PTEN functions by recruitment to cytoplasmic vesicles

New insights into the regulation and roles of phosphatidylinositol 3,4-bisphosphate

Phosphoinositides (PIPs) are phospholipids and components of the cellular membrane. In mammals, seven phosphorylated derivatives of PIPs have been identified. Among them, phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2] is produced by lipid phosphatases (e.g., SHIP2) or by lipid kinases PI3KC2α and PI3KC2β. Although PI(3,4)P2 is undetectable in normal mouse or human tissues and common cell lines, it appears in a mouse prostate cancer model and in cells exposed to oxidative stress, indicating that PI(3,4)P2 is involved in the pathogenesis of some diseases. Here, I summarize recent findings on the cellular roles and pathophysiological significance of PI(3,4)P2.

Dietary pro-oxidant therapy by a vitamin K precursor targets PI 3-kinase VPS34 function

Men taking antioxidant vitamin E supplements have increased prostate cancer (PC) risk. However, whether pro-oxidants protect from PC remained unclear. In this work, we show that a pro-oxidant vitamin K precursor [menadione sodium bisulfite (MSB)] suppresses PC progression in mice, killing cells through an oxidative cell death: MSB antagonizes the essential class III phosphatidylinositol (PI) 3-kinase VPS34-the regulator of endosome identity and sorting-through oxidation of key cysteines, pointing to a redox checkpoint in sorting. Testing MSB in a myotubular myopathy model that is driven by loss of MTM1-the phosphatase antagonist of VPS34-we show that dietary MSB improved muscle histology and function and extended life span. These findings enhance our understanding of pro-oxidant selectivity and show how definition of the pathways they impinge on can give rise to unexpected therapeutic opportunities.

Pten regulates endocytic trafficking of cell adhesion and Wnt signaling molecules to pattern the retina

The retina is exquisitely patterned, with neuronal somata positioned at regular intervals to completely sample the visual field. Here, we show that phosphatase and tensin homolog (Pten) controls starburst amacrine cell spacing by modulating vesicular trafficking of cell adhesion molecules and Wnt proteins. Single-cell transcriptomics and double-mutant analyses revealed that Pten and Down syndrome cell adhesion molecule Dscam) are co-expressed and function additively to pattern starburst amacrine cell mosaics. Mechanistically, Pten loss accelerates the endocytic trafficking of DSCAM, FAT3, and MEGF10 off the cell membrane and into endocytic vesicles in amacrine cells. Accordingly, the vesicular proteome, a molecular signature of the cell of origin, is enriched in exocytosis, vesicle-mediated transport, and receptor internalization proteins in Pten conditional knockout (PtencKO) retinas. Wnt signaling molecules are also enriched in PtencKO retinal vesicles, and the genetic or pharmacological disruption of Wnt signaling phenocopies amacrine cell patterning defects. Pten thus controls vesicular trafficking of cell adhesion and signaling molecules to establish retinal amacrine cell mosaics.

Thrombomodulin: a multifunctional receptor modulating the endothelial quiescence

Thrombomodulin (TM) is a type 1 receptor best known for its function as an anticoagulant cofactor for thrombin activation of protein C on the surface of vascular endothelial cells. In addition to its anticoagulant cofactor function, TM also regulates fibrinolysis, complement, and inflammatory pathways. TM is a multidomain receptor protein with a lectin-like domain at its N-terminus that has been shown to exhibit direct anti-inflammatory functions. This domain is followed by 6 epidermal growth factor-like domains that support the interaction of TM with thrombin. The interaction inhibits the procoagulant function of thrombin and enables the protease to regulate the anticoagulant and fibrinolytic pathways by activating protein C and thrombin-activatable fibrinolysis inhibitor. TM has a Thr/Ser-rich region immediately above the membrane surface that harbors chondroitin sulfate glycosaminoglycans, and this region is followed by a single-spanning transmembrane and a C-terminal cytoplasmic domain. The structure and physiological function of the extracellular domains of TM have been extensively studied, and numerous excellent review articles have been published. However, the physiological function of the cytoplasmic domain of TM has remained poorly understood. Recent data from our laboratory suggest that intracellular signaling by the cytoplasmic domain of TM plays key roles in maintaining quiescence by modulating phosphatase and tensin homolog signaling in endothelial cells. This article briefly reviews the structure and function of extracellular domains of TM and focuses on the mechanism and possible physiological importance of the cytoplasmic domain of TM in modulating phosphatase and tensin homolog signaling in endothelial cells.

Distinct early role of PTEN regulation during HCMV infection of monocytes

Human cytomegalovirus (HCMV) infection of monocytes is essential for viral dissemination and persistence. We previously identified that HCMV entry/internalization and subsequent productive infection of this clinically relevant cell type is distinct when compared to other infected cells. We showed that internalization and productive infection required activation of epidermal growth factor receptor (EGFR) and integrin/c-Src, via binding of viral glycoprotein B to EGFR, and the pentamer complex to β1/β3 integrins. To understand how virus attachment drives entry, we compared infection of monocytes with viruses containing the pentamer vs. those without the pentamer and then used a phosphoproteomic screen to identify potential phosphorylated proteins that influence HCMV entry and trafficking. The screen revealed that the most prominent pentamer-biased phosphorylated protein was the lipid- and protein-phosphatase phosphatase and tensin homolog (PTEN). PTEN knockdown with siRNA or PTEN inhibition with a PTEN inhibitor decreased pentamer-mediated HCMV entry, without affecting trimer-mediated entry. Inhibition of PTEN activity affected lipid metabolism and interfered with the onset of the endocytic processes required for HCMV entry. PTEN inactivation was sufficient to rescue pentamer-null HCMV from lysosomal degradation. We next examined dephosphorylation of a PTEN substrate Rab7, a regulator of endosomal maturation. Inhibition of PTEN activity prevented dephosphorylation of Rab7. Phosphorylated Rab7, in turn, blocked early endosome to late endosome maturation and promoted nuclear localization of the virus and productive infection.

PTEN loss in glioma cell lines leads to increased extracellular vesicles biogenesis and PD-L1 cargo in a PI3K-dependent manner

Phosphatase and Tensin Homologue (PTEN) is one of the most frequently lost tumor suppressors in cancer and the predominant negative regulator of the PI3K/AKT signaling axis. A growing body of evidence has highlighted the loss of PTEN with immuno-modulatory functions including the upregulation of the programmed death ligand-1 (PD-L1), an altered tumor derived secretome that drives an immunosuppressive tumor immune microenvironment (TIME), and resistance to certain immunotherapies. Given their roles in immunosuppression and tumor growth, we examined whether the loss of PTEN would impact the biogenesis, cargo, and function of extracellular vesicles (EVs) in the context of the anti-tumor associated cytokine interferon-γ (IFN-γ). Through genetic and pharmacological approaches, we show that PD-L1 expression is regulated by JAK/STAT signaling, not PI3K signaling. Instead, we observe that PTEN loss positively upregulates cell surface levels of PD-L1 and enhances the biogenesis of EVs enriched with PD-L1 in a PI3K-dependent manner. We demonstrate that because of these changes, EVs derived from glioma cells lacking PTEN have a greater ability to suppress T cell receptor (TCR) signaling. Taken together, these findings provide important new insights into how the loss of PTEN can contribute to an immunosuppressive TIME, facilitate immune evasion, and highlight a novel role for PI3K signaling in the regulation of EV biogenesis and the cargo they contain.

Phosphocode-dependent glutamyl-prolyl-tRNA synthetase 1 signaling in immunity, metabolism, and disease

Ubiquitously expressed aminoacyl-tRNA synthetases play essential roles in decoding genetic information required for protein synthesis in every living species. Growing evidence suggests that they also function as crossover mediators of multiple biological processes required for homeostasis. In humans, eight cytoplasmic tRNA synthetases form a central machinery called the multi-tRNA synthetase complex (MSC). The formation of MSCs appears to be essential for life, although the role of MSCs remains unclear. Glutamyl-prolyl-tRNA synthetase 1 (EPRS1) is the most evolutionarily derived component within the MSC that plays a critical role in immunity and metabolism (beyond its catalytic role in translation) via stimulus-dependent phosphorylation events. This review focuses on the role of EPRS1 signaling in inflammation resolution and metabolic modulation. The involvement of EPRS1 in diseases such as cancer is also discussed.

