Synapse loss induced by interleukin-1β requires pre- and post-synaptic mechanisms

Neuroinflammation, microglial activation, and glucose metabolism in neurodegenerative diseases

Alzheimer's disease is characterized by aggregated amyloid beta plaques and neurofibrillary tangles. Apart from the plaques and tangles, microglial activation plays a significant role in neurodegeneration and neuronal function. This review discusses the way in which microglial activation influences neurodegeneration and how systemic inflammation, type 2 diabetes mellitus, obesity and hypercholesterolemia influence neuroinflammation. Also reviewed is how systemic inflammation influences microglial activation along with the relationship between microglial activation and glucose metabolism.

Neuroimmune System as a Driving Force for Plasticity Following CNS Injury

Following an injury to the central nervous system (CNS), spontaneous plasticity is observed throughout the neuraxis and affects multiple key circuits. Much of this spontaneous plasticity can elicit beneficial and deleterious functional outcomes, depending on the context of plasticity and circuit affected. Injury-induced activation of the neuroimmune system has been proposed to be a major factor in driving this plasticity, as neuroimmune and inflammatory factors have been shown to influence cellular, synaptic, structural, and anatomical plasticity. Here, we will review the mechanisms through which the neuroimmune system mediates plasticity after CNS injury. Understanding the role of specific neuroimmune factors in driving adaptive and maladaptive plasticity may offer valuable therapeutic insight into how to promote adaptive plasticity and/or diminish maladaptive plasticity, respectively.

Acute exposure to low-level light at night is sufficient to induce neurological changes and depressive-like behavior

The advent and wide-spread adoption of electric lighting over the past century has profoundly affected the circadian organization of physiology and behavior for many individuals in industrialized nations; electric lighting in homes, work environments, and public areas have extended daytime activities into the evening, thus, increasing night-time exposure to light. Although initially assumed to be innocuous, chronic exposure to light at night (LAN) is now associated with increased incidence of cancer, metabolic disorders, and affective problems in humans. However, little is known about potential acute effects of LAN. To determine whether acute exposure to low-level LAN alters brain function, adult male, and female mice were housed in either light days and dark nights (LD; 14 h of 150 lux:10 h of 0 lux) or light days and low level light at night (LAN; 14 h of 150 lux:10 h of 5 lux). Mice exposed to LAN on three consecutive nights increased depressive-like responses compared to mice housed in dark nights. In addition, female mice exposed to LAN increased central tendency in the open field. LAN was associated with reduced hippocampal vascular endothelial growth factor-A (VEGF-A) in both male and female mice, as well as increased VEGFR1 and interleukin-1β mRNA expression in females, and reduced brain derived neurotrophic factor mRNA in males. Further, LAN significantly altered circadian rhythms (activity and temperature) and circadian gene expression in female and male mice, respectively. Altogether, this study demonstrates that acute exposure to LAN alters brain physiology and can be detrimental to well-being in otherwise healthy individuals.

Histamine Induces Microglia Activation and the Release of Proinflammatory Mediators in Rat Brain Via H1R or H4R

Histamine is a major peripheral inflammatory mediator and a neurotransmitter in the central nervous system. We have reported that histamine induces microglia activation and releases proinflammatory factors in primary cultured microglia. Whether histamine has similar effects in vivo is unknown. In the present study, we aimed to investigate the role of histamine and its receptors in the release of inflammatory mediators and activation of microglia in rat brain. We site-directed injected histamine, histamine receptor agonists or histamine receptor antagonists in the rat lateral ventricle using stereotaxic techniques. Flow cytometry was employed to determine histamine receptor expression in rat microglia. Microglia activation was assessed by Iba1 immunohistochemistry. The levels of tumour necrosis factor-alpha (TNF-α), interleukin-1beta (IL-1β) and interleukin-10 (IL-10) were measured with commercial enzyme-linked immunosorbent assay (ELISA) kits, TNF-α, IL-1β and IL-10 mRNA expressions were determined with Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR). We found that all four types of histamine receptors were expressed in rat brain microglia. Histamine was able to induce microglia activation and subsequent production of the inflammatory factors TNF-α, IL-1β and IL-10, and these effects were partially abolished by H₁R and H₄R antagonists. However, H₂R and H₃R antagonists significantly increased production of TNF-α and IL-1β, and decreased IL-10 levels. The H₁R or H₄R agonists stimulated the production of TNF-α and IL-1β, while the H₂R or H₃R agonists increased IL-10 release. Our results demonstrate that histamine induces microglia activation and the release of both proinflammatory and anti-inflammatory factors in rat brain, thus contributing to the development of inflammation in the brain. Graphical Abstract Histamine induces microglia activation and the release of both proinflammatory (TNF-α and IL-1β) and anti-inflammatory factors (IL-10) in rat brain, thus contributing to the development of inflammation in the brain.

Automated Live-Cell Imaging of Synapses in Rat and Human Neuronal Cultures

Synapse loss and dendritic damage correlate with cognitive decline in many neurodegenerative diseases, underlie neurodevelopmental disorders, and are associated with environmental and drug-induced CNS toxicities. However, screening assays designed to measure loss of synaptic connections between live cells are lacking. Here, we describe the design and validation of automated synaptic imaging assay (ASIA), an efficient approach to label, image, and analyze synapses between live neurons. Using viral transduction to express fluorescent proteins that label synapses and an automated computer-controlled microscope, we developed a method to identify agents that regulate synapse number. ASIA is compatible with both confocal and wide-field microscopy; wide-field image acquisition is faster but requires a deconvolution step in the analysis. Both types of images feed into batch processing analysis software that can be run on ImageJ, CellProfiler, and MetaMorph platforms. Primary analysis endpoints are the number of structural synapses and cell viability. Thus, overt cell death is differentiated from subtle changes in synapse density, an important distinction when studying neurodegenerative processes. In rat hippocampal cultures treated for 24 h with 100 μM 2-bromopalmitic acid (2-BP), a compound that prevents clustering of postsynaptic density 95 (PSD95), ASIA reliably detected loss of postsynaptic density 95-enhanced green fluorescent protein (PSD95-eGFP)-labeled synapses in the absence of cell death. In contrast, treatment with 100 μM glutamate produced synapse loss and significant cell death, determined from morphological changes in a binary image created from co-expressed mCherry. Treatment with 3 mM lithium for 24 h significantly increased the number of fluorescent puncta, showing that ASIA also detects synaptogenesis. Proof of concept studies show that cell-specific promoters enable the selective study of inhibitory or principal neurons and that alternative reporter constructs enable quantification of GABAergic or glutamatergic synapses. ASIA can also be used to study synapse loss between human induced pluripotent stem cell (iPSC)-derived cortical neurons. Significant synapse loss in the absence of cell death was detected in the iPSC-derived neuronal cultures treated with either 100 μM 2-BP or 100 μM glutamate for 24 h, while 300 μM glutamate produced synapse loss and cell death. ASIA shows promise for identifying agents that evoke synaptic toxicities and screening for compounds that prevent or reverse synapse loss.

