Neoculin, a taste-modifying sweet protein, accumulates in ripening fruits of cultivated Curculigo latifolia

A review on natural sweeteners, sweet taste modulators and bitter masking compounds: structure-activity strategies for the discovery of novel taste molecules

Growing demand for the tasty and healthy food has driven the development of low-calorie sweeteners, sweet taste modulators, and bitter masking compounds originated from natural sources. With the discovery of human taste receptors, increasing numbers of sweet taste modulators have been identified through human taste response and molecular docking techniques. However, the discovery of novel taste-active molecules in nature can be accelerated by using advanced spectrometry technologies based on structure-activity relationships (SARs). SARs explain why structurally similar compounds can elicit similar taste qualities. Given the characterization of structural information from reported data, strategies employing SAR techniques to find structurally similar compounds become an innovative approach to expand knowledge of sweeteners. This review aims to summarize the structural patterns of known natural non-nutritive sweeteners, sweet taste enhancers, and bitter masking compounds. Innovative SAR-based approaches to explore sweetener derivatives are also discussed. Most sweet-tasting flavonoids belong to either the flavanonols or the dihydrochalcones and known bitter masking molecules are flavanones. Based on SAR findings that structural similarities are related to the sensory properties, innovative methodologies described in this paper can be applied to screen and discover the derivatives of taste-active compounds or potential taste modulators.

Pharmacognostical and Phytochemical Studies and Biological Activity of Curculigo latifolia Plant Organs for Natural Skin-Whitening Compound Candidate

Curculigo latifolia (family Amaryllidaceae) is used empirically for medicinal purposes. It is distributed throughout Asian countries, especially Indonesia. This study aimed at standardizing the C. latifolia plant, analyzing its phytochemical profile, and evaluating its pharmacological effects. The powder from each organ (root, stem, and leaves) was standardized organoleptically and microscopically. Samples were extracted by graded maceration using hexane, ethyl acetate, and ethanol. The extracts were determined for total phenolic content (TPC) and total flavonoid content (TFC). Antioxidant (radical scavenging and metal ion reduction) and antityrosinase activities were determined by spectrophotometric methods. Extracts were analysed for phytochemical profiles by LC-ESI-MS. The highest TPC and TFC were found in the ethanolic extract of the root organ (68.63 ± 2.97 mg GAE/g) and the ethyl acetate extract of the stem (14.33 ± 0.71 mg QE/g extract). High antioxidant activities were found in the ethanolic root extract (20.42 ± 0.33 µg/mL) and ethanolic stem extract (45.65 ± 0.77 µg/mL) by DPPH• and NO• assays, respectively. The ion reduction activity (by CUPRAC assay) was most significant in the ethyl acetate stem extract (390.42 ± 14.49 µmol GAEAC/g extract). Ethanolic root extract was the most active in inhibiting tyrosinase (IC50 value of 108.5 µg/mL). The correlation matrix between TPC and antioxidant activities showed a moderate to robust correlation, whereas the TPC and antityrosinase activity showed a robust correlation. The TFC and antioxidant or antityrosinase activities showed a weak to moderate correlation. The LC-ESI-MS data identified major phenols in the active extracts, including methyl 3-hydroxy-4-methoxy-benzoate, quercetin, 4-O-caffeoylquinic acid-1, and curculigoside. Overall, this study suggests that extracts from the C. latifolia plant offer potent antioxidant and antityrosinase activities, allowing them to be used as natural antioxidants and candidates for skin-lightening compounds.

De novo transcriptome analysis and comparative expression profiling of genes associated with the taste-modifying protein neoculin in Curculigo latifolia and Curculigo capitulata fruits

Background

Curculigo latifolia is a perennial plant endogenous to Southeast Asia whose fruits contain the taste-modifying protein neoculin, which binds to sweet receptors and makes sour fruits taste sweet. Although similar to snowdrop (Galanthus nivalis) agglutinin (GNA), which contains mannose-binding sites in its sequence and 3D structure, neoculin lacks such sites and has no lectin activity. Whether the fruits of C. latifolia and other Curculigo plants contain neoculin and/or GNA family members was unclear.

