The Role of Neuronal Factors in the Epigenetic Reprogramming of Microglia in the Normal and Diseased Central Nervous System

MicroRNA biomarkers as next-generation diagnostic tools for neurodegenerative diseases: a comprehensive review

Neurodegenerative diseases (NDs) are characterized by abnormalities within neurons of the brain or spinal cord that gradually lose function, eventually leading to cell death. Upon examination of affected tissue, pathological changes reveal a loss of synapses, misfolded proteins, and activation of immune cells-all indicative of disease progression-before severe clinical symptoms become apparent. Early detection of NDs is crucial for potentially administering targeted medications that may delay disease advancement. Given their complex pathophysiological features and diverse clinical symptoms, there is a pressing need for sensitive and effective diagnostic methods for NDs. Biomarkers such as microRNAs (miRNAs) have been identified as potential tools for detecting these diseases. We explore the pivotal role of miRNAs in the context of NDs, focusing on Alzheimer's disease, Parkinson's disease, Multiple sclerosis, Huntington's disease, and Amyotrophic Lateral Sclerosis. The review delves into the intricate relationship between aging and NDs, highlighting structural and functional alterations in the aging brain and their implications for disease development. It elucidates how miRNAs and RNA-binding proteins are implicated in the pathogenesis of NDs and underscores the importance of investigating their expression and function in aging. Significantly, miRNAs exert substantial influence on post-translational modifications (PTMs), impacting not just the nervous system but a wide array of tissues and cell types as well. Specific miRNAs have been found to target proteins involved in ubiquitination or de-ubiquitination processes, which play a significant role in regulating protein function and stability. We discuss the link between miRNA, PTM, and NDs. Additionally, the review discusses the significance of miRNAs as biomarkers for early disease detection, offering insights into diagnostic strategies.

The Role of Glial Cells in Neurobiology and Prion Neuropathology

Prion diseases are rare and neurodegenerative diseases that are characterized by the misfolding and infectious spread of the prion protein in the brain, causing progressive and irreversible neuronal loss and associated clinical and behavioral manifestations in humans and animals, ultimately leading to death. The brain has a complex network of neurons and glial cells whose crosstalk is critical for function and homeostasis. Although it is established that prion infection of neurons is necessary for clinical disease to occur, debate remains in the field as to the role played by glial cells, namely astrocytes and microglia, and whether these cells are beneficial to the host or further accelerate disease. Here, we review the current literature assessing the complex morphologies of astrocytes and microglia, and the crosstalk between these two cell types, in the prion-infected brain.

Complement C4-deficient mice have a high mortality rate during PTZ-induced epileptic seizures, which correlates with cognitive problems and the deficiency in the expression of Egr1 and other immediate early genes

Complement system plays an important role in the immune defense against pathogens; however, recent studies demonstrated an important role of complement subunits C1q, C4, and C3 in normal functions of the central nervous system (CNS) such as non-functional synapse elimination (synapse pruning), and during various neurologic pathologies. Humans have two forms of C4 protein encoded by C4A and C4B genes that share 99.5% homology, while mice have only one C4B gene that is functionally active in the complement cascade. Overexpression of the human C4A gene was shown to contribute to the development of schizophrenia by mediating extensive synapse pruning through the activation C1q-C4-C3 pathway, while C4B deficiency or low levels of C4B expression were shown to relate to the development of schizophrenia and autism spectrum disorders possibly via other mechanisms not related to synapse elimination. To investigate the potential role of C4B in neuronal functions not related to synapse pruning, we compared wildtype (WT) mice with C3- and C4B- deficient animals for their susceptibility to pentylenetetrazole (PTZ)- induced epileptic seizures. We found that C4B (but not C3)-deficient mice were highly susceptible to convulsant and subconvulsant doses of PTZ when compared to WT controls. Further gene expression analysis revealed that in contrast to WT or C3-deficient animals, C4B-deficient mice failed to upregulate expressions of multiple immediate early genes (IEGs) Egrs1-4, c-Fos, c-Jus, FosB, Npas4, and Nur77 during epileptic seizures. Moreover, C4B-deficient mice had low levels of baseline expression of Egr1 on mRNA and protein levels, which was correlated with the cognitive problems of these animals. C4-deficient animals also failed to upregulate several genes downstream of IEGs such as BDNF and pro-inflammatory cytokines IL-1β, IL-6, and TNF. Taken together, our study demonstrates a new role of C4B in the regulation of expression of IEGs and their downstream targets during CNS insults such as epileptic seizures.

