Cellular localisation and quantification of Camptothecin in different plant parts of Nothapodytes nimmoniana (J. Graham) Mabberley of Sri Lankan origin

INTRODUCTION

Nothapodytes nimmoniana (Icacinaceae) is a rich source of Camptothecin (CPT), an anti-cancer prodrug. Efficient extraction of CPT from various plant parts is crucial for better recovery of this pre-drug.

OBJECTIVES

To investigate the distribution of CPT in plant parts and to compare the methods of extraction on CPT yield to evaluate how cellular localisation affects the efficiency of extraction methods.

METHODS

Transverse sections of plant parts were observed under a ultraviolet (UV)-fluorescence microscope for the fluorescence that the CPT molecule emits when exposed to UV radiation. Dried plant parts were extracted using 90% methanol with ultrasonic assistance, hot ethanol (61% ethanol at 60°C), and chloroform-methanol (4:1, v/v). The CPT in plant parts were detected by thin-layer chromatography (TLC) and confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and high-performance liquid chromatography (HPLC). Quantification was carried out by HPLC.

RESULTS

Blue fluorescence indicated a prominent accumulation of CPT in roots compared to leaf with petiole, twigs, and stembark. This accumulation was observed in upper and lower epidermis of the leaf, isolated strands of fibres in the phloem in the petiole, and groups of idioblast cells in the cortex. The ultrasonic-assisted extraction with 90% methanol showed the highest CPT yield in the root (1.91 ± 0.02 mg/g of dry weight), followed by stembark and the least in leaves [0.02 ± 0.01 mg/g (dry weight)] irrespective of the method of extraction. However, hot ethanol extraction gave the highest CPT yield for twig and leaf, indicating the necessity of tissue-specific extraction methods for better recovery of CPT.

Diaporthe sp. F18; a new source of camptothecin-producing endophytic fungus from Nothapodytes nimmoniana growing in Sri Lanka

An endophytic fungus producing camptothecin (CPT) was isolated from the leaf of Nothapodytes nimmoniana (Sri Lanka), and culture conditions were optimised to enhance the yield of CPT. The TLC, HPLC-PDA, LC-MS/MS and spectroscopic data were used to identify and quantify CPT. Solvent extraction (chloroform: methanol 4:1 v/v) of submerged cultures in Sabouraud Dextrose Broth (SDB) detected CPT in the mycelial extract but not in the culture broth. The fungus was (KX212080) closely related to Diaporthe guangxiensis (MK335772) with 99% sequence similarity, thus tentatively identified as Diaporthe sp. F18. A significantly high CPT content (72.0 ± 0.2 µg/g) was produced in SDB, pH, 5.6 incubated at 30 °C under shake flask condition (150 rpm) for 14 days. Tryptophan significantly (p > 0.05) enhanced CPT production while ethanol increased it by 8-fold. This endophytic source produced higher CPT content than what has been reported hitherto in the literature, with fairly stable production up to sixth subculture generations.

Transcriptome-wide identification, characterization, and phylogenomic analysis of cytochrome P450s from Nothapodytes nimmoniana reveal candidate genes involved in the camptothecin biosynthetic pathway

The plant Nothapodytes nimmoniana is an important source of camptothecin (CPT), an anticancer compound widely used in the treatment of colorectal, lung, and ovarian cancers. CPT is biosynthesized by the combination of the seco-iridoid and indole pathways in plants. The majority of the biosynthetic steps and associated genes still remain unknown. Certain reactions in the seco-iridoid pathway are catalyzed by cytochrome P450 enzymes. Hence, identifying transcriptionally active cytochrome P450 genes becomes essential in the elucidation of the CPT biosynthetic pathway. Here, we report the identification of 94 cytochrome P450s from the assembled transcriptomic data from leaf and root tissues of N. nimmoniana. The identified cytochrome P450 genes were full length and possessed all four conserved characteristic signature motifs of cytochrome P450 genes. Phylogenetic analysis of the protein sequences revealed their evolution and diversification and further categorized them into A-type (52.12%) and non-A-type (47.87%) cytochrome P450s. These 94 sequences represent 38 families and 63 subfamilies of cytochrome P450s. We also compared the transcriptional activity of identified cytochrome P450s with the expression of their homologs in the CPT-producing plant Ophiorrhiza pumila. Based on expression profiles and quantitative PCR validation, we propose NnCYP81CB1 and NnCYP89R1 as candidate cytochrome P450 genes involved in camptothecin biosynthesis in N. nimmoniana.