Amyloid-beta impairs TOM1-mediated IL-1R1 signaling

Defects in interleukin-1β (IL-1β)-mediated cellular responses contribute to Alzheimer's disease (AD). To decipher the mechanism associated with its pathogenesis, we investigated the molecular events associated with the termination of IL-1β inflammatory responses by focusing on the role played by the target of Myb1 (TOM1), a negative regulator of the interleukin-1β receptor-1 (IL-1R1). We first show that TOM1 steady-state levels are reduced in human AD hippocampi and in the brain of an AD mouse model versus respective controls. Experimentally reducing TOM1 affected microglia activity, substantially increased amyloid-beta levels, and impaired cognition, whereas enhancing its levels was therapeutic. These data show that reparation of the TOM1-signaling pathway represents a therapeutic target for brain inflammatory disorders such as AD. A better understanding of the age-related changes in the immune system will allow us to craft therapies to limit detrimental aspects of inflammation, with the broader purpose of sharply reducing the number of people afflicted by AD.

Ethanol and Cytokines in the Central Nervous System

The innate immune system plays a critical role in the ethanol-induced neuroimmune response in the brain. Ethanol initiates the innate immune response via activation of the innate immune receptors Toll-like receptors (TLRs, e.g., TLR4, TLR3, TLR7) and NOD-like receptors (inflammasome NLRs) leading to a release of a plethora of chemokines and cytokines and development of the innate immune response. Cytokines and chemokines can have pro- or anti-inflammatory properties through which they regulate the immune response. In this chapter, we will focus on key cytokines (e.g., IL-1, IL-6, TNF-α) and chemokines (e.g., MCP-1/CCL2) that mediate the ethanol-induced neuroimmune responses. In this regard, we will use IL-1β, as an example cytokine, to discuss the neuromodulatory properties of cytokines on cellular properties and synaptic transmission. We will discuss their involvement through a set of evidence: (1) changes in gene and protein expression following ethanol exposure, (2) association of gene polymorphisms (humans) and alterations in gene expression (animal models) with increased alcohol intake, and (3) modulation of alcohol-related behaviors by transgenic or pharmacological manipulations of chemokine and cytokine systems. Over the last years, our understanding of the molecular mechanisms mediating cytokine- and chemokine-dependent regulation of immune responses has advanced tremendously, and we review evidence pointing to cytokines and chemokines serving as neuromodulators and regulators of neurotransmission.

Lipopolysaccharide-induced neuroinflammation induces presynaptic disruption through a direct action on brain tissue involving microglia-derived interleukin 1 beta

BACKGROUND

Systemic inflammation has been linked to synapse loss and cognitive decline in human patients and animal models. A role for microglial release of pro-inflammatory cytokines has been proposed based on in vivo and primary culture studies. However, mechanisms are hard to study in vivo as specific microglial ablation is challenging and the extracellular fluid cannot be sampled without invasive methods. Primary cultures have different limitations as the intricate multicellular architecture in the brain is not fully reproduced. It is essential to confirm proposed brain-specific mechanisms of inflammatory synapse loss directly in brain tissue. Organotypic hippocampal slice cultures (OHSCs) retain much of the in vivo neuronal architecture, synaptic connections and diversity of cell types whilst providing convenient access to manipulate and sample the culture medium and observe cellular reactions.

METHODS

OHSCs were generated from P6-P9 C57BL/6 mice. Inflammation was induced via addition of lipopolysaccharide (LPS), and cultures were analysed for changes in synaptic proteins, gene expression and protein secretion. Microglia were selectively depleted using clodronate, and the effect of IL1β was assessed using a specific neutralising monoclonal antibody.

RESULTS

LPS treatment induced loss of the presynaptic protein synaptophysin without altering PSD95 or Aβ protein levels. Depletion of microglia prior to LPS application prevented the loss of synaptophysin, whilst microglia depletion after the inflammatory insult was partially effective, although less so than pre-emptive treatment, indicating a time-critical window in which microglia can induce synaptic damage. IL1β protein and mRNA were increased after LPS addition, with these effects also prevented by microglia depletion. Direct application of IL1β to OHSCs resulted in synaptophysin loss whilst pre-treatment with IL1β neutralising antibody prior to LPS addition prevented a significant loss of synaptophysin but may also impact basal synaptic levels.

CONCLUSIONS

The loss of synaptophysin in this system confirms LPS can act directly within brain tissue to disrupt synapses, and we show that microglia are the relevant cellular target when all major CNS cell types are present. By overcoming limitations of primary culture and in vivo work, our study strengthens the evidence for a key role of microglia-derived IL1β in synaptic dysfunction after inflammatory insult.

MicroRNAs in Microglia: How do MicroRNAs Affect Activation, Inflammation, Polarization of Microglia and Mediate the Interaction Between Microglia and Glioma?

