Characterization of two sorting nexins : sorting nexin-11 and sorting nexin-30
The intracellular milieu is housed by countless numbers of intracellular molecules travelling by diffusion or along transient paths, which are regulated by specific trafficking proteins.  Among these traffic regulators are the sorting nexins that determine the fate of internalized proteins by directing toward a defined path, which can lead to either degradation or recycling.  To date, 33 sorting nexins (Snx1-33) have been identified, which all share a common characteristic, the presence of a PX (PHOX-homology) domain.  The PX domain is a phosphatidylinositol phosphate (PIP)-binding domain, which helps bring sorting nexins to PIP-enriched areas of lipid membranes.  For example, during receptor endocytosis, the surrounding membrane becomes transiently occupied by PI(4,5)P2.  PI(4,5)P2 is then recognized by Snx9, which contributes to the progression of endocytosis by linking the receptor complex to the actin cytoskeleton. The research presented in this thesis is the first to investigate the functions for two sorting nexins, Snx11 and Snx30, during embryogenesis and endosomal protein trafficking.  Results obtained from knockdown experiments in the frog (Xenopus laevis) were combined with data from cell culture and biomolecular experiments to propose a function for these two proteins.  The data presented here suggest that Snx11 is involved in somitogenesis, and regulates actin-dependent and -independent processes.  Snx11 could serve as a scaffolding protein, linking the extra-cellular matrix to the actin cytoskeleton and could also function in actin-independent receptor recycling.  On the other hand, Snx30 is implicated in early cardiogenesis and promotes the commitment of a population of mesoderm cells to the cardiac lineage.  It does so through the inhibition of Wnt/β-catenin signaling but the underlying mechanism is still unclear.  Expression of Snx30 in Xenopus coincides with this critical period of cardiac specification and knockdown of Snx30 results in cardiac malformations as well as other defects in mesoderm- and endoderm-derived tissue.  In addition, data from both Xenopus and HEK293T cell culture show that knockdown of Snx30 increases Wnt/β-catenin signaling. This work provides the basis for future studies on Snx11 and Snx30.  Interestingly, Snx11 and Snx30 seem to act as fine-tuners of signaling pathways.  These proteins could potentially become interesting therapeutic targets due to their specificity and relatively subtle impact when knocked-down.  As such, interference with their function could be useful to re-balance a cellular disequilibrium while minimizing side effects.
Sorting nexin Wnt/β-catenin Actin Sorting nexin Protein trafficking Embryogenesis Cardiogenesis Somitogenesis Wnt/β-catenin