How do Astrocytes Release ATP?

About This Grant

Project Summary Astrocytes respond to neuronal activity via various neurotransmitter receptors, triggering intracellular calcium increases in astrocytes that are both necessary and sufficient to induce changes in neural circuit activity and behavior. Substantial evidence from several model organisms has suggested that purinergic signaling is required downstream of astrocyte calcium events, supporting the idea that ATP is released from astrocytes, broken down to adenosine, and then signals to neurons through adenosine receptors to regulate neural circuit function. However, we still do not understand the precise mechanisms by which ATP may be released. To investigate how ATP and adenosine may be involved in astrocyte regulation of neural circuits, I performed preliminary experiments in Drosophila using ex vivo live imaging of a fluorescent biosensor of extracellular ATP expressed in astrocyte membranes. In contrast to the predictions of the current model of astrocyte neuromodulation, induction of calcium activity in astrocytes via bath application of neurotransmitters did not result in increases in extracellular ATP. However, I did observe ATP events evoked by neuronal activation with K+ bath application or electrical stimulation. These events were blocked by the pannexin/innexin blocker, probenecid. I will build on these preliminary results by investigating the signals that lead to ATP release and identifying the molecule responsible for induced ATP release. In Aim 1, I will block synaptic release and astrocyte calcium elevations to determine if either is required for ATP release. I also will perform chemogenetic activation of neuronal subtypes (e.g. glutamatergic, GABAergic) to determine which (if any) are responsible for inducing ATP release. I also will use primary astrocyte cell culture to evaluate if astrocytes have cell-type intrinsic responses to K+ elevations. In Aim 2, I will use cell-type specific genetic knockdown/knockout of known targets of probenecid, members of the Innexin gene family, in combination with live imaging to identify the molecule responsible for induced ATP release in the fly brain. Under the mentorship of Dr. Marc Freeman and aided by the fantastic intellectual and material resources available at the Vollum and OHSU, I will gain additional technical expertise in live imaging as well as developing new skills in cloning and transgenesis, and culture of mammalian astrocytes. I will also further my skills in scientific communication, leadership, and teaching and mentorship. This training plan will provide abilities crucial for me to reach my long-term goal of becoming an independent researcher studying the fundamental signaling mechanisms in the brain. The proposed research plan will provide a new mechanistic understanding of how ATP and adenosine signaling – the primary means by which astrocytes regulate neuronal activity – is regulated in vivo. Given the deep conservation of astrocyte neuromodulation and adrenergic signaling from flies, to zebrafish to mouse, I expect my work will provide important new insights into astrocyte adrenergic signaling in the human brain.