Naomi P. Visanji

The prion hypothesis in Parkinson's disease: Braak to the future

Parkinson’s disease (PD) is a progressive neurodegenerative disorder typified by the presence of intraneuronal inclusions containing aggregated alpha synuclein (αsyn). The progression of parkinsonian pathology and clinical phenotype has been broadly demonstrated to follow a specific pattern, most notably described by Braak and colleagues. In more recent times it has been hypothesized that αsyn itself may be a critical factor in mediating transmission of disease pathology from one brain area to another. Here we investigate the growing body of evidence demonstrating the ability of αsyn to spread transcellularly and induce pathological aggregation affecting neurons by permissive templating and provide a critical analysis of some irregularities in the hypothesis that the progression of PD pathology may be mediated by such a prion-like process. Finally we discuss some key questions that remain unanswered which are vital to determining the potential contribution of a prion-like process to the pathogenesis of PD.

Targeted delivery of an Mecp2 transgene to forebrain neurons improves the behavior of female Mecp2-deficient mice

Rett syndrome is an X-linked neurological condition affecting almost exclusively girls that is caused by mutations of the MECP2 gene. Recent studies have shown that transgenic delivery of MeCP2 function to Mecp2-deficient male mice can improve their Rett-like behavior. However, as the brain of a Rett girl contains a mosaic of MeCP2 expressing and non-expressing neurons, and the over-expression of MeCP2 in neurons can induce a severe progressive neurological phenotype, testing whether functional rescue can be achieved by gene re-introduction strategies in a female model of Rett syndrome is warranted. To address this, we generated transgenic mice expressing an epitope-tagged Mecp2 transgene in forebrain neurons. These mice over-express MeCP2 protein at about 1.6 times normal levels in cortex and develop impaired motor behavior by 9-12 months of age. To test whether forebrain-targeted MeCP2 restoration would improve behavior in female Mecp2(-/+) mice, we crossed these transgenics with Mecp2(-/+) mice and examined the behavioral properties of the female rescue mice for 1 year. These assessments revealed that the diminished rearing activity, impaired mobility and the diminished locomotive activity of female Mecp2(-/+) mice were restored to wild-type levels in the rescue mice. These results show that improvement of Rett-like behavior can be achieved in Mecp2(-/+) females by targeted gene re-introduction without inducing deficits relating to MeCP2 over-expression.

Colonic mucosal α-synuclein lacks specificity as a biomarker for Parkinson disease

Objective: To determine the utility of detecting α-synuclein (αSyn) in colonic mucosal biopsy tissue as a potential diagnostic biomarker for Parkinson disease (PD). Methods: We used the paraffin-embedded tissue (PET) blot, which degrades physiologic nonaggregated αSyn using proteinase K and enhances antigen retrieval allowing sensitive and selective detection of remaining protein aggregates, to detect αSyn in colonic mucosal biopsies from 15 patients with early PD (<3 years), 7 patients with later PD (>5 years), and 11 individuals without PD. αSyn and serine 129–phosphorylated αSyn (Ser129p-αSyn) were assessed by PET blot and conventional immunohistochemistry. Results: PET blot–resistant aggregated αSyn and Ser129p-αSyn was present in 12 of 15 individuals with early PD, 7 of 7 individuals with later PD, and 11 of 11 control subjects. The number of biopsies positive by PET blot relative to conventional immunohistochemistry was significantly lower in both PD groups compared with the control group for both αSyn and Ser129p-αSyn, whereas routine immunohistochemistry was positive more often in PD, but was positive in as many as 9 of 11 control individuals. Conclusion: Strong evidence of the presence of aggregated hyperphosphorylated αSyn in individuals with and without PD, using such a sensitive and specific method as the PET blot, suggests that colonic deposition of αSyn is not a useful diagnostic test for PD. The utility of detecting αSyn in the colon as a biomarker in combination with other assessments remains to be determined.

Dopamine D3 receptor stimulation underlies the development of L-DOPA-induced dyskinesia in animal models of Parkinson's disease.

Development of L-DOPA-induced dyskinesia (LID) remains a major problem in the long-term treatment of Parkinsons disease (PD). Sensitization to L-DOPA correlates with ectopic expression of D3 dopamine receptors in the striatum, implicating D3 receptors in development of LID. We demonstrate that the selective D3 antagonist S33084 abolishes development of LID over 30 days in MPTP-lesioned marmosets without effecting the anti-parkinsonian actions of L-DOPA. Furthermore, following a 14 day washout, when challenged with L-DOPA in the absence of S33084, these animals continued to exhibit reduced LID. In the 6-OHDA-lesioned rat, S33084 similarly attenuated development of behavioural sensitization to L-DOPA. Additionally, L-DOPA-induced elevations in striatal pre-proenkephalin-A (PPE-A) (but not PPE-B, phospho[Thr(34)]DARPP-32, D1, and D2 receptor mRNA or D3 receptor levels) were reduced in S33084 treated animals. Our data suggest a role for D3 receptors in the development of LID and suggest that initiating L-DOPA treatment with a D3 antagonist may reduce the development of LID in PD.

