A trimeric metazoan Rab7 GEF complex is crucial for endocytosis and scavenger function.

Endosome biogenesis in eukaryotic cells is critical for nutrient uptake and plasma membrane integrity. Early endosomes initially contain Rab5, which is replaced by Rab7 on late endosomes prior to their fusion with lysosomes. Recruitment of Rab7 to endosomes requires the Mon1-Ccz1 guanine-nucleotide-exchange factor (GEF). Here, we show that full function of the Drosophila Mon1-Ccz1 complex requires a third stoichiometric subunit, termed Bulli (encoded by CG8270). Bulli localises to Rab7-positive endosomes, in agreement with its function in the GEF complex. Using Drosophila nephrocytes as a model system, we observe that absence of Bulli results in (i) reduced endocytosis, (ii) Rab5 accumulation within non-acidified enlarged endosomes, (iii) defective Rab7 localisation and (iv) impaired endosomal maturation. Moreover, longevity of animals lacking bulli is affected. Both the Mon1-Ccz1 dimer and a Bulli-containing trimer display Rab7 GEF activity. In summary, this suggests a key role for Bulli in the Rab5 to Rab7 transition during endosomal maturation rather than a direct influence on the GEF activity of Mon1-Ccz1.

Drosophila pericardial nephrocyte ultrastructure changes during ageing.

Here we show that a labyrinth channel compartment and slit diaphragms, which are the histological structures enabling insect nephrocytes ultrafiltration, are established during embryogenesis first by the garland nephrocytes (GCNs). The later pericardial nephrocytes, which represent the majority of functional nephrocytes in larvae and adults, lack these characteristic features at the embryonic stage. During larval development, a subpopulation of the pericardial cells survives and matures into functional nephrocytes (PCNs) displaying a fully differentiated slit diaphragm and a labyrinth channel compartment. Likely the embryonic pericardial cells have primary functions other than ultrafiltration (e.g. in production and secretion of ECM constituents). We also show, for the first time, that PCNs in the adult fly undergo dramatic histological degeneration upon ageing. The slit diaphragms disappear, the labyrinth channel system degenerates and the lysosomal compartment becomes highly enriched with electron-dense material. When using nephrocytes as a model for genetic screening purposes or to investigate the specific role of genes involved in endocytosis, histological changes occurring upon ageing need to be taken into account when interpreting structural data.