Early acquisition of typical metabolic features upon differentiation of mouse neural stem cells into astrocytes

The Effects of Cuprizone on Murine Subventricular Zone-Derived Neural Stem Cells and Progenitor Cells Grown as Neurospheres

Despite the extensive use of the cuprizone (CPZ) demyelination animal model, there is little evidence regarding the effects of CPZ on a cellular level. Initial studies have suggested that oligodendrocytes (OL) are the main cell targets for CPZ toxicity. However, recent data have revealed additional effects on neural stem cells and progenitor cells (NSC/NPC), which constitute a reservoir for OL regeneration during brain remyelination. We cultured NSC/NPC as neurospheres to investigate CPZ effects on cell mechanisms which are thought to be involved in demyelination and remyelination processes in vivo. Proliferating NSC/NPC cultures exposed to CPZ showed overproduction of intracellular reactive oxygen species and increased progenitor migration at the expense of a significant inhibition of cell proliferation. Although NSC/NPC survival was not affected by CPZ in proliferative conditions, we found that CPZ-treated cultures undergoing cell differentiation were more prone to cell death than controls. The commitment and cell differentiation towards neural lineages did not seem to be affected by CPZ, as shown by the conserved proportions of OL, astrocytes, and neurons. Nevertheless, when CPZ treatment was performed after cell differentiation, we detected a significant reduction in the number and the morphological complexity of OL, astrogliosis, and neuronal damage. We conclude that, in addition to damaging mature OL, CPZ also reduces NSC/NPC proliferation and activates progenitor migration. These results shed light on CPZ direct effects on NSC proliferation and the progression of in vitro differentiation.

Cystatin B-deficiency triggers ectopic histone H3 tail cleavage during neurogenesis

Cystatin B (CSTB) acts as an inhibitor of cysteine proteases of the cathepsin family and loss-of-function mutations result in human brain diseases with a genotype-phenotype correlation. In the most severe case, CSTB-deficiency disrupts brain development, and yet the molecular basis of this mechanism is missing. Here, we establish CSTB as a regulator of chromatin structure during neural stem cell renewal and differentiation. Murine neural precursor cells (NPCs) undergo transient proteolytic cleavage of the N-terminal histone H3 tail by cathepsins B and L upon induction of differentiation into neurons and glia. In contrast, CSTB-deficiency triggers premature H3 tail cleavage in undifferentiated self-renewing NPCs and sustained H3 tail proteolysis in differentiating neural cells. This leads to significant transcriptional changes in NPCs, particularly of nuclear-encoded mitochondrial genes. In turn, these transcriptional alterations impair the enhanced mitochondrial respiration that is induced upon neural stem cell differentiation. Collectively, our findings reveal the basis of epigenetic regulation in the molecular pathogenesis of CSTB deficiency.

Ganglioside GM1 Targets Astrocytes to Stimulate Cerebral Energy Metabolism

Gangliosides are major constituents of the plasma membrane and are known to promote a number of physiological actions in the brain, including synaptic plasticity and neuroprotection. In particular, the ganglioside GM1 was found to have a wide range of preclinical and clinical benefits in brain diseases such as spinal cord injury, Huntington’s disease and Parkinson’s disease. However, little is known about the underlying cellular and molecular mechanisms of GM1 in the brain. In the present study, we show that GM1 exerts its actions through the promotion of glycolysis in astrocytes, which leads to glucose uptake and lactate release by these cells. In astrocytes, GM1 stimulates the expression of several genes involved in the regulation of glucose metabolism. GM1 also enhances neuronal mitochondrial activity and triggers the expression of neuroprotection genes when neurons are cultured in the presence of astrocytes. Finally, GM1 leads to a neuroprotective effect in astrocyte-neuron co-culture. Together, these data identify a previously unrecognized mechanism mediated by astrocytes by which GM1 exerts its metabolic and neuroprotective effects.

NF-κB and TNF Affect the Astrocytic Differentiation from Neural Stem Cells

The NF-κB signaling pathway is crucial during development and inflammatory processes. We have previously shown that NF-κB activation induces dedifferentiation of astrocytes into neural progenitor cells (NPCs). Here, we provide evidence  that the NF-κB pathway plays also a fundamental role during the differentiation of NPCs into astrocytes. First, we show that the NF-κB pathway is essential to initiate astrocytic differentiation as its early inhibition induces NPC apoptosis and impedes their differentiation. Second, we demonstrate that persistent NF-κB activation affects NPC-derived astrocyte differentiation. Tumor necrosis factor (TNF)-treated NPCs show NF-κB activation, maintain their multipotential and proliferation properties, display persistent expression of immature markers and inhibit astrocyte markers. Third, we analyze the effect of  NF-κB activation on the main known astrocytic differentiation pathways, such as NOTCH and JAK-STAT. Our findings suggest that the NF-κB pathway plays a dual fundamental role during NPC differentiation into astrocytes: it promotes astrocyte specification, but its persistent activation impedes their differentiation.

