CHAMPAIGN, Ill. — An experiment to understand how chemokine peptides dock to a receptor on a cell wall – a pivotal connection that allows HIV to infect healthy cells – has yielded an unexpected fundamental discovery and a possible new way to block AIDS.
“There is still a lot to be done, but our findings provide new insights about the receptor’s flexibility and the potential mechanism of the interaction with HIV at the entry point of healthy cells,” said Ziwei Huang, a professor of biochemistry at the University of Illinois at Urbana-Champaign.
Huang was the principal investigator of the study, which appears in the May 17 issue of the Journal of Biological Chemistry. Huang’s team included researchers at the Kimmel Cancer Center at Thomas Jefferson University in Philadelphia and Dana-Farber Cancer Institute at the Harvard Medical School in Boston.
Researchers used computer modeling to predict the structure of chemokine receptor-ligand bonding. Then they synthesized reverse-chirality D-peptides, based on a portion of amino-acid sequences of two natural L-proteins that bond to CXCR4, a chemokine receptor to HIV discovered in the mid-1990s. CXCR4 and CD4, a receptor identifed in the 1980s, are thought to act together to let HIV bond and insert its replicating machinery into cells.
Only in rare cases – and never involving membrane-class proteins – have reverse-chirality peptides bound at identical receptors. “We didn’t have any expectation that our synthesized peptide would bind to the CXCR4 receptor,” Huang said. “If you switch the chirality, you have a mirror-image change. You’d expect a different lineup of key amino-acid sidechains. We looked at this out of pure curiosity. We just wanted to see what would happen.
“Unexpectedly,” he said, “these D-peptides displayed strong binding and antagonistic activity toward CXCR4, thus revealing that the peptide binding site on CXCR4 is tolerant of changes in the chirality of ligands,” Huang said. “Normally, ligand receptor-bindings are very specific, just like putting your left hand in your left pants pocket.”
Not only were the D-peptides still recognized, they fit into the receptor more snugly than their natural L-peptide counterparts, blocking the virus from bonding. Subsequent experiments, documented in the paper, showed the synthetic D-peptides have a longer half life, making them less susceptible to biological decay than L-peptides.
In making the discovery, Huang designed and synthesized the D-peptide and tested its bonding to the receptor at Illinois. Testing against HIV-1 was done at Dana-Farber in Dr. Joseph Sodroski’s laboratory and by a group led by Hideko Kaji at Jefferson Medical College, where Huang had previously worked and had begun his research.
Traditional drug therapies have targeted protease or reverse transcriptase activity that occur after HIV has entered healthy cells. None stop HIV from docking with cells.
“Researchers realize that this approach is not enough,” Huang said. “It would be nice to have other targets. Our discovery suggests the possibility of a whole new therapeutic weapon, which alone or in combination with existing therapies could work better to block AIDS.”
Illinois authors on the paper, along with Huang, were James Pesavento, Chang-Zhi Dong, Youli Wang and Jing An. Co-authors from Thomas Jefferson University Medical College were Kaji, Naiming Zhou, Zhaowen Luo, Jiansong Luo, Xuejun Fan, Dongxiang Liu, Xiaobing Han and Megumi Hiraoka. Dana-Farber researchers were Mark Cayabyab and Sodroski.
The National Institutes of Health, Center for AIDS Research, G. Harold and Leila Y. Mathers Charitable Foundation, William F. McCarty-Cooper, Douglas and Judity Krupp, and Japan Science Foundation funded the research.