Characterization of two alternately spliced forms of phospholipase D1. Activation of the purified enzymes by phosphatidylinositol 4,5-bisphosphate, ADP-ribosylation factor, and Rho family monomeric GTP-binding proteins and protein kinase C-alpha

Increased mRNA expression of phospholipase D (PLD) isozymes during granulocytic differentiation of HL60 cells

In response to dibutyryl cyclic AMP (dbcAMP) and all-trans retinoic acid (ATRA), HL60 cells differentiate into granulocyte-like cells. Membrane-associated phospholipase D (PLD) activity in response to guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) or phorbol myristate acetate (PMA) was upregulated by these treatments. Reverse transcriptase-polymerase chain reaction (RT-PCR) analyses revealed that both hPLD1a and hPLD1b mRNAs were expressed in HL60 cells and that their expression levels increased during differentiation. hPLD2 mRNA levels rose dramatically during differentiation. These results suggest that the PLD genes undergo changes in transcriptional regulation during granulocytic differentiation of HL60 cells.

Relocalization of phospholipase D activity mediates membrane formation during meiosis

Phospholipase D (PLD) enzymes catalyze the hydrolysis of phosphatidylcholine and are involved in membrane trafficking and cytoskeletal reorganization. The Saccharomyces cerevisiae SPO14 gene encodes a PLD that is essential for meiosis. We have analyzed the role of PLD in meiosis by examining two mutant proteins, one with a point mutation in a conserved residue (Spo14pK--> H) and one with an amino-terminal deletion (Spo14pDeltaN), neither of which can restore meiosis in a spo14 deletion strain. Spo14pK--> H is enzymatically inactive, indicating that PLD activity is required, whereas Spo14pDeltaN retains PLD catalytic activity in vitro, indicating that PLD activity is not sufficient for meiosis. To explore other aspects of Spo14 function, we followed the localization of the enzyme during meiosis. Spo14p is initially distributed throughout the cell, becomes concentrated at the spindle pole bodies after the meiosis I division, and at meiosis II localizes to the new spore membrane as it surrounds the nuclei and then expands to encapsulate the associated cytoplasm during the formation of spores. The catalytically inactive protein also undergoes relocalization during meiosis; however, in the absence of PLD activity, no membrane is formed. In contrast, Spo14pDeltaN does not relocalize properly, indicating that the failure of this protein to complement a spo14 mutant is due to its inability to localize its PLD activity. Furthermore, we find that Spo14p movement is correlated with phosphorylation of the protein. These experiments indicate that PLD participates in regulated membrane formation during meiosis, and that both its catalytic activity and subcellular redistribution are essential for this function.

Different domains of mammalian ADP-ribosylation factor 1 mediate interaction with selected target proteins

Mammalian ADP-ribosylation factor 1 (mARF1) is a small GTP-binding protein that is activated by a Golgi guanine nucleotide exchange factor. Once bound to the Golgi membranes in the GTP form, mARF1 initiates the recruitment of the adaptor protein 1 (AP-1) complex and coatomer (COPI) onto these membranes and activates phospholipase D1 (PLD1). To map the domains of mARF1 that are important for these activities, we constructed chimeras between mARF1 and Saccharomyces cerevisiae ARF2, which functions poorly in all of these processes except COPI recruitment. The carboxyl half of mARF1 (amino acids 95-181) was essential for activation by the Golgi guanine nucleotide exchange factor, whereas a separate domain (residues 35-94) was required to effectively activate PLD1 and to promote efficient AP-1 recruitment. Since residues 35-94 of mARF1 are critical for optimal activity in both PLD1 activation and AP-1 recruitment, we hypothesize that this region of ARF contains residues that interact with effector molecules.

Activation of ADP-ribosylation factor 1 GTPase-activating protein by phosphatidylcholine-derived diacylglycerols

Disassembly of the coatomer from Golgi vesicles requires that the small GTP-binding protein ADP-ribosylation factor 1 (ARF1) hydrolyzes its bound GTP by the action of a GTPase-activating protein. In vitro, the binding of the ARF1 GTPase-activating protein to lipid vesicles and its activity on membrane-bound ARF1GTP are increased by diacylglycerols with monounsaturated acyl chains, such as those arising in vivo as secondary products from the hydrolysis of phosphatidylcholine by ARF-activated phospholipase D. Thus, the phospholipase D pathway may provide a feedback mechanism that promotes GTP hydrolysis on ARF1 and the consequent uncoating of vesicles.

