Development of an Efficient Extraction Method for Harvesting Gymnodimine-A from Large-Scale Cultures of Karenia selliformis

Five Karenia species along the Chinese coast: With the description of a new species, Karenia hui sp. nov. (Kareniaceae, Dinophyta)

Dinoflagellates within the genus Karenia are well known for their potential to cause harmful algal blooms and induce detrimental ecological consequences. In this study, five Karenia species, Karenia longicanalis, Karenia papilionacea, Karenia mikimotoi, Karenia selliformis, and a new species, Karenia hui sp. nov., were isolated from Chinese coastal waters. The new species exhibits the typical characteristics of the genus Karenia, including a linear apical groove and butanoyl-oxyfucoxanthin as the major accessory pigment. It is distinguished from the other Karenia species by a wide-open sulcal intrusion onto the epicone, a conical epicone with an apical crest formed by the rim of the apical groove, and a hunchbacked hypocone. It is most closely related to Karenia cristata, with a genetic divergence of 3.16 % (22 bp out of 883 bp of LSU rDNA). Acute toxicity tests indicated that the five Karenia species from China are all toxic to marine medaka Oryzias melastigma. Karenia selliformis and K. hui were very toxic to O. melastigma, resulting in 100 % mortality within 4 h and 24 h, respectively. Further analysis by high-performance liquid chromatography revealed that four species, K. selliformis, K. longicanalis, K. papilionacea and K. mikimotoi were capable of producing Gymnodimine-A (GYM-A). The highest GYM-A content was in K. selliformis (strain HK-43), in which the value was 889 fg/cell. No GYM-A was detected in the new species K. hui, however and its toxin remains unknown. Below we provide a comprehensive report of the morphology, phylogeny, pigment composition, and toxicity profiles of Karenia species along the Chinese coast. These findings contribute new insights for monitoring of Karenia species, with important toxicological and ecological implications.

The Cyclic Imine Core Common to the Marine Macrocyclic Toxins Is Sufficient to Dictate Nicotinic Acetylcholine Receptor Antagonism

Macrocyclic imine phycotoxins are an emerging class of chemical compounds associated with harmful algal blooms and shellfish toxicity. Earlier binding and electrophysiology experiments on nAChR subtypes and their soluble AChBP surrogates evidenced common trends for substantial antagonism, binding affinities, and receptor-subtype selectivity. Earlier, complementary crystal structures of AChBP complexes showed that common determinants within the binding nest at each subunit interface confer high-affinity toxin binding, while distinctive determinants from the flexible loop C, and either capping the nest or extending toward peripheral subsites, dictate broad versus narrow receptor subtype selectivity. From these data, small spiroimine enantiomers mimicking the functional core motif of phycotoxins were chemically synthesized and characterized. Voltage-clamp analyses involving three nAChR subtypes revealed preserved antagonism for both enantiomers, despite lower subtype specificity and binding affinities associated with faster reversibility compared with their macrocyclic relatives. Binding and structural analyses involving two AChBPs pointed to modest affinities and positional variability of the spiroimines, along with a range of AChBP loop-C conformations denoting a prevalence of antagonistic properties. These data highlight the major contribution of the spiroimine core to binding within the nAChR nest and confirm the need for an extended interaction network as established by the macrocyclic toxins to define high affinities and marked subtype specificity. This study identifies a minimal set of functional pharmacophores and binding determinants as templates for designing new antagonists targeting disease-associated nAChR subtypes.

Apoptosis and oxidative stress of mouse breast carcinoma 4T1 and human intestinal epithelial Caco-2 cell lines caused by the phycotoxin gymnodimine-A

Gymnodimine-A (GYM-A) is a cyclic imine phycotoxin produced by some marine dinoflagellates. It can cause rapid death of mice via intraperitoneal administration and frequently accumulate in shellfish potentially threatening human health. In this study, four different cell lines were exposed to GYM-A for the viability assessment. Results showed that GYM-A was cytotoxic with concentration-dependent pattern to each cell type, with mean IC50 values ranging from 1.39 to 2.79 μmol L-1. Results suggested that the loss of cell viability of 4T1 and Caco-2 cells was attributed to apoptosis. Furthermore, the collapse of mitochondrial membrane potential and caspases activation were observed in the GYM-A-treated cells. Reactive oxygen species (ROS) and lipid peroxides (LPO) levels were markedly increased in 4T1 and Caco-2 cells exposed to GYM-A at 2 μmol L-1, and the oxidative stress in 4T1 cells was more obvious than that in Caco-2 cells. Additionally, unusual ultrastructure impairment on mitochondria and mitophagosomes occurred in the GYM-A-treated cells. These results suggested that an ROS-mediated mitochondrial pathway for apoptosis and mitophagy was implicated in the cytotoxic effects induced by GYM-A. This is the first report to explore the cytotoxic mechanisms of GYM-A through apoptosis and oxidative stress, and it will provide theoretical foundations for the potential therapeutic applications of GYM-A.

