The immune system of mucosal tissues must effectively protect the host from pathogen invasion, while facilitating homeostatic interactions with a diverse colonizing microbiota. A clear understanding of the key molecules and mechanisms that achieve this delicate balance remains incomplete, leaving a gap in critical knowledge. This investigation, AI32438-16, has focused on a-defensins HD5 (DEFA5) and HD6 (DEFA6) as key effector molecules of innate immunity. These a-defensins are the most abundant antimicrobial peptides of human Paneth cells and compelling evidence shows that dysrupted Paneth cell function increases susceptibility to enteric pathogens, as well as to chronic inflammatory bowel disease. While much is known about the structure, microbicidal activities, and biological functions of HD5, comparatively little is known about its small intestinl partner, HD6 - an abundant ?-defensin that is exceptionally well-conserved among primates. We recently discovered that HD6 has a novel mechanism of protective action. Unlike HD5 and other defensin peptides that protect the host through microbicidal activity, HD6 does not kill microbes but rather protects the intestinal mucosa by blocking microbial invasion. Our preliminary and newly published data support a mechanism that involves initial binding of HD6 to surface proteins of microbes, followed by self-assembly of additional HD6 peptide molecules to form microbe-entangling nanofibrils and nanonets, which prevent penetration into host cells. The objectives for this application are 1) to define the molecular target on the microbe surface that initiates initial binding of HD6 peptides; 2) elucidate the structural features of HD6 that promote self-assembly; and 3) determine how HD6 mediates its protective function at the epithelial surface. We propose a combination of complementary approaches to accomplish these aims. By determining the fundamental features of HD6 function, these experiments will characterize a novel mechanism of protection afforded by the innate immune system. Successful completion of these studies will have broad impact on our mechanistic understanding of innate immunity, as identification of the target for HD6 binding may reveal a new pathogen-associated molecular pattern (PAMP), and the structure-function analyses of HD6 will yield fundamental knowledge on mechanisms of peptide self-assembly. Our goal to elucidate the essential determinants of HD6 mucosal protection will fill a void in knowledge on a conserved, highly abundant human defensin.

Public Health Relevance

Defensins are fundamental immune molecules found throughout nature that both provide critical host defense from pathogenic microbes and help maintain homeostasis at mucosal surfaces of the body. Relatively little is known about human defensin 6 (HD6, DEFA6), a highly expressed ?-defensin that is released into the lumen of the small intestine by specialized secretory (Paneth) cells. By elucidating the mechanism by which HD6 functions, these studies will provide a frame work for how ?-defensins can self-assemble into protective macromolecular complexes and should yield insight into why Paneth cell dysfunction is associated with susceptibility to infectious disease and pathogenesis of chronic inflammatory disease.

Agency
National Institute of Health (NIH)
Institute
National Institute of Allergy and Infectious Diseases (NIAID)
Type
Method to Extend Research in Time (MERIT) Award (R37)
Project #
5R37AI032738-18
Application #
9207722
Study Section
Host Interactions with Bacterial Pathogens Study Section (HIBP)
Program Officer
Rothermel, Annette L
Project Start
1993-07-01
Project End
2020-01-31
Budget Start
2017-02-01
Budget End
2018-01-31
Support Year
18
Fiscal Year
2017
Total Cost
$382,052
Indirect Cost
$105,302
Name
University of California Davis
Department
Microbiology/Immun/Virology
Type
Schools of Medicine
DUNS #
047120084
City
Davis
State
CA
Country
United States
Zip Code
95618
Bergman, P; Roan, N R; Römling, U et al. (2016) Amyloid formation: functional friend or fearful foe? J Intern Med 280:139-52
Underwood, Mark A; Arriola, Jennifer; Gerber, Colin W et al. (2014) Bifidobacterium longum subsp. infantis in experimental necrotizing enterocolitis: alterations in inflammation, innate immune response, and the microbiota. Pediatr Res 76:326-33
Leonard, Brian C; Marks, Stanley L; Outerbridge, Catherine A et al. (2012) Activity, expression and genetic variation of canine β-defensin 103: a multifunctional antimicrobial peptide in the skin of domestic dogs. J Innate Immun 4:248-59
Underwood, Mark A; Kananurak, Anchasa; Coursodon, Christine F et al. (2012) Bifidobacterium bifidum in a rat model of necrotizing enterocolitis: antimicrobial peptide and protein responses. Pediatr Res 71:546-51
Chu, Hiutung; Pazgier, Marzena; Jung, Grace et al. (2012) Human α-defensin 6 promotes mucosal innate immunity through self-assembled peptide nanonets. Science 337:477-81
Bevins, Charles L; Salzman, Nita H (2011) Paneth cells, antimicrobial peptides and maintenance of intestinal homeostasis. Nat Rev Microbiol 9:356-68
Biswas, Amlan; Liu, Yuen-Joyce; Hao, Liming et al. (2010) Induction and rescue of Nod2-dependent Th1-driven granulomatous inflammation of the ileum. Proc Natl Acad Sci U S A 107:14739-44
Salzman, Nita H; Hung, Kuiechun; Haribhai, Dipica et al. (2010) Enteric defensins are essential regulators of intestinal microbial ecology. Nat Immunol 11:76-83