HIV-1, chemokines and neurogenesis

The Dialogue Between Neuroinflammation and Adult Neurogenesis: Mechanisms Involved and Alterations in Neurological Diseases

Adult neurogenesis occurs mainly in the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles. Evidence supports the critical role of adult neurogenesis in various conditions, including cognitive dysfunction, Alzheimer's disease (AD), and Parkinson's disease (PD). Several factors can alter adult neurogenesis, including genetic, epigenetic, age, physical activity, diet, sleep status, sex hormones, and central nervous system (CNS) disorders, exerting either pro-neurogenic or anti-neurogenic effects. Compelling evidence suggests that any insult or injury to the CNS, such as traumatic brain injury (TBI), infectious diseases, or neurodegenerative disorders, can provoke an inflammatory response in the CNS. This inflammation could either promote or inhibit neurogenesis, depending on various factors, such as chronicity and severity of the inflammation and underlying neurological disorders. Notably, neuroinflammation, driven by different immune components such as activated glia, cytokines, chemokines, and reactive oxygen species, can regulate every step of adult neurogenesis, including cell proliferation, differentiation, migration, survival of newborn neurons, maturation, synaptogenesis, and neuritogenesis. Therefore, this review aims to present recent findings regarding the effects of various components of the immune system on adult neurogenesis and to provide a better understanding of the role of neuroinflammation and neurogenesis in the context of neurological disorders, including AD, PD, ischemic stroke (IS), seizure/epilepsy, TBI, sleep deprivation, cognitive impairment, and anxiety- and depressive-like behaviors. For each disorder, some of the most recent therapeutic candidates, such as curcumin, ginseng, astragaloside, boswellic acids, andrographolide, caffeine, royal jelly, estrogen, metformin, and minocycline, have been discussed based on the available preclinical and clinical evidence.

CXCR4 knockout induces neuropathological changes in the MPTP-lesioned model of Parkinson's disease

C-X-C chemokine receptor type 4 (CXCR4) is highly expressed in Parkinson's disease (PD) mice's brains and is related to astrocyte signaling and microglial activation. This makes CXCR4 related to neuroinflammation and also makes CXCR4 considered to be the PD development mechanism and possible therapeutic targets. Therefore, it is worth studying the effect of CXCR4 on neuropathological changes and its potential therapeutic value for PD. This study aimed to investigate the effect of CXCR4 knockout on neuropathological changes in the mouse model of PD and its mechanism. In this study, CXCR4-WT and CXCR4^+/- C57BL mice were used to make Parkinson's model. Behavioral experiments, dopaminergic neuron markers, neuroinflammation, and blood-brain barrier damage were detected to verify the effect of CXCR4 knockout on neuropathological changes. CXCR4 knockout improved the behavioral results and tyrosine hydroxylase (TH) expression of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mice. In the substantia nigra (SN) area of the brain of PD mouse model, the number of Iba1-positive (p = 0.0004) and GFAP-positive cells (p = 0.0349) was significantly lower in CXCR4 knockout group than CXCR4-WT group. CXCR4 knockout reduced MPTP-induced infiltration of peripheral immune cells and the expression of pro-inflammatory cytokines. CXCR4 knockout also protected blood-brain barrier (BBB) from MPTP-induced damage. In conclusion, CXCR4 knockout inhibits the degeneration of dopamine neurons, microglial and astrocyte activation, neuroinflammation, and BBB damages in the MPTP-lesioned PD mice.

Opioid and neuroHIV Comorbidity - Current and Future Perspectives

With the current national opioid crisis, it is critical to examine the mechanisms underlying pathophysiologic interactions between human immunodeficiency virus (HIV) and opioids in the central nervous system (CNS). Recent advances in experimental models, methodology, and our understanding of disease processes at the molecular and cellular levels reveal opioid-HIV interactions with increasing clarity. However, despite the substantial new insight, the unique impact of opioids on the severity, progression, and prognosis of neuroHIV and HIV-associated neurocognitive disorders (HAND) are not fully understood. In this review, we explore, in detail, what is currently known about mechanisms underlying opioid interactions with HIV, with emphasis on individual HIV-1-expressed gene products at the molecular, cellular and systems levels. Furthermore, we review preclinical and clinical studies with a focus on key considerations when addressing questions of whether opioid-HIV interactive pathogenesis results in unique structural or functional deficits not seen with either disease alone. These considerations include, understanding the combined consequences of HIV-1 genetic variants, host variants, and μ-opioid receptor (MOR) and HIV chemokine co-receptor interactions on the comorbidity. Lastly, we present topics that need to be considered in the future to better understand the unique contributions of opioids to the pathophysiology of neuroHIV. Graphical Abstract Blood-brain barrier and the neurovascular unit. With HIV and opiate co-exposure (represented below the dotted line), there is breakdown of tight junction proteins and increased leakage of paracellular compounds into the brain. Despite this, opiate exposure selectively increases the expression of some efflux transporters, thereby restricting brain penetration of specific drugs.

