HIV-1 coreceptors CCR5 and CXCR4 both mediate neuronal cell death but CCR5 paradoxically can also contribute to protection

PTEN gene silencing prevents HIV-1 gp120(IIIB)-induced degeneration of striatal neurons

To assess the role of the phosphatase and tensin homologue on chromosome 10 (PTEN) in mediating envelope glycoprotein 120 (gp120)-induced neurotoxicity in the striatum, PTEN was silenced using short interfering RNA (siRNA) vectors. PTEN activity directs multiple downstream pathways implicated in gp120-induced neuronal injury and death. PTEN is a negative regulator of Akt (protein kinase B) phosphorylation, but has also been shown to directly activate extrasynaptic NMDA receptors and dephosphorylate focal adhesion kinase. Rodent striatal neurons were nucleofected with green fluorescent protein (GFP)-expressing siRNA constructs to silence PTEN (PTENsi-GFP) or with negative-control (NCsi-GFP) vectors, and exposed to HIV-1 gp120(IIIB) using rigorously controlled, cell culture conditions including computerized time-lapse microscopy to track the fate of individual neurons following gp120 exposure. Immunofluorescence labeling showed that subpopulations of striatal neurons possess CXCR4 and CCR5 co-receptor immunoreactivity and that gp120(IIIB) was intrinsically neurotoxic to isolated striatal neurons. Importantly, PTENsi-GFP, but not control NCsi-GFP, constructs markedly decreased PTEN mRNA and protein levels and significantly attenuated gp120-induced death. These findings implicate PTEN as a critical factor in mediating the direct neurotoxic effects of HIV-1 gp120, and suggest that effectors downstream of PTEN such as Akt or other targets are potentially affected. The selective abatement of PTEN activity in neurons may represent a potential therapeutic strategy for the CNS complications of HIV-1.

The chemokine receptor CCR5 in the central nervous system

The expression and the role of the chemokine receptor CCR5 have been mainly studied in the context of HIV infection. However, this protein is also expressed in the brain, where it can be crucial in determining the outcome in response to different insults. CCR5 expression can be deleterious or protective in controlling the progression of certain infections in the CNS, but it is also emerging that it could play a role in non-infectious diseases. In particular, it appears that, in addition to modulating immune responses, CCR5 can influence neuronal survival. Here, we summarize the present knowledge about the expression of CCR5 in the brain and highlight recent findings suggesting its possible involvement in neuroprotective mechanisms.

Neurotrophins modulate the expression of chemokine receptors in the brain

In the central nervous system, chemokines are primarily mediators of inflammatory processes. Their receptors, in particular, CXCR4 and CCR5, serve as co-factors along with CD4 that permit Human immunodeficiency virus-1 (HIV) infection. Moreover, experimental evidence has shown that CXCR4 and CCR5 mediate the neurotoxic effects of the HIV envelope protein gp120, suggesting that these receptors could also promote the neuropathogenesis observed in HIV-positive individuals. Therefore, a better understanding of the molecular mechanisms governing the expression of chemokine receptors in the brain may lead to improved therapies that reduce HIV neurotoxicity. This study presents evidence that the expression of chemokine receptors in the brain is modulated by two neurotrophins in an area-specific manner. This new evidence suggests that the neurotrophins may be an adjunct therapy to reduce HIV-mediated neuronal injury evoked by chemokine receptor activation.

Morphine induces the release of CCL5 from astrocytes: potential neuroprotective mechanism against the HIV protein gp120

A number of human immunodeficiency virus type-1 (HIV) positive subjects are also opiate abusers. These individuals are at high risk to develop neurological complications. However, little is still known about the molecular mechanism(s) linking opiates and HIV neurotoxicity. To learn more, we exposed rat neuronal/glial cultures prepared from different brain areas to opiate agonists and HIV envelope glycoproteins gp120IIIB or BaL. These strains bind to CXCR4 and CCR5 chemokine receptors, respectively, and promote neuronal death. Morphine did not synergize the toxic effect of gp120IIIB but inhibited the cytotoxic property of gp120BaL. This effect was blocked by naloxone and reproduced by the mu opioid receptor agonist DAMGO. To examine the potential mechanism(s) of neuroprotection, we determined the effect of morphine on the release of chemokines CCL5 and CXCL12 in neurons, astrocytes, and microglia cultures. CCL5 has been shown to prevent gp120BaL neurotoxicity while CXCL12 decreases neuronal survival. Morphine elicited a time-dependent release of CCL5 but failed to affect the release of CXCL12. This effect was observed only in primary cultures of astrocytes. To examine the role of endogenous CCL5 in the neuroprotective activity of morphine, mixed cerebellar neurons/glial cells were immunoneutralized against CCL5 prior to morphine and gp120 treatment. In these cells the neuroprotective effect of opiate agonists was blocked. Our data suggest that morphine may exhibit a neuroprotective activity against M-tropic gp120 through the release of CCL5 from astrocytes.