Regulation of Phosphoinositide Signaling by Scaffolds at Cytoplasmic Membranes

Cytoplasmic phosphoinositides (PI) are critical regulators of the membrane-cytosol interface that control a myriad of cellular functions despite their low abundance among phospholipids. The metabolic cycle that generates different PI species is crucial to their regulatory role, controlling membrane dynamics, vesicular trafficking, signal transduction, and other key cellular events. The synthesis of phosphatidylinositol (3,4,5)-triphosphate (PI3,4,5P3) in the cytoplamic PI3K/Akt pathway is central to the life and death of a cell. This review will focus on the emerging evidence that scaffold proteins regulate the PI3K/Akt pathway in distinct membrane structures in response to diverse stimuli, challenging the belief that the plasma membrane is the predominant site for PI3k/Akt signaling. In addition, we will discuss how PIs regulate the recruitment of specific scaffolding complexes to membrane structures to coordinate vesicle formation, fusion, and reformation during autophagy as well as a novel lysosome repair pathway.

The membrane surface as a platform that organizes cellular and biochemical processes

Membranes are essential for life. They act as semi-permeable boundaries that define cells and organelles. In addition, their surfaces actively participate in biochemical reaction networks, where they confine proteins, align reaction partners, and directly control enzymatic activities. Membrane-localized reactions shape cellular membranes, define the identity of organelles, compartmentalize biochemical processes, and can even be the source of signaling gradients that originate at the plasma membrane and reach into the cytoplasm and nucleus. The membrane surface is, therefore, an essential platform upon which myriad cellular processes are scaffolded. In this review, we summarize our current understanding of the biophysics and biochemistry of membrane-localized reactions with particular focus on insights derived from reconstituted and cellular systems. We discuss how the interplay of cellular factors results in their self-organization, condensation, assembly, and activity, and the emergent properties derived from them.

Phosphoinositide 3-kinase (PI3K) classes: From cell signaling to endocytic recycling and autophagy

Lipid signaling is defined as any biological signaling action in which a lipid messenger binds to a protein target, converting its effects to specific cellular responses. In this complex biological pathway, the family of phosphoinositide 3-kinase (PI3K) represents a pivotal role and affects many aspects of cellular biology from cell survival, proliferation, and migration to endocytosis, intracellular trafficking, metabolism, and autophagy. While yeasts have a single isoform of phosphoinositide 3-kinase (PI3K), mammals possess eight PI3K types divided into three classes. The class I PI3Ks have set the stage to widen research interest in the field of cancer biology. The aberrant activation of class I PI3Ks has been identified in 30-50% of human tumors, and activating mutations in PIK3CA is one of the most frequent oncogenes in human cancer. In addition to indirect participation in cell signaling, class II and III PI3Ks primarily regulate vesicle trafficking. Class III PI3Ks are also responsible for autophagosome formation and autophagy flux. The current review aims to discuss the original data obtained from international research laboratories on the latest discoveries regarding PI3Ks-mediated cell biological processes. Also, we unravel the mechanisms by which pools of the same phosphoinositides (PIs) derived from different PI3K types act differently.

Endosomal trafficking protein TBC-2 modulates stress resistance and lifespan through DAF-16-dependent and independent mechanisms

The FOXO transcription factor, DAF-16, plays an integral role in insulin/IGF-1 signaling (IIS) and stress response. In conditions of stress or decreased IIS, DAF-16 moves to the nucleus where it activates genes that promote survival. To gain insight into the role of endosomal trafficking in resistance to stress, we disrupted tbc-2, which encodes a GTPase activating protein that inhibits RAB-5 and RAB-7. We found that tbc-2 mutants have decreased nuclear localization of DAF-16 in response to heat stress, anoxia, and bacterial pathogen stress, but increased nuclear localization of DAF-16 in response to chronic oxidative stress and osmotic stress. tbc-2 mutants also exhibit decreased upregulation of DAF-16 target genes in response to stress. To determine whether the rate of nuclear localization of DAF-16 affected stress resistance in these animals, we examined survival after exposure to multiple exogenous stressors. Disruption of tbc-2 decreased resistance to heat stress, anoxia, and bacterial pathogen stress in both wild-type worms and stress-resistant daf-2 insulin/IGF-1 receptor mutants. Similarly, deletion of tbc-2 decreases lifespan in both wild-type worms and daf-2 mutants. When DAF-16 is absent, the loss of tbc-2 is still able to decrease lifespan but has little or no impact on resistance to most stresses. Combined, this suggests that disruption of tbc-2 affects lifespan through both DAF-16-dependent and DAF-16-independent pathways, while the effect of tbc-2 deletion on resistance to stress is primarily DAF-16-dependent. Overall, this work demonstrates the importance of endosomal trafficking for the proper nuclear localization of DAF-16 during stress and that perturbation of normal endosomal trafficking is sufficient to decrease both stress resistance and lifespan.

Chemerin and Chemokine-like Receptor 1 Expression Are Associated with Hepatocellular Carcinoma Progression in European Patients

The chemoattractant protein chemerin is protective in experimental hepatocellular carcinoma (HCC), and high expression in HCC tissues of Asian patients was related to a favorable prognosis. Studies from Asia found reduced expression of chemerin in HCC compared to para-tumor tissues while our previous analysis observed the opposite. Aim of this study was to correlate chemerin expression in HCC tissues with disease severity of European patients Hepatocyte chemerin protein expression was assessed by immunohistochemistry in HCC tissue of 383 patients, and was low in 24%, moderate in 49% and high in 27%. High chemerin protein in the HCC tissues was related to the T stage, vessel invasion, histologic grade, Union for International Cancer Control (UICC) stage and tumor size. Chemokine-like receptor 1 (CMKLR1) is a functional chemerin receptor. CMKLR1 protein in hepatocytes was low expressed in HCC tissues of 36%, moderate in tissues of 32% and high in 32% of the HCCs. Tumor CMKLR1 was associated with the T stage, vessel invasion, histologic grade and UICC stage. Notably, sex-specific analysis revealed that associations of chemerin and CMKLR1 expression with HCC progression were significant in males but not in females. The tumor chemerin and CMKLR1 protein expression were not related to steatosis, inflammation and fibrosis grades. In summary, chemerin as well as CMKLR1 protein were related to disease severity of European HCC patients, and this was significant in males. This observation is in contrast to Asian patients where higher chemerin in the tumors was protective. Current analysis provides evidence for ethnicity and sex-related differences of tumor expressed chemerin and HCC severity.

Targeting Class I-II-III PI3Ks in Cancer Therapy: Recent Advances in Tumor Biology and Preclinical Research

Phosphatidylinositol-3-kinase (PI3K) enzymes, producing signaling phosphoinositides at plasma and intracellular membranes, are key in intracellular signaling and vesicular trafficking pathways. PI3K is a family of eight enzymes divided into three classes with various functions in physiology and largely deregulated in cancer. Here, we will review the recent evidence obtained during the last 5 years on the roles of PI3K class I, II and III isoforms in tumor biology and on the anti-tumoral action of PI3K inhibitors in preclinical cancer models. The dependency of tumors to PI3K isoforms is dictated by both genetics and context (e.g., the microenvironment). The understanding of class II/III isoforms in cancer development and progression remains scarce. Nonetheless, the limited available data are consistent and reveal that there is an interdependency between the pathways controlled by all PI3K class members in their role to promote cancer cell proliferation, survival, growth, migration and metabolism. It is unknown whether this feature contributes to partial treatment failure with isoform-selective PI3K inhibitors. Hence, a better understanding of class II/III functions to efficiently inhibit their positive and negative interactions with class I PI3Ks is needed. This research will provide the proof-of-concept to develop combination treatment strategies targeting several PI3K isoforms simultaneously.

