Expression of BDNF–TrkB–AKT1 pathway components and apoptosis-related factors across yak brain regions at low and high altitudes

INTRODUCTION: This study investigated the expression of brain-derived neurotrophic factor (BDNF) signaling components (BDNF-TrkB-AKT1) and apoptosis-related factors (Bcl-2 and Bax) in yak brain regions at different altitudes. MATERIAL AND METHODS: The cerebral cortex, cerebellum, hippocampus, thalamus, and medulla oblongata were collected from 3-year-old yaks living at low and high altitudes. The relative mRNA expression of BDNF, TrkB, AKT1, Bcl-2, and Bax was assessed by qRT-PCR. Protein abundance and cellular localization of BDNF, TrkB, AKT1, Bcl-2, and Bax were evaluated by Western blotting and immunohistochemistry, with immunoreactivity quantified by optical density analysis. RESULTS: Within each altitude group, BDNF, TrkB, AKT1, and Bcl-2 mRNA expression and the corresponding protein levels (BDNF, TrkB, AKT1, and Bcl-2) were significantly higher in the cerebral cortex and hippocampus than in the cerebellum, thalamus, and medulla oblongata (P < 0.05). In contrast, Bax mRNA and Bax protein levels did not differ significantly among the five regions. Compared with low-altitude yaks, high-altitude yaks showed significantly higher BDNF, TrkB, AKT1, and Bcl-2 mRNA expression and higher BDNF, TrkB, AKT1, and Bcl-2 protein levels in brain tissues (P < 0.05), whereas Bax protein expression did not differ between altitude groups. Immunohistochemistry revealed immunoreactivity for BDNF, TrkB, AKT1, Bcl-2, and Bax in both altitude groups, with prominent labeling in cortical pyramidal neurons and across the pyramidal cell layer in the hippocampal CA region. Immunoreactivity was also detected in large neurons of the thalamus and medulla oblongata. In the cerebellum, labeling was strongest in Purkinje cells, with weaker signals in the granule cell layer and molecular layer. CONCLUSIONS: BDNF-TrkB-AKT1 pathway components and Bcl-2 showed relatively higher expression in the cerebral cortex and hippocampus within each altitude group, whereas Bax expression did not vary across regions. These patterns are consistent with an association between BDNF-TrkB-AKT1 signaling and increased Bcl-2 expression without a corresponding increase in Bax, which may support neuronal adaptation in the cerebral cortex and hippocampus. Elevated expression of BDNF, TrkB, AKT1, and Bcl-2 at high altitude suggests enhanced adaptation to hypoxia in high-altitude yaks; the underlying mechanisms require further investigation.