New insights into the effects of caffeine on adult hippocampal neurogenesis in stressed mice: Inhibition of CORT-induced microglia activation

Chronic stress-evoked depression has been implied to associate with the decline of adult hippocampal neurogenesis. Caffeine has been known to combat stress-evoked depression. Herein, we aim to investigate whether the protective effect of caffeine on depression is related with improving adult hippocampus neurogenesis and explore the mechanisms. Mouse chronic water immersion restraint stress (CWIRS) model, corticosterone (CORT)-established cell stress model, a coculture system containing CORT-treated BV-2 cells and hippocampal neural stem cells (NSCs) were utilized. Results showed that CWIRS caused obvious depressive-like disorders, abnormal 5-HT signaling, and elevated-plasma CORT levels. Notably, microglia activation-evoked brain inflammation and inhibited neurogenesis were also observed in the hippocampus of stressed mice. In comparison, intragastric administration of caffeine (10 and 20 mg/kg, 28 days) significantly reverted CWIRS-induced depressive behaviors, neurogenesis recession and microglia activation in the hippocampus. Further evidences from both in vivo and in vitro mechanistic experiments demonstrated that caffeine treatment significantly suppressed microglia activation via the A2AR/MEK/ERK/NF-κB signaling pathway. The results suggested that CORT-induced microglia activation contributes to stress-mediated neurogenesis recession. The antidepression effect of caffeine was associated with unlocking microglia activation-induced neurogenesis inhibition.

GSEA of mouse and human mitochondriomes reveals fatty acid oxidation in astrocytes

The prevalent view in neuroenergetics is that glucose is the main brain fuel, with neurons being mostly oxidative and astrocytes glycolytic. Evidence supporting that astrocyte mitochondria are functional has been overlooked. Here we sought to determine what is unique about astrocyte mitochondria by performing unbiased statistical comparisons of the mitochondriome in astrocytes and neurons. Using MitoCarta, a compendium of mitochondrial proteins, together with transcriptomes of mouse neurons and astrocytes, we generated cell-specific databases of nuclear genes encoding for mitochondrion proteins, ranked according to relative expression. Standard and in-house Gene Set Enrichment Analyses (GSEA) of five mouse transcriptomes revealed that genes encoding for enzymes involved in fatty acid oxidation (FAO) and amino acid catabolism are consistently more expressed in astrocytes than in neurons. FAO and oxidative-metabolism-related genes are also up-regulated in human cortical astrocytes versus the whole cortex, and in adult astrocytes versus fetal astrocytes. We thus present the first evidence of FAO in human astrocytes. Further, as shown in vitro, FAO coexists with glycolysis in astrocytes and is inhibited by glutamate. Altogether, these analyses provide arguments against the glucose-centered view of energy metabolism in astrocytes and reveal mitochondria as specialized organelles in these cells.

RNA Nanotherapeutics for the Amelioration of Astroglial Reactivity

In response to injuries to the CNS, astrocytes enter a reactive state known as astrogliosis, which is believed to be deleterious in some contexts. Activated astrocytes overexpress intermediate filaments including glial fibrillary acidic protein (GFAP) and vimentin (Vim), resulting in entangled cells that inhibit neurite growth and functional recovery. Reactive astrocytes also secrete inflammatory molecules such as Lipocalin 2 (Lcn2), which perpetuate reactivity and adversely affect other cells of the CNS. Herein, we report proof-of-concept use of the packaging RNA (pRNA)-derived three-way junction (3WJ) motif as a platform for the delivery of siRNAs to downregulate such reactivity-associated genes. In vitro, siRNA-3WJs induced a significant knockdown of Gfap, Vim, and Lcn2 in a model of astroglial activation, with a concomitant reduction in protein expression. Knockdown of Lcn2 also led to reduced protein secretion from reactive astroglial cells, significantly impeding the perpetuation of inflammation in otherwise quiescent astrocytes. Intralesional injection of anti-Lcn2-3WJs in mice with contusion spinal cord injury led to knockdown of Lcn2 at mRNA and protein levels in vivo. Our results provide evidence for siRNA-3WJs as a promising platform for ameliorating astroglial reactivity, with significant potential for further functionalization and adaptation for therapeutic applications in the CNS.

Increase of aquaporin 9 expression in astrocytes participates in astrogliosis

Here we assess the potential functional role of increased aquaporin 9 (APQ9) in astrocytes. Increased AQP9 expression was achieved in primary astrocyte cultures by transfection of a plasmid-containing green fluorescent protein fused to either wild-type or mutated human AQP9. Increased AQP9 expression and phosphorylation at Ser222 were associated with a significant change in astrocyte morphology, mainly with a higher number of processes. Similar phenotypic changes are observed in astrogliosis processes after injury. In parallel, we observed that in vivo, thrombin preconditioning before ischemic stroke induced an early increase in AQP9 expression in the male mouse brain. This increased AQP9 expression was also associated with astrocyte morphological changes, especially in the white matter tract. Astrocyte reactivity is debated as being either beneficial or deleterious. As thrombin preconditioning leads to a decrease in lesion size after stroke, our data suggest that the early increase in AQP9 concomitant with astrocyte reactivity leads to a beneficial effect. © 2017 Wiley Periodicals, Inc.

A comparative transcriptomic analysis of astrocytes differentiation from human neural progenitor cells

Astrocytes are a morphologically and functionally heterogeneous population of cells that play critical roles in neurodevelopment and in the regulation of central nervous system homeostasis. Studies of human astrocytes have been hampered by the lack of specific molecular markers and by the difficulties associated with purifying and culturing astrocytes from adult human brains. Human neural progenitor cells (NPCs) with self-renewal and multipotent properties represent an appealing model system to gain insight into the developmental genetics and function of human astrocytes, but a comprehensive molecular characterization that confirms the validity of this cellular system is still missing. Here we used an unbiased transcriptomic analysis to characterize in vitro culture of human NPCs and to define the gene expression programs activated during the differentiation of these cells into astrocytes using FBS or the combination of CNTF and BMP4. Our results demonstrate that in vitro cultures of human NPCs isolated during the gliogenic phase of neurodevelopment mainly consist of radial glial cells (RGCs) and glia-restricted progenitor cells. In these cells the combination of CNTF and BMP4 activates the JAK/STAT and SMAD signaling cascades, leading to the inhibition of oligodendrocytes lineage commitment and activation of astrocytes differentiation. On the other hand, FBS-derived astrocytes have properties of reactive astrocytes. Our work suggests that in vitro culture of human NPCs represents a valuable cellular system to study human disorders characterized by impairment of astrocytes development and function. Our datasets represent an important resource for researchers studying human astrocytes development and might set the basis for the discovery of novel human-specific astrocyte markers.

