Neurons modulate oxytocin receptor expression in rat cultured astrocytes: involvement of TGF-beta and membrane components

The Role of Oxytocin in Abnormal Brain Development: Effect on Glial Cells and Neuroinflammation

The neonatal period is critical for brain development and determinant for long-term brain trajectory. Yet, this time concurs with a sensitivity and risk for numerous brain injuries following perinatal complications such as preterm birth. Brain injury in premature infants leads to a complex amalgam of primary destructive diseases and secondary maturational and trophic disturbances and, as a consequence, to long-term neurocognitive and behavioral problems. Neuroinflammation is an important common factor in these complications, which contributes to the adverse effects on brain development. Mediating this inflammatory response forms a key therapeutic target in protecting the vulnerable developing brain when complications arise. The neuropeptide oxytocin (OT) plays an important role in the perinatal period, and its importance for lactation and social bonding in early life are well-recognized. Yet, novel functions of OT for the developing brain are increasingly emerging. In particular, OT seems able to modulate glial activity in neuroinflammatory states, but the exact mechanisms underlying this connection are largely unknown. The current review provides an overview of the oxytocinergic system and its early life development across rodent and human. Moreover, we cover the most up-to-date understanding of the role of OT in neonatal brain development and the potential neuroprotective effects it holds when adverse neural events arise in association with neuroinflammation. A detailed assessment of the underlying mechanisms between OT treatment and astrocyte and microglia reactivity is given, as well as a focus on the amygdala, a brain region of crucial importance for socio-emotional behavior, particularly in infants born preterm.

Targeting the Oxytocinergic System: A Possible Pharmacological Strategy for the Treatment of Inflammation Occurring in Different Chronic Diseases

Unresolved inflammation represents a central feature of different human pathologies including neuropsychiatric, cardiovascular, and metabolic diseases. The epidemiologic relevance of such disorders justifies the increasing interest in further understanding the mechanisms underpinning the inflammatory process occurring in such chronic diseases to provide potential novel pharmacological approaches. The most common and effective therapies for controlling inflammation are glucocorticoids; however, a variety of other molecules have been demonstrated to have an anti-inflammatory potential, including neuropeptides. In recent years, the oxytocinergic system has seen an explosion of scientific studies, demonstrating its potential to contribute to a variety of physiological processes including inflammation. Therefore, the aim of the present review was to understand the role of oxytocin in the modulation of inflammation occurring in different chronic diseases. The criterion we used to select the diseases was based on the emerging literature showing a putative involvement of the oxytocinergic system in inflammatory processes in a variety of pathologies including neurological, gastrointestinal and cardiovascular disorders, diabetes and obesity. The evidence reviewed here supports a beneficial role of oxytocin in the control of both peripheral and central inflammatory response happening in the aforementioned pathologies. Although future studies are necessary to elucidate the mechanistic details underlying such regulation, this review supports the idea that the modulation of the endogenous oxytocinergic system might represent a new potential pharmacological approach for the treatment of inflammation.

Oxytocin and Fear Memory Extinction: Possible Implications for the Therapy of Fear Disorders?

Several psychiatric conditions such as phobias, generalized anxiety, and post-traumatic stress disorder (PTSD) are characterized by pathological fear and anxiety. The main therapeutic approach used in the management of these disorders is exposure-based therapy, which is conceptually based upon fear extinction with the formation of a new safe memory association, allowing the reduction in behavioral conditioned fear responses. Nevertheless, this approach is only partially resolutive, since many patients have difficulty following the demanding and long process, and relapses are frequently observed over time. One strategy to improve the efficacy of the cognitive therapy is the combination with pharmacological agents. Therefore, the identification of compounds able to strengthen the formation and persistence of the inhibitory associations is a key goal. Recently, growing interest has been aroused by the neuropeptide oxytocin (OXT), which has been shown to have anxiolytic effects. Furthermore, OXT receptors and binding sites have been found in the critical brain structures involved in fear extinction. In this review, the recent literature addressing the complex effects of OXT on fear extinction at preclinical and clinical levels is discussed. These studies suggest that the OXT roles in fear behavior are due to its local effects in several brain regions, most notably, distinct amygdaloid regions.

