Saturday, October 11, 2014

Organizer Without a Brain: the Centrosome

The microtubule organizing center of eukaryotic cells, which guides mitosis and nucleates cilia.

One of the more immediate wonders of biology is the division of cells and the organized march of chromsomes that accompanies it, girdled by a beautiful web of microtubules. These microtubules all emanate from something called the microtubule organizing center (MTOC), or centrosome, which is itself strictly duplicated during cell division. It all looks so alive. Yet the process is one of molecules each doing their little thing in blind fashion to make a greater and very dynamic whole.

In this image of a large eukaryotic cell, DNA is stained blue, microtubules are stained green, and the centrosome is stained red / orange. At some point during the mitotic process, the kinetochores (possibly the pink/lavender spots) would line up strictly along the midline between the two centrosomes.

Centrosomes are special to eukaryotes- there is no trace of them in bacteria. They also have no DNA or other sign of being descended from some symbiotic or other bacterial forebear, unlike the mitochondrion or chloroplast, perhaps even the peroxisome. (Though some researchers find they contain a few special RNAs and argue for an endosymbiotic origin on that basis.) Yet they are quite complex organelles, based on the same tubulin that composes the microtubules of the cell generally, but with hundreds of other proteins added in. They organize not only the microtubule asters that drive mitosis, but also form the basal bodies of cilia- the flaggella of eukaryotic cells, again composed of microtubules. These cilia in turn have important, though only recently-discovered roles in mechanisensory and chemical signaling.

The centrosome and its component centrioles. The micrograph cross section shows parts of each centriole from one centrosome.

The centrosome is composed of two centrioles, one of which is the mother, and the other the daugher. At each cell cycle, they split apart and each grows another daughter centriole to reconstitute a full centrosome. Each centriole resembles a microtubule, which is a tube of nine paired tubulin tubules, but has an extra set of tubules, so nine triple tubules of tubulin. At the base of each centriole is something called a "cartwheel", which is made up mostly of a non-tubulin protein (Sas6) that forms a nine-fold symmetric ring that nucleates growth of the centriole, which in turn nucleates growth of the microtubule.

Detailed structure of microtubules (top) and the centriole (bottom). Note the two pairs of tubules in the microtubule, but triplets in the centriole, or basal body. The third tubule appears to template or make space for the dynein arms, which are the motors that allow cilia, for instance, to wave and move.

Models of the molecular structure of the "Cartwheel" protein complex, which  (d) templates and nucleates the centriole structure. Microtubules (purple) fit to the outside spokes of the cartwheel (blue). In the electron micrograph (e), three cartwheel stacks can be seen at the bottom, at the base of the centriole, and higher up, with lower electron density, the microtubule.

"Studies using human cells have revealed a mechanism that regulates cartwheel assembly through controlling the amount of SAS-6 in the cell. SAS-6 starts to accumulate at the end of the G1 phase and decreases in anaphase through proteasomal degradation mediated by APC/CCdh1 [19]. When the SAS-6 level is artificially increased by overexpression of non-degradable SAS-6, excess procentrioles are formed on the mother centriole."

Another electron micrograph of the centrosomes and their microtubule junctions.

The main component of all these structures is tubulin, the building block of all microtubules, which, along with actin, make up the cellular cytoskeleton of eukaryotes. Actin is smaller and free to form more flexible structures, (as well as being used to form the structural scaffold of muscle action), while microtubules make bigger structures (in cellular terms) like that which spans the whole cell for division, making cilia / flagella, and conducting cargoes along the enormous distances of nerve cell axons. Not all eukaryotes have centrosomes, (notably plants and yeast don't), but all have some form of microtubule nucleating center, (MTOC), of sometimes more modest construction. While microtubules are made of alpha and beta tubulin, the centrioles have a third form, gamma tubulin, which only exists at the nucleating base, not in the bulk structure.

Diagram of nucleation from a cartwheel (red) to a centriole (blue).

Unfortunately, less is known about how all this works dynamically than about its structural components. A recent paper covered two key proteins (Cnn and DSpd-2, in fruit flies) that seem to be all that are required to turn a bare centriole into a busy MTOC with hundreds of other proteins in a nimbus around it (the pericentriolar material). These key proteins are controlled by phosphorylation which is a common element of the cell cycle, and when turned on, come into the most interior precincts of the centrosome/centriole, and then migrate slowly outwards, with new molecules arriving at the center as long as the cell is in the requisite state.
"Mimicking phosphorylation allows the PReM domain [part of Cnn] to multimerize in vitro and Cnn to spontaneously assemble into cytosolic scaffolds in vivo that can organize MTs. Conversely, ablating phosphorylation does not interfere with Cnn recruitment to centrioles, but inhibits Cnn scaffold assembly. We speculate that, like Cnn, DSpd-2 can assemble into a scaffold and that this assembly is regulated in vivo so that it only occurs around mother centrioles."

At the other end, a good deal more is known about how microtubules attach to the kinetochores (structures at the center of chromosomes -at DNA centromeres- that anchor them to the mitotic spindle). This is not really in the scope of the post, but again, phosphorylation of key proteins is the theme, as is regulated assembly and disassembly of the microtubule itself. Disassembly right where it abuts the kinetochore causes shortening, thus tightening of the whole spindle, as long as the kinetochore has properly captured microtubules on/from both sides, i.e. both centrosomes, which is another complicated and phosphorylation-regulated process.

Example of a model for microtubule dynamics during one part of mitosis, with microtubules in purple,  (and unconnected microtubules in yellow, cell edge in gray), the centrosome in pink dots, and relevant motor proteins in green and salmon. The point is that, if relevant motors and attachment functions are regulated and happen at the right times, the system can accomplish complex tasks.

It is worthwhile to note that while the replication of the centrosome and its component centrioles is strictly regulated with the cell cycle, the same core system is used to create the basal bodies of cilia, and can create organizing centers from scratch. It does not require a pre-existing centrosome (unlike mitochondria, chloroplasts, and cells, which always come from pre-existing respective cells or cell-ish organelles). Sometimes, the basal body is the same as the centrosome; when cell division is done it migrates to the membrane. But many cells have multiple cilia, and each one has a newly created basal body at its base, whipped up without the complicated replication process. So it appears that replication is more a regulated restriction of MTOC creation than it is a necessary precondition of MTOC birth.

The science of the microtubule nucleation systems is still underway, but the direction is clear, as in any other area of molecular biology that has been laid bare to date. Molecules of some complexity, equipped with various means of regulation, such as regulated transcription and synthesis, outright destruction, interaction with various partners, or covalent modification like phosphorylation, neddylation, methylation, palmitoylation, GPI-anchoring, etc., can find their places and times of action in chemically explicable ways within a cell that is a vast, if microscopic, machine.


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