Receptor-mediated endocytosis in kidney proximal tubules: recent advances and hypothesis

Preparation of kidney proximal tubules in suspension allows the study of receptor-mediated endocytosis, protein reabsorption, and traffic of endosomal vesicles. The study of tubular protein transport in vitro coupled with that of the function of endosomal preparation offers a unique opportunity to investigate a receptor-mediated endocytosis pathway under physiological and pathological conditions. We assume that receptor-mediated endocytosis of albumin in kidney proximal tubules in situ and in vitro can be regulated, on the one hand, by the components of the acidification machinery (V-type H+-ATPase, Cl(-)-channel and Na+/H+-exchanger), giving rise to formation and dissipation of a proton gradient in endosomal vesicles, and, on the other hand, by small GTPases of the ADP-ribosylation factor (Arf)-family. In this paper we thus analyze the recent advances of the studies of cellular and molecular mechanisms underlying the identification, localization, and function of the acidification machinery (V-type H+-ATPase, Cl(-)-channel) as well as Arf-family small GTPases and phospholipase D in the endocytotic pathway of kidney proximal tubules. Also, we explore the possible functional interaction between the acidification machinery and Arf-family small GTPases. Finally, we propose the hypothesis of the regulation of translocation of Arf-family small GTPases by an endosomal acidification process and its role during receptor-mediated endocytosis in kidney proximal tubules. The results of this study will not only enhance our understanding of the receptor-mediated endocytosis pathway in kidney proximal tubules under physiological conditions but will also have important implications with respect to the functional consequences under some pathological circumstances. Furthermore, it may suggest novel targets and approaches in the prevention and treatment of various diseases (cystic fibrosis, Dent's disease, diabetes and autosomal dominant polycystic kidney disease).

Cloning and characterization of phospholipase D from rat brain

The regulation of phospholipase D cloned from rat brain (rPLD) was examined in vivo and in vitro. The enzyme was a shorter splice variant of human phospholipase D 1 (Hammond, S. M., Altshuller, Y. M. , Sung, T.-C., Rudge, S. M., Rose, K., Engebrecht, J. A., Morris, A. J., and Frohman, M. A. (1995) J. Biol. Chem. 270, 29640-29643). Its expression in COS-7 cells led to increased phospholipase D (PLD) activity that was further stimulated by constitutively active V14RhoA. V14RhoA had no effect on the endogenous PLD of the COS-7 cells, but constitutively active L71ARF3 increased its activity. In contrast, L71ARF3 did not activate rPLD expressed in the cells. Addition of phorbol ester markedly increased the endogenous PLD activity of COS-7 cells, and there was a further increase in the cells expressing rPLD. In membranes from COS-7 cells expressing rPLD, addition of myristoylated ADP-ribosylation factor (ARF) and RhoA in vitro stimulated PLD activity. The effect of ARF was greater than that of RhoA, although the concentrations for half-maximal stimulation (0.08-0.2 microM) were similar. Membranes isolated from cells expressing rPLD plus L71ARF3 and/or V14RhoA also showed higher PLD activity but no synergism between the two G proteins. Addition of phorbol ester and protein kinase C alpha (PKCalpha) also stimulated PLD activity in membranes from COS-7 cells expressing rPLD, but it had no effect on the activity in control (vector) membranes and did not enhance the effects of constitutively active ARF or Rho. The stimulation by PKCalpha did not require ATP and was not increased by addition of this nucleotide. No synergism between ARF and Rho and between these and PKCalpha on PLD activity was observed when these were added to membranes from cells expressing rPLD. Oleate inhibited the PLD activity of membranes from both control and rPLD-expressing cells. In summary, these results indicate that in vitro, rPLD is stimulated by ARF, RhoA, and PKCalpha and inhibited by oleate. However, in intact COS-7 cells, ARF activates endogenous PLD but not rPLD, whereas the reverse is true for RhoA. In addition, the effects of phorbol ester are much greater in the intact cells. It is concluded that the regulation of rPLD in intact COS-7 cells differs significantly from that seen in vitro; possible reasons for this are discussed.

