Six billion neurons lost in AIDS. A stereological study of the neocortex

Untreated Patients Dying With AIDS Have Loss of Neocortical Neurons and Glia Cells

Untreated human immunodeficiency virus (HIV) depletes its host CD4 cells, ultimately leading to acquired immunodeficiency syndrome (AIDS). In brain, the HIV confines itself to astrocytes and microglia, the resident brain macrophages, but does not infect oligodendrocytes and neurons. Nonetheless, cognitive symptoms associated with HIV and AIDS are attributed to loss of axons and white matter damage. We used design-based stereology to estimate the numbers of neocortical neurons and glial cells (astrocytes, oligodendrocytes, and microglia), in a series of 12 patients dying with AIDS before the era of retroviral treatments, and in 13 age-matched control brains. Relative to the control material, there was a 19% loss of neocortical neuron (p = 0.04) and a 29% reduction of oligodendrocytes (p = 0.003) in the patients with AIDS, whereas astrocyte and microglia numbers did not differ between patients and controls. Furthermore, we saw a 17% reduction in mean hemispheric volume in the AIDS group (p = 0.0015), which was driven by neocortical and white matter loss (p < 0.05), while the archicortex, subcortical gray matter, and ventricular volumes were within normal limits. Our results confirm previous reports of neuronal loss in AIDS. The new finding of oligodendrocyte loss supports the proposal that HIV in the brain provokes demyelination and axonal dysfunction and suggests that remyelination treatment strategies may be beneficial to patients suffering from HIV-associated neurocognitive deficits.

Neocortical Development in Brain of Young Children-A Stereological Study

The early postnatal development of neuron and glia numbers is poorly documented in human brain. Therefore we estimated using design-based stereological methods the regional volumes of neocortex and the numbers of neocortical neurons and glial cells for 10 children (4 girls and 6 boys), ranging from neonate to 3 years of age. The 10 infants had a mean of 20.7 × 109 neocortical neurons (range 18.0-24.8 × 109) estimated with a coefficient of variation (CV) = 0.11; this range is similar to adult neuron numbers. The glia populations were 10.5 × 109 oligodendrocytes (range 5.0-16.0 × 109; CV = 0.40); 5.3 × 109 astrocytes (range 2.7-8.3 × 109, CV = 0.39); and 0.32 × 109 microglia (range 0.15-0.43 × 109, CV = 0.31). Thus, the estimated mean composite number of neocortical neuron and glial cells was 36.8 × 109 (range 26.8-48.3 × 109, CV = 0.21), of which approximately one-half were glial cells. There was a significant linear increase in oligodendrocyte and astrocyte numbers during the first 3 years of life, but no change in the total number of neurons. This is in line with our expectation that the total number of neocortical neurons is already determined in mid-fetal life.

Stereological estimation of total cell numbers in the human cerebral and cerebellar cortex

Our knowledge of the relationship between brain structure and cognitive function is still limited. Human brains and individual cortical areas vary considerably in size and shape. Studies of brain cell numbers have historically been based on biased methods, which did not always result in correct estimates and were often very time-consuming. Within the last 20-30 years, it has become possible to rely on more advanced and unbiased methods. These methods have provided us with information about fetal brain development, differences in cell numbers between men and women, the effect of age on selected brain cell populations, and disease-related changes associated with a loss of function. In that this article concerns normal brain rather than brain disorders, it focuses on normal brain development in humans and age related changes in terms of cell numbers. For comparative purposes a few examples of neocortical neuron number in other mammals are also presented.

Mapping cerebellar degeneration in HIV/AIDS

Progressive brain atrophy in HIV/AIDS is associated with impaired psychomotor performance, perhaps partly reflecting cerebellar degeneration; yet little is known about how HIV/AIDS affects the cerebellum. We visualized the three-dimensional profile of atrophy in 19 HIV-positive patients (age: 42.9+/-8.3 years) versus 15 healthy controls (age: 38.5+/-12.0 years). We localized consistent patterns of subregional atrophy with an image analysis method that automatically deforms each patient's scan, in three dimensions, to match a reference image. Atrophy was greatest in the posterior cerebellar vermis (14.9% deficit) and correlated with depression severity (P=0.009, corrected), but not with dementia, alcohol/substance abuse, CD4+T-cell counts, or viral load. Profound cerebellar deficits in HIV/AIDS (P=0.007, corrected) were associated with depression, suggesting a surrogate disease marker for antiretroviral trials.

