Transition Support CDA Raman Das

Summary of fellowship aims: Newborn neurons in the spinal cord of the developing embryo shed their tips to move to their final location. As a result of this shedding event, these newborn neurons lose key proteins that allow them to distinguish their front from their back, causing them to lose polarity. Following this, the neuron must now rapidly repolarise itself. This repolarisation is crucial, as it allows the neuron to extend a long cell-process, called an axon, which makes connections with its targets, which could be muscles or other neurons. In the developing embryo, the neuron repolarises in response to external cues from the surrounding tissue. These external cues determine the position of this repolarisation and therefore determine the direction in which the axon will travel, or if it forms at all. This is thus a critical event that is essential for the formation of functional neuronal circuitry. My fellowship will study the molecular and cellular mechanisms that direct this key cell-biological process. In addition to neuron repolarisation, many cancer cells also undergo a very similar repolarisation process during metastasis, which allows them to migrate to different locations and form more tumours. It is likely that the cancer cells do this in a way similar to newborn neurons, so understanding neuron repolarisation will potentially also help us to understand how cancer cells migrate to form secondary tumours. Thus understanding how cells repolarise in response to surrounding tissues has far-reaching consequences.

Progress made so far: As a consequence of shedding their tips, newborn neurons also lose their cellular antenna, called the primary cilium. We have shown that as a result of this shedding, these cells are able to stop dividing and enter the later steps of neuronal differentiation. However, as these cells repolarise, they must be able to receive and respond to signals from the surrounding tissue to extend their axons in the correct direction. Our work has demonstrated that newborn neurons re-grow a new primary cilium that now allows the cell to respond to the signalling environment and directs repolarisation of the newborn neuron and extension of a nascent axon. This work has recently been published in the journal Science Advances.

How transition funding will advance my career: My aim is to be in a strong position to apply for an MRC senior fellowship. Although I have established a unique research niche, I need more publications to make me competitive for this. Transition funding will allow me to capitalise on this strong foundation by allowing me to retain my postdoc, therefore facilitating publication of two further papers in the two-year transition period, which will cement the position of my lab as the best possible environment to study how cells polarise in a tissue context. In addition, being awarded transition support would provide me with greater job stability as I would be employed by the University of Manchester on a permanent basis. This support will also be transformative for my postdoc, who also intends to apply for her own fellowships to establish her independent research career.

Technical Summary
Differentiating neurons in the developing spinal cord undergo acute loss of apical polarity through the process of apical abscission. The nascent neuron must now rapidly re-establish polarity to determine the correct position of axon outgrowth, leading to formation of functional neural circuitry. My fellowship aims to study the molecular and cell biological mechanisms that direct the important process of neuron repolarisation using cutting-edge methods to image spinal cord neurogenesis in real time at high spatial and temporal resolution in ex-vivo tissue slices.
With my MRC CDA-funded postdoc, I have established the role of the primary cilium during neuronal differentiation. Briefly, we have uncovered a novel cell biological mechanism, called primary cilium remodelling, that leads to a switch in the cellular response to Sonic Hedgehog (Shh) signalling, which now directs correct axon extension. This work has been published at Science Advances, and directly leads to two further novel lines of investigation that will produce two strategic publications and set up my long-term research programme as a tenured academic:
Objective 1: Determine the mechanisms directing polarised microtubule rearrangements.
Objective 2: Determine the mechanisms of non-canonical Shh signal transduction.
This work will uncover the novel mechanisms through which neurons translate extracellular signalling cues to achieve the polarised cytoskeletal rearrangements characteristic of axon extension and will consolidate the research niche I have established. Furthermore this work will directly lead on to the objectives of my senior fellowship, which are to investigate the mechanisms of primary cilium remodelling and how this allows cells to dynamically translate and integrate the diverse extracellular signalling cues that direct the stepwise process of neuron polarisation