Vps35 in cooperation with LRRK2 regulates synaptic vesicle endocytosis through the endosomal pathway in Drosophila

Mutant mice with rod-specific VPS35 deletion exhibit retinal α-synuclein pathology-associated degeneration

Vacuolar protein sorting 35 (VPS35), the core component of the retromer complex which regulates endosomal trafficking, is genetically linked with Parkinson's disease (PD). Impaired vision is a common non-motor manifestation of PD. Here, we show mouse retinas with VPS35-deficient rods exhibit synapse loss and visual deficit, followed by progressive degeneration concomitant with the emergence of Lewy body-like inclusions and phospho-α-synuclein (P-αSyn) aggregation. Ultrastructural analyses reveal VPS35-deficient rods accumulate aggregates in late endosomes, deposited as lipofuscins bound to P-αSyn. Mechanistically, we uncover a protein network of VPS35 and its interaction with HSC70. VPS35 deficiency promotes sequestration of HSC70 and P-αSyn aggregation in late endosomes. Microglia which engulf lipofuscins and P-αSyn aggregates are activated, displaying autofluorescence, observed as bright dots in fundus imaging of live animals, coinciding with pathology onset and progression. The Rod∆Vps35 mouse line is a valuable tool for further mechanistic investigation of αSyn lesions and retinal degenerative diseases.

Exploring the triad: VPS35, neurogenesis, and neurodegenerative diseases

Vacuolar protein sorting 35 (VPS35), a critical component of the retromer complex, plays a pivotal role in the pathogenesis of neurodegenerative diseases (NDs). It is involved in protein transmembrane sorting, facilitating the transport from endosomes to the trans-Golgi network (TGN) and plasma membrane. Recent investigations have compellingly associated mutations in the VPS35 gene with neurodegenerative disorders such as Parkinson's and Alzheimer's disease. These genetic alterations are implicated in protein misfolding, disrupted autophagic processes, mitochondrial dysregulation, and synaptic impairment. Furthermore, VPS35 exerts a notable impact on neurogenesis by influencing neuronal functionality, protein conveyance, and synaptic performance. Dysregulation or mutation of VPS35 may escalate the progression of neurodegenerative conditions, underscoring its pivotal role in safeguarding neuronal integrity. This review comprehensively discusses the role of VPS35 and its functional impairments in NDs. Furthermore, we provide an overview of the impact of VPS35 on neurogenesis and further explore the intricate relationship between neurogenesis and NDs. These research advancements offer novel perspectives and valuable insights for identifying potential therapeutic targets in the treatment of NDs.

Retromer-dependent lysosomal stress in Parkinson's disease

While causative mutations in complex disorders are rare, they can be used to extract a biological pathway whose pathogenicity can generalize to common forms of the disease. Here we begin by relying on the biological consequences of mutations in LRRK2 and VPS35, genetic causes of autosomal-dominant Parkinson's disease, to hypothesize that 'Retromer-dependent lysosomal stress' represents a pathway that can generalize to idiopathic Parkinson's disease. Next, we outline a series of studies that can test this hypothesis, including the development of biomarkers of pathway dysfunction. If validated, the hypothesis can suggest a unified mechanism of disease and might inform future diagnostic and therapeutic investigations. This article is part of a discussion meeting issue 'Understanding the endo-lysosomal network in neurodegeneration'.

The exocyst subunit Sec15 is critical for proper synaptic development and function at the Drosophila NMJ

The exocyst protein complex is important for targeted vesicle fusion in a variety of cell types, however, its function in neurons is still not entirely known. We found that presynaptic knockdown (KD) of the exocyst component sec15 by transgenic RNAi expression caused a number of unexpected morphological and physiological defects in the synapse. These include the development of active zones (AZ) devoid of essential presynaptic proteins, an increase in the branching of the presynaptic arbor, the appearance of satellite boutons, and a decrease in the amplitude of stimulated postsynaptic currents as well as a decrease in the frequency of spontaneous synaptic vesicle release. We also found the release of extracellular vesicles from the presynaptic neuron was greatly diminished in the Sec15 KDs. These effects were mimicked by presynaptic knockdown of Rab11, a protein known to interact with the exocyst. sec15 RNAi expression caused an increase in phosphorylated Mothers against decapentaplegic (pMad) in the presynaptic terminal, an indication of enhanced bone morphogenic protein (BMP) signaling. Some morphological phenotypes caused by Sec15 knockdown were reduced by attenuation of BMP signaling through knockdown of wishful thinking (Wit), while other phenotypes were unaffected. Individual knockdown of multiple proteins of the exocyst complex also displayed a morphological phenotype similar to Sec15 KD. We conclude that Sec15, functioning as part of the exocyst complex, is critically important for proper formation and function of neuronal synapses. We propose a model in which Sec15 is involved in the trafficking of vesicles from the recycling endosome to the cell membrane as well as possibly trafficking extracellular vesicles for presynaptic release and these processes are necessary for the correct structure and function of the synapse.

The involvement of α-synucleinopathy in the disruption of microglial homeostasis contributes to the pathogenesis of Parkinson's disease

The intracellular deposition and intercellular transmission of α-synuclein (α-syn) are shared pathological characteristics among neurodegenerative disorders collectively known as α-synucleinopathies, including Parkinson's disease (PD). Although the precise triggers of α-synucleinopathies remain unclear, recent findings indicate that disruption of microglial homeostasis contributes to the pathogenesis of PD. Microglia play a crucial role in maintaining optimal neuronal function by ensuring a homeostatic environment, but this function is disrupted during the progression of α-syn pathology. The involvement of microglia in the accumulation, uptake, and clearance of aggregated proteins is critical for managing disease spread and progression caused by α-syn pathology. This review summarizes current knowledge on the interrelationships between microglia and α-synucleinopathies, focusing on the remarkable ability of microglia to recognize and internalize extracellular α-syn through diverse pathways. Microglia process α-syn intracellularly and intercellularly to facilitate the α-syn neuronal aggregation and cell-to-cell propagation. The conformational state of α-synuclein distinctly influences microglial inflammation, which can affect peripheral immune cells such as macrophages and lymphocytes and may regulate the pathogenesis of α-synucleinopathies. We also discuss ongoing research efforts to identify potential therapeutic approaches targeting both α-syn accumulation and inflammation in PD. Video Abstract.

The role of membrane trafficking and retromer complex in Parkinson's and Alzheimer's disease

Membrane trafficking is a physiological process encompassing different pathways involved in transporting cellular products across cell membranes to specific cell locations via encapsulated vesicles. This process is required for cells to mature and function properly, allowing them to adapt to their surroundings. The retromer complex is a complex composed of nexin proteins and peptides that play a vital role in the endosomal pathway of membrane trafficking. In humans, any interference in normal membrane trafficking or retromer complex can cause profound changes such as those seen in neurodegenerative disorders such as Alzheimer's and Parkinson's. Several studies have explored the potential causative mechanisms in developing both disease processes; however, the role of retromer trafficking in their pathogenesis is becoming increasingly significant with promising therapeutic applications. This manuscript describes the processes involved in membrane transport and the roles of the retromer in the onset and progression of Alzheimer's and Parkinson's. Moreover, we will also explore how these aberrant mechanisms may serve as possible avenues for treatment development in both diseases and the prospect of its future application.

