Terpenoid balance in Aspergillus nidulans unveiled by heterologous squalene synthase expression

Filamentous fungi produce numerous uncharacterized natural products (NPs) that are often challenging to characterize because of cryptic expression in laboratory conditions. Previously, we have successfully isolated novel NPs by expressing fungal artificial chromosomes (FACs) from a variety of fungal species into Aspergillus nidulans. Here, we demonstrate a twist to FAC utility wherein heterologous expression of a Pseudogymnoascus destructans FAC in A. nidulans altered endogenous terpene biosynthetic pathways. In contrast to wild type, the FAC transformant produced increased levels of squalene and aspernidine type compounds, including three new nidulenes (1- 2, and 5), and lost nearly all ability to synthesize the major A. nidulans characteristic terpene, austinol. Deletion of a squalene synthase gene in the FAC restored wild-type chemical profiles. The altered squalene to farnesyl pyrophosphate ratio leading to synthesis of nidulenes and aspernidines at the expense of farnesyl pyrophosphate-derived austinols provides unexpected insight into routes of terpene synthesis in fungi.

The Promiscuity of Squalene Synthase-Like Enzyme: Dehydrosqualene Synthase, a Natural Squalene Hyperproducer?

Dehydrosqualene synthase (CrtM), as a squalene synthase-like enzyme from Staphylococcus aureus, can naturally utilize farnesyl diphosphate to produce dehydrosqualene (C30H48). However, no study has documented the natural production of squalene (C30H50) by CrtM. Here, based on an HPLC-Q-Orbitrap-MS/MS study, we report that the expression of crtM in vitro or in Bacillus subtilis 168 both results in the output of squalene, dehydrosqualene, and phytoene (C40H64). Notably, wild-type CrtM exhibits a significantly higher squalene yield compared to squalene synthase (SQS) from Bacillus megaterium with an approximately 2.4-fold increase. Moreover, the examination of presqualene diphosphate's stereostructures in both CrtM and SQS enzymes provides further understanding into the presence of multiple identified terpenoids. In summary, this study not only provides insights into the promiscuity demonstrated by squalene synthase-like enzymes but also highlights a new strategy of utilizing CrtM as a potential replacement for SQS in cell factories, thereby enhancing squalene production.

Intramembrane protease SPP defines a cholesterol-regulated abundance control of the mevalonate pathway enzyme squalene synthase

Intramembrane proteolysis regulates important processes such as signaling and transcriptional and posttranslational abundance control of proteins with key functions in metabolic pathways. This includes transcriptional control of mevalonate pathway genes, thereby ensuring balanced biosynthesis of cholesterol and other isoprenoids. Our work shows that, at high cholesterol levels, signal peptide peptidase (SPP) cleaves squalene synthase (SQS), an enzyme that defines the branching point for allocation of isoprenoids to the sterol and nonsterol arms of the mevalonate pathway. This intramembrane cleavage releases SQS from the membrane and targets it for proteasomal degradation. Regulation of this mechanism is achieved by the E3 ubiquitin ligase TRC8 that, in addition to ubiquitinating SQS in response to cholesterol levels, acts as an allosteric activator of SPP-catalyzed intramembrane cleavage of SQS. Cellular cholesterol levels increase in the absence of SPP activity. We infer from these results that, SPP-TRC8 mediated abundance control of SQS acts as a regulation step within the mevalonate pathway.

Transcription Factors ZjMYB39 and ZjMYB4 Regulate Farnesyl Diphosphate Synthase- and Squalene Synthase-Mediated Triterpenoid Biosynthesis in Jujube

Jujube (Ziziphus jujuba Mill.) is rich in valuable bioactive triterpenoids. However, the regulatory mechanism underlying triterpenoid biosynthesis in jujube remains poorly studied. Here, we characterized the triterpenoid content in wild jujube and cultivated jujube. The triterpenoid content was higher in wild jujube than in cultivated jujube, triterpenoids were most abundant in young leaves, buds, and later stages of development. The transcriptome analysis and correlation analysis showed that differentially expressed genes (DEGs) were enriched in the terpenoid synthesis pathways, and triterpenoids content was strongly correlated with farnesyl diphosphate synthase (ZjFPS), squalene synthase (ZjSQS), and transcription factors ZjMYB39 and ZjMYB4 expression. Gene overexpression and silencing analysis indicated that ZjFPS and ZjSQS were key genes in triterpenoid biosynthesis and transcription factors ZjMYB39 and ZjMYB4 regulated triterpenoid biosynthesis. Subcellular localization experiments showed that ZjFPS and ZjSQS were localized to the nucleus and endoplasmic reticulum and ZjMYB39 and ZjMYB4 were localized to the nucleus. Yeast one-hybrid, glucuronidase activity, and dual-luciferase activity assays suggested that ZjMYB39 and ZjMYB4 regulate triterpenoid biosynthesis by directly binding and activating the promoters of ZjFPS and ZjSQS. These findings provide insights into the underlying regulatory network of triterpenoids metabolism in jujube and lay theoretical and practical foundation for molecular breeding.

[Effect of light intensity on growth, accumulation of ginsenosides, and expression of related enzyme genes of Panax quinquefolius]

Appropriate light intensity is favorable for the photosynthesis, biomass accumulation, key enzyme activity, and secondary metabolite synthesis of medicinal plants. This study aims to explore the influence of light intensity on growth and quality of Panax quinquefolius. To be specific, sand culture experiment was carried out in a greenhouse under the light intensity of 40, 80, 120, and 160 μmol·m~(-2)·s~(-1), respectively. The growth indexes, photosynthetic characteristics, content of 6 ginsenosides of the 3-year-old P. quinquefolius were determined, and the expression of ginsenoside synthesis-related enzyme genes in leaves, main roots, and fibrous roots was determined. The results showed that the P. quinquefolius growing at 80 μmol·m~(-2)·s~(-1) light intensity had the most biomass and the highest net photosynthetic rate. The total biomass of P. quinquefolius treated with 120 μmol·m~(-2)·s~(-1) light intensity was slightly lower than that with 80 μmol·m~(-2)·s~(-1). The root-to-shoot ratio in the treatment with 120 μmol·m~(-2)·s~(-1) light intensity was up to 6.86, higher than those in other treatments(P<0.05),and the ginsenoside content in both aboveground and underground parts of P. quinquefolius in this treatment was the highest, which was possibly associated with the high expression of farnesylpyrophosphate synthase(FPS), squalene synthase(SQS), squalene epoxidase(SQE), oxidosqualene cyclase(OSC), dammarenediol-Ⅱ synthase(DS), and P450 genes in leaves and SQE and DS genes in main roots. In addition, light intensities of 120 and 160 μmol·m~(-2)·s~(-1) could promote PPD-type ginsenoside synthesis in leaves by triggering up-regulation of the expression of upstream ginsenoside synthesis genes. The decrease in underground biomass accumulation of the P. quinquefolius grown under weak light(40 μmol·m~(-2)·s~(-1)) and strong light(160 μmol·m~(-2)·s~(-1)) was possibly attributed to the low net photosynthetic rate, stomatal conductance, and transpiration rate in leaves. In the meantime, the low expression of SQS, SQE, OSC, and DS genes in the main roots might led to the decrease in ginsenoside content. However, there was no significant correlation between the ginsenoside content and the expression of synthesis-related genes in the fibrous roots of P. quinquefolius. Therefore, the light intensity of 80 and 120 μmol·m~(-2)·s~(-1) is beneficial to improving yield and quality of P. quinquefolius. The above findings contributed to a theoretical basis for reasonable shading in P. quinquefolius cultivation, which is of great significance for improving the yield and quality of P. quinquefolius through light regulation.

