Molecular events in thymocyte apoptosis

Steroid hormone control of cell death and cell survival: molecular insights using RNAi

The insect steroid hormone ecdysone triggers programmed cell death of obsolete larval tissues during metamorphosis and provides a model system for understanding steroid hormone control of cell death and cell survival. Previous genome-wide expression studies of Drosophila larval salivary glands resulted in the identification of many genes associated with ecdysone-induced cell death and cell survival, but functional verification was lacking. In this study, we test functionally 460 of these genes using RNA interference in ecdysone-treated Drosophila l(2)mbn cells. Cell viability, cell morphology, cell proliferation, and apoptosis assays confirmed the effects of known genes and additionally resulted in the identification of six new pro-death related genes, including sorting nexin-like gene SH3PX1 and Sox box protein Sox14, and 18 new pro-survival genes. Identified genes were further characterized to determine their ecdysone dependency and potential function in cell death regulation. We found that the pro-survival function of five genes (Ras85D, Cp1, CG13784, CG32016, and CG33087), was dependent on ecdysone signaling. The TUNEL assay revealed an additional two genes (Kap-alpha3 and Smr) with an ecdysone-dependent cell survival function that was associated with reduced cell death. In vitro, Sox14 RNAi reduced the percentage of TUNEL-positive l(2)mbn cells (p<0.05) following ecdysone treatment, and Sox14 overexpression was sufficient to induce apoptosis. In vivo analyses of Sox14-RNAi animals revealed multiple phenotypes characteristic of aberrant or reduced ecdysone signaling, including defects in larval midgut and salivary gland destruction. These studies identify Sox14 as a positive regulator of ecdysone-mediated cell death and provide new insights into the molecular mechanisms underlying the ecdysone signaling network governing cell death and cell survival.

A balance between the diap1 death inhibitor and reaper and hid death inducers controls steroid-triggered cell death in Drosophila

The steroid hormone ecdysone directs the massive destruction of obsolete larval tissues during Drosophila metamorphosis, providing a model system for defining the molecular mechanisms of steroid-regulated programmed cell death. Although earlier studies have identified an ecdysone triggered genetic cascade that immediately precedes larval tissue cell death, no death regulatory genes have been functionally linked to this death response. We show here that ecdysone-induced expression of the death activator genes reaper (rpr) and head involution defective (hid) is required for destruction of the larval midgut and salivary glands during metamorphosis, with hid playing a primary role in the salivary glands and rpr and hid acting in a redundant manner in the midguts. We also identify the Drosophila inhibitor of apoptosis 1 as a survival factor in the larval cell death pathway, delaying death until its inhibitory effect is overcome by rpr and hid. This study reveals functional interactions between rpr and hid in Drosophila cell death responses and provides evidence that the precise timing of larval tissue cell death during metamorphosis is achieved through a steroid-triggered shift in the balance between the Drosophila inhibitor of apoptosis 1 and the rpr and hid death activators.

Programmed cell death (apoptosis) in cord blood lymphocytes

Cord blood lymphocytes are functionally immature and have deficient immune responses. In order to determine whether the process of programmed cell death is distinct between cord blood and peripheral blood lymphocytes, we analyzed the expression of fas and bax (apoptosis promoting genes) and bcl-2 and bcl-xL (apoptosis inhibiting genes) at protein or mRNA levels using flow cytometry and quantitative PCR methods, respectively. The susceptibility of T cell subsets from cord blood and adult peripheral blood to undergo apoptosis following restimulation with anti-CD3 or anti-Fas monoclonal antibodies was also studied. We observed that cord blood T cell subsets expressed lower levels of Fas and Bcl-2, a low bcl-2/bax ratio, and higher bcl-xL compared to peripheral blood. Additionally, upon primary stimulation with anti-CD3, cord blood T cell subsets were more resistant to apoptosis compared to peripheral blood. In contrast, rechallenge of previously stimulated lymphocytes with anti-CD3 monoclonal antibody triggered apoptosis in a larger proportion of T cells from cord blood as compared to peripheral blood, whereas the number of T cells undergoing anti-Fas-induced programmed cell death were lower in cord blood compared to peripheral blood.