Abnormal alpha-synuclein interactions with rab3a and rabphilin in diffuse Lewy body disease

Cell-to-cell transmitted alpha-synuclein recapitulates experimental Parkinson's disease

Parkinson's disease is characterized by a progressive accumulation of alpha-Synuclein (αSyn) neuronal inclusions called Lewy bodies in the nervous system. Lewy bodies can arise from the cell-to-cell propagation of αSyn, which can occur via sequential steps of secretion and uptake. Here, by fusing a removable short signal peptide to the N-terminus of αSyn, we developed a novel mouse model with enhanced αSyn secretion and cell-to-cell transmission. Expression of the secreted αSyn in the mouse brain was under the control of a novel hybrid promoter in combination with adeno-associated virus serotype 9 (AAV9). This combination of promoter and viral vector induced a robust expression in neurons but not in the glia of injected mice. Biochemical characterization of the secreted αSyn revealed that, in cultured cells, this protein is released to the extracellular milieu via conventional secretion. The released αSyn is then internalized and processed by acceptor cells via the endosome-lysosome pathway indicating that the secreted αSyn is cell-to-cell transmitted. The secreted αSyn is aggregation-prone and amyloidogenic, and when expressed in the brain of wild-type non-transgenic mice, it induces a Parkinson's disease-like phenotype that includes a robust αSyn pathology in the substantia nigra, neuronal loss, neuroinflammation, and motor deficits, all the key features of experimental animal models of Parkinson's disease. In summary, a novel animal model of Parkinson's disease based on enhanced cell-to-cell transmission of αSyn was developed. The neuron-produced cell-to-cell transmitted αSyn triggers all phenotypic features of experimental Parkinson's disease in mice.

An Update on the Interplay between LRRK2, Rab GTPases and Parkinson's Disease

Over the last decades, research on the pathobiology of neurodegenerative diseases has greatly evolved, revealing potential targets and mechanisms linked to their pathogenesis. Parkinson's disease (PD) is no exception, and recent studies point to the involvement of endolysosomal defects in PD. The endolysosomal system, which tightly controls a flow of endocytosed vesicles targeted either for degradation or recycling, is regulated by a number of Rab GTPases. Their associations with leucine-rich repeat kinase 2 (LRRK2), a major causative and risk protein of PD, has also been one of the hot topics in the field. Understanding their interactions and functions is critical for unraveling their contribution to PD pathogenesis. In this review, we summarize recent studies on LRRK2 and Rab GTPases and attempt to provide more insight into the interaction of LRRK2 with each Rab and its relationship to PD.

A Proteome-Wide Effect of PHF8 Knockdown on Cortical Neurons Shows Downregulation of Parkinson's Disease-Associated Protein Alpha-Synuclein and Its Interactors

Synaptic dysfunction may underlie the pathophysiology of Parkinson's disease (PD), a presently incurable condition characterized by motor and cognitive symptoms. Here, we used quantitative proteomics to study the role of PHD Finger Protein 8 (PHF8), a histone demethylating enzyme found to be mutated in X-linked intellectual disability and identified as a genetic marker of PD, in regulating the expression of PD-related synaptic plasticity proteins. Amongst the list of proteins found to be affected by PHF8 knockdown were Parkinson's-disease-associated SNCA (alpha synuclein) and PD-linked genes DNAJC6 (auxilin), SYNJ1 (synaptojanin 1), and the PD risk gene SH3GL2 (endophilin A1). Findings in this study show that depletion of PHF8 in cortical neurons affects the activity-induced expression of proteins involved in synaptic plasticity, synaptic structure, vesicular release and membrane trafficking, spanning the spectrum of pre-synaptic and post-synaptic transmission. Given that the depletion of even a single chromatin-modifying enzyme can affect synaptic protein expression in such a concerted manner, more in-depth studies will be needed to show whether such a mechanism can be exploited as a potential disease-modifying therapeutic drug target in PD.

Endosomal recycling protein Rab11 in Parkin and Pink1 signaling in Drosophila model of Parkinson's disease

Neurodegenerative diseases are progressive disorders of the nervous system primarily affecting the loss of neuronal cells present in the brain. Although most neurodegenerative cases are sporadic, some familial genes are found to be involved in the neurodegenerative diseases. The extensively studied parkin and pink1 gene products are known to be involved in the removal of damaged mitochondria via autophagy (mitophagy), a quality control process. If the function of any of these genes is somehow disrupted, accumulation of damaged mitochondria occurs in the forms of protein aggregates in the cytoplasm, leading to formation of the Lewy-bodies. Autophagy is an important catabolic process where the endosomal Rab proteins are seen to be involved. Rab11, an endosomal recycling protein, serves as an ATG9A carrier that helps in autophagosome formation and maturation. Earlier studies have reported that loss of Rab11 prevents the fusion of autophagosomes with the late endosomes hampering the autophagy pathway resulting in apoptosis of cells. In this study, we have emphasized on the importance and functional role of Rab11 in the molecular pathway of Parkin/Pink1 in Parkinson's disease.

The Role of Rab Proteins in Parkinson's Disease Synaptopathy

In patients affected by Parkinson's disease (PD), the most common neurodegenerative movement disorder, the brain is characterized by the loss of dopaminergic neurons in the nigrostriatal system, leading to dyshomeostasis of the basal ganglia network activity that is linked to motility dysfunction. PD mostly arises as an age-associated sporadic disease, but several genetic forms also exist. Compelling evidence supports that synaptic damage and dysfunction characterize the very early phases of either sporadic or genetic forms of PD and that this early PD synaptopathy drives retrograde terminal-to-cell body degeneration, culminating in neuronal loss. The Ras-associated binding protein (Rab) family of small GTPases, which is involved in the maintenance of neuronal vesicular trafficking, synaptic architecture and function in the central nervous system, has recently emerged among the major players in PD synaptopathy. In this manuscript, we provide an overview of the main findings supporting the involvement of Rabs in either sporadic or genetic PD pathophysiology, and we highlight how Rab alterations participate in the onset of early synaptic damage and dysfunction.

Rabphilin-3A as a Novel Target to Reverse α-synuclein-induced Synaptic Loss in Parkinson's Disease

Toxic aggregates of α-synuclein (αsyn) are considered key drivers of Parkinson's disease (PD) pathology. In early PD, αsyn induces synaptic dysfunction also modulating the glutamatergic neurotransmission. However, a more detailed understanding of the molecular mechanisms underlying αsyn-triggered synaptic failure is required to design novel therapeutic interventions. Here, we described the role of Rabphilin-3A (Rph3A) as novel target to counteract αsyn-induced synaptic loss in PD. Rph3A is a synaptic protein interacting with αsyn and involved in stabilizing dendritic spines and in promoting the synaptic retention of NMDA-type glutamate receptors. We found that in vivo intrastriatal injection of αsyn-preformed fibrils in mice induces the early loss of striatal synapses associated with decreased synaptic levels of Rph3A and impaired Rph3A/NMDA receptors interaction. Modulating Rph3A striatal expression or interfering with the Rph3A/αsyn complex with a small molecule prevented dendritic spine loss and rescued associated early motor defects in αsyn-injected mice. Notably, the same experimental approaches prevented αsyn-induced synaptic loss in vitro in primary hippocampal neurons. Overall, these findings indicate that approaches aimed at restoring Rph3A synaptic functions can slow down the early synaptic detrimental effects of αsyn aggregates in PD.

