Ethayaraja Mani

Amyloid β (Aβ40) is a class of amyloidogenic proteins known to aggregate into a fibrillar network. The rate of aggregation and fibril yield is sensitive to external energy input, such as shear. In this work, simple shear and shaking experiments are performed on Aβ40 solution using a Couette cell and an orbital shaker, respectively. Experiments show that, under uniform shear, both the mass of fibrils and aggregation rate increase with the shear rate. In the case of orbital shaking, the lag time decreases with the rotational speed of the shaker, but the final fibril mass is the same for all agitation speeds. To explain this contrasting behavior of aggregation kinetics, a population balance model is developed to account for the effect of shear on the aggregation of Aβ. The kinetic model includes primary nucleation, secondary nucleation, elongation, fragmentation, and depolymerization steps. The effect of steady uniform shear is encoded in the depolymerization rate constant (kd), and it is shown that kd decreases with shear rate initially and saturates at high shear rates. A competition between elongation and depolymerization rates yields different equilibrium masses of fibril at different shear rates. The model results agree quantitatively well with experimental data on the rate of aggregation and mass of fibrils as a function of shear rate. The modeling framework can be used to explain the shear rate-dependent aggregation of other amyloidogenic proteins.

Extracellular plaques of amyloid beta (Aβ) fibrils and neurofibrillary tangles are known to be associated with neurological diseases such as Alzheimer's disease. Studies have shown that spherical nanoparticles inhibit the formation of Aβ fibrils by intercepting the nucleation and growth pathways of fibrillation. In this report, gold nanorods (AuNRs) are used to inhibit the formation of Aβ fibrils and the shape-dependent plasmonic properties of AuNRs are exploited to faciliate faster dissolution of mature Aβ fibrils. Negatively charged, lipid (DMPC) stabilized AuNRs inhibit the formation of fibrils due to selective binding to the positevly charged amyloidogenic sequence of Aβ protein. The kinetics of inhibition is characterized by thioflavin T (ThT) fluorescence, transmission electronic microscopy (TEM), atomic force microscopy (AFM), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). An increase in the aspect ratio of DMPC-AuNR in the range of 2.2-4.2 decreased the fibrils content proportionally. Further, the fibrils content is decreased by increasing the concentration of AuNR for all aspect ratios. As AuNR absorb near-infrared (NIR) light and creates a localized hotspot, NIR laser (800 nm) is applied for 2 min to facilitate the thermal dissolution of mature Aβ fibrils. Majority of Aβ fibrils are disintegrated into smaller fragments after exposure to NIR in the presence of AuNR. Thus, the DMPC-AuNRs exhibit a dual effect: inhibition of fibrillation and NIR laser facilitated dissolution of mature amyloid fibrils. This study essentially provides guidelines to design efficient nanoparticle-based therapeutics for neurodegenerative diseases.

Plaques of amyloid beta (Aβ) protein are associated with neurodegenerative diseases, and preventing their formation and dissolution of plaques are essential to the development of therapeutics. In this study, silver triangular nanoplates (AgTNPs) are shown to dissolve mature Aβ fibrils because of their plasmonic photothermal property. Mature Aβ fibrils treated with AgTNPs under near-infrared (NIR)-illuminated conditions are dissolved in less than 1 h, while an equal concentration of silver spherical nanoparticles took about 70 h. The concentration of the fibrils decreased from 10 to 0.3 μM upon treating the amyloid fibrils with AgTNPs under NIR. AgTNPs are also shown to prevent the formation of Aβ fibrils by selective binding to the positively charged amyloidogenic sequence of the Aβ monomer. The kinetics of inhibition by AgTNPs follows the predictions of the detailed kinetic model (Ramesh et al., Langmuir 2018, 34, 4004-4012). The kinetics of dissolution and inhibition are characterized by Congo red/ThT assay, transmission electronic microscopy, atomic force microscopy, and attenuated total reflectance Fourier transform-infrared spectroscopy. Cell viability studies on SH-SY5Y and BE-(2)-C cells using 3-[4,5-dimethy-lthi-azol-2-yl]-2,5-diphenyl-tetrazdium bromide and lactate dehydrogenase assay show that the viability of the cells increased from 33 to 70% on treating the cells with AgTNP-incubated Aβ fibrils compared to the mature Aβ fibrils. The study provides new insights to design plasmonic nanoparticle-based therapeutics to cure neurodegenerative diseases.

Experiments have shown that charged nanoparticles (NP) inhibit, partially or completely, the aggregation of Aβ protein monomers into fibrils. The equilibrium fibril content is found to be inversely proportional to the concentration of NP. In this work, we report a kinetic model for the fibrillation of Aβ protein in the presence of NP. In the model, apart from nucleation, elongation and fragmentation processes, the effect of NP is considered to cause a conformational change to the protein monomer, making the latter incompatible for aggregation. The simulated results explain the growth kinetics of pure Aβ (1-40) protein, and the kinetics in the presence of NP. The NP-monomer interaction considered in the model captures the significant effect of NP on the fibrillation process at a very molar ratio (NP to Aβ monomer) as low as 10-4. The model predictions are compared with two different NP systems, namely, gold and silica NP. The model can be applied to explain the inhibitory effect of other additives such as small molecules, NP, lipids, and surfactants that show a similar inhibition trend for fibril formation of Aβ and other proteins.