Toxicity and potential underlying mechanism of Karenia selliformis to the fish Oryzias melastigma

Karenia selliformis can produce toxins such as gymnodimines, and form microalgal blooms causing massive mortality of marine life such as fish and shellfish, and resulting in serious economic losses. However, there are a few of studies on the toxic effects of K. selliformis on marine organisms and the underlying mechanisms, and it is not clear whether the toxins produced by K. selliformis affect fish survival through the food chain. In this study, a food chain was simulated and composed by K. selliformis-brine shrimp-marine medaka to investigate the possibility of K. selliformis toxicity transmission through the food chain, in which fish behavior, histopathology and transcriptomics changes were observed after direct or indirect exposure (through the food chain) of K. selliformis. We found that both direct and indirect exposure of K. selliformis could affect the swimming behavior of medaka, manifested as decreased swimming performance and increased "frozen events". Meanwhile, exposure to K. selliformis caused pathological damage to the intestine and liver tissues of medaka to different degree. The effect of direct exposure to K. selliformis on swimming behavior and damage to fish tissues was more severe. In addition, K. selliformis exposure induced significant changes in the expression of genes related to energy metabolism, metabolic detoxification and immune system in medaka. These results suggest that toxins produced by K. selliformis can be transferred through the food chain, and that K. selliformis can destroy the intestinal integrity of medaka and increase the absorption of toxins, leading to energy metabolism disorders in fish, affecting the metabolic detoxification capacity of the liver. Our finding provides novel insight into the toxicity of K. selliformis to marine fish.

Stability and Chemical Conversion of the Purified Reference Material of Gymnodimine-A under Different Temperature and pH Conditions

Gymnodimines (GYMs) are a group of fast-acting phycotoxins and their toxicological effects on human beings are still unclear due to the lack of sufficiently well-characterized large quantities of purified toxins for toxicology studies. In this study, a certified reference material (CRM) of GYM-A was prepared from the dinoflagellate Karenia selliformis, followed by multi-step chromatography separation and purification. Subsequently, the stability of GYM-A in methanolic media was evaluated at different temperature (-20, 4, and 20 °C) and pH (3, 5, and 7) conditions for 8 months, and the conversion products of GYM-A were explored by liquid chromatography-high resolution mass spectrometry (LC-HRMS). The results show that the stability of GYM-A decreased with increasing temperature and pH values. The GYM-A was stable during storage at -20 °C regardless of pH, but it decreased rapidly (81.8% ± 9.3%) at 20 °C in pH 7 solution after 8 months. Moreover, the concentrations of GYM-A did not significantly change at all temperatures in solutions with pH 3 (p > 0.05). It is recommended that GYM-A should be stored at low temperature (≤-20 °C) and pH (≤3) conditions for long-term storage in aqueous methanolic media. In addition, two conversion products of GYM-A, tentatively named as GYM-K (m/z 540) and GYM-L (m/z 524), were identified in the samples stored at high levels of pH and temperature. Based on the LC-HRMS data, the hypothetical chemical structures of both converting derivatives were proposed. A useful strategy for long-term storage of GYM-A CRM in aqueous methanolic media was suggested and two hypothesized conversion products of GYM-A were discovered in this study.

The Isolation of Specialty Compounds from Amphidinium carterae Biomass by Two-Step Solid-Phase and Liquid-Liquid Extraction

The two main methods for partitioning crude methanolic extract from Amphidinium carterae biomass were compared. The objective was to obtain three enriched fractions containing amphidinols (APDs), carotenoids, and fatty acids. Since the most valuable bioproducts are APDs, their recovery was the principal goal. The first method consisted of a solid-phase extraction (SPE) in reverse phase that, for the first time, was optimized to fractionate organic methanolic extracts from Amphidinium carterae biomass using reverse-phase C18 as the adsorbent. The second method consisted of a two-step liquid-liquid extraction coupled with SPE and, alternatively, with solvent partitioning. The SPE method allowed the recovery of the biologically-active fraction (containing the APDs) by eluting with methanol (MeOH): water (H₂O) (80:20 v/v). Alternatively, an APD purification strategy using solvent partitioning proved to be a better approach for providing APDs in a clear-cut way. When using n-butanol, APDs were obtained at a 70% concentration (w/w), whereas for the SPE method, the most concentrated fraction was only 18% (w/w). For the other fractions (carotenoids and fatty acids), a two-step liquid-liquid extraction (LLE) method coupled with the solvent partitioning method presented the best results.

Comparative study on the esterification of gymnodimine in different shellfish exposed to the dissolved toxin in seawater

Some lipophilic phycotoxins dissolved in seawater can be accumulated by bivalves via the filtering process. To explore the relationship between the bioaccumulation of gymnodimine-A (GYM-A) and free fatty acids (FFAs) of shellfish, three species of bivalves (venus clam Meretrix meretrix, mussel Mytilus galloprovincialis, and ark shell Anadara kagoshimensis) were exposed to dissolved GYM-A for 7 days in the same seawater system. Results indicated that GYM-A can be accumulated by these bivalves from the dissolved phase and esterified with FFAs reaching over 90% in most tissues of bivalves. Gymnodimine-A and its esters mainly distributed in the gills of shellfish, and the highest concentration of toxins occurred in mussel, followed by ark shell and venus clam. Similar percent of different fatty acid esters occurred in the experimental shellfish, in which the C16:0, C17:0, C18:0, C18:1, C20:1, C20:2, C22:2, and C22:6-GYM-A esters were the main metabolites of GYM-A. The binding capacity of fatty acids and GYM-A varied in different FFAs, which can explain why the C20:1-GYM-A ester dominated the ester profile while C16:0 was the most abundant fatty acid in all samples. Comparing with the FFA profile of shellfish in the control groups, overexpression of some FFAs occurred in the tissues of shellfish exposed to GYM-A in the experimental groups, which suggested that biosynthesis of FFAs was affected by the accumulation and metabolism of GYM-A in bivalves. Multiple fatty acids including some valuably nutritional FFAs such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were consumed in the esterification metabolism of GYM-A, which hinted that the lipid metabolism and nutritional quality of shellfish affected by the contamination of GYMs should be explored and assessed in future works.