Transcriptome Assembly of Asian Buffalo Leech Hirudinaria Manillensis and An Accurate Identification of Genes Involved in Anticoagulant Biosynthesis

doi:10.21203/rs.3.rs-67619/v1

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Research article

Transcriptome Assembly of Asian Buffalo Leech Hirudinaria Manillensis and An Accurate Identification of Genes Involved in Anticoagulant Biosynthesis

https://doi.org/10.21203/rs.3.rs-67619/v1

This work is licensed under a CC BY 4.0 License

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posted 04 Sep, 2020

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Background: The Asian buffalo leech Hirudinaria manillensis is a valuable animal widely used in traditional Chinese medicine to cure blood-clotting. However, although the content of the anticoagulant genes in H. manillensis were identified, their actual expression profile remains undetermined. Herein, in this study we conducted transcriptome analysis on this species to address this subject.

Results: A high qualified accurate gene expression profile of H. manillensis transcripts was obtained. By identifying genes upregulated during feeding, anticoagulant-related genes and signaling pathways were identified. The assembled Unigenes were compared in various mainstream databases, and a total of 16,682 Unigenes received annotation information. In addition, the Unigenes were evaluated in terms of length distribution, GC content, and expression level. The data showed good sequencing quality and high reliability. A total of 155 anticoagulant-related genes were found in this study, including those involved in different types of degradation of the extracellular matrix and inhibition of platelet aggregation.

Conclusions：Substantial transcriptome information was obtained by transcriptome sequencing of H. manillensis. This information should help to provide a further molecular theoretical basis for functional gene analysis, genomics, genetic diversity analysis, and molecular marker development of H. manillensis and for the anticoagulation-related genes of this species and its medicinal value as an anticoagulant.

Epigenetics & Genomics

Hirudinaria manillensis

Transcriptome sequencing

comparative transcriptomic analysis

Anticoagulation genes

Differential gene expression

Hirudinaria manillensis (common name Asian buffalo leech, Poecilobdella manillensis) is a leech of the Arhynchobdellida from the order Hirudinea. It is also a type species of the family Poecilobdella. These large bloodsucking leeches live in paddies, ditches, rivers, or ponds and survive by feeding on the blood of mammals, which has high nutritional value. They also exhibit clinical anticoagulant, thrombolytic, and antiplatelet activities. They are animals with medicinal value and have great potential for further uses because of their effects on aggregation, lowering blood lipids, and improving blood circulation [1].

In recent years, a new generation of RNA sequencing technology (RNA-seq) has changed the way research is performed away from the model of traditional molecular biology. This technology provides an important means of analyzing gene expression levels, and performing differential gene expression screening, functional gene mining, and evolutionary analysis [2–7]. Transcriptome sequencing can enrich our understanding of the leech genome. For example, as described in reports published in 2013 and 2016, Kvist et al. used 454 pyrosequencing to obtain transcriptome data from the salivary secretions of Haemadipsa interrupta [8] and Pontobdella macrothela, revealing the correlations of anticoagulants contained in these two leeches and exploring the evolution of these anticoagulants in different leeches.

Leeches have become increasingly widely used in medical applications due to their natural and high-efficiency anticoagulant capacity [9–12]. However, to date, research on H. manillensis has only focused on its classification, breeding, and exploration of the level of anticoagulant activity. The lack of transcriptome and genome-related research has restricted our understanding of this important medicinal animal and medicinal research on it. In view of this, we carried out transcriptome sequencing of H. manillensis, the results of which can supplement the information in genetic databases on this species and strongly promote related research on other leeches. The identified anticoagulant-related genes can also be investigated by others. Related research on bloodsucking animals provides a reference for promoting the development and utilization of medicines by humans.

Transcriptome assembly

A total of 41.80 Gb of clean data was obtained from the transcriptome sequencing analysis of six transcriptome samples of H. manillensis. The clean data of each sample reached 6.88 Gb, the expected coverage exceeded 200×, and the Q20 base percentage was over 94.90%. The Q30 base percentage was over 88.83%. Clean reads were subjected to filtering out of the joints with the joints, as well as the removal of low-quality reads (unknown N > 1% and quality values Q < 15 reads), so there was no need to further use Trimmomatic or remove other impurities. Sequencing statistics of all six transcriptome samples are shown in Table 1.

