DAG second messengers: molecular switches and growth control

The Endothelial Glycocalyx as a Key Mediator of Albumin Handling and the Development of Diabetic Nephropathy

The endothelial glycocalyx is a complex mesh of proteoglycans, glycoproteins and other soluble components, which cover the vascular endothelium. It plays an important role in many physiological processes including vascular permeability, transduction of shear stress and interaction of blood cells and other molecules with the vascular wall. Its complex structure makes its precise assessment challenging, and many different visualization techniques have been used with varying results. Diabetes, one of the main disease models where disorders of the glycocalyx are present, causes degradation of the glycocalyx through a variety of molecular pathways and especially through oxidative stress due to the action of reactive oxygen species. As the glycocalyx has been primarily studied in the glomerular endothelium, more evidence points towards a vital role in albumin handling and, consequently, in diabetic nephropathy. Therefore, the maintenance or restoration of the integrity of the glycocalyx seems a promising therapeutic target. In this review, we consider the structural and functional capacities of the endothelial glycocalyx, the available methods for its evaluation, the mechanisms through which diabetes leads to glycocalyx degradation and albuminuria, and possible treatment options targeting the glycocalyx.

Optogenetic toolkit for precise control of calcium signaling

Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated 'ON' and 'OFF' reactions in mammalian cells, pharmacological tools and 'caged' compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as 'genetically encoded Ca2+ actuators', or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.

Regulatory mechanism of gallic acid against advanced glycation end products induced cardiac remodeling in experimental rats

Advanced glycation end products (AGEs) play a major role in the development of cardiovascular disorders in diabetic patients. Recent studies evidenced the beneficial role of phytochemicals in reducing the risk of cardiovascular diseases. Hence the present study was framed to investigate the protective role of Gallic acid (GA) on AGEs induced cardiac fibrosis. Rats were infused with in vitro prepared AGEs (50mg/kg BW-intravenous injection) for 30 days. Further, GA (25mg/kgBW) was administered to rats along with AGEs. On infusion of AGEs, induction of fibrotic markers, collagen deposition, oxidative marker NADPH oxidase (NOX-p47 phox subunit), AGE receptor (RAGE) and cytokines expression was evaluated in the heart tissues using RT-PCR, Western blot and immunostaining methods. AGEs infusion significantly (P<0.01) increased the HW/BW ratio and fibrosis (4-fold) with increased expression of matrix genes MMP-2 and -9 (P<0.01, respectively) in the heart tissues. Whereas, administration of GA along with AGEs infusion prevented the fibrosis induced by AGEs. Further, GA treatment effectively prevented the AGEs mediated up-regulation of pro-fibrotic genes and ECM proteins such as TNF-α, TGF-β, MMP-2 and -9 expression. In addition, the increased expression of NOX (P<0.01), RAGE (P<0.01), NF-κB (P<0.01) and ERK 1/2 on AGEs infusion were normalized by GA treatment. Thus the present study shows the protective effect of GA on the fibrotic response and cardiac remodeling process induced by advanced glycation end products from external sources.

The renal endothelium in diabetic nephropathy

Diabetic nephropathy is the leading cause of end-stage renal disease. Diabetes mellitus is characterized by generalized endothelial dysfunction. However, recent data also emphasizes the role of local renal endothelium dysfunction in the pathogenesis of diabetic nephropathy. Hyperglycemia triggers a complex network of signal-transduction molecules, transcription factors, and mediators that culminate in endothelial dysfunction. In the glomerulus, vascular endothelial growth factor-A (VEGF)-induced neoangiogenesis may contribute to the initial hyperfiltration and microalbuminuria due to increased filtration area and immaturity of the neovessels, respectively. However, subsequent decrease in podocytes number decreases VEGF production resulting in capillary rarefaction and decreased glomerular filtration rate (GFR). Decreased nitric oxide availability also plays a significant role in the development of advanced lesions of diabetic nephropathy through disruption of glomerular autoregulation, uncontrolled VEGF action, release of prothrombotic substances by endothelial cells and angiotensin-II-independent aldosterone production. In addition, disturbances in endothelial glycocalyx contribute to decreased permselectivity and microalbuminuria; whereas there are recent evidences that reduced glomerular fenestral endothelium leads to decreased GFR levels. Endothelial repair mechanisms are also impaired in diabetes, since circulating endothelial progenitor cells number is decreased in diabetic patients with microalbuminuria. Finally, in the context of elevated profibrotic cytokine transforming growth factor-β levels, endothelial cells also confer to the deteriorating process of fibrosis in advanced diabetic nephropathy through endothelial to mesenchymal transition.

