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Vol. 301, Issue 3, 820-829, June 2002
College of Pharmacy and Upjohn Center for Clinical Pharmacology
(C.S., H.S., N.S.T., P.J.L., D.E.S.), and Departments of Neurosurgery
and Physiology (R.F.K.), University of Michigan, Ann Arbor, Michigan
Recent studies have established the functional and molecular presence
of a high-affinity peptide transporter, PEPT2, in whole tissue rat
choroid plexus. However, the precise membrane location and
directionality of PEPT2-mediated transport is uncertain at present. In
this study, we examined the transport kinetics of a model dipeptide,
glycylsarcosine (GlySar), along with the protein expression of PEPT2
using primary cell cultures of choroidal epithelium from neonatal rats.
GlySar accumulation and transepithelial transport were 3 to 4 times
higher when introduced from the apical as opposed to the basal side of
the monolayers. GlySar apical uptake was also stimulated by an inwardly
directed proton gradient. The uptake of GlySar was inhibited by
di/tripeptides, carnosine, and
-amino cephalosporins but was
unaffected by amino acids, cephalosporins lacking an
-amino group,
and organic anions and cations. The Michaelis constant
(Km) of GlySar was 59.6 µM for apical
uptake and 1.4 mM for basal uptake; this is consistent with the
high-affinity properties of PEPT2 at the apical membrane. Immunoblot
analyses and immunofluorescent confocal microscopy demonstrated the
presence of PEPT2, but not PEPT1, in rat choroid plexus epithelial
cells. Moreover, PEPT2 was present in the apical and subapical regions of the cell but was absent in the basolateral membrane. These findings
demonstrate, for the first time, that PEPT2 protein is present at the
apical membrane of choroidal epithelial cells and that it is
functionally active at this membrane surface. The results suggest that
PEPT2 may have a role in the efflux of peptides and/or mimetics from
cerebrospinal fluid to the blood.
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