Marcus Kaul

Balance between synaptic versus extrasynaptic NMDA receptor activity influences inclusions and neurotoxicity of mutant huntingtin

Huntingtons disease is caused by an expanded CAG repeat in the gene encoding huntingtin (HTT), resulting in loss of striatal and cortical neurons. Given that the gene product is widely expressed, it remains unclear why neurons are selectively targeted. Here we show the relationship between synaptic and extrasynaptic activity, inclusion formation of mutant huntingtin protein (mtHtt) and neuronal survival. Synaptic N-methyl-D-aspartate–type glutamate receptor (NMDAR) activity induces mtHtt inclusions via a T complex-1 (TCP-1) ring complex (TRiC)-dependent mechanism, rendering neurons more resistant to mtHtt-mediated cell death. In contrast, stimulation of extrasynaptic NMDARs increases the vulnerability of mtHtt-containing neurons to cell death by impairing the neuroprotective cyclic AMP response element–binding protein (CREB)–peroxisome proliferator–activated receptor-γ coactivator-1α (PGC-1α) cascade and increasing the level of the small guanine nucleotide–binding protein Rhes, which is known to sumoylate and disaggregate mtHtt. Treatment of transgenic mice expressing a yeast artificial chromosome containing 128 CAG repeats (YAC128) with low-dose memantine blocks extrasynaptic (but not synaptic) NMDARs and ameliorates neuropathological and behavioral manifestations. By contrast, high-dose memantine, which blocks both extrasynaptic and synaptic NMDAR activity, decreases neuronal inclusions and worsens these outcomes. Our findings offer a rational therapeutic approach for protecting susceptible neurons in Huntingtons disease.

Caspase Cascades in Human Immunodeficiency Virus-Associated Neurodegeneration

Many patients infected with human immunodeficiency virus-1 (HIV-1) develop a syndrome of neurologic deterioration known as HIV-associated dementia (HAD). Neurons are not productively infected by HIV-1; thus, the mechanism of HIV-induced neuronal injury remains incompletely understood. Several investigators have observed evidence of neuronal injury, including dendritic degeneration, and apoptosis in CNS tissue from patients with HAD. Caspase enzymes, proteases associated with the process of apoptosis, are synthesized as inactive proenzymes and are activated in a proteolytic cascade after exposure to apoptotic signals. Here we demonstrate that HAD is associated with active caspase-3-like immunoreactivity that is localized to the soma and dendrites of neurons in affected regions of the human brain. Additionally, the cascade of caspase activation was studied using anin vitro model of HIV-induced neuronal apoptosis. Increased caspase-3 proteolytic activity and mitochondrial release of cytochrome c were observed in cerebrocortical cultures exposed to the HIV coat protein gp120. Specific inhibitors of both the Fas/tumor necrosis factor-α/death receptor pathway and the mitochondrial caspase pathway prevented gp120-induced neuronal apoptosis. Caspase inhibition also prevented the dendrite degeneration observed in vivo in transgenic mice with CNS expression of HIV/gp120. These findings suggest that pharmacologic interventions aimed at the caspase enzyme pathways may be beneficial for the prevention or treatment of HAD.

Mechanisms of neuroimmunity and neurodegeneration associated with HIV-1 infection and AIDS.

Infection with the human immunodeficiency virus-1 (HIV-1) and acquired immunodeficiency syndrome (AIDS) are a persistent health problem worldwide. HIV-1 seems to enter the brain very soon after peripheral infection and can induce severe and debilitating neurological problems that include behavioral abnormalities, motor dysfunction, and frank dementia. Infected peripheral immune-competent cells, in particular macrophages, appear to infiltrate the CNS and provoke a neuropathological response involving all cell types in the brain. The course of HIV-1 disease is strongly influenced by viral and host factors, such as the viral strain and the response of the hosts immune system. In addition, HIV-1-dependent disease processes in the periphery have a substantial effect on the pathological changes in the central nervous system (CNS), although the brain eventually harbors a distinctive viral population of its own. In the CNS, HIV-1 also incites activation of chemokine receptors, inflammatory mediators, extracellular matrix-degrading enzymes, and glutamate receptor-mediated excitotoxicity, all of which can initiate numerous downstream signaling pathways and disturb neuronal and glial function. Although there have been many major improvements in the control of viral infection in the periphery, an effective therapy for HIV-1-associated dementia (HAD) is still not available. This article addresses recently uncovered pathologic neuroimmune and degenerative mechanisms contributing to neuronal damage induced by HIV-1 and discusses experimental and potentially future therapeutic approaches.

Molecular mechanisms of neuroinvasion by monocytes-macrophages in HIV-1 infection

HIV associated neurocognitive disorders and their histopathological correlates largely depend on the continuous seeding of the central nervous system with immune activated leukocytes, mainly monocytes/macrophages from the periphery. The blood-brain-barrier plays a critical role in this never stopping neuroinvasion, although it appears unaltered until the late stage of HIV encephalitis. HIV flux that moves toward the brain thus relies on hijacking and exacerbating the physiological mechanisms that govern blood brain barrier crossing rather than barrier disruption. This review will summarize the recent data describing neuroinvasion by HIV with a focus on the molecular mechanisms involved.

Cytokine-Stimulated, But Not HIV-Infected, Human Monocyte-Derived Macrophages Produce Neurotoxic Levels of l-Cysteine

Approximately one-quarter of individuals with AIDS develop neuropathological symptoms that are attributable to infection of the brain with HIV. The cognitive manifestations have been termed HIV-associated dementia. The mechanisms underlying HIV-associated neuronal injury are incompletely understood, but various studies have confirmed the release of neurotoxins by macrophages/microglia infected with HIV-1 or stimulated by viral proteins, including the envelope glycoprotein gp120. In the present study, we investigated the possibility that l-cysteine, a neurotoxin acting at the N-methyl-d-aspartate subtype of glutamate receptor, could contribute to HIV-associated neuronal injury. Picomolar concentrations of gp120 were found to stimulate cysteine release from human monocyte-derived macrophages (hMDM) in amounts sufficient to injure cultured rat cerebrocortical neurons. TNF-α and IL-1β, known to be increased in HIV-encephalitic brains, as well as a cellular product of cytokine stimulation, ceramide, were also shown to induce release of cysteine from hMDM in a dose-dependent manner. A TNF-α-neutralizing Ab and an IL-1βR antagonist partially blocked gp120-induced cysteine release, suggesting that these cytokines may mediate the actions of gp120. Interestingly, hMDM infected with HIV-1 produced significantly less cysteine than uninfected cells following stimulation with TNF-α. Our findings imply that cysteine may play a role in the pathogenesis of neuronal injury in HIV-associated dementia due to its release from immune-activated macrophages but not virus-infected macrophages. Such uninfected cells comprise the vast majority of mononuclear phagocytes (macrophages and microglia) found in HIV-encephalitic brains.

BAG1 over-expression in brain protects against stroke.

The co‐chaperone BAG1 binds and regulates 70 kDa heat shock proteins (Hsp70/Hsc70) and exhibits cytoprotective activity in cell culture models. Recently, we observed that BAG1 expression is induced during neuronal differentiation in the developing brain. However, the in vivo effects of BAG1 during development and after maturation of the central nervous system have never been examined. We generated transgenic mice over‐expressing BAG1 in neurons. While brain development was essentially normal, cultured cortical neurons from transgenic animals exhibited resistance to glutamate‐induced, apoptotic neuronal death. Moreover, in an in vivo stroke model involving transient middle cerebral artery occlusion, BAG1 transgenic mice demonstrated decreased mortality and substantially reduced infarct volumes compared to wild‐type littermates. Interestingly, brain tissue from BAG1 transgenic mice contained higher levels of neuroprotective Hsp70/Hsc70 protein but not mRNA, suggesting a potential mechanism whereby BAG1 exerts its anti‐apoptotic effects. In summary, BAG1 displays potent neuroprotective activity in vivo against stroke, and therefore represents an interesting target for developing new therapeutic strategies including gene therapy and small‐molecule drugs for reducing brain injury during cerebral ischemia and neurodegenerative diseases.

