PLD1 Regulates mTOR Signaling and Mediates Cdc42 Activation of S6K1

Effects of phospholipase D1-inhibitory peptide on the growth and metastasis of gastric cancer cells

Phospholipase D1 (PLD1) contributes to cancer development and progression through its effects on cell proliferation, survival, invasion, metastasis, angiogenesis, drug resistance, and modulation of the tumor microenvironment. Its central role in these processes makes it a promising target for novel cancer treatments aimed at inhibiting its activity and disrupting the signaling pathways it regulates. In this study, we aimed to investigate the effect of PLD1 inhibition on gastric cancer cell growth using a novel peptide inhibitor, TAT-TVTSP. PLD1, which plays a role in cancer progression, catalyzes the conversion of phosphatidylcholine into choline and phosphatidic acid through hydrolysis. To effectively target PLD1 in cells, we engineered TAT-TVTSP by fusing a PLD1-inhibitory peptide (TVTSP) with a cell-penetrating peptide (TAT). We observed that TAT-TVTSP effectively inhibited PLD1 activity in AGS gastric cancer cells. Moreover, TAT-TVTSP significantly inhibited the mammalian target of the rapamycin signaling pathway, including the phosphorylation of key downstream targets such as S6K1, AKT, S473, glycogen synthase kinase-3b, and forkhead box O1. TAT-TVTSP did not induce cell death, but it triggered cell cycle arrest by activating p21 and p27 via AKT phosphorylation. Functional assays revealed that TAT-TVTSP significantly impaired the colony-forming ability of AGS cells, thus inhibiting cell proliferation. Transwell and wound-healing assays revealed that this peptide disrupted the cellular behaviors critical to cancer progression, such as migration and invasion. In vivo, TAT-TVTSP significantly reduced tumor growth in the xenograft model of gastric cancer without any toxicity. Overall, our results suggest that TAT-TVTSP is a novel therapeutic agent for PLD1-mediated cancers.

A multiprotein signaling complex sustains AKT and mTOR/S6K activity necessary for the survival of cancer cells undergoing stress

The ability of cancer cells to survive microenvironmental stresses is critical for tumor progression and metastasis; however, how they survive these challenges is not fully understood. Here, we describe a novel multiprotein complex (DockTOR) essential for the survival of cancer cells under stress, triggered by the GTPase Cdc42 and a signaling partner Dock7, which includes AKT, mTOR, and the mTOR regulators TSC1, TSC2, and Rheb. DockTOR enables cancer cells to maintain a low but critical mTORC2-dependent phosphorylation of AKT during serum deprivation by preventing AKT dephosphorylation through an interaction between phospho-AKT and the Dock7 DHR1 domain. This activity stimulates a Raptor-independent but Rapamycin-sensitive mTOR/S6K activity necessary for survival. These findings address long-standing questions of how Cdc42 signals result in mTOR activation and demonstrate how cancer cells survive conditions when growth factor-dependent activation of mTORC1 is off. Determining how cancer cells survive stress conditions could identify vulnerabilities that lead to new therapeutic strategies.

PLD1 promotes spindle assembly and migration through regulating autophagy in mouse oocyte meiosis

PLD1 has been implicated in cytoskeletal reorganization and vesicle trafficking in somatic cells; however, its function remains unclear in oocyte meiosis. Herein, we found PLD1 stably expresses in mouse oocytes meiosis, with direct interaction with spindle, RAB11A+ vesicles and macroautophagic/autophagic vacuoles. The genetic or chemical inhibition of PLD1 disturbed MTOC clustering, spindle assembly and its cortical migration, also decreased PtdIns(4,5)P2, phosphorylated CFL1 (p-CFL1 [Ser3]) and ACTR2, and their local distribution on MTOC, spindle and vesicles. Furthermore in PLD1-suppressed oocytes, vesicle size was significantly reduced while F-actin density was dramatically increased in the cytoplasm, the asymmetric distribution of autophagic vacuoles was broken and the whole autophagic process was substantially enhanced, as illustrated with characteristic changes in autophagosomes, autolysosome formation and levels of ATG5, BECN1, LC3-II, SQSTM1 and UB. Exogenous administration of PtdIns(4,5)P2 or overexpression of CFL1 hyperphosphorylation mutant (CFL1S3E) could significantly improve polar MTOC focusing and spindle structure in PLD1-depleted oocytes, whereas overexpression of ACTR2 could rescue not only MTOC clustering, and spindle assembly but also its asymmetric positioning. Interestingly, autophagy activation induced similar defects in spindle structure and positioning; instead, its inhibition alleviated the alterations in PLD1-depleted oocytes, and this was highly attributed to the restored levels of PtdIns(4,5)P2, ACTR2 and p-CFL1 (Ser3). Together, PLD1 promotes spindle assembly and migration in oocyte meiosis, by maintaining rational levels of ACTR2, PtdIns(4,5)P2 and p-CFL1 (Ser3) in a manner of modulating autophagy flux. This study for the first time introduces a unique perspective on autophagic activity and function in oocyte meiotic development.Abbreviations: ACTR2/ARP2: actin related protein 2; ACTR3/ARP3: actin related protein 3; ATG5: autophagy related 5; Baf-A1: bafilomycin A1; BFA: brefeldin A; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GOLGA2/GM130: golgin A2; GV: germinal vesicle; GVBD: germinal vesicle breakdown; IVM: in vitro maturation; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MI: metaphase of meiosis I; MII: metaphase of meiosis II; MO: morpholino; MTOC: microtubule-organizing center; MTOR: mechanistic target of rapamycin kinase; PB1: first polar body; PLA: proximity ligation assay; PLD1: phospholipase D1; PtdIns(4,5)P2/PIP2: phosphatidylinositol 4,5-bisphosphate; RAB11A: RAB11A, member RAS oncogene family; RPS6KB1/S6K1: ribosomal protein S6 kinase B1; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscopy; TUBA/α-tubulin: tubulin alpha; TUBG/γ-tubulin: tubulin gamma; UB: ubiquitin; WASL/N-WASP: WASP like actin nucleation promoting factor.

β-Hydroxy-β-methylbutyrate (HMB) leads to phospholipase D2 (PLD2) activation and alters circadian rhythms in myotubes

β-Hydroxy-β-methylbutyrate (HMB) is a breakdown product of leucine, which promotes muscle growth. Although some studies indicate that HMB activates AKT and mTOR, others show activation of the downstream effectors, P70S6K and S6, independent of mTOR. Our aim was to study the metabolic effect of HMB around the circadian clock in order to determine more accurately the signaling pathway involved. C2C12 myotubes were treated with HMB and clock, metabolic and myogenic markers were measured around the clock. HMB-treated C2C12 myotubes showed no activation of AKT and mTOR, but did show activation of P70S6K and S6. Activation of P70S6K and S6 was also found when myotubes were treated with HMB combined with metformin, an indirect mTOR inhibitor, or rapamycin, a direct mTOR inhibitor. The activation of the P70S6K and S6 independent of AKT and mTOR, was accompanied by increased activation of phospholipase D2 (PLD). In addition, HMB led to high amplitude and advanced circadian rhythms. In conclusion, HMB induces myogenesis in C2C12 by activating P70S6K and S6 via PLD2, rather than AKT and mTOR, leading to high amplitude advanced rhythms.

Targeting the Metabolic Paradigms in Cancer and Diabetes

Dysregulated metabolic dynamics are evident in both cancer and diabetes, with metabolic alterations representing a facet of the myriad changes observed in these conditions. This review delves into the commonalities in metabolism between cancer and type 2 diabetes (T2D), focusing specifically on the contrasting roles of oxidative phosphorylation (OXPHOS) and glycolysis as primary energy-generating pathways within cells. Building on earlier research, we explore how a shift towards one pathway over the other serves as a foundational aspect in the development of cancer and T2D. Unlike previous reviews, we posit that this shift may occur in seemingly opposing yet complementary directions, akin to the Yin and Yang concept. These metabolic fluctuations reveal an intricate network of underlying defective signaling pathways, orchestrating the pathogenesis and progression of each disease. The Warburg phenomenon, characterized by the prevalence of aerobic glycolysis over minimal to no OXPHOS, emerges as the predominant metabolic phenotype in cancer. Conversely, in T2D, the prevailing metabolic paradigm has traditionally been perceived in terms of discrete irregularities rather than an OXPHOS-to-glycolysis shift. Throughout T2D pathogenesis, OXPHOS remains consistently heightened due to chronic hyperglycemia or hyperinsulinemia. In advanced insulin resistance and T2D, the metabolic landscape becomes more complex, featuring differential tissue-specific alterations that affect OXPHOS. Recent findings suggest that addressing the metabolic imbalance in both cancer and diabetes could offer an effective treatment strategy. Numerous pharmaceutical and nutritional modalities exhibiting therapeutic effects in both conditions ultimately modulate the OXPHOS-glycolysis axis. Noteworthy nutritional adjuncts, such as alpha-lipoic acid, flavonoids, and glutamine, demonstrate the ability to reprogram metabolism, exerting anti-tumor and anti-diabetic effects. Similarly, pharmacological agents like metformin exhibit therapeutic efficacy in both T2D and cancer. This review discusses the molecular mechanisms underlying these metabolic shifts and explores promising therapeutic strategies aimed at reversing the metabolic imbalance in both disease scenarios.

