Neuron loss and axon reorganization in the dentate gyrus of cats infected with the feline immunodeficiency virus

HIV-1 and drug abuse comorbidity: Lessons learned from the animal models of NeuroHIV

Various research studies that have investigated the association between HIV infection and addiction underpin the role of various drugs of abuse in impairing immunological and non-immunological pathways of the host system, ultimately leading to augmentation of HIV infection and disease progression. These studies have included both in vitro and in vivo animal models wherein investigators have assessed the effects of various drugs on several disease parameters to decipher the impact of drugs on both HIV infection and progression of HIV-associated neurocognitive disorders (HAND). However, given the inherent limitations in the existing animal models of HAND, these investigations only recapitulated specific aspects of the disease but not the complex human syndrome. Despite the inability of HIV to infect rodents over the last 30 years, multiple strategies have been employed to develop several rodent models of HAND. While none of these models can accurately mimic the overall pathophysiology of HAND, they serve the purpose of modeling some unique aspects of HAND. This review provides an overview of various animal models used in the field and a careful evaluation of methodological strengths and limitations inherent in both the model systems and study designs to understand better how the various animal models complement one another.

Proportional loss of parvalbumin-immunoreactive synaptic boutons and granule cells from the hippocampus of sea lions with temporal lobe epilepsy

One in 26 people develop epilepsy and in these temporal lobe epilepsy (TLE) is common. Many patients display a pattern of neuron loss called hippocampal sclerosis. Seizures usually start in the hippocampus but underlying mechanisms remain unclear. One possibility is insufficient inhibition of dentate granule cells. Normally parvalbumin-immunoreactive (PV) interneurons strongly inhibit granule cells. Humans with TLE display loss of PV interneurons in the dentate gyrus but questions persist. To address this, we evaluated PV interneuron and bouton numbers in California sea lions (Zalophus californianus) that naturally develop TLE after exposure to domoic acid, a neurotoxin that enters the marine food chain during harmful algal blooms. Sclerotic hippocampi were identified by the loss of Nissl-stained hilar neurons. Stereological methods were used to estimate the number of granule cells and PV interneurons per dentate gyrus. Sclerotic hippocampi contained fewer granule cells, fewer PV interneurons, and fewer PV synaptic boutons, and the ratio of granule cells to PV interneurons was higher than in controls. To test whether fewer boutons was attributable to loss versus reduced immunoreactivity, expression of synaptotagmin-2 (syt2) was evaluated. Syt2 is also expressed in boutons of PV interneurons. Sclerotic hippocampi displayed proportional losses of syt2-immunoreactive boutons, PV boutons, and granule cells. There was no significant difference in the average numbers of PV- or syt2-positive boutons per granule cell between control and sclerotic hippocampi. These findings do not address functionality of surviving synapses but suggest reduced granule cell inhibition in TLE is not attributable to anatomical loss of PV boutons.

Neurologic disease in feline immunodeficiency virus infection: disease mechanisms and therapeutic interventions for NeuroAIDS

Feline immunodeficiency virus (FIV) is a lentivirus that causes immunosuppression through virus-mediated CD4+ T cell depletion in feline species. FIV infection is complicated by virus-induced disease in the nervous system. FIV enters the brain soon after primary infection and is detected as FIV-encoded RNA, DNA, and proteins in microglia, macrophages, and astrocytes. FIV infection activates neuroinflammatory pathways including cytokines, chemokines, proteases, and ROS with accompanying neuronal injury and loss. Neurobehavioral deficits during FIV infection are manifested as impaired motor and cognitive functions. Several treatment strategies have emerged from studies of FIV neuropathogenesis including the therapeutic benefits of antiretroviral therapies, other protease inhibitors, anti-inflammatory, and neurotrophic compounds. Recently, insulin's antiviral, anti-inflammatory, and neuroprotective effects were investigated in models of lentivirus brain infection. Insulin suppressed HIV-1 replication in human microglia as well as FIV replication of lymphocytes. Insulin treatment diminished cytokine and chemokine activation in HIV-infected microglia while also protecting neurons from HIV-1 Vpr protein-mediated neurotoxicity. Intranasal (IN) insulin delivery for 6 weeks suppressed FIV expression in the brains of treated cats. IN insulin also reduced neuroinflammation and protected neurons in the hippocampus, striatum, and neocortex of FIV-infected animals. These morphological and molecular effects of IN insulin were confirmed by neurobehavioral studies that showed IN insulin-treated FIV-infected animals displayed improved motor and cognitive performance compared to sham-treated FIV-infected animals. Thus, FIV infection of the nervous system provides a valuable comparative in vivo model for discovering and evaluating disease mechanisms as well as developing therapeutic strategies for NeuroAIDS in humans.

