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60 Wisden, W. et al. (1992) The distribution of 13 GABAA receptor subunit mRNAs in the rat brain. I. Telencephalon, diencephalon, mesencephalon. J. Neurosci. 12, 1040–1062 61 Fritschy, J-M. and Mohler, H. (1994) GABAAreceptor heterogeneity in the adult rat brain: differential regional and cellular distribution of seven major subunits. J. Comp. Neurol. 359, 154–194 62 Nusser, Z. et al. (1996) Differential synaptic localization of two major gamma-aminobutyric acid type A receptor alpha subunits on hippocampal pyramidal cells. Proc. Natl. Acad. Sci. U. S. A. 93, 11939–11944 63 Nusser, Z. et al. (1998) Segregation of different GABAA receptors to synaptic and extrasynaptic membranes of cerebellar granule cells. J. Neurosci. 18, 1693–1703 64 Nusser, Z. et al. (1997) Differences in synaptic

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Retroviral diseases of the nervous system: pathogenic host response or viral gene-mediated neurovirulence? Christopher Power Retroviruses represent an important group of RNA viruses that cause a spectrum of nervous system diseases. Furthermore, newly recognized retroviral infections of the nervous system and some retroviral vectors or proteins used for gene delivery raise potential safety concerns. This article highlights different retroviruses and their causative mechanisms of nervous system disease, or neurovirulence. Specific sequences within retroviral genes might determine the development of neurovirulence. Conversely, neurovirulent retroviruses also activate host immune responses, resulting in a neuropathogenic cascade that is mediated by pro-inflammatory and neurotoxic molecules, ultimately culminating in neuronal death. Thus, retroviral infections of the nervous system illustrate a molecular interplay between distinct infectious agents and pathogenic host responses, which results in neurovirulence.

Christopher Power Neurovirology Laboratory, Neuroscience Research Group, Departments of Clinical Neuroscience, Microbiology and Infectious Diseases, 3330 Hospital Drive NW, Calgary, Alberta, Canada T2N 4N1. e-mail: power@ ucalgary.ca

Viral infections of the nervous system are increasingly important, as evidenced by the emergence of several global neurovirological epidemics over the past five years1. Moreover, there is an increasing interest in using viral vectors for gene expression and therapy in the brain2. Neurovirological infections are defined by the following attributes: neuroinvasiveness or viral entry of the nervous system; NEUROTROPISM (see Glossary) or viral infection of brain cells including the selective infection of neurons, termed neuronotropism; and NEUROVIRULENCE or virus-induced nervous system disease. Although many RETROVIRUSES fulfill the above criteria, several retroviral properties contribute to

their complex neurobiology, including (1) predilection to genomic mutation, (2) ability to induce of both innate and adaptive immune responses within the nervous system, and (3) the ability to cause disease outside the nervous system. Retroviral structure and life cycle

Retroviruses represent a large family of enveloped RNA viruses found in many species (Fig. 1a)3. These RNA viruses are termed retroviruses because, uniquely, they undergo reverse transcription from RNA to a complementary DNA PROVIRUS, which subsequently integrates into the host chromosomal DNA (Fig. 1b). Following integration, the provirus synthesizes viral transcripts, potentially for the duration of the survival of the cell, which depend on transcription factors produced by the host cell or the virus. All replication-competent retroviruses contain three major OPEN READING FRAMES (within gag, pol and env genes – see Glossary) (Fig. 1c). However, some retrovirus groups, such as the LENTIVIRUSES, contain several accessory genes that regulate viral replication, infectivity and transcription. The untranslated LONG TERMINAL REPEAT (LTR), which flanks the open reading frames and acts as a promoter domain, is required for transcription of the entire viral genome and induction of individual viral gene transcripts. The retroviral genome comprises a

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(a)

HSRV

Spumavirus

MuLV HERV-C HIV-1 STV FIV VMV SMRV

(b)

Lentivirus D-type

MMTV HERV-K IAP

B-type

RSV

ALV-like

HTLV-1 HTLV-2

HTLV

Envelope RNA Capsid Attachment

Maturation Structural proteins

Penetration

Budding Assembly

Regulatory proteins

Uncoating

New genome

Reverse transcription Gene expression

DNA

(c)