The Protein Tyrosine Kinase Inhibitor Tyrphostin 23 Strongly Accelerates Glycolytic Lactate Production in Cultured Primary Astrocytes

Tyrphostin 23 (T23) is a well-known inhibitor of protein tyrosine kinases. To investigate potential acute effects of T23 on the viability and the glucose metabolism of brain cells, we exposed cultured primary rat astrocytes to T23 for up to 4 h. While the viability and the morphology of the cultured astrocytes were not acutely affected by the presence of T23 in concentrations of up to 300 µM, this compound caused a rapid, time- and concentration-dependent increase in glucose consumption and lactate release. Maximal effects on glycolytic flux were found for incubations with 100 µM T23 for 2 h which doubled both glucose consumption and lactate production. The stimulation of glycolytic flux by T23 was reversible, completely abolished upon removal of the compound and not found in presence of other known inhibitors of endocytosis. Structurally related compounds such as tyrphostin 25 and catechol or modulators of AMP kinase activity did neither affect the basal nor the T23-stimulated lactate production by astrocytes. In contrast, the presence of the phosphatase inhibitor vanadate completely abolished the stimulation by T23 of astrocytic lactate production in a concentration-dependent manner. These data suggest that T23-sensitive phosphorylation/dephosphorylation events are involved in the regulation of astrocytic glycolysis.

A cellular perspective on brain energy metabolism and functional imaging

The energy demands of the brain are high: they account for at least 20% of the body's energy consumption. Evolutionary studies indicate that the emergence of higher cognitive functions in humans is associated with an increased glucose utilization and expression of energy metabolism genes. Functional brain imaging techniques such as fMRI and PET, which are widely used in human neuroscience studies, detect signals that monitor energy delivery and use in register with neuronal activity. Recent technological advances in metabolic studies with cellular resolution have afforded decisive insights into the understanding of the cellular and molecular bases of the coupling between neuronal activity and energy metabolism and point at a key role of neuron-astrocyte metabolic interactions. This article reviews some of the most salient features emerging from recent studies and aims at providing an integration of brain energy metabolism across resolution scales.

Neural Stem Cells in the Adult Subventricular Zone Oxidize Fatty Acids to Produce Energy and Support Neurogenic Activity

Neural activity is tightly coupled to energy consumption, particularly sugars such as glucose. However, we find that, unlike mature neurons and astrocytes, neural stem/progenitor cells (NSPCs) do not require glucose to sustain aerobic respiration. NSPCs within the adult subventricular zone (SVZ) express enzymes required for fatty acid oxidation and show sustained increases in oxygen consumption upon treatment with a polyunsaturated fatty acid. NSPCs also demonstrate sustained decreases in oxygen consumption upon treatment with etomoxir, an inhibitor of fatty acid oxidation. In addition, etomoxir decreases the proliferation of SVZ NSPCs without affecting cellular survival. Finally, higher levels of neurogenesis can be achieved in aged mice by ectopically expressing proliferator‐activated receptor gamma coactivator 1 alpha (PGC1α), a factor that increases cellular aerobic capacity by promoting mitochondrial biogenesis and metabolic gene transcription. Regulation of metabolic fuel availability could prove a powerful tool in promoting or limiting cellular proliferation in the central nervous system. Stem Cells2015;33:2306–2319

Expression of the Parkinson's disease protein DJ-1 during the differentiation of neural stem cells

DJ-1 is a key neuroprotective factor and its loss-of-function mutations cause an autosomal recessive, early-onset form of familial Parkinson's disease at the chromosomal PARK7 locus. However, the expression of DJ-1 during the differentiation of neural stem cells has not been fully elucidated. In this study, we investigated the expression of DJ-1 quantitatively using fluorescence immunocytochemistry and flow cytometry for differentiated neural stem cells from the cortex of the 14-day mouse embryos. We found that DJ-1 was co-expressed with the neuron-specific enolase and glial fibrillary acidic proteins, and also its expression was significantly increased in neurons and astrocytes with a prolonged differentiation period. These findings strongly suggest that DJ-1 is closely associated with the differentiation of neural stem cells.

Derivation of microglia-free astrocyte cultures from neural stem cells

Astrocytes play fundamental roles in the establishment and maintenance of tissue homeostasis in the central nervous system (CNS). To examine these different functions in vitro, it is important to be able to generate pure astrocyte cultures. While many "enriched" astrocyte cultures have been described, these are complicated by the presence of other contaminating cell types, such as microglia. In this chapter, we describe a method in which microglia-free astrocyte cultures are generated from neurospheres and also include an immunocytochemical approach to demonstrate the purity of these cultures.