Table 1

Data on *H. manillensis* transcriptome sequencing results
Sample ID	Raw Reads	Clean Reads	Raw Bases (Gb)	Clean Bases (Gb)	Effective Rate(%)	Read Length	Q20 (%)	Q30 (%)	GC (%)
ST1	46,812,410	45,953,050	7.02	6.89	98.16	150×2	97.70	94.19	43.74
ST2	47,683,582	46,840,394	7.15	7.03	98.23	150×2	97.76	94.29	43.51
ST3	48,116,230	47,844,864	7.58	7.11	93.77	150×2	97.43	93.69	43.05
AF1	47,116,220	46,275,578	7.07	6.94	98.22	150×2	97.82	94.44	42.65
AF2	46,678,946	45,885,834	7.00	6.88	98.30	150×2	97.45	93.66	43.44
AF3	48,826,824	47,301,362	7.55	7.13	94.43	150×2	94.90	88.83	42.66

Note: Sample ID: sample name ST refers to before blood sucking, while AF refers to after blood sucking. Effective rate: The amount of valid data, the ratio of clean reads to raw reads. Q20: The number of reads achieving a sequencing error quality value of less than 1%. Q30: The number of reads achieving a sequencing error quality value of less than 0.1%.

Functional annotation of genes

To obtain complete functional annotations, all assembled single gene sequences were used as search queries in various databases, and the assembled Unigene uses GO (Gene Ontology), KEGG (Kyoto Encyclopedia of Genes and Genomes), KOG (Clusters of Orthologous Groups), P-fam (Protein family), Swiss-Prot (a manually annotated and reviewed protein sequence database), and Nr (NCBI Non-redundant Protein Sequences) to understand the gene types and expression. The notes on each database are shown in Table 2; according to this table, a total of 16,682 Unigenes received annotation information, of which for 9397 information was posted to the Unigenes in the GO database; 7838 in the KEGG database; 11,534 in the KOG database; 14,273 Unigene in the P-fam database; whilst 15,933 Unigene in the Swiss-Prot database and 13,461 Unigenes annotated to the Nr database, accounting for 56.33%, 46.98%, 69.14%, 85.56%, 95.51% and 80.69% of the total number of annotated Unigenes respectively. The clean reads of each sample were sequence-matched with the designated reference genome. Based on the alignment results, alternative splicing prediction analysis, gene structure optimization analysis, and new gene discovery were performed, and 992 new genes were discovered, of which 394 were functionally annotated. The structure of 7,592 genes was optimized(table S1).

Table 2

Gene function annotation statistics
Database	Annotated number	Percentage (%)
COG	5837	34.99
GO	9397	56.33
KEGG	7838	46.98
KOG	11,534	69.14
P-fam	14,273	85.56
Swiss-Prot	15,933	95.51
eggNOG	14,604	87.54
Nr	13,461	80.69
All Annotated	16,682	100

Note: Annotation database: Functional annotation database; Annotated number: Number of genes annotated to the corresponding database; Percentage (%): Percentage of genes annotated in the database as a percentage of total genes.

KOG, GO, and KEGG analyses

Classification of KOG functions

The H. manillensis Unigenes were annotated into the KOG database and classified based on the orthologous relationship. A total of 11,534 Unigenes were annotated in this database and are involved in 25 functional categories. There was a large number of Unigene annotations with only general functional predictions. This may be because the current database and transcriptome data of leeches are very limited. Among the KOG functional categories of H. manillensis of all Unigenes, the top ten most involved in the number of Unigenes show signal transduction mechanisms. These categories include signal transduction mechanisms; General function prediction only; Post-translational modification, protein turnover chaperones; Function unknown; Intracellular trafficking, secretion, and vesicular transport; Inorganic ion transport and metabolism; RNA processing and modification; Cytoskeleton; and Translation, Ribosome structure and biogenesis, with corresponding numbers of Unigenes of 2184, 2127, 1183, 871, 856, 608, 502, 499, 471, and 464 respectively(Table S2). Among them, 856 Unigenes with unknown function were found. We speculate that these sequences may be new genes unique to H. manillensis.

