Attenuation of the norepinephrine transporter activity and trafficking via interactions with α-synuclein

From Synaptic Physiology to Synaptic Pathology: The Enigma of α-Synuclein

Alpha-synuclein (α-syn) has gained significant attention due to its involvement in neurodegenerative diseases, particularly Parkinson's disease. However, its normal function in the human brain is equally fascinating. The α-syn protein is highly dynamic and can adapt to various conformational stages, which differ in their interaction with synaptic elements, their propensity to drive pathological aggregation, and their toxicity. This review will delve into the multifaceted role of α-syn in different types of synapses, shedding light on contributions to neurotransmission and overall brain function. We describe the physiological role of α-syn at central synapses, including the bidirectional interaction between α-syn and neurotransmitter systems.

The Role of α-Synuclein in the Regulation of Serotonin System: Physiological and Pathological Features

In patients affected by Parkinson's disease (PD), up to 50% of them experience cognitive changes, and psychiatric disturbances, such as anxiety and depression, often precede the onset of motor symptoms and have a negative impact on their quality of life. Pathologically, PD is characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the presence of intracellular inclusions, called Lewy bodies and Lewy neurites, composed mostly of α-synuclein (α-Syn). Much of PD research has focused on the role of α-Syn aggregates in the degeneration of SNc DA neurons due to the impact of striatal DA deficits on classical motor phenotypes. However, abundant Lewy pathology is also found in other brain regions including the midbrain raphe nuclei, which may contribute to non-motor symptoms. Indeed, dysfunction of the serotonergic (5-HT) system, which regulates mood and emotional pathways, occurs during the premotor phase of PD. However, little is known about the functional consequences of α-Syn inclusions in this neuronal population other than DA neurons. Here, we provide an overview of the current knowledge of α-Syn and its role in regulating the 5-HT function in health and disease. Understanding the relative contributions to α-Syn-linked alterations in the 5-HT system may provide a basis for identifying PD patients at risk for developing depression and could lead to a more targeted therapeutic approach.

Novel PET Biomarkers to Disentangle Molecular Pathways across Age-Related Neurodegenerative Diseases

There is a need to disentangle the etiological puzzle of age-related neurodegenerative diseases, whose clinical phenotypes arise from known, and as yet unknown, pathways that can act distinctly or in concert. Enhanced sub-phenotyping and the identification of in vivo biomarker-driven signature profiles could improve the stratification of patients into clinical trials and, potentially, help to drive the treatment landscape towards the precision medicine paradigm. The rapidly growing field of neuroimaging offers valuable tools to investigate disease pathophysiology and molecular pathways in humans, with the potential to capture the whole disease course starting from preclinical stages. Positron emission tomography (PET) combines the advantages of a versatile imaging technique with the ability to quantify, to nanomolar sensitivity, molecular targets in vivo. This review will discuss current research and available imaging biomarkers evaluating dysregulation of the main molecular pathways across age-related neurodegenerative diseases. The molecular pathways focused on in this review involve mitochondrial dysfunction and energy dysregulation; neuroinflammation; protein misfolding; aggregation and the concepts of pathobiology, synaptic dysfunction, neurotransmitter dysregulation and dysfunction of the glymphatic system. The use of PET imaging to dissect these molecular pathways and the potential to aid sub-phenotyping will be discussed, with a focus on novel PET biomarkers.

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.

Alpha-Synuclein RNA Expression is Increased in Major Depression

Alpha-synuclein (SNCA) is a small membrane protein that plays an important role in neuro-psychiatric diseases. It is best known for its abnormal subcellular aggregation in Lewy bodies that serves as a hallmark of Parkinson’s disease (PD). Due to the high comorbidity of PD with depression, we investigated the role of SNCA in patients suffering from major depressive disorder (MDD). SNCA mRNA expression levels were analyzed in peripheral blood cells of MDD patients and a healthy control group. SNCA mRNA expression was positively correlated with severity of depression as indicated by psychometric assessment. We found a significant increase in SNCA mRNA expression levels in severely depressed patients compared with controls. Thus, SNCA analysis could be a helpful target in the search for biomarkers of MDD.

Living in Promiscuity: The Multiple Partners of Alpha-Synuclein at the Synapse in Physiology and Pathology

Alpha-synuclein (α-syn) is a small protein that, in neurons, localizes predominantly to presynaptic terminals. Due to elevated conformational plasticity, which can be affected by environmental factors, in addition to undergoing disorder-to-order transition upon interaction with different interactants, α-syn is counted among the intrinsically disordered proteins (IDPs) family. As with many other IDPs, α-syn is considered a hub protein. This function is particularly relevant at synaptic sites, where α-syn is abundant and interacts with many partners, such as monoamine transporters, cytoskeletal components, lipid membranes, chaperones and synaptic vesicles (SV)-associated proteins. These protein–protein and protein–lipid membrane interactions are crucial for synaptic functional homeostasis, and alterations in α-syn can cause disruption of this complex network, and thus a failure of the synaptic machinery. Alterations of the synaptic environment or post-translational modification of α-syn can induce its misfolding, resulting in the formation of oligomers or fibrillary aggregates. These α-syn species are thought to play a pathological role in neurodegenerative disorders with α-syn deposits such as Parkinson’s disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), which are referred to as synucleinopathies. Here, we aim at revising the complex and promiscuous role of α-syn at synaptic terminals in order to decipher whether α-syn molecular interactants may influence its conformational state, contributing to its aggregation, or whether they are just affected by it.

α-Synuclein and Noradrenergic Modulation of Immune Cells in Parkinson's Disease Pathogenesis

α-synuclein (α-syn) pathology and loss of noradrenergic neurons in the locus coeruleus (LC) are among the most ubiquitous features of Parkinson's disease (PD). While noradrenergic dysfunction is associated with non-motor symptoms of PD, preclinical research suggests that the loss of LC norepinephrine (NE), and subsequently its immune modulatory and neuroprotective actions, may exacerbate or even accelerate disease progression. In this review, we discuss the mechanisms by which α-syn pathology and loss of central NE may directly impact brain health by interrupting neurotrophic factor signaling, exacerbating neuroinflammation, and altering regulation of innate and adaptive immune cells.

