Peter J. Crack

Suppressor of cytokine signaling 1 negatively regulates Toll-like receptor signaling by mediating Mal degradation

Toll-like receptor (TLR) signals that initiate innate immune responses to pathogens must be tightly regulated to prevent excessive inflammatory damage to the host. The adaptor protein Mal is specifically involved in signaling via TLR2 and TLR4. We demonstrate here that after TLR2 and TLR4 stimulation Mal becomes phosphorylated by Brutons tyrosine kinase (Btk) and then interacts with SOCS-1, which results in Mal polyubiquitination and subsequent degradation. Removal of SOCS-1 regulation potentiates Mal-dependent p65 phosphorylation and transactivation of NF-κB, leading to amplified inflammatory responses. These data identify a target of SOCS-1 that regulates TLR signaling via a mechanism distinct from an autocrine cytokine response. The transient activation of Mal and subsequent SOCS-1–mediated degradation is a rapid and selective means of limiting primary innate immune response.

Reactive oxygen species enhance insulin sensitivity

Chronic reactive oxygen species (ROS) production by mitochondria may contribute to the development of insulin resistance, a primary feature of type 2 diabetes. In recent years it has become apparent that ROS generation in response to physiological stimuli such as insulin may also facilitate signaling by reversibly oxidizing and inhibiting protein tyrosine phosphatases (PTPs). Here we report that mice lacking one of the key enzymes involved in the elimination of physiological ROS, glutathione peroxidase 1 (Gpx1), were protected from high-fat-diet-induced insulin resistance. The increased insulin sensitivity in Gpx1(-/-) mice was attributed to insulin-induced phosphatidylinositol-3-kinase/Akt signaling and glucose uptake in muscle and could be reversed by the antioxidant N-acetylcysteine. Increased insulin signaling correlated with enhanced oxidation of the PTP family member PTEN, which terminates signals generated by phosphatidylinositol-3-kinase. These studies provide causal evidence for the enhancement of insulin signaling by ROS in vivo.

Bacterial membrane vesicles deliver peptidoglycan to NOD1 in epithelial cells

Gram‐negative bacterial peptidoglycan is specifically recognized by the host intracellular sensor NOD1, resulting in the generation of innate immune responses. Although epithelial cells are normally refractory to external stimulation with peptidoglycan, these cells have been shown to respond in a NOD1‐dependent manner to Gram‐negative pathogens that can either invade or secrete factors into host cells. In the present work, we report that Gram‐negative bacteria can deliver peptidoglycan to cytosolic NOD1 in host cells via a novel mechanism involving outer membrane vesicles (OMVs). We purified OMVs from the Gram‐negative mucosal pathogens: Helicobacter pylori, Pseudomonas aeruginosa and Neisseria gonorrhoea and demonstrated that these peptidoglycan containing OMVs upregulated NF‐κB and NOD1‐dependent responses in vitro. These OMVs entered epithelial cells through lipid rafts thereby inducing NOD1‐dependent responses in vitro. Moreover, OMVs delivered intragastrically to mice‐induced innate and adaptive immune responses via a NOD1‐dependent but TLR‐independent mechanism. Collectively, our findings identify OMVs as a generalized mechanism whereby Gram‐negative bacteria deliver peptidoglycan to cytosolic NOD1. We propose that OMVs released by bacteria in vivo may promote inflammation and pathology in infected hosts.

Increased infarct size and exacerbated apoptosis in the glutathione peroxidase-1 (Gpx-1) knockout mouse brain in response to ischemia/reperfusion injury

Glutathione peroxidase is an antioxidant enzyme that is involved in the control of cellular oxidative state. Recently, unregulated oxidative state has been implicated as detrimental to neural cell viability and involved in both acute and chronic neurodegeneration. In this study we have addressed the importance of a functional glutathione peroxidase in a mouse ischemia/reperfusion model. Two hours of focal cerebral ischemia followed by 24 h of reperfusion was induced via the intraluminal suture method. Infarct volume was increased three‐fold in the glutathione peroxidase‐1 (Gpx‐1) –/– mouse compared with the wild‐type mouse; this was mirrored by an increase in the level of apoptosis found at 24 h in the Gpx‐1 –/– mouse compared with the wild‐type mouse. Neuronal deficit scores correlated to the histologic data. We also found that activated caspase‐3 expression is present at an earlier time point in the Gpx‐1 –/– mice when compared with the wild‐type mice, which suggests an enhanced susceptibility to apoptosis in the Gpx‐1 –/– mouse. This is the first known report of such a dramatic increase, both temporally and in level of apoptosis in a mouse stroke model. Our results suggest that Gpx‐1 plays an important regulatory role in the protection of neural cells in response to the extreme oxidative stress that is released during ischemia/reperfusion injury.

