Normal brain development requires the elaboration of neuronal structures and formation of synaptic connections in order to assemble the neuronal circuits required for cognitive function. Increasing evidence suggests that disorders in circuit formation during brain development contribute to the etiology of several neurological diseases, including schizophrenia, autism, epilepsy, and bipolar disease, as well as X-linked mental retardation syndromes. An initial analysis of Fragile X, an X-linked form of mental retardation in which the Fragile X mental retardation protein (FMRP) is not made, suggests that local protein synthesis within neuronal dendrites is essential for brain development. FMRP is an RNA binding protein that associates with ribonucleoprotein (RNP) granules and ribosomes in neuronal dendrites.
Anatomical studies of brains of Fragile X patients and mouse knockouts of FMRP indicate that their neurons do not development a normal dendritic arbor structure. Consequently, neuronal circuits governing cognitive and behavioral functions do not develop normally. The function of local protein synthesis in regulating the development of dendritic structure and neuronal circuits is not known. Using a combination of molecular biology, gene transfection and in vivo time-lapse imaging with the 2-photon microscope, the proposed experiments will test whether dendritic protein synthesis is required for the elaboration of the neuronal dendritic arbor and formation of neuronal circuits in vivo.
We will perform the experiments using in vivo time-lapse 2-photon imaging of neurons in the visual system of Xenopus tadpoles. These animals are transparent, so that the structure of single fluorescently labeled neurons can be imaged with high resolution over periods up to 2 weeks in the intact animal. The time-lapse movies of dendritic arbor development generated from these images have allowed us to determine many of the cellular mechanisms and molecular components required for normal neuronal development. Furthermore, we have recently shown that a brief period of enhanced visual experience increases dendritic arbor development and increases visual responsiveness in the intact animal. This type of plasticity in the visual responsiveness may require protein synthesis. We now propose to use this versatile system to test whether transport of mRNA into dendrites and activity-regulated local protein synthesis are required for the development of dendritic arbors and the formation of the neuronal circuits in the optic tectum that are required for receiving and processing visual information.