Inositides in the nucleus: regulation of nuclear PI-PLCbeta1

Decoding how receptor tyrosine kinases (RTKs) mediate nuclear calcium signaling

Calcium (Ca2+) is a highly versatile intracellular messenger that regulates several cellular processes. Although it is unclear how a single-second messenger coordinates various effects within a cell, there is growing evidence that spatial patterns of Ca2+ signals play an essential role in determining their specificity. Ca2+ signaling patterns can differ in various cell regions, and Ca2+ signals in the nuclear and cytoplasmic compartments have been observed to occur independently. The initiation and function of Ca2+ signaling within the nucleus are not yet fully understood. Receptor tyrosine kinases (RTKs) induce Ca2+ signaling resulting from phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and inositol 1,4,5-trisphosphate (InsP3) formation within the nucleus. This signaling mechanism may be responsible for the effects of specific growth factors on cell proliferation and gene transcription. This review highlights the recent advances in RTK trafficking to the nucleus and explains how these receptors initiate nuclear calcium signaling.

The role of phospholipase Cβ on the plasma membrane and in the cytosol: How modular domains enable novel functions

Phospholipase Cβ (PLCβ) is a signaling enzyme activated by G proteins to generate calcium signals. The catalytic core of PLCβ is surrounded by modular domains that mediate the interaction of the enzyme with known protein partners on the plasma membrane. The C-terminal region PLCβ contains a novel coiled-coil domain that is required for Gαq binding and activation. Recent work has shown that this domain also binds a number of cytosolic proteins that regulate protein translation, and that these proteins compete with Gαq for PLCβ binding. The ability of PLCβ to shuttle between the cytosol to impact protein translation and the plasma membrane to mediate calcium signals puts PLCβ in a central role in cell function.

Phospholipase Cβ-TRAX Association Is Required for PC12 Cell Differentiation

When treated with nerve growth factor, PC12 cells will differentiate over the course of several days. Here, we have followed changes during differentiation in the cellular levels of phosphoinositide-specific phospholipase Cβ (PLCβ) and its activator, Gα_q, which together mediate Ca²⁺ release. We also followed changes in the level of the novel PLCβ binding partner TRAX (translin-associated factor X), which promotes RNA-induced gene silencing. We find that the level of PLCβ increases 4-fold within 24 h, whereas Gα_q increases only 1.4-fold, and this increase occurs ∼24 h later than PLCβ. Alternately, the level of TRAX remains constant over the 72 h tested. When PLCβ1 or TRAX is down-regulated, differentiation does not occur. The impact of PLCβ on differentiation appears independent of Gα_q as down-regulating Gα_q at constant PLCβ does not affect differentiation. Förster resonance energy transfer studies after PLCβ association with its partners indicate that PLCβ induced soon after nerve growth factor treatment associates with TRAX rather than Gα_q Functional measurements of Ca²⁺ signals to assess the activity of PLCβ-Gα_q complexes and measurements of the reversal of siRNA(GAPDH) to assess the activity of PLCβ-TRAX complexes additionally suggest that the newly synthesized PLCβ associates with TRAX to impact RNA-induced silencing. Taken together, our studies show that PLCβ, through its ability to bind TRAX and reverse RNA silencing of specific genes, plays a key role in switching PC12 cells to their differentiated state.

Phospholipase Cβ connects G protein signaling with RNA interference

Phosphoinositide-specific-phospholipase Cβ (PLCβ) is the main effector of Gαq stimulation which is coupled to receptors that bind acetylcholine, bradykinin, dopamine, angiotensin II as well as other hormones and neurotransmitters. Using a yeast two-hybrid and other approaches, we have recently found that the same region of PLCβ that binds Gαq also interacts with Component 3 Promoter of RNA induced silencing complex (C3PO), which is required for efficient activity of the RNA-induced silencing complex. In purified form, C3PO competes with Gαq for PLCβ binding and at high concentrations can quench PLCβ activation. Additionally, we have found that the binding of PLCβ to C3PO inhibits its nuclease activity leading to reversal of RNA-induced silencing of specific genes. In cells, we found that PLCβ distributes between the plasma membrane where it localizes with Gαq, and in the cytosol where it localizes with C3PO. When cells are actively processing small interfering RNAs the interaction between PLCβ and C3PO gets stronger and leads to changes in the cellular distribution of PLCβ. The magnitude of attenuation is specific for different silencing RNAs. Our studies imply a direct link between calcium responses mediated through Gαq and post-transcriptional gene regulation through PLCβ.

