| This thesis describes two lines of research that explore the transcriptional basis of astrocyte physiology in situ. The first line examines the role of astrocytes in supporting neuronal metabolism. Compartmentalized metabolic pathways characterize metabolic support of neurons by astrocytes, and result in distinct metabolite profiles between astrocytes and neurons. Evidence supporting compartmentalized metabolism derives mainly from in vitro studies, since it is technically challenging to study metabolic processes in vitro in single astrocytes or neurons. The lack of evidence from in vitro has led to much debate around the cell type specific compartmentalization of metabolic pathways, and thus, to uncertainty about metabolic processes during behavioral rest and activity. The first part of this thesis aims to clarify the compartmentalized nature of metabolic pathways between astrocytes and neurons in situ . The results demonstrate that astrocytes, much like neurons, have a robust capacity for oxidative metabolism, and confirm the compartmentalized nature of lactate and glutamate metabolism in situ. Thus, astrocytes exhibit pathways for metabolic support of neurons, however, both neurons and astrocytes display self-sufficiency for glucose oxidation.;The second line of research investigates the role of astrocytes in paracrine purinergic signaling. Over the past decade new roles of astrocytes have emerged. Astrocytes are chemically equipped with receptors for neurotransmitters that enable them to receive information about ongoing synaptic activity. In response to activity astrocytes can release chemical substances, termed gliotransmitters, which include the purine ATP. As a gliotransmitter, ATP was first described as eliciting autocrine effects in cultured astrocytes. However, recent evidence has indicated that ATP is released in situ in amounts sufficient to modify neuronal transmission. This observation suggests that the roles of astrocytes are not limited to homeostatic support of neurons. To investigate the paracrine nature of ATP, I first present the paracrine purinergic transcriptome. This allows, for the first time, a comprehensive analysis of the cellular expression pattern of purinergic signaling molecules from astrocytes, neurons, microglia, endothelial cells, oligodendrocyte precursor cells, and vascular smooth muscle cells in situ. These results indicate that the purinergic signals, ATP and its metabolite ADP, are mainly involved in signaling between glia, while excitatory neurons are poor in ATP/ADP signaling. In contrast, neurons have the A1 adenosine receptor, which is known to mediate synaptic depression. Because the extracellular environment exhibits potent enzymatic activities that rapidly can degrade astrocytic released ATP into adenosine, I next asked whether ATP is a source of adenosine mediating synaptic depression under tonic and activity-dependent excitatory activity. The results demonstrate that the ectoenzyme CD73 is crucial for AMP to adenosine conversion in the brain, and upon manipulating its activity, I found that ATP is a source of adenosine during tonic, but not during activity-dependent, Al-receptor mediated synaptic depression. Altogether, these results suggest that astrocytes in situ provide metabolic support for neurons, and that paracrine purinergic signaling between astrocytes and neurons is a tonic phenomenon. |
