Sandra Dovero

Increased D1 dopamine receptor signaling in levodopa-induced dyskinesia

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa therapy for Parkinsons disease. Although changes affecting D1 and D2 dopamine receptors have been studied in association with this condition, no causal relationship has yet been established. Taking advantage of a monkey brain bank constituted to study levodopa‐induced dyskinesia, we report changes affecting D1 and D2 dopamine receptors within the striatum of normal, parkinsonian, nondyskinetic levodopa‐treated parkinsonian, and dyskinetic levodopa‐treated parkinsonian animals. Whereas D1 receptor expression itself is not related to dyskinesia, D1 sensitivity per D1 receptor measured by D1 agonist‐induced [35S]GTPγS binding is linearly related to dyskinesia. Moreover, the striata of dyskinetic animals show higher levels of cyclin‐dependent kinase 5 (Cdk5) and of the dopamine‐ and cAMP‐regulated phosphoprotein of 32kDa (DARPP‐32). Our data suggest that levodopa‐induced dyskinesia results from increased dopamine D1 receptor–mediated transmission at the level of the direct pathway. Ann Neurol 2004

Lewy body extracts from Parkinson disease brains trigger α‐synuclein pathology and neurodegeneration in mice and monkeys

Mounting evidence suggests that α‐synuclein, a major protein component of Lewy bodies (LB), may be responsible for initiating and spreading the pathological process in Parkinson disease (PD). Supporting this concept, intracerebral inoculation of synthetic recombinant α‐synuclein fibrils can trigger α‐synuclein pathology in mice. However, it remains uncertain whether the pathogenic effects of recombinant synthetic α‐synuclein may apply to PD‐linked pathological α‐synuclein and occur in species closer to humans.

Absence of MPTP-Induced Neuronal Death in Mice Lacking the Dopamine Transporter

MPTP has been shown to induce parkinsonism both in human and in nonhuman primates. The precise mechanism of dopaminergic cell death induced following MPTP treatment is still subject to intense debate. MPP+, which is the oxidation product of MPTP, is actively transported into presynaptic dopaminergic nerve terminals through the plasma membrane dopamine transporter (DAT). In this study, we used mice lacking the DAT by homologous recombination and demonstrated that the MPTP-induced dopaminergic cell loss is dependent on the presence of the DAT. For this we have used tyrosine hydroxylase immunoreactivity (TH-IR) labeling of dopamine cells of the substantia nigra compacta in wild-type, heterozygote, and homozygote mice that were given either saline or MPTP treatments (two ip injections of 30 mg/kg, 10 h apart). Our results show a significant loss of TH-IR in wild type (34.4%), less loss in heterozygotes (22.5%), and no loss in homozygote animals. Thus dopamine cell loss is related to levels of the DAT. These results shed light on the degenerative process of dopamine neurons and suggest that individual differences in developing Parkinsons disease in human may be related to differences of uptake through the DAT of a yet unidentified neurotoxin.

Maladaptive plasticity of serotonin axon terminals in levodopa-induced dyskinesia.

Striatal serotonin projections have been implicated in levodopa‐induced dyskinesia by providing an unregulated source of dopamine release. We set out to determine whether these projections are affected by levodopa treatment in a way that would favor the occurrence of dyskinesia.

Coordinated reset has sustained aftereffects in Parkinsonian monkeys

Coordinated reset neuromodulation consists of the application of consecutive brief high‐frequency pulse trains through the different contacts of the stimulation electrode. In theoretical studies, by achieving unlearning of abnormal connectivity between neurons, coordinated reset neuromodulation reduces pathological synchronization, a hallmark feature of Parkinsons disease pathophysiology. Here we show that coordinated reset neuromodulation of the subthalamic nucleus has both acute and sustained long‐lasting aftereffects on motor function in parkinsonian nonhuman primates. Long‐lasting aftereffects were not observed with classical deep brain stimulation. These observations encourage further development of coordinated reset neuromodulation for treating motor symptoms in Parkinson disease patients. ANN NEUROL 2012;72:816–820

