Podosomes in migrating microglia: components and matrix degradation

Cooperative interactions between neurotrophin receptors and CXCR4 regulate macrophage phenotype and susceptibility to activation by HIV

Neural damage due to inflammatory activation of macrophages and microglia is a consequence of HIV infection that leads to cognitive dysfunction. The damage is due, in part, to the release of factors that impair neuronal function but the mechanisms that control their release are poorly understood. Previous studies have shown that mature nerve growth factor (NGF) binding to tropomyosin receptor kinase A (TrkA), and proNGF acting through the p75 neurotrophin receptor (p75NTR) differentially control the phenotype of macrophages in response to HIV. However, the mechanisms responsible for these actions are unclear. The current studies demonstrated that in human monocyte-derived macrophages, CCR5 tropic HIV virions interact with the CXCR4 receptor to promote a neurotoxic macrophage phenotype. TrkA cooperatively interacted with CXCR4 to promote quick and dynamic changes in CXCR4 phosphorylation and more stable downstream actin remodeling in the form of membrane ruffles. TrkA signaling also promoted increased moacrophage calcium spiking, and low neurotoxic activity. Disruption of these interactions by HIV led to an alternative podosome-bearing phenotype with minimal calcium signaling and enhanced toxicity. Neurotrophin receptors provide an independent yet cooperative pathway for modifying the actin cytoskeleton in response to chemokines and subsequent degenerative activity. The strong opposing effects of mature and proneurotrophins may provide the opportunity to develop novel therapies that regulate the phenotype of macrophages in the context of HIV infection and perhaps other degenerative diseases.

Understanding Actin Remodeling in Neuronal Cells Through Podosomes

Cytoskeletal dysregulation forms an important aspect of many neurodegenerative diseases such as Alzheimer's disease. Cytoskeletal functions require the dynamic activity of the cytoskeletal proteins-actin, tubulin, and the associated proteins. One of such important phenomena is that of actin remodeling, which helps the cell to migrate, navigate, and interact with extracellular materials. Podosomes are complex actin-rich cytoskeletal structures, abundant in proteins that interact and degrade the extracellular matrix, enabling cells to displace and migrate. The formation of podosomes requires extensive actin networks and remodeling. Here we present a novel immunofluorescence-based approach to study actin remodeling in neurons through the medium of podosomes.

The kynurenic acid analog SZR104 induces cytomorphological changes associated with the anti-inflammatory phenotype in cultured microglia

We previously showed the anti-inflammatory effects of kynurenic acid (KYNA) and its brain-penetrable analog N-(2-(dimethylamino)ethyl)-3-(morpholinomethyl)-4-hydroxyquinoline-2-carboxamide (SZR104) both in vivo and in vitro. Here, we identified the cytomorphological effects of KYNA and SZR104 in secondary microglial cultures established from newborn rat forebrains. We quantitatively analyzed selected morphological aspects of microglia in control (unchallenged), lipopolysaccharide (LPS)-treated (challenged), KYNA- or SZR104-treated, and LPS + KYNA or LPS + SZR104-treated cultures. Multicolor immunofluorescence labeling followed by morphometric analysis (area, perimeter, transformation index, lacunarity, density, span ratio, maximum span across the convex hull, hull circularity, hull area, hull perimeter, max/min radii, mean radius, diameter of bounding circle, fractal dimension, roughness, circularity) on binary (digital) silhouettes of the microglia revealed their morphological plasticity under experimental conditions. SZR104 and, to a lesser degree, KYNA inhibited proinflammatory phenotypic changes. For example, SZR104 treatment resulted in hypertrophied microglia characterized by a swollen cell body, enlarged perimeter, increased transformation index/decreased circularity, increased convex hull values (area, perimeter, mean radius, maximum span, diameter of the bounding circle and hull circularity), altered box-counting parameters (such as fractal dimension), and increased roughness/decreased density. Taken together, analysis of cytomorphological features could contribute to the characterization of the anti-inflammatory activity of SZR104 on cultured microglia.

Tau aggregates improve the purinergic receptor P2Y12-associated podosome rearrangements in microglial cells

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is associated with protein misfolding, plaque accumulation, neuronal dysfunction, synaptic loss, and cognitive decline. The pathological cascade of AD includes the intracellular Tau hyperphosphorylation and its subsequent aggregation, extracellular Amyloid-β plaque formation and microglia-mediated neuroinflammation. The extracellular release of aggregated Tau is sensed by surveilling microglia through the involvement of various cell surface receptors. Among all, purinergic P2Y12R signaling is involved in microglial chemotaxis towards the damaged neurons. Microglial migration is highly linked with membrane-associated actin remodeling leading to the phagocytosis of extracellular Tau species. Here, we studied the formation of various actin structures such as podosome, lamellipodia and filopodia, in response to extracellular Tau monomers and aggregates. Microglial podosomes are colocalized with actin nucleator protein WASP, Arp2 and TKS5 adaptor protein during Tau-mediated migration. Moreover, the P2Y12 receptors were associated with F-actin-rich podosome structures, which signify the potential of Tau aggregates in microglial chemotaxis through the involvement of actin remodeling.

Biomolecular Liquid-Liquid Phase Separation for Biotechnology

The liquid-liquid phase separation (LLPS) of biomolecules induces condensed assemblies called liquid droplets or membrane-less organelles. In contrast to organelles with lipid membrane barriers, the liquid droplets induced by LLPS do not have distinct barriers (lipid bilayer). Biomolecular LLPS in cells has attracted considerable attention in broad research fields from cellular biology to soft matter physics. The physical and chemical properties of LLPS exert a variety of functions in living cells: activating and deactivating biomolecules involving enzymes; controlling the localization, condensation, and concentration of biomolecules; the filtration and purification of biomolecules; and sensing environmental factors for fast, adaptive, and reversible responses. The versatility of LLPS plays an essential role in various biological processes, such as controlling the central dogma and the onset mechanism of pathological diseases. Moreover, biomolecular LLPS could be critical for developing new biotechnologies such as the condensation, purification, and activation of a series of biomolecules. In this review article, we introduce some fundamental aspects and recent progress of biomolecular LLPS in living cells and test tubes. Then, we discuss applications of biomolecular LLPS toward biotechnologies.

Src family kinases (SFKs): critical regulators of microglial homeostatic functions and neurodegeneration in Parkinson's and Alzheimer's diseases

c-Src was the first protein kinase to be described as capable of phosphorylating tyrosine residues. Subsequent identification of other tyrosine-phosphorylating protein kinases with a similar structure to c-Src gave rise to the concept of Src family kinases (SFKs). Microglia are the resident innate immune cell population of the CNS. Under physiological conditions, microglia actively participate in brain tissue homeostasis, continuously patrolling the neuronal parenchyma and exerting neuroprotective actions. Activation of pathogen-associated molecular pattern (PAMP) and damage-associated molecular pattern (DAMP) receptors induces microglial proliferation, migration toward pathological foci, phagocytosis, and changes in gene expression, concurrent with the secretion of cytokines, chemokines, and growth factors. A significant body of literature shows that SFK stimulation positively associates with microglial activation and neuropathological conditions, including Alzheimer's and Parkinson's diseases. Here, we review essential microglial homeostatic functions regulated by SFKs, including phagocytosis, environmental sensing, and secretion of inflammatory mediators. In addition, we discuss the potential of SFK modulation for microglial homeostasis in Parkinson's and Alzheimer's diseases.

