Does the ionic distribution in the electrical double layer modify second harmonic scattering?

Surface-specific nonlinear optical techniques are ideally suited to investigate the complex structure of aqueous interfaces. For colloidal particles dispersed in aqueous solutions, interfacial properties can be retrieved with angle-resolved second harmonic scattering (AR-SHS). The mathematical framework of AR-SHS does not require a priori knowledge on the electrostatic distribution in the first few nanometers close to the interface, therefore allowing us to formulate a molecular-level description of the electrical double layer (EDL) based on the experimental data. However, farther away from the interface, an analytical form of the electrostatic potential decay is necessary to account for the distance dependence of the surface electrostatic field propagating into the solution. This requirement is especially important at low ionic strengths, where the electrostatic field is not efficiently screened by counterions. Here, we examine to what extent the analytical form of the electrostatic potential decay impacts the AR-SHS data analysis. We analyze the effect of different functions on the scattering form factors, on the integrated AR-SHS signal intensity, and on the surface parameters extracted from fitting the AR-SHS data. We find that the trends of the surface parameters remain similar regardless of the chosen function, demonstrating the robustness of our approach to establish a molecular-level picture of the EDL. At ionic strengths <10-4M for 100-nm diameter particles, a functional form that physically represents counterions packed more densely in the vicinity of the surface than in the case of the Poisson-Boltzmann distribution has the largest impact, resulting in an overestimation of the obtained surface potential.

Evolution of the electrical double layer with electrolyte concentration probed by second harmonic scattering.

Investigating the electrical double layer (EDL) structure has been a long-standing challenge and has seen the emergence of several sophisticated techniques able to probe selectively the few molecular layers of a solid/water interface. While a qualitative estimation of the thickness of the EDL can be obtained using simple theoretical models, following experimentally its evolution is not straightforward and can be even more complicated in nano- or microscale systems, particularly when changing the ionic concentration by several orders of magnitude. Here, we bring insight into the structure of the EDL of SiO2 nanoparticle suspensions and its evolution with increasing ionic concentration using angle-resolved second harmonic scattering (AR-SHS). Below millimolar salt concentrations, we can successively characterize inner-sphere adsorption, diffuse layer formation, and outer-sphere adsorption. Moreover, we show for the first time that, by appropriately selecting the nanoparticle size, it is possible to retrieve information also in the millimolar range. There, we observe a decrease in the magnitude of the surface potential corresponding to a compression in the EDL thickness, which agrees with the results of several other electroanalytical and optical techniques. Molecular dynamics simulations suggest that the EDL compression mainly results from the diffuse layer compression rather than outer-sphere ions (Stern plane) moving closer to the surface.

Size dependence of second-harmonic scattering from nanoparticles: Disentangling surface and electrostatic contributions.

Polarimetric angle-resolved second-harmonic scattering (AR-SHS) is an all-optical tool enabling the study of unlabeled interfaces of nano-sized particles in an aqueous solution. As the second harmonic signal is modulated by interference between nonlinear contributions originating at the particle's surface and those originating in the bulk electrolyte solution due to the presence of a surface electrostatic field, the AR-SHS patterns give insight into the structure of the electrical double layer. The mathematical framework of AR-SHS has been previously established, in particular regarding changes in probing depth with ionic strength. However, other experimental factors may influence the AR-SHS patterns. Here, we calculate the size dependence of the surface and electrostatic geometric form factors for nonlinear scattering, together with their relative contribution to the AR-SHS patterns. We show that the electrostatic term is stronger in the forward scattering direction for smaller particle sizes, while the ratio of the electrostatic to surface terms decreases with increasing size. Besides this competing effect, the total AR-SHS signal intensity is also weighted by the particle's surface characteristics, given by the surface potential Φ0 and the second-order surface susceptibility χs,2 2. The weighting effect is experimentally demonstrated by comparing SiO2 particles of different sizes in NaCl and NaOH solutions of varying ionic strengths. For NaOH, the larger χs,2 2 values generated by deprotonation of surface silanol groups prevail over the electrostatic screening occurring at high ionic strengths; however, only for larger particle sizes. This study establishes a better connection between the AR-SHS patterns and surface properties and predicts trends for arbitrarily-sized particles.

Map of Enteropathogenic Escherichia coli Targets Mitochondria and Triggers DRP-1-Mediated Mitochondrial Fission and Cell Apoptosis in Bovine Mastitis.