Impaired neurogenesis by HIV-1-Gp120 is rescued by genetic deletion of fatty acid amide hydrolase enzyme

Background and Purpose

The HIV-envelope glycoprotein Gp120 is involved in neuronal injury and is associated with neuro-AIDS pathogenesis in the brain. Endocannabinoids are important lipid ligands in the CNS regulating neural functions, and their degeneration is controlled by hydrolysing enzymes such as the fatty acid amide hydrolase (FAAH). Here, we examined whether in vivo genetic deletion of Faah gene prevents HIV-1 Gp120-mediated effects on neurogenesis.

Experimental Approach

We generated new GFAP/Gp120 transgenic (Tg) mice that have genetic deletion of Faah gene by mating glial fribillary acidic protein (GFAP)/Gp120 Tg mice with Faah−/− mice. Neurogenesis and cell death were assessed by immunocytochemical analysis.

Key Results

Endocannabinoid levels in the brain of the double GFAP/Gp120//Faah−/− mice were similar to those observed in Faah−/− mice. However, unlike the impaired neurogenesis observed in GFAP/Gp120 Tg mice and Faah−/− mice, these GFAP/Gp120//Faah-/ mice showed significantly improved neurogenesis in the hippocampus, indicated by a significant increase in neuroblasts and neuronal cells, an increase in BrdU⁺ cells and doublecortin positive cells (DCX⁺), and an increase in the number of PCNA. Furthermore, a significant decrease in astrogliosis and gliogenesis was observed in GFAP/Gp120//Faah−/−mice and neurogenesis was stimulated by neural progenitor cells (NPCs) and/or the newly formed NPC niches characterized by increased COX-2 expression and elevated levels of PGE₂.

Conclusions and Implications

In vivo genetic ablation of Faah, resulted in enhanced neurogenesis through modulation of the newly generated NPC niches in GFAP/Gp120//Faah−/− mice. This suggests a novel approach of using FAAH inhibitors to enhance neurogenesis in HIV-1 infected brain.

Of mice and monkeys: can animal models be utilized to study neurological consequences of pediatric HIV-1 infection?

Pediatric human immunodeficiency virus (HIV-1) infection remains a global health crisis. Children are much more susceptible to HIV-1 neurological impairments than adults, which can be exacerbated by coinfections. Neurological characteristics of pediatric HIV-1 infection suggest dysfunction in the frontal cortex as well as the hippocampus; limited MRI data indicate global cerebral atrophy, and pathological data suggest accelerated neuronal apoptosis in the cortex. An obstacle to pediatric HIV-1 research is a human representative model system. Host-species specificity of HIV-1 limits the ability to model neurological consequences of pediatric HIV-1 infection in animals. Several models have been proposed including neonatal intracranial injections of HIV-1 viral proteins in rats and perinatal simian immunodeficiency virus (SIV) infection of infant macaques. Nonhuman primate models recapitulate the complexity of pediatric HIV-1 neuropathogenesis while rodent models are able to elucidate the role specific viral proteins exert on neurodevelopment. Nonhuman primate models show similar behavioral and neuropathological characteristics to pediatric HIV-1 infection and offer a stage to investigate early viral mechanisms, latency reservoirs, and therapeutic interventions. Here we review the relative strengths and limitations of pediatric HIV-1 model systems.

Reduction of pyramidal and immature hippocampal neurons in pediatric simian immunodeficiency virus infection

Pediatric HIV infection remains a global health crisis with a worldwide infection rate of 2.5 million (WHO, Geneva Switzerland, 2009). Children are much more susceptible to HIV-1 neurological impairments compared with adults, which is exacerbated by coinfections. A major obstacle in pediatric HIV research is sample access. The proposed studies take advantage of ongoing pediatric simian immunodeficiency virus (SIV) pathogenesis and vaccine studies to test the hypothesis that pediatric SIV infection diminishes neuronal populations and neurogenesis in the hippocampus. Newborn rhesus macaques (Macaca mulatta) that received intravenous inoculation of highly virulent SIVmac251 (n=3) or vehicle (control n=4) were used in this study. After a 6-18-week survival time, the animals were euthanized and the brains prepared for quantitative histopathological analysis. Systematic sections through the hippocampus were either Nissl stained or immunostained for doublecortin (DCX+), a putative marker of immature neurons. Using design-based stereology, we report a 42% reduction in the pyramidal neuron population of the CA1, CA2, and CA3 fields of the hippocampus (P<0.05) in SIV-infected infants. The DCX+ neuronal population was also significantly reduced within the dentate gyrus of the hippocampus. The loss of hippocampal neurons and neurogenic capacity may contribute to the rapid neurocognitive decline associated with pediatric HIV infection. These data suggest that pediatric SIV infection, which leads to significant neuronal loss in the hippocampus within 3 months, closely models a subset of pediatric HIV infections with rapid progression.

