Mucin Exocytosis
The glycoprotein mucin forms a gel layer that lines the respiratory airways, the lumen of the stomach and intestines, and other epithelial surfaces. Mucin is released from specialized cells known as goblet cells through a process of vesicle exocytosis. In this process, small vesicles densely packed with mucin fuse with the cell's membrane and open into the extracellular space. The mucin expands from the vesicle into this space at a very rapid rate. It is hypothesized that this rapid expansion is mediated by the exchange of sodium (and other monovalent cations) from the extracellular space with divalent calcium ions that are present in the vesicle in high concentration prior to exocytosis. The idea is that calcium, by cross-shielding negative charges on nearby mucin molecules, allows these molecules to be tightly packed in the vesicle, and that when calcium is exchanged with sodium, the cross-shielding is greatly diminished and the gel expands rapidly.
We have formulated novel mathematical models of gel swelling that properly account for the chemical potentials of both the network and solvent phases, that incorporate the effect of cross-shielding by divalent ions. The net effect of this is that we are able to follow the swelling kinetics of a vesicle following a change in the bath chemical concentrations. We are also able to study the effect of changes in affinities for ion species on the kinetics and equilibrium state of the mucin gel.
Publications on Mucin exocytosis
J. P. Keener, S. Sircar and A. L. Fogelson, Kinetics of swelling gels, SIAM Journal of Applied Mathematics, 71 (2011), 854—875.
James P. Keener, Sarthok Sircar, and Aaron L. Fogelson Influence of the standard free energy on swelling kinetics of gels Physical Review E 83 (2011).
S. Sircar, J. P. Keener, and A. L. Fogelson, The Effect of Divalent vs. Monovalent ions on the Swelling of Mucin-like Polyelectrolyte Gels, Journal of Chemical Physics, 138 (2013).