MuLV-like

MuLV

LTR

HIV-1

env vif

LTR

gag

vpr

0

2

4

LTR

env tat

pol vpu

rev

6

8

nef

10 kb TRENDS in Neurosciences

Fig. 1. Retroviral phylogenetic tree, life cycle and representative genomic structures. (a) Within the phylogenetic tree, several groups of neurotropic retroviruses are shown including the Type C retroviruses [murine (MuLV) and avian (ALV) leukemia viruses, and human T-cell lymphotropic virus (HTLV)], lentiviruses [human (HIV-1), simian (SIV), feline (FIV) immunodeficiency viruses and visnamaedi virus (VMV)], spumaviruses, and human endogenous retroviruses (HERV-K and -C).(b) The life cycle of retroviruses is defined by the binding of virion to the host cell receptor, cell entry, followed by reverse transcription and subsequent integration into the host genome. Full length viral transcripts and individual viral mRNAs are synthesized in the nucleus, which subsequently encode both structural and regulatory proteins, permitting viral assembly and eventual budding of new virions. (c) Representative retroviral genomes of Type C retroviruses (MuLV) and lentiviruses (HIV-1) showing the major structural genes of retroviruses (gag, pol, env), and showing the greater complexity of the lentiviral genomes because of the accessory genes, including rev, tat, vif, vpu, nef, vpr.

positive-sense, linear RNA molecule, of approximately 10 kb (Ref. 3), which exists in duplicate in the intact viral particle or virion. Specific retroviral proteins such as reverse transcriptase, integrase and protease http://tins.trends.com

are encoded by pol and are essential for viral replication, integration and viral assembly, respectively. The gag gene encodes proteins necessary for intracellular viral assembly and release from the cell. The env gene encodes structural proteins (surface unit and transmembrane unit) that are necessary for binding to cell-surface molecules that serve as receptors and mediate cell entry. Among some retroviruses, it is these latter proteins that are responsible for neurovirulence by killing neurons directly or through indirect mechanisms4 after infection of glial cells, which subsequently release neurotoxic molecules. The indirect mechanisms can also involve activation (and infection) of cells associated with INNATE IMMUNITY (macrophages, microglia, astrocytes) or ADAPTIVE IMMUNITY (T or B lymphocytes) within the nervous system. The immunological disease caused by some retroviruses, outside the nervous system results in secondary infections or malignant processes within the nervous system. However, this article is concerned only with retroviruses that directly cause neurovirulence, focusing on the mechanisms by which neurotropism and neurovirulence occur. Both Type C (also termed oncogenic) retroviruses, including the murine leukemia (MuLV) and the human T-cell lymphotropic viruses (HTLVs), and the lentiviruses, including human immunodeficiency viruses are discussed. Neurotropic strains of these retroviruses have been studied extensively and can give rise to serious health problems in humans. Type C or oncogenic retroviruses

LTR

gag pol pro

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Type C or oncogenic retroviruses are associated with neurovirulence in a range of host species, including humans, cats, rodents and birds4 (Table 1), with or without concurrent immunological disease. The genomic structure of this group of retroviruses is simpler than the lentiviruses, as their genomes are comprised of the three structural genes, two LTRs and, in some cases, an accessory gene. Neurotropic Type C retroviruses include the murine (MuLV), feline (FeLV) and avian (ALV) leukemia viruses and human T-cell lymphotropic viruses types 1 and 2 (HTVL-1 and HTLV-2). MuLVs

MuLVs comprise the largest group of neurotropic retroviruses and are associated with a range of behavioral phenotypes, usually manifested by tremor, hindlimb paralysis and lack of coordination5. Depending on the individual MuLV strain, glial cells such as microglia, astrocytes and oligodendrocytes are the primary infected cells in the brain, although several MuLVs infect neurons. Several cell-surface receptors for MuLVs have been identified3, including a cationic amino acid transporter, mCAT-1 (Ref. 6), but it is not known if this molecule serves as a receptor in the nervous system. MuLVs are assumed to access the brain under natural conditions by

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Table 1. Representative neurotropic retroviruses: viral strains, neuropathology and cell tropism Retrovirus group

Virus

Neuropathologya

In vivo infected neural cells

Type C

CasBrE

Spongiform, gliosis (SC)

Endothelia, neurons, glia

Lentiviruses

NE-8

Spongiform, gliosis (SC, BS, Cbl)

Endothelia, astrocytes, neurons

FrCasE

Spongiform, gliosis (SC, BS, Cbl)

Endothelia, microglia, neurons, oligodendrocytes

Fr 98D

Spongiform, gliosis (Cbl, WM, GM)

Microglia, neurons (?)

