1. Synapse formation and plasticity

The brain consists of a number of neurons each of which has a unique character.　However, the basis of higher brain function is not the variety of neurons, but it depends on the complex and accurate neuronal network. The position of neurons in the brain tissue, and the highly polarized morphology of neurons, which have axons and dendrites, play important roles for the formation and maintenance of the neuronal network. We are mainly interested in how the actin-based signal transduction regulates the above phenomena.

(a) Dendritic spine formation

Dendritic spines, which are receptive regions of mature excitatory synapses, develop via dendritic filopodia. However, the molecular mechanisms of spine morphogenesis are poorly understood. The actin cytoskeleton predominates in spines, and regulates their morphological plasticity and the anchoring of certain postsynaptic molecules. Consequently, the actin cytoskeleton has been proposed to be a key player in spine morphogenesis. We currently hypothesize that the developmentally-regulated reorganization of postsynaptic actin cytoskeleton by drebrin is an essential process of spine morphogenesis. In order to examine this hypothesis, we used immunocytochemistry of cultured hippocampal neurons and unique cell-biological manipulations of their drebrin-expression to address the three following questions: (1) How does the actin cytoskeleton change during spine morphogenesis; (2) Is drebrin involved in the developmental changes of the actin cytoskeleton; and (3) Do the developmental changes of PSD components depend on those of the actin cytoskeleton

(b) Actin-cytoskeleton-based Spine Morphology and Synaptic Function.

The main cytoskeleton in neuronal dendritic spines is actin filament, and this actin filament regulates spine morphology and synaptic function. The actin filament is regulated by actin-associated-proteins. Drebrin, one of the actin-binding-proteins, is highly enriched in mature dendritic spines, and is known to regulate spine morphology. Now we analyze the function of drebrin A isoform (adult type isoform; it specifically exists in neurons) using knock down technique of drebrin A. We have already developed an assay system to decrease drebrin A expression by using antisense oligonucleotides in cultured neurons. Our present results indicate drebrin-actin cytoskeletons regulate not only spine morphology but also synaptic function.

(c) Study of drebrin to in vivo level

To extend the study of drebrin to in vivo level, we have developed transgenic mice which express EGFP-drebrin A in the matured forebrain under CaMKII promotor transcriptional regulation. With the use of this model, we are investigating the drebrin A functions in synaptic plasticity or in higher functions such as memory and learning.