Bovine mastitis seriously affects bovine health and dairy product quality. Escherichia coli is the most important pathogen in the environment and dairy products. Enteropathogenic Escherichia coli (EPEC) is a zoonotic pathogen, which seriously threatens the health of people and dairy cows. We recently reported that E. coli can induce endogenous apoptosis in bovine mammary epithelial cells. However, the mechanism of EPEC-damaged mitochondria and -induced bovine mastitis is unclear. In this study, we found that EPEC can induce DRP-1-dependent mitochondrial fission and apoptosis. This was verified by the application of Mdivi, a DRP-1 inhibitor. Meanwhile, in order to verify the role of the Map virulence factor in EPEC-induced bovine mastitis, we constructed a map mutant, complementary strain, and recombinant plasmid MapHis. In the present study, we find that Map induced DRP-1-mediated mitochondrial fission, resulting in mitochondrial dysfunction and apoptosis. These inferences were further verified in vivo by establishing a mouse mastitis model. After the map gene was knocked out, breast inflammation and apoptosis in mice were significantly alleviated. All results show that EPEC targets mitochondria by secreting the Map virulence factor to induce DRP-1-mediated mitochondrial fission, mitochondrial dysfunction, and endogenous apoptosis in bovine mastitis.

Butyrate-mediated autophagy inhibition limits cytosolic Salmonella Infantis replication in the colon of pigs treated with a mixture of Lactobacillus and Bacillus.

Probiotics as an effective and safe strategy for controlling Salmonella infection are much sought after, while autophagy is a central issue in eliminating intracellular pathogens of intestinal epithelial cells. In this study, an animal model of colitis has been developed by infecting weaned pigs orally with a strain of Salmonella Infantis in order to illuminate the potential efficacy of a mixture of Lactobacillus and Bacillus (CBB-MIX) in the resistance to Salmonella infection by regulating butyrate-mediated autophagy. We found that CBB-MIX alleviated S. Infantis-induced colitis and tissue damage. Autophagy markers ATG5, Beclin-1, and the LC3-II/I ratio were significantly enhanced by S. Infantis infection, while treatment with CBB-MIX suppressed S. Infantis-induced autophagy. Additionally, S. Infantis-induced colonic microbial dysbiosis was restored by this treatment, which also preserved the abundance of the butyrate-producing bacteria and the butyrate concentration in the colon. A Caco-2 cell model of S. Infantis infection showed that butyrate had the same effect as the CBB-MIX in restraining S. Infantis-induced autophagy activation. Further, the intracellular S. Infantis load assay indicated that butyrate restricted the replication of cytosolic S. Infantis rather than that in Salmonella-containing vacuoles. Suppression of autophagy by knockdown of ATG5 also attenuated S. Infantis-induced cell injury. Moreover, hyper-replication of cytosolic S. Infantis in Caco-2 cells was significantly decreased when autophagy was inhibited. Our data demonstrated that Salmonella may benefit from autophagy for cytosolic replication and butyrate-mediated autophagy inhibition reduced the intracellular Salmonella load in pigs treated with a probiotic mixture of Lactobacillus and Bacillus.

Persistent activation of Nrf2 in a p62-dependent non-canonical manner aggravates lead-induced kidney injury by promoting apoptosis and inhibiting autophagy.

INTRODUCTION: Lead (Pb) is an environmental toxicant that poses severe health risks to humans and animals, especially renal disorders. Pb-induced nephrotoxicity has been attributed to oxidative stress, in which apoptosis and autophagy are core events. OBJECTIVES: Nuclear factor erythroid 2-related factor 2 (Nrf2) acts as a major contributor to counteract oxidative damage, while hyperactivation or depletion of Nrf2 pathway can cause the redox imbalance to induce tissue injury. This study was performed to clarify the function and mechanism of Nrf2 in Pb-triggered kidney injury. METHODS AND RESULTS: First, data showed that Pb exposure activates Nrf2 pathway in primary rat proximal tubular cells. Next, Pb-induced Nrf2 activation was effectively regulated by pharmacological modulation or siRNA-mediated knockdown in vitro and in vivo assays. Notably, Pb-triggered cytotoxicity, renal injury and concomitant apoptosis were improved by Nrf2 downregulation, confirming that Pb-induced persistent Nrf2 activation contributes to nephrotoxicity. Additionally, Pb-triggered autophagy blockage was relieved by Nrf2 downregulation. Mechanistically, we found that Pb-induced persistent Nrf2 activation is attributed to reduced Nrf2 ubiquitination and nuclear-cytoplasmic loss of Keap1 in a p62-dependent manner. CONCLUSIONS: In conclusion, these findings highlight the dark side of persistent Nrf2 activation and potential crosstalk among Pb-induced persistent Nrf2 activation, apoptosis and autophagy blockage in Pb-triggered nephrotoxicity.

