Phospholipase C Isoforms Are Localized at the Cleavage Furrow during Cytokinesis

It has recently been demonstrated that phosphatidylinositol 4,5-bisphosphate (PIP2) is localized at the cleavage furrow in dividing cells and its hydrolysis is required for complete cytokinesis, suggesting a pivotal role of PIP2 in cytokinesis. Here, we report that at least three mammalian isoforms of phosphoinositide-specific phospholipase C (PLC), PLCdelta1, PLCdelta3 and PLCbeta1, are localized to the cleavage furrow during cytokinesis. Targeting of the delta1 isoform to the furrow depends on the specific interaction between the PH domain and PIP2 in the plasma membrane. The necessity of active PLC in animal cell cytokinesis was confirmed using the specific inhibitors for PIP2 hydrolysis. These results support the model that activation of selected PLC isoforms at the cleavage furrow controls progression of cytokinesis through regulation of PIP2 levels: induction of the cleavage furrow by a contractile ring consisting of actomyosin is regulated by PIP2-dependent actin-binding proteins and formation of specific lipid domains required for membrane separation is affected by alterations in the lipid composition of the furrow.

Crystal structure of Rac1 bound to its effector phospholipase C-β2

Although diverse signaling cascades require the coordinated regulation of heterotrimeric G proteins and small GTPases, these connections remain poorly understood. We present the crystal structure of the GTPase Rac1 bound to phospholipase C-beta2 (PLC-beta2), a classic effector of heterotrimeric G proteins. Rac1 engages the pleckstrin-homology (PH) domain of PLC-beta2 to optimize its orientation for substrate membranes. Gbetagamma also engages the PH domain to activate PLC-beta2, and these two activation events are compatible, leading to additive stimulation of phospholipase activity. In contrast to PLC-delta, the PH domain of PLC-beta2 cannot bind phosphoinositides, eliminating this mode of regulation. The structure of the Rac1-PLC-beta2 complex reveals determinants that dictate selectivity of PLC-beta isozymes for Rac GTPases over other Rho-family GTPases, and substitutions within PLC-beta2 abrogate its stimulation by Rac1 but not by Gbetagamma, allowing for functional dissection of this integral signaling node.

Influence of differential stability of G protein βγ dimers containing the γ11 subunit on functional activity at the M1 muscarinic receptor, A1 adenosine receptor, and phospholipase C-β

Ggamma11 is an unusual guanine nucleotide-binding regulatory protein (G protein) subunit. To study the effect of different Gbeta-binding partners on gamma11 function, four recombinant betagamma dimers, beta1gamma2, beta4gamma2, beta1gamma11, and beta4gamma11, were characterized in a receptor reconstitution assay with the G(q)-linked M1 muscarinic and the G(i1)-linked A1 adenosine receptors. The beta4gamma11 dimer was up to 30-fold less efficient than beta4gamma2 at promoting agonist-dependent binding of [35S]GTPgammaS to either alpha(q) or alpha(i1). Using a competition assay to measure relative affinities of purified betagamma dimers for alpha, the beta4gamma11 dimer had a 15-fold lower affinity for G(i1) alpha than beta4gamma2. Chromatographic characterization of the beta4gamma11 dimer revealed that the betagamma is stable in a heterotrimeric complex with G(i1) alpha; however, upon activation of alpha with MgCl2 and GTPgammaS under nondenaturing conditions, the beta4 and gamma11 subunits dissociate. Activation of purified G(i1) alpha:beta4gamma11 with Mg+2/GTPgammaS following reconstitution into lipid vesicles and incubation with phospholipase C (PLC)-beta resulted in stimulation of PLC-beta activity; however, when this activation preceded reconstitution into vesicles, PLC-beta activity was markedly diminished. In a membrane coupling assay designed to measure the ability of G protein to promote a high-affinity agonist-binding conformation of the A1 adenosine receptor, beta4gamma11 was as effective as beta4gamma2 when coexpressed with G(i1) alpha and receptor. However, G(i1) alpha:beta4gamma11-induced high-affinity binding was up to 20-fold more sensitive to GTPgammaS than G(i1) alpha:beta4gamma2-induced high-affinity binding. These results suggest that the stability of the beta4gamma11 dimer can modulate G protein activity at the receptor and effector.

