Rescuing neurotrophic signalling through Rab10: a novel therapeutic target for Alzheimer's disease.

Dementia is a mental health condition that affects memory, reasoning, perception and communication. More than 850,000 people live with dementia in the UK and 60% of the cases are caused by Alzheimer's disease (AD). Dementia costs society £26 billion per year in the UK and constitutes one of the most important health problems for the ageing population. While significant efforts have been devoted to understand multiple genetic causes and pathological mechanisms of AD, no treatments have been able to date to stop or reverse its progression. Importantly, the shape and connectivity of neurons are progressively affected by AD, and together with the accumulation of Abeta peptide and abnormal protein Tau found in post-mortem brains, they are main clues to try to understand what causes AD and how the brain is damaged in AD and other neurodegenerative conditions.

Since survival, shape and connectivity of many types of brain cells largely depend on a family of molecules called neurotrophins, these factors have been proposed to play a key role in neurodegenerative diseases and are primary therapeutic targets in AD. One of the most important members of the neurotrophins family is the brain-derived neurotrophic factor (BDNF), which binds to a receptor called TrkB on the surface of neurons. Upon BDNF binding, this receptor is internalised to generate intracellular signals with extraordinary spatial and temporal precision. BDNF/TrkB signalling is decreased in AD patients and animal models of AD. Furthermore, several therapies involving reprogramming of brain cells to produce BDNF or drugs that mimic its mode of action have shown promising results and are being tested in clinical trials.

The big challenge still to be addressed is bringing the signal generated by neurotrophins to the specific sites where it is required. When BDNF is secreted by a neuron, axon terminals of other connected neurons are exposed to the neurotrophin, causing the activation, internalisation and transport of TrkB receptors along the axon towards the cell body. The arrival of the activated receptors in the cell body triggers signalling cascades that regulate shape, connectivity and gene expression. Recent results from our laboratory indicate that the small GTPase Rab10 is crucial for the sorting of internalised TrkB receptors to long-range axonal transport. Interestingly, Rab10 has been found concentrated at sites of accumulation of the protein Tau and around senile plaques in post-mortem brains of AD patients. Furthermore, a recent whole genome study revealed that in families carrying the ApoE epsilon4 allele, which increase the risk of AD, a rare variant of the Rab10 gene confers protection against dementia.

This project has been designed to determine whether the upregulation of Rab10 activity in neurons can overcome the trafficking impairments that characterise AD and restore normal neurotrophic signalling. Here, we propose to manipulate the activity of Rab10 with high spatial and temporal precision by using light-inducible probes in AD neurons both in vitro and in vivo to test whether Rab10 modulation can protect neurons from the disease. Since Rab10 is also a target of anti-diabetic drugs that are already available on the market, we aim to carry out a pilot study to determine whether increasing pharmacologically Rab10 function in AD neurons and mice improves disease outcome, therefore accelerating the translation of our pre-clinical research into new therapies tackling dementia.

AD is the most prevalent neurodegenerative disease and the first cause of dementia. While significant efforts have been devoted to understand its multiple genetic causes and pathological mechanisms, no current treatment have been able to stop or reverse disease progression. Several aspects of the pathophysiology of AD are regulated by neurotrophins, including production of Abeta, phosphorylation of Tau, synaptic plasticity, dendritic branching and intracellular transport; suggesting that neurotrophins play a central role in the pathological cascade. Since neurotrophic factors are secreted by diverse neuronal targets, retrograde axonal transport of signalling complexes is crucial for neurons, as exemplified by the involvement of axonal transport in several neurodegenerative diseases.

Our preliminary results indicate that the GTPase Rab10 plays a key role in retrograde transport of TrkB receptors in neurons. Interestingly, recent evidence shows that phosphorylated Rab10 co-localise with Tau aggregates and around the site of senile plaques in humans. Moreover, a rare single nucleotide polymorphism of Rab10 confers protection to ApoE epsilon4 allele carriers. In this proposal we will test the hypothesis that modulation of Rab10 can overcome trafficking abnormalities in AD, restoring neurotrophic signalling and function.

We propose an integrated approach to manipulate Rab10 both in vitro and in vivo, gaining a better insight into AD pathomechanisms and the therapeutic potential of modulating Rab10 function. To make our pre-clinical research program more translatable, we will explore the potential of the anti-diabetic drug metformin, a well-established regulator of Rab10-positive organelles, to regulate neurotrophic signalling in neurons in vitro and in vivo. This project is expected to further our understanding on the mechanisms of neurotrophic dysfunction in AD and test a novel therapeutic target for AD amenable to drug discovery and repositioning.

This project has been designed to provide evidence on the role of a novel cellular mechanism of regulation of neurotrophic signalling in the pathophysiology of Alzheimer's disease (AD), one of the most relevant and widespread neurological disorders. That crossover of basic cell biology and pre-clinical research attracts the interest of a diverse academic community, including cell biologists, medical researchers and mental health experts from different backgrounds (i.e. public health, social sciences, policy making). An early career scientist and additional students are going to be trained in new technologies and experimental approaches in the next three years of this project.

The essential role of Rab10 in the retrograde axonal transport of neurotrophin receptors has yet not been explored, hence this project is novel and represents a great opportunity to gain a better understanding on the regulatory steps controlling the sorting of receptors at axon terminals, the nature of signalling endosomes and the molecular signatures that determine their final destination in the cell body. These are long-standing questions in the field of cellular neurobiology that will be addressed in this investigation.

Neurotrophic signalling and axonal transport are affected in many neurological diseases, including AD, Parkinson's and Huntington's diseases, Amyotrophic Lateral Sclerosis, Frontotemporal Dementia, Charcot-Marie-Tooth and Niemann-Pick's disease. These pathologies affect a total of around 900,000 individuals in the UK alone. Our research will help to identify common mechanisms and novel therapeutic targets, which are acutely needed in the field to develop effective treatments for these disorders. Therefore, our results will have an impact beyond academic researchers, reaching the pharmaceutical industry. We will optimise optogenetic methods that have the potential to be used in human experimental medicine, an area bound to experience a massive growth in the next few years. The fact that this proposal is based on human genetic and histological data on the involvement of Rab10 in AD, together with our plans of testing the ability of the widely available anti-diabetic drug metformin to regulate Rab10 in primary neurons and animal models of dementia, show our commitment to rapid translatability for future clinical studies.

The impact of this project is not limited to neurology, since Rab10 activity has been shown to be implicated in diverse human diseases. This small GTPase is a well-documented regulator of insulin sensitivity, therefore our research is expected to have an impact in type-2 diabetes, a condition that affects more than 4 million people in the UK. Rab10 and neurotrophic factors have been also linked to different types of cancer, another key area of clinical interest.

Prime Minister's Challenge on Dementia 2020 has prioritised advance on risk reduction as well as health and care of patients. Our project is aligned with this challenge and also with the strategic aims of the Medical Research Council, particularly regarding the objectives of advancing our understanding on the biology of aging and neurodegeneration, exploring risk factors in mental health, and promoting translation of knowledge from regenerative medicine into new therapies for chronic diseases.