Role of phospholipase C-β in RNA interference

Phospholipase C-β (PLCβ) enzymes are activated by G proteins in response to agents such as hormones and neurotransmitters, and have been implicated in leukemias and neurological disorders. PLCβ activity causes an increase in intracellular calcium which ultimately leads to profound changes in the cell. PLCβ localizes to three cellular compartments: the plasma membrane, the cytosol and the nucleus. Under most cell conditions, the majority of PLCβ localizes to the plasma membrane where it interacts with G proteins. In trying to determine the factors that localize PLCβ to the cytosol and nucleus, we have recently identified the binding partner, TRAX. TRAX is a nuclease and part of the machinery involved in RNA interference. This review discusses the interaction between PLCβ and TRAX, and its repercussions in G protein signaling and RNA silencing.

Phospholipase Cβ1 is linked to RNA interference of specific genes through translin-associated factor X

Phospholipase Cβ1 (PLCβ1) is a G-protein-regulated enzyme whose activity results in proliferative and mitogenic changes in the cell. We have previously found that in solution PLCβ1 binds to the RNA processing protein translin-associated factor X (TRAX) with nanomolar affinity and that this binding competes with G proteins. Here, we show that endogenous PLCβ1 and TRAX interact in SK-N-SH cells and also in HEK293 cells induced to overexpress PLCβ1. In HEK293 cells, TRAX overexpression ablates Ca(2+) signals generated by G protein-PLCβ1 activation. TRAX plays a key role in down-regulation of proteins by small, interfering RNA, and PLCβ1 overexpression completely reverses the 2- to 4-fold down-regulation of GAPDH by siRNA in HEK293 and HeLa cells as seen by an ∼4-fold recovery in both the transcript and protein levels. Also, down-regulation of endogenous PLCβ1 in HEK293 and HeLa cells allows for an ∼20% increase in siRNA(GAPDH) silencing. While PLCβ1 overexpression results in a 50% reversal of cell death caused by siRNA(LDH), it does not affect cell survival or silencing of other genes (e.g., cyclophilin, Hsp90, translin). PLCβ1 overexpression in HEK293 and HeLa cells causes a 30% reduction in the total amount of small RNAs. LDH and GAPDH are part of a complex that promotes H2B synthesis that allows cells to progress through the S phase. We find that PLCβ1 reverses the cell death and completely rescues H2B levels caused by siRNA knockdown of LDH or GAPDH. Taken together, our study shows a novel role of PLCβ1 in gene regulation through TRAX association.

Proteins tightly bound to DNA: new data and old problems

Proteins tightly bound to DNA (TBP) comprise a group of proteins that remain bound to DNA after usual deproteinization procedures such as salting out and treatment with phenol or chloroform. TBP bind to DNA by covalent phosphotriester and noncovalent ionic and hydrogen bonds. Some TBP are conservative, and they are usually covalently bound to DNA. However, the TBP composition is very diverse and significantly different in different tissues and in different organisms. TBP include transcription factors, enzymes of the ubiquitin-proteasome system, phosphatases, protein kinases, serpins, and proteins of retrotransposons. Their distribution within the genome is nonrandom. However, the DNA primary structure or DNA curvatures do not define the affinity of TBP to DNA. But there are repetitive DNA sequences with which TBP interact more often. The TBP distribution within genes and chromosomes depends on a cell's physiological state, differentiation type, and stage of organism development. TBP do not interact with DNA in the sites of its association with nuclear matrix and most likely they are not components of the latter.

Identification of a novel binding partner of phospholipase cβ1: translin-associated factor X

Mammalian phospholipase Cβ1 (PLCβ1) is activated by the ubiquitous Gα_q family of G proteins on the surface of the inner leaflet of plasma membrane where it catalyzes the hydrolysis of phosphatidylinositol 4,5 bisphosphate. In general, PLCβ1 is mainly localized on the cytosolic plasma membrane surface, although a substantial fraction is also found in the cytosol and, under some conditions, in the nucleus. The factors that localize PLCβ1in these other compartments are unknown. Here, we identified a novel binding partner, translin-associated factor X (TRAX). TRAX is a cytosolic protein that can transit into the nucleus. In purified form, PLCβ1 binds strongly to TRAX with an affinity that is only ten-fold weaker than its affinity for its functional partner, Gα_q. In solution, TRAX has little effect on the membrane association or the catalytic activity of PLCβ1. However, TRAX directly competes with Gα_q for PLCβ1 binding, and excess TRAX reverses Gα_q activation of PLCβ1. In C6 glia cells, endogenous PLCβ1 and TRAX colocalize in the cytosol and the nucleus, but not on the plasma membrane where TRAX is absent. In Neuro2A cells expressing enhanced yellow and cyano fluorescent proteins (i.e., eYFP- PLCβ1 and eCFP-TRAX), Förster resonance energy transfer (FRET) is observed mostly in the cytosol and a small amount is seen in the nucleus. FRET does not occur at the plasma membrane where TRAX is not found. Our studies show that TRAX, localized in the cytosol and nucleus, competes with plasma-membrane bound Gα_q for PLCβ1 binding thus stabilizing PLCβ1 in other cellular compartments.