Sphingomyelin and its role in cellular signaling

Sphingolipid de novo biosynthesis is related with metabolic diseases. However, the mechanism is still not quite clear. Sphingolipids are ubiquitous and critical components of biological membranes. Their biosynthesis starts with soluble precursors in the endoplasmic reticulum and culminates in the Golgi complex and plasma membrane. The interaction of sphingomyelin, cholesterol, and glycosphingolipid drives the formation of plasma membrane rafts. Lipid rafts have been shown to be involved in cell -signaling, lipid and protein sorting, and membrane trafficking. It is well known that toll-like receptors, class A and B scavenger receptors, and insulin receptor are located in lipid rafts. Sphingomyelin is also a reservoir for other sphingolipids. So, sphingomyelin has important impact in cell -signaling through its structural role in lipid rafts or its catabolic inter-mediators, such as ceramide and glycoceramide. In this chapter, we will discuss both aspects.

A glimpse of various pathogenetic mechanisms of diabetic nephropathy

Diabetic nephropathy is a well-known complication of diabetes and is a leading cause of chronic renal failure in the Western world. It is characterized by the accumulation of extracellular matrix in the glomerular and tubulointerstitial compartments and by the thickening and hyalinization of intrarenal vasculature. The various cellular events and signaling pathways activated during diabetic nephropathy may be similar in different cell types. Such cellular events include excessive channeling of glucose intermediaries into various metabolic pathways with generation of advanced glycation products, activation of protein kinase C, increased expression of transforming growth factor β and GTP-binding proteins, and generation of reactive oxygen species. In addition to these metabolic and biochemical derangements, changes in the intraglomerular hemodynamics, modulated in part by local activation of the renin-angiotensin system, compound the hyperglycemia-induced injury. Events involving various intersecting pathways occur in most cell types of the kidney.

Genetic networks of liver metabolism revealed by integration of metabolic and transcriptional profiling

Although numerous quantitative trait loci (QTL) influencing disease-related phenotypes have been detected through gene mapping and positional cloning, identification of the individual gene(s) and molecular pathways leading to those phenotypes is often elusive. One way to improve understanding of genetic architecture is to classify phenotypes in greater depth by including transcriptional and metabolic profiling. In the current study, we have generated and analyzed mRNA expression and metabolic profiles in liver samples obtained in an F2 intercross between the diabetes-resistant C57BL/6 leptin^ob/ob and the diabetes-susceptible BTBR leptin^ob/ob mouse strains. This cross, which segregates for genotype and physiological traits, was previously used to identify several diabetes-related QTL. Our current investigation includes microarray analysis of over 40,000 probe sets, plus quantitative mass spectrometry-based measurements of sixty-seven intermediary metabolites in three different classes (amino acids, organic acids, and acyl-carnitines). We show that liver metabolites map to distinct genetic regions, thereby indicating that tissue metabolites are heritable. We also demonstrate that genomic analysis can be integrated with liver mRNA expression and metabolite profiling data to construct causal networks for control of specific metabolic processes in liver. As a proof of principle of the practical significance of this integrative approach, we illustrate the construction of a specific causal network that links gene expression and metabolic changes in the context of glutamate metabolism, and demonstrate its validity by showing that genes in the network respond to changes in glutamine and glutamate availability. Thus, the methods described here have the potential to reveal regulatory networks that contribute to chronic, complex, and highly prevalent diseases and conditions such as obesity and diabetes.

Although numerous quantitative trait loci (QTL) influencing disease-related phenotypes have been detected through gene mapping and positional cloning, identifying individual genes and their potential roles in molecular pathways leading to disease remains a challenge. In this study, we include transcriptional and metabolic profiling in genomic analyses to address this limitation. We investigated an F2 intercross between the diabetes-resistant C57BL/6 leptin^ob/ob and the diabetes-susceptible BTBR leptin^ob/ob mouse strains that segregates for genotype and diabetes-related physiological traits; blood glucose, plasma insulin and body weight. Our study shows that liver metabolites (comprised of amino acids, organic acids, and acyl-carnitines) map to distinct genetic regions, thereby indicating that tissue metabolites are heritable. We also demonstrate that genomic analysis can be integrated with liver mRNA expression and metabolite profiling data to construct causal, testable networks for control of specific metabolic processes in liver. We apply an in vitro study to confirm the validity of this integrative method, and thus provide a novel approach to reveal regulatory networks that contribute to chronic, complex, and highly prevalent diseases and conditions such as obesity and diabetes.