The essential roles of microglia in maintaining homeostasis in the healthy brain and contributing to neuropathology are well documented. Emerging evidence suggests that epigenetic modulation regulates microglial behavior in both physiological and pathological conditions. MicroRNAs (miRNAs) are short, non-coding epigenetic regulators that repress target gene expression mostly via binding to 3'-untranslated region (3'-UTR) of mRNA in a Dicer-dependent manner. Dysregulation of certain miRNAs can contribute to microglial hyper-activation, persistent neuroinflammation, and abnormal macrophage polarization in the brain. These abnormal conditions can support the pathogenesis of neurological disorders such as glioma, Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), stroke, ischemia, and spinal cord injury (SCI). However, the roles of miRNAs in microglia in health and neurological disease have not been systematically summarized. This review will first report the role of Dicer, a key endoribonulease that is responsible for most miRNA biogenesis in microglia. Second, we will focus on recent research about the function of miRNAs in activation, inflammation and polarization of microglia, respectively. In addition, potential crosstalk between microglia and glioma cells via miRNAs will be discussed in this part. Finally, the role of two essential miRNAs, miR-124, and miR-155, in microglia will be highlighted.

The TSPO Ligands 2-Cl-MGV-1, MGV-1, and PK11195 Differentially Suppress the Inflammatory Response of BV-2 Microglial Cell to LPS

The 18 kDa Translocator Protein (TSPO) is a marker for microglial activation as its expression is enhanced in activated microglia during neuroinflammation. TSPO ligands can attenuate neuroinflammation and neurotoxicity. In the present study, we examined the efficacy of new TSPO ligands designed by our laboratory, MGV-1 and 2-Cl-MGV-1, in mitigating an in vitro neuroinflammatory process compared to the classic TSPO ligand, PK 11195. We exposed BV-2 microglial cells to lipopolysaccharide (LPS) for 24 h to induce inflammatory response and added the three TSPO ligands: (1) one hour before LPS treatment (pretreatment), (2) simultaneously with LPS (cotreatment), and (3) one hour after LPS exposure (post-treatment). We evaluated the capability of TSPO ligands to reduce the levels of three glial inflammatory markers: cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and nitric oxide (NO). We compared the effects of the two novel ligands to PK 11195. Both 2-Cl-MGV-1 and MGV-1 reduced the levels of glial COX-2, iNOS, and NO in LPS-treated BV-2 cells more efficiently than PK 11195. Notably, even when added after exposure to LPS, all ligands were able to suppress the inflammatory response. Due to their pronounced anti-inflammatory activity, 2-Cl-MGV-1 and MGV-1 may serve as potential therapeutics in neuroinflammatory and neurodegenerative diseases.

Principles of inflammasome priming and inhibition: Implications for psychiatric disorders

The production of inflammatory proteins by the innate immune system is a tightly orchestrated procedure that allows the body to efficiently respond to exogenous and endogenous threats. Recently, accumulating evidence has indicated that disturbances in the inflammatory response system not only provoke autoimmune disorders, but also can have deleterious effects on neuronal function and mental health. As inflammation in the brain is primarily mediated by microglia, there has been an expanding focus on the mechanisms through which these cells initiate and propagate neuroinflammation. Microglia can enter persistently active states upon their initial recognition of an environmental stressor and are thereafter prone to elicit amplified and persistent inflammatory responses following subsequent exposures to stressors. A recent focus on why primed microglia cells are susceptible to environmental insults has been the NLRP3 inflammasome. Its function within the innate immune system is regulated in such a manner that supports a role for the complex in gating neuroinflammatory responses. The activation of NLRP3 inflammasome in microglia results in the cleavage of zymogen inflammatory interleukins into functional forms that elicit a number of consequential effects in the local neuronal environment. There is evidence to support the principle that within primed neuroimmune systems a lowered threshold for NLRP3 activation can cause persistent neuroinflammation or the amplified production of inflammatory cytokines, such as IL-1β and IL-18. Over the course of an individual's lifetime, persistent neuroinflammation can subsequently lead to the pathophysiological signatures that define psychological disorders. Therefore, targeting the NLRP3 inflammasome complex may represent an innovative and consequential approach to limit neuroinflammatory states in psychiatric disorders, such as major depressive disorder.

The Communication Between the Immune and Nervous Systems: The Role of IL-1β in Synaptopathies

In the last 15 years, groundbreaking genetic progress has underlined a convergence onto coherent synaptic pathways for most psychiatric and neurodevelopmental disorders, which are now collectively called “synaptopathies.” However, the modest size of inheritance detected so far indicates a multifactorial etiology for these disorders, underlining the key contribution of environmental effects to them. Inflammation is known to influence the risk and/or severity of a variety of synaptopathies. In particular, pro-inflammatory cytokines, produced and released in the brain by activated astrocytes and microglia, may play a pivotal role in these pathologies. Although the link between immune system activation and defects in cognitive processes is nowadays clearly established, the knowledge of the molecular mechanisms by which inflammatory mediators specifically hit synaptic components implicated in synaptopathies is still in its infancy. This review summarizes recent evidence showing that the pro-inflammatory cytokine interleukin-1β (IL-1β) specifically targets synaptopathy molecular substrate, leading to memory defects and pathological processes. In particular, we describe three specific pathways through which IL-1β affects (1) synaptic maintenance/dendritic complexity, (2) spine morphology, and (3) the excitatory/inhibitory balance. We coin the term immune synaptopathies to identify this class of diseases.

Scaling Synapses in the Presence of HIV

A defining feature of HIV-associated neurocognitive disorder (HAND) is the loss of excitatory synaptic connections. Synaptic changes that occur during exposure to HIV appear to result, in part, from a homeostatic scaling response. Here we discuss the mechanisms of these changes from the perspective that they might be part of a coping mechanism that reduces synapses to prevent excitotoxicity. In transgenic animals expressing the HIV proteins Tat or gp120, the loss of synaptic markers precedes changes in neuronal number. In vitro studies have shown that HIV-induced synapse loss and cell death are mediated by distinct mechanisms. Both in vitro and animal studies suggest that HIV-induced synaptic scaling engages new mechanisms that suppress network connectivity and that these processes might be amenable to therapeutic intervention. Indeed, pharmacological reversal of synapse loss induced by HIV Tat restores cognitive function. In summary, studies indicate that there are temporal, mechanistic and pharmacological features of HIV-induced synapse loss that are consistent with homeostatic plasticity. The increasingly well delineated signaling mechanisms that regulate synaptic scaling may reveal pharmacological targets suitable for normalizing synaptic function in chronic neuroinflammatory states such as HAND.

