Modulation of neuritogenesis by astrocyte muscarinic receptors

The Effect of Donepezil Hydrochloride in the Twitcher Mouse Model of Krabbe Disease

Krabbe disease (KD) is a rare demyelinating disorder characterized by demyelination caused by mutations in the GALC gene, resulting in toxic accumulation of psychosine. Psychosine has been identified as detrimental to oligodendrocytes, leading to demyelination through diverse hypothesized pathways. Reducing demyelination is essential to maintain neurological function in KD; however, therapeutic interventions are currently limited. Acetylcholinesterase inhibitors (AChEi) are commonly used for symptomatic management of Alzheimer's Disease and are suggested to have potential disease-modifying effects, including regulating myelin state. In particular, donepezil, an AChEi, has demonstrated promising effects in cellular and animal models, including promotion of the expression of myelin-related genes and reduction of glial cell reactivity. This drug also acts as an agonist for sigma-1 receptors (Sig-1R), which are implicated in demyelination diseases. In the context of drug repurposing, here, we demonstrate that administration of donepezil has protective effects in the twitcher mouse model of KD. We provide data showing that donepezil preserves myelin and reduces glial cell reactivity in the brains of twitcher mice. Moreover, donepezil also improves behavioral phenotypes and increases lifespan in twitcher animals. These findings suggest that donepezil, with its dual activity as an AChE inhibitor and Sig-1R agonist, may hold promise as a therapeutic candidate for demyelinating diseases, including KD.

Astrocyte extracellular matrix modulates neuronal dendritic development

Major developmental events occurring in the hippocampus during the third trimester of human gestation and neonatally in altricial rodents include rapid and synchronized dendritic arborization and astrocyte proliferation and maturation. We tested the hypothesis that signals sent by developing astrocytes to developing neurons modulate dendritic development in vivo. We altered neuronal development by neonatal (third trimester-equivalent) ethanol exposure in mice; this treatment increased dendritic arborization in hippocampal pyramidal neurons. We next assessed concurrent changes in the mouse astrocyte translatome by translating ribosomal affinity purification (TRAP)-seq. We followed up on ethanol-inhibition of astrocyte Chpf2 and Chsy1 gene translation because these genes encode for biosynthetic enzymes of chondroitin sulfate glycosaminoglycan (CS-GAG) chains (extracellular matrix components that inhibit neuronal development and plasticity) and have not been explored before for their roles in dendritic arborization. We report that Chpf2 and Chsy1 are enriched in astrocytes and their translation is inhibited by ethanol, which also reduces the levels of CS-GAGs measured by Liquid Chromatography/Mass Spectrometry. Finally, astrocyte-conditioned medium derived from Chfp2-silenced astrocytes increased neurite branching of hippocampal neurons in vitro. These results demonstrate that CS-GAG biosynthetic enzymes in astrocytes regulates dendritic arborization in developing neurons.

Synaptogenesis by Cholinergic Stimulation of Astrocytes

Astrocytes release numerous factors known to contribute to the process of synaptogenesis, yet knowledge about the signals that control their release is limited. We hypothesized that neuron-derived signals stimulate astrocytes, which respond to neurons through the modulation of astrocyte-released synaptogenic factors. Here we investigate the effect of cholinergic stimulation of astrocytes on synaptogenesis in co-cultured neurons. Using a culture system where primary rat astrocytes and primary rat neurons are first grown separately allowed us to independently manipulate astrocyte cholinergic signaling. Subsequent co-culture of pre-stimulated astrocytes with naïve neurons enabled us to assess how prior stimulation of astrocyte acetylcholine receptors uniquely modulates neuronal synapse formation. Pre-treatment of astrocytes with the acetylcholine receptor agonist carbachol increased the expression of synaptic proteins, the number of pre- and postsynaptic puncta, and the number of functional synapses in hippocampal neurons after 24 h in co-culture. Astrocyte secretion of the synaptogenic protein thrombospondin-1 increased after cholinergic stimulation and inhibition of the receptor for thrombospondins prevented the increase in neuronal synaptic structures. Thus, we identified a novel mechanism of neuron-astrocyte-neuron communication, where neuronal release of acetylcholine stimulates astrocytes to release synaptogenic proteins leading to increased synaptogenesis in neurons. This study provides new insights into the role of neurotransmitter receptors in developing astrocytes and into our understanding of the modulation of astrocyte-induced synaptogenesis.

Receptor-receptor interactions and microvesicle exchange as mechanisms modulating signaling between neurons and astrocytes

It is well known that astrocytes play a significant metabolic role in the nervous tissue, maintaining the homeostasis of the extracellular space and of the blood-brain barrier, and providing trophic support to neurons. In addition, however, evidence exists indicating astrocytes as important elements for brain activity through signaling exchange with neurons. Astrocytes, indeed, can sense synaptic activity and their molecular machinery responds to neurotransmitters released by neurons with cytoplasmic Ca²⁺ elevations that, in turn, stimulate the release of neuroactive substances (gliotransmitters) influencing nearby neurons. In both cell types the recognition and transduction of this complex pattern of signals is mediated by specific receptors that are also involved in mechanisms tuning the intercellular cross-talk between astrocytes and neurons. Two of these mechanisms are the focus of the present discussion. The first concerns direct receptor-receptor interactions leading to the formation at the cell membrane of multimeric receptor complexes. The cooperativity that emerges in the actions of orthosteric and allosteric ligands of the monomers forming the assembly provides the cell decoding apparatus with sophisticated and flexible dynamics in terms of recognition and signal transduction pathways. A further mechanism of plasticity involving receptors is based on the transfer of elements of the cellular signaling apparatus via extracellular microvesicles acting as protective containers, which can lead to transient changes in the transmitting/decoding capabilities of the target cell.

Synaptogenesis by Cholinergic Stimulation of Astrocytes

Astrocytes release numerous factors known to contribute to the process of synaptogenesis, yet knowledge about the signals that control their release is limited. We hypothesized that neuron-derived signals stimulate astrocytes, which respond by signaling back to neurons through the modulation of astrocyte-released synaptogenic factors. Here we investigate the effect of cholinergic stimulation of astrocytes on synaptogenesis in co-cultured neurons. Using a culture system where primary rat astrocytes and primary rat neurons are first grown separately allowed us to independently manipulate astrocyte cholinergic signaling. Subsequent co-culture of pre-stimulated astrocytes with naïve neurons enabled us to assess how prior stimulation of astrocyte acetylcholine receptors uniquely modulates neuronal synapse formation. Pre-treatment of astrocytes with the acetylcholine receptor agonist carbachol increased the expression of synaptic proteins, the number of pre- and postsynaptic puncta, and the number of functional synapses in hippocampal neurons after 24 hours in co-culture. Astrocyte secretion of the synaptogenic protein thrombospondin-1 increased after cholinergic stimulation and the inhibition of the target receptor for thrombospondins prevented the observed increase in neuronal synaptic structures. Thus, we identified a novel mechanism of neuron-astrocyte-neuron communication, i.e. , neuronal release of acetylcholine stimulates astrocytes to release synaptogenic proteins leading to increased synaptogenesis in neurons. This study provides new insights into the role of neurotransmitter receptors in developing astrocytes and into our understanding of the modulation of astrocyte-induced synaptogenesis.

