Heterogeneity in astrocyte responses after acute injury in vitro and in vivo - a podcast by Ludwig-Maximilians-Universität München

Astrocytes present a major population of glial cells in the adult mammalian brain. The heterogeneity of astrocytes in different regions of the healthy central nervous system (CNS) and their physiological functions are well understood. In contrast, rather little is known about the diversity of astrocyte reactions under pathological conditions. After CNS injury the reaction of astrocytes, also termed ‘reactive astrogliosis’, is characterized by morphological and molecular changes such as hypertrophy, polarization, migration and up-regulation of intermediate filaments. So far, it was unknown whether all astrocytes undergo these changes, or whether only specific subpopulations of reactive astrocytes possess special plasticity. Since some quiescent, postmitotic astrocytes in the cortical gray matter apparently de-differentiate and re-enter the cell cycle upon injury, reactive astrocytes have the ability to acquire restrictive stem cell potential. However, the mechanisms leading to increased astrocyte numbers after acute injury, e.g. proliferation and migration, had not been investigated live in vivo. For the first time, recently established in vivo imaging using 2-photon laser scanning microscopy (2pLSM) allowed to follow single GFP-labeled astrocytes for days and weeks after cortical stab wound injury. Tracing morphological changes during the transition from a quiescent to reactive state, these live observations revealed a heterogeneous behavior of reactive astrocytes depending on the lesion size. Different subsets of astrocytes either became hypertrophic, polarized and/ or divided, but never migrated towards the injury. Intriguingly, the lack of astrocyte migration was not only contradictory to what had been predicted based on in vitro and in situ studies, but was also in stark contrast to the motility of other glial cells. Additionally, live imaging provided first evidence that only a small subset of reactive astrocytes in juxtavascular positions re-gains proliferative capacity after injury. While astrocyte proliferation was affected by conditional deletion of RhoGTPase Cdc42 – a key regulator of cell polarity –, the vascular niche was preserved, indicating that juxtavascular astrocytes are uniquely suited for proliferation after injury. Following the behavior of cdc42-deficient astrocytes by live imaging using an in vitro scratch wound assay, cell-autonomous effects including disturbed polarity and impaired directional migration confirmed a crucial role of Cdc42 signaling in reactive astrocytes after acute injury in vitro and in vivo.
These novel insights revise current concepts of reactive astrocytes involved in glial scar formation by assigning regenerative potential to a minor pool of proliferative, juxtavascular astrocytes, and suggesting specific functions of different astrocyte subsets after CNS trauma.

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