Unconventional integrin secretion: Its role in epithelium integrity and tumour formation.

The cancers of epithelial tissues, the carcinomas, are characterized by the formation of tumours that occurs by over-proliferation of epithelial cells after the epithelium loses its integrity. Therefore, the molecular principles underlying the establishment and maintenance of the epithelial integrity need to be understood at the molecular level to prevent its loss.
Epithelial integrity depends on cell-cell junctions (tight/septate junctions and the adherens junctions), but also on adhesion of the cells to the extracellular matrix (ECM). This is mediated by the extracellular domain of integrins that are alpha/beta integral plasma membrane heterodimers. On the cytoplasmic side, integrins interact with cytoskeletal and signalling proteins to form so-called focal adhesion sites. It is here that signalling is transduced from within and from outside the cell. By integrating signals from within and outside the cell, integrins are involved in a large variety of cellular processes that, when uncontrolled, can result in pathological situations, such as cancer. During organogenesis, epithelial sheets need to modulate their adhesion. Conversely, when epithelial remodelling is achieved, new adhesion sites are generated. In this respect, the transport of newly synthesized integrins to the plasma membrane is crucial.
As transmembrane proteins, integrins use the typical secretory pathway from the Endoplasmic Reticulum (ER) to the Golgi apparatus to the plasma membrane. However, what triggers and regulates the deposition of integrin in relation to organogenesis and tissue morphogenesis, and how their plasma membrane deposition occurs, remains largely unexplored. However, this needs to be tightly regulated, as their inappropriate deposition may provide tissues aberrant adhesion properties, thus inducing uncontrolled signalling pathways, which can be involved in the onset of malignant tumour formation. In particular metastasis is a process that is believed to require activation of the transcriptional programme that alter cell-cell and cell-ECM adhesion and stimulate survival, proliferation and migration. Strikingly, integrins can be upregulated in carcinomas. In this context, we have recently discovered that during epithelium remodelling in Drosophila, the alpha subunits of integrins are deposited to the basal plasma membrane using an unconventional secretion route that is built de novo. This new route mediates their direct transport from the ER to the plasma membrane without passing through the Golgi, a very unusual feature for transmembrane proteins. Crucially, this route depends on the re-localisation of a small subset of Golgi resident proteins, including dGRASP, to the plasma membrane, through a process involving the up-regulation and targeting of their mRNAs to the same location. Accordingly, in a dgrasp mutant, several epithelia are severely disorganised because the alpha integrin subunits are retained intracellularly. Strinkingly, the same unconventional pathway seems to be also built at the onset of epithelial tumour formation.
The main aim of this proposal is to understand how the unconventional integrin secretion pathway is generated and what is its role during epithelium remodelling in physiological conditions AND at the onset of tumour formation. This proposal combines areas of cell biology, signalling, and developmental biology that will be integrated to shed new light on epithelium integrity and re-arrrangement, including events taking place at the onset of tumour formation, both in Drosophila and in mammals. It comprises 3 independent parts that will use a combination of Drosophila genetics, microscopy (immunofluorescence and fluorescence in situ hybridisation), biochemistry and mammalian cell culture.
1. How does epithelial remodelling induce the de novo biogenesis of the dGRASP dependent unconventional integrin secretion pathway? Identification of the signalling pathways required for the biogenesis of the unconventional sec