Structural Determinants for Phosphatidic Acid Regulation of Phospholipase C-β1

Signaling from G protein-coupled receptors to phospholipase C-beta (PLC-beta) is regulated by coordinate interactions among multiple intracellular signaling molecules. Phosphatidic acid (PA), a signaling phospholipid, binds to and stimulates PLC-beta(1) through a mechanism that requires the PLC-beta(1) C-terminal domain. PA also modulates Galpha(q) stimulation of PLC-beta(1). These data suggest that PA may have a key role in the regulation of PLC-beta(1) signaling in cells. The present studies addressed the structural requirements and the mechanism for PA regulation of PLC-beta(1). We used a combination of enzymatic assays, PA-binding assays, and circular dichroism spectroscopy to evaluate the interaction of PA with wild-type and mutant PLC-beta(1) proteins and with fragments of the Galpha(q) binding domain. The results identify a region that includes the alphaA helix and flexible loop of the Galpha(q)-binding domain as necessary for PA regulation. A mutant PLC-beta(1) with multiple alanine/glycine replacements for residues (944)LIKEHTTKYNEIQN(957) was markedly impaired in PA regulation. The high affinity and low affinity component of PA stimulation was reduced 70% and PA binding was reduced 45% in this mutant. Relative PLC stimulation by PA increased with PLC-beta(1) concentration in a manner suggesting cooperative binding to PA. Similar concentration dependence was observed in the PLC-beta(1) mutant. These data are consistent with a model for PA regulation of PLC-beta(1) that involves cooperative interactions, probably PLC homodimerization, that require the flexible loop region, as is consistent with the dimeric structure of the Galpha(q)-binding domain. PA regulation of PLC-beta(1) requires unique residues that are not required for Galpha(q) stimulation or GTPase-activating protein activity.

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.

Superantigens: supersignalers?

Some bacterial and viral proteins are potent activators of the immune response, earning them the title of superantigens (SAgs). Infection with pathogens containing these proteins can produce massive T cell activation and can result in various potentially fatal conditions, such as toxic shock and food poisoning. Unlike conventional peptide antigens, SAgs bind promiscuously to the external faces of class II major histocompatibility complex (MHC) molecules and families of T cell receptors (TCRs), thereby activating large numbers of T cells simultaneously. The manner in which SAgs bind MHC and TCR differs from the way in which peptide antigens interact with these structures. Nevertheless, because they simultaneously engage MHC and TCR, SAgs were assumed to activate T cells through the canonical signaling pathway that has been described for T cell activation by TCR engagement of peptide-MHC complexes. However, recent research shows that SAgs also activate an alternative signaling pathway in T cells. This study shows that SAgs can stimulate T cells in the absence of the Src family kinase, Lck, by activating a heterotrimeric guanine nucleotide-binding protein (G protein), Galpha(11). Galpha(11) activates phospholipase C-beta (PLC-beta), rather than the more abundant PLC-gamma1, and, by this means, links SAg signaling to the phosphatidylinositol and protein kinase C signaling pathways. The discovery of a signaling pathway specifically activated by SAgs, and not by conventional peptide antigens, opens the possibility of developing therapeutic reagents that may help control diseases caused by these agents.

