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

HIV-1 gp120 Impairs Spatial Memory Through Cyclic AMP Response Element-Binding Protein

HIV-associated neurocognitive disorders (HAND) remain an unsolved problem that persists despite using antiretroviral therapy. We have obtained data showing that HIV-gp120 protein contributes to neurodegeneration through metabolic reprogramming. This led to decreased ATP levels, lower mitochondrial DNA copy numbers, and loss of mitochondria cristae, all-important for mitochondrial biogenesis. gp120 protein also disrupted mitochondrial movement and synaptic plasticity. Searching for the mechanisms involved, we found that gp120 alters the cyclic AMP response element-binding protein (CREB) phosphorylation on serine residue 133 necessary for its function as a transcription factor. Since CREB regulates the promoters of PGC1α and BDNF genes, we found that CREB dephosphorylation causes PGC1α and BDNF loss of functions. The data was validated in vitro and in vivo. The negative effect of gp120 was alleviated in cells and animals in the presence of rolipram, an inhibitor of phosphodiesterase protein 4 (PDE4), restoring CREB phosphorylation. We concluded that HIV-gp120 protein contributes to HAND via inhibition of CREB protein function.

CaMKII-Mediated CREB Phosphorylation Is Involved in Ca2+-Induced BDNF mRNA Transcription and Neurite Outgrowth Promoted by Electrical Stimulation

Electrical stimulation (ES)-triggered up-regulation of brain-derived neurotrophic factor (BDNF) and neurite outgrowth in cultured rat postnatal dorsal root ganglion neurons (DRGNs) is calcium (Ca²⁺)-dependent. The effects of increased Ca²⁺ on BDNF up-regulation and neurite outgrowth remain unclear. We showed here that ES increased phosphorylation of the cAMP-response element binding protein (CREB). Blockade of Ca²⁺ suppressed CREB phosphorylation and neurite outgrowth. Down-regulation of phosphorylated (p)-CREB reduced BDNF transcription and neurite outgrowth triggered by ES. Furthermore, blockade of calmodulin-dependent protein kinase II (CaMKII) using the inhibitors KN93 or KN62 reduced p-CREB, and specific knockdown of the CaMKIIα or CaMKIIβ subunit was sufficient to suppress p-CREB. Recombinant BDNF or hyperforin reversed the effects of Ca²⁺ blockade and CaMKII knockdown. Taken together, these data establish a potential signaling pathway of Ca²⁺-CaMKII-CREB in neuronal activation. To our knowledge, this is the first report of the mechanisms of Ca²⁺-dependent BDNF transcription and neurite outgrowth triggered by ES. These findings might help further investigation of complex molecular signaling networks in ES-triggered nerve regeneration in vivo.

AMPK Plays a Dual Role in Regulation of CREB/BDNF Pathway in Mouse Primary Hippocampal Cells

The hippocampus is vulnerable to damage under conditions of ischemia and hypoxia, causing multiple mental illnesses. cAMP response element-binding protein (CREB) plays a pivotal role in preventing the apoptosis of neurons and many other cells. Here, we found that AMP-activated protein kinase (AMPK) and CREB are oppositely regulated in mouse primary hippocampal neurons impaired by hypoxia-hypoglycemia. AMPK overexpression reduced the CREB level by upregulating SIRT1 and was negatively posttranscriptionally regulated by miR-134, suggesting a negative regulatory role of AMPK in the expression of CREB. Interestingly, the downstream genes of CREB, brain-derived neurotrophic factor (BDNF), and Bcl-2 remained unchanged when CREB was downregulated by AMPK expression. In addition, in AMPK(-/-) primary hippocampal neurons, comparisons between the effect of upregulation and silencing of miR-134 on the expression of CREB, BDNF, and Bcl-2 were made. The results reveal that AMPK is crucial for the activation of CREB via phosphorylation. Therefore, AMPK plays a dual role in the regulation of CREB in mouse primary hippocampal cells: a negative effect on total CREB expression by elevating SIRT1/miR-134 and a positive effect on activity via phosphorylation.

