Are protein plaques the last line of defense against neurodegenerative diseases?
Neurofibrillary tangles in Alzheimer's disease are not killers, but defenders of cells?
Vera Bashmakova, "Elements".
Like most neurodegenerative diseases, Alzheimer's disease is characterized by the appearance of intracellular protein inclusions in the affected areas of the brain. Previously, it was believed that it was these inclusions that lead the cell to death. However, recent work has shown that, most likely, protein aggregates are, on the contrary, an attempt by a cell to take pathological processes under control and avoid death.
One of the common features of almost all neurodegenerative diseases is the occurrence in the neurons of the affected areas of protein aggregates formed by incorrectly folded proteins. Such proteins can bring a lot of troubles to the cell. For example, long strands of the huntingtin protein, consisting of many glutamine repeats, are something like a fishing net, in which the contents of the cytoplasm can become entangled. And an improperly folded prion protein causes the "rearrangement" of a similar protein from the host cell, which eventually leads to its death. Therefore, the correct folding of proteins is an extremely important process in the life of a cell.
The newly synthesized protein comes off the ribosome simply in the form of a long chain. Various amino acids of this chain, depending on their properties, begin to attract each other or, conversely, repel, and as a result, the protein collapses, acquiring the necessary conformation. Some proteins are not able to form themselves in the right way right away, they need help: for this there is a special class of proteins in the cell – chaperones. The most "stubborn" proteins that do not agree to fold correctly in any way will sooner or later be destroyed by the cell – most likely through lysosomes (one of the functions of which is autophagy, that is, the destruction of structures unnecessary to the cell or the digestion of proteins and other substances produced inside the cell itself) or the ubiquitin-proteasome system.
If, despite all efforts, it is not possible to cope with incorrectly folded proteins, they begin to accumulate, "sticking together" with each other. This is how intracellular protein inclusions appear in the cell. Depending on which protein problems have arisen, protein aggregates appear in various parts of the nervous system and neurodegenerative diseases develop (why the nervous system is almost always affected is a question that still remains without a confirmed answer). Levi's corpuscles from alpha-synuclein protein in the black substance (see Substantia nigra) is a characteristic feature of Parkinsonism, Huntington's chorea develops inclusions from huntingtin protein in the striatum and so on.
For a long time it was believed that protein aggregates destroy neurons – after all, they were found exactly where most cells died, and they contained extremely dangerous improperly folded proteins. However, another hypothesis is gradually gaining strength, according to which protein inclusions are an attempt by a cell to escape from incorrectly folded proteins by putting them all together. A few years ago, such a protective function was shown for aggregates of the huntingtin protein, which plays a key role in the pathogenesis of Huntington's chorea. And now there is evidence that the tangles of tau protein formed in Alzheimer's disease are not "killers" of cells, but their defenders.
The brain of an elderly person is normal (left) and with Alzheimer's disease (right).
Alzheimer's disease is an incurable fatal disease that manifests itself in a progressive impairment of cognitive functions (in other words, loss of the ability to think). Patients have a gradual loss of neurons in the cerebral cortex, as well as in some subcortical areas. When studying the brain affected by Alzheimer's disease under a microscope, amyloid plaques on the surface of neurons formed by the short peptide beta-amyloid and intracellular tangles of tau protein are clearly visible. Normally, tau protein is very important for the cell. It can be in a phosphorylated (active) and dephosphorylated (inactive) state. Being phosphorylated, tau protein stabilizes microtubules and thus regulates intracellular transport. However, hyperphosphorylation of tau has terrible consequences – its molecules become insoluble and stick together, forming tangles, and the cell has huge problems due to disruption of microtubular transport.
A few years ago, it was shown that the tau protein is cleaved by caspases. Caspases are proteases that perform the role of "executioners" in the cell: they trigger the mechanism of programmed cell death – apoptosis. Caspases are divided into two types – initiatory and effector. Initiator caspases (8, 9 and 10) cleave effector caspases (3, 6 and 7) and thus activate them. Once activated, effector caspases "go wild" – they trigger cascades of reactions that cause DNA fragmentation and cell destruction. The tau protein is "cut" by effector caspases, and the resulting "trimmed" tau aggregates much faster than usual. It is logical to assume that cell death in Alzheimer's disease is somehow related to the interaction between caspases and tau protein.
To test this assumption, a group of researchers from the USA and France conducted a series of experiments.
Experiments were conducted on transgenic Tg4510 mice expressing mutant human tau protein (it is more susceptible to hyperphosphorylation and therefore aggregates better than conventional tau). These mice at the age of seven months (which is quite respectable for mice) show some signs of Alzheimer's disease: they have neurofibrillary tangles, and there is also a loss of a significant number of neurons.