Neuroinflammation and oxidative stress: Co-conspirators in the pathology of Parkinson’s disease

Parkinsons disease (PD) is a complex disease, with genetics and environment contributing to the disease onset. Recent studies of causative PD genes have confirmed the involvement of cellular mechanisms engaged in mitochondrial and UPS dysfunction, oxidative stress and apoptosis in the progressive degeneration of the dopaminergic neurons in PD. In addition, clinical, epidemiological and experimental evidence has implicated neuroinflammation in the disease progression. This review will discuss neuroinflammation in PD, with particular focus on the genetic and toxin-based models of the disease. These studies have confirmed elevated oxidative stress and the pro-inflammatory response occurs early in the disease and these processes contribute to and/or exacerbate the nigro-striatal degeneration. In addition, the experimental models discussed here have also provided strong evidence that these pathways are an important link between the familial and sporadic causes of PD. The potential application of anti-inflammatory interventions in limiting the dopaminergic neuronal cell death in these models is discussed with evidence suggesting that the further investigation of their use as part of multi-targeted clinical trials is warranted.

Reduction of cerebral infarct volume by apocynin requires pretreatment and is absent in Nox2-deficient mice

Background and purpose:  Reactive oxygen species (ROS) derived from Nox2‐containing reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity is reportedly detrimental in cerebrovascular disease. However, ROS generation by other Nox isoforms may have a physiological role. No Nox2‐selective inhibitors have yet been identified, and thus it is unclear whether isoform non‐selective Nox inhibitors would necessarily improve outcome after stroke. We assessed the effect of apocynin on cerebrovascular ROS production and also on outcome following cerebral ischaemia when administered either before ischaemia or after cerebral reperfusion. The involvement of Nox2‐containing NADPH oxidase in the effects of apocynin was assessed using Nox2−/− mice.

The contribution of neuroinflammation to amyloid toxicity in Alzheimer's disease

Alzheimers disease (AD) is a progressive neurodegenerative disease and the most common cause of dementia. Deposition of amyloid‐β (Aβ) remains a hallmark feature of the disease, yet the precise mechanism(s) by which this peptide induces neurotoxicity remain unknown. Neuroinflammation has long been implicated in AD pathology, yet its contribution to disease progression is still not understood. Recent evidence suggests that various Aβ complexes interact with microglial and astrocytic expressed pattern recognition receptors that initiate innate immunity. This process involves secretion of pro‐inflammatory cytokines, chemokines and generation of reactive oxygen species that, in excess, drive a dysregulated immune response that contributes to neurodegeneration. The mechanisms by which a neuroinflammatory response can influence Aβ production, aggregation and eventual clearance are now becoming key areas where future therapeutic intervention may slow progression of AD. This review will focus on evidence supporting the combined neuroinflammatory‐amyloid hypothesis for pathogenesis of AD, describing the key cell types, pathways and mediators involved.

Neural injury following stroke: are Toll-like receptors the link between the immune system and the CNS?

The CNS can exhibit features of inflammation in response to injury, infection or disease, whereby resident cells generate inflammatory mediators, including cytokines, prostaglandins, free radicals and complement, chemokines and adhesion molecules that recruit immune cells, and activate glia and microglia. Cerebral ischaemia triggers acute inflammation, which exacerbates primary brain damage. The regulation of inflammation after stroke is multifaceted and comprises vascular effects, distinct cellular responses, apoptosis and chemotaxis. There are many cell types that are affected including neurons, astrocytes, microglia and endothelial cells, all responding to the resultant neuroinflammation in different ways. Over the past 20 years, researchers examining brain tissue at various time intervals after stroke observed the presence of inflammatory cells, neutrophils and monocytes at the site of injury, as well as the activation of endogenous glia and microglia. This review examines the involvement of these cells in the progression of neural injury and proposes that the Toll‐like receptors (TLRs) are likely to be an integral component in the communication between the CNS and the periphery. This receptor system is the archetypal pathogen sensing receptor system and its presence and signalling in the brain following neural injury suggests a more diverse role. We propose that the TLR system presents excellent pharmacological targets for the design of a new generation of therapeutic agents to modulate the inflammation that accompanies neural injury.

Toll-like receptors in the brain and their potential roles in neuropathology

The explosion of the toll‐like receptors (TLRs) over the past decade has touched almost every field of mammalian biology and neuroscience is not an exception. The current advent of research papers examining the TLRs in the central nervous system (CNS) signifies that these receptors are not only involved in peripheral innate immunity but may also play a role in the development and regulation of CNS inflammation, neurodegeneration and brain trauma. This review addresses the potential role of TLRs in the brain and how they may be involved in various neuropathologies.

Oxidative stress and neural dysfunction in Down syndrome.

Total or partial trisomy of chromosome 21 occurs with relatively high frequency and is responsible for the occurrence of Down syndrome. Phenotypically, individuals with Down syndrome display characteristic morphological features and a variety of clinical disorders. One of the challenges for researchers in this field has been to ascertain and understand the relationship between the Down syndrome phenotype with the gene dosage effect resulting from trisomy of chromosome 21. Much attention therefore, has been given towards investigating the consequences of overexpressing chromosome 21-linked genes. In particular, an extensive analysis of SOD1 and APP have provided important insights as to how perturbations in the expression of their respective genes may contribute to the Down syndrome phenotype. In this review we will highlight studies which support a key role for SOD1 and APP in the pathogenesis of neural abnormalities observed in individuals with Down syndrome. Central to this relationship is how the redox state of the cell is affected and its consequences to neural function and integrity.