Defining the role of post-translational modifications of Frizzled-5 receptor: implications in cell signalling and synapse formation

Communication between cells is crucial for a range of biological functions, from tissue formation and growth, to the formation of functional circuits in the brain. Cells communicate by releasing signalling molecules that act on neighboring cells. Importantly, recipient cells are able to detect these released signals because of molecules located at the cell surface, called receptors, that act as antennas. Once the receptor, or antennas, detect the released signals, they activate a series of molecular events inside the cell, called signalling cascades. The intensity and duration of these cascades determine the type of cellular response. Thus, understanding how signalling molecules act on recipient cells and how receptors behave on the cell surface are fundamental processes that concerns cell and developmental biologists, immunologists and neuroscientists.

In the brain, the assembly of synapses, sites of contact between nerve cells, or neurons, is a crucial step in the formation of functional circuits. Great progress has been made in understanding the cellular and molecular mechanisms that contributes to synapse formation. Previous work from our lab has led to the discovery that specific factors released by neurons, called Wnt proteins, play a critical role in the formation of brain circuits. Wnt proteins elicit different responses through their interaction with surface receptors that transmit information inside cells. For instance, our studies led to the identification of Frizzled-5 (Fz5), a key receptor for Wnt7a, a Wnt protein that promotes synapse formation. We also found that neurons missing the Fz5 receptor are unable to respond to Wnt7a to form synapses. More recently, we discovered a novel mechanism that modulates the level of Fz5 receptors at the cell surface and its ability to promote the formation of synapses.

In this project, we will examine the precise molecular mechanisms by which Fz5 levels are regulated at the cell surface. We will use a multidisciplinary approach that uses biochemical approaches, state-of-the-art live-cell imaging techniques combined with modulation of gene function in the developing brain. This project will provide novel mechanistic insights into how receptors are localised and retained to the cell surface to control fundamental cell functions.

Synaptic assembly is a critical step in the formation of functional neuronal circuits. Several signalling molecules have been identified as key players that modulate synapse formation. Extensive work has been done in elucidating how secreted signals act on their receptors to promote synaptogenesis. However, the mechanisms controlling the trafficking and location of surface receptors and their ability to signal to downstream targets remain poorly characterised.
Our lab has been studying the role of Wnt signalling in synaptic connectivity in the vertebrate nervous system. We found that Wnt7a promotes the formation of synapses in the hippocampus. Presynaptically, Wnt7a requires Frizzled-5 (Fz5), a seven transmembrane receptor, to promote synapse formation. Importantly, loss of function of Fz5 blocks the response of neurons to Wnt7a to induce synapse formation. Moreover, we found that Wnts are required for activity-mediated trafficking of Fz5 to the cell surface. Importantly, we discovered a novel post-translational modification in Fz5 receptor that control its surface expression, its ability to activate the Wnt signalling cascade and to promote synapse formation.

In this project, we will examine how this novel post-translational modification regulates Fz5 trafficking to the cell surface and its ability to activate the Wnt pathway. We will take a multidisciplinary approach that combines biochemical analyses, time-lapse imaging of fluorescently labeled receptors to visualize their trafficking, super-resolution microscopy to evaluate interactions with components of the Wnt signalosome, and in vivo analyses of receptor function during synapse formation. Our studies will shed new light into novel molecular mechanisms that control surface receptor levels and their ability to modulate downstream signalling. Our work will also have a impact in the field of embryonic development, cell biology and neuronal connectivity where the Wnt signalling pathway plays a key role.

An outstanding question in biology is how the levels of surface receptors are regulated and how this process influences downstream events controlling diverse cellular functions from cell differentiation and cell migration to tissue organisation and the formation of complex neuronal circuits in the brain.
In this project, we aim to define the molecular mechanisms that control the trafficking and location of a receptor for Wnt7a, a member of the Wnt family of secreted proteins, and its impact on synapse formation in the developing mammalian brain. Given the role of Frizzled receptors in diverse celullar processes, our project will have a significant impact in the way we think about how these receptors modulate celullar functions such as cell determination, cell migration and the assembly of neuronal circuits.

Our results will provide the bases for developing strategies to modulate receptor function. Therefore, this work will be of great interest to pharmacological industries seeking to design drugs for regulating diverse cellular functions from developmental disorders to synapse formation and function in diverse neurological conditions.

The research carried out under this project will give us the opportunity to bring pupils from in primary and secondary schools to work in our lab. Indeed, as part of our outreach activities we provide master science classes and we have been involved in a program (In2ScienceUK), which aims to bring secondary school children (age 16-17) from disadvantaged backgrounds to work in research laboratories during the summer period. This program is extremely useful to bring the work of scientists into schools and colleges and we will keep participate in this initiative.

Other examples of our outreach program are:
-Gave public lectures to increase awareness on how fundamental research contributes to the development of therapeutic approaches and national health.
-Participated at UCL events that allow the interaction of scientists with the government like the UCL Parliamentary Reception organised a few years ago at the House of Lords or events organised by funding bodies aiming at increasing the interaction with lawmakers.
-Conducted interviews at local radios in the UK, including BBC World Service and Bloomberg, The New Scientist and Alzforum (website on Alzheimer's research) as well as interviews at newspapers from other countries.
-Contributed with scientific materials to the BBC programs such as "The Human Body" aired in 2011, to the BBC website, to the Royal Society Program on Women in Science featuring Professor Dame Athene Donald and Professor Sarah-Jane Blakemore and to the exhibition "The Beautiful Brain: the Drawings of Santiago Ramon y Cajal" Grey Art NYU Gallery (January-March 2018).
-Wrote a review on gender equality: Salinas P.C. and Bagni C. (2017) Gender equality from a European perspective: myth and reality. Neuron. 96: 721-729.This provided a valuate source of data on gender equality and possible solutions on how to narrow the gap on gender inequality.
-Gave lectures on gender equality at UK Universities.