Research Projects

Molecular Mechanisms Regulating Development of Neuronal Circuits

While there has been impressive progress in understanding the molecular control of axonal and dendritic development during the last decade, we know very little about the mechanisms that allow neurons to select appropriate synaptic partners. This problem, which may be the most fundamental unsolved problem in neural development, is the focus of a major research project in the lab. We are using the hippocampus as a model system to study the mechanisms of synapse formation and synaptic specificity because of its highly structured connectivity. The goal of this project is to use identify and characterize molecular signals that regulate synaptic development, specificity, and function.

Regulation of Circuit Organization by Calcium-dependent Transcription

Synaptic activity leads to lasting changes in neuronal function by regulating post-translational modification of synaptic proteins as well as by inducing changes in gene expression.  Many of the effects of neuronal activity, such as activity-dependent dendritic growth, long-term plasticity in sensory systems, and memory consolidation, require calcium-dependent transcription.

Much of our understanding of calcium-dependent transcription has come from studies of Immediate Early Genes (IEGs), such as c-fos, which are rapidly induced upon calcium influx. Among the major insights from these studies has been the recognition that a CREB-CBP complex plays a central role in regulating calcium-dependent transcription, and several studies support a role for CREB in regulating aspects of synaptic plasticity.

In an effort to gain further insight into the role of activity-dependent transcription in regulating neuronal development and plasticity, we developed a strategy called Transactivator Trap to clone new calcium-regulated transactivators (Aizawa et al., 2004).  The first gene we cloned using this screen was a novel gene we called CREST (for calcium-regulated transactivator). We have found that CREST plays an important role in regulating dendritic development and synaptic function.  We are currently exploring the possibility that CREST-mediated transcription regulates AMPA and NMDA currents and thereby regulates the plasticity-state of a neuron.

We are also investigating the function of NeuroD2, which was also identified in the Transactivator Trap screen. We have discovered that mutations in NeuroD2 lead to defects in segregation of thalamocortical axons in barrel cortex. The NeuroD2 phenotype is particularly interesting since it also has a defect in synaptic maturation. We are currently investigating the role of NeuroD2 in hippocampal development and function.

Functional Analysis of Cortical Circuits

In a recently-initiated project we are developing molecular genetic approaches to study the role of specific cell types in the development and function of cortical circuits.  We are exploring the possibility that GABAergic inputs, which regulate postsynaptic depolarization with great spatial and temporal precision, may play a critical role in determining cortical connectivity and function.  GABAergic inputs are ideally suited to mediate input selectivity since different classes of GABAergic neurons innervate different parts of the principal (pyramidal) neurons and serve different functions. By selectively silencing the activity of different GABAergic subpopulations in vivo we should be able to assess their contribution to the development and refinement of cortical circuits.

Differentiation and Functional Integration of human ES and iPS cell-derived Neurons

The newest area of research in our lab concerns the differentiation and integration of human ES and iPS cells. We are developing conditions to drive the differentiation of human ES and iPS cells into forebrain neurons. The ability of these neurons to integrate into functional circuits would then be examined using co-culture and cell transplantation approaches.  In addition we are exploring using hES and iPS cells to develop an in vitro model of synapse loss associated with Alzheimer’s Disease.

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