Familial Forms Of Parkinson's Disease Research Articles

Altered Expression of DJ-1 and PINK1 in Sporadic ALS and in the SOD1G93AALS Mouse Model

Mitochondrial dysfunction is an important mechanism in the pathogenesis of neurodegenerative diseases such as Parkinson disease and amyotrophic lateral sclerosis (ALS). DJ-1 and PTEN-induced putative kinase 1 (PINK1) are important proteins for the maintenance of mitochondrial function and protection against cell death. Mutations in the genes coding for these proteins cause familial forms of Parkinson disease. Recent studies have postulated that changes in the expression of both proteins are also involved in pathologic mechanisms in ALS mouse models. Here, we studied the mRNA and protein expression of PINK1 and DJ-1 in postmortem brain and spinal cord tissue and muscle biopsy samples from ALS patients and controls and in brain, spinal cord, and gastrocnemius muscle of SOD1(G93A) ALS mice at different disease stages. We found significant decreases of PINK1 and DJ-1 mRNA levels in muscle tissue of SOD1(G93A) mice. Together with the significant decrease of PINK1 mRNA levels in human ALS muscle tissue, statistically nonsignificant reduction of DJ-1 mRNA levels, and reduced immunostaining for PINK1 in human ALS muscle, the results suggest potential pathophysiologic roles for these proteins in both mutant SOD1 transgenic mice and in sporadic ALS(G93A).

Phosphatase and Tensin Homolog (PTEN)-induced Putative Kinase 1 (PINK1)-dependent Ubiquitination of Endogenous Parkin Attenuates Mitophagy: STUDY IN HUMAN PRIMARY FIBROBLASTS AND INDUCED PLURIPOTENT STEM CELL-DERIVED NEURONS

Mutations in the E3 ubiquitin ligase Parkin and the mitochondrial PTEN-induced putative kinase 1 (PINK1) have been identified to cause autosomal recessive forms of familial Parkinson disease, with PINK1 functioning upstream of Parkin in a pathway important for the maintenance of mitochondrial function and morphology. Upon the loss of the mitochondrial membrane potential, Parkin translocates to mitochondria in a PINK1-dependent manner to ubiquitinate mitochondrial proteins. Parkin-mediated polyubiquitination of outer mitochondrial membrane (OMM) proteins recruits the ubiquitin- and LC3-binding adaptor protein p62 to mitochondria and induces mitophagy. Although previous studies examined mitophagy in established cell lines through overexpression approaches, there is an imperative to study the role of endogenous Parkin and PINK1 in human-derived and biologically relevant cell models. Here, we demonstrate in human primary fibroblasts and induced pluripotent stem-derived neurons from controls and PINK1 mutation carriers that endogenous levels of Parkin are not sufficient to initiate mitophagy upon loss of the mitochondrial membrane potential, caused by its (self-)ubiquitination, followed by degradation via the ubiquitin proteasome system. Next, we showed differential PINK1-dependent, Parkin-mediated ubiquitination of OMM proteins, which is Parkin dose-dependent and affects primarily OMM proteins of higher molecular mass. In contrast to the situation fibroblasts, we did not detect mitophagy in induced pluripotent stem-derived neurons even upon overexpression of Parkin. Taken together, our data demonstrate that mitophagy differs between human non-neuronal and neuronal cells and between "endogenous" and "Parkin-overexpressing" cellular models.

SNCA (α-synuclein)-induced toxicity in yeast cells is dependent on Sir2-mediated mitophagy

SNCA (α-synuclein) misfolding and aggregation is strongly associated with both idiopathic and familial forms of Parkinson disease (PD). Evidence suggests that SNCA has an impact on cell clearance routes and protein quality control systems such as the ubiquitin-proteasome system (UPS) and autophagy. Recent advances in the key role of the autosomal recessive PARK2/PARKIN and PINK1 genes in mitophagy, highlighted this process as a prominent new pathogenic mechanism. Nevertheless, the role of autophagy/mitophagy in the pathogenesis of sporadic and autosomal dominant familial forms of PD is still enigmatic. The yeast Saccharomyces cerevisiae is a powerful “empty room” model that has been exploited to clarify different molecular aspects associated with SNCA toxicity, which combines the advantage of being an established system for aging research. The contribution of autophagy/mitophagy for the toxicity induced by the heterologous expression of the human wild-type SNCA gene and the clinical A53T mutant during yeast chronological life span (CLS) was explored. A reduced CLS together with an increase of autophagy and mitophagy activities were observed in cells expressing both forms of SNCA. Impairment of mitophagy by deletion of ATG11 or ATG32 resulted in a CLS extension, further implicating mitophagy in the SNCA toxicity. Deletion of SIR2, essential for SNCA toxicity, abolished autophagy and mitophagy, thereby rescuing cells. These data show that Sir2 functions as a regulator of autophagy, like its mammalian homolog, SIRT1, but also of mitophagy. Our work highlights that increased mitophagy activity, mediated by the regulation of ATG32 by Sir2, is an important phenomenon linked to SNCA-induced toxicity during aging.