Molecular Mechanisms of AMPA Receptor Trafficking in the Nervous System

Synaptic plasticity enhances or reduces connections between neurons, affecting learning and memory. Postsynaptic AMPARs mediate greater than 90% of the rapid excitatory synaptic transmission in glutamatergic neurons. The number and subunit composition of AMPARs are fundamental to synaptic plasticity and the formation of entire neural networks. Accordingly, the insertion and functionalization of AMPARs at the postsynaptic membrane have become a core issue related to neural circuit formation and information processing in the central nervous system. In this review, we summarize current knowledge regarding the related mechanisms of AMPAR expression and trafficking. The proteins related to AMPAR trafficking are discussed in detail, including vesicle-related proteins, cytoskeletal proteins, synaptic proteins, and protein kinases. Furthermore, significant emphasis was placed on the pivotal role of the actin cytoskeleton, which spans throughout the entire transport process in AMPAR transport, indicating that the actin cytoskeleton may serve as a fundamental basis for AMPAR trafficking. Additionally, we summarize the proteases involved in AMPAR post-translational modifications. Moreover, we provide an overview of AMPAR transport and localization to the postsynaptic membrane. Understanding the assembly, trafficking, and dynamic synaptic expression mechanisms of AMPAR may provide valuable insights into the cognitive decline associated with neurodegenerative diseases.

Inhibition of LRRK2 kinase activity rescues deficits in striatal dopamine physiology in VPS35 p.D620N knock-in mice

Dysregulation of dopamine neurotransmission profoundly affects motor, motivation and learning behaviors, and can be observed during the prodromal phase of Parkinson's disease (PD). However, the mechanism underlying these pathophysiological changes remains to be elucidated. Mutations in vacuolar protein sorting 35 (VPS35) and leucine-rich repeat kinase 2 (LRRK2) both lead to autosomal dominant PD, and VPS35 and LRRK2 may physically interact to govern the trafficking of synaptic cargos within the endo-lysosomal network in a kinase-dependent manner. To better understand the functional role of VPS35 and LRRK2 on dopamine physiology, we examined Vps35 haploinsufficient (Haplo) and Vps35 p.D620N knock-in (VKI) mice and how their behavior, dopamine kinetics and biochemistry are influenced by LRRK2 kinase inhibitors. We found Vps35 p.D620N significantly elevates LRRK2-mediated phosphorylation of Rab10, Rab12 and Rab29. In contrast, Vps35 haploinsufficiency reduces phosphorylation of Rab12. While striatal dopamine transporter (DAT) expression and function is similarly impaired in both VKI and Haplo mice, that physiology is normalized in VKI by treatment with the LRRK2 kinase inhibitor, MLi-2. As a corollary, VKI animals show a significant increase in amphetamine induced hyperlocomotion, compared to Haplo mice, that is also abolished by MLi-2. Taken together, these data show Vps35 p.D620N confers a gain-of-function with respect to LRRK2 kinase activity, and that VPS35 and LRRK2 functionally interact to regulate DAT function and striatal dopamine transmission.

Drosophila Atlastin regulates synaptic vesicle mobilization independent of bone morphogenetic protein signaling

BACKGROUND

The endoplasmic reticulum (ER) contacts endosomes in all parts of a motor neuron, including the axon and presynaptic terminal, to move structural proteins, proteins that send signals, and lipids over long distances. Atlastin (Atl), a large GTPase, is required for membrane fusion and the structural dynamics of the ER tubules. Atl mutations are the second most common cause of Hereditary Spastic Paraplegia (HSP), which causes spasticity in both sexes' lower extremities. Through an unknown mechanism, Atl mutations stimulate the BMP (bone morphogenetic protein) pathway in vertebrates and Drosophila. Synaptic defects are caused by atl mutations, which affect the abundance and distribution of synaptic vesicles (SV) in the bouton. We hypothesize that BMP signaling, does not cause Atl-dependent SV abnormalities in Drosophila.

RESULTS

We show that atl knockdown in motor neurons (Atl-KD) increases synaptic and satellite boutons in the same way that constitutively activating the BMP-receptor Tkv (thick veins) (Tkv-CA) increases the bouton number. The SV proteins Cysteine string protein (CSP) and glutamate vesicular transporter are reduced in Atl-KD and Tkv-CA larvae. Reducing the activity of the BMP receptor Wishful thinking (wit) can rescue both phenotypes. Unlike Tkv-CA larvae, Atl-KD larvae display altered activity-dependent distributions of CSP staining. Furthermore, Atl-KD larvae display an increased FM 1-43 unload than Control and Tkv-CA larvae. As decreasing wit function does not reduce the phenotype, our hypothesis that BMP signaling is not involved is supported. We also found that Rab11/CSP colocalization increased in Atl-KD larvae, which supports the concept that late recycling endosomes regulate SV movements.

CONCLUSIONS

Our findings reveal that Atl modulates neurotransmitter release in motor neurons via SV distribution independently of BMP signaling, which could explain the observed SV accumulation and synaptic dysfunction. Our data suggest that Atl is involved in membrane traffic as well as formation and/or recycling of the late endosome.

Leucine-rich repeat kinase 2 at a glance

Leucine-rich repeat kinase 2 (LRRK2) is a multidomain scaffolding protein with dual guanosine triphosphatase (GTPase) and kinase enzymatic activities, providing this protein with the capacity to regulate a multitude of signalling pathways and act as a key mediator of diverse cellular processes. Much of the interest in LRRK2 derives from mutations in the LRRK2 gene being the most common genetic cause of Parkinson's disease, and from the association of the LRRK2 locus with a number of other human diseases, including inflammatory bowel disease. Therefore, the LRRK2 research field has focused on the link between LRRK2 and pathology, with the aim of uncovering the underlying mechanisms and, ultimately, finding novel therapies and treatments to combat them. From the biochemical and cellular functions of LRRK2, to its relevance to distinct disease mechanisms, this Cell Science at a Glance article and the accompanying poster deliver a snapshot of our current understanding of LRRK2 function, dysfunction and links to disease.

Retromer Stabilization Improves Cognitive Function and Synaptic Plasticity in a Mouse Model of Down Syndrome

BACKGROUND

Retromer complex proteins are decreased in postmortem brain tissues from Down syndrome subjects and inversely correlate with the Alzheimer's disease-like neuropathology. However, whether targeting in vivo the retromer system affects cognitive deficits and synaptic function in Down syndrome remains unknown.

