Lactoferrin receptors in Gram-negative bacteria: an evolutionary perspective

Iron-based microbial interactions: the role of iron metabolism in the cheese ecosystem

Iron is involved in various microbial metabolisms and interactions and is an essential micronutrient for most microorganisms. This review focuses on the cheese ecosystem, in which iron is sparse (median concentration of 2.9 mg/kg based on a literature survey) and of limited bioavailability due to the presence of various metal-binding agents in the cheese matrix. Cheese microorganisms overcome this low bioavailability of iron by producing and/or importing ferric iron-specific chelators called siderophores. We introduce these siderophores and their specific transporters, which play a key role in ecological interactions and microbial metabolism. We discuss the impact of iron on all the major taxa (fungi, bacteria, and viruses) and functional groups (starters, ripening microorganisms, and pathogens) present and interacting in cheese, from the community to individual levels. We describe the ways in which cheese-ripening microorganisms use iron and the effects of iron limitation on major metabolic pathways, including the tricarboxylic acid (TCA) cycle and amino-acid biosynthesis. The cheese ecosystem is a relevant in situ model for improving our understanding of iron biochemistry and its putative role in microbe-microbe interactions. Yet, this review highlights critical gaps in our understanding of iron's role in cheese from fundamental ecological and biochemical perspectives to applied microbiology, with broader implications for the quality, safety, and organoleptic properties of cheese.

Repurposing pharmaceuticals for Alzheimer's treatment via adjusting the lactoferrin interacting proteins

Alzheimer's disease (AD), the most common neurodegenerative disease in humans, has been a major medical challenge. Lactoferrin (Ltf) in salivary glands might be identified as a potential detectable biomarker in AD and a therapeutic target for AD. Pharmaceutical studies directly addressing this biomarker, though, are scarce. Using a computational strategy for drug repurposing, we explored the proximal neighborhood of Ltf by exploring its interactome and regulatory constellations. We aimed to focus on the discovery of potential therapeutic agents for AD. Based on extensive analytical evaluation comprising structural congruence scales, profiling disease clusters, pathway enrichment analyses as well as molecular docking, SPR, in vivo studies, and immunofluorescence assays, our research identified three candidate repurposed drugs: Lovastatin, SU-11652, and SB-239063. Taken together, these results highlight strong binding affinities of the drug candidates to Ltf. In vitro studies showed that such compounds decrease β-amyloid (Aβ) production by increasing the fluorescence signal emitted by Ltf in N2a-sw cells, and that they act by modulating the expression of amyloidogenic pathway-associated enzymes (BACE1 and APH1α). In addition, in vivo studies showed a concomitant reduction in the expression levels of amyloidogenic pathway-related enzymes (BACE1 or APH1α). Thus, computational studies have focused on Ltf interactions that may recommend drug repurposing strategies and options for AD.

Molecular Characterization and Expression of Lactoferrin Receptor (LfR) in Different Regions of the Brain Responding to Lactoferrin Intervention

Lactoferrin (LF), an iron-binding glycoprotein rich in human milk, promotes neurodevelopment and cognition, but whether it acts through the LF receptor (LfR) and its expression profile in the brain remains unknown. We characterized 972 bp of piglet brain LfR cDNA and found LfR mRNA was expressed all brain regions being highest in the frontal lobe, followed by parietal lobe, brainstem, occipital lobe, cingulate gyrus, subventricular zone, olfactory bulb, hippocampus, amygdala, cerebellum, and thalamus. LfR mRNA and protein in different regions of the brain responded to low (155 mg/kg/day) and high (285 mg/kg/day) LF supplementation of piglets from postnatal days 3 to 38. By postnatal day 39, the low LF diet significantly increased LfR protein expression in the occipital lobe compared to controls, but not the high LF diet. LfR protein in the subventricular zone of the high LF group was 42% and 38% higher than that of the low LF group and controls, respectively. There was a trend for a dose-response relationship between LF intervention and LfR protein expression only in the prefrontal and parietal lobes. LF supplementation significantly improved piglet working memory for a difficult task, which was positively correlated with LfR protein in the prefrontal, parietal, and occipital lobes, but no dose response. Brain LfR responds to dietary LF supplementation, a mechanism by which LF can promote learning and working memory through its receptor. LfR is expressed in the whole brain, and its expression level is anatomic region specific.

