One of the main pathological hallmarks of Alzheimer’s disease (AD) and other neurodegenerative diseases called tauopathies is the presence of large protein tangles within brain cells called neurons. The primary component of these tangles is a protein known as tau which has undergone a chemical change known as phosphorylation. Phosphorylation of tau makes it more susceptible to aggregation and it is unable to carry out its normal function in the neuron. The accumulation of aggregated tau in neurons then causes the cell to degenerate and die by a process that is still relatively unclear. Mouse models recapitulating the aggregation and toxicity of tau have been generated and are valuable laboratory tools. Mechanisms to prevent tau phosphorylation and aggregation and/or promote the clearance of phosphorylated tau are strategies currently being pursued for the prevention and treatment of AD and other tauopathies.
One such strategy to prevent the aggregation of phosphorylated tau is the use of antibodies, common therapeutic agents that can specifically bind to proteins involved in disease and help neutralise them. The advantage of antibodies as therapeutics is that they have very high specificity and binding affinity for their target. Initial evidence shows that tau pathology can be reduced and disease progression slowed by immunisation with a phosphorylated tau peptide in transgenic mice. Likewise, immunisation with a phosphorylated tau-specific antibody has also shown promise. However, as phosphorylated tau is localised largely within the neuron, and antibodies are unable to enter the cell, it is likely that these antibodies are only targeting a small population of pathogenic tau that has been released from the cell. These studies could therefore be improved by targeting the major population of pathogenic tau within the neuron where is primarily localised.
This project aims to overcome the problems associated with the previously studied antibodies by developing intracellular antibodies, known as intrabodies. Intrabodies are small fragments of antibodies that can be delivered as a gene to a cell. Once delivered, the cell can then continually turn the gene into a functioning intrabody within the cell. Furthermore, intrabodies can be engineered to localise within a specific intracellular compartment and contain signals which can direct the inrabody’s target to the cell’s protein degradation machinery for clearance. Intrabodies have previously been shown to be successful therapeutic strategies for the treatment of Huntington’s disease and Parkinson’s disease, both of which are neurodegenerative diseases caused by misfolded proteins within neurons. Novel antibody fragments have been successfully selected against phosphorylated tau in our laboratory from a large, human, antibody fragment library and their genes have been sequenced. The gene sequences are currently being modified to include signal peptides that will direct the intrabody and the bound phosphorylated tau to the cell’s protein degradation machinery. This project aims to test the intrabodies in a transgenic mouse model of tau aggregation and dementia. It is hypothesised that the intrabodies will bind to the phosphorylated, diseaseassociated form of tau within the neuron, leaving the normal tau to remain functional. This will prevent phosphorylated tau aggregation and enhance its clearance, thereby slowing the disease progression. It is anticipated that a successful tau-specific intrabody generated from this project could be further developed for potential use as a therapeutic for the treatment of Alzheimer’s disease and other tau-related dementias.