Activation of septal OXTr neurons induces anxiety- but not depressive-like behaviors

The neuropeptide oxytocin (OXT) is well recognized for eliciting anxiolytic effects and promoting social reward. However, emerging evidence shows that OXT increases aversive events. These seemingly inconsistent results may be attributable to the broad OXT receptor (OXTr) expression in the central nervous system. This study selectively activated septal neurons expressing OXTr using chemogenetics. We found that chemogenetic activation of septal OXTr neurons induced anxiety- but not depressive-like behavior. In addition, septal OXTr neurons projected dense fibers to the horizontal diagonal band of Broca (HDB), and selective stimulation of those HDB projections also elicited anxiety-like behaviors. We also found that septal OXTr neurons express the vesicular GABA transporter (vGAT) protein and optogenetic stimulation of septal OXTr projections to the HDB inactivated HDB neurons. Our data collectively reveal that septal OXTr neurons increase anxiety by projecting inhibitory GABAergic inputs to the HDB.

Extrahypothalamic oxytocin neurons drive stress-induced social vigilance and avoidance

Oxytocin increases the salience of both positive and negative social contexts and it is thought that these diverse actions on behavior are mediated in part through circuit-specific action. This hypothesis is based primarily on manipulations of oxytocin receptor function, leaving open the question of whether different populations of oxytocin neurons mediate different effects on behavior. Here we inhibited oxytocin synthesis in a stress-sensitive population of oxytocin neurons specifically within the medioventral bed nucleus of the stria terminalis (BNSTmv). Oxytocin knockdown prevented social stress-induced increases in social vigilance and decreases in social approach. Viral tracing of BNSTmv oxytocin neurons revealed fibers in regions controlling defensive behaviors, including lateral hypothalamus, anterior hypothalamus, and anteromedial BNST (BNSTam). Oxytocin infusion into BNSTam in stress naïve mice increased social vigilance and reduced social approach. These results show that a population of extrahypothalamic oxytocin neurons plays a key role in controlling stress-induced social anxiety behaviors.

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

Social approach and social vigilance are differentially regulated by oxytocin receptors in the nucleus accumbens

Oxytocin is currently being considered as a novel therapeutic for anxiety disorders due to its ability to promote affiliative behaviors. In the nucleus accumbens (NAc) activation of oxytocin receptors (OTR) promotes social approach (time spent near an unfamiliar individual). Here, we show that stressful social experiences reduce the expression of NAc OTR mRNA, coinciding with decreases in social approach. Social stressors also increase social vigilance, characterized as orienting to an unfamiliar individual without approaching. Vigilance is a key component of behavioral inhibition, a personality trait that is a risk factor for anxiety disorders. To understand whether NAc OTR can modulate both social approach and vigilance, we use pharmacological approaches to assess the impact of activation or inhibition of NAc OTR downstream pathways on these behaviors. First, we show that in unstressed male and female California mice, inhibition of OTR by an unbiased antagonist (L-368,899) reduces social approach but does not induce social vigilance. Next, we show that infusion of Atosiban, an OTR-Gq antagonist/OTR-Gi agonist, has the same effect in unstressed females. Finally, we show that Carbetocin, a biased OTR-Gq agonist, increases social approach in stressed females while simultaneously inhibiting social vigilance. Taken together these data suggest that OTR in the NAc differentially modulate social approach and social vigilance, primarily through an OTR-Gq mechanism. Importantly, pharmacological inhibition of OTR alone is insufficient to induce vigilance in unstressed mice, suggesting that mechanisms modulating social approach may be distinct from mechanisms modulating social vigilance.

