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Neurodegenerative Diseases

Interrupted communication

Blocking of chemical signal emitted by the healthy version of the prion could result in Alzheimer’s disease and brain tumor treatments

RICCARDO CASSIANI-INGONI / SCIENCE PHOTO LIBRARYBeautiful and aggressive: astrocyte, brain cell that reproduces itself uncontrollably in a glioblastomaRICCARDO CASSIANI-INGONI / SCIENCE PHOTO LIBRARY

Cell biologists Marco Prado and Glaucia Hajj spent last March 13in a dark room of a gothic palazzo on the banks of the Grand Canal, Venice’s busiest transportation route. They sat for nearly three hours at the Veneto Institute of Science, Letters and Art, listening to foreign researchers give presentations on their research projects, focused on Alzheimer’s Disease. The researchers spoke about their recent findings, which associates the origin of Alzheimer’s Disease to the interaction between the beta-amyloid oligomer, a clump of protein fragments that are toxic to the brain cells, and the cellular prion, a protein naturally produced by the body. The cellular prion protects the central nervous system.

Marco and Glaucia were not surprised. The Brazilian researchers and their collaborators in São Paulo and in Rio de Janeiro had already shown, in the last few years, that the cellular prion (PrPC) is crucial for the healthy development and survival of neurons. Last year, the group observed that the beta-amyloid prevents the proper functioning of the PrPC, a phenomenon that seems to be common in the early stages of Alzheimer, before the cells degrade and die.

As is often the case, none of the speakers mentioned the Brazilian research studies. At the end of the presentations, Marco concluded that “the speakers are seeing something we had observed years ago. At some point they will realize that they´re reinventing the wheel.” He decided to remain silent to avoid drawing the attention of research groups working at bigger research institutions and that have more experience with Alzheimer’s Disease. “In this case, I preferred to keep quiet and listen,” the researcher commented when he went back to Canada, where he heads a laboratory at the University of Western Ontario.

Marco and his Brazilian colleagues have good reasons to avoid exposure at this time. He and biochemist Vilma Martins, of São Paulo’s A. C. Camargo Hospital, are waiting for two major papers on the role of the cellular prion in brain diseases to be published in the next couple of months. One of these goes one step beyond the ideas discussed in Venice. In this paper – about which Vilma and Marco do not give any details – they present evidence that interfering with the communication between the beta-amyloid and the PrPC might avoid the toxic effects caused by the oligomer that is formed during the initial stages of Alzheimer’s Disease.

In the studies published in the Journal of Biological Chemistry and in the Faseb Journal, they described how cell signaling, intermediated by the PrPC, i , involves the participation of other proteins of the membrane that play a major role in Alzheimer’s Disease.

The Brazilians were the first researchers to investigate the proteins that, like beta-amyloid, also bind with PrPC – especially the stress inducible protein-1 or STI-1. Vilma has studied this protein since the 1990s, when she began to work with oncologist Ricardo Brentani; she pioneered the production of the protein’s synthetic version. At the beginning of this year, Vilma and Marco obtained a temporary patent in the United States, to use the protein as a neuroprotector.

By means of experiments conducted in the last 15 years, Vilma and her team have shown that the STI-1 cannot be separated from the cellular prion. This protein, produced by the astrocyte, another brain cell, travels throughout the extracellular medium up to the surface of the neuron, where it binds with the cellular prion protein and triggers chemical commands that allow the cell to survive. Vilma is now trying to use this protein to block the toxic effect of beta-amyloid.

When combined with other groups, these results generate a more complex and comprehensive understanding of how neurodegenerative diseases are triggered and evolve in association with the malfunctioning of the cellular prion protein.

057_Prion_194Tiago CirilloThe Brazilian believes that the PrPC acts like an extra-cellular information manager. Molecules from the extracellular medium, such as beta-amyloid or STI-1, bind with the PrPC to form a complex that slips through the cell’s membrane and interacts with the other proteins on the surface of the neuron, namely, the cell receptors responsible for making information from the extracellular medium travel to the inside of the cell. The effects can be protective or toxic, depending on which molecule binds with the PrPC.

This hypothesis, presented by the Brazilian researchers five years ago, has led the search for new strategies to deal with diseases such as Alzheimer’s and spongiform encephalopathy – including mad cow’s disease and the human form of this disease – and Creutzfeldt-Jakob disease, caused by a deformity of the PrPC.

Under the coordination of Rafael Linden, from the Federal University of Rio de Janeiro (UFRJ), Vilma, Marco, Iván Izquierdo, from the Catholic University of Rio Grande do Sul, and Ricardo Brentani, former president of the foundation that runs the A. C. Camargo Hospital and former director-president of FAPESP (Brentani passed away last November) conducted a broad review of the role of the cellular prion protein. In the paper, published in 2008 in Physiological Reviews, they suggested that the death of neurons in diseases caused by prions is not only due to the toxic effect of deformed PrPC but also to the loss of the protection provided by the cellular prion.

The Brazilians suggest that the approach to those diseases might also be applicable to the early stages of Alzheimer’s. The link between the prion-caused diseases and Alzheimer’s is that in both cases the signaling of the PrPC is cut off, but for different reasons. In the case of cellular prion-caused diseases, the reason is the defect in the PrPC itself. In the case of Alzheimer’s Disease, it is that the action of the cellular prion protein is blocked by the beta-amyloid. “We are not stating that toxicity does not kill the cell,” says Vilma. “We believe that, in addition to this process, the cell also dies because the cellular prion is no longer protecting it.”