Secretory cells of the airway express molecules of the chemoreceptive cascade

Airway secretion is maintained by specialized non-ciliated epithelial cells whose phenotype varies with their topographical location. In addition, specialized epithelial cells located in the airway contain the molecular machinery of chemoreceptive elements. Our aim has been to evaluate whether the secretory cells themselves possess a chemoreceptive capability, which requires the simultaneous presence of chemosensory and secretory mechanisms. We performed immunohistochemical analysis with antibodies against the Clara-cell-specific secretory proteins, CC10 and CC26, as secretory markers. As chemoreceptive markers, we employed antibodies against alpha-gustducin and phospholipase C beta 2 (PLCbeta2), two components of the taste transduction pathway. We also attempted to characterize further the secretory cell type by using a marker of chloride secretion, cystic fibrosis transmembrane regulator (CFTR). We found alpha-gustducin localized in non-ciliated cells of the epithelium lining the trachea and bronchioles of adult rats, where it was also co-expressed with CC10 and CC26. Ultrastructural immunohistochemistry revealed alpha-gustducin in the apical cytoplasm of secretory cells, concentrated around and inside the granules. CFTR was also observed in a subpopulation of non-ciliated epithelial cells, co-localized with some alpha-gustducin- and PLCbeta2-immunoreactive cells, at all levels of the airway epithelium. We conclude that non-ciliated epithelial cells of the rat airway express components of distinct signaling mechanisms and suggest that secretory events are driven by a molecular mechanism activated by the binding of luminal substances to G-protein-coupled receptors.

Characterization of S1P1 and S1P2 receptor function in smooth muscle by receptor silencing and receptor protection

Sphingosine-1-phosphate (S1P) induces an initial Ca(2+)-dependent contraction followed by a sustained Ca(2+)-independent, RhoA-mediated contraction in rabbit gastric smooth muscle cells. The cells coexpress S1P(1) and S1P(2) receptors, but the signaling pathways initiated by each receptor type and the involvement of one or both receptors in contraction are not known. Lentiviral vectors encoding small interfering RNAs were transiently transfected into cultured smooth muscle cells to silence S1P(1) or S1P(2) receptors. Phospholipase C (PLC)-beta activity and Rho kinase activity were used as markers of pathways mediating initial and sustained contraction, respectively. Silencing of S1P(1) receptors abolished S1P-stimulated activation of Galpha(i3) and partially inhibited activation of Galpha(i1), whereas silencing of S1P(2) receptors abolished activation of Galpha(q), Galpha(13), and Galpha(i2) and partially inhibited activation of Galpha(i1). Silencing of S1P(2) but not S1P(1) receptors suppressed S1P-stimulated PLC-beta and Rho kinase activities, implying that both signaling pathways were mediated by S1P(2) receptors. The results obtained by receptor silencing were corroborated by receptor inactivation. The selective S1P(1) receptor agonist SEW2871 did not stimulate PLC-beta or Rho kinase activity or induce initial and sustained contraction; when this agonist was used to protect S1P(1) receptors so as to enable chemical inactivation of S1P(2) receptors, S1P did not elicit contraction, confirming that initial and sustained contraction was mediated by S1P(2) receptors. Thus S1P(1) and S1P(2) receptors are coupled to distinct complements of G proteins. Only S1P(2) receptors activate PLC-beta and Rho kinase and mediate initial and sustained contraction.

Thyrotropin-releasing hormone increases GABA release in rat hippocampus

Thyrotropin-releasing hormone (TRH) is a tripeptide that is widely distributed in the brain including the hippocampus where TRH receptors are also expressed. TRH has anti-epileptic effects and regulates arousal, sleep, cognition, locomotion and mood. However, the cellular mechanisms underlying such effects remain to be determined. We examined the effects of TRH on GABAergic transmission in the hippocampus and found that TRH increased the frequency of GABAA receptor-mediated spontaneous IPSCs in each region of the hippocampus but had no effects on miniature IPSCs or evoked IPSCs. TRH increased the action potential firing frequency recorded from GABAergic interneurons in CA1 stratum radiatum and induced membrane depolarization suggesting that TRH increases the excitability of interneurons to facilitate GABA release. TRH-induced inward current had a reversal potential close to the K+ reversal potential suggesting that TRH inhibits resting K+ channels. The involved K+ channels were sensitive to Ba2+ but resistant to other classical K+ channel blockers, suggesting that TRH inhibits the two-pore domain K+ channels. Because the effects of TRH were mediated via Galphaq/11, but were independent of its known downstream effectors, a direct coupling may exist between Galphaq/11 and K+ channels. Inhibition of the function of dynamin slowed the desensitization of TRH responses. TRH inhibited seizure activity induced by Mg2+ deprivation, but not that generated by picrotoxin, suggesting that TRH-mediated increase in GABA release contributes to its anti-epileptic effects. Our results demonstrate a novel mechanism to explain some of the hippocampal actions of TRH.

