D1 cap region involved in the receptor recognition and neural cell survival activity of human ciliary neurotrophic factor

Role of IL-6 in the regulation of neuronal development, survival and function

The pleiotropic cytokine interleukin-6 (IL-6) is emerging as a molecule with both beneficial and destructive potentials. It can exert opposing actions triggering either neuron survival after injury or causing neurodegeneration and cell death in neurodegenerative or neuropathic disorders. Importantly, neurons respond differently to IL-6 and this critically depends on their environment and whether they are located in the peripheral or the central nervous system. In addition to its hub regulator role in inflammation, IL-6 is recently emerging as an important regulator of neuron function in health and disease, offering exciting possibilities for more mechanistic insight into the pathogenesis of mental, neurodegenerative and pain disorders and for developing novel therapies for diseases with neuroimmune and neurogenic pathogenic components.

Enhancement of neurogenesis and memory by a neurotrophic peptide in mild to moderate traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is a risk factor for Alzheimer disease (AD), a neurocognitive disorder with similar cellular abnormalities. We recently discovered a small molecule (Peptide 6) corresponding to an active region of human ciliary neurotrophic factor, with neurogenic and neurotrophic properties in mouse models of AD and Down syndrome.

OBJECTIVE

To describe hippocampal abnormalities in a mouse model of mild to moderate TBI and their reversal by Peptide 6.

METHODS

TBI was induced in adult C57Bl6 mice using controlled cortical impact with 1.5 mm of cortical penetration. The animals were treated with 50 nmol/d of Peptide 6 or saline solution for 30 days. Dentate gyrus neurogenesis, dendritic and synaptic density, and AD biomarkers were quantitatively analyzed, and behavioral tests were performed.

RESULTS

Ipsilateral neuronal loss in CA1 and the parietal cortex and increase in Alzheimer-type hyperphosphorylated tau and A-β were seen in TBI mice. Compared with saline solution, Peptide 6 treatment increased the number of newborn neurons, but not uncommitted progenitor cells, in dentate gyrus by 80%. Peptide 6 treatment also reversed TBI-induced dendritic and synaptic density loss while increasing activity in tri-synaptic hippocampal circuitry, ultimately leading to improvement in memory recall on behavioral testing.

CONCLUSION

Long-term treatment with Peptide 6 enhances the pool of newborn neurons in the dentate gyrus, prevents neuronal loss in CA1 and parietal cortex, preserves the dendritic and synaptic architecture in the hippocampus, and improves performance on a hippocampus-dependent memory task in TBI mice. These findings necessitate further inquiry into the therapeutic potential of small molecules based on neurotrophic factors.

Molecular interactions within the IL-6/IL-12 cytokine/receptor superfamily

Production of cytokines by immune cells in response to stimuli and the binding of cytokines to specific receptors on target cells is a central feature of the immune response. The IL-12 cytokine family is particularly influential in determining the fate of T cells and is characterized by the sharing of cytokine and receptor subunits. A thorough understanding of the molecular interactions within this family will be a key to the development of therapeutic inhibitors or enhancers of IL-12 family function. While the current structural and molecular data for IL-12 family members is limited, there is ample information on the structurally related IL-6 cytokine family. This review will summarize the current structural and mutagenesis data within the IL-12 family and will attempt to utilize similarities between the IL-6 and IL-12 families to understand molecular interactions between IL-12 family subunits and with receptor components.

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.

Shared cytokine signaling receptors: structural insights from the gp130 system

The vast majority of cytokine signaling is mediated by "shared" receptors that form central signaling components of higher-order complexes incorporating ligand-specific receptors. These include the common gamma chain (gamma(c)), common beta chain (beta(c)), and gp130, as well as others. These receptors have the dual tasks of cross-reactive cytokine recognition, and formation of precisely oriented multimeric signaling assemblies. Currently, detailed structural information on a shared receptor complex exists only for gp130, which is a highly pleiotropic shared cytokine signaling receptor essential for mammalian cell growth and homeostasis. To date, more than 10 different four-helix bundle ligands have been identified that incorporate gp130, or one of its close relatives such as LIF receptor, into functional oligomeric signaling complexes. In this review we summarize our current knowledge of shared receptor recognition and activation, with a focus on gp130. We discuss recent structural and functional information to analyze overall architectural assemblies of gp130 cytokine complexes and probe the basis for the extreme cross-reactivity of gp130 for its multiple cytokine ligands.

Mapping of the Leptin Binding Sites and Design of a Leptin Antagonist

The leptin/leptin receptor system shows strong similarities to the long-chain cytokine interleukin-6 (IL-6) and granulocyte colony-stimulating factor cytokine/receptor systems. The IL-6 family cytokines interact with their receptors through three different binding sites I-III. The leptin structure was superposed on the crystal structures of several long-chain cytokines, and a series of leptin mutants was generated focusing on binding sites I-III. The effect of the mutations on leptin receptor (LR) signaling and on binding to the membrane proximal cytokine receptor homology domain (CRH2) of the LR was determined. Mutations in binding site I at the C terminus of helix D show a modest effect on signaling and do not affect binding to CRH2. Binding site II is composed of residues at the surface of helices A and C. Mutations in this site impair binding to CRH2 but have only limited effect on signaling. Site III mutations around the N terminus of helix D impair receptor activation without affecting binding to CRH2. We identified an S120A/T121A mutant in binding site III, which lacks any signaling capacity, but which still binds to CRH2 with wild type affinity. This leptin mutant behaves as a potent leptin antagonist both in vitro and in vivo.

Leukemia inhibitory factor (LIF), cardiotrophin-1, and oncostatin M share structural binding determinants in the immunoglobulin-like domain of LIF receptor

Leukemia inhibitory factor (LIF), cardiotrophin-1 (CT-1), and oncostatin M (OSM) are four helix bundle cytokines acting through a common heterodimeric receptor composed of gp130 and LIF receptor (LIFR). Binding to LIFR occurs through a binding site characterized by an FXXK motif located at the N terminus of helix D (site III). The immunoglobulin (Ig)-like domain of LIFR was modeled, and the physico-chemical properties of its Connolly surface were analyzed. This analysis revealed an area displaying properties complementary to those of the LIF site III. Two residues of the Ig-like domain of LIFR, Asp214 and Phe284, formed a mirror image of the FXXK motif. Engineered LIFR mutants in which either or both of these two residues were mutated to alanine were transfected in Ba/F3 cells already containing gp130. The F284A mutation impaired the biological response induced by LIF and CT-1, whereas the response to OSM remained unchanged. The Asp214 mutation did not alter the functional responses. The D214A/F284A double mutation, however, totally impaired cellular proliferation to LIF and CT-1 and partially impaired OSM-induced proliferation with a 20-fold increase in EC50. These results were corroborated by the analysis of STAT3 phosphorylation and Scatchard analysis of cytokine binding to Ba/F3 cells. Molecular modeling of the complex of LIF with the Ig-like domain of LIFR provides a clue for the superadditivity of the D214A/F284A double mutation. Our results indicate that LIF, CT-1, and OSM share an overlapping binding site located in the Ig-like domain of LIFR. The different behaviors of LIF and CT-1, on one side, and of OSM, on the other side, can be related to the different affinity of their site III for LIFR.

A structural template for gp130-cytokine signaling assemblies

The gp130-cytokine system has been fertile ground for protein structure-function studies aimed at elucidating the basis of ligand recognition and receptor activation. A number of longstanding questions involve the mechanism of the stepwise assembly of the active signaling complexes, as well as the structure of the gp130-cytokine complexes. It has been clear from functional studies that the paradigm of gp130-cyokine recognition will differ substantially from the classical homo-dimeric systems, typified by human growth hormone (hGH) and its receptor. Recently, a crystal structure of a viral interleukin-6 (vIL-6), complexed with the D1D2D3 domains of the gp130 extracellular domain, has resolved many of these questions, and reconciled much of the functional and mutagenesis data which have existed for a variety of gp130-cytokines. In this review, we discuss the structure of the vIL-6/gp130 complex in some detail and suggest that the geometry of this complex will be a common structural template utilized by other gp130-cytokines, as well as cytokines from distinct signaling systems.

The ciliary neurotrophic factor receptor alpha component induces the secretion of and is required for functional responses to cardiotrophin-like cytokine

Ciliary neurotrophic factor (CNTF) is involved in the survival of a number of different neural cell types, including motor neurons. CNTF functional responses are mediated through a tripartite membrane receptor composed of two signalling receptor chains, gp130 and the leukaemia inhibitory factor receptor (LIFR), associated with a non-signalling CNTF binding receptor alpha component (CNTFR). CNTFR-deficient mice show profound neuronal deficits at birth, leading to a lethal phenotype. In contrast, inactivation of the CNTF gene leads only to a slight muscle weakness, mainly during adulthood, suggesting that CNTFR binds to a second ligand that is important for development. Modelling studies of the interleukin-6 family member cardiotrophin-like cytokine (CLC) revealed structural similarities with CNTF, including the conservation of a site I domain involved in binding to CNTFR. Co-expression of CLC and CNTFR in mammalian cells generates a secreted composite cytokine, displaying activities on cells expressing the gp130-LIFR complex on their surface. Correspondingly, CLC-CNTFR activates gp130, LIFR and STAT3 signalling components, and enhances motor neuron survival. Together, these observations demonstrate that CNTFR induces the secretion of CLC, as well as mediating the functional responses of CLC.