Feline Immunodeficiency Virus Neuropathogenesis: A Model for HIV-Induced CNS Inflammation and Neurodegeneration

Feline Immunodeficiency virus (FIV), similar to its human analog human immunodeficiency virus (HIV), enters the central nervous system (CNS) soon after infection and establishes a protected viral reservoir. The ensuing inflammation and damage give rise to varying degrees of cognitive decline collectively known as HIV-associated neurocognitive disorders (HAND). Because of the similarities to HIV infection and disease, FIV has provided a useful model for both in vitro and in vivo studies of CNS infection, inflammation and pathology. This mini review summarizes insights gained from studies of early infection, immune cell trafficking, inflammation and the mechanisms of neuropathogenesis. Advances in our understanding of these processes have contributed to the development of therapeutic interventions designed to protect neurons and regulate inflammatory activity.

Hippocampal neuropathology of domoic acid-induced epilepsy in California sea lions (Zalophus californianus)

California sea lions (Zalophus californianus) are abundant human-sized carnivores with large gyrencephalic brains. They develop epilepsy after experiencing status epilepticus when naturally exposed to domoic acid. We tested whether sea lions previously exposed to DA (chronic DA sea lions) display hippocampal neuropathology similar to that of human patients with temporal lobe epilepsy. Hippocampi were obtained from control and chronic DA sea lions. Stereology was used to estimate numbers of Nissl-stained neurons per hippocampus in the granule cell layer, hilus, and pyramidal cell layer of CA3, CA2, and CA1 subfields. Adjacent sections were processed for somatostatin immunoreactivity or Timm-stained, and the extent of mossy fiber sprouting was measured stereologically. Chronic DA sea lions displayed hippocampal neuron loss in patterns and extents similar but not identical to those reported previously for human patients with temporal lobe epilepsy. Similar to human patients, hippocampal sclerosis in sea lions was unilateral in 79% of cases, mossy fiber sprouting was a common neuropathological abnormality, and somatostatin-immunoreactive axons were exuberant in the dentate gyrus despite loss of immunopositive hilar neurons. Thus, hippocampal neuropathology of chronic DA sea lions is similar to that of human patients with temporal lobe epilepsy.

Animal models of retroviral encephalopathies: feline model

Human immunodeficiency virus infection in children and adults results in a progressive neurodegenerative disease consistent with a predominant subcortical mediated dementia. Techniques for developing a feline model of the early stages of lentiviral-associated neurodegeneration are presented. The behavioral, neurophysiologic, immunologic, virologic, and neuropathologic aspects of this model are also described.

Somatostatin in the dentate gyrus

The neuropeptide somatostatin (SST) is expressed in a discrete population of interneurons in the dentate gyrus. These interneurons have their soma in the hilus and project to the outer molecular layer onto dendrites of dentate granule cells, adjacent to perforant path input. SST-containing interneurons are very sensitive to excitotoxicty, and thus are vulnerable to a variety of neurological diseases and insults, including epilepsy, Alzheimer's disease, traumatic brain injury, and ischemia. The SST gene contains a prototypical cyclic AMP response element (CRE) site. Such a regulatory site confers activity-dependence to the gene, such that it is turned on when neuronal activity is high. Thus SST expression is increased by pathological conditions such as seizures and by natural stimulation such as environmental enrichment. SST may play an important role in cognition by modulating the response of neurons to synaptic input. In the dentate, SST and the related peptide cortistatin (CST) reduce the likelihood of generating long-term potentiation, a cellular process involved in learning and memory. Thus these neuropeptides would increase the threshold of input required for acquisition of new memories, increasing "signal to noise" to filter out irrelevant environmental cues. The major mechanism through which SST inhibits LTP is likely through inhibition of voltage-gated Ca(2+) channels on dentate granule cell dendrites. Transgenic overexpression of CST in the dentate leads to profound deficits in spatial learning and memory, validating its role in cognitive processing. A reduction of synaptic potentiation by SST and CST in dentate may also contribute to the well-characterized antiepileptic properties of these neuropeptides. Thus SST and CST are important neuromodulators in the dentate gyrus, and disruption of this signaling system may have major impact on hippocampal function.