Involvement of the phospholipase C beta1 pathway in desensitization of the carbachol-activated nonselective cationic current in murine gastric myocytes

In murine gastrointestinal myocytes muscarinic stimulation activates nonselective cation channels via a G-protein and Ca2+-dependent pathway. We recorded inward cationic currents following application of carbachol (ICCh) to murine gastric myocytes held at -60 mV, using the whole-cell patch-clamp method. The properties of the inward cationic currents were similar to those of the nonselective cation channels activated by muscarinic stimulation in other gastrointestinal smooth muscle cells. CCh-induced ICCh and spontaneous decay of ICCh (desensitization of ICCh) occurred. Unlike the situation in guinea pig gastric myocytes, desensitization was not affected by varying [EGTA]i. Pretreatment with the PLC inhibitor (U73122) blocked the activation of ICCh, and desensitization of ICCh was attenuated in PLC beta1 knock-out mice. These results suggest that the desensitization of ICCh in murine gastric myocytes is not due to a pathway dependent on intracellular Ca2+ but to the PLC beta1 pathway.

An essential role for endocytosis of rhodopsin through interaction of visual arrestin with the AP-2 adaptor

Previously, we have identified a class of retinal degeneration mutants in Drosophila in which the normally transient interaction between arrestin2 (Arr2) and rhodopsin is stabilized and the complexes are rapidly internalized into the cell body by receptor-mediated endocytosis. The accumulation of protein complexes in the cytoplasm eventually results in photoreceptor cell death. We now show that the endocytic adapter protein AP-2 is essential for rhodopsin endocytosis through an Arr2-AP-2β interaction, and mutations in Arr2 that disrupt its interaction with the β subunit of AP-2 prevent endocytosis-induced retinal degeneration. We further demonstrate that if the interaction between Arr2 and AP-2 is blocked, this also results in retinal degeneration in an otherwise wild-type background. This indicates that the Arr2-AP-2 interaction is necessary for the pathology observed in a number of Drosophila visual system mutants, and suggests that regular rhodopsin turnover in wild-type photoreceptor cells by Arr2-mediated endocytosis is essential for photoreceptor cell maintenance.

Stable Association between Gαq and Phospholipase Cβ1 in Living Cells

Signal transduction through G alpha(q) involves stimulation of phospholipase C beta (PLC beta) that results in increased intracellular Ca2+ and activation of protein kinase C. We have measured complex formation between G alpha(q) and PLC beta1 in vitro and in living PC12 and HEK293 cells by fluorescence resonance energy transfer. In vitro measurements show that PLC beta1 will bind to G alpha(q)(guanosine 5'-3-O-(thio)triphosphate) and also to G alpha(q)(GDP), and the latter association has a different protein-protein orientation. In cells, image analysis of fluorescent-tagged proteins shows that G alpha(q) is localized almost entirely to the plasma membrane, whereas PLC beta1 has a significant cytosolic population. By using fluorescence resonance energy transfer, we found that these proteins are pre-associated in the unstimulated state in PC12 and HEK293 cells. By determining the cellular levels of the two proteins in transfected versus nontransfected cells, we found that under our conditions overexpression should not significantly promote complex formation. G alpha(q)-PLC beta1 complexes are observed in both single cell measurements and measurements of a large (i.e. 10(6)) cell suspension. The high level (approximately 40% maximum) of FRET is surprising considering that G alpha(q) is more highly expressed than PLC beta1 and that not all PLC beta1 is plasma membrane-localized. Our measurements suggest a model in which G proteins and effectors can exist in stable complexes prior to activation and that activation is achieved through changes in intermolecular interactions rather than diffusion and association. These pre-formed complexes in turn give rise to rapid, localized signals.