Nuclear calcium signaling: a cell within a cell

Calcium (Ca2+) is a versatile second messenger that regulates a wide range of cellular functions. Although it is not established how a single second messenger coordinates diverse effects within a cell, there is increasing evidence that the spatial patterns of Ca2+ signals may determine their specificity. Ca2+ signaling patterns can vary in different regions of the cell and Ca2+ signals in nuclear and cytoplasmic compartments have been reported to occur independently. No general paradigm has been established yet to explain whether, how, or when Ca2+ signals are initiated within the nucleus or their function. Here we highlight that receptor tyrosine kinases rapidly translocate to the nucleus. Ca2+ signals that are induced by growth factors result from phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate formation within the nucleus rather than within the cytoplasm. This novel signaling mechanism may be responsible for growth factor effects on cell proliferation.

Phospholipase C beta 4 in mouse hepatocytes: rhythmic expression and cellular distribution

BACKGROUND

Circadian regulated physiological processes have been well documented in the mammalian liver. Phospholipases are important mediators of both cytoplasmic and nuclear signaling mechanisms in hepatocytes, and despite a potentially critical role for these enzymes in regulating the temporal aspect of hepatic physiology, their involvement in the circadian liver clock has not been the subject of much investigation. The phospholipase C beta4 (PLCbeta4) enzyme is of particular interest as it has been linked to circadian clock function. In general, there is no knowledge of the role of the PLCbeta4 isozyme in mammalian hepatocytes as this is the first report of its expression in the mammalian liver.

RESULTS

We found that in the liver of mice housed on a light:dark cycle, PLCbeta4 protein underwent a significant circadian rhythm with a peak occurring during the early night. In constant darkness, the protein rhythm was more robust and peaked around dusk. We also observed a significant oscillation in plcbeta4 gene expression in the livers of mice housed in both photoperiodic and constant dark conditions. The cellular distribution of the protein in hepatocytes varied over the course of the circadian day with PLCbeta4 primarily cytoplasmic around dusk and nuclear at dawn.

CONCLUSION

Our results indicate that PLCbeta4 gene and protein expression is regulated by a circadian clock in the mouse liver and is not dependent on the external photoperiod. A light-independent daily translocation of PLCbeta4 implies that it may play a key role in nuclear signaling in hepatocytes and serve as a daily temporal cue for physiological processes in the liver.

Spontaneous calcium oscillations and nuclear PLC-β1 in human GV oocytes

Our aim was to investigate if human oocytes, like mouse oocytes, exhibit spontaneous Ca(2+) oscillations and nuclear translocation of PLC-beta1 prior to germinal vesicle breakdown (GVBD), and to correlate these events with the evolution of chromatin configuration as a landmark for the meiosis resumption kinetics. Human germinal vesicle (GV) oocytes were either loaded with Fluo-3 probe to record Ca(2+) signals or fixed for subsequent fluorescent labeling of both chromatin and PLC-beta1, and immunogold labeling of PLC-beta1. Here for the first time, we show that human oocytes at the GV-stage exhibit spontaneous Ca(2+) oscillations. Interestingly, only oocytes with a large diameter and characterized by a compact chromatin surrounding the nucleolus of the GV could reveal these kind of oscillations. We also observed a translocation of PLC-beta1 from the cytoplasm towards the nucleus during in vitro maturation of human oocytes. Spontaneous calcium oscillations and nuclear translocation of PLC-beta1 may reflect some degree of oocyte maturity. The impact of our results may be very helpful to understand and resolve many enigmatic problems usually encountered during the in vitro meiotic maturation of human GV oocytes.

