Prion disease: exponential growth requires membrane binding

Behavioral and Neuronal Characterizations, across Ages, of the TgSwDI Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder that currently affects as many as 50 million people worldwide. It is neurochemically characterized by an aggregation of β-amyloid plaques and tau neurofibrillary tangles that result in neuronal dysfunction, cognitive decline, and a progressive loss of brain function. TgSwDI is a well-studied transgenic mouse model of AD, but no longitudinal studies have been performed to characterize cognitive deficits or β-amyloid plaque accumulation for use as a baseline reference in future research. Thus, we use behavioral tests (T-Maze, Novel Object Recognition (NOR), Novel Object Location (NOL)) to study long-term and working memory, and immunostaining to study β-amyloid plaque deposits, as well as brain size, in hippocampal, cerebellum, and cortical slices in TgSwDI and wild-type (WT) mice at 3, 5, 8, and 12 months old. The behavioral results show that TgSwDI mice exhibit deficits in their long-term spatial memory starting at 8 months old and in long-term recognition memory at all ages, but no deficits in their working memory. Immunohistochemistry showed an exponential increase in β-amyloid plaque in the hippocampus and cortex of TgSwDI mice over time, whereas there was no significant accumulation of plaque in WT mice at any age. Staining showed a smaller hippocampus and cerebellum starting at 8 months old for the TgSwDI compared to WT mice. Our data show how TgSwDI mice differ from WT mice in their baseline levels of cognitive function and β-amyloid plaque load throughout their lives.

Kinetics of α-synuclein prions preceding neuropathological inclusions in multiple system atrophy

Multiple system atrophy (MSA), a progressive neurodegenerative disease characterized by autonomic dysfunction and motor impairment, is caused by the self-templated misfolding of the protein α-synuclein. With no treatment currently available, we sought to characterize the spread of α-synuclein in a transgenic mouse model of MSA prion propagation to support drug discovery programs for synucleinopathies. Brain homogenates from MSA patient samples or mouse-passaged MSA were inoculated either by standard freehand injection or stereotactically into TgM83+/- mice, which express human α-synuclein with the A53T mutation. Following disease onset, brains from the mice were tested for biologically active α-synuclein prions using a cell-based assay and examined for α-synuclein neuropathology. Inoculation studies using homogenates prepared from brain regions lacking detectable α-synuclein neuropathology transmitted neurological disease to mice. Terminal animals contained similar concentrations of α-synuclein prions; however, a time-course study where mice were terminated every five days through disease progression revealed that the kinetics of α-synuclein prion replication in the mice were variable. Stereotactic inoculation into the thalamus reduced variability in disease onset in the mice, although incubation times were consistent with standard inoculations. Using human samples with and without neuropathological lesions, we observed that α-synuclein prion formation precedes neuropathology in the brain, suggesting that disease in patients is not limited to brain regions containing neuropathological lesions.

Aggregation of PrP106-126 on surfaces of neutral and negatively charged membranes studied by molecular dynamics simulations

Prion diseases are neurodegenerative disorders characterized by the aggregation of an abnormal form of prion protein. The interaction of prion protein and cellular membrane is crucial to elucidate the occurrence and development of prion diseases. Its fragment, residues 106-126, has been proven to maintain the pathological properties of misfolded prion and was used as a model peptide. In this study, explicit solvent molecular dynamics (MD) simulations were carried out to investigate the adsorption, folding and aggregation of PrP106-126 with different sizes (2-peptides, 4-peptides and 6-peptides) on the surface of both pure neutral POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and negatively charged POPC/POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol) (3:1) lipids. MD simulation results show that PrP106-126 display strong affinity with POPC/POPG but does not interact with pure POPC. The positively charged and polar residues participating hydrogen bonding with membrane promote the adsorption of PrP106-126. The presence of POPC and POPC/POPG exert limited influence on the secondary structures of PrP106-126 and random coil structures are predominant in all simulation systems. Upon the adsorption on the POPC/POPG surface, the aggregation states of PrP106-126 have been changed and more small oligomers were observed. This work provides insights into the interactions of PrP106-126 and membranes with different compositions in atomic level, which expand our understanding the role membrane plays in the development of prion diseases. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Engineering amyloid fibrils from β-solenoid proteins for biomaterials applications