Amphetamine Reverses Escalated Cocaine Intake via Restoration of Dopamine Transporter Conformation

Cocaine abuse disrupts dopamine system function, and reduces cocaine inhibition of the dopamine transporter (DAT), which results in tolerance. Although tolerance is a hallmark of cocaine addiction and a DSM-V criterion for substance abuse disorders, the molecular adaptations producing tolerance are unknown, and testing the impact of DAT changes on drug taking behaviors has proven difficult. In regard to treatment, amphetamine has shown efficacy in reducing cocaine intake; however, the mechanisms underlying these effects have not been explored. The goals of this study were twofold; we sought to (1) identify the molecular mechanisms by which cocaine exposure produces tolerance and (2) determine whether amphetamine-induced reductions in cocaine intake are connected to these mechanisms. Using cocaine self-administration and fast-scan cyclic voltammetry in male rats, we show that low-dose, continuous amphetamine treatment, during self-administration or abstinence, completely reversed cocaine tolerance. Amphetamine treatment also reversed escalated cocaine intake and decreased motivation to obtain cocaine as measured in a behavioral economics task, thereby linking tolerance to multiple facets of cocaine use. Finally, using fluorescence resonance energy transfer imaging, we found that cocaine tolerance is associated with the formation of DAT-DAT complexes, and that amphetamine disperses these complexes. In addition to extending our basic understanding of DATs and their role in cocaine reinforcement, we serendipitously identified a novel therapeutic target: DAT oligomer complexes. We show that dispersion of oligomers is concomitant with reduced cocaine intake, and propose that pharmacotherapeutics aimed at these complexes may have potential for cocaine addiction treatment.SIGNIFICANCE STATEMENT Tolerance to cocaine's subjective effects is a cardinal symptom of cocaine addiction and a DSM-V criterion for substance abuse disorders. However, elucidating the molecular adaptions that produce tolerance and determining its behavioral impact have proven difficult. Using cocaine self-administration in rats, we link tolerance to cocaine effects at the dopamine transporter (DAT) with aberrant cocaine-taking behaviors. Further, tolerance was associated with multi-DAT complexes, which formed after cocaine exposure. Treatment with amphetamine deconstructed DAT complexes, reversed tolerance, and decreased cocaine seeking. These data describe the behavioral consequence of cocaine tolerance, provide a putative mechanism for its development, and suggest that compounds that disperse DAT complexes may be efficacious treatments for cocaine addiction.

Synucleinopathies: common features and hippocampal manifestations

Parkinson's disease (PD), dementia with Lewy Bodies (DLB), and multiple system atrophy (MSA) are three major synucleinopathies characterized by α-synuclein-containing inclusions in the brains of patients. Because the cell types and brain structures that are affected vary markedly between the disorders, the patients have different clinical manifestations in addition to some overlapping symptoms, which are the basis for differential diagnosis. Cognitive impairment and depression associated with hippocampal dysfunction are frequently observed in these disorders. While various α-synuclein-containing inclusions are found in the hippocampal formation, increasing evidence supports that small α-synuclein aggregates or oligomers may be the real culprit, causing deficits in neurotransmission and neurogenesis in the hippocampus and related brain regions, which constitute the major mechanism for the hippocampal dysfunctions and associated neuropsychiatric manifestations in synucleinopathies.

Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters

Modulation of neurotransmission by the monoamines dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is critical for normal nervous system function. Precise temporal and spatial control of this signaling in mediated in large part by the actions of monoamine transporters (DAT, NET, and SERT, respectively). These transporters act to recapture their respective neurotransmitters after release, and disruption of clearance and reuptake has significant effects on physiology and behavior and has been linked to a number of neuropsychiatric disorders. To ensure adequate and dynamic control of these transporters, multiple modes of control have evolved to regulate their activity and trafficking. Central to many of these modes of control are the actions of protein kinases, whose actions can be direct or indirectly mediated by kinase-modulated protein interactions. Here, we summarize the current state of our understanding of how protein kinases regulate monoamine transporters through changes in activity, trafficking, phosphorylation state, and interacting partners. We highlight genetic, biochemical, and pharmacological evidence for kinase-linked control of DAT, NET, and SERT and, where applicable, provide evidence for endogenous activators of these pathways. We hope our discussion can lead to a more nuanced and integrated understanding of how neurotransmitter transporters are controlled and may contribute to disorders that feature perturbed monoamine signaling, with an ultimate goal of developing better therapeutic strategies.

Chlorella sorokiniana Extract Improves Short-Term Memory in Rats

Increasing evidence shows that eukaryotic microalgae and, in particular, the green microalga Chlorella, can be used as natural sources to obtain a whole variety of compounds, such as omega (ω)-3 and ω-6 polyunsatured fatty acids (PUFAs). Although either beneficial or toxic effects of Chlorella sorokiniana have been mainly attributed to its specific ω-3 and ω-6 PUFAs content, the underlying molecular pathways remain to be elucidated yet. Here, we investigate the effects of an acute oral administration of a lipid extract of Chlorella sorokiniana, containing mainly ω-3 and ω-6 PUFAs, on cognitive, emotional and social behaviour in rats, analysing possible underlying neurochemical alterations. Our results showed improved short-term memory in Chlorella sorokiniana-treated rats compared to controls, without any differences in exploratory performance, locomotor activity, anxiety profile and depressive-like behaviour. On the other hand, while the social behaviour of Chlorella sorokiniana-treated animals was significantly decreased, no effects on aggressivity were observed. Neurochemical investigations showed region-specific effects, consisting in an elevation of noradrenaline (NA) and serotonin (5-HT) content in hippocampus, but not in the prefrontal cortex and striatum. In conclusion, our results point towards a beneficial effect of Chlorella sorokiniana extract on short-term memory, but also highlight the need of caution in the use of this natural supplement due to its possible masked toxic effects.

Dopamine Transporter Activity Is Modulated by α-Synuclein

The duration and strength of the dopaminergic signal are regulated by the dopamine transporter (DAT). Drug addiction and neurodegenerative and neuropsychiatric diseases have all been associated with altered DAT activity. The membrane localization and the activity of DAT are regulated by a number of intracellular proteins. α-Synuclein, a protein partner of DAT, is implicated in neurodegenerative disease and drug addiction. Little is known about the regulatory mechanisms of the interaction between DAT and α-synuclein, the cellular location of this interaction, and the functional consequences of this interaction on the basal, amphetamine-induced DAT-mediated dopamine efflux, and membrane microdomain distribution of the transporter. Here, we found that the majority of DAT·α-synuclein protein complexes are found at the plasma membrane of dopaminergic neurons or mammalian cells and that the amphetamine-mediated increase in DAT activity enhances the association of these proteins at the plasma membrane. Further examination of the interaction of DAT and α-synuclein revealed a transient interaction between these two proteins at the plasma membrane. Additionally, we found DAT-induced membrane depolarization enhances plasma membrane localization of α-synuclein, which in turn increases dopamine efflux and enhances DAT localization in cholesterol-rich membrane microdomains.

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.

