Phosphoinositide-specific inositol polyphosphate 5-phosphatase IV inhibits Akt/protein kinase B phosphorylation and leads to apoptotic cell death

4.1N-Mediated Interactions and Functions in Nerve System and Cancer

Scaffolding protein 4.1N is a neuron-enriched 4.1 homologue. 4.1N contains three conserved domains, including the N-terminal 4.1-ezrin-radixin-moesin (FERM) domain, internal spectrin–actin–binding (SAB) domain, and C-terminal domain (CTD). Interspersed between the three domains are nonconserved domains, including U1, U2, and U3. The role of 4.1N was first reported in the nerve system. Then, extensive studies reported the role of 4.1N in cancers and other diseases. 4.1N performs numerous vital functions in signaling transduction by interacting, locating, supporting, and coordinating different partners and is involved in the molecular pathogenesis of various diseases. In this review, recent studies on the interactions between 4.1N and its contactors (including the α7AChr, IP3R1, GluR1/4, GluK1/2/3, mGluR8, KCC2, D2/3Rs, CASK, NuMA, PIKE, IP6K2, CAM 1/3, βII spectrin, flotillin-1, pp1, and 14-3-3) and the 4.1N-related biological functions in the nerve system and cancers are specifically and comprehensively discussed. This review provides critical detailed mechanistic insights into the role of 4.1N in disease relationships.

Binding to PI(4,5)P2 is indispensable for secretion of B-cell clonogenic HIV-1 matrix protein p17 variants

HIV-1 matrix protein p17 variants (vp17s) derived from non-Hodgkin's lymphoma (NHL) tissues of HIV-1-seropositive (HIV+) patients promote B-cell growth by activating the Akt signaling pathway. It is fundamental to understand the role played by vp17s in producing a microenvironment that fosters lymphoma development and progression. Therefore, we asked whether vp17s could be secreted from infected cells in their biologically active form. In this study, we show that two B-cell growth-promoting vp17s, NHL-a101 and NHL-a102, characterized by amino acid insertions at position 117 to 118 (Ala-Ala) or 125 to 126 (Gly-Asn), respectively, are secreted from HIV-1-infected Jurkat T cells during the active phase of viral replication. Secretion of biologically active vp17s also occurred in HeLa cells nucleofected with a plasmid expressing the entire Gag gene, following proteolytic cleavage of the Gag precursor polyprotein (Pr55Gag) by cellular aspartyl proteases. Binding of Pr55Gag to phosphatidylinositol-(4,5)-bisphosphate was indispensable for allowing the unconventional secretion of both wildtype p17 and vp17s. Indeed, here we demonstrate that inhibition of Pr55Gag binding to phosphatidylinositol-(4,5)-bisphosphate by using neomycin, or its enzymatic depletion achieved by overexpression of 5ptaseIV, significantly impair the secretion of p17s. We also demonstrated that heparan sulfate proteoglycans were involved in tethering p17s at the cell surface. This finding opens up an interesting way for investigating whether tethered p17s on the surface of HIV-1 reservoirs may represent a likely target for immune-mediated killing.

A transient role of the ciliary gene Inpp5e in controlling direct versus indirect neurogenesis in cortical development

During the development of the cerebral cortex, neurons are generated directly from radial glial cells or indirectly via basal progenitors. The balance between these division modes determines the number and types of neurons formed in the cortex thereby affecting cortical functioning. Here, we investigate the role of primary cilia in controlling the decision between forming neurons directly or indirectly. We show that a mutation in the ciliary gene Inpp5e leads to a transient increase in direct neurogenesis and subsequently to an overproduction of layer V neurons in newborn mice. Loss of Inpp5e also affects ciliary structure coinciding with reduced Gli3 repressor levels. Genetically restoring Gli3 repressor rescues the decreased indirect neurogenesis in Inpp5e mutants. Overall, our analyses reveal how primary cilia determine neuronal subtype composition of the cortex by controlling direct versus indirect neurogenesis. These findings have implications for understanding cortical malformations in ciliopathies with INPP5E mutations.

Cellular signalling by primary cilia in development, organ function and disease

Primary cilia project in a single copy from the surface of most vertebrate cell types; they detect and transmit extracellular cues to regulate diverse cellular processes during development and to maintain tissue homeostasis. The sensory capacity of primary cilia relies on the coordinated trafficking and temporal localization of specific receptors and associated signal transduction modules in the cilium. The canonical Hedgehog (HH) pathway, for example, is a bona fide ciliary signalling system that regulates cell fate and self-renewal in development and tissue homeostasis. Specific receptors and associated signal transduction proteins can also localize to primary cilia in a cell type-dependent manner; available evidence suggests that the ciliary constellation of these proteins can temporally change to allow the cell to adapt to specific developmental and homeostatic cues. Consistent with important roles for primary cilia in signalling, mutations that lead to their dysfunction underlie a pleiotropic group of diseases and syndromic disorders termed ciliopathies, which affect many different tissues and organs of the body. In this Review, we highlight central mechanisms by which primary cilia coordinate HH, G protein-coupled receptor, WNT, receptor tyrosine kinase and transforming growth factor-β (TGFβ)/bone morphogenetic protein (BMP) signalling and illustrate how defects in the balanced output of ciliary signalling events are coupled to developmental disorders and disease progression.

COMMD1 and PtdIns(4,5)P2 interaction maintain ATP7B copper transporter trafficking fidelity in HepG2 cells

Copper-responsive intracellular ATP7B trafficking is crucial for maintaining the copper balance in mammalian hepatocytes and thus copper levels in organs. The copper metabolism domain-containing protein 1 (COMMD1) binds both the ATP7B copper transporter and phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P₂], whereas COMMD1 loss causes hepatocyte copper accumulation. Although it is clear that COMMD1 is localized to endocytic trafficking complexes, a direct function for COMMD1 in ATP7B trafficking has not yet been defined. In this study, experiments using quantitative colocalization analysis reveal that COMMD1 modulates copper-responsive ATP7B trafficking through recruitment to PtdIns(4,5)P₂ Decreased COMMD1 abundance results in loss of ATP7B from lysosomes and the trans-Golgi network (TGN) in high copper conditions, although excess expression of COMMD1 also disrupts ATP7B trafficking and TGN structure. Overexpression of COMMD1 mutated to inhibit PtdIns(4,5)P₂ binding has little impact on ATP7B trafficking. A mechanistic PtdIns(4,5)P₂-mediated function for COMMD1 is proposed that is consistent with decreased cellular copper export as a result of disruption of the ATP7B trafficking itinerary and early endosome accumulation when COMMD1 is depleted. PtdIns(4,5)P₂ interaction with COMMD1 as well as COMMD1 abundance could both be important in maintenance of specific membrane protein trafficking pathways.

The impact of phosphoinositide 5-phosphatases on phosphoinositides in cell function and human disease

Phosphoinositides (PIs) are recognized as major signaling molecules in many different functions of eukaryotic cells. PIs can be dephosphorylated by multiple phosphatase activities at the 5-, 4-, and 3- positions. Human PI 5-phosphatases belong to a family of 10 members. Except for inositol polyphosphate 5-phosphatase A, they all catalyze the dephosphorylation of PI(4,5)P₂ and/or PI(3,4,5)P₃ at the 5- position. PI 5-phosphatases thus directly control the levels of PI(3,4,5)P₃ and participate in the fine-tuning regulatory mechanisms of PI(3,4)P₂ and PI(4,5)P₂ Second messenger functions have been demonstrated for PI(3,4)P₂ in invadopodium maturation and lamellipodia formation. PI 5-phosphatases can use several substrates on isolated enzymes, and it has been challenging to establish their real substrate in vivo. PI(4,5)P₂ has multiple functions in signaling, including interacting with scaffold proteins, ion channels, and cytoskeleton proteins. PI 5-phosphatase isoenzymes have been individually implicated in human diseases, such as the oculocerebrorenal syndrome of Lowe, through mechanisms that include lipid control. Oncogenic and tumor-suppressive functions of PI 5-phosphatases have also been reported in different cell contexts. The mechanisms responsible for genetic diseases and for oncogenic or tumor-suppressive functions are not fully understood. The regulation of PI 5-phosphatases is thus crucial in understanding cell functions.