Modulation of Ca2+ signals by phosphatidylinositol-linked novel D1 dopamine receptor in hippocampal neurons

Recent evidence indicates the existence of a putative novel phosphatidylinositol-linked D1 dopamine receptor in brain that mediates phosphatidylinositol hydrolysis via activation of phospholipase Cbeta. The present work was designed to characterize the Ca(2+) signals regulated by this phosphatidylinositol-linked D(1) dopamine receptor in primary cultures of hippocampal neurons. The results indicated that stimulation of phosphatidylinositol-linked D1 dopamine receptor by its newly identified selective agonist SKF83959 induced a long-lasting increase in basal [Ca(2+)](i) in a time- and dose-dependent manner. Stimulation was observable at 0.1 microm and reached the maximal effect at 30 microm. The [Ca(2+)](i) increase induced by 1 microm SKF83959 reached a plateau in 5 +/- 2.13 min, an average 96 +/- 5.6% increase over control. The sustained elevation of [Ca(2+)](i) was due to both intracellular calcium release and calcium influx. The initial component of Ca(2+) increase through release from intracellular stores was necessary for triggering the late component of Ca(2+) rise through influx. We further demonstrated that activation of phospholipase Cbeta/inositol triphosphate was responsible for SKF83959-induced Ca(2+) release from intracellular stores. Moreover, inhibition of voltage-operated calcium channel or NMDA receptor-gated calcium channel strongly attenuated SKF83959-induced Ca(2+) influx, indicating that both voltage-operated calcium channel and NMDA receptor contribute to phosphatidylinositol-linked D(1) receptor regulation of [Ca(2+)](i).

Taste Receptor Cells Express Voltage-Dependent Potassium Channels in a Cell Age-Specific Manner

Two voltage-dependent potassium channels, KCNQ1 and KCNH2, are expressed in the taste buds and were identified as strong candidates involved in the repolarization of taste receptor cells expressing phospholipase C-beta2 and TRPM5 (beta2/M5-TRCs). In cell type-specific expression, KCNQ1 was expressed in most taste bud cells, including beta2/M5-TRCs, whereas KCNH2 was expressed in a subset of beta2/M5-TRCs with no correlation with their taste modality, such as sweet or bitter taste reception. Expression of KCNH2 was restricted to young beta2/M5-TRCs. These results suggest that taste bud cells other than beta2/M5-TRCs are depolarized by some stimuli and also that beta2/M5-TRCs have cell age-dependent molecular mechanisms of repolarization.

Taste bud contains both short-lived and long-lived cell populations

Taste bud cells undergo continual turnover even in adulthood, and their average lifespan has been estimated as approximately 10 days. However, it is not clear whether this figure can be applied to all the different cell types contained in a taste bud. Here, we describe the age and life cycle of taste bud cells in rat circumvallate papillae, and indicate that the lifespan is heterogeneous, ranging from 2 days to over 3 weeks. Taste bud cells were incorporated from the basal proliferative layer in 1-2 days after birth. After incorporation, approximately half of the cells were eliminated within 2-3 days, and the remaining half were maintained with gradual decrease, suggesting that there are at least two types of cells; short-lived cells and long-lived cells. Moreover, above 10% of the incorporated cells were maintained at 3 weeks. In order to gain information about the relationship between the cell functions and the cell age, we carried out double-labeling experiments using 5-bromo-2'-deoxyuridine and each of two markers for in situ hybridization: mammalian achaete-scute homolog 1 (Mash1) and phospholipase C beta 2 (PLCbeta2) as markers of early differentiation and functional taste signaling, respectively. Mash1 expression began immediately after the incorporation and reached a maximum at 5-6 days after birth. Fewer but distinct Mash1-positive cells were still observed after 3 weeks. PLCbeta2 expression was observed from day 5, reached a maximum at day 12, and continued over 3 weeks. Taken together, a taste bud contains both short-lived and long-lived cells: the short-lived cells are eliminated in a time course similar to the surrounding epithelial cells, and the long-lived cells including taste receptor cells have a lifespan longer than the previous estimation.

