[Preparation and application of rabbit polyclonal antibody against human glutamine fructose-6-phosphate amidotransferase 1(GFPT1)]

Objective To prepare rabbit polyclonal antibody specifically against human glutamine fructose-6-phosphate amidotransferase 1(GFPT1). Methods The protein sequences of GFPT1 and its highly homologous isozyme GFPT2 were compared. Two peptides for the specific sequence of GFPT1 were designed and synthesized. New Zealand rabbits were immunized by peptide coupled with Keyhole Limpet hemocyanin (KLH) as antigen. Antiserum was obtained after 3 booster immunizations. The titer of the antiserum against GFPT1 were detected by ELISA. The E.coli expression vectors of GFPT1 and GFPT2 were constructed, and the recombinant proteins of GFPT1 and GFPT2 were obtained by induced expression. GFPT1 and GFPT2 recombinant proteins were analyzed by Western blot to verify the specificity of the antiserum. Immuno-fluorescence cytochemical staining for GFPT1 expression in 786-O cells was verified as for whether the obtained antiserum could recognize the endogenously expressed GFPT1. Results Polyclonal antibody specifically recognizing GFPT1 was obtained and the titer of polyclonal antibody reached 1:1 458 000. Conclusion The experiment successfully prepared the specific rabbit polyclonal antibody against GFPT1.

Evidence coupling increased hexosamine biosynthesis pathway activity to membrane cholesterol toxicity and cortical filamentous actin derangement contributing to cellular insulin resistance

Hyperinsulinemia is known to promote the progression/worsening of insulin resistance. Evidence reveals a hidden cost of hyperinsulinemia on plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP(2))-regulated filamentous actin (F-actin) structure, components critical to the normal operation of the insulin-regulated glucose transport system. Here we delineated whether increased glucose flux through the hexosamine biosynthesis pathway (HBP) causes PIP(2)/F-actin dysregulation and subsequent insulin resistance. Increased glycosylation events were detected in 3T3-L1 adipocytes cultured under conditions closely resembling physiological hyperinsulinemia (5 nm insulin; 12 h) and in cells in which HBP activity was amplified by 2 mm glucosamine (GlcN). Both the physiological hyperinsulinemia and experimental GlcN challenge induced comparable losses of PIP(2) and F-actin. In addition to protecting against the insulin-induced membrane/cytoskeletal abnormality and insulin-resistant state, exogenous PIP(2) corrected the GlcN-induced insult on these parameters. Moreover, in accordance with HBP flux directly weakening PIP(2)/F-actin structure, pharmacological inhibition of the rate-limiting HBP enzyme [glutamine-fructose-6-phosphate amidotransferase (GFAT)] restored PIP(2)-regulated F-actin structure and insulin responsiveness. Conversely, overexpression of GFAT was associated with a loss of detectable PM PIP(2) and insulin sensitivity. Even less invasive challenges with glucose, in the absence of insulin, also led to PIP(2)/F-actin dysregulation. Mechanistically we found that increased HBP activity increased PM cholesterol, the removal of which normalized PIP(2)/F-actin levels. Accordingly, these data suggest that glucose transporter-4 functionality, dependent on PIP(2) and/or F-actin status, can be critically compromised by inappropriate HBP activity. Furthermore, these data are consistent with the PM cholesterol accrual/toxicity as a mechanistic basis of the HBP-induced defects in PIP(2)/F-actin structure and impaired glucose transporter-4 regulation.

Glucose induces increases in levels of the transcriptional repressor Id2 via the hexosamine pathway

Changes in glucose levels are known to directly alter gene expression. A number of previous studies have found that these effects are in part mediated by modulating the levels and the activity of transcription factors. We have investigated an alternative mechanism by which glucose might regulate gene expression by modulating levels of a transcriptional repressor. We have focused on Id2, which is a protein that indirectly regulates gene expression by sequestering certain transcription factors and preventing them from forming functional dimers. Id2 targets include the class A basic helix-loop-helix transcription factors and the sterol regulatory element-binding protein (SREBP)-1. We demonstrate that increases in glucose levels cause a rapid increase in levels of Id2 in J774.2 macrophages, and a number of lines of evidence indicate that this is via the hexosamine pathway because 1) the effect of glucose requires glutamine; 2) the effect of glucose is mimicked by low levels of glucosamine; 3) the effect of glucose is inhibited by azaserine, an inhibitor of glutamine:fructose-6-phosphate amidotransferase (GFAT); and 4) adenoviral mediated overexpression of GFAT increases levels of Id2. We go on to show that increases in Id2 can have functional effects on metabolic genes, because Id2 blocked the SREBP-1-induced induction of hormone-sensitive lipase (HSL) promoter activity, whereas Id2 alone does not modulate activity of the HSL promoter. In summary, these studies define a new mechanism by which glucose uses the hexosamine pathway to regulate gene expression by increasing levels of a transcriptional repressor.

