Fluoride does not inhibit enamel protease activity

ERS Mediated by GRP-78/PERK/CHOP Signaling Is Involved in Fluoride-Induced Ameloblast Apoptosis

Fluoride can be widely ingested from the environment, and its excessive intake could result in adverse effects. Dental fluorosis is an early sign of fluoride toxicity which can cause esthetic and functional problems. Though apoptosis in ameloblasts is one of the potential mechanisms, the specific signal cascade is in-conclusive. High-throughput sequencing and molecular biological techniques were used in this study to explore the underlying pathogenesis of dental fluorosis, for its prevention and treatment. A fluorosis cell model was established. Viability and apoptosis rate of mouse ameloblast-derived cell line (LS8 cells) was measured using cell counting kit-8 (CCK-8) assay and flow cytometry analysis. Cells were harvested with or without 2-mM sodium fluoride (NaF) stimulation for high-throughput sequencing. Based on the sequencing data, subcellular structures, endoplasmic reticulum stress (ERS), and apoptosis related biomarkers were verified using transmission electron microscopy, quantitative real-time polymerase chain reaction, and Western blotting techniques. Expression of ERS markers, apoptosis related proteins, and enamel formation enzymes were detected using Western blotting after addition of 4-phenylbutyrate (4-PBA). NaF-inhibited LS8 cells displayed time- and dose- dependent viability. Additionally, apoptosis and morphological changes were observed. RNA-sequencing data showed that protein processing in endoplasmic reticulum was obviously affected. ERS and apoptosis were induced by excessive NaF. Downregulation of kallikrein-related peptidase 4 (KLK4) was also observed. Inhibition of ERS by 4-PBA rescued the apoptotic and functional protein changes in cells. Excessive fluoride induces apoptosis by activating ERS, which is mediated by GRP-78/PERK/CHOP signaling. Key proteinase is present in maturation-stage enamel; KLK4 was also affected by fluoride, but rescued by 4-PBA. This study presents a possibility for therapeutic strategies for dental fluorosis, while further exploration is required.

Sodium/(calcium + potassium) exchanger NCKX4 optimizes KLK4 activity in the enamel matrix microenvironment to regulate ECM modeling

Enamel development is a process in which extracellular matrix models from a soft proteinaceous matrix to the most mineralized tissue in vertebrates. Patients with mutant NCKX4, a gene encoding a K+-dependent Na+/Ca2+-exchanger, develop a hypomineralized and hypomature enamel. How NCKX4 regulates enamel protein removal to achieve an almost protein-free enamel is unknown. We characterized the upregulation pattern of Nckx4 in the progressively differentiating enamel-forming ameloblasts by qPCR, and as well as confirmed NCKX4 protein to primarily localize at the apical surface of wild-type ruffle-ended maturation ameloblasts by immunostaining of the continuously growing mouse incisors, posing the entire developmental trajectory of enamel. In contrast to the normal mature enamel, where ECM proteins are hydrolyzed and removed, we found significant protein retention in the maturation stage of Nckx4 -/- mouse enamel. The Nckx4 -/- enamel held less Ca2+ and K+ but more Na+ than the Nckx4 +/+ enamel did, as measured by EDX. The alternating acidic and neutral pH zones at the surface of mineralizing Nckx4 +/+ enamel were replaced by a largely neutral pH matrix in the Nckx4 -/- enamel. In situ zymography revealed a reduced kallikrein-related peptidase 4 (KLK4) activity in the Nckx4 -/- enamel. We showed that KLK4 took on 90% of proteinase activity in the maturation stage of normal enamel, and that recombinant KLK4 as well as native mouse enamel KLK4 both performed less effectively in a buffer with increased [Na+] and pH, conditions found in the Nckx4 -/- developing enamel. This study, for the first time to our knowledge, provides evidence demonstrating the impaired in situ KLK4 activity in Nckx4 -/- enamel and suggests a novel function of NCKX4 in facilitating KLK4-mediated hydrolysis and removal of ECM proteins, warranting the completion of enamel matrix modeling.