Measurement of phospholipase D activity

Phosphodiesteric cleavage of phosphatidylcholine by members of a growing family of phospholipases D produces choline and phosphatidic acid. These enzymes can also catalyse a transphosphatidylation reaction in which the aliphatic chain of a primary alcohol is transferred to the phosphatidyl moiety of the phosphatidic acid product. PLD enzymes are found in a variety of organisms including bacteria, yeast, plants, and vertebrates. In mammalian systems, biochemical and cell biological approaches have identified phosphatidic acid as a mediator (or progenitor of mediators) that play important roles in the transduction of extracellular signals. Phosphatidic acid or its metabolites may be regulators of key cellular processes such as the control of intracellular protein trafficking, secretion, and alterations in cell morphology and motility. This review discusses methods for the determination of PLD activity both in vitro and in intact cells.

Cloning and expression analysis of murine phospholipase D1

Activation of phosphatidylcholine-specific phospholipase D(PLD) occurs as part of the complex signal-transduction cascade initiated by agonist stimulation of tyrosine kinase and G-protein-coupled receptors. A variety of mammalian PLD activities have been described, and cDNAs for two PLDs recently reported (human PLD1 and murine PLD2). We describe here the cloning and chromosomal localization of murine PLD1. Northern-blot hybridization and RNase protection analyses were used to examine the expression of murine PLD1 and PLD2 ina variety of cell lines and tissues. PLD1 and PLD2 were expressed in all RNA samples examined, although the absolute expression of each isoform varied, as well as the ratio of PLD1 to PLD2. Moreover, in situ hybridization of adult brain and murine embryo sections revealed high levels of expression of individual PLDs in some cell types and no detectable expression in others. Thus the two PLDs probably carry out distinct roles in restricted subsets of cells rather than ubiquitous roles in all cells.

Characteristics of protein-kinase-C- and ADP-ribosylation-factor-stimulated phospholipase D activities in human embryonic kidney cells

Phospholipase D (PLD) activity in human embryonic kidney (HEK) cells is stimulated by phorbol-ester-activated protein kinase C (PKC) and by membrane receptors, the latter apparently acting via the GTP-binding proteins, ADP-ribosylation factor (ARF) and Rho. In the present study, performed in cell-free preparations, we have characterized and compared the regulation of HEK cell PLD activity by the stable GTP analogue, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]), and the phorbol ester, phorbol 12-myristate 13-acetate (PMA). In digitonin-permeabilized HEK cells, prelabeled with [3H]oleic acid, GTP[S] and PMA caused an approximately threefold concentration-dependent increase in the formation of [3H]phosphatidylethanol, measured in the presence of ethanol. Neomycin, which is known to complex with the PLD cofactor, phosphatidylinositol 4,5-bisphosphate, decreased basal and GTP[S]- or PMA-stimulated PLD activities with similar sensitivity. GDP and its analogue, guanosine 5'-O-[beta-thio]diphosphate, inhibited the stimulatory effect of GTP[S], whereas the PMA response was prevented by the nonselective PKC inhibitor, staurosporine, but not vice versa. PLD stimulation by GTP[S], but not by PMA, was markedly reduced upon cytosol depletion and reconstituted by purified recombinant ARF1. In HEK cell membranes, addition of purified recombinant ARNO, a guanine-nucleotide-exchange factor for ARF1. potentiated the GTP[S]-stimulated PLD activity. PLD stimulation by PMA in HEK cell membranes required MgATP and was largely prevented by the selective PKC inhibitors Goe 6976 and bisindolylmaleimide I. Immunoblot analysis demonstrated that both conventional PKC (alpha, beta, gamma) and atypical PKC isozymes (zeta, tau) were present in HEK cell membranes. The results indicate that phorbol ester stimulation of PLD activity in HEK cells apparently occurs by a phosphorylation-dependent mechanism involving membrane-associated PKC isozymes but not ARF proteins, the major targets of GTP[S]' action.

Role of protein-phospholipid interactions in the activation of ARF1 by the guanine nucleotide exchange factor Arno

