Enamel proteins and proteases in Mmp20 and Klk4 null and double-null mice

Matrix metalloproteinase 20 (MMP20) and kallikrein-related peptidase 4 (KLK4) are thought to be necessary to clear proteins from the enamel matrix of developing teeth. We characterized Mmp20 and Klk4 null mice to better understand their roles in matrix degradation and removal. Histological examination showed retained organic matrix in Mmp20, Klk4, and Mmp20/Klk4 double-null mouse enamel matrix, but not in the wild-type. X-gal histostaining of Mmp20 null mice heterozygous for the Klk4 knockout/lacZ knockin showed that Klk4 is expressed normally in the Mmp20 null background. This finding was corroborated by zymogram and western blotting, which discovered a 40-kDa protease induced in the maturation stage of Mmp20 null mice. Proteins were extracted from secretory-stage or maturation-stage maxillary first molars from wild-type, Mmp20 null, Klk4 null, and Mmp20/Klk4 double-null mice and were analyzed by SDS-PAGE and western blotting. Only intact amelogenins and ameloblastin were observed in secretory-stage enamel of Mmp20 null mice, whereas the secretory-stage matrix from Klk4 null mice was identical to the matrix from wild-type mice. More residual matrix was observed in the double-null mice compared with either of the single-null mice. These results support the importance of MMP20 during the secretory stage and of KLK4 during the maturation stage and show there is only limited functional redundancy for these enzymes.     

Kallikrein-related peptidase 4, matrix metalloproteinase 20, and the maturation of murine and porcine enamel

The crowns of matrix metalloproteinase 20 (Mmp20) null mice fracture at the dentino-enamel junction (DEJ), whereas the crowns of kallikrein-related peptidase 4 (Klk4) null mice fracture in the deep enamel just above the DEJ. We used backscatter scanning electron microscopy to assess enamel mineralization in incisors from 9-wk-old wild-type, Klk4 null, and Mmp20 null mice, and in developing pig molars. We observed a line of hypermineralization along the DEJ in developing wild-type mouse and pig teeth. This line was discernible from the early secretory stage until the enamel in the maturation stage reached a similar density. The line was apparent in Klk4 null mice, but absent in Mmp20 null mice. Enamel in the Klk4 null mice matured normally at the surface, but was progressively less mineralized with depth. Enamel in the Mmp20 null mice formed as a mineral bilayer, with neither layer looking like true enamel. The most superficial mineral layer expanded during the maturation stage and formed irregular surface nodules. A surprising finding was the observation of electron backscatter from mid-maturation wild-type ameloblasts, which we attributed to the accumulation and release of iron. We conclude that enamel breaks in the deep enamel of Klk4 null mice because of decreasing enamel maturation with depth, and at the DEJ in Mmp20 null mice because of hypomineralization at the DEJ.     

Relationships between protein and mineral during enamel development in normal and genetically altered mice

The purpose of this study was to quantify and compare the amounts of volatiles (mostly protein) and mineral present in developing incisor enamel in normal mice and in those genetically engineered for absence of intact enamelin, ameloblastin, matrix metalloproteinase 20 (MMP20) or kallikrein-related peptidase 4 (KLK4). Data indicated that all mice showed peaks in the gross weight of volatiles and a similar weight of mineral at locations on incisors normally associated with early maturation. Thereafter, the content of volatiles on normal incisors declined rapidly by as much as 62%, but not by 100%, over 2 mm, accompanied by increases of ≈ threefold in mineral weights. Enamelin heterozygous mice (lower incisors) showed a decrease in volatile content across the maturation stage, yet mineral failed to increase significantly. Mmp20 null mice showed no significant loss of volatiles from maturing enamel, yet the amount of mineral increased. Klk4 null mice showed normal mineral acquisition up to early maturation, but the input of new volatiles in mid to late maturation caused the final mineralization to slow below normal levels. These results suggest that it is not only the amount of protein but also the nature or type of protein or fragments present in the local crystallite environment that affects their volumetric expansion as they mature.     