Analysis of genes causing hypertension and stroke in spontaneously hypertensive rats: gene expression profiles in the brain

Spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) are frequently used as rat models not only of essential hypertension and stroke, but also of attention-deficit hyperactivity disorder (ADHD). Normotensive Wistar-Kyoto rats (WKY) are used as the control rats in these cases. An increasing number of studies has demonstrated the critical role of the central nervous system in the development and maintenance of hypertension. In a previous study, we analyzed the gene expression profiles in the adrenal glands of SHR. Thus, in this study, we analyzed gene expression profiles in the brains of SHR in order to identify the genes responsible for causing hypertension and stroke, as well as those involved in ADHD. Using genome-wide microarray technology, we examined the gene expression profiles in the brains of 3 rat strains (SHR, SHRSP and WKY) when the rats were 3 and 6 weeks of age, a period in which the rats are considered to be in a pre-hypertensive state. Gene expression profiles in the brain were compared between SHR and WKY, and between SHRSP and SHR. A total of 179 genes showing a >4- or <-4-fold change in expression were isolated, and candidate genes were selected using two different web tools: the first tool was the Database for Annotation, Visualization and Integrated Discovery (DAVID), which was used to search for significantly enriched genes, and categorized them using Gene Ontology (GO) terms, and the second was the network explorer of Ingenuity Pathway Analysis (IPA), which was used to search for interaction networks among SHR- and SHRSP-specific genes. The IPA of SHR-specific genes revealed that prostaglandin E receptor 4 (Ptger4) is one of the candidate genes responsible for causing hypertension in SHR, and that albumin (Alb) and chymase 1 (Cma1) are also responsible for causing hypertension in SHR in the presence of angiotensinogen (Agt). Similar analyses of SHRSP-specific genes revealed that the angiotensin II receptor-associated gene (Agtrap) interacts with the FBJ osteosarcoma oncogene (Fos), and with the angiotensin II receptor type-1b (Agtr1b). As Agtrap and Agtr1b not only participate in the 'uptake of norepinephrine' and 'blood pressure', but also in the 'behavior' of SHRSP at 6 weeks of age, our data demonstrate a close association between hypertension and ADHD.

Norepinephrine transporter variant A457P knock-in mice display key features of human postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is a common autonomic disorder of largely unknown etiology that presents with sustained tachycardia on standing, syncope and elevated norepinephrine spillover. Some individuals with POTS experience anxiety, depression and cognitive dysfunction. Previously, we identified a mutation, A457P, in the norepinephrine (NE; also known as noradrenaline) transporter (NET; encoded by SLC6A2) in POTS patients. NET is expressed at presynaptic sites in NE neurons and plays a crucial role in regulating NE signaling and homeostasis through NE reuptake into noradrenergic nerve terminals. Our in vitro studies demonstrate that A457P reduces both NET surface trafficking and NE transport and exerts a dominant-negative impact on wild-type NET proteins. Here we report the generation and characterization of NET A457P mice, demonstrating the ability of A457P to drive the POTS phenotype and behaviors that are consistent with reported comorbidities. Mice carrying one A457P allele (NET(+/P)) exhibited reduced brain and sympathetic NE transport levels compared with wild-type (NET(+/+)) mice, whereas transport activity in mice carrying two A457P alleles (NET(P/P)) was nearly abolished. NET(+/P) and NET(P/P) mice exhibited elevations in plasma and urine NE levels, reduced 3,4-dihydroxyphenylglycol (DHPG), and reduced DHPG:NE ratios, consistent with a decrease in sympathetic nerve terminal NE reuptake. Radiotelemetry in unanesthetized mice revealed tachycardia in NET(+/P) mice without a change in blood pressure or baroreceptor sensitivity, consistent with studies of human NET A457P carriers. NET(+/P) mice also demonstrated behavioral changes consistent with CNS NET dysfunction. Our findings support that NET dysfunction is sufficient to produce a POTS phenotype and introduces the first genetic model suitable for more detailed mechanistic studies of the disorder and its comorbidities.

Age-dependent effects of A53T alpha-synuclein on behavior and dopaminergic function

Expression of A53T mutant human alpha-synuclein under the mouse prion promoter is among the most successful transgenic models of Parkinson's disease. Accumulation of A53T alpha-synuclein causes adult mice to develop severe motor impairment resulting in early death at 8–12 months of age. In younger, pre-symptomatic animals, altered motor activity and anxiety-like behaviors have also been reported. These behavioral changes, which precede severe neuropathology, may stem from non-pathological functions of alpha-synuclein, including modulation of monoamine neurotransmission. Our analysis over the adult life-span of motor activity, anxiety-like, and depressive-like behaviors identifies perturbations both before and after the onset of disease. Young A53T mice had increased distribution of the dopamine transporter (DAT) to the membrane that was associated with increased striatal re-uptake function. DAT function decreased with aging, and was associated with neurochemical alterations that included increased expression of beta-synuclein and gamma synuclein. Prior to normalization of dopamine uptake, transient activation of Tau kinases and hyperphosphorylation of Tau in the striatum were also observed. Aged A53T mice had reduced neuron counts in the substantia nigra pars compacta, yet striatal medium spiny neuron dendritic spine density was largely maintained. These findings highlight the involvement of the synuclein family of proteins and phosphorylation of Tau in the response to dopaminergic dysfunction of the nigrostriatal pathway.

From α-synuclein to synaptic dysfunctions: new insights into the pathophysiology of Parkinson's disease

Alpha-synuclein is a natively unfolded protein playing a key role in the regulation of several neuronal synaptic functions in physiological and pathological conditions. Many studies, over the past years, have shown that it is actively involved in PD pathophysiology. Alpha-synuclein is integrated in a complex network of neuronal processes through the interaction with cytosolic and synaptic proteins. Hence, it is not the sole α-synuclein pathology but its effects on diverse protein partners and specific cellular pathways in the membrane and/or cytosolic districts such as endoplasmic reticulum/Golgi, axonal and synaptic compartments of dopaminergic neurons, that may cause the onset of neuronal cell dysfunction and degeneration which are among the key pathological features of the PD brain. Here we summarize a series of experimental data supporting that α-synuclein aggregation may induce dysfunction and degeneration of synapses via these multiple mechanisms. Taken together, these data add new insights into the complex mechanisms underlying synaptic derangement in PD and other α-synucleinopathies. This article is part of a Special Issue entitled: Brain Integration.