Feline immunodeficiency virus neuropathogenesis: from cats to calcium

Invasion of human immunodeficiency virus (HIV) into the central and peripheral nervous system produces a wide range of neurological symptoms, which continue to persist even with adequate therapeutic suppression of the systemic viremia. The development of therapies designed to prevent the neurological complications of HIV require a detailed understanding of the mechanisms of virus penetration into the nervous system, infection, and subsequent neuropathogenesis. These processes, however, are difficult to study in humans. The identification of animal lentiviruses similar to HIV has provided useful models of HIV infection that have greatly facilitated these efforts. This review summarizes contributions made from in vitro and in vivo studies on the infectious and pathological interactions of feline immunodeficiency virus (FIV) with the nervous system. In vivo studies on FIV have provided insights into the natural progression of CNS disease as well as the contribution of various risk factors. In vitro studies have contributed to our understanding of immune cell trafficking, CNS infection and neuropathogenesis. Together, these studies have made unique contributions to our understanding of (1) lentiviral interactions at the blood-cerebrospinal fluid (CSF) barrier within the choroid plexus, (2) early FIV invasion and pathogenesis in the brain, and (3) lentiviral effects on intracellular calcium deregulation and neuronal dysfunction. The ability to combine in vitro and in vivo studies on FIV offers enormous potential to explore neuropathogenic mechanisms and generate information necessary for the development of effective therapeutic interventions.

Neuropathology associated with feline immunodeficiency virus infection highlights prominent lymphocyte trafficking through both the blood-brain and blood-choroid plexus barriers

Feline immunodeficiency virus (FIV) infection in the cat is a well-evaluated model of human immunodeficiency virus (HIV)-1 infection in man with both viruses associated with significant neuropathology. Although studies in both HIV and FIV infections have shown that virus enters the brain in the acute stages of disease, little is known of the mechanisms of viral entry. The dissection of this stage is fundamental to the development of therapies that may prevent or modulate central nervous system (CNS) infection. The present study was designed to characterize the early sequential neuropathological changes following infection with FIV(GL8), a strain known to enter the CNS in acute infection. Cats were infected either by the intraperitoneal (n = 13) or intravenous (n = 12) route with 2000 cat infectious units of virus. Histopathological assessments following intraperitoneal infections were at 4 (n = 2), 5 (n = 1), 8 (n = 3), 10 (n = 1), 16 (n = 1), 32 (n = 2), 52 (n = 2), and 104 (n = 1) weeks post infection whereas animals infected intravenously were examined (n = 3) at 1, 4, 10, and 23 weeks post infection. The most significant lesions following both routes of infection were lymphocyte-rich perivascular infiltrates within cerebral and cerebellar meninges, in choroid plexus and spinal cord dura mater and within epineurium of the sciatic nerve. In addition, following intravenous infection perivascular infiltrations were noted in parenchymal blood vessels primarily of cerebral white matter. Infiltrates were composed of CD79+ B cells and CD3+ T cells. The latter population contained a mixture of CD4+ and CD8+ cells. The severity of lesions increased in intensity in the 8-to 16-week period following infection and then began to wane. The evaluation of this large group of cats at multiple time points revealed pathology comparable with that of early stage HIV-1-associated encephalitis. Moreover, in contrast to previous FIV neuropathology studies, transient meningeal, choroid plexus, and parenchymal vascular pathology were consistent significant findings suggesting that, as in HIV-1 infection, blood-brain barrier and choroid plexus brain barrier integrity are both compromised in early infection.