Exploring heme and iron acquisition strategies of Porphyromonas gingivalis-current facts and hypotheses

Iron and heme are crucial for pathogenic bacteria living in the human host but are not available in free form due to their binding by iron- and heme-sequestering proteins. Porphyromonas gingivalis causes dysbiosis in the oral microbiome and is considered a keystone pathogen in the onset and progression of periodontal diseases. Its ability to infect and multiply in host cells and its presence in distant tissues and fluids highlights its pathogenic versatility and explains the relationship between periodontal diseases and systemic or neurodegenerative diseases. Porphyromonas gingivalis has evolved specialized mechanisms that allow it to thrive in the host under adverse nutrient-limited conditions. This review presents the updated summary of the mechanisms of iron and heme acquisition by P. gingivalis, with a central role played by gingipains and the unique Hmu system. The potential role of other iron and heme acquisition systems, such as Hus and Iht, indicates the importance of the partially conserved heme biosynthesis pathway, involving homologs of the HemN, HemG, and HemH proteins. In light of increasing antibiotic resistance, difficulties with diagnosis, and drug administration, targeting the mechanisms of heme and iron acquisition of P. gingivalis represents a promising target for developing diagnostic tests, preventive or therapeutic strategies.

How prevalent are lactoferrin receptors in Gram-negative bacteria?

Surface receptors in Gram-negative bacteria that bind and extract iron from the host glycoproteins transferrin (Tf) or lactoferrin (Lf) was discovered 35 years ago in pathogenic Neisseria species and subsequently was discovered in other pathogens of humans and food production animals. These bacterial species reside exclusively on the mucosal surfaces of the respiratory or genitourinary tract of their mammalian host and rely on their host specific Tf and Lf receptors to acquire iron for survival. Since the specificity of the bacterial Tf receptors was shown to be due to selective pressures on the host Tf, their presence in bacteria that reside in both mammals and birds indicates that they arose over 320 million years ago. Once Lf arose in mammals due to a gene duplication event, Lf receptors subsequently arose from Tf receptors. The focus on pathogens for discovery of these receptors has led to a limited understanding of how prevalent the Tf and Lf receptors are in commensal species and raises the question whether they are present in additional bacterial lineages. Since the Lf receptor provides a secondary iron acquisition system plus can provide protection from cationic peptides its presence varies in bacterial lineages.

Mechanisms of host adaptation by bacterial pathogens

The emergence of new infectious diseases poses a major threat to humans, animals, and broader ecosystems. Defining factors that govern the ability of pathogens to adapt to new host species is therefore a crucial research imperative. Pathogenic bacteria are of particular concern, given dwindling treatment options amid the continued expansion of antimicrobial resistance. In this review, we summarize recent advancements in the understanding of bacterial host species adaptation, with an emphasis on pathogens of humans and related mammals. We focus particularly on molecular mechanisms underlying key steps of bacterial host adaptation including colonization, nutrient acquisition, and immune evasion, as well as suggest key areas for future investigation. By developing a greater understanding of the mechanisms of host adaptation in pathogenic bacteria, we may uncover new strategies to target these microbes for the treatment and prevention of infectious diseases in humans, animals, and the broader environment.