Molecular and Functional Interactions of Alpha-Synuclein with Rab3a

Alpha-synuclein (a-Syn) is a presynaptic protein, the misfolding of which is associated with Parkinson’s disease. Rab GTPases are small guanine nucleotide binding proteins that play key roles in vesicle trafficking and have been associated with a-Syn function and dysfunction. a-Syn is enriched on synaptic vesicles, where it has been reported to interact with GTP-bound Rab3a, a master regulator of synaptic vesicle trafficking. a-Syn is known to bind weakly to Rab8a in solution via a positively charged patch, but the physiological implications of such interactions have not been explored. Here, we investigate direct interactions between a-Syn and Rab3a in solution and on lipid membranes using NMR spectroscopy. We find that the C terminus of a-Syn interacts with Rab3a in a manner similar to its previously reported interaction with Rab8a. While weak in solution, we demonstrate that this interaction becomes stronger when the proteins are bound to a membrane surface. The Rab3a binding site for a-Syn is similar to the surface that contacts the Rab3a effector rabphilin-3A, which modulates the enzymatic activity of Rab3a. Accordingly, we show that a-Syn inhibits GTP hydrolysis by Rab3a and that inhibition is more potent on the membrane surface, suggesting that their interaction may be functionally relevant. Finally, we show that phosphorylation of a-Syn residue Ser 129, a modification associated with Parkinson’s disease pathology, enhances its interactions with Rab3a and increases its ability to inhibit Rab3a GTP hydrolysis. These results represent the first observation of a functional role for synuclein-Rab interactions and for a-Syn Ser 129 phosphorylation.

Role of Rph3A in brain injury induced by experimental cerebral ischemia-reperfusion model in rats

Aim

The aim was to study the role of Rph3A in neuronal injury induced by cerebral ischemia‐reperfusion.

Methods

The protein and mRNA levels of Rph3A in penumbra were detected by Western blot. The localization of Rph3A in different cell types in penumbra was detected by immunofluorescence. Apoptosis in the brain was detected by TUNEL staining. We tested neurobehavioral evaluation using rotarod test, adhesive‐removal test, and Morris Water maze test. We examined the expression and localization of Rph3A in cultured neurons and astrocytes in vitro by Western blot and ELISA, respectively.

Results

The mRNA and protein levels of Rph3A had significantly increased in brain penumbra of the rat MCAO/R model. Rph3A was mainly distributed in neurons and astrocytes and was significantly increased by MCAO/R. We downregulated Rph3A and found that it further worsened the cerebral infarct, neuronal death and behavioral, cognitive, and memory impairments in rats after MCAO/R. We also found that ischemia‐reperfusion upregulated the in vitro protein level and secretion of Rph3A in astrocytes but led to a decrease in the protein level of Rph3A in neurons.

Conclusion

The increase in Rph3A in the brain penumbra may be an endogenous protective mechanism against ischemia‐reperfusion injury, which is mainly dominated by astrocytes.

Alpha-synuclein and cortico-striatal plasticity in animal models of Parkinson disease

Alpha-synuclein (α-synuclein) is a small, acidic protein containing 140 amino acids, highly expressed in the brain and primarily localized in the presynaptic terminals. It is found in high concentrations in Lewy Bodies, proteinaceous aggregates that constitute a typical histopathologic hallmark of Parkinson's disease. Altered environmental conditions, genetic mutations and post-translational changes can trigger abnormal aggregation processes with the increased frequency of oligomers, protofibrils, and fibrils formation that perturbs the neuronal homeostasis leading to cell death. Relevant to neuronal activity, a function of α-synuclein that has been extensively detailed is its regulatory actions in the trafficking of synaptic vesicles, including the processes of exocytosis, endocytosis and neurotransmitter release. Most recently, increasing attention has been paid to the possible role that α-synuclein plays at a postsynaptic level by interacting with selective subunits of the glutamate N-methyl-d-aspartate receptor, altering the corticostriatal plasticity of distinct neuronal populations.

Alpha-Synuclein and the Endolysosomal System in Parkinson's Disease: Guilty by Association

Abnormal accumulation of the protein α- synuclein (α-syn) into proteinaceous inclusions called Lewy bodies (LB) is the neuropathological hallmark of Parkinson's disease (PD) and related disorders. Interestingly, a growing body of evidence suggests that LB are also composed of other cellular components such as cellular membrane fragments and vesicular structures, suggesting that dysfunction of the endolysosomal system might also play a role in LB formation and neuronal degeneration. Yet the link between α-syn aggregation and the endolysosomal system disruption is not fully elucidated. In this review, we discuss the potential interaction between α-syn and the endolysosomal system and its impact on PD pathogenesis. We propose that the accumulation of monomeric and aggregated α-syn disrupt vesicles trafficking, docking, and recycling, leading to the impairment of the endolysosomal system, notably the autophagy-lysosomal degradation pathway. Reciprocally, PD-linked mutations in key endosomal/lysosomal machinery genes (LRRK2, GBA, ATP13A2) also contribute to increasing α-syn aggregation and LB formation. Altogether, these observations suggest a potential synergistic role of α-syn and the endolysosomal system in PD pathogenesis and represent a viable target for the development of disease-modifying treatment for PD and related disorders.

Dysfunction of RAB39B-Mediated Vesicular Trafficking in Lewy Body Diseases

Intracellular vesicular trafficking is essential for neuronal development, function, and homeostasis and serves to process, direct, and sort proteins, lipids, and other cargo throughout the cell. This intricate system of membrane trafficking between different compartments is tightly orchestrated by Ras analog in brain (RAB) GTPases and their effectors. Of the 66 members of the RAB family in humans, many have been implicated in neurodegenerative diseases and impairment of their functions contributes to cellular stress, protein aggregation, and death. Critically, RAB39B loss-of-function mutations are known to be associated with X-linked intellectual disability and with rare early-onset Parkinson's disease. Moreover, recent studies have highlighted altered RAB39B expression in idiopathic cases of several Lewy body diseases (LBDs). This review contextualizes the role of RAB proteins in LBDs and highlights the consequences of RAB39B impairment in terms of endosomal trafficking, neurite outgrowth, synaptic maturation, autophagy, as well as alpha-synuclein homeostasis. Additionally, the potential for therapeutic intervention is examined via a discussion of the recent progress towards the development of specific RAB modulators. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Cortical Proteins Associated With Cognitive Resilience in Community-Dwelling Older Persons

Importance

Identifying genes and proteins for cognitive resilience (ie, targets that may be associated with slowing or preventing cognitive decline regardless of the presence, number, or combination of common neuropathologic conditions) provides a complementary approach to developing novel therapeutics for the treatment and prevention of Alzheimer disease and related dementias.

Objective

To identify proteins associated with cognitive resilience via a proteome-wide association study of the human dorsolateral prefrontal cortex.

Design, Setting, and Participants

This study used data from 391 community-dwelling older persons who participated in the Religious Orders Study and the Rush Memory and Aging Project. The Religious Orders Study began enrollment January 1, 1994, and the Rush Memory and Aging Project began enrollment September 1, 1997, and data were collected and analyzed through October 23, 2019.

Exposures

Participants had undergone annual detailed clinical examinations, postmortem evaluations, and tandem mass tag proteomics analyses.

Main Outcomes and Measures

The outcome of cognitive resilience was defined as a longitudinal change in cognition over time after controlling for common age-related neuropathologic indices, including Alzheimer disease, Lewy bodies, transactive response DNA-binding protein 43, hippocampal sclerosis, infarcts, and vessel diseases. More than 8000 high abundance proteins were quantified from frozen dorsolateral prefrontal cortex tissue using tandem mass tag and liquid chromatography-mass spectrometry.