α-Synuclein synaptic pathology and its implications in the development of novel therapeutic approaches to cure Parkinson's disease

Parkinson's disease (PD) is characterized by a progressive loss of dopamine (DA) neurons of the nigrostriatal system and by the presence of Lewy bodies (LB), proteinaceous inclusions mainly composed of filamentous α-synuclein aggregates. Alpha-synuclein is a natively unfolded protein which plays a central role in the control of dopaminergic neuronal functions and which is thought to be critically implicated in PD pathophysiology. Indeed, besides the fact that α-synuclein is the main protein component of LB, genetic studies showed that mutations and multiplications of the α-synuclein gene are responsible for the onset of familial forms of PD. A large body of evidence indicates that α-synuclein pathology at dopaminergic synapses may underlie the onset of neuronal cell dysfunction and degeneration in the PD brain. Thus, since the available therapeutic approaches to cure this disease are still limited, we hypothesized that the analysis of the α-synuclein synaptic proteome/lipidome may represent a tool to identify novel potential therapeutic targets to cure this disorder. We thus performed a critical review of studies describing α-synuclein pathophysiology at synaptic sites in experimental models of PD and in this paper we outline the most relevant findings regarding the specific modulatory effects exerted by α-synuclein in the control of synaptic functions in physiological and pathological conditions. The conclusions of these studies allow to single out novel potential therapeutic targets among the α-synuclein synaptic partners. These targets may be considered for the development of new pharmacological and gene-based strategies to cure PD.

Proteins interacting with monoamine transporters: current state and future challenges

Plasma membrane and vesicular transporters for the biogenic amines, dopamine, norepinephrine, and serotonin, represent a group of proteins that play a crucial role in the regulation of neurotransmission. Clinically, mono amine transporters are the primary targets for the actions of many therapeutic agents used to treat mood disorders, as well as the site of action for highly addictive psychostimulants such as cocaine, amphetamine, methamphetamine, and 3,4-methylenedioxymethamphetamine. Over the past decade, the use of approaches such as yeast two-hybrid and proteomics has identified a multitude of transporter interacting proteins, suggesting that the function and regulation of these transporters are more complex than previously anticipated. With the increasing number of interacting proteins, the rules dictating transporter synthesis, assembly, targeting, trafficking, and function are beginning to be deciphered. Although many of these protein interactions have yet to be fully characterized, current knowledge is beginning to shed light on novel transporter mechanisms involved in monoamine homeostasis, the molecular actions of psychostimulants, and potential disease mechanisms. While future studies resolving the spatial and temporal resolution of these, and yet unknown, interactions will be needed, the realization that monoamine transporters do not work alone opens the path to a plethora of possible pharmacological interventions.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

Synuclein modulation of monoamine transporters

Although well-studied in the context of neurodegenerative disease, a clear biological function for the synuclein proteins remains elusive. Emerging data indicate a role for synucleins in monoamine neurotransmitter homeostasis. A key regulatory component of monoamine neurotransmission is re-uptake of neurotransmitter by the dopamine transporter, norepinephrine transporter, and serotonin transporter, which are common drug targets in the treatment of depression and other mood disorders. Through interactions with these transporters, the neuronal cytoskeleton, and pre-synaptic scaffolding proteins, α-synuclein, β-synuclein, and γ-synuclein modulate trafficking, expression and function of monoamine transporters at the cell surface, thus playing a central role in regulating monoamine re-uptake.

Biochemical and morphological consequences of human α-synuclein expression in a mouse α-synuclein null background

A consensus about the functions of human wild-type or mutated α-synuclein (αSYN) is lacking. Both forms of αSYN are implicated in Parkinson's disease, whereas the wild-type form is implicated in substance abuse. Interactions with other cellular proteins and organelles may meditate its functions. We developed a series of congenic mouse lines containing various allele doses or combinations of the human wild-type αSYN (hwαSYN) or a doubly mutated (A30P*A53T) αSYN (hm(2) αSYN) in a C57Bl/6J line spontaneously deleted in mouse αSYN (C57BL/6JOla). Both transgenes had a functional role in the nigrostriatal system, demonstrated by significant elevations in striatal catecholamines, metabolites and the enzyme tyrosine hydroxylase compared with null-mice without a transgene. Consequences occurred when the transgenes were expressed at a fraction of the endogenous level. Hemizygous congenic mice did not exhibit any change in the number or size of dopaminergic neurons in the ventral midbrain at 9 months of age. Human αSYN was predominantly located in neuronal cell bodies, neurites, synapses, and in intraneuronal/intraneuritic aggregates. The hm(2) αSYN transgene resulted in more aggregates and dystrophic neurites than did the hw5 transgene. The hwαSYN transgene resulted in higher expression of two striatal proteins, synaptogamin 7 and UCHL1, compared with the levels of the hm(2) αSYN transgene. These observations suggest that mutations in αSYN may impair specific functional domains, leaving others intact. These lines may be useful for exploring interactions between hαSYN and environmental or genetic risk factors in dopamine-related disorders using a mouse model.

NACP-Rep1 relates to Beck Depression Inventory scores in healthy humans

Alpha-synuclein (SNCA) is associated with a range of psychiatric diseases including neurodegeneration, alcohol craving, and depression. It regulates cellular homeostasis by virtue of its ability to interfere in dopaminergic, serotonergic, and noradrenergic pathways. To date, it is unclear whether the previously described association between SNCA and depressive symptomatology is limited to females with eating disorders or whether it could be extended to include healthy individuals. We included 105 women and 108 men. Genetic data and mRNA expression analyses were drawn from peripheral blood and the severity of depressive symptoms was quantified by the Beck's Depression Inventory (BDI). We found a significant association between the NACP-Rep1 length polymorphism and the BDI score (p = 0.004). Moreover, there was a significant gender dimorphism regarding mRNA expression of SNCA (p = 0.011). Our analysis revealed no further association between the In4 polymorphism or between the mRNA expression of SNCA and the BDI score. Since this investigation was limited to healthy individuals, conclusions concerning depression according to ICD-10 or DSM-IV cannot be drawn. The reported results may contribute to a better understanding of the molecular mechanisms linked to depressive symptoms.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

P25α / TPPP expression increases plasma membrane presentation of the dopamine transporter and enhances cellular sensitivity to dopamine toxicity

Parkinson's disease is characterized by preferential degeneration of the dopamine-producing neurons of the brain stem substantia nigra. Imbalances between mechanisms governing dopamine transport across the plasma membrane and cellular storage vesicles increase the level of toxic pro-oxidative cytosolic dopamine. The microtubule-stabilizing protein p25α accumulates in dopaminergic neurons in Parkinson's disease. We hypothesized that p25α modulates the subcellular localization of the dopamine transporter via effects on sorting vesicles, and thereby indirectly affects its cellular activity. Here we show that co-expression of the dopamine transporter with p25α in HEK-293-MSR cells increases dopamine uptake via increased plasma membrane presentation of the transporter. No direct interaction between p25α and the dopamine transporter was demonstrated, but they co-fractionated during subcellular fractionation of brain tissue from striatum, and direct binding of p25α peptides to brain vesicles was demonstrated. Truncations of the p25α peptide revealed that the requirement for stimulating dopamine uptake is located in the central core and were similar to those required for vesicle binding. Co-expression of p25α and the dopamine transporter in HEK-293-MSR cells sensitized them to the toxicity of extracellular dopamine. Neuronal expression of p25α thus holds the potential to sensitize the cells toward dopamine and toxins carried by the dopamine transporter.

