Role of Grb2 in growth and differentiation of embryonic stem cells
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Embryonic stem (ES) cells are derived from the inner cell mass of the blastocyst stage embryo. They exhibit unlimited proliferation in culture and have the ability to differentiate into all three germ layers of the developing organism, a property defined as pluripotency. Previously it was reported that growth factor-bound protein 2 (Grb2) is required for differentiation of the epiblast, the embryonic tissue that harbours the pluripotent founder cells of the foetus. GRB2 is an adapter protein involved in the activation of the mitogen-activated protein kinase (MAPK) pathway in response to extracellular signals. It has also been implicated in the activation of the phosphoinositol-3-kinase (PI3K) pathway in response to fibroblast growth factor (FGF) signaling. The work presented in this thesis examines the role of Grb2 in ES cells and describes previously unreported contributions of this adaptor protein in regulating ES cell growth and differentiation. It has been previously been shown by others that Grb2 deficient (Grb2-/-) cells grow relatively normally in ES growth medium containing serum. However, in serum free conditions (N2B27 medium) in this project, proliferation of Grb2-/- cells is reduced compared with wild type and “restored” Grb2-/- cells stably expressing a Grb2 cDNA mini gene. Under serum free conditions, Grb2-/- cells grow in tight, refractive colonies. Nanog expression was uniformly upregulated, in contrast to the heterogeneous pattern reported in serum-based medium. Colony expansion on the substratum appears to be compromised, although there is no apparent defect in the initial attachment of Grb2-/- cells. Cell cycle analysis indicates that the slower growth of Grb2-/- cells in serum free medium could be due to lengthening of the G1 phase of the ES cell cycle. In an attempt to identify the signalling deficiency responsible for the growth defect of Grb2-/- cells, MAPK activation was restored by two methods, PMA a ligand that bypasses the requirement for Grb2, and Raf-ER, a conditionally regulated component of the MAPK pathway that acts downstream of Grb2 in the MAPK pathway. Although both approaches increased MAPK signalling they were unable to rescue the growth defect. This suggests that MAPK is not required or alone is not sufficient. Inhibition of Glycogen synthase kinase 3 β (GSK3 β ) is known to augment growth of ES cells under MAPK inhibition. Surprisingly, GSK3 β inhibition did not enhance Grb2-/- cell growth. Under GSK3 β inhibition, Grb2-/- ES cells fail to thrive. It is hypothesised that under these conditions cells undergo hyper-self-renewal at the cost of growth. Grb2-/- ES cells are reported to exhibit limited differentiation potential. To examine the potency of Grb2-/- cells, these cells were subjected to embryoid body (EB) and monolayer differentiation. Analysis of EBs showed a loss of Gata4, Gata6 and endoderm marker gene expression. However, markers of ectoderm (Sox1, Pax6, MAP2), the late epiblast/nascent mesoderm (Brachyury) and markers associated with gastrulation (Twist and Snail) were expressed. Outgrowths of morphologically and immunohistochemically identifiable neuronal cells confirmed differentiation of ectodermal cell types, indicating Grb2 is not required for neuronal differentiation. However, beating cardiomyocytes could not be identified in Grb2-/- EBs, though readily found in restored Grb2-/- cells expressing the Grb2 cDNA. This suggests that there is an essential role for Grb2 in the mesoderm/cardiomyocyte differentiation pathway. This may be due to a defect in GATA factor expression since these factors are essential for cardiogenesis. In serum-free monolayer differentiation, Grb2-/- cells formed neuronal cells. Additional inhibition of the MAPK pathway using a small chemical inhibitor failed to prevent this differentiation. However, biochemical analysis of the cells indicates that this occurs when ERK activation is very low, indicating differentiation was not MAPK-independent. Grb2 mediates FGF-MAPK induced exit from the naïve ground state. These data suggest a Grb2-independent pathway can also facilitate this transition. Grb2 is dispensable for differentiation in to some lineages. However as differentiation of Grb2-/- ES cells is restricted, this indicates Grb2 is required for true pluripotency.