Phosphoinositide signaling pathways in nuclei are associated with nuclear speckles containing pre-mRNA processing factors

The nucleus of HeLa cells contains tubular structures for Ca2+ signaling with the involvement of mitochondria

Calcium is an important messenger that controls many nuclear functions such as gene expression in mammalian cells but the regulation of nuclear Ca(2+) remains unclear. It has long been thought that Ca(2+) is translocated from the cytosol by a long distance to the nucleus through the nuclear pore complexes to activate or suppress gene transcription. However, this model is at best an incomplete one. With an aid of confocal and transmission electron microscopy, we demonstrated here that tubules, in a vertical or horizontal orientation, extended deep inside the nucleus of HeLa cells. These nuclear tubules (NTs) are double-membraned invaginations of the nuclear envelope and are usually associated with nucleolus. Also, membrane bound vesicles are found inside and inositol 1,4,5 trisphosphate (IP(3)) receptors are enriched in some but not all of these tubular structures. Interestingly, shuttling of mitochondria was observed in the NT and cytoplasm of the HeLa cells loaded with dihydro-rhod-2/AM. After stimulation with histamine that increases cytosolic [Ca(2+)] through IP(3) production, a slow rise of dihydro-rhod-2 fluorescence for the measurement of intra-mitochondrial Ca(2+) was observed in the area of NT indicating that Ca(2+) was sequestered by mitochondria inside the tubular invagination. Our work therefore suggests that the NTs and mitochondrial activities represent a specialized compartment and dynamic process involved in the regulation of Ca(2+) inside the cell nucleus.

Regulation of PI4,5P2 synthesis by nuclear-cytoplasmic shuttling of the Mss4 lipid kinase

The essential phospholipid PI4,5P(2) is generated by a well conserved PI4P 5-kinase, Mss4, in yeast. Balanced production and turnover of PI4,5P(2) is important for normal organization of the actin cytoskeleton and cell viability. Previous studies have shown that multiple PI phosphatases can regulate PI4,5P(2) levels. We report a new, unexpected regulatory mechanism for PI4,5P(2) homeostasis, directed by nuclear-cytoplasmic shuttling of the lipid kinase. We show that Mss4 is a phosphoprotein, which contains a functional nuclear localization signal (NLS) and can shuttle between the cytoplasm and the nucleus. Temperature-conditional mss4 cells that accumulate Mss4 protein in the nucleus exhibit reduced levels of PI4,5P(2), depolarization of the actin cytoskeleton and a block in Mss4 phosphorylation, suggesting an essential role for phosphorylated Mss4 at the plasma membrane. Through the isolation of gene dosage-dependent suppressors of mss4 mutants, we identified Bcp1, a protein enriched in the nucleus, which is required for Mss4 nuclear export and is related to the mammalian BRCA2-interacting protein BCCIP. Together, these studies suggest a new mechanism for lipid kinase regulation through regulated nuclear-cytoplasmic shuttling.

DYRK1A accumulates in splicing speckles through a novel targeting signal and induces speckle disassembly

The protein kinase DYRK1A is distributed throughout the nucleoplasm, accumulating in speckle-like regions. We have found that this punctuated nuclear distribution is determined by the contribution of several elements. Although the nuclear import is mediated by two distinct nuclear localization signals, one at the N-terminus and the other located in the linker region, between subdomains X and XI of the catalytic domain, the accumulation in speckles that are SC35 positive depends on a sequence motif that is located C-terminal to the kinase domain and comprises a histidine tail. A similar sequence is also responsible for the targeting of cyclin T1. Therefore the histidine-rich region represents a novel splicing speckle targeting signal. Moreover, overexpression of DYRK1A induces speckle disassembly. Such disassembly is DYRK1A activity specific, since the overexpression of a DYRK1A kinase inactive mutant, the paralogous DYRK1B or a chimeric protein DYRK1B that has been directed to the speckles via the DYRK1A targeting signal, leaves the SC35 speckle pattern untouched. Thus DYRK1A protein kinase may play a role in regulating the biogenesis of the splicing speckle compartment.

Nuclear speckles: a model for nuclear organelles

Speckles are subnuclear structures that are enriched in pre-messenger RNA splicing factors and are located in the interchromatin regions of the nucleoplasm of mammalian cells. At the fluorescence-microscope level they appear as irregular, punctate structures, which vary in size and shape, and when examined by electron microscopy they are seen as clusters of interchromatin granules. Speckles are dynamic structures, and both their protein and RNA-protein components can cycle continuously between speckles and other nuclear locations, including active transcription sites. Studies on the composition, structure and behaviour of speckles have provided a model for understanding the functional compartmentalization of the nucleus and the organization of the gene-expression machinery.

The PHD finger of the chromatin-associated protein ING2 functions as a nuclear phosphoinositide receptor

Phosphoinositides (PtdInsPs) play critical roles in cytoplasmic signal transduction pathways. However, their functions in the nucleus are unclear, as specific nuclear receptors for PtdInsPs have not been identified. Here, we show that ING2, a candidate tumor suppressor protein, is a nuclear PtdInsP receptor. ING2 contains a plant homeodomain (PHD) finger, a motif common to many chromatin-regulatory proteins. We find that the PHD fingers of ING2 and other diverse nuclear proteins bind in vitro to PtdInsPs, including the rare PtdInsP species, phosphatidylinositol 5-phosphate (PtdIns(5)P). Further, we demonstrate that the ING2 PHD finger interacts with PtdIns(5)P in vivo and provide evidence that this interaction regulates the ability of ING2 to activate p53 and p53-dependent apoptotic pathways. Together, our data identify the PHD finger as a phosphoinositide binding module and a nuclear PtdInsP receptor, and suggest that PHD-phosphoinositide interactions directly regulate nuclear responses to DNA damage.