Does HIV-1/AIDS-associated frontotemporal neuropathology following perinatal infection influence the development of moral behaviour?

While HIV encephalopathy and the AIDS dementia complex are considered hallmark neurologic manifestations of HIV-1 infection, increasing evidence of a continuum of nervous system involvement indicates the existence of an unrecognized number of individuals with milder, mostly cognitive and/or behavioural effects. Questions are raised whether HIV-related frontotemporal neuropathology during critical developmental stages could affect development of the brain networks documented to be involved in moral decisions, and whether this could contribute to the phenomenon of delinquency in an unknown percentage of the current generation of approximately 18-25 year old survivors of early childhood or vertically acquired HIV infection. Carefully planned and executed long term, prospective controlled studies using environmental, clinical, neurological, behavioural, genetic, immune and functional neuroimaging correlates would be required to elucidate whether HIV-specific neuropathology could indeed act as an independent risk factor for the development of a frontotemporal sociopathy syndrome. If such an association is proven, the accelerated development of neurospecific therapies should be a priority, especially for clinically and immunologically stable HIV-infected children. It may be necessary to institute such treatment as early as possible in perinatally infected cases, and maybe even during intrauterine life if HIV-1 is demonstrated to also act as a neurobehavioural teratogen for the developing fetal brain. It may, however, prove to be difficult to separate primary neurobiological from environmental factors, since the epigenetic effects on the host genome of retroviral insertion influencing behavioural gene expression characteristics, and altered gene expression following early life stresses may involve overlapping neurodevelopmental gene regulatory networks. In the meantime it remains necessary to prevent or ameliorate frequent neuropsychiatric morbidity from whatever causes.

Primary visual cortex volume and total neuron number are reduced in schizophrenia

A number of studies that assessed the visual system in subjects with schizophrenia found impairments in early visual processing. Furthermore, functional imaging studies suggested changes in primary visual cortex activity in subjects with schizophrenia. Interestingly, postmortem studies of subjects with schizophrenia reported an increased density of neurons in the primary visual cortex (Brodmann's area 17, BA17). The observed changes in visual processing may thus be reflected in structural changes in the circuitry of BA17. To characterize the structural changes further we used stereological methods based on unbiased principles of sampling (Cavalieri's principle and the optical fractionator) to estimate the total volume and neuron number of BA17 in postmortem brains from 10 subjects with schizophrenia and 10 matched normal comparison subjects. In addition, we assessed cortical thickness. We found a marked and significant reduction in total neuron number (25%) and volume (22%) of BA17 in the schizophrenia group relative to the normal comparison subjects. In contrast, we found no changes in neuronal density or cortical thickness between the two groups. Subjects with schizophrenia therefore have a smaller cortical area allocated to primary visual perception. This finding suggests the existence of a schizophrenia-related change in cortical parcellation.

Intracerebroventricular infusions of gp120 inhibit weight gain and induce atrophy in the hippocampus and neostriatum without affecting cognition

The HIV envelope protein, gp120, has been proposed to be a key agent in the development of AIDS dementia complex (ADC). To elucidate CNS effects that gp120 alone may be inducing in ADC, the present study investigated changes in weight, motor activity, cognitive function and corresponding neuropathology in rats given daily bilateral infusions of gp120 intracerebroventricularly for 7 days. gp120 inhibited weight gain, but had no measurable effects on motor activity or water maze cognitive performance. Nonetheless, gp120 infusions did induce both hippocampal and neostriatal atrophy. Thus, gp120 alone can cause ADC-related neuropathologic and weight changes, but gp120 alone was not sufficient to induce impairments in spatial learning and memory.

3D mapping of ventricular and corpus callosum abnormalities in HIV/AIDS

OBJECTIVE

40 million people worldwide are now infected with HIV/AIDS, an illness that often leads to rapidly progressing dementia and death. Even so, little is known about how AIDS affects the brain. Using computational anatomy techniques, we mapped how AIDS impacts the corpus callosum (CC) and ventricular system, two systems that show prominent changes on MRI. We (1) identified regions with greatest differences between AIDS patients and healthy controls and (2) correlated specific 3D patterns of structural differences with measures of immune system deterioration and cognitive decline.

