Signal Transduction
C3G - an essential regulator of cerebral cortex development
AK Voss, MP Dixon, BN Sheikh, C Collin, T Thomas in collaboration with SS Tan, JM Britto (Howard Florey Institute) Pub ref: 144
The six-layered cerebral cortex is unique to mammals. The cortex develops through a period of neuron production and active neuronal migration, at the end of which cortical neurons are located in precise positions within the six layers of the cortex. Defects in cortical neuron migration lead to mental disorders such as lissencephaly with severe mental disability and epilepsy. Interaction of neurons with their extracellular environment regulate cortical neuron migration through cell surface receptors. However, it is unclear how the signals from extracellular proteins are transduced intracellularly. We have discovered that mouse embryos lacking the Ras family guanine nucleotide exchange factor, C3G (Rapgef1, Grf2), exhibit a cortical neuron migration defect resulting in a failure to split the preplate into the cortical layer 1 and subplate and a failure to form a cortical plate. C3G-deficient cortical neurons fail to migrate in vivo and in vitro. Instead, they arrest in a multipolar state and accumulate below the unsplit preplate. The basement membrane is also disrupted and radial glial processes are disorganised and lack attachment in C3G-deficient brains. C3G is activated in cortical neurons in response to signalling from the extracellular matrix protein, reelin, which, in turn, leads to activation of the small GTPase Rap1. In C3G-deficient cells, Rap1 GTP loading in response to reelin stimulation is reduced. In conclusion, the Ras family regulator C3G is essential for two aspects of cortex development, namely radial glial attachment and neuronal migration.
C3G deficient cortical neurons fail to migrate in vitro and in vivo. (A, B) E10.5 dorsal telencephalic explants of stained for the neuronal marker type III β-tubulin (red) and counterstained with bisbenzimide (blue). (C, D) E13.5 developing cerebral cortex with layer I (I) neurons and subplate (SP) neurons stained for calretinin (brown) and (E, F) the pial basement membrane laminin (red) and the radial glial cell marker RC2 (green). Wild type (A, C, E) and C3G deficient (B, D, F). Note migrating neurons (arrows A, but not in B), preplate cells split into layer I and subplate (arrows in C, but unsplit in D) and ventricular to pial orientation of radial glial processes (arrows in E, but not in F), as well as disruption of the laminin deposits (F, but not E). CP, cortical plate.
Biochemical and molecular characterisation of a novel kinase-like protein
IJ Majewski, JM Murphy, GM Tannahill, J Collinge, JG Zhang, DJ Hilton in collaboration with M Bahlo, M Wakefield (Bioinformatics Division), E Hatchell, D Stockwell, AA Hilton, WS Alexander (Cancer and Haematology Division)
An ENU-mutagenesis screen was performed in mice to identify novel genes that regulate platelet production. Using this approach, we identified a kinase-like protein, termed PLT15, that regulates important signal pathways in megakaryocytes and stem cells. Mutations in this gene increase platelet production yet cause a significant impairment in stem cell function. Loss-of-function studies have demonstrated that PLT15 influences signalling through the JAK-STAT pathway and we are currently investigating its involvement in other key pathways. More broadly, we are applying molecular, biochemical and bioinformatic techniques to better characterise the role of this gene in haemopoiesis.
Dr Benjamin Kile and Dr Kylie Mason explain the mechanism of platelet life span to the then Shadow Minister for Industry, Innovation, Science and Research, Senator Kim Carr