Sirtuins, Sirtuin 2 and Neurodegeneration

Sirtuins, Sirtuin 2 and Neurodegeneration

Sirtuins, ageing and disease

Disease prevention and treatment has always been of interest to humans, but nowadays with an ageing population, age related disease and effects of ageing have become even more of interest. Why do we age? And what makes us more likely to develop diseases as we age? 

One of the most vital parts of our bodies is our brain, in the central nervous system: a system that is crucial for our quality of life. Cells within this system are called the oligodendrocyte precursor cells and the molecules inside them, sirtuins, hold some of the answers and possible solutions to ageing. 

What are sirtuins?

Our bodies are driven by molecules that have a specific function that are linked to each other. 

Sirtuins are defined as ‘’ a type of protein involved in regulating cellular processes including the aging and death of cells and their resistance to stress’’.  In recent studies, sirtuins have been generating a large interest due to their relationship in biological functions related to metabolism, lifespan, transcription, genome stability and neural activity- all of which are of interest to humans because of their impact on one’s quality of life. Malfunction of these processes can lead to series illnesses, including cancers, diabetes, Parkinson's disease, and multiple sclerosis. If scientists dive deeper into understanding the biological role of the mechanisms of how sirtuins work and variations between different tissues and diseases, the outcome could be the innovation of novel treatments for a range of diseases. 

Sirtuins are a type of nicotinamide adenine dinucleotide (NAD+) consuming enzyme (quite a mouthful!). In this case, consuming means that NAD+ is needed for the protein to function. In mammals, including humans, seven types, called isoforms, of sirtuin are found. These are conveniently named SIRT1—7. The first sirtuin that was discovered was the silencing regulator 2 (Sir2). This protein was discovered in yeast. Research has shown that this sirtuin is an enzyme that is needed for the catalysation of the hydrolysis of acetyl-lysine in histones, with the use of NAD+ as a co-substrate. Following research showed that many other sirtuins were found to perform this activity. Enzymes are proteins that are found naturally in organisms and are needed to drive reactions. Without them, reactions in our bodies would take a lot longer and would also most likely need higher temperatures. Enzymes are not only found in our bodies, but these proteins have also been manipulated to be used in industrial processes, for example in biological detergents to break down stains and reduce the temperature needed for effective clothes washing. A substrate is the molecule that the enzyme reacts with (in the case of detergents, this would be the stain). 

If we look at the structures of SIRT1–7, each of these proteins share the same core and needs as NAD+, in addition to a catalytic core. However, these proteins differ in the activity of their genes, their geography inside the cell and what they react with. This means that they perform different functions, using distinct molecules. SIRT2 is the dominant sirtuin found in oligodendrocyte precursor cells and will be the sirtuin that is discussed throughout this article. SIRT2 was not found to be present in other cell types tested within the central nervous system, which included astrocytes, neurons, microglia, and endothelial cells. In postnatal oligodendrocyte precursor cells, SIRT2 is found in the nucleus, then when these cells grow and mature, the SIRT2 can be found in the cytosol, which is the fluid inside the cell. The location of SIRT2 is important as it relates to the protein’s function.  In the cytosol it regulates cell division and proliferation (by deacetylation of microtubule proteins). The protein is also found to be a tumour suppressor and linked to neurodegeneration, and it promotes lipid formation and the prevention of differentiation in mature adipocytes.

This characteristic has been the basis for studies that look at the mechanism behind the activation and inhibition of SIRT2, to develop tools for the treatment of such diseases. 

A recent (March 2022) study that has been published in Nature, by Xiao-Ru Ma et al., from the Department of Pathology of Sir Run Shaw Hospital, China, showed that the effects on myelination in ageing are brought about by SIRT2 and NAD+ and that SIRT2 is required for the restoration of oligodendrocyte precursor cells. The study concluded that when SIRT2 and NAD+ levels are increased in the oligodendrocyte precursor cells, the potential for the cells to reform returns to similar levels of younger ages. This information can be used for the design of treatments and protection of neural cells during ageing. 

What are oligodendrocyte precursor cells and oligodendrocytes?

