Microtubule assembly is directly affected by MPP(+)in vitro

Is there a special relationship between complex I activity and nigral neuronal loss in Parkinson's disease? A critical reappraisal

Parkinson’s disease (PD) is a progressive neurodegenerative disease manifesting both motor and non-motor symptoms. The motor features are generally ascribed to the selective loss of dopamine neurons within the substantia nigra pars compacta. While the precise etiology of PD remains elusive, multiple genetic and environmental elements have emerged as contributing factors. The discovery of MPTP-induced parkinsonism directed intense inquiry towards mitochondrial pathways, with a specific focus on mitochondrial complex I. Consisting of more than 40 subunits, complex I is the first enzyme of the electron transport chain that is required for mitochondrial ATP production. In this review, we present a critical analysis of studies assessing the prevalence and specificity of mitochondrial complex I deficiency in PD. In addition, we take the novel view of incorporating the features of genetically-defined bona fide complex I disorders and the prevalence of nigral involvement in such cases. Through this innovative bi-directional view, we consider both complex I changes in a disease of the substantia nigra and nigral changes in diseases of complex I. We assess the strength of association between nigral cell loss and complex I deficits, as well as the oft under-appreciated heterogeneity of complex I deficiency disorders and the variability of the PD data.

N-palmitoylethanolamide Prevents Parkinsonian Phenotypes in Aged Mice

Parkinson's disease (PD) is a neurodegenerative disease characterized by degeneration of dopaminergic neurons. Aging is a major risk factor for idiopathic PD. Several prior studies examined the neuroprotective effects of palmitoylethanolamide (PEA), alone or combined with antioxidants, in a model of PD induced by the dopaminergic toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Here, we analyzed the pretreatment effect of micronized PEA (PEAm) on neuroinflammation and neuronal cell death in the MPTP model. Male CD mice (21 months of age) were pre-treated for 60 days with PEAm. After this time, they received four intraperitoneal injections of MPTP over a 24-h period and were killed 7 days later. On the 8th day, brains were processed. Pretreatment with PEAm ameliorated behavioral deficits and the reductions in expression of tyrosine hydroxylase and dopamine transporter, while blunting the upregulation of α-synuclein and β3-tubulin in the substantia nigra after MPTP induction. Moreover, PEAm reduced proinflammatory cytokine expression and showed a pro-neurogenic effect in hippocampus. These findings propose this strategy as a valid approach to prevent neurodegenerative diseases associated with old age.

Microtubule Destabilization Paves the Way to Parkinson's Disease

Microtubules are dynamic structures normally associated to the cell division, during which they form the mitotic spindle, as well as to the initial phases of specification and polarization of various cell types, including neurons. Although microtubules could have a role in the death of many cells and tissues, the microtubule-based degenerative mechanisms have been poorly investigated; nevertheless, during the last two decades, many clues have been accumulated suggesting the importance of the microtubule system during neurodegeneration. Thus, the aim of this review is to analyse how the changes of the microtubule cytoskeleton, in terms of organization and dynamics, as well as the failure of the microtubule-dependent neuronal processes, as axonal transport, may play a pivotal role in the chain of events leading to Parkinson's disease. Last but not least, since disease-modifying or neuroprotective strategies are a clinical priority in Parkinson's disease, we will also present the hints about the concrete possibility of a microtubule-targeted therapy, which would have the potentiality to block the running degenerative events and to prompt the regeneration of the lost tissues.

Microtubule: A Common Target for Parkin and Parkinson’s Disease Toxins

Parkinson’s disease (PD) is characterized by the selective loss of nigral dopaminergic (DA) neurons, which have long axons enriched with microtubules. Depolymerization of microtubules by PD toxins such as rotenone disrupts vesicular transport. The ensuing accumulation of vesicles in the cell body leads to increased cytosolic concentration of dopamine due to leakage of the vesicles. Elevated oxidative stress induced by dopamine oxidation may thus trigger the selective demise of DA neurons. Many strategies have been developed to protect DA neurons by stabilizing microtubules either directly or through intracellular signaling cascades. On the other hand, parkin, one of the most frequently mutated genes in PD, encodes for a protein-ubiquitin E3 ligase that strongly binds to microtubules. Parkin stabilizes microtubules through three domains that provide strong and independent interactions with tubulin and microtubules. These interactions anchor parkin on microtubules and may facilitate its E3 ligase activity on misfolded proteins transported along microtubules. Thus, parkin and rotenone, two prominent genetic and environmental factors linked to PD, act in an opposing manner on the same molecular target in the cell, microtubules, whose destruction underlies the selective vulnerability of dopaminergic neurons.

