Biology Reference
In-Depth Information
centrosome and its position becomes less centralized (Burakov et al.
2003
). Based
on observations from other cell types, we have posited that the reason why the
centrosome relocates toward a particular neurite, if it does, is that the machinery
that transports microtubules strongly favors that particular neurite at that particular
moment in development (Baas
1996
). Thus, the microtubules that are released
from the centrosome would flow into the relevant neurite but those that are not yet
released would react to the motor-driven forces by pulling the centrosome toward
that particular neurite. Thus, in neurons, we would speculate that the degree to
which the location of the centrosome is predictive of where an axon or dendrite
emerges from the cell body probably relates to how active the centrosome is in
nucleating microtubules as well as the degree to which or rate at which the
microtubules are released once nucleated. In other words, if the centrosome is not
very active at nucleating microtubules, it would not be relocated toward any
particular neurite. If the centrosome is highly active at nucleating microtubules but
most or all of the microtubules are almost immediately released upon nucleation,
the centrosome would not be relocated toward any particular neurite. Also, if the
relevant motors do not favor any particular neurite, the centrosome would not be
relocated toward any particular neurite. These points are schematically illustrated
in Figs.
18.1
and
18.2
b. It is difficult to imagine a scenario by which the centro-
some would relocate without being active at nucleating microtubules.
Whether or not it is functionally important that the centrosome is located where
it is, in various types of neurons at particular stages of development, remains to be
seen. Certainly, if there are multiple options for where the microtubules released
from the centrosome could be transported, a location near the hillock of the
relevant neurite would be an advantage for directing microtubules into that neurite.
Another possibility is that the location of the centrosome could be functionally
important but for other reasons, such as providing a flow of Golgi-derived vesicles.
Support for this idea comes from work showing in cultured hippocampal neurons a
particularly robust flow of membranous elements into the immature process that
develops into the axon (Bradke and Dotti
1997
). Yet another possibility is that the
centrosome is important for reasons related to the various proteins that gather
together to form the pericentriolar material. For example, the pericentriolar
material is rich in kinases (Hames et al.
2005
), and hence the centrosome could act
as a processing center to phosphorylate functionally important proteins. Alterna-
tively, the pericentriolar material might act as a sink for various proteins that
would otherwise, and under certain circumstances, be widely distributed in the
neuron. This could apply not only to proteins such as kinases, but also to proteins
directly related to microtubule nucleation. In such a scenario, it may not be
essential that the centrosome nucleates microtubules, but by sequestering the
proteins needed for microtubule nucleation, the centrosome ensures that micro-
tubule nucleation does not occur in other locales, where it would be problematic.
Whatever the case, it may become important, as we ponder the entirety of the data
on the neuronal centrosome, to think more expansively on the potential roles that it
may play in organizing the cytoplasm and directing various events relevant to the
axon and dendrites.