Monoacylglycerol lipase inhibitor JZL184 prevents HIV-1 gp120-induced synapse loss by altering endocannabinoid signaling

Monoacylglycerol lipase (MGL) hydrolyzes 2-arachidonoylglycerol to arachidonic acid and glycerol. Inhibition of MGL may attenuate neuroinflammation by enhancing endocannabinoid signaling and decreasing prostaglandin (PG) production. Almost half of HIV infected individuals are afflicted with HIV-associated neurocognitive disorder (HAND), a neuroinflammatory disease in which cognitive decline correlates with synapse loss. HIV infected cells shed the envelope protein gp120 which is a potent neurotoxin that induces synapse loss. Here, we tested whether inhibition of MGL, using the selective inhibitor JZL184, would prevent synapse loss induced by gp120. The number of synapses between rat hippocampal neurons in culture was quantified by imaging clusters of a GFP-tagged antibody-like protein that selectively binds to the postsynaptic scaffolding protein, PSD95. JZL184 completely blocked gp120-induced synapse loss. Inhibition of MGL decreased gp120-induced interleukin-1β (IL-1β) production and subsequent potentiation of NMDA receptor-mediated calcium influx. JZL184-mediated protection of synapses was reversed by a selective cannabinoid type 2 receptor (CB2R) inverse agonist/antagonist. JZL184 also reduced gp120-induced prostaglandin E2 (PGE2) production; PG signaling was required for gp120-induced IL-1β expression and synapse loss. Inhibition of MGL prevented gp120-induced synapse loss by activating CB2R resulting in decreased production of the inflammatory cytokine IL-1β. Because PG signaling was required for gp120-induced synapse loss, JZL184-induced decreases in PGE2 levels may also protect synapses. MGL presents a promising target for preventing synapse loss in neuroinflammatory conditions such as HAND.

A GluN2B-Selective NMDAR Antagonist Reverses Synapse Loss and Cognitive Impairment Produced by the HIV-1 Protein Tat

HIV-associated neurocognitive disorder (HAND) affects approximately half of HIV-infected patients. Loss of synaptic connections is a hallmark of many neurocognitive disorders, including HAND. The HIV-1 protein transactivator of transcription (Tat) disrupts synaptic connections both in vitro and in vivo and has been linked to impaired neurocognitive function in humans. In vitro studies have shown that ifenprodil, an antagonist selective for GluN2B-containing NMDARs, reverses synapse loss when applied after Tat. Here, we tested the hypothesis that Tat-induced loss and ifenprodil-mediated rescue of synaptic spines in vivo would predict impairment and rescue of cognitive function. Using intracranial multiphoton imaging, we found that infusion of 100 ng of HIV-1 Tat into the lateral ventricle of yellow fluorescent protein-expressing transgenic mice produced a 17 ± 1% loss of dendritic spines in layer 1 of retrosplenial cortex. Repeated imaging of the same dendrites over 3 weeks enabled longitudinal experiments that demonstrated sustained spine loss after Tat infusion and transient rescue after ifenprodil administration (10 mg/kg, i.p.). Parallel trace fear conditioning experiments showed that spine loss predicted learning deficits and that the time course of ifenprodil-induced rescue of spine density correlated with restoration of cognitive function. These results show for the first time that, during exposure to an HIV-1 neurotoxin in vivo, alteration of GluN2B-containing NMDAR signaling suppresses spine density and impairs learning. Pharmacological inhibition of these NMDARs rescued spines and restored cognitive function. Drugs that rescue synapses may improve neurocognitive function in HAND.SIGNIFICANCE STATEMENT Synaptodendritic damage correlates with cognitive decline in HIV-associated neurocognitive disorder (HAND) patients. We developed an in vivo imaging approach for longitudinal tracking of spine density that enabled correlation of synaptic changes with behavioral outcomes in a model of HAND. We show for the first time that spine loss after exposure to an HIV-1 protein can be reversed pharmacologically and that loss and recovery of dendritic spines predict impairment and restoration of cognitive function, respectively. Therefore, synapse loss, the hallmark of cognitive decline in HAND, is reversible. Drugs that restore spine density may have broad application for improving cognitive function during the early phases of neurodegenerative diseases.

The X-Linked Intellectual Disability Protein IL1RAPL1 Regulates Dendrite Complexity

Mutations and deletions of the interleukin-1 receptor accessory protein like 1 (IL1RAPL1) gene, located on the X chromosome, are associated with intellectual disability (ID) and autism spectrum disorder (ASD). IL1RAPL1 protein is located at the postsynaptic compartment of excitatory synapses and plays a role in synapse formation and stabilization. Here, using primary neuronal cultures and Il1rapl1-KO mice, we characterized the role of IL1RAPL1 in regulating dendrite morphology. In Il1rapl1-KO mice we identified an increased number of dendrite branching points in CA1 and CA2 hippocampal neurons associated to hippocampal cognitive impairment. Similarly, induced pluripotent stem cell-derived neurons from a patient carrying a null mutation of the IL1RAPL1 gene had more dendrites. In hippocampal neurons, the overexpression of full-length IL1RAPL1 and mutants lacking part of C-terminal domains leads to simplified neuronal arborization. This effect is abolished when we overexpressed mutants lacking part of N-terminal domains, indicating that the IL1RAPL1 extracellular domain is required for regulating dendrite development. We also demonstrate that PTPδ interaction is not required for this activity, while IL1RAPL1 mediates the activity of IL-1β on dendrite morphology. Our data reveal a novel specific function for IL1RAPL1 in regulating dendrite morphology that can help clarify how changes in IL1RAPL1-regulated pathways can lead to cognitive disorders in humans.SIGNIFICANCE STATEMENT Abnormalities in the architecture of dendrites have been observed in a variety of neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Here we show that the X-linked intellectual disability protein interleukin-1 receptor accessory protein like 1 (IL1RAPL1) regulates dendrite morphology of mice hippocampal neurons and induced pluripotent stem cell-derived neurons from a patient carrying a null mutation of IL1RAPL1 gene. We also found that the extracellular domain of IL1RAPL1 is required for this effect, independently of the interaction with PTPδ, but IL1RAPL1 mediates the activity of IL-1β on dendrite morphology. Our data reveal a novel specific function for IL1RAPL1 in regulating dendrite morphology that can help clarify how changes in IL1RAPL1-regulated pathways can lead to cognitive disorders in humans.

