The transition of microglia to a ramified phenotype is associated with the formation of stable acetylated and detyrosinated microtubules

Downregulation of Fidgetin-Like 2 Increases Microglial Function: The Relationship Between Microtubules, Morphology, and Activity

The microtubule cytoskeleton regulates microglial morphology, motility, and effector functions. The microtubule-severing enzyme, fidgetin-like 2 (FL2), negatively regulates cell motility and nerve regeneration, making it a promising therapeutic target for central nervous system injury. Microglia perform important functions in response to inflammation and injury, but how FL2 affects microglia is unclear. In this study, we investigated the role of FL2 in microglial morphology and injury responses in vitro. We first determined that the pro-inflammatory stimulus, lipopolysaccharide (LPS), induced a dose- and time-dependent reduction in FL2 expression associated with reduced microglial ramification. We then administered nanoparticle-encapuslated FL2 siRNA to knockdown FL2 and assess microglial functions compared to negative control siRNA and vehicle controls. Time-lapse live-cell microscopy showed that FL2 knockdown increased the velocity of microglial motility. After incubation with fluorescently labeled IgG-opsonized beads, FL2 knockdown increased phagocytosis. Microglia were exposed to low-dose LPS after nanoparticle treatment to model injury-induced cytokine secretion. FL2 knockdown enhanced LPS-induced cytokine secretion of IL-1α, IL-1β, and TNFα. These results identify FL2 as a regulator of microglial morphology and suggest that FL2 can be targeted to increase or accelerate microglial injury responses.

A cytoskeleton symphony: Actin and microtubules in microglia dynamics and aging

Microglia dynamically reorganize their cytoskeleton to perform essential functions such as phagocytosis of toxic protein aggregates, surveillance of the brain parenchyma, and regulation of synaptic plasticity during neuronal activity bursts. Recent studies have shed light on the critical role of the microtubule cytoskeleton in microglial reactivity and function, revealing key regulators like cyclin-dependent kinase 1 and centrosomal nucleation in the remodeling of microtubules in activated microglia. Concurrently, the role of the actin cytoskeleton is also pivotal, particularly in the context of small GTPases like RhoA, Rac1, and Cdc42 and actin-binding molecules such as profilin-1 and cofilin. This article delves into the intricate molecular landscape of actin and microtubules, exploring their synergistic roles in driving microglial cytoskeletal dynamics. We propose a more integrated view of actin and microtubule cooperation, which is fundamental to understanding the functional coherence of the microglial cytoskeleton and its pivotal role in propelling brain homeostasis. Furthermore, we discuss how alterations in microglial cytoskeleton dynamics during aging and in disease states could have far-reaching implications for brain function. By unraveling the complexities of microglia cytoskeletal dynamics, we can deepen our understanding of microglial functional states and their implications in health and disease, offering insights into potential therapeutic interventions for neurologic disorders.

Polarized microtubule remodeling transforms the morphology of reactive microglia and drives cytokine release

Microglial reactivity is a pathological hallmark in many neurodegenerative diseases. During stimulation, microglia undergo complex morphological changes, including loss of their characteristic ramified morphology, which is routinely used to detect and quantify inflammation in the brain. However, the underlying molecular mechanisms and the relation between microglial morphology and their pathophysiological function are unknown. Here, proteomic profiling of lipopolysaccharide (LPS)-reactive microglia identifies microtubule remodeling pathways as an early factor that drives the morphological change and subsequently controls cytokine responses. We find that LPS-reactive microglia reorganize their microtubules to form a stable and centrosomally-anchored array to facilitate efficient cytokine trafficking and release. We identify cyclin-dependent kinase 1 (Cdk-1) as a critical upstream regulator of microtubule remodeling and morphological change in-vitro and in-situ. Cdk-1 inhibition also rescues tau and amyloid fibril-induced morphology changes. These results demonstrate a critical role for microtubule dynamics and reorganization in microglial reactivity and modulating cytokine-mediated inflammatory responses.

Special issue on "Cytoskeletal Proteins in Health and Neurodegenerative Disease: Concepts and Methods"

Since 2016, when we compiled a very well-received special issue on "Cytoskeletal Proteins in Health and Neurodegenerative Disease" for Brain Research Bulletin, the field has rapidly evolved, to a large part thanks to the development and maturation of new methods including super-resolution microscopy. Being asked to create a sequel, we therefore decided to keep the main topic, but focus on emerging concepts and novel methods. As before, we compiled nine articles on the role of the neuronal cytoskeleton in both physiological and pathological conditions. Seven of the contributions present current concepts and discuss how cytoskeletal components develop and are maintained throughout a neuron's long lifespan, and also, how they may contribute to physiology and neurodegenerative diseases. Two contributions focus on novel methodological developments and how these techniques can be used to analyze the structure and function of the neuronal cytoskeleton in new ways. The compilation of the articles makes it clear that future approaches must consider the functional relationships between the individual filament systems and the influence different signal transduction mechanisms have on the cytoskeleton and vice versa, in order to adequately explore the causes and consequences of the role of cytoskeletal proteins in health and disease. We hope that this compilation will help in the design of appropriate experiments, aided by new methods, to test critical hypotheses in the field.

