Reconstitution of membrane fusion between pancreatic islet secretory granules and plasma membranes: catalysis by a protein constituent recognized by monoclonal antibodies directed against glyceraldehyde-3-phosphate dehydrogenase

GAPDH controls extracellular vesicle biogenesis and enhances the therapeutic potential of EV mediated siRNA delivery to the brain

Extracellular vesicles (EVs) are biological nanoparticles with important roles in intercellular communication, and potential as drug delivery vehicles. Here we demonstrate a role for the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in EV assembly and secretion. We observe high levels of GAPDH binding to the outer surface of EVs via a phosphatidylserine binding motif (G58), which promotes extensive EV clustering. Further studies in a Drosophila EV biogenesis model reveal that GAPDH is required for the normal generation of intraluminal vesicles in endosomal compartments, and promotes vesicle clustering. Fusion of the GAPDH-derived G58 peptide to dsRNA-binding motifs enables highly efficient loading of small interfering RNA (siRNA) onto the EV surface. Such vesicles efficiently deliver siRNA to multiple anatomical regions of the brain in a Huntington's disease mouse model after systemic injection, resulting in silencing of the huntingtin gene in different regions of the brain.

D-Glyceraldehyde-3-Phosphate Dehydrogenase Structure and Function

Aside from its well-established role in glycolysis, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been shown to possess many key functions in cells. These functions are regulated by protein oligomerization , biomolecular interactions, post-translational modifications , and variations in subcellular localization . Several GAPDH functions and regulatory mechanisms overlap with one another and converge around its role in intermediary metabolism. Several structural determinants of the protein dictate its function and regulation. GAPDH is ubiquitously expressed and is found in all domains of life. GAPDH has been implicated in many diseases, including those of pathogenic, cardiovascular, degenerative, diabetic, and tumorigenic origins. Understanding the mechanisms by which GAPDH can switch between its functions and how these functions are regulated can provide insights into ways the protein can be modulated for therapeutic outcomes.

Functional diversity

There is increasing evidence to support a gene economy model that is fully based on the principles of evolution in which a limited number of proteins does not necessarily reflect a finite number of biochemical processes. The concept of 'gene sharing' proposes that a single protein can have alternate functions that are typically attributed to other proteins. GAPDH appears to play this role quite well in that it exhibits more than one function. GAPDH represents the prototype for this new paradigm of protein multi-functionality. The chapter discusses the diverse functions of GAPDH among three broad categories: cell structure, gene expression and signal transduction. Protein function is curiously re-specified given the cell's unique needs. GAPDH provides the cell with the means of linking metabolic activity to various cellular processes. While interpretations may often lead to GAPDH's role in meeting focal energy demands, this chapter discusses several other very distinct GAPDH functions (i.e. membrane fusogenic properties) that are quite different from its ability to catalyze oxidative phosphorylation of the triose, glyceraldehyde 3-phosphate. It is suggested that a single protein participates in multiple processes in the structural organization of the cell, controls the transmission of genetic information (i.e. GAPDH's involvement may not be finite) and mediates intracellular signaling.

Comparative secretome investigation of Magnaporthe oryzae proteins responsive to nitrogen starvation

Magnaporthe oryzae is a fungal pathogen that causes blast disease in rice. During its early infection process, during which starvation of nutrients, including nitrogen, prevails before establishment of successful infection, the fungally secreted proteins play an important role in the pathogenicity and stress response. In this study, M. oryzae-secreted proteins were investigated in an N-deficient minimal medium using two-dimensional gel electrophoresis (2-DGE) coupled with mass spectrometry analysis (MALDI-TOF-MS and μLC-ESI-MS/MS). The 2-DGE analysis of secreted proteins detected 89 differentially expressed protein spots (14 downregulated and 75 upregulated) responsive to N starvation. Eighty five of the protein spots were identified by mass spectrometry analyses. Identified proteins were mainly cell wall hydrolase enzymes (22.4%), protein and lipid hydrolases (24.7%), reactive oxygen species detoxifying proteins (22.4%), and proteins with unknown function (14.1%), suggesting early production of prerequisite proteins for successful infection of the host. SignalP analysis predicted the presence of signal peptides in 67% of the identified proteins, suggesting that in addition to the classical Golgi/endoplasmic reticulum secretory pathway, M. oryzae might possess other, as yet undefined, secretory pathways. Those nonclassical or leaderless secretion proteins accounted for 25.9% of the total identified proteins by TatP and SecretomeP predictions. Semiquantitative reverse transcriptase polymerase chain reaction of seven randomly selected N-responsive secreted proteins also revealed a good correlation between RNA and protein levels. Taken together, the establishment of the M. oryzae secretome that is responsive to N starvation provides the first evidence of the secretion of 60 unreported and 25 previously known proteins. This developed protein inventory could be exploited to improve our understanding of the secretory mechanisms of M. oryzae and its invasive growth process in rice tissue.

