Uncovering astrocyte roles at the blood brain barrier in the healthy and concussed brain

Abstract

The blood-brain barrier (BBB) is regulated by factors that can be secreted by multiple cell types, including astrocytes, that maintain the BBB in health and promote repair after injury. However, astrocyte contributions to the BBB are largely assumed from transplantation studies in which astrocyte progenitor grafts conferred BBB-like properties to tissues that normally lack a BBB. To determine if astrocytes contribute an essential and non-redundant function in maintaining the healthy BBB, I genetically ablated a small number of astrocytes using a conditional, tamoxifen-inducible mouse model. Within 2 hours after induction, I observed sparse astrocyte death in the cortex and leakage of the small molecule Cadaverine and large plasma protein fibrinogen, which are normally contained by a functional BBB. Vessels within regions of ablated astrocytes showed reduced expression of the tight junction protein zonula occludens-1, indicating impairment of the physical barrier formed between endothelial cells. Cadaverine leakage persisted for weeks, a feature I also found in mice after mild concussive traumatic brain injury (cTBI), thus highlighting the potential for revealing astrocyte roles in post-injury repair. Unlike the genetic ablation model, astrocytes within Cadaverine leakage areas did not undergo cell death after cTBI and instead downregulated homeostatic proteins. Our preliminary results show this atypical phenotype appearing 10 minutes after cTBI, along with severe vessel rupture, BBB leakage, and disruption of endfoot and basement membrane proteins. This damage persists for months, suggesting that the BBB fails to repair in these areas. Our results provide direct in-vivo evidence for essential astrocyte roles in the maintenance of the healthy BBB. Maintenance and/or repair fail after mild concussive cTBI, possibly contributing to irreversible progression to neurodegenerative diseases.