Electrophysiological evidence of monosynaptic excitatory transmission between granule cells after seizure-induced mossy fiber sprouting

Mossy fiber sprouting is a form of synaptic reorganization in the dentate gyrus that occurs in human temporal lobe epilepsy and animal models of epilepsy. The axons of dentate gyrus granule cells, called mossy fibers, develop collaterals that grow into an abnormal location, the inner third of the dentate gyrus molecular layer. Electron microscopy has shown that sprouted fibers from synapses on both spines and dendritic shafts in the inner molecular layer, which are likely to represent the dendrites of granule cells and inhibitory neurons. One of the controversies about this phenomenon is whether mossy fiber sprouting contributes to seizures by forming novel recurrent excitatory circuits among granule cells. To date, there is a great deal of indirect evidence that suggests this is the case, but there are also counterarguments. The purpose of this study was to determine whether functional monosynaptic connections exist between granule cells after mossy fiber sprouting. Using simultaneous recordings from granule cells, we obtained direct evidence that granule cells in epileptic rats have monosynaptic excitatory connections with other granule cells. Such connections were not obtained when age-matched, saline control rats were examined. The results suggest that indeed mossy fiber sprouting provides a substrate for monosynaptic recurrent excitation among granule cells in the dentate gyrus. Interestingly, the characteristics of the excitatory connections that were found indicate that the pathway is only weakly excitatory. These characteristics may contribute to the empirical observation that the sprouted dentate gyrus does not normally generate epileptiform discharges.

Neuronal injury in hippocampus with human immunodeficiency virus transactivating protein, Tat

Patients with human immunodeficiency virus infection may develop a dementing illness. Using both in vitro and in vivo models, we investigated the susceptibility of the hippocampal formation to the Tat protein of human immunodeficiency virus. We also determined the pattern of hippocampal injury in patients with human immunodeficiency virus encephalitis. Following exposure of hippocampal slices to Tat, marked susceptibility of CA3 region with relative insensitivity of the CA1/2 region was observed. Injection of Tat into different regions of the rat hippocampus produced similar neuronal loss in both CA3 region and the dentate gyrus. In animals administered Tat, lesions were dose-dependent and immunohistochemical staining showed marked gliosis and loss of microtubule associated protein-2 in the affected areas at 3 days post-injection. Interestingly, synaptophysin staining was relatively preserved. In hippocampal tissue from patients with human immunodeficiency virus encephalitis, loss of microtubule-associated protein-2 staining was reduced in the molecular layer of the dentate gyrus. The results of our experiments demonstrate a unique pattern of hippocampal injury in organotypic culture and rats exposed to Tat. Our observations that patients with human immunodeficiency virus reveal a similar pattern of damage suggests that Tat protein may be pathophysiological relevant in human immunodeficiency virus encephalitis.

The feline model of neuroAIDS: understanding the progression towards AIDS dementia

Feline immunodeficiency virus (FIV) is a neurotropic lentivirus that produces a protracted state of immunodeficiency and encephalopathy in the cat. Recent evidence has shown several similarities to the natural progression of human immunodeficiency virus infection (HIV-1) associated degenerative effects on the central and peripheral nervous systems. Similar to HIV-1, FIV-induced encephalopathy neurovirulence is strain dependent, results in progressive immunodeficiency and increasing early peripheral but not brain viral load, preferentially affects the developing nervous system, produces quantifiable behavioural and neurophysiological impairment that is not directly linked to neuronal infectivity, and induces neuronal injury and loss both in vivo and in vitro. This paper highlights the cumulative scientific body of evidence supporting the use of the feline model of neuroAIDS.

Hippocampal injury and alterations in neuronal chemokine co-receptor expression in patients with AIDS

Hippocampal neurons express high levels of HIV chemokine co-receptors, activation of which causes injury or death in vitro. To determine if their in vivo expression correlates with injury, we evaluated neuronal CXCR4 and CCR5 immunoreactivity and reactive gliosis in autopsy hippocampus of 10 control cases, 11 AIDS cases without HIV encephalitis (HIVnE) or opportunistic infections/lymphomas (OI/L), and 11 AIDS cases with HIV encephalitis (HIVE). All groups had higher CXCR4 and CCR5 expression in CA3 and CA4 neurons than CA1 neurons (p < 0.05). HIVE cases had increased neuronal CXCR4 and decreased neuronal CCR5 expression as well as increased numbers of hippocampal GFAP-positive astrocytes and LN3-positive microglia. Changes were most severe in CA3 and CA4 and lowest in CA1 regions. These findings also were noted in the 4 HIVE cases with neither hippocampal HIVE nor brain OI/L and in the HIVnE groups. This study quantitates the regional distribution of hippocampal neuronal CXCR4 and CCR5 and shows their respective increase and decrease in AIDS. It suggests a relationship between neuronal loss and gliosis with intensity of neuronal chemokine expression and raises the possibility of a selective vulnerability of hippocampal neurons to AIDS-related injury.