The extracellular protein Spätzle is required for activation of the Toll signaling pathway in the embryonic development and innate immune defense of Drosophila. Spätzle is synthesized as a pro-protein and is processed to a functional form by a serine protease. We show here that the mature form of Spätzle triggers a Toll-dependent immune response after injection into the hemolymph of flies. Spätzle specifically bound to Drosophila cells and to Cos-7 cells expressing Toll. Furthermore, in vitro experiments showed that the mature form of Spätzle bound to the Toll ectodomain with high affinity and with a stoichiometry of one Spätzle dimer to two receptors. The Spätzle pro-protein was inactive in all these assays, indicating that the pro-domain sequence, which is natively unstructured, acts to prevent interaction of the cytokine and its receptor Toll. These results show that, in contrast to the human Toll-like receptors, Drosophila Toll requires only an endogenous protein ligand for activation and signaling.

Ciliary neurotrophic factor (CNTF) is a cytokine supporting the differentiation and survival of various cell types in the peripheral and central nervous systems. Its receptor complex consists of a non-signaling alpha chain, CNTFR, and two signaling beta chains, gp130 and the leukemia inhibitory factor receptor (LIFR). Striking phenotypic differences between CNTF- and CNTFR-deficient mice suggest that CNTFR serves as a receptor for a second, developmentally important ligand. We have identified this factor as a stable secreted complex of cardiotrophin-like cytokine (CLC) and the soluble receptor cytokine-like factor-1 (CLF). CLF expression was required for CLC secretion, and the complex acted only on cells expressing functional CNTF receptors. The CLF/CLC complex activated gp130, LIFR and signal transducer and activator of transcription 3 (STAT3) and supported motor neuron survival. Our results indicate that the CLF/CLC complex is a second ligand for CNTFR with potentially important implications in nervous system development.

We describe a novel cytokine receptor named GP130 Like receptor, or GPL, that displays similarities with the interleukin-6 and interleukin-12 family of signaling receptors. Four different isoforms diverging in their carboxyl terminus were isolated, corresponding to proteins encompassing 560, 610, 626, and 745 amino acids. Sequences included a signal peptide of 32 amino acids, followed by a cytokine binding domain containing four conserved cysteines, a WSDWS motif, and a region consisting of three fibronectin type III domain repeats. No immunoglobulin-like module was identified in the GPL sequences. The intracellular part of longer isoforms contained a proline-rich region defining a box1 motif for interaction with the Janus kinases. The Gpl gene is organized in 15 exons and is located on 5q11.2 in tandem with the gp130 gene. Both genes were only separated by 24 kilobases, with opposite transcriptional orientations. The GPL receptor displayed a 28% identity with gp130. Specific GPL transcripts were observed in tissues involved in reproduction. Transcripts were also found in blood cells and in bone marrow, revealing expression of GPL in all of the myelomonocytic lineage, from hematopoietic stem cells to activated dendritic cells. In monocytes and dendritic cells, expression of GPL was strongly up-regulated by interferon-␥, indicating a possible involvement of GPL in Th1-type immune responses. The molecular basis of cell signaling mediated by GPL was studied using chimeric receptors where external portions of ␣ or ␤ interleukin-5 receptor subunits were fused to the internal portion of GPL or of related receptors. Results indicated that association of GPL to the intracellular portions of gp130, or LIF receptor, allowed the signaling cascade.