Mevalonate Kinase Deficiency and Squalene Synthase Inhibitor (TAK-475): The Balance to Extinguish the Inflammation

Mevalonate Kinase Deficiency (MKD) is a rare inborn disease belonging to the family of periodic fever syndromes. The MKD phenotype is characterized by systemic inflammation involving multiple organs, including the nervous system. Current anti-inflammatory approaches to MKD are only partially effective and do not act specifically on neural inflammation. According to the new emerging pharmacology trends, the repositioning of drugs from the indication for which they were originally intended to another one can make mechanistic-based medications easily available to treat rare diseases. According to this perspective, the squalene synthase inhibitor Lapaquistat (TAK-475), originally developed as a cholesterol-lowering drug, might find a new indication in MKD, by modulating the mevalonate cholesterol pathway, increasing the availability of anti-inflammatory isoprenoid intermediates. Using an in vitro model for MKD, we mimicked the blockade of the cholesterol pathway and evaluated the potential anti-inflammatory effect of Lapaquistat. The results obtained showed anti-inflammatory effects of Lapaquistat in association with a low blockade of the metabolic pathway, while this effect did not remain with a tighter blockade. On these bases, Lapaquistat could be configured as an effective treatment for MKD’s mild forms, in which the residual enzymatic activity is only reduced and not almost completely absent as in the severe forms.

Ubiquitin specific peptidase 32 acts as an oncogene in epithelial ovarian cancer by deubiquitylating farnesyl-diphosphate farnesyltransferase 1

Epithelial ovarian cancer (EOC) is the seventh most common cancer worldwide and the deadliest gynecological malignancy because of its aggressiveness and high recurrence rate. To discover new therapeutic targets for EOC, we combined public EOC microarray datasets with our previous in vivo shRNA screening dataset. The top-ranked gene ubiquitin specific peptidase 32 (USP32), coding a deubiquitinating enzyme, is a component of the ubiquitin proteasome system. Clinically, USP32 is expressed in primary ovarian cancer, especially in metastatic peritoneal tumors, and negatively impacts the survival outcome. USP32 regulates proliferative and epithelial mesenchymal transition capacities that are associated with EOC progression. Proteomic analysis identified farnesyl-diphosphate farnesyltransferase 1 (FDFT1) as a novel substrate of USP32 that is an enzyme in the mevalonate pathway, essentially associated with cell proliferation and stemness. USP32 and FDFT1 expression was higher in tumor spheres than in adherent cells. Inhibition of USP32, FDFT1, or mevalonate pathway considerably suppressed tumor sphere formation, which was restored by adding squalene, a downstream product of FDFT1. These findings suggested that USP32-FDFT1 axis contributes to EOC progression, and could be novel therapeutic targets for EOC treatment.

Roles of Farnesyl-Diphosphate Farnesyltransferase 1 in Tumour and Tumour Microenvironments

Farnesyl-diphosphate farnesyltransferase 1 (FDFT1, squalene synthase), a membrane-associated enzyme, synthesizes squalene via condensation of two molecules of farnesyl pyrophosphate. Accumulating evidence has noted that FDFT1 plays a critical role in cancer, particularly in metabolic reprogramming, cell proliferation, and invasion. Based on these advances in our knowledge, FDFT1 could be a potential target for cancer treatment. This review focuses on the contribution of FDFT1 to the hallmarks of cancer, and further, we discuss the applicability of FDFT1 as a cancer prognostic marker and target for anticancer therapy.

Origin and Evolution of Polycyclic Triterpene Synthesis

Polycyclic triterpenes are members of the terpene family produced by the cyclization of squalene. The most representative polycyclic triterpenes are hopanoids and sterols, the former are mostly found in bacteria, whereas the latter are largely limited to eukaryotes, albeit with a growing number of bacterial exceptions. Given their important role and omnipresence in most eukaryotes, contrasting with their scant representation in bacteria, sterol biosynthesis was long thought to be a eukaryotic innovation. Thus, their presence in some bacteria was deemed to be the result of lateral gene transfer from eukaryotes. Elucidating the origin and evolution of the polycyclic triterpene synthetic pathways is important to understand the role of these compounds in eukaryogenesis and their geobiological value as biomarkers in fossil records. Here, we have revisited the phylogenies of the main enzymes involved in triterpene synthesis, performing gene neighborhood analysis and phylogenetic profiling. Squalene can be biosynthesized by two different pathways containing the HpnCDE or Sqs proteins. Our results suggest that the HpnCDE enzymes are derived from carotenoid biosynthesis ones and that they assembled in an ancestral squalene pathway in bacteria, while remaining metabolically versatile. Conversely, the Sqs enzyme is prone to be involved in lateral gene transfer, and its emergence is possibly related to the specialization of squalene biosynthesis. The biosynthesis of hopanoids seems to be ancestral in the Bacteria domain. Moreover, no triterpene cyclases are found in Archaea, invoking a potential scenario in which eukaryotic genes for sterol biosynthesis assembled from ancestral bacterial contributions in early eukaryotic lineages.

Production of Squalene in Bacillus subtilis by Squalene Synthase Screening and Metabolic Engineering

Squalene synthase (SQS) catalyzes the conversion of two farnesyl pyrophosphates to squalene, an important intermediate in between isoprene and valuable triterpenoids. In this study, we have constructed a novel biosynthesis pathway for squalene in Bacillus subtilis and performed metabolic engineering aiming at facilitating further exploitation and production of squalene-derived triterpenoids. Therefore, systematic studies and analysis were performed including selection of multiple SQS candidates from various organisms, comparison of expression vectors, optimization of cultivation temperatures, and examination of rate-limiting factors within the synthetic pathway. We were, for the first time, able to obtain squalene synthesis in B. subtilis. Furthermore, we achieved a 29-fold increase of squalene yield (0.26-7.5 mg/L) by expressing SQS from Bacillus megaterium and eliminating bottlenecks within the upstream methylerythritol-phosphate pathway. Moreover, our findings showed that also ispA could positively affect the production of squalene.

Structure and Location Studies on Key Enzymes in Saponins Biosynthesis of Panax notoginseng

Panax notoginseng is one of the most widely used traditional herbs for the treatment of various diseases, in which saponins were the main active components. At present, the research of P. notoginseng mainly focused on the discovery of new compounds and pharmacology. However, there were few studies on the molecular mechanism of the synthesis of secondary metabolites of P. notoginseng. In our study, four coding sequences (CDS) encoding the key enzymes involved in saponin biosynthesis were cloned, namely farnesyl diphosphate synthase (FPS), squalene synthase (SS), squalene epoxidase (SE), and dammarenediol-II synthase (DS), which contained open reading frame (ORF) of 1029 bp, 1248 bp, 1614 bp, and 2310 bp, and coded 342, 415, 537, and 769 amino acids, respectively. At the same time, their domains, secondary structures, three-dimensional structures, and phylogenetics trees were analyzed by kinds of bioinformatics tools. Their phylogenetics relationships were also analyzed. In addition, GFP (Green fluorescent protein) fusion genes were constructed by the plasmid transformation system to determine the subcellular localization. The results of subcellular localization showed that FPS, SE, and DS were mainly located in cytomembrane and its surrounding, while SS was located both in cytoplasm and cytomembrane. Our findings provided data demonstrating the expression patterns of genes involved in saponin biosynthesis and would facilitate efforts to further elucidate the biosynthesis of the bioactive components in P. notoginseng.

Isolation and functional analysis of squalene synthase gene in tea plant Camellia sinensis

Tea contains high quantities and diverse types of triterpenoids, particularly in the form of saponins. However, little is yet known about the molecular basis of triterpenoid biosynthesis in tea plant. Here we report on isolation and functional analysis of squalene synthase (SQS) gene from tea plant (Camellia sinensis var. sinensis), which controls the biosynthesis of triterpenoids precursor. First, a full-length cDNA of squalene synthase, designated CsSQS, was isolated from tea plant. The protein is highly homologous to SQSs from other plants. Using CsSQS-reporter assays, CsSQS was demonstrated to be endoplasmic reticulum membrane-bound. The coding region of CsSQS excluding transmemberane sequence was expressed in Escherichia coli. Recombinant CsSQS catalyzed the formation of squalene using farnesyl-pyrophosphate (FPP) as substrate with NADPH and Mg2+. In tea plant leaves, CsSQS expression was significantly induced by both herbivore and mechanical damages. Consistent with the stronger induction of CsSQS expression by mechanical damage than herbivory, tea plants injured mechanically released squalene as a volatile compound, which however was not detected from herbivore-damaged tea plants. Furthermore, it was found that the flowers of another tea plant cultivar Camellia sinensis var. assamica contain higher concentrations of squalene than the cultivar sinensis, indicating variations among tea plant varieties. With the identification and molecular characterization of squalene synthase in tea plant, next, we can ask the questions about the roles of squalene as a volatile product as well as a precursor for triterpenoids, which may promote product development from diverse tea materials and mining of excellent tea germplasm resources.

