Investigating lysosomal dysfunction as a unifying pathological mechanism in hereditary spastic paraplegia

Hereditary spastic paraplegias (HSPs) are genetic conditions that cause progressive leg paralysis. They are currently untreatable, so it is critical that we understand the problems inside cells that are causing the condition, as this may suggest treatment options. The foundation for this project is our work linking the function of many HSP genes to a single vital
process. This work identified abnormalities in the lysosome (the cellular degradation and recycling centre) in several different genetic subtypes of HSP. Importantly, abnormal lysosomes were present in the nerve cells affected by HSP, where they accumulated in abnormal axonal swellings. Axons are the main cellular structures that degenerate to cause HSP; thus these abnormal lysosomes are directly located at the site of disease, making them prime candidates to be involved in its causation. The aim of our project is to understand the mechanisms by which these lysosomal problems could cause axonal degeneration, and to determine if improving lysosomal function could provide a treatment strategy for HSP.

Axonal health is necessary for a functional nervous system and it is vital to understand how this is disrupted in axonopathies. Insights into this are provided by genetic conditions causing selective axonal degeneration, of which hereditary spastic paraplegias (HSPs) are exemplars; HSPs are characterised by distal "dying back" axonal degeneration in the corticospinal tracts.
Through work funded by our present MRC grant, we discovered characteristic lysosomal abnormalities in several genetic subtypes of HSP, including the most common form, spastin-HSP. This led us to propose that lysosomal dysfunction underlies HSP axonopathy. Building on these findings, we aim to test the hypothesis that lysosomal dysfunction is a unifying and potentially treatable pathological mechanism in HSP. We will:
i) Investigate how the lysosomal abnormality affects downstream cellular pathways that require functional lysosomes, thereby identifying mechanisms by which lysosomal dysfunction could cause axonopathy. We will prioritise effects on endosomal lipid metabolism, BMP signalling and autophagy, as preliminary evidence highlights these as strong candidate pathways to be involved in HSP.
ii) Test a causal relationship between lysosomal dysfunction and axonopathy by determining whether strategies to improve lysosomal function can ameliorate abnormal lysosomal morphological phenotypes, effects on downstream lysosomal pathways [identified at i)] and HSP-associated axonal phenotypes. These experiments may also identify potential therapeutic approaches.
iii) Determine whether lysosome dysfunction is a broadly unifying mechanism for HSP, by examining whether it is present in representative examples of additional key functional groups of HSP proteins, beyond those that we have already shown to be involved.
Through this work we will better understand how lysosomal dysfunction contributes to HSP pathogenesis, and how this might be targeted therapeutic benefit.

Who will benefit from this research and how?
This project focuses on understanding the cell biology and axonal pathology associated with hereditary spastic paraplegia. For this reason the most immediate beneficiaries (within 3 years) will be national and international researchers working in related areas, as described in the "Academic Beneficiaries" section.

In the medium-longer term (5-15 years), understanding the molecular and cellular processes that control axonal maintenance and degeneration will have wider implications for health and economic performance. The research may provide the foundation to identify new therapeutic strategies for axonal degenerative diseases. As well as the >5000 people in the UK and >500,000 people worldwide who suffer from HSP, this could be of benefit to the millions of patients who have common neurological disorders that include axonopathy as a feature; for example, patients with some subtypes of multiple sclerosis have a morphologically identical axonopathy to that seen in HSP. In addition, a morphologically highly similar lysosomal phenotype to that we have seen in HSP has recently been described in animal models of Alzheimer's dementia, and so understanding its pathological roles could have implications for treating this very common disease. Thus our work could be of commercial interest and benefit to UK-based pharmaceutical companies, with whom we would actively seek collaborations. In this we will be aided by the University of Cambridge Office for Translational Research, by Cambridge Enterprise and by the recently launched Milner Therapeutics Institute (on the Addenbrooke's Campus), whose mission is to lower the barriers in translating basic research to therapeutics.

Our work may be of relevance to the "healthy" general public. As well as the neurodegenerative diseases mentioned above, axonal degeneration is an important feature of declining neurological function in normal ageing. Our work will elucidate molecular factors that are critical in keeping axons alive and which would be strong candidates to be affected by the ageing process, and which perhaps could be targeted by therapeutics to slow the process. The fact that many HSPs show age-dependant penetrance indicates that there is an interplay between onset of the disease and age.

This proposal will also develop the scientific, management and communications skills of the post-doctoral researcher employed. She will receive training in a variety of laboratory skills that cut across several related disciplines, including cutting edge microscopy on living and fixed cells, lipidomics/proteomics, and work with neuronal models of disease. In addition, the researcher will receive further training and experience in writing scientific papers, presenting work at scientific meetings and helping to run a scientific group. Thus the project will help build UK scientific capacity in life sciences, a key part of the UK industrial strategy, and develop a highly trained workforce.