Cleavage Of Phosphatidylinositol Research Articles

Pathological Eye Growth in Myopia: a Limited Review on Endocrine Physiology for Future Research

Purpose: Myopia or short-sightedness: first manifests in school-children (early-onset), and also in young adults (late-onset). Pathological myopia can lead to stretching of the eyeball to the point of blindness from a detached retina. This title presents a preliminary understanding of endocrine, juxtacrine, and paracrine factors: hoping to stimulate further research and enable early therapeutic intervention. Background: The optical stimulus for eye growth is a blurred image on the retina that results from under-focused accommodation of the internal crystalline lens. Neural signals and biochemical messaging from the retina and vascular choroid provide local impetus to alter collagen structure in the outer white sclera of the eye. In persons that develop highly myopic refractive error, additional factors from blood and tissue chemistry, might influence intra-ocular pressure (IOP), posited to influence eyeball elongation. New research shows that GABA and dopamine are involved, but other factors remain unexplored. Eye Physiology: Human sclera has developmental origination same as any other connective tissue derived from the neural crest and mesoderm. Bundles of collagen fibers exist in the white opaque sclera and are more stringently oriented in the dome of the clear, transparent cornea. “Deformation compliance” of the sclera is governed by complexity of the collagen architecture, and “reduced scleral rigidity” is a known factor for myopic elongation, but developmental physiology is not well understood. Muscular contraction is required for sustained eye focusing to objects located at normal working distances. Short-term contractions can reduce IOP but increased muscle density from sustained near accommodation can promote progression of myopia. This implicates nitric oxide and calcium channels, and a role for associated biochemical and nutritional agonists and antagonists. Endocrine Influences: In his 1954 lecture to the Royal College of Surgeons, Dr. Howard C. Naffziger pointed to all forms of “ocular protrusion,” not just “thyroid exophthalmos,” as being physiologically related. A 2016 review paper on autonomic nervous physiology, aside from sympathetic (adrenergic) and parasympathic (collinergic) control, also implicates Substance P (SP), Vasoactive Intestinal Peptide (VIP), and Phospholipase C (PC). These topics are further described in limited detail. SP is a neuropeptide of the tachykinin family, is released from the terminals of sensory nerves, and regulates emotion (hypothalamus, amygdala, and the periaqueductal gray). It is also a vasodilator dependent on release of nitric oxide. Experimental eye physiology and clinical research pertaining to these topics appears warranted. VIP regulates cardiac contractions, relaxes smooth muscle of the trachea, and affects prolactin secretion by the pituitary gland. Cardiac contractions lead to pulsations of eye pressure about three times in two seconds, and pulmonary effects on IOP occur once every five seconds. Research into these mechanisms requires cross-disciplinary collaborations. PC is a membrane-associated enzyme that cleaves dietary phospholipids and has downstream effects on calcium channels. Binding to blood platelets, of agonists such as thrombin, epinephrine, or collagen: activates PC to release arachidonic acid from cleavage of phosphatidylinositol and phosphatidyl choline. Behavioral effects include visual attention-span and memory: both of which also concern education of children, and effectiveness of adults at work. Conclusions: The author is pleased to report growing interest and evidence for endocrine influences upon the outer coat of the eye (cornea and sclera). A 2022 study on “keratoconus” from Texas and Oklahoma provides in-vitro as well as in-vivo data, suggesting the existence of what they call, the Hypothalamic-Pituitary-Adrenal-Corneal axis. Effective communication across university disciplines and clinic doctors appears necessary for further progress. Funding: The author received no funds from Federal or State granting mechanisms, and no wages from corporate employers. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Identifying Motifs for Phosphatidylcholine Activation of Bacterial Phosphatidylinositol-Specific Phospholipase C Enzymes

Phosphatidylinositol-specific phospholipase Cs (PI-PLCs) secreted by pathogenic bacteria are often virulence factors. For one well-studied example, Bacillus thuringiensis PI-PLC (BtPI-PLC), the presence of the non-substrate lipid phosphatidylcholine (PC) in membranes enhances both membrane binding and enzymatic activity towards phosphatidylinositol (PI). A strip of four surface-exposed Tyr residues near the rim of the α/β-barrel are involved in this enhancement, and it has been proposed that this strip contributes to a specific PC binding site. The related PI-PLC from Staphylococcus aureus (SaPI-PLC), has similar kinetic characteristics to BtPI-PLC, but has only two Tyr residues in the same region. While SaPI-PLC can also be specifically activated towards PI cleavage by incorporation of PC into assay systems, SaPI-PLC membrane binding as a function of PC content is considerably weaker than that of BtPI-PLC. Mutagenesis of the two SaPI-PLC Tyr residues (Y253S/Y255S) reduces interfacial activity of SaPI-PLC. Adding the two ‘missing’ Tyr residues (N254Y/H258Y) did not further enhance SaPI-PLC specific activity but it dramatically enhanced binding of the protein to PC-rich vesicles. This suggests that PC activation is more complicated than simply binding the protein to membranes. Comparing enzyme kinetics and the effects of mutations on BtPI-PLC, SaPI-PLC and the PI-PLC from an intracellular pathogen (Listeria monocytogenes) suggests a rationale for the acquisition of a discrete PC binding site in some, but not all, PI-PLC enzymes.

