Cellular mechanism for transporting calcium in the formation of dental enamel cells

The team found that the main calcium influx pathway involved in the mineralization of enamel [called the CRAC (Ca2+ release-activated Ca2+) channel -- the main type of SOCE (Store-operated Ca2+ entry) channel -- is critical for controlling calcium uptake, which is necessary for the development of tooth enamel. Despite calcium's central role in the development of enamel, it was not previously understood how it was transported from the bloodstream to the zone where enamel crystals grow.

The finding has important implications for people who suffer from abnormal tooth enamel due to mutations in the genes that control the activity of these channels.

"One of the main characteristics of enamel is its durability, which it owes to the particularly high amount of calcium it contains as well as other minerals," says Dr. Lacruz. "But calcium has to reach the area where crystals are forming. If this action is impeded, which happens when there are mutations in the genes that form the core of the CRAC channel, enamel is severely affected."

In this study, the team, which included Dr. Meerim Nurbaeva and Miriam Eckstein in Dr. Lacruz's lab, for the first time used freshly dissected enamel cells (ameloblasts) from rodent teeth to modulate physiological processes in order to understand the contribution of CRAC channels in enamel calcium signaling. The study builds on previous genome research that identified the genes involved in the maturation stage of enamel and other studies which showed that mutations in the genes ORAI1 and STIM1, the main components of the CRAC channel, can affect enamel development. The new study demonstrates a physiological mechanism for calcium influx in enamel cells and shows how it can be modulated.

"By turning off calcium pumps in the endoplasmic reticulum we were able to see the contribution of CRAC channels to calcium uptake and signaling, and more specifically to how they modulate enamel gene function," says Dr. Lacruz.

Gene mutations involved in enamel mineralization through the CRAC channels were first seen in people with a type of severe combined immunodeficiency, a disorder in which -- among other symptoms -- tooth enamel may fall off the teeth within five or six years after birth, and the dentine becomes exposed. The number of cases of these severe genetic disorders is less than 20 worldwide, but depending on where the mutation occurs, individuals may experience tooth enamel defects combined with either minor or severe immune deficiencies in the body. Moreover, teeth might become a biomarker to identify defects in the immune system.

Coauthor Dr. Stefan Feske, of the New York University School of Medicine Department of Pathology, in 2006 discovered ORAI channels, which open and close to allow calcium into cells. This discovery has been critical to solve an important mystery regarding the role of calcium influx in enamel mineralization in which Dr. Feske has also played an important role.

Calcium is stored in the endoplasmic reticulum (ER) of cells until it is needed. In many cells, the protein STIM1 acts as a sensor in the ER, ensuring the right balance of calcium inside the ER and raising an alarm when calcium levels are low. STIM1 interacts with the ORAI channel in the membrane of the cell, allowing calcium from the blood into the cell to restore balance. Enamel defects can be caused by genetic mutations affecting either the STIM1 calcium sensor in the endoplasm reticulum or in the ORAI channel in the cell membrane.

According to Dr. Lacruz, this new frontier in enamel biology brings closer to reality the possibility of regenerating enamel which in the long run will benefit people who suffer from enamel-formation disorders. "You need to know the ingredients required and the mechanisms involved in the transport of calcium. In all likelihood, this research will contribute to the field of tissue regeneration. You have to know how the tissue works in order to mimic its properties."

Additional coauthors included Dr. Axel Concepcion, Department of Pathology, NYU School of Medicine; Dr. Charles Smith, Department of Anatomy & Cell Biology and Faculty of Dentistry, McGill University, Facility for Electron Microscopy Research; Drs. Sonal Srikanth and Yousang Gwack, Department of Physiology, University of California, Los Angeles; Dr. Michael Paine, Center for Craniofacial Molecular Biology, Ostrow School of Dentistry, University of Southern California; and Dr. Michael Hubbard, Departments of Paediatrics and Pharmacology, The University of Melbourne.

This work was funded by NIH/NIDCR K99/R00 award (DE022799) to RSL, NIH grant AI097302 to SF, NIH/NIDCR (DE019629) to MLP, NIH grant AI083432 to YG, and by the Melbourne Research Unit for Facial Disorders to MJH. ARC is funded by a postdoctoral fellowship from the Alfonso Martin Escudero Foundation.