The proper functioning of the cellular prion is essential to maintain the neurons alive. Over the last decade, Vilma, Marco, Brentani, Rafael and other Brazilian researchers have gathered evidence that the cellular prion triggers chemical reactions that protect the cells from programmed cell death and stimulate the development of the neurites, the branches that connect the neurons to each other. In addition, the cellular prion is crucial to form memory (see Pesquisa FAPESP issue 148).

The beneficial effects are only seen in healthy bodies. As the body ages, it begins to process, abnormally, a protein that goes through the neurons’ membrane; more specifically, this is the amyloid precursor protein. The result is an accumulation of fragments (peptides) that bind to each other and form small groups, referred to as beta-amyloid oligomers.

Healthy neurons, with preserved synapses, and neurons affected by beta-amyloid (in red)

GLAUCIA HAJJ / HOSPITAL A.C.CAMARGO Healthy neurons, with preserved synapses, and neurons affected by beta-amyloid (in red)GLAUCIA HAJJ / HOSPITAL A.C.CAMARGO

In 2009, the group headed by Yale University’s Stephen Strittmatter –one of the speakers at the event in Venice – showed that these agglomerated fragments bind with the cellular prion. This finding had a huge impact, because it established an unexpected connection between prion-caused diseases – which are rare in human beings – and Alzheimer’s Disease, the most common neurodegenerative disease in elderly people.

The paper, published in Nature, provided more breathing space for the American and European laboratories that had been investigating the infectious action of prions and whose research projects had been sidelined because of the global economic crisis that blew up in 2008. However, the paper failed to answer an important question: what happens after the beta-amyloid binds with the cellular prion?

The research teams headed by Marco and Vilma found the answer through tests conducted in cooperation with Fernanda De Felice and Sergio Ferreira – researchers from the UFRJ who study the origins of Alzheimer’s Disease.

The beta-amyloid corrupts the transmission of information that comes from outside into the neuron. When it adheres to the PrPC, the beta-amyloid prevents it from being dragged by the neuron, in a temporary plunge that advises the cell to branch out. In 2011, Fabiana Caetano, Flavio Beraldo and Glaucia published a paper in the Journal of Neurochemistry where they showed that the protective effects may disappear if the plunge does not occur.

The evidence showing that the beta-amyloid blocks PrPC on the outside of the membrane reinforced the hypothesis that, in the case of Alzheimer’s Disease, especially in the initial stages, the toxic effect of the oligomers is preceded by alterations in the functions of the cellular prion. Other papers support this hypothesis. In an article scheduled for publishing in July in the Prion journal, Nigel Hooper, from England’s University of Leeds (another researcher who attended the event in Venice) states that he had been able to detect lower levels of PrPC in the brains of people with Alzheimer’s Disease – but only in cases of spontaneous Alzheimer’s Disease, i.e., non-hereditary ones.

“Loss or corruption of a function is not the only factor, but it is an important one,” Vilma explains. Recently, she and Marco began a study to verify the efficacy of STI-1 in inhibiting the beta-amyloid’s adhesion to the cellular prion in animals. They intend to induce genetically modified mice to produce the symptoms of Alzheimer’s Disease to check whether it is possible to block the progress of the disease.

Vilma is also exploring the interaction between the STI-1 and the cellular prion to treat glioblastoma, another serious disease of the central nervous system. This aggressive brain tumor, which leads to death in a few months, results from the uncontrolled proliferation of cells derived from astrocytes that nourish and protect the central nervous system from invading microorganisms.

The astrocytes launch this prion-activating protein into the extracellular medium; this protein acts on the neurons and on the astrocytes themselves. While it promotes the differentiation of the neurons and the self-renovation of the neural precursor cells, as observed by Tiago Santos, from the A.C. Camargo Hospital, and by Marilene Lopes, from the University of São Paulo, the STI-1 blocks the reproduction of the astrocytes in a healthy brain. Physician Rafael Erlich, while working at the laboratory run by biochemist Vivaldo Moura Neto at the UFRJ, noticed that glioblastoma cells also secrete STI-1. In this case, however, the protein triggers the proliferation of tumor cells.

The strategy imagined by the research group is to block the cellular prion’s activity – without which the cell is unable to proliferate – by means of a chemical competition, but without using the STI-1, which is at the root of the problem. To deal with this issue, the researchers decided to use a synthetic fragment of this protein that adheres to the PrPC without activating it. The peptide, which was patented by Vilma when she was working at the Ludwig Institute for Cancer Research, has already been tested on mice with human glioblastoma. The results look promising. The peptide delayed the growth of the tumor and preserved the animals’ cognitive capacity; the cognitive capacity is altered at the advanced stages of the disease. For the time being, however, it is impossible to predict whether these strategies will lead to the development of a medical drug. Vilma points out that “things that work on animals don’t always produce the same effects on people.”

The Project
Mechanisms associated with the function of the prion protein and its STI-1/Hop binder: therapeutical approaches (nº 2009/14027-2); Modality Thematic Project; Coordinator Vilma Regina Martins – A.C. Camargo Hospital; Investment R$ 1,700,557.50 (FAPESP)