Human immunodeficiency virus type 1 in the central nervous system leads to decreased dopamine in different regions of postmortem human brains

Human immunodeficiency virus type 1 (HIV-1) invades the central nervous system (CNS) shortly after infection and becomes localized in varying concentrations in different brain regions, the most vulnerable is the basal ganglia (BG). It is hypothesized that HIV-1-mediated neuropathogenesis involves degeneration of dopaminergic neurons in the substantia nigra and the loss of dopaminergic terminals in the BG, leading to deficits in the central dopaminergic activity, resulting in progressive impairment of neurocognitive and motor functions. In the era of highly active antiretroviral therapy (HAART), although the incidence of HIV-associated dementia (HAD) has decreased, the neurocognitive and neuropsychological deficits continue to persist after HAART. In this study, We investigated the impact of HIV-1 on dopaminergic activity with respect to concentrations of dopamine (DA) and homovanillic acid (HVA) in different regions of postmortem human brains of HIV-1-negative and HIV-1+ individuals and their relationship to neurocognitive impairment. We found that in HIV-1+ as well as HIV-negative cases, dopamine and HVA concentrations in ranged widely in different brain regions. In HIV-negative brain regions, the highest concentration of DA was found in putamen, caudate, substantia nigra, and the basal ganglia. In HIV-1+ cases, there was a significant decrease in DA levels in caudate nucleus, putamen, globus pallidus, and substantia nigra compared to that in HIV-negative cases. In HIV-1+ cases, a strong correlation was found between DA levels in substantia nigra and other brain regions. Concentration of HVA in HIV-negative cases was also highest in the regions containing high dopamine levels. However, no significant decrease in regional HVA levels was found in HIV-1+ cases. HIV-1 RNA load (nondetectable [ND] to log10 6.9 copies/g tissue) also ranged widely in the same brain regions of HIV-1+ cases. Interestingly, the brain regions having the highest HIV-1 RNA had the maximum decrease in DA levels. Age, gender, ethnicity, and postmortem interval were not correlated with decrease in DA levels. Profile of DA, HVA, and HIV-1 RNA levels in the brain regions of HIV-1+ individuals treated with HAART was similar to those not treated with HAART. A majority of HIV-1+ individuals had variable degrees of neurocognitive impairments, but no specific relationship was found between the regional DA content and severity of neurocognitive deficits. These findings suggest widespread deficits in dopamine in different brain regions of HIV-1-infected cases, and that these deficits may be the results of HIV-1-induced neurodegeneration in the subcortical regions of human brain.

Activation of p38 MAPK is required in monocytic and neuronal cells for HIV glycoprotein 120-induced neurotoxicity

HIV-1 envelope protein gp120 has been implicated in neurotoxin production by monocytic cells (i.e., macrophages and microglia), as well as in the pathogenesis of HIV-1-associated neurocognitive disorders. We previously showed in cerebrocortical cell cultures from rodents containing microglia, astrocytes, and neurons that overall inhibition of p38 MAPK signaling abrogated the neurotoxic effect of HIV-1 gp120. However, the time course of p38 MAPK activation and the contribution of this kinase in the various cell types remained unknown. In this study, we found that active p38 MAPK is required in monocytic lineage cells (i.e., macrophages and microglia) and neuronal cells for HIV gp120-induced neurotoxicity to occur. In cerebrocortical cell cultures, HIV-1 gp120 stimulated a time-dependent overall increase in active p38 MAPK, and the activated kinase was primarily detected in microglia and neurons. Interestingly, increased activation of p38 MAPK and neuronal death in response to gp120 were prevented by prior depletion of microglia or the presence of CCR5 ligand CCL4 or p38 MAPK inhibitors. In human monocytic THP-1 cells and primary monocyte-derived macrophages, HIV gp120-stimulated production of neurotoxins was abrogated by prior introduction into the cells of a dominant-negative p38 MAPK mutant or p38 MAPK small interfering RNA. In addition, the neurotoxic effects of cell-free supernatants from gp120-stimulated monocytic THP-1 cells were prevented in microglia-depleted cerebrocortical cells pretreated with a pharmacological inhibitor of p38 MAPK. Thus, p38 MAPK signaling was critical, upon exposure to HIV gp120, for the neurotoxic phenotype of monocytic cells and subsequent toxin-initiated neuronal apoptosis.

