Dense-core granule formation and function
Some years ago, we showed that DCG 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 DCG compartment formation. When males mate, they rapidly secrete the contents of approximately four of the ten secretory compartments in secondary cells, stimulating more BMP signalling and promoting biogenesis of new dense cores, so that the accessory gland lumen can be rapidly replenished.
More recently, we have characterised many different trafficking events involved in making DCG compartments, following each step using live-cell imaging. We have also shown that the protein aggregation events driving DCG formation are membrane-dependent and require a key input from Rab11-positive recycling endosomes. This mechanism has since been confirmed in human cells, where there is also increasing evidence that DCG compartments contain ILVs, as we have found for secondary cells. 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.
In a subsequent study, we found that transmembrane Amyloid Precursor Protein (APP) is a key player in membrane-primed protein aggregation events that drive DCG biogenesis. This role for Drosophila APP can be substituted by human APP. Critically, these APP-dependent DCG aggregation events are disrupted by Aβ-peptides, the aberrant cleavage product of APP, which induces neurodegeneration in AD; unpublished data suggest that these events also malfunction when cytoskeletal tau is expressed. Aβ-containing extracellular amyloid plaques and tau-containing intracellular neurofibrillary tangles are the two hallmarks of AD, but it remains unclear how they are linked and whether that link leads to the cellular defects that drive degeneration. Our findings suggest that regulated secretion is a missing piece in the puzzle and when it malfunctions, the cell’s endolysosomal system is disrupted and this defect propagates to other cells, two key features of early-stage AD. The mechanisms we have identified help to account for several observations made in AD models, which until now, have been difficult to explain, eg. how can regulated secretion, endosomes and exosomes all be involved in Aβ-induced AD pathology – Answer: they all meet in regulated secretory compartments. We are now identifying genetic manipulations that suppress different pathological phenotypes produced by AD-relevant treatments and starting to test whether they block neurodegeneration in fly models of AD.
Biological relevance
Our work has linked together endosomal trafficking, regulated secretion, protein aggregation events in DCG biogenesis and the formation of Rab11-exosomes. This brings together four basic biological processes that were not previously connected, but play critical roles in cell biology. The secondary cell system continues to allow us to unravel the biological significance of these associations and the mechanisms involved.
Disease relevance
Some of the proteins that control DCG formation in secondary cells have already been implicated in AD, other neurodegenerative diseases and/or endocrinological disorders. By identifying genetic manipulations that suppress secretory phenotypes in secondary cells, which may provide novel leads in devising new treatments in humans.
Image Description: Dense-core granule compartments in secondary cells
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.