This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Human immunodeficiency virus (HIV) infection is a global health burden, affecting an estimated 40 million people worldwide. Over half those infected suffer from neurological symptoms, ranging from mild cognitive deficits to severe dementia and motor disorders;however the cellular mechanisms underlying neurological alterations following infection with HIV remain unclear. This project makes use of tissue samples collected from rhesus macaque monkeys inoculated with simian immunodeficiency virus (SIV) as a non-human primate model for HIV.
The aim of this study is to examine neuronal dendrite morphology as a predictor of neuronal connectivity, and to establish a time-course for changes in morphology as related to the progression of SIV. Our studies employ Golgi staining, a classic but unparalleled staining technique for neurons which enables measurements of key features of architecture known to be associated with alterations in connectivity and excitability. We have now collected tissue from several brain regions that are likely to contribute to neurological deficits observed in patients with HIV. These include the prefrontal cortex (associated with working memory), the thalamus (associated with attentional functions) and the hippocampus (associated with spatial memory and mnemonic functions). While we intend to characterize alterations of dendrite morphology within each region in full, our analyses to date have concentrated on the hippocampus: this region has been the focus of extensive studies of learning and memory, and its anatomy and physiology are well defined. Our analyses have concentrated on three hippocampal regions, namely the dentate gyrus, CA3 and CA1;together, these regions represent the three principal regions of the tri-synaptic pathway within the hippocampus, and the site of long-term potentiation, a form of synaptic plasticity thought to be the basis for hippocampal learning. Extensive evidence exists to show that disruption of hippocampal function leads to cognitive impairment in animals and humans: in animal models, disruption of cellular events supporting LTP, and structural alterations in hippocampal dendrites, are associated with inability to perform a variety of cognitive tasks. Studies of patients with HIV consistently demonstrate alterations in functional markers within the hippocampus. Thus, understanding the temporal pattern of neurological changes within the hippocampus following SIV infection represents an important opportunity to design strategies to limit or even reverse cognitive disturbances associated with HIV. To date, we have compared indices of dendrite morphology in three treatment groups: controls, animals infected with SHIV, and animals infected with SIV. For these studies, tissues were collected from animals following chronic infection (on average 1 year after inoculation);however future studies aim to examine whether altered dendrite morphology is a feature of acute infection. Our current studies demonstrate that inoculation with SIV or SHIV results in alterations in hippocampal dendrite morphology that would not be predicted from results of existing post-mortem studies performed at disease endpoints within human patients. Within the granule cells of the dentate gyrus -- the first neurons within the trisynaptic pathway -- we observed increases in dendritic segment number and length following inoculation with SIV or SHIV, compared to controls. The increases in dendritic complexity in response to SHIV inoculation were highly significant, whereas alterations in arbor following SIV are more modest, and failed to reach statistical significance within our current sample size. Preliminary studies of morphology of CA3 pyramidal neurons (representing the second synapse within the trisynaptic pathway) are somewhat hampered by the greater variability in dendrite morphology within this region;however, our preliminary data suggest that SIV and SHIV inoculation results in increased amounts of dendritic arbor within basal dendrites, but reduces the extent of arbor within apical dendrites. While greater sample sizes will be required to verify this pattern of alterations, these results are consistent with increased connectivity within the tri-synaptic path, but reduced connectivity from extra-hippocampal regions, such as thalamus and entorhinal cortex, that are known to synapse directly with CA3 apical dendrites. This pattern is essentially repeated within the CA1 region, the terminus of the tri-synaptic pathway: here, robust increases (approximately 50%) in dendrite length were observed within basal dendrites. Thus, the patterns of alterations in dendritic complexity that we observe are consistent with discrete, anatomically defined changes in activity within specific hippocampal circuits, and further suggest that drug therapies targeted at restoring the balance of hippocampal activity and connectivity may be useful in reversing cognitive deficits. The importance of these results is demonstrated by the lack of congruence between our results and those of human post-mortem studies, in which by necessity measurements of dendrite morphology are made very late in the progression of the disease. In contrast, to our findings, postmortem studies within the hippocampus have demonstrated loss of hippocampal dendritic arbor. Thus, strategies aimed solely at regrowth of dendritic arbor may be ineffectual in preventing or reversing abnormalities in dendritic connectivity, particularly during early stages of the disease. Our on-going studies aim to increase sample sizes in order to provide confirmation of these patterns of dendrite morphology, to examine acute vs. chronic effects on dendritic arbor, and to extend our studies to other regions of the brain likely to support cognitive functions. We have also begun measurements of dendritic arbor within the prefrontal cortex, an area in which loss of function is associated with reduced inhibitory behaviors, a possible contributing factor to increased risk-taking that occurs in many patients with HIV.
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