Enhanced apoptosis through farnesol inhibition of phospholipase D signal transduction

Farnesol is a catabolite of the cholesterol biosynthetic pathway that preferentially causes apoptosis in tumorigenic cells. Phosphatidylcholine (PC), phosphatidic acid (PA), and diacylglycerol (DAG) were able to prevent induction of apoptosis by farnesol. Primary alcohol inhibition of PC catabolism by phospholipase D augmented farnesol-induced apoptosis. Exogenous PC was unable to prevent the increase in farnesol-induced apoptosis by primary alcohols, whereas DAG was protective. Farnesol-mediated apoptosis was prevented by transformation with a plasmid coding for the PA phosphatase LPP3, but not by an inactive LPP3 point mutant. Farnesol did not directly inhibit LPP3 PA phosphatase enzyme activity in an in vitro mixed micelle assay. We propose that farnesol inhibits the action of a DAG pool generated by phospholipase D signal transduction that normally activates an antiapoptotic/pro-proliferative target.

Role of endogenous TRPC6 channels in Ca2+ signal generation in A7r5 smooth muscle cells

The ubiquitously expressed canonical transient receptor potential (TRPC) ion channels are considered important in Ca2+ signal generation, but their mechanisms of activation and roles remain elusive. Whereas most studies have examined overexpressed TRPC channels, we used molecular, biochemical, and electrophysiological approaches to assess the expression and function of endogenous TRPC channels in A7r5 smooth muscle cells. Real time PCR and Western analyses reveal TRPC6 as the only member of the diacylglycerol-responsive TRPC3/6/7 subfamily of channels expressed at significant levels in A7r5 cells. TRPC1, TRPC4, and TRPC5 were also abundant. An outwardly rectifying, nonselective cation current was activated by phospholipase C-coupled vasopressin receptor activation or by the diacylglycerol analogue, oleoyl-2-acetyl-sn-glycerol (OAG). Introduction of TRPC6 small interfering RNA sequences into A7r5 cells by electroporation led to 90% reduction of TRPC6 transcript and 80% reduction of TRPC6 protein without any detectable compensatory changes in the expression of other TRPC channels. The OAG-activated nonselective cation current was similarly reduced by TRPC6 RNA interference. Intracellular Ca2+ measurements using fura-2 revealed that thapsigargin-induced store-operated Ca2+ entry was unaffected by TRPC6 knockdown, whereas vasopressin-induced Ca2+ entry was suppressed by more than 50%. In contrast, OAG-induced Ca2+ transients were unaffected by TRPC6 knockdown. Nevertheless, OAG-induced Ca2+ entry bore the hallmarks of TRPC6 function; it was inhibited by protein kinase C and blocked by the Src-kinase inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Importantly, OAG-induced Ca2+ entry was blocked by the potent L-type Ca2+ channel inhibitor, *nimodipine. Thus, TRPC6 activation probably results primarily in Na ion entry and depolarization, leading to activation of L-type channels as the mediators of Ca2+ entry. Calculations reveal that even 90% reduction of TRPC6 channels would allow depolarization sufficient to activate L-type channels. This tight coupling between TRPC6 and L-type channels is probably important in mediating smooth muscle cell membrane potential and muscle contraction.

Purification, characterization, and identification of a sphingomyelin synthase from Pseudomonas aeruginosa. PlcH is a multifunctional enzyme

