bims-auttor 2019-09-08 papers

Issue of 2019–09–08
five papers selected by
Viktor Korolchuk, Newcastle University

Mol Cell. 2019 Aug 12. pii: S1097-2765(19)30551-9. [Epub ahead of print]

mTORC1 Activation Requires DRAM-1 by Facilitating Lysosomal Amino Acid Efflux.

Florian Beaumatin, Jim O'Prey, Valentin J A Barthet, Barbara Zunino, Jean-Philippe Parvy, Alexis Maximilien Bachmann, Margaret O'Prey, Elżbieta Kania, Pablo Sierra Gonzalez, Robin Macintosh, Laurence Y Lao, Colin Nixon, Jonathan Lopez, Jaclyn S Long, Stephen W G Tait, Kevin M Ryan.

Sensing nutrient availability is essential for appropriate cellular growth, and mTORC1 is a major regulator of this process. Mechanisms causing mTORC1 activation are, however, complex and diverse. We report here an additional important step in the activation of mTORC1, which regulates the efflux of amino acids from lysosomes into the cytoplasm. This process requires DRAM-1, which binds the membrane carrier protein SCAMP3 and the amino acid transporters SLC1A5 and LAT1, directing them to lysosomes and permitting efficient mTORC1 activation. Consequently, we show that loss of DRAM-1 also impacts pathways regulated by mTORC1, including insulin signaling, glycemic balance, and adipocyte differentiation. Interestingly, although DRAM-1 can promote autophagy, this effect on mTORC1 is autophagy independent, and autophagy only becomes important for mTORC1 activation when DRAM-1 is deleted. These findings provide important insights into mTORC1 activation and highlight the importance of DRAM-1 in growth control, metabolic homeostasis, and differentiation.

Keywords: DRAM-1; SCAMP3; amino acid transporters; and adipocyte differentiation; autophagy; insulin signaling; mTOR

DOI: https://doi.org/10.1016/j.molcel.2019.07.021

Autophagy. 2019 Sep 02.

The autophagic protein LC3 translocates to the nucleus and localizes in the nucleolus associated to NUFIP1 in response to cyclic mechanical stress.

Myoung Sup Shim, April Nettesheim, Joshua Hirt, Paloma B Liton.

The trabecular meshwork (TM) is a key regulatory tissue of intraocular pressure (IOP) in the anterior chamber of eye. Dysfunction of the TM causes resistance to outflow of aqueous humor, which in turn leads to elevated IOP, a main risk factor of glaucomatous neurodegeneration. Due to variations in IOP, TM cells are continuously exposed to mechanical deformations. We previously reported activation of macroautophagy/autophagy, as one of the physiological responses elicited in TM cells following mechanical strain application. By using biochemical fractionation analysis and imaging techniques, we demonstrate here for the first time the nuclear accumulation of the autophagic marker MAP1LC3/LC3 (microtubule associated protein1 light chain 3)-II, endogenous and exogenously added (AdGFP-LC3, AdtfLC3), in response to cyclic mechanical stress (CMS). Wheat germ agglutinin (WGA) and leptomycin B treatment suggest LC3 to enter the nucleus by passive diffusion, but to exit in an XPO1/CRM1 (exportin 1)-dependent manner in human TM (hTM) cells. While blockage of nuclear export leads to accumulation of LC3 with promyelocytic leukemia (PML) bodies, nuclear LC3 localizes in the nucleolus in cells under CMS. Moreover, nuclear LC3 co-immunoprecipitated with NUFIP1, a ribosome receptor for starvation-induced ribophagy. More interestingly, we further demonstrate that NUFIP1 translocates from the nucleus to LAMP2 (lysosomal associated membrane protein 2)-positive organelles in the stretched cells without triggering ribophagy, suggesting a more general role of NUFIP1 as a selective autophagy receptor for another yet-to-be-identified target in CMS and a surveillance role of nuclear LC3 against stretch-induced damage.

Keywords: LC3; NUFIP1; PML bodies; autophagy; glaucoma; mechanical stress; nuclear LC3; nucleolus; stretching; trabecular meshwork

DOI: https://doi.org/10.1080/15548627.2019.1662584

Proc Natl Acad Sci U S A. 2019 Sep 04. pii: 201904563. [Epub ahead of print]

Nmnat restores neuronal integrity by neutralizing mutant Huntingtin aggregate-induced progressive toxicity.

Yi Zhu, Chong Li, Xianzun Tao, Jennifer M Brazill, Joun Park, Zoraida Diaz-Perez, R Grace Zhai.