Neurologic disease in HIV-infected children and the impact of combination antiretroviral therapy

The prevalence of HIV-associated neurocognitive impairment in perinatally HIV-infected children has declined since the introduction of combination antiretroviral therapy (cART). Early initiation of cART in infancy has been shown to positively impact neurodevelopment; however, children continue to be diagnosed with HIV outside of the early infancy period and can experience subtle to severe neurocognitive deficits despite cART. The causes of these neurocognitive deficits despite effective cART are multifactorial and likely include continued viral replication in the CNS, ongoing neuroinflammation, irreversible CNS injury prior to cART initiation, neurotoxic effects of cART, and socioeconomic and psychosocial effects. Many aspects of our understanding of HIV-associated neurocognitive disorders have emerged from research in adult patients, but perinatally HIV-infected children represent a very different population. These children were exposed to HIV during a period of rapid brain development and have lifelong infection and potential lifelong cART exposure. HIV is no longer a rapidly fatal disease, and most HIV-infected children in resource-rich countries are living into adulthood. It is therefore critical to optimize neurocognitive outcomes of these youth. This review summarizes current understanding of the pathogenesis of HIV-associated CNS infection and the impact of cART on neurocognitive function in children and adolescents and discusses important areas for future research.

Implementing neuronal plasticity in NeuroAIDS: the experience of brain-derived neurotrophic factor and other neurotrophic factors

Human immunodeficiency virus type-1 (HIV) causes mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when HIV patients receive antiretroviral therapy. Thus, novel adjunctive therapies are necessary to reduce or abolish the neurotoxic effect of HIV. However, new therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity. HAND subjects are characterized by being profoundly depressed, and they experience deficits in memory, learning and movements. Experimental evidence has also shown that HIV reduces neurogenesis. These deficits resemble those occurring in premature brain aging or in a brain with impaired neural repair properties. Thus, it appears that HIV diminishes neuronal survival, along with reduced neuronal connections. These two phenomena should not occur in the adult and developing brain when synaptic plasticity is promoted by neurotrophic factors, polypeptides that are present in adult synapses. This review will outline experimental evidence as well as present emerging concepts for the use of neurotrophic factors and in particular brain-derived neurotrophic factor as an adjunct therapy to prevent HIV-mediated neuronal degeneration and restore the loss of synaptic connections.

Synaptic dysfunction in human immunodeficiency virus type-1-positive subjects: inflammation or impaired neuronal plasticity?

Many people infected with the human immunodeficiency virus type-1 (HIV) exhibit mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when receiving antiretroviral therapy. Thus, novel adjunctive therapies must be developed to overcome the neurotoxic effect of HIV. New therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity and the risk factors that, besides inflammation and T-cell depletion and drugs of abuse, render the central nervous system (CNS) a target of HIV-induced neurotoxicity. HIV appears to impair neuronal plasticity, which refers to the innate ability of the CNS respond to injury and promote recovery of function. The availability of brain-derived neurotrophic factor (BDNF), a potent neurotrophic factor that is present in abundance in the adult brain, is essential for neuronal plasticity. BDNF acts through a receptor system composed of Trk and p75NTR. Here, we present experimental evidence that some of the clinical features of HIV-mediated neurological impairment could result from altered BDNF/TrkB/p75NTR regulation and function.

Impact of Theiler's virus infection on hippocampal neuronal progenitor cells: differential effects in two mouse strains

AIMS

Disease-associated alterations in hippocampal neurogenesis are discussed as an important factor contributing to long-term consequences of central nervous system diseases. Therefore, the study aimed to determine the impact of Theiler's murine encephalomyelitis virus infection on hippocampal cell proliferation, neuronal progenitor cells and neurogenesis as well as the influence of microglia on respective disease-associated alterations.

METHODS

The impact of the infection was evaluated in two mouse strains which differ in the disease course, with an acute polioencephalitis followed by virus elimination in C57BL/6 mice and a chronic demyelinating disease in SJL/J mice.