Gsalpha is involved in sugar perception in Drosophila melanogaster

In Drosophila melanogaster, gustatory receptor genes (Grs) encode G-protein-coupled receptors (GPCRs) in gustatory receptor neurons (GRNs) and some olfactory receptor neurons. One of the Gr genes, Gr5a, encodes a sugar receptor that is expressed in a subset of GRNs and has been most extensively studied both molecularly and physiologically, but the G-protein alpha subunit (Galpha) that is coupled to this sugar receptor remains unknown. Here, we propose that Gs is the Galpha that is responsible for Gr5a-mediated sugar-taste transduction, based on the following findings: First, immunoreactivities against Gs were detected in a subset of GRNs including all Gr5a-expressing neurons. Second, trehalose-intake is reduced in flies heterozygous for null mutations in DGsalpha, a homolog of mammalian Gs, and trehalose-induced electrical activities in sugar-sensitive GRNs were depressed in those flies. Furthermore, expression of wild-type DGsalpha in sugar-sensitive GRNs in heterozygotic DGsalpha mutant flies rescued those impairments. Third, expression of double-stranded RNA for DGsalpha in sugar-sensitive GRNs depressed both behavioral and electrophysiological responses to trehalose. Together, these findings indicate that DGsalpha is involved in trehalose perception. We suggest that sugar-taste signals are processed through the Gsalpha-mediating signal transduction pathway in sugar-sensitive GRNs in Drosophila.

Bacterial Superantigens Bypass Lck-Dependent T Cell Receptor Signaling by Activating a Gα11-Dependent, PLC-β-Mediated Pathway

The paradigm to explain antigen-dependent T cell receptor (TCR) signaling is based on the activation of the CD4 or CD8 coreceptor-associated kinase Lck. It is widely assumed that this paradigm is also applicable to signaling by bacterial superantigens. However, these bacterial toxins can activate human T cells lacking Lck, suggesting the existence of an additional pathway of TCR signaling. Here we showed that this alternative pathway operates in the absence of Lck-dependent tyrosine-phosphorylation events and was initiated by the TCR-dependent activation of raft-enriched heterotrimeric Galpha11 proteins. This event, in turn, activated a phospholipase C-beta and protein kinase C-mediated cascade that turned on the mitogen-activated protein kinases ERK-1 and ERK-2, triggered Ca(2+) influx, and translocated the transcription factors NF-AT and NF-kappaB to the nucleus, ultimately inducing the production of interleukin-2 in Lck-deficient T cells. The triggering of this alternative pathway by superantigens suggests that these toxins use a G protein-coupled receptor as a coreceptor on T cells.

Transgenic labeling of taste receptor cells in model fish under the control of the 5'-upstream region of medaka phospholipase C-beta 2 gene

Vertebrate taste receptor cells express signaling molecules such as taste receptors and effectors to convert taste stimuli to inner cellular signals. Phospholipase C-beta2 (PLC-beta2) is an effector enzyme that is necessary to transduce taste signals in the mouse. It was shown that a subset of the plc-beta2 expressing cells also express taste receptor molecules, T1Rs or T2Rs, in mammals and fish. To label plc-beta2 expressing cells in the model fish species, we constructed a transgene by linking the 5'-upstream region of the medaka plc-beta2 gene to a green fluorescent protein (GFP) gene. The resulting transgenic medaka exhibited GFP signals in taste buds of the lips and the pharyngeal region. Detailed observation revealed that the GFP signals were in a subpopulation of taste bud cells, and co-localized with the transcript of endogenous plc-beta2 gene. Zebrafish introduced with the same transgene showed GFP signals in a subpopulation of taste bud cells of the lips and the pharyngeal region as in the case of medaka. This is the first report of successful labeling of taste receptor cells in two model fish species under the control of the plc-beta2 promoter. This promoter will be a useful genetic tool to study the vertebrate taste system in general.