Multi-omic profiling reveals the ataxia protein sacsin is required for integrin trafficking and synaptic organization

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset cerebellar ataxia caused by mutations in SACS, which encodes the protein sacsin. Cellular ARSACS phenotypes include mitochondrial dysfunction, intermediate filament disorganization, and progressive death of cerebellar Purkinje neurons. It is unclear why the loss of sacsin causes these deficits or why they manifest as cerebellar ataxia. Here, we perform multi-omic profiling in sacsin knockout (KO) cells and identify alterations in microtubule dynamics and mislocalization of focal adhesion (FA) proteins, including multiple integrins. Deficits in FA structure, signaling, and function can be rescued by targeting PTEN, a negative regulator of FA signaling. ARSACS mice possess mislocalization of ITGA1 in Purkinje neurons and synaptic disorganization in the deep cerebellar nucleus (DCN). The sacsin interactome reveals that sacsin regulates interactions between cytoskeletal and synaptic adhesion proteins. Our findings suggest that disrupted trafficking of synaptic adhesion proteins is a causal molecular deficit in ARSACS.

Glutamyl-prolyl-tRNA synthetase 1 coordinates early endosomal anti-inflammatory AKT signaling

The AKT signaling pathway plays critical roles in the resolution of inflammation. However, the underlying mechanisms of anti-inflammatory regulation and signal coordination remain unclear. Here, we report that anti-inflammatory AKT signaling is coordinated by glutamyl-prolyl-tRNA synthetase 1 (EPRS1). Upon inflammatory activation, AKT specifically phosphorylates Ser999 of EPRS1 in the cytoplasmic multi-tRNA synthetase complex, inducing release of EPRS1. EPRS1 compartmentalizes AKT to early endosomes via selective binding to the endosomal membrane lipid phosphatidylinositol 3-phosphate and assembles an AKT signaling complex specific for anti-inflammatory activity. These events promote AKT activation-mediated GSK3β phosphorylation, which increase anti-inflammatory cytokine production. EPRS1-deficient macrophages do not assemble the early endosomal complex and consequently exacerbate inflammation, decreasing the survival of EPRS1-deficient mice undergoing septic shock and ulcerative colitis. Collectively, our findings show that the housekeeping protein EPRS1 acts as a mediator of inflammatory homeostasis by coordinating compartment-specific AKT signaling.

The orchestrated signaling by PI3Kα and PTEN at the membrane interface

The oncogene PI3Kα and the tumor suppressor PTEN represent two antagonistic enzymatic activities that regulate the interconversion of the phosphoinositide lipids PI(4,5)P2 and PI(3,4,5)P3 in membranes. As such, they are defining components of phosphoinositide-based cellular signaling and membrane trafficking pathways that regulate cell survival, growth, and proliferation, and are often deregulated in cancer. In this review, we highlight aspects of PI3Kα and PTEN interplay at the intersection of signaling and membrane trafficking. We also discuss the mechanisms of PI3Kα- and PTEN- membrane interaction and catalytic activation, which are fundamental for our understanding of the structural and allosteric implications on signaling at the membrane interface and may aid current efforts in pharmacological targeting of these proteins.

The Rab GTPase activating protein TBC-2 regulates endosomal localization of DAF-16 FOXO and lifespan

FOXO transcription factors have been shown to regulate longevity in model organisms and are associated with longevity in humans. To gain insight into how FOXO functions to increase lifespan, we examined the subcellular localization of DAF-16 in C. elegans. We show that DAF-16 is localized to endosomes and that this endosomal localization is increased by the insulin-IGF signaling (IIS) pathway. Endosomal localization of DAF-16 is modulated by endosomal trafficking proteins. Disruption of the Rab GTPase activating protein TBC-2 increases endosomal localization of DAF-16, while inhibition of TBC-2 targets, RAB-5 or RAB-7 GTPases, decreases endosomal localization of DAF-16. Importantly, the amount of DAF-16 that is localized to endosomes has functional consequences as increasing endosomal localization through mutations in tbc-2 reduced the lifespan of long-lived daf-2 IGFR mutants, depleted their fat stores, and DAF-16 target gene expression. Overall, this work identifies endosomal localization as a mechanism regulating DAF-16 FOXO, which is important for its functions in metabolism and aging.

FOXO transcription factors have been shown to modulate lifespan in multiple model organisms and to be associated with longevity in humans. Here we describe a new localization of the C. elegans FOXO transcription factor, called DAF-16. We report that DAF-16 localizes to endosomes, membrane compartments internalized from the plasma membrane at the cell surface. We demonstrate that expansion of these endosome compartments by disruption of an endosomal regulator called TBC-2 results in increased localization of DAF-16 on endosomes at the expense of nuclear localization in the intestinal cells. This results in altered expression of DAF-16 target genes, reduced fat storage and decreased lifespan. These results demonstrate the importance of endosomal trafficking for proper localization of DAF-16 and suggest that the endosome is an important site of FOXO regulation. An intriguing possibility based on our results is that storage of FOXO on endosomes facilitates the mobilization of FOXO as a rapid response to environmental stress.

PTEN differentially regulates endocytosis, migration, and proliferation in the enteric protozoan parasite Entamoeba histolytica

PTEN is a lipid phosphatase that is highly conserved and involved in a broad range of biological processes including cytoskeletal reorganization, endocytosis, signal transduction, and cell migration in all eukaryotes. Although regulation of phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] signaling via PTEN has been well established in model organisms and mammals, it remains elusive in the parasitic protist E. histolytica, which heavily relies on PtdIns phosphate(s)-dependent membrane traffic, migration, and phago- and trogocytosis for its pathogenesis. In this study, we characterized the major PTEN from E. histolytica, EhPTEN1, which shows the highest expression at the transcript level in the trophozoite stage among 6 possible PTENs, to understand the significance of PtdIns(3,4,5)P3 signaling in this parasite. Live imaging of GFP-EhPTEN1 expressing amebic trophozoites showed localization mainly in the cytosol with a higher concentration at pseudopods and the extending edge of the phago- and trogocytic cups. Furthermore, quantitative analysis of phago- and trogocytosis using a confocal image cytometer showed that overexpression of EhPTEN1 caused reduction in trogo- and phagocytosis while transcriptional gene silencing of EhPTEN1 gene caused opposite phenotypes. These data suggest that EhPTEN1 has an inhibitory role in these biological processes. Conversely, EhPTEN1 acts as a positive regulator for fluid-phase and receptor-mediated endocytosis in E. histolytica trophozoites. Moreover, we showed that EhPTEN1 was required for optimal growth and migration of this parasite. Finally, the phosphatase activity of EhPTEN1 towards PtdIns(3,4,5)P3 was demonstrated, suggesting that the biological roles of EhPTEN1 are likely linked to its catalytic function. Taken together, these results indicate that EhPTEN1 differentially regulates multiple cellular activities essential for proliferation and pathogenesis of the organism, via PtdIns(3,4,5)P3 signaling. Elucidation of biological roles of PTEN and PtdIns(3,4,5)P3 signaling at the molecular levels promotes our understanding of the pathogenesis of this parasite.

PI3K/AKT Signaling Tips the Balance of Cytoskeletal Forces for Cancer Progression

The PI3K/AKT signaling pathway plays essential roles in multiple cellular processes, which include cell growth, survival, metabolism, and motility. In response to internal and external stimuli, the PI3K/AKT signaling pathway co-opts other signaling pathways, cellular components, and cytoskeletal proteins to reshape individual cells. The cytoskeletal network comprises three main components, which are namely the microfilaments, microtubules, and intermediate filaments. Collectively, they are essential for many fundamental structures and cellular processes. In cancer, aberrant activation of the PI3K/AKT signaling cascade and alteration of cytoskeletal structures have been observed to be highly prevalent, and eventually contribute to many cancer hallmarks. Due to their critical roles in tumor progression, pharmacological agents targeting PI3K/AKT, along with cytoskeletal components, have been developed for better intervention strategies against cancer. In our review, we first discuss existing evidence in-depth and then build on recent advances to propose new directions for therapeutic intervention.