Microglia reactivity entails microtubule remodeling from acentrosomal to centrosomal arrays

Microglia reactivity entails a large-scale remodeling of cellular geometry, but the behavior of the microtubule cytoskeleton during these changes remains unexplored. Here we show that activated microglia provide an example of microtubule reorganization from a non-centrosomal array of parallel and stable microtubules to a radial array of more dynamic microtubules. While in the homeostatic state, microglia nucleate microtubules at Golgi outposts, and activating signaling induces recruitment of nucleating material nearby the centrosome, a process inhibited by microtubule stabilization. Our results demonstrate that a hallmark of microglia reactivity is a striking remodeling of the microtubule cytoskeleton and suggest that while pericentrosomal microtubule nucleation may serve as a distinct marker of microglia activation, inhibition of microtubule dynamics may provide a different strategy to reduce microglia reactivity in inflammatory disease.

Microtubule organization and dynamics in oligodendrocytes, astrocytes, and microglia

Though much is known about microtubule organization and microtubule-based transport in neurons, the development and function of microtubules in glia are more enigmatic. In this review, we provide an overview of the literature on microtubules in ramified brain cells, including oligodendrocytes, astrocytes, and microglia. We focus on normal cell biology-how structure relates to function in these cells. In oligodendrocytes, microtubules are important for extension of processes that contact axons and for elongating the myelin sheath. Recent studies demonstrate that new microtubules can form outside of the oligodendrocyte cell body off of Golgi outpost organelles. In astrocytes and microglia, changes in cell shape and ramification can be influenced by neighboring cells and the extracellular milieu. Finally, we highlight key papers implicating glial microtubule defects in neurological injury and disease and discuss how microtubules may contribute to invasiveness in gliomas. Thus, future research on the mechanisms underlying microtubule organization in normal glial cell function may yield valuable insights on neurological disease pathology.

Quantifying the Local Mechanical Properties of Cells in a Fibrous Three-Dimensional Microenvironment

Measurements of the mechanical response of biological cells are critical for understanding injury and disease, for developing diagnostic tools, and for computational models in mechanobiology. Although it is well known that cells are sensitive to the topography of their microenvironment, the current paradigm in mechanical testing of adherent cells is mostly limited to specimens grown on flat two-dimensional substrates. In this study, we introduce a technique in which cellular indentation via optical trapping is performed on cells at a high spatial resolution to obtain their regional mechanical properties while they exist in a more favorable three-dimensional microenvironment. We combine our approach with nonlinear contact mechanics theory to consider the effects of a large deformation. This allows us to probe length scales that are relevant for obtaining overall cell stiffness values. The experimental results herein provide the hyperelastic material properties at both high (∼100 s^-1) and low (∼1-10 s^-1) strain rates of murine central nervous system glial cells. The limitations due to possible misalignment of the indenter in the three-dimensional space are examined using a computational model.

Epothilone B Benefits Nigral Dopaminergic Neurons by Attenuating Microglia Activation in the 6-Hydroxydopamine Lesion Mouse Model of Parkinson's Disease

Parkinson’s disease (PD) is characterized by loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and a subsequent reduction in striatal DA levels. Recent studies have shown that systemic administration of subtoxic doses of epothilone B (EpoB), a microtubule stabilizing agent, enhances axonal regeneration. However, the underlying alterations in cellular mechanisms remain undetermined. In the present study, we investigated the neuroprotective effects of EpoB on DA neurons in mouse model of PD induced by 6-hydroxyDA (6-OHDA) and in vitro. The results indicated that EpoB improved behavioral deficits, protected the nigrostriatal dopaminergic projections and restored DA level in the striatum of mice exposed to 6-OHDA. Meanwhile, EpoB attenuated microglia activation in the SNc of PD mice. Furthermore, EpoB treatment ameliorated 6-OHDA induced cytotoxicity to MN9D dopaminergic cells in a co-culture transwell system of BV2/MN9D cells, and redistributed the cytoskeleton of microglial BV2 and caused the morphological transition, inhibited the polarization to the M1 phenotype by suppressing expression of pro-inflammatory factors including interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α. Overall, our study suggested that EpoB treatment protects nigral DA neurons and projections through limiting the cytotoxicity of activated microglia in 6-OHDA lesioned mice.

