Astrocytes In Multiple Sclerosis

Dense Astrocytic scarring in chronic Multiple Sclerosis (MS) plaques produces an inhibitory environment which can impede tissue repair.

Animal studies have shown that the AntiGenic phenotype of the most abundant cell type in the Brain, the Astrocyte, varies depending on Astrocyte type and location.

To identify the phenotype of Scar Astrocytes (SAs) in chronic lesions, markers of reactive Astrocytes characterized in animal studies were investigated. To date these are the only established markers.

Cerebral SubVentricular deep White Matter from normal control, MS Normal-Appearing White Matter and Lesions (acute, subacute and chronic) were examined by ImmunoHistoChemistry and ImmunoBlotting.

The AntiGenic profile of SAs revealed significant modification of Astrocyte protein expression in chronic MS lesions.

SAs express Nestin, embryonic Neural Cell Adhesion Molecule, Fibroblast Growth Factor Receptor 4, Epidermal Growth Factor Receptor, Nerve Growth Factor and a subpopulation of SAs also express basic Fibroblast Growth Factor.

These are in addition to the expected markers: Glial Fibrillary Acidic Protein, Vimentin, and the Tenascins C and R.

Therefore, an SA AntiGenic phenotype has now been defined. This knowledge may allow the development of therapeutic strategies that prevent scar formation and promote tissue repair.

Understanding how Neurons and Glia sort and deliver Cell Adhesion Molecules to their cell surface should provide important clues as to how such molecules participate in dynamic Neuronal functions in the developing and adult Brain.

The present study examines translocation of PolySialylated Neural Cell Adhesion Molecule (PSA-NCAM), a negative regulator of Cell Adhesion, in cells of the rat HypoThalamo-NeuroHypophysial System.

In which it is expressed throughout life and which undergo Morphological remodelling in response to stimulation.

PSA-NCAM expression in this system does not vary markedly in relation to different conditions of regulated NeuroSecretion, suggesting that the GlycoProtein reaches cell surfaces via the Constitutive Pathway.

To study this more directly, we here used ImmunoFluorescence for PSA on NCAM in live, unpermeabilized cells to monitor PSA-NCAM surface expression in organotypic slice cultures from postnatal rat HypoThalami.

Subsequent Immunolabelling for Oxytocin confirmed that the cultures included MagnoCellular OxytoCinergic Neurons displaying many properties of adult NeuroSecretory Neurons in situ.

In the cultures, ImmunoReaction for PSA-NCAM was visible on the surface of OxytoCinergic and Non-OxytoCinergic Axons.

This reaction disappeared after exposure of the cultures to EndoNeurAminidase, an Enzyme which specifically cleaves alpha-2-8-linked PSA from NCAM. PSA-NCAM reappeared on Axonal surfaces 4h after Enzyme washout.

Such re-expression was visibly not affected by Neuronal activity inhibition (blockade of Ca2⁺ channels with Mn2⁺), of Na⁺ channels with TetrodoToxin.

Or, of Glutamate Receptors with 6-Cyano-7-NitroQuinoxaline-2,3-Dione Or D-2-Amino-5-PhosphonoPentanoic Acid) or facilitation (K⁺ depolarization or GABA-A Receptor blockade with Bicuculline).

In contrast, PSA-NCAM surface translocation was inhibited reversibly by cooling the cultures at 20 degrees C.

A procedure which blocks constitutive secretion and which resulted in accumulation of PSA-NCAM in the Cytoplasm of OxytoCinergic and Non-OxytoCinergic Neurons.

This treatment also revealed PSA-NCAM in the Cytoplasm of underlying Astrocytes.

Our observations provide direct evidence that PSA-NCAM reaches the cell surface of HypoThalamic Neurons and Astrocytes via the Constitutive Pathway, independently of Ca2⁺ entry and enhanced Neuronal activity.

Thus, PSA-NCAM in the HypoThalamo-NeuroHypophysial System would be continuously available to permit its cells to undergo remodelling whenever the proper stimulus intervenes.

Astrocytes are the major Glial Cell within the Central Nervous System (CNS) and have a number of important physiological properties related to CNS homeostasis.

The aspect of Astrocyte biology addressed in this review article is the Astrocyte as an ImmunoCompetent Cell within the Brain.

The capacity of Astrocytes to express Class II Major Histocompatibility Complex (MHC) Antigens and Costimulatory molecules (B7 and CD40) that are critical for Antigen Presentation and T-Cell activation are discussed.

The functional role of Astrocytes as Immune Effector Cells and how this may influence aspects of inflammation and Immune reactivity within the Brain follows, emphasizing the involvement of Astrocytes in promoting Th2 Responses.