Let's break this information down, starting with the oligodendrocyte precursor cells. The body is made up of different cells, each having a specific function. Throughout development, some cells mature into cells that have different characteristics. In the case of the oligodendrocyte precursor cells, cells that are found in the central nervous system, which includes the brain together with the spinal cord. The central nervous system has two main groups of cells, neurological and non-neurological. The oligodendrocyte precursor cells are a subtype of glial cells, which are the non-neurological cells, meaning they do not produce electrical impulses. These cells mature into oligodendrocytes and are used to cover axons or nerve fibres, in the form of myelin. Myelin is important in conducting cells because it insulates the nerve fibres allowing faster electrical transmission. If these cells decrease, myelination also decreases, which results in the damage of neurological functions. 

What is the function of myelin and remyelination in the central nervous system?

The discoveries on brain myelination mechanisms and links of cells’ degeneration due to diseases, such as autoimmune diseases, inflammatory diseases, as well as ageing, have driven research on the possibilities of reactivating this natural process. In the process of remyelination, the oligodendrocyte precursor cells are regenerated. This is followed by the cells’ growth and maturation into oligodendrocytes and new myelin sheaths are formed around the demyelinated axons (when the myelin sheath is damaged or degenerated) in the central nervous system. The new myelin formed is usually thinner than the original myelin. The reason for this is still unclear. In multiple sclerosis, the oligodendrocyte precursor cells fail to become remyelination oligodendrocytes, leaving some axons myelinated and subject to further degeneration. 

In the study mentioned by Xiao-Ru Ma, the supplementation of β-nicotinamide mononucleotide (NMN), aided in the nuclear entry of SIRT2 in oligodendrocyte precursor cells, amazingly, remyelination was revived in ageing mice. 

β-nicotinamide mononucleotide, is derived from the molecule niacin, a form of vitamin B3, that can convert into nicotinamide adenine dinucleotide (NAD+). Nicotinamide adenine dinucleotide (NAD) is a molecule needed for metabolism in the cells of all living organisms. The molecule is made up of two nucleotide groups, linked with a phosphate group. When a hydrogen atom is lost (oxidation), the molecule becomes charged: NAD+. For NAD to form, the amino acid tryptophan or aspartic acid are needed. 

Sirtuins need NAD+ for the reaction of deacetylation and this allows sirtuins to respond to changes in the metabolism inside a cell. A basic pathway of the NAD formation:  tryptophan →niacin →NMN →NAD. β-nicotinamide mononucleotide is also found in vegetables including edamame, cabbage, cucumber, avocado and broccoli. 

What happens when SIRT2 is lacking in oligodendrocyte precursor cells?

The research by Xiao-Ru Ma showed that the decline of NAD+ and SIRT2, was a feature in aged oligodendrocyte precursor cells, and the supplementation of mice using this, resulted in the reproduction of SIRT2 on the oligodendrocyte precursor cells, allowing for entry in the nucleus. This, in turn, led to the production of mature oligodendrocytes needed for myelin generation, in a central nervous system that was aged. The research also indicated that the sirtuin protein plays a major role in this mechanism. This is because, once the entry of SIRT2 was enhanced in the nucleus, myelin ageing was delayed in the mice with a normal central nervous system (delayed ageing) and in aged mice, myelination was boosted. 

Another experiment that highlighted the importance of the SIRT2 protein, was seeing what happens to the oligodendrocyte precursor cells when the gene that makes the SIRT2 protein is removed (Sirt2). This was done using oligodendrocyte precursor cells grown in the lab, injecting them with a molecule called lysolecithin, which is used to promote demyelination - damaging the cells. After the treatment, the cells were observed using electron microscopy. The results showed that in the damaged cells, where SIRT2 was removed, reproduction and specialisation of the cells into mature oligodendrocytes, that become myelin was weakened, therefore could not be repaired. When the same experiment was performed on mice, rather than cells, the same results were observed. This concluded that mice that lacked the SIRT2 protein, showed damaged remyelination and any myelin produced was thin and loose when compared to new myelin. The myelin damage observed was three times more severe than in old mice. The experiments confirm that SIRT2 is vital for remyelination and that the effects of a lack of remyelination are long-lasting. 

What can be done to increase sirtuin production, effecting ageing and neurodegeneration? 

