Zsuzsanna Környei

Role of CX3CR1 (Fractalkine Receptor) in Brain Damage and Inflammation Induced by Focal Cerebral Ischemia in Mouse

CX3CR1 (fractalkine receptor) is important for sustaining normal microglial activity in the brain. Lack of CX3CR1 reportedly results in neurotoxic microglial phenotype in disease models. The objective of this study was to test the hypothesis that the absence of CX3CR1 worsens the outcome in cerebral ischemia. We observed significantly smaller (56%) infarcts and blood—brain barrier damage in CX3CR1-deficient (CX3CR1−/−) animals compared with CX3CR1 +/− and wild-type mice after transient occlusion of the middle cerebral artery (MCAo). Functional recovery of CX3CR1−/−animals was enhanced, while less number of apoptotic cells and infiltrating leukocytes were found in the ipsilateral hemisphere. Expression of IL-1β mRNA, protein, and interleukin (IL)-1Ra and tumor necrosis factor (TNF)-α mRNAs was lower in CX3CR1−/− mice, whereas no difference was observed in the number of IL-1β-expressing microglia or plasma IL-1β concentration. We observed early IL-1β expression in astrocytes in vivo after MCAo and after oxygen—glucose deprivation in vitro, which might contribute to the ischemic damage. Our findings indicate that lack of CX3CR1 does not result in microglial neurotoxicity after MCAo, but rather significantly reduces ischemic damage and inflammation. Reduced IL-1β and TNFα expression as well as decreased leukocyte infiltration might be involved in the development of smaller infarcts in CX3CR1−/− animals.

Microglia protect against brain injury and their selective elimination dysregulates neuronal network activity after stroke

Microglia are the main immune cells of the brain and contribute to common brain diseases. However, it is unclear how microglia influence neuronal activity and survival in the injured brain in vivo. Here we develop a precisely controlled model of brain injury induced by cerebral ischaemia combined with fast in vivo two-photon calcium imaging and selective microglial manipulation. We show that selective elimination of microglia leads to a striking, 60% increase in infarct size, which is reversed by microglial repopulation. Microglia-mediated protection includes reduction of excitotoxic injury, since an absence of microglia leads to dysregulated neuronal calcium responses, calcium overload and increased neuronal death. Furthermore, the incidence of spreading depolarization (SD) is markedly reduced in the absence of microglia. Thus, microglia are involved in changes in neuronal network activity and SD after brain injury in vivo that could have important implications for common brain diseases.

Proliferative and migratory responses of astrocytes to in vitro injury

An in vitro “scratch‐wound” model was used to evoke and investigate some astroglial responses to mechanical injury. The changes in the morphology, locomotion, and proliferation of injured astrocytes were analysed under culture conditions devoid of blood‐derived cells responsible for activating the inflammatory cascade. The rate of proliferation was determined by immunocytochemical detection of BrdU‐incorporating cells located next to or far from the wound. The motility of individual cells and the mass‐advancement of cell‐assemblies were monitored by computer controlled video‐microscopy both in injured monolayers and in preparations of single cells or aggregates of astrocytes. The large sets of digitalized data allowed a reliable statistical evaluation of changes in cell positions providing a quantitative approach for studies on dynamics of cell locomotion. The results indicated that cultivated astrocytes respond to injury (1) with enhanced nestin immunoreactivity at the expanding processes, (2) with increased mitotic activity exceeding the rate caused by the liberation from contact inhibition, but (3) without specific, injury‐induced activation of cell locomotion. Some advantages and drawbacks of “scratch‐wound” models of astrocytic responses to mechanical injury are presented and discussed. J. Neurosci. Res. 61:421–429, 2000.

