514
the end, they identified 63 differentially expressed proteins, 21 of which had
decreased expression (Cao et al. 2012 ). The overlap between the studies was
rather unimpressive. Cao et al. identified six (50 %) of the differentially regulated
proteins reported by Vitale et al.; tens of identified differentially expressed pro-
teins are in a stark contrast with transcriptome profiling, which typically identifies
ten to hundred times more transcripts. However, despite much lower resolution,
proteomic analysis is important for understanding OET because it monitors func-
tional (final) gene products, not mere mRNA intermediates. In addition, it allows
for identifying posttranslational modifications, which are undetectable by tran-
scriptome profiling. At the same time, these data illustrate limitations of pro-
teomic studies relying on protein mass, which cannot be amplified like nucleic
acids. Thus, further advancement of mammalian OET proteomics requires either
isolation of large amounts of individual stages or dramatically increased resolu-
tion in terms of protein detection and quantification.
So far, the most comprehensive and informative analysis of mouse OET was
produced by Wang et al., who collected 7000 oocytes (fully grown and MII) and
zygotes and subjected them to semiquantitative mass spectrometry (Wang et al.
2010 ). Two to three thousand proteins were identified, providing the first solid
insight into proteome changes during OET before zygotic genome activation (Wang
et al. 2010 ). Notably, maternal proteins were quickly degraded after fertilization as
suggested by 50 % fewer peptides identified in zygotes relative to MII oocytes
despite the same number of zygotes and oocytes being analyzed (Wang et al. 2010 ).
Remarkably, this observation correlates nicely with 30-year-old data showing that
protein half-lives become reduced after fertilization from ~18 to ~13 h between the
1-cell and cleavage stages (Merz et al. 1981 ).
Taken together, research on maternal protein degradation is coming of age. Our
current knowledge of protein-degrading mechanisms and genes expressed during
OET opens new directions for analyzing, for example, roles of individual F-box
proteins in selective protein degradation by ubiquitin-proteasome pathway during
OET. In addition, development of more sensitive instruments and new methods for
proteomic analysis brings hopes that comprehensive proteome profiling during
mammalian OET will become more feasible in the near future.
10.4 Erasure of Parental Epigenetic Marks
Epigenetic information is heritable information not encoded in the DNA
sequence. The two key molecular mechanisms discussed further below concern
chemical alteration of DNA (DNA methylation) and posttranslational modifica-
tion of histones (histone marks) (for further reading, see Allis et al. 2007 ). The
zygote is formed upon fusion of two differentiated gametes. Each of them carries
its own pattern of DNA methylation and histone modifications, both of which are
extensively remodeled during OET (reviewed in detail in Kimmins and Sassone-
Corsi 2005 ; Burton and Torres-Padilla 2010 , 2014 ; Gill et al. 2012 ; Rivera and
P. Svoboda et al.