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Biochemistry

Preserved memories

Insulin protects the brain from Alzheimer

marcelo vieira/ufrjGlimmer of hope: hormone might prevent the disease that wipes out the pastmarcelo vieira/ufrj

Recently, neuroscientist Fernanda De Felice and biochemist Sergio Teixeira Ferreira had to hone the skills that always helped the meticulous detective Sherlock Holmes solve his cases:  namely, observation and deductive reasoning. Of course the pair of researchers from the Federal University of Rio de Janeiro, were not trying to solve crimes without using a magnifying glass or a pipe. They were pursuing something possibly more complicated.  They were seeking to understand the biochemical mechanisms through which Alzheimer’s Disease, one of the most devastating diseases discovered in the last century, destroys the neurons, the brain cells responsible for transmitting and storing information, leading to a definitive loss of memory.

Based on the work conducted in their own lab and on clues raised by other teams, last year Fernanda Ferreira confirmed the existence of a connection – unsuspected until a short time ago – between Alzheimer´s and type 2 diabetes, another disease common to elderly people. In both cases, the diseases occur because the cells become unable to react adequately to the insulin hormone, which has different functions in different parts of the body – in the muscles, it facilitates the absorption of glucose, the body’s main source of energy, while in the brain it helps form the memory.

Now, in partnership with a team from the United States, the Rio researchers showed that this same insulin, which is not fully taken advantage of in the case of Alzheimer’s, can prevent damage to the neurons. In lab tests, this protective effect is intensified when the insulin is associated with a drug called Rosiglitazone, used for the treatment of type 2 diabetes, the group reported in an article published in February in the Proceedings of the National Academy of Sciences (PNAS).

These results provide a clearer and more detailed understanding of how Alzheimer’s begins and progresses.  In its initial stage, the disease manifests itself by means of short memory lapses, such as forgetting where one left one’s wallet; as it progresses, it destroys even the deepest memories, and entirely erases the past. The findings have also created the opportunity of developing more efficient and less aggressive alternatives for the treatment of this disease, the incidence of which is increasing as people live longer – according to the World Health Organization, there are currently 18 million people worldwide suffering from Alzheimer’s;  this number is expected to double in the next 15 years.

“In spite of this progress, we are far from being able to offer a new therapy for patients”, says Ferreira. First, they will have to verify whether the protective effect of insulin that he and Fernanda observed in the neurons isolated on a glass slide occurs in live beings. If the tests they plan to initiate this year on animals are successful, then it will be possible to evaluate this treatment strategy on humans. Before, however, it will be necessary to find an efficient way of making the insulin reach the brain, because only part of the  hormone injected into the blood passes through the membrane that protects the brain and other parts of the central nervous system. “We have years of work ahead of us”, says the biochemist.

The first signs of a connection between Alzheimer’s and diabetes appeared five years ago. In the United States, a team coordinated by Juliette Janson, from the Mayo Clinic, and Peter Butler, from the University of Southern California, analyzed samples of  the brain and pancreas of 105 people – one group with Alzheimer’s, a second group with type 2 diabetes, and a third group comprised of healthy people. They noticed that diabetes was twice as frequent in Alzheimer patients as in the healthy people. They also verified that patients with Alzheimer’s had lesions in the pancreas similar to the lesions that this neurodegenerative disease leaves in the brain, although they did not find more damage in the neurons of the type 2 diabetes patients than in non-diabetic people, according to an article published in 2004 in Diabetes. Apparently, diabetes and the death of the insulin-producing cells were a consequence of Alzheimer’s.

The following year, the group led by Suzanne de la Monte, from the US’s Brown University, presented findings that reinforced this connection, except that the findings pointed in the opposite direction. The researchers knew that the brain of Alzheimer’s patients had more difficulty to use glucose, the sugar that is converted into energy by the cells.  Suzanne verified that the problem was not the lack of glucose or insulin.  In a brain affected by Alzheimer’s, the insulin did not produce the desired effects because the pathway was blocked. Produced by a special group of pancreatic cells and released in the blood, the insulin needs to adhere to a protein on the surface of the cells, the so-called insulin receptor, to open up the way for the glucose or, in the brain, trigger the necessary chemical reactions to acquire and consolidate memory.  If the insulin fails to find the receptor, this does not happen.

