CARD9-Dependent Neutrophil Recruitment Protects against Fungal Invasion of the Central Nervous System

Candida is the most common human fungal pathogen and causes systemic infections that require neutrophils for effective host defense. Humans deficient in the C-type lectin pathway adaptor protein CARD9 develop spontaneous fungal disease that targets the central nervous system (CNS). However, how CARD9 promotes protective antifungal immunity in the CNS remains unclear. Here, we show that a patient with CARD9 deficiency had impaired neutrophil accumulation and induction of neutrophil-recruiting CXC chemokines in the cerebrospinal fluid despite uncontrolled CNS Candida infection. We phenocopied the human susceptibility in Card9 ^-/- mice, which develop uncontrolled brain candidiasis with diminished neutrophil accumulation. The induction of neutrophil-recruiting CXC chemokines is significantly impaired in infected Card9 ^-/- brains, from both myeloid and resident glial cellular sources, whereas cell-intrinsic neutrophil chemotaxis is Card9-independent. Taken together, our data highlight the critical role of CARD9-dependent neutrophil trafficking into the CNS and provide novel insight into the CNS fungal susceptibility of CARD9-deficient humans.     

Toll信号通路在中枢神经系统损伤中的作用

Toll样受体(Toll-like receptors,TLR)是先天性免疫反应识别病原体的一个重要分子,在免疫系统中发挥关键作用.其家族各种成员的主要功能是识别入侵病原体表面的各种不同分子模式,随后启动免疫反应,达到保护机体作用.在大脑中,小胶质细胞可以作为抗原提呈细胞,参与脑内免疫反应,也可以通过分泌各种促炎症因子启动或促进免疫反应,而TLR家族在中枢神经免疫系统的作用仍存在争议,它既可以通过促进神经免疫反应枢纽因子的表达来增强免疫,也可因免疫过度而损伤神经细胞.总之,Toll信号通路对中枢神经系统疾病有一定的调控作用.     

Wnt Signaling in the Vertebrate Central Nervous System: From Axon Guidance to Synaptic Function

Regulation of cell signaling by Wnt proteins is critical for the formation of neuronal circuits. Wnts modulate axon pathfinding, dendritic development, and synaptic assembly. Through different receptors, Wnts activate diverse signaling pathways that lead to local changes on the cytoskeleton or global cellular changes involving nuclear function. Recently, a link between neuronal activity, essential for the formation and refinement of neuronal connections, and Wnt signaling has been uncovered. Indeed, neuronal activity regulates the release of Wnt and the localization of their receptors. Wnts mediate synaptic structural changes induced by neuronal activity or experience. New emerging evidence suggests that dysfunction in Wnt signaling contributes to neurological disorders. In this article, the attention is focused on the function of Wnt signaling in the formation of neuronal circuits in the vertebrate central nervous system.The formation of neuronal connections requires the navigation of axons to their appropriate synaptic targets, the formation of terminal branches, and the assembly of functional synapses. These processes greatly depend on the proper dialogue between axons and their environment as they navigate to their target, and between axons and their postsynaptic dendrites during synapse assembly. A combination of secreted molecules and transmembrane proteins modulates these processes. Studies over the last 10 years have revealed an essential role for Wnt signaling in axon pathfinding, dendritic development, and synapse assembly in both central and peripheral nervous systems. Wnts also modulate basal synaptic transmission and the structural and functional plasticity of synapses in the central nervous system. Studies of Wnts in the nervous system have significantly contributed to our current understanding of the molecular mechanisms that control neuronal circuit assembly. These studies have also shed light into fundamental aspects of cell signaling such as novel mechanisms of protein secretion (Korkut et al. 2009) and receptor dynamics (Sahores et al. 2010). Here I review the mechanisms by which Wnts modulate axon guidance and synapse formation in the vertebrate central nervous system. I also discuss the increasing evidence in support for a role of Wnts in basal synaptic transmission, synaptic plasticity, and neurological disorders.     

