One protein hardens enamel while the other weakens it, say USC researchers

A gene critical to tooth formation expresses a protein that is then cleaved into two proteins with seemingly opposite functions, according to a University of Southern California (USC)-led team of dental researchers.

The team's study of the two proteins, dentin sialoprotein (DSP) and dentin phosphoprotein (DPP), has been accepted by the Journal of Biological Chemistry and is available on the journal's Website.

Lead author Michael Paine of the USC School of Dentistry said both proteins derive from the gene for dentin sialophosphoprotein, which plays an important role in the formation of the tooth coverings enamel and its softer internal cousin dentin.

"We were able to dissect this gene into two different proteins and look at them individually," Paine said.

The researchers conducted animal studies in which either DSP or DPP were over-expressed in forming enamel during the period of tooth development. They found that over-expression of DSP increased the hardness of enamel and its rate of formation, while over-expression of DPP created pitted and chalky enamel that was more prone to fracture and wear.

In normal teeth, DSP is expressed only in dentin and a very thin layer of enamel at the junction with dentin. This thin enamel layer also appears to be considerably harder than the bulk enamel of teeth, Paine said. He suggested that DSP could have the potential to become a protective agent in dental care.

If the protein could be incorporated into the entire layer of enamel, Paine said, "then it might act in a similar way to fluoride in water" by making teeth harder and more resistant to decay.

Paine cautioned that, just as heavy fluoridation can weaken teeth, excessive expression of DSP could be detrimental.

"There might be a point where if you increase the hardness anymore, teeth might be too brittle."

While the other protein, DPP, appears to weaken enamel, it too is necessary for proper tooth formation.

"All the data suggests that it [DPP] is one of the few proteins that seems to be involved with the very early stages of mineralization," Paine said.

The fine balance between DSP and DPP highlights the delicacy of the critical dentin-enamel junction, where the softer dentin is joined securely to the outer, ceramic-like enamel covering.

Dental researchers sometimes liken dentin and enamel to a bed mattress and a glass plate, respectively, Paine said, with the difference that the supple dentin-enamel junction prevents the enamel from shattering over an individual's lifetime of chewing and grinding.

The study built on the work of co-author Mary MacDougall, a former USC researcher who in 1997 was the first to show that DSP and DPP came from the same gene.