Results

There were 391 participants (273 women); their mean (SD) age was 79.7 (6.7) years at baseline and 89.2 (6.5) years at death. Eight cortical proteins were identified in association with cognitive resilience: a higher level of NRN1 (estimate, 0.140; SE, 0.024; P = 7.35 × 10-9), ACTN4 (estimate, 0.321; SE, 0.065; P = 9.94 × 10-7), EPHX4 (estimate, 0.198; SE, 0.042; P = 2.13 × 10-6), RPH3A (estimate, 0.148; SE, 0.031; P = 2.58 × 10-6), SGTB (estimate, 0.211; SE, 0.045; P = 3.28 × 10-6), CPLX1 (estimate, 0.136; SE, 0.029; P = 4.06 × 10-6), and SH3GL1 (estimate, 0.179; SE, 0.039; P = 4.21 × 10-6) and a lower level of UBA1 (estimate, -0.366; SE, 0.076; P = 1.43 × 10-6) were associated with greater resilience.

Conclusions and Relevance

These protein signals may represent novel targets for the maintenance of cognition in old age.

In Search of Effective Treatments Targeting α-Synuclein Toxicity in Synucleinopathies: Pros and Cons

Parkinson's disease (PD), multiple system atrophy (MSA) and Dementia with Lewy bodies (DLB) represent pathologically similar, progressive neurodegenerative disorders characterized by the pathological aggregation of the neuronal protein α-synuclein. PD and DLB are characterized by the abnormal accumulation and aggregation of α-synuclein in proteinaceous inclusions within neurons named Lewy bodies (LBs) and Lewy neurites (LNs), whereas in MSA α-synuclein inclusions are mainly detected within oligodendrocytes named glial cytoplasmic inclusions (GCIs). The presence of pathologically aggregated α-synuclein along with components of the protein degradation machinery, such as ubiquitin and p62, in LBs and GCIs is considered to underlie the pathogenic cascade that eventually leads to the severe neurodegeneration and neuroinflammation that characterizes these diseases. Importantly, α-synuclein is proposed to undergo pathogenic misfolding and oligomerization into higher-order structures, revealing self-templating conformations, and to exert the ability of "prion-like" spreading between cells. Therefore, the manner in which the protein is produced, is modified within neural cells and is degraded, represents a major focus of current research efforts in the field. Given that α-synuclein protein load is critical to disease pathogenesis, the identification of means to limit intracellular protein burden and halt α-synuclein propagation represents an obvious therapeutic approach in synucleinopathies. However, up to date the development of effective therapeutic strategies to prevent degeneration in synucleinopathies is limited, due to the lack of knowledge regarding the precise mechanisms underlying the observed pathology. This review critically summarizes the recent developed strategies to counteract α-synuclein toxicity, including those aimed to increase protein degradation, to prevent protein aggregation and cell-to-cell propagation, or to engage antibodies against α-synuclein and discuss open questions and unknowns for future therapeutic approaches.

Parkinson's: A Disease of Aberrant Vesicle Trafficking

Parkinson's disease (PD) is a leading cause of neurodegeneration that is defined by the selective loss of dopaminergic neurons and the accumulation of protein aggregates called Lewy bodies (LBs). The unequivocal identification of Mendelian inherited mutations in 13 genes in PD has provided transforming insights into the pathogenesis of this disease. The mechanistic analysis of several PD genes, including α-synuclein (α-syn), leucine-rich repeat kinase 2 (LRRK2), PTEN-induced kinase 1 (PINK1), and Parkin, has revealed central roles for protein aggregation, mitochondrial damage, and defects in endolysosomal trafficking in PD neurodegeneration. In this review, we outline recent advances in our understanding of these gene pathways with a focus on the emergent role of Rab (Ras analog in brain) GTPases and vesicular trafficking as a common mechanism that underpins how mutations in PD genes lead to neuronal loss. These advances have led to previously distinct genes such as vacuolar protein-sorting-associated protein 35 (VPS35) and LRRK2 being implicated in a common signaling pathway. A greater understanding of these common nodes of vesicular trafficking will be crucial for linking other PD genes and improving patient stratification in clinical trials underway against α-syn and LRRK2 targets.

The GTPase Rab27b regulates the release, autophagic clearance, and toxicity of α-synuclein

α-Synuclein (αsyn) is the primary component of proteinaceous aggregates termed Lewy bodies that pathologically define synucleinopathies including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). αsyn is hypothesized to spread through the brain in a prion-like fashion by misfolded protein forming a template for aggregation of endogenous αsyn. The cell-to-cell release and uptake of αsyn are considered important processes for its prion-like spread. Rab27b is one of several GTPases essential to the endosomal-lysosomal pathway and is implicated in protein secretion and clearance, but its role in αsyn spread has yet to be characterized. In this study, we used a paracrine αsyn in vitro neuronal model to test the impact of Rab27b on αsyn release, clearance, and toxicity. shRNA-mediated knockdown (KD) of Rab27b increased αsyn-mediated paracrine toxicity. Rab27b reduced αsyn release primarily through nonexosomal pathways, but the αsyn released after Rab27b KD was of higher-molecular-weight species, as determined by size-exclusion chromatography. Rab27b KD increased intracellular levels of insoluble αsyn and led to an accumulation of endogenous light chain 3 (LC3)-positive puncta. Rab27b KD also decreased LC3 turnover after treatment with an autophagosome-lysosome fusion inhibitor, chloroquine, indicating that Rab27b KD induces a defect in autophagic flux. Rab27b protein levels were increased in brain lysates obtained from postmortem tissues of individuals with PD and DLB compared with healthy controls. These data indicate a role for Rab27b in the release, clearance, and toxicity of αsyn and, ultimately, in the pathogenesis of synucleinopathies.

Insights into the pathogenesis of multiple system atrophy: focus on glial cytoplasmic inclusions

Multiple system atrophy (MSA) is a debilitating and fatal neurodegenerative disorder. The disease severity warrants urgent development of disease-modifying therapy, but the disease pathogenesis is still enigmatic. Neurodegeneration in MSA brains is preceded by the emergence of glial cytoplasmic inclusions (GCIs), which are insoluble α-synuclein accumulations within oligodendrocytes (OLGs). Thus, preventive strategies against GCI formation may suppress disease progression. However, although numerous studies have tried to elucidate the molecular pathogenesis of GCI formation, difficulty remains in understanding the pathological interaction between the two pivotal aspects of GCIs; α-synuclein and OLGs. The difficulty originates from several enigmas: 1) what triggers the initial generation and possible propagation of pathogenic α-synuclein species? 2) what contributes to OLG-specific accumulation of α-synuclein, which is abundantly expressed in neurons but not in OLGs? and 3) how are OLGs and other glial cells affected and contribute to neurodegeneration? The primary pathogenesis of GCIs may involve myelin dysfunction and dyshomeostasis of the oligodendroglial cellular environment such as autophagy and iron metabolism. We have previously reported that oligodendrocyte precursor cells are more prone to develop intracellular inclusions in the presence of extracellular fibrillary α-synuclein. This finding implies a possibility that the propagation of GCI pathology in MSA brains is mediated through the internalization of pathological α-synuclein into oligodendrocyte precursor cells. In this review, in order to discuss the pathogenesis of GCIs, we will focus on the composition of neuronal and oligodendroglial inclusions in synucleinopathies. Furthermore, we will introduce some hypotheses on how α-synuclein pathology spreads among OLGs in MSA brains, in the light of our data from the experiments with primary oligodendrocyte lineage cell culture. While various reports have focused on the mysterious source of α-synuclein in GCIs, insights into the mechanism which regulates the uptake of pathological α-synuclein into oligodendroglial cells may yield the development of the disease-modifying therapy for MSA. The interaction between glial cells and α-synuclein is also highlighted with previous studies of post-mortem human brains, cultured cells, and animal models, which provide comprehensive insight into GCIs and the MSA pathomechanisms.