Targeting Intracellular Bacteria with Dual Drug-loaded Lactoferrin Nanoparticles

Treatment of microbial infections is becoming daunting because of widespread antimicrobial resistance. The treatment challenge is further exacerbated by the fact that certain infectious bacteria invade and localize within host cells, protecting the bacteria from antimicrobial treatments and the host's immune response. To survive in the intracellular niche, such bacteria deploy surface receptors similar to host cell receptors to sequester iron, an essential nutrient for their virulence, from host iron-binding proteins, in particular lactoferrin and transferrin. In this context, we aimed to target lactoferrin receptors expressed by macrophages and bacteria; as such, we prepared and characterized lactoferrin nanoparticles (Lf-NPs) loaded with a dual drug combination of antimicrobial natural alkaloids, berberine or sanguinarine, with vancomycin or imipenem. We observed increased uptake of drug-loaded Lf-NPs by differentiated THP-1 cells with up to 90% proportion of fluorescent cells, which decreased to about 60% in the presence of free lactoferrin, demonstrating the targeting ability of Lf-NPs. The encapsulated antibiotic drug cocktail efficiently cleared intracellular Staphylococcus aureus (Newman strain) compared to the free drug combinations. However, the encapsulated drugs and the free drugs alike exhibited a bacteriostatic effect against the hard-to-treat Mycobacterium abscessus (smooth variant). In conclusion, the results of this study demonstrate the potential of lactoferrin nanoparticles for the targeted delivery of antibiotic drug cocktails for the treatment of intracellular bacteria.

Lactoferrin, Osteopontin and Lactoferrin–Osteopontin Complex: A Critical Look on Their Role in Perinatal Period and Cardiometabolic Disorders

Milk-derived bioactive proteins have increasingly gained attention and consideration throughout the world due to their high-quality amino acids and multiple health-promoting attributes. Apparently, being at the forefront of functional foods, these bioactive proteins are also suggested as potential alternatives for the management of various complex diseases. In this review, we will focus on lactoferrin (LF) and osteopontin (OPN), two multifunctional dairy proteins, as well as to their naturally occurring bioactive LF–OPN complex. While describing their wide variety of physiological, biochemical, and nutritional functionalities, we will emphasize their specific roles in the perinatal period. Afterwards, we will evaluate their ability to control oxidative stress, inflammation, gut mucosal barrier, and intestinal microbiota in link with cardiometabolic disorders (CMD) (obesity, insulin resistance, dyslipidemia, and hypertension) and associated complications (diabetes and atherosclerosis). This review will not only attempt to highlight the mechanisms of action, but it will critically discuss the potential therapeutic applications of the underlined bioactive proteins in CMD.

Insights from targeting transferrin receptors to develop vaccines for pathogens of humans and food production animals

While developing vaccines targeting surface transferrin receptor proteins in Gram-negative pathogens of humans and food production animals, the common features derived from their evolutionary origins has provided us with insights on how improvements could be implemented in the various stages of research and vaccine development. These pathogens are adapted to live exclusively on the mucosal surfaces of the upper respiratory or genitourinary tract of their host and rely on their receptors to acquire iron from transferrin for survival, indicating that there likely are common mechanisms for delivering transferrin to the mucosal surfaces that should be explored. The modern-day receptors are derived from those present in bacteria that lived over 320 million years ago. The pathogens represent the most host adapted members of their bacterial lineages and may possess factors that enable them to have strong association with the mucosal epithelial cells, thus likely reside in a different niche than the commensal members of the bacterial lineage. The bacterial pathogens normally lead a commensal lifestyle which presents challenges for development of relevant infection models as most infection models either exclude the early stages of colonization or subsequent disease development, and the immune mechanisms at the mucosal surface that would prevent disease are not evident. Development of infection models emulating natural horizontal disease transmission are also lacking. Our aim is to share our insights from the study of pathogens of humans and food production animals with individuals involved in vaccine development, maintaining health or regulation of products in the human and animal health sectors.

Lactoferrin: A glycoprotein that plays an active role in human health

Lactoferrin (Lf), existing widely in human and mammalian milk, is a multifunctional glycoprotein with many functions, such as immune regulation, anti-inflammation, antibacterial, antiviral, and antioxidant. These extensive functions largely attribute to its ability to chelate iron and interfere with the cellular receptors of pathogenic microorganisms and their hosts. Moreover, it is non-toxic and has good compatibility with other supplements. Thus, Lf has been widely used in food nutrition, drug carriers, biotechnology, and feed development. Although Lf has been continuously explored and studied, a more comprehensive and systematic compendium is still required. This review presents the recent advances in the structure and physicochemical properties of Lf as well as clinical studies on human diseases, with the aim of providing a reference for further research of Lf and the development of its related functional products.

