Recent molecular approaches to understanding astrocyte function in vivo

Necroptosis in CNS diseases: Focus on astrocytes

In the last few years, necroptosis, a recently described type of cell death, has been reported to play an important role in the development of various brain pathologies. Necroptosis is a cell death mechanism that has morphological characteristics similar to necrosis but is mediated by fundamentally different molecular pathways. Necroptosis is initiated by signaling through the interaction of RIP1/RIP3/MLKL proteins (receptor-interacting protein kinase 1/receptor-interacting protein kinase 3/mixed lineage kinase domain-like protein). RIPK1 kinase is usually inactive under physiological conditions. It is activated by stimulation of death receptors (TNFR1, TNFR2, TLR3, and 4, Fas-ligand) by external signals. Phosphorylation of RIPK1 results in the formation of its complex with death receptors. Further, complexes with the second member of the RIP3 and MLKL cascade appear, and the necroptosome is formed. There is enough evidence that necroptosis plays an important role in the pathogenesis of brain ischemia and neurodegenerative diseases. In recent years, a point of view that both neurons and glial cells can play a key role in the development of the central nervous system (CNS) pathologies finds more and more confirmation. Astrocytes play complex roles during neurodegeneration and ischemic brain damage initiating both impair and protective processes. However, the cellular and molecular mechanisms that induce pathogenic activity of astrocytes remain veiled. In this review, we consider these processes in terms of the initiation of necroptosis. On the other hand, it is important to remember that like other types of programmed cell death, necroptosis plays an important role for the organism, as it induces a strong immune response and is involved in the control of cancerogenesis. In this review, we provide an overview of the complex role of necroptosis as an important pathogenetic component of neuronal and astrocyte death in neurodegenerative diseases, epileptogenesis, and ischemic brain damage.

Optogenetic and chemogenetic modulation of astroglial secretory phenotype

Astrocytes play a major role in brain function and alterations in astrocyte function that contribute to the pathogenesis of many brain disorders. The astrocytes are attractive cellular targets for neuroprotection and brain tissue regeneration. Development of novel approaches to monitor and to control astroglial function is of great importance for further progress in basic neurobiology and in clinical neurology, as well as psychiatry. Recently developed advanced optogenetic and chemogenetic techniques enable precise stimulation of astrocytes in vitro and in vivo, which can be achieved by the expression of light-sensitive channels and receptors, or by expression of receptors exclusively activated by designer drugs. Optogenetic stimulation of astrocytes leads to dramatic changes in intracellular calcium concentrations and causes the release of gliotransmitters. Optogenetic and chemogenetic protocols for astrocyte activation aid in extracting novel information regarding the function of brain's neurovascular unit. This review summarizes current data obtained by this approach and discusses a potential mechanistic connection between astrocyte stimulation and changes in brain physiology.

Connexin43 Expression and Associated Chronic Inflammation Presages the Development of Cerebral Radiation Necrosis

Cerebral radiation necrosis (CRN) is a delayed complication of radiosurgery that can result in severe neurological deficits. The biological changes leading to necrotic damage may identify therapeutic targets for this complication. Connexin43 expression associated with chronic inflammation may presage the development of CRN. A mouse model of delayed CRN was used. The left hemispheres of adult female mice were irradiated with single-fraction, high-dose radiation using a Leksell Gamma Knife. The brains were collected 1 and 4 days, and 1-3 weeks after the radiation. The expression of connexin43, interleukin-1β (IL-1β), GFAP, isolectin B-4, and fibrinogen was evaluated using immunohistochemical staining and image analysis. Compared with the baseline, the area of connexin43 and IL-1β staining was increased in ipsilateral hemispheres 4 days after radiation. Over the following 3 weeks, the density of connexin43 gradually increased in parallel with progressive increases in GFAP, isolectin B-4, and fibrinogen labeling. The overexpression of connexin43 in parallel with IL-1β spread into the affected brain regions first. Further intensified upregulation of connexin43 was associated with escalated astrocytosis, microgliosis, and blood-brain barrier breach. Connexin43-mediated inflammation may underlie radiation necrosis and further investigation of connexin43 hemichannel blockage is merited for the treatment of CRN.

Astrocytes: From the Physiology to the Disease

Astrocytes are key cells for adequate brain formation and regulation of cerebral blood flow as well as for the maintenance of neuronal metabolism, neurotransmitter synthesis and exocytosis, and synaptic transmission. Many of these functions are intrinsically related to neurodegeneration, allowing refocusing on the role of astrocytes in physiological and neurodegenerative states. Indeed, emerging evidence in the field indicates that abnormalities in the astrocytic function are involved in the pathogenesis of multiple neurodegenerative diseases, including Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). In the present review, we highlight the physiological role of astrocytes in the CNS, including their communication with other cells in the brain. Furthermore, we discuss exciting findings and novel experimental approaches that elucidate the role of astrocytes in multiple neurological disorders.

NDRG2 attenuates ischemia-induced astrocyte necroptosis via the repression of RIPK1

Cerebral ischemia results in severe brain damage, and is a leading cause of death and long-term disability. Previous studies have investigated methods to activate astrocytes in order to promote repair in injured brain tissue and inhibit cell death. It has previously been shown that N-myc downstream-regulated gene 2 (NDRG2) was highly expressed in astrocytes and associated with cell activity, but the underlying mechanism is largely unknown. The present study generated NDRG2 conditional knockout (Ndrg2-/-) mice to investigate whether NDRG2 can block ischemia-induced astrocyte necroptosis by suppressing receptor interacting protein kinase 1 (RIPK1) expression. This study investigated astrocyte activity in cerebral ischemia, and identified that ischemic brain injuries could trigger RIP-dependent astrocyte necroptosis. The depletion of NDRG2 was found to accelerate permanent middle cerebral artery occlusion-induced necroptosis in the brain tissue of Ndrg2-/- mice, indicating that NDRG2 may act as a neuroprotector during cerebral ischemic injury. The present study suggested that NDRG2 attenuated astrocytic cell death via the suppression of RIPK1. The pharmacological inhibition of astrocyte necroptosis by necrostatin-1 provided neuroprotection against ischemic brain injuries after NDRG2 knockdown. Therefore, NDRG2 could be considered as a potential target for the treatment of cerebral ischemia.

A stocked toolbox for understanding the role of astrocytes in disease

Our understanding of astrocytes and their role in neurological diseases has increased considerably over the past two decades as the diverse roles of these cells have become recognized. Our evolving understanding of these cells suggests that they are more than support cells for neurons and that they play important roles in CNS homeostasis under normal conditions, in neuroprotection and in disease exacerbation. These multiple functions make them excellent candidates for targeted therapies to treat neurological disorders. New technological advances, including in vivo imaging, optogenetics and chemogenetics, have allowed us to examine astrocytic functions in ways that have uncovered new insights into the dynamic roles of these cells. Furthermore, the use of induced pluripotent stem cell-derived astrocytes from patients with a host of neurological disorders can help to tease out the contributions of astrocytes to human disease. In this Review, we explore some of the technological advances developed over the past decade that have aided our understanding of astrocyte function. We also highlight neurological disorders in which astrocyte function or dysfunction is believed to have a role in disease pathogenesis or propagation and discuss how the technological advances have been and could be used to study each of these diseases.

Modeling vanishing white matter disease with patient-derived induced pluripotent stem cells reveals astrocytic dysfunction

Aims

Vanishing white matter disease (VWM) is an inherited leukoencephalopathy in children attributed to mutations in EIF2B1–5, encoding five subunits of eukaryotic translation initiation factor 2B (eIF2B). Although the defects are in the housekeeping genes, glial cells are selectively involved in VWM. Several studies have suggested that astrocytes are central in the pathogenesis of VWM. However, the exact pathomechanism remains unknown, and no model for VWM induced pluripotent stem cells (iPSCs) has been established.

Methods

Fibroblasts from two VWM children were reprogrammed into iPSCs by using a virus‐free nonintegrating episomal vector system. Control and VWM iPSCs were sequentially differentiated into neural stem cells (NSCs) and then into neural cells, including neurons, oligodendrocytes (OLs), and astrocytes.

Results

Vanishing white matter disease iPSC‐derived NSCs can normally differentiate into neurons, oligodendrocytes precursor cells (OPCs), and oligodendrocytes in vitro. By contrast, VWM astrocytes were dysmorphic and characterized by shorter processes. Moreover, δ‐GFAP and αB‐Crystalline were significantly increased in addition to increased early and total apoptosis.

Conclusion

The results provided further evidence supporting the central role of astrocytic dysfunction. The establishment of VWM‐specific iPSC models provides a platform for exploring the pathogenesis of VWM and future drug screening.

The Astrocyte-Neuron Interface: An Overview on Molecular and Cellular Dynamics Controlling Formation and Maintenance of the Tripartite Synapse

Astrocytes are known to provide trophic support to neurons and were originally thought to be passive space-filling cells in the brain. However, recent advances in astrocyte development and functions have highlighted their active roles in controlling brain functions by modulating synaptic transmission. A bidirectional cross talk between astrocytic processes and neuronal synapses define the concept of tripartite synapse. Any change in astrocytic structure/function influences neuronal activity which could lead to neurodevelopmental and neurodegenerative disorders. In this chapter, we briefly overview the methodologies used in deciphering the mechanisms of dynamic interplay between astrocytes and neurons.

