An investigation into the role of mitochondrial dysfunction in South African Parkinson’s disease patients

Van Der Merwe, Celia (2012-12)

Thesis (MScMedSC)--Stellenbosch University, 2012.

Bibliography

Thesis

ENGLISH ABSTRACT: Parkinson’s disease (PD) is a neurodegenerative movement disorder characterized by the loss of dopaminergic neurons in the substantia nigra of the midbrain. Although the aetiology of PD is still not fully understood, it is thought to involve a combination of environmental (such as exposure to pesticides and neurotoxins) and genetic factors. A number of PD-causing genes have been found including SNCA, LRRK2, EIF4G1 and VPS35 (for autosomal dominant forms of PD) and parkin, PINK1, DJ-1 and ATP13A2 (for autosomal recessive forms of PD – arPD). Mutations in the parkin gene are the predominant cause of arPD. Parkin plays a role in the ubiquitin-proteasomal system which degrades damaged and unwanted proteins in the cell and it is also thought to be involved in maintaining healthy mitochondria. Numerous studies have implicated mitochondrial function in the pathogenesis of PD. Therefore the aim of the present study was to investigate the role of mitochondrial dysfunction in PD patients with parkin-null mutations. Four South African PD patients, each harbouring two parkin-null mutations, were recruited for this study. A muscle biopsy was performed for analysis of mitochondrial morphology using histology and transmission electron microscopy (TEM). Skin biopsies were taken, from which fibroblasts were cultured. These fibroblasts were used in i) mitochondrial morphological assessments using TEM, ii) mitochondrial network analysis, iii) functional studies via ROS measurement and iv) analysis of the proteome using a LTQ Orbitrap Velos mass spectrometer. In addition, RNA was isolated from peripheral blood samples for gene expression studies using the RT² Profiler PCR Array (SABiosciences, USA) and the RT² PCR Primer Assay (SABiosciences, USA). Heterozygous family members (carriers) and wild-type controls were also recruited for this study. Results from the histological and TEM analysis from the muscle biopsy observed subtle mitochondrial changes including the presence of type II fibres, atrophic fibres, the presence of lipids, and wrinkling of the sarcolemmal membrane. Enlarged mitochondria were also observed in one patient. TEM analysis on the patient’s fibroblasts observed an increase in the number of electron dense vacuoles, speculated to be autolysosomes. The mitochondrial network in two of the patients’ fibroblasts showed fragmented and dot-like networks which are indicative of damaged mitochondria. An increase in mitochondrial ROS levels was observed in three of the four patients. Expression studies found down-regulation of 14 genes from four of the five mitochondrial complexes and a total of 688 proteins were found only in the control and not in the patient fibroblasts. Some of these proteins are known to be part of the ‘mitochondrial dysfunction’ pathway. Taken together, these results indicate that the absence of parkin results in a number of mitochondrial alterations. Based on these findings, a model of PD was proposed: It is speculated that when parkin is absent, electron transport chain complex genes are down-regulated. This results in impaired oxidative phosphorylation, causing an increase in the production of mitochondrial ROS and subsequent oxidative stress. Mitochondria are then damaged; resulting in the fragmentation of the mitochondrial network. The impaired mitochondria are thus tagged for degradation, causing the recruitment of autolysosomes which engulf the mitochondria via mitophagy. Ultimately, as the compensatory mechanisms fail, this triggers the consequential cascade of cellular apoptotic events. This study has elucidated the effect of parkin on the mitochondria, and can act as a ‘stepping stone’ towards future development of therapeutic strategies and/or biochemical markers that will benefit not only patients with PD but also other neurodegenerative disorders.

