Differential signalling of the chemokine receptor CXCR4 by stromal cell‐derived factor 1 and the HIV glycoprotein in rat neurons and astrocytes
ABSTRACT CXCR4 is the Gi protein-linked seven-transmembrane receptor for the alpha chemokine stromal cell-derived factor 1 (SDF-1), a chemoattractant for lymphocytes. This receptor is highly conserved between human and rodent. CXCR4 is also a coreceptor for entry of human immunodeficiency virus (HIV) in T cells and is expressed in the CNS. To investigate how these CXCR4 ligands influence CNS development and/or function, we have examined the expression and signalling of this chemokine receptor in rat neurons and astrocytes in vitro. CXCR4 transcripts and protein are synthesized by both cell types and in E15 brain neuronal progenitors. In these progenitors, SDF-1, but not gp120 (the HIV glycoprotein), induced activation of extracellular signal regulated kinases (ERKs) 1/2 and a dose-dependent chemotactic response. This chemotaxis was inhibited by Pertussis toxin, which uncouples Gi proteins and the bicyclam AMD3100, a highly selective CXCR4 antagonist, as well as by an inhibitor of the MAP kinase pathway. In differentiated neurons, both SDF-1 and the glycoprotein of HIV, gp120, triggered activation of ERKs with similar kinetics. These effects were significantly inhibited by Pertussis toxin and the CXCR4 antagonist. Rat astrocytes also responded to SDF-1 signalling by phosphorylation of ERKs but, in contrast to cortical neurons, no kinase activation was induced by gp120. Thus neurons and astrocytes can respond differently to signalling by SDF-1 and/or gp120. As SDF-1 triggers directed migration of neuronal progenitors, this alpha chemokine may play a role in cortex development. In differentiated neurons, both natural and viral ligands of CXCR4 activate ERKs and may therefore influence neuronal function.
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ABSTRACT: Chemokine receptors, in particular, CXCR4 and CCR5, mediate human immunodeficiency virus type 1 (HIV-1) infection of immunocompetent cells and the apoptosis of these cells. However, the virus does not infect neurons. Yet through a variety of mechanisms, HIV promotes glial cell activation, synaptodendritic alterations, and neuronal loss that ultimately lead to motor and cognitive impairment. Chemokines and chemokine receptors are abundant in the adult central nervous system and play a role in neuronal apoptosis evoked by HIV proteins. Thus, reducing the availability of chemokine receptors may prevent the neuronal degeneration seen in HIV-positive patients. In this article, we present and discuss a recent experimental approach aimed at testing effective neuroprotective therapies against HIV-mediated neuronal degeneration.Journal of Neuroscience Research 03/2008; 86(2):243-55. · 2.97 Impact Factor
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ABSTRACT: Hypoxia/ischemia (H/I) induces rapid and massive brain damage in neonatal rat brain, resulting in long-term consequences on structural and functional maturation of the central nervous system. Inflammatory mediators contribute to these permanent pathological changes, which are sensitive to corticoid treatments. Since the chemokine receptor CXCR4, specific for the SDF-1 alpha/CXCL12 ligand, regulates both apoptotic and neuroregeneration processes, this receptor was quantified 2 days following H/I in neonatal rat brain in relation with dexamethasone (DEX) treatment. Seven-day-old male rats were exposed to a 90-min hypoxia following unilateral carotid ligation (H/I) and were sacrificed 48 h later. Glucocorticoid-pretreated animals were injected subcutaneously 5 h prior to hypoxia with 0.5 microg/g DEX. Glial fibrillary acidic protein and cresyl violet staining were used for assessing the extent of brain lesion subdivided into necrotic and penumbra-like areas. The density of CXCR4 receptors was determined by quantitative autoradiography using [(125)I]SDF-1 alpha as a ligand. The H/I resulted in a massive lesion ipsilateral to the carotid ligation, which was extended to cortical, striatal, hippocampal and thalamic areas, while the contralateral hemisphere remained apparently unaffected. DEX decreased the lesion size by reducing mainly the necrotic area. H/I induced a marked increase in CXCR4 receptor binding in the penumbra-like areas. DEX pretreatment decreased CXCR4 receptor density in the penumbra and attenuated astrocytosis. Furthermore, DEX strongly lowered mortality rate and reduced functional recovery time right after hypoxia. The rapid enhancement in CXCR4 chemokine receptor binding in the affected brain areas suggests that SDF-1 alpha/CXCR4 may play a role in the hypoxia-induced inflammatory reaction in the neonatal brain. Attenuation of CXCR4 expression and astrogliosis could contribute to the neuroprotective effect of DEX pretreatment via influencing the inflammatory cascade induced by H/I in the neonatal brain.NeuroImmunoModulation 02/2004; 11(6):404-13. · 1.84 Impact Factor
