Yvonne M. Nolan

The age‐related attenuation in long‐term potentiation is associated with microglial activation

It is well established that inflammatory changes contribute to brain ageing, and an increased concentration of proinflammatory cytokine, interleukin‐1β (IL‐1β), has been reported in the aged brain associated with a deficit in long‐term potentiation (LTP) in rat hippocampus. The precise age at which changes are initiated is unclear. In this study, we investigate parallel changes in markers of inflammation and LTP in 3‐, 9‐ and 15‐month‐old rats. We report evidence of increased hippocampal concentrations of the proinflammatory cytokines IL‐1α, IL‐18 and interferon‐γ (IFNγ), which are accompanied by deficits in LTP in the older rats. We also show an increase in expression of markers of microglial activation, CD86, CD40 and intercellular adhesion molecules (ICAM). Associated with these changes, we observed a significant impairment of hippocampal LTP in the same rats. The importance of microglial activation in the attenuation of long‐term potentiation (LTP) was demonstrated using an inhibitor of microglial activation, minocycline; partial restoration of LTP in 15‐month‐old rats was observed following administration of minocycline. We propose that signs of neuroinflammation are observed in middle age and that these changes, which are characterized by microglial activation, may be triggered by IL‐18.

The influence of microglia on the pathogenesis of Parkinson's disease

Parkinsons disease (PD) is characterised by degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Inflammation may be associated with the neuropathology of PD due to the following accumulating evidence: excessive microglial activation and increased levels of the pro-inflammatory cytokines tumour necrosis factor-alpha and interleukin-1beta in the SNpc of patients with PD; the emergence of PD-like symptoms following influenza infection; the increased susceptibility to PD associated with bacterial vaginosis; the presence of inflammatory mediators and activators in animal models of PD; the ability of anti-inflammatory drugs to decrease susceptibility to PD; and the emerging possibility of the use of microglial activation inhibitors as a therapy in PD. In this review, we will discuss the role of inflammation in PD. We will focus on the influence of microglia in the pathogenesis of PD and discuss potential therapeutic interventions for PD, that target microglia.

CD200 Ligand–Receptor Interaction Modulates Microglial Activation In Vivo and In Vitro: A Role for IL-4

Deficits in cognitive function are associated with neuroinflammatory changes, typified by activation of glial cells and an alteration of the pro- and anti-inflammatory cytokine balance in the brain. Although there is evidence to suggest that activation of microglia is regulated by interaction with other cell types in the brain, the mechanism(s) involved is poorly understood. Here, we provide evidence that interaction between CD200 and its receptor plays a role in modulating microglial activation under conditions of chronic and acute inflammation of the brain. We report that interleukin-4 (IL-4) plays a central role in modulating expression of CD200 and identify a mechanism by which IL-4 directly controls microglial cell activation. Our findings provide the first demonstration of a role for IL-4 in modulating CD200 expression and suggest a mechanism for regulation of microglial activation in the intact CNS under inflammatory conditions.

Contributions of central and systemic inflammation to the pathophysiology of Parkinson's disease

Idiopathic Parkinsons disease (PD) represents a complex interaction between the inherent vulnerability of the nigrostriatal dopaminergic system, a possible genetic predisposition, and exposure to environmental toxins including inflammatory triggers. Evidence now suggests that chronic neuroinflammation is consistently associated with the pathophysiology of PD. Activation of microglia and increased levels of pro-inflammatory mediators such as TNF-α, IL-1β and IL-6, reactive oxygen species and eicosanoids has been reported after post-mortem analysis of the substantia nigra from PD patients and in animal models of PD. It is hypothesised that chronically activated microglia secrete high levels of pro-inflammatory mediators which damage neurons and further activate microglia, resulting in a feed forward cycle promoting further inflammation and neurodegeneration. Moreover, nigrostriatal dopaminergic neurons are more vulnerable to pro-inflammatory and oxidative mediators than other cell types because of their low intracellular glutathione concentration. Systemic inflammation has also been suggested to contribute to neurodegeneration in PD, as lymphocyte infiltration has been observed in brains of PD patients and in animal models of PD, substantiating the current theory of a fundamental role of inflammation in neurodegeneration. We will examine the current evidence in the literature which offers insight into the premise that both central and systemic inflammation may contribute to neurodegeneration in PD. We will discuss the emerging possibility of the use of diagnostic tools such as imaging technologies for PD patients. Finally, we will present the immunomodulatory therapeutic strategies that are now under investigation and in clinical trials as potential neuroprotective drugs for PD.