Role of glyceraldehyde-3-phosphate dehydrogenase in vesicular transport from golgi apparatus to endoplasmic reticulum

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a well-studied glycolytic protein with energy production as its implied occupation. It has established itself lately as a multifunctional protein. Recent studies have found GAPDH to be involved in a variety of nuclear and cytosolic pathways ranging from its role in apoptosis and regulation of gene expression to its involvement in regulation of Ca2+ influx from endoplasmic reticulum. Numerous studies also indicate that GAPDH interacts with microtubules and participates in cell membrane fusion. This review is focused on the cytosolic functions of the protein related to vesicular transport. Suggestions for future directions as well as the model of protein polymer structure and possible post-translational modifications as a basis for its multifunctional activities in the early secretory pathway are given.

Potential mechanisms contributing to sulfatide depletion at the earliest clinically recognizable stage of Alzheimer's disease: a tale of shotgun lipidomics

Shotgun lipidomics is a rapidly developing technology, which identifies and quantifies individual lipid molecular species directly from lipid extracts of biological samples. Alterations in lipid molecular species in the brain induced by neurodegenerative diseases, such as Alzheimer's disease (AD) could provide fundamental clues to disease pathogenesis. To date, the cause(s) leading to AD pathogenesis are still unknown and apolipoprotein E (apoE) allele 4 is the only known major risk factor for this devastating disease. By utilizing shotgun lipidomics, we have recently shown that a substantial and specific depletion of sulfatide (a class of specialized myelin sphingolipids) is present in postmortem brains from subjects at the earliest clinically recognizable stage of AD. In subsequent studies to identify the biochemical mechanisms underlying sulfatide depletion at this very mild stage of AD, we have found that apoE is associated with sulfatide transport and mediates sulfatide homeostasis in the nervous system through lipoprotein metabolism pathways and that alterations in apoE-mediated sulfatide trafficking can lead to sulfatide depletion in the brain. Thus, a working model related to the potential biochemical mechanisms underlying sulfatide depletion in AD can be derived based on these results. Collectively, the results obtained from lipidomic analyses of brain samples provide important insights into the biochemical mechanisms underlying AD pathogenesis.

Shotgun lipidomics identifies a paired rule for the presence of isomeric ether phospholipid molecular species

Background

Ether phospholipids are abundant membrane constituents present in electrically active tissues (e.g., heart and the brain) that play important roles in cellular function. Alterations of ether phospholipid molecular species contents are associated with a number of genetic disorders and human diseases.

Methodology/Principal Findings

Herein, the power of shotgun lipidomics, in combination with high mass accuracy/high resolution mass spectrometry, was explored to identify a paired rule for the presence of isomeric ether phospholipid molecular species in cellular lipidomes. The rule predicts that if an ether phospholipid A′-B is present in a lipidome, its isomeric counterpart B′-A is also present (where the ′ represents an ether linkage). The biochemical basis of this rule results from the fact that the enzymes which participate in either the sequential oxidation of aliphatic alcohols to fatty acids, or the reduction of long chain fatty acids to aliphatic alcohols (metabolic precursors of ether lipid synthesis), are not entirely selective with respect to acyl chain length or degree of unsaturation. Moreover, the enzymatic selectivity for the incorporation of different aliphatic chains into the obligatory precursor of ether lipids (i.e., 1-O-alkyl-glycero-3-phosphate) is also limited.

