ANAT3231 Cell Biology Neuronal Degeneration in Parkinson’s Disease

1. ANAT3231 Cell BiologyNeuronal Degeneration in Parkinson’s Disease Blay, Nick; Lee, James; Suy, Sophy; Introduction Mitochondrial Dysfunction alpha - Synuclein Neuronal Cell Death Parkinson’s Disease (PD) is a progressive neurological disorder involving the basal ganglia. A marked loss in the dopaminergic neurons (DA) of the Substantia Nigra (SN) is indicative of PD at a cellular level. Symptomatically it is characterized by important diagnostic features such as rest tremors, bradykinesia, rigidity and postural instability. In Australia, it is estimated that approximately 30,000 people have PD with the greatest prevalence in people aged 50-75. The cause of PD is not clear, but it is thought that genetic and environmental factors (such as toxins), or a combination of both, lead to most cases of the disease. Despite the cause not being known, several pathological processes that may lead to the degeneration of the neurons have been identified. However, definitive research on which process is actually involved has so far been inconclusive. Figure 1.Macroscopic Microscopic pale SN in PD patientsnormal pigmented SN Figure 2. Schematic representation of MD Figure 4. Schematic representation of Apoptosis: -Synuclein is a highly conserved protein present in all vertebrates,and is found in the brains of both affected and unaffected individuals It is implicated in PD because of its high concentration in Lewy Bodies, which are distinctive neuronal inclusions which have traditionally been the histopathological hall mark of PD Although some rare inherited forms of PD have been linked to mutations in the gene for -Synuclein,most PD sufferers do not have any mutation. The exogenous addition of -Synuclein to primary and immortalised neuronal cells reduced cell viability by 50% within 30 minutes(compared to control) -Synuclein is transported into the neuron via Ras related protein Rab5A,which is a transmembrane protein involved in the regulation of endocytosis. Cells which have had Rab5A disabled experienced a reduced cytotoxicity towards -Synuclein . • Chromatin compaction • Convolution of nuclear and cellular membranes • Fragmentation of cytoplasm and nucleus • Formation of ‘apoptotic bodies’ • Apoptotic bodies engulfed by neighbouring cells Necrosis Apoptosis Oxidative Stress Current and future treatments Three of the possible pathways contributing to oxidative cell death Figure 3. -Synuclein and Lewy Bodies Small, dense proteinaceous deposits, termed Lewy bodies are a major feature in the brains of patients with Parkinson's disease. The three sections of brain tissue shown here depict how the protein, -synuclein (stained brown), packs these deposits. Drug Therapy Drug treatments focus on restoring dopamine levels in the Substantia Nigra or assisting its passage within the brain. Levodopa and dopamine agonists are examples of a current drug therapy. Clinical trials have found some treatments are beneficial in prolonging the life expectancy of Parkinsons patients, with many not showing significant clinical features of the disease if the drug is administered prior to a particular stage in the disease.]However, drug therapy has draw backs such as toxicity to the Substantia Nigra, physical side effects and biochemical factors influencing the effectiveness of the drugs in different patients. Stem Cell Therapy An exciting example of future treatment of Parkinsons Disease is the use of embryonic stem cells as a source of dopamine neurons that can be transplanted in patients. Mesencephalic precursor cells from the developing midbrain of fetuses have been used as a source of dopaminergic neurons and have been transplanted into Parkinsons rat models. These cells have successfully developed into functional dopaminergic neurons. However, many ethical, medical and practical problems are limiting this treatments potential use. For example, one patient would require the cells from at least five fetuses for success. As well as this dopamine neurons have a poor survival rate of less than 10% and tumour development is a risk associated with stem cell therapy. Despite these drawbacks, some successful trials have highlighted an area with enormous potential in future treatment options. trigger production of nitric oxide, maybe due to oxidised