New insights on the mitochondrial proteome plasticity in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases whose relentless progression results in severe disability. Although PD aetiology is unknown, growing evidences point to the mitochondrial involvement in the pathobiology of this disorder. So, it seems imperative to understand the means by which the molecular pathways harboured in this organelle are regulated. With the advances in MS-based proteomics, there is a substantial expectation in the increased knowledge of mitochondrial protein dynamics. Still, few studies have been performed on mitochondrial protein profiling in the context of PD. In order to integrate data from these studies, network analyses were performed taking into consideration variables such as model of PD, cell line, or tissue origin. Overall, data retrieved from these analyses highlighted the modulation of the biological processes related with "generation of energy," "cellular metabolism," and "mitochondrial transport" in PD. However, it was noted that the impact of sample type and/or PD model on the biological processes was modulated by the disease. Moreover, technical considerations related to protein characterization using gel-based or gel-free MS approaches should be considered in data comparison among different studies. Data from the present review will help to envisage future studies targeting these mechanisms.

MPTP's pathway of toxicity indicates central role of transcription factor SP1

Deriving a Pathway of Toxicity from transcriptomic data remains a challenging task. We explore the use of weighted gene correlation network analysis (WGCNA) to extract an initial network from a small microarray study of MPTP toxicity in mice. Five modules were statistically significant; each module was analyzed for gene signatures in the Chemical and Genetic Perturbation subset of the Molecular Signatures Database as well as for over-represented transcription factor binding sites and WGCNA clustered probes by function and captured pathways relevant to neurodegenerative disorders. The resulting network was analyzed for transcription factor candidates, which were narrowed down via text-mining for relevance to the disease model, and then combined with the large-scale interaction FANTOM4 database to generate a genetic regulatory network. Modules were enriched for transcription factors relevant to Parkinson's disease. Transcription factors significantly improved the number of genes that could be connected in a given component. For each module, the transcription factor that had, by far, the highest number of interactions was SP1, and it also had substantial experimental evidence of interactions. This analysis both captures much of the known biology of MPTP toxicity and suggests several candidates for further study. Furthermore, the analysis strongly suggests that SP1 plays a central role in coordinating the cellular response to MPTP toxicity.

Vasonatrin peptide, a novel protector of dopaminergic neurons against the injuries induced by n-methyl-4-phenylpyridiniums

Vasonatrin peptide (VNP), a novel manmade natriuretic peptide, is known as a cardiovascular active substance. However, its neuroeffects are largely unknown. Here, cultured dopaminergic neurons from ventral mesencephalon of mouse were exposed to N-methyl-4-phenylpyridinium (MPP(+)), and the effects of VNP on the neurotoxicity of MPP(+) were investigated. As a result, MPP(+) caused injuries in the dopaminergic neurons. VNP significantly reduced the cytotoxicity of MPP(+) by increasing axon number and length of dopaminergic neurons, and by enhancing the cell viability. Also, the MPP(+)-induced depolymerization of β-Tubulin III was attenuated by the treatment of VNP. In addition, VNP significantly increased the intracellular levels of cGMP. These effects of VNP were mimicked by 8-br-cGMP (a cell-permeable analog of cGMP), whereas inhibited by HS-142-1 (the antagonist of the particulate guanylyl cyclase-coupled natriuretic peptide receptors), or KT-5823 (a cGMP-dependent protein kinase inhibitor). Taken together, VNP attenuates the neurotoxicity of MPP(+) via guanylyl cyclase-coupled NPR/cGMP/PKG pathway, indicating that VNP might be a new effective reagent in the treatment of neuron degeneration of dopaminergic neurons in Parkinson's disease (PD).