A modified in vitro method to obtain pure astrocyte cultures induced from mouse hippocampal neural stem cells using clonal expansion

The aim of the present study was to produce astrocyte cultures of high purity from mouse hippocampal neural stem cells and to compare their in vitro properties with those isolated from enriched mixed glial cultures prepared from mouse hippocampus, which are commonly contaminated by microglia. We produced primary cultures of newborn mouse hippocampal neural stem cells, which have the potential to differentiate into astrocytes, neurons, and oligodendrocytes. We produced monoclonal neural stem cell colonies by limiting dilution. We induced astrocyte differentiation by plating the colonies on poly-L: -lysine and culturing them in induction medium consisting of minimum essential medium/F12 supplemented with 10% fetal bovine serum and 100 ng/ml ciliary neurotrophic factor. We then further purified the cells by differential adherence and shaking at a constant temperature, followed by a second round of limiting dilution. Immunocytochemistry for glial fibrillary acidic protein showed that our method yielded 99.4 ± 0.5% pure astrocytes, whereas traditionally enriched mixed glial cultures yielded 94.2 ± 2% pure astrocytes. Induced cells resembled primary astrocyte cultures in functional properties such as cell proliferation rates and lack of tumorigenicity and p53, and expression of epidermal growth factor receptor, bystin, and nitric oxygen synthase. Our novel method of culture and purification of neural stem cells can therefore be used routinely for the primary culture of highly purified astrocytes from mouse hippocampus.

Transforming growth factor α transforms astrocytes to a growth-supportive phenotype after spinal cord injury

Astrocytes are both detrimental and beneficial for repair and recovery after spinal cord injury (SCI). These dynamic cells are primary contributors to the growth-inhibitory glial scar, yet they are also neuroprotective and can form growth-supportive bridges on which axons traverse. We have shown that intrathecal administration of transforming growth factor α (TGFα) to the contused mouse spinal cord can enhance astrocyte infiltration and axonal growth within the injury site, but the mechanisms of these effects are not well understood. The present studies demonstrate that the epidermal growth factor receptor (EGFR) is upregulated primarily by astrocytes and glial progenitors early after SCI. TGFα directly activates the EGFR on these cells in vitro, inducing their proliferation, migration, and transformation to a phenotype that supports robust neurite outgrowth. Overexpression of TGFα in vivo by intraparenchymal adeno-associated virus injection adjacent to the injury site enhances cell proliferation, alters astrocyte distribution, and facilitates increased axonal penetration at the rostral lesion border. To determine whether endogenous EGFR activation is required after injury, SCI was also performed on Velvet (C57BL/6J-Egfr(Vel)/J) mice, a mutant strain with defective EGFR activity. The affected mice exhibited malformed glial borders, larger lesions, and impaired recovery of function, indicating that intrinsic EGFR activation is necessary for neuroprotection and normal glial scar formation after SCI. By further stimulating precursor proliferation and modifying glial activation to promote a growth-permissive environment, controlled stimulation of EGFR at the lesion border may be considered in the context of future strategies to enhance endogenous cellular repair after injury.

Blood-brain barrier modeling with co-cultured neural progenitor cell-derived astrocytes and neurons

In vitro blood-brain barrier (BBB) models often consist of brain microvascular endothelial cells (BMECs) that are co-cultured with other cells of the neurovascular unit, such as astrocytes and neurons, to enhance BBB properties. Obtaining primary astrocytes and neurons for co-culture models can be laborious, while yield and heterogeneity of primary isolations can also be limiting. Neural progenitor cells (NPCs), because of their self-renewal capacity and ability to reproducibly differentiate into tunable mixtures of neurons and astrocytes, represent a facile, readily scalable alternative. To this end, differentiated rat NPCs were co-cultured with rat BMECs and shown to induce BBB properties such as elevated trans-endothelial electrical resistance, improved tight junction continuity, polarized p-glycoprotein efflux, and low passive permeability at levels indistinguishable from those induced by primary rat astrocyte co-culture. An NPC differentiation time of 12 days, with the presence of 10% fetal bovine serum, was found to be crucial for generating NPC-derived progeny capable of inducing the optimal response. This approach could also be extended to human NPC-derived astrocytes and neurons which similarly regulated BBB induction. The distribution of rat or human NPC-derived progeny under these conditions was found to be a roughly 3 : 1 mixture of astrocytes to neurons with varying degrees of cellular maturity. BMEC gene expression analysis was conducted using a BBB gene panel, and it was determined that 23 of 26 genes were similarly regulated by either differentiated rat NPC or rat astrocyte co-culture while three genes were differentially altered by the rat NPC-derived progeny. Taken together, these results demonstrate that NPCs are an attractive alternative to primary neural cells for use in BBB co-culture models.

Transplantation of specific human astrocytes promotes functional recovery after spinal cord injury

Repairing trauma to the central nervous system by replacement of glial support cells is an increasingly attractive therapeutic strategy. We have focused on the less-studied replacement of astrocytes, the major support cell in the central nervous system, by generating astrocytes from embryonic human glial precursor cells using two different astrocyte differentiation inducing factors. The resulting astrocytes differed in expression of multiple proteins thought to either promote or inhibit central nervous system homeostasis and regeneration. When transplanted into acute transection injuries of the adult rat spinal cord, astrocytes generated by exposing human glial precursor cells to bone morphogenetic protein promoted significant recovery of volitional foot placement, axonal growth and notably robust increases in neuronal survival in multiple spinal cord laminae. In marked contrast, human glial precursor cells and astrocytes generated from these cells by exposure to ciliary neurotrophic factor both failed to promote significant behavioral recovery or similarly robust neuronal survival and support of axon growth at sites of injury. Our studies thus demonstrate functional differences between human astrocyte populations and suggest that pre-differentiation of precursor cells into a specific astrocyte subtype is required to optimize astrocyte replacement therapies. To our knowledge, this study is the first to show functional differences in ability to promote repair of the injured adult central nervous system between two distinct subtypes of human astrocytes derived from a common fetal glial precursor population. These findings are consistent with our previous studies of transplanting specific subtypes of rodent glial precursor derived astrocytes into sites of spinal cord injury, and indicate a remarkable conservation from rat to human of functional differences between astrocyte subtypes. In addition, our studies provide a specific population of human astrocytes that appears to be particularly suitable for further development towards clinical application in treating the traumatically injured or diseased human central nervous system.