VEGF-Trap Modulates Retinal Inflammation in the Murine Oxygen-Induced Retinopathy (OIR) Model

Anti-Vascular Endothelial Growth Factor (VEGF) agents are the first-line treatment for retinal neovascular diseases, which represent the most prevalent causes of acquired vision loss world-wide. VEGF-Trap (Aflibercept, AFL), a recombinant decoy receptor recognizing ligands of both VEGFR-1 and -2, was recently reported to be highly efficient in improving visual acuity and preserving retinal anatomy in individuals affected by diabetic macular edema. However, the precise molecular and cell biological mechanisms underlying the beneficial effects of this novel tool have yet to be elucidated. Using the mouse oxygen-induced retinopathy (OIR) model as a surrogate of retinopathies with sterile post-ischemic inflammation, such as late proliferative diabetic retinopathy (PDR), retinopathy of prematurity (ROP), and diabetic macular edema (DME), we provide evidence that AFL modulates inflammation in response to hypoxia by regulating the morphology of microglial cells, a parameter commonly used as a proxy for changes in their activation state. We show that AFL administration during the hypoxic period of OIR leads to an increased number of ramified Iba1⁺ microglial cells/macrophages while subsequently limiting the accumulation of these cells in particular retinal layers. Our results suggest that, beyond its well-documented beneficial effects on microvascular regeneration, AFL might exert important modulatory effects on post-ischemic retinal inflammation.

Interactions between glial cells and the blood-brain barrier and their role in Alzheimer's disease

Alzheimer's disease (AD), which is an irreversible neurodegenerative disorder characterized by senile plaques and neurofibrillary tangles, is the most common form of dementia worldwide. However, currently, there are no satisfying curative therapies for AD. The blood-brain barrier (BBB) acts as a selective physical barrier and plays protective roles in maintaining brain homeostasis. BBB dysfunction as an upstream or downstream event promotes the onset and progression of AD. Moreover, the pathogenesis of AD caused by BBB injury hasn't been well elucidated. Glial cells, BBB compartments and neurons form a minimal functional unit called the neurovascular unit (NVU). Emerging evidence suggests that glial cells are regulators in maintaining the BBB integrity and neuronal function. Illustrating the regulatory mechanism of glial cells in the BBB assists us in drawing a glial-vascular coupling diagram of AD, which may offer new insight into the pathogenesis of AD and early intervention strategies for AD. This review aims to summarize our current knowledge of glial-BBB interactions and their pathological implications in AD and to provide new therapeutic potentials for future investigations.

Polygenic risk for traumatic loss-related PTSD in US military veterans: Protective effect of secure attachment style

OBJECTIVES

To examine whether attachment style moderates the relationship between polygenic risk scores (PRS) for posttraumatic stress disorder (PTSD) re-experiencing (PTSD_REX) symptoms and the severity of and positive screen for traumatic loss-related PTSD.

METHODS

Data were analysed from 631 US veterans who endorsed 'unexpected death of a loved one' as their 'worst' traumatic event. Multivariable models evaluated the association between PRS for PTSD_REX, attachment style, and their interaction in predicting severity and positive screen for PTSD. A gene enrichment analysis was conducted to identify possible molecular mechanisms underlying the association between PTSD_REX PRS and PTSD.

RESULTS

PTSD_REX PRS (β = 0.17; odds ratio [OR] = 1.85), attachment style (β= -0.33; OR = 0.14), and PTSD_REX PRS × attachment style interaction (β= -0.12; OR = 0.53) were significant predictors of the severity and positive screen for PTSD. The most significant gene set detected was the gene ontology (GO) cellular component podosome set (GO:0002102, p < 3.95 × 10^-5).

CONCLUSIONS

Having a secure attachment style may help mitigate polygenic risk for developing traumatic loss-related PTSD in US veterans. Podosomes, which are implicated in inflammatory and neuroplasticity processes, may contribute to the genetic liability to developing loss-related PTSD. Psychological treatments targeting attachment security may help mitigate increased polygenic risk for loss-related PTSD in this population.

The extracellular HDAC6 ZnF UBP domain modulates the actin network and post-translational modifications of Tau

BACKGROUND

Microtubule-associated protein Tau undergoes aggregation in Alzheimer`s disease (AD) and a group of other related diseases collectively known as Tauopathies. In AD, Tau forms aggregates, which are deposited intracellularly as neurofibrillary tangles. Histone deacetylase-6 (HDAC6) plays an important role in aggresome formation, where it recruits polyubiquitinated aggregates to the motor protein dynein.

METHODS

Here, we have studied the effects of HDAC6 ZnF UBP on Tau phosphorylation, ApoE localization, GSK-3β regulation and cytoskeletal organization in neuronal cells by immunocytochemical analysis. This analysis reveals that the cell exposure to the UBP-type zinc finger domain of HDAC6 (HDAC6 ZnF UBP) can modulate Tau phosphorylation and actin cytoskeleton organization.

RESULTS

HDAC6 ZnF UBP treatment to cells did not affect their viability and resulted in enhanced neurite extension and formation of structures similar to podosomes, lamellipodia and podonuts suggesting the role of this domain in actin re-organization. Also, HDAC6 ZnF UBP treatment caused increase in nuclear localization of ApoE and tubulin localization in microtubule organizing centre (MTOC). Therefore, our studies suggest the regulatory role of this domain in different aspects of neurodegenerative diseases. Upon HDAC6 ZnF UBP treatment, inactive phosphorylated form of GSK-3β increases without any change in total GSK-3β level.

CONCLUSIONS

HDAC6 ZnF UBP was found to be involved in cytoskeletal re-organization by modulating actin dynamics and tubulin localization. Overall, our study suggests that ZnF domain of HDAC6 performs various regulatory functions apart from its classical function in aggresome formation in protein misfolding diseases. Video abstract.

Matrix Stiffness Modulates Mechanical Interactions and Promotes Contact between Motile Cells

The mechanical micro-environment of cells and tissues influences key aspects of cell structure and function, including cell motility. For proper tissue development, cells need to migrate, interact, and form contacts. Cells are known to exert contractile forces on underlying soft substrates and sense deformations in them. Here, we propose and analyze a minimal biophysical model for cell migration and long-range cell–cell interactions through mutual mechanical deformations of the substrate. We compute key metrics of cell motile behavior, such as the number of cell-cell contacts over a given time, the dispersion of cell trajectories, and the probability of permanent cell contact, and analyze how these depend on a cell motility parameter and substrate stiffness. Our results elucidate how cells may sense each other mechanically and generate coordinated movements and provide an extensible framework to further address both mechanical and short-range biophysical interactions.

Phosphoinositides: Roles in the Development of Microglial-Mediated Neuroinflammation and Neurodegeneration

Microglia are increasingly recognized as vital players in the pathology of a variety of neurodegenerative conditions including Alzheimer’s (AD) and Parkinson’s (PD) disease. While microglia have a protective role in the brain, their dysfunction can lead to neuroinflammation and contributes to disease progression. Also, a growing body of literature highlights the seven phosphoinositides, or PIPs, as key players in the regulation of microglial-mediated neuroinflammation. These small signaling lipids are phosphorylated derivates of phosphatidylinositol, are enriched in the brain, and have well-established roles in both homeostasis and disease.Disrupted PIP levels and signaling has been detected in a variety of dementias. Moreover, many known AD disease modifiers identified via genetic studies are expressed in microglia and are involved in phospholipid metabolism. One of these, the enzyme PLCγ2 that hydrolyzes the PIP species PI(4,5)P₂, displays altered expression in AD and PD and is currently being investigated as a potential therapeutic target.Perhaps unsurprisingly, neurodegenerative conditions exhibiting PIP dyshomeostasis also tend to show alterations in aspects of microglial function regulated by these lipids. In particular, phosphoinositides regulate the activities of proteins and enzymes required for endocytosis, toll-like receptor signaling, purinergic signaling, chemotaxis, and migration, all of which are affected in a variety of neurodegenerative conditions. These functions are crucial to allow microglia to adequately survey the brain and respond appropriately to invading pathogens and other abnormalities, including misfolded proteins. AD and PD therapies are being developed to target many of the above pathways, and although not yet investigated, simultaneous PIP manipulation might enhance the beneficial effects observed. Currently, only limited therapeutics are available for dementia, and although these show some benefits for symptom severity and progression, they are far from curative. Given the importance of microglia and PIPs in dementia development, this review summarizes current research and asks whether we can exploit this information to design more targeted, or perhaps combined, dementia therapeutics. More work is needed to fully characterize the pathways discussed in this review, but given the strength of the current literature, insights in this area could be invaluable for the future of neurodegenerative disease research.