METHODS

51 3D brain MRI scans from 30 non-demented AIDS patients (age: 43.4 years +/- 7.6 SD) and 21 HIV-seronegative controls (age: 39.5 years +/- 12.2) were aligned to ICBM standard space. 3D surface mesh reconstructions of the lateral ventricles and CC were spatially averaged and compared across diagnostic groups. Structural alterations were correlated with viral load, T cell counts, and cognitive impairment.

RESULTS

Statistical maps revealed the 3D profile of ventricular expansion and callosal thinning in AIDS. Specific 3D ventricular changes were linked with immune system decline (CD4+ T cell counts; P < 0.001) and cognitive impairment (P < 0.009), but not viral load. Frontal horn maps distinguished AIDS patients from controls better than occipital and temporal horn measures. T cell decline linked with callosal thinning in anterior regions connecting frontal areas with greatest cortical atrophy.

CONCLUSION

These maps (1) reveal how brain changes in HIV/AIDS relate to immune decline and impaired cognition, and, after further validation and testing, (2) may offer possible neuroimaging markers for anti-viral drug trials, which gauge how well treatments oppose disease progression in the brain.

Thinning of the cerebral cortex visualized in HIV/AIDS reflects CD4+ T lymphocyte decline

HIV/AIDS infection is the fourth leading cause of death worldwide, and one in every 100 adults aged 15-49 years is HIV-infected. Forty percent of AIDS patients suffer from neurological symptoms, but the selective profile of damage caused by HIV in the brain is not well understood. Here, we report 3D maps revealing how AIDS affects the human cerebral cortex, identifying the most vulnerable regions and where deficits link with cognitive decline and immune-system suppression. With high-resolution brain MRI scans, we created composite maps of cortical gray-matter thickness in 26 AIDS patients and 14 healthy controls to establish the selective pattern of brain deficits in AIDS. In AIDS, primary sensory, motor, and premotor cortices were 15% thinner. Thinner frontopolar and language cortex correlated with immune system deterioration measured through blood levels of CD4+ T lymphocytes. Prefrontal and parietal tissue loss correlated with cognitive/motor deficits. T cell depletion and cognitive impairment are, therefore, associated with specific 3D brain-deficit patterns visualized with MRI. These quantitative MRI-based maps reveal that HIV selectively damages the cortex. They provide an approach to gauge the impact of AIDS on the living brain and show that the brain is still vulnerable to infection even when patients are receiving antiretroviral therapy.

Severe subcortical degeneration in macaques infected with neurovirulent simian immunodeficiency virus

Infection with human immunodeficiency virus-1 (HIV-1), the causative agent of acquired immunodeficiency syndrome (AIDS) in humans, causes a spectrum of neuropathology that includes alterations in behavior, changes in evoked potentials, and neuronal degeneration. In the simian immunodeficiency virus (SIV) model of HIV infection, affected monkeys show clinical symptoms and neurological complications that mimic those observed in human neuro-AIDS. To investigate the relationship between morphological correlates and neurophysiological deficits, unbiased stereology was used to assess total neuron number, volume, and neuronal density for all neurons in the globus pallidus (GP) and for dopamine (DA)-containing neurons in the substantia nigra (SN) in eight macaques inoculated with macrophage-tropic, neurovirulent SIV (SIVmac R71/17E), and five control animals. There was a significant difference between rapid progressors and controls for both neuron number (P < .01) and neuronal density (P < .05) in the GP, and for neuron number (P < .05) in the SN. Neuron loss ranged from 6% to 70% in the GP and from 10% to 50% in the SN. Neuropathological analyses confirmed neuroAIDS-like changes in brain, including microglial nodules, extensive perivascular cuffing and/or the presence of multinucleated giant cells, and alterations in neuronal morphology in the majority of the rapid progressors. By comparison, slow progressors showed little, if any, neuropathology. These neuropathological changes in SIV-infected monkeys indicate that neuron death and morphological alterations in the basal ganglia may contribute to the motor impairments reported in the SIV model and, by analogy, in the subset of patients afflicted with motor impairment in human neuro-AIDS.

No evidence for loss of hippocampal neurons in non-Alzheimer dementia patients

OBJECTIVE

To use stereological methods for estimating the total number of neurons in hippocampi of non-Alzheimer demented patients.

