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 336

“yang” effects of the mTORC1 and mTORC2 signaling complexes on the
BTB dynamics that regulate BTB restructuring during the seminiferous
epithelial cycle of spermatogenesis, which is being critically evaluated in
the following sections.
4.2. Regulation of BTB Dynamics by mTORC1
In the seminiferous epithelium of adult rat testes, rpS6, a crucial down-
stream signaling molecule of mTORC1 (Section 3.2.2. ) was found to be
highly expressed in the basal compartment of the seminiferous epithe-
lium in all stages of the epithelial cycle, consistent with its localization
at the BTB, implicating the likely involvement of mTORC1 signaling
complex in BTB dynamics ( Mok et al., 2012c ). Interestingly, p-rpS6, the
activated form of rpS6, was highly expressed at the BTB and colocalized
with putative BTB proteins ZO-1, N-cadherin and Arp3, but restrictive
to late stage VIII-IX, coinciding with the time of BTB restructuring to
facilitate the transit of preleptotene spermatocytes at the site ( Mok et al.,
2012c ). This timely upregulation in the phosphorylated and activated form
of rpS6 at the BTB suggests that rpS6 may take part in the “opening”
of the BTB for the transit of spermatocytes from the basal to the apical
compartment. To confirm this postulate, rpS6 phosphorylation was abol-
ished by inactivating mTORC1 signaling in cultured Sertoli cells with
an established TJ-permeability barrier by either treatment of cells with
rapamycin or a knockdown of rpS6 by RNAi, both approaches was shown
to promote the Sertoli cell TJ barrier by making the BTB “tighter” fol-
lowing a blockade rpS6 activation or its knockdown ( Mok et al., 2012c ).
In addition, the expression of TJ proteins, such as claudin-11, were upregu-
lated with claudin-11 being redistributed and localized more intensely to
the Sertoli cell-cell interface ( Mok et al., 2012c ), possibly being used to
“strengthen” the TJ barrier. Furthermore, changes in the F-actin organi-
zation was detected with more actin filaments were found at the Sertoli
cell-cell interface ( Mok et al., 2012c ), possibly being used to strengthen
the Sertoli cell TJ barrier. In short, these findings illustrate that rpS6 was
specifically activated and highly expressed at the site of the BTB in the
seminiferous epithelium during its restructuring at stage VIII-IX of the
epithelial cycle, whereas a suppression of rpS6 or its knockdown in Sertoli
cells led to a “tightening” of the TJ barrier. These findings thus support
the notion that the rpS6 activation is crucial to elicit BTB restructur-
ing, such as at stage VIII-IX of the epithelial cycle. An earlier study has
shown that mouse embryonic fibroblasts (MEFs, also known as feeder