Zooming into protein oligomerization in neurodegeneration using BiFC

Angiotensin type 1 receptor activation promotes neuronal and glial alpha-synuclein aggregation and transmission

The brain renin-angiotensin system (RAS) has been related to dopaminergic degeneration, and high expression of the angiotensin II (AngII) type 1 receptor (AT1) gene is a marker of the most vulnerable neurons in humans. However, it is unknown whether AngII/AT1 overactivation affects α-synuclein aggregation and transmission. In vitro, AngII/AT1 activation increased α-synuclein aggregation in dopaminergic neurons and microglial cells, which was related to AngII-induced NADPH-oxidase activation and intracellular calcium raising. In mice, AngII/AT1 activation was involved in MPTP-induced increase in α-synuclein expression and aggregation, as they significantly decreased in mice treated with the AT1 blocker telmisartan and AT1 knockout mice. Cell co-cultures (transwells) revealed strong transmission of α-synuclein from dopaminergic neurons to astrocytes and microglia. AngII induced a higher α-synuclein uptake by microglial cells and an increase in the transfer of α-synuclein among astroglial cells. However, AngII did not increase the release of α-synuclein by neurons. The results further support brain RAS dysregulation as a major mechanism for the progression of Parkinson's disease, and AT1 inhibition and RAS modulation as therapeutic targets.

From Monomers to Hexamers: A Theoretical Probability of the Neighbor Density Approach to Dissect Protein Oligomerization in Cells

Deciphering the oligomeric state of proteins within cells is pivotal to understanding their role in intricate cellular processes. With the recent advances in single-molecule localization microscopy, previous efforts have harnessed protein location density approaches, coupled with simulations, to extract membrane protein oligomeric states in cells, highlighting the value of such techniques. However, a comprehensive theoretical approach that can be universally applied across different proteins (e.g., membrane and cytosolic proteins) remains elusive. Here, we introduce the theoretical probability of neighbor density (PND) as a robust tool to discern protein oligomeric states in cellular environments. Utilizing our approach, the theoretical PND was validated against simulated data for both membrane and cytosolic proteins, consistently aligning with experimental baselines for membrane proteins. This congruence was maintained even when adjusting for protein concentrations or exploring proteins of various oligomeric states. The strength of our method lies not only in its precision but also in its adaptability, accommodating diverse cellular protein scenarios without compromising the accuracy. The development and validation of the theoretical PND facilitate accurate protein oligomeric state determination and bolster our understanding of protein-mediated cellular functions.

Uric acid regulates α-synuclein transmission in Parkinsonian models

Ample evidence demonstrates that α-synuclein (α-syn) has a critical role in the pathogenesis of Parkinson's disease (PD) with evidence indicating that its propagation from one area of the brain to others may be the primary mechanism for disease progression. Uric acid (UA), a natural antioxidant, has been proposed as a potential disease modifying candidate in PD. In the present study, we investigated whether UA treatment modulates cell-to-cell transmission of extracellular α-syn and protects dopaminergic neurons in the α-syn-enriched model. In a cellular model, UA treatment decreased internalized cytosolic α-syn levels and neuron-to-neuron transmission of α-syn in donor-acceptor cell models by modulating dynamin-mediated and clathrin-mediated endocytosis. Moreover, UA elevation in α-syn-inoculated mice inhibited propagation of extracellular α-syn which decreased expression of phosphorylated α-syn in the dopaminergic neurons of the substantia nigra leading to their increased survival. UA treatment did not lead to change in markers related with autophagolysosomal and microglial activity under the same experimental conditions. These findings suggest UA may control the pathological conditions of PD via additive mechanisms which modulate the propagation of α-syn.

Chemical and Biological Strategies for Profiling Protein-Protein Interactions in Living Cells

Protein-protein interactions (PPIs) play critical roles in almost all cellular signal transduction events. Characterization of PPIs without interfering with the functions of intact cells is very important for basic biology study and drug developments. However, the ability to profile PPIs especially those weak/transient interactions in their native states remains quite challenging. To this end, many endeavors are being made in developing new methods with high efficiency and strong operability. By coupling with advanced fluorescent microscopy and mass spectroscopy techniques, these strategies not only allow us to visualize the subcellular locations and monitor the functions of protein of interest (POI) in real time, but also enable the profiling and identification of potential unknown interacting partners in high-throughput manner, which greatly facilitates the elucidation of molecular mechanisms underlying numerous pathophysiological processes. In this review, we will summarize the typical methods for PPIs identification in living cells and their principles, advantages and limitations will also be discussed in detail.

