The latest research from USTC: Delaying aging starts with saving mitochondria!

Mitochondrion is a semi-autonomous organelle present in most eukaryotic cells. It contains limited genetic material and is also the place where energy is produced in cells and provides power for cells. In addition, mitochondria are also involved in physiological processes such as cell differentiation, intercellular information transmission and cell apoptosis, which are essential for the normal physiological functions of various cells in our body. The structure of mitochondria can be divided from the outside to the inside into the outer membrane, the membrane gap, the inner membrane, the mitochondrial cristae (a structure formed by the folding of the mitochondrial inner membrane into the mitochondrial matrix. The formation of mitochondrial cristae increases the surface area of ​​the mitochondrial inner membrane) and the mitochondrial matrix (Figure 1).

Figure 1 Mitochondrial structure

Aging is an inevitable process for each of us. During the aging process, the brain structure will undergo irreversible degenerative changes, such as the reduction in the volume of the cerebral cortex. This structural change will undoubtedly affect the normal functioning of the brain, which is reflected in our daily lives, such as decreased reaction ability, slow movement, or changes in psychological emotions. Similarly, with the onset of aging, the structure of mitochondria in each of our cells will also change, including the gradual retraction of mitochondrial cristae to the inner mitochondrial membrane, fragmentation of mitochondrial matrix, formation of vesicle-like structures in the inner mitochondrial membrane, and dissociation of the dimer structure of ATPase on the inner membrane into monomers, and finally the rupture of the outer membrane, releasing apoptotic factors into the cytoplasm, leading to cell death [1]. However, the molecular mechanism of these abnormal changes in mitochondrial structure caused by aging is still unclear.

Recently, Liu Qiang's team from the University of Science and Technology of China published a research paper titled "Aging-induced tRNAGlu-derived fragment impairs glutamate biosynthesis by targeting mitochondrial translation-dependent cristae organization" in the journal Cell Metabolism [2]. The paper found that glutamate transfer RNA (tRNAGlu) released from the nuclei of glutamatergic neurons in the aging brain is cleaved into microRNA fragments (transfer-RNA derived small RNA, tsRNA) - Glu-5'tsRNA-CTC and aggregated in the mitochondria, thereby interrupting the binding of mitochondrial leucine transfer RNA (mt-tRNALeu) and leucine transfer RNA synthetase 2 (Leucyl-tRNA synthetase2, LaRs2), impairing the aminoacylation of mitochondrial leucine transfer RNA and the translation of mitochondrial encoded proteins. Moreover, the aggregation of Glu-5'tsRNA-CTC also leads to changes in mitochondrial cristae structure, impairing the glutaminase (GLS)-dependent glutamate synthesis process, reducing the content of synaptic glutamate, and leading to the appearance of aging-related manifestations, such as memory loss. In addition, in addition to revealing the importance of mitochondrial ultrastructure in maintaining glutamate homeostasis under physiological conditions, this study also discovered the pathological role of microRNA fragments derived from transfer RNA in aging and aging-related diseases.

The research team first discovered through RNA sequencing that compared with young mice, the Glu-5'tsRNA-CTC in the brains of aged mice increased significantly, and the increase in Glu-5'tsRNA-CTC was caused by angiogenin (ANG, a ribonuclease of the RNase A family, responsible for shearing transfer RNA). When young, angiogenin is mainly distributed in the cell nucleus in a phosphorylated form. With the onset of aging, angiogenin gradually becomes dephosphorylated and gradually distributed in the cytoplasm to shear transfer RNA, resulting in the production of a large number of microRNA fragments. It is worth mentioning that the serine 28 site (Ser28) on angiogenin is the site that mediates angiogenin phosphorylation.