Neuropeptide Y has a protective role during murine retrovirus-induced neurological disease

Viral infections in the central nervous system (CNS) can lead to neurological disease either directly by infection of neurons or indirectly through activation of glial cells and production of neurotoxic molecules. Understanding the effects of virus-mediated insults on neuronal responses and neurotrophic support is important in elucidating the underlying mechanisms of viral diseases of the CNS. In the current study, we examined the expression of neurotrophin- and neurotransmitter-related genes during infection of mice with neurovirulent polytropic retrovirus. In this model, virus-induced neuropathogenesis is indirect, as the virus predominantly infects macrophages and microglia and does not productively infect neurons or astrocytes. Virus infection is associated with glial cell activation and the production of proinflammatory cytokines in the CNS. In the current study, we identified increased expression of neuropeptide Y (NPY), a pleiotropic growth factor which can regulate both immune cells and neuronal cells, as a correlate with neurovirulent virus infection. Increased levels of Npy mRNA were consistently associated with neurological disease in multiple strains of mice and were induced only by neurovirulent, not avirulent, virus infection. NPY protein expression was primarily detected in neurons near areas of virus-infected cells. Interestingly, mice deficient in NPY developed neurological disease at a faster rate than wild-type mice, indicating a protective role for NPY. Analysis of NPY-deficient mice indicated that NPY may have multiple mechanisms by which it influences virus-induced neurological disease, including regulating the entry of virus-infected cells into the CNS.

M-tropic HIV envelope protein gp120 exhibits a different neuropathological profile than T-tropic gp120 in rat striatum

Most early human immunodeficiency virus type 1 (HIV-1) strains are macrophage (M)-tropic HIV variants and use the chemokine receptor CCR5 for infection. Neuronal loss and dementia are less severe among individuals infected with M-tropic strains. However, after several years, the T-cell (T)-tropic HIV strain, which uses the CXCR4 variant, can emerge in conjunction with brain abnormalities, suggesting strain-specific differences in neuropathogenicity. The molecular and cellular mechanisms of such diversity remain under investigation. We have previously demonstrated that HIV envelope protein gp120IIIB, which binds to CXCR4, causes neuronal apoptosis in rodents. Thus, we have used a similar experimental model to examine the neurotoxic effects of M-tropic gp120BaL. gp120BaL was microinjected in the rat striatum and neuronal apoptosis was examined in the striatum, as well as in anatomically connected areas, such as the somatosensory cortex and the substantia nigra. gp120BaL promoted neuronal apoptosis and tissue loss that were confined to the striatum. Apoptosis was associated with microglial activation and increased levels of interleukin-1beta. Intriguingly, gp120BaL increased brain-derived neurotrophic factor in the striatum. Overall, our data show that gp120BaL demonstrates a different neuropathological profile than gp120IIIB. A better understanding of the pathogenic mechanisms mediating HIV neurotoxicity is vital for developing effective neuroprotective therapies against AIDS-associated dementia complex.

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.

TNF-alpha-dependent regulation of CXCR3 expression modulates neuronal survival during West Nile virus encephalitis

The chemokine CXCL10 exerts antiviral effects within the central nervous system (CNS) through the recruitment of virus-specific T cells. However, elevated levels of CXCL10 may induce neuronal apoptosis given its receptor, CXCR3, is expressed by neurons. Using a murine model of West Nile virus (WNV) encephalitis, we determined that WNV-infected neurons express TNF-alpha, which down-regulates neuronal CXCR3 expression via signaling through TNFR1. Down-regulation of neuronal CXCR3 decreased CXCL10-mediated calcium transients and delayed Caspase 3 activation. Loss of CXCR3 activation, via CXCR3-deficiency or pretreatment with TNF-alpha prevented neuronal apoptosis during in vitro WNV infection. These results suggest that neuronal TNF-alpha expression during WNV encephalitis may be an adaptive response to diminish CXCL10-induced death.

