Expression and localization of sulphated glycoprotein-2 mRNA in the rat incisor tooth ameloblasts: relationships with apoptosis.

The expression of sulphated glycoprotein-2 (SGP-2) is associated with the onset of cellular atrophy and death in many rodent tissues. This gene has a multifunctional involvement that includes apoptosis, spermatogenesis, promotion of cell-cell interactions, modulation of complement systems and tissue regeneration and remodelling. Using decalcified mandibles, mRNA for SGP-2 in rat incisor tooth ameloblasts was examined by in situ hybridization using 35S riboprobes. The rat incisor is unique in that, at one time, all stages of the complex life cycle of the ameloblasts are represented along the length of the enamel forming aspect of the tooth. The pre-ameloblasts only secrete enamel matrix after mitosis. When the full thickness of the enamel has been formed, a remarkable transition in phenotype takes place in the ameloblast. This transition is accompanied by apoptosis or programmed cell death of approximately 25% of ameloblasts. An additional 25% of ameloblasts undergo apoptosis when maturation of enamel matrix takes place with removal of water and protein from the increasingly mineralized matrix. In the present study, expression of SGP-2 was localized most often in the post-secretory transition and maturation ameloblasts. In contrast, the presecretory and secretory ameloblasts did not demonstrate specific hybridization signals. Consistently, neither the odontoblasts nor the pulp demonstrated hybridization signals. Hence our results support other published results which show that increased expression of SGP-2 is associated with apoptosis. The exact function of the SGP-2 gene and its products is not fully defined.(ABSTRACT TRUNCATED AT 250 WORDS)