Staurosporine induces dopaminergic neurite outgrowth through AMP-activated protein kinase/mammalian target of rapamycin signaling pathway

Axonal degeneration of dopaminergic neurons is one of the pathological features in the early stages of Parkinson disease. Promotion of axonal outgrowth of the remaining dopaminergic neurons leads to the recovery of the nigrostriatal pathway. Staurosporine (STS), a wide-spectrum kinase inhibitor, induces neurite outgrowth in various cell types, although its mechanism of action remains elusive. In this study, we analyzed which protein kinase is involved in STS-induced neurite outgrowth. We have previously established the method to measure the length of dopaminergic neurites that extend from a mesencephalic cell region, which is formed on a coverslip by an isolation wall. By means of this method, we clarified that STS treatment causes dopaminergic axonal outgrowth in mesencephalic primary cultures. Among the specific protein kinase inhibitors we tested, compound C (C.C), an AMP-activated protein kinase (AMPK) inhibitor, promoted dopaminergic neurite outgrowth. STS as well as C.C elevated the phosphorylation level of 70-kDa ribosomal protein S6 kinase, a downstream target of mammalian target of rapamycin (mTOR) signaling pathway. The STS- and C.C-induced dopaminergic neurite outgrowth was suppressed by rapamycin, an mTOR inhibitor. Furthermore, the application of C.C rescued 1-methyl-4-phenylpyridinium ion (MPP(+))-induced dopaminergic neurite degeneration. These results suggest that STS induces dopaminergic axonal outgrowth through mTOR signaling pathway activation as a consequence of AMPK inhibition.

Effects of potential environmental interferents on kinesin-powered molecular shuttles

Biomolecular motor-powered active transport represents an alternate means for analyte processing in nanoscale biosensors and bioanalytical devices. For example, a prototype "smart dust" biosensor has recently been reported in which the motor protein kinesin processes antibody-functionalized microtubules (MTs) to capture and separate optically tagged protein analytes. A potential limitation of this technology, however, involves the inhibition of transport function by interfering compounds that may be present in raw samples. Here we characterized the response of kinesin-MT transport to a range of potential interferents including solvents, acids, oxidizers, and environmental contaminants. The results of kinesin motility assays suggest that, among the tested interferents, only acetic acid and sodium hypochlorite adversely affected MT transport, primarily due to depolymerization of MT filaments. While negative effects were not observed for the remaining compounds tested, enhancement in motility was observed in the presence of acetone, antifreeze, and organic matter. Overall, the data suggest that kinesin-MT transport is resilient against a variety of common interferents, but primarily susceptible to failure due to significant changes in pH or the presence of an oxidizer.

WldS but not Nmnat1 protects dopaminergic neurites from MPP+ neurotoxicity

BACKGROUND

The WldS mouse mutant ("Wallerian degeneration-slow") delays axonal degeneration in a variety of disorders including in vivo models of Parkinson's disease. The mechanisms underlying WldS -mediated axonal protection are unclear, although many studies have attributed WldS neuroprotection to the NAD+-synthesizing Nmnat1 portion of the fusion protein. Here, we used dissociated dopaminergic cultures to test the hypothesis that catalytically active Nmnat1 protects dopaminergic neurons from toxin-mediated axonal injury.

RESULTS

Using mutant mice and lentiviral transduction of dopaminergic neurons, the present findings demonstrate that WldS but not Nmnat1, Nmnat3, or cytoplasmically-targeted Nmnat1 protects dopamine axons from the parkinsonian mimetic N-methyl-4-phenylpyridinium (MPP+). Moreover, NAD+ synthesis is not required since enzymatically-inactive WldS still protects. In addition, NAD+ by itself is axonally protective and together with WldS is additive in the MPP+ model.