Classification of GO functions

A total of 9,397 Unigenes were annotated in the GO database and divided into three categories: cellular components, molecular functions, and biological processes (Figure 2). In the 17 functional categories of the main category cellular component (table S2), the main annotations of the Unigenes were cell, cell part, and organelle components. In the 15 functional categories of the main category molecular function, the main annotations of the Unigenes were binding, catalytic activity, and transporter activity. Finally, in the 16 functional categories in the main category of biological process, the main annotations of the Unigenes were metabolic processes, cellular processes, and single-organism processes.

KEGG pathway enrichment analysis

The obtained H. manillensis Unigene sequences were analyzed in the KEGG database for comprehensive metabolic pathway analysis. A total of 7,838 Unigenes were annotated in this database, which participate in 253 KEGG pathways (table S2). The top ten pathways most strongly associated with the Unigenes were Spliceosome (199 Unigenes); Purine metabolism (196 Unigenes); Ubiquitin-mediated proteolysis (178 Unigenes); RNA transport (175 Unigenes); Endocytosis (158 Unigenes); Protein processing in endoplasmic (151 Unigenes); Ribosome (135 Unigenes); Neuroactive ligand-receptor interaction Ribosome (131 Unigenes); Pyrimidine metabolism (123 Unigenes); and Lysosome (122 Unigenes).

Analysis of differentially expressed genes of H. manillensis before and after bloodsucking

DEGs screening

The screening of differentially expressed genes (DEGs) before and after vaccination of H. manillensis was carried out in accordance with strict screening criteria (FDR ≤ 0.01 and FC ≥ 2). The screening results obtained are shown in Figure 4. The results show that there are 3048 DEGs (table S3) between before and after bloodsucking（ST and AF）. Compared with the levels in the ST group, the expression levels of 2020 genes were upregulated in AF, while the expression levels of 1028 genes were downregulated. After the feeding of H. manillensis, the number of upregulated genes was higher than that of the downregulated genes. This indicates that the bodily functions of this species do not decrease to reduce consumption after a single feeding, and more genes are instead activated to participate in metabolic mechanisms. The identified DEGs should provide an abundance of molecular information for further exploration of the bloodsucking habits of H.manillensis.

GO function analysis of DEGs

The selected DEGs were annotated with GO functions to reveal the associations between DEGs and certain biological functions. The results showed that a total of 1210 DEGs were annotated in the GO data (table S3). The GO annotation information results of the DEGs are shown in Figure 5. According to these results, DEGs were classified into 48 functional categories in the three major categories of biological processes, cellular components, and molecular functions. Within the category of biological processes, involving 16 GO function categories, the differentially expressed genes were particularly associated with metabolic processes, cellular processes, and single-organism processes, while the remaining DEGs were mainly involved in response to stimulus, among others. Within the category of cellular component, involving 17 GO function categories, the categories of membrane, membrane part, cell, and cell part had the most differentially expressed genes. Within the 15 functional categories of molecular function, the differentially expressed genes were mainly involved in binding, catalytic activity, and transporter activity. The remaining DEGs were mainly involved in enzyme regulatory activity and electron carrier activity, among others. From this, we can conclude that metabolic activity, protein synthesis activity, and stress response of H. manillensis change significantly between before and after blood sucking.

Then, the study enriched the GO of the differential gene in this process, the results of which are shown in Figure 6. In the results of the GO term enrichment of H. manillensis, the main identified categories were protein degradation, plasma membrane, serine endopeptidase, response to wounding and cell redox homeostasis. These uniquely enriched GO terms are related to damage repair and inflammation of the body. These results show that there are diverse anti-inflammatory and analgesic effects on the host in H. manillensis. It is speculated that these may be related to the size of the wound and the physiological characteristics of the bloodsucking.

Functional analysis of KEGG pathways associated with DEGs

Using the KEGG database, pathway enrichment analysis on the DEGs was performed. The results revealed a total of 889 DEGs that participate in 173 metabolic pathways (table S3). There was a total of 20 KEGG pathways with significant enrichment, including Metabolism of xenobiotics by cytochrome, Apoptosis, Glutathione metabolism, Vitamin B6 metabolism, VEGF signaling pathway, Phenylalanine and so on. The statistical results are shown in Table 3.

For the KEGG pathway results, this study found that, among the KEGG pathways enriched with the differentially expressed genes of H. manillensis, the particularly well-represented pathways included Neuroactive ligand-receptor interaction, Dorsoventral axis formation, cysteine and methionine metabolism. Most of these genes are not directly or only indirectly related to anticoagulation, but are related to developmental regulation. Therefore, it is speculated that the activated metabolism of H. manillensis is the main genetic change that occurs after feeding, and there is relatively limited regulation of anticoagulation-related genes.