MOBILE pipeline enables identification of context-specific networks and regulatory mechanisms

Robust identification of context-specific network features that control cellular phenotypes remains a challenge. We here introduce MOBILE (Multi-Omics Binary Integration via Lasso Ensembles) to nominate molecular features associated with cellular phenotypes and pathways. First, we use MOBILE to nominate mechanisms of interferon-γ (IFNγ) regulated PD-L1 expression. Our analyses suggest that IFNγ-controlled PD-L1 expression involves BST2, CLIC2, FAM83D, ACSL5, and HIST2H2AA3 genes, which were supported by prior literature. We also compare networks activated by related family members transforming growth factor-beta 1 (TGFβ1) and bone morphogenetic protein 2 (BMP2) and find that differences in ligand-induced changes in cell size and clustering properties are related to differences in laminin/collagen pathway activity. Finally, we demonstrate the broad applicability and adaptability of MOBILE by analyzing publicly available molecular datasets to investigate breast cancer subtype specific networks. Given the ever-growing availability of multi-omics datasets, we envision that MOBILE will be broadly useful for identification of context-specific molecular features and pathways.

Activity-based directed evolution of a membrane editor in mammalian cells

Cellular membranes contain numerous lipid species, and efforts to understand the biological functions of individual lipids have been stymied by a lack of approaches for controlled modulation of membrane composition in situ. Here we present a strategy for editing phospholipids, the most abundant lipids in biological membranes. Our membrane editor is based on a bacterial phospholipase D (PLD), which exchanges phospholipid head groups through hydrolysis or transphosphatidylation of phosphatidylcholine with water or exogenous alcohols. Exploiting activity-dependent directed enzyme evolution in mammalian cells, we have developed and structurally characterized a family of 'superPLDs' with up to a 100-fold enhancement in intracellular activity. We demonstrate the utility of superPLDs for both optogenetics-enabled editing of phospholipids within specific organelle membranes in live cells and biocatalytic synthesis of natural and unnatural designer phospholipids in vitro. Beyond the superPLDs, activity-based directed enzyme evolution in mammalian cells is a generalizable approach to engineer additional chemoenzymatic biomolecule editors.

Vaginal epithelial dysfunction is mediated by the microbiome, metabolome, and mTOR signaling

Bacterial vaginosis (BV) is characterized by depletion of Lactobacillus and overgrowth of anaerobic and facultative bacteria, leading to increased mucosal inflammation, epithelial disruption, and poor reproductive health outcomes. However, the molecular mediators contributing to vaginal epithelial dysfunction are poorly understood. Here we utilize proteomic, transcriptomic, and metabolomic analyses to characterize biological features underlying BV in 405 African women and explore functional mechanisms in vitro. We identify five major vaginal microbiome groups: L. crispatus (21%), L. iners (18%), Lactobacillus (9%), Gardnerella (30%), and polymicrobial (22%). Using multi-omics we show that BV-associated epithelial disruption and mucosal inflammation link to the mammalian target of rapamycin (mTOR) pathway and associate with Gardnerella, M. mulieris, and specific metabolites including imidazole propionate. Experiments in vitro confirm that type strain G. vaginalis and M. mulieris supernatants and imidazole propionate directly affect epithelial barrier function and activation of mTOR pathways. These results find that the microbiome-mTOR axis is a central feature of epithelial dysfunction in BV.

Phosphorylation of LKB1 by PDK1 Inhibits Cell Proliferation and Organ Growth by Decreased Activation of AMPK

The master kinase LKB1 is a key regulator of se veral cellular processes, including cell proliferation, cell polarity and cellular metabolism. It phosphorylates and activates several downstream kinases, including AMP-dependent kinase, AMPK. Activation of AMPK by low energy supply and phosphorylation of LKB1 results in an inhibition of mTOR, thus decreasing energy-consuming processes, in particular translation and, thus, cell growth. LKB1 itself is a constitutively active kinase, which is regulated by posttranslational modifications and direct binding to phospholipids of the plasma membrane. Here, we report that LKB1 binds to Phosphoinositide-dependent kinase (PDK1) by a conserved binding motif. Furthermore, a PDK1-consensus motif is located within the kinase domain of LKB1 and LKB1 gets phosphorylated by PDK1 in vitro. In Drosophila, knockin of phosphorylation-deficient LKB1 results in normal survival of the flies, but an increased activation of LKB1, whereas a phospho-mimetic LKB1 variant displays decreased AMPK activation. As a functional consequence, cell growth as well as organism size is decreased in phosphorylation-deficient LKB1. Molecular dynamics simulations of PDK1-mediated LKB1 phosphorylation revealed changes in the ATP binding pocket, suggesting a conformational change upon phosphorylation, which in turn can alter LKB1's kinase activity. Thus, phosphorylation of LKB1 by PDK1 results in an inhibition of LKB1, decreased activation of AMPK and enhanced cell growth.

The interaction between circadian rhythm and epilepsy

Evidence about the interaction between circadian rhythms (CR) and epilepsy has been expanded with the application of advanced detection technology. An adequate understanding of how circadian system and epilepsy interact with each other could contribute to more accurate seizure prediction as well as rapid development of potential treatment timed to specific phases of CR. In this review, we present the reciprocal relationship between CR and epileptic activities from aspects of sleep effect, genetic modulation and brain biochemistry. It has been found that sleep-wake patterns, circadian timing systems and multidien rhythms have essential roles in seizure activities and interictal epileptiform discharge (IED). For instance, specific distribution patterns of seizures and IED have been reported, i.e., lighter non-rapid eye movement (NREM) sleep stage (stage 2) induces seizures while deeper NREM sleep stage (stage 3) activates IEDs. Furthermore, the epilepsy type, seizure type and seizure onset zone can significantly affect the rhythms of seizure occurrence. Apart from the common seizure types, several specific epilepsy syndromes also have a close correlation with sleep-wakefulness patterns. Sleep influences the epilepsy rhythm, and conversely, epilepsy alters the sleep rhythm through multiple pathways. Clock genes accompanied by two feedback loops of regulation have an important role in cortical excitability and seizure occurrence, which may be involved in the mTORopathy. The suprachiasmatic nuclei (SCN) has a rhythm of melatonin and cortisol secretion under the circadian pattern, and then these hormones can feed back into a central oscillator to affect the SCN-dependent rhythms, leading to variable but prominent influence on epilepsy. Furthermore, we discuss the precise predictive algorithms and chronotherapy strategies based on different temporal patterns of seizure occurrence for patients with epilepsy, which may offer a valuable indication for non-invasive closed-loop treatment system. Optimization of the time and dose of antiseizure medications, and resynchronization of disturbed CR (by hormone therapy, light exposure, ketogenic diet, novel small molecules) would be beneficial for epileptic patients in the future. Before formal clinical practice, future large-scale studies are urgently needed to assist prediction and treatment of circadian seizure activities and address unsolved restrictions.

Phosphatidic acid binds to and stimulates the activity of ARGAH2 from Arabidopsis

Phosphatidic acid (PA) has emerged as an important lipid signal during abiotic and biotic stress conditions such as drought, salinity, freezing, nutrient starvation, wounding and microbial elicitation. PA acts during stress responses primarily via binding and translocating target proteins or through modulating their activity. Owing to the importance of PA during stress signaling and developmental stages, it is imperative to identify PA interacting proteins and decipher their specific roles. In the present study, we have identified PA binding proteins from the leaves of Arabidopsis thaliana. Mass spectroscopy analysis led to the identification of 21 PA binding proteins with known roles in various cellular processes. One of the PA-binding proteins identified during this study, AtARGAH2, was further studied to unravel the role of PA interaction. Recombinant AtARGAH2 binding with immobilized PA on a solid support validated PA-AtARGAH2 binding invitro. PA binding to AtARGAH2 leads to the enhancement of arginase enzymatic activity in a dose dependent manner. Enzyme kinetics of recombinant AtARGAH2 demonstrated a lower K_m value in presence of PA, suggesting role of PA in efficient enzyme-substrate binding. This simple approach could systematically be applied to perform an inclusive study on lipid binding proteins to elucidate their role in physiology of plants.