G-CSF (Granulocyte-Colony Stimulating Factor) in the Central Nervous System

G-CSF (Granulocyte-colony stimulating factor) is a hematopoietic growth factor that has been known for 20 years, and has been named for its role in the proliferation and differentiation of cells of the myeloic lineage. We have uncovered a novel spectrum of activities of G-CSF in the central nervous system. G-CSF and its receptor are expressed by neurons in many brain regions, and are upregulated upon experimental stroke. In neurons, G-CSF acts anti-apoptotically by activating several protective pathways. In vivo, G-CSF decreases infarct volumes in acute stroke models in rodents. Moreover, G-CSF stimulates neuronal differentiation of adult neural stem cells in the brain, and improves long-term recovery in more chronic stroke models. Thus, G-CSF is a novel neurotrophic factor, and a highly attractive candidate for the treatment of neurodegenerative conditions. Here we discuss this new property of G-CSF in contrast to its known functions in the hematopoietic system, summarize data from other groups on G-CSF’s actions in cerebral ischemia, compare G-CSF to Erythropoietin (EPO) in the CNS and highlight clinical implications.     

Delayed Cell Death Signaling in Traumatized Central Nervous System: Hypoxia

There are two different ways for cells to die: necrosis and apoptosis. Cell death has traditionally been described as necrotic or apoptotic based on morphological criteria. There are controversy about the respective roles of apoptosis and necrosis in cell death resulting from trauma to the central nervous system (CNS). An evaluation of work published since 1997 in which electron microscopy was applied to ascertain the role of apoptosis and necrosis in: spinal cord injury, stroke, and hypoxia/ischemia (H/I) showed evidence for necrosis and apoptosis based on DNA degradation, presence of histones in cytoplasm, and morphological evidence in spinal cord. In the aftermath of stroke, many of the biochemical markers for apoptosis were present but the morphological determinations suggested that necrosis is the major source of post-traumatic cell death. This was not the case in H/I where both biochemical assays and the morphological studies gave more consistent results in a manner similar to the spinal cord injury studies. After H/I, major factors affecting cell death outcomes are DNA damage and repair processes, expression of bcl-like gene products and inflammation-triggered cytokine production.     

Peripheral Nerve Injury Modulates Neurotrophin Signaling in the Peripheral and Central Nervous System

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75^NTR and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.     

TWEAK and the Central Nervous System

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a member of the tumor necrosis factor superfamily that acts on responsive cells via binding to a cell surface receptor named fibroblast growth factor-inducible 14 (Fn14). TWEAK can regulate numerous cellular responses in vitro and in vivo. Recent studies have indicated that TWEAK and Fn14 are expressed in the central nervous system (CNS), and that in response to a variety of stimuli, including cerebral ischemia, there is an increase in TWEAK and Fn14 expression in perivascular astrocytes, microglia, endothelial cells, and neurons with subsequent increase in the permeability of the blood–brain barrier (BBB) and cell death. Furthermore, there is a growing body of evidence indicating that TWEAK induces the activation of the NF-κB in the CNS with release of proinflammatory cytokines and matrix metalloproteinases. In addition, inhibition of TWEAK activity by either treatment with a Fn14-Fc fusion protein or neutralizing anti-TWEAK antibodies has shown therapeutic efficacy in animal models of ischemic stroke, cerebral edema, and multiple sclerosis.     

Vimentin in the Central Nervous System

Intermediate filament proteins were identified by two-dimensional gel electrophoresis in urea extracts of rat optic nerves undergoing Wallerian degeneration and in cytoskeletal preparations of rat brain and spinal cord during postnatal development. The glial fibrillary acidic (GFA) protein and vimentin were the major optic nerve proteins following Wallerian degeneration. Vimentin was a major cytoskeletal component of newborn central nervous system (CNS) and then progressively decreased until it became barely identifiable in mature brain and spinal cord. The decrease of vimentin occurred concomitantly with an increase in GFA protein. A protein with the apparent molecular weight of 61,000 and isoelectric point of 5.6 was identified in both cytoskeletal preparations of brain and spinal cord, and in urea extracts of normal optic nerves. The protein disappeared together with the polypeptides forming the neurofilament triplet in degenerated optic nerves.     

