Mitochondrial dysfunction is implicated in age‐related degenerative disorders such as Alzheimer's disease (AD). Maintenance of mitochondrial dynamics is essential for regulating mitochondrial function. Aβ oligomers (AβOs), the typical cause of AD, lead to mitochondrial dysfunction and neuronal loss. AβOs have been shown to induce mitochondrial fragmentation, and their inhibition suppresses mitochondrial dysfunction and neuronal cell death. Oxidative stress is one of the earliest hallmarks of AD. Cyclin‐dependent kinase 5 (Cdk5) may cause oxidative stress by disrupting the antioxidant system, including Prx2. Cdk5 is also regarded as a modulator of mitochondrial fission; however, a precise mechanistic link between Cdk5 and mitochondrial dynamics is lacking. We estimated mitochondrial morphology and alterations in mitochondrial morphology‐related proteins in Neuro‐2a (N2a) cells stably expressing the Swedish mutation of amyloid precursor protein (APP), which is known to increase AβO production. We demonstrated that mitochondrial fragmentation by AβOs accompanies reduced mitofusin 1 and 2 (Mfn1/2) levels. Interestingly, the Cdk5 pathway, including phosphorylation of the Prx2‐related oxidative stress, has been shown to regulate Mfn1 and Mfn2 levels. Furthermore, Mfn2, but not Mfn1, over‐expression significantly inhibits the AβO‐mediated cell death pathway. Therefore, these results indicate that AβO‐mediated oxidative stress triggers mitochondrial fragmentation via decreased Mfn2 expression by activating Cdk5‐induced Prx2 phosphorylation.
Serotonin (5‐hydroxytryptamine, 5‐HT) has been implicated to play critical roles in early neural development. Recent reports have suggested that perinatal exposure to selective serotonin reuptake inhibitors (SSRIs) resulted in cortical network miswiring, abnormal social behavior, callosal myelin malformation, as well as oligodendrocyte (OL) pathology in rats. To gain further insight into the cellular and molecular mechanisms underlying SSRIs‐induced OL and myelin abnormalities, we investigated the effect of 5‐HT exposure on OL development, cell death, and myelination in cell culture models. First, we showed that 5‐HT receptor 1A and 2A subtypes were expressed in OL lineages, using immunocytochemistry, Western blot, as well as intracellular Ca2+ measurement. We then assessed the effect of serotonin exposure on the lineage development, expression of myelin proteins, cell death, and myelination, in purified OL and neuron‐OL myelination cultures. For pure OL cultures, our results showed that 5‐HT exposure led to disturbance of OL development, as indicated by aberrant process outgrowth and reduced myelin proteins expression. At higher doses, such exposure triggered a development‐dependent cell death, as immature OLs exhibited increasing susceptibility to 5‐HT treatment compared to OL progenitor cells (OPC). We showed further that 5‐HT‐induced immature OL death was mediated at least partially via 5‐HT2A receptor, since cell death could be mimicked by 5‐HT2A receptor agonist 1‐(2,5‐dimethoxy‐4‐iodophenyl)‐2‐aminopropane hydrochloride, (±)‐2,5‐dimethoxy‐4‐iodoamphetamine hydrochloride, but atten‐uated by pre‐treatment with 5‐HT2A receptor antagonist ritanserin. Utilizing a neuron‐OL myelination co‐culture model, our data showed that 5‐HT exposure significantly reduced the number of myelinated internodes. In contrast to cell injury observed in pure OL cultures, 5‐HT exposure did not lead to OL death or reduced OL density in neuron‐OL co‐cultures. However, abnormal patterns of contactin‐associated protein (Caspr) clustering were observed at the sites of Node of Ranvier, suggesting that 5‐HT exposure may affect other axon‐derived factors for myelination. In summary, this is the first study to demonstrate that manipulation of serotonin levels affects OL development and myelination, which may contribute to altered neural connectivity noted in SSRIs‐treated animals.
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