Results

Through de novo RNA-seq assembly of the fruits of C. latifolia and the related C. capitulata and detailed analysis of the expression patterns of neoculin and neoculin-like genes in both species, we assembled 85,697 transcripts from C. latifolia and 76,775 from C. capitulata using Trinity and annotated them using public databases. We identified 70,371 unigenes in C. latifolia and 63,704 in C. capitulata. In total, 38.6% of unigenes from C. latifolia and 42.6% from C. capitulata shared high similarity between the two species. We identified ten neoculin-related transcripts in C. latifolia and 15 in C. capitulata, encoding both the basic and acidic subunits of neoculin in both plants. We aligned these 25 transcripts and generated a phylogenetic tree. Many orthologs in the two species shared high similarity, despite the low number of common genes, suggesting that these genes likely existed before the two species diverged. The relative expression levels of these genes differed considerably between the two species: the transcripts per million (TPM) values of neoculin genes were 60 times higher in C. latifolia than in C. capitulata, whereas those of GNA family members were 15,000 times lower in C. latifolia than in C. capitulata.

Conclusions

The genetic diversity of neoculin-related genes strongly suggests that neoculin genes underwent duplication during evolution. The marked differences in their expression profiles between C. latifolia and C. capitulata may be due to mutations in regions involved in transcriptional regulation. Comprehensive analysis of the genes expressed in the fruits of these two Curculigo species helped elucidate the origin of neoculin at the molecular level.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07674-3.

An efficient in vitro plantlet regeneration from shoot tip cultures of Curculigo latifolia, a medicinal plant

A procedure was developed for in vitro propagation of Curculigo latifolia through shoot tip culture. Direct regeneration and indirect scalp induction of Curculigo latifolia were obtained from shoot tip grown on MS medium supplemented with different concentrations and combinations of thidiazuron and indole-3-butyric acid. Maximum response for direct regeneration in terms of percentage of explants producing shoot, shoot number, and shoot length was obtained on MS medium supplemented with combination of thidiazuron (0.5 mg L⁻¹) and indole-3-butyric acid (0.25 mg L⁻¹) after both 10 and 14 weeks of cultures. Indole-3-butyric acid in combination with thidiazuron exhibited a synergistic effect on shoot regeneration. The shoot tips were able to induce maximum scalp from basal end of explants on the medium with 2 mg L⁻¹ thidiazuron. Cultures showed that shoot number, shoot length, and scalp size increased significantly after 14 weeks of culture. Transferring of the shoots onto the MS medium devoid of growth regulators resulted in the highest percentage of root induction and longer roots, while medium supplemented with 0.25 mg L⁻¹ IBA produced more numbers of roots.

Medicinal plants of genus Curculigo: traditional uses and a phytochemical and ethnopharmacological review

ETHNOPHARMACOLOGICAL RELEVANCE

In the genus Curculigo, Curculigo orchioides Gaertn, Curculigo capitulata (Lour) O. Ktze and Curculigo pilosa (Schumach. & Thonn.) Engl are often used in traditional medicine. Curculigo orchioides is used for the treatment of impotence, limb limpness, arthritis of the lumbar and knee joints, and watery diarrhea in traditional Chinese medicine, and also used as a potent immunomodulator and aphrodisiac in the Ayurvedic medical system. Curculigo capitulata is used for the treatment of consumptive cough, kidney asthenia, impotence and spermatorrhea, hemorrhoids, asthma, jaundice, diarrhea, colic and gonorrhea in traditional Chinese and India medicine, and to treat urinary tract infection, acute renal pelvis and nephritis, nephritis-edema, cystitis, nephrolithiasis, hypertension and rheumatic arthritis in traditional Dai medicine. Curculigo pilosa are applied to treat gastrointestinal and heart diseases in Africa.

AIM OF THE REVIEW

This review aims to exhibit up-to-date and comprehensive information about traditional uses, phytochemistry, pharmacology and toxicology of medicinal plants in the genus Curculigo, and has an insight into the opportunities for the future research and development of Curculigo plant.

METHODS

A bibliographic investigation was performed by analyzing the information available on Curculigo plant from worldwide accepted scientific databases (Pubmed, Scopus and Web of Science, SciFinder, Google Scholar, Yahoo). Furthermore, information also was obtained from some local and foreign books on ethnobotany and ethnomedicines.