Novel nuclear translocation of inositol polyphosphate 4-phosphatase is associated with cell cycle, proliferation and survival

Inositol polyphosphate 4 phosphatase type I enzyme (INPP4A) has a well-documented function in the cytoplasm where it terminates the phosphatidylinositol 3-kinase (PI 3-K) pathway by acting as a negative regulator. In this study, we demonstrate for the first time that INPP4A shuttles between the cytoplasm and the nucleus. Nuclear INPP4A is enzymatically active and in dynamic equilibrium between the nucleus and cytoplasm depending on the cell cycle stage, with highest amounts detected in the nucleus during the G0/G1 phase. Moreover, nuclear INPP4A is found to have direct proliferation suppressive activity. Cells constitutively overexpressing nuclear INPP4A exhibit massive apoptosis. In human tissues as well as cell lines, lower nuclear localization of INPP4A correlate with cancerous growth. Together, our findings suggest that nuclear compartmentalization of INPP4A may be a mechanism to regulate cell cycle progression, proliferation and apoptosis. Our results imply a role for nuclear-localized INPP4A in tumor suppression in humans.

A compartmentalized phosphoinositide signaling axis at cilia is regulated by INPP5E to maintain cilia and promote Sonic Hedgehog medulloblastoma

Sonic Hedgehog (SHH) signaling at primary cilia drives the proliferation and progression of a subset of medulloblastomas, the most common malignant paediatric brain tumor. Severe side effects associated with conventional treatments and resistance to targeted therapies has led to the need for new strategies. SHH signaling is dependent on primary cilia for signal transduction suggesting the potential for cilia destabilizing mechanisms as a therapeutic target. INPP5E is an inositol polyphosphate 5-phosphatase that hydrolyses PtdIns(4,5)P₂ and more potently, the phosphoinositide (PI) 3-kinase product PtdIns(3,4,5)P₃. INPP5E promotes SHH signaling during embryonic development via PtdIns(4,5)P₂ hydrolysis at cilia, that in turn regulates the cilia recruitment of the SHH suppressor GPR161. However, the role INPP5E plays in cancer is unknown and the contribution of PI3-kinase signaling to cilia function is little characterized. Here, we reveal INPP5E promotes SHH signaling in SHH medulloblastoma by negatively regulating a cilia-compartmentalized PI3-kinase signaling axis that maintains primary cilia on tumor cells. Conditional deletion of Inpp5e in a murine model of constitutively active Smoothened-driven medulloblastoma slowed tumor progression, suppressed cell proliferation, reduced SHH signaling and promoted tumor cell cilia loss. PtdIns(3,4,5)P₃, its effector pAKT and the target pGSK3β, which when non-phosphorylated promotes cilia assembly/stability, localized to tumor cell cilia. The number of PtdIns(3,4,5)P₃/pAKT/pGSK3β-positive cilia was increased in cultured Inpp5e-null tumor cells relative to controls. PI3-kinase inhibition or expression of wild-type, but not catalytically inactive HA-INPP5E partially rescued cilia loss in Inpp5e-null tumor cells in vitro. INPP5E mRNA and copy number were reduced in human SHH medulloblastoma compared to other molecular subtypes and consistent with the murine model, reduced INPP5E was associated with improved overall survival. Therefore our study identifies a compartmentalized PtdIns(3,4,5)P₃/AKT/GSK3β signaling axis at cilia in SHH-dependent medulloblastoma that is regulated by INPP5E to maintain tumor cell cilia, promote SHH signaling and thereby medulloblastoma progression.

INPP5E Preserves Genomic Stability through Regulation of Mitosis

The partially understood phosphoinositide signaling cascade regulates multiple aspects of cellular metabolism. Previous studies revealed that INPP5E, the inositol polyphosphate-5-phosphatase that is mutated in the developmental disorders Joubert and MORM syndromes, is essential for the function of the primary cilium and maintenance of phosphoinositide balance in nondividing cells. Here, we report that INPP5E further contributes to cellular homeostasis by regulating cell division. We found that silencing or genetic knockout of INPP5E in human and murine cells impairs the spindle assembly checkpoint, centrosome and spindle function, and maintenance of chromosomal integrity. Consistent with a cell cycle regulatory role, we found that INPP5E expression is cell cycle dependent, peaking at mitotic entry. INPP5E localizes to centrosomes, chromosomes, and kinetochores in early mitosis and shuttles to the midzone spindle at mitotic exit. Our findings identify the previously unknown, essential role of INPP5E in mitosis and prevention of aneuploidy, providing a new perspective on the function of this phosphoinositide phosphatase in health and development.

INPP5E regulates phosphoinositide-dependent cilia transition zone function

Dyson et al. demonstrate that the inositol polyphosphate 5-phosphatase INPP5E is essential for Hedgehog-dependent embryonic development. By regulating PI(4,5)P₂ and PI(3,4,5)P₃ signals at cilia, INPP5E contributes to cilia transition zone function and thereby Smoothened accumulation at cilia.

Human ciliopathies, including Joubert syndrome (JBTS), arise from cilia dysfunction. The inositol polyphosphate 5-phosphatase INPP5E localizes to cilia and is mutated in JBTS. Murine Inpp5e ablation is embryonically lethal and recapitulates JBTS, including neural tube defects and polydactyly; however, the underlying defects in cilia signaling and the function of INPP5E at cilia are still emerging. We report Inpp5e^−/− embryos exhibit aberrant Hedgehog-dependent patterning with reduced Hedgehog signaling. Using mouse genetics, we show increasing Hedgehog signaling via Smoothened M2 expression rescues some Inpp5e^−/− ciliopathy phenotypes and “normalizes” Hedgehog signaling. INPP5E’s phosphoinositide substrates PI(4,5)P₂ and PI(3,4,5)P₃ accumulated at the transition zone (TZ) in Hedgehog-stimulated Inpp5e^−/− cells, which was associated with reduced recruitment of TZ scaffolding proteins and reduced Smoothened levels at cilia. Expression of wild-type, but not 5-phosphatase-dead, INPP5E restored TZ molecular organization and Smoothened accumulation at cilia. Therefore, we identify INPP5E as an essential point of convergence between Hedgehog and phosphoinositide signaling at cilia that maintains TZ function and Hedgehog-dependent embryonic development.

Cellular aspartyl proteases promote the unconventional secretion of biologically active HIV-1 matrix protein p17

The human immune deficiency virus type 1 (HIV-1) matrix protein p17 (p17), although devoid of a signal sequence, is released by infected cells and detected in blood and in different organs and tissues even in HIV-1-infected patients undergoing successful combined antiretroviral therapy (cART). Extracellularly, p17 deregulates the function of different cells involved in AIDS pathogenesis. The mechanism of p17 secretion, particularly during HIV-1 latency, still remains to be elucidated. A recent study showed that HIV-1-infected cells can produce Gag without spreading infection in a model of viral latency. Here we show that in Gag-expressing cells, secretion of biologically active p17 takes place at the plasma membrane and occurs following its interaction with phosphatidylinositol-(4,5)-bisphosphate and its subsequent cleavage from the precursor Gag (Pr55^Gag) operated by cellular aspartyl proteases. These enzymes operate a more complex Gag polypeptide proteolysis than the HIV-1 protease, thus hypothetically generating slightly truncated or elongated p17s in their C-terminus. A 17 C-terminal residues excised p17 was found to be structurally and functionally identical to the full-length p17 demonstrating that the final C-terminal region of p17 is irrelevant for the protein’s biological activity. These findings offer new opportunities to identify treatment strategies for inhibiting p17 release in the extracellular microenvironment.

INPP4B is upregulated and functions as an oncogenic driver through SGK3 in a subset of melanomas

Inositol polyphosphate 4-phosphatase type II (INPP4B) negatively regulates PI3K/Akt signalling and has a tumour suppressive role in some types of cancers. However, we have found that it is upregulated in a subset of melanomas. Here we report that INPP4B can function as an oncogenic driver through activation of serum- and glucocorticoid-regulated kinase 3 (SGK3) in melanoma. While INPP4B knockdown inhibited melanoma cell proliferation and retarded melanoma xenograft growth, overexpression of INPP4B enhanced melanoma cell and melanocyte proliferation and triggered anchorage-independent growth of melanocytes. Noticeably, INPP4B-mediated melanoma cell proliferation was not related to activation of Akt, but was mediated by SGK3. Upregulation of INPP4B in melanoma cells was associated with loss of miRNA (miR)-494 and/or miR-599 due to gene copy number reduction. Indeed, overexpression of miR-494 or miR-599 downregulated INPP4B, reduced SGK3 activation, and inhibited melanoma cell proliferation, whereas introduction of anti-miR-494 or anti-miR-599 upregulated INPP4B, enhanced SGK3 activation, and promoted melanoma cell proliferation. Collectively, these results identify upregulation of INPP4B as an oncogenic mechanism through activation of SGK3 in a subset of melanomas, with implications for targeting INPP4B and restoring miR-494 and miR-599 as novel approaches in the treatment of melanomas with high INPP4B expression.

