Purpose of review The present review discusses current concepts of HIV-associated neurocognitive disorders (HAND) in the era of antiretroviral therapy (ART). As the HIV epidemic enters its fourth decade (the second decade of ART), research must address evolving factors in HAND pathogenesis. These include persistent systemic and central nervous system (CNS) inflammation, aging in the HIV-infected brain, HIV subtype (clade) distribution, concomitant use of drugs of abuse, and potential neurotoxicity of ART drugs. Recent findings Although the severest form of HAND, HIV-associated dementia (HAD), is now rare due to ART, the persistence of milder, functionally important HAND forms persist in up to half of HIV-infected individuals. HAND prevalence may be higher in areas of Africa where different HIV subtypes predominate, and ART regimens that are more effective in suppressing CNS HIV replication can improve neurological outcomes. HAND are correlated with persistent systemic and CNS inflammation, and enhanced neuronal injury due to stimulant abuse (cocaine and methamphetamine), aging, and possibly ART drugs themselves. Summary Prevention and treatment of HAND requires strategies aimed at suppressing CNS HIV replication and effects of systemic and CNS inflammation in aging and substance-abusing HIV populations. Use of improved CNS-penetrating ART must be accompanied by evaluation of potential ART neurotoxicity.

Postmitotic neurons need to keep their cell cycle under control to survive and maintain a differentiated state. This study aims to test the hypothesis that the chemokine CXCL12 regulates neuronal survival and differentiation by promoting Rb function, as suggested by previous studies showing that CXCL12 protects neurons from apoptosis induced by Rb loss. To this end, the effect of CXCL12 on Rb expression and transcriptional activity and the role of Rb in CXCL12-induced neuronal survival were studied. CXCL12 increases Rb protein and RNA levels in rat cortical neurons. The chemokine also stimulates an exogenous Rb promoter expressed in these neurons and counteracts the inhibition of the Rb promoter induced by E2F1 overexpression. Furthermore CXCL12 stimulates Rb activity as a transcription repressor. The effects of CXCL12 are mediated by its specific receptor CXCR4, and do not require the presence of glia. Finally, shRNA studies show that Rb expression is crucial to the neuroprotective activity of CXCL12 as indicated by NMDA-neurotoxicity assays. These findings suggest that proper CXCR4 stimulation in the mature CNS can prevent impairment of the Rb-E2F pathway and support neuronal survival. This is important to maintain CNS integrity in physiological conditions and prevent neuronal injury and loss typical of many neurodegenerative and neuroinflammatory conditions. Cell Death and Differentiation ( 1,2 Stimulation of CXCR4 by its endogenous ligand leads to activation of intracellular pathways affecting neuronal survival, migration, and neurotransmission.1,3 For instance, CXCR4 stimulates the PI3K/Akt pathway and regulates cell-cycle proteins in neurons. [3][4][5][6] Under pathological conditions at least some of these essential CXCR4 functions are compromised, leading to neuronal dysfunction/death. 1,7 Thus, a complete understanding of the effects of CXCR4 activation in the brain has important physiological and pathological implications.Rb is a well-known transcriptional repressor, which controls cell-cycle progression, differentiation/survival, and genomic integrity. Rb is also implicated in fundamental CNS developmental processes such as, neuronal migration, differentiation, and neurite extension. 8,9 The effects of Rb on cell survival are primarily due to regulation of members of the E2F family of transcription factors, though Rb can also directly inactivate proapoptotic proteins, such as pp32.10 E2F proteins stimulate expression of genes promoting cell-cycle progression in proliferating cells, whereas they induce apoptotic genes in differentiated cells, including neurons.11 Rb is phosphorylated at several phosphoacceptor sites by specific kinases. 12 Hence, the immediate effect of Rb phosphorylation is to remove Rb from its specific promoters, leading to inhibition of Rb-dependent gene repression. Rb phosphorylation results in enhanced E2F1 transcriptional activity culminating in death of postmitotic neurons.11 Alterations of Rb/E2F pathway were reported in various neuropathologies, such as Parkinson's and Alzhei...

