John D. Bartlett, PhD
Senior Member of the Staff
Department of Cytokine Biology
Assistant Professor, Department of Oral & Developmental Biology
Harvard School of Dental Medicine
University of New Hampshire, M.S., 1984, Microbiology
University of Vermont, Ph.D., 1990, Cell & Molecular Biology
Dental enamel is the hardest substance produced by the human body, but it does not start out that way. When it first begins forming on an unerupted developing tooth, enamel tissue is as soft as cheese. As the enamel matures, it becomes progressively harder and develops ultimately into a mineralized product. Researchers in the Bartlett laboratory study the processes regulating enamel development and are studying the role of a proteinase that is essential for normal enamel development. Work is also underway to characterize the molecular mechanisms that cause dental fluorosis.
Research on Proteinases in Enamel Development
Enamel matrix proteins, such as amelogenin, ameloblastin, and enamelin, are cleaved by proteinases soon after they are secreted. The cleavage products accumulate in the deeper, more mature enamel layers while the full-length proteins are only observed at the surface. This observation suggests that proteinases are necessary for "activating" enamel proteins so the parent proteins and their cleavage products may perform different functions.
We have cloned a novel matrix metalloproteinase, enamelysin (MMP-20), from tooth tissues and have shown that it localizes primarily within the most recently formed enamel. We have found that MMP-20 transcripts are produced early in development in both the ameloblasts of the enamel organ and the odontoblasts of the pulp organ. These cell types are located adjacent to their respective mineralizing tissues; enamel and dentin, and each cell type is responsible for producing the organic components of its mineralizing matrix.
The MMP-20 gene has been mapped to human chromosome 11q22 and is clustered with at least seven other members of the MMP family. MMP-20, however, is a unique matrix metalloproteinase, in part because it has a highly restricted pattern of expression. Most MMPs are expressed in a wide variety of tissues, but MMP-20 is observed exclusively in the enamel and pulp organs of developing teeth and is present only as an active enzyme. We have demonstrated that MMP-20 can cleave a wide variety of proteins so we believe its highly restricted pattern of expression is important so that essential proteins (i.e., type V collagen) important for tissue integrity are not degraded.
The known normal function of MMP-20 is to cleave the most abundant enamel resident protein, amelogenin. Previous studies have demonstrated that mice devoid of either the amelogenin (Amelx) or the MMP-20 genes have seriously defective dental enamel. Mutations in each of these genes also cause malformed dental enamel in people. Therefore, we can state with certainty that MMP-20 is an enzyme that is absolutely required for healthy dental enamel formation.
Research on Dental Fluorosis
Fluoride in drinking water is very effective for protecting people from getting cavities in their teeth. However, if too much fluoride is ingested, dental fluorosis may result. Fluorosed dental enamel is softer and more porous than normal. Mildly fluorosed teeth have white spots while severely fluorosed teeth can become darkly stained.
Currently, no one knows how fluoride causes dental fluorosis. We are characterizing the molecular responses of cells that are exposed to fluoride treatment. Our laboratory was the first to demonstrate that fluoride causes endoplasmic reticulum (ER) stress in ameloblasts responsible for dental enamel formation.
ER-stress occurs if the ER cannot properly fold the proteins it manufactures. Proteins must be correctly folded and assembled in the ER prior to transit to organelles or the cell surface. The ER can be thought of as a processing plant for protein folding and post-translational processing, and the ER is exquisitely sensitive to alterations in homeostasis. Perturbation of calcium levels, redox status, glucose deprivation, overload of cholesterol, and elevated secretory protein synthesis can each interfere with protein folding, resulting in accumulation of unfolded or misfolded proteins in the ER lumen. This causes ER-stress. ER-stress initiates highly specific signaling pathways, termed the unfolded protein response (U PR), to cope with the misfolded proteins. Depending on the ER-stress intensity, the UPR can induce chaperone protein expression to help fold proteins, direct reduced overall protein synthesis, degrade misfolded proteins; and if all else fails, initiate apoptosis [reviewed in: Zhang & Kaufman, J. Biol. Chem. 279:25935–38, 2004].
Our goal is to characterize the fluoride-induced ER-stress response and determine if ER-stress contributes to dental fluorosis.
Once we truly understand how dental enamel is formed, we may be able to replicate the process of enamel development in a test tube. This could generate a ceramic with unique properties potentially useful in a variety of applications, including dental cavity fillings or other applications. More importantly, we may be able to help individuals suffering from amelogenesis imperfecta, a disease in which dental enamel is improperly formed. Our aim in these cases would be to replace the malformed enamel with a suitable synthetic enamel substitute. However, this goal can only be attained once we have a more thorough understanding of how enamel normally forms (i.e., the role of MMP20 in making healthy dental enamel) and a more thorough understanding of how enamel formation can be perturbed (i.e., how fluoride causes dental fluorosis). Through advances such as these, we look forward to an enlightened future.
