GP Ibα expression, megakaryocyte proliferation, and differentiation. (A) Results suggest a shift in the PI3K/Akt axis of TPO stimulation. A hypothesis is presented whereby the cytoplasmic tail of GP Ibα sequesters signaling proteins, such as 14-3-3ξ and PI3K, and down-regulates the Akt-dependent pathway. In the truncated GP Ibα variant, hTg^Y605X, a shift in the PI3K/Akt axis results in increased Akt activation and downstream consequences of increased endomitosis and accumulation of a greater percentage of high-ploidy megakaryocytes. (B) A schematic model is presented to illustrate how an increase in Akt phosphorylation results in a megakaryocyte population with increased ploidy. The precursor cell proliferation (2n → 4n → 2n) is more active in the hTg^WT model (Table 1) and is depicted by larger arrows compared with proliferation in the hTg^Y605X model. However, Akt phosphorylation in response to TPO is increased in the hTg^Y605X model (Figure 6A) and corresponds to an increase in the accumulation of cells of higher ploidy (Figures 3B, 4). Thus, repeated endomitotic divisions in the hTg^Y605X marrow are depicted by larger arrows compared with a similar process in the hTg^WT model. The increased proliferative potential of hTg^WT cells and the potential for hTg^Y605X cells to generate megakaryocytes of a higher ploidy class provide mechanisms whereby both models produce similar levels of platelet. Results highlight the independence of ploidy and platelet release in response to stimuli. GP Ibα expression increases during megakaryocyte maturation and, as such, becomes a critical timing factor for normal megakaryocyte maturation.