Anàlisi de les funcions de la proteïna NCAM2 en desenvolupament i plasticitat neuronal

Abstract

[eng] The Central Nervous System (NS) is a fascinating and complex structure formed by millions of interconnected neurons that facilitate cognitive tasks such as memory and learning. The development and maintenance of this system rely on genetic programs that regulate the spatial and temporal expression of numerous genes. Cell adhesion molecules (CAMs) are key proteins in the development and maintenance of the adult CNS participating in the architecture of the system and in signal transduction. These proteins exhibit a wide range of structural and functional diversity, allowing them to regulate different processes, including the renewal and proliferation of neural stem cells (NSCs), neuronal migration and differentiation, as well as the integration of neurons into functional circuits. The present thesis investigates the role of neuronal cell adhesion molecule 2 (NCAM2) in neuronal development and the regulation of adult neuronal plasticity. NCAM2, also known as OCAM and RNCAM, is a glycoprotein located in the cell membrane from the NCAM family. The NCAM family comprises two members, NCAM1 and NCAM2, which are generated by genomic duplication. Alternative splicing of Ncam2 produces two isoforms of the protein: NCAM2.1 and NCAM2.2. Both isoforms share an extracellular domain composed of five immunoglobulin-like modules (IgI-IgV) and two fibronectin type III modules (FnIII1-2). Through the extracellular domain, they establish trans homophilic interactions that are responsible for a large part of their functions. The intracellular domain varies depending on the isoform, while NCAM2.1 has a transmembrane domain and a cytoplasmic tail, NCAM2.2 is attached to the membrane via a glycosylphosphatidylinositol (GPI) anchor. The different structure of the isoforms diversifies their cellular localization, interactions, and biological functions. The implications of NCAM2 have been extensively studied in the olfactory bulb, where it participates in neurite growth, axo-dendritic compartmentalization, and selective axon fasciculation. However, its role outside of this region largely unexplored to date. The main objective of this thesis has been to determine the functions of the NCAM2 protein in the cortex and hippocampus. It has been observed that NCAM2 controls the process of neuronal polarization and morphogenesis. Silencing the protein during neuronal polarization events leads to aberrant dendritic phenotypes and the formation of shorter and more branched axons. The mechanisms through which NCAM2 mediates its functions in neuronal morphogenesis include interaction with cytoskeletal proteins (actin, tubulin, and neurofilaments), cytoskeleton-associated proteins (MAP1B, MAP2, or CAPZ), and other intercellular effectors (CAMKII or 14-3-3). Moreover, the effect of the NCAM2 protein on the regulation of adult neural stem cells (NSCs) and synaptic plasticity in the hippocampus has been analyzed. The results in the context of neurogenesis revealed that regulated levels of Ncam2 are necessary for the proper activation of quiescent NSCs, their division, and neuronal differentiation. Increased levels of NCAM2 lead to cell arrest at a neural progenitor stage and hinder the

formation of new neurons. In synaptic plasticity, our data show that the protein is necessary for synaptic formation and its proper maintenance during adulthood. Silencing NCAM2 leads to a constriction and a reduction in density of spines. Conversely, upregulated levels may promote contact stabilization by increasing the size of dendritic spines. In summary, the results obtained in this thesis highlight the relevance of NCAM2 in neuronal development and plasticity, thus reinforcing the crucial role of CAMs in the proper functioning of the CNS. The NCAM2 gene has been associated with neurodevelopmental disorders or neurodegenerative diseases such as autism, Down syndrome, or Alzheimer's disease. The evidence provided in the thesis, could enhance our understanding of the neurogenic and synaptic deficits that occur in these pathologies and opening new promising avenues for future research.