When we talk about the brain or the nervous system, the cell type that we most hear about are neurons – the cells that transmit signals throughout the brain. However, in the past couple of decades, there has been enough evidence to show that glial cells (or glia, as they are commonly called) have major roles in providing nutrition and support to neurons. In humans, glia make up approximately 90% of the brain.
There are majorly five types of glia in mammals – Schwann cells, oligodendroglia, astrocytes , microglia and satellite cells – which together provide support and insulation to the neurons, in addition to clearing out the dead cells. Interestingly, glia perform similar roles in the fruit fly Drosophila melanogaster, which is a classic genetic model system for biologists. Therefore, studying glia in Drosophila can contribute immensely to our understanding of the roles of these prominent cell types in our nervous system.
In Drosophila, the number of glia in adults is maintained at a constant number of approximately 650 cells. This could either mean that these cells never die or that their turnover is kept constant such that the dead ones are replaced by an exact number of glial cells again. Led by Prof. Stephen Cohen from the Institute of Molecular and Cell Biology in Singapore, Foo and colleagues aimed to look at which of these situations is true.
In an article recently published in EMBO, the authors identified a subset of bipotent adult neural progenitors characterised by the expression of a micro RNA (miR-31a). MicroRNAs are small biological molecules that are non-coding in nature i.e. they do not carry information for making proteins. Instead, they have regulatory roles such as preventing the expression of certain proteins etc. In an elegant lineage trace experiment tagging cells that express the miR-31a with a heritable expression of green fluorescent protein, the authors show that these cells can give rise to both neurons and glia, but predominantly glia. Contrary to the expectation that flies lacking miR-31a will have fewer glial cells from an early stage in development or may struggle during growth, the mutant flies are both viable and have no developmental defects.
Interestingly, they observe that there is no difference in glial numbers during pupal stages but by day 7 in mutant adult flies, there is a significant decrease in the number of astrocytes, a type of glial cell. This decrease is rescued by day 21 so that the number of glia is restored back to normal. This observation suggests that miR-31a is not responsible for the formation of glia during early development but is important in the adult stages. This also provides solid evidence supporting the theory that new glia are, in fact, produced in adult brain. When the authors deleted some glia by inducing cell death, they were able to show that this is compensated again by glial cell generation and growth, thereby maintaining cell number. However, the later the deletion of glia occurred, the poorer the glial recovery, suggesting loss of plasticity with age. These findings provide interesting mechanistic insights in understanding how the cell number may be maintained in adult brain.
This article was written by Aishwarya Sivakumar and edited by Bonnie Nicholson.