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 337

cells) from rpS6 p−/− mice displayed a higher rate of global protein synthesis
( Ruvinsky and Meyuhas, 2006 ), suggesting that a decline in phosphory-
lated rpS6 might trigger de novo synthesis of TJ proteins, which is consis-
tent with our findings that a knockdown of rpS6 in Sertoli cell epithelium
induced claudin-11 expression ( Mok et al., 2012c ). Furthermore, rpS6
might take part in regulating actin cytoskeleton similar to its upstream acti-
vator S6K1 since actin filament rearrangement was shown to be stimulated
following a knockdown of rpS6; and to further support the role of rpS6
in actin dynamics, phosphorylated rpS6 was found to structurally interact
with actin as demonstrated by coimmunoprecipitation ( Mok et al., 2012c ).
Taking these findings collectively, it is clear that the promotion of the
Sertoli cell TJ-barrier function after a suppression of rpS6 likely leads to
an increase in the synthesis of TJ proteins (e.g. claudin-11), which coupled
with redistribution and/or relocalization of BTB proteins to the Sertoli
cell-cell interface, supported by an increase in F-actin bundles at the corti-
cal region of the Sertoli cells in the epithelium, thereby strengthening the
BTB integrity. In short, during the epithelial cycle of spermatogenesis,
the timely activation of mTORC1 at stage VIII-IX that leads to phos-
phorylation of rpS6 during BTB restructuring may facilitate this process
by transiently downregulating TJ proteins, and perturbing the supportive
F-actin network underneath cell adhesion complexes that facilitates their
endocytosis. In short, BTB is transiently “opened” above the preleptotene
spermatocytes in transit at the BTB induced by an upregulation of p-rpS6,
which facilitates the migration of these spermatocytes across the BTB to
enter the adluminal compartment to prepare for meiosis I/II.
4.3. Regulation of BTB Dynamics by mTORC2
For mTORC2, its key binding partner rictor was shown to be highly
expressed at the BTB from stages I-VI of the seminiferous epithelial cycle,
however, it was downregulated from late stage VII and it was considerably
diminished and barely detectable at stage IX ( Mok et al., 2012a ) ( Fig. 6.4 ).
This suggests that mTORC2 signaling may be involved in maintaining the
BTB integrity during all the stages of the epithelial cycle of spermatogenesis
except at stage VIII-IX when it is downregulated when the BTB is under
restructuring ( Mok et al., 2012a ). To confirm this postulate, studies were
performed in which a knockdown of rictor by RNAi in cultured Sertoli
cells with an established TJ-permeability barrier was found to disrupt the TJ
barrier, and this event was also associated with a reduced phosphorylation
of PKC-α, but not PKB ( Mok et al., 2012a ). Thus, the Raf-1-MEK-ERK