Diacylglycerol kinase-theta is localized in the speckle domains of the nucleus

It is well established that the nucleus is endowed with enzymes that are involved in lipid-dependent signal transduction pathways. Diacylglycerol (DAG) is a fundamental lipid second messenger that is produced in the nucleus. Previous reports have shown that the nucleus contains diacylglycerol kinases (DGKs), i.e., the enzymes that, by converting DAG into phosphatidic acid (PA), terminate DAG-dependent events. Here, we show, by immunofluorescence staining and confocal analysis, that DGK-theta localizes mainly to the nucleus of various cell lines, such as MDA-MB-453, MCF-7, PC12, and HeLa. Nuclear DGK-theta co-localizes with phosphatidylinositol 4,5-bisphosphate (PIP(2)) in domains that correspond to nuclear speckles, as revealed by the use of an antibody to the splicing factor SC-35, a well-established marker for these structures. The spatial distribution of nuclear DGK-theta was dynamic in that it was affected by inhibition of mRNA transcription with alpha-amanitin. Immuno-electron microscopy analysis demonstrated that DGK-theta, PIP(2), and phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1) associated with electron-dense particles within the nucleus that correspond to interchromatin granule clusters. Cell fractionation experiments performed in MDA-MB-453, HeLa, and PC12 cells showed a preferential association of DGK-theta with the nucleus. Western blots demonstrated that DGK-theta was enriched in the nuclear matrix fraction prepared from MDA-MB-453 cells. Immunoprecipitation experiments with an antibody to PLCbeta1 revealed in MDA-MB-453 cells an association between this enzyme and both DGK-theta and phosphatidylinositol phosphate kinase Ialpha (PIPKIalpha). Our findings strengthen the contention that speckles represent a crucial site for the nuclear-based inositol lipid cycle. We may speculate that nuclear speckle-located DGK-theta, on cell stimulation with an agonist, converts to PA the DAG derived from PLCbeta1-dependent PIP(2) hydrolysis.

Membrane ruffling requires coordination between type Ialpha phosphatidylinositol phosphate kinase and Rac signaling

Membrane ruffle formation requires remodeling of cortical actin filaments, a process dependent upon the small G-protein Rac. Growth factors stimulate actin remodeling and membrane ruffling by integration of signaling pathways that regulate actin-binding proteins. Phosphatidylinositol 4,5-bisphosphate (PIP2) regulates the activity of many actin-binding proteins and is produced by the type I phosphatidylinositol phosphate kinases (PIPKIs). Here we show in MG-63 cells that only the PIPKIalpha isoform is localized to platelet-derived growth factor (PDGF)-induced membrane ruffles. Further, expression of kinase dead PIPKIalpha, which acts as a dominant negative mutant, blocked membrane ruffling, suggesting that PIPKIalpha and PIP2 participate in ruffling. To explore this, PIPKIalpha was overexpressed in serum-starved cells and stimulated with PDGF. In serum-starved cells, PIPKIalpha expression did not stimulate actin remodeling, but when these cells were stimulated with PDGF, actin rapidly reorganized into foci but not membrane ruffles. PIPKIalpha-mediated formation of actin foci was independent of both Rac1 and phosphatidylinositol 3-kinase activities. Significantly, coexpression of dominant active Rac1 with PIPKIalpha in PDGF-stimulated cells resulted in membrane ruffling. The PDGF- and Rac1-stimulated ruffling was inhibited by expression of kinase-dead PIPKIalpha. Combined, these data support a model where the localized production of PIP2 by PIPKIalpha is necessary for actin remodeling, whereas formation of membrane ruffles required Rac signaling.

Use of expression constructs to dissect the functional domains of the CHS/beige protein: identification of multiple phenotypes

The Chediak-Higashi Syndrome (CHS) and the orthologous murine disorder beige are characterized at the cellular level by the presence of giant lysosomes. The CHS1/Beige protein is a 3787 amino acid protein of unknown function. To determine functional domains of the CHS1/Beige protein, we generated truncated constructs of the gene/protein. These truncated proteins were transiently expressed in Cos-7 or HeLa cells and their effect on membrane trafficking was examined. Beige is apparently a cytosolic protein, as are most transiently expressed truncated Beige constructs. Expression of the Beige construct FM (amino acids 1-2037) in wild-type cells led to enlarged lysosomes. Similarly, expression of a 5.5-kb region (amino acids 2035-3787) of the carboxyl terminal of Beige (22B) also resulted in enlarged lysosomes. Expression of FM solely affected lysosome size, whereas expression of 22B led to alterations in lysosome size, changes in the Golgi and eventually cell death. The two constructs could be used to further dissect phenotypes resulting from loss of the Beige protein. CHS or beigej fibroblasts show an absence of nuclear staining using a monoclonal antibody directed against phosphatidylinositol 4,5 bisphosphate [PtdIns(4,5) P2]. Transformation of beige j fibroblasts with a YAC containing the full-length Beige gene resulted in the normalization of lysosome size and nuclear PtdIns(4,5)P2 staining. Expression of the carboxyl dominant negative construct 22B led to loss of nuclear PtdIns(4,5)P2 staining. Expression of the FM dominant negative clone did not alter nuclear PtdIns(4,5) P2 localization. These results suggest that the Beige protein interacts with at least two different partners and that the Beige protein affects cellular events, such as nuclear PtdIns(4,5)P2 localization, in addition to lysosome size.

Profilin I colocalizes with speckles and Cajal bodies: a possible role in pre-mRNA splicing

Profilin is one of the major components controlling actin polymerization. Here, profilin I was located in fibroblasts and HeLa cells by the use of two different sets of affinity-purified antibodies. Both antibody preparations labeled nuclei in a speckle-like pattern and displayed extensive colocalization with small nuclear ribonucleoprotein particle (snRNP)-core proteins and p80 coilin-containing Cajal bodies. Treatment with actinomycin D led to largely similar reorganizations of snRNPs and profilin, while profilin and Cajal bodies separated under these conditions. One of the profilin antibodies interfered with pre-mRNA splicing in vitro, further indicating a role for profilin during pre-mRNA processing.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Genetic evidence for a role of phospholipase C at the budding yeast kinetochore

Chromosome segregation during mitosis requires kinetochores, specialized organelles that mediate chromosome attachment to spindle microtubules. We have shown previously that in budding yeast, Plc1p (phosphoinositide-specific phospholipase C) localizes to centromeric loci, associates with the kinetochore proteins Ndc10p and Cep3p, and affects the function of kinetochores. Deletion of PLC1 results in nocodazole sensitivity, mitotic delay, and a higher frequency of chromosome loss. We report here that despite the nocodazole sensitivity of plc1Delta cells, Plc1p is not required for the spindle checkpoint. However, plc1Delta cells require a functional BUB1/BUB3-dependent spindle checkpoint for viability. PLC1 displays strong genetic interactions with genes encoding components of the inner kinetochore, including NDC10, SKP1, MIF2, CEP1, CEP3, and CTF13. Furthermore, plc1Delta cells display alterations in chromatin structure in the core centromere. Chromatin immunoprecipitation experiments indicate that Plc1p localizes to centromeric loci independently of microtubules, and accumulates at the centromeres during G(2)/M stage of cell cycle. These results are consistent with the view that Plc1p affects kinetochore function, possibly by modulating the structure of centromeric chromatin.

