Researchers publishing in Cellular Signaling have explained how the protein AP2A1 affects stress fibers that change with cellular senescence.
Stress fibers
Stress fibers naturally hold cells into their proper shape. They are made out of the common protein actin and linked together by α-actinin and a form of myosin that is not directly related to muscles. Mesenchymal cells, which are found in the extracellular matrix (ECM), use these stress fibers to pull the ECM into the necessary shapes [2]. These miniature mechanical processes affect how cells develop [3] and have even been implicated in cancer [4].
These fibers change with senescence [5], and these researchers have recently discovered how considerable the changes are: out of 135 proteins, the researchers found that 63 of them were upregulated as cells become senescent [6]. Many of these proteins have been thoroughly researched, but despite being generally known in its biological functions and implicated in multiple diseases [7], AP2A1 had not previously been investigated in the context of senescence.
This work began by allowing human fibroblasts to divide 30 times, which is when they were considered aged and approached replicative senescence, with the typical changes in morphology and senescence-related biomarkers. Fibroblasts passaged for 10 and 20 times were referred to as young and adult, respectively.
Part of why senescent cells look different
Consistent with the protein upregulation that these researchers previously found, the stress fibers in the aged fibroblasts were thicker than their young and adult counterparts. The natural turnover rate of these fibers was also significantly lower. These cells also kept, rather than recycled, the structural protein integrin β1, which is used to bolster fiber thickness. Additionally, the researchers confirmed that these senescent cells lacked the motility that the younger cells had.
Senescent fibroblasts were found to adhere differently, and more firmly, to the ECM than their younger counterparts. In younger cells, two proteins related to focal adhesion, vinculin and paxillin, were located at the edges, as were stress fibers; meanwhile, in older cells, these cellular features were located more centrally.
AP2A1 was found to increase with age in both proteomic and gene expression analyses. While in younger cells, this protein is diffusely spread throughout the cellular structure, it is aligned along the fibers of senescent cells. AP2A1 is known to affect endocytosis, the process that transports materials into the cell, and this process was found to also be increased with senescence. The movement of AP2A1 within the cell was found to be slowed down with age as well.
These age-related changes were confirmed to be associated with multiple forms of senescence. In addition to repeated replication, cells can be driven senescent by radiation or chemicals. Using either of these approaches led to the same increases in AP2A1, and associated changes in morphology, as replicative senescence did.
A two-way street
While senescence clearly affects AP2A1, the researchers also wanted to know whether this relationship also works in reverse. Using silencing RNA (siRNA), the researchers stopped the aged cells in their culture from expressing AP2A1. Unsurprisingly, the modified cells were smaller and had fewer stress fibers, but most critically, they also had reduced levels of senescence biomarkers, including p53, p21, and the well-known SA-β-gal. Cellular proliferation, which declines with senescence, was enhanced by this removal.
Overexpression, on the other hand, appeared to lead to senescence. Young cells that were induced to express more AP2A1 had the characteristic increases in stress fibers and overall size, and their senescence biomarkers were increased as well.
The researchers believe that these facts make AP2A1 a good target for further study. Still, this is a cellular study in fibroblasts, and its findings have not been confirmed in animal models. As this protein appears to be a fundamental building block of cellular function, broad reductions may have significant side effects in living animals. However, if it can be more precisely targeted, this protein may be of key interest to research groups looking how to mitigate the increase in senescence that comes with aging.
Literature
[1] Burridge, K., & Wittchen, E. S. (2013). The tension mounts: stress fibers as force-generating mechanotransducers. Journal of Cell Biology, 200(1), 9-19.
[2] Burridge, K., & Guilluy, C. (2016). Focal adhesions, stress fibers and mechanical tension. Experimental cell research, 343(1), 14-20.
[3] Ridley, A. J., Schwartz, M. A., Burridge, K., Firtel, R. A., Ginsberg, M. H., Borisy, G., … & Horwitz, A. R. (2003). Cell migration: integrating signals from front to back. Science, 302(5651), 1704-1709.
[4] Tojkander, S., Gateva, G., & Lappalainen, P. (2012). Actin stress fibers–assembly, dynamics and biological roles. Journal of cell science, 125(8), 1855-1864.
[5] Chen, Q. M., Tu, V. C., Catania, J., Burton, M., Toussaint, O., & Dilley, T. (2000). Involvement of Rb family proteins, focal adhesion proteins and protein synthesis in senescent morphogenesis induced by hydrogen peroxide. Journal of cell science, 113(22), 4087-4097.
[6] Liu, S., Matsui, T. S., Kang, N., & Deguchi, S. (2022). Analysis of senescence-responsive stress fiber proteome reveals reorganization of stress fibers mediated by elongation factor eEF2 in HFF-1 cells. Molecular biology of the cell, 33(1), ar10.
[7] Wang, C., Zhao, D., Shah, S. Z. A., Yang, W., Li, C., & Yang, L. (2017). Proteome analysis of potential synaptic vesicle cycle biomarkers in the cerebrospinal fluid of patients with sporadic Creutzfeldt–Jakob disease. Molecular Neurobiology, 54, 5177-5191.