Snca and Bdnf gene expression in the VTA and raphe nuclei of midbrain in chronically victorious and defeated male mice

Background

Alpha-synuclein (α-Syn) is a small neuronal protein that has been found to be expressed throughout the brain. It has been shown that α-Syn regulates the homeostasis of monoamine neurotransmitters and is involved in various degenerative and affective disorders. There is indication that α-Syn may regulate expression of the brain-derived neurotropic factor (BDNF) which plays an important role in the mood disorders.

Methodology/Principal Findings

The study aimed to analyze the mRNA levels of Snca and Bdnf genes in the ventral tegmental area (VTA) and raphe nuclei of the midbrain in male mice that had each won or defeated 20 encounters (20-time winners and 20-time losers, respectively) in daily agonistic interactions. Groups of animals that had the same winning and losing track record followed by a no-fight period for 14 days (no-fighting winners and no-fighting losers) were also studied. Snca mRNA levels were increased in the raphe nuclei in the 20-time losers and in the VTA of the 20-time winners. After no-fight period Snca mRNA levels decreased in both groups. Snca mRNA levels were similar to the control level in the VTA of the 20-time losers and in the raphe nuclei of the 20-time winners. However Snca gene expression increased in these areas in the no-fighting winners and no-fighting losers in comparison with respective mRNA levels in animals before no-fight period. Bdnf mRNA levels increased in VTA of 20-time winners. Significant positive correlations were found between the mRNA levels of Snca and Bdnf genes in the raphe nuclei.

Conclusions/Significance

Social experience affects Snca gene expression depending on brain areas and functional activity of monoaminergic systems in chronically victorious or defeated mice. These findings may be useful for understanding the mechanisms of forming different alpha-synucleinopathies.

Insulin reveals Akt signaling as a novel regulator of norepinephrine transporter trafficking and norepinephrine homeostasis

Noradrenergic signaling in the CNS plays an essential role in circuits involving attention, mood, memory, and stress as well as providing pivotal support for autonomic function in the peripheral nervous system. The high-affinity norepinephrine (NE) transporter (NET) is the primary mechanism by which noradrenergic synaptic transmission is terminated. Data indicate that NET function is regulated by insulin, a hormone critical for the regulation of metabolism. Given the high comorbidity of metabolic disorders such as diabetes and obesity with mental disorders such as depression and schizophrenia, we sought to determine how insulin signaling regulates NET function and thus noradrenergic homeostasis. Here, we show that acute insulin treatment, through the downstream kinase protein kinase B (Akt), significantly decreases NET surface expression in mouse hippocampal slices and superior cervical ganglion neuron boutons (sites of synaptic NE release). In vivo manipulation of insulin/Akt signaling, with streptozotocin, a drug that induces a type 1-like diabetic state in mice, also results in aberrant NET function and NE homeostasis. Notably, we also demonstrate that Akt inhibition or stimulation, independent of insulin, is capable of altering NET surface availability. These data suggest that aberrant states of Akt signaling such as in diabetes and obesity have the potential to alter NET function and noradrenergic tone in the brain. Furthermore, they provide one potential molecular mechanism by which Akt, a candidate gene for mood disorders such as schizophrenia and depression, can impact brain monoamine homeostasis.

Rab11 supports amphetamine-stimulated norepinephrine transporter trafficking

The norepinephrine transporter (NET) is a presynaptic plasma membrane protein that mediates reuptake of synaptically released norepinephrine. NET is also a major target for medications used for the treatment of depression, attention deficit/hyperactivity disorder, narcolepsy, and obesity. NET is regulated by numerous mechanisms, including catalytic activation and membrane trafficking. Amphetamine (AMPH), a psychostimulant and NET substrate, has also been shown to induce NET trafficking. However, neither the molecular basis nor the nature of the relevant membrane compartments of AMPH-modulated NET trafficking has been defined. Indeed, direct visualization of drug-modulated NET trafficking in neurons has yet to be demonstrated. In this study, we used a recently developed NET antibody and the presence of large presynaptic boutons in sympathetic neurons to examine basal and AMPH-modulated NET trafficking. Specifically, we establish a role for Rab11 in AMPH-induced NET trafficking. First, we found that, in cortical slices, AMPH induces a reduction in surface NET. Next, we observed AMPH-induced accumulation and colocalization of NET with Rab11a and Rab4 in presynaptic boutons of cultured neurons. Using tagged proteins, we demonstrated that NET and a truncated Rab11 effector (FIP2DeltaC2) do not redistribute in synchrony, whereas NET and wild-type Rab11a do. Analysis of various Rab11a/b mutants further demonstrates that Rab11 regulates NET trafficking. Expression of the truncated Rab11a effector (FIP2DeltaC2) attenuates endogenous Rab11 function and prevented AMPH-induced NET internalization as does GDP-locked Rab4 S22N. Our data demonstrate that AMPH leads to an increase of NET in endosomes of single boutons and varicosities in a Rab11-dependent manner.

Mice expressing the A53T mutant form of human alpha-synuclein exhibit hyperactivity and reduced anxiety-like behavior

Genetic mutations associated with alpha-synuclein (alpha-Syn) are implicated in the pathogenesis of Parkinson's disease (PD). PD is primarily a movement disorder, but patients are known to experience anxiety and other mood disorders. In this study, we examined the effect of the hA53T mutation during development by analyzing the protein expression of norepinephrine (NET), serotonin (SERT), and dopamine (DAT) transporters in addition to assessing locomotor and anxiety-like behavior. We observed significant decreases in DAT expression at 8 months in transgenic animals compared with normal and younger mice. We used the elevated plus maze, open-field test, and rotarod apparatus to evaluate wild-type and hA53T hemizygous mice at 2, 8, and 12 months of age. Our results showed that 12-month-old transgenic mice spend more time in the open arms and display a greater number of open entries of the elevated plus maze compared with wild-type controls and younger mice. Open-field test results showed that 12-month-old mice travel a greater distance overall and travel more in the inner zone than either wild-type or younger mice. Rotarod testing showed that 8- and 12-month-old transgenic mice perform better than either wild-type controls or younger mice. Overall, 8-12-month-old transgenic mice showed a trend toward reduced anxiety-like behavior and increased hyperactivity. These results indicate a possible role of the A53T alpha-Syn mutation in anxiety-like and hyperactive behaviors in a PD mouse model, suggesting that these behaviors might be comorbid with this disease.