OBJECTIVE

The aim of the current study was to examine the effects of pharmacological retromer stabilization on cognitive and synaptic functions in a mouse model of Down syndrome.

METHODS

Ts65dn mice were administered the pharmacological chaperone, TPT-172, or vehicle from 4 to 9 months of age and then assessed for changes in cognitive function. To assess the effects of TPT-172 on synaptic plasticity, hippocampal slices from Ts65dn mice were incubated in TPT-172 and used for field potential recordings.

RESULTS

Chronic TPT-172 treatment improved performance in cognitive function tests, its incubation with hippocampal slices ameliorated synaptic function response.

CONCLUSION

Pharmacological stabilization of the retromer complex improves synaptic plasticity and memory in a mouse model of Down syndrome. These results support the therapeutic potential of pharmacological retromer stabilization for individual with Down syndrome.

GTPase-activating protein TBC1D5 coordinates with retromer to constrain synaptic growth by inhibiting BMP signaling

Formation and plasticity of neural circuits rely on precise regulation of synaptic growth. At Drosophila neuromuscular junction (NMJ), Bone Morphogenetic Protein (BMP) signaling is critical for many aspects of synapse formation and function. The evolutionarily conserved retromer complex and its associated GTPase-activating protein TBC1D5 are critical regulators of membrane trafficking and cellular signaling. However, their functions in regulating the formation of NMJ are less understood. Here, we report that TBC1D5 is required for inhibition of synaptic growth, and loss of TBC1D5 leads to abnormal presynaptic terminal development, including excessive satellite boutons and branch formation. Ultrastructure analysis reveals that the size of synaptic vesicles and the density of subsynaptic reticulum are increased in TBC1D5 mutant boutons. Disruption of interactions of TBC1D5 with Rab7 and retromer phenocopies the loss of TBC1D5. Unexpectedly, we find that TBC1D5 is functionally linked to Rab6, in addition to Rab7, to regulate synaptic growth. Mechanistically, we show that loss of TBC1D5 leads to upregulated BMP signaling by increasing the protein level of BMP type II receptor Wishful Thinking (Wit) at NMJ. Overall, our data establish that TBC1D5 in coordination with retromer constrains synaptic growth by regulating Rab7 activity, which negatively regulates BMP signaling through inhibiting Wit level.

Sphingolipids in neurodegenerative diseases

Neurodegenerative Diseases (NDDs) are a group of disorders that cause progressive deficits of neuronal function. Recent evidence argues that sphingolipid metabolism is affected in a surprisingly broad set of NDDs. These include some lysosomal storage diseases (LSDs), hereditary sensory and autonomous neuropathy (HSAN), hereditary spastic paraplegia (HSP), infantile neuroaxonal dystrophy (INAD), Friedreich's ataxia (FRDA), as well as some forms of amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD). Many of these diseases have been modeled in Drosophila melanogaster and are associated with elevated levels of ceramides. Similar changes have also been reported in vertebrate cells and mouse models. Here, we summarize studies using fly models and/or patient samples which demonstrate the nature of the defects in sphingolipid metabolism, the organelles that are implicated, the cell types that are initially affected, and potential therapeutics for these diseases.

Rab11 and Its Role in Neurodegenerative Diseases

Vesicles mediate the trafficking of membranes/proteins in the endocytic and secretory pathways. These pathways are regulated by small GTPases of the Rab family. Rab proteins belong to the Ras superfamily of GTPases, which are significantly involved in various intracellular trafficking and signaling processes in the nervous system. Rab11 is known to play a key role especially in recycling many proteins, including receptors important for signal transduction and preservation of functional activities of nerve cells. Rab11 activity is controlled by GEFs (guanine exchange factors) and GAPs (GTPase activating proteins), which regulate its function through modulating GTP/GDP exchange and the intrinsic GTPase activity, respectively. Rab11 is involved in the transport of several growth factor molecules important for the development and repair of neurons. Overexpression of Rab11 has been shown to significantly enhance vesicle trafficking. On the other hand, a reduced expression of Rab11 was observed in several neurodegenerative diseases. Current evidence appears to support the notion that Rab11 and its cognate proteins may be potential targets for therapeutic intervention. In this review, we briefly discuss the function of Rab11 and its related interaction partners in intracellular pathways that may be involved in neurodegenerative processes.

Studies of neurodegenerative diseases using Drosophila and the development of novel approaches for their analysis

The use of Drosophila in neurodegenerative disease research has contributed to the identification of modifier genes for the pathology. The basis for neurodegenerative disease occurrence in Drosophila is the conservation of genes across species and the ability to perform rapid genetic analysis using a compact brain. Genetic findings previously discovered in Drosophila can reveal molecular pathologies involved in human neurological diseases in later years. Disease models using Drosophila began to be generated during the development of genetic engineering. In recent years, results of reverse translational research using Drosophila have been reported. In this review, we discuss research on neurodegenerative diseases; moreover, we introduce various methods for quantifying neurodegeneration in Drosophila.

Oxidative stress and synaptic dysfunction in rodent models of Parkinson's disease

Parkinson's disease (PD) is a multifactorial disorder involving a complex interplay between a variety of genetic and environmental factors. In this scenario, mitochondrial impairment and oxidative stress are widely accepted as crucial neuropathogenic mechanisms, as also evidenced by the identification of PD-associated genes that are directly involved in mitochondrial function. The concept of mitochondrial dysfunction is closely linked to that of synaptic dysfunction. Indeed, compelling evidence supports the role of mitochondria in synaptic transmission and plasticity, although many aspects have not yet been fully elucidated. Here, we will provide a brief overview of the most relevant evidence obtained in different neurotoxin-based and genetic rodent models of PD, focusing on mitochondrial impairment and synaptopathy, an early central event preceding overt nigrostriatal neurodegeneration. The identification of early deficits occurring in PD pathogenesis is crucial in view of the development of potential disease-modifying therapeutic strategies.

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.

Reduced synaptic activity and dysregulated extracellular matrix pathways in midbrain neurons from Parkinson's disease patients

Several mutations that cause Parkinson's disease (PD) have been identified over the past decade. These account for 15-25% of PD cases; the rest of the cases are considered sporadic. Currently, it is accepted that PD is not a single monolithic disease but rather a constellation of diseases with some common phenotypes. While rodent models exist for some of the PD-causing mutations, research on the sporadic forms of PD is lagging due to a lack of cellular models. In our study, we differentiated PD patient-derived dopaminergic (DA) neurons from the induced pluripotent stem cells (iPSCs) of several PD-causing mutations as well as from sporadic PD patients. Strikingly, we observed a common neurophysiological phenotype: neurons derived from PD patients had a severe reduction in the rate of synaptic currents compared to those derived from healthy controls. While the relationship between mutations in genes such as the SNCA and LRRK2 and a reduction in synaptic transmission has been investigated before, here we show evidence that the pathogenesis of the synapses in neurons is a general phenotype in PD. Analysis of RNA sequencing results displayed changes in gene expression in different synaptic mechanisms as well as other affected pathways such as extracellular matrix-related pathways. Some of these dysregulated pathways are common to all PD patients (monogenic or idiopathic). Our data, therefore, show changes that are central and convergent to PD and suggest a strong involvement of the tetra-partite synapse in PD pathophysiology.