Mechanistic Target of Rapamycin Complex 1: From a Nutrient Sensor to a Key Regulator of Metabolism and Health

Mechanistic target of rapamycin complex 1 (mTORC1) is a multi-protein complex widely found in eukaryotes. It serves as a central signaling node to coordinate cell growth and metabolism by sensing diverse extracellular and intracellular inputs, including amino acid-, growth factor-, glucose-, and nucleotide-related signals. It is well documented that mTORC1 is recruited to the lysosomal surface, where it is activated and, accordingly, modulates downstream effectors involved in regulating protein, lipid, and glucose metabolism. mTORC1 is thus the central node for coordinating the storage and mobilization of nutrients and energy across various tissues. However, emerging evidence indicated that the overactivation of mTORC1 induced by nutritional disorders leads to the occurrence of a variety of metabolic diseases, including obesity and type 2 diabetes, as well as cancer, neurodegenerative disorders, and aging. That the mTORC1 pathway plays a crucial role in regulating the occurrence of metabolic diseases renders it a prime target for the development of effective therapeutic strategies. Here, we focus on recent advances in our understanding of the regulatory mechanisms underlying how mTORC1 integrates metabolic inputs as well as the role of mTORC1 in the regulation of nutritional and metabolic diseases.

Interactome analysis of human phospholipase D and phosphatidic acid-associated protein network

Mammalian phospholipase D (PLD) enzyme family consists of six members. Among them, PLD1/2/6 catalyzes phosphatidic acid (PA) production, while PLD3/4/5 has no catalytic activities. Deregulation of the PLD-PA lipid signaling has been associated with various human diseases including cancer. However, a comprehensive analysis of the regulators and effectors for this crucial lipid metabolic pathway has not been fully achieved. Using a proteomic approach, we defined the protein interaction network for the human PLD family of enzymes and PA and revealed diverse cellular signaling events involving them. Through it, we identified PJA2 as a novel E3 ubiquitin ligase for PLD1 involved in control of the PLD1-mediated mammalian target of rapamycin signaling. Additionally, we showed that PA interacted with and positively regulated sphingosine kinase 1. Taken together, our study not only generates a rich interactome resource for further characterizing the human PLD-PA lipid signaling but also connects this important metabolic pathway with numerous biological processes.

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Defining the interactome of human phospholipase D enzymes and phosphatidic acid.

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PJA2 functions as an E3 ubiquitin ligase of phospholipase D1.

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Phosphatidic acid interacts with and positively regulates sphingosine kinase 1.

AMPK-mTOR Signaling and Cellular Adaptations in Hypoxia

Cellular energy is primarily provided by the oxidative degradation of nutrients coupled with mitochondrial respiration, in which oxygen participates in the mitochondrial electron transport chain to enable electron flow through the chain complex (I-IV), leading to ATP production. Therefore, oxygen supply is an indispensable chapter in intracellular bioenergetics. In mammals, oxygen is delivered by the bloodstream. Accordingly, the decrease in cellular oxygen level (hypoxia) is accompanied by nutrient starvation, thereby integrating hypoxic signaling and nutrient signaling at the cellular level. Importantly, hypoxia profoundly affects cellular metabolism and many relevant physiological reactions induce cellular adaptations of hypoxia-inducible gene expression, metabolism, reactive oxygen species, and autophagy. Here, we introduce the current knowledge of hypoxia signaling with two-well known cellular energy and nutrient sensing pathways, AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin complex 1 (mTORC1). Additionally, the molecular crosstalk between hypoxic signaling and AMPK/mTOR pathways in various hypoxic cellular adaptions is discussed.

Integrated Analysis of Transcriptome and Histone Modifications in Granulosa Cells During Ovulation in Female Mice

The ovulatory luteinizing hormone (LH) surge induces rapid changes of gene expression and cellular functions in granulosa cells (GCs) undergoing luteinization. However, it remains unclear how the changes in genome-wide gene expression are regulated. H3K4me3 histone modifications are involved in the rapid alteration of gene expression. In this study, we investigated genome-wide changes of transcriptome and H3K4me3 status in mouse GCs undergoing luteinization. GCs were obtained from mice treated with equine chorionic gonadotropin (hCG) before, 4 hours, and 12 hours after human chorionic gonadotropin injection. RNA-sequencing identified a number of upregulated and downregulated genes, which could be classified into 8 patterns according to the time-course changes of gene expression. Many genes were transiently upregulated or downregulated at 4 hours after hCG stimulation. Gene Ontology terms associated with these genes included steroidogenesis, ovulation, cumulus-oocyte complex (COC) expansion, angiogenesis, immune system, reactive oxygen species (ROS) metabolism, inflammatory response, metabolism, and autophagy. The cellular functions of DNA repair and cell growth were newly identified as being activated during ovulation. Chromatin immunoprecipitation-sequencing revealed a genome-wide and rapid change in H3K4me3 during ovulation. Integration of transcriptome and H3K4me3 data identified many H3K4me3-associated genes that are involved in steroidogenesis, ovulation, COC expansion, angiogenesis, inflammatory response, immune system, ROS metabolism, lipid and glucose metabolism, autophagy, and regulation of cell size. The present results suggest that genome-wide changes in H3K4me3 after the LH surge are associated with rapid changes in gene expression in GCs, which enables GCs to acquire a lot of cellular functions within a short time that are required for ovulation and luteinization.

mTOR deletion in neural crest cells disrupts cardiac outflow tract remodeling and causes a spectrum of cardiac defects through the mTORC1 pathway

A critical cell type participating in cardiac outflow tract development is a subpopulation of the neural crest cells, the cardiac neural crest cells (NCCs), whose defect causes a spectrum of cardiovascular abnormalities. Accumulating evidence indicates that mTOR, which belongs to the PI3K-related kinase family and impacts multiple signaling pathways in a variety of contexts, plays a pivotal role for NCC development. Here, we investigated functional roles of mTOR for cardiac neural crest development using several lines of mouse genetic models. We found that disruption of mTOR caused NCC defects and failure of cardiac outflow tract separation, which resulted in a spectrum of cardiac defects including persistent truncus arteriosus, ventricular septal defect and ventricular wall defect. Specifically, mutant neural crest cells showed reduced migration into the cardiac OFT and prematurely exited the cell cycle. A number of critical factors and fundamental signaling pathways, which are important for neural crest and cardiomyocyte development, were impaired. Moreover, actin dynamics was disrupted by mTOR deletion. Finally, by phenotyping the neural crest Rptor and Rictor knockout mice respectively, we demonstrate that mTOR acts principally through the mTORC1 pathway for cardiac neural crest cells. Altogether, these data established essential roles of mTOR for cardiac NCC development and imply that dysregulation of mTOR in NCCs may underline a spectrum of cardiac defects.

The Role of mTOR Signaling as a Therapeutic Target in Cancer

The aim of this review was to summarize current available information about the role of phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling in cancer as a potential target for new therapy options. The mTOR and PI3K/AKT/mTORC1 (mTOR complex 1) signaling are critical for the regulation of many fundamental cell processes including protein synthesis, cell growth, metabolism, survival, catabolism, and autophagy, and deregulated mTOR signaling is implicated in cancer, metabolic dysregulation, and the aging process. In this review, we summarize the information about the structure and function of the mTOR pathway and discuss the mechanisms of its deregulation in human cancers including genetic alterations of PI3K/AKT/mTOR pathway components. We also present recent data regarding the PI3K/AKT/mTOR inhibitors in clinical studies and the treatment of cancer, as well the attendant problems of resistance and adverse effects.

Ror2-mediated non-canonical Wnt signaling regulates Cdc42 and cell proliferation during tooth root development

The control of size and shape is an important part of regulatory process during organogenesis. Tooth formation is a highly complex process that fine-tunes the size and shape of the tooth, which are crucial for its physiological functions. Each tooth consists of a crown and one or more roots. Despite comprehensive knowledge of the mechanism that regulates early tooth crown development, we have limited understanding of the mechanism regulating root patterning and size during development. Here, we show that Ror2-mediated non-canonical Wnt signaling in the dental mesenchyme plays a crucial role in cell proliferation, and thereby regulates root development size in mouse molars. Furthermore, Cdc42 acts as a potential downstream mediator of Ror2 signaling in root formation. Importantly, activation of Cdc42 can restore cell proliferation and partially rescue the root development size defects in Ror2 mutant mice. Collectively, our findings provide novel insights into the function of Ror2-mediated non-canonical Wnt signaling in regulating tooth morphogenesis, and suggest potential avenues for dental tissue engineering.