From Determining Brain Insulin Resistance in a Sporadic Alzheimer's Disease Model to Exploring the Region-Dependent Effect of Intranasal Insulin

Impaired response to insulin has been linked to many neurodegenerative disorders like Alzheimer's disease (AD). Animal model of sporadic AD has been developed by intracerebroventricular (icv) administration of streptozotocin (STZ), which given peripherally causes insulin resistance. Difficulty in demonstrating insulin resistance in this model led to our aim: to determine brain regional and peripheral response after intranasal (IN) administration of insulin in control and STZ-icv rats, by exploring peripheral and central metabolic parameters. One month after STZ-icv or vehicle-icv administration to 3-month-old male Wistar rats, cognitive status was determined after which rats received 2 IU of fast-acting insulin aspart intranasally (CTR + INS; STZ + INS) or saline only (CTR and STZ). Rats were sacrificed 2 h after administration and metabolic and glutamatergic parameters were measured in plasma, CSF, and the brain. Insulin and STZ increased amyloid-β concentration in plasma (CTR + INS and STZ vs CTR), while there was no effect on glucose and insulin plasma and CSF levels. INS normalized the levels of c-fos in temporal cortex of STZ + INS vs STZ (co-localized with neurons), while hypothalamic c-fos was found co-localized with the microglial marker. STZ and insulin brain region specifically altered the levels and activity of proteins involved in cell metabolism and glutamate signaling. Central changes found after INS in STZ-icv rats suggest hippocampal and cortical insulin sensitivity. Altered hypothalamic metabolic parameters of STZ-icv rats were not normalized by INS, indicating possible hypothalamic insulin insensitivity. Brain insulin sensitivity depends on the affected brain region and presence of metabolic dysfunction induced by STZ-icv administration.

Microglial immune regulation by epigenetic reprogramming through histone H3K27 acetylation in neuroinflammation

Epigenetic reprogramming is the ability of innate immune cells to form memories of environmental stimuli (priming), allowing for heightened responses to secondary stressors. Herein, we explored microglial epigenetic marks using the known inflammagen LPS as a memory priming trigger and Parkinsonian-linked environmental neurotoxic stressor manganese (Mn) as the secondary environmental trigger. To mimic physiological responses, the memory priming trigger LPS treatment was removed by triple-washing to allow the cells' acute inflammatory response to reset back before applying the secondary insult. Our results show that after the secondary Mn insult, levels of key proinflammatory markers, including nitrite release, iNOS mRNA and protein expression, Il-6, Il-α and cytokines were exaggerated in LPS-primed microglia. Our paradigm implies primed microglia retain immune memory that can be reprogrammed to augment inflammatory response by secondary environmental stress. To ascertain the molecular underpinning of this neuroimmune memory, we further hypothesize that epigenetic reprogramming contributes to the retention of a heightened immune response. Interestingly, Mn-exposed, LPS-primed microglia showed enhanced deposition of H3K27ac and H3K4me3 along with H3K4me1. We further confirmed the results using a PD mouse model (MitoPark) and postmortem human PD brains, thereby adding clinical relevance to our findings. Co-treatment with the p300/H3K27ac inhibitor GNE-049 reduced p300 expression and H3K27ac deposition, decreased iNOS, and increased ARG1 and IRF4 levels. Lastly, since mitochondrial stress is a driver of environmentally linked Parkinson's disease (PD) progression, we examined the effects of GNE-049 on primary trigger-induced mitochondrial stress. GNE-049 reduced mitochondrial superoxide, mitochondrial circularity and stress, and mitochondrial membrane depolarization, suggesting beneficial consequences of GNE-049 on mitochondrial function. Collectively, our findings demonstrate that proinflammatory primary triggers can shape microglial memory via the epigenetic mark H3K27ac and that inhibiting H3K27ac deposition can prevent primary trigger immune memory formation and attenuate subsequent secondary inflammatory responses.