Conclusions/Significance

This intrinsic amplification of the number of lipid molecular species present in biological membranes predicted by this rule and demonstrated in this study greatly expands the number of ether lipid molecular species present in cellular lipidomes. Application of this rule to mass spectrometric analyses provides predictive clues to the presence of specific molecular species and greatly expands the number of identifiable and quantifiable ether lipid species present in biological samples. Through appropriate alterations in the database, use of the paired rule increases the number of identifiable metabolites in metabolic networks, thereby facilitating identification of biomarkers presaging disease states.

Interaction of glyceraldehyde-3-phosphate dehydrogenase in the light-induced rod alpha-transducin translocation

The light-dependent subcellular translocation of rod alpha-transducin (GNAT-1, or rod Talpha) has been well documented. In dark-adapted animals, rod Talpha (rTalpha) is predominantly located in the rod outer segment (ROS) and translocates into the rod inner segment (RIS) upon exposure to the light. Neither the molecular participants nor the mechanism(s) involved in this protein trafficking are known. We hypothesized that other proteins must interact with rTalpha to affect the translocations. Using the MBP-rTalpha fusion pulldown assay, the yeast two-hybrid assay and the co-immunoprecipitation assay, we identified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and rTalpha as interacting proteins. Immunoprecipitation also showed beta-actin associates with rTalpha in the dark but not in the light. To further investigate the involvement of GAPDH in light-induced rod Talpha translocation, GAPDH mRNA was knocked down in vivo by transient expression of siRNAs in rat photoreceptor cells. Under completely dark- and light-adapted conditions, the translocation of rTalpha was not significantly different within the 'GAPDH knock-down photoreceptor cells' compared to the non-transfected control cells. However, under partial dark-adaptation, rTalpha translocated more slowly in the 'GAPDH knock-down cells' supporting the conclusion that GAPDH is involved in rTalpha translocation from the RIS to the ROS during dark adaptation.

Insulin secretory responses and phospholipid composition of pancreatic islets from mice that do not express Group VIA phospholipase A2 and effects of metabolic stress on glucose homeostasis

Studies involving pharmacologic or molecular biologic manipulation of Group VIA phospholipase A(2) (iPLA(2)beta) activity in pancreatic islets and insulinoma cells suggest that iPLA(2)beta participates in insulin secretion. It has also been suggested that iPLA(2)beta is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels and arachidonate incorporation into phosphatidylcholine (PC). We have generated iPLA(2)beta-null mice by homologous recombination and have reported that they exhibit reduced male fertility and defective motility of spermatozoa. Here we report that pancreatic islets from iPLA(2)beta-null mice have impaired insulin secretory responses to D-glucose and forskolin. Electrospray ionization mass spectrometric analyses indicate that the abundance of arachidonate-containing PC species of islets, brain, and other tissues from iPLA(2)beta-null mice is virtually identical to that of wild-type mice, and no iPLA(2)beta mRNA was observed in any tissue from iPLA(2)beta-null mice at any age. Despite the insulin secretory abnormalities of isolated islets, fasting and fed blood glucose concentrations of iPLA(2)beta-null and wild-type mice are essentially identical under normal circumstances, but iPLA(2)beta-null mice develop more severe hyperglycemia than wild-type mice after administration of multiple low doses of the beta-cell toxin streptozotocin, suggesting an impaired islet secretory reserve. A high fat diet also induces more severe glucose intolerance in iPLA(2)beta-null mice than in wild-type mice, but PLA(2)beta-null mice have greater responsiveness to exogenous insulin than do wild-type mice fed a high fat diet. These and previous findings thus indicate that iPLA(2)beta-null mice exhibit phenotypic abnormalities in pancreatic islets in addition to testes and macrophages.