Accumulative aggregation of mutant Huntingtin (Htt) is a primary neuropathological hallmark of Huntington's disease (HD). Currently, mechanistic understanding of the cytotoxicity of mutant Htt aggregates remains limited, and neuroprotective strategies combating mutant Htt-induced neurodegeneration are lacking. Here, we show that in Drosophila models of HD, neuronal compartment-specific accumulation of mutant Htt aggregates causes neurodegenerative phenotypes. In addition to the increase in the number and size, we discovered an age-dependent acquisition of thioflavin S+, amyloid-like adhesive properties of mutant Htt aggregates and a concomitant progressive clustering of aggregates with mitochondria and synaptic proteins, indicating that the amyloid-like adhesive property underlies the neurotoxicity of mutant Htt aggregation. Importantly, nicotinamide mononucleotide adenylyltransferase (NMNAT), an evolutionarily conserved nicotinamide adenine dinucleotide (NAD+) synthase and neuroprotective factor, significantly mitigates mutant Htt-induced neurodegeneration by reducing mutant Htt aggregation through promoting autophagic clearance. Additionally, Nmnat overexpression reduces progressive accumulation of amyloid-like Htt aggregates, neutralizes adhesiveness, and inhibits the clustering of mutant Htt with mitochondria and synaptic proteins, thereby restoring neuronal function. Conversely, partial loss of endogenous Nmnat exacerbates mutant Htt-induced neurodegeneration through enhancing mutant Htt aggregation and adhesive property. Finally, conditional expression of Nmnat after the onset of degenerative phenotypes significantly delays the progression of neurodegeneration, revealing the therapeutic potential of Nmnat-mediated neuroprotection at advanced stages of HD. Our study uncovers essential mechanistic insights to the neurotoxicity of mutant Htt aggregation and describes the molecular basis of Nmnat-mediated neuroprotection in HD.

Keywords: Huntington’s disease; Nmnat; aggregate; amyloid; mitochondria

DOI: https://doi.org/10.1073/pnas.1904563116

Exp Mol Med. 2019 Sep 06. 51(9): 102

Mechanisms and disease implications of sirtuin-mediated autophagic regulation.

In Hye Lee.

Accumulating evidence has indicated that sirtuins are key components of diverse physiological processes, including metabolism and aging. Sirtuins confer protection from a wide array of metabolic and age-related diseases, such as cancer, cardiovascular and neurodegenerative diseases. Recent studies have also suggested that sirtuins regulate autophagy, a protective cellular process for homeostatic maintenance in response to environmental stresses. Here, we describe various biological and pathophysiological processes regulated by sirtuin-mediated autophagy, focusing on cancer, heart, and liver diseases, as well as stem cell biology. This review also emphasizes key molecular mechanisms by which sirtuins regulate autophagy. Finally, we discuss novel insights into how new therapeutics targeting sirtuin and autophagy may potentially lead to effective strategies to combat aging and aging-related diseases.

DOI: https://doi.org/10.1038/s12276-019-0302-7

J Cell Physiol. 2019 Sep 06.

Accumulation of sphingomyelin in Niemann-Pick disease type C cells disrupts Rab9-dependent vesicular trafficking of cholesterol.

Masahiro Wanikawa, Hiroyuki Nakamura, Shunsuke Emori, Naohiro Hashimoto, Toshihiko Murayama.

Niemann-Pick disease type C (NPC) is a genetic disorder in which patient cells have endosomal/lysosomal accumulation of cholesterol and sphingolipids. However, the relationship between sphingolipids and cholesterol accumulation in NPC cells has not been established. Here, we investigated the role of sphingomyelin (SM) on the accumulation of cholesterol in NPC cells. Reduction of SM by inhibition of the ceramide transfer protein CERT decreased the cholesterol accumulation in NPC cells. The accumulation of SM in NPC cells inhibited the transport of cholesterol to the endoplasmic reticulum. Overexpression of Rab9 in NPC cells reduced the cholesterol accumulation, which was recovered by treatment with SM. In NPC cells that overexpressed a Rab9 constitutively active mutant, SM treatment did not lead to the cholesterol accumulation. These results indicate that SM negatively regulates the Rab9-dependent vesicular trafficking of cholesterol, and a reduction in SM levels in NPC cells recovers the Rab9-dependent vesicular trafficking defect.

Keywords: ACAT; CERT; LDL; cholesterol/trafficking; lipid transfer proteins; sphingolipids

DOI: https://doi.org/10.1002/jcp.29137