There and back again: functional outcomes of reciprocal neuron-astrocyte signalling

Neurons do not exist in isolation in the central nervous system, and there is a growing appreciation that the interactions between neuronal and non-neuronal cells are fundamentally important for nervous system function. A major family of non-neuronal cells are the astrocytes, with a surge of recent work suggesting the relationship between neurons and astrocytes is bidirectional and highly complex. In my thesis I seek to further uncover the nature of this intimate relationship between neurons and astrocytes of the cortex. One well-established role of astrocytes is the collection of neuronal glutamate via their high affinity excitatory amino acid transporters, with dysfunctions in this system being linked to numerous neurological diseases. Previous reports suggest that neurons may regulate the expression of these astrocytic glutamate transporters, through an as yet unknown pathway. In my thesis I first investigate the nature of this non-cell-autonomous neuronal control of astrocytes. I begin by using results from the lab’s novel mixed-species RNA-sequencing dataset to explore how neurons regulate astrocytic gene expression, finding that they upregulated the astrocytic glutamate transporters. By electrophysiological recording I show a corresponding functional increase in the astrocytes’ ability to collect glutamate, before demonstrating that neurons upregulate the astrocytic transporters through Notch signalling. I then investigate whether continuous Notch signalling is required to maintain these transporters’ expression and function, finding that removal of Notch signalling after the establishment of transporter expression significantly reduces the transporters’ activity. For the remainder of my thesis I explore how cortical astrocytes may in turn control cortical neuronal function. Using RNA-seq data generated in the lab I discover a host of neuronal genes that are regulated by astrocytes. Amongst these genes were the functionally important K+ inward rectifying channel family, which were strongly downregulated in neurons by astrocytes, an observation hitherto unseen. I hypothesise that this downregulation will result in alterations to neuronal membrane properties which will enhance neuronal excitability, and that this may in turn have down-stream consequences on neuronal activity and synaptogenesis. I find that cortical neurons are rendered more excitable by astrocytes, leading to an enhancement of neuronal activity, driven by the astrocyte-induced decrease in K+ inward rectifiers. Although I do not see an increase in baseline synaptogenesis, I show a range of homeostatic neuronal responses emerge in the presence of astrocytes. This work suggests that astrocytes play a central role in regulating neuronal activity