Peri-Infarct Upregulation of the Oxytocin Receptor in Vascular Dementia

Vascular dementia (VaD) is cognitive decline linked to reduced cerebral blood perfusion, yet there are few therapeutic options to protect cognitive function following cerebrovascular accidents. The purpose of this study was to profile gene expression changes unique to VaD to identify and characterize disease relevant changes that could offer clues for future therapeutic direction. Microarray-based profiling and validation studies of postmortem frontal cortex samples from VaD, Alzheimer disease, and age-matched control subjects revealed that the oxytocin receptor (OXTR) was strongly and differentially upregulated in VaD. Further characterization in fixed tissue from the same cases showed that OXTR upregulation occurs de novo around and within microinfarcts in peri-infarct reactive astrocytes as well as within vascular profiles, likely on microvascular endothelial cells. These results indicate that increased OXTR expression in peri-infarct regions may be a specific response to microvascular insults. Given the established OXTR signaling cascades that elicit antioxidant, anti-inflammatory, and pro-angiogenic responses, the present findings suggest that de novo OXTR expression in the peri-infarct space is a tissue-protective response by astroglial and vascular cells in the wake of ischemic damage that could be exploited as a therapeutic option for the preservation of cognition following cerebrovascular insults.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

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.

Sex Differences in the Embryonic Development of the Central Oxytocin System in Mice

Recent studies suggest that oxytocin (OXT) may be important for organising the neural circuitry that underlies adult social behaviour. Although most of the work exploring these effects has focused on early postnatal development, there is evidence that OXT may also be important during foetal development. However, without an understanding of how the OXT system develops, the ability to functionally link OXT in foetal life to adult behaviour is limited. To understand where and when OXT could be acting during embryonic development to affect the organisation of neural substrates, we examined the development of the mouse OXT system from embryonic day (E) 12.5 through postnatal day (PND) 2 using OXT receptor (OXTR) binding and a quantitative polymerase chain reaction. In both males and females, OXTR binding was observed by E16.5 in the ventricular and subventricular zones, as well as the developing amygdala. In males, OXT mRNA was not detectable until PND2, whereas it was detectable by E16.5 in females. OXTR mRNA was detected by E12.5 in both sexes, although females appear to have more OXTR mRNA during foetal development than males. The present study is significant because it is the first to reveal an unexpected sex difference in the development of the OXT system and supports the possibility that OXT during foetal development may contribute to sex differences in adult behaviour.

Role of TGFβ signaling in the pathogenesis of Alzheimer's disease

Aging is the main risk factor for Alzheimer’s disease (AD); being associated with conspicuous changes on microglia activation. Aged microglia exhibit an increased expression of cytokines, exacerbated reactivity to various stimuli, oxidative stress, and reduced phagocytosis of β-amyloid (Aβ). Whereas normal inflammation is protective, it becomes dysregulated in the presence of a persistent stimulus, or in the context of an inflammatory environment, as observed in aging. Thus, neuroinflammation can be a self-perpetuating deleterious response, becoming a source of additional injury to host cells in neurodegenerative diseases. In aged individuals, although transforming growth factor β (TGFβ) is upregulated, its canonical Smad3 signaling is greatly reduced and neuroinflammation persists. This age-related Smad3 impairment reduces protective activation while facilitating cytotoxic activation of microglia through several cellular mechanisms, potentiating microglia-mediated neurodegeneration. Here, we critically discuss the role of TGFβ-Smad signaling on the cytotoxic activation of microglia and its relevance in the pathogenesis of AD. Other protective functions, such as phagocytosis, although observed in aged animals, are not further induced by inflammatory stimuli and TGFβ1. Analysis in silico revealed that increased expression of receptor scavenger receptor (SR)-A, involved in Aβ uptake and cell activation, by microglia exposed to TGFβ, through a Smad3-dependent mechanism could be mediated by transcriptional co-factors Smad2/3 over the MSR1 gene. We discuss that changes of TGFβ-mediated regulation could at least partially mediate age-associated microglia changes, and, together with other changes on inflammatory response, could result in the reduction of protective activation and the potentiation of cytotoxicity of microglia, resulting in the promotion of neurodegenerative diseases.

