Purkinje cells are the principal cells of the cerebellar cortex and have a large and highly branched dendritic tree. Our laboratory has a long-standing interest in the molecules and mechanisms regulating growth and development of Purkinje cell dendrites. We have identified Protein Kinase C (PKC) activity as a major regulator of Purkinje cell dendritic growth. Protein kinase C is also linked to SCA-14, a hereditary cerebellar disease which is characterized by cerebellar dysfunction and Purkinje cell degeneration and which is caused by point mutations or deletions in the PKC? gene. The mechanisms by which the mutations in the PKC? gene lead to Purkinje cell dysfunction in SCA-14 are still not known. We have generated a transgenic mouse model with a SCA-14 mutation located in the catalytic domain of PKC? (S361G). We could show that the presence of the mutated protein in mouse Purkinje cells induces a striking reduction of Purkinje cell dendritic outgrowth indicating that the mutated protein has constitutive kinase activity and is biologically active in Purkinje cells. The finding of a strongly reduced dendritic growth caused by a mutation from spinocerebellar ataxia has also linked the disturbance of dendritic development of Purkinje cells to the pathogenesis of spinocerebellar ataxias. The Purkinje cell-specific expression of the S361G mutation has allowed us to use this this mouse model for identifying molecules which are transcriptionally regulated in Purkinje cells carrying this mutation and which might be involved in the signaling mechanisms eventually resulting in the reduction of dendritic growth. As a first candidate molecule we have identified Car8 as a potential mediator of PKC signaling. Further work in our group has now identified more potential candidate molecules which have an altered expression in S361G Purkinje cells, which might interact directly or indirectly with PKC? and which are linked to PKC? signaling. Besides Car8, these include the IP3 receptor 1 (IP3R1), the Plasma membrane calcium ATPase 2 (PMCA2), the Collapsin response mediator protein 2 (CRMP2) and the Inositol polyphosphate-5-phosphatase (InPP5a). Two more candidate molecules have been identified on the basis of their transcriptional regulation not only in SCA14 but also in two other forms of spinocerebellar ataxia, these are the serine/threonine kinase 17b (STK17b) and the regulator of G- protein signaling RGS8. We have by now shown for all the mentioned candidate molecules that they are expressed in Purkinje cells and that their expression is changed in the S361G mouse model. The major aim of this project is to better define the interaction of these molecules with PKC? and their role for PKC signaling, Purkinje cell dendritic development and Purkinje cell dysfunction in spinocerebellar ataxia. These studies aim at a better understanding of the role of PKC? signaling for Purkinje cell dendritic development and for dysfunction in spinocerebellar ataxia as the basis for developing novel therapeutic strategies for cerebellar diseases.