Inhibition of Na+/H+exchanger modulates microglial activation and scar formation following microelectrode implantation

Objective.Intracortical microelectrodes are an important tool for neuroscience research and have great potential for clinical use. However, the use of microelectrode arrays to treat neurological disorders and control prosthetics is limited by biological challenges such as glial scarring, which can impair chronic recording performance. Microglia activation is an early and prominent contributor to glial scarring. After insertion of an intracortical microelectrode, nearby microglia transition into a state of activation, migrate, and encapsulate the device. Na⁺/H⁺exchanger isoform-1 (NHE-1) is involved in various microglial functions, including their polarity and motility, and has been implicated in pro-inflammatory responses to tissue injury. HOE-642 (cariporide) is an inhibitor of NHE-1 and has been shown to depress microglial activation and inflammatory response in brain injury models.Approach.In this study, the effects of HOE-642 treatment on microglial interactions to intracortical microelectrodes was evaluated using two-photon microscopyin vivo.Main results.The rate at which microglia processes and soma migrate in response to electrode implantation was unaffected by HOE-642 administration. However, HOE-642 administration effectively reduced the radius of microglia activation at 72 h post-implantation from 222.2µm to 177.9µm. Furthermore, treatment with HOE-642 significantly reduced microglial encapsulation of implanted devices at 5 h post-insertion from 50.7 ± 6.0% to 8.9 ± 6.1%, which suggests an NHE-1-specific mechanism mediating microglia reactivity and gliosis during implantation injury.Significance.This study implicates NHE-1 as a potential target of interest in microglial reactivity and HOE-642 as a potential treatment to attenuate the glial response and scar formation around implanted intracortical microelectrodes.

Quantitative morphometric and cell-type-specific population analysis of microglia-enriched cultures subcloned to high purity from newborn rat brains

Morphological and functional characterizations of cultured microglia are essential for the improved understanding of their roles in neuronal health and disease. Although some studies (phenotype analysis, phagocytosis) can be carried out in mixed or microglia-enriched cultures, in others (gene expression) pure microglia must be used. If the use of genetically modified microglial cells is not feasible, isolation of resident microglia from nervous tissue must be carried out. In this study, mixed primary cultures were established from the forebrains of newborn rats. Secondary microglia-enriched cultures were then prepared by shaking off these cells from the primary cultures, which were subsequently used to establish tertiary cultures by further shaking off the easily detachable microglia. The composition of these cultures was quantitatively analyzed by immunocytochemistry of microglia-, astrocyte-, oligodendrocyte- and neuron-specific markers to determine yield and purity. Microglia were quantitatively characterized regarding morphological and proliferation aspects. Secondary and tertiary cultures typically exhibited 73.3% ± 17.8% and 93.1% ± 6.0% purity for microglia, respectively, although the total number of microglia in the latter was much smaller. One in seven attempts of culturing the tertiary cultures had ~99% purity for microglia. The overall yield from the number of cells plated at DIV0 to the Iba1-positive microglia in tertiary cultures was ~1%. Astrocytic and neuronal contamination progressively decreased during subcloning, while oligodendrocytes were found sporadically throughout culturing. Although the tertiary microglia cultures had a low yield, they produced consistently high purity for microglia; after validation, such cultures are suitable for purity-sensitive functional screenings (gene/protein expression).

Myelination of regenerating optic nerve axons occurs in conjunction with an increase of the oligodendrocyte precursor cell population in the adult mice

Recently, regeneration of CNS tracts has been partially accomplished by strategies of intrinsic neuronal growth stimulation. However, restoration of function is dependent on proper myelination of regenerating axons. Previous work from our group (Goulart et al., 2018) has shown an increase in oligodendrocyte staining in the regenerating optic nerve, 2 weeks after crush, in animals that were submitted to conditional deletion of pten gene in retinal ganglion cells and intravitreal injection of zymosan + cAMP. Thus, in the present study we aimed to investigate the maturation of the oligodendroglial lineage and myelination during the regeneration of the optic nerve under the same conditions of our previous work. We showed that the combined treatment promoted an increase of myelinated fibers within the optic nerve, 12 weeks after lesion, as well as an increase in Sox10 positive cells. Early-OPCs, positive to A2B5, were also increased at 12 weeks, whereas O4 positive, late-OPCs, were increased from 2 until 12 weeks after crush. At 12 weeks after crush, the optic nerve of Regenerating group presented more CC1 positive oligodendrocytes and increased MRF positive myelinating oligodendrocytes, culminating in CTB traced regenerating axons superimposed to MBP staining, suggestive of myelination. Thus, our work showed that conditional deletion of pten gene in retinal ganglion cells and intravitreal inflammatory stimuli + cAMP stimulate full maturation of the olidodendroglial lineage, from OPC proliferation and differentiation to myelination of regenerating CNS axons.

Voltage gated sodium channels in cancer and their potential mechanisms of action

Voltage gated sodium channels (VGSC) are implicated in cancer cell invasion and metastasis. However, the mechanism by which VGSC increase cell invasiveness and probability of metastasis is still unknown. In this review we outline lesser known functions of VGSC outside of action potential propagation, and the current understanding of the effects of VGSC in cancer. Finally, we discuss possible downstream effects of VGSC activation in cancer cells. After extensive review of the literature, the most likely role of VGSC in cancer is in the invadopodia, the leading edge of metastatic cancer cells. Sodium gradients are used to drive many biological processes in the body, and invadopodia may be similar. The function of the sodium hydrogen exchanger (NHE) and sodium calcium exchanger (NCX) are driven by sodium gradients. Voltage gated calcium channels, activated by membrane depolarization, are also capable of becoming activated in response to VGSC activity. Changes to hydrogen ion exchange or calcium handling have functional consequences for invadopodia and would explain the relationship between VGSC expression and invasiveness of cancer cells.

Activation of FAK/Rac1/Cdc42-GTPase signaling ameliorates impaired microglial migration response to Aβ42 in triggering receptor expressed on myeloid cells 2 loss-of-function murine models

Mutation of Triggering receptor expressed on myeloid cells 2 (TREM2) impairs the response of microglia to amyloid-β (Aβ) pathology in Alzheimer's disease (AD), although the mechanism governing TREM2-regulated microglia recruitment to Aβ plaques remains unresolved. Here, we confirm that TREM2 mutation attenuates microglial migration. Then, using Trem2^-/- mice and an R47H variant mouse model for AD generated for this study, we show that TREM2 deficiency or the AD-associated R47H mutation results in inhibition of FAK and Rac1/Cdc42-GTPase signaling critical for cell migration. Intriguingly, treatment with CN04, a Rac1/Cdc42-GTPase activator, partially enhances microglial migration in response to oligomeric Aβ₄₂ in Trem2^-/- or R47H microglia both in vitro and in vivo. Our study shows that the dysfunction of microglial migration in the AD-associated TREM2 R47H variant is caused by FAK/Rac1/Cdc42 signaling disruption, and that activation of this signaling ameliorates impaired microglial migration response to Aβ₄₂ , suggesting a therapeutic target for R47H-bearing patients with high risk of AD.

Embryonic microglia influence developing hypothalamic glial populations

Background

Although historically microglia were thought to be immature in the fetal brain, evidence of purposeful interactions between these immune cells and nearby neural progenitors is becoming established. Here, we examined the influence of embryonic microglia on gliogenesis within the developing tuberal hypothalamus, a region later important for energy balance, reproduction, and thermoregulation.

Methods

We used immunohistochemistry to quantify the location and numbers of glial cells in the embryonic brain (E13.5–E17.5), as well as a pharmacological approach (i.e., PLX5622) to knock down fetal microglia. We also conducted cytokine and chemokine analyses on embryonic brains in the presence or absence of microglia, and a neurosphere assay to test the effects of the altered cytokines on hypothalamic progenitor behaviors.