Gamma Interferon Signaling in Macrophage Lineage Cells Regulates Central Nervous System Inflammation and Chemokine Production

Intracranial (i.c.) infection of mice with lymphocytic choriomeningitis virus (LCMV) results in anorexic weight loss, mediated by T cells and gamma interferon (IFN-γ). Here, we assessed the role of CD4⁺ T cells and IFN-γ on immune cell recruitment and proinflammatory cytokine/chemokine production in the central nervous system (CNS) after i.c. LCMV infection. We found that T-cell-depleted mice had decreased recruitment of hematopoietic cells to the CNS and diminished levels of IFN-γ, CCL2 (MCP-1), CCL3 (MIP-1α), and CCL5 (RANTES) in the cerebrospinal fluid (CSF). Mice deficient in IFN-γ had decreased CSF levels of CCL3, CCL5, and CXCL10 (IP-10), and decreased activation of both resident CNS and infiltrating antigen-presenting cells (APCs). The effects of IFN-γ signaling on macrophage lineage cells was assessed using transgenic mice, called “macrophages insensitive to interferon gamma” (MIIG) mice, that express a dominant-negative IFN-γ receptor under the control of the CD68 promoter. MIIG mice had decreased levels of CCL2, CCL3, CCL5, and CXCL10 compared to controls despite having normal numbers of LCMV-specific CD4⁺ T cells in the CNS. MIIG mice also had decreased recruitment of infiltrating macrophages and decreased activation of both resident CNS and infiltrating APCs. Finally, MIIG mice were significantly protected from LCMV-induced anorexia and weight loss. Thus, these data suggest that CD4⁺ T-cell production of IFN-γ promotes signaling in macrophage lineage cells, which control (i) the production of proinflammatory cytokines and chemokines, (ii) the recruitment of macrophages to the CNS, (iii) the activation of resident CNS and infiltrating APC populations, and (iv) anorexic weight loss.Immune cell recruitment to and infiltration of the central nervous system (CNS) is central to the pathology of a variety of inflammatory neurological diseases, including infectious meningoencephalitis, multiple sclerosis, and cerebral ischemia (59, 60). Chemokines have been shown to be highly upregulated in both human diseases and animal models of neuroinflammation and are thought to be important mediators of immune cell entry into the CNS (59, 60). For example, during experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis (MS), the chemokines CCL2 (monocyte chemoattractant protein 1 [MCP-1α]), CCL3 (macrophage inflammatory protein 1α [MIP-1α]), CCL5 (regulated upon activation, T-cell expressed and secreted [RANTES]), and CXCL10 (gamma interferon [IFN-γ]-inducible protein 10 [IP-10]) are produced by either resident CNS cells or infiltrating cells (27) and serve to amplify the ongoing inflammatory response (25, 28). However, in some EAE studies, neither CCL3 nor CXCL10 were required for disease (72, 73). During CNS viral infection, CXCL10 and CCL5 are highly produced in several models (2, 41, 48, 82). In addition, mice deficient in CCR5, which binds (among others) CCL3 and CCL5, do not display impaired CNS inflammation after certain viral infections (13). Thus, the role of chemokines in CNS inflammation is likely complex and dissimilar between autoimmune and viral infection models.IFN-γ is present in the CNS during autoimmunity and infection (7, 54, 69). Several studies suggest that IFN-γ can be a potent inducer of CNS chemokine expression. Adenoviral expression of IFN-γ in the CNS strongly induced CCL5 and CXCL10 mRNA and protein, and this induction was dependent on the presence of the IFN-γ receptor (50). In EAE and Toxoplasma infection, mice deficient in IFN-γ or the IFN-γ receptor demonstrated reduced expression of several chemokines, including CCL2, CCL3, CCL5, and CXCL10 (26, 69). However, given the near-ubiquitous expression of the IFN-γ receptor (44), the mechanisms by which IFN-γ regulates CNS chemokine production remain to be elucidated.We studied neuroinflammation and immune-mediated disease using a well-studied mouse model of infection with lymphocytic choriomeningitis virus (LCMV). Intracranial (i.c.) injection of mice with LCMV results in seizures and death 6 to 8 days after inoculation. The onset of symptoms is associated with a massive influx of mononuclear cells into the cerebrospinal fluid (CSF), meninges, choroid plexus, and ependymal membranes (6, 8, 18), as well as the presence of proinflammatory cytokines (7, 38). The immune response is critical for disease, since infection of irradiated or T-cell-depleted mice leads to persistent infection with very high levels of virus in multiple tissues without the development of lethal meningitis (18, 34, 64). i.c. LCMV infection of β₂-microglobulin-deficient mice (β₂m^−/− mice) also results in meningitis and production of proinflammatory cytokines and chemokines; however, meningitis occurs with a later onset and lower severity compared to wild-type mice (17, 24, 53, 57). Interestingly, i.c. LCMV infection of these mice also causes severe anorexia and weight loss (33, 38, 46, 52, 57) that is mediated by major histocompatibility complex (MHC) class II-restricted, CD4⁺ T cells (17, 46, 53, 57). Anorexia and weight loss are also observed in wild-type mice, but they succumb to lethal meningitis shortly thereafter (33), making study of this particular aspect of disease difficult. LCMV-induced weight loss, similar to what we have observed in β₂m^−/− mice also occurs in perforin-deficient mice, which possess CD8⁺ T cells (37). Although some reports have observed weight loss after peripheral LCMV infection (11, 45), we note that these studies used high doses of the clone 13 strain of LCMV, in contrast to our studies which have used the Armstrong strain of LCMV and orders of magnitude less virus (33, 38, 46, 52, 57). Although we cannot exclude a contribution of peripheral cells to weight loss in our i.c. Armstrong infection model, we previously showed that this weight loss does not occur with peripheral infection with LCMV Armstrong (33, 38), indicating that interactions between the CNS and the immune system are contribute substantially to disease.During LCMV infection, there is biphasic production of IFN-γ: a small, early peak of IFN-γ (most likely produced by NK or NKT cells), followed by T-cell-mediated production of IFN-γ (23, 75). Further, both CD4⁺ T cells and CD8⁺ T cells produce large amounts of IFN-γ after LCMV infection and T-cell production of IFN-γ is critical for LCMV-induced weight loss (35). Chemokines, especially CXCL10, CCL5, and CCL2, and their receptors, are upregulated in the brain after i.c. LCMV infection (2, 13). Brain chemokine mRNA expression after i.c. LCMV infection is reduced in IFN-γ-deficient mice and relatively absent in athymic mice (2). However, the mechanism(s) by which T cells and IFN-γ mediate the effects on CNS chemokine expression, cellular infiltration into the CNS, and LCMV-induced anorexic weight loss remain unclear.In the present study, we focused on two major questions. The first question concerned the role of IFN-γ on immune cell recruitment to and chemokine/cytokine production within the CNS? We found that macrophages and myeloid dendritic cells (DCs) require IFN-γ for their accumulation within the CNS. Second, since macrophages and myeloid DCs are the predominant cellular infiltrate, we sought to determine whether IFN-γ signaling on these cells was direct with regard to their recruitment and to chemokine/cytokine production. We found that IFN-γ signaling in macrophage lineage cells contributes significantly to their recruitment, to chemokine production in the CNS, and to anorexic weight loss. Together, these data suggest that much of the proinflammatory effects of IFN-γ in the CNS are mediated by the effects of IFN-γ on CD68-bearing cells.     