Brain water mobility decreases after astrocytic aquaporin-4 inhibition using RNA interference

Neuroimaging with diffusion-weighted imaging is routinely used for clinical diagnosis/prognosis. Its quantitative parameter, the apparent diffusion coefficient (ADC), is thought to reflect water mobility in brain tissues. After injury, reduced ADC values are thought to be secondary to decreases in the extracellular space caused by cell swelling. However, the physiological mechanisms associated with such changes remain uncertain. Aquaporins (AQPs) facilitate water diffusion through the plasma membrane and provide a unique opportunity to examine the molecular mechanisms underlying water mobility. Because of this critical role and the recognition that brain AQP4 is distributed within astrocytic cell membranes, we hypothesized that AQP4 contributes to the regulation of water diffusion and variations in its expression would alter ADC values in normal brain. Using RNA interference in the rodent brain, we acutely knocked down AQP4 expression and observed that a 27% AQP4-specific silencing induced a 50% decrease in ADC values, without modification of tissue histology. Our results demonstrate that ADC values in normal brain are modulated by astrocytic AQP4. These findings have major clinical relevance as they suggest that imaging changes seen in acute neurologic disorders such as stroke and trauma are in part due to changes in tissue AQP4 levels.

Glycogen metabolism as a marker of astrocyte differentiation

Glycogen is a hallmark of mature astrocytes, but its emergence during astrocytic differentiation is unclear. Differentiation of E14 mouse neurospheres into astrocytes was induced with fetal bovine serum (FBS), Leukemia Inhibitory Factor (LIF), or Ciliary Neurotrophic Factor (CNTF). Cytochemical and enzymatic analyses showed that glycogen is present in FBS- or LIF- but not in CNTF-differentiated astrocytes. Glycogenolysis was induced in FBS- and LIF-differentiated astrocytes but glycogen resynthesis was observed only with FBS. Protein targeting to glycogen mRNA expression appeared with glial fibrillary acidic protein and S100beta in FBS and LIF conditions but not with CNTF. These results show that glycogen metabolism constitutes a useful marker of astrocyte differentiation.

Metabolic effects of blocking lactate transport in brain cortical tissue slices using an inhibitor specific to MCT1 and MCT2

A novel inhibitor of lactate transport, AR-C122982, was used to study the effect of inhibiting the monocarboxylate transporters MCT1 and MCT2 on cortical brain slice metabolism. We studied metabolism of L-[3-13C]lactate, and D-[1-13C]glucose under a range of conditions. Experiments using L-[3-13C]lactate showed that the inhibitor AR-C122982 altered exchange of lactate. Under depolarizing conditions, net flux of label from D-[1-13C]glucose was barely altered by 10 or 100 nM AR-C122982. In the presence of AMPA or glutamate there were increases in net flux of label and metabolic pool sizes. These data suggest lactate may supply compartments in the brain not usually directly accessed by glucose. In general, it would appear that movement of lactate between cell types is not essential for metabolic activity, with the heavy metabolic workloads imposed being unaffected by inhibition of MCT1 and MCT2. Further experiments investigating the mechanism of operation of AR-C122982 are necessary to corroborate this finding.

Gene expression profiling of human neural progenitor cells following the serum-induced astrocyte differentiation

Neural stem cells (NSC) with self-renewal and multipotent properties could provide an ideal cell source for transplantation to treat spinal cord injury, stroke, and neurodegenerative diseases. However, the majority of transplanted NSC and neural progenitor cells (NPC) differentiate into astrocytes in vivo under pathological environments in the central nervous system, which potentially cause reactive gliosis. Because the serum is a potent inducer of astrocyte differentiation of rodent NPC in culture, we studied the effect of the serum on gene expression profile of cultured human NPC to identify the gene signature of astrocyte differentiation of human NPC. Human NPC spheres maintained in the serum-free culture medium were exposed to 10% fetal bovine serum (FBS) for 72 h, and processed for analyzing on a Whole Human Genome Microarray of 41,000 genes, and the microarray data were validated by real-time RT-PCR. The serum elevated the levels of expression of 45 genes, including ID1, ID2, ID3, CTGF, TGFA, METRN, GFAP, CRYAB and CSPG3, whereas it reduced the expression of 23 genes, such as DLL1, DLL3, PDGFRA, SOX4, CSPG4, GAS1 and HES5. Thus, the serum-induced astrocyte differentiation of human NPC is characterized by a counteraction of ID family genes on Delta family genes. Coimmunoprecipitation analysis identified ID1 as a direct binding partner of a proneural basic helix-loop-helix (bHLH) transcription factor MASH1. Luciferase assay indicated that activation of the DLL1 promoter by MASH1 was counteracted by ID1. Bone morphogenetic protein 4 (BMP4) elevated the levels of ID1 and GFAP expression in NPC under the serum-free culture conditions. Because the serum contains BMP4, these results suggest that the serum factor(s), most probably BMP4, induces astrocyte differentiation by upregulating the expression of ID family genes that repress the proneural bHLH protein-mediated Delta expression in human NPC.

