C3G: a signal transduction protein integrating extracellular signals to control neural progenitor pr
The Ras signalling pathway regulates a number of important cellular functions, such as cell proliferation, migration and differentiation in health and disease. The guanine exchange factor, C3G, is a regulator of Ras family proteins. C3G predominantly activates Rap1 and does so when signalling occurs through growth factor receptors and extracellular matrix receptors, particularly integrin β1 (Figure 4).
Figure 4: Schematic diagram of C3G function based on data published by our group and others (left) and the protein domain structure of C3G (right). C3G has a carboxy-terminal catalytic domain for GTP exchange on Ras family members and internal proline-rich domains, which convey binding to the focal adhesion molecule p130cas and to the adapter molecule Crk.
The C3G gene is expressed widely at low levels and at higher levels in the developing nervous system. Studying C3G deficient mice we have shown that, amongst a number of functions in different tissues, this protein has a major role in regulating the growth of the precursor cell population, which forms the cerebral cortex (Figure 5). We observed an overproliferation of the C3G deficient cerebrocortical neuroepithelium. Labelling of cells in the cell cycle and additional labelling of cells in DNA synthesis phase showed that C3G deficient cells are retained in the cell cycle instead of exiting the cell cycle normally.
Figure 5: C3G is required to restrict neural precursor proliferation. Expression of a lacZ reporter gene in the C3G locus shows C3G gene activity in the neural tube (blue) at embryonic day 9.5 (A). In the C3G deficient cerebrocortical neuroepithelium (Ctx in C) compared to wild type controls (B) neural precursor cells overproliferate, such that the neuroepithelium forms folds (arrow in C). C3G deficient neuroepithelium (VZ in E) fails to generate differentiating neurons (labelled with the DNA synthesis marker BrdU (green) 24 h earlier and marked with asterix in the control (D). Increase in phosphorylated Akt/PKB (pAkt) and Erk2 (pErk2) in C3G deficient neural precursor cells (MT) as compared to control cells (WT) indicate overactivation (F, H) and increase in phosphoryated Gsk3β (pGsk3β) indicates inhibition (F, G) in response to growth factor stimulation.
In C3G deficient cultured neural precursor cells we observed a significant overactivation of the Ras signalling pathway components Erk1/2 and Akt/PKB with a concomitant inhibition of Gsk3β, as shown by increased levels of phosphorylation on these proteins (Figure 5). Our data show that C3G promotes cell cycle exit and that signalling through FGF2 and the EGF-receptor, besides activating proliferation-promoting pathways, concomitantly activate a proliferation-restricting mechanism, which crucially requires C3G. Thus C3G is a critical regulator of the balance between neural precursor proliferation and differentiation.