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Cargo motion through the cell--all shook up!?

by Andre on 1 April 2009

Here is a post from friend-of-the-blog Igor Kulic. He describes the road that led to our paper on intracellular transport last year. If you don’t have a subscription, you can get the pdf from Igor’s site or free from PubMed Central. You can see from his fun, conspiratorial style why it’s such a pleasure to work with him. It also doesn’t hurt to have a talented theorist around to sharpen your intuitions!

-Andre

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We all know how intracellular transport works from textbooks—at least roughly. Molecular motors, kinsein and dynein pull their cargos (organelles and molecules) along microtubules from point A to B within the cell.

During this motor-hauling-a-cargo transport process, microtubules are assumed to be comparably immobile, at least far less mobile than the molecular motors along them. However newer data seems to sharply contradict this simplistic picture. And maybe we even have to overhaul it entirely. The road/vehicle distinction in the case of microtubules and their motors seems to be an artificial and inaccurate approximation. In fact the whole microtubule cytoskeleton seems constantly in a strange turmoil—a random very fast back and forth motion mixing and pumping everything through the cell interior.

Here is how it came about.

The story began some years ago when we analyzed the movies from the Gelfand and Selvin labs of fluorescently labeled organelles moving along microtubules in drosophila S2 cells. This is a nice well controlled actin-free model cell where microtubules are strongly polarized with their “plus” ends pointing radially outwards in small bundles (processes) from the cell nucleus to the periphery. After first visual inspections of some of the movies from these cells one thing hit the eye: The organelles were moving in a weird manner: erratic, back and forth like a drunken sailor along a line (the microtubule bundles). This happened seemingly without any plan, or bias towards any microtubule end.

As it soon turned out such a motion was actually well documented in the literature and the term bidirectional transport was coined for it. While nobody really knew what it was, people commonly speculated that it could be a peculiar form of competition—an epic struggle between the molecular motors kinesin and dynein sticking to the same cargo, fighting for the direction of motion. Even if we took this explanation for truth, it is hard to anticipate why the motion is so symmetric with kinesin and dynein being such two very unequal rivals having different biochemistry, running speeds, and forces. If you go into the details of this popular “coordinated kinesin-dynein tug-of-war” model you quickly notice that the explanation becomes increasingly elusive (with a similar incomprehensible voo-doo flavor of e.g. string theory).

The question we asked was, could this motion be the signature of something completely different? We had some initial idea, but were hesitant to push the heresy without hard evidence. So we went on the journey to better document the strange phenomenon. When we looked then more carefully at fluorescently labeled cargos we realized that some of them seemed to move in unison.

The strongly correlated motion of distant objects (several microns apart) allowed only one logical conclusion: somehow the “immobile” background—the frame of reference—must be in motion. However when looking at the whole cell and the microscope cover-slide we were sure that the motion was not due to a global displacement of the cell or any such artifact. Furthermore, the motion was only along the apparent microtubule direction.

Eventually we had collected enough data to conclude: The roads—the microtubules themselves—were moving in a very rapid random-oscillatory fashion with large speeds (up to 10s of microns / sec) and amplitudes (several microns). Despite its randomness, this microtubule motion is biochemically active (ATP consuming) and kinesin motor dependent—as we showed by knocking down kinesin.

We could in fact visualize this motion directly by staining the microtubules in chic mCherry-red. [see movie in less chic grayscale -AB] So microtubule motion was the reason for cargo correlations…

Interestingly however, sometimes coherently moving cargo pairs lost their correlation and distance between them ceased to be constant. We concluded that cargos did bind and unbind from such shaking microtubules and engage something that we called “hitchhiking” mode of transport. This motion resembles a randomly moving conveyer belt (microtubules) on which cargo is loaded and unloaded also in a random fashion. This eventually leads to a long range distribution of cargo throughout the whole system. [As Igor showed previously [arXiv] -AB]

So is all the transport in the cell “shook up” in this manner? We analyzed the statistical properties of cargo positions in time and found some characteristic power-law signatures of the microtubule shaking process. Using this method we came to the preliminary answer: Not all, but a substantial portion (>50-70%) of cargos in drosophila S2 cells is engaging in this exotic non-textbook form of transport.

How universal is the hitchhiking in other cells? There are some first hints that it could be very common in many cell types. The present answer is: we don’t know yet for sure… In any case we learned one simple lesson: Often you see things by … just watching.

More (and bigger) surprises are waiting to be revealed by cellular paparazzi yet to come. Never mind the umbrellas initially flying at your head, trust your eyes, rely on intuition.

It’s your turn now.



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