Biocompatible and colloidally stable gold nanorods (GNRs) with well-defined plasmonic properties are essential for biomedical and theranostic applications. The as-synthesized GNRs using the seed-mediated method are stabilized by the surfactant, cetyltrimethylammonium bromide (CTAB), which is known for its cytotoxicity in many cell lines. Biocompatible GNRs synthesized using known protocols exhibit some extent of cytotoxicity and colloidal instability because of the incomplete removal of CTAB. We report a facile method for the efficient removal of CTAB molecules with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) phospholipid molecules, which are naturally present in cell membranes. The kinetics of the ligand exchange process is studied using surface-enhanced Raman scattering (SERS) and corroborated with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. From colloidal stability studies using dynamic light scattering (DLS) and UV-Vis spectroscopy, the optimal lipid concentration and duration required for the successful ligand exchange of CTAB by DMPC are reported. Using thermogravimetric analysis, the surface concentration of DMPC on colloidally stable GNRs is found to be approximately 9 molecules per nm2. The 3-(4,5-dimethylthiozol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assays show that the surface-modified DMPC-GNRs have significantly better biocompatibility than those of CTAB-GNRs. Studies on the ligand exchange, colloidal stability and biocompatibility of DMPC-GNRs with aspect ratios ranging from 2.2 to 4.2 demonstrate the robustness of the proposed method. The results provide insights into the important factors to be considered while designing biocompatible GNRs suitable for applications in nanomedicine.

Several kinds of small molecules and nanoparticles (NP) are known for effective inhibition of amyloid fibrillation, which is known as a precursor for many neurodegenerative diseases. We address the role of surface charge of NP in this process from a systematic study using charged NP and surfactants. The fibrillation kinetics is investigated using time-resolved Thioflavin T fluorescence and transmission electron microscopy. It is found that if the protein residues corresponding to the β-sheet (key secondary structure for the formation of fibrils) are charged, addition of oppositely charged NP will inhibit the fibrillation process, irrespective of the material composition of the NP. Molecular dynamics simulations show that electrostatic interaction between the β-sheet forming residues and the NP is responsible for this effect. The finding is further validated by using oppositely charged surfactants: like-charged surfactants and NP do not affect the fibrillation kinetics. Further, even the pre-formed fibrils are dissociated into protein aggregates upon adding oppositely charged NP/surfactants. These results suggest that NP can be used as therapeutic agents even after the fibrils are formed, and that they can be dissolved into soluble aggregates. The inhibitory effect is essentially decided by the charge of the inhibitor, irrespective of the size, shape and material composition. The study provides new insight for the rational design of nanoparticle-based therapeutics, with appropriate surface charge, to inhibit the onset of amyloid β fibrillation and to dissociate the pre-formed fibrils.

The utilization of eco-friendly and sustainable approaches in the synthesis of nanoparticles has gained significant attention due to their potential applications in various fields. In this study, we present a green synthesis method for the synthesis of silver nanoparticles (AgNPs) using the leaf extract of Cichorium intybus. Cichorium intybus commonly known as cichory, a widely available and easily cultivable plant, is rich in phytochemicals with inherent reducing and stabilizing properties. The leaves of Cichorium intybus were collected and authenticated. The aqueous leaf extract of Cichorium intybuswas prepared using cold maceration method and the extract was packedtightly. The Physicochemical parameters and phytochemical screening of extract were assessed. The green synthesis process was conducted by mixing aqueous silver nitrate (AgNO3) with Cichorium intybus leaf extract under controlled reaction conditions. The reduction of Ag+ ions to AgNPs was monitored by changes in the solution's color, confirming the formation of nanoparticles. The resulting AgNPs were characterized using various analytical techniques such as UV-Visible spectroscopy, zeta sizer, zeta potential, scanning electron microscopy(SEM), transmission electron microscopy (TEM). The green synthesis of AgNPs using Cichorium intybus leaf extract showcases a sustainable and environmentally friendly approach to nanoparticle production. This study contributes to the growing the knowledge on green nanotechnology and highlights the potential of Cichorium intybus as a valuable source for nanoparticle synthesis.

Marine snow floc refers to coagulation of microbes and marine debris in the upper ocean layers, bound together by bio-polymers such as transparent exopolymer particles (TEP) secreted by microbes. The stickiness of TEP plays a crucial role in determining the rate of marine snow floc formation. Additionally, the effect of TEP on the size distribution of marine snow influences the sinking velocity of the flocs. Using a surrogate material system, we study the kinetics of marine snow using a custom-built experimental setup, which allows direct measurement of floc size, floc number density, and floc sinking velocity as a function of TEP concentration. By comparing the experimental floc size with Smoluchowski coagulation theory, we obtain stickiness index, which increases with TEP concentration first, reaches maximum around 0.3 g/L of TEP and decreases upon further increase in TEP concentration. The experimental sinking velocity scales with floc size as ws=adb, with b ranging from 0.57 to 0.68. The exponent is slightly higher than that of 0.5 expected in the Stokes limit. This study establishes a clear link between stickiness index, sinking velocity, and polymer concentration, providing valuable insights for modelling of marine snow dynamics in deep ocean conditions. These findings contribute to a better understanding of the kinetics of marine snow formation, essential for predicting carbon sequestration within the biological carbon pump.