Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 113

assembly occur in a closed compartment requires the transport of normally
cytoplasmic contents into the nucleus, such as tubulin and many spindle-
associated proteins. Both Giardia intestinalis and C. elegans , undergo a semi-
closed (or semiopen depending on how optimistic you are) mitosis, in which
chromosomes remain within a permeabilized or partially broken down
nuclear envelope ( Fig. 3.8 B). In Giardia, MTs external to the nucleus pen-
etrate through fenestrae to form the mitotic spindle ( Sagolla et al., 2006 ),
while in C. elegans cytoplasmic proteins can penetrate the more permeable
nuclear envelope ( Hayashi et al., 2012 ). Again, in these cases, the various
non-MT structures may physically support spindle assembly and spatial
localization of spindle components. To speculate, this external support from
the nuclear envelope and relatively small genomes may preclude the need for
large arrays of MTs to organize around the chromatin to establish a mechan-
ically sound structure. However, this remains to be explicitly tested. The
development of new systems and a comprehensive assessment of spindle
architecture in the context of different nuclear envelope structural support
systems and different genome sizes are needed.
In a variation on closed mitosis, spindles formed in a Drosophila embryo
appear to have a compliant meshwork of large proteins that surround the
spindle, termed the spindle matrix ( Johansen and Johansen, 2009 ;
Fig. 3.8 C). Disruption of the spindle matrix protein Chromator can increase
mitotic index and is homozygous lethal ( Ding et al., 2009 ). The spindle
matrix may provide structural support similar to remnant or full nuclear
envelopes of nonmetazoans, but since the Drosophila embryo spindle archi-
tecture appears to be large and robust, an alternative function may be to
maintain some level of compartmentalization of the spindle away from
the rest of the cytoplasm. In the context of a Drosophila embryo syncytium,
a means of compartmentalizing the components of adjacent spindles may be
critical prior to cellularization. In Xenopus egg extract spindles, there appears
to be no isotropic strong spindle matrix ( Gatlin et al., 2010 ). However, a
meshwork of lamin proteins is associated with the spindle and remains intact
after MT depolymerization ( Tsai et al., 2006 ). Whether the lamin mesh-
work is necessary or sufficient to sequester cytoplasmic components or to
provide force resisting anchor sites remains to be elucidated in detail, but
there is support for both hypotheses ( Goodman et al., 2010 ).
Finally, at least three nonprotein macromolecules are implicated in spin-
dle assembly in Xenopus egg extracts. These are RNA, poly-ADP ribose, and
glycogen ( Blower et al., 2005; Chang et al., 2004; Groen et al., 2011 ). While
the molecular mechanisms by which these factors act to promote spindle