Polymorphism of the multiple hemoglobins in blood clam Tegillarca granosa and its association with disease resistance to Vibrio parahaemolyticus

Hemoglobin wonders: a fascinating gas transporter dive into molluscs

Hemoglobin (Hb) has been identified in at least 14 molluscan taxa so far. Research spanning over 130 years on molluscan Hbs focuses on their genes, protein structures, functions, and evolution. Molluscan Hbs are categorized into single-, two-, and multiple-domain chains, including red blood cell, gill, and extracellular Hbs, based on the number of globin domains and their respective locations. These Hbs exhibit variation in assembly, ranging from monomeric and dimeric to higher-order multimeric forms. Typically, molluscan Hbs display moderately high oxygen affinity, weak cooperativity, and varying pH sensitivity. Hb's potential role in antimicrobial pathways could augment the immune defense of bivalves, which may be a complement to their lack of adaptive immunity. The role of Hb as a respiratory protein in bivalves likely originated from the substitution of hemocyanin. Molluscan Hbs demonstrate adaptive evolution in response to environmental changes via various strategies (e.g. increasing Hb types, multimerization, and amino acid residue substitutions at key sites), enhancing or altering functional properties for habitat adaptation. Concurrently, an increase in Hb assembly diversity, coupled with a downward trend in oxygen affinity, is observed during molluscan differentiation and evolution. Hb in Protobranchia, Heteroconchia, and Pteriomorphia bivalves originated from separate ancestors, with Protobranchia inheriting a relative ancient molluscan Hb gene. In bivalves, extracellular Hbs share a common origin, while gill Hbs likely emerged from convergent evolution. In summary, research on molluscan Hbs offers valuable insights into the origins, biological variations, and adaptive evolution of animal Hbs.

Immune responses of hemocytes in the blood clam Tegillarca granosa in response to in vivo Vibrio harveyi infection

Aquaculture of the blood clam Tegillarca granosa accounts for approximately 50% of Arcidae (ark shell) production in China. Vibrio infection severely threatens the sustainability of the clam aquaculture industry. Exposure to Vibrio induces an immune response in blood clams. However, the underlying mechanism remains poorly understood. In this study, immune responses of hemocytes in blood clams were detected after Vibrio infection; the immersion method was used in vivo to mimic the clam's natural infection process. After 24 h of exposure to Vibrio infection, the Vibrio load in hemolymph fluid in both the treatment Ⅰ (25,033.33 ± 19,563.11 CFU/mL) and treatment Ⅱ (122,163.33 ± 194,409.49 CFU/mL) groups were significantly higher, than that in the control group (13.67 ± 37.73 CFU/mL) (P < 0.05). Correspondingly, the production of intracellular reactive oxygen species was approximately 1.40 (treatment Ⅰ) and 2.12 (treatment Ⅱ) fold higher than that in the control group (P < 0.05), and the induced DNA damage showed a similar trend (P < 0.05). Vibrio infection also significantly increased lysozyme content, adenosine triphosphate content, and peroxidase isozyme activity, in both the serum and hemocyte lysates (P < 0.05). The expression of immune-associated genes (ABCA3, c-Myc, Caspase 3, and HSP70) was upregulated under infection conditions. The phagocytic activity was approximately 1.99 (treatment Ⅰ) and 2.57 (treatment Ⅱ) fold that in control clams (P < 0.05). In addition, the total hemocyte count and red granulocyte percentage both significantly decreased by approximately 75-90% after Vibrio infection. These results provided novel insights into the mechanism of hemocyte immunity in T. granosa against Vibrio infection, which may aid in the future prevention and control of Vibrio infection in vivo.

