Chemistry Reference
In-Depth Information
during oogenesis [88]. At the beginning gurken is localized at the future posterior pole
of the oocyte, sending a signal back to the oocyte to initiate the formation of the
anteroposterior axis. The signal leads to the repolarization of the oocytemicrotubules
and the migration of the oocyte nucleus to the dorsoanterior corner of the oocyte [89].
When gurken is localized in an anterodorsal cap near the oocyte nucleus the second
round of signaling initiates the formation of the dorsoventral axis. The overlying
follicle cells acquire dorsal fates, leading later to secretion of correct eggshell
structures [88, 90]. The gurken mRNA
first moves across the internal oocyte to the
anterior and then turns towards the nucleus in the anterodorsal position. Both steps
require dynein and microtubules, but they rely on different microtubule net-
works [91]. RNA binding proteins such as Squid and Hrp48 are involved in gurken
dorsal movements [92] and its localized translation is restricted to the dorsal anterior
region [91, 93]. After the initial signal from gurken, the further development of the
anterior
-
posterior symmetry of the oocyte involves several other localized transcripts
in addition to gurken: bicoid for anterior speci
cation and oskar and nanos mRNAs,
both localized in the posterior. This axial polarity is established by opposite gradients
of these proteins maintained in the oocyte by the maternal determinants (from
ovarian nurse cells), transported on cytoskeletal networks to their destination and
then anchored and translated. At the anterior pole bicoid is recruited in two phases: an
earlier phase inmid-oogenesis whenmicrotubules are polarized towards the anterior
pole, and a later phase, after nurse cell dumping, when ooplasmic streaming is
thought to facilitate the mixing of the incoming material [94]. In the
rst phase,
microtubules and the binding of the trans-acting factor Exuperentia are essential
for the localization at the anterior pole [15]. Also in the late phase bicoid is localized
by active transport [94] instead of diffusion and trapping as is the case with nanos
(see below). This involves the binding of Staufen protein to bicoid, before nurse cell
dumping, and transport of the complex on a subset of microtubules that originates at
the anterior pole. Microtubules and actin
filaments are responsible for the enrich-
ment of bicoid at this pole not through anchoring, but instead by a continuous active
dynein-driven transport [94].
At the posterior pole oskar is one of the
first mRNAs recruited, probably by kinesin
I. Interestingly, proteins in the exon-junction-complex (EJC) and the splicing reaction
per se seem to be involved in its localization [95, 96]. Oskar protein in turn is required
for nanos mRNA localization. The peculiarity of nanos localization is how the speci
c
expression in the posterior pole is achieved. Indeed, nanos enters the oocyte during
nurse cell dumping and is dispersed in the ooplasmby streamingmovements and by
diffusion in the whole oocyte [97]. Once at the posterior, it is anchored to the actin
cytoskeleton and translated [97]. In contrast, outside this region nanos is transla-
tionally repressed by Glorund in the oocyte [98] and in the embryo by Smaug [99, 100]
or also degraded [101]. Another mRNAwhich becomes localized at the posterior pole
by degradation outside its target region is hsp83 [102]. Both these posterior enrich-
ments require two distinct cis-acting elements in the 3
0
untranslated region (UTR) of
the RNA: a degradation element that targets the mRNA for destruction in all regions
of the egg or embryo, and a protection element that stabilizes the mRNA at the
posterior [101].
