The mammalian embryo develops as a quasi-stem cell system whose differentiation and pluripotentiality in vitro is controlled by a single regulatory factor, Differentiation Inhibiting Activity/Leukemia Inhibitory Factor (DIA/LIF). DIA/LIF is expressed in two distinct functional forms, derived from the use of alternate transcriptional start sites, one of which is freely diffusible and the other tightly associated with the extracellular matrix. The dissemination of the DIA/LIF signal is therefore under specific molecular control. The expression of DIA/LIF in vitro is both developmentally programmed and controlled by the action of other growth factors, the most notable of which are members of the fibroblast growth factor family expressed by the stem cells themselves. This indicates that differentiation and proliferation in early development of the mouse are controlled, at least in part, by an interactive network of specific growth and differentiation regulatory factors.

The development of most organisms is characterised by two features: the progressive elaboration of different cell types from common precursors and the generation of tissue and body architecture in a defined pattern. These processes in many situations are thought to depend upon specific cellular responses to environmental cues, relating either to relative position within the developing organism or interactions with other cells or groups of cells. Two key issues in modern embryology are accordingly the molecular identity of such environmental signals and the mechanisms by which such signals are interpreted into changes in gene expression and cell behaviour.

There is now very good evidence that intercellular signalling in many developmental systems is mediated by specific polypeptide growth and differentiation factors, often initially identified by their biological activity in adult or pathological situations. This evidence comes from three main sources. Firstly, analysis of expression in vivo by in situ hybridisation and immunohistochemistry techniques has revealed that many growth factors are principally expressed in the embryonic stages of development. Where detailed information is available, e.g. for TGFβ (Lehnert and Ackhurst, 1988; Heine et al. 1987), TGFβ and BMP2A (Lyons et al. 1989), IGF2 (Beck et al. 1987) and Int-2 (Wilkinson et al. 1988), expression is often found in multiple sites and at different stages of development.

This evidence suggests that growth factors have some kind of activity in embryological development, and the often complex patterns of expression observed suggest that either individual growth factors perform some specific function in a diversity of situations or that they have diverse functions which depend upon the cellular context in which they act. The second type of evidence comes from experimental situations in which it has been possible to demonstrate that exogenously applied growth factors can mimic the effects of normal intercellular interactions in controlling developmental decisions. Examples would include the effect of members of the FGF gene family in inducing ventral-type mesoderm differentiation in the animal pole cells of early Xenopus embryos (Slack et al. 1987), or the action of TGFβ and the TGFβ-like XTC mesoderm inducing factor in the induction of dorsal mesoderm differentiation in the same system (Smith, 1987; Rosa et al. 1988). The third line of evidence comes from the characterisation of developmental mutants (particularly in Drosophila’) where genes which can be shown, by genetic methods, to control specific pathways of differentiation and tissue pattern are found to encode molecules which are very similar to growth factor receptors (eg. sevenless and torpedo, reviewed by Hafen and Basler, this volume) or growth factors themselves (e.g. wingless, Rijsewick et al. 1987).

In this paper we shall be concerned with the identity and function of growth factors controlling developmental decisions and gene expression in the early development of the mouse. The mouse offers certain advantages for this problem which principally arise from the strategy of early mouse development. Abundant experimental evidence (reviewed by Gardner and Beddington, 1988) has shown that early mouse development occurs as a quasi-stem cell system whereby the major differentiated cell types formed in the early mouse embryo are withdrawn by differentiation from a pool of cells, leaving a residual ‘stem cell’ population which retains multipotential properties and from whom subsequent differentiated cell types are formed. This multipotent stem cell population only exists transiently during the course of normal development, although it has been hypothesised that the residue of this multipotent stem cell pool goes on to form the germ cell lineage (Heath, 1978). Each successive differentiation event in this situation occurs as a consequence of cellular responses to environmental cues such as relative location or proximity to other cell types. A historical difficulty with the direct molecular analysis of these signals has been the small size and inaccessibility of the embryo as development unfolds. However, a major experimental advantage is the availability of cultured cell lines derived either by tumourgenic conversion of the pluripotent quasi-stem cell population present in the normal embryo, embryonal carcinoma (EC) cells, or directly from it, embryonic stem (ES) cells, as reviewed by Robertson (1989). The key feature of ES cell lines, in particular, is that upon réintroduction into a host preimplantation embryo they will participate in normal development and give rise to progeny in all the tissues of the adult mouse, including the germ cell lineage (Bradley et al. 1984).