Inpp5e suppresses polycystic kidney disease via inhibition of PI3K/Akt-dependent mTORC1 signaling

Polycystic kidney disease (PKD) is a common cause of renal failure with few effective treatments. INPP5E is an inositol polyphosphate 5-phosphatase that dephosphorylates phosphoinositide 3-kinase (PI3K)-generated PI(3,4,5)P₃ and is mutated in ciliopathy syndromes. Germline Inpp5e deletion is embryonically lethal, attributed to cilia stability defects, and is associated with polycystic kidneys. However, the molecular mechanisms responsible for PKD development upon Inpp5e loss remain unknown. Here, we show conditional inactivation of Inpp5e in mouse kidney epithelium results in severe PKD and renal failure, associated with a partial reduction in cilia number and hyperactivation of PI3K/Akt and downstream mammalian target of rapamycin complex 1 (mTORC1) signaling. Treatment with an mTORC1 inhibitor improved kidney morphology and function, but did not affect cilia number or length. Therefore, we identify Inpp5e as an essential inhibitor of the PI3K/Akt/mTORC1 signaling axis in renal epithelial cells, and demonstrate a critical role for Inpp5e-dependent mTORC1 regulation in PKD suppression.

Suppression of SHIP2 contributes to tumorigenesis and proliferation of gastric cancer cells via activation of Akt

BACKGROUND

The Src homology 2-containing inositol 5-phosphatase 2 (SHIP2) is implicated in diabetes, arthrosclerosis, and cancer. However, the role of SHIP2 in human gastric cancer remains unclear.

METHODS

The expression levels of SHIP2 in gastric cancer tissues, a panel of gastric cancer cell lines, and normal gastric epithelial cells were analyzed by immunohistochemistry (IHC), Western blot, and real-time quantitative RT-PCR (qRT-PCR). Gastric cancer cells with either overexpressed SHIP2 or co-overexpressed SHIP2 and Akt were analyzed to determine cell proliferation, colony formation, apoptosis, cell migration, and invasion assays. Normal gastric epithelial cells with knockdown SHIP2 or co-knockdown SHIP2 and Akt were subjected by anchorage-independent growth assays. The effect of SHIP2 on tumor growth in vivo was detected by xenograft tumorigenesis assays.

RESULTS

SHIP2 was commonly downregulated in gastric cancer compared with normal gastric mucosa, and overexpression of SHIP2 inhibited cell proliferation, induced apoptosis, suppressed cell motility and invasion in gastric cancer cells in vitro, and retarded the growth of xenograft gastric tumors in vivo, while knockdown of SHIP2 in normal gastric epithelial cells promoted anchorage-independent growth. Moreover, overexpression of SHIP2 inactivated Akt, and upregulated p21, p27, and the pro-apoptotic protein Bim. Restoring Akt activation in gastric cancer cells largely blocked the inhibition of PI3K/Akt signaling by SHIP2 and reversed the inhibitory effect of SHIP2 on tumorigenesis and proliferation.

CONCLUSIONS

This study demonstrates, for the first time, that SHIP2 is frequently downregulated in gastric cancer, and reduced SHIP2 expression promotes tumorigenesis and proliferation of gastric cancer via activation of the PI3K/Akt signaling.

Regulation of PtdIns(3,4,5)P3/Akt signalling by inositol polyphosphate 5-phosphatases

The phosphoinositide 3-kinase (PI3K) generated lipid signals, PtdIns(3,4,5)P3 and PtdIns(3,4)P2, are both required for the maximal activation of the serine/threonine kinase proto-oncogene Akt. The inositol polyphosphate 5-phosphatases (5-phosphatases) hydrolyse the 5-position phosphate from the inositol head group of PtdIns(3,4,5)P3 to yield PtdIns(3,4)P2. Extensive work has revealed several 5-phosphatases inhibit PI3K-driven Akt signalling, by decreasing PtdIns(3,4,5)P3 despite increasing cellular levels of PtdIns(3,4)P2. The roles that 5-phosphatases play in suppressing cell proliferation and transformation are slow to emerge; however, the 5-phosphatase PIPP [proline-rich inositol polyphosphate 5-phosphatase; inositol polyphosphate 5-phosphatase (INPP5J)] has recently been identified as a putative tumour suppressor in melanoma and breast cancer and SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase 1] inhibits haematopoietic cell proliferation. INPP5E regulates cilia stability and INPP5E mutations have been implicated ciliopathy syndromes. This review will examine 5-phosphatase regulation of PI3K/Akt signalling, focussing on the role PtdIns(3,4,5)P3 5-phosphatases play in developmental diseases and cancer.

Investigation of the Lipid Binding Properties of the Marburg Virus Matrix Protein VP40

Marburg virus (MARV), which belongs to the virus family Filoviridae, causes hemorrhagic fever in humans and nonhuman primates that is often fatal. MARV is a lipid-enveloped virus that during the replication process extracts its lipid coat from the plasma membrane of the host cell it infects. MARV carries seven genes, one of which encodes its matrix protein VP40 (mVP40), which regulates the assembly and budding of the virions. Currently, little information is available on mVP40 lipid binding properties. Here, we have investigated the in vitro and cellular mechanisms by which mVP40 associates with lipid membranes. mVP40 associates with anionic membranes in a nonspecific manner that is dependent upon the anionic charge density of the membrane. These results are consistent with recent structural determination of mVP40, which elucidated an mVP40 dimer with a flat and extensive cationic lipid binding interface.

IMPORTANCE

Marburg virus (MARV) is a lipid-enveloped filamentous virus from the family Filoviridae. MARV was discovered in 1967, and yet little is known about how its seven genes are used to assemble and form a new viral particle in the host cell it infects. The MARV matrix protein VP40 (mVP40) underlies the inner leaflet of the virus and regulates budding from the host cell plasma membrane. In vitro and cellular assays in this study investigated the mechanism by which mVP40 associates with lipids. The results demonstrate that mVP40 interactions with lipid vesicles or the inner leaflet of the plasma membrane are electrostatic but nonspecific in nature and are dependent on the anionic charge density of the membrane surface. Small molecules that can disrupt lipid trafficking or reduce the anionic charge of the plasma membrane interface may be useful in inhibiting assembly and budding of MARV.

4.1N suppresses hypoxia-induced epithelial-mesenchymal transition in epithelial ovarian cancer cells

Protein 4.1N (4.1N) is a member of the protein 4.1 family and is essential for the regulation of cell adhesion, motility and signaling. Previous studies have suggested that 4.1N may serve a tumor suppressor role. However, the molecular mechanisms remain unclear. In the current study, the role of 4.1N in the downregulation of hypoxia‑induced factor 1α (HIF‑1α) under hypoxic conditions and therefore the suppression of hypoxia induced epithelial‑mesenchymal transition (EMT) was investigated. The data were obtained from overexpressed and knockdown 4.1N epithelial ovarian cancer (EOC) cell lines. It was identified that 4.1N was capable of regulating the sub‑cellular localization and expression levels of HIF‑1α, by which 4.1N served a dominant role in the suppression of hypoxia‑induced EMT and associated genes. Collectively, the data of the current study identified 4.1N as an inhibitor of hypoxia‑induced tumor progression in EOC cells and highlighted its potential role in EOC therapy.

INPP4B is an oncogenic regulator in human colon cancer

Inositol polyphosphate 4-phosphatase type II (INPP4B) negatively regulates phosphatidylinositol 3-kinase signaling and is a tumor suppressor in some types of cancers. However, we have found that it is frequently upregulated in human colon cancer cells. Here we show that silencing of INPP4B blocks activation of Akt and serum- and glucocorticoid-regulated kinase 3 (SGK3), inhibits colon cancer cell proliferation and retards colon cancer xenograft growth. Conversely, overexpression of INPP4B increases proliferation and triggers anchorage-independent growth of normal colon epithelial cells. Moreover, we demonstrate that the effect of INPP4B on Akt and SGK3 is associated with inactivation of phosphate and tensin homolog through its protein phosphatase activity and that the increase in INPP4B is due to Ets-1-mediated transcriptional upregulation in colon cancer cells. Collectively, these results suggest that INPP4B may function as an oncogenic driver in colon cancer, with potential implications for targeting INPP4B as a novel approach to treat this disease.

