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

PIP2-Effector Protein MPRIP Regulates RNA Polymerase II Condensation and Transcription

The specific post-translational modifications of the C-terminal domain (CTD) of the Rpb1 subunit of RNA polymerase II (RNAPII) correlate with different stages of transcription. The phosphorylation of the Ser5 residues of this domain associates with the initiation condensates, which are formed through liquid-liquid phase separation (LLPS). The subsequent Tyr1 phosphorylation of the CTD peaks at the promoter-proximal region and is involved in the pause-release of RNAPII. By implementing super-resolution microscopy techniques, we previously reported that the nuclear Phosphatidylinositol 4,5-bisphosphate (PIP2) associates with the Ser5-phosphorylated-RNAPII complex and facilitates the RNAPII transcription. In this study, we identified Myosin Phosphatase Rho-Interacting Protein (MPRIP) as a novel regulator of the RNAPII transcription that recruits Tyr1-phosphorylated CTD (Tyr1P-CTD) to nuclear PIP2-containing structures. The depletion of MPRIP increases the number of the initiation condensates, indicating a defect in the transcription. We hypothesize that MPRIP regulates the condensation and transcription through affecting the association of the RNAPII complex with nuclear PIP2-rich structures. The identification of Tyr1P-CTD as an interactor of PIP2 and MPRIP further points to a regulatory role in RNAPII pause-release, where the susceptibility of the transcriptional complex to leave the initiation condensate depends on its association with nuclear PIP2-rich structures. Moreover, the N-terminal domain of MPRIP, which is responsible for the interaction with the Tyr1P-CTD, contains an F-actin binding region that offers an explanation of how nuclear F-actin formations can affect the RNAPII transcription and condensation. Overall, our findings shed light on the role of PIP2 in RNAPII transcription through identifying the F-actin binding protein MPRIP as a transcription regulator and a determinant of the condensation of RNAPII.

Endocannabinoid 2-Arachidonoylglycerol Synthesis and Metabolism at Neuronal Nuclear Matrix Fractions Derived from Adult Rat Brain Cortex

In this report, we describe the kinetics characteristics of the diacylglycerol lipase-α (DGLα) located at the nuclear matrix of nuclei derived from adult cortical neurons. Thus, using high-resolution fluorescence microscopy, classical biochemical subcellular fractionation, and Western blot techniques, we demonstrate that the DGLα enzyme is located in the matrix of neuronal nuclei. Furthermore, by quantifying the 2-arachidonoylglycerol (2-AG) level by liquid chromatography and mass spectrometry when 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) was exogenously added as substrate, we describe the presence of a mechanism for 2-AG production through DGLα dependent biosynthesis with an apparent K_m (K_m^app) of 180 µM and a V_max of 1.3 pmol min^-1 µg^-1 protein. We also examined the presence of enzymes with hydrolytic and oxygenase activities that are able to use 2-AG as substrate, and described the localization and compartmentalization of the major 2-AG degradation enzymes, namely monoacylglycerol lipase (MGL), fatty acid amide hydrolase (FAAH), α/β-hydrolase domain 12 protein (ABHD12) and cyclooxygenase-2 (COX2). Of these, only ABHD12 exhibited the same distribution with respect to chromatin, lamin B1, SC-35 and NeuN as that described for DGLα. When 2-AG was exogenously added, we observed the production of arachidonic acid (AA), which was prevented by inhibitors (but not specific MGL or ABHD6 inhibitors) of the ABHD family. Overall, our results expand knowledge about the subcellular distribution of neuronal DGLα, and provide biochemical and morphological evidence to ensure that 2-AG is produced in the neuronal nuclear matrix. Thus, this work paves the way for proposing a working hypothesis about the role of 2-AG produced in neuronal nuclei.

The basis of nuclear phospholipase C in cell proliferation

Ca²⁺ is a highly versatile intracellular signal that regulates many biological processes such as cell death and proliferation. Broad Ca²⁺-signaling machinery is used to assemble signaling systems with a precise spatial and temporal resolution to achieve this versatility. Ca²⁺-signaling components can be organized in different regions of the cell and local increases in Ca²⁺ within the nucleus can regulate different cellular functions from the increases in cytosolic Ca²⁺. However, the mechanisms and pathways that promote localized increases in Ca²⁺ levels in the nucleus are still under investigation. This review presents evidence that the nucleus has its own Ca²⁺ stores and signaling machinery, which modulate processes such as cell proliferation and tumor growth. We focus on what is known about the functions of nuclear Phospholipase C (PLC) in the generation of nuclear Ca²⁺ transients that are involved in cell proliferation.

Nuclear Phosphatidylinositol 3,4,5-Trisphosphate Interactome Uncovers an Enrichment in Nucleolar Proteins

Polyphosphoinositides (PPIns) play essential roles as lipid signaling molecules, and many of their functions have been elucidated in the cytoplasm. However, PPIns are also intranuclear where they contribute to chromatin remodeling, transcription, and mRNA splicing. The PPIn, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P₃), has been mapped to the nucleus and nucleoli, but its role remains unclear in this subcellular compartment. To gain further insights into the nuclear functions of PtdIns(3,4,5)P₃, we applied a previously developed quantitative MS-based approach to identify the targets of PtdIns(3,4,5)P₃ from isolated nuclei. We identified 179 potential PtdIns(3,4,5)P₃-interacting partners, and gene ontology analysis for the biological functions of this dataset revealed an enrichment in RNA processing/splicing, cytokinesis, protein folding, and DNA repair. Interestingly, about half of these interactors were common to nucleolar protein datasets, some of which had dual functions in rRNA processes and DNA repair, including poly(ADP-ribose) polymerase 1 (PARP1, now referred as ADP-ribosyltransferase 1). PARP1 was found to interact directly with PPIn via three polybasic regions in the DNA-binding domain and the linker located N-terminal of the catalytic region. PARP1 was shown to bind to PtdIns(3,4,5)P₃ as well as phosphatidylinositol 3,4-bisphosphate in vitro and to colocalize with PtdIns(3,4,5)P₃ in the nucleolus and with phosphatidylinositol 3,4-bisphosphate in nucleoplasmic foci. In conclusion, the PtdIns(3,4,5)P₃ interactome reported here will serve as a resource to further investigate the molecular mechanisms underlying PtdIns(3,4,5)P₃-mediated interactions in the nucleus and nucleolus.

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Phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P₃) localizes to nucleoli.

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PtdIns(3,4,5)P₃ interactomics from isolated nuclei identifies nucleolar proteins.

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PARP1 interacts directly with polyphosphoinositides via several polybasic regions.

