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Anirban Siddhanta

Bio: Anirban Siddhanta is an academic researcher from University of Calcutta. The author has contributed to research in topics: Golgi apparatus & Phosphatidic acid. The author has an hindex of 11, co-authored 17 publications receiving 967 citations. Previous affiliations of Anirban Siddhanta include Albert Einstein College of Medicine.

Papers
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Journal ArticleDOI
TL;DR: It is demonstrated that immunoaffinity-purified human PLD1 stimulated nascent secretory vesicle budding from the TGN and ARF-1 stimulated endogenous PLD activity in Golgi membranes approximately threefold and this activation correlated with its enhancement of vesicles budding.
Abstract: Phospholipase D (PLD) is a phospholipid hydrolyzing enzyme whose activation has been implicated in mediating signal transduction pathways, cell growth, and membrane trafficking in mammalian cells. Several laboratories have demonstrated that small GTP-binding proteins including ADP-ribosylation factor (ARF) can stimulate PLD activity in vitro and an ARF-activated PLD activity has been found in Golgi membranes. Since ARF-1 has also been shown to enhance release of nascent secretory vesicles from the TGN of endocrine cells, we hypothesized that this reaction occurred via PLD activation. Using a permeabilized cell system derived from growth hormone and prolactin-secreting pituitary GH3 cells, we demonstrate that immunoaffinity-purified human PLD1 stimulated nascent secretory vesicle budding from the TGN approximately twofold. In contrast, a similarly purified but enzymatically inactive mutant form of PLD1, designated Lys898Arg, had no effect on vesicle budding when added to the permeabilized cells. The release of nascent secretory vesicles from the TGN was sensitive to 1% 1-butanol, a concentration that inhibited PLD-catalyzed formation of phosphatidic acid. Furthermore, ARF-1 stimulated endogenous PLD activity in Golgi membranes approximately threefold and this activation correlated with its enhancement of vesicle budding. Our results suggest that ARF regulation of PLD activity plays an important role in the release of nascent secretory vesicles from the TGN.

258 citations

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TL;DR: The data show that the intracellular localization of PLD1 is consistent with a role in vesicle trafficking from the Golgi apparatus and suggest that it also functions in the cell nucleus.
Abstract: Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid. In mammalian cells this reaction has been implicated in the recruitment of coatomer to Golgi membranes and release of nascent secretory vesicles from the trans-Golgi network. These observations suggest that PLD is associated with the Golgi complex; however, to date, because of its low abundance, the intracellular localization of PLD has been characterized only indirectly through overexpression of chimeric proteins. We have used highly sensitive antibodies to PLD1 together with immunofluorescence and immunogold electron microscopy as well as cell fractionation to identify the intracellular localization of endogenous PLD1 in several cell types. Although PLD1 had a diffuse staining pattern, it was enriched significantly in the Golgi apparatus and was also present in cell nuclei. On fragmentation of the Golgi apparatus by treatment with nocodazole, PLD1 closely associated with membrane fragments, whereas after inhibition of PA synthesis, PLD1 dissociated from the membranes. Overexpression of an hemagglutinin-tagged form of PLD1 resulted in displacement of the endogenous enzyme from its perinuclear localization to large vesicular structures. Surprisingly, when the Golgi apparatus collapsed in response to brefeldin A, the nuclear localization of PLD1 was enhanced significantly. Our data show that the intracellular localization of PLD1 is consistent with a role in vesicle trafficking from the Golgi apparatus and suggest that it also functions in the cell nucleus.

178 citations

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TL;DR: The data demonstrate that in mammalian cells accumulation of PA rather than DAG is a key step in regulating budding of secretory vesicles from the trans-Golgi network.

138 citations

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TL;DR: The results suggest that PA stimulation of PtdIns(4,5)P2 synthesis is required for maintaining the structural integrity and function of the Golgi apparatus.

113 citations

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TL;DR: Recent observations on PLD localization to the Golgi apparatus are discussed and how members of this enzyme family might play a role in regulating the structure of this organelle.

