Traumatic brain injury (TBI) results in permanent neurological deficits and impacts an estimated 69 million people around the globe each year. Though mammals retain endogenous neural stem cells (NSCs) into adulthood, these NSCs do not regenerate lost neurons post-TBI. Stimulating mammalian NSCs to increase regeneration may be a potential therapy for patient suffering from TBI. However, the cell responses governing NSCs in vivo post-TBI are unclear. In contrast, the zebrafish is an excellent model to study in vivo brain regeneration because its NSCs produce new neurons post-TBI. The Unfolded Protein Response (UPR) is a vertebrate conserved stress response that may regulate NSC-driven regeneration post-TBI. Inositol requiring enzyme 1 (IRE1) is a vital UPR protein that regulates mammalian neurons, astrocytes, and microglia post-TBI. However, whether IRE1 activity changes during successful regeneration post-TBI remains unknown. The objective of this study is to characterize IRE1 signaling during the proliferative stages of brain regeneration using a larval zebrafish model.

Larval Tg(xbp1s:eGFP) transgenic fish that express GFP upon IRE1 activation were studied at 5 days post-fertilization, when IRE1 activation appeared to stabilize during early development. Structure of the larval telencephalon was assessed along the rostral-caudal axis so that a novel larval TBI model could be designed to accurately create a lesion in a single hemisphere of the larval telencephalon. Upon inducing TBI, larvae were bathed with the proliferative marker, EdU, before sacrifice for immunostaining. Forebrain tissue was analyzed for GFP expression and NSC proliferation (EdU+/Sox2+). Analysis of GFP signal showed that IRE1 activity rapidly increased by 1-hour post-TBI but significantly decreased by 1-day post-TBI. Proliferation increased by 2 days post-TBI. This study yielded valuable insight into the dynamics of IRE1 activity induced by TBI in a model of successful neuroregeneration.