Irina Kaverina, Ph.D., reviews study information in her lab with graduate student Paul Miller. (photo by Anne Rayner)
Microtubules help Golgi get itself together
Microtubules are the main component of the cell's internal scaffolding (cytoskeleton). One of their main functions is to transport organelles and proteins required for a cell to move and divide.
While the majority of microtubules originate from a small structure near the nucleus called the centrosome, Irina Kaverina, Ph.D., and colleagues previously identified a unique subset of microtubules that originate from the Golgi complex — a structure inside the cell that processes and packages proteins for transport.
In the September issue of Nature Cell Biology, the researchers define the functions of these Golgi-derived microtubules, including their importance in organizing the Golgi itself and in facilitating directional transport of proteins. These functions have important implications for cell movement and possibly for cancer cell invasion and metastasis.
Like a cellular post office, the Golgi complex processes and packages macromolecules (proteins and lipids) made by the cell and then ships the parcels out to their cellular destinations.
Kaverina and colleagues identified the Golgi-derived microtubules in 2007 and found that these microtubules were directed at the leading edge of migrating cells — unlike centrosomal microtubules, which radiate out in all directions. This suggested that the two microtubules subsets may have very different functions.
“One of our goals was to find out the functions of this specific microtubule subset,” said Kaverina, an assistant professor of Cell and Developmental Biology and senior author on the paper. “We assumed that if there are two subsets of microtubules, there should be two distinct functions and they should somehow cooperate.”
From their previous work, the researchers knew that proteins called CLASPs were required for the formation of Golgi-derived microtubules, but not for centrosomal microtubules.
Therefore, they depleted CLASPs in retinal pigment epithelial cells — a type of migrating epithelial cell — to eliminate Golgi microtubules in order to study their functions. Paul Miller, a graduate student in Kaverina's lab, led these experiments.
Destroying this set of microtubules disrupted the formation of the Golgi complex, they found. Instead of the continuous ribbon-like structure seen in normal cells, they observed an abnormal, highly fragmented Golgi complex.
The findings revealed that normally the assembly of the large Golgi complex by microtubules happens in two stages. In the first stage, Golgi-derived microtubules act in a “search and capture” mechanism, drawing the smaller Golgi stacks together into clusters at the cell periphery. In the second stage, centrosomal microtubules bring the Golgi clusters together at the center of the cell into a single large ribbon-like structure.
In the absence of Golgi-derived microtubules, the first stage was missing. Stacks were collected around the centrosome but were not able to properly fuse into an organized complex.
Kaverina and colleagues also found that CLASP-depleted cells lacking Golgi microtubules lost the ability to send their protein parcels in a single direction; instead, the packages were sent out in all directions.
“They're still functional in terms of protein processing within these little stacks. So for protein processing, (Golgi- derived) microtubules are not needed,” said Kaverina. “But if you want these proteins to be exported to a certain location in a cell, then they would not know where to go.”
This directional protein trafficking is important because cell migration requires the delivery of numerous proteins and membrane to the “leading edge” of a migrating cell in order for that cell to move. Determining the role of Golgi-derived microtubules in protein trafficking and cell movement may offer insight into how cancer cells invade distant tissues.
And since many chemotherapy drugs work by altering microtubules, the research could have implications for the development of chemotherapy drugs with fewer toxic side effects.
Other Vanderbilt authors on the paper were: Andrew Folkmann, Ana Rita Maia, Nadia Efimova and Andrey Efimov. The research was supported by the National Institutes of Health.