Tsuchiya M, Sharma R, Tye CE, Sugiyama T, Bartlett JD. 2009. Transforming growth factor-beta1 expression is up-regulated in maturation-stage enamel organ and may induce ameloblast apoptosis. Eur J Oral Sci. 117(2):105-12.
Tye CE, PHam CT, Simmer JP, Bartlett JD. 2009. DPPI may activate KLK3 during enamel formation. J Dent Res. 88(4):323-7.
Tsuchiya M, Tye CE, Sharma R, Smith CE, Bartlett JD. 2008. XBP1 May Determine the Size of teh Ameloblast Endoplasmic Reticulum. J Dent Res. 87(11):1058-62.
Sharma R, Tsuchiya M, Bartlett JD. 2008. Fluoride induces endoplasmic reticulum stress and inhibits protein synthesis and secretion. Environ Health Perspect. 116(9):1142-6.
Lu Y, Pagaerakis P, Yamakoshi Y, Hu JC, Bartlett JD, Simmer JP. 2008. Functions of KLK4 and MMP-20 in dental enamel formation. Biol Chem. 389(6):695-700.
Bartlett JD, Ball RL, Kawai T, Tye FE, Tsuchiya M, Simmer JP. 2006. Origin, splicing, and expression of rodent amelogenin exon 8. J. Dent. Res. 85(10) :894-899.
Bartlett JD, Ganss B, Goldberg M, Moradian-Oldak J, Paine ML, Snead ML, We X,White SN, Zhou YL. 2006. 3. Protein-Protein Interactions of the Developing Enamel Matrix. Curr. Top. Dev. Biol. 74:57–115.
Turk BE, Lee DH, Yamakoshi Y, Klingenhoff A, Reichenberger E, Wright JT, Simmer JP, Komisarof JA, Lewis C, Cantley LC, Bartlett JD. 2006. MMP20 is predominately a tooth-specific enzyme with a deep catalytic pocket that hydrolyzes type V collagen. Biochemistry 45(1 2):3863–3874.
Bartlett JD, Dwyer SE, Beniash E, Skobe Z, Payne-Ferreira TL. 2005. Fluorosis: A new model and new insights. J. Dent. Res. 84(9) :832–836.
Bartlett JD. 2005. Making the cut in dental enamel—The discovery of enamelysin (MMP-20). J. Dent. Res. 84(11):986–988.
Kubota K, Lee DH, Tsuchiya M, Young CS, Everett ET, Martinez-Mier EA, Snead ML, Nguyen L, Urano F, Bartlett JD. 2005. Fluoride induces ER-stress in ameloblasts responsible for dental enamel formation. J. Biol. Chem. 280(24) :23194–23202.
Kim JW, Simmer JP, Hart TC, Hart PS, Ramaswami MD, Bartlett JD, Hu JC. 2005. MMP-20 mutation in autosomal recessive pigmented hypomaturation amelogenesis imperfecta. J. Med. Genet. 42 (3) :271–275.
Bartlett JD, Beniash E, Lee DH, Smith CE. 2004. Decreased mineral content in MMP-20 null mouse enamel is prominent during the maturation stage. J. Dent. Res. 83(3) :909–913.
Simmer JP, Bartlett JD. 2004. Kallikrein 4 is a secreted protein. Cancer Res. 64(22):8481 –8483.
Bartlett JD. 2004. Enamelysin. In: Barrett AJ, Rawlings ND, Woessner JF. (eds). Handbook of Proteoloytic Enzymes, 2nd Edition, Chapter 144, pp. 61–64. Amsterdam: Elsevier Academic Press.
Bartlett JD, Zhou Z, Skobe Z, Dobeck JM, Tryggvason K. 2003. Delayed tooth eruption in membrane type-1 matrix metalloproteinase deficient mice. Connect. Tissue Res. 44(Suppl. 1) :300–304.
Caterina JJ, Skobe Z, Shi J, Ding Y, Simmer JP, Birdedal-Hansen H, Bartlett JD. 2002. Enamelysin (MMP-20) deficient mice display an amelogenesis imperfecta phenotype. J. Biol. Chem. 277(51) :49598–49604.
Bartlett JD, Simmer JP. 1999. Proteinases in developing dental enamel. Crit. Rev. Oral Biol. Med. 10(4):425–441.
Bartlett JD, Simmer JP, Xue J, Margolis HC, Moreno EC. 1996. Molecular cloning and mRNA tissue distribution of a novel matrix metalloproteinase isolated from porcine enamel organ. Gene proteins. 183(1-2):123-128
Ramaswamy Sharma, Ph.D.
Coralee Tye, Ph.D.