A recent article in PLoS One (Thanks Daniel!) suggests that the amyloid beta (Abeta) proteins that aggregate to form fibrous plaques in the brain tissue of Alzheimer victims, function as typical defensive anti-microbial peptides (AMPs), similar to the LL-37 cathelicidin implicated in facial tissue in rosacea. The structural and functional similarities of Abeta and LL-37 suggest to me that Alzheimer’s and rosacea may also be similar in initiation and treatment. Let’s compare amyloids and AMPs.
[The figure shows a model protein (from ref.) used to examine stain binding to amyloids. The stains appear to bind to aromatic amino acids spaced evenly between adjacent proteins, but adjacent basic amino acids (blue) are spaced the same way and provide sites for heparin binding.]
- Amyloid proteins/peptides align into stacks and fibers
- Stacked beta sheets bind amyloid stains: Congo Red, Thioflavin-T
- Fibers form on anionic polymers: heparin, DNA
- Short amyloid stacks are toxic to cells
- Proteases produce multiple sizes of amyloid peptides
- AMPS typically contain heparin-binding domains -- basic peptides/ plus charge
- Some AMPs, e.g. LL-37, form fibers on DNA, heparin (stain with amyloid stains)
- Toxic to cell membranes
- Kallikrein stimulated by gut flora migrates to face and clips LL-37 to a smaller peptide that binds to host DNA and stimulates the TLR receptor to produce inflammation
- Stomach pepsin hydrolyzes dietary proteins into anti-microbial peptides (heparin is secreted by mast cells onto to the intestinal surface to protect from any amyloid-like effects)
- Defensins, cathelicidins and other AMPs are under transcriptional control of vitamin D receptor
Abeta Is Anti-microbial Like LL-37
Amyloid beta is the well-known source of the fibrous plaques forming brain lesions in Alzheimer’s disease. The normal function of Abeta has not been firmly established. The recent article shows data to support Abeta as an anti-microbial peptide comparable to LL-37 against several pathogenic bacteria and yeast. Knock-out mice deprived of a gene corresponding to Abeta are susceptible to bacterial infections. The anti-microbial activity present in extracts from Alzheimer’s disease brains was inactivated by anti-Abeta antibodies.
Implications of Abeta as an AMP Like LL-37
The similarities between AMPs and amyloid peptides suggest some implications for both Alzheimer’s disease and rosacea. Vitamin D is a hormone that binds to a cytoplasmic receptor and the vitD/receptor complex then acts as a transcription factor that controls the expression of defensins in the intestines, LL-37 in facial skin and perhaps Abeta in brains.
Amyloids form fibers on a scaffolding of heparan sulfate (HS). There is usually an excess of HS on the surface of cells and the HS is rapidly recycled back into cells. During inflammation, mast cells release heparin, short fragments of HS, that should also inhibit amyloid fiber formation on HS. Chronic inflammation, however, reduces HS production and may set the stage for amyloid fiber formation. HS metabolism of the brain may be vitally important to the development of Alzheimer’s disease, especially since the increasing chronic inflammation of aging people should deplete brain HS.
LL-37 forms complexes with DNA from damaged host cells in rosacea skin. The LL-37/DNA complexes trigger TLRs and inflammation. LL-37 may normally bind to cell surface HS and chronic inflammation of the skin may cause the shift to pathogenic autoinflammation. Topical application or perhaps low dose IV heparin may be effective in disrupting the autoinflammation due to LL-37. Part of the toxicity of LL-37 in the skin may be due to amyloid like structures that could form with inadequate HS and overabundant LL-37 production. Vitamin D metabolism should also be very important, since LL-37 synthesis is controlled by vitamin D. This is consistent with the benefits that some rosaceans observe with high doses of vitamin D3 supplements.
Soscia SJ, Kirby JE, Washicosky KJ, Tucker SM, Ingelsson M, Hyman B, Burton MA, Goldstein LE, Duong S, Tanzi RE, Moir RD. The Alzheimer's disease-associated amyloid beta-protein is an antimicrobial peptide. PLoS One. 2010 Mar 3;5(3):e9505.
Abedini A, Tracz SM, Cho JH, Raleigh DP. Characterization of the heparin binding site in the N-terminus of human pro-islet amyloid polypeptide: implications for amyloid formation. Biochemistry. 2006 Aug 1;45(30):9228-37.
Biancalana M, Makabe K, Koide A, Koide S. Molecular mechanism of thioflavin-T binding to the surface of beta-rich peptide self-assemblies. J Mol Biol. 2009 Jan 30;385(4):1052-63. Epub 2008 Nov 14.