Current HIV Research (v.13, #1)

Conditional Tat Protein Brain Expression in the GT-tg Bigenic Mouse Induces Cerebral Fractional Anisotropy Abnormalities by Amanda N. Carey, Xiaoxu Liu, Dionyssios Mintzopoulos, Jason J. Paris, Jay P. McLaughlin, Marc J. Kaufman (3-9).
Cerebral white matter changes including tissue water diffusion abnormalities detected with diffusion tensor magnetic resonance imaging (DTI) are commonly found in humans with Human Immunodeficiency Virus (HIV) infection, as well as in animal models of the disorder. The severities of some of these abnormalities have been reported to correlate with measures of disease progression or severity, or with the degree of cognitive dysfunction. Accordingly, DTI may be a useful translational biomarker. HIV-Tat protein appears to be an important factor in the viral pathogenesis of HIV-associated neurotoxicity. We previously reported cerebral gray matter density reductions in the GT-tg bigenic mouse treated with doxycycline (Dox) to conditionally induce Tat protein expression. Presently, we administered intraperitoneal (i.p.) Dox (100 mg/kg/day) for 7 days to GT-tg mice to determine whether induction of conditional Tat expression led to the development of cerebral DTI abnormalities. Perfused and fixed brains from eight GT-tg mice administered Dox and eight control mice administered saline i.p. were extracted and underwent DTI scans on a 9.4 Tesla scanner. A whole brain analysis detected fractional anisotropy (FA) reductions in several areas including insular and endopiriform regions, as well as within the dorsal striatum. These findings suggest that exposure to Tat protein is sufficient to induce FA abnormalities, and further support the use of the GT-tg mouse to model some effects of HIV.

Chronic HIV-1 Tat and HIV Reduce Rbfox3/NeuN: Evidence for Sex- Related Effects by Yun Kyung Hahn, Ruturaj R. Masvekar, Ruqiang Xu, Kurt F. Hauser, Pamela E. Knapp (10-20).
The NeuN antibody has been widely used to identify and quantify neurons in normal and disease situations based on binding to a nuclear epitope in most types of neurons. This epitope was recently identified as the RNA-binding, feminizing locus on X-3 (Rbfox3), a member of the larger, mammalian Fox1 family of RNA binding proteins. Fox1 proteins recognize a unique UGCAUG mRNA motif and regulate alternative splicing of precursor mRNA to control post-transcriptional events important in neuronal differentiation and central nervous system development. Recent clinical findings show that Rbfox3/NeuN gene dosage is altered in certain human neurodevelopmental disorders, and redistribution has been noted in HIV+ tissue. We hypothesized that HIV-1 Tat might affect Rbfox3/NeuN expression, and examined this question in vivo using inducible transgenic mice, and in vitro using human mesencephalic-derived neurons. Rbfox3/NeuN expression and localization in HIV+ basal ganglia and hippocampus was also examined. Chronic Tat exposure reduced Rbfox3/NeuN protein levels and increased cytoplasmic localization, similar to the effect of HIV exposure. Cytoplasmic Rbfox3/NeuN signal has occasionally been reported, although the meaning or function of cytoplasmic versus nuclear localization remains speculative. Importantly, Rbfox3/NeuN reductions were more significant in male mice. Although Rbfox3/NeuN-expressing cells were significantly decreased by Tat exposure, stereology showed that Nissl+ neuron numbers remained normal. Thus, loss of Rbfox3/NeuN may relate more to functional change than to neuron loss. The effects of Tat by itself are highly relevant to HIV+ individuals maintained on antiretroviral therapy, since Tat is released from infected cells even when viral replication is inhibited.

HIV-1 Proteins, Tat and gp120, Target the Developing Dopamine System by Sylvia Fitting, Rosemarie M. Booze, Charles F. Mactutus (21-42).
In 2014, 3.2 million children (< 15 years of age) were estimated to be living with HIV and AIDS worldwide, with the 240,000 newly infected children in the past year, i.e., another child infected approximately every two minutes [1]. The primary mode of HIV infection is through mother-to-child transmission (MTCT), occurring either in utero, intrapartum, or during breastfeeding. The effects of HIV-1 on the central nervous system (CNS) are putatively accepted to be mediated, in part, via viral proteins, such as Tat and gp120. The current review focuses on the targets of HIV-1 proteins during the development of the dopamine (DA) system, which appears to be specifically susceptible in HIV-1-infected children. Collectively, the data suggest that the DA system is a clinically relevant target in chronic HIV-1 infection, is one of the major targets in pediatric HIV-1 CNS infection, and may be specifically susceptible during development. The present review discusses the development of the DA system, follows the possible targets of the HIV-1 proteins during the development of the DA system, and suggests potential therapeutic approaches. By coupling our growing understanding of the development of the CNS with the pronounced age-related differences in disease progression, new light may be shed on the neurological and neurocognitive deficits that follow HIV-1 infection.