The Fifth Transmembrane Domain of Angiotensin II Type 1 Receptor Participates in the Formation of the Ligand-binding Pocket and Undergoes a Counterclockwise Rotation upon Receptor Activation

The octapeptide hormone angiotensin II exerts a wide variety of cardiovascular effects through the activation of the angiotensin II Type 1 (AT(1)) receptor, which belongs to the G protein-coupled receptor superfamily. Like other G protein- coupled receptors, the AT(1) receptor possesses seven transmembrane domains that provide structural support for the formation of the ligand-binding pocket. The role of the fifth transmembrane domain (TMD5) was investigated using the substituted cysteine accessibility method. All of the residues within Thr-190 to Leu-217 region were mutated one at a time to cysteine, and after expression in COS-7 cells, the mutant receptors were treated with the sulfhydryl-specific alkylating agent methanethiosulfonate-ethylammonium (MTSEA). MTSEA reacts selectively with water-accessible, free sulfhydryl groups of endogenous or introduced point mutation cysteines. If a cysteine is found in the binding pocket, the covalent modification will affect the binding kinetics of the ligand. MTSEA substantially decreased the binding affinity of L197C-AT(1), N200C-AT(1), I201C-AT(1), G203C-AT(1), and F204C-AT(1) mutant receptors, which suggests that these residues orient themselves within the water-accessible binding pocket of the AT(1) receptor. Interestingly, this pattern of acquired MTSEA sensitivity was altered for TMD5 reporter cysteines engineered in a constitutively active N111G-AT(1) receptor background. Indeed, mutant I201C-N111G-AT(1) became more sensitive to MTSEA, whereas mutant G203C-N111G-AT(1) lost some sensitivity. Our results suggest that constitutive activation of AT(1) receptor causes an apparent counterclockwise rotation of TMD5 as viewed from the extracellular side.

Modulation of Bacillus thuringiensis Phosphatidylinositol-specific Phospholipase C Activity by Mutations in the Putative Dimerization Interface

Cleavage of phosphatidylinositol (PI) to inositol 1,2-(cyclic)-phosphate (cIP) and cIP hydrolysis to inositol 1-phosphate by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C are activated by the enzyme binding to phosphatidylcholine (PC) surfaces. Part of this reflects improved binding of the protein to interfaces. However, crystallographic analysis of an interfacially impaired phosphatidylinositol-specific phospholipase (W47A/W242A) suggested protein dimerization might occur on the membrane. In the W47A/W242A dimer, four tyrosine residues from one monomer interact with the same tyrosine cluster of the other, forming a tight dimer interface close to the membrane binding regions. We have constructed mutant proteins in which two or more of these tyrosine residues have been replaced with serine. Phospholipid binding and enzymatic activity of these mutants have been examined to assess the importance of these residues to enzyme function. Replacing two tyrosines had small effects on enzyme activity. However, removal of three or four tyrosine residues weakened PC binding and reduced PI cleavage by the enzyme as well as PC activation of cIP hydrolysis. Crystal structures of Y247S/Y251S in the absence and presence of myo-inositol as well as Y246S/Y247S/Y248S/Y251S indicate that both mutant proteins crystallized as monomers, were very similar to one another, and had no change in the active site region. Kinetic assays, lipid binding, and structural results indicate that either (i) a specific PC binding site, critical for vesicle activities and cIP activation, has been impaired, or (ii) the reduced dimerization potential for Y246S/Y247S/Y248S and Y246S/Y247S/Y248S/Y251S is responsible for their reduced catalytic activity in all assay systems.

