Cell Junctions, Structure, Function, and Regulation
363
protein composition of the tight junction
strand may control the selectivity of the
epithelial barrier. In 1998, two novel in-
tegral membrane proteins, claudin 1 and
claudin 2, were identiFed from TJ enriched
fractions. When expressed in Fbroblasts,
these proteins were able to produce TJ
strands that were morphologically similar
to those of epithelial cells. Since that time,
over 20 claudin isoforms have been iden-
tiFed. Claudins are 20 to 27 kD proteins
with 4 transmembrane domains that form
two extracellular loops and whose N- and
C-termini are localized on the cytoplas-
mic side of the plasma membrane. The
C-terminal tail is long and is divergent be-
tween isoforms suggesting that anchorage
to the cytoskeleton or scaffolding to signal-
ing molecules may also be important in
regulating claudin function.
Claudins were found to play an im-
portant role in determining the TER in
MDCK1 and MDCK2 cells. Expression of
claudin 1 in MDCK cells was found to
increase the TER 4 to 5-fold over wild-
type MDCK. Expression of claudin 2 in
highly resistant MDCK1 cells, which nor-
mally express claudins 1 and 4, resulted
in a decrease in TER and tight junc-
tion morphology similar to low-resistive
MDCK2 that typically express claudin 2 in
addition to claudin 1 and 4. This study
demonstrated that strands containing dif-
ferent claudin isoforms are responsible
for the variable barrier properties of differ-
ent tight junction strands. Claudins have
also been found to regulate ion selectiv-
ity. Indeed, expression of claudin 4 in
MDCK will increase TER by decreasing
Na
+
permeability without changing the
permeability to Cl
, the flux of mannitol,
or the rank order of permeabilities of alkali
metal cations. These studies all demon-
strate that claudins are the proteins in the
tight junction strand that are responsible
for regulating the selective permeability of
epithelial and endothelial cells.
How do different claudin isoforms cre-
ate barriers with different selectivity? Al-
though the mechanisms involved are not
fully understood, current evidence sup-
ports the hypothesis that the speciFc struc-
tural characteristics of different claudin
isoforms play a role in regulating the selec-
tivity and barrier properties of TJs. Studies
using expression of different claudin iso-
fo
rmsbo
ths
ingu
la
r
lyaswe
l
lasinpa
i
rs
have found that claudins can form mixed
strands containing more than one isoform
in each strand. In addition, claudins have
been shown to bind in both a homophilic
and a heterophilic manner. Heterotypic in-
teractions only occur with speciFc pairs of
claudin isoforms. ±or example, cells ex-
pressing claudin 1 or claudin 3 form tight
junction strands when placed in mixed
culture, whereas cells expressing claudin 2
will not form tight junction strands when
mixed with either claudin 1-or claudin
3-expressing cells. It has been proposed
that mixed strands containing claudin 1
and 3 would form a very restrictive bar-
rier as these two isoforms bind to each
other in a homotypic and heterotypic man-
ner as shown (±ig. 14b). In contrast, a
strand containing claudin 1 and claudin 2
would form a less restrictive barrier. In this
strand, claudin 1 would bind to claudin 1
and claudin 2 would bind to claudin 2 in
a homotypic manner, creating a barrier.
However, at certain points in the strand,
claudin 1 would oppose claudin 2 forming
a pore that would allow the transport of
molecules through the strand (±ig. 14b).
As over 20 claudin isoforms have been
identiFed, a number of different pairings
could exist, allowing for different barrier
properties in speciFc tissues.
Since the discovery of claudin, a num-
ber of inherited human diseases have been
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