What the urinary tract’s front-line defenses can teach us about our innate ability to self-heal…and thwart antibiotic resistance.
Antibiotic resistance is on the rise, health care-acquired infections are becoming harder to treat and even simple infectious illnesses account for billions of dollars per year in spending in the United States alone. As with most health science challenges, there is no magic bullet, no one-size-fits-all solution. But there is an increasingly attractive opportunity for a solution to these problems: the body’s ability to self-heal.
Imagine a therapy that boosts our own natural abilities to combat bacteria and defeat infection. That’s the hope of many researchers studying human antimicrobial peptides, a group of cell types with potent antibiotic activity that naturally occur in the skin, colon and a number of other organs. These killer cells shield the body from daily assaults by infection- causing pathogens, helping the body heal itself or even prevent illness altogether.
Antimicrobial peptides are champions of innate immune function. Although they often exist only in small concentrations, they can be lethal to a wide range of enemies. They are potent and effective and, even when they succumb to invaders, they go down fighting, ensuring a coordinated response from the acquired immune system.
Innate immunity’s role as a first-line defense for the body is well known, but some of its many mechanisms still evade scientific understanding. Antimicrobial peptides are increasingly being studied for their ability to teach us two things: how the body protects itself and how we can design therapies with the power to thwart antibiotic resistance.
More than 23,000 people in the United States die each year from infections caused by antibiotic-resistant bacteria. An endogenous defender that kills even drug-resistant bacteria at concentrations far smaller than those of typical antibiotic medications would seem too good to be true. And yet, researchers across the country are studying just that.
Present in everything from algae and agriculture to rabbits and human beings, antimicrobial peptides — AMPs for short — are a highly preserved defense mechanism. The first animal AMPs were discovered in silk moths and rabbits. But our current understanding of AMPs owes its origin to novel work with an equally unexpected creature: the African clawed frog, Xenopus laevis. In the 1980s, Michael Zasloff, MD, PhD, then a scientist at the National Institutes of Health, was studying eukaryote RNA expression in the ovaries of frogs. He noticed that the surgical sites of the amphibians rarely got infected, despite non-sterile surgical procedures and the less-than-sanitary tank water the animals inhabited.
The AMP group he discovered, which he named magainins, offered researchers a glimpse into the arsenal of the animal kingdom’s defenses against microorganisms. The field exploded. Human counterparts were identified almost immediately and the ranks of known AMPs in all species has since swelled to over 2,000.
Less than 15 years after his landmark discovery of magainins, Dr. Zasloff, now a professor of surgery at Georgetown University School of Medicine, began another milestone in AMP history: he is turning a frog-derived AMP into a therapeutic for human use. The antimicrobial skin cream, called Pexiganan, targets infected foot ulcers in diabetics and is on track to become one of the first medicinal applications of AMP research approved by the Food and Drug Administration.
The therapeutic potential of AMPs may be broadly recognized, but scientists’ understanding of the peptides’ various functions remains limited. For instance, researchers don’t know why, if killer AMPs are truly as potent as they suspect, people still fall prey to infection.
They also don’t know the best way to harness AMPs for medicines that boost the body’s self-immunity or if over-expressing AMPs to fight an infection could be harmful. In short, scientists have a lot to learn.
According to research on the human urinary tract by John David Spencer, MD, andBrian Becknell, MD, PhD, nephrologists at Nationwide Children’s Hospital, a bit of urine may help resolve some of AMPs’ most fundamental mysteries.
The human kidneys and urinary tract are bombarded with nasty bacteria, but only about 3 percent of all children develop urinary tract infections each year. Although that accounts for more than 1 million visits to the pediatrician annually, Dr. Zasloff theorizes the number would be much higher without the urinary tract’s family of front-line defenders.
“I think AMPs are pretty much the reason why most of us — men, women and children — do not have daily infections of the urinary tract,” says Dr. Zasloff, who also is scientific director of Medstar Georgetown Transplant Institute. He credits much of that knowledge to the work of Drs. Spencer and Becknell, who have spent the past five years characterizing the innate immune defense of the urinary tract. They started with only the basic knowledge that the kidneys housed an AMP known as beta-defensin, identified from an isolated project from another teamabout a decade ago. Now, their studies of the human and mouse urinary tracts have revealed a new family of ribonucleases. Called the RNase A Superfamily, these enzymes have a number of appealing microbicidal functions that could be exploited for drug development.
“It’s an absolutely fundamental discovery,” says Dr. Zasloff of their research, begun under the guidance of Andrew Schwaderer, MD, research director of Nephrology at Nationwide Children’s, and former colleague David S. Hains, MD, now program director of nephrology fellowship training at Le Bonheur Children’s Hospital. “They’ve shown that in a state of health, the kidney is protected from microbial invasion by its ability to produce very high concentrations of some very powerful antimicrobial agents.”
Clearly, many people still suffer from UTIs, some of whom contract recurrent, often-serious infections. Drs. Spencer and Becknell, both principal investigators in theCenter for Clinical and Translational Research at The Research Institute, believe that AMP production — or lack thereof — may be at the root of that problem, too.
“There is incredible genetic diversity in AMP production among humans,” Dr. Becknell explains. “Sometimes the differences can be so great that certain individuals might not express a certain AMP at all. And if that AMP is required by the body to shield against infection, that might be the individual who’s getting UTIs.”
Nephrologists may have an advantage in studying the range of human AMP diversity and production. Patients with chronic kidney infections often require hospitalization and catheterization, so urine samples are easy to collect. Scientists could study AMP activity during and after infection as well as variation among patients or from day to day — even hour to hour — within individuals. Catheterization for these patients also presents a potential fix for another AMP challenge: mode of administration.