Neuropeptide Y in the olfactory microvillar cells

This paper examines a possible role of microvillar cells in coordinating cell death and regeneration of olfactory epithelial neurons. The olfactory neuroepithelium of mammals is a highly dynamic organ. Olfactory neurons periodically degenerate by apoptosis and as a consequence of chemical or physical damage. To compensate for this loss of cells, the olfactory epithelium maintains a lifelong ability to regenerate from a pool of resident multipotent stem cells. To assure functional continuity and histological integrity of the olfactory epithelium over a period of many decades, apoptosis and regeneration require to be precisely coordinated. Among the factors that have been implicated in mediating this regulation is the neuropeptide Y (NPY). Knockout mice that lack functional expression of this neurogenic peptide show defects in embryonic development of the olfactory epithelium and in its ability to regenerate in the adult. Here we show that, in postnatal olfactory epithelia, NPY is exclusively expressed by a specific population of microvillar cells. We previously characterized these cells as a novel type of putative chemosensory cells, which are provided with a phosphatidyl-inositol-mediated signal transduction cascade. Our findings allow for the first time to suggest that microvillar cells are involved in connecting apoptosis to neuronal regeneration by stimulus-induced release of NPY.

The pleckstrin homology domain of phospholipase Cbeta transmits enzymatic activation through modulation of the membrane-domain orientation

Phospholipase Cbeta (PLCbeta) enzymes are activated by Galpha q and Gbetagamma subunits and catalyze the hydrolysis of the minor membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Activation of PLCbeta2 by Gbetagamma subunits has been shown to be conferred through its N-terminal pleckstrin homology (PH) domain, although the underlying mechanism is unclear. Also unclear are observations that the extent of Gbetagamma activation differs on different membrane surfaces. In this study, we have identified a unique region of the PH domain of the PLCbeta2 domain (residues 71-88) which, when added to the enzyme as a peptide, causes enzyme activation similar to that with Gbetagamma subunits. This PH domain segment interacts strongly with membranes composed of lipid mixtures but not those containing lipids with electrically neutral zwitterionic headgroups. Also, addition of this segment perturbs interaction of the catalytic domain, but not the PH domain, with membrane surfaces. We monitored the orientation of the PH and catalytic domains of PLC by intermolecular fluorescence resonance energy transfer (FRET) using the Gbetagamma activatable mutant, PLCbeta2/delta1(C193S). We find an increase in the level of FRET with binding to membranes with mixed lipids but not to those containing only lipids with electrically neutral headgroups. These results suggest that enzymatic activation can be conferred through optimal association of the PHbeta71-88 region to specific membrane surfaces. These studies allow us to understand the basis of variations of Gbetagamma activation on different membrane surfaces.

Inhibition of phospholipase C-beta1-mediated signaling by O-GlcNAc modification

Here we report inhibition of phospholipase C-beta1 (PLC-beta1)-mediated signaling by post-translational glycosylation with beta-N-acetylglucosamine (O-GlcNAc modification). In C2C12 myoblasts, isoform-specific knock-down experiments using siRNA showed that activation of bradykinin (BK) receptor led to stimulation of PLC-beta1 and subsequent intracellular Ca2+ mobilization. In C2C12 myotubes, O-GlcNAc modification of PLC-beta1 was markedly enhanced in response to treatment with glucosamine (GlcNH2), an inhibitor of O-GlcNAase (PUGNAc) and hyperglycemia. This was associated with more than 50% inhibition of intracellular production of IP3 and Ca2+ mobilization in response to BK. Since the abundance of PLC-beta1 remained unchanged, these data suggest that O-GlcNAc modification of PLC-beta1 led to inhibition of its activity. Moreover, glucose uptake stimulated by BK was significantly blunted by treatment with PUGNAc. These data support the notion that O-GlcNAc modification negatively modulates the activity of PLC-beta1.