Parkin and Parkinson Disease

Featured Article: Shimura H, Hattori N, Kubo S, Mizuno Y, Asakawa S, Minoshima S, et al. Familial Parkinson disease gene product, parkin, is a ubiquitin-protein ligase. Nat Genet 2000;25:302–5.5 Since the 1970s, Japanese neurologists have described patients with autosomal recessive forms of familial Parkinson disease (PD),6 which have been termed “autosomal recessive juvenile parkinsonism” and “early-onset parkinsonism with diurnal fluctuation,” both of which have become known as “PARK2” (1). We attempted to identify the gene responsible for autosomal recessive familial PD. In 1997, we identified, along with our collaborators, an autosomal recessive familial PD gene between D6S437 and D6S264 (2), and in 1998 we found that mutations in that gene were linked to autosomal recessive familial PD. We designated the gene, formerly known as parkin , as PARK2 7 \[parkinson protein 2, E3 ubiquitin protein ligase (parkin)\] (3). Parkin is a 465-amino-acid protein containing an N-terminal …

Remodeling of Lipid Vesicles into Cylindrical Micelles by α-Synuclein in an Extended α-Helical Conformation

α-Synuclein (αS) is a protein with multiple conformations and interactions. Natively unfolded in solution, αS accumulates as amyloid in neurological tissue in Parkinson disease and interacts with membranes under both physiological and pathological conditions. Here, we used cryoelectron microscopy in conjunction with electron paramagnetic resonance (EPR) and other techniques to characterize the ability of αS to remodel vesicles. At molar ratios of 1:5 to 1:40 for protein/lipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol), large spherical vesicles are converted into cylindrical micelles ~50 Å in diameter. Other lipids of the same charge (negative) exhibit generally similar behavior, although bilayer tubes of 150-500 Å in width are also produced, depending on the lipid acyl chains. At higher protein/lipid ratios, discoid particles, 70-100 Å across, are formed. EPR data show that, on cylindrical micelles, αS adopts an extended amphipathic α-helical conformation, with its long axis aligned with the tube axis. The observed geometrical relationship between αS and the micelle suggests that the wedging of its long α-helix into the outer leaflet of a membrane may cause curvature and an anisotropic partition of lipids, leading to tube formation.

The ubiquitin E3 ligase parkin regulates the proapoptotic function of Bax

Autosomal recessive loss-of-function mutations within the PARK2 gene functionally inactivate the E3 ubiquitin ligase parkin, resulting in neurodegeneration of catecholaminergic neurons and a familial form of Parkinson disease. Current evidence suggests both a mitochondrial function for parkin and a neuroprotective role, which may in fact be interrelated. The antiapoptotic effects of parkin have been widely reported, and may involve fundamental changes in the threshold for apoptotic cytochrome c release, but the substrate(s) involved in parkin dependent protection had not been identified. Here, we demonstrate the parkin-dependent ubiquitination of endogenous Bax comparing primary cultured neurons from WT and parkin KO mice and using multiple parkin-overexpressing cell culture systems. The direct ubiquitination of purified Bax was also observed in vitro following incubation with recombinant parkin. We found that parkin prevented basal and apoptotic stress-induced translocation of Bax to the mitochondria. Moreover, an engineered ubiquitination-resistant form of Bax retained its apoptotic function, but Bax KO cells complemented with lysine-mutant Bax did not manifest the antiapoptotic effects of parkin that were observed in cells expressing WT Bax. These data suggest that Bax is the primary substrate responsible for the antiapoptotic effects of parkin, and provide mechanistic insight into at least a subset of the mitochondrial effects of parkin.

