Heterogeneity in astrocyte responses after acute injury in vitro and in vivo

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.