Keeping Synapses in Shape: Degradation Pathways in the Healthy and Aging Brain

Synapses maintain their molecular composition, plasticity and function through the concerted action of protein synthesis and removal. The complex and polarized neuronal architecture poses specific challenges to the logistics of protein and organelle turnover since protein synthesis and degradation mainly happen in the cell soma. In addition, post-mitotic neurons accumulate damage over a lifetime, challenging neuronal degradative pathways and making them particularly susceptible to the effects of aging. This review will summarize the current knowledge on neuronal protein turnover mechanisms with a particular focus on the presynapse, including the proteasome, autophagy and the endolysosomal route and their roles in regulating presynaptic proteostasis and function. In addition, the author will discuss how physiological brain aging, which entails a progressive decline in cognitive functions, affects synapses and the degradative machinery.

Expression Analysis of Genes Involved in Transport Processes in Mice with MPTP-Induced Model of Parkinson’s Disease

Processes of intracellular and extracellular transport play one of the most important roles in the functioning of cells. Changes to transport mechanisms in a neuron can lead to the disruption of many cellular processes and even to cell death. It was shown that disruption of the processes of vesicular, axonal, and synaptic transport can lead to a number of diseases of the central nervous system, including Parkinson’s disease (PD). Here, we studied changes in the expression of genes whose protein products are involved in the transport processes (Snca, Drd2, Rab5a, Anxa2, and Nsf) in the brain tissues and peripheral blood of mice with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced models of PD. We detected changes in the expressions of Drd2, Anxa2, and Nsf at the earliest modeling stages. Additionally, we have identified conspicuous changes in the expression level of Anxa2 in the striatum and substantia nigra of mice with MPTP-induced models of PD in its early stages. These data clearly suggest the involvement of protein products in these genes in the earliest stages of the pathogenesis of PD.

Understanding the contributions of VPS35 and the retromer in neurodegenerative disease

Perturbations of the endolysosomal pathway have been suggested to play an important role in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease (PD) and Alzheimer's disease (AD). Specifically, VPS35 and the retromer complex play an important role in the endolysosomal system and are implicated in the pathophysiology of these diseases. A single missense mutation in VPS35, Asp620Asn (D620N), is known to cause late-onset, autosomal dominant familial PD. In this review, we focus on the emerging role of the PD-linked D620N mutation in causing retromer dysfunction and dissect its implications in neurodegeneration. Additionally, we will discuss how VPS35 and the retromer are linked to AD, amyotrophic lateral sclerosis, and primary tauopathies. Interestingly, reduced levels of VPS35 and other retromer components have been observed in post-mortem brain tissue, suggesting a role for the retromer in the pathophysiology of these diseases. This review will provide a comprehensive dive into the mechanisms of VPS35 dysfunction in neurodegenerative diseases. Furthermore, we will highlight outstanding questions in the field and the retromer as a therapeutic target for neurodegenerative disease at large.

Beware of Misdelivery: Multifaceted Role of Retromer Transport in Neurodegenerative Diseases

Retromer is a highly integrated multimeric protein complex that mediates retrograde cargo sorting from endosomal compartments. In concert with its accessory proteins, the retromer drives packaged cargoes to tubular and vesicular structures, thereby transferring them to the trans-Golgi network or to the plasma membrane. In addition to the endosomal trafficking, the retromer machinery participates in mitochondrial dynamics and autophagic processes and thus contributes to cellular homeostasis. The retromer components and their associated molecules are expressed in different types of cells including neurons and glial cells, and accumulating evidence from genetic and biochemical studies suggests that retromer dysfunction is profoundly involved in the pathogenesis of neurodegenerative diseases including Alzheimer’s Disease and Parkinson’s disease. Moreover, targeting retromer components could alleviate the neurodegenerative process, suggesting that the retromer complex may serve as a promising therapeutic target. In this review, we will provide the latest insight into the regulatory mechanisms of retromer and discuss how its dysfunction influences the pathological process leading to neurodegeneration.

Local regulation of extracellular vesicle traffic by the synaptic endocytic machinery

Neuronal extracellular vesicles (EVs) are locally released from presynaptic terminals, carrying cargoes critical for intercellular signaling and disease. EVs are derived from endosomes, but it is unknown how these cargoes are directed to the EV pathway rather than for conventional endolysosomal degradation. Here, we find that endocytic machinery plays an unexpected role in maintaining a release-competent pool of EV cargoes at synapses. Endocytic mutants, including nervous wreck (nwk), shibire/dynamin, and AP-2, unexpectedly exhibit local presynaptic depletion specifically of EV cargoes. Accordingly, nwk mutants phenocopy synaptic plasticity defects associated with loss of the EV cargo synaptotagmin-4 (Syt4) and suppress lethality upon overexpression of the EV cargo amyloid precursor protein (APP). These EV defects are genetically separable from canonical endocytic functions in synaptic vesicle recycling and synaptic growth. Endocytic machinery opposes the endosomal retromer complex to regulate EV cargo levels and acts upstream of synaptic cargo removal by retrograde axonal transport. Our data suggest a novel molecular mechanism that locally promotes cargo loading into synaptic EVs.

Organization of Presynaptic Autophagy-Related Processes

Brain synapses pose special challenges on the quality control of their protein machineries as they are far away from the neuronal soma, display a high potential for plastic adaptation and have a high energy demand to fulfill their physiological tasks. This applies in particular to the presynaptic part where neurotransmitter is released from synaptic vesicles, which in turn have to be recycled and refilled in a complex membrane trafficking cycle. Pathways to remove outdated and damaged proteins include the ubiquitin-proteasome system acting in the cytoplasm as well as membrane-associated endolysosomal and the autophagy systems. Here we focus on the latter systems and review what is known about the spatial organization of autophagy and endolysomal processes within the presynapse. We provide an inventory of which components of these degradative systems were found to be present in presynaptic boutons and where they might be anchored to the presynaptic apparatus. We identify three presynaptic structures reported to interact with known constituents of membrane-based protein-degradation pathways and therefore may serve as docking stations. These are (i) scaffolding proteins of the cytomatrix at the active zone, such as Bassoon or Clarinet, (ii) the endocytic machinery localized mainly at the peri-active zone, and (iii) synaptic vesicles. Finally, we sketch scenarios, how presynaptic autophagic cargos are tagged and recruited and which cellular mechanisms may govern membrane-associated protein turnover in the presynapse.