Results

We identified a subpopulation of activated microglia that congregated adjacent to the third ventricle alongside embryonic Olig2+ neural progenitor cells (NPCs) that are destined to give rise to oligodendrocyte and astrocyte populations. In the absence of microglia, we observed an increase in Olig2+ glial progenitor cells that remained at the ventricle by E17.5 and a concomitant decrease of these Olig2+ cells in the mantle zone, indicative of a delay in migration of these precursor cells. A further examination of maturing oligodendrocytes in the hypothalamic grey and white matter area in the absence of microglia revealed migrating oligodendrocyte progenitor cells (OPCs) within the grey matter at E17.5, a time point when OPCs begin to slow their migration. Finally, quantification of cytokine and chemokine signaling in ex vivo E15.5 hypothalamic cultures +/− microglia revealed decreases in the protein levels of several cytokines in the absence of microglia. We assayed the influence of two downregulated cytokines (CCL2 and CXCL10) on neurosphere-forming capacity and lineage commitment of hypothalamic NPCs in culture and showed an increase in NPC proliferation as well as neuronal and oligodendrocyte differentiation.

Conclusion

These data demonstrate that microglia influence gliogenesis in the developing tuberal hypothalamus.

Role of Glia in the Regulation of Sleep in Health and Disease

Sleep is a naturally occurring physiological state that is required to sustain physical and mental health. Traditionally viewed as strictly regulated by top-down control mechanisms, sleep is now known to also originate locally. Glial cells are emerging as important contributors to the regulation of sleep-wake cycles, locally and among dedicated neural circuits. A few pioneering studies revealed that astrocytes and microglia may influence sleep pressure, duration as well as intensity, but the precise involvement of these two glial cells in the regulation of sleep remains to be fully addressed, across contexts of health and disease. In this overview article, we will first summarize the literature pertaining to the role of astrocytes and microglia in the regulation of sleep under normal physiological conditions. Afterward, we will discuss the beneficial and deleterious consequences of glia-mediated neuroinflammation, whether it is acute, or chronic and associated with brain diseases, on the regulation of sleep. Sleep disturbances are a main comorbidity in neurodegenerative diseases, and in several brain diseases that include pain, epilepsy, and cancer. Identifying the relationships between glia-mediated neuroinflammation, sleep-wake rhythm disruption and brain diseases may have important implications for the treatment of several disorders. © 2020 American Physiological Society. Compr Physiol 10:687-712, 2020.

Silencing the Cytoskeleton Protein Iba1 (Ionized Calcium Binding Adapter Protein 1) Interferes with BV2 Microglia Functioning

Iba1 (ionized calcium binding adapter protein 1) is a cytoskeleton protein specific only for microglia and macrophages, where it acts as an actin-cross linking protein. Although frequently regarded as a marker of activation, its involvement in cell migration, membrane ruffling, phagocytosis or in microglia remodeling during immunological surveillance of the brain suggest that Iba1 is not a simple cytoskeleton protein, but a signaling molecule involved in specific signaling pathways. In this study we investigated if Iba1 could also represent a drug target, and tested the hypothesis that its specific silencing with customized Iba1-siRNA can modulate microglia functioning. The results showed that Iba1-silenced BV2 microglia migrate less due to reduced proliferation and cell adhesion, while their phagocytic activity and P2x7 functioning was significantly increased. Our data are the proof of concept that Iba1 protein is a new microglia target, which opens a new therapeutic avenue for modulating microglia behavior.

Carnosine Suppresses Human Colorectal Cell Migration and Intravasation by Regulating EMT and MMP Expression

Carnosine is an endogenous dipeptide found in the vertebrate skeletal muscles that is usually obtained through the diet. To investigate the mechanism by which carnosine regulates the migration and intravasation of human colorectal cancer (CRC) cells, we used cultured HCT-116 cells as an experimental model in this study. We examined HCT-116 cell migratory and intravasive abilities and expression of epithelial-mesenchymal transition (EMT)-associated molecules and matrix metalloproteinases (MMPs) after carnosine treatment. The results showed that both migration and invasion were inhibited in cells treated with carnosine. We found significant decreases in Twist-1 protein levels and increases in E-cadherin protein levels in HCT-116 cells after carnosine exposure. Although plasminogen activator (uPA) and MMP-9 mRNA and protein levels were decreased, TIMP-1 mRNA and protein levels were increased. Furthermore, the cytosolic levels of phosphorylated I κ B (p-I κ B) and NF- κ B DNA-binding activity were reduced after carnosine treatment. These results indicate that carnosine inhibits the migration and intravasation of human CRC cells. The regulatory mechanism may occur by suppressing NF- κ B activity and modulating MMP and EMT-related gene expression in HCT-116 cells.

Microglia: Brain cells on the move

In the last decade, tremendous progress has been made in understanding the biology of microglia - i.e. the fascinating immigrated resident immune cell population of the central nervous system (CNS). Recent literature reviews have largely dealt with the plentiful functions of microglia in CNS homeostasis, development and pathology, and the influences of sex and the microbiome. In this review, the intriguing aspect of their physical plasticity during CNS development will get specific attention. Microglia move around (mobility) and reshape their processes (motility). Microglial migration into and inside the CNS is most prominent throughout development and consequently most of the data described in this review concern mobility and motility in the changing environment of the developing brain. Here, we first define microglia based on their highly specialized age- and region-dependent gene expression signature and associated functional heterogeneity. Next, we describe their origin, the migration route of immature microglial cells towards the CNS, the mechanisms underlying their invasion of the CNS, and their spatiotemporal localization and surveying behaviour inside the developing CNS. These processes are dependent on microglial mobility and motility which are determined by the microenvironment of the CNS. Therefore, we further zoom in on the changing environment during CNS development. We elaborate on the extracellular matrix and the respective integrin receptors on microglia and we discuss the purinergic and molecular signalling in microglial mobility. In the last section, we discuss the physiological and pathological functions of microglia in which mobility and motility are involved to stress the importance of microglial 'movement'.

Microglia Responses to Pro-inflammatory Stimuli (LPS, IFNγ+TNFα) and Reprogramming by Resolving Cytokines (IL-4, IL-10)

Microglia respond to CNS injuries and diseases with complex reactions, often called "activation." A pro-inflammatory phenotype (also called classical or M1 activation) lies at one extreme of the reactivity spectrum. There were several motivations for this study. First, bacterial endotoxin (lipopolysaccharide, LPS) is the most commonly used pro-inflammatory stimulus for microglia, both in vitro and in vivo; however, pro-inflammatory cytokines (e.g., IFNγ, TNFα) rather than LPS will be encountered with sterile CNS damage and disease. We lack direct comparisons of responses between LPS and such cytokines. Second, while transcriptional profiling is providing substantial data on microglial responses to LPS, these studies mainly use mouse cells and models, and there is increasing evidence that responses of rat microglia can differ. Third, the cytokine milieu is dynamic after acute CNS damage, and an important question in microglial biology is: How malleable are their responses? There are very few studies of effects of resolving cytokines, particularly for rat microglia, and much of the work has focused on pro-inflammatory outcomes. Here, we first exposed primary rat microglia to LPS or to IFNγ+TNFα (I+T) and compared hallmark functional (nitric oxide production, migration) and molecular responses (almost 100 genes), including surface receptors that can be considered part of the sensome. Protein changes for exemplary molecules were also quantified: ARG1, CD206/MRC1, COX-2, iNOS, and PYK2. Despite some similarities, there were notable differences in responses to LPS and I+T. For instance, LPS often evoked higher pro-inflammatory gene expression and also increased several anti-inflammatory genes. Second, we compared the ability of two anti-inflammatory, resolving cytokines (IL-4, IL-10), to counteract responses to LPS and I+T. IL-4 was more effective after I+T than after LPS, and IL-10 was surprisingly ineffective after either stimulus. These results should prove useful in modeling microglial reactivity in vitro; and comparing transcriptional responses to sterile CNS inflammation in vivo.

