Characterization of receptors for ciliary neurotrophic factor on rat hippocampal astrocytes

Osteopontin facilitates West Nile virus neuroinvasion via neutrophil "Trojan horse" transport

West Nile virus (WNV) can cause severe human neurological diseases including encephalitis and meningitis. The mechanisms by which WNV enters the central nervous system (CNS) and host-factors that are involved in WNV neuroinvasion are not completely understood. The proinflammatory chemokine osteopontin (OPN) is induced in multiple neuroinflammatory diseases and is responsible for leukocyte recruitment to sites of its expression. In this study, we found that WNV infection induced OPN expression in both human and mouse cells. Interestingly, WNV-infected OPN deficient (Opn ^-/-) mice exhibited a higher survival rate (70%) than wild type (WT) control mice (30%), suggesting OPN plays a deleterious role in WNV infection. Despite comparable levels of viral load in circulating blood cells and peripheral organs in the two groups, WNV-infected polymorphonuclear neutrophil (PMN) infiltration and viral burden in brain of Opn ^-/- mice were significantly lower than in WT mice. Importantly, intracerebral administration of recombinant OPN into the brains of Opn ^-/- mice resulted in increased WNV-infected PMN infiltration and viral burden in the brain, which was coupled to increased mortality. The overall results suggest that OPN facilitates WNV neuroinvasion by recruiting WNV-infected PMNs into the brain.

Enhancement of dentate gyrus neurogenesis, dendritic and synaptic plasticity and memory by a neurotrophic peptide

Pharmacological enhancement of hippocampal neurogenesis is a therapeutic approach for improvement of cognition in learning and memory disorders such as Alzheimer's disease. Here we report the development of an 11-mer peptide that we designed based on a biologically active region of the ciliary neurotrophic factor. This peptide, Peptide 6, induced proliferation and increased survival and maturation of neural progenitor cells into neurons in the dentate gyrus of normal adult C57BL6 mice. Furthermore, Peptide 6 increased the MAP2 and synaptophysin immunoreactivity in the dentate gyrus. Thirty-day treatment of the mice with a slow release bolus of the peptide implanted subcutaneously improved reference memory of the mice in Morris water maze. Peptide 6 has a plasma half life of over 6 h, is blood-brain barrier permeable, and acts by competitively inhibiting the leukemia inhibitory factor signaling. The fact that Peptide 6 is both neurogenic and neurotrophic and that this peptide is effective when given peripherally, demonstrates its potential for prevention and treatment of learning and memory disorders.

Differential stimulation-induced receptor localization in lipid rafts for interleukin-6 family cytokines signaling through the gp130/leukemia inhibitory factor receptor complex

Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) are cytokines which signal through receptor complexes that include the receptor subunits glycoprotein 130 (gp130) and the LIF receptor (LIFR), but CNTF also requires the non-signal transducing CNTF receptor (CNTFR) for binding. We show here that in IMR-32 neuronal cells endogenously expressing the receptor subunits for LIF and CNTF, CNTFR, but not gp130 or LIFR, is found in detergent-resistant lipid rafts. In addition, stimulation of these cells with CNTF resulted in a rapid translocation of a portion of gp130 and LIFR into detergent-resistant lipid rafts while an equivalent stimulation with LIF did not. Disruption of lipid rafts by cholesterol depletion of cell membranes blocked the CNTF-induced translocation of LIFR and gp130. Interestingly, while cholesterol-depletion did not inhibit signal transducer and activator of transcription 3 phosphorylation by either CNTF or LIF stimulation, it strongly inhibited both CNTF- and LIF-mediated phosphorylation of extracellular signal-regulated kinases 1 and 2 and Akt. LIF and CNTF generally appear to have redundant effects in cells responsive to both cytokines. Intriguingly, the data presented here suggest a possible mechanism whereby CNTF or other cytokines that signal through CNTFR could generate signals distinct from those elicited by cytokines such as LIF which utilize a LIFR/gp130 heterodimer, via association with or exclusion from lipid rafts.