Mammalian stress granules represent sites of accumulation of stalled translation initiation complexes

In eukaryotic cells subjected to environmental stress, untranslated mRNA accumulates in discrete cytoplasmic foci that have been termed stress granules. Recent studies have shown that in addition to mRNA, stress granules also contain 40S ribosomal subunits and various translation initiation factors, including the mRNA binding proteins eIF4E and eIF4G. However, eIF2, the protein that transfers initiator methionyl-tRNA(i) (Met-tRNA(i)) to the 40S ribosomal subunit, has not been detected in stress granules. This result is surprising because the eIF2. GTP. Met-tRNA(i) complex is thought to bind to the 40S ribosomal subunit before the eIF4G. eIF4E. mRNA complex. In the present study, we show in both NIH-3T3 cells and mouse embryo fibroblasts that stress granules contain not only eIF2 but also the guanine nucleotide exchange factor for eIF2, eIF2B. Moreover, we show that phosphorylation of the alpha-subunit of eIF2 is necessary and sufficient for stress granule formation during the unfolded protein response. Finally, we also show that stress granules contain many, if not all, of the components of the 48S preinitiation complex, but not 60S ribosomal subunits, suggesting that they represent stalled translation initiation complexes.

Epsins: adaptors in endocytosis?

Endocytic adaptor proteins select specific cargo for internalization by endocytosis through clathrin-coated pits or vesicles. Recent studies indicate that epsins might also be classified as adaptors.

Inhibition of phosphatidylinositol-4-phosphate 5-kinase Ialpha impairs localized actin remodeling and suppresses phagocytosis

Actin polymerization drives the extension of pseudopods required for phagocytosis. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is thought to play a central role in this process, because it interacts with several actin-regulatory proteins and undergoes acute and localized changes at sites of phagocytosis. We therefore studied whether phosphatidylinositol-4-phosphate 5-kinase (PIPK), the enzyme responsible for the generation of PIP(2) from phosphatidylinositol 4-phosphate, is involved in the control of phagocytosis. PIPKIalpha was found to accumulate transiently on forming phagosomes. To test the functional involvement of PIPKIalpha in particle engulfment, we generated a double mutant (D309N/R427Q) that lacks kinase activity. When ectopically expressed in cultured cells, this mutant is targeted to the plasma membrane and accumulates at the phagosomal cup during particle engulfment. Expression of PIP5KIalpha D309N/R427Q impaired phagocytosis in RAW264.7 macrophages and in engineered phagocytes generated by transfection of Fc receptors in Chinese hamster ovary cells. Inhibition of phagocytosis could not be attributed to defects in particle binding or receptor clustering, which was monitored using green fluorescent protein-tagged Fcgamma receptors. Instead, expression of the inactive kinase diminished the accumulation of PIP(2) and of F-actin in the phagosomal cup. These data suggest that PIPKIalpha activity is involved in the actin remodeling that is a prerequisite for efficient phagocytosis. PIPKIalpha appears to contribute to the transient changes in PIP(2) levels that are associated with, and likely required for, the recruitment and regulation of actin-modulating proteins.

Phospholipase D2 is localized to the rims of the Golgi apparatus in mammalian cells

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid, a molecule known to have multiple physiological roles, including release of nascent secretory vesicles from the trans-Golgi network. In mammalian cells two forms of the enzyme, PLD1 and PLD2, have been described. We recently demonstrated that PLD1 is localized to the Golgi apparatus, nuclei, and to a lesser extent, plasma membrane. Due to its low abundance, the intracellular localization of PLD2 has been characterized only indirectly through overexpression of chimeric proteins. Using antibodies specific to PLD2, together with immunofluorescence microscopy, herein we demonstrate that a significant fraction of endogenous PLD2 localized to the perinuclear Golgi region and was also distributed throughout cells in dense cytoplasmic puncta; a fraction of which colocalized with caveolin-1 and the plasma membrane. On treatment with brefeldin A, PLD2 translocated into the nucleus in a manner similar to PLD1, suggesting a potential role in nuclear signaling. Most significantly, cryoimmunogold electron microscopy demonstrated that in pituitary GH(3) cells >90% of PLD2 present in the Golgi apparatus was localized to cisternal rims and peri-Golgi vesicles exclusively. The data are consistent with a model whereby PLD2 plays a role in Golgi vesicular transport.

The nuclear phosphoinositide 3-kinase/AKT pathway: a new second messenger system

Lipid second messengers, particularly those derived from the polyphosphoinositide cycle, play a pivotal role in several cell signaling networks. Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in a plethora of cell functions. One of the best-characterized targets of PI3K lipid products is the serine/threonine protein kinase Akt. Recent findings have implicated Akt in cancer progression because it stimulates cell proliferation and suppresses apoptosis. Evidence accumulated over the past 15 years has highlighted the presence of an autonomous nuclear inositol lipid metabolism, and suggests that lipid molecules are important components of signaling pathways operating within the nucleus. PI3Ks, their lipid products, and Akt have also been identified at the nuclear level. In this review, we shall summarize the most updated findings about these molecules in relationship with the nuclear compartment and provide an overview of the possible mechanisms by which they regulate important cell functions.

Nuclear lipid signaling

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.

Phosphatidylinositol 3-kinases are involved in the all-trans retinoic acid-induced upregulation of CD38 antigen on human haematopoietic cells

All-trans retinoic acid (ATRA) is a specific inducer of CD38 antigen on marrow CD34+ cells as well as on blast cells in acute promyelocytic and myeloblastic leukaemia. The CD38 antigen contributes to the control of blast cell proliferation, and the upregulation of CD38 might constitute an element in the pathogenesis of retinoic acid syndrome. The aim of this study was to determine whether phosphoinositide 3-kinase (PI3-K) is involved in the modification of CD38 antigen expression on myeloid cells, as PI3-K plays a major role in the ATRA-induced granulocytic differentiation of HL-60 cells. We evaluated the effects of PI3-K inhibitors (wortmannin and LY294002) on the levels of CD38 antigen and mRNA in HL-60 and normal marrow CD34+ cells exposed to ATRA (1 micromol/l). The inhibitors prevented increase in CD38 mRNA expression and the overexpression of membrane CD38 antigen, without modification of the cytoplasmic level of this antigen. Interestingly, PI3-K activity was also necessary for CD38 expression on normal marrow CD34+ cells and for the ATRA-induced upregulation of CD157, a CD38-related antigen. In conclusion, PI3-K activity plays an essential role in the regulation of CD38 expression on human haematopoietic cells, and might constitute an interesting therapeutic target in haematological disorders involving CD38 overexpression.