MoMuLV-ts1

Spongiform, gliosis (SC, BS, TH, GM, WM)

Endothelia, astrocytes, microglia

PCV-211

Spongiform (Cbr, SC, BS Cbl, WM)

Endothelia

TR1.3

Hemorrhage (Cbr)

Endothelia

LP-BM5

Gliosis (Cbr, Cbl, WM)

Astrocytes, microglia, choroid plexus

WB91-GV

Gliosis

Oligodendrocytes, astrocytes

HTLV-I/II

Demyelination, gliosis, lymphocyte infiltration (SC)

Lymphocytes, astrocytes

Human immunodeficiency virus

Gliosis, microglial nodules, multinucleated giant cells; diffuse myelin pallor (SC, Cbr, Cbl, WM, GM, PN)

Macrophages, microglia, astrocytes

Visna-maedi virus

Gliosis, perivascular inflammation, demyelination (WM)

Macrophages, microglia, astrocytes

Feline immunodeficiency virus

Perivascular inflammation, gliosis, microglial nodules (WM, GM, Cbr, SC, PN)

Macrophages, microglia

Caprine arthritis– encephalitis virus

Gliosis, demyelination (WM)

Macrophages and microglia

Equine infectious anemia virus

Ependymitis, perivascular inflammation (Cbr)

Perivascular and meningeal cells

Bovine immunodeficiency virus

Perivascular inflammation (WM)

Glial cells

Simian immunodeficiency virus

Perivascular inflammation, gliosis, multinucleated giant cells (WM, Cbr, GM, SC)

Macrophages, microglia, astrocytes, endothelia

aNeuropathological features are based on histopathological analysis. Spongiform changes and gliosis might accompany neuronal injury and loss that is not apparent on histopathology.

Abbreviations: BS, brainstem; Cbl, cerebellum; Cbr, cerebrum; GM, gray matter; PN, peripheral neuropathy; SC, spinal cord; TH, thalamus; WM, white matter.

infected leucocytes trafficking through the nervous system or by infection of brain endothelial cells. The neuropathology caused by MuLVs also depends on the individual MuLV strain although ‘spongiform’ changes, or circumscribed loss of CNS parenchyma, in the spinal cord and brainstem are the most common findings. Nonetheless, one strain of MuLV causes a hemorrhagic stroke-like pathology, which reflects its selective tropism for endothelial cells (Table 1). The neuropathogenic mechanisms in MuLV infections exemplify interactions between sequences within the retroviral genome, usually located in the env or gag genes, coupled with abnormal host responses that are influenced by the mouse genetic background. For example, within the Fr-98 MuLV strain, two separate domains within the env gene regulate the development of neurovirulence in specific mouse lines7, defined by neurobehavioral http://tins.trends.com

abnormalities leading to death. In contrast, its parent retrovirus has a similar cell tropism (microglia) but is avirulent in the nervous system. Altered intracellular processing and expression of MuLV env-encoded proteins8 and levels of viral replication in the brain7 appear to be contributing factors to neurovirulence in some MuLV models. The pathogenic host responses initiated by MuLV infections of the nervous remain largely undefined but increased pro-inflammatory cytokine production by glial cells has been correlated with neurovirulence9. Other abnormal host responses induced by MuLV infections include altered permeability of the blood–brain barrier and increased extracellular glutamate concentrations, resulting in neuronal death through an excitotoxic mechanism10. Resistance to MuLV-induced neurological disease has been mapped to several host genes including FV-1, FV-4 and Akvr-1, which express endogenous

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Brain Microglia MMPs, TNF-α, Tat, RANTES, gp120

Microglia

NMDA-R/AMPA-R

Macrophage Neuron NO, PAF, QA, cysteine, gp120, Tat

Astrocyte

TNF-α, Tat, gp120

Monocyte PVC

CD4 Monocyte

?