LRRK2, alpha-synuclein, and tau: partners in crime or unfortunate bystanders?

The identification of genetic forms of Parkinson's disease (PD) has tremendously expanded our understanding of the players and mechanisms involved. Mutations in the genes encoding for alpha-synuclein (aSyn), LRRK2, and tau have been associated with familial and sporadic forms of the disease. aSyn is the major component of Lewy bodies and Lewy neurites, which are pathognomonic protein inclusions in PD. Hyperphosphorylated tau protein accumulates in neurofibrillary tangles in the brains of Alzheimer's disease patients but is also seen in the brains of PD patients. LRRK2 is a complex multi-domain protein with kinase and GTPase enzymatic activity. Since aSyn and tau are phosphoproteins, we review the possible interplay between the three proteins. Understanding the interplay between LRRK2, aSyn and tau is extremely important, as this may enable the identification of novel targets and pathways for therapeutic intervention.

Statins and the Brain: More than Lipid Lowering Agents?

Background:

Statins represent a class of medications widely prescribed to efficiently treat dyslipidemia. These drugs inhibit 3-βhydroxy 3β-methylglutaryl Coenzyme A reductase (HMGR), the rate-limiting enzyme of mevalonate (MVA) pathway. Besides cholesterol, MVA pathway leads to the production of several other compounds, which are essen-tial in the regulation of a plethora of biological activities, including in the central nervous system. For these reasons, statins are able to induce pleiotropic actions, and acquire increased interest as potential and novel modulators in brain processes, es-pecially during pathological conditions.

Objective:

The purpose of this review is to summarize and examine the current knowledge about pharmacokinetic and phar-macodynamic properties of statins in the brain. In addition, effects of statin on brain diseases are discussed providing the most up-to-date information.

Methods:

Relevant scientific information was identified from PubMed database using the following keywords: statins and brain, central nervous system, neurological diseases, neurodegeneration, brain tumors, mood, stroke.

Results:

315 scientific articles were selected and analyzed for the writing of this review article. Several papers highlighted that statin treatment is effective in preventing or ameliorating the symptomatology of a number of brain pathologies. Howev-er, other studies failed to demonstrate a neuroprotective effect.

Conclusion:

Even though considerable research studies suggest pivotal functional outcomes induced by statin therapy, addi-tional investigation is required to better determine the pharmacological effectiveness of statins in the brain, and support their clinical use in the management of different neuropathologies.

Soluble α-synuclein facilitates priming and fusion by releasing Ca2+ from the thapsigargin-sensitive Ca2+ pool in PC12 cells

α-Synuclein is associated with Parkinson's disease, and is mainly localized in presynaptic terminals and regulates exocytosis, but its physiological roles remain controversial. Here, we studied the effects of soluble and aggregated α-synuclein on exocytosis, and explored the molecular mechanism by which α-synuclein interacts with regulatory proteins, including Rab3A, Munc13-1 (also known as Unc13a) and Munc18-1 (also known as STXBP1), in order to regulate exocytosis. Through fluorescence recovery after photobleaching experiments, overexpressed α-synuclein in PC12 cells was found to be in a monomeric form, which promotes exocytosis. In contrast, aggregated α-synuclein induced by lactacystin treatment inhibits exocytosis. Our results show that α-synuclein is involved in vesicle priming and fusion. α-Synuclein and phorbol 12-myristate 13-acetate (PMA), which is known to enhance vesicle priming mediated by Rab3A, Munc13-1 and Munc18-1, act on the same population of vesicles, but regulate priming independently. Furthermore, the results show a novel effects of α-synuclein on mobilizing Ca²⁺ release from thapsigargin-sensitive Ca²⁺ pools to enhance the ATP-induced [Ca²⁺]_i increase, which enhances vesicle fusion. Our results provide a detailed understanding of the action of α-synuclein during the final steps of exocytosis.

Salivary biomarkers for the diagnosis and monitoring of neurological diseases

Current research efforts on neurological diseases are focused on identifying novel disease biomarkers to aid in diagnosis, provide accurate prognostic information and monitor disease progression. With advances in detection and quantification methods in genomics, proteomics and metabolomics, saliva has emerged as a good source of samples for detection of disease biomarkers. Obtaining a sample of saliva offers multiple advantages over the currently tested biological fluids as it is a non-invasive, painless and simple procedure that does not require expert training or harbour undesirable side effects for the patients. Here, we review the existing literature on salivary biomarkers and examine their validity in diagnosing and monitoring neurodegenerative and neuropsychiatric disorders such as autism and Alzheimer's, Parkinson's and Huntington's disease. Based on the available research, amyloid beta peptide, tau protein, lactoferrin, alpha-synuclein, DJ-1 protein, chromogranin A, huntingtin protein, DNA methylation disruptions, and micro-RNA profiles provide display a reliable degree of consistency and validity as disease biomarkers.

Synapsin III is a key component of α-synuclein fibrils in Lewy bodies of PD brains

Lewy bodies (LB) and Lewy neurites (LN), which are primarily composed of α-synuclein (α-syn), are neuropathological hallmarks of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). We recently found that the neuronal phosphoprotein synapsin III (syn III) controls dopamine release via cooperation with α-syn and modulates α-syn aggregation. Here, we observed that LB and LN, in the substantia nigra of PD patients and hippocampus of one subject with DLB, displayed a marked immunopositivity for syn III. The in situ proximity ligation assay revealed the accumulation of numerous proteinase K-resistant neuropathological inclusions that contained both α-syn and syn III in tight association in the brain of affected subjects. Most strikingly, syn III was identified as a component of α-syn-positive fibrils in LB-enriched protein extracts from PD brains. Finally, a positive correlation between syn III and α-syn levels was detected in the caudate putamen of PD subjects. Collectively, these findings indicate that syn III is a crucial α-syn interactant and a key component of LB fibrils in the brain of patients affected by PD.

