Vertebrate Development Maternal to Zygotic Control (Advances in Experimental Medicine and Biology)

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towards the dorsal side (Houliston and Elinson 1991 ; Olson et al. 2015 ). Live imag-
ing studies indicate that the arrays associate and move with the deeper cytoplasm
(Houliston 1994 ; Larabell et al. 1996 ; Olson et al. 2015 ). These microtubule arrays
are transient and are progressively depolymerized upon first cleavage under the
control of MPF activation (Marrari et al. 2003 ), thus terminating cortical rotation.
Surprisingly little is known about the regulation of microtubule activity during
cortical rotation. Generalized kinesin-related protein activity in the cortex proper is
thought to tether the microtubule array to the cortex and facilitate movement
(Marrari et al. 2003 ). This has been assessed using function-blocking antibodies but
specific roles for individual kinesins have not been identified. Kinesin1/Kif5b
appears dispensable in Xenopus (Marrari et al. 2000 , 2003 ), and Dynein has been
shown to act early in rotation, as shown by injection of the antagonistic Dctn2
(Dynamitin/p50; Marrari et al. 2004 ). Recently, a suite of mRNAs localized to the
vegetal pole in oocytes has also been implicated in regulating microtubule assem-
bly. Maternal mRNA depletion experiments show that reductions in perilipin2
(plin2; Chan et al. 2007 ), tripartite motif containing 36 (trim36; Cuykendall and
Houston 2009 ), and dead end homolog 1 (dnd1; Mei et al. 2013 ) lead to abnormal

cortex

kinesins

microtubules plus ends

towards

dorsal

cortex layer rotates

~30 ̊ relative to inner

yoly cytoplasm transport of

cortex sep dorsalizing activity

V D

egg post-fert.~ 40’ post-fert.~ 60’

emtb-gfp emtb-gfp emtb-gfp

trim36, wnt11b

Fig. 6.3 Events of cortical rotation in Xenopus. Microtubules are disassembled during oocyte
maturation, and are absent from the egg cortex (left panels). Certain RNAs are localized to the
vegetal cortex during oogenesis (blue) and encode proteins critical for cortical rotation and dorsal-
ization (e.g., trim36, wnt11b). After fertilization, the incoming sperm pronucleus and associated
centrosome initiate astral microtubule assembly. Cortical microtubule assembly also begins, form-
ing a network by 40 min post-fertilization. A shear zone forms and microtubules associate with the
yolky cytoplasmic core (not shown) and cortical rotation begins, under the action of kinesin-like
proteins (kinesins). Relative cortical movement occurs dorsally, possibly the result of nudging by
ventrally positioned astral microtubules, and rapidly orients microtubule plus ends dorsally (Olson
et al. 2015 ) (middle panel). Microtubule assembly and organization becomes robust by 60 min
post-fertilization and full cortical rotation commences, continuing until first cleavage. Rapid trans-
port of dorsalizing activity occurs along parallel microtubule arrays using kinesin-like motors
(right panel). The corresponding bottom panels show live images of microtubules labeled with
Enconsin microtubule-binding domain tagged GFP (EMTB-GFP), showing progressive assembly
and alignment during cortical rotation (Olson et al. 2015 )

D.W. Houston

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