Microglial cell dysregulation in brain aging and neurodegeneration

Aging is the main risk factor for neurodegenerative diseases. In aging, microglia undergoes phenotypic changes compatible with their activation. Glial activation can lead to neuroinflammation, which is increasingly accepted as part of the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD). We hypothesize that in aging, aberrant microglia activation leads to a deleterious environment and neurodegeneration. In aged mice, microglia exhibit an increased expression of cytokines and an exacerbated inflammatory response to pathological changes. Whereas LPS increases nitric oxide (NO) secretion in microglia from young mice, induction of reactive oxygen species (ROS) predominates in older mice. Furthermore, there is accumulation of DNA oxidative damage in mitochondria of microglia during aging, and also an increased intracellular ROS production. Increased ROS activates the redox-sensitive nuclear factor kappa B, which promotes more neuroinflammation, and can be translated in functional deficits, such as cognitive impairment. Mitochondria-derived ROS and cathepsin B, are also necessary for the microglial cell production of interleukin-1β, a key inflammatory cytokine. Interestingly, whereas the regulatory cytokine TGFβ1 is also increased in the aged brain, neuroinflammation persists. Assessing this apparent contradiction, we have reported that TGFβ1 induction and activation of Smad3 signaling after inflammatory stimulation are reduced in adult mice. Other protective functions, such as phagocytosis, although observed in aged animals, become not inducible by inflammatory stimuli and TGFβ1. Here, we discuss data suggesting that mitochondrial and endolysosomal dysfunction could at least partially mediate age-associated microglial cell changes, and, together with the impairment of the TGFβ1-Smad3 pathway, could result in the reduction of protective activation and the facilitation of cytotoxic activation of microglia, resulting in the promotion of neurodegenerative diseases.

Missing and Possible Link between Neuroendocrine Factors, Neuropsychiatric Disorders, and Microglia

Endocrine systems have long been suggested to be one of the important factors in neuropsychiatric disorders, while the underlying mechanisms have not been well understood. Traditionally, neuropsychiatric disorders have been mainly considered the consequence of abnormal conditions in neural circuitry. Beyond the neuronal doctrine, microglia, one of the glial cells with inflammatory/immunological functions in the central nervous system (CNS), have recently been suggested to play important roles in neuropsychiatric disorders. However, the crosstalk between neuroendocrine factors, neuropsychiatric disorders, and microglia has been unsolved. Therefore, we herein introduce and discuss a missing and possible link between these three factors; especially highlighting the following hormones; (1) Hypothalamic-Pituitary-Adrenal (HPA) axis-related hormones such as corticotropin-releasing hormone (CRH) and glucocorticoids, (2) sex-related hormones such as estrogen and progesterone, and (3) oxytocin. A growing body of evidence has suggested that these hormones have a direct effect on microglia. We hypothesize that hormone-induced microglial activation and the following microglia-derived mediators may lead to maladaptive neuronal networks including synaptic dysfunctions, causing neuropsychiatric disorders. Future investigations to clarify the correlation between neuroendocrine factors and microglia may contribute to a novel understanding of the pathophysiology of neuropsychiatric disorders.

Reactive astrogliosis after spinal cord injury-beneficial and detrimental effects

Reactive astrogliosis is a pathologic hallmark of spinal cord injury (SCI). It is characterised by profound morphological, molecular, and functional changes in astrocytes that occur within hours of SCI and evolves as time elapses after injury. Astrogliosis is a defense mechanism to minimize and repair the initial damage but eventually leads to some detrimental effects. Reactive astrocytes secrete a plethora of both growth promoting and inhibitory factors after SCI. However, the production of inhibitory components surpasses the growth stimulating factors, thus, causing inhibitory effects. In severe cases of injury, astrogliosis results in the formation of irreversible glial scarring that acts as regeneration barrier due to the expression of inhibitory components such as chondroitin sulfate proteoglycans. Scar formation was therefore recognized from a negative perspective for many years. Accumulating evidence from pharmacological and genetic studies now signifies the importance of astrogliosis and its timing for spinal cord repair. These studies have advanced our knowledge regarding signaling pathways and molecular mediators, which trigger and modulate reactive astrocytes and scar formation. In this review, we discuss the recent advances in this field. We also review therapeutic strategies that have been developed to target astrocytes reactivity and glial scaring in the environment of SCI. Astrocytes play pivotal roles in governing SCI mechanisms, and it is therefore crucial to understand how their activities can be targeted efficiently to harness their potential for repair and regeneration after SCI.

