Reactive Glia in the Injured Brain Acquire Stem Cell Properties in Response to Sonic Hedgehog

Abstract

As a result of brain injury, astrocytes become activated and start to proliferate in the vicinity of the injury site. Recently, we had demonstrated that these reactive astrocytes, or glia, can form self-renewing and multipotent neurospheres in vitro. In the present study, we demonstrate that it is only invasive injury, such as stab wounding or cerebral ischemia, and not noninvasive injury conditions, such as chronic amyloidosis or induced neuronal death, that can elicit this increase in plasticity. Furthermore, we find that Sonic hedgehog (SHH) is the signal that acts directly on the astrocytes and is necessary and sufficient to elicit the stem cell response both in vitro and in vivo. These findings provide a molecular basis for how cells with neural stem cell lineage emerge at sites of brain injury and imply that the high levels of SHH known to enter the brain from extraneural sources after invasive injury can trigger this response.

Astrocyte reactivity after brain injury—: The role of galectins 1 and 3

“…After transcardial perfusion with phosphate buffered saline (PBS) followed by 4% paraformaldehyde (PFA) in PBS, brains were removed, postfixed in 4% PFA for 4 h at 4°C and processed for immunostainings or in situ hybridization (ISH), as described previously (Brill et al, 2009; Sirko et al 2013). Nuclei were visualized with 4′,6′‐diamidino‐2‐phenylindole (DAPI; 0.1 µg/mL; Sigma Aldrich).…”

“…Neurospheres were prepared from cells of the injured GM of C57BL/6J ( n  = 6) and Lgals1 − / − Lgals3 − / − ( n  = 6) mice at 5 dpi as described in Sirko et al (2013) (see also Supp. Info.…”

“…Indeed, proliferating astrocytes are critical for restricting the injury size and the number of infiltrating cells and inflammation, since their elimination has been shown to aggravate brain damage after lesion (Burda and Sofroniew, 2014). Interestingly, astrocyte proliferation in the GM is highly injury‐dependent and does not occur upon amyloid plaque deposition or even pronounced neuronal cell death, in spite of profound microglia activation and proliferation (Behrendt et al, 2013; Sirko et al, 2013). Instead, it is selectively elicited upon injury involving alterations of the blood brain barrier, such as traumatic, ischemic, and demyelinating injuries (Behrendt et al, 2013; Dimou and Götz, 2014; Gadea et al, 2008; Götz and Sirko, 2013; Kamphuis et al, 2012).…”

“…Instead, it is selectively elicited upon injury involving alterations of the blood brain barrier, such as traumatic, ischemic, and demyelinating injuries (Behrendt et al, 2013; Dimou and Götz, 2014; Gadea et al, 2008; Götz and Sirko, 2013; Kamphuis et al, 2012). These injury‐specific differences led to the identification of signals regulating reactive astrocyte proliferation, including endothelin‐1, sonic hedgehog and fibroblast growth factor (FGF) signaling (Gadea et al, 2008; Kang et al, 2014; Sirko et al, 2013; Zamanian et al, 2012). To obtain a more comprehensive view on the key regulators of reactive astrocyte proliferation, we set out to examine the pattern of gene expression in reactive astrocytes at the peak of their proliferation following stab wound injury in comparison to nonproliferative astrocytes in the intact adult cerebral cortex GM.…”

“…As a subset of proliferating reactive astrocytes acquire neural stem cell (NSC) potential after injury, monitored by the ability to form multipotent, self‐renewing neurospheres (Buffo et al, 2008; Grande et al, 2013; Sirko et al, 2013), this prompts the question how much of the gene expression changes of reactive astrocytes may be shared with NSCs. Only genomewide expression analysis comparing reactive astrocytes, NSCs and nonreactive astrocytes allow determining the degree of similarity between NSCs and reactive astrocytes and the extent of injury‐specific gene expression.…”