Increased 5-HT2A receptors in the temporal cortex of parkinsonian patients with visual hallucinations†

Well‐formed visual hallucinations (VH) are common in patients with Parkinsons disease (PD). The pathophysiology of VH in PD is unknown but may involve structures mediating visual processing such as the inferior temporal cortex. Serotonergic type 2A (5‐HT2A) receptors have been linked to many psychiatric disorders, including psychosis. We hypothesized that enhanced 5‐HT2A receptor levels may be involved in VH in PD. Autoradiographic binding using [3H]‐ketanserin and spiperone, to define 5‐HT2A receptors, was performed in 6 PD patients with VH, 6 PD patients without VH, and 5 healthy, age‐matched controls. The cerebral regions studied included the orbitofrontal cortex, inferolateral temporal cortex, motor cortex, striatum, and substantia nigra. There was a significant (45.6%) increase in the levels of [3H]‐ketanserin binding in the inferolateral temporal cortex of PD patients with VH when compared with PD patients without VH (54.3 ± 5.2 fmol/mg vs. 37.3 ± 4.3 fmol/mg, P = 0.039). Additionally, there was a significant increase in the levels of 5‐HT2A receptors in the motor cortex of all PD patients taken as a group when compared with controls (57.8 ± 5.7 fmol/mg vs. 41.2 ± 2.6 fmol/mg, P = 0.0297). These results suggest that enhanced 5‐HT2A‐mediated neurotransmission in the inferolateral temporal cortex, a critical structure in visual processing, might be associated with the development of VH in PD. Our results provide new insights into the pathophysiology of VH in PD and provide an anatomical basis to explain why compounds with 5‐HT2A antagonist activity are effective at alleviating this debilitating complication.

Pharmacological characterization of psychosis-like behavior in the MPTP-lesioned nonhuman primate model of Parkinson's disease

Investigation of the pathophysiology of psychosis in Parkinsons disease (PD), as well as the assessment of potential novel therapeutics, has been limited by the lack of a well‐validated animal model. MPTP‐lesioned primates exhibit abnormal behaviors that are distinct from dyskinesia and parkinsonism and may represent behavioral correlates of neural processes related to psychosis in PD. Here we assess four types of behavior—agitation, hallucinatory‐like responses to nonapparent stimuli, obsessive grooming, and stereotypies that are termed “psychosis‐like”—and define their pharmacology using a psychosis‐like behavior rating scale. By assessing the actions of drugs known to enhance or attenuate psychosis in PD patients, we find that the pharmacology of these behaviors recapitulates, in several respects, the pharmacology of psychosis in PD. Thus, levodopa and apomorphine elicited psychosis‐like behaviors. Amantadine significantly decreased levodopa‐induced dyskinesia but exacerbated psychosis‐like behaviors. Haloperidol reduced psychosis‐like behaviors but at the expense of increased parkinsonian disability while the atypical neuroleptics clozapine and quetiapine reduced psychosis‐like behaviors without significant effect on parkinsonian disability. The response of different components of the psychotomimetic behavior suggested the involvement of both dopaminergic and nondopaminergic mechanisms in their expression.

PYM50028, a novel, orally active, nonpeptide neurotrophic factor inducer, prevents and reverses neuronal damage induced by MPP+ in mesencephalic neurons and by MPTP in a mouse model of Parkinson’s disease

Many experimental data support the enhancement of neurotrophic factors as a means to modify neurodegeneration in Parkinsons disease. However, the translation of this to the clinic has proven problematic. This is likely due to the complex nature of the surgical gene delivery and cell‐based approaches adopted to deliver proteinaceous neurotrophic factors to targets within the central nervous system. We investigated the ability of a novel, orally active, nonpeptide neurotrophic factor inducer, PYM50028 (Cogane), to restore dopaminergic function after 1‐methyl‐4‐phenylpyridinium (MPP+) ‐induced damage to mesencephalic neurons in vitro and in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) ‐lesioned mice. In rat mesencephalic neurons, administration of PYM50028, either before or after MPP+, significantly prevented and reversed both MPP+‐induced neuronal atrophy and cell loss. These effects were potent and of a magnitude equivalent to those achieved by a combination of brain‐derived neurotrophic factor (BDNF) and glial‐derived neurotrophic factor (GDNF). Oral admin istration of PYM50028 (10 mg/kg/day for 60 days) to MPTP‐lesioned mice, commencing after a striatal impairment was evident, resulted in a significant elevation of striatal GDNF (297%) and BDNF (511%), and attenuated the loss of striatal dopaminergic transporter levels and dopaminergic neurons in the substantia nigra. PYM50028 did not inhibit monoamine oxidase B in vitro, nor did it alter brain levels of MPP+ in vivo. PYM50028 has neuroprotective and neurorestorative potential and is in clinical development for the treat ment of neurodegenerative disorders, including Parkinsons disease.—Visanji, N. P., Orsi, A., Johnston, T. H., Howson, P. A., Dixon, K., Callizot, N., Brotchie, J. M., Rees, D. D. PYM50028, a novel, orally active, nonpeptide neurotrophic factor inducer, prevents and reverses neuronal damage induced by MPP+ in mesencephalic neurons and by MPTP in a mouse model of Parkinsons disease. FASEB J. 22, 2488–2497 (2008)