The INPP4B paradox: Like PTEN, but different

Cancer is a complex and heterogeneous disease marked by the dysregulation of cancer driver genes historically classified as oncogenes or tumour suppressors according to their ability to promote or inhibit tumour development and growth, respectively. Certain genes display both oncogenic and tumour suppressor functions depending on the biological context, and as such have been termed dual-role cancer driver genes. However, because of their context-dependent behaviour, the tumourigenic mechanism of many dual-role genes is elusive and remains a significant knowledge gap in our effort to understand and treat cancer. Inositol polyphosphate 4-phosphatase type II (INPP4B) is an emerging dual-role cancer driver gene, primarily known for its role as a negative regulator of the phosphoinositide 3-kinase (PI3K)/AKT signalling pathway. In response to growth factor stimulation, class I PI3K generates PtdIns(3,4,5)P₃ at the plasma membrane. PtdIns(3,4,5)P₃ can be hydrolysed by inositol polyphosphate 5-phosphatases to generate PtdIns(3,4)P₂, which, together with PtdIns(3,4,5)P₃, facilitates the activation of AKT to promote cell proliferation, survival, migration, and metabolism. Phosphatase and tensin homology on chromosome 10 (PTEN) and INPP4B are dual-specificity phosphatases that hydrolyse PtdIns(3,4,5)P₃ and PtdIns(3,4)P₂, respectively, and thus negatively regulate PI3K/AKT signalling. PTEN is a bona fide tumour suppressor that is frequently lost in human tumours. INPP4B was initially characterised as a tumour suppressor akin to PTEN, and has been implicated as such in a number of cancers, including prostate, thyroid, and basal-like breast cancers. However, evidence has since emerged revealing INPP4B as a paradoxical oncogene in several malignancies, with increased INPP4B expression reported in AML, melanoma and colon cancers among others. Although the tumour suppressive function of INPP4B has been mostly ascribed to its ability to negatively regulate PI3K/AKT signalling, its oncogenic function remains less clear, with proposed mechanisms including promotion of PtdIns(3)P-dependent SGK3 signalling, inhibition of PTEN-dependent AKT activation, and enhancing DNA repair mechanisms to confer chemoresistance. Nevertheless, research is ongoing to identify the factors that dictate the tumourigenic output of INPP4B in different human cancers. In this review we discuss the dualistic role that INPP4B plays in the context of cancer development, progression and treatment, drawing comparisons to PTEN to explore how their similarities and, importantly, their differences may account for their diverging roles in tumourigenesis.

Structural analysis of the PTEN:P-Rex2 signaling complex reveals how cancer-associated mutations coordinate to hyperactivate Rac1

The dual-specificity phosphatase PTEN functions as a tumor suppressor by hydrolyzing PI(3,4,5)P₃ to PI(4,5)P₂ to inhibit PI3K-AKT signaling and cellular proliferation. P-Rex2 is a guanine nucleotide exchange factor for Rho GTPases and can be activated by Gβγ subunits downstream of G protein-coupled receptor signaling and by PI(3,4,5)P₃ downstream of receptor tyrosine kinases. The PTEN:P-Rex2 complex is a commonly mutated signaling node in metastatic cancer. Assembly of the PTEN:P-Rex2 complex inhibits the activity of both proteins, and its dysregulation can drive PI3K-AKT signaling and cellular proliferation. Here, using cross-linking mass spectrometry and functional studies, we gained mechanistic insights into PTEN:P-Rex2 complex assembly and coinhibition. We found that PTEN was anchored to P-Rex2 by interactions between the PDZ-interacting motif in the PTEN C-terminal tail and the second PDZ domain of P-Rex2. This interaction bridged PTEN across the P-Rex2 surface, preventing PI(3,4,5)P₃ hydrolysis. Conversely, PTEN both allosterically promoted an autoinhibited conformation of P-Rex2 and blocked its binding to Gβγ. In addition, we observed that the PTEN-deactivating mutations and P-Rex2 truncations combined to drive Rac1 activation to a greater extent than did either single variant alone. These insights enabled us to propose a class of gain-of-function, cancer-associated mutations within the PTEN:P-Rex2 interface that uncouple PTEN from the inhibition of Rac1 signaling.

TUBB3 is associated with PTEN, neuroendocrine differentiation, and castration resistance in prostate cancer

BACKGROUND

Tubulin-β3 encoded by the Tubulin-β3 (TUBB3) gene is a microtubule protein. Previous studies have shown that TUBB3 expression is upregulated in castration-resistant prostate cancer (CaP) and is involved in taxane resistance. However, the biological mechanism of TUBB3 involvement in the progression to castration-resistant CaP is not fully elucidated. This study aimed to analyze the expression and function of TUBB3 in localized and metastatic CaP.

METHODS

TUBB3 expression was determined using immunohistochemistry in localized and metastatic CaP. We also investigated the association between TUBB3, phosphatase and tensin homolog (PTEN), and neuroendocrine differentiation and examined the involvement of TUBB3 in new antiandrogen drugs (enzalutamide and apalutamide) resistance in metastatic CaP.

RESULTS

In 155 cases of localized CaP, immunohistochemistry showed that 5 (3.2%) of the CaP cases were positive for tubulin-β3. Kaplan-Meier analysis showed that high expression of tubulin-β3 was associated with poor prostate-specific antigen recurrence-free survival after radical prostatectomy. In 57 cases of metastatic CaP, immunohistochemistry showed that 14 (25%) cases were positive for tubulin-β3. Tubulin-β3 expression was higher in metastatic CaP than in localized CaP. High tubulin-β3 expression was correlated with negative PTEN expression. TUBB3 expression was increased in neuroendocrine CaP based on several public databases. PTEN knockout decreased the sensitivity to enzalutamide and apalutamide in 22Rv-1 cells. TUBB3 knockdown reversed the sensitivity to enzalutamide and apalutamide in PTEN-CRISPR 22Rv-1 cells. High expression of tubulin-β3 and negative expression of PTEN were significantly associated with poor overall survival in metastatic CaP treated with androgen deprivation therapy.

CONCLUSIONS

These results suggest that TUBB3 may be a useful predictive biomarker for survival and play an essential role in antiandrogen resistance in CaP.

Organismal roles for the PI3Kα and β isoforms: their specificity, redundancy or cooperation is context-dependent

PI3Ks are important lipid kinases that produce phosphoinositides phosphorylated in position 3 of the inositol ring. There are three classes of PI3Ks: class I PI3Ks produce PIP3 at plasma membrane level. Although D. melanogaster and C. elegans have only one form of class I PI3K, vertebrates have four class I PI3Ks called isoforms despite being encoded by four different genes. Hence, duplication of these genes coincides with the acquisition of coordinated multi-organ development. Of the class I PI3Ks, PI3Kα and PI3Kβ, encoded by PIK3CA and PIK3CB, are ubiquitously expressed. They present similar putative protein domains and share PI(4,5)P2 lipid substrate specificity. Fifteen years after publication of their first isoform-selective pharmacological inhibitors and genetically engineered mouse models (GEMMs) that mimic their complete and specific pharmacological inhibition, we review the knowledge gathered in relation to the redundant and selective roles of PI3Kα and PI3Kβ. Recent data suggest that, further to their redundancy, they cooperate for the integration of organ-specific and context-specific signal cues, to orchestrate organ development, physiology, and disease. This knowledge reinforces the importance of isoform-selective inhibitors in clinical settings.