NMDARs Adapt to Neurotoxic HIV Protein Tat Downstream of a GluN2A-Ubiquitin Ligase Signaling Pathway

HIV-associated neurocognitive disorder (HAND) affects approximately half of HIV-infected patients. Infected non-neuronal cells release neurotoxic factors such as the viral protein transactivator of transcription (Tat) that potentiate NMDAR function. NMDARs regulate synaptic changes observed after exposure to HIV proteins, which may underlie cognitive impairment in HAND patients. Here, we used patch-clamp recording to measure NMDAR-mediated currents in rat hippocampal cultures after exposure to Tat. Tat (4-16 h) potentiated NMDA-evoked whole-cell current and increased the NMDAR:AMPAR ratio of evoked EPSCs. Potentiated currents adapted back to baseline amplitudes after 24 h of exposure to Tat. Pharmacological inhibition of GluN2A-containing NMDARs prevented adaptation, but inhibition of GluN2B-containing NMDARs did not. Pharmacological and genetic approaches determined that potentiated NMDARs activated the kinase Akt, which then activated the E3 ubiquitin ligase Mdm2. Inhibition of protein synthesis prevented adaptation, suggesting that Mdm2 altered gene expression, possibly through its well known target p53. Expression of GFP-tagged GluN1 subunits resulted in fluorescent puncta that colocalized with synaptic markers. Tat (24 h) caused an Mdm2-dependent loss of NMDAR puncta on a timescale similar to adaption of NMDAR function. Activation of the Mdm2 pathway degrades PSD-95, a scaffolding protein that clusters NMDARs at the synapse and enhances their function. Adaptation to the continued presence of excitotoxins that potentiate NMDARs such as HIV Tat may protect from excessive NMDAR activation while also contributing to the synaptic loss that correlates with cognitive decline in HAND.

SIGNIFICANCE STATEMENT

Synaptodendritic damage correlates with cognitive decline in HIV-associated neurocognitive disorder (HAND). In a cell culture model, we show that the HIV protein transactivator of transcription (Tat) initially potentiates NMDARs that then adapt to the presence of the toxin. Adaptation of NMDAR function was mediated by a GluN2A/Akt/Mdm2 pathway not previously linked to neuroinflammatory disorders such as HAND. Activation of this pathway caused a loss of synaptic NMDAR clusters. Decreased NMDAR function may result from a homeostatic response gone awry and underlie impaired synaptic function in HAND.

Long-term depression-associated signaling is required for an in vitro model of NMDA receptor-dependent synapse pruning

Activity-dependent pruning of synaptic contacts plays a critical role in shaping neuronal circuitry in response to the environment during postnatal brain development. Although there is compelling evidence that shrinkage of dendritic spines coincides with synaptic long-term depression (LTD), and that LTD is accompanied by synapse loss, whether NMDA receptor (NMDAR)-dependent LTD is a required step in the progression toward synapse pruning is still unknown. Using repeated applications of NMDA to induce LTD in dissociated rat neuronal cultures, we found that synapse density, as measured by colocalization of fluorescent markers for pre- and postsynaptic structures, was decreased irrespective of the presynaptic marker used, post-treatment recovery time, and the dendritic location of synapses. Consistent with previous studies, we found that synapse loss could occur without apparent net spine loss or cell death. Furthermore, synapse loss was unlikely to require direct contact with microglia, as the number of these cells was minimal in our culture preparations. Supporting a model by which NMDAR-LTD is required for synapse loss, the effect of NMDA on fluorescence colocalization was prevented by phosphatase and caspase inhibitors. In addition, gene transcription and protein translation also appeared to be required for loss of putative synapses. These data support the idea that NMDAR-dependent LTD is a required step in synapse pruning and contribute to our understanding of the basic mechanisms of this developmental process.

Age-dependent neuroinflammation and cognitive decline in a novel Ala152Thr-Tau transgenic mouse model of PSP and AD

Introduction

Mutations of Tau are associated with several neurodegenerative disorders. Recently, the Tau mutation A152T was described as a novel risk factor for frontotemporal dementia spectrum disorders and Alzheimer disease. In vitro Tau-A152T shows a decreased binding to microtubules and a reduced tendency to form abnormal fibers.

Results

To study the effects of this mutation we generated a mouse model expressing human full-length Tau with this mutation (hTau40^AT). At young age (2–3 months) immunohistological analysis reveals pathological Tau conformation and Tau-hyperphosphorylation combined with Tau missorting into the somatodendritic compartment of neurons. With increasing age there is Tau aggregation including co-aggregates of endogenous mouse Tau and exogenous human Tau, accompanied by loss of synapses (especially presynaptic failure) and neurons. From ~10 months onwards the mice show a prominent neuroinflammatory response as judged by activation of microglia and astrocytes. This progressive neuroinflammation becomes visible by in vivo bioluminescence imaging after crossbreeding of hTau40^AT mice and Gfap-luciferase reporter mice. In contrast to other Tau-transgenic models and Alzheimer disease patients with reduced protein clearance, hTau40^AT mice show a strong induction of autophagy. Although Tau-hyperphosphorylation and aggregation is also present in spinal cord and motor cortex (due to the Thy1.2 promoter), neuromotor performance is not affected. Deficits in spatial reference memory are manifest at ~16 months and are accompanied by neuronal death.

Conclusions

The hTau40^AT mice mimic pathological hallmarks of tauopathies including a cognitive phenotype combined with pronounced neuroinflammation visible by bioluminescence. Thus the mice are suitable for mechanistic studies of Tau induced toxicity and in vivo validation of neuroprotective compounds.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-016-0281-z) contains supplementary material, which is available to authorized users.