Monitoring α-synuclein aggregation

Synucleinopathies, including Parkinson's disease (PD), dementia with Lewy Bodies (DLB), and multiple system atrophy (MSA), are characterized by the misfolding and subsequent aggregation of alpha-synuclein (α-syn) that accumulates in cytoplasmic inclusions bodies in the cells of affected brain regions. Since the seminal report of likely-aggregated α-syn presence within the Lewy bodies by Spillantini et al. in 1997, the keyword "synuclein aggregation" has appeared in over 6000 papers (Source: PubMed October 2022). Studying, observing, describing, and quantifying α-syn aggregation is therefore of paramount importance, whether it happens in tubo, in vitro, in post-mortem samples, or in vivo. The past few years have witnessed tremendous progress in understanding aggregation mechanisms and identifying various polymorphs. In this context of growing complexity, it is of utmost importance to understand what tools we possess, what exact information they provide, and in what context they may be applied. Nonetheless, it is also crucial to rationalize the relevance of the information and the limitations of these methods for gauging the final result. In this review, we present the main techniques that have shaped the current views about α-syn structure and dynamics, with particular emphasis on the recent breakthroughs that may change our understanding of synucleinopathies.

Monitoring alpha-synuclein oligomerization and aggregation using bimolecular fluorescence complementation assays: What you see is not always what you get

Bimolecular fluorescence complementation (BiFC) was introduced a decade ago as a method to monitor alpha‐synuclein (α‐syn) oligomerization in intact cells. Since then, several α‐syn BiFC cellular assays and animal models have been developed based on the assumption that an increase in the fluorescent signal correlates with increased α‐syn oligomerization or aggregation. Despite the increasing use of these assays and models in mechanistic studies, target validation and drug screening, there have been no reports that (1) validate the extent to which the BiFC fluorescent signal correlates with α‐syn oligomerization at the biochemical level; (2) provide a structural characterization of the oligomers and aggregates formed by the BiFC. To address this knowledge gap, we first analysed the expression level and oligomerization properties of the individual constituents of α‐syn‐Venus, one of the most commonly used BiFC systems, in HEK‐293 & SH‐SY5Y cells from three different laboratories using multiple biochemical approaches and techniques. Next, we investigated the biochemical and aggregation properties of α‐syn upon co‐expression of both BiFC fragments. Our results show that (1) the C‐terminal‐Venus fused to α‐syn (α‐syn‐Vc) is present in much lower abundance than its counterpart with N‐terminal‐Venus fused to α‐syn (Vn‐α‐syn); (2) Vn‐α‐syn exhibits a high propensity to form oligomers and higher‐order aggregates; and (3) the expression of either or both fragments does not result in the formation of α‐syn fibrils or cellular inclusions. Furthermore, our results suggest that only a small fraction of Vn‐α‐syn is involved in the formation of the fluorescent BiFC complex and that some of the fluorescent signal may arise from the association or entrapment of α‐syn‐Vc in Vn‐α‐syn aggregates. The fact that the N‐terminal fragment exists predominantly in an aggregated state also indicates that one must exercise caution when using this system to investigate α‐syn oligomerization in cells or in vivo. Altogether, our results suggest that cellular and animal models of oligomerization, aggregation and cell‐to‐cell transmission based on the α‐syn BiFC systems should be thoroughly characterized at the biochemical level to ensure that they reproduce the process of interest and measure what they are intended to measure.

Effects of pharmacological modulators of α-synuclein and tau aggregation and internalization

Parkinson's disease (PD) and Alzheimer's disease (AD) are common neurodegenerative disorders of the elderly and, therefore, affect a growing number of patients worldwide. Both diseases share, as a common hallmark, the accumulation of characteristic protein aggregates, known as Lewy bodies (LB) in PD, and neurofibrillary tangles in AD. LBs are primarily composed of misfolded α-synuclein (aSyn), and neurofibrillary tangles are primarily composed of tau protein. Importantly, upon pathological evaluation, most AD and PD/Lewy body dementia cases exhibit mixed pathology, with the co-occurrence of both LB and neurofibrillary tangles, among other protein inclusions. Recent studies suggest that both aSyn and tau pathology can spread and propagate through neuronal connections. Therefore, it is important to investigate the mechanisms underlying aggregation and propagation of these proteins for the development of novel therapeutic strategies. Here, we assessed the effects of different pharmacological interventions on the aggregation and internalization of tau and aSyn. We found that anle138b and fulvic acid decrease aSyn and tau aggregation, that epigallocatechin gallate decreases aSyn aggregation, and that dynasore reduces tau internalization. Establishing the effects of small molecules with different chemical properties on the aggregation and spreading of aSyn and tau will be important for the development of future therapeutic interventions.

