Surface-enhanced Raman scattering sensing platform for detecting amyloid-ß peptide interaction with an aggregation inhibitor.

Soluble, small amyloid-ß oligomers (AßO) are recognized as significant contributors to the pathology of Alzheimer's disease (AD). Although drugs for treating AD symptoms have been approved, no therapy targeting amyloid-ß (Aß) capable of modifying the course of the disease is available. In an effort to develop a label-free method for screening new anti-AD therapeutic agents, we show the use of a surface-enhanced Raman scattering (SERS) active substrate for detecting the interactions between Aß peptides and spin-labeled fluorine (SLF), a peptide aggregation inhibitor. Changes in the peak positions and intensity ratios of two spectral peaks near 1600cm-1 and 2900cm-1 can be used to monitor the molecular interactions between SLF and Aß. This study demonstrates the potential of SERS spectroscopy for rapidly screening and identifying new anti-Aß therapeutic agents.

Kv1.3 inhibition as a potential microglia-targeted therapy for Alzheimer's disease: preclinical proof of concept.

Microglia significantly contribute to the pathophysiology of Alzheimer's disease but an effective microglia-targeted therapeutic approach is not yet available clinically. The potassium channels Kv1.3 and Kir2.1 play important roles in regulating immune cell functions and have been implicated by in vitro studies in the 'M1-like pro-inflammatory' or 'M2-like anti-inflammatory' state of microglia, respectively. We here found that amyloid-ß oligomer-induced expression of Kv1.3 and Kir2.1 in cultured primary microglia. Likewise, ex vivo microglia acutely isolated from the Alzheimer's model 5xFAD mice co-expressed Kv1.3 and Kir2.1 as well as markers traditionally associated with M1 and M2 activation suggesting that amyloid-ß oligomer induces a microglial activation state that is more complex than previously thought. Using the orally available, brain penetrant small molecule Kv1.3 blocker PAP-1 as a tool, we showed that pro-inflammatory and neurotoxic microglial responses induced by amyloid-ß oligomer required Kv1.3 activity in vitro and in hippocampal slices. Since we further observed that Kv1.3 was highly expressed in microglia of transgenic Alzheimer's mouse models and human Alzheimer's disease brains, we hypothesized that pharmacological Kv1.3 inhibition could mitigate the pathology induced by amyloid-ß aggregates. Indeed, treating APP/PS1 transgenic mice with a 5-month oral regimen of PAP-1, starting at 9 months of age, when the animals already manifest cognitive deficits and amyloid pathology, reduced neuroinflammation, decreased cerebral amyloid load, enhanced hippocampal neuronal plasticity, and improved behavioural deficits. The observed decrease in cerebral amyloid deposition was consistent with the in vitro finding that PAP-1 enhanced amyloid-ß uptake by microglia. Collectively, these results provide proof-of-concept data to advance Kv1.3 blockers to Alzheimer's disease clinical trials.

A Bifunctional Anti-Amyloid Blocks Oxidative Stress and the Accumulation of Intraneuronal Amyloid-Beta.

There is growing recognition regarding the role of intracellular amyloid beta (Aß) in the Alzheimer's disease process, which has been linked with aberrant signaling and the disruption of protein degradation mechanisms. Most notably, intraneuronal Aß likely underlies the oxidative stress and mitochondrial dysfunction that have been identified as key elements of disease progression. In this study, we employed fluorescence imaging to explore the ability of a bifunctional small molecule to reduce aggregates of intracellular Aß and attenuate oxidative stress. Structurally, this small molecule is comprised of a nitroxide spin label linked to an amyloidophilic fluorene and is known as spin-labeled fluorene (SLF). The effect of the SLF on intracellular Aß accumulation and oxidative stress was measured in MC65 cells, a human neuronal cell line with inducible expression of the amyloid precursor protein and in the N2a neuronal cell line treated with exogenous Aß. Super-resolution microscopy imaging showed SLF decreases the accumulation of intracellular Aß. Confocal microscopy imaging of MC65 cells treated with a reactive oxygen species (ROS)-sensitive dye demonstrated SLF significantly reduces the intracellular Aß-induced ROS signal. In order to determine the contributions of the separate SLF moieties to these protective activities, experiments were also carried out on cells with nitroxides lacking the Aß targeting domain or fluorene derivatives lacking the nitroxide functionality. The findings support a synergistic effect of SLF in counteracting both the conformational toxicity of both endogenous and exogenous Aß, its promotion of ROS, and Aß metabolism. Furthermore, these studies demonstrate an intimate link between ROS production and Aß oligomer formation.