Mechanisms of HIV-1 Tat Neurotoxicity via CDK5 Translocation and Hyper-Activation: Role in HIV-Associated Neurocognitive Disorders by Jerel Adam Fields, Wilmar Dumaop, Leslie Crews, Anthony Adame, Brian Spencer, Jeff Metcalf, Johnny He, Edward Rockenstein, Eliezer Masliah (43-54).
The advent of more effective antiretroviral therapies has reduced the frequency of HIV dementia, however the prevalence of milder HIV associated neurocognitive disorders [HAND] is actually rising. Neurodegenerative mechanisms in HAND might include toxicity by secreted HIV-1 proteins such as Tat, gp120 and Nef that could activate neuro-inflammatory pathways, block autophagy, promote excitotoxicity, oxidative stress, mitochondrial dysfunction and dysregulation of signaling pathways. Recent studies have shown that Tat could interfere with several signal transduction mechanisms involved in cytoskeletal regulation, cell survival and cell cycle re-entry. Among them, Tat has been shown to hyper-activate cyclin-dependent kinase [CDK] 5, a member of the Ser/Thr CDKs involved in cell migration, angiogenesis, neurogenesis and synaptic plasticity. CDK5 is activated by binding to its regulatory subunit, p35 or p39. For this manuscript we review evidence showing that Tat, via calcium dysregulation, promotes calpain-1 cleavage of p35 to p25, which in turn hyper-activates CDK5 resulting in abnormal phosphorylation of downstream targets such as Tau, collapsin response mediator protein-2 [CRMP2], doublecortin [DCX] and MEF2. We also present new data showing that Tat interferes with the trafficking of CDK5 between the nucleus and cytoplasm. This results in prolonged presence of CDK5 in the cytoplasm leading to accumulation of aberrantly phosphorylated cytoplasmic targets [e.g.: Tau, CRMP2, DCX] that impair neuronal function and eventually lead to cell death. Novel therapeutic approaches with compounds that block Tat mediated hyper-activation of CDK5 might be of value in the management of HAND.

Human immunodeficiency virus type 1 (HIV-1) Tat protein is a major pathogenic factor in HIV-associated neurological diseases; it exhibits direct neurotoxicity and indirect astrocyte-mediated neurotoxicity. We have shown that Tat alone is capable of activating glial fibrillary acidic protein (GFAP) expression and inducing astrocytosis involving sequential activation of early growth response protein 1 (Egr-1) and p300. In this study, we determined the roles of signal transducer and activator of transcription 3 (STAT3) in Tat-induced GFAP transactivation. STAT3 expression and phosphorylation led to significant increases in GFAP transcription and protein expression. Tat expression was associated with increased STAT3 expression and phosphorylation in Tat-expressing astrocytes and HIV-infected astrocytes. GFAP, Egr-1 and p300 transcription and protein expression all showed positive response to STAT3 and its phosphorylation. Importantly, knockdown of STAT3 resulted in significant decreases in Tat-induced GFAP and Egr-1 transcription and protein expression. Taken together, these findings show that STAT3 is involved in and acts upstream of Egr1 and p300 in the Tat-induced GFAP transactivation cascade and suggest important roles of STAT3 in controlling astrocyte proliferation and activation in the HIV-infected central nervous system.

Didehydro-Cortistatin A Inhibits HIV-1 Tat Mediated Neuroinflammation and Prevents Potentiation of Cocaine Reward in Tat Transgenic Mice by Sonia Mediouni, Joseph Jablonski, Jason J. Paris, Mark A. Clementz, Suzie Thenin-Houssier, Jay P. McLaughlin, Susana T. Valente (64-79).
HIV-1 Tat protein has been shown to have a crucial role in HIV-1-associated neurocognitive disorders (HAND), which includes a group of syndromes ranging from undetectable neurocognitive impairment to dementia. The abuse of psychostimulants, such as cocaine, by HIV infected individuals, may accelerate and intensify neurological damage. On the other hand, exposure to Tat potentiates cocaine-mediated reward mechanisms, which further promotes HAND. Here, we show that didehydro-Cortistatin A (dCA), an analog of a natural steroidal alkaloid, crosses the blood-brain barrier, cross-neutralizes Tat activity from several HIV-1 clades and decreases Tat uptake by glial cell lines. In addition, dCA potently inhibits Tat mediated dysregulation of IL-1β, TNF-α and MCP-1, key neuroinflammatory signaling proteins. Importantly, using a mouse model where doxycycline induces Tat expression, we demonstrate that dCA reverses the potentiation of cocaine-mediated reward. Our results suggest that adding a Tat inhibitor, such as dCA, to current antiretroviral therapy may reduce HIV-1-related neuropathogenesis.

Cortical Consequences of HIV-1 Tat Exposure in Rats are Enhanced by Chronic Cocaine by Wesley N. Wayman, Lihua Chen, Amanda L. Persons, T. Celeste Napier (80-87).
The life span of individuals that are sero-positive for human immunodeficiency virus (HIV) has greatly improved; however, complications involving the central nervous system (CNS) remain a concern. While HIV does not directly infect neurons, the proteins produced by the virus, including HIV transactivator of transcription (Tat), are released from infected glia; these proteins can be neurotoxic. This neurotoxicity is thought to mediate the pathology underlying HIVassociated neurological impairments. Cocaine abuse is common among HIV infected individuals, and this abuse augments HIV-associated neurological deficits. The brain regions and pathophysiological mechanisms that are dysregulated by both chronic cocaine and Tat are the focus of the current review.