Heat-Stressed Brains: Epigenetic and Mitochondrial Signatures in Zebrafish

Rising water temperatures due to climate change pose significant risks to aquatic species, particularly during early neurodevelopment. By the end of the century, global temperatures are projected to rise, with similar increases expected in aquatic ecosystems. Here, we investigated the effects of elevated temperatures (28 degrees C, 31 degrees C, and 34 degrees C) on zebrafish (Danio rerio) larvae, focusing on morphology, brain histopathology, oxidative stress, and key molecular pathways. Heat exposure caused a temperature-dependent increase in developmental malformations, reduced survival, and histologically confirmed neurodegeneration. Immunohistochemistry revealed elevated 8-hydroxy-2 '-deoxyguanosine (8-OHdG) and 4-hydroxynonenal (4-HNE), indicating oxidative stress in brain tissues. Molecular analyses showed upregulation of epigenetic regulators (dnmt3a, hdac1) with concurrent downregulation of tet1, increased expression of the neuroendocrine stress marker pomca, and unfolded protein response mediators (atf4, xbp1). Mitochondrial genes (cox1 downregulated, cox4 variable) exhibited temperature-dependent changes. Quantitative RT-PCR revealed progressive upregulation of GFAP and Nfl with increasing temperature. Biochemical measurements indicated simultaneous increases in ATP and ROS levels. ELISA analysis showed temperature-dependent elevation of 5-mC, cortisol, norepinephrine, BDNF, IL-6, HSP-70, and TNF-alpha. Together, these findings indicate that heat stress elicits a coordinated response involving oxidative stress, epigenetic remodeling, ER stress, mitochondrial dysfunction, glial activation, and neuronal injury. The results highlight the vulnerability of multiple biological compartments in the developing zebrafish brain and provide mechanistic insight into how rising global temperatures may compromise neurodevelopment in aquatic organisms.