Elevated tauopathy and alpha-synuclein pathology in postmortem Parkinson's disease brains with and without dementia

Parkinson's disease (PD), a progressive neurodegenerative disease, results in abnormal accumulation of insoluble alpha-synuclein (alpha-Syn) in dopaminergic neurons. Here we examined tauopathic changes and the alpha-Syn/p-GSK-3beta/proteasome pathway in postmortem striata and inferior frontal gyri (IFG) from patients with PD and PD with dementia (PDD). In both PD and PDD, alpha-Syn levels were high, especially the insoluble form of this protein; in PDD, insoluble alpha-Syn levels were persistently higher than PD across both brain regions. Levels of p-GSK-3beta phosphorylated at Tyr 216, which hyperphosphorylates Tau to produce toxic pathological forms of p-Tau, were higher in striata of both PD and PDD compared to controls, but were unaltered in IFG. While proteasomal activity was unchanged in striatum of PD and PDD, such activity was diminished in the IFG of both PD and PDD. A decrease in 19S subunit of the proteasomes was seen in IFG of PDD, while lower levels of 20S subunits were seen in striatum and IFG of both PD and PDD patients. Parkin levels were similar in PD and PDD, suggesting lack of involvement of this protein. Most interestingly, tauopathic changes were noted only in striatum of PD and PDD, with increased hyperphosphorylation seen at Ser262 and Ser396/404; increases in Ser202 levels were seen only in PD but not in PDD striatum. We were unable to detect any tauopathy in IFG in either PD or PDD despite increased levels of alpha-Syn, and decreased proteasomal activity, and is probably due to lack of increase in p-GSK-3beta in IFG. Unlike Alzheimer's disease where tauopathy is more globally observed in diverse brain regions, our data demonstrates restricted expression of tauopathy in brains of PD and PDD, probably limited to dopaminergic neurons of the nigrostriatal region.

Prosurvival effect of human wild-type alpha-synuclein on MPTP-induced toxicity to central but not peripheral catecholaminergic neurons isolated from transgenic mice

In the present work we report the generation of a new line of alpha-synuclein (alpha-SYN) transgenic mice in which the human wild-type alpha-SYN cDNA is expressed under the control of a tyrosine hydroxylase (TH) promoter. We provide evidence that the ectopic protein is found in TH expressing neurons of both central and peripheral nervous systems. The transgene is expressed very early in development coinciding with the activity of the TH promoter and in the adult brain the human protein distributes normally to the nerve endings and cell bodies of dopaminergic nigral neurons without any evidence of abnormal aggregation. Our results indicate that expression of human wild-type alpha-SYN does not affect normal development or maintenance of TH immunoreactive nigral neurons, striatal dopamine content, or locomotor activity. Systemic administration of the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces a loss of TH immunoreactive nigral neurons and terminals and of dopamine levels to the same degree in both transgenic and non-transgenic adult mice. Intoxication also results in a similar loss of cardiac noradrenaline in both genotypes. Surprisingly, cultured transgenic ventral mesencephalic fetal dopaminergic neurons exhibit complete resistance to cell death induced by 1-methyl-4-phenylpyridinium ion (MPP(+)) intoxication, without changes in dopamine transporter (DAT) surface levels. Interestingly, this protection is not observed in other populations of catecholaminergic neurons such as peripheral sympathetic neurons, despite their high sensitivity to MPP(+)in vitro.

Vulnerability of peripheral catecholaminergic neurons to MPTP is not regulated by alpha-synuclein

Although generally considered a prototypical movement disorder, Parkinson's disease is commonly associated with a broad-spectrum of non-motor symptoms, including autonomic dysfunctions caused by significant alterations in catecholaminergic neurons of the peripheral sympathetic nervous system. Here we present evidence that alpha-synuclein is highly expressed by sympathetic ganglion neurons throughout embryonic and postnatal life and that it is found in tyrosine hydroxylase-positive sympathetic fibers innervating the heart of adult mice. However, mice deficient in alpha-synuclein do not exhibit any apparent alterations in sympathetic development. Sympathetic neurons isolated from mouse embryos and early postnatal mice are sensitive to the parkinsonian drug MPTP/MPP(+) and intoxication requires entry of the neurotoxin through the noradrenaline transporter. Furthermore, recovery of noradrenaline from cardiac sympathetic fibers is reduced in adult mice treated with MPTP systemically. However, MPP(+)-induced sympathetic neuron loss in vitro or MPTP-induced cardiac noradrenaline depletion in vivo is not modified in mice lacking alpha-synuclein. This is in clear contrast with the observation that dopaminergic neurons of the central nervous system are significantly less vulnerable to MPTP/MPP(+) in the absence of alpha-synuclein, suggesting different actions of this molecule in central and peripheral catecholaminergic neurons.

Desipramine induced changes in the norepinephrine transporter, alpha- and gamma-synuclein in the hippocampus, amygdala and striatum

The high incidence of depression in Parkinson's disease (PD) has been well documented in the clinic; however, the underlying molecular mechanisms of these overlapping pathologies remain elusive. Using a rodent model of depression, the Wistar-Kyoto (WKY) rat, we previously demonstrated that in the frontal cortex the altered expression and protein interactions of alpha- and gamma-synuclein (alpha-Syn, gamma-Syn) were associated with dysregulated trafficking of the norepinephrine transporter (NET). Chronic treatment with desipramine (DMI), a NET-selective antidepressant, caused a disappearance of depressive-like behavior that was accompanied by a change in alpha-Syn and gamma-Syn expression and their trafficking of NET. Using this same model, we examined the expression of NET, alpha-Syn and gamma-Syn in the hippocampus, amygdale, brainstem, and striatum, all regions implicated in the development or maintenance of depression or PD pathology. Following chronic treatment with DMI, we observed a significant decrease in NET in the hippocampus, amygdala, and brainstem; decrease in gamma-Syn in the hippocampus and amygdala; and, increase in alpha-Syn in the hippocampus and amygdala. Unexpectedly, we observed a significant decrease in alpha-Syn expression in the striatum of the WKY following chronic DMI treatment. The altered expression of NET, alpha-Syn and gamma-Syn in different brain suggest that DMI's ability to improve depressive-like behavior in a rodent is associated with region-specific changes in the regulation of NET by alpha- and gamma-Syn.