Astrocytes integrate and drive action potential firing in inhibitory subnetworks

Many brain functions depend on the ability of neural networks to temporally integrate transient inputs to produce sustained discharges. This can occur through cell-autonomous mechanisms in individual neurons and through reverberating activity in recurrently connected neural networks. We report a third mechanism involving temporal integration of neural activity by a network of astrocytes. Previously, we showed that some types of interneurons can generate long-lasting trains of action potentials (barrage firing) following repeated depolarizing stimuli. Here we show that calcium signaling in an astrocytic network correlates with barrage firing; that active depolarization of astrocyte networks by chemical or optogenetic stimulation enhances; and that chelating internal calcium, inhibiting release from internal stores, or inhibiting GABA transporters or metabotropic glutamate receptors inhibits barrage firing. Thus, networks of astrocytes influence the spatiotemporal dynamics of neural networks by directly integrating neural activity and driving barrages of action potentials in some populations of inhibitory interneurons.

Specific types of inhibitory neurons exhibit prolonged, high-frequency barrages of action potentials. Here, the authors show that astrocytes might mediate such barrage firing.

Astrocytes, the rising stars of the glioblastoma microenvironment

Glioblastoma (GBM) is an aggressive primary tumor, causing thousands of deaths worldwide every year. The mean survival of patients with GBM remains below 20 months despite current available therapies. GBM cells' interactions with their stromal counterparts are crucial for tumor development. Astrocytes are glial cells that comprise ~50% of all brain cells and are therefore likely to establish direct contact with GBM cells. As other tumor cell types can hijack fibroblasts or immune cells to facilitate tumor growth, GBM cells can actually activate astrocytes, namely, the tumor associated astrocytes (TAAs), to promote GBM invasion in the healthy tissue. TAAs have thus been shown to be involved in GBM cells growth and limited response to radiation or chemotherapy (i.e., Temozolomide). Nevertheless, even though the interest in the cancer research community is increasing, the role of TAAs during GBM development is still overlooked. Yet, obtaining an in-depth understanding of the mechanisms by which TAAs influence GBM progression might lead to the development of new therapeutic strategies. This article therefore reports the different levels of GBM progression at which TAAs have been recently described to be involved in, including tumor cells' proliferation/invasion and resistance to therapies, especially through the activity of extracellular vesicles.

Dopamine D2 receptor restricts astrocytic NLRP3 inflammasome activation via enhancing the interaction of β-arrestin2 and NLRP3

Astrocytes are involved in the neuroinflammation of neurodegenerative diseases, such as Parkinson's disease (PD). Among the numerous inflammatory cytokines, interleukin-1β (IL-1β) produced by astrocytic Nod-like receptor protein (NLRP) inflammasome is crucial in the pathogenesis of PD. β-arrestin2-mediated dopamine D2 receptor (Drd2) signal transduction has been regarded as a potential anti-inflammatory target. Our previous study revealed that astrocytic Drd2 suppresses neuroinflammation in the central nervous system. However, the role of Drd2 in astrocytic NLRP3 inflammasome activation and subsequent IL-1β production remains unclear. In the present study, we used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced PD mouse model to investigate whether Drd2 could suppress astrocytic NLRP3 inflammasome activation. We showed that Drd2 agonist inhibited NLRP3 inflammasome activation, evidenced by decreased caspase-1 expression and reduced IL-1β release in the midbrain of wild type mice. The anti-inflammasome effect of Drd2 was abolished in β-arrestin2 knockout and β-arrestin2 small interfering RNA-injected mice, suggesting a critical role of β-arrestin2 in Drd2-regulated NLRP3 inflammasome activation. We also found that Drd2 agonists suppressed the upregulation of caspase-1 and IL-1β expression in primary cultured mouse astrocytes in response to the activation of NLRP3 inflammasome induced by lipopolysaccharide plus adenosine triphosphate. Furthermore, we demonstrated that β-arrestin2 mediated the inhibitory effect of Drd2 on NLRP3 inflammasome activation via interacting with NLRP3 and interfering the inflammasome assembly. Collectively, our study illustrates that astrocytic Drd2 inhibits NLRP3 inflammasome activation through a β-arrestin2-dependent mechanism, and provides a new strategy for treatment of PD.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

3,5,6,7,8,3',4'-Heptamethoxyflavone Ameliorates Depressive-Like Behavior and Hippocampal Neurochemical Changes in Chronic Unpredictable Mild Stressed Mice by Regulating the Brain-Derived Neurotrophic Factor: Requirement for ERK Activation

