Activation of α7 nicotinic acetylcholine receptor ameliorates HIV-associated neurology and neuropathology

α7 nicotinic receptor activation mitigates herpes simplex virus type 1 infection in microglia cells

Herpes simplex virus type 1 (HSV-1), a neurotropic DNA virus, establishes latency in neural tissues, with reactivation causing severe consequences like encephalitis. Emerging evidence links HSV-1 infection to chronic neuroinflammation and neurodegenerative diseases. Microglia, the central nervous system's (CNS) immune sentinels, express diverse receptors, including α7 nicotinic acetylcholine receptors (α7 nAChRs), critical for immune regulation. Recent studies suggest α7 nAChR activation protects against viral infections. Here, we show that α7 nAChR agonists, choline and PNU-282987, significantly inhibit HSV-1 replication in microglial BV2 cells. Notably, this inhibition is independent of the traditional ionotropic nAChR signaling pathway. mRNA profiling revealed that choline stimulates the expression of antiviral factors, IL-1β and Nos2, and down-regulates the apoptosis genes and type A Lamins in BV2 cells. These findings suggest a novel mechanism by which microglial α7 nAChRs restrict viral infections by regulating innate immune responses.

The brain-liver cholinergic anti-inflammatory pathway and viral infections

Efferent cholinergic signaling is a critical and targetable source of immunoregulation. The vagus nerve (VN) is the primary source of cholinergic signaling in the body, and partially innervates hepatic functionality through the liver-brain axis. Virus-induced disruption of cholinergic signaling may promote pathogenesis in hepatotropic and neurotropic viruses. Therefore, restoring VN functionality could be a novel therapeutic strategy to alleviate pathogenic inflammation in hepatotropic and neurotropic viral infections alike. In this minireview, we discuss the physiological importance of cholinergic signaling in maintaining liver-brain axis homeostasis. Next, we explore mechanisms by which the VN is perturbed by viral infections, and how non-invasive restoration of cholinergic signaling pathways with bioelectronic medicine (BEM) might ameliorate hepatic inflammation and neuroinflammation in certain viral infections.

Long COVID as a Tauopathy: Of "Brain Fog" and "Fusogen Storms"

Long COVID, also called post-acute sequelae of SARS-CoV-2, is characterized by a multitude of lingering symptoms, including impaired cognition, that can last for many months. This symptom, often called "brain fog", affects the life quality of numerous individuals, increasing medical complications as well as healthcare expenditures. The etiopathogenesis of SARS-CoV-2-induced cognitive deficit is unclear, but the most likely cause is chronic inflammation maintained by a viral remnant thriving in select body reservoirs. These viral sanctuaries are likely comprised of fused, senescent cells, including microglia and astrocytes, that the pathogen can convert into neurotoxic phenotypes. Moreover, as the enteric nervous system contains neurons and glia, the virus likely lingers in the gastrointestinal tract as well, accounting for the intestinal symptoms of long COVID. Fusogens are proteins that can overcome the repulsive forces between cell membranes, allowing the virus to coalesce with host cells and enter the cytoplasm. In the intracellular compartment, the pathogen hijacks the actin cytoskeleton, fusing host cells with each other and engendering pathological syncytia. Cell-cell fusion enables the virus to infect the healthy neighboring cells. We surmise that syncytia formation drives cognitive impairment by facilitating the "seeding" of hyperphosphorylated Tau, documented in COVID-19. In our previous work, we hypothesized that the SARS-CoV-2 virus induces premature endothelial senescence, increasing the permeability of the intestinal and blood-brain barrier. This enables the migration of gastrointestinal tract microbes and/or their components into the host circulation, eventually reaching the brain where they may induce cognitive dysfunction. For example, translocated lipopolysaccharides or microbial DNA can induce Tau hyperphosphorylation, likely accounting for memory problems. In this perspective article, we examine the pathogenetic mechanisms and potential biomarkers of long COVID, including microbial cell-free DNA, interleukin 22, and phosphorylated Tau, as well as the beneficial effect of transcutaneous vagal nerve stimulation.

Vagal-mAChR4 signaling promotes Friend virus complex (FV)-induced acute erythroleukemia

