A paper published in GeroScience has described a gene responsible for a key biomarker of cellular senescence.
A widely used biomarker
Senescence-associated beta-galactosidase (SA-ß-gal) is the most widely used and most definitive biomarker of cellular senescence [1]. However, unlike other senescence-related biomarkers, such as p21 [2], SA-ß-gal testing requires tissue harvesting, making it difficult to use on living organisms [3].
However, SA-ß-gal’s actual relationship to senescence processes has not been fully explored. In order to explore its potential as a biomarker, these researchers developed an RNA-binding protein that restricts SA-ß-gal expression both in C.elegans worms and in human cell cultures, and their experiments provided some insights into how this compound works.
Testing the value of SA-ß-gal
To begin their experiments, the researchers first tested to make sure that SA-ß-gal indeed increases in C.elegans worms with age. As these worms normally live for less than two weeks, these experiments were rapid, showing increasing levels of SA-ß-gal every two days. The researchers knocked out two related gene expressions in these worms, bgal-1 and bgal-2, with the former being primarily responsible for the staining reaction used in their testing.
The researchers also made the important finding that this was only a biomarker. Knocking out these two genes in these worms had no impact on their lifespan, which was in accordance with previous testing in mammalian cells. These researchers conducted further experiments in this area, finding that knocking out the mammalian equivalent in cells (Regnase-1) reduced SA-ß-gal but did not affect the cells’ lifespan; however, knocking out this gene in mice causes them to die soon after birth [5].
Only some longevity interventions affect SA-ß-gal
Anti-senescence drugs were found to reduce SA-ß-gal in C.elegans. One of them is quercetin, a widely known senolytic, which was previously found to increase lifespan in these worms [4]. Navitoclax, another commonly studied senolytic, also significantly decreased SA-ß-gal, as did rapamycin and metformin.
On the other hand, the p53 pathway, which is associated with cellular death by apoptosis, did not decrease SA-ß-gal, as found in worms that were mutated not to express this protein. Further experiments with apoptosis also found no changes in SA-ß-gal.
Irradiating mammalian cells with UV radiation, which is known to trigger senescence, did not increase SA-ß-gal in those cells. Subjecting worms to caloric restriction decreased SA-ß-gal at day four of their lifespan decreased SA-ß-gal at day seven.
The lysosome, which is responsible for consuming cellular junk (autophagy), is the repository of SA-ß-gal. Expectedly, lysosomal activity was found to be strongly related to SA-ß-gal accumulation.
The researchers tested multiple mutations of C.elegans worms that are known to increase lifespan. Many of these mutations affected SA-ß-gal, but many did not. The researchers noted that mutations that increase mitochondrial longevity and affected mTORC1 also decreased the accumulation of SA-ß-gal with age. While the evidence was not fully conclusive (the researchers described it as “tantalizing”), they found a specific mTORC1-related pathway associated with SA-ß-gal.
Valuable information beyond biomarkers
The researchers believe that they have discovered a link between SA-ß-gal and a previously discovered pathway that connects metformin, mitochondria, and mTORC1 [6]. Specifically, metformin increases ACAD10, a factor that decreases SA-ß-gal. Decreasing ACAD10, accordingly, increases SA-ß-gal. The researchers also noted existing links between Regnase-1 and human senescence-related responses, inclduing stress and inflammation [7].
In total, this paper explains many key reasons behind exactly why SA-ß-gal is so thoroughly connected to cellular senescence. The researchers also hold that their findings have opened new paths towards investigating cellular senescence.
Literature
[1] Dimri, G. P., Lee, X., Basile, G., Acosta, M., Scott, G., Roskelley, C., … & Pereira-Smith, O. (1995). A biomarker that identifies senescent human cells in culture and in aging skin in vivo. Proceedings of the National Academy of Sciences, 92(20), 9363-9367.
[2] Wang, B., Wang, L., Gasek, N. S., Zhou, Y., Kim, T., Guo, C., … & Xu, M. (2021). An inducible p21-Cre mouse model to monitor and manipulate p21-highly-expressing senescent cells in vivo. Nature aging, 1(10), 962-973.
[3] Beck, J., Horikawa, I., & Harris, C. (2020). Cellular senescence: mechanisms, morphology, and mouse models. Veterinary Pathology, 57(6), 747-757.
[4] Kampkötter, A., Timpel, C., Zurawski, R. F., Ruhl, S., Chovolou, Y., Proksch, P., & Wätjen, W. (2008). Increase of stress resistance and lifespan of Caenorhabditis elegans by quercetin. Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 149(2), 314-323.
[5] Matsushita, K., Takeuchi, O., Standley, D. M., Kumagai, Y., Kawagoe, T., Miyake, T., … & Akira, S. (2009). Zc3h12a is an RNase essential for controlling immune responses by regulating mRNA decay. Nature, 458(7242), 1185-1190.
[6] Wu, L., Zhou, B., Oshiro-Rapley, N., Li, M., Paulo, J. A., Webster, C. M., … & Soukas, A. A. (2016). An ancient, unified mechanism for metformin growth inhibition in C. elegans and cancer. Cell, 167(7), 1705-1718.
[7] Mao, R., Yang, R., Chen, X., Harhaj, E. W., Wang, X., & Fan, Y. (2017). Regnase-1, a rapid response ribonuclease regulating inflammation and stress responses. Cellular & Molecular Immunology, 14(5), 412-422.
