DESCRIPTION: The objective of this proposal is to understand the mechanisms that control axonal and dendritic differentiation and morphology in granule neurons. To determine how cues intrinsic to the cell, or present in the extracellular environment, direct the differentiation and maturation of granule cell axons and dendrites, the following hypotheses will be tested: 1) The initial site of unipolar process extension is determined by the cytoplasmic location of the Golgi apparatus; after unipolar extension, the GA re-orients within the cytoplasm to support the growth of the second axonal process. To test this hypothesis, time-lapse confocal microscopy will be used to determine the dynamic behavior of the GA during the initial stages of granule clel axon outgrowth in. 2) After initial bipolar parallel fiber (PF) axon outgrowth is completed, diffusible or contact-mediated signals from developing Purkinje cells (PC) and/or granule cells within the molecular layer (ML) cause a down-regulation in expression of the axonal cell adhesion molecule, TAG-1, on granule cells located at the deep external granule layer (EGL)/ML border. To test this hypothesis, it will determined whether granule cell/PC contact in , or conditions that mimic the onset of synaptic activity between granule cells and Pcs, provide an "off-signal" for TAG-1 expression on initial granule cell axons. 3) Exposure of immature granule cells to depolarizing conditions inhibits axonogenesis and promotes dendritic differentiation. To test this hypothesis, immature granule cells will be exposed to depolarizing condition from the time of plating in . 4) Complete granule cell dendritic differentiation, which is characterized by the formation of characteristic claw-like dendrites, proceeds only after direct cell-cell contact with mossy fiber afferents. To test this hypothesis, granule cells will be co-cultured with pontine explants to provide a source of mossy fiber afferent input, to determine whether direct cell-cell contact with mossy fibers 'drives' further dendritic differentiation. Understanding the mechanisms that control the normal differentiation of granule neurons is critical in understanding the etiology of the various cerebellar ataxias and birth defects such as fetal alcohol syndrome.