Epothilone D inhibits microglia-mediated spread of alpha-synuclein aggregates

Multiple System Atrophy (MSA) is a progressive neurodegenerative disease characterized by chronic neuroinflammation and widespread α-synuclein (α-syn) cytoplasmic inclusions. Neuroinflammation associated with microglial cells is typically located in brain regions with α-syn deposits. The potential link between microglial cell migration and the transport of pathological α-syn protein in MSA was investigated. Qualitative analysis via immunofluorescence of MSA cases (n = 4) revealed microglial cells bearing α-syn inclusions distal from oligodendrocytes bearing α-syn cytoplasmic inclusions, as well as close interactions between microglia and oligodendrocytes bearing α-syn, suggestive of a potential transfer mechanism between microglia and α-syn bearing cells in MSA and the possibility of microglia acting as a mobile vehicle to spread α-syn between anatomically connected brain regions. Further In vitro experiments using microglial-like differentiated THP-1 cells were conducted to investigate if microglial cells could act as potential transporters of α-syn. Monomeric or aggregated α-syn was immobilized at the centre of glass coverslips and treated with either cell free medium, undifferentiated THP-1 cells or microglial-like phorbol-12-myristate-13-acetate differentiated THP-1 cells (48 h; n = 3). A significant difference in residual immobilized α-syn density was observed between cell free controls and differentiated (p = 0.016) as well as undifferentiated and differentiated THP-1 cells (p = 0.032) when analysed by quantitative immunofluorescence. Furthermore, a significantly greater proportion of differentiated cells were observed bearing α-syn aggregates distal from the immobilized protein than their non-differentiated counterparts (p = 0.025). Similar results were observed with Highly Aggressive Proliferating Immortalised (HAPI) microglial cells, with cells exposed to aggregated α-syn yielding lower residual immobilized α-syn (p = 0.004) and a higher proportion of α-syn positive distal cells (p = 0.001) than cells exposed to monomeric α-syn. Co-treatment of THP-1 groups with the tubulin depolymerisation inhibitor, Epothilone D (EpoD; 10 nM), was conducted to investigate if inhibition of microtubule activity had an effect on cell migration and residual immobilized α-syn density. There was a significant increase in both residual immobilized α-syn between EpoD treated and non-treated differentiated cells exposed to monomeric (p = 0.037) and aggregated (p = 0.018) α-syn, but not with undifferentiated cells. Differentiated THP-1 cells exposed to immobilized aggregated α-syn showed a significant difference in the proportion of distal aggregate bearing cells between EpoD treated and untreated (p = 0.027). The results suggest microglia could play a role in α-syn transport in MSA, a role which could potentially be inhibited therapeutically by EpoD.

Microtubule dynamics and the neurodegenerative triad of Alzheimer's disease: The hidden connection

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is, on a histopathological level, characterized by the presence of extracellular amyloid plaques composed of the protein fragment Aβ, and intracellular neurofibrillary tangles, which contain the microtubule-associated protein tau in a hyperphosphorylated state. In AD defects in microtubule (MT) assembly and organization have also been reported; however, it is unclear whether MT abnormalities have a causal and early role in the disease process or represent a common end point downstream of the neurodegenerative cascade. Recent evidence indicates that microtubule-stabilizing drugs prevent axonopathy in animal models of tauopathies and reverse Aβ-induced loss of synaptic connectivity in an ex vivo model of amyloidosis. This could suggest that MT dysfunction connects some of the degenerative events and provides a useful target to simultaneously prevent several neurodegenerative processes in AD. Here, we describe how changes in the structure and dynamics of MTs are involved in the different aspects of the neurodegenerative triad of AD. We discuss evidence that MTs are affected both by tau-dependent and tau-independent mechanisms but appear to be regulated in a distinct way in different neuronal compartments. We argue that modulation of MT dynamics could be of potential benefit but needs to be precisely controlled in a cell and compartment-specific manner to avoid harmful side effects. This article is part of the series "Beyond Amyloid".

Glial Tau Pathology in Tauopathies: Functional Consequences

Tauopathies are a class of neurodegenerative diseases characterized by the presence of hyperphosphorylated and aggregated tau pathology in neuronal and glial cells. Though the ratio of neuronal and glial tau aggregates varies across diseases, glial tau aggregates can populate the same degenerating brain regions as neuronal tau aggregates. While much is known about the deleterious consequences of tau pathology in neurons, the relative contribution of glial tau pathology to these diseases is less clear. Recent studies using a number of model systems implicate glial tau pathology in contributing to tauopathy pathogenesis. This review aims to highlight the functional consequences of tau overexpression in glial cells and explore the potential contribution of glial tau pathology in the pathogenesis of neurodegenerative tauopathies.

Functional diversity of microglia - how heterogeneous are they to begin with?

Microglia serve in the surveillance and maintenance, protection and restoration of the central nervous system (CNS) homeostasis. By their parenchymal location they differ from other CNS-associated myeloid cells, and by origin as well as functional characteristics they are also–at least in part–distinct from extraneural tissue macrophages. Nevertheless, microglia themselves may not comprise a uniform cell type. CNS regions vary by cellular and chemical composition, including white matter (myelin) content, blood–brain barrier properties or prevailing neurotransmitters. Such a micromilieu could instruct as well as require local adaptions of microglial features. Yet even cells within circumscribed populations may reveal some specialization by subtypes, regarding house-keeping duties and functional capacities upon challenges. While diversity of reactive phenotypes has been established still little is known as to whether all activated cells would respond with the same program of induced genes and functions or whether responder subsets have individual contributions. Preferential synthesis of a key cytokine could asign a master control to certain cells among a pool of activated microglia. Critical functions could be sequestered to discrete microglial subtypes in order to avoid interference, such as clearance of endogenous material and presentation of antigens. Indeed, several and especially a number of recent studies provide evidence for the constitutive and reactive heterogeneity of microglia by and within CNS regions. While such a principle of “division of labor” would influence the basic notion of “the” microglia, it could come with the practival value of addressing separate microglia types in experimental and therapeutic manipulations.

