Molecules Involved in Purkinje Cell Dendritic Development and Spinocerebellar Ataxias

Spinocerebellar ataxias (SCAs) are a group of hereditary neurodegenerative diseases which are caused by diverse genetic mutations in a variety of different genes. The genetic background of SCAs can be classified into two groups: Group I repeat expansion SCAs, are caused by dynamic repeat expansion mutations, such as SCA1 and SCA7, and Group II conventional mutation SCAs are caused by mutations in specific genes, such as Protein Kinase C gamma (PKCγ) in SCA14. In the moment it is still unclear by which mechanisms the genetic mutations cause the disease. Several genes have been identified to be dysregulated in SCAs, but it is not well understood how these genes contribute to the pathogenesis of the disease.
An important open question is whether there are some key genes which are dysregulated in both Group I and Group II types of SCAs. Such genes might help us to pinpoint potential pathological mechanisms shared by all SCAs. As a starting point of my thesis I have compared microarray data from published SCA1 and SCA7 mouse models (both Group I) and a transgenic mouse model of SCA14 from our own lab (Group II) with a constitutive activation of the PKCγ kinase (PKCγ S361G mice). With this comparison, I have identified three potential key molecules (RGS8, STK17B and INPP5A) dysregulated in all three SCA mouse models. Recently, INPP5A has been reported to contribute to neuropathology of SCA17, supporting our concept of key molecules being involved in SCA pathogenesis.
In this thesis, the potential key molecules RGS8 and STK17B have been studied in more detail and their role for Purkinje cell dendritic development and for SCA pathogenesis has been explored. We showed that RGS8 is specifically expressed in Purkinje cells in the mouse cerebellum. Further studies showed that increased expression of RGS8 in Purkinje cells is associated with an increased activity of the mGluR1-PKCγ signaling pathway. Functional experiments showed that RGS8 overexpression could protect Purkinje cells from the negative effects of mGluR1 activation on dendritic growth. Our results indicate that RGS8 is an important mediator of mGluR1 pathway dysregulation in Purkinje cells and it is well known that abnormal mGluR1 signaling is found in several different types of SCAs.
STK17B is one of the molecules downstream of PKCγ signaling. STK17B is strongly expressed in Purkinje cells starting at early postnatal development. The expression of the STK17B protein was reduced in dissociated cerebellar cultures of SCA14 S361G mice in parallel with a reduction of the dendritic tree size of Purkinje cells in this mouse model. We showed that PMA treatment induced a strong decrease of STK17B expression in Purkinje cells. As STK17B is known to be a substrate and phosphorylated by PKCγ, we overexpressed the phosphorylation mimetic form of STK17B(S351D) in Purkinje cells. We found that overexpression STK17B(S351D) in Purkinje cells did indeed inhibit dendritic growth of Purkinje cells. The regulation of its expression and our functional studies suggest that STK17B downregulation in the SCA14 mouse model might be a protective reaction protecting Purkinje cells from the constant activation of PKCγ signaling in the SCA14 PKCγ S361G mouse model.
Our findings show that the study of molecules with a common dysregulation in SCAs caused by mutations in different genes is a fruitful strategy to learn more about the pathological mechanisms causing SCAs.