Table 3

KEGG pathways with significant enrichment of differentially expressed genes
Pathway-term	Gene_number
Metabolism of xenobiotics by cytochrome Apoptosis	6 5
Glutathione metabolism	13
Vitamin B6 metabolism	2
VEGF signaling pathway	2
Ubiquinone and other terpenoid-quinone biosynthesis	3
Starch and sucrose metabolism	6
Phenylalanine, tyrosine and tryptophan biosynthesis	3
Phenylalanine metabolism	4
Nitrogen metabolism Neuroactive ligand-receptor interaction MAPK signaling pathway Glyoxylate and dicarboxylate metabolism Drug metabolism-cytochrome P450 Dorso-ventral axis formation DNA replication Cysteine and methionine metabolism Biosynthesis of unsaturated fatty acids Arginine and proline metabolism Adipocytokine signaling pathway	6 20 8 7 4 10 7 11 5 11 3

Note: Pathway-term: KEGG path name; Gene number: Number of genes enriched

Functional gene mining related to bloodsucking characteristics of H. manillensis

Screening of anticoagulant genes in H. manillensis

The P-fam protein family were annotated with antithrombin collected by querying the interproscan website [13, 14]. All genes annotated to the same P-fam family were then screened as anticoagulant-related genes in H. manillensis. The results showed that there were 155 anticoagulant-related gene sequences in the H. manillensis genome. These genes were categorized using anticoagulant and anticoagulant-related gene types (analgesic and anti-inflammatory, inhibition of platelet, extracellular matrix degradation, and direct anticoagulation). The results are shown in Table 4. The number of anticoagulant and anticoagulant-related loci in H. manillensis suggests the possible anticoagulant effects and medicinal value.

Table 4

Classification of anticoagulant and anticoagulant genes in *H. manillensis*
Functional classifications	Gene description	Obtained gene number
Analgesic and anti-inflammatory	Antistasin family	34
	Interleukin-17	2
	Kunitz/Bovine pancreatic trypsin inhibitor domain	6
	Hemerythrin HHE cation binding domain Macin	3 2
	Spider toxin	1
	Potato inhibitor I family	17
Extracellular matrix degradation	Inter-alpha-trypsin inhibitor heavy chain C-terminus Matrixin	1 21
Inhibition of platelets	Apyrase	8
Inhibition of platelets	Destabilase Disintegrin	8 51
Anticoagulant	Hirudin	1

Note: Functional classification: Functional annotation classification; Obtained gene number: Number of genes belonging to this classification

Anticoagulant-related genes of H. manillensis

Antistasin family-related genes

The antistasin family of proteins is a class of cysteine-rich serine protease inhibitors that can markedly inhibit the coagulation process, albeit through different mechanisms of action [15-17]. Antistasin can also respond to host inflammatory responses, exerting an anti-inflammatory effect. Among the genes related to the antistasin family identified in this study, 10 related genes were upregulated and 5 were downregulated upon blood sucking in H. manillensis. This indicates that the antistasin protein family plays an important role in the anticoagulant process after the blood sucking of H. manillensis.

ATP diphosphate hydrolase (apyrase)-related genes

ATP diphosphate hydrolase (apyrase) is an antiplatelet factor. The mechanism by which apyrase in the saliva of bloodsucking arthropods works is as follows: ADP present in the host after tissue damage promotes platelet activation, but apyrase can hydrolyze ATP and ADP to AMP and inorganic phosphorus, which hinders platelet aggregation [18-22]. This process makes it difficult for a bitten host to form a thrombus, which facilitates the feeding of bloodsucking animals.

In this study, three apyrase-related genes were shown to be downregulated after blood sucking by H. manillensis and one was downregulated. This indicates that apyrase participates in the anticoagulation process after H. manillensis bloodsucking.

Destabilase-related genes

Destabilase acts as a thrombolytic agent in leech; it cleaves peptide bonds and promotes fibrinolysis, thereby inhibiting coagulation activity [23-26]. As a multifunctional enzyme, destabilase can also exert antibacterial function. In addition, the salivary glands of the leech can also secrete other kinds of enzyme, which together with destabilase play a bacteriostatic role.

