The Prion Protein Controls Polysialylation of Neural Cell Adhesion Molecule 1 during Cellular Morphogenesis

The cellular prion protein (PrPC) is one of the most studied mammalian proteins. Despite the ongoing efforts and the ubiquitous expression of PrPC in vertebrate cells, the main function of this protein has remained enigmatic to date. Several lines of evidence pointed towards the involvement of PrPC in the morphogenetic reprogramming underlying the epithelial-tomesenchymal transition (EMT). To address this, we applied CRISPR-Cas9 genome engineering technique to establish multiple PrPC knockout cell models (Chapter 2). The follow-up studies uncovered a critical role of PrPC in EMT and showed that PrPC controls the polysialic acid (polySia) modification of neural cell adhesion molecule 1 (NCAM1). Surprisingly, this effect of PrPC on NCAM1 polysialylation relies on a novel signaling loop that modulates expression of ST8SIA2, one of the two polysialyltransferase enzymes responsible for adding polySia on NCAM1 (Chapter 3). Intriguingly, a comparative analysis revealed that PrPC levels correlate directly or inversely with ST8SIA2 levels in different cell models. Deep global proteome analyses of multiple PrP-deficient models not only uncovered surprising cell model-specific proteome shifts but also showed robust changes in the levels of MARCKS and BASP protein families at the plasma membrane. Further investigation of their involvement in a biology that controls NCAM1 polysialylation revealed that reduced levels of MARCKSL1 also led to a significant reduction of polySia-modified NCAM1 (Chapter 4). Data from this study emphasize a iii thus far underappreciated coordinated biology of PrP and NCAM1. A close comparison of knockout phenotypes in mice is consistent with a model, whereby several of the known phenotypes in PrP-deficient mice might be explained by the contribution of PrPC to NCAM1 polysialylation. Attempts to validate this model and to assert its significance for determining if perturbed PrPC signaling contributes to toxicity in prion diseases are ongoing and have not been included in this thesis at this time.Ph.D