The Proinflammatory RAGE/NF-κB Pathway Is Involved in Neuronal Damage and Reactive Gliosis in a Model of Sleep Apnea by Intermittent Hypoxia

Sleep apnea (SA) causes long-lasting changes in neuronal circuitry, which persist even in patients successfully treated for the acute effects of the disease. Evidence obtained from the intermittent hypoxia (IH) experimental model of SA has shown neuronal death, impairment in learning and memory and reactive gliosis that may account for cognitive and structural alterations observed in human patients. However, little is known about the mechanism controlling these deleterious effects that may be useful as therapeutic targets in SA. The Receptor for Advanced Glycation End products (RAGE) and its downstream effector Nuclear Factor Kappa B (NF-κB) have been related to neuronal death and astroglial conversion to the pro-inflammatory neurodegenerative phenotype. RAGE expression and its ligand S100B were shown to be increased in experimental models of SA. We here used dissociated mixed hippocampal cell cultures and male Wistar rats exposed to IH cycles and observed that NF-κB is activated in glial cells and neurons after IH. To disclose the relative contribution of the S100B/RAGE/NF-κB pathway to neuronal damage and reactive gliosis after IH we performed sequential loss of function studies using RAGE or S100B neutralizing antibodies, a herpes simplex virus (HSV)-derived amplicon vector that induces the expression of RAGEΔcyto (dominant negative RAGE) and a chemical blocker of NF-κB. Our results show that NF-κB activation peaks 3 days after IH exposure, and that RAGE or NF-κB blockage during this critical period significantly improves neuronal survival and reduces reactive gliosis. Both in vitro and in vivo, S100B blockage altered reactive gliosis but did not have significant effects on neuronal survival. We conclude that both RAGE and downstream NF-κB signaling are centrally involved in the neuronal alterations found in SA models, and that blockage of these pathways is a tempting strategy for preventing neuronal degeneration and reactive gliosis in SA.     

Thioredoxin 1 overexpression attenuated diabetes-induced endoplasmic reticulum stress in Müller cells via apoptosis signal-regulating kinase 1

As one of the common and serious chronic complications of diabetes mellitus (DM), the related mechanism of diabetic retinopathy (DR) has not been fully understood. Müller cell reactive gliosis is one of the early pathophysiological features of DR. Therefore, exploring the manner to reduce diabetes-induced Müller cell damage is essential to delay DR. Thioredoxin 1 (Trx1), one of the ubiquitous redox enzymes, plays a vital role in redox homeostasis via protein–protein interactions, including apoptosis signal-regulating kinase 1 (ASK1). Previous studies have shown that upregulation of Trx by some drugs can attenuate endoplasmic reticulum stress (ERS) in DR, but the related mechanism was unclear. In this study, we used DM mouse and high glucose (HG)-cultured human Müller cells as models to clarify the effect of Trx1 on ERS and the underlying mechanism. The data showed that the diabetes-induced Müller cell damage was increased significantly. Moreover, the expression of ERS and reactive gliosis was also upregulated in diabetes in vivo and in vitro. However, it was reversed after Trx1 overexpression. Besides, ERS-related protein expression, reactive gliosis, and apoptosis were decreased after transfection with ASK1 small-interfering RNA in stable Trx1 overexpression Müller cells after HG treatment. Taken together, Trx1 could protect Müller cells from diabetes-induced damage, and the underlying mechanism was related to inhibited ERS via ASK1.     

Genetic dissection reveals two separate pathways for rod and cone regeneration in the teleost retina

Development of therapies to treat visual system dystrophies resulting from the degeneration of rod and cone photoreceptors may directly benefit from studies of animal models, such as the zebrafish, that display continuous retinal neurogenesis and the capacity for injury-induced regeneration. Previous studies of retinal regeneration in fish have been conducted on adult animals and have relied on methods that cause acute damage to both rods and cones, as well as other retinal cell types. We report here the use of a genetic approach to study progenitor cell responses to photoreceptor degeneration in the larval and adult zebrafish retina. We have compared the responses to selective rod or cone degeneration using, respectively, the XOPS-mCFP transgenic line and zebrafish with a null mutation in the pde6c gene. Notably, rod degeneration induces increased proliferation of progenitors in the outer nuclear layer (ONL) and is not associated with proliferation or reactive gliosis in the inner nuclear layer (INL). Molecular characterization of the rod progenitor cells demonstrated that they are committed to the rod photoreceptor fate while they are still mitotic. In contrast, cone degeneration induces both Müller cell proliferation and reactive gliosis, with little change in proliferation in the ONL. We found that in both lines, proliferative responses to photoreceptor degeneration can be observed as 7 days post fertilization (dpf). These two genetic models therefore offer new opportunities for investigating the molecular mechanisms of selective degeneration and regeneration of rods and cones.     