We previously reported that the subcutaneous administration of 3,5,6,7,8,3',4'-heptamethoxyflavone (HMF), a citrus polymethoxyflavone, attenuated depressive-like behavior and increased the expression of brain-derived neurotrophic factor (BDNF) in the hippocampus of a corticosterone-induced depression-like mouse model. We herein demonstrated that (1) HMF was detectable in the brain 10 and 30 min after its oral administration, (2) orally administered HMF improved chronic unpredictable mild stress (CUMS)-induced pathological conditions, including body weight loss and depressive-like behavior, and CUMS-induced neurochemical changes, such as reduction in BDNF expression, decrease in neurogenesis, and decreased level of phosphorylated calcium-calmodulin-dependent protein kinase II in the hippocampus, and (3) these effects of HMF were inhibited by the pre-administration of U0126, a mitogen-activated protein (MAP) kinase inhibitor. These results suggest that orally administered HMF is beneficial for the upregulation of BDNF in the hippocampus via the extracellular signal-regulated kinase1/2 (ERK1/2)/MAP system, which may account for its antidepression effects.

Hippocampal Astrocyte Cultures from Adult and Aged Rats Reproduce Changes in Glial Functionality Observed in the Aging Brain

Astrocytes are dynamic cells that maintain brain homeostasis, regulate neurotransmitter systems, and process synaptic information, energy metabolism, antioxidant defenses, and inflammatory response. Aging is a biological process that is closely associated with hippocampal astrocyte dysfunction. In this sense, we demonstrated that hippocampal astrocytes from adult and aged Wistar rats reproduce the glial functionality alterations observed in aging by evaluating several senescence, glutamatergic, oxidative and inflammatory parameters commonly associated with the aging process. Here, we show that the p21 senescence-associated gene and classical astrocyte markers, such as glial fibrillary acidic protein (GFAP), vimentin, and actin, changed their expressions in adult and aged astrocytes. Age-dependent changes were also observed in glutamate transporters (glutamate aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1)) and glutamine synthetase immunolabeling and activity. Additionally, according to in vivo aging, astrocytes from adult and aged rats showed an increase in oxidative/nitrosative stress with mitochondrial dysfunction, an increase in RNA oxidation, NADPH oxidase (NOX) activity, superoxide levels, and inducible nitric oxide synthase (iNOS) expression levels. Changes in antioxidant defenses were also observed. Hippocampal astrocytes also displayed age-dependent inflammatory response with augmentation of proinflammatory cytokine levels, such as TNF-α, IL-1β, IL-6, IL-18, and messenger RNA (mRNA) levels of cyclo-oxygenase 2 (COX-2). Furthermore, these cells secrete neurotrophic factors, including glia-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), S100 calcium-binding protein B (S100B) protein, and transforming growth factor-β (TGF-β), which changed in an age-dependent manner. Classical signaling pathways associated with aging, such as nuclear factor erythroid-derived 2-like 2 (Nrf2), nuclear factor kappa B (NFκB), heme oxygenase-1 (HO-1), and p38 mitogen-activated protein kinase (MAPK), were also changed in adult and aged astrocytes and are probably related to the changes observed in senescence marker, glutamatergic metabolism, mitochondrial dysfunction, oxidative/nitrosative stress, antioxidant defenses, inflammatory response, and trophic factors release. Together, our results reinforce the role of hippocampal astrocytes as a target for understanding the mechanisms involved in aging and provide an innovative tool for studies of astrocyte roles in physiological and pathological aging brain.

The nature of early astroglial protection-Fast activation and signaling

Our present review is focusing on the uniqueness of balanced astroglial signaling. The balance of excitatory and inhibitory signaling within the CNS is mainly determined by sharp synaptic transients of excitatory glutamate (Glu) and inhibitory γ-aminobutyrate (GABA) acting on the sub-second timescale. Astroglia is involved in excitatory chemical transmission by taking up i) Glu through neurotransmitter-sodium transporters, ii) K⁺ released due to presynaptic action potential generation, and iii) water keeping osmotic pressure. Glu uptake-coupled Na⁺ influx may either ignite long-range astroglial Ca²⁺ transients or locally counteract over-excitation via astroglial GABA release and increased tonic inhibition. Imbalance of excitatory and inhibitory drives is associated with a number of disease conditions, including prevalent traumatic and ischaemic injuries or the emergence of epilepsy. Therefore, when addressing the potential of early therapeutic intervention, astroglial signaling functions combating progress of Glu excitotoxicity is of critical importance. We suggest, that excitotoxicity is linked primarily to over-excitation induced by the impairment of astroglial Glu uptake and/or GABA release. Within this framework, we discuss the acute alterations of Glu-cycling and metabolism and conjecture the therapeutic promise of regulation. We also confer the role played by key carrier proteins and enzymes as well as their interplay at the molecular, cellular, and organ levels. Moreover, based on our former studies, we offer potential prospect on the emerging theme of astroglial succinate sensing in course of Glu excitotoxicity.