Dysregulation of neuroprotective astrocytes, a spectrum of microglial activation states, and altered hippocampal neurogenesis are revealed by single-cell RNA sequencing in prion disease

Prion diseases are neurodegenerative disorders with long asymptomatic incubation periods, followed by a rapid progression of cognitive and functional decline culminating in death. The complexity of intercellular interactions in the brain is challenging to unravel and the basis of disease pathobiology remains poorly understood. In this study, we employed single cell RNA sequencing (scRNAseq) to produce an atlas of 147,536 single cell transcriptomes from cortex and hippocampus of mice infected with prions and showing clinical signs. We identified transcriptionally distinct populations and sub-populations of all the major brain cell-types. Disease-related transcription was highly specific to not only overarching cell-types, but also to sub-populations of glia and neurons. Most striking was an apparent decrease in relative frequency of astrocytes expressing genes that are required for brain homeostasis such as lipid synthesis, glutamate clearance, synaptic modulation and regulation of blood flow. Additionally, we described a spectrum of microglial activation states that suggest delineation of phagocytic and neuroinflammatory functions in different cell subsets. Differential responses of immature and mature neuron populations were also observed, alongside abnormal hippocampal neurogenesis. Our scRNAseq library provides a new layer of knowledge on single cell gene expression in prion disease, and is a basis for a more detailed understanding of cellular interplay that leads to neurodegeneration.

Cell Reprogramming for Regeneration and Repair of the Nervous System

A persistent barrier to the cure and treatment of neurological diseases is the limited ability of the central and peripheral nervous systems to undergo neuroregeneration and repair. Recent efforts have turned to regeneration of various cell types through cellular reprogramming of native cells as a promising therapy to replenish lost or diminished cell populations in various neurological diseases. This review provides an in-depth analysis of the current viral vectors, genes of interest, and target cellular populations that have been studied, as well as the challenges and future directions of these novel therapies. Furthermore, the mechanisms by which cellular reprogramming could be optimized as treatment in neurological diseases and a review of the most recent cellular reprogramming in vitro and in vivo studies will also be discussed.

Neuroimmune Mechanisms Underlying Neuropathic Pain: The Potential Role of TNF-α-Necroptosis Pathway

The neuroimmune mechanism underlying neuropathic pain has been extensively studied. Tumor necrosis factor-alpha (TNF-α), a key pro-inflammatory cytokine that drives cytokine storm and stimulates a cascade of other cytokines in pain-related pathways, induces and modulates neuropathic pain by facilitating peripheral (primary afferents) and central (spinal cord) sensitization. Functionally, TNF-α controls the balance between cell survival and death by inducing an inflammatory response and two programmed cell death mechanisms (apoptosis and necroptosis). Necroptosis, a novel form of programmed cell death, is receiving increasing attraction and may trigger neuroinflammation to promote neuropathic pain. Chronic pain is often accompanied by adverse pain-associated emotional reactions and cognitive disorders. Overproduction of TNF-α in supraspinal structures such as the anterior cingulate cortex (ACC) and hippocampus plays an important role in pain-associated emotional disorders and memory deficits and also participates in the modulation of pain transduction. At present, studies reporting on the role of the TNF-α–necroptosis pathway in pain-related disorders are lacking. This review indicates the important research prospects of this pathway in pain modulation based on its role in anxiety, depression and memory deficits associated with other neurodegenerative diseases. In addition, we have summarized studies related to the underlying mechanisms of neuropathic pain mediated by TNF-α and discussed the role of the TNF-α–necroptosis pathway in detail, which may represent an avenue for future therapeutic intervention.