AFRIKAANSE OPSOMMING: Parkinson se siekte (PS) is ‘n neurodegeneratiewe bewegings-afwyking gedefineer deur die verlies van dopaminergiese neurone in die substantia nigra van die midde brein. Alhoewel die spesifieke oorsprong van die afwyking nog nie ten volle begryp is nie, word bydraes van beide omgewings faktore (bv. blootstelling aan plaagdoders en neurotoksienes) asook genetiese faktore gespekuleer. Vanuit ‘n genetiese aspek is ‘n aantal gene al geassosieer met PS. Hierdie gene sluit in SNCA, LRRK2, EIF4G1 en VPS35 (vir outosomale dominante vorms van PS) en parkin, PINK1, DJ-1, en ATP13A2 (vir outosomale resessiewe vorms van PS - orPS). Mutasies in die parkin geen is aangedui as die hoof oorsaak van orPS. Parkin speel ‘n rol in die ubiquitine-proteasomale sisteem wat beskadige en ongewensde proteïne binne in die sel verwyder en is verdink om by te dra tot die instandhouding van gesonde mitokondria. Mitokondriese wanfunksionering is ook deur talle studies gewys as ‘n bydraende faktor in die patologie van PS. Die doel van die studie is om ondersoek in te stel tot die spesifieke rol wat mitokondriese wanfunsionering speel in PS pasiënte met parkin-nul mutasies. Vier Suid-Afrikaanse PS-pasiënte, elk met twee parkin-nul mutasies, is gebruik vir die studie. Deur middel van spierbiopsies is monsters verkry vir mitokondriese morfologiese analises met behulp van histologiese en elektron-oordrag mikroskopie tegnieke (TEM). Vel biopsies is ook geneem en fibroblaste is gekweek vir die gebruik in: i) mitokondriese morfologiese assesering; ii) mitokondriese netwerk analiese; iii) funksionele studies waar vlakke van reaktiewe suurstof spesies (ROS) gemeet is; iv) proteoom analiese met behup van ‘n LTQ Orbitrap Velos massa spektrometer. RNA is ook geisoleer vanaf perifere bloedmonsters vir die gebruik in geen-uitdrukkings studies met behulp van ‘n RT² Profiler PCR Array en ‘n RT² Primer Assay. Selle vanaf famielie lede wat heterosigotiese draers is van die mutasie, asook normale (geen parkin mutasie) selle is gebruik as kontroles in die studie. TEM resultate vanaf die spier monsters het subtiele mitokondriese veranderinge getoon. Hierdie sluit in die teenwoordigheid van tipe II vesels, atrofiese vesels, teenwoordigheid van lipiedes, assook waarnemings van rimpeling van die sarcolemmal membraan. Vergrote mitokondrias is ook in een van die pasiënte opgelet. TEM resultate vanaf die fibroblaste het toename in die aantal elektron-digte vakuole vertoon, moontlik geidentifiseer as autolisosome. Gefragmenteerde en onderbreekte mitokondria netwerke is gelet tydens netwerk analiese van die fibroblaste, ‘n indikasie van beskadigde mitokondria. ‘n Toename in mitokondriese ROS vlakke is gevind in drie van die vier pasiënte. Af-regulering van 14 gene, geassosieerd met vier uit die vyf mitokondria komplekse, is verneem tydens die geen-uitdrukkings studie. Saam met dit is ‘n totaal van 688 proteïene geidentifiseer wat slegs teenwoordig is in die kontrole monsters en nie in die pasiënt monsters nie. Hierdie proteïene is almal uitgedruk en betrokke in die mitokondriese wanfunsionerings-weë. Hierdie resultate dui dat die afwesigheid van parkin mitokondriese afwykings tot gevolg het wat kan lei tot die afsterwing van selle. Dit dra ook by tot die vorming van ‘n beter-verstaande siekte-model vir PS: Mutasies in parkin (wat lei tot die afwesigheid van parkin) kan dus moontlik lei tot die af-regulasie van gene geassosieerd met die elektron-vervoer ketting komplekse in die mitokondria. Dit lei tot gebrekkige oksidatiewe fosforilering en veroorsaak ‘n toename in die vorming van ROS, wat dan ‘n toename in oksidatiewe stres binne in die sel tot gevolg het. Uiteindelik lei dit dus tot die beskadiging van die mitokondria wat gepaard gaan met fragmentering van die mitokondriese netwerk. Beskadigde mitokondrias word geetiketeer vir afbraking. Hierdie etiketering aktiveer omringende autophagosome wat die beskadigde mitokondrias dan verwyder deur middel van ‘n verswelgende proses genaamd mitophagy. Dit veroorsaak die aktivering van ‘n aantal gekorreleerde sellulêre prosesse wat lei tot apoptose (afsterwing van die sel). Hierdie studie dra by tot die verklaring van die spesifieke effek wat parkin mutasies het op die funksionering van die mitokondria. Resultate hier lê ook die grondslag vir toekomstige studies met die doel tot die ontwikkeling van terapeutiese strategeë en biochemiese merkers wat kan bydrae tot die genesing van beide pasiënte met PS, asook pasiënte met ander neurodegeneratiewe afwykings.

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