The ability of Astrocytes to produce a wide array of Chemokines and Cytokines is discussed, with an emphasis on the Immunological properties of these mediators.

The significance of Astrocytic Antigen Presentation and Chemokine/Cytokine production to Neurological Diseases with an Immunological component is described.

Copyright 2001 Wiley-Liss, Inc.

Nitric Oxide generated by the inducible form of Nitric Oxide Synthase (INOS) may contribute to the pathogenesis of Multiple Sclerosis (MS).

In this report, we studied postmortem tissues of MS patients for the expression of iNOS by in situ hybridization and ImmunoCytoChemistry.

ImmunoCytoChemistry for NitroTyrosine, a putative footprint for Peroxynitrite formation was also performed.

In acute MS lesions, intense reactivity for iNOS mRNA and protein was detected in reactive Astrocytes throughout the lesion and in adjacent Normal-Appearing White Matter.

Staining of Macrophages, inflammatory cell infiltrates, and Endothelial Cells was variable from case to case, but generally detected only in acute lesions.

In chronic MS lesions reactive Astrocytes at the lesion edge were positive for INOS whereas the lesion center was nonreactive.

Normal-Appearing White Matter demonstrated little reactivity, as did tissues from noninflamed control Brains.

Staining for NitroTyrosine was also detected in acute but not chronic MS lesions, and displayed a diffuse Parenchymal, membranous, and PeriVascular pattern of ImmunoReactivity.

These results support the conclusion that iNOS is induced in multiple cell types in MS lesions and that Astrocyte-derived Nitric Oxide could be important in orchestrating inflammatory responses in MS, particularly at the Blood-Brain Barrier.

Intermediate Filaments (IFs) are a major component of the CytoSkeleton in Astrocytes. Their role is far from being completely understood.

Immature Astrocytes play a major role in Neuronal migration and NeuritoGenesis, and their IFs are mainly composed of Vimentin. In mature differentiated Astrocytes, Vimentin is replaced by the IF protein Glial Fibrillary Acidic Protein (GFAP).

In response to injury of the CNS in the adult, Astrocytes become reactive, upregulate the expression of GFAP, and re-express Vimentin.

These modifications contribute to the formation of a Glial scar that is obstructive to Axonal regeneration. Nevertheless, Astrocytes in vitro are considered to be the ideal substratum for the growth of embryonic CNS Axons.

In the present study, we have examined the potential role of these two major IF proteins in both Neuronal survival and Neurite growth.

For this purpose, we cocultured wild-type Neurons on Astrocytes from three types of knock-out (KO) mice for GFAP or/and Vimentin in a Neuron-Astrocyte coculture model.

We show that the double KO Astrocytes present many features of immaturity and greatly improve survival and Neurite growth of cocultured Neurons by increasing cell-cell contact and secreting diffusible factors.

Moreover, our data suggest that the absence of Vimentin is not a key element in the permissivity of the mutant Astrocytes.

Finally, we show that only the absence of GFAP is associated with an increased expression of some ExtraCellular Matrix and Adhesion Molecules.

To conclude, our results suggest that GFAP expression is able to modulate key biochemical properties of Astrocytes that are implicated in their permissivity.

To initiate the inflammatory cascade leading to DeMyelination in Multiple Sclerosis (MS) T-Cells have to recognize their specific Myelin Antigen.

Which needs to be presented in the context of Major HistoCompatibility (MHC) Class II molecules expressed on Antigen-Presenting Cells.

Whether Astrocytes can express MHC Class II molecules in vivo is a controversial issue.

We performed double labeling ImmunoHistoChemistry in postmortem samples from nine patients with MS, three patients with a Cerebral infarction and six controls.

Astrocytes in controls, in Normal-Appearing White Matter in MS, and at the boundary of infarctions were MHC Class II negative.

In contrast, a subset of Astrocytes in active chronic plaques ImmunoStained for MHC Class II, indicating potential Antigen-Presenting interactions of Astrocytes in MS.

In the present study the distribution of the Inhibitory ExtraCellular molecules Tenascin-R (TN-R) and Tenascin- C (TN-C) was examined by ImmunoCytoChemistry during evolution of the Multiple Sclerosis (MS) lesion, in which AstroGliosis is a prominent feature.

Sections were cut from five control cases and from 22 blocks containing lesions representing different pathological stages in 18 cases of Secondary/Progressive MS.

Widespread expression of TN-R was found in the normal human Central Nervous System (CNS), while that of TN-C was in general restricted to White Matter.

