TARGETING PARKIN & MITOCHONDRIAL DYNAMICS IN HUNTINGTON'S DISEASE

Huntington's disease is an incurable, fatal neurodegenerative disorder that is characterised by the loss of vulnerable neurons in the brains of patients. The disease is caused by an increase in the size of a particular DNA sequence (a trinucleotide CAG repeat) which encodes for the amino acid glutamine in the huntingtin (HTT) gene. If the number of glutamines in the HTT protein increases beyond a critical length it misfolds and forms protein tangles, which can disrupt vital cellular processes. An increased number of CAG repeat units in the HTT gene is associated with an earlier age of Huntington's onset. However, there is substantial variability in the onset of symptoms in individuals with the same number of repeats, and studies have shown that ~40 % of this variation is due to other genes. This suggests that there are many potential therapeutic targets capable of significantly altering age of disease onset in the human genome.

Previous work by our laboratory and others has found that mitochondria - the energy powerhouses of cells - do not function properly in several ways in Huntington's disease. Interestingly, we have recently found that the protein parkin, which is critical for clearing unhealthy mitochondria for the cell, can improve disease symptoms in a fruit fly model of Huntington's. Notably, mutations in the gene encoding parkin cause hereditary forms of Parkinson's disease. Here we propose to dissect the mechanisms underlying parkin protection. As parkin is involved in controlling the shape of mitochondria, we will interrogate mitochondrial morphology using various microscopy-based techniques, in order to see if in the context of Huntington's model flies parkin influences this process. In parallel, we will also study mitochondrial function in these flies using several biochemical approaches. We will also extend this work to systematically test several proteins related to parkin function and mitochondrial dynamics, in order to better understand how parkin is influencing disease "symptoms" in Huntington's flies. In total, this work will inform the mechanisms underlying Huntington's disease, will may ultimately lead to the development of novel therapeutic strategies for this disorder. As mitochondrial dysfunction is linked to several neurodegenerative diseases, any compelling insights may ultimately have broader significance.

Huntington's disease (HD) is an incurable, fatal neurodegenerative disorder caused by the expansion of a polyglutamine tract in the huntingtin (HTT) protein. This mutation causes HTT to misfold and aggregate, leading to widespread dysfunction and death of vulnerable neurons. As with several other neurodegenerative disorders, the pathogenesis of HD has been closely linked with mitochondrial dysfunction, which includes alterations in mitochondrial dynamics (e.g. increased mitochondrial fragmentation). In preliminary studies we have recently found that the Parkinson's-linked protein parkin alleviates several disease phenotypes in HD flies when overexpressed. Parkin plays a role in clearance of damaged mitochondria (mitophagy), via modulation of mitochondrial dynamics. Here we propose to further investigate the mechanisms underlying parkin protection in HD fruit flies and a PC12 cell model, and also explore the role of mitochondrial dynamics and mitophagy in pathogenesis of this disorder. We will use genetic approaches to modulate parkin expression in HD models - as well as related proteins (e.g. PINK1, Mfn, Opa1, Drp1) - and ascertain the influence upon mitochondrial phenotypes (e.g. mitochondrial morphology, mitochondrial respiration). Relatedly, as parkin has been found to participate in protein degradation and autophagy of aggregated molecules and reduce proteotoxicity, we will also explore the relationship between parkin and HTT. In total, this work will explore the protective role of parkin in HD, and interrogate mitophagy and mitochondrial dynamics in this disorder. These studies may ultimately contribute to therapeutic strategies for HD.

This proposal has direct implications for individuals suffering from Huntington's disease (HD), and their families. Though several drugs are available for symptomatic management of HD, no treatments are available that halt or delay progression or onset of HD and very few new disease-modifying therapies are in the clinical pipeline. It is estimated that HD costs up to $1 billion per year in medical and nursing costs in the United States alone. Furthermore the common neurodegenerative disorders affect more than 700,000 people in the UK, and cases are predicted to double in the next 25 years, costing the UK economy a staggering £50 billion. In addition to the obvious patient benefits, the identification of an effective treatment for HD and other neurodegenerative disorders could dramatically cut these expenditures world-wide.

We have considerable expertise in the use of fly and mammalian cell models for HD in order to understand the mechanisms underlying this disease, as well as identifying novel candidate therapeutic target. Our previous identification of kynurenine 3-monoxygenase (KMO) as a candidate therapeutic target for Huntington's disease speaks strongly for our approach. Several inhibitors of KMO have been, and are currently being, developed with the aim of entering clinical trials in the near future. Importantly, KMO inhibitors will likely also have relevance for AD and PD. In addition, we have recently found that a compound which mimics the antioxidant activity of glutathione peroxidases is robustly protective in fly and mammalian cell models of HD. Notably, this compound - ebselen - is well-tolerated in humans, giving further impetus for future preclinical analyses. Thus, the proposed research will likely provide additional insights into pathogenesis and treatment for HD, and has a high chance of delivering upon this promise. As mutations in parkin are associated with some familial forms of Parkinson's disease, and alterations in mitochondrial dynamics and mitochondrial impairment are observed in several neurodegenerative disorders, the proposed work may also inform these diseases.