The Aging Kidney and Acute Kidney Injury

Aging is unavoidable and driven, at least in part, by the biologic process termed senescence. Senescence is characterized by cell cycle arrest incurred by one or more stressors wherein there is cellular upregulation of cyclin dependent kinase inhibitors, p16Ink4a and/or p21Cip1. Senescent cells are metabolically active, resistant to apoptosis, have increased β-galactosidase activity, and express a senescence-associated secretory phenotype (SASP); the SASP produces an array of inflammatory, cytotoxic, fibrotic, and vasoactive factors. Senescence is a diffusible process as, largely through the SASP, a focus of senescent cells can recruit adjacent cells and cells in distant organs, thereby expanding its scope way beyond its original focus. Senescence predisposes not only to chronic diseases (including CKD) but also acute processes (including AKI). This review provides evidence that age increases the risk of human AKI and summarizes clinical factors that predispose older individuals to AKI. It provides an overview of the biology of senescence, and it discusses the role of intrarenal senescent cells in the pathogenesis of AKI; the significance of the extra-renal milieu and distant organs in predisposing to AKI with age; heme as a pro-senescent species; and the role of senescence in the AKI to CKD transition. The risk for AKI with aging, in large part, reflects the fact that virtually all processes implicated in AKI, irrespective of age, occur as the kidney ages. Processes discussed include senescent cells, vascular responses, diabetes, impaired NAD+ content, sirtuin expression, mitochondrial dysfunction, impaired autophagy, epigenetic changes, telomere shortening, impaired nephroprotectant expression, inflammaging, and immunosenescence. The review concludes by discussing the basis for senolytics (agents that kill senescent cells), senomorphics (agents that block SASP factors), and other aspects of senotherapies.