Regrowing Teeth?

A sorry fact of humanity is the shared fate of adult teeth. Once gone, they never return. As humanity progressed from the ancient to the antibiotic, dentistry danced the same progressive rhythm. Dental technology went from primitive ‘dental drills’ to much more advanced dental drills, treatments from frogs bathed in moonlight to a complex ‘classification system of periodontal diseases’. Two recent breakthroughs have increased this gap, with researchers from Harvard University and then King’s College London making leaps in regenerative medicine in the last decade. Restorative dentistry has always been performed artificially: with crowns, fillings and root canal treatments (RCTs) used to combat the damage done to teeth. Dental caries (decay and cavities) is the seemingly unavoidable fate of permanent teeth, with 92% of adults uncomfortably familiar with the pain they cause. The researchers’ new findings could spell the end of fillings, introducing a simpler way to repair the cavities caused by dental caries. But there is still a significant leap between repairing teeth and regenerating them completely. Dental caries may well be overcome, but the inevitability of tooth loss still reigns supreme against modern medicine. Assuming the researchers’ breakthroughs are viable and pass clinical trials, the ability to regrow whole teeth might not be so far away. But in order to predict the likelihood of tooth regrowth in some distant tomorrow, it would be useful to understand the field’s most recent breakthroughs of today.

Harvard’s Research: A Bright Idea Researchers used low-powered lasers to stimulate oral stem cells to differentiate into dentin: a major mineral component of the tooth below the enamel. The laser generates reactive oxygen species (ROS) in the area they illuminate: the superoxide (O2•) free radical and the hydrogen peroxide (H2O2) nonradical. The superoxide free radical is formed by the addition of an unpaired electron onto an oxygen molecule. The hydrogen peroxide nonradical is formed by the addition of another unpaired electron onto that. In either case, an unstable ROS is formed, venturing out into the void with a desire to steal an electron from some unsuspecting molecule. These reactive species activate transforming growth factor-β (TGF-β) molecules: handy polypeptides that provide a motivational boost to dividing cells. The activated TGF-β molecules initiate the signalling pathway responsible for dentin production. During TGF- β signalling, the activated TGF- β molecules attach to a receptor-complex on the cell-surface membrane of an oral stem cell. Kinases (phosphorylating enzymes) on the receptor phosphorylate SMAD proteins (transcription factors that regulate genetic expression) in the cytoplasm, which are activated in turn and eventually form SMAD complexes of their own. These SMAD complexes enter the nucleus and latch on to TGF- β target genes within the DNA of the stem cells. It is these target genes that give stem cells purpose as they jerk into life: a few coughs and splutters later and fully-fledged odontoblasts are born. This sudden army of odontoblasts within the tooth collectively secrete enough dentin to repair the dental caries. To recap, reactive oxygen species activate the TGF-β signalling pathway, which directs the differentiation of stem cells towards dentine-producing odontoblasts. These then promptly go about their business repairing the dental caries in the tooth. All that from a low-powered laser.