Glial connexin expression and function in the context of Alzheimer's disease

A hallmark of neurodegenerative diseases is the reactive gliosis characterized by a phenotypic change in astrocytes and microglia. This glial response is associated with modifications in the expression and function of connexins (Cxs), the proteins forming gap junction channels and hemichannels. Increased Cx expression is detected in most reactive astrocytes located at amyloid plaques, the histopathological lesions typically present in the brain of Alzheimer's patients and animal models of the disease. The activity of Cx channels analyzed in vivo as well as in vitro after treatment with the amyloid β peptide is also modified and, in particular, hemichannel activation may contribute to neuronal damage. In this review, we summarize and discuss recent data that suggest glial Cx channels participate in the neurodegenerative process of Alzheimer's disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.     

Reciprocal Modulation Between Microglia and Astrocyte in Reactive Gliosis Following the CNS Injury

Reactive gliosis, also known as glial scar formation, is an inflammatory response characterized by the proliferation of microglia and astrocytes as well as astrocytic hypertrophy following injury in the central nervous system (CNS). The glial scar forms a physical and molecular barrier to isolate the injured area from adjacent normal nervous tissue for re-establishing the integrity of the CNS. It prevents the further spread of cellular damage but represents an obstacle to regrowing axons. In this review, we integrated the current findings to elucidate the tightly reciprocal modulation between activated microglia and astrocytes in reactive gliosis and proposed that modification of cellular response to the injury or cellular reprogramming in the glial scar could lead advances in axon regeneration and functional recovery after the CNS injury.     

Translation of the Prion Protein mRNA Is Robust in Astrocytes but Does Not Amplify during Reactive Astrocytosis in the Mouse Brain

Prion diseases induce neurodegeneration in specific brain areas for undetermined reasons. A thorough understanding of the localization of the disease-causing molecule, the prion protein (PrP), could inform on this issue but previous studies have generated conflicting conclusions. One of the more intriguing disagreements is whether PrP is synthesized by astrocytes. We developed a knock-in reporter mouse line in which the coding sequence of the PrP expressing gene (Prnp), was replaced with that for green fluorescent protein (GFP). Native GFP fluorescence intensity varied between and within brain regions. GFP was present in astrocytes but did not increase during reactive gliosis induced by scrapie prion infection. Therefore, reactive gliosis associated with prion diseases does not cause an acceleration of local PrP production. In addition to aiding in Prnp gene activity studies, this reporter mouse line will likely prove useful for analysis of chimeric animals produced by stem cell and tissue transplantation experiments.     

Inhibition of reactive gliosis attenuates excitotoxicity-mediated death of retinal ganglion cells

Reactive gliosis is a hallmark of many retinal neurodegenerative conditions, including glaucoma. Although a majority of studies to date have concentrated on reactive gliosis in the optic nerve head, very few studies have been initiated to investigate the role of reactive gliosis in the retina. We have previously shown that reactive glial cells synthesize elevated levels of proteases, and these proteases, in turn, promote the death of retinal ganglion cells (RGCs). In this investigation, we have used two glial toxins to inhibit reactive gliosis and have evaluated their effect on protease-mediated death of RGCs. Kainic acid was injected into the vitreous humor of C57BL/6 mice to induce reactive gliosis and death of RGCs. C57BL/6 mice were also treated with glial toxins, alpha-aminoadipic acid (AAA) or Neurostatin, along with KA. Reactive gliosis was assessed by immunostaining of retinal cross sections and retinal flat-mounts with glial fibrillary acidic protein (GFAP) and vimentin antibodies. Apoptotic cell death was assessed by TUNEL assays. Loss of RGCs was determined by immunostaining of flat-mounted retinas with Brn3a antibodies. Proteolytic activities of matrix metalloproteinase-9 (MMP-9), tissue plasminogen activator (tPA), and urokinase plasminogen activator (uPA) were assessed by zymography assays. GFAP-immunoreactivity indicated that KA induced reactive gliosis in both retinal astrocytes and in Muller cells. AAA alone or in combination with KA decreased GFAP and vimentin-immunoreactivity in Mϋller cells, but not in astrocytes. In addition AAA failed to decrease KA-mediated protease levels and apoptotic death of RGCs. In contrast, Neurostatin either alone or in combination with KA, decreased reactive gliosis in both astrocytes and Mϋller cells. Furthermore, Neurostatin decreased protease levels and prevented apoptotic death of RGCs. Our findings, for the first time, indicate that inhibition of reactive gliosis decreases protease levels in the retina, prevents apoptotic death of retinal neurons, and provides substantial neuroprotection.     