HIV-1 interaction with human mannose receptor (hMR) induces production of matrix metalloproteinase 2 (MMP-2) through hMR-mediated intracellular signaling in astrocytes

Astrocytes are susceptible to HIV-1 infection. We have recently demonstrated that human mannose receptor (hMR) is directly involved in CD4-independent HIV-1 infection of astrocytes. The apparent paradox between the vivid binding affinity of HIV-1 gp120 protein to hMR and the low efficiency of hMR-mediated HIV-1 infection raises the possibility that HIV-1 binding to hMR alone may negatively affect astrocyte function. In this study, we examined the relationship between HIV-1 interaction with hMR and the production of matrix metalloproteinases (MMPs) in astrocytes. We took advantage of an astroglial cell line U87.MR stably expressing hMR as an in vitro astrocyte model system and human primary astrocytes, and demonstrated that HIV-1 binding to astrocytes induced the production of MMP-2. This induction appeared to be most potent with M-tropic HIV-1 viruses. Increased MMP-2 production was not due to hMR-mediated HIV-1 entry and/or HIV-1 viral gene expression, as the transfection of HIV-1 proviral DNA did not result in MMP-2 production, and the infection of AT-2-treated HIV-1 viruses did not inhibit MMP-2 production. Direct involvement of hMR in HIV-induced MMP-2 production was confirmed by the inhibition of the yeast mannan, an hMR ligand antagonist, and an anti-hMR serum. Furthermore, HIV-induced MMP-2 production in astrocytes was shown to involve hMR-mediated intracellular signaling. Taken together, these results suggest that HIV-1 binding to astrocytes in the absence of HIV-1 viral entry is sufficient to alter astrocyte function through hMR-mediated intracellular signaling. In addition, these results provide new evidence to support the notion that hMR is capable of eliciting intracellular signaling upon ligand binding.

The complex of ciliary neurotrophic factor-ciliary neurotrophic factor receptor alpha up-regulates connexin43 and intercellular coupling in astrocytes via the Janus tyrosine kinase/signal transducer and activator of transcription pathway

Cytokines regulate numerous cell processes, including connexin expression and gap junctional coupling. In this study, we examined the effect of ciliary neurotrophic factor (CNTF) on connexin43 (Cx43) expression and intercellular coupling in astrocytes. Murine cortical astrocytes matured in vitro were treated with CNTF (20 ng/ml), soluble ciliary neurotrophic factor receptor alpha (CNTFRalpha) (200 ng/ml), or CNTF-CNTFRalpha. Although CNTF and CNTFRalpha alone had no effect on Cx43 expression, the heterodimer CNTF-CNTFRalpha significantly increased both Cx43 mRNA and protein levels. Cx43 immunostaining correlated with increased intercellular coupling as determined by dye transfer analysis. By using the pharmacological inhibitor alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide (AG490), the increase in Cx43 was found to be dependent on the Janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Immunocytochemical analysis revealed that CNTF-CNTFRalpha treatment produced nuclear localization of phosphorylated STAT3, whereas CNTF treatment alone did not. Transient transfection of constructs containing various sequences of the Cx43 promoter tagged to a LacZ reporter into ROS 17/2.8 cells confirmed that the promoter region between -838 to -1693 was deemed necessary for CNTF-CNTFRalpha to induce heightened expression. CNTF-CNTFRalpha did not alter Cx30 mRNA levels, suggesting selectivity of CNTF-CNTFRalpha for connexin signaling. Together in the presence of soluble receptor, CNTF activates the JAK/STAT pathway leading to enhanced Cx43 expression and intercellular coupling.

Astroglial expression of the P-glycoprotein is controlled by intracellular CNTF

BACKGROUND

The P-glycoprotein (P-gp), an ATP binding cassette transmembrane transporter, is expressed by astrocytes in the adult brain, and is positively modulated during astrogliosis. In a search for factors involved in this modulation, P-gp overexpression was studied in long-term in vitro astroglial cultures.

RESULTS

Surprisingly, most factors that are known to induce astroglial activation in astroglial cultures failed to increase P-gp expression. The only effective proteins were IFNgamma and those belonging to the IL-6 family of cytokines (IL-6, LIF, CT-1 and CNTF). As well as P-gp expression, the IL-6 type cytokines (but not IFNgamma) stimulated the expression of endogenous CNTF in astrocytes. In order to see whether an increased intracellular level of CNTF was necessary for induction of P-gp overexpression by IL-6 type cytokines, by the same cytokines analysis was carried out on astrocytes obtained from CNTF knockout mice. In these conditions, IFNgamma produced increased P-gp expression, but no overexpression of P-gp was observed with either IL-6, LIF or CT-1, pointing to a role of CNTF in the intracellular signalling pathway leading to P-gp overexpression. In agreement with this suggestion, application of exogenous CNTF (which is internalised with its receptor) produced an overexpression of P-gp in CNTF-deficient astrocytes.

