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That sirtuin something Since the discovery that the life extending benefits of caloric restriction in yeast require Sir2, a protein originally isolated in a screen for gene silencing factors (and called silent information regulator 2), there has been considerable interest in the family of proteins collectively known as the sirtuins. This week Toren Finkel, Chu-Xia Deng and Raul Mostoslavsky review recent advances in sirtuin biology. These NAD-dependent enzymes have been implicated in a wide array of cell-fate decisions, in maintaining genomic stability and regulating overall energy metabolism. Increasing evidence also implicates the sirtuins in the progression of a number of disease states ranging from diabetes to cancer. These observations, coupled with the recent progress in the rationale design of small molecule sirtuin activators, raise the possibility of new therapies targeted to a host of age-related diseases, as well as potential pharmacological strategies to slow mammalian ageing. The sirtuins are a highly conserved family of NAD + -dependent enzymes that regulate lifespan in lower organisms. Recently, the mammalian sirtuins have been connected to an ever widening circle of activities that encompass cellular stress resistance, genomic stability, tumorigenesis and energy metabolism. Here we review the recent progress in sirtuin biology, the role these proteins have in various age-related diseases and the tantalizing notion that the activity of this family of enzymes somehow regulates how long we live.

Sun et al . describe how to image and quantify mitophagy in both living cells and tissues, using the pH-sensitive fluorescent reporter mt-Keima. This protocol provides information for analysis by both confocal and super-resolution microscopy. Mitophagy is a cellular process that selectively removes damaged, old or dysfunctional mitochondria. Defective mitophagy is thought to contribute to normal aging and to various neurodegenerative and cardiovascular diseases. Previous methods used to detect mitophagy in vivo were cumbersome, insensitive and difficult to quantify. We created a transgenic mouse model that expresses the pH-dependent fluorescent protein mt-Keima in order to more readily assess mitophagy. Keima is a pH-sensitive, dual-excitation ratiometric fluorescent protein that also exhibits resistance to lysosomal proteases. At the physiological pH of the mitochondria (pH 8.0), the shorter-wavelength excitation predominates. Within the acidic lysosome (pH 4.5) after mitophagy, mt-Keima undergoes a gradual shift to longer-wavelength excitation. In this protocol, we describe how to monitor mitophagic flux in living cells over an 18-h time frame, as well as how to quantify mitophagy using the mt-Keima probe. This protocol also describes how to use confocal microscopy to visualize mitophagy in living tissues obtained from mt-Keima transgenic mice. With this protocol, the mt-Keima probe can reliably be imaged within the first 60 min after tissue collection. We also describe how to apply mt-Keima with stimulated emission depletion (STED) microscopy, which can potentially provide substantially higher-resolution images. Typically, the approximate time frame for time-lapse fluorescence imaging of mt-Keima is 20 h for living cells. For confocal analysis of tissue from an mt-Keima mouse, the whole procedure generally takes no longer than 60 min, and the STED imaging usually takes <2 h.

The capacity of cells to alter bioenergetics in response to the demands of various biological processes is essential for normal physiology. The coordination of energy sensing and production with highly energy-demanding cellular processes, such as cell division, is poorly understood. Here, we show that a cell cycle-dependent mitochondrial Ca 2+ transient connects energy sensing to mitochondrial activity for mitotic progression. The mitochondrial Ca 2+ uniporter (MCU) mediates a rapid mitochondrial Ca 2+ transient during mitosis. Inhibition of mitochondrial Ca 2+ transients via MCU depletion causes spindle checkpoint-dependent mitotic delay. Cellular ATP levels drop during early mitosis, and the mitochondrial Ca 2+ transients boost mitochondrial respiration to restore energy homeostasis. This is achieved through mitosis-specific MCU phosphorylation and activation by the mitochondrial translocation of energy sensor AMP-activated protein kinase (AMPK). Our results establish a critical role for AMPK- and MCU-dependent mitochondrial Ca 2+ signalling in mitosis and reveal a mechanism of mitochondrial metabolic adaptation to acute cellular energy stress. Zhao et al. find that AMPK phosphorylates and activates the mitochondrial Ca 2+ uniporter in response to low energy status in mitosis, allowing Ca 2+ entry into mitochondria to boost mitochondrial respiration.

Toren Finkel reviews how metabolism and aging are connected, and highlights pathways that could be pharmacologically targeted to combat aging and age-related disease. Here we review the environmental and genetic manipulations that link cellular and organismal metabolism to aging. In particular, we explore how nutrients are sensed and how various intracellular energy nodes seem to coordinate distinct metabolic alterations linked to extended longevity. In addition, the role of mitochondria as both a metabolic and signaling organelle is discussed. Finally, we review a host of new targeted pharmacological approaches that attempt to exploit the connection between aging and metabolism to treat a wide range of age-related diseases. Together, these insights are beginning to reveal answers to century-old mysteries and are providing a future road map for the rational extension of lifespan.

Sirtuins are highly conserved enzymes with key roles in life extension in multiple organisms. A study now describes selective autophagic degradation of nuclear SIRT1 in senescent cells. These observations suggest that blocking sirtuin degradation could be a potential approach for anti-ageing therapies.

