511results revealed that autophagy is essential for a specific phase of OET but not for
mammalian postimplantation development. As the authors note, mammalian pre-
implantation development progresses slowly without extracellular nutrient stores
in contrast to birds, fish, and amphibians; thus, autophagy may be a unique strategy
to support mammalian development (Tsukamoto et al. 2008 ). The importance of
autophagy for OET in zebrafish or Xenopus remains unknown.
10.3.2 Ubiquitin-Proteasome Pathway During OET
The ubiquitin-proteasome pathway is the main pathway for regulated protein
degradation in eukaryotic cells (reviewed in Yao and Ndoja 2012 ). It is initiated
by covalent attachment of ubiquitin chain to a targeted protein, which is followed
by recognition of ubiquitinated protein and its degradation by the proteasome.
Proteasome assembly requires a set of dedicated chaperones (reviewed in Murata
et al. 2009 ). Mouse oocytes and zygotes express a zygote-specific proteasome
assembly chaperone, whose expression peaks at the 2-cell stage and enhances the
biogenesis of 20S proteasome, thus presumably augmenting degradation of
maternal proteins (Shin et al. 2013 ).
The ubiquitin proteolytic system is selective. Selectivity is mediated by a multi-
meric ubiquitin-ligating protein (E3) complex, which contains a variable substrate
recognition factor targeting a specific protein for ubiquitination. A common sub-
strate recognition factor is a member of the F-box family of proteins (reviewed in
Jin et al. 2004 ). GNF SymAtlas (BioGPS, Su et al. 2004 ) contains expression data
for approximately 100 mouse genes encoding F-box proteins. Of them, approxi-
mately three quarters show detectable expression in the oocyte, and 21 of them
show at least four times higher expression in the oocyte than a mean expression
value in the GNF1M set of somatic tissues (Su et al. 2004 ). In agreement with tran-
scriptome profiling, proteome profiling of mouse oocytes identified a large group of
maternally expressed F-box proteins (Wang et al. 2010 ), which match F-box
protein- encoding genes found in the GNF SymAtlas.
A common tool to specifically and reversibly inhibit the ubiquitin-proteasome
pathway is the proteasomal inhibitor MG132. Treatment of oocytes and embryos
with MG132 shows that ubiquitin-proteasome pathway-mediated protein degrada-
tion has numerous roles during OET. MG132-treated (75 μM) zebrafish eggs showed
delayed completion of MII and extrusion of the second polar body. An embryotoxic
effect was found when matured eggs were held in 100 μM MG132 (Siripattarapravat
et al. 2009 ). Inhibition of the ubiquitin-proteasome pathway with MG132 (up to
20 μM) in mouse embryos delayed the onset of ZGA (Shin et al. 2010 ).
Taken together, the ubiquitin-proteasome pathway is active in mammalian
oocytes and early embryos and plays multiple roles by selectively targeting pro-
teins for degradation. The number of F-box proteins expressed during OET
suggests that a complex system of protein degradation exists, which would
provide another layer of control over OET.
10 Clearance of Parental Products