Astrocyte heterogeneity: an underappreciated topic in neurobiology

Astrocytes, one of the most numerous types of cells in the central nervous system, are crucial for potassium homeostasis, neurotransmitter uptake, synapse formation, regulation of blood-brain-barrier, and the development of the nervous system. Historically, astrocytes have been studied as a homogeneous group of cells. However, evidence has accumulated that suggests heterogeneity of astrocytes across brain regions as well as within the same brain regions. Astrocytes differ in their morphology, developmental origin, gene expression profile, physiological properties, function, and response to injury and disease. A better understanding of the heterogeneity of astrocytes will greatly aid investigation of the function of astrocytes in normal brain as well as the roles of astrocytes in neurological disorders.

Aging-dependent changes of microglial cells and their relevance for neurodegenerative disorders

Among multiple structural and functional brain changes, aging is accompanied by an increase of inflammatory signaling in the nervous system as well as a dysfunction of the immune system elsewhere. Although the long-held view that aging involves neurocognitive impairment is now dismissed, aging is a major risk factor for neurodegenerative diseases such as Alzheimer;s disease, Parkinson;s disease and Huntington's disease, among others. There are many age-related changes affecting the brain, contributing both to certain declining in function and increased frailty, which could singly and collectively affect neuronal viability and vulnerability. Among those changes, both inflammatory responses in aged brains and the altered regulation of toll like receptors, which appears to be relevant for understanding susceptibility to neurodegenerative processes, are linked to pathogenic mechanisms of several diseases. Here, we review how aging and pro-inflammatory environment could modulate microglial phenotype and its reactivity and contribute to the genesis of neurodegenerative processes. Data support our idea that age-related microglial cell changes, by inducing cytotoxicity in contrast to neuroprotection, could contribute to the onset of neurodegenerative changes. This view can have important implications for the development of new therapeutic approaches.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

The role of thrombospondin-1 and transforming growth factor-beta after spinal cord injury in the rat

Spinal cord injury (SCI) continues to result in high morbidity and mortality throughout the world. An effective neuroprotective agent is still not available to counteract secondary damage caused by traumatic injury. Thrombospondin-1 (TSP-1) and transforming growth factor-beta (TGF-beta) have a role in angiogenesis, scar deposition, inflammation and may affect astrocyte phenotype and mobility. We investigated the role of TSP-1 and TGF-beta in a model of spinal cord injury in rats. Forty female Sprague-Dawley rats were randomly divided into two equal groups: the experimental group was subject to SCI using an impactor and the sham-operated group was not subject to SCI. These animals were sacrificed at 12 h and 24 h after SCI for immunochemistry and Western blot analysis of the injured spinal segment for the expression of the TSP-1 and TGF-beta proteins. We found that TSP-1 and TGF-beta expression increased immediately after SCI in the injured segment. After 12 h, TSP-1 concentrations increased more rapidly and dramatically than TGF-beta in the injured segment of the spinal cord. Elevations in TSP-1 and TGF-beta concentrations persisted for 24 h after injury. These results show that elevated expression of TSP-1 and TGF-beta can be detected in the injured segment of the spinal cord 12 and 24 h after injury. Thus, TSP-1 and TGF-beta may have a role in SCI.

