Astrocyte polarization in perinatal white matter injury and its contribution to disease outcomes

Introduction:

White matter injury (WMI) is the most common form of brain injury in preterm infants and a major cause of long-term neurological morbidity. WMI results from inflammatory and hypoxic/ischemic insults to the developing brain during a peak period of vulnerability and is characterized by reactive microgliosis and astrocytosis, delayed oligodendrocyte differentiation, and in severe cases, neuronal death. Two different types of reactive astrocytes are recognized in brain injury, A1 astrocytes (A1 s), which promote neurodegeneration and A2 astrocytes (A2 s), which support neuronal survival and tissue repair. At present, the specific nature of astrocyte reactivity after WMI (A1 s, A2 s, or other) remains obscure. Given recent findings that A1 formation is induced by reactive microglia and that these astrocytes delay oligodendrocyte differentiation and promote neuronal death, we hypothesize that A1 s play a central role in WMI and may be an exciting therapeutic target for this disease. Here we report the results of experiments aimed to investigate the formation of A1 astrocytes in WMI.

Materials and Methods:

WMI was induced in 2 day-old rat pups using a combination of hypoxic-ischemic and inflammatory insults. In situ hybridization with probes for A1-specific mRNA transcripts was performed on brain tissue from injured and control neonatal rat brains at multiple post-injury timepoints. We used immunopanning to purify astrocytes from brains of injured and control rats. mRNA isolated from these cells was used for qRT-PCR analysis.

Results:

In situ hybridization experiments demonstrate a significant increase in the prevalence of A1 astrocytes in subcortical white matter tracts after WMI in our rodent model. An astrocyte immunopanning protocol optimized for our disease model yields acutely purified viable primary astrocytes from injured and control neonatal rat brains. qRT-PCR using mRNA from these cells reveals regulation of A1-specific transcripts over time after injury.

Conclusion:

We demonstrate the formation of A1 reactive astrocytes in a rodent model of WMI. This result is an important step towards understanding astrocyte polarization in WMI. Should we confirm our hypothesis that A1 astrocyte formation drives WMI outcomes, our findings will open the door to novel therapeutic strategies targeting astrocyte polarization for improving neonatal neurologic outcomes in preterm birth.