HIV-1 Tat-induced changes in synaptically-driven network activity adapt during prolonged exposure

HIV-associated neurocognitive disorders (HAND) afflict approximately half of HIV-infected patients. The HIV-1 transactivator of transcription (Tat) protein is released by infected cells and contributes to the pathogenesis of HAND, but many of the underlying mechanisms remain poorly understood. Here we used fura-2-based Ca(2+) imaging and whole-cell patch-clamp recording to study the effects of Tat on the spontaneous synaptic activity that occurs in networked rat hippocampal neurons in culture. Tat triggered aberrant network activity that exhibited a decrease in the frequency of spontaneous action potential bursts and Ca(2+) spikes with a simultaneous increase in burst duration and Ca(2+) spike amplitude. These network changes were apparent after 4 h treatment with Tat and required the low-density lipoprotein receptor-related protein (LRP). Interestingly, Tat-induced changes in network activity adapted during 24 h exposure. The activity returned to control levels in the maintained presence of Tat for 24 h. These observations indicate that Tat causes aberrant network activity, which is dependent on LRP, and adapts following prolonged exposure. Changes in network excitability may contribute to Tat-induced neurotoxicity in vitro and seizure disorders in vivo. Adaptation of neural networks may be a neuroprotective response to the sustained presence of the neurotoxic protein Tat and could underlie the behavioral and electrophysiological changes observed in HAND.

Induction of Interleukin-1β by Human Immunodeficiency Virus-1 Viral Proteins Leads to Increased Levels of Neuronal Ferritin Heavy Chain, Synaptic Injury, and Deficits in Flexible Attention

Synaptodendritic pruning and alterations in neurotransmission are the main underlying causes of HIV-associated neurocognitive disorders (HAND). Our studies in humans and nonhuman primates indicated that the protein ferritin heavy chain (FHC) is a critical player in neuronal changes and ensuing cognitive deficit observed in these patients. Here we focus on the effect of HIV proteins and inflammatory cytokines implicated in HAND on neuronal FHC levels, dendritic changes, and neurocognitive behavior. In two well characterized models of HAND (HIV transgenic and gp120-treated rats), we report reductions in spine density and dendritic branches in prefrontal cortex pyramidal neurons compared with age-matched controls. FHC brain levels are elevated in these animals, which also show deficits in reversal learning. Moreover, IL-1β, TNF-α, and HIV gp120 upregulate FHC in rat cortical neurons. However, although the inflammatory cytokines directly altered neuronal FHC, gp120 only caused significant FHC upregulation in neuronal/glial cocultures, suggesting that glia are necessary for sustained elevation of neuronal FHC by the viral protein. Although the envelope protein induced secretion of IL-1β and TNF-α in cocultures, TNF-α blockade did not affect gp120-mediated induction of FHC. Conversely, studies with an IL-1β neutralizing antibody or specific IL-1 receptor antagonist revealed the primary involvement of IL-1β in gp120-induced FHC changes. Furthermore, silencing of neuronal FHC abrogates the effect of gp120 on spines, and spine density correlates negatively with FHC levels or cognitive deficit. These results demonstrate that viral and host components of HIV infection increase brain expression of FHC, leading to cellular and functional changes, and point to IL-1β-targeted strategies for prevention of these alterations. Significance statement: This work demonstrates the key role of the cytokine IL-1β in the regulation of a novel intracellular mediator [i.e., the protein ferritin heavy chain (FHC)] of HIV-induced dendritic damage and the resulting neurocognitive impairment. This is also the first study that systematically investigates dendritic damage in layer II/III prefrontal cortex neurons of two different non-infectious models of HIV-associated neurocognitive disorders (HAND) and reveals a precise correlation of these structural changes with specific biochemical and functional alterations also reported in HIV patients. Overall, these data suggest that targeting the IL-1β-dependent FHC increase may represent a valid strategy for neuroprotective adjuvant therapies in HAND.

IL-1 interacts with ethanol effects on GABAergic transmission in the mouse central amygdala

Neuroinflammation is hypothesized to enhance alcohol consumption and contribute to the development of alcoholism. GABAergic transmission in the central amygdala (CeA) plays an important role in the transition to alcohol dependence. Therefore, we studied the effects of interleukin-1β (IL-1β), a proinflammatory cytokine mediating ethanol-induced neuroinflammation, and its interaction with ethanol on CeA GABAegic transmission in B6129SF2/J mice. We also assessed ethanol intake in B6129SF2/J mice. Intake with unlimited (24 h) ethanol access was 9.2–12.7 g/kg (3–15% ethanol), while limited (2 h) access produced an intake of 4.1 ± 0.5 g/kg (15% ethanol). In our electrophysiology experiments, we found that recombinant IL-1β (50 and 100 ng/ml) significantly decreased the amplitude of evoked inhibitory postsynaptic potentials (eIPSPs), with no significant effects on paired-pulse facilitation (PPF). IL-1β (50 ng/ml) had dual effects on spontaneous miniature inhibitory postsynaptic currents (mIPSCs): increasing mIPSC frequencies in most CeA neurons, but decreasing both mIPSC frequencies and amplitudes in a few cells. The IL-1β receptor antagonist (IL-1ra; 100 ng/ml) also had dual effects on mIPSCs and prevented the actions of IL-1β on mIPSC frequencies. These results suggest that IL-1β can alter CeA GABAergic transmission at pre- and postsynaptic sites. Ethanol (44 mM) significantly increased eIPSP amplitudes, decreased PPFs, and increased mIPSC frequencies. IL-1β did not alter ethanol’s enhancement of the eIPSP amplitude, but, in IL-1β-responsive neurons, the ethanol effects on mIPSC frequencies were lost. Overall, our data suggest that the IL-1 system is involved in basal GABAergic transmission and that IL-1β interacts with the ethanol-induced facilitation of CeA GABAergic transmission.

Diet-induced obesity and low testosterone increase neuroinflammation and impair neural function

BACKGROUND

Low testosterone and obesity are independent risk factors for dysfunction of the nervous system including neurodegenerative disorders such as Alzheimer's disease (AD). In this study, we investigate the independent and cooperative interactions of testosterone and diet-induced obesity on metabolic, inflammatory, and neural health indices in the central and peripheral nervous systems.

METHODS

Male C57B6/J mice were maintained on normal or high-fat diet under varying testosterone conditions for a four-month treatment period, after which metabolic indices were measured and RNA isolated from cerebral cortex and sciatic nerve. Cortices were used to generate mixed glial cultures, upon which embryonic cerebrocortical neurons were co-cultured for assessment of neuron survival and neurite outgrowth. Peripheral nerve damage was determined using paw-withdrawal assay, myelin sheath protein expression levels, and Na+,K+-ATPase activity levels.