Altered ion-responsive gene expression in Mmp20 null mice

During enamel maturation, hydroxyapatite crystallites expand in volume, releasing protons that acidify the developing enamel. This acidity is neutralized by the buffering activity of carbonic anhydrases and ion transporters. Less hydroxyapatite forms in matrix metalloproteinase-20 null (Mmp20(-/-)) mouse incisors, because enamel thickness is reduced by approximately 50%. We therefore asked if ion regulation was altered in Mmp20(-/-) mouse enamel. Staining of wild-type and Mmp20(-/-) incisors with pH indicators demonstrated that wild-type mice had pronounced changes in enamel pH as development progressed. These pH changes were greatly attenuated in Mmp20(-/-) mice. Expression of 4 ion-regulatory genes (Atp2b4, Slc4a2, Car6, Cftr) was significantly decreased in enamel organs from Mmp20(-/-) mice. Notably, expression of secreted carbonic anhydrase (Car6) was reduced to almost undetectable levels in the null enamel organ. In contrast, Odam and Klk4 expression was unaffected. We concluded that a feedback mechanism regulates ion-responsive gene expression during enamel development.     

同源异型盒基因Msx—1在小鼠牙齿发育生物矿化过程中的表达研究

目的：研究同源异型盒基因Msx-1在牙齿硬组织形成过程中的表达和意义。方法：采用原位杂交技术检测Msx-1mRNA在出生后1天、7天和14天昆明小鼠磨牙和切牙牙轴质和牙本质形成 的表达。结果：磨牙中,Msx-1mRNA主要表达于生后1天到7天正在极化的前成轴细胞和前成牙本质细胞,处于分泌期的成釉细胞和成牙本质细胞,7天时信号最强;随后其表达随细胞分化的成熟和牙釉质、牙本质基质形成的进展而逐渐下降。切牙中,牙冠部细胞中的表达与磨牙基本相似;但根部分唇则未分化的颈环上皮和外胚间充质细胞始终呈Msx-1阳性表达,结论同源异型盒基因Msx-1转录主要发生于硬组织形成早期阶段,即成釉细胞和成牙本质细胞的极化和分泌阶段,提示Msx-1可能与了小鼠牙胚硬组织形成过程中细胞分化和生物矿化。     

Protective properties of salivary pellicles from two different intraoral sites on enamel erosion

The purpose of this study was to investigate the protective effect and ultrastructure of salivary pellicles formed in vivo near the orifices of the ducts of parotid and submandibular/sublingual salivary glands. Pellicles were formed by exposing bovine enamel slabs to the oral environment at the buccal aspect of the upper first molars and at the lingual aspect of the lower incisors in 3 subjects over periods of 24 h. Enamel specimens with and without 24-hour pellicles were immersed in citric acid (0.1 and 1%) for periods ranging from 30 s to 5 min, and processed for measurement of surface microhardness (SMH) and transmission electron microscopy (TEM). In comparison to uncovered enamel specimen significantly less decrease in SMH due to acid exposure was observed in pellicle-coated enamel specimens. Pellicles formed at the buccal aspect of the upper molars were less effective in protecting the enamel against acid-induced softening as compared to pellicles formed at the lingual aspect of the lower incisors only after 5 min exposure in 1% citric acid. TEM analysis showed that pellicle layers were dissolved continuously due to acid exposure. However, even after 5 min exposure to 1% citric acid, a residual pellicle layer could be detected on the enamel surface. In conclusion, site-dependent differences of buccally and lingually in vivo formed 24-hour pellicles have minor importance concerning the pellicle-induced protection of the enamel surface against erosive changes.     

Excessive fluoride reduces Foxo1 expression in dental epithelial cells of the rat incisor

Enamel fluorosis is characterized by hypomineralization, and forkhead box O1 (Foxo1) is essential for mouse enamel biomineralization. This study investigated the effect of fluoride on Foxo1 expression and its implications for enamel fluorosis. Mandibular incisors were extracted from Sprague Dawley rats treated for 3 months with water containing 0, 50, or 100 p.p.m. F^?. Immunohistochemistry was used to localize and quantify FOXO1 expression in dental epithelial layer cells of the incisors. The effect of fluoride on expression of Foxo1, kallikrein‐4 (Klk4), and amelotin (Amtn) mRNAs was analyzed by real‐time RT‐PCR, and western blotting was used to measure total and nuclear FOXO1 protein levels in mature dental epithelial cells. The results revealed that nuclear FOXO1 was mainly localized in the transition and the mature ameloblasts and exhibited weaker expression in the rats exposed to fluoride. In addition to the reduced levels of Foxo1, Klk4, and AmtnmRNAs, the protein levels of total and nuclearFOXO1 were decreased in the mature dental epithelial cells exposed to fluoride. Thus, excessive fluoride may have an effect on the expression levels of Foxo1 in dental epithelial cells and thereby affect hypomineralization of the enamel during fluorosis.     

