REPRODUCTIONREVIEW Leukaemia inhibitory factor in implantation and uterine biology Susan J Kimber Faculty of Life Sciences, University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT, UK Correspondence should be addressed to S J Kimber; Email: [email protected] Abstract Leukaemia inhibitory factor (LIF) is one of the most important cytokines in the reproductive tract. Without expression of LIF in the uterus, implantation of a blastocyst cannot begin. Yet, 13 years after publication of the phenotype of the LIF knockout mouse we are only just beginning to understand how LIF functions in the uterus. This review addresses our knowledge of the role of LIF in regulating implantation through its influence on the luminal epithelium and stromal decidualization, but also its influence on reproductive tract cells such as leukocytes and glandular epithelium, during the pre-implantation phase of pregnancy. Reproduction (2005) 130 131–145 Introduction of an embryo. Decidualization involves changes in the expression of a large number of genes (Farrar & Carson Blastocyst implantation is a unique feature of mammalian 1992, Paria et al. 2001) and is marked by a rapid increase reproduction and a tightly regulated process. Changes in in vascular permeability with resulting oedema in the ovarian steroids during the pre-implantation period stroma around the implantation site (Abrahamsohn & instigate the required maturation of the endometrial Zorn 1993). In spite of the well-established requirement epithelium and stroma. Following priming with preovula- for oestrogen priming before blastocyst signals can estab- tory oestrogen, increasing ovarian progesterone produces lish a decidual response, in mice decidualization has a sensitized uterus. In rodents, the ability of the uterus to been shown to occur in the absence of a functional oes- support and facilitate the implantation of the blastocyst is trogen receptor-a (Curtis et al. 1999, Paria et al. 1999, regulated by the downstream effects of a subsequent small Curtis-Hewitt et al. 2002). Following the receptive period, transient increase in oestrogen (Finn & Martin 1970). Oes- the uterus becomes refractory, no longer allowing implan- trogen induces changes which allow the uterus to support tation of any remaining blastocysts. Thus tightly regulated implantation of the blastocyst over a short period of the synchrony between embryonic development and uterine reproductive cycle, the receptive period. This spans about maturation is essential for successful pregnancy. 18–24 h in rodents and probably several days in It is clear that implantation of the mammalian embryo the human (Navot et al. 1991) although experimentally requires co-ordinated interaction between the embryo and the amount of oestrogen can influence the duration of the the uterus. Although it has long been known that ovarian receptive phase (Ma et al. 2003). The endometrial steroids regulate this process, it is only in the last couple response involves differentiation of the endometrial epi- of decades that some of the local factors have been ident- thelium and stroma to a phenotype favouring interaction ified. Steroidal regulation of uterine function is mediated with the trophectoderm. Changes in the luminal epi- to a large extent through the action of growth factors and thelium (LE), with which the trophectoderm of the acti- cytokines on their receptors (for review see Sharkey 1998, vated blastocyst first interacts, are essential for the Saito 2001). initiation of implantation (Kimber & Spanswick 2000, Aplin & Kimber 2004). The steroid-prepared stroma must also undergo a process of further differentiation, known as Leukaemia inhibitory factor (LIF) decidualization, triggered by an embryonic signal (or arti- One cytokine which is essential for successful implantation ficial mimic). Decidualization is induced in stroma around is Leukaemia Inhibitory Factor (LIF), a member of the inter- the implantation site from late on day 4 of pregnancy in leukin (IL)-6 family. LIF is a highly glycosylated 40–50 kDa mice. In women, decidual changes occur in the absence glycoprotein with a range of biological functions (Haines q 2005 Society for Reproduction and Fertility DOI: 10.1530/rep.1.00304 ISSN 1470–1626 (paper) 1741–7899 (online) Online version via www.reproduction-online.org Downloaded from Bioscientifica.com at 10/02/2021 11:09:40AM via free access 132 S J Kimber et al. 2000). It is expressed in various embryonic and adult LIF and implantation tissues (Hilton & Gough 1991, Schafer-Somi 2003) with par- In inbred female C57Bl mice, null for the LIF gene, ticularly high levels in the uterus. At the cell surface, LIF embryos develop to the blastocyst stage but do not implant receptor-b (LIF-Rb) binds the glycoprotein gp-130 (the com- (Stewart et al. 1992, Cheng et al. 2002) and the uteri show mon signalling receptor for IL-6 family cytokines) to form a little evidence of decidualization (Chen et al. 2000). We