Matthew J. Bellizzi

Tumor Necrosis Factor-Alpha in Normal and Diseased Brain: Conflicting Effects Via Intraneuronal Receptor Crosstalk?

Tumor necrosis factor-alpha (TNF-α) is pleiotropic mediator of a diverse array of physiological and neurological functions, including both normal regulatory functions and immune responses to infectious agents. Its role in the nervous system is prominent but paradoxical. Studies on uninflamed or “normal” brain have generally attributed TNF-α a neuromodulatory effect. In contrast, in inflamed or diseased brain, the abundance of evidence suggests that TNF-α has an overall neurotoxic effect, which may be particularly pronounced for virally mediated neurological disease. Still others have found TNF-α to be protective under some conditions of neurological insult. It is still uncertain exactly how TNF-α is able to induce these opposing effects through receptor activation of only a limited set of cell signaling pathways. In this paper, we provide support from the literature to advance our hypothesis that one mechanism by which TNF-α can exert its paradoxical effects in the brain is via crosstalk with signaling pathways of growth factors or other cytokines.

Synaptic activity becomes excitotoxic in neurons exposed to elevated levels of platelet-activating factor

Neurologic impairment in HIV-1-associated dementia (HAD) and other neuroinflammatory diseases correlates with injury to dendrites and synapses, but how such injury occurs is not known. We hypothesized that neuroinflammation makes dendrites susceptible to excitotoxic injury following synaptic activity. We report that platelet-activating factor, an inflammatory phospholipid that mediates synaptic plasticity and neurotoxicity and is dramatically elevated in the brain during HAD, promotes dendrite injury following elevated synaptic activity and can replicate HIV-1-associated dendritic pathology. In hippocampal slices exposed to a stable platelet-activating factor analogue, tetanic stimulation that normally induces long-term synaptic potentiation instead promoted development of calcium- and caspase-dependent dendritic beading. Chemical preconditioning with diazoxide, a mitochondrial ATP-sensitive potassium channel agonist, prevented dendritic beading and restored long-term potentiation. In contrast to models invoking excessive glutamate release, these results suggest that physiologic synaptic activity may trigger excitotoxic dendritic injury during chronic neuroinflammation. Furthermore, preconditioning may represent a novel therapeutic strategy for preventing excitotoxic injury while preserving physiologic plasticity.

Protecting the Synapse: Evidence for a Rational Strategy to Treat HIV-1 Associated Neurologic Disease

Loss of synaptic integrity and function appears to underlie neurologic deficits in patients with HIV-1-associated dementia (HAD) and other chronic neurodegenerative diseases. Because synaptic injury often long precedes neuronal death and surviving neurons possess a remarkable capacity for synaptic repair and functional recovery, we hypothesize that therapeutic intervention to protect synapses has great potential to improve neurologic function in HAD and other diseases. We discuss findings from both HAD and Alzheimers disease to demonstrate that the disruption of synaptic structure and function that can occur during excitotoxic injury and neuroinflammation represents a likely substrate for neurologic deficits. Based on available evidence, we provide a rationale for future studies aimed at identifying molecular targets for synaptic protection in neurodegenerative disease. Whereas patients with HAD beginning antiretroviral therapy have shown reversal of neurologic symptoms that is unique for patients with chronic neurodegenerative conditions, we propose that the potential for such reversal is not unique.

Platelet-Activating Factor Receptors Mediate Excitatory Postsynaptic Hippocampal Injury in Experimental Autoimmune Encephalomyelitis.