MATERIAL AND METHODS

Hippocampi from six women with severely impaired memory but without Alzheimer pathology were compared with six mentally intact age-matched female controls. The total number of neurons was estimated in the granule cell layer of the dentate gyrus, the hilus of the dentate gyrus, the pyramidal cell layer of CA3 and CA2, the pyramidal cell layer of CA1 and the cellular layer of subiculum using the optical fractionator.

RESULTS

The total neuron number was the same in the dementia cases, 22.4 x 106, compared with 22.7 x 106 in the controls (P = 0.85). No region-specific group differences or side difference were found. Two cases without clinical signs of dementia but with abundant plaques and tangles in hippocampus and neocortex had total neuron numbers within normal limits.

CONCLUSION

Our results indicate that severely impaired memory can occur in the presence of intact numbers of hippocampal neurons in non-Alzheimer dementia and that nerve cell loss in the hippocampus might be characteristic for Alzheimer's disease, and perhaps other forms of primary cortical dementia.

Ethanol pre-exposure suppresses HIV-1 glycoprotein 120-induced neuronal degeneration by abrogating endogenous glutamate/Ca2+-mediated neurotoxicity

The neurotoxic mechanism of HIV-1 envelope glycoprotein 120 (gp120) involves glutamatergic (NMDA) receptor/Ca2+-dependent excitotoxicity, mediated in part via glia. Pro-inflammatory cytokines also may have roles. We have reported that pre-exposure of brain cultures to 'physiological' ethanol concentrations (20-30 mM) protects against neuronal damage from HIV-1 gp120, but not from the direct receptor agonist, NMDA. Using lactate dehydrogenase assays and propidium iodide staining of rat organotypic hippocampal-entorhinal cortical slice cultures we determined that ethanol's suppression of gp120 neurotoxicity required at least 4 days of pretreatment. The gp120-induced neurotoxicity was accompanied by interleukin-6 elevations that were not affected by the pretreatment. However, gp120 induced substantial, early increases in extracellular glutamate levels that were blocked by ethanol pretreatment, conceivably abrogating excitotoxicity. Consistent with abrogation of excitotoxic pathways, fura-2 imaging showed selective deficits in gp120-dependent intracellular Ca2+ responses in ethanol-pretreated slices. Gp120 is believed to increase glutamate levels by both stimulating release and inhibiting (re)uptake. Results with a labeled glutamate analog, D-[3H]aspartate, revealed that gp120's inhibition of glutamate uptake, rather than its stimulation of release, was abolished after ethanol. Further studies indicated that two converging effects of ethanol pretreatment may underlie the abolishment of gp120-mediated glutamate uptake inhibition: (a) blockade of gp120-induced release (ostensibly from glia) of arachidonic acid, an inhibitor of astroglial glutamate reuptake, and (b) modest proliferation and activation of astroglia upon gp120 stimulation--which are likely to augment glutamate transporters. Thus, as with gp120 itself, glia and glutamate/arachidonic acid regulation appear to be important targets for ethanol. Since moderate ethanol consumption is as common among HIV-infected individuals as in the general population, this newly recognized neuroprotective (and apparently anti-excitotoxic) effect of ethanol withdrawal in vitro could be important, but it requires further study before its significance, if any, is understood.

Microglial activation and neurological symptoms in the SIV model of NeuroAIDS: association of MHC-II and MMP-9 expression with behavioral deficits and evoked potential changes

HIV-1 causes cognitive and motor deficits and HIV encephalitis (HIVE) in a significant proportion of AIDS patients. Neurological impairment and HIVE are thought to result from release of cytokines and other harmful substances from infected, activated microglia. In this study, the quantitative relationship between microglial activation and neurological impairment was examined in the simian immunodeficiency model of HIVE. Macaque monkeys were infected with a passaged, neurovirulent strain of simian immunodeficiency virus, SIV(mac)239(R71/17E). In concurrent studies, functional impairment was assessed by motor and auditory brainstem evoked potentials and by measurements of cognitive and motor behavioral deficits. Brain tissue was examined by immunohistochemistry using two markers of microglia activation, MHC-II and matrix metalloproteinase-9 (MMP-9). The inoculated animals formed two groups: rapid progressors, which survived 6-14 weeks postinoculation, and slow progressors, which survived 87-109 weeks. In the rapid progressors, two patterns of MHC-II expression were present: (1) a widely disseminated pattern of MHC-II expressing microglia and microglial nodules in cortical gray matter and subcortical white matter, and (2) a more focal pattern in which MHC-II expressing microglia were concentrated into white matter. Animals exhibiting both patterns of microglial activation showed mild to severe changes in cognitive and motor behavior and evoked potentials. All rapid progressors showed expression of MMP-9 in microglia located in subcortical white matter. In the slow progressors MHC-II and MMP-9 staining was similar to uninoculated control macaques, and there was little or no evidence of HIVE. These animals showed behavioral deficits at the end of the disease course, but little changes in evoked potentials. Thus, increases in MHC-II and MMP-9 expression are associated with development of cognitive and motor deficits, alterations in evoked potentials, and rapid disease progression.

