Exocytosis of mast cell secretory granules

Question

I've been doing a bit of reading about mast cell degranulation and have become thoroughly lost while trying to understand how the secretory granules are actually secreted.

I understand that there are many ways in which mast cells can be stimulated to degranulate, but I have been focusing on the example of IgE bound to FcεRI receptors. I believe I'm right in saying that antigen binding causes cross-linking of FcεRI receptors (since the antigen epitopes bind with the IgE paratopes and the IgE antibodies are themselves bound to the FcεRI receptors.) This recruits Lyn which phosphorylates ITAM motifs on the β and γ chains of the FcεRI receptors. This recruits Syk which is phosphorylated and activates LAT. This phosphorylates PLCγ which catalyzes the breakdown of PIP2 into IP3 and DAG.

All of this leads to the activation of PKC. I understand that PKC is (at least in part) responsible for the exoctyosis of SG but my question is by what means? Wikipedia, KEGG and this handy resource are all very cursory, whereas the academic papers which google turns up are way over my head.

So, if I might, what is it that PKC does and, in general, how are the secretory granules transferred from within the cell to the plasma membrane and out into the environment around the cell?

Answer 1

Mast cells contain a number of large, granular structures that are regarded as “secretory lysosomes,” yet they share several features in common with large dense-core vesicles, particularly with respect to the mechanisms of exocytosis. Upon activation, mast cells "degranulate" and exotcytose the contents of granules.

SNARE-dependent mechanism mediates exocytosis of neurotransmitters, hormones, and cytokines. Exocytosis of MCG is also SNARE-dependent. SNARE proteins are a family of evolutionarily conserved proteins that facilitate docking of apposing membranes and facilitate the fusion of membranes to accomplish exocytosis. The characteristic feature is a SNARE motif, a stretch of 60-70 amino acids, arranged in heptad repeat. This feature allows SNARE motifs of the individual SNARE proteins (from opposing membranes) to zipper together, forming a core complex of a four helix bundle. The major SNARE proteins involved in mast cell degranulation include VAMP-8 (integrated in mast cell membrane), SNAP-23 and syntaxin-4 (both plasma membrane bound)⁠. There are other layers of regulation of course. The SNARE accessory proteins synaptotagmin 2 (a calcium sensor), Munc13-1/2, Munc18-2 and complexin-2, exert negative or positive modulatory effects on mast cell granule exocytosis (Woska & Gillespie 2012).

EDIT: I don't know about PKC's involvement, but I can speculate that it's part of the signalling pathway after activation of the Fc receptor and upstream of events that initiate mobilization of intracellular calcium, which would then immediately trigger exocytosis. As for movement, if they're like dense-core vesicles, move along actin filaments using a myosin motor (Va or Vb, for example), and in association with Rab-3. This is speculation though, and I'd suggest looking into the literature for more detailed analyses as you are asking for a broad literature review.

Answer 2

You've already learned much more than I know about cellular cascades in mast cells, but I wanted to be sure you knew about this handy resource - a free, searchable "Molecular Biology of the Cell" textbook: http://www.ncbi.nlm.nih.gov/books/NBK21054/

For general explanations about cell biology, it's thorough and readable. For instance:

• Transport from the Trans Golgi Network to the Cell Exterior: Exocytosis
• B Cells and Antibodies

Carry on learning.

Answer 3

Here is a paper reviewing PKC action in exocytosis. Although it dates from several years ago it should be a good start for accessing the literature on this topic since it has been cited 90 times.

Morgan A. et al. (2005) Regulation of exocytosis by protein kinase C. Biochem Soc Trans. 33:1341-4.

PKC (protein kinase C) has been known for many years to modulate regulated exocytosis in a wide variety of cell types. In neurons and neuroendocrine cells, PKC regulates several different stages of the exocytotic process, suggesting that these multiple actions of PKC are mediated by phosphorylation of distinct protein targets. In recent years, a variety of exocytotic proteins have been identified as PKC substrates, the best characterized of which are SNAP-25 (25 kDa synaptosome-associated protein) and Munc18. In the present study, we review recent evidence suggesting that site-specific phosphorylation of SNAP-25 and Munc18 by PKC regulates distinct stages of exocytosis.

Answer 4

Having returned to this topic, I found this very handy journal article:

http://journal.frontiersin.org/Journal/10.3389/fimmu.2012.00130/full

Much of it, I'm afraid, is very difficult for me to follow, but I still managed to take a lot from it and I hope others may find it useful. I would summarise what I have found thus:

PKC and the increased levels of Ca²⁺ combine to direct the reorganising of the cytoskeleton through various intermediaries. The cytoskeleton is like the skeleton of the cell and it also provides the structure along which the secretory granules move to the cell membrane. The vesicle will bind with the cell wall and disperses its contents.

I have read suggestions that PKC may be involved in the process of the secretory granules fusing with the cell membrane. Wikipedia suggests that PKC is involved in severing the links between the surface of the secretory granules and the exoskeleton, allowing it to merge with the cell membrane.

Any corrections are obviously welcome.