CONCLUSION

These results reveal two different pathways regulating P-gp expression and activity in reactive astrocytes, one of which depends upon the intracellular concentration of CNTF. This regulation of P-gp may be one of the long searched for physiological roles of CNTF.

Ciliary neurotrophic factor activates spinal cord astrocytes, stimulating their production and release of fibroblast growth factor-2, to increase motor neuron survival

At focal CNS injury sites, several cytokines accumulate, including ciliary neurotrophic factor (CNTF) and interleukin-1beta (IL-1beta). Additionally, the CNTF alpha receptor is induced on astrocytes, establishing an autocrine/paracrine loop. How astrocyte function is altered as a result of CNTF stimulation remains incompletely characterized. Here, we demonstrate that direct injection of CNTF into the spinal cord increases GFAP expression and astroglial size and that primary cultures of spinal cord astrocytes treated with CNTF, IL-1beta, or leukemia inhibitory factor exhibit nuclear hypertrophy comparable to that observed in vivo. Using a coculture bioassay, we further demonstrate that CNTF treatment of astrocytes increases their ability to support ChAT(+) ventral spinal cord neurons (presumably motor neurons) more than twofold compared with untreated astrocytes. Also, the complexity of neurites was significantly increased in neurons cultured with CNTF-treated astrocytes compared with untreated astrocytes. RT-PCR analysis demonstrated that CNTF increased levels of FGF-2 and nerve growth factor (NGF) mRNA and that IL-1beta increased NGF and hepatocyte growth factor mRNA levels. Furthermore, both CNTF and IL-1beta stimulated the release of FGF-2 from cultured spinal cord astrocytes. These findings demonstrate that cytokine-activated astrocytes better support CNS neuron survival via the production of neurotrophic molecules. We also show that CNTF synergizes with FGF-2, but not epidermal growth factor, to promote DNA synthesis in spinal cord astrocyte cultures. The significance of these findings is discussed by presenting a new model depicting the sequential activation of astrocytes by cytokines and growth factors in the context of CNS injury and repair.

Ciliary neurotrophic factor may activate mature astrocytes via binding with the leukemia inhibitory factor receptor

Ciliary neurotrophic factor (CNTF) acts on immature astrocytes that express its trimeric receptor. In contrast, mature astrocytes do not significantly express the specific CNTFalpha receptor subunit, yet they respond to CNTF administration in vivo. Here we show that this controversy may be solved by a shift in astroglial sensitivity to CNTF over time, related to a change in the type of receptor bound by the cytokine on mature astrocytes. A convergent set of results supports the hypothesis that the CNTF effect is due to the illegitimate binding on the leukemia inhibitory factor receptor (LIFR): (i) it requires high concentration of recombinant rat CNTF; (ii) it involves the Jak/Stat and Ras-MAPK pathways; (iii) it is preserved in CNTFRalpha-/- cells; (iv) it is potentiated by soluble CNTFRalpha added to the medium; and (v) it is significantly decreased by a partial antagonist of LIFR. On these bases, we propose a mechanistic model in which, in the adult brain, a CNTF/LIFR interglial system may be modulated by neurons that synthesize CNTFRalpha.

Neurotrophic factors in Alzheimer's and Parkinson's disease brain

The biomedical literature on the subject of neurotrophic growth factors has expanded prodigiously. This essay reviews neurotrophic factors (NTF) and their receptors in Alzheimer's disease (AD) and Parkinson's disease (PD) brain and recent updates on receptor signaling. The hypotheses for specific NTF involvement in neurodegenerative diseases in human and as potential therapy are based mainly on experimental animal and in vitro models. There are wide gaps in information on regional synthesis and cell contents of NTFs and their receptors in human brain. Observations on AD brain indicate increases in NGF and decreases in BDNF in surviving neurons of hippocampus and certain neocortical regions and decreases in TrkA in cortex and nucleus basalis. In PD brain, the few data available indicate decreases in neuronal content of GDNF and bFGF in surviving substantia nigra dopaminergic neurons. There are very few data regarding age-dependent effects on NTFs and on their receptors in human brain. Since NTFs in neurons are subject to retrograde and, in at least some cases, to anterograde transport from and to target neurons, their effects may be related to synthesis in local or remote sites or to changes in axoplasmic transport. Also, certain NTFs and their receptors are found to be expressed in activated glia. Thus, comparative in situ data for transcription levels and protein contents for NTFs and their receptors in both sites of neuronal origin and termination in human brain are needed to understand their potential roles in treating human diseases.