Summary: The function of Ror2-mediated non-canonical Wnt signaling and its effect on Cdc42 activation is crucial in regulating progenitor cell proliferation, odontoblast differentiation and Hertwig's epithelial root sheath formation during tooth root morphogenesis.

Aminoacyl-tRNA synthetases and amino acid signaling

Aminoacyl-tRNA synthetases (ARSs) are a family of evolutionarily conserved housekeeping enzymes used for protein synthesis that have pivotal roles in the ligation of tRNA with their cognate amino acids. Recent advances in the structural and functional studies of ARSs have revealed many previously unknown biological functions beyond the classical catalytic roles. Sensing the sufficiency of intracellular nutrients such as amino acids, ATP, and fatty acids is a crucial aspect for every living organism, and it is closely connected to the regulation of diverse cellular physiologies. Notably, among ARSs, leucyl-tRNA synthetase 1 (LARS1) has been identified to perform specifically as a leucine sensor upstream of the amino acid-sensing pathway and thus participates in the coordinated control of protein synthesis and autophagy for cell growth. In addition to LARS1, other types of ARSs are also likely involved in the sensing and signaling of their cognate amino acids inside cells. Collectively, this review focuses on the mechanisms of ARSs interacting within amino acid signaling and proposes the possible role of ARSs as general intracellular amino acid sensors.

Regulation of Body Size and Growth Control

The control of body and organ growth is essential for the development of adults with proper size and proportions, which is important for survival and reproduction. In animals, adult body size is determined by the rate and duration of juvenile growth, which are influenced by the environment. In nutrient-scarce environments in which more time is needed for growth, the juvenile growth period can be extended by delaying maturation, whereas juvenile development is rapidly completed in nutrient-rich conditions. This flexibility requires the integration of environmental cues with developmental signals that govern internal checkpoints to ensure that maturation does not begin until sufficient tissue growth has occurred to reach a proper adult size. The Target of Rapamycin (TOR) pathway is the primary cell-autonomous nutrient sensor, while circulating hormones such as steroids and insulin-like growth factors are the main systemic regulators of growth and maturation in animals. We discuss recent findings in Drosophila melanogaster showing that cell-autonomous environment and growth-sensing mechanisms, involving TOR and other growth-regulatory pathways, that converge on insulin and steroid relay centers are responsible for adjusting systemic growth, and development, in response to external and internal conditions. In addition to this, proper organ growth is also monitored and coordinated with whole-body growth and the timing of maturation through modulation of steroid signaling. This coordination involves interorgan communication mediated by Drosophila insulin-like peptide 8 in response to tissue growth status. Together, these multiple nutritional and developmental cues feed into neuroendocrine hubs controlling insulin and steroid signaling, serving as checkpoints at which developmental progression toward maturation can be delayed. This review focuses on these mechanisms by which external and internal conditions can modulate developmental growth and ensure proper adult body size, and highlights the conserved architecture of this system, which has made Drosophila a prime model for understanding the coordination of growth and maturation in animals.

Angiotensin-Converting Enzyme Inhibitor Rapidly Ameliorates Depressive-Type Behaviors via Bradykinin-Dependent Activation of Mammalian Target of Rapamycin Complex 1

BACKGROUND

Angiotensin-converting enzyme inhibitors (ACEIs) are widely prescribed antihypertensive agents. Intriguingly, case reports and clinical trials have indicated that ACEIs, including captopril and lisinopril, may have a rapid mood-elevating effect in certain patients, but few experimental studies have investigated their value as fast-onset antidepressants.

METHODS

The present study consisted of a series of experiments using biochemical assays, immunohistochemistry, and behavioral techniques to examine the effect and mechanism of captopril on depressive-like behavior in 2 animal models, the chronic unpredictable stress model and the chronic social defeat stress model.

RESULTS

Captopril (19.5 or 39 mg/kg, intraperitoneal injection) exerted rapid antidepressant activity in mice treated under the chronic unpredictable stress model and mice treated under the chronic social defeat stress model. Pharmacokinetic analysis revealed that captopril crossed the blood-brain barrier and that lisinopril, another ACEI with better blood-brain barrier permeability, exerted a faster and longer-lasting effect at a same molar equivalent dose. This antidepressant effect seemed to be independent of the renin-angiotensin system, but dependent on the bradykinin (BK) system, since the decreased BK detected in the stressed mice could be reversed by captopril. The hypofunction of the downstream effector of BK, Cdc42 (cell division control protein 42) homolog, contributed to the stress-induced loss of dendritic spines, which was rapidly reversed by captopril via activating the mTORC1 (mammalian target of rapamycin complex 1) pathway.

CONCLUSIONS

Our findings indicate that the BK-dependent activation of mTORC1 may represent a promising mechanism underlying antidepressant pharmacology. Considering their affordability and availability, ACEIs may emerge as a novel fast-onset antidepressant, especially for patients with comorbid depression and hypertension.

The two splice variant forms of Cdc42 exert distinct and essential functions in neurogenesis

The small GTPase cell division cycle 42 (CDC42) plays essential roles in neurogenesis and brain development. Previously, using murine embryonic P19 cells as a model system, we showed that CDC42 stimulates mTOR complex 1 (mTORC1) activity and thereby up-regulates transcription factors required for the formation of neural progenitor cells. However, paradoxically, although endogenous CDC42 is required for both the initial transition of undifferentiated P19 cells to neural progenitors and their ultimate terminal differentiation into neurons, ectopic CDC42 overexpression promotes only the first stage of neurogenesis (i.e. the formation of neuroprogenitors) and not the second phase (differentiation into neurons). Here, using both P19 cells and mouse embryonic stem cells, we resolve this paradox, demonstrating that two splice variants of CDC42, differing only in nine amino acid residues in their very C-terminal regions, play distinct roles in neurogenesis. We found that a CDC42 splice variant that has a ubiquitous tissue distribution, termed here as CDC42u, specifically drives the formation of neuroprogenitor cells, whereas a brain-specific CDC42 variant, CDC42b, is essential for promoting the transition of neuroprogenitor cells to neurons. We further show that the specific roles of CDC42u and CDC42b in neurogenesis are due to their opposing effects on mTORC1 activity. Specifically, CDC42u stimulated mTORC1 activity and thereby induced neuroprogenitor formation, whereas CDC42b worked together with activated CDC42-associated kinase (ACK) in down-regulating mTOR expression and promoting neuronal differentiation. These findings highlight the remarkable functional specificities of two highly similar CDC42 splice variants in regulating distinct stages of neurogenesis.

Chronobiology of limbic seizures: Potential mechanisms and prospects of chronotherapy for mesial temporal lobe epilepsy

Mesial Temporal Lobe Epilepsy (mTLE) characterized by progressive development of complex partial seizures originating from the hippocampus is the most prevalent and refractory type of epilepsy. One of the remarkable features of mTLE is the rhythmic pattern of occurrence of spontaneous seizures, implying a dependence on the endogenous clock system for seizure threshold. Conversely, circadian rhythms are affected by epilepsy too. Comprehending how the circadian system and seizures interact with each other is essential for understanding the pathophysiology of epilepsy as well as for developing innovative therapies that are efficacious for better seizure control. In this review, we confer how the temporal dysregulation of the circadian clock in the hippocampus combined with multiple uncoupled oscillators could lead to periodic seizure occurrences and comorbidities. Unraveling these associations with additional research would help in developing chronotherapy for mTLE, based on the chronobiology of spontaneous seizures. Notably, differential dosing of antiepileptic drugs over the circadian period and/or strategies that resynchronize biological rhythms may substantially improve the management of seizures in mTLE patients.

