The amyloid protein conformation, a structure that propagates as insoluble fibrils of aggregated β-strand-containing peptides, is a common feature of many neurodegenerative diseases. In Alzheimer, Parkinson, and Huntington diseases, as well as in prion diseases, toxic amyloids or their precursors are blamed for all manner of neuropathology. In fact, most researchers have never met a nonpathogenic amyloid in higher animals and tend to view the protein fold as a bad seed, a carryover from evolution with no physiologically redeeming features.
That view may have to change, now that Jeffery Kelly and colleagues at the Scripps Research Institute in San Diego have discovered an abundant amyloid protein structure that functions in normal pigment production in mammalian cells. In the 29 November PloS online, the researchers report that amyloid fibers of the Pmel17 protein pack melanosomes, organelles that produce the protective pigment melanin. The amyloid provides a scaffold for the polymerization of tyrosine derivatives into melanin, accelerating the process and possibly protecting cells from the toxic effects of reactive melanin precursors. These results show for the first time a nonpathological role of amyloid, and suggest that this much-maligned conformation may, in fact, be an evolutionarily conserved motif with a number of physiological functions.
Melanin, the stuff of suntans and the body’s main defense against ionizing radiation, is produced inside membrane-bound melanosomes that are then transferred intact to neighboring cells. Melanosome maturation requires Pmel17, a transmembrane glycoprotein that is processed in melanosomes to release fragments (Mα), which then self-assemble into fibers. Present in many organisms, from fungi and insects to humans, Pmel17 is known to be essential for pigmentation in mice, chicken, and zebrafish.
Douglas Fowler and Atanas Koulov, co-first authors on the paper, used dye binding and biophysical techniques to show that Pmel17 fibers in melanosomes adopt an amyloid structure. In purified melanosomes from cow retina, Mα fibers bound both thioflavin S and Congo red, two amyloidophilic dyes. The fibers were detergent-resistant, but could be destroyed by boiling in 10 percent SDS, characteristic of a stable amyloid structure.
When the researchers expressed and purified recombinant Mα in bacteria, they found that the fragment spontaneously self-assembled into amyloid in vitro. The fibrillization was fast, at least four orders of magnitude faster than observed with the Alzheimer Aβ peptide or synuclein. Biophysical measurements, including electron microscopy, x-ray diffraction, and circular dichroism measurements were all consistent with the presence of the amyloid cross-β structure.
The function of the Pmel17 amyloid was revealed by experiments that simulated melanin production in vitro. Melanin is formed by polymerization of highly reactive tyrosine derivates, and the presence of Mα fibers sped up this polymerization. Interestingly, the structure of the melanin precursor indole-5,6-quinone (DHQ) resembles the core structure of thioflavin T, which might explain its avid binding to the Pmel17 amyloid. Also, Aβ and synuclein amyloids could also accelerate melanin production in vivo, suggesting that the presence of the cross-β sheet structure was the salient feature that enhanced polymerization.
Since amyloid formation is a highly toxic process for cells, why don’t melanocytes suffer consequences from harboring such a nasty player? The authors speculate that the secret lies in the way that functional amyloid formation occurs under optimized, controlled circumstances. The trafficking Pmel17 restricts its cleavage to melanosomes, thus sequestering the amyloidogenic Mα peptide inside a membrane-limited organelle. Also, the rapid kinetics of amyloid formation may serve to protect cells from damage by amyloid precursors. Compared to Pmel17’s tidy process, the extracellular formation of amyloid in neurodegenerative diseases seems downright disorderly, and more studies of this contrast may lead to new insights into pathogenic amyloid formation. Right off, the discovery of physiological amyloid in mammalian cells raises some new questions about possible pitfalls to using nonselective inhibitors of amyloid formation to treat such diseases.
The authors believe that Pmel17 is just the tip of a functional amyloid iceberg. They propose using the name “amyloidin” for nontoxic, useful amyloid, with the expectation that many more examples of this type of structure will be forthcoming.—Pat McCaffrey
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- Fowler DM, Koulov AV, Alory-Jost C, Marks MS, Balch WE, Kelly JW. Functional amyloid formation within mammalian tissue. PLoS Biol. 2006 Jan;4(1):e6. PubMed.