Human munc13 is a diacylglycerol receptor that induces apoptosis and may contribute to renal cell injury in hyperglycemia

Triglycerides Stabilize Water/Organic Interfaces of Changing Area via Conformational Flexibility

The role of triglycerides (TGs) in both natural and synthetic biological membranes has long been the subject of study, involving metabolism, disease, and colloidal synthesis. TGs have been found to be critical components for successful liposomal encapsulation via a water/oil/water double emulsion, which this work endeavors to explain. TGs can occupy multiple positions in biological membranes. The glycerol backbone can reside at the water/organic interface, adjacent to phospholipid headgroups ("m" conformation), typically with relatively low (<3%) solubility. The glycerol backbone can also occupy hydrophobic regions, where it is isolated from water ("h" or "oil" conformation). This can occur in either midmembrane positions or phospholipid-coated lipid droplets (LDs). These conformations can be distinguished using 13C-nuclear magnetic resonance spectroscopy (NMR), which determines the degree of hydration of the TG backbone. Using this method, it was revealed that TGs transition from "m" to "h" conformation as the organic solvent is removed via evaporation. A new transitional TG backbone position has been identified with a level of hydration between "m" and "h". These results suggest that TGs can temporarily coat and stabilize the large water/organic interfaces present after emulsification. As the organic solvent is removed and interfaces shrink, the TGs recede into midmembrane spaces or bud off into LDs, which are confirmed via transmission electron microscopy (TEM) and can be removed via centrifugation. Encapsulation efficiency is found to be inversely related to both the saturation and length of the TG acyl chains, indicating that membrane fluidization is a key property arising from the presence of TGs. Beyond clarification of a mechanism for high-efficiency liposomal encapsulation, these results implicate TGs as components that are able to stabilize biological membrane transitions involving a changing interfacial area and curvature. This role for TGs may be of use in the formulation of drug delivery systems as well as in the investigation of membrane transitions in life sciences.

Beyond Lipid Signaling: Pleiotropic Effects of Diacylglycerol Kinases in Cellular Signaling

The diacylglycerol kinase family, which can attenuate diacylglycerol signaling and activate phosphatidic acid signaling, regulates various signaling transductions in the mammalian cells. Studies on the regulation of diacylglycerol and phosphatidic acid levels by various enzymes, the identification and characterization of various diacylglycerol and phosphatidic acid-regulated proteins, and the overlap of different diacylglycerol and phosphatidic acid metabolic and signaling processes have revealed the complex and non-redundant roles of diacylglycerol kinases in regulating multiple biochemical and biological networks. In this review article, we summarized recent progress in the complex and non-redundant roles of diacylglycerol kinases, which is expected to aid in restoring dysregulated biochemical and biological networks in various pathological conditions at the bed side.

The Association of UNC13B Gene Polymorphisms and Diabetic Kidney Disease in a Chinese Han Population

BACKGROUND Polymorphisms in the UNC13B gene are associated with diabetic kidney disease (DKD) in the European population. Asian populations are more likely to suffer from complications of type 2 diabetes mellitus (T2DM), including diabetic kidney disease (DKD). This case-control study aimed to investigate the association between UNC13B gene polymorphisms and DKD in a Chinese Han population. MATERIAL AND METHODS Five single nucleotide polymorphism (SNP) loci (rs13293564, rs17360668, rs10114937, rs661712, and rs2281999) were genotyped in the UNC13B gene in 600 Chinese Han subjects. The study population included patients with T2DM with DKD (N=292) and control patients with T2DM without DKD (N=308). SNP genotyping was performed using a Sequenom MassARRAY system using chip-based matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). RESULTS There were no significant differences in the distribution of allele or genotype frequencies in the five UNC13B SNP markers (rs13293564, rs17360668, rs10114937, rs661712, and rs2281999) between the DKD group and control group of patients with T2DM. Haplotype analysis identified eight haplotypes for the combined effect of the five SNP markers in the UNC13B gene. The haplotype GGCCG was significantly associated with an increased risk of DKD. CONCLUSIONS This was the first study to demonstrate an association between UNC13B gene polymorphisms and the susceptibility to DKD in a Chinese Han population with T2DM. The haplotype GGCCG was significantly associated with an increased risk of DKD. The findings highlight the joint effect of SNP markers in the pathogenesis of DKD.

Strain-Dependent Effects of Acute Alcohol on Synaptic Vesicle Recycling and Post-Tetanic Potentiation in Medial Glutamate Inputs to the Mouse Basolateral Amygdala

BACKGROUND

Inbred mouse strains are differentially sensitive to the acute effects of ethanol (EtOH) and are useful tools for examining how unique genomes differentially affect alcohol-related behaviors and physiology. DBA/2J mice have been shown to be sensitive to the acute anxiolytic effects of alcohol as well as the anxiogenic effects of withdrawal from chronic alcohol exposure, while B6 mice are resistant to both. Considering that the basolateral amygdala (BLA) is an important brain region for the acute and chronic effects of EtOH on fear and anxiety related behaviors, we hypothesized that there would be strain-dependent differences in the acute effects of EtOH in BLA slices.

METHODS

We utilized patch clamp electrophysiology in BLA coronal slices from 4 inbred mouse strains (A/J, BALBcJ, C57BL/6J, and DBA/2J) to examine how genetic background influences acute EtOH effects on synaptic vesicle recycling and post-tetanic potentiation (PTP) in response to low (2 Hz)- and high (40 Hz)-frequency stimulation.