Sulphur dioxide and fluoride co-exposure induce incisor hypomineralization and amelogenin upregulation via YAP/RUNX2 signaling pathway

Sulphur dioxide (SO₂) and fluoride are among the most common environmental pollutants affecting human health, and both co-exist in areas predominantly consuming coal. It is vital to analyse the combined toxicity of SO₂ and fluoride, and their effects on health and the underlying mechanisms of their co-exposure have not yet been adequately assessed. In the present study, we used ICR mice and LS8 cells to investigate the toxicity of SO₂ and fluoride exposure to the enamel, alone or in combination. Factorial design analysis was used to reveal the combined toxicity in vitro and in vivo. Co-exposure to SO₂ and fluoride exacerbated enamel injury, resulting in more severe hypomineralization of incisor, and enamel structure disorders in mice, and could induce the accumulation of protein residue in the matrix of the enamel. Amelogenin expression was increased upon exposure to SO₂ and fluoride, but enamel matrix proteases were not affected. Consistent with our in vivo results, co-exposure of SO₂ and fluoride aggravated amelogenin expression in LS8 cells, and increased the YAP and RUNX2 levels. Co-exposure to SO₂ and fluoride resulted in greater toxicity than individual exposure, both in vitro and in vivo, indicating that residents of areas exposed to SO₂ and fluoride may have an increased risk of developing enamel damage.

Fluoride Alters Klk4 Expression in Maturation Ameloblasts through Androgen and Progesterone Receptor Signaling

Fluorosed maturation stage enamel is hypomineralized in part due to a delay in the removal of matrix proteins to inhibit final crystal growth. The delay in protein removal is likely related to reduced expression of kallikrein-related peptidase 4 (KLK4), resulting in a reduced matrix proteinase activity that found in fluorosed enamel. Klk4 transcription is known to be regulated in other cell types by androgen receptor (AR) and progesterone receptors (PR). In this study, we determined the possible role of fluoride in down-regulation of KLK4 expression through changes in AR and PR. Immunohistochemical localization showed that both AR and PR nuclear translocation was suppressed in fluoride exposed mice. However, when AR signaling was silenced in mouse ameloblast-lineage cells (ALCs), expression of both Pgr and Klk4 were increased. Similar to the effect from AR silencing, fluoride also upregulated Pgr in ALCs, but downregulated Klk4. This finding suggests that though suppression of AR transactivation by fluoride increases Prg expression, inhibition of PR transactivation by fluoride has a much greater effect, ultimately resulting in downregulation of Klk4 expression. These findings indicate that in ameloblasts, PR has a dominant role in regulating Klk4 expression. We found that when AR was retained in the cytoplasm in the presence of fluoride, that co-localized with heat shock protein 90 (HSP90), a well-known chaperone for steroid hormone receptors. HSP90 also known to regulate TGF-β signaling. Consistent with the effect of fluoride on AR and HSP90, we found evidence of reduced TGF-β signaling activity in fluorosed ameloblasts as reduced immunolocalization of TGFB1 and TGFBR-2 and a significant increase in Cyclin D1 mRNA expression, which also possibly contributes to the reduced AR signaling activity. In vitro, when serum was removed from the media, aluminum was required for fluoride to inhibit the dissociation of HSP90 from AR. In conclusion, fluoride related downregulation of Klk4 is associated with reduced nuclear translocation of AR and PR, and also reduced TGF-β signaling activity, all of which are regulated by HSP90. We suggest that a common mechanism by which fluoride affects AR, PR, and TGF-β signaling is through inhibiting ATP-dependent conformational cycling of HSP90.

The Unfolded Protein Response in Amelogenesis and Enamel Pathologies

During the secretory phase of their life-cycle, ameloblasts are highly specialized secretory cells whose role is to elaborate an extracellular matrix that ultimately confers both form and function to dental enamel, the most highly mineralized of all mammalian tissues. In common with many other “professional” secretory cells, ameloblasts employ the unfolded protein response (UPR) to help them cope with the large secretory cargo of extracellular matrix proteins transiting their ER (endoplasmic reticulum)/Golgi complex and so minimize ER stress. However, the UPR is a double-edged sword, and, in cases where ER stress is severe and prolonged, the UPR switches from pro-survival to pro-apoptotic mode. The purpose of this review is to consider the role of the ameloblast UPR in the biology and pathology of amelogenesis; specifically in respect of amelogenesis imperfecta (AI) and fluorosis. Some forms of AI appear to correspond to classic proteopathies, where pathological intra-cellular accumulations of protein tip the UPR toward apoptosis. Fluorosis also involves the UPR and, while not of itself a classic proteopathic disease, shares some common elements through the involvement of the UPR. The possibility of therapeutic intervention by pharmacological modulation of the UPR in AI and fluorosis is also discussed.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Mineralization-defects are comparable in fluorotic impacted human teeth and fluorotic mouse incisors

OBJECTIVE

Fluoride excess of 0.05-0.07mgF/kgbw/day in water or food additives like salt is the principal cause of endemic dental fluorosis. How fluoride causes these defects is not clear yet. Recent studies in rodents suggest that development of enamel fluorosis is associated with insufficient neutralization of protons released during the formation of hypermineralized lines.