Arylsulfatase A, a genetic modifier of Parkinson's disease, is an α-synuclein chaperone

Mutations in lysosomal genes increase the risk of neurodegenerative diseases, as is the case for Parkinson's disease. Here, we found that pathogenic and protective mutations in arylsulfatase A (ARSA), a gene responsible for metachromatic leukodystrophy, a lysosomal storage disorder, are linked to Parkinson's disease. Plasma ARSA protein levels were changed in Parkinson's disease patients. ARSA deficiency caused increases in α-synuclein aggregation and secretion, and increases in α-synuclein propagation in cells and nematodes. Despite being a lysosomal protein, ARSA directly interacts with α-synuclein in the cytosol. The interaction was more extensive with protective ARSA variant and less with pathogenic ARSA variant than wild-type. ARSA inhibited the in vitro fibrillation of α-synuclein in a dose-dependent manner. Ectopic expression of ARSA reversed the α-synuclein phenotypes in both cell and fly models of synucleinopathy, the effects correlating with the extent of the physical interaction between these molecules. Collectively, these results suggest that ARSA is a genetic modifier of Parkinson's disease pathogenesis, acting as a molecular chaperone for α-synuclein.

Characterization of the activity, aggregation, and toxicity of heterodimers of WT and ALS-associated mutant Sod1

Aggregation of the antioxidant enzyme Sod1 represents common factors of both familial (fALS) and sporadic cases of ALS, a fatal neurodegenerative disease. Although many ALS studies have focused on Sod1 homodimers/homomers, the investigation of Sod1 heterodimers/heteromers remains controversial and has mostly been performed with recombinant proteins in vitro, in the absence of a cellular environment. By using living cells, this study sheds light into a critical issue in the context of fALS, the high toxicity of the WT–mutant heteromeric inclusions, especially WT–A4V heteromers which accumulate both in human cells as well as in chronologically aged yeast cells. Besides the aggregation, we proposed that an inefficient heteromer response against oxidative conditions might contribute to fALS-linked mutant hSod1 toxicity.

Mutations in Cu/Zn superoxide dismutase (Sod1) have been reported in both familial and sporadic amyotrophic lateral sclerosis (ALS). In this study, we investigated the behavior of heteromeric combinations of wild-type (WT) and mutant Sod1 proteins A4V, L38V, G93A, and G93C in human cells. We showed that both WT and mutant Sod1 formed dimers and oligomers, but only mutant Sod1 accumulated in intracellular inclusions. Coexpression of WT and hSod1 mutants resulted in the formation of a larger number of intracellular inclusions per cell than that observed in cells coexpressing WT or mutant hSod1. The number of inclusions was greater in cells expressing A4V hSod1. To eliminate the contribution of endogenous Sod1, and better evaluate the effect of ALS-associated mutant Sod1 expression, we expressed human Sod1 WT and mutants in human cells knocked down for endogenous Sod1 (Sod1-KD), and in sod1Δ yeast cells. Using Sod1-KD cells we found that the WT–A4V heteromers formed higher molecular weight species compared with A4V and WT homomers. Using the yeast model, in conditions of chronological aging, we concluded that cells expressing Sod1 heterodimers showed decreased antioxidant activity, increased oxidative damage, reduced longevity, and oxidative stress-induced mutant Sod1 aggregation. In addition, we also found that ALS-associated Sod1 mutations reduced nuclear localization and, consequently, impaired the antioxidant response, suggesting this change in localization may contribute to disease in familial ALS. Overall, our study provides insight into the molecular underpinnings of ALS and may open avenues for the design of future therapeutic strategies.

Iron-mediated aggregation and toxicity in a novel neuronal cell culture model with inducible alpha-synuclein expression

Parkinson's disease (PD) represents an increasing problem in society. The oligomerization of alpha-synuclein (αSyn) is a suggested key event in its pathogenesis, yet the pathological modes of action remain to be fully elucidated. To identify potential disease-modifying therapeutics and to study αSyn-mediated toxic mechanisms, we established cell lines with inducible overexpression of different αSyn constructs: αSyn, αSyn coupled to the fluorescence protein Venus (αSyn-Venus), and αSyn coupled to the N-terminal or C-terminal part of Venus (V1S and SV2, respectively) for a bimolecular fluorescence complementation assay (BiFC). Inducibility was achieved by applying modified GAL4-UAS or Cre-loxP systems and addition of tebufenozide or 4-OH-tamoxifen, respectively. Expression constructs were stably integrated into the host genome of H4 neuroglioma cells by lentiviral transduction. We here demonstrate a detailed investigation of the expression characteristics of inducible H4 cells showing low background expression and high inducibility. We observed increased protein load and aggregation of αSyn upon incubation with DMSO and FeCl₃ along with an increase in cytotoxicity. In summary, we present a system for the creation of inducibly αSyn-overexpressing cell lines holding high potential for the screening for modulators of αSyn aggregation and αSyn-mediated toxicity.

