Regulation of synaptic function by intramembrane proteolysis

Neurons are the fundamental units of the brain that send and receive signals called neurotransmitters. This communication process is called neurotransmission, and it occurs at closely apposed junctions between two neurons called synapses. Upon receipt of a stimulus, neurotransmitter is released from the pre-synapse of a signal-sending neuron into the synaptic cleft. At the post-synapse, neurotransmitter is bound by receptors on a signal receiving neuron, which ultimately leads to relay of a signal. Ultimately this basic process is repeated iteratively across numerous synapses to wire the brain.

For neurotransmission to occur efficiently and correctly, both sides of a synapse need to be aligned precisely. This alignment is co-ordinated by adhesion proteins that act as a glue for the pre- and post-synapse, and also act as a "sighting device" that ensures specific neurotransmitters are released opposite to specific postsynaptic receptors. This arrangement forms the basis of long-term memory, where the strength of synapses is reinforced by synapse adhesion over time. In reality however, synaptic connections grow and shrink according to how often they are used. This is a challenge for neurons. For instance, when neurotransmitter release at a presynapse is turned down, there is a requirement to downscale synaptic adhesion so that postsynaptic receptors are concentrated in the right place to maximise signal reception. How this occurs is not known. It is very important to understand though, because during ageing the shape, size and number of synapses fluctuates - and synapses are often lost in neurodegenerative conditions such as Alzheimer's Disease. Finding ways to control or limit synapse loss might therefore be a way forward to curing neurodegenerative disorders.

Our lab study enzymes called rhomboid proteases, which exist in every kingdom of life but we have yet to fully discover what they do in humans. In an exciting discovery, we have found that rhomboids can cut important adhesion proteins that align the pre- and post-synapse, ultimately removing them from synapses. We have found that rhomboids are found at high levels in neurons, especially high when synapses are formed. In this project, we will test our hypothesis that rhomboids instruct where neurotransmission takes place within synapses by shaping synaptic connections. As part of this project, we will use super-resolution microscopes that allow us to look at how single molecules of rhomboids and adhesion proteins move at the surface of synapses in living neurons. This will give us information on how rhomboids choose where and when to remove adhesion proteins from synapses. By knowing how this process occurs, we will be able to assess whether rhomboids represent a promising drug target to treat neurodegeneration.

The main function of neurons is neurotransmission. Efficient and tuneable neurotransmission requires alignment of the pre and postsynapse, which is controlled by adhesion proteins that form discrete transsynaptic columns. These transsynaptic complexes are under continual flux in relation to demand: synaptic activity alters transsynaptic column size and content. This necessitates post-translational regulation of synaptic membrane protein content, which is a challenging task: the postsynapse is densely populated and exists beyond membraneous diffusion barriers at dendritic spines. This led us to ask how abundance of transsynaptic adhesion proteins is selectively regulated at synapses.

We propose that an answer is rhomboid intramembrane proteolysis. Rhomboids are intramembrane proteases that cleave transmembrane domains, but the physiological function of the majority of mammalian rhomboids is not known. Interestingly, cell surface rhomboids are highly expressed - some almost exclusively, in neurons. As rhomboid activity is irreversible, it is a potentially powerful and uncharacterised form of neurobiological regulation. Rhomboids have unique diffusion properties that enable access and cleavage of synaptic adhesion proteins. In this project, we aim to delineate this novel mechanism for neurotransmission and synaptic plasticity control. To do so, we will employ a range of cutting-edge techniques. These include biochemical characterisation of substrate cleavage mechanisms, proteomic identification of regulators of membrane protein diffusion, advanced super-resolution microscopy for characterisation of synapse nanoarchitecture, and (electro)physiological readouts of neurotransmission. Overall, we will shed light on the molecular, cellular and neurophysiological roles of rhomboids by providing a mechanism for control of synaptic architecture and function. In doing so, we may gain insight into the molecular basis of ageing of the human brain, and neurodegeneration.