CONCLUSIONS

Our data suggest that NAD+ and WldS act through separate and possibly parallel mechanisms to protect dopamine axons. As MPP+ is thought to impair mitochondrial function, these results suggest that WldS might be involved in preserving mitochondrial health or maintaining cellular metabolism.

Ubiquitin/proteasome pathway impairment in neurodegeneration: therapeutic implications

The ubiquitin/proteasome pathway is the major proteolytic quality control system in cells. In this review we discuss the impact of a deregulation of this pathway on neuronal function and its causal relationship to the intracellular deposition of ubiquitin protein conjugates in pathological inclusion bodies in all the major chronic neurodegenerative disorders, such as Alzheimer's, Parkinson's and Huntington's diseases as well as amyotrophic lateral sclerosis. We describe the intricate nature of the ubiquitin/proteasome pathway and discuss the paradox of protein aggregation, i.e. its potential toxic/protective effect in neurodegeneration. The relations between some of the dysfunctional components of the pathway and neurodegeneration are presented. We highlight possible ubiquitin/proteasome pathway-targeting therapeutic approaches, such as activating the proteasome, enhancing ubiquitination and promoting SUMOylation that might be important to slow/treat the progression of neurodegeneration. Finally, a model time line is presented for neurodegeneration starting at the initial injurious events up to protein aggregation and cell death, with potential time points for therapeutic intervention.

Mitochondrial Dysfunction: The Road to Alpha-Synuclein Oligomerization in PD

While the etiology of Parkinson's disease remains largely elusive, there is accumulating evidence suggesting that mitochondrial dysfunction occurs prior to the onset of symptoms in Parkinson's disease. Mitochondria are remarkably primed to play a vital role in neuronal cell survival since they are key regulators of energy metabolism (as ATP producers), of intracellular calcium homeostasis, of NAD(+)/NADH ratio, and of endogenous reactive oxygen species production and programmed cell death. In this paper, we focus on mitochondrial dysfunction-mediated alpha-synuclein aggregation. We highlight some of the findings that provide proof of evidence for a mitochondrial metabolism control in Parkinson's disease, namely, mitochondrial regulation of microtubule-dependent cellular traffic and autophagic lysosomal pathway. The knowledge that microtubule alterations may lead to autophagic deficiency and may compromise the cellular degradation mechanisms that culminate in the progressive accumulation of aberrant protein aggregates shields new insights to the way we address Parkinson's disease. In line with this knowledge, an innovative window for new therapeutic strategies aimed to restore microtubule network may be unlocked.

Human dental pulp stem cells protect mouse dopaminergic neurons against MPP+ or rotenone

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive death of substantia nigra dopaminergic neurons that results in a regional loss of striatal dopamine (DA) levels. Dental pulp contains ex vivo-expandable cells called dental pulp stem cells (DPSCs), with the capacity to differentiate into multiple cell lineages. More interestingly, due to their embryonic origin, DPSCs express neurotrophic factors such as brain-derived neurotrophic factor, nerve growth factor and glial cell-derived neurotrophic factor. The aim of the present study was to investigate the neuroprotective effects of DPSCs against MPP+ (2.5, 5, and 10 μM) and rotenone (0.25, 0.5 and 1 μM) in an in vitro model of PD, using an indirect co-culture system with mesencephalic cell cultures. When mesencephalic cultures were challenged with MPP+ or rotenone, in the presence of DPSCs a statistically significant protective effect was observed at all the tested doses in terms of DA uptake. DPSCs protective effect on DA neurons was also confirmed by immunocytochemistry: an increased number of spared tyrosine hydroxylase (TH)+ cells was observed in co-culture conditions compared to controls, and neurons showed longer processes in comparison with mesencephalic cells grown without DPSCs. In conclusion, the co-culture with DPSCs significantly attenuated MPP+ or rotenone-induced toxicity in primary cultures of mesencephalic neurons. Considering that the direct contact between the two cell types was prevented, it can be speculated that neuroprotection could be due to soluble factors such as BDNF and NGF, released by DPSCs. Blocking BDNF and NGF with neutralizing antibodies, the neuroprotecting effect of DPSCs was completely abolished. Therefore DPSCs can be viewed as possible candidates for studies on cell-based therapy in neurodegenerative disorders.