This phenomenon has three significant implications. Firstly, it permits the use of ES cells as a cellular vector for genetic manipulation of the mouse genome, providing the means to generate defined gain of function or loss of function mutations for experimental analysis of gene function (reviewed by Frohman and Martin, 1989). Secondly, ES cells provide an experimentally accessible resource for the molecular characterisation of growth factor expression and action: the demonstrated pluripotentiality of ES cells argues that ES cells (and their differentiated progeny) are capable of both generating and responding correctly to all the environmental signals that occur in the course of normal development. Thirdly, the extent of tissue colonisation occurring upon réintroduction of ES cells into the embryo provides a rigorous biological assay for the action of regulatory agents on ES cell function.

1. Control of stem cell function

An empirical finding of the first workers to isolate ES cells, Martin (1981) and Evans and Kaufman (1981), was, that like many EC cell lines (Martin and Evans, 1975), effective propagation and maintenance of ES cell pluripotentiality required that the cells be grown in association with a heterologous feeder cell layer (usually irradiated fibroblasts). Whilst this was technically inconvenient it strongly implied that maintenance of ES cell pluripotentiality and inhibition of their differentiation depended upon some interaction with the feeder cell layer; in other words some type of feeder-derived signal was continuously required to actively maintain ES cell function in vitro. A number of workers (Smith and Hooper, 1983; Koopman and Cotton, 1984; Smith and Hooper, 1987) observed that the requirement for a functional feeder cell layer to suppress initially EC, and subsequently ES, cell differentiation could be replaced by culture media conditioned by a variety of cell types such as STO fibroblasts (Smith and Hooper, 1983) and the BRL rat liver cell line (Smith and Hooper, 1987). The differentiation of ES cells in vitro can therefore be suppressed by soluble factors secreted by heterologous cell types.

The principal agent responsible for these effects of feeder-conditioned media on ES cell function is a single polypeptide regulatory factor DIA/LIF. This was conclusively demonstrated by the finding that recombinant DIA/LIF from either bacterial or mammalian sources can completely substitute for heterologous feeder cells or feeder-conditioned medium, both for maintenance of ES cell pluripotentiality in continuous culture (Smith et al. 1988; Williams et al. 1988) and for isolation of pluripotent ES cell lines de novo from the normal mouse embryo (L. Williams and S. Pease, personal communication; A. G. Smith and J. Nichols, unpublished observations). These observations suggest that DIA/LIF is a major determinant of pluripotent stem cell function in the normal embryo, and the pluripotent stem cell state is maintained as long as DIA/LIF is present. It follows that an understanding of the intercellular mechanisms of stem cell differentiation require analysis of DIA/LIF action at the molecular level and that the transient existence of the stem cell state in the course of normal development (Gardner and Beddington, 1988) is likely to be controlled by the exact timing and form of the delivery of the DIA/LIF signal to the cells in vivo.

2. Molecular and biological characteristics

DIA/LIF is a molecule of about 43 × 103Mr derived by extensive glycosylation of a core 19× 103Mr polypeptide chain (Smith et al. 1988). Although the extent of glycosylation can depend upon the identity of the cellular source, it does not appear that the carbohydrate moiety is essential for biological function since activity is manifest in non-glycosylated material derived from expression in bacteria (Gearing et al. 1989). It is present in ‘natural’ sources, biologically active at sub-nanomolar concentrations and exerts its biological activity through association with specific, high affinity cell surface receptors expressed at between 300 –1500 sites per cell (Smith et al. 1988; Hilton et al. 1988). The structural characterisation of DIA/LIF receptor(s) will be an important step towards defining the intercellular mechanism controlling ES cell differentiation and function. However, the molecular nature of the DIA/LIF receptor(s) is at present unclear, although indirect arguments (see below) suggest that they may be of the PIN-type form typical of certain lymphokines such as 11-6 (reviewed by Guy et al. 1990) rather than the tyrosine kinase or G-Protein linked serpentine receptors associated with many growth factors and mitogens.