Engineering linear, branched-chain triterpene metabolism in monocots

Triterpenes are thirty-carbon compounds derived from the universal five-carbon prenyl precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Normally, triterpenes are synthesized via the mevalonate (MVA) pathway operating in the cytoplasm of eukaryotes where DMAPP is condensed with two IPPs to yield farnesyl diphosphate (FPP), catalyzed by FPP synthase (FPS). Squalene synthase (SQS) condenses two molecules of FPP to generate the symmetrical product squalene, the first committed precursor to sterols and most other triterpenes. In the green algae Botryococcus braunii, two FPP molecules can also be condensed in an asymmetric manner yielding the more highly branched triterpene, botryococcene. Botryococcene is an attractive molecule because of its potential as a biofuel and petrochemical feedstock. Because B. braunii, the only native host for botryococcene biosynthesis, is difficult to grow, there have been efforts to move botryococcene biosynthesis into organisms more amenable to large-scale production. Here, we report the genetic engineering of the model monocot, Brachypodium distachyon, for botryococcene biosynthesis and accumulation. A subcellular targeting strategy was used, directing the enzymes (botryococcene synthase [BS] and FPS) to either the cytosol or the plastid. High titres of botryococcene (>1 mg/g FW in T₀ mature plants) were obtained using the cytosolic-targeting strategy. Plastid-targeted BS + FPS lines accumulated botryococcene (albeit in lesser amounts than the cytosolic BS + FPS lines), but they showed a detrimental phenotype dependent on plastid-targeted FPS, and could not proliferate and survive to set seed under phototrophic conditions. These results highlight intriguing differences in isoprenoid metabolism between dicots and monocots.

Virus-induced gene silencing of the two squalene synthase isoforms of apple tree (Malus × domestica L.) negatively impacts phytosterol biosynthesis, plastid pigmentation and leaf growth

The use of a VIGS approach to silence the newly characterized apple tree SQS isoforms points out the biological function of phytosterols in plastid pigmentation and leaf development. Triterpenoids are beneficial health compounds highly accumulated in apple; however, their metabolic regulation is poorly understood. Squalene synthase (SQS) is a key branch point enzyme involved in both phytosterol and triterpene biosynthesis. In this study, two SQS isoforms were identified in apple tree genome. Both isoforms are located at the endoplasmic reticulum surface and were demonstrated to be functional SQS enzymes using an in vitro activity assay. MdSQS1 and MdSQS2 display specificities in their expression profiles with respect to plant organs and environmental constraints. This indicates a possible preferential involvement of each isoform in phytosterol and/or triterpene metabolic pathways as further argued using RNAseq meta-transcriptomic analyses. Finally, a virus-induced gene silencing (VIGS) approach was used to silence MdSQS1 and MdSQS2. The concomitant down-regulation of both MdSQS isoforms strongly affected phytosterol synthesis without alteration in triterpene accumulation, since triterpene-specific oxidosqualene synthases were found to be up-regulated to compensate metabolic flux reduction. Phytosterol deficiencies in silenced plants clearly disturbed chloroplast pigmentation and led to abnormal development impacting leaf division rather than elongation or differentiation. In conclusion, beyond the characterization of two SQS isoforms in apple tree, this work brings clues for a specific involvement of each isoform in phytosterol and triterpene pathways and emphasizes the biological function of phytosterols in development and chloroplast integrity. Our report also opens the door to metabolism studies in Malus domestica using the apple latent spherical virus-based VIGS method.

Molecular and biochemical characterization of squalene synthase from Siraitia grosvenorii

OBJECTIVES

To clone and characterize the squalene synthase from Siraitia grosvenorii (SgSQS).

RESULTS

The gene encoding SgSQS was cloned. SgSQS has 417 amino acid residues with an pI of 7.3. There are 32 phosphorylation sites in its sequence: S⁴⁸ as well as S¹⁹⁶ play important roles in regulation of enzyme activity. The enzyme is a monomeric protein with a cave-like active center formed by α helixes and has two transmembrane domains at its C-terminus. SgSQS mRNA expression in stem and root were about twice as much as that in leaf and peel. Full-length SgSQS with measurable catalytic activity was expressed in Escherichia coli. SgSQS activity was optimal at 37 °C and pH 7.5 respectively.

CONCLUSION

SgSQS gene was cloned, and the molecular structure and biochemical function of SgSQS were characterized.

Microbial modulation of bacoside A biosynthetic pathway and systemic defense mechanism in Bacopa monnieri under Meloidogyne incognita stress

Plant-associated beneficial microbes have been explored to fulfill the imperative function for plant health. However, their impact on the host secondary metabolite production and nematode disease management remains elusive. Our present work has shown that chitinolytic microbes viz., Chitiniphilus sp. MTN22 and Streptomyces sp. MTN14 singly as well as in combination modulated the biosynthetic pathway of bacoside A and systemic defense mechanism against Meloidogyne incognita in Bacopa monnieri. Interestingly, expression of bacoside biosynthetic pathway genes (3-Hydroxy-3-methylglutaryl coenzyme A reductase, mevalonate diphosphate decarboxylase, and squalene synthase) were upregulated in plants treated with the microbial combination in the presence as well as in absence of M. incognita stress. These microbes not only augmented bacoside A production (1.5 fold) but also strengthened host resistance via enhancement in chlorophyll a, defense enzymes and phenolic compounds like gallic acid, syringic acid, ferulic acid and cinnamic acid. Furthermore, elevated lignification and callose deposition in the microbial combination treated plants corroborate well with the above findings. Overall, the results provide novel insights into the underlying mechanisms of priming by beneficial microbes and underscore their capacity to trigger bacoside A production in B. monnieri under biotic stress.

Lactobacillus acidophilus K301 Inhibits Atherogenesis via Induction of 24 (S), 25-Epoxycholesterol-Mediated ABCA1 and ABCG1 Production and Cholesterol Efflux in Macrophages

Lactobacillus acidophilus species are well-known probiotics with the beneficial activity of regulating cholesterol levels. In this study, we showed that L. acidophilus K301 reduced the level of cholesterol through reverse transport in macrophages. L. acidophilus K301 upregulated the mRNA and protein levels of genes such as ATP-binding cassette A1 (ABCA1) and ATP-binding cassette G1 (ABCG1) under the control of liver X receptor (LXR), resulting in increased apoA-I-dependent cholesterol efflux in phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 cells. L. acidophilus K301 induced both ABCA1 and ABCG1 through the endogenous LXR agonist 24(S), 25-epoxcycholesterol, which is synthesized by intracellular cholesterol synthetic pathways. In vivo studies using L. acidophilus K301-treated ApoE^-/- mice showed reduced accumulation of lipoproteins in the arterial lumen. The inhibitory effects of L. acidophilus K301 on accumulation of lipoprotein in atherosclerotic plaques were mediated by the induction of squalene reductase (SQLE) and oxidosqualene cyclase (OSC) and resulted in ABCA1-mediated cholesterol efflux. Taken together, our findings revealed that Lactobacillus acidophilus K301 regulates the expression of genes related to cholesterol reverse transport via the induction of endogenous LXR agonist, suggesting the therapeutic potential of Lactobacillus acidophilus K301 as an anti-atherosclerotic agent.

[Cloning and expression analysis of squalene synthase gene in Tripterygium wilfordii]

In this paper, we cloned the full-length cDNA of TwSQS from Tripterygium wilfordii suspension cells(Gen Bank: KR401220) and performed the bioinformation and m RNA expression analysis. The expression after methyl jasmonate(MJ) treatment of the gene was detected by RT-PCR. The full-length cDNA of TwSQS was 1 800 bp containing a 1 242 bp open reading frame(ORF) encoding a polypeptide of 413 amino acids. The theoretical isoelectric point(p I) was 7.94 and the calculate molecular weight was about 47.20 k D. The relative expression level of TwSQS was deduced by MJ and reached the highest at 4 h after the treatment. The gene information we got in this study enriched the biosynthesis pathway of triterpenoids in Tripterygium wilfordii and laid foundation for further studies.