Phosphatidylinositolphosphate phosphatase activities and cancer

Signaling through the phosphoinositide 3-kinase pathways mediates the actions of a plethora of hormones, growth factors, cytokines, and neurotransmitters upon their target cells following receptor occupation. Overactivation of these pathways has been implicated in a number of pathologies, in particular a range of malignancies. The tight regulation of signaling pathways necessitates the involvement of both stimulatory and terminating enzymes; inappropriate activation of a pathway can thus result from activation or inhibition of the two signaling arms. The focus of this review is to discuss, in detail, the activities of the identified families of phosphoinositide phosphatase expressed in humans, and how they regulate the levels of phosphoinositides implicated in promoting malignancy.

Molecular control of PtdIns(3,4,5)P3 signaling in neutrophils

Neutrophils play critical roles in innate immunity and host defense. However, excessive neutrophil accumulation or hyper-responsiveness of neutrophils can be detrimental to the host system. Thus, the response of neutrophils to inflammatory stimuli needs to be tightly controlled. Many cellular processes in neutrophils are mediated by localized formation of an inositol phospholipid, phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3), at the plasma membrane. The PtdIns(3,4,5)P3 signaling pathway is negatively regulated by lipid phosphatases and inositol phosphates, which consequently play a critical role in controlling neutrophil function and would be expected to act as ideal therapeutic targets for enhancing or suppressing innate immune responses. Here, we comprehensively review current understanding about the action of lipid phosphatases and inositol phosphates in the control of neutrophil function in infection and inflammation.

INPP5E interacts with AURKA, linking phosphoinositide signaling to primary cilium stability

Mutations in inositol polyphosphate 5-phosphatase E (INPP5E) cause the ciliopathies known as Joubert and MORM syndromes; however, the role of INPP5E in ciliary biology is not well understood. Here, we describe an interaction between INPP5E and AURKA, a centrosomal kinase that regulates mitosis and ciliary disassembly, and we show that this interaction is important for the stability of primary cilia. Furthermore, AURKA phosphorylates INPP5E and thereby increases its 5-phosphatase activity, which in turn promotes transcriptional downregulation of AURKA, partly through an AKT-dependent mechanism. These findings establish the first direct link between AURKA and phosphoinositide signaling and suggest that the function of INPP5E in cilia is at least partly mediated by its interactions with AURKA.

Loss of PI(4,5)P2 5-Phosphatase A Contributes to Resistance of Human Melanoma Cells to RAF/MEK Inhibitors

Past studies have shown that the inositol polyphosphate 5-phosphatase, phosphatidylinositol 4,5-bisphosphate 5-phosphatase (PIB5PA), is commonly downregulated or lost in melanomas, which contributes to elevated activation of phosphatidylinositol 3-kinase (PI3K)/Akt in melanoma cells. In this report, we provide evidence that PIB5PA deficiency plays a role in resistance of melanoma cells to RAF/mitogen-activated protein kinase kinase (MEK) inhibitors. Ectopic expression of PIB5PA enhanced apoptosis induced by the RAF inhibitor PLX4720 in BRAF(V600E) and by the MEK inhibitor U0126 in both BRAF(V600E) and wild-type BRAF melanoma cells. This was due to inhibition of PI3K/Akt, as co-introduction of an active form of Akt (myr-Akt) abolished the effect of overexpression of PIB5PA on apoptosis induced by PLX4720 or U0126. While overexpression of PIB5PA triggered activation of Bad and down-regulation of Mcl-1, knockdown of Bad or overexpression of Mcl-1 recapitulated, at least in part, the effect of myr-Akt, suggesting that regulation of Bad and Mcl-1 is involved in PIB5PA-mediated sensitization of melanoma cells to the inhibitors. The role of PIB5PA deficiency in BRAF inhibitor resistance was confirmed by knockdown of PIB5PA, which led to increased growth of BRAF(V600E) melanoma cells selected for resistance to PLX4720. Consistent with its role in vitro, overexpression of PIB5PA and the MEK inhibitor selumetinib cooperatively inhibited melanoma tumor growth in a xenograft model. Taken together, these results identify loss of PIB5PA as a novel resistance mechanism of melanoma to RAF/MEK inhibitors and suggest that restoration of PIB5PA may be a useful strategy to improve the therapeutic efficacy of the inhibitors in the treatment of melanoma.

PI(4,5)P2 5-phosphatase A regulates PI3K/Akt signalling and has a tumour suppressive role in human melanoma

Inositol polyphosphate 5-phosphatases can terminate downstream signalling of phosphatidylinositol-3 kinase; however, their biological role in the pathogenesis of cancer is controversial. Here we report that the inositol polyphosphate 5-phosphatase, phosphatidylinositol 4,5-bisphosphate 5-phosphatase, has a tumour suppressive role in melanoma. Although it is commonly downregulated in melanoma, overexpression of phosphatidylinositol 4,5-bisphosphate 5-phosphatase blocks Akt activation, inhibits proliferation and undermines survival of melanoma cells in vitro, and retards melanoma growth in a xenograft model. In contrast, knockdown of phosphatidylinositol 4,5-bisphosphate 5-phosphatase results in increased proliferation and anchorage-independent growth of melanocytes. Although DNA copy number loss is responsible for downregulation of phosphatidylinositol 4,5-bisphosphate 5-phosphatase in a proportion of melanomas, histone hypoacetylation mediated by histone deacetylases HDAC2 and HDAC3 through binding to the transcription factor Sp1 at the PIB5PA gene promoter appears to be another commonly involved mechanism. Collectively, these results establish the tumour suppressive role of phosphatidylinositol 4,5-bisphosphate 5-phosphatase and reveal mechanisms involved in its downregulation in melanoma.

Inositol polyphosphate 5-phosphatases, such as PIB5PA, terminate signalling downstream of phosophoinositide-3 kinase; however, their biological roles remain unclear. Here the authors report that PIB5PA has a tumour suppressive role in melanoma.

Inositol polyphosphate phosphatases in human disease

Phosphoinositide signalling molecules interact with a plethora of effector proteins to regulate cell proliferation and survival, vesicular trafficking, metabolism, actin dynamics and many other cellular functions. The generation of specific phosphoinositide species is achieved by the activity of phosphoinositide kinases and phosphatases, which phosphorylate and dephosphorylate, respectively, the inositol headgroup of phosphoinositide molecules. The phosphoinositide phosphatases can be classified as 3-, 4- and 5-phosphatases based on their specificity for dephosphorylating phosphates from specific positions on the inositol head group. The SAC phosphatases show less specificity for the position of the phosphate on the inositol ring. The phosphoinositide phosphatases regulate PI3K/Akt signalling, insulin signalling, endocytosis, vesicle trafficking, cell migration, proliferation and apoptosis. Mouse knockout models of several of the phosphoinositide phosphatases have revealed significant physiological roles for these enzymes, including the regulation of embryonic development, fertility, neurological function, the immune system and insulin sensitivity. Importantly, several phosphoinositide phosphatases have been directly associated with a range of human diseases. Genetic mutations in the 5-phosphatase INPP5E are causative of the ciliopathy syndromes Joubert and MORM, and mutations in the 5-phosphatase OCRL result in Lowe's syndrome and Dent 2 disease. Additionally, polymorphisms in the 5-phosphatase SHIP2 confer diabetes susceptibility in specific populations, whereas reduced protein expression of SHIP1 is reported in several human leukaemias. The 4-phosphatase, INPP4B, has recently been identified as a tumour suppressor in human breast and prostate cancer. Mutations in one SAC phosphatase, SAC3/FIG4, results in the degenerative neuropathy, Charcot-Marie-Tooth disease. Indeed, an understanding of the precise functions of phosphoinositide phosphatases is not only important in the context of normal human physiology, but to reveal the mechanisms by which these enzyme families are implicated in an increasing repertoire of human diseases.