Phosphorylation of αB-crystallin supports reactive astrogliosis in demyelination

The small heat shock protein αB-crystallin (CRYAB) has been implicated in multiple sclerosis (MS) pathogenesis. Earlier studies have indicated that CRYAB inhibits inflammation and attenuates clinical disease when administered in the experimental autoimmune encephalomyelitis model of MS. In this study, we evaluated the role of CRYAB in primary demyelinating events. Using the cuprizone model of demyelination, a noninflammatory model that allows the analysis of glial responses in MS, we show that endogenous CRYAB expression is associated with increased severity of demyelination. Moreover, we demonstrate a strong correlation between the expression of CRYAB and the extent of reactive astrogliosis in demyelinating areas and in in vitro assays. In addition, we reveal that CRYAB is differentially phosphorylated in astrocytes in active demyelinating MS lesions, as well as in cuprizone-induced lesions, and that this phosphorylation is required for the reactive astrocyte response associated with demyelination. Furthermore, taking a proteomics approach to identify proteins that are bound by the phosphorylated forms of CRYAB in primary cultured astrocytes, we show that there is clear differential binding of protein targets due to the specific phosphorylation of CRYAB. Subsequent Ingenuity Pathway Analysis of these targets reveals implications for intracellular pathways and biological processes that could be affected by these modifications. Together, these findings demonstrate that astrocytes play a pivotal role in demyelination, making them a potential target for therapeutic intervention, and that phosphorylation of CRYAB is a key factor supporting the pathogenic response of astrocytes to oligodendrocyte injury.

New Transgenic Mouse Lines for Selectively Targeting Astrocytes and Studying Calcium Signals in Astrocyte Processes In Situ and In Vivo

Astrocytes exist throughout the nervous system and are proposed to affect neural circuits and behavior. However, studying astrocytes has proven difficult because of the lack of tools permitting astrocyte-selective genetic manipulations. Here, we report the generation of Aldh1l1-Cre/ERT2 transgenic mice to selectively target astrocytes in vivo. We characterized Aldh1l1-Cre/ERT2 mice using imaging, immunohistochemistry, AAV-FLEX-GFP microinjections, and crosses to RiboTag, Ai95, and new Cre-dependent membrane-tethered Lck-GCaMP6f knockin mice that we also generated. Two to three weeks after tamoxifen induction, Aldh1l1-Cre/ERT2 selectively targeted essentially all adult (P80) brain astrocytes with no detectable neuronal contamination, resulting in expression of cytosolic and Lck-GCaMP6f, and permitting subcellular astrocyte calcium imaging during startle responses in vivo. Crosses with RiboTag mice allowed sequencing of actively translated mRNAs and determination of the adult cortical astrocyte transcriptome. Thus, we provide well-characterized, easy-to-use resources with which to selectively study astrocytes in situ and in vivo in multiple experimental scenarios.

String Vessel Formation is Increased in the Brain of Parkinson Disease

BACKGROUND

String vessels are collapsed basement membrane without endothelium and have no function in circulation. String vessel formation contributes to vascular degeneration in Alzheimer disease. By comparing to age-matched control cases we have recently reported endothelial degeneration in brain capillaries of human Parkinson disease (PD).

OBJECTIVE

Current study evaluated changes of basement membrane of capillaries, string vessel formation and their association with astrocytes, blood-brain-barrier integrity and neuronal degeneration in PD.

METHODS

Brain tissue from human cases of PD and age-matched controls was used. Immunohistochemical staining for collagen IV, GFAP, NeuN, tyrosine hydroxylase, fibrinogen and Factor VIII was evaluated by image analysis in the substantia nigra, caudate nucleus and middle frontal gyrus.

RESULTS

While the basement-membrane-associated vessel density was similar between the two groups, the density of string vessels was significantly increased in the PD cases, particularly in the substantia nigra. Neuronal degeneration was found in all brain regions. Astrocytes and fibrinogen were increased in the caudate nuclei of PD cases compared with control cases.

CONCLUSIONS

Endothelial degeneration and preservation of basement membrane result in an increase of string vessel formation in PD. The data may suggest a possible role for cerebral hypoperfusion in the neuronal degeneration characteristic of PD, which needs further investigation. Elevated astrocytosis in the caudate nucleus of PD cases could be associated with disruption of the blood-brain barrier in this brain region.

