513accessible OET system for proteomic studies has been that of Xenopus. In 2014,
three proteomic studies provided insights into Xenopus oocyte proteome
composition and its dynamics during OET.
Sun et al. implemented isobaric tags for relative and absolute quantitation
(iTRAQ) analysis of four Xenopus early development stages from egg to neurula
and provided protein dynamics for nearly 4000 proteins classified in six clusters
according to their expression pattern (Sun et al. 2014 ). Two clusters harbored mater-
nal proteins degraded after fertilization where one protein cluster became destabi-
lized upon fertilization (179 proteins) and one protein cluster remained stable until
MBT/ZGA and then was degraded (190 proteins). In addition, they identified two
additional clusters of proteins, which were encoded by maternal mRNAs and
became synthesized upon fertilization either in a transient manner (88 proteins) or
as a complement to sustain levels of embryonic expression (Sun et al. 2014 ).
Smits et al. performed single-cell proteomic analysis of Xenopus eggs using
mass spectrometry (Smits et al. 2014 ). They quantified over 5800 proteins and pro-
vided a proteomic comparison of oocytes and gastrula. It showed that proteomes of
oocytes and early embryos highly correlate (r = 0.90) and that less than 10 % pro-
teins were significantly regulated (including 313 significantly downregulated mater-
nal proteins) (Smits et al. 2014 ). Remarkably, authors observed minimal if any
correlation between transcript and protein levels suggesting that posttranscriptional
control is the major control layer determining protein abundance.
Wuhr et al. provided the most extensive proteome catalogue of Xenopus oocytes
with more than 11,000 identified proteins, many of which were found even in the
absence of corresponding mRNAs, being possibly acquired from blood plasma
together with yolk (Wuhr et al. 2014 ). Thus, uptake of maternal non-yolk proteins
might form another layer of developmental control executed in the embryo by
maternal proteins not expressed from oocyte’s genome (Wuhr et al. 2014 ).
In mammals, proteomic analyses of oocytes and early embryos are limited by the
amount of available material, which is two to three orders of magnitude lower than
in zebrafish or Xenopus oocytes. For example, a proteomic analysis of ~3000 mouse
MII oocytes using unlabeled proteins resolved by 2D electrophoresis revealed sev-
eral hundred proteins, but only dozens of them were identified by tandem mass
spectrometry (Calvert et al. 2003 ). However, the catalogue of the maternal proteome
continued to grow. A similar later study used 80 μg of total protein isolated from
MII oocytes and identified 869 spots resolved by 2D electrophoresis corresponding
to 380 unique proteins (Ma et al. 2008b). Zhang et al. used 2700 oocytes and identi-
fied 627 different proteins in MII oocytes (Zhang et al. 2009 ), and Pfeiffer et al.
reported a catalogue of 3699 proteins (Pfeiffer et al. 2011 ).
Several studies also addressed proteome dynamics during OET. Vitale et al.
analyzed proteome dynamics during meiotic maturation (Vitale et al. 2007 ).
Using 500 fully grown and MII oocytes profiled by 2D electrophoresis, they iden-
tified 12 differentially expressed proteins, which were identified by mass spec-
trometry. Of the 12 proteins, seven were downregulated during meiosis. A similar
later study by Cao et al. comparing fully grown and MII oocytes recognized 1114
protein spots, of which ~10 % were differentially expressed (fold change >1.5). In
10 Clearance of Parental Products