Challenging Periodontitis With Innate Immunity

A little more than 30 years ago, Dr. Anne C. R. Tanner and her colleagues at the Forsyth Institute isolated the oral bacterium now named Tannerella forsythia. Despite later being listed as one of the 3 Red Complex pathogens that typically trigger chronic periodontitis, T forsythia flew mostly below the investigative radar in the 1980s and 1990s. It was too difficult to culture and not amenable to standard laboratory tricks. But 5 years ago, Dr. Tanner and a Forsyth colleague published a timely review and prediction, entitled, "Tannerella forsythia: A periodontal pathogen entering the genomic era." As the authors explained, the bacterium's genomic sequence was now publicly available. By systematically studying the genes stitched into its DNA, scientists soon would discover the biocircuitry that makes this microbe tick, including the extent of its biochemical weaponry that likely contribute to periodontal disease.
In the journal Molecular Microbiology (January 2011), a National Institute of Dental and Craniofacial Research (NIDCR)-supported researcher and colleagues take another fascinating look at a previously unknown component of T forsythia's weaponry. It's a unique protease that they found encoded in T forsythia's genome and later named karilysin. Proteases are scissor-like enzymes that all forms of life use to snip specific cellular proteins and start, stop, or otherwise synchronize a range of biological activities. But microbes also secrete proteases as weapons, for instance, to degrade peptides that their host produces as a first line of defense against infection, called innate immunity. These pathogenic proteases, which poisonous snakes and insects also secrete in their venom, often belong to a family of enzymes called matrix metalloproteinases (MMPs). The researchers reported previously that karilysin closely resembles an MMP structurally. They also found that the protease inactivates the LL-37 antimicrobial peptide, suggesting one way that karilysin may contribute to chronic periodontitis. Now, they go further and elucidate the structure of one of karilysin's main catalytic domains, called Kly18, and take a remarkable turn into evolutionary biology. Comparing similar gene sequences across species, they found that the structure of Kly18 is evolutionarily much closer to the MMPs found in winged insects or mammals than those in bacteria. They concluded that the gene sequence encoding Kly18 must have been transferred from humans or another animal or insect to the bacterium, a biological phenomenon called horizontal gene transfer (HGT). "This proposed example of HGT entailing the shuffling of a metazoan [part of the animal kingdom] MMP to a human pathogenic bacterium, which involves distinct domains of life, is both a short circuit of Darwinian evolution and a testimony to the compatibility of proteins and cellular mechanisms despite evolutionary divergence over billions of years."

 


(Source: NIDCR, Science News in Brief, February 7, 2011)

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