RESULTS

We found that EtOH inhibited synaptic vesicle recycling in a strain- and stimulation frequency-dependent manner. Vesicle recycling in DBA/2J and BALBcJ cells was inhibited by acute EtOH during both low- and high-frequency stimulation, while recycling measured from A/J cells was sensitive only during high-frequency stimulation. Recycling at C57BL/6J synapses was insensitive to EtOH regardless of stimulation frequency. We additionally found that cells from DBA/2J and BALBcJ mice were sensitive to EtOH-mediated inhibition of PTP.

CONCLUSIONS

Acute EtOH application inhibited vesicle recycling and PTP at glutamatergic synapses in both a strain- and frequency-dependent fashion. Several presynaptic proteins that contribute to synaptic vesicle priming in addition to PTP have been implicated in alcohol-related behaviors, including Munc13, Munc18, and RIM proteins, making them potential candidates for the molecular mechanism controlling these effects.

The genetics of diabetic complications

The rising global prevalence of diabetes mellitus is accompanied by an increasing burden of morbidity and mortality that is attributable to the complications of chronic hyperglycaemia. These complications include blindness, renal failure and cardiovascular disease. Current therapeutic options for chronic hyperglycaemia reduce, but do not eradicate, the risk of these complications. Success in defining new preventative and therapeutic strategies hinges on an improved understanding of the molecular processes involved in the development of these complications. This Review explores the role of human genetics in delivering such insights, and describes progress in characterizing the sequence variants that influence individual predisposition to diabetic kidney disease, retinopathy, neuropathy and accelerated cardiovascular disease. Numerous risk variants for microvascular complications of diabetes have been reported, but very few have shown robust replication. Furthermore, only limited evidence exists of a difference in the repertoire of risk variants influencing macrovascular disease between those with and those without diabetes. Here, we outline the challenges associated with the genetic analysis of diabetic complications and highlight ongoing efforts to deliver biological insights that can drive translational benefits.

Lipid requirements for entry of protein toxins into cells

The plant toxin ricin and the bacterial toxin Shiga toxin both belong to a group of protein toxins having one moiety that binds to the cell surface, and another, enzymatically active moiety, that enters the cytosol and inhibits protein synthesis by inactivating ribosomes. Both toxins travel all the way from the cell surface to endosomes, the Golgi apparatus and the ER before the ribosome-inactivating moiety enters the cytosol. Shiga toxin binds to the neutral glycosphingolipid Gb3 at the cell surface and is therefore dependent on this lipid for transport into the cells, whereas ricin binds both glycoproteins and glycolipids with terminal galactose. The different steps of transport used by these toxins have specific requirements for lipid species, and with the recent developments in mass spectrometry analysis of lipids and microscopical and biochemical dissection of transport in cells, we are starting to see the complexity of endocytosis and intracellular transport. In this article we describe lipid requirements and the consequences of lipid changes for the entry and intoxication with ricin and Shiga toxin. These toxins can be a threat to human health, but can also be exploited for diagnosis and therapy, and have proven valuable as tools to study intracellular transport.

Regulation of Lipid Signaling by Diacylglycerol Kinases during T Cell Development and Function

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.

p23/Tmp21 differentially targets the Rac-GAP beta2-chimaerin and protein kinase C via their C1 domains

The ER/Golgi protein p23/Tmp21 acts as a C1 domain-docking protein that mediates perinuclear translocation of β-chimaerin. C1 domains from PKC isozymes can also interact with p23/Tmp21. Our study highlights the relevance of C1 domains in protein-protein interactions in addition to their well-established lipid-binding properties.

The C1 domains in protein kinase C (PKC) isozymes and other signaling molecules are responsible for binding the lipid second messenger diacylglycerol and phorbol esters, and for mediating translocation to membranes. Previous studies revealed that the C1 domain in α- and β-chimaerins, diacylglycerol-regulated Rac-GAPs, interacts with the endoplasmic reticulum/Golgi protein p23/Tmp21. Here, we found that p23/Tmp21 acts as a C1 domain-docking protein that mediates perinuclear translocation of β2-chimaerin. Glu227 and Leu248 in the β2-chimaerin C1 domain are crucial for binding p23/Tmp21 and perinuclear targeting. Interestingly, isolated C1 domains from individual PKC isozymes differentially interact with p23/Tmp21. For PKCε, it interacts with p23/Tmp21 specifically via its C1b domain; however, this association is lost in response to phorbol esters. These results demonstrate that p23/Tmp21 acts as an anchor that distinctively modulates compartmentalization of C1 domain-containing proteins, and it plays an essential role in β2-chimaerin relocalization. Our study also highlights the relevance of C1 domains in protein–protein interactions in addition to their well-established lipid-binding properties.

The histone deacetylase inhibitor LBH589 inhibits expression of mitotic genes causing G2/M arrest and cell death in head and neck squamous cell carcinoma cell lines

Head and neck squamous cell carcinoma represents a complex set of neoplasms arising in diverse anatomical locations. The site and stage of the cancer determine whether patients will be treated with single or multi-modality therapy. The HDAC inhibitor LBH589 is effective in treating some haematological neoplasms and shows promise for certain epithelial neoplasms. As with other human cancer cell lines, LBH589 causes up-regulation of p21, G2/M cell cycle arrest, and cell death of human HNSCC cell lines, as measured using flow cytometry and cDNA microarrays. Global RNA expression studies following treatment of the HNSCC cell line FaDu with LBH589 reveal down-regulation of genes required for chromosome congression and segregation (SMC2L1), sister chromatid cohesion (DDX11), and kinetochore structure (CENP-A, CENP-F, and CENP-M); these LBH589-induced changes in gene expression coupled with the down-regulation of MYC and BIRC5 (survivin) provide a plausible explanation for the early mitotic arrest and cell death observed. When LBH589-induced changes in gene expression were compared with gene expression profiles of 41 primary HNSCC samples, many of the genes that were down-regulated by LBH589 showed increased expression in primary HNSCC, suggesting that some patients with HNSCC may respond to treatment with LBH589.

