Participation of d-serine in the development and reproduction of the silkworm Bombyx mori

Abstract

The silkworm Bombyx mori contains high concentrations of free d-serine, an optical isomer of l-serine. To elucidate its function, we first investigated the localization of d-serine in various organs of silkworm larvae, pupae, and adult moths. Using immunohistochemical analysis with an anti-d-serine antibody, we found d-serine in the microvilli of midgut goblet and cylindrical cells and in peripheral matrix components of testicular and ovarian cells. By spectrophotometric analysis, d-serine was also found in the hemolymph and fat body. d-Alanine was not detected in the various organs by immunohistochemistry. Serine racemase, which catalyzes the inter-conversion of l- and d-serine, was found to co-localize with d-serine, and d-serine production from l-serine by intrinsic serine racemase was suggested. O-Phospho-l-serine is an inhibitor of serine racemase, and it was administered to the larvae to reduce the d-serine level. This reagent decreased the midgut caspase-3 level and caused a delay in spermatogenesis and oogenesis. The reagent also decreased mature sperm and egg numbers, suggesting d-serine participation in these processes. d-Serine administration induced an increase in pyruvate levels in testis, midgut, and fat body, indicating conversion of d-serine to pyruvate. On the basis of these results, together with our previous investigation of ATP biosynthesis in testis, we consider the possible involvement of d-serine in ATP synthesis for metamorphosis and reproduction.