Cellular phosphoinositides and the maturation of bluetongue virus, a non-enveloped capsid virus

Background

Bluetongue virus (BTV), a member of Orbivirus genus in the Reoviridae family is a double capsid virus enclosing a genome of 10 double-stranded RNA segments. A non-structural protein of BTV, NS3, which is associated with cellular membranes and interacts with outer capsid proteins, has been shown to be involved in virus morphogenesis in infected cells. In addition, studies have also shown that during the later stages of virus infection NS3 behaves similarly to HIV protein Gag, an enveloped viral protein. Since Gag protein is known to interact with membrane lipid phosphatidylinositol (4,5) bisphosphate [PI(4,5)P₂] and one of the known binding partners of NS3, cellular protein p11 also interacts with annexin a PI(4,5)P₂ interacting protein, this study was designed to understand the role of this negatively charged membrane lipid in BTV assembly and maturation.

Methods

Over expression of cellular enzymes that either depleted cells of PI(4,5)P₂ or altered the distribution of PI(4,5)P₂, were used to analyze the effect of the lipid on BTV maturation at different times post-infection. The production of mature virus particles was monitored by plaque assay. Microscopic techniques such as confocal microscopy and electron microscopy (EM) were also undertaken to study localization of virus proteins and virus particles in cells, respectively.

Results

Initially, confocal microscopic analysis demonstrated that PI(4,5)P₂ not only co-localized with NS3, but it also co-localized with VP5, one of the outer capsid proteins of BTV. Subsequently, experiments involving depletion of cellular PI(4,5)P₂ or its relocation demonstrated an inhibitory effect on normal BTV maturation and it also led to a redistribution of BTV proteins within the cell. The data was supported further by EM visualization showing that modulation of PI(4,5)P₂ in cells indeed resulted in less particle production.

Conclusion

This study to our knowledge, is the first report demonstrating involvement of PI(4,5)P₂ in a non-enveloped virus assembly and release. As BTV does not have lipid envelope, this finding is unique for this group of viruses and it suggests that the maturation of capsid and enveloped viruses may be more closely related than previously thought.

Alterations in the MA and NC domains modulate phosphoinositide-dependent plasma membrane localization of the Rous sarcoma virus Gag protein

Retroviral Gag proteins direct virus particle assembly from the plasma membrane (PM). Phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)] plays a role in PM targeting of several retroviral Gag proteins. Here we report that depletion of intracellular PI(4,5)P(2) and phosphatidylinositol-(3,4,5)-triphosphate [PI(3,4,5)P(3)] levels impaired Rous sarcoma virus (RSV) Gag PM localization. Gag mutants deficient in nuclear trafficking were less sensitive to reduction of intracellular PI(4,5)P(2) and PI(3,4,5)P(3), suggesting a possible connection between Gag nuclear trafficking and phosphoinositide-dependent PM targeting.

Phosphoinositide 5-phosphatases: How do they affect tumourigenesis?

The activity of biological molecules is often affected by their phosphorylation state. Regulatory phosphorylation operates as a binary switch and is usually controlled by counteracting kinases and phosphatases. However, phosphatidylinositol (PtdIns) has three phosphorylation sites on its inositol ring. The phosphorylation status of PtdIns is controlled by multiple kinases and phosphatases with distinct substrate specificities, serving as a 'lipid code' or 'phosphoinositide code'. Class I phosphoinositide 3-kinase (PI3K) converts PtdIns(4,5)P₂ to PtdIns(3,4,5)P₃, which plays a pivotal role in signals controlling glucose uptake, cytoskeletal reorganization, cell proliferation and apoptosis. PI3K is pro-oncogenic, whereas phosphoinositide phosphatases that degrade PtdIns(3,4,5)P₃ are not always anti-oncogenic. Recent studies have revealed the unique characteristics of phosphoinositide 5-phosphatases.

Evidence of a role of inositol polyphosphate 5-phosphatase INPP5E in cilia formation in zebrafish

Inositol phosphatases are important regulators of cell signaling and membrane trafficking. Mutations in inositol polyphosphate 5-phosphatase, INPP5E, have been identified in Joubert syndrome, a rare congenital disorder characterized by midbrain malformation, retinitis pigmentosa, renal cysts, and polydactyly. Previous studies have implicated primary cilia abnormalities in Joubert syndrome, yet the role of INPP5E in cilia formation is not well understood. In this study, we examined the function of INPP5E in cilia development in zebrafish. Using specific antisense morpholino oligonucleotides to knockdown Inpp5e expression, we observed phenotypes of microphthalmia, pronephros cysts, pericardial effusion, and left-right body axis asymmetry. The Inpp5e morphant zebrafish exhibited shortened and decreased cilia formation in the Kupffer's vesicle and pronephric ducts as compared to controls. Epinephrine-stimulated melanosome trafficking was delayed in the Inpp5e zebrafish morphants. Expression of human INPP5E expression rescued the phenotypic defects in the Inpp5e morphants. Taken together, we showed that INPP5E is critical for the cilia development in zebrafish.

Identification of inositol polyphosphate 4-phosphatase type II as a novel tumor resistance biomarker in human laryngeal cancer HEp-2 cells

Although tumor resistance remains a significant impediment to successful radiotherapy, associated regulatory markers and detailed molecular mechanisms underlying this phenomenon are not well defined. In this study, we identified inositol polyphosphate 4-phosphatase type II (INPP4B) as a novel marker of radioresistance by systematically analyzing Unigene libraries of laryngeal cancer. INPP4B was highly expressed in radioresistant laryngeal cancer cells and was induced by treatment with either radiation or anticancer drugs in various types of cancer cells. Ectopic INPP4B overexpression increased radioresistance and anticancer drug resistance by suppressing apoptosis in HEp-2 cells. Conversely, INPP4B depletion with small interfering RNA resensitized HEp-2 as well as A549 and H1299 cells to radiation- and anticancer drug-induced apoptosis. Furthermore, radiation-induced INPP4B expression was blocked by inhibition of extracellular signal-regulated kinase (ERK). INPP4B depletion significantly attenuated radiation-induced increases in Akt phosphorylation, indicating an association of INPP4B-mediated radioresistance with Akt survival signaling. Taken together, our data suggest that ERK-dependent induction of INPP4B triggers the development of a tumor-resistance phenotype via Akt signaling and identify INPP4B as a potentially important target molecule for resolving the radioresistance of cancer cells.

Inositol polyphosphate 5-phosphatases; new players in the regulation of cilia and ciliopathies

Phosphoinositides regulate numerous cellular events via the recruitment and activation of multiple lipid-binding effector proteins. The precise temporal and spatial regulation of phosphoinositide signals by the co-ordinated activities of phosphoinositide kinases and phosphatases is essential for homeostasis and development. Mutations in two inositol polyphosphate 5-phosphatases, INPP5E and OCRL, cause the cerebrorenal syndromes of Joubert and Lowe's, respectively. INPP5E and OCRL exhibit overlapping phosphoinositide substrate specificity and subcellular localisation, including an association with the primary cilia. Here, we review recent studies that identify a new role for these enzymes in the regulation of primary cilia function. Joubert syndrome has been extensively linked to primary cilia defects, and Lowe's may represent a new class of 'ciliopathy associated' syndromes.

An introduction to phosphoinositides

Phosphoinositides (PIs) are minor components of cellular membranes that play critical regulatory roles in several intracellular functions. This chapter describes the main enzymes regulating the turnover of each of the seven PIs in mammalian cells and introduces to some of their intracellular functions and to some evidences of their involvement in human diseases. Due to the complex interrelation between the distinct PIs and the plethora of functions that they can regulate inside a cell, this chapter is not meant to be a comprehensive coverage of all aspects of PI signalling but rather an introduction to this complex signalling field. For more details of their regulation/functions and extensive description of their intracellular roles, more detailed reviews are suggested on each single topic.

Influence of proline-rich inositol polyphosphate 5-phosphatase, on early development of fertilized mouse eggs, via inhibition of phosphorylation of Akt

OBJECTIVES

Proline-rich inositol polyphosphate 5-phosphatase (PIPP) is one of the signal-modifying enzymes that play pivotal regulatory roles in PI3K signalling pathway. The aim of this study was to determine the role of PIPP in early development of fertilized mouse eggs, via inhibition of Akt activity and subsequent downstream signalling events.

MATERIALS AND METHODS

The mRNA transcript levels of endogenous PIPP and Akt1, Akt2, Akt3 were detected in G(1) , S, G(2) and M phases of fertilized mouse eggs by RT-PCR. Levels of exogenous PIPP, phosphorylated Akt at Ser473, dephosphorylated cdc2 at Tyr15 and levels of CCNB1, were detected respectively by immunoblotting. Changes in Akt localization were observed by fluoroimmunoassay; meanwhile, changes in activity of Akt and its downstream MPF were detected. Percentages of cells undergoing division were determined by counting, using a dissecting microscope.

