Microtubules as a gyroscope for cells on the go

Free advice never hurt anyone. And when Harry Malech couldn't decide between research on cell motility or infectious disease, his fellowship mentor Richard Root offered up this gem: “You know how to combine that, don't you? You ought to be interested in neutrophils—they are the fastest cells alive.”

So began a study of neutrophil chemotaxis that became the first clear demonstration that microtubules oriented and organized the internal structure of migrating cells (Malech et al., 1977). In an earlier study, Goldman (1971) had noted that after destruction of microtubules with colchicine “no one ruffling edge [of a motile cell] seems to be capable of taking over as the leading edge,” suggesting that these “fibers may be involved in determining which ruffling edge becomes the leading edge.” But a dynamic picture of such a process was missing. The work also showed that migration and pseudopod formation required functioning actin filaments. While many experts had postulated various roles for the two major classes of cytoskeletal filaments, the study clarified their distinct, yet complimentary, functions in migration.

“[The study] of white cell locomotion at that time was very descriptive,” says John Gallin, who teamed up with Malech and Root. “In response to a chemoattractant, was there a rudder? A skeletal structure that gave it shape? I was thrilled that we could use cells from humans to demonstrate this.”

Malech had been setting up “chemotaxis chambers” with porous filters that neutrophils could migrate through in response to a toxin-activated serum gradient. He would then embed the whole filter with cells in resin for sectioning and electron microscopy. On one “serendipitous” day, Malech says he grabbed the wrong size filters, with pores too tiny for whole cell migration. He didn't realize his mistake until he looked at the sections and “was amazed to see these cells lined up like little soldiers in frustrated chemotaxis.”

He realized the accidental assay would make it easy to ask what was common about the immobilized cells trying to move in the same direction. When he reversed the direction of the chemoattractant, he observed that the positions of the nucleus and centriole and orientation of microtubules also reversed rapidly.

When he added cytochalasin B to disrupt actin filaments, the neutrophils could no longer migrate toward a reversed chemical gradient and failed to put out pseudopods. Yet their internal organelle structure still shifted in response to the gradient reversal. Adding colchicine, the microtubule inhibitor, had roughly the opposite effect—random migration was not impaired but the organization of internal structures broke down. And, without microtubules, attractant-directed migration became severely handicapped, “like a drunken man who can't walk a straight line,” says Malech, now chief of the Laboratory of Host Defenses at the National Institute for Allergy and Infectious Disease (Bethesda, MD).

The study hit home the point that microtubules' role in migration is similar to how “a gyroscope keeps a moving vehicle on course,” says Malech. “It's not the motile force or the steering wheel, but it provides stability by giving the direction of which way [the vehicle] is going and has been going.” Malech and Gallin extended their study of neutrophil microtubules into clinical applications like the study of Chediak-Higashi syndrome, a rare disorder involving recurrent infections downstream of neutrophil dysfunction (Gallin et al., 1980).