Cell Signalling Genes
The stem cells of the embryo are intinially aranged in a very simple ball of cells: a structure from which they create complex tissues of multiple cell types and shapes, connected by the systems of the body. The interplay of cells, through signals produced and received by one another, underpins these processes. One signal or set of signals can cause a number of effects downstream as the cells differentiate or begin new interactions with neighbouring cells (see Chapter 3). In this way a surprisingly small number of signalling factors are able to coordinate the early stages of embryonic development.
Some signals can diffuse short distances across embryonic tissues and bind to receptors on the surfaces of target cells (paracrine signalling, Figure 4.1), and some signals require direct cell‐to‐cell contact (juxtacrine signalling, Figure 4.2). Transmembrane receptor proteins undergo conformational changes when the extracellular domain binds a ligand; this modification usually causes the intracellular portion to dissociate and trigger a cascade of events resulting in the binding of transcription factors to DNA and changes to gene expression.
Paracrine signalling is relatively conserved between species and, in general, uses four groups of signalling proteins, namely Wnt, Hedgehog, fibroblast growth factor (FGF) and transforming growth factor beta (TGF‐β), all of which are intricately involved in development. Juxtacrine signalling can occur between adjacent cells or between a cell and the extracellular matrix. The direct contact and bonds that form have been shown to be vital in a number of developmental processes, including the neural system and the heart.
Notch is an example of a cell membrane protein receptor that once bound to its juxtacrine ligand on an adjacent cell (another cell membrane protein such as Serrate or Delta) undergoes a conformational change, is cleaved by an enzyme and the dissociated intracellular portion binds to a dormant transcription factor that affects gene expression. A number of varieties of Notch and its associated ligands are involved in human embryonic development.
Transcription factors are proteins that can bind to DNA and affect the transcription of nearby regions. They can promote or inhibit (up‐or downregulate) the transcription of a particular gene from DNA to mRNA. The Hox proteins are transcription factors involved in body pattern formation and segmentation and their genes have been highly conserved during evolution, having similar roles in very different species (see Chapter 21). To classify as a transcription factor a protein must contain a section that can bind directly to DNA. There are other proteins involved in regulating DNA expression that are not transcription factors as they cannot bind to DNA.
There are over 200 types of modification that can occur to a protein post‐translation, but all are involved in determining the biological properties of a protein. Commonly these include chemical alterations including hydroxylation, methylation, sulphation, phosphorylation andglycosylation. Altering aprotein post‐translationally can affect protein shape, activity, turnover, interactions with other proteins and localization. Through interactions with enzymes these proteins can have functional groups added or taken away, such as sugars, lipids and proteins. The post‐translational modification of certain proteins in signalling pathways ha n to affect cell‐to‐cell interactions during development.
Wnt protein are vertebrate version of the Wingless gene that was first identified in the fruit fly (Drosophila melanogaster), so named because mutating the gene caused flies to develop without wings. The Wnt signals bind to Frizzled transmembrane cell surface receptors, and they are involved in body patterning (see Chapter 21), cell fate, proliferation and migration. Failures of these systems in humans result in limb, eye, genitourinary and bone development disorders. Soluble forms of Frizzled known as sFrps (secreted Frizzled‐like proteins) bind Wnt proteins and inhibit Wnt signalling.
Early research into Notch was also performed in Drosophila, where this gene was initially discovered. In development this gene has great importance in cell differentiation and inducing specific cell clusters that lead to neurone, endothelial, cardiocyte and T cell development, to name a few. Notch has also been shown to help maintain stem cell populations.
Hedgehogs are a family of proteins that work much like a skeleton key (one key that can open a lot of different doors). Their effect on a cell depends on cell type, dosage and how differentiated the cell is. The Hedgehog gene was first identified in Drosophila and in addition to its body patterning role in the early embryo mutations of this gene caused naturally occurring spiky denticles to occur in a solid region rather than in stripes, and made the embryo shorter, giving the embryo a hedgehog‐like appearance. The Hedgehog family in humans includes Sonic hedgehog homologue (SHH), Indian hedgehog homologue (IHH) and Desert hedgehog homologue (DHH). Sonic hedgehog has a key role in neural, bone, limb and kidney development; muscle patterning; and lung branching. It is also involved in the development of the special sense organs. It binds to a cell surface transmembrane receptor called Patched.
There are currently 22 varieties of FGF with a number of func- tions in adult tissues. During embryological development they are key players in a wide range of processes including limb and neural development, angiogenesis, very early patterning and induction of mesoderm development. FGFs bind FGF receptors (FGFRs) and heparan sulphate proteoglycans are part of the signal transduction process. The many different FGF types and receptor combinations allow for a variety of effects in different situations, and interruption of these effects during development causes a number of developmental abnormalities.
TGF‐β is described as part of a superfamily of signalling factors, and includes the bone morphogenetic proteins (BMPs). When originally discovered it was associated with tumour development, but a number of structurally similar molecules have since been identified and implicated in many events during embryological development and in adult tissues. Three forms of TGF‐β and 15 types of BMPs have been discovered. TGF‐β ligands bind to a type II TGF‐β receptor that recruits a type I receptor, triggering a SMAD cascade and ultimately a change in DNA transcription.