“Whether we end up administering a drug that contains synthesized AMPs or something to stimulate the body’s own AMP defenses, we need to find the best way to deliver it,” Dr. Spencer explains.
As with any pharmaceutical, the mode of administration matters, but localized or targeted delivery may be especially important for these antimicrobial forces. Upregulating them throughout the body may not be ideal, Drs. Spencer and Becknell theorize, so direct administration of AMPs or an upregulator — on a catheter already being placed for individuals with kidney infections, for example — could offer a way to avoid systemic effects.
“It’s a simple but compelling vision to try to induce your own antimicrobial forces against infection,” Dr. Becknell says. “But it’s also predicated on this assumption that more AMPs are actually good for us.”
Not ones to rely on assumptions, Drs. Becknell and Spencer are currently studying AMP upregulation in mice to determine whether high concentrations are toxic to the host. Certain diseases, such as psoriasis, have been tentatively linked to overexpression of these innate immunity defenders. If AMPs do have negative impacts when over-activated, local upregulation may become a crucial requirement of turning these defense mechanisms into therapeutics.
“Could AMP upregulation alter the natural bacterial flora of certain organs? Could it harm our healthy, native tissue?” Dr. Spencer asks. “We don’t know yet.”
Learning how AMPs truly function in vivo is a critical first step in developing the concept of these defenders as self-healing medicine, the team says.
Thankfully, the challenge of targeted administration has been overcome before. Drugs to treat UTIs, for example, tend to work best when they are metabolized by the kidneys instead of the liver, and their effects are concentrated in the urinary tract.
“The one advantage AMPs have over all conventional antibiotics is that we make them,” Dr. Zasloff says, “And in principle, we could turn them on or up to higher levels should we need to do so.” Even in very specific areas, he suggests. If researchers can design a pill that upregulates or synthesizes AMPs and primarily affects the kidneys and bladder, it could minimize systemic effects and target the urinary tract for receipt of its medicinal payload.
Once such barriers are overcome, AMP researchers believe the potential of these peptides to evade the modern problem of antibiotic resistance is immense.
Unlike most antibiotic drugs, AMPs target and defeat invading bacteria by attacking parts of the pathogens’ membranes that cannot be easily altered. That means that AMPs may not lose potency due to antibiotic resistance.
“If these things have been there since Genesis Chapter One and they’re still having efficacy,” Dr. Spencer says, “the chances of pathogens developing resistance is probably lower.”
These killer cells are also active against a broad spectrum of bacteria, which could make upregulation of even a single AMP an effective treatment for multiple pathogens, he says. But before the nation defects from antibiotics and starts clamoring for synthesized AMP pills, researchers need to determine whether it’s best to maintain a higher level of defenses or to simply boost our response during infection.
“Your innate immune system is there at baseline, keeping things at bay,” Dr. Spencer says. “But it’s like any machine — it’s probably not good to leave it revved up all the time. Maybe it could burn out, resistance could develop to it or being so fired up all the time could harm the body or the tissue it’s trying to protect.”
For these reasons, the researchers believe that AMP therapeutics will likely take the shape of short-course treatments rather than a long-term drug, although they are unwilling to rule that option out.
“This is just another reason why kidney patients are great candidates for helping us open the door to AMP-based therapeutics,” says Dr. Becknell. “They would be some of the patients most likely to benefit from something that could replace preventive antibiotic drugs.”
And although the search for AMP therapeutics is just getting started, Dr. Becknell is hopeful that the journey won’t take too long. “There may be an existing FDA-approved drug that upregulates AMPs and we just don’t know it yet — it could be a drug for blood pressure, diabetes, an existing antibiotic or even a vitamin,” he says.
The team is currently working to connect the dots of the AMP profile of the urinary tract and individual AMPs’ effectiveness at preventing urinary tract infections. They are also beginning to explore mechanisms and tools for upregulating AMP production and whether high-risk patient populations for these infections have a decreased production of any particular AMPs.
Whatever the challenges, these AMP researchers are optimistic. “I think traditional antibiotics will always have a place in medicine,” Dr. Spencer says, “But I don’t think it is farfetched to imagine a future where AMPs have a significant role in the treatment of a wide range of infections.”
Join the conversation. What would you still want to know about AMPs before prescribing an upregulator to your patients?
1. Becknell B, Eichler TE, Beceiro S, Li B, et al. Ribonucleases 6 and 7 have antimicrobial function in the human and murine urinary tract. Kidney International. 2014 Jul 30. [Epub ahead of print.]
2. Becknell B, Spencer JD, Carpenter AR, Chen X, et al. Expression and antimicrobial function of beta-defensin 1 in the lower urinary tract. PLoS One. 2013 Oct 21, 8(10):e77714.
3. Spencer JD, Hains DS, Porter E, Bevins CL, et al. Human alpha defensin 5 expression in the human kidney and urinary tract. PLoS One. 2012, 7(2):e31712.
4. Spencer JD, Schwaderer AL, Becknell B, Watson J, Hains DS. The innate immune response during urinary tract infection and pyelonephritis. Pediatric Nephrology. 2014 Jul, 29(7):1139-49.
5. Zasloff M. Magainins, a class of antimicrobial peptides from Xenopus skin: Isolation, characterization of two active forms, and partial cDNA sequence of a precursor. Proceedings of the National Academy of Sciences of the United States of America. 1987 Aug, 84(15):5449-53.
Correction Appended: The print magazine issue of Pediatrics Nationwide listed Michael A. Zasloff as Michael A. Zasloff, PhD, and this title has been corrected in the online version of the article to reflect both his medical and doctoral degrees, listing him as Michael A. Zasloff, MD, PhD.