Transcriptome analysis of amoeboid and ramified microglia isolated from the corpus callosum of rat brain

Background

Microglia, the resident immune cells of the central nervous system (CNS), have two distinct phenotypes in the developing brain: amoeboid form, known to be amoeboid microglial cells (AMC) and ramified form, known to be ramified microglial cells (RMC). The AMC are characterized by being proliferative, phagocytic and migratory whereas the RMC are quiescent and exhibit a slow turnover rate. The AMC transform into RMC with advancing age, and this transformation is indicative of the gradual shift in the microglial functions. Both AMC and RMC respond to CNS inflammation, and they become hypertrophic when activated by trauma, infection or neurodegenerative stimuli. The molecular mechanisms and functional significance of morphological transformation of microglia during normal development and in disease conditions is not clear. It is hypothesized that AMC and RMC are functionally regulated by a specific set of genes encoding various signaling molecules and transcription factors.

Results

To address this, we carried out cDNA microarray analysis using lectin-labeled AMC and RMC isolated from frozen tissue sections of the corpus callosum of 5-day and 4-week old rat brain respectively, by laser capture microdissection. The global gene expression profiles of both microglial phenotypes were compared and the differentially expressed genes in AMC and RMC were clustered based on their functional annotations. This genome wide comparative analysis identified genes that are specific to AMC and RMC.

Conclusions

The novel and specific molecules identified from the trancriptome explains the quiescent state functioning of microglia in its two distinct morphological states.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Lipid rafts in mast cell biology

Mast cells have long been recognized to have a direct and critical role in allergic and inflammatory reactions. In allergic diseases, these cells exert both local and systemic responses, including allergic rhinitis and anaphylaxis. Mast cell mediators are also related to many chronic inflammatory conditions. Besides the roles in pathological conditions, the biological functions of mast cells include roles in innate immunity, involvement in host defense mechanisms against parasites, immunomodulation of the immune system, tissue repair, and angiogenesis. Despite their growing significance in physiological and pathological conditions, much still remains to be learned about mast cell biology. This paper presents evidence that lipid rafts or raft components modulate many of the biological processes in mast cells, such as degranulation and endocytosis, play a role in mast cell development and recruitment, and contribute to the overall preservation of mast cell structure and organization.

Gangliosides are important for the preservation of the structure and organization of RBL-2H3 mast cells

Gangliosides are known to be important in many biological processes. However, details concerning the exact function of these glycosphingolipids in cell physiology are poorly understood. In this study, the role of gangliosides present on the surface of rodent mast cells in maintaining cell structure was examined using RBL-2H3 mast cells and two mutant cell lines (E5 and D1) deficient in the gangliosides, GM(1) and the alpha-galactosyl derivatives of the ganglioside GD(1b). The two deficient cell lines were morphologically different from each other as well as from the parental RBL-2H3 cells. Actin filaments in RBL-2H3 and E5 cells were under the plasma membrane following the spindle shape of the cells, whereas in D1 cells, they were concentrated in large membrane ruffles. Microtubules in RBL-2H3 and E5 cells radiated from the centrosome and were organized into long, straight bundles. The bundles in D1 cells were thicker and organized circumferentially under the plasma membrane. The endoplasmic reticulum, the Golgi complex, and the secretory granule matrix were also altered in the mutant cell lines. These results suggest that the mast cell-specific alpha-galactosyl derivatives of ganglioside GD(1b) and GM(1) are important in maintaining normal cell morphology.

Tubulin cofactor B regulates microtubule densities during microglia transition to the reactive states

Microglia are highly dynamic cells of the CNS that continuously survey the welfare of the neural parenchyma and play key roles modulating neurogenesis and neuronal cell death. In response to injury or pathogen invasion parenchymal microglia transforms into a more active cell that proliferates, migrates and behaves as a macrophage. The acquisition of these extra skills implicates enormous modifications of the microtubule and actin cytoskeletons. Here we show that tubulin cofactor B (TBCB), which has been found to contribute to various aspects of microtubule dynamics in vivo, is also implicated in microglial cytoskeletal changes. We find that TBCB is upregulated in post-lesion reactive parenchymal microglia/macrophages, in interferon treated BV-2 microglial cells, and in neonate amoeboid microglia where the microtubule densities are remarkably low. Our data demonstrate that upon TBCB downregulation both, after microglia differentiation to the ramified phenotype in vivo and in vitro, or after TBCB gene silencing, microtubule densities are restored in these cells. Taken together these observations support the view that TBCB functions as a microtubule density regulator in microglia during activation, and provide an insight into the understanding of the complex mechanisms controlling microtubule reorganization during microglial transition between the amoeboid, ramified, and reactive phenotypes.

GM1 induces p38 and microtubule dependent ramification of rat primary microglia in vitro

Microglia are immunologically competent cells in the central nervous system and considered to be a key player in brain inflammation. The morphological change of microglia has been shown to be linked to functional phenotypes both in vivo and in vitro. As an attempt to identify factors that regulate microglial morphology, we investigated the effect of gangliosides on microglial ramification in vitro. Brain gangliosides mixture and GM1 induced typical ramification of cultured rat primary microglia, however, GD1a and GT1b did not. Although GM1 significantly induced the expression of neurotrophin-3 (NT-3), NT-3 did not induce typical morphological changes in cultured rat primary microglia. SB203580 (an inhibitor of p38), and paclitaxel and nocodazole (microtubule-disrupting drugs) inhibited GM1-induced microglial ramification, but Jaki (an inhibitor of JAK), PD98059 (an inhibitor of Erk1/2), SP600125 (an inhibitor of JNK), and cytochalasin B and latrunculin B (actin polymerization inhibitors) did not, suggesting that GM1 induced ramification of microglia in p38- and microtubule-dependent manner. This in vitro system would be helpful in understanding the mechanisms of microglial ramification and physiological roles of gangliosides in microglia.

