As cells can only arise from cells that already exist, continuity of life depends
on the highly regulated sequence of events that control cell division. This process
is mediated by a complex macromolecular structure called the mitotic spindle. The
most conspicuous components of the spindle are microtubules, which are made of tubulin
and other associated proteins. In most animal cells—body cells and male germline cells
(spermatocytes)—spindle assembly is orchestrated by organelles called centrosomes,
which actively polymerize (that is, add tubulin subunits) and stabilize microtubules.
The spindles found in these cells are known as astral because of the star-shaped asters—structures
made of centrosome-anchored microtubules—that can be observed associating with each
spindle pole. Some cells—such as the cells of the female germline (oocytes)—do not
contain centrosomes, and the chromosomes themselves seem to arrange and stabilize
the microtubules into spindles. These spindles are referred to as anastral.
To gain insight into the mechanisms of spindle assembly, scientists are increasingly
relying on techniques that allow them to directly observe dynamic, complex processes
in the living cell. Using time-lapse microscopy of fluorescently labeled fruitfly
(Drosophila melanogaster) spermatocytes, Cayetano Gonzalez and his colleagues at the
European Molecular Biology Laboratory in Germany (and now at the Centro Nacional de
Investigaciones Oncológicas in Spain) have been able to observe the assembly and sorting
of microtubules of noncentrosomal origin in cells that contain centrosomes. The task
of flagging such microtubules is complicated by the fact that centrosomes become quite
active microtubule organizers once cell division begins. Thus, as soon as the membrane
around the nucleus breaks down, microtubules from the centrosome invade the nuclear
region, making it hard to identify any noncentrosomal microtubules that might appear.
To get around this problem, Elena Rebollo in the Gonzalez lab set up two experimental
conditions under which centrosomes remain functional but are kept affixed to the cell
membrane—and, therefore, away from the nucleus—in Drosophila spermatocytes. One takes
advantage of a genetic mutation (called asp, for abnormal spindle); the other uses
a transient treatment with a drug (called colcemid) that depolymerizes microtubules.
In these modified cells, microtubules can be seen growing not only over the membrane-bound
centrosomes, as expected, but also over the nuclear region, away from the centrosomes.
Nucleation, or formation, of such noncentrosomal microtubules has a relatively late
onset, starting only once chromosomes are condensed, and takes place on the inner
side of the remnants of the nuclear envelope. In a fraction of cells, these microtubules
are sorted into bipolar spindle-shaped structures, highly reminiscent of the anastral
spindles found in oocytes. Chromosome segregation—a critical stage of cell division—and
cell division itself tend to be aberrant in these cells.
These results, Rebollo et al. propose, strongly suggest that microtubules of noncentrosomal
origin may significantly contribute to spindle assembly even in cells that contain
active centrosomes. Moreover, by facilitating the nucleation of such noncentrosomal
microtubules, the degraded nuclear envelope may play a previously unsuspected role
in spindle assembly in Drosophila spermatocytes. It is unlikely, the researchers also
conclude, that the anastral spindles they have observed can fill in as a backup to
ensure successful cell division. More likely, they argue, both centrosomal and noncentrosomal
microtubules are required for proper spindle assembly and robust cell division in
cells with centrosomes. As the authors point out, Drosophila is a rich model system
that should help scientists further investigate the intricacies of spindle assembly.
The answers will help us understand how the cell executes one of its most important
duties: safeguarding genomic stability for future generations.