Branch Pathway Blocking in Artemisia annua is a Useful Method for Obtaining High Yield Artemisinin

There are many biosynthetic pathways competing for the metabolic flux with the artemisinin biosynthetic pathway in Artemisia annua L. To study the relationship between genes encoding enzymes at branching points and the artemisinin biosynthetic pathway, β-caryophyllene, β-farnesene and squalene were sprayed on young seedlings of A. annua. Transient expression assays indicated that the transcription levels of β-caryophyllene synthase (CPS), β-farnesene synthase (BFS) and squalene synthase (SQS) were inhibited by β-caryophyllene, β-farnesene and squalene, respectively, while expression of some artemisinin biosynthetic pathway genes increased. Thus, inhibition of these genes encoding enzymes at branching points may be helpful to improve the artemisinin content. For further study, the expression levels of four branch pathway genes CPS, BFS, germacrene A synthase (GAS) and SQS were down-regulated by the antisense method in A. annua. In anti-CPS transgenic plants, mRNA levels of BFS and ADS were increased, and the contents of β-farnesene, artemisinin and dihydroartemisinic acid (DHAA) were increased by 212, 77 and 132%, respectively. The expression levels of CPS, SQS, GAS, amorpha-4,11-diene synthase (ADS), amorphadiene 12-hydroxylase (CYP71AV1) and aldehyde dehydrogenase 1 (ALDH1) were increased in anti-BFS transgenic plants and, at the same time, the contents of artemisinin and DHAA were increased by 77% and 54%, respectively, and the content of squalene was increased by 235%. In anti-GAS transgenic plants, mRNA levels of CPS, BFS, ADS and ALDH1 were increased. The contents of artemisinin and DHAA were enhanced by 103% and 130%, respectively. In anti-SQS transgenic plants, the transcription levels of BFS, GAS, CPS, ADS, CYP71AV1 and ALDH1 were all increased. Contents of artemisinin and DHAA were enhanced by 71% and 223%, respectively, while β-farnesene was raised to 123%. The mRNA level of artemisinic aldehyde Δ11(13) reductase (DBR2) had changed little in almost all transgenic plants.

Engineering Triterpene and Methylated Triterpene Production in Plants Provides Biochemical and Physiological Insights into Terpene Metabolism

Linear, branch-chained triterpenes, including squalene (C30), botryococcene (C30), and their methylated derivatives (C31-C37), generated by the green alga Botryococcus braunii race B have received significant attention because of their utility as chemical and biofuel feedstocks. However, the slow growth habit of B. braunii makes it impractical as a production system. In this study, we evaluated the potential of generating high levels of botryococcene in tobacco (Nicotiana tabacum) plants by diverting carbon flux from the cytosolic mevalonate pathway or the plastidic methylerythritol phosphate pathway by the targeted overexpression of an avian farnesyl diphosphate synthase along with two versions of botryococcene synthases. Up to 544 µg g(-1) fresh weight of botryococcene was achieved when this metabolism was directed to the chloroplasts, which is approximately 90 times greater than that accumulating in plants engineered for cytosolic production. To test if methylated triterpenes could be produced in tobacco, we also engineered triterpene methyltransferases (TMTs) from B. braunii into wild-type plants and transgenic lines selected for high-level triterpene accumulation. Up to 91% of the total triterpene contents could be converted to methylated forms (C31 and C32) by cotargeting the TMTs and triterpene biosynthesis to the chloroplasts, whereas only 4% to 14% of total triterpenes were methylated when this metabolism was directed to the cytoplasm. When the TMTs were overexpressed in the cytoplasm of wild-type plants, up to 72% of the total squalene was methylated, and total triterpene (C30+C31+C32) content was elevated 7-fold. Altogether, these results point to innate mechanisms controlling metabolite fluxes, including a homeostatic role for squalene.

Volatile squalene from a nonseed plant Selaginella moellendorffii: Emission and biosynthesis

The triterpene squalene is a key metabolic intermediate for sterols, hopanoids and various other triterpenoids. The biosynthesis of squalene is catalyzed by squalene synthase (SQS), which converts two molecules of farnesyl diphosphate to squalene. In this study, a lycophyte Selaginella moellendorffii was found to emit squalene as a volatile compound under a number of conditions that mimic biotic stresses. Searching the genome sequence of S. moellendorffii led to the identification of a putative squalene synthase gene. It was designated as SmSQS. SmSQS is homologous to known squalene synthases from other plants and animals at both the amino acid level and structural level. Recombinant SmSQS expressed in Escherichia coli catalyzed the formation of squalene using farnesyl diphosphate as substrate. The expression of SmSQS was significantly induced by the same set of stress factors that induced the emission of volatile squalene from S. moellendorffii plants. Taken together, these results support that SmSQS is responsible for the biosynthesis of volatile squalene and volatile squalene may have a role in the defense of S. moellendorffii plants against biotic stresses.

Triterpenoid profiling and functional characterization of the initial genes involved in isoprenoid biosynthesis in neem (Azadirachta indica)

BACKGROUND

Neem tree (Azadirachta indica) is one of the richest sources of skeletally diverse triterpenoids and they are well-known for their broad-spectrum pharmacological and insecticidal properties. However, the abundance of Neem triterpenoids varies among the tissues. Here, we delineate quantitative profiling of fifteen major triterpenoids across various tissues including developmental stages of kernel and pericarp, flower, leaf, stem and bark using UPLC-ESI(+)-HRMS based profiling. Transcriptome analysis was used to identify the initial genes involved in isoprenoid biosynthesis. Based on transcriptome analysis, two short-chain prenyltransferases and squalene synthase (AiSQS) were cloned and functionally characterized.

RESULTS

Quantitative profiling revealed differential abundance of both total and individual triterpenoid content across various tissues. RNA from tissues with high triterpenoid content (fruit, flower and leaf) were pooled to generate 79.08 million paired-end reads using Illumina GA ΙΙ platform. 41,140 transcripts were generated by d e novo assembly. Transcriptome annotation led to the identification of the putative genes involved in isoprenoid biosynthesis. Two short-chain prenyltransferases, geranyl diphosphate synthase (AiGDS) and farnesyl diphosphate synthase (AiFDS) and squalene synthase (AiSQS) were cloned and functionally characterized using transcriptome data. RT-PCR studies indicated five-fold and ten-fold higher relative expression level of AiSQS in fruits as compared to leaves and flowers, respectively.

CONCLUSIONS

Triterpenoid profiling indicated that there is tissue specific variation in their abundance. The mature seed kernel and initial stages of pericarp were found to contain the highest amount of limonoids. Furthermore, a wide diversity of triterpenoids, especially C-seco triterpenoids were observed in kernel as compared to the other tissues. Pericarp, flower and leaf contained mainly ring-intact triterpenoids. The initial genes such as AiGDS, AiFDS and AiSQS involved in the isoprenoids biosynthesis have been functionally characterized. The expression levels of AiFDS and AiSQS were found to be in correlation with the total triterpenoid content in individual tissues.

[Cloning and prokaryotic expression analysis of squalene synthase 2 (SQS2) from Salvia miltiorrhiza f. alba]

According to the designed specific primers of gene fragment based on the Salvia miltiorrhiza transcriptome data, a full-length cDNA sequence of SQS2 from S. miltiorrhiza f. alba was cloned by the method of reverse transcription polymerase chain reaction (RT-PCR). The SmSQS2 cDNA sequence was obtained, this sequence is named SmSQS2 and its GenBank registration number is KM244731. The full length of SmSQS2 cDNA was 1245 bp, encoding 414 amino acids including 5'UTR 115 bp and 3'UTR 237 bp. Sequence alignment and phylogenetic analysis demonstrated that SmSQS2 had relative close relationship to the SQS2 of S. miltiorrhiza. The induction of E. coli [pET28-SQS2] in different temperature, induction time, IPTG concentrations and density of inducing host bacterium (A600) were performed, Shaking the culture at 30 degrees C until the A600 is approximately 0.6 and add IPTG to final concentration of 0.2 mmol x L(-1), and then the optimal expression of SmSQS2 recombinant protein were accumulated after the induction time of 20 h. The research provided important base for the study of sterol and terpene biosynthesis of SQS2 in S. miltiorrhiza f. alba.