Rab34 and its effector munc13-2 constitute a new pathway modulating protein secretion in the cellular response to hyperglycemia

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease requiring dialysis in the Western world. Clinical studies reveal that stringent control of blood glucose levels reduces the risk of most diabetic complications, underscoring the importance of understanding the cellular response to hyperglycemia. Our work identifies a new pathway of potential significance in this response, linking hyperglycemia to the stimulation of constitutive protein secretion via a pathway involving munc13 and rab34. These two proteins have previously been shown to interact at the Golgi via the munc13 homology domain 2 (MHD2). In the present study, using cultured rat mesangial cells (RMC), we show that high glucose-induced upregulation of endogenous munc13-2 increases secretion of the model protein, vesicular stomatitis virus glycoprotein-green fluorescent protein (VSVG-GFP), while small interfering (si)RNA-mediated knockdown of either munc13-2 or rab34 abolishes this effect. Similarly, increased secretion of VSVG-GFP is observed following transfection of HeLa cells with wild-type munc13-2, but not when HeLa cells are transfected with a mutant protein in which the MHD2 domain is deleted. Finally, we show that high glucose-stimulated secretion of fibronectin in RMC is abolished by siRNA knockdown of munc13-2. Collectively, our results demonstrate that the mechanistic basis for our observed high glucose-induced protein secretion is through interaction of munc13 and rab34, indicating a potentially critical role for this newly described pathway in the pathogenesis of DN.

Alteration of lipid composition of rat thymus during thymic atrophy by whole-body X-irradiation

PURPOSE

Thymic atrophy induced by irradiation is well known, but in vivo lipid metabolism during the atrophy has not been studied in detail. We determined the lipid composition of rat thymus during the progress of thymic atrophy induced by whole-body X-irradiation.

MATERIALS AND METHODS

The lipid analysis of total lipid of rat thymus after 5 Gy whole-body X-irradiation was performed by high performance liquid chromatography and gas chromatography equipped with mass spectrometry.

RESULTS

Major changes observed were a 16.2-fold elevation of cholesterol ester (CE) during a 48-h post-irradiation period and a 6.1-fold increase of alkyldiacylglycerol (ADG) at 24 h. Other significant changes detected were an increase in lysophosphatidylcholine and a transient increase in ceramide and phosphatidic acid. Acyl chain analysis revealed a substantial elevation of arachidonate composition of CE and an unusually high content of polyunsaturated fatty acids (71.5%, mol/mol) in ADG.

CONCLUSION

Lipid analysis shows that the thymic atrophy by X-irradiation was accompanied by a significant change in thymic lipids. This in vivo result opens up new vistas of the role of lipids in apoptosis and phagocytosis during thymic atrophy.

G/T substitution in intron 1 of the UNC13B gene is associated with increased risk of nephropathy in patients with type 1 diabetes

OBJECTIVE

Genetic and environmental factors modulate the susceptibility to diabetic nephropathy, as initiating and/or progression factors. The objective of the European Rational Approach for the Genetics of Diabetic Complications (EURAGEDIC) study is to identify nephropathy susceptibility genes. We report molecular genetic studies for 127 candidate genes for nephropathy.

RESEARCH DESIGN AND METHODS

Polymorphisms were identified through sequencing of promoter, exon, and flanking intron gene regions and a database search. A total of 344 nonredundant SNPs and nonsynonymous variants were tested for association with diabetic nephropathy (persistent albuminuria >/=300 mg/24 h) in a large type 1 diabetes case/control (1,176/1,323) study from three European populations.

RESULTS

Only one SNP, rs2281999, located in the UNC13B gene, was significantly associated with nephropathy after correction for multiple testing. Analyses of 21 additional markers fully characterizing the haplotypic variability of the UNC13B gene showed consistent association of SNP rs13293564 (G/T) located in intron 1 of the gene with nephropathy in the three populations. The odds ratio (OR) for nephropathy associated with the TT genotype was 1.68 (95% CI 1.29-2.19) (P = 1.0 x 10(-4)). This association was replicated in an independent population of 412 case subjects and 614 control subjects (combined OR of 1.63 [95% CI 1.30-2.05], P = 2.3 x 10(-5)).

CONCLUSIONS

We identified a polymorphism in the UNC13B gene associated with nephropathy. UNC13B mediates apopotosis in glomerular cells in the presence of hyperglycemia, an event occurring early in the development of nephropathy. We propose that this polymorphism could be a marker for the initiation of nephropathy. However, further studies are needed to clarify the role of UNC13B in nephropathy.

