Heat shock and aging
What does a molecular thermometer look like? This question is much more complicated than it might seem at first glance. Apparently, the "thermometer" used by the cell, which plays one of the most important roles in maintaining the stability of the cell proteome, is a system of transcription factors and specialized chaperone proteins, including heat shock proteins, reacting not only to an increase in temperature (this is only the first of the discovered functions of this class of proteins), but also other physiological effects that damage the cell.
Chaperones are a class of proteins whose main function is to restore the correct tertiary structure of damaged proteins, as well as the formation and dissociation of protein complexes.
The chaperone system reacts to the damage that occurs during the vital activity of the cell and ensures the correct passage of folding – folding of amino acid chains descending from the ribosomal "assembly line" into three-dimensional structures. Despite the evidence of the exceptional importance of this system, for a long time none of the specialists involved in its study even imagined that this molecular thermometer is also a kind of "source of youth" of the cell, and its study provides an opportunity to look at a number of diseases from a new, previously unknown side.
Proteins, which are the main product of the functioning of the genome, not only form the structure, but also ensure the work of all cells, tissues and organs. The absence of failures in the processes of synthesis of amino acid sequences; the formation, assembly and transportation of protein molecules, as well as the removal of damaged proteins is an important aspect of maintaining the health of both individual cells and the whole organism. Proteins are also the material necessary for the formation and effective functioning of "molecular machines" that ensure the processes of biosynthesis, a process critical for ensuring the longevity of the body. Many problems are caused by violations of the fundamental process of protein folding. Violations of the "OTC", represented by heat shock proteins and chaperones, lead to the appearance and accumulation of errors. These errors disrupt the work of molecular mechanisms, which can lead to the development of various diseases. The occurrence of such errors in neurons is fraught with truly terrible consequences, manifested by the development of neurodegenerative diseases such as multiple sclerosis, as well as Huntington's, Parkinson's and Alzheimer's diseases.
Discovered in 1962 by Ferruccio Ritossa, the heat shock reaction is described as a temperature-induced change in the organization of tightly packed chromosomes in the salivary gland cells of fruit flies, leading to the formation of so-called "bloating". Such swellings, which look like cotton balls squeezed between tightly packed sections of chromosomes under the microscope, also appear when exposed to dinitrophenol, ethanol and salicylic acid salts.
It turned out that chromosome swellings are new transcription regions that begin the synthesis of new informational RNAs within a few minutes after their occurrence. Protein products of this process are now widely known as heat shock proteins, the most studied of which are Hsp90 and Hsp70. Proteins of this family regulate the folding of amino acid chains and prevent the appearance of improperly formed protein molecules in the cells of all living organisms.
In the late 1970s and early 1980s, with the help of an original technique of cellular biochemistry, which allows to increase the number of informational RNAs encoding the sequences of the corresponding proteins, scientists managed to clone the first genes of heat shock of the drosophila fly. At that time, experts were of the opinion that the reaction of heat shock is characteristic exclusively for the body of fruit flies. At this stage, Richard Morimoto made his first contribution to the study of heat shock proteins. He collected an extensive collection of DNA of multicellular organisms and using the southern blotting method demonstrated that all of them contain almost identical analogues of the Hsp70 gene in structure. Around the same time, Jim Bardwell and Betty Craig from the University of Wisconsin at Madison identified the DnaK gene in the genome of Escherichia coli (Escherichia coli), which is also an analogue of Hsp70. The result of further detailed study of this issue was the understanding that the genes of heat shock in an almost unchanged form in the course of evolution are represented in the genomes of representatives of all five kingdoms of the living world.
The next achievement in the chain of events that followed was the identification of a family of transcription factors that control the launch of the first stage of the heat shock reaction. Several research groups from different universities took part in this work, including the Morimoto group. Scientists have demonstrated that an increase in cell temperature causes a change in the shape of these transcription factors, which contributes to their binding to heat shock gene promoters that initiate the synthesis of heat shock proteins. Moreover, it turned out that unlike yeast, fruit flies and nematodes Caenorhabditis elegans, which have only one transcription factor of heat shock genes, there are as many as three such factors in human cells. Such a complex scheme of regulation of the expression of the studied genes led scientists to the idea of their multifunctionality, which requires additional study.
Further studies have shown that heat shock proteins themselves regulate the functioning of the transcription factor that initiates their production in cell nuclei. It also became obvious that heat shock proteins perform the functions of molecular chaperones – they control the folding of amino acid chains, ensuring the formation of correct spatial conformations of protein molecules, as well as identify and eliminate failures in this process. Thus, it turned out that the cell thermometer not only measures the temperature, but also monitors the appearance of incorrectly formed and damaged proteins in the cell. Heat shock and other stressful effects fill the cell with abnormal proteins, to which chaperones react by binding these proteins and releasing heat shock transcription factor-1 (Hsf1). Molecules of this factor spontaneously form trimers (complexes of three molecules) that bind to the corresponding regions of the genome, in turn triggering the synthesis of heat shock proteins. The subsequent increase in the concentration of heat shock proteins to the required level on the feedback principle suppresses the transcriptional activity of the Hsf1 transcription factor.