TGF-beta in neural stem cells and in tumors of the central nervous system

Mechanisms that regulate neural stem cell activity in the adult brain are tightly coordinated. They provide new neurons and glia in regions associated with high cellular and functional plasticity, after injury, or during neurodegeneration. Because of the proliferative and plastic potential of neural stem cells, they are currently thought to escape their physiological control mechanisms and transform to cancer stem cells. Signals provided by proteins of the transforming growth factor (TGF)-beta family might represent a system by which neural stem cells are controlled under physiological conditions but released from this control after transformation to cancer stem cells. TGF-beta is a multifunctional cytokine involved in various physiological and patho-physiological processes of the brain. It is induced in the adult brain after injury or hypoxia and during neurodegeneration when it modulates and dampens inflammatory responses. After injury, although TGF-beta is neuroprotective, it may limit the self-repair of the brain by inhibiting neural stem cell proliferation. Similar to its effect on neural stem cells, TGF-beta reveals anti-proliferative control on most cell types; however, paradoxically, many brain tumors escape from TGF-beta control. Moreover, brain tumors develop mechanisms that change the anti-proliferative influence of TGF-beta into oncogenic cues, mainly by orchestrating a multitude of TGF-beta-mediated effects upon matrix, migration and invasion, angiogenesis, and, most importantly, immune escape mechanisms. Thus, TGF-beta is involved in tumor progression. This review focuses on TGF-beta and its role in the regulation and control of neural and of brain-cancer stem cells.

TGF-β1/SMAD signaling induces astrocyte fate commitmentin vitro: Implications for radial glia development

Radial glial (RG) cells are specialized type of cell, which functions as neuronal precursors and scaffolding guides to migrating neurons during cerebral cortex development. After neurogenesis and migration are completed, most of RG cells transform into astrocytes. Mechanism and molecules involved in this process are not completely elucidated. We previously demonstrated that neurons activate the promoter of the astrocyte maturation marker GFAP in astrocytes by secretion of transforming growth factor beta 1 (TGF-beta1) in vitro. Here, we studied the role of neurons and TGF-beta1 pathway in RG differentiation. To address this question, we employed cortical progenitor cultures enriched in GLAST/nestin double-labeled cells, markers of RG cells. TGF-beta1 and conditioned medium derived from neuron-astrocyte cocultures (CM) decreased the number of cells expressing the precursor marker nestin and increased that expressing GFAP in cortical progenitor cultures. These events were impaired by addition of neutralizing antibodies against TGF-beta1. Increase in the number of GFAP positive cells was associated with Smads 2/3 nuclear translocation, a hallmark of TGF-beta1 pathway activation. PCR-assays revealed a decrease in the levels of mRNA for the RG marker, BLBP (brain lipid binding protein), due to TGF-beta1 and CM treatment. We further identified TGF-beta1 receptor in cortical progenitor cultures suggesting that these cells might be target for TGF-beta1 during development. Our work provides strong evidence that TGF-beta1 might be a novel factor involved in RG-astrocyte transformation and highlights the role of neuron-glia interaction in this process.

Interactive properties of human glioblastoma cells with brain neurons in culture and neuronal modulation of glial laminin organization

The harmonious development of the central nervous system depends on the interactions of the neuronal and glial cells. Extracellular matrix elements play important roles in these interactions, especially laminin produced by astrocytes, which has been shown to be a good substrate for neuron growth and axonal guidance. Glioblastomas are the most common subtypes of primary brain tumors and may be astrocytes in origin. As normal laminin-producing glial cells are the preferential substrate for neurons, and glial tumors have been shown to produce laminin, we questioned whether glioblastoma retained the same normal glial-neuron interactive properties with respect to neuronal growth and differentiation. Then, rat neurons were co-cultured onto rat normal astrocytes or onto three human glioblastoma cell lines obtained from neurosurgery. The co-culture confirmed that human glioblastoma cells as well as astrocytes maintained the ability to support neuritogenesis, but non-neural normal or tumoral cells failed to do so. However, glioblastoma cells did not distinguish embryonic from post-natal neurons in relation to neurite pattern in the co-cultures, as normal astrocytes did. Further, the laminin organization on both normal and tumoral glial cells was altered from a filamentous arrangement to a mixed punctuate/filamentous pattern when in co-culture with neurons. Together, these results suggest that glioblastoma cells could identify neuronal cells as partners, to support their growth and induce complex neurites, but they lost the normal glia property to distinguish neuronal age. In addition, our results show for the first time that neurons modulate the organization of astrocytes and glioblastoma laminin on the extracellular matrix.