Riboflavin Homeostasis in the Central Nervous System

Abstract: The mechanisms by which riboflavin, which is not synthesized in mammals, enters and leaves brain, CSF, and choroid plexus were investigated by injecting [¹⁴C]riboflavin intravenously or intraventricularly. Tracer amounts of [¹⁴C]riboflavin with or without FMN were infused intravenously at a constant rate into normal, starved, or probenecid-pretreated rabbits. At 3 h, [¹⁴C]riboflavin readily entered choroid plexus and brain, and, to a much lesser extent, CSF. Over 85% of the [¹⁴C]riboflavin in brain and choroid plexus was present as [¹⁴C]FMN and [¹⁴C]FAD. The addition of 0.2 mmol/kg FMN to the infusate markedly depressed the relative entry of [¹⁴C]riboflavin into brain, choroid plexus, and, less so, CSF, whereas starvation increased the relative entry of [¹⁴C]riboflavin into brain and choroid plexus. After intraventricular injection (2 h), most of the [¹⁴C]riboflavin was extremely rapidly cleared from CSF into blood. Some of the [¹⁴C]riboflavin entered brain, where over 85% of the ¹⁴C was present as [¹⁴C]FMN plus [¹⁴C]FAD. The addition of 1.2³μmol FAD (which was rapidly hydrolyzed to riboflavin) to the injectate decreased the clearance of [¹⁴C]riboflavin from CSF and the phosphorylation of [¹⁴C]riboflavin in brain. Probenecid in the injectate also decreased the clearance of [¹⁴C]riboflavin from CSF. These results show that the control of entry and exit of riboflavin is the mechanism, at least in part, by which total riboflavin levels in brain cells and CSF are regulated. Penetration of riboflavin through the blood-brain barrier, saturable efflux of riboflavin from CSF, and saturable entry of riboflavin into brain cells are three distinct parts of the homeostatic system for total riboflavin in the central nervous system.