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PARP1 colocalizes with PtdIns(3,4,5)P₃ in the nucleolus.

De novo phosphoinositide synthesis in zebrafish is required for triad formation but not essential for myogenesis

Phosphoinositides (PIPs) and their regulatory enzymes are key players in many cellular processes and are required for aspects of vertebrate development. Dysregulated PIP metabolism has been implicated in several human diseases, including a subset of skeletal myopathies that feature structural defects in the triad. The role of PIPs in skeletal muscle formation, and particularly triad biogenesis, has yet to be determined. CDP-diacylglycerol-inositol 3-phosphatidyltransferase (CDIPT) catalyzes the formation of phosphatidylinositol, which is the base of all PIP species. Loss of CDIPT should, in theory, result in the failure to produce PIPs, and thus provide a strategy for establishing the requirement for PIPs during embryogenesis. In this study, we generated cdipt mutant zebrafish and determined the impact on skeletal myogenesis. Analysis of cdipt mutant muscle revealed no apparent global effect on early muscle development. However, small but significant defects were observed in triad size, with T-tubule area, inter terminal cisternae distance and gap width being smaller in cdipt mutants. This was associated with a decrease in motor performance. Overall, these data suggest that myogenesis in zebrafish does not require de novo PIP synthesis but does implicate a role for CDIPT in triad formation.

Subcellular Localization Relevance and Cancer-Associated Mechanisms of Diacylglycerol Kinases

An increasing number of reports suggests a significant involvement of the phosphoinositide (PI) cycle in cancer development and progression. Diacylglycerol kinases (DGKs) are very active in the PI cycle. They are a family of ten members that convert diacylglycerol (DAG) into phosphatidic acid (PA), two-second messengers with versatile cellular functions. Notably, some DGK isoforms, such as DGKα, have been reported to possess promising therapeutic potential in cancer therapy. However, further studies are needed in order to better comprehend their involvement in cancer. In this review, we highlight that DGKs are an essential component of the PI cycle that localize within several subcellular compartments, including the nucleus and plasma membrane, together with their PI substrates and that they are involved in mediating major cancer cell mechanisms such as growth and metastasis. DGKs control cancer cell survival, proliferation, and angiogenesis by regulating Akt/mTOR and MAPK/ERK pathways. In addition, some DGKs control cancer cell migration by regulating the activities of the Rho GTPases Rac1 and RhoA.

Protein Kinase C Theta Modulates PCMT1 through hnRNPL to Regulate FOXP3 Stability in Regulatory T Cells

T cell receptor signaling, together with cytokine-induced signals, can differentially regulate RNA processing to influence T helper versus regulatory T cell fate. Protein kinase C family members have been shown to function in alternative splicing and RNA processing in various cell types. T cell-specific protein kinase C theta, a molecular regulator of T cell receptor downstream signaling, has been shown to phosphorylate splicing factors and affect post-transcriptional control of T cell gene expression. In this study, we explored how using a synthetic cell-penetrating peptide mimic for intracellular anti-protein kinase C theta delivery fine-tunes differentiation of induced regulatory T cells through its differential effects on RNA processing. We identified protein kinase C theta signaling as a critical modulator of two key RNA regulatory factors, heterogeneous nuclear ribonucleoprotein L (hnRNPL) and protein-l-isoaspartate O-methyltransferase-1 (PCMT1), and loss of protein kinase C theta function initiated a "switch" in post-transcriptional organization in induced regulatory T cells. More interestingly, we discovered that protein-l-isoaspartate O- methyltransferase-1 acts as an instability factor in induced regulatory T cells, by methylating the forkhead box P3 (FOXP3) promoter. Targeting protein-l-isoaspartate O-methyltransferase-1 using a cell-penetrating antibody revealed an efficient means of modulating RNA processing to confer a stable regulatory T cell phenotype.

Nuclear Inositides and Inositide-Dependent Signaling Pathways in Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are a heterogeneous group of hematological malignancies characterized by peripheral blood cytopenia and abnormal myeloproliferation, as well as a variable risk of evolution into acute myeloid leukemia (AML). The nucleus is a highly organized organelle with several distinct domains where nuclear inositides localize to mediate essential cellular events. Nuclear inositides play a critical role in the modulation of erythropoiesis or myelopoiesis. Here, we briefly review the nuclear structure, the localization of inositides and their metabolic enzymes in subnuclear compartments, and the molecular aspects of nuclear inositides in MDS.

The nuclear phosphoinositide response to stress

Accumulating evidence reveals that nuclear phosphoinositides (PIs) serve as central signaling hubs that control a multitude of nuclear processes by regulating the activity of nuclear proteins. In response to cellular stressors, PIs accumulate in the nucleus and multiple PI isomers are synthesized by the actions of PI-metabolizing enzymes, kinases, phosphatases and phospholipases. By directly interacting with effector proteins, phosphoinositide signals transduce changes in cellular functions. Here we describe nuclear phosphoinositide signaling in multiple sub-nuclear compartments and summarize the literature that demonstrates roles for specific kinases, phosphatases, and phospholipases in the orchestration of nuclear phosphoinositide signaling in response to cellular stress. Additionally, we discuss the specific PI-protein complexes through which these lipids execute their functions by regulating the configuration, stability, and transcription activity of their effector proteins. Overall, our review provides a detailed landscape of the current understanding of the nuclear PI-protein interactome and its role in shaping the coordinated response to cellular stress.

Nuclear Phosphoinositides-Versatile Regulators of Genome Functions

The many functions of phosphoinositides in cytosolic signaling were extensively studied; however, their activities in the cell nucleus are much less clear. In this review, we summarize data about their nuclear localization and metabolism, and review the available literature on their involvements in chromatin remodeling, gene transcription, and RNA processing. We discuss the molecular mechanisms via which nuclear phosphoinositides, in particular phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2), modulate nuclear processes. We focus on PI(4,5)P2’s role in the modulation of RNA polymerase I activity, and functions of the nuclear lipid islets—recently described nucleoplasmic PI(4,5)P2-rich compartment involved in RNA polymerase II transcription. In conclusion, the high impact of the phosphoinositide–protein complexes on nuclear organization and genome functions is only now emerging and deserves further thorough studies.