Structure of an extracellular gp130 cytokine receptor signaling complex

The activation of gp130, a shared signal-transducing receptor for a family of cytokines, is initiated by recognition of ligand followed by oligomerization into a higher order signaling complex. Kaposi's sarcoma-associated herpesvirus encodes a functional homolog of human interleukin-6 (IL-6) that activates human gp130. In the 2.4 angstrom crystal structure of the extracellular signaling assembly between viral IL-6 and human gp130, two complexes are cross-linked into a tetramer through direct interactions between the immunoglobulin domain of gp130 and site III of viral IL-6, which is necessary for receptor activation. Unlike human IL-6 (which uses many hydrophilic residues), the viral cytokine largely uses hydrophobic amino acids to contact gp130, which enhances the complementarity of the viral IL-6-gp130 binding interfaces. The cross-reactivity of gp130 is apparently due to a chemical plasticity evident in the amphipathic gp130 cytokine-binding sites.

Receptor recognition by gp130 cytokines

Cytokines of the gp130 family exert their diverse biological effects by formation of stable high affinity transmembrane receptor complexes that are characterized by the presence of the shared transmembrane signalling receptor gp130. Different gp130 ligands form signalling complexes that vary in both composition and stoichiometry. Analysis of the three-dimensional structure of selected ligands and receptor elements indicates that ligands display three topologically conserved receptor recognition epitopes that interact with complementary ligand recognition elements. The composition of the signalling complex and downstream biological responses is defined by the relative affinity of different receptor components for these epitopes. The detailed structure of receptor recognition epitopes indicates that the generation of small molecule cytokine mimetics may be a feasible objective.

Solution structure of leukemia inhibitory factor

The solution structure of a murine-human chimera of leukemia inhibitory factor (LIF), a 180-residue cytokine with a molecular mass of 20 kDa, has been determined using multidimensional heteronuclear NMR techniques. The protein contains four alpha-helices, the relative orientations of which are well defined on the basis of long-range interhelical nuclear Overhauser effects. The helices are arranged in an up-up-down-down orientation, as found in other four-helix bundle cytokines, and the overall topology of the chimera is similar to that of the crystal structure of murine LIF (Robinson, R. C., Grey, L. M., Staunton, D., Vankelecom, H. Vernallis, A. B., Moreau, J. F., Stuart, D. I., Heath, J. K., and Jones, E. Y. (1994) Cell 77, 1101-1116). Differences between the structures are evident in the N-terminal region, where the peptide bond preceding Pro17 has a trans-conformation in solution but a cis-conformation in the crystal, and in the small antiparallel beta-sheet encompassing residues in the N terminus and the CD loop in the crystal structure, which is not apparent in solution. There are also minor differences in the extent of the helices. Other than at the N terminus, the main difference between the two structures occurs at the C-terminal end of the CD loop. As this loop is close to a receptor-binding site on LIF that makes a major contribution to high affinity binding to the LIF receptor alpha-chain, these differences between the solution and crystal structures should be taken into account in structural models of LIF receptor interactions.

Agonistic and antagonistic variants of ciliary neurotrophic factor (CNTF) reveal functional differences between membrane-bound and soluble CNTF alpha-receptor

Ciliary neurotrophic factor (CNTF) drives the sequential assembly of a receptor complex containing the ligand-specific alpha-receptor subunit (CNTFR) and the signal-transducing beta-subunits gp130 and leukemia inhibitory factor receptor-beta (LIFR). CNTFR can function in either membrane-bound or soluble forms. The membrane-bound form mediates the neuronal actions of CNTF, whereas the soluble form serves to confer cytokine responsiveness to non-neuronal cells expressing gp130 and LIFR. The objective of this work was to analyze whether the two receptor isoforms differ in their ability to interact functionally with CNTF and related proteins. Two new types of CNTF variants, characterized by weakened interactions with either CNTFR or both LIFR and gp130, were developed, and the biological activities of these and other mutants were determined in non-neuronal versus neuronal cells, as well as in non-neuronal cells transfected with an expression vector for CNTFR. Membrane anchoring of CNTFR was found to render the CNTF receptor complex relatively insensitive to changes in agonist affinity for either alpha- or beta-receptor subunits and to promote a more efficient interaction with a gp130-depleting antagonistic variant of CNTF. As a result of this phenomenon, which can be rationalized in terms of the multivalent nature of CNTF receptor interaction, CNTF variants display striking changes in receptor selectivity.

