Small proteins highlight problems in cellular activity
Rho-family guanosine triphosphatases (GTPases) are small proteins that regulate numerous aspects of intracellular behaviour such as cell proliferation, cell death and gene expression. Cdc42, Rac1 and RhoA are three such proteins whose proper functioning is necessary to avoid disruptions in normal human development and the onset of disease. A key group of Rho-family GTPases are the Rho guanine nucleotide exchange factor (GEF) proteins, a large number of which are encoded by the human genome and play a role in stimulating Rho GTPase activation. The 'The role of RhoGEFs in epithelial polarity using a three-dimensional model' (RhoGEFs in epithelia) project has set out to perform an unbiased large-scale functional analysis of all the Rho GEF proteins. The end goal is to identify Rho-family GEF proteins involved in the cancerous growth of epithelial cells, as their disrupted regulation can lead to malignant transformation. Rho GEF functioning is being studied in an MDCK (Madin-Darby Canine Kidney Cells, a virus tissue culture) 3D system. Having generated a library of short hairpin RNAs (shRNAs) targeting Rho-family GEFs, the project partners produced viruses, collecting and filtering them for infection of MDCK cells. The infected cells were examined and plated to allow for cyst formation, and then fixed and stained for further study. The results of this reagent treatment approach were not satisfactory and researchers opted to take a candidate screening approach to knockdown or reduce the gene expression of specific GEFs that are expressed in MDCK cells. With this new system, Cdc42 specific GEF Intersectin2 (ITSN2) has been shown to be a candidate for regulating the Cdc42 activity that controls the formation of lumen. This is the interior of a tubular structure such as urinary collecting tubes or kidney ducts. Investigation of ITSN2 knockdown cells revealed a reduction in levels of Cdc42-GTP, explaining the aberrant phenotype in development of cancerous cysts. ITSN2 has also been shown to localise at centrosomes: this is where microtubles are produced, and an excess volume of these are often found in cancer cells. Other study results have so far shown that ITSN2 and Cdc42 are necessary for normal cell division processes.