Astroglia-derived retinoic acid is a key factor in glia-induced neurogenesis

Astroglial cells are essential components of the neurogenic niches within the central nervous system. Emerging evidence suggests that they are among the key regulators of postnatal neurogenesis. Although astrocytes have been demonstrated to possess the potential to instruct stem cells to adopt a neuronal fate, little is known about the nature of the glia‐derived instructive signals. Here we propose that all‐trans reti‐noic acid, one of the most powerful morphogenic molecules regulating neuronal cell fate commitment, may be one of the glia‐derived factors directing astro‐glia‐induced neurogenesis. According to data obtained from several complementary approaches, we show that cultured astrocytes express the key enzyme mRNAs of retinoic acid biosynthesis and actively produce all‐trans retinoic acid. We show that blockage of retinoic acid signaling by the pan‐RAR antagonist AGN193109 prevents glia‐induced neuron formation by noncommitted stem cells. Therefore, we provide strong in vitro evidence for retinoic acid action in astroglia‐induced neuronal differentiation.—Környei, Z., Gócza, E., Rühl, R., Vörös, E., Orsolits, B., Szabo, B., Vágovits, B., Madarász, E. Astroglia‐derived retinoic acid is a key factor in glia‐induced neurogenesis. FASEB J. 21, 2496–2509 (2007)

Humoral and contact interactions in astroglia/stem cell co-cultures in the course of glia-induced neurogenesis.

Astroglial cells support or restrict the migration and differentiation of neural stem cells depending on the developmental stage of the progenitors and the physiological state of the astrocytes. In the present study, we show that astroglial cells instruct noncommitted, immortalized neuroectodermal stem cells to adopt a neuronal fate, while they fail to induce neuronal differentiation of embryonic stem cells under similar culture conditions. Astrocytes induce neuron formation by neuroectodermal progenitors both through direct cell‐to‐cell contacts and via short‐range acting humoral factors. Neuron formation takes place inside compact stem cell assemblies formed 30– 60 h after the onset of glial induction. Statistical analyses of time‐lapse microscopic recordings show that direct contacts with astrocytes hinder the migration of neuroectodermal progenitors, while astroglia‐derived humoral factors increase their motility. In non‐contact co‐cultures with astrocytes, altered adhesiveness prevents the separation of frequently colliding neural stem cells. By contrast, in contact co‐cultures with astrocytes, the restricted migration on glial surfaces keeps the cell progenies together, resulting in the formation of clonally proliferating stem cell aggregates. The data indicate that in vitro maintained parenchymal astrocytes (1) secrete factors, which initiate neuronal differentiation of neuroectodermal stem cells; and (2) provide a cellular microenvironment where stem cell/stem cell interactions can develop and the sorting out of the future neurons can proceed. In contrast to noncommitted progenitors, postmitotic neuronal precursors leave the stem cell clusters, indicating that astroglial cells selectively support the migration of maturing neurons as well as the elongation of neurites.

Cultured astrocytes react to LPS with increased cyclooxygenase activity and phagocytosis

Phagocytosis and prostaglandin E(2) production were investigated in purified cultures of perinatal rat forebrain astrocytes. Light and electron microscopic data indicated that astrocytes respond to bacterial endotoxin, lipopolysaccharide (LPS) by increased phagocytosis and by activating the cyclooxygenase enzyme-pathway. LPS-inducible phagocytosis of astrocytes was demonstrated by electron microscopic studies on colloidal gold uptake and by photometric determination of fluorescent bead ingestion. The internalisation of fragments of the plasma membrane was shown by histochemical detection of membrane-bound ecto-ATPase activity within intracellular vesicles. Activation of the cyclooxygenase pathway, a characteristic reaction of immune cells under inflammatory conditions, was also detected in astroglial cells upon treatment with LPS. The increased prostaglandin E(2) (PGE(2)) production by astrocytes in response to LPS was reduced by the non-steroid anti-inflammatory drug, indomethacin. Our data indicate that astrocytes display some tissue-protective reactions in response to inflammation inducing factors, even in the absence of peripheral immune cells or central microglia. The role of inducible astrocytic phagocytosis in a non-immune protection-pathway is discussed.