Lipopolysaccharide‐induced increase in signalling in hippocampus is abrogated by IL‐10 – a role for IL‐1β?

Parenterally administered lipopolysaccharide (LPS) increases the concentration of the pro‐inflammatory cytokine interleukin‐1β (IL‐1β) in the rat hippocampus and evidence suggests that this effect plays a significant role in inhibiting long‐term potentiation (LTP). The anti‐inflammatory cytokine IL‐10, antagonizes certain effects of IL‐1β, so if the effects of LPS are mediated through an increase in IL‐1β, it might be predicted that IL‐10 would also abrogate the effect of LPS. Here, we report that IL‐10 reversed the inhibitory effect of LPS on LTP and the data couple this with an inhibitory effect on the LPS‐induced increase in IL‐1β. LPS treatment increased hippocampal expression of IL‐1 receptor Type I protein. Consistent with the LPS‐induced increases in IL‐1β concentration and receptor expression, were downstream changes which included enhanced phosphorylation of IRAK and the stress‐activated kinases, JNK and p38; these LPS‐induced changes were reversed by IL‐10, which concurs with the idea that these events are triggered by increased activation of IL‐1RI by IL‐1β. We provide evidence which indicates that LPS treatment leads to evidence of cell death and this was reversed in hippocampus prepared from LPS‐treated rats which received IL‐10. The evidence is therefore consistent with the idea that IL‐10 acts to protect neuronal tissue from the detrimental effects induced by LPS.

Downregulation of IL-4-induced signalling in hippocampus contributes to deficits in LTP in the aged rat.

Ageing is characterized by deficits in learning and memory and by a deficit in long-term potentiation (LTP) in hippocampus. Several age-related changes, including dysfunction of calcium homeostatic mechanisms and upregulation of inflammatory processes are likely to contribute to these deficits. Here we exploited the fact that aged rats fall into a subgroup which fail to sustain LTP in perforant path granule cell synapses as a result of tetanic stimulation, and a subgroup which sustains LTP in a manner indistinguishable from young rats, in an effort to identify differential changes in the two subgroups. The age-related increase in IL-1beta concentration and IL-1beta-induced signalling was more profound in aged rats which failed to sustain LTP. We demonstrate that functional IL-4 receptors are expressed in rat hippocampus and that age is associated with a decrease in IL-4 concentration accompanied by a decrease in phosphorylation of JAK-1 and STAT-6. We propose that the imbalance between pro-inflammatory and anti-inflammatory cytokines in the aged brain significantly contributes to age-related deficits in synaptic function.

Tumour necrosis factor-α impairs neuronal differentiation but not proliferation of hippocampal neural precursor cells: Role of Hes1

Tumour necrosis factor-alpha (TNFalpha) is a pro-inflammatory cytokine, which influences neuronal survival and function yet there is limited information available on its effects on hippocampal neural precursor cells (NPCs). We show that TNFalpha treatment during proliferation had no effect on the percentage of proliferating cells prepared from embryonic rat hippocampal neurosphere cultures, nor did it affect cell fate towards either an astrocytic or neuronal lineage when cells were then allowed to differentiate. However, when cells were differentiated in the presence of TNFalpha, significantly reduced percentages of newly born and post-mitotic neurons, significantly increased percentages of astrocytes and increased expression of TNFalpha receptors, TNF-R1 and TNF-R2, as well as expression of the anti-neurogenic Hes1 gene, were observed. These data indicate that exposure of hippocampal NPCs to TNFalpha when they are undergoing differentiation but not proliferation has a detrimental effect on their neuronal lineage fate, which may be mediated through increased expression of Hes1.

