TGFβ1-Smad3 signaling mediates the formation of a stable serine racemase dimer in microglia

D-serine is synthesized by serine racemase (SR), a fold type II class of pyridoxal-5'-phosphate (PLP)-dependent enzyme. Whereas X-ray crystallography reveals that SR can be monomeric, reversible dimers having the highest racemase activity, or stable SR dimers resistant to both denaturation and reductive treatment, showing reduced racemase activity have been detected in microglia and astrocytes; the latter especially in oxidative or inflammatory environments. The microglial inflammatory environment depends largely on the TGF beta 1-mediated regulation of inflammatory cytokines such as TNF alpha and IL1 beta. Here we evaluated the participation of TGF beta 1 in the regulation of SR, and whether that regulation is associated with the induction of stable SR dimers in the microglia from adult mice. In contrast to the effect of lipopolysaccharide (LPS), TGF beta 1 increased the formation of stable SR dimers and reduced the detection of monomers in microglia in culture. LPS or TGF beta 1 did not change the amount of total SR. The increase of stable SR dimer was abolished when TGF beta 1 treatment was done in the presence of the Smad inhibitor SIS3, showing that Smad3 has a role in the induction of stable dimers. Treatment with TGF beta 1 + SIS3 also reduced total SR, indicating that the canonical TGF beta 1 pathway participates in the regulation of the synthesis or degradation of SR. In addition, the decrease of IL1 beta, but not the decrease of TNF alpha induced by TGF beta 1, was mediated by Smad3. Our results reveal a mechanism for the regulation of D-serine through the induction of stable SR dimers mediated by TGF beta 1-Smad3 signaling in microglia.