Erythroleukemia belongs to acute myeloid leukemia (AML) type 6 (M6), and treatment remains difficult due to the poor prognosis of the disease. Friend virus (FV) is a complex of two viruses: Friend murine leukemia virus (F-MuLV) strain along with a defective spleen focus forming virus (SFFV), which can induce acute erythroleukemia in mice. We have previously reported that activation of vagal α7 nicotinic acetylcholine receptor (nAChR) signaling promotes HIV-1 transcription. Whether vagal muscarinic signaling mediates FV-induced erythroleukemia and the underlying mechanisms remain unclear. In this study, sham and vagotomized mice were intraperitoneally injected with FV. FV infection caused anemia in sham mice, and vagotomy reversed this change. FV infection increased erythroblasts ProE, EryA, and EryB cells in the spleen, and these changes were blocked by vagotomy. In bone marrow, FV infection reduced EryC cells in sham mice, an effect that was counteracted by vagotomy. FV infection increased choline acetyltransferase (ChAT) expression in splenic CD4⁺ and CD8⁺ T cells, and this change was reversed by vagotomy. Furthermore, the increase of EryA and EryB cells in spleen of FV-infected wild-type mice was reversed after deletion of ChAT in CD4⁺ T cells. In bone marrow, FV infection reduced EryB and EryC cells in sham mice, whereas lack of ChAT in CD4⁺ T cells did not affect this change. Activation of muscarinic acetylcholine receptor 4 (mAChR4) by clozapine N-oxide (CNO) significantly increased EryB in the spleen but decreased the EryC cell population in the bone marrow of FV-infected mice. Thus, vagal-mAChR4 signaling in the spleen and bone marrow synergistically promotes the pathogenesis of acute erythroleukemia. We uncover an unrecognized mechanism of neuromodulation in erythroleukemia.

Nicotinic acetylcholine receptors: Key targets for attenuating neurodegenerative diseases

Nicotinic acetylcholine receptors (nAChRs) are master regulators of immune functions via the cholinergic anti-inflammatory pathway and are expressed in microglia, the brain's resident immune cells. There is an extensive dialogue between the neurons and the glial cells around them from which microglia are tasked with monitoring, nurturing, and defending their microenvironment. Dysregulation of any of these processes can have devastating and long-lasting consequences involving microglia-mediated neuroinflammation associated with neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, amongst others. Disease-associated microglia acquire a distinguishing phenotype that emphasizes scavenging and defence functions while nurturing and repairing functions become muted. Attempts to resolve this critical imbalance remain a key focus of research. Furthermore, cholinergic modulation of neuroinflammation represents a promising avenue for treatment.

α7- and α9-Containing Nicotinic Acetylcholine Receptors in the Functioning of Immune System and in Pain

Nicotinic acetylcholine receptors (nAChRs) present as many different subtypes in the nervous and immune systems, muscles and on the cells of other organs. In the immune system, inflammation is regulated via the vagus nerve through the activation of the non-neuronal α7 nAChR subtype, affecting the production of cytokines. The analgesic properties of α7 nAChR-selective compounds are mostly based on the activation of the cholinergic anti-inflammatory pathway. The molecular mechanism of neuropathic pain relief mediated by the inhibition of α9-containing nAChRs is not fully understood yet, but the role of immune factors in this process is becoming evident. To obtain appropriate drugs, a search of selective agonists, antagonists and modulators of α7- and α9-containing nAChRs is underway. The naturally occurring three-finger snake α-neurotoxins and mammalian Ly6/uPAR proteins, as well as neurotoxic peptides α-conotoxins, are not only sophisticated tools in research on nAChRs but are also considered as potential medicines. In particular, the inhibition of the α9-containing nAChRs by α-conotoxins may be a pathway to alleviate neuropathic pain. nAChRs are involved in the inflammation processes during AIDS and other viral infections; thus they can also be means used in drug design. In this review, we discuss the role of α7- and α9-containing nAChRs in the immune processes and in pain.

Kynurenic acid blunts A1 astrocyte activation against neurodegeneration in HIV-associated neurocognitive disorders

Despite extensive astrocyte activation in patients suffering from HIV-associated neurocognitive disorders (HAND), little is known about the contribution of astrocytes to HAND neuropathology. Here, we report that the robust activation of neurotoxic astrocytes (A1 astrocytes) in the CNS promotes neuron damage and cognitive deficits in HIV-1 gp120 transgenic mice. Notably, knockout of α7 nicotinic acetylcholine receptors (α7nAChR) blunted A1 astrocyte responses, ultimately facilitating neuronal and cognitive improvement in the gp120tg mice. Furthermore, we provide evidence that Kynurenic acid (KYNA), a tryptophan metabolite with α7nAChR inhibitory properties, attenuates gp120-induced A1 astrocyte formation through the blockade of α7nAChR/JAK2/STAT3 signaling activation. Meanwhile, compared with gp120tg mice, mice fed with tryptophan showed dramatic improvement in cognitive performance, which was related to the inhibition of A1 astrocyte responses. These initial and determinant findings mark a turning point in our understanding of the role of α7nAChR in gp120-mediated A1 astrocyte activation, opening up new opportunities to control neurotoxic astrocyte generation through KYNA and tryptophan administration.

