Advisor(s)

Jay P. McLaughlin

Contributor(s)

Martin L. Block, William A, Carlezon, Marcelo Febo, Marc J. Kaufman, Richard H. Melloni

Date of Award

2010

Date Accepted

6-2010

Degree Grantor

Northeastern University

Degree Level

Ph.D.

Degree Name

Doctor of Philosophy

Department or Academic Unit

College of Arts and Sciences. Department of Psychology.

Keywords

psychology, dementia, HIV, MRI, NeuroAIDS, Tat protein

Subject Categories

HIV (Viruses), Protein-protein interactions, Gene expression, Behavior genetics, Behavior modification

Disciplines

Psychology

Abstract

This dissertation investigated the role of HIV-Tat protein in mediating behavioral dysfunction and neurodegeneration associated with NeuroAIDS. NeuroAIDS is the syndrome consisting of neuropsychological manifestations and central nervous system pathology resulting from HIV-1 infection. The syndrome is characterized by the progressive loss of cognitive and motor function, and is also linked to neuropsychological disturbances that include changes in mood, "personality", and drug abuse. Research suggests that the cognitive and neuropsychological dysfunction may have a common etiological and biological basis; it is theorized that the release of toxic viral proteins from HIV-infected cells is a key mechanism by which HIV-1 indirectly and adversely affects neurons. HIV-1 Tat viral protein has been implicated in the pathogenesis of HIV-1 neurological complications, but direct behavioral demonstrations are limited. The central hypothesis tested in this thesis is that HIV-Tat protein, when expressed in brain, mediates behavioral dysfunction and neurodegeneration in an exposure-dependent manner. Four studies tested this hypothesis using the GT-tg bigenic mouse. The GT-tg mouse possesses an inducible and brain-selective tat gene that codes for Tat protein. These mice were uniquely suited for this study, as the expression of Tat protein is under the control of the researcher through the administration of Doxycycline (Dox). The dosage and number of days mice are administered Dox may be varied in order to control the amount and the length of time of Tat expression.

The first study tested if Tat was sufficient to impair learning and memory processes in Tat-induced GT-tg mice compared to the C57Bl/6J parent strain of mice and/or the uninduced GT-tg littermates. The Barnes maze was used to assess spatial learning and memory. The GT-tg mice induced to express Tat demonstrated longer latencies to find an escape hole, committed more total, reference, and working memory errors, and favored a less efficient search strategy during acquisition compared to their uninduced GT-tg littermates, suggesting impaired spatial learning. Tat-induced mice also demonstrated poor probe trial performance, suggesting impaired spatial memory. Furthermore, reversal learning was impaired in Tat-induced mice, as the mice learned a new escape location slower than uninduced littermates. Tat-induced mice also demonstrated deficiencies in novel object recognition (NOR), a behavioral model for a different type of learning and memory performance than the Barnes maze. Tat-induced mice displayed long-lasting (i.e., up to one month) NOR deficits. Furthermore, NOR impairment was dependent on the dose and duration of Dox exposure, suggesting that the amount and duration of exposure to Tat progressively mediated deficits. The results of the Barnes maze and NOR testing provide evidence that Tat protein may mediate cognitive deficits seen in HIV-infected individuals and may be responsible for some of the behavioral pathology seen in HAD.

The next study tested the hypothesis that Tat can result in increased anxiety depending on the extent of Tat protein expression. Uninduced and Tat-induced GT-tg mice were initially tested in the open field test for anxiety-like behavior 48 h and one week after the completion of Tat induction to determine if any increases in anxiety were transient or persistent. Increases in anxiety-like behavior were apparent in mice induced to express Tat compared to controls, but mice administered the maximum induction regimen (Dox, 100 mg/kg, 7 days) were noted to have a more complex behavioral profile. These data were confirmed in the light-dark box. Mice induced to express Tat using various Dox induction regimens were also tested in the elevated plus maze. Interestingly, Tat-induced mice demonstrated behavioral changes in the elevated plus maze that were suggestive not of anxiety, but of behavioral disinhibition, a behavior commonly observed in patients suffering from dementia. Together, this battery of tests was used to characterize the behavioral profile of the transgenic mice induced to express varying amounts of Tat for different lengths of time. These studies demonstrated the complex relationship between Tat and behavioral dysfunction that could potentially mimic the progression of the mood disorders and cognitive decline in the disease process.

