Molecular Mechanisms Regulating Spermatogonial Stem Cell Fate Decisions - Male Germline Stem Cells Developmental and Regenerative Potential - page 145

expression by siRNA treatment followed by functional transplantation analysis
revealed impaired SSC maintenance in vitro , suggesting a role in SSC self-renewal
and survival (Oatley et al. 2007 ). In vivo , LHX1 expression is localized to indi-
vidual spermatogonia in the testes of both pre-pubertal and adult mice (Oatley et al.
2007 ). Unfortunately, further investigation of LHX1 importance in SSC function
in vivo has been challenging due to severe defects in embryonic development of
Lhx1 −/− mice. Deletion of Lhx1 expression in the germline specifically via methods
such as Cre/Lox technology will be required to further examine the role of this
molecule in SSC functions in vivo .

7.3.2.4 OCT6

Individual members of the octamer (OCT)-binding family of transcription factors,
also referred to as POU domain transcription factors, display a spatiotemporal
expression pattern and have diverse roles in cellular processes and stem cell func-
tions. One POU domain protein expressed by SSCs and up-regulated by GDNF
stimulation is POU3f1, also referred to as TST-1 or OCT6. Expression of this tran-
scription factor was first identified in the testis (Meijer et al. 1990 ) and has been
identified as a regulator of neural cell development (He et al. 1989 ; Suzuki et al.
1990 ). In the male mouse germline, OCT6 expression is localized to spermatogonia
and regulated by GDNF through the phosphoinositide-3-kinase (PI3K)/AKT path-
way (Wu et al. 2010 ). In addition, transient reduction of Oct6 expression by siRNA
treatment impairs GDNF-induced maintenance of SSCs in vitro and increases apop-
tosis (Wu et al. 2010 ). Thus, OCT6 is another GDNF-regulated transcription factor
playing a role in the regulation of SSC maintenance.

7.3.3 Non-GDNF Regulated Transcription/Translation Factors

7.3.3.1 MILI

The PIWI family of proteins regulate translation in a variety of cell types and are
essential for maintenance of germline stem cells in the ovary and testis of
Drosophila melanogaster (Lin and Spradling 1997 ; Cox et al. 1998 ; Lin 2007 ). The
murine homologs of PIWI, termed MIWI and MILI (PIWIL1 and PIWIL2, respec-
tively), are expressed in the germline of mouse testes and disruption of either mol-
ecule causes impaired spermatogenesis (Deng and Lin 2002 ; Unhavaithaya et al.
2009 ). MIWI expression is localized specifically to meiotic spermatocytes and
spermatids rather than spermatogonia or SSCs (Deng and Lin 2002 ), and deletion
in mice causes arrested germ cell development at the spermatid stage indicating a
role in spermiogenesis, but not stem cell function (Deng and Lin 2002 ). MILI
expression is localized to gonocytes in neonatal mice, and spermatogonia and early
spermatocytes in the adult testis (Unhavaithaya et al. 2009 ). Spermatogenesis in