Talking to the Brain at the Blood-Brain Barrier through Inflammation-Induced Prostaglandin E2

Abstract

The immune-to-brain signaling is a critical survival factor when the body is confronted by pathogens, and in particular by microorganisms. During infections, the ability of the immune system to engage the central nervous system (CNS) in the management of the inflammatory response is just as important as its ability to mount a specific immune response against the pathogen, since the CNS can provide a systemic negative feed-back to the immune activation by release of stress hormones and also can prioritize the usage of the energy resources by the vital organs. Prostaglandin E2 (PGE2) and pro-inflammatory cytokines were among the first mediators to be identified to participate in the immune-to-brain signaling, a process that is clinically recognized by the development of manifestations of common illness such as fever, anorexia, decreased social interactions, lethargy, sleepiness, and hyperalgesia.In this thesis the contribution of PGE2 to the immune-to-brain signaling was further characterized at the blood-brain-barrier (BBB) and in the anterior preoptic area (POA) of the hypothalamus (i.e. the thermoregulatory region or, in sickness, the fever generating region).BBB is the major interface region between peripheral circulating cytokines and the neuronal parenchyma and a critical source of PGE2. Using chimeric mice lacking the inducible enzyme for PGE2 synthesis, microsomal PGE synthase-1 (mPGES-1), in either hematopoietic or nonhematopoietic cells, we demonstrate in paper I that brain endothelial cells are the critical source of PGE2 in BBB during peripheral inflammation. For the demonstration of the mPGES-1 expression in the BBB cells we developed in paper I a method for enzymatic dissociation of these cells, followed by fluorescence activated cell sorting (FACS). Using the same method, we show in paper II that the BBB response to immune stimuli is towards an increased production of PGE2 in endothelial cells and an increased sensitivity of these cells for proinflammatory cytokines. These changes are supported by decreased PGE2 degradation and decreased synthesis of other prostanoids in perivascular macrophages, which hence respond in concordance with the endothelial cells in enhancing PGE2 signaling.Once released in the neuronal tissue, PGE2 has been shown to be critical for the fever response by acting on the type 3 PGE2 receptors (EP3) within POA. By laser capture microdissection (LCM) we extracted the EP3 receptor expressing region in POA, defined by in situ hybridization histochemistry, from mouse brain sections. We demonstrate in paper III that the predominant subtypes of the EP3 receptor in POA are EP3α and EP3γ. In paper IV we further analyze the effect of PGE2 on the LCM dissected EP-rich POA using gene expression microarrays. We demonstrate that PGE2 has a limited effect on the gene expression changes within POA, suggesting that the neuronal activity is modulated by PGE2 in a transcriptionindependent manner and that the profound gene expression changes that are seen in the CNS during inflammation ...