This study identified three genes related to destabilase that were upregulated after the blood sucking of H. manillensis and two that were downregulated. The results show that destabilase plays a role in the anticoagulant and antibacterial processes upon the sucking of blood by H. manillensis.

Disintegrin-related genes

Disintegrin contains a sperm-glycoside-aspartic acid sequence or a lysine-aspartate sequence and is rich in cysteine. It binds to the glycoprotein fibrinogen receptor on the platelet membrane, competitively antagonizing the binding of fibrin to platelets, and prevents platelet activation and changes in glycoprotein receptor conformation, blocking the binding of receptors to multiple ligands. In this way, it inhibits the final common pathway of platelet aggregation and blocks the formation of a thrombus.

This study identified 15 genes related to disintegrin that were upregulated after blood sucking by H. manillensis and 9 that were downregulated. This indicates that disintegrin plays an important role in the anticoagulation process after the blood sucking of this species.

Gene analysis of bloodsucking characteristics of H. manillensis

Finally, 155 genes related to anticoagulation were screened, corresponding to 13 active substances, including antistasin, apyrase, destabilase, and disintegrin. According to their functions and effects, these genes are classified into those involved in platelet inhibition and those directly encoding anticoagulants. Among them, the expression level of the antistasin family, which has both anticoagulant and anti-inflammatory effects, was generally increased. In terms of the expression trends of genes related to the inhibitors of platelet aggregation apyrase, destabilase, and disintegrin, these were upregulated. At the same time, the expression levels of related genes involved in the anti-inflammatory response of host wounds also showed an upward trend. This shows that, in the process of blood sucking, H. manillensis produces a variety of active anticoagulant substances, and directly or indirectly, these together inhibit the host’s blood coagulation process, accompanied by an anti-inflammatory response. Taken together, these findings indicate that the anticoagulant process is a complex biological pathway in which multiple substances cooperate and interact.

Upon consuming blood, bloodsucking animals can also take in infectious bacteria from the surface of the wound and immune proteins from the blood of the host. Therefore, they need to effectively protect themselves against bacterial invasion and immune protein attack. Destabilase has a strong inhibitory effect on bacteria. Hyaluronidase also acts as an active bacteriostatic protein, which dissolves the envelope of bacteria and forms antibodies, effectively inhibiting the activity of various bacteria and fungi. Some of the functional genes associated with the immune response, such as those annotated as hyaluronidase, were shown to be upregulated after the blood sucking of H. manillensis .

After bloodsucking animals ingest host blood, the hemoglobin in the blood tends to hydrolyze to produce a large amount of hemoglobin. Heme is a potent oxidant that produces hydroxyl radicals with excellent oxidizing ability. Studies have shown that heme can cause DNA damage in mitochondria and alter the expression of apoptotic proteins in these organelles. Hemoglobin also causes a certain degree of damage to the genome. In H. manillensis, hemoglobin is likely to cause great damage during the process of digesting blood. Therefore, this species needs to cope with the oxidative stress generated during blood digestion. Citelli et al. [15] asserted that a variety of proteins with oxidoreductase activity, such as superoxide dismutase, glutathione S-transferase, and thioredoxin, are involved in this antioxidant mechanism. In the process of blood sucking by animals such as ticks, the stress response and non-specific immunity play important roles. After the blood sucking of H. manillensis, the expression of related genes involved in the antioxidant process was found to be upregulated and the antioxidant capacity was significantly enhanced.

Upon blood sucking, H. manillensis needs to produce a large amount of active anticoagulant substances to ensure the fluidity of the host blood. At the same time, it also produces antibacterial and immune-related substances to protect against foreign substances, and proteins having oxidoreductase activity are produced by antioxidation to cope with oxidative stress generated by hemoglobin during blood digestion.

Functional analysis of KEGG pathway in anticoagulation-related DEGs

We found 155 anticoagulant-related genes based on the transcriptome data of H. manillensis and explored their related pathways. The Notch signaling pathway, nicotinate, and nicotinamide metabolism associated with anticoagulant genes are involved in cell growth. It is of great significance to help to detect the occurrence and development of tumors and to treat them in a targeted manner by excavating these pathways.