Maternal antibodies and developing blood-brain barrier

We briefly review the protective role of maternal antibodies during fetal development and at early postnatal stages. We describe antibody delivery to fetuses, particularly in the context of the developing blood-brain barrier (BBB), and present the essential concepts regarding the adult BBB, together with existing information on the prenatal developing BBB. We focus on maternal antibody transfer to the developing brain and the consequences of the presence of pathogenic antibodies at early stages of brain development on subsequent brain dysfunction.

Neuroprotective effects of human umbilical cord-derived mesenchymal stromal cells combined with nimodipine against radiation-induced brain injury through inhibition of apoptosis

BACKGROUND AIMS

Mesenchymal stromal cells (MSCs) possess the ability to repair brain injuries. Additionally, nimodipine is a neuroprotective agent that increases cerebral blood flow and may help with the homing of MSCs to the injury site. Here we investigate the effectiveness of a combined human umbilical cord-derived MSCs and nimodipine therapy in radiation-induced brain injury (RIBI).

METHODS

Female mice received whole brain irradiation (WBI) and were treated with saline, nimodipine, hUC-MSCs, or hUC-MSCs combined with nimodipine. Body weight was measured weekly. An open field test for locomotor activity and a step-down avoidance test for learning and memory function were conducted at week 4 and week 12 post-WBI. The histological damage was evaluated by hematoxylin and eosin staining and glial fibrillary acidic protein immunohistochemistry. Quantitative polymerase chain reaction and Western blotting were used to detect apoptosis-related mediators (p53, Bax and Bcl-2).

RESULTS

In mice receiving the hUC-MSCs or the combined treatment, their body weight recovered, their locomotor and cognitive ability improved, and the percentage of necrotic neurons and astrocytes was reduced. The combined therapy was significantly (P < 0.05) more effective than hUC-MSCs alone; these mice showed decreased expression of pro-apoptotic indicators (p53, Bax) and increased expression of an anti-apoptotic indicator (Bcl-2), which may protect brain cells.

CONCLUSIONS

We demonstrated that hUC-MSCs therapy helps recover body weight loss and behavior dysfunction in a mice model of RIBI. Moreover, the effectiveness of the combined hUC-MSCs and nimodipine therapy is due to apoptosis inhibition and enhancing homing of MSCs to the injured brain.

Theoretical Approaches to Lentiviral Mediated Neurotrophin Delivery in Potential Treatments of Parkinson's Disease

Parkinson's disease is a late-onset neurodegenerative disease, characterized by both motor and non-motor symptoms. Motor symptoms include postural instability, rigidity, and tremor, while non-motor symptoms include anxiety, dementia, and depression. In this integrative review, we discuss PD disease pathophysiology in detail and introduce how neurotrophic growth factor delivery via a retroviral-based system can be used as efficacious tools for targeted gene therapy.

SDF-1 and CXCR4 play an important role in adult SVZ lineage cell proliferation and differentiation

Evidence has shown that stromal cell-derived factor (SDF-1/CXCL12) plays an important role in maintaining adult neural progenitor cells (NPCs). SDF-1 is also known to enhance recovery by recruiting NPCs to damaged regions and recent studies have revealed that SDF-1α exhibits pleiotropism, thereby differentially affecting NPC subpopulations. In this study, we investigated the role of SDF-1 in in vitro NPC self-renewal, proliferation and differentiation, following treatment with different concentrations of SDF-1 or a CXCR4 antagonist, AMD3100. We observed that AMD3100 inhibited the formation of primary neurospheres. However, SDF-1 and AMD3100 exhibited no effect on proliferation upon inclusion of growth factors in the media. Following growth factor withdrawal, AMD3100 and SDF-1 treatment resulted in differential effects on NPC proliferation. SDF-1, at a concentration of 500ng/ml, resulted in an increase in the relative proportion of oligodendrocytes following growth factor withdrawal-induced differentiation. Using CXCR4 knockout mice, we observed that SDF-1 affected NPC proliferation in the sub-ventricular zone (SVZ). We also investigated the occurrence of differential CXCR4 expression at different stages during lineage progression. These results clearly indicate that signaling interactions between SDF-1 and CXCR4 play an important role in adult SVZ lineage cell proliferation and differentiation.

