Understanding how cell polarity regulators control endocytosis, vesicle acidification and cell signalling
We have discovered that the cell polarity regulator, Lgl, regulates the Notch signalling pathway by effecting vesicle acidification and g-secretase activity (Parsons et al., 2014; Portela et al., 2015). We found that in lgl mutant tissue, early and MVB endosomes accumulate and vesicle acidification occurs leading to increased γ−secretase activity, which depends upon low pH. Activated γ−secretase then leads to increased cleavage of Notch-ext (Next) to form the transcriptionally active Nicd form. Over-expression of Next in Drosophila tissues induces overgrowth of the thorax and perturbs eye development, which is dependent on g-secretase activity. Over-expression of Lgl suppresses the Next-induced overgrowth and eye phenotypes, but does not prevent Nicd-induced overgrowth, which bypasses the γ−secretase step.
This data shows that Lgl acts at the level of g-secretase activity to regulate Notch signalling. Using mass spec analysis (in collaboration with Dr Alexey Veraksa, University or Massachusetts, Boston, USA) we have identified proteins that might be involved in linking Lgl to these processes, and are currently investigating these using genetic, cell biological and biochemical approaches.