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JAIDS Journal of Acquired Immune Deficiency Syndromes:
Brief Report: Clinical Science

Potential Role for White Matter Lysosome Expansion in HIV-Associated Dementia

Gelman, Benjamin B MD, PhD*†; Soukup, Vicki M PhD‡; Holzer, Charles E III PhD§; Fabian, Roderic H MD‡; Schuenke, Kimberly W PhD*; Keherly, Michael J PhD*; Richey, Frances J RN*; Lahart, Christopher J MD∥

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Author Information

From the Departments of *Pathology, †Neuroscience and Cell Biology, ‡Neurology, and §Psychiatry, University of Texas Medical Branch, Galveston, TX; and ∥Internal Medicine, Baylor College of Medicine, Houston, TX.

Received for publication November 10, 2004; accepted March 21, 2005.

Specimens, data, and support were provided by the National NeuroAIDS Tissue Consortium (NS 45491, NS 38841, MH 59724, MH 59745) and from DC 04970 and MH 69200.

Reprints: Benjamin B. Gelman, Texas NeuroAIDS Research Center, Department of Pathology, Rt. 0785, University of Texas Medical Branch, Galveston, TX 77555-0785 (e-mail: bgelman@utmb.edu).

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Abstract

Expansion of the lysosomal apparatus occurs in subcortical white matter in brains from persons with AIDS. This study examined whether HIV-associated subcortical dementia (HAD) is significantly related to this lysosomal anomaly. Brain cortex and adjacent white matter from the middle frontal gyrus were obtained from the National NeuroAIDS Tissue Consortium. Lysosomal hydrolase activity was assayed in 57 subjects who underwent neuropsychological testing within 6 months prior to autopsy. Decedents were evaluated from 4 geographical sites in the United States: Galveston/Houston, Texas (n = 36), Los Angeles, California (n = 5), New York, New York (n = 5), and San Diego, California (n = 11). Increased β-glucuronidase activity, a representative lysosomal glycosidase, was correlated with the amount of neurocognitive impairment. Significant correlation was present in 5 of 7 functional testing domains, including some that draw upon frontal lobe output (r = 0.419; P < 0.002). The biochemical anomaly was negligible in cerebral cortex and cerebrospinal fluid and was not correlated with brain dysfunction in those compartments. Glycosidase activation was associated significantly with increased HIV RNA concentration in brain tissue (r = 0.469; P < 0.021) and possibly with HIV RNA in cerebrospinal fluid (r = 0.266; P < 0.067). HIV RNA in blood plasma was not correlated. These results support the suggestion that abnormal metabolism in white matter glial cells contributes to cognitive slowing in persons with HAD. Because membrane turnover is routed through the endosome-lysosome apparatus, these data are in agreement with brain spectroscopic data that have suggested that there is an increase in membrane turnover in white matter glia.

Brain white matter injury in HIV-1 infection may be clinically important because infected persons develop a subcortical type of neurocognitive impairment termed HIV-associated dementia (HAD).1-3 The nature of subcortical white matter changes in HIV-1-infected persons is not well characterized. A blood-brain barrier disturbance occurs in cerebral white matter that can produce abnormal histologic staining of myelinated nerve fibers in HIV-infected persons (“white matter pallor”).4-6 Histochemical data show structural damage of white matter axons,7 and magnetic resonance imaging shows that white matter volume is decreased, perhaps as a result of destruction of myelinated nerve fibers.8,9 Brain spectroscopy (MRS) shows that choline-containing compounds and myoinositol are increased in the white matter of HIV-infected persons, which is believed to reflect increased membrane turnover in white matter glia.10,11 These MRS anomalies may be driven by HIV infection, because they were reversed when HIV replication was suppressed.12 Diffusion tensor imaging in HIV-infected persons has revealed highly subtle structural changes in nerve fibers of white matter.13-15 Brain HIV-1 RNA loads are high in subcortical white matter relative to brain cortex,16 and there is preferential white matter brain pathology in monkeys with lentivirus encephalitis.17,18 There is an accumulation of lysosomes in white matter astrocytes, macrophages, and microglial and glial cells that is linked with the presence of HIV proviral DNA in brain tissue.19 Because membrane turnover is routed through the lysosomal compartment primarily,20 this expansion of the lysosomal apparatus is consonant with data suggesting that membrane turnover is increased in white matter.11,21 The relationship of white matter anomalies overall to brain dysfunction still is not clear. Here we show that an index of lysosomal expansion in white matter measured biochemically is indeed correlated with HIV-associated neurocognitive impairment.

