Inhibition of SARS-CoV-2 Infection in Vero Cells by Bovine Lactoferrin under Different Iron-Saturation States

Despite the rapid mass vaccination against COVID-19, the emergence of new SARS-CoV-2 variants of concern, such as omicron, is still a great distress, and new therapeutic options are needed. Bovine lactoferrin (bLf), a multifunctional iron-binding glycoprotein available in unsaturated (apo-bLf) and saturated (holo-bLf) forms, has been shown to exert broad-spectrum antiviral activity against many viruses. In this study, we evaluated the efficacy of both forms of bLf at 1 mg/mL against infection of Vero cells by SARS-CoV-2. As assessed with antiviral assays, an equivalent significant reduction in virus infection by about 70% was observed when either form of bLf was present throughout the infection procedure with the SARS-CoV-2 ancestral or omicron strain. This inhibitory effect seemed to be concentrated during the early steps of virus infection, since a significant reduction in its efficiency by about 60% was observed when apo- or holo-bLf were incubated with the cells before or during virus addition, with no significant difference between the antiviral effects of the distinct iron-saturation states of the protein. However, an ultrastructural analysis of bLf treatment during the early steps of virus infection revealed that holo-bLf was somewhat more effective than apo-bLf in inhibiting virus entry. Together, these data suggest that bLf mainly acts in the early events of SARS-CoV-2 infection and is effective against the ancestral virus as well as its omicron variant. Considering that there are no effective treatments to COVID-19 with tolerable toxicity yet, bLf shows up as a promising candidate.

Two-Step In Vitro Model to Evaluate the Cellular Immune Response to SARS-CoV-2

The cellular immune response plays an important role in COVID-19, caused by SARS-CoV-2. This feature makes use of in vitro models' useful tools to evaluate vaccines and biopharmaceutical effects. Here, we developed a two-step model to evaluate the cellular immune response after SARS-CoV-2 infection-induced or spike protein stimulation in peripheral blood mononuclear cells (PBMC) from both unexposed and COVID-19 (primo-infected) individuals (Step1). Moreover, the supernatants of these cultures were used to evaluate its effects on lung cell lines (A549) (Step2). When PBMC from the unexposed were infected by SARS-CoV-2, cytotoxic natural killer and nonclassical monocytes expressing inflammatory cytokines genes were raised. The supernatant of these cells can induce apoptosis of A549 cells (mock vs. Step2 [mean]: 6.4% × 17.7%). Meanwhile, PBMCs from primo-infected presented their memory CD4+ T cells activated with a high production of IFNG and antiviral genes. Supernatant from past COVID-19 subjects contributed to reduce apoptosis (mock vs. Step2 [ratio]: 7.2 × 1.4) and to elevate the antiviral activity (iNOS) of A549 cells (mock vs. Step2 [mean]: 31.5% × 55.7%). Our findings showed features of immune primary cells and lung cell lines response after SARS-CoV-2 or spike protein stimulation that can be used as an in vitro model to study the immunity effects after SARS-CoV-2 antigen exposure.

Conformational analysis of Pneumococcal Surface Antigen A (PsaA) upon zinc binding by fluorescence spectroscopy

PsaA (pneumococcal surface antigen A) is a S. pneumoniae virulence factor that belongs to the metal transport system. The Manganese PsaA binding has been associated with oxidative stress resistance becoming a pivotal element in the bacteria virulence. It has been shown that Zinc inhibits the Manganese acquisition and promotes bacteria toxicity. We have performed a PsaA conformational analysis both in the presence (Zn-rPsaA) and in the absence of Zinc (free-rPsaA). We performed experiments in the presence of different Zinc concentrations to determine the metal minimum concentration which induced a conformational change. The protein in free and Zn-binding condition was also studied in pH ranging 2.6-8.0 and in temperature ranging 25oC-85oC. pH experiments showed a decrease of fluorescence intensity only in acidic medium. Analysis of the heat-induced denaturation demonstrated that Zinc-binding promoted an increase in melting temperature from 55oC (free-rPsaA) to 78.8oC (Zn-rPsaA) according to fluorescence measurements. In addition, the rPsaA stabilization by Zinc was verified through analysis of urea and guanidine hydrochloride denaturation. Data showed that Zinc promoted an increase in the rPsaA stability and its removal by EDTA can lead to a PsaA intermediate conformation. These findings can be considered in the development of vaccines containing PsaA as antigen.

The p53 core domain is a molten globule at low pH: functional implications of a partially unfolded structure

p53 is a transcription factor that maintains genome integrity, and its function is lost in 50% of human cancers. The majority of p53 mutations are clustered within the core domain. Here, we investigate the effects of low pH on the structure of the wild-type (wt) p53 core domain (p53C) and the R248Q mutant. At low pH, the tryptophan residue is partially exposed to the solvent, suggesting a fluctuating tertiary structure. On the other hand, the secondary structure increases, as determined by circular dichroism. Binding of the probe bis-ANS (bis-8-anilinonaphthalene-1-sulfonate) indicates that there is an increase in the exposure of hydrophobic pockets for both wt and mutant p53C at low pH. This behavior is accompanied by a lack of cooperativity under urea denaturation and decreased stability under pressure when p53C is in acidic pH. Together, these results indicate that p53C acquires a partially unfolded conformation (molten-globule state) at low pH (5.0). The hydrodynamic properties of this conformation are intermediate between the native and denatured conformation. ¹H-¹⁵N HSQC NMR spectroscopy confirms that the protein has a typical molten-globule structure at acidic pH when compared with pH 7.2. Human breast cells in culture (MCF-7) transfected with p53-GFP revealed localization of p53 in acidic vesicles, suggesting that the low pH conformation is present in the cell. Low pH stress also tends to favor high levels of p53 in the cells. Taken together, all of these data suggest that p53 may play physiological or pathological roles in acidic microenvironments.

Cognate DNA stabilizes the tumor suppressor p53 and prevents misfolding and aggregation

The tumor suppressor protein p53 is a nuclear protein that serves as an important transcription factor. The region responsible for sequence-specific DNA interaction is located in its core domain (p53C). Although full-length p53 binds to DNA as a tetramer, p53C binds as a monomer since it lacks the oligomerization domain. It has been previously demonstrated that two core domains have a dimerization interface and undergo conformational change when bound to DNA. Here we demonstrate that the interaction with a consensus DNA sequence provides the core domain of p53 with enhanced conformational stability at physiological salt concentrations (0.15 M). This stability could be either increased or abolished at low (0.01 M) or high (0.3 M) salt concentrations, respectively. In addition, interaction with the cognate sequence prevents aggregation of p53C into an amyloid-like structure, whereas binding to a nonconsensus DNA sequence has no effect on p53C stability, even at low ionic strength. Strikingly, sequence-specific DNA binding also resulted in a large stabilization of full-length p53, whereas nonspecific sequence binding led to no stabilization. The effects of cognate DNA could be mimicked by high concentrations of osmolytes such as glycerol, which implies that the stabilization is caused by the exclusion of water. Taken together, our results show an enhancement in protein stability driven by specific DNA recognition. When cognate DNA was added to misfolded protein obtained after a pressurization cycle, the original conformation was mostly recovered. Our results may aid the development of therapeutic approaches to prevent misfolded species of p53.