Modulation of cardiac PIP2 by cardioactive hormones and other physiologically relevant interventions

Phosphatidylinositol 4,5-bisphosphate (PIP2) affects profoundly several cardiac ion channels and transporters, and studies of PIP2-sensitive currents in excised patches suggest that PIP2 can be synthesized and broken down within 30 s. To test when, and if, total phosphatidylinositol 4-phosphate (PIP) and PIP(2) levels actually change in intact heart, we used a new, nonradioactive HPLC method to quantify anionic phospholipids. Total PIP and PIP2 levels (10-30 micromol/kg wet weight) do not change, or even increase, with activation of Galpha(q)/phospholipase C (PLC)-dependent pathways by carbachol (50 microM), phenylephrine (50 microM), and endothelin-1 (0.3 microM). Adenosine (0.2 mM) and phorbol 12-myristate 13-acetate (1microM) both cause 30% reduction of PIP2 in ventricles, suggesting that diacylglycerol (DAG)-dependent mechanisms negatively regulate cardiac PIP2. PIP2, but not PIP, increases reversibly by 30% during electrical stimulation (2 Hz for 5 min) in guinea pig left atria; the increase is blocked by nickel (2 mM). Both PIP and PIP2 increase within 3 min in hypertonic solutions, roughly in proportion to osmolarity, and similar effects occur in multiple cell lines. Inhibitors of several volume-sensitive signaling mechanisms do not affect these responses, suggesting that PIP2 metabolism might be sensitive to membrane tension, per se.

HIV-1 Tat protein-mediated transactivation of the HIV-1 long terminal repeat promoter is potentiated by a novel nuclear Tat-interacting protein of 110 kDa, Tip110

Human immunodeficiency virus type 1 (HIV-1) gene expression and replication is highly dependent on and modulated by interactions between viral and host cellular factors. Tat protein, encoded by one of the HIV-1 regulatory genes, tat, is essential for HIV-1 gene expression. A number of host cellular factors have been shown to interact with Tat in this process. During our attempts to determine the molecular mechanisms of Tat interaction with brain cells, we isolated a cDNA clone that encodes a novel Tat-interacting protein of 110 kDa or Tip110 from a human fetal brain cDNA library. GenBank BLAST search revealed that Tip110 was almost identical to a previously cloned KIAA0156 gene with unknown functions. In vivo binding of Tip110 with Tat was confirmed by immunoprecipitation and Western blotting, in combination with mutagenesis. The yeast three-hybrid RNA-protein interaction assay indicated no direct interaction of Tip110 with Tat transactivating response element RNA. Nevertheless, Tip110 strongly synergized with Tat on Tat-mediated chloramphenicol acetyltransferase reporter gene expression and HIV-1 virus production, whereas down-modulation of constitutive Tip110 expression inhibited HIV-1 virus production. Northern blot analysis showed that Tip110 mRNA was expressed in a variety of human tissues and cells. Moreover, digital fluorescence microscopic imaging revealed that Tip110 was expressed exclusively in the nucleus, and within a nuclear speckle structure that has recently been described for human cyclin T and CDK9, two critical components for Tat transactivation function on HIV-1 long terminal repeat promoter. Taken together, these data demonstrate that Tip110 regulates Tat transactivation activity through direct interaction, and suggest that Tip110 is an important cellular factor for HIV-1 gene expression and viral replication.

A monoclonal antibody to visualize PtdIns(3,4,5)P(3) in cells

Phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] is a second messenger produced in response to agonist stimulation. Traditionally, visualization of phosphoinositide polyphosphates (PtdInsP(n)) in living cells is accomplished using chimeric green fluorescent protein (GFP)-pleckstrin homology (PH) domain proteins, while PtdInsP(n) quantitation is accomplished by extraction and separation of radiolabeled cellular PtdInsP(n)s. Here we describe preparation of a covalent protein-PtdIns(3,4,5)P(3) immunogen, characterization of binding selectivity of an anti-PtdIns(3,4,5)P(3) IgM, and immunodetection of PtdIns(3,4,5)P(3) in stimulated mammalian cells. This antibody has greater than three orders of magnitude selectivity for binding PtdIns(3,4,5)P(3) relative to its precursor, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), and is therefore optimal for studies of cell function. The immunodetection in platelet-derived growth factor (PDGF)-stimulated NIH 3T3 cells was benchmarked against HPLC analysis of [3H]-myo-inositol-labeled cellular PtdInsP(n)s. In addition, the changes in subcellular amounts and localizations of both PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2) in stimulated NIH 3T3 fibroblasts and human neutrophils were observed by immunofluorescence. In insulin- or PDGF-stimulated fibroblasts, PtdIns(3,4,5)P(3) levels increased in the cytoplasm, peaking at 10 min. In contrast, increases in the PtdIns(4,5)P(2) levels were detected in nuclei, corresponding to the production of new substrate following depletion by phosphoinositide (PI) 3-kinase.

Nuclear localization of phosphatidylinositol 4-kinase β

Whereas most phosphatidylinositol 4-kinase (PtdIns 4-kinase) activity is localized in the cytoplasm, PtdIns 4-kinase activity has also been detected in membranedepleted nuclei of rat liver and mouse NIH 3T3 cells. Here we have characterized the PtdIns 4-kinase that is present in nuclei from NIH 3T3 cells. Both type II and type III PtdIns 4-kinase activity were observed in the detergent-insoluble fraction of NIH 3T3 cells. Dissection of this fraction into cytoplasmic actin filaments and nuclear lamina-pore complexes revealed that the actin filament fraction contains solely type II PtdIns 4-kinase,whereas lamina-pore complexes contain type III PtdIns 4-kinase activity. Using specific antibodies, the nuclear PtdIns 4-kinase was identified as PtdIns 4-kinase β. Inhibition of nuclear export by leptomycin B resulted in an accumulation of PtdIns 4-kinase β in the nucleus. These data demonstrate that PtdIns 4-kinase β is present in the nuclei of NIH 3T3 fibroblasts,suggesting a specific function for this kinase in nuclear processes.

Type I PIPkinases interact with and are regulated by the retinoblastoma susceptibility gene product-pRB

Inositide signaling at the plasma membrane has been implicated in the regulation of numerous cellular processes including cytoskeletal dynamics, vesicle trafficking, and gene transcription. Studies have also shown that a distinct inositide pathway exists in nuclei, where it may regulate nuclear processes such as mRNA export, cell cycle progression, gene transcription, and DNA repair. We previously demonstrated that nuclear PtdIns(4,5)P(2) synthesis is stimulated during progression from G1 through S phase, although mechanistic details of how cell cycle progression impinges on the regulation of nuclear inositides is unknown. In this study, we demonstrate that pRB, which regulates progression of cells from G1 through S phase interacts both in vitro and in vivo with Type I PIPkinases, the enzymes responsible for nuclear PtdIns(4,5)P(2) synthesis. Moreover, this interaction stimulates the activity of Type Ialpha PIPkinase in an in vitro assay. Using murine erythroleukamia (MEL) cells expressing a temperature-sensitive mutant of large T antigen (LTA), we demonstrate changes in vivo in nuclear PtdIns(4,5)P(2) levels that are consistent with the ability of LTA to disrupt pRB/Type I interactions. This study, for the first time, provides a potential mechanism for how cell cycle progression could regulate the levels of nuclear inositides.