Disruption of Phospholipase Cδ4 Gene Modulates the Liver Regeneration in Cooperation with Nuclear Protein Kinase C

Phospholipase Cdelta4 (PLC delta4) gene has been cloned from the cDNA library of regenerating rat liver. Using PLC delta4 gene-disrupted mice (PLC delta4(-/-)), we studied a role of PLC delta4 during liver regeneration after partial hepatectomy (PH). In PLC delta4(-/-), liver regeneration occurred in an apparently normal way. However, BrdU-indices indicated that PLC delta4 gene disruption delayed the onset of DNA synthesis by 2 h. Noticeably, the BrdU-indices in PLC delta4(+/+) remained rather constant throughout S phase, 25-35%, whereas in PLC delta4(-/-), it fluctuated drastically from 25% at 34 h to 65% at late S, 42 h after PH. This fact showed that PLC delta4 gene disruption caused a higher synchronization of cell proliferation. The mRNA for PLC delta4 in PLC delta4(+/+) appeared at late G1, and the expression continued throughout S phase. PLC activity increased transiently in chromatin at the late G1 and S phases in only PLC delta4(+/+), but not in PLC delta4(-/-). The specific increases in PLC activity well correlated with the transient increases of protein kinase C (PKC) alpha in chromatin of PLC delta4(+/+). PKC epsilon also increased transiently in chromatin from PLC delta4(+/+) at late S. It is concluded that PLC delta4 regulates the liver regeneration in cooperation with nuclear PKC alpha and epsilon.

Phosphoinositide-specific phospholipase C (PI-PLC) β1 and nuclear lipid-dependent signaling

Over the last years, evidence has suggested that phosphoinositides, which are involved in the regulation of a large variety of cellular processes both in the cytoplasm and in the plasma membrane, are present also within the nucleus. A number of advances has resulted in the discovery that phosphoinositide-specific phospholipase C signalling in the nucleus is involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Even though nuclear inositol lipids hydrolysis generates second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. Among phosphoinositide-specific phospholipase C, the beta(1) isoform appears to be one of the key players of the nuclear lipid signaling. This review aims at highlighting the most significant and up-dated findings about phosphoinositide-specific phospholipase C beta(1) in the nucleus.

Nuclear phospholipase C beta1, regulation of the cell cycle and progression of acute myeloid leukemia

A large number of observations have hinted at the fact that location impinges on function of some of the main players of nuclear inositol lipid cycle. PLC beta1 is a well-known example, given that it has been shown that only the enzyme located in the nucleus targets the cyclin D3/cdk4 complex, playing, in turn, a key role in the control of normal progression through the G1 phase of the cell cycle. The PLC beta1 gene, which is constituted of 36 small exons and large introns, maps on the short arm of human chromosome 20 (20pl2, nearby markers D20S917 and D20S177) with the specific probe (PAC clone HS881E24) spanning from exon 19 to 32 of the gene itself. The chromosome band 20pl2 has been shown to be rearranged in human diseases such as solid tumors without a more accurate definition of the alteration, maybe because of the absence of candidate genes or specific probes. Moreover, non-specific alterations in chromosome 20 have been found in patients affected by MDS and acute myeloid leukemia AML. MDS is an adult hematological disease that evolves into AML in about 30% of the cases. The availability of a highly specific probe gave an opportunity to perform in patients affected with MDS/AML, associated with normal karyotype, painting and FISH analysis aimed to check the PLC beta1 gene, given that this signaling molecule is a key player in the control of some checkpoints of the normal progression through the cell cycle. FISH analysis disclosed in a small group of MDS/AML patients with normal karyotype the monoallelic deletion of the PLC beta1 gene. In contrast, PLC beta4, another gene coding for a signaling molecule, located on 20pl2.3 at a distance as far as less than 1 Mb from PLC beta1, is unaffected in MDS patients with the deletion of PLC beta1 gene, hinting at an interstitial deletion. The MDS patients, bearing the deletion, rapidly evolved to AML, whilst the normal karyotype MDS patients, showing non-deletion of PLC beta1 gene, are still alive at least 24 months after the diagnosis. The immunocytochemical analysis using an anti PLC beta1 monoclonal antibody showed that all the AML/MDS patients who were normal at FISH analysis also had normal staining of the nucleus, which is a preferential site for PLC beta1. In contrast, the monoallelic deletion gave rise to a dramatic decrease of the nuclear staining suggesting a decreased expression of the nuclear PLC beta1. The reported data strengthen the contention of a key role played by PLC beta1 in the nucleus, suggest a possible involvement of PLC beta1 in the progression of MDS to AML and pave the way for a larger investigation aimed at identifying a possible high risk group among MDS patients with a normal karyotype.

