Dense-core granule formation and function
We have shown that dense-core granule biogenesis and release in secondary cells is controlled by Bone Morphogenetic Protein (BMP) signalling. The BMP ligand, DPP, is loaded into the cores and activates BMP signalling when released, increasing the rate of new dense-core granule compartment formation. When males mate, they rapidly secrete approximately four dense cores, stimulating more BMP signalling and promoting biogenesis of new dense cores, so that the accessory gland lumen can be rapidly replenished. We have also characterised some of the subcellular trafficking steps involved in making these compartments.
More recently, we have shown that if in some genetic manipulations where Rab11-exosomes fail to form normally, dense cores are either very small or are not made. Our hypothesis, which we are currently testing, is that these vesicles act as primers for the aggregation of proteins that form the dense core, an idea consistent with several studies that suggest protein aggregation and amyloidogenesis are catalysed by intraluminal vesicles in endosomal compartments. In a collaborative study with Prof Matthew Wood and Prof Deborah Goberdhan, we have shown that the glycolytic enzyme GAPDH, which is known to be trafficked to the external surface of some exosomes, is involved in clustering exosomes in secondary cells, a process that also appears to be required for normal dense core biogenesis.
Secondary cells produce dense-core granules, which are so large that when they are fluorescently labelled, they can be visualised with a simple fluorescence stereomicroscope. This is allowing us to develop simple genetic screens to identify other regulators of dense-core granule formation that might further explain their biogenesis.
Our work has linked together endosomal trafficking, protein aggregation events in dense-core granule biogenesis and the formation of Rab11-exosomes. This brings together three basic biological processes that play critical roles in health and disease, and the secondary cell system continues to allow us to unravel the biological significance of these associations and the mechanisms involved.
Some of the proteins that control dense-core granule formation in secondary cells have already been implicated in endocrinological diseases and amyloidogenic degenerative disease. We are developing models to screen for genetic manipulations that suppress disease phenotypes that we induce in secondary cells, which may provide novel leads in devising new treatments in humans.
Image Description: Dense-core granule compartments in secondary cells
Confocal image of living secondary cell expressing a GFP construct, which marks membranes (including the plasma membrane and compartment limiting membranes) and dense cores, and stained with LysoTracker Red to label acidic compartments. One dense-core granule compartment is marked by an arrow. Scale bar = 10 µm. From Redhai et al., 2016.