Neuron–Microglia Contacts Govern the PGE2 Tolerance through TLR4-Mediated de Novo Protein Synthesis

Cellular and molecular mechanisms of the peripheral immune system (e.g., macrophage and monocyte) in programming endotoxin tolerance (ET) have been well studied. However, regulatory mechanism in development of brain immune tolerance remains unclear. The inducible COX-2/PGE₂ axis in microglia, the primary innate immune cells of the brain, is a pivotal feature in causing inflammation and neuronal injury, both in acute excitotoxic insults and chronic neurodegenerative diseases. This present study investigated the regulatory mechanism of PGE₂ tolerance in microglia. Multiple reconstituted primary brain cells cultures, including neuron–glial (NG), mixed glial (MG), neuron-enriched, and microglia-enriched cultures, were performed and consequently applied to a treatment regimen for ET induction. Our results revealed that the levels of COX-2 mRNA and supernatant PGE₂ in NG cultures, but not in microglia-enriched and MG cultures, were drastically reduced in response to the ET challenge, suggesting that the presence of neurons, rather than astroglia, is required for PGE₂ tolerance in microglia. Furthermore, our data showed that neural contact, instead of its soluble factors, is sufficient for developing microglial PGE₂ tolerance. Simultaneously, this finding determined how neurons regulated microglial PGE₂ tolerance. Moreover, by inhibiting TLR4 activation and de novo protein synthesis by LPS-binding protein (LBP) manipulation and cycloheximide, our data showed that the TLR4 signal and de novo protein synthesis are necessary for microglia to develop PGE₂ tolerance in NG cells under the ET challenge. Altogether, our findings demonstrated that neuron–microglia contacts are indispensable in emerging PGE₂ tolerance through the regulation of TLR4-mediated de novo protein synthesis.

Transcriptome analysis reveals the contribution of oligodendrocyte and radial glia-derived cues for maintenance of microglia identity

Microglia are increasingly being recognized as druggable targets in neurodegenerative disorders, and good in vitro models are crucial to address cell biological questions. Major challenges are to recapitulate the complex microglial morphology and their in vivo transcriptome. We have therefore exposed primary microglia from adult rhesus macaques to a variety of different culture conditions including exposure to soluble factors as M-CSF, IL-34, and TGF-β as well as serum replacement approaches, and compared their morphologies and transcriptomes to those of mature, homeostatic in vivo microglia. This enabled us to develop a new, partially serum-free, monoculture protocol, that yields high numbers of ramified cells. We also demonstrate that exposure of adult microglia to M-CSF or IL-34 induces similar transcriptomes, and that exposure to TGF-β has much less pronounced effects than it does on rodent microglia. However, regardless of culture conditions, the transcriptomes of in vitro and in vivo microglia remained substantially different. Analysis of differentially expressed genes inspired us to perform 3D-spherical coculture experiments of microglia with oligodendrocytes and radial glia. In such spheres, microglia signature genes were strongly induced, even in the absence of neurons and astrocytes. These data reveal a novel role for oligodendrocyte and radial glia-derived cues in the maintenance of microglial identity, providing new anchor points to study microglia in health and disease.

Early-life thermal stress mediates long-term alterations in hypothalamic microglia

Stressful environmental events in early life have long-lasting consequences on later stress responses. We previously showed that heat conditioning of 3-day-old chicks during the critical period of heat-response development leads to heat vulnerability later in life. Here we assessed the role of early-life heat stress on the inflammatory response in the chick anterior hypothalamus (AH), focusing on hypothalamic microglia. We identified the microglial cell population in the chick AH using anti-KUL01 and anti-CD45 antibodies. Specific microglial features were also confirmed by expression of their signature genes. Under normal environmental conditions, hypothalamic microglia isolated from lipopolysaccharide (LPS)-injected chicks displayed a classical activated proinflammatory profile accompanied by a decreased homeostatic signature, demonstrating similarity of immune response with mammalian microglial cells. In accordance with our previous observations, conditioning of 3-day-old chicks under high ambient temperature decreased the number of newborn cells in the AH, among them microglial precursors. Although heat exposure did not affect microglial cell viability, it had a long-term impact on LPS-induced inflammatory response. Exposure to harsh heat led to heat vulnerability, and attenuated recruitment of peripheral monocytes and T cells into the AH following LPS challenge. Moreover, heat conditioning altered microglial reactivity, manifested as suppressed microglial activation in response to LPS. Innate immune memory developed by heat conditioning might underlie suppression of the microglial response to LPS challenge. We describe alterations in genome-wide CpG methylation profile of hypothalamic microglia, demonstrating probable epigenetic involvement in the reprogramming of microglial function, leading to heat-induced inflammatory cross-tolerance.