Emerging roles for TGF-beta1 in nervous system development

Transforming growth factor betas (TGF-betas) are known as multifunctional growth factors, which participate in the regulation of key events of development, disease and tissue repair. In central nervous system (CNS), TGF-beta1 has been widely recognized as an injury-related cytokine, specially associated with astrocyte scar formation in response to brain injury. TGF-betas family is represented by three isoforms: TGF-beta1, -beta 2 and -beta 3, all produced by both glial and neuronal cells. They are involved in essential tissue functions, including cell-cycle control, regulation of early development and differentiation, neuron survival and astrocyte differentiation. TGF-beta signaling is mediated mainly by two serine threonine kinase receptors, TGFRI and TGFRII, which activate Smad 2/3 and Smad 4 transcription factors. Phosphorylation and activation of these proteins is followed by formation of Smad 2/3-4 complex, which translocates to the nucleus regulating transcriptional responses to TGF-beta. Very few data are available concerning the intracellular pathway required for the effect of TGF-beta in brain cells. Recently, emerging data on TGF-beta1 and its signaling molecules have been suggesting that besides its role in brain injury, TGF-beta1 might be a crucial regulator of CNS development. In this review, we will focus on TGF-betas members, specially TGF-beta1, in neuron and astrocyte development. We will discuss some advances concerning the emerging scenario of TGF-beta1 and its signaling pathways as putative modulators of astrocyte biology and their implications as a novel mediator of cellular interactions in the CNS.

Transforming growth factor-β1 produced by hippocampal cells modulates microglial reactivity in culture

Activated microglia produce superoxide anion (O2-) and nitric oxide (NO), both of which can be neurotoxic. To identify regulatory mechanisms that might modulate over-activation of microglia, we evaluated the inhibition of microglial activation by factors secreted by hippocampal cells. Supernatants from hippocampal cell cultures (Hippocampal-Cm) prevented microglial O2- and NO production. LAP-TGF beta1 was present in the Hippocampal-Cm as shown by immunoblot and a TGF beta1-dependent proliferation-inhibition bioassay. LAP-TGF beta1 and TGFbeta activity increased in hippocampal cultures exposed to proinflammatory conditions (LPS and Interferon-gamma). The inhibition of O2- and NO production by Hippocampal-Cm was mimicked by the addition of recombinant TGF beta1. Treating Hippocampal-Cm with an antibody against TGF beta1 to neutralize its activity eliminated its ability to inhibit O2- and NO production. Our findings suggest that the TGF beta1 secreted by hippocampal cells modulated microglial activity. We propose that in pathological conditions, impairment of this modulatory mechanism could enhance microglia-mediated neurotoxicity.

Glial fibrillary acidic protein gene promoter is differently modulated by transforming growth factor-beta 1 in astrocytes from distinct brain regions

The expression of glial fibrillary acidic protein (GFAP), the major intermediate filament protein of mature astrocytes, is regulated under developmental and pathological conditions. Recently, we have investigated GFAP gene modulation by using a transgenic mouse bearing part of the GFAP gene promoter linked to the beta-galactosidase reporter gene. We demonstrated that cerebral cortex neurons activate the GFAP gene promoter, inducing transforming growth factor-beta 1 (TGF-beta 1) secretion by astrocytes. Here, we report that cortical neurons or conditioned medium derived from them do not activate the GFAP gene promoter of transgenic astrocytes derived from midbrain and cerebellum suggesting a neuroanatomical regional specificity of this phenomenon. Surprisingly, they do induce synthesis of TGF-beta 1 by these cells. Western blot and immunocytochemistry assays revealed wild distribution of TGF receptor in all subpopulations of astrocytes and expression of TGF-beta 1 in neurons derived from all regions, thus indicating that the unresponsiveness of the cerebellar and midbrain GFAP gene to TGF-beta 1 is not due to a defect in TGF-beta 1 signalling. Together, our data highlight the great complexity of neuron-glia interactions and might suggest a distinct mechanism underlying modulation of the GFAP gene in the heterogeneous population of astrocytes throughout the central nervous system.

