The saliva of the medicinal leech Hirudo medicinalis contains a wide range of biologically active compounds, including multiple anticoagulants. Previously, we identified a novel cysteine-rich anticoagulant protein (CRA) from leech saliva and produced it recombinantly in Escherichia coli, demonstrating its potential as a basis for new anticoagulant drugs. In this study, we developed an optimized procedure for scalable production and purification of recombinant CRA. The purified protein was analyzed for common contaminants originating from E. coli, such as endotoxins, bacterial proteins, and DNA, and its anticoagulant properties were evaluated using standard clotting assays. Across three independent experiments, the yield of purified CRA ranged from 3.7 to 5.5 mg per liter of bacterial culture, with impurity levels per milligram of protein ranging from 7.1–31.2 ng of bacterial proteins, 1.2–15.1 ng of DNA, and 60–1445 EU of endotoxins. The purified CRA displayed electrophoretic and chromatographic homogeneity and retained strong anticoagulant activity. Additionally, a truncated form of CRA lacking the C-terminal region was produced and characterized. This variant lost membrane affinity and showed altered activity profiles, with higher thrombin time activity but reduced prothrombin time and activated partial thromboplastin time activities compared with the full-length protein.

The growing threat of infectious diseases requires novel therapeutics with different mechanisms of action. Antimicrobial peptides (AMPs), which are crucial for innate immunity, are a promising research area. The medicinal leech (Hirudo medicinalis) is a potential source of bioactive AMPs that are vital while interacting with microorganisms. This study aims to investigate the antimicrobial properties of peptides found in the H. medicinalis genome using a novel high-throughput screening method based on the expression of recombinant AMP genes in Escherichia coli. This approach enables the direct detection of AMP activity within cells, skipping the synthesis and purification steps, while allowing the simultaneous analysis of multiple peptides. The application of this method to the first identified candidate AMPs from H. medicinalis resulted in the discovery of three novel peptides: LBrHM1, NrlHM1 and NrlHM2. These peptides, which belong to the lumbricin and macin families, exhibit significant activity against E. coli. Two fragments of the new LBrHM1 homologue were synthesised and studied: a unique N-terminal fragment (residues 1–23) and a fragment (residues 27–55) coinciding with the active site of lumbricin I. Both fragments exhibited antimicrobial activity in a liquid medium against Bacillus subtilis. Notably, the N-terminal fragment lacks homologues among previously described AMPs.

Blood-sucking organisms produce various anticoagulant proteins that prevent blood clotting in their prey. Even in well-studied species like Hirudo medicinalis, many such proteins remain unidentified. We previously described a novel cysteine-rich anticoagulant (CRA), a distant homolog of antistasin. Later, we discovered another, much larger homolog in the medicinal leech. Its amino acid sequence is also highly cysteine-rich. Analysis of cysteine patterns showed four antistasin-like domain motifs, with one of them strongly disrupted. Since both antistasin and CRA contain two such domains, the new protein represents a duplicated antistasin-like structure. We cloned its cDNA, expressed the recombinant protein in Escherichia coli, purified it by metal-chelate chromatography, refolded it, and tested its anticoagulant properties. Using standard clinical assays—activated partial thromboplastin time, prothrombin time, and thrombin time—we found that the protein inhibited coagulation in all tests, though to varying degrees. These findings suggest that different antistasin-like anticoagulants in the leech enable it to block both intrinsic and extrinsic coagulation pathways, while hirudin inhibits the final step of clot formation. The combination of different anticoagulant proteins allows the leech to effectively prevent the prey’s blood from clotting during feeding.

Background/Objectives: The global rise in antibiotic resistance necessitates the development of novel antimicrobial agents. Antimicrobial peptides (AMPs), key components of innate immunity, are promising candidates. This study aimed to develop novel therapeutic peptides with enhanced properties through the mutagenesis of natural AMPs and high-throughput screening. Methods: We constructed mutant libraries of three broad-spectrum AMPs—melittin, cecropin, and Hm-AMP2—using mutagenesis with partially degenerate oligonucleotides. Libraries were expressed in Escherichia coli, and antimicrobial activity was assessed through bacterial growth kinetics and droplet serial dilution assays. Candidate molecules were identified by DNA sequencing, and the most promising variants were chemically synthesized. Antimicrobial activity was determined by minimal inhibitory concentration (MIC) against E. coli and Bacillus subtilis, while cytotoxicity was evaluated in human Expi293F cells (IC₉₀) viability. The therapeutic index was calculated as the ratio of an AMP’s cytotoxic concentration to its effective antimicrobial concentration. Results: Mutant forms of melittin (MR1P7, MR1P8) showed significantly reduced cytotoxicity while retaining antimicrobial activity. Cecropin mutants exhibited reduced efficacy against E. coli, but variants CR2P2, CR2P7, and CR2P8 gained activity against Gram-positive bacteria. Mutagenesis of Hm-AMP2 generally decreased activity against E. coli, though two variants (A2R1P5 and A2R3P6) showed retained or enhanced efficacy against B. subtilis while maintaining low cytotoxicity. Conclusions: The proposed strategy successfully generated peptides with improved therapeutic profiles, including reduced toxicity or a broader spectrum of antimicrobial activity, despite not improving all parameters. This approach enables the discovery of novel bioactive peptides to combat antibiotic-resistant pathogens.