Mutant Protein A30P α-Synuclein Adopts Wild-type Fibril Structure, Despite Slower Fibrillation Kinetics

α-Synuclein (AS) is associated with both sporadic and familial forms of Parkinson disease (PD). In sporadic disease, wild-type AS fibrillates and accumulates as Lewy bodies within dopaminergic neurons of the substantia nigra. The accumulation of misfolded AS is associated with the death of these neurons, which underlies many of the clinical features of PD. In addition, a rare missense mutation in AS, A30P, is associated with highly penetrant, autosomal dominant PD, although the pathogenic mechanism is unclear. A30P AS fibrillates more slowly than the wild-type (WT) protein in vitro and has been reported to preferentially adopt a soluble, protofibrillar conformation. This has led to speculation that A30P forms aggregates that are distinct in structure compared with wild-type AS. Here, we perform a detailed comparison of the chemical shifts and secondary structures of these fibrillar species, based upon our recent characterization of full-length WT fibrils. We have assigned A30P AS fibril chemical shifts de novo and used them to determine its secondary structure empirically. Our results illustrate that although A30P forms fibrils more slowly than WT in vitro, the chemical shifts and secondary structure of the resultant fibrils are in high agreement, demonstrating a conserved β-sheet core.

Neurodegenerative phenotypes in an A53T -synuclein transgenic mouse model are independent of LRRK2

Mutations in the genes encoding LRRK2 and α-synuclein cause autosomal dominant forms of familial Parkinson's disease (PD). Fibrillar forms of α-synuclein are a major component of Lewy bodies, the intracytoplasmic proteinaceous inclusions that are a pathological hallmark of idiopathic and certain familial forms of PD. LRRK2 mutations cause late-onset familial PD with a clinical, neurochemical and, for the most part, neuropathological phenotype that is indistinguishable from idiopathic PD. Importantly, α-synuclein-positive Lewy bodies are the most common pathology identified in the brains of PD subjects harboring LRRK2 mutations. These observations may suggest that LRRK2 functions in a common pathway with α-synuclein to regulate its aggregation. To explore the potential pathophysiological interaction between LRRK2 and α-synuclein in vivo, we modulated LRRK2 expression in a well-established human A53T α-synuclein transgenic mouse model with transgene expression driven by the hindbrain-selective prion protein promoter. Deletion of LRRK2 or overexpression of human G2019S-LRRK2 has minimal impact on the lethal neurodegenerative phenotype that develops in A53T α-synuclein transgenic mice, including premature lethality, pre-symptomatic behavioral deficits and human α-synuclein or glial neuropathology. We also find that endogenous or human LRRK2 and A53T α-synuclein do not interact together to influence the number of nigrostriatal dopaminergic neurons. Taken together, our data suggest that α-synuclein-related pathology, which occurs predominantly in the hindbrain of this A53T α-synuclein mouse model, occurs largely independently from LRRK2 expression. These observations fail to provide support for a pathophysiological interaction of LRRK2 and α-synuclein in vivo, at least within neurons of the mouse hindbrain.

Parkinson's disease: Molecular risk factors

Parkinson's disease (PD) is a progressive neurodegenerative disorder second only to Alzheimer's disease. Diagnosis remains clinical, based on phenotypic patterns. In the last decade many attempts to develop early differential pre-clinical markers have been reported. In this presentation, the molecular risk factors that may link between the etiopathogenesis leading to PD and peripheral markers will be discussed. Genetic variation known to be involved in familial forms of PD will be shown to be linked to sporadic cases, as for example leucine-rich repeat kinase 2 (LRRK2) that was found to regulate microRNA-mediated translation regulation. In addition postmortem microarray findings of transcription alterations will be compared to the peripheral findings of mRNA profiles. Molecular processes involved in ubiquitination and proteasome, autophagy, mitochondrial dysfunction and the nicotinic and adenosine A2 protection will be discussed. The question of what time-point should be used measuring the different markers and the course of the disease considered, and the future possibilities in exploring these techniques will be debated.

Variant in the 3′ region of SNCA associated with Parkinson’s disease and serum α-synuclein levels

Parkinson's disease (PD) is the second most common neurodegenerative disorder. The presence of Lewy bodies is a major pathological change of PD. α-synuclein is the main component of Lewy bodies and is encoded by the SNCA gene. Mutations in the SNCA gene mainly result in rare familial forms of PD, while genetic variability in the SNCA gene modulates susceptibility to sporadic PD. Recent studies have suggested that levels of α-synuclein in extracellular biological fluid are associated with PD and implicated α-synuclein as a potential biomarker for PD diagnosis and severity. We studied serum α-synuclein concentration and two polymorphic variants of SNCA (Rep1 and rs11931074) in 110 sporadic PD patients and 136 controls. We further explored the influence of the two polymorphisms on the expression levels of serum α-synuclein. Soluble α-synuclein was detected in serum in all subjects, with no statistically significant difference between PD patients and controls (p=0.611). Different Rep1 alleles and genotypes did not influence the expression of serum α-synuclein. The frequency of allele T of rs11931074 was significantly elevated in PD patients (p=0.041), and was correlated with decreased serum α-synuclein in both dominant (p=0.011) and additive (p=0.008) models of association.