Increased brain damage after ischaemic stroke in mice lacking the chemokine receptor CCR5

BACKGROUND AND PURPOSE

The chemokine receptor CCR5 is well known for its function in immune cells; however, it is also expressed in the brain, where its specific role remains to be elucidated. Because genetic factors may influence the risk of developing cerebral ischaemia or affect its clinical outcome, we have analysed the role of CCR5 in experimental stroke.

EXPERIMENTAL APPROACH

Permanent cerebral ischaemia was performed by occlusion of the middle cerebral artery in wild-type and CCR5-deficient mice. Locomotor behaviour, infarct size and histochemical alterations were analysed at different time points after occlusion.

KEY RESULTS

The cerebral vasculature was comparable in wild-type and CCR5-deficient mice. However, the size of the infarct and the motor deficits after occlusion were markedly increased in CCR5-deficient mice as compared with wild type. No differences between wild-type and CCR5-deficient mice were elicited by occlusion with respect to the morphology and abundance of astrocytes and microglia. Seven days after occlusion the majority of CCR5-deficient mice displayed neutrophil invasion in the infarct region, which was not observed in wild type. As compared with wild type, the infarct regions of CCR5-deficient mice were characterized by increased neuronal death.

CONCLUSIONS AND IMPLICATIONS

Lack of CCR5 increased the severity of brain injury following occlusion of the middle cerebral artery. This is of particular interest with respect to the relatively frequent occurrence of CCR5 deficiency in the human population (1-2% of the Caucasian population) and the advent of CCR5 inhibitors as novel drugs.

CSF chemokine alterations related to the clinical course of amyotrophic lateral sclerosis

We measured the levels of 27 cytokines/chemokines and growth factors in cerebrospinal fluid (CSF) from 42 patients with sporadic amyotrophic lateral sclerosis (ALS), 12 patients with lower motor neuron disease (LMND), and 34 control patients with non-inflammatory neurological diseases (OND), using a multiplexed fluorescent bead-based immunoassay. Among cytokines/chemokines elevated in ALS, CCL2 and CXCL8 levels were negatively correlated with the revised ALS functional rating scale (ALSFRS-R) score, while CCL4 showed a positive correlation with ALSFRS-R score. CCL4 and CXCL10 showed negative correlations with disease progression rate. These chemokine alterations are assumed to somehow correlate with the clinical course of ALS.

Molecular mechanisms involving sigma receptor-mediated induction of MCP-1: implication for increased monocyte transmigration

Cocaine abuse hastens the neurodegeneration often associated with advanced HIV-1 infection. The mechanisms, in part, revolve around the neuroinflammatory processes mediated by the chemokine monocyte chemotactic protein-1 (MCP-1/CCL2). Understanding factors that modulate MCP-1 and, in turn, facilitate monocyte extravasation in the brain is thus of paramount importance. We now demonstrate that cocaine induces MCP-1 in rodent microglia through translocation of the sigma receptor to the lipid raft microdomains of the plasma membrane. Sequential activation of Src, mitogen-activated protein kinases (MAPKs), and phosphatidylinositol-3' kinase (PI3K)/Akt and nuclear factor kappaB (NF-kappaB) pathways resulted in increased MCP-1 expression. Furthermore, conditioned media from cocaine-exposed microglia increased monocyte transmigration, and thus was blocked by antagonists for CCR2 or sigma receptor. These findings were corroborated by demonstrating increased monocyte transmigration in mice exposed to cocaine, which was attenuated by pretreatment of mice with the sigma receptor antagonist. Interestingly, cocaine-mediated transmigratory effects were not observed in CCR2 knockout mice. We conclude that cocaine-mediated induction of MCP-1 accelerates monocyte extravasation across the endothelium. Understanding the regulation of MCP-1 expression and functional changes by cocaine/sigma receptor system may provide insights into the development of potential therapeutic targets for HIV-1-associated neurocognitive disorders.