RESULTS

PIPP and Akt1 transcripts were detectable in G(1), S, G(2) and M phases of fertilized mouse eggs, but Akt2 and Akt3 were not. We also observed that overexpression of PIPP in fertilized eggs decreased expression of phosphorylated Akt at Ser473 and altered membrane localization of phosphorylated Akt at Ser473 specifically. Furthermore, overexpression of PIPP resulted in decreases in mitosis-phase promoting factor activity, level of dephosphorylated cdc2 at Tyr15 and cleavage rate of fertilized mouse eggs.

CONCLUSIONS

Our data suggest, for the first time, that PIPP may affect development of fertilized mouse eggs by inhibition of level of phosphorylated Akt at Ser473 and subsequent inhibition of downstream signal cascades.

Synergistic effect of non-transmissible Sendai virus vector encoding the c-myc suppressor FUSE-binding protein-interacting repressor plus cisplatin in the treatment of malignant pleural mesothelioma

Human malignant pleural mesothelioma (HMPM) is highly resistant to conventional therapy, and therefore novel therapies are required. We previously reported that overexpression of the FUSE-binding protein-interacting repressor (FIR), a c-myc transcriptional repressor, induces apoptosis via c-Myc suppression, and is thus a suitable cancer therapy. In the current preclinical trial, a fusion gene deleted non-transmissible Sendai virus vector encoding FIR (SeV/ΔF/FIR) was prepared and its cytotoxic activity against an orthotopic xenograft model of HMPM, in combination with cisplatin, was assessed. SeV/ΔF/FIR and a fusion gene deleted non-transmissible Sendai virus vector encoding green fluorescent protein (SeV/ΔF/GFP) were prepared. The transduction efficiency of these agents in terms of dose-dependent cytotoxicity and/or apoptosis induction was then assessed in a few HMPM cells. Combination therapy with SeV/ΔF/FIR plus cisplatin was evaluated in vitro and in a mouse model. SeV/ΔF/FIR significantly reduced cell viability in three HMPM cell lines but was less effective in non-tumor immortalized mesothelial cells. SeV/ΔF/FIR cytotoxicity was partly due to apoptosis induction via c-Myc suppression. In addition, SeV/ΔF/FIR showed synergistic antitumor effects in combination with cisplatin, as was revealed by isobologram analysis in MSTO-211H. Moreover, combination therapy with SeV/ΔF/FIR plus cisplatin demonstrated significant tumor reduction and improvement in survival rate in an animal model. Combination therapy with SeV/ΔF/FIR plus cisplatin has therapeutic potential against HMPM. SeV/ΔF/FIR plus cisplatin will be an attractive modality against HMPM in the future.

Phosphoinositides direct equine infectious anemia virus gag trafficking and release

Phosphatidylinositol 4,5-biphosphate [PI(4,5)P(2) ], the predominant phosphoinositide (PI) on the plasma membrane, binds the matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) and equine infectious anemia virus (EIAV) with similar affinities in vitro. Interaction with PI(4,5)P(2) is critical for HIV-1 assembly on the plasma membrane. EIAV has been shown to localize in internal compartments; hence, the significance of its interaction with PI(4,5)P(2) is unclear. We therefore investigated the binding in vitro of other PIs to EIAV MA and whether intracellular association with compartments bearing these PIs was important for assembly and release of virus-like particles (VLPs) formed by Gag. In vitro, EIAV MA bound phosphatidylinositol 3-phosphate [PI(3)P] with higher affinity than PI(4,5)P(2) as revealed by nuclear magnetic resonance (NMR) spectra upon lipid titration. Gag was detected on the plasma membrane and in compartments enriched in phosphatidylinositol 3,5-biphosphate [PI(3,5)P(2) ]. Treatment of cells with YM201636, a kinase inhibitor that blocks production of PI(3,5)P(2) from PI(3)P, caused Gag to colocalize with aberrant compartments and inhibited VLP release. In contrast to HIV-1, release of EIAV VLPs was not significantly diminished by coexpression with 5-phosphatase IV, an enzyme that specifically depletes PI(4,5)P(2) from the plasma membrane. However, coexpression with synaptojanin 2, a phosphatase with broader specificity, diminished VLP production. PI-binding pocket mutations caused striking budding defects, as revealed by electron microscopy. One of the mutations also modified Gag-Gag interaction, as suggested by altered bimolecular fluorescence complementation. We conclude that PI-mediated targeting to peripheral and internal membranes is a critical factor in EIAV assembly and release.

Assembly and replication of HIV-1 in T cells with low levels of phosphatidylinositol-(4,5)-bisphosphate

HIV-1 Gag assembles into virus particles predominantly at the plasma membrane (PM). Previously, we observed that phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)] is essential for Gag binding to the plasma membrane and virus release in HeLa cells. In the current study, we found that PI(4,5)P(2) also facilitates Gag binding to the PM and efficient virus release in T cells. Notably, serial passage of HIV-1 in an A3.01 clone that expresses polyphosphoinositide 5-phosphatase IV (5ptaseIV), which depletes cellular PI(4,5)P(2), yielded an adapted mutant with a Leu-to-Arg change at matrix residue 74 (74LR). Virus replication in T cells expressing 5ptaseIV was accelerated by the 74LR mutation relative to replication of wild type HIV-1 (WT). This accelerated replication of the 74LR mutant was not due to improved virus release. In control T cells, the 74LR mutant releases virus less efficiently than does the WT, whereas in cells expressing 5ptaseIV, the WT and the 74LR mutant are similarly inefficient in virus release. Unexpectedly, we found that the 74LR mutation increased virus infectivity and compensated for the inefficient virus release. Altogether, these results indicate that PI(4,5)P(2) is essential for Gag-membrane binding, targeting of Gag to the PM, and efficient virus release in T cells, which in turn likely promotes efficient virus spread in T cell cultures. In T cells with low PI(4,5)P(2) levels, however, the reduced virus particle production can be compensated for by a mutation that enhances virus infectivity.

New frontiers in the treatment of liposarcoma, a therapeutically resistant malignant cohort

The adipogenic origin-derived liposarcoma (LPS) family is the most common soft tissue sarcoma histological subtype. This group is composed of three categories as per the 2002 WHO guidelines: (1) well-differentiated and dedifferentiated liposarcoma (WDLPS/DDLPS); (2) myxoid and round cell liposarcoma (MLS and RCL); and (3) pleomorphic liposarcoma (PLS). While clustered together, these histological subtypes are widely diverse in their clinical, pathological, and molecular characteristics. In general, surgery still remains the mainstay of LPS therapy and the only approach offering the potential of cure. Effective therapeutic strategies for locally advanced and metastatic disease are currently lacking and are crucially needed. With the current gradually increasing knowledge of LPS genetic- and epigenetic-associated deregulations, the ultimate goal is to develop drugs that can specifically eliminate LPS cells while sparing normal tissues. This tumor-tailored target-orientated approach will hopefully result in a significant improvement in the outcome of patients suffering from these poor prognosis malignancies.

Phosphoinositide signalling in cancer: beyond PI3K and PTEN

There are numerous studies that suggest multiple links between the cellular phosphoinositide system and cancer. As key roles in cancer have been established for PI3K and PTEN - enzymes that regulate the levels of phosphatidylinositol-3,4,5-trisphosphate - compounds targeting this pathway are entering the clinic at a rapid pace. Several other phosphoinositide-modifying enzymes, including phosphoinositide kinases, phosphatases and phospholipase C enzymes, have been implicated in the generation and progression of tumours. Studies of these enzymes are providing new insights into the mechanisms and the extent of their involvement in cancer, highlighting new potential targets for therapeutic intervention.