Drug discovery in Parkinson's disease: update and developments in the use of cellular models

Parkinson's disease (PD) is the second most common neurodegenerative disorder and is characterized by the degeneration of dopaminergic (DA) neurons within the substantia nigra. Dopamine replacement drugs remain the most effective PD treatment but only provide temporary symptomatic relief. New therapies are urgently needed, but the search for a disease-modifying treatment and a definitive understanding of the underlying mechanisms of PD has been limited by the lack of physiologically relevant models that recapitulate the disease phenotype. The use of immortalized cell lines as in vitro model systems for drug discovery has met with limited success, since efficacy and safety too often fail to translate successfully in human clinical trials. Drug discoverers are shifting their focus to more physiologically relevant cellular models, including primary neurons and stem cells. The recent discovery of induced pluripotent stem (iPS) cell technology presents an exciting opportunity to derive human DA neurons from patients with sporadic and familial forms of PD. We anticipate that these human DA models will recapitulate key features of the PD phenotype. In parallel, high-content screening platforms, which extract information on multiple cellular features within individual neurons, provide a network-based approach that can resolve temporal and spatial relationships underlying mechanisms of neurodegeneration and drug perturbations. These emerging technologies have the potential to establish highly predictive cellular models that could bring about a desperately needed revolution in PD drug discovery.

A pilot trial of the microtubule-interacting peptide (NAP) in mice overexpressing alpha-synuclein shows improvement in motor function and reduction of alpha-synuclein inclusions

Abnormal accumulation of α-synuclein is associated with several neurodegenerative disorders (synucleinopathies), including sporadic Parkinson's disease (PD). Genetic mutations and multiplication of α-synuclein cause familial forms of PD and polymorphisms in the α-synuclein gene are associated with PD risk. Overexpression of α-synuclein can impair essential functions within the cell such as microtubule-dependent transport, suggesting that compounds that act on the microtubule system may have therapeutic benefit for synucleinopathies. In this study, mice overexpressing human wildtype α-synuclein under the Thy1 promoter (Thy1-aSyn) and littermate wildtype control mice were administered daily the microtubule-interacting peptide NAPVSIPQ (NAP; also known as davunetide or AL-108) intranasally for 2 months starting at 1 month of age, in a regimen known to produce effective concentrations of the peptide in mouse brain. Motor performance, coordination, and activity were assessed at the end of treatment. Olfactory function, which is altered in PD, was measured 1 month later. Mice were sacrificed at 4.5 months of age, and their brains examined for proteinase K-resistant α-synuclein inclusions in the substantia nigra and olfactory bulb. NAP-treated Thy1-aSyn mice showed a 38% decrease in the number of errors per step in the challenging beam traversal test and a reduction in proteinase K-resistant α-synuclein inclusions in the substantia nigra compared to vehicle treated transgenics. The data indicate a significant behavioral benefit and a long lasting improvement of α-synuclein pathology following administration of a short term (2 months) NAP administration in a mouse model of synucleinopathy.

Toxin-Induced and Genetic Animal Models of Parkinson's Disease

Parkinson's disease (PD) is a common progressive neurodegenerative disorder. The major pathological hallmarks of PD are the selective loss of nigrostriatal dopaminergic neurons and the presence of intraneuronal aggregates termed Lewy bodies (LBs), but the pathophysiological mechanisms are not fully understood. Epidemiologically, environmental neurotoxins such as pesticides are promising candidates for causative factors of PD. Oxidative stress and mitochondrial dysfunction induced by these toxins could contribute to the progression of PD. While most cases of PD are sporadic, specific mutations in genes that cause familial forms of PD have led to provide new insights into its pathogenesis. This paper focuses on animal models of both toxin-induced and genetically determined PD that have provided significant insight for understanding this disease. We also discuss the validity, benefits, and limitations of representative models.