Lipid research picks up speed on the slopes of Taos

The 2002 Keystone Symposium on "Regulation of Cellular Responses by Lipid Mediators" provided a lively and active forum to discuss research in lipid signaling. This meeting review can provide only a glimpse into the diversity of research presented. Here we have chosen to highlight a group of exciting presentations describing novel features of the temporal and spatial regulation of phosphoinositides and their downstream targets.

Nuclear phosphoinositides could bring FYVE alive

Phosphoinositide signalling systems exist in all eukaryotes. A high degree of evolutionary conservation is found at the functional level, but distinct phylogenetic differences are also becoming evident. Although the nuclear phosphoinositide system is likely to be a primordial forerunner of the plasma membrane system, relatively little is known about it. However, nuclear phosphoinositides might have far more diverse roles than hitherto envisaged and interact specifically with regulatory proteins containing phosphoinositide-binding domains. A novel family of proteins, so far only identified in plants, display domain structures that might link phosphoinositide metabolism to nuclear function in an unexpected way.

Stereo-specific substrate recognition by phosphatidylinositol phosphate kinases is swapped by changing a single amino acid residue

Type I and type II phosphatidylinositol phosphate (PIP) kinases generate the lipid second messenger phosphatidylinositol (PtdIns) 4,5-bisphosphate and thus play fundamental roles in the regulation of many cellular processes. Although the two kinase families are highly homologous, they phosphorylate distinct substrates and are functionally non-redundant. Type I PIP kinases phosphorylate PtdIns 4-phosphate at the D-5 hydroxyl group and are consequently PtdIns 4-phosphate 5-kinases. By contrast, type II PIP kinases are PtdIns 5-phosphate 4-kinases that phosphorylate PtdIns 5-phosphate at the D-4 position. Type I PIP kinases, in addition, also phosphorylate other phosphoinositides in vitro and in vivo and thus have the potential to generate multiple lipid second messengers. To understand how these enzymes differentiate between stereoisomeric substrates, we used a site-directed mutagenesis approach. We show that a single amino acid substitution in the activation loop, A381E in IIbeta and the corresponding mutation E362A in Ibeta, is sufficient to swap substrate specificity between these PIP kinases. In addition to its role in substrate specificity, the type I activation loop is also key in subcellular targeting. The Ibeta(E362A) mutant and other mutants with reduced PtdIns 4-phosphate binding affinity were largely cytosolic when expressed in mammalian cells in contrast to wild-type Ibeta which targets to the plasma membrane. These results clearly establish the role of the activation loop in determining both signaling specificity and plasma membrane targeting of type I PIP kinases.

A direct role of SRY and SOX proteins in pre-mRNA splicing

The mammalian testis determining factor SRY and its related Sox factors are critical developmental regulators. They share significant similarity in their high mobility group (HMG) domain and display discrete patterns of tissue-specific expression. Here we show that SRY and the Sox protein SOX6 colocalize with splicing factors in the nucleus and are dynamically redistributed following the blockage of splicing in living cells. Anti-SOX6 antibodies supershift the spliceosomal complex from assembled splicing reactions and inhibit splicing in vitro of multiple pre-mRNA substrates. Most importantly, SOX6-depleted nuclear extracts have impaired splicing activity, which is efficiently restored by addition of the recombinant SOX6 HMG domain and also by recombinant SRY and the SOX9 HMG domain. These results reveal an unexpected biological function of the SRY, SOX6, and SOX9 gene products and provide a functional link to the biochemical mechanisms operating in mammalian sex determination and in other developmental processes regulated by Sox genes.

Disassembly of interchromatin granule clusters alters the coordination of transcription and pre-mRNA splicing

To examine the involvement of interchromatin granule clusters (IGCs) in transcription and pre-mRNA splicing in mammalian cell nuclei, the serine-arginine (SR) protein kinase cdc2-like kinase (Clk)/STY was used as a tool to manipulate IGC integrity in vivo. Both immunofluorescence and transmission electron microscopy analyses of cells overexpressing Clk/STY indicate that IGC components are completely redistributed to a diffuse nuclear localization, leaving no residual structure. Conversely, overexpression of a catalytically inactive mutant, Clk/STY(K190R), causes retention of hypophosphorylated SR proteins in nuclear speckles. Our data suggest that the protein-protein interactions responsible for the clustering of interchromatin granules are disrupted when SR proteins are hyperphosphorylated and stabilized when SR proteins are hypophosphorylated. Interestingly, cells without intact IGCs continue to synthesize nascent transcripts. However, both the accumulation of splicing factors at sites of pre-mRNA synthesis as well as pre-mRNA splicing are dramatically reduced, demonstrating that IGC disassembly perturbs coordination between transcription and pre-mRNA splicing in mammalian cell nuclei.

Phosphatidylinositol 3-kinase c2alpha contains a nuclear localization sequence and associates with nuclear speckles

Phosphoinositide 3-kinase C2alpha (PI3K-C2alpha) belongs to the class II phosphatidylinositol 3-kinases, which are defined by their in vitro usage of phosphatidylinositol and phosphatidylinositol 4-phosphate as substrates. All type II phosphatidylinositol 3-kinases contain at their C terminus a C2-like domain. Here we demonstrate that Homo sapiens phosphoinositide 3-kinase C2alpha (HsPI3K-C2alpha) has dual cellular localization present in the cytoplasm and in the nucleus. A distinct nuclear localization signal sequence was identified by expressing HsPI3K-C2alpha-green fluorescent protein fusion proteins in HeLa cells. The nuclear localization signal was mapped to a stretch of 11 amino acids (KRKTKISRKTR) located within C2-like domain of the kinase. In the cytoplasm and the nucleus HsPI3K-C2alpha associates with macromolecular complexes that are resistant to detergent extraction. Indirect immunofluorescence reveals that in the nucleus HsPI3K-C2alpha is enriched at distinct subnuclear domains known as nuclear speckles, which contain pre-mRNA processing factors and are functionally connected to RNA metabolism. Phosphorylation of HsPI3K-C2alpha is induced by inhibition of RNA polymerase II-dependent transcription and coincides with enlargement and rounding up of the nuclear speckles. The results suggest that phosphorylation of HsPI3K-C2alpha is inversely linked to mRNA transcription and supports the importance of phosphoinositides for nuclear activity.