Single Point Mutation and Its Role in Specific Pathogenicity to Reveal the Mechanism of Related Protein Families.

Pyolysin (PLO) is secreted by Trueperella pyogenes as a water-soluble monomer after forming transmembrane ß-barrel channels in the cell membrane by binding cholesterol. Two significantly conserved residues at domain 1 of PLO are mutated, which provides novel evidence of a relationship between conformational change and interaction with the cell membrane and uncovers the pore formation mechanism of the cholesterol-dependent cytolysin (CDC) family. Moreover, PLO is a special member of the CDCs, which the percentage of sequence identities between PLO and other CDC members is from 31% to 45%, while others are usually from 40% to 70%. It is important to understand that at very low sequence identities, models can be different in the pathogenic mechanisms of these CDC members, which are dedicated to a large number of Gram-positive bacterial pathogens. Our studies, for the first time, located and mutated two different highly conserved structural sites in the primary structure critical for PLO structure and function that proved the importance of these sites. Together, novel and repeatable observations into the pore formation mechanism of CDCs are provided by our findings. IMPORTANCE Postpartum disease of dairy cows caused by persistent bacterial infection is a global disease, which has a serious impact on the development of the dairy industry and brings huge economic losses. As one of the most relevant pathogenic bacteria for postpartum diseases in dairy cows, Trueperella pyogenes can secrete pyolysin (PLO), a member of the cholesterol-dependent cytolysin (CDC) family and recognized as the most important toxin of T. pyogenes. However, the current research work on PLO is still insufficient. The pathogenic mechanism of this toxin can be fully explored by changing the local structure and overall function of the toxin by a previously unidentified single point mutation. These studies lay the groundwork for future studies that will explore the contribution of this large family of CDC proteins to microbial survival and human disease.

Map, but not EspF, induces breast epithelial cell apoptosis through ERK/DRP-1 pathway.

The incidence of bovine mastitis induced by enteropathogenic Escherichia coli (EPEC) in the environment has increased, but the mechanism of effector-Map and -EspF secreted by EPEC in breast epithelial cells is not clear. Therefore, this study focused on Map and EspF to explore the role of these two virulence factors in EPEC-induced bovine mastitis. We established Δmap and/or ΔEspF mutant strains to explore their role in EPEC-induced bovine mastitis. It was found that both Map and EspF could affect the mitochondrial membrane potential, apoptosis, and tight junctions in MAC-T cells. In addition, we also found that Map could affect the ERK signaling pathway. In order to further verify the effect of Map on the ERK signaling pathway, we introduced an ERK inhibitor (PD98059) and pc DNA3.1 plasmid with the map gene. It was found that DRP-1 and apoptosis were no longer affected by Map. In summary, the study implies that E. coli can promote breast epithelial cell apoptosis and destroy tight junctions through Map and EspF. However, Map, but not EspF, induces breast epithelial cell apoptosis through the ERK-DRP-1 pathway.

Lactobacillus rhamnosus Ameliorates Multi-Drug-Resistant Bacillus cereus-Induced Cell Damage through Inhibition of NLRP3 Inflammasomes and Apoptosis in Bovine Endometritis.

Bacillus cereus, considered a worldwide human food-borne pathogen, has brought serious health risks to humans and animals and huge losses to animal husbandry. The plethora of diverse toxins and drug resistance are the focus for B. cereus. As an alternative treatment to antibiotics, probiotics can effectively alleviate the hazards of super bacteria, food safety, and antibiotic resistance. This study aimed to investigate the frequency and distribution of B. cereus in dairy cows and to evaluate the effects of Lactobacillus rhamnosus in a model of endometritis induced by multi-drug-resistant B. cereus. A strong poisonous strain with a variety of drug resistances was used to establish an endometrial epithelial cell infection model. B. cereus was shown to cause damage to the internal structure, impair the integrity of cells, and activate the inflammatory response, while L. rhamnosus could inhibit cell apoptosis and alleviate this damage. This study indicates that the B. cereus-induced activation of the NLRP3 signal pathway involves K+ efflux. We conclude that LGR-1 may relieve cell destruction by reducing K+ efflux to the extracellular caused by the perforation of the toxins secreted by B. cereus on the cell membrane surface.