Viral and cellular factors underlying neuropathogenesis in HIV associated neurocognitive disorders (HAND)

As the HIV-1 epidemic enters its fourth decade, HIV-1 associated neurological disorders (HAND) continue to be a major concern in the infected population, despite the widespread use of anti-retroviral therapy. Advancing age and increased life expectancy of the HIV-1 infected population have been shown to increase the risk of cognitive dysfunction. Over the past 10 years, there has been a significant progress in our understanding of the mechanisms and the risk factors involved in the development of HAND. Key events that lead up to neuronal damage in HIV-1 infected individuals can be categorized based on the interaction of HIV-1 with the various cell types, including but not limited to macrophages, brain endothelial cells, microglia, astrocytes and the neurons. This review attempts to decipher these interactions, beginning with HIV-1 infection of macrophages and ultimately resulting in the release of neurotoxic viral and host products. These include: interaction with endothelial cells, resulting in the impairment of the blood brain barrier; interaction with the astrocytes, leading to metabolic and neurotransmitter imbalance; interactions with resident immune cells in the brain, leading to release of toxic cytokines and chemokines. We also review the mechanisms underlying neuronal damage caused by the factors mentioned above. We have attempted to bring together recent findings in these areas to help appreciate the viral and host factors that bring about neurological dysfunction. In addition, we review host factors and viral genotypic differences that affect phenotypic pathological outcomes, as well as recent advances in treatment options to specifically address the neurotoxic mechanisms in play.

Upregulation of CREM-1 relates to retinal ganglion cells apoptosis after light-induced damage in vivo

Previous studies have shown activation of cyclic AMP response element-binding protein (CREB) family is involved in the retinal ganglion cells (RGCs) protection. However, the function of cyclic AMP response element modulator-1 (CREM-1), one member of the CREB family, is still with limited acquaintance. To investigate whether CREM-1 is involved in RGCs death, we performed a light-induced retinal damage model in adult rats. Upregulation of CREM-1 was observed in retina after light-induced damage by performing western blot. Immunofluorescent labeling indicated that upregulated CREM-1 was localized mainly in the RGCs. We also investigated co-localization of CREM-1 with active-caspase-3 and TUNEL (apoptotic markers) in the retina after light-induced damage. In addition, the expression patterns of B cell lymphoma/leukemia-2 and Bcl-2 associated X protein were parallel with that of CREM-1. Collectively, we hypothesized upregulation of CREM-1 in the retina was associated with RGCs death after light-induced damage.

High levels of divergent HIV-1 quasispecies in patients with neurological opportunistic infections in China

Despite the fact that the survival of people infected with human immunodeficiency virus (HIV) has improved worldwide because of the increasingly powerful and highly active antiretroviral therapy, opportunistic infections (OIs) of the central nervous system (CNS) remain a serious burden. HIV-1 is capable of entering the CNS through infected peripheral monocytes, but its effect on OIs of CNS remains unclear. In this study, we investigated the characteristics of HIV-1 in acquired immunodeficiency syndrome (AIDS) patients with CNS OIs. A total of 24 patients with CNS OIs and 16 non-CNS OIs (control) cases were selected. These AIDS patients were infected with HIV-1 by paid blood donors in China. HIV-1 loads in plasma and cerebrospinal fluid (CSF) were detected using RT-PCR, and the C2-V5 region of HIV-1 envelope gene was amplified from viral quasispecies isolated from CSF using nested PCR. The CSF HIV-1 load of CNS OIs was higher than that of non-CNS OIs, but plasma HIV-1 load of CNS OIs was not higher than that of non-CNS OIs. The nucleotide sequence of C2-V5 region of the HIV-1 quasispecies isolated from the CSF of CNS OIs had a high diversity, and the HIV-1 quasispecies isolated from the CSF of CNS OIs revealed R5 tropism as 11/25 charge rule. These results suggest that high levels of divergent HIV-1 quasispecies in the CNS probably contribute to opportunistic infections.

