The protein kinase DYRK1A regulates cell cycle progression, neuronal differentiation, and stability of the proteins p27 Kip1, Cyclin D1 and Septin4

Dual specificity tyrosine-(Y)-phosphorylation regulated kinase 1A (DYRK1A) is encoded on human chromosome 21 and thus 1.5-fold overexpressed in Down syndrome (DS). Reports from animal models provide strong evidence that an altered DYRK1A gene dosage results in disturbed brain development. Neuronal differentiation is regulated by an accurate control of G1-G0 cell cycle exit, and a disturbed expression of DYRK1A is assumed to deregulate this process. Hence, normalization of elevated DYRK1A kinase activity is currently considered as a possible therapeutic option in DS. Nonetheless, the underlying mechanisms how DYRK1A affects cell cycle regulation and neuronal development remain largely unknown. Therefore, the main part of this study aimed to elucidate the hypothesis that DYRK1A overexpression alters neuronal cell cycle progression and differentiation, and to identify the underlying molecular mechanisms. To investigate effects of DYRK1A overexpression in neuronal cell cycle progression a human cell model with controllable overexpression of DYRK1A was developed. This study shows that DYRK1A overexpression arrested SH SY5Y neuroblastoma cells in G1-phase dependent on its kinase activity and expression levels. Long-term overexpression of DYRK1A led to G0 cell cycle exit and premature neuronal differentiation. Moreover, DYRK1A affected protein stability of the cell cycle regulators p27Kip1 and Cyclin D1 in G1-phase by phosphorylation of their regulatory sites Ser10 and Thr286, respectively. Furthermore, DYRK1A contributed to p27Kip1 Ser10 phosphorylation in differentiating primary mouse neurons and in developing embryonic mouse brain. In summary, the study provides new mechanistic evidence that DYRK1A mediates neuronal cell cycle progression by regulation of G1-S-phase transition and G0 cell cycle exit. The reported findings support the assumption that DYRK1A overexpression contributes to the neurodevelopmental alterations that are present in DS. Septins are a family of GTP-binding and filament forming proteins that fulfill important cellular functions by formation of scaffolds or diffusion barriers. Septin4 was found in neurodegeneration related protein aggregates and is required for proper migration of cortical neurons. Nonetheless, the functional regulation of Septin4 remains unclear. Therefore, the second part of this thesis reports a novel mechanism to regulate Septin4 function. It is shown that DYRK1A phosphorylated Septin4 at Ser107 and that Ser107 phosphorylation affected Septin4 protein stability. These findings strongly indicated a functional relation of DYRK1A and Septin4. Therefore, Septin4 fusion proteins for future analysis of Ser107 phosphorylation in the regulation of protein stability and filament assembly were constructed