Pathogenesis And Treatment Of Neurodegenerative Disease

Neurological Disorders And Stroke

Investigators

Linked publications & trials

Abstract

ETB studies of the pathogenesis of dopamine neuron death in Parkinson's disease examined the close correspondence between the distribution of alpha-synuclein and that of muscarinic M1 and M3 receptors. To explore a possible role for this protein in cholinergic transmission, we completed an evaluation of the effect of muscarinic receptor stimulation on alpha-synuclein in SH-SY5Y cells, a human dopaminergic cell line that expresses alpha-synuclein. Under basal conditions, alpha-synuclein was detected in all subcellular compartments isolated: plasma membrane, cytoplasm, nucleus, and two vesicle fractions. The lipid fractions contained only a 45-kDa alpha-synuclein oligomer, whereas the cytoplasmic and nuclear fractions contained both the oligomer and the monomer, suggesting alpha-synuclein exists physiologically as a lipid-bound oligomer and a soluble monomer. Muscarinic stimulation by carbachol reduced the alpha-synuclein oligomer in plasma membrane, with a concomitant increase of both the oligomer and the monomer in the cytoplasmic fraction. The oligomer was associated with a light vesicle fraction in cytoplasm that contains uncoated endocytotic vesicles. Translocation of the alpha-synuclein oligomer in response to carbachol stimulation corresponds closely with the time course of ligand-stimulated muscarinic receptor endocytosis. The data suggest that the muscarine receptor stimulated release of the alpha-synuclein oligomer from plasma membrane and its subsequent association with the endocytotic vesicle fraction may contribute to muscarine receptor endocytosis. Conceivably, its function may be a transient release of membrane-bound phospholipase D2 from alpha-synuclein inhibition, thus allowing this lipase to participate in muscarinic receptor endocytosis. Prostaglandin A(1) (PGA1) reportedly inhibits NF-kappaB activation and induces expression of heat shock proteins. Since both these effects could be neuroprotective, the therapeutic potential of PGA1 in neurodegenerative disorders, such as Parkinson's or Huntington's disease, where excitotoxicity may contribute to pathogenesis, was evaluated in rat striatal neurons exposed to the NMDA receptor agonist quinolinic acid (QA). Intrastriatal administration of PGA1 attenuated QA-induced internucleosomal DNA fragmentation. The inhibitory effects of a single dose of PGA1 on QA-induced DNA fragmentation were observed 12 to 48 h after treatment. PGA1 also attenuated QA-induced DNA fragmentation when administered up to 4 h after QA exposure. PGA1 decreased the loss of D1 dopamine receptors and GAD(67) mRNA in QA-injected striatum, suggesting that it reduced the neuronal loss induced by QA. PGA1 significantly inhibited QA-induced NF-kappaB activation by blocking inhibitory kappaB-alpha degradation. PGA1 treatment substantially increased striatal 70- and 72-kDa heat shock protein. These results indicate that PGA1 blunts NMDA receptor-mediated neuronal apoptosis by a mechanism possibly involving the up-regulation of neuroprotective heat shock proteins and inhibition of NF-kappaB activation. In view of its potent neuroprotective activity, PGA1 could prove useful in the treatment of certain neurodegenerative disorders related to excitotoxicity. Previous Branch findings indicated that the motor response complication syndrome that ultimately disables most patients with advanced Parkinson's disease results from the intermittent stimulation of striatal dopamine receptors by all standard therapies. As part of our continuing attempts to remedy this problem, we conducted two controlled clinical studies to evaluate a novel approach involving a lipid-soluble, transdermally administered, D2 dopamine receptor agonist, N-0923, which earlier ETB research had shown to posses potent antiparkinsonian activity when given intravenously. One study was a small, inpatient proof-of-concept evaluation consisting of a 2-week dose escalation phase followed by a 2-week dose maintenance phase at the highest dose. Each individual's levodopa dose was back-titrated as feasible. The median daily levodopa dose requirement declined by 71% in association with a significant reduction in "off" time. The second study evaluated N-0923 in 85 parkinsonian outpatients randomized to placebo or one of four doses of N-0923. At the maximum allowable dose, N-0923 co-administration again was found to permit a significant reduction in levodopa dosage. N-0923 was deemed safe and well tolerated in both studies. The results suggest that transdermal N-0923 is an effective approach to the continuous treatment of Parkinson's disease, allowing a lower levodopa dosage and thus a reduction in the harmful effects of intermittent dopaminergic stimulation without loss of antiparkinsonian efficacy. In addition to its therapeutic benefits, N-0923 will allow the first test of the ETB hypothesis that continuous dopaminergic stimulation from the onset of therapy will prevent the appearance of motor complications. Based on earlier ETB studies, sensitization of NMDA receptors on dendrites of striatal medium spiny neurons as a consequence of the intermittent stimulation of coexpressed dopamine receptors has been implicated in the pathogenesis of the motor complication syndrome. We have found that NMDA receptor antagonists can palliate parkinsonian signs as well as motor complication in animal models and patients with Parkinson's disease. During the past year controlled evaluations of the effects of the noncompetitive NMDA antagonist, amantadine, on levodopa-induced dyskinesias were completed. Parkinsonian patients with severe motor fluctuations and peak dose dyskinesias received their first morning levodopa dose, followed by a 2-hour intravenous infusion of amantadine or placebo. Intravenous amantadine reduced dyskinesias by about 50% without compromising the anti-parkinsonian benefis of levodopa. These results confirm our previous preclinical and clinical observations and strengthen the rationale for using this or other antiglutamatergic drugs in the treatment of Parkinson's disease. In a further evaluation of this possibility, an assessment of the safety and efficacy of another non-competitive glutamate receptor antagonist was completed in 279 levodopa treated patients with motor fluctuations. Remacemide, in a multicenter, controlled, dose-ranging study, appeared well tolerated and tended to prolong "on" time and improve parkinsonian scores compared with placebo. These findings help validate observations in non-human primates and develop an algorithm regarding which glutamate antagonists confer the greatest therapeutic benefit to parkinsonian patients.