Synergistic cooperation between methamphetamine and HIV-1 gsp120 through the P13K/Akt pathway induces IL-6 but not IL-8 expression in astrocytes

HIV-1 envelope protein gp120 has been extensively studied for neurotoxic effects that have been attributed to the increased expression of various proinflammatory cytokines in the CNS. Recently we have shown that methamphetamine (MA) also increases expression of proinflammatory cytokines in astrocytes. However, combined effect of gp120 and MA is not known. The present study was undertaken to determine cumulative effect and the mechanism(s)/pathways involved in the functional interaction between gp120 and MA in SVGA astrocytes. Our results clearly suggest that gp120 and MA affect IL-6 but not IL-8 in a synergistic manner and this synergy was mediated by PI3K/Akt and NF-κB pathways. Inhibition of either of these pathways could abrogate the increased expression of IL-6 due to MA or gp120 alone, as well as the increased expression of IL-6 when the astrocytes were treated with both gp120 and MA. These results were confirmed by both, using chemical inhibitors/siRNA as well as western blotting. This study therefore provides novel information regarding the interaction between MA and gp120 in terms of the expression of IL-6 and the mechanisms underlying potential synergy between MA and gp120 in astrocytes.

Accumulation of nuclear and mitochondrial DNA damage in the frontal cortex cells of patients with HIV-associated neurocognitive disorders

Oxidative stress has been suggested to play a key role in the neuropathogenesis of HIV infection. HIV proteins (gp120, Tat) and proinflammatory cytokines can trigger the production of reactive oxygen species (ROS), resulting in DNA and RNA lesions. Among all the lesions induced by ROS, one of the most abundant lesions in DNA and RNA is 8-hydroxydeoxyguanosine (8-oxoG). Here, we studied accumulated DNA oxidative damage induced by ROS in the central nervous system (CNS) in tissue from neuro-AIDS patients. The frontal cortex of autopsy tissue from HIV-1 infected patients was adopted for analysis for HIV-1 subtype, nuclear and mitochondrial DNA lesions by immunofluorescence staining, qPCR and sequencing of PCR cloning. This study provides evidence that HIV infection in the CNS leads to nuclear and mitochondrial genomic DNA damage in the brain. High level of nuclear and mtDNA 8-oxoG damage were identified in the cortex autopsy tissue of HAND patients. Increased accumulation of mtDNA mutations and depletion occurs in brain tissue in a subset of HAND cases, and is significantly different from that observed in control cases. These findings suggest that higher level of ROS in the CNS of HAND patients would contribute to the HIV induced neuro-inflammation and apoptosis of neuronal and glial cells.

Activation of cannabinoid type 2 receptors inhibits HIV-1 envelope glycoprotein gp120-induced synapse loss

HIV-1 infection of the central nervous system is associated with dendritic and synaptic damage that correlates with cognitive decline in patients with HIV-1-associated dementia (HAD). HAD is due in part to the release of viral proteins from infected cells. Because cannabinoids modulate neurotoxic and inflammatory processes, we investigated their effects on changes in synaptic connections induced by the HIV-1 envelope glycoprotein gp120. Morphology and synapses between cultured hippocampal neurons were visualized by confocal imaging of neurons expressing DsRed2 and postsynaptic density protein 95 fused to green fluorescent protein (PSD95-GFP). Twenty-four-hour treatment with gp120 IIIB decreased the number of PSD95-GFP puncta by 37 ± 4%. The decrease was concentration-dependent (EC₅₀ = 153 ± 50 pM). Synapse loss preceded cell death as defined by retention of DsRed2 fluorescence gp120 activated CXCR4 on microglia to evoke interleukin-1β (IL-1β) release. Pharmacological studies determined that sequential activation of CXCR4, the IL-1β receptor, and the N-methyl-d-aspartate receptor was required. Expression of alternative reading frame polypeptide, which inhibits the ubiquitin ligase murine double minute 2, protected synapses, implicating the ubiquitin-proteasome pathway. Cannabimimetic drugs are of particular relevance to HAD because of their clinical and illicit use in patients with AIDS. The cannabinoid receptor full agonist [(R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate salt] (Win55,212-2) inhibited gp120-induced IL-1β production and synapse in a manner reversed by a cannabinoid type 2 receptor antagonist. In contrast, Win55,212-2 did not inhibit synapse loss elicited by exposure to the HIV-1 protein Tat. These results indicate that cannabinoids prevent the impairment of network function produced by gp120 and, thus, might have therapeutic potential in HAD.

