Cell death in neural precursor cells and neurons before neurite formation prevents the emergence of abnormal neural structures in the Drosophila optic lobe.

Programmed cell death is a conserved strategy for neural development both in vertebrates and invertebrates and is recognized at various developmental stages in the brain from neurogenesis to adulthood. To understand the development of the central nervous system, it is essential to reveal not only molecular mechanisms but also the role of neural cell death (Pinto-Teixeira et al., 2016). To understand the role of cell death in neural development, we investigated the effect of inhibition of cell death on optic lobe development. Our data demonstrate that, in the optic lobe of Drosophila, cell death occurs in neural precursor cells and neurons before neurite formation and functions to prevent various developmental abnormalities. When neuronal cell death was inhibited by an effector caspase inhibitor, p35, multiple abnormal neuropil structures arose during optic lobe development-e.g., enlarged or fused neuropils, misrouted neurons and abnormal neurite lumps. Inhibition of cell death also induced morphogenetic defects in the lamina and medulla development-e.g., failures in the separation of the lamina and medulla cortices and the medulla rotation. These defects were reproduced in the mutant of an initiator caspase, dronc. If cell death was a mechanism for removing the abnormal neuropil structures, we would also expect to observe them in mutants defective for corpse clearance. However, they were not observed in these mutants. When dead cell-membranes were visualized with Apoliner, they were observed only in cortices and not in neuropils. These results suggest that the cell death occurs before mature neurite formation. Moreover, we found that inhibition of cell death induced ectopic neuroepithelial cells, neuroblasts and ganglion mother cells in late pupal stages, at sites where the outer and inner proliferation centers were located at earlier developmental stages. Caspase-3 activation was observed in the neuroepithelial cells and neuroblasts in the proliferation centers. These results indicate that cell death is required for elimination of the precursor cells composing the proliferation centers. This study substantiates an essential role of early neural cell death for ensuring normal development of the central nervous system.

The role of the effector caspases drICE and dcp-1 for cell death and corpse clearance in the developing optic lobe in Drosophila.

In the developing Drosophila optic lobe, cell death occurs via apoptosis and in a distinctive spatio-temporal pattern of dying cell clusters. We analyzed the role of effector caspases drICE and dcp-1 in optic lobe cell death and subsequent corpse clearance using mutants. Neurons in many clusters required either drICE or dcp-1 and each one is sufficient. This suggests that drICE and dcp-1 function in cell death redundantly. However, dying neurons in a few clusters strictly required drICE but not dcp-1, but required drICE and dcp-1 when drICE activity was reduced via hypomorphic mutation. In addition, analysis of the mutants suggests an important role of effecter caspases in corpse clearance. In both null and hypomorphic drICE mutants, greater number of TUNEL-positive cells were observed than in wild type, and many TUNEL-positive cells remained until later stages. Lysotracker staining showed that there was a defect in corpse clearance in these mutants. All the results suggested that drICE plays an important role in activating corpse clearance in dying cells, and that an additional function of effector caspases is required for the activation of corpse clearance as well as that for carrying out cell death.

Ecdysone-dependent and ecdysone-independent programmed cell death in the developing optic lobe of Drosophila.

The adult optic lobe of Drosophila develops from the primordium during metamorphosis from mid-3rd larval stage to adult. Many cells die during development of the optic lobe with a peak of the number of dying cells at 24 h after puparium formation (h APF). Dying cells were observed in spatio-temporal specific clusters. Here, we analyzed the function of a component of the insect steroid hormone receptor, EcR, in this cell death. We examined expression patterns of two EcR isoforms, EcR-A and EcR-B1, in the optic lobe. Expression of each isoform altered during development in isoform-specific manner. EcR-B1 was not expressed in optic lobe neurons from 0 to 6h APF, but was expressed between 9 and 48 h APF and then disappeared by 60 h APF. In each cortex, its expression was stronger in older glia-ensheathed neurons than in younger ones. EcR-B1 was also expressed in some types of glia. EcR-A was expressed in optic lobe neurons and many types of glia from 0 to 60 h APF in a different pattern from EcR-B1. Then, we genetically analyzed EcR function in the optic lobe cell death. At 0 h APF, the optic lobe cell death was independent of any EcR isoforms. In contrast, EcR-B1 was required for most optic lobe cell death after 24 h APF. It was suggested that cell death cell-autonomously required EcR-B1 expressed after puparium formation. ßFTZ-F1 was also involved in cell death in many dying-cell clusters, but not in some of them at 24 h APF. Altogether, the optic lobe cell death occurred in ecdysone-independent manner at prepupal stage and ecdysone-dependent manner after 24 h APF. The acquisition of ecdysone-dependence was not directly correlated with the initiation or increase of EcR-B1 expression.

Spatio-temporal pattern of programmed cell death in the developing Drosophila optic lobe.

A large number of cells die via programmed cell death during the normal development of the Drosophila optic lobe. In this study, we report the precise spatial and temporal pattern of cell death in this organ. Cell death in the developing optic lobe occurs in two distinct phases. The first phase extends from the start of metamorphosis to the mid-pupal stage. During this phase, a large number of cells die in the optic lobe as a whole, with a peak of cell death at an early pupal stage in the lamina and medulla cortices and the region of the T2/T3/C neurons, and a smaller number of dead cells observed in the lobula plate cortex. The second phase extends from the mid-pupal stage to eclosion. Throughout this period, a small number of dying cells can be observed, with a small peak at a late pupal stage. Most of the dying cells are neurons. During the first phase, dying cells are distributed in specific patterns in cortices. The lamina cortex contains two distinct clusters of dying cells; the medulla cortex, four clusters; the lobula plate cortex, one cluster; and the region of the T2/T3/C neurons, one cluster. Many of the clusters maintain their distinct positions in the optic lobe but others extend the region they cover during development. The presence of distinct clusters of dying cells at different phases suggests that distinct mechanisms control cell death during different stages of optic lobe development in Drosophila.