The Most Important Class of Enzyme-Linked Cell Surface Receptors: Receptor Tyrosine Kinases

September 24, 2010

Receptor tyrosine kinases (RTKs) are cell membrane associated receptors with an intracellular and an extracellular domain. The extracellular domain binds different kinds of ligands that induce cellular growth by a signal transduction pathway I would like to explain here. Sort variations of RTKs are mainly based on different extracellular domains (always N-terminus) and their ligand affinity. Ligands for RTKs include soluble or membrane bound peptide or protein hormones such as insulin, epidermal growth factor (EGF), platelet derived growth factor (PDGF), fibroblast growth factor (FGF) and many more GFs. The intracellular domain (C-terminus) carries an enzyme which is known as tyrosine kinase and is thus able to phosphorylate tyrosines.

A specialty of all RTKs is that the ligand-binding extracellular domain is connected to the catalytic intracellular domain only by a single transmembrane α-helix. That means that it is not possible for cytosolic substances to transmit signals across the cell membrane. Since information transduction is the ultimate task of receptors there is an evolutionary solution to this problem: Upon ligand binding two adjacent RTKs dimerize and their catalytic domains come into contact and cross-phosphorylate each other at their multiple tyrosine residues (autophosphorylation). Fig. 1 depicts a simplified and general model of RTK structure and downstream functioning.

Fig. 1: Binding of the ligand to the monomeric receptor structure induces dimerization which is followed by autophosphorylation of the catalytic intracellular domains at tyrosine residues. The activated domains now serve as binding sites for several proteins, especially the Grb-Sos heterodimer. Attached to the inner cell membrane is the Ras G protein which is bound by activated Sos and exchanges GTP for GDP, thereby becoming active. Activated Ras now becomes a binding site itself and binds the proteins kinase Raf which relays the signal to a number MAP kinases and finally leads to a cellular response through transcription factor activation (not shown) (adapted from 1).

In the following I want to concentrate on one interesting example of RTKs: the fibroblast growth factor receptor (FGFR) its structure, function and downstream signal. Personally I find this type of receptor very interesting because it stands out as an example how even small amino acid modifications can distort receptor function and lead to developmental errors. In the case of FGFR a consequence can be Kallmann syndrome (KS) based on inhibited and incomplete migrations of axons. In particular olfactory neurons are affected. Since certain other neurons, such as neurons that later synthesize gonadotropin releasing hormones (GnRH), depend on the formation of olfactory neurons because they have to migrate alongside them KS has a phenotype which constitutes olfactory bulb dysgenesis (no sense of smell) and hypogonadotrophic hypogonadism (delayed or no puberty) as consequences.

So far I can summarize the FGFR specific signal transduction pathway as FGF binding to FGFR which then dimerizes and relays the signal to the downstream effectors FRS2 and Grb2/Sos which activate Ras. Ras then phosphorylates Raf which transfers the signal to a mitogen activated kinase kinase (MAPKK) and subsequently to MAPK which activates nuclear transcription factors that can potentially lead to cellular differentiation and proliferation 2. In the case of KS especially the genetic regulation of actin mediated neuronal migration does not seems to be functional.

In order to understand these processes in more detail and to introduce the multiple factors that influence FGFR function it is necessary to focus on this type of receptor in a more integrated way (Fig. 2). Many contributing elements such as extracellular cofactors and intracellular downstream signalling effectors contribute to FGFR regulation, but are at the same time targets for mutations which can lead to diseases such as KS.

Fig. 2: Summary of the signalling pathways associated with FGFR function and extra- and intracellular factors that either positively or negatively infer with FGFR. Most prominently the MAPK pathway regulates FGF developmental regulation (see text and Fig. 1). Secondly the Erk and PI3 kinase (PI3K)/Akt pathways can be activated by Grb2 action. A third signal transduction pathway which directly activated by FGFR associated phospholipase C (PLC-γ)  (thus not via Grb2) is the calcium/protein kinase C (PKC) pathway. When focussing on KS it is important to highlight that the earlier mentioned Crk/Shc/Sos adaptor complex is responsible for p38 and Jun MAPK activation which is associated with neuronal migration. In addition the complex activates cytoskeletal rearrangement by Rho via Ras and Rac which might also play a role in FGFR associated KS. Important to note are also FGFR’s interactions with upstream elements such as heparan sulphate (HPSG), anosemin and a number of membrane associated proteins (adapted from 3).

Later I will continue with a focus on the 3D structure of elements that participate in downstream signalling of FGFR. A paper by Alloy and Russel 4 is an excellent basis for this approach. In addition I want to give examples of the most prominent sites that can be mutated on FGFR1 and that cause KS. Noteworthy is especially the IgIII subunit of FGFR as depicted in Fig. 2 which serves as the predominant binding site for FGF.

1.  Lincoln & Taiz Gene Expression and Signal Transduction. Plant Physiology 26-29 (2006).

2.  Eswarakumar, Lax & Schlessinger Cellular signaling by fibroblast growth factor receptors. Cytokine Growth Factor Rev 16, 139-149 (2005).

3.  Mason Initiation to end point: the multiple roles of fibroblast growth factors in neural development. Nat Rev Neurosci 8, 583-596 (2007).

4.  Aloy & Russell Structural systems biology: modelling protein interactions. Nat Rev Mol Cell Biol 7, 188-197 (2006).

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