Neural progenitor cells were isolated from rat fetal telencephalon and proliferate as neurospheres in the presence of EGF, FGF-2, and heparin. In the absence of these growth factors, neurospheres differentiate into neurons, astrocytes, and oligodendrocytes. Using an embryonal carcinoma cell line as in vitro differentiation model, we have already demonstrated the presence of an autocrine loop system between kinin-B2 receptor activity and secretion of its ligand bradykinin (BK) as prerequisites for final neuronal differentiation (Martins et al., J Biol Chem 2005; 280: 19576-19586). The aim of this study was to verify the activity of the kallikrein-kinin system (KKS) during neural progenitor cell differentiation. Immunofluorescence studies and flow cytometry analysis revealed increases in glial fibrillary acidic protein and b-3 tubulin expression and decrease in the number of nestin-positive cells along neurospheres differentiation, indicating the transition of neural progenitor cells to astrocytes and neurons. Kinin-B2 receptor expression and activity, secretion of BK into the medium, and presence of highmolecular weight kininogen suggest the participation of the KKS in neurosphere differentiation. Functional kinin-B2 receptors and BK secretion indicate an autocrine loop during neurosphere differentiation to neurons, astrocytes, and oligodendrocytes, reflecting events occurring during early brain development. ' 2008 International Society for
Analytical CytologyKey terms kinin-B2 receptor; neural differentiation; neurosphere; kallikrein-kinin system BRADYKININ (BK), kallidin, des-Arg 9 -BK, and des-Arg 9 -kallidin are the biological active peptides of the kallikrein-kinin system (KKS). BK and kallidin as ligands of Gprotein-coupled kinin-B2 receptors (B2BKR) are generated upon proteolytic cleavage of high-or low-molecular weight kininogen (HMWK or LMWK) by plasma-or tissue-kallikrein serine protease, respectively. Carboxy-terminal arginines are removed from these biological active peptides by carboxypeptidases M or N to originate the kinin-B1 receptor (B1BKR) agonists des-Arg 9 -BK and des-Arg 9 -kallidin (Fig. 1). Stimulation of the B2BKR by its agonists results in the activation of phospholipase C-b (PLC-b), generating diacyl glycerol and inositol 1,4,5-triphosphate (IP 3 ) and resulting in release of Ca 21 from intracellular IP 3 -sensitive stores. Furthermore, BK mediates the activation of endothelial nitric oxide synthase (1) and stimulates the phospholipase A2 activity (2). B2BKR also activates proteins with tyrosine kinase activity (3), and the receptor can directly interact with neuronal and endothelial nitric oxide synthetase (nNOS and eNOS), resulting in NO production (4). Both B1BKR and B2BKR are coupled to G aq and G ai proteins (5,6) and triggered the same signaling pathways, but differ in their expression pattern and intensities of receptorinduced calcium responses and receptor-desensitization rates (7).B2BKR is constitutively expressed and broadly distributed throughout the tissues, and B1BK...