Neuron-conditioned media differentially affect the survival of activated or unstimulated microglia: evidence for neuronal control on apoptotic elimination of activated microglia

It is presently unknown what types of neuronal signals maintain microglial cells resting in the normal brain or control their activation in neuropathology. Recent data suggest that microglia activation induces apoptosis and that healthy neurons are controllers of the activation state and immune functions of microglia. In the present study we have evaluated, on microglial cells in cultures, whether neurons are able to affect their survival in resting conditions or upon activation with the bacterial endotoxin, lipopolysaccharide (LPS). We report that neuron-conditioned culture media induced apoptosis of LPS-stimulated, but not of unstimulated, microglia. This effect was, however, only present when conditioned media had been exposed to differentiated neurons and not to immature ones, and was absent when glutamate receptors had been pharmacologically blocked in neuronal cultures. The effect was also blocked by heat-inactivation of the conditioned media. Media conditioned with either differentiated or undifferentiated cerebellar granule neurons positively affected the survival of unstimulated microglial cells when the standard concentration of fetal bovine serum (10%) was included in the culture media. Our results highlight the ability of differentiated neurons to maintain a controlled inflammatory state through production of factor(s) favoring the apoptotic elimination of activated microglia. They also suggest that immature neurons may, on the contrary, favor the survival of microglia during development.

Neuro-glia interaction effects on GFAP gene: a novel role for transforming growth factor-beta1

Central nervous system (CNS) development is highly guided by microenvironment cues specially provided by neuron-glia interactions. By using a transgenic mouse bearing part of the gene promoter of the astrocytic maturation marker GFAP (glial fibrillary acidic protein) linked to the beta-galactosidase (beta-Gal) reporter gene, we previously demonstrated that cerebral cortical neurons increase transgenic beta-Gal astrocyte number and activate GFAP gene promoter by secretion of soluble factors in vitro. Here, we identified TGF-beta1 as the major mediator of this event. Identification of TGF-beta1 in neuronal and astrocyte extracts revealed that both cell types might synthesize this factor, however, addition of neurons to astrocyte monolayers greatly increased TGF-beta1 synthesis and secretion by astrocytes. Further, by exploiting the advantages of cell culture system we investigated the influence of neuron and astrocyte developmental stage on such interaction. We demonstrated that younger neurons derived from 14 embryonic days wild-type mice were more efficient in promoting astrocyte differentiation than those derived from 18 embryonic days mice. Similarly, astrocytes also exhibited timed-schedule developed responsiveness to neuronal influence with embryonic astrocytes being more responsive to neurons than newborn and late postnatal astrocytes. RT-PCR assays identified TGF-beta1 transcripts in young but not in old neurons, suggesting that inability to induce astrocyte differentiation is related to TGF-beta1 synthesis and secretion. Our work reveals an important role for neuron-glia interactions in astrocyte development and strongly implicates the involvement of TGF-beta1 in this event.

Effects of progesterone on gastric emptying and intestinal transit in male rats

AIM: To study the dose-dependent of progesterone (P) effect and the interaction between the oxytocin (OT) and P on gastrointestinal motility.

METHODS: In order to monitor the gastric emptying and intestinal transit, the SD male rats were intubated via a catheter with normal saline (3 mL/kg) containing Na₂⁵¹CrO₄ (0.5 μCi/mL) and 10% charcoal. OT was dissolved into normal saline and P was dissolved into 75% alcohol.

RESULTS: Low does of P (1 mg/kg, i.p.) enhanced the gastric emptying (75% ± 3%, P < 0.05) and high dose of P (5 mg/kg, i.p.) inhibit it (42% ± 11.2%, P < 0.01). P (1 mg/kg) increased the intestinal transit (4.2 ± 0.3, P < 0.05) while the higher dose (10-20 mg/kg) had no effect. OT (0.8 mg/kg, i.p.) inhibited the gastric emptying (23.5% ± 9.8%, P < 0.01). The inhibitory effects of P (20 mg/kg) (32% ± 9.7%, P < 0.05) and OT (0.8 mg/kg) on gastric emptying enhanced each other when the two chemicals were administrated simultaneously (17% ± 9.4%, P < 0.01).

CONCLUSION: Low dose of P increased GI motility while high dose of P decreased it. During the later period of pregnancy, elevated plasma level of OT may also participate in the gastrointestinal inhibition.