The endoplasmic reticulum of dorsal root ganglion neurons contains functional TRPV1 channels

Transient receptor potential vanilloid type 1 (TRPV1) is a plasma membrane Ca(2+) channel involved in transduction of painful stimuli. Dorsal root ganglion (DRG) neurons express ectopic but functional TRPV1 channels in the endoplasmic reticulum (ER) (TRPV1(ER)). We have studied the properties of TRPV1(ER) in DRG neurons and HEK293T cells expressing TRPV1. Activation of TRPV1(ER) with capsaicin or other vanilloids produced an increase of cytosolic Ca(2+) due to Ca(2+) release from the ER. The decrease of [Ca(2+)](ER) was directly revealed by an ER-targeted aequorin Ca(2+) probe, expressed in DRG neurons using a herpes amplicon virus. The sensitivity of TRPV1(ER) to capsaicin was smaller than the sensitivity of the plasma membrane TRPV1 channels. The low affinity of TRPV1(ER) was not related to protein kinase A- or C-mediated phosphorylations, but it was due to inactivation by cytosolic Ca(2+) because the sensitivity to capsaicin was increased by loading the cells with the Ca(2+) chelator BAPTA. Decreasing [Ca(2+)](ER) did not affect the sensitivity of TRPV1(ER) to capsaicin. Disruption of the TRPV1 calmodulin-binding domains at either the C terminus (Delta35AA) or the N terminus (K155A) increased 10-fold the affinity of TRPV1(ER) for capsaicin, suggesting that calmodulin is involved in the inactivation. The lack of TRPV1 sensitizers, such as phosphatidylinositol 4,5-bisphosphate, in the ER could contribute to decrease the affinity for capsaicin. The low sensitivity of TRPV1(ER) to agonists may be critical for neuron health, because otherwise Ca(2+) depletion of ER could lead to ER stress, unfolding protein response, and cell death.

Mutations in INPP5E, encoding inositol polyphosphate-5-phosphatase E, link phosphatidyl inositol signaling to the ciliopathies

Phosphotidylinositol (PtdIns) signaling is tightly regulated, both spatially and temporally, by subcellularly localized PtdIns kinases and phosphatases that dynamically alter downstream signaling events 1. Joubert Syndrome (JS) characterized by a specific midbrain-hindbrain malformation (“molar tooth sign”) and variably associated retinal dystrophy, nephronophthisis, liver fibrosis and polydactyly 2, and is included in the newly emerging group of “ciliopathies”. In patients linking to JBTS1, we identified mutations in the INPP5E gene, encoding inositol polyphosphate-5-phosphatase E, which hydrolyzes the 5-phosphate of PtdIns(3,4,5)P3 and PtdIns(4,5)P2. Mutations clustered in the phosphatase domain and impaired 5-phosphatase activity, resulting in altered cellular PtdIns ratios. INPP5E localized to cilia in major organs affected in JS, and mutations promoted premature destabilization of cilia in response to stimulation. Thus, these data links PtdIns signaling to the primary cilium, a cellular structure that is becoming increasingly appreciated for its role in mediating cell signals and neuronal function.

INPP5E mutations cause primary cilium signaling defects, ciliary instability and ciliopathies in human and mouse

The primary cilium is an antenna-like structure that protrudes from the cell surface of quiescent/differentiated cells and participates in extracellular signal processing. Here, we report that mice deficient for the lipid 5-phosphatase Inpp5e develop a multiorgan disorder associated with structural defects of the primary cilium. In ciliated mouse embryonic fibroblasts, Inpp5e is concentrated in the axoneme of the primary cilium. Inpp5e inactivation did not impair ciliary assembly but altered the stability of pre-established cilia after serum addition. Blocking phosphoinositide 3-kinase (PI3K) activity or ciliary platelet-derived growth factor receptor alpha (PDGFRalpha) restored ciliary stability. In human INPP5E, we identified a mutation affecting INPP5E ciliary localization and cilium stability in a family with MORM syndrome, a condition related to Bardet-Biedl syndrome. Together, our results show that INPP5E plays an essential role in the primary cilium by controlling ciliary growth factor and PI3K signaling and stability, and highlight the consequences of INPP5E dysfunction.

Regulation of PI(3)K/Akt signalling and cellular transformation by inositol polyphosphate 4-phosphatase-1

Akt is a crucial phosphoinositide 3-kinase (PI(3)K) effector that regulates cell proliferation and survival. PI(3)K-generated signals, PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2), direct Akt plasma membrane engagement. Pathological Akt plasma membrane association promotes oncogenesis. PtdIns(3,4)P(2) is degraded by inositol polyphosphate 4-phosphatase-1 (4-ptase-1) forming PtdIns(3)P; however, the role of 4-ptase-1 in regulating the activation and function of Akt is unclear. In mouse embryonic fibroblasts lacking 4-ptase-1 ((-/-)MEFs), the Akt-pleckstrin homology (PH) domain was constitutively membrane-associated both in serum-starved and agonist-stimulated cells, in contrast to (+/+)MEFs, in which it was detected only at the plasma membrane following serum stimulation. Epidermal growth factor (EGF) stimulation resulted in increased Ser(473) and Thr(308)-Akt phosphorylation and activation of Akt-dependent signalling in (-/-)MEFs, relative to (+/+)MEFs. Significantly, loss of 4-ptase-1 resulted in increased cell proliferation and decreased apoptosis. SV40-transformed (-/-)MEFs showed increased anchorage-independent cell growth and formed tumours in nude mice. This study provides the first evidence, to our knowledge, that 4-ptase-1 controls the activation of Akt and thereby cell proliferation, survival and tumorigenesis.

Anx2 interacts with HIV-1 Gag at phosphatidylinositol (4,5) bisphosphate-containing lipid rafts and increases viral production in 293T cells

The neuronal damage characteristic of HIV-1-mediated CNS diseases is inflicted by HIV-1 infected brain macrophages. Several steps of viral replication, including assembly and budding, differ between macrophages and T cells; it is likely that cell-specific host factors mediate these differences. We previously defined Annexin 2 (Anx2) as an HIV Gag binding partner in human monocyte-derived macrophages (MDMs) that promotes proper viral assembly. Anx2, a calcium-dependent membrane-binding protein that can aggregate phospholipid-containing lipid rafts, is expressed to high levels in macrophages, but not in T lymphocytes or the 293T cell line. Here, we use bimolecular fluorescence complementation in the 293T cell model to demonstrate that Anx2 and HIV-1 Gag interact at the phosphatidylinositol (4,5) bisphosphate-containing lipid raft membrane domains at which Gag mediates viral assembly. Furthermore, we demonstrate that Anx2 expression in 293T cells increases Gag processing and HIV-1 production. These data provide new evidence that Anx2, by interacting with Gag at the membranes that support viral assembly, functions in the late stages of HIV-1 replication.

The role of the inositol polyphosphate 5-phosphatases in cellular function and human disease

Phosphoinositides are membrane-bound signalling molecules that regulate cell proliferation and survival, cytoskeletal reorganization and vesicular trafficking by recruiting effector proteins to cellular membranes. Growth factor or insulin stimulation induces a canonical cascade resulting in the transient phosphorylation of PtdIns(4,5)P(2) by PI3K (phosphoinositide 3-kinase) to form PtdIns(3,4,5)P(3), which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) back to PtdIns(4,5)P(2), or by the 5-ptases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). The 5-ptases also hydrolyse PtdIns(4,5)P(2), forming PtdIns4P. Ten mammalian 5-ptases have been identified, which share a catalytic mechanism similar to that of the apurinic/apyrimidinic endonucleases. Gene-targeted deletion of 5-ptases in mice has revealed that these enzymes regulate haemopoietic cell proliferation, synaptic vesicle recycling, insulin signalling, endocytosis, vesicular trafficking and actin polymerization. Several studies have revealed that the molecular basis of Lowe's syndrome is due to mutations in the 5-ptase OCRL (oculocerebrorenal syndrome of Lowe). Futhermore, the 5-ptases SHIP [SH2 (Src homology 2)-domain-containing inositol phosphatase] 2, SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) and 72-5ptase (72 kDa 5-ptase)/Type IV/Inpp5e (inositol polyphosphate 5-phosphatase E) are implicated in negatively regulating insulin signalling and glucose homoeostasis in specific tissues. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. Gene profiling studies have identified changes in the expression of various 5-ptases in specific cancers. In addition, 5-ptases such as SHIP1, SHIP2 and 72-5ptase/Type IV/Inpp5e regulate macrophage phagocytosis, and SHIP1 also controls haemopoietic cell proliferation. Therefore the 5-ptases are a significant family of signal-modulating enzymes that govern a plethora of cellular functions by regulating the levels of specific phosphoinositides. Emerging studies have implicated their loss or gain of function in human disease.