Interactions among p22, glyceraldehyde-3-phosphate dehydrogenase and microtubules

Previously, we have shown that p22, an EF-hand Ca2+-binding protein, interacts indirectly with microtubules in an N-myristoylation-dependent and Ca2+-independent manner. In the present study, we report that N-myristoylated p22 interacts with several microtubule-associated proteins within the 30-100 kDa range using overlay blots of microtubule pellets containing cytosolic proteins. One of those p22-binding partners, a 35-40 kDa microtubule-binding protein, has been identified by MS as GAPDH (glyceraldehyde-3-phosphate dehydrogenase). Several lines of evidence suggest a functional relationship between GAPDH and p22. First, endogenous p22 interacts with GAPDH by immunoprecipitation. Secondly, p22 and GAPDH align along microtubule tracks in analogous punctate structures in BHK cells. Thirdly, GAPDH facilitates the p22-dependent interactions between microtubules and microsomal membranes, by increasing the ability of p22 to bind microtubules but not membranes. We have also shown a direct interaction between N-myristoylated p22 and GAPDH in vitro with a K(D) of approximately 0.5 microM. The removal of either the N-myristoyl group or the last six C-terminal amino acids abolishes the binding of p22 to GAPDH and reduces the ability of p22 to associate with microtubules. In summary, we report that GAPDH is involved in the ability of p22 to facilitate microtubule-membrane interactions by affecting the p22-microtubule, but not the p22-membrane, association.

Glucokinase is an integral component of the insulin granules in glucose-responsive insulin secretory cells and does not translocate during glucose stimulation

The association of glucokinase with insulin secretory granules has been shown by cell microscopy techniques. We used MIN6 insulin-secretory cells and organelle fractionation to determine the effects of glucose on the subcellular distribution of glucokinase. After permeabilization with digitonin, 50% of total glucokinase remained bound intracellularly, while 30% was associated with the 13,000g particulate fraction. After density gradient fractionation of the organelles, immunoreactive glucokinase was distributed approximately equally between dense insulin granules and low-density organelles that cofractionate with mitochondria. Although MIN6 cells show glucose-responsive insulin secretion, glucokinase association with the granules and low-density organelles was not affected by glucose. Subfractionation of the insulin granule components by hypotonic lysis followed by sucrose gradient centrifugation showed that glucokinase colocalized with the granule membrane marker phogrin and not with insulin. PFK2 (6-phosphofructo-2-kinase-2/fructose-2,6-bisphosphatase)/FDPase-2, a glucokinase-binding protein, and glyceraldehyde phosphate dehydrogenase, which has been implicated in granule fusion, also colocalized with glucokinase after hypotonic lysis or detergent extaction of the granules. The results suggest that glucokinase is an integral component of the granule and does not translocate during glucose stimulation.

The N'-terminal domain of glyceraldehyde-3-phosphate dehydrogenase of the apicomplexan Plasmodium falciparum mediates GTPase Rab2-dependent recruitment to membranes

Spatial and temporal distribution of the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase (pfGAPDH) and aldolase (pfAldolase) of Plasmodium falciparum were investigated using specific mAbs and indirect immunofluorescence analysis (IFA). Both glycolytic enzymes were co-localized during ring and trophozoite stages of both liver and asexual blood stage parasites. During schizogony, pfGAPDH became associated with the periphery of the parasites and eventually accumulated in the apical region of merozoites, while pfAldolase showed no segregation. Subcellular fractionation experiments demonstrated that pfGAPDH was found in both the membrane-containing pellet and the supernatant fraction of parasite lysates. In contrast, pfAldolase was only found in the supernatant fraction. A quantitative binding assay showed that pfGAPDH could be recruited to HeLa cell microsomal membranes in response to mammalian GTPase Rab2, indicating that Rab2-dependent recruitment of cytosolic components to membranes is conserved in evolution. Two overlapping fragments of pfGAPDH (residues 1-192 and 133-337) were evaluated in the microsomal binding assay. We found that the N'-terminal fragment competitively inhibited Rab2-stimulated pfGAPDH recruitment. Thus, the domain mediating the evolutionarily conserved Rab2-dependent membrane recruitment is located in the N'-terminus of GAPDH. Together, these results suggest that pfGAPDH exerts non-glycolytic function(s) in P. falciparum, possibly including a role in vesicular transport and biogenesis of apical organelles.

