Capturing the HIV virus on film - Cellular Biology

USA Today (Society for the Advancement of Education), June, 2003

In stunning color images using time-lapse microscopy, scientists at the University of Illinois at Chicago for the first time have captured the very earliest stages of HIV infection in living cells. The researchers filmed individual HIV particles as they traveled to the nucleus of a human cell and began taking over its genetic machinery--the first step in the destruction of the body's immune system that leads to AIDS.

The movies not only offer tantalizing glimpses of HIV in action, but provide visual proof that HIV enlists the assistance of its host to wreak havoc on the body's defenses. The virus can be seen traveling along a part of the host cell's skeletal framework of microtubules as it makes its way from the outer membrane to the nucleus. The virus hitches a ride aboard a multiunit protein called dynein, commonly referred to as a molecular motor. "Dynein is like a tractor trailer; the microtubules are the highway; and the HIV particles are the cargo," notes David McDonald, assistant professor of microbiology and immunology.

Until recently, little was known about how HIV enters a cell. The virus is made of an outer shell, or envelope, and a core, referred to as a particle, which is composed of proteins and genetic material. When the virus attacks an immune cell, it fuses with the cell's membrane and releases its particle core inside. Yet, what those particles do once they are inside--in particular, how they arrive at the nucleus to hijack the cell's genetic machinery and begin reproducing their own DNA--had remained a mystery.

The tiny particles, only about 12-millionths of a centimeter in diameter, have to cross a distance that is up to 500 times their size to reach the nucleus. Moreover, the way is blocked by all kinds of cellular structures, from energy-generating mitochondria to packets of proteins. How do the particles get through this obstacle course? The scientists were able to visualize individual HIV particles by attaching green fluorescent protein to one of their components. Derived from jellyfish, the protein has only recently been discovered as a means of tagging individual molecules inside a living cell. When blue light shines on the protein, it gives off a green glow. The scientists also made the microtubules of the host cells glow a deep red by incorporating another fluorescent prorein into their building blocks.

Pictures of living cells infected with HIV were taken under a microscope at intervals as short as 15 seconds, creating a movie of the viruses' activities as they traversed the microtubular highway toward their destination in the nucleus. "They don't make a beeline for the nucleus," McDonald explains. "Their progress is somewhat halting. They appear to jump from one microtubule to another, moving in a jagged path, even sometimes moving backward. But they eventually reach their destination." The journey to the nucleus takes about two to four hours.

At the periphery of the nuclei, the scientists then saw the viruses form complexes with genetic material of the host cells--appropriating the tools that HIV needs to reproduce. Dynein's role was confirmed by injecting an off-the-shelf antibody into the cells that prevents the molecular motors from working properly. When the motors stop, the viral particles are found scattered throughout the host cells, not congregated around the cells' nuclei.