Phospholipase C-β3 and -β1 Form Homodimers, but Not Heterodimers, through Catalytic and Carboxyl-Terminal Domains

Phospholipase C-beta (PLC-beta) isoenzymes are key effectors in G protein-coupled signaling pathways. Prior research suggests that some isoforms of PLC-beta may exist and function as dimers. Using coimmunoprecipitation assays of differentially tagged PLC-beta constructs and size-exclusion chromatography of native PLC-beta, we observed homodimerization of PLC-beta3 and PLC-beta1 isoenzymes but failed to detect heterodimerization of these isoenzymes. Size-exclusion chromatography data suggest that PLC-beta3 and PLC-beta1 form higher affinity homodimers than PLC-beta2. Evidence supportive of limited PLC-beta monomer-homodimer equilibrium appears at < or =100 nM. Further assessment of homodimerization status by coimmunoprecipitation assays with differentially tagged PLC-beta3 fragments demonstrated that at least two subdomains of PLC-beta3 are involved in dimer formation, one in the catalytic X and Y domains and the other in the G protein-regulated carboxyl-terminal domain. In addition, we provide evidence consistent with the existence of PLC-beta homodimers in a whole-cell context, using fluorescent protein-tagged constructs and microscopic fluorescence resonance energy transfer assays.

Protein kinase C modulates ecdysteroidogenesis in the prothoracic gland of the tobacco hornworm, Manduca sexta

The prothoracic gland is the primary source of ecdysteroid hormones in the immature insect. Ecdysteroids coordinate gene expression necessary for growth, molting and metamorphosis. Prothoracicotropic hormone (PTTH), a brain neuropeptide, regulates ecdysteroid synthesis in the prothoracic gland. PTTH stimulates ecdysteroid synthesis through a signal transduction cascade that involves at least four protein kinases: protein kinase A (PKA), p70 S6 kinase, an unidentified tyrosine kinase, and the extracellular signal-regulated kinase (ERK). In this report, the participation of protein kinase C (PKC) in PTTH signalling is demonstrated and characterized. PTTH stimulates PKC activity through a PLC and Ca(2+)-dependent pathway that is not cAMP regulated. Inhibition of PKC inhibits PTTH-stimulated ecdysteroidogenesis as well as PTTH-stimulated phosphorylation of ERK and its upstream regulator, MAP/ERK kinase (MEK). These observations reveal that the acute regulation of prothoracic gland steroidogenesis is dependent on a web of interacting kinase pathways, which probably converge on factors that regulate translation.

Regulation of cardiomyocyte signaling by RGS proteins: differential selectivity towards G proteins and susceptibility to regulation

Many signals that regulate cardiomyocyte growth, differentiation and function are mediated via heterotrimeric G proteins, which are under the control of RGS proteins (Regulators of G protein Signaling). Several RGS proteins are expressed in the heart, but so far little is known about their function and regulation. Using adenoviral gene transfer, we conducted the first comprehensive analysis of the capacity and selectivity of the major cardiac RGS proteins (RGS2-RGS5) to regulate central G protein-mediated signaling pathways in adult ventricular myocytes (AVM). All four RGS proteins potently inhibited Gq/11-mediated phospholipase C beta stimulation and cell growth (assessed in neonatal myocytes). Importantly, RGS2 selectively inhibited Gq/11 signaling, whereas RGS3, RGS4 and RGS5 had the capacity to regulate both Gq/11 and Gi/o signaling (carbachol-induced cAMP inhibition). Gs signaling was unaffected, and, contrary to reports in other cell lines, RGS2-RGS5 did not appear to regulate adenylate cyclase directly in AVM. Since RGS proteins can be highly regulated in their expression by many different stimuli, we also tested the hypothesis that RGS expression is subject to G protein-mediated regulation in AVM and determined the specificity with which enhanced G protein signaling alters endogenous RGS expression in AVM. RGS2 mRNA and protein were markedly but transiently up-regulated by enhanced Gq/11 signaling (alpha1-adrenergic stimulation or Galphaq* overexpression), possibly by a negative feedback mechanism. In contrast, the other negative regulators of Gq/11 signaling (RGS3-RGS5) were unchanged. Endogenous RGS2 (but not RGS3-RGS5) expression was also up-regulated in cells with enhanced AC signaling (beta-adrenergic or forskolin stimulation). Taken together, these findings suggest diverse roles of RGS proteins in regulating myocyte signaling. RGS2 emerged as the only selective and highly regulated inhibitor of Gq/11 signaling that could potentially become a promising target for ameliorating Gq/11-mediated signaling and growth.