Participation of a fusogenic protein, glyceraldehyde-3-phosphate dehydrogenase, in nuclear membrane assembly

We found an autoimmune serum, K199, that strongly suppresses nuclear membrane assembly in a cell-free system involving a Xenopus egg extract. Four different antibodies that suppress nuclear assembly were affinity-purified from the serum using Xenopus egg cytosol proteins. Three proteins recognized by these antibodies were identified by partial amino acid sequencing to be glyceraldehyde-3-phosphate dehydrogenase (GAPDH), fructose-1,6-bisphosphate aldolase, and the regulator of chromatin condensation 1. GAPDH is known to be a fusogenic protein. To verify the participation of GAPDH in nuclear membrane fusion, authentic antibodies against human and rat GAPDH were applied, and strong suppression of nuclear assembly at the nuclear membrane fusion step was observed. The nuclear assembly activity suppressed by antibodies was recovered on the addition of purified chicken GAPDH. A peptide with the sequence of amino acid residues 70-94 of GAPDH, which inhibits GAPDH-induced phospholipid vesicle fusion, inhibited nuclear assembly at the nuclear membrane fusion step. We propose that GAPDH plays a crucial role in the membrane fusion step in nuclear assembly in a Xenopus egg extract cell-free system.

Tubulin is the endogenous inhibitor of the glyceraldehyde 3-phosphate dehydrogenase isoform that catalyzes membrane fusion: Implications for the coordinated regulation of glycolysis and membrane fusion

Previously we demonstrated that specific chromatographically resolvable isoforms of rabbit brain GAPDH catalyze either glycolytic flux or membrane fusion activity (but not both). Moreover, GAPDH membrane fusion activity was latent until it was separated from an endogenous cytosolic inhibitor by anion-exchange chromatography. Herein we demonstrate that the cytosolic inhibitor is nondialyzable, heat-labile, and trypsin-sensitive, thereby identifying it as a cytosolic protein constituent. Chromatographic purification of the rabbit-brain cytosolic protein inhibitor of GAPDH isoform-catalyzed membrane fusion identified a predominant 55-kDa doublet that contained an internal 15-aa peptide identical to a sequence present in alpha-tubulin (residues 65-79). The identity of the 55-kDa doublet as tubulin was substantiated through Western blot analysis and inhibition of GAPDH-catalyzed membrane fusion by authentic tubulin. Stopped-flow kinetic analysis demonstrated the high-affinity, rapid, and direct modulation of GAPDH-catalyzed fusion activity by tubulin. Because GTP-activated Rab 2 recruits GAPDH to membranes about to undergo fusion [Tisdale, E. J. (2001) J. Biol. Chem. 276, 2480-2486] and protein kinase Ciota/lambda phosphorylates GAPDH modulating its interactions with tubulin [Tisdale, E. J. (2001) J. Biol. Chem. 277, 3334-3341], the present study suggests a coordinated mechanism through which membrane trafficking and cellular signaling can be integrated with glycolytic flux.

Plasmalogen deficiency in early Alzheimer's disease subjects and in animal models: molecular characterization using electrospray ionization mass spectrometry

To explore the hypothesis that alterations in ethanolamine plasmalogen may be directly related to the severity of dementia in Alzheimer's disease (AD), we performed a systematic examination of plasmalogen content in cellular membranes of gray and white matter from different regions of human subjects with a spectrum of AD clinical dementia ratings (CDR) using electrospray ionization mass spectrometry (ESI/MS). The results demonstrate: (1) a dramatic decrease in plasmalogen content (up to 40 mol% of total plasmalogen) in white matter at a very early stage of AD (i.e. CDR 0.5); (2) a correlation of the deficiency in gray matter plasmalogen content with the AD CDR (i.e. approximately 10 mol% of deficiency at CDR 0.5 (very mild dementia) to approximately 30 mol% of deficiency at CDR 3 (severe dementia); (3) an absence of alterations of plasmalogen content and molecular species in cerebellar gray matter at any CDR despite dramatic alterations of plasmalogen content in cerebellar white matter. Alterations of ethanolamine plasmalogen content in two mouse models of AD, APP(V717F) and APPsw, were also examined by ESI/MS. A plasmalogen deficiency was present (up to 10 mol% of total plasmalogen at the age of 18 months) in cerebral cortices, but was absent in cerebella from both animal models. These results suggest plasmalogen deficiency may play an important role in the AD pathogenesis, particularly in the white matter, and suggest that altered plasmalogen content may contribute to neurodegeneration, synapse loss and synaptic dysfunction in AD.