Functional analysis of mammalian phospholipase D enzymes

Phosphatidylcholine (PC)-specific phospholipase D (PLD) hydrolyzes the phosphodiester bond of the PC to generate phosphatidic acid (PA) and regulates several subcellular functions. Mammalian genomes contain two genes encoding distinct isoforms of PLD in contrast with invertebrate genomes that include a single PLD gene. However, the significance of two genes within a genome encoding the same biochemical activity remains unclear. Recently, loss of function in the only PLD gene in Drosophila was reported to result in reduced PA levels and a PA-dependent collapse of the photoreceptor plasma membrane due to defects in vesicular transport. Phylogenetic analysis reveals that human PLD1 (hPLD1) is evolutionarily closer to dPLD than human PLD2 (hPLD2). In the present study, we expressed hPLD1 and hPLD2 in Drosophila and found that while reconstitution of hPLD1 is able to completely rescue retinal degeneration in a loss of function dPLD mutant, hPLD2 was less effective in its ability to mediate a rescue. Using a newly developed analytical method, we determined the acyl chain composition of PA species produced by each enzyme. While dPLD was able to restore the levels of most PA species in dPLD^3.1 cells, hPLD1 and hPLD2 each were unable to restore the levels of a subset of unique species of PA. Finally, we found that in contrast with hPLD2, dPLD and hPLD1 are uniquely distributed to the subplasma membrane region in photoreceptors. In summary, hPLD1 likely represents the ancestral PLD in mammalian genomes while hPLD2 represents neofunctionalization to generate PA at distinct subcellular membranes.

mTOR Inhibition via Displacement of Phosphatidic Acid Induces Enhanced Cytotoxicity Specifically in Cancer Cells

The mTOR is a central regulator of cell growth and is highly activated in cancer cells to allow rapid tumor growth. The use of mTOR inhibitors as anticancer therapy has been approved for some types of tumors, albeit with modest results. We recently reported the synthesis of ICSN3250, a halitulin analogue with enhanced cytotoxicity. We report here that ICSN3250 is a specific mTOR inhibitor that operates through a mechanism distinct from those described for previous mTOR inhibitors. ICSN3250 competed with and displaced phosphatidic acid from the FRB domain in mTOR, thus preventing mTOR activation and leading to cytotoxicity. Docking and molecular dynamics simulations evidenced not only the high conformational plasticity of the FRB domain, but also the specific interactions of both ICSN3250 and phosphatidic acid with the FRB domain in mTOR. Furthermore, ICSN3250 toxicity was shown to act specifically in cancer cells, as noncancer cells showed up to 100-fold less sensitivity to ICSN3250, in contrast to other mTOR inhibitors that did not show selectivity. Thus, our results define ICSN3250 as a new class of mTOR inhibitors that specifically targets cancer cells.Significance: ICSN3250 defines a new class of mTORC1 inhibitors that displaces phosphatidic acid at the FRB domain of mTOR, inducing cell death specifically in cancer cells but not in noncancer cells. Cancer Res; 78(18); 5384-97. ©2018 AACR.

Therapeutic potential of a phospholipase D1 inhibitory peptide fused with a cell-penetrating peptide as a novel anti-asthmatic drug in a Der f 2-induced airway inflammation model

Asthma is a chronic lung disease that causes airflow obstruction due to airway inflammation. However, its therapeutics remain inadequate. We previously reported that phospholipase D1 (PLD1) is a key enzyme involved in the production of pro-inflammatory cytokines in airway inflammation induced by the house dust mite allergen Dermatophagoides farinae 2 (Der f 2). We also revealed that PLD1 is specifically inactivated by AP180 (assembly protein, 180 kDa) and identified the PLD1-specific binding motif (TVTSP) of AP180. Therefore, the aims of this study were to develop a novel anti-asthmatic agent that could suppress airway inflammation by inhibiting PLD1 and examine its acute and chronic toxicity. We designed TAT-TVTSP, a PLD1-inhibitory peptide fused with a cell-penetrating peptide (CPP) delivery system. TAT-TVTSP was efficiently delivered to bronchial epithelial cells and significantly reduced Der f 2-induced PLD activation and Interleukin 13 (IL-13) production. Intranasally administered TAT-TVTSP was also efficiently transferred to airway tissues and ameliorated airway inflammation in a Der f 2-induced allergic asthma mouse model. Moreover, we investigated the safety of TAT-TVTSP as a therapeutic agent through single- and repeated-dose toxicity studies in a mouse model. Taken together, these results indicated that a PLD1-inhibitory peptide fused with a cell-penetrating peptide may be useful for treating allergic inflammatory asthma induced by house dust mites (HDMs).

A drug that targets a key enzyme involved in airway tissue inflammation shows promise in the treatment of allergic asthma. The enzyme phospholipase D1 (PLD1) triggers airway inflammation in allergic asthma brought on by house dust mites. Joong-Soo Han at Hanyang University in Seoul, Eung-Gook Kim at Chungbuk National University, Cheongju, South Korea, and co-workers have developed a treatment aimed at suppressing PLD1 and trialed the drug on mouse models of dust-mite allergy. The team designed a carrier system capable of accurately delivering a PLD1-inhibitory peptide to airway tissues and cells. They found that airway inflammation was significantly reduced in the treated mice. The drug appeared to be relatively safe when used in repeated doses, although further investigations are needed to verify this. The team hope their treatment will improve therapies for allergic asthma.

CDC42-related genes are upregulated in helper T cells from obese asthmatic children

BACKGROUND

Pediatric obesity-related asthma is more severe and less responsive to medications than asthma in normal-weight children. Obese asthmatic children have nonatopic TH1-polarized systemic inflammation that correlates with pulmonary function deficits, but the pathways underlying TH1-polarized inflammation are not well understood.

OBJECTIVE

We compared the CD4+ T-cell transcriptome in obese children with asthma with that in normal-weight children with asthma to identify key differentially expressed genes associated with TH1-polarized inflammation.

METHODS

CD4+ T-cell transcriptome-wide differential gene expression was compared between 21 obese and 21 normal-weight children by using directional RNA sequencing. High-confidence differentially expressed genes were verified in the first cohort and validated in a second cohort of 20 children (10 obese and 10 normal-weight children) by using quantitative RT-PCR.

RESULTS

Transcriptome-wide differential gene expression among obese asthmatic children was enriched for genes, including VAV2, DOCK5, PAK3, PLD1, CDC42EP4, and CDC42PBB, which are associated with CDC42, a small guanosine triphosphate protein linked to T-cell activation. Upregulation of MLK3 and PLD1, genes downstream of CDC42 in the mitogen-activated protein kinase and mammalian target of rapamycin pathways and the inverse correlation of CDC42EP4 and DOCK5 transcript counts with FEV1/FVC ratio together support a role of CDC42 in the TH1 polarization and pulmonary function deficits found in patients with obesity-related asthma.

CONCLUSIONS

Our study identifies the CDC42 pathway as a novel target that is upregulated in TH cells of obese asthmatic children, suggesting its role in nonatopic TH1-polarized systemic inflammation and pulmonary function deficits found in patients with pediatric obesity-related asthma.

A Unique Homeostatic Signaling Pathway Links Synaptic Inactivity to Postsynaptic mTORC1

mTORC1-dependent translational control plays a key role in several enduring forms of synaptic plasticity such as long term potentiation (LTP) and mGluR-dependent long term depression. Recent evidence demonstrates an additional role in regulating synaptic homeostasis in response to inactivity, where dendritic mTORC1 serves to modulate presynaptic function via retrograde signaling. Presently, it is unclear whether LTP and homeostatic plasticity use a common route to mTORC1-dependent signaling or whether each engage mTORC1 through distinct pathways. Here, we report a unique signaling pathway that specifically couples homeostatic signaling to postsynaptic mTORC1 after loss of excitatory synaptic input. We find that AMPAR blockade, but not LTP-inducing stimulation, induces phospholipase D (PLD)-dependent synthesis of the lipid second messenger phosphatidic acid (PA) in rat cultured hippocampal neurons of either sex. Pharmacological blockade of PLD1/2 or pharmacogenetic disruption of PA interactions with mTOR eliminates mTORC1 signaling and presynaptic compensation driven by AMPAR blockade, but does not alter mTORC1 activation or functional changes during chemical LTP (cLTP). Overexpression of PLD1, but not PLD2, recapitulates both functional synaptic changes as well as signature cellular adaptations associated with homeostatic plasticity. Finally, transient application of exogenous PA is sufficient to drive rapid presynaptic compensation requiring mTORC1-dependent translation of BDNF in the postsynaptic compartment. These results thus define a unique homeostatic signaling pathway coupling mTORC1 activation to changes in excitatory synaptic drive. Our results further imply that more than one canonical mTORC1 activation pathway may be relevant for the design of novel therapeutic approaches against neurodevelopmental disorders associated with mTORC1 dysregulation.SIGNIFICANCE STATEMENT Homeostatic and Hebbian forms of synaptic plasticity are thought to play complementary roles in regulating neural circuit function, but we know little about how these forms of plasticity are distinguished at the single neuron level. Here, we define a signaling pathway that uniquely links mTORC1 with homeostatic signaling in neurons.

