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Review
, 280 (2), 139-52

Amyloid Formation: Functional Friend or Fearful Foe?

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Review

Amyloid Formation: Functional Friend or Fearful Foe?

P Bergman et al. J Intern Med.

Abstract

Amyloid formation has been most studied in the context of neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, as well as in amyloidosis. However, it is becoming increasingly clear that amyloid is also present in the healthy setting; for example nontoxic amyloid formation is important for melanin synthesis and in innate immunity. Furthermore, bacteria have mechanisms to produce functional amyloid structures with important roles in bacterial physiology and interaction with host cells. Here, we will discuss some novel aspects of fibril-forming proteins in humans and bacteria. First, the amyloid-forming properties of the antimicrobial peptide human defensin 6 (HD6) will be considered. Intriguingly, unlike other antimicrobial peptides, HD6 does not kill bacteria. However, recent data show that HD6 can form amyloid structures at the gut mucosa with strong affinity for bacterial surfaces. These so-called nanonets block bacterial invasion by entangling the bacteria in net-like structures. Next, the role of functional amyloid fibrils in human semen will be discussed. These fibrils were discovered through their property to enhance HIV infection but they may also have other yet unknown functions. Finally, the role of amyloid formation in bacteria will be reviewed. The recent finding that bacteria can make amyloid in a controlled fashion without toxic effects is of particular interest and may have implications for human disease. The role of amyloid in health and disease is beginning to be unravelled, and here, we will review some of the most recent findings in this exciting area.

Keywords: HIV; Paneth; amyloid; defensin; nets; semen.

Conflict of interest statement

Conflict of interest statement: The authors have no conflicts of interest to declare.

Figures

Fig. 1
Fig. 1
Human defensin 6 (HD6) nanonets entangle enteric pathogens in human ileal fluid. Scanning electron microscopy image of Salmonella typhimurium treated ex vivo with HD6 (10 μg/mL) in clarified human small intestinal luminal fluid aspirate (reproduced from [9]).
Fig. 2
Fig. 2
Mechanism of semen-derived enhancer of viral infection (SEVI)-mediated enhancement of HIV-1 infection. SEVI amyloid fibrils (black) sequester HIV-1 particles (red) and bind to the surface of target cells (yellow), thereby increasing viral attachment and fusion rates. The image was generated by Walther Mothes, Joseph Luna, and Pradeep Uchil, using a Nikon TE2000 microscope. Z-stacks of images were deconvoluted by using Volocity software from Improvision, and the final image was pseudocolored in Adobe Photoshop.
Fig. 3
Fig. 3
Salmonella typhimurium strain 4037/65 producing curli and cellulose (left) and a curli-only producing mutant (right). Reproduced from Werner Bokranz, PhD thesis.

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