Drosophila: A Model to Study the Pathogenesis of Parkinson's Disease

Human Central Nervous System (CNS) is the complex part of the human body, which regulates multiple cellular and molecular events taking place simultaneously. Parkinsons Disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease (AD). The pathological hallmarks of PD are loss of dopaminergic neurons in the substantianigra (SN) pars compacta (SNpc) and accumulation of misfolded α-synuclein, in intra-cytoplasmic inclusions called Lewy bodies (LBs). So far, there is no cure for PD, due to the complexities of molecular mechanisms and events taking place during the pathogenesis of PD. Drosophila melanogaster is an appropriate model organism to unravel the pathogenicity not only behind PD but also other NDs. In this context as numerous biological functions are preserved between Drosophila and humans. Apart from sharing 75% of human disease-causing genes homolog in Drosophila, behavioral responses like memory-based tests, negative geotaxis, courtship and mating are also well studied. The genetic, as well as environmental factors, can be studied in Drosophila to understand the geneenvironment interactions behind the disease condition. Through genetic manipulation, mutant flies can be generated harboring human orthologs, which can prove to be an excellent model to understand the effect of the mutant protein on the pathogenicity of NDs.

Synaptic Vesicle Recycling and the Endolysosomal System: A Reappraisal of Form and Function

The endolysosomal system is present in all cell types. Within these cells, it performs a series of essential roles, such as trafficking and sorting of membrane cargo, intracellular signaling, control of metabolism and degradation. A specific compartment within central neurons, called the presynapse, mediates inter-neuronal communication via the fusion of neurotransmitter-containing synaptic vesicles (SVs). The localized recycling of SVs and their organization into functional pools is widely assumed to be a discrete mechanism, that only intersects with the endolysosomal system at specific points. However, evidence is emerging that molecules essential for endolysosomal function also have key roles within the SV life cycle, suggesting that they form a continuum rather than being isolated processes. In this review, we summarize the evidence for key endolysosomal molecules in SV recycling and propose an alternative model for membrane trafficking at the presynapse. This includes the hypotheses that endolysosomal intermediates represent specific functional SV pools, that sorting of cargo to SVs is mediated via the endolysosomal system and that manipulation of this process can result in both plastic changes to neurotransmitter release and pathophysiology via neurodegeneration.

Increased expression of pathological markers in Parkinson's disease dementia post-mortem brains compared to dementia with Lewy bodies

BACKGROUND

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are common age-related neurodegenerative diseases comprising Lewy body spectrum disorders associated with cortical and subcortical Lewy body pathology. Over 30% of PD patients develop PD dementia (PDD), which describes dementia arising in the context of established idiopathic PD. Furthermore, Lewy bodies frequently accompany the amyloid plaque and neurofibrillary tangle pathology of Alzheimer's disease (AD), where they are observed in the amygdala of approximately 60% of sporadic and familial AD. While PDD and DLB share similar pathological substrates, they differ in the temporal onset of motor and cognitive symptoms; however, protein markers to distinguish them are still lacking.

METHODS

Here, we systematically studied a series of AD and PD pathogenesis markers, as well as mitochondria, mitophagy, and neuroinflammation-related indicators, in the substantia nigra (SN), temporal cortex (TC), and caudate and putamen (CP) regions of human post-mortem brain samples from individuals with PDD and DLB and condition-matched controls.

RESULTS

We found that p-APPT668 (TC), α-synuclein (CP), and LC3II (CP) are all increased while the tyrosine hydroxylase (TH) (CP) is decreased in both PDD and DLB compared to control. Also, the levels of Aβ42 and DD2R, IBA1, and p-LRRK2S935 are all elevated in PDD compared to control. Interestingly, protein levels of p-TauS199/202 in CP and DD2R, DRP1, and VPS35 in TC are all increased in PDD compared to DLB.

CONCLUSIONS

Together, our comprehensive and systematic study identified a set of signature proteins that will help to understand the pathology and etiology of PDD and DLB at the molecular level.

Neurodegenerative Disorders and the Current State, Pathophysiology, and Management of Parkinson's Disease

In the last few decades, major knowledge has been gained about pathophysiological aspects and molecular pathways behind Parkinson's Disease (PD). Based on neurotoxicological studies and postmortem investigations, there is a general concept of how environmental toxicants (neurotoxins, pesticides, insecticides) and genetic factors (genetic mutations in PD-associated proteins) cause depletion of dopamine from substantia nigra pars compacta region of the midbrain and modulate cellular processes leading to the pathogenesis of PD. α-Synuclein, a neuronal protein accumulation in oligomeric form, called protofibrils, is associated with cellular dysfunction and neuronal death, thus possibly contributing to PD propagation. With advances made in identifying loci that contribute to PD, molecular pathways involved in disease pathogenesis are now clear, and introducing therapeutic strategy at the right time may delay the progression. Biomarkers for PD have helped monitor PD progression; therefore, personalized therapeutic strategies can be facilitated. In order to further improve PD diagnostic and prognostic accuracy, independent validation of biomarkers is required.

Crossroads between membrane trafficking machinery and copper homeostasis in the nerve system

Imbalanced copper homeostasis and perturbation of membrane trafficking are two common symptoms that have been associated with the pathogenesis of neurodegenerative and neurodevelopmental diseases. Accumulating evidence from biophysical, cellular and in vivo studies suggest that membrane trafficking orchestrates both copper homeostasis and neural functions-however, a systematic review of how copper homeostasis and membrane trafficking interplays in neurons remains lacking. Here, we summarize current knowledge of the general trafficking itineraries for copper transporters and highlight several critical membrane trafficking regulators in maintaining copper homeostasis. We discuss how membrane trafficking regulators may alter copper transporter distribution in different membrane compartments to regulate intracellular copper homeostasis. Using Parkinson's disease and MEDNIK as examples, we further elaborate how misregulated trafficking regulators may interplay parallelly or synergistically with copper dyshomeostasis in devastating pathogenesis in neurodegenerative diseases. Finally, we explore multiple unsolved questions and highlight the existing challenges to understand how copper homeostasis is modulated through membrane trafficking.

Parkinson's disease and microRNAs - Lessons from model organisms and human studies

Parkinson's disease (PD) is a progressive, age-associated neurodegenerative disorder that affects an estimated 10 million people worldwide. PD is characterized by proteinaceous, cytoplasmic inclusions containing α-synuclein, called Lewy Bodies, which form in dopaminergic neurons in an age-dependent manner, and are associated with the emergence of characteristic PD symptoms such as resting tremor, rigidity, slow movements and postural instability. Although considerable progress has been made in recent years in identifying genetic and environmental factors that are associated with PD, early diagnosis and therapeutic options remain severely lacking. Recently, microRNAs (miRNAs) have emerged as novel therapeutic targets in various diseases, such as cancer and neurodegenerative diseases. MiRNAs have been shown to play roles in various aging and neurodegenerative disease models across phyla. More recently, studies have identified specific roles for miRNAs and their targets in the pathogenesis and progression of PD in several model organisms. Here, we discuss the evolving field of miRNAs, their association with PD, and the outlook for the future.