Subcellular localization of the antidepressant-sensitive norepinephrine transporter

BACKGROUND

Reuptake of synaptic norepinephrine (NE) via the antidepressant-sensitive NE transporter (NET) supports efficient noradrenergic signaling and presynaptic NE homeostasis. Limited, and somewhat contradictory, information currently describes the axonal transport and localization of NET in neurons.

RESULTS

We elucidate NET localization in brain and superior cervical ganglion (SCG) neurons, aided by a new NET monoclonal antibody, subcellular immunoisolation techniques and quantitative immunofluorescence approaches. We present evidence that axonal NET extensively colocalizes with syntaxin 1A, and to a limited degree with SCAMP2 and synaptophysin. Intracellular NET in SCG axons and boutons also quantitatively segregates from the vesicular monoamine transporter 2 (VMAT2), findings corroborated by organelle isolation studies. At the surface of SCG boutons, NET resides in both lipid raft and non-lipid raft subdomains and colocalizes with syntaxin 1A.

CONCLUSION

Our findings support the hypothesis that SCG NET is segregated prior to transport from the cell body from proteins comprising large dense core vesicles. Once localized to presynaptic boutons, NET does not recycle via VMAT2-positive, small dense core vesicles. Finally, once NET reaches presynaptic plasma membranes, the transporter localizes to syntaxin 1A-rich plasma membrane domains, with a portion found in cholera toxin-demarcated lipid rafts. Our findings indicate that activity-dependent insertion of NET into the SCG plasma membrane derives from vesicles distinct from those that deliver NE. Moreover, NET is localized in presynaptic membranes in a manner that can take advantage of regulatory processes targeting lipid raft subdomains.

Partial regulation of serotonin transporter function by gamma-synuclein

Human alpha-synuclein (alpha-Syn) is instrumental in maintaining homeostasis of monoamine neurotransmitters in brain, through its trafficking, and regulation of the cell surface expression and, thereby, activity of dopamine, serotonin and norepinephrine transporters. Here we have investigated whether other members of the synuclein family of proteins, gamma-synuclein (gamma-Syn) and beta-synuclein (beta-Syn) can similarly modulate the serotonin transporter (SERT). In Ltk(-) cells co-transfected with SERT and gamma-Syn, gamma-Syn reduced [(3)H]5-HT uptake, in a manner dependent on its expression levels. The decrease in SERT activity was via decreased V(max) of the transporter, without change in K(m), compared to cells expressing only SERT. By contrast, beta-Syn co-expression failed to alter SERT uptake activity, and neither the V(max) nor the K(m) was changed in the presence of beta-Syn. gamma-Syn modulation of SERT was only partial, with a maximal approximately 27% decrease in SERT activity seen even at high expression levels of gamma-Syn. By contrast, alpha-Syn attenuated SERT activity by approximately 65% at identical expression levels as gamma-Syn. Co-immunoprecipitation studies showed the presence of heteromeric protein:protein complexes between gamma-Syn or alpha-Syn and SERT, while beta-Syn failed to physically interact with SERT. Both alpha-Syn and gamma-Syn colocalized with SERT in rat primary raphae nuclei neurons. These studies document a novel physiological role for gamma-Syn in regulating 5-HT synaptic availability and homeostasis, and may be of relevance in depression and mood disorders, where SERT function is dysregulated.

Nigral burden of alpha-synuclein correlates with striatal dopamine deficit

Aggregated alpha-synuclein is the hallmark of Parkinson's disease (PD), diffuse Lewy body disease (DLBD), and multiple system atrophy (MSA). Physiologically, alpha-synuclein ensures normal functions of dopamine transporter (DAT) and tyrosine hydoxylase. In alpha-synucleinopathies, it accumulates in neuronal cytoplasm and neurites through several stages. It is unclear whether the accumulation of pathological alpha-synuclein in the substantia nigra in PD correlates with the dopaminergic deficit in the striatal target. We evaluated the impact of the nigral burden of pathological alpha-synuclein immunoreactivity in 27 alpha-synucleinopathy brains by morphometric immunohistochemistry. DAT immunoreactivity in the striatum inversely correlates with the total alpha-synuclein burden in the substantia nigra but not with cytoplasmic inclusion counts only. This result has implications for imaging, clinicopathological correlative studies, and staging of the disease process.

Desipramine modulation of alpha-, gamma-synuclein, and the norepinephrine transporter in an animal model of depression

The mechanisms underlying depression remain elusive. We previously determined that alpha-synuclein (alpha-Syn) modulates the activity and trafficking of the norepinephrine transporter (NET) in a manner that is dependent on its interactions with microtubules (MTs). Here we sought to determine if alpha-Syn, or the other synuclein family members, beta-synuclein (beta-Syn) and gamma-synuclein (gamma-Syn), modulate NET activity in an animal model of depression, the Wistar-Kyoto (WKY) rat. The NET-selective antidepressant desipramine (DMI) was chronically administered for 14 days to WKY rats and the strain from which it was outbred that does not show depressive-like behavior, the Wistar rat. This drug regimen induced significant behavioral improvements in the WKY, but not the Wistar rat, in the forced swim test. In WKY rats there was an overexpression of gamma-Syn which was reduced following DMI treatment. In parallel, DMI caused an increase in both alpha-Syn and NET in the frontal cortex. Frontal cortex synaptosomes from WKY rats were not sensitive to nocodazole, a compound that promotes MT destabilization. However, in WKYs treated with DMI, nocodazole induced an increase in [(3)H]-NE uptake. This trend was reversed in Wistars. Underlying these DMI-induced changes were alterations in the protein interactions between the synucleins and NET with the tubulins. These results are the first to implicate alpha-Syn or gamma-Syn in the pathophysiology of depression and suggest that targeting synucleins may provide a new therapeutic option for depression.