Vital Role of Glutamate Dehydrogenase Gene in Ammonia Detoxification and the Association Between its SNPs and Ammonia Tolerance in Sinonovacula constricta

Increasing evidence has revealed accumulated ammonia will cause adverse effects on the growth, reproduction, and survival of aquatic animals. As a marine benthic mollusk, the razor clam Sinonovacula constricta shows better growth and survival under high ammonia nitrogen environment. However, little is known about its adaptation mechanisms to high ammonia stress in an integrated mariculture system. In this study, we analyzed the association between the polymorphism of glutamate dehydrogenase gene (GDH), a key gene involved in ammonia nitrogen detoxification, and ammonia tolerance. The results showed that 26 and 22 single-nucleotide polymorphisms (SNPs) of GDH in S. constricta (denoted as Sc-GDH) were identified from two geographical populations, respectively. Among them, two SNPs (c.323T > C and c.620C > T) exhibited a significant and strong association with ammonia tolerance, suggesting that Sc-GDH gene could serve as a potential genetic marker for molecular marker–assisted selection to increase survival rate and production of S. constricta. To observe the histological morphology and explore the histocellular localization of Sc-GDH, by paraffin section and hematoxylin–eosin staining, the gills were divided into gill filament (contains columnar and flattened cells) and gill cilia, whereas hepatopancreas was made up of individual hepatocytes. The results of immunohistochemistry indicated that the columnar cells of gill filaments and the endothelial cells of hepatocytes were the major sites for Sc-GDH secretion. Under ammonia stress (180 mg/L), the expression levels of Sc-GDH were extremely significantly downregulated at 24, 48, 72, and 96 h (P < 0.01) after RNA interference. Thus, we can speculate that Sc-GDH gene may play an important role in the defense process against ammonia stress. Overall, these findings laid a foundation for further research on the adaptive mechanisms to ammonia–nitrogen tolerance for S. constricta.

Laser-Induced Breakdown Spectroscopy for Rapid Discrimination of Heavy-Metal-Contaminated Seafood Tegillarca granosa

Tegillarca granosa samples contaminated artificially by three kinds of toxic heavy metals including zinc (Zn), cadmium (Cd), and lead (Pb) were attempted to be distinguished using laser-induced breakdown spectroscopy (LIBS) technology and pattern recognition methods in this study. The measured spectra were firstly processed by a wavelet transform algorithm (WTA), then the generated characteristic information was subsequently expressed by an information gain algorithm (IGA). As a result, 30 variables obtained were used as input variables for three classifiers: partial least square discriminant analysis (PLS-DA), support vector machine (SVM), and random forest (RF), among which the RF model exhibited the best performance, with 93.3% discrimination accuracy among those classifiers. Besides, the extracted characteristic information was used to reconstruct the original spectra by inverse WTA, and the corresponding attribution of the reconstructed spectra was then discussed. This work indicates that the healthy shellfish samples of Tegillarca granosa could be distinguished from the toxic heavy-metal-contaminated ones by pattern recognition analysis combined with LIBS technology, which only requires minimal pretreatments.

The anti-hyperplasia of mammary gland effect of protein extract HSS from Tegillarca granosa

Tegillarca granosa Linnaeus, possesses various biological functions and has been used a Chinese traditional medicine more than one century, but there is no report about anti-hyperplasia of mammary gland (HMG) activity of drugs from T. granosa. In this study, we investigated the anti-HMG effect of protein extract named HSS from T. granosa. The HMG model of virgin female Sprague Dawley rats was prepared by injecting estrogen in the thigh muscle of the rats and progestogen consecutively. HMG rats were treated with either HSS or positive control drug by i.g. for 35 consecutive days. In order to evaluate anti-HMG activity of HSS, Changes of nipple height and diameter, serum sex hormones levels, organ indexes and pathologic changes of mammary gland were performed. Body weight, food intake, pathomorphology examination of organs (heart, liver, spleen, lung, kidney), hematological and biochemical analysis were performed to evaluate the toxicity of HSS. HSS could significantly reduce nipples height and diameter, increase P concentration of HMG rat serum, spleen and thymus index, decrease uterus index, and has therapeutic effect on rat HMG and no toxicity at 500mg/kg/day. The anti-HMG mechanism of HSS may be related to AP-2α and P53. HSS has protective and therapeutic effects on HMG rats, and may be a promising agent for treating hyperplasia of mammary glands.

