The Actin Superhighway
Posted on by Dr. Francis Collins
What looks like a traffic grid filled with roundabouts is nothing of the sort: It’s actually a peek inside a tiny microchamber that models a complex system operating in many of our cells. The system is a molecular transportation network made of the protein actin, and researchers have reconstructed it in the lab to study its rules of the road and, when things go wrong, how it can lead to molecular traffic accidents.
This 3D super-resolution image shows the model’s silicone beads (circles) positioned in a tiny microfluidic-chamber. Suspended from the beads are actin filaments that form some of the main cytoskeletal roadways in our cells. Interestingly, a single dye creates the photo’s beautiful colors, which arise from the different vertical dimensions of a microscopic image: 300 nanometers below the focus (red), at focus (green), and 300 nanometers above the focus (blue). When a component spans multiple dimensions—such as the spherical beads—all the colors of the rainbow are visible. The technique is called 3D stochastic optical reconstruction microscopy, or STORM .
The image comes from then-graduate student Andrew Lombardo in the NIH-supported lab of David Warshaw at the University of Vermont, Burlington. Lombardo fashioned the microchamber to study at very close range how motor-like proteins called myosin push cargo along these actin highways. It’s not as straightforward as you might think. Like streets in a busy city, these cellular highways feature intersections, which present a directional and physical challenge for the myosin motors to navigate. And there are no stop lights.
By using STORM, Lombardo could view in 3D how myosin molecular motors push their cargo, piloting “overpasses” and “underpasses” along the actin highway without disengaging or crashing. Creating a 3D actin highway and looking at the transport process with 3D STORM allowed Lombardo and his colleagues to unravel how myosin motors zipped across actin highway intersections, making a straight shot for their destination . These destinations are measured in nanometers, or billionths of a meter.
Although the photo was a finalist in the 2018 Art of Science Image Contest, sponsored by the Biophysical Society, Bethesda, MD, it is much more than a pretty picture. The photo contributes data on important health issues, such as how insulin particles make their way in pancreatic β-cells for secretion into the bloodstream, and how genetic mutations to actin lead to heart failure and even sudden death. Meanwhile, Lombardo has earned his Ph.D., and he’s headed down a new road as a postdoc at Cornell University, Ithaca, NY.
 Whole-cell 3D STORM reveals interactions between cellular structures with nanometer-scale resolution. Huang B, Jones SA, Brandenburg B, Zhuang X. Nat Methods. 2008 Dec;5(12):1047-52.
 Myosin Va molecular motors manoeuvre liposome cargo through suspended actin filament intersections in vitro. Lombardo AT, Nelson SR, Ali MY, Kennedy GG, Trybus KM, Walcott S, Warshaw DM. Nat Commun. 2017 Jun 1;8:15692.
Warshaw Molecular Motors Group (University of Vermont, Burlington)
Video: Simulation of myosin moving along an actin highway (University of Vermont)
NIH Support: National Institute of General Medical Sciences; National Heart, Lung, and Blood Institute; National Institute of Arthritis and Musculoskeletal and Skin Diseases