DIA/LIF exemplifies the multiple biological functions ascribed to polypeptide regulatory factors described above. It was initially observed as an activity which induced macrophage differentiation in the M1 mouse leukaemia cell line (Tomida et al. 1984) and this property enabled isolation of mouse and human cDNA clones encoding DIA/LIF (Gearing et al. 1987; Gough et al. 1988). It was independently observed that the activity present in BRL-conditioned media which acted to suppress differentiation of ES cells was active as a maintenance factor for the DAla murine leukaemia cell line. Exploitation of this latter activity led to the isolation of human cDNA clones encoding DIA/LIF (Smith et al. 1988; Moreau et al. 1988). These findings created a link between stem cells of the early mouse embryo and cell differentiation and proliferation in the haemopoetic system. Following these initial discoveries, DIA/LIF has emerged as a molecule with distinct bioregulatory actions in a diversity of cellular systems including bone, liver and the nervous system (Table 1). This strongly indicates that, specifically in the case of DIA/LIF, the exact biological effects of a polypeptide regulatory agent in vivo depend on the precise circumstances in which the signal is delivered to the target cell and the cellular context in which the signal is interpreted. A second feature of the range of DIA/LIF biological activity is an intriguing congruity between the actions of DIA/LIF and Interleukin 6 in systems such as MI, DAla cells and hepatocytes (although 11-6 does not affect ES cell differentiation in vitro, A. G. Smith, unpublished results). This may be coincidental, but could indicate that both agents operate through analogous signal transduction pathways in which ligand specificity is determined by expression of appropriate receptors.

Table 1.

Bioregulatory actions of DIA/LIF in various cellular systems

Bioregulatory actions of DIA/LIF in various cellular systems
Bioregulatory actions of DIA/LIF in various cellular systems

It has been argued above that a key issue in understanding the role of DIA/LIF action in vivo is the way in which the signal is delivered. It is of considerable significance therefore that recent findings indicate that DIA/LIF exists in two functional forms which are distinguished by their range of action in vivo (Rathjen et al. 1990). Detailed examination of DIA/LIF gene transcription in cultured cells has shown that two classes of DIA/LIF mRNAs exist which diverge in sequence at their 5′ ends. Molecular cloning of cDNAs encoding these two forms of DIA/LIF mRNA shows that they encode two distinct forms of DIA/LIF protein which differ in sequence at the amino terminus (Fig. 1). The 5 ′ sequence divergence is generated by initiation of transcription at two distinct promoters, thereby creating two distinct first exons which are subsequently spliced onto common second and third exons to generate the two distinct mRNA species. The functional significance of this arrangement is that one form of the DIA/LIF protein (containing the amino-terminal sequence MKVLA) is expressed in a form which is freely diffusible and the alternative form (containing the amino-terminal sequence MRCR) is expressed in a form which is specifically associated with the extracellular matrix of the expressing cells.

Fig. 1.

Nucleotide sequence (upper) and predicted amino acid sequence (1ower) of 5′ ends of alternative DIA/LIF cDNA clones. Coding regions are underlined, the position of exonl/exon2 boundaries are indicated by arrows and amino-terminal differences in bold type.

Fig. 1.

Nucleotide sequence (upper) and predicted amino acid sequence (1ower) of 5′ ends of alternative DIA/LIF cDNA clones. Coding regions are underlined, the position of exonl/exon2 boundaries are indicated by arrows and amino-terminal differences in bold type.

Although it is not clear how the amino-terminal sequence determines the localisation of the parent protein, the use of alternative transcripts dictates whether a DIA/LIF signal can act over a distance or whether its sphere of action is confined to cells in intimate physical contact. In essence, the way a DIA/LIF signal is delivered is of paramount importance in understanding its function in vivo and a molecular mechanism exists to control tightly the delivery of the signal by exploitation of the extracellular matrix to limit dissemination.

The biological importance of this distinction can be envisaged by consideration of a hypothetical multilayer tissue (Fig. 2). Expression of the ECM-associated form of DIA/LIF would confine its action to cells in the adjacent layer, whereas expression of the diffusible form would permit control of cell behaviour in cell layers remote from the signal source. This behaviour represents a simple mechanism for creating pattern in a multilayered tissue, and is intriguingly reminiscent of the concept of stem cell ‘niche’ (reviewed by Lajtha, 1979) which proposes that stem cells are inhibited from differentiation into more mature cell types by sequestration in a specific physical compartment. According to this view, differentiation induction occurs as a consequence of cell movement out of the ‘niche’ (or away from the source of signal). It should be emphasised that the use of multiple protein forms to control the dissemination of a signal is not confined to DIA/LIF. Recent studies of int-2 expression, for example, have revealed the existence of multiple amino termini whose identity directs localisation of the int-2 molecule to the nucleus, endoplasmic reticulum or extracellular matrix (Acland et al. 1990; Dickson et al. this volume).