The Role of Mammalian Target of Rapamycin (mTOR) in Insulin Signaling

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that controls a wide spectrum of cellular processes, including cell growth, differentiation, and metabolism. mTOR forms two distinct multiprotein complexes known as mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), which are characterized by the presence of raptor and rictor, respectively. mTOR controls insulin signaling by regulating several downstream components such as growth factor receptor-bound protein 10 (Grb10), insulin receptor substrate (IRS-1), F-box/WD repeat-containing protein 8 (Fbw8), and insulin like growth factor 1 receptor/insulin receptor (IGF-IR/IR). In addition, mTORC1 and mTORC2 regulate each other through a feedback loop to control cell growth. This review outlines the current understanding of mTOR regulation in insulin signaling in the context of whole body metabolism.

Co-ordinated activation of classical and novel PKC isoforms is required for PMA-induced mTORC1 activation

Protein kinase C (PKC) has been shown to activate the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, a central hub in the regulation of cell metabolism, growth and proliferation. However, the mechanisms by which PKCs activate mTORC1 are still ambiguous. Our previous study revealed that activation of classical PKCs (cPKC) results in the perinuclear accumulation of cPKC and phospholipase D2 (PLD2) in recycling endosomes in a PLD2-dependent manner. Here, we report that mTORC1 activation by phorbol 12,13-myristate acetate (PMA) requires both classic, cPKC, and novel PKC (nPKC) isoforms, specifically PKCη, acting through distinct pathways. The translocation of mTOR to perinuclear lysosomes was detected after treatment of PKC activators, which was not colocalized with PKCα- or RAB11-positive endosomes and was not inhibited by PLD inhibitors. We found that PKCη inhibition by siRNA or bisindolylmaleimide I effectively decreased mTOR accumulation in lysosomes and its activity. Also, we identified that PKCη plays a role upstream of the v-ATPase/Ragulator/Rag pathway in response to PMA. These data provides a spatial aspect to the regulation of mTORC1 by sustained activation of PKC, requiring co-ordinated activation of two distinct elements, the perinuclear accumulation of cPKC- and PLD-containing endosomes and the nPKC-dependent translation of of mTOR in the perinuclear lysosomes. The close proximity of these two distinct compartments shown in this study suggests the possibility that transcompartment signaling may be a factor in the regulation of mTORC1 activity and also underscores the importance of PKCη as a potential therapeutic target of mTORC-related disorders.

Simultaneous high expression of PLD1 and Sp1 predicts a poor prognosis for pancreatic ductal adenocarcinoma patients

Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease with few therapeutic options. Recently, insight into cancer biology suggested abnormal lipid metabolism to be a risk factor for human malignancies. As a key enzyme implicated in lipid metabolism, PLD1 was elevated in various human cancer associating with malignant phenotypes. However, little was known about its expression and function in PDAC. We showed that PLD1 was elevated in both the cell lines and clinical samples of PDAC, and it positively correlated with vascular invasion (p = 0.041) and responsible for a poor prognosis (p = 0.009). Meanwhile, we also found Sp1 to be elevated in the disease, correlating with vascular invasion (p = 0.007). Moreover, the correlation assay suggested that PLD1 positively correlated with Sp1 in the clinical sample (r = 0.390; p < 0.001) and the cell lines. Finally, we showed that co-high expression of both the factors confers the poorest prognosis for the patients, and that their simultaneous high expression might be an independent prognostic factor (p = 0.001; HR = 3.427; 95% CI 1.629-7.211).

PLD1 regulates adipogenic differentiation through mTOR - IRS-1 phosphorylation at serine 636/639

Phospholipase D1 (PLD1) plays a known role in several differentiation processes, but its role in adipogenic differentiation remains unknown. In the present study, we identified PLD1 as a negative regulator of adipogenic differentiation. We showed that PLD activity was downregulated by both 3-Isobutyl-1-methylxanthine (IBMX) and insulin upon induction of differentiation in 3T3-L1 adipogenic cells. In line with this observation, PLD activity decreased in both high fat diet (HFD)-fed mice and ob/ob mice. We also found that differentiation of 3T3-L1 preadipocytes was enhanced by the depletion of PLD1 levels or inhibition of PLD1 activity by VU0155069, a PLD1-specific inhibitor. Conversely, treatment with phosphatidic acid (PA), a PLD product, and overexpression of PLD1 both caused a decrease in adipogenic differentiation. Moreover, the elevated differentiation in PLD1-knockdown 3T3-L1 cells was reduced by either PA treatment or PLD1 expression, confirming negative roles of PLD1 and PA in adipogenic differentiation. Further investigation revealed that PA displaces DEP domain-containing mTOR-interacting protein (DEPTOR) from mTORC1, which subsequently phosphorylates insulin receptor substrate-1 (IRS-1) at serine 636/639 in 3T3-L1 cells. Taken together, our findings provide convincing evidence for a direct role of PLD1 in adipogenic differentiation by regulating IRS-1 phosphorylation at serine 636/639 through DEPTOR displacement and mTOR activation.

FLIM FRET Visualization of Cdc42 Activation by Netrin-1 in Embryonic Spinal Commissural Neuron Growth Cones

Netrin-1 is an essential extracellular chemoattractant that signals through its receptor DCC to guide commissural axon extension in the embryonic spinal cord. DCC directs the organization of F-actin in growth cones by activating an intracellular protein complex that includes the Rho GTPase Cdc42, a critical regulator of cell polarity and directional migration. To address the spatial distribution of signaling events downstream of netrin-1, we expressed the FRET biosensor Raichu-Cdc42 in cultured embryonic rat spinal commissural neurons. Using FLIM-FRET imaging we detected rapid activation of Cdc42 in neuronal growth cones following application of netrin-1. Investigating the signaling mechanisms that control Cdc42 activation by netrin-1, we demonstrate that netrin-1 rapidly enriches DCC at the leading edge of commissural neuron growth cones and that netrin-1 induced activation of Cdc42 in the growth cone is blocked by inhibiting src family kinase signaling. These findings reveal the activation of Cdc42 in embryonic spinal commissural axon growth cones and support the conclusion that src family kinase activation downstream of DCC is required for Cdc42 activation by netrin-1.

Leucyl-tRNA Synthetase Activates Vps34 in Amino Acid-Sensing mTORC1 Signaling

Amino acid availability activates signaling by the mammalian target of rapamycin (mTOR) complex 1, mTORC1, a master regulator of cell growth. The class III PI-3-kinase Vps34 mediates amino acid signaling to mTORC1 by regulating lysosomal translocation and activation of the phospholipase PLD1. Here, we identify leucyl-tRNA synthetase (LRS) as a leucine sensor for the activation of Vps34-PLD1 upstream of mTORC1. LRS is necessary for amino acid-induced Vps34 activation, cellular PI(3)P level increase, PLD1 activation, and PLD1 lysosomal translocation. Leucine binding, but not tRNA charging activity of LRS, is required for this regulation. Moreover, LRS physically interacts with Vps34 in amino acid-stimulatable non-autophagic complexes. Finally, purified LRS protein activates Vps34 kinase in vitro in a leucine-dependent manner. Collectively, our findings provide compelling evidence for a direct role of LRS in amino acid activation of Vps34 via a non-canonical mechanism and fill a gap in the amino acid-sensing mTORC1 signaling network.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Oscillatory mTOR inhibition and Treg increase in kidney transplantation

Intracellular metabolic pathways dependent upon the mammalian target of rapamycin (mTOR) play a key role in immune-tolerance control. In this study, we focused on long-term mTOR-dependent immune-modulating effects in kidney transplant recipients undergoing conversion from calcineurin inhibitors (CNI) to mTOR inhibitors (everolimus) in a 1-year follow-up. The conversion to everolimus is associated with a decrease of neutrophils and of CD8(+) T cells. In addition, we observed a reduced production of interferon (IFN)-γ by CD8(+) T cells and of interleukin (IL)-17 by CD4(+) T lymphocytes. An increase in CD4(+) CD25(+) forkhead box protein 3 (FoxP3)(+) [regulatory T cell [(Treg)] numbers was also seen. Treg increase correlated with a higher proliferation rate of this regulatory subpopulation when compared with the CD4(+) FoxP3(-) effector counterpart. Basal phosphorylation level of S6 kinase, a major mTOR-dependent molecular target, was substantially maintained in patients treated with everolimus. Moreover, oscillations in serum concentration of everolimus were associated with changes in basal and activation-dependent S6 kinase phosphorylation of CD4(+) and CD8(+) T cells. Indeed, T cell receptor (TCR) triggering was observed to induce significantly higher S6 kinase phosphorylation in the presence of lower everolimus serum concentrations. These results unveil the complex mTOR-dependent immune-metabolic network leading to long-term immune-modulation and might have relevance for novel therapeutic settings in kidney transplants.

