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For example, TJ strands formed by claudin-1 are highly branched network
while claudin-11-based TJ strands, as those found in Sertoli cells, are mostly
parallel strands with little branching ( Gow et al., 1999 ; Morita et al., 1999b ).
Furthermore, the selectivity of ions and solutes of a permeability barrier is
also dependent on the composition of claudins as illustrated by gain-or-loss
function studies in animals, humans or cell lines involving specific claudins.
For instance, overexpression of claudin-2, but not claudin-3, in MDCK I cells
which express only claudin-1 and -4, leads to a “leaky” TJ barrier, as shown
by a decrease in transepithelial electrical resistance (TER) across the cell epi-
thelium. This thus reflects the differential ability among different claudins in
conferring the TJ-barrier function ( Furuse et al., 2001 ). Furthermore, in clau-
din-15 knockout mice, the small intestine displayed malabsorption of glucose
due to a disruption of paracellular transport of Na + ions across the TJ barrier
( Tamura et al., 2011 ). Claudin-16, however, was shown to be important to
paracellular transport of Mg 2+ across the TJ barrier ( Simon et al., 1999 ).
Claudins also play an important role in maintaining the BTB function
during spermatogenesis. In fact, TJ strands at the BTB is contributed signifi-
cantly by claudin-11 since deletion of claudin-11 leads to a loss of the BTB
ultrastructure, resulting in the lack of TJ strands between Sertoli cells ( Gow
et al., 1999 ). Interestingly, Sertoli cells, which normally cease to divide after
postnatal day 15, are found to be proliferating in adult claudin-11 knockout
mice ( Gow et al., 1999 ). This is probably due to the loss of contact inhibi-
tion after the disappearance of TJs. This thus suggests that the permeability
barrier imposed by claudin-11 also has a role in regulating cell cycle func-
tion in Sertoli cells. Furthermore, a recent report has shown that claudin-3
may be a crucial protein involving in the intermediate compartment during
translocation of spermatocytes across the BTB ( Komljenovic et al., 2009 ).
Immunofluorescence staining illustrated that during the transit of prelep-
totene spermatocytes across the BTB at stage VII-IX in mice, localization
of claudin-3 at the BTB was found apically to preleptotene spermatocytes
(“old” BTB) at stage VII; however, at stage VIII-early IX, claudin-3 was
detected at both apically (“old” BTB) and basally (“new” BTB) of the trans-
locating spermatocytes; and finally claudin-3 was detected only at the basal
side (“new” BTB) of leptotene spermatocytes transformed from prelepto-
tene spermatocytes ( Komljenovic et al., 2009 ). Despite this stage-specific
localization of claudin-3 coinciding with the intermediate compartment,
this observation requires further verification by functional studies, such as
if its knockdown would indeed impede the migration of spermatocytes at
the BTB. Additionally, the role of claudin-3 may be species-specific since
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