Walking the line, using a microscope

While I oft discuss evolution and autism on my blog, I research neither.  My primary interest is actually cell biology, along with a large dose of microscopy.  Once in a while a paper will come along which really hits on both of these topics, and its damned well time I blogged about one.  So, for once, a posting about a really cool cell-biology/microscopy paper.

It was published just a few months ago (OK, I'm a little behind on this posting) in PLOS Biology:  Single molecule imaging reveals differences in microtubule track selection between Kinesin motors.

This paper combines some interesting cell biology along with some cutting-edge microscopy.  But before we hit the paper, a bit of background.

Things need to get transported around inside of our cells.  For example, proteins meant to detect extracellular signals like hormones must move to the cell surface; otherwise they won't work.  Much of this cargo gets moved through small balloon-like structures called vesicles.  Rather than drifting randomly, these "balloons" move along tracks in the cell called microtubules; long, filamentous proteins that form a skeleton within the cell.  The image to the right shows this cytoskeleton; microtubules are in green (blue is the cells DNA, red is another part of the skeleton called actin).

Like a train, these "balloons" require a motor to pull them along the microtubule tracks.  In a cell this job is mediated by motor proteins.  While there are a few kinds of motor proteins, this paper deals with one kind called kinesins.  Kinesins are proteins which tend to be involved in the movement of proteins from where they are made (ER and golgi) to the plasma membrane.  Kinesins are odd proteins, that "walk" along the length of the microtubule in a way that looks kinda like a drunk cowboy stumbling out of a saloon (see the video below).

One outstanding question in the biology of kinesins is how do they know where to go - as you can see in the picture of the cytoskeleton at the beginning of this post, microtubules go everywhere, which makes it hard to understand how things can be selectively moved to specific points in the cell.  As it turns out, microtubules are not quite as simple as I outlined here - they're dynamic, as in they continually grow and shrink.  But among those ever-changing tracks there are a small number of microtubules that are modified in a way which makes them stable.  Furthermore, these stable microtubules do tend to go to specific places - for example, in neurons they lead to the junctions between one neuron and the next.  Perhaps these modified microtubules act as highways that allow cells to specifically move proteins to important places.

That is the question asked by this paper.

The real image geekery comes into play with their methods.  They used one of my favourite imaging tricks - single particle tracking (SPT) - to follow kinesins while the pull their cargo around the inside of the cell.  How SPT works is a blog post or two in itself, but the coles notes version is SPT is a method we can use to monitor the movement of single proteins inside of a cell.  Combined with basic microscopy, this method allowed this group to look at the movement of kinesins along microtubules, and to determine which kinds of microtubules these kinesins prefer.

The long and short of how every experiment in this paper was preformed is this group first preformed single particle tracking of the kinesins, overlayed the map of those track with staining of the microtubules, and then used that overlay to determine which microtubules those kinesins are using (click the pic to the right for an example from their paper).

This study looked at three of the kinesins (out of a total of 14), and found that one of them - kinesin 1 - only moves along the modified (stable) microtubules, while two others (kinesins 2&3) moved along all types of microtubules - including those actively growing.  Even better, they also tracked the movement of proteins moved by these kinesins, to show that not only the motor, but the cargo the motor carries, follows along the respective type of microtubule.

This paper has confirmed what we long suspected - that some motor proteins follow specific "tracks" in the cell.  And while we're a long ways away form understanding exactly how a cell directs this traffic, this paper has provided us our first view of one way that cells direct some products to specific regions of the cell - in this case by using kinesin 1 to move proteins down specific tracks in the cell.

Cai D, McEwen DP, Martens JR, Meyhofer E, & Verhey KJ (2009). Single molecule imaging reveals differences in microtubule track selection between Kinesin motors. PLoS biology, 7 (10) PMID: 19823565