Mechanisms of long-range retrograde signal propagation and morphological plasticity in hippocampal neurons

The aim of this work is to understand how long-range growth factor signalling shapes neurons of the brain. Throughout life we encounter new experiences, meet new people, learn and forget. All this information is encoded within neuronal networks of the brain, and the ability of the brain to remodel and adapt these networks to store new information is termed plasticity. To restructure neuronal networks, components of the network, i.e. individual neurons change their shape. They extend processes where new synaptic connections are built, and retract other processes where old ones are abolished. This is a strictly controlled and tightly balanced procedure important in normal brain development, and its deregulation contributes to cognitive dysfunction.
Neurons are specialised cells that extend axons and dendrites over very large distances. Particularly the axon, the "delivering end" of the neuron, can reach up to several centimeters in the human brain, and up to over a meter in motor and sensory neurons, from the spinal cord to the toes. This poses multiple logistical challenges, for example with regards to information processing within the neuron. Neurons communicate with their environment via electrical signal propagation and neurotransmitter release, or via a slower mode of communication, involving growth factors. These molecules are detected by receptors that sit on the neuronal surface and can prompt signalling cascades inside the cell. As their names suggest, growth factors are key to the structural organisation of the cell. In contrast to electrical signals, growth factor signalling is not by default unidirectional, and therefore allows the cell body to gather information about events happening at the axon terminal. This direction of information flow is termed "retrograde". Each growth factor detected near the axon terminal ("distally") may initiate signals that are processed locally, to be translated into e.g. "start building a new synapse at this place". Alternatively, it can be sent back to the cell body, to instruct the entire neuron to start growing. This retrograde long-distance communication from the terminal back to the cell body is crucial for neuronal health, and defects in this process have been found associated with a large number of diseases, including motor neuron degeneration and dementia. However, surprisingly little is know about retrograde signalling within neurons of the brain.
Brain-derived neurotrophic factor (BDNF) is a major growth factor of the brain that is first detected early postnatally and is expressed throughout life, and is known to promote network plasticity. The work proposed here will address how signals activated by BDNF at the distal axon are processed within the neuron, and how this relates to changes in cell shape and thus neuronal plasticity. We are particularly keen to understand what information is sent back to the cell body, where it can alter gene expression and affect the makeup of the entire neuron, and how this process of sending back information works on a mechanistic level. We will address this question by growing hippocampal neurons in specialised devices that allow isolation and controlled perfusion of the axon with BDNF. We chose the hippocampus as it is a highly plastic area of the brain, crucial for learning and remembering. We will identify the intracellular mechanisms that are activated locally and those relayed back to the soma using this approach, and monitor how interfering with select information impacts on changes in the shape of individual neurons. This will help understand how all signals, supportive or adverse, are communicated over distances in neurons of the brain, and why this process is so crucial for the proper function of individual nerve cells. A broader knowledge of mechanisms required for normal maintenance and plasticity of individual neurons will also provide information about processes that are failing in cognitive dysfunction.

Neuronal networks are maintained and remodelled in a controlled manner throughout life, a process called plasticity that is tightly coupled to neuronal activity. BDNF is an example of a growth factor that impacts morphology and is released at synaptic areas in response to activity. This stimulus may be processed locally, or alternatively the soma detects and reacts to such signals initiated at distal axons; our previous studies suggest that a combination of both is the case. The neuron then translates this information into defined changes in morphology to maintain appropriate connectivity, a process that remains poorly understood.
Retrograde signal propagation is critical to neuronal function in both central and peripheral neurons. My preliminary work suggests that there are key differences in retrograde receptor trafficking between central and peripheral neurons, which implies also differences in long-range signalling. We have recently identified a broad range of signals generated by BDNF in central neurons using quantitative mass spectrometry, and characterised a mechanism of BDNF-mediated axon branching. Combining my experience in neuronal signalling and membrane trafficking and the results of our proteomic screen with the local expertise in bioengineering, we will now ask how retrograde signal propagation impacts the morphology of hippocampal neurons. We will develop microfluidic devices that allow us to investigate retrograde axonal signalling in response to tightly controlled stimulation (spatial and temporal) using Western blotting to identify signals and live cell imaging approaches to monitor morphological effects over time.
This work will begin to illuminate retrograde signal propagation along axons of brain-derived neurons, and define underlying mechanisms how retrograde signalling impacts on the morphology of these neurons, both outstanding questions in neuronal cell biology.

Pharmaceutical Industries and Charities: The project addresses fundamental cell biological/ neurobiological questions of signalling and morphological plasticity. However, there is a tight correlation of reduced BDNF levels with a number of neurodegenerative diseases and mental health dysfunctions, and observations exists that for example (i) genetically raising BDNF levels ameliorates symptoms in mouse models of Huntington's disease and (ii) antidepressants generally raise BDNF levels. Combined with the fact that BDNF itself does not cross the blood-brain barrier and no functional biomimetics exist, advancing the understanding basic BDNF cell biology may help identify alternative strategies and highlight new potentially druggable targets and processes that may ultimately lead to therapies ameliorating disease progression. I will engage with the translational community at Southampton General Hospital through the Southampton Neuroscience Group and joint seminars to explore this further.
Economic Impact: As addressed above, the proposed project is basic neurobiology research, and as such the future economic impact is difficult to assess. However, publications and communications arising from this work are expected to be of interest to the international research community and contribute to the competitiveness of the UK Higher Education Institutions. Further, in line with the potential impact on general health, mental health disorders affect a significant, often relatively young, part of the population and their immediate relatives, and as such negatively impact on the skilled workforce, while neurodegenerative disorders place a huge burden on the health system. Thus any potential new, druggable avenues uncovered by advancing the basic understanding of morphological plasticity may also ultimately, indirectly, have a significant economic impact.
General Public/ Quality of Life: A better general understanding of molecular mechanisms underlying brain plasticity may lead to a broader acceptance of mental health disorders as a true illness, and thus help de-stigmatise those suffering from these disorders. In addition, given the high number of people affected by neurodegenerative diseases or mental health disorders, any advances in the basic understanding of neuronal plasticity that may ultimately lead to new therapeutic approaches have the potential to significantly contribute to the overall general health and well-being of the population. I will engage with the general public through local outreach activities such as "Bringing Research to Life. University of Southampton Roadshow" and the Science and Engineering day open house.