To find out how the activity of caspases is related to the formation of neurofibrillary tangles and cell death, scientists used the technique of multiphoton microscopy (see Multiphoton Excitation Microscopy). This method allows you to observe live neurons of living animals through a hole in the skull through a microscope. In order to "see" neurofibrillary tangles and caspase activity in cells, special dyes are dripped onto the surface of the brain.
As a result of research, it turned out that most (87%) of "caspase" neurons contain tangles. At the same time, caspase activity was registered in only 10% of neurons containing tangles. All this confirmed the connection between caspase activity and tau aggregation, but left many questions. Then the scientists decided to trace the fate of "glomerular" and "caspase" neurons for several days and find out what the dynamics of glomerular life and caspase activity are.
We managed to find out the following. If a tangle has appeared in a neuron, it does not go anywhere and does not "dissolve". However, the cell at the same time continues to live and, apparently, feels tolerable. Something else is interesting. Postmortem studies of "caspase-glomerular" neurons have shown that the nucleus in these cells is not damaged, which means that the program of cell death has not been completed. That is, it looks like if there is a neurofibrillary tangle in the cell, the caspases are powerless and can no longer "kill" the cell.
Indicator on neurofibrillary tangles of thioflavin S (green) and caspase indicator (red) on the first (a, b) and second (c) days after the addition of dyes to the brain surface. The arrow shows that on the second day a neurofibrillary tangle appears in the "caspase-tubeless" neuron.
d – the histogram shows that tangles appear in the majority (88%) of "caspase-tubeless" neurons during the day.
The length of the scale ruler is 10 microns. Image from the discussed article in Nature
But the most stunning results were obtained on "caspase-tubeless" neurons. These cells were extremely rare: In seven animals, only 41 such neurons were found in the study of 56 brain regions. What was the surprise of the researchers when they found that the next day a neurofibrillary tangle appeared in 36 of these cells! At the same time, the occurrence of a tangle in a "caspase–free" neuron is very rare, and is observed in only 1.7% of cases (and, most likely, short-term caspase activity was simply missed in these neurons). In other words, activation of caspases causes the formation of tangles in cells in less than a day. And since in most of the "glomerular" neurons, the caspases were "turned off", it is quite possible that it is the appearance of tangles that helps the cell survive after the caspase "attack".
Okay, but then what triggered the activity of caspases in the cells? Apparently, this "switch" was the presence of a mutant tau protein in the neurons. In its absence, the caspases are silent – at least, no caspase activity was detected in either wild-type mice or mice expressing APP (a precursor of beta-amyloid forming amyloid plaques). If the expression of mutant tau is "turned off" (transgenic Tg4510 mice were bred in such a way that this expression could be "turned off" with the help of the antibiotic doxycycline), then the activity of caspases drops sharply – after six weeks of "turning off" tau by almost 20 times!
Conversely, if an adenovirus is used to induce the expression of non-mutant tau in the brains of wild-type mice, then caspase activity appears in some neurons, and postmortem studies show the presence of tau "trimmed" by caspases in cells. The "stripped-down" protein is also found in mice overexpressing human non-mutant tau. Taken together, these data suggest that caspases are "turned on" during overexpression of tau protein in neurons.
There remains one more important question: for what reason does the tau protein begin to aggregate after the caspases "go on the warpath"? In vitro, it was shown that the very presence of tau "trimmed" by caspases is able to "turn on" aggregation. To test whether this principle works in vivo, scientists introduced a virus encoding "stripped-down" tau to wild-type mice. It turned out that a fairly solid percentage of neurons expressing "truncated" tau showed Alzheimer-like changes. Moreover, the endogenous ("native") cellular tau changes its location and localizes together with the "truncated" one. This is a serious argument in favor of the fact that a "stripped-down" tau protein can trigger the formation of protein aggregates and "recruit" normal tau molecules to participate in them.
Of course, many questions remain unanswered. How do neurons manage to avoid death after a caspase attack? What are neurofibrillary tangles – protection for the cell, a side result of caspase activity, or a "slow weapon" that gradually kills neurons? Nevertheless, the results obtained significantly clarify the picture of pathogenesis in Alzheimer's disease.
Source: A. de Calignon et al., Caspase activation precedes and leads to tangles // Nature. V. 464. P. 1201-1204 (22 April 2010).
Portal "Eternal youth" http://vechnayamolodost.ru17.05.2010