Mechanisms of α-Synuclein Induced Synaptopathy in Parkinson's Disease

Parkinson's disease (PD) is characterized by intracellular inclusions of aggregated and misfolded α-Synuclein (α-Syn), and the loss of dopaminergic (DA) neurons in the brain. The resulting motor abnormalities mark the progression of PD, while non-motor symptoms can already be identified during early, prodromal stages of disease. Recent studies provide evidence that during this early prodromal phase, synaptic and axonal abnormalities occur before the degenerative loss of neuronal cell bodies. These early phenotypes can be attributed to synaptic accumulation of toxic α-Syn. Under physiological conditions, α-Syn functions in its native conformation as a soluble monomer. However, PD patient brains are characterized by intracellular inclusions of insoluble fibrils. Yet, oligomers and protofibrils of α-Syn have been identified to be the most toxic species, with their accumulation at presynaptic terminals affecting several steps of neurotransmitter release. First, high levels of α-Syn alter the size of synaptic vesicle pools and impair their trafficking. Second, α-Syn overexpression can either misregulate or redistribute proteins of the presynaptic SNARE complex. This leads to deficient tethering, docking, priming and fusion of synaptic vesicles at the active zone (AZ). Third, α-Syn inclusions are found within the presynaptic AZ, accompanied by a decrease in AZ protein levels. Furthermore, α-Syn overexpression reduces the endocytic retrieval of synaptic vesicle membranes during vesicle recycling. These presynaptic alterations mediated by accumulation of α-Syn, together impair neurotransmitter exocytosis and neuronal communication. Although α-Syn is expressed throughout the brain and enriched at presynaptic terminals, DA neurons are the most vulnerable in PD, likely because α-Syn directly regulates dopamine levels. Indeed, evidence suggests that α-Syn is a negative modulator of dopamine by inhibiting enzymes responsible for its synthesis. In addition, α-Syn is able to interact with and reduce the activity of VMAT2 and DAT. The resulting dysregulation of dopamine levels directly contributes to the formation of toxic α-Syn oligomers. Together these data suggest a vicious cycle of accumulating α-Syn and deregulated dopamine that triggers synaptic dysfunction and impaired neuronal communication, ultimately causing synaptopathy and progressive neurodegeneration in Parkinson's disease.

Molecular Insights into the Roles of Rab Proteins in Intracellular Dynamics and Neurodegenerative Diseases

In eukaryotes, the cellular functions are segregated to membrane-bound organelles. This inherently requires sorting of metabolites to membrane-limited locations. Sorting the metabolites from ribosomes to various organelles along the intracellular trafficking pathways involves several integral cellular processes, including an energy-dependent step, in which the sorting of metabolites between organelles is catalyzed by membrane-anchoring protein Rab-GTPases (Rab). They contribute to relaying the switching of the secretory proteins between hydrophobic and hydrophilic environments. The intracellular trafficking routes include exocytic and endocytic pathways. In these pathways, numerous Rab-GTPases are participating in discrete shuttling of cargoes. Long-distance trafficking of cargoes is essential for neuronal functions, and Rabs are critical for these functions, including the transport of membranes and essential proteins for the development of axons and neurites. Rabs are also the key players in exocytosis of neurotransmitters and recycling of neurotransmitter receptors. Thus, Rabs are critical for maintaining neuronal communication, as well as for normal cellular physiology. Therefore, cellular defects of Rab components involved in neural functions, which severely affect normal brain functions, can produce neurological complications, including several neurodegenerative diseases. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways of Rab proteins and the impact of their defects on different neurodegenerative diseases. The insights gathered into the dynamics of Rabs that are described in this review provide new avenues for developing effective treatments for neurodegenerative diseases-associated with Rab defects.

Sisyphus in Neverland

The study of life and living organisms and the way in which these interact and organize to form social communities have been central to my career. I have been fascinated by biology, neurology, and neuropathology, but also by history, sociology, and art. Certain current historical, political, and social events, some occurring proximally but others affecting people in apparently distant places, have had an impact on me. Epicurus, Seneca, and Camus shared their philosophical positions which I learned from. Many scientists from various disciplines have been exciting sources of knowledge as well. I have created a world of hypothesis and experiments but I have also got carried away by serendipity following unexpected observations. It has not been an easy path; errors and wanderings are not uncommon, and opponents close to home much more abundant than one might imagine. Ambition, imagination, resilience, and endurance have been useful in moving ahead in response to setbacks. In the end, I have enjoyed my dedication to science and I am grateful to have glimpsed beauty in it. These are brief memories of a Spanish neuropathologist born and raised in Barcelona, EU.

Endolysosomal dysfunction in Parkinson's disease: Recent developments and future challenges

Increasingly, genetic, cell biological, and in vivo work emphasizes the role of the endolysosomal system dysfunction in Parkinson's disease pathogenesis. Yet many questions remain about the mechanisms by which primary endolysosomal dysfunction causes PD as well as how the endolysosomal system interacts with α-synuclein-mediated neurotoxicity. We recently described a new mouse model of parkinsonism in which loss of the endolysosomal protein Atp13a2 causes behavioral, neuropathological, and biochemical changes similar to those present in human subjects with ATP13A2 mutations. In this Scientific Perspectives, we revisit the evidence implicating the endolysosomal system in PD, current hypotheses of disease pathogenesis, and how recent studies refine these hypotheses and raise new questions for future research. © 2016 International Parkinson and Movement Disorder Society.

Rabphilin 3A: A novel target for the treatment of levodopa-induced dyskinesias

N-methyl-d-aspartate receptor (NMDAR) subunit composition strictly commands receptor function and pharmacological responses. Changes in NMDAR subunit composition have been documented in brain disorders such as Parkinson's disease (PD) and levodopa (L-DOPA)-induced dyskinesias (LIDs), where an increase of NMDAR GluN2A/GluN2B subunit ratio at striatal synapses has been observed. A therapeutic approach aimed at rebalancing NMDAR synaptic composition represents a valuable strategy for PD and LIDs. To this, the comprehension of the molecular mechanisms regulating the synaptic localization of different NMDAR subtypes is required. We have recently demonstrated that Rabphilin 3A (Rph3A) is a new binding partner of NMDARs containing the GluN2A subunit and that it plays a crucial function in the synaptic stabilization of these receptors. Considering that protein-protein interactions govern the synaptic retention of NMDARs, the purpose of this work was to analyse the role of Rph3A and Rph3A/NMDAR complex in PD and LIDs, and to modulate Rph3A/GluN2A interaction to counteract the aberrant motor behaviour associated to chronic L-DOPA administration. Thus, an array of biochemical, immunohistochemical and pharmacological tools together with electron microscopy were applied in this study. Here we found that Rph3A is localized at the striatal postsynaptic density where it interacts with GluN2A. Notably, Rph3A expression at the synapse and its interaction with GluN2A-containing NMDARs were increased in parkinsonian rats displaying a dyskinetic profile. Acute treatment of dyskinetic animals with a cell-permeable peptide able to interfere with Rph3A/GluN2A binding significantly reduced their abnormal motor behaviour. Altogether, our findings indicate that Rph3A activity is linked to the aberrant synaptic localization of GluN2A-expressing NMDARs characterizing LIDs. Thus, we suggest that Rph3A/GluN2A complex could represent an innovative therapeutic target for those pathological conditions where NMDAR composition is significantly altered.

PIF* promotes brain re-myelination locally while regulating systemic inflammation- clinically relevant multiple sclerosis M.smegmatis model

Neurologic disease diagnosis and treatment is challenging. Multiple Sclerosis (MS) is a demyelinating autoimmune disease with few clinical forms and uncertain etiology. Current studies suggest that it is likely caused by infection(s) triggering a systemic immune response resulting in antigen/non-antigen-related autoimmune response in central nervous system (CNS). New therapeutic approaches are needed. Secreted by viable embryos, PreImplantation Factor (PIF) possesses a local and systemic immunity regulatory role. Synthetic PIF (PIF) duplicates endogenous peptide's protective effect in pre-clinical autoimmune and transplantation models. PIF protects against brain hypoxia-ischemia by directly targeting microglia and neurons. In chronic experimental autoimmune encephalitis (EAE) model PIF reverses paralysis while promoting neural repair. Herein we report that PIF directly promotes brain re-myelination and reverses paralysis in relapsing remitting EAE MS model. PIF crosses the blood-brain barrier targeting microglia. Systemically, PIF decreases pro-inflammatory IL23/IL17 cytokines, while preserving CNS-specific T-cell repertoire. Global brain gene analysis revealed that PIF regulates critical Na⁺/K⁺/Ca⁺⁺ ions, amino acid and glucose transport genes expression. Further, PIF modulates oxidative stress, DNA methylation, cell cycle regulation, and protein ubiquitination while regulating multiple genes. In cultured astrocytes, PIF promotes BDNF-myelin synthesis promoter and SLC2A1 (glucose transport) while reducing deleterious E2F5, and HSP90ab1 (oxidative stress) genes expression. In cultured microglia, PIF increases anti-inflammatory IL10 while reducing pro-inflammatory IFNγ expression. Collectively, PIF promotes brain re-myelination and neuroprotection in relapsing remitting EAE MS model. Coupled with ongoing, Fast-Track FDA approved clinical trial, NCT#02239562 (immune disorder), current data supports PIF's translation for neurodegenerative disorders therapy.