Inhibition of Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1) signaling in the striatum reverts motor symptoms associated with l-dopa–induced dyskinesia

l-dopa–induced dyskinesia (LID) is a common debilitating complication of dopamine replacement therapy in Parkinsons disease. Recent evidence suggests that LID may be linked causally to a hyperactivation of the Ras–ERK signaling cascade in the basal ganglia. We set out to determine whether specific targeting of Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1), a brain-specific activator of the Ras–ERK pathway, may provide a therapy for LID. On the rodent abnormal involuntary movements scale, Ras-GRF1–deficient mice were significantly resistant to the development of dyskinesia during chronic l-dopa treatment. Furthermore, in a nonhuman primate model of LID, lentiviral vectors expressing dominant negative forms of Ras-GRF1 caused a dramatic reversion of dyskinesia severity leaving intact the therapeutic effect of l-dopa. These data reveal the central role of Ras-GRF1 in governing striatal adaptations to dopamine replacement therapy and validate a viable treatment for LID based on intracellular signaling modulation.

Lentiviral Overexpression of GRK6 Alleviates l-Dopa–Induced Dyskinesia in Experimental Parkinson’s Disease

G protein–coupled receptor kinase 6, which promotes desensitization of the dopamine receptor, alleviates dyskinesia without compromising the antiparkinsonian effect of l-dopa. Treatment for Tremors Without Side Effects As neurodegenerative diseases go, Parkinson’s disease is fairly treatable. Oral doses of l-dopa can still the tremors and normalize a patient’s movements—for a time. Eventually, however, most patients develop involuntary aimless gestures call dyskinesias, thought to be a result of oversensitive dopamine responses in the brain, caused by years of taking l-dopa. Now, Bezard and his colleagues have taken aim at a regulator of the dopamine receptor, G protein–coupled receptor kinase 6 (GRK6), to combat these disturbing side effects. The dopamine receptor, like others in its family, will desensitize after use. In this state, the receptor can no longer be activated and is taken up by the cell. The first step in desensitization is the phosphorylation of the receptor by GRK6. After many years of l-dopa, the amount of GRK in the brain starts to decline and the machinery that desensitizes the receptor does not work properly, leading, it is believed, to the uncontrolled movements of dyskinesia. The authors reinstated GRKs with gene therapy in mice that had an induced parkinsonian syndrome and showed that the dyskinesia-like movements of the mice were much reduced and, as expected, desensitization of the dopamine receptor was normalized. Repeating this experiment in macaque monkeys, in which a Parkinson-like disease had been artificially induced by a toxic agent, gave similar results: Increasing GRK6 expression in the brain could markedly improve the dyskinesia-like side effects of long-term l-dopa treatment, likely by correcting the desensitization of dopamine receptors. Notably, correction of GRK6 did not interfere with the therapeutic effects of l-dopa—an important attribute for the eventual application of such a therapy. These authors have identified a signaling pathway that seems to be responsible for the worst side effect of the standard treatment for Parkinson’s disease. Manipulation of one of its members, GRK6, or other components of dopamine receptor sensitization may prove to be an effective treatment for these side effects without hindering the efficacy of one of the most useful drugs in the neurologist’s armamentarium. Parkinson’s disease is caused primarily by degeneration of brain dopaminergic neurons in the substantia nigra and the consequent deficit of dopamine in the striatum. Dopamine replacement therapy with the dopamine precursor l-dopa is the mainstay of current treatment. After several years, however, the patients develop l-dopa–induced dyskinesia, or abnormal involuntary movements, thought to be due to excessive signaling via dopamine receptors. G protein–coupled receptor kinases (GRKs) control desensitization of dopamine receptors. We found that dyskinesia is attenuated by lentivirus-mediated overexpression of GRK6 in the striatum in rodent and primate models of Parkinson’s disease. Conversely, reduction of GRK6 concentration by microRNA delivered with lentiviral vector exacerbated dyskinesia in parkinsonian rats. GRK6 suppressed dyskinesia in monkeys without compromising the antiparkinsonian effects of l-dopa and even prolonged the antiparkinsonian effect of a lower dose of l-dopa. Our finding that increased availability of GRK6 ameliorates dyskinesia and increases duration of the antiparkinsonian action of l-dopa suggests a promising approach for controlling both dyskinesia and motor fluctuations in Parkinson’s disease.