Protective spin-labeled fluorenes maintain amyloid beta peptide in small oligomers and limit transitions in secondary structure.

Alzheimer's disease is characterized by the presence of extracellular plaques comprised of amyloid beta (Aß) peptides. Soluble oligomers of the Aß peptide underlie a cascade of neuronal loss and dysfunction associated with Alzheimer's disease. Single particle analyses of Aß oligomers in solution by fluorescence correlation spectroscopy (FCS) were used to provide real-time descriptions of how spin-labeled fluorenes (SLFs; bi-functional small molecules that block the toxicity of Aß) prevent and disrupt oligomeric assemblies of Aß in solution. Furthermore, the circular dichroism (CD) spectrum of untreated Aß shows a continuous, progressive change over a 24-hour period, while the spectrum of Aß treated with SLF remains relatively constant following initial incubation. These findings suggest the conformation of Aß within the oligomer provides a complementary determinant of Aß toxicity in addition to oligomer growth and size. Although SLF does not produce a dominant state of secondary structure in Aß, it does induce a net reduction in beta secondary content compared to untreated samples of Aß. The FCS results, combined with electron paramagnetic resonance spectroscopy and CD spectroscopy, demonstrate SLFs can inhibit the growth of Aß oligomers and disrupt existing oligomers, while retaining Aß as a population of smaller, yet largely disordered oligomers.

Molecular crowding impacts the structure of apolipoprotein A-I with potential implications on in vivo metabolism and function.

The effect molecular crowding, defined as the volume exclusion exerted by one soluble inert molecule upon another soluble molecule, has on the structure and self-interaction of lipid-free apoA-I were explored. The influence of molecular crowding on lipid-free apoA-I oligomerization and internal dynamics has been analyzed using electron paramagnetic resonance (EPR) spectroscopy measurements of nitroxide spin label at selected positions throughout the protein sequence and at varying concentrations of the crowding agent Ficoll-70. The targeted positions include sites previously shown to be sensitive for detecting intermolecular interaction via spin-spin coupling. Circular dichroism was used to study secondary structural changes in lipid-free apoA-I imposed by increasing concentrations of the crowding agent. Crosslinking and SDS-PAGE gel analysis was employed to further characterize the role molecular crowding plays in inducing apoA-I oligomerization. It was concluded that the dynamic apoA-I structure and oligomeric state was altered in the presence of the crowding agent. It was also found that the C-terminal was slightly more sensitive to molecular crowding. Finally, the data described the region around residue 217 in the C-terminal domain of apoA-I as the most sensitive reporter of the crowding-induced self-association of apoA-I. The implications of this behavior to in vivo functionality are discussed. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 683-692, 2016.

Binding of apolipoprotein E inhibits the oligomer growth of amyloid-ß peptide in solution as determined by fluorescence cross-correlation spectroscopy.

One of the primary neuropathological hallmarks of Alzheimer disease is the presence of extracellular amyloid plaques resulting from the aggregation of amyloid-ß (Aß) peptides. The intrinsic disorder of the Aß peptide drives self-association and progressive reordering of the conformation in solution, and this dynamic distribution of Aß complicates biophysical studies. This property poses a challenge for understanding the interaction of Aß with apolipoprotein E (apoE). ApoE plays a pivotal role in the aggregation and clearance of Aß peptides in the brain, and the ε4 allele of APOE is the most significant known genetic modulator of Alzheimer risk. Understanding the interaction between apoE and Aß will provide insight into the mechanism by which different apoE isoforms determine Alzheimer disease risk. Here we applied alternating laser excitation fluorescence cross-correlation spectroscopy to observe the single molecule interaction of Aß with apoE in the hydrated state. The diffusion time of freely diffusing Aß in the absence of apoE shows significant self-aggregation, whereas in the presence of apoE, binding of the protein results in a more stable complex. These results show that apoE slows down the oligomerization of Aß in solution and provide direct insight into the process by which apoE influences the deposition and clearance of Aß peptides in the brain. Furthermore, by developing an approach to remove signals arising from very large Aß aggregates, we show that real-time single particle observations provide access to information regarding the fraction of apoE bound and the stoichiometry of apoE and Aß in the complex.

Analysis of lipid phase behavior and protein conformational changes in nanolipoprotein particles upon entrapment in sol-gel-derived silica.

