Regulation of urea cycle by reversible high-stoichiometry lysine succinylation.

The post-translational modification lysine succinylation is implicated in the regulation of various metabolic pathways. However, its biological relevance remains uncertain due to methodological difficulties in determining high-impact succinylation sites. Here, using stable isotope labelling and data-independent acquisition mass spectrometry, we quantified lysine succinylation stoichiometries in mouse livers. Despite the low overall stoichiometry of lysine succinylation, several high-stoichiometry sites were identified, especially upon deletion of the desuccinylase SIRT5. In particular, multiple high-stoichiometry lysine sites identified in argininosuccinate synthase (ASS1), a key enzyme in the urea cycle, are regulated by SIRT5. Mutation of the high-stoichiometry lysine in ASS1 to succinyl-mimetic glutamic acid significantly decreased its enzymatic activity. Metabolomics profiling confirms that SIRT5 deficiency decreases urea cycle activity in liver. Importantly, SIRT5 deficiency compromises ammonia tolerance, which can be reversed by the overexpression of wild-type, but not succinyl-mimetic, ASS1. Therefore, lysine succinylation is functionally important in ammonia metabolism.

CD38 regulates ovarian function and fecundity via NAD+ metabolism.

Mammalian female reproductive lifespan is typically significantly shorter than life expectancy and is associated with a decrease in ovarian NAD+ levels. However, the mechanisms underlying this loss of ovarian NAD+ are unclear. Here, we show that CD38, an NAD+ consuming enzyme, is expressed in the ovarian extrafollicular space, primarily in immune cells, and its levels increase with reproductive age. Reproductively young mice lacking CD38 exhibit larger primordial follicle pools, elevated ovarian NAD+ levels, and increased fecundity relative to wild type controls. This larger ovarian reserve results from a prolonged window of follicle formation during early development. However, the beneficial effect of CD38 loss on reproductive function is not maintained at advanced age. Our results demonstrate a novel role of CD38 in regulating ovarian NAD+ metabolism and establishing the ovarian reserve, a critical process that dictates a female's reproductive lifespan.

Senescent cells promote tissue NAD+ decline during ageing via the activation of CD38+ macrophages.

Declining tissue nicotinamide adenine dinucleotide (NAD) levels are linked to ageing and its associated diseases. However, the mechanism for this decline is unclear. Here, we show that pro-inflammatory M1-like macrophages, but not naive or M2 macrophages, accumulate in metabolic tissues, including visceral white adipose tissue and liver, during ageing and acute responses to inflammation. These M1-like macrophages express high levels of the NAD-consuming enzyme CD38 and have enhanced CD38-dependent NADase activity, thereby reducing tissue NAD levels. We also find that senescent cells progressively accumulate in visceral white adipose tissue and liver during ageing and that inflammatory cytokines secreted by senescent cells (the senescence-associated secretory phenotype, SASP) induce macrophages to proliferate and express CD38. These results uncover a new causal link among resident tissue macrophages, cellular senescence and tissue NAD decline during ageing and offer novel therapeutic opportunities to maintain NAD levels during ageing.

Alpha-Ketoglutarate, an Endogenous Metabolite, Extends Lifespan and Compresses Morbidity in Aging Mice.

Metabolism and aging are tightly connected. Alpha-ketoglutarate is a key metabolite in the tricarboxylic acid (TCA) cycle, and its levels change upon fasting, exercise, and aging. Here, we investigate the effect of alpha-ketoglutarate (delivered in the form of a calcium salt, CaAKG) on healthspan and lifespan in C57BL/6 mice. To probe the relationship between healthspan and lifespan extension in mammals, we performed a series of longitudinal, clinically relevant measurements. We find that CaAKG promotes a longer, healthier life associated with a decrease in levels of systemic inflammatory cytokines. We propose that induction of IL-10 by dietary AKG suppresses chronic inflammation, leading to health benefits. By simultaneously reducing frailty and enhancing longevity, AKG, at least in the murine model, results in a compression of morbidity.

MANF regulates metabolic and immune homeostasis in ageing and protects against liver damage.

Aging is accompanied by altered intercellular communication, deregulated metabolic function, and inflammation. Interventions that restore a youthful state delay or reverse these processes, prompting the search for systemic regulators of metabolic and immune homeostasis. Here we identify MANF, a secreted stress-response protein with immune modulatory properties, as an evolutionarily conserved regulator of systemic and in particular liver metabolic homeostasis. We show that MANF levels decline with age in flies, mice and humans, and MANF overexpression extends lifespan in flies. MANF deficient flies exhibit enhanced inflammation and shorter lifespans, and MANF heterozygous mice exhibit inflammatory phenotypes in various tissues, as well as progressive liver damage, fibrosis, and steatosis. We show that immune cell-derived MANF protects against liver inflammation and fibrosis, while hepatocyte-derived MANF prevents hepatosteatosis. Liver rejuvenation by heterochronic parabiosis in mice further depends on MANF, while MANF supplementation ameliorates several hallmarks of liver aging, prevents hepatosteatosis induced by diet, and improves age-related metabolic dysfunction. Our findings identify MANF as a systemic regulator of homeostasis in young animals, suggesting a therapeutic application for MANF in age-related metabolic diseases.

mTORC1 Activation during Repeated Regeneration Impairs Somatic Stem Cell Maintenance.

