Another reason why the increase in colocalisation between the CIMPR and TGN46 is relatively modest is possibly due to the increase in CIMPR levels after TBC1D5 knockdown that is detectable both by increased CIMPR fluorescence staining and also by western blotting. This increase in CIMPR levels may actually result in elevated endosomal CIMPR due to saturation of the retrieval machinery (as would be observed if the CIMPR were simply overexpressed), thereby masking a more pronounced gain in TGN-localised CIMPR. Consistent with a gain of function for retromer after TBC1D5 knockdown is the observation that processing of APP to Aβ is reduced. This is, in some ways, very similar to the report that a pharmacological chaperone that stabilises the retromer CSC and enhances levels of membrane-associated VPS35 and VPS26 can also reduce APP processing to Aβ (Mecozzi et al., 2014) and confirms the importance of the retromer CSC in regulating the processing of APP.