Golgi-bound Rab34 is a novel member of the secretory pathway

Golgi-localized Rab34 has been implicated in repositioning lysosomes and activation of macropinocytosis. Using HeLa cells, we undertook a detailed investigation of Rab34 involvement in intracellular vesicle transport. Immunoelectron microscopy and immunocytochemistry confirmed that Rab34 is localized to the Golgi stack and that active Rab34 shifts lysosomes to the cell center. Contrary to a previous report, we found that Rab34 is not concentrated at membrane ruffles and is not involved in fluid-phase uptake. Also, Rab34-induced repositioning of lysosomes does not affect mannose 6-phosphate receptor trafficking. Most strikingly, HeLa cells depleted of Rab34 by transfection with dominant-negative Rab34 or after RNA interference, failed to transport the temperature-sensitive vesicular stomatitis virus G-protein (VSVG) fused to green fluorescent protein (VSVG-GFP) from the Golgi to the plasma membrane. Transfection with mouse Rab34 rescued this defect. Using endogenous major histocompatibility complex class I (MHCI) as a marker, an endoglycosidase H resistance assay showed that endoplasmic reticulum (ER) to medial Golgi traffic remains intact in knockdown cells, indicating that Rab34 specifically functions downstream of the ER. Further, brefeldin A treatment revealed that Rab34 effects intra-Golgi transport, not exit from the trans-Golgi network. Collectively, these results define Rab34 as a novel member of the secretory pathway acting at the Golgi.

Protein kinase C delta regulates anti-apoptotic alphaB-crystallin in the retina of type 2 diabetes

We investigated the relationship between phosphorylation of alphaB-crystallin (alphaBC) and retinal apoptosis in type 2 diabetes. The retinas of male Otsuka Long-Evans Tokushima fatty (OLETF) rats at 24 and 35 weeks were used as an animal model for type 2 diabetes and sex- and age-matched Long-Evans Tokushima Otsuka (LETO) rats were used as controls. In the retinas of 35-week OLETF rats, the interaction between alphaBC and protein kinase C delta (PKC delta) among the PKC isozymes, alphaBC phosphorylation at Ser45 (S45p-alphaBC), TUNEL-positive apoptotic ganglion cells, several apoptotic signs, and co-localization of S45p-alphaBC and TUNEL significantly increased as compared with other groups while the alphaBC-Bax interaction greatly decreased. These changes were abolished by rottlerin treatment, a highly specific PKC delta inhibitor. These results suggest that PKC delta is involved in regulation of anti-apoptotic function of alphaBC in the retina of type 2 diabetes.

The lipid second messenger diacylglycerol as a negative regulator of Rac signalling

We have established a novel role for the second messenger DAG (diacylglycerol), a product of PtdIns(4,5)P2 hydrolysis by PLC (phospholipase C). In addition to its well-known function as a protein kinase C activator, DAG produced by stimulation of the epidermal growth factor receptor causes the redistribution of the Rac-GAP (GTPase-activating protein) beta2-chimaerin to the plasma membrane, where it associates with the active form of Rac1 and promotes the inactivation of this small G-protein. This represents the first example of a Rac-GAP regulated directly by DAG in response to the activation of a tyrosine kinase receptor, and suggests a previously unappreciated role for this lipid as a negative modulator of Rac signalling.

Cytotoxic action of phorbol esters on human pancreatic cancer cells

We previously showed that phorbol esters are cytotoxic to human thyroid epithelial cells expressing a mutant RAS oncogene. Here we explore the generality of this finding using cells derived from pancreatic cancer, which, like thyroid, shows a high frequency of RAS mutation, but is a much greater cause of cancer mortality. The response to phorbol myristate acetate (PMA) and related agents was assessed on a panel of 9 pancreatic cancer cell lines, using a range of assays for cell growth and death in vitro and in vivo. In most lines, PMA induced non-apoptotic cell death which was, surprisingly, independent of its "classic" target, protein kinase C. With 24 hr exposure, the IC(50) in the most sensitive line (Aspc-1) was <1 ng/ml (1.6 nM), with survival undetectable at concentrations >/=>/=16 nM, and after only 1 hr exposure the IC(50) was still </=</=16 nM. Interestingly, the efficacy of a second phorbol ester, phorbol dibutyrate, was much lower, and the PMA analogue bryostatin-1, which is in clinical trials against other tumour types, was totally inactive. Pre-treatment of Aspc-1 cells with PMA before subcutaneous inoculation into nude mice prevented, or greatly retarded, subsequent xenograft tumour growth. Furthermore, treatment of established tumours with a single peri-tumoral injection of PMA induced extensive cell death and arrested tumour development. Taken together with recent Phase 1 clinical studies, these data suggest that activity against pancreatic cancer will be attainable by systemic administration of PMA, and point to potential novel therapeutic targets for this highly aggressive cancer.

Lymphoblastoid cell lines differing in p53 status show clear differences in basal gene expression with minor changes after irradiation

BACKGROUND AND PURPOSE

The genetic profile as determined by microarray is considered to be an ideal marker of the individual radiosensitivity. However, it is still an open question, whether this profile has to be determined prior to or only after irradiation, since the expression of some genes is affected by irradiation. These changes are induced mainly due to a p53-dependent transactivation.

MATERIALS AND METHODS

In this study gene expression profiles were measured for 3 lymphoblastoid cell lines differing in p53 status (p53 wt: TK6; p53null: TK6E6, p53mut: WTK1) measured either prior to or 3h after exposure to 2Gy. The gene expression profile was determined using the Affymetrix Human HG U133A GeneChip and for selective genes, variation in gene expression was validated by qRT-PCR. In addition, different assays were used to characterize the radioresponse of these three strains.

