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Selenium is a naturally found trace element, which provides multiple benefits including antioxidant, anticancer, and antiaging, as well as boosting immunity. One unique feature of selenium is its incorporation as selenocysteine, a rare 21st amino acid, into selenoproteins. Twenty-five human selenoproteins have been discovered, and a majority of these serve as crucial antioxidant enzymes for redox homeostasis. Unlike other amino acids, incorporation of selenocysteine requires a distinctive UGA stop codon recoding mechanism. Although many studies correlating selenium, selenoproteins, aging, and senescence have been performed, it has not yet been explored if the upstream events regulating selenoprotein synthesis play a role in senescence-associated pathologies. The epitranscriptomic writer alkylation repair homolog 8 (ALKBH8) is critical for selenoprotein production, and its deficiency can significantly decrease levels of selenoproteins that are essential for reactive oxygen species (ROS) detoxification, and increase oxidative stress, one of the major drivers of cellular senescence. Here, we review the potential role of epitranscriptomic marks that govern selenocysteine utilization in regulating the senescence program.

Cellular senescence has evolved as a protective mechanism to arrest growth of cells with oncogenic potential but is accompanied by the often pathologically deleterious senescence-associated secretory phenotype (SASP). Here we demonstrate an H2O2-dependent functional disruption controlling senescence-associated Ca2+ homeostasis and the SASP. Senescent cells fail to respond to H2O2-dependent plasma lamellar Ca2+ entry when compared to pre-senescent cells. Limiting exposure to senescence-associated H2O2 restores H2O2-dependent Ca2+ entry as well as transient receptor potential cation channel subfamily C member 6 (TRPC6) function. SA-TRPC6 and SASP expression is blocked by restoring Ca2+ entry with the TRP channel antagonist SKF-96365 or by the mTOR inhibitors rapamycin and Ku0063794. Together, our findings provide compelling evidence that redox and mTOR-mediated regulation of Ca2+ entry through TRPC6 modulates SASP gene expression and approaches which preserve normal Ca2+ homeostasis may prove useful in disrupting SASP activity. Impact statement Through its ability to evoke responses from cells in a paracrine fashion, the senescence-associated secretory phenotype (SASP) has been linked to numerous age-associated disease pathologies including tumor invasion, cardiovascular dysfunction, neuroinflammation, osteoarthritis, and renal disease. Strategies which limit the amplitude and duration of SASP serve to delay age-related degenerative decline. Here we demonstrate that the SASP regulation is linked to shifts in intracellular Ca2+ homeostasis and strategies which rescue redox-dependent calcium entry including enzymatic H2O2 scavenging, TRP modulation, or mTOR inhibition block SASP and TRPC6 gene expression. As Ca2+ is indispensable for secretion from both secretory and non-secretory cells, it is exciting to speculate that the expression of plasma lamellar TRP channels critical for the maintenance of intracellular Ca2+ homeostasis may be coordinately regulated with the SASP.

Autoimmune uveitis is a sight-threatening ocular inflammatory condition in which the retina and uveal tissues become a target of autoreactive immune cells. The CD47 is a ubiquitously expressed transmembrane protein which plays multiple roles in fundamental cellular functions including phagocytosis, proliferation, and adhesion. Signal regulatory protein alpha (SIRPα), one of the CD47 ligands, is predominantly expressed in myeloid lineage cells such as dendritic cells (DCs) or macrophages, and CD47-SIRPα signaling pathway is implicated in the development of autoimmune diseases. Our current study demonstrates how CD47 depletion is effective in the prevention of experimental autoimmune uveitis (EAU), an animal model of human autoimmune uveitis, in animals deficient of CD47 (CD47-/-). Systemic suppression of SIRPα+ DCs in animals deficient in CD47 resulted in the inability of autoreactive CD4+ T cells to develop, which is crucial to induction of EAU. Of interest, retinal microglia, the resident immune cell of the retina, express SIRPα, however these cells were not operative in EAU suppression in response to CD47 depletion. These results identify CD47 as a significant regulator in the development of SIRPα+ DCs that is vital to disease induction in EAU.