Phospholipase D1 promotes astrocytic differentiation through the FAK/AURKA/STAT3 signaling pathway in hippocampal neural stem/progenitor cells

Phospholipase D1 (PLD1) plays a crucial role in cell differentiation of different cell types. However, the involvement of PLD1 in astrocytic differentiation remains uncertain. In the present study, we investigate the possible role of PLD1 and its product phosphatidic acid (PA) in astrocytic differentiation of hippocampal neural stem/progenitor cells (NSPCs) from hippocampi of embryonic day 16.5 rat embryos. We showed that overexpression of PLD1 increased the expression level of glial fibrillary acidic protein (GFAP), an astrocyte marker, and the number of GFAP-positive cells. Knockdown of PLD1 by transfection with Pld1 shRNA inhibited astrocytic differentiation. Moreover, PLD1 deletion (Pld1^-/-) suppressed the level of GFAP in the mouse hippocampus. These results indicate that PLD1 plays a crucial role in regulating astrocytic differentiation in hippocampal NSPCs. Interestingly, PA itself was sufficient to promote astrocytic differentiation. PA-induced GFAP expression was decreased by inhibition of signal transducer and activation of transcription 3 (STAT3) using siRNA. Furthermore, PA-induced STAT3 activation and astrocytic differentiation were regulated by the focal adhesion kinase (FAK)/aurora kinase A (AURKA) pathway. Taken together, our findings suggest that PLD1 is an important modulator of astrocytic differentiation in hippocampal NSPCs via the FAK/AURKA/STAT3 signaling pathway.

Emerging Role of Neuron-Glia in Neurological Disorders: At a Glance

Based on the diverse physiological influence, the impact of glial cells has become much more evident on neurological illnesses, resulting in the origins of many diseases appearing to be more convoluted than previously happened. Since neurological disorders are often random and unknown, hence the construction of animal models is difficult to build, representing a small fraction of people with a gene mutation. As a result, an immediate necessity is grown to work within in vitro techniques for examining these illnesses. As the scientific community recognizes cell-autonomous contributions to a variety of central nervous system illnesses, therapeutic techniques involving stem cells for treating neurological diseases are gaining traction. The use of stem cells derived from a variety of sources is increasingly being used to replace both neuronal and glial tissue. The brain's energy demands necessitate the reliance of neurons on glial cells in order for it to function properly. Furthermore, glial cells have diverse functions in terms of regulating their own metabolic activities, as well as collaborating with neurons via secreted signaling or guidance molecules, forming a complex network of neuron-glial connections in health and sickness. Emerging data reveals that metabolic changes in glial cells can cause morphological and functional changes in conjunction with neuronal dysfunction under disease situations, highlighting the importance of neuron-glia interactions in the pathophysiology of neurological illnesses. In this context, it is required to improve our understanding of disease mechanisms and create potential novel therapeutics. According to research, synaptic malfunction is one of the features of various mental diseases, and glial cells are acting as key ingredients not only in synapse formation, growth, and plasticity but also in neuroinflammation and synaptic homeostasis which creates critical physiological capacity in the focused sensory system. The goal of this review article is to elaborate state-of-the-art information on a few glial cell types situated in the central nervous system (CNS) and highlight their role in the onset and progression of neurological disorders.

Ubiquitin ligase Triad1 promotes neurite outgrowth by inhibiting MDM2-mediated ubiquitination of the neuroprotective factor pleiotropin

Spinal cord injury (SCI) is the most severe result of spine injury, but no effective therapy exists to treat SCI. We have previously shown that the E3 ubiquitin ligase Two RING fingers and DRIL 1 (Triad1) promotes neurite outgrowth after SCI. However, the mechanism by which Triad1 affects neuron growth and the potential involvement of its ubiquitination activity is unclear. Neuroprotective cytokine pleiotrophin (PTN) can promote microglia proliferation and neurotrophic factor secretion to achieve neuroprotection. We find using immunostaining and behavioral assays in rats that the expression of Triad1 and the PTN was peaked at 1 day after SCI and Triad1 improved motor function and histomorphological injury after SCI. We show using flow cytometry and astrocyte/neuronal coculture assays that Triad1 overexpression promoted PTN protein levels, neurotrophic growth factor (NGF) expression, brain-derived neurotrophic factor (BDNF) expression, astrocyte and neuronal viability, and neurite outgrowth but suppressed astrocyte apoptosis, while shRNA-mediated knockdown of Triad1 and PTN had the opposite effects. Ubiquitin ligase murine double mutant 2 (MDM2) has previously been demonstrated to participate in the process of neurite outgrowth and mediate ubiquitination of p53. Furthermore, we demonstrate overexpression of MDM2 downregulated PTN protein levels, NGF expression and BDNF expression in astrocytes, and inhibited neurite outgrowth of neurons. In addition, MDM2 facilitated PTN ubiquitination, which was reversed by Triad1. Finally, we show simultaneous sh-PTN and MDM2 overexpression attenuated the neurite outgrowth-promoting effect of Triad1 overexpression. In conclusion, we propose Triad1 promotes astrocyte-dependent neurite outgrowth to accelerate recovery after SCI by inhibiting MDM2-mediated PTN ubiquitination.

Crosstalk between neurons and glia through G-protein coupled receptors: Insights from Caenorhabditis elegans

The past decades have witnessed a dogmatic shift from glia as supporting cells in the nervous system to their active roles in neurocentric functions. Neurons and glia communicate and show bidirectional responses through tripartite synapses. Studies across species indicate that neurotransmitters released by neurons are perceived by glial receptors, which allow for gliotransmitter release. These gliotransmitters can result in activation of neurons via neuronal GPCR receptors. However, studies of these molecular interactions are in their infancy. Caenorhabditis elegans has a conserved neuron-glia architectural repertoire with molecular and functional resemblance to mammals. Further, glia in C. elegans can be manipulated through ablation and mutations allowing for deciphering of glial dependent processes in vivo at single glial resolutions. Here, we will review recent findings from vertebrate and invertebrate organisms with a focus on how C. elegans can be used to advance our understanding of neuron-glia interactions through GPCRs.

Astrocyte tissue plasminogen activator expression during brain development and its role in pyramidal neuron neurite outgrowth

The serine protease tissue plasminogen activator (tPA), encoded by the gene Plat, exerts a wide range of proteolysis-dependent and proteolysis-independent functions. In the developing brain, tPA is involved in neuronal development via the modulation of the proteolytic degradation of the extracellular matrix (ECM). Both lack of and excessive tPA are associated with neurodevelopmental disorders and with brain pathology. Astrocytes play a major role in neurite outgrowth of developing neurons as they are major producers of ECM proteins and ECM proteases. In this study we investigated the expression of Plat in developing and mature hippocampal and cortical astrocytes of Aldh1l1-EGFP-Rpl10a mice in vivo following Translating Ribosome Affinity Purification (TRAP) and the role of tPA in modulating astrocyte-mediated neurite outgrowth in an in vitro astrocyte-neuron co-culture system. We show that Plat is highly enriched in astrocytes in the developing, but not in the mature, hippocampus and cortex. Both the silencing of tPA expression in astrocytes and astrocyte exposure to recombinant tPA reduce neuritogenesis in co-cultured hippocampal neurons. These results suggest that astrocyte tPA is involved in modulating neuronal development and that tight control of astrocyte tPA expression is important for normal neuronal development, with both experimentally elevated and reduced levels of this proteolytic enzyme impairing neurite outgrowth. These results are consistent with the hypothesis that the ECM, by serving as adhesive substrate, enables neurite outgrowth, but that controlled proteolysis of the ECM is needed for growth cone advancement.