As we age, the production of sirtuins decreases, and nutraceuticals and pharmacological agents, including supplements, can aid in boosting sirtuin activity and treating these age-related conditions. By elevating the β-nicotinamide mononucleotide, the levels of NAD+ are increased, this helped increase the SIRT2 entry into the nucleus in aged mice with the presence of central nervous system damage. The addition of β-nicotinamide mononucleotide as a supplement is shown to also increase the density of the oligodendrocyte precursor cells and their specialisation into mature oligodendrocytes. In mice with damaged telomeres, the addition of β-nicotinamide mononucleotide, led to full restoration of the impaired cells. Genetic material is condensed in the form of chromosomes and telomeres are the structures at the end of chromosome, which protect the genetic material during cell replication. Over time, the length of the telomeres, decreases, resulting in DNA damage. Another sign of ageing is this reduction of telomeres in DNA. 

How are the experiments performed?

The experiments are performed throughout this study were done on mice cells, human cells, and mice. Mice are used because they are mammals and as their evolution is close to humans. They are also useful as model organisms due to their relatively short lifespans, making them ideal to study the effects of ageing. Many previous preliminary experiments were first performed on smaller organisms before moving to mammals, these included yeast, fruit flies and nematodes. 

Conclusion

What does this research mean for humans? The research shows that an introduction and long-term supplementation of β-nicotinamide mononucleotide elevates NAD+, enhancing the production of SIRT2 and its entry into the nucleus of oligodendrocyte precursor cells. This leads to a delay in ageing and neurodegeneration because of sirtuins’ role in the restoration of myelin in the central nervous system. The supplementation also makes more myelin compact, making it a more effective at protecting the cells it covers. The research done on mice and other mammals is being further studied in humans. These results have a positive indication towards to importance of β-nicotinamide mononucleotide supplements in the delay of ageing consequences, DNA damage and neurodegenerative diseases. 

References and further reading

(1) Franklin RJM, french-Constant C. Remyelination in the CNS: from biology to therapy. Nature Reviews Neuroscience 2008;9(11):839-855.

(2) Bogan KL, Brenner C. Nicotinic Acid, Nicotinamide, and Nicotinamide Riboside: A Molecular Evaluation of NAD+ Precursor Vitamins in Human Nutrition. Annu Rev Nutr 2008;28(1):115-130.

(3) Sauve AA, Youn DY. Sirtuins: NAD+-dependent deacetylase mechanism and regulation. Curr Opin Chem Biol 2012;16(5):535-543.

(4) Buller B, Chopp M, Ueno Y, Zhang L, Zhang RL, Morris D, et al. Regulation of serum response factor by miRNA-200 and miRNA-9 modulates oligodendrocyte progenitor cell differentiation. Glia 2012;60(12):1906-1914.

(5) Ma X, Zhu X, Xiao Y, Gu H, Zheng S, Li L, et al. Restoring nuclear entry of Sirtuin 2 in oligodendrocyte progenitor cells promotes remyelination during ageing. Nature Communications 2022;13(1):1225.

(6) Guarente L. Sirtuins in aging and disease. Cold Spring Harb Symp Quant Biol 2007;72:483-488.

(7) Nielsen AL, Rajabi N, Kudo N, Lundø K, Moreno-Yruela C, Bæk M, et al. Mechanism-based inhibitors of SIRT2: structure–activity relationship, X-ray structures, target engagement, regulation of α-tubulin acetylation and inhibition of breast cancer cell migration. RSC Chem Biol 2021;2(2):612-626.

(8) Morris BJ. Chapter 4 - Sirtuins and aging. In: Maiese K, editor. Sirtuin Biology in Medicine: Academic Press; 2021. p. 49-77.

(9) Lin H. Chapter 4 - The Enzymatic Activities of Sirtuins. In: Guarente L, Mostoslavsky R, Kazantsev A, editors. Introductory Review on Sirtuins in Biology, Aging, and Disease: Academic Press; 2018. p. 45-62.

(10) Rajabi N, Galleano I, Madsen AS, Olsen CA. Chapter Two - Targeting Sirtuins: Substrate Specificity and Inhibitor Design. Progress in Molecular Biology and Translational Science 2018;154:25-69.


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