Arno is a 47-kDa human protein recently identified as a guanine nucleotide exchange factor for ADP ribosylation factor 1 (ARF1) with a central Sec7 domain responsible for the exchange activity and a carboxyl-terminal pleckstrin homology (PH) domain (Chardin, P., Paris, S., Antonny, B., Robineau, S., Béraud-Dufour, S., Jackson, C. L., and Chabre, M. (1996) Nature 384, 481-484). Binding of the PH domain to phosphatidylinositol 4,5-bisphosphate (PIP2) greatly enhances Arno-mediated activation of myristoylated ARF1. We show here that in the absence of phospholipids, Arno promotes nucleotide exchange on [Delta17]ARF1, a soluble mutant of ARF1 lacking the first 17 amino acids. This reaction is unaffected by PIP2, which suggests that the PIP2-PH domain interaction does not directly regulate the catalytic activity of Arno but rather serves to recruit Arno to membranes. Arno catalyzes the release of GDP more efficiently than that of GTP from [Delta17]ARF1, and a stable complex between Arno Sec7 domain and nucleotide-free [Delta17]ARF1 can be isolated. In contrast to [Delta17]ARF1, full-length unmyristoylated ARF1 is not readily activated by Arno in solution. Its activation requires the presence of phospholipids and a reduction of ionic strength and Mg2+ concentration. PIP2 is strongly stimulatory, indicating that binding of Arno to phospholipids is involved, but in addition, electrostatic interactions between phospholipids and the amino-terminal portion of unmyristoylated ARF1GDP seem to be important. We conclude that efficient activation of full-length ARF1 by Arno requires a membrane surface and two distinct protein-phospholipid interactions: one between the PH domain of Arno and PIP2, and the other between amino-terminal cationic residues of ARF1 and anionic phospholipids. The latter interaction is normally induced by insertion of the amino-terminal myristate into the bilayer but can also be artificially facilitated by decreasing Mg2+ and salt concentrations.

Phorbol myristate acetate-dependent association of protein kinase C alpha with phospholipase D1 in intact cells

A phospholipase D1 (PLD1) was purified from rat brain by the use of antibody-coupled protein A Sepharose. We found that protein kinase C alp (PKCalpha) stimulated PLD1 activity in the presence of phorbol myristate acetate (PMA). PMA-dependent association of PKCalpha with PLD1 was verified in NIH-3T3 fibroblast cells, and COS7 cells transiently expressing PLD1 as well as in vitro suggesting that the activation of PLD1 resulted from direct association of PKCalpha with PLD1.

Human phospholipase D1 can be tyrosine-phosphorylated in HL-60 granulocytes

The human phospholipase D1 (hPLD1) has recently been cloned. Although recent data have implicated PLD in receptor-stimulated secretion, the regulation of the activity of PLD enzymes remains to be clarified. Purified hPLD1 is activated by several cytosolic cofactors among which are protein kinase Calpha, ARF, and RhoA. In human granulocytes, a strong correlation between tyrosine phosphorylation of proteins and PLD activity has been established. In this study, the presence of hPLD1 in HL-60 granulocytes and its phosphorylation on tyrosine residues have been studied. We generated antipeptide antibodies (Abs) specific for hPLD1 but not PLD2 as shown by Western blotting (WB) of recombinant PLD1 and PLD2. These Abs identified the presence of hPLD1 in HL-60 cells with the bulk of it being detected in the membranes and only a minor fraction in the cytosol. The hPLD1 Abs detected a major band at 120 kDa (PLD1a) and a minor band at 115 kDa (PLD1b). The specificity of the Abs was confirmed using PLD antisera neutralized with the immunizing peptides. The two forms of hPLD1 were consistently detected by immunoprecipitation under nondenaturing and denaturing conditions following a WB analysis with hPLD1 Abs. Following exposure of HL-60 cells to peroxides of vanadate (V4+-OOH), an inhibitor of tyrosine phosphatases, hPLD1 was immunoprecipitated under nondenaturing conditions from HL-60 cell lysates and assayed for tyrosine phosphorylation by WB. hPLD1 comigrated with a 120-kDa tyrosine phosphorylated protein by gel electrophoresis. Other tyrosine-phosphorylated peptides of 160, 140, 135, 90, and 75-80 kDa were also detected in hPLD1 immune complexes. hPLD1 and the associated tyrosine-phosphorylated proteins were not immunoprecipitated by neutralized hPLD1 Abs. Using denaturing conditions, the PLD immunoprecipitates were sequentially immunoblotted with anti-PLD1 and anti-phosphotyrosine Abs. PLD1a and PLD1b were detected, and the major PLD1a protein was superimposable with a major tyrosine-phosphorylated protein detected at 120 kDa. Conversely, PLD1a and PLD1b were recovered, at least in part, in the anti-phosphotyrosine immunoprecipitates. These results provide evidence that two PLD1 forms are expressed in human granulocytes. Furthermore, in response to stimulation by V4+-OOH, PLD1 was tyrosine-phosphorylated and associated with several, presently undefined, tyrosine-phosphorylated proteins.

