Spring 2008, Issue 8

Bioengineering New Teeth

Pamel YelickPamela Yelick, PhD, joined Tufts University School of Dental Medicine in 2006. She is director of the Division of Craniofacial and Molecular Genetics in the Department of Oral and Maxillofacial Pathology and is a member of the Sackler School’s Genetics Program and its Cell, Molecular, and Developmental Biology Program. Yelick’s research group investigates development and regeneration in craniofacial cartilage, bone, and teeth by taking two different but complementary approaches: using zebrafish as a genetic and development model and using postnatal dental progenitor cells for whole-tooth tissue engineering.

After earning a PhD in molecular biology from Tufts University in 1989, where she studied with Norman Hecht and Susan Ernst, Yelick pursued postdoctoral training at the Dana–Farber Cancer Institute and worked as a research associate at Harvard University. She continues to lecture at Harvard School of Dental Medicine, the Boston University Goldman School of Dental Medicine, and the Department of Continuing Education at Harvard Medical School, where she lectures on tissue engineering.

Zebrafish Genetic and Development Model

Zebrafish (Danio rerio) shed and regenerate teeth continuously during their two-year life span. These little minnows have significant nucleotide and amino acid sequence identity to higher vertebrates (including humans), they are highly fecund, and they grow rapidly and are transparent—attributes that make them attractive not only as a model for tooth development but also for other genetic and development questions. Yelick plans to grow her supply of approximately 5,000 fish to more than 20,000 within the next year, allowing her to extend their use, and her group’s expertise, to other Tufts investigators.

The first stage of development for teeth in zebrafish as well as humans is the formation of tooth buds. Whereas zebrafish retain their ability to form tooth buds throughout life, humans lose this ability once permanent tooth buds are formed below the primary teeth. Yelick’s group is working to understand the molecular cues that induce zebrafish tooth regeneration. One avenue of investigation is to identify a line of mutant zebrafish that cannot regenerate teeth, then manipulate the genome in such a way as to reinstate the ability. “Wild-type zebrafish have a full set of 11 teeth, on each of two bilateral supporting arches,” says Yelick. “One mutant we’ve identified has only two teeth on each side. We know the main gene mutation in that mutant, but we want to learn what the downstream signaling factors are, and to see whether we can rescue replacement-tooth formation in this mutant.” The research group is using saturation mutagenesis screens to identify genes acting within replacement-tooth signaling pathways.

Yelick’s group is examining two signaling cascades involved in tooth development—transforming growth factor-ß and fibroblast growth factor 8. They have identified four zebrafish receptor genes, including the novel type I receptor activin-like kinase 8 (alk8). Studies of alk8 contributed to the identification of the zebrafish alk8 mutant, lost-a-fin (laf/alk8), which the Yelick group is using for in vivo analyses of alk8 function. “While homozygous recessive laf/alk8 mutants exhibit an early lethal phenotype, heterozygous laf/alk8 animals are viable and fertile but do not form replacement teeth,” says Yelick. “We anticipate that studies of laf/alk8 and other mutants will reveal molecular targets that may facilitate replacement-tooth induction in humans.”

One unusual technique Yelick’s research group applies is in vivo mineralized tissue staining. Zebrafish embryos are placed in a staining solution and the stain gets incorporated into mineralized tissue. The embryos are then returned to the fish tank to develop. Live, transparent fish can be anesthetized periodically, monitored under fluorescent light for tooth development, and returned to the tank.

Oral Tissue Engineering with Postnatal Dental Progenitor Cells

Postnatal dental progenitor cells are available from primary and adult tooth pulp, and from wisdom tooth tissues. Yelick’s research group is working to develop tooth replacement therapies in which a new tooth develops from a bioengineered biodegradable scaffold that is seeded with a patient’s own tooth progenitor cells and implanted in the patient’s jaw. The group is testing several scaffold materials and designs to identify best characteristics for growing oral tissues, including whole teeth. David Kaplan, Stern Family Professor in Engineering and chair of biomedical engineering at the School of Engineering and director of the Tissue Engineering Resource Center, is collaborating with Yelick on this project—in particular on using silk scaffolds. The Yelick group is also testing a new nanofabricated material called biorubber, in collaboration with Robert Langer’s team at Massachusetts Institute of Technology. Long-term collaborator Joseph Vacanti, director of the Laboratory for Tissue Engineering and Organ Fabrication at Massachusetts General Hospital, initiated tooth tissue-engineering studies with Yelick about seven years ago.

Yelick is wrapping up a study of pig dental pulp cells that she began before coming to Tufts. “We isolated immature tooth buds from a pig, dissociated the cells into single-cell suspensions and cultured them in vitro for about a week, then seeded them onto polyester scaffolds and implanted the scaffolds into an adult rat host,” Yelick explains. “When we removed this implant we saw small developing tooth structures that look just like natural teeth. When you cross section the implants, you find a little tooth farm—here’s a molar-like structure, here’s an incisor, and there’s pulp and predentin. This means that the single cells were able to reorganize themselves into what appears to be a whole tooth.” The scaffold, however, was filled with many small teeth instead of one full-sized tooth. Yelick’s research group is currently modifying the scaffold material and design to be able to guide the cells to organize into a single large tooth. Yelick would like to continue working with pig tooth bud cells, possibly collaborating with researchers at the Cummings School of Veterinary Medicine. She is also looking forward to working with human tooth tissues in collaboration with clinicians at the dental school.

“What I hope to do is establish collaborations with many researchers at the university,” says Yelick. “We have a beautiful setup here, including microinjection stations, microscopy, and a state-of-the-art fish facility. We are currently working on three or four collaborative research projects. We want to really maximize use of the zebrafish facility to its full extent.”

For more information, please see http://www.tufts.edu/sackler/genetics/facultypages/yelick.

 

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