Multi-parametric analysis of 57 SYNGAP1 variants reveal impacts on GTPase signaling, localization, and protein stability

SYNGAP1 is a neuronal Ras and Rap GTPase-activating protein with important roles in regulating excitatory synaptic plasticity. While many SYNGAP1 missense and nonsense mutations have been associated with intellectual disability, epilepsy, schizophrenia, and autism spectrum disorder (ASD), whether and how they contribute to individual disease phenotypes is often unknown. Here, we characterize 57 variants in seven assays that examine multiple aspects of SYNGAP1 function. Specifically, we used multiplex phospho-flow cytometry to measure variant impact on protein stability, pERK, pGSK3β, pp38, pCREB, and high-content imaging to examine subcellular localization. We find variants ranging from complete loss-of-function (LoF) to wild-type (WT)-like in their regulation of pERK and pGSK3β, while all variants retain at least partial ability to dephosphorylate pCREB. Interestingly, our assays reveal that a larger proportion of variants located within the disordered domain of unknown function (DUF) comprising the C-terminal half of SYNGAP1 exhibited higher LoF, compared to variants within the better studied catalytic domain. Moreover, we find protein instability to be a major contributor to dysfunction for only two missense variants, both located within the catalytic domain. Using high-content imaging, we find variants located within the C2 domain known to mediate membrane lipid interactions exhibit significantly larger cytoplasmic speckles than WT SYNGAP1. Moreover, this subcellular phenotype shows both correlation with altered catalytic activity and unique deviation from signaling assay results, highlighting multiple independent molecular mechanisms underlying variant dysfunction. Our multidimensional dataset allows clustering of variants based on functional phenotypes and provides high-confidence, multi-functional measures for making pathogenicity predictions.

Neddylation of PTEN regulates its nuclear import and promotes tumor development

PTEN tumor suppressor opposes the PI3K/Akt signaling pathway in the cytoplasm and maintains chromosomal integrity in the nucleus. Nucleus-cytoplasm shuttling of PTEN is regulated by ubiquitylation, SUMOylation and phosphorylation, and nuclear PTEN has been proposed to exhibit tumor-suppressive functions. Here we show that PTEN is conjugated by Nedd8 under high glucose conditions, which induces PTEN nuclear import without effects on PTEN stability. PTEN neddylation is promoted by the XIAP ligase and removed by the NEDP1 deneddylase. We identify Lys197 and Lys402 as major neddylation sites on PTEN. Neddylated PTEN accumulates predominantly in the nucleus and promotes rather than suppresses cell proliferation and metabolism. The nuclear neddylated PTEN dephosphorylates the fatty acid synthase (FASN) protein, inhibits the TRIM21-mediated ubiquitylation and degradation of FASN, and then promotes de novo fatty acid synthesis. In human breast cancer tissues, neddylated PTEN correlates with tumor progression and poor prognosis. Therefore, we demonstrate a previously unidentified pool of nuclear PTEN in the Nedd8-conjugated form and an unexpected tumor-promoting role of neddylated PTEN.

Phosphatidylinositol-3-OH kinase signalling is spatially organized at endosomal compartments by microtubule-associated protein 4

The canonical model of agonist-stimulated phosphatidylinositol-3-OH kinase (PI3K)-Akt signalling proposes that PI3K is activated at the plasma membrane, where receptors are activated and phosphatidylinositol-4,5-bisphosphate is concentrated. Here we show that phosphatidylinositol-3,4,5-trisphosphate generation and activated Akt are instead largely confined to intracellular membranes upon receptor tyrosine kinase activation. Microtubule-associated protein 4 (MAP4) interacts with and controls localization of membrane vesicle-associated PI3Kα to microtubules. The microtubule-binding domain of MAP4 binds directly to the C2 domain of the p110α catalytic subunit. MAP4 controls the interaction of PI3Kα with activated receptors at endosomal compartments along microtubules. Loss of MAP4 results in the loss of PI3Kα targeting and loss of PI3K-Akt signalling downstream of multiple agonists. The MAP4-PI3Kα assembly defines a mechanism for spatial control of agonist-stimulated PI3K-Akt signalling at internal membrane compartments linked to the microtubule network.

PTEN: Bridging Endocytosis and Signaling

The transduction of signals in the PTEN/PI3-kinase (PI3K) pathway is built around a phosphoinositide (PIP) lipid messenger, phosphatidylinositol trisphosphate, PI(3,4,5)P₃ or PIP₃ Another, more ancient role of this family of messengers is the control of endocytosis, where a handful of separate PIPs act like postal codes. Prominent among them is PI(3)P, which helps to ensure that endocytic vesicles, their cargo, and membranes themselves reach their correct destinations. Traditionally, the cancer and the endocytic functions of the PI3K signaling pathway have been studied by cancer and membrane biologists, respectively, with some notable but overall minimal overlap. Modern microscopy has enabled monitoring of the PTEN/PI3K pathway in action. Here, we explore the flurry of groundbreaking concepts emerging from those efforts. The discovery that PTEN contains an autonomous PI(3)P reader domain, fused to the catalytic PIP₃ eraser domain has prompted us to explore the relationship between PI3K signaling and endocytosis. This revealed how PTEN can achieve signal termination in a precisely controlled fashion, because endocytosis can package the PIP₃ signal into discrete units that PTEN will erase. We explore how PTEN can bridge the worlds of endocytosis and PI3K signaling and discuss progress on how PI3K/AKT signaling can be acting from internal membranes. We discuss how the PTEN/PI3K system for growth control may have emerged from principles of endocytosis, and how this development could have affected the evolution of multicellular organisms.

Multi-model functionalization of disease-associated PTEN missense mutations identifies multiple molecular mechanisms underlying protein dysfunction

Functional variomics provides the foundation for personalized medicine by linking genetic variation to disease expression, outcome and treatment, yet its utility is dependent on appropriate assays to evaluate mutation impact on protein function. To fully assess the effects of 106 missense and nonsense variants of PTEN associated with autism spectrum disorder, somatic cancer and PTEN hamartoma syndrome (PHTS), we take a deep phenotypic profiling approach using 18 assays in 5 model systems spanning diverse cellular environments ranging from molecular function to neuronal morphogenesis and behavior. Variants inducing instability occur across the protein, resulting in partial-to-complete loss-of-function (LoF), which is well correlated across models. However, assays are selectively sensitive to variants located in substrate binding and catalytic domains, which exhibit complete LoF or dominant negativity independent of effects on stability. Our results indicate that full characterization of variant impact requires assays sensitive to instability and a range of protein functions.

Mutations in PTEN have been associated with various human disease, including autism spectrum disorder (ASD) and cancer. Here, the authors assess the function of 106 PTEN variants in yeast, invertebrate models and cell culture and report that PTEN variants generally decrease protein stability.

Autism-associated PTEN missense mutation leads to enhanced nuclear localization and neurite outgrowth in an induced pluripotent stem cell line

Germline mutation in the PTEN gene is the genetic basis of PTEN hamartoma tumor syndrome with the affected individuals harboring features of autism spectrum disorders. Characterizing a panel of 14 autism‐associated PTEN missense mutations revealed reduced protein stability, catalytic activity, and subcellular distribution. Nine out of 14 (64%) PTEN missense mutants had reduced protein expression with most mutations confined to the C2 domain. Selected mutants displayed enhanced polyubiquitination and shortened protein half‐life, but that did not appear to involve the polyubiquitination sites at lysine residues at codon 13 or 289. Analyzing their intrinsic lipid phosphatase activities revealed that 78% (11 out of 14) of these mutants had twofold to 10‐fold reduction in catalytic activity toward phosphatidylinositol phosphate substrates. Analyzing the subcellular localization of the PTEN missense mutants showed that 64% (nine out of 14) had altered nuclear‐to‐cytosol ratios with four mutants (G44D, H123Q, E157G, and D326N) showing greater nuclear localization. The E157G mutant was knocked‐in to an induced pluripotent stem cell line and recapitulated a similar nuclear targeting preference. Furthermore, iPSCs expressing the E157G mutant were more proliferative at the neural progenitor cell stage but exhibited more extensive dendritic outgrowth. In summary, the combination of biological changes in PTEN is expected to contribute to the behavioral and cellular features of this neurodevelopmental disorder.

