Corticotropin-releasing hormone-binding protein is up-regulated by brain-derived neurotrophic factor and is secreted in an activity-dependent manner in rat cerebral cortical neurons

A recent study revealed that corticotropin-releasing hormone (CRH) in the cerebral cortex (CTX) plays a regulatory role in emotional behaviors in rodents. Given the functional interaction between brain-derived neurotrophic factor (BDNF) and the CRH-signaling pathway in the hypothalamic-pituitary-adrenal axis, we hypothesized that BDNF may regulate gene expression of CRH and its related molecules in the CTX. Findings of real-time quantitative PCR (RT-qPCR) indicated that stimulation of cultured rat cortical neurons with BDNF led to marked elevations in the mRNA levels of CRH and CRH-binding protein (CRH-BP). The BDNF-induced up-regulation of CRH-BP mRNA was attenuated by inhibitors of tropomyosin related kinase (Trk) and MEK, but not by an inhibitor for PI3K and Phospholipase C gamma (PLCγ). The up-regulation was partially blocked by an inhibitor of lysine-specific demethylase (KDM) 6B. Fluorescent imaging identified the vesicular pattern of pH-sensitive green fluorescent protein-fused CRH-BP (CRH-BP-pHluorin), which co-localized with mCherry-tagged BDNF in cortical neurons. In addition, live-cell imaging detected drastic increases of pHluorin fluorescence in neurites upon membrane depolarization. Finally, we confirmed that tetrodotoxin partially attenuated the BDNF-induced up-regulation of CRH-BP mRNA, but not that of the protein. These observations indicate the following: In cortical neurons, BDNF led to gene expression of CRH-BP and CRH. TrkB, MEK, presumably ERK, and KDM6B are involved in the BDNF-induced gene expression of CRH-BP, and BDNF is able to induce the up-regulation in a neuronal activity-independent manner. It is suggested that CRH-BP is stored into BDNF-containing secretory granules in cortical neurons, and is secreted in response to membrane depolarization.