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The research presented in this thesis focuses on glial fibrillary acidic protein (GFAP), the main intermediate filament (IF) in astrocytes and astrocyte subpopulations such as neural stem cells (NSCs). In neurodegenerative diseases or upon brain damage, astrocytes respond to an injury with an upregulation of IF proteins such as GFAP. Expression of GFAP is also highly regulated during development of the mammalian brain, where initiation of GFAP expression induces astrocyte differentiation in NSCs. The GFAP gene is alternatively spliced, and the canonical isoform GFAPα and an alternative spliced form GFAPδ represent the most abundant isoforms. In contrast to GFAPα, GFAPδ is assembly compromised and its expression levels are a crucial determinant of IF network assembly. Our research has shown that a specialized GFAP network, induced by expression of an alternatively spliced isoform GFAPδ, modulates astrocyte motility, adhesion, and production of extracellular matrix proteins. Hence, modulation of the GFAP isoform signature modulates astrocyte function, which highlights the importance of a tight regulation of GFAP alternative splicing. In this respect, we unraveled that epigenetic modifications such as histone acetylation and transcriptional regulators, like Notch, are integral components in the control of GFAP isoform expression. Future research will focus on the mechanism of how GFAP, as part of the cytoskeleton, acts as a signaling platform in the cell to regulate vital cellular functions such as proliferation, motility, and adhesion of astrocytes.

Glial fibrillary acidic protein (GFAP) is the main intermediate filament (IF) in astrocytes. The GFAP gene can give rise to different splice isoforms, of which GFAPα is the canonical isoform. GFAPδ is an isoform which in the human SVZ is expressed in specific astrocytes; the adult neural stem cells. Expression differences between these isoforms lead to the hypothesis that GFAPδ may play a functional role in specific subtypes of astrocytes. In overexpression studies, we investigated the effects of GFAP isoform expression on the IF network and functional features of neurogenic astrocytes. We confirmed that GFAPδ expression collapses the GFAP network which altered cell morphology but not motility or proliferation rate. Since there is a constant exchange of proteins from an insoluble form to soluble form in the cytoplasm we assessed the dynamic properties of GFAPα and GFAPδ. The exchange rate of GFAPδ was lower compared to GFAPα and the exchange rates of both isoforms were decreased in a collapsed IF network. A role for GFAPδ in both cell motility and cell adhesion was found in experiments where the endogenous GFAP isoform levels were altered with specific isoform knockdown. Moreover, the expression levels of integrins, plectin and Laminin1a were regulated in cells with a high GFAPδ:GFAPɑ ratio. The molecular mechanisms underlying the influence of GFAPδ on cell motility, adhesion, and extracellular matrix (ECM) related protein expression is still unknown. Future research should focus on the link between the IF network, the ECM, integrin signalling and the role of GFAPδ therein.