Selective noradrenergic vulnerability in α-synuclein transgenic mice

Classical pathological signs of Parkinson's disease (PD) include loss of dopaminergic neurons in substantia nigra (SN) and noradrenergic neurons in locus coeruleus (LC), and deposition of Lewy bodies rich in the presynaptic protein alpha-synuclein (ASYN). Mammalian genetic models based on ASYN overexpression, however, have generally not reproduced the profound dopaminergic deficit of PD and do not display classical PD phenotypes. In the current study we examined these catecholaminergic systems in transgenic (Tg) mice expressing the A53T mutant of human ASYN under the Prion promoter. Surprisingly we detected a substantial reduction in norepinephrine (NE), but not dopamine (DA), levels in spinal cord, olfactory bulb and striatum of aged (15-month-old), but not young (4-month-old) transgenic compared to control mice. In spinal cord and olfactory bulb of 15-month-old Tg mice there was an age-dependent decrease in tyrosine hydroxylase (TH) protein levels, which in spinal cord was accompanied by a decrease in TH-positive terminals detected by immunohistochemistry. There was no difference in the number of TH-positive neuron cell bodies in SN or LC between Tg and control mice. We conclude that aberrant ASYN, expressed in both SN and LC, induces preferential degeneration of noradrenergic terminals. These observations suggest that in mice the NE may be more vulnerable than the DA system to the toxic effects of aberrant alpha-synuclein, and are in line with the major damage to the NE system that occurs in patients with PD.

Gating deficits in isolation-reared rats are correlated with alterations in protein expression in nucleus accumbens

The isolation-rearing (IR) paradigm, consisting of the social deprivation for 6-9 weeks after weaning, induces a spectrum of aberrant behaviors in adult rats. Some of these alterations such as sensorimotor gating deficits are reminiscent of the dysfunctions observed in schizophrenia patients. Although gating impairments in IR rats have been linked to impairments in the cortico-mesolimbic system, the specific molecular mechanisms underlying this relation are unclear. To elucidate the neurochemical modifications underlying the gating disturbances exhibited by IR rats, we compared their pre-pulse inhibition (PPI) of the acoustic startle reflex with that of socially reared (SR) controls, and correlated this index to the results of proteomic analyses in prefrontal cortex and nucleus accumbens from both groups. As expected, IR rats exhibited significantly lower startle amplitude and PPI than their SR counterparts. Following behavioral testing, IR and SR rats were killed and protein expression profiles of their brain regions were examined using two-dimensional electrophoresis based proteomics. Image analysis in the Coomassie blue-stained gel revealed that three protein spots were differentially expressed in the nucleus accumbens of IR and SR rats. Mass spectrometry (matrix-assisted laser desorption ionization-time of flight and MS/MS) identified these spots as heat shock protein 60 (HSP60), alpha-synuclein (alpha-syn), and 14-3-3 protein zeta/delta. While accumbal levels of HSP60 was decreased in IR rats, alpha-syn and 14-3-3 proteins were significantly increased in IR in comparison with SR controls. Notably, these two last alterations were significantly correlated with different loudness intensity-specific PPI deficits in IR rats. In view of the role of these proteins in synaptic trafficking and dopaminergic regulation, these findings might provide a neurochemical foundation for the gating alterations and psychotic-like behaviors in IR rats.

The effect of alpha-synuclein knockdown on MPP+ toxicity in models of human neurons

The protein alpha-synuclein is central to the pathophysiology of Parkinson's disease (PD) but its role in the development of neurodegeneration remains unclear. alpha-Synuclein-knockout mice develop without gross abnormality and are resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a mitochondrial inhibitor widely used to model parkinsonism. Here we show that differentiated human dopaminergic neuron-like cells also have increased resistance to 1-methyl-4-phenylpyridine (MPP+), the active metabolite of MPTP, when alpha-synuclein is knocked down using RNA interference. In attempting to understand how this occurred we found that lowering alpha-synuclein levels caused changes to intracellular vesicles, dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2), each of which is known to be an important component of the early events leading to MPP+ toxicity. Knockdown of alpha-synuclein reduced the availability of DAT on the neuronal surface by 50%, decreased the total number of intracellular vesicles by 37% but increased the density of VMAT2 molecules per vesicle by 2.8-fold. However, these changes were not associated with any reduction in MPP+ -induced superoxide production, suggesting that alpha-synuclein knockdown may have other downstream effects which are important. We then showed that alpha-synuclein knockdown prevented MPP+ -induced activation of nitric oxide synthase (NOS). Activation of NOS is an essential step in MPTP toxicity and increasing evidence points to nitrosative stress as being important in neurodegeneration. Overall, these results show that as well as having a number of effects on cellular events upstream of mitochondrial dysfunction alpha-synuclein affects pathways downstream of superoxide production, possibly involving regulation of NOS activity.

Regulated interactions of the norepineprhine transporter by the actin and microtubule cytoskeletons

One role of the actin cytoskeleton is to maintain the structural morphology and activity of the pre-synaptic terminal. We sought to determine if the actin cytoskeleton plays a role in regulating interactions between the norepinephrine transporter (NET) and alpha-Synuclein (alpha-Syn), two proteins expressed in the pre-synaptic terminal. In cells transfected with either 0.5 microg/mL or 3 microg/mL of alpha-Syn and 1 microg/mL of NET DNA, treatment with cytochalasin D, an actin depolymerizing agent, caused a dose-dependent decrease and increase, respectively, in [3H]-NE uptake. Protein interactions between NET, beta-actin, and alpha-Syn were modified, along with levels of surface transporters. Treatment of primary brainstem neurons and frontal cortex synaptosomes with cytochalasin D caused a 115% and 28% increase, respectively, in NET activity. Depolymerization of both actin and microtubules did not alter NET activity in cells with 0.5 microg/mL alpha-Syn, but caused an increase in [3H]-NE uptake in cells transfected with 3 microg/mL of alpha-Syn and primary neurons. This is the first direct demonstration of NET activity being regulated via actin and modulated by interactions with alpha-Syn.

Regulation of the norepinephrine transporter by alpha-synuclein-mediated interactions with microtubules

alpha-Synuclein (alpha-Syn) regulates catecholaminergic neurotransmission. We demonstrate that alpha-Syn regulates the activity and surface expression of the norepinephrine transporter (NET), depending on its expression levels. In cells co-transfected with NET and low amounts of alpha-Syn, NET activity and cell surface expression were increased and protein interactions with alpha-Syn decreased, compared with cells transfected with NET alone. Converse effects were observed at higher levels of alpha-Syn expression. Treatment with nocodazole and other microtubule (MT) destabilizers abolished the expression-dependent bimodal regulation of NET by alpha-Syn. At low alpha-Syn levels, nocodazole had no effect on NET surface expression or protein interactions, while inducing increases in these measures at higher levels. Cells that were transfected with NET alone displayed no sensitivity to nocodazole, indicating that alpha-Syn expression was necessary for the MT-dependent changes in NET activity. MT destabilizers also caused a significant increase in [(3)H]-NE uptake in brainstem primary neurons and synaptosomes from the frontal cortex, but not striatal synaptosomes. These findings suggest that the surface localization and activity of NET is modulated by alpha-Syn in a manner that is both dependent on interactions with the MT cytoskeleton and varies across brain regions.