Adult neurogenesis is regulated by a number of cellular players within the neurogenic niche. Astrocytes participate actively in brain development, regulation of the mature central nervous system (CNS), and brain plasticity. They are important regulators of the local environment in adult neurogenic niches through the secretion of diffusible morphogenic factors, such as Wnts. Astrocytes control the neurogenic niche also through membrane-associated factors, however, the identity of these factors and the mechanisms involved are largely unknown. In this study, we sought to determine the mechanisms underlying our earlier finding of increased neuronal differentiation of neural progenitor cells when cocultured with astrocytes lacking glial fibrillary acidic protein (GFAP) and vimentin (GFAP 2/2 Vim 2/2 ). We used primary astrocyte and neurosphere cocultures to demonstrate that astrocytes inhibit neuronal differentiation through a cell-cell contact. GFAP 2/2 Vim 2/2 astrocytes showed reduced endocytosis of Notch ligand Jagged1, reduced Notch signaling, and increased neuronal differentiation of neurosphere cultures. This effect of GFAP 2/2 Vim 2/2 astrocytes was abrogated in the presence of immobilized Jagged1 in a manner dependent on the activity of c-secretase. Finally, we used GFAP 2/2 Vim 2/2 mice to show that in the absence of GFAP and vimentin, hippocampal neurogenesis under basal conditions as well as after injury is increased. We conclude that astrocytes negatively regulate neurogenesis through the Notch pathway, and endocytosis of Notch ligand Jagged1 in astrocytes and Notch signaling from astrocytes to neural stem/progenitor cells depends on the intermediate filament proteins GFAP and vimentin.

ABSTRACT

Intermediate filaments (also termed nanofilaments) are involved in many cellular functions and play important roles in cellular responses to stress. The upregulation of glial fibrillary acidic protein (GFAP) and vimentin (Vim), intermediate filament proteins of astrocytes, is the hallmark of astrocyte activation and reactive gliosis in response to injury, ischemia or neurodegeneration. Reactive gliosis is essential for the protective role of astrocytes at acute stages of neurotrauma or ischemic stroke. However, GFAP and Vim were also linked to neural plasticity and regenerative responses in healthy and injured brain. Mice deficient for GFAP and vimentin (GFAP−/−Vim−/−) exhibit increased post-traumatic synaptic plasticity and increased basal and post-traumatic hippocampal neurogenesis. Here we assessed the locomotor and exploratory behavior of GFAP−/−Vim−/− mice, their learning, memory and memory extinction, by using the open field, object recognition and Morris water maze tests, trace fear conditioning, and by recording reversal learning in IntelliCages. While the locomotion, exploratory behavior and learning of GFAP−/−Vim−/− mice, as assessed by object recognition, the Morris water maze, and trace fear conditioning tests, were comparable to wildtype mice, GFAP−/−Vim−/− mice showed more pronounced memory extinction when tested in IntelliCages, a finding compatible with the scenario of an increased rate of reorganization of the hippocampal circuitry.