The balance between self-renewal and differentiation ensures long-term maintenance of stem cell (SC) pools in regenerating epithelial tissues. This balance is challenged during periods of high regenerative pressure and is often compromised in aged animals. Here, we show that target of rapamycin (TOR) signaling is a key regulator of SC loss during repeated regenerative episodes. In response to regenerative stimuli, SCs in the intestinal epithelium of the fly and in the tracheal epithelium of mice exhibit transient activation of TOR signaling. Although this activation is required for SCs to rapidly proliferate in response to damage, repeated rounds of damage lead to SC loss. Consistently, age-related SC loss in the mouse trachea and in muscle can be prevented by pharmacologic or genetic inhibition, respectively, of mammalian target of rapamycin complex 1 (mTORC1) signaling. These findings highlight an evolutionarily conserved role of TOR signaling in SC function and identify repeated rounds of mTORC1 activation as a driver of age-related SC decline.

PGAM5 promotes lasting FoxO activation after developmental mitochondrial stress and extends lifespan in Drosophila.

The mitochondrial unfolded protein response (UPRmt) has been associated with long lifespan across metazoans. In Caenorhabditis elegans, mild developmental mitochondrial stress activates UPRmt reporters and extends lifespan. We show that similar developmental stress is necessary and sufficient to extend Drosophila lifespan, and identify Phosphoglycerate Mutase 5 (PGAM5) as a mediator of this response. Developmental mitochondrial stress leads to activation of FoxO, via Apoptosis Signal-regulating Kinase 1 (ASK1) and Jun-N-terminal Kinase (JNK). This activation persists into adulthood and induces a select set of chaperones, many of which have been implicated in lifespan extension in flies. Persistent FoxO activation can be reversed by a high-protein diet in adulthood, through mTORC1 and GCN-2 activity. Accordingly, the observed lifespan extension is prevented on a high-protein diet and in FoxO-null flies. The diet-sensitivity of this pathway has important implications for interventions that seek to engage the UPRmt to improve metabolic health and longevity.

Immune modulation by MANF promotes tissue repair and regenerative success in the retina.

Regenerative therapies are limited by unfavorable environments in aging and diseased tissues. A promising strategy to improve success is to balance inflammatory and anti-inflammatory signals and enhance endogenous tissue repair mechanisms. Here, we identified a conserved immune modulatory mechanism that governs the interaction between damaged retinal cells and immune cells to promote tissue repair. In damaged retina of flies and mice, platelet-derived growth factor (PDGF)-like signaling induced mesencephalic astrocyte-derived neurotrophic factor (MANF) in innate immune cells. MANF promoted alternative activation of innate immune cells, enhanced neuroprotection and tissue repair, and improved the success of photoreceptor replacement therapies. Thus, immune modulation is required during tissue repair and regeneration. This approach may improve the efficacy of stem-cell-based regenerative therapies.

The combination of lithium and l-Dopa/Carbidopa reduces MPTP-induced abnormal involuntary movements (AIMs) via calpain-1 inhibition in a mouse model: Relevance for Parkinson׳s disease therapy.

Lithium has recently been suggested to have neuroprotective effects in several models of neurodegenerative disease including Parkinson׳s disease (PD). Levodopa (l-Dopa) replacement therapy remains the most common and effective treatment for PD, although it induces the complication of l-Dopa induced dyskinesia after years of use. Here we examined the potential use of lithium in combination with l-Dopa/Carbidopa for both reducing MPTP-induced abnormal involuntary movements (AIMs) as well as protecting against cell death in MPTP-lesioned mice. Chronic lithium administration (0.127% LiCl in the feed) in the presence of daily l-Dopa/Carbidopa injection for a period of 2 months was sufficient to effectively reduce MPTP-induced AIMs in mice. Mechanistically, lithium was found to suppress MPTP-induced calpain activities in vivo coinciding with down-regulation of calpain-1 but not calpain-2 expression in both the striatum (ST) and the brain stem (BS). Calpain inhibition has previously been associated with increased levels of the rate-limiting enzyme in dopamine synthesis, tyrosine hydroxylase (TH), which is probably mediated by the up-regulation of the transcription factors MEF-2A and 2D. Lithium was found to induce up-regulation of TH expression in the ST and the BS, as well as in N27 rat dopaminergic cells. Further, histone acetyltransferase (HAT) expression was substantially up-regulated by lithium treatment in vitro. These results suggest the potential use of lithium in combination with l-Dopa/Carbidopa not only as a neuroprotectant, but also for reducing AIMs and possibly alleviating potential side-effects associated with the current treatment for PD.

Quantitative mapping of reversible mitochondrial Complex I cysteine oxidation in a Parkinson disease mouse model.