The entrapment of nanolipoprotein particles (NLPs) and liposomes in transparent, nanoporous silica gel derived from the precursor tetramethylorthosilicate was investigated. NLPs are discoidal patches of lipid bilayer that are belted by amphiphilic scaffold proteins and have an average thickness of 5 nm. The NLPs in this work had a diameter of roughly 15 nm and utilized membrane scaffold protein (MSP), a genetically altered variant of apolipoprotein A-I. Liposomes have previously been examined inside of silica sol-gels and have been shown to exhibit instability. This is attributed to their size (∼150 nm) and altered structure and constrained lipid dynamics upon entrapment within the nanometer-scale pores (5-50 nm) of the silica gel. By contrast, the dimensional match of NLPs with the intrinsic pore sizes of silica gel opens the possibility for their entrapment without disruption. Here we demonstrate that NLPs are more compatible with the nanometer-scale size of the porous environment by analysis of lipid phase behavior via fluorescence anisotropy and analysis of scaffold protein secondary structure via circular dichroism spectroscopy. Our results showed that the lipid phase behavior of NLPs entrapped inside of silica gel display closer resemblance to its solution behavior, more so than liposomes, and that the MSP in the NLPs maintain the high degree of α-helix secondary structure associated with functional protein-lipid interactions after entrapment. We also examined the effects of residual methanol on lipid phase behavior and the size of NLPs and found that it exerts different influences in solution and in silica gel; unlike in free solution, silica entrapment may be inhibiting NLP size increase and/or aggregation. These findings set precedence for a bioinorganic hybrid nanomaterial that could incorporate functional integral membrane proteins.

Novel Stilbene-Nitroxyl Hybrid Compounds Display Discrete Modulation of Amyloid Beta Toxicity and Structure.

Several neurodegenerative diseases are driven by misfolded proteins that assemble into soluble aggregates. These "toxic oligomers" have been associated with a plethora of cellular dysfunction and dysregulation, however the structural features underlying their toxicity are poorly understood. A major impediment to answering this question relates to the heterogeneous nature of the oligomers, both in terms of structural disorder and oligomer size. This not only complicates elucidating the molecular etiology of these disorders, but also the druggability of these targets as well. We have synthesized a class of bifunctional stilbenes to modulate both the conformational toxicity within amyloid beta oligomers (AßO) and the oxidative stress elicited by AßO. Using a neuronal culture model, we demonstrate this bifunctional approach has the potential to counter the molecular pathogenesis of Alzheimer's disease in a powerful, synergistic manner. Examination of AßO structure by various biophysical tools shows that each stilbene candidate uniquely alters AßO conformation and toxicity, providing insight towards the future development of structural correctors for AßO. Correlations of AßO structural modulation and bioactivity displayed by each provides insights for future testing in vivo. The multi-target activity of these hybrid molecules represents a highly advantageous feature for disease modification in Alzheimer's, which displays a complex, multifactorial etiology. Importantly, these novel small molecules intervene with intraneuronal AßO, a necessary feature to counter the cycle of dysregulation, oxidative stress and inflammation triggered during the earliest stages of disease progression.

Identification of amyloid beta in small extracellular vesicles via Raman spectroscopy.

One of the hallmarks of Alzheimer's disease (AD) pathogenesis is believed to be the production and deposition of amyloid-beta (Aß) peptide into extracellular plaques. Existing research indicates that extracellular vesicles (EVs) can carry Aß associated with AD. However, characterization of the EVs-associated Aß and its conformational variants has yet to be realized. Raman spectroscopy is a label-free and non-destructive method that is able to assess the biochemical composition of EVs. This study reports for the first time the Raman spectroscopic fingerprint of the Aß present in the molecular cargo of small extracellular vesicles (sEVs). Raman spectra were measured from sEVs isolated from Alzheimer's disease cell culture model, where secretion of Aß is regulated by tetracycline promoter, and from midbrain organoids. The averaged spectra of each sEV group showed considerable variation as a reflection of the biochemical content of sEVs. Spectral analysis identified more intense Raman peaks at 1650 cm-1 and 2930 cm-1 attributable to the Aß peptide incorporated in sEVs produced by the Alzheimer's cell culture model. Subsequent analysis of the spectra by principal component analysis differentiated the sEVs of the Alzheimer's disease cell culture model from the control groups of sEVs. Moreover, the results indicate that Aß associated with secreted sEVs has a α-helical secondary structure and the size of a monomer or small oligomer. Furthermore, by analyzing the lipid content of sEVs we identified altered fatty acid chain lengths in sEVs that carry Aß that may affect the fluidity of the EV membrane. Overall, our findings provide evidence supporting the use of Raman spectroscopy for the identification and characterization of sEVs associated with potential biomarkers of neurological disorders such as toxic proteins.