Exposure of Microglia to Interleukin-4 Represses NF-κB-Dependent Transcription of Toll-Like Receptor-Induced Cytokines

Interleukin (IL)-4 is a cytokine that affects both adaptive and innate immune responses. In the central nervous system, microglia express IL-4 receptors and it has been described that IL-4-exposed microglia acquire anti-inflammatory properties. We here demonstrate that IL-4 exposure induces changes in the cell surface protein expression profile of primary rhesus macaque microglia and enhances their potential to induce proliferation of T cells with a regulatory signature. Moreover, we show that Toll like receptor (TLR)-induced cytokine production is broadly impaired in IL-4-exposed microglia at the transcriptional level. IL-4 type 2 receptor-mediated signaling is shown to be crucial for the inhibition of microglial innate immune responses. TLR-induced nuclear translocalization of NF-κB appeared intact, and we found no evidence for epigenetic modulation of target genes. By contrast, nuclear extracts from IL-4-exposed microglia contained significantly less NF-κB capable of binding to its DNA consensus site. Further identification of the molecular mechanisms that underlie the inhibition of TLR-induced responses in IL-4-exposed microglia may aid the design of strategies that aim to modulate innate immune responses in the brain, for example in gliomas.

Activation of Three Major Signaling Pathways After Endurance Training and Spinal Cord Injury

We aimed to investigate the effects of endurance training on expression of growth factors (GFs) and stimulation of neurotrophin-dependent signaling pathways (PI3k/Akt, PLCγ/PKC, PLCγ/CAMKII, Ras-Erk1/2 and Rac1-Cdc42) responsible for neuroplasticity, neuroregeneration, survival and growth after spinal cord injury (SCI). Wistar rats were divided into four groups: (i) intact controls; (ii) 6 weeks of endurance training; (iii) SCI; (iv) pre-training + SCI. The animals survived for 6 weeks after SCI. Firstly, endurance training markedly upregulated mRNA expression and protein levels (up to four times) of growth factors (BDNF, GDNF) and their receptors (TrkB, Gfrα) in low thoracic segments (Th8–Th10) compared to levels in untrained animals. Secondly, we found that spontaneous neuroplasticity seen in the SCI alone group was GF-specific and was activated through both PLCγ-PKC and PLC-CAMKII signaling pathways. In addition, training prior to SCI markedly increased the activity of PLCγ-PKC signaling at both transcript and protein levels at and around the lesion site. Similar effects were seen in expression of PI3k/Akt and Ras/Erk1/2 signaling responsible for cell survival and regeneration. Thirdly, rats which underwent physical activity prior to SCI were more active and had significantly better neurological scores at the 14th and 42nd days of survival. These results suggest that regular physical activity could play an important role after SCI, as it maintains increased expression of GFs in spinal cord tissue 6 weeks post-SCI. The BDNF- and/or BDNF + GDNF-dependent signaling pathways were significantly affected in pre-trained SCI animals. In contrast, GDNF-dependent Rac1-Cdc42 signaling was not involved in training-affected SCI response.

