Dopamine Neurons In Parkinson's Disease Research Articles

Role of mitochondria in cell death of dopamine neurons in Parkinson disease

Role of mitochondria in cell death of dopamine neurons in Parkinson disease

Recent advances in research on Parkinson disease: synuclein and parkin.

Until recently, most research effort on Parkinson disease (PD) was focused on possible environmental causes. With the discovery of mutations in two genes, synuclein and parkin, which are responsible for rare familial forms of the disease, there has been a major change in emphasis. The first genetic cause of PD to be identified was in the gene for synuclein, resulting in an alanine to threonine substitution at position 53. The likely pathogenetic significance of this mutation was supported by the discovery of a second mutation, and the presence of synuclein in Lewy bodies in sporadic PD cases. The synuclein protein has a tendency to self aggregate, and this tendency is increased in the mutants, and by oxidative injury to the protein. While there is growing evidence in animal models that overexpression of wildtype or mutant synuclein may lead to intracytoplasmic inclusions, and dysfunction of dopamine neurons, no animal models in rodents have yet replicated all important features of the disease. Deletions or point mutations in the gene for parkin cause an autosomal recessive, early onset form of parkinsonism. The parkin protein functions as an E3 ubiquitin-protein ligase, and it is involved in the degradation of cellular proteins by the proteasomal pathway. It is hypothesized that the loss of this function results in the toxic accumulation of its target proteins. Research on these inherited forms of PD is pointing towards a common theme, that disturbances of cellular protein handling can lead to the death of dopamine neurons in PD.

New insights into Parkinson's disease.

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Recent advances in genetics and pathophysiology have led to new insights into the pathogenesis of PD. Ten loci have been linked to hereditary PD. Mutations in alpha-synuclein and ubiquitin carboxy hydrolase L1 (UchL1) cause autosomal dominant PD and mutations in parkin and DJ-1 cause autosomal recessive PD. alpha-Synuclein has emerged as an important protein in the pathogenesis of PD, as it appears to be the major structural component of Lewy bodies and its accumulation/aggregation seems to play a prominent role in sporadic PD. Mutations in parkin are the most common cause of hereditary PD, and mutations in parkin are thought to lead to a loss of parkin's ubiquitin E3 ligase activity. Derangements in parkin function as well as mutations in UCH-L1 fit with the notion that derangements in the ubiquitin proteasomal pathway (UPP) may play important roles in the demise of dopamine neurons in PD. DJ-1 is a protein of unknown function that is linked to autosomal recessive PD. Oxidative stress and impairment in mitochondrial complex I activity are important in sporadic PD, and there is emerging interest in the role of herbicides, fungicides and insecticides that inhibit mitochondrial complex I activity and their role in contributing to the development of PD. These important findings serve as the foundation for discovering new pathways that may lead to the development of new therapies for PD.

Parkinson's disease: changes in apoptosis-related factors suggesting possible gene therapy.

Specific degeneration of the nigrostriatal dopamine (DA) neurons of the substantia nigra pars compacta and the resulting loss of nerve terminals accompanied by DA deficiency in the striatum are responsible for most of the movement disturbances called parkinsonism, i.e., muscle rigidity, akinesia, and resting tremor, observed in Parkinson's disease (PD). We and other workers have found changes in the levels of cytokines, neurotrophins, and other apoptosis-related factors in the nigro-striatal region of postmortem brain and/or in the cerebrospinal fluid (CSF) from PD patients, or from animal models of PD such as 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP)-induced PD in mice or 6-hydroxydopamine (6-OHDA)-induced PD in rats. The most remarkable changes observed specifically in the nigrostriatal region were decreased levels of neurotrophins supporting DA neurons. These results indicate that the process of cell death in the nigrostriatal DA neurons in PD may be the so-called programmed cell death, i.e., apoptosis. Thus gene therapy for PD should aim both at supplementing the decreased striatal DA level by introducing the genes of DA-synthesizing enzymes into non-DA cells in the striatum and at supporting or restoring DA neurons by preventing apoptosis by introducing genes that block the process of apoptosis.

NAIP protects the nigrostriatal dopamine pathway in an intrastriatal 6-OHDA rat model of Parkinson's disease.

Parkinson's disease (PD) is a progressive neurodegenerative disorder of the basal ganglia, associated with the inappropriate death of dopaminergic neurons of the substantia nigra pars compacta (SNc). Here, we show that adenovirally mediated expression of neuronal apoptosis inhibitor protein (NAIP) ameliorates the loss of nigrostriatal function following intrastriatal 6-OHDA administration by attenuating the death of dopamine neurons and dopaminergic fibres in the striatum. In addition, we also addressed the role of the cysteine protease caspase-3 activity in this adult 6-OHDA model, because a role for caspases has been implicated in the loss of dopamine neurons in PD, and because NAIP is also a reputed inhibitor of caspase-3. Although caspase-3-like proteolysis was induced in the SNc dopamine neurons of juvenile rats lesioned with 6-OHDA and in adult rats following axotomy of the medial forebrain bundle, caspase-3 is not induced in the dopamine neurons of adult 6-OHDA-lesioned animals. Taken together, these results suggest that therapeutic strategies based on NAIP may have potential value for the treatment of PD.