Superhydrophobic bowl-like SERS substrates patterned from CMOS sensors for extracellular vesicle characterization.

Using a regular CMOS sensor as a template, we are able to fabricate a simple but highly effective superhydrophobic SERS substrate. Specifically, we decorated the microlens layer of the sensor with 7 µm polystyrene beads to obtain a PDMS patterned replica. The process resulted in a uniform pattern of voids in the PDMS (denoted nanobowls) that are intercalated with a few larger voids (denoted here microbowls). The voids act as superhydrophobic substrates with analyte concentration capabilities in bigger bowl-like structures. Silver nanoparticles were directly grown on the patterned PDMS substrate inside both the nano- and microbowls, and serve as strong electromagnetic field enhancers for the SERS substrate. After systematic characterization of the fabricated SERS substrate by atomic force microscopy and scanning electron microscopy, we demonstrated its SERS performance using 4-aminothiophenol as a reporter molecule. Finally, we employed this innovative substrate to concentrate and analyze extracellular vesicles (EVs) isolated from an MC65 neural cell line in an ultralow sample volume. This substrate can be further exploited for the investigation of various EV biomarkers for early diagnosis of different diseases using liquid biopsy.

Oligomerization Alters Binding Affinity Between Amyloid Beta and a Modulator of Peptide Aggregation.

The soluble oligomeric form of the amyloid beta (Aß) peptide is the major causative agent in the molecular pathogenesis of Alzheimer's disease (AD). We have previously developed a pyrroline-nitroxyl fluorene compound (SLF) that blocks the toxicity of Aß. Here we introduce the multi-parametric surface plasmon resonance (MP-SPR) approach to quantify SLF binding and effect on the self-association of the peptide via a label-free, real-time approach. Kinetic analysis of SLF binding to Aß and measurements of layer thickness alterations inform on the mechanism underlying the ability of SLF to inhibit Aß toxicity and its progression towards larger oligomeric assemblies. Depending on the oligomeric state of Aß, distinct binding affinities for SLF are revealed. The Aß monomer and dimer uniquely possess sub-nanomolar affinity for SLF via a non-specific mode of binding. SLF binding is weaker in oligomeric Aß, which displays an affinity for SLF on the order of 100 µM. To complement these experiments we carried out molecular docking and molecular dynamics simulations to explore how SLF interacts with the Aß peptide. The MP-SPR results together with in silico modeling provide affinity data for the SLF-Aß interaction and allow us to develop a new general method for examining protein aggregation.

A Metal-Free Method for Producing MRI Contrast at Amyloid-ß.

Alzheimer's disease (AD) is characterized by depositions of the amyloid-ß (Aß) peptide in the brain. The disease process develops over decades, with substantial neurological loss occurring before a clinical diagnosis of dementia can be rendered. It is therefore imperative to develop methods that permit early detection and monitoring of disease progression. In addition, the multifactorial pathogenesis of AD has identified several potential avenues for AD intervention. Thus, evaluation of therapeutic candidates over lengthy trial periods also demands a practical, noninvasive method for measuring Aß in the brain. Magnetic resonance imaging (MRI) is the obvious choice for such measurements, but contrast enhancement for Aß has only been achieved using Gd(III)-based agents. There is great interest in gadolinium-free methods to image the brain. In this study, we provide the first demonstration that a nitroxide-based small-molecule produces MRI contrast in brain specimens with elevated levels of Aß. The molecule is comprised of a  fluorene (a molecule with high affinity for Aß) and a nitroxide spin label (a paramagnetic MRI contrast species). Labeling of brain specimens with the spin-labeled fluorene produces negative contrast in samples from AD model mice whereas no negative contrast is seen in specimens harvested from wild-type mice. Injection of spin-labeled fluorene into live mice resulted in good brain penetration, with the compound able to generate contrast 24-h post injection. These results provide a proof of concept method that can be used for early, noninvasive, gadolinium-free detection of amyloid plaques by MRI.

Spectroscopic Characterization of Structural Changes in Membrane Scaffold Proteins Entrapped within Mesoporous Silica Gel Monoliths.

