CYTOSKELETAL TARGETS OF NEURONAL SIGNALING

这一建议解决了神经元细胞骨架的分子控制

This proposal addresses the molecular control of neuronal cytoskeletal proteins by extracellular signals. Understanding how the neuronal cytoskeleton is regulated is important because several disease processes, including Alzheimer's disease and schizophrenia are thought to affect the cytoskeleton and thereby alter neuronal function. During outgrowth, development, and synaptogenesis, the morphology of neurons is controlled by local environmental cues like growth factors and neurotransmitters. The microstructure of adult neurons is also thought to be modified in response to synaptic or neurohumoral activity. We propose to examine the cellular basis for morphological plasticity in neurons by focusing on the regulation of the phosphorylation state and function of the microtubule- associated protein MAP2. MAP2 is thought to play an important role in the development and maintenance of dendritic morphology via its interaction with microtubules and other cytoskeletal elements. Multiple protein kinases and protein phosphatases are known to modify MAP2 phosphorylation and MAP2 function in vitro, but little is known about the properties of MAP2 phosphorylation in vivo. Our previous studies revealed that one signal which alters MAP2 phosphorylation in intact neurons is activation of excitatory amino acid receptors. MAP2 becomes rapidly and selectively dephosphorylated as a result of N-methyl-D- aspartate receptor activation. This effect appears to involve a novel signal transduction pathway for NMDA receptors. In the proposed studies, we will define the signal transduction pathways which modulate the phosphorylation state of MAP2 in hippocampal slices and cultured hippocampal neurons. Hippocampal cells metabolically labeled with 32P- orthophosphate will be used to determine the mechanism, receptor specificity and temporal properties of excitatory amino acid-induced dephosphorylation of MAP2. The stimulation of MAP2 phosphorylation by neurotransmitter and growth factor-dependent protein kinase activity will also be examined. Two forms of MAP2 will be studied: conventional MAP2 found in adult neurons, and an alternatively spliced variant unique to immature neurons called MAP2c. This variant lacks a large middle portion of adult MAP2, and thus contains a relatively small number of phosphorylation sites. The immature form of MAP2 will help us to define which phosphorylation sites are under the control of NMDA receptors. Once we identify the kinase and phosphatase pathways for MAP2 and MAP2c regulation, we will determine the precise location of phosphorylation sites on the smaller protein MAP2c. Such information will be important in eventually determining the role of phosphorylation of MAP2 and MAP2c in regulating neuronal morphology. These results will contribute to an understanding of how extracellular signals regulate MAP2 function in vivo. More generally, these experiments will allow formulation of a molecular model for transmembrane regulation of the neuronal cytoskeleton.