A paper published in this week’s issue of PLoS Biology describes a study that has reactivated a dormant gene found in humans and coaxed it – in tissue culture – to produce an antiviral peptide. Lead scientist Alexander Cole used aminoglycosides – drugs commonly used to fight bacterial infections – to trigger the production of the protein, which is encoded by the dormant human defensin gene that he calls “retrocyclin”. The authors hope that this research might ultimately lead to the development of a treatment that would activate the gene in a person’s own cells, for example, and thereby prevent infection with viruses in the treated tissue.
Defensins are a large family of small antimicrobial peptides that contribute to host defense against a broad spectrum of pathogens. In primates, defensins are divided into three subfamilies – alpha, beta, and theta – on the basis of their disulfide bonding pattern. Theta-defensins were the most recently identified defensin subfamily, isolated initially from white blood cells and bone marrow of rhesus monkeys. They are the only known cyclic peptides in mammals and act primarily by preventing viruses such as HIV-1 from entering cells. Whereas theta-defensin genes are intact in Old World monkeys, in humans they have a premature stop codon that prevents their expression; they thus exist as pseudogenes. On correction of the premature termination codon in theta-defensin pseudogenes, human myeloid cells produce cyclic, antiviral peptides, indicating that the cells retain the intact machinery to make cyclic peptides. Given that the endogenous production of retrocyclins could also be restored in human tissues, the possibility exists that that aminoglycoside-based topical microbicides might be useful in preventing sexual transmission of HIV-1.
Dozens of scientists around the world are looking for ways to prevent the transmission of viruses such as HIV. Cole and colleagues have previously discovered that retrocyclin proteins found in some primates appeared to prevent HIV infections in cell cultures. The same gene exists in humans, but because of a mutation that interrupts the gene sequence, it no longer produces protein. Now, a collaboration between researchers has found that restoring the production of retrocyclins prevents HIV entry into human cells. The scientists have found a way to get the gene to produce the retrocyclin and then showed that the retrocyclin appears to prevent the transmission of HIV in cells cultured in the laboratory. They applied aminoglycoside antibiotics to vaginal tissues and cervical cells in the lab and found the antibiotic appears to stimulate those cells and tissues to produce retrocyclins on their own. There is a possibility the aminoglycoside antibiotics will be used in a cream or gel format that could someday be a simple way to prevent the transmission of HIV. Much more work would be required before this would be possible, including taking the result in tissue culture and showing the same effect in whole organisms.
Reawakening retrocyclins: ancestral human defensins active against HIV-1. 2009 PLoS Biol 7(4):e1000095
Human alpha and beta defensins contribute substantially to innate immune defenses against microbial and viral infections. Certain nonhuman primates also produce theta-defensins—18 residue cyclic peptides that act as HIV-1 entry inhibitors. Multiple human theta-defensin genes exist, but they harbor a premature termination codon that blocks translation. Consequently, the theta-defensins (retrocyclins) encoded within the human genome are not expressed as peptides. In vivo production of theta-defensins in rhesus macaques involves the post-translational ligation of two nonapeptides, each derived from a 12-residue ‘‘demidefensin’’ precursor. Neither the mechanism of this unique process nor its existence in human cells is known. To ascertain if human cells retained the ability to process demidefensins, we transfected human promyelocytic cells with plasmids containing repaired retrocyclin-like genes. The expected peptides were isolated, their sequences were verified by mass spectrometric analyses, and their anti-HIV-1 activity was confirmed in vitro. Our study reveals for the first time, to our knowledge, that human cells have the ability to make cyclic theta-defensins. Given this evidence that human cells could make theta-defensins, we attempted to restore endogenous expression of retrocyclin peptides. Since human theta-defensin genes are transcribed, we used aminoglycosides to read-through the premature termination codon found in the mRNA transcripts. This treatment induced the production of intact, bioactive retrocyclin-1 peptide by human epithelial cells and cervicovaginal tissues. The ability to reawaken retrocyclin genes from their 7 million years of slumber using aminoglycosides could provide a novel way to secure enhanced resistance to HIV-1 infection.
Related:
Assessing the quality of information on the internet is all about filtering – whether it’s a blog post or a peer-reviewed journal. I don’t want to shock you, but not everything published in peer-reviewed journals is correct, you still need to use your judgement. And some blogs provide some of the best information out there. The best example of this I’ve come across recently is How do adamantane drugs block M2? by Michael Clarkson:
