Overexpression of Leap2 impairs Xenopus embryonic development and modulates FGF and activin signals

Identification of antibacterial activity of LEAP2 from Antarctic icefish Chionodraco hamatus

Liver-expressed antimicrobial peptide 2 (LEAP2) is a small peptide, which is consisted of signal peptide, pro-peptide and the bioactive mature peptide. Mature LEAP2 is an antibacterial peptide with four highly conserved cysteines forming two intramolecular disulfide bonds. Chionodraco hamatus, an Antarctic notothenioid fish that lives in the coldest water, has white blood unlike most fish of the world. In this study, the LEAP2 coding sequence was cloned from C. hamatus, including a 29 amino acids signal peptide and mature peptide of 46 amino acids. High levels of LEAP2 mRNA were detected in the skin and liver. Mature peptide was obtained by chemical synthesis in vitro, displayed selective antimicrobial activities against Escherichia coli, Aeromonas hydrophila, Staphylococcus aureus and Streptococcus agalactiae. Liver-expressed antimicrobial peptide 2 showed bactericidal activity by destroying the cell membrane integrity and robustly combined with bacterial genomic DNA. In addition, overexpression of the Tol-LEAP2-EGFP in zebrafish larva showed stronger antimicrobial activity in C. hamatus than in zebrafish, accompanied by lower bacterial load and expression of pro-inflammatory factors. This is the first demonstration of the antimicrobial activity of LEAP2 from C. hamatus, which is of useful value in improving resistance to pathogens.

Host defence peptide LEAP2 contributes to antimicrobial activity in a mustache toad (Leptobrachium liui)

BACKGROUND

The liver-expressed antimicrobial peptide 2 (LEAP2) is essential in host immunity against harmful pathogens and is only known to act as an extracellular modulator to regulate embryonic development in amphibians. However, there is a dearth of information on the antimicrobial function of amphibian LEAP2. Hence, a LEAP2 homologue from Leptobrachium liui was identified, characterized, and chemically synthesized, and its antibacterial activities and mechanisms were determined.

RESULTS

In this study, LEAP2 gene (Ll-LEAP2) cDNA was cloned and sequenced from the Chong'an Moustache Toad (Leptobrachium liui). The predicted amino acid sequence of Ll-LEAP2 comprises a signal peptide, a mature peptide, and a prodomain. From sequence analysis, it was revealed that Ll-LEAP2 belongs to the cluster of amphibian LEAP2 and displays high similarity to the Tropical Clawed Frog (Xenopus tropicalis)'s LEAP2. Our study revealed that LEAP2 protein was found in different tissues, with the highest concentration in the kidney and liver of L. liui; and Ll-LEAP2 mRNA transcripts were expressed in various tissues with the kidney having the highest mRNA expression level. As a result of Aeromonas hydrophila infection, Ll-LEAP2 underwent a noticeable up-regulation in the skin while it was down-regulated in the intestines. The chemically synthesized Ll-LEAP2 mature peptide was selective in its antimicrobial activity against several in vitro bacteria including both gram-positive and negative bacteria. Additionally, Ll-LEAP2 can kill specific bacteria by disrupting bacterial membrane and hydrolyzing bacterial gDNA.

CONCLUSIONS

This study is the first report on the antibacterial activity and mechanism of amphibian LEAP2. With more to uncover, the immunomodulatory functions and wound-healing activities of Ll-LEAP2 holds great potential for future research.

Molecular cloning and analysis of the ghrelin/GHSR system in Xenopus tropicalis

Ghrelin is a gut-derived peptide with several physiological functions, including feeding, gastrointestinal motility, and hormonal secretion. Recently, a host defense peptide, liver-expressed antimicrobial peptide-2 (LEAP2), was reported as an endogenous antagonist of growth hormone secretagogue receptor (GHS-R). The physiological relevance of the molecular LEAP2-GHS-R interaction in mammals has been explored; however, studies on non-mammals are limited. Here, we report the identification and functional characterization of ghrelin and its related molecules in Western clawed frog (Xenopus tropicalis), a known model organism. We first identified cDNA encoding X. tropicalis ghrelin and GHS-R. RT-qPCR revealed that ghrelin mRNA expression was most abundant in the stomach. GHS-R mRNA was widely distributed in the brain and peripheral tissues, and a relatively strong signal was observed in the stomach and intestine. In addition, LEAP2 was mainly expressed in intestinal tissues at higher levels than in the liver. In functional analysis, X. tropicalis ghrelin and human ghrelin induced intracellular Ca²⁺ mobilization with EC₅₀ values in the low nanomolar range in CHO-K1 cells expressing X. tropicalis GHS-R. Furthermore, ghrelin-induced GHS-R activation was antagonized with IC₅₀ values in the nanomolar range by heterologous human LEAP2. We also validated the expression of ghrelin and feeding-related factors under fasting conditions. After 2 days of fasting, no changes in ghrelin mRNA levels were observed in the stomach, but GHS-R mRNA levels were significantly increased, associated with significant downregulation of nucb2. In addition, LEAP2 upregulation was observed in the duodenum. These results provide the first evidence that LEAP2 functions as an antagonist of GHS-R in the anuran amphibian X. tropicalis. It has also been suggested that the ghrelin/GHS-R/LEAP2 system may be involved in energy homeostasis in X. tropicalis.

Characterization and functional analysis of liver-expressed antimicrobial peptide-2 (LEAP-2) from golden pompano Trachinotus ovatus (Linnaeus 1758)

The liver-expressed antimicrobial peptide-2 (LEAP-2) is an important component of the innate immune defense system and plays an important role in resisting the invasion of pathogenic microorganisms. In this study, LEAP-2 from golden pompano (Trachinotus ovatus) was characterized and its expression in response to Photobacterium damselae was investigated. The full-length LEAP-2 cDNA was 1758 bp, which comprised a 5'-UTR of 250 bp, an ORF of 321 bp, and a 3'-UTR of 1187 bp, encoding 106 amino acids. LEAP-2 consisted of a conserved saposin B domain and four conserved cysteines that formed two pairs of disulphide bonds. The genomic organization of LEAP-2 was also determined and shown to consisted of three introns and two exons. The predicted promoter region of ToLEAP-2 contained several putative transcription factor binding sites. Quantitative real-time (qRT-PCR) analysis indicated that LEAP-2 was ubiquitously expressed in all examined tissues, with higher mRNA levels observed in the muscle, liver, spleen, and kidney. After P. damselae stimulation, the expression level of LEAP-2 mRNA was significantly upregulated in various tissues of golden pompano. In addition, SDS-PAGE showed that the molecular mass of recombinant LEAP-2 expressed in pET-32a was approximately 23 kDa. The purified recombinant protein showed antibacterial activity against Gram-positive and Gram-negative bacteria. Luciferase reporters were constructed for five deletion fragments of different lengths from the promoter region (-1575 bp to +251 bp), and the results showed that L3 (-659 bp to +251 bp) presented the highest activity, and it was therefore defined as the core region of the LEAP-2 promoter. The seven predicted transcription factor binding sites were deleted by using PCR technology, and the results showed that the mutation of the USF transcription factor binding site caused the activity to significantly decrease. The results indicate that golden pompano LEAP-2 potentially exhibits antimicrobial effects in fish innate immunity.