Leukemia inhibitory factor signaling in Xenopus embryo: Insights from gain of function analysis and dominant negative mutant of the receptor.

Leukemia inhibitory factor (LIF) is a cytokine member of the interleukin 6 family (IL6) of cytokines. It signals through a heterodimer receptor complex that consists of the LIF receptor (or LIFR formerly known as gp190) and the Interleukin 6 signal transducer (or IL6ST formerly known as gp130). LIF signaling is mediated mainly by signal transducer and activator of transcription 3 (STAT3) and has a wide variety of biological activities with pleiotropic effects on many cell types and organs among which are stem cell renewal and implantation process in mammalian embryo. Despite the wealth of data on LIF in mammalian cells, there is a paucity of information on its functions in lower vertebrates. Here, we provide information on the status and the function of LIF signaling in Xenopus amphibian. The IL6 cytokine family is highly conserved in Xenopus genome both at ligands and receptors levels. All cytokines and receptors of the family, except oncostatin M (OSM) and IL27, can be identified in the genome including the orthologs of LIF, cardiotrophin 1 (CTF1), ciliary neurotrophic factor (CNTF), cardiotrophin like cytokine factor 1 (CLCF1), LIFR, IL6ST, IL6R, IL11RA and CNTFR. Lif mRNA is zygotically expressed after midblastula transition while lifr and il6st are maternally expressed. We have investigated the functions of LIF in Xenopus early development with a gain-of-function analysis combined to the use of a dominant negative form of the receptor. The overexpression of Xenopus lif in embryo activates STAT3 phosphorylation and induces a dramatic phenotype where embryos are ventralised and show a reduction of anterior structures with microcephaly. This results mainly from BMP signal stimulation and antagonism towards IGF signals. In addition, most embryos develop tumor-like cell masses according to both autonomous and non-autonomous processes. Through the use of a dominant negative form of the receptor, we demonstrate for the first time that a functional LIF signaling is required for normal vertebrate kidney development. Owing to its experimental advantages, the Xenopus embryo constitutes a useful model to identify the molecular actors that may account for the pleiotropic functions of LIF and their role in vertebrate development.