Isabelle Gloaguen

CNTF VARIANTS WITH INCREASED BIOLOGICAL POTENCY AND RECEPTOR SELECTIVITY DEFINE A FUNCTIONAL SITE OF RECEPTOR INTERACTION

Human CNTF is a neurocytokine that elicits potent neurotrophic effects by activating a receptor complex composed of the ligand‐specific alpha‐receptor subunit (CNTFR alpha) and two signal transducing proteins, which together constitute a receptor for leukemia inhibitory factor (LIFR). At high concentrations, CNTF can also activate the LIFR and possibly other cross‐reactive cytokine receptors in the absence of CNTFR alpha. To gain a better understanding of its structure‐function relationships and to develop analogs with increased receptor specificity, the cytokine was submitted to affinity maturation using phage display technology. Variants with greatly increased CNTFR alpha affinity were selected from a phage‐displayed library of CNTF variants carrying random amino acid substitutions in the putative D helix. Selected variants contained substitutions of the wild‐type Gln167 residue, either alone or in combination with neighboring mutations. These results provide evidence for an important functional role of the mutagenized region in CNTFR alpha binding. Affinity enhancing mutations conferred to CNTF increased potency to trigger biological effects mediated by CNTFR alpha and enhanced neurotrophic activity on chicken ciliary neurons. In contrast, the same mutations did not potentiate the CNTFR alpha‐independent receptor actions of CNTF. These CNTF analogs thus represent receptor‐specific superagonists, which should help to elucidate the mechanisms underlying the pleiotropic actions of the neurocytokine.

Functional phage display of ciliary neurotrophic factor.

We report the display of human ciliary neurotrophic factor (hCNTF), a survival factor for neuronal cells belonging to the alpha-helical cytokine superfamily, on the surface of the filamentous bacteriophage fd. The hCNTF cDNA was fused to a DNA sequence encoding the C-terminal domain of pIII, a minor coat protein exposed at one end of fd. Gene fusions were cloned into a plasmid containing the ColE1 plasmid and fd origins of replication, and were packaged into phagemid particles upon superinfection with M13KO7 helper phage. The resulting fusion phage bound specifically to anti-CNTF antibodies and to the recombinant soluble CNTF alpha-receptor. Moreover, phage-displayed hCNTF was found to possess biological activity at concentrations comparable to those of the soluble cytokine. These results demonstrate that CNTF can be displayed on phage in a correctly folded and functionally active form. Binding of fusion phage to immobilized CNTF alpha-receptor and subsequent elution at low pH resulted in affinity purification of CNTF-displaying virions. Utilization of this technology should enable the selection of high-affinity variants from libraries of CNTF mutants displayed on phage.

In vivo localization and characterization of functional ciliary neurotrophic factor receptors which utilize JAK-STAT signaling

The ciliary neurotrophic factor receptor is critically involved in embryonic motor neuron development. Postnatally, it may contribute to neuronal maintenance and regeneration. In addition, pharmacological stimulation of the receptor may slow the progression of several neurodegenerative disorders. The widespread nervous system expression of ciliary neurotrophic factor receptor components and the effects of low ciliary neurotrophic factor concentrations on a wide variety of cells in culture combine to suggest that functional ciliary neurotrophic factor receptors are expressed by many classes of neurons in vivo. However, the in vivo signaling properties and distribution of functional ciliary neurotrophic factor receptors have not been directly determined. We developed a novel in vivo assay of functional ciliary neurotrophic factor receptors which revealed that, in the adult nervous system, cranial and spinal motor neurons are very sensitive to ciliary neurotrophic factor and display a rapid, robust increase in phospho-STAT3 in their dendrites, cell bodies and nuclei, which is specifically blocked by the ciliary neurotrophic factor receptor antagonist, AADH-CNTF. In distinct contrast, several other classes of ciliary neurotrophic factor receptor expressing neurons fail to increase phospho-STAT3 levels following ciliary neurotrophic factor treatment, even when ciliary neurotrophic factor is applied at high concentrations. Leukemia inhibitory factor and epidermal growth factor elicit the same cell-type-dependent pattern of phospho-STAT3 increases. Responsive and non-responsive neurons express comparable levels of STAT3.Therefore, in vivo ciliary neurotrophic factor receptor-initiated STAT3 signal transduction is regulated in a very cell-type-dependent manner. The present data suggest that at least some of this regulation occurs at the STAT3 tyrosine phosphorylation step. These unexpected results also suggest that other forms of receptor-initiated STAT3 signal transduction may be similarly regulated.

Centrally Mediated Inhibition of Local Inflammation by Ciliary Neurotrophic Factor

Since ciliary neurotrophic factor (CNTF) inhibits the production of TNF and activates the hypothalamus-pituitary-adrenal axis (HPAA), we investigated whether CNTF can produce antiinflammatory actions and whether it may act through a central mechanism, using the murine air pouch model of inflammation. In this model, inflammation is evaluated by measuring the induction of TNF and IL-6 as well as cell recruitment in the pouch fluid 24 h after carrageenan. Intracerebroventricular injection, but not intravenous or local injection of CNTF markedly inhibited inflammation. This was associated with high serum corticosterone levels, and antiinflammatory action was not observed in adrenalectomized mice, indicating that an intact HPAA is required. A CNTF receptor antagonist increased carrageenan inflammation, suggesting that endogenous CNTF might have a centrally mediated antiinflammatory role.

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 α-receptor subunit (CNTFR) and the signal-transducing β-subunits gp130 and leukemia inhibitory factor receptor-β (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 α- or β-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.