Accumulation of squalene in a microalga Chlamydomonas reinhardtii by genetic modification of squalene synthase and squalene epoxidase genes

Several microalgae accumulate high levels of squalene, and as such provide a potentially valuable source of this useful compound. However, the molecular mechanism of squalene biosynthesis in microalgae is still largely unknown. We obtained the sequences of two enzymes involved in squalene synthesis and metabolism, squalene synthase (CrSQS) and squalene epoxidase (CrSQE), from the model green alga Chlamydomonas reinhardtii. CrSQS was functionally characterized by expression in Escherichia coli and CrSQE by complementation of a budding yeast erg1 mutant. Transient expression of CrSQS and CrSQE fused with fluorescent proteins in onion epidermal tissue suggested that both proteins were co-localized in the endoplasmic reticulum. CrSQS-overexpression increased the rate of conversion of 14C-labeled farnesylpyrophosphate into squalene but did not lead to over-accumulation of squalene. Addition of terbinafine caused the accumulation of squalene and suppression of cell survival. On the other hand, in CrSQE-knockdown lines, the expression level of CrSQE was reduced by 59-76% of that in wild-type cells, and significant levels of squalene (0.9-1.1 μg mg-1 cell dry weight) accumulated without any growth inhibition. In co-transformation lines with CrSQS-overexpression and CrSQE-knockdown, the level of squalene was not increased significantly compared with that in solitary CrSQE-knockdown lines. These results indicated that partial knockdown of CrSQE is an effective strategy to increase squalene production in C. reinhardtii cells.

Developmentally regulated sesquiterpene production confers resistance to Colletotrichum gloeosporioides in ripe pepper fruits

Sesquiterpenoid capsidiol, exhibiting antifungal activity against pathogenic fungus, is accumulated in infected ripe pepper fruits. In this study, we found a negative relation between the capsidiol level and lesion size in fruits infected with Colletotrichum gloeosporioides, depending on the stage of ripening. To understand the developmental regulation of capsidiol biosynthesis, fungal-induced gene expressions in the isoprenoid biosynthetic pathways were examined in unripe and ripe pepper fruits. The sterol biosynthetic pathway was almost shut down in healthy ripe fruits, showing very low expression of hydroxymethyl glutaryl CoA reductase (HMGR) and squalene synthase (SS) genes. In contrast, genes in the carotenoid pathway were highly expressed in ripe fruits. In the sesquiterpene pathway, 5-epi-aristolochene synthase (EAS), belonging to a sesquiterpene cyclase (STC) family, was significantly induced in the ripe fruits upon fungal infection. Immunoblot and enzyme activity analyses showed that the STCs were induced both in the infected unripe and ripe fruits, while capsidiol was synthesized discriminatively in the ripe fruits, implying diverse enzymatic specificity of multiple STCs. Thereby, to divert sterol biosynthesis into sesquiterpene production, infected fruits were pretreated with an SS inhibitor, zaragozic acid (ZA), resulting in increased levels of capsidiol by more than 2-fold in the ripe fruits, with concurrent reduction of phytosterols. Taken together, the present results suggest that the enhanced expression and activity of EAS in the ripe fruits play an important role in capsidiol production, contributing to the incompatibility between the anthracnose fungus and the ripe pepper fruits.

Enhancing the accumulation of beta-amyrin in Saccharomyces cerevisiae by co-expression of Glycyrrhiza uralensis squalene synthase 1 and beta-amyrin synthase genes

Glycyrrhiza uralensis Fisch. ex DC is widely used in traditional Chinese medicine (TCM). Among its various active components, glycyrrhizic acid is believed to be the marker component. Squalene synthase (SQS) and beta-amyrin synthase (beta-AS) are key enzymes in the biosynthetic pathway of glycyrrhizic acid in G uralensis. To reveal the effects of co-expression of SQS1 and beta-AS genes on this pathway, 7 yeast expression vectors harboring different SQS1 variants and beta-AS were constructed and expressed in Saccharomyces cerevisiae as fusion proteins. TLC and GC-MS results showed that co-expression of SQS1 and beta-AS enhanced the accumulation of beta-amyrin. The effects of SQS12 were more obvious than the other two SQS1 variants. This study is significant for further investigations concerned with exploring the biosynthesis of glycyrrhizic acid in vitro and strengthening the efficacy of G. uralensis by means of increasing the content of glycyrrhizic acid.

Squalene synthase as a target for Chagas disease therapeutics

Trypanosomatid parasites are the causative agents of many neglected tropical diseases and there is currently considerable interest in targeting endogenous sterol biosynthesis in these organisms as a route to the development of novel anti-infective drugs. Here, we report the first x-ray crystallographic structures of the enzyme squalene synthase (SQS) from a trypanosomatid parasite, Trypanosoma cruzi, the causative agent of Chagas disease. We obtained five structures of T. cruzi SQS and eight structures of human SQS with four classes of inhibitors: the substrate-analog S-thiolo-farnesyl diphosphate, the quinuclidines E5700 and ER119884, several lipophilic bisphosphonates, and the thiocyanate WC-9, with the structures of the two very potent quinuclidines suggesting strategies for selective inhibitor development. We also show that the lipophilic bisphosphonates have low nM activity against T. cruzi and inhibit endogenous sterol biosynthesis and that E5700 acts synergistically with the azole drug, posaconazole. The determination of the structures of trypanosomatid and human SQS enzymes with a diverse set of inhibitors active in cells provides insights into SQS inhibition, of interest in the context of the development of drugs against Chagas disease.

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi and affects eight million individuals, primarily in Latin America. Currently there is no cure for chronic T. cruzi infections. Unlike humans, this parasite use a variety of sterols (e.g. ergosterol, 24-ethyl-cholesta-5,7,22-trien-3 beta ol, and its 22-dihydro analogs), rather than cholesterol in their cell membranes, so inhibiting endogenous sterol biosynthesis is an important therapeutic target. Here, we report the first structure of the parasite's squalene synthase, which catalyzes the first committed step in sterol biosynthesis, as well as the structures of a broad range of squalene synthase inhibitors active against the clinically relevant intracellular stages, opening the way to new approaches to treating this neglected tropical disease.

Molecular characterization of Glycine max squalene synthase genes in seed phytosterol biosynthesis

The reaction catalyzed by squalene synthase (EC.2.5.1.21) that converts two molecules of farnesyl pyrophosphate to squalene represents a crucial branch point of the isoprenoid pathway in diverting carbon flux towards the biosynthesis of sterols. In the present study two soybean squalene synthase genes, GmSQS1 and GmSQS2, were identified in the soybean genome and functionally characterized for their roles in sterol biosynthesis. Both genes encode a deduced protein of 413 amino acids. Complementation assays showed that the two genes were able to convert yeast sterol auxotrophy erg9 mutant to sterol prototrophy. Expression of GmSQS1 and GmSQS2 was ubiquitous in roots, stem, leaves, flower and young seeds of soybean, however GmSQS1 transcript was preferential in roots while GmSQS2 transcript was more in leaves. Their expression was lower in response to dehydration treatments suggesting they might be negative regulators of water stress adaptation. Transgenic Arabidopsis plants overexpressing GmSQS1 driven by either constitutive or seed-specific promoters showed increases in the major end product sterols: campesterol, sitosterol and stigmasterol, which resulted in up to 50% increase in total sterol content in the seeds. The increase in the end product sterols by GmSQS1 overexpression was at the level achievable by previously reported overexpression of individual or combination of other key enzymes in the sterol pathway. Together the data demonstrate that soybean SQS genes play an important role in diverting carbon flux to the biosynthesis of the end product sterols in the seeds.