The third study evaluated the ability of Tat to modulate the rewarding and sensitizing effects of cocaine. Once expressed, Tat produced sensitization to the locomotor effects of acutely administered cocaine, as the locomotor effects induced by a single injection of cocaine were significantly potentiated, a response typically seen only after repeated exposure to cocaine. Furthermore, mice expressing Tat showed a greatly potentiated cocaine-conditioned place preference response as compared to C57Bl/6J or uninduced mice, suggesting that brain expression of Tat protein enhanced the rewarding effects of cocaine. The magnitude of this effect was dependent on the dose and duration of Dox administered and the corresponding Tat induction. We also confirmed that the potentiated preference response was not a transient event, and became extinct at the same rate as that of uninduced littermates. Experiments further demonstrated the effects of Tat on an established reward state, with the induction of Tat protein significantly potentiating the effect of an additional exposure to place conditioning over both prior preference and the response of mice that were uninduced. Together, these experiments suggest that induction of Tat protein can potentiate reward in subjects with no previous experience with cocaine and in drug non-naïve subjects. Finally, we demonstrated that after extinction of cocaine preference in untreated GT-tg mice, induction of Tat protein directly resulted in the reinstatement of cocaine-seeking behavior.

The final study tested the hypotheses that Tat protein expression in the central nervous system is associated with decreases in gray matter density, white matter microstructural changes, and histological abnormalities in brain areas known to be affected in NeuroAIDS patients. In the study, brains were excised from Tat-induced, uninduced, and C57Bl/6J mice for ex vivo anatomical histology-grade magnetic resonance imaging using a 9.4 Tesla Varian magnet. Reductions in gray matter density in the amygdala, amygdala-hippocampal area, piriform, perirhinal, and entorhinal cortices were found in the brains of 5 day Tat-induced mice compared to Dox-treated C57Bl/6J mice. Significant loss of white matter microstructural integrity in the insula, endopiriform nucleus, and part of the striatum was observed in the 7 day Tat-induced mice compared to uninduced GT-tg mice, suggesting a progressive effect of Tat protein on white matter, with increasing exposure to Tat resulting in more severe damage. Western blot analysis was run in parallel to confirm Tat protein expression in whole brain as well as specific brain areas. Additionally, limited analysis of histology staining used to detect activated microglia and cell death was performed to correlate in vitro evidence of the toxicity of Tat to in vivo demonstrations of neurodegeneration in our animal model.

In conclusion, this thesis addressed an under-investigated topic, identifying the contribution of HIV-1 Tat protein to neurodegeneration and neuropsychological impairments associated with NeuroAIDS, correlating cognitive and neuropsychological dysfunction with Tat-induced abnormalities in brain gray and white matter. Notably, this research expands on current literature evidence of Tat-induced dysfunction in the dopamine system with possible behavioral consequences of this dysfunction, such as the potentiation of psychostimulant effects. Evaluating the consequences of Tat activity in the brain may improve our understanding of the neurological underpinnings of NeuroAIDS and the neurodegeneration associated with HIV-1 infection. Moreover, understanding the effects of Tat and utilizing this model could one day facilitate the identification and development of preventative therapies or adjunctive countermeasures for the treatment of the neurodegeneration and behavioral dysfunction that persists in the HIV-1-infected population even with the use of current antiretroviral drugs.

Document Type

Dissertation

Rights Holder

Amanda Natalie Carey


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