CXCR4, the specific receptor for the chemokine SDF-1α that also binds CXCR4-using HIV gp120s, affects survival of different cell types, including neurons. However, current data show that the outcome of CXCR4 activation on neuronal survival may vary depending on the ligand and/or the cellular conditions. In this study, we have systematically compared the effects of SDF-1α and gp120 IIIB (with or without CD4) on several intracellular pathways involved in cell survival, including MAP kinases and Akt-dependent pathways. Our data show that gp120 IIIB and SDF-1α are both potent activators of MAP kinases in neuronal and non-neuronal cells, though the kinetic of these responses is slightly different. Furthermore, unlike SDF-1α, and independently of CD4, gp120 IIIB is unable to stimulate Akt and some of its antiapoptotic targets (NF-κB and MDM2)-despite its ability to activate other signaling pathways in the same conditions. Finally, the viral protein is more efficient in recruiting some effectors (e.g., JNK) than others in comparison with SDF-1α (EC 50 = 0.1 vs. 0.6 nM). We conclude that the intrinsic efficacy of the two ligands is significantly different and is pathway dependent. These findings have important implications for our understanding of CXCR4-mediated responses in the CNS, as well as the role of this coreceptor in HIV neuropathogenesis.

The chemokine receptor CXCR4 regulates neuronal survival and differentiation and is involved in a number of pathologies, including cancer and human immunodeficiency virus (HIV). Recent data suggest that chemokines act in concert with neurotransmitters and neuropeptides, such as opioids. This study aimed to determine whether μ-opioid agonists alter the effect of CXCL12 (the specific CXCR4 ligand) on central neurons. Neuronal expression of CXCR4 and μ-opioid receptors (MORs) was analyzed by Western blot, immunostaining, and flow cytometry. Single-cell studies showed that all CXCR4-positive neurons coexpress MORs. Treatment of neuronal cultures with the selective MOR agonist DAMGO or the endogenous peptide endomorphin-1 inhibited intracellular signaling pathways (ERK1/2 and Akt) activated by CXCL12. Furthermore, DAMGO abolished the neuroprotective effect of CXCL12 in N-methyl-d-aspartate (NMDA) neurotoxicity studies. The effects of DAMGO and endomorphin-1 were inhibited by a general or a μ-specific opioid receptor antagonist, and not caused by changes in neuronal CXCR4 levels. DAMGO did not affect CXCL12-induced internalization of CXCR4. The authors propose that interactions between MOR and CXCR4 signaling can modulate the action of CXCL12 on neuronal survival-which may have important implications to neuroAIDS as well as other neuroinflammatory disorders.

Abnormal activation of CXCR4 during inflammatory/infectious states may lead to neuronal dysfunction or damage. The major goal of this study was to determine the coupling of CXCR4 to p53-dependent survival pathways in primary neurons. Neurons were stimulated with the HIV envelope protein gp120 IIIB or the endogenous CXCR4 agonist, SDF-1α. We found that gp120 stimulates p53 activity and induces expression of the p53 pro-apoptotic target Apaf-1 in cultured neurons. Inhibition of CXCR4 by AMD3100 abrogates the effect of gp120 on both p53 and Apaf-1. Moreover, gp120 neurotoxicity is markedly reduced by the p53-inhibitor, pifithrin-α. The viral protein also regulates p53 phosphorylation and expression of other p53-responsive genes, such as MDM2 and p21. Conversely, SDF-1α, which can promote neuronal survival, increases p53 acetylation and p21 expression in neurons. Thus, the stimulation of different p53 targets could be instrumental in determining the outcome of CXCR4 activation on neuronal survival in neuroinflammatory disorders.