Altering Cell-Cell Interaction in Prenatal Alcohol Exposure Models: Insight on Cell-Adhesion Molecules During Brain Development

Alcohol exposure during pregnancy disrupts the development of the brain and produces long lasting behavioral and cognitive impairments collectively known as Fetal Alcohol Spectrum Disorders (FASDs). FASDs are characterized by alterations in learning, working memory, social behavior and executive function. A large body of literature using preclinical prenatal alcohol exposure models reports alcohol-induced changes in architecture and activity in specific brain regions affecting cognition. While multiple putative mechanisms of alcohol’s long-lasting effects on morphology and behavior have been investigated, an area that has received less attention is the effect of alcohol on cell adhesion molecules (CAMs). The embryo/fetal development represents a crucial period for Central Nervous System (CNS) development during which the cell-cell interaction plays an important role. CAMs play a critical role in neuronal migration and differentiation, synaptic organization and function which may be disrupted by alcohol. In this review, we summarize the physiological structure and role of CAMs involved in brain development, review the current literature on prenatal alcohol exposure effects on CAM function in different experimental models and pinpoint areas needed for future study to better understand how CAMs may mediate the morphological, sensory and behavioral outcomes in FASDs.

Regulators of cholinergic signaling in disorders of the central nervous system

Cholinergic signaling is crucial in cognitive processes, and degenerating cholinergic projections are a pathological hallmark in dementia. Use of cholinesterase inhibitors is currently the main treatment option to alleviate symptoms of Alzheimer's disease and has been postulated as a therapeutic strategy in acute brain damage (stroke and traumatic brain injury). However, the benefits of this treatment are still not clear. Importantly, cholinergic receptors are expressed both by neurons and by astrocytes and microglia, and binding of acetylcholine to the α7 nicotinic receptor in glial cells results in anti-inflammatory response. Similarly, the brain fine-tunes the peripheral immune response over the cholinergic anti-inflammatory axis. All of these processes are of importance for the outcome of acute and chronic neurological disease. Here, we summarize the main findings about the role of cholinergic signaling in brain disorders and provide insights into the complexity of molecular regulators of cholinergic responses, such as microRNAs and transfer RNA fragments, both of which may fine-tune the orchestra of cholinergic mRNAs. The available data suggest that these small noncoding RNA regulators may include promising biomarkers for predicting disease course and assessing treatment responses and might also serve as drug targets to attenuate signaling cascades during overwhelming inflammation and to ameliorate regenerative capacities of neuroinflammation.

Effects of ethanol-and choline-treated astrocytes on hippocampal neuron neurite outgrowth in vitro

Exposure to ethanol in utero can result in Fetal Alcohol Spectrum Disorders, which may cause long-lasting cognitive and behavioral abnormalities. Preclinical studies indicate that choline ameliorates the behavioral effects of developmental alcohol exposure in rodents, and clinical studies on the effectiveness of choline, administered early in pregnancy, showed that the adverse effects of heavy prenatal alcohol exposure on postnatal growth, and cognition in human infants were mitigated. However, little is known on the mechanisms behind the effects of choline. We have previously reported that astrocyte pre-treatment with 75 mM ethanol, in vitro, reduces neurite outgrowth in hippocampal neurons co-cultured with the pre-treated astrocytes. Our in vitro system allows us to study the effects of chemicals on astrocyte functions, able to modulate neuronal development. The main objective was to test the hypothesis that choline can ameliorate the astrocyte-mediated effects of ethanol on neurite growth. In this study, we exposed primary rat cortical astrocytes to ethanol, choline, ethanol plus choline, or control conditions for 24 h. Culture media was then removed, replaced with fresh media containing no ethanol or choline treatments and primary rat hippocampal neurons were plated on top of the astrocyte monolayer and cultured for 16 h. Neurons were then stained for β-III Tubulin and neurite outgrowth was measured. Astrocyte exposure to ethanol (25, 50, and 75 mM) decreases neurite outgrowth in co-cultured hippocampal pyramidal neurons, while astrocyte treatment with choline had no effect. Astrocyte treatment with ethanol and choline in combination, however, prevented the effect of ethanol, leading to levels of neurite outgrowth similar the control condition. Choline prevents the inhibitory effect of ethanol-treated astrocytes on neurite outgrowth while not altering normal neuronal development. These results suggest a new, astrocyte-mediated mechanism by which choline ameliorates the effects of developmental alcohol exposure.

Characterization of Glycosaminoglycan Disaccharide Composition in Astrocyte Primary Cultures and the Cortex of Neonatal Rats

Astrocytes are major producers of the extracellular matrix (ECM), which is involved in the plasticity of the developing brain. In utero alcohol exposure alters neuronal plasticity. Glycosaminoglycans (GAGs) are a family of polysaccharides present in the extracellular space; chondroitin sulfate (CS)- and heparan sulfate (HS)-GAGs are covalently bound to core proteins to form proteoglycans (PGs). Hyaluronic acid (HA)-GAGs are not bound to core proteins. In this study we investigated the contribution of astrocytes to CS-, HS-, and HA-GAG production by comparing the makeup of these GAGs in cortical astrocyte cultures and the neonatal rat cortex. We also explored alterations induced by ethanol in GAG and core protein levels in astrocytes. Finally, we investigated the relative expression in astrocytes of CS-PGs of the lectican family of proteins, major components of the brain ECM, in vivo using translating ribosome affinity purification (TRAP) (in Aldh1l1-EGFP-Rpl10a mice. Cortical astrocytes produce low levels of HA and show low expression of genes involved in HA biosynthesis compared to the whole developing cortex. Astrocytes have high levels of chondroitin-0-sulfate (C0S)-GAGs (possibly because of a higher sulfatase enzyme expression) and HS-GAGs. Ethanol upregulates C4S-GAGs as well as brain-specific lecticans neurocan and brevican, which are highly enriched in astrocytes of the developing cortex in vivo. These results begin to elucidate the role of astrocytes in the biosynthesis of CS- HS- and HA-GAGs, and suggest that ethanol-induced alterations of neuronal development may be in part mediated by increased astrocyte GAG levels and neurocan and brevican expression.

Altered cleavage plane orientation with increased genomic aneuploidy produced by receptor-mediated lysophosphatidic acid (LPA) signaling in mouse cerebral cortical neural progenitor cells

The brain is composed of cells having distinct genomic DNA sequences that arise post-zygotically, known as somatic genomic mosaicism (SGM). One form of SGM is aneuploidy-the gain and/or loss of chromosomes-which is associated with mitotic spindle defects. The mitotic spindle orientation determines cleavage plane positioning and, therefore, neural progenitor cell (NPC) fate during cerebral cortical development. Here we report receptor-mediated signaling by lysophosphatidic acid (LPA) as a novel extracellular signal that influences cleavage plane orientation and produces alterations in SGM by inducing aneuploidy during murine cortical neurogenesis. LPA is a bioactive lipid whose actions are mediated by six G protein-coupled receptors, LPA₁-LPA₆. RNAscope and qPCR assessment of all six LPA receptor genes, and exogenous LPA exposure in LPA receptor (Lpar)-null mice, revealed involvement of Lpar1 and Lpar2 in the orientation of the mitotic spindle. Lpar1 signaling increased non-vertical cleavage in vivo by disrupting cell-cell adhesion, leading to breakdown of the ependymal cell layer. In addition, genomic alterations were significantly increased after LPA exposure, through production of chromosomal aneuploidy in NPCs. These results identify LPA as a receptor-mediated signal that alters both NPC fate and genomes during cortical neurogenesis, thus representing an extracellular signaling mechanism that can produce stable genomic changes in NPCs and their progeny. Normal LPA signaling in early life could therefore influence both the developing and adult brain, whereas its pathological disruption could contribute to a range of neurological and psychiatric diseases, via long-lasting somatic genomic alterations.