Chymotrypsin C (caldecrin) is associated with enamel development

Two main proteases cleave enamel extracellular matrix proteins during amelogenesis. Matrix metalloprotease-20 (Mmp20) is the predominant enzyme expressed during the secretory stage, while kallikrein-related peptidase-4 (Klk4) is predominantly expressed during maturation. Mutations to both Mmp20 and Klk4 result in abnormal enamel phenotypes. During a recent whole-genome microarray analysis of rat incisor enamel organ cells derived from the secretory and maturation stages of amelogenesis, the serine protease chymotrypsin C (caldecrin, Ctrc) was identified as significantly up-regulated (> 11-fold) during enamel maturation. Prior reports indicate that Ctrc expression is pancreas-specific, albeit low levels were also noted in brain. We here report on the expression of Ctrc in the enamel organ. Quantitative PCR (qPCR) and Western blot analysis were used to confirm the expression of Ctrc in the developing enamel organ. The expression profile of Ctrc is similar to that of Klk4, increasing markedly during the maturation stage relative to the secretory stage, although levels of Ctrc mRNA are lower than for Klk4. The discovery of a new serine protease possibly involved in enamel development has important implications for our understanding of the factors that regulate enamel biomineralization.     

The relationship between the termination of cell proliferation and expression of heat-shock protein-25 in the rat developing tooth germ

Odontoblast- and ameloblast-lineage cells acquire heat-shock protein (HSP)-25 immunoreactivity after they complete cell division during postnatal odontogenesis in rat molars. However, there are no data available concerning the relationship between the termination of cell proliferation and HSP-25 immunoreactivity during tooth morphogenesis. We compared the expression of HSP-25 in tooth germs with their proliferative activity in the rat prenatal to perinatal molar and postnatal incisor to clarify the functional significance of HSP-25 during tooth morphogenesis by immunohistochemistry using anti-HSP-25 and anti-Ki67/5-bromo-2'-deoxyuridine (BrdU). Numerous proliferating cells in developing molars were distributed throughout the tooth germ and HSP-25 immunoreactivity was recognizable in the dental epithelial and mesenchymal cells after they completed cell division. However, both cell proliferation and immunoreaction for HSP-25 are absent in the enamel knots. The distribution pattern of the proliferating cells in the incisors was basically identical to that in the prenatal molars except for the lack of non-proliferating secondary enamel knots and the sparse distribution of proliferating cells in the apical bud. Thus, HSP-25 protein is suggested to act as a switch between cell proliferation and terminal cyto-differentiation during odontogenesis.     

Ameloblastin and amelogenin expression in posnatal developing mouse molars

Ameloblastin and amelogenin are structural proteins present in the enamel matrix of developing teeth. Here we report the results of in situ hybridization analyses with DNA probes of ameloblastin and amelogenin expression in the mandibular first molars of ICR/Jcl mice from postnatal day 1 to day 15. Ameloblastin mRNA expression was observed in ameloblasts at day 2 while amelogenin mRNA was detected in secretory ameloblasts at day 3. Significant expression of both molecules was observed at days 4, 5 and 6, after which the levels decreased. Amelogenin expression ended on day 10, while ameloblastin mRNA was only weakly detected on day 12. Neither amelogenin nor ameloblastin expression was observed in day 15 mouse molars. Amelogenin and ameloblastin mRNAs were restricted to ameloblasts. We conclude that amelogenin and ameloblastin expression is enamel-specific, and suggest that these genes might be involved in the mineralization of enamel. It is possible that ameloblastin could participate in the attachment of ameloblasts to the enamel surface. In this case, the downregulation of expression may indicate the beginning of the maturation stage in which the ameloblasts tend to detach from the enamel layer.     