RESULTS

Curculigo orchioides, Curculigo capitulata and Curculigo pilosa have been used as traditional medicine to treat kinds of diseases such as impotence, limb limpness, gastrointestinal and heart diseases, etc. Phytochemical investigation of eight species of the genus Curculigo has resulted in identification of more than 110 compounds. The content of curculigoside is used as an indicator to evaluate the quality of rhizome of Curculigo orchioides. The medicinal plants have showed a wide spectrum pharmacological activities, including adaptive, immunostimulatory, taste-modifying and sweet-tasting, antioxidant, mast cell stabilization, antihistaminic and antiasthmatic, hepatoprotective and neuroprotective activity. Toxicological test indicated that Curculigo orchioides at the dose of 120 g/kg after administrating rats for 180 days may cause injury of liver and kidney.

CONCLUSION

The medicinal plants of genus Curculigo have emerged as a good source of the traditional medicines. Some uses of these plants in the traditional medicines have been validated by pharmacological investigation. However, the mechanism of their actions should be further elucidated; the particular constituent responsible for toxicity should be isolated and identified, and the target tissue and mechanism of toxic ingredients also deserve to be further investigated; more reference substances should be prepared, and sophisticated analytical technologies should be developed to comprehensively assess the quality of Curculigo herbs. These investigations will be helpful for further utilization of the plants of genus Curculigo.

Dissimilar sweet proteins from plants: oddities or normal components?

The fruits of a few tropical plants contain intensely sweet proteins. Their common property points to a protein family. Generally, proteins belonging to the same family share similar folds, similar sequences and, at least in part, similar function but sweet proteins constitute an exception to this rule. Apart from sharing the rather unusual taste function, they show no obvious similarities either in their sequences or in three-dimensional structures. In this review we describe the nature, structure and mechanism of action of the best known sweet tasting proteins, including two taste modifying proteins. Sweet proteins stand out among sweet molecules because their volume is not compatible with an interaction with orthosteric active sites of the sweet taste receptor. The best explanation of their mechanism of action is the interaction with the external surface of the sweet taste receptor, according to a model that has been named "wedge model". It is hypothesized that this mode of action may be related to the ability of other members of their protein families to inhibit different enzymes.

[History of the Nippon Shinyaku Institute for Botanical Research]

Soon after its foundation in 1919, Nippon Shinyaku Co., Ltd began to develop the domestic production of Santonin, an anthelmintic agent, which, until then, had been totally imported from Russia. In 1927, Artemisia maritima ssp. monogyna was introduced from Europe and confirmed to contain Santonin. This European aster plant was named Mibu-yomogi after the place name of the headquarters of Nippon Shinyaku. In 1934, Yamashina Experimental Farm was founded to breed Mibu-yomogi cultivars of high quality as a plant material for Santonin production in Japan. In 1953, the Experimental Farm was reorganized into the Institute for Botanical Research for the continuous breeding of Santonin-containing aster plants and for the development of any new medicines from medicinal plants. Through the breeding of Santonin-containing aster plants, many cultivars including Yamashina No. 2 from Mibu-yomogi, Penta-yomogi and Hexa-yomogi which were crosssed with Mibu-yomogi and A. kurramensis, were bred. Furthermore, we still have four ethical drug products originated from medicinal plants. Since 1994, the Institute has become a botanical garden in order to maintain, develop and exhibit the plant collection and for the cultivation studies of rare plants.

Characterization of the beta-D-glucopyranoside binding site of the human bitter taste receptor hTAS2R16

G-protein-coupled receptors mediate the senses of taste, smell, and vision in mammals. Humans recognize thousands of compounds as bitter, and this response is mediated by the hTAS2R family, which is one of the G-protein-coupled receptors composed of only 25 receptors. However, structural information on these receptors is limited. To address the molecular basis of bitter tastant discrimination by the hTAS2Rs, we performed ligand docking simulation and functional analysis using a series of point mutants of hTAS2R16 to identify its binding sites. The docking simulation predicted two candidate binding structures for a salicin-hTAS2R16 complex, and at least seven amino acid residues in transmembrane 3 (TM3), TM5, and TM6 were shown to be involved in ligand recognition. We also identified the probable salicin-hTAS2R16 binding mode using a mutated receptor experiment. This study characterizes the molecular interaction between hTAS2R16 and β-d-glucopyranoside and will also facilitate rational design of bitter blockers.