Synaptic and circuit pathology in a mouse model of AP4 deficiency syndrome

Nerve cells (neurons) are critical for brain function and when they go wrong this leads to neurological diseases. Key to their function is the nerve cells specialised shape which includes a large dendritic tree which receives connections from other neurons and a long thin axonal process which can connect to downstream neurons. Nerve cells can communicate with each other via specialised cell-cell contact sites called synapses where they release and detect neurotransmitters. The correct formation of axons and dendrites is a critical step that allows synapses to wire up and operate correctly. Synapses can change their strength in response to changes in neural activity, an important property for key brain functions such as cognition and learning and memory. Moreover if synapses do not wire correctly during development (for example because axons and dendrites don't properly form) or if the synapses go wrong later in life this can lead to devastating diseases including intellectual disability and epilepsy in addition to neuropsychiatric disorders like Schizophrenia and autism and neurodegenerative disorders like Alzheimer's diseases.

Cells contain a series of lipid membrane bound 'organelle' compartments that play a key role in compartmentalising important processes at discrete locations within cell. Proteins, membrane and other cell constituents often need to be transported between different organelle compartments which is often carried out in transport carriers called vesicles by a process of membrane trafficking or 'cargo delivery'. By controlling the levels of signalling receptors on the neuronal surface and hence neuronal signalling vesicular trafficking is important for key steps during the developmental wiring up of the brain and then the operation of the synaptic connections. One key family of receptors that are transported to where they are needed in vesicles are the receptors for neurotransmitters (the chemical signalling molecules in the brain) and have a specialised role to bind neurotransmitters like GABA and glutamate that are released at synapses.

A key goal of this proposal is to investigate how the regulation of the delivery of cargo between different organelles is performed, characterising the molecular properties of the protein AP4 which we think is crucial for regulating the homeostasis of organelle networks and for the regulation of receptor signalling. When the AP4 complex goes wrong in humans this leads to a devastiating neurological disorder, causing intellectual disability, brain structural alterations, epilepsy, in addition to spastic paraplegia and gait problems. However we still don't know how AP4 complex dysfunction leads to this devastating disease. In particular we want to know how AP4 complexes help brain cells to regulate membrane delivery and the number, activity and location of the key membrane proteins (such as neurotransmitter receptors) in the plasma membrane of brain cells to control neuronal signalling. Studying the molecular mechanisms that underlie these regulatory processes will allow us to better understand how the brain works under healthy conditions. In addition because disrupted cargo delivery is implicated in many neurodegenerative and neuropsychiatric diseases, our proposed work may also lead to an improved understanding of diseases where membrane trafficking and synaptic function are altered such as in epilepsy, stroke, Alzheimer's disease, motor neuron disease, Huntington's disease, schizophrenia and autism.

Vesicular trafficking is key for correct brain wiring and function, and implicated in many physiologically important processes in neurons including neuronal migration, axon pathfinding, growth cone dynamics, the establishment and maintenance of axons and dendritic arborisations and the function and plasticity of synapses. The AP4 complex is highly expressed in the nervous system where it may play an important role in the vesicular trafficking of key cargo. Mutations in the subunits of the AP4 membrane trafficking complex in humans lead to loss of AP4 subunit protein and an associated spectrum of severe neurological problems including intellectual disability, epilepsy and spastic paraplegia but very little is know regarding the function of the AP4 complex in the brain. We will use a combination of molecular, cell biological, mouse transgenic, imaging and electrophysiological techniques to determine how AP4 complexes contribute to neuronal membrane trafficking and brain function. A key goal will be to better understand how AP4 contributes to the correct development and maintenance of neuronal and synaptic architecture and the strength of synapses. This will help to better understand how AP4 loss leads to disease and will also provide new insights regarding the circuit and synaptic pathology that underpins intellectual disability and epilepsy and may be more widely relevant to other neurological disorders including Alzheimer's disease.

Who will benefit from this research?

Academic beneficiaries.
Public health.
UK science base.
Pharmaceutical industry.
General public.


How will they benefit from this research?

Pharma and Public Health: The proposed programme of work will address major deficiencies in our knowledge pertaining to the mechanisms that underpin neuronal dysfunction in patients with mutations that cause intellectual disability, epilepsy and spastic paraplegia, disorders with a very significant associated disease burden. More generally, major neurological diseases that represent a very significant disease burden to the U.K. include components of altered neuronal membrane trafficking and altered glutamatergic and GABAergic signaling. Research on diseases such as stroke, epilepsy, motor neuron disease, Alzheimer's disease, schizophrenia and autism will directly benefit from a detailed understanding of the molecular mechanisms that control signaling and trafficking in nerve cells. Our research will therefore in the longer-term benefit quality of life and the pharmaceutical industry. Indeed, several of our recently published studies impact on our understanding of neuronal dysfunction in neurological diseases including work in models of ischemia / stroke, Parkinson's and Huntington's disease, autism and schizophrenia.

Training the next generation of scientists: The academic community and the UK Science base will directly benefit from this programme of research through the scientific and professional training of the post doctoral scientists on the project. The high level of training given within the Department of Neuroscience, Physiology and Pharmacology at UCL also has the potential to directly benefit the pharmaceutical and biotech industries as research skills learned during the project would be readily transferable to research and development projects in the pharma and biotech sectors. Transferable skills gained by the postdoctoral researchers during the project are also relevant to the wider UK economy, therefore potentially benefiting the wider UK commercial sector and general public.


What will be done to ensure that they benefit from this research?

Our main aim is to publish our work in high impact international journals as an excellent means of contributing to UK research competitiveness and disseminating our findings and data to the widest academic and pharmaceutical audiences. We will also disseminate our data at earlier stages prior to publication in the form of presentations of the data at invited lectures at universities and pharmaceutical companies and at national and international meetings and symposia. I am active in organising symposia and conferences both in the UK and abroad (e.g. Mitochondrial Trafficking and Function in Neuronal Health and Disease, 2012; GABAergic signalling in Health and Disease 2014) and will aim to organise a more focused symposium on vesicular trafficking in the nervous system which will allow discussion and exchange of ideas and data with others in the field.

We will also engage the public regarding our key findings. 'UCL Neuroscience' (www.ucl.ac.uk/neuroscience) which brings together neuroscience research at UCL has a very active public engagement policy. UCL was named one of the six 'Beacons of Public Engagement' nationwide. My lab is actively engaged in communicating with UCL Public Engagement to disseminate our research to a wider public audience. Work from my lab has been previously showcased during Brain Awareness Week and in the news sections of various research and funding websites including the MRC website and the MRC Network magazine. The lab also takes part every year in the In2ScienceUK program, which offers underprivileged students currently at high school in deprived schools the opportunity to work alongside practising scientists, giving them an insight into scientific research and development.