Metabolic and transcriptional analyses in response to potent inhibitors establish MEP pathway as major route for camptothecin biosynthesis in Nothapodytes nimmoniana (Graham) Mabb

BACKGROUND

Nothapodytes nimmoniana, a plant of pivotal medicinal significance is a source of potent anticancer monoterpene indole alkaloid (MIA) camptothecin (CPT). This compound owes its potency due to topoisomerase-I inhibitory activity. However, biosynthetic and regulatory aspects of CPT biosynthesis so far remain elusive. Production of CPT is also constrained due to unavailability of suitable in vitro experimental system. Contextually, there are two routes for the biosynthesis of MIAs: the mevalonate (MVA) pathway operating in cytosol and the methylerythritol phosphate (MEP) pathway in the plastids. Determination of relative precursor flux through either of these pathways may provide a new vista for manipulating the enhanced CPT production.

RESULTS

In present study, specific enzyme inhibitors of MVA (lovastatin) and MEP pathways (fosmidomycin) were used to perturb the metabolic flux in N. nimmoniana. Interaction of both these pathways was investigated at transcriptional level by using qRT-PCR and at metabolite level by evaluating secologanin, tryptamine and CPT contents. In fosmidomycin treated plants, highly significant reduction was observed in both secologanin and CPT accumulation in the range 40-57% and 64-71.5% respectively, while 4.61-7.69% increase was observed in tryptamine content as compared to control. Lovastatin treatment showed reduction in CPT (7-11%) and secologanin (7.5%) accumulation while tryptamine registered slight increase (3.84%) in comparison to control. These inhibitor mediated changes were reflected at transcriptional level via altering expression levels of deoxy-xylulose-5-phosphate reductoisomerase (DXR) and hydroxymethylglutaryl-CoA reductase (HMG). Further, mRNA expression of four more genes downstream to DXR and HMG of MEP and MVA pathways respectively were also investigated. Expression analysis also included secologanin synthase (SLS) and strictosidine synthase (STR) of seco-iridoid pathway. Present investigation also entailed development of an efficient in vitro multiplication system as a precursor to pathway flux studies. Further, a robust Agrobacterium-mediated transformed hairy root protocol was also developed for its amenability for up-scaling as a future prospect.

CONCLUSIONS

Metabolic and transcriptional changes reveal differential efficacy of cytosolic and plastidial inhibitors in context to pathway flux perturbations on seco-iridoid end-product camptothecin. MEP pathway plausibly is the major precursor contributor towards CPT production. These empirical findings allude towards developing suitable biotechnological interventions for enhanced CPT production.

Proteome study of somatic embryogenesis in Nothapodytes nimmoniana (J. Graham) Mabberly

Somatic embryogenesis (SE) is the most suitable biotechnological tool for the rapid clonal propagation of endangered woody plants, but many bottlenecks limit understanding its molecular and physiological processes in Nothapodytes nimmoniana. Combinations of two-dimensional electrophoresis (2-DE) and mass spectrometry (MaSp) were used to study proteomic expression changes during SE of the forest tree. Callus was induced from mature seed embryos, and embryogenic callus (EC) obtained at very low frequency after about 6 month culture. Globular embryos were induced from the seed embryo-derived EC and the subsequent stages of the SE. Analysis of the extracted proteins from globular, heart/torpedo-shaped, and maturing embryo stages resolved in the 2-DE gels showed increased protein expression across developmental stages of the somatic embryos. The mass spectrometric analysis with database search aided identification of 55 out of 100 and 54 selected protein spots. Identified proteins classified by the cellular role which they perform are involved in aspects of stress responses, energy metabolism, carbon fixation, secondary metabolism, and other metabolic functions, while three proteins are of unknown cellular role. The putative role of the expressed proteins during SE provided insight into the physiology of somatic embryo development in N. nimmoniana.

De novo transcriptome analyses reveals putative pathway genes involved in biosynthesis and regulation of camptothecin in Nothapodytes nimmoniana (Graham) Mabb