Phospholipase D stimulates release of nascent secretory vesicles from the trans-Golgi network

Phospholipase D (PLD) is a phospholipid hydrolyzing enzyme whose activation has been implicated in mediating signal transduction pathways, cell growth, and membrane trafficking in mammalian cells. Several laboratories have demonstrated that small GTP-binding proteins including ADP-ribosylation factor (ARF) can stimulate PLD activity in vitro and an ARF-activated PLD activity has been found in Golgi membranes. Since ARF-1 has also been shown to enhance release of nascent secretory vesicles from the TGN of endocrine cells, we hypothesized that this reaction occurred via PLD activation. Using a permeabilized cell system derived from growth hormone and prolactin-secreting pituitary GH3 cells, we demonstrate that immunoaffinity-purified human PLD1 stimulated nascent secretory vesicle budding from the TGN approximately twofold. In contrast, a similarly purified but enzymatically inactive mutant form of PLD1, designated Lys898Arg, had no effect on vesicle budding when added to the permeabilized cells. The release of nascent secretory vesicles from the TGN was sensitive to 1% 1-butanol, a concentration that inhibited PLD-catalyzed formation of phosphatidic acid. Furthermore, ARF-1 stimulated endogenous PLD activity in Golgi membranes approximately threefold and this activation correlated with its enhancement of vesicle budding. Our results suggest that ARF regulation of PLD activity plays an important role in the release of nascent secretory vesicles from the TGN.

ADP-ribosylation-factor-regulated phospholipase D activity localizes to secretory vesicles and mobilizes to the plasma membrane following N-formylmethionyl-leucyl-phenylalanine stimulation of human neutrophils

Phospholipase D (PLD) is responsible for the hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. Human neutrophils contain PLD activity which is regulated by the small GTPases, ADP-ribosylation factor (ARF) and Rho proteins. In this study we have examined the subcellular localization of the ARF-regulated PLD activity in non-activated neutrophils and cells 'primed' with N-formylmethionyl-leucyl-phenylalanine (fMetLeuPhe). We report that PLD activity is localized at the secretory vesicles in control cells and is mobilized to the plasma membrane upon stimulation with fMetLeuPhe. We conclude that the ARF-regulated PLD activity is translocated to the plasma membrane by secretory vesicles upon stimulation of neutrophils with fMetLeuPhe in inflammatory/priming doses. We propose that this relocalization of PLD is important for the subsequent events occurring during neutrophil activation.

RalA interacts directly with the Arf-responsive, PIP2-dependent phospholipase D1

RalA GTPase associates with a phospholipase D (PLD) that is activated in v-Src- and v-Ras-transformed cells. Two mammalian PLDs were recently cloned: PLD1, which is activated by Arf family GTPases and dependent upon phosphatidylinositol-4,5-bisphosphate (PIP2), and PLD2, which is also dependent upon PIP2, but not stimulated by Arf. Another PLD has been described that is stimulated by oleate. Evidence is provided that the RalA-assiciated PLD is PLD1. First, the PLD precipitated by RalA from murine fibroblasts was stimulated by Arf, dependent upon PIP2, and inhibited by oleate. Second, immobilized RalA precipitated PLD1 from sf9 insect cells overexpressing PLD1. Third, a series of RalA mutants precipitated PLD activity from both PLD1-expressing insect cells and murine fibroblasts with the same efficiency. And finally, immobilized RalA precipitated PLD1 from a purified PLD1 preparation. These data argue that RalA associates directly with the Arf-responsive, PIP2-dependent PLD1.

Inhibition of phospholipase D by clathrin assembly protein 3 (AP3)

In the accompanying paper (Chung, J.-K., Sekiya, F., Kang, H.-S., Lee, C., Han, J.-S., Kim, S. R., Bae, Y. S., Morris, A. J., and Rhee, S. G. (1997) J. Biol. Chem. 272, 15980-15985), synaptojanin is identified as a protein that inhibits phospholipase D (PLD) activity stimulated by ADP-ribosylation factor and phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2). Here, the purification from rat brain cytosol of another PLD-inhibitory protein that is immunologically distinct from synaptojanin is described, and this protein is identified as clathrin assembly protein 3 (AP3) by peptide sequencing and immunoblot analysis. AP3 binds both inositol hexakisphosphate and preassembled clathrin cages with high affinity. However, neither inositol hexakisphosphate binding nor clathrin cage binding affected the ability of AP3 to inhibit PLD. AP3 also binds to PI(4,5)P2 with low affinity. But the PI(4,5)P2 binding was not responsible for PLD inhibition, because the potency and efficacy of AP3 as an inhibitor of PLD were similar in the absence and presence of PI(4,5)P2. A bacterially expressed fusion protein, glutathione S-transferase-AP3 (GST-AP3), also inhibited PLD with a potency equal to that of brain AP3. The inhibitory effect of AP3 appeared to be the result of direct interaction between AP3 and PLD because PLD bound GST-AP3 in an in vitro binding assay. Using GST fusion proteins containing various AP3 sequences, we found that the sequence extending from residues Pro-290 to Lys-320 of AP3 is critical for both inhibition of and binding to PLD. The fact that AP3 is a synapse-specific protein indicates that the AP3-dependent inhibition of PLD might play a regulatory role that is restricted to the rapid cycling of synaptic vesicles.