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METHODS

Subjects and Neuropsychological Testing

Fifty-seven HIV-infected persons who had end-stage AIDS were studied. The subjects were treated with highly active antiretroviral therapy and underwent autopsy at 1 of 4 National NeuroAIDS Tissue Consortium (NNTC) brain repository sites, located in Galveston, Texas, Los Angeles, California, New York, New York, and San Diego, California (all in the United States). NNTC sites employed identical protocols for neuropsychological testing and brain dissection.22,23 Subjects underwent neuropsychological testing within 6 months prior to death using the NNTC neuropsychological test battery. Documentation of comorbid conditions, interrater reliability, general reliability, and sensitivity are reported elsewhere.2,23-25Table 1 details the cognitive domains that were tested. Raw scores were transformed into age-, education-, or ethnicity-corrected standardized scores; clinical impairment ratings were computed for each domain; a global clinical rating (GCR) and neurocognitive diagnosis was provided according to Heaton et al.2

Table 1
Table 1
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Autopsy, Enzymology, and Virologic Assay

Subjects received a complete autopsy within 24 hours postmortem.22 Frozen samples of brain cortex and underlying gyral white matter from Brodmann area 8 were assayed enzymatically for β-glucuronidase activity.19 Brain specimens and correlative clinical data used in this study are available to the worldwide research community (available at: http://www.hivbrainbanks.org). Total RNA was isolated from frozen brain samples with UltraSpec RNAzol (Biotecx Laboratories, Houston, TX), and HIV RNA was assayed at the Johns Hopkins Medical Laboratories in Baltimore, MD, using the Amplicor HIV-1 Monitor UltraSensitive assay system (Roche Diagnostics, Indianapolis, IN). Blood and cerebrospinal fluid (CSF) were sent directly to the testing laboratory for HIV RNA assay. Pearson coefficients were used to compare neurocognitive impairment ratings with lysosomal enzyme activity.26

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RESULTS

The GCR is a composite index of neurocognitive impairment as detected using the NNTC battery of tests. Fifty-one of 57 subjects (89%) had GCR scores >4, which is the cutoff for a minimal amount of neuropsychological impairment. Five subjects (9%) were not impaired. Four (7%) had subtle impairment that was not clinically apparent (“subsyndromic”). Sixteen (28%) had minor cognitive and motor disturbance. In 15 (26%), HAD was diagnosed. Comorbid conditions occur frequently in the NNTC population, so we studied 17 subjects who had comorbid conditions (diagnosed NPI-O for “neurocognitively impaired, other”). HIV-1 infection is still a potential cause of impairment in persons who were NPI-O; comorbid factors could have contributed in whole or part. We included these 17 subjects a priori in the analysis to determine whether they differed from those without comorbidity. Statistical validity of the hypotheses tested was not influenced by including them, and conclusions made were replicated with, and without, their inclusion.

Several enzymes of the lysosomal class were assayed; all enzymes tested in this class were increased in AIDS brain specimens. 19 To avoid redundancy we focus on β-glucuronidase activity as illustrative of the results overall. β-Glucuronidase in white matter was positively correlated with 5 of 7 neurocognitive domains that were tested, and also with the GCR. Activity in gray matter showed hardly any correlation with neurocognitive test results (Table 1). Selectivity of white matter enzyme activity for significant correlation is shown in Figure 1. β-Glucuronidase was compared with the concentration of HIV-1 RNA in 3 body compartments in subjects for whom we could obtain those data (Fig. 2). HIV-1 RNA in brain frontal lobe was positively correlated with white matter β-glucuronidase (r = 0.4688, P < 0.021). A possible association between HIV-1 RNA in CSF and white matter β-glucuronidase was also found (r = 0.2657; P < 0.0677). Plasma HIV-1 RNA was not correlated. About 25% of the cohort had mildly increased β-glucuronidase in CSF, but it was not correlated with impairment (not illustrated).

Figure 1
Figure 1
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Figure 2
Figure 2
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DISCUSSION

The index of lysosome expansion in white matter was indeed correlated with HIV-associated neurocognitive impairment and with brain HIV RNA loading. These data add new biochemical evidence that white matter changes contribute to HIV-associated impairment. The scenario of metabolic activation of white matter glia in response to HIV-1 infection agrees with brain MRS data showing abnormal cellular metabolism of white matter27-29 that are believed to reflect increased membrane turnover.10,11,21 Because membrane turnover is routed almost exclusively through the endosome-lysosome system,20 expansion of the lysosome compartment could reflect increased membrane trafficking and organelle synthesis as suggested by MRS data. The anomaly is highly prevalent in autopsy specimens and it shows some relationship to assay of HIV-1 DNA and mRNA in brain tissue.19 MRS anomalies also are prevalent with HIV-1 infection and, because they improve after highly active antiretroviral therapy,12 also are related to HIV-1 replication.

One scenario that explains lysosome expansion is that white matter macrophages and microglia undergo increased lysosomal enzyme production and secretion when infected by HIV-1-a rudimentary sign of immunologic activation.30-33 In vitro data support that suggestion, as HIV-1 infection has a strong influence on the process of endosome trafficking, phagolysosome fusion, and virus budding.30,31,34,35 White matter astrocytes also exhibit lysosomal expansion19 and undergo limited infection by HIV-136; these cells also could participate in increased traffic through endosomal/lysosome pathways. The fact that white matter lysosome expansion is multicellular19 suggests that metabolic activation in white matter glial cells is broad based. That scenario is consistent with the theory that multicellular inflammatory cascades drive brain dysfunction and fits with the fact that inflammation seems to exert more influence on brain dysfunction than does the concentration of HIV-1 in the brain.37-39 Metabolic anomalies in white matter such as lysosomal expansion could drive neurocognitive impairment and may be visible in patients using MRS. The biochemistry of white matter and correlation with spectroscopic anomalies in HIV infection deserve further study.

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Keywords:

AIDS; dementia; HIV encephalitis; lysosome; myelin; white matter

© 2005 Lippincott Williams & Wilkins, Inc.

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