人β-防御素-2在涎腺肿瘤及涎腺炎症中的表达及其意义

目的 研究涎腺良性肿瘤组织、恶性肿瘤组织和涎腺炎症中人β-防御素-2（HBD-2）mRNA和蛋白的表达特征。方法 对不同涎腺组织,采用聚合酶链反应（PCR）、实时聚合酶链反应（Real-Time PCR）和免疫组化检测HBD-2的表达,并分析HBD-2 mRNA和蛋白在涎腺良性肿瘤组织、恶性肿瘤组织、炎症组织和正常涎腺组织中的表达差异。结果 与涎腺正常组织比较,良性肿瘤组HBD-2 mRNA表达量为其6.468倍,显著高于涎腺正常组织组（P<0.05）;恶性肿瘤组为其0.334倍,显著低于涎腺正常组织组（P<0.05）;涎腺炎症组为其10.563倍,显著高于涎腺正常组织组（P<0.05）。HBD-2不仅在这些组织的细胞质中有表达,而且在恶性组织中的细胞核也有表达。结论 HBD-2在涎腺良性肿瘤组织及涎腺炎症组织中高表达,在涎腺恶性肿瘤组织中低表达,其蛋白发生核转移。     

Transgenic rescue of enamel phenotype in Ambn null mice

Ameloblastin null mice fail to make an enamel layer, but the defects could be due to an absence of functional ameloblastin or to the secretion of a potentially toxic mutant ameloblastin. We hypothesized that the enamel phenotype could be rescued by the transgenic expression of normal ameloblastin in Ambn mutant mice. We established and analyzed 5 transgenic lines that expressed ameloblastin from the amelogenin (AmelX) promoter and identified transgenic lines that express virtually no transgene, slightly less than normal (Tg+), somewhat higher than normal (Tg++), and much higher than normal (Tg+++) levels of ameloblastin. All lines expressing detectable levels of ameloblastin at least partially recovered the enamel phenotype. When ameloblastin expression was only somewhat higher than normal, the enamel covering the molars and incisors was of normal thickness, had clearly defined rod and interrod enamel, and held up well in function. We conclude that ameloblastin is essential for dental enamel formation.     

Immunolocalization of transforming growth factor beta 1 and epidermal growth factor receptor epitopes in mouse incisors and molars with a demonstration of in vitro production of transforming activity.

Day-14 lower incisors and day-18 first lower molars of mouse embryos produced in vitro transforming activities for non-confluent NRK cells co-cultured in agar, and mitogenic activities for exponentially growing NRK and BHK cells. The patterns of distribution of TGF beta 1 and EGF receptor, both known to regulate cell proliferation, differentiation and transformation in vitro and suspected to play important roles in developmental processes, were studied during mouse odontogenesis by means of indirect immunofluorescence on fixed or frozen fixed sections. TGF beta 1 epitopes were detected in the stellate reticulum of day-13 to day-16 incisors and of molars from day-17 onwards. Dental mesenchyme of day-14 incisors and postnatal molars, and peridental mesenchyme of bud and cap stage molars and incisors were also stained by TGF beta 1 antibodies. EGF receptor was localized in the enamel organs of incisors and molars; the inner dental epithelium and later the outer dental epithelium rapidly became negative while the stellate reticulum remained stained. Incisor apical mesenchyme showed an intense reaction with EGF receptor antibodies after birth.     

Ameloblasts require active RhoA to generate normal dental enamel

RhoA plays a fundamental role in regulation of the actin cytoskeleton, intercellular attachment, and cell proliferation. During amelogenesis, ameloblasts (which produce the enamel proteins) undergo dramatic cytoskeletal changes and the RhoA protein level is up‐regulated. Transgenic mice were generated that express a dominant‐negative RhoA transgene in ameloblasts using amelogenin gene‐regulatory sequences. Transgenic and wild‐type (WT) molar tooth germs were incubated with sodium fluoride (NaF) or sodium chloride (NaCl) in organ culture. Filamentous actin (F‐actin) stained with phalloidin was elevated significantly in WT ameloblasts treated with NaF compared with WT ameloblasts treated with NaCl or with transgenic ameloblasts treated with NaF, thereby confirming a block in the RhoA/Rho‐associated protein kinase (ROCK) pathway in the transgenic mice. Little difference in quantitative fluorescence (an estimation of fluorosis) was observed between WT and transgenic incisors from mice provided with drinking water containing NaF. We subsequently found reduced transgene expression in incisors compared with molars. Transgenic molar teeth had reduced amelogenin, E‐cadherin, and Ki67 compared with WT molar teeth. Hypoplastic enamel in transgenic mice correlates with reduced expression of the enamel protein, amelogenin, and E‐cadherin and cell proliferation are regulated by RhoA in other tissues. Together these findings reveal deficits in molar ameloblast function when RhoA activity is inhibited.     