A novel method to establish microglia-free astrocyte cultures: comparison of matrix metalloproteinase expression profiles in pure cultures of astrocytes and microglia

Increased matrix metalloproteinase (MMP) proteolytic activity contributes to the pathogenesis of many neuroinflammatory and neurodegenerative conditions in the CNS. To fully understand this process, it is important to define the MMP expression profile of specific cell types, including the CNS-resident cells astrocytes and microglia. While previous studies have characterized astrocyte MMP expression by using mixed glial cultures, these results are likely complicated by the presence of contaminating microglia within these cultures. In the current study, we sought to clarify this complexity, by taking a novel approach to prepare pure astrocyte cultures entirely devoid of microglia, by promoting neural stem cell (NSC) differentiation into astrocytes. The MMP expression profile of mixed glial cultures, neurosphere-derived astrocytes, and pure microglia was characterized by RNase protection assay. This revealed that MMP gene expression is largely cell-type specific. Astrocytes constitutively expressed MMP-11, MMP-14, and MMP-2 and showed induction of MMP-3 in response to IL-1beta but did not respond to lipopolysaccharide (LPS). In contrast, microglia constitutively expressed high levels of MMP-12 and showed strong induction of MMP-9 and MMP-14 in response to LPS. Gelatin zymography confirmed that LPS and TNF-alpha induced strong expression of MMP-9 in microglia but not astrocytes. In summary, these studies demonstrate that neurosphere-derived astrocytes represent an attractive alternative system in which to study astrocyte behavior in vitro. Using this system, we have shown that astrocytes and microglia express distinct sets of MMP genes and that microglia, not astrocytes, are the major source of MMP-9 in response to LPS or TNF-alpha.

Brorin, a Novel Secreted Bone Morphogenetic Protein Antagonist, Promotes Neurogenesis in Mouse Neural Precursor Cells

We identified a gene encoding a novel secreted protein in mice and humans and named it Brorin. Mouse Brorin consists of 324 amino acids with a putative secreted signal sequence at its amino terminus and two cysteine-rich domains in its core region. Positions of 10 cysteine residues in the domains of Brorin are similar to those in the cysteine-rich domains of members of the Chordin family. However, the amino acid sequence of Brorin is not significantly similar to that of any other member of the Chordin family, indicating that Brorin is a unique member of the family. Mouse Brorin protein produced in cultured cells was efficiently secreted into the culture medium. The protein inhibited the activity of bone morphogenetic protein 2 (BMP2) and BMP6 in mouse preosteoblastic MC3T3-E1 cells. Mouse Brorin was predominantly expressed in neural tissues in embryos and also predominantly expressed in the adult brain. In the brain, the expression was detected in neurons, but not glial cells. The neural tissue-specific expression profile of Brorin is quite distinct from that of any other member of the Chordin family. Brorin protein promoted neurogenesis, but not astrogenesis, in mouse neural precursor cells. The present findings indicate that Brorin is a novel secreted BMP antagonist that potentially plays roles in neural development and functions.

Glial fibrillary acidic protein-expressing cells in the neurogenic regions in normal and injured adult brains

In the adult brain, neurogenic stem cells are prevalent in the subventricular zone (SVZ) of the lateral ventricle wall and the subgranular zone (SGZ) in the dentate gyrus. Cells that have structural and molecular characteristics of astrocytes function as neurogenic stem cells in these regions, in which these cells also participate in the creation of the microenvironment that stimulates neurogenesis. In the present paper, we review the phenotypic properties, subpopulations, and proliferation of glial fibrillary acidic protein (GFAP)-expressing cells in these two neurogenic regions and their responses to different brain injuries. Cells fulfilling the criteria for astrocytes, i.e., expressing GFAP, in the SVZ and SGZ respond differently to brain injuries or neurogenic stimuli. The importance of guidance by astrocytes of newly formed neuronal cells is emphasized. The assessment of GFAP-expressing cells in the neurogenic regions is of great importance for understanding the mechanism underlying the response of neural stem cells to brain injury.

Activity-dependent regulation of energy metabolism by astrocytes: An update

Astrocytes play a critical role in the regulation of brain metabolic responses to activity. One detailed mechanism proposed to describe the role of astrocytes in some of these responses has come to be known as the astrocyte-neuron lactate shuttle hypothesis (ANLSH). Although controversial, the original concept of a coupling mechanism between neuronal activity and glucose utilization that involves an activation of aerobic glycolysis in astrocytes and lactate consumption by neurons provides a heuristically valid framework for experimental studies. In this context, it is necessary to provide a survey of recent developments and data pertaining to this model. Thus, here, we review very recent experimental evidence as well as theoretical arguments strongly supporting the original model and in some cases extending it. Aspects revisited include the existence of glutamate-induced glycolysis in astrocytes in vitro, ex vivo, and in vivo, lactate as a preferential oxidative substrate for neurons, and the notion of net lactate transfer between astrocytes and neurons in vivo. Inclusion of a role for glycogen in the ANLSH is discussed in the light of a possible extension of the astrocyte-neuron lactate shuttle (ANLS) concept rather than as a competing hypothesis. New perspectives offered by the application of this concept include a better understanding of the basis of signals used in functional brain imaging, a role for neuron-glia metabolic interactions in glucose sensing and diabetes, as well as novel strategies to develop therapies against neurodegenerative diseases based upon improving astrocyte-neuron coupled energetics.

