Stuart M. Allan

Cytokines and acute neurodegeneration

Cytokines have been implicated as mediators and inhibitors of diverse forms of neurodegeneration. They are induced in response to brain injury and have diverse actions that can cause, exacerbate, mediate and/or inhibit cellular injury and repair. Here we review evidence for the contribution of cytokines to acute neurodegeneration, focusing primarily on interleukin 1 (IL-1), tumour necrosis factor-α (TNFα) and transforming growth factor-β (TGFβ). TGFβ seems to exert primarily neuroprotective actions, whereas TNFα might contribute to neuronal injury and exert protective effects. IL-1 mediates ischaemic, excitotoxic and traumatic brain injury, probably through multiple actions on glia, neurons and the vasculature. Understanding cytokine action in acute neurodegeneration could lead to novel and effective therapeutic strategies, some of which are already in clinical trials.

Interleukin-1 and neuronal injury

Interleukin-1 is a pro-inflammatory cytokine that has numerous biological effects, including activation of many inflammatory processes (through activation of T cells, for example), induction of expression of acute-phase proteins, an important function in neuroimmune responses and direct effects on the brain itself. There is now extensive evidence to support the direct involvement of interleukin-1 in the neuronal injury that occurs in both acute and chronic neurodegenerative disorders. This article discusses the key evidence of a role for interleukin-1 in acute neurodegeneration — for example, stroke and brain trauma — and provides a rationale for targeting the interleukin-1 system as a therapeutic strategy.

The role of interleukin 1 in acute neurodegeneration and stroke: pathophysiological and therapeutic implications.

The last 5 yr have witnessed significant changes in research direction and new discoveries about the mechanisms of neurodegeneration. Earlier studies on neuronal death focused on neurons, neuronal pathways, and neurotransmitters. Now similar interest is directed toward glia and vascular cells, peptides, and inflammatory processes in the brain. This research has already revealed several potential therapeutic targets for the treatment of acute brain damage such as stroke and brain injury. The brain fails to exhibit a classical “inflammatory response,” generally characterized by early invasion of macrophages and leukocytes. Nevertheless, the brain can exhibit many of the hallmarks of inflammation when subjected to ischemia, injury, or infection. Such responses include edema, activation of resident phagocytic cells (microglia), invasion of circulating immune cells, and activation or induction of numerous inflammatory molecules including cyclooxygenase products, kinins, complement, acute phase proteins, and proinflammatory cytokines. Of the cytokines, we have identified IL-1 as a mediator of diverse forms of acute neurodegeneration. IL-1 has also been implicated in chronic neurological conditions.

Systemic inflammatory stimulus potentiates the acute phase and CXC chemokine responses to experimental stroke and exacerbates brain damage via interleukin-1- and neutrophil-dependent mechanisms.

Systemic inflammatory stimuli, such as infection, increase the risk of stroke and are associated with poorer clinical outcome. The mechanisms underlying the impact of systemic inflammatory stimuli on stroke are not well defined. We investigated the impact of systemic inflammation on experimental stroke and potential mechanisms involved. Focal cerebral ischemia was induced by intraluminal filament occlusion of the middle cerebral artery (MCAo). Brain damage and neurological deficit 24 h after MCAo were exacerbated by systemic lipopolysaccharide (LPS) administration. This exacerbation was critically dependent on interleukin (IL)-1, because coadministration of IL-1 receptor antagonist abolished the effect of LPS on brain damage. Systemic administration of IL-1 increased ischemic damage to a similar extent as LPS and also exacerbated brain edema. IL-1 markedly potentiated circulating levels of the acute phase proteins, serum amyloid A and IL-6, and the neutrophil-selective CXC chemokines, KC and macrophage inflammatory protein-2. Neutrophil mobilization and cortical neutrophil infiltration were aggravated by IL-1 before changes in ischemic damage. Neutropenia abolished the damaging effects of systemic IL-1. These data show for the first time that an acute systemic inflammatory stimulus is detrimental to outcome after experimental stroke and highlight IL-1 as a critical mediator in this paradigm. Our data suggest IL-1-induced potentiation of neutrophil mobilization via CXC chemokine induction is a putative mechanism underlying this effect. Our results may help to explain the poorer outcome in stroke patients presenting with infection and may have implications for neurodegenerative diseases involving neurovascular alterations, such as Alzheimers disease, in which systemic inflammation can modulate disease progression.