Behavior of in vitro cultured ameboid microglial cells migrating on Müller cell end-feet in the quail embryo retina

Ameboid microglial cells migrate tangentially on the vitreal part of quail embryo retinas by crawling on Müller cell end-feet (MCEF) to which they adhere. These microglial cells can be cultured immediately after dissection of the eye and isolation of sheets containing the inner limiting membrane (ILM) covered by a carpet of MCEF (ILM/MCEF sheets), to which the cells remain adhered. Morphological changes of microglial cells cultured on ILM/MCEF sheets for 4 days were characterized in this study. During the first minutes in vitro, lamellipodia-bearing bipolar microglial cells became rounded in shape. From 1 to 24 h in vitro (hiv), microglial cells swept and phagocytosed the MCEF on which they were initially adhered, becoming directly adhered on the ILM. MCEF sweep was dependent on active cell motility, as shown by inhibition of sweep after cytochalasin D treatment. From 24 hiv on, after MCEF phagocytosis, microglial cells became more flattened, increasing the surface area of their adhesion to substrate, and expressed the beta1 subunit of integrins on their membrane. Morphological evidence suggested that microglial cells migrated for short distances on ILM/MCEF sheets, leaving tracks produced by their strong adhesion to the substrate. The simplicity of the isolation method, the immediate availability of cultured microglial cells, and the presence of multiple functional processes (phagocytosis, migration, upregulation of surface molecules, etc.) make cultures of microglial cells on ILM/MCEF sheets a valuable model system for in vitro experimental investigation of microglial cell functions.

The influence of the peptide NAP on Mac-1-deficient mice following closed head injury

A single administration of the neuroprotective peptide NAP was previously shown to protect against death associated with closed head injury (CHI) and enhance recovery of the surviving mice. The protective effect was accompanied by down-regulation of the relative mRNA content of the complement receptor 3 (Mac-1, a marker for inflammation) as measured about a month after the injury. In contrast, the mRNA transcripts for activity-dependent neuroprotective protein (ADNP, the NAP containing protein) were shown to increase 29 days post CHI in the injured hemisphere of Mac-1 expressing mice. The present study was set out to investigate: (1) are Mac-1-deficient mice less susceptible to the adverse outcome of traumatic head injury; (2) does NAP treatment affect Mac-1-deficient mice subjected to head injury; and (3) is Mac-1 expression associated with ADNP expression. Results showed that (1) Mac-1-deficient mice were partially protected against death associated with severe head injury as compared to Mac-1 expressing mice. (2) Significant protection against death was observed in NAP-treated mice and an increase in recovery was observed in the NAP-treated Mac-1 mice 4 weeks after injury. (3) ADNP expression did not change in the Mac-1-deficient mice following head injury. Our working hypothesis is that a month following injury, gene expression in the injured brain is altered and competing proteins are expressed such as Mac-1 that is associated with inflammation and ADNP that is associated with neuroprotection. Obviously, this plasticity in gene expression is intimately interwoven with the genetic background of the animal. NAP treatment tilts the balance toward neuroprotection.

Cytoplasmic Domain of proEGF Affects Distribution and Post-Translational Modification of Microtubuli and Increases Microtubule-Associated Proteins 1b and 2 Production in Human Thyroid Carcinoma Cells

We studied the distribution of transcripts encoding the cytoplasmic domain of the membrane-anchored precursor epidermal growth factor (proEGFcyt) and a novel cytoplasmic proEGF splice isoform with a deleted exon 23 and an out-of-frame fusion of exon 24 (proEGFdel23) in human normal and neoplastic thyroid tissues. In papillary thyroid carcinoma (PTC), coexpression of transcripts encoding for both proEGFcyt and proEGFdel23 correlated with poor differentiation of PTC. To determine potential roles of the cytoplasmic proEGF domain in human thyroid cells, we generated stable transfectants of the human follicular thyroid carcinoma cell line FTC-133 overexpressing the normal cytoplasmic domain proEGFcyt, a truncated proEGFcyt composed of the peptide sequence encoded by exons 22 and 23 (proEGF22.23) and proEGFdel23. The proEGFcyt and proEGF22.23 transfectants displayed significantly reduced proliferation rates, an enlarged cellular phenotype, and alterations in the distribution and post-translational modification of the microtubular system. These transfectants also displayed increased production of microtubule-associated proteins 1b and 2c, which was absent in FTC-133-proEGFdel23 or FTC-133-empty plasmid transfectants. This is the first evidence of an involvement of proEGF cytoplasmic domain in microtubular stability in the human thyroid carcinoma cell line FTC-133 and may suggest a specific role for the cytoplasmic domain of membrane-anchored proEGF, particularly exon 23, in thyroid carcinoma. The up-regulation of proEGFdel23 in poorly differentiated PTC and the exclusive detection of both proEGF isoforms in undifferentiated thyroid carcinoma may indicate an involvement of this novel truncated proEGFdel23 cytoplasmic domain during dedifferentiation processes of human thyroid cells.