Structural and functional insights into iron acquisition from lactoferrin and transferrin in Gram-negative bacterial pathogens

Iron is an essential element for various lifeforms but is largely insoluble due to the oxygenation of Earth’s atmosphere and oceans during the Proterozoic era. Metazoans evolved iron transport glycoproteins, like transferrin (Tf) and lactoferrin (Lf), to keep iron in a non-toxic, usable form, while maintaining a low free iron concentration in the body that is unable to sustain bacterial growth. To survive on the mucosal surfaces of the human respiratory tract where it exclusively resides, the Gram-negative bacterial pathogen Moraxella catarrhalis utilizes surface receptors for acquiring iron directly from human Tf and Lf. The receptors are comprised of a surface lipoprotein to capture iron-loaded Tf or Lf and deliver it to a TonB-dependent transporter (TBDT) for removal of iron and transport across the outer membrane. The subsequent transport of iron into the cell is normally mediated by a periplasmic iron-binding protein and inner membrane transport complex, which has yet to be determined for Moraxella catarrhalis. We identified two potential periplasm to cytoplasm transport systems and performed structural and functional studies with the periplasmic binding proteins (FbpA and AfeA) to evaluate their role. Growth studies with strains deleted in the fbpA or afeA gene demonstrated that FbpA, but not AfeA, was required for growth on human Tf or Lf. The crystal structure of FbpA with bound iron in the open conformation was obtained, identifying three tyrosine ligands that were required for growth on Tf or Lf. Computational modeling of the YfeA homologue, AfeA, revealed conserved residues involved in metal binding.

Targeting bacterial transferrin and lactoferrin receptors for vaccines

A substantial disease burden in vertebrates is due to Gram-negative bacteria that exclusively inhabit the upper respiratory or genitourinary tracts of their hosts and rely on directly acquiring iron from the host iron-binding glycoproteins through surface receptor proteins. The receptors enable these bacteria to proliferate independently from their neighbors on the mucosal surface and during invasive infection of the host. The diversity in these receptors evolved over millions of years of evolution, which thus bodes well for long-lasting vaccine coverage. Experiments in food production animals provide proof of concept for the use of engineered antigens derived from the receptor proteins to prevent colonization and invasive infection in the natural host, strongly supporting development of these vaccines for use in humans.

TbpBY167A-Based Vaccine Can Protect Pigs against Glässer’s Disease Triggered by Glaesserella parasuis SV7 Expressing TbpB Cluster I

Glaesserella parasuis is the etiological agent of Glässer’s disease (GD), one of the most important diseases afflicting pigs in the nursery phase. We analyzed the genetic and immunological properties of the TbpB protein naturally expressed by 27 different clinical isolates of G. parasuis that were typed as serovar 7 and isolated from pigs suffering from GD. All the strains were classified as virulent by LS-PCR. The phylogenetic analyses demonstrated high similarity within the amino acid sequence of TbpB from 24 clinical strains all belonging to cluster III of TbpB, as does the protective antigen TbpBY167A. Three G. parasuis isolates expressed cluster I TbpBs, indicating antigenic diversity within the SV7 group of G. parasuis. The antigenic analysis demonstrated the presence of common epitopes on all variants of the TbpB protein, which could be recognized by an in vitro analysis using pig IgG induced by a TbpBY167A-based vaccine. The proof of concept of the complete cross-protection between clusters I and III was performed in SPF pigs immunized with the TbpBY167A-based vaccine (cluster III) and challenged with G. parasuis SV7, strains LM 360.18 (cluster I). Additionally, pigs immunized with a whole-cell inactivated vaccine based on G. parasuis SV5 (Nagasaki strain) did not survive the challenge performed with SV7 (strain 360.18), demonstrating the absence of cross-protection between these two serovars. Based on these results, we propose that a properly formulated TbpBY167A-based vaccine may elicit a protective antibody response against all strains of G. parasuis SV7, despite TbpB antigenic diversity, and this might be extrapolated to other serovars. This result highlights the promising use of the TbpBY167A antigen in a future commercial vaccine for GD prevention.

Natural resources to control COVID-19: could lactoferrin amend SARS-CoV-2 infectivity?