Endothelial cells CCR5 Lymphocyte

Lentivirus Blood

CXCR4 TRENDS in Neurosciences

Fig. 2. Cell types involved in lentivirus neuropathogenesis. The cell types involved in lentivirus neuropathogenesis are shown by initial infection of blood lymphocytes and monocytes, followed by trafficking of infected cells across the blood–brain barrier. It is assumed that the main infected cell type entering the nervous system is the monocyte/macrophage, which subsequently permits infection of perivascular macrophages (PVC), microglia, and astrocytes. Infected and activated macrophages and microglia produce pro-inflammatory molecules including cytokines, chemokines, and matrix metalloproteinases (MMPs), which contribute to neuronal death. Neuronal function is also impaired by potential neurotoxins, such as quinolinic acid (QA), glutamate, cysteine, platelet-activating factor (PAF), and nitric oxide (NO). Viral proteins including Tat and the env-encoded gp120 or gp41 might act directly and indirectly to cause neuronal death. Infected or activated astrocytes exhibit diminished uptake of glutamate, thereby increasing the likelihood of neuronal death via excitotoxic mechanisms. Chemokine receptors serve as co-receptors for infection of lymphocytes and macrophages/microglia and might also modulate neuronal survival and function. Abbreviations: AMPA-R, AMPA receptor; NMDA-R, NMDA receptor; TNF-α, tumor necrosis factor α.

retroviral env and gag genes that prevent neurovirulence by blocking the putative viral receptor or inhibiting a post-entry step5. In addition, neurovirulence caused by several MuLVs is age dependent, with neonates exhibiting increased susceptibility because of the relative absence of protective immunity. HTLVs

HTLV-1 and HTLV-2 are retroviruses that cause a neurological syndrome or malignancy in humans although both conditions very rarely co-exist11. A subset of HTLV-1- or HTLV-2-infected adult patients (<1%) will develop a progressive MYELOPATHY, characterized by inflammation, principally in the thoracic region of the spinal cord. Viral proteins and genome12 have been detected in astrocytes and lymphocytes within spinal cord lesions, although the viral receptor required for cell entry remains unknown. It is assumed that, like MuLVs, HTVL-1 or HTLV-2 enters the nervous system by way of trafficking lymphocytes. Certain human leukocyte antigen (HLA) haplotypes have been correlated with http://tins.trends.com

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increased susceptibility to HTLV-induced myelopathy, which is accompanied by enhanced intrathecal immune responses to the virus13. The neuropathological features are defined by the infiltration of lymphocytes into a demyelinating lesion in the spinal cord and surrounding meninges, accompanied by axonal injury and loss. The mechanism by which HTLV-1 causes the damage to axons and myelin associated with HTLV-1 infection is probably dependent on the intense inflammation within the lesion. Cytotoxic (CD8+) T lymphocytes (CTLs) have been isolated from biopsied tissue, suggesting a role for CTL-mediated injury. Although there is little evidence for HTLV-1 or HTLV-2 straindependent neurovirulence, individual viral genes appear to contribute to the occurrence of inflammation. From in vitro studies, the tax gene, encoded by HTLV-1, is known to transactivate several host genes, including those that encode cytokines and matrix metalloproteinases in lymphocytes and astrocytes, which have been hypothesized to damage the myelin and axons directly or alter blood–brain barrier permeability14. Lentiviruses

The lentivirus group of retroviruses has attracted increased attention because of the HIV–AIDS epidemic and the high frequency with which nervous system disease occurs during these infections15. Lentiviruses include the immunodeficiency viruses, human (HIV), simian (SIV), feline (FIV), bovine (BIV) and several other ungulate lentiviruses, such as visna-maedi virus (VMV)16. More recently, several groups have constructed SIV–HIV chimeras (SHIV), which are also neurotropic17. Despite immense molecular diversity within several lentiviral genes, the lentiviruses share many genetic and biological aspects, including the essential retroviral genome organization described above. However, all lentivirus genomes also contain multiple accessory genes that are essential for viral infectivity and replication. Lentiviruses are distinguished by their ability to replicate in non-dividing cells such as macrophages18, and lentiviral neurotropism predominantly involves cells of bone marrow lineage, such as microglia and macrophages, whereas astrocytes and endothelial cells can be infected to a much lesser extent19,20. It is widely assumed that lentiviruses enter the CNS through infected macrophages crossing the blood–brain barrier and subsequent infection of microglial cells21 (Fig. 2). For some lentiviruses, such as HIV, SIV and FIV, the cell-surface viral receptors are known both inside and outside the nervous system, including CD4 or several chemokine receptors, or both, such as CXCR4 and CCR5 (Ref. 22). Clinical–neurobehavioral and neuropathological features

The neuropathological and clinical–neurobehavioral manifestations of lentivirus infections vary widely,