Structural Mechanisms of PTEN Regulation

The tumor suppressor phosphatase and tensin homolog on chromosome 10 (PTEN) is a tightly regulated enzyme responsible for dephosphorylating the progrowth lipid messenger molecule phosphatidylinositol 3,4,5-trisphosphate (PIP₃) on the plasma membrane. The carboxy-terminal tail (CTT) of PTEN is key for regulation of the enzyme. When phosphorylated, the unstructured CTT interacts with the phosphatase-C2 superdomain to inactivate the enzyme by preventing membrane association. PTEN mutations associated with cancer also inactivate the enzyme. Alternate translation-initiation sites generate extended isoforms of PTEN, such as PTEN-L that has multiple roles in cells. The extended amino-terminal region bears a signal sequence and a polyarginine sequence to facilitate exit from and entry into cells, respectively, and a membrane-binding helix that activates the enzyme. This amino-terminal region also facilitates mitochondrial and nucleolar localization. This review explores PTEN structure and its impact on localization and regulation.

Phosphatidylinositol 3,4-bisphosphate synthesis and turnover are spatially segregated in the endocytic pathway

Phosphoinositides play crucial roles in intracellular membrane dynamics and cell signaling, with phosphatidylinositol (PI) 3-phosphates being the predominant phosphoinositide lipids at endosomes and lysosomes, whereas PI 4-phosphates, such as phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂), are enriched at the cell surface including sites of endocytosis. How PI 4-phosphates and PI 3-phosphates are dynamically interconverted within the endocytic pathway and how this is controlled in space and time remains poorly understood. Here, combining live imaging, genome engineering, and acute chemical and genetic manipulations, we found that local synthesis of PI(3,4)P₂ by phosphatidylinositol 3-kinase C2α at plasma membrane clathrin-coated pits is spatially segregated from its hydrolysis by the PI(3,4)P₂-specific inositol polyphosphate 4-phosphatase 4A (INPP4A). We observed that INPP4A is dispensable for clathrin-mediated endocytosis and is undetectable in endocytic clathrin-coated pits. Instead, we found that INPP4A partially localizes to endosomes and that loss of INPP4A in HAP1 cancer cells perturbs signaling via AKT kinase and mTOR complex 1. These results reveal a function for INPP4-mediated PI(3,4)P₂ hydrolysis in local regulation of growth factor and nutrient signals at endosomes in cancer cells. They further suggest a model whereby synthesis and turnover of PI(3,4)P₂ are spatially segregated within the endocytic pathway to couple endocytic membrane traffic to growth factor and nutrient signaling.

The Cytoskeleton as Regulator of Cell Signaling Pathways

During interphase, filamentous actin, microtubules, and intermediate filaments regulate cell shape, motility, transport, and interactions with the environment. These activities rely on signaling events that control cytoskeleton properties. Recent studies uncovered mechanisms that go far beyond this one-directional flow of information. Thus, the three branches of the cytoskeleton impinge on signaling pathways to determine their activities. We propose that this regulatory role of the cytoskeleton provides sophisticated mechanisms to control the spatiotemporal output and the intensity of signaling events. Specific examples emphasize these emerging contributions of the cytoskeleton to cell physiology. In our opinion, further exploration of these pathways will uncover new concepts of cellular communication that originate from the cytoskeleton.

Multifaceted Regulation of PTEN Subcellular Distributions and Biological Functions

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a tumor suppressor gene frequently found to be inactivated in over 30% of human cancers. PTEN encodes a 54-kDa lipid phosphatase that serves as a gatekeeper of the phosphoinositide 3-kinase pathway involved in the promotion of multiple pro-tumorigenic phenotypes. Although the PTEN protein plays a pivotal role in carcinogenesis, cumulative evidence has implicated it as a key signaling molecule in several other diseases as well, such as diabetes, Alzheimer's disease, and autism spectrum disorders. This finding suggests that diverse cell types, especially differentiated cells, express PTEN. At the cellular level, PTEN is widely distributed in all subcellular compartments and organelles. Surprisingly, the cytoplasmic compartment, not the plasma membrane, is the predominant subcellular location of PTEN. More recently, the finding of a secreted 'long' isoform of PTEN and the presence of PTEN in the cell nucleus further revealed unexpected biological functions of this multifaceted molecule. At the regulatory level, PTEN activity, stability, and subcellular distribution are modulated by a fascinating array of post-translational modification events, including phosphorylation, ubiquitination, and sumoylation. Dysregulation of these regulatory mechanisms has been observed in various human diseases. In this review, we provide an up-to-date overview of the knowledge gained in the last decade on how different functional domains of PTEN regulate its biological functions, with special emphasis on its subcellular distribution. This review also highlights the findings of published studies that have reported how mutational alterations in specific PTEN domains can lead to pathogenesis in humans.

Role of the RAB7 Protein in Tumor Progression and Cisplatin Chemoresistance

RAB7 is a small guanosine triphosphatase (GTPase) extensively studied as regulator of vesicular trafficking. Indeed, its role is fundamental in several steps of the late endocytic pathway, including endosome maturation, transport from early endosomes to late endosomes and lysosomes, clustering and fusion of late endosomes and lysosomes in the perinuclear region and lysosomal biogenesis. Besides endocytosis, RAB7 is important for a number of other cellular processes among which, autophagy, apoptosis, signaling, and cell migration. Given the importance of RAB7 in these cellular processes, the interest to study the role of RAB7 in cancer progression is widely grown. Here, we describe the current understanding of oncogenic and oncosuppressor functions of RAB7 analyzing cellular context and other environmental factors in which it elicits pro and/or antitumorigenic effects. We also discuss the role of RAB7 in cisplatin resistance associated with its ability to regulate the late endosomal pathway, lysosomal biogenesis and extracellular vesicle secretion. Finally, we examined the potential cancer therapeutic strategies targeting the different molecular events in which RAB7 is involved.

Perspective: Potential Impact and Therapeutic Implications of Oncogenic PI3K Activation on Chromosomal Instability

Genetic activation of the class I PI3K pathway is very common in cancer. This mostly results from oncogenic mutations in PIK3CA, the gene encoding the ubiquitously expressed PI3Kα catalytic subunit, or from inactivation of the PTEN tumour suppressor, a lipid phosphatase that opposes class I PI3K signalling. The clinical impact of PI3K inhibitors in solid tumours, aimed at dampening cancer-cell-intrinsic PI3K activity, has thus far been limited. Challenges include poor drug tolerance, incomplete pathway inhibition and pre-existing or inhibitor-induced resistance. The principle of pharmacologically targeting cancer-cell-intrinsic PI3K activity also assumes that all cancer-promoting effects of PI3K activation are reversible, which might not be the case. Emerging evidence suggests that genetic PI3K pathway activation can induce and/or allow cells to tolerate chromosomal instability, which-even if occurring in a low fraction of the cell population-might help to facilitate and/or drive tumour evolution. While it is clear that such genomic events cannot be reverted pharmacologically, a role for PI3K in the regulation of chromosomal instability could be exploited by using PI3K pathway inhibitors to prevent those genomic events from happening and/or reduce the pace at which they are occurring, thereby dampening cancer development or progression. Such an impact might be most effective in tumours with clonal PI3K activation and achievable at lower drug doses than the maximum-tolerated doses of PI3K inhibitors currently used in the clinic.

The enemy of my enemy: PTEN and PLCXD collude to fight endosomal PtdIns(4,5)P2

Loss of the phosphoinositide 5-phosphatase OCRL causes accumulation of PtdIns(4,5)P2 on membranes and, ultimately, Lowe syndrome. In this issue, Mondin et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201805155) discover that a surprising partnership between PTEN and the phospholipase PLCXD can compensate for OCRL to suppress endosomal PtdIns(4,5)P2 accumulation.

