GDF15 promotes simultaneous astrocyte remodeling and tight junction strengthening at the blood–brain barrier

Perivascular astrocyte processes (PAP) surround cerebral endothelial cells (ECs) and modulate the strengthening of tight junctions to influence blood–brain barrier (BBB) permeability. Morphologically altered astrocytes may affect barrier properties and trigger the onset of brain pathologies. However, astrocyte-dependent mediators of these events remain poorly studied. Here, we show a pharmacologically driven elevated expression and release of growth/differentiation factor 15 (GDF15) in rat primary astrocytes and cerebral PAP. GDF15 has been shown to possess trophic properties for motor neurons, prompting us to hypothesize similar effects on astrocytes. Indeed, its increased expression and release occurred simultaneously to morphological changes of astrocytes in vitro and PAP, suggesting modulatory effects of GDF15 on these cells, but also neighboring EC. Administration of recombinant GDF15 was sufficient to promote astrocyte remodeling and enhance barrier properties between ECs in vitro, whereas its pharmacogenetic abrogation prevented these effects. We validated our findings in male high anxiety-related behavior rats, an animal model of depressive-like behavior, with shrunk PAP associated with reduced expression of the junctional protein claudin-5, which were both restored by a pharmacologically induced increase in GDF15 expression. Thus, we identified GDF15 as an astrocyte-derived trigger of astrocyte process remodeling linked to enhanced tight junction strengthening at the BBB.     

Central nervous system-infiltrated immune cells induce calcium increase in astrocytes via astroglial purinergic signaling

Interaction between autoreactive immune cells and astroglia is an important part of the pathologic processes that fuel neurodegeneration in multiple sclerosis. In this inflammatory disease, immune cells enter into the central nervous system (CNS) and they spread through CNS parenchyma, but the impact of these autoreactive immune cells on the activity pattern of astrocytes has not been defined. By exploiting naïve astrocytes in culture and CNS-infiltrated immune cells (CNS IICs) isolated from rat with experimental autoimmune encephalomyelitis (EAE), here we demonstrate previously unrecognized properties of immune cell–astrocyte interaction. We show that CNS IICs but not the peripheral immune cell application, evokes a rapid and vigorous intracellular Ca²⁺ increase in astrocytes by promoting glial release of ATP. ATP propagated Ca²⁺ elevation through glial purinergic P2X7 receptor activation by the hemichannel-dependent nucleotide release mechanism. Astrocyte Ca²⁺ increase is specifically triggered by the autoreactive CD4⁺ T-cell application and these two cell types exhibit close spatial interaction in EAE. Therefore, Ca²⁺ signals may mediate a rapid astroglial response to the autoreactive immune cells in their local environment. This property of immune cell–astrocyte interaction may be important to consider in studies interrogating CNS autoimmune disease.     

Mechanism of grape seed proanthocyanidin oligomers inhibiting polarization of type Al astrocytes北大核心CSCD

Aim To explore the mechanism of grape seed proanthocyanidins (GSPs) targeting astrocytes (AS), so as to regulate the phenotype and function of AS and maximize their neuroprotective effect. Methods The effects of GSPs on the phenotype, secretion of pro-inflammatory factors and neurotrophic factors of Al AS induced by TNF-α, IL-1α and Clq were investigated by RT-PCR, Elisa and Western blot in vitro. And JNK phosphorylation was determined using Western blot. Results GSPs significantly reduced the expression of C3d and Clq of Al AS markers and inhibited the phosphorylation of JNK. Moreover, compared with the model group, GSPs could significantly inhibit the release of pro-inflammatory cytokines IL-6, IL-1 α, IL-17 and H 2 O 2, and promote the secretion of anti-inflammatory cytokine TGF-β and neurotrophic factor CNTF. Conclusions GSPs can inhibit the polarization of AS to Al and improve the inflammatory micro-environment, which are related to the inhibition of JNK phosphorylation. © 2023 Publication Centre of Anhui Medical University. All rights reserved.     