Exopolysaccharides from Bifidobacterium animalis Ameliorate Escherichia coli-Induced IPEC-J2 Cell Damage via Inhibiting Apoptosis and Restoring Autophagy.

Enteropathogenic Escherichia coli (EPEC) is a common zoonotic pathogen that causes acute infectious diarrhea. Probiotics like Bifidobacterium are known to help prevent pathogen infections. The protective effects of Bifidobacterium are closely associated with its secretory products exopolysaccharides (EPS). We explored the effects of the EPS from Bifidobacterium animalis subsp. lactis (B. lactis) on ameliorating the damage of an intestinal porcine epithelial cell line (IPEC-J2) during EPEC infection. Pretreatment with EPS alleviated EPEC-induced apoptosis through the restoration of cell morphology and the downregulation of protein expressions of cleaved-caspase 8, cleaved-caspase 3, and cleaved-PARP. EPS-mediated remission of apoptosis significantly improved cell viability during EPEC infection. EPEC infection also resulted in impaired autophagy, as demonstrated by decreased expressions of autophagy-related proteins Beclin 1, ATG5, and microtubule-binding protein light chain-3B (LC3B) and the increased expression of p62 through western blot analysis. However, EPS reversed these effects which indicated that EPS promoted autophagosome formation. Furthermore, EPS prevented the lysosome damage induced by EPEC as it enhanced lysosomal acidification and raised lysosome-associated protein levels, thus promoted autophagosome degradation. Our findings suggest that the amelioration of EPEC-induced cell damages by EPS is associated with the limitation of detrimental apoptosis and the promotion of autophagy flux.

Lactobacillus johnsonii L531 Alleviates the Damage Caused by Salmonella Typhimurium via Inhibiting TLR4, NF-κB, and NLRP3 Inflammasome Signaling Pathways.

Salmonella Typhimurium (S. Typhimurium) is an aggressive zoonotic pathogen that causes enteritis and diarrhea. Antibiotic therapy is still the primary method at present. However, the increasing emergence of multi-drug resistant bacteria weakens the therapeutic efficacy of antibiotics. Probiotics have been widely studied as an alternative antibiotic therapy. In this study, we established an IPEC-J2 cell model of S. Typhimurium infection, aiming to determine the protective effect of Lactobacillus johnsonii L531 (L. johnsonii L531) on S. Typhimurium infection. As our data showed, S. Typhimurium infection resulted in a robust inflammatory response demonstrated by promoted protein levels of the inflammatory-related pathway (TLR4, MyD88, p-IκBα, and p-p65), increased cytokine levels of IL-6, IL-1ß, IL-18, and TNF-α, and activated the NLRP3 inflammasome via promoting its assembly. However, L. johnsonii L531 pre-incubation inhibited the activation of the above inflammatory signaling pathways and reduced the expression levels of pro-inflammatory cytokines. In addition, L. johnsonii L531 alleviated the damage of S. Typhimurium to tight junctions ZO-1, Occludin, and Claudin-1. In summary, our findings suggested that L. johnsonii L531 alleviated S. Typhimurium-induced tight junction injury by inhibiting the TLR4/NF-κB/NLRP3 inflammasome signaling pathway.

Lactobacillus rhamnosus GR-1 Prevents Escherichia coli-Induced Apoptosis Through PINK1/Parkin-Mediated Mitophagy in Bovine Mastitis.