Cholinergic Modulation of Glial Function During Aging and Chronic Neuroinflammation

Aging is a complex biological process that increases the risk of age-related cognitive degenerative diseases such as dementia, including Alzheimer’s disease (AD), Lewy Body Dementia (LBD), and mild cognitive impairment (MCI). Even non-pathological aging of the brain can involve chronic oxidative and inflammatory stress, which disrupts the communication and balance between the brain and the immune system. There has been an increasingly strong connection found between chronic neuroinflammation and impaired memory, especially in AD. While microglia and astrocytes, the resident immune cells of the central nervous system (CNS), exerting beneficial effects during the acute inflammatory phase, during chronic neuroinflammation they can become more detrimental. Central cholinergic circuits are involved in maintaining normal cognitive function and regulating signaling within the entire cerebral cortex. While neuronal-glial cholinergic signaling is anti-inflammatory and anti-oxidative, central cholinergic neuronal degeneration is implicated in impaired learning, memory sleep regulation, and attention. Although there is evidence of cholinergic involvement in memory, fewer studies have linked the cholinergic anti-inflammatory and anti-oxidant pathways to memory processes during development, normal aging, and disease states. This review will summarize the current knowledge of cholinergic effects on microglia and astroglia, and their role in both anti-inflammatory and anti-oxidant mechanisms, concerning normal aging and chronic neuroinflammation. We provided details on how stimulation of α7 nicotinic acetylcholine (α7nACh) receptors can be neuroprotective by increasing amyloid-β phagocytosis, decreasing inflammation and reducing oxidative stress by promoting the nuclear factor erythroid 2-related factor 2 (Nrf2) pathways and decreasing the release of pro-inflammatory cytokines. There is also evidence for astroglial α7nACh receptor stimulation mediating anti-inflammatory and antioxidant effects by inhibiting the nuclear factor-κB (NF-κB) pathway and activating the Nrf2 pathway respectively. We conclude that targeting cholinergic glial interactions between neurons and glial cells via α7nACh receptors could regulate neuroinflammation and oxidative stress, relevant to the treatment of several neurodegenerative diseases.

G-Protein-Coupled Receptors in Astrocyte-Neuron Communication

Astrocytes, a major type of glial cell, are known to play key supportive roles in brain function, contributing to ion and neurotransmitter homeostasis, maintaining the blood-brain barrier and providing trophic and metabolic support for neurons. Besides these support functions, astrocytes are emerging as important elements in brain physiology through signaling exchange with neurons at tripartite synapses. Astrocytes express a wide variety of neurotransmitter transporters and receptors that allow them to sense and respond to synaptic activity. Principal among them are the G-protein-coupled receptors (GPCRs) in astrocytes because their activation by synaptically released neurotransmitters leads to mobilization of intracellular calcium. In turn, activated astrocytes release neuroactive substances called gliotransmitters, such as glutamate, GABA, and ATP/adenosine that lead to synaptic regulation through activation of neuronal GPCRs. In this review we will present and discuss recent evidence demonstrating the critical roles played by GPCRs in the bidirectional astrocyte-neuron signaling, and their crucial involvement in the astrocyte-mediated regulation of synaptic transmission and plasticity.

Neurodevelopmental Outcomes of Prenatal Preeclampsia Exposure

Preeclampsia is a dangerous hypertensive disorder of pregnancy with known links to negative child health outcomes. Here, we review epidemiological and basic neuroscience work from the past several decades linking prenatal preeclampsia to altered neurodevelopment. This work demonstrates increased rates of neuropsychiatric disorders [e.g., increased autism spectrum disorder, attention deficit hyperactivity disorder (ADHD)] in children of preeclamptic pregnancies, as well as increased rates of cognitive impairments [e.g., decreased intelligence quotient (IQ), academic performance] and neurological disease (e.g., stroke and epilepsy). We also review findings from multiple animal models of preeclampsia. Manipulation of key clinical preeclampsia processes in these models (e.g., placental hypoxia, immune dysfunction, angiogenesis, oxidative stress) causes various disruptions in offspring, including ones in white matter/glia, glucocorticoid receptors, neuroimmune outcomes, cerebrovascular structure, and cognition/behavior. This animal work implicates potentially high-yield targets that may be leveraged in the future for clinical application.

Anchoring a dynamic in vitro model of human neuronal differentiation to key processes of early brain development in vivo

We characterize temporal pathway dynamics of differentiation in an in vitro neurotoxicity model with the aim of informing design and interpretation of toxicological assays. Human neural progenitor cells (hNPCs) were cultured in differentiation conditions up to 21 days. Genes significantly changed through time were identified and grouped according to temporal dynamics. Quantitative pathway analysis identified gene ontology (GO) terms enriched among significantly changed genes and provided a temporal roadmap of pathway trends in vitro. Gene expression in hNPCs was compared with publicly available gene expression data from developing human brain tissue in vivo. Quantitative pathway analysis of significantly changed genes and targeted analysis of specific pathways of interest identified concordance between in vivo and in vitro expression associated with proliferation, migration, differentiation, synapse formation, and neurotransmission. Our analysis anchors gene expression patterns in vitro to sensitive windows of in vivo development, helping to define appropriate applications of the model.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Acetylcholine suppresses the increase of glia fibrillary acidic protein expression via acetylcholine receptors in cAMP-induced astrocytic differentiation of rat C6 glioma cells

Astrocytes, the most common glial cells in the central nervous system, maintain neuronal functions and have roles in neurological diseases. Acetylcholine (ACh) is one of the most essential neurotransmitters, and ACh receptor (AChR) ligands were recently reported to influence astrocyte functions. However, the functions of ACh, the only endogenous agonist of AChR, in astrocytogenesis and in the expression of astrocytic marker genes have not been known. We previously demonstrated that the inhibition of acetylcholine esterase (AChE) suppressed the differentiation of rat glioma C6 cells, an astrocyte differentiation model, and we observed a suppressive effect of ACh agonists on astrocyte differentiation. Our present study revealed that in the cAMP-induced differentiation of C6 cells, an AChR antagonist alleviated the expression of glia fibrillary acidic protein (GFAP) that had been suppressed by dichlorvos (DDVP), an organophosphate and an AChE inhibitor. Our findings also demonstrated a direct effect of ACh on the GFAP expression, and that muscarinic AChR is involved in the suppressive effect of ACh on the GFAP expression in differentiation-induced C6 cells. This is the first report indicating that ACh the only endogenous agonist for AChRs functions as a mediator of astrocyte differentiation.

Advances in understanding the association between Down syndrome and Hirschsprung disease (DS-HSCR)

The clinical association between Trisomy 21 (Down syndrome) and aganglionosis (Hirschsprung disease; DS-HSCR) is well-established, being of the order of 5% and remains the most common congenital association with Hirschsprung disease. However, little consensus exists as to the possible etiologic and genetic factors influencing this association. Recent research has identified a number of levels at which development of the enteric nervous system is potentially affected in Trisomy 21. These include a decreased central pool of available neuroblasts for migration into the enteric nervous system, abnormal neuroblast type, poor synaptic nerve function and early germline gene-related influences on the migrating neuroblasts due to genetic mutations of a number of important developmental genes, and possible somatic mutations resulting from alterations in the local tissue microenvironment. In this paper, we review available evidence for this association. In addition, we provide evidence of both germline and somatic gene mutations suggesting causation. Although the picture is complex, recent associations between specific RET proto-oncogene variations have been shown to be significant in Down syndrome patients with Hirschsprung disease, as they probably interfere with vital RET functions in the development of the autonomic and enteric nervous systems, increasing the risk of disturbed normal function. In addition, we explore potential role of other facilitatory influence of other susceptibility genes as well as potential other chromosome 21 gene actions and the microenvironment on the Down syndrome gastro-intestinal tract. The various ways in which trisomy of chromosome influences the enteric nervous system are becoming clearer. The sum of these effects influences the outcome of surgery in Down syndrome patients with Hirschsprung Disease.