RESULTS

Infection with the low neurovirulent BeAn strain did not exert significant acute effects regardless of the mouse strain. In the chronic phase, the number of neuronal progenitor cells and early postmitotic neurones was significantly reduced in infected SJL/J mice, whereas no long-term alterations were observed in C57BL/6 mice. A contrasting course of microglia activation was observed in the two mouse strains, with an early increase in the number of activated microglia cells in SJL/J mice and a delayed increase in C57BL/6 mice. Quantitative analysis did not confirm a correlation between the number of activated microglia and the number of neuronal progenitor cells and early postmitotic neurones. However, flow cytometric analyses revealed alterations in the functional state of microglial cells which might have affected the generation of neuronal progenitor cells.

CONCLUSIONS

Theiler's murine encephalomyelitis virus infection can exert delayed effects on the hippocampal neuronal progenitor population with long-term alterations evident 3 months following infection. These alterations proved to depend on strain susceptibility and might contribute to detrimental consequences of virus encephalitis such as cognitive impairment.

Distemper virus encephalitis exerts detrimental effects on hippocampal neurogenesis

AIMS

Despite knowledge about the impact of brain inflammation on hippocampal neurogenesis, data on the influence of virus encephalitis on dentate granule cell neurogenesis are so far limited. Canine distemper is considered an interesting model of virus encephalitis, which can be associated with a chronic progressing disease course and can cause symptomatic seizures.

METHODS

To determine the impact of canine distemper virus (CDV) infection on hippocampal neurogenesis, we compared post-mortem tissue from dogs with infection with and without seizures, from epileptic dogs with non-viral aetiology and from dogs without central nervous system diseases.

RESULTS

The majority of animals with infection and with epilepsy of non-viral aetiology exhibited neuronal progenitor numbers below the age average in controls. Virus infection with and without seizures significantly decreased the mean number of neuronal progenitor cells by 43% and 76% as compared to age-matched controls. Ki-67 labelling demonstrated that hippocampal cell proliferation was neither affected by infection nor by epilepsy of non-viral aetiology. Analysis of CDV infection in cells expressing caspase-3, doublecortin or Ki-67 indicated that infection of neuronal progenitor cells is extremely rare and suggests that infection might damage non-differentiated progenitor cells, hamper neuronal differentiation and promote glial differentiation. A high inter-individual variance in the number of lectin-reactive microglial cells was evident in dogs with distemper infection. Statistical analyses did not reveal a correlation between the number of lectin-reactive microglia cells and neuronal progenitor cells.

CONCLUSIONS

Our data demonstrate that virus encephalitis with and without seizures can exert detrimental effects on hippocampal neurogenesis, which might contribute to long-term consequences of the disease. The lack of a significant impact of distemper virus on Ki-67-labelled cells indicates that the infection affected neuronal differentiation and survival of newborn cells rather than hippocampal cell proliferation.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Abrogated inflammatory response promotes neurogenesis in a murine model of Japanese encephalitis

Background

Japanese encephalitis virus (JEV) induces neuroinflammation with typical features of viral encephalitis, including inflammatory cell infiltration, activation of microglia, and neuronal degeneration. The detrimental effects of inflammation on neurogenesis have been reported in various models of acute and chronic inflammation. We investigated whether JEV-induced inflammation has similar adverse effects on neurogenesis and whether those effects can be reversed using an anti-inflammatory compound minocycline.

Methodology/Principal Findings

Here, using in vitro studies and mouse models, we observed that an acute inflammatory milieu is created in the subventricular neurogenic niche following Japanese encephalitis (JE) and a resultant impairment in neurogenesis occurs, which can be reversed with minocycline treatment. Immunohistological studies showed that proliferating cells were replenished and the population of migrating neuroblasts was restored in the niche following minocycline treatment. In vitro, we checked for the efficacy of minocycline as an anti-inflammatory compound and cytokine bead array showed that production of cyto/chemokines decreased in JEV-activated BV2 cells. Furthermore, mouse neurospheres grown in the conditioned media from JEV-activated microglia exhibit arrest in both proliferation and differentiation of the spheres compared to conditioned media from control microglia. These effects were completely reversed when conditioned media from JEV-activated and minocycline treated microglia was used.

Conclusion/Significance

This study provides conclusive evidence that JEV-activated microglia and the resultant inflammatory molecules are anti-proliferative and anti-neurogenic for NSPCs growth and development, and therefore contribute to the viral neuropathogenesis. The role of minocycline in restoring neurogenesis may implicate enhanced neuronal repair and attenuation of the neuropsychiatric sequelae in JE survivors.