Selective LRRK2 kinase inhibition reduces phosphorylation of endogenous Rab10 and Rab12 in human peripheral mononuclear blood cells

Genetic variation in the leucine-rich repeat kinase 2 (LRRK2) gene is associated with risk of familial and sporadic Parkinson’s disease (PD). To support clinical development of LRRK2 inhibitors as disease-modifying treatment in PD biomarkers for kinase activity, target engagement and kinase inhibition are prerequisite tools. In a combined proteomics and phosphoproteomics study on human peripheral mononuclear blood cells (PBMCs) treated with the LRRK2 inhibitor Lu AF58786 a number of putative biomarkers were identified. Among the phospho-site hits were known LRRK2 sites as well as two phospho-sites on human Rab10 and Rab12. LRRK2 dependent phosphorylation of human Rab10 and human Rab12 at positions Thr73 and Ser106, respectively, was confirmed in HEK293 and, more importantly, Rab10-pThr73 inhibition was validated in immune stimulated human PBMCs using two distinct LRRK2 inhibitors. In addition, in non-stimulated human PBMCs acute inhibition of LRRK2 with two distinct LRRK2 inhibitor compounds reduced Rab10-Thr73 phosphorylation in a concentration-dependent manner with apparent IC₅₀’s equivalent to IC₅₀’s on LRRK2-pSer935. The identification of Rab10 phosphorylated at Thr73 as a LRRK2 inhibition marker in human PBMCs strongly support inclusion of assays quantifying Rab10-pThr73 levels in upcoming clinical trials evaluating LRRK2 kinase inhibition as a disease-modifying treatment principle in PD.

Impaired endo-lysosomal membrane integrity accelerates the seeding progression of α-synuclein aggregates

In neurodegenerative diseases, seeding is a process initiated by the internalization of exogenous protein aggregates. Multiple pathways for internalization of aggregates have been proposed, including direct membrane penetration and endocytosis. To decipher the seeding mechanisms of alpha-synuclein (αS) aggregates in human cells, we visualized αS aggregation, endo-lysosome distribution, and endo-lysosome rupture in real-time. Our data suggest that exogenous αS can seed endogenous cytoplasmic αS by either directly penetrating the plasma membrane or via endocytosis-mediated endo-lysosome rupture, leading to formation of endo-lysosome-free or endo-lysosome-associated αS aggregates, respectively. Further, we demonstrate that αS aggregates isolated from postmortem human brains with diffuse Lewy body disease (DLBD) preferentially show endocytosis-mediated seeding associated with endo-lysosome rupture and have significantly reduced seeding activity compared to recombinant αS aggregates. Colocalization of αS pathology with galectin-3 (a marker of endo-lysosomal membrane rupture) in the basal forebrain of DLBD, but not in age-matched controls, suggests endo-lysosome rupture is involved in the formation of αS pathology in humans. Interestingly, cells with endo-lysosomal membrane permeabilization (LMP) are more vulnerable to the seeding effects of αS aggregates. This study suggests that endo-lysosomal impairment in neurons might play an important role in PD progression.

Synaptic Vesicle-Recycling Machinery Components as Potential Therapeutic Targets

Presynaptic nerve terminals are highly specialized vesicle-trafficking machines. Neurotransmitter release from these terminals is sustained by constant local recycling of synaptic vesicles independent from the neuronal cell body. This independence places significant constraints on maintenance of synaptic protein complexes and scaffolds. Key events during the synaptic vesicle cycle-such as exocytosis and endocytosis-require formation and disassembly of protein complexes. This extremely dynamic environment poses unique challenges for proteostasis at synaptic terminals. Therefore, it is not surprising that subtle alterations in synaptic vesicle cycle-associated proteins directly or indirectly contribute to pathophysiology seen in several neurologic and psychiatric diseases. In contrast to the increasing number of examples in which presynaptic dysfunction causes neurologic symptoms or cognitive deficits associated with multiple brain disorders, synaptic vesicle-recycling machinery remains an underexplored drug target. In addition, irrespective of the involvement of presynaptic function in the disease process, presynaptic machinery may also prove to be a viable therapeutic target because subtle alterations in the neurotransmitter release may counter disease mechanisms, correct, or compensate for synaptic communication deficits without the need to interfere with postsynaptic receptor signaling. In this article, we will overview critical properties of presynaptic release machinery to help elucidate novel presynaptic avenues for the development of therapeutic strategies against neurologic and neuropsychiatric disorders.

Transcriptional network analysis in frontal cortex in Lewy body diseases with focus on dementia with Lewy bodies

The present study investigates global transcriptional changes in frontal cortex area 8 in incidental Lewy Body disease (iLBD), Parkinson disease (PD) and Dementia with Lewy bodies (DLB). We identified different coexpressed gene sets associated with disease stages, and gene ontology categories enriched in gene modules and differentially expressed genes including modules or gene clusters correlated to iLBD comprising upregulated dynein genes and taste receptors, and downregulated innate inflammation. Focusing on DLB, we found modules with genes significantly enriched in functions related to RNA and DNA production, mitochondria and energy metabolism, purine metabolism, chaperone and protein folding system and synapses and neurotransmission (particularly the GABAergic system). The expression of more than fifty selected genes was assessed with real time quantitative polymerase chain reaction. Our findings provide, for the first time, evidence of molecular cortical alterations in iLBD and involvement of several key metabolic pathways and gene hubs in DLB which may underlie cognitive impairment and dementia.

Dementia with Lewy Bodies: Molecular Pathology in the Frontal Cortex in Typical and Rapidly Progressive Forms

Objectives

The goal of this study was to assess mitochondrial function, energy, and purine metabolism, protein synthesis machinery from the nucleolus to the ribosome, inflammation, and expression of newly identified ectopic olfactory receptors (ORs) and taste receptors (TASRs) in the frontal cortex of typical cases of dementia with Lewy bodies (DLB) and cases with rapid clinical course (rpDLB: 2 years or less) compared with middle-aged non-affected individuals, in order to learn about the biochemical abnormalities underlying Lewy body pathology.

Methods

Real-time quantitative PCR, mitochondrial enzymatic assays, and analysis of β-amyloid, tau, and synuclein species were used.