Neurochemistry Changes Associated with Mutations in Familial Parkinsons Disease

Parkinson's disease (PD), a common neurodegenerative disease, is characterized by the progressive loss of dopamine neurons and the accumulation of Lewy bodies and neurites. The exact role of genetic and environmental factors in the pathogenesis of PD has frequently been debated. The association of MPTP (methyl-4-phenyl-1, 2, 3, 6- tetrahydropyridine) and toxins (such as rotenone) with parkinsonism highlights the potential etiologic role of environmental toxins in disease causation. The recent discoveries of monogenic (such as α-synuclein, Parkin, UCHL1, PINK1, DJ-1, LRRK2) forms of PD have provided considerable insights into its pathophysiology. Parkin, an ubiquitin protein ligase assists in the degradation of toxic substrates via the ubiquitin proteasome system. It can also mediate a nondegradative form of ubiquitination. PINK1 and LRRK2 are possibly involved in the phosphorylation of substrates important for various cellular functions. Some toxins could interact with α-synuclein, an endogenous protein that is implicated in pathology of PD. Increasing in vitro and in vivo studies suggest that deficits in mitochondrial function, oxidative and nitrosative stress, the accumulation of aberrant or misfolded proteins, and ubiquitin-proteasome system dysfunction underpin the pathogenesis of sporadic and familial forms of PD. Elucidation of the functions of the proteins encoded by the diseasecausing genes will provide an opportunity for identification of specific pathways that could be targeted in neurotherapeutics.

Genetic variability at the PARK16 locus

Parkinson's disease (PD) is a complex neurodegenerative disease which is clinically heterogeneous and pathologically consists of loss of dopaminergic neurons in the substantia nigra and intracytoplasmic neuronal inclusions containing alpha-synuclein aggregations known as Lewy bodies. Although the majority of PD is idiopathic, pathogenic mutations in several mendelian genes have been successfully identified through linkage analyses. To identify susceptibility loci for idiopathic PD, several genome-wide association studies (GWAS) within different populations have recently been conducted in both idiopathic and familial forms of PD. These analyses have confirmed SNCA and MAPT as loci harboring PD susceptibility. In addition, the GWAS identified several other genetic loci suggestively associated with the risk of PD; among these, only one was replicated by two different studies of European and Asian ancestries. Hence, we investigated this novel locus known as PARK16 for coding mutations in a large series of idiopathic pathologically proven PD cases, and also conducted an association study in a case-control cohort from the United Kingdom. An association between a novel RAB7L1 mutation, c.379-12insT, and disease (P-value=0.0325) was identified. Two novel coding variants present only in the PD cohort were also identified within the RAB7L1 (p.K157R) and SLC41A1 (p.A350V) genes. No copy number variation analyses have yet been performed within this recently identified locus. We concluded that, although both coding variants and risk alleles within the PARK16 locus seem to be rare, further molecular analyses within the PARK16 locus and within different populations are required in order to examine its biochemical role in the disease process.

Parkinson Disease Insights

A study in cultured cells by scientists in Japan and the United States provides new information on interactions between 2 genes known to be involved in the development of familial forms of Parkinson disease (Matsuda N et al. J Cell Biol. 2010;189[2]:211-221).

α-Synuclein overexpression increases dopamine toxicity in BE(2)-M17 cells

BackgroundOxidative stress has been proposed to be involved in the pathogenesis of Parkinson's disease (PD). A plausible source of oxidative stress in nigral dopaminergic neurons is the redox reactions that specifically involve dopamine and produce various toxic molecules, i.e., free radicals and quinone species. α-Synuclein, a protein found in Lewy bodies characteristic of PD, is also thought to be involved in the pathogenesis of PD and point mutations and multiplications in the gene coding for α-synuclein have been found in familial forms of PD.ResultsWe used dopaminergic human neuroblastoma BE(2)-M17 cell lines stably transfected with WT or A30P mutant α-synuclein to characterize the effect of α-synuclein on dopamine toxicity. Cellular toxicity was analyzed by lactate dehydrogenase assay and by fluorescence-activated cell sorter analysis. Increased expression of either wild-type or mutant α-synuclein enhances the cellular toxicity induced by the accumulation of intracellular dopamine or DOPA.ConclusionsOur results suggest that an interplay between dopamine and α-synuclein can cause cell death in a neuron-like background. The data presented here are compatible with several models of cytotoxicity, including the formation of α-synuclein oligomers and impairment of the lysosomal degradation.

