In stunning color images using time-lapse microscopy, researchers at the University of Illinois at Chicago have for the first time 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 own skeletal framework of microtubules as it makes its way from the outer membrane to the nucleus. The virus hitches a ride aboard a multi-unit 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," said David McDonald, assistant professor of microbiology and immunology at UIC.
McDonald and Thomas Hope, associate professor of microbiology and immunology at UIC, are co-authors of the study, which appeared Nov. 11 in the Journal of Cell Biology. Science magazine named the paper an "editor's choice" in its Nov. 22 issue, and it will be featured in an upcoming issue of Nature Cell Biology.
An editorial accompanying the paper said, "With the powerful approaches developed by McDonald et al. and the incredible progress in imaging single fluorescent molecules in living cells, ... important and fascinating questions of HIV cell biology can now be addressed."
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.
But 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 researchers 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 researchers also made the microtubules of the host cells glow a deep red by incorporating another fluorescent protein 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 said. "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, he said.
At the periphery of the nuclei, the scientists 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. When the motors stop, the viral particles are found scattered throughout the host cells, not congregated around the cells' nuclei.
The paper represents four years of research, begun when Hope was a researcher at the Salk Institute for Biological Sciences in La Jolla, Calif.
"This work is confirmation of the dynamic new methods we are using to study HIV," Hope said. "We hope this basic research will one day lead to new targets for drug therapy in the longstanding battle against AIDS."
Hope said he plans to extend the technique developed in this HIV research to study Ebola, one of the deadliest viruses known and one that could be used in a bioterrorist attack. Little is understood about Ebola's basic biology, including how it enters cells.
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