Distinct Changes in cAMP and Extracellular Signal-Regulated Protein Kinase Signalling in L-DOPA-Induced Dyskinesia

Background In rodents, the development of dyskinesia produced by L-DOPA in the dopamine-depleted striatum occurs in response to increased dopamine D1 receptor-mediated activation of the cAMP - protein kinase A and of the Ras-extracellular signal-regulated kinase (ERK) signalling pathways. However, very little is known, in non-human primates, about the regulation of these signalling cascades and their association with the induction, manifestation and/or maintenance of dyskinesia. Methodology/Results We here studied, in the gold-standard non-human primate model of Parkinsons disease, the changes in PKA-dependent phosphorylation of DARPP-32 and GluR1 AMPA receptor, as well as in ERK and ribosomal protein S6 (S6) phosphorylation, associated to acute and chronic administration of L-DOPA. Increased phosphorylation of DARPP-32 and GluR1 was observed in both L-DOPA first-ever exposed and chronically-treated dyskinetic parkinsonian monkeys. In contrast, phosphorylation of ERK and S6 was enhanced preferentially after acute L-DOPA administration and decreased during the course of chronic treatment. Conclusion Dysregulation of cAMP signalling is maintained during the course of chronic L-DOPA administration, while abnormal ERK signalling peaks during the initial phase of L-DOPA treatment and decreases following prolonged exposure. While cAMP signalling enhancement is associated with dyskinesia, abnormal ERK signalling is associated with priming.

Levodopa-induced dyskinesia in MPTP-treated macaques is not dependent on the extent and pattern of nigrostrial lesioning

The extent of nigrostriatal denervation is presumed to play a role in the genesis of levodopa‐induced dyskinesia. Yet some parkinsonian patients who have been treated over a long period do not develop dyskinesia, raising the possibility that the pattern of denervation is as important as the extent of lesioning as a risk factor. Here we study the extent and pattern of nigrostriatal denervation in a homogeneous population of parkinsonian macaque monkeys chronically treated with levodopa. Based on the characteristics of the lesioning, non‐dyskinetic animals could not be differentiated from those with dyskinesia. Indeed, the number of tyrosine‐hydroxylase (TH)‐immunopositive neurons in the substantia nigra pars compacta, striatal dopamine transporter (DAT) binding and TH immunostaining, as well as the overall TH striatal content measured by Western blotting were identical. Moreover, the patterns of lesioning assessed by a detailed analysis of the TH‐ and DAT‐immunopositive striatal fibers were comparable in all functional quadrants and at all rostro‐caudal levels considered. These data indicate that neither the extent nor the pattern of nigrostriatal lesioning are sufficient to explain the occurrence of levodopa‐induced dyskinesia.

Involvement of Sensorimotor, Limbic, and Associative Basal Ganglia Domains in L-3,4-Dihydroxyphenylalanine-Induced Dyskinesia

Dyskinesia represents a debilitating complication of l-3,4-dihydroxyphenylalanine (l-dopa) therapy for Parkinsons disease. Such motor manifestations are attributed to pathological activity in the motor parts of basal ganglia. However, because consistent funneling of information takes place between the sensorimotor, limbic, and associative basal ganglia domains, we hypothesized that nonmotor domains play a role in these manifestations. Here we report the changes in 2-deoxyglucose (2-DG) accumulation in the sensorimotor, limbic, and associative domains of basal ganglia and thalamic nuclei of four groups of nonhuman primates: normal, parkinsonian, parkinsonian chronically treated with l-dopa without exhibiting dyskinesia, and parkinsonian chronically treated with l-dopa and exhibiting overt dyskinesia. Although nondyskinetic animals display a rather normalized metabolic activity, dyskinetic animals are distinguished by significant changes in 2-DG accumulation in limbic- and associative-related structures and not simply in sensorimotor-related ones, suggesting that dyskinesia is linked to a pathological processing of limbic and cognitive information. We propose that these metabolic changes reflect the underlying neural mechanisms of not simply motor dyskinesias but also affective, motivational, and cognitive disorders associated with long-term exposure to l-dopa.