(1) Notch signaling pathway

In the KEGG database annotation, there are 48 Unigene annotations in the Notch signaling pathway, with three DEGs participating in the pathway. The Notch signaling pathway is important for cell-to-cell communication, which regulates cell differentiation, apoptosis, proliferation, and morphogenesis. It is also critical for the growth and development of organisms [27-32]. The Notch signaling pathway is mainly composed of Notch-related receptors, Notch-associated ligands, Notch downstream signal transduction molecules, and nuclear response factors. TACE (ADAM17) is a key enzyme in the activation of the Notch signaling pathway and has a significant limiting effect on its activation [33-35, 39]. Simultaneous activation of ADAM17 induces GPIbα digestion [36], and platelet membrane glycoprotein (GP) Ibα digestion reduces the expression of platelet surface functional receptors, resulting in weakened platelet adhesion. This affects the formation of blood clots and inhibits the aggregation function of platelets, being a recognized negative regulatory mechanism of platelets. After the feeding of H. manillensis , the expression of TACE-related gene (ADAM 17-like protease) is upregulated, which inhibits platelet aggregation, blocks thrombus formation, affects the activation of the Notch signaling pathway, and participates in the anticoagulant process.

Notch-mediated signaling plays a key role in the development of the cardiovascular system and cardiovascular disease, and is closely related to the regulation of immune system function, pancreatic cancer, and medulloblastoma [37-39]. Notch signaling may play different roles in different environments, times, or cell lines, and its over- or under-expression may have a negative impact on the organism. Therefore, an in-depth understanding of Notch signaling and its interaction with various pathways can provide a foundation for more research on the treatment of cardiovascular diseases and tumors, and for carrying out targeted and efficient clinical treatment.

(2) Nicotinate and nicotinamide metabolism

ATP diphosphate hydrolase (Apyrase) blocks platelet aggregation. The apyrase-related gene detected in this study is involved in the nicotinate and nicotinamide metabolism pathway, and for three enzymes there was upregulated gene expression after vaccination with H. manillensis, while for one enzyme there was downregulated gene expression.

In the KEGG database annotation, there are 27 Unigene annotations on the Notch signaling pathway, with four DEGs participating in the pathway. Nicotinamide phosphoribosyltransferase (Nampt) is the rate-limiting enzyme of this pathway [40-42]; it also has physiological functions such as promoting angiogenesis, anti-apoptosis, participating in the body’s inflammatory response, and promoting the proliferation, differentiation, and maturation of various cells. After the feeding of H. manillensis, apyrase-related genes are involved in the anticoagulant process, inhibiting platelet aggregation, and downregulating the expression of Nampt-related genes, affecting the niacin and nicotinamide metabolic pathways.

The concentration of Nampt in the blood also significantly increases in many tumor patients, so an inhibitor of the activity of this enzyme is an anti-tumor drug with great potential for clinical application [41-43]. However, in this context, there are still many problems to be resolved. The role of anticoagulation genes related to H. manillensis before and after blood sucking is discussed to provide a basis for the better application of tumor treatment.

SNP analysis

Single-nucleotide polymorphism(SNP）is mainly a DNA sequence polymorphism caused by DNA replication, a genetic marker formed by a single-nucleotide variation in the genome. These variations have become commonly used as molecular markers, and in disease diagnosis, paternity testing, plant breeding, and genetic mapping [44, 45]. By identifying potential SNP sites, it is possible to analyze whether these sites affect the expression level of a gene of interest or the particular protein that is produced.

SNPs can be divided into two types: transitions and transversions. Depending on the number of alleles at the SNP site, the SNP can be divided into a homozygous SNP site (only one allele) or a heterozygous SNP site (two or more alleles). There is a difference in the proportion of heterozygous SNPs in different species. SNP molecular markers were developed from H. manillensis transcriptome data. In the obtained 17,051 Unigenes of H. manillensis transcripts, an average of 154,399 SNP sites were found, of which 109,917 were transitions, accounting for 71.19%, and 44,482 were transversions, accounting for 28.81% (Table 5).