RESULTS

The three strains were found to be different in all aspects of radiosensitivity studied. Cells with p53wt showed more apoptosis, slightly stronger arrest in G1, but less lethal aberrations and a lower viability when compared to cells with mutated p53, whereas cells absent in p53 are characterized by an intermediate response. The gene expression profile measured prior to irradiation already revealed huge differences. Significance analysis of microarrays (SAM) identified 141 genes that changed expression twofold or more with a false discovery rate (FDR) of 5.4%. When compared to p53null cell line with p53wt showed a twofold difference in up- or down-regulation in 28 genes. A much higher variation was even found when p53mut cells were compared with p53null cells with a twofold difference in even 123 genes. The respective genes were found to be involved mainly in apoptosis, cell cycle regulation, metabolisms and signalling but with only one gene relevant for DNA repair. Radiation was found to affect this profile solely for cells with p53wt with a twofold significant up-regulation in only five genes. For selective genes (BCL2, CASP1, CCND2, DDB2, XPC, RAD51C, SESN1, FUCA1, CDKN1A, MDM2, XPC) array data were confirmed by qRT-PCR.

CONCLUSION

The result, that the gene expression profile of lymphoblastoid cells differing in p53 status already displayed clear differences when measured prior to irradiation with only few changes after irradiation, which are solely seen for p53wt cells, suggests, that the differences in radiosensitivity observed for these cells are primarily determined by the variation in expression profile present already prior to irradiation.

Identification of differentially expressed genes in psoriasis using expression profiling approaches

To identify differentially expressed genes which play causal roles in pathogenesis and maintenance for psoriasis, we used BodyMapping and introduced amplified fragment length polymorphism approaches. From the BodyMap database, we selected 2007 genes which specifically expressed in epithelial tissues. Among 2007 genes, we surveyed genes which differentially expressed in involved or uninvolved psoriatic lesional skin samples compared with atopic dermatitis, mycosis fungoides, and normal skin samples. As a result of surveying 2007 genes, 241 genes were differentially expressed only in involved psoriatic skin but not in the other samples. Hierarchical cluster analysis of gene expression profiles showed that 13 independent psoriatic-involved skin samples clustered tightly together, reflecting highly similar expression profiles. Using the same 2007 gene set, we examined gene expression levels in five serial lesions from distal uninvolved psoriatic skin to involved psoriatic plaque. We identified seven genes such as alpha-1-microglobulin/bikunin precursor, calnexin, claudin 1, leucine zipper down-regulated in cancer 1, tyrosinase-related protein 1, Yes-associated protein 1, and unc-13-like protein (Coleonyx elegans) which show high-expression levels only in uninvolved psoriatic lesions. These seven genes, which were reported to be related to apoptosis or antiproliferation, might have causal roles in pathophysiology in psoriasis.

Diacylglycerol-activated Hmunc13 serves as an effector of the GTPase Rab34

Hmunc13 is a cytosolic diacylglycerol (DAG)-binding protein, which is upregulated in renal cortical tubule and mesangial cells by hyperglycemia. In response to DAG activation, hmunc13 translocates to the Golgi. To investigate how this may relate to its function, we used a bacterial two-hybrid screen to look for hmunc13-interacting proteins. Full-length Rab34 was specifically isolated from a human kidney cDNA library. Co-expression of the two proteins confirmed Rab34 as a Golgi-associated protein, which was immunoprecipitated from cell lysates by hmunc13. Glutathione S-transferase fusion proteins of WT, Q111L (GTP bound), and T66N (GDP bound) mutants were created, and their GTP-binding activity verified by radioactive overlay assay. Binding of hmunc13 was observed with Q111L, barely detectable with T66N and enhanced with Rab34WT loaded with GTPgammaS compared with GDP loaded. Deletion of munc homolgy domain (MHD)-2, eliminated the hmunc13/Rab34 interaction. The Q111L mutant localized to the Golgi apparatus, but T66N was cytosolic. Localization of both mutants and Rab34WT was unchanged by DAG activation. The data suggest that DAG activation of hmunc13 causes it to be translocated to the Golgi, where it binds to GTP-bound Rab34 via MHD-2. Because Rab34 is known to regulate intracellular lysosome positioning, we propose that hmunc13 serves as an effector of Rab34, mediating lysosome-Golgi trafficking.

Apoptosis and lipoapoptosis in the fission yeast Schizosaccharomyces pombe

Yeasts being simple eukaryotes are established genetic systems that are often employed to solve important biological questions. Recently, it has become evident that certain cell death programs exist in these unicellular organisms. For example, it has been shown recently that strains of the fission yeast Schizosaccharomyces pombe deficient in triacylglycerol synthesis undergo cell death with prominent apoptotic markers. This minireview is intended to discuss key developments that have rendered fission yeast useful both as a tool and as a model for apoptosis and lipoapoptosis research. It is attempted to delineate a putative signaling pathway leading to the execution of lipoapoptosis in the fission yeast. Although in its infancy, apoptosis research in the fission yeast promises exciting breakthroughs in the near future.

Schizosaccharomyces pombe cells deficient in triacylglycerols synthesis undergo apoptosis upon entry into the stationary phase

Triacylglycerols (TAG) are important energy storage molecules for nearly all eukaryotic organisms. In this study, we found that two gene products (Plh1p and Dga1p) are responsible for the terminal step of TAG synthesis in the fission yeast Schizosaccharomyces pombe through two different mechanisms: Plh1p is a phospholipid diacylglycerol acyltransferase, whereas Dga1p is an acyl-CoA:diacylglycerol acyltransferase. Cells with both dga1+ and plh1+ deleted (DKO cells) lost viability upon entry into the stationary phase and demonstrated prominent apoptotic markers. Exponentially growing DKO cells also underwent dramatic apoptosis when briefly treated with diacylglycerols (DAGs) or free fatty acids. We provide strong evidence suggesting that DAG, not sphingolipids, mediates fatty acids-induced lipoapoptosis in yeast. Lastly, we show that generation of reactive oxygen species is essential to lipoapoptosis.