Amyotrophic lateral sclerosis as a protein level, non-genomic disease: Therapy with S2RM exosome released molecules

Amyotrophic lateral sclerosis (ALS) is a rapidly progressing neurodegenerative disease that leads to death. No effective treatments are currently available. Based on data from epidemiological, etiological, laboratory, and clinical studies, I offer a new way of thinking about ALS and its treatment. This paper describes a host of extrinsic factors, including the exposome, that disrupt the extracellular matrix and protein function such that a spreading, prion-like disease leads to neurodegeneration in the motor tracts. A treatment regimen is described using the stem cell released molecules from a number of types of adult stem cells to provide tissue dependent molecules that restore homeostasis, including proteostasis, in the ALS patient. Because stem cells themselves as a therapeutic are cumbersome and expensive, and when implanted in a host cause aging of the host tissue and often fail to engraft or remain viable, only the S2RM molecules are used. Rebuilding of the extracellular matrix and repair of the dysfunctional proteins in the ALS patient ensues.

Microglia express ABI3 in the brains of Alzheimer's disease and Nasu-Hakola disease

Nasu-Hakola disease (NHD) is a rare autosomal recessive leukoencephalopathy caused by a loss-of-function mutation of either TYROBP (DAP12) or TREM2 expressed in microglia. A rare variant of the TREM2 gene encoding p.Arg47His causes a 3-fold increase in the risk for late-onset Alzheimer's disease (LOAD). A recent study demonstrated that a rare coding variant p.Ser209Phe in the ABI family member 3 (ABI3) gene, a regulator of actin cytoskeleton organization, confers risk of developing of LOAD, although the pattern of ABI3 expression in AD and NHD brains with relevance to microglial pathology remains to be characterized. We investigated the cell type-specific expression of ABI3 in the brains derived from four non-neurological controls (NC), ten AD and five NHD cases by immunohistochemistry. We identified an intense ABI3 immunoreactivity chiefly on a subset of microglia with ramified or amoeboid morphology located in the grey matter and the white matter of the frontal cortex and the hippocampus of NC, AD, and NHD cases. The immunolabeled area of ABI3-positive microglia was not significantly different among NC, AD, and NHD cases due to great variability from case to case. The clusters of ABI3-immunoreactive microglia were found exclusively in AD brains and they were associated with amyloid plaques. Although these observations do not actively support the view that ABI3-immunoreactive microglia play a central role in the development of leukoencephalopathy in NHD brains and the neurodegeneration in AD brains, the intense expression of ABI3 on microglia might regulate their migration under conditions of health and disease in the central nervous system (CNS).

Responses of rat and mouse primary microglia to pro- and anti-inflammatory stimuli: molecular profiles, K+ channels and migration

BACKGROUND

Acute CNS damage is commonly studied using rat and mouse models, but increasingly, molecular analysis is finding species differences that might affect the ability to translate findings to humans. Microglia can undergo complex molecular and functional changes, often studied by in vitro responses to discrete activating stimuli. There is considerable evidence that pro-inflammatory (M1) activation can exacerbate tissue damage, while anti-inflammatory (M2) states help resolve inflammation and promote tissue repair. However, in assessing potential therapeutic targets for controlling inflammation, it is crucial to determine whether rat and mouse microglia respond the same.

METHODS

Primary microglia from Sprague-Dawley rats and C57BL/6 mice were cultured, then stimulated with interferon-γ + tumor necrosis factor-α (I + T; M1 activation), interleukin (IL)-4 (M2a, alternative activation), or IL-10 (M2c, acquired deactivation). To profile their activation responses, NanoString was used to monitor messenger RNA (mRNA) expression of numerous pro- and anti-inflammatory mediators, microglial markers, immunomodulators, and other molecules. Western analysis was used to measure selected proteins. Two potential targets for controlling inflammation-inward- and outward-rectifier K⁺ channels (Kir2.1, Kv1.3)-were examined (mRNA, currents) and specific channel blockers were applied to determine their contributions to microglial migration in the different activation states.

RESULTS

Pro-inflammatory molecules increased after I + T treatment but there were several qualitative and quantitative differences between the species (e.g., iNOS and nitric oxide, COX-2). Several molecules commonly associated with an M2a state differed between species or they were induced in additional activation states (e.g., CD206, ARG1). Resting levels and/or responses of several microglial markers (Iba1, CD11b, CD68) differed with the activation state, species, or both. Transcripts for several Kir2 and Kv1 family members were detected in both species. However, the current amplitudes (mainly Kir2.1 and Kv1.3) depended on activation state and species. Treatment-induced changes in morphology and migratory capacity were similar between the species (migration reduced by I + T, increased by IL-4 or IL-10). In both species, Kir2.1 block reduced migration and Kv1.3 block increased it, regardless of activation state; thus, these channels might affect microglial migration to damage sites.

CONCLUSIONS

Caution is recommended in generalizing molecular and functional responses of microglia to activating stimuli between species.

Transcriptomic analysis of purified human cortical microglia reveals age-associated changes

Microglia are essential for CNS homeostasis and innate neuroimmune function, and play important roles in neurodegeneration and brain aging. Here we present gene expression profiles of purified microglia isolated at autopsy from the parietal cortex of 39 human subjects with intact cognition. Overall, genes expressed by human microglia were similar to those in mouse, including established microglial genes CX3CR1, P2RY12 and ITGAM (CD11B). However, a number of immune genes, not identified as part of the mouse microglial signature, were abundantly expressed in human microglia, including TLR, Fcγ and SIGLEC receptors, as well as TAL1 and IFI16, regulators of proliferation and cell cycle. Age-associated changes in human microglia were enriched for genes involved in cell adhesion, axonal guidance, cell surface receptor expression and actin (dis)assembly. Limited overlap was observed in microglial genes regulated during aging between mice and humans, indicating that human and mouse microglia age differently.

Integrin-Kindlin3 requirements for microglial motility in vivo are distinct from those for macrophages

Microglia play a critical role in the development and homeostasis of the CNS. While mobilization of microglia is critical for a number of pathologies, understanding of the mechanisms of their migration in vivo is limited and often based on similarities to macrophages. Kindlin3 deficiency as well as Kindlin3 mutations of integrin-binding sites abolish both integrin inside-out and outside-in signaling in microglia, thereby resulting in severe deficiencies in cell adhesion, polarization, and migration in vitro, which are similar to the defects observed in macrophages. In contrast, while Kindlin3 mutations impaired macrophage mobilization in vivo, they had no effect either on the population of microglia in the CNS during development or on mobilization of microglia and subsequent microgliosis in a model of multiple sclerosis. At the same time, acute microglial response to laser-induced injury was impaired by the lack of Kindlin3-integrin interactions. Based on 2-photon imaging of microglia in the brain, Kindlin3 is required for elongation of microglial processes toward the injury site and formation of phagosomes in response to brain injury. Thus, while Kindlin3 deficiency in human subjects is not expected to diminish the presence of microglia within CNS, it might delay the recovery process after injury, thereby exacerbating its complications.

Intramembranous processing by γ-secretase regulates reverse signaling of ephrin-B2 in migration of microglia

The Eph-ephrin system plays pivotal roles in cell adhesion and migration. The receptor-like functions of the ephrin ligands allow the regulation of intracellular processes via reverse signaling. γ-Secretase mediated processing of ephrin-B has previously been linked to activation of Src, a kinase crucial for focal adhesion and podosome phosphorylation. Here, we analyzed the role of γ-secretase in the stimulation of reverse ephrin-B2 signaling in the migration of mouse embryonic stem cell derived microglia. The proteolytic generation of the ephrin-B2 intracellular domain (ICD) by γ-secretase stimulates Src and focal adhesion kinase (FAK). Inhibition of γ-secretase decreased the phosphorylation of Src and FAK, and reduced cell motility. These effects were associated with enlargement of the podosomal surface. Interestingly, expression of ephrin-B2 ICD could rescue these effects, indicating that this proteolytic fragment mediates the activation of Src and FAK, and thereby regulates podosomal dynamics in microglial cells. Together, these results identify γ-secretase as well as ephrin-B2 as regulators of microglial migration.