Experimental and potential future therapeutic approaches for HIV-1 associated dementia targeting receptors for chemokines, glutamate and erythropoietin

Severe and debilitating neurological problems that include behavioral abnormalities, motor dysfunction and frank dementia can occur after infection with the human immunodeficiency virus-1 (HIV-1). Infected peripheral immune-competent cells, in particular macrophages, infiltrate the central nervous system (CNS) and provoke a neuropathological response involving all cell types in the brain. HIV-1 infection results in activation of chemokine receptors, inflammatory mediators, extracellular matrix-degrading enzymes and glutamate receptor-mediated excitotoxicity, all of which can trigger numerous downstream signaling pathways that result in disruption of neuronal and glial function. Despite many major improvements in the control of viral infection in the periphery, a truly effective therapy for HIV-1 associated dementia is currently not available. This review will discuss experimental and potentially future therapeutic strategies based on recently uncovered pathologic mechanisms contributing to neuronal damage induced by HIV-1.

Signaling pathways to neuronal damage and apoptosis in human immunodeficiency virus type 1-associated dementia: Chemokine receptors, excitotoxicity, and beyond

Dementia can occur as a debilitating consequence of human immunodeficiency virus-1 (HIV-1) infection. The neuropathology incited by HIV infection involves activation of chemokine receptors, inflammatory factors, and N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity, all of which can activate several downstream mechanisms. This article discusses recently identified pathways to neuronal damage triggered by HIV-1 and efforts aimed at development of applicable therapeutic intervention.

HIV-1-Associated Dementia

Dementia constitutes one devastating consequence of infection with human immunodeficiency virus-1 (HIV-1). While the mechanism of the HIV-1 induced neuropathology remains incompletely understood, several lines of evidence suggest the involvement of chemokine receptors, inflammatory factors and NMDA receptor-mediated excitotoxicity. All of these can in tum trigger several downstream mechanisms, including excessive influx of Ca2+ ions, activation of stress-associated protein kinases, caspases, and transcription factors, and production of cytotoxic free radicals. The chapter on HIV-1 associated dementia will review recently identified pathways to neuronal injury and death triggered by the virus or parts of it, and potential approaches to development of applicable therapeutic intervention.

Neuroinflammation and Excitotoxicity in Neurobiology of HIV-1 Infection and AIDS: Targets for Neuroprotection

Infection with the human immunodeficiency virus-1 (HIV-1) and acquired immunodeficiency syndrome (AIDS) pose a persistent health problem worldwide. Infected peripheral immune-competent cells, in particular macrophages, appear to infiltrate the central nervous system (CNS) and provoke a neuropathological and inflammatory response involving all cell types in the brain. In fact, HIV-1 seems to enter the brain very soon after infection and can subsequently induce severe and debilitating neurological problems that range from mild behavioral abnormalities and motor dysfunction to frank dementia. The course of HIV-1 disease is strongly influenced by viral and host factors, such as the viral strain and the response of the host’s immune system. In addition, HIV-1-dependent disease processes in the periphery, such as inflammation, have a substantial effect on the pathological changes in the CNS, despite the fact that the brain seems to harbor a distinctive viral population of its own. In the CNS, HIV-1 incites activation of chemokine receptors, inflammatory mediators, extracellular matrix-degrading enzymes, and glutamate receptor-mediated excitotoxicity, all of which can initiate numerous downstream signaling pathways and perturb neuronal and glial function. Despite many major improvements in the control of viral infection in the periphery, an effective therapy for HIV-1 associated dementia remains to be developed. This chapter will address recently uncovered pathologic and degenerative mechanisms contributing to neuronal damage induced by HIV-1 and discuss experimental and potential future therapeutic approaches.