Plasminogen activator system homeostasis and its dysregulation by ethanol in astrocyte cultures and the developing brain

In utero alcohol exposure can cause fetal alcohol spectrum disorders (FASD), characterized by structural brain abnormalities and long-lasting behavioral and cognitive dysfunction. Neuronal plasticity is affected by in utero alcohol exposure and can be modulated by extracellular proteolysis. Plasmin is a major extracellular serine-protease whose activation is tightly regulated by the plasminogen activator (PA) system. In the present study we explored the effect of ethanol on the expression of the main components of the brain PA system in sex-specific cortical astrocyte primary cultures in vitro and in the cortex and hippocampus of post-natal day (PD) 9 male and female rats. We find that ethanol alters the PA system in astrocytes and in the developing brain. In particular, the expression of tissue-type PA (tPA), encoded by the gene Plat, is consistently upregulated by ethanol in astrocytes in vitro and in the cortex and hippocampus in vivo. Astrocytes exhibit endogenous plasmin activity that is increased by ethanol and recombinant tPA and inhibited by tPA silencing. We also find that tPA is expressed by astrocytes of the developing cortex and hippocampus in vivo. All components of the PA system investigated, with the exception of Neuroserpin/Serpini1, are expressed at higher levels in astrocyte cultures than in the developing brain, suggesting that astrocytes are major producers of these proteins in the brain. In conclusion, astrocyte PA system may play a major role in the modulation of neuronal plasticity; ethanol-induced upregulation of tPA levels and plasmin activity may be responsible for altered neuronal plasticity in FASD.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Contributions of polyunsaturated fatty acids (PUFA) on cerebral neurobiology: an integrated omics approach with epigenomic focus

The epigenetic landscape is vulnerable to diets. Here, we investigated the influence of different polyunsaturated fatty acids (PUFA) dietary supplements on rodents' nervous system development and functions and potential consequences to neurodegenerative disorders. Our previous nutrigenomics study showed significant impact of high n-3 PUFA-enriched diet (ERD) on synaptogenesis and various neuromodulators. The present study introduced a second equicaloric diet with n-6 PUFA balanced by n-3 PUFA (BLD). The typical lab diet with high n-6 PUFA was the baseline. Transcriptomic and epigenetic investigations, namely microRNA (miRNA) and DNA methylation assays, were carried out on the hemibrains of the C57BL/6j mice fed on any of these three diets from their neonatal age to midlife. Integrating the multiomics data, we focused on the genes encoding both hypermethylated CpG islands and suppressed transcripts. In addition, miRNA:mRNA pairs were screened to identify those overexpressed miRNAs that reduced transcriptional expressions. The majority of miRNAs overexpressed by BLD are associated with Alzheimer's and schizophrenia. BLD implicated long-term potentiation, memory, cognition and learning, primarily via hypermethylation of those genes that enrich the calcium-releasing neurotransmitters. ERD caused hypermethylation of those genes that enrich cytoskeletal development networks and promote the formation of neuronal precursors. We drew the present observations in light of our limited knowledge regarding the epigenetic influences on biofunctions. A more comprehensive study is essential to understand the broad influences of dietary supplements and to suggest optimal dietary solutions for neurological disorders.

The effect of titanium dioxide nanoparticles on neuroinflammation response in rat brain

Titanium dioxide nanoparticles (TiO₂ NPs) are widely used for their whiteness and opacity in several applications such as food colorants, drug additives, biomedical ceramic, and implanted biomaterials. Research on the neurobiological response to orally administered TiO₂ NPs is still limited. In our study, we investigate the effects of anatase TiO₂ NPs on the brain of Wistar rats after oral intake. After daily intragastric administration of anatase TiO₂ NPs (5-10 nm) at 0, 50, 100, and 200 mg/kg body weight (BW) for 60 days, the coefficient of the brain, acethylcholinesterase (AChE) activities, the level of interleukin 6 (IL-6), and the expression of glial fibrillary acidic protein (GFAP) were assessed to quantify the brain damage. The results showed that high-dose anatase TiO₂ NPs could induce a downregulated level of AChE activities and showed an increase in plasmatic IL-6 level as compared to the control group accompanied by a dose-dependent decrease inter-doses, associated to an increase in the cerebral IL-6 level as a response to a local inflammation in brain. Furthermore, we observed elevated levels of immunoreactivity to GFAP in rat cerebral cortex. We concluded that oral intake of anatase TiO₂ NPs can induce neuroinflammation and could be neurotoxic and hazardous to health.

Ethanol-induced changes in poly (ADP ribose) polymerase and neuronal developmental gene expression

Prenatal alcohol exposure has profound effects on neuronal growth and development. Poly-ADP Ribose Polymerase (PARP) enzymes are perhaps unique in the field of epigenetics in that they directly participate in histone modifications, transcription factor modifications, DNA methylation/demethylation and are highly inducible by ethanol. It was our hypothesis that ethanol would induce PARP enzymatic activity leading to alterations in neurodevelopmental gene expression. Mouse E18 cortical neurons were treated with ethanol, PARP inhibitors, and nuclear hormone receptor transcription factor PPARγ agonists and antagonists. Subsequently, we measured PARP activity and changes in Bdnf, OKSM (Oct4, Klf4, Sox2, c-Myc), DNA methylating/demethylating factors, and Pparγ mRNA expression, promoter 5-methylcytosine (5MC) and 5-hydroxymethylcytosine (5HMC), and PPARγ promoter binding. We found that ethanol reduced Bdnf4, 9a, and Klf4 mRNA expression, and increased c-Myc expression. These changes were reversed with a PARP inhibitor. In agreement with its role in DNA demethylation PARP inhibition increased 5MC levels at the c-Myc promoter. In addition, we found that inhibition of PARP enzymatic activity increased PPARγ promoter binding, and this corresponded to increased Bdnf and Klf4 mRNA expression. Our results suggest that PARP participates in DNA demethylation and reduces PPARγ promoter binding. The current study underscores the importance of PARP in ethanol-induced changes to neurodevelopmental gene expression.

Gadd45b and N-methyl-D-aspartate induced DNA demethylation in postmitotic neurons

AIM

In nondividing neurons examine the role of Gadd45b in active 5-methylcytosine (5MC) and 5-hydroxymethylcytosine (5HMC) removal at a gene promoter highly implicated in mental illnesses and cognition, Bdnf.

MATERIALS & METHODS

Mouse primary cortical neuronal cultures with and without Gadd45b siRNA transfection were treated with N-methyl-d-aspartate (NMDA). Expression changes of genes reportedly involved in DNA demethylation, Bdnf mRNA and protein and 5MC and 5HMC at Bdnf promoters were measured.

RESULTS

Gadd45b siRNA transfection in neurons abolishes the NMDA-induced increase in Bdnf IXa mRNA and reductions in 5MC and 5HMC at the Bdnf IXa promoter.

CONCLUSION

These results contribute to our understanding of DNA demethylation mechanisms in neurons, and its role in regulating NMDA responsive genes implicated in mental illnesses.

Astrocyte Intermediaries of Septal Cholinergic Modulation in the Hippocampus

The neurotransmitter acetylcholine, derived from the medial septum/diagonal band of Broca complex, has been accorded an important role in hippocampal learning and memory processes. However, the precise mechanisms whereby acetylcholine released from septohippocampal cholinergic neurons acts to modulate hippocampal microcircuits remain unknown. Here, we show that acetylcholine release from cholinergic septohippocampal projections causes a long-lasting GABAergic inhibition of hippocampal dentate granule cells in vivo and in vitro. This inhibition is caused by cholinergic activation of hilar astrocytes, which provide glutamatergic excitation of hilar inhibitory interneurons. These results demonstrate that acetylcholine release can cause slow inhibition of principal neuronal activity via astrocyte intermediaries.