Viral infection and neural stem/progenitor cell's fate: implications in brain development and neurological disorders

Viral infections in the prenatal (during pregnancy) and perinatal period have been a common cause of brain malformation. Besides the immediate neurological dysfunctions, virus infections may critically affect CNS development culminating in long-term cognitive deficits. Most of these neurotropic viruses are most damaging at a critical stage of the host, when the brain is in a dynamic stage of development. The neuropathology can be attributed to the massive neuronal loss induced by the virus as well as lack of CNS repair owing to a deficit in the neural stem/progenitor cell (NSPC) pool or aberrant formation of new neurons from NSPCs. Being one of the mitotically active populations in the post natal brain, the NSPCs have emerged as the potential targets of neurotropic viruses. The NSPCs are self-renewing and multipotent cells residing in the neurogenic niches of the brain, and, therefore, hampering the developmental fate of these cells may adversely affect the overall neurogenesis pattern. A number of neurotropic viruses utilize NSPCs as their cellular reservoirs and often establish latent and persistent infection in them. Both HIV and Herpes virus infect NSPCs over long periods of time and reactivation of the virus may occur later in life. The virus infected NSPCs either undergoes cell cycle arrest or impaired neuronal or glial differentiation, all of which leads to impaired neurogenesis. The disturbances in neurogenesis and CNS development following neurotropic virus infections have direct implications in the viral pathogenesis and long-term neurobehavioral outcome in infected individuals.

Neurotrophins modulate the expression of chemokine receptors in the brain

In the central nervous system, chemokines are primarily mediators of inflammatory processes. Their receptors, in particular, CXCR4 and CCR5, serve as co-factors along with CD4 that permit Human immunodeficiency virus-1 (HIV) infection. Moreover, experimental evidence has shown that CXCR4 and CCR5 mediate the neurotoxic effects of the HIV envelope protein gp120, suggesting that these receptors could also promote the neuropathogenesis observed in HIV-positive individuals. Therefore, a better understanding of the molecular mechanisms governing the expression of chemokine receptors in the brain may lead to improved therapies that reduce HIV neurotoxicity. This study presents evidence that the expression of chemokine receptors in the brain is modulated by two neurotrophins in an area-specific manner. This new evidence suggests that the neurotrophins may be an adjunct therapy to reduce HIV-mediated neuronal injury evoked by chemokine receptor activation.

Morphine induces the release of CCL5 from astrocytes: potential neuroprotective mechanism against the HIV protein gp120

A number of human immunodeficiency virus type-1 (HIV) positive subjects are also opiate abusers. These individuals are at high risk to develop neurological complications. However, little is still known about the molecular mechanism(s) linking opiates and HIV neurotoxicity. To learn more, we exposed rat neuronal/glial cultures prepared from different brain areas to opiate agonists and HIV envelope glycoproteins gp120IIIB or BaL. These strains bind to CXCR4 and CCR5 chemokine receptors, respectively, and promote neuronal death. Morphine did not synergize the toxic effect of gp120IIIB but inhibited the cytotoxic property of gp120BaL. This effect was blocked by naloxone and reproduced by the mu opioid receptor agonist DAMGO. To examine the potential mechanism(s) of neuroprotection, we determined the effect of morphine on the release of chemokines CCL5 and CXCL12 in neurons, astrocytes, and microglia cultures. CCL5 has been shown to prevent gp120BaL neurotoxicity while CXCL12 decreases neuronal survival. Morphine elicited a time-dependent release of CCL5 but failed to affect the release of CXCL12. This effect was observed only in primary cultures of astrocytes. To examine the role of endogenous CCL5 in the neuroprotective activity of morphine, mixed cerebellar neurons/glial cells were immunoneutralized against CCL5 prior to morphine and gp120 treatment. In these cells the neuroprotective effect of opiate agonists was blocked. Our data suggest that morphine may exhibit a neuroprotective activity against M-tropic gp120 through the release of CCL5 from astrocytes.