Early P2X7R-related astrogliosis in autoimmune encephalomyelitis

Astrocytes are the main cells responsible for maintenance of brain homeostasis. Undisturbed action and signaling with other cells are crucial for proper functioning of the central nervous system (CNS). Dysfunctional astrocytes may determine the degree of neuronal injury and are associated with several brain pathologies, among which are multiple sclerosis (MS) and the animal model of this disease which is known as experimental autoimmune encephalomyelitis (EAE). One of the many functions of astrocytes is their response to CNS damage when they undergo reactive gliosis. Our data reveal that activation of astrocytes occurs in forebrains of immunized rats at a very early stage of EAE, well before the symptomatic phase of the disease. We have noted enhanced expression of GFAP and S100β starting from day 4 post-immunization. Temporal coincidence between the expression of astrocyte activation markers and the expression of connexin 43 and purinergic P2X7 receptor (P2X7R) was also observed. Administration of Brilliant blue G, an antagonist of P2X7R, significantly decreases astrogliosis as confirmed by immunohistochemical analysis and observation of decreased levels of GFAP and S100β. The condition of the treated animals was improved and the neurological symptoms of the disease were alleviated. With the knowledge that cerebral astroglia represent the main source of ATP and glutamate which are potentially neurotoxic substances released through P2X7R and connexin hemichannels, we suggest that astroglia may be involved in pathogenesis of MS/EAE at a very early stage through the purinergic/glutamatergic mechanisms.

Targeted gene transfer to the brain via the delivery of brain-penetrating DNA nanoparticles with focused ultrasound

Gene therapy holds promise for the treatment of many pathologies of the central nervous system (CNS), including brain tumors and neurodegenerative diseases. However, the delivery of systemically administered gene carriers to the CNS is hindered by both the blood-brain barrier (BBB) and the nanoporous and electrostatically charged brain extracelluar matrix (ECM), which acts as a steric and adhesive barrier. We have previously shown that these physiological barriers may be overcome by, respectively, opening the BBB with MR image-guided focused ultrasound (FUS) and microbubbles and using highly compact "brain penetrating" nanoparticles (BPN) coated with a dense polyethylene glycol corona that prevents adhesion to ECM components. Here, we tested whether this combined approach could be utilized to deliver systemically administered DNA-bearing BPN (DNA-BPN) across the BBB and mediate localized, robust, and sustained transgene expression in the rat brain. Systemically administered DNA-BPN delivered through the BBB with FUS led to dose-dependent transgene expression only in the FUS-treated region that was evident as early as 24h post administration and lasted for at least 28days. In the FUS-treated region ~42% of all cells, including neurons and astrocytes, were transfected, while less than 6% were transfected in the contralateral non-FUS treated hemisphere. Importantly, this was achieved without any sign of toxicity or astrocyte activation. We conclude that the image-guided delivery of DNA-BPN with FUS and microbubbles constitutes a safe and non-invasive strategy for targeted gene therapy to the brain.

Elusive roles for reactive astrocytes in neurodegenerative diseases

Astrocytes play crucial roles in the brain and are involved in the neuroinflammatory response. They become reactive in response to virtually all pathological situations in the brain such as axotomy, ischemia, infection, and neurodegenerative diseases (ND). Astrocyte reactivity was originally characterized by morphological changes (hypertrophy, remodeling of processes) and the overexpression of the intermediate filament glial fibrillary acidic protein (GFAP). However, it is unclear how the normal supportive functions of astrocytes are altered by their reactive state. In ND, in which neuronal dysfunction and astrocyte reactivity take place over several years or decades, the issue is even more complex and highly debated, with several conflicting reports published recently. In this review, we discuss studies addressing the contribution of reactive astrocytes to ND. We describe the molecular triggers leading to astrocyte reactivity during ND, examine how some key astrocyte functions may be enhanced or altered during the disease process, and discuss how astrocyte reactivity may globally affect ND progression. Finally we will consider the anticipated developments in this important field. With this review, we aim to show that the detailed study of reactive astrocytes may open new perspectives for ND.

Specific gene expression in mouse cortical astrocytes is mediated by a 1740bp-GFAP promoter-driven combined adeno-associated virus 2/5/7/8/9

We sought to demonstrate the in vivo transduction efficiency and tropism range in astrocytes of a combined-serotype adeno associated virus (AAV2/5/7/8/9). To control expression of enhanced green fluorescent protein (EGFP), a 1740bp glial fibrillary acidic protein (GFAP) promoter was obtained and ligated into vectors of each AAV serotype (2/5/7/8/9). Purified AAVs were then injected into the somatosensory cortex of C57BL/6J mice. Cell-type specific antibodies and subsequent immunofluorescence were used to identify astrocytes (GFAP), neurons (neuronal nuclear antigen, NeuN), microglia (ionized calcium-binding adapter molecule 1, Iba1), and oligodendrocytes (myelin basic protein, MBP), whereby, EGFP expression was measured in each cell type at 1-4 weeks post-injection. Our results indicated that the majority of astrocytes expressed EGFP, while only a small number of neurons expressed EGFP. Both microglia and oligodendrocytes lacked EGFP expression after viral injection. Quantitative analyses revealed that the percentage of EGFP-positive astrocytes was about 98% after viral injection, while the EGFP-positive neuronal percentage was less than 2%. Thus, this study shows that using a combined-serotype AAV carrying a 1740bp GFAP promoter results in successful, cell-type specific infection of the central nervous system, with robust gene expression in murine astrocytes.