Escherichia coli is one of the most important pathogens that cause clinical mastitis in dairy cattle worldwide and lead to severe economic losses. Antibiotics are often used to treat this inflammatory disease; however, antimicrobial resistance and environmental pollution cannot be ignored. Probiotic is the best alternative; however, its mechanisms of action to prevent mastitis remain unclear. Moreover, the role of probiotics in regulating mitophagy, a selective autophagy that maintains mitochondrial quality, needs to be explored. E. coli infection induced NOD-like receptor family member pyrin domain-containing protein 3 (NLRP3) inflammasome assembly, Caspase-1 activation, and apoptosis in MAC-T cells. Infection also resulted in mitochondrial damage and subsequent increase in reactive oxygen species (ROS) production. Moreover, inhibition of ROS release by scavenger N-acetyl-L-cysteine (NAC) abrogated the importance of ROS in NLRP3 assembly and apoptosis in MAC-T cells. Pretreatment with Lactobacillus rhamnosus GR-1 (LGR-1), a probiotic, alleviated E. coli-induced NLRP3 inflammasome activation and apoptosis via ROS inhibition. Besides, E. coli infection inhibited mitophagy while LGR-1 pretreatment augmented PINK1/Parkin-mediated mitophagy activation, which further blocked ROS generation. To explore the effect of LGR-1 in vivo, a mouse mastitis model was established. The results showed that LGR-1 pretreatment had preventive and protective effects on E. coli induced mastitis, and could reduce cytokines levels such as IL-1ß and TNF-α. In accordance with the results in vitro, E. coli can inhibit mitophagy and activate NLRP3 inflammasome and apoptosis, while LGR-1 can weaken the effect of E. coli. Taken together, our data indicated that LGR-1 pretreatment induced PINK1/Parkin-mediated mitophagy that eliminated damaged mitochondria and reduced ROS production and NLRP3 inflammasome activation, which subsequently decreased E. coli-induced apoptosis. To conclude, our study suggests that therapeutic strategies aiming at the upregulation of mitophagy under E. coli-induced mastitis may preserve mitochondrial function and provide theoretical support for the application of probiotics in bovine mastitis.

Salmonella Infantis Delays the Death of Infected Epithelial Cells to Aggravate Bacterial Load by Intermittent Phosphorylation of Akt With SopB.

Salmonella Infantis has emerged as a major clinical pathogen causing gastroenteritis worldwide in recent years. As an intracellular pathogen, Salmonella has evolved to manipulate and benefit from the cell death signaling pathway. In this study, we discovered that S. Infantis inhibited apoptosis of infected Caco-2 cells by phosphorylating Akt. Notably, Akt phosphorylation was observed in a discontinuous manner: immediately 0.5 h after the invasion, then before peak cytosolic replication. Single-cell analysis revealed that the second phase was only induced by cytosolic hyper-replicating bacteria at 3-4 hpi. Next, Akt-mediated apoptosis inhibition was found to be initiated by Salmonella SopB. Furthermore, Akt phosphorylation increased mitochondrial localization of Bcl-2 to prevent Bax oligomerization on the mitochondrial membrane, maintaining the mitochondrial network homeostasis to resist apoptosis. In addition, S. Infantis induced pyroptosis, as evidenced by increased caspase-1 (p10) and GSDMS-N levels. In contrast, cells infected with the ΔSopB strain displayed faster but less severe pyroptosis and had less bacterial load. The results indicated that S. Infantis SopB-mediated Akt phosphorylation delayed pyroptosis, but aggravated its severity. The wild-type strain also caused more severe diarrhea and intestinal inflammatory damage than the ΔSopB strain in mice. These findings revealed that S. Infantis delayed the cells' death by intermittent activation of Akt, allowing sufficient time for replication, thereby causing more severe inflammation.

Interplay between autophagy and apoptosis in lead(II)-induced cytotoxicity of primary rat proximal tubular cells.

Autophagy and apoptosis are two different biological processes that determine cell fates. We previously reported that autophagy inhibition and apoptosis induction are involved in lead(II)-induced cytotoxicity in primary rat proximal tubular (rPT) cells, but the interplay between them remains to be elucidated. Firstly, data showed that lead(II)-induced elevation of LC3-II protein levels can be significantly modulated by 3-methyladenine or rapamycin; moreover, protein levels of Autophagy-related protein 5 (Atg5) and Beclin-1 were markedly up-regulated by lead(II) treatment, demonstrating that lead(II) could promote the autophagosomes formation in rPT cells. Next, we applied three pharmacological agents and genetic method targeting the early stage of autophagy to validate that enhancement of autophagosomes formation can inhibit lead(II)-induced apoptotic cell death in rPT cells. Simultaneously, lead(II) inhibited the autophagic degradation of rPT cells, while the addition of autophagic degradation inhibitor bafilomycin A1 aggravated lead(II)-induced apoptotic death in rPT cells. Collectively, this study provided us a good model to know about the dynamic process of lead(II)-induced autophagy in rPT cells, and the interplay between autophagy and apoptosis highlights a new sight into the mechanism of lead(II)-induced nephrotoxicity.