CPEB1 modulates lipopolysaccharide-mediated iNOS induction in rat primary astrocytes

Upon CNS damage, astrocytes undergo a series of biological changes including increased proliferation, production of inflammatory mediators and morphological changes, in a response collectively called reactive gliosis. This process is an essential part of the brains response to injury, yet much is unknown about the molecular mechanism(s) that induce these changes. In this study, we investigated the role of cytoplasmic polyadenylation element binding protein 1 (CPEB1) in the regulation of inflammatory responses in a model of reactive gliosis, lipopolysaccharide-stimulated astrocytes. CPEB1 is an mRNA-binding protein recently shown to be expressed in astrocytes that may play a role in astrocytes migration. After LPS stimulation, the expression and phosphorylation of CPEB1 was increased in rat primary astrocytes in a JNK-dependent process. siRNA-induced knockdown of CPEB1 expression inhibited the LPS-induced up-regulation of iNOS as well as NO and ROS production, a hallmark of immunological activation of astrocytes. The results from the study suggest that CPEB1 is actively involved in the regulation of inflammatory responses in astrocytes, which might provide new insights into the regulatory mechanism after brain injury.     

Cannabidiol reduces intestinal inflammation through the control of neuroimmune axis

Enteric glial cells (EGC) actively mediate acute and chronic inflammation in the gut; EGC proliferate and release neurotrophins, growth factors, and pro-inflammatory cytokines which, in turn, may amplify the immune response, representing a very important link between the nervous and immune systems in the intestine. Cannabidiol (CBD) is an interesting compound because of its ability to control reactive gliosis in the CNS, without any unwanted psychotropic effects. Therefore the rationale of our study was to investigate the effect of CBD on intestinal biopsies from patients with ulcerative colitis (UC) and from intestinal segments of mice with LPS-induced intestinal inflammation. CBD markedly counteracted reactive enteric gliosis in LPS-mice trough the massive reduction of astroglial signalling neurotrophin S100B. Histological, biochemical and immunohistochemical data demonstrated that S100B decrease was associated with a considerable decrease in mast cell and macrophages in the intestine of LPS-treated mice after CBD treatment. Moreover the treatment of LPS-mice with CBD reduced TNF-α expression and the presence of cleaved caspase-3. Similar results were obtained in ex vivo cultured human derived colonic biopsies. In biopsies of UC patients, both during active inflammation and in remission stimulated with LPS+INF-γ, an increased glial cell activation and intestinal damage were evidenced. CBD reduced the expression of S100B and iNOS proteins in the human biopsies confirming its well documented effect in septic mice. The activity of CBD is, at least partly, mediated via the selective PPAR-gamma receptor pathway. CBD targets enteric reactive gliosis, counteracts the inflammatory environment induced by LPS in mice and in human colonic cultures derived from UC patients. These actions lead to a reduction of intestinal damage mediated by PPARgamma receptor pathway. Our results therefore indicate that CBD indeed unravels a new therapeutic strategy to treat inflammatory bowel diseases.     

Hepatic stellate cells and astrocytes

Scar formation inhibits tissue repair and regeneration in the liver and central nervous system. Activation of hepatic stellate cells (HSCs) after liver injury or of astrocytes after nervous system damage is considered to drive scar formation. HSCs are the fibrotic cells of the liver, as they undergo activation and acquire fibrogenic properties after liver injury. HSC activation has been compared to reactive gliosis of astrocytes, which acquire a reactive phenotype and contribute to scar formation after nervous system injury, much like HSCs after liver injury. It is intriguing that a wide range of neuroglia-related molecules are expressed by HSCs. We identified an unexpected role for the p75 neurotrophin receptor in regulating HSC activation and liver repair. Here we discuss the molecular mechanisms that regulate HSC activation and reactive gliosis and their contributions to scar formation and tissue repair. Juxtaposing key mechanistic and functional similarities in HSC and astrocyte activation might provide novel insight into liver regeneration and nervous system repair.