MTMR9 increases MTMR6 enzyme activity, stability, and role in apoptosis

Myotubularin-related protein 6 (MTMR6) is a catalytically active member of the myotubularin (MTM) family, which is composed of 14 proteins. Catalytically active myotubularins possess 3-phosphatase activity dephosphorylating phosphatidylinositol-3-phoshate and phosphatidylinositol-3,5-bisphosphate, and some members have been shown to form homomers or heteromeric complexes with catalytically inactive myotubularins. We demonstrate that human MTMR6 forms a heteromer with an enzymatically inactive member myotubularin-related protein 9 (MTMR9), both in vitro and in cells. MTMR9 increased the binding of MTMR6 to phospholipids without changing the lipid binding profile. MTMR9 increased the 3-phosphatase activity of MTMR6 up to 6-fold. We determined that MTMR6 is activated up to 28-fold in the presence of phosphatidylserine liposomes. Together, MTMR6 activity in the presence of MTMR9 and assayed in phosphatidylserine liposomes increased 84-fold. Moreover, the formation of this heteromer in cells resulted in increased protein levels of both MTMR6 and MTMR9, probably due to the inhibition of degradation of both proteins. Furthermore, co-expression of MTMR6 and MTMR9 decreased etoposide-induced apoptosis, whereas decreasing both MTMR6 and MTMR9 by RNA interference led to increased cell death in response to etoposide treatment when compared with that seen with RNA interference of MTMR6 alone. Thus, MTMR9 greatly enhances the functions of MTMR6.

Regulation of PLCβ1a membrane anchoring by its substrate phosphatidylinositol (4,5)-bisphosphate

Basic knowledge as to the subcellular location and dynamics of PLCβ isozymes is scant. Here, we report on the subcellular location of GFP-PLCβ1a and the use of total internal reflection fluorescence (TIRF) microscopy to examine the dynamics of GFP-PLCβ1a at the plasma membrane upon stimulation of Gq-coupled receptors. Using this technique, we observed PLCβ1a dissociation from the plasma membrane upon addition of agonist. An increase in intracellular calcium and a decrease in PtdIns(4,5)P2 both coincided with a translocation of PLCβ1a from the plasma membrane into the cytosol. In order to differentiate between calcium and PtdIns(4,5)P2, rapamycin-induced heterodimerization of FRB and FKBP12 fused to 5-phosphatase IV was used to instantaneously convert PtdIns(4,5)P2 into PtdIns(4)P. Addition of rapamycin caused PLCβ1a to dissociate from the plasma membrane, indicating that removal of PtdIns(4,5)P2 is sufficient to cause translocation of PLCβ1a from the plasma membrane. In conclusion, PLCβ1a localization is regulated by its own substrate.

Hypoosmotic- and pressure-induced membrane stretch activate TRPC5 channels

Transient receptor potential (TRP) channels mediate a wide array of sensory functions. We investigated the role of TRPC5, a poorly characterized channel widely expressed in the central and peripheral nervous system, as a potential osmosensory protein. Here we show that hypoosmotic stimulation activates TRPC5 channels resulting in a large calcium influx. The response to osmotically induced membrane stretch is blocked by GsMTx-4, an inhibitor of stretch activated ion channels. Direct hypoosmotic activation of TRPC5 is independent of phospholipase C function. However, the osmotic response is inhibited in a cell line in which PIP(2) levels are reduced by regulated overexpression of a lipid phosphatase. The response was restored by increasing intracellular PIP(2) levels through the patch pipette. The mechano-sensitivity of the channel was probed in the whole-cell configuration by application of steps of positive pressure through the patch pipette. Pressure-induced membrane stretch also activated TRPC5 channels, suggesting its role as a transducer of osmo-mechanical stimuli. We also demonstrated the expression of TRPC5 in sensory neurones which together with the osmo-mechanical characteristics of TRPC5 channels suggest its putative role in mechanosensory transduction events.

Structure of the myristylated human immunodeficiency virus type 2 matrix protein and the role of phosphatidylinositol-(4,5)-bisphosphate in membrane targeting

During the late phase of retroviral replication, newly synthesized Gag proteins are targeted to the plasma membrane (PM), where they assemble and bud to form immature virus particles. Membrane targeting by human immunodeficiency virus type 1 (HIV-1) Gag is mediated by the PM marker molecule phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)], which is capable of binding to the matrix (MA) domain of Gag in an extended lipid conformation and of triggering myristate exposure. Here, we show that, as observed previously for HIV-1 MA, the myristyl group of HIV-2 MA is partially sequestered within a narrow hydrophobic tunnel formed by side chains of helices 1, 2, 3, and 5. However, the myristate of HIV-2 MA is more tightly sequestered than that of the HIV-1 protein and does not exhibit concentration-dependent exposure. Soluble PI(4,5)P(2) analogs containing truncated acyl chains bind HIV-2 MA and induce minor long-range structural changes but do not trigger myristate exposure. Despite these differences, the site of HIV-2 assembly in vivo can be manipulated by enzymes that regulate PI(4,5)P(2) localization. Our findings indicate that HIV-1 and HIV-2 are both targeted to the PM for assembly via a PI(4,5)P(2)-dependent mechanism, despite differences in the sensitivity of the MA myristyl switch, and suggest a potential mechanism that may contribute to the poor replication kinetics of HIV-2.

A phosphatidylinositol phosphate-specific myo-inositol polyphosphate 5-phosphatase required for seedling growth

The phosphatidylinositol phosphate signaling pathway is involved in many crucial cellular functions. The myo-inositol polyphosphate 5-phosphatases (5PTases) (E.C. 3.1.3.56) comprise a large protein family that hydrolyze 5-phosphates from a variety of phosphatidylinositol phosphate and inositol phosphate substrates. We previously reported that the At5PTase11 enzyme (At1g47510), which is one of the smallest predicted 5PTases found in any organism, encodes an active 5PTase whose activity is restricted to tris- and bis-, but not mono-phosphorylated phosphatidylinositol phosphate substrates containing a 5-phosphate. This is in contrast to other unrestricted Arabidopsis 5PTases, which also hydrolyze tris- and bis inositol phosphate molecules. To further explore the function of At5PTase11, we have characterized two T-DNA mutants in the At5PTase11 gene, and have complemented this mutant. Seed from 5ptase11 mutants germinate slower than wildtype seed and mutant seedlings have decreased hypocotyl growth as compared to wildtype seedlings when grown in the dark. This phenotype is the opposite of the increased hypocotyl growth phenotype previously described for other 5ptase mutants defective in inositol phosphate-specific 5PTase enzymes. By labeling the endogenous myo-inositol pool in 5ptase11 mutants, we correlated these hypocotyl growth changes with a small increase in the 5PTase11 substrate, phosphatidylinositol (4,5) bisphosphate, and decreases in the potential products of 5PTase11, phosphatidylinositol (3) phosphate and phosphatidylinositol (4) phosphate. Surprisingly, we also found that dark-grown 5ptase11 mutants contain increases in inositol (1,4,5) trisphosphate and an inositol bisphosphate that is not a substrate for recombinant 5PTase11. We present a model for regulation of hypocotyl growth by specific molecules found in this pathway.

Soft tissue sarcoma cells are highly sensitive to AKT blockade: a role for p53-independent up-regulation of GADD45 alpha

The AKT signaling pathway is activated in soft tissue sarcoma (STS). However, AKT blockade has not yet been studied as a potential targeted therapeutic approach. Here, we examined the in vitro and in vivo effects of AKT inhibition in STS cells. Western blot analysis was used to evaluate the expression of AKT pathway components and the effect of AKT stimulation and inhibition on their phosphorylation. Cell culture assays were used to assess the effect of AKT blockade (using a phosphatidylinositol 3-kinase inhibitor and a specific AKT inhibitor) on STS cell growth, cell cycle, and apoptosis. Oligoarrays were used to determine gene expression changes in response to AKT inhibition. Reverse transcription-PCR was used for array validation. Specific small inhibitory RNA was used to knockdown GADD45 alpha. Human STS xenografts in nude mice were used for in vivo studies, and immunohistochemistry was used to assess the effect of treatment on GADD45 alpha expression, proliferation, and apoptosis. Multiple STS cell lines expressed activated AKT. AKT inhibition decreased STS downstream target phosphorylation and growth in vitro; G(2) cell cycle arrest and apoptosis were also observed. AKT inhibition induced GADD45 alpha mRNA and protein expression in all STS cells treated independent of p53 mutational status. GADD45 alpha knockdown attenuated the G(2) arrest induced by AKT inhibition. In vivo, AKT inhibition led to decreased STS xenograft growth. AKT plays a critical role in survival and proliferation of STS cells. Modulation of AKT kinase activity may provide a novel molecularly based strategy for STS-targeted therapies.