Phospholipase D activity in PC12 cells. Effects of overexpression of alpha2A-adrenergic receptors

PC12 neuronal cells express a membrane phospholipase D (PLD) activity that is detected at similar levels in undifferentiated or differentiated cells. The regulation of this activity by agonists was explored. Membrane phospholipase D activity was increased by treatment of cells with the phorbol ester phorbol 12-myristate 13-acetate (PMA) or with nerve growth factor. The ability of PMA to activate PLD was confirmed in intact PC12 cells. Basal activity of PLD in membranes was reduced in RG20, a PC12 cell line overexpressing the human alpha2A-adrenergic receptor. PMA did not increase PLD activity in RG20 cells, as assessed both in membrane preparations and in intact cells. Cyclic AMP levels did not regulate phospholipase D activity in either cell type. However, incubation of RG20 cells with the alpha2-adrenergic antagonist rauwolscine or with pertussis toxin increased membrane PLD activity and restored activation of PLD by PMA. These data suggest that the effects of the overexpressed alpha2A-adrenergic receptor on PLD activity are mediated by precoupling of the receptor to the heterotrimeric GTP-binding protein, Gi, but are independent of adenylate cyclase regulation. The results of this study suggest that membrane phospholipase D activity can be negatively regulated via Gi in PC12 cells.

Phospholipase D2, a distinct phospholipase D isoform with novel regulatory properties that provokes cytoskeletal reorganization

BACKGROUND

Activation of phospholipase D (PLD) is an important but poorly understood component of receptor-mediated signal transduction responses and regulated secretion. We recently reported the cloning of the human gene encoding PLD1; this enzyme has low basal activity and is activated by protein kinase C and the small GTP-binding proteins, ADP-ribosylation factor (ARF), Rho, Rac and Cdc42. Biochemical and cell biological studies suggest, however, that additional and distinct PLD activities exist in cells, so a search was carried out for novel mammalian genes related to PLD1.

RESULTS

We have cloned the gene for a second PLD family member and characterized the protein product, which appears to be regulated differently from PLD1: PLD2 is constitutively active and may be modulated in vivo by inhibition. Unexpectedly, PLD2 localizes primarily to the plasma membrane, in contrast to PLD1 which localizes solely to peri-nuclear regions (the endoplasmic reticulum, Golgi apparatus and late endosomes), where PLD activity has been shown to promote ARF-mediated coated-vesicle formation. PLD2 provokes cortical reorganization and undergoes redistribution in serum-stimulated cells, suggesting that it may have a role in signal-induced cytoskeletal regulation and/or endocytosis.

CONCLUSIONS

PLD2 is a newly identified mammalian PLD isoform with novel regulatory properties. Our findings suggest that regulated secretion and morphological reorganization, the two most frequently proposed biological roles for PLD, are likely to be effected separately by PLD1 and PLD2.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Post-traumatic oedema of the foot after tibial fracture

A total of 97 patients with diaphyseal tibial fractures treated with functional bracing were studied prospectively. Persistent ipsilateral foot swelling was present in 84.5 per cent of the patients. Most of the swellings subsided with time, but a small percentage of them persisted for a duration of 2 years or more after injury. The time for disappearance of the swelling in 50 per cent of the patients was 18.6 weeks. The development of oedema is not related to the age and sex of the patients, the configuration, type and level of the fractures, or the association of a fibular fracture. The bone healed quicker in those who did not have swelling of the foot. Once the swelling has developed, it seems to run its own course and its disappearance is not related to the age and sex, the configuration, type and level of fractures, the association of a fibular fracture, or the time for fracture healing. This complication does not have any adverse effect on the functional recovery of the patients.