conditions. Both uPAR-mediated signaling and plasmin-catalyzed extracellular proteolysis may contribute to pathogenesis. To evaluate the involvement of plasminogen in such circumstances, we have taken advantage of transgenic mouse models in which overexpression of uPA and/or uPAR in enamel epithelium, basal epidermis, and hair follicles leads to a pathological phenotype; uPA transgenic mice have chalky-white incisors and, when uPAR is co-expressed, develop extensive alopecia, epidermal thickening, and subepidermal blisters. We report here that when these transgenic mice were backcrossed into a plasminogen-deficient (Plg-/-) background, the dental and skin phenotypes appeared completely normal. Heterozygous Plg+/- transgenic mice exhibited a haplo-insufficiency, with an intermediate or normal phenotype. These results do not argue in favor of a role for uPAR-mediated signaling in our experimental model; rather, they demonstrate an essential, dose-dependent, requirement for plasminogen in uPA-mediated tissue alterations. They also support the hypothesis that plasminogen could play a part in certain skin diseases.

Molecular cloning, chromosomal mapping, and characteristic expression in tooth organ of rat and mouse Krox-25

A novel member of the Krox family of proteins, designated Krox-25, was identified by screening clones from the cDNA libraries of a rat incisor and mouse embryo craniofacial tissue. Rat and mouse Krox-25 mRNAs are about 2.4 kb long, encoding 225 and 224 amino acids, respectively. Krox-25 consists of five zinc finger motifs homologous to the Drosophila Krüppel segmentation gene and also contains several consensus amino acid sequences for a protein kinase C binding domain. Northern blot analysis revealed an intense expression of Krox-25 mRNA in rat and mouse teeth, although it was expressed weakly in other tissues, including calvaria, brain, lung, thymus, kidney, and submandibular gland of mouse. In situ hybridization showed that Krox-25 mRNA began to be expressed weakly in the early odontogenic mesenchyme and primitive enamel epithelium located at the apical end of the rat incisor, and Krox-25 expression increased in the presecretory ameloblasts and became intense in the secretory ameloblasts. This expression was also similar to the results of immunohistochemistry and Western blot, especially the Krox-25 localization in the nuclei of enamel epithelial cells. These results suggest that Krox-25 plays an important role as a transcription factor for the cytodifferentiation and amelogenesis of enamel epithelium.

Smad3 is required for enamel biomineralization

Smad3 is an intracellular signaling molecule that mediates the signal from transforming growth factor-beta (TGF-beta) and activin receptors. In this study, we reveal hypomineralized enamel in mice with the targeted deletion of the Smad3 gene. The Smad3 (-/-) mice had chalky white incisor enamel, while the enamel of the wild-type or Smad3 (+/-) mice was yellow-brown. Histological analysis of the undecalcified sections showed that the enamel thickness of the maxillary incisors in the Smad3 (-/-) mice was similar to that of the wild-type and Smad3 (+/-) mice while that the enamel of the maxillary molars in Smad3 (-/-) mice was disrupted in places. Microcomputed tomography (microCT) analysis revealed that the mineralization of the maxillary incisors and mandibular molars in the Smad3 (-/-) mice showed significant reduction in the degree of mineralization when compared to that of the wild-type and Smad3 (+/-) mice. Scanning electron microscopic (SEM) analysis of the mandibular incisors revealed that the enamel surface of the Smad3 (-/-) mice was irregular and disrupted in places and showed images similar to decalcified mature enamel. The histological analysis of the decalcified sections showed that distinct morphological changes in the ameloblasts at the secretory and maturational stages were not observed between the Smad3 (-/-) and Smad3 (+/-) or wild-type mice, while the enamel matrix was observed in the decalcified sections of the mandibular molars in the Smad3 (-/-) mice. These results suggested that Smad3 was required for enamel biomineralization, and TGF-beta and activin signaling might be critical for its process.

Tumor cell budding and laminin-5 expression in colorectal carcinoma can be modulated by the tissue micro-environment

Expression of laminin-5 alpha3, beta3 and gamma2 protein subunits was investigated in colorectal adenocarcinomas using immunostaining and confocal microscopy. The laminin-5 heterotrimer was found in basement membranes and as extracellular deposits in tumor stroma. In contrast to the alpha3 subunit, which was under-expressed, the gamma2 and beta3 subunits were detected in the cytoplasm of carcinoma cells dissociating (budding) from neoplastic tubules, suggestive of focal alterations in laminin-5 assembly and secretion. Laminin-5 gamma2 or beta3 subunit-reactive budding carcinoma cells expressed cytokeratins but not vimentin; they did not proliferate and were not apoptotic. Furthermore, expression of laminin-5 gamma2 and beta3 subunits in budding cells was associated with focal under-expression of the E-cadherin-beta-catenin complex. Results from xenograft experiments showed that budding activity in colorectal adenocarcinomas could be suppressed when these tumors grew at ectopic s.c. sites in nude mice. In vitro, cultured colon carcinoma cells, but not adenoma-derived tumor cells, shared the laminin-5 phenotype expressed by carcinoma cells in vivo. Using colon carcinoma cell lines implanted orthotopically and invading the cecum of nude mice, the laminin-5-associated budding was restored, indicating that this phenotype is not only determined by tumor cell properties but also dependent on the tissue micro-environment. Our results indicate that both laminin-5 alpha3 subunit expression and cell-cell cohesiveness are altered in budding carcinoma cells, which we consider to be actively invading. We propose that the local tissue micro-environment contributes to these events.