HIV-1 associated dementia: update on pathological mechanisms and therapeutic approaches

PURPOSE OF REVIEW

Infection with HIV-1 can induce dementia despite successful administration of life-prolonging highly active antiretroviral therapy. This review will discuss recent progress toward a better understanding of the pathogenesis and an improved design of therapies for HIV-associated neurocognitive disorders.

RECENT FINDINGS

Highly active antiretroviral therapy prolongs the lives of HIV patients, but the incidence of HIV-associated dementia as an AIDS-defining illness has increased and the brain is now recognized as a viral sanctuary that requires additional therapeutic effort. The neuropathology of HIV infection also has changed due to improved therapy, and while more similarities with other neurodegenerative diseases are being reported, predictive biomarkers remain elusive. However, improvements of in-vivo imaging technology and progress in uncovering the molecular mechanisms of HIV disease keep providing new insights. As such it appears that a prolonged activation of the immune system by HIV eventually leads to AIDS, and several lines of evidence indicate that simultaneously neurotoxic processes and impairment of neurogenesis both contribute to the development of HIV-associated neurocognitive disorders.

SUMMARY

The improved understanding of the interaction between HIV and its human host provides hope that adjunctive therapies to antiretroviral treatment can be developed for HIV-associated neurocognitive disorders.

RANTES modulates the release of glutamate in human neocortex

The effects of the recombinant chemokine human RANTES (hRANTES) on the release of glutamate from human neocortex glutamatergic nerve endings were investigated. hRANTES facilitated the spontaneous release of d [(3)H]D-aspartate ([(3)H]DASP-) by binding Pertussis toxin-sensitive G-protein-coupled receptors (GPCRs), whose activation caused Ca(2+) mobilization from inositol trisphosphate-sensitive stores and cytosolic tyrosine kinase-mediated phosphorylations. Facilitation of release switched to inhibition when the effects of hRANTES on the 12 mM K(+)-evoked [(3)H]D-ASP exocytosis were studied. Inhibition of exocytosis relied on activation of Pertussis toxin-sensitive GPCRs negatively coupled to adenylyl cyclase. Both hRANTES effects were prevented by met-RANTES, an antagonist at the chemokine receptors (CCRs) of the CCR1, CCR3, and CCR5 subtypes. Interestingly, human neocortex glutamatergic nerve endings seem to possess all three receptor subtypes. Blockade of CCR1 and CCR5 by antibodies against the extracellular domain of CCRs prevented both the hRANTES effect on [(3)H]D-ASP release, whereas blockade of CCR3 prevented inhibition, but not facilitation, of release. The effects of RANTES on the spontaneous and the evoked release of [(3)H]D-ASP were also observed in experiments with mouse cortical synaptosomes, which may therefore represent an appropriate animal model to study RANTES-induced effects on neurotransmission. It is concluded that glutamate transmission can be modulated in opposite directions by RANTES acting at distinct CCR receptor subtypes coupled to different transduction pathways, consistent with the multiple and sometimes contrasting effects of the chemokine.

CCR5 deficiency induces astrocyte activation, Abeta deposit and impaired memory function

Activation of astrocytes has been known to be associated with amyloid-beta (Abeta) deposit and production of pro-inflammatory cytokines and chemokines that lead to neuronal cell death in the pathogenesis of Alzheimer disease (AD). In the present study, we investigated whether the absence of CC chemokine receptor 5 (CCR5) results in activation of astrocytes, Abeta deposit and memory dysfunction in CCR5 knock (CCR5(-/-)) out mice. We found that long-term and spatial memory functions were impaired in CCR5(-/-) mice. There was a significant increased expression of glial fibrillary acidic protein (GFAP) in the brain of CCR5(-/-) mice as compared with that of wild type of CCR5 (CCR5(+/+)) mice. The expression of CCR5 was observed in CCR5(+/+) astrocytes, but was reduced in the CCR5(-/-) astrocytes even though the expression of GFAP was much higher. Paralleling with the activation of astorcytes, the Abeta(1-42) level was higher in the brains of CCR5(-/-) mice than that of CCR5(+/+) mice. Expression of beta-secretase (BACE1) and its product C99 was significantly elevated in CCR5(-/-) mice. The activation of CC chemokine receptor 2 (CCR2) causes activation of astrocytes that leads to Abeta deposit and memory dysfunction in CCR5(-/-) mice. In CCR5(-/-) mice, CCR2 expression was high and co-localized with GFAP. These findings suggest that the absence of CCR5 increases expression of CCR2, which leads to the activation of astrocytes causing Abeta deposit, and thereby impairs memory function. These results suggest that CCR5 may be a critical suppressor of the development and progression of AD pathology.