Differential cysteine oxidation within mitochondrial Complex I has been quantified in an in vivo oxidative stress model of Parkinson disease. We developed a strategy that incorporates rapid and efficient immunoaffinity purification of Complex I followed by differential alkylation and quantitative detection using sensitive mass spectrometry techniques. This method allowed us to quantify the reversible cysteine oxidation status of 34 distinct cysteine residues out of a total 130 present in murine Complex I. Six Complex I cysteine residues were found to display an increase in oxidation relative to controls in brains from mice undergoing in vivo glutathione depletion. Three of these residues were found to reside within iron-sulfur clusters of Complex I, suggesting that their redox state may affect electron transport function.

Mitochondrial-dependent Ca2+ handling in Huntington's disease striatal cells: effect of histone deacetylase inhibitors.

Evidence suggests that neuronal dysfunction in Huntington's disease (HD) striatum involves deficits in mitochondrial function and in Ca2+ handling. However, the relationship between mitochondria and Ca2+ handling has been incompletely studied in intact HD striatal cells. Treatment with histone deacetylase (HDAC) inhibitors reduces cell death in HD models, but the effects of this promising therapy on cellular function are mostly unknown. Here, we use real-time functional imaging of intracellular Ca2+ and mitochondrial membrane potential to explore the role of in situ HD mitochondria in Ca2+ handling. Immortalized striatal (STHdh) cells and striatal neurons from transgenic mice, expressing full-length mutant huntingtin (Htt), were used to model HD. We show that (1) active glycolysis in STHdh cells occludes the mitochondrial role in Ca2+ handling as well as the effects of mitochondrial inhibitors, (2) STHdh cells and striatal neurons in the absence of glycolysis are critically dependent on oxidative phosphorylation for energy-dependent Ca2+ handling, (3) expression of full-length mutant Htt is associated with deficits in mitochondrial-dependent Ca2+ handling that can be ameliorated by treatment with HDAC inhibitors (treatment with trichostatin A or sodium butyrate decreases the proportion of STHdh cells losing Ca2+ homeostasis after Ca2+-ionophore challenging, and accelerates the restoration of intracellular Ca2+ in striatal neurons challenged with NMDA), and (4) neurons with different response patterns to NMDA receptor activation exhibit different average somatic areas and are differentially affected by treatment with HDAC inhibitors, suggesting subpopulation or functional state specificity. These findings indicate that neuroprotection induced by HDAC inhibitors involves more efficient Ca2+ handling, thus improving the neuronal ability to cope with excitotoxic stimuli.

A PMLRARA transgene results in a retinoid-deficient phenotype associated with enhanced susceptibility to skin tumorigenesis.

The construction of transgenic FVB/N mice targeting the PMLRARA fusion gene under the control of a human MRP8 promoter recapitulated the phenotype of acute promyelocytic leukemia but had the unexpected result of multiple squamous papillomas of the skin (Brown et al., PROC: Natl. Acad. Sci. USA, 94:2551-2556, 1997). In addition, transgenic MRP8-PMLRARA mice exhibited a skin phenotype characteristic of vitamin A deficiency. The severity of the skin phenotype and spontaneous papilloma development correlated with the level of transgene expression. Papilloma formation was preceded by follicular hyperplasia and the expression of epidermal differentiation markers in the follicular epithelium. Mutations in the Ha or Ki alleles of ras were not detected in papillomas that developed on transgenic skin, and papilloma formation was accentuated on the C57/Bl6 background, unlike the usual resistance of this strain to skin tumor induction. Analysis of liver extracts from transgenic mice indicated a deficiency in the production of retinoic acid. Furthermore, affected transgenic epidermis had reduced levels of retinoic acid receptoralpha (RARalpha) and retinoic X receptor (RXRalpha), and supplementation with exogenous retinoic acid prevented the skin phenotype. When transgenic keratinocytes were grafted to nude mice, the resulting integument was normal, and conversely, when transgenic bone marrow was grafted to normal mice, a skin phenotype did not develop. Together these results suggest that local interruption of PML and RARalpha signaling in the skin, together with a systemic retinoid deficiency, initiates a tumor induction pathway that is independent of ras activation.

Dissection of human papillomavirus E6 and E7 function in transgenic mouse models of cervical carcinogenesis.

Human cervix cancer is caused by high-risk human papillomaviruses encoding E6 and E7 oncoproteins, each of which alter function of distinct targets regulating the cell cycle, apoptosis, and differentiation. Here we determined the molecular contribution of E6 or E7 to neoplastic progression and malignant growth in a transgenic mouse model of cervical carcinogenesis. E7 increased proliferation and centrosome copy number, and produced progression to multifocal microinvasive cervical cancers. E6 elevated centrosome copy number and eliminated detectable p53 protein, but did not produce neoplasia or cancer. E6 plus E7 additionally elevated centrosome copy number and created large, extensively invasive cancers. Centrosome copy number increases and p53 loss likely contributed to malignant growth; however, dysregulated proliferation and differentiation were required for carcinogenic progression.