MICA: Optogenetic dissection of homeostatic and Hebbian components of cortical plasticity

The cerebral cortex is a vitally important part of the brain; it is most highly developed in humans and endows us with many of our uniquely human qualities. When the cortex malfunctions in psychiatric conditions such as schizophrenia and autism, or degenerates in conditions such as Alzheimer's disease, those uniquely human qualities are lost or degraded. Ideas about how we might tackle these debilitating conditions have changed over the years. As pointed out by the head of the US National Institute for Mental Health, the way we used to understand mental health disorders as "chemical imbalances" or as "social constructs" seem strangely outdated and have given way amidst a flurry of discoveries to the newer ideas of mental health conditions as disorders of "brain circuits" arising from a combination of genetic risk factors and environmental effects(1). Perhaps it is not entirely surprising that we still only understand the operation of the cortical "brain circuit" at a very rudimentary level at present, after all it is extremely complex. However, a number of powerful molecular and optogenetic tools are now available for unpicking this complicated knot (2) .

Our approach is to look at a relatively simple part of the cortex known as the barrel cortex, where the basic architecture of the cortex can be seen using simple histochemical stains or physiological imaging methods, and to try to understand the cortical circuits involved. The elementary modules that make up the cortex are known as cortical columns and consist of groups of interconnected neurones arranged in six layers. Higher order animals with more complicated cortex have more columns than lower order animals, but the basic architecture of the columns are similar in both cases. We can therefore understand a great deal about the human cortical circuitry by analysing the mouse barrel cortex circuitry. We want to know how information is processed within these circuits and to understand the molecular mechanisms that allow the circuits to change in response to altered experience via their synaptic plasticity mechanisms. Our present work on the circuitry and plasticity of this system has already yielded insights into schizophrenia and AIDS-related dementia. In the present work we combine the new technologies of optogenetic pathway stimulation, dendritic spine imaging and optical manipulation of synapses with our substantial knowledge and experience of the operation of the barrel cortex to understand the cortical circuit. Our immediate plan includes provision to test whether our findings generalise to visual cortex.

We recently made a ground breaking discovery in finding that two major subdivisions of the layer 5 cells, the regular spiking (RS) and intrinsic bursting (IB) cells show fundamentally different plasticity. These cells are the major output cells in the cortex. To a first order analysis, RS cells show depression but not potentiation while IB cells show potentiation but not depression. To a second order analysis, RS cells showed potentiation to subcortical but not intracortical input. We have now discovered a further difference at the molecular level: RS cells exhibit homeostatic (TNFalpha-dependent) plasticity in response to depression, while IB cells show Hebbian CaMKII dependent potentiation. We need to know how these plasticity differences arise, whether they are general to other cortical areas and how they fit into the general response of the cortical circuit to changes in sensory input.

1. T.R.Insel Translating scientific opportunity into public health impact: a strategic plan for research on mental illness. Arch Gen Psychiatry 66,128 (2009)

2. L. Madisen et al. A toolboox of cre-dependent optogenetic transgenic mice for light induced activation and silencing. Nat. Neurosci 15,793 (2012)

Synaptic plasticity is thought to endow the organism with vital functions such as circuit formation during development, learning and memory in adulthood, and recovery of function following trauma. Defective synaptic plasticity either during development or adulthood is thought to occur in many common forms of psychosis and dementia. Our long term goal is to understand synaptic plasticity, not only for fundamental scientific reasons, but also for understanding and ultimately treating debilitating neurological and mental health conditions. Two factors are bottlenecks to progress in this respect (1) understanding the normal circuitry of the cerebral cortex and (2) understanding the diversity of plasticity among cellular subsystems. Recently we found that cortical layer 5 RS and IB cells have fundamentally different plasticity properties. RS cells exhibit little potentiation per se but do show homeostatic rebound from depression. Conversely, IB cells show little depression but do show CaMKII dependent potentiation. Our medium term goal is therefore to determine the pathways and mechanisms underlying these differences and to see if they are general features of cortical architecture. To achieve these objectives we have devised the following 8 experiments:

1 Decomposition of projection pathways to L5 RS cells for sensory transmission

2 Identification of homeostatic pathway(s) operating in RS cells

3 Decomposition of projection pathways to L5 IB cells for sensory transmission

4 Identification of Hebbian pathway(s) operating in IB cells

5 Determination of the dependence of functional plasticity in L5 IB cells on structural plasticity in L5 apical dendrites or L2/3 cells

6 Determination of whether L5 RS and IB plasticity distinctions generalise to visual cortex

7 Determination of the dependence of visual cortex plasticity on structural plasticity

8 Identification of structural differences between homeostatic and Hebbian plasticity

The current grant application runs in parallel with work in the lab that has exposed plasticity defects in AIDS-related dementia and in a model of schizophrenia. Differences in circuit function are likely to arise from abnormalities in plasticity processes. We believe that the research will ultimately be of benefit to pharmaceutical companies in understanding how the normal cortical circuit is configured and hence how abnormal cortex may differ in a number of psychiatric and disease conditions. Understanding the heterogeneity of synaptic plasticity functions should help aid production of therapeutic targets for ameliorating symptoms and possibly speeding recovery of function in some cases.

The current project will capitalise on a recent breakthrough in distinguishing two different types of plasticity present in two different types of cortical output cell. The potential impact is that it could provide insight into treating three major health problems - psychiatric disease, dementia and stroke. This project will help understand developmental psychiatric conditions by providing a comparison between the inherent plasticity and functional circuitry of normal cortex and cortices in animal models of psychiatric diseases. This in turn could provide a platform to tests potential therapies using the natural plasticity mechanisms present in the cortex. This would result in substantially increased quality of life and improvements in health if realised given that the incidence of mental health problems are extremely high in the community. The early stages of Alzheimer's disease involve a loss of plasticity. By understanding the plasticity mechanisms present in the cortex we may be able to find ways to prevent loss of plasticity or enhance the plasticity present in the remaining cells. Given the increasing incidence of dementia in an ageing population, such plasticity enhancement could provide a major health benefit. Finally, this project focuses on plasticity in layer 5 cells, which provide the major output cells of the cortex. One of the challenges in stroke is to recover function in cortical cells often to restore movement. Since the output cells need to re-wire in stroke to regain function, their plasticity processes are fundamental to this objective. Our project will provide insight into the different plasticity processes present in subcortical projecting cells versus cortico-cortically projecting cells. This will enable better design of therapeutic agents able to enhance plasticity during recovery from stroke. The potential impact is therefore to aid the speed and degree of recovery from stroke.

This project ties in with a second MRC project in the lab that is explicitly a collaboration with Japanese colleagues at Tokyo University. This project will therefore help promote research ties between the two countries and aid interchange of innovative methodologies, particularly in the field of advanced microscopy. This should aid the UK knowledge economy both by increasing the adoption of novel methodologies, but also by increasing awareness of potential future UK interactions in the scientific and commercial communities.

The current application will promote the training of individuals within the laboratory in advanced multi-photon imaging and molecular techniques. This includes the existing post-doctoral workers in the lab but also any PhD students in the lab. This training will be further enhanced by visiting and hosting Japanese colleagues in the lab and spending some time working in Japan. This will help enhance research capacity in the UK in the under-represented area of in vivo studies and also develop expertise among UK scientists in advanced imaging combined with electrophysiology in vivo.