Vanishing white matter: a leukodystrophy due to astrocytic dysfunction

VWM is one of the most prevalent leukodystrophies with unique clinical, pathological and molecular features. It mostly affects children, but may develop at all ages, from birth to senescence. It is dominated by cerebellar ataxia and susceptible to stresses that act as factors provoking disease onset or episodes of rapid neurological deterioration possibly leading to death. VWM is caused by mutations in any of the genes encoding the five subunits of the eukaryotic translation initiation factor 2B (eIF2B). Although eIF2B is ubiquitously expressed, VWM primarily manifests as a leukodystrophy with increasing white matter rarefaction and cystic degeneration, meager astrogliosis with no glial scarring and dysmorphic immature astrocytes and increased numbers of oligodendrocyte progenitor cells that are restrained from maturing into myelin‐forming cells. Recent findings point to a central role for astrocytes in driving the brain pathology, with secondary effects on both oligodendroglia and axons. In this, VWM belongs to the growing group of astrocytopathies, in which loss of essential astrocytic functions and gain of detrimental functions drive degeneration of the white matter. Additional disease mechanisms include activation of the unfolded protein response with constitutive predisposition to cellular stress, failure of astrocyte‐microglia crosstalk and possibly secondary effects on the oxidative phosphorylation. VWM involves a translation initiation factor. The group of leukodystrophies due to defects in mRNA translation is also growing, suggesting that this may be a common disease mechanism. The combination of all these features makes VWM an intriguing natural model to understand the biology and pathology of the white matter.     

Aging‐related tau astrogliopathy (ARTAG): not only tau phosphorylation in astrocytes

Aging‐related tau astrogliopathy (ARTAG) is defined by the presence of two types of tau‐bearing astrocytes: thorn‐shaped astrocytes (TSAs) and granular/fuzzy astrocytes in the brain of old‐aged individuals. The present study is focused on TSAs in rare forms of ARTAG with no neuronal tau pathology or restricted to entorhinal and transentorhinal cortices, to avoid bias from associated tauopathies. TSAs show 4Rtau phosphorylation at several specific sites and abnormal tau conformation, but they lack ubiquitin and they are not immunostained with tau‐C3 antibodies which recognize truncated tau at Asp421. Astrocytes in ARTAG have atrophic processes, reduced glial fibrillary acidic protein (GFAP) and increased superoxide dismutase 2 (SOD2) immunoreactivity. Gel electrophoresis and western blotting of sarkosyl‐insoluble fractions reveal a pattern of phospho‐tau in ARTAG characterized by two bands of 68 and 64 kDa, and several middle bands between 35 and 50 kDa which differ from what is seen in AD. Phosphoproteomics of dissected vulnerable regions identifies an increase of phosphorylation marks in a large number of proteins in ARTAG compared with controls. GFAP, aquaporin 4, several serine‐threonine kinases, microtubule associated proteins and other neuronal proteins are among the differentially phosphorylated proteins in ARTAG thus suggesting a hyper‐phosphorylation background that affects several molecules, including many kinases and proteins from several cell compartments and various cell types. Finally, present results show for the first time that tau seeding is produced in neurons of the hippocampal complex, astrocytes, oligodendroglia and along fibers of the corpus callosum, fimbria and fornix following inoculation into the hippocampus of wild type mice of sarkosyl‐insoluble fractions enriched in hyper‐phosphorylated tau from selected ARTAG cases. These findings show astrocytes as crucial players of tau seeding in tauopathies.     

Sequential activation of microglia and astrocyte cytokine expression precedes increased iba‐1 or GFAP immunoreactivity following systemic immune challenge

