Tau- and α-synuclein-targeted gold nanoparticles: applications, opportunities, and future outlooks in the diagnosis and therapy of neurodegenerative diseases.

The use of nanomaterials in medicine offers multiple opportunities to address neurodegenerative disorders such as Alzheimer's and Parkinson's disease. These diseases are a significant burden for society and the health system, affecting millions of people worldwide without sensitive and selective diagnostic methodologies or effective treatments to stop their progression. In this sense, the use of gold nanoparticles is a promising tool due to their unique properties at the nanometric level. They can be functionalized with specific molecules to selectively target pathological proteins such as Tau and α-synuclein for Alzheimer's and Parkinson's disease, respectively. Additionally, these proteins are used as diagnostic biomarkers, wherein gold nanoparticles play a key role in enhancing their signal, even at the low concentrations present in biological samples such as blood or cerebrospinal fluid, thus enabling an early and accurate diagnosis. On the other hand, gold nanoparticles act as drug delivery platforms, bringing therapeutic agents directly into the brain, improving treatment efficiency and precision, and reducing side effects in healthy tissues. However, despite the exciting potential of gold nanoparticles, it is crucial to address the challenges and issues associated with their use in the medical field before they can be widely applied in clinical settings. It is critical to ensure the safety and biocompatibility of these nanomaterials in the context of the central nervous system. Therefore, rigorous preclinical and clinical studies are needed to assess the efficacy and feasibility of these strategies in patients. Since there is scarce and sometimes contradictory literature about their use in this context, the main aim of this review is to discuss and analyze the current state-of-the-art of gold nanoparticles in relation to delivery, diagnosis, and therapy for Alzheimer's and Parkinson's disease, as well as recent research about their use in preclinical, clinical, and emerging research areas.

The curvature of gold nanoparticles influences the exposure of amyloid-ß and modulates its aggregation process.

Gold nanoparticles (GNP) are tunable nanomaterials that can be used to develop rational therapeutic inhibitors against the formation of pathological aggregates of proteins. In the case of the pathological aggregation of the amyloid-ß protein (Aß), the shape of the GNP can slow down or accelerate its aggregation kinetics. However, there is a lack of elementary knowledge about how the curvature of GNP alters the interaction with the Aß peptide and how this interaction modifies key molecular steps of fibril formation. In this study, we analysed the effect of flat gold nanoprisms (GNPr) and curved gold nanospheres (GNS) on in vitro Aß42 fibril formation kinetics by using the thioflavin-based kinetic assay and global fitting analysis, with several models of aggregation. Whereas GNPr accelerate the aggregation process and maintain the molecular mechanism of aggregation, GNS slow down this process and modify the molecular mechanism to one of fragmentation/secondary nucleation, with respect to controls. These results can be explained by a differential interaction between the Aß peptide and GNP observed by Raman spectroscopy. While flat GNPr expose key hydrophobic residues involved in the Aß peptide aggregation, curved GNS hide these residues from the solvent. Thus, this study provides mechanistic insights to improve the rational design of GNP nanomaterials for biomedical applications in the field of amyloid-related aggregation.

Oligoarginine Peptide Conjugated to BSA Improves Cell Penetration of Gold Nanorods and Nanoprisms for Biomedical Applications.

Gold nanoparticles (AuNPs) have been shown to be outstanding tools for drug delivery and biomedical applications, mainly owing to their colloidal stability, surface chemistry, and photothermal properties. The biocompatibility and stability of nanoparticles can be improved by capping the nanoparticles with endogenous proteins, such as albumin. Notably, protein coating of nanoparticles can interfere with and decrease their cell penetration. Therefore, in the present study, we functionalized albumin with the r8 peptide (All-D, octaarginine) and used it for coating NIR-plasmonic anisotropic gold nanoparticles. Gold nanoprisms (AuNPrs) and gold nanorods (AuNRs) were coated with bovine serum albumin (BSA) previously functionalized using a cell penetrating peptide (CPP) with the r8 sequence (BSA-r8). The effect of the coated and r8-functionalized AuNPs on HeLa cell viability was assessed by the MTS assay, showing a low effect on cell viability after BSA coating. Moreover, the internalization of the nanostructures into HeLa cells was assessed by confocal microscopy and transmission electron microscopy (TEM). As a result, both nanoconstructs showed an improved internalization level after being capped with BSA-r8, in contrast to the BSA-functionalized control, suggesting the predominant role of CPP functionalization in cell internalization. Thus, our results validate both novel nanoconstructs as potential candidates to be coated by endogenous proteins and functionalized with a CPP to optimize cell internalization. In a further approach, coating AuNPs with CPP-functionalized BSA can broaden the possibilities for biomedical applications by combining their optical properties, biocompatibility, and cell-penetration abilities.