An Emerging Role for Phosphoinositides in the Pathophysiology of Parkinson’s Disease

Recent data support an involvement of defects in homeostasis of phosphoinositides (PIPs) in the pathophysiology of Parkinson’s disease (PD). Genetic mutations have been identified in genes encoding for PIP-regulating and PIP-interacting proteins, that are associated with familial and sporadic PD. Many of these proteins are implicated in vesicular membrane trafficking, mechanisms that were recently highlighted for their close associations with PD. PIPs are phosphorylated forms of the membrane phospholipid, phosphatidylinositol. Their composition in the vesicle’s membrane of origin, as well as membrane of destination, controls vesicular membrane trafficking. We review the converging evidence that points to the involvement of PIPs in PD. The review describes PD- and PIP-associated proteins implicated in clathrin-mediated endocytosis and autophagy, and highlights the involvement of α-synuclein in these mechanisms.

Endosomal traffic and glutamate synapse activity are increased in VPS35 D620N mutant knock-in mouse neurons, and resistant to LRRK2 kinase inhibition

Vacuolar protein sorting 35 (VPS35) regulates neurotransmitter receptor recycling from endosomes. A missense mutation (D620N) in VPS35 leads to autosomal-dominant, late-onset Parkinson's disease. Here, we study the basic neurobiology of VPS35 and Parkinson's disease mutation effects in the D620N knock-in mouse and the effect of leucine-rich repeat kinase 2 (LRRK2) inhibition on synaptic phenotypes. The study was conducted using a VPS35 D620N knock-in mouse that expresses VPS35 at endogenous levels. Protein levels, phosphorylation states, and binding ratios in brain lysates from knock-in mice and wild-type littermates were assayed by co-immunoprecipitation and western blot. Dendritic protein co-localization, AMPA receptor surface expression, synapse density, and glutamatergic synapse activity in primary cortical cultures from knock-in and wild-type littermates were assayed using immunocytochemistry and whole-cell patch clamp electrophysiology. In brain tissue, we confirm VPS35 forms complexes with LRRK2 and AMPA-type glutamate receptor GluA1 subunits, in addition to NMDA-type glutamate receptor GluN1 subunits and D2-type dopamine receptors. Receptor and LRRK2 binding was unaltered in D620N knock-in mice, but we confirm the mutation results in reduced binding of VPS35 with WASH complex member FAM21, and increases phosphorylation of the LRRK2 kinase substrate Rab10, which is reversed by LRRK2 kinase inhibition in vivo. In cultured cortical neurons from knock-in mice, pRab10 is also increased, and reversed by LRRK2 inhibition. The mutation also results in increased endosomal recycling protein cluster density (VPS35-FAM21 co-clusters and Rab11 clusters), glutamate transmission, and GluA1 surface expression. LRRK2 kinase inhibition, which reversed Rab10 hyper-phosphorylation, did not rescue elevated glutamate release or surface GluA1 expression in knock-in neurons, but did alter AMPAR traffic in wild-type cells. The results improve our understanding of the cell biology of VPS35, and the consequences of the D620N mutation in developing neuronal networks. Together the data support a chronic synaptopathy model for latent neurodegeneration, providing phenotypes and candidate pathophysiological stresses that may drive eventual transition to late-stage parkinsonism in VPS35 PD. The study demonstrates the VPS35 mutation has effects that are independent of ongoing LRRK2 kinase activity, and that LRRK2 kinase inhibition alters basal physiology of glutamate synapses in vitro.

The understudied links of the retromer complex to age-related pathways

Neuronal aging is associated with numerous diseases resulting in memory impairment and functional decline. A common hallmark of these disorders is the accumulation of intracellular and extracellular protein aggregates. The retromer complex plays a central role in sorting proteins by marking them for reuse rather than degradation. Retromer dysfunction has been shown to induce protein aggregates and neurodegeneration, suggesting that it may be important for age-related neuronal decline and disease progression. Despite this, little is known about how aging influences retromer stability and the proteins with which it interacts. Detailed insights into age-dependent changes in retromer structure and function could provide valuable information towards treating and preventing many age-related neurodegenerative disorders. Here, we visit age-related pathways which interact with retromer function that ought to be further explored to determine its role in age-related neurodegeneration.

Opposing functions for retromer and Rab11 in extracellular vesicle traffic at presynaptic terminals

Neuronal extracellular vesicles (EVs) play important roles in intercellular communication and pathogenic protein propagation in neurological disease. However, it remains unclear how cargoes are selectively packaged into neuronal EVs. Here, we show that loss of the endosomal retromer complex leads to accumulation of EV cargoes including amyloid precursor protein (APP), synaptotagmin-4 (Syt4), and neuroglian (Nrg) at Drosophila motor neuron presynaptic terminals, resulting in increased release of these cargoes in EVs. By systematically exploring known retromer-dependent trafficking mechanisms, we show that EV regulation is separable from several previously identified roles of neuronal retromer. Conversely, mutations in rab11 and rab4, regulators of endosome-plasma membrane recycling, cause reduced EV cargo levels, and rab11 suppresses cargo accumulation in retromer mutants. Thus, EV traffic reflects a balance between Rab4/Rab11 recycling and retromer-dependent removal from EV precursor compartments. Our data shed light on previous studies implicating Rab11 and retromer in competing pathways in Alzheimer's disease, and suggest that misregulated EV traffic may be an underlying defect.

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.

The cell biology of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder resulting from the death of dopamine neurons in the substantia nigra pars compacta. Our understanding of PD biology has been enriched by the identification of genes involved in its rare, inheritable forms, termed PARK genes. These genes encode proteins including α-syn, LRRK2, VPS35, parkin, PINK1, and DJ1, which can cause monogenetic PD when mutated. Investigating the cellular functions of these proteins has been instrumental in identifying signaling pathways that mediate pathology in PD and neuroprotective mechanisms active during homeostatic and pathological conditions. It is now evident that many PD-associated proteins perform multiple functions in PD-associated signaling pathways in neurons. Furthermore, several PARK proteins contribute to non-cell-autonomous mechanisms of neuron death, such as neuroinflammation. A comprehensive understanding of cell-autonomous and non-cell-autonomous pathways involved in PD is essential for developing therapeutics that may slow or halt its progression.