2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside suppresses human colorectal cancer cell metastasis through inhibiting NF-κB activation

2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (THSG), a major component of Polygonum multiflorum Thunb (He-Shou-Wu), has been reported to exhibit antioxidant and anti-inflammatory effects. However, its anti-metastatic effect against colorectal cancer is still unclear. In this study, cell migration, invasion and adhesion abilities as well as metastasis-associated protein and NF-κB pathway signaling factor expression were analyzed after treating HT-29 cells with THSG. According to the results, the migration and invasiveness of HT-29 cells were reduced after treatment with 5 or 10 mM THSG (p<0.05). Additionally, the levels of matrix metalloproteinase-2 (MMP-2) and phosphorylated VE-cadherin in HT-29 cells were reduced and the transepithelial electrical resistance (TEER) of EA.hy926 endothelial cell monolayers was increased after incubation in THSG for 24 h (p<0.05). Cell adhesion ability and the E-selectin and intercellular adhesion molecule-1 (ICAM-1) protein levels were reduced when EA.hy926 endothelial cells were treated with THSG (p<0.05). In addition, the cytoplasmic phosphorylation of IκB, the nuclear p65 level and the DNA-binding activity of NF-κB were reduced after treating HT-29 or EA.hy926 cells with 5 or 10 mM THSG (p<0.05). These results suggest that THSG inhibits HT-29 cell metastasis by suppressing cell migration, invasion and adhesion. Furthermore, THSG inhibits metastasis-associated protein expression by suppressing NF-κB pathway activation.

Effects of Panax notoginseng on the Metastasis of Human Colorectal Cancer Cells

The goal of this study was to investigate the effect of the Panax notoginseng ethanol extract (PNEE) on the regulation of human colorectal cancer (CRC) metastasis. The migratory, invasive, and adhesive abilities and the expression of metastasis-associated regulatory molecules in cultured human CRC cells (HCT-116) treated with the PNEE were analyzed in this study. The migratory and invasive abilities of HCT-116 cells were reduced after PNEE treatment. The incubation of HCT-116 cells with the PNEE for 24 h decreased MMP-9 expression and increased E-cadherin expression compared with the control group. The adhesion reaction assay indicated that treatment with the PNEE led to significantly decreased HCT-116 adhesion to endothelial cells (EA.hy926 cells). The integrin-1 protein levels in HCT-116 cells were significantly decreased following treatment with the PNEE. Similarly, the protein levels of E-selectin and intercellular adhesion molecule-1 (ICAM-1) were significantly decreased by treatment of the EA.hy926 endothelial cells with PNEE. A scanning electron microscope (SEM) examination indicated that HCT-116 cells treated with LPS combined with the PNEE had a less flattened and retracted shape compared with LPS-treated cells, and this change in shape was found to be a phenomenon of extravasation invasion. The transepithelial electrical resistance (TEER) of the EA.hy926 endothelial cell monolayer increased after incubation with the PNEE for 24 h. A cell-cell permeability assay indicated that HCT-116 cells treated with the PNEE displayed significantly reduced levels of phosphorylated VE-cadherin (p-VE-cadherin). These results demonstrate the antimetastatic properties of the PNEE and show that the PNEE affects cells by inhibiting cell migration, invasion, and adhesion and regulating the expression of metastasis-associated signaling molecules.

Enhancement of Chemotactic Cell Aggregation by Haptotactic Cell-To-Cell Interaction

The crawling of biological cell is a complex phenomenon involving various biochemical and mechanical processes. Some of these processes are intrinsic to individual cells, while others pertain to cell-to-cell interactions and to their responses to extrinsically imposed cues. Here, we report an interesting aggregation dynamics of mathematical model cells, when they perform chemotaxis in response to an externally imposed global chemical gradient while they influence each other through a haptotaxis-mediated social interaction, which confers intriguing trail patterns. In the absence of the cell-to-cell interaction, the equilibrium population density profile fits well to that of a simple Keller-Segal population dynamic model, in which a chemotactic current density J→chemo∼∇p competes with a normal diffusive current density J→diff∼∇ρ, where p and ρ refer to the concentration of chemoattractant and population density, respectively. We find that the cell-to-cell interaction confers a far more compact aggregation resulting in a much higher peak equilibrium cell density. The mathematical model system is applicable to many biological systems such as swarming microglia and neutrophils or accumulating ants towards a localized food source.

Calmodulin inhibition regulates morphological and functional changes related to the actin cytoskeleton in pure microglial cells

The roles of calmodulin (CaM), a multifunctional intracellular calcium receptor protein, as concerns selected morphological and functional characteristics of pure microglial cells derived from mixed primary cultures from embryonal forebrains of rats, were investigated through use of the CaM antagonists calmidazolium (CALMID) and trifluoperazine (TFP). The intracellular localization of the CaM protein relative to phalloidin, a bicyclic heptapeptide that binds only to filamentous actin, and the ionized calcium-binding adaptor molecule 1 (Iba1), a microglia-specific actin-binding protein, was determined by immunocytochemistry, with quantitative analysis by immunoblotting. In unchallenged and untreated (control) microglia, high concentrations of CaM protein were found mainly perinuclearly in ameboid microglia, while the cell cortex had a smaller CaM content that diminished progressively deeper into the branches in the ramified microglia. The amounts and intracellular distributions of both Iba1 and CaM proteins were altered after lipopolysaccharide (LPS) challenge in activated microglia. CALMID and TFP exerted different, sometimes opposing, effects on many morphological, cytoskeletal and functional characteristics of the microglial cells. They affected the CaM and Iba1 protein expressions and their intracellular localizations differently, inhibited cell proliferation, viability and fluid-phase phagocytosis to different degrees both in unchallenged and in LPS-treated (immunologically challenged) cells, and differentially affected the reorganization of the actin cytoskeleton in the microglial cell cortex, influencing lamellipodia, filopodia and podosome formation. In summary, these CaM antagonists altered different aspects of filamentous actin-based cell morphology and related functions with variable efficacy, which could be important in deciphering the roles of CaM in regulating microglial functions in health and disease.

Podosomes in space: macrophage migration and matrix degradation in 2D and 3D settings

Migration of macrophages is a key process for a variety of physiological functions, such as pathogen clearance or tissue homeostasis. However, it can also be part of pathological scenarios, as in the case of tumor-associated macrophages. This review presents an overview of the different migration modes macrophages can adopt, depending on the physical and chemical properties of specific environments, and the constraints they impose upon cells. We discuss the importance of these environmental and also of cellular parameters, as well as their relative impact on macrophage migration and on the formation of matrix-lytic podosomes in 2D and 3D. Moreover, we present an overview of routinely used and also newly developed assays for the study of macrophage migration in both 2D and 3D contexts, their respective advantages and limitations, and also their potential to reliably mimic in vivo situations.