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Table 2. Neuromodulatory and neurotoxic molecules implicated in retroviral neuropathogenesis Source of molecules

Molecules

Viral proteins

Env-encoded proteins (gp 120, gp41), Tat, Nef

Macrophage/microglialderived molecules

Low molecular weight toxic factors: quinolinic acid, glutamate, cysteine, nitric oxide, super anion, peroxynitrite, Ntox Arachidonic acid metabolites (prostaglandin E2 and F2; leukotriene B4 and D4; thromboxane B2; plateletactivating factor) Cytokines: TNF-α, IL-1β, IL-6, IFN-α Chemokines: MCP-1, RANTES, MIP-1α Matrix metalloproteinases: MMP-2, -7, -9

Abbreviations: IFN, interferon; IL, interleukin; MCP, macrophage chemoattractant protein; MIP, macrophage inhibitory protein; TNF, tumor necrosis factor.

depending on the individual lentivirus and host factors such as age and systemic immune status. Involvement of the brain, resulting in dementia or ENCEPHALOPATHY, is common to all lentiviruses. Neuronal injury and death accompanied by inflammation appear to be pivotal determinants of the clinical–behavioral syndromes that accompany lentiviral infections20,23. Select neuronal populations in the basal ganglia and frontal cortex, which are the chief sites of infection and macrophage infiltration, are particularly vulnerable to lentiviral-induced damage24,25. Morphological changes include neuronal loss, dendritic vacuolization, axonal injury with coexisting gliosis, macrophage infiltration, multinucleated giant cells, and altered blood–brain barrier structure and function26 28. However, for both HIV and the animal lentiviral infections, there is often a dichotomy within the host between the severity of neuropathological abnormalities (ENCEPHALITIS) and the clinical–behavioral manifestations (encephalopathy)29,30. Viral determinants of lentivirus neurovirulence

Neuronal injury and death probably result from lentivirus infection and activation of surrounding glia and macrophages, causing the release of potential neurotoxins (Table 2)31. A causative relationship between lentiviral gene expression and neurovirulence is derived from multiple lines of evidence. These include reports of lentiviral strainassociated neurovirulence in the SIV (Ref. 32), VMV (Ref. 33) and FIV (Ref. 34) models, and neurotoxicity studies using viral proteins derived from HIV, SIV, FIV and VMV (Ref. 35). For example, specific sequences or mutations within the env gene have been shown to influence directly the development of SIV- and FIV-associated neurovirulence. In the SIV model, distinct sequences within the transmembrane domain, gp41, are crucial for neurovirulence, but in the FIV model, the surface unit, gp95, appears chiefly responsible for neurovirulence. These latter studies are mirrored by reports showing that specific mutations within brain- and http://tins.trends.com

blood-derived HIV env sequences are associated with HIV-associated dementia (HAD)36,37. Functional studies indicate that infection of macrophages by recombinant viruses containing brain-derived env sequences from patients with HAD result in the release of neurotoxic molecules in vitro38, possibly through a mechanism involving induction of the signal transducer and activator of transcription 1–Janus kinase 1 (STAT1–JAK1) pathway and matrix metalloproteinases39. Domains within lentiviral env-encoded proteins have also been shown to be directly and indirectly neurotoxic in vitro and in vivo40,41, such as the CD4-binding42 and the hypervariable regions43 within HIV-1 gp120, possibly through an excitotoxic mechanism involving glutamate receptors and voltage-operated Ca2+channel activation. Transgenic mice expressing HIV gp120 in astrocytes display neuropathological findings including astrogliosis, neuronal loss and dendritic vacuolizations, resembling HIV encephalitis44. Other lentiviral proteins, including HIV and VMV Tat, HIV gp41 and HIV Nef, have been shown to be neurotoxic in vitro or cause induction expression of host genes35. The Tat protein has been the focus of many neuropathogenesis studies because of the remarkable range of its effects on viral transcription, activation of host genes and ability to cause neuronal death45. Lentivirus copy number or ‘viral load’ in plasma is a strong predictor of immunological disease progression46,47, but a correlation between the presence of HAD or lentivirus encephalopathy in animals and viral load in the brain remains undefined34,48, although encephalitic lesions frequently exhibit lentiviral antigens20,49,50 and cerebrospinal fluid viral load can be correlated with lentivirus neurovirulence51,52. However, an important caveat derived from the in vivo studies of lentivirus infections is that systemic immune suppression is also an important predictor of neurological disease34,53,54; careful interpretation of in vitro studies must therefore be made. Host determinants of lentivirus neurovirulence