Results

The main alterations in DLB and rpDLB, which are more marked in the rapidly progressive forms, include (i) deregulated expression of several mRNAs and proteins of mitochondrial subunits, and reduced activity of complexes I, II, III, and IV of the mitochondrial respiratory chain; (ii) reduced expression of selected molecules involved in energy metabolism and increased expression of enzymes involved in purine metabolism; (iii) abnormal expression of nucleolar proteins, rRNA18S, genes encoding ribosomal proteins, and initiation factors of the transcription at the ribosome; (iv) discrete inflammation; and (v) marked deregulation of brain ORs and TASRs, respectively. Severe mitochondrial dysfunction involving activity of four complexes, minimal inflammatory responses, and dramatic altered expression of ORs and TASRs discriminate DLB from Alzheimer’s disease. Altered solubility and aggregation of α-synuclein, increased β-amyloid bound to membranes, and absence of soluble tau oligomers are common in DLB and rpDLB. Low levels of soluble β-amyloid are found in DLB. However, increased soluble β-amyloid 1–40 and β-amyloid 1–42, and increased TNFα mRNA and protein expression, distinguish rpDLB.

Conclusion

Molecular alterations in frontal cortex in DLB involve key biochemical pathways such as mitochondria and energy metabolism, protein synthesis, purine metabolism, among others and are accompanied by discrete innate inflammatory response.

Synaptic proteins in CSF relate to Parkinson's disease stage markers

Recent findings of morphological and functional changes in Parkinson's disease brains have shown altered synapse formation, but their role in cognitive decline is still an area under exploration. Here we measured the concentration of three key synaptic proteins, Rab3A, SNAP25 and neurogranin by enzyme-linked immunosorbent assay, in cerebrospinal fluid from a total of 139 participants (87 controls and 52 Parkinson's disease patients out of which 30 were drug-naïve) and explored their associations with motor and cognitive symptoms. Associations with motor disease stage (assessed by Hoehn and Yahr scale) and cognitive performance (assessed by the Montreal Cognitive Assessment scores) were explored. An overall increase in the concentration of SNAP25 was found in Parkinson's disease patients (p = 0.032). Increased neurogranin levels were found in the drug naïve patients subgroup (p = 0.023). Significant associations were observed between increased concentration of neurogranin and cognitive impairment in total Parkinson's disease group (p = 0.017), as well as in the drug naïve (p = 0.021) and with motor disease stage (p = 0.041). There were no significant disease-driven changes observed in the concentration of Rab3a. Concentrations SNAP25 and neurogranin were increased in cerebrospinal fluid of Parkinson's disease patients in a disease specific manner and related to cognitive and motor symptom severity. Future longitudinal studies should explore whether cerebrospinal fluid synaptic proteins can predict cognitive decline in Parkinson's disease.

Proteome Analysis of Potential Synaptic Vesicle Cycle Biomarkers in the Cerebrospinal Fluid of Patients with Sporadic Creutzfeldt-Jakob Disease

Sporadic Creutzfeldt-Jakob disease (sCJD) is the most frequent fatal human prion disease with a rapid progression and unknown mechanism. The synaptic vesicle (SV) cycle pathway has been a hot research field associated with many neurodegenerative diseases that affect synaptic function and thus may affect pathogenesis of the disorder. Here, we used the iTRAQ-based proteomic method and a KEGG pathway enrichment analysis to meticulously analyze all pathways involved in sCJD disease. In total, 1670 proteins were validated in pooled cerebrospinal fluid (CSF) from 20 patients with sCJD compared with that from 13 patients without CJD. The demographic analysis demonstrated that 557 proteins were upregulated and 595 proteins were downregulated with a 1.5-fold change, and 690 proteins involved in 39 pathways changed significantly (p ≤ 0.05) according to the enrichment analysis. The SV cycle pathway and proteins involved were further evaluated, and 14 proteins were confirmed to participate in the SV cycle pathway due to increased expression. Six key proteins, such as AP2A1, SYT1, SNAP25, STXBP1, CLTB, and Rab3a, showed the same trend by western blot as detected by iTRAQ. This is the first study to use high-throughput proteomics to accurately identify and quantify proteins in the SV cycle pathway of a neurodegenerative disease. These results will help define the mechanism and provide new insight into the pathogenetic factors involved in the SV cycle pathway in patients with sCJD. We hope that promising biomarkers can be identified in the CSF of patients with sCJD and other neurodegenerative disorders to help predict disease progression.

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.

Functions of Rab Proteins at Presynaptic Sites

Presynaptic neurotransmitter release is dominated by the synaptic vesicle (SV) cycle and entails the biogenesis, fusion, recycling, reformation or turnover of synaptic vesicles-a process involving bulk movement of membrane and proteins. As key mediators of membrane trafficking, small GTPases from the Rab family of proteins play critical roles in this process by acting as molecular switches that dynamically interact with and regulate the functions of different sets of macromolecular complexes involved in each stage of the cycle. Importantly, mutations affecting Rabs, and their regulators or effectors have now been identified that are implicated in severe neurological and neurodevelopmental disorders. Here, we summarize the roles and functions of presynaptic Rabs and discuss their involvement in the regulation of presynaptic function.

Strain-Specific Altered Regulatory Response of Rab7a and Tau in Creutzfeldt-Jakob Disease and Alzheimer's Disease

There is an increasing demand for the understanding of pathophysiology on neurodegeneration diseases at early stages. Changes in endocytic machinery and the cytoskeleton-associated response are the first alterations observed in Creutzfeldt-Jakob disease (CJD) and Alzheimer's disease AD brain. In this study, we performed a targeted search for endocytic pathway proteins in the different regions of the brain. We found late endosome marker Rab7a which was significantly upregulated in the frontal cortex region in the rapid progressive CJD form (MM1) and rapid progressive AD (rpAD) forms. However, Rab9 expression was significantly downregulated only in CJD-MM1 brain frontal cortex region. In the cerebellum, Rab7a expression showed significant upregulation in both subtype MM1 and VV2 CJD forms, in contrast to Rab9 which showed significant downregulation in both subtype MM1 and VV2 CJD forms at terminal stage of the disease. To check regulatory response at pre-symptomatic stage of the disease, we checked the regulatory interactive response of Rab7a, Rab9, and known biomarkers PrP^C and tau forms in frontal cortex at pre-symptomatic stage of the disease in tg340 mice expressing about fourfold of human PrP-M129 with PrP-null background that had been inoculated with human sCJD MM1 brain tissue homogenates (sCJD MM1 mice). In addition, we analyzed 5XFAD mice, exhibiting five mutations in the APP and presenilin genes related to familial Alzheimer's disease (FAD), to validate specific regulatory response of Rab7a, Rab9, tau, and phosphorylated form of tau by immunostaining 5XFAD mice in comparison with the wild-type age-matched mice brain. The cortical region of 5XFAD mice brain showed accumulated form of Rab7a in puncta that co-label for p-Tau, indicating colocalization by using confocal laser-scanning microscopy and was confirmed by using reverse co-immunoprecipitation. Furthermore, synthetic RNA (siRNA) against the Rab7a gene decreased expression of Rab7a protein, in cortical primary neuronal cultures of PrP^C wild type. This depleted expression of Rab7a led to the increased accumulation of PrP^C in Rab9-positive endosomal compartments and consequently an increased co-localization between PrP^C/Rab9; however, total tau level decreased. Interestingly, siRNA against tau gene in cortical primary neuronal cultures of PrP^C wild-type mice showed enhanced Rab7a and Rab9 expression and increase formation of dendritic spines. The work described highlighted the selective involvement of late endosomal compartment marker Rab7a in CJD, slow and rapid progressive forms of AD pathogenesis.

