Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 140

cells that facilitate sperm entry in teleosts. The mitochondrial cloud (Balbiani
body), which expresses Buc protein and RNA, fails to assemble in buc
mutants ( Bontems et al., 2009 ), and germ plasm-localized mRNAs (e.g.,
dazl ) also fail to localize vegetally. Animally restricted (e.g., cyclinB ) RNAs
expand vegetally, suggesting an overall loss of vegetal polarity and expansion
of animal polarity. Additionally, the formation of multiple micropyles,
which are follicle cell derivatives, in buc mutants is indirect evidence for
oocyte-follicle signaling in establishing animal-vegetal polarity ( Bontems
et al., 2009 ). It cannot be excluded, however, whether Buc may function
independently in both tissues, responding to the same polarizing cue.
A second oocyte polarity mutant identified, magellan ( mgn ), also showed
defects in the mitochondrial cloud (Balbiani body), resulting in
mispositioning of both the nucleus and the cloud in early oocytes ( Dosch
et al., 2004; Gupta et al., 2010 ). During subsequent stages of oogensis, the
mitochondrial cloud became enlarged and failed to migrate to the cortex,
and vegetal mRNAs did not properly localize. ER,mitochondria, andmicro-
tubules also failed to become enriched in the cortex. The genetic defect in
magellan was identified as a deletion in the microtubule actin cross-linking factor
1 ( macf1 ) leading to a premature stop codon ( Gupta et al., 2010 ). Macf1 is a
conserved cytoskeletal regulator belonging to the spectraplakin family of pro-
teins, which can interact with both the actin and microtubule cytoskeletons
( Suozzi et al., 2012 ). The main defects in mgm mutants appear to be a failure
to tether microtubules to actin filaments in the cortex (which are unaffected),
resulting in a global mispositioning of organelles and possibly organelle
transport to and from the mitochondrial cloud. Interestingly, the Drosophila
homologue of mcaf1 , shortstop , is a component of the fusome and is required
for microtubule organization and transport ( Bottenberg et al., 2009 ). It is
thus tempting to speculate that Mcaf1 could function in a manner similar to
spectrosome components to maintain polarity in the early oocyte.

2.4.3 Early polarity in mammalian oocytes
Mouse oocytes are also derived from germline cyst divisions. These are only
present perinatally ( Pepling and Spradling, 2001 ) and female germ cells can
also aggregate to form nonclonal clusters ( Mork et al., 2012 ), suggesting that
mammals may be less dependent on cyst divisions. Genetic deletion in
mouse of a ring canal component (Tex14) resulted in normal oocyte spec-
ification, although fewer oocytes were produced ( Greenbaum et al., 2009 ),
further indicating that intracellular bridges are not essential for

mammalian oogenesis.