A Dictyostelium nuclear phosphatidylinositol phosphate kinase required for developmental gene expression

The generation of diacylglycerol (DAG) in response to receptor stimulation is a well-documented signalling mechanism that leads to activation of protein kinase C (PKC). Putative alternative effectors contain sequences that interact with DAGs, but the mechanisms of signal transduction are unknown. We have identified a Dictyostelium gene encoding a novel protein which contains a domain with high identity to the DAG-binding domain of PKC. It does not encode a PKC homologue as the conservation does not extend outside this region. We confirm that the proposed DAG-binding domain is sufficient to mediate interaction of a fusion protein with vesicles containing DAG. The protein also shows significant homology to mammalian phosphatidylinositol phosphate (PIP) kinases and we show that this domain has PIP kinase activity. The protein, PIPkinA, is enriched in the nucleus and abrogation of gene function by homologous recombination inhibits early developmental gene expression, blocking development at an early stage. Thus, we have identified a PIP kinase from Dictyostelium which is required for development, is a candidate effector for DAG and has the potential to synthesize nuclear PIP(2).

G-protein-coupled receptor activation induces the membrane translocation and activation of phosphatidylinositol-4-phosphate 5-kinase I alpha by a Rac- and Rho-dependent pathway

Phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) mediates cell motility and changes in cell shape in response to extracellular stimuli. In platelets, it is synthesized from PI4P by PIP5K in response to stimulation of a G-protein-coupled receptor by an agonist, such as the thrombin. In the present study, we have addressed the pathway that induces PIP5K I alpha activation following the addition of thrombin. Under resting condition expressed PIP5K I alpha was predominantly localized in a perinuclear distribution. After stimulation of the thrombin receptor, PAR1, or overexpression of a constitutively active variant of G alpha(q), PIP5K I alpha translocated to the plasma membrane. Movement of PIP5K I alpha to the cell membrane was dependent on both GTP-bound Rac and Rho, but not Arf, because: 1) inactive GDP-bound variants of either Rac or Rho blocked the translocation induced by constitutively active G alpha(q), 2) constitutively GTP-bound active variants of Rac or Rho induced PIP5K I alpha translocation in the absence of other stimuli, and 3) constitutively active variants of Arf1 or Arf6 failed to induce membrane translocation of PIP5K I alpha. In addition, a dominant negative variant of Rho blocked the PIP5K I alpha membrane translocation induced by constitutively active Rac, whereas dominant negative variants of either Rac or Arf6 failed to block PIP5K I alpha membrane translocation induced by constitutively active Rho. This implies that the effect on PIP5K I alpha by Rac is indirect, and requires the activation of Rho. In contrast to the findings with PIP5K I alpha, the related lipid kinase PIP4K failed to undergo translocation after stimulation by small GTP-binding proteins Rac or Rho. We also tested whether membrane localization of PIP5K I alpha correlated with an increase in its lipid kinase activity and found that co-expressing of PIP5K I alpha with either constitutively active G alpha(q), Rac, or Rho led to a 5- to 7-fold increase in PIP5K I alpha activity. Thus, these findings suggest that stimulation of a G-protein-coupled receptor (PAR1) leads to the sequential activation of G alpha(q), Rac, Rho, and PIP5K I alpha. Once activated and translocated to the cell membrane, PIP5K I alpha becomes available to phosphorylate PI4P to generate PI4,5P(2) on the plasma membrane.

Inositol lipids are regulated during cell cycle progression in the nuclei of murine erythroleukaemia cells

Previous data suggest the existence of discrete pools of inositol lipids, which are components of a nuclear phosphoinositide (PI) cycle. However, it is not known whether the contents of these pools are regulated during cell proliferation. In the present study we demonstrate that the mass levels of three important constituents of the nuclear PI cycle are regulated during the cell cycle. Radioactive label incorporation into PtdIns(4,5)P(2) was seen to increase dramatically as synchronized cells entered S-phase. This did not coincide with any significant changes in the nuclear mass levels of this lipid, suggesting that the rate of turnover of this molecule was increased. Levels of PtdIns4P, the major substrate for PtdIns(4,5)P(2) production by Type I PtdInsP kinases (PIPkins), were regulated during the cell cycle and indicated a complex relationship between these two lipids. An alternative substrate for PtdIns(4,5)P(2), PtdIns5P, phosphorylated by Type II PIPkins, was present in nuclei at much smaller amounts than the PtdIns4P, and thus is unlikely to contribute significantly to PtdIns(4,5)P(2) turnover. However, a large increase in nuclear PtdIns5P mass was observed when murine erythroleukaemia cells are in G(1), and this could represent a potential pool of nuclear inositol lipid that has a specific signalling role. Analysis of extracted lipid fractions indicated the absence of any PtdIns3P in these nuclei.

Nuclear PtdIns(4,5)P2 assembles in a mitotically regulated particle involved in pre-mRNA splicing

Phosphoinositide turnover regulates multiple cellular processes. Compared with their well-known cytosolic roles, limited information is available on the functions of nuclear phosphoinositides. Here, we show that phosphatidylinositol(4,5)-bisphosphate (PtdIns(4,5)P2) stably associates with electron-dense particles within the nucleus that resemble interchromatin granule clusters. These PtdIns(4,5)P2-containing structures have a distribution which is cell-cycle dependent and contain components of both the transcriptional and pre-mRNA processing machinery, including RNA polymerase II and the splicing factor SC-35. Immunodepletion and add-back experiments demonstrate that PtdIns(4,5)P2 and associated factors are necessary but not sufficient for pre-mRNA splicing in vitro, indicating a crucial role for PtdIns(4,5)P2-containing complexes in nuclear pre-mRNA processing.