Neudesin, a secreted factor, promotes neural cell proliferation and neuronal differentiation in mouse neural precursor cells

Neudesin encodes a secreted signal with neurotrophic activity in neurons. Most neurotrophic factors are involved in neural cell proliferation and/or differentiation. However, the role of neudesin in neural development remains to be elucidated. We examined the expression of neudesin in mouse embryonic cerebral cortex and cultured mouse neural precursor cells and its roles in neural development. Neudesin was expressed in the embryonic cerebral cortex early in development. Its expression was observed mainly in the preplate, where mostly postmitotic neural cells existed. Because neudesin mRNA was expressed in the neural precursor cells before the appearance of neurons, the roles of neudesin in neural development were examined by using the precursor cells. Neudesin significantly promoted neuronal differentiation and overrode the undifferentiated state of the neural precursor cells sustained by fibroblast growth factor 2 (FGF2). In contrast, it inhibited the differentiation of astrocytes. In addition, neudesin transiently promoted neural cell proliferation early in the developmental process. The effect on cell proliferation was distinct from that of FGF2, a self-renewal-promoting factor for neural precursor cells. The differentiation was mediated though activation of the protein kinase A (PKA) and phosphatidylinositol-3 kinase (PI-3K) pathways. In contrast, the proliferation was mediated through the mitogen-activated protein kinase and PKA pathways. The expression profile and activity indicate that neudesin plays unique roles in neural development. The present findings have revealed new potential roles of neudesin in neural cell proliferation and neuronal differentiation.

Metabolic properties of astrocytes differentiated from rat neurospheres

The metabolic properties of astroglia differentiated from neurospheres have not been fully assessed. In this study, the glycolytic and oxidative metabolism of glucose in astroglia differentiated from rat tertiary neurospheres (astroglia(NS)) was compared with that in astroglia prepared from the striata of embryonic day 16 rats (astroglia(ST)). In addition to the basal condition, we also investigated energy metabolism under Na+,K+-ATPase activation. Furthermore, the effects of glucose concentration in the culture medium were assessed. No significant differences in 2-deoxy-D-[1-(14)C]glucose phosphorylation were observed between astroglia(NS) and astrogliaST. The rates of L-[U-14C]lactate ([14C]lactate) and D-[U-14C]glucose ([14C]glucose) oxidation were 5.74+/-0.82 and 2.83+/-0.4 pmol/60 min/microg protein, respectively, in astrogliaNS grown in low glucose (2 mM) and 3.01+/-1.03 and 1.77+/-0.23 pmol/60 min/microg protein, respectively, in astrogliaNS grown in high glucose (22 mM). Neither the [14C]lactate nor the [14C]glucose oxidation rates in astrogliaNS were significantly different from those in astrogliaST. D-aspartate (500 microM) significantly increased the [14C]lactate and [14C]glucose oxidation rates by 127% and 62%, respectively, in astrogliaNS grown in low glucose and by 217% and 115%, respectively, in astroglia(NS) grown in high glucose. D-aspartate also increased the oxidation of [14C]lactate and [14C]glucose to 236% and 147% of the control values, respectively, in astrogliaST grown in low glucose and to 174% and 144%, respectively, in astrogliaST grown in high glucose. Rat astroglia differentiated from neurospheres might possess an equivalent capacity for utilizing energy substrates under both basal and activated conditions to that of astroglia prepared from striatum.

Reactive astrogliosis induces astrocytic differentiation of adult neural stem/progenitor cells in vitro

Neural stem cells reside in defined areas of the adult mammalian brain, including the dentate gyrus of the hippocampus. Rat neural stem/progenitor cells (NSPCs) isolated from this region retain their multipotency in vitro and in vivo after grafting into the adult brain. Recent studies have shown that endogenous or grafted NSPCs are activated after an injury and migrate toward lesioned areas. In these areas, reactive astrocytes are present and secrete numerous molecules and growth factors; however, it is not currently known whether reactive astrocytes can influence the lineage selection of NSPCs. We investigated whether reactive astrocytes could affect the differentiation, proliferation, and survival of adult NSPCs by modelling astrogliosis in vitro, using mechanical lesion of primary astrocytes. Initially, it was found that conditioned medium from lesioned astrocytes induced astrocytic differentiation of NSPCs without affecting neuronal or oligodendrocytic differentiation. In addition, NSPCs in coculture with lesioned astrocytes also displayed increased astrocytic differentiation and some of these NSPC-derived astrocytes participated in glial scar formation in vitro. When proliferation and survival of NSPCs were analyzed, no differential effects were observed between lesioned and nonlesioned astrocytes. To investigate the molecular mechanisms of the astrocyte-inducing activity, the expression of two potent inducers of astroglial differentiation, ciliary neurotrophic factor and leukemia inhibitory factor, was analyzed by Western blot and shown to be up-regulated in conditioned medium from lesioned astrocytes. These results demonstrate that lesioned astrocytes can induce astroglial differentiation of NSPCs and provide a mechanism for astroglial differentiation of these cells following brain injury.

Bilirubin-induced inflammatory response, glutamate release, and cell death in rat cortical astrocytes are enhanced in younger cells

Unconjugated bilirubin (UCB) encephalopathy is a predominantly early life condition resulting from the impairment of several cellular functions in the brain of severely jaundiced infants. However, only few data exist on the age-dependent effects of UCB and their association with increased vulnerability of premature newborns, particularly in a sepsis condition. We investigated cell death, glutamate efflux, and inflammatory cytokine dynamics after exposure of astrocytes at different stages of differentiation to clinically relevant concentrations of UCB and/or lipopolysaccharide (LPS). Younger astrocytes were more prone to UCB-induced cell death, glutamate efflux, and inflammatory response than older ones. Furthermore, in immature cells, LPS exacerbated UCB effects, such as cell death by necrosis. These findings provide a basis for the increased susceptibility of premature newborns to UCB deleterious effects, namely when associated with sepsis, and underline how crucial the course of cell maturation can be to UCB encephalopathy during moderate to severe neonatal jaundice.