Glutamate and GABA in Microglia-Neuron Cross-Talk in Alzheimer's Disease

The physiological balance between excitation and inhibition in the brain is significantly affected in Alzheimer’s disease (AD). Several neuroactive compounds and their signaling pathways through various types of receptors are crucial in brain homeostasis, among them glutamate and γ-aminobutyric acid (GABA). Activation of microglial receptors regulates the immunological response of these cells, which in AD could be neuroprotective or neurotoxic. The novel research approaches revealed the complexity of microglial function, including the interplay with other cells during neuroinflammation and in the AD brain. The purpose of this review is to describe the role of several proteins and multiple receptors on microglia and neurons, and their involvement in a communication network between cells that could lead to different metabolic loops and cell death/survival. Our review is focused on the role of glutamatergic, GABAergic signaling in microglia–neuronal cross-talk in AD and neuroinflammation. Moreover, the significance of AD-related neurotoxic proteins in glutamate/GABA-mediated dialogue between microglia and neurons was analyzed in search of novel targets in neuroprotection, and advanced pharmacological approaches.

miR-31 promotes neural stem cell proliferation and restores motor function after spinal cord injury

This study aims to examine whether miR-31 promotes endogenous NSC proliferation and be used for spinal cord injury management. In the present study, the morpholino knockdown of miR-31 induced abnormal neuronal apoptosis in zebrafish, resulting in impaired development of the tail. miR-31 agomir transfection in NSCs increased Nestin expression and decreased ChAT and GFAP expression levels. miR-31 induced the proliferation of mouse NSCs by upregulating the Notch signaling pathway, and more NSCs entered G1; Notch was inhibited by miR-31 inactivation. Injection of a miR-31 agomir into mouse models of spinal cord injury could effectively restore motor functions after spinal cord injury, which was achieved by promoting the proliferation of endogenous NSCs. After the injection of a miR-31 agomir in spinal cord injury mice, the expression of Nestin and GFAP increased, while GFAP expression decreased. In conclusion, the zebrafish experiments prove that a lack of miR-31 will block nervous system development. In spinal cord injury mouse models, miR-31 overexpression might promote spinal cord injury repair.

Dietary Nutrients and Prevention of Alzheimer's disease

Alzheimer's disease is an irrevocable, progressive brain disorder that gradually destroys memory and cognitive skills. One of the extensively studied method of preventing Alzheimer's disease (AD) disease progression is by providing nutritional diet. Several reports have shown that intake of nutritional elements as huperzine A, ursolic acid, vitamins etc. can directly influence pathogenesis of AD. Surprisingly, occurrence of metabolic disorders due to unhealthy diet has been known to be a major environmental causes for AD. It has been noted that AD disease severity can be controlled by supplementing dietary supplements containing huge amounts of health-promoting ingredients. These elements promote cell health, regeneration, and the anti-aging process that specifically interrupt the pathogenic pathways in AD development. Fortunately, incorporating changes in the nutritional content is inexpensive, easy, acceptable, safe, effective, and in most cases free from major adverse events. Many nutritional phytoconstituents such as flavonoids, alkaloids, and terpenoids are still being evaluated in the hope of identifying a successful therapy for AD. This review discusses the therapeutical potential of several key nutrients that have been researched for treating AD treatment and the method of their neuroprotective intervention.

Laurent CD, McCord KA, Bains A, Sidhu G, Sarkar S, Plemel JR, Macauley MS. The CD33 short isoform is a gain-of-function variant that enhances Aβ1-42 phagocytosis in microglia

BACKGROUND

CD33 is genetically linked to Alzheimer's disease (AD) susceptibility through differential expression of isoforms in microglia. The role of the human CD33 short isoform (hCD33m), preferentially encoded by an AD-protective CD33 allele (rs12459419T), is unknown. Here, we test whether hCD33m represents a loss-of-function or gain-of-function variant.

METHODS

We have developed two models to test the role of hCD33m. The first is a new strain of transgenic mice expressing hCD33m in the microglial cell lineage. The second is U937 cells where the CD33 gene was disrupted by CRISPR/Cas9 and complemented with different variants of hCD33. Primary microglia and U937 cells were tested in phagocytosis assays and single cell RNA sequencing (scRNAseq) was carried out on the primary microglia. Furthermore, a new monoclonal antibody was developed to detect hCD33m more efficiently.