Mapping Transient Protein Interactions at the Nanoscale in Living Mammalian Cells

Protein-protein interactions (PPIs) form the basis of cellular processes, regulating cell behavior and fate. PPIs can be extremely transient in nature, which hinders their detection. In addition, traditional biochemical methods provided limited information on the spatial distribution and temporal dynamics of PPIs that is crucial for their regulation in the crowded cellular environment. Given the pivotal role of membrane micro- and nanodomains in the regulation of PPIs at the plasma membrane, the development of methods to visualize PPIs with a high spatial resolution is imperative. Here, we present a super-resolution fluorescence microscopy technique that can detect and map short-lived transient protein-protein interactions on a nanometer scale in the cellular environment. This imaging method is based on single-molecule fluorescence microscopy and exploits the effect of the difference in the mobility between cytosolic and membrane-bound proteins in the recorded fluorescence signals. After the development of the proof of concept using a model system based on membrane-bound modular protein domains and fluorescently labeled peptides, we applied this imaging approach to investigate the interactions of cytosolic proteins involved in the epidermal growth factor signaling pathway (namely, Grb2, c-Raf, and PLCγ1). The detected clusters of Grb2 and c-Raf were correlated with the distribution of the receptor at the plasma membrane. Additionally, the interactions of wild type PLCγ1 were compared with those detected with truncated mutants, which provided important information regarding the role played by specific domains in the interaction with the membrane. The results presented here demonstrate the potential of this technique to unravel the role of membrane heterogeneity in the spatiotemporal regulation of cell signaling.

Implications of fALS Mutations on Sod1 Function and Oligomerization in Cell Models

Among the familial forms of amyotrophic lateral sclerosis (fALS), 20% are associated with the Cu,Zn-superoxide dismutase (Sod1). fALS is characterized by the accumulation of aggregated proteins and the increase in oxidative stress markers. Here, we used the non-invasive bimolecular fluorescence complementation (BiFC) assay in human H4 cells to investigate the kinetics of aggregation and subcellular localization of Sod1 mutants. We also studied the effect of the different Sod1 mutants to respond against oxidative stress by following the levels of reactive oxygen species (ROS) after treatment with hydrogen peroxide. Our results showed that only 30% of cells transfected with A4VSod1 showed no inclusions while for the other Sod1 mutants tested (L38V, G93A and G93C), this percentage was at least 70%. In addition, we found that 10% of cells transfected with A4VSod1 displayed more than five inclusions per cell and that A4V and G93A Sod1 formed inclusions more rapidly than L38V and G93C Sod1. Expression of WTSod1 significantly decreased the intracellular oxidation levels in comparison with expression of fALS Sod1 mutants, suggesting the mutations induce a functional impairment. All fALS mutations impaired nuclear localization of Sod1, which is important for maintaining genomic stability. Consistently, expression of WTSod1, but not of fALS Sod1 mutants, reduced DNA damage, as measured by the comet assay. Altogether, our study sheds light into the effects of fALS Sod1 mutations on inclusion formation, dynamics, and localization as well as on antioxidant response, opening novel avenues for investigating the role of fALS Sod1 mutations in pathogenesis.

Tau deletion promotes brain insulin resistance

The molecular pathways underlying tau pathology-induced synaptic/cognitive deficits and neurodegeneration are poorly understood. One prevalent hypothesis is that hyperphosphorylation, misfolding, and fibrillization of tau impair synaptic plasticity and cause degeneration. However, tau pathology may also result in the loss of specific physiological tau functions, which are largely unknown but could contribute to neuronal dysfunction. In the present study, we uncovered a novel function of tau in its ability to regulate brain insulin signaling. We found that tau deletion leads to an impaired hippocampal response to insulin, caused by altered IRS-1 and PTEN (phosphatase and tensin homologue on chromosome 10) activities. Our data also demonstrate that tau knockout mice exhibit an impaired hypothalamic anorexigenic effect of insulin that is associated with energy metabolism alterations. Consistently, we found that tau haplotypes are associated with glycemic traits in humans. The present data have far-reaching clinical implications and raise the hypothesis that pathophysiological tau loss-of-function favors brain insulin resistance, which is instrumental for cognitive and metabolic impairments in Alzheimer's disease patients.

Cellular models as tools for the study of the role of alpha-synuclein in Parkinson's disease

Neurodegenerative diseases are highly debilitating conditions characterised primarily by progressive neuronal loss and impairment of the nervous system. Parkinson's disease (PD) is one of the most common of these disorders, affecting 1-2% of the population above the age of 65. Although the underlying mechanisms of PD have been extensively studied, we still lack a full understanding of the molecular underpinnings of the disease. Thus, the in vitro and in vivo models currently used are able to only partially recapitulate the typical phenotypes of the disease. Here, we review various cell culture models currently used to study the molecular basis of PD, with a focus on alpha-synuclein-associated molecular pathologies. We also discuss how different cell models may constitute powerful tools for high-throughput screening of molecules capable of modulating alpha-synuclein toxicity.