Without insulin, plaques of peptides, in red, adhere to the neuron and eliminate the synapses (green dots). With insulin: plaques (red dots) disappear and the neuron preserves active synapses

Withdrawn antennas
The team from Brown University also noticed that the genes that contain the information to produce the insulin receptors were less active in the neurons of people with Alzheimer’s than in the healthy individuals.  Through an unknown mechanism, the brain affected by Alzheimer’s no longer produces these receptors that work like radio antennas capturing the chemical signs brought by the insulin and retransmitting them to the inside of the cells. The production of the insulin receptors dropped even more as the disease progressed.

In view of these clues, Fernanda and Ferreira decided to find out what was disconnecting the gene of the insulin receptor and making these neurons put away their antennas that maintain them in contact with the outer surface.  They then had a hunch. They figured that the reduction of the receptors was associated with the existence of plaques of a protein fragment – called beta amyloidal peptide – that form in the brain of Alzheimer’s patients. Produced by the normal degradation of a protein that is important for the functioning of the neurons, this peptide bonds with similar molecules on the outside surface of the cells, forming the so-called oligomers.

But could it actually be the oligomers that caused the reduction of the insulin receptors? During the period that she spent working in William Klein’s laboratory at Chicago’s Northwestern University from  2005 to 2007, Fernanda prepared a simple test to clarify the doubt. She placed mice neurons on a glass slide and added a red compound that adhered to the insulin receptors.  Then she added the beta amyloidal oligomers, marked with a green protein, and waited to see what would happen.  She noted that the plaques adhered to the surface of the neuron closest to the insulin receptors.  Half an hour after beginning the experiment, 22% of these receptors had been collected by the cell, and three hours later, 70% of them had disappeared.

As a result, the neurons had become immune to the action of the insulin, or, as physicians describe it, they became insulin-resistant, showing the same symptom that characterizes type 2 diabetes. Even in the midst of a high content of this protein, cells become incapable of interpreting the commands of insulin, which is to absorb glucose in the muscles or transform an experience into a memory in the neurons.

The most serious effect appeared the next day. After 24 hours in the presence of the oligomers, the extensions that connect one neuron to another had lost most of their contact points (synapses) with the nearby cells, the researchers reported in PNAS. “This gradual damage to the neurons might explain what happens during the initial stages of Alzheimer’s, when people begin to show memory flaws before the brain cells die”, says Fernanda.

If the oligomers common in Alzheimer’s lead the neurons to behave in the same way as other cells in a body with diabetes, then what would happen if the brain cells got the same treatment given to diabetics? Fernanda and Ferreira came back to Brazil with this question in their minds and went back to work at their lab. This time they subjected the neurons to three treatments – insulin, Rosiglitazone, or a mixture of both – used to control resistance to insulin. Then they added the beta amyloidal plaques to the culture.  The result was astonishing.  The insulin bath prevented the oligomers from adhering to the neurons and triggering cell death.  In this experiment, conducted together with Marcelo Vieira, Theresa Bonfim and Helena Decker, Ferreira and Fernanda also noticed that the protective effect of insulin was enhanced by the Rosiglitazone. But neither the insulin nor the Rosiglitazone prevented the adhesion of the oligomers when the receptors were not functioning adequately, stated the group from Rio in a paper prepared together with William Klein’s team. In Ferreira’s opinion, this fact suggests that insulin might be efficient  only during the initial stages of Alzheimer’s, or perhaps even before the onset of the disease.

Although it is necessary to conduct tests on animals and on human beings to confirm insulin’s protective role, Ferreira is excited about this new therapeutic possibility.  The two classes of medical drugs available to treat Alzheimer’s – acetyl chlorinestase inhibitors and glutamate receptor inhibitors – do not prevent the neurons from dying.  They help reduce memory loss and work on a small proportion of people with this disease, and only for a few months.

Scientific article
DE FELICE, F. G. et al. Protection of synapses against Alzheimer’s-linked toxins: insulin signaling prevents the pathogenic binding of Abeta oligomers. PNAS. v. 106, n. 6, p. 1971-1976. 10 Feb. 2009.

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