Phosphopeptide Analysis Reveals Two Discrete Clusters of Phosphorylation in the N-Terminus and the Roc Domain of the Parkinson-Disease Associated Protein Kinase LRRK2

Mutations in leucine-rich repeat kinase 2 (LRRK2) that increase its kinase activity associate with familial forms of Parkinson disease (PD). As phosphorylation determines the functional state of most protein kinases, we systematically mapped LRRK2 phosphorylation sites by mass spectrometry. Our analysis revealed a high degree of constitutive phosphorylation in a narrow serine-rich region preceding the LRR-domain. Allowing de novo autophosphorylation of purified LRRK2 in an in vitro autokinase assay prior to mass spectrometric analysis, we discovered multiple sites of autophosphorylation. Solely serine and threonine residues were found phosphorylated suggesting LRRK2 as a true serine threonine kinase. Autophosphorylation mainly targets the ROC GTPase domain and its clustering around the GTP binding pocket of ROC suggests cross-regulatory activity between kinase and Roc domain. In conclusion, the phosphoprotein LRRK2 functions as an autocatalytically active serine threonine kinase. Clustering of phosphosites within two discrete domains suggest that phosphorylation may regulate its biological functions in a yet unknown fashion.

Extensive enteric nervous system abnormalities in mice transgenic for artificial chromosomes containing Parkinson disease-associated α-synuclein gene mutations precede central nervous system changes

Parkinson disease (PD) is a neurodegenerative disease with motor as well as non-motor signs in the gastrointestinal tract that include dysphagia, gastroparesis, prolonged gastrointestinal transit time, constipation and difficulty with defecation. The gastrointestinal dysfunction commonly precedes the motor symptoms by decades. Most PD is sporadic and of unknown etiology, but a fraction is familial. Among familial forms of PD, a small fraction is caused by missense (A53T, A30P and E46K) and copy number mutations in SNCA which encodes alpha-synuclein, a primary protein constituent of Lewy bodies, the pathognomonic protein aggregates found in neurons in PD. We set out to develop transgenic mice expressing mutant alpha-synuclein (either A53T or A30P) from insertions of an entire human SNCA gene as models for the familial disease. Both the A53T and A30P lines show robust abnormalities in enteric nervous system (ENS) function and synuclein-immunoreactive aggregates in ENS ganglia by 3 months of age. The A53T line also has abnormal motor behavior but neither demonstrates cardiac autonomic abnormalities, olfactory dysfunction, dopaminergic neurotransmitter deficits, Lewy body inclusions or neurodegeneration. These animals recapitulate the early gastrointestinal abnormalities seen in human PD. The animals also serve as an in vivo system in which to investigate therapies for reversing the neurological dysfunction that target alpha-synuclein toxicity at its earliest stages.

A Biochemical and Functional Protein Complex Involving Dopamine Synthesis and Transport into Synaptic Vesicles

Synaptic transmission depends on neurotransmitter pools stored within vesicles that undergo regulated exocytosis. In the brain, the vesicular monoamine transporter-2 (VMAT(2)) is responsible for the loading of dopamine (DA) and other monoamines into synaptic vesicles. Prior to storage within vesicles, DA synthesis occurs at the synaptic terminal in a two-step enzymatic process. First, the rate-limiting enzyme tyrosine hydroxylase (TH) converts tyrosine to di-OH-phenylalanine. Aromatic amino acid decarboxylase (AADC) then converts di-OH-phenylalanine into DA. Here, we provide evidence that VMAT(2) physically and functionally interacts with the enzymes responsible for DA synthesis. In rat striata, TH and AADC co-immunoprecipitate with VMAT(2), whereas in PC 12 cells, TH co-immunoprecipitates with the closely related VMAT(1) and with overexpressed VMAT(2). GST pull-down assays further identified three cytosolic domains of VMAT(2) involved in the interaction with TH and AADC. Furthermore, in vitro binding assays demonstrated that TH directly interacts with VMAT(2). Additionally, using fractionation and immunoisolation approaches, we demonstrate that TH and AADC associate with VMAT(2)-containing synaptic vesicles from rat brain. These vesicles exhibited specific TH activity. Finally, the coupling between synthesis and transport of DA into vesicles was impaired in the presence of fragments involved in the VMAT(2)/TH/AADC interaction. Taken together, our results indicate that DA synthesis can occur at the synaptic vesicle membrane, where it is physically and functionally coupled to VMAT(2)-mediated transport into vesicles.