Estradiol, in the CNS, targets several physiologically relevant membrane-associated proteins

We will describe the identity and function of two unexpected estrogen binding proteins from rat brain cell membranes in search for the putative membrane estrogen receptor (mER). An E-6-BSA column retained a distinctive 37-kDa protein that showed 100% homology with glyceraldehyde-3-phosphate dehydrogenase (GAPDH). A P-3-BSA column also retained the same protein but with less affinity. E-6-BSA bound to GAPDH with an IC50 of 50 nM, whereas the IC50 for P-3-BSA was about 500 nM. A dose of 10 nM 17beta-estradiol stimulated the catalysis of GAPDH, whereas progesterone at 100 nM inhibited it. Other steroids were ineffective. We examined if GAPDH activity would change during the rat estrous cycle, and what would be the effect of ovariectomy and estrogen treatment. The hippocampus and cerebellum were collected and GAPDH catalysis in both cytosolic and plasmalemmal-microsomal fractions was tested. The highest activity was found in Proestrus morning and the lowest in Estrus in both fractions. After ovariectomy (3 weeks) the hippocampus membrane fraction showed significantly reduced activity compared to that of Diestrus. An injection of estradiol in ovariectomized rats (10 microg/rat, s.c.) increased GAPDH activity in the hippocampus membrane fractions close to 60% from that of ovariectomized oil-treated controls 24 h after treatment maintaining similar levels by 48 h. No changes were detected in the preparations from the cerebellum of the same rats. The other protein retained by E-BSA columns was a 55-kDa protein identified as beta-tubulin. Two other proteins were also co-purified from the rat hippocampus: a 37-kDa (GAPDH) and a 45-kDa (actin). A purified brain tubulin (Cytoskeleton) was also retained with high affinity by the E-6-BSA, but with less affinity by an E-17-BSA column and not retained by either BSA, P-3-BSA or C-21-BSA columns. E-6-[125I]BSA bound with high affinity to tubulin (1 microg) and 17beta-estradiol completely displaced the binding at 10(-7) M. 17alpha-estradiol was ineffective and neither progesterone, corticosterone, DES nor 2-methoxyestradiol (2-ME) was able to displace the ligand. The T-3-[125I]BSA also bound to tubulin. But it seems to interact with another binding site, because colchicine at 10(-5) M completely eliminated the binding of T-3-[125 I]BSA to tubulin but did not displace the E-6-BSA site. Taxol competed off both ligands but only by 50%. None of the two ligands bound actin. These novel findings add new information to be considered in the intracellular actions of estradiol, particularly in the remodeling and functions of the cytoskeleton.

Glyceraldehyde-3-phosphate dehydrogenase is required for vesicular transport in the early secretory pathway

Protein transport in the early secretory pathway requires Rab2 GTPase. This protein promotes the recruitment of soluble components that participate in protein sorting and recycling from pre-Golgi intermediates (vesicular tubular clusters (VTCs)). We previously reported that a constitutively activated form of Rab2 (Q65L) as well as Rab2 wild type promoted vesicle formation from VTCs. These vesicles contained Rab2, beta-COP, p53/gp58, and protein kinase Ciota/lambda but lacked anterograde-directed cargo. To identify other candidate Rab2 effectors, the polypeptide composition of the vesicles was further analyzed. We found that vesicles released in response to Rab2 also contained the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). To study the relationship of this enzyme to Rab2 function, we performed a quantitative binding assay to measure recruitment of GAPDH to membrane when incubated with Rab2. Rab2-treated microsomes showed a 5-10-fold increase in the level of membrane-associated GAPDH. We generated an affinity-purified anti-GAPDH polyclonal to study the biochemical role of GAPDH in the early secretory pathway. The antibody arrests transport of a reporter molecule in an assay that reconstitutes ER to Golgi traffic. Furthermore, the affinity-purified antibody blocked the ability of Rab2 to recruit GAPDH to membrane. However, the antibody did not interfere with Rab2 stimulated vesicle release. These data suggest that GAPDH is required for ER to Golgi transport. We propose that membranes incubated with anti-GAPDH and Rab2 form "dead end" vesicles that are unable to transport and fuse with the acceptor compartment.