DESIGN

Here we examined whether hypermineralization could also be assessed by MicroCT in developing molar enamel of humans exposed to fluoride.

RESULT

Micro-CT analysis of hypomineralized enamel from human fluorotic molars graded by the Thylstrup-Fejerskov (TF) Index as III-IV showed weak hypermineralized lines and hypermineralized patches not seen in TF-I/II grade enamel. The mesio-distal sides of these molar teeth were significantly smaller (∼18%, p=0.02) than in TF-I/II teeth.

CONCLUSION

The patterns of changes observed in human fluorotic teeth were similar to those in fluorotic rodent incisors. The data are consistent with the hypothesis that also in developing human teeth fluoride-stimulated local acidification of enamel could be a mechanism for developing fluorotic enamel.

Fluoride affects enamel protein content via TGF-β1-mediated KLK4 inhibition

Dental fluorosis is caused by chronic high-level fluoride (F(-)) exposure during enamel development, and fluorosed enamel has a higher than normal protein content. Matrix metalloproteinase 20 cleaves enamel matrix proteins during the secretory stage, and KLK4 further cleaves these proteins during the maturation stage so that the proteins can be reabsorbed from the hardening enamel. We show that transforming growth factor β1 (TGF-β1) can induce Klk4 expression, and we examine the effect of F(-) on TGF-β1 and KLK4 expression. We found that in vivo F(-) inhibits Klk4 but not Mmp20 transcript levels. LacZ-C57BL/6-Klk4 (+/LacZ) mice have LacZ inserted in frame at the Klk4 translation initiation site so that the endogenous Klk4 promoter drives LacZ expression in the same temporal/spatial way as it does for Klk4. KLK4 protein levels in rat enamel and β-galactosidase staining in LacZ-C57BL/6-Klk4 (+/LacZ) mouse enamel were both significantly reduced by F(-) treatment. Since TGF-β1 induces KLK4 expression, we tested and found that F(-) significantly reduced Tgf-β1 transcript levels in rat enamel organ. These data suggest that F(-)-mediated downregulation of TGF-β1 expression contributes to reduced KLK4 protein levels in fluorosed enamel and provides an explanation for why fluorosed enamel has a higher than normal protein content.

Stress response pathways in ameloblasts: implications for amelogenesis and dental fluorosis

Human enamel development of the permanent teeth takes place during childhood and stresses encountered during this period can have lasting effects on the appearance and structural integrity of the enamel. One of the most common examples of this is the development of dental fluorosis after childhood exposure to excess fluoride, an elemental agent used to increase enamel hardness and prevent dental caries. Currently the molecular mechanism responsible for dental fluorosis remains unknown; however, recent work suggests dental fluorosis may be the result of activated stress response pathways in ameloblasts during the development of permanent teeth. Using fluorosis as an example, the role of stress response pathways during enamel maturation is discussed.

Assessment of dental fluorosis in Mmp20 +/- mice

The molecular mechanisms that underlie dental fluorosis are poorly understood. The retention of enamel proteins hallmarking fluorotic enamel may result from impaired hydrolysis and/or removal of enamel proteins. Previous studies have suggested that partial inhibition of Mmp20 expression is involved in the etiology of dental fluorosis. Here we ask if mice expressing only one functional Mmp20 allele are more susceptible to fluorosis. We demonstrate that Mmp20 (+/-) mice express approximately half the amount of MMP20 as do wild-type mice. The Mmp20 heterozygous mice have normal-appearing enamel, with Vickers microhardness values similar to those of wild-type control enamel. Therefore, reduced MMP20 expression is not solely responsible for dental fluorosis. With 50-ppm-fluoride (F(-)) treatment ad libitum, the Mmp20 (+/-) mice had F(-) tissue levels similar to those of Mmp20 (+/+) mice. No significant difference in enamel hardness was observed between the F(-)-treated heterozygous and wild-type mice. Interestingly, we did find a small but significant difference in quantitative fluorescence between these two groups, which may be attributable to slightly higher protein content in the Mmp20 (+/-) mouse enamel. We conclude that MMP20 plays a nominal role in dental enamel fluorosis.