King’s Research: High Hopes Researchers made the breakthrough after much hard work pointed them in the direction of one particular drug. The drug: ‘Tideglusib’, was originally a treatment for Alzheimer’s before scientists suspected its uses in encouraging tooth regeneration. This was useful as it meant the drug had already passed clinical trials, increasing the chances of a modified version being fit for human consumption. The drug contains ‘small molecule GSK-3 antagonists’, which upregulate necessary cell-signalling within the tooth to stimulate stem cells. Dentin covers the ‘soft inner pulp tissue’ of the tooth. Once dental caries penetrates and damage the pulp tissue, Wnt/β-catenin signalling is responsible for upregulating dentin levels within the tooth to partially repair the cavity. β-catenin levels are regulated in the body by its incorporation into a destruction complex, which uses the Ubiquitin protein to mark β-catenin for proteasomal degradation. Simply put, β-catenin levels in stem cells are normally kept low. However, the introduction of Wnt – when the pulp tissue is damaged – encourages the destruction complex to migrate towards the cell-surface membrane. The destruction complex contains the Dishevelled (Dvl) protein, which is phosphorylated by the LRP receptor at the membrane. This phosphorylation activates the Dvl to inhibit the reduction of β-catenin. In short, the presence of Wnt inhibits the complex, resulting in an increase in β-catenin levels. The raised levels of β-catenin promote the transcription of Wnt target genes within the nucleus, which encourages differentiation into dentin – just as the TGF-β target genes do with Harvard’s laser method. This process occurs naturally, but the dentin produced cannot close the dental caries entirely. The Tideglusib drug is effective because it takes this natural process and speeds it up. The drug contains targeted inhibitors to the GSK-3 enzymes present in the destruction complex. The GSK-3 enzyme limits the amount of reparative dentine produced via phosphorylation (the addition of a phosphoryl group) of β-catenin, resulting in the constant ubiquitination and degradation that keeps cellular β-catenin levels low. Inhibitors work by distorting the bonds – hydrogen or otherwise - within the enzyme by binding to either its active or allosteric site, being structurally similar to the substrate. This distorts the active site and renders the enzyme inactive. The inhibitors to GSK-3 are able to bind to the enzyme due to similarities in functional groups, disrupting the substrate from forming a complex with and being catalysed by the enzyme. These targeted inhibitors therefore block its phosphorylation of β-catenin by distorting the active site – either competitively or non-competitively – so it can no longer catalyse the appropriate reactions. β-catenin is therefore upregulated: free from its purgative cycle of ubiquitination and degradation, and therefore able to promote the transcription of Wnt target genes in the nucleus and stimulate stem cells to differentiate into dentine. These artificial inhibitors perform this task faster than the natural inhibition of the destruction complex does, meaning that more of the dentine lost to dental caries can be restored in a given time.

Conclusion

After exploring recent breakthroughs in dental regeneration, it is almost certain that dental caries is soon to become an easily curable annoyance rather than anything chronic. It can be assumed, then, that the regrowth of permanent teeth is only a breakthrough away from becoming a reality. Stem cells appear to be pivotal to such a breakthrough, with both techniques above utilising them to repair regenerate dentin. Surely those same stem cells could be directed to regenerate other parts of the tooth? The roots and the enamel, for example, are other vital sections that would need to be regrown. Unfortunately, it doesn’t appear to be that simple. In 2010, Professor Paul Sharpe – who helped pioneer the GSK-3 antagonists technique – put the brakes on any optimistic customers thinking of swapping dentures for dentin. He addressed the many hurdles to be overcome before permanent teeth regrowth in humans can be attempted: issues with accessing appropriate quantities of necessary cells and the lack of speed with which human teeth regenerate compared to rodents. Tooth regrowth in mice has been achieved but doing the same thing in humans is an entirely different matter. Since 2010, these hurdles still haven’t been overcome, with the techniques above regenerating dentin exclusively in our rodent cousins. Despite promising progress from recent breakthroughs, it remains unlikely that teeth regrowth will become an option to us in the near future. Headway has been made, but it all falls short of being practically applicable. That isn’t to say a spark hasn’t been struck: much research and excitement has been ignited in the hopes of regrowing a whole human tooth. But until that tempting tomorrow, the dentist chair will remain a necessary experience for us all. All the lasers and drugs in the world won’t spur teeth upwards just yet. The sorry fact of humanity will remain – at least for now – the shared fate of all adult teeth.

Sources:

P. R. Arany, A. Cho, T. D. Hunt, G. Sidhu; Photoactivation of Endogenous Latent Transforming Growth Factor–β1 Directs Dental Stem Cell Differentiation for Regeneration. Science Translational Medicine; vol 6 no. 238 (May 2014); pages 1-22

V. C. M. Neves, R. Babb, D. Chandrasekaran, P. T. Sharpe; Promotion of natural tooth repair by small molecule GSK3 antagonists. Scientific Reports; vol 7 (January 2017); pages 1-7

R. Lui, L. P. Desai; Reciprocal regulation of TGF-β and reactive oxygen species: A perverse cycle for fibrosis. Redox Biology; vol 6 (Dec 2015) pages 565-577

B. T. MacDonald, K. Tamai and X. He; Wnt/β-catenin signaling: components, mechanisms, and diseases. Developmental Cell; vol 17 no. 1 (July 2009); pages 9-26

J. L. Stamos, W. I. Weis; The β-Catenin Destruction Complex. Cold Spring Harbor Perspectives in Biology; vol 5 no. 1 (Jan 2013); pages 1-16

A. A. Valponi, Y. Pang, P. T. Sharpe; Stem cell-based biological tooth repair and regeneration. Trends in Cell Biology; vol 20 no. 12 (Dec 2010); pages 715-722