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Ion pumps
in polarized epithelia: Sorting and function. The
cell surface membranes of epithelial cells and neurons are divided into distinct
domains, characterized by unique protein compositions and functional properties.
We are interested in the molecular signals and cellular machinery involved in
generating this polarity. We have recently found that members of the tetraspan
family of membrane proteins interact with a wide variety of ion transport systems.
Through direct associations, members of this family appear to play very significant
roles in determining the subcellular distributions and dynamic properties of a
number of different transport systems. Cystic fibrosis is a genetic disease that
is attributable to the mis-trafficking of a membrane protein. Most cases result
from mutations in the CFTR chloride channel that cause this protein to be retained
in the endoplasmic reticulum. We are developing a new small molecule approach
to liberating mutant CFTR protein from the ER such that it can achieve at least
partial function at the cell surface. We have identified a compound that is non-toxic
and appears to be effective in mouse models of cystic fibrosis. Our studies suggest
that this compound exerts its effects by altering the interaction of CFTR with
the chaperone proteins that retain it within the cell. 
Figure
caption: Polarized pig kidney epithelial cells
have been transfected with a cDNA encoding a chimeric ion pump composed of portions
of the Na,K-ATPase and the gastric H,K-ATPase. The chimera and the endogenous
population of Na,K-ATPase were localized by immunfluorescence confocal microscopy.
The distribution of the chimera (red staining) is restricted to the apical membranes
of these epithelial cells (shown in cross-section), whereas the Na,K-ATPase (green
staining) is limited to the basolateral surfaces. Despite ~85% amino acid sequence
identity, therefore, the chimera and the Na,K-ATPase are differentially sorted
by these cells. Recent publications: Grimm,
D.H., Y. Cai, V. Chauvet, V. Rajendran, R. Zeltner, L. Geng, E.D. Avner, W. Sweeney,
S. Somlo, and M.J. Caplan. Polycystin-1
distribution is modulated by polycystin-2 expression in mammalian cells. J.
Biol. Chem., 278: 36786-36793, 2003.
Duffield, A., E.J. Kamsteeg, A.N. Brown, P. Pagel and M.J. Caplan. The
tetraspsanin CD63 enhances the internalization of the H,K-ATPase ²-subunit.
Proc. Nat. Acad. Sci. (USA) 100: 15560-15565, 2003.
Egan, M.E., M. Pearson, S.A. Weiner, V. Rajendran, D. Rubin, J. Gloeckner-Pagel,
S. Canny, K. Du, G. Lukacs and M.J. Caplan. Curcumin,
a major constitutent of turmeric, corrects cystic fibrosis defects. Science,
304: 600-602, 2004. Egan,
M.E., J. Gloeckner-Pagel, C.A. Ambrose, P.A. Cahill, L. Papoe, N. Balamuth, E.
Cho, S. Canny, C.A. Wagner, J. Geibel and M.J. Caplan. Calcium-pump
inhibitors induce functional surface expression of ?F508-CFTR protein in cystic
fibrosis epithelial cells. Nature Medicine 8:485-492, 2002. Caplan
MJ. Ion
pump sorting in polarized renal epithelial cells. Kidney Int. 2001 Aug;60(2):427-30.
Dunbar LA, Caplan MJ. Ion
pumps in polarized cells: sorting and regulation of the Na+,K+- and H+,K+-atpases.
J Biol Chem. 2001 Aug 10;276(32):29617-20.
McCarthy JB, Lim ST, Elkind
NB, Trimmer JS, Duvoisin RM, Rodriguez-Boulan E, Caplan MJ. The
C-terminal tail of the metabotropic glutamate receptor subtype 7 is necessary
but not sufficient for cell surface delivery and polarized targeting in neurons
and epithelia. J Biol Chem. 2001 Mar 23;276(12):9133-40.
Reinhardt
J, Grishin AV, Oberleithner H, Caplan MJ. Differential
localization of human nongastric H+-K+-ATPase ATP1AL1 in polarized renal epithelial
cells. Am J Physiol Renal Physiol. 2000 Sep;279(3):F417-25. Dunbar
LA, Caplan MJ. The
cell biology of ion pumps: sorting and regulation. Eur J Cell Biol. 2000 Aug;79(8):557-63.
michael.caplan@yale.edu 
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