Proteolytic processing of dentin sialophosphoprotein plays essential role in dentinogenesis

Objectives: Dentin sialophosphoprotein (DSPP) is a large precursor protein that is proteolytically processed into the NH₂-terminal fragment (DSP and DSP-PG) and COOH-terminal fragment (DPP). The abundance of DSP, DSP-PG and DPP, along with the scarcity of DSPP in the extracellular matrix (ECM) of dentin suggests that the proteolytic processing of DSPP may be an activation step. The primary objective of this study was to test if the proteolytic processing DSPP is an activation event essential for dentinogenesis. 

Methods: To test this hypothesis we generated transgenic mice expressing DSPP with a substitution of Asp⁴⁵², a residue at a key cleavage site (these mice were designated as “D452A-DSPP”). We believe that Asp⁴⁵² is critical for mouse DSPP processing and substitution of this residue will block the processing of this protein. We also generated transgenic mice expressing normal DSPP (designated as “normal-DSPP”). We crossbred both types of transgenic mice with Dspp null mice to remove the endogenous DSPP and to investigate whether or not the expression of the transgenes can rescue the phenotypic changes of the Dspp null mice.  Biochemical, morphological and radiological approaches were used to characterize the teeth of the transgenic mice D452A-DSPP in the Dspp-null background, in comparison with the transgenic mice expressing normal DSPP in the Dspp-null background, the Dspp-null mice and the wild type mice. 

Results: 1. D452A substitution blocked the proteolytic processing of DSPP in the transgenic mice expressing D452A-DSPP in the Dspp null background. 2. The expression of D452A-DSPP mice was unable to rescue the phenotypic changes of the Dspp null mice while the expression of normal DSPP could. 

Conclusions: The loss of DSPP function resulting from a failure in the cleavage of DSPP indicates that the proteolytic processing of DSPP is an activation step that is essential for dentinogenesis.