In Vivo Measurement of Microtubule Dynamics Using Stable Isotope Labeling with Heavy Water

Microtubules are dynamic polymers with central roles in the mitotic checkpoint, mitotic spindle assembly, and chromosome segregation. Agents that block mitotic progression and cell proliferation by interfering with microtubule dynamics (microtubule-targeted tubulin-polymerizing agents (MTPAs)) are powerful antitumor agents. Effects of MTPAs (e.g. paclitaxel) on microtubule dynamics have not yet been directly demonstrated in intact animals, however. Here we describe a method that measures microtubule dynamics as an exchange of tubulin dimers into microtubules in vivo. The incorporation of deuterium ((2)H(2)) from heavy water ((2)H(2)O) into tubulin dimers and polymers is measured by gas chromatography/mass spectrometry. In cultured human lung and breast cancer cell lines, or in tumors implanted into nude mice, tubulin dimers and polymerized microtubules exhibited nearly identical label incorporation rates, reflecting their rapid exchange. Administration of paclitaxel during 24 h of (2)H(2)O labeling in vivo reduced (2)H labeling in polymers while increasing (2)H in dimers, indicating diminished flux of dimers into polymers (i.e. inhibition of microtubule dynamic equilibrium). In vivo inhibition of microtubule dynamics was dose-dependent and correlated with inhibition of DNA replication, a stable isotopic measure of tumor cell growth. In contrast, microtubule polymers from sciatic nerve of untreated mice were not in dynamic equilibrium with tubulin dimers, and paclitaxel increased label incorporation into polymers. Our results directly demonstrate altered microtubule dynamics as an important action of MTPAs in vivo. This sensitive and quantitative in vivo assay of microtubule dynamics may prove useful for pre-clinical and clinical development of the next generation of MTPAs as anticancer drugs.

Interaction of tau with the neural membrane cortex is regulated by phosphorylation at sites that are modified in paired helical filaments

The axonal microtubule-associated phosphoprotein tau interacts with neural plasma membrane (PM) components during neuronal development (Brandt, R., Léger, J., and Lee, G. (1995) J. Cell Biol. 131, 1327-1340). To analyze the mechanism and potential regulation of tau's PM association, a method was developed to isolate PM-associated tau using microsphere separation of surface-biotinylated cells. We show that tau's PM association requires an intact membrane cortex and that PM-associated tau and cytosolic tau are differentially phosphorylated at sites detected by several Alzheimer's disease (AD) diagnostic antibodies (Ser(199)/Ser(202), Thr(231), and Ser(396)/Ser(404)). In polar neurons, the association of endogenous tau phosphoisoforms with the membrane cortex correlates with an enrichment in the axonal compartment. To test for a direct effect of AD-specific tau modifications in determining tau's interactions, a phosphomutant that simulates an AD-like hyperphosphorylation of tau was produced by site-directed mutagenesis of Ser/Thr residues to negatively charged amino acids (Glu). These mutations completely abolish tau's association with the membrane cortex; however, the construct retains its capability to bind to microtubules. The data suggest that a loss of tau's association with the membrane cortex as a result of phosphorylation at sites that are modified during disease contributes to somatodendritic tau accumulation, axonal microtubule disintegration, and neuronal death characteristic for AD.

Activation of mouse microglial cells affects P2 receptor signaling

Microglial cells are the immunocompetent cells of the CNS, which are known to exist in several activation states. Here we investigated the impact of microglial activation on the P2 receptor-mediated intracellular calcium ([Ca(2+)](i)) signaling by means of fluo-3 based Ca(2+)-imaging. Cultured mouse microglial cells were treated with either astrocyte-conditioned medium to induce a ramified morphology or LPS to shift the cells toward the fully activated stage. The extracellular application of ATP (100 microM) induced a [Ca(2+)](i) elevation in 85% of both untreated and ramified microglial cells, whereas only 50% of the LPS-activated cells responded to the stimulus. To characterise the pharmacological profile of microglial P2 receptors we investigated the effects of various P2 agonists on [Ca(2+)](i) in cultured microglial cells. Untreated and ramified microglial cells demonstrated a very similar sensitivity to the different P2 agonists. In contrast, in LPS-activated microglia, a sharp decrease of responses to P2 agonist stimulation was seen. This indicates that microglial activation influences the capability of microglial cells to generate [Ca(2+)](i) signals upon P2 receptor activation.