Rheb signaling and tumorigenesis: mTORC1 and new horizons

Rheb is a conserved small GTPase that belongs to the Ras superfamily, and is mainly involved in activation of cell growth through stimulation of mTORC1 activity. Because deregulation of the Rheb/mTORC1 signaling is associated with proliferative disorders and cancer, inhibition of mTORC1 has been therapeutically approached. Although this therapy has proven antitumor activity, its efficacy is not as expected. Here, we will review the main work on the identification of the role of Rheb in cell growth, and on the relevance of Rheb in proliferative disorders, including cancer. We will also review the Rheb functions that could explain tumor resistance to therapies with mTORC1 inhibitors, and will mainly focus our discussion on mTORC1-independent Rheb functions that could also be implicated in cancer cell survival and tumorigenesis. The current progress on the understanding of the noncanonical Rheb functions prompts future studies to establish their relevance in cancer and in the context of current cancer therapies.

PLD1 activation mediates Amb a 1-induced Th2-associated cytokine expression via the JNK/ATF-2 pathway in BEAS-2B cells

The purpose of this study was to identify the role of phospholipase D1 (PLD1) in Amb a 1-induced IL-5 and IL-13 expression. When BEAS-2B cells were stimulated with Amb a 1, PLD activity increased, and knockdown of PLD1 decreased Amb a 1-induced IL-5 and IL-13 expression. Amb a 1 also activated the PLCγ/p70S6K/JNK pathway. Furthermore, Amb a 1-induced PLD activation was also attenuated by PLCγ inhibition, and knockdown of PLD1 decreased Amb a 1-induced activation of P70S6K and JNK. When ATF-2 activity was blocked with ATF-2 siRNA, Amb a 1-induced IL-5 and IL-13 expression was completely abolished, indicating that ATF-2 is a transcriptional factor required for the expression of IL-5 and IL-13 in response to Amb a 1. Taken together, we suggest that PLD1 acts as an important regulator in Amb a 1-induced expression of IL-5 and IL-13 via a PLCγ/p70S6K/JNK/ATF-2 pathway in BEAS-2B cells.

Active mechanistic target of rapamycin plays an ancillary rather than essential role in zebrafish CNS axon regeneration

The developmental decrease of the intrinsic regenerative ability of the mammalian central nervous system (CNS) is associated with reduced activity of mechanistic target of rapamycin (mTOR) in mature neurons such as retinal ganglion cells (RGCs). While mTOR activity is further decreased upon axonal injury, maintenance of its pre-injury level, for instance by genetic deletion of the phosphatase and tensin homolog (PTEN), markedly promotes axon regeneration in mammals. The current study now addressed the question whether active mTOR might generally play a central role in axon regeneration by analyzing its requirement in regeneration-competent zebrafish. Remarkably, regulation of mTOR activity after optic nerve injury in zebrafish is fundamentally different compared to mammals. Hardly any activity was detected in naïve RGCs, whereas it was markedly increased upon axotomy in vivo as well as in dissociated cell cultures. After a short burst, mTOR activity was quickly attenuated, which is contrary to the requirements for axon regeneration in mammals. Surprisingly, mTOR activity was not essential for axonal growth per se, but correlated with cytokine- and PTEN inhibitor-induced neurite extension in vitro. Moreover, inhibition of mTOR using rapamycin significantly reduced axon regeneration in vivo and compromised functional recovery after optic nerve injury. Therefore, axotomy-induced mTOR activity is involved in CNS axon regeneration in zebrafish similar to mammals, although it plays an ancillary rather than essential role in this regeneration-competent species.

Early Steps of Jaagsiekte Sheep Retrovirus-Mediated Cell Transformation Involve the Interaction between Env and the RALBP1 Cellular Protein

Ovine pulmonary adenocarcinoma is a naturally occurring lung cancer in sheep induced by the Jaagsiekte sheep retrovirus (JSRV). Its envelope glycoprotein (Env) carries oncogenic properties, and its expression is sufficient to induce in vitro cell transformation and in vivo lung adenocarcinoma. The identification of cellular partners of the JSRV envelope remains crucial for deciphering mechanisms leading to cell transformation. We initially identified RALBP1 (RalA binding protein 1; also known as RLIP76 or RIP), a cellular protein implicated in the ras pathway, as a partner of JSRV Env by yeast two-hybrid screening and confirmed formation of RALBP1/Env complexes in mammalian cells. Expression of the RALBP1 protein was repressed in tumoral lungs and in tumor-derived alveolar type II cells. Through its inhibition using specific small interfering RNA (siRNA), we showed that RALBP1 was involved in envelope-induced cell transformation and in modulation of the mTOR (mammalian target of rapamycin)/p70S6K pathway by the retroviral envelope.

IMPORTANCE

JSRV-induced lung adenocarcinoma is of importance for the sheep industry. While the envelope has been reported as the oncogenic determinant of the virus, the cellular proteins directly interacting with Env are still not known. Our report on the formation of RALBP/Env complexes and the role of this interaction in cell transformation opens up a new hypothesis for the dysregulation observed upon virus infection in sheep.

Rapid mitogenic regulation of the mTORC1 inhibitor, DEPTOR, by phosphatidic acid

The mammalian target of rapamycin complex 1 (mTORC1) is regulated, in part, by the endogenous inhibitor DEPTOR. However, the mechanism of DEPTOR regulation with regard to rapid mTORC1 activation remains unknown. We report that DEPTOR is rapidly and temporarily dissociated from mTORC1 upon mitogenic stimulation, suggesting a mechanism underlying acute mTORC1 activation. This mitogen-stimulated DEPTOR dissociation is blocked by inhibition or depletion of the mTORC1 regulator, phospholipase D (PLD), and recapitulated with the addition of the PLD product phosphatidic acid (PA). Our mass spectrometry analysis has independently identified DEPTOR as an mTOR binding partner dissociated by PA. Interestingly, only PA species with unsaturated fatty acid chains, such as those produced by PLD, are capable of displacing DEPTOR and activating mTORC1, with high affinity for the FRB domain of mTOR. Our findings reveal a mechanism of mTOR regulation and provide a molecular explanation for the exquisite specificity of PA function.

Inhibited insulin signaling in mouse hepatocytes is associated with increased phosphatidic acid but not diacylglycerol

Although an elevated triacylglycerol content in non-adipose tissues is often associated with insulin resistance, the mechanistic relationship remains unclear. The data support roles for intermediates in the glycerol-3-phosphate pathway of triacylglycerol synthesis: diacylglycerol (DAG), which may cause insulin resistance in liver by activating PKCϵ, and phosphatidic acid (PA), which inhibits insulin action in hepatocytes by disrupting the assembly of mTOR and rictor. To determine whether increases in DAG and PA impair insulin signaling when produced by pathways other than that of de novo synthesis, we examined primary mouse hepatocytes after enzymatically manipulating the cellular content of DAG or PA. Overexpressing phospholipase D1 or phospholipase D2 inhibited insulin signaling and was accompanied by an elevated cellular content of total PA, without a change in total DAG. Overexpression of diacylglycerol kinase-θ inhibited insulin signaling and was accompanied by an elevated cellular content of total PA and a decreased cellular content of total DAG. Overexpressing glycerol-3-phosphate acyltransferase-1 or -4 inhibited insulin signaling and increased the cellular content of both PA and DAG. Insulin signaling impairment caused by overexpression of phospholipase D1/D2 or diacylglycerol kinase-θ was always accompanied by disassociation of mTOR/rictor and reduction of mTORC2 kinase activity. However, although the protein ratio of membrane to cytosolic PKCϵ increased, PKC activity itself was unaltered. These data suggest that PA, but not DAG, is associated with impaired insulin action in mouse hepatocytes.

Phosphoinositides: Key modulators of energy metabolism

Phosphoinositides are key players in many trafficking and signaling pathways. Recent advances regarding the synthesis, location and functions of these lipids have dramatically improved our understanding of how and when these lipids are generated and what their roles are in animal physiology. In particular, phosphoinositides play a central role in insulin signaling, and manipulation of PtdIns(3,4,5)P₃levels in particular, may be an important potential therapeutic target for the alleviation of insulin resistance associated with obesity and the metabolic syndrome. In this article we review the metabolism, regulation and functional roles of phosphoinositides in insulin signaling and the regulation of energy metabolism. This article is part of a Special Issue entitled Phosphoinositides.