cPKC-dependent sequestration of membrane-recycling components in a subset of recycling endosomes

In addition to the classical role of protein kinase C (PKC) as a mediator of transmembrane signals initiated at the plasma membrane, there is also significant evidence to suggest that a more sustained PKC activity is necessary for a variety of long term cellular responses. To date, the subcellular localization of PKC during sustained activation has not been extensively studied. We report here that long term activation of PKC (1 h) leads to the selective translocation of classical PKC isoenzymes, alpha and betaII, to a juxtanuclear compartment. Juxtanuclear translocation of PKC required an intact C1 and C2 domain, and occurred in a microtubule-dependent manner. This juxtanuclear compartment was localized close to the Golgi complex but displayed no overlap with Golgi markers, and was resistant to dispersal with Golgi disrupting agents, brefeldin A and nocodazole. Further characterization revealed that PKCalpha and betaII translocated to a compartment that colocalized with the small GTPase, rab11, which is a marker for the subset of recycling endosomes concentrated around the microtubule-organizing center/centrosome. Analysis of the functional consequence of cPKC translocation on membrane recycling demonstrated a cPKC-dependent sequestration of transferrin, a marker of membrane recycling, in the cPKC compartment. These results identify a novel site for cPKC translocation and define a novel function for the sustained activation of PKCalpha and betaII in regulation of recycling components.

Diabetes-induced overexpression of endothelin-1 and endothelin receptors in the rat renal cortex is mediated via poly(ADP-ribose) polymerase activation

We hypothesize that poly (ADP-ribosyl)ation, that is, poly (ADP-ribose) polymerase (PARP)-dependent transfer of ADP-ribose moieties from NAD to nuclear proteins, plays a role in diabetic nephropathy. We evaluated whether PARP activation is present and whether two unrelated PARP inhibitors, 3-aminobenzamide (ABA) and 1,5-isoquinolinediol (ISO), counteract overexpression of endothelin-1 (ET-1) and ET receptors in the renal cortex in short-term diabetes. The studies were performed in control rats and streptozotocin-diabetic rats treated with/without ABA or ISO (30 and 3 mg x kg(-1) x day(-1), intraperitoneally, for 2 weeks after 2 weeks of diabetes). Poly (ADP-ribose) immunoreactivity was increased in tubuli, but not glomeruli, of diabetic rats and this increase was corrected by ISO, whereas ABA had a weaker effect. ET-1 concentration (ELISA) was increased in diabetic rats, and this elevation was blunted by ISO. ET-1, ET(A), and ET(B) mRNA (ribonuclease protection assay), but not ET-3 mRNA (RT/PCR), abundance was increased in diabetic rats, and three variables were, at least, partially corrected by ISO. ABA produced a trend towards normalization of ET-1 concentration and ET-1, ET(A), and ET(B) mRNA abundance, but the differences with untreated diabetic group were not significant. Poly(ADP-ribosyl)ation is involved in diabetes-induced renal overexpression of ET-1 and ET receptors. PARP inhibitors could provide a novel therapeutic approach for diabetic complications including nephropathy, and other diseases that involve the endothelin system.

The use of fluorescent phorbol esters in studies of protein kinase C-membrane interactions

The family of protein kinase C (PKC) isozymes belongs to a growing class of proteins that become active by associating with membranes containing anionic phospholipids, such as phosphatidylserine. Depending on the particular PKC isoform, this process is mediated by Ca(2+)-binding to a C2 domain and interaction of activators such as 1,2-diacyl-sn-glycerol or phorbol esters with tandem C1 domains. This cooperation between the C1 and C2 domains in inducing the association of PKC with lipid membranes provides the energy for a conformational change that consists of the release of a pseudosubstrate sequence from the active site, culminating in activation. Thus, the properties of the interactions of the C1 and C2 domains with membranes, both as isolated domains, and as modules in the full length PKC isoforms, have been the subject of intense scrutiny. Here, we review the findings of studies in which fluorescent phorbol esters have been utilized to probe the properties of the C1 domains of PKC with respect to the interaction with activators, the subsequent interaction with membranes, and the role of the activating conformational change that leads to activation.

Novel "nonkinase" phorbol ester receptors: the C1 domain connection

In recent years, there have been great advances in our understanding of the pharmacology and biology of the receptors for the phorbol ester tumor promoters and the second messenger diacylglycerol (DAG). The traditional view of protein kinase C (PKC) as the sole receptor for the phorbol esters has been challenged with the discovery of proteins unrelated to PKC that bind phorbol esters with high affinity, suggesting a high degree of complexity in the signaling pathways activated by DAG. These novel "nonkinase" phorbol ester receptors include chimaerins (a family of Rac GTPase activating proteins), RasGRPs (exchange factors for Ras/Rap1), and Munc13 isoforms (scaffolding proteins involved in exocytosis). In all cases, phorbol ester binding occurs at the single C1 domain present in these proteins and, as in PKC isozymes, ligand binding is a phospholipid-dependent event. Moreover, the novel phorbol ester receptors are also subject to subcellular redistribution or "translocation" by phorbol esters, leading to their association to different effector and/or regulatory molecules. Clearly, the use of phorbol esters as specific activators of PKC in cellular models is questionable. Alternative pharmacological and molecular approaches are therefore needed to dissect the involvement of each receptor class as a mediator of phorbol ester/DAG responses.

