Molecular Mechanisms Regulating Spermatogonial Stem Cell Fate Decisions - Male Germline Stem Cells Developmental and Regenerative Potential - page 143

lymphoma 6, member b ( Bcl6b ; also termed Bazf ), Ets variant gene 5 ( Etv5 ; also
termed Erm ), and Lim homeobox protein 1 ( Lhx1 , also termed Lim1 ) (Oatley et al.
2006 ). Transient reduction of Bcl6b , Etv5 , or Lhx1 transcript levels with siRNA
treatment impaired SSC maintenance in vitro demonstrated by transplantation
analysis showing loss of SSCs after one self-renewal cycle in vitro (Oatley et al.
2006 ). GDNF-regulated gene expression was also examined in cultured GFRa1+
germ cells isolated from testes of pre-pubertal mice (Hofmann et al. 2005 ;
Braydich-Stolle et al. 2007 ; He et al. 2008 ). In those studies, up-regulation of
several genes including Numb , a component of the notch signaling pathway, was
identified in the cultured cells. Additionally, c-Fos and n-Myc gene expressions
were up-regulated in cultured GFRa1+ testis cells after exposure to GDNF
(Braydich-Stolle et al. 2007 ; He et al. 2008 ). In contrast, expression of either Numb ,
c-Fos , or n-Myc was not affected by GDNF stimulation in cultured THY1+ germ
cell populations proven to contain SSCs by functional transplantation (Oatley et al.
2006 ). Unfortunately, the SSC composition of cultured GFRa1+ cell populations
used in previous studies was not determined. Thus, while these factors are poten-
tially regulators of SSC functions the true importance of GDNF-regulated Numb,
c-Fos, or n-Myc expression is difficult to assess based on the analyses reported to
date. The remainder of this chapter will focus on transcription factors with demon-
strated regulation or function in SSCs.

7.3.2 GDNF-Regulated Transcription Factors

7.3.2.1 BCL6B

The transcriptional repressor BCL6B is a homolog of BCL6 in both the mouse
(Okabe et al. 1998 ) and human (Sakashita et al. 2002 ). In the lymphocyte lineage,
BCL6B is important for activating naive CD4+ T cells and mediating proliferation
of CD8+ memory T cells (Manders et al. 2005 ). In cultured mouse SSCs, Bcl6b
gene expression is down-regulated upon GDNF withdrawal and up-regulated by
GDNF stimulation, suggesting a key role in SSC self-renewal (Oatley et al. 2006 ).
To study a biological importance of BCL6B in SSC function, expression was
experimentally reduced by siRNA treatment in cultured SSCs (Oatley et al. 2006 ).
Using functional transplantation as an assay, reduction of BCL6B expression was
shown to impair SSC maintenance in vitro over one self-renewal cycle of 7 days.
Further examination showed that BCL6B siRNA treatment induced apoptosis in the
cultured mouse SSCs (Oatley et al. 2006 ). Inactivation of BCL6B in vivo causes a
sub-fertile phenotype in male mice (Oatley et al. 2006 ). At 3 months of age, testes
of Bcl6b −/− mice are smaller compared to those of wild-type litter mates and contain
varying percentages of seminiferous tubules with degenerating spermatogenesis
including spermatozoa-only and Sertoli-cell-only phenotypes (Oatley et al. 2006 ).
Collectively, these observations suggest that BCL6B plays an important role in
promoting SSC survival.