Sphingomyelin synthase is the enzyme that synthesizes sphingomyelin (SM) in mammalian cells by transferring a phosphorylcholine moiety from phosphatidylcholine to ceramide. Despite its importance, the gene and/or the protein responsible for this activity has not yet been identified. Here we report the purification, identification, and biochemical characterization of an enzymatic activity that synthesizes SM in Pseudomonas aeruginosa. SM synthase-like activity was found secreted in the culture medium of P. aeruginosa, strains PA01 and PAK, whereas it could not be detected in cultures of Escherichia coli. From the medium of PAK cultures, SM synthase was purified through sequential chromatographic columns. After separation on polyacrylamide-SDS gels and visualization by silver staining, the purified enzyme showed two bands, one of approximately 75 kDa and one of 30-35 kDa. Interestingly, the highly purified SM synthase preparation also showed neutral sphingomyelinase activity. We therefore investigated whether the protein we purified as SM synthase could actually be the previously identified PlcH, a 78-kDa phospholipase C known to hydrolyze phosphatidylcholine and SM in P. aeruginosa. First, the purified SM synthase preparation contained a 78-kDa protein that reacted with monoclonal antibodies raised against purified PlcH. Second, purified PlcH showed SM synthase activity. Third, using different knockout mutant strains for the PlcH operon, PlcH was found to be necessary for SM synthase activity in P. aeruginosa. Interestingly, SM synthase activity was specific to the Pseudomonas PlcH as other bacterial phospholipases did not display SM synthase activity. Biochemical studies on the Pseudomonas SM synthase confirmed that it is a transferase, similar to the mammalian enzyme, that specifically recognizes the choline head-group and the primary hydroxyl on ceramide. This SM synthase did not have reverse transferase activity. In conclusion, the Pseudomonas PlcH also exerts SM synthase activity; therefore, for the first time, we have identified a structural gene for a SM synthase.

Regulation of canonical transient receptor potential (TRPC) channel function by diacylglycerol and protein kinase C

The mechanism of receptor-induced activation of the ubiquitously expressed family of mammalian canonical transient receptor potential (TRPC) channels has been the focus of intense study. Primarily responding to phospholipase C (PLC)-coupled receptors, the channels are reported to receive modulatory input from diacylglycerol, endoplasmic reticulum inositol 1,4,5-trisphosphate receptors and Ca2+ stores. Analysis of TRPC5 channels transfected within DT40 B cells and deletion mutants thereof revealed efficient activation in response to PLC-beta or PLC-gamma activation, which was independent of inositol 1,4,5-trisphoshate receptors or the content of stores. In both HEK293 cells and DT40 cells, TRPC5 and TRPC3 channel responses to PLC activation were highly analogous, but only TRPC3 and not TRPC5 channels responded to the addition of the permeant diacylglycerol (DAG) analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG). However, OAG application or elevated endogenous DAG, resulting from either DAG lipase or DAG kinase inhibition, completely prevented TRPC5 or TRPC4 activation. This inhibitory action of DAG on TRPC5 and TRPC4 channels was clearly mediated by protein kinase C (PKC), in distinction to the stimulatory action of DAG on TRPC3, which is established to be PKC-independent. PKC activation totally blocked TRPC3 channel activation in response to OAG, and the activation was restored by PKC-blockade. PKC inhibition resulted in decreased TRPC3 channel deactivation. Store-operated Ca2+ entry in response to PLC-coupled receptor activation was substantially reduced by OAG or DAG-lipase inhibition in a PKC-dependent manner. However, store-operated Ca2+ entry in response to the pump blocker, thapsigargin, was unaffected by PKC. The results reveal that each TRPC subtype is strongly inhibited by DAG-induced PKC activation, reflecting a likely universal feedback control on TRPCs, and that DAG-mediated PKC-independent activation of TRPC channels is highly subtype-specific. The profound yet distinct control by PKC and DAG of the activation of TRPC channel subtypes is likely the basis of a spectrum of regulatory phenotypes of expressed TRPC channels.

Hormone-sensitive lipase deficiency in mice causes diglyceride accumulation in adipose tissue, muscle, and testis

Hormone-sensitive lipase (HSL) is expressed predominantly in white and brown adipose tissue where it is believed to play a crucial role in the lipolysis of stored triglycerides (TG), thereby providing the body with energy substrate in the form of free fatty acids (FFA). From in vitro assays, HSL is known to hydrolyze TG, diglycerides (DG), cholesteryl esters, and retinyl esters. In the current study we have generated HSL knock-out mice and demonstrate three lines of evidence that HSL is instrumental in the catabolism of DG in vivo. First, HSL deficiency in mice causes the accumulation of DG in white adipose tissue, brown adipose tissue, skeletal muscle, cardiac muscle, and testis. Second, when tissue extracts were used in an in vitro lipase assay, a reduced FFA release and the accumulation of DG was observed in HSL knock-out mice which did not occur when tissue extracts from control mice were used. Third, in vitro lipolysis experiments with HSL-deficient fat pads demonstrated that the isoproterenol-stimulated release of FFA was decreased and DG accumulated intracellularly resulting in the essential absence of the isoproterenol-stimulated glycerol formation typically observed in control fat pads. Additionally, the absence of HSL in white adipose tissue caused a shift of the fatty acid composition of the TG moiety toward increased long chain fatty acids implying a substrate specificity of the enzyme in vivo. From these in vivo results we conclude that HSL is the rate-limiting enzyme for the cellular catabolism of DG in adipose tissue and muscle.

