Neuronal expression of cytokines is an area of active investigation in the contexts of development, disease, and normal neural function. Although cultured rat sympathetic neurons respond very weakly to exogenous interleukin 6 (IL-6), we find that addition of soluble IL-6 receptor (sIL-6R) and IL-6 enhances neuronal survival in the absence of nerve growth factor. Neutralizing monoclonal antibodies against IL-6 block these effects. Addition of IL-6 and sIL-6R also induces a subset of neuropeptide and transmitter synthetic enzyme mRNAs identical to that demonstrated for leukemia inhibitory factor, ciliary neurotrophic factor, and oncostatin M. Both of these effects are duplicated by addition of a highly active fusion protein of sIL-6R and IL-6, covalently linked by a f lexible peptide chain, which is designated H-IL-6. In addition, we show that sympathetic neurons produce IL-6. In situ hybridization indicates a neuronal localization of IL-6 mRNA in superior cervical ganglia, and bioactive IL-6 protein is detected in ganglion culture supernatants. Interestingly, the IL-6 produced by sympathetic neurons does not lead to survival of these cells in culture unless sIL-6R is added. Thus, sympathetic neurons can produce IL-6 and may respond to it in an autocrine͞paracrine manner if sIL-6R is present. Moreover, the prior findings of sIL-6R in serum and inf lammatory f luids now have added interest in the context of neuroimmune interactions.

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Interleukin-6 (IL-6) on target cells binds to the specific IL-6 receptor (IL-6R) and subsequently induces homodimerization of the signal-transducing protein gp130. Cells which express gp130 but no IL-6R and which therefore do not respond to IL-6 can be stimulated by the complex of IL-6 and soluble IL-6R (slL-6R). Here we show that on rat pheochromocytoma cells (PC12), the combination of IL-6 and slL-6R but not IL-6 alone induces expression of c-fos, GAP-43 and neuron-specific enolase followed by neuron-specific differentiation and formation of a neuronal network. The differentiation was dose-and time-dependent and followed the same kinetics as nerve-growth factor (NGF)-induced differentiation. The responses of PC12 cells to IL-6/sIL-6R and NGF were additive, suggesting independent signaling pathways. We demonstrate that activation of gp130 generates a neuronal differentiation signal that is equivalent to and independent of trk/NGF receptor tyrosine kinase. Interestingly, the failure of IL-6 to induce differentiation of PC12 cells is not due to lack of surface expression of IL-6R as IL-6 alone triggered expression of GAP-43 mRNA and protein. We hypothesize that PC12 cells express more gp130 than IL-6R and that the extent of activated gp130 molecules determines the quality of the response.

Cytokines of the interleukin-6 (IL-6) family participate in regulatory and inflammatory processes within the nervous system. IL-6, ciliary neurotrophic factor (CNTF) and IL-11 act via specific membrane receptors which, together with their ligands, associate with signal-transducing receptor subunits thereby initiating cytoplasmic signalling. Cells which only express signal-transducing receptor subunits but no ligand binding subunits for IL-6, CNTF and IL-11 are refractory to these cytokines. An unusual feature of the IL-6 cytokine family is that the soluble forms of the ligand binding receptor subunits generated by one cell type in complex with their ligands can directly stimulate the signal-transducing receptor subunits on different cell types which lack ligand binding receptor subunits. This process has been named transsignalling. This article focuses on the importance of transsignalling events in neuronal differentiation and survival responses.