Synaptic dysfunction and septin protein family members in neurodegenerative diseases

Cognitive decline and disease progression in different neurodegenerative diseases typically involves synaptic dysfunction preceding the neuronal loss. The synaptic dysfunction is suggested to be caused by imbalanced synaptic plasticity i.e. enhanced induction of long-term depression and concomitantly decreased long-term potentiation accompanied with excess stimulation of extrasynaptic N-Methyl-D-aspartate (NMDA) receptors due to various disturbances in pre- and postsynaptic sites. Recent research has identified neurodegenerative disease-related changes in protein accumulation and aggregation, gene expression, and protein functions, which may contribute to imbalanced synaptic function. Nevertheless, a comprehensive understanding of the mechanisms regulating synaptic plasticity in health and disease is still lacking and therefore characterization of new candidates involved in these mechanisms is needed. Septins, a highly conserved group of guanosine-5'-triphosphate (GTP)-binding proteins, show high neuronal expression and are implicated in the regulation of synaptic vesicle trafficking and neurotransmitter release. In this review, we first summarize the evidence how synaptic dysfunction is related to the pathogenesis of Alzheimer's, Parkinson's and Huntington's disease and frontotemporal lobar degeneration. Then, we discuss different aspects of the potential involvement of the septin family members in the regulation of synaptic function in relation to the pathogenesis of neurodegenerative diseases.

Rab11 modulates α-synuclein-mediated defects in synaptic transmission and behaviour

A central pathological hallmark of Parkinson's disease (PD) is the presence of proteinaceous depositions known as Lewy bodies, which consist largely of the protein α-synuclein (aSyn). Mutations, multiplications and polymorphisms in the gene encoding aSyn are associated with familial forms of PD and susceptibility to idiopathic PD. Alterations in aSyn impair neuronal vesicle formation/transport, and likely contribute to PD pathogenesis by neuronal dysfunction and degeneration. aSyn is functionally associated with several Rab family GTPases, which perform various roles in vesicle trafficking. Here, we explore the role of the endosomal recycling factor Rab11 in the pathogenesis of PD using Drosophila models of aSyn toxicity. We find that aSyn induces synaptic potentiation at the larval neuromuscular junction by increasing synaptic vesicle (SV) size, and that these alterations are reversed by Rab11 overexpression. Furthermore, Rab11 decreases aSyn aggregation and ameliorates several aSyn-dependent phenotypes in both larvae and adult fruit flies, including locomotor activity, degeneration of dopaminergic neurons and shortened lifespan. This work emphasizes the importance of Rab11 in the modulation of SV size and consequent enhancement of synaptic function. Our results suggest that targeting Rab11 activity could have a therapeutic value in PD.

Alpha-synuclein function and dysfunction on cellular membranes

Alpha-synuclein is a small neuronal protein that is closely associated with the etiology of Parkinson's disease. Mutations in and alterations in expression levels of alpha-synuclein cause autosomal dominant early onset heredity forms of Parkinson's disease, and sporadic Parkinson's disease is defined in part by the presence of Lewy bodies and Lewy neurites that are composed primarily of alpha-synuclein deposited in an aggregated amyloid fibril state. The normal function of alpha-synuclein is poorly understood, and the precise mechanisms by which it leads to toxicity and cell death are also unclear. Although alpha-synuclein is a highly soluble, cytoplasmic protein, it binds to a variety of cellular membranes of different properties and compositions. These interactions are considered critical for at least some normal functions of alpha-synuclein, and may well play critical roles in both the aggregation of the protein and its mechanisms of toxicity. Here we review the known features of alpha-synuclein membrane interactions in the context of both the putative functions of the protein and of its pathological roles in disease.

α-Synuclein interacts with the switch region of Rab8a in a Ser129 phosphorylation-dependent manner

Alpha-synuclein (αS) misfolding is associated with Parkinson's disease (PD) but little is known about the mechanisms underlying αS toxicity. Increasing evidence suggests that defects in membrane transport play an important role in neuronal dysfunction. Here we demonstrate that the GTPase Rab8a interacts with αS in rodent brain. NMR spectroscopy reveals that the C-terminus of αS binds to the functionally important switch region as well as the C-terminal tail of Rab8a. In line with a direct Rab8a/αS interaction, Rab8a enhanced αS aggregation and reduced αS-induced cellular toxicity. In addition, Rab8 - the Drosophila ortholog of Rab8a - ameliorated αS-oligomer specific locomotor impairment and neuron loss in fruit flies. In support of the pathogenic relevance of the αS-Rab8a interaction, phosphorylation of αS at S129 enhanced binding to Rab8a, increased formation of insoluble αS aggregates and reduced cellular toxicity. Our study provides novel mechanistic insights into the interplay of the GTPase Rab8a and αS cytotoxicity, and underscores the therapeutic potential of targeting this interaction.

Decreased rabphilin 3A immunoreactivity in Alzheimer's disease is associated with Aβ burden

Synaptic dysfunction, together with neuritic plaques, neurofibrillary tangles and cholinergic neuron loss is an established finding in the Alzheimer's disease (AD) neocortex. The synaptopathology of AD is known to involve both pre- and postsynaptic components. However, the status of rabphilin 3A (RPH3A), which interacts with the SNARE complex and regulates synaptic vesicle exocytosis and Ca(2+)-triggered neurotransmitter release, is at present unclear. In this study, we measured RPH3A and its ligand Rab3A as well as several SNARE proteins in postmortem neocortex of patients with AD, and found specific reductions of RPH3A immunoreactivity compared with aged controls. RPH3A loss correlated with dementia severity, cholinergic deafferentation, and increased β-amyloid (Aβ) concentrations. Furthermore, RPH3A expression is selectively downregulated in cultured neurons treated with Aβ25-35 peptides. Our data suggest that presynaptic SNARE dysfunction forms part of the synaptopathology of AD.

The function of α-synuclein

Human genetics has indicated a causal role for the protein α-synuclein in the pathogenesis of familial Parkinson's disease (PD), and the aggregation of synuclein in essentially all patients with PD suggests a central role for this protein in the sporadic disorder. Indeed, the accumulation of misfolded α-synuclein now defines multiple forms of neural degeneration. Like many of the proteins that accumulate in other neurodegenerative disorders, however, the normal function of synuclein remains poorly understood. In this article, we review the role of synuclein at the nerve terminal and in membrane remodeling. We also consider the prion-like propagation of misfolded synuclein as a mechanism for the spread of degeneration through the neuraxis.

α-Synuclein in Parkinson's disease: pathogenic function and translation into animal models

Parkinson's disease is a common neurodegenerative disease characterised by the loss of dopaminergic neurons in the substantia nigra pars compacta and the formation of α-synuclein aggregates found in Lewy bodies throughout the brain. Several α-synuclein transgenic mouse models have been generated, as well as viral-mediated overexpression of wild-type and mutated α-synuclein to mimic the disease and to delineate the pathogenic pathway of α-synuclein-mediated toxicity and neurodegeneration. In this review, we will recapitulate what we have learned about the function of α-synuclein and α-synuclein-mediated toxicity through studies of transgenic animal models, inducible animal models and viral-based models.

The Rab family of proteins: 25 years on

Intracellular membrane trafficking requires the complex interplay of several classes of trafficking proteins. Rab proteins, the largest subfamily of the Ras superfamily of small G-proteins, are central regulators of all aspects of intracellular trafficking processes including vesicle budding and uncoating, motility, tethering and fusion. In the present paper, we discuss the discovery, evolution and characterization of the Rab GTPase family. We examine their basic functional roles, their important structural features and the regulatory proteins which mediate Rab function. We speculate on outstanding issues in the field, such as the mechanisms of Rab membrane association and the co-ordinated interplay between distinct Rab proteins. Finally, we summarize the data implicating Rab proteins in an ever increasing number of diseases.