The relative role of PLCβ and PI3Kγ in platelet activation

Stimulation of platelet G protein-coupled receptors results in the cleavage of phosphatidylinositol 4,5-trisphosphate (PIP(2)) into inositol 1,4,5-trisphosphate and 1,2-diacylglycerol by phospholipase C (PLCbeta). It also results in the phosphorylation of PIP2 by the gamma isoform of phosphatidylinositol 3-kinase (PI3Kgamma) to synthesize phosphatidylinositol 3,4,5-trisphosphate. To understand the role of PIP2 in platelet signaling, we evaluated knock-out mice lacking 2 isoforms of PLCbeta (PLCbeta2 and PLCbeta3) or lacking the G(betagamma)-activated isoform of PI3K (PI3Kgamma). Both knock-out mice were unable to form stable thrombi in a carotid injury model. To provide a functional explanation, knock-out platelets were studied ex vivo. PLCbeta2/beta3-/- platelets failed to assemble filamentous actin, had defects in both secretion and mobilization of intracellular calcium, and were unable to form stable aggregates following low doses of agonists. Platelets lacking PI3Kgamma disaggregated following low-dose adenosine diphosphate (ADP) and had a mildly impaired ability to mobilize intracellular calcium. Yet, they exhibited essentially normal actin assembly and secretion. Remarkably, both PLCbeta2/beta3-/- and PI3Kgamma-/- platelets spread more slowly upon fibrinogen. These results suggest substantial redundancy in platelet signaling pathways. Nonetheless, the diminished ability of knock-out platelets to normally spread after adhesion and to form stable thrombi in vivo suggests that both PLCbeta2/beta3 and PI3Kgamma play vital roles in platelet cytoskeletal dynamics.

Signaling Via the T Cell Receptor

Upon T cell receptor (TCR) engagement, Lck (a member of the Src family of protein tyrosine kinases) phosphorylates immunoreceptor tyrosine-based activation motifs (ITAMs) contained within the cytoplasmic domains of the chains of the CD3 complex. Subsequently, Zap70 (a member of the Syk family of kinases) is recruited by binding of its Src homology 2 (SH2) domains to the phosphorylated ITAM sites. Activated Zap70 propagates signal transduction through the phosphorylation of downstream targets, including the adaptor molecules LAT and SLP-76. These adaptors facilitate phospholipase C-γ1 (PLCγ1) activation, resulting in the cleavage of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ] to inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG). The known signaling pathways that lead to T cell activation upon TCR engagement rely on IP 3 and DAG second messengers. IP 3 triggers calcium mobilization, which leads to activation of the transcription factor nuclear factor of activated T cells (NF-AT). DAG activates Ras guanine nucleotide-releasing protein (RasGRP) and protein kinase C θ (PKCθ), which in turn lead to activation of the Ras-mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) pathways, respectively. CD45 and Csk have been identified as key proximal regulators of T cell signal transduction by modulating phosphorylation and, hence, enzymatic activity of the Src family kinases. The Connections Maps Pathway emphasizes the signaling events that lead to changes in gene expression in response to T cell activation. Science Viewpoint A. L. Singer, G. A. Koretzky, Control of T cell function by positive and negative regulators. Science 296 , 1639-1640 (2002). [ Abstract ] [ Full Text ]

Leaky vesicle fusion induced by phosphatidylinositol-specific phospholipase C: observation of mixing of vesicular inner monolayers.

Large unilamellar vesicles containing phosphatidylinositol (PI), neutral phospholipids, and cholesterol are induced to fuse by the catalytic activity of phosphatidylinositol-specific phospholipase C (PI-PLC). PI cleavage by PI-PLC is followed by vesicle aggregation, intervesicular lipid mixing, and mixing of vesicular aqueous contents. An average of 2-3 vesicles merge into a large one in the fusion process. Vesicle fusion is accompanied by leakage of vesicular contents. A novel method has been developed to monitor mixing of lipids located in the inner monolayers of the vesicles involved in fusion. Using this method, the mixing of inner monolayer lipids and that of vesicular aqueous contents are seen to occur simultaneously, thus giving rise to the fusion pore. Kinetic studies show, for fusing vesicles, second-order dependence of lipid mixing on diacylglycerol concentration in the bilayer. Varying proportions of PI in the liposomal formulation lead to different physical effects of PI-PLC. Specifically, 30-40 mol % PI lead to vesicle fusion, while with 5-10 mol % PI only hemifusion is detected, i.e., mixing of outer monolayer lipids without mixing of aqueous contents. However, when diacylglycerol is included in the bilayers containing 5 mol % PI, PI-PLC activity leads to complete fusion.