Plasmonic Nanoparticles as Optical Sensing Probes for the Detection of Alzheimer's Disease.

Alzheimer's disease (AD), considered a common type of dementia, is mainly characterized by a progressive loss of memory and cognitive functions. Although its cause is multifactorial, it has been associated with the accumulation of toxic aggregates of the amyloid-ß peptide (Aß) and neurofibrillary tangles (NFTs) of tau protein. At present, the development of highly sensitive, high cost-effective, and non-invasive diagnostic tools for AD remains a challenge. In the last decades, nanomaterials have emerged as an interesting and useful tool in nanomedicine for diagnostics and therapy. In particular, plasmonic nanoparticles are well-known to display unique optical properties derived from their localized surface plasmon resonance (LSPR), allowing their use as transducers in various sensing configurations and enhancing detection sensitivity. Herein, this review focuses on current advances in in vitro sensing techniques such as Surface-enhanced Raman scattering (SERS), Surface-enhanced fluorescence (SEF), colorimetric, and LSPR using plasmonic nanoparticles for improving the sensitivity in the detection of main biomarkers related to AD in body fluids. Additionally, we refer to the use of plasmonic nanoparticles for in vivo imaging studies in AD.

Functionalization with PEG/Angiopep-2 peptide to improve the delivery of gold nanoprisms to central nervous system: in vitro and in vivo studies.

One of main drawbacks for the treatment of neurodegenerative pathologies is ensuring the delivery of therapeutic agents into the central nervous system (CNS). Nowadays, gold nanoprisms (GNPr) have become an emerging nanomaterial with a localized surface plasmon resonance in the biological window, showing applications in both detection and treatment of diseases. In this work, GNPr were functionalized with polyethylene glycol (PEG) and Angiopep-2 (Ang2) peptide to obtain a new highly stable nanomaterial and evaluate its toxicity and ability to cross the blood-brain barrier (BBB) in a zebrafish larvae model. The success in the functionalization was confirmed by a full characterization that showed the physicochemical changes at each step. In turn, the colloidal stability of GNPr-PEG-Ang2 in biologically relevant media also was demonstrated. The toxicity assays of GNPr-PEG-Ang2 performed on SH-SY5Y neuroblastoma cell line and on zebrafish larvae showed no effects both in vitro and in vivo. GNPr delivery to the CNS was studied in zebrafish larvae by immersion. We confirmed that functionalization with PEG-Ang2 improved the crossing through the BBB in this model compared with GNPr functionalized only with PEG. Notably, our nanomaterial was not detected in the CNS of zebrafish larvae 24 h after exposure that correlates with an adequate clearance of GNPr-PEG-Ang2 from the brain. This report is the first study of GNPr in the in vivo model of zebrafish larvae demonstrating that its functionalization with Ang2 allows the crossing of the BBB. Moreover, considering the stability achieved of the GNPr-PEG-Ang2 and the results of in vitro and in vivo studies, this work becomes a high contribution to the design of new nanomaterials with potential biomedical applications for CNS-related diseases.

Intranasal administration of gold nanoparticles designed to target the central nervous system: Fabrication and comparison between nanospheres and nanoprisms.

The presence of the blood-brain barrier (BBB) limit gold nanoparticles (GNP) accumulation in central nervous system (CNS) after intravenous (IV) administration. The intranasal (IN) route has been suggested as a good strategy for circumventing the BBB. In this report, we used gold nanoprisms (78 nm) and nanospheres (47 nm), of comparable surface areas (8000 vs 7235 nm2) functionalized with a polyethylene glycol (PEG) and D1 peptide (GNPr-D1 and GNS-D1, respectively) to evaluate their delivery to the CNS after IN administration. Cell viability assay showed that GNPr-D1 and GNS-D1 were not cytotoxic at concentrations ranged between 0.05 and 0.5 nM. IN administration of GNPr-D1 and GNS-D1 demonstrated a significant difference between the two types of GNP, in which the latter reached the CNS in higher levels. Pharmacokinetic study showed that the peak brain level of gold was 0.75 h after IN administration of GNS-D1. After IN and IV administrations of GNS-D1, gold concentrations found in brain were 55 times higher via the IN route compared to IV administration. Data revealed that the IN route is more effective for targeting gold to the brain than IV administration. Finally, no significant difference was observed between the IN and IV routes in the distribution of GNS-D1 in the various brain areas.