Severe hypoxia exposure inhibits larval brain development but does not affect the capacity to mount a cortisol stress response in zebrafish.

Fish nursery habitats are increasingly hypoxic and the brain is recognized as highly hypoxia sensitive, yet there is a lack of information on the effects of hypoxia on the development and function of the larval fish brain. Here, we tested the hypothesis that by inhibiting brain development, larval exposure to severe hypoxia has persistent functional effects on the cortisol stress response in zebrafish (Danio rerio). Exposing 5 days post-fertilization (dpf) larvae to 10% dissolved O2 (DO) for 16 h only marginally reduced survival, but it decreased forebrain neural proliferation by 55%, and reduced the expression of neurod1, gfap and mbpa, markers of determined neurons, glia and oligodendrocytes, respectively. The 5 dpf hypoxic exposure also elicited transient increases in whole-body cortisol and in crf, uts1 and hsd20b2 expression, key regulators of the endocrine stress response. Hypoxia exposure at 5 dpf also inhibited the cortisol stress response to hypoxia in 10 dpf larvae and increased hypoxia tolerance. However, 10% DO exposure at 5 dpf for 16 h did not affect the cortisol stress response to a novel stressor in 10 dpf larvae or the cortisol stress response to hypoxia in adult fish. Therefore, while larval exposure to severe hypoxia can inhibit brain development, it also increases hypoxia tolerance. These effects may transiently reduce the impact of hypoxia on the cortisol stress response but not its functional capacity to respond to novel stressors. We conclude that the larval cortisol stress response in zebrafish has a high capacity to cope with severe hypoxia-induced neurogenic impairment.

MRI-guided focused ultrasound enhances drug delivery in experimental diffuse intrinsic pontine glioma.

Diffuse intrinsic pontine glioma (DIPG) is a surgically unresectable and devasting tumour in children. To date, there are no effective chemotherapeutics despite a myriad of clinical trials. The intact blood-brain barrier (BBB) is likely responsible for the limited clinical response to chemotherapy. MRI-guided focused ultrasound (MRgFUS) is a promising non-invasive method for treating CNS tumours. Moreover, MRgFUS allows for the temporary and repeated disruption of the BBB. Our group previously reported the feasibility of temporary BBB opening within the normal murine brainstem using MRgFUS following intravenous (IV) administration of microbubbles. In the current study, we set out to test the effectiveness of targeted chemotherapy when paired with MRgFUS in murine models of DIPG. Doxorubicin was selected from a drug screen consisting of conventional chemotherapeutics tested on patient-derived cell lines. We studied the RCAS/Tv-a model where RCAS-Cre, RCAS-PDGFB, and RCAS-H3.3K27M were used to drive tumourigenesis upon injection in the pons. We also used orthotopically injected SU-DIPG-6 and SU-DIPG-17 xenografts which demonstrated a diffusely infiltrative tumour growth pattern similar to human DIPG. In our study, SU-DIPG-17 xenografts were more representative of human DIPG with an intact BBB. Following IV administration of doxorubicin, MRgFUS-treated animals exhibited a 4-fold higher concentration of drug within the SU-DIPG-17 brainstem tumours compared to controls. Moreover, the volumetric tumour growth rate was significantly suppressed in MRgFUS-treated animals whose tumours also exhibited decreased Ki-67 expression. Herein, we provide evidence for the ability of MRgFUS to enhance drug delivery in a mouse model of DIPG. These data provide critical support for clinical trials investigating MRgFUS-mediated BBB opening, which may ameliorate DIPG chemotherapeutic approaches in children.