Solution structure of recombinant human interleukin-6

Interleukin-6 (IL-6) is a 185 amino acid cytokine which exerts multiple biological effects in vivo and whose dysregulation underlies several disease processes. The solution structure of recombinant human interleukin-6 has now been determined using heteronuclear three and four-dimensional NMR spectroscopy. The structure of the molecule was determined using 3044 distance and torsion restraints derived by NMR spectroscopy to generate an ensemble of 32 structures using a combined distance geometry/simulated annealing protocol. The protein contains five alpha-helices interspersed with variable-length loops; four of these helices constitute a classical four-helix bundle with the fifth helix located in the CD loop. There were no distance violations greater than 0.3 A in any of the final 32 structures and the ensemble has an average-to-the-mean backbone root-mean-square deviation of 0.50 A for the core four-helix bundle. Although the amino-terminal 19 amino acids are disordered in solution, the remainder of the molecule has a well defined structure that shares many features displayed by other long-chain four-helix bundle cytokines. The high-resolution NMR structure of hIL-6 is used to rationalize available mutagenesis data in terms of a heteromeric receptor complex.

Interleukin-6: structure-function relationships

Interleukin-6 (IL-6) is a multifunctional cytokine that plays a central role in host defense due to its wide range of immune and hematopoietic activities and its potent ability to induce the acute phase response. Overexpression of IL-6 has been implicated in the pathology of a number of diseases including multiple myeloma, rheumatoid arthritis, Castleman's disease, psoriasis, and post-menopausal osteoporosis. Hence, selective antagonists of IL-6 action may offer therapeutic benefits. IL-6 is a member of the family of cytokines that includes interleukin-11, leukemia inhibitory factor, oncostatin M, cardiotrophin-1, and ciliary neurotrophic factor. Like the other members of this family, IL-6 induces growth or differentiation via a receptor-system that involves a specific receptor and the use of a shared signaling subunit, gp130. Identification of the regions of IL-6 that are involved in the interactions with the IL-6 receptor, and gp130 is an important first step in the rational manipulation of the effects of this cytokine for therapeutic benefit. In this review, we focus on the sites on IL-6 which interact with its low-affinity specific receptor, the IL-6 receptor, and the high-affinity converter gp130. A tentative model for the IL-6 hexameric receptor ligand complex is presented and discussed with respect to the mechanism of action of the other members of the IL-6 family of cytokines.

Identification of ciliary neurotrophic factor (CNTF) residues essential for leukemia inhibitory factor receptor binding and generation of CNTF receptor antagonists

Ciliary neurotrophic factor (CNTF) drives the sequential assembly of a receptor complex containing the ligand-specific alpha-receptor subunit (CNTFR alpha) and the signal transducers gp130 and leukemia inhibitory factor receptor-beta (LIFR). The D1 structural motif, located at the beginning of the D-helix of human CNTF, contains two amino acid residues, F152 and K155, which are conserved among all cytokines that signal through LIFR. The functional importance of these residues was assessed by alanine mutagenesis. Substitution of either F152 or K155 with alanine was found to specifically inhibit cytokine interaction with LIFR without affecting binding to CNTFR alpha or gp130. The resulting variants behaved as partial agonists with varying degrees of residual bioactivity in different cell-based assays. Simultaneous alanine substitution of both F152 and K155 totally abolished biological activity. Combining these mutations with amino acid substitutions in the D-helix, which enhance binding affinity for the CNTFR alpha, gave rise to a potent competitive CNTF receptor antagonist. This protein constitutes a new tool for studies of CNTF function in normal physiology and disease.

Activating mechanism of CNTF and related cytokines

Ciliary neurotrophic factor (CNTF) shares structural and functional properties with members of the hematopoietic cytokine family. It is composed of a four-helix bundle structure and shares the transmembrane signal transducing proteins, glycoprotein-130 (gp130) and leukemia inhibitory factor receptor (LIF-R). Structure-function analysis showed that the gp130-interactive proteins bind in a similar manner to that of growth hormone (site I and II). In addition, gp130-interactive proteins and granulocyte colony-stimulating factor (G-CSF) utilize another binding site (site III) at the boundary between CD loop and helix D. CNTF triggers the association of receptor components, resulting in activation of a signal transduction cascade mediated by specific intracellular protein tyrosine kinases. The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) and Ras/mitogen-activated protein kinase (MAPK) signaling pathways have been characterized in terms of gp130-interactive protein, and there should be other pathways and some crosstalk between them to enhance, prolong, or specify the signals.