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Brain-specific angiogenesis inhibitor 2 (BAI2) is a member of adhesion G protein-coupled receptors (GPCRs). The expression of BAI2 is specific in the brain, especially at the neurons and astrocytes in the hippocampus, amygdala, and cerebral cortex.
BAI2 activated by proteolytic processing
Adhesion-GPCRs, including BAI2, generally to have a long N-terminal extracellular region (ECR) comprising a GPCR proteolytic site (GPS), and it is suspected that cleavage of ECR in the GPS domain is important for its function.
To date, we know several cleaved end fragments of BAI2 has been identified in mouse hippocampus. Daisuke et al. confirmed that like other adhesion-GPCRs, the GPS domain is important for proteolytic processing of BAI2, suggesting that BAI2 may be cleaved in the GPS domain. Several conserved residues in the GPS domain including Trp, Cys, and Ser (Trp889, Cys908 and Ser912 in hBAI2) are important for the proteolytic processing of adhesion-GPCRs. Mutation in these site can abort cleavage activities. Results indicate that the GPS domain was essential for the proteolytic processing of BAI2, as reported in other adhesion-GPCRs. Additionally, the C-terminal fragment cleaved at the GPS domain specifically activated the nuclear factor of activated T-cell (NFAT) pathway. These results suggest that BAI2 is a functional GPCR regulated by proteolytic processing and activates the NFAT pathway.
To test potential effects of the BAI2 mutation on receptor insertion in the plasma membrane, Ryan et al. assessed total and cell surface expression of the full-length and truncated (ΔNT) forms of the both the WT and mutant receptors using a surface biotinylation approach. These studies revealed that transfection of full-length BAI2 into HEK-293T cells did not result in activation of either reporter, either for the WT or mutant forms of the receptor. However, BAI2ΔNT robustly activated NFAT-luciferase. The BAI2ΔNT signal to NFAT luciferase was strongly inhibited by the Gβγ subunit inhibitor gallein. Neither WT- nor mutant BAI2ΔNT induced signaling was inhibited by the PLCβ inhibitor U73122, which blocks signaling downstream of Gαq-coupled receptors, and only signaling by the mutant receptor BAI2ΔNT showed statistically significant sensitivity to pertussis toxin (PTX), which inhibits Gαi/o-mediated signaling. And the activation of NFAT luciferase by both WT and mutant BAI2ΔNT was almost completely blocked by the calcium channel inhibitor SKF96365. These results indicate that the NFAT reporter activation by BAI2ΔNT is almost entirely due to Gβγ liberation and activation of a calcium channel.
Fig. 1. The proposed model of the proteolytic processing and the activation mechanism of BAI2. (Michael et al. Journal of Receptors and Signal Transduction. 2017).
BAI2 regulates VEGF through GABP that acts as a transcriptional repressor
Angiogenesis plays an important role in a wide range of physiological and pathological processes. Angiogenesis is controlled by the balance between angiogenic factors and inhibitors. Angiogenesis occurs when angiogenic factors such as vascular endothelial growth factor (VEGF) or basic fibroblast growth factor are superior to angiogenesis inhibitors. ETS transcription factor GA-binding protein (GABP) controls gene expression in cell cycle control, protein synthesis, and cellular metabolism. GABP is unique in the ETS factor because the active complex is an obligate heterotetramer composed of two different proteins. GABPα contains the ETS DNA binding domain, while two alternate subunits, GABPβ or GABPγ, contain a kyrin repeats and the transcriptional activation domain.
BAI2 is involved in the early stages of neovascularization of the cerebral cortex after ischemia. Furthermore, an increase in VEGF expression observed in antisense BAI2 cDNA transfected cells and a potential negative effect of angiogenesis inhibitor BAI2 on angiogenic vascular endothelial growth factor were observed during focal ischemia after BAI2 reduction. Both GABPβ and GABPγ contain nuclear localization signals (NLS), which are mainly located in the nucleus regardless of the presence or absence of GABPα. GABPα lacks NLS and its entry into the nucleus absolutely requires the expression of GABPβ or GABPγ. Therefore, GABPα acts as a transcription factor only after forming a complex with its partner protein GABPβ or GABPγ. In this regard, Jeong et al. suggest that inhibition of available GABP by inactivation of BAI2 after ischemia leads to depletion of available GABPα for nuclear GABPα2/β2 heterotetramer formation, GABPα/γ heterodimer formation, both acting as transcriptional repressors of VEGF It helps to form new blood vessels after ischemia. Collectively, there exists a reciprocal relationship between BAI2 and VEGF in the brain angiogenesis in which angiostatic BAI2 regulates VEGF expression through GABPs after cerebral ischemia as well as under normal conditions.
Fig. 2. A schematic diagram showing the transcriptional regulation of VEGF expression. (Jeong et al. FEBS Lett. 2006).