Jianbin Wang

Evolution of the Aging Brain Transcriptome and Synaptic Regulation

Alzheimers disease and other neurodegenerative disorders of aging are characterized by clinical and pathological features that are relatively specific to humans. To obtain greater insight into how brain aging has evolved, we compared age-related gene expression changes in the cortex of humans, rhesus macaques, and mice on a genome-wide scale. A small subset of gene expression changes are conserved in all three species, including robust age-dependent upregulation of the neuroprotective gene apolipoprotein D (APOD) and downregulation of the synaptic cAMP signaling gene calcium/calmodulin-dependent protein kinase IV (CAMK4). However, analysis of gene ontology and cell type localization shows that humans and rhesus macaques have diverged from mice due to a dramatic increase in age-dependent repression of neuronal genes. Many of these age-regulated neuronal genes are associated with synaptic function. Notably, genes associated with GABA-ergic inhibitory function are robustly age-downregulated in humans but not in mice at the level of both mRNA and protein. Gene downregulation was not associated with overall neuronal or synaptic loss. Thus, repression of neuronal gene expression is a prominent and recently evolved feature of brain aging in humans and rhesus macaques that may alter neural networks and contribute to age-related cognitive changes.

The HIV Env variant N283 enhances macrophage tropism and is associated with brain infection and dementia

HIV infects tissue macrophages and brain microglia, which express lower levels of CD4 and CCR5 than CD4+ T cells in peripheral blood. Mechanisms that enhance HIV tropism for macrophages in the CNS and other tissues are not well understood. Here, we identify an HIV envelope glycoprotein (Env) variant in the CD4-binding site of gp120, Asn 283 (N283), that is present at a high frequency in brain tissues from AIDS patients with HIV-associated dementia (HAD). N283 increases gp120 affinity for CD4 by decreasing the gp120-CD4 dissociation rate, enhancing the capacity of HIV Envs to use low levels of CD4 for virus entry and increasing viral replication in macrophages and microglia. Structural modeling suggests that the enhanced ability of Envs with N283 to use low levels of CD4 is due to a hydrogen bond formed with Gln 40 of CD4. N283 is significantly more frequent in brain-derived Envs from HAD patients (41%; n = 330) compared with non-HAD patients (8%; n = 151; P < 0.001). These findings suggest that the macrophage-tropic HIV Env variant N283 is associated with brain infection and dementia in vivo, representing an example of a HIV variant associated with a specific AIDS-related complication.

CD16+ monocytes produce IL-6, CCL2, and matrix metalloproteinase-9 upon interaction with CX3CL1-expressing endothelial cells

The CD16+ subset of peripheral blood monocytes (Mo) is expanded dramatically during inflammatory conditions including sepsis, HIV‐1 infection, and cancer. CD16+ express high levels of CX3CR1, which mediates arrest onto CX3CL1‐expressing endothelial cells (EC) under flow conditions. In contrast, attachment of CD16− Mo onto cytokine‐activated EC is independent of CX3CL1. Here, we investigate the ability of CD16+ and CD16− Mo to produce proinflammatory cytokines upon interaction with CX3CL1‐expressing HUVEC. We demonstrate that CD16+ but not CD16− Mo produce high levels of IL‐6, CCL2, and matrix metalloproteinase (MMP)‐9 when cocultured with TNF/IFN‐γ‐activated HUVEC or nonactivated HUVEC expressing CX3CL1. Furthermore, supernatants from Mo cocultured with cytokine‐activated HUVEC induce neuronal death in vitro. These results suggest that membrane‐bound CX3CL1 stimulates production of IL‐6, CCL2, and MMP‐9 by CD16+ Mo, likely via engagement of CX3CR1. Thus, expansion of CD16+ Mo and their accumulation onto CX3CL1‐expressing EC may result in recruitment of Mo and T cell subsets at sites of inflammation in response to CCL2, IL‐6‐induced cell activation and/or differentiation, and MMP‐9‐mediated vascular and tissue injury.

Mechanisms of HIV-1 Neurotropism

Human immunodeficiency virus type 1 (HIV) infects macrophages and microglia in the CNS and frequently causes neurocognitive impairment. Although antiviral therapy generally reduces the viral load in the CNS and improves HIV-associated neurological dysfunction, most current antiviral drugs have poor CNS penetrance and cannot completely suppress viral replication. Furthermore, drug-resistance mutations can evolve independently in the CNS. Thus, a long-lived viral reservoir persists in macrophages and microglia in the brain despite antiviral therapy. This review discusses mechanisms underlying the neurotropism of HIV, focusing on the role of the HIV envelope glycoproteins and their interactions with CD4 and the chemokine receptors CCR5 and CXCR4. We review data from studies of neurotropic HIV derived from the brains of patients with HIV-associated neurocognitive impairment as well as studies of nonhuman primate models. Understanding mechanisms that underlie HIV neurotropism and neurovirulence is critical for development of therapeutics to inhibit CNS infection and preventing neurological injury in HIV-infected individuals.

