SIRT1 (Sir2) is an NAD + -dependent deacetylase that plays critical roles in a broad range of biological events, including metabolism, immune response, and aging 1 – 5 . While there is strong interest in stimulating SIRT1 catalytic activity, the homeostasis of SIRT1 at the protein level is poorly understood. Here, we report that macroautophagy (hereafter referred as autophagy), a catabolic membrane trafficking pathway that degrades cellular components through autophagosomes and lysosomes, mediates downregulation of mammalian SIRT1 protein during senescence and in vivo aging. In senescence, nuclear SIRT1 is recognized as an autophagy substrate and is subjected to cytoplasmic autophagosome-lysosome degradation, via the autophagy protein LC3. Importantly, the autophagy-lysosome pathway contributes to loss of SIRT1 during aging of several tissues related to the immune and hematopoietic system in mice, including spleen, thymus, and hematopoietic stem and progenitor cells, and in CD8 + CD28 - T cells from aged human donors. Our study reveals a mechanism in regulating the protein homeostasis of SIRT1, and suggests a potential strategy to stabilize SIRT1 to promote productive aging.

Acetone-butanol-ethanol (ABE) fermentation with a hyper-butanol producing Clostridium acetobutylicum JB200 was studied for its potential to produce a high titer of butanol that can be readily recovered with gas stripping. In batch fermentation without gas stripping, a final butanol concentration of 19.1 g/L was produced from 86.4 g/L glucose consumed in 78 h, and butanol productivity and yield were 0.24 g/L h and 0.21 g/g, respectively. In contrast, when gas stripping was applied intermittently in fed-batch fermentation, 172 g/L ABE (113.3 g/L butanol, 49.2 g/L acetone, 9.7 g/L ethanol) were produced from 474.9 g/L glucose in six feeding cycles over 326 h. The overall productivity and yield were 0.53 g/L h and 0.36 g/g for ABE and 0.35 g/L h and 0.24 g/g for butanol, respectively. The higher productivity was attributed to the reduced butanol concentration in the fermentation broth by gas stripping that alleviated butanol inhibition, whereas the increased butanol yield could be attributed to the reduced acids accumulation as most acids produced in acidogenesis were reassimilated by cells for ABE production. The intermittent gas stripping produced a highly concentrated condensate containing 195.9 g/L ABE or 150.5 g/L butanol that far exceeded butanol solubility in water. After liquid-liquid demixing or phase separation, a final product containing ~610 g/L butanol, ~40 g/L acetone, ~10 g/L ethanol, and no acids was obtained. Compared to conventional ABE fermentation, the fed-batch fermentation with intermittent gas stripping has the potential to reduce at least 90% of energy consumption and water usage in n-butanol production from glucose.

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