[The construction of over-expression vector for Panax notoginseng SS gene and its transformation]

PNS (Panax notoginseng saponins) is the main medical bioactive component in Panax notoginseng. The medical value of PNS cannot be extended because of its low production. With the deep study of saponins biosynthetic pathway, the control of PNS biosynthesis through metabolic engineering has gradually become possible. In this study, the Squalene synthase (SS) over-expression vector was established. By the way of agrobacterium-mediated method, the vector was transfered and integrated into the Panax notoginseng genome. The result of the PCR detection and the saponin content detection shows that over-expression SS is able to produce high level of Panax notoginseng saponins, and confirms the regulatory function of SS in the biosynthesis of ginsenosides in Panax notoginseng. It provides a theoretical basis and technical basis for the construction of PNS homologous or heterologous efficient expression system in the future.

[Researches on influence of squalene synthase gene polymorphism on catalytic efficiency of its encode enzyme in Glycyrrhiza uralensis]

OBJECTIVE

To analyse the polymorphism of squalene synthase gene and reveal the influence of squalene synthase (SQS) gene polymorphism on the catalytic efficiency of its encode enzyme in Glycyrrhiza uralensi.

METHOD

The total RNA was extracted. PCR was used to amplify the coding sequences of squalene synthase gene, which were sequenced and analysed. The expression vectors containing different SQS gene sequences, including SQS1C, SQS1F, SQS2A, SQS2B, were constructed and transformed into Escherichia coli BL21. The fusion protein was induced to express by IPTG, then was isolated, purified and used to carry out the enzymatic reaction in vitro. GC-MS was used to analyse the production.

RESULT

There were three kinds of gene polymorphism existing in SQS1 gene of G. uralensis, including single nucleotide polymorphism (SNPs), insertion/deletion length polymorphism (InDels) and level of amino acid, the proportion of conservative replace of SQS1 was 53.94%, and there were 2 mutational sites in structural domains. The proportion of conservative replace of SQS2 was 60%, and there was 1 mutational site in structural domains. The production squalene could be detected by GC-MS in all the 4 kinds of enzymatic reactions. The capacity of accumulating squalene of SQS1F was higher than other SQS genes.

CONCLUSION

The polymorphism of SQS gene was quite abundant in G. uralensis, which maybe the molecular foundation of the formation of high-quality liquorice.

Cloning and expression analysis of squalene synthase, a key enzyme involved in antifungal steroidal glycoalkaloids biosynthesis from Solanum nigrum

Steroidal glycoalkaloids (SGAs) are a family of nitrogenous secondary metabolites produced in solanaceous plants. In our present study, γ-solamargine and its aglycone solasodine from Solanum nigrum were found to inhibit hyphae formation of Fusarium oxysporum. As phytoalexins, the formation of SGAs was significantly increased in the plants when infected with the spore of F. oxysporum. In order to understand this inducible defense mechanism, the rate-limiting enzyme squalene synthase in the biosynthesis process of SGAs was investigated well. A full-length cDNA encoding squalene synthase was isolated from S. nigrum (the squalene synthase in S. nigrum was designated as SnSS). The full-length cDNA of SnSS was 1,765 bp and contained a 1,236 bp open reading frame (ORF) encoding a polypeptide of 411 amino acids. Bioinformatic analysis revealed that the deduced SnSS protein had a high similarity with other plant squalene synthases. Real-time RT-PCR analysis showed that SnSS was expressed constitutively in all tested tissues, with the highest expression in stems. After treatment with the spore of F. oxysporum, the mRNA level of SnSS was significantly increased in the infected plants in accordance with the change of SGAs.

[Analyze on the expression differences of key genes in saponins biosynthesize in Eleutherococcus senticosus and its callus]

OBJECTIVE

To analyze the expression differences of SS, SE and bAS in leaf, petiole and callus from leaf and petiole of Eleutherococcus senticosus.

METHODS

Calluses were induced from explants of E. senticosus leaf and petiole. The expression amount of SS, SE and bAS gene was detected by real time PCR.

RESULTS

The expression amount of SS, SE and bAS genes in callus from leaf were 89.3%, 73.8% and 83.4% of that in leaf, respectively. The expression amount of SS, SE and bAS genes in callus from petiole were 80.2%, 87.7% and 70.9% of that in petiole, respectively.

CONCLUSION

The expression amount of SS, SE and bAS gene in callus from E. senticosus are significantly lower than that in plant.

[Cloning and analysis of squalene synthase (HsSQS1) gene in Huperzia serrata]

Squalene synthase (SQS) is a key enzyme in plant terpenoid biosynthetic pathway. This study focused on cloning and analysis of Huperzia serrata SQS (HsSQS1) gene. After searching the transcriptome dataset of H serrata, one unique sequence encoding SQS was discovered. The primers were designed according to the transcript sequence of HsSQS1 from the H. serrata transcriptome dataset. The open reading frame of HsSQS1 was cloned using RT-PCR strategy. The bioinformatic analysis of this gene and its corresponding protein were performed. The cDNA (named as HsSQS1) contains a 1263 bp open reading frame and encodes a predicted protein of 420 amino acids. The GenBank accession number for this gene is JQ004938. HsSQS1 contains two transmembrane regions, without signal peptide. The conserved domain of squalene synthase was presented in HsSQS1. HsSQS1 was more abundant in H. serrata root than in leaf and stem. This study cloned and analyzed squalene synthase gene from H. serrata for the first time. The result will provide a foundation for exploring the mechanism ofterpenoid biosynthesis in H. serrata plants.

Induction of potato steroidal glycoalkaloid biosynthetic pathway by overexpression of cDNA encoding primary metabolism HMG-CoA reductase and squalene synthase

Potato steroidal glycoalkaloids (SGAs) are toxic secondary metabolites whose total content in tubers must be regulated. SGAs are biosynthesized by the sterol branch of the mevalonic acid/isoprenoid pathway. In a previous study, we showed a correlation between SGA levels and the abundance of transcript coding for HMG-CoA reductase 1 (HMG1) and squalene synthase 1 (SQS1) in potato tissues and potato genotypes varying in SGA content. Here, Solanum tuberosum cv. Desirée (low SGA producer) was transformed with a gene construct containing the coding region of either HMG1 or SQS1 of Solanum chacoense Bitt. clone 8380-1, a high SGA producer. SGA levels in transgenic HMG-plants were either greater than (in eight of 14 plants) or no different from untransformed controls, whereas only four of 12 SQS-transgenics had greater SGA levels than control, as determined by HPLC. Quantitative real-time PCR was used to estimate relative steady-state transcript levels of isoprenoid-, steroid-, and SGA-related genes in leaves of the transgenic plants compared to nontransgenic controls. HMG-transgenic plants exhibited increased transcript accumulation of SQS1, sterol C24-methyltransferase type1 (SMT1), and solanidine glycosyltransferase 2 (SGT2), whereas SQS-transgenic plants, had consistently lower transcript levels of HMG1 and variable SMT1 and SGT2 transcript abundance among different transgenics. HMG-transgenic plants exhibited changes in transcript accumulation for some sterol biosynthetic genes as well. Taken together, the data suggest coordinated regulation of isoprenoid metabolism and SGA secondary metabolism.

Induction of root colonization by Piriformospora indica leads to enhanced asiaticoside production in Centella asiatica

Centella asiatica (Indian pennywort) has wide application in Indian and Chinese traditional medicines with documented evidence for wound healing and neuroprotective and anti-aging potential. Asiaticoside, a trisaccharide triterpene, is the most medicinally active compound in the plant. β-Amyrin synthase and squalene synthase have been identified as the two key genes in the triterpenoid pathway which regulate the production of asiaticoside in C. asiatica. The paper reports salient findings of our study utilizing the growth-promoting endophytic fungus Piriformospora indica to successfully colonize roots of C. asiatica in vitro cultures for investigating the effect of the mutualistic association on asiaticoside production. Co-cultivation of P. indica resulted in the rapid enhancement of root and shoot biomass of host plant, which was visible after 7 days of culture and continued up to 45 days. P. indica co-cultivation also favored the synthesis of asiaticosides, as evidenced by HPLC analysis which indicated about twofold increase (0.53% (w/w) in leaves and 0.23% (w/w) in whole plant) over control (0.33% (w/w) in leaves and 0.14% (w/w) in whole plant). Real-time PCR results confirmed the strong upregulation of squalene synthase and β-amyrin synthase transcripts in P. indica-challenged plants compared with the control. Our data demonstrate the potential use of P. indica as a means to enhance plant secondary metabolite production in planta with scope for further field evaluation.