Chemokine receptors and virus entry in the central nervous system

Several members of the chemokine receptor family are used as coreceptors together with CD4 for HIV and SIV entry in the central nervous system (CNS). CCR5 is the major coreceptor for HIV-1 infection of macrophages and microglia, the major target cells for HIV-1 infection in the CNS. CXCR4 and CCR3 are also expressed on microglia and can mediate infection by certain HIV-1 isolates but at lower efficiency than CCR5. Additional chemokine receptors that can function as HIV-1 and SIV coreceptors for a subset of viruses are expressed in the brain (i.e. Apj, CX3CR1, STRL33/BONZO, and gpr1), but their role in CNS infection has not been defined. The expression of CXCR4, and possibly other chemokine receptors, on subpopulations of neurons and glial cells may contribute to mechanisms of CNS injury that are independent of viral infection. Understanding the role of chemokine receptors and their chemokine ligands in HIV-1 and SIV infection of the CNS will elucidate mechanisms of viral tropism and pathogenesis and advance the development of new therapeutic strategies.

Regulation of CC Chemokine Receptor 5 and CD4 Expression and Human Immunodeficiency Virus Type 1 Replication in Human Macrophages and Microglia by T Helper Type 2 Cytokines

Macrophages, microglia, and other mononuclear phagocytes serve as cellular reservoirs for viral persistence in patients with acquired immunodeficiency syndrome. To understand host mechanisms that affect human immunodeficiency virus type 1 (HIV-1) pathogenesis by modulating expression of coreceptors, cytokine regulation of CC chemokine receptor 5 (CCR5) and CD4 expression on monocytes, monocyte-derived macrophages (MDMs), and microglia was investigated. Interleukin (IL)-4 and IL-10 enhanced the entry and replication of HIV-1 in microglia through up-regulation of CD4 and CCR5 expression, respectively. IL-4 stimulated HIV-1 replication in MDMs but down-regulated CD4 and CCR5 expression and inhibited virus entry, whereas IL-10 had the opposite effects. Thus, mechanisms independent of CCR5 and CD4 expression levels are involved in pathways that regulate HIV-1 replication in MDMs. CCR5 up-regulation by IL-10 was associated with increased migration of microglia in response to macrophage inflammatory protein-1beta. These findings suggest that increased production of T helper type 2 cytokines in the later stages of disease can enhance virus entry and replication in mononuclear phagocytes and facilitate chemotactic migration.

Reconstitution of human immunodeficiency virus–induced neurodegeneration using isolated populations of human neurons, astrocytes, and microglia and neuroprotection mediated by insulin-like growth factors

Primary human neuron cultures are an important in vitro model system for studies on mechanisms involved in human immunodeficiency virus (HIV)-associated dementia (HAD) and other neurological disorders. Here, more than 80 cell surface antigens were screened to identify a marker that could readily distinguish between neurons and astrocytes and found that neurons lack CD44 surface expression, whereas astrocytes and other cell types in brain are CD44+. Neurons and astrocytes were isolated from human fetal brain based on differential expression of CD44. Using purified neurons cocultured with astrocytes and/or microglia, it was demonstrated that HIV infection of microglia induces cellular activation and production of soluble factors that activate uninfected microglia and astrocytes and induce neuronal cell death. Activated astrocytes promoted HIV replication in microglia, thereby amplifying HIV-induced neurotoxicity. A screen for 120 cytokine/proteins detected upregulation of insulin-like growth factor (IGF)-binding protein (IGFBP)-2, interleukin (IL)-6, and CCL8/MCP-2 (monocyte chemoattractant protein 2) in supernatants of HIV-infected brain cell cultures. IGF-1 and -2 increased neuronal survival in HIV-infected brain cell cultures, whereas IGFBP-2 inhibited prosurvival effects of these growth factors. These findings identify CD44 as a marker that can be used to sort neurons from other cell types in brain, suggest the importance of microglia-astrocyte interactions in neurodegenerative mechanisms associated with HIV infection, and indicate a role for insulin-like growth factors in neuroprotection from HIV-induced neurodegeneration. The ability to reconstitute brain cultures using isolated populations of neurons, astrocytes, and microglia will be valuable for studies on pathogenic mechanisms in HAD and other neurological disorders, and will also facilitate neuroactive drug discovery.

Chapter 5.3 – HIV-1-Associated Dementia

Publisher Summary This chapter discusses the role of chemokine receptors and their ligands in the disease mechanisms. HIV-1-associated dementia (HAD) results from complex interactions between viral and host factors. Four chemokine receptors and their ligands (CXCR4/SDF-1, CCR5/MIP-1α and β, RANTES, CCR2/MCP-1, and CX3CR1/fractalkine) are of particular importance in HAD. HIV-1 infection of macrophages and microglia in the CNS is primarily mediated by CCR5. A subset of strains can infect macrophages and microglia via CXCR4. Increased monocyte trafficking into the CNS of patients with HAD is likely to result from increased expression of MCP-1, in addition to CCR5 chemokines. Neurodegenerative mechanisms in HAD may involve interactions between the HIV-1 envelope glycoproteins (Env) and CXCR4, or possibly other chemokine receptors on neurons, and possibly other CNS cell types. CCR5 and CXCR4 are promising targets for the prevention and treatment of HAD. Several compounds that selectively block virus entry without affecting physiological functions of these receptors are being developed, and some are in early clinical trials. Understanding the role of CCR5, CXCR4 and other chemokine receptors and their ligands in HIV-1 neuropathogenesis will be important for advancing the development of new therapeutic strategies for the prevention and treatment of HAD and other neurological complications of HIV-1 infection.