The world population is still facing the second wave of the COVID-19 pandemic. Such a challenge requires complicated tools to control, namely vaccines, effective cures, and complementary agents. Here we present one candidate for the role of an effective cure and/or complementary agent: lactoferrin. It is the cross-talking mediator between many organs/cellular systems in the body. It serves as a physiological, immunological, and anti-microbial barrier, and acts as a regulator molecule. Furthermore, lactoferrin has receptors on most tissues cells, and is a rich source for bioactive peptides, particularly in the digestive system. In the past months, in vitro and in vivo evidence has accumulated regarding lactoferrin's ability to control SARS-CoV-2 infectivity in different indicated scenarios. Also, lactoferrin or whey milk (of human or other mammal's origin) is a cheap, easily available, and safe agent, the use of which can produce promising results. Pharmaceutical and/or food supplementary formulas of lactoferrin could be particularly effective in controlling the gastrointestinal COVID-19-associated symptoms and could limit the fecal-oral viral infection transmission, through mechanisms that mimic that of norovirus infection control by lactoferrin via induction of intestinal innate immunity. This natural avenue may be effective not only in symptomatic patients, but could also be more helpful in asymptomatic patients as a main or adjuvant treatment.

Development of a non-biased, high-throughput ELISA for the rapid evaluation of immunogenicity and cross-reactivity

Traditional ELISA-based protein analysis has been predicated on the assumption that proteins bind randomly to the solid surface of the ELISA plate polymer (polystyrene or polyvinyl chloride). Random adherence to the plate ensures equal access to all faces of the protein, an important consideration when evaluating immunogenicity of polyclonal serum samples as well as when examining the cross-reactivity of immune serum against different antigenic variants of a protein. In this study we demonstrate that the soluble form of the surface lipoprotein transferrin binding protein B (TbpB) from three different bacterial pathogens (Neisseria meningitidis, Actinobacillus pleuropneumoniae, and Mannheimia haemolytica) bind the ELISA plate in a manner that consistently obscures the transferrin binding face of the proteins' N-lobe. In order to develop a non-biased ELISA where all faces of the protein are accessible, the strong interaction between biotin and avidin has been exploited by adding a biotin tag to these proteins during Escherichia coli-based cytoplasmic expression and utilizing streptavidin or neutravidin coated ELISA plates for protein capture and display. The use of avidin coated ELISA plates also allows for rapid purification of biotin-tagged proteins from crude E. coli lysates, removing the requirement of prior affinity purification of each protein to be included in the ELISA-based analyses. In proof of concept experiments we demonstrate the utility of this approach for evaluating immunogenicity and cross-reactivity of serum from mice and pigs immunized with TbpBs from human and porcine pathogens.

Proof of Concept for Prevention of Natural Colonization by Oral Needle-Free Administration of a Microparticle Vaccine

There has been substantial interest in the development of needle-free vaccine administration that has led to a variety of approaches for delivery through the skin for induction of a systemic immune response. The mucosal administration of vaccines has inherently been needle-free, but the simple application of vaccines on the mucosal surface by itself does not lead to mucosal immunity. Since many important bacterial infections develop after initial colonization of the upper respiratory tract of the host, prevention of colonization could not only prevent infection but also eliminate the reservoir of pathogens that reside exclusively in that ecologic niche. This study was designed to provide proof of concept for a needle-free immunization approach that would reduce or eliminate colonization and prevent infection. In order to accomplish this a microparticle vaccine preparation was delivered just below the oral mucosal epithelial cell layer where it would lead to a robust immune response. A vaccine antigen (mutant transferrin binding protein B) shown to be capable of preventing infection in pigs was incorporated into a polyphosphazene microparticle preparation and delivered by a needle-free device to the oral sub-epithelial space of pigs. This vaccination regimen not only provided complete protection from infection after intranasal challenge by Glaesserella parasuis but also eliminated natural colonization by this bacterium. Notably, the complete prevention of natural colonization was dependent upon delivery of the microparticle preparation below the epithelial layer in the oral mucosa as intradermal or intramuscular delivery was not as effective at preventing natural colonization. This study also demonstrated that a primary immunization in the presence of maternal antibody limited the resulting antibody response but a robust antibody response after the second immunization indicated that maternal antibody did not prevent induction of B-cell memory.