In acute MS plaques however, there was a similar striking loss of both TN-R and TN-C up to the edge of the lesion, where the Macrophage density is greatest, extending into the Apparently Normal White Matter.

In subacute lesions a TN-C and/or TN-R-ImmunoPositive reactive Astrocyte subpopulation was prominent.

Reflecting synthesis of ExtraCellular Matrix molecules. Both Tenascins were expressed throughout chronic MS plaques at levels similar to those seen in adjacent White Matter.

The loss of TN-R and TN-C in acute plaques is indicative of Enzyme-mediated breakdown of the Matrix which may be a marker of Blood-Brain Barrier breakdown and Leucocyte extravasation.

Subsequent production of Tenascins by reactive Astrocytes may result in Glial scar formation impeding ReMyelination and Axonal repair in MS lesions.

Astrocytes are derived from the Ventricular and SubVentricular Zones of the Neural Plate, though there is controversy over their derivation from Astrocyte-specific Precursor Cells or Radial Glia intermediates.

Astrocytes are the most abundant cell type in the Brain and contribute to Brain homeostasis in several ways, including buffering of ExtraCellular K⁺, regulating NeuroTransmitter release, forming the Blood-Brain Barrier (BBB), releasing Growth Factors, and regulating the Brain Immune Response.

In addition, Astrocytes have been shown to release ApolipoProtein E (ApoE), which has been shown to regulate NeuroTransmission, Growth Factor release, and Immune Responses.

Due to the diverse functions of Astrocytes, they may play a role in a variety of diseases such as Hepatic Encephalopathy, Multiple Sclerosis, Epilepsy, and age-related diseases including Alzheimer's Disease and Parkinson's Disease.

This review highlights many of the diverse roles played by Astrocytes in regulating Brain homeostasis and discusses their potential role in a variety of disorders.

The 14-3-3 protein family consists of acidic 30-kd proteins expressed at high levels in Neurons of the Central Nervous System.

Seven isoforms form a dimeric complex that acts as a molecular chaperone that interacts with key signaling components.

Recent studies indicated that the 14-3-3 protein identified in the CerebroSpinal Fluid of various Neurological Diseases including Multiple Sclerosis (MS) is a marker for extensive Brain destruction.

However, it remains unknown whether the 14-3-3 protein plays an active role in the pathological process of MS.

To investigate the differential expression of seven 14-3-3 isoforms in MS lesions, brain tissues of four progressive cases were ImmunoLabeled with a panel of isoform-specific AntiBodies.

Reactive Astrocytes in chronic DeMyelinating lesions intensely expressed beta, epsilon, zeta, eta, and sigma isoforms, among which the epsilon isoform is a highly specific marker for reactive Astrocytes.

Furthermore, protein overlay, mass spectrometry, ImmunoPrecipitation, and double-ImmunoLabeling analysis showed that the 14-3-3 protein interacts with both Vimentin and Glial Fibrillary Acidic Protein in cultured human Astrocytes.

These results suggest that the 14-3-3 protein plays an organizing role in the intermediate filament network in reactive Astrocytes at the site of DeMyelinating lesions in MS.

In the adult CNS, antibodies to the NG2 Chondroitin Sulphate ProteoGlycan (CSPG) label a large population of Glia that have the antigenic phenotype of Oligodendrocyte Progenitor Cells (OPC).

However, NG2 expressing Glia have the morphological phenotype of Astrocytes, not OPC.

We propose adult NG2 expressing Glia are a distinct mature Glial type, which we have called Syantocytes or Synantoglia after the Greek 'to contact', because they specifically contact Neurons and Axons at Synapses and Nodes Of Ranvier, respectively.

Synantocytes are highly complex cells that elaborate multiple branching processes and are an equally significant population in both White and Gray Matter.

We provide evidence that phenotypically distinct Synantocytes develop postnatally and that neither postnatal nor adult Synantocytes depend on Axons for their survival, indicating they respond with markedly different behaviours to the environmental cues and Axonal signals that control the differentiation of OPC into Oligodendrocytes.

The primary response of Synantocytes to changes in the CNS environment is a rapid and localized reactive Gliosis.

Reactive Synantocytes interact intimately with Astrocytes and Macrophages at lesion sites, consistent with them playing a key role in the orchestration of scar formation that protects the underlying Neural tissue.

It is our hypothesis that Synantocytes are specialised to monitor and respond to changes in the integrity of the CNS, by way of their cellular contacts, repertoire of Plasmalemmal Receptors and the NG2 molecule itself.

To paraphrase Del Rio Hortega, we propose that Synantocytes are the fifth element in the CNS, in addition to Neurons, Astrocytes, Oligodendrocytes and Microglia.