Switching from astrocytic neuroprotection to neurodegeneration by cytokine stimulation

Astrocytes, the largest cell population in the human brain, are powerful inflammatory effectors. Several studies have examined the interaction of activated astrocytes with neurons, but little is known yet about human neurotoxicity under such situations and about strategies of neuronal rescue. To address this question, immortalized murine astrocytes (IMA) were combined with human LUHMES neurons and stimulated with an inflammatory (TNF, IL-1) cytokine mix (CM). Neurotoxicity was studied both in co-cultures and in monocultures after transfer of conditioned medium from activated IMA. Interventions with >20 drugs were used to profile the model system. Control IMA supported neurons and protected them from neurotoxicants. Inflammatory activation reduced this protection, and prolonged exposure of co-cultures to CM triggered neurotoxicity. Neither the added cytokines nor the release of NO from astrocytes were involved in this neurodegeneration. The neurotoxicity-mediating effect of IMA was faithfully reproduced by human astrocytes. Moreover, glia-dependent toxicity was also observed, when IMA cultures were stimulated with CM, and the culture medium was transferred to neurons. Such neurotoxicity was prevented when astrocytes were treated by p38 kinase inhibitors or dexamethasone, whereas such compounds had no effect when added to neurons. Conversely, treatment of neurons with five different drugs, including resveratrol and CEP1347, prevented toxicity of astrocyte supernatants. Thus, the sequential IMA-LUHMES neuroinflammation model is suitable for separate profiling of both glial-directed and directly neuroprotective strategies. Moreover, direct evaluation in co-cultures of the same cells allows for testing of therapeutic effectiveness in more complex settings, in which astrocytes affect pharmacological properties of neurons.

Fetal Alcohol Spectrum Disorders: An Overview from the Glia Perspective

Alcohol consumption during pregnancy can produce a variety of central nervous system (CNS) abnormalities in the offspring resulting in a broad spectrum of cognitive and behavioral impairments that constitute the most severe and long-lasting effects observed in fetal alcohol spectrum disorders (FASD). Alcohol-induced abnormalities in glial cells have been suspected of contributing to the adverse effects of alcohol on the developing brain for several years, although much research still needs to be done to causally link the effects of alcohol on specific brain structures and behavior to alterations in glial cell development and function. Damage to radial glia due to prenatal alcohol exposure may underlie observations of abnormal neuronal and glial migration in humans with Fetal Alcohol Syndrome (FAS), as well as primate and rodent models of FAS. A reduction in cell number and altered development has been reported for several glial cell types in animal models of FAS. In utero alcohol exposure can cause microencephaly when alcohol exposure occurs during the brain growth spurt a period characterized by rapid astrocyte proliferation and maturation; since astrocytes are the most abundant cells in the brain, microenchephaly may be caused by reduced astrocyte proliferation or survival, as observed in in vitro and in vivo studies. Delayed oligodendrocyte development and increased oligodendrocyte precursor apoptosis has also been reported in experimental models of FASD, which may be linked to altered myelination/white matter integrity found in FASD children. Children with FAS exhibit hypoplasia of the corpus callosum and anterior commissure, two areas requiring guidance from glial cells and proper maturation of oligodendrocytes. Finally, developmental alcohol exposure disrupts microglial function and induces microglial apoptosis; given the role of microglia in synaptic pruning during brain development, the effects of alcohol on microglia may be involved in the abnormal brain plasticity reported in FASD. The consequences of prenatal alcohol exposure on glial cells, including radial glia and other transient glial structures present in the developing brain, astrocytes, oligodendrocytes and their precursors, and microglia contributes to abnormal neuronal development, reduced neuron survival and disrupted brain architecture and connectivity. This review highlights the CNS structural abnormalities caused by in utero alcohol exposure and outlines which abnormalities are likely mediated by alcohol effects on glial cell development and function.

LPA-primed astrocytes induce axonal outgrowth of cortical progenitors by activating PKA signaling pathways and modulating extracellular matrix proteins

Lysophosphatidic acid (LPA) is one of the main membrane-derived lysophospholipids, inducing diverse cellular responses like cell proliferation, cell death inhibition, and cytoskeletal rearrangement, and thus is important in many biological processes. In the central nervous system (CNS), post-mitotic neurons release LPA extracellularly whereas astrocytes do not. Astrocytes play a key role in brain development and pathology, producing various cytokines, chemokines, growth factors, and extracellular matrix (ECM) components that act as molecular coordinators of neuron-glia communication. However, many molecular mechanisms underlying these events remain unclear-in particular, how the multifaceted interplay between the signaling pathways regulated by lysophospholipids is integrated in the complex nature of the CNS. Previously we showed that LPA-primed astrocytes induce neuronal commitment by activating LPA1-LPA2 receptors. Further, we revealed that these events were mediated by modulation and organization of laminin levels by astrocytes, through the induction of the epidermal growth factor receptor (EGFR) signaling pathway and the activation of the mitogen-activated protein (MAP) kinase (MAPK) cascade in response to LPA (Spohr et al., 2008, 2011). In the present work, we aimed to answer whether LPA affects astrocytic production and rearrangement of fibronectin, and to investigate the mechanisms involved in neuronal differentiation and maturation of cortical neurons induced by LPA-primed astrocytes. We show that PKA activation is required for LPA-primed astrocytes to induce neurite outgrowth and neuronal maturation and to rearrange and enhance the production of fibronectin and laminin. We propose a potential mechanism by which neurons and astrocytes communicate, as well as how such interactions drive cellular events such as neurite outgrowth, cell fate commitment, and maturation.

Role of astroglia in Down's syndrome revealed by patient-derived human-induced pluripotent stem cells

Down's syndrome (DS), caused by trisomy of human chromosome 21, is the most common genetic cause of intellectual disability. Here we use induced pluripotent stem cells (iPSCs) derived from DS patients to identify a role for astrocytes in DS pathogenesis. DS astroglia exhibit higher levels of reactive oxygen species and lower levels of synaptogenic molecules. Astrocyte-conditioned medium collected from DS astroglia causes toxicity to neurons, and fails to promote neuronal ion channel maturation and synapse formation. Transplantation studies show that DS astroglia do not promote neurogenesis of endogenous neural stem cells in vivo. We also observed abnormal gene expression profiles from DS astroglia. Finally, we show that the FDA-approved antibiotic drug, minocycline, partially corrects the pathological phenotypes of DS astroglia by specifically modulating the expression of S100B, GFAP, inducible nitric oxide synthase, and thrombospondins 1 and 2 in DS astroglia. Our studies shed light on the pathogenesis and possible treatment of DS by targeting astrocytes with a clinically available drug.

Synaptic structure quantification in cultured neurons

Behavioral problems (e.g., learning and memory) following developmental exposure to toxicants suggests that dysregulation of the process of synapse formation and function may occur. The ability to assess these changes is thus of value. This unit describes a method to investigate toxicant-induced changes to synaptic structure formation in primary hippocampal neurons using immunocytochemical labeling of the pre- and post-synaptic markers synaptophysin and PSD-95, confocal imaging, and three-dimensional object analysis. Protocols for the long-term culturing of primary hippocampal neurons and of primary cortical astrocytes, as well as their co-culture, are included. While the described methods focus on how astrocytes influence synapse formation and how toxicants may interfere in this process, modifications to the experimental plan can easily be implemented. This would allow for the investigation of the effects of toxicants after treating neurons alone, or both astrocytes and neurons in co-culture. With the common endpoint of synapse structure formation, differences between varying treatment paradigms can expand the understanding of the influence of particular toxicants on these diverse cell types and provide insight into potential mechanisms of effect and the contributions of each to synapse formation.