Microtubule dysfunction precedes transport impairment and mitochondria damage in MPP+ -induced neurodegeneration

Dysfunction of the microtubule (MT) system is an emerging theme in the pathogenesis of Parkinson's disease. This study was designed to investigate the putative role of MT dysfunction in dopaminergic neuron death induced by the neurotoxin 1-methyl-4-phenylpiridinium (MPP(+)). In nerve growth factor-differentiated PC12 cells, we have analyzed post-translational modifications of tubulin known to be associated with differently dynamic MTs and show that MPP(+) causes a selective loss of dynamic MTs and a concomitant enrichment of stable MTs. Through a direct live cell imaging approach, we show a significant reduction of MT dynamics following exposure to MPP(+) and a reorientation of MTs. Furthermore, these alterations precede the impairment of intracellular transport as revealed by changes in mitochondria movements along neurites and their accumulation into varicosities. We have also analyzed activation of caspase 3 and mitochondrial injury, well-known alterations induced by MPP(+), and found that they are noticeable only when MT dysfunction is already established. These data provide the first evidence that axonal transport impairment and mitochondrial damage might be a consequence of MT dysfunction in MPP(+) -induced neurodegeneration, lending support to the concept that alterations of MT organization and dynamics could play a pivotal role in neuronal death in Parkinson's disease.

Cell death pathways in Parkinson's disease: proximal triggers, distal effectors, and final steps

Parkinson's disease (PD) is a common neurodegenerative disorder. Neuronal cell death in PD is still poorly understood, despite a wealth of potential pathogenic mechanisms and pathways. Defects in several cellular systems have been implicated as early triggers that start cells down the road toward neuronal death. These include abnormal protein accumulation, particularly of alpha-synuclein; altered protein degradation via multiple pathways; mitochondrial dysfunction; oxidative stress; neuroinflammation; and dysregulated kinase signaling. As dysfunction in these systems mounts, pathways that are more explicitly involved in cell death become recruited. These include JNK signaling, p53 activation, cell cycle re-activation, and signaling through bcl-2 family proteins. Eventually, neurons become overwhelmed and degenerate; however, even the mechanism of final cell death in PD is still unsettled. In this review, we will discuss cell death triggers and effectors that are relevant to PD, highlighting important unresolved issues and implications for the development of neuroprotective therapies.

Cytoskeleton/Endoplasmic Reticulum Collapse Induced by Prostaglandin J2 Parallels Centrosomal Deposition of Ubiquitinated Protein Aggregates

Many neurodegenerative disorders, such as Parkinson disease, exhibit inclusion bodies containing ubiquitinated proteins. The mechanisms implicated in this aberrant protein deposition remain elusive. In these disorders signs of inflammation are also apparent in the affected central nervous system areas. We show that prostaglandin J2 (PGJ2), an endogenous product of inflammation, disrupts the cytoskeleton in neuronal cells. Furthermore, PGJ2 perturbed microtubule polymerization in vitro and decreased the number of free sulfhydryl groups on tubulin cysteines. A direct effect of PGJ2 on actin was not apparent, although actin filaments were altered in cells treated with PGJ2. This cyclopentenone prostaglandin triggered endoplasmic reticulum (ER) collapse and the redistribution of ER proteins, such as calnexin and catechol-O-methyltransferase, into a large centrosomal aggregate containing ubiquitinated proteins and alpha-synuclein. The PGJ2-dependent cytoskeletal rearrangement paralleled the development of the large centrosomal aggregate. Both of these events were replicated by treating cells with colchicine, which disrupts the microtubule/ER network, but not with brefeldin A, which impairs ER/Golgi transport. PGJ2 also perturbed 26 S proteasome assembly and activity, which preceded the accumulation of ubiquitinated proteins as detergent/salt-insoluble aggregates. Our data support a mechanism by which, upon PGJ2 treatment, cytoskeleton/ER collapse coincides with the relocation of ER proteins, other potentially neighboring proteins, and ubiquitinated proteins into centrosomal aggregates. Development of these large perinuclear aggregates is associated with disruption of the microtubule/ER network. This aberrant protein deposition, triggered by a product of inflammation, may be common to other compounds that disrupt microtubules and induce protein aggregation, such as MPP+ and rotenone, found to be associated with neurodegeneration.