Neurons express a variety of chemokine receptors that regulate neuronal signaling and survival, including CXCR4 and CCR5, the two major human immunodeficiency virus (HIV) coreceptors. However, the role of chemokine receptors in HIV neuropathology and neuroinflammatory disorders is still unclear. This study aims to determine whether chemokine receptors regulate the activity of cell-cycle proteins in neurons and evaluate the possibility that alterations of these proteins are involved in HIV neuropathogenesis. The authors studied the effect of the chemokine stromal cell-derived factor (SDF)-1alpha, the natural CXCR4 ligand, and an X4-using variant of gp120 on the activity of cell-cycle proteins involved in neuronal apoptosis and differentiation, such as Rb and E2F-1. Changes in expression, localization, and phosphorylation/activation of Rb and E2F-1 induced by SDF-1alpha (20 nM) gp120(IIIB) (200 pM) were analyzed in primary cultures of rat neurons and in a human cell line expressing recombinant CXCR4. The data indicate that changes in the nuclear and cytosolic levels of Rb--which result in the functional loss of this protein--are associated with apoptosis in hippocampal or cerebellar granule neurons and in cell lines. SDF-1alpha, which is able to rescue these neurons from apoptosis, induces a time-dependent increase of total Rb expression while decreasing the nuclear content of phosphorylated (Ser780/Ser795) Rb and the transcriptional activity of E2F-1. The HIV envelope protein gp120(IIIB) exerts opposite effects at the nuclear level. These data indicate that CXCR4 affects cell-cycle proteins in neurons and raise the possibility that chemokines may contribute to neuronal survival by repressing the activity of E2F-dependent apoptotic genes and maintaining neurons in a highly differentiated and quiescent state. This state may be altered during neuroinflammatory conditions and/or by HIV-derived proteins.

This study sought to determine the role of the transcription factor E2F1 in CXCR4-mediated neurotoxicity and HIV neuropathology. We studied the effect of the HIV envelope protein gp120 on the expression of E2F1-dependent apoptotic proteins in human and rodent neurons and examined the expression pattern of E2F1 in the brain of HIVinfected individuals. Our findings suggest that in cultured neurons gp120 increased E2F1 levels in the nucleus, stimulated its transcriptional activity and enhanced the expression of the E2F1 target proteins Cdc2 and Puma. Studies with neuronal cultures from E2F1 deficient mice demonstrated that the transcription factor is required for gp120-induced neurotoxicity and up-regulation of Cdc2 and Puma. Levels of E2F1 protein were greater in the nucleus of neurons in brains of HIVinfected patients exhibiting dementia when compared to HIV-negative subjects or HIV-positive neurologically normal patients. Overall, these studies indicate that E2F1 is primarily involved in CXCR4-mediated neurotoxicity and HIV neuropathogenesis.

Neurons express a variety of chemokine receptors that regulate neuronal signaling and survival, including CXCR4 and CCR5, the two major human immunodeficiency virus (HIV) coreceptors. However, the role of chemokine receptors in HIV neuropathology and neuroinflammatory disorders is still unclear. This study aims to determine whether chemokine receptors regulate the activity of cell-cycle proteins in neurons and evaluate the possibility that alterations of these proteins are involved in HIV neuropathogenesis. The authors studied the effect of the chemokine stromal cellderived factor (SDF)-1α, the natural CXCR4 ligand, and an X4-using variant of gp120 on the activity of cell-cycle proteins involved in neuronal apoptosis and differentiation, such as Rb and E2F-1. Changes in expression, localization, and phosphorylation/activation of Rb and E2F-1 induced by SDF-1α (20 nM) gp120 IIIB (200 pM) were analyzed in primary cultures of rat neurons and in a human cell line expressing recombinant CXCR4. The data indicate that changes in the nuclear and cytosolic levels of Rb-which result in the functional loss of this protein-are associated with apoptosis in hippocampal or cerebellar granule neurons and in cell lines. SDF-1α, which is able to rescue these neurons from apoptosis, induces a time-dependent increase of total Rb expression while decreasing the nuclear content of phosphorylated (Ser780/Ser795) Rb and the transcriptional activity of E2F-1. The HIV envelope protein gp120 IIIB exerts opposite effects at the nuclear level. These data indicate that CXCR4 affects cell-cycle proteins in neurons and raise the possibility that chemokines may contribute to neuronal survival by repressing the activity of E2F-dependent apoptotic genes and maintaining neurons in a highly differentiated and quiescent state. This state may be altered during neuroinflammatory conditions and/or by HIV-derived proteins.