high affinity receptor through which LIF signalling is trig- observed a similar phenotype on an outbred, MF1, back- gered (Heinrich et al. 2003). Transduction of the signal can ground (Sherwin et al. 2004, Fouladi-Nashta et al. 2005). occur by activation of several pathways, the main ones In spite of the presence of anti-mesometrially located blas- being the JAK/STAT pathway, the Src homology 2-domain- tocysts in the uterine lumen, no decidual reaction is evi- containing tyrosine phoshatase (SHP-2)/Ras/extracellular dent, nor is there penetration of LE by trophoblast, even by day 7 of pregnancy (Fig. 1). LIF 2/2 embryos can implant signal-regulated kinase (ERK) pathway or the phosphatidyl- in the uteri of wild-type female mice and delivery of LIF, inositol-3-kinase (PI3K)/Akt pathways, the relative import- either by a micro-osmotic pump or injection on day 4, ance of which varies with tissue. Signalling can be inhibited restores implantation capacity to homozygous mutant by suppressors of cytokine signalling or protein inhibitor of females (Stewart et al. 1992, Chen et al. 2000, Sherwin activated STAT (PIAS) proteins (Chung et al. 1997, Bousquet et al. 2004). Since LIF-null embryos develop to term in het- et al. 1999, Duval et al. 2000). erozygous dams and can implant after transfer to wild-type Figure 1 Histological features of the uterus in LIF-null and wild-type mice. Semi-thin resin sections from days 5 and 6 of pregnancy were stained with toluidine blue. The insets show high magnification of differentiated polygonal stromal cells in the wild-type and undifferen- tiated fibroblast-like stroma cells in the LIF-null mice on day 6 of pregnancy. E ¼ embryo, scale bars ¼ 50 mm (Reprinted from Developmental Biology, vol 281, Fouladi-Nashta et al. Charac- terization of the uterine phenotype during the peri-implantation period for LIF-null, MF1 strain mice, pp 1–21, 2005, with permission from Elsevier). Reproduction (2005) 130 131–145 www.reproduction-online.org Downloaded from Bioscientifica.com at 10/02/2021 11:09:40AM via free access LIF in implantation and uterine biology 133 females of appropriate endocrine status it is clear that the precise level of LIF may be important. It has been proposed major implantation defect is on the maternal side. This is that heterozygous mutations in the LIF gene may account supported by the absence of an implantation defect in for some incidences of infertility (Giess et al. 1999) with embryos lacking components of the LIF signalling cascade. reduced amounts or LIF activity in the uterus leading to For instance, gp-130-null embryos die only during the failure of implantation. However, out of 50 women with second half of gestation with multiple abnormalities unexplained infertility, only one was reported with a het- including in heart and haematopoietic progenitors (Yoshida erozygous mutation in the LIF gene and she achieved a et al. 1996) while LIF-Rb knockout animals implant nor- pregnancy after ovarian stimulation (Steck et al. 2004). The mally but have, for instance, placental defects and motor authors concluded that although LIF mutations may well neuron degeneration and die at birth (Ware et al. 1995). play a role in infertility, screening for them would not be Deletion of the signal transducer and activator of transcrip- feasible because of their low prevalence. Another group tion 3 (Stat-3) is embryo lethal but only after implantation (Inagaki et al. 2003) found no statistically significant differ- has occurred (Takeda et al. 1997). The lack of implantation ence between LIF levels in patients with recurrent implan- in females carrying the LIF-null mutation suggests that the tation failure and control multiparous women. action of LIF in the uterus is essential for even early events Nevertheless, defects in LIF expression have in some in this process. However, the precise role of maternal LIF at studies been associated with recurrent miscarriage and the molecular level is still unclear in any species. some conditions of unexplained infertility consistent with role(s) in early events in pregnancy (Hambartsoumian 1998, Lass et al. 2001). LIF in the human uterus LIF probably plays a role in endometrial function in Regulation of LIF expression in mice humans (Vogiagis et al. 1996, Hambartsoumian 1998, Lass In mice, the highest levels of LIF mRNA are found prior to et al. 2001) and domestic species (Vogiagis et al. 1997, implantation in glandular epithelium following the nida- Modric et al. 2000, Oshima et al. 2003, Schafer-Somi tory surge of oestrogen on the morning of day 4 of preg- 2003). In humans, LIF mRNA and protein are expressed in nancy (Bhatt et al. 1991, Chen et al. 2000). Highest levels the endometrial glands during the luteal phase of the men- of LIF protein have also been reported on day 4, mainly in strual cycle when implantation would occur (Charnock- the epithelium but some in stroma (Yang et al. 1995, Fou- Jones et al. 1994, Arici et al.