T lymphocytes are powerful immune cells that can destroy tumors, but cancers have developed tricks to evade killing. Vodnala et al. found that potassium ions in the tumor microenvironment serve a dual role of influencing T cell effector function and stemness (see the Perspective by Baixauli Celda et al. ). Increased potassium impairs T cell metabolism and nutrient uptake, resulting in a starvation state known as autophagy. The increased potassium can also preserve T cells in a stem-like state where they retain the capacity to divide. These seemingly divergent processes are linked to the cellular distribution of acetyl–coenzyme A, which, when manipulated, can restore the ability of human T cells to eliminate tumors in mice. Science , this issue p. eaau0135 ; see also p. 1395 Potassium in the tumor microenvironment metabolically reprograms tumor-infiltrating immunological T cells. A paradox of tumor immunology is that tumor-infiltrating lymphocytes are dysfunctional in situ, yet are capable of stem cell–like behavior including self-renewal, expansion, and multipotency, resulting in the eradication of large metastatic tumors. We find that the overabundance of potassium in the tumor microenvironment underlies this dichotomy, triggering suppression of T cell effector function while preserving stemness. High levels of extracellular potassium constrain T cell effector programs by limiting nutrient uptake, thereby inducing autophagy and reduction of histone acetylation at effector and exhaustion loci, which in turn produces CD8 + T cells with improved in vivo persistence, multipotency, and tumor clearance. This mechanistic knowledge advances our understanding of T cell dysfunction and may lead to novel approaches that enable the development of enhanced T cell strategies for cancer immunotherapy.

SARS-CoV-2 (2019-nCoV) is the pathogenic coronavirus responsible for the global pandemic of COVID-19 disease. The Spike (S) protein of SARS-CoV-2 attaches to host lung epithelial cells through the cell surface receptor ACE2, a process dependent on host proteases including TMPRSS2. Here, we identify small molecules that reduce surface expression of TMPRSS2 using a library of 2,560 FDA-approved or current clinical trial compounds. We identify homoharringtonine and halofuginone as the most attractive agents, reducing endogenous TMPRSS2 expression at sub-micromolar concentrations. These effects appear to be mediated by a drug-induced alteration in TMPRSS2 protein stability. We further demonstrate that halofuginone modulates TMPRSS2 levels through proteasomal-mediated degradation that involves the E3 ubiquitin ligase component DDB1- and CUL4-associated factor 1 (DCAF1). Finally, cells exposed to homoharringtonine and halofuginone, at concentrations of drug known to be achievable in human plasma, demonstrate marked resistance to SARS-CoV-2 infection in both live and pseudoviral in vitro models. Given the safety and pharmacokinetic data already available for the compounds identified in our screen, these results should help expedite the rational design of human clinical trials designed to combat active COVID-19 infection. The serine protease TMPRSS2 primes SARS-CoV-2 glycoprotein for cell entry. Here, the authors perform a screen to identify drugs that reduce TMPRSS2 expression and find that halofuginone modulates proteasome-mediated degradation of TMPRSS2 and reduces entry of SARS-CoV-2.

At present, it remains difficult to deconvolute serum in order to identify the cell or tissue origin of a given circulating protein. Here, by exploiting the properties of proximity biotinylation, we describe a mouse model that enables the elucidation of the in vivo tissue-specific secretome. As an example, we demonstrate how we can readily identify in vivo endothelial-specific secretion as well as how this model allows for the characterization of muscle-derived serum proteins that either increase or decrease with exercise. This genetic platform should, therefore, be of wide utility in understanding normal and disease physiology and for the rational design of tissue-specific disease biomarkers.

GWAS cannot identify functional SNPs (fSNP) from disease-associated SNPs in linkage disequilibrium (LD). Here, we report developing three sequential methodologies including Reel-seq (Regulatory element-sequencing) to identify fSNPs in a high-throughput fashion, SDCP-MS (SNP-specific DNA competition pulldown-mass spectrometry) to identify fSNP-bound proteins and AIDP-Wb (allele-imbalanced DNA pulldown-Western blot) to detect allele-specific protein:fSNP binding. We first apply Reel-seq to screen a library containing 4316 breast cancer-associated SNPs and identify 521 candidate fSNPs. As proof of principle, we verify candidate fSNPs on three well-characterized loci: FGFR2 , MAP3K1 and BABAM1 . Next, using SDCP-MS and AIDP-Wb, we rapidly identify multiple regulatory factors that specifically bind in an allele-imbalanced manner to the fSNPs on the FGFR2 locus. We finally demonstrate that the factors identified by SDCP-MS can regulate risk gene expression. These data suggest that the sequential application of Reel-seq, SDCP-MS, and AIDP-Wb can greatly help to translate large sets of GWAS data into biologically relevant information. It is often difficult to identify functional SNPs from disease-associated SNPs in linkage disequilibrium. Here, the authors present Reel-seq, SDCP-MS and AIDP-Wb, three sequential methodologies for fSNP identification and characterization.

Mitochondrial calcium has been postulated to regulate a wide range of processes from bioenergetics to cell death. Here, we characterize a mouse model that lacks expression of the recently discovered mitochondrial calcium uniporter (MCU). Mitochondria derived from MCU −/− mice have no apparent capacity to rapidly uptake calcium. Whereas basal metabolism seems unaffected, the skeletal muscle of MCU −/− mice exhibited alterations in the phosphorylation and activity of pyruvate dehydrogenase. In addition, MCU −/− mice exhibited marked impairment in their ability to perform strenuous work. We further show that mitochondria from MCU −/− mice lacked evidence for calcium-induced permeability transition pore (PTP) opening. The lack of PTP opening does not seem to protect MCU −/− cells and tissues from cell death, although MCU −/− hearts fail to respond to the PTP inhibitor cyclosporin A. Taken together, these results clarify how acute alterations in mitochondrial matrix calcium can regulate mammalian physiology. Until the recent discovery of the mitochondrial calcium uniporter (MCU), the effect of increases in mitochondrial calcium levels could not be tested in vivo . Finkel and colleagues have knocked out the gene coding for MCU in adult mice, and show that MCU is required for transport of calcium into the mitochondria. They also show that, in its absence, the function of skeletal muscle is altered; however, surprisingly, no effects are observed on the sensitivity to cell-death-inducing agents.