Polyphosphoinositides in the nucleus: Roadmap of their effectors and mechanisms of interaction

Biomolecular interactions between proteins and polyphosphoinositides (PPIn) are essential in the regulation of the vast majority of cellular processes. Consequently, alteration of these interactions is implicated in the development of many diseases. PPIn are phosphorylated derivatives of phosphatidylinositol and consist of seven species with different phosphate combinations. PPIn signal by recruiting proteins via canonical domains or short polybasic motifs. Although their actions are predominantly documented on cytoplasmic membranes, six of the seven PPIn are present within the nucleus together with the PPIn kinases, phosphatases and phospholipases that regulate their turnover. Importantly, the contribution of nuclear PPIn in the regulation of nuclear processes has led to an increased recognition of their importance compared to their more accepted cytoplasmic roles. This review summarises our knowledge on the identification and functional characterisation of nuclear PPIn-effector proteins as well as their mode of interactions, which tend to favour polybasic motifs.

C3G dynamically associates with nuclear speckles and regulates mRNA splicing

The first example of a Ras family GTPase and its exchange factor C3G localizing to nuclear speckles and regulating mRNA splicing is presented.

C3G (Crk SH3 domain binding guanine nucleotide releasing factor) (Rap guanine nucleotide exchange factor 1), essential for mammalian embryonic development, is ubiquitously expressed and undergoes regulated nucleocytoplasmic exchange. Here we show that C3G localizes to SC35-positive nuclear speckles and regulates splicing activity. Reversible association of C3G with speckles was seen on inhibition of transcription and splicing. C3G shows partial colocalization with SC35 and is recruited to a chromatin and RNase-sensitive fraction of speckles. Its presence in speckles is dependent on intact cellular actin cytoskeleton and is lost on expression of the kinase Clk1. Rap1, a substrate of C3G, is also present in nuclear speckles, and inactivation of Rap signaling by expression of GFP-Rap1GAP alters speckle morphology and number. Enhanced association of C3G with speckles is seen on glycogen synthase kinase 3 beta inhibition or differentiation of C2C12 cells to myotubes. CRISPR/Cas9-mediated knockdown of C3G resulted in altered splicing activity of an artificial gene as well as endogenous CD44. C3G knockout clones of C2C12 as well as MDA-MB-231 cells showed reduced protein levels of several splicing factors compared with control cells. Our results identify C3G and Rap1 as novel components of nuclear speckles and a role for C3G in regulating cellular RNA splicing activity.

Nuclear lipid mediators: Role of nuclear sphingolipids and sphingosine-1-phosphate signaling in epigenetic regulation of inflammation and gene expression

Phospholipids, sphingolipids, and cholesterol are integral components of eukaryotic cell organelles, including the nucleus. Recent evidence shows characteristic features of nuclear lipid composition and signaling, which are distinct from that of the cytoplasm and plasma membrane. While the nuclear phosphoinositol lipid signaling in cell cycle regulation and differentiation has been well described, there is a paucity on the role of nuclear sphingolipids and sphingolipid signaling in different physiological and pathophysiological human conditions. In this prospective, we describe the role of sphingolipids and specifically focus on the sphingoid bases, such as sphingosine, ceramide, and sphingosine-1-phosphate (S1P) generation and catabolism in nuclear signaling and function. Particularly, S1P generated in the nucleus by phosphorylation of SPHK2 modulates HDAC activity either by direct binding or through activation of nuclear reactive oxygen species and regulates cell cycle and pro-inflammatory gene expression. Potential implication of association of SPHK2 with the co-repressor complexes and generation of S1P in the nucleus on chromatin remodeling under normal and pathological conditions is discussed. A better understanding of sphingolipid signaling in the nucleus will facilitate the design and development of new and novel therapeutic approaches to modulate expression of pro-inflammatory and cell cycle dependent genes in human pathologies such as cancer, bacterial lung infection, neurodegeneration, and cystic fibrosis.

Nuclear speckles: molecular organization, biological function and role in disease

The nucleoplasm is not homogenous; it consists of many types of nuclear bodies, also known as nuclear domains or nuclear subcompartments. These self-organizing structures gather machinery involved in various nuclear activities. Nuclear speckles (NSs) or splicing speckles, also called interchromatin granule clusters, were discovered as sites for splicing factor storage and modification. Further studies on transcription and mRNA maturation and export revealed a more general role for splicing speckles in RNA metabolism. Here, we discuss the functional implications of the localization of numerous proteins crucial for epigenetic regulation, chromatin organization, DNA repair and RNA modification to nuclear speckles. We highlight recent advances suggesting that NSs facilitate integrated regulation of gene expression. In addition, we consider the influence of abundant regulatory and signaling proteins, i.e. protein kinases and proteins involved in protein ubiquitination, phosphoinositide signaling and nucleoskeletal organization, on pre-mRNA synthesis and maturation. While many of these regulatory proteins act within NSs, direct evidence for mRNA metabolism events occurring in NSs is still lacking. NSs contribute to numerous human diseases, including cancers and viral infections. In addition, recent data have demonstrated close relationships between these structures and the development of neurological disorders.

Arachidonoyl-Specific Diacylglycerol Kinase ε and the Endoplasmic Reticulum

The endoplasmic reticulum (ER) comprises an interconnected membrane network, which is made up of lipid bilayer and associated proteins. This organelle plays a central role in the protein synthesis and sorting. In addition, it represents the synthetic machinery of phospholipids, the major constituents of the biological membrane. In this process, phosphatidic acid (PA) serves as a precursor of all phospholipids, suggesting that PA synthetic activity is closely associated with the ER function. One enzyme responsible for PA synthesis is diacylglycerol kinase (DGK) that phosphorylates diacylglycerol (DG) to PA. DGK is composed of a family of enzymes with distinct features assigned to each isozyme in terms of structure, enzymology, and subcellular localization. Of DGKs, DGKε uniquely exhibits substrate specificity toward arachidonate-containing DG and is shown to reside in the ER. Arachidonic acid, a precursor of bioactive eicosanoids, is usually acylated at the sn-2 position of phospholipids, being especially enriched in phosphoinositide. In this review, we focus on arachidonoyl-specific DGKε with respect to the historical context, molecular basis of the substrate specificity and ER-targeting, and functional implications in the ER.

DGK-θ: Structure, Enzymology, and Physiological Roles

Diacylglycerol kinases (DGKs) are a family of enzymes that catalyze the ATP-dependent phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PtdOH). The recognition of the importance of these enzymes has been increasing ever since it was determined that they played a role in the phosphatidylinositol (PtdIns) cycle and a number of excellent reviews have already been written [(see van Blitterswijk and Houssa, 2000; Kanoh et al., 2002; Mérida et al., 2008; Tu-Sekine and Raben, 2009, 2011; Shulga et al., 2011; Tu-Sekine et al., 2013) among others]. We now know there are ten mammalian DGKs that are organized into five classes. DGK-θ is the lone member of the Type V class of DGKs and remains as one of the least studied. This review focuses on our current understanding of the structure, enzymology, regulation, and physiological roles of this DGK and suggests some future areas of research to understand this DGK isoform.