Characterization of a G protein-activated phosphoinositide 3-kinase in vascular smooth muscle cell nuclei

Recent studies highlight the existence of an autonomous nuclear polyphosphoinositide metabolism related to cellular proliferation and differentiation. However, only few data document the nuclear production of the putative second messengers, the 3-phosphorylated phosphoinositides, by the phosphoinositide 3-kinase (PI3K). In the present paper, we examine whether GTP-binding proteins can directly modulate 3-phosphorylated phosphoinositide metabolism in membrane-free nuclei isolated from pig aorta smooth muscle cells (VSMCs). In vitro PI3K assays performed without the addition of any exogenous substrates revealed that guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) specifically stimulated the nuclear synthesis of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), whereas guanosine 5'-(beta-thio)diphosphate was ineffective. PI3K inhibitors wortmannin and LY294002 prevented GTPgammaS-induced PtdIns(3,4,5)P(3) synthesis. Moreover, pertussis toxin inhibited partially PtdIns(3,4,5)P(3) accumulation, suggesting that nuclear G(i)/G(0) proteins are involved in the activation of PI3K. Immunoblot experiments showed the presence of Galpha(0) proteins in VSMC nuclei. In contrast with previous reports, immunoblots and indirect immunofluorescence failed to detect the p85alpha subunit of the heterodimeric PI3K within VSMC nuclei. By contrast, we have detected the presence of a 117-kDa protein immunologically related to the PI3Kgamma. These results indicate the existence of a G protein-activated PI3K inside VSMC nucleus that might be involved in the control of VSMC proliferation and in the pathogenesis of vascular proliferative disorders.

Phosphoinositides: key players in cell signalling, in time and space

Over the last few years, many reports have extended our knowledge of the inositol lipid metabolism and brought out some exciting information about the location, the variety and the role of phosphoinositides (PIs). Besides the so-called "canonical PI pathway" leading to the production of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), the precursor of the intracellular second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG), many other metabolic pathways have been identified to produce seven different polyphosphoinositides. Several of these quantitatively minor lipid molecules appear to be specifically involved in the control of cellular events, such as the spatial and temporal organisation of key signalling pathways, the rearrangement of the actin cytoskeleton or the intracellular vesicle trafficking. This is consistent with the fact that many of the enzymes, such as kinases and phosphatases, involved in the tight control of the intracellular level of polyphosphoinositides, are regulated and/or relocated through cell surface receptors for extracellular ligands. The remarkable feature of PIs, which can be rapidly synthesised and degraded in discrete membrane domains or even subnuclear structures, places them as ideal regulators and integrators of very dynamic mechanisms of cell regulation. In this review, we will summarise recent studies on the potential location, the metabolic pathways and the role of the different PIs. Some aspects of the temporal synthesis of D3 PIs will also be discussed.

Functional contacts with a range of splicing proteins suggest a central role for Brr2p in the dynamic control of the order of events in spliceosomes of Saccharomyces cerevisiae

Mapping of functional protein interactions will help in understanding conformational rearrangements that occur within large complexes like spliceosomes. Because the U5 snRNP plays a central role in pre-mRNA splicing, we undertook exhaustive two-hybrid screening with Brr2p, Prp8p, and other U5 snRNP-associated proteins. DExH-box protein Brr2p interacted specifically with five splicing factors: Prp8p, DEAH-box protein Prp16p, U1 snRNP protein Snp1p, second-step factor Slu7p, and U4/U6.U5 tri-snRNP protein Snu66p, which is required for splicing at low temperatures. Co-immunoprecipitation experiments confirmed direct or indirect interactions of Prp16p, Prp8p, Snu66p, and Snp1p with Brr2p and led us to propose that Brr2p mediates the recruitment of Prp16p to the spliceosome. We provide evidence that the prp8-1 allele disrupts an interaction with Brr2p, and we propose that Prp8p modulates U4/U6 snRNA duplex unwinding through another interaction with Brr2p. The interactions of Brr2p with a wide range of proteins suggest a particular function for the C-terminal half, bringing forward the hypothesis that, apart from U4/U6 duplex unwinding, Brr2p promotes other RNA rearrangements, acting synergistically with other spliceosomal proteins, including the structurally related Prp2p and Prp16p. Overall, these protein interaction studies shed light on how splicing factors regulate the order of events in the large spliceosome complex.

Intracellular localization of phospholipase D1 in mammalian cells

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.

Highly saturated endonuclear phosphatidylcholine is synthesized in situ and colocated with CDP-choline pathway enzymes

Chromatin-associated phospholipids are well recognized. A report that catalytically active endonuclear CTP:choline-phosphate cytidylyltransferase alpha is necessary for cell survival questions whether endonuclear, CDP-choline pathway phosphatidylcholine synthesis may occur in situ. We report that chromatin from human IMR-32 neuroblastoma cells possesses such a biosynthetic pathway. First, membrane-free nuclei retain all three CDP-choline pathway enzymes in proportions comparable with the content of chromatin-associated phosphatidylcholine. Second, following supplementation of cells with deuterated choline and using electrospray ionization mass spectrometry, both the time course and molecular species labeling pattern of newly synthesized endonuclear and whole cell phosphatidylcholine revealed the operation of spatially separate, compositionally distinct biosynthetic routes. Specifically, endogenous and newly synthesized endonuclear phosphatidylcholine species are both characterized by a high degree of diacyl/alkylacyl chain saturation. This unusual species content and synthetic pattern (evident within 10 min of supplementation) are maintained through cell growth arrest by serum depletion and when proliferation is restored, suggesting that endonuclear disaturated phosphatidylcholine enrichment is essential and closely regulated. We propose that endonuclear phosphatidylcholine synthesis may regulate periodic nuclear accumulations of phosphatidylcholine-derived lipid second messengers. Furthermore, our estimates of saturated phosphatidylcholine nuclear volume occupancy of around 10% may imply a significant additional role in regulating chromatin structure.

Jun, Fos and Krox in the hippocampus after noxious stimulation: simultaneous-input-dependent expression and nuclear speckling

Stimulation of sensory C-fibres produces extensive expression of the Fos, Jun and Krox families of inducible transcription factors (ITFs) in many nociceptive CNS areas [28]. In the hippocampus, however, c-Fos is only weakly induced by such stimulation, and expression of the other ITFs has not been studied. Here we examine the effects of single, repeated and simultaneous C-fibre inputs on ITF expressions in the rat hippocampus. A brief, strong electrical stimulation of sciatic nerve C-fibres induced little or no expression of c-Fos or Krox-20. In contrast, FosB was induced and continued to rise in all areas, whereas the basal expressions of c-Jun and Krox-24 were initially reduced but then returned during the subsequent 36 h. A weak noxious cutaneous stimulus applied to one hindpaw induced only weak expressions of the ITFs. However, if the sciatic stimulation was applied contralaterally and 6 h beforehand, this weak stimulus strongly induced Krox-24, but not other ITFs, i.e. there was a potentiation of Krox-24 expression. When these two stimuli were applied simultaneously a few c-Fos labelled cells did appear, and there was and an increased Krox-24 expression. There was also a strong potentiation of FosB and a strong reduction in c-Jun expression. This simultaneous stimulation was the only type of stimulation to induce expression of Krox-20. Also after simultaneous stimulation the majority of the nuclear labelling for FosB, but not of the other ITFs, had a speckled appearance. MK-801 blocked these changes in ITF expressions, but it could also cause the C-fibre stimulations to induce c-Fos and c-Jun in specific areas of the hippocampus. Thus C-fibre stimulation does affect transcription factor activity in the hippocampus; and the strong responses of some ITFs to simultaneous inputs points to their having a role as 'genetic coincidence detectors' in the hippocampus.