Association of TrkA and APP Is Promoted by NGF and Reduced by Cell Death-Promoting Agents

The amyloid precursor protein (APP) interacts with the tropomyosin receptor kinase A (TrkA) in normal rat, mouse, and human brain tissue but not in Alzheimer's disease (AD) brain tissue. However, it has not been reported whether the two proteins interact directly, and if so, which domains are involved. Clarifying these points will increase our understanding of the role and regulation of the TrkA/APP interaction in normal brain functioning as well as in AD. Here we addressed these questions using bimolecular fluorescence complementation (BiFC) and the proximity ligation assay (PLA). We demonstrated that exogenously expressed APP and TrkA associate through their juxtamembrane/transmembrane domains, to form a complex that localizes mainly to the plasma membrane, endoplasmic reticulum (ER) and Golgi. Formation of the complex was inhibited by p75NTR, ShcC and Mint-2. Importantly, we demonstrated that the association between endogenous APP and TrkA in primary septal neurons were modified by NGF, or by drugs that either inhibit ER-to-Golgi transport or perturb microtubules and microfilaments. Interestingly, several agents that induce cell death [amyloid β (Aβ)-peptide, staurosporine and rapamycin], albeit via different mechanisms, all caused dissociation of APP/TrkA complexes and increased production of C-terminal fragment (β-CTF) APP fragment. These findings open new perspectives for investigating the interplay between these proteins during neurodegeneration and AD.

Traffic jams and the complex role of α-Synuclein aggregation in Parkinson disease

A common pathological event among various neurodegenerative disorders (NDs) is the misfolding and aggregation of different proteins in the brain. This is thought to potentiate aberrant protein-protein interactions that culminate in the disruption of several biological processes and, ultimately, in neuronal cell loss. Although protein aggregates are a common hallmark in several disorders, the molecular pathways leading to their generation remain unclear. The misfolding and aggregation of α-Synuclein (aSyn) is the pathological hallmark of Parkinson disease (PD), the second most common age related ND. It has been postulated that oligomeric species of aSyn, rather than more mature aggregated forms of the protein, are the causative agents of cytotoxicity. In recent years, we have been investigating the molecular mechanisms underlying the initial steps of aSyn accumulation in living cells. Using an unbiased genome-wide lentiviral RNAi screen we identified trafficking and kinase genes as modulators of aSyn oligomerization, aggregation, and toxicity. Among those, Rab8b, Rab11a, Rab13 and Slp5 were found to promote the clearance of aSyn inclusions and reduce aSyn toxicity. Moreover, we found that endocytic recycling and secretion of aSyn was enhanced upon expression of Rab11a or Rab13 in cells accumulating aSyn inclusions. Altogether, our findings suggest specific trafficking steps may prove beneficial as targets for therapeutic intervention in synucleinopathies, and should be further investigated in other models.

Intracellular formation of α-synuclein oligomers and the effect of heat shock protein 70 characterized by confocal single particle spectroscopy

Synucleinopathies such as dementia with Lewy bodies or Parkinson's disease are characterized by intracellular deposition of pathologically aggregated α-synuclein. The details of the molecular pathogenesis of PD and especially the conditions that lead to intracellular aggregation of α-synuclein and the role of these aggregates in cell death remain unknown. In cell free in vitro systems considerable knowledge about the aggregation processes has been gathered. In comparison, the knowledge about these aggregation processes in cells is far behind. In cells α-synuclein aggregates can be toxic. However, the crucial particle species responsible for decisive steps in pathogenesis such as seeding a continuing aggregation process and triggering cell death remain to be identified. In order to understand the complex nature of intracellular α-synuclein aggregate formation, we analyzed fluorescent particles formed by venus and α-synuclein-venus fusion proteins and α-synuclein-hemi-venus fusion proteins derived from gently lyzed cells. With these techniques we were able to identify and characterize α-synuclein oligomers formed in cells. Especially the use of α-synuclein-hemi-venus fusion proteins enabled us to identify very small α-synuclein oligomers with high sensitivity. Furthermore, we were able to study the molecular effect of heat shock protein 70, which is known to inhibit α-synuclein aggregation in cells. Heat shock protein 70 does not only influence the size of α-synuclein oligomers, but also their quantity. In summary, this approach based on fluorescence single particle spectroscopy, that is suited for high throughput measurements, can be used to detect and characterize intracellularly formed α-synuclein aggregates and characterize the effect of molecules that interfere with α-synuclein aggregate formation.