The changes in the orientation and conformation of three different membrane scaffold proteins (MSPs) upon entrapment in sol-gel-derived mesoporous silica monoliths were investigated. MSPs were examined in either a lipid-free or a lipid-bound conformation, where the proteins were associated with lipids to form nanolipoprotein particles (NLPs). NLPs are water-soluble, disk-shaped patches of a lipid bilayer that have amphiphilic MSPs shielding the hydrophobic lipid tails. The NLPs in this work had an average thickness of 5 nm and diameters of 9.2, 9.7, and 14.8 nm. We have previously demonstrated that NLPs are more suitable lipid-based structures for silica gel entrapment than liposomes because of their size compatibility with the mesoporous network (2-50 nm) and minimally altered structure after encapsulation. Here we further elaborate on that work by using a variety of spectroscopic techniques to elucidate whether or not different MSPs maintain their protein-lipid interactions after encapsulation. Fluorescence spectroscopy and quenching of the tryptophan residues with acrylamide, 5-DOXYL-stearic acid, and 16-DOXYL-stearic acid were used to determine the MSP orientation. We also utilized fluorescence anisotropy of tryptophans to measure the relative size of the NLPs and MSP aggregates after entrapment. Finally, circular dichroism spectroscopy was used to examine the secondary structure of the MSPs. Our results showed that, after entrapment, all of the lipid-bound MSPs maintained orientations that were minimally changed and indicative of association with lipids in NLPs. The tryptophan residues appeared to remain buried within the hydrophobic core of the lipid tails in the NLPs and appropriately spaced from the bilayer center. Also, after entrapment, lipid-bound MSPs maintained a high degree of α-helical content, a secondary structure associated with protein-lipid interactions. These findings demonstrate that NLPs are capable of serving as viable hosts for functional integral membrane proteins in the synthesis of sol-gel-derived bioinorganic hybrid nanomaterials.

Identification of streptococcal m-protein cardiopathogenic epitopes in experimental autoimmune valvulitis.

The M protein of rheumatogenic group A streptococci induces carditis and valvulitis in Lewis rats and may play a role in pathogenesis of rheumatic heart disease. To identify the epitopes of M5 protein that produce valvulitis, synthetic peptides spanning A, B, and C repeat regions contained within the extracellular domain of the streptococcal M5 protein were investigated. A repeat region peptides NT4, NT5/6, and NT7 induced valvulitis similar to the intact pepsin fragment of M5 protein. T cell lines from rats with valvulitis recognized M5 peptides NT5/6 and NT6. Passive transfer of an NT5/6-specific T cell line into naïve rats produced valvulitis characterized by infiltration of CD4+ cells and upregulation of VCAM-1, while an NT6-specific T cell line did not target the valve. Our new data suggests that M protein-specific T cells may be important mediators of valvulitis in the Lewis rat model of rheumatic carditis.

EPR assessment of protein sites for incorporation of Gd(III) MRI contrast labels.

We have engineered apolipoprotein A-I (apoA-I), a major protein constituent of high-density lipoprotein (HDL), to contain DOTA-chelated Gd(III) as an MRI contrast agent for the purpose of imaging reconstituted HDL (rHDL) biodistribution, metabolism and regulation in vivo. This protein contrast agent was obtained by attaching the thiol-reactive Gd[MTS-ADO3A] label at Cys residues replaced at four distinct positions (52, 55, 76 and 80) in apoA-I. MRI of infused mice previously showed that the Gd-labeled apoA-I migrates to both the liver and the kidney, the organs responsible for HDL catabolism; however, the contrast properties of apoA-I are superior when the ADO3A moiety is located at position 55, compared with the protein labeled at positions 52, 76 or 80. It is shown here that continuous wave X-band (9 GHz) electron paramagnetic resonance (EPR) spectroscopy is capable of detecting differences in the Gd(III) signal when comparing the labeled protein in the lipid-free with the rHDL state. Furthermore, the values of NMR relaxivity obtained for labeled variants in both the lipid-free and rHDL states correlate to the product of the X-band Gd(III) spectral width and the collision frequency between a nitroxide spin label and a polar relaxation agent. Consistent with its superior relaxivity measured by NMR, the rHDL-associated apoA-I containing the Gd[MTS-ADO3A] probe attached to position 55 displays favorable dynamic and water accessibility properties as determined by X-band EPR. While room temperature EPR requires >1 m m Gd(III)-labeled and only >10 µ m nitroxide-labeled protein to resolve the spectrum, the volume requirement is exceptionally low (~5 µl). Thus, X-band EPR provides a practical assessment for the suitability of imaging candidates containing the site-directed ADO3A contrast probe.