Critically important to the maintenance of the glutathione (GSH) redox cycle are the activities of many selenocysteine-containing GSH metabolizing enzymes whose translation is controlled by the epitranscriptomic writer alkylation repair homolog 8 (ALKBH8). ALKBH8 is a tRNA methyltransferase that methylates the wobble uridine of specific tRNAs to regulate the synthesis of selenoproteins. Here we demonstrate that a deficiency in the writer ALKBH8 (Alkbh8def), alters selenoprotein levels and engages senescence, regulates stress response genes and promotes mitochondrial reprogramming. Alkbh8def mouse embryonic fibroblasts (MEFs) increase many hallmarks of senescence, including senescence associated β-galactosidase, heterocromatic foci, the cyclin dependent kinase inhibitor p16Ink4a, markers of mitochondrial dynamics as well as the senescence associated secretory phenotype (SASP). Alkbh8def cells also acquire a stress resistance phenotype that is accompanied by an increase in a number redox-modifying transcripts. In addition, Alkbh8def MEFs undergo a metabolic shift that is highlighted by a striking increase in the level of uncoupling protein 2 (UCP2) which enhances oxygen consumption and promotes a reliance on glycolytic metabolism. Finally, we have shown that the Alkbh8 deficiency can be exploited and corresponding MEFs are killed by glycolytic inhibition. Our work demonstrates that defects in an epitransciptomic writer promote senescence and mitochondrial reprogramming and unveils a novel adaptive mechanism for coping with defects in selenocysteine utilization., Graphical abstract Image 1, Highlights • Deficiencies in selenocysteine utilization engages cellular senescence and the senescence associated secretory phenotype. • Alkbh8 deficiency promotes mitochondrial elongation, increased oxygen consumption and a reliance on glycolytic metabolism. • Cellular adaptions to Alkbh8 deficiency confer stress resistance.

Cellular senescence has evolved as a protective mechanism to arrest growth of cells with oncogenic potential. While senescent cells have lost the ability to divide, they remain metabolically active and adapt a deleterious senescence associated secretory phenotype (SASP) central to the progression of several age-associated disease pathologies. The SASP is mechanistically regulated by the pro-inflammatory cytokine interleukin-1 alpha (IL-1α) whose expression and activity is responsive to the senescence associated (SA) oxidant production and the accompanying disruption of calcium (Ca2+) homeostasis. Using primary IMR-90 human fetal lung fibroblasts as a model of replicative senescence, we explored the molecular underpinnings facilitating increased Ca2+ entry in senescent cells. We establish that the redox-responsive Transient Receptor Potential TRPC6 channel is compromised due to desensitization owing to SA increases in steady state hydrogen peroxide (H2O2) production. SA dysregulation of Ca2+ is also accompanied by loss of response to H2O2-induced Ca2+ influx that can be rescued with catalase pre-treatments. Senescent cells are also insensitive to Ca2+ entry induced by hyperforin, a specific activator of TRPC6 that can be restored by catalase pre-treatments, further suggesting redox regulation of TRPC6 in senescence. Inhibition of TRPC6 channel activity restores ability of senescent cells to respond to peroxide-induced Ca2+ in addition to suppressing SASP gene expression. Furthermore, mammalian target of rapamycin (mTOR) signaling regulates SASP by means of modulating TRPC6 channel expression. Together, our findings provide compelling evidence that H2O2 and mTOR-mediated regulation of TRPC6 channel modulates SASP gene expression. As TRP channels emerge as targets of pharmacologic intervention for numerous disease pathologies, it is exciting to speculate that effect of TRP interventions may be attributed in part to inhibition of senescence and the SASP.

Over 14,000 Americans die each year from kidney cancer with clear-cell Renal Cell Carcinoma (ccRCC) being the most prevalent and malignant type of kidney cancer. Recent work has implicated disruptions in glutathione (GSH) metabolism as predictors of poor survival in ccRCC. Maintenance of the GSH redox cycle is reliant on the activities of selenocysteine-containing GSH metabolizing enzymes whose translation is controlled by the alkylation repair homolog 8 (Alkbh8) tRNA methyltransferase. In cells from Alkbh8-/- mice the absence of the tRNA methyltransferase Alkbh8 decreases the levels of many GSH metabolizing selenoproteins, promotes increased ROS and DNA damage levels, and confers enhanced sensitivity to oxidizing agents. Surprisingly the Alkbh8-/- mice reproduce, thrive normally but display phenotypic characteristics of premature ageing such as weight loss, reduced subcutaneous fat, alopecia (hair loss), kyphosis (curvature of the spine) when compared to their aged-matched WT control. Alkbh8-deficiency also triggers a senescence and metabolic gene signature that is predictive (Hazard Ratio 2.94, p