SDF-1alpha/CXCL12 enhances GABA and glutamate synaptic activity at serotonin neurons in the rat dorsal raphe nucleus

The serotonin (5-hydroxytryptamine; 5-HT) system has a well-characterized role in depression. Recent reports describe comorbidities of mood-immune disorders, suggesting an immunological component may contribute to the pathogenesis of depression as well. Chemokines, immune proteins which mediate leukocyte trafficking, and their receptors are widely distributed in the brain, mediate neuronal patterning, and modulate various neuropathologies. The purpose of this study was to investigate the neuroanatomical relationship and functional impact of the chemokine stromal cell-derived factor-1alpha/CXCL12 and its receptor, CXCR4, on the serotonin dorsal raphe nucleus (DRN) system in the rat using anatomical and electrophysiological techniques. Immunohistochemical analysis indicates that over 70% of 5-HT neurons colocalize with CXCL12 and CXCR4. At a subcellular level, CXCL12 localizes throughout the cytoplasm whereas CXCR4 concentrates to the outer membrane and processes of 5-HT neurons. CXCL12 and CXCR4 also colocalize on individual DRN cells. Furthermore, electrophysiological studies demonstrate CXCL12 depolarization of 5-HT neurons indirectly via glutamate synaptic inputs. CXCL12 also enhances the frequency of spontaneous inhibitory and excitatory postsynaptic currents (sIPSC and sEPSC). CXCL12 concentration-dependently increases evoked IPSC amplitude and decreases evoked IPSC paired-pulse ratio selectively in 5-HT neurons, effects blocked by the CXCR4 antagonist AMD3100. These data indicate presynaptic enhancement of GABA and glutamate release at 5-HT DRN neurons by CXCL12. Immunohistochemical analysis further shows CXCR4 localization to DRN GABA neurons, providing an anatomical basis for CXCL12 effects on GABA release. Thus, CXCL12 indirectly modulates 5-HT neurotransmission via GABA and glutamate synaptic afferents. Future therapies targeting CXCL12 and other chemokines may treat serotonin related mood disorders, particularly depression experienced by immune-compromised individuals.

Molecular pathology of neuro-AIDS (CNS-HIV)

The cognitive deficits in patients with HIV profoundly affect the quality of life of people living with this disease and have often been linked to the neuro-inflammatory condition known as HIV encephalitis (HIVE). With the advent of more effective anti-retroviral therapies, HIVE has shifted from a sub-acute to a chronic condition. The neurodegenerative process in patients with HIVE is characterized by synaptic and dendritic damage to pyramidal neurons, loss of calbindin-immunoreactive interneurons and myelin loss. The mechanisms leading to neurodegeneration in HIVE might involve a variety of pathways, and several lines of investigation have found that interference with signaling factors mediating neuroprotection might play an important role. These signaling pathways include, among others, the GSK3beta, CDK5, ERK, Pyk2, p38 and JNK cascades. Of these, GSK3beta has been a primary focus of many previous studies showing that in infected patients, HIV proteins and neurotoxins secreted by immune-activated cells in the brain abnormally activate this pathway, which is otherwise regulated by growth factors such as FGF. Interestingly, modulation of the GSK3beta signaling pathway by FGF1 or GSK3beta inhibitors (lithium, valproic acid) is protective against HIV neurotoxicity, and several pilot clinical trials have demonstrated cognitive improvements in HIV patients treated with GSK3beta inhibitors. In addition to the GSK3beta pathway, the CDK5 pathway has recently been implicated as a mediator of neurotoxicity in HIV, and HIV proteins might activate this pathway and subsequently disrupt the diverse processes that CDK5 regulates, including synapse formation and plasticity and neurogenesis. Taken together, the GSK3beta and CDK5 signaling pathways are important regulators of neurotoxicity in HIV, and modulation of these factors might have therapeutic potential in the treatment of patients suffering from HIVE. In this context, the subsequent sections will focus on reviewing the involvement of the GSK3beta and CDK5 pathways in neurodegeneration in HIV.

Neuronal injury in simian immunodeficiency virus and other animal models of neuroAIDS

The success of antiretroviral therapy has reduced the incidence of severe neurological complication resulting from human immunodeficiency virus (HIV) infection. However, increased patient survival has been associated with an increased prevalence of protracted forms of HIV encephalitis leading to moderate cognitive impairment. NeuroAIDS remains a great challenge to patients, their families, and our society. Thus development of preclinical models that will be suitable for testing promising new compounds with neurotrophic and neuroprotective capabilities is of critical importance. The simian immunodeficiency virus (SIV)-infected macaque is the premiere model to study HIV neuropathogenesis. This model was central to the seminal work of Dr. Opendra "Bill" Narayan. Similar to patients with HIV encephalitis, in the SIV model there is injury to the synaptodendritic structure of excitatory pyramidal neurons and inhibitory calbindin-immunoreactive interneurons. This article, which is part of a special issue of the Journal of NeuroVirology in honor of Dr. Bill Narayan, discusses the most important neurodegenerative features in preclinical models of neuroAIDS and their potential for treatment development.

