Role of ions in stem cells signalling

Resumen

The cell signalling process allows cells to develop a coordinated action during embryogenesis and assimilate coherently the signals received through the environment. Some of the most currently used signalling molecules in clinics and research are growth factors and cytokines. However, there is a growing trend in the use of other types of molecules, such as metal ions. Some ions such as calcium and zinc are able to carry out secondary messenger functions, transmitting signals in cascade. Others ions, such as lithium, are capable to inactivate protein kinases altering signalling pathways. During the development of this doctoral thesis, we investigated the effect of zinc on mouse muscle cells (myoblasts), the role of zinc in embryonic stem cells (ESCs) self-renewal, and the role of lithium in the differentiation of ESCs. In the first chapter, we showed that zinc is able to increase the differentiation of myoblasts. The analysis of intracellular zinc indicated that the differentiated myoblasts were capable to harbour higher concentration of intracellular zinc than undifferentiated ones. Addition of high concentration of extracellular zinc increased protein kinase Akt phosphorylation and activation. Akt activity is critical for myoblasts differentiation and has been well studied by other authors. Our results indicated that zinc transporter Zip7 was critical for zinc-mediated cell differentiation. It was prior demonstrated that the activation of this transporter by extracellular zinc increased the phosphorylation of Akt. The inhibition of Zip7 by interfering RNA resulted in a lower phosphorylation of Akt and reduced differentiation of the myoblasts exposed to extracellular zinc. These results demonstrated that high concentration of extracellular zinc enhances the differentiation of myoblasts through activation of Akt mediated by Zip7. In the second chapter, we have analysed the effect of zinc on ESCs. Leukaemia inhibitory factor (LIF) was used as pluripotency sustaining factor. We observed that extracellular supplementation of 100 ¿M zinc produced an immediate increase of the intracellular concentration, which resulted in the activation of Akt mediated by Zip7 transporter. ESCs treated with high concentrations of zinc maintained self-renewal. The role of Akt on ESCs self-renewal has been well established in the literature. To demonstrate that this effect is associated with the activation of Akt mediated by Zip7, we inhibited Akt phosphorylation and silenced the expression of Zip7. Both approaches resulted in an increase in the differentiation levels of the ESCs treated with zinc. We further demonstrated that ESCs treated with zinc during 30 days were able to retain their pluripotency, while the control condition cultured 30 days without zinc presented evident traits of spontaneous cellular differentiation. Finally, the combination of LIF with zinc produced a synergistic-like increase in the effect of LIF on ESCs self-renewal. Finally, we addressed the effect of lithium on the differentiation of ESCs. Lithium is an inhibitor of glycogen synthase kinase 3 beta (GSK3ß). In terms of ESCs, GSK3ß activates differentiation mechanisms. Our results indicated that high concentration of lithium (10 mM) was able to phosphorylate and strongly inhibit the activity of GSK3ß. However, instead of maintaining pluripotency, ESCs differentiated into the mesoderm lineage after 3 days of culture. After a total of 6 days, ESCs treated with 10 mM lithium showed haemogenic endothelium characteristics, expressing CD31, Sca-1 and CD31/Sca-1 positive cells. The phosphorylation of GSK3ß resulted in the activation of the ß-catenin protein, whose transcriptional activity is necessary for embryonic hematogenesis. The ability of endothelial cells with hemogenic potential obtained from lithium-treated ESCs to derive into hematopoietic stem cells was confirmed after maturation of these cells, resulting in rounded cell aggregates positive for Sox17.