Herein, we report how we cloned human uroplakin Ib cDNA and found
that it has two polyadenylation sites, tagged by
GS8043 and
GS8103. These two transcripts are present in approximately
equal intensities in normal human urothelium examined by Northern blot
analysis.
19 By RT-PCR, we found that these two transcripts
are also present in the normal ocular surface epithelia of humans. When
we searched against database (db) EST, however, we found that
the expressions of the two GS sequences were quite different.
GS8043 matched five of EST’s material sources: invasive
kidney tumor (GenBank accession numbers AA513869 and AA484313),
well-differentiated endometrial adenocarcinoma (AI632869), poorly
differentiated endometrial adenocarcinoma (AI811548), and gallbladder
(AA345,951);
GS8103 was not found at all. Judging by this,
the transcript corresponding to
GS8043 may be more highly
expressed in carcinoma tissues than in normal tissues.
Using RT-PCR, we clearly detected uroplakin Ib and faintly detected
uroplakin II in corneal and conjunctival epithelia. Uroplakins Ia and
III, were not detected. Our results therefore seem to indicate a
difference between ocular surface epithelia and bladder epithelium,
because it is known that uroplakin Ib makes a heterodimer with
uroplakin III in bladder epithelium.
22 Uroplakins Ia
and Ib are members of the TM4 family of proteins and have been
implicated in diverse cellular processes, including cell activation,
proliferation, differentiation, adhesion, and motility by facilitating
specific interactions between cell surface proteins.
23 24 Recent studies have shown that CD9, CD63, and CD81 form complexes with
integrins on the cell surface.
25 26 To clarify the
possible function of uroplakin Ib in the ocular surface, it would be
useful to investigate its possible association with other proteins.
In the corneal epithelium, uroplakin Ib was identified by
immunofluorescence confocal microscopy in the membranes of cells in the
superficial half of this multicellular layer. This localization is
generally similar to that in bladder epithelium, except that in bladder
it is the superficial epithelial cells that label the
strongest.
27 As in corneal epithelium, in conjunctival
epithelium and limbal epithelium, uroplakin Ib was found in the cell
membranes in all epithelial layers, albeit with lower levels of
immunostaining. It is interesting that uroplakin Ib protein was not
detected in the most superficial cells in the ocular surface
epithelium, even when immunofluorescence confocal microscopy was
performed after treatment with neuraminidase or acetylcysteine to avoid
an effect of heavily glycosylated proteins (data not shown).
When we consider the tissue distribution of uroplakin Ib as described
in dbEST, it is apparent that it exists in several nonkeratinized
epithelia at the tissue–fluid interface. In the ocular surface,
apolipoprotein J and mucin proteins are known to localize at the
tissue–fluid interface.
28 29 Similar to these proteins,
uroplakin Ib may function to maintain ocular surface homeostasis at the
tear–ocular surface interface. Uroplakin Ib is one of the major
components of bladder, where it is thought to prevent rupturing during
distension.
5 6 7 We reason that if uroplakin Ib has a
similar mechanical function in ocular surface, it may help resist
mechanical forces on the cornea by intraocular pressure or external
pressures such as those due to blinking.
As an ocular surface defense system and to prevent bacterial infection,
tear fluid contains secretory IgA and IgG, complement components,
lactoferrins, and lysozyme.
30 Recently, it has been
indicated that antimicrobial peptides called defensins are produced by
the cornea, conjunctiva, and lacrimal gland.
31 In the
corneal epithelium, there are numerous solid junctions, and the
permeability of this cellular layer is much less than that of
conjunctival epithelium.
32 However, if the defense system
is compromised and bacteria invade the corneal epithelium, the
immunologic defense of the cornea is lower than that of the
conjunctiva.
30 With this in mind, it is interesting to
note that uroplakin Ia and Ib are able to bind to
E. coli type I pili and that this results in exfoliation of host bladder
epithelial cells as part of an innate host defense
system.
13 14 We postulate that uroplakin Ib expressed in
ocular surface epithelium, especially the apical side of corneal
epithelium, may prevent bacterial invasion into deeper tissues. The
most superficial cells may not need uroplakin Ib, because they fall off
in a short time, whether bacteria attach them or not. The reason the
uroplakin Ib, not uroplakin Ia, is necessary in ocular surface
epithelium is not clear at present and is the subject of proposed
studies.
The authors thank Satoshi Kawasaki for technical help and Tung-Tien
Sun, New York University, and Andrew Quantock, Cardiff University,
United Kingdom, for critical reading and comments on the manuscript.