Electrospray ionization mass spectrometric analyses of changes in tissue phospholipid molecular species during the evolution of hyperlipidemia and hyperglycemia in Zucker diabetic fatty rats

The Zucker diabetic fatty (ZDF) rat is a genetic model of type II diabetes mellitus in which males homozygous for nonfunctional leptin receptors (fa/fa) develop obesity, hyperlipidemia, and hyperglycemia, but rats homozygous for normal receptors (+/+) remain lean and normoglycemic. Insulin resistance develops in young fa/fa rats and is followed by evolution of an insulin secretory defect that triggers hyperglycemia. Because insulin secretion and insulin sensitivity are affected by membrane phospholipid fatty acid composition, we have determined whether metabolic abnormalities in fa/fa rats are associated with changes in tissue phospholipids. Electrospray ionization mass spectrometric analyses of glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) molecular species from tissues of prediabetic (6 wk of age) and overtly diabetic (12 wk) fa/fa rats and from +/+ rats of the same ages indicate that arachidonate-containing species from heart, aorta, and liver of prediabetic fa/fa rats made a smaller contribution to GPC total ion current than was the case for +/+ rats. There was a correspondingly larger contribution from species with sn-2 oleate or linoleate substituents in fa/fa heart and aorta. The relative contributions of arachidonate-containing GPC species increased in these tissues as fa/fa rats aged and were equal to or greater than those for +/+ rats by 12 wk. For heart and aorta, relative contributions from GPE species with sn-2 arachidonate or docosahexaenoate substituents to the total ion current increased and those from species with sn-2 oleate or linoleate substituents fell as fa/fa rats aged, but these tissue lipid profiles changed little with age in +/+ rats. GPC and GPE profiles for brain, kidney, sciatic nerve, and red blood cells were similar among fa/fa and +/+ rats at 6 and 12 wk of age, and pancreatic islets from fa/fa and +/+ rats exhibited similar GPC and GPE profiles at 12 wk of age. Under-representation of arachidonate-containing GPC and GPE species in some fa/fa rat tissues at 6 wk could contribute to insulin resistance, but depletion of islet arachidonate-containing GPC and GPE species is unlikely to explain the evolution of the insulin secretory defect that is well-developed by 12 wk of age.

New insights into an old protein: the functional diversity of mammalian glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was considered a classical glycolytic protein examined for its pivotal role in energy production. It was also used as a model protein for analysis of protein structure and enzyme mechanisms. The GAPDH gene was utilized as a prototype for studies of genetic organization, expression and regulation. However, recent evidence demonstrates that mammalian GAPDH displays a number of diverse activities unrelated to its glycolytic function. These include its role in membrane fusion, microtubule bundling, phosphotransferase activity, nuclear RNA export, DNA replication and DNA repair. These new activities may be related to the subcellular localization and oligomeric structure of GAPDH in vivo. Furthermore, other investigations suggest that GAPDH is involved in apoptosis, age-related neurodegenerative disease, prostate cancer and viral pathogenesis. Intriguingly, GAPDH is also a unique target of nitric oxide. This review discusses the functional diversity of GAPDH in relation to its protein structure. The mechanisms through which mammalian cells may utilize GAPDH amino acid sequences to provide these new functions and to determine its intracellular localization are considered. The interrelationship between new GAPDH activities and its role in cell pathologies is addressed.

Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-induced arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.