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ABSTRACT: Chemokines and their receptors are expressed both, in the developing humans and in the adult central nervous system (CNS). Numbers of papers showed that the CXCL12 and CXCR4 receptor are differentially expressed; CXCL12 is highly expressed in glial cells (mostly on astrocytes) and CXCR4 in both glial (astrocytes and microglia) and neuronal cells. The pattern of distribution supports the idea that the CXCL12/CXCR4 system is involved in the fast bidirectional communication system between astrocytes and neurons. Infact, local application of CXCL12 in cultured and in situ astrocytes induces calcium-dependent glutamate release through a direct activation of the G-protein coupled receptor (GPCRs) CXCR4. The chemokine-mediated glutamate release process in astrocytes seems to involve a long chain of intracellular and extracellular events related to the release of two chemical mediators, the tumor necrosis factor-alpha (TNFα) and prostaglandins (PGs). The mechanisms of glutamate release from astrocytes have been extensively studied during the last years. The first evidence for the existence of a calcium-dependent exocytosis pathway had been provided few years ago when by complementing the ultrastructural studies in situ with dynamic total internal reflection fluorescence (TIRF) real-time imaging studies in cultured astrocytes it has been shown that astrocytes express glutammatergic vesicles able to undergo regulated exocytosis. In our recent paper we investigated the physiological role of chemokines by studying whether the CXCL12/CXCR4 system triggers release of glutamate by regulated exocytosis. The activation of CXCR4 receptor induces a burst of exocytosis that occurs in the order of a few hundred milliseconds and is mediated by the release of calcium from internal stores. The exocytotic burst unfolds in a time scale much shorter than that of dense-core granules in neurosecretory cells, which are governed by voltage-gated calcium channels. Taking into account that astrocytes are electrically non-excitable and that their release rely only on the activation of GPCRs, the rapid exocytotic pathway is of a difficult interpretation without considering the existence of a local morphological and functional interaction between vesicles and site of calcium release. Indeed, by looking at the spatial organization of endoplasmic reticulum tubules and sites of vesicles docking with TIRF illumination, one can find that endoplasmic reticulum tubules, together with plasma membrane and docked vesicles form complex and peculiar structures of sub-micrometer spaces that provide the structural basis for local calcium signaling. Upon stimulation of group I of mGluR, sub-micrometer localized calcium elevations (hot spots) are generated in the sub-plasma membrane domains of ER. Interestingly, in most cases the sub-membrane calcium events occurred at/near sites where SLMVs underwent exocytosis (interdistance: ≤280 nm), and were in strict temporal correlation with the fusion events. Recent works have shown that glutamate released from astrocytes during physiological synaptic activity targets neuronal receptors in either axonal terminals or dendrites, exerting a neuromodulatory action. In particular in a recent paper it has been shown that at the perforant path–granule cell (PP–GC) synapses in the hippocampal dentate gyrus, astrocytes of the outer molecular layer sense synaptic activity, elevate their intracellular calcium and release glutamate via exocytosis of glutammatergic vesicles. Astrocytic glutamate is released at presynaptic level in close proximity of NR2B-containing NMDA receptors; the activation of such receptors results in an increased synaptic transmitter release and in the strengthening of synaptic transmission. The appreciation that brain activity involves interactive signaling between neurons and glia opens new perspectives for understanding the pathogenesis of brain diseases with strong inflammatory components (such as Alzheimer’s disease and AIDS dementia complex). In particular, in the presence of inflammatory cytokines such as interleukin-1β (IL-1β) and TNFα, changes in the expression of junctional proteins, in propagation of intracellular calcium waves and glutamate release, have been observed. Thus, when local inflammatory reaction is triggered in the brain, microglial cells rapidly migrate to the site of injury become activated and start releasing a number of mediators such as PGs and TNFα, deeply altering the properties of calcium-dependent glutamate release from astrocytes.12/2009: pages 271-300;