Autolysosomal exocytosis of lipids protect neurons from ferroptosis.

During oxidative stress neurons release lipids that are internalized by glia. Defects in this coordinated process play an important role in several neurodegenerative diseases. Yet, the mechanisms of lipid release and its consequences on neuronal health are unclear. Here, we demonstrate that lipid-protein particle release by autolysosome exocytosis protects neurons from ferroptosis, a form of cell death driven by lipid peroxidation. We show that during oxidative stress, peroxidated lipids and iron are released from neurons by autolysosomal exocytosis which requires the exocytic machinery VAMP7 and syntaxin 4. We observe membrane-bound lipid-protein particles by TEM and demonstrate that these particles are released from neurons using cryoEM. Failure to release these lipid-protein particles causes lipid hydroperoxide and iron accumulation and sensitizes neurons to ferroptosis. Our results reveal how neurons protect themselves from peroxidated lipids. Given the number of brain pathologies that involve ferroptosis, defects in this pathway likely play a key role in the pathophysiology of neurodegenerative disease.

Reovirus uses temporospatial compartmentalization to orchestrate core versus outercapsid assembly.

Reoviridae virus family members, such as mammalian orthoreovirus (reovirus), encounter a unique challenge during replication. To hide the dsRNA from host recognition, the genome remains encapsidated in transcriptionally active proteinaceous core capsids that transcribe and release +RNA. De novo +RNAs and core proteins must repeatedly assemble into new progeny cores in order to logarithmically amplify replication. Reoviruses also produce outercapsid (OC) proteins µ1, σ3 and σ1 that assemble onto cores to create highly stable infectious full virions. Current models of reovirus replication position amplification of transcriptionally-active cores and assembly of infectious virions in shared factories, but we hypothesized that since assembly of OC proteins would halt core amplification, OC assembly is somehow regulated. Kinetic analysis of virus +RNA production, core versus OC protein expression, and core particles versus whole virus particle accumulation, indicated that assembly of OC proteins onto core particles was temporally delayed. All viral RNAs and proteins were made simultaneously, eliminating the possibility that delayed OC RNAs or proteins account for delayed OC assembly. High resolution fluorescence and electron microscopy revealed that core amplification occurred early during infection at peripheral core-only factories, while all OC proteins associated with lipid droplets (LDs) that coalesced near the nucleus in a µ1-dependent manner. Core-only factories transitioned towards the nucleus despite cycloheximide-mediated halting of new protein expression, while new core-only factories developed in the periphery. As infection progressed, OC assembly occurred at LD-and nuclear-proximal factories. Silencing of OC µ1 expression with siRNAs led to large factories that remained further from the nucleus, implicating µ1 in the transition to perinuclear factories. Moreover, late during infection, +RNA pools largely contributed to the production of de-novo viral proteins and fully-assembled infectious viruses. Altogether the results suggest an advanced model of reovirus replication with spatiotemporal segregation of core amplification, OC complexes and fully assembled virions.

SUMOylation regulates insulin exocytosis downstream of secretory granule docking in rodents and humans.

OBJECTIVE: The reversible attachment of small ubiquitin-like modifier (SUMO) proteins controls target localization and function. We examined an acute role for the SUMOylation pathway in downstream events mediating insulin secretion. RESEARCH DESIGN AND METHODS: We studied islets and ß-cells from mice and human donors, as well as INS-1 832/13 cells. Insulin secretion, intracellular Ca(2+), and ß-cell exocytosis were monitored after manipulation of the SUMOylation machinery. Granule localization was imaged by total internal reflection fluorescence and electron microscopy; immunoprecipitation and Western blotting were used to examine the soluble NSF attachment receptor (SNARE) complex formation and SUMO1 interaction with synaptotagmin VII. RESULTS: SUMO1 impairs glucose-stimulated insulin secretion by blunting the ß-cell exocytotic response to Ca(2+). The effect of SUMO1 to impair insulin secretion and ß-cell exocytosis is rapid and does not require altered gene expression or insulin content, is downstream of granule docking at the plasma membrane, and is dependent on SUMO-conjugation because the deSUMOylating enzyme, sentrin/SUMO-specific protease (SENP)-1, rescues exocytosis. SUMO1 coimmunoprecipitates with the Ca(2+) sensor synaptotagmin VII, and this is transiently lost upon glucose stimulation. SENP1 overexpression also disrupts the association of SUMO1 with synaptotagmin VII and mimics the effect of glucose to enhance exocytosis. Conversely, SENP1 knockdown impairs exocytosis at stimulatory glucose levels and blunts glucose-dependent insulin secretion from mouse and human islets. CONCLUSIONS: SUMOylation acutely regulates insulin secretion by the direct and reversible inhibition of ß-cell exocytosis in response to intracellular Ca(2+) elevation. The SUMO protease, SENP1, is required for glucose-dependent insulin secretion.

