Antimicrobial peptides (AMPs), critical components of the innate immune system, are widely distributed throughout the animal and plant kingdoms. They can protect against a broad array of infection-causing agents, such as bacteria, fungi, parasites, viruses, and tumor cells, and also exhibit immunomodulatory activity. AMPs exert antimicrobial activities primarily through mechanisms involving membrane disruption, so they have a lower likelihood of inducing drug resistance. Extensive studies on the structure-activity relationship have revealed that net charge, hydrophobicity, and amphipathicity are the most important physicochemical and structural determinants endowing AMPs with antimicrobial potency and cell selectivity. This review summarizes the recent advances in AMPs development with respect to characteristics, structure-activity relationships, functions, antimicrobial mechanisms, expression regulation, and applications in food, medicine, and animals. K E Y W O R D S antimicrobial peptides, application, mechanism, structure-activity relationship Med Res Rev. 2019;39:831-859.wileyonlinelibrary.com/journal/med

Poor proteolytic resistance is an urgent problem to be solved in the clinical application of antimicrobial peptides (AMPs), yet common solutions, such as complicated chemical modifications and utilization of d-amino acids, greatly increase the difficulty and cost of producing AMPs. In this work, a set of novel peptides was synthesized based on an antitrypsin/antichymotrypsin hydrolytic peptide structure unit (XYPX) n (X represents I, L, and V; Y represents R and K), which was designed using a systematic natural amino acid arrangement. Of these peptides, 16 with seven repeat units had the highest average selectivity index (GMSI = 99.07) for all of the Gram-negative bacteria tested and remained highly effective in combating Escherichia coli infection in vivo. Importantly, 16 also had dramatic resistance to a high concentration of trypsin/chymotrypsin hydrolysis and exerted bactericidal activity through a membrane-disruptive mechanism. Overall, these findings provide new approaches for the development of antiprotease hydrolytic peptides that target Gram-negative bacteria.

Antimicrobial peptides are an important weapon against invading pathogens and are potential candidates as novel antibacterial agents, but their antifungal activities are not fully developed. In this study, a set of imperfectly amphipathic peptides was developed based on the imperfectly amphipathic palindromic structure R (XRXXXRX)R ( n = 1, 2; X represents L, I, F, or W), and the engineered peptides exhibited high antimicrobial activities against all fungi and bacteria tested (including fluconazole-resistant Candida albicans), with geometric mean (GM) MICs ranging from 2.2 to 6.62 μM. Of such peptides, 13 (I6) (RRIRIIIRIRR-NH) that was Ile rich in its hydrophobic face had the highest antifungal activity (GM = 1.64 μM) while showing low toxicity and high salt and serum tolerance. It also had dramatic LPS-neutralizing propensity and a potent membrane-disruptive mechanism against microbial cells. In summary, these findings were useful for short AMPs design to combat the growing threat of drug-resistant fungal and bacterial infections.

Inflammatory bowel disease (IBD) develops as a result of complicated interactions between genetic susceptibility, excessive innate immunity, and environmental factors, which are mainly related to the gut microbiota. The present study aimed to elucidate the protective effects and underlying mechanisms of a short-chain fatty acid salt, sodium butyrate, on colonic inflammation induced by dextran sulfate sodium (DSS) in mice. Pretreatment with sodium butyrate attenuated colitis, as demonstrated by the decreased disease activity index (DAI), colon length shortening, spleen tumidness, and histopathology scores, while maintaining intestinal barrier integrity, as observed by H&E staining and electron microscopy. 16S rRNA sequence analysis revealed that sodium butyrate caused a remarkable alteration of the gut microbiota. Bacteroides, Lachnospiraceae, the Lachnospiraceae NK4A136 group, and Ruminiclostridium 6 presented dramatic differences after sodium butyrate supplementation. This work verifies that sodium butyrate can improve mouse colitis via microbe–host interactions by regulating the microbial community. Taken together, the findings demonstrate that sodium butyrate shows great potential as a probiotic agent for ameliorating colitis.