From the baker to the bedside: yeast models of Parkinson's disease

The baker’s yeast Saccharomyces cerevisiae has been extensively explored for our understanding of fundamental cell biology processes highly conserved in the eukaryotic kingdom. In this context, they have proven invaluable in the study of complex mechanisms such as those involved in a variety of human disorders. Here, we first provide a brief historical perspective on the emergence of yeast as an experimental model and on how the field evolved to exploit the potential of the model for tackling the intricacies of various human diseases. In particular, we focus on existing yeast models of the molecular underpinnings of Parkinson’s disease (PD), focusing primarily on the central role of protein quality control systems. Finally, we compile and discuss the major discoveries derived from these studies, highlighting their far-reaching impact on the elucidation of PD-associated mechanisms as well as in the identification of candidate therapeutic targets and compounds with therapeutic potential.

LRRK2 Promotes Tau Accumulation, Aggregation and Release

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are known as the most frequent cause of familial Parkinson's disease (PD), but are also present in sporadic cases. The G2019S-LRRK2 mutation is located in the kinase domain of the protein, and has consistently been reported to promote a gain of kinase function. Several proteins have been reported as LRRK2 substrates and/or interactors, suggesting possible pathways involved in neurodegeneration in PD. Hyperphosphorylated Tau protein accumulates in neurofibrillary tangles, a typical pathological hallmark in Alzheimer's disease and frontotemporal dementia. In addition, it is also frequently found in the brains of PD patients. Although LRRK2 is a kinase, it appears that a putative interaction with Tau is phosphorylation-independent. However, the underlying mechanisms and the cellular consequences of this interaction are still unclear. In this study, we demonstrate an interaction between LRRK2 and Tau and that LRRK2 promotes the accumulation of non-monomeric and high-molecular weight (HMW) Tau species independent of its kinase activity. Interestingly, we found that LRRK2 increases Tau secretion, possibly as a consequence of an impairment of Tau proteasomal degradation. Our data highlight a mechanism through which LRRK2 regulates intracellular Tau levels, contributing to the progression of the pathology caused by the LRRK2-mediated proteasome impairment. In total, our findings suggest that the interplay between LRRK2 and proteasome activity might constitute a valid target for therapeutic intervention in PD.

Glucocerebrosidase depletion enhances cell-to-cell transmission of α-synuclein

Deposition of α-synuclein aggregates occurs widely in the central and peripheral nervous systems in Parkinson's disease (PD). Although recent evidence has suggested that cell-to-cell transmission of α-synuclein aggregates is associated with the progression of PD, the mechanism by which α-synuclein aggregates spread remains undefined. Here, we show that α-synuclein aggregates are transmitted from cell to cell through a cycle involving uptake of external aggregates, co-aggregation with endogenous α-synuclein and exocytosis of the co-aggregates. Moreover, we find that glucocerebrosidase depletion, which has previously been strongly associated with PD and increased cognitive impairment, promotes propagation of α-synuclein aggregates. These studies define how α-synuclein aggregates spread among neuronal cells and may provide an explanation for how glucocerebrosidase mutations increase the risk of developing PD and other synucleinopathies.

DJ-1 interactions with α-synuclein attenuate aggregation and cellular toxicity in models of Parkinson's disease

Parkinson's disease (PD) is a devastating neurodegenerative disorder characterized by the loss of neurons in the substantia nigra pars compacta and the presence of Lewy bodies in surviving neurons. These intracellular protein inclusions are primarily composed of misfolded α-synuclein (aSyn), which has also been genetically linked to familial and sporadic forms of PD. DJ-1 is a small ubiquitously expressed protein implicated in several pathways associated with PD pathogenesis. Although mutations in the gene encoding DJ-1 lead to familial early-onset PD, the exact mechanisms responsible for its role in PD pathogenesis are still elusive. Previous work has found that DJ-1--which has protein chaperone-like activity--modulates aSyn aggregation. Here, we investigated possible physical interactions between aSyn and DJ-1 and any consequent functional and pathological relevance. We found that DJ-1 interacts directly with aSyn monomers and oligomers in vitro, and that this also occurs in living cells. Notably, several PD-causing mutations in DJ-1 constrain this interaction. In addition, we found that overexpression of DJ-1 reduces aSyn dimerization, whereas mutant forms of DJ-1 impair this process. Finally, we found that human DJ-1 as well as yeast orthologs of DJ-1 reversed aSyn-dependent cellular toxicity in Saccharomyces cerevisiae. Taken together, these data suggest that direct interactions between DJ-1 and aSyn constitute the basis for a neuroprotective mechanism and that familial mutations in DJ-1 may contribute to PD by disrupting these interactions.

Visualization and quantification of APP intracellular domain-mediated nuclear signaling by bimolecular fluorescence complementation

BACKGROUND

The amyloid precursor protein (APP) intracellular domain (AICD) is released from full-length APP upon sequential cleavage by either α- or β-secretase followed by γ-secretase. Together with the adaptor protein Fe65 and the histone acetyltransferase Tip60, AICD forms nuclear multiprotein complexes (AFT complexes) that function in transcriptional regulation.