HIV Nef Expression Down-modulated GFAP Expression and Altered Glutamate Uptake and Release and Proliferation in Astrocytes

HIV infection of astrocytes leads to restricted gene expression and replication but abundant expression of HIV early genes Tat, Nef and Rev. A great deal of neuroHIV research has so far been focused on Tat protein, its effects on astrocytes, and its roles in neuroHIV. In the current study, we aimed to determine effects of Nef expression on astrocytes and their function. Using transfection or infection of VSVG-pseudotyped HIV viruses, we showed that Nef expression down-modulated glial fibrillary acidic protein (GFAP) expression. We then showed that Nef expression also led to decreased GFAP mRNA expression. The transcriptional regulation was further confirmed using a GFAP promoter-driven reporter gene assay. We performed transcription factor profiling array to compare the expression of transcription factors between Nef-intact and Nef-deficient HIV-infected cells and identified eight transcription factors with expression changes of 1.5-fold or higher: three up-regulated by Nef (Stat1, Stat5, and TFIID), and five down-regulated by Nef (AR, GAS/ISRE, HIF, Sp1, and p53). We then demonstrated that removal of the Sp1 binding sites from the GFAP promoter resulted in a much lower level of the promoter activity and reversal of Nef effects on the GFAP promoter, confirming important roles of Sp1 in the GFAP promoter activity and for Nef-induced GFAP expression. Lastly, we showed that Nef expression led to increased glutamate uptake and decreased glutamate release by astrocytes and increased astrocyte proliferation. Taken together, these results indicate that Nef leads to down-modulation of GFAP expression and alteration of glutamate metabolism in astrocytes, and astrocyte proliferation and could be an important contributor to neuroHIV.

Activation of α7 nicotinic acetylcholine receptor promotes HIV-1 transcription

Alpha7 nicotinic acetylcholine receptor (α7 nAChR), a hub of the cholinergic anti-inflammatory pathway (CAP), is required for the treatment of inflammatory diseases. HIV-1 infection can upregulate the expression of α7 nAChR in T lymphocytes and affect the role of CAP. However, whether α7 nAChR regulates HIV-1 infection in CD4+ T cells is unclear. In this study, we first found that activation of α7 nAChR by GTS-21 (an α7 nAChR agonist) can promote the transcription of HIV-1 proviral DNA. Then, through transcriptome sequencing analysis, we found that p38 MAPK signaling was enriched in GTS-21 treated HIV-latent T cells. Mechanistically, activation of α7 nAChR could increase reactive oxygen species (ROS), reduce DUSP1 and DUSP6, and consequently enhance the phosphorylation of p38 MAPK. By co-immunoprecipitation and liquid chromatography tandem mass spectrometry, we found that p-p38 MAPK interacted with Lamin B1 (LMNB1). Activation of α7 nAChR increased the binding between p-p38 MAPK and LMNB1. We confirmed that knockdown of MAPK14 significantly downregulated NFATC4, a key activator of HIV-1 transcription. Taken together, activation of the α7 nAChR could trigger ROS/p-p38 MAPK/LMNB1/NFATC4 signaling pathway enhancing HIV-1 transcription. We have revealed an unrecognized mechanism of α7 nAChR-mediated neuroimmune regulation of HIV infection.

HIV Tat and cocaine interactively alter genome-wide DNA methylation and gene expression and exacerbate learning and memory impairments

Cocaine use is a major comorbidity of HIV-associated neurocognitive disorder (HAND). In this study, we show that cocaine exposure worsens the learning and memory of doxycycline-inducible and brain-specific HIV Tat transgenic mice (iTat) and results in 14,838 hypermethylated CpG-related differentially methylated regions (DMRs) and 15,800 hypomethylated CpG-related DMRs, which are linked to 52 down- and 127 upregulated genes, respectively, in the hippocampus of iTat mice. These genes are mostly enriched at the neuronal function-, cell morphology-, and synapse formation-related extracellular matrix (ECM) receptor-ligand interaction pathway and mostly impacted in microglia. The accompanying neuropathological changes include swollen dendritic spines, increased synaptophysin expression, and diminished glial activation. We also find that sex (female) and age additively worsen the behavioral and pathological changes. These findings together indicate that chronic cocaine and long-term Tat expression interactively contribute to HAND, likely involving changes of DNA methylation and ECM receptor-ligand interactions.

Homomeric and Heteromeric α7 Nicotinic Acetylcholine Receptors in Health and Some Central Nervous System Diseases

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels involved in the modulation of essential brain functions such as memory, learning, and attention. Homomeric α7 nAChR, formed exclusively by five identical α7 subunits, is involved in rapid synaptic transmission, whereas the heteromeric oligomers composed of α7 in combination with β subunits display metabotropic properties and operate in slower time frames. At the cellular level, the activation of nAChRs allows the entry of Na⁺ and Ca²⁺; the two cations depolarize the membrane and trigger diverse cellular signals, depending on the type of nAChR pentamer and neurons involved, the location of the intervening cells, and the networks of which these neuronal cells form part. These features make the α7 nAChR a central player in neurotransmission, metabolically associated Ca²⁺-mediated signaling, and modulation of diverse fundamental processes operated by other neurotransmitters in the brain. Due to its ubiquitous distribution and the multiple functions it displays in the brain, the α7 nAChR is associated with a variety of neurological and neuropsychiatric disorders whose exact etiopathogenic mechanisms are still elusive.