The Deadly Mechanism of Amyloids

Amyloids are clusters of proteins that generate very stable fibrous structures, which are insoluble and thus lead to the formation of precipitates in an aqueous environment, as is the cells in our body. Amyloids are notorious mainly because of their involvement in neurodegenerative disorders of the nervous system, such as Alzheimer's and Parkinson's diseases, both of which are characterized by the accumulation of clumps of these precipitates in brain cells. In contrast to the amyloids related to these diseases, microorganisms, such as bacteria, produce functional amyloids that are involved in many activities that benefit the organism producing them.

Recently, investigators hypothesized that these functional amyloids take part in the disease mechanism of the violent Staphylococcus aureus bacteria, which cause severe infections. The bacteria secrete amyloid-forming proteins into the bloodstream that kill blood and immune cells. One of these proteins, the highly toxic 3alphaPSM, is related to the infectious potential of the Staphylococcus bacteria. Led by Assistant Professor Meytal Landau, researchers from the Faculty of Biology at the Technion unraveled a key step in this process and discovered a surprising finding regarding these bacterial amyloids.

Like all proteins, 3alphaPSM also consists of a chain of amino acids arranged like beads on a string. The protein’s function is determined, among other things, by the way this chain is organized into its three-dimensional structure. Two of the most common protein structural formations include a helix, in which the chain is organized spirally; and a sheet, in which the chain is organized as a flat structure.

Fibrils of amyloids involved in neurodegenerative diseases are constructed from flat sheets. However, when PhD student Einav Tayeb-Fligelman and Dr. Orly Tabachnikov used X-ray crystallography to examine the structure of this bacterial protein in detail, they were surprised to find that the fibers actually consisted of helical proteins.

Prof. Meytal Landau (left) with Einav Tayeb-Fligelman (center) and Dr. Orly Tabachnikov | Photograph: Nitzan Zohar, Technion

To understand the relationship between the amyloid protein structure and its toxicity, the researchers created mutations in the DNA of bacteria, resulting in the replacement of some amino acids in the protein chain, thus preventing fibers from being formed. Testing these mutated proteins on immune cell cultures revealed that most of their toxicity was lost, but when the fibers were allowed to be formed, the protein killed the immune cells in the culture. The researchers concluded that the helical structure is important for creating the fibers, and thus for the toxicity of the bacteria, although the spiral structure by itself, without the formation of protein fibers, resulted in a significantly reduced toxicity.

To prove that this was not a special case of toxicity against the cells of the immune system, the researchers repeated the experiment with another type of human cell – fetal kidney cells, a model system used in many other studies – and obtained similar results. In a paper published in the journal Science, they suggest that this activity is not specific to particular cells, but rather, is a general mechanism that allows bacteria to attack cells with the help of these functional amyloids.

A fiber constructed of sheets (on the right) and one constructed of helices. At the top: an illustration of the fibers attacking cells | Source: Dimedia, Technion

New Antibiotics

“What prompted us to investigate the atomic structure of 3alphaPSM was the desire to explore amyloid functioning in bacteria and to see how similar their structure is to the amyloid associated with Alzheimer's disease,” said Landau to the Davidson Institute website. “Although both types of amyloids form fibers, after we determined the structure of the bacterial protein at the atomic level, we found, to our surprise, that despite their general similarity to disease-related amyloids, the bacterial amyloid protein creates a fibrous structure that was not previously known.”

The next step for Landau and her colleagues is to find ways to use this discovery as the basis for searching for new antibiotics. “It is possible that unlocking the structure of the functional amyloid in bacteria will allow designing inhibitors that will disrupt its three-dimensional structure and thus neutralize its toxicity. Unlike antibiotics, this will not kill the bacteria but rather, will curb their violence and harm. It may also help overcome the problem of antibiotic resistance which already exists and is exacerbated as bacteria become more and more violent,” she says. “We still do not exactly know how the fibers kill the cells, but that should not prevent us from developing substances that will thwart their formation, and thus greatly reduce the bacterium’s lethal potential.”

And what about the amyloids involved in Alzheimer's disease? Although their fibers are composed of proteins in sheet structures, the researchers have not ruled out the possibility that the process of their formation includes an intermediate helical structure, which may contribute to their toxicity towards brain cells. “Hopefully, our discovery may also open doors to the study of mechanisms associated with Alzheimer's disease,” says Landau.

Watch the Technion’s video about the study: