Receptor Signalling

Head of laboratory: Nick Dibb (PhD)

Current lab members and sources of funding:

Jessica Taylor PhD student. BBSRC
Nicola Brownlow PhD student. BBSRC

Current research interests:

Oncogenic mutations of the PDGF receptor family.
This family consists of a set of five closely related tyrosine kinase receptors: KIT (SCF); FMS (CSF-1 or M-CSF); PDGF a and ß (PDGF)
and FLT3 (FLT3) that are activated by the specific growth factors shown in brackets. Oncogenic mutations of these receptors contribute to a wide range of cancers including leukaemia and stomach cancer. These oncogenic mutations either disrupt the juxtamembrane region (JMR) of the receptors or they specifically mutate an aspartate residue within the activation segment of the kinase domain. We were involved in the discovery of the kinase domain mutation and in collaboration with Cliff Mol we have recently proposed a molecular explanation as to why mutations of this aspartate residue cause illicit kinase activation. The KIT receptor can be inhibited by the ATP analogue Imatinib, which is now used successfully to treat those patients with stomach cancer that have mutations of the JMR of KIT. Unfortunately, mutations of the asparate residue of the kinase domain of KIT make the receptor resistant to Imatinib and so this drug cannot be used to treat patients with mast cell leukaemia, who usually have this particular mutation. We are interested in the mechanism by which oncogenic mutations confer resistance to kinase inhibitors and are also particularly keen to identify a clinically useful inhibitor of the FMS receptor. 

Structure of a kinase domain which illustrates residues of the M-CSF receptor that are important for downregulation. We found that kinase-inactivating mutations of the M-CSF receptor either prevent receptor downreguation (K616A); allow downreguation (G591A, E633H); or induce downreguation (Y809G, E825A, R900A). The oncogenic mutation D802V and the insulin receptor mutations R1164Q and W1193L also induce downregulation. The mutations that induce downregulation would be expected to disrupt the structure of the activating loop (shown in black) and these mutations alter residues that are either within or that interact with the activating loop (fig 1B). We have suggested that downregulation of tyrosine kinase receptors is initiated by some aspect of the known structural movement of the activating loop in response to ligand. According to this model, K616A inhibits downreguation because it prevents the structural alteration of the A-loop necessary for downregulation, whereas the kinase-inactivating mutations G591A and E633H do not prevent this change.

 

Receptor downregulation

MAPKinase signalling specificity
The MAPKinase signalling pathway seems essential for most of the great variety of effects of ligands upon cell function and fate. This creates the problem of explaining how the same signalling pathway can mediate so many different cell responses. We have discovered and characterised a protein called Mae (modulator of the activity of Ets transcription factors) that may provide part of the answer to this problem (Baker et al., 2001). The mae gene is essential for normal Drosophila development and viability and it has a key role in EGF receptor signalling. Mae binds to the Drosophila Ets transcription factors Yan and Pointed-2. The binding of Mae allows the key regulatory serine residue of Yan to be phosphorylated by MAPkinase. There are other sites on Yan that can be directly phosphorylated by MAPkinase, but the key site can only be phosphorylated if Yan first binds Mae. Our results indicate that the MAPkinase signalling pathway can be coupled to different substrates in different cell types, so conferring specificity to MAPKinase signalling.

Genomics
The ongoing genome sequencing projects are generating a mass of information of relevance to gene evolution. One of the outstanding questions about genes is why some of them have introns. It is now generally agreed that the majority of introns were inserted into genes relatively late in evolution. However, there is an ongoing debate as to whether a significant minority of introns might be much older than this. We have recently discovered that cryptic splice sites were preferred targets for the insertion of some of the introns of the actin gene family during evolution. This result raises the possibility that even if ancestral genes were not split by introns, they may still have been able to use cryptic splice sites for the purpose of alternative splicing. We would like to establish whether introns were inserted into the cryptic splice sites of other genes and whether most introns were inserted into cryptic splice sites.

Key references

Dibb, N. J., Dilworth, S. M. & Mol, C (2004). Switching on kinases- the oncogenic activation of B-RAF and the PDGF receptor family. Nature Reviews Cancer 4, 718-727.

Sadusky, T., Newman, A.J. & Dibb, N. J. (2004). Exon junction sequences as cryptic splice sites: implications for intron origin. Current Biology 14, 505-509.

Baker, D.A., Mille-Baker, B., Wainwright, S.M., Ish-Horowicz, D. & Dibb, N.J (2001) Mae mediates MAP kinase phosphorylation of Ets transcription factors in Drosophila. Nature 411, 330-334.

Uden, M., Morley, G. & Dibb, N.J. (1999). Evidence that downregulation of the M-CSF receptor is not dependent upon receptor kinase activity. Oncogene 18, 3846-3851.