Insulin selectively stimulates nuclear phosphoinositide-specific phospholipase C (PI-PLC) beta1 activity through a mitogen-activated protein (MAP) kinase-dependent serine phosphorylation

Using NIH 3T3 cells, we have investigated nuclear phosphoinositide metabolism in response to insulin, a molecule which acts as a proliferating factor for this cell line and which is known as a powerful activator of the mitogen-activated protein (MAP) kinase pathway. Insulin stimulated inositol lipid metabolism in the nucleus, as demonstrated by measurement of the diacylglycerol mass produced in vivo and by in vitro nuclear phosphoinositide-specific phospholipase C (PI-PLC) activity assay. Despite the fact that nuclei of NIH 3T3 cells contained all of the four isozymes of the beta family of PI-PLC (i.e. beta1, beta2, beta3, and beta4), insulin only activated the beta1 isoform. Insulin also induced nuclear translocation of MAP kinase, as demonstrated by Western blotting analysis, enzyme activity assays, and immunofluorescence staining, and this translocation was blocked by the specific MAP kinase kinase inhibitor PD98059. By means of both a monoclonal antibody recognizing phosphoserine and in vivo labeling with [(32)P]orthophosphate, we ascertained that nuclear PI-PLC-beta1 (and in particular the b subtype) was phosphorylated on serine residues in response to insulin. Both phosphorylation and activation of nuclear PI-PLC-beta1 were substantially reduced by PD98059. Our results conclusively demonstrate that activation of nuclear PI-PLC-beta1 strictly depends on its phosphorylation which is mediated through the MAP kinase pathway.

Intracellular delivery of phosphoinositides and inositol phosphates using polyamine carriers

Phosphoinositide signaling regulates events in endocytosis and exocytosis, vesicular trafficking of proteins, transduction of extracellular signals, remodeling of the actin cytoskeleton, regulation of calcium flux, and apoptosis. Obtaining mechanistic insights in living cells is impeded by the membrane impermeability of these anionic lipids. We describe a carrier system for intracellular delivery of phosphoinositide polyphosphates (PIP(n)s) and fluorescently labeled PIP(n)s into living cells, such that intracellular localization can be directly observed. Preincubation of PIP(n)s or inositol phosphates with carrier polyamines produced complexes that entered mammalian, plant, yeast, bacterial, and protozoal cells in seconds to minutes via a nonendocytic mechanism. Time-dependent transit of both PIP(n)s and the carrier to specific cytosolic and nuclear compartments was readily visualized by fluorescence microscopy. Platelet-derived growth factor treatment of NIH 3T3 fibroblasts containing carrier-delivered phosphatidylinositol 4,5-bisphosphate [PtdIns(4, 5)P(2)]-7-nitrobenz-2-oxa-1,3-diazole resulted in the redistribution of the fluorescent signal, suggesting that fluorescent PtdIns(4, 5)P(2) was a substrate for phospholipase C. We also observed a calcium flux in NIH 3T3 cells when complexes of carrier and PtdIns(4, 5)P(2) or inositol 1,4,5-trisphosphate were added extracellularly. This simple intracellular delivery system allows for the efficient translocation of biologically active PIP(n)s, inositol phosphates, and their fluorescent derivatives into living cells in a physiologically relevant context.

Properties and functions of diacylglycerol kinases

Diacylglycerol kinases (DGKs) phosphorylate the second-messenger diacylglycerol (DAG) to phosphatidic acid (PA). The family of DGKs is well conserved among most species. Nine mammalian isotypes have been identified, and are classified into five subgroups based on their primary structure. DGKs contain a conserved catalytic domain and an array of other conserved motifs that are likely to play a role in lipid-protein and protein-protein interactions in various signalling pathways dependent on DAG and/or PA production. DGK is therefore believed to be activated at the (plasma) membrane where DAG is generated. Some isotypes are found associated with and/or regulated by small GTPases of the Rho family, presumably acting in cytoskeletal rearrangements. Others are (also) found in the nucleus, in association with other regulatory enzymes of the phosphoinositide cycle, and have an effect on cell cycle progression. Most DGK isotypes show high expression in the brain, often in distinct brain regions, suggesting that each individual isotype has a unique function.

Nuclear lipid signaling

There is now abundant evidence for the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope, but are actually within the nucleus, presumably not in a bilayer membrane but instead forming proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol and inositol-1,4,5-trisphosphate [Ins(1,4,5)P(3)]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet clear. Generation of diacylglycerol within the nucleus is believed to recruit protein kinase C to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P(3) may mobilize Ca(2+) from the space between the nuclear membranes and thus increase nucleoplasmic Ca(2+). Less well understood are an increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus: (i) two separate routes of synthesis of phosphatidylinositol-4,5-bisphosphate; (ii) two different sources of diacylglycerol, one being from the PI-PLC pathway, and the other probably from phosphatidylcholine; (iii) several different isoforms of PKC translocating to the nuclei; (iv) increases in activity of the PI-PLC pathway at two different points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P(3), which may have several functions, including a role in the transfer of messenger RNA (mRNA) out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and 3-phosphorylated inositol lipids.

Localization of phosphatidylinositol 3-phosphate in yeast and mammalian cells

Phosphatidylinositol 3-kinase (PI3K) regulates several vital cellular processes, including signal transduction and membrane trafficking. In order to study the intracellular localization of the PI3K product, phosphatidylinositol 3-phosphate [PI(3)P], we constructed a probe consisting of two PI(3)P-binding FYVE domains. The probe was found to bind specifically, and with high affinity, to PI(3)P both in vitro and in vivo. When expressed in fibroblasts, a tagged probe localized to endosomes, as detected by fluorescence microscopy. Electron microscopy of untransfected fibroblasts showed that PI(3)P is highly enriched on early endosomes and in the internal vesicles of multivesicular endosomes. While yeast cells deficient in PI3K activity (vps15 and vps34 mutants) were not labelled, PI(3)P was found on intralumenal vesicles of endosomes and vacuoles of wild-type yeast. vps27Delta yeast cells, which have impaired endosome to vacuole trafficking, showed a decreased vacuolar labelling and increased endosome labelling. Thus PI(3)P follows a conserved intralumenal degradation pathway, and its generation, accessibility and turnover are likely to play a crucial role in defining the early endosome and the subsequent steps leading to multivesicular endosome formation.