Gray matter degeneration contributes to progressive disability in multiple sclerosis (MS) and can occur out of proportion to measures of white matter disease. Although white matter pathology, including demyelination and axon injury, can lead to secondary gray matter changes, we hypothesized that neurons can undergo direct excitatory injury within the gray matter independent of these. We tested this using a model of experimental autoimmune encephalomyelitis (EAE) with hippocampal degeneration in C57BL/6 mice, in which immunofluorescent staining showed a 28% loss of PSD95-positive excitatory postsynaptic puncta in hippocampal area CA1 compared with sham-immunized controls, despite preservation of myelin and VGLUT1-positive excitatory axon terminals. Loss of postsynaptic structures was accompanied by appearance of PSD95-positive debris that colocalized with the processes of activated microglia at 25 d after immunization, and clearance of debris was followed by persistently reduced synaptic density at 55 d. In vitro, addition of activated BV2 microglial cells to hippocampal cultures increased neuronal vulnerability to excitotoxic dendritic damage following a burst of synaptic activity in a manner dependent on platelet-activating factor receptor (PAFR) signaling. In vivo treatment with PAFR antagonist BN52021 prevented PSD95-positive synapse loss in hippocampi of mice with EAE but did not affect development of EAE or local microglial activation. These results demonstrate that postsynaptic structures can be a primary target of injury within the gray matter in autoimmune neuroinflammatory disease, and suggest that this may occur via PAFR-mediated modulation of activity-dependent synaptic physiology downstream of microglial activation. SIGNIFICANCE STATEMENT Unraveling gray matter degeneration is critical for developing treatments for progressive disability and cognitive impairment in multiple sclerosis (MS). In a mouse model of MS, we show that neurons can undergo injury at their synaptic connections within the gray matter, independent of the white matter pathology, demyelination, and axon injury that have been the focus of most current and emerging treatments. Damage to excitatory synapses in the hippocampus occurs in association with activated microglia, which can promote excitotoxic injury via activation of receptors for platelet-activating factor, a proinflammatory signaling molecule elevated in the brain in MS. Platelet-activating factor receptor blockade protected synapses in the mouse model, identifying a potential target for neuroprotective treatments in MS.

HIV Tat causes synapse loss in a mouse model of HIV-associated neurocognitive disorder that is independent of the classical complement cascade component C1q

Microglial activation, increased proinflammatory cytokine production, and a reduction in synaptic density are key pathological features associated with HIV‐associated neurocognitive disorders (HAND). Even with combination antiretroviral therapy (cART), more than 50% of HIV‐positive individuals experience some type of cognitive impairment. Although viral replication is inhibited by cART, HIV proteins such as Tat are still produced within the nervous system that are neurotoxic, involved in synapse elimination, and provoke enduring neuroinflammation. As complement deposition on synapses followed by microglial engulfment has been shown during normal development and disease to be a mechanism for pruning synapses, we have tested whether complement is required for the loss of synapses that occurs after a cortical Tat injection mouse model of HAND. In Tat‐injected animals evaluated 7 or 28 days after injection, levels of early complement pathway components, C1q and C3, are significantly elevated and associated with microgliosis and a loss of synapses. However, C1qa knockout mice have the same level of Tat‐induced synapse loss as wild‐type (WT) mice, showing that the C1q‐initiated classical complement cascade is not driving synapse removal during HIV1 Tat‐induced neuroinflammation.

Infectious Agents in Neurodegenerative Disease

Diminution in synaptic function and loss of normal synaptic architecture are likely to be the biologically relevant substrate for neurologic deficits in patients with HIV-1-associated dementia (HAD) and patients suffering from the prionoses. Because synaptic injury frequently precedes neuronal apoptosis and surviving neurons retain a remarkable capacity for synaptic repair and functional recovery, synaptic protection with neuroprotective agents may have great potential to improve neurologic function in HAD. In this article we focus on the evidence to support these claims, as well as discuss findings that explain how disruption of synaptic structure and function that can occur during excitotoxic injury with neuroinflammation represents a likely substrate for neurologic deficits. Based on available data, we provide a rationale for identification of molecular targets for synaptic protection in neurodegenerative disease that accompanies HIV-1 infection of the nervous system. While patients with HAD beginning antiretroviral therapy have shown reversal of neurologic symptoms that is unique for patients with chronic neurodegenerative conditions, we propose that the therapeutic potential for such reversal may not be limited to this disease.