The Importance of Drosophila melanogaster Research to UnCover Cellular Pathways Underlying Parkinson's Disease

Parkinson's disease (PD) is a complex neurodegenerative disorder that is currently incurable. As a consequence of an incomplete understanding of the etiology of the disease, therapeutic strategies mainly focus on symptomatic treatment. Even though the majority of PD cases remain idiopathic (~90%), several genes have been identified to be causative for PD, facilitating the generation of animal models that are a good alternative to study disease pathways and to increase our understanding of the underlying mechanisms of PD. Drosophila melanogaster has proven to be an excellent model in these studies. In this review, we will discuss the different PD models in flies and key findings identified in flies in different affected pathways in PD. Several molecular changes have been identified, of which mitochondrial dysfunction and a defective endo-lysosomal pathway emerge to be the most relevant for PD pathogenesis. Studies in flies have significantly contributed to our knowledge of how disease genes affect and interact in these pathways enabling a better understanding of the disease etiology and providing possible therapeutic targets for the treatment of PD, some of which have already resulted in clinical trials.

Dynamic control of the dopamine transporter in neurotransmission and homeostasis

The dopamine transporter (DAT) transports extracellular dopamine into the intracellular space contributing to the regulation of dopamine neurotransmission. A reduction of DAT density is implicated in Parkinson's disease (PD) by neuroimaging; dopamine turnover is dopamine turnover is elevated in early symptomatic PD and in presymptomatic individuals with monogenic mutations causal for parkinsonism. As an integral plasma membrane protein, DAT surface expression is dynamically regulated through endocytic trafficking, enabling flexible control of dopamine signaling in time and space, which in turn critically modulates movement, motivation and learning behavior. Yet the cellular machinery and functional implications of DAT trafficking remain enigmatic. In this review we summarize mechanisms governing DAT trafficking under normal physiological conditions and discuss how PD-linked mutations may disturb DAT homeostasis. We highlight the complexity of DAT trafficking and reveal DAT dysregulation as a common theme in genetic models of parkinsonism.

Mechanisms of VPS35-Mediated Neurodegeneration in Parkinson's Disease

Parkinson's disease is a sporadic and common neurodegenerative movement disorder resulting from the complex interplay between genetic risk, aging and environmental exposure. Familial forms of PD account for ~10% of cases and are known to result from the inheritance of mutations in at least 15 genes. Mutations in the vacuolar protein sorting 35 ortholog (VPS35) gene cause late-onset, autosomal dominant familial PD. VPS35 is a key suunit of the pentameric retromer complex that plays a role in the retrograde sorting and recycling of transmembrane cargo proteins from endosomes to the plasma membrane and trans-Golgi network. A single heterozygous Asp620Asn (D620N) mutation in VPS35 has been identified in multiple families that segregates with PD, and a number of experimental cellular and animal models have been developed to understand its pathogenic effects. At the molecular level, the D620N mutation has been shown to impair the interaction of VPS35 with the WASH complex, that plays an accessory function in retromer-dependent sorting. In addition, the D620N mutation has been linked to the abnormal sorting of retromer cargo, including CI-M6PR, AMPA receptor subunits, MUL1, LAMP2a and ATG9A, as well as to LRRK2 hyperactivation. At the cellular level, data support an impact of D620N VPS35 on mitochondrial function, the autophagy-lysosomal pathway, Wnt signaling and neurotransmission via altered endosomal sorting. The relevance of abnormal retromer sorting and cellular pathways to PD-related neurodegenerative phenotypes induced by D620N VPS35 in rodent models is not yet clear. There is also uncertainty regarding the mechanism-of-action of the D620N mutation and whether it manifests pathogenic effects in animal models and PD through a gain-of-function and/or a partial dominant-negative mechanism. Here, we discuss the emerging molecular and cellular mechanisms underlying PD induced by familial VPS35 mutations, going from structure to cellular function to neuropathology. We further discuss studies linking reduced retromer function to other neurodegenerative diseases and potential therapeutic strategies to normalize retromer function to mitigate disease.

Mitochondrial Dysfunction and Mitophagy in Parkinson's Disease: From Mechanism to Therapy

Mitochondrial dysfunction has been associated with neurodegeneration in Parkinson's disease (PD) for over 30 years. Despite this, the role of mitochondrial dysfunction as an initiator, propagator, or bystander remains undetermined. The discovery of the role of the PD familial genes PTEN-induced putative kinase 1 (PINK1) and parkin (PRKN) in mediating mitochondrial degradation (mitophagy) reaffirmed the importance of this process in PD aetiology. Recently, progress has been made in understanding the upstream and downstream regulators of canonical PINK1/parkin-mediated mitophagy, alongside noncanonical PINK1/parkin mitophagy, in response to mitochondrial damage. Progress has also been made in understanding the role of PD-associated genes, such as SNCA, LRRK2, and CHCHD2, in mitochondrial dysfunction and their overlap with sporadic PD (sPD), opening opportunities for therapeutically targeting mitochondria in PD.

A Critical LRRK at the Synapse? The Neurobiological Function and Pathophysiological Dysfunction of LRRK2

Since the discovery of LRRK2 mutations causal to Parkinson's disease (PD) in the early 2000s, the LRRK2 protein has been implicated in a plethora of cellular processes in which pathogenesis could occur, yet its physiological function remains elusive. The development of genetic models of LRRK2 PD has helped identify the etiological and pathophysiological underpinnings of the disease, and may identify early points of intervention. An important role for LRRK2 in synaptic function has emerged in recent years, which links LRRK2 to other genetic forms of PD, most notably those caused by mutations in the synaptic protein α-synuclein. This point of convergence may provide useful clues as to what drives dysfunction in the basal ganglia circuitry and eventual death of substantia nigra (SN) neurons. Here, we discuss the evolution and current state of the literature placing LRRK2 at the synapse, through the lens of knock-out, overexpression, and knock-in animal models. We hope that a deeper understanding of LRRK2 neurobiology, at the synapse and beyond, will aid the eventual development of neuroprotective interventions for PD, and the advancement of useful treatments in the interim.

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.

Experimental studies of mitochondrial and lysosomal function in in vitro and in vivo models relevant to Parkinson's disease genetic risk

Several studies have identified the involvement of mitochondrial and lysosomal dysfunction in Parkinson's disease (PD) pathology. In this review we discuss recent work that has identified deficits in mitophagy, mitochondrial network formation, increased sensitivity to mitochondrial stressors and alterations in proteins regulating mitochondrial fission and fusion associated with patient-derived fibroblasts harboring mutations in LRRK2 gene and from sporadic PD patient cells. We further focus on alterations of lysosomal enzymes, in particular glucocerebrosidase activity, and resultant lipid dyshomeostasis in PD and aging, in human tissue and in vivo rodent models. Future studies aimed at understanding the convergence of mitochondrial and lysosomal pathways will be of essence for the identification of unique cellular defects in PD and for the development of new treatments.

