In mammals, embryo development takes place inside the maternal uterus and the foetus is permanently linked to the mother though the placenta, ensuring a source of nutrients during the whole pregnancy. The early phases of development take place before the embryo attaches or implants into the uterus, during the pre-implantation period. The correct advancement of this period is crucial for the embryo to be able to successfully implant and progress though pregnancy. Indeed, implantation failure remains the limiting step in assisted reproduction as only around 25% of human embryos implant and develop to term. Therefore, understanding how embryo development proceeds prior to implantation is a fundamental question not only in developmental biology but also in the clinic.
Just before implantation, the mammalian embryo is composed of cells that will either form the placenta or that have the ability to turn into all cell types found in the embryo proper. The later are called embryonic stem cells (ESCs) and as the embryo develops they gradually form more specialized cell types that are no longer able to contribute to all body tissues. This process is called cell differentiation. The ability of embryo cells to form any cell type is maintained for a short period of time after implantation, however the characteristics of ESCs change as the embryo implants so they become ready to for differentiation. This change is necessary for development to proceed and what determines it remains incompletely understood. ESCs can be isolated from embryos and cultured in the laboratory under specific conditions. By modifying these conditions we can induce their differentiation to form specific tissues, such as liver, lung or neurons, mimicking what occurs in the embryo. This makes them a very powerful tool in regenerative medicine. However, to be able to use ESCs in a safe and efficient manner a complete understanding of how their differentiation occurs during normal embryo development is necessary.
All processes taking place at early developmental stages are very tightly controlled by instructions encoded in the genes. Each gene codes for a specific protein using an intermediate product called messenger RNA (mRNA), and it is the final protein repertoire of a particular cell which determines its identity and how it behaves within the embryo (for instance if it will detach from its neighbour cells or change shape). Each gene is divided into fragments called exons, and some of the exons can be used or not to form the mRNA that will be translated into the protein. Thus, depending on whether a particular exon is used the resulting protein will be slightly different and can exert different functions within the cell. To fully understand how cell identity and behaviour is regulated we have to take into account which particular fragments (exons) are being used to form the proteins in each cell type.
The main objective of this project was to use data from pre-implantation embryos as well as ESCs cultured in the laboratory, to generate a database that allow us to identify proteins that despite being encoded by the same gene differ in the exons used for their production. Using this information, the second objective was to experimentally test to what extend the usage of a particular exon to form a protein has an impact on the final protein function during development and in stem cells. We have successfully completed both objectives and the results obtained will be ready for publication in the near future.