RESULTS

In both primary microglia and U937 cells, we find that hCD33m enhances phagocytosis. This contrasts with the human CD33 long isoform (hCD33M) that represses phagocytosis, as previously demonstrated. As revealed by scRNAseq, hCD33m+ microglia are enriched in a cluster of cells defined by an upregulated expression and gene regulatory network of immediate early genes, which was further validated within microglia in situ. Using a new hCD33m-specific antibody enabled hCD33m expression to be examined, demonstrating a preference for an intracellular location. Moreover, this newly discovered gain-of-function role for hCD33m is dependent on its cytoplasmic signaling motifs, dominant over hCD33M, and not due to loss of glycan ligand binding.

CONCLUSIONS

These results provide strong support that hCD33m represents a gain-of-function isoform and offers insight into what it may take to therapeutically capture the AD-protective CD33 allele.

Epigenetics and Communication Mechanisms in Microglia Activation with a View on Technological Approaches

Microglial cells, the immune cells of the central nervous system (CNS), play a crucial role for the proper brain development and function and in CNS homeostasis. While in physiological conditions, microglia continuously check the state of brain parenchyma, in pathological conditions, microglia can show different activated phenotypes: In the early phases, microglia acquire the M2 phenotype, increasing phagocytosis and releasing neurotrophic and neuroprotective factors. In advanced phases, they acquire the M1 phenotype, becoming neurotoxic and contributing to neurodegeneration. Underlying this phenotypic change, there is a switch in the expression of specific microglial genes, in turn modulated by epigenetic changes, such as DNA methylation, histones post-translational modifications and activity of miRNAs. New roles are attributed to microglial cells, including specific communication with neurons, both through direct cell–cell contact and by release of many different molecules, either directly or indirectly, through extracellular vesicles. In this review, recent findings on the bidirectional interaction between neurons and microglia, in both physiological and pathological conditions, are highlighted, with a focus on the complex field of microglia immunomodulation through epigenetic mechanisms and/or released factors. In addition, advanced technologies used to study these mechanisms, such as microfluidic, 3D culture and in vivo imaging, are presented.

Effects of acute and chronic arecoline in adult zebrafish: Anxiolytic-like activity, elevated brain monoamines and the potential role of microglia

Arecoline is a naturally occurring psychoactive alkaloid with partial agonism at nicotinic and muscarinic acetylcholine receptors. Arecoline consumption is widespread, making it the fourth (after alcohol, nicotine and caffeine) most used substance by humans. However, the mechanisms of acute and chronic action of arecoline in-vivo remain poorly understood. Animal models are a valuable tool for CNS disease modeling and drug screening. Complementing rodent studies, the zebrafish (Danio rerio) emerges as a promising novel model organism for neuroscience research. Here, we assessed the effects of acute and chronic arecoline on adult zebrafish behavior and physiology. Overall, acute and chronic arecoline treatments produced overt anxiolytic-like behavior (without affecting general locomotor activity and whole-body cortisol levels), with similar effects also caused by areca nut water extracts. Acute arecoline at 10 mg/L disrupted shoaling, increased social preference, elevated brain norepinephrine and serotonin levels and reduced serotonin turnover. Acute arecoline also upregulated early protooncogenes c-fos and c-jun in the brain, whereas chronic treatment with 1 mg/L elevated brain expression of microglia-specific biomarker genes egr2 and ym1 (thus, implicating microglial mechanisms in potential effects of long-term arecoline use). Finally, acute 2-h discontinuation of chronic arecoline treatment evoked withdrawal-like anxiogenic behavior in zebrafish. In general, these findings support high sensitivity of zebrafish screens to arecoline and related compounds, and reinforce the growing utility of zebrafish for probing molecular mechanisms of CNS drugs. Our study also suggests that novel anxiolytic drugs can eventually be developed based on arecoline-like molecules, whose integrative mechanisms of CNS action may involve monoaminergic and neuro-immune modulation.