Glutathione Production of hydrogen peroxide from L dopa and dopamine Gene Defect Normally pigmented SN Decreased pigments and neurons in PD Mitochondrial Dysfunction Altered activity of ubiquitin-proteasome system No recognition of proteins Levels of oxidised Glutathione Release of mitochondrial cytochrome Activation of caspase -3 Loss of membrane integrity Antioxidant system overloaded Hydroxyl radical forms Lipid hydroperoxides form Proteins altered, unrecognised by proteasomes DNA fragments, depleting NAD+ stores. Mitochondrial function and ATP formation impaired Activation of caspases -8,-9 &-3 Peroxynitrite forms Hydroxyl radicals form Single-strand DNA breakage Cleavage of NAD+ into ADP-ribose and nicotinamide NAD+ store depleted Mitochondrial function, glycolysis and ATP formation impaired Energy production The mechanisms of cell death in Parkinson's disease are largely unknown. Some of the factors that have been implicated in neuronal degeneration in PD include mitochondrial dysfunction (MD), oxidative stress, and aggregation of the protein α-synuclein. The mitochondria is the 'powerhouse' of the cell ,responsible for energy production. In MD, complex-I activity is impaired, resulting in energy failure and the death of DA neurons. Neuronal Cell Death • The modes of neuronal cell death in PD may include apoptosis and necrosis, which can be induced by factors such as mitochondrial dysfunction and oxidative stress. • Apoptosis is a form of programmed cell death or cell 'suicide'. The mitochondria plays an important part in the apoptosis-inducing pathway. Sitting along the inner and outer membranes of the mitochondria is a multiprotein pore- the mitochondrial permeability transition pore. Opening or closing of this pore by factors such as oxidative stress, results in the release of death proteins e.g. cytochrome C. These death proteins activate the caspase cascade, which send out apoptotic signals. • Upon receiving these signals, DA neurons undergo a variety of morphologic changes (refer to figure 4). The morphologies of these neurons range from normal appearance to severe distortion, with a combination of abnormal features such as: • Lewy Body formation • shape and size alterations of the cell body and nucleus (cell shrinkage) • organelle fragmentation • dispersion of Nissl bodies • cytoplasmic vacuolization • chromatin condensation • The DA neurons are transformed into clusters of membrane-bound particles, which are eventually engulfed by neighbouring macrophages. • The process of MD can be summarized as follows (refer to figure 2): • Inhibition of complex-I(attached to the inner mitochondria membrane) by MPTP • MPTP is a potent neurotoxin captured into acidic organelles, e.g.astrocytes • Astrocytes contain monoamine oxidase B(MAO-B), which converts MPTP to MPP+. • MPP+, reaches the extracellular fluid and is transported into DA nerve terminals by the DA transporter. • Accumulation of MPP+ causes the inhibition of complex-I activity, which impairs ATP formation and causes disruption of calcium homeostasis. • This results in the disturbances of the normal cell function, leading to cellular damage and cell death. • MPP+ causes neuronal death that is marked by cellular shrinkage, chromatin condensation against the nuclear envelope, and preservation of organelles, as well as DNA fragmentation. response Acknowledgements 1. Guttman, G., Kish, S.J., Furukawa, Y., ÒCurrent Concepts in the Diagnosis and Management of Parkinsons DiseaseÓ, Canadian Medical Association Journal, 2003, 168, 293. 2.Klein, J.A., Ackerman, S.L., (2003), ‘Oxidative stress, cell cycle, and neurodegeneration’, Journal of Clinical Investigation, vol.111, no.6, pp.785-793. 3.Okano, H., et al., ÒIsolation and Transplantation of Dopeminergic Neurons and Neural Stem CellsÓ., Parkinsonism and Related Disorders, 2002, 9, 23-28. 4.Przedborski, S., Vila, M., Jackson-Lewis, V., (2003), 'Neurodegeneration: What is it and where are we?', The Journal of Clinical Investigation, vol.111, no.1, pp.3-10 5.Rajput, A.H., ÒLevodopa prolongs life expectancy and is non toxic to Substantia NigraÓ, Parkinsonism and Related Disorders, 2001, 8, 95-100. 6.Savitz,S.I., Rosenbaum, D.M., (1998), 'Apoptosis in Neurological Disease', Neurosurgery Online, vol.42, no.3, pp.555-574. 7. 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