Expression and contributions of the Kir2.1 inward-rectifier K(+) channel to proliferation, migration and chemotaxis of microglia in unstimulated and anti-inflammatory states

When microglia respond to CNS damage, they can range from pro-inflammatory (classical, M1) to anti-inflammatory, alternative (M2) and acquired deactivation states. It is important to determine how microglial functions are affected by these activation states, and to identify molecules that regulate their behavior. Microglial proliferation and migration are crucial during development and following damage in the adult, and both functions are Ca²⁺-dependent. In many cell types, the membrane potential and driving force for Ca²⁺ influx are regulated by inward-rectifier K⁺ channels, including Kir2.1, which is prevalent in microglia. However, it is not known whether Kir2.1 expression and contributions are altered in anti-inflammatory states. We tested the hypothesis that Kir2.1 contributes to Ca²⁺ entry, proliferation and migration of rat microglia. Kir2.1 (KCNJ2) transcript expression, current amplitude, and proliferation were comparable in unstimulated microglia and following alternative activation (IL-4 stimulated) and acquired deactivation (IL-10 stimulated). To examine functional roles of Kir2.1 in microglia, we first determined that ML133 was more effective than the commonly used blocker, Ba²⁺; i.e., ML133 was potent (IC₅₀ = 3.5 μM) and voltage independent. Both blockers slightly increased proliferation in unstimulated or IL-4 (but not IL-10)-stimulated microglia. Stimulation with IL-4 or IL-10 increased migration and ATP-induced chemotaxis, and blocking Kir2.1 greatly reduced both but ML133 was more effective. In all three activation states, blocking Kir2.1 with ML133 dramatically reduced Ca²⁺ influx through Ca²⁺-release-activated Ca²⁺ (CRAC) channels. Thus, Kir2.1 channel activity is necessary for microglial Ca²⁺ signaling and migration under resting and anti-inflammatory states but the channel weakly inhibits proliferation.

Expression and contributions of TRPM7 and KCa2.3/SK3 channels to the increased migration and invasion of microglia in anti-inflammatory activation states

Microglia rapidly respond to CNS injury and disease and can assume a spectrum of activation states. While changes in gene expression and production of inflammatory mediators have been extensively described after classical (LPS-induced) and alternative (IL4-induced) microglial activation, less is known about acquired de-activation in response to IL10. It is important to understand how microglial activation states affect their migration and invasion; crucial functions after injury and in the developing CNS. We reported that LPS-treated rat microglia migrate very poorly, while IL4-treated cells migrate and invade much better. Having discovered that the lamellum of migrating microglia contains a large ring of podosomes – microscopic structures that are thought to mediate adhesion, migration and invasion – we hypothesized that IL4 and IL10 would differentially affect podosome expression, gene induction, migration and invasion. Further, based on the enrichment of the KCa2.3/SK3 Ca²⁺-activated potassium channel in microglial podosomes, we predicted that it regulates migration and invasion. We found both similarities and differences in gene induction by IL4 and IL10 and, while both cytokines increased migration and invasion, only IL10 affected podosome expression. KCa2.3 currents were recorded in microglia under all three activation conditions and KCNN3 (KCa2.3) expression was similar. Surprisingly then, of three KCa2.3 inhibitors (apamin, tamapin, NS8593), only NS8593 abrogated the increased migration and invasion of IL4 and IL10-treated microglia (and invasion of unstimulated microglia). This discrepancy was explained by the observed block of TRPM7 currents in microglia by NS8593, which occurred under all three activation conditions. A similar inhibition of both migration and invasion was seen with a TRPM7 inhibitor (AA-861) that does not block KCa2.3 channels. Thus, we conclude that TRPM7 (not KCa2.3) contributes to the enhanced ability of microglia to migrate and invade when in anti-inflammatory states. This will be an important consideration in developing TRPM7 inhibitors for treating CNS injury.

IL-4 type 1 receptor signaling up-regulates KCNN4 expression, and increases the KCa3.1 current and its contribution to migration of alternative-activated microglia

The Ca²⁺-activated K⁺ channel, KCa3.1 (KCNN4/IK1/SK4), contributes to “classical,” pro-inflammatory activation of microglia, and KCa3.1 blockers have improved the outcome in several rodent models of CNS damage. For instance, blocking KCa3.1 with TRAM-34 rescued retinal ganglion neurons after optic nerve damage in vivo and, reduced p38 MAP kinase activation, production of reactive oxygen and nitrogen species, and neurotoxicity by microglia in vitro. In pursuing the therapeutic potential of KCa3.1 blockers, it is crucial to assess KCa3.1 contributions to other microglial functions and activation states, especially the IL-4-induced “alternative” activation state that can counteract pro-inflammatory states. We recently found that IL-4 increases microglia migration – a crucial function in the healthy and damaged CNS – and that KCa3.1 contributes to P₂Y₂ receptor-stimulated migration. Here, we discovered that KCa3.1 is greatly increased in alternative-activated rat microglia and then contributes to an enhanced migratory capacity. IL-4 up-regulated KCNN4 mRNA (by 6 h) and greatly increased the KCa3.1 current by 1 day, and this required de novo protein synthesis. The increase in current was sustained for at least 6 days. IL-4 increased microglial migration and this was reversed by blocking KCa3.1 with TRAM-34. A panel of inhibitors of signal-transduction mediators was used to analyze contributions of IL-4-related signaling pathways. Induction of KCNN4 mRNA and KCa3.1 current was mediated specifically through IL-4 binding to the type I receptor and, surprisingly, it required JAK3, Ras/MEK/ERK signaling and the transcription factor, activator protein-1, rather than JAK2, STAT6, or phosphatidylinositol 3-kinase.The same receptor subtype and pathway were required for the enhanced KCa3.1-dependent migration. In providing the first direct signaling link between an IL-4 receptor, expression and roles of an ion channel, this study also highlights the potential importance of KCa3.1 in alternative-activated microglia.

Podosomes in adhesion, migration, mechanosensing and matrix remodeling

Cells use various actin-based motile structures to allow them to move across and through matrix of varying density and composition. Podosomes are actin cytoskeletal structures that form in motile cells and that mediate adhesion to substrate, migration, and other specialized functions such as transmigration through cell and matrix barriers. The podosome is a unique and interesting entity, which appears in the light microscope as an individual punctum, but is linked to other podosomes like a node on a network of the underlying cytoskeleton. Here, we discuss the signals that control podosome assembly and dynamics in different cell types and the actin organising proteins that regulate both the inner actin core and integrin-rich surrounding ring structures. We review the structure and composition of podosomes and also their functions in various cell types of both myeloid and endothelial lineage. We also discuss the emerging idea that podosomes can sense matrix stiffness and enable cells to respond to their environment.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Caveolin isoform switching as a molecular, structural, and metabolic regulator of microglia

Microglia are ramified cells that serve as central nervous system (CNS) guardians, capable of proliferation, migration, and generation of inflammatory cytokines. In non-pathological states, these cells exhibit ramified morphology with processes intermingling with neurons and astrocytes. Under pathological conditions, they acquire a rounded amoeboid morphology and proliferative and migratory capabilities. Such morphological changes require cytoskeleton rearrangements. The molecular control points for polymerization states of microtubules and actin are still under investigation. Caveolins (Cavs), membrane/lipid raft proteins, are expressed in inflammatory cells, yet the role of caveolin isoforms in microglia physiology is debatable. We propose that caveolins provide a necessary control point in the regulation of cytoskeletal dynamics, and thus investigated a role for caveolins in microglia biology. We detected mRNA and protein for both Cav-1 and Cav-3. Cav-1 protein was significantly less and localized to plasmalemma (PM) and cytoplasmic vesicles (CVs) in the microglial inactive state, while the active (amoeboid-shaped) microglia exhibited increased Cav-1 expression. In contrast, Cav-3 was highly expressed in the inactive state and localized with cellular processes and perinuclear regions and was detected in active amoeboid microglia. Pharmacological manipulation of the cytoskeleton in the active or non-active state altered caveolin expression. Additionally, increased Cav-1 expression also increased mitochondrial respiration, suggesting possible regulatory roles in cell metabolism necessary to facilitate the morphological changes. The present findings strongly suggest that regulation of microglial morphology and activity are in part due to caveolin isoforms, providing promising novel therapeutic targets in CNS injury or disease.