Table 5

SNP statistics
Sample ID	SNP Number	Genic SNP	Intergenic SNP	Transition	Transversion	Heterozygosity
ST1	147115	124889	22226	71.41%	28.59%	51.76%
ST2	169399	140997	28402	70.70%	29.30%	49.96%
ST3	132107	111179	20928	71.54%	28.46%	51.54%
AF1	194328	159091	35237	70.21%	29.79%	52.70%
AF2	173142	145623	27519	70.70%	29.30%	51.13%
AF3	110302	94053	16249	72.57%	27.43%	47.29%

Note：Sample ID：Sample analysis number; SNP Number: Total number of SNP sites; Genic SNP: Total number of SNP sites in the gene region; Intergenic SNP: Total number of SNP sites in the intergenic region; Transition: The percentage of the number of conversion-type SNP sites within the total number of SNP sites; Transversion: The percentage of the number of SNP sites of the transversion type within the total number of SNP sites; Heterozygosity: The percentage of heterozygous SNP sites in the total number of SNP sites.

H. manillensis from the order Hirudinea is a bloodsucking leech. It has a wide range of feeding habits, a large body size, and sucks a large amount of blood. We used the Illumina Hiseq sequencing platform to sequence the transcriptome of this species. We obtained a total of 41.80 Gb of clean data for analysis, with 16,682 annotated Unigenes. We used the six databases GO, KEGG, KOG, P-fam, Swiss-Prot, and Nr to annotate the H. manillensis transcript Unigenes, which corresponded to 9397, 7838, 11,534, 14,273, 15,933, and 13,461, respectively, and found 155 anticoagulant-related genes. Using the KEGG database, we also performed pathway enrichment analysis on the DEGs. Finally, we identified a total of 889 DEGs participating in 173 metabolic pathways. We explored the anticoagulant-related genes and pathways based on H. manillensis transcriptome data. After sucking blood, H. manillensis produces active anticoagulant substances, and antibacterial and immune-related substances. The protein having oxidoreductase activity produces an oxidative stress by hemoglobin during blood digestion by antioxidation.

Transcriptomes are important resources for genetic diversity analysis, comparative genomics, and potential molecular marker-assisted selection. SNP molecular markers were developed from H. manillensis transcriptome data. In the obtained 17,051 Unigenes of H. manillensis transcripts, an average of 154,399 SNP sites were found, of which 109,917 were transitions, accounting for 71.19%; and 44,482 were transversions, accounting for 28.81%. Among all variant types, G/A and C/T occurred most frequently. The 154,399 SNP sites identified in this transcriptome should be a valuable resource as molecular markers in H. manillensis.

The transcriptome dataset of H. manillensis is valuable for further research on functional gene analysis and molecular marker development of this species. At the same time, it provides a theoretical molecular basis for the study of anticoagulant-related genes and the anticoagulant medicinal value of H. manillensis.

Material

All leeches collected in this study were in vivo. At least 20 individuals of H. manillensis were collected from Hechi City, Guangxi Province. They were kept in mineral water and brought back to the laboratory. Then, each individual was subjected to morphological identification, and after species determination they were selected for the experiment and reared in a dark and ventilated place.

RNA extraction, detection, library construction, and sequencing

The RNA required for transcriptome construction was extracted using Qiagen kit (QIAGEN RNeasy mini kit). First, the live leeches were directly frozen in liquid nitrogen, then pulverized in a mortar, and a small amount of RNAiso lysate was added and centrifuged immediately. After the precipitate had been obtained, a sufficient amount of lysate was added and digested for at least 2 h. Next, total RNA was extracted from the lysed sample in accordance with the standard procedure on the kit. The quality of the final extracted RNA was accurately quantified using 1% agarose gel electrophoresis, qubit, and an Agilent 2100 bioanalyzer. Since the transcriptome was designed for two expression states per species, which were set to pre-feeding and 3 h after feeding, for each transcriptome sequencing sample, the total RNA of an individual was extracted, which was stored in liquid nitrogen and used for subsequent sequencing.

The transcriptome sequencing was carried out by Beijing Baimaike Biotech Co., Ltd. For transcriptome sequencing, two RNA libraries were constructed for H. manillensis using the Hiseq 4000 platform. Library construction used oligomagnetic beads to physically enrich mRNA; the mRNA was then eluted and the random six-mer primers were reverse-transcribed to form a cDNA library. The cDNA library was then subjected to ultrasonication, fragment recovery (300 bp), end repair, poly-A addition, ligation, library recovery, and PCR preamplification. After the library quality test had been passed, bridge PCR was carried out for chip fixation, and PE 150 bp (PE 150) was sequenced. The generated data were then used for subsequent analysis.

