The role of astrocytes in experience dependent plasticity

We sense the features of our external world using special types of cells, such as those that can detect light for vision, sound for hearing or pressure for touch. The signals from these cells are sent along nerve fibres to the brain's cerebral cortex, where the information carried from our sensory cells is deciphered and "presented to us" as sensations of vision, hearing or touch. In the middle of the last century it was discovered that the connections between nerve cells in the cortex were not permanently fixed but could be modified, either by changes to the activity of the special sensory cells, but also due to brain damage. It became clear that these cortex changes were part of the brain's adaptation to changes in the environment. Soon after, similar changes were found elsewhere in the brain and it was realised that these processes were similar to the changes in nerve cell interconnections which occur during learning and in memory. This has become known as 'plasticity' and it is vital to understand its mechanisms, as this is key to understanding how the brain retains information. Moreover, if we understand the mechanisms of plasticity we may also be able to control brain plasticity. We believe that the ability to manipulate plasticity will enable us to improve learning and memory as well as offer new way of "repairing" the brain after damage, caused by either accident, stroke or epilepsy. For mice or rats, in contrast to humans, the sense of vision is not very well developed, so in every day life they rely on touch and use their whiskers to recognise subtle features of their immediate environment. Hence, a large part of rodents' cortex is devoted to receiving and deciphering the signals from the whiskers. For many years scientists have been investigating plastic changes that happen to nerve connections in the cortex when some, or all of the rodent's whiskers are cut, and a lot of progress has been made in understanding the mechanisms involved. In recent years however it has been realised that apart from nerve cells, there are also other types of cells in the brain that could be involved in the mechanisms of nerve cell plasticity. Interest has focused especially on a type of brain cell called an astrocyte, as it was discovered that astrocytes can transmit and receive signals to and from nerve cells. Importantly, we have recently found that astrocytes can also undergo plastic changes; therefore it is essential to understand how important astrocytic plasticity is to nerve cell plasticity in the cortex of living animals. In our experiments we will therefore cut whiskers and measure the extent of plastic changes happening to astrocytes compared to those happening to nerve cells and whether blocking or increasing astrocytic plasticity also modifies nerve cell plasticity. Finally, using a newly developed approach we can use optical stimulation to make the astrocytes undergo plasticity and so can test if that process in turn affects nerve cell plasticity. Improved understanding of how astrocytes are involved in neuronal plasticity will not only shed light on how our brain retains information, but will also lay a foundation for the development of new medical treatments that can help with memory loss and damage due to conditions such as stroke and Alzheimer's.

Changing the whisker complement in rodents induces experience-dependent plastic changes in neurones of the somatosensory cortex. Much is known about the mechanisms for experience-dependent plasticity (EDP), but very little about the potential role of astrocytes in this phenomenon. The potential role of astrocytes may be significant since it has recently been demonstrated that astrocytes regulate synaptic plasticity induced in vitro and in the somatosensory system can undergo plastic changes themselves, as well as modulating neuronal activity. We will therefore evaluate the degree of the contribution astrocytes might make to neuronal EDP. We will use the single-whisker experience (SWE) model to ask whether our recently discovered plasticity of astrocytic glutamate release, known as long-term enhancement (LTE) is experience-dependent, whether astrocytes are necessary for the induction and the maintenance of EDP, and finally whether concurrent whisker and astrocytic stimulation are sufficient to induce EDP. The study will comprise of in vitro, in vivo and ex vivo experiments. The magnitude of LTE will be determined using patch-clamp recordings from neurones and Calcium imaging from astrocytes. The magnitude of EDP will be determined by single-unit recordings, and use of FosGFP transgenic mice. The experiments delineating the role of astrocytes in EDP will employ genetically modified animals, in which astrocytic signalling is down-regulated (IP3-R2 KO) or up-regulated by lentiviral expression of the OPTO-XR fusion gene under a GFAP promoter into the barrel cortex and optical stimulation. Should we find that astrocytes and/or astrocytic plasticity are involved in EDP, then this will promise major advances in our understanding of basic brain function and may offer new mechanisms by which brain damage due to sensory function loss, epilepsy, or memory deficits, can be therapeutically targetted.

The immediate direct beneficiaries of this project will be the two research assistants that will take part in the project. They will receive training in the rapidly advancing field of astrocyte - neuron interactions and the cutting edge techniques of electrophysiology, calcium imaging and optogenetics, and so will acquire a highly relevant skill set. They will also gain from being part of a multi level collaboration between the two laboratories, which will provide them with a firm foundation for their future careers. The academic research communities at Aston and Keele will benefit with the interaction with those involved in this exciting proposal. In addition, undergraduate and postgraduate students that we teach in both Aston and Keele will benefit, as our teaching of neuroscience is directly informed by our research. Such content inspires students and increases their awareness of the limitless possibilities of scientific research and development.
The interested general public will also gain. We still do not know precisely how the healthy cerebral cortex operates and how it responds to changes in the environment. There is currently a resurgence of interest in science, notable by the success of large science festivals such as The Cheltenham and British Science festivals and the growth in popular science programmes and celebrity scientists on television. There is a special interest in brain function and increasing brain plasticity through "brain training" games which is coming to the attention of neuroscientists (Herculano-Houzel S. (2003) What does the public want to know about the brain Nat. Neurosci., 6:325). We believe the public and non-neuroscientists will be fascinated by our research and findings as it will increase knowledge of brain function and shed light on new mechanisms of learning.
Equipment that we will purchase for the selective light activation of OPTO-XR has been developed by CAIRN Research, which is based in the UK, in Kent. We have had a good rapport with this leading British concern for a number of years and so apart from the initial purchase of the system, CAIRN will benefit from our feedback on specific light activation strategies of astrocytic signalling, and from this be able to produce tailor made systems in the future to address such questions. This will feed forward to benefit future researchers entering the astrocyte signalling field.
The proposal is directed towards a previously unexplored area of research that we expect to reveal new mechanism of plasticity that could be therapeutically targeted. Our proposal to test the malleability of plasticity by light activation of astrocytes, therefore has many potential applications - to increase plasticity where there are deficits such as increasing cognition, or even in disease states. We believe that we could understand eventually how to reduce plasticity in other distressing states such as phantom limb syndrome.
Benefits to the research assistants will range from the time of commencement of the project, right through their long term careers. Undergraduate students and the general public will also benefit during the project lifetime as well as following the project when work is being published and promoted. Companies such as CAIRN will benefit immediately from our communication and may be expected to benefit commercially in a few years. Through the next decade, we expect this proposal to contribute to clinical interventions, as well as the field of neurological research.