Programmed cell death acts at different stages of Drosophila neurodevelopment to shape the central nervous system

Abstract

Nervous system development is a process that integrates cell proliferation, differentiation, and programmed cell death (PCD). PCD is an evolutionary conserved mechanism and a fundamental developmental process by which the final cell number in a nervous system is established. In vertebrates and invertebrates, PCD can be determined intrinsically by cell lineage and age, as well as extrinsically by nutritional, metabolic, and hormonal states. Drosophila has been an instrumental model for understanding how this mechanism is regulated. We review the role of PCD in Drosophila central nervous system development from neural progenitors to neurons, its molecular mechanism and function, how it is regulated and implemented, and how it ultimately shapes the fly central nervous system from the embryo to the adult. Finally, we discuss ideas that emerged while integrating this information.

Keywords: Drosophila; apoptosis; neurodevelopment.

Figure 1. Reaper, Hid, Grim and Sickle orchestrate apoptosis in Drosophila

(A) Reaper, Hid, Grim, and Sickle (RHG) integrate signals from different sources, and trigger apoptosis. RHG act through two independent pathways. They either inhibit DIAP1, and/or other inhibitors of apoptosis, or they act by promoting mitochondria permeabilization. Both pathways ultimate lead to the activation of caspases, cysteine proteases that implement the apoptotic program. Solid arrows represent proven interactions and dashed lines represent unproven interactions. (B) Schematic representation of the genomic locus of the third chromosome where RHG are located. The neuroblast regulatory region (NBRR) is situated between rpr and grim and is responsible for the integration of developmental signals that control reaper, grim and sickle expression, and, hence, apoptosis. A cis-regulatory element (enh-1) is responsible for the restricted expression of the pro-apoptotic genes in the abdominal neuroblasts. Three main genomic deletions that have been used to study apoptosis in flies. In deficiency Df(3l)H99, rpr, hid, and grim are removed, while in Df(3l)XR38, rpr and skl are not present. Finally, rpr⁸⁷ only removes rpr. Black arrows represent the deleted regions, and red bars the pro-apoptotic genes. The NBRR is represented in green and, within it, the enh-1 enhancer in light green.

Figure 2. Neuroblast fate is regulated by Hox gene expression and differs between abdominal and thoracic segments

(A) In the embryo, the identity of each segment depends on Hox gene activity. Sex combs reduced (Scr) and Antennapedia are active in the thoracic segments, while Ubx, Abdominal-A, and Abdominal-B are active in the abdominal and caudal segments, in a colinear manner with respect to their positions in the Hox cluster. The ventral nerve chord (VNC) is similarly subdivided into 17 neuromeres, 3 gnathal, 3 thoracic, 7 abdominal, and 4 caudal. (B) Upon delamination, 30 neuroblasts are observed in each thoracic and abdominal hemisegment. At the end of embryogenesis some thoracic and most of the abdominal neuroblasts are eliminated by PCD. When the larva hatches, 23 post embryonic neuroblasts per hemisegment are found in the thorax, 12 in A1, 4 in A2, and 3 in each of the A3-A8 abdominal hemisegments. Neuroblasts that are maintained to the larvae are labeled in black, apoptotic neuroblasts are labeled in red, and potential thoracic apoptotic neuroblasts are labeled in dashed red.

Figure 3. Patterning of neuroblast mitotic activity and apoptosis throughout development

Patterning of neuroblast activity and apoptosis during embryonic, larval and pupal development. Colored bars represent the mitotic activity of the neuroblasts depicted in the left cartoon (gnathal neuroblasts are not visible). Red dashed lines indicate the timing of neuroblast apoptosis. Note that gnathal neuroblasts division is approximate and deduced based on limited description in the literature (see main text). The timing of the female caudal neuroblast apoptosis and gnathal neuroblasts is approximate (see main text). Neuroblast number is representative and does not correspond to the real number. Abbreviations: MB, Mushroom body; OL, Optic Lobe; Gn, Gnathal; T,Thorax; A, Abdominal.

Figure 4. Neuroblast tTFs control apoptosis in both neuroblasts and neurons

(A) Abdominal neuroblasts have to transit trough the tTFs to schedule apoptosis. The tTF Castor downregulates the expression of Dichaete that inhibits precocious RHG activation and upregulates Grh. Grh installs the competence to respond to a pulse of AbdA that triggers apoptosis. (B) Neuroblasts tTFs controls Notch mediated neuronal apoptosis. Two pathways determine the binary life-or-death fate of neurons. tOPC neuroblasts tTFs control both the identity of the neurons produced at each time window and their survival by specifying the death of Notch^ON neurons in a first phase and of Notch^OFF neurons in a second phase. The red arrows indicate the dying neurons.