Interactions between the NO-Citrulline Cycle and Brain-derived Neurotrophic Factor in Differentiation of Neural Stem Cells

The diffusible messenger NO plays multiple roles in neuroprotection, neurodegeneration, and brain plasticity. Argininosuccinate synthase (AS) is a ubiquitous enzyme in mammals and the key enzyme of the NO-citrulline cycle, because it provides the substrate l-arginine for subsequent NO synthesis by inducible, endothelial, and neuronal NO synthase (NOS). Here, we provide evidence for the participation of AS and of the NO-citrulline cycle in the progress of differentiation of neural stem cells (NSC) into neurons, astrocytes, and oligodendrocytes. AS expression and activity and neuronal NOS expression, as well as l-arginine and NOx production, increased along neural differentiation, whereas endothelial NOS expression was augmented in conditions of chronic NOS inhibition during differentiation, indicating that this NOS isoform is amenable to modulation by extracellular cues. AS and NOS inhibition caused a delay in the progress of neural differentiation, as suggested by the decreased percentage of terminally differentiated cells. On the other hand, BDNF reversed the delay of neural differentiation of NSC caused by inhibition of NOx production. A likely cause is the lack of NO, which up-regulated p75 neurotrophin receptor expression, a receptor required for BDNF-induced differentiation of NSC. We conclude that the NO-citrulline cycle acts together with BDNF for maintaining the progress of neural differentiation. Background: NO and BDNF are responsible for numerous functions in the CNS; however, joint actions exerted by these factors have not been studied. Results: BDNF reversed the block on neural differentiation caused by insufficient NO signaling. Conclusion: The NO-citrulline cycle and BDNF through up-regulation of p75 expression interact for restoring normal NO signaling and promoting neural differentiation. Significance: New insights are provided for BDNF and NO-citrulline cycle actions in neurogenesis.