Ca2+-activated but Not G Protein-mediated Inositol Phosphate Responses in Rat Neonatal Cardiomyocytes Involve Inositol 1,4,5-Trisphosphate Generation

Inositol phosphate (InsP) responses to receptor activation are assumed to involve phospholipase C cleavage of phosphatidylinositol 4,5-bisphosphate to generate Ins(1,4,5)P(3). However, in [(3)H]inositol-labeled rat neonatal cardiomyocytes (NCM) both initial and sustained [(3)H]InsP responses to alpha(1)-adrenergic receptor stimulation with norepinephrine (100 microM) were insensitive to the phosphatidylinositol 4,5-bisphosphate-binding agent neomycin (5 mM). Introduction of 300 microM unlabeled Ins(1,4, 5)P(3) into guanosine 5'-3-O-(thio)triphosphate (GTPgammaS)-stimulated, permeabilized [(3)H]inositol-labeled NCM increased [(3)H]Ins(1,4,5)P(3) slightly but did not significantly reduce levels of its metabolites [(3)H]Ins(1,4)P(2) and [(3)H]Ins(4)P, suggesting that these [(3)H]InsPs are not formed principally from [(3)H]Ins(1,4,5)P(3). In contrast, the calcium ionophore A23187 (10 microM) provoked [(3)H]InsP responses in intact NCM which were sensitive to neomycin, and elevation of free calcium in permeabilized NCM led to [(3)H]InsP responses characterized by marked increases in [(3)H]Ins(1,4,5)P(3) (2.9 +/- 0.2% of total [(3)H]InsPs after 20 min of high Ca(2+) treatment in comparison to 0. 21 +/- 0.05% of total [(3)H]InsPs accumulated after 20 min of GTPgammaS stimulation). These data provide evidence that Ins(1,4, 5)P(3) generation is not a major contributor to G protein-coupled InsP responses in NCM, but that substantial Ins(1,4,5)P(3) generation occurs under conditions of Ca(2+) overload. Thus in NCM, Ca(2+)-induced Ins(1,4,5)P(3) generation has the potential to worsen Ca(2+) overload and thereby aggravate Ca(2+)-induced electrophysiological perturbations.

Conditional expression of glycosylphosphatidylinositol phospholipase C in Trypanosoma brucei

Trypanosoma brucei glycosylphosphatidylinositol phospholipase C (GPIPLC) is expressed in the bloodstream stage of the life cycle, but not in the procyclic form. It is capable of hydrolyzing GPI-anchored proteins and phosphatidylinositol (PI) in vitro. Several roles have been proposed for GPIPLC in vivo, in the release of variant surface glycoprotein during differentiation or in the regulation of GPI and PI levels, but none has been substantiated. To explore GPIPLC function in vivo, tetracycline-inducible GPIPLC gene ( GPIPLC) conditional knock-out bloodstream form and tetracycline-inducible GPIPLC-expressing procyclic cell lines were constructed. We were unable to generate GPIPLC null mutants. Cleavage of GPI-anchored proteins was abolished in extracts from uninduced conditional knock-outs and was restored upon induction. Despite the barely detectable level of GPIPLC activity in uninduced conditional knock-out bloodstream forms, their growth was not affected. GPI-protein cleavage activity could be induced in procyclic cell extracts, up to wild-type bloodstream levels. Myo-[ 3H]inositol incorporation into [ 3H]inositol monophosphate was about 14-fold lower in GPIPLC conditional knock-out bloodstream forms than in the wild type. Procyclic cells expressing GPIPLC showed a 28-fold increase in myo-[ 3H]inositol incorporation into [ 3H]inositol monophosphate and a 1.5-fold increase in [ 3H]inositol trisphosphate levels, suggesting that GPIPLC may regulate levels of inositol phosphates, by cleavage of PI and phosphatidylinositol 4,5-bisphosphate.

Catecholamines in cardiac hypertrophy

There has been intense interest in the roles catecholamines may play in compensatory myocardial hypertrophy. This article reviews the following: (1) chronic infusions of catecholamines in experimental animals result in cardiac hypertrophy, but in many of the studies mechanical factors have played a role; (2) experiments using isolated papillary muscles and isolated hearts, stretched isolated myocytes, and denervated hearts in vivo demonstrate that mechanical activity is sufficient to cause increased protein synthesis and cell growth; (3) in neonatal myocyte cell cultures, α-adrenergic agonists are powerful stimulants for protein synthesis and cell growth. Beta-adrenergic stimulation of nonmyocyte myocardial cells causes release of a factor that promotes protein synthesis in neonatal myocytes. Either α or β stimulation, probably through different mechanisms, appears to have growth-promoting effects on isolated adult myocytes in culture; (4) α stimulation is transduced through the Gq pathway and its activation of phospholipase C, cleavage of phosphatidylinositol (4,5)-bisphosphate, and then further through the ras/raf, mitogen-activated protein (MAP) kinase system; (5) transgenic mice with upregulation of catecholamine-related systems have not clarified the independent role of either the α- or β-adrenergic pathway; and (6) observations in humans suggest that mechanical factors predominate in the development and regression of cardiac hypertrophy. Humoral mechanisms, including catecholamines, may play a role, but their quantitative importance has not been determined. It is hypothesized that catecholamines may play a role in transition from the adaptive to the maladaptive state.

