Sinead M. Gibney

The immune theory of psychiatric diseases : a key role for activated microglia and circulating monocytes

This review describes a key role for mononuclear phagocytes in the pathogenesis of major psychiatric disorders. There is accumulating evidence for activation of microglia (histopathology and PET scans) and circulating monocytes (enhanced gene expression of immune genes, an overproduction of monocyte/macrophage‐related cytokines) in patients with bipolar disorder, major depressive disorder, and schizophrenia. These data are strengthened by observations in animal models, such as the MIA models, the chronic stress models, and the NOD mouse model. In these animal models of depressive‐, anxiety‐, and schizophrenia‐like behavior, similar activations of microglia and circulating monocytes can be found. These animal models also make in‐depth pathogenic studies possible and show that microglia activation impacts neuronal development and function in brain areas congruent with the altered depressive and schizophrenia‐like behaviors.

Poly I:C-induced activation of the immune response is accompanied by depression and anxiety-like behaviours, kynurenine pathway activation and reduced BDNF expression.

In this study we characterised the ability of the viral mimetic poly I:C to induce a neuroinflammatory response and induce symptoms of depression and anxiety in rats. Furthermore, the ability of poly I:C to deplete central tryptophan and serotonin via induction of indolamine 2,3 dioxygenase (IDO), and also the ability of poly I:C to impact upon expression of the neurotrophin BDNF and its receptor TrkB were examined as potential mechanisms to link inflammation to depression. Poly I:C induced a neuroinflammatory response characterised by increased expression of IL-1β, IL-6, TNF-α and CD11b in frontal cortex and hippocampus. In the first 24h following poly I:C administration rats displayed sickness behaviour characterised by reduced locomotor activity and weight gain. Anhedonia measured using the saccharin preference test was used as an indicator of depressive behaviour, and poly I:C induced depressive behaviour that persisted for up to 72h following administration. Anxiety was measured using the open field test and anxious behaviour was observed 24h following poly I:C, a time-point when sickness behaviour had resolved. These behavioural changes were accompanied by decreased expression of BDNF and TrkB in hippocampus and frontal cortex. In addition, poly I:C increased central IDO expression and increased concentrations of tryptophan, and its metabolite kynurenine. However this activation of the kynurenine pathway did not result in reduced central serotonin concentrations. These findings suggest that depressive and anxiety-like behaviours elicited by poly I:C are associated with a reduction in BDNF signalling, and activation of the kynurenine pathway, but not a reduction in serotonin.

Evidence for a Dysregulated Immune System in the Etiology of Psychiatric Disorders

There is extensive bi-directional communication between the brain and the immune system in both health and disease. In recent years, the role of an altered immune system in the etiology of major psychiatric disorders has become more apparent. Studies have demonstrated that some patients with major psychiatric disorders exhibit characteristic signs of immune dysregulation and that this may be a common pathophysiological mechanism that underlies the development and progression of these disorders. Furthermore, many psychiatric disorders are also often accompanied by chronic medical conditions related to immune dysfunction such as autoimmune diseases, diabetes and atherosclerosis. One of the major psychiatric disorders that has been associated with an altered immune system is schizophrenia, with approximately one third of patients with this disorder showing immunological abnormalities such as an altered cytokine profile in serum and cerebrospinal fluid. An altered cytokine profile is also found in a proportion of patients with major depressive disorder and is thought to be potentially related to the pathophysiology of this disorder. Emerging evidence suggests that altered immune parameters may also be implicated in the neurobiological etiology of autism spectrum disorders. Further support for a role of immune dysregulation in the pathophysiology of these psychiatric disorders comes from studies showing the immunomodulating effects of antipsychotics and antidepressants, and the mood altering effects of anti-inflammatory therapies. This review will not attempt to discuss all of the psychiatric disorders that have been associated with an augmented immune system, but will instead focus on several key disorders where dysregulation of this system has been implicated in their pathophysiology including depression, schizophrenia and autism spectrum disorder.

Inhibition of stress-induced hepatic tryptophan 2,3-dioxygenase exhibits antidepressant activity in an animal model of depressive behaviour