Circulating monocytes in HIV-1-infected viremic subjects exhibit an antiapoptosis gene signature and virus- and host-mediated apoptosis resistance

Mechanisms that may allow circulating monocytes to persist as CD4 T cells diminish in HIV-1 infection have not been investigated. We have characterized steady-state gene expression signatures in circulating monocytes from HIV-infected subjects and have identified a stable antiapoptosis gene signature comprised of 38 genes associated with p53, CD40L, TNF, and MAPK signaling networks. The significance of this gene signature is indicated by our demonstration of cadmium chloride- or Fas ligand-induced apoptosis resistance in circulating monocytes in contrast to increasing apoptosis in CD4 T cells from the same infected subjects. As potential mechanisms in vivo, we show that monocyte CCR5 binding by HIV-1 virus or agonist chemokines serves as independent viral and host modulators resulting in increased monocyte apoptosis resistance in vitro. We also show evidence for concordance between circulating monocyte apoptosis-related gene expression in HIV-1 infection in vivo and available datasets following viral infection or envelope exposure in monocyte-derived macrophages in vitro. The identification of in vivo gene expression associated with monocyte resistance to apoptosis is of relevance to AIDS pathogenesis since it would contribute to: 1) maintaining viability of infection targets and long-term reservoirs of HIV-1 infection in the monocyte/macrophage populations, and 2) protecting a cell subset critical to host survival despite sustained high viral replication.

Involvement of TRPC channels in CCL2-mediated neuroprotection against tat toxicity

Chemokine (C-C motif) ligand 2 (CCL2), also known as monocyte chemoattractant protein-1, plays a critical role in leukocyte recruitment and activation. In the present study, we identify an additional role for CCL2 that of neuroprotection against HIV-1 transactivator protein (Tat) toxicity in rat primary midbrain neurons. Furthermore, we report the involvement of transient receptor potential canonical (TRPC) channels in CCL2-mediated neuroprotection. TRPC are Ca(2+)-permeable, nonselective cation channels with a variety of physiological functions. Blockage of TRPC channels resulted in suppression of both CCL2-mediated neuroprotection and intracellular Ca(2+) elevations. Parallel but distinct extracellular signal-regulated kinase (ERK)/cAMP response element-binding protein (CREB) and Akt/nuclear factor kappaB (NF-kappaB) pathways were involved in the CCL2-mediated neuroprotection. Blocking TRPC channels and specific downregulation of TRPC channels 1 and 5 resulted in suppression of CCL2-induced ERK/CREB activation but not Akt/NF-kappaB activation. In vivo relevance of these findings was further corroborated in wild-type and CCR2 knock-out mice. In the wild-type but not CCR2 knock-out mice, exogenous CCL2 exerted neuroprotection against intrastriatal injection of HIV-1 Tat. These findings clearly demonstrate a novel role of TRPC channels in the protection of neurons against Tat through the CCL2/CCR2 axis.

G-protein-coupled receptor screen reveals a role for chemokine receptor CCR5 in suppressing microglial neurotoxicity

G-protein-coupled receptors (GPCRs) form the largest superfamily of membrane proteins, and several GPCRs have been implicated in signaling between neurons and glia to protect neurons from pathological stresses. Here, we have used a screening strategy to investigate GPCRs that are involved in neuronal protection. The real-time PCR was performed using 274 primers targeting nonsensory GPCR mRNAs, which were listed on the database. The cDNAs from control and nerve-injured hypoglossal nuclei of mouse brain were used, and the alterations of PCR products were compared. This screen and the subsequent in situ hybridization screen exhibited six GPCR mRNAs which were prominently and convincingly induced in nerve-injured hypoglossal nuclei. Among these candidates, the chemokine receptor CCR5 was selected, based on the marked induction in CCR5 mRNA in microglia after nerve injury. The mRNA expression of ligands for CCR5, such as regulated on activation normal T-cell expressed and secreted (RANTES/CCL5), MIP-1alpha, and MIP-1beta, were induced in injured motor neurons, indicating that CCR5 and its ligands were expressed in microglia and neurons, respectively, in response to nerve injury. In vitro, lipopolysaccharide (LPS)-induced expression of mRNAs for inflammatory cytokines (IL-1beta, IL-6, and tumor necrosis factor-alpha) and inducible nitric oxide synthase (iNOS) in microglia were all suppressed by RANTES. Those suppressions were not observed in microglia from CCR5 null mice. In addition, nerve injury-induced motor neuron death seen in wild type C56BL/6J mice was accelerated in CCR5 knock-out C57BL/6J. These results may suggest that CCR5-mediated neuron-glia signaling functions to protect neurons by suppressing microglia toxicity.