In vivo tactile stimulation-evoked responses in Caenorhabditis elegans amphid sheath glia

Glial cells are important components of the nervous system. However, how they respond to physiological stimuli in vivo remains largely unknown. In this study, we investigated the electrophysiological activities and Ca²⁺ responses of the C. elegans amphid sheath glia (AMsh glia) to tactile stimulation in vivo. We recorded robust inward currents and Ca²⁺ elevation in the AMsh cell with the delivery of tactile stimuli of varying displacements to the nose tip of the worm. Compared to the adjacent mechanoreceptor ASH neuron, the AMsh cell showed greater sensitivity to tactile stimulation. Amiloride, an epithelial Na⁺ channel blocker, blocked the touch-induced currents and Ca²⁺ signaling in the ASH neuron, but not those in the AMsh cell. Taken together, our results revealed that AMsh glial cells actively respond to in vivo tactile stimulation and likely function cell-autonomously as mechanoreceptors.

Neuroglia and their roles in central respiratory control; an overview

While once viewed as mere housekeepers, providing structural and metabolic support for neurons, it is now clear that neuroglia do much more. Phylogenetically, they have undergone enormous proliferation and diversification as central nervous systems grew in their complexity. In addition, they: i) are morphologically and functionally diverse; ii) play numerous, vital roles in maintaining CNS homeostasis; iii) are key players in brain development and responses to injury; and, iv) via gliotransmission, are likely participants in information processing. In this review, we discuss the diverse roles of neuroglia in maintaining homeostasis in the CNS, their evolutionary origins, the different types of neuroglia and their functional significance for respiratory control, and finally consider evidence that they contribute to the processing of chemosensory information in the respiratory network and the homeostatic control of blood gases.

Astrocyte calcium signaling: from observations to functions and the challenges therein

We provide an overview of recent progress on the study of astrocyte intracellular Ca(2+) signaling. We consider the methods that have been used to monitor astrocyte Ca(2+) signals, the various types of Ca(2+) signals that have been discovered (waves, microdomains, and intrinsic fluctuations), the approaches used to broadly trigger and block Ca(2+) signals, and, where possible, the proposed and demonstrated physiological roles for astrocyte Ca(2+) signals within neuronal microcircuits. Although important progress has been made, we suggest that further detailed work is needed to explore the biophysics and molecular mechanisms of Ca(2+) signaling within entire astrocytes, including their fine distal extensions, such as processes that interact spatially with neurons and blood vessels. Improved methods are also needed to mimic and block molecularly defined types of Ca(2+) signals within genetically specified populations of astrocytes. Moreover, it will be essential to study astrocyte Ca(2+) activity in vivo to distinguish between pharmacological and physiological activity, and to study Ca(2+) activity in situ to rigorously explore mechanisms. Once methods to reliably measure, mimic, and block specific astrocyte Ca(2+) signals with high temporal and spatial precision are available, researchers will be able to carefully explore the correlative and causative roles that Ca(2+) signals may play in the functions of astrocytes, blood vessels, neurons, and microcircuits in the healthy and diseased brain.

Imaging intracellular Ca²⁺ signals in striatal astrocytes from adult mice using genetically-encoded calcium indicators

Astrocytes display spontaneous intracellular Ca(2+) concentration fluctuations ([Ca(2+)]i) and in several settings respond to neuronal excitation with enhanced [Ca(2+)]i signals. It has been proposed that astrocytes in turn regulate neurons and blood vessels through calcium-dependent mechanisms, such as the release of signaling molecules. However, [Ca(2+)]i imaging in entire astrocytes has only recently become feasible with genetically encoded calcium indicators (GECIs) such as the GCaMP series. The use of GECIs in astrocytes now provides opportunities to study astrocyte [Ca(2+)]i signals in detail within model microcircuits such as the striatum, which is the largest nucleus of the basal ganglia. In the present report, detailed surgical methods to express GECIs in astrocytes in vivo, and confocal imaging approaches to record [Ca(2+)]i signals in striatal astrocytes in situ, are described. We highlight precautions, necessary controls and tests to determine if GECI expression is selective for astrocytes and to evaluate signs of overt astrocyte reactivity. We also describe brain slice and imaging conditions in detail that permit reliable [Ca(2+)]i imaging in striatal astrocytes in situ. The use of these approaches revealed the entire territories of single striatal astrocytes and spontaneous [Ca(2+)]i signals within their somata, branches and branchlets. The further use and expansion of these approaches in the striatum will allow for the detailed study of astrocyte [Ca(2+)]i signals in the striatal microcircuitry.