PKC-Theta is a Novel SC35 Splicing Factor Regulator in Response to T Cell Activation

Alternative splicing of nuclear pre-mRNA is essential for generating protein diversity and regulating gene expression. While many immunologically relevant genes undergo alternative splicing, the role of regulated splicing in T cell immune responses is largely unexplored, and the signaling pathways and splicing factors that regulate alternative splicing in T cells are poorly defined. Here, we show using a combination of Jurkat T cells, human primary T cells, and ex vivo naïve and effector virus-specific T cells isolated after influenza A virus infection that SC35 phosphorylation is induced in response to stimulatory signals. We show that SC35 colocalizes with RNA polymerase II in activated T cells and spatially overlaps with H3K27ac and H3K4me3, which mark transcriptionally active genes. Interestingly, SC35 remains coupled to the active histone marks in the absence of continuing stimulatory signals. We show for the first time that nuclear PKC-θ co-exists with SC35 in the context of the chromatin template and is a key regulator of SC35 in T cells, directly phosphorylating SC35 peptide residues at RNA recognition motif and RS domains. Collectively, our findings suggest that nuclear PKC-θ is a novel regulator of the key splicing factor SC35 in T cells.

Nuclear diacylglycerol lipase-α in rat brain cortical neurons: evidence of 2-arachidonoylglycerol production in concert with phospholipase C-β activity

In this report, we describe the localization of diacylglycerol lipase-α (DAGLα) in nuclei from adult cortical neurons, as assessed by double-immunofluorescence staining of rat brain cortical sections and purified intact nuclei and by western blot analysis of subnuclear fractions. Double-labeling assays using the anti-DAGLα antibody and NeuN combined with Hoechst staining showed that only nuclei of neuronal origin were DAGLα positive. At high resolution, DAGLα-signal displayed a punctate pattern in nuclear subdomains poor in Hoechst's chromatin and lamin B1 staining. In contrast, SC-35- and NeuN-signals (markers of the nuclear speckles) showed a high overlap with DAGLα within specific subdomains of the nuclear matrix. Among the members of the phospholipase C-β (PLCβ) family, PLCβ1, PLCβ2, and PLCβ4 exhibited the same distribution with respect to chromatin, lamin B1, SC-35, and NeuN as that described for DAGLα. Furthermore, by quantifying the basal levels of 2-arachidonoylglycerol (2-AG) by liquid chromatography and mass spectrometry (LC-MS), and by characterizing the pharmacology of its accumulation, we describe the presence of a mechanism for 2-AG production, and its PLCβ/DAGLα-dependent biosynthesis in isolated nuclei. These results extend our knowledge about subcellular distribution of neuronal DAGLα, providing biochemical grounds to hypothesize a role for 2-AG locally produced within the neuronal nucleus.

Diacylglycerol kinase as a possible therapeutic target for neuronal diseases

Diacylglycerol kinase (DGK) is a lipid kinase converting diacylglycerol to phosphatidic acid, and regulates many enzymes including protein kinase C, phosphatidylinositol 4-phosphate 5-kinase, and mTOR. To date, ten mammalian DGK subtypes have been cloned and divided into five groups, and they show subtype-specific tissue distribution. Therefore, each DGK subtype is thought to be involved in respective cellular responses by regulating balance of the two lipid messengers, diacylglycerol and phosphatidic acid. Indeed, the recent researches using DGK knockout mice have clearly demonstrated the importance of DGK in the immune system and its pathophysiological roles in heart and insulin resistance in diabetes. Especially, most subtypes show high expression in brain with subtype specific regional distribution, suggesting that each subtype has important and unique functions in brain. Recently, neuronal functions of some DGK subtypes have accumulated. Here, we introduce DGKs with their structural motifs, summarize the enzymatic properties and neuronal functions, and discuss the possibility of DGKs as a therapeutic target of the neuronal diseases.

Nuclear phospholipase C-β1 and diacylglycerol LIPASE-α in brain cortical neurons

Phosphoinositide (PtdIns) signaling involves the generation of lipid second messengers in response to stimuli in a receptor-mediated manner at the plasma membrane. In neuronal cells of adult brain, the standard model proposes that activation of metabotropic receptors coupled to Phospholipase C-β1 (PLC-β1) is linked to endocannabinoid signaling through the production of diacylglycerol (DAG), which could be systematically metabolized by 1,2-diacylglycerol Lipases (DAGL) to produce an increase of 2-arachidonoyl-glycerol (2-AG), the most abundant endocannabinoid in the brain. However, the existence of a nuclear PtdIns metabolism independent from that occurring elsewhere in the cell is now widely accepted, suggesting that the nucleus constitutes both a functional and a distinct compartment for PtdIns metabolism. In this review, we shall highlight the main achievements in the field of neuronal nuclear inositol lipid metabolism with particular attention to progress made linked to the 2-AG biosynthesis. Our aim has been to identify potential sites of 2-AG synthesis other than the neuronal cytoplasmic compartment by determining the subcellular localization of PLC-β1 and DAGL-α, which is much more abundant than DAGL-β in brain. Our data show that PLC-β1 and DAGL-α are detected in discrete brain regions, with a marked predominance of pyramidal morphologies of positive cortical cells, consistent with their role in the biosynthesis and release of 2-AG by pyramidal neurons to control their synaptic inputs. However, as novelty, we showed here an integrated description of the localization of PLC-β1 and DAGL-α in the neuronal nuclear compartment. We discuss our comparative analysis of the expression patterns of PLC-β1 and DAGL-α, providing some insight into the potential autocrine role of 2-AG production in the neuronal nuclear compartment that probably subserve additional roles to the recognized activation of the CB1 cannabinoid receptor.

Nuclear PI-PLCβ1: an appraisal on targets and pathology

Lipid signalling molecules are essential components of the processes that allow one extracellular signal to be transferred inside the nucleus, where specific lipid second messengers elicit reactions capable of regulating gene transcription, DNA replication or repair and DNA cleavage, eventually resulting in cell growth, differentiation, apoptosis or many other cell functions. Nuclear inositides are independently regulated, suggesting that the nucleus constitutes a functionally distinct compartment of inositol lipids metabolism. Indeed, nuclear inositol lipids themselves can modulate nuclear processes, such as transcription and pre-mRNA splicing, growth, proliferation, cell cycle regulation and differentiation. Nuclear PI-PLCβ1 is a key molecule for nuclear inositide signalling, where it plays a role in cell cycle progression, proliferation and differentiation. Here we review the targets and possible involvement of nuclear PI-PLCβ1 in human physiology and pathology.