Herpes simplex virus-mediated expression of the axonal protein tau in human model neurons (NT2-N cells)

The establishment of axonal-somatodendritic polarity is an important event during neuronal development. The analysis of the underlying molecular events requires experimental models that display characteristic steps in the development of polarity and that are accessible for experimental manipulations. Here we show that human model neurons (NT2-N cells) can be efficiently infected with an amplicon-based herpes simplex virus (HSV) system that expresses the axonal microtubule-associated protein tau. We demonstrate that the neurons express a high level of exogenous tau, which persists for several days, thus allowing us to analyze the morphological effects of the expressed protein. The intracellular interactions of tau and the effects on the microtubule structure of infected neurons, which were processed for immunocytochemistry, were determined using laser scanning microscopy (LSM). Exogenous tau expression does not result in an increased axon growth of the neurons but promotes neuronal microtubule assembly as indicated by an increased amount of total microtubule polymer as well as a labile, detyrosinated microtubule subpopulation. In contrast, tau expression does not induce a significant microtubule stabilization as judged from the quantitation of acetylated microtubule staining 24 hours after infection. The data demonstrate that HSV-mediated expression of proteins in human model neurons provides a useful system for analysis of the effect of neuronal proteins on the morphology and cytoskeletal organization of terminally differentiated polar neurons. In addition, it suggests a role for tau as a factor which locally promotes tubulin polymerization while the dynamics of axonal microtubules are preserved.

Voltage-gated proton currents in microglia of distinct morphology and functional state

Whole-cell patch-clamp measurements were performed to investigate voltage-gated proton currents (I(PR)) in cultured murine microglia of distinct morphology and functional state. We studied I(PR) in ameboid microglia of untreated cultures, in ameboid microglia which had been activated by lipopolysaccharide, and in ramified microglia which had been exposed to astrocyte-conditioned medium. Proton currents of these three microglia populations did not differ regarding their activation threshold or the voltage dependence of steady-state activation. Moreover, pharmacological properties of I(PR) were similar: proton currents were sensitive to extracellularly applied Zn2+ or La3+, and could be abolished by each of those at a concentration of 100 microM. In the presence of extracellular Na+, I(PR) was decreased to a similar small extent due to activity of the Na+/H+ exchanger in all microglial populations. In contrast, proton currents of microglia differed between the three cell populations with respect to their current density and their time-course of activation: in comparison with untreated microglia, the current density of I(PR) was reduced by about 50% in microglia after their treatment with either lipopolysaccharide or astrocyte-conditioned medium. Moreover, I(PR) activated significantly more slowly in cells exposed to lipopolysaccharide or astrocyte-conditioned medium than in untreated cells. It can be concluded that the distinct H+ current characteristics of the three microglial populations do not correlate with the functional state of the cells.

Induction of potassium channels in mouse brain microglia: cells acquire responsiveness to pneumococcal cell wall components during late development

Lipopolysaccharides derived from cell walls of Gram-negative bacteria have proven a useful tool to simulate bacterial infection of the central nervous system. Rapid activation of microglia within the brain parenchyma as well as in vitro has thereby been shown to be an early event upon bacterial or lipopolysaccharide challenges. Less is known about microglial responses to a contact with Gram-positive bacteria, such as Streptococcus pneumoniae, a lethal pathogen causing meningitis with a 30% mortality rate. In the present study, we compared lipopolysaccharide-induced microglial activation in vitro with that induced by preparations of pneumococcal cell walls. As a readout of microglial activation, we studied by patch-clamp recording the expression of outward rectifying potassium currents (IK+OR), which are known to be induced by lipopolysaccharide. We found that pneumococcal cell walls and lipopolysaccharide induced a similar type of IK+OR. Stimulation of IK+OR by pneumococcal cell walls and lipopolysaccharide involved protein synthesis since it was not induced in the presence of cycloheximide. Pharmacological characterization of the pneumococcal cell wall- and lipopolysaccharide-induced currents with specific ion channel blockers indicated for both cases expression of the charybdotoxin/margatoxin-sensitive Kv1.3 subtype of the Shaker family of voltage-dependent potassium channels. Activation of the outward currents by pneumococcal cell walls depended on the developmental stage: while lipopolysaccharide triggered IK+OR in both embryonal and postnatal microglial cells, pneumococcal cell walls had only a marginal effect on embryonal cells. This, however, does not imply that embryonic microglial cells are unresponsive to pneumococcal cell walls. In both embryonic and postnatal cells, (i) the amplitude of the constitutively expressed inward rectifying potassium current was significantly reduced, (ii) tumor necrosis factor-a was released and (iii) the cells changed their morphology, similarly as it was induced by lipopolysaccharide treatment. Thus, embryonic microglial cells are sensitive to pneumococcal cell wall challenges, but respond with a distinctly different pattern of physiological reactions. The expression of IK+OR could thus be a suitable tool to study signalling cascades selectively involved in the activation of microglia by Gram-negative and -positive cell wall components and to functionally distinguish between populations of microglial cells.

Proteasome blockers inhibit TNF-alpha release by lipopolysaccharide stimulated macrophages and microglia: implications for HIV-1 dementia

HIV-1 infection of the central nervous system can cause severe neurologic disease although only microglial cells and brain macrophages are susceptible to productive viral infection. Substances secreted by infected cells are thought to cause disease indirectly. Tumor necrosis factor alpha (TNF-alpha) is one candidate neurotoxin and is upregulated during HIV-1 infection of the brain, likely via activation of the transcription factor NF-kappaB. We used the proteasome inhibitors, MG132 and ALLN (N-acetyl-Leu-Leu-Norleucinal), to inhibit NF-kappaB activation in primary human fetal microglia (PHFM) and primary monocyte derived-macrophages, and showed that they could block TNF-alpha release stimulated by lipopolysaccharide (LPS) or TNF-alpha. In addition, we performed electrophoretic mobility shift analysis and determined that in microglia, the p50/p65 heterodimer of NF-kappaB is activated by LPS stimulation, and is inhibited by MG132. Thus, blockade of NF-kappaB activation in microglia in vitro can decrease production of TNF-alpha and may prove to be a novel strategy for treating HIV-1 dementia.