The study of the functioning of heat shock proteins on cell lines severely limited the capabilities of researchers, since it did not provide information about the accompanying changes occurring throughout the body. Therefore, around 1999, Morimoto and his colleagues decided to switch to a new model – C.elegans roundworms. They were especially inspired by the work published in 1994 by Max Perutz, who established that the cause of a serious neurodegenerative disease – Huntington's disease – is a special mutation of a gene called huntingtin. This mutation leads to the synthesis of a protein variant containing an additional fragment from a long chain of the amino acid glutamine, apparently disrupting the normal folding process. Aggregation of such abnormal protein molecules in neurons leads to the development of Huntington's disease. The researchers suggested that the study of proteins, the formation of molecules of which is disrupted due to the expression of polyglutamine or similar reasons, will help to understand the work of the molecular thermometer.
While working on the creation of animal models of expression in neurons and muscle cells of proteins containing excess polyglutamine sequences, the researchers found that the degree of aggregation and associated toxicity of such proteins is proportional to their length and age of the organism. This led them to the idea that the suppression of the insulin-mediated signaling mechanism that regulates the lifespan of the body can affect the aggregation process of polyglutamine-containing proteins. The results of further studies confirmed the existence of the alleged relationship, and also demonstrated that the effect of the functioning of the Hsf1 transcription factor on the lifespan of the body is mediated by an insulin-dependent signaling mechanism. These observations made it obvious that the reaction of heat shock is equally important both for the survival of the organism under acute stress, and for the constant neutralization of the toxic effect of proteins, which negatively affects the functioning and lifespan of cells.
The use of living organisms as an experimental model allowed scientists to transfer research to a qualitatively new level. They began to pay attention to the mechanisms by which the body perceives and integrates information coming from outside at the molecular level. If stress affects the aging process, it is logical to assume that heat shock proteins that register the appearance and prevent the accumulation of damaged proteins in the cell are quite capable of slowing down the development of aging effects.
The fact that many diseases associated with the accumulation of proteins prone to aggregation are characterized by symptoms of aging, and all diseases based on violations of the formation of protein molecules are associated with aging suggests that temperature-sensitive metastable proteins lose their functionality as the body ages. Indeed, experiments on C.elegans have shown that the functioning of the mechanism triggered by the Hsf1 transcription factor, as well as other cell defense systems, begins to fade almost immediately after the body reaches maturity. However, it turned out that the activation of the Hsf1 transcription factor in the early stages of development can prevent the disruption of the stability of protein molecules (proteostasis).
Perhaps this observation, which suggests very intriguing possibilities, does not apply to more complex multicellular organisms, but all living things consist of proteins, so the results obtained in experiments on roundworms with a high degree of probability can help scientists understand the mechanisms of human aging.
However, this is not the end of the story. The results of the work recently conducted under the supervision of Professor Morimoto indicate the existence of mechanisms for correcting proteostasis that do not require direct intervention in the functioning of the Hsf1 transcription factor. The researchers decided to conduct a classic genetic screening of C.elegans mutants, demonstrating violations of the formation of protein molecules in muscle cells. As a result, they found that the mutation affecting this process is in the gene of the transcription factor controlling the production of the neurotransmitter gamma-aminobutyric acid (GABA). GABA controls the functioning of arousal neurotransmitters and regulates muscle tone. An interesting fact is that any violation of the stability of the GABA-mediated mechanisms leads to hyperstimulation, forcing postsynaptic muscle cells to respond to non-existent stress, which leads to disruption of the formation of protein molecules. In other words, it turned out that the activity of neurons can affect the functioning of molecular thermometers of other cells of the body, which further complicated the emerging picture.
If this mechanism also applies to humans, then perhaps scientists will be able to develop a method of influencing neurons, leading to the activation of heat shock proteins in skeletal muscle cells and contributing to the elimination of symptoms of muscular dystrophy and other diseases of motor neurons. Perhaps manipulations of these mechanisms will also allow controlling the accumulation of damaged proteins associated with aging. However, unfortunately, not everything is as simple as we would like. In C.elegans, the development of a heat shock reaction in all adult somatic cells is controlled by one pair of neurons. Apparently, the activity of these neurons and the feedback mechanism allow cells and tissues to activate heat shock proteins according to their specific needs. The fact is that different tissues are characterized by different activity of protein biosynthesis, as well as different severity and nature of external influences. Therefore, a universal approach to managing the heat shock reaction is in principle impossible.
Armed with the results of their work and promising ideas, Morimoto and several of his colleagues founded the company Proteostasis Therapeutics, the purpose of which is to identify therapeutic small molecules capable of correcting the pathological effects of the accumulation of improperly formed protein molecules. This approach is associated with a rather high degree of risk, since the level of heat shock proteins increases in many malignant diseases. However, Morimoto and his associates believe that the direction they are developing has too much potential to ignore.
About the author Professor Richard Morimoto, after defending his doctoral dissertation, fully devoted his work to studying the functioning of heat shock proteins and their role in the aging of the body.
Morimoto took his first steps in the direction he chose at Harvard University under the guidance of Dr. Matt Meselson. Currently, Richard Morimoto is the director of the Rice Institute for Biomedical Research, part of Northwestern University in Evanston, Illinois, and one of the founders of Proteostasis Therapeutics (Cambridge, Massachusetts).
Evgeniya Ryabtseva
Portal "Eternal youth" http://vechnayamolodost.ru based on the materials of The Scientist: Richard Morimoto, "Shock and Age".
07.06.2010