Regulation of Blood–Testis Barrier (BTB) Dynamics during Spermatogenesis via the “Yin” and “Yang” Effects of Mammalian Target of Rapamycin Complex 1 (mTORC1) and mTORC2 - International Review of Cell and Molecular Biology - page 297

region, to be followed by discrete AJs and DS, which constitute the junctional
complex ( Fig. 6.1 ). In addition, GJs are located basal to the junctional com-
plex ( Hartsock and Nelson, 2008 ; Miyoshi and Takai, 2008 ) ( Fig. 6.1 ). In these
blood-tissue barriers, the permeability barrier is created almost exclusively by
TJs which seal the intercellular space between adjacent membranes and confer
cell polarity to restrict paracellular and transcellular transport of substances
( Steed et al., 2010 ; Tsukita et al., 2001 ), whereas AJs which connect to a dense
actin filament network confer the adhesion property ( Harris andTepass, 2010 ).
Thus, the coexisting TJs, basal ES and GJs which contribute to the barrier and
adhesion function of the BTB as an entity is in fact a unique feature amongst
all the blood-tissue barriers ( Fig. 6.1 ). Since TJs, basal ES and GJs are all linked
to underlying actin cytoskeleton via corresponding adaptors, changes in the
organization of actin filaments at the BTB during the epithelial cycle play a
significant role in its restructuring. In this section, we briefly discuss each junc-
tion type at the BTB and how these junctions associate with the underlying
F-actin cytoskeleton, interacting with each other.
2.1. Tight Junction
TJs appear as “kisses” between adjacent epithelial or endothelial cells under
electron microscope where two plasma membranes fuse together as illus-
trated in the Sertoli cell BTB ( Cheng and Mruk, 2010b ; Steed et al., 2010 ;
Tsukita et al., 2001 ). In other blood-tissue barriers, TJs are located apically in
an epithelium or endothelium and act as “fences” that divide the membranes
into apical and basolateral domains. Since integral membrane proteins are
freely diffusible in plasma membrane, this “fence” function of the TJ restricts
proteins to their respective apical or basal location ( Steed et al., 2010 ; Tsukita
et al., 2001 ), generating apicobasal polarity in an epithelium and to prevent
transcellular transport of substances across the barrier. Although the inter-
cellular space is sealed by TJs in which the TJ strands from two neighbor-
ing plasma membranes associate laterally with each other to form a “gate,”
selected ions and/or solutes can pass through these “gates” via paracellular
transport, which is dependent on their charge and size ( Steed et al., 2010 ;
Tsukita et al., 2001 ). This “gate” function of TJs varies among cell types due
to the differences in the relative proportions of different TJ proteins ( Steed
et al., 2010 ; Tsukita et al., 2001 ). In addition, differences in TJ-strand density
also affect permeability of the TJ ( Steed et al., 2010 ; Tsukita et al., 2001 ).
While the “fence” and “gate” functions imply TJs are considerably rigid in
nature,TJs are actually dynamic ultrastructures by adjusting their permeability
barrier function in response to changes in environment and/or physiological