Functional annotation of genes

The NR[46],Swiss-Prot[47]༌GO[48]༌COG[49]༌KOG[50]༌Pfam[51] and KEGG[52] gene annotations were used for Blast[53] alignment using the downloaded databases, and the E-value threshold was set to 1e − 01. The KEGG pathway map was obtained using KAAS[54].

Screening and analysis of anticoagulant genes in H. manillensis

A number of records related to anticoagulant-related proteins and polypeptides in leech are available for download. By downloading all of these previously reported leeches with anticoagulant function and protein and polypeptide sequences associated with anticoagulation, from NCBI's protein. DESeq[55] was used for differential expression analysis between sample groups to obtain the differentially expressed gene set between the two biological conditions

SNP detection

In this study, GATK[56] software was used to identify the single base mismatch between the sequencing sample and the reference genome based on the comparison result of TopHat2 of reads of each sample and the reference genome sequence, so as to identify potential SNP loci.Then it can be analyzed whether these SNP sites affect the expression level of genes or the type of protein products. Meanwhile, SnpEff [57] were by annotated variation (SNP) and predict variation effects.

NR:non-redundant protein sequence database;Swiss-Prot:A manually annotated, non-redundant protein sequence database;GO:Gene Ontology database;COG:The database of Clusters of Orthologous Groups of proteins;KOG:The database of Clusters of Protein homology;Pfam:The database of Homologous protein family;KEGG:The database of Kyoto Encyclopedia of Genes and Genomes;DEGs:differentially expressed genes; Nampt: Nicotinamide phosphoribosyltransferase ;SNP: Single-nucleotide polymorphism

Acknowledgements

Not applicable.

Funding

This work was supported financially by the Excellent Doctor Innovation Project of Shaanxi Normal University (S2015YB03), Fundamental Research Funds for the Central Universities (2019CSLZ009), and Fundamental Research Funds for the Central Universities (GK201604008; GK201702010; GK201701006;). In addition, this work was partly supported by the National Natural Science Foundation of China (no. 31372250).

Availability of data and materials

The RNA sequence dataset supporting the present results is available in the repository of NCBI Sequence Read Archive (SRA) with the following accession Number: SRX6035209 ~ SRX6035214.

Authors’ contributions

S-Q. X, and H-H. T conceived the study and designed the experiments. NNA and J-H L performed sequencing and bioinformatics analysis, C-X F, L-W and D-L. G analyzed the data. C-X F and D-L. G wrote the manuscript. S-Q. X and H-H. T revised the manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participate

No permission was required for Shaanxi Normal University employees to perform insect collections.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests

Author details

College of Life Sciences, Shaanxi Normal University, Xi’an, China

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posted 04 Sep, 2020

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Transcriptome Assembly of Asian Buffalo Leech Hirudinaria Manillensis and An Accurate Identification of Genes Involved in Anticoagulant Biosynthesis

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Version 1

Abstract

Figures

Background

Results

Transcriptome assembly

Functional annotation of genes

KOG, GO, and KEGG analyses

Classification of KOG functions

Classification of GO functions

KEGG pathway enrichment analysis

Analysis of differentially expressed genes of H. manillensis before and after bloodsucking

DEGs screening

GO function analysis of DEGs

Functional analysis of KEGG pathways associated with DEGs

Functional gene mining related to bloodsucking characteristics of H. manillensis

Screening of anticoagulant genes in H. manillensis

Anticoagulant-related genes of H. manillensis

Antistasin family-related genes

ATP diphosphate hydrolase (apyrase)-related genes

Destabilase-related genes

Disintegrin-related genes

Gene analysis of bloodsucking characteristics of H. manillensis

Functional analysis of KEGG pathway in anticoagulation-related DEGs

(1) Notch signaling pathway

(2) Nicotinate and nicotinamide metabolism

SNP analysis

Discussion

Materials And Methods

Material

RNA extraction, detection, library construction, and sequencing

Functional annotation of genes

Screening and analysis of anticoagulant genes in H. manillensis

SNP detection

Abbreviations

Declarations

Acknowledgements

Funding

Availability of data and materials

Authors’ contributions

Ethics approval and consent to participate

Consent for publication

Competing interests

Author details

References

Supplementary Files

Status:

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