CCL3L1 prevents gp120-induced neuron death via the CREB cell signaling pathway

Neurodegeneration is a characteristic feature of AIDS dementia complex and is commonly associated with neuronal death in the brains of both pediatric and adult patients. Neuronal death associated with AIDS dementia complex can be induced by the HIV-1 protein gp120, but the underlying signal transduction mechanism remains unclear, especially for HIV-1 subtypes commonly seen in China. We have now demonstrated that the human CC ligand 3-like protein 1 (CCL3L1), a member of the CC chemokine family, appears to protect neuronal cultures through its ability to attenuate gp120-induced neuronal death. We found that (i) both pCREB levels and Bcl-2 expression are up-regulated in neuronal culture following treatment with CCL3L1 plus gp120; (ii) CCL3L1 induces cell survival via phosphorylation of CREB by way of the PKA and CaMKI/CaMKIV cell signaling pathways; (iii) transcription of the cell survival gene bcl-2 is induced by pCREB; and (iv) CCL3L1 protects cultured neurons against CCR5-mediated excitotoxicity induced by gp120. Thus, the CCL3L1/bcl-2-regulated anti-apoptotic pathway significantly contributes to reduction of HIV-1/gp120-induced neuronal apoptosis, and therefore, CCL3L1 should be further investigated as a potential chemokine to protect against neuronal injury in gp120-related neuronal toxicity.

HIV/gp120 decreases adult neural progenitor cell proliferation via checkpoint kinase-mediated cell-cycle withdrawal and G1 arrest

Impaired adult neurogenesis has been observed in several neurodegenerative diseases, including human immunodeficiency virus (HIV-1)-associated dementia (HAD). Here we report that the HIV-envelope glycoprotein gp120, which is associated with HAD pathogenesis, inhibits proliferation of adult neural progenitor cells (aNPCs) in vitro and in vivo in the dentate gyrus of the hippocampus of HIV/gp120-transgenic mice. We demonstrate that HIV/gp120 arrests cell-cycle progression of aNPCs at the G1 phase via a cascade consisting of p38 mitogen-activated protein kinase (MAPK) --> MAPK-activated protein kinase 2 (a cell-cycle checkpoint kinase) --> Cdc25B/C. Our findings define a molecular mechanism that compromises adult neurogenesis in this neurodegenerative disorder.

CXCR4-CCR5: a couple modulating T cell functions

Chemokines and their receptors direct leukocyte migration among blood, lymph and tissues. Evidence has recently accumulated that, besides their chemotactic functions, chemokine receptors are highly versatile players that fine tune immune responses. During human T cell activation by antigen-presenting cells, the chemokine receptors CCR5 and CXCR4 are recruited into the immunological synapse, where they deliver costimulatory signals. However, the molecular mechanisms allowing signaling versatility of chemokine receptors are unknown. Here, we describe the functional interaction between CXCR4 and CCR5 to exert specific biological functions and modulate T lymphocyte responses. We demonstrate that simultaneous expression and cooperation between CCR5 and CXCR4 are required for chemokine-induced T cell costimulation at the immunological synapse. In addition, we provide evidence for a physical association of the two receptors in a signaling complex that activates distinct T cell functions. We suggest that cooperation between receptors represents one key strategy for the functional plasticity of chemokines.