Phytosterols play a key role in plant innate immunity against bacterial pathogens by regulating nutrient efflux into the apoplast

Bacterial pathogens colonize a host plant by growing between the cells by utilizing the nutrients present in apoplastic space. While successful pathogens manipulate the plant cell membrane to retrieve more nutrients from the cell, the counteracting plant defense mechanism against nonhost pathogens to restrict the nutrient efflux into the apoplast is not clear. To identify the genes involved in nonhost resistance against bacterial pathogens, we developed a virus-induced gene-silencing-based fast-forward genetics screen in Nicotiana benthamiana. Silencing of N. benthamiana SQUALENE SYNTHASE, a key gene in phytosterol biosynthesis, not only compromised nonhost resistance to few pathovars of Pseudomonas syringae and Xanthomonas campestris, but also enhanced the growth of the host pathogen P. syringae pv tabaci by increasing nutrient efflux into the apoplast. An Arabidopsis (Arabidopsis thaliana) sterol methyltransferase mutant (sterol methyltransferase2) involved in sterol biosynthesis also compromised plant innate immunity against bacterial pathogens. The Arabidopsis cytochrome P450 CYP710A1, which encodes C22-sterol desaturase that converts β-sitosterol to stigmasterol, was dramatically induced upon inoculation with nonhost pathogens. An Arabidopsis Atcyp710A1 null mutant compromised both nonhost and basal resistance while overexpressors of AtCYP710A1 enhanced resistance to host pathogens. Our data implicate the involvement of sterols in plant innate immunity against bacterial infections by regulating nutrient efflux into the apoplast.

Molecular cloning and expression of squalene synthase and 2,3-oxidosqualene cyclase genes in persimmon (Diospyros kaki L.) fruits

Oleanolic acid (OA) and ursolic acid (UA) are the main triterpene acids in persimmon fruit, and squalene synthase and 2,3-oxidosqualene cyclases are important enzymes in pentacyclic triterpene biosynthesis. In order to study their relationship, DkSQS and DkOSC were cloned from persimmon fruits in the present study. The full-length cDNA of DkSQS was 1647 bp, containing an open reading frame (ORF) of 1245 bp that encoded a peptide of 415 amino acids (AA). The 3'-end of DkOSC cDNA fragment contained 522 bp, including a partial ORF of 298 bp, a full poly A tail that encoded 98 AA. Two cultivars of persimmon, i.e. cv. Nishimurawase and cv. Niuxinshi, were used to study the content of OA and UA and the related gene expression. Results showed that OA and UA contents changed in both cultivars during fruit development, the difference in cv. Nishimurawase was greater than that in cv. Niuxinshi. The expression of DkSQS and DkOSC had no obvious correlation with the biosynthesis of OA and UA in the flesh. There may be two main reasons. Firstly, different enzymes involved in the biosynthesis of triterpenes and mutual adjustment were existed in different gene expressions. Secondly, it was not clear that the DkOSC cloned in this research belonged to which subfamily. Therefore, the real relationship between triterpenes and DkSQS and DkOSC in persimmon fruits is still to be revealed.

RNAi-mediated disruption of squalene synthase improves drought tolerance and yield in rice

About one-third of the world's rice area is in rain-fed lowlands and most are prone to water shortage. The identification of genes imparting tolerance to drought in the model cereal plant, rice, is an attractive strategy to engineer improved drought tolerance not only rice but other cereals as well. It is demonstrated that RNAi-mediated disruption of a rice farnesyltransferase/squalene synthase (SQS) by maize squalene synthase improves drought tolerance at both the vegetative and reproductive stages. Twenty-day-old seedlings of wild type (Nipponbare) and seven independent events of transgenic RNAi lines showed no difference in morphology. When subjected to water stress for a period of 32 d under growth chamber conditions, transgenic positives showed delayed wilting, conserved more soil water, and improved recovery. When five independent events along with wild-type plants were subjected to drought at the reproductive stage under greenhouse conditions, the transgenic plants lost water more slowly compared with the wild type, through reduced stomatal conductance and the retention of high leaf relative water content (RWC). After 28 d of slow progressive soil drying, transgenic plants recovered better and flowered earlier than wild-type plants. The yield of water-stressed transgenic positive plants ranged from 14-39% higher than wild-type plants. When grown in plates with Yoshida's nutrient solution with 1.2% agar, transgenic positives from three independent events showed increased root length and an enhanced number of lateral roots. The RNAi-mediated inactivation produced reduced stomatal conductance and subsequent drought tolerance.

Gene regulation patterns in triterpene biosynthetic pathway driven by overexpression of squalene synthase and methyl jasmonate elicitation in Bupleurum falcatum

The root of Bupleurum falcatum L. (Apiaceae) has long been one of the most important traditional herbal medicines in Asian countries. A group of triterpene saponins (saikosaponins) are the major constituents of this plant. Squalene synthase (SS) may play a regulatory role in directing triterpene intermediates and sterol pathways. Here, we investigated the regulatory role of the squalene synthase (BfSS1) gene in the biosynthesis of phytosterol and triterpene in B. falcatum. BfSS1 mRNA accumulated ubiquitously in plant organs and markedly increased in roots after treatment with methyl jasmonate (MeJA), ABA and ethephon. Transgenic B. falcatum constructs overexpressing BfSS1 in the sense and antisense orientations were assembled using the Agrobacterium-mediated method. Transgenic roots overexpressing BfSS1 in the sense orientation resulted in enhanced production of both phytosterol and saikosaponins. Overexpression of the BfSS1 gene in the sense orientation increased the mRNA accumulation of downstream genes such as squalene epoxidase and cycloartenol synthase but unexpectedly decreased the mRNA levels of β-amyrin synthase (β-AS), a triterpene synthase mRNA. MeJA treatment of wild-type roots strongly stimulated β-AS mRNA accumulation and saikosaponin production but suppressed phytosterol production. MeJA treatment of transgenic roots overexpressing BfSS1 in the sense orientation failed to stimulate β-AS mRNA accumulation but still enhanced saikosaponin and phytosterol production. These results indicate that overexpression of BfSS1 in B. falcatum regulates more powerfully the downstream genes than elicitor (MeJA) treatment in triterpene and phytosterol biosynthesis.

Functional microdomains in bacterial membranes

The membranes of eukaryotic cells harbor microdomains known as lipid rafts that contain a variety of signaling and transport proteins. Here we show that bacterial membranes contain microdomains functionally similar to those of eukaryotic cells. These membrane microdomains from diverse bacteria harbor homologs of Flotillin-1, a eukaryotic protein found exclusively in lipid rafts, along with proteins involved in signaling and transport. Inhibition of lipid raft formation through the action of zaragozic acid--a known inhibitor of squalene synthases--impaired biofilm formation and protein secretion but not cell viability. The orchestration of physiological processes in microdomains may be a more widespread feature of membranes than previously appreciated.

Key enzymes of triterpenoid saponin biosynthesis and the induction of their activities and gene expressions in plants

Triterpenoid saponins are one of the key active components of many medicinal plants. The biosynthetic pathway of triterpenoid saponins in higher plants and a lot of experimental results both indicated that the key enzymes involved in triterpenoid saponin synthesis are squalene synthase (SS), squalene epoxidase (SE), lupeol synthase (LS), dammarenediol synthase (DS), beta-amyrin synthase (beta-AS), cytochrome P450-dependent monooxygenase (PDMO), and glycosyltransferase (GT). The activities and coding genes of the key enzymes could be induced by a range of factors in various plant species. However, the effects of the factors on the content and composition of the triterpenoid saponins in specific plants are not certainly coincident, and different factors appear to induce the gene expressions of the key enzymes by different signal pathways and at different levels. This paper could provide a reference for strengthening the triterpenoid saponin-synthesizing capability of specific medicinal plants at enzyme and/or gene expression levels in order to improve the plants' commercial values.