RESULTS

Our results demonstrate that detrimental effects on both metabolic (blood glucose, insulin sensitivity) and proinflammatory (cytokine expression) responses caused by diet-induced obesity are exacerbated by testosterone depletion. Mixed glial cultures generated from obese mice retain elevated cytokine expression, although low testosterone effects do not persist ex vivo. Primary neurons co-cultured with glial cultures generated from high-fat fed animals exhibit reduced survival and poorer neurite outgrowth. In addition, low testosterone and diet-induced obesity combine to increase inflammation and evidence of nerve damage in the peripheral nervous system.

CONCLUSIONS

Testosterone and diet-induced obesity independently and cooperatively regulate neuroinflammation in central and peripheral nervous systems, which may contribute to observed impairments in neural health. Together, our findings suggest that low testosterone and obesity are interactive regulators of neuroinflammation that, in combination with adipose-derived inflammatory pathways and other factors, increase the risk of downstream disorders including type 2 diabetes and Alzheimer's disease.

Critical role of inflammatory cytokines in impairing biochemical processes for learning and memory after surgery in rats

Background

Patients with postoperative cognitive dysfunction have poor outcomes. Neuroinflammation may be the underlying pathophysiology for this dysfunction. We determined whether proinflammatory cytokines affect the trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors to the plasma membrane, a fundamental biochemical process for learning and memory.

Methods

Four-month-old male Fischer 344 rats were subjected to right carotid exposure under isoflurane anesthesia. Some rats received intravenous lidocaine infusion during anesthesia. Rats were tested two weeks later by Barnes maze. The hippocampus was harvested six hours after the surgery for western blotting of interleukin (IL)-1β or IL-6. Hippocampal slices were prepared from control rats or rats subjected to surgery two weeks previously. They were incubated with tetraethylammonium, an agent that can induce long term potentiation, for determining the trafficking of GluR1, an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit.

Results

Surgery or anesthesia increased the time to identify the target box during the Barnes maze test training sessions and one day after the training sessions. Surgery also prolonged the time to identify the target box eight days after the training sessions. Surgery increased IL-1β and IL-6 in the hippocampus. The tetraethylammonium–induced GluR1 phosphorylation and trafficking were abolished in the hippocampal slices of rats after surgery. These surgical effects were partly inhibited by lidocaine. The incubation of control hippocampal slices with IL-1β and IL-6 abolished tetraethylammonium–induced GluR1 trafficking and phosphorylation. Lidocaine minimally affected the effects of IL-1β on GluR1 trafficking.

Conclusions

Our results suggest that surgery increases proinflammatory cytokines that then inhibit GluR1 trafficking, leading to learning and memory impairment.

Upregulation of insulin-like growth factor and interleukin 1β occurs in neurons but not in glial cells in the cochlear nucleus following cochlear ablation

One of the main mechanisms used by neurons and glial cells to promote repair following brain injury is to upregulate activity-dependent molecules such as insulin-like growth factor 1 (IGF-1) and interleukin-1β (IL-1β). In the auditory system, IGF-1 is crucial for restoring synaptic transmission following hearing loss; however, whether IL-1β is also involved in this process is unknown. In this study, we evaluated the expression of IGF-1 and IL-1β within neurons and glial cells of the ventral cochlear nucleus in adult rats at 1, 7, 15, and 30 days following bilateral cochlear ablation. After the lesion, significant increases in both the overall mean gray levels of IGF-1 immunostaining and the mean gray levels within cells of the cochlear nucleus were observed at 1, 7, and 15 days compared with control animals. The expression and distribution of IL-1β in the ventral cochlear nucleus of ablated animals was temporally and spatially correlated with IGF-1. We also observed a lack of colocalization between IGF-1 and IL-1β with either astrocytes or microglia at any of the time points following ablation. These results suggest that the upregulation of IGF-1 and IL-1β levels within neurons-but not within glial cells-may reflect a plastic mechanism involved in repairing synaptic homeostasis of the overall cellular environment of the cochlear nucleus following bilateral cochlear ablation.

Perspectives on neuroinflammation and excitotoxicity: a neurotoxic conspiracy?

Emerging evidences underline the ability of several environmental contaminants to induce an inflammatory response within the central nervous system, named neuroinflammation. This can occur as a consequence of a direct action of the neurotoxicant to the CNS and/or as a response secondary to the activation of the peripheral inflammatory response. In both cases, neuroinflammation is driven by the release of several soluble factors among which pro-inflammatory cytokines. IL-1β and TNF-α have been extensively studied for their effects within the CNS and emerged for their role in the modulation of the neuronal response, which allow the immune response to integrate with specific neuronal functions, as neurotransmission and synaptic plasticity. In particular, it has been evidenced a potential detrimental link between these cytokines and the glutamatergic system that seems to be part of increased brain excitability and excitotoxicity occurring in different pathological conditions. Aim of this mini-review will be to present experimental evidence on the way IL-1β and TNF-α impact neurons, focusing on the glutamatergic signalling, to provide a perspective on novel pathways possibly involved in environmental contaminants neurotoxicity.

α-Asarone Ameliorates Memory Deficit in Lipopolysaccharide-Treated Mice via Suppression of Pro-Inflammatory Cytokines and Microglial Activation

α-Asarone exhibits a number of pharmacological actions including neuroprotective, anti-oxidative, anticonvulsive, and cognitive enhancing action. The present study investigated the effects of α-asarone on pro-inflammatory cytokines mRNA, microglial activation, and neuronal damage in the hippocampus and on learning and memory deficits in systemic lipopolysaccharide (LPS)-treated C57BL/6 mice. Varying doses of α-asarone was orally administered (7.5, 15, or 30 mg/kg) once a day for 3 days before the LPS (3 mg/kg) injection. α-Asarone significantly reduced TNF-α and IL-1β mRNA at 4 and 24 hours after the LPS injection at dose of 30 mg/kg. At 24 hours after the LPS injection, the loss of CA1 neurons, the increase of TUNEL-labeled cells, and the up-regulation of BACE1 expression in the hippocampus were attenuated by 30 mg/kg of α-asarone treatment. α-Asarone significantly reduced Iba1 protein expression in the hippocampal tissue at a dose of 30 mg/kg. α-Asarone did not reduce the number of Iba1-expressing microglia on immunohistochemistry but the average cell size and percentage areas of Iba1-expressing microglia in the hippocampus were significantly decreased by 30 mg/kg of α-asarone treatment. In the Morris water maze test, α-asarone significantly prolonged the swimming time spent in the target and peri-target zones. α-Asarone also significantly increased the number of target heading and memory score in the Morris water maze. The results suggest that inhibition of pro-inflammatory cytokines and microglial activation in the hippocampus by α-asarone may be one of the mechanisms for the α-asarone-mediated ameliorating effect on memory deficits.

