Deterioration of adult stem cells accounts for much of aging-associated compromised tissue maintenance. How stem cells maintain metabolic homeostasis remains elusive. Here, we identified a regulatory branch of the mitochondrial unfolded protein response (UPRmt), which is mediated by the interplay of SIRT7 and NRF1 and is coupled to cellular energy metabolism and proliferation. SIRT7 inactivation caused reduced quiescence, increased mitochondrial protein folding stress (PFSmt), and compromised regenerative capacity of hematopoietic stem cells (HSCs). SIRT7 expression was reduced in aged HSCs, and SIRT7 up-regulation improved the regenerative capacity of aged HSCs. These findings define the deregulation of a UPRmt-mediated metabolic checkpoint as a reversible contributing factor for HSC aging.

SUMMARY Nonalcoholic fatty liver disease is the most common chronic liver disorder in developed countries. Its pathogenesis is poorly understood, and therapeutic options are limited. Here we show that SIRT7, an NAD+-dependent H3K18Ac deacetylase, functions at chromatin to suppress ER stress and prevents the development of fatty liver disease. SIRT7 is induced upon ER stress and is stabilized at the promoters of ribosomal proteins through its interaction with the transcription factor Myc to silence gene expression and to relieve ER stress. SIRT7 deficient mice develop chronic hepatosteatosis resembling human fatty liver disease. Myc inactivation or pharmacological suppression of ER stress alleviates fatty liver caused by SIRT7 deficiency. Importantly, SIRT7 suppresses ER stress and reverts the fatty liver disease in diet-induced obese mice. Our study identifies SIRT7 as a cofactor of Myc for transcriptional repression and delineates a druggable regulatory branch of the ER stress response that prevents and reverts fatty liver disease.

It is well documented that the rate of aging can be slowed, but it remains unclear to which extent agingassociated conditions can be reversed. How the interface of immunity and metabolism impinges upon the diabetes pandemic is largely unknown. Here, we show that NLRP3, a pattern recognition receptor, is modified by acetylation in macrophages and is deacetylated by SIRT2, an NAD + -dependent deacetylase and a metabolic sensor. We have developed a cellbased system that models aging-associated inflammation, a defined co-culture system that simulates the effects of inflammatory milieu on insulin resistance in metabolic tissues during aging, and aging mouse models; and demonstrate that SIRT2 and NLRP3 deacetylation prevent, and can be targeted to reverse, aging-associated inflammation and insulin resistance. These results establish the dysregulation of the acetylation switch of the NLRP3 inflammasome as an origin of aging-associated chronic inflammation and highlight the reversibility of aging-associated chronic inflammation and insulin resistance.

SUMMARY Stem cells balance cellular fates through asymmetric and symmetric divisions in order to self-renew or to generate downstream progenitors. Symmetric commitment divisions in stem cells are required for rapid regeneration during tissue damage and stress. The control of symmetric commitment remains poorly defined. Using single-cell RNA sequencing (scRNA-seq) in combination with transcriptomic profiling of HSPCs (hematopoietic stem and progenitor cells) from control and m6A methyltransferase Mettl3 conditional knockout mice, we found that m6A-deficient hematopoietic stem cells (HSCs) fail to symmetrically differentiate. Dividing HSCs are expanded and are blocked in an intermediate state that molecularly and functionally resembles multipotent progenitors. Mechanistically, RNA methylation controls Myc mRNA abundance in differentiating HSCs. We identified MYC as a marker for HSC asymmetric and symmetric commitment. Overall, our results indicate that RNA methylation controls symmetric commitment and cell identity of HSCs and may provide a general mechanism for how stem cells regulate differentiation fate choice.

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