Restricting availability of essential amino acids (EAAs) limits aminoacylation of tRNAs by their cognate EAAs and activates the nutrient-sensing kinase, general control nonderepressible 2 (GCN2). Activated GCN2 phosphorylates eukaryotic initiation factor 2 (eIF2), altering gene-specific translation and initiating a transcriptional program collectively described as the integrated stress response (ISR). Central GCN2 activation by EAA deprivation is also linked to an acute aversive feeding response. Dietary methionine restriction (MR) produces a well-documented series of physiological responses (increased energy intake and expenditure, decreased adiposity, and increased insulin sensitivity), but the role of GCN2 in mediating them is unknown. Using Gcn2−/− mice, we found that the absence of GCN2 had no effect on the ability of MR to reduce body weight or adiposity, increase energy intake and expenditure, increase hepatic transcription and release of fibroblast growth factor 21, or improve insulin sensitivity. Interestingly, hepatic eIF2 phosphorylation by MR was uncompromised in Gcn2−/− mice. Instead, protein kinase R–like endoplasmic reticulum (ER) kinase (PERK) was activated in both intact and Gcn2−/− mice. PERK activation corresponded with induction of the ISR and the nuclear respiratory factor 2 antioxidant program but not ER stress. These data uncover a novel glutathione-sensing mechanism that functions independently of GCN2 to link dietary MR to its metabolic phenotype.

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Extracellular vesicles (EVs) are emerging as a universal means of cell-to-cell communication and hold great potential in diagnostics and regenerative therapies [1]. An urgent need in the field is a fundamental understanding of physiological mechanisms driving EV generation and function. Ciliary EVs act as signaling devices in Chlamydomonas and C. elegans [2-4]. Mammalian cilia shed EVs to eliminate unwanted receptors [5] or to retract cilia before entering the cell cycle [6]. Here we used our established C. elegans model to study sensoryevoked ciliary EV release and targeting using a fluorescently labeled EV cargo polycystin-2 (PKD-2). In C. elegans and mammals, the Autosomal Dominant Polycystic Kidney Disease (ADPKD) gene products polycystin-1 and polycystin-2 localize to cilia and EVs, act in the same genetic pathway, and function in a sensory capacity, suggesting ancient conservation [7]. We find that males deposit PKD-2-carrying EVs onto the vulva of the hermaphrodite during mating. We also show that mechanical stimulation triggers release of PKD-2-carrying EVs from cilia. To our knowledge this is the first report of mechanoresponsive nature of the ciliary EV release and of ciliary EV directional transfer from one animal to another animal. Since the polycystins are evolutionarily conserved ciliary EV cargoes, our findings suggest that similar mechanisms for EV release and targeting may occur in other systems and biological contexts. C. elegans male mating involves stereotyped behavioral steps including response to hermaphrodite contact, location of the hermaphrodite's vulva, spicule insertion, and sperm transfer to the hermaphrodite's uterus [7]. To examine male-hermaphrodite EV-mediated interactions during mating, we paired fluorescently labeled transgenic adult males with unlabeled hermaphrodites for 24 hours (Figure 1A). Male sperm transfer was visualized with MitoTracker dye, whereas ciliary EVs were tracked via the PKD-2::GFP EV cargo protein. In all mated hermaphrodites inseminated with MitoTracker labeled sperm, we observed deposition of male-derived PKD-2::GFP EVs on the hermaphrodite vulvae (Figure 1B-C). No PKD-2::GFP EVs were found inside the hermaphrodite uterus. Location of the male-deposited EVs at the hermaphrodite's vulva is consistent with the position of a male tail during mating and suggests that EVs were released in the timeframe between successful location of the vulva and retraction of spicules post-copulation. This timeframe represents the closest contact between the male tail and the vulva area of the hermaphrodite, suggesting that the vulva may provide mechanical or chemical cues to stimulate ciliary EV release from the male. Living C. elegans males release EVs when mounted between an agarose-layered slide and a bare glass coverslip [3], with EVs usually floating close to the surface of the coverslip. To

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