The role of hepatic tryptophan 2,3 dioxygenase (TDO) was assessed in the provocation of stress-induced depression-related behaviour in the rat. TDO drives tryptophan metabolism via the kynurenine pathway (KP) and leads to the production of neuroactive metabolites including kynurenine. A single 2 h period of restraint stress in adult male Sprague-Dawley rats provoked an increase in circulating concentrations of the glucocorticoid corticosterone and induction of hepatic TDO expression and activity. Repeated exposure to stress (10 d of 2 h restraint each day) provoked an increase in immobility in the forced swimming test (FST) indicative of depression-related behaviour. Immobility was accompanied by an increase in the circulating corticosterone concentrations, expression and activity of hepatic TDO and increase in the expression of TDO in the cerebral cortex. Increased TDO activity was associated with raised circulating kynurenine concentrations and a reduction in circulating tryptophan concentrations indicative of KP activation. Co-treatment with the TDO inhibitor allopurinol (20 mg/kg, i.p.), attenuated the chronic stress-related increase in immobility in the FST and the accompanying increase in circulating kynurenine concentrations. These findings indicate that stress-induced corticosterone and consequent activation of hepatic TDO, tryptophan metabolism and production of kynurenine provoke a depression-related behavioural phenotype. Inhibition of stress-related hepatic TDO activity promotes antidepressant activity. TDO may therefore represent a promising target for the treatment of depression associated with stress-related disorders in which there is evidence for KP activation.

Noradrenaline acting on astrocytic β2-adrenoceptors induces neurite outgrowth in primary cortical neurons

The neurotransmitter noradrenaline (NA) has anti-inflammatory properties and promotes expression of neurotrophic factors in the central nervous system (CNS) via activation of glial adrenoceptors. Here we examined the ability of conditioned media (CM) from NA-treated glial cells to impact upon neuronal complexity. Primary rat cortical neurons were treated either directly with NA (1-10 μM), or treated with CM from NA-stimulated primary mixed glial cells. Neuronal complexity was assessed using Sholl analysis. Exposure of neurons to CM from NA-stimulated glial cells increased all indices of neuronal complexity, whereas direct exposure of neurons to NA did not. CM from NA-stimulated astrocytes, but not microglia, also increased neuronal complexity indicating a key role for astrocytes. The β-adrenergic subtype was implicated in this response as the increase was blocked by the β-adrenoceptor antagonist propanolol, but not by the α-adrenoceptor antagonist phentolamine. CM from glial cells treated with the β₂-adrenoceptor agonists salmeterol and clenbuterol, but not the β₁-adrenoceptor agonist xamoterol, mimicked the ability of NA to increase neuronal complexity. NA induced expression of a range of growth factors (BDNF, NGF-β, GDNF, FGF-2 and IL-6) in glial cells. In addition to this, the phosphatidylinositol 3-kinase (PI3K), mitogen activated protein kinase (MAPK) and JAK-STAT signalling pathways are implicated in NA CM-induced neuritic growth as inhibition of these pathways attenuated NA CM-induced neuritic growth. In conclusion, this study indicates a novel role for NA acting at glial β₂-adrenoceptors to induce neuritic growth through the expression of soluble factors that elicit a neurotrophic action and increase neuronal complexity.

Exaggerated Increases in Microglia Proliferation, Brain Inflammatory Response and Sickness Behaviour upon Lipopolysaccharide Stimulation in Non-Obese Diabetic Mice

The non-obese diabetic (NOD) mouse, an established model for autoimmune diabetes, shows an exaggerated reaction of pancreas macrophages to inflammatory stimuli. NOD mice also display anxiety when immune-stimulated. Chronic mild brain inflammation and a pro-inflammatory microglial activation is critical in psychiatric behaviour. Objective: To explore brain/microglial activation and behaviour in NOD mice at steady state and after systemic lipopolysaccharide (LPS) injection. Methods: Affymetrix analysis on purified microglia of pre-diabetic NOD mice (8-10 weeks) and control mice (C57BL/6 and CD1 mice, the parental non-autoimmune strain) at steady state and after systemic LPS (100 μg/kg) administration. Quantitative PCR was performed on the hypothalamus for immune activation markers (IL-1β, IFNγ and TNFα) and growth factors (BDNF and PDGF). Behavioural profiling of NOD, CD1, BALB/c and C57BL/6 mice at steady state was conducted and sickness behaviour/anxiety in NOD and CD1 mice was monitored before and after LPS injection. Results: Genome analysis revealed cell cycle/cell death and survival aberrancies of NOD microglia, substantiated as higher proliferation on BrdU staining. Inflammation signs were absent. NOD mice had a hyper-reactive response to novel environments with some signs of anxiety. LPS injection induced a higher expression of microglial activation markers, a higher brain pro-inflammatory set point (IFNγ, IDO) and a reduced expression of BDNF and PDGF after immune stimulation in NOD mice. NOD mice displayed exaggerated and prolonged sickness behaviour after LPS administration. Conclusion: After stimulation with LPS, NOD mice display an increased microglial proliferation and an exaggerated inflammatory brain response with reduced BDNF and PDGF expression and increased sickness behaviour as compared to controls.