Evidence that lipopolysaccharide-induced cell death is mediated by accumulation of reactive oxygen species and activation of p38 in rat cortex and hippocampus

Lipopolysaccharide (LPS) administration stimulates immune activation, inflammation and deterioration in cell function. Neuronal tissue in cortex and hippocampus are particularly susceptible. In this study, we report that LPS induces cell death as measured by caspase-3 activation and DNA fragmentation and that this is coupled with stimulation of the mitogen-activated protein kinase, p38. We provide evidence of co-localization of activated p38 and caspase-3 in cells prepared from cortical and hippocampal tissue after LPS treatment. Furthermore, administration of the p38 inhibitor, SB203580, abolished the LPS-induced increase in caspase-3 activation. We observed that LPS treatment provoked accumulation of reactive oxygen species (ROS) while in vitro incubation of cortical and hippocampal tissue with H(2)O(2) increased p38 activity. In addition, H(2)O(2)-induced activation of caspase-3 was abrogated by SB203580. We propose, based on the data presented, that the action of LPS to induce cell death in cortex and hippocampus may be mediated by ROS accumulation and activation of p38.

Imaging of neurosphere oxygenation with phosphorescent probes

Multicellular spheroids are useful models of mammalian tissue for studies of cell proliferation, differentiation, replacement therapies and drug action. Having a size of 100-500 μm they mimic in vivo micro-environment and characteristic gradients of O2, pH and nutrients. We describe the use of cell-penetrating O2 probes based on phosphorescent Pt-porphyrins to perform high-resolution 2D and 3D mapping of O2 in spheroid structures by live cell fluorescence imaging technique. Optimised procedures for preparation of neurospheres from cortical neural cells isolated from embryonic rat brain, their staining with the phosphorescent O2 probes NanO2 and MM2 and subsequent analysis of oxygenation on different live cell imaging platforms, including widefield and confocal phosphorescence lifetime imaging microscopy (PLIM), conventional confocal and two-photon ratiometric intensity based O2 detection are presented. This is followed by a series of physiological experiments in which oxygenation patterns of the neurospheres are correlated with culturing conditions (atmospheric hypoxia and hyperoxia, size, growth factors), distribution of stem cells, mature neurons and astrocytes, HIF-2α stabilisation and responses to metabolic stimulation. The O2 imaging method allows multiplexing with many conventional fluorescent probes to perform multi-parametric imaging analysis of cells in 3D microenvironment. It can be applied to other types of spheroids and 3D tissue models.

A role for interleukin-1β in determining the lineage fate of embryonic rat hippocampal neural precursor cells.

Neurogenesis occurs in the hippocampus of the developing and adult brain due to the presence of multipotent stem cells and restricted precursor cells at different stages of differentiation. It has been proposed that they may be of potential benefit for use in cell transplantation approaches for neurodegenerative disorders and trauma. Prolonged release of interleukin-1β (IL-1β) from activated microglia has a deleterious effect on hippocampal neurons and is implicated in the impaired neurogenesis and cognitive dysfunction associated with aging, Alzheimers disease and depression. This study assessed the effect of IL-1β on the proliferation and differentiation of embryonic rat hippocampal NPCs in vitro. We show that IL-1R1 is expressed on proliferating NPCs and that IL-1β treatment decreases cell proliferation and neurosphere growth. When NPCs were differentiated in the presence of IL-1β, a significant reduction in the percentages of newly-born neurons and post-mitotic neurons and a significant increase in the percentage of astrocytes was observed in these cultures. These effects were attenuated by IL-1 receptor antagonist. These data reveal that IL-1β exerts an anti-proliferative, anti-neurogenic and pro-gliogenic effect on embryonic hippocampal NPCs, which is mediated by IL-1R1. The present results emphasise the consequences of an inflammatory environment during NPC development, and indicate that strategies to inhibit IL-1β signalling may be necessary to facilitate effective cell transplantation approaches or in conditions where endogenous hippocampal neurogenesis is impaired.