PTEN reduces endosomal PtdIns(4,5)P2 in a phosphatase-independent manner via a PLC pathway

This work reveals that the tumor suppressor PTEN acts through a PLC to reduce levels of endosomal PtdIns(4,5)P₂, its own enzymatic product. This pathway can be chemically activated to rescue OCRL1 depletion in several disease models of the Lowe syndrome, a rare multisystemic genetic disease.

The tumor suppressor PTEN dephosphorylates PtdIns(3,4,5)P₃ into PtdIns(4,5)P₂. Here, we make the unexpected discovery that in Drosophila melanogaster PTEN reduces PtdIns(4,5)P₂ levels on endosomes, independently of its phosphatase activity. This new PTEN function requires the enzymatic action of dPLCXD, an atypical phospholipase C. Importantly, we discovered that this novel PTEN/dPLCXD pathway can compensate for depletion of dOCRL, a PtdIns(4,5)P₂ phosphatase. Mutation of OCRL1, the human orthologue of dOCRL, causes oculocerebrorenal Lowe syndrome, a rare multisystemic genetic disease. Both OCRL1 and dOCRL loss have been shown to promote accumulation of PtdIns(4,5)P₂ on endosomes and cytokinesis defects. Here, we show that PTEN or dPLCXD overexpression prevents these defects. In addition, we found that chemical activation of this pathway restores normal cytokinesis in human Lowe syndrome cells and rescues OCRL phenotypes in a zebrafish Lowe syndrome model. Our findings identify a novel PTEN/dPLCXD pathway that controls PtdIns(4,5)P₂ levels on endosomes. They also point to a potential new strategy for the treatment of Lowe syndrome.

ZFYVE21 is a complement-induced Rab5 effector that activates non-canonical NF-κB via phosphoinosotide remodeling of endosomes

Complement promotes vascular inflammation in transplant organ rejection and connective tissue diseases. Here we identify ZFYVE21 as a complement-induced Rab5 effector that induces non-canonical NF-κB in endothelial cells (EC). In response to membrane attack complexes (MAC), ZFYVE21 is post-translationally stabilized on MAC+Rab5+ endosomes in a Rab5- and PI(3)P-dependent manner. ZFYVE21 promotes SMURF2-mediated polyubiquitinylation and proteasome-dependent degradation of endosome-associated PTEN to induce vesicular enrichment of PI(3,4,5)P3 and sequential recruitment of activated Akt and NF-κB-inducing kinase (NIK). Pharmacologic alteration of cellular phosphoinositide content with miltefosine reduces ZFYVE21 induction, EC activation, and allograft vasculopathy in a humanized mouse model. ZFYVE21 induction distinctly occurs in response to MAC and is detected in human renal and synovial tissues. Our data identifies ZFYVE21 as a Rab5 effector, defines a Rab5-ZFYVE21-SMURF2-pAkt axis by which it mediates EC activation, and demonstrates a role for this pathway in complement-mediated conditions.

Complement activation contributes to vascular inflammation in the contexts of allograft rejection and connective tissue disease. Here Fang et al. identify ZFYVE21 as a novel effector of Rab5 and find it regulates pro-inflammatory NF-κB signaling in endothelial cells in response to complement activation.

Phosphoinositide spatially free AKT/PKB activation to all membrane compartments

Ser and Thr kinase AKT also known as protein kinase B (PKB) was discovered more than two and half decades ago and is one of the key downstream molecules in the phosphoinositide 3-kinase signaling pathways. The pleiotropic effects of this kinase have attracted intense interest and limelight in cancer biology, cancer therapy, diabetes, and cardiovascular diseases. Authors may refer to other more comprehensive and recent reviews on AKT/PKB (Manning and Cantley, 2007; Manning and Toker, 2017). AKT/PKB is one of the most enigmatic and most studied signaling molecule in cancers and is a significant therapeutic target (Brown and Banerji, 2017). Yet, how AKT/PKB activation couples with its downstream target/substrate molecules that function in diverse subcellular compartments remains obscure. Recent studies indicate the continuous interaction of AKT/PKB with PI3,4,5P₃ or PI3,4P₂ in a lipid membrane is required for its activation throughout the cells (Ebner et al., 2017). Here, we summarize the recent progress on the mechanism for phosphoinositide (PI3,4,5P₃ and PI3,4P₂) spatial control of AKT/PKB activation on the plasma membrane and endomembrane compartments.

Getting the Akt Together: Guiding Intracellular Akt Activity by PI3K

Intracellular signaling pathways mediate the rapid response of cells to environmental cues. To control the fidelity of these responses, cells coordinate the activities of signaling enzymes with the strength, timing, and localization of the upstream stimuli. Protein kinase Akt links the PI3K-coupled receptors to cellular anabolic processes by phosphorylating multiple substrates. How the cells ensure that Akt activity remains proportional to upstream signals and control its substrate specificity is unclear. In this review, I examine how cell-autonomous and intrinsic allosteric mechanisms cooperate to ensure localized, context-specific signaling in the PI3K/Akt axis.

A high-avidity biosensor reveals plasma membrane PI(3,4)P2 is predominantly a class I PI3K signaling product

Class I phosphoinositide 3-OH kinase (PI3K) signaling is central to animal growth and metabolism, and pathological disruption of this pathway affects cancer and diabetes. However, the specific spatial/temporal dynamics and signaling roles of its minor lipid messenger, phosphatidylinositol (3,4)-bisphosphate (PI(3,4)P₂), are not well understood. This owes principally to a lack of tools to study this scarce lipid. Here we developed a high-sensitivity genetically encoded biosensor for PI(3,4)P₂, demonstrating high selectivity and specificity of the sensor for the lipid. We show that despite clear evidence for class II PI3K in PI(3,4)P₂-driven function, the overwhelming majority of the lipid accumulates through degradation of class I PI3K-produced PIP₃ However, we show that PI(3,4)P₂ is also subject to hydrolysis by the tumor suppressor lipid phosphatase PTEN. Collectively, our results show that PI(3,4)P₂ is potentially an important driver of class I PI3K-driven signaling and provides powerful new tools to begin to resolve the biological functions of this lipid downstream of class I and II PI3K.

Insight into the PTEN - p85α interaction and lipid binding properties of the p85α BH domain

The phosphatidylinositol 3-kinase (PI3K) pathway plays a key role in regulating cell growth and cell survival and is frequently deregulated in cancer cells. p85α regulates the p110α lipid kinase, and also stabilizes and stimulates PTEN, the lipid phosphatase that downregulates this pathway. In this report, we determined that the p85α BH domain binds several phosphorylated phosphoinositide lipids, an interaction that could help localize p85α to membranes rich in these lipids. We also identified key residues responsible for mediating PTEN – p85α complex formation. Based on these experimental results, a docking model for the PTEN – p85α BH domain complex was developed that is consistent with the known binding interactions for both PTEN and p85α. This model involves extensive side-chain and peptide backbone contacts between both the PASE and C2 domains of PTEN with the p85α BH domains. The p85α BH domain residues shown to be important for PTEN binding were p85α residues E212, Q221, K225, R228 and H234. We also verified experimentally the importance of PTEN-E91 in mediating the interaction with the p85α BH domain. These results shed new light on the mechanism of PTEN regulation by p85α.