The microglial activation state regulates migration and roles of matrix-dissolving enzymes for invasion

Background

Microglial cells are highly mobile under many circumstances and, after central nervous system (CNS) damage, they must contend with the dense extracellular matrix (ECM) in order to reach their target sites. In response to damage or disease, microglia undergo complex activation processes that can be modulated by environmental cues and culminate in either detrimental or beneficial outcomes. Thus, there is considerable interest in comparing their pro-inflammatory (‘classical’ activation) and resolving ‘alternative’ activation states. Almost nothing is known about how these activation states affect the ability of microglia to migrate and degrade ECM, or the enzymes used for substrate degradation. This is the subject of the present study.

Methods

Primary cultured rat microglial cells were exposed to lipopolysaccharide (LPS) to evoke classical activation or IL4 to evoke alternative activation. High-resolution microscopy was used to monitor changes in cell morphology and aspects of the cytoskeleton. We quantified migration in a scratch-wound assay and through open filter holes, and invasion through Matrigel™. A panel of inhibitors was used to analyze contributions of different matrix-degrading enzymes to migration and invasion, and quantitative real-time reverse transcriptase PCR (qRT-PCR) was used to assess changes in their expression.

Results

Vinculin- and F-actin-rich lamellae were prominent in untreated and IL4-treated microglia (but not after LPS). IL4 increased the migratory capacity of microglia but eliminated the preferential anterior nuclear-centrosomal axis polarity and location of the microtubule organizing center (MTOC). Microglia degraded fibronectin, regardless of treatment, but LPS-treated cells were relatively immobile and IL4-treated cells invaded much more effectively through Matrigel™. For invasion, untreated microglia primarily used cysteine proteases, but IL4-treated cells used a wider range of enzymes (cysteine proteases, cathepsin S and K, heparanase, and matrix metalloproteases). Untreated microglia expressed MMP2, MMP12, heparanase, and four cathepsins (B, K, L1, and S). Each activation stimulus upregulated a different subset of enzymes. IL4 increased MMP2 and cathepsins S and K; whereas LPS increased MMP9, MMP12, MMP14 (MT1-MMP), heparanase, and cathepsin L1.

Conclusions

Microglial cells migrate during CNS development and after CNS damage or disease. Thus, there are broad implications of the finding that classically and alternatively activated microglia differ in morphology, cytoskeleton, migratory and invasive capacity, and in the usage of ECM-degrading enzymes.

Selective activation of KCa3.1 and CRAC channels by P2Y2 receptors promotes Ca(2+) signaling, store refilling and migration of rat microglial cells

Microglial activation involves Ca(2+) signaling, and numerous receptors can evoke elevation of intracellular Ca(2+). ATP released from damaged brain cells can activate ionotropic and metabotropic purinergic receptors, and act as a chemoattractant for microglia. Metabotropic P2Y receptors evoke a Ca(2+) rise through release from intracellular Ca(2+) stores and store-operated Ca(2+) entry, and some have been implicated in microglial migration. This Ca(2+) rise is expected to activate small-conductance Ca(2+)-dependent K(+) (SK) channels, if present. We previously found that SK3 (KCa2.3) and KCa3.1 (SK4/IK1) are expressed in rat microglia and contribute to LPS-mediated activation and neurotoxicity. However, neither current has been studied by elevating Ca(2+) during whole-cell recordings. We hypothesized that, rather than responding only to Ca(2+), each channel type might be coupled to different receptor-mediated pathways. Here, our objective was to determine whether the channels are differentially activated by P2Y receptors, and, if so, whether they play differing roles. We used primary rat microglia and a rat microglial cell line (MLS-9) in which riluzole robustly activates both SK3 and KCa3.1 currents. Using electrophysiological, Ca(2+) imaging and pharmacological approaches, we show selective functional coupling of KCa3.1 to UTP-mediated P2Y2 receptor activation. KCa3.1 current is activated by Ca(2+) entry through Ca(2+)-release-activated Ca(2+) (CRAC/Orai1) channels, and both CRAC/Orai1 and KCa3.1 channels facilitate refilling of Ca(2+) stores. The Ca(2+) dependence of KCa3.1 channel activation was skewed to abnormally high concentrations, and we present evidence for a close physical association of the two channel types. Finally, migration of primary rat microglia was stimulated by UTP and inhibited by blocking either KCa3.1 or CRAC/Orai1 channels. This is the first report of selective coupling of one type of SK channel to purinergic stimulation of microglia, transactivation of KCa3.1 channels by CRAC/Orai1, and coordinated roles for both channels in store refilling, Ca(2+) signaling and microglial migration.

Microglia: key elements in neural development, plasticity, and pathology

A century after Cajal identified a "third element" of the nervous system, many issues have been clarified about the identity and function of one of its major components, the microglia. Here, we review recent findings by microgliologists, highlighting results from imaging studies that are helping provide new views of microglial behavior and function. In vivo imaging in the intact adult rodent CNS has revolutionized our understanding of microglial behaviors in situ and has raised speculation about their function in the uninjured adult brain. Imaging studies in ex vivo mammalian tissue preparations and in intact model organisms including zebrafish are providing insights into microglial behaviors during brain development. These data suggest that microglia play important developmental roles in synapse remodeling, developmental apoptosis, phagocytic clearance, and angiogenesis. Because microglia also contribute to pathology, including neurodevelopmental and neurobehavioral disorders, ischemic injury, and neuropathic pain, promising new results raise the possibility of leveraging microglia for therapeutic roles. Finally, exciting recent work is addressing unanswered questions regarding the nature of microglial-neuronal communication. While it is now apparent that microglia play diverse roles in neural development, behavior, and pathology, future research using neuroimaging techniques will be essential to more fully exploit these intriguing cellular targets for effective therapeutic intervention applied to a variety of conditions.

Regulation of podosome formation, microglial migration and invasion by Ca(2+)-signaling molecules expressed in podosomes

BACKGROUND

Microglia migrate during brain development and after CNS injury, but it is not known how they degrade the extracellular matrix (ECM) to accomplish this. Podosomes are tiny structures with the unique ability to adhere to and dissolve ECM. Podosomes have a two-part architecture: a core that is rich in F-actin and actin-regulatory molecules (for example, Arp2/3), surrounded by a ring with adhesion and structural proteins (for example, talin, vinculin). We recently discovered that the lamellum at the leading edge of migrating microglia contains a large F-actin-rich superstructure ('podonut') composed of many podosomes. Microglia that expressed podosomes could degrade ECM molecules. Finely tuned Ca(2+) signaling is important for cell migration, cell-substrate adhesion and contraction of the actomyosin network. Here, we hypothesized that podosomes contain Ca(2+)-signaling machinery, and that podosome expression and function depend on Ca(2+) influx and specific ion channels.

METHODS

High-resolution immunocytochemistry was used on rat microglia to identify podosomes and novel molecular components. A pharmacological toolbox was applied to functional assays. We analyzed roles of Ca(2+)-entry pathways and ion channels in podosome expression, microglial migration into a scratch-wound, transmigration through pores in a filter, and invasion through Matrigel™-coated filters.

RESULTS

Microglial podosomes were identified using well-known components of the core (F-actin, Arp2) and ring (talin, vinculin). We discovered four novel podosome components related to Ca(2+) signaling. The core contained calcium release activated calcium (CRAC; Orai1) channels, calmodulin, small-conductance Ca(2+)-activated SK3 channels, and ionized Ca(2+) binding adapter molecule 1 (Iba1), which is used to identify microglia in the CNS. The Orai1 accessory molecule, STIM1, was also present in and around podosomes. Podosome formation was inhibited by removing external Ca(2+) or blocking CRAC channels. Blockers of CRAC channels inhibited migration and invasion, and SK3 inhibition reduced invasion.

CONCLUSIONS

Microglia podosome formation, migration and/or invasion require Ca(2+) influx, CRAC, and SK3 channels. Both channels were present in microglial podosomes along with the Ca(2+)-regulated molecules, calmodulin, Iba1 and STIM1. These results suggest that the podosome is a hub for sub-cellular Ca(2+)-signaling to regulate ECM degradation and cell migration. The findings have broad implications for understanding migration mechanisms of cells that adhere to, and dissolve ECM.