Targeting axonal transport regulation by neurotrophic factors to treat peripheral nerve degeneration

Charcot-Marie-Tooth disease (CMT) is an inherited nervous system disease that affects 1 in about 2,500 people, which makes it one of the most common diseases to affect both nerves and muscles. The symptoms of CMT usually begin during teenage years, but the disease does not affect survival, therefore it causes life-long disability, for which the estimated societal cost is approximately £0.5 billion per year in the UK alone. People with CMT have mobility issues and difficulties with sensation, because two different types of nerve cell, known as motor neurons and sensory neurons, malfunction and progressively deteriorate. It is unknown why these peripheral nerves are affected, which has made it difficult to develop treatments for CMT. Mutations in more than 100 different genes cause CMT, and the collection of genes linked to the most CMT subtypes all produce enzymes known as aminoacyl tRNA-synthetases (ARSs). The ARS enzymes are essential to all cells for making protein. To better understand how CMT is caused, we study two ARS genes - GARS1, which produces GlyRS protein and is linked to CMT type 2D (CMT2D) and YARS1, which produces TyrRS protein and is linked to a subtype known as DI-CMTC. Using cell and mouse models, we study CMT2D and DI-CMTC to understand why motor and sensory neurons deteriorate, because this will enable us to design better treatments to combat the disease. CMT-causing mutations in GARS1 and YARS1 affect the structure of the proteins they produce. My laboratory discovered that this causes mutant GlyRS and mutant TyrRS to mis-interact with a series of important proteins called Trk receptors, which are found on the surface of nerve cells and normally bind to survival molecules called neurotrophins. Recently, we have shown that the aberrant association between mutant ARS proteins and Trk receptors impairs a critical and continually active process in nerve cells known as axonal transport. Nerve cells have a long, thin pipe-like structure called an axon, which helps to deliver electrical signals and cellular components over large distances. For nerves to function correctly and survive, they require many substances to be delivered up and down axons by the process of axonal transport. In particular, motor and sensory neurons rely on this process to deliver neurotrophins from muscles towards the brain. We therefore boosted the levels of one of these key neurotrophins, BDNF, in muscles of CMT mice and showed that it can treat the disruption in axonal transport and improve other features of the disease. We now propose to examine in detail the role that neurotrophins and other similar neurotrophic factors (NTFs) play in regulating the process of axonal transport in motor and sensory nerves. Doing so, will not only allow us to better understand why these peripheral nerves deteriorate in CMT, but it will enable us to achieve our main goal, which is to develop a multi-pronged approach to treat the symptoms of CMT and improve patient quality of life. We will accomplish this through four main objectives: 1) We will assess the structures of ARS and Trk receptor proteins, identifying exactly how they mis-interact, so that we can design treatments blocking specifically these aberrant associations. 2) To identify key proteins other than NTFs that can be targeted to treat defects in axonal transport, we will generate and study new models of CMT2D and DI-CMTC using human motor neurons. 3) By testing a collection of NTFs, we will identify whether molecules other than BDNF can be supplied to muscle to alleviate the axonal transport impairment in both human neurons and mice. 4) We will adapt our treatment strategy to enable the continual and long-term delivery of BDNF, and other NTFs identified in Aim 3, to all muscles using harmless viruses and then test the effectiveness of these therapies in mouse models of several different subtypes of CMT.