Characterization of phosphatidylinositol, phosphatidylinositol-4-phosphate, and phosphatidylinositol-4,5-bisphosphate by electrospray ionization tandem mass spectrometry: a mechanistic study

Structural characterization of phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PI-4P), and phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2) by collisionally activated dissociation (CAD) tandem mass spectrometry with electrospray ionization is described. In negative ion mode, the major fragmentation pathways under low energy CAD for PI arise from neutral loss of free fatty acid substituents ([M - H - RxCO2H]-) and neutral loss of the corresponding ketenes ([M - H - R'xCH=C=O]-), followed by consecutive loss of the inositol head group. The intensities of the ions arising from neutral loss of the sn-2 substituent as a free fatty acid ([M - H - R2CO2H]-) or as a ketene ([M - H - R'2CH=C=O] ) are greater than those of ions reflecting corresponding losses of the sn-1 substutient. This is consistent with our recent finding that ions reflecting those losses arise from charge-driven processes that occur preferentially at the sn-2 position. These features permit assignment of the position of the fatty acid substituents on the glycerol backbone. Nucleophilic attack of the anionic phosphate onto the C-1 or the C-2 of the glycerol to which the fatty acids attached expels sn-1 (R1CO2-) or sn-2 (R2CO2-) carboxylate anion, respectively. This pathway is sterically more favorable at sn-2 than at sn-1. However, further dissociations of [M - H - RxCO2H - inositol] , [M - H - RxCO2H]-, and [M - H - RxCH=C=O]- precursor ions also yield RxCO2- ions, whose abundance are affected by the collision energy applied. Therefore, relative intensities of the RxCO2- ions in the spectrum do not reflect their positions on the glycerol backbone and determination of their regiospecificities based on their ion intensities is not reliable. The spectra also contain specific ions at m/z 315, 279, 259, 241, and 223, reflecting the inositol head group. The last three ions are also observed in the tandem spectra of the [M - H]- ions of phosphatidylinositol monophosphate (PI-P) and phosphatidylinositol bisphosphate (PI-P2), in addition to the ions at m/z 321 and 303, reflecting the doubly phosphorylated inositol ions. The PI-P2 also contains unique ions at m/z 401 and 383 that reflect the triply phosphorylated inositol ions. The [M - H]- ions of PI-P and PI-P2 undergo fragmentation pathways similar to that of PI upon CAD. However, the doubly charged ([M - 2H]2-) molecular ions undergo fragmentation pathways that are typical of the [M - H]- ions of glycerophosphoethanolamine, which are basic. These results suggest that the further deprotonated gaseous [M - 2H]2 ions of PI-P and PI-P2 are basic precursors.

Phospholipase D, tumor promoters, proliferation and prostaglandins

Phosphatidylcholine hydrolysis by phospholipase D is a widespread response to cellular stimulation. However, the downstream signaling events subsequent to phosphatidylcholine hydrolysis are just beginning to be determined. Initially it was proposed that diglyceride formation by phospholipase D and phosphatidate phosphohydrolase resulted in long-term stimulation of protein kinase C. However, recent studies indicate that phosphatidic acid is the relevant signaling molecule in some signaling pathways. The present review will summarize studies of phospholipase D in the response of cells to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, which causes cells to mimic the phenotype of oncogenic transformation. The role of phospholipase D in stimulation of Raf-1 and prostaglandin H synthase type-2 is emphasized.

Ceramide in apoptosis--does it really matter?

During recent years, ceramide has received a lot of attention as a possible mediator of the cellular responses to a variety of apoptotic stimuli. In a manner analogous to generation of its sibling diacylglycerol, ceramide is generated by a phospholipase-C-type reaction from its lipid precursor sphingomyelin. Two observations led to the proposal that ceramide plays a role in apoptosis: (1) treatment of cells with tumor necrosis factor or other inducers of apoptosis leads to activation of sphingomyelinases and to an increase in cellular ceramide levels; (2) ectopic generation or administration of ceramide can mimic apoptotic cell death. Recently, several observations have challenged the notion that ceramide is an important cell-death mediator and have prompted a re-evaluation of previously published results.