This story originally ran on Dec. 22.
A team led by researchers at the University of California, San Francisco, has identified a family of amyloidogenic peptides in semen that may significantly enhance HIV infection.
Their findings, which were detailed in the current edition of Cell Host & Microbe, provide new insight into the possible role of semen in promoting HIV infection and suggest routes by which preventative agents like anti-HIV microbicides might be improved, Warner Greene, professor of translational medicine at UCSF’s Gladstone Institute of Virology and Immunology and one of the study’s authors, told ProteoMonitor.
Using MALDI mass spec on an AB Sciex 4800 MALDI TOF-TOF Proteomics Analyzer, the UCSF researchers identified a series of peptides from semenogelins – the predominant proteins in semen – capable of forming amyloid fibrils. These amyloid fibrils, Greene said, are thought to enhance HIV infection by promoting attachment of the HIV virions to the surface of target cells.
The research builds upon a 2007 study published in Cell that identified a different set of amyloidogenic peptides in semen that similarly enhance HIV infection (PM 12/20/2007). That work isolated peptides derived from prostatic acid phosphatase – a key player in the semen liquefaction process – that formed amyloids the authors termed semen-derived enhancers of viral infection, or SEVI.
The UCSF researchers’ investigation began with their own attempt at isolating SEVI from human semen to confirm these factors’ role in enhancing HIV. However, Greene noted, when they analyzed their samples, they identified not SEVI factors but the semenogelin, or SEM, peptides instead.
“We were asking the question, ‘Is SEVI responsible for all of the enhancing activity that is present in semen?’” Green said. “We fully anticipated that SEVI would be the only amyloid fibril present. But, surprisingly, we came up with the peptides from seminogelin that were amyloidogenic, as well.”
It was well known, he said, that cleavage of seminogelin proteins into peptides by the protease prostate specific antigen, or PSA, is important in the semen liquefaction process that enables fertilization. “Entirely new,” however, is the discovery “that some of the peptides from that breakdown are highly amyloidogenic and can enhance HIV infection quite effectively,” Greene noted.
In vitro studies examining the SEVI factors found they could enhance HIV infection by several hundred thousand fold, Greene said, and while such measurements have not yet been done with the SEM peptides, the functional effects of the two factors “appear very comparable,” he said.
He cautioned, however, that the effect will likely be less pronounced in in vivo studies. The UCSF researchers, Greene said, are currently planning in vivo experiments to test the SEVI and SEM agents’ enhancing properties in Rhesus macaques.
“We’re envisioning a trial with 16 Rhesus macaques, challenging the animals repeatedly with low amounts of virus,” he said. “Eight monkeys will get just the virus, and eight monkeys will get the virus mixed with semen [containing the SEVI and SEM agents]. The idea is that we will see much earlier infection of the monkey in the presence of semen as opposed to just in the presence of the virus alone.”
“We need to move beyond these somewhat artificial in vitro experiments to experiments with natural semen, and see how much of an enhancing effect we see,” he added.
One question surrounding the in vitro work is how sample storage might affect the levels of amyloidogenic peptides found in semen – a factor that Green noted might be responsible for the different peptides discovered by his team and by the researchers behind the original Cell paper.
He said he was unsure as to why the SEVI researchers had not also discovered the SEM peptides in their analysis, but suggested that it might be due to use of samples at different stages of the liquefaction process.
“A key issue is that in the process of liquefaction, you generate these [SEM] peptides, and then the [infection-enhancing] fibril has a chance to form,” Greene said. “But if you continue to incubate [the peptides] in the presence of PSA, the PSA will cut the peptides yet again, converting it into a non-fibril-forming peptide. Depending on how the semen was treated in the first study, it’s possible that it was allowed to incubate long enough that you may have lost these types of amyloidogenic peptides.”
The UCSF researchers’ failure to detect SEVI in their study, he suggested, was perhaps due to masking by the higher abundance SEM peptides. The authors noted that the concentration of full-length SEM proteins is estimated at between 10 mg/ml and 20 mg/ml in semen while the concentration of PAP is between 1 mg/ml and 2 mg/ml.
Despite not isolating the SEVI factors in their work, “we still believe that SEVI plays a role, and we certainly make no priority claims” regarding SEVI and SEM’s relative importance in enhancing HIV infection, Greene said. However, he added, “we wonder now if seminogelin might be biologically more important because it is present at higher concentrations.”
Assuming the SEVI and SEM factors’ effects hold up in in vivo experiments, these peptides could be promising targets for anti-HIV microbicidal gels, Greene said.
“Microbicide development for HIV has been very checkered,” he said, adding that “the first several microbicides tested didn’t block HIV infection in women but, in fact, propelled it.”
Recently, a microbicide gel using tenofovir – marketed as Viread by Gilead Sciences – showed some success in Phase IIb clinical trials, but even in that case it reduced HIV infection only by an estimated 39 percent, which, Greene said, is “really not the kind of protection we’re looking for.” The hope, he said, is that such agents can be improved by including compounds targeting both HIV itself and infection-enhancing semen factors like SEVI and SEM.
“It’s our hypothesis that if you have a combination microbicide that targets both the virus and the semen factors that enhance infection, you’d have a far better microbicide,” he said. “Ideally we’d like to have small molecules that would disrupt the amyloid fibrils.”
Such agents have yet to be identified, he noted. Anti-amyloid Alzheimer’s therapies under development at drug companies including Eli Lilly and Pfizer could have potential given the fact that amyloid fibrils are implicated in both diseases. However, Greene said, his team’s work with such agents met with limited success.
“We tried some of the agents that have been reported to disassemble Alzheimer’s fibrils, and at high doses we could see some disassembly, but it was an experiment plagued with a fair amount of toxicity,” he said. “So we certainly don’t have any magic defibrilizing agents in hand at the moment.”
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