Chia-Wei Lin

Luteolin (3',4',5,7-tetrahydroxyflavone), a food-derived flavonoid, has been reported to possess antioxidant, anti-inflammatory, and anticancer activities. In this work, we assessed whether luteolin has neurotrophic activity, namely, the ability to induce neurite outgrowth and to attenuate serum withdrawal-induced cytotoxicity in PC12 cells. Our results show that luteolin significantly induced neurite outgrowth along with increased expression of the differentiation marker, growth-associated protein-43 (GAP-43), in PC12 cells dose-dependently. Incubation of serum-deprived PC12 cells with luteolin prevented apoptosis, increased the expression of heme oxygenase-1 (HO-1) mRNA and protein levels, and enhanced the binding of nuclear factor E2-related factor 2 (Nrf2) to antioxidant response element (ARE), which works as an enhancer sequence in the HO-1 promoter. Addition of zinc protoporphyrin (Znpp), a selective HO-1 competitive inhibitor, significantly reduced the cytoprotective ability of luteolin, indicating the vital role of HO-1. Luteolin also persistently activated extracellular signal-regulated protein kinase 1/2 (ERK1/2); while the addition of U0126, a pharmacological MEK/ERK inhibitor, attenuated luteolin-induced Nrf2 binding activity, HO-1 expression, cytoprotective effect, and neurite outgrowth. Taken together, the above findings suggest that luteolin induces neurite outgrowth and augments cellular antioxidant defense capacity, at least in part, through the activation of the ERK signaling pathway.

During the process of integration into brain circuits, new neurons develop both input and output synapses with their appropriate targets. The vast majority of neurons in the mammalian brain are generated before birth and integrate into immature circuits while these are being assembled. In contrast, adult-generated neurons face an additional challenge as they integrate into a mature, fully functional circuit. Here, we examined how synapses of a single neuronal type, the granule cell in the olfactory bulb, develop during their integration into the immature circuit of the newborn and the fully mature circuit of the adult rat. We used a genetic method to label pre and postsynaptic sites in granule neurons and observed a stereotypical development of synapses in specific dendritic domains. In adult-generated neurons, synapses appeared sequentially in different dendritic domains with glutamatergic input synapses that developed first at the proximal dendritic domain, followed several days later by the development of input-output synapses in the distal domain and additional input synapses in the basal domain. In contrast, for neurons generated in neonatal animals, input and input-output synapses appeared simultaneously in the proximal and distal domains, respectively, followed by the later appearance of input synapses to the basal domain. The sequential formation of synapses in adult-born neurons, with input synapses appearing before output synapses, may represent a cellular mechanism to minimize the disruption caused by the integration of new neurons into a mature circuit in the adult brain.

New neurons integrate in large numbers into the mature olfactory bulb circuit throughout life. The factors controlling the synaptic development of adult-born neurons and their connectivity remain essentially unknown. We examined the role of activity-dependent mechanisms in the synaptic development of adult-born neurons by genetic labeling of synapses while manipulating sensory input or cell-intrinsic excitability. Sensory deprivation induced marked changes in the density of input and output synapses during the period when new neurons develop most of their synapses. In contrast, when sensory deprivation started after synaptic formation was complete, input synapses increased in one domain without detectable changes in the other dendritic domains. We then investigated the effects of genetically raising the intrinsic excitability of new neurons on their synaptic development by delivering a voltage-gated sodium channel that triggers long depolarizations. Surprisingly, genetically increasing excitability did not affect synaptic development but rescued the changes in glutamatergic input synapses caused by sensory deprivation. These experiments show that, during adult neurogenesis in the olfactory bulb, synaptic plasticity is primarily restricted to an early period during the maturation of new neurons when they are still forming synapses. The addition of cells endowed with such an initial short-lived flexibility and long-term stability may enable the processing of information by the olfactory bulb to be both versatile and reliable in the face of changing behavioral demands.