MPTP induces intranuclear rodlet formation in midbrain dopaminergic neurons

Neuronal intranuclear rodlets (INRs; rodlets of Roncoroni) have been known to neuroanatomists since the turn of the century. However, the functional and/or pathological significance of these structures has remained enigmatic. We recently demonstrated that these structures are immunoreactive for class III beta tubulin and for glucocorticoid receptor. Moreover, they are markedly reduced in the temporal cortex of patients with Alzheimer's disease relative to age-matched controls and those with dementia with Lewy bodies, thereby implicating these structures in neurodegenerative disease pathogenesis. The present report represents an experimental pilot study to investigate the possible involvement of INRs in Parkinson's disease (PD). Specifically, we demonstrate significantly increased INRs in dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area in mice treated with the selective catecholaminergic neurotoxin MPTP, relative to saline-treated controls. We have hypothesized that INRs represent an intranuclear sequestrum of monomeric beta-tubulin and that their alteration in neurodegeneration may reflect disrupted or abnormal microtubule dynamics. We propose that the increased formation of INRs is related to the demonstrated ability of MPTP to cause microtubule disruption. Because tubulin has also been implicated in the pathogenesis of human PD, it is possible that the results of this study will have important implications for this most common neurodegenerative movement disorder.

Selective vulnerability of dopaminergic neurons to microtubule depolymerization

Parkinson disease (PD) is characterized by the specific degeneration of dopaminergic (DA) neurons in substantia nigra and has been linked to a variety of environmental and genetic factors. Rotenone, an environmental PD toxin, exhibited much greater toxicity to DA neurons in midbrain neuronal cultures than to non-DA neurons. The effect was significantly decreased by the microtubule-stabilizing drug taxol and mimicked by microtubule-depolymerizing agents such as colchicine or nocodazole. Microtubule depolymerization disrupted vesicular transport along microtubules and caused the accumulation of dopamine vesicles in the soma. This led to increased oxidative stress due to oxidation of cytosolic dopamine leaked from vesicles. Inhibition of dopamine metabolism significantly reduced rotenone toxicity. Thus, our results suggest that microtubule depolymerization induced by PD toxins such as rotenone plays a key role in the selective death of dopaminergic neurons.

The parkinsonism producing neurotoxin MPP+affects microtubule dynamics by acting as a destabilising factor

Dysfunction of the microtubule system is emerging as a contributing factor in a number of neurodegenerative diseases. Looking for the potential role played by the microtubule cytoskeleton in neuron degeneration underlying Parkinson's disease (PD), we investigate the influence of the parkinsonism producing neurotoxin 1-methyl-4-phenylpyridinium (MPP+) on microtubule dynamics. We find that it acts as a strong catastrophe promoter causing a decrease of the average length of microtubules assembled from purified tubulin. We also find that it reduces the number of microtubules nucleated from purified centrosomes. Finally, binding assays demonstrate that the neurotoxin binds specifically to tubulin in the microtubule lattice in a close to stoichiometric manner. This paper provides the first evidence that dynamic instability of microtubules is specifically affected by MPP+ and suggests that it could play a role in neuronal cell death underlying PD.