Action of Phosphatidylinositol-specific Phospholipase Cγ1 on Soluble and Micellar Substrates: SEPARATING EFFECTS ON CATALYSIS FROM MODULATION OF THE SURFACE

The kinetics of PI-PLCgamma1 toward a water-soluble substrate (inositol 1,2-cyclic phosphate, cIP) and phosphatidylinositol (PI) in detergent mixed micelles were monitored by 31P NMR spectroscopy. That cIP is also a substrate (Km = approximately 15 mM) implies a two-step mechanism (intramolecular phosphotransferase reaction to form cIP followed by cyclic phosphodiesterase activity to form inositol-1-phosphate (I-1-P)). PI is cleaved by PI-PLCgamma1 to form cIP and I-1-P with the enzyme specific activity and ratio of products (cIP/I-1-P) regulated by assay temperature, pH, Ca2+, and other amphiphilic additives. Cleavage of both cIP and PI by the enzyme is optimal at pH 5. The effect of Ca2+ on PI-PLCgamma1 activity is unique compared with other isozymes enzymes: Ca2+ is necessary for the activity and low Ca2+ activates the enzyme; however, high Ca2+ inhibits PI-PLCgamma1 hydrolysis of phosphoinositides (but not cIP) with the extent of inhibition dependent on pH, substrate identity (cIP or PI), substrate presentation (e.g. detergent matrix), and substrate surface concentration. This inhibition of PI-PLCgamma1 by high Ca2+ is proposed to derive from the divalent metal ion-inducing clustering of the PI and reducing its accessibility to the enzyme. Amphiphilic additives such as phosphatidic acid, fatty acid, and sodium dodecylsulfate enhance PI cleavage in micelles at pH 7.5 but not at pH 5.0; they have no effect on cIP hydrolysis at either pH value. These different kinetic patterns are used to propose a model for regulation of the enzyme. A key hypothesis is that there is a pH-dependent conformational change in the enzyme that controls accessibility of the active site to both water-soluble cIP and interfacially organized PI. The low activity enzyme at pH 7.5 can be activated by PA (or phosphorylation by tyrosine kinase). However, this activation requires lipophilic substrate (PI) present because cIP hydrolysis is not enhanced in the presence of PA.

Altered Postreceptor Signal Transduction of Formyl-Met-Leu-Phe Receptors in Polymorphonuclear Leukocytes of Patients with Non-Insulin-Dependent Diabetes Mellitus

The signal transduction of the formyl-Met-Leu-Phe (FMLP) receptor in polymorphonuclear leukocytes (PMNLs) from patients with non-insulin-dependent diabetes mellitus (NIDDM) was compared to that of PMNLs obtained from healthy volunteers. According to our previous studies in this group of patients neither the decrease in insulin binding capacity nor the enhanced insulin-degrading enzyme activity was involved. In control PMNLs, 10 nMFMLP induced a pertussis toxin-sensitive increase in phosphatidyl inositol (PI) cleavage and a subsequent Ca2+signaling from the intracellular pools. On the other hand, the FMLP-induced protein kinase C (PKC) activation and translocation into the membrane could not be detected in these cells via the measurement of32P incorporation into histone. In contrast, in PMNLs of this special group of patients suffering from NIDDM the FMLP stimulus produced a significantly low increase in PI cleavage and Ca2+signaling from the intracellular pools. Moreover, in resting PMNLs of these patients with NIDDM, not only the [Ca2+]ibut also the membrane-bound PKC activity was found to be significantly increased. In addition, PKC translocation into the cell membrane of diabetic PMNLs could be further triggered with FMLP as judged by the measurement of32P incorporation into histone. Based on these results, it appears that the signaling of FMLP receptors in PMNLs of some NIDDM patients may have an alternative pathway through Ca2+influx from extracellular medium, arachidonic acid cascade, and PKC activation.