Tumor Necrosis Factor α Is Toxic to Embryonic Mesencephalic Dopamine Neurons

Levels of the proinflammatory cytokine tumor necrosis factor α (TNFα) are increased in postmortem brain and cerebral spinal fluid from patients with Parkinson's disease (PD). This observation provides a basis for associating TNFα with neurodegeneration, but a specific toxicity in dopamine (DA) neurons has not been firmly established. Therefore, we investigated TNFα-induced toxicity in DA neurons by utilizing primary cultures of embryonic rat mesencephalon. Exposure to TNFα resulted in a dose-dependent decrease in DA neurons as evidenced by decreased numbers of tyrosine hydroxylase-immunoreactive (THir) cells. TNFα toxicity was selective for DA neurons in that neither glial cell counts nor the total number of neurons was decreased and no general cytotoxicity was evidenced by lactate dehydrogenase assay. Many of the cells which remained immunoreactive for TH had shrunken and rounded cell bodies with broken, blunted, or absent processes. However, TNFα-treated cultures also contained some THir cells which appeared to be undamaged and possibly resistant to TNFα-induced toxicity. Additionally, immunocytochemistry revealed basal expression of TNFα receptor 1 (p55, R1) and TNFα receptor 2 (p75, R2) on all cells within the mesencephalic cultures to some degree, even though only DA neurons were affected by TNFα treatment. These data strongly suggest that TNFα mediates cell death in a sensitive population of DA neurons and support the potential involvement of proinflammatory cytokines in the degeneration of DA neurons in PD.

Disrupted mitochondrial electron transport function increases expression of anti-apoptotic bcl-2 and bcl-X(L) proteins in SH-SY5Y neuroblastoma and in Parkinson disease cybrid cells through oxidative stress.

Death of dopamine neurons in Parkinson disease (PD) may arise from consequences of the complex I (C-I) defect in the mitochondrial electron transport chain (ETC). Whether cells activate programmed death (apoptosis) pathways derives, in part, from relative activities of proteins such as bcl-2 and bcl-X(L), that have anti-apoptotic actions. We studied the responses of bcl-2 and bcl-X(L) genes in pharmacologic (acute incubation with methylpyridinium (MPP+)) and mitochondrial transgenic ("cybrid") models of Parkinson disease C-I defects. MPP+ incubation increased levels of bcl-2 and bcl-X(L) proteins in native SH-SY5Y cells but not in rho(0) cells devoid of ETC activity. MPP+ increased bcl-2 mRNA levels by 40% at 8 hr. Confocal microscopic imaging showed that the intracellular distribution of immunoreactive bcl-2 was not significantly associated with mitochondrial membranes at baseline but was associated with mitochondria after 12 hr of MPP+. Immunoreactive bcl-X(L) protein was significantly and equally associated with mitochondrial membranes both at baseline and after MPP+. PD cybrids showed increased basal levels of bcl-2 and bcl-X(L) proteins, similar to the maximum levels found after MPP+ treatment of control SY5Y cells. After MPP+ exposure, bcl-2 protein levels increased in control cybrids but did not increase further in PD cybrids. Both pharmacologically generated and transgenically induced C-I inhibition increases levels of anti-apoptotic bcl proteins, possibly from increased gene transcription. Augmentation of bcl-2 and bcl-X(L) expression may delay neurodegeneration in PD.

Melatonin rescues dopamine neurons from cell death in tissue culture models of oxidative stress

Dopamine (DA) neurons are uniquely vulnerable to damage and disease. Their loss in humans is associated with diseases of the aged, most notably, Parkinson's Disease (PD). There is now a great deal of evidence to suggest that the destruction of DA neurons in PD involves the accumulation of harmful oxygen free radicals. Since the antioxidant hormone, melatonin, is one of the most potent endogenous scavengers of these toxic radicals, we tested its ability to rescue DA neurons from damage/death in several laboratory models associated with oxidative stress. In the first model, cells were grown in low density on serum-free media. Under these conditions, nearly all cells died, presumably due to the lack of essential growth factors. Treatment with 250 μM melatonin rescued nearly all dying cells (100% tau + neurons), including tyrosine hydroxylase immunopositive DA neurons, for at least 7 days following growth factor deprivation. This effect was dose and time dependent and was mimicked by other antioxidants such as 2-iodomelatonin and vitamin E. Similarly, in the second model of oxidative stress, 250 μM melatonn produced a near total recovery from the usual 50% loss of DA neurons caused by neurotoxic injury from 2.5 μM 1-methyl-4-phenylpyridine (MPP +). These results indicate that melatonin possesses the remarkable ability to rescue DA neurons from cell death in several experimental paradigms associated with oxidative stress.

Lazaroids improve the survival of grafted rat embryonic dopamine neurons.

In rodent models of Parkinson disease in which transplants of dissociated rodent and human embryonic mesencephalic tissue, rich in dopamine neurons, have been studied, only 5-20% of the dopamine neurons survive the implantation procedure. We have investigated the effects of inhibiting free radical generation with two lazaroids, U-74389G and U-83836E, on the survival of embryonic rat dopamine neurons. U-74389G is a 21-aminosteroid, and U-83836E combines the piperazinyl pyrimidine portion of 21-aminosteroids with the antioxidant ring of alpha-tocopherol. In an initial study, we found that the lazaroids markedly prolonged the period after tissue dissociation that an embryonic mesencephalic cell suspension exhibits high cell viability in vitro, as assessed by using a dye exclusion method. In a second series of experiments, addition of lazaroids to dissociated mesencephalic graft tissue increased the yield of surviving rat dopamine neurons 2.6-fold after implantation in the dopamine-denervated rat striatum. The improved survival correlated with an earlier onset of graft-induced functional effects in the amphetamine-induced rotation test. Thus, inhibition of free radical generation can significantly increase the yield of grafted embryonic dopamine neurons. Addition of lazaroids to the graft preparation is a relatively simple modification of the transplantation protocol and could readily be applied in a clinical setting. Moreover, since iron-dependent lipid peroxidation has been suggested to play a role in the death of nigral dopamine neurons in Parkinson disease and lazaroids are particularly potent inhibitors of such processes, the findings may have implications for the pathogenesis of this disease.