Multiple molecules released by macrophages and microglia have been implicated in lentivirus neuropathogenesis (Table 2). Although the induction of pro-inflammatory molecules by lentiviruses is associated with neurovirulence, specific mutations or polymorphisms within the same host proinflammatory genes have not been implicated to date. In contrast, HIV-infected individuals heterozygous for a 32 bp deletion in the β-chemokine receptor, CCR5, are at less risk of HIV-associated dementia, compared with individuals with AIDS who do not carry this polymorphism55. As CCR5 is crucial for HIV and SIV infection of the microglia and macrophages within the brain, and also modulates intracellular signaling of many pro-inflammatory pathways such as the STAT–JAK pathways56, this

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receptor might be a key component in the development of lentivirus neurovirulence (Fig. 2). Most cells in the CNS are capable of producing cytokines, but the major sources are activated glial cells such as macrophages, microglia and astrocytes57. Tumor necrosis factor α is an inflammatory cytokine that has been highlighted in lentiviral CNS infections because of its increased levels in the brains of HIV-, FIV- and SIV-infected hosts, and because of its ability to influence the release of other inflammatory molecules and to modulate excitotoxic neurotransmitter levels58. With increased understanding of chemokine receptors as co-receptors for lentiviral infections, there has also been an expanding interest in the pathogenic actions of chemokines in the nervous system59. Studies show that macrophage inhibitory protein 1α (MIP1α) and RANTES (regulated on activation, normal T cell expressed and secreted) (Table 2) are able to block gp120-induced neuronal death, might affect Ca2+ signaling in neurons, inhibit infection of lentiviruses and also serve as chemoattractant molecules for infected or activated macrophages, or both. These pro-inflammatory molecules represent elements of a local intra- and extracellular cascade that involves interactions between cells of the monocyte–macrophage lineage and astrocytes, which contribute to neuronal injury.

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properties of integrating into the host genome and replicating in non-dividing cells66. The concurrent safety concerns include the potential development of virulent retroviral vectors67 or the potential toxicity of a lentiviral protein such as Tat (Ref. 68), which is a proposed vehicle for gene delivery. Furthermore, nonhuman lentiviruses and derived vectors infect human cells and could cause death of human lymphocytes, macrophages and neurons69. Nonetheless, recombinant non-replicating retroviral vectors with restricted infection of specific cells hold promise for the treatment of neurological disorders such as Parkinson’s disease, stroke and brain tumors. Other emerging retroviral infections include the endogenous retroviruses70 and spuma viruses3. Although spuma viruses are associated with a relative paucity of disease in their natural hosts, their ability to cause neurodegeneration when expressed as transgenes in mice and to infect different species including humans makes their potential use as viral vectors concerning71. The endogenous retroviruses (ERVs) are an intriguing group of retroviruses that are found in most mammalian species, including humans, mice, primates and felines, and could represent up to 1% of the host genome72. ERVs are usually replication defective but some might recombine with their exogenous homolog to generate replication-competent viruses, as shown for FeLVs and MuLVs. ERVs have been implicated in human

Mechanisms of neuronal death in lentivirus infections

The intracellular pathways by which neurons are damaged or killed, or both, during lentiviral infections are receiving increased attention60. Neuronal death has been shown to be apoptotic in some studies but it is unclear at present whether programmed cell death is the major mechanism that underlies neuronal death61. In vitro studies suggest that the HIV env-encoded protein, gp120, can induce apoptosis in human fetal neurons through activation of neuronal JNK and extracellular-regulated kinase pathways62. Other reports indicate that gp120, derived from a T-cell tropic HIV strain, induces neuronal apoptosis that is mediated by p38 mitogenactivated protein kinase63. In vitro studies using HIV Tat have shown that the neuronal glycogen synthase kinase 3β and caspase 9 also regulate neuronal survival64. From these studies, it appears multiple pathways might determine the mechanism and frequency of HIV-related neuronal death. Future considerations

Retroviruses are a major health issue because of the preference of this group of viruses for infection of the nervous and immune systems. Immediate issues include the growing HIV–AIDS epidemic and potential xeno-infections arising from the use of porcine or nonhuman primate donor organs for human transplantation65. For the development of viral vectors in which sustained gene expression is sought, many retroviruses have the desirable http://tins.trends.com