Impaired adult neurogenesis in the dentate gyrus of a triple transgenic mouse model of Alzheimer's disease.

It has become generally accepted that new neurones are added and integrated mainly in two areas of the mammalian CNS, the subventricular zone and the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus, which is of central importance in learning and memory. The newly generated cells display neuronal morphology, are able to generate action potentials and receive functional synaptic inputs, i.e. their properties are similar to those found in mature neurones. Alzheimers disease (AD) is the primary and widespread cause of dementia and is an age-related, progressive and irreversible neurodegenerative disease that deteriorates cognitive functions. Here, we have used male and female triple transgenic mice (3xTg-AD) harbouring three mutant genes (β-amyloid precursor protein, presenilin-1 and tau) and their respective non-transgenic (non-Tg) controls at 2, 3, 4, 6, 9 and 12 months of age to establish the link between AD and neurogenesis. Using immunohistochemistry we determined the area density of proliferating cells within the SGZ of the DG, measured by the presence of phosphorylated Histone H3 (HH3), and their possible co-localisation with GFAP to exclude a glial phenotype. Less than 1% of the HH3 labeled cells co-localised with GFAP. Both non-Tg and 3xTg-AD showed an age-dependent decrease in neurogenesis. However, male 3xTg-AD mice demonstrated a further reduction in the production of new neurones from 9 months of age (73% decrease) and a complete depletion at 12 months, when compared to controls. In addition, female 3xTg-AD mice showed an earlier but equivalent decrease in neurogenesis at 4 months (reduction of 63%) with an almost inexistent rate at 12 months (88% decrease) compared to controls. This reduction in neurogenesis was directly associated with the presence of β-amyloid plaques and an increase in the number of β-amyloid containing neurones in the hippocampus; which in the case of 3xgTg females was directly correlated. These results suggest that 3xTg-AD mice have an impaired ability to generate new neurones in the DG of the hippocampus, the severity of which increases with age and might be directly associated with the known cognitive impairment observed from 6 months of age onwards . The earlier reduction of neurogenesis in females, from 4 months, is in agreement with the higher prevalence of AD in women than in men. Thus it is conceivable to speculate that a recovery in neurogenesis rates in AD could help to rescue cognitive impairment.

Systemic Inflammation Alters the Kinetics of Cerebrovascular Tight Junction Disruption after Experimental Stroke in Mice

Systemic inflammatory events, such as infection, increase the risk of stroke and are associated with worse outcome, but the mediators of this clinically important effect are unknown. Our aim here was to elucidate mechanisms contributing to the detrimental effects of systemic inflammation on mild ischemic brain injury in mice. Systemic inflammation was induced in mice by peripheral interleukin-1β (IL-1β) challenge and focal cerebral ischemia by transient middle cerebral artery occlusion (MCAo). Systemic inflammation caused an alteration in the kinetics of blood–brain barrier (BBB) disruption through conversion of a transient to a sustained disruption of the tight junction protein, claudin-5, and also markedly exacerbated disruption to the cerebrovascular basal lamina protein, collagen-IV. These alterations were associated with a systemic inflammation-induced increase in neurovascular gelatinolytic activity that was mediated by a fivefold increase in neutrophil-derived matrix metalloproteinase-9 (MMP-9) in the brains of IL-1β-challenged mice after MCAo. Specific inhibition of MMP-9 abrogated the effects of systemic inflammation on the sustained but not the acute disruption of claudin-5, which was associated with phosphorylation of cerebrovascular myosin light chain. MMP-9 inhibition also attenuated the deleterious impact of systemic inflammation on brain damage, edema, neurological deficit, and incidence of hemorrhagic transformation. These data indicate that a transformation from transient to sustained BBB disruption caused by enhanced neutrophil-derived neurovascular MMP-9 activity is a critical mechanism underlying the exacerbation of ischemic brain injury by systemic inflammation. These mechanisms may contribute to the poor clinical outcome in stroke patients presenting with antecedent infection.

