Abstract by Emil Jakobsen

The second messenger cyclic AMP (cAMP) is a ubiquitous signaling molecule, regulating a large variety of biological processes. cAMP is produced by adenylyl cyclases (ACs), it can stimulate four different types of cAMP effector proteins, and are being degraded by a class of enzymes known as phosphodiesterases. As such, cAMP signaling may be seen as a simple canonical pathway. However, research over the last four decades revealed that cAMP signaling is highly organized and operates in tightly controlled cAMP compartments. This compartmentalization ensures specificity in cAMP signaling, linking upstream signals to specific downstream effects.

The brain is an extremely complex organ, consisting of several regions and cell types, carrying out different functions to collaboratively allow the amazing functions of the brain. One of these cell types, the astrocyte, plays an important role in restoring homeostasis in the brain after neuronal activity and thereby support neuronal function. The brain consumes large amounts of energy and dependents on constant delivery of glucose and oxygen from the blood stream. Astrocytes contains glycogen, an intracellular storage of glucose. The function of this storage has long been neglected and considered irrelevant due to the brains dependency on blood-borne glucose. However, research over the last couple of decades revealed that glycogen metabolism in astrocytes is occurring during physiological neuronal function. The importance of glycogen metabolism was highlighted by a series of studies revealing that memory formation was prevented if glycogen degradation was inhibited.

While it is well known that cAMP signals can induce glycogen degradation, the molecular details of the signaling pathway that couples neuronal activity to glycogen degradation in astrocytes is currently not fully elucidated, although some pathways involving the Ca²⁺-activated AC8 and soluble adenylyl cyclase (sAC) has been suggested.

The aims of this PhD thesis was to 1) investigate what is known about cAMP compartmentalization in astrocytes, 2) Characterize the functional output of inhibiting sAC in astrocytes and thereby elucidate its possible roles in astrocytic energy- and glycogen metabolism and 3) Investigate if AC8-mediated cAMP signals are capable of inducing glycogen degradation.

First we reviewed the current literature of cAMP compartmentalization in astrocytes. We found that surprisingly little is known about the details of which ACs and/or cAMP domains that are involved in mediated known cAMP effects in astrocytes.

Next, we demonstrated how inhibition of sAC in primary cultured astrocytes decreased oxidative phosphorylation, which verified that astrocytes contains a mitochondrial cAMP domain depending on sAC-produced cAMP. This decreased oxidative phosphorylation led to activation of AMP-activated protein kinase (AMPK) and degradation of glycogen.

Lastly, we demonstrated how stimulation of AC8, either via stimulation of Gα_q-coupled receptors or by direct stimulation, with a newly discovered activator, induced glycogen degradation in U87 MG glioblastoma cells. This response was eliminated in U87 AC8KO cells.

The work presented in this thesis demonstrates that little is currently known about cAMP compartmentalization in astrocytes. Our findings of the role of sAC in astrocytic energy metabolism and the role of AC8 in U87 glycogen metabolism, adds small pieces of information towards a better understanding of this compartmentalization.