Chimaerins, novel non-protein kinase C phorbol ester receptors, associate with Tmp21-I (p23): evidence for a novel anchoring mechanism involving the chimaerin C1 domain

The regulation and function of chimaerins, a family of "non-protein kinase C" (PKC) phorbol ester/diacylglycerol receptors with Rac-GAP activity, is largely unknown. In a search for chimaerin-interacting proteins, we isolated Tmp21-I (p23), a protein localized at the perinuclear Golgi area. Remarkably, phorbol esters translocate beta2-chimaerin to the perinuclear region and promote its association with Tmp21-I in a PKC-independent manner. A deletional analysis revealed that the C1 domain in chimaerins is required for the interaction with Tmp21-I, thereby implying a novel function for this domain in protein-protein associations in addition to its role in lipid and phorbol ester binding. Our results support the emerging concept that multiple pathways transduce signaling by phorbol esters and revealed that, like PKC isozymes, chimaerins are subject to a positional regulation. In this setting, Tmp21-I serves as an anchoring protein that determines the intracellular localization of these novel phorbol ester receptors.

Gene expression and identification of gene therapy targets in diabetic nephropathy

A number of novel genes that are up-regulated in diabetic kidneys have been identified. Recently, transforming growth factor-beta (TGF-beta)--driven secreted proteins, i.e., connective tissue growth factor (CTGF) and gremlin, were identified. They are up-regulated in kidneys of diabetic animals and modulate the biology of mesangial cells. CTGF mediates TGF-beta--induced matrix overproduction by the mesangial cells. Gremlin is a putative antagonist of bone morphogenetic protein-2 that blocks mesangial cell proliferation. Thus, gremlin may modulate the biology of mesangium by stimulating mesangial cell proliferation and in turn production of matrix. In addition, transcriptionally regulated kinases, serum glucocorticoid-regulated kinase and munc-13 have been identified. The former stimulates renal tubular Na+ transport and is involved in hyperfiltraion of diabetic kidneys by a Na+ transport feedback mechanism. Munc-13 has been shown to induce apoptosis in hyperglycemic state via diacylglycerol-activated, PKC-independent signaling pathway. Another pathway relevant to diabetic nephropathy is polyol pathway, where glucose is reduced to sorbitol by aldose reductase. Recently, a renal-specific reductase of the aldo-keto reductase family was isolated. It is up-regulated in diabetic mice, and this could serve as a suitable target for gene therapy in renal complications of diabetes. Several mitochondrial genome-encoded genes, such as, cytochrome oxidase and NADH dehydrogenase, are up-regulated in diabetic kidneys. A novel nuclear-encoded mitochondrial gene, i.e., translocase inner mitochondrial membrane 44 (Tim44), is up-regulated in diabetic kidneys, and it may also serve as another target for molecular therapeutic intervention at the core storage energy sites, i.e., mitochondria. In this review, these novel differentially regulated genes that respond to hyperglycemic stress are described, and they may serve as possible targets for gene therapy in the treatment of diabetic nephropathy.

Synaptic exocytosis and nervous system development impaired in Caenorhabditis elegans unc-13 mutants

C. elegans mutants defective in unc-13 exhibited severe behavioral abnormalities including paralyzed locomotion and slow pharyngeal pumping and irregular defecation cycle. Consistent with the phenotypes, the mutants accumulated abnormally high levels of the neurotransmitter acetylcholine and were resistant to acetylcholinesterase inhibitors. The unc-13 gene was expressed in most, if not all, neurons when analyzed by using chimeric constructs consisting of the unc-13 promoter and green fluorescence protein or beta-galactosidase reporter gene. While Ca(2+)-regulated acetylcholine release is lacking, the mutants were still able to release acetylcholine in vivo and in vitro at similar levels to that mediated by the regulated mechanism. Double mutants defective in both unc-13 and other genes involved in synaptic transmission showed the Unc-13 phenotype, rather than other mutant phenotypes, in terms of locomotion as well as of acetylcholine accumulation. Furthermore, electron microscopic reconstruction of the mutant nervous system uncovered that a majority of neurons developed and connected as those in the wild type except for subtle abnormalities including inappropriate connections through gap junctions and morphological alterations of neurons. These results demonstrate that the unc-13 gene product plays an essential role at a late stage in Ca(2+)-regulated synaptic exocytosis. Neurotransmitters released through the Ca(2+)-regulated mechanism are required for, but do not play major roles in the nervous system development. The large amount of Ca(2+)-independent neurotransmitter release observed in the unc-13 mutants suggests that there may be a distinct mechanism from evoked or spontaneous release in neurotransmission.

Phorbol esters and related analogs regulate the subcellular localization of beta 2-chimaerin, a non-protein kinase C phorbol ester receptor