Statins are potent inhibitors of the hydroxy-methyl-glutaryl-coenzyme A reductase, the rate limiting enzyme for cholesterol biosynthesis. Experimental and clinical studies with statins suggest that they have beneficial effects on neurodegenerative disorders. Thus, it was of interest to characterize the direct effects of statins on CNS neurons and glial cells. We have treated defined cultures of neurons and astrocytes of newborn rats with two lipophilic statins, atorvastatin and simvastatin, and analyzed their effects on morphology and survival. Treatment of astrocytes with statins induced a time- and dose-dependent stellation, followed by apoptosis. Similarly, statins elicited programmed cell death of cerebellar granule neurons but with a higher sensitivity. Analysis of different signaling cascades revealed that statins fail to influence classical pathways such as Akt or MAP kinases, known to be activated in CNS cells. In addition, astrocyte stellation triggered by statins resembled dibutryl-cyclic AMP (db-cAMP) induced morphological differentiation. However, in contrast to db-cAMP, statins induced upregulation of low-density lipoprotein receptors, without affecting GFAP expression, indicating separate underlying mechanisms. Analysis of the cholesterol biosynthetic pathway revealed that lack of mevalonate and of its downstream metabolites, mainly geranylgeranyl-pyrophosphate (GGPP), is responsible for the statin-induced apoptosis of neurons and astrocytes. Moreover, astrocytic stellation triggered by statins was inhibited by mevalonate and GGPP. Interestingly, neuronal cell death was significantly reduced in astrocyte/neuron co-cultures treated with statins. We postulate that under these conditions signals provided by astrocytes, e.g., isoprenoids play a key role in neuronal survival.

The cytokine interleukin-6 (IL-6) has multiple functions in the immune and hematopoietic systems. IL-6 is related to ciliary neurotrophic factor (CNTF), a trophic factor for motoneurons, sensory dorsal root ganglion (DRG) neurons, and other neuronal subpopulations. Both act via related receptor complexes, consisting of one ligand-specific alpha-receptor subunit (IL-6R and CNTFR, respectively) and two signal-transducing receptor components. Even though IL-6 is expressed by neurons and glia, the functions of IL-6 in the nervous system are poorly understood. Here, we report that exogenous human IL-6 promotes the survival of dissociated newborn rat DRG neurons in vitro if supplemented with soluble human IL-6-alpha-receptor. The dosages of human IL-6 and soluble human IL-6R necessary to achieve neurotrophic effects could be reduced markedly by linking ligand and alpha-receptor component in a designer cytokine. Furthermore, we show that newborn rat DRG neurons express and secrete bioactive IL-6. Endogenously secreted IL-6 does not enhance survival of these neurons in vitro, suggesting that DRG neurons do not sufficiently express cell surface IL-6R. Exogenously added soluble rat IL-6R rendered DRG neurons responsive to secreted IL-6. Our results indicate an autocrine function of IL-6 in DRG neuron survival which depends on membrane-bound or soluble IL-6R as a neurotrophic cofactor.

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Human herpes virus‐8 (HHV8) encodes a cytokine named viral interleukin‐6 (vIL‐6) that shares 25% amino‐acid identity with its human homologue. Human IL‐6 is known to be a growth and differentiation factor of lymphatic cells and plays a potential role in the pathophysiology of various lymphoproliferative diseases. vIL‐6 is expressed in HHV8‐associated‐diseases including Kaposi's sarcoma, Body‐cavity‐based‐lymphoma and Castleman's disease, suggesting a pathogenetic involvement in the malignant growth of B‐cell associated diseases and other malignant tumours. We expressed vIL‐6 in Escherichia coli as a fusion protein with recombinant periplasmic maltose binding protein. After cleavage from the maltose binding protein moiety and purification, vIL‐6 was shown to be correctly folded using circular dichroism spectroscopy. A rabbit antiserum was raised against the recombinant vIL‐6 protein. vIL‐6 turned out to be active on cells that expressed gp130 but no IL‐6 receptor (IL‐6‐R) suggesting that, in contrast to human IL‐6, vIL‐6 stimulated gp130 directly. Accordingly, vIL‐6 activity could be inhibited by a soluble gp130 Fc Fusion protein. vIL‐6 was shown to induce neuronal differentiation of rat pheochromocytoma cells and to stimulate colony formation of human hematopoietic progenitor cells. Thus, vIL‐6 exhibits biologic activity that has only been observed for the IL‐6/soluble IL‐6‐R complex but not for IL‐6 alone. These properties are important for the evaluation of the pathophysiological potential of vIL‐6.