Effect of D-serine on spermatogenesis and extracellular signal-regulated protein kinase (ERK) phosphorylation in the testis of the silkworm, Bombyx mori

摘要

Although the pupae and larvae of Bombyx mori possess especially large amounts of free D-serine, the physiological role of the amino acid in the silkworm is unknown. We investigated the effect of D-serine on spermatogenesis. A lowered D-serine level throughout larval development caused a delay in spermatogenesis and resulted in reduced numbers of eupyrene sperm. Administration of D-serine transiently increased the activation of extracellular signal-regulated protein kinase1/2 (ERK1/2; hereafter, ERK) by approximately 25% in the testis of day 3 fifth instar larvae. L-Serine had no effect on ERK activation, and other organs did not respond to D-serine. The effect of D-serine on ERK activation was confirmed by administering D-serine dehydratase, an enzyme that specifically degrades D-serine, and the enzyme's inhibitor, hydroxylamine. ERK phosphorylation in the testis was significantly inhibited by Go6983 and U0126, inhibitors of protein kinase C (PKC) and mitogen-associated protein kinase kinase 1/2 (MEK), respectively, but not by H-89, a protein kinase A (PICA) inhibitor, indicating that ERK was activated in the testis via PKC and MEK but not via PICA. The inhibition of ERK phosphorylation by Go6983 or U0126 was reduced by 20-30% by D-serine. Roughly 30% of c-Raf phosphorylation at an inhibitory site (Ser259) was decreased by the addition of D-serine. These results suggest that D-serine activates ERK in the testis of silkworms through a pathway including c-Raf but not PKC or MEK. Immunohistochemistry confirmed D-serine-induced ERK phosphorylation in the testis and revealed the presence of phospho-ERK in the nuclei of spermatocytes and spermatids