Preferential loss of large neocortical neurons during HIV infection: a study of the size distribution of neocortical neurons in the human brain

The infection with human immunodeficiency virus (HIV) is associated with a global and severe loss of neocortical neurons. However, there is limited knowledge concerning whether all neurons are equally susceptible to damage during HIV infection. Other studies have reported low vulnerability of small interneurons and high vulnerability of large motor neurons. Thus, it is natural to suggest that HIV infection, which causes damage to neurons in several ways, may predominantly affect large neurons in the neocortex. In this study we have used three unbiased stereological probes: Cavalieri's principle, the optical dissector and the rotator method, to obtain both total neocortical neuron number and their size distribution in formalin-fixed brains from six male acquired immunodeficiency syndrome (AIDS) patients and six male controls. The material is a selection of a large material choosing the youngest. The number of neurons in neocortex was reduced by 25% from 24.4 x 10(9) in controls to 18.3 x 10(9) in the AIDS patients; the reduction is similar to that of 27% found in the large material. In the normal size distribution of the neocortical neurons most neurons were smaller than 5000 micron3 and no sampled neurons were larger than 28,000 micron3. In addition, the absolute size distribution of neocortical neurons showed a significant decrease of the largest group of neurons by 50% (2p = 0.01) in the AIDS group, while there was no significant difference between controls and AIDS patients in the number of small neurons. The pattern of reduction in the number of large neocortical neurons was found in frontal, temporal, parietal as well as in occipital regions. This suggests that large neurons are more sensitive than small neurons to the destruction caused by the HIV infection.

DNA breaks detected by in situ end-labelling in dorsal root ganglia of patients with AIDS

Distal sensory axonal polyneuropathy (DSP) is the most frequent HIV-associated peripheral neuropathy. DSPs tend to occur in full-blown AIDS and worsen as CD4 cell counts decrease in blood. To assess a possible role for apoptosis in the pathogenesis of the neuropathy, we used in situ end-labelling (ISEL) detecting DNA strand breaks in DRG neurons of 19 HIV-infected patients, of whom nine had axonal polyneuropathy, and 11 controls. Sensory neurons with ISEL-assessed DNA breaks were observed in 9/19 patients with AIDS, 0/3 patients with pre-AIDS, and 1/11 controls. The prevalence of DNA breaks in neurons was higher in AIDS patients than in controls (P < 0.05). Among AIDS patients, DNA breaks in neurons were more abundant in patients with peripheral neuropathy (P < 0.04). It is possible that DNA breaks of DRG neurons induce the axonopathy and consequently play a role in the pathogenesis of DSP. It cannot be excluded, however, that DNA breaks could represent the result rather than the cause of axonopathy. We suggest that ISEL may detect neurons that were primed to apoptosis before death in relation with the HIV infection, and undergo DNA fragmentation at time of death, rather than neurons that underwent premortem both priming and triggering steps of the apoptotic process. This hypothesis could explain why most ISEL-positive neurons lack typical apoptotic morphology and why normal controls do not show ISEL positive cells.

Neuronal Loss in FIV-MD Infected Cats

Previously, this laboratory has shown that the Maryland strain of feline immunodeficiency virus (FIV-MD) causes neurological disease in cats similar to human immunodeficiency virus type 1 (HIV-1) in people. Using morphometrical methods on neocortical histologic sections we now show a significant loss of neurons in FIV-MD infected cats compared to age-matched uninfected controls. The neuronal populations affected resembles those lost in HIV-1 infection of the brain in published reports, providing further evidence for the utility of FIV-MD infection as a model for HIV-1 infections of the brain.