Understanding aquaporin-4 relocalisation in the central nervous system

The human body is approximately 60% water, as such, control of the water content of different parts of our body is vital for health. Osmosis is the fundamental process by which water moves into and out of cells. Living cells control osmosis using the flow of water through aquaporin water channel proteins. These proteins exist in the cell membranes of all living organisms, facilitating the passage of water across the membrane. Aquaporin-4 (AQP4) is the water channel protein in the brain, where it is enriched at the interface between blood vessels and brain tissue (the blood-brain barrier), and allows water to move between the blood and the brain tissue. Following stroke or head injury, control of this water movement is disrupted, which can lead to influx of water into the brain. This excess water causes the brain to swell and press against the skull, which can be fatal or lead to long-term disability. In healthy brains, the movement of water through AQP4 appears to be important for some kinds of learning and memory formation, but it is not understood why.

I have discovered that AQP4 can move between the inside of cells and the membrane, and that this process is activated following injury, leading to accumulation of AQP4 in the membrane and increasing the rate at which water can pass through the membrane (and thereby into the brain tissue). Using animal models of spinal cord injury and brain injury, I have found that drugs that stop AQP4 from moving to the membrane can reduce swelling and lead to greatly improved post-injury recovery. This suggests that limiting membrane localisation of AQP4 could help trauma and stroke patients.

This is a completely new approach to developing drugs for channel proteins. The traditional approach is to try to find a drug that physically blocks the channel; my approach is to prevent the channel from getting to the membrane, removing the necessity to block it at all. However, the drugs I have used in my experiments so far also block lots of other useful processes in the body; whilst they provide proof-of-principle, they also have obvious potential for unwanted side-effects when used in patients.

In this project I have two key goals. The first is to understand, at the molecular level, the process of AQP4 relocalisation to the membrane. The second is to create a realistic model of the human blood brain barrier in the lab, in a way that allows me to test a large number of drugs (this is known as a 'high-throughput screening' approach).

Achieving these objectives will put me in a position to begin the search for new, specific drugs and drug targets that ONLY limit membrane localisation of AQP4, with the goal of specific inhibition of AQP4 relocalisation, to treat brain injury, spinal cord injury, and stroke patients, and to understand the role of AQP4 in the healthy functioning of the brain.

Aquaporin-4 (AQP4) is the main water channel protein in the mammalian brain, where it is enriched in astrocytes at the blood-brain barrier. As such, AQP4 controls the flow of water between the vasculature and the brain parenchyma. Hypoxia in the brain, as a result of stroke, traumatic injury, or cancer, causes a loss of control of ion homeostasis and influx of osmotically-obliged water into the brain parenchyma, facilitated by AQP4. As such, AQP4 is recognised as an attractive drug target. However, this recognition is yet to be exploited for patient benefit, due the difficulties associated with development of AQP4 pore-blocking drugs.

I have recently discovered that AQP4 can rapidly relocalise from intracellular vesicles to the plasma membrane, and that targeting this relocalisation is a viable therapeutic strategy to prevent or minimise brain and spinal cord oedema. However, the drugs I have used so far are inhibitors of signalling proteins with a broad range of functions, so have obvious potential for side effects.

In this project, I will investigate the molecular and structural biology of AQP4 relocalisation, to discover new drug targets for anti-oedema therapies, and new tool compounds to understand the role of AQP4 in the healthy brain. I will also develop an organ-on-a-chip model of the blood-brain barrier in which AQP4 relocalisation can be measured, as a platform for future high-throughput screening projects.