NR4A orphan nuclear receptors as mediators of CREB-dependent neuroprotection

Induced expression of neuroprotective genes is essential for maintaining neuronal integrity after stressful insults to the brain. Here we show that NR4A nuclear orphan receptors are induced after excitotoxic and oxidative stress in neurons, up-regulate neuroprotective genes, and increase neuronal survival. Moreover, we show that NR4A proteins are induced by cAMP response element binding protein (CREB) in neurons exposed to stressful insults and that they function as mediators of CREB-induced neuronal survival. Animals with null mutations in three of six NR4A alleles show increased oxidative damage, blunted induction of neuroprotective genes, and increased vulnerability in the hippocampus after treatment with kainic acid. We also demonstrate that NR4A and the transcriptional coactivator PGC-1alpha independently regulate distinct CREB-dependent neuroprotective gene programs. These data identify NR4A nuclear orphan receptors as essential mediators of neuroprotection after exposure to neuropathological stress.

Differential roles of Epac in regulating cell death in neuronal and myocardial cells

Cell survival and death play critical roles in tissues composed of post-mitotic cells. Cyclic AMP (cAMP) has been known to exert a distinct effect on cell susceptibility to apoptosis, protecting neuronal cells and deteriorating myocardial cells. These effects are primarily studied using protein kinase A activation. In this study we show the differential roles of Epac, an exchange protein activated by cAMP and a new effector molecule of cAMP signaling, in regulating apoptosis in these cell types. Both stimulation of Epac by 8-p-methoxyphenylthon-2'-O-methyl-cAMP and overexpression of Epac significantly increased DNA fragmentation and TUNEL (terminal deoxynucleotidyltransferase-mediated biotin nick end-labeling)-positive cell counts in mouse cortical neurons but not in cardiac myocytes. In contrast, stimulation of protein kinase A increased apoptosis in cardiac myocytes but not in neuronal cells. In cortical neurons the expression of the Bcl-2 interacting member protein (Bim) was increased by stimulation of Epac at the transcriptional level and was decreased in mice with genetic disruption of Epac1. Epac-induced neuronal apoptosis was attenuated by the silencing of Bim. Furthermore, Epac1 disruption in vivo abolished the 3-nitropropionic acid-induced neuronal apoptosis that occurs in wild-type mice. These results suggest that Epac induces neuron-specific apoptosis through increasing Bim expression. Because the disruption of Epac exerted a protective effect on neuronal apoptosis in vivo, the inhibition of Epac may be a consideration in designing a therapeutic strategy for the treatment of neurodegenerative diseases.

Effective post-insult neuroprotection by a novel Ca(2+)/ calmodulin-dependent protein kinase II (CaMKII) inhibitor

Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of physiological glutamate signaling involved in higher brain functions. Here, we show CaMKII involvement in pathological glutamate signaling relevant in stroke. The novel inhibitor tatCN21 was neuroprotective even when added hours after glutamate insults. By contrast, the "traditional" inhibitor KN93 attenuated excitotoxicity only when present during the insult. Both inhibitors efficiently blocked Ca(2+)/CaM-stimulated CaMKII activity, CaMKII interaction with NR2B and aggregation of CaMKII holoenzymes. However, only tatCN21 but not KN93 blocked the Ca(2+)-independent "autonomous" activity generated by Thr-286 autophosphorylation, the hallmark feature of CaMKII regulation. Mutational analysis further validated autonomous CaMKII activity as the drug target crucial for post-insult neuroprotection. Overexpression of CaMKII wild type but not the autonomy-deficient T286A mutant significantly increased glutamate-induced neuronal death. Maybe most importantly, tatCN21 also significantly reduced infarct size in a mouse stroke model (middle cerebral arterial occlusion) when injected (1 mg/kg intravenously) 1 h after onset of arterial occlusion. Together, these data demonstrate that inhibition of autonomous CaMKII activity provides a promising therapeutic avenue for post-insult neuro-protection after stroke.