Dr. James Matthew Lipton, 1938-2016

Dr. Jim Lipton died on the 10th of July this year. It was a great loss not only to his family and friends but also to the scientific world. The facts and dates of his life are well expressed in the obituary written by his daughter reprinted above, with permission. Many years ago, when Jim and his buddy, S.M. ‘Don’ McCann were both Professors of Physiology in Dallas, they suggested we start a new NIM journal. I was opposed to the idea, because there were already too many journals proliferating at a dizzying pace. The success of the ISNIM journal proved that they were right and I was wrong. One could not imagine two more disparate personalities, but they always managed to work together harmoniously. Both were geniuses, Jim soft-spoken and modest ... and Don, with an amazing photographic memory, boisterous (Don was first my professor at the University of Pennsylvania, and later, my student and honorary lecturer and awardee of the Novera Spector lectureship). A book needs to be written about Don, but most observers agree that he should have shared at least one Nobel Prize. Both remained my good friends all their lives. At one point I went to an international congress organized by Jim, only to find that he had dedicated this meeting to me! I hope that his friends and especially his wife, Luby, will I have been asked to write an obituary-memoriam for the cofounder of Neuroimmunomodulation (NIM) , who died on the 10th of July this year. Below is an obituary written by Jim’s daughter and reprinted with her permission, followed by some informal remembrances from Luby, his widow, and me.

96. Anti-depressants and kynurenine pathway inhibition: A therapeutic strategy for neuroprotection in vitro?

The kynurenine pathway (KP) is the main pathway by which tryptophan is metabolized. Its rate limiting enzyme indolamine 2, 3-dioxygenase (IDO) catalyses the production of kynurenine, which in turn is metabolised by either kynurenine 3-hydroxylase (KMO) which leads to the production of quinolinic acid (neurotoxic), or by kynurenine aminotransferase (KAT II) which yields kynurenic acid (neuroprotective). These enzymes are localised in microglia and astrocytes respectively. The present studies demonstrated that IFN-γ induces expression of IDO, KMO and kynase along with the production of neurotoxic molecules from BV-2 microglia. This was determined using a cell culture system whereby microglia were activated with IFN-γ and this conditioned medium was then placed on primary cortical neurons. Inhibition of the kynurenine pathway with 1-(D)-methyl-tryptophan (IDO inhibitor) and Ro 61-8048 (KMO inhibitor) completely attenuated this microglial-mediated neurotoxicity, as did the NMDA antagonist MK-801. Furthermore the anti-depressant fluoxetine partially attenuated this neurotoxicity and IFN-γ induced increases in IDO and kynase mRNA expression. Inhibition of classical neurotoxic factors released from microglia i.e. iNOS and the enzyme NADPH oxidase had no effect on this microglial mediated neurotoxicity, and neither did the anti-inflammatory agent dexamethasone. These results propose IFN-γ activated microglia induce neurotoxicity by activating the KP, and this can be attenuated using fluoxetine, suggesting a possible role for anti-depressants in ameliorating neuronal damage and in influencing the KP balance.

154. The serotonin transporter and monoamine oxidase A as novel pharmacological targets to inhibit microglial activation

Clenbuterol is a brain penetrant beta2-adrenoceptor agonist that has anti-inflammatory actions and elicits neuroprotective effects. Here we evaluated the efficacy of clenbuterol in combating the inflammatory response in an acute brain injury model induced by intrastriatal administration of the inflammatory cytokine IL-1beta. Clenbuterol (0.5 mg/kg; i.p.) was administered to rats one hour prior to intra-striatal microinjection of IL-1beta (100 ng). Four hours postinjection, rats were deeply anaesthetised with urethane, blood samples were collected via cardiac puncture for plasma chemokine analysis and ipsilateral striatum and liver tissue were harvested for mRNA expression analysis. Intrastriatal IL-1beta microinjection auto-induced its own mRNA expression in the striatum, and increased expression of the inflammatory cytokine TNF-alpha, the chemokines CINC-1, MCP-1 and IP-10, and cell-adhesion molecules ICAM-1 and VCAM-1. Pre-treatment with clenbuterol inhibited this increased expression of IL-1beta, TNF-alpha, MCP-1, IP-10, ICAM-1 and VCAM-1 in the striatum, but did not inhibit striatal CINC-1 expression. Clenbuterol also inhibited the IL-1beta-induced increase in TNF-alpha, MCP-1 and IP-10 expression in the liver. However, clenbuterol treatment alone provoked CINC-1 expression in the liver. Moreover, with the exception of CINC-1, clenbuterol suppressed IL1beta-induced hepatic expression of the chemokines. Clenbuterol also suppressed the level of circulating CINC-1 protein. This study provides evidence that the beta2-adrenoceptor agonist clenbuterol has anti-inflammatory effects in an acute brain injury model, and suppresses the acute phase response to brain injury. Supported by HRB Ireland.