[Culture of transgenic Glycyrrhiza uralensis hairy root with licorice squalene synthase (SQS) gene]

A squalene synthase gene cloned (GuSQS1, accession number in GenBank database: AM182329) from Glycyrrhiza uralensis was transferred into G. uralensis via Agrobacterium rhizogenes A4 for investigating biosynthesis pathway and enhancing synthesis of glycyrrhizic acid (GA). Hypocotyl explants from G. uralensis were infected with A. rhizogenes A4 containing GuSQS1 gene to induce the hairy roots. The hairy root lines established were selected in medium containing 0.8 mg x L(-1) phosphinothricin (PPT) and analyzed by PCR and southern blotting. The transgenic hairy roots were cultured in liquid MS medium. GA contents in transgenic hairy roots were detected by HPLC. Results showed that maximal GA content in transgenic hairy root lines was 3.6 times as high as in wild type hairy roots.

Molecular consequences of altered neuronal cholesterol biosynthesis

The first dedicated step in de novo cholesterol biosynthesis begins with formation of squalene and ends with the reduction of 7-dehydrocholesterol by 7-dehydrocholesterol reductase (Dhcr7) into cholesterol, which is an essential structural and signaling molecule. Mutations in the Dhcr7 gene lead to Smith-Lemli-Opitz syndrome (SLOS), which is characterized by developmental deformities, incomplete myelination, and mental retardation. To understand better the molecular consequences of Dhcr7 deficiency in neuronal tissue, we analyzed the effect of cholesterol deficiency on the transcriptome in Neuro2a cells. Transient down-regulation of Dhcr7 by siRNA led to altered expression of multiple molecules that play critical roles in intracellular signaling or vesicular transport or are inserted into membrane rafts (e.g. Egr1, Snx, and Adam19). A similar down-regulation was also observed in stable Dhrc7-shRNA-transfected cell lines, and the findings were verified by qPCR. Furthermore, we investigated the Dhcr7-deficient and control cells for the expression of several critical genes involved in lipid biosynthesis. Among these, fatty acid synthase, sterol-regulatory element binding protein 2, SREBF chaperone, site-1 protease, and squalene synthase showed a significant down-regulation, suggesting that, in a neuronal cell line, Dhcr7 is a potent regulator of lipid biosynthesis. Importantly, the gene expression changes were present in both lipid-containing and cholesterol-deficient media, suggesting that intrinsic cholesterol biosynthesis is necessary for normal neuronal function and cannot be supplemented from extrinsic sources.

[Cloning and characterization of cDNA encoding Psammosilene tunicoides squalene synthase]

The total triterpene saponins of Psammosilene tunicoides have significant pharmacologic activity. Psammosilene tunicoides squalene synthase (PSS) is a gateway enzyme to regulate the biosynthesis of total triterpene saponins extracted from the root of Psammosilene tunicoides which is an endangered species. In this paper, cDNA encoding of PSS was cloned by the degenerate primer PCR and rapid-amplification of cDNA ends (RACE). The full-length of cDNA of PSS is 1663 bp, with an open reading frame (ORF) of 1 245 bp, encoding 414 amino acid polypeptide (calculated molecular mass, 47.69 kDa), 5'UTR (untranslated region) and 3'UTR are 260 bp and 158 bp, respectively. The deduced amino acid sequence of PSS has higher homology with the known squalene synthases of several species such as Panax notoginseng (83%), Panax ginseng (82%) and Glycyrrhiza glabra (82%) than that with Schizosacharomyces pombe (35%), Candida albicans (39%) and Homo sapiens (47%). The characterization of PSS was done by a series of methods, such as prokaryotic expression, the activity of enzyme in vitro, capillary gas chromatography (GC) and capillary gas chromatography mass spectrometry (GC-MS). The results showed that the cell-free extract of E. coli transformed with the recombinant plasmid can effectively convert farnesyl diphosphate into squalene in vitro. GenBank accession number is EF585250. Our research provided important base for the study of Psammosilene tunicoides secondary metabolism and metabolic engineering.

Quantitative transcript profiling reveals down-regulation of A sterol pathway relevant gene and overexpression of artemisinin biogenetic genes in transgenic Artemisia annua plants

To investigate the dynamic fluctuation of terpenoid relevant transcriptomics in transgenic ARTEMISIA ANNUA plants that express the genomic integrated antisense squalene synthase gene ( ASSS), we have quantified the transcript levels of the sterol anabolic SS gene as well as artemisinin biogenetic amorphadiene synthase (ADS), cytochrome P450 monooxygenase (CYP71AV1) and cytochrome P450 reductase (CPR) genes by real-time fluorescent quantitative polymerase chain reaction (RFQ-PCR). The SS mRNA level in transgenic plants sharply droped to 7.4 % - 55.3 % (i. e., 44.7 - 92.6 % reduction as the wild-type control), strongly implying that the expression of endogenous SS gene is significantly suppressed by the exogenous ASSS gene. In a synchronous fashion, ADS, CYP71AV1 and CPR mRNA levels elevated with the decline of SS mRNA level in transgenic plants, and the maximal ADS, CYP71AV1 and CPR mRNA levels in transgenic plants were 3.0-, 4.4- and 2.5-fold, respectively, higher than those in the control. Without a lethal effect but with a distinguishable impact on the organogenesis and morphology of transgenic plants, the down-regulation of SS gene has also led to the coordinated overexpression of ADS, CYP71AV1 and CPR genes together with the overproduction of artemisinin although no fully perfect correlation among the available experimental data has been shown.

Arabidopsis thaliana contains a single gene encoding squalene synthase

Squalene synthase (SQS) catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to produce squalene (SQ), the first committed precursor for sterol, brassinosteroid, and triterpene biosynthesis. Arabidopsis thaliana contains two SQS-annotated genomic sequences, At4g34640 (SQS1) and At4g34650 (SQS2), organized in a tandem array. Here we report that the SQS1 gene is widely expressed in all tissues throughout plant development, whereas SQS2 is primarily expressed in the vascular tissue of leaf and cotyledon petioles, and the hypocotyl of seedlings. Neither the complete A. thaliana SQS2 protein nor the chimeric SQS resulting from the replacement of the 69 C-terminal residues of SQS2 by the 111 C-terminal residues of the Schizosaccharomyces pombe SQS were able to confer ergosterol prototrophy to a Saccharomyces cerevisiae erg9 mutant strain lacking SQS activity. A soluble form of SQS2 expressed in Escherichia coli and purified was unable to synthesize SQ from FPP in the presence of NADPH and either Mg2+ or Mn2+. These results demonstrated that SQS2 has no SQS activity, so that SQS1 is the only functional SQS in A. thaliana. Mutational studies revealed that the lack of SQS activity of SQS2 cannot be exclusively attributed to the presence of an unusual Ser replacing the highly conserved Phe at position 287. Expression of green fluorescent protein (GFP)-tagged versions of SQS1 in onion epidermal cells demonstrated that SQS1 is targeted to the endoplasmic reticulum (ER) membrane and that this location is exclusively dependent on the presence of the SQS1 C-terminal hydrophobic trans-membrane domain.

New targets and emerging therapies for reducing LDL cholesterol

PURPOSE OF REVIEW

Effective therapies for lowering LDL-cholesterol reduce the incidence of cardiovascular disease and provide associated decreases in morbidity and mortality. Progress in our understanding of metabolism and innovations in drug design have jointly identified promising new drug targets and alternative approaches to old targets. This review focuses on the mechanism, safety and efficacy of emerging LDL-cholesterol lowering therapies.

RECENT FINDINGS

Decreasing apolipoprotein B expression or preventing the formation of a stable lipoprotein structure by inhibiting microsomal triglyceride transfer protein attenuates the secretion of atherogenic lipoproteins containing apolipoprotein B into the plasma. Increases in LDL receptor-mediated cholesterol clearance occur when hepatic cholesterol stores are reduced secondary to inhibition of squalene synthase or LDL receptor degradation is disrupted by reduced activity of proprotein convertase subtilisin kexin type 9. Each of these developing therapies demonstrably reduces LDL-cholesterol levels.

SUMMARY

The emergence of modalities that act in series and in parallel with available agents may allow more effective LDL-cholesterol lowering in those patients intolerant of current therapy, and may permit decremental reductions in LDL-cholesterol for those unable to achieve aggressive LDL-cholesterol goals using existing agents.