NolR controls expression of the Rhizobium meliloti nodulation genes involved in the core Nod factor synthesis

The synthesis of Rhizobium meliloti Nod signal molecules, encoded by the nod gene products, is finely regulated. A negative control of plasmid-borne nod gene expression is provided by the NolR repressor encoded by the chromosomal nolR gene. NolR was previously shown to downregulate the expression of the activator nodD1 gene and the common nodABC operon by binding to an overlapping region of the two promoters adjacent to the n1 nod-box (Kondorosi et al., 1989). We demonstrate here that NolR also controls the expression of two additional genes, nodD2 and nodM, but does not directly regulate the expression of the host-specific nod genes located downstream of the n2, n3 and n5 nod-boxes. Thus, the nod genes are differentially regulated by NolR and only those providing common nodulation functions, by determining the synthesis of the core Nod factor structure, are subjected to this negative regulation. Furthermore, NolR has a strong negative effect on the production of Nod metabolites, the level of which may serve as a fine-tuning mechanism for optimal nodulation, specific to host-plant genotypes. In addition, it elicits preferential synthesis of Nod factors carrying unsaturated C16 fatty acids. Expression of nolR was high both in the free-living bacterium and in the bacteroid and it was downregulated by its own product and by the nod gene inducer luteolin.

Rhizobium nodM and nodN genes are common nod genes: nodM encodes functions for efficiency of nod signal production and bacteroid maturation

Earlier, we showed that Rhizobium meliloti nodM codes for glucosamine synthase and that nodM and nodN mutants produce strongly reduced root hair deformation activity and display delayed nodulation of Medicago sativa (Baev et al., Mol. Gen. Genet. 228:113-124, 1991). Here, we demonstrate that nodM and nodN genes from Rhizobium leguminosarum biovar viciae restore the root hair deformation activity of exudates of the corresponding R. meliloti mutant strains. Partial restoration of the nodulation phenotypes of these two strains was also observed. In nodulation assays, galactosamine and N-acetylglucosamine could substitute for glucosamine in the suppression of the R. meliloti nodM mutation, although N-acetylglucosamine was less efficient. We observed that in nodules induced by nodM mutants, the bacteroids did not show complete development or were deteriorated, resulting in decreased nitrogen fixation and, consequently, lower dry weights of the plants. This mutant phenotype could also be suppressed by exogenously supplied glucosamine, N-acetylglucosamine, and galactosamine and to a lesser extent by glucosamine-6-phosphate, indicating that the nodM mutant bacteroids are limited for glucosamine. In addition, by using derivatives of the wild type and a nodM mutant in which the nod genes are expressed at a high constitutive level, it was shown that the nodM mutant produces significantly fewer Nod factors than the wild-type strain but that their chemical structures are unchanged. However, the relative amounts of analogs of the cognate Nod signals were elevated, and this may explain the observed host range effects of the nodM mutation. Our data indicate that both the nodM and nodN genes of the two species have common functions and confirm that NodM is a glucosamine synthase with the biochemical role of providing sufficient amounts of the sugar moiety for the synthesis of the glucosamine oligosaccharide signal molecules.

The NodD protein does not bind to the promoters of inducible nodulation genes in extracts of bacteroids of Rhizobium leguminosarum biovar viciae

In a previous study, we showed that in bacteroids, transcription of the inducible nod genes does not occur and expression of nodD is decreased by 65% (H. R. M. Schlaman, B. Horvath, E. Vijgenboom, R.J.H. Okker, and B. J. J. Lugtenberg, J. Bacteriol. 173:4277-4287, 1991). In the present study, we show, using gel retardation, that in crude extracts of bacteroids of Rhizobium leguminosarum biovar (bv.) viciae, NodD protein does not bind to the nodF, nodM, and nodO box and that it binds only weakly to the nodA box. Binding of NodD from bacteroids to nod box DNA could be restored by mild proteinase K treatment, indicating that NodD is present in bacteroids in an altered form or complex which prevents its binding to nod box DNA. In addition, a novel nodA box DNA-protein complex was found which is specific for the nodA promoter region. This novel complex was formed neither with material from cultured bacterial cells nor with an extract from uninfected roots, and it did not contain NodD but another protein. These results are consistent with the hypothesis that the protein present in the novel retardation complex acts as a transcriptional repressor causing the decreased nodD expression in bacteroids. Such a repressor also explains the lack of nodABCIJ transcription despite the weak NodD binding to the nodA box.

A cultivar-specific interaction between Rhizobium leguminosarum bv. trifolii and subterranean clover is controlled by nodM, other bacterial cultivar specificity genes, and a single recessive host gene

Insertion mutagenesis identified two negatively acting gene loci which restrict the ability of Rhizobium leguminosarum bv. trifolii TA1 to infect the homologous host Trifolium subterraneum cv. Woogenellup. One locus was confirmed by DNA sequence analysis as the nodM gene, while the other locus, designated csn-1 (cultivar-specific nodulation), is not located on the symbiosis plasmid. The presence of these cultivar specificity loci could be suppressed by the introduction of the nodT gene from ANU843, a related R. leguminosarum bv. trifolii strain. Other nod genes, present in R. leguminosarum bv. viciae (including nodX) and R. meliloti, were capable of complementing R. leguminosarum bv. trifolii TA1 for nodulation on cultivar Woogenellup. Nodulation studies conducted with F2 seedlings from a cross between cultivar Geraldton and cultivar Woogenellup indicated that a single recessive gene, designated rwt1, is responsible for the Nod- association between strain TA1 and cultivar Woogenellup. Parallels can be drawn between this association and gene-for-gene systems common in interactions between plants and biotrophic pathogens.