Stabilization of protein-loaded starch microgel by polyelectrolytes

Complexation between antimicrobial peptides and polyelectrolytes

As a result of increasing bacterial resistance against antibiotics, we are facing an emerging health crisis, in which 'simple' infections may no longer be treatable. One class of molecules attracting interest in this context is antimicrobial peptides (AMPs), and considerable research efforts have been directed to identifying selective and potent AMPs. In addition, since in vivo delivery of AMPs is challenging, there is an emerging awareness that successful development of AMP therapeutics can be facilitated by careful design of AMPs delivery systems. In the present overview, we discuss polyelectrolyte complexation as a strategy to deliver AMPs. In doing so, key factors for AMP-polyelectrolyte complexation are illustrated for AMP-polyelectrolyte nanoparticle formation, as well as for AMP incorporation in polyelectrolyte microgels and multilayer structures, and consequences of these for functional performance exemplified.

Counterion Exchange in Peptide-Complexed Core-Shell Microgels

The complexation of polyvalent macroions with oppositely charged polyelectrolyte microgels can lead to core-shell structures. The shell is believed to be highly deswollen with a high concentration of counter-macroions. The core is believed to be relatively free of macroions but under a uniform compressive stress due to the deswollen shell. We use cryo-scanning electron microscopy (SEM) with X-ray microanalysis to confirm this understanding. We study poly(acrylic acid) (PAA) microgels which form a core-shell structure when complexed with a small cationic antimicrobial peptide (L5). We follow the spatial distribution of polymer, water, Na counterions, and peptide based on the characteristic X-ray intensities of C, O, Na, and N, respectively. Frozen-hydrated microgel suspensions include buffers of known composition from which calibration curves can be generated and used to quantify both the microgel water and sodium concentrations, the latter with a minimum quantifiable concentration less than 0.048 M. We find that as-synthesized PAA microgels are enriched in Na relative to the surrounding buffer as anticipated from established ideas of counterion shielding of electrostatic charge. The shell in L5-complexed microgels is depleted in Na and enriched in peptide and contains relatively little water. Our measurements furthermore show that shell/core interface is diffuse over a length scale of a few micrometers. Within the limits of detection, the core Na concentration is the same as that in as-synthesized microgels, and the core is free of peptide. The core has a slightly lower water concentration than as-synthesized controls, consistent with the hypothesis that the core is under compression from the shell.

The delivery of sensitive food bioactive ingredients: Absorption mechanisms, influencing factors, encapsulation techniques and evaluation models

Food-sourced bioactive compounds have drawn much attention due to their health benefits such as anti-oxidant, anti-cancer, anti-diabetes and cardiovascular disease-preventing functions. However, the poor solubility, low stability and limited bioavailability of sensitive bioactive compounds greatly limited their application in food industry. Therefore, numbers of carriers were developed for improving their dispersibility, stability and bioavailability. This review addresses the digestion and absorption mechanisms of bioactive compounds in epithelial cells based on several well-known in vitro and in vivo models. Factors such as environmental stimuli, stomach conditions and mucus barrier influencing the utilization efficacy of the bioactive compounds are discussed. Delivery systems with enhanced utilization efficacy, such as complex coacervates, cross-linked polysaccharides, self-assembled micro-/nano-particles and Pickering emulsions are compared. It is a comprehensive multidisciplinary review which provides useful guidelines for application of bioactive compounds in food industry.

Development of bicistronic expression system for the enhanced and reliable production of recombinant proteins in Leuconostoc citreum

The lactic acid bacteria (LAB) Leuconostoc citreum are non-sporulating hetero-fermentative bacteria that play an important role in the fermented food industry. In this study, for the enhanced and reliable production of recombinant proteins in L. citreum, we developed a bicistronic design (BCD) expression system which includes a short leader peptide (1^st cistron) followed by target genes (2^nd cistron) under the control of a single promoter. Using superfolder green fluorescent protein (sfGFP) as a reporter, the functionality of BCD in L. citreum was verified. Further, to improve the expression in BCD, we tried to engineer a Shine-Dalgarno sequence (SD2) for the 2^nd cistron and a promoter by FACS screening of random libraries, and both strong SD2 (eSD2) and promoter (P_710V4) were successfully isolated. The usefulness of the engineered BCD with P_710V4 and eSD2 was further validated using three model proteins—glutathione-s-transferase, human growth hormone, and α-amylase. All examined proteins were successfully produced with levels highly increased compared with those in the original BCD as well as the monocistronic design (MCD) expression system.