The expression of diacylglycerol kinase theta during the organogenesis of mouse embryos

Background

Diacylglycerol kinase (DGK) is a key enzyme that regulates diacylglycerol (DG) turnover and is involved in a variety of physiological functions. The isoform DGKθ has a unique domain structure and is the sole member of type V DGK. To reveal the spatial and temporal expression of DGKθ we performed immunohistochemical staining on paraffin sections of mouse embryos.

Results

At an early stage of development (E10.5 and 11.5), the expression of DGKθ was prominently detected in the brain, spinal cord, dorsal root ganglion, and limb bud, and was also moderately detected in the bulbus cordis and the primordium of the liver and gut. At later stages (E12.5 and 14.5), DGKθ expression persisted or increased in the neocortex, epithalamus, hypothalamus, medulla oblongata, and pons. DGKθ was also evident in the epidermis, and nearly all epithelia of the oropharyngeal membrane, digestive tract, and bronchea. At prenatal developmental stages (E16.5 and E18.5), the expression pattern of DGKθ was maintained in the central nervous system, intestine, and kidney, but was attenuated in the differentiated epidermis.

Conclusion

These results suggest that DGKθ may play important physiological roles not only in the brain, but also in diverse organs and tissues during the embryonic stages.

cAMP-stimulated transcription of DGKθ requires steroidogenic factor 1 and sterol regulatory element binding protein 1

Diacylglycerol kinase (DGK)θ is a lipid kinase that phosphorylates diacylglycerol to form phosphatidic acid (PA). We have previously shown that PA is a ligand for the nuclear receptor steroidogenic factor 1 (SF1) and that cAMP-stimulated expression of SF1 target genes requires DGKθ. In this study, we sought to investigate the role of cAMP signaling in regulating DGKθ gene expression. Real time RT-PCR and Western blot analysis revealed that dibutyryl cAMP (Bt2cAMP) increased the mRNA and protein expression, respectively, of DGKθ in H295R human adrenocortical cells. SF1 and sterol regulatory element binding protein 1 (SREBP1) increased the transcriptional activity of a reporter plasmid containing 1.5 kb of the DGKθ promoter fused to the luciferase gene. Mutation of putative cAMP responsive sequences abolished SF1- and SREBP-dependent DGKθ reporter gene activation. Consistent with this finding, chromatin immunoprecipitation assay demonstrated that Bt2cAMP signaling increased the recruitment of SF1 and SREBP1 to the DGKθ promoter. Coimmunoprecipitation assay revealed that SF1 and SREBP1 interact, suggesting that the two transcription factors form a complex on the DGKθ promoter. Finally, silencing SF1 and SREBP1 abolished cAMP-stimulated DGKθ expression. Taken together, we demonstrate that SF1 and SREBP1 activate DGKθ transcription in a cAMP-dependent manner in human adrenocortical cells.

Dual regulation of diacylglycerol kinase (DGK)-θ: polybasic proteins promote activation by phospholipids and increase substrate affinity

Diacylglycerol kinases are important mediators of lipid signaling cascades, and insight into their regulation is of increasing interest. Using purified DGK-θ, we show that this isoform is subject to dual regulation and that the previously characterized stimulation by acidic phospholipids is dependent on the presence of a positively charged protein or peptide. Polybasic cofactors lowered the K(m) for diacylglycerol at the membrane surface (K(m)((surf))), and worked synergistically with acidic phospholipids to increase activity 10- to 30-fold, suggesting that the purified enzyme is autoinhibited. Vesicle pulldown studies showed that acidic phospholipids recruit polybasic cofactors to the vesicle surface but have little effect on the membrane association of DGK-θ, suggesting that a triad of enzyme, acidic lipid and basic protein are necessary for interfacial activity. Importantly, these data demonstrate that the interfacial association and catalytic activity of DGK-θ are independently regulated. Finally, we show that DGK-θ directly interacts with, and is activated by, basic proteins such as histone H1 and Tau with nm affinity, consistent with a potential role for a polybasic protein or protein domain in the activation of this enzyme.

Cellular neurochemical characterization and subcellular localization of phospholipase C β1 in rat brain

The present study describes a complete and detailed neuroanatomical distribution map of the phospholipase C beta1 (PLCβ1) isoform along the adult rat neuraxis, and defines the phenotype of cells expressing PLCβ1, along with its subcellular localization in cortical neurons as assessed by double-immunofluorescence staining and confocal laser scanning. Immunohistochemical labeling revealed a considerable morphological heterogeneity among PLCβ1-positive cells in the cortex, even though there was a marked predominance of pyramidal morphologies. As an exception to the general non-matching distribution of GFAP and PLCβ1, a high degree of co-expression was observed in radial glia-like processes of the spinal cord white matter. In the somatosensory cortex, the proportion of GABAergic neurons co-stained with PLCβ1 was similar (around 2/3) in layers I, II-III, IV and VI, and considerably lower in layer V (around 2/5). Double immunofluorescence against PLCβ1 and nuclear speckle markers SC-35 and NeuN/Fox3 in isolated nuclei from the rat cortex showed a high overlap of both markers with PLCβ1 within the nuclear matrix. In contrast, there was no apparent co-localization with markers of the nuclear envelope and lamina. Finally, to assess whether the subcellular expression pattern of PLCβ1 involved specifically one of the two splice variants of PLCβ1, we carried out Western blot experiments in cortical subcellular fractions. Notably, PLCβ1a/1b ratios were statistically higher in the cytoplasm than in the nuclear and plasma membrane fractions. These results provide a deeper knowledge of the cellular distribution of the PLCβ1 isoform in different cell subtypes of the rat brain, and of its presence in the neuronal nuclear compartment.

