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actin cytoskeleton for the maintenance of the BTB integrity is best illustrated
in studies using actin regulating proteins Eps8 and Arp3 ( Lie et al., 2010 ,
2009 ). It was reported that after in vitro knockdown of Eps8 in Sertoli cells
with an established functional TJ-permeability barrier by RNAi, actin disor-
ganization was detected, leading to the redistribution of occludin and ZO-1
from the cell-cell interface into the cell cytosol ( Lie et al., 2009 ). Moreover,
in vivo knockdown of Eps8 in testis also led to truncation and mislocaliza-
tion of F-actin and occludin, respectively, contributing to the disruption of
the BTB integrity when assessed by an in vivo BTB functional assay ( Lie
et al., 2009 ). Furthermore, in a study using wiskostatin to block Arp3 activa-
tion in cultured Sertoli cells, the inhibition of branched actin polymerization
that resulted in deposition of actin filament bundles at the cell-cell interface,
led to a promotion of the Sertoli cell TJ-permeability barrier function ( Lie
et al., 2010 ). Indeed, one of the most important findings from the above
studies was that it illustrated the two actin regulating proteins Eps8 and Arp3
that exhibited stage-specific and restrictive spatiotemporal expression at the
BTB during the seminiferous epithelial cycle provided the means for cyclic
reorganization of the actin cytoskeleton at the Sertoli cell BTB ( Lie et al.,
2010 , 2009 ). In fact, besides binding to AJs, TJs and actin, adaptor proteins
ZO-1/2/3 also bind to GJs, polarity proteins (e.g. PATJ), actin-binding pro-
teins (e.g. cortactin, AF-6) and a variety of signaling molecules, such as kinases
(e.g. c-Src, PKC), transcription factors (e.g. ZONAB, c-Jun) and G proteins
(e.g. G protein α subunit) ( Gonzalez-Mariscal et al., 2000 ; Tsukita et al.,
2009 ). Thus, these adaptor proteins also act as scaffolding proteins at the TJ
barrier by recruiting other regulatory proteins to the site and to provide cross
talks among coexisting junctions at the BTB including TJs, basal ES and GJs.
2.2. Ectoplasmic Specialization (ES)
In epithelia and endothelia, AJ is localized below TJ in the basolateral region
of two adjacent cells. It is a discrete structure physically segregated from
TJ and is primarily responsible for cell-cell adhesion by connecting to a
dense actin cytoskeleton that create a plaque-like ultrastructure known as
zonula adherens ( Hartsock and Nelson, 2008 ; Miyoshi and Takai, 2008 ).
In the testis, however, AJ is distinctly different from those found in other
epithelia/endothelia, instead a testis-specific ultrastructure known as ES is
found. There are two ESs in the seminiferous epithelium dependent on
its location. The ES that is found near the basement membrane between
adjacent Sertoli cells, and is localized at the BTB is the basal ES, it coex-
ists with TJ and GJ, and is responsible for Sertoli cell-cell adhesion ( Cheng
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