Ionizing Radiation Induces Altered Neuronal Differentiation by mGluR1 through PI3K-STAT3 Signaling in C17.2 Mouse Neural Stem-Like Cells

Abstract

Most studies of IR effects on neural cells and tissues in the brain are still focused on loss of neural stem cells. On the other hand, the effects of IR on neuronal differentiation and its implication in IR-induced brain damage are not well defined. To investigate the effects of IR on C17.2 mouse neural stem-like cells and mouse primary neural stem cells, neurite outgrowth and expression of neuronal markers and neuronal function-related genes were examined. To understand this process, the signaling pathways including PI3K, STAT3, metabotrophic glutamate receptor 1 (mGluR1) and p53 were investigated. In C17.2 cells, irradiation significantly increased the neurite outgrowth, a morphological hallmark of neuronal differentiation, in a dose-dependent manner. Also, the expression levels of neuronal marker proteins, β-III tubulin were increased by IR. To investigate whether IR-induced differentiation is normal, the expression of neuronal function-related genes including synaptophysin, a synaptic vesicle forming proteins, synaptotagmin1, a calcium ion sensor, γ-aminobutyric acid (GABA) receptors, inhibitory neurotransmitter receptors and glutamate receptors, excitatory neurotransmitter receptors was examined and compared to that of neurotrophin-stimulated differentiation. IR increased the expression of synaptophysin, synaptotagmin1 and GABA receptors mRNA similarly to normal differentiation by stimulation of neurotrophin. Interestingly, the overall expression of glutamate receptors was significantly higher in irradiated group than normal differentiation group, suggesting that the IR-induced neuronal differentiation may cause altered neuronal function in C17.2 cells. Next, the molecular mechanism of the altered neuronal differentiation induced by IR was studied by investigating signaling pathways including p53, mGluR1, STAT3 and PI3K. Increases of neurite outgrowth, neuronal marker and neuronal function-related gene expressions by IR were abolished by inhibition of p53, mGluR-1, STAT3 or PI3K. The inhibition of PI3K blocked both p53 signaling and STAT3-mGluR1 signaling but inhibition of p53 did not affect STAT3-mGluR1 signaling in irradiated C17.2 cells. Finally, these results of the IR-induced altered differentiation in C17.2 cells were verified in ex vivo experiments using mouse primary neural stem cells. In conclusion, the results of this study demonstrated that IR is able to trigger the altered neuronal differentiation in undifferentiated neural stem-like cells through PI3K-STAT3-mGluR1 and PI3K-p53 signaling. It is suggested that the IR-induced altered neuronal differentiation may play a role in the brain dysfunction caused by IR.

p53 drives premature neuronal differentiation in response to radiation-induced DNA damage during early neurogenesis

“…A potential role of p53 in radiation-induced neuronal differentiation had previously also been suggested based on in vitro experiments 37,38 . We confirmed these results, and showed that irradiation of primary mouse NPCs (with 1 Gy of X-rays) diminished proliferation and induced differentiation from bipolar cells into multipolar neurons (Fig.…”

“…Radiation-induced neuronal differentiation has been demonstrated in vitro 37,38,57,58 although the underlying mechanisms remained largely unclear. To our knowledge, this is the first study demonstrating the activation of a p53-dependent transcriptional program as a mechanism to induce neuronal differentiation in vivo , although a role for p53 in cellular differentiation in vivo has been previously proposed in mammary stem cells, the airway epithelium and cancer stem cells 6 .…”

Chronic irradiation with low-dose-rate 137Cs-γ rays inhibits NGF-induced neurite extension of PC12 cells via Ca2+/calmodulin-dependent kinase II activation

“…Previously, it was reported that a transient irradiation with X-rays (dose rate: 0.1 Gy/min, total dose: 500 mGy) promoted NGF-induced neurite extension in PC12 cells [ 3 ]. Similarly, low-dose-rate (0.95 Gy/min, total dose: 6 Gy) 137 Cs-γ radiation facilitated neurite extension in neural stem–like cells [ 4 ]. The cause of the difference between the results of promoting neuronal differentiation in the above-mentioned reports and the results of suppression of neuronal differentiation obtained in reports included in this study is not quite clear.…”

“…The cause of the difference between the results of promoting neuronal differentiation in the above-mentioned reports and the results of suppression of neuronal differentiation obtained in reports included in this study is not quite clear. The total irradiation doses used in studies on irradiation-induced inhibition of neurite extension, including the present study, range from 0.01 to 2.1 Gy [ 1 , 2 ], overlapping with those used in studies on the promotion of neurite extension, which range from 0.1 to 6 Gy [ 3 , 4 ]. In contrast, irradiation dose rates used in reports on the irradiation-induced inhibition of neurite extension range from 0.4 to 15 mGy/h [ 1 , 2 ], whereas those used in reports on the promotion of neurite extension range from 5 to 57 Gy/h [ 3 , 4 ].…”

“…In contrast, we have previously reported that transient irradiation with X-rays promotes NGF-induced neurite extension in PC12 cells [ 3 ]. Similarly, low-dose-rate 137 Cs-γ radiation has been reported to facilitate neurite extension in mouse neural stem–like cells [ 4 ]. These findings suggest that low-dose or low-dose-rate radiation have substantial impacts on the differentiation of neural cells and may produce completely opposite effects, according to dose amounts or dose rates.…”