In this study, we identified the molecular mechanisms that induce cellular senescence in DRG neurons and the inflammation-induced phenotype of aged DRG neurons that have undergone cellular senescence (Fig. 6).
We found that cellular senescence in DRG neurons was induced by increased expression of CDKN2A, which encodes the cellular senescence-inducing factor p16, which in turn was caused by increased expression of Srebf1, a transcription factor involved in lipid and cholesterol metabolism. Although Srebf1 expression increases with aging, the senescence-induced gene network to which Srebf1 belongs primarily includes genes whose expression decreases with aging. This gene network includes Fdps and Pmvk, the leading-edge genes of the pathways involved in cholesterol metabolism and biosynthesis, which are significantly suppressed by aging. Furthermore, Mapk3, a hub gene that plays a central role in this network, was suppressed by aging, suggesting that cholesterol metabolism and biosynthesis are regulated by Mapk3. The conflicting results between Srebf1 and Mapk3 expression may be attributed to the possibility that Srebf1 expression may have increased to compensate for the Mapk3-mediated decline in cholesterol metabolism and biosynthetic functions associated with aging. Mapk3 is a target gene for cancer therapy, and its increased expression activates cell proliferation and differentiation, whereas its suppression induces the expression of the cellular senescence marker p16 and accumulation of aged cells [27, 28]. Srebf1 has been identified as a marker of cellular senescence in patients with bladder [29] and esophageal cancers [30]. Although decreased Mapk3 expression and increased Srebf1 expression are involved in cellular senescence, the pathways involved are unclear. However, in this study, two independent changes in gene expression thought to induce cellular senescence can be viewed as a series of molecular mechanisms associated with aging. Interestingly, reduced cholesterol metabolism and biosynthetic function, which is key to the molecular response that induces cellular senescence in DRG neurons, occurs only in the axons of DRG neurons and not the cell body. In DRG neurons, axons are subject to various stresses, which may induce cellular senescence of DRG neurons. In summary, the p16-mediated cellular senescence process in DRG neurons may be explained by the compensatory upregulation of Srebf1 for cholesterol metabolism and biosynthetic dysfunction via Mapk3 downregulation in axons.
We also identified a senescence phenotype that characterizes aged DRG neurons. In this phenotype, Ctss functions as a hub gene and promotes the immune response by increasing the expression of antigen-presenting cell activators (Ncf2, Itgam, nckap1l, Itgb2, and Tyrobp). The molecular response that results in chronic inflammation in aged DRG neurons also occurs specifically in the axons. This is because T cells that recognizes the antigens presented by activated antigen-presenting cells infiltrate the SN but rarely infiltrate the DRG. Zhou et al. showed that CD8+ T-cell infiltration of sensory nerve axons occurs when aging and releases inflammatory cytokines, leading to age-related axonal degeneration and prolonged injury healing. Our study also identified CD8+ T cells as an infiltrating T-cell type that significantly increases with age in SNs [31]. In addition, Ctss, the hub gene identified in this study, is a plasma protein marker that predicts migratory deficits in elderly individuals, suggesting a link with aging [32]. In summary, aged DRG neurons may contribute to chronic inflammation in sensory nerve axons by activating antigen-presenting cells through Ctss upregulation, which in turn causes activation of CD8+ T cells. Chronic inflammation of nerve tissue accelerates tissue aging [33–35]. Our findings may provide the basis for research on the molecular mechanism underlying the “Inflammaging” of peripheral sensory nerves.
A limitation of this study is that the inferred molecular mechanisms underlying the cellular senescence process and senescence phenotype have not been validated through in vitro and in vivo experiments. In particular, it remains to be verified whether the upregulation of Srebf1 in the cell body during the cellular senescence process is a compensatory response to reduced cholesterol metabolism in axons, specifically due to the inactivation of the mevalonate pathway. However, there is some evidence that a reduction in the mevalonate pathway causes the cellular aging process. This is evidenced by the use of statins, which inhibit the mevalonate pathway, leading to age-like declines in peripheral nerve function [36, 37] and the involvement of cell membrane fluidity, which is regulated by the mevalonate pathway, in the cellular senescence process [38, 39].