Perhaps the most remarkable feature of the developing brain is its ability to self-organize into functional circuits. How does this happen? We know that the formation of appropriate connections requires the targeting of axons and dendrites to specific regions of the brain and the selection of appropriate synaptic partners within those regions. Our lab is interested in understanding the mechanisms that mediate these decisions, and in the past few years we have identified a number of extracellular factors as well as transcription factors that regulate different aspects of hippocampal and cortical connectivity. The major areas of current research are described below.
While there has been significant progress in our understanding of 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 is a fundamental unsolved problem in neural development, is investigated in several projects in the lab. We are using the hippocampus as a model system to study the mechanisms of synaptic specificity because of its highly structured connectivity. We are using electrophysiological and molecular approaches to identify and characterize molecules that regulate synaptic specificity during establishment of neural circuits.
The establishment of functional neuronal circuits relies on the formation of excess synapses, followed by the elimination of inappropriate connections. Although the stabilization of presynaptic inputs is critical for the development and maintenance of functional circuits, the signals that regulate presynaptic stability are not known. We have found that synapse formation in cortical and hippocampal cultures is highly dynamic and involves the stabilization of a subset of synapses in a backdrop of a high rate of synapse formation and elimination. During the peak of synaptogenesis, only about 50% of putative synapses are stable over an hour. We have found that presynaptic stability is strongly correlated with the presence of postsynaptic AMPA but not NMDA receptors. We are examining the mechanisms by which postsynaptic AMPA receptors regulate presynaptic stability.
Once the initial connections are formed, neuronal activity exerts a major influence on the organization of neuronal circuits by regulating changes in synaptic strength. At many synapses, the direction and extent of change in synaptic strength depends on the stimulus parameters. For example, at the CA3-CA1 Schaffer collateral synapse in the hippocampus, low frequency stimulation leads to long term depression (LTD) and high frequency stimulation leads to long term potentiation (LTP). The relationship between stimulus frequency and change in synaptic strength is often depicted by a function called the Bienenstock, Cooper, Munro (BCM) function. This function itself can be modified by various manipulations, and the resulting shift in the BCM function is a measure of metaplasticity.
Despite the importance of metaplasticity, the molecular mechanisms that regulate metaplasticity are not well understood. We are examining the hypothesis that activity-dependent transcription plays a critical role in regulating metaplasticity of synapses by controlling the AMPA: NMDA ratio. We have found that CREST, a transcription factor cloned in our lab, exerts a significant influence in regulating AMPA and NMDA receptor levels.
Another area of recent investigation in the lab is to use molecular genetic approaches to study the role of specific cell types in the development 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 by influencing input selectivity. 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. While the dendritic inhibitory inputs locally regulate the amplitude of excitatory postsynaptic potentials (EPSPs), the perisomatic inhibitory inputs regulate the overall firing rates of neurons. In principle each of these inputs could influence which excitatory inputs are stabilized, but whether they serve such a function is not known. By selectively silencing the activity of different GABAergic subpopulations we should be able to assess their contribution to the function of cortical circuits.
A new area of research in our lab concerns the differentiation human ES and iPS cells into forebrain neurons, and using these neurons to develop stem cell-based models of neurological and psychiatric disorders. Our specific interests are in developing models of synaptic dysfunction associated with autism and Alzheimer’s Disease.