A comparison of the ability of myo-inositol and epi-inositol to attenuate lithium-plocarpine seizures in rats

A major hypothesis for the mechanism of action of lithium suggests that lithium affects cellular signalling in neuronal systems linked to the phosphatidylinositol (PI) cycle. Activation of phospholipase C (PLC) results in the cleavage of phosphatidylinositol 4, 5-bisphosphate (PIP2) and the production of two second messengers, inositol trisphosphate (IP3) and diacylglycerol. Lithium uncompetitively inhibits inositol monophosphatase in rat brain (Hallcher and Sherman 1980) and in erythrocytes of bipolar patients (Moscovich et al 1990), elevating levels of inositol monophosphate and reducing the level of inositol in the rat cortex (Allison and Stewart 1971; Sherman et al 1985). Cholinergic stimulation in animals that have been pretreated with a therapeutic dose of lithium elicits a reduction of rnyo-inositol levels and buildup of inositol monophosphate that are tenfold greater than the effects of either the cholinergic agonist or lithium alone (Honchar et al 1983); however, the ability of lithium to significantly affect inositol levels after chronic treatment has been questioned (Sherman et al 1981; Whitworth and Kendall 1989). Several in vitro electrophysiologic and pharmacologic effects of lithium were reversed by myo-inositol, but not by epi-inositol (Busa and Gimiich 1989; Mason and Biello 1992). Reversal of several lithium-induced behaviors, such as inhibition of rearing (Kofman and Belmaker 1990), lithium-pilocarpine seizures

The effect of fructose metabolism on the accumulation of inositol phosphates in rat pancreatic islets.

The mechanism by which glucose recognition of B cells results in the release of inositol 1,4,5-trisphosphate is not known at present. In pancreatic islets, fructose shares a common metabolic pathway with glucose from the second step of glycolysis and can augment insulin secretion at stimulatory glucose levels. To evaluate the impact of glycolysis on the release of inositol 1,4,5-trisphosphate, we studied the effect of glucose and fructose metabolism on insulin secretion and the activation of inositol-specific phospholipase C, using collagenase digested rat pancreatic islets incorporated with 3H-labelled myo-inositol. Inositol phosphates, generated by the cleavage of phosphatidyl inositol by inositol phospholipase C, were analyzed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mmol 1(-1) glucose for 45 min in the absence or presence of 10, 20 or 30 mmol 1(-1) fructose, and the amount of insulin released into the medium was measured. Intracellular inositol phosphate accumulation was measured under the same glucose concentrations with 0, 10 and 30 mmol 1(-1) fructose. As expected, fructose alone had no insulinotropic effect, but potentiated the glucose-induced (5.5 and 12 mmol 1(-1)) insulin secretion at concentrations of 10-30 mmol 1(-1). Glucose (12 vs. 3.3 mmol 1(-1)) significantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-trisphosphate. Fructose, however, had no potentiating effects on the accumulation of inositol phosphates. It is therefore supposed that glucose does not activate inositol-specific phospholipase C via the glycolysis. Further, since fructose did not activate inositol-specific phospholipase C, this stimulation is likely to be induced by glucose as such.

Phosphatidylinositol-specific phospholipase C from Bacillus cereus at the lipid-water interface: interfacial binding, catalysis, and activation.

Binding characteristics of phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus binding to the phospholipid-water interface were determined by spectroscopic methods and correlated with PI-PLC's catalytic properties. Binding of the enzyme to micelles and bilayers of zwitterionic phosphocholines is accompanied by an increase in the fluorescence emission from tryptophan, whereas a decrease in the emission is observed with synthetic anionic lipids containing a phosphomethanol head group. A similar decrease in the tryptophan emission is observed with phosphatidylinositol (PI) analogues containing the phospho-D-1-myo-inositol head group, but not with the enantiomeric L-1-myo-inositol. In covesicles of PI and phosphatidylcholine (PC), the rate of cleavage of PI is reduced because, as a neutral diluent, PC effectively reduces the surface concentration of PI that the bound enzyme "sees" in the interface. This permits determination of the interfacial Michaelis constant (KM*) as 0.26 mol fraction for PI as substrate. On the other hand, ditetradecylglycerophosphomethanol (DTPM) acts as a kinetic competitive inhibitor in the covesicles. The spectroscopic and catalytic activity data taken together show that PI-PLC binds to the interface of aqueous dispersions of phospholipids with an apparent Kd (in terms of the lipid monomers) of about 10-50 microM. However, only lipids with an anionic head group, such as phosphomethanol and phospho-D-1-myo-inositol, are able to bind as single molecules into the active site of the enzyme at the interface. Enantiomeric phospho-L-1-myo-inositol or the zwitterionic phosphocholine head group has little affinity for the enzyme at the interface. Thus, PI-PLC appears to obey the two-stage, Michaelis-Menten adaptation of interfacial catalysis, according to which the binding of the enzyme to the interface precedes the steps of the catalytic turnover at the interface. Limit estimates suggest that on PI or PI/PC vesicles the catalysis occurs in the "scooting" mode with a moderate processivity. DTPM vesicles also inhibit the activity of PI-PLC toward the synthetic water-soluble substrate myo-inositol 1-(4-nitrophenyl phosphate), but the activity is enhanced severalfold in the presence of vesicles of zwitterionic phosphatidylcholine. Several possible explanations of this interfacial activation are considered within the general context of the kinetic scheme for interfacial catalysis. The kinetic results for the action of PI-PLC bound to vesicles are consistent with a model in which the interface acts as an "allosteric" effector of the catalytic rate constant, kcat, without affecting the substrate binding.