Kinase inhibition of G2019S-LRRK2 enhances autolysosome formation and function to reduce endogenous alpha-synuclein intracellular inclusions

The Parkinson's disease (PD)-associated kinase Leucine-Rich Repeat Kinase 2 (LRRK2) is a crucial modulator of the autophagy-lysosome pathway, but unclarity exists on the precise mechanics of its role and the direction of this modulation. In particular, LRRK2 is involved in the degradation of pathological alpha-synuclein, with pathogenic mutations precipitating neuropathology in cellular and animal models of PD, and a significant proportion of LRRK2 patients presenting Lewy neuropathology. Defects in autophagic processing and lysosomal degradation of alpha-synuclein have been postulated to underlie its accumulation and onset of neuropathology. Thus, it is critical to obtain a comprehensive knowledge on LRRK2-associated pathology. Here, we investigated a G2019S-LRRK2 recombinant cell line exhibiting accumulation of endogenous, phosphorylated alpha-synuclein. We found that G2019S-LRRK2 leads to accumulation of LC3 and abnormalities in lysosome morphology and proteolytic activity in a kinase-dependent fashion, but independent from constitutively active Rab10. Notably, LRRK2 inhibition was ineffective upon upstream blockade of autophagosome-lysosome fusion events, highlighting this step as critical for alpha-synuclein clearance.

Guilt-by-Association - Functional Insights Gained From Studying the LRRK2 Interactome

The Parkinson's disease-associated Leucine-rich repeat kinase 2 (LRRK2) is a complex multi-domain protein belonging to the Roco protein family, a unique group of G-proteins. Variants of this gene are associated with an increased risk of Parkinson's disease. Besides its well-characterized enzymatic activities, conferred by its GTPase and kinase domains, and a central dimerization domain, it contains four predicted repeat domains, which are, based on their structure, commonly involved in protein-protein interactions (PPIs). In the past decades, tremendous progress has been made in determining comprehensive interactome maps for the human proteome. Knowledge of PPIs has been instrumental in assigning functions to proteins involved in human disease and helped to understand the connectivity between different disease pathways and also significantly contributed to the functional understanding of LRRK2. In addition to an increased kinase activity observed for proteins containing PD-associated variants, various studies helped to establish LRRK2 as a large scaffold protein in the interface between cytoskeletal dynamics and the vesicular transport. This review first discusses a number of specific LRRK2-associated PPIs for which a functional consequence can at least be speculated upon, and then considers the representation of LRRK2 protein interactions in public repositories, providing an outlook on open research questions and challenges in this field.

Alpha-Synuclein and LRRK2 in Synaptic Autophagy: Linking Early Dysfunction to Late-Stage Pathology in Parkinson's Disease

The lack of effective disease-modifying strategies is the major unmet clinical need in Parkinson’s disease. Several experimental approaches have attempted to validate cellular targets and processes. Of these, autophagy has received considerable attention in the last 20 years due to its involvement in the clearance of pathologic protein aggregates and maintenance of neuronal homeostasis. However, this strategy mainly addresses a very late stage of the disease, when neuropathology and neurodegeneration have likely “tipped over the edge” and disease modification is extremely difficult. Very recently, autophagy has been demonstrated to modulate synaptic activity, a process distinct from its catabolic function. Abnormalities in synaptic transmission are an early event in neurodegeneration with Leucine-Rich Repeat Kinase 2 (LRRK2) and alpha-synuclein strongly implicated. In this review, we analyzed these processes separately and then discussed the unification of these biomolecular fields with the aim of reconstructing a potential “molecular timeline” of disease onset and progression. We postulate that the elucidation of these pathogenic mechanisms will form a critical basis for the design of novel, effective disease-modifying therapies that could be applied early in the disease process.

Rab11-mediated recycling endosome role in nervous system development and neurodegenerative diseases

STUDY

Membrane trafficking process is significant for the complex and precise regulatory of the nervous system. Rab11, as a small GTPase of the Rab superfamily, controls endocytic vesicular trafficking to the cell surface after sorting in recycling endosome (RE), coordinating with its receptors to maintain neurological function.

MATERIALS AND METHODS

This article reviewed the literature of Rab11 in nervous system.

RESULTS

Rab11-positive vesicles targeted transport growth-related molecules, such as integrins, protrudin, tropomyosin receptor kinase (Trk) A/B receptor and AMPA receptor (AMPAR) to membrane surface to promote the regeneration capacity of axon and dendrites and maintain synaptic plasticity. In addition, many studies have shown that the expression of Rab11 is decreased in multiple neurodegenerative diseases, which is highly correlated with the process of production, transport and clearance of disease-related pathological proteins. And overexpression or increased activity of Rab11 can greatly improve the defect of membrane trafficking and slow down the disease process.

CONCLUSION

With increasing research efforts on Rab11-dependent membrane trafficking mechanisms, a potential target for nerve regeneration and neurodegenerative diseases will be provided.

The Role of VPS35 in the Pathobiology of Parkinson's Disease

The vacuolar protein sorting 35 (VPS35) gene located on chromosome 16 has recently emerged as a cause of late-onset familial Parkinson's disease (PD) (PARK17). The gene encodes a 796-residue protein nearly ubiquitously expressed in human tissues. The protein localizes on endosomes where it assembles with other peripheral membrane proteins to form the retromer complex. How VPS35 mutations induce dopaminergic neuron degeneration in humans is still unclear. Because the retromer complex recycles the receptors that mediate the transport of hydrolase to lysosome, it has been suggested that VPS35 mutations lead to impaired lysosomal and autophagy function. Recent studies also demonstrated that VPS35 and the retromer complex influence mitochondrial homeostasis, suggesting that VPS35 mutations elicit mitochondrial dysfunction. More recent studies have identified a key role of VPS35 in neurotransmission, whilst others reported a functional interaction between VPS35 and other genes associated with familial PD, including α-SYNUCLEIN-PARKIN-LRRK2. Here, we review the biological role of VPS35 protein, the VPS35 mutations identified in human PD patients, and the potential molecular mechanism by which VPS35 mutations can induce progressive neurodegeneration in PD.

Retromer subunit, VPS29, regulates synaptic transmission and is required for endolysosomal function in the aging brain

Retromer, including Vps35, Vps26, and Vps29, is a protein complex responsible for recycling proteins within the endolysosomal pathway. Although implicated in both Parkinson's and Alzheimer's disease, our understanding of retromer function in the adult brain remains limited, in part because Vps35 and Vps26 are essential for development. In Drosophila, we find that Vps29 is dispensable for embryogenesis but required for retromer function in aging adults, including for synaptic transmission, survival, and locomotion. Unexpectedly, in Vps29 mutants, Vps35 and Vps26 proteins are normally expressed and associated, but retromer is mislocalized from neuropil to soma with the Rab7 GTPase. Further, Vps29 phenotypes are suppressed by reducing Rab7 or overexpressing the GTPase activating protein, TBC1D5. With aging, retromer insufficiency triggers progressive endolysosomal dysfunction, with ultrastructural evidence of impaired substrate clearance and lysosomal stress. Our results reveal the role of Vps29 in retromer localization and function, highlighting requirements for brain homeostasis in aging.