Immunological properties of oxygen-transport proteins: hemoglobin, hemocyanin and hemerythrin

It is now well documented that peptides with enhanced or alternative functionality (termed cryptides) can be liberated from larger, and sometimes inactive, proteins. A primary example of this phenomenon is the oxygen-transport protein hemoglobin. Aside from respiration, hemoglobin and hemoglobin-derived peptides have been associated with immune modulation, hematopoiesis, signal transduction and microbicidal activities in metazoans. Likewise, the functional equivalents to hemoglobin in invertebrates, namely hemocyanin and hemerythrin, act as potent immune effectors under certain physiological conditions. The purpose of this review is to evaluate the true extent of oxygen-transport protein dynamics in innate immunity, and to impress upon the reader the multi-functionality of these ancient proteins on the basis of their structures. In this context, erythrocyte-pathogen antibiosis and the immune competences of various erythroid cells are compared across diverse taxa.

A preliminary study on the antibacterial mechanism of Tegillarca granosa hemoglobin by derived peptides and peroxidase activity

The blood clam, Tegillarca granosa, is one of the few bivalve molluscs containing hemoglobin (Hb). In the present study, we purified two types of T. granosa hemoglobin, Tg-HbI and Tg-HbII, using size exclusion chromatography and measured their antibacterial and peroxidase activities. We also tested antibacterial activities of peptides prepared by trypsin digestion of purified Tg-Hb and reversed-phase high-performance liquid chromatography purification. Purified Tg-HbI and Tg-HbII showed antibacterial activity against Escherichia coli, Pseudomonas putida, Bacillus subtilis, and Bacillus firmus, with differences in minimal inhibitory concentrations (MICs), but lacked antibacterial activity against Vibrio alginolyticus, Vibrio parahaemolyticus, Vibrio harveyi and Staphylococcus aureus. In contrast, 7 Tg-Hb derived peptides exhibited varying degrees of antibacterial activity against V. alginolyticus (MICs: 12-200 μg/ml), V. parahaemolyticus (11-100 μg/ml) and V. harveyi (1-200 μg/ml). The antibacterial activity of Hb derived peptides was confirmed by fluorescence microscopy. In addition, peroxidase activity was detected in Tg-HbI and Tg-HbII. The results indicated that in addition to functioning as a respiratory protein T. granosa hemoglobins likely play a role in host antibacterial defense probably via a peroxidase activity of native molecules and some internal peptides released from the proteins.

Development of Vibrio spp. infection resistance related SNP markers using multiplex SNaPshot genotyping method in the clam Meretrix meretrix

The clam Meretrix meretrix is a commercially important mollusc species in the coastal areas of South and Southeast Asia. In the present study, large-scale SNPs were genotyped by the Multiplex SNaPshot genotyping method among the stocks of M. meretrix with different Vibrio spp. infection resistance profile. Firstly, the AUTOSNP software was applied to mine SNPs from M. meretrix transcriptome, and 323 SNP loci (including 120 indels) located on 64 contigs were selected based on Uniprot-GO associations. Then, 38 polymorphic SNP loci located on 15 contigs were genotyped successfully in the clam stocks with different resistance to Vibrio parahaemolyticus infection (11-R and 11-S groups). Pearson's Chi-square test was applied to compare the allele and genotype frequency distributions of the SNPs between the different stocks, and seven SNP markers located on three contigs were found to be associated with V. parahaemolyticus infection resistance trait. Haplotype-association analysis showed that six haplotypes had significantly different frequency distributions in 11-S and 11-R (P < 0.05). With selective genotyping between 09-R and 09-C populations, which had different resistance to Vibrio harveyi infection, four out of the seven selected SNPs had significantly different distributions (P < 0.05) and therefore they were considered to be associated with Vibrio spp. infection resistance. Sequence alignments and annotations indicated that the contigs containing the associated SNPs had high similarity to the immune related genes. All these results would be useful for the future marker-assisted selection of M. meretrix strains with high Vibrio spp. infection resistance.