Star-PAP control of BIK expression and apoptosis is regulated by nuclear PIPKIα and PKCδ signaling

BIK protein is an initiator of mitochondrial apoptosis, and BIK expression is induced by proapoptotic signals, including DNA damage. Here, we demonstrate that 3' end processing and expression of BIK mRNA are controlled by the nuclear PI4,5P(2)-regulated poly(A) polymerase Star-PAP downstream of DNA damage. Nuclear PKCδ is a key mediator of apoptosis, and DNA damage stimulates PKCδ association with the Star-PAP complex where PKCδ is required for Star-PAP-dependent BIK expression. PKCδ binds the PI4,5P(2)-generating enzyme PIPKIα, which is essential for PKCδ interaction with the Star-PAP complex, and PKCδ activity is directly stimulated by PI4,5P(2). Features in the BIK 3' UTR uniquely define Star-PAP specificity and may block canonical PAP activity toward BIK mRNA. This reveals a nuclear phosphoinositide signaling nexus where PIPKIα, PI4,5P(2), and PKCδ regulate Star-PAP control of BIK expression and induction of apoptosis. This pathway is distinct from the Star-PAP-mediated oxidative stress pathway indicating signal-specific regulation of mRNA 3' end processing.

c-Abl tyrosine kinase regulates serum-induced nuclear export of diacylglycerol kinase α by phosphorylation at Tyr-218

c-Abl is a tyrosine kinase involved in many cellular processes, including cell cycle control and proliferation. However, little is known about its substrates. Here, we show that c-Abl directly phosphorylates diacylglycerol kinase α (DGKα), an important regulator of many cellular events through its conversion of diacylglycerol to phosphatidic acid. We found that DGKα was transported from the cytoplasm to the nucleus in response to serum starvation, and serum restoration induced the nuclear export of the enzyme to the cytoplasm. This serum-induced export involves two tyrosine kinases, c-Src and c-Abl. The latter, c-Abl, is activated by c-Src, phosphorylates DGKα, and shuttles between the nucleus and the cytoplasm in a direction opposite to that of DGKα in response to serum restoration. Moreover, an in vitro phosphorylation assay using purified mutants of DGKα identified Tyr-218 as a site of phosphorylation by c-Abl. We confirmed these results for endogenous DGKα using an antibody specific for phospho-Tyr-218, and this phosphorylation was necessary for the serum-induced export of DGKα. These results demonstrate that the nucleo-cytoplasmic shuttling of DGKα is orchestrated by tyrosine phosphorylation by the Src-activated tyrosine kinase c-Abl and that this phosphorylation is important for regulating the function of cytoplasmic and/or nuclear DGKα.

Nuclear sphingolipid metabolism

Nuclear lipid metabolism is implicated in various processes, including transcription, splicing, and DNA repair. Sphingolipids play roles in numerous cellular functions, and an emerging body of literature has identified roles for these lipid mediators in distinct nuclear processes. Different sphingolipid species are localized in various subnuclear domains, including chromatin, the nuclear matrix, and the nuclear envelope, where sphingolipids exert specific regulatory and structural functions. Sphingomyelin, the most abundant nuclear sphingolipid, plays both structural and regulatory roles in chromatin assembly and dynamics in addition to being an integral component of the nuclear matrix. Sphingosine-1-phosphate modulates histone acetylation, sphingosine is a ligand for steroidogenic factor 1, and nuclear accumulation of ceramide has been implicated in apoptosis. Finally, nuclear membrane-associated ganglioside GM1 plays a pivotal role in Ca(2+) homeostasis. This review highlights research on the factors that control nuclear sphingolipid metabolism and summarizes the roles of these lipids in various nuclear processes.

Regulation and roles of neuronal diacylglycerol kinases: a lipid perspective

Diacylglycerol kinases (DGKs) are a class of enzymes that catalyze the ATP-dependent conversion of diacylglycerol (DAG) to phosphatidic acid (PtdOH), resulting in the coordinate regulation of these two lipid second messengers. This regulation is particularly important in the nervous system where it is now well-established that DAG and PtdOH serve very important roles in modulating a variety of neurological functions. There are currently 10 identified mammalian DGKs, organized into five classes or "Types" based upon similarities in their primary sequences. A number of studies have identified eight of these isoforms in various regions of the mammalian central nervous system (CNS): DGK-α, DGK-β, DGK-γ, DGK-η, DGK-ζ, DGK-ι, DGK-ϵ, and DGK-θ. Further studies have provided compelling evidence supporting roles for these enzymes in neuronal spine density, myelination, synaptic activity, neuronal plasticity, epileptogenesis and neurotransmitter release. The physiological regulation of these enzymes is less clear. Like all interfacial enzymes, DGKs metabolize their hydrophobic substrate (DAG) at a membrane-aqueous interface. Therefore, these enzymes can be regulated by alterations in their subcellular localization, enzymatic activity, and/or membrane association. In this review, we summarize what is currently understood about the localization and regulation of the neuronal DGKs in the mammalian CNS.

Nuclear localization of phospholipase D1 mediates the activation of nuclear protein kinase C(alpha) and extracellular signal-regulated kinase signaling pathways

Recent studies highlight the existence of a nuclear lipid metabolism related to cellular proliferation. However, the importance of nuclear phosphatidylcholine (PC) metabolism is poorly understood. Therefore, we were interested in nuclear PC as a source of second messengers and, particularly, nuclear localization of PC-specific phospholipase D (PLD). In the present study we have identified the nuclear localization sequence (NLS) of PLD1 whose mutation abolished its nuclear import. Recently, we reported that caspase-mediated cleavage of PLD1 generates the N-terminal fragment (NF-PLD1) and C-terminal fragment (CF-PLD1). Here we show that CF-PLD1 but not NF-PLD1, is exclusively imported into the nucleus via its functional NLS, whereas only some portions of intact PLD1 were localized into the nucleus. The NLS of intact PLD1 or CF-PLD1 is required for interaction with importin-β, which is known to mediate nuclear import. The amount of intact PLD1 or CF-PLD1 translocated into nucleus is correlated with its binding affinity with importin-β. Ultimately, nuclear localization of intact PLD1 but not CF-PLD1 mediates the activation of nuclear protein kinase Cα and extracellular signal-regulated kinase signaling pathways. Taken together, we propose that nuclear localization of PLD1 via the NLS and its interaction with importin-β may provide new insights on the functional role of nuclear PLD1 signaling.

