Invited review: Modeling milk stability

Novel insights into the stability of milk and milk products during storage and processing result from describing caseins near neutral pH as hydrophilic, intrinsically disordered, proteins. Casein solubility is strongly influenced by pH and multivalent ion binding. Solubility is high at neutral pH or above but decreases as casein net charge approaches zero, allowing a condensed casein phase or gel to form then increases at lower pH. Of particular importance for casein micelle stability near neutral pH is the proportion of free caseins in the micelle (i.e., caseins not bound directly to nanoclusters of calcium phosphate). Free caseins are more soluble and better able to act as molecular chaperones (to prevent casein and whey protein aggregation) than bound caseins. Some free caseins are highly phosphorylated and can also act as mineral chaperones to inhibit the growth of calcium phosphate phases and prevent mineralized deposits from forming on membranes or heat exchangers. Thus, casein micelle stability is reduced when free caseins bind to amyloid fibrils, destabilized whey proteins or calcium phosphate. The multivalent-binding model of the casein micelle quantitatively describes these and other factors affecting the stability of milk and milk protein products during manufacture and storage.

Biophysical and biochemical nature of amorphous protein oligomers determines the strength of immune response and the generation of T-cell memory

Here, we used domain 3 of dengue virus serotype 3 envelope protein (D3ED3), a natively folded globular low-immunogenicity protein, to ask whether the biophysical nature of amorphous oligomers can affect immunogenicity. We prepared nearly identical 30 ~ 50 nm-sized amorphous oligomers in five distinct ways and looked at any correlation between their biophysical properties and immunogenicity. One oligomer type was produced using our SCP tag (solubility controlling peptide) made of 5 isoleucines (C5I). The others were prepared by miss-shuffling the SS bonds (Ms), heating (Ht), stirring (St) and freeze-thaw (FT). Dynamic light scattering showed that all five formulations contained oligomers of approximately identical sizes with hydrodynamic radii (Rh) between 30 and 55 nm. Circular dichroism (cd) indicated that the secondary structure content of oligomers formed by stirring and freeze-thaw was essentially identical to that of the native monomeric D3ED3. The secondary structure content of the Ms showed moderate changes, whereas the C5I and heat-induced (Ht) oligomers exhibited a significant change. The Ms contained D3ED3 with intermolecular SS bonds as assessed by nonreducing size exclusion chromatography (SEC). Immunization in JcL:ICR mice showed that both C5I and Ms significantly increased the anti-D3ED3 IgG titre. Ht, St and FT were only mildly immunogenic, similar to the monomeric D3ED3. Cell surface CD marker analysis by flow cytometry confirmed that immunization with Ms generated a strong central and effector T-cell memory. Our observations indeed suggest that controlled oligomerization can provide a new, adjuvant-free method for increasing a protein's immunogenicity, yielding a potentially powerful platform for protein-based (subunit) vaccines.

Possible mechanisms of polyphosphate-induced amyloid fibril formation of β2-microglobulin

Polyphosphate (polyP), which is found in various microorganisms and human cells, is an anionic biopolymer consisting of inorganic phosphates linked by high-energy phosphate bonds. Previous studies revealed that polyPs strongly promoted the amyloid formation of several amyloidogenic proteins; however, the mechanism of polyP-induced amyloid formation remains unclear. In the present study using β₂-microglobulin (β2m), a protein responsible for dialysis-related amyloidosis, we investigated amyloid formation in the presence of various chain lengths of polyPs at different concentrations under both acidic (pH 2.0 to 2.5) and neutral pH (pH 7.0 to 7.5) conditions. We found that the amyloid formation of β2m at acidic pH was significantly accelerated by the addition of polyPs at an optimal polyP concentration, which decreased with an increase in chain length. The results obtained indicated that electrostatic interactions between positively charged β2m and negatively charged polyPs play a major role in amyloid formation. Under neutral pH conditions, long polyP with 60 to 70 phosphates induced the amyloid formation of β2m at several micromoles per liter, a similar concentration range to that in vivo. Since β2m with an isoelectric point of 6.4 has a slightly negative net charge at pH 7, polyPs were unlikely to interact with β2m electrostatically. PolyPs appear to dehydrate water molecules around β2m under the unfolded conformation, leading to the preferential stabilization of less water-exposed amyloid fibrils. These results not only revealed the pH-dependent mechanism of the amyloid formation of β2m but also suggested that polyPs play an important role in the development of dialysis-related amyloidosis.

Amorphous aggregation of tau in the presence of titanium dioxide nanoparticles: biophysical, computational, and cellular studies

Background

Nanoparticles (NPs) when injected into the body can reach target tissues like nervous system and interact with tau proteins and neurons. This can trigger conformational changes of tau and may affect NP toxicity.

Methods

In this study, we used several biophysical techniques (extrinsic and intrinsic fluorescence spectroscopy, circular dichroism (CD) spectroscopy, ultraviolet (UV)-visible spectroscopy), transmission electron microscopy (TEM) investigations, molecular docking and molecular dynamics studies, and cellular assays [3-(4,5-Dimethylthiazol-2-Yl)-2,5-Diphenyltetrazolium Bromide (MTT) and flow cytometry) to reveal how structural changes of tau protein can change the cytotoxicity of titanium dioxide (TiO₂) NPs against neuron-like cells (SH-SY5Y) cells.

Results

It was shown that TiO₂ NPs result in hydrophilic interactions, secondary and tertiary structural changes, and the formation of amorphous tau aggregates. Conformational changes of tau increased the induced cytotoxicity by TiO₂ NPs. These data revealed that the denatured adsorbed protein on the NP surface may enhance NP cytotoxicity.

Conclusion

Therefore, this study provides useful insights on the NP-protein interactions and discusses how the protein corona can increase cytotoxicity to determine the efficacy of targeted delivery of nanosystems.