Researchers have published a method of rescuing cells from damaged mitochondria and cellular senescence, potentially alleviating major aspects of aging.
Bad mitochondria must be consumed
A core part of autophagy involves selective autophagy receptors (SARs), which build the autophagosomes in which the organelles are consumed [1]. Mitophagy is a subset of autophagy that refers to the consumption of mitochondria. When these tiny power plants become damaged and dysfunctional, they need to be cleared and replaced, and failure to do this drives age-related diseases [2]. Mitochondrial autophagy is primarily spurred by the PINK1/Parkin pathway, which has been extensively studied [3]. PINK1 on the surface of damaged mitochondria leads to the formation of downstream targets, which spur SAR activity through multiple biochemical pathways [4].
However, much previous research into mitophagy has been done in models of intense mitochondrial stress. This research team has also done work demonstrating that a lack of mitophagy is a driver of cellular senescence [5], and this paper builds on that work, elaborating on a pathway fundamental to mitophagy and a small molecule that encourages it.
Mitophagy in human cells
These researchers used a genetically engineered human cell line that expresses a fluorescent reporter compound when mitophagy is conducted, along with a separate reporter that offers real-time information [6]. They then exposed these cells to ionizing radiation to drive them into senescence, according to well-established biomarkers and a halting of the cell cycle. Interestingly, this did not stop autophagy as a whole; in fact, general autophagy was increased, which is in line with previous research [7].
However, mitophagy was greatly suppressed with radiation-induced senescence, which still held true on a different group of cells driven senescent through hydrogen peroxide. Pre-senescent human dermal fibroblasts (HDFs) derived from older people also had reduced mitophagy compared to their younger counterparts.
Mitochondrial superoxide, unlike other reactive oxygen species (ROS), was found to be significantlly downregulated in radiation, hydrogen peroxide exposure, and natural aging. This superoxide is one method by which mitochondria are encouraged to be consumed in mitophagy [8]. The researchers discovered that this was attributable to mitochondrial fusion: instead of being consumed, the mitochondria were fusing together in cells.
This process was amenable to chemical intervention. Exposing cells to paraquat, which encourages superoxide production, also encouraged mitophagy. Targeting the cells with the well-known mitochondrial ROS scavenger mitoquinone (MitoQ) discouraged mitophagy, and exposing HDFs to MitoQ over 11 days drove them into senescence. Rather than being entirely negative, therefore, some ROS are clearly required for proper mitochondrial function.
Multiple other elements were found to be related to this superoxide-induced mitophagy, including the PINK1/Parkin pathway and the autophagy receptor p62. Knocking down p62 suppressed autophagy in proliferating HDFs.
Potential treatments
The researchers investigated whether NAD precusors, including nicotinamide and NR, along with the well-known compound rapamycin could rescue mitophagy, and they found positive results for all of these compounds. Some of the markers associated with senescence were recovered, but it did not restore the cells’ ability to proliferate. NAD precursors were able to encourage mitophagy even in cells that had p62 knocked down.
The researchers then performed a long series of experiments involving p62 and various mutant forms. They found that one particular small molecule, STOCK1N-57534, strongly encouraged p62 to oligomerize, which encouraged mitophagy. Most importantly, applying STOCK1N-57534 to the HDFs derived from older people restored much of their function, decreasing senescence markers and increasing the motility and activity of these cells and their mitochondria.
The researchers clearly believe that they have discovered a potential approach to age-related diseases of mitochondria and senescence. However, this is only a cellular study. Preclinical work in animals will need to be done before this small molecule, or any derivative, can be considered for the clinical trial process.
Literature
[1] Conway, O., Akpinar, H. A., Rogov, V. V., & Kirkin, V. (2020). Selective autophagy receptors in neuronal health and disease. Journal of molecular biology, 432(8), 2483-2509.
[2] Sedlackova, L., & Korolchuk, V. I. (2019). Mitochondrial quality control as a key determinant of cell survival. Biochimica et Biophysica Acta (BBA)-Molecular Cell Research, 1866(4), 575-587.
[3] Onishi, M., Yamano, K., Sato, M., Matsuda, N., & Okamoto, K. (2021). Molecular mechanisms and physiological functions of mitophagy. The EMBO journal, 40(3), e104705.
[4] Lazarou, M., Sliter, D. A., Kane, L. A., Sarraf, S. A., Wang, C., Burman, J. L., … & Youle, R. J. (2015). The ubiquitin kinase PINK1 recruits autophagy receptors to induce mitophagy. Nature, 524(7565), 309-314.
[5] Korolchuk, V. I., Miwa, S., Carroll, B., & Von Zglinicki, T. (2017). Mitochondria in cell senescence: is mitophagy the weakest link?. EBioMedicine, 21, 7-13.
[6] Sun, N., Yun, J., Liu, J., Malide, D., Liu, C., Rovira, I. I., … & Finkel, T. (2015). Measuring in vivo mitophagy. Molecular cell, 60(4), 685-696.
[7] Young, A. R., Narita, M., Ferreira, M., Kirschner, K., Sadaie, M., Darot, J. F., … & Narita, M. (2009). Autophagy mediates the mitotic senescence transition. Genes & development, 23(7), 798-803.
[8] Kataura, T., Otten, E. G., Rabanal‐Ruiz, Y., Adriaenssens, E., Urselli, F., Scialo, F., … & Korolchuk, V. I. (2023). NDP52 acts as a redox sensor in PINK1/Parkin‐mediated mitophagy. The EMBO Journal, 42(5), e111372.
