The effect of sphingomyelin synthase 2 (SMS2) deficiency on the expression of drug transporters in mouse brain.

Sphingomyelin synthase (SMS), the last enzyme involved in the biosynthesis of sphingomyelin (SM), plays a critical role in the constitution of cell membrane and has impact on the expression of membrane proteins. SMS2, one of two SMS enzymes, is predominantly located in the plasma membrane, and is mainly expressed in the brain. Therefore, it is conceivable that SMS2 deficiency may have impact on expression of some membrane proteins, such as membrane-bound drug transporters. Using SMS2 gene deficient mouse brain tissues, we studied the gene and protein expression profiles of drug transporters, ERM proteins (ezrin/radixin/moesin) and the cytoskeleton protein, β-actin, in mouse brain by RT-PCR, western blot and immunohistochemistry analysis. We found that the mRNA expression of Mdr1 rather than the other drug transporters was significantly decreased in the SMS2 deficient brain. Accordingly, the expression and the function of Pgp (Mdr1/P-glycoprotein) were significantly downregulated in brain. In addition, the substantially downregulated expression of ezrin and β-actin was also observed in the SMS2 deficient brain. The immunohistochemistry analysis further revealed the suppressed expression of Pgp, ezrin and β-actin in both cortex and paraventricular areas of SMS2 knockout mice. Furthermore, both Pgp and β-actin were found to be co-immunoprecipitated with ezrin from the total brain lysate, suggesting the association between Pgp, ezrin and β-actin in the brain. These results indicate that SMS2 participates in the expression regulation of drug transporters, particularly Pgp, and suggest that SMS2 may be a potential target for enhancing drug access to the brain.