Insulin action on polyunsaturated phosphatidic acid formation in rat brain: an "in vitro" model with synaptic endings from cerebral cortex and hippocampus

The highly efficient formation of phosphatidic acid from exogenous 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) in rat brain synaptic nerve endings (synaptosomes) from cerebral cortex and hippocampus is reported. Phosphatidic acid synthesized from SAG or 1,2-dipalmitoyl-sn-glycerol (DPG) was 17.5 or 2.5 times higher, respectively, than from endogenous synaptosomal diacylglycerides. Insulin increased diacylglycerol kinase (DAGK) action on endogenous substrate in synaptic terminals from hippocampus and cerebral cortex by 199 and 97%, respectively. Insulin preferentially increased SAG phosphorylation from hippocampal membranes. In CC synaptosomes insulin increased phosphatidic acid (PA) synthesis from SAG by 100% with respect to controls. Genistein (a tyrosine kinase inhibitor) inhibited this stimulatory insulin effect. Okadaic acid or cyclosporine, used as Ser/Threo protein phosphatase inhibitors, failed to increase insulin effect on PA formation. GTP gamma S and particularly NaF were potent stimulators of PA formation from polyunsaturated diacylglycerol but failed to increase this phosphorylation when added after 5 min of insulin exposure. GTP gamma S and NaF increased phosphatidylinositol 4,5 bisphosphate (PIP2) labeling with respect to controls when SAG was present. On the contrary, they decreased polyphosphoinositide labeling with respect to controls in the presence of DPG. Our results indicate that a DAGK type 3 (DAGKepsilon) which preferentially, but not selectively, utilizes 1-acyl-2-arachidonoyl-sn-glycerol and which could be associated with polyphosphoinositide resynthesis, participates in synaptic insulin signaling. GTP gamma S and NaF appear to be G protein activators related to insulin and the insulin receptor, both affecting the signaling mechanism that augments phosphatidic acid formation.

Diacylglycerol kinase zeta is associated with chromatin, but dissociates from condensed chromatin during mitotic phase in NIH3T3 cells

Diacylglycerol kinase (DGK) converts diacylglycerol (DG) to phosphatidic acid, both of which act as second messengers to mediate a variety of cellular mechanisms. Therefore, DGK contributes to the regulation of these messengers in cellular signal transduction. Of DGK isozymes cloned, DGKzeta is characterized by a nuclear localization signal that overlaps with a sequence similar to the myristoylated alanine-rich C-kinase substrate. Previous studies showed that nuclear DG is differentially regulated from plasma membrane DG and that the nuclear DG levels fluctuate in correlation with cell cycle progression, suggesting the importance of nuclear DG in cell cycle control. In this connection, DGKzeta has been shown to localize to the nucleus in fully differentiated cells, such as neurons and lung cells, although it remains elusive how DGK behaves during the cell cycle in proliferating cells. Here we demonstrate that DGKzeta localizes to the nucleus during interphase including G1, S, and G2 phases and is associated with chromatin although it dissociates from condensed chromatin during mitotic phase in NIH3T3 cells. Furthermore, this localization pattern is also observed in proliferating spermatogonia in the testis. These results suggest a reversible association of DGKzeta with histone or its related proteins in cell cycle, plausibly dependent on their post-translational modifications.

Paraspeckle protein p54nrb links Sox9-mediated transcription with RNA processing during chondrogenesis in mice

The Sox9 transcription factor plays an essential role in promoting chondrogenesis and regulating expression of chondrocyte extracellular-matrix genes. To identify genes that interact with Sox9 in promoting chondrocyte differentiation, we screened a cDNA library generated from the murine chondrogenic ATDC5 cell line to identify activators of the collagen, type II, alpha 1 (Col2a1) promoter. Here we have shown that paraspeckle regulatory protein 54-kDa nuclear RNA-binding protein (p54nrb) is an essential link between Sox9-regulated transcription and maturation of Sox9-target gene mRNA. We found that p54nrb physically interacted with Sox9 and enhanced Sox9-dependent transcriptional activation of the Col2a1 promoter. In ATDC5 cells, p54nrb colocalized with Sox9 protein in nuclear paraspeckle bodies, and knockdown of p54(nrb) suppressed Sox9-dependent Col2a1 expression and promoter activity. We generated a p54nrb mutant construct lacking RNA recognition motifs, and overexpression of mutant p54nrb in ATDC5 cells markedly altered the appearance of paraspeckle bodies and inhibited the maturation of Col2a1 mRNA. The mutant p54nrb inhibited chondrocyte differentiation of mesenchymal cells and mouse metatarsal explants. Furthermore, transgenic mice expressing the mutant p54nrb in the chondrocyte lineage exhibited dwarfism associated with impairment of chondrogenesis. These data suggest that p54nrb plays an important role in the regulation of Sox9 function and the formation of paraspeckle bodies during chondrogenesis.

Insights into nuclear localization and dynamic association of CD38 in Raji and K562 cells

CD38 is a type II transmembrane glycoprotein found mainly on the plasma membrane involved in the metabolism of cADPR and NAADP, two nucleotides with calcium mobilizing activity independent of inositol trisphosphate. Recent data report the presence of CD38 in different cellular compartments raising new questions about its effective role in cellular metabolism. In rat hepatocyte nuclei, CD38 has been proposed as a responsive to cADPR integral inner membrane protein suggesting that the nuclear envelope may also be an important source of Ca2+ stores. Further reports indicating that CD38 is localized in nuclear compartments in a variety of cell types and tissues including brain, liver, eye, spleen, and bone raise the condition of resolving the question concerning the effective presence of CD38 within the nucleus. Here we report data supporting the presence of CD38 at nuclear level independently of expression of surface CD38. We utilized two different human leukemia cell lines expressing or not expressing CD38 molecule on their cell surface. The morphological and biochemical results including enzymatic activity and proteomic determinations explain the effective nuclear localization of CD38 in human Raji and K562 cells. Since cell nucleus is a complex and highly dynamic environment with many functionally specialized regions, the nuclear localization of specific proteins represents an important mechanism in signal transduction. The presence of CD38 at the interchromatin region whether linked to nuclear scaffold or stored in nuclear structures as micronuclei and Cajal bodies co-localizing with coilin, suggests its involvement in nuclear processes including transcription, replication, repairing and splicing.

Cyclic AMP-stimulated interaction between steroidogenic factor 1 and diacylglycerol kinase theta facilitates induction of CYP17

In the human adrenal cortex, adrenocorticotropin (ACTH) activates CYP17 transcription by promoting the binding of the nuclear receptor steroidogenic factor 1 (SF1) (Ad4BP, NR5A1) to the promoter. We recently found that sphingosine is an antagonist for SF1 and inhibits cyclic AMP (cAMP)-dependent CYP17 gene transcription. The aim of the current study was to identify phospholipids that bind to SF1 and to characterize the mechanism by which ACTH/cAMP regulates the biosynthesis of this molecule(s). Using tandem mass spectrometry, we show that in H295R human adrenocortical cells, SF1 is bound to phosphatidic acid (PA). Activation of the ACTH/cAMP signal transduction cascade rapidly increases nuclear diacylglycerol kinase (DGK) activity and PA production. PA stimulates SF1-dependent transcription of CYP17 reporter plasmids, promotes coactivator recruitment, and induces the mRNA expression of CYP17 and several other steroidogenic genes. Inhibition of DGK activity attenuates the binding of SF1 to the CYP17 promoter, and silencing of DGK-theta expression inhibits cAMP-dependent CYP17 transcription. LXXLL motifs in DGK-theta mediate a direct interaction of SF1 with the kinase and may facilitate binding of PA to the receptor. We conclude that ACTH/cAMP stimulates PA production in the nucleus of H295R cells and that this increase in PA concentrations facilitates CYP17 induction.