Activation of the peripheral immune system elicits a coordinated response from the central nervous system. Key to this immune to brain communication is that glia, microglia, and astrocytes, interpret and propagate inflammatory signals in the brain that influence physiological and behavioral responses. One issue in glial biology is that morphological analysis alone is used to report on glial activation state. Therefore, our objective was to compare behavioral responses after in vivo immune (lipopolysaccharide, LPS) challenge to glial specific mRNA and morphological profiles. Here, LPS challenge induced an immediate but transient sickness response with decreased locomotion and social interaction. Corresponding with active sickness behavior (2–12 h), inflammatory cytokine mRNA expression was elevated in enriched microglia and astrocytes. Although proinflammatory cytokine expression in microglia peaked 2‐4 h after LPS, astrocyte cytokine, and chemokine induction was delayed and peaked at 12 h. Morphological alterations in microglia (Iba‐1⁺) and astrocytes (GFAP⁺), however, were undetected during this 2–12 h timeframe. Increased Iba‐1 immunoreactivity and de‐ramified microglia were evident 24 and 48 h after LPS but corresponded to the resolution phase of activation. Morphological alterations in astrocytes were undetected after LPS. Additionally, glial cytokine expression did not correlate with morphology after four repeated LPS injections. In fact, repeated LPS challenge was associated with immune and behavioral tolerance and a less inflammatory microglial profile compared with acute LPS challenge. Overall, induction of glial cytokine expression was sequential, aligned with active sickness behavior, and preceded increased Iba‐1 or GFAP immunoreactivity after LPS challenge. GLIA 2016;64:300–316     

SSEA‐4 and YKL‐40 positive progenitor subtypes in the subventricular zone of developing human neocortex

The glycosphingolipid SSEA‐4 and the glycoprotein YKL‐40 have both been associated with human embryonic and neural stem cell differentiation. We investigated the distribution of SSEA‐4 and YKL‐40 positive cells in proliferative zones of human fetal forebrain using immunohistochemistry and double‐labeling immunofluorescence. A few small rounded SSEA‐4 and YKL‐40 labeled cells were present in the radial glial BLBP positive proliferative zones adjacent to the lateral ganglionic eminence from 12th week post conception. With increasing age, a similarly stained cell population appeared more widespread in the subventricular zone. At midgestation, the entire subventricular zone showed patches of SSEA‐4, YKL‐40, and BLBP positive cells. Co‐labeling with markers for radial glial cells (RGCs) and neuronal, glial, and microglial markers tested the lineage identity of this subpopulation of radial glial descendants. Adjacent to the ventricular zone, a minor fraction showed overlap with GFAP but not with nestin, Olig2, NG2, or S100. No co‐localization was found with neuronal markers NeuN, calbindin, DCX or with markers for microglial cells (Iba‐1, CD68). Moreover, the SSEA‐4 and YKL‐40 positive cell population in subventricular zone was largely devoid of Tbr2, a marker for intermediate neuronal progenitor cells descending from RGCs. YKL‐40 has recently been found in astrocytes in the neuron‐free fimbria, and both SSEA‐4 and YKL‐40 are present in malignant astroglial brain tumors. We suggest that the population of cells characterized by immunohistochemical combination of antibodies against SSEA‐4 and YKL‐40 and devoid of neuronal and microglial markers represent a yet unexplored astrogenic lineage illustrating the complexity of astroglial development. GLIA 2016;64:90–104     

β1‐integrin restricts astrocytic differentiation of adult hippocampal neural stem cells

Integrins are transmembrane receptors that mediate cell‐extracellular matrix and cell–cell interactions. The β1‐integrin subunit is highly expressed by embryonic neural stem cells (NSCs) and is critical for NSC maintenance in the developing nervous system, but its role in the adult hippocampal niche remains unexplored. We show that β1‐integrin expression in the adult mouse dentate gyrus (DG) is localized to radial NSCs and early progenitors, but is lost in more mature progeny. Although NSCs in the hippocampal subgranular zone (SGZ) normally only infrequently differentiate into astrocytes, deletion of β1‐integrin significantly enhanced astrocyte differentiation. Ablation of β1‐integrin also led to reduced neurogenesis as well as depletion of the radial NSC population. Activation of integrin‐linked kinase (ILK) in cultured adult NSCs from β1‐integrin knockout mice reduced astrocyte differentiation, suggesting that at least some of the inhibitory effects of β1‐integrin on astrocytic differentiation are mediated through ILK. In addition, β1‐integrin conditional knockout also resulted in extensive cellular disorganization of the SGZ as well as non‐neurogenic regions of the DG. The effects of β1‐integrin ablation on DG structure and astrogliogenesis show sex‐specific differences, with the effects following a substantially slower time‐course in males. β1‐integrin thus plays a dual role in maintaining the adult hippocampal NSC population by supporting the structural integrity of the NSC niche and by inhibiting astrocytic lineage commitment. GLIA 2016;64:1235–1251