Identification of MINUS, a small polypeptide that functions as a microtubule nucleation suppressor

In eukaryotic cells, tubulin polymerization must be regulated precisely during cell division and differentiation. To identify new mechanisms involved in cellular microtubule formation, we isolated an activity that suppresses microtubule nucleation in vitro. The activity was due to a small acidic polypeptide of 4.7 kDa which we named MINUS (microtubule nucleation suppressor). MINUS inhibited tau- and taxol-mediated microtubule assembly in vitro and was inactivated by dephosphorylation. The protein was purified to homogeneity from cultured neural (PC12) cells and bovine brain. Microinjection of MINUS caused a transient loss of dynamic microtubules in Vero cells. The results suggest that MINUS acts with a novel mechanism on tubulin polymerization, thus regulating microtubule formation in living cells.

Involvement of stretch-activated Cl- channels in ramification of murine microglia

A stretch-activated Cl- current (ICl) was investigated in cultured murine microglia using the whole-cell configuration of the patch-clamp technique. After application of membrane stretch, a Cl- current appeared within seconds, and its amplitude increased further within 3-8 min. ICl underwent rundown, which was prevented by addition of 4 mM ATP to the intracellular perfusing solution. The stretch-activated Cl- current exhibited outward rectification and did not show any voltage-dependent gating. Lowering the concentration of extracellular Cl- from 142 to 12 mM by equimolar substitution of Cl- with gluconate shifted the reversal potential of ICl by 41.6 +/- 1.8 mV in the depolarizing direction. 4, 4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) blocked ICl in a voltage- and time-dependent manner. At a test potential of +40 mV, a half-maximal blockade at 16.1 microM DIDS and at 71.0 microM SITS was determined for ICl. At a concentration of 200 microM, 5-nitro-2-(3-phenylpropylamino)benzoic acid or flufenamic acid blocked ICl by 88% and 75%, respectively. Each of these four Cl- channel blockers reversibly inhibited the ramification process of microglia, whereas blockers of voltage-gated Na+ and K+ channels did not affect the transformation of microglia from their ameboid into the ramified phenotype. It is suggested that in microglia functional stretch-activated Cl- channels are required for the induction of ramification but not for maintaining the ramified shape.

Changes in proton currents in murine microglia induced by cytoskeletal disruptive agents

Voltage-gated proton currents (IPR) were investigated in cultured murine microglia using the whole-cell configuration of the patch clamp technique. At a gradient of 1.5 between intracellular (pHi = 6.0) and extracellular pH (pHo = 7.5) values, outward IPR were detected at depolarizing potentials, while the activation threshold of IPR was -40 mV. Time-dependent activation of IPR was fitted by a single exponential with a time constant of 661 ms at +40 mV. An increase in the activation time constant of IPR was seen after exposure of microglia to the cytoskeletal disruptive agents cytochalasin D or colchicine. Moreover, the current density of IPR was significantly reduced by 49% in cells treated with cytochalasin D and by 27% in cells treated with colchicine for 24 h. In contrast, voltage-dependence of steady-state activation of IPR was unchanged after disruption of the cytoskeleton. Exposure of microglia to the cytoskeletal stabilizers phalloidin and taxol did not affect IPR of microglia.

Tangential migration of ameboid microglia in the developing quail retina: mechanism of migration and migratory behavior

Long distance migration of microglial precursors within the central nervous system is essential for microglial colonization of the nervous parenchyma. We studied morphological features of ameboid microglial cells migrating tangentially in the developing quail retina to shed light on the mechanism of migration and migratory behavior of microglial precursors. Many microglial precursors remained attached on retinal sheets containing the inner limiting membrane covered by a carpet of Müller cell endfeet. This demonstrates that most ameboid microglial cells migrate tangentially on Müller cell endfeet. Many of these cells showed a central-to-peripheral polarized morphology, with extensive lamellipodia spreading through grooves flanked by Müller cell radial processes, to which they were frequently anchored. Low protuberances from the vitreal face of microglial precursors were firmly attached to the subjacent basal lamina, which was accessible through gaps in the carpet of Müller cell endfeet. These results suggest a mechanism of migration involving polarized extension of lamellipodia at the leading edge of the cell, strong cell-to-substrate attachment, translocation of the cell body forward, and retraction of the rear of the cell. Other ameboid cells were multipolar, with lamellipodial projections radiating in all directions from the cell body, suggesting that microglial precursors explore the surrounding environment to orient their movement. Central-to-peripheral migration of microglial precursors in the retina does not follow a straight path; instead, these cells perform forward, backward, and sideways movements, as suggested by the occurrence of (a) V-shaped bipolar ameboid cells with their vertex pointing toward either the center or the periphery of the retina, and (b) threadlike processes projecting from either the periphery-facing edge or the center-facing edge of ameboid microglial cells.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.