Pluripotent Stem Cells from the Postnatal Testis: Unlocking the Potential of Spermatogonial Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 30

SSCs into pluripotent stem cells. Of note, however, other pluripotency associated
genes have been found to be expressed in the adult testis. For example, Lin28 ,
previously associated with regulation of let-7 precursor microRNA processing, was
found to induce pluripotency of somatic cells (when introduced ectopically in con-
junction with Oct4 , Sox2 , and Nanog ) (Viswanathan et al. 2008 ; Yu et al. 2007 ).
Recently, Lin28 was found to be expressed in adult undifferentiated spermatogonia
(Zheng et al. 2009 ). It is not known whether Lin28 can constitute an upstream
signal leading to expression of other core pluripotency genes.

2.4 Epigenetic Factors that Could Predispose to Pluripotency

The state of chromatin in the development of the male germline represents a key
distinguishing feature from somatic cell types and one that is crucial for reproduc-
tive success. Deficiencies of genes that drive chromatin modifications such as DNA
methylation can result in male sterility (Kaneda et al. 2004 ). Likewise, the unique
chromatin state could also represent a predisposing factor for premature acquisition
of pluripotency. Murine PGCs, unlike somatic lineages, undergo erasure of recently
acquired DNA methylation in both imprinted and nonimprinted loci around the
time of entry into the gonads by about E12.5 (Hajkova et al. 2002 ). Subsequently,
male imprinting patterns become reestablished during the remainder of the prenatal
period and into early postnatal life (Davis et al. 1999 ; Li et al. 2004 ; Schaefer et al.
2007 ; Oakes et al. 2007 ). However, Farthing et al. ( 2008 ) recently found unex-
pected similarities in the global promoter methylation status between ES cells, EG
cells, and sperm, suggesting that male germline cells could activate transcription of
pluripotency-associated genes more easily than somatic cells (Farthing et al. 2008 ).
ES cells have recently been shown to exhibit characteristic sets of histone methyl
marks, linked to their pluripotent status (Bernstein et al. 2006 ). The histone methy-
lation profile of postnatal SSCs is poorly characterized but a distinctive pattern of
perinuclear histone H3 lysine 9 and H4 lysine 20 tri-methylation was recently
described on postnatal undifferentiated spermatogonia, although the patterns at
specific loci were not examined (Payne and Braun 2006 ). Recently, the acquisition
of pluripotency in mouse PGCs at E8.5 was linked to DNA demethylation, with
subsequent loss of pluripotency following histone replacement after E11.5 (Hajkova
et al. 2008 ).
Since pluripotency is acquired in vitro , the chromatin status of cultured SSCs
could affect the stability of lineage commitment and predispose the cells to pluri-
potency. The Shinohara Laboratory found that neonatal SSCs bear the expected
androgenetic pattern of methylation at imprinted genes, which was stable in long-
term culture (Kanatsu-Shinohara et al. 2004, 2005b ). When pluripotency-associated
genes were examined specifically, both sperm and cultured SSCs exhibited relative
hypomethylation of regulatory regions in a number of such genes, although this was
not the case for specific key genes, such as Sox2 , and did not necessarily correlate
with the presence of the corresponding protein (Imamura et al. 2006 ). The authors