Co-loading and intestine-specific delivery of multiple antioxidants in pH-responsive microspheres based on TEMPO-oxidized polysaccharides

In this study, pH-responsive microspheres loaded with multiple antioxidants were developed for intestine-specific delivery and exhibited synergistic activity. They consist of chitosan (CS)-coated microspheres made of TEMPO-oxidized Konjac glucomannan (OKGM) polymers, of which the carboxyl (COO^-) groups are cross-linked via ferric ions (Fe³⁺), allowing the hydrophobic (β-carotene) and hydrophilic (anthocyanins) antioxidants to be simultaneously incorporated. CLSM images showed successful co-encapsulation of β-carotene and anthocyanins. The in vitro release kinetics of co-loaded CS-OKGM microspheres in simulated GI fluids indicated that the microspheres retain the dual antioxidants in an acidic gastric environment and release them at intestinal pH. Free radical scavenging experiments demonstrated that multiple antioxidants loaded into OKGM system had synergistic activity with enhanced stability against heat. The multi-functional CS-OKGM microspheres showed great potential for multiple antioxidants and intestine-specific delivery with enhanced stability.

Controlled uptake and release of lysozyme from glycerol diglycidyl ether cross-linked oxidized starch microgel

A biodegradable microgel system based on glycerol-1,3-diglycidyl ether (GDGE) cross-linked TEMPO-oxidized potato starch polymers was developed for controlled uptake and release of proteins. A series of microgels were prepared with a wide range of charge density and cross-link density. We found both swelling capacity (SWw) and lysozyme uptake at saturation (Γsat) increased with increasing degree of oxidation (DO) and decreasing cross-link density. Microgel of DO100% with a low cross-link density (RGDGE/polymer (w/w) of 0.025) was selected to be the optimum gel type for lysozyme absorption; Γsat increased with increasing pH and decreasing ionic strength. It suggests that the binding strength was the strongest at high pH and low ionic strength, which was recognized as the optimum absorption conditions. The lysozyme release was promoted at low pH and high ionic strength, which were considered to be the most suitable conditions for triggering protein release. These results may provide useful information for the controlled uptake and release of proteins by oxidized starch microgels.

A study of controlled uptake and release of anthocyanins by oxidized starch microgels

Anthocyanins are well-known antioxidants, but they are sensitive to environmental conditions. Herein we used oxidized starch microgel to prevent their early degradation and deliver them to the target place. The aim of this study was to investigate the uptake and the release ability of anthocyanins by the oxidized starch microgels and measure their in vitro gastrointestinal release. The gels were made of oxidized potato starch polymers, which were chemically cross-linked by sodium trimetaphosphate (STMP). In this study, the uptake and release behaviors of anthocyanins by starch microgel were investigated under various pH and salt concentrations. The microgel of high degree of oxidation and high cross-link density had a high uptake capacity for anthocyanins at low pH and salt concentration; 62 mg anthocyanins had been absorbed per gram of dry DO100% (degree of oxidation 100%) microgel at pH 3 with ionic strength 0.05M. The in vitro study of the release was investigated under stimulated gastrointestinal fluid. The anthocyanins were identified and quantified by UV/vis detection. The results indicated that the oxidized starch microgels had a potential for being a carrier system for protecting anthocyanins from degradation in the upper gastric tract and for delivering them to the intestine. This paper provides a good reference for an intestinal-targeted delivery system of vulnerable functional ingredients by oxidized starch microgel.