[Abstract unavailable

Peroxisomes are highly metabolic, autonomously replicating organelles that generate ROS as a by product of fatty acid β-oxidation. Consequently, cells must maintain peroxisome homeostasis, or risk pathologies associated with too few peroxisomes, such as peroxisome biogenesis disorders, or too many peroxisomes, inducing oxidative damage and promoting diseases such as cancer. We report that the PEX5 peroxisome import receptor binds ataxia-telangiectasia mutated (ATM) and localizes this kinase to the peroxisome. In response to reactive oxygen species (ROS), ATM signaling activates ULK1 and inhibits mTORC1 to induce autophagy. Specificity for autophagy of peroxisomes (pexophagy) is provided by ATM phosphorylation of PEX5 at Ser141, which promotes PEX5 mono-ubiquitination at K209, and recognition of ubiquitinated PEX5 by the autophagy adapter protein p62, directing the autophagosome to peroxisomes to induce pexophagy. These data reveal an important new role for ATM in metabolism as a sensor of ROS that regulates pexophagy.

SUMMARY Posttranslational modifications (PTMs) of tubulin specify microtubules for specialized cellular functions and comprise what is termed a “tubulin code”. PTMs of histones comprise an analogous “histone code”, although the “readers, writers and erasers” of the cytoskeleton and epigenome have heretofore been distinct. We show that methylation is a PTM of dynamic microtubules, and that the histone methytransferase, SETD2, which is responsible for H3 lysine 36 trimethylation (H3K36me3) of histones, also methylates α-tubulin at lysine 40, the same lysine that is marked by acetylation on microtubules. Methylation of microtubules occurs during mitosis and cytokinesis, and can be ablated by SETD2 deletion, which causes mitotic spindle and cytokinesis defects, micronuclei and polyploidy. These data now identify SETD2 as a dual function methyltransferase for both chromatin and the cytoskeleton, and show a requirement for methylation in maintenance of genomic stability and the integrity of both the tubulin and histone codes.

Subcellular localization is emerging as an important mechanism for mTORC1 regulation. We report that the tuberous sclerosis complex (TSC) signaling node, TSC1, TSC2 and Rheb, localizes to peroxisomes, where it regulates mTORC1 in response to reactive oxygen species (ROS). TSC1 and TSC2 were bound by PEX19 and PEX5, respectively, and peroxisome-localized TSC functioned as a Rheb GAP to suppress mTORC1 and induce autophagy. Naturally occurring pathogenic mutations in TSC2 decreased PEX5 binding, abrogated peroxisome localization, Rheb GAP activity, and suppression of mTORC1 by ROS. Cells lacking peroxisomes were deficient in mTORC1 repression by ROS and peroxisome-localization deficient TSC2 mutants caused polarity defects and formation of multiple axons in neurons. These data identify a role for TSC in Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms COMPETING FINANCIAL INTERESTSThe authors declare that they have no competing financial interests. HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript responding to ROS at the peroxisome, and identify the peroxisome as a signaling organelle involved in regulation of mTORC1.Tuberous sclerosis complex (TSC) is a hereditary hamartoma syndrome caused by defects in either the TSC1 or TSC2 genes 1, 2 . The TSC tumor suppressor is a heterodimer comprised of tuberin (TSC2), a GTPase activating protein (GAP), and its activation partner hamartin (TSC1), which localizes the TSC tumor suppressor to endomembranes and protects TSC2 from proteasomal degradation 3,4 . TSC inhibits the activity of the small GTPase Rheb to repress mammalian target of rapamycin complex 1 (mTORC1) signaling, a negative regulator of autophagy [5][6][7][8][9][10][11][12] . mTORC1 is regulated by a variety of cellular stimuli including amino acids, mitogens such as insulin, glucose, and energy stress [13][14][15] . In the case of amino acids, which do not signal through TSC-Rheb pathway 15 , mTORC1 activity is regulated by the Rag GTPases, which form the Ragulator complex that localizes mTORC1 to the late endosome or lysosome compartment of cells [13][14][15][16][17][18] . We recently reported that TSC functions in a signaling node downstream of ataxia telangiectasia mutated (ATM) to repress mTORC1 in response to reactive oxygen species (ROS) 19 . However, identification of the specific subcellular compartment(s) in which the TSC tumor suppressor functions to regulate mTORC1 in response to ROS has heretofore remained elusive.Peroxisomes, carry out key metabolic functions in the cell including β-oxidation of fatty acids, and are a major source of cellular ROS 20,21 . Like mitochondria, peroxisomes are autonomously replicating organelles. Peroxisome biogenesis requires peroxin (PEX) proteins, which are essential for assembly of functional peroxisomes 22 . Specific PEX pro...

Reactive intermediates such as reactive nitrogen species play essential roles in the cell as signaling molecules but, in excess, constitute a major source of cellular damage. We found that nitrosative stress induced by steady-state nitric oxide (NO) caused rapid activation of an ATM damage-response pathway leading to downstream signaling by this stress kinase to LKB1 and AMPK kinases, and activation of the TSC tumor suppressor. As a result, in an ATM-, LKB1-, TSC-dependent fashion, mTORC1 was repressed, as evidenced by decreased phosphorylation of S6K, 4E-BP1, and ULK1, direct targets of the mTORC1 kinase. Decreased ULK1 phosphorylation by mTORC1 at S757 and activation of AMPK to phosphorylate ULK1 at S317 in response to nitrosative stress resulted in increased autophagy: the LC3-II/LC3-I ratio increased as did GFP-LC3 puncta and acidic vesicles; p62 levels decreased in a lysosomedependent manner, confirming an NO-induced increase in autophagic flux. Induction of autophagy by NO correlated with loss of cell viability, suggesting that, in this setting, autophagy was functioning primarily as a cytotoxic response to excess nitrosative stress. These data identify a nitrosative-stress signaling pathway that engages ATM and the LKB1 and TSC2 tumor suppressors to repress mTORC1 and regulate autophagy. As cancer cells are particularly sensitive to nitrosative stress, these data open another path for therapies capitalizing on the ability of reactive nitrogen species to induce autophagy-mediated cell death. signal transduction | cancer therapyA utophagy is a self-digestion process by which a eukaryotic cell degrades and recycles aggregate-prone proteins, macromolecules, and organelles. During autophagy, cytoplasmic contents are sequestered in double-membrane bound vesicles called autophagosomes and delivered to lysosomes for degradation, thereby allowing cells to eliminate and recycle the contents (1-3). Autophagy participates in both prosurvival (recycling of cellular building blocks) and prodeath (excess catalysis) pathways. A comprehensive understanding of signaling pathways that regulate autophagy holds great promise for new therapeutic opportunities by opening the possibility to compromise prosurvival autophagic pathways that enable tumor cells to evade therapy, or by promoting prodeath autophagic pathways to kill cancer cells.The classical pathway regulating autophagy in mammalian cells involves the serine/threonine kinase, mammalian target of rapamycin (mTOR). Active mTOR kinase in the mTORC1 complex phosphorylates and inhibits ULK1, a key proautophagy adapter involved in nucleation of the autophagophore membrane. Inactivation of mTORC1, either pharmacologically with rapamycin or via activation of the tuberous sclerosis complex (TSC) tumor suppressor, leads to downstream dephosphorylation events, including loss of ULK1 phosphorylation at S757. The TSC1/2 heterodimer is itself regulated by upstream kinases, including the AMP-activated protein kinase (AMPK), which regulates several metabolic processes and activates t...