Glycerol-3-phosphate acyltransferase-4-deficient mice are protected from diet-induced insulin resistance by the enhanced association of mTOR and rictor

Glycerol-3-phosphate acyltransferase (GPAT) activity is highly induced in obese individuals with insulin resistance, suggesting a correlation between GPAT function, triacylglycerol accumulation, and insulin resistance. We asked whether microsomal GPAT4, an isoform regulated by insulin, might contribute to the development of hepatic insulin resistance. Compared with control mice fed a high fat diet, Gpat4(-/-) mice were more glucose tolerant and were protected from insulin resistance. Overexpression of GPAT4 in mouse hepatocytes impaired insulin-suppressed gluconeogenesis and insulin-stimulated glycogen synthesis. Impaired glucose homeostasis was coupled to inhibited insulin-stimulated phosphorylation of Akt(Ser⁴⁷³) and Akt(Thr³⁰⁸). GPAT4 overexpression inhibited rictor's association with the mammalian target of rapamycin (mTOR), and mTOR complex 2 (mTORC2) activity. Compared with overexpressed GPAT3 in mouse hepatocytes, GPAT4 overexpression increased phosphatidic acid (PA), especially di16:0-PA. Conversely, in Gpat4(-/-) hepatocytes, both mTOR/rictor association and mTORC2 activity increased, and the content of PA in Gpat4(-/-) hepatocytes was lower than in controls, with the greatest decrease in 16:0-PA species. Compared with controls, liver and skeletal muscle from Gpat4(-/-)-deficient mice fed a high-fat diet were more insulin sensitive and had a lower hepatic content of di16:0-PA. Taken together, these data demonstrate that a GPAT4-derived lipid signal, likely di16:0-PA, impairs insulin signaling in mouse liver and contributes to hepatic insulin resistance.

Leptin increases TNF-α expression and production through phospholipase D1 in Raw 264.7 cells

Epidemiological evidence suggests that obesity is associated with inflammation of the respiratory tract and the pathogenesis of asthma. The purpose of this study was to examine the role of phospholipase D1 (PLD1) in leptin-induced expression of the proinflammatory cytokine, tumor necrosis factor (TNF)-α, and to suggest a molecular link between obesity and respiratory tract inflammation. We investigated whether leptin, a typical adipocytokine, plays a role in the expression of TNF-α through increased PLD1 activity in Raw 264.7. Leptin enhanced the activity of PLD1 through activation of PLCγ and Src, while PLD1 siRNA decreased the leptin-induced expression and production of TNF-α. Leptin-induced PLD activation was also inhibited by a PLCγ inhibitor (PAO) and Src kinase inhibitor (PP2), indicating that PLCγ and Src kinase are upstream activators of PLD1. Down-regulation of PLD1 also completely blocked activation of p70S6K, an activator of JNK. Leptin-induced expression of TNF-α was also prevented by inhibition of p70S6K and JNK. Taken together, these results indicate that PLD1 acts as an important regulator of leptin-induced expression of TNF-α by participating in the PLCγ/Src/PLD1/PA/p70S6K/JNK pathway.

A neurotoxic glycerophosphocholine impacts PtdIns-4, 5-bisphosphate and TORC2 signaling by altering ceramide biosynthesis in yeast

Unbiased lipidomic approaches have identified impairments in glycerophosphocholine second messenger metabolism in patients with Alzheimer's disease. Specifically, we have shown that amyloid-β42 signals the intraneuronal accumulation of PC(O-16:0/2:0) which is associated with neurotoxicity. Similar to neuronal cells, intracellular accumulation of PC(O-16:0/2:0) is also toxic to Saccharomyces cerevisiae, making yeast an excellent model to decipher the pathological effects of this lipid. We previously reported that phospholipase D, a phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂)-binding protein, was relocalized in response to PC(O-16:0/2:0), suggesting that this neurotoxic lipid may remodel lipid signaling networks. Here we show that PC(O-16:0/2:0) regulates the distribution of the PtdIns(4)P 5-kinase Mss4 and its product PtdIns(4,5)P₂ leading to the formation of invaginations at the plasma membrane (PM). We further demonstrate that the effects of PC(O-16:0/2:0) on the distribution of PM PtdIns(4,5)P₂ pools are in part mediated by changes in the biosynthesis of long chain bases (LCBs) and ceramides. A combination of genetic, biochemical and cell imaging approaches revealed that PC(O-16:0/2:0) is also a potent inhibitor of signaling through the Target of rampamycin complex 2 (TORC2). Together, these data provide mechanistic insight into how specific disruptions in phosphocholine second messenger metabolism associated with Alzheimer's disease may trigger larger network-wide disruptions in ceramide and phosphoinositide second messenger biosynthesis and signaling which have been previously implicated in disease progression.

Accelerated cognitive decline in Alzheimer's patients is associated with distinct changes in the abundance of choline-containing lipids belonging to the platelet activating factor family. In particular, PC(O-16:0/2:0) or C16:0 platelet activating factor (PAF), is specifically elevated in brains of Alzheimer's patients. Since elevated intraneuronal levels of PC(O-16:0/2:0) are thought to contribute to the loss of neuronal cells it is imperative to identify the underlying mechanisms contributing to the toxic effects of PC(O-16:0/2:0). In this study, we have determined that elevated levels of PC(O-16:0/2:0) has negative effects upon the distribution of phosphoinositides at the plasma membrane leading to a potent inhibition of target of rapamycin (TOR) signaling. We further show that the changes in phosphoinositide distribution are due to changes in ceramide metabolism. In conclusion, our study suggests that the toxicity associated with aberrant metabolism of glycerophosphocholine lipids species is likely due to the remodeling of phosphoinositide and ceramide metabolism and that therapeutic strategies which target these disruptions may be effective in ameliorating Alzheimer's Disease pathology.

Complete genomic landscape of a recurring sporadic parathyroid carcinoma

Parathyroid carcinoma is a rare endocrine malignancy with an estimated incidence of less than 1 per million population. Excessive secretion of parathyroid hormone, extremely high serum calcium level, and the deleterious effects of hypercalcaemia are the clinical manifestations of the disease. Up to 60% of patients develop multiple disease recurrences and although long-term survival is possible with palliative surgery, permanent remission is rarely achieved. Molecular drivers of sporadic parathyroid carcinoma have remained largely unknown. Previous studies, mostly based on familial cases of the disease, suggested potential roles for the tumour suppressor MEN1 and proto-oncogene RET in benign parathyroid tumourigenesis, while the tumour suppressor HRPT2 and proto-oncogene CCND1 may also act as drivers in parathyroid cancer. Here, we report the complete genomic analysis of a sporadic and recurring parathyroid carcinoma. Mutational landscapes of the primary and recurrent tumour specimens were analysed using high-throughput sequencing technologies. Such molecular profiling allowed for identification of somatic mutations never previously identified in this malignancy. These included single nucleotide point mutations in well-characterized cancer genes such as mTOR, MLL2, CDKN2C, and PIK3CA. Comparison of acquired mutations in patient-matched primary and recurrent tumours revealed loss of PIK3CA activating mutation during the evolution of the tumour from the primary to the recurrence. Structural variations leading to gene fusions and regions of copy loss and gain were identified at a single-base resolution. Loss of the short arm of chromosome 1, along with somatic missense and truncating mutations in CDKN2C and THRAP3, respectively, provides new evidence for the potential role of these genes as tumour suppressors in parathyroid cancer. The key somatic mutations identified in this study can serve as novel diagnostic markers as well as therapeutic targets.

Phospholipase D regulates the size of skeletal muscle cells through the activation of mTOR signaling

mTOR is a major actor of skeletal muscle mass regulation in situations of atrophy or hypertrophy. It is established that Phospholipase D (PLD) activates mTOR signaling, through the binding of its product phosphatidic acid (PA) to mTOR protein. An influence of PLD on muscle cell size could thus be suspected. We explored the consequences of altered expression and activity of PLD isoforms in differentiated L6 myotubes. Inhibition or down-regulation of the PLD1 isoform markedly decreased myotube size and muscle specific protein content. Conversely, PLD1 overexpression induced muscle cell hypertrophy, both in vitro in myotubes and in vivo in mouse gastrocnemius. In the presence of atrophy-promoting dexamethasone, PLD1 overexpression or addition of exogenous PA protected myotubes against atrophy. Similarly, exogenous PA protected myotubes against TNFα-induced atrophy. Moreover, the modulation of PLD expression or activity in myotubes showed that PLD1 negatively regulates the expression of factors involved in muscle protein degradation, such as the E3-ubiquitin ligases Murf1 and Atrogin-1, and the Foxo3 transcription factor. Inhibition of mTOR by PP242 abolished the positive effects of PLD1 on myotubes, whereas modulating PLD influenced the phosphorylation of both S6K1 and Akt, which are respectively substrates of mTORC1 and mTORC2 complexes. These observations suggest that PLD1 acts through the activation of both mTORC1 and mTORC2 to induce positive trophic effects on muscle cells. This pathway may offer interesting therapeutic potentialities in the treatment of muscle wasting.