Low-dose cannabinoid receptor 2 agonist induces microglial activation in a cancer pain-morphine tolerance rat model

AIMS

Cancer pain seriously affects the life quality of patients. Morphine is commonly used for cancer pain, but tolerance development limits its clinical administration. Central immune signaling is important in the development of cancer pain and morphine tolerance. Cannabinoid receptor 2 (CB2) inhibits cancer pain and morphine tolerance by regulating central immune signaling. In the present study, we investigated the mechanisms of central immune signaling involved in morphine tolerance inhibition by the CB2 agonist AM1241 in cancer pain treatment.

MAIN METHODS

Rats were implanted with tumor cells and divided into 4 groups: Vehicle (PBS), 0.07 μg AM1241, 0.03 μg AM1241, and AM630 (10 μg) + AM1241 (0.07 μg). All groups received morphine (20 μg/day, i.t.) for 8 days. AM630 (CB2 antagonist) was intrathecally injected 30 min before AM1241, and AM1241 was intrathecally injected 30 min before morphine. The spinal cord (SC) and dorsal root ganglion (DRG) were collected to determine the expression of Toll-like receptor 4 (TLR4), the p38 mitogen-activated protein kinase (MAPK), microglial markers, interleukin (IL)-1β, and tumor necrosis factor (TNF)-α.

KEY FINDINGS

The expression of TLR4, p38 MAPK, microglial markers, IL-1β, and TNF-α was significantly higher in AM1241-pretreated groups than in the vehicle group (P < 0.05). No difference in microglial markers, IL-1β, and TNF-α expression was detected in the AM630 + AM1241 group compared with the vehicle group.

SIGNIFICANCE

Our results suggest that in a cancer pain-morphine tolerance model, an i.t. non-analgesic dose of AM1241 induces microglial activation and IL-1β TNF-α upregulation in SC and DRG via the CB2 receptor pathway.

Role of Microglia in Modulating Adult Neurogenesis in Health and Neurodegeneration

Microglia are the resident immune cells of the brain, constituting the powerhouse of brain innate immunity. They originate from hematopoietic precursors that infiltrate the developing brain during different stages of embryogenesis, acquiring a phenotype characterized by the presence of dense ramifications. Microglial cells play key roles in maintaining brain homeostasis and regulating brain immune responses. They continuously scan and sense the brain environment to detect any occurring changes. Upon detection of a signal related to physiological or pathological processes, the cells are activated and transform to an amoeboid-like phenotype, mounting adequate responses that range from phagocytosis to secretion of inflammatory and trophic factors. The overwhelming evidence suggests that microglia are crucially implicated in influencing neuronal proliferation and differentiation, as well as synaptic connections, and thereby cognitive and behavioral functions. Here, we review the role of microglia in adult neurogenesis under physiological conditions, and how this role is affected in neurodegenerative diseases.

Enduring and Sex-specific Changes in Hippocampal Gene Expression after a Subchronic Immune Challenge

Major illnesses, including heart attack and sepsis, can cause cognitive impairments, depression, and progressive memory decline that persist long after recovery from the original illness. In rodent models of sepsis or subchronic immune challenge, memory deficits also persist for weeks or months, even in the absence of ongoing neuroimmune activation. This raises the question of what mechanisms in the brain mediate such persistent changes in neural function. Here, we used RNA-sequencing as a large-scale, unbiased approach to identify changes in hippocampal gene expression long after a subchronic immune challenge previously established to cause persistent memory impairments in both males and females. We observed enduring dysregulation of gene expression three months after the end of a subchronic immune challenge. Surprisingly, there were striking sex differences in both the magnitude of changes and the specific genes and pathways altered, where males showed persistent changes in both immune- and plasticity-related genes three months after immune challenge, whereas females showed few such changes. In contrast, females showed striking differential gene expression in response to a subsequent immune challenge. Thus, immune activation has enduring and sex-specific consequences for hippocampal gene expression and the transcriptional response to subsequent stimuli. Together with findings of long-lasting memory impairments after immune challenge, these data suggest that illnesses can cause enduring vulnerability to, cognitive decline, affective disorders, and memory impairments via dysregulation of transcriptional processes in the brain.