Comprehensive transcriptome analysis of leaf and root tissues of Nothapodytes nimmoniana unravels several putative pathway genes, transcription factors and CYPs related to camptothecin (CPT) biosynthesis. Additionally, post-transcriptional suppression by artificial microRNA (aMIR) of NnCYP76B6 (geraniol 10-hydroxylase) suggests its role in CPT biosynthesis. Tissue-specific LC-MS/MS analysis revealed the presence of secologanin as the central intermediate of MIA pathway in N. nimmoniana. Nothapodytes nimmoniana is a rich source of potent anticancer drug camptothecin (CPT) whose biosynthetic pathway is unresolved due to the lack of genomic and transcriptomic information. Present investigation entails deep transcriptome analysis of N. nimmoniana which led to identification of putative pathway genes and regulatory components involved in CPT biosynthesis. Using Illumina HiSeq 2500 sequencing platform a total of 31,172,889 (6.23 Gb) and 31,218,626 (6.24 Gb) raw reads were generated from leaf and root wood, respectively. These were assembled de novo into 138,183 unique contigs. Additionally, 16 cytochrome P450 transcripts related to secondary metabolism were also identified. Further, transcriptome data pool presented 1683 putative transcription factors of which transcripts corresponding to WRKY TFs were the most abundant (14.14%). A total of 2741 transcripts were differentially expressed out of which 478 contigs showed downregulation in root wood and 2263 contigs were up-regulated. Further, comparative analyses of 17 genes involved in CPT biosynthetic pathway were validated by qRT-PCR. On basis of intermediates, two distinct seco-iridoid pathways are involved in the biosynthesis of monoterpene indole alkaloids either through multiple isomers of strictosidinic acid or strictosidine. Tissue-specific LC-MS/MS analysis revealed the presence of secologanin as the central intermediate of MIA pathway in N. nimmoniana. Geraniol-10 hydroxylase (NnCYP76B6) an important enzyme in CPT biosynthesis which specifically shunts geraniol into the secologanin pathway was also cloned from the trancriptome resource. In planta transient expression of NnCYP76B6 showed a significant enhancement in mRNA transcript levels coincident with enhanced CPT accumulation. Further, artificial microRNA (aMIR) mediated downregulation of NnCYP76B6 resulted in reduction of mRNA transcript levels as well as CPT content in comparison to control. These empirical results suggest a plausible regulatory role for NnCYP76B6 in CPT biosynthesis and also establish a valuable repository for deciphering various structural, rate limiting and regulatory genes of CPT biosynthetic pathway.

Ultrasound-assisted rapid extraction and kinetic modelling of influential factors: Extraction of camptothecin from Nothapodytes nimmoniana plant

Ultrasound-assisted extraction (UAE) of commercially important natural product camptothecin (CPT) from Nothapodytes nimmoniana plant has been investigated. The influences of process factors such as electric acoustic intensity, solid to liquid ratio, duty cycle, temperature and particle size on the maximum extraction yield and kinetic mechanisms of the entire extraction process have been investigated. The kinetics results showed that increasing the intensity, duty cycle, solid to liquid ratio and decreasing the particle size lead to substantial increase in extraction yields compared to classical stirring extraction. Different kinetic models were applied to fit the experimental data. The second order rate model appears to be the best. The extraction rate constant, initial extraction rate and the equilibrium concentration for all experimental conditions have been calculated. SEM analysis of spent plant material clearly showed hollow openings on cell structure, which could be directly correlated to explosive disruption by the action of ultrasound waves. Overall 1.7-fold increase in extraction yields of CPT (0.32% w/w) and decrease in time from 6h to 18min was observed over the stirring method.

Micropropagation and validation of genetic and biochemical fidelity among regenerants of Nothapodytes nimmoniana (Graham) Mabb. employing ISSR markers and HPLC

An in vitro protocol has been established for clonal propagation of Nothapodytes nimmoniana which is an important source of Camptothecin (CPT). Elite source was identified based on the chemical potency to accumulate the optimum level of CPT. Different types and concentrations of plant growth regulators were used to study their effect on inducing multiple shoots from the explants regenerated from embryos of N. nimmoniana. Of these, a combination of N6-benzyladenine (0.2 mg L-1) and Indole-3-butyric acid (IBA) (0.1 mg L-1) proved optimum for differentiating multiple shoots in 90.6 % of the cultures with an average of 10.24 shoots per explant obtained within 8 weeks of inoculation. Nearly, 92 % of the excised in vitro shoots rooted on half strength Murashige and Skoog (MS) medium containing 0.05 % activated charcoal, supplemented with 1-naphthaleneacetic acid and IBA at 0.1 mg L-1 each. The micropropagated plants were evaluated for their genetic fidelity by employing inter simple sequence repeats (ISSR) markers. Ten individuals, randomly chosen from a population of 145 regenerants, were compared with the donor plant. The regenerated plants were also evaluated for their chemical potency using high-performance liquid chromatography (HPLC) analysis of CPT content. The true-to-type nature of the micropropagated plants was confirmed based on their monomorphic banding profiles with that of the mother plants using ISSR markers. Besides, HPLC evaluation of the CPT content confirmed the existence of chemical uniformity among the regenerated plants and the elite mother plant.

Restoration of camptothecine production in attenuated endophytic fungus on re-inoculation into host plant and treatment with DNA methyltransferase inhibitor

Fungal endophytes inhabit living tissues of plants without any apparent symptoms and in many cases are known to produce secondary metabolites similar to those produced by their respective host plants. However on sub-culture, the endophytic fungi gradually attenuate their ability to produce the metabolites. Attenuation has been a major constraint in realizing the potential of endophytic fungi as an alternative source of plant secondary metabolites. In this study, we report attempts to restore camptothecine (CPT) production in attenuated endophytic fungi isolated from CPT producing plants, Nothapodytes nimmoniana and Miquelia dentata when they are passed through their host plant or plants that produce CPT and when treated with a DNA methyl transferase inhibitor. Attenuated endophytic fungi that traversed through their host tissue or plants capable of synthesizing CPT, produced significantly higher CPT compared to the attenuated fungi. Attenuated fungus cultured in the presence of 5-azacytidine, a DNA methyltransferase inhibitor, had an enhanced CPT content compared to untreated attenuated fungus. These results indicate that the attenuation of CPT production in endophytic fungi could in principle be reversed by eliciting some signals from plant tissue, most likely that which prevents the methylation or silencing of the genes responsible for CPT biosynthesis.