Diacylglycerol kinases: Why so many of them?

Diacylglycerol (DAG) kinase (DGK) modulates the balance between the two signaling lipids, DAG and phosphatidic acid (PA), by phosphorylating DAG to yield PA. To date, ten mammalian DGK isozymes have been identified. In addition to the C1 domains (protein kinase C-like zinc finger structures) conserved commonly in all DGKs, these isoforms possess a variety of regulatory domains of known and/or predicted functions, such as a pair of EF-hand motifs, a pleckstrin homology domain, a sterile alpha motif domain and ankyrin repeats. Beyond our expectations, recent studies have revealed that DGK isozymes play pivotal roles in a wide variety of signal transduction pathways conducting development, neural and immune responses, cytoskeleton reorganization and carcinogenesis. Moreover, there has been rapidly growing evidence indicating that individual DGK isoforms exert their specific roles through interactions with unique partner proteins such as protein kinase Cs, Ras guanyl nucleotide-releasing protein, chimaerins and phosphatidylinositol-4-phosphate 5-kinase. Therefore, an emerging paradigm for DGK is that the individual DGK isoforms assembled in their own signaling complexes should carry out spatio-temporally segregated tasks for a wide range of biological processes via regulating local, but not global, concentrations of DAG and/or PA.

Role of PI3-kinase and PI4-kinase in actin polymerization during bovine sperm capacitation

We have recently demonstrated the involvement of phospholipase D (PLD) in actin polymerization during mammalian sperm capacitation. In the present study, we investigated the involvement of phosphatidylinositol 3- and 4-kinases (PI3K and PI4K) in actin polymerization, as well as the production of PIP(2(4,5)), which is a known cofactor for PLD activation, during bovine sperm capacitation. PIK3R1 (p85 alpha regulatory subunit of PI3K) and PIKCB (PI4K beta) in bovine sperm were detected by Western blotting and immunocytochemistry. Wortmannin (WT) inhibited PI3K and PI4K type III at concentrations of 10 nM and 10 microM, respectively. PI4K activity and PIP(2(4,5)) production were blocked by 10 microM WT but not by 10 nM WT, whereas PI3K activity and PIP(3(3,4,5)) production were blocked by 10 nM WT. Moreover, spermine, which is a known PI4K activator and a component of semen, activated sperm PI4K, resulting in increased cellular PIP(2(4,5)) and F-actin formation. The increases in PIP(2(4,5)) and F-actin intracellular levels during sperm capacitation were mediated by PI4K but not by PI3K activity. Activation of protein kinase A (PKA) by dibutyryl cAMP enhanced PIP(2(4,5)), PIP(3(3,4,5)), and F-actin formation, and these effects were mediated through PI3K. On the other hand, activation of PKC by phorbol myristate acetate enhanced PIP(2(4,5)) and F-actin formation mediated by PI4K activity, while the PI3K activity and intracellular PIP(3(3,4,5)) levels were reduced. These results suggest that two alternative pathways lead to PI4K activation: indirect activation by PKA, which is mediated by PI3K; and activation by PKC, which is independent of PI3K activity. Our results also suggest that spermine, which is present in the ejaculate, regulates PI4K activity during the capacitation process in vivo.

Proteomic-based analysis of nuclear signaling: PLCbeta1 affects the expression of the splicing factor SRp20 in Friend erythroleukemia cells

An extensive body of evidence links inositide-specific phospholipase C (PLC) to the nucleus and the main isoform located in the nucleus is PLCbeta(1). Constitutive overexpression of nuclear PLCbeta(1) has been previously shown to inhibit Friend erythroleukemia cells differentiation and to induce cell cycle progression targeting cyclin D3. The aim of this study was to identify new proteins regulated by PLCbeta(1) overexpression, given the role exerted by its signaling in the nucleus during cell growth and differentiation. To identify novel downstream effectors of nuclear PLCbeta(1)-dependent signaling in Friend erythroleukemia cells, we performed the high-resolution 2-DE-based proteomic analysis. Using a proteomic approach we found that SRp20, a member of the highly conserved SR family of splicing regulators, was down-regulated in cells overexpressing nuclear PLCbeta(1) as compared with wild-type cells. Reduction in SRp20 was confirmed by 2-D Western blotting. Moreover, we have shown that nuclear PLCbeta(1) is bound to the SRp20 splicing factor. Indeed, by immunoprecipitation and subcellular fractioning, we have demonstrated that endogenous PLCbeta(1) and SRp20 physically interact in the nucleus. Here we show the existence of a PLCbeta(1)-specific target, the splicing factor SRp20, whose expression is specifically down-regulated by the nuclear signaling evoked by PLCbeta(1).

Subnuclear localization and differentiation-dependent increased expression of DGK-zeta in C2C12 mouse myoblasts

Diacylglycerol kinases (DGKs) catalyze phosphorylation of diacylglycerol (DG) to yield phosphatidic acid (PA). Previous evidence has shown that the nucleus contains several DGK isoforms. In this study, we have analyzed the expression and subnuclear localization of DGK-zeta employing C2C12 mouse myoblasts. Immunocytochemistry coupled to confocal laser scanning microscopy showed that both endogenous and green fluorescent protein-tagged overexpressed DGK-zeta localized mostly to the nucleus. In contrast, overexpressed DGK-alpha, -beta, -delta, and -iota did not migrate to the nucleus. DGK-zeta was present in the nuclear speckle domains, as also revealed by immuno-electron microscopy analysis. Moreover, DGK-zeta co-localized and interacted with phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1), that is involved in inositide-dependent signaling pathways important for the regulation of cell proliferation and differentiation. Furthermore, we report that DGK-zeta associated with nuclear matrix, the fundamental organizing principle of the nucleus where many cell functions take place, including DNA replication, gene expression, and protein phosphorylation. Nuclear DGK-zeta increased during myogenic differentiation of C2C12 cells, while DGK-zeta down-regulation by siRNA markedly impaired differentiation. Overall, our findings further support the importance of speckles and nuclear matrix in lipid-dependent signaling and suggest that nuclear DGK-zeta might play some fundamental role during myogenic differentiation of C2C12 cells.