Through marker staining experiments, the research team found that Glu-5'tsRNA-CTCs aggregated in the mitochondria of neurons. After carefully comparing the content of Glu-5'tsRNA-CTCs in various mitochondrial structures, it was determined that Glu-5'tsRNA-CTCs were mainly concentrated in the mitochondrial matrix. Surprisingly, there was no angiogenin in the mitochondria at all, which means that Glu-5'tsRNA-CTCs must have entered the mitochondria in other ways, rather than being cut directly in the mitochondria. RNA probe and mass spectrometry analysis gave the answer. Leucine transfer RNA synthetase 2 synthesized in the cytoplasm is the "accomplice" that binds to Glu-5'tsRNA-CTC to help it enter the mitochondria. After leucine transfer RNA synthetase 2 binds to the UUA site of Glu-5'tsRNA-CTC, Glu-5'tsRNA-CTC is engulfed by the mitochondria into the mitochondrial matrix with the help of leucine transfer RNA synthetase 2. However, this binding prevents the leucine transfer RNA synthetase 2 from interacting with its original "partner" - mitochondrial leucine transfer RNA, because Glu-5'tsRNA-CTC occupies the binding site in advance, making it impossible for mitochondrial leucine transfer RNA to be aminoacylated normally (Figure 2).

Figure 2 Schematic diagram of competitive binding between Glu-5'tsRNA-CTC and mitochondrial leucine transfer RNA

Leucine is present in many proteins encoded by the mitochondrial genome. Due to the interference of Glu-5'tsRNA-CTC, leucine cannot be normally aminoacylated, resulting in the inability to translate mitochondrial proteins normally. As mentioned in the previous article, with the aging process, mitochondrial cristae will gradually retract to the inner membrane of mitochondria. Using Glu-5'tsRNA-CTC knockout mice and transmission electron microscopy (TEM), the research team found that the retraction of mitochondrial cristae is caused by the inability of mitochondrial proteins to be translated normally due to Glu-5'tsRNA-CTC. This ultimately affects mitochondrial function.

Glutamate is the most common excitatory neurotransmitter in interneuronal signaling. It is released from the presynaptic membrane and recognized by the glutamate receptors on the postsynaptic membrane to activate downstream signals. The last link in the synthesis of glutamate in the presynaptic membrane is the conversion of glutamine to glutamate catalyzed by glutaminase, and glutaminase is located on the mitochondrial cristae. Therefore, when aging eventually leads to abnormal mitochondrial cristae structure, the ubiquitination of glutaminase increases, resulting in a large amount of glutaminase being degraded and the content decreasing, which ultimately leads to a decrease in the glutamate content between synapses and a significant decrease in the memory ability of mice. In addition to directly indicating that the increase in Glu-5'tsRNA-CTC is directly related to memory defects caused by aging, the above results also suggest that reducing the content of Glu-5'tsRNA-CTC can protect aged mice from mitochondrial dysfunction, glutamate metabolism disorders and memory dysfunction (Figure 3).

Figure 3 Schematic diagram of the pathological effects of Glu-5'tsRNA-CTC

In addition to a series of studies on mice, the research team also found an increase in the content of Glu-5'tsRNA-CTC in the brains of elderly primates (rhesus monkeys and humans). These phenomena further suggest that Glu-5'tsRNA-CTC may be a potential target for the treatment of aging-related memory disorders. Therefore, the research team designed antisense oligonucleotides (ASOs) targeting Glu-5'tsRNA-CTC for treatment. Antisense oligonucleotides are chemically synthesized oligonucleotides, usually 12-30 nucleotides in length, which can pair and bind to target RNA through base pairing rules, thereby causing the target RNA to degrade or break, or occupy the binding site of the target RNA, making it unable to translate normally, thereby playing an inhibitory role. The research team successfully used Glu-5'tsRNA-CTC antisense oligonucleotides to achieve ideal therapeutic effects on mice. Of course, although the research team has made a major breakthrough in the study of Glu-5'tsRNA-CTC, a microRNA fragment derived from transfer RNA, there are actually many microRNA fragments in cells. It is not ruled out that other microRNA fragments also play an important role in physiological or pathological conditions. Perhaps in the future, the combination of antisense oligonucleotides of multiple microRNA fragments will be a better treatment method.

References:

[1] Daum B, Walter A, Horst A, Osiewacz HD, Kühlbrandt W. Age-dependent dissociation of ATP synthase dimers and loss of inner-membrane cristae in mitochondria. Proc Natl Acad Sci US A. 2013 Sep 17;110(38):15301-6. doi: 10.1073/pnas.1305462110. Epub 2013 Sep 4. PMID: 24006361; PMCID: PMC3780843.

[2] Li D, Gao X, Ma 10.1016/j.cmet.2024.02.011. Epub ahead of print. PMID: 38458203.