79 citations


Cited by
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Journal ArticleDOI
TL;DR: The current knowledge of the CD28 and CTLA‐4 signaling mechanisms [involving phosphatidylinositol 3 kinase (PI3K), growth factor receptor‐bound protein 2 (Grb2), Filamin A, protein kinase C θ (PKCθ), and phosphatases] that control T‐cell immunity are outlined.
Abstract: T-cell activation is mediated by antigen-specific signals from the TCRζ/CD3 and CD4–CD8–p56lck complexes in combination with additional co-signals provided by coreceptors such as CD28, inducible costimulator (ICOS), cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death (PD-1), and others. CD28 and ICOS provide positive signals that promote and sustain T-cell responses, while CTLA-4 and PD-1 limit responses. The balance between stimulatory and inhibitory co-signals determines the ultimate nature of T-cell responses where response to foreign pathogen is achieved without excess inflammation and autoimmunity. In this review, we outline the current knowledge of the CD28 and CTLA-4 signaling mechanisms [involving phosphatidylinositol 3 kinase (PI3K), growth factor receptor-bound protein 2 (Grb2), Filamin A, protein kinase C θ (PKCθ), and phosphatases] that control T-cell immunity. We also present recent findings on T-cell receptor-interacting molecule (TRIM) regulation of CTLA-4 surface expression, and a signaling pathway involving CTLA-4 activation of PI3K and protein kinase B (PKB)/AKT by which cell survival is ensured under conditions of anergy induction.

739 citations

Journal ArticleDOI
TL;DR: Abnormal choline metabolism is emerging as a metabolic hallmark that is associated with oncogenesis and tumour progression, and increased levels of these compounds provide a non-invasive biomarker of transformation, staging and response to therapy.
Abstract: Abnormal choline metabolism is emerging as a metabolic hallmark that is associated with oncogenesis and tumour progression. Following transformation, the modulation of enzymes that control anabolic and catabolic pathways causes increased levels of choline-containing precursors and breakdown products of membrane phospholipids. These increased levels are associated with proliferation, and recent studies emphasize the complex reciprocal interactions between oncogenic signalling and choline metabolism. Because choline-containing compounds are detected by non-invasive magnetic resonance spectroscopy (MRS), increased levels of these compounds provide a non-invasive biomarker of transformation, staging and response to therapy. Furthermore, enzymes of choline metabolism, such as choline kinase, present novel targets for image-guided cancer therapy.

650 citations

Journal ArticleDOI
08 Aug 2003-Cell
TL;DR: It is proposed that PI4KIIalpha establishes the Golgi's unique lipid-defined organelle identity by generating PI(4)P-rich domains that specify the docking of the AP-1 coat machinery.

530 citations

Journal ArticleDOI
TL;DR: This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel.
Abstract: Secretory granules are specialized intracellular organelles that serve as a storage pool for selected secretory products. The exocytosis of secretory granules is markedly amplified under physiologically stimulated conditions. While granules have been recognized as post-Golgi carriers for almost 40 years, the molecular mechanisms involved in their formation from the trans-Golgi network are only beginning to be defined. This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel. In the first half of the review, the emerging role of immature secretory granules in protein sorting is highlighted. The second half of the review summarizes what is known about the composition of granule membranes. The numerous similarities and relatively limited differences identified between granule membranes and other vesicular carriers that convey products to and from the plasmalemma, serve as a basis for examining how granule membrane composition might be established and how its unique functions interface with general post-Golgi membrane traffic. Studies of granule formation in vitro offer additional new insights, but also important challenges for future efforts to understand how regulated secretory pathways are constructed and maintained.

514 citations

Journal ArticleDOI
TL;DR: Common themes of localized signal generation and the spatially localized recruitment of effector proteins appear to underlie mechanisms employed in signal transduction, cytoskeletal, and membrane trafficking events.
Abstract: Signaling roles for phosphoinositides that involve their regulated hydrolysis to generate second messengers have been well characterized. Recent work has revealed additional signaling roles for phosphoinositides that do not involve their hydrolysis. PtdIns 3-P, PtdIns 3,4,5-P3, and PtdIns 4,5-P2 function as site-specific signals on membranes that recruit and/or activate proteins for the assembly of spatially localized functional complexes. A large number of phosphoinositide-binding proteins have been identified as the potential effectors for phosphoinositide signals. Common themes of localized signal generation and the spatially localized recruitment of effector proteins appear to underlie mechanisms employed in signal transduction, cytoskeletal, and membrane trafficking events.

493 citations