Ionizing radiation induces neuronal differentiation of Neuro-2a cells via PI3-kinase and p53-dependent pathways

AOP report: Development of an adverse outcome pathway for deposition of energy leading to learning and memory impairment

Understanding radiation-induced non-cancer effects on the central nervous system (CNS) is essential for the risk assessment of medical (e.g., radiotherapy) and occupational (e.g., nuclear workers and astronauts) exposures. Herein, the adverse outcome pathway (AOP) approach was used to consolidate relevant studies in the area of cognitive decline for identification of research gaps, countermeasure development, and for eventual use in risk assessments. AOPs are an analytical construct describing critical events to an adverse outcome (AO) in a simplified form beginning with a molecular initiating event (MIE). An AOP was constructed utilizing mechanistic information to build empirical support for the key event relationships (KERs) between the MIE of deposition of energy to the AO of learning and memory impairment through multiple key events (KEs). The evidence for the AOP was acquired through a documented scoping review of the literature. In this AOP, the MIE is connected to the AO via six KEs: increased oxidative stress, increased deoxyribonucleic acid (DNA) strand breaks, altered stress response signaling, tissue resident cell activation, increased pro-inflammatory mediators, and abnormal neural remodeling that encompasses atypical structural and functional alterations of neural cells and surrounding environment. Deposition of energy directly leads to oxidative stress, increased DNA strand breaks, an increase of pro-inflammatory mediators and tissue resident cell activation. These KEs, which are themselves interconnected, can lead to abnormal neural remodeling impacting learning and memory processes. Identified knowledge gaps include improving quantitative understanding of the AOP across several KERs and additional testing of proposed modulating factors through experimental work. Broadly, it is envisioned that the outcome of these efforts could be extended to other cognitive disorders and complement ongoing work by international radiation governing bodies in their review of the system of radiological protection.

Radiotherapy Side Effects: Comprehensive Proteomic Study Unraveled Neural Stem Cell Degenerative Differentiation upon Ionizing Radiation

Cranial radiation therapy is one of the most effective treatments for childhood brain cancers. Despite the ameliorated survival rate of juvenile patients, radiation exposure-induced brain neurogenic region injury could markedly impair patients' cognitive functions and even their quality of life. Determining the mechanism underlying neural stem cells (NSCs) response to irradiation stress is a crucial therapeutic strategy for cognitive impairment. The present study demonstrated that X-ray irradiation arrested NSCs' cell cycle and impacted cell differentiation. To further characterize irradiation-induced molecular alterations in NSCs, two-dimensional high-resolution mass spectrometry-based quantitative proteomics analyses were conducted to explore the mechanism underlying ionizing radiation's influence on stem cell differentiation. We observed that ionizing radiation suppressed intracellular protein transport, neuron projection development, etc., particularly in differentiated cells. Redox proteomics was performed for the quantification of cysteine thiol modifications in order to profile the oxidation-reduction status of proteins in stem cells that underwent ionizing radiation treatment. Via conjoint screening of protein expression abundance and redox status datasets, several significantly expressed and oxidized proteins were identified in differentiating NSCs subjected to X-ray irradiation. Among these proteins, succinate dehydrogenase [ubiquinone] flavoprotein subunit, mitochondrial (sdha) and the acyl carrier protein, mitochondrial (Ndufab1) were highly related to