Pancreatic acinar cells are responsible for secreting the majority of digestive enzymes into the gut. They accomplish this by packaging enzyme precursors into compartments known as zymogen granules. These granules fuse with the plasma membrane, releasing their enzymatic contents to the outside of the cell in a mechanism known as exocytosis. As these zymogen granules are large enough to see with light microscopy, acinar cells are an ideal model cell to study the cellular process of exocytosis. The majority of my work has used two-photon fluorescence microscopy to investigate two different aspects of exocytosis - protons inside zymogen granules and their fate following exocytosis, and Soluble NSF Attachment Protein REceptor (SNARE) proteins.
Protons are present in a variety of exocytotic compartments where they are required to drive uptake of negatively-charged neurotransmitters or to act as a charge screen between proteins. There is some disagreement over the pH of the environment inside granules. As the majority of enzymes synthesised by the pancreas are pH-sensitive, the pH of the granule has potential consequences for premature enzyme activation and pancreatitis. Any protons present in the granule would also be released upon exocytosis and free to diffuse into the lumen of the duct where they could affect neighbouring acinar cells and down-stream duct cells. I have been able to show that zymogen granules are acidic, and to quantify the protons as they are released into the external environment. I show these releases of protons cause significant pH changes and can affect the calcium oscillations inside the acinar cell.
In the second part of my project, I used the two-photon method to track the process of compound exocytosis, and investigate the role of a particular SNARE protein, Vesicle-Associated Membrane Protein (VAMP)-8. SNARE proteins are involved in one of the final steps of exocytosis, acting like a lock and key between the granule and the plasma membrane bringing them in a close enough proximity to enable fusion to occur. In the case of VAMP-8, this protein appears to be responsible for fusion between granules themselves, a type of exocytosis known as compound exocytosis. In compound exocytosis, granules do not fuse directly with the plasma membrane, but instead with other granules that have already fused.
The final part of my project involved the development of a novel zebrafish (Danio rerio) line which expressed a pH-sensitive fluorescent protein specifically in the exocrine pancreas. Using this line, I was able to image the pancreas in vivo using two-photon and confocal microscopy.