Proteomic modeling for HIV-1 infected microglia-astrocyte crosstalk

Background

HIV-1-infected and immune competent brain mononuclear phagocytes (MP; macrophages and microglia) secrete cellular and viral toxins that affect neuronal damage during advanced disease. In contrast, astrocytes can affect disease by modulating the nervous system's microenvironment. Interestingly, little is known how astrocytes communicate with MP to influence disease.

Methods and Findings

MP-astrocyte crosstalk was investigated by a proteomic platform analysis using vesicular stomatitis virus pseudotyped HIV infected murine microglia. The microglial-astrocyte dialogue was significant and affected microglial cytoskeleton by modulation of cell death and migratory pathways. These were mediated, in part, through F-actin polymerization and filament formation. Astrocyte secretions attenuated HIV-1 infected microglia neurotoxicity and viral growth linked to the regulation of reactive oxygen species.

Conclusions

These observations provide unique insights into glial crosstalk during disease by supporting astrocyte-mediated regulation of microglial function and its influence on the onset and progression of neuroAIDS. The results open new insights into previously undisclosed pathogenic mechanisms and open the potential for biomarker discovery and therapeutics that may influence the course of HIV-1-mediated neurodegeneration.

HIV's double strike at the brain: neuronal toxicity and compromised neurogenesis

Infection with the human immunodeficiency virus-1 (HIV-1) and acquired immunodeficiency syndrome (AIDS) are often associated with severe and debilitating neurological problems that include behavioral abnormalities, motor dysfunction and frank dementia. HIV-1-infected peripheral immune cells, in particular macrophages, appear to infiltrate the CNS, release neurotoxins and provoke a neuropathological response involving all cell types in the brain. In the CNS, HIV-1 and its components initiate activation of chemokine receptors, inflammatory mediators and glutamate receptor-mediated excitotoxicity, all of which can activate numerous downstream signaling pathways and disturb neuronal and glial function. Recent experimental evidence suggests that disturbance by HIV-1 results not only in neuronal injury and death but also in impairment of neurogenesis. This article will review recently identified pathological mechanisms which potentially contribute to the development of neurocognitive impairment and dementia in association with HIV-1 infection.

Human immunodeficiency virus-1/surface glycoprotein 120 induces apoptosis through RNA-activated protein kinase signaling in neurons

Previous work has demonstrated that the surface glycoprotein (gp120) of human immunodeficiency virus-1 (HIV-1) can induce damage and apoptosis of neurons both in vitro and in vivo. In this report, we provide evidence that double-stranded RNA-activated protein kinase (PKR), a stress kinase, is involved in HIV/gp120-associated neurodegeneration. In cultures of mixed cortical cells, HIV/gp120 increased the protein level of PKR. Additionally, PKR was phosphorylated in neurons but not glia after exposure to gp120. The use of two independent pharmacological inhibitors of PKR activity abrogated neuronal cell death induced by gp120. Cortical neurons from PKR knock-out mice were significantly protected from neurotoxicity induced by gp120, further validating the pivotal proapoptotic function of PKR. gp120-induced phosphorylated PKR localized prominently to neuronal nuclei; PKR inhibition or the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate] abrogated this effect. PKR inactivation also inhibited gp120-induced caspase-3 activation, consistent with its neuroprotective effect. Finally, brain tissue from individuals diagnosed with HIV-associated dementia (HAD), but not HIV infection alone, contained the activated form of PKR, which localized predominantly to neuronal nuclei. Together, these results identify PKR as a critical mediator of gp120 neurotoxicity, suggesting that activation of PKR contributes to the neuronal injury and cell death observed in HAD.

Neuronal chemokines: versatile messengers in central nervous system cell interaction

Whereas chemokines are well known for their ability to induce cell migration, only recently it became evident that chemokines also control a variety of other cell functions and are versatile messengers in the interaction between a diversity of cell types. In the central nervous system (CNS), chemokines are generally found under both physiological and pathological conditions. Whereas many reports describe chemokine expression in astrocytes and microglia and their role in the migration of leukocytes into the CNS, only few studies describe chemokine expression in neurons. Nevertheless, the expression of neuronal chemokines and the corresponding chemokine receptors in CNS cells under physiological and pathological conditions indicates that neuronal chemokines contribute to CNS cell interaction. In this study, we review recent studies describing neuronal chemokine expression and discuss potential roles of neuronal chemokines in neuron-astrocyte, neuron-microglia, and neuron-neuron interaction.