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Transcriptional expression of a gene or genes is absolutely required for induction of glucocorticoid-induced thymocyte apoptosis. We have previously shown that expression of T cell death-associated gene 8 (TDAG8) is quickly induced exclusively in the thymus after dexamethasone (DEX) treatment. Here, we present data that TDAG8 expression is induced prior to induction of DEX-mediated apoptosis. In contrast, TDAG8 expression in thymocytes was not induced in the process of gamma-irradiation-mediated apoptosis. TDAG8 expression accelerated only DEX-induced, but not TCR-mediated or gamma-irradiation-induced, thymocyte apoptosis in transgenic mice overexpressing TDAG8. Interestingly, these effects were specifically detected in CD4(+)CD8(+) double-positive thymocytes. Moreover, activation of caspase-3, -8 and -9 was enhanced in thymocytes of TDAG8 transgenic mice after DEX stimulation. In conclusion, TDAG8 expression is involved in glucocorticoid-induced signals to activate caspase-9, -8 and -3 for subsequent apoptosis induction in CD4(+)CD8(+) double-positive thymocytes.

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How melanosomal proteins such as enzymic proteins (tyrosinase and tyrosinase-related proteins, Tyrps) and structural protein (gp100) are transported from Golgi to melanosomal compartments is not yet fully understood. A number of small GTPases have been found to be associated with melanosomes and we have identified one of them, Rab7, a regulator of vesicular transport, organelle motility, phospholipid signaling and cytosolic degradative machinery, as being involved in the transport of Tyrp1 from Golgi to stage I melanosomes. This study further characterizes the role of Rab7 as a regulator of differential sorting of melanosomal proteins in this process. Murine melanocytes were transiently transfected with a plasmid encoding either wild-type (Rab7WT), constitutively active (Rab7Q67L) or dominant-negative (Rab7N125I and Rab7T22N) Rab7. Through immunocytostaining and confocal laser scanning microscopy, we quantitatively compared the bio-distribution of melanosomal proteins between Rab7WT-expressing cells and mutant Rab7-expressing cells. We also characterized their differential elimination from melanosomal compartments by Rab7 by utilizing a proteasome inhibitor, MG132. Our findings indicate that Rab7 plays an important role in differential sorting of tyrosinase, Tyrp1 and gp100 in early melanogenesis cascade, and that it is more specifically involved with Tyrp1 than tyrosinase and gp100 in the trafficking from Golgi to melanosomes and the specific exit from the degradative process.

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