The chemokine SDF-1/CXCL12 regulates the migration of melanocyte progenitors in mouse hair follicles

Mouse skin melanocytes originate from the neural crest and subsequently invade the epidermis and migrate into the hair follicles (HF) where they proliferate and differentiate. Here we demonstrate a role for the chemokine SDF-1/CXCL12 and its receptor CXCR4 in regulating the migration and positioning of melanoblasts during HF formation and cycling. CXCR4 expression by melanoblasts was upregulated during the anagen phase of the HF cycle. CXCR4-expressing cells in the HF also expressed the stem cell markers nestin and LEX, the neural crest marker SOX10 and the cell proliferation marker PCNA. SDF-1 was widely expressed along the path taken by migrating CXCR4-expressing cells in the outer root sheath (ORS), suggesting that SDF-1-mediated signaling might be required for the migration of CXCR4 cells. Skin sections from CXCR4-deficient mice, and skin explants treated with the CXCR4 antagonist AMD3100, contained melanoblasts abnormally concentrated in the epidermis, consistent with a defect in their migration. SDF-1 acted as a chemoattractant for FACS-sorted cells isolated from the anagen skin of CXCR4-EGFP transgenic mice in vitro, and AMD3100 inhibited the SDF-1-induced migratory response. Together, these data demonstrate an important role for SDF-1/CXCR4 signaling in directing the migration and positioning of melanoblasts in the HF.

Neurogliogenesis in the mature olfactory system: a possible protective role against infection and toxic dust

The outpost position of the olfactory bulb (OB) between the direct inputs from sensory neurons of the nasal epithelium and other parts of the brain suggests its highest vulnerability among all brain structures to penetration of exogenous agents. A number of neurotropic viruses have been found to invade the brain through the OB. There is growing evidence that microscopic particles of toxic dusts can propagate from the nasal epithelium to the OB and further into the brain. These harmful agents impair cellular elements of the brain. Apparently, cells in the OB are the most affected, as they are the first to encounter viral infections and toxic particles. It is well known that neuronal and glial progenitors are continuously generated from neuronal stem cells in the subventricular zone of the adult brain and then migrate predominantly into the OB. Therefore, it is feasible to suggest that substitution of injured or dead cells in the OB by new-born neurons, differentiating from progenitors, plays a role in protecting the OB neuronal microcircuits from destruction. Furthermore, some cytokines and chemokines released in response to infection and/or intoxication can modulate different stages of neurogenesis (proliferation, migration, and differentiation). We hypothesize that continuous neurogenesis in the olfactory system throughout adulthood evolved as a protective mechanism to prevent impairment of the most ancient but vitally important sensory system. In addition, differentiation of a substantial portion of progenitors to glial cells, including macrophages and microglia, may create an additional barrier to exogenous agents on their way deep to the brain.

Inflammation: a new candidate in modulating adult neurogenesis

Any pathological perturbation to the brain provokes a cascade of molecular and cellular events, which manifests in the form of microglial activation and release of various proinflammatory molecules. This eventually culminates in a profound neuroinflammatory reaction that characterizes the brain's response to stress, injury, or infection. The inflammatory cascade is an attempt by the system to eliminate the challenge imposed on the brain, clear the system of the dead and damaged neurons, and rescue the normal functioning of this vital organ. However, during the process of microglial activation, the proinflammatory mediators released exert certain detrimental effects, and neural stem cells and progenitor cells are likely to be affected. Here we review how the proliferation, maturation, and migration of the neural stem cells are modulated under such an inflammatory condition. The fate of the noncommitted neural stem cells and its differentiation potency are often under strict regulation, and these proinflammatory mediators seem to disrupt this critical balance and finely tune the neurogenesis pattern in the two niches of neurogenesis, the subventricular zone and the subgranular zone of the hippocampus. Moreover, the migration ability of these stem cells, which is important for localization to the proper site, is also affected in a major way by the chemokines released following inflammation.

Apelin, an endogenous neuronal peptide, protects hippocampal neurons against excitotoxic injury

Several G protein-coupled receptors (GPCRs) mediate neuronal cell migration and survival upon activation by their native peptide ligands but activate death-signaling pathways when activated by certain non-native ligands. In cultured neurons, we recently described expression of the unique seven-transmembrane (7TM) -G protein-coupled receptor, APJ, which is also strongly expressed in neurons in the brain and various cell types in other tissues. We now demonstrate that the endogenous APJ peptide ligand apelin activates signaling pathways in rat hippocampal neurons and modulates neuronal survival. We found that (i) both APJ and apelin are expressed in hippocampal neurons; (ii) apelin peptides induce phosphorylation of the cell survival kinases AKT and Raf/ERK-1/2 in hippocampal neurons; and (iii) apelin peptides protect hippocampal neurons against NMDA receptor-mediated excitotoxicity, including that induced by human immunodeficiency virus type 1. Thus, apelin/APJ signaling likely represents an endogenous hippocampal neuronal survival response, and therefore apelin should be further investigated as a potential neuroprotectant against hippocampal injury.

