Biology Reference
In-Depth Information
One of the most enduring mysteries of neuronal polarity is why most neurons
have a single axon and how it is that the formation of additional axons is sup-
pressed. It is interesting in this regard that neurons also most typically have a
single centrosome. It is tempting to propose that the singularity of the axon and the
singularity of the centrosome are somehow related. In the unusual case of the
cerebellar granule neurons with two axons, a single centrosome changes location
to serve both. However, in the recent paper by de Anda et al. (
2005
), they observed
a small number of hippocampal neurons with two centrosomes and such neurons
consistently formed two axons. On the other hand, it should be noted that cultured
sympathetic neurons initially differentiate several axons after which they re-craft
their morphology into a single axon and multiple dendrites (Bruckenstein and
Higgins
1988
); and yet, despite initially forming several axons, they only have one
centrosome (Yu et al.
1993
). Unlike the case with the axon, dendrites are almost
always multiple in numbers, and it would be hard to fathom that the centrosome
could be so mobile in the cell body as to move from dendrite to dendrite and then
back to the axon to serve each neurite one at a time. Interestingly, we reported
several years ago what appears to be streams of microtubules flowing from the
centrosome into developing dendrites of cultured hippocampal neurons, with a
location roughly centralized among the dendrites (Sharp et al.
1995
; also see
Fig.
18.2
). No such flow of microtubules was observed between the centrosome
and the axon at this stage of development. Taken together, these several findings
indicate that there is no ''one size fits all'' scenario for the location of the neuronal
centrosome. Even so, it would certainly appear that the centrosome is an important
structure in the neuron, at least for the early stages of development.
18.3 Why is the Neuronal Centrosome Important?
The centrosome is best known in eukaryotic cells as a microtubule-organizing
center that organizes microtubules by virtue of its microtubule-nucleating prop-
erties. The centrosome consists of two barrel-shaped centrioles surrounded by
amorphous pericentriolar material (Alberts et al.
2007
). Among the components of
the pericentriolar material are structures known as c-TuRCs (gamma-tubulin ring
complexes). Each c-TuRC, which consists of gamma-tubulin together with several
other proteins, is a template for nucleating a microtubule. Microtubules are
nucleated from the c-TuRCs in such a way that the plus ends of the microtubules
grow away from the centrosome. Thus, if the microtubules remain attached to the
centrosome, they form a radial array of uniform polarity orientation (Euteneuer
and McIntosh
1981
; Schiebel
2000
; Teixido-Travesa et al.
2010
). Such a radial
array, typical of simple interphase cells, is able to direct organelle traffic by virtue
of the tendency of different types of organelles to interact with specific motors that
move toward either plus or minus ends of microtubules. This is why, for example,
the Golgi apparatus tends to cluster at the centrosome; because membranous
elements that comprise the Golgi are transported by cytoplasmic dynein toward
