Cellular tragedies, part 3

Shock!

Polina Loseva, "The Attic"

The inner life of the cell is eventful no less than the human one. It is full of passions, dangers and, like any life, ends sooner or later. Polina Loseva understands what plots occur in the fate of cells and how their development affects you and me. This topic will be cellular shock, which only vaguely resembles human shock, but brings much more harm.

"Cellular tragedies" is a large series of articles about cells, which continues to be replenished. Read others: about death and suicide or stress.

What unites Parkinsonism, prion diseases, congenital bone fragility and aging? The molecular underside. At the heart of all these conditions is a problem familiar to most girls – improper styling. Only in this case we are talking about intracellular proteins.

Fatal curls

The cherished dream of every lady is that her hair should lie by itself and she did not have to spend priceless morning hours taming the obstinate. In this sense, the cells were more fortunate: in the sequence of any protein, self-folding into the final structure is programmed. This usually happens during protein synthesis on the ribosome: the protein strand is gradually squeezed out, like paste from a tube, and immediately begins to curl. This happens due to electromagnetic interaction: positively and negatively charged sections of the filament attract, as well as uncharged (hydrophobic) regions stick together. As a result of this twisting, the protein acquires a certain conformation – the structure necessary to perform its functions.

But let's imagine that something goes wrong. A young inexperienced lady in an attempt to achieve a perfect perm keeps her hair on a curling iron for too long. They overheat, and instead of the long-awaited curls, straw turns out, which no longer holds any shape. A similar situation in cells is called heat shock. When heated, the proteins lose their conformation, as the water molecules in the solution begin to move faster and destroy the bonds between the sections of the protein strand. If the structure of proteins is not restored in any way, then the consequences for the cell will be deplorable. Shock itself is not the direct cause of cell death – it has a paralyzing effect on the cell. Like a person who has fallen into a stupor and lost control of what is happening, the cell ceases to regulate the work of its vital processes. And as soon as the heat shock touches the proteins involved in energy production, the cell will begin to die. In the absence of energy, it will not be able to regulate transport through the membrane, water will begin to flow in uncontrollably, and cellular contents will come out.

After the discovery of heat shock, it turned out that other stressful factors, not only temperature, can lead to a similar situation. Among them are oxidative stress (accumulation of active molecules with free radicals in the cell), infection, poisoning with toxins and heavy metals. All of them lead to a violation of the normal conformation of proteins. Therefore, now they are more often talking about proteotoxic shock – a state when cellular proteins are folded incorrectly for one reason or another. Proteotoxic shock can develop in two ways. The first one we have already described above: it is associated with the unwinding of proteins and entails cell death. The second, in a sense more dangerous, leads to the accumulation of incorrectly folded proteins in the cell (and sometimes, as a consequence, between cells).

Imagine that when styling her hair, our lady unsuccessfully pulled the curling iron, and the twisted strands tangled with each other. Instead of neat individual curls, a shapeless tangle appeared. If it is not untangled, it will grow, clinging to new and new strands. The situation can develop even more deplorably if the hair has been previously smeared with some kind of adhesive cosmetics. Many proteins have such a sticky area – hydrophobic areas. Due to the lack of charge, water is not attracted to them, so they actively stick together, regardless of whether they are on the same protein or on different ones. Therefore, if the proteins fold incorrectly or unfold, they can stick together. In addition, incorrectly twisted protein strands can get tangled like hair. As a result, protein aggregates are formed.

Accumulation of proteins in the nervous tissue. On the left is Creutzfeld–Jakob syndrome, pale clusters in the center are caused by prion proteins. On the right – Parkinson's disease, dark colored clusters of proteins.

A cell accumulating protein aggregates may not die immediately. For a while they will just accumulate in her and complicate her life. A serious threat arises when such an aggregate adheres to the membrane, because it breaks from this. The cell dies, and the proteins go out into the intercellular space and continue to accumulate there. Then problems begin already at the tissue level (access of nutrients to cells is disrupted), and then the body (cells begin to die en masse). There are many diseases that develop by this mechanism. Among them are the following.

• Osteogenesis imperfecta, aka "crystal man's disease". It is associated with a congenital defect of collagen, a protein that forms, among other things, the basis of the extracellular substance of bones. Instead of forming a strong skeleton, collagen accumulates in the form of aggregates, and bone strength decreases.

• Parkinson's and Alzheimer's diseases. Despite the fact that the exact causes of these diseases are still unknown, we already know that they are accompanied by the accumulation of protein aggregates in the nervous system. This causes the death of neurons and, consequently, impaired motor and cognitive functions.

• Huntington's disease. Here the mechanism is known for sure, and the reason is also known. In the gene of the huntingtin protein, there are repeats of three nucleotides that, with a certain mutation, gain the ability to self-replicate, thus lengthening both the gene and the protein. Starting with a certain length of thread, huntingtin ceases to fold normally and forms aggregates, again irreversibly damaging the nervous system and other organs and tissues.