How astrocytes feed hungry neurons

For years glucose was thought to constitute the sole energy substrate for neurons; it was believed to be directly provided to neurons via the extracellular space by the cerebral circulation. It was recently proposed that in addition to glucose, neurons might rely on lactate to sustain their activity. Therefore, it was demonstrated that lactate is a preferred oxidative substrate for neurons not only in vitro but also in vivo. Moreover, the presence of specific monocarboxylate transporters on neurons as well as on astrocytes is consistent with the hypothesis of a transfer of lactate from astrocytes to neurons. Evidence has been provided for a mechanism whereby astrocytes respond to glutamatergic activity by enhancing their glycolytic activity, resulting in increased lactate release. This is accomplished via the uptake of glutamate by glial glutamate transporters, leading to activation of the Na+/K+ ATPase and a stimulation of astrocytic glycolysis. Several recent observations obtained both in vitro and in vivo with different approaches have reinforced this view of brain energetics. Such an understanding might be critically important, not only because it forms the basis of some classical functional brain imaging techniques but also because several neurodegenerative diseases exhibit diverse alterations in energy metabolism.

Ampakine CX546 bolsters energetic response of astrocytes: a novel target for cognitive-enhancing drugs acting as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor modulators

Glutamate was previously shown to enhance aerobic glycolysis i.e. increase glucose utilization and lactate production with no change in oxygen levels, in mouse cortical astrocytes by a mechanism involving glutamate uptake. It is reported here that a similar response is produced in both hippocampal and cerebellar astrocytes. Application of the cognitive-enhancing drug CX546 promoted further enhancement of glucose utilization by astrocytes from each brain area following glutamate exposure. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors represent the purported molecular target of cognitive-enhancing drugs such as CX546, and the presence of AMPA receptor subunits GluR1-4 was evidenced in astrocytes from all three regions by immunocytochemistry. AMPA itself did not stimulate aerobic glycolysis, but in the presence of CX546, a strong enhancement of glucose utilization and lactate production was obtained in cortical, hippocampal and cerebellar astrocytes. The effect of CX546 was concentration-dependent, with an EC(50) of 93.2 microm in cortical astrocytes. AMPA-induced glucose utilization in the presence of CX546 was prevented by the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the negative modulator GYKI 52466. In addition, the metabolic effect of CX546 in the presence of AMPA was mimicked by the AMPA receptor modulator cyclothiazide. Our data suggest that astrocyte energetics represents a novel target for cognitive-enhancing drugs acting as AMPA receptor modulators.

In vitro differentiated neural stem cells express functional glial glutamate transporters

The possibility to isolate stem cells from the adult central nervous system and to maintain and propagate these cells in vitro has raised a general interest with regards to their use in cell replacement therapy for degenerative brain diseases. Considering the critical role played by astrocytes in the control of glutamate homeostasis, we have characterised the expression of functional glutamate transporters in neural stem cells exposed to selected culture conditions favouring their differentiation into astrocytes. Commonly, neural stem cells proliferate in suspension as neurospheres in serum-free medium. The addition of serum or a supplement of growth factors (G5) to the culture medium was found to trigger cell adhesion on coated surfaces and to favour their differentiation. Indeed, after 7 days in these conditions, the vast majority of the cells adopted markedly distinct morphologies corresponding to protoplasmic (with serum) or fibrous (with G5 supplement) astrocytes and approximately 35-40% acquired the expression of the glial fibrillary acidic protein (GFAP). Immunocytochemical analysis also revealed that the treatments with serum or with the G5 supplement triggered the expression of the glial glutamate transporters GLT-1 (35 and 21%, respectively) and GLAST (29 and 69%, respectively). This effect was correlated with a robust increase in the Na+ -dependent [3H]-d-aspartate uptake, which was partially inhibited by dihydrokainate, a selective blocker of GLT-1. Together, these results indicate that in vitro differentiation of cultured neural stem cells can give rise to distinct populations of astrocytes expressing functional glutamate transporters.

Radial glial cells, proliferating periventricular cells, and microglia might contribute to successful structural repair in the cerebral cortex of the lizard Gallotia galloti

Reptiles are the only amniotic vertebrates known to be capable of spontaneous regeneration of the central nervous system (CNS). In this study, we analyzed the reactive changes of glial cells in response to a unilateral physical lesion in the cerebral cortex of the lizard Gallotia galloti, at 1, 3, 15, 30, 120, and 240 days postlesion. The glial cell markers glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), S100 protein, and tomato lectin, as well as proliferating cell nuclear antigen (PCNA) were used to evaluate glial changes occurring because of cortical lesions. A transitory and unilateral upregulation of GFAP and GS in reactive radial glial cells were observed from 15 to 120 days postlesion. In addition, reactive lectin-positive macrophage/microglia were observed from 1 to 120 days postlesion, whereas the expression of S100 protein remained unchanged throughout the examined postlesion period. The matricial zones closest to the lesion site, the sulcus lateralis (SL) and the sulcus septomedialis (SSM), showed significantly increased numbers of dividing cells at 30 days postlesion. At 240 days postlesion, the staining pattern for PCNA, GFAP, GS, and tomato lectin in the lesion site became similar to that observed in unlesioned controls. In addition, ultrastructural data of the lesioned cortex at 240 days postlesion indicated a structural repair process. We conclude that restoration of the glial framework and generation of new neurons and glial cells in the ventricular wall play a key role in the successful structural repair of the cerebral cortex of the adult lizard.