Glossary Adaptive immunity: host immune response mediated by B and T cells, for example, antibodies or cytotoxic (CD8+) T lymphocytes. Encephalitis: morphological changes in the brain associated with inflammation and infection, including leukocyte infiltration and glial cell activation; frequently associated with viral genome or antigen presence. Encephalopathy: disease of the brain that results in behavioral, cognitive or motor abnormalities. env: retroviral gene encoding envelope proteins such as the surface and transmembrane unit. gag: retroviral gene encoding capsid and nucleocapsid proteins. Innate immunity: host immune response mediated by macrophages, microglia, dendritic cells, mast cells and natural killer cells. Lentivirus: complex retrovirus characterized by its ability to infect non-dividing cells, for example, HIV, SIV and FIV. Long terminal repeat (LTR): untranslated region of retrovirus that flanks the structural genes and contains transcriptional regulatory elements. Myelopathy: disease of the spinal cord that results in motor and sensory dysfunction. Neurotropism: infection of the nervous system by a virus. Neurovirulence: disease in the nervous system caused by virus. Open reading frame: protein-encoding domain within a gene. pol: retroviral gene encoding reverse transcriptase (DNA polymerase), integrase and protease enzymes. In some instances, the protease can be encoded by gag. Provirus: DNA intermediate of retrovirus, usually integrated into the host DNA. Retrovirus: positive-sense RNA virus, capable of reverse transcription. Type C retrovirus: simple retrovirus, frequently associated with oncogenesis, for example, MuLV and HTLV-1 or HTLV-2.

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Acknowledgements We thank Quentin Pittman, Bruce Chesebro, Richard T. Johnson, Voon Wee Yong and the members of the Neurovirology laboratory for helpful discussions and suggestions.

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disease, as suggested by the recent association of human ERVs with the development of multiple sclerosis and other autoimmune diseases with neurological manifestations73. Neurodegeneration, arising because of chronic inflammation, is a cardinal feature of neurotropic retroviral infections. Many retroviruses are now available as defined infectious molecular clones, and thus, the potential exists to dissect the individual signaling pathways in vivo induced by the

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retroviruses that result in neurodegeneration. Proinflammatory and neurotoxic host molecules that participate in the cascade of events resulting in retroviral neurovirulence have also been implicated in other neurodegenerative diseases such as multiple sclerosis, stroke, Alzheimer’s disease and brain trauma74. Hence, the mechanisms by which retroviruses induce brain inflammation and injury could provide useful insights into the pathogenesis of other brain diseases that involve inflammation.

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Clues to the cochlear amplifier from the turtle ear Robert Fettiplace, Anthony J. Ricci and Carole M. Hackney Sound stimuli are detected in the cochlea by vibration of hair bundles on sensory hair cells, which activates mechanotransducer ion channels and generates an electrical signal. Remarkably, the process can also work in reverse with additional force being produced by the ion channels as they open and close, evoking active movements of the hair bundle. These movements could supplement the energy of the sound stimuli but to be effective they would need to be very fast. New measurements in the turtle ear have shown that such active bundle movements occur with delays of less than a millisecond, and are triggered by the entry of Ca2++ into the cell via the mechanotransducer channel. Furthermore, their speed depends on the frequency to which the hair cell is most sensitive, suggesting that such movements could be important in cochlear amplification and frequency discrimination.

In the vertebrate inner ear, sound stimuli are detected by hair cells of the cochlea via deformation of their mechanically sensitive hair bundles (Fig. 1). Although much has been learnt about this process over the past twenty years, several issues are still

unresolved. One is the molecular identity of the mechanotransducer channel, a mechanosensitive ion channel responsible for converting vibrations of the sensory hair bundles into electrical signals1–3. Another is the precise mechanism underlying cochlear amplification, whereby sound-induced vibrations of the cochlear membranes are boosted by energy supplied by the hair cells (Fig. 1). This amplification is necessary to overcome the damping effects of the cochlear fluids, and is central to explaining not only the cochlea’s high sensitivity but also its frequency selectivity, whereby each cell responds best to a narrow range of sound frequencies4. A variety of evidence implicates the outer hair cells as the site of force-generation in the mammalian cochlea4, and the discovery that these cells can contract rapidly in response to changes in membrane potential has raised a possible mechanism for this process5–7. However, studies on lower

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