Identification of palmitoylated mitochondrial proteins using a bio-orthogonal azido-palmitate analogue.

Increased levels of circulating saturated free fatty acids, such as palmitate, have been implicated in the etiology of type II diabetes and cancer. In addition to being a constituent of glycerolipids and a source of energy, palmitate also covalently attaches to numerous cellular proteins via a process named palmitoylation. Recognized for its roles in membrane tethering, cellular signaling, and protein trafficking, palmitoylation is also emerging as a potential regulator of metabolism. Indeed, we showed previously that the acylation of two mitochondrial proteins at their active site cysteine residues result in their inhibition. Herein, we sought to identify other palmitoylated proteins in mitochondria using a nonradioactive bio-orthogonal azido-palmitate analog that can be selectively derivatized with various tagged triarylphosphines. Our results show that, like palmitate, incorporation of azido-palmitate occurred on mitochondrial proteins via thioester bonds at sites that could be competed out by palmitoyl-CoA. Using this method, we identified 21 putative palmitoylated proteins in the rat liver mitochondrial matrix, a compartment not recognized for its content in palmitoylated proteins, and confirmed the palmitoylation of newly identified mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase. We postulate that covalent modification and perhaps inhibition of various mitochondrial enzymes by palmitoyl-CoA could lead to the metabolic impairments found in obesity-related diseases.

Posttranslational myristoylation of caspase-activated p21-activated protein kinase 2 (PAK2) potentiates late apoptotic events.

p21-activated protein kinase (PAK) 2 is a small GTPase-activated serine/threonine kinase regulating various cytoskeletal functions and is cleaved by caspase-3 during apoptosis. We demonstrate that the caspase-cleaved PAK2 C-terminal kinase fragment (C-t-PAK2) is posttranslationally myristoylated, although myristoylation is typically a cotranslational process. Myristoylation and an adjacent polybasic domain of C-t-PAK2 are sufficient to redirect EGFP from the cytosol to membrane ruffles and internal membranes. Membrane localization and the ability of C-t-PAK2 to induce cell death are significantly reduced when myristoylation is abolished. In addition, the proper myristoylation-dependent membrane localization of C-t-PAK2 significantly increased signaling through the stress-activated c-Jun N-terminal kinase signaling pathway, which often regulates apoptosis. Interestingly, C-t-PAK2 promoted cell death without compromising mitochondrial integrity. Posttranslational myristoylation of caspase-cleaved proteins involved in cytoskeletal dynamics (e.g., PAK2, actin, and gelsolin) might be part of a unique series of mechanisms involved in the regulation of the later events of apoptosis.

Activation of serine/threonine protein phosphatase-1 is required for ceramide-induced survival of sympathetic neurons.

In sympathetic neurons, C6-ceramide, as well as endogenous ceramides, blocks apoptosis elicited by NGF (nerve growth factor) deprivation. The mechanism(s) involved in ceramide-induced neuronal survival are poorly understood. Few direct targets for the diverse cellular effects of ceramide have been identified. Amongst those proposed is PP-1c, the catalytic subunit of serine/threonine PP-1 (protein phosphatase-1). Here, we present the first evidence of PP-1c activation by ceramide in live cells, namely NGF-deprived sympathetic neurons. We first determined PP activity in cellular lysates from sympathetic neurons treated with exogenous ceramide and demonstrated a 2-3-fold increase in PP activity. PP activation was completely blocked by the addition of the specific type-1 PP inhibitor protein I-2 as well as by tautomycin, but unaffected by 2 nM okadaic acid, strongly indicating that the ceramide-activated phosphatase activity was PP-1c. Inhibition of PP activity by phosphatidic acid (which has been reported to be a selective inhibitor of PP-1c) and tautomycin (a PP-1 and PP-2A inhibitor), but not by 10 nM okadaic acid, abolished the anti-apoptotic effect of ceramide in NGF-deprived neurons, suggesting that activation of PP-1c is required for ceramide-induced neuronal survival. Ceramide was able to prevent pRb (retinoblastoma gene product) hyperphosphorylation by a mechanism dependent on PP-1c activation, suggesting that two consequences of NGF deprivation in sympathetic neurons are inhibition of PP-1c and subsequent hyperphosphorylation of pRb protein. These findings suggest a novel mechanism for ceramide-induced survival, and implicate the involvement of PPs in apoptosis induced by NGF deprivation.