Nuclear diacylglycerol kinase-theta is activated in response to nerve growth factor stimulation of PC12 cells

Previous evidence from independent laboratories has shown that the nucleus contains diacylglycerol kinase (DGK) isoforms, i.e., the enzymes, which yield phosphatidic acid from diacylglycerol, thus terminating protein kinase C-mediated signaling events. A DGK isoform, which resides in the nucleus of PC12 cells, is DGK-theta. Here, we show that nerve growth factor (NGF) treatment of serum-starved PC12 cells results in the stimulation of both a cytoplasmic and a nuclear DGK activity. However, time course analysis shows that cytoplasmic DGK activity peaked earlier than its nuclear counterpart. While nuclear DGK activity was dramatically down-regulated by a monoclonal antibody known for selectively inhibiting DGK-theta, cytoplasmic DGK activity was not. Moreover, nuclear DGK activity was stimulated by phosphatidylserine, an anionic phospholipid that had no effect on cytoplasmic DGK activity. Upon NGF stimulation, the amount and the activity of DGK-theta, which was bound to the insoluble nuclear matrix fraction, substantially increased. Epidermal growth factor up-regulated a nuclear DGK activity insensitive to anti-DGK-theta monoclonal antibody. Overall, our findings identify nuclear DGK-theta as a down-stream target of NGF signaling in PC12 cells.

Nuclear matrix bound fibroblast growth factor receptor is associated with splicing factor rich and transcriptionally active nuclear speckles

We have used confocal microscopy combined with computer image analysis to evaluate the functional significance of a constitutively expressed form of the receptor tyrosine kinase FGFR1 (fibroblast growth factor receptor 1) in the nucleus of rapidly proliferating serum stimulated TE 671 cells, a medullobastoma human cell line. Our results demonstrate a limited number of large sites and numerous smaller sites of FGFR1 in the nuclear interior. The larger sites showed virtually complete colocalization (>90%) with splicing factor rich nuclear speckles while the smaller sites showed very limited overlap (<20%). Similar results were found for several other proliferating cell lines grown in culture. An in situ transcription assay was used to determine colocalization with transcription sites by incorporating 5-bromouridine triphosphate (BrUTP) followed by dual staining for BrUTP and FGFR1. These results combined with those from using an antibody against the large subunit of RNA polymerase II suggest a significant degree of colocalization (26-38%) over both the large and small sites. No colocalization was detected with sites of DNA replication. The spatial arrangements of FGFR1 sites and colocalization with nuclear speckles were maintained following extraction for nuclear matrix. Moreover, immunoblots indicated a significant enrichment of FGFR1 in the nuclear matrix fraction. Our findings suggest an involvement of a nuclear matrix bound FGFR1 in transcriptional and RNA processing events in the cell nucleus. We further propose that nuclear speckles, aside from a role in transcriptional/RNA processing events, may serve as fundamental regulatory factories for the integration of diverse signaling and regulatory factors that impact transcription and cellular regulation.

Nuclear inositides: facts and perspectives

Strong evidence has been accumulating over the last 15 years suggesting that phosphoinositides, which are involved in the regulation of a large variety of cellular processes in the cytoplasm and in the plasma membrane, are present within the nucleus. Several advances have resulted in the discovery that nuclear phosphoinositides are involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Although nuclear inositol lipids generate second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. This review aims at highlighting the most significant and updated findings about inositol lipid metabolism in the nucleus.

Expression of phospholipase C beta family isoenzymes in C2C12 myoblasts during terminal differentiation

In the present work, we have analyzed the expression and subcellular localization of all the members of inositide-specific phospholipase C (PLCbeta) family in muscle differentiation, given that nuclear PLCbeta1 has been shown to be related to the differentiative process. Cell cultures of C2C12 myoblasts were induced to differentiate towards the phenotype of myotubes, which are also indicated as differentiated C2C12 cells. By means of immunochemical and immunocytochemical analysis, the expression and subcellular localization of PLCbeta1, beta2, beta3, beta4 have been assessed. As further characterization, we investigated the localization of PLCbeta isoenzymes in C2C12 cells by fusing their cDNA to enhanced green fluorescent protein (GFP). In myoblast culture, PLCbeta4 was the most expressed isoform in the cytoplasm, whereas PLCbeta1 and beta3 exhibited a lesser expression in this cell compartment. In nuclei of differentiated myotube culture, PLCbeta1 isoform was expressed at the highest extent. A marked decrease of PLCbeta4 expression in the cytoplasm of differentiated C2C12 cells was detected as compared to myoblasts. No relevant differences were evidenced as regards the expression of PLCbeta3 at both cytoplasmatic and nuclear level, whilst PLCbeta2 expression was almost undetectable. Therefore, we propose that the different subcellular expression of these PLC isoforms, namely the increase of nuclear PLCbeta1 and the decrease of cytoplasmatic PLCbeta4, during the establishment of myotube differentiation, is related to a spatial-temporal signaling event, involved in myogenic differentiation. Once again the subcellular localization appears to be a key step for the diverse signaling activity of PLCbetas.