OBJECTIVE

To develop a medium-throughput machine-based assay for visualization and quantification of AFT complex formation in cultured cells.

METHODS

We used cotransfection of bimolecular fluorescence complementation (BiFC) fusion constructs of APP and Tip60 for analysis of subcellular localization by confocal microscopy and quantification by flow cytometry (FC).

RESULTS

Our novel BiFC-constructs show a nuclear localization of AFT complexes that is identical to conventional fluorescence-tagged constructs. Production of the BiFC signal is dependent on the adaptor protein Fe65 resulting in fluorescence complementation only after Fe65-mediated nuclear translocation of AICD and interaction with Tip60. We applied the AFT-BiFC system to show that the Swedish APP familial Alzheimer's disease mutation increases AFT complex formation, consistent with the notion that AICD mediated nuclear signaling mainly occurs following APP processing through the amyloidogenic β-secretase pathway. Next, we studied the impact of posttranslational modifications of AICD on AFT complex formation. Mutation of tyrosine 682 in the YENPTY motif of AICD to phenylalanine prevents phosphorylation resulting in increased nuclear AFT-BiFC signals. This is consistent with the negative impact of tyrosine phosphorylation on Fe65 binding to AICD. Finally, we studied the effect of oxidative stress. Our data shows that oxidative stress, at a level that also causes cell death, leads to a reduction in AFT-BiFC signals.

CONCLUSION

We established a new method for visualization and FC quantification of the interaction between AICD, Fe65 and Tip60 in the nucleus based on BiFC. It enables flow cytometric analysis of AICD nuclear signaling and is characterized by scalability and low background fluorescence.

Subcellular localization and self-interaction of plant-specific Nt-4/1 protein

The Nicotiana tabacum Nt-4/1 protein is a plant-specific protein of unknown function. Analysis of bacterially expressed Nt-4/1 protein in vitro revealed that the protein secondary structure is mostly alpha-helical and suggested that it could consist of three structural domains. Earlier studies of At-4/1, the Arabidopsis thaliana-encoded ortholog of Nt-4/1, demonstrated that GFP-fused At-4/1 was capable of polar localization in plant cells, association with plasmodesmata, and cell-to-cell transport. Together with the At-4/1 ability to interact with a plant virus movement protein, these data supported the hypothesis of the At-4/1 protein involvement in viral transport through plasmodesmata. Studies of the Nt-4/1-GFP fusion protein reported in this paper revealed that the protein was localized to cytoplasmic bodies, which were co-aligned with actin filaments and capable of actin-dependent intracellular movement. The Nt-4/1-GFP bodies, being non-membrane structures, were found in association with the plasma membrane, the tubular endoplasmic reticulum and endosome-like structures. Bimolecular fluorescence complementation experiments and inhibition of nuclear export showed that the Nt-4/1 protein was capable of nuclear-cytoplasmic transport. The nuclear export signal (NES) was identified in the Nt-4/1 protein by site-directed mutagenesis. The Nt-4/1 NES mutant was localized to the nucleoplasm forming spherical bodies. Immunogold labeling and electron microscopy of cytoplasmic Nt-4/1-containing bodies and nuclear structures containing the Nt-4/1 NES mutant revealed differences in their fine structure. In mammalian cells, Nt-4/1-GFP formed cytoplasmic spherical bodies similar to those found for the Nt-4/1 NES mutant in plant cell nuclei. Using dynamic laser light scattering and electron microscopy, the Nt-4/1 protein was found to form multimeric complexes in vitro.

Live-cell imaging of p53 interactions using a novel Venus-based bimolecular fluorescence complementation system

p53 plays an important role in regulating a wide variety of cellular processes, such as cell cycle arrest and/or apoptosis. Dysfunction of p53 is frequently associated with several pathologies, such as cancer and neurodegenerative diseases. In recent years substantial progress has been made in developing novel p53-activating molecules. Importantly, modulation of p53 interaction with its main inhibitor, Mdm2, has been highlighted as a promising therapeutic target. In this regard, bimolecular fluorescence complementation (BiFC) analysis, by providing direct visualization of protein interactions in living cells, offers a straightforward method to identify potential modulators of protein interactions. In this study, we developed a simple and robust Venus-based BiFC system to screen for modulators of p53-p53 and p53-Mdm2 interactions in live mammalian cells. We used nutlin-3, a well-known disruptor of p53-Mdm2 interaction, to validate the specificity of the assay. The reduction of BiFC signal mediated by nutlin-3 was correlated with an increase in Puma transactivation, PARP cleavage, and cell death. Finally, this novel BiFC approach was exploited to identify potential modulators of p53-Mdm2 complex formation among a commercially available chemical library of 33 protein phosphatase inhibitors. Our results constitute "proof-of-concept" that this model has strong potential as an alternative to traditional target-based drug discovery strategies. Identification of new modulators of p53-p53 and p53-Mdm2 interactions will be useful to achieve synergistic drug efficacy with currently used anti-tumor therapies.