• Prion diseases. This is a group of diseases caused by the ingestion of proteins of incorrect conformation. Meeting with healthy proteins, prions also force them to change their conformation, which again results in their accumulation in tissues.

Guardian Squirrels

What if every cell is filled with fatal curls that can get out of control at any moment? Get other, life-saving, squirrels. It turns out that if you heat the cells of any eukaryotic (containing nuclei) organism, from yeast to humans, then the number of proteins of a certain group increases in them. They were called heat shock proteins. And then, naturally, it turned out that they react to any proteotoxic shock, not necessarily caused by temperature. This is a whole group with diverse functions, but in general they use three strategies to combat the accumulation of proteins.

Firstly, any lady knows that the best remedy for tangles is a comb. This tool separates the hair from each other, giving each strand its own track between the teeth. Heat shock proteins from the chaperone group work in a similar way. This name reflects the English meaning of the word chaperone" – "mentor", "companion". They are barrels with a hydrophobic cavity, inside which protein can fit. Thus, they restrict him from interacting with other proteins (as companions prevented excessive contacts of their wards, especially young ladies, with others) and give him the opportunity to twist independently of them.

A molecular model of the chaperone complex. A hydrophobic cavity is visible in the center. Image Author: P99am Wikimedia Commons CC BY-SA 3.0

Secondly, for cases with which the comb does not cope, there are scissors. You can try to cut off a separate strand to unravel the rest. In a cell, proteasomes serve to destroy proteins of incorrect conformation. These are large molecular complexes that break down proteins. Some chaperones can serve as a label that the protein is folded incorrectly. If a protein is bound to a chaperone, then the proteasome is more likely to destroy it.

Thirdly, finally, if small scissors do not help, then you can take up the typewriter and shave the whole thing off. At the cell level, this means the launch of autophagy, that is, self-eating (it was for a detailed description of this mechanism, by the way, that the Nobel Prize in Physiology and Medicine was awarded last year). Heat shock proteins can contribute to the digestion of individual parts of the cell (for example, protein aggregates) by lysosomes – vesicles with digestive enzymes.

In fact, the functions of heat shock proteins are not limited to this. From time to time, scientists discover their connection with a variety of processes. It turns out that they block apoptosis, preventing the cell from committing suicide. What if the breakdown can still be repaired? They participate in reproduction, and mice deprived of them are infertile. They also play a role in the growth of embryos. Therefore, it is still a long time before the final clarification of the situation. The regulation of their work in multicellular organisms also remains mysterious. While in cell cultures, under the influence of stressful factors, their work starts automatically, some other mechanism seems to work in the body. For example, the roundworm C. Elegans has special neurons that send a signal about heat shock. But in humans, everything is not so obvious. At least in Alzheimer's and Parkinson's diseases, heat shock proteins do not begin to work more intensively, despite the accumulation of protein aggregates. Scientists have yet to discover a signal that could stimulate their work.

Vaccine from a comb 

Despite all the mysteries surrounding heat shock proteins, there are already proposals to use them in medicine. For example, it is known that as the body ages, the number of chaperones decreases. It can be assumed that by increasing their number, it is possible to slow down the deterioration of the body and the accompanying diseases. Therefore, scientists suggest taking activators of heat shock proteins, for example celastrol, as a prevention of aging (although such therapy is not carried out anywhere yet). In addition, it has already been possible, by increasing the number of chaperones, to improve the condition of rats with Alzheimer's disease.

It is also interesting to see what happens to heat shock proteins in cancer cells. In them, on the contrary, their number is growing. This is due to the appearance of mutant proteins and a high level of oxidative stress. Therefore, cancer cells need chaperones to properly fold proteins and prevent apoptosis. In laboratory conditions, it even turned out to kill human cancer cells by blocking the work of heat shock proteins there.

However, the greatest interest now is another application of these proteins. One of the problems that arise in cancer is that the immune system "does not see" tumor cells, that is, it does not react to them as to foreign ones. To "incite" lymphocytes on tumor cells, you need to introduce them to some mutant tumor protein. And here it is very useful to recall that mutant proteins in cancer cells are associated with chaperones. At the same time, it turns out that they are connected quite firmly. Therefore, you can take a tumor biopsy, isolate chaperones from there, and get mutant proteins with them. Then take a certain number of lymphocytes from the patient, grow them and add a mixture of proteins with chaperones to the culture. After that, returning to the patient's body, the lymphocytes will have a much better idea of the enemy they have to fight. Officially, this procedure has not yet been approved, but the number of clinical trials of such a vaccine is measured in dozens. So everything is probably still ahead.

Portal "Eternal youth" http://vechnayamolodost.ru