Nutritional regulation of endogenous antimicrobial peptide (AMP) expression is considered a promising nonantibiotic approach to suppressing intestinal infection of pathogen. The current study investigated the effects of l-arginine on LPS-induced intestinal inflammation and barrier dysfunction in vivo and in vitro. The results revealed that l-arginine attenuated LPS-induced inflammatory response, inhibited the downregulation of tight junction proteins (TJP) (p < 0.05) by LPS, and maintained intestinal integrity. In porcine intestinal epithelial cells (IPEC-J2), l-arginine obviously suppressed (p < 0.05) the levels of IL-6 (220.63 ± 2.82), IL-8 (333.95 ± 3.75), IL-1β (693.08 ± 2.38), and TNF-α (258.04 ± 4.14) induced by LPS. Furthermore, l-arginine diminished the LPS-induced expression of Toll-like receptor 4 (TLR4) and inhibited activation of TLR4-mediated nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways. Importantly, we proposed a new mechanism that l-arginine had the ability to stimulate the expression of porcine epithelial β-defensins through activating the mammalian target of the rapamycin (mTOR) pathway, which exerts anti-inflammatory influence. Moreover, pBD-1 gene overexpression decreased (p < 0.05) the TNF-α level stimulated by LPS in IPEC-J2 cells (4.22 ± 1.64). The present study indicated that l-arginine enhanced disease resistance through inhibiting the TLR4/NF-κB and MAPK pathways and partially, possibly through increasing the intestinal β-defensin expression.

BackgroundLipid rafts are major structural components in plasma membranes that play critical roles in many biological processes including virus infection. However, few reports have described the relationship between lipid rafts and porcine rotavirus (PRV) infection. In this study, we investigated whether or not the locally high concentrations (3–5 fold) of cholesterol present in lipid rafts are required for PRV infection, and further examined which stages of the infection process are most affected.ResultsWhen cellular cholesterol was depleted by methyl-β-cyclodextrin (MβCD), PRV infectivity significantly declined in a dose-dependent manner. This inhibition was partially reversed upon reintroduction of cholesterol into the system. This was corroborated by the co-localization of PRV with a recombinant, GPI-anchored green fluorescent protein, which functioned as a marker for membranous regions high in cholesterol and indicative of lipid rafts. Changes in virus titer and western blot analyses indicated that depletion of cellular cholesterol with MβCD had no apparent effect on PRV adsorption; however, depletion of cholesterol significantly restricted entry and post-entry of PRV into the cell. Both points of inhibition were restored to near normal levels by the addition of exogenous cholesterol.ConclusionsWe conclude from these studies that membrane-based cholesterol and in particular that localized to lipid rafts, is an indispensable biomolecule for PRV infection, and that cholesterol-based control of the infection process takes place during entry and immediately post-entry into the cell.

Coiled-coil, a basic folding pattern of native proteins, was previously demonstrated to be associated with the specific spatial recognition, association, and dissociation of proteins and can be used to perfect engineering peptide model. Thus, in this study, a series of amphiphiles composed of heptads repeats with coiled-coil structures was constructed, and the designed peptides exhibited a broad spectrum of antimicrobial activities. Circular dichroism and biological assays showed that the heptad repeats and length of the linker between the heptads largely influenced the amphiphile's helical propensity and cell selectivity. The engineered amphiphiles were also found to efficiently reduce sessile P. aeruginosa biofilm biomass, neutralize endotoxins, inhibit the inflammatory response, and remain active under physiological salt concentrations. In summary, these findings are helpful for short AMP design with a highly therapeutic index to treat bacteria-induced infection.

The gastrointestinal tract forms a robust line of defense against invading pathogens through the production of endogenous antimicrobial peptides (AMPs), which are crucial molecules of the innate defense system. Tryptophan could modulate intestinal immunity through promoting the expression of AMPs. However, the precise mechanism needs to be further clarified. In this study, we show that treatment with tryptophan for 24 h triggers (p < 0.05) the expression of porcine β-defensin (pBD) 1 (62.67 ± 3.10 pg/mL) and pBD2 (74.41 ± 1.33 pg/mL) in the porcine intestinal epithelial cells (IPEC-J2) though calcium-sensing receptor (CaSR)-tryptophan metabolic pathways. Meanwhile, tryptophan alleviates (p < 0.05) intestinal inflammation induced by lipopolysaccharide (LPS) through induction of the defensins and activation of the CaSR-AMP-activated protein kinase (AMPK) pathways in vitro and in vivo. Moreover, the activation of CaSR induces the expression of defensins and decreases the levels of IL-1β (75.26 ± 2.74 pg/mL) and TNF-α (449.8 ± 23.31 pg/mL) induced by LPS (p < 0.05). Importantly, tryptophan maintains kynurenine homeostasis through the activation of CaSR during the inflammatory response. To that end, the work identifies a regulatory circuit between CaSR signaling and tryptophan metabolic pathways involved in the tryptophan-trigged AMP expression, which contributes to improving intestinal immune defense.