Targeted Next-Generation Sequencing Identifies Actionable Targets in Estrogen Receptor Positive and Estrogen Receptor Negative Endometriod Endometrial Cancer

Endometrioid endometrial cancer (EEC) is the commonest form of endometrial cancer and can be divided into estrogen receptor (ER) positive and negative subtypes. The mutational profiles of EEC have been shown to aid in tailoring treatment; however, little is known about the differences between the gene mutation profiles between these two subtypes. This study aims to investigate the gene mutation profile in ER positive and negative EEC, and to further elucidate the role of WHSC1 mutations in this cancer. EEC and normal endometrial tissues were obtained from 29 patients and subjected to next-generation sequencing (NGS) using Ion Ampliseq Comprehensive Cancer PanelTM targeting 409 cancer related. A total of 741 non-synonymous alterations were identified from 272 genes in ER positive subtype while 448 non-synonymous variants were identified from 221 genes in ER negative subtype. PTEN is the most frequently altered gene in ER positive subtype (64%, 7/11) while ARID1A is the most frequently altered gene in ER negative subtype (50%, 4/8). We also identified alterations in ERRB3 (36%, 4/11), GNAS (36%, 4/11), and WHSC1 (27%, 3/11) in the ER positive subtype. WHSC1 R1126H and L1268P were shown to significantly increase cell viability, proliferation, migration, and survival. In addition, reduction in ER expression sensitized EEC-1 cell with WHSC1 L1268P mutant to Fulvestrant treatment. We revealed the mutational spectra of ER positive and ER negative EEC that could lead to better understanding of the biological mechanisms of endometrial cancer and may ultimately result in improvement of treatment options and patient prognosis.

Multiplex assessment of protein variant abundance by massively parallel sequencing

Determining the pathogenicity of genetic variants is a critical challenge, and functional assessment is often the only option. Experimentally characterizing millions of possible missense variants in thousands of clinically important genes requires generalizable, scalable assays. We describe variant abundance by massively parallel sequencing (VAMP-seq), which measures the effects of thousands of missense variants of a protein on intracellular abundance simultaneously. We apply VAMP-seq to quantify the abundance of 7,801 single-amino-acid variants of PTEN and TPMT, proteins in which functional variants are clinically actionable. We identify 1,138 PTEN and 777 TPMT variants that result in low protein abundance, and may be pathogenic or alter drug metabolism, respectively. We observe selection for low-abundance PTEN variants in cancer, and show that p.Pro38Ser, which accounts for ~10% of PTEN missense variants in melanoma, functions via a dominant-negative mechanism. Finally, we demonstrate that VAMP-seq is applicable to other genes, highlighting its generalizability.

Loss of PTEN promotes formation of signaling-capable clathrin-coated pits

Defective endocytosis and vesicular trafficking of signaling receptors has recently emerged as a multifaceted hallmark of malignant cells. Clathrin-coated pits (CCPs) display highly heterogeneous dynamics on the plasma membrane where they can take from 20 s to over 1 min to form cytosolic coated vesicles. Despite the large number of cargo molecules that traffic through CCPs, it is not well understood whether signaling receptors activated in cancer, such as epidermal growth factor receptor (EGFR), are regulated through a specific subset of CCPs. The signaling lipid phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P₃], which is dephosphorylated by phosphatase and tensin homolog (PTEN), is a potent tumorigenic signaling lipid. By using total internal reflection fluorescence microscopy and automated tracking and detection of CCPs, we found that EGF-bound EGFR and PTEN are enriched in a distinct subset of short-lived CCPs that correspond with clathrin-dependent EGF-induced signaling. We demonstrated that PTEN plays a role in the regulation of CCP dynamics. Furthermore, increased PI(3,4,5)P₃ resulted in higher proportion of short-lived CCPs, an effect that recapitulates PTEN deletion. Altogether, our findings provide evidence for the existence of short-lived 'signaling-capable' CCPs.

Mitochondrial Complex I Inhibitors Expose a Vulnerability for Selective Killing of Pten-Null Cells

A hallmark of advanced prostate cancer (PC) is the concomitant loss of PTEN and p53 function. To selectively eliminate such cells, we screened cytotoxic compounds on Pten-/-;Trp53-/- fibroblasts and their Pten-WT reference. Highly selective killing of Pten-null cells can be achieved by deguelin, a natural insecticide. Deguelin eliminates Pten-deficient cells through inhibition of mitochondrial complex I (CI). Five hundred-fold higher drug doses are needed to obtain the same killing of Pten-WT cells, even though deguelin blocks their electron transport chain equally well. Selectivity arises because mitochondria of Pten-null cells consume ATP through complex V, instead of producing it. The resulting glucose dependency can be exploited to selectively kill Pten-null cells with clinically relevant CI inhibitors, especially if they are lipophilic. In vivo, deguelin suppressed disease in our genetically engineered mouse model for metastatic PC. Our data thus introduce a vulnerability for highly selective targeting of incurable PC with inhibitors of CI.

Identification of a PTEN mutation with reduced protein stability, phosphatase activity, and nuclear localization in Hong Kong patients with autistic features, neurodevelopmental delays, and macrocephaly

PTEN is a tumor suppressor gene inactivated in over 30% of human cancers. It encodes a lipid phosphatase that serves as a gatekeeper of the phosphoinositide 3-kinase signaling pathway. Germline mutation frequently occurs in this gene in patients diagnosed with PTEN Hamartoma Tumor Syndrome (PHTS). PHTS individuals are characterized by macrocephaly, benign growth of multiple tissues and increased tumor risk. In addition, autistic phenotypes are found in 10-20% of individuals carrying the germline PTEN mutation with macrocephaly. In this report, 13 suspected PHTS patients were screened for mutation in the PTEN gene. A missense variant (c. 302T > C) substituting the isoleucine at codon 101 to a threonine, a single nucleotide insertion (c. 327-328insC) causing a frame shift mutation and termination at codon 109, and a nonsense variant (c. 1003C > T) truncated the protein at codon 335 were identified. The I101T mutation significantly reduced PTEN protein expression levels by 2.5- to 4.0-fold. Mechanistically, I101T reduced the protein half-life of PTEN possibly due to enhanced polyubiquitination at Lysine 13. However, the I101T mutant retained almost 30% of the lipid phosphatase activity of the wild-type protein. Finally, the I101T mutant has reduced phosphorylation at a PTEN auto-dephosphorylation site at Threonine 366 and a lowered ratio of nuclear to cytosolic protein level. These partial losses of multiple PTEN biochemical functions may contribute to the tissue overgrowth and autistic features of this PHTS patient. Autism Res 2018, 11: 1098-1109. © 2018 The Authors Autism Research published by International Society for Autism Research and Wiley Periodicals, Inc. LAY SUMMARY: The genetics of autism spectrum disorders is highly complex with individual risk influenced by both genetic and environmental factors. Mutation in the human PTEN gene confers a high risk of developing autistic behavior. This report revealed that PTEN mutations occurred in 23% of a selected group of Hong Kong patients harboring autistic features with gross overgrowth symptoms. Detailed characterization of a PTEN mutation revealed reduced protein stability as one of the underlying mechanisms responsible for reduced PTEN activity.

Cystic Fibrosis Transmembrane Conductance Regulator Attaches Tumor Suppressor PTEN to the Membrane and Promotes Anti Pseudomonas aeruginosa Immunity

The tumor suppressor PTEN controls cell proliferation by regulating phosphatidylinositol-3-kinase (PI3K) activity, but the participation of PTEN in host defense against bacterial infection is less well understood. Anti-inflammatory PI3K-Akt signaling is suppressed in patients with cystic fibrosis (CF), a disease characterized by hyper-inflammatory responses to airway infection. We found that Ptenl-/- mice, which lack the NH2-amino terminal splice variant of PTEN, were unable to eradicate Pseudomonas aeruginosa from the airways and could not generate sufficient anti-inflammatory PI3K activity, similar to what is observed in CF. PTEN and the CF transmembrane conductance regulator (CFTR) interacted directly and this interaction was necessary to position PTEN at the membrane. CF patients under corrector-potentiator therapy, which enhances CFTR transport to the membrane, have increased PTEN amounts. These findings suggest that improved CFTR trafficking could enhance P. aeruginosa clearance from the CF airway by activating PTEN-mediated anti-bacterial responses and might represent a therapeutic strategy.