Therapeutic Effects of a Sonic Hedgehog Agonist on Developmental Cerebellar Brain Injury
The cerebellum undergoes rapid growth during the third trimester and it is known to be vulnerable to injury in preterm infants. Factors associated with preterm cerebellar hypoplasia include chronic lung disease (causing intermittent hypoxia) and postnatal glucocorticoid (GC) administration. In this work I modeled hypoxia and GC administration to study cerebellar and cell type-specific effects leading to injury. I utilized a chronic hypoxic model of postnatal mouse pups to show permanent decreased cerebellar volume. This is compounded by administration of the synthetic GC Prednisolone (Pred) at clinically relevant doses as shown by greater volume loss and a selective vulnerability of the Purkinje neuronal cell population. These effects are both alleviated by administration of a small molecule Smoothened (Hedgehog pathway) agonist (SAG), which preserves cerebellar volume and protects against Purkinje cell death. Interestingly, the protective effects of SAG were observed even when given almost a week after hypoxia + Pred. To determine cell type-specific roles for the hypoxia inducible factor (HIF) pathway I performed conditional knockout of the von Hippel Lindau (VHL) gene to hyperactivate HIF1 in two different neuronal populations: cerebellar granule neurons (CGNs) using Math1-cre transgenic mice or Purkinje neurons using L7-cre transgenic mice. HIF overactivation in CGNs given Pred resulted in cerebellar hypoplasia, whereas in Purkinje cells with Pred this caused cell death. Together, these findings indicate that hypoxia/HIF + postnatal GC administration act on distinct cellular pathways to cause cerebellar injury. In addition to previous work showing how SAG can prevent GC-induced neonatal cerebellar injury and continuing work exploring the role of SAG in protecting against an animal model of postnatal ischemic stroke, this work demonstrates that SAG can be an effective neuroprotective therapeutic agent in the setting of complex neonatal brain injury.