The novel phorbol ester receptor beta2-chimaerin is a Rac-GAP protein possessing a single copy of the C1 domain, a 50-amino acid motif initially identified in protein kinase C (PKC) isozymes that is involved in phorbol ester and diacylglycerol binding. We have previously shown that, like PKCs, beta2-chimaerin binds phorbol esters with high affinity in a phospholipid-dependent manner (Caloca, M. J., Fernandez, M. N., Lewin, N. E., Ching, D., Modali, R., Blumberg, P. M., and Kazanietz, M. G. (1997) J. Biol. Chem. 272, 26488-26496). In this paper we report that like PKC isozymes, beta2-chimaerin is translocated by phorbol esters from the cytosolic to particulate fraction. Phorbol esters also induce translocation of alpha1 (n)- and beta1-chimaerins, suggesting common regulatory mechanisms for all chimaerin isoforms. The subcellular redistribution of beta2-chimaerin by phorbol esters is entirely dependent on the C1 domain, as revealed by deletional analysis and site-directed mutagenesis. Interestingly, beta2-chimaerin translocates to the Golgi apparatus after phorbol ester treatment, as revealed by co-staining with the Golgi marker BODIPY-TR-ceramide. Structure relationship analysis of translocation using a series of PKC ligands revealed substantial differences between translocation of beta2-chimaerin and PKCalpha. Strikingly, the mezerein analog thymeleatoxin is not able to translocate beta2-chimaerin, although it very efficiently translocates PKCalpha. Phorbol esters also promote the association of beta2-chimaerin with Rac in cells. These data suggest that chimaerins can be positionally regulated by phorbol esters and that each phorbol ester receptor class has distinct pharmacological properties and targeting mechanisms. The identification of selective ligands for each phorbol ester receptor class represents an important step in dissecting their specific cellular functions.

Pharmacology of the receptors for the phorbol ester tumor promoters: multiple receptors with different biochemical properties

The phorbol ester tumor promoters and related analogs are widely used as potent activators of protein kinase C (PKC). The phorbol esters mimic the action of the lipid second messenger diacylglycerol (DAG). The aim of this commentary is to highlight a series of important and controversial concepts in the pharmacology and regulation of phorbol ester receptors. First, phorbol ester analogs have marked differences in their biological properties. This may be related to a differential regulation of PKC isozymes by distinct analogs. Moreover, it seems that marked differences exist in the ligand recognition properties of the C1 domains, the phorbol ester/DAG binding sites in PKC isozymes. Second, an emerging theme that we discuss here is that phorbol esters also target receptors unrelated to PKC isozymes, a concept that has been largely ignored. These novel receptors lacking kinase activity include chimaerins (a family of Rac-GTPase-activating proteins), RasGRP (a Ras exchange factor), and Unc-13/Munc-13 (a family of proteins involved in exocytosis). Unlike the classical and novel PKCs, these "non-kinase" phorbol ester receptors possess a single copy of the C1 domain. Interestingly, each receptor class has unique pharmacological properties and biochemical regulation. Lastly, it is well established that phorbol esters and related analogs can translocate each receptor to different intracellular compartments. The differential pharmacological properties of the phorbol ester receptors can be exploited to generate specific agonists and antagonists that will be helpful tools to dissect their cellular function.

Expression of multiple UNC-13 proteins in the Caenorhabditis elegans nervous system

The Caenorhabditis elegans UNC-13 protein and its mammalian homologues are important for normal neurotransmitter release. We have identified a set of transcripts from the unc-13 locus in C. elegans resulting from alternative splicing and apparent alternative promoters. These transcripts encode proteins that are identical in their C-terminal regions but that vary in their N-terminal regions. The most abundant protein form is localized to most or all synapses. We have analyzed the sequence alterations, immunostaining patterns, and behavioral phenotypes of 31 independent unc-13 alleles. Many of these mutations are transcript-specific; their phenotypes suggest that the different UNC-13 forms have different cellular functions. We have also isolated a deletion allele that is predicted to disrupt all UNC-13 protein products; animals homozygous for this null allele are able to complete embryogenesis and hatch, but they die as paralyzed first-stage larvae. Transgenic expression of the entire gene rescues the behavior of mutants fully; transgenic overexpression of one of the transcripts can partially compensate for the genetic loss of another. This finding suggests some degree of functional overlap of the different protein products.

Regulation of transmitter release by Unc-13 and its homologues

Neurotransmitters are released by Ca(2+)-triggered exocytotic fusion of synaptic vesicles. Before fusion, vesicles dock at a specialised presynaptic plasma membrane region, the active zone, where they are primed to a fusion competent state. The nature of this priming reaction has long been enigmatic. Recent evidence demonstrates that priming is an essential and rate-limiting step in secretion from neurons and neuroendocrine cells. Members of the Unc-13 protein family, which are highly conserved during evolution and act as novel targets of the diacylglycerol second-messenger pathway, have been identified to play an essential role in this process.

Eyes wide shut: protein kinase C isozymes are not the only receptors for the phorbol ester tumor promoters

In addition to the well-characterized interaction with classical and novel protein kinase C (PKC) isozymes, the phorbol ester tumor promoters bind to other receptors lacking kinase activity. Among these novel phorbol ester receptors, two families of proteins may play a role in the regulation of cell growth and malignant transformation: chimaerins and ras guanyl-releasing protein (ras-GRP). These proteins possess a single copy of the C1 domain that is involved in binding of phorbol esters and the lipid second messenger diacylglycerol. Four isoforms of chimaerins (alpha1-, alpha2-, beta1-, and beta2-chimaerins) have been isolated to-date, all of them possessing GTPase-activating protein activity for Rac, a small GTP-binding protein that controls actin cytoskeleton organization, cell-cycle progression, adhesion, and migration. Ras-GRP is a guanine nucleotide exchange factor for ras and promotes malignant transformation in fibroblasts in a phorbol ester-dependent manner. The C1 domain in Ras-GRP may, therefore, have a dominant role in Ras-GRP activation and is essential for phorbol ester-dependent activation of downstream effectors of ras, i.e., the mitogen-activated protein kinase cascade. Thus, a novel concept emerges in which phorbol esters may exert cellular responses through pathways not involving phorbol ester-responsive PKC isozymes. The discovery of "nonPKC" phorbol ester receptors adds an additional level of complexity to the understanding of phorbol ester effects and the molecular mechanisms of carcinogenesis.