Nature provides numerous examples of self-assembly that can potentially be implemented for materials applications. Considerable attention has been given to one-dimensional cross-β or amyloid structures that can serve as templates for wire growth or strengthen materials such as glue or cement. Here, we demonstrate controlled amyloid self-assembly based on modifications of β-solenoid proteins. They occur naturally in several contexts (e.g., antifreeze proteins, drug resistance proteins) but do not aggregate in vivo due to capping structures or distortions at their ends. Removal of these capping structures and regularization of the ends of the spruce budworm and rye grass antifreeze proteins yield micron length amyloid fibrils with predictable heights, which can be a platform for biomaterial-based self-assembly. The design process, including all-atom molecular dynamics simulations, purification, and self-assembly procedures are described. Fibril formation with the predicted characteristics is supported by evidence from thioflavin-T fluorescence, circular dichroism, dynamic light scattering, and atomic force microscopy. Additionally, we find evidence for lateral assembly of the modified spruce budworm antifreeze fibrils with sufficient incubation time. The kinetics of polymerization are consistent with those for other amyloid formation reactions and are relatively fast due to the preformed nature of the polymerization nucleus.

Lipid interaction triggering Septin2 to assembly into β-sheet structures investigated by Langmuir monolayers and PM-IRRAS

The molecular mechanisms responsible for protein structural changes in the central nervous system leading to Alzheimer's disease are unknown, but there is evidence that a family of proteins known as septins may be involved. Septins are a conserved group of GTP-binding proteins which participate in various cellular processes, including polarity determination and membrane dynamics. SEPT1, SEPT4, and SEPT2 have been found in deposits known as neurofibrillary tangles and glial fibrils in Alzheimer's disease. In this study, we provide molecular-level information for the interaction of SEPT2 with Langmuir monolayers at the air/water interface, which are used as simplified membrane models. The high surface activity of SEPT2 causes it to adsorb onto distinct types of lipid Langmuir monolayers, namely dipalmitoylphosphatidylcholine and PtdIns(4,5)P2. However, the interaction with PtdIns(4,5)P2 is much stronger, not only leading to a higher adsorption, but also to SEPT2 remaining inserted within the membrane at high surface pressures. Most importantly, in situ polarization-modulated infrared reflection absorption spectroscopy results indicated that the native secondary structure of SEPT2 is preserved upon interacting with PtdIns(4,5)P2, but not when dipalmitoylphosphatidylcholine is at the air/water interface. Taken together, the results presented here suggest that the interaction between SEPT2 and the cell membrane may play an important role in the assembly of SEPT2 into amyloid-like fibers.

Allosteric function and dysfunction of the prion protein

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases associated with progressive oligo- and multimerization of the prion protein (PrP(C)), its conformational conversion, aggregation and precipitation. We recently proposed that PrP(C) serves as a cell surface scaffold protein for a variety of signaling modules, the effects of which translate into wide-range functional consequences. Here we review evidence for allosteric functions of PrP(C), which constitute a common property of scaffold proteins. The available data suggest that allosteric effects among PrP(C) and its partners are involved in the assembly of multi-component signaling modules at the cell surface, impose upon both physiological and pathological conformational responses of PrP(C), and that allosteric dysfunction of PrP(C) has the potential to entail progressive signal corruption. These properties may be germane both to physiological roles of PrP(C), as well as to the pathogenesis of the TSEs and other degenerative/non-communicable diseases.

Investigating the conformational stability of prion strains through a kinetic replication model

Prion proteins are known to misfold into a range of different aggregated forms, showing different phenotypic and pathological states. Understanding strain specificities is an important problem in the field of prion disease. Little is known about which PrP^Sc structural properties and molecular mechanisms determine prion replication, disease progression and strain phenotype. The aim of this work is to investigate, through a mathematical model, how the structural stability of different aggregated forms can influence the kinetics of prion replication. The model-based results suggest that prion strains with different conformational stability undergoing in vivo replication are characterizable in primis by means of different rates of breakage. A further role seems to be played by the aggregation rate (i.e. the rate at which a prion fibril grows). The kinetic variability introduced in the model by these two parameters allows us to reproduce the different characteristic features of the various strains (e.g., fibrils' mean length) and is coherent with all experimental observations concerning strain-specific behavior.