Astrocytes protect against diazinon- and diazoxon-induced inhibition of neurite outgrowth by regulating neuronal glutathione

Evidence demonstrating that human exposure to various organophosphorus insecticides (OPs) is associated with neurobehavioral deficits in children continues to emerge. The present study focused on diazinon (DZ) and its active oxygen metabolite, diazoxon (DZO), and explored their ability to impair neurite outgrowth in rat primary hippocampal neurons as a mechanism of developmental neurotoxicity. Both DZ and DZO (0.5-10 μM) significantly inhibited neurite outgrowth in hippocampal neurons, at concentrations devoid of any cyototoxicity. These effects appeared to be mediated by oxidative stress, as they were prevented by antioxidants (melatonin, N-t-butyl-alpha-phenylnitrone, and glutathione ethyl ester). Inhibition of neurite outgrowth was observed at concentrations below those required to inhibit the catalytic activity of acetylcholinesterase. The presence of astrocytes in the culture was able to provide protection against inhibition of neurite outgrowth by DZ and DZO. Astrocytes increased neuronal glutathione (GSH) in neurons, to levels comparable to those of GSH ethyl ester. Astrocytes depleted of GSH by L-buthionine-(S,R)-sulfoximine no longer conferred protection against DZ- and DZO-induced inhibition of neurite outgrowth. The findings indicate that DZ and DZO inhibit neurite outgrowth in hippocampal neurons by mechanisms involving oxidative stress, and that these effects can be modulated by astrocytes and astrocyte-derived GSH. Oxidative stress from other chemical exposures, as well as genetic abnormalities that result in deficiencies in GSH synthesis and regulation, may render individuals more susceptible to these developmental neurotoxic effects of OPs.

Diazinon and diazoxon impair the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons

Evidence from in vivo and epidemiological studies suggests that organophosphorus insecticides (OPs) are developmental neurotoxicants, but possible underlying mechanisms are still unclear. Astrocytes are increasingly recognized for their active role in normal neuronal development. This study sought to investigate whether the widely-used OP diazinon (DZ), and its oxygen metabolite diazoxon (DZO), would affect glial-neuronal interactions as a potential mechanism of developmental neurotoxicity. Specifically, we investigated the effects of DZ and DZO on the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons. The results show that both DZ and DZO adversely affect astrocyte function, resulting in inhibited neurite outgrowth in hippocampal neurons. This effect appears to be mediated by oxidative stress, as indicated by OP-induced increased reactive oxygen species production in astrocytes and prevention of neurite outgrowth inhibition by antioxidants. The concentrations of OPs were devoid of cytotoxicity, and cause limited acetylcholinesterase inhibition in astrocytes (18 and 25% for DZ and DZO, respectively). Among astrocytic neuritogenic factors, the most important one is the extracellular matrix protein fibronectin. DZ and DZO decreased levels of fibronectin in astrocytes, and this effect was also attenuated by antioxidants. Underscoring the importance of fibronectin in this context, adding exogenous fibronectin to the co-culture system successfully prevented inhibition of neurite outgrowth caused by DZ and DZO. These results indicate that DZ and DZO increase oxidative stress in astrocytes, and this in turn modulates astrocytic fibronectin, leading to impaired neurite outgrowth in hippocampal neurons.

Glia and neurodevelopment: focus on fetal alcohol spectrum disorders

During the last 20 years, new and exciting roles for glial cells in brain development have been described. Moreover, several recent studies implicated glial cells in the pathogenesis of neurodevelopmental disorders including Down syndrome, Fragile X syndrome, Rett Syndrome, Autism Spectrum Disorders, and Fetal Alcohol Spectrum Disorders (FASD). Abnormalities in glial cell development and proliferation and increased glial cell apoptosis contribute to the adverse effects of ethanol on the developing brain and it is becoming apparent that the effects of fetal alcohol are due, at least in part, to effects on glial cells affecting their ability to modulate neuronal development and function. The three major classes of glial cells, astrocytes, oligodendrocytes, and microglia as well as their precursors are affected by ethanol during brain development. Alterations in glial cell functions by ethanol dramatically affect neuronal development, survival, and function and ultimately impair the development of the proper brain architecture and connectivity. For instance, ethanol inhibits astrocyte-mediated neuritogenesis and oligodendrocyte development, survival and myelination; furthermore, ethanol induces microglia activation and oxidative stress leading to the exacerbation of ethanol-induced neuronal cell death. This review article describes the most significant recent findings pertaining the effects of ethanol on glial cells and their significance in the pathophysiology of FASD and other neurodevelopmental disorders.

Arylsulfatase B modulates neurite outgrowth via astrocyte chondroitin-4-sulfate: dysregulation by ethanol

In utero ethanol exposure causes fetal alcohol spectrum disorders, associated with reduced brain plasticity; the mechanisms of these effects are not well understood, particularly with respect to glial involvement. Astrocytes release factors that modulate neurite outgrowth. We explored the hypothesis that ethanol inhibits neurite outgrowth by increasing the levels of inhibitory chondroitin sulfate proteoglycans (CSPGs) in astrocytes. Astrocyte treatment with ethanol inhibited the activity of arylsulfatase B (ARSB), the enzyme that removes sulfate groups from chondroitin-4-sulfate (C4S) and triggers the degradation of C4S, increased total sulfated glycosaminoglycans (GAGs), C4S, and neurocan core-protein content and inhibited neurite outgrowth in neurons cocultured with ethanol-treated astrocytes in vitro, effects reversed by treatment with recombinant ARSB. Ethanol also inhibited ARSB activity and increased sulfate GAG and neurocan levels in the developing hippocampus after in vivo ethanol exposure. ARSB silencing increased the levels of sulfated GAGs, C4S, and neurocan in astrocytes and inhibited neurite outgrowth in cocultured neurons, indicating that ARSB activity directly regulates C4S and affects neurocan expression. In summary, this study reports two major findings: ARSB modulates sulfated GAG and neurocan levels in astrocytes and astrocyte-mediated neurite outgrowth in cocultured neurons; and ethanol inhibits the activity of ARSB, increases sulfated GAG, C4S, and neurocan levels, and thereby inhibits astrocyte-mediated neurite outgrowth. An unscheduled increase in CSPGs in the developing brain may lead to altered brain connectivity and to premature decrease in neuronal plasticity and therefore represents a novel mechanism by which ethanol can exert its neurodevelopmental effects.

Phospholipase D is a target for inhibition of astroglial proliferation by ethanol

The proliferation of astrocytes during early brain development is driven by growth factors and is accompanied by the activation of phospholipase D (PLD). Ethanol disrupts PLD signaling in astrocytes, a process which may contribute to delayed brain growth of fetuses exposed to alcohol during pregnancy. We here report that insulin-like growth factor 1 (IGF-1) is a strong mitogen for rat astrocytes (EC50 0.2 μg/ml) and a strong stimulator of astroglial PLD activity; both effects are inhibited by ethanol and 1-butanol, but not t-butanol, suggesting participation of PLD. Downregulation of PLD1 and exposure to the PLD1 inhibitor VU0359595 attenuated PLD activity and strongly reduced the mitogenic activity of serum and IGF-1. The PLD2 inhibitor VU0285655-1 also reduced PLD activity but had lesser effects on IGF-1-driven proliferation. PLD2 down-regulation affected serum - but not IGF-1-induced proliferation. In separate experiments, alcohol treatment of murine astrocytes taken from PLD-deficient animals revealed an insensitivity of PLD1(-/-) cells to 1-butanol whereas PLD2(-/-) cells were not affected. We conclude that astroglial proliferation induced by IGF-1 is critically dependent on the PLD signaling pathway, with a stronger contribution from PLD1 than PLD2. The teratogenic effects of ethanol may be explained, at least in part, by disruption of the IGF1-PLD signaling pathway.