Alimentary, my dear Watson? The challenges of enteric α-synuclein as a Parkinson's disease biomarker.

An accurate early diagnostic test for Parkinsons disease (PD) is a critical unmet need. Recently, independent groups using different histological techniques have reported that the presence of alpha‐synuclein (α‐syn) in colonic biopsy tissue is able to distinguish living patients with PD from those without the disease. In addition, a further study has suggested that the presence of α‐syn in colonic biopsy tissue may be evident in early or even prodromal PD. However, several questions remain regarding the translation of these findings into using the assessment of α‐syn deposition in the enteric nervous system as a diagnostic biomarker for prodromal PD. Here we address critical issues related to the location and quantification of enteric α‐syn, detection of α‐syn with currently available histological techniques, timing of detection of α‐syn deposition, and, most crucially, whether enteric α‐syn can distinguish those with PD from both healthy individuals and individuals with other related diseases. We conclude that, although enteric α‐syn is a very exciting prospect, further studies will be vital to determine whether enteric α‐syn deposition has the potential to be the biomarker for prodromal PD that the field so desperately seeks.

α-Synuclein membrane association is regulated by the Rab3a recycling machinery and presynaptic activity

Background: α-Synuclein is known to undergo exchange between membrane and cytosolic compartments. Results: α-Synuclein interacts with GTP-bound Rab3a on synaptic vesicles, and its dissociation is mediated by GDI/Hsp90. Conclusion: α-Synucleins membrane association and dissociation cycle is linked to synaptic activity by the Rab3a recycling machinery. Significance: Significance: Impairments to α-synuclein interactions with vesicles and with the Rab3a recycling machinery may affect neurodegeneration. α-Synuclein is an abundant presynaptic protein and a primary component of Lewy bodies in Parkinson disease. Although its pathogenic role remains unclear, in healthy nerve terminals α-synuclein undergoes a cycle of membrane binding and dissociation. An α-synuclein binding assay was used to screen for vesicle proteins involved in α-synuclein membrane interactions and showed that antibodies directed to the Ras-related GTPase Rab3a and its chaperone RabGDI abrogated α-synuclein membrane binding. Biochemical analyses, including density gradient sedimentation and co-immunoprecipitation, suggested that α-synuclein interacts with membrane-associated GTP-bound Rab3a but not to cytosolic GDP-Rab3a. Accumulation of membrane-bound α-synuclein was induced by the expression of a GTPase-deficient Rab3a mutant, by a dominant-negative GDP dissociation inhibitor mutant unable to recycle Rab3a off membranes, and by Hsp90 inhibitors, radicicol and geldanamycin, which are known to inhibit Rab3a dissociation from membranes. Thus, all treatments that inhibited Rab3a recycling also increased α-synuclein sequestration on intracellular membranes. Our results suggest that membrane-bound GTP-Rab3a stabilizes α-synuclein on synaptic vesicles and that the GDP dissociation inhibitor·Hsp90 complex that controls Rab3a membrane dissociation also regulates α-synuclein dissociation during synaptic activity.

The nociceptin/orphanin FQ (NOP) receptor antagonist J-113397 enhances the effects of levodopa in the MPTP-lesioned nonhuman primate model of Parkinson's disease.

The anti‐parkinsonian and levodopa‐sparing potential of the nociceptin/orphanin FQ receptor (NOP) antagonist J‐113397 has been demonstrated in rodent models of Parkinsons disease. Here, we describe the levodopa‐sparing potential of J‐113397 in MPTP‐lesioned marmosets. Coadministration of J‐113397 (30 mg/kg) with a sub‐therapeutic dose of levodopa (12.5 mg/kg) produced an anti‐parkinsonian action equivalent to that of a therapeutic dose of levodopa. However, these effects were accompanied by an equivalent level of dyskinesia. The actions of NOP antagonists seen in rodents translate to nonhuman primates. However, the present study raises the possibility that these levodopa‐sparing benefits may be offset by a propensity to exacerbate dyskinesia.