Inflammation and brain injury: Acute cerebral ischaemia, peripheral and central inflammation

Inflammation is a classical host defence response to infection and injury that has many beneficial effects. However, inappropriate (in time, place and magnitude) inflammation is increasingly implicated in diverse disease states, now including cancer, diabetes, obesity, atherosclerosis, heart disease and, most relevant here, CNS disease. A growing literature shows strong correlations between inflammatory status and the risk of cerebral ischaemia (CI, most commonly stroke), as well as with outcome from an ischaemic event. Intervention studies to demonstrate a causal link between inflammation and CI (or its consequences) are limited but are beginning to emerge, while experimental studies of CI have provided direct evidence that key inflammatory mediators (cytokines, chemokines and inflammatory cells) contribute directly to ischaemic brain injury. However, it remains to be determined what the relative importance of systemic (largely peripheral) versus CNS inflammation is in CI. Animal models in which CI is driven by a CNS intervention may not accurately reflect the clinical condition; stroke being typically induced by atherosclerosis or cardiac dysfunction, and hence current experimental paradigms may underestimate the contribution of peripheral inflammation. Experimental studies have already identified a number of potential anti-inflammatory therapeutic interventions that may limit ischaemic brain damage, some of which have been tested in early clinical trials with potentially promising results. However, a greater understanding of the contribution of inflammation to CI is still required, and this review highlights some of the key mechanism that may offer future therapeutic targets.

Proliferating resident microglia after focal cerebral ischaemia in mice.

Cerebral ischaemia usually results in the rapid death of neurons within the immediate territory of the affected artery. Neuronal loss is accompanied by a sequence of events, including brain oedema, blood-brain barrier (BBB) breakdown, and neuroinflammation, all of which contribute to further neuronal death. Although the role of macrophages and mononuclear phagocytes in the expansion of ischaemic injury has been widely studied, the relative contribution of these cells, either of exogenous or intrinsic central nervous system (CNS) origin is still not entirely clear. The purpose of this study, therefore, was to use different durations of transient middle cerebral artery occlusion (tMCAo) in the mouse to investigate fully post-occlusion BBB permeability and cellular changes in the brain during the 72 h post-MCAo period. This was achieved using in vivo magnetic resonance imaging (MRI) and cell labelling techniques. Our results show that BBB breakdown and formation of the primary ischaemic damage after tMCAo is not associated with significant infiltration of neutrophils, although more are observed with longer periods of MCAo. In addition, we observe very few infiltrating exogenous macrophages over a 72 h period after 30 or 60 mins of occlusion, instead a profound increase in proliferating resident microglia cells was observed. Interestingly, the more severe injury associated with 60 mins of MCAo leads to a markedly reduced proliferation of resident microglial cells, suggesting that these cells may play a protective function, possibly through phagocytosis of infiltrating neutrophils. These data further support possible beneficial actions of microglial cells in the injured brain.

Systemic infection, inflammation and acute ischemic stroke

Extensive evidence implicates inflammation in multiple phases of stroke etiology and pathology. In particular, there is growing awareness that inflammatory events outside the brain have an important impact on stroke susceptibility and outcome. Numerous conditions, including infection and chronic non-infectious diseases, that are established risk factors for stroke are associated with an elevated systemic inflammatory profile. Recent clinical and pre-clinical studies support the concept that the systemic inflammatory status prior to and at the time of stroke is a key determinant of acute outcome and long-term prognosis. Here, we provide an overview of the impact of systemic inflammation on stroke susceptibility and outcome. We discuss potential mechanisms underlying the impact on ischemic brain injury and highlight the implications for stroke prevention, therapy and modeling.

Interleukin-1beta and the interleukin-1 receptor antagonist act in the striatum to modify excitotoxic brain damage in the rat.

The cytokine interleukin‐1 (IL‐1) has been implicated in ischaemic, traumatic and excitotoxic brain damage. The results presented here reveal novel actions of IL‐1 in the striatum which markedly exacerbate cortical neuronal damage elicited by local excitotoxins in the striatum or cortex.