Regulation of DNA methylation by ethanol induces tissue plasminogen activator expression in astrocytes

Alcohol exposure affects neuronal plasticity in the adult and developing brain. Astrocytes play a major role in modulating neuronal plasticity and are a target of ethanol. Tissue plasminogen activator (tPA) is involved in modulating neuronal plasticity by degrading the extracellular matrix proteins including fibronectin and laminin and is up-regulated by ethanol in vivo. In this study we explored the hypothesis that ethanol affects DNA methylation in astrocytes thereby increasing expression and release of tPA. It was found that ethanol increased tPA mRNA levels, an effect mimicked by an inhibitor of DNA methyltransferase (DNMT) activity. Ethanol also increased tPA protein expression and release, and inhibited DNMT activity with a corresponding decrease in DNA methylation levels of the tPA promoter. Furthermore, it was observed that protein levels of DNMT3A, but not DNMT1, were reduced in astrocytes after ethanol exposure. These novel studies show that ethanol inhibits DNA methylation in astrocytes leading to increased tPA expression and release; this effect may be involved in astrocyte-mediated inhibition of neuronal plasticity by alcohol.

Effect of cholinergic signaling on neuronal cell bioenergetics

Alzheimer's disease (AD) patients have reduced brain acetylcholine and reversing this deficit yields clinical benefits. In this study we explored how increased cholinergic tone impacts cell bioenergetics, which are also perturbed in AD. We treated SH-SY5Y neuroblastoma cells with carbachol, a cholinergic agonist, and tested for bioenergetic flux and bioenergetic infrastructure changes. Carbachol rapidly increased both oxidative phosphorylation and glycolysis fluxes. ATP levels rose slightly, as did cell energy demand, and AMPK phosphorylation occurred. At least some of these effects depended on muscarinic receptor activation, ER calcium release, and ER calcium re-uptake. Our data show that increasing cholinergic signaling enhances cell bioenergetics, and reveal mechanisms that mediate this effect. Phenomena we observed could potentially explain why cholinesterase inhibitor therapy increases AD brain glucose utilization and N-acetyl aspartate levels. The question of whether cholinesterase inhibitors have a disease modifying effect in AD has long been debated; our data suggest a theoretical mechanism through which such an effect could potentially arise.

The effects of caffeine on sleep and maturational markers in the rat

Adolescence is a critical period for brain maturation during which a massive reorganization of cortical connectivity takes place. In humans, slow wave activity (<4.5 Hz) during NREM sleep was proposed to reflect cortical maturation which relies on use-dependent processes. A stimulant like caffeine, whose consumption has recently increased especially in adolescents, is known to affect sleep wake regulation.

The goal of this study was to establish a rat model allowing to assess the relationship between cortical maturation and sleep and to further investigate how these parameters are affected by caffeine consumption. To do so, we assessed sleep and markers of maturation by electrophysiological recordings, behavioral and structural readouts in the juvenile rat. Our results show that sleep slow wave activity follows a similar inverted U-shape trajectory as already known in humans. Caffeine treatment exerted short-term stimulating effects and altered the trajectory of slow wave activity. Moreover, caffeine affected behavioral and structural markers of maturation. Thus, caffeine consumption during a critical developmental period shows long lasting effects on sleep and brain maturation.

Acetylcholine esterase is a regulator of GFAP expression and a target of dichlorvos in astrocytic differentiation of rat glioma C6 cells

The main target of neurotoxins is neurons because they comprise the main part of neural function, but glial cells may be indirect targets because they support the function of neurons. Among the glial cells, astrocytes in particular act as "nurse cells", regulating neuronal survival and functions. In the present study, to reveal whether a known neurotoxic substance, organophosphate dichlorvos (DDVP), affects the differentiation of astrocytes, we used an astrocyte differentiation model in rat glioma C6 cells. Morphological change and induction of GFAP expression in the differentiating C6 cells were suppressed by DDVP treatment. The known potential targets of DDVP are acetylcholine esterase (AChE), fatty acid amide hydrolase and methyl guanine methyl transferase. Among the specific inhibitors against these enzymes, the AChE inhibitor paraoxon successfully suppressed the cellular morphological changes and the induction of GFAP expression in differentiating C6 cells. These results indicate that DDVP inhibits differentiation in the C6 astrocyte-differentiation model, in which at least AChE inhibition is involved and that AChE is a potent regulator of the differentiation. Furthermore, considering that the main substrate of AChE is ACh, thus, ACh may act as regulators of astrocyte differentiation.

Valproic acid induces astrocyte-dependent neurite outgrowth from cultured rat primary cortical neuron via modulation of tPA/PAI-1 activity

Tissue plasminogen activator (tPA) is expressed in several regions of brain and plays regulatory roles such as neurite outgrowth, synaptic plasticity and long term potentiation. The activity of tPA is regulated by an endogenous inhibitor plasminogen activator inhibitor-1 (PAI-1), which is expressed mainly in astrocytes. Valproic acid (VPA), a histone deacetylase inhibitor that is used for the treatment of epilepsy and bipolar disorders, promotes neurite extension, neuronal growth and has neuroprotective effect in neurodegenerative diseases. In this study, we examined whether the neurite extension effects of VPA is mediated by modulating tPA/PAI-1 system. VPA dose-dependently increased tPA activity and decreased PAI-1 activity in rat primary astrocytes but not in neurons. PAI-1 protein level secreted into the culture medium but not tPA per se was decreased by VPA. In co-culture system or in neuronal culture stimulated with astrocyte conditioned media but not in pure neuronal cell culture, VPA induced neurite outgrowth via increased tPA activity due to the decreased PAI-1 activity in astrocytes. The decrease in PAI-1 activity and increased neurite extension was regulated via JNK mediated post-transcriptional pathway. The essential role of tPA/PAI-1 system in the regulation of VPA-mediated neurite extension was further demonstrated by experiments using astrocyte conditioned media obtained from tPA or PAI-1 knockout mice. Regulation of PAI-1 activity in astrocyte by VPA may affect both physiological and pathological processes in brain by upregulating tPA activity.

Nicotine uses neuron-glia communication to enhance hippocampal synaptic transmission and long-term memory

Nicotine enhances synaptic transmission and facilitates long-term memory. Now it is known that bi-directional glia-neuron interactions play important roles in the physiology of the brain. However, the involvement of glial cells in the effects of nicotine has not been considered until now. In particular, the gliotransmitter D-serine, an endogenous co-agonist of NMDA receptors, enables different types of synaptic plasticity and memory in the hippocampus. Here, we report that hippocampal long-term synaptic plasticity induced by nicotine was annulled by an enzyme that degrades endogenous D-serine, or by an NMDA receptor antagonist that acts at the D-serine binding site. Accordingly, both effects of nicotine: the enhancement of synaptic transmission and facilitation of long-term memory were eliminated by impairing glial cells with fluoroacetate, and were restored with exogenous D-serine. Together, these results show that glial D-serine is essential for the long-term effects of nicotine on synaptic plasticity and memory, and they highlight the roles of glial cells as key participants in brain functions.