The function of gametocyte specific factor 1 (GTSF1) in mammalian oocyte and ovarian follicle development

A detailed understanding of the genes and mechanisms that regulate oocyte growth and
maturation underpins the development of improved methods of assisted conception.
Gametocyte specific factor 1 (GTSF1)-a putative marker of gamete developmental
competence, is highly conserved across species but in mammals demonstrates a sexual
dimorphism in its function. In mice, male mutants for Gtsf1 have an infertile phenotype,
whereas female mutants appear to have normal ovarian function. It is hypothesised that
GTSF1 regulates oocyte development in monovular species such as the sheep.
Initial studies characterised the expression and cellular distribution of GTSF1 across
cDNA libraries spanning ovine oogenesis and embryogenesis and by using in situ
hybridisation of fixed tissue. GTSF1 expression was confined to gonadal and embryonic
tissues with highest expression in the ooplasm of germinal vesicle (GV)-staged
secondary oocytes. The gene sequence of GTSF1 was obtained and the gene and
predicted protein sequences revealed close homology across many species with two
conserved CHHC zinc finger domains known to bind RNA.
Functional analysis of the role of GTSF1 during sheep oocyte maturation was conducted
using short interference RNA (siRNA injection) in conjunction with oocyte in vitro
maturation (IVM) and oocytectomised cumulus shell co-culture. This system was
validated using siRNA knockdown (kd) for a known oocyte-specific gene, Growth
differentiation factor 9 (GDF9). The effect on GTSF1 kd was evaluated following the
microinjection of 770 GV oocytes with siRNA target against the sixth exon of ovine
GTSF1. The effects of GTSF1 kd were evaluated in 57 MII oocytes and cumulus shells.
Targeted kd of GTSF1 in GV oocytes followed by IVM and cumulus shell co-culture
did not affect oocyte meiotic progression or cumulus expansion. Microarray analysis
using the bovine GeneChip Affymetrix array revealed that 6 down-regulated genes
(TCOF1, RPS8, CACNA1D, SREK1IP1, TIMP1, MYL9) following the GTSF1 kd were
associated with developmental competence, RNA storage, post-transcriptional
modifications and translation. Immunofluorescent studies localized GTSF1 protein to
the P-body in GV ovine oocytes.
Collectively these results suggest a possible role of GTSF1 in post-transcriptional
control of RNA processing, translational regulation and RNA storage which may impact
on oocyte developmental competence.