Immunohistochemical studies of organic anion transporters and urate transporter 1 expression in human salivary gland

Abstract

Background. Various substances including uric acid, organic acids and drugs are transported by organic anion transporters (OATs) in the kidney. In addition, a member of the OAT family, urate transporter 1 (URAT1), is involved in the reabsorption of uric acid from the renal tubule. Benzbromarone inhibits URAT1 to block uric acid reabsorption. Methods. Our group previously observed higher salivary uric acid levels than serum levels in patients taking benzbromarone, and reported the possible existence of URAT1-like uric acid excretion mechanism in the salivary gland. The purpose of this study was to elucidate the uric acid excretion mechanism in salivary gland tissues using rabbit anti-human OAT1-4 and URAT1 polyclonal antibodies with EnVision^? system. Results. In the salivary gland, OAT1 was expressed in ductal cells. OAT2 was found in both ductal cells and serous acinar cells and weak expression was also observed in several nuclei. OAT3 expression was observed in serous acinar cells and nuclei and OAT4 was expressed only in ductal cells. URAT1 expression was observed in the cytoplasm of ductal cells and strong punctuate staining was seen in part of the supra-nuclear cytoplasm. The number of cells expressing URAT1 was smaller compared with OATs. In the kidney, however, OAT1-4 and URAT1 were strongly expressed on proximal renal tubules. Conclusions. The present study confirmed the existence of OAT1-4 and URAT1 in the salivary gland. These results may support the previous speculation that benzbromarone inhibits URAT1 to block uric acid reabsorption in the salivary gland, resulting in higher salivary uric acid levels than serum levels.     

腺病毒介导CTLA4Ig对舍格伦综合征小鼠颌下腺AQP5表达的影响

目的：观察CTLA4Ig-重组腺病毒载体（Ad-CTLA4Ig）对SS颌下腺AQP5表达的影响。方法：采用完全弗氏佐剂＋同种鼠颌下腺抗原激发诱导小鼠舍格伦综合征（SS）。实验组在激发前30min予以Ad-CTLA4Ig腹腔注射,未转CTLA4IgcDNA的腺病毒载体（Adv）及SS作对照。比较各组动物颌下腺形态学改变,并采用Western blot分析SS小鼠颌下腺AQP5表达变化。结果：CTLA4Ig-重组腺病毒实验组颌下腺病理学改变不明显。并且删表达量明显高于对照组（P〈0．05）。结论：Ad-CTLA4Ig可正向调节小鼠颌下腺AQP5的表达,并可有效抑制小鼠合格伦综合征的发生。     

Consequences for enamel development and mineralization resulting from loss of function of ameloblastin or enamelin

Although the nonamelogenin proteins, ameloblastin and enamelin, are both low-abundance and rapidly degrading components of forming enamel, they seem to serve essential developmental functions, as suggested by findings that an enamel layer fails to appear on teeth of mice genetically engineered to produce either a truncated form of ameloblastin (exons 5 and 6 deleted) or no enamelin at all (null). The purpose of this study was to characterize, by direct micro weighing, changes in enamel mineralization occurring on maxillary and mandibular incisors of mice bred for these alterations in nonamelogenin function ( Ambn ^{+/+ , +/−5 , 6 , −5 , 6/−5 , 6} , Enam ^{+/+ , +/− , −/−} ). The results indicated similar changes to enamel-mineralization patterns within the altered genotypes, including significant decreases by as much as 50% in the mineral content of maturing enamel from heterozygous mice and the formation of a thin, crusty, and disorganized mineralized layer, rather than true enamel, on the labial (occlusal) surfaces of incisors and molars along with ectopic calcifications within enamel organ cells in Ambn ^{−5 , 6/−5 , 6} and Enam ^−/− homozygous mice. These findings confirm that both ameloblastin and enamelin are required by ameloblasts to create an enamel layer by appositional growth as well as to assist in achieving its unique high level of mineralization.