Diversity in genetic in vivo methods for protein-protein interaction studies: from the yeast two-hybrid system to the mammalian split-luciferase system

The yeast two-hybrid system pioneered the field of in vivo protein-protein interaction methods and undisputedly gave rise to a palette of ingenious techniques that are constantly pushing further the limits of the original method. Sensitivity and selectivity have improved because of various technical tricks and experimental designs. Here we present an exhaustive overview of the genetic approaches available to study in vivo binary protein interactions, based on two-hybrid and protein fragment complementation assays. These methods have been engineered and employed successfully in microorganisms such as Saccharomyces cerevisiae and Escherichia coli, but also in higher eukaryotes. From single binary pairwise interactions to whole-genome interactome mapping, the self-reassembly concept has been employed widely. Innovative studies report the use of proteins such as ubiquitin, dihydrofolate reductase, and adenylate cyclase as reconstituted reporters. Protein fragment complementation assays have extended the possibilities in protein-protein interaction studies, with technologies that enable spatial and temporal analyses of protein complexes. In addition, one-hybrid and three-hybrid systems have broadened the types of interactions that can be studied and the findings that can be obtained. Applications of these technologies are discussed, together with the advantages and limitations of the available assays.

Quantitative analysis of α-synuclein solubility in living cells using split GFP complementation

Presently incurable, Parkinson's disease (PD) is the most common neurodegenerative movement disorder and affects 1% of the population over 60 years of age. The hallmarks of PD pathogenesis are the loss of dopaminergic neurons in the substantia nigra pars compacta, and the occurrence of proteinaceous cytoplasmic inclusions (Lewy bodies) in surviving neurons. Lewy bodies are mainly composed of the pre-synaptic protein alpha-synuclein (αsyn), an intrinsically unstructured, misfolding-prone protein with high propensity to aggregate. Quantifying the pool of soluble αsyn and monitoring αsyn aggregation in living cells is fundamental to study the molecular mechanisms of αsyn-induced cytotoxicity and develop therapeutic strategies to prevent αsyn aggregation. In this study, we report the use of a split GFP complementation assay to quantify αsyn solubility. Particularly, we investigated a series of naturally occurring and rationally designed αsyn variants and showed that this method can be used to study how αsyn sequence specificity affects its solubility. Furthermore, we demonstrated the utility of this assay to explore the influence of the cellular folding network on αsyn solubility. The results presented underscore the utility of the split GFP assay to quantify αsyn solubility in living cells.

Formation and toxicity of soluble polyglutamine oligomers in living cells

BACKGROUND

Aggregation and cytotoxicity of mutant proteins containing an expanded number of polyglutamine (polyQ) repeats is a hallmark of several diseases, including Huntington's disease (HD). Within cells, mutant Huntingtin (mHtt) and other polyglutamine expansion mutant proteins exist as monomers, soluble oligomers, and insoluble inclusion bodies (IBs). Determining which of these forms constitute a toxic species has proven difficult. Recent studies support a role for IBs as a cellular coping mechanism to sequester levels of potentially toxic soluble monomeric and oligomeric species of mHtt.

METHODOLOGY/PRINCIPAL FINDINGS

When fused to a fluorescent reporter (GFP) and expressed in cells, the soluble monomeric and oligomeric polyglutamine species are visually indistinguishable. Here, we describe two complementary biophysical fluorescence microscopy techniques to directly detect soluble polyglutamine oligomers (using Htt exon 1 or Htt(ex1)) and monitor their fates in live cells. Photobleaching analyses revealed a significant reduction in the mobilities of mHtt(ex1) variants consistent with their incorporation into soluble microcomplexes. Similarly, when fused to split-GFP constructs, both wildtype and mHtt(ex1) formed oligomers, as evidenced by the formation of a fluorescent reporter. Only the mHtt(ex1) split-GFP oligomers assembled into IBs. Both FRAP and split-GFP approaches confirmed the ability of mHtt(ex1) to bind and incorporate wildtype Htt into soluble oligomers. We exploited the irreversible binding of split-GFP fragments to forcibly increase levels of soluble oligomeric mHtt(ex1). A corresponding increase in the rate of IBs formation and the number formed was observed. Importantly, higher levels of soluble mHtt(ex1) oligomers significantly correlated with increased mutant cytotoxicity, independent of the presence of IBs.

CONCLUSIONS/SIGNIFICANCE

Our study describes powerful and sensitive tools for investigating soluble oligomeric forms of expanded polyglutamine proteins, and their impact on cell viability. Moreover, these methods should be applicable for the detection of soluble oligomers of a wide variety of aggregation prone proteins.