Mouse Alzheimer's under the microscope: day by day

Alexey Levin, "Elements" 

Harvard researchers have published the results of experiments that make an important contribution to the long-term discussions about the mechanism of Alzheimer's disease.

In recent years, this pathology has become notorious as the most frightening representative of neurodegenerative diseases (that is, diseases that lead to the death of certain groups of nerve cells). Alzheimer's disease mainly affects the neurons of the temporoparietal cortex, where the centers of memory and intelligence are located. Therefore, it inevitably leads to progressive dementia, in other words, dementia.

Although Alzheimer's disease has been known for more than a hundred years, the causes of neuronal death are still in question. Microscopic examination of the brain tissue sections of the victims of this disease makes it easy to notice specific plaques surrounded by dead cells. They mainly consist of an insoluble fibrous peptide known as beta-amyloid. These peptide chains arise as a result of the sequential work of two enzymes – beta- and gamma-secretase, which cut into pieces the molecules of the complex glycoprotein APP (amyloid precursor protein, "protein precursor of amyloid"), which is part of cell membranes. Although APP is present in many tissues, it is mainly concentrated at the junctions between neurons – synapses.

Many experts have believed and believe that the death of neurons in Alzheimer's disease is caused by the accumulation of beta-amyloid plaques, which act as strong neurotoxins. This model has a lot of confirmations. For example, it has been strictly proven that certain mutations of the gene that encodes the APP protein greatly increase the risk of developing Alzheimer's disease. In addition, amyloid proteins (and beta-amyloid is only a variety of them) are generally suspected of being associated with a whole bunch of diseases, in particular spongiform encephalopathy (Creutzfeldt–Jakob disease) and type II diabetes mellitus. So the amyloid hypothesis is quite plausible.

However, there are other explanations. In the same areas of the brain where beta-amyloid plaques are deposited and neurons die, clusters of very curious brain inhabitants – glial macrophages (they are also called microglia) are invariably found. This is one of the types of neuroglia – auxiliary cells of nervous tissue that fill the space between neurons and capillaries feeding them. Neuroglia performs many different functions, including immune ones. They are implemented just by glial macrophages. Like all macrophages, they have the ability to phagocytosis, absorption of bacteria and other foreign objects. Therefore, there was an assumption that the neurons are primarily attacked by microglia, which then somehow contributes to the formation of amyloid deposits.

Finally, spiral-shaped strands of abnormal tau protein are observed in the affected neurons themselves, which is also seen as a possible culprit of Alzheimer's disease.

Harvard neuroscientists decided to check what exactly appears first – beta-amyloids or microglia. They worked with genetically engineered mice of the APPswe/PS1d9xYFP line (also known as B6C3-YFP). The first three letters of such a long coded name just mean that the body of these rodents intensively produces an abnormal kind of protein precursor amyloid (more precisely, it is a chimeric mouse-human protein, which is indicated by the letters swe). This precursor of amyloid serves as a raw material for the formation of beta-amyloid plaques, which are deposited in the brain of these rodents in the fifth to sixth month of life. Such mice are used as a laboratory biomodel of Alzheimer's disease.

YFP is an abbreviation of the name yellow fluorescent protein, a yellow phosphorescent protein. It is synthesized by the body of mice (thanks to a specially built-in gene) and deposited precisely in those parts of the brain that are affected by Alzheimer's disease. The presence of this protein makes it possible to observe the brain tissues of living mice through specially cut windows using a wonderful optoelectronic device – a multiphoton laser confocal microscope.

Scheme of amyloid plaque formation in 5-6-month-old mice. Day 1 is the day when the mouse first noticed a tiny extracellular deposition of amyloid – microblake. Then the deposition begins to grow rapidly, neighboring neurons and dendrites change, astroglia and microglia contract to the growing plaque. On the 3rd day, axon damage is already noticeable. On the 7th day, the plaque reaches maturity and its structure stabilizes. Fig. from the article by Eliezer Masliah. Neuropathology: Alzheimer's in real time // Nature. V. 451. P. 638-639Professor of Biology Bradley Hyman and his colleagues have received very interesting results.

They found out that amyloid plaques are formed in the cerebral cortex first, and at a very high rate. Previously, it was believed that even in mice, this process stretches for weeks or months (and in humans, of course, for years). It turned out that in experimental mice, plaques are deposited much faster, in just a day. After another day or two, microglia is tightened to them, and one or two weeks later, the deformation of neurons begins. These processes are clearly visible on the attached microphotographs.

That's not all. Harvard researchers have found out that amyloid deposits begin with the appearance of tiny germ – microblocks. Although their size is very small, they damage the processes of nerve cells, axons and dendrites, through which nerve impulses are transmitted. Perhaps this explains the sad fact that Alzheimer's disease damages synaptic connections between neurons.

This is how the formation of an amyloid plaque occurs (shown by the white arrow). On the 6th day, neuron dystrophy is already visible (triangular arrow). Blue color – amyloid deposits, green – neurons; the length of the scale ruler is 20 microns; the images were taken using a multiphoton microscope. Image from the discussed article in NatureOf course, the neurophysiology of mice differs from that of humans, so it's too early to draw final conclusions.

Nevertheless, the work of Professor Hyman and his collaborators strongly supported the hypothesis of the beta-amyloid nature of Alzheimer's disease. Their results are presented in an article that appeared in the journal Nature on February 7.

Source: Bradley T. Hyman et al. Rapid appearance and local toxicity of amyloid-β plaques in a mouse model of Alzheimer's disease // Nature. 2008. V. 451. P. 20–724.

Portal "Eternal youth"  www.vechnayamolodost.ru14.02.2008