Docosahexaenoic Acid Reduces Amyloid-β Induced Toxicity in Cells of the Neurovascular Unit

Alzheimers disease (AD) is characterized by the accumulation of amyloid-β peptides (Aβ) as perivascular deposits and senile plaques in the brain. The intake of the polyunsaturated fatty acid docosahexaenoic acid (DHA) has been associated with decreased amyloid deposition and reduced risk in AD in several epidemiological trials; however the exact underlying molecular mechanism remains to be elucidated. The aim of the study was to test whether DHA can exert a direct protective effect on the elements of the neurovascular unit, such as neurons, glial cells, brain endothelial cells, and pericytes, treated with Aβ42 (15 μM). A dose-dependent high cellular toxicity was found in viability assays in all cell types and on acute hippocampal slices after treatment with Aβ42 small oligomers prepared in situ from an isopeptide precursor. The cell morphology also changed dramatically in all cell types. In brain endothelial cells, damaged barrier function and increased para- and transcellular permeability were observed after peptide treatment. The production of reactive oxygen species was elevated in pericytes and endothelial and glial cells. DHA (30 μM) significantly decreased the Aβ42-induced toxic effects in all cell types measured by viability assays, and protected the barrier integrity and functions of brain endothelial cells. DHA also decreased the elevated rhodamine 123 accumulation in brain endothelial cells pre-treated with Aβ42 indicating an effect on efflux pump activity. These results indicate for the first time that DHA can protect not only neurons but also the other elements of the neurovascular unit from the toxic effects of Aβ42 and this effect may be beneficial in AD.

Atomic force microscopy of height fluctuations of fibroblast cells

We investigated the nanometer scale height fluctuations of 3T3 fibroblast cells with the atomic force microscope under physiological conditions. A correlation between these fluctuations and lateral cellular motility can be observed. Fluctuations measured on leading edges appear to be predominantly related to actin polymerization-depolymerization processes. We found fast (5 Hz) pulsatory behavior with 1-2 nm amplitude on a cell with low motility showing emphasized structure of stress fibers. Myosin driven contractions of stress fibers are thought to induce this pulsation.

Astroglia genesis in vitro: distinct effects of retinoic acid in different phases of neural stem cell differentiaion

In the developing CNS, the manifestation of the macroglial phenotypes is delayed behind the formation of neurons. The “neurons first – glia second” principle seems to be valid for neural tissue differentiation throughout the neuraxis, but the reasons behind are far from clear. In the presented study, the mechanisms of this timing were investigated in vitro, in the course of the neural differentiation of one cell derived NE‐4C neuroectodermal stem and P19 embryonic carcinoma cells. The data demonstrated that astrocyte formation coincided in time with the maturation of postmitotic neurons, but the close vicinity of neurons did not initate astrocyte formation before schedule. All‐trans retinoic acid, a well‐known inducer of neuronal differentiation, on the other hand, blocked effectively the astroglia production if present in defined stages of the in vitro neuroectodermal cell differentiation. According to the data, retinoic acid plays at least a dual role in astrogliogenesis: while it is needed for committing neural progenitors for a future production of astrocytes, it prevents premature astrogliogenesis by inhibiting the differentiation of primed glial progenitors.

Dynamics of re-constitution of the human nuclear proteome after cell division is regulated by NLS-adjacent phosphorylation.

Phosphorylation by the cyclin-dependent kinase 1 (Cdk1) adjacent to nuclear localization signals (NLSs) is an important mechanism of regulation of nucleocytoplasmic transport. However, no systematic survey has yet been performed in human cells to analyze this regulatory process, and the corresponding cell-cycle dynamics have not yet been investigated. Here, we focused on the human proteome and found that numerous proteins, previously not identified in this context, are associated with Cdk1-dependent phosphorylation sites adjacent to their NLSs. Interestingly, these proteins are involved in key regulatory events of DNA repair, epigenetics, or RNA editing and splicing. This finding indicates that cell-cycle dependent events of genome editing and gene expression profiling may be controlled by nucleocytoplasmic trafficking. For in-depth investigations, we selected a number of these proteins and analyzed how point mutations, expected to modify the phosphorylation ability of the NLS segments, perturb nucleocytoplasmic localization. In each case, we found that mutations mimicking hyper-phosphorylation abolish nuclear import processes. To understand the mechanism underlying these phenomena, we performed a video microscopy-based kinetic analysis to obtain information on cell-cycle dynamics on a model protein, dUTPase. We show that the NLS-adjacent phosphorylation by Cdk1 of human dUTPase, an enzyme essential for genomic integrity, results in dynamic cell cycle-dependent distribution of the protein. Non-phosphorylatable mutants have drastically altered protein re-import characteristics into the nucleus during the G1 phase. Our results suggest a dynamic Cdk1-driven mechanism of regulation of the nuclear proteome composition during the cell cycle.