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A Drosophila in vivo model of the glial repair response

Final Activity Report Summary - GRR (A Drosophila in vivo model of the glial repair response)

Devastating spinal cord injury and brain damage, e.g. injury, demyelinating diseases and stroke, cannot be cured, are human tragedies and represent a great society burden. Damage to the spinal cord or brain induces the proliferation of glial cells conducive to homeostasis maintenance and remyelination, known as the glial repair response (GRR). Although this response is not sufficient to repair the injured nervous system so to enable normal function and full locomotion, it reveals an endogenous tendency of the nervous system to repair itself. Understanding of the molecular mechanism underlying the GRR is a golden opportunity to learn how to manipulate glial cells to promote repair.

The most promising approach to treat the broken spinal cord and brain damage is the transplantation of stem cells or glial progenitors to the site of injury. Transplantation of olfactory ensheathing glia to the site of damage in animal models of spinal cord injury is sufficient to recover full locomotion, revealing that replenishing glial cell populations is sufficient to enable repair. However, the mechanisms controlling the way in which glial progenitor cells divide are largely unknown except for:

1. notch signalling activated by jagged 1 from axons maintains oligodendrocyte progenitors quiescent and able to proliferate
2. release of tumour necrosis factor alpha (TNFa) upon injury triggers proliferation of oligodendrocte progenitors.

The way in which notch and TNF implement these functions, and whether they are related, remains unknown. In the absence of such knowledge, transplantation of stem cells or glial progenitors may not result in repair and can instead induce tumours and cancer. Unravelling the molecular mechanism underlying the proliferation of glial cells is therefore absolutely essential for the therapeutic implementation of repair.

Drosophila is the most powerful model organism for in vivo functional analysis of gene networks. Drosophila led to the discovery of major gene networks involved in human development and disease. During past work, we showed that:

1. the maintenance of glial quiescence in Drosophila required notch, like in vertebrates
2. this also involved the gene prospero
3. glial proliferation upon injury in adult flies required the drosophila TNF homologue eiger.

The aim of this project was to investigate if the functions of notch, prospero and TNF and eiger were linked in a common gene network controlling the GRR to central nervous system (CNS) injury. The project involved standard approaches including genetics, immunohistochemisty, confocal microscopy and statisctical analysis, along with two innovative approaches. A novel experimental paradigm to test the GRR to CNS injury was established in the course of this project. The ventral nerve cords were stabbed by a fine needle and cultured for about 20 hours. After fixation, brains were stained by immunohistochemistry and analysed at the confocal microscope. In collaboration with a post-doc engineer in the Hidalgo group, Dr Forero, a software programme was developed for automatic counting of all glial cells in the larval CNS. This software made it feasible to objectively quantify hundreds to thousands of cells in the CNS in several minutes. These experimental paradigm and analytical tool enabled us to assess the role of genes in the GRR upon neuronal injury by examining the influence of loss and gain of genes’ function.

Our project revealed a gene network involving the functions of the genes notch, prospero, eiger and TNF, and genes controlling the proliferation and cell cycle in GRR. By tweaking with the distinct contribution of each of these genes, glial proliferation could be altered from repair to glioma. It was anticipated that the potential evolutionary conservation of this gene network would provide important insights into the control of mammalian glial progenitor proliferation and repair.