Parkin Stabilizes Microtubules through Strong Binding Mediated by Three Independent Domains

Mutations of parkin, a protein-ubiquitin isopeptide ligase (E3), appear to be the most frequent cause of familial Parkinson's disease (PD). Our previous studies have demonstrated that parkin binds strongly to alpha/beta tubulin heterodimers and microtubules. Here we show that the strong binding between parkin and tubulin, as well as that between parkin and microtubules, was mediated by three independent domains: linker, RING1, and RING2. These redundant strong interactions made it virtually impossible to separate parkin from microtubules by high concentrations of salt (3.8 m) or urea (0.5 m). Parkin co-purified with tubulin and was found in highly purified tubulin preparation. Expression of either full-length parkin or any of its three microtubule-binding domains significantly attenuated colchicine-induced microtubule depolymerization. The abilities of parkin to bind to and stabilize microtubules were not affected by PD-linked mutations that abrogate its E3 ligase activity. Thus, the tubulin/microtubule-binding activity of parkin and its E3 ligase activity are independent. The strong binding between parkin and tubulin/microtubules through three redundant interaction domains may not only stabilize microtubules but also guarantee the anchorage of this E3 ligase on microtubules. Because many misfolded proteins are transported on microtubules, the localization of parkin on microtubules may provide an important environment for its E3 ligase activity toward misfolded substrates.

Linkage disequilibrium and haplotype tagging polymorphisms in the Tau H1 haplotype

We and others have previously detected association of the Tau H1 haplotype on chromosome 17 with risk of idiopathic Parkinson disease (PD). The H1 haplotype appears to have a fundamental importance in neurodegeneration, as multiple studies have shown it is also associated with an increased risk for progressive supranuclear palsy, corticobasal degeneration, frontotemporal lobar degeneration syndromes, and primary progressive aphasia. Therefore, to divide the H1 haplotype into sub-haplotypes that could be more significantly associated with the risk of developing PD, and to delimit the genes lying in the H1 haplotype, we analyzed 34 single nucleotide polymorphisms (SNPs) spanning over 3.15 megabases in the region containing Tau. These SNPs are located in or flank the corticotropin-releasing hormone receptor 1, presenilin homolog 2, Tau, Saitohin, and KIAA1267 genes. Analysis of linkage disequilibrium (LD) using these 34 SNPs suggests that the H1 haplotype extends over about 1.3 megabases, making it the largest region of LD reported to date. Of the 29 SNPs lying in this region of LD, 5 were identified as "haplotype tagging" SNPs (htSNPs), capturing 96% of the sample's haplotype diversity. Association analysis with these htSNPs revealed a new H1 sub-haplotype that is significantly associated with PD ( P<0.02). These results define the genes and regulatory regions included in this region of LD, containing an important susceptibility allele contributing to increased risk of neurodegeneration.

Parkin binds to alpha/beta tubulin and increases their ubiquitination and degradation

In addition to inhibiting the mitochondrial respiratory chain, toxins known to cause Parkinson's disease (PD), such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and rotenone, also strongly depolymerize microtubules and increase tubulin degradation. Microtubules are polymers of tubulin alpha/beta heterodimers, whose correct folding requires coordinated actions of cellular chaperonins and cofactors. Misfolded tubulin monomers are highly toxic and quickly degraded through a hitherto unknown mechanism. Here we report that parkin, a protein-ubiquitin E3 ligase linked to PD, was tightly bound to microtubules in taxol-mediated microtubule coassembly assays. In lysates from the rat brain or transfected human embryonic kidney (HEK) 293 cells, alpha-tubulin and beta-tubulin were strongly coimmunoprecipitated with parkin at 4 degrees C in the presence of colchicine, a condition in which tubulin exits as alpha/beta heterodimers. At the subcellular level, parkin exhibited punctate immunostaining along microtubules in rat brain sections, cultured primary neurons, glial cells, and cell lines. This pattern of subcellular localization was abolished in cells treated with the microtubule-depolymerizing drug colchicine. The binding between parkin and tubulin apparently led to increased ubiquitination and accelerated degradation of alpha- and beta-tubulins in HEK293 cells. Similarly ubiquitinated tubulins were also observed in rat brain lysates. Furthermore, parkin mutants found in PD patients did not ubiquitinate or degrade either tubulin. Taken together, our results show that parkin is a novel tubulin-binding protein, as well as a microtubule-associated protein. Its ability to enhance the ubiquitination and degradation of misfolded tubulins may play a significant role in protecting neurons from toxins that cause PD.