Network of signal transduction pathways involving lipids: protein kinase C-dependent and -independent pathways.

The cellular glycerol- and sphingolipids are turning out to play not only fundamental roles in cellular and organellar membranes but also new and exciting roles in cellular regulation. The great diversity and complexity of the glycerolipids and sphingolipids present in mammalian cells had long portended important cellular functions, yet few molecular clues were uncovered until protein kinase C was discovered by Nishizuka (1986) to be regulated by sn-l,2-diacylglycerol (DAG), a second messenger derived by cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2)- Glycerolipids turned out to play two major roles in protein kinase C regulation: i) as essential phospholipid cofactors and ii) as lipid second messenger activators, DAG (Bell, 1986). In 1985, detailed structure-activity studies on protein kinase C using advanced mixed micellar methods to investigate specificity and stoichiometry of phospholipid cofactor and DAG activator function produced molecular insight into the mechanism of regulation (Hannun et al., 1985); these studies also led to the discovery that sphingosine inhibits protein kinase C activity and blocks [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding (Hannun et al., 1986). The work opened up a growing field of research involving the role of sphingolipid breakdown products in normal cellular regulation and perturbation of these normal pathways in disease and pathobiology (Hannun and Bell, 1987; 1989).

High-level expression in Escherichia coli and rapid purification of phosphatidylinositol-specific phospholipase C from Bacillus cereus and Bacillus thuringiensis

The construction of four vectors for high-level expression in Escherichia coli of the phosphatidylinositol-specific phospholipase C from Bacillus cereus or Bacillus thuringiensis is described. In all constructs the coding sequence for the mature phospholipase is precisely fused to the E. coli heat-stable enterotoxin II signal sequence for targeting of the protein to the periplasm. In one set of plasmids expression of the B. cereus or B. thuringiensis enzyme is under control of the E. coli alkaline phosphatase promoter, while in a second set of plasmids expression is under control of a lac-tac-tac triple tandem promoter. A simple and rapid procedure for complete purification of the phospholipase C overproduced in E. coli, involving isolation of the periplasmic proteins by osmotic shock followed by a single column chromatography step, is described. The largest quantity of purified enzyme, 40–60 mg per liter culture, is obtained with the plasmid expressing the B. cereus enzyme under control of the lac-tac-tac promoter. Lower quantities are obtained with the plasmids containing the alkaline phosphatase promoter (15–20 and 4–6 mg/liter for the B. cereus and B. thuringiensis enzymes, respectively) and with the plasmid expressing the B. thuringiensis phospholipase under control of the lac-tac-tac promoter (15–20 mg/liter). A comparison of the functional properties of the recombinant phospholipases with the native enzymes isolated from B. cereus or B. thuringiensis culture supernatant shows that they are identical with respect to their catalytic functions, viz., cleavage of phosphatidylinositol and cleavage of the glycosyl-phosphatidylinositol membrane anchor of bovine erythrocyte acetylcholinesterase.

Phosphatidylinositol-specific phospholipase C from Bacillus cereus combines intrinsic phosphotransferase and cyclic phosphodiesterase activities: a phosphorus-31 NMR study