Prion diseases are caused by the accumulation of a cellular prion protein with an altered conformation, which acts as a catalyst for the further recruitment and the modification of the normal form of the protein. Protein polymerization appears to have a central role in the progression of the disease, an aspect shared with several other neurodegenerative diseases. The aim of this work is to investigate at the kinetic level the “prion strain phenomenon”, i.e., the ability of prion proteins to misfold into a range of different aggregated forms exhibiting different replication and propagation properties. The dynamics of prion replication is investigated with the help of a mathematical model. We relate a measurement accessible in vitro (prion structural stability) to a mathematical description of the fibrils' kinetics in vivo. The analysis of the model suggests that the replication kinetics of the different prion strains is characterizable by means of two parameters, representing the rates of breakage and aggregation. This result is coherent with various experimental findings concerning strain-specific behavior, such as, for example, the observation of the fibril mean length of the various strains.

Hydration profiles of amyloidogenic molecular structures

Hydration shells of normal proteins display regions of highly structured water as well as patches of less structured bulk-like water. Recent studies suggest that isomers with larger surface densities of patches of bulk-like water have an increased propensity to aggregate. These aggregates are toxic to the cellular environment. Hence, the early detection of these toxic deposits is of paramount medical importance. We show that various morphological states of association of such isomers can be differentiated from the normal protein background based on the characteristic partition between bulk, caged, and surface hydration water and the magnetic resonance (MR) signals of this water. We derive simple mathematical equations relating the compartmentalization of water to the local hydration fraction and the packing density of the newly formed molecular assemblies. Then, we employ these equations to predict the MR response of water constrained by protein aggregation. Our results indicate that single units and compact aggregates that contain no water between constituents induce a shift of the MR signal from normal protein background to values in the hyperintensity domain (bright spots), corresponding to bulk water. In contrast, large plaques that cage significant amounts of water between constituents are likely to generate MR responses in the hypointensity domain (dark spots), typical for strongly correlated water. The implication of these results is that amyloids can display both dark and bright spots when compared to the normal gray background tissue on MR images. In addition, our findings predict that the bright spots are more likely to correspond to amyloids in their early stage of development. The results help explain the MR contrast patterns of amyloids and suggest a new approach for identifying unusual protein aggregation related to disease.

A gamma-secretase inhibitor and quinacrine reduce prions and prevent dendritic degeneration in murine brains

In prion-infected mice, both the Notch-1 intracellular domain transcription factor (NICD) and the disease-causing prion protein (PrP(Sc)) increase in the brain preceding dendritic atrophy and loss. Because the drug LY411575 inhibits the gamma-secretase-catalyzed cleavage of Notch-1 that produces NICD, we asked whether this gamma-secretase inhibitor (GSI) might prevent dendritic degeneration in mice with scrapie. At 50 d postinoculation with Rocky Mountain Laboratory (RML) prions, mice were given GSI orally for 43-60 d. Because we did not expect GSI to produce a reduction of PrP(Sc) levels in brain, we added quinacrine (Qa) to the treatment regimen. Qa inhibits PrP(Sc) formation in cultured cells. The combination of GSI and Qa reduced PrP(Sc) by approximately 95% in the neocortex and hippocampus but only approximately 50% in the thalamus at the site of prion inoculation. The GSI plus Qa combination prevented dendritic atrophy and loss, but GSI alone did not. Even though GSI reduced NICD levels to a greater extent than GSI plus Qa, it was unable to prevent dendritic degeneration. Whether a balance between NICD and dendrite growth-stimulating factors was achieved with GSI plus Qa but not GSI alone remains to be determined. Although the combination of GSI and Qa diminished PrP(Sc) in the brains of RML-infected mice, GSI toxicity prevented us from being able to assess the effect the GSI plus Qa combination on incubation times. Whether less toxic GSIs can be used in place of LY411575 to prolong survival remains to be determined.

Kinetics of amyloid formation and membrane interaction with amyloidogenic proteins

Interest in amyloidogenesis has exploded in recent years, as scientists recognize the role of amyloid protein aggregates in degenerative diseases such as Alzheimer's and Parkinson's disease. Assembly of proteins or peptides into mature amyloid fibrils is a multistep process initiated by conformational changes, during which intermediate aggregation states such as oligomers, protofibrils, and filaments are sampled. Although once it was assumed that the mature fibril was the biologically toxic species, more recently it has been widely speculated that soluble intermediates are the most damaging. Because of its relevance to mechanism of disease, the paths traversed during fibrillogenesis, and the kinetics of the process, are of considerable interest. In this review we discuss various kinetic models used to describe amyloidogenesis. Although significant advances have been made, construction of rigorous, detailed, and experimentally validated quantitative models remains a work in progress. We briefly review recent literature that illustrates the interplay between kinetics and amyloid-membrane interactions: how do different intermediates interact with lipid bilayers, and how does the lipid bilayer affect kinetics of amyloidogenesis?