Molecular chaperone function and motor neuron degeneration

In order to co-ordinate movement, signals must be sent from the brain to the muscles along specialized nerve cells called motor neurons. Some of these motor neurons are very large and send signals over several feet, this makes their biology very challenging and means that they can be susceptible to stress and disease. When these motor neurons die it leads to paralysis. In some families this paralysis can be inherited because of genetic changes. Recently, it was discovered that inherited changes in a stress response protein called HSJ1 lead to paralysis through the death of motor neurons. We have been studying this protein for many years and know that it can protect nerve cells, including motor neurons, from stress. To function normally all cells need to balance their production of new proteins with their removal and this is especially important at times of stress, when proteins can get damaged. Therefore, evolution has developed a family of proteins called molecular chaperones to help maintain this balance and HSJ1 is an important molecular chaperone in nerve cells. To investigate how HSJ1 functions in nerve cells and understand why it is so critical to motor neurons, we have now reprogrammed cells from patients with inherited paralysis and will use them to study human motor neurons. We also have developed an animal model that mimics the human disease. In this project we will use these models to discover the molecular basis for paralysis in patients. It is likely that the events that cause paralysis in this disease will also affect how motor neurons function in other diseases and therefore we think this will tell us more about other types of motor neuron disease and nerve cell death in general. We will also use this detailed molecular understanding to develop new treatments that might be used for this type of paralysis and potentially other forms of nerve cell death such as motor neuron disease, Parkinson's and Alzheimer's disease.

Many neurodegenerative diseases are characterised by intracellular proteinacious inclusions of aggregated, misfolded protein, suggesting that imbalances of protein homeostasis (proteostasis) are a factor in disease pathogenesis. Molecular chaperones are important regulators of proteostasis and the neuronal chaperone HSJ1 (DnaJB2) can protect against several types of neurodegeneration, including polyglutamine expanded huntingtin and mutant SOD1 in vivo. Furthermore, mutations that cause loss of HSJ1 function lead to distal hereditary motor neuropathy (dHMN) and Charcot-Marie-Tooth type 2 (CMT2), suggesting HSJ1 is particularly important for maintaining motor neuron proteostasis. This project aims to discover the precise molecular mechanisms that underlie the death of motor neurons caused by HSJ1 mutations. We have successfully reprogrammed HSJ1 patient cells to an induced pluripotent stem cell (iPSC) state and have developed a conditional HSJ1 knock-out (KO) mouse. The constitutive KO recapitulates the human phenotype, with death of motor neurons in the spinal cord. Here we will combine these models to understand motor neuron degeneration caused by loss of HSJ1. We will use a multidisciplinary approach to investigate specific functional deficits in iPSC motor neurons. Subsequently, we will rescue the cellular phenotype, by viral delivery of HSJ1 isoforms, to confirm the specificity of these functional defects, in parallel with investigating isoform specific mouse models, to define the HSJ1 isoform requirements for motor neuron survival. The client proteins of HSJ1 in motor neurons will be identified by a combined proteomic approach of identifying co-purifying proteins and monitoring changes in the proteome following loss of HSJ1. The interaction of putative clients with HSJ1 will be defined, and their importance for motor neuron survival tested. Finally, this mechanistic understanding will be used to test potential therapies for HSJ1 mediated dHMN.

This research is directly relevant to the general public as neurodegeneration is a disease that affects most of us in some way, either directly or through affecting a loved one. What we discover about how motor neurons die could have a broader relevance to many forms of neurodegeneration. More specifically this research will be important to those affected by the many forms of motor neuron disease and the patient orientated groups that fund raise and motivate for funding to find effective treatments, such as the Motor Neurone Disease Association.
Furthermore, there are links between athletic excellence and achievement and motor neuron dysfunction and this research could be directly relevant to influencing how we can ensure that exercise does not have negative effects on nerve cell function. We will learn more about how neurons respond to environmental stress and this will be of interest to how we consider the environment and our interaction with it.
The research will also have impact on the biotech community. There are several small recently established Biotech companies that are pursuing potential pharmacological manipulation of molecular chaperones, protein folding and the cell stress machinery (heat shock response and unfolded protein response). Large Pharma are also interested in the potential of manipulating these pathways, either as targets in oncology or for their potential in alleviating neuronal dysfunction. Importantly, GSK is a collaborator on this project providing exclusive compounds for manipulation of the unfolded protein response and other Pharma have provided drugs (e.g. Novartis and HSP990). The data generated by this research project could have important impact for these potential applications in the future.