Maturing neurons are selectively sensitive to human immunodeficiency virus type 1 exposure in differentiating human neuroepithelial progenitor cell cultures

Human immunodeficiency virus type 1 (HIV-1) infection of the brain is associated with neuronal injury manifested by dendritic pruning, aberrant neurofilament metabolism, and decreased synaptic density. The central nervous system (CNS) responds to neuronal injury by differentiating new neurons and astrocytes from resident populations of multipotent neuroepithelial progenitor cells (NEP) located in regions such as the subventricular zone or hippocampus. In vitro studies have demonstrated that the HIV-1 virion or envelope glycoprotein gp120 can injure differentiated human neurons and astrocytes, suggesting that HIV-1 proteins could similarly injure NEP or NEP-derived glial and neuronal lineage-committed precursor cells. To answer this question, human fetal brain-derived "neurospheres" containing NEP and NEP-derived precursor cells were cultured in low serum differentiation medium containing lymphotropic HIV-1(SF2), macrophage-tropic HIV-1(SF128A), or recombinant gp120SF2 from HIV-1(SF2). These experiments indicate that exposure to HIV-1 does not affect the ability of the NEP to differentiate into cells expressing either astrocyte-specific or neuron-specific cytoskeletal antigens. However prolonged exposure to HIV-1 does selectively decrease expression of neuronal antigens (microtubule beta-III-tubulin and intermediate filament neurofilament-L) but not astrocyte antigens (intermediate filament glial fibrillary acidic protein). The effects of continuous exposure to HIV-1 or gp120 may result from injury to developing neurons and/or impairment of the neuronal developmental process itself. By depressing neuronal microtubule and neurofilament protein expression, HIV-1 and gp120 exposure compromise the potential for postmitotic neuronal dendrite and axon development.

HIV-1 neuropathogenesis: glial mechanisms revealed through substance abuse

Neuronal dysfunction and degeneration are ultimately responsible for the neurocognitive impairment and dementia manifest in neuroAIDS. Despite overt neuronal pathology, HIV-1 does not directly infect neurons; rather, neuronal dysfunction or death is largely an indirect consequence of disrupted glial function and the cellular and viral toxins released by infected glia. A role for glia in HIV-1 neuropathogenesis is revealed in experimental and clinical studies examining substance abuse-HIV-1 interactions. Current evidence suggests that glia are direct targets of substance abuse and that glia contribute markedly to the accelerated neurodegeneration seen with substance abuse in HIV-1 infected individuals. Moreover, maladaptive neuroplastic responses to chronic drug abuse might create a latent susceptibility to CNS disorders such as HIV-1. In this review, we consider astroglial and microglial interactions and dysfunction in the pathogenesis of HIV-1 infection and examine how drug actions in glia contribute to neuroAIDS.

Neurotoxins and neurotoxicity mechanisms. an overview

Neurotoxins represent unique chemical tools, providing a means to 1) gain insight into cellular mechanisms of apopotosis and necrosis, 2) achieve a morphological template for studies otherwise unattainable, 3) specifically produce a singular phenotype of denervation, and 4) provide the starting point to delve into processes and mechanisms of nerve regeneration and sprouting. There are many other notable uses of neurotoxins in neuroscience research, and ever more being discovered each year. The objective of this review paper is to highlight the broad areas of neuroscience in which neurotoxins and neurotoxicity mechanism come into play. This shifts the focus away from neurotoxins per se, and onto the major problems under study today. Neurotoxins broadly defined are used to explore neurodegenerative disorders, psychiatric disorders and substance use disorders. Neurotoxic mechanisms relating to protein aggregates are indigenous to Alzheimer disease, Parkinson's disease. NeuroAIDS is a disorder in which microglia and macrophages have enormous import. The gap between the immune system and nervous system has been bridged, as neuroinflammation is now considered to be part of the neurodegenerative process. Related mechanisms now arise in the process of neurogenesis. Accordingly, the entire spectrum of neuroscience is within the purview of neurotoxins and neurotoxicity mechanisms. Highlights on discoveries in the areas noted, and on selective neurotoxins, are included, mainly from the past 2 to 3 years.