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Apoptosis of the midline glia during Drosophila embryogenesis: a correlation with axon contact
1Department of Biology, McMaster University, Hamilton, Ontario, Canada.
Abstract:
We have examined cell death within lineages in the midline of Drosophila embryos. Approximately 50% of cells within the anterior, middle and posterior midline glial (MGA, MGM and MGP) lineages died by apoptosis after separation of the commissural axon tracts. Glial apoptosis is blocked in embryos deficient for reaper, where greater than wild-type numbers of midline glia (MG) are present after stage 12. Quantitative studies revealed that MG death followed a consistent temporal pattern during embryogenesis. Apoptotic MG were expelled from the central nervous system and were subsequently engulfed by phagocytic haemocytes. MGA and MGM survival was apparently dependent upon proper axonal contact. In embryos mutant for the commissureless gene, a decrease in axon-glia contact correlated with a decrease in MGA and MGM survival and accelerated the time course of MG death. In embryos mutant for the slit gene, MGA and MGM maintained contact with longitudinally and contralaterally projecting axons and MG survival was comparable to that in wild-type embryos. The initial number of MG within individual ventral nerve cord segments was increased by ectopic expression of the rhomboid gene, without changing axon number. Extra MGA and MGM were eliminated from the ventral nerve cord by apoptosis to restore wild-type numbers of midline glia. Ectopic rhomboid expression also shifted MGA and MGM cell death to an earlier stage of embryogenesis. One possible explanation is that axon-glia contact or communication promotes survival of the MG and that MG death may result from a competition for available axon contact.
Insights
Midline glia (MG) in Drosophila embryos undergo apoptosis, with survival depending on axon contact. Disrupting this contact increases MG death, suggesting competition for axons drives glial cell elimination.
Area of Science:
- Developmental Biology
- Neuroscience
- Cell Biology
Background:
- Midline glial cells (MG) are crucial for central nervous system development in Drosophila.
- Apoptosis plays a role in regulating cell numbers during embryogenesis.
Purpose of the Study:
- To investigate the mechanisms regulating cell death in Drosophila midline glial lineages.
- To determine the role of axon-glia interactions in midline glial survival.
Main Methods:
- Analysis of cell death in wild-type and mutant Drosophila embryos.
- Genetic manipulation including gene deficiency (reaper), mutations (commissureless, slit), and ectopic gene expression (rhomboid).
- Quantitative assessment of glial cell numbers and apoptosis timing.
Main Results:
- Approximately 50% of anterior, middle, and posterior midline glial (MGA, MGM, MGP) cells undergo apoptosis after commissural axon tract separation.
- Glial apoptosis is inhibited in reaper-deficient embryos.
- Midline glial survival is dependent on axon-glia contact; reduced contact (commissureless mutants) increases death, while maintained contact (slit mutants) preserves survival.
- Ectopic rhomboid expression increases initial MG numbers but triggers apoptosis to restore wild-type levels, shifting death to an earlier stage.
Conclusions:
- Axon-glia contact or communication is essential for midline glial survival.
- Midline glial cell death may result from competition for available axon contacts.
- Apoptosis acts as a mechanism to regulate midline glial populations based on axonal interactions.