Pyrrolidine dithiocarbamate attenuates surgery-induced neuroinflammation and cognitive dysfunction possibly via inhibition of nuclear factor κB

Surgery induces learning and memory impairment. Neuroinflammation may contribute to this impairment. Nuclear factor κB (NF-κB) is an important transcription factor to regulate the expression of inflammatory cytokines. We hypothesize that inhibition of NF-κB by pyrrolidine dithiocarbamate (PDTC) reduces neuroinflammation and the impairment of learning and memory. To test this hypothesis, four-month-old male Fischer 344 rats were subjected to right carotid exploration under propofol and buprenorphine anesthesia. Some rats received two doses of 50mg/kg PDTC given intraperitoneally 30min before and 6h after the surgery. Rats were tested in the Barnes maze and fear conditioning paradigm begun 6days after the surgery. Expression of various proteins related to inflammation was examined in the hippocampus at 24h or 21days after the surgery. Here, surgery, but not anesthesia alone, had a significant effect on prolonging the time needed to identify the target hole during the training sessions of the Barnes maze. Surgery also increased the time for identifying the target hole in the long-term memory test and decreased context-related learning and memory in fear conditioning test. Also, surgery increased nuclear expression of p65, a NF-κB component, decreased cytoplasmic amount of inhibitor of NF-κB, and increased the expression of interleukin-1β, interleukin-6, ionized calcium binding adaptor molecule 1 and active matrix metalloproteinase 9 (MMP-9). Finally, surgery enhanced IgG extravasation in the hippocampus. These surgical effects were attenuated by PDTC. These results suggest that surgery, but not propofol-based anesthesia, induces neuroinflammation and impairment of learning and memory. PDTC attenuates these effects possibly by inhibiting NF-κB activation and the downstream MMP-9 activity.

Glycyrrhizin alleviates neuroinflammation and memory deficit induced by systemic lipopolysaccharide treatment in mice

The present study investigated the effects of glycyrrhizin (GRZ) on neuroinflammation and memory deficit in systemic lipopolysaccharide (LPS)-treated C57BL/6 mice. Varying doses of GRZ was orally administered (10, 30, or 50 mg/kg) once a day for 3 days before the LPS (3 mg/kg) injection. At 24 h after the LPS injection, GRZ significantly reduced TNF-α and IL-1β mRNA at doses of 30 and 50 mg/kg. COX-2 and iNOS protein expressions were significantly reduced by GRZ at doses of 30 and 50 mg/kg. In the Morris water maze test, GRZ (30 mg/kg) significantly prolonged the swimming time spent in the target and peri-target zones. GRZ also significantly increased the target heading and memory score numbers. In the hippocampal tissue, GRZ significantly reduced the up-regulated Iba1 protein expression and the average cell size of Iba1-expressing microglia induced by LPS. The results indicate that GRZ ameliorated the memory deficit induced by systemic LPS treatment and the effect of GRZ was found to be mediated through the inhibition of pro-inflammatory mediators and microglial activation in the brain tissue. This study supports that GRZ may be a putative therapeutic drug on neurodegenerative diseases associated with cognitive deficits and neuroinflammation such as Alzheimer's disease.

Neuroinflammation: the role and consequences

Neuroinflammation is central to the common pathology of several acute and chronic brain diseases. This review examines the consequences of excessive and prolonged neuroinflammation, particularly its damaging effects on cellular and/or brain function, as well as its relevance to disease progression and possible interventions. The evidence gathered here indicates that neuroinflammation causes and accelerates long-term neurodegenerative disease, playing a central role in the very early development of chronic conditions including dementia. The wide scope and numerous complexities of neuroinflammation suggest that combinations of different preventative and therapeutic approaches may be efficacious.

Human immunodeficiency virus-1 protein Tat induces excitotoxic loss of presynaptic terminals in hippocampal cultures

Human immunodeficiency virus (HIV) infection of the CNS produces dendritic damage that correlates with cognitive decline in patients with HIV-associated neurocognitive disorders (HAND). HIV-induced neurotoxicity results in part from viral proteins shed from infected cells, including the HIV transactivator of transcription (Tat). We previously showed that Tat binds to the low density lipoprotein receptor-related protein (LRP), resulting in overactivation of NMDA receptors, activation of the ubiquitin-proteasome pathway, and subsequent loss of postsynaptic densities. Here, we show that Tat also induces a loss of presynaptic terminals. The number of presynaptic terminals was quantified using confocal imaging of synaptophysin fused to green fluorescent protein (Syn-GFP). Tat-induced loss of presynaptic terminals was secondary to excitatory postsynaptic mechanisms because treatment with an LRP antagonist or an NMDA receptor antagonist inhibited this loss. Treatment with nutlin-3, an E3 ligase inhibitor, prevented Tat-induced loss of presynaptic terminals. These data suggest that Tat-induced loss of presynaptic terminals is a consequence of excitotoxic postsynaptic activity. We previously found that ifenprodil, an NR2B subunit-selective NMDA receptor antagonist, induced recovery of postsynaptic densities. Here we show that Tat-induced loss of presynaptic terminals was reversed by ifenprodil treatment. Thus, Tat-induced loss of presynaptic terminals is reversible, and this recovery can be initiated by inhibiting a subset of postsynaptic NMDA receptors. Understanding the dynamics of synaptic changes in response to HIV infection of the CNS may lead to the design of improved pharmacotherapies for HAND patients.