MAMMALIAN OLIGOPEPTIDE TRANSPORTERS - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 112

dipeptide derivative probe (D-Ala-Lys-AMCA) in the inner cortex and outer stripe in
rat kidney. 47 Interestingly, the uptake of the substrate was competitively inhibited by
cefadroxil and Gly-Gln, indicative of PepT2-mediated absorption. 47 However, PepT2
is much more widely distributed than just the kidney and as such must play a more
important role in peptide absorption and accumulation in other tissues as well.
In particular, PepT2 transcripts, 45 protein, 54 , 55 and functional activity 56 , 57 have
been shown in choroid plexus and brain, as nicely reviewed by Smith et al. 58 As such,
this transporter is believed to play a significant role in neuropeptide homeostasis and
the efflux of peptides and peptidomimetic xenobiotics from cerebrospinal fluid. Im-
munoblotting analysis determined the neural PepT2 tissue localization, with protein
expression apparent in cerebral cortex, olfactory bulb, basal ganglia, cerebellum, and
hindbrain sections of adult brain. 59 The strongest signals were found in the cere-
bral cortex, while PepT1 protein expression was not found in brain. 59 Interestingly,
expression levels were maximal in the fetus and declined with age, indicative of a
complex regulatory mechanism to be discussed later. Also, the protein was expressed
exclusively on the apical membrane (CSF-facing), indicative of its role in peptide
homeostasis and transport. 59
Shen et al. demonstrated that PepT2 is the primary POT member responsible for
Gly-Sar uptake in the choroid plexus of PepT2 knockout mice. 60 However, the lack
of an observable pathological phenotype indicates that other systems are able to com-
pensate for the loss of activity. 60 Furthermore, while Gly-Sar is a model substrate for
PepT transporters, recent studies in our laboratory have demonstrated that it is not a
substrate for the human PHT1 isoform in transiently transfected COS-7 cells. 61 The rat
PHT1 isoform was first suggested to be expressed in the apical membranes of the rat
choroid plexus, 20 although our results may not reflect species differences in the func-
tion of the PHT1 isoforms. Taken together, these data suggest that redundancies may
exist for the transport of di- and tripeptides at the choroid plexus and presumably,
other biological barriers.
Interestingly, although many studies have demonstrated that PepT2 is not expressed
in the gastrointestinal tract, R uhl et al. investigated peptide transport activity in the
neuromuscular layers of whole-mount preparations from mouse, rat, and guinea pig
stomach and small and large intestines. 62 Surprisingly, D-Ala-Lys-AMCA specifi-
cally accumulated in both ganglionic layers of the enteric nervous system, and its
accumulation was inhibited by Gly-Sar, D-Phe-Ala, Gly-Gln, and cefadroxil, but
not free histidine or benzylpenicillin (a PepT2 substrate). 62 Immunohistochemical
analyses demonstrated that the dipeptide uptake was localized to enteric glial cells
[as demonstrated by 100% uptake in of glial fibrillary acidic protein (GFAP + ) and
S100 + cells] and periganglionic tissue-resident macrophages. 62 The researchers sur-
mised that PepT2-mediated dipeptide transport in enteric glia could contribute to the
clearance of neuropeptides in the enteric nervous system. 62
The findings described above underscore the importance of POT members with
respect to peptide homeostasis, uptake, and accumulation. Recent evidence indicates
that enteric glia play a significant role in modulation of gastrointestinal functions
and may be involved in signaling processes of the enteric nervous system. 62 Loss of