Structural insight of DNA topoisomerases I from camptothecin-producing plants revealed by molecular dynamics simulations

DNA topoisomerase I (Top1) catalyzes changes in DNA topology by cleaving and rejoining one strand of the double stranded (ds)DNA. Eukaryotic Top1s are the cellular target of the plant-derived anticancer indole alkaloid camptothecin (CPT), which reversibly stabilizes the Top1-dsDNA complex. However, CPT-producing plants, including Camptotheca acuminata, Ophiorrhiza pumila and Ophiorrhiza liukiuensis, are highly resistant to CPT because they possess point-mutated Top1. Here, the adaptive convergent evolution is reported between CPT production ability and mutations in their Top1, as a universal resistance mechanism found in all tested CPT-producing plants. This includes Nothapodytes nimmoniana, one of the major sources of CPT. To obtain a structural insight of the resistance mechanism, molecular dynamics simulations of CPT- resistant and -sensitive plant Top1s complexed with dsDNA and topotecan (a CPT derivative) were performed, these being compared to that for the CPT-sensitive human Top1. As a result, two mutations, Val617Gly and Asp710Gly, were identified in O. pumila Top1 and C. acuminata Top1, respectively. The substitutions at these two positions, surprisingly, are the same as those found in a CPT derivative-resistant human colon adenocarcinoma cell line. The results also demonstrated an increased linker flexibility of the CPT-resistant Top1, providing an additional explanation for the resistance mechanism found in CPT-producing plants. These mutations could reflect the long evolutionary adaptation of CPT-producing plant Top1s to confer a higher degree of resistance.

Evaluating Nothapodytes nimmoniana population from three localities of Western Ghats using camptothecin as phytochemical marker and selection of elites using a new-content range chart method

Background:

Nothapodytes nimmoniana (Grah.) Mabb. is a high valued medicinal plant endemic to Western Ghats of India, distributed in fragmented populations. The plant is valued for potent anticancer drug camptothecin (CPT).

Objective:

The study compares and expounds variation in CPT content from leaves and stems of N. nimmoniana obtained from three populations of Western Ghats, India. The study also describes a method for categorizing these populations using content range chart (CRC) method for percent yield of CPT.

Materials and Methods:

A total of 60 samples were investigated including ten each of leaves and stems from three localities. Micro-extraction method was implemented to extract CPT. reversed phase ultra-performance liquid chromatography photo diode array technique was used to quantify CPT.

Results:

Leaf samples of an individual collected from Joida, yielded lowest CPT content (0.002 ± 0.000 g/100 g), whereas a stem sample from Amgaon, yielded highest CPT content (0.123 ± 0.006 g/100 g). The findings suggest great variation in individuals producing and accumulating CPT. Using this data along with earlier published work, five categories of CPT yielding plants were made viz. I: Very low: <0.020, II: Low: 0.021-0.039, III: Moderate: 0.040-0.059, IV: High: 0.060-0.079 and V: Very high: >0.080. Based on CPT content in leaves, majority of individuals were under very low category (I^st) and on the other hand stem samples were in ‘II’ category. Besides, very few individuals were observed in category ‘V’.

Conclusion:

The study expounds use of CRC method for identifying elite population and suggests the need for its conservation.

Seasonal variation in content of camptothecin from the bark of Nothapodytes nimmoniana (Grah.) Mabb., using HPLC analysis

Objective:

To study and compare seasonal variation in camptothecin (CPT) content from bark samples of Nothapodytes nimmoniana obtained from geographically and climatologically isolated populations.

Methods:

A standard High Performance Liquid Chromatography methodology was used to analyze and quantify CPT from bark samples of N. nimmoniana.

Results:

Sample collected from Amboli yielded highest CPT content 1.337 g/100 g dry bark powder during the monsoon compared to other localities in study. Monsoon (August) showed to accumulate higher levels of CPT in barks of N. nimmoniana as compared to summer (May). Amboli averaged highest accumulation of CPT compared to other localities under study.

Conclusion:

These findings indicate season to have control over accumulation of CPT. Locality Amboli has highest CPT content in all seasons and were the elite population during the study. The study also suggests the need for further investigation in lights of biosynthesis in the plant.