The inositol phosphate products formed during the cleavage of phosphatidylinositol by phosphatidylinositol-specific phospholipase C from Bacillus cereus were analyzed by 31P NMR. 31P NMR spectroscopy can distinguish between the inositol phosphate species and phosphatidylinositol. Chemical shift values (with reference to phosphoric acid) observed are 0.41, 3.62, 4.45, and 16.30 ppm for phosphatidylinositol, myo-inositol 1-monophosphate, myo-inositol 2-monophosphate, and myo-inositol 1,2-cyclic monophosphate, respectively. It is shown that under a variety of experimental conditions this phospholipase C cleaves phosphatidylinositol via an intramolecular phosphotransfer reaction producing diacylglycerol and D-myo-inositol 1,2-cyclic monophosphate. We also report the new and unexpected observation that the phosphatidylinositol-specific phospholipase C from B. cereus is able to hydrolyze the inositol cyclic phosphate to form D-myo-inositol 1-monophosphate. The enzyme, therefore, possesses phosphotransferase and cyclic phosphodiesterase activities. The second reaction requires thousandfold higher enzyme concentrations to be observed by 31P NMR. This reaction was shown to be regiospecific in that only the 1-phosphate was produced and stereospecific in that only D-myo-inositol 1,2-cyclic monophosphate was hydrolyzed. Inhibition with a monoclonal antibody specific for the B. cereus phospholipase C showed that the cyclic phosphodiesterase activity is intrinsic to the bacterial enzyme. We propose a two-step mechanism for the phosphatidyl-inositol-specific phospholipase C from B. cereus involving sequential phosphotransferase and cyclic phosphodiesterase activities. This mechanism bears a resemblance to the well-known two-step mechanism of pancreatic ribonuclease, RNase A.

Kinetic analysis of the formation of inositol 1:2-cyclic phosphate in carbachol-stimulated pancreatic minilobules: Half is formed by direct phosphodiesteratic cleavage of phosphatidylinositol

The issue as to whether there is direct phosphodiesteratic cleavage of phosphatidylinositol (PI), in addition to that of phosphatidylinositol 4,5-bisphosphate (PIP2), on agonist stimulation of cells has been controversial. In an attempt to resolve this issue, we have studied the kinetics of the formation and breakdown of the cyclic inositol phosphates. This approach is fairly straightforward, since the turnover of the cyclic inositol phosphates is very slow as compared to that of the noncyclic inositol phosphates and proceeds from inositol 1:2-cyclic 4,5-trisphosphate to inositol 1:2-cyclic phosphate (I(c1:2)P) directly by dephosphorylation without any branching pathways, in contrast to the multiple branchpoints of the noncyclic inositol phosphate pathway. Mouse pancreatic minilobules were prelabeled with [3H]inositol for 30 min, followed by washing to remove free inositol. They were then stimulated with carbachol for 30 min. The inositol cyclic polyphosphates reached steady state at 10-15 min, and I(c1:2)P reached steady state at 25 min. We blocked the action of carbachol by addition of an excess of atropine at 30 min, and the rate of disappearance of the three cyclic inositol phosphates was measured. From these data, the contribution of the inositol cyclic polyphosphate pathway to I(c1:2)P was calculated, which was 40-50% of total I(c1:2)P formation. Thus, 40-50% of the I(c1:2)P formed must have been derived from direct phosphodiesteratic cleavage of PI. This approach should prove useful in measuring the relative contributions of PI hydrolysis and PI phosphorylation (phosphatidylinositol 4,5-bisphosphate hydrolysis) in the overall PI cascade.

Angiotensin II-stimulated phosphatidylinositol turnover in rat adrenal glomerulosa cells has a complex dependence on calcium.

The stimulation of phosphatidylinositol (PI) turnover by angiotensin II in rat adrenal glomerulosa cells has been studied in detail and shown to have a complex dependence on Ca2+. After the addition of angiotensin II, inositol monophosphate, inositol bisphosphate, and inositol trisphosphate increased rapidly and transiently. The transient increase was followed by a slower sustained rise, which continued for up to 30 min. Inositol phosphate accumulation during the sustained phase was decreased when experiments were performed in Ca2+-free medium. The initial transient response was not altered. Addition of the Ca2+ ionophore A23187 enhanced the angiotensin II response at 20 min, but not the 15 sec response. The sustained response, but not the transient response, was attenuated by the Ca2+ channel blocker nifedipine, indicating that the effect of Ca2+ required uptake through voltage-dependent Ca2+ channels. Subsequent studies showed that cAMP decreased inositol phosphate accumulation at 20 min while having no effect at 15 sec. Also, incubation with phorbol 12-myristate 13-acetate produced a more effective inhibition of the sustained response than of the initial transient response. However, while the transient and sustained phases of PI turnover were differently affected by Ca2+ and inhibitory compounds, the profiles of inositol phosphates generated were similar. At both 15 sec and 20 min inositol-(1,4,5) trisphosphate was detected, indicating sustained cleavage of PI-(4,5) bisphosphate. Taken together, the results suggest that while the initial PI turnover response is independent of Ca2+ and presumably initiates the rise in cytosolic Ca2+, sustained response requires entry of Ca2+ to maintain elevated cytosolic Ca2+ concentrations. Thus, while the increase in cytosolic Ca2+ may have a direct role in the stimulation of aldosterone synthesis, it is also required to sustain the PI turnover response to angiotensin II.