Fig. 2.

Different forms of DIA/LIF have different range of actions. MKVLA-DIA/LIF (upward facing arrows) is freely diffusible and can affect the behaviour of cells remote from the site of expression. MRCR-DIA/LIF (downward facing arrows) is deposited in the extracellular matrix of expressing cells, and only those cells in direct physical contact are capable of responding.

Fig. 2.

Different forms of DIA/LIF have different range of actions. MKVLA-DIA/LIF (upward facing arrows) is freely diffusible and can affect the behaviour of cells remote from the site of expression. MRCR-DIA/LIF (downward facing arrows) is deposited in the extracellular matrix of expressing cells, and only those cells in direct physical contact are capable of responding.

Embryonic stem cells not only modify their behaviour in response to environmental cues such as DIA/LIF, but are themselves a source of potent regulatory factors. It has been known for some time that EC cells and ES cells express multiple growth factor species with important functional consequences. The major growth factor species expressed by ES and EC cells have been characterised by protein purification and recombinant DNA techniques (Heath et al. 1989). These prove to be secreted (and soluble) members of the FGF family of growth factors: K-FGF, also called Ks. (Delli-bovi et al. 1987) or hst-1 (Taira et. al, 1987) and a second FGF-like growth factor, closely related to K-FGF in biochemical properties and biological function, whose primary sequence is at present undefined (Heath et al. 1989). It has been established (mainly from analysis of EC cell differentiation systems) that these stem cell-derived growth factors are capable of controlling the multiplication of their differentiated progenitors (Heath and Rees, 1985) suggesting the existence of a form of ‘feedforward control’ whereby stem cells control the behaviour of the differentiated progeny via the action of these secreted factors (reviewed by Heath and Smith, 1988).

The importance of stem cell-derived growth factors goes beyond their mitogenic action on differentiated progeny. The expression of DIA/LIF (both soluble and matrix associated forms) in a variety of cell types has been found to depend upon the presence of exogenous growth factors, including members of the FGF family (Rathjen et al. 1990). Furthermore, expression in many cases is enhanced in the presence of modulators of the TGFβ family which are also known to be expressed (albeit in the latent form) by EC cells and their differentiated derivatives (Mummery et al. 1990). DIA/LIF expression (soluble and matrix associated forms) is also found to increase greatly as ES cells undergo differentiation in vitro in the absence of exogenous DIA/LIF (Rathjen et al. unpublished data).

These findings have two implications. Firstly, they suggest that the natural source of DIA/LIF made available to stem cells in vivo is their immediate differentiated progeny; the stem cell ‘pool’ described above would in this view be maintained as a consequence of initial differentiation events. This provides further support for a ‘feedback’ relationship between differentiated progeny and stem cell parents (Heath and Smith, 1988). Secondly, the inducibility of DIA/LIF expression by stem cell-derived factors in certain cell types provides a potential patterning mechanism, since stem cell differentiation (and manifestations of DIA/LIF action in other sites) would be controlled by the inducibility of other cell types in the immediate vicinity. The specific case of the feeder effect in ES cells may well be due to this phenomenon where ES cells locally induce responsive heterologous feeder cells to express DIA/LIF.

This review has focused on the role of differentiation regulatory factors in early mouse development. The exploitation of cultured stem cell lines has led to the identification of both a stem cell regulatory factor (DIA/LIF) and stem cell-derived growth factors such as K-FGF. A closer examination of the action of these agents has revealed two specific issues which may have broader application outside the early phases of mammalian development. Firstly, the way a regulatory signal is delivered to a responding cell and how it is subsequently interpreted are key issues in fully understanding the role of bioregulatory factors in controlling patterning and the generation of form. In the case of DIA/LIF (and other embryonic growth and differentiation factors) mechanisms exist which control the dissemination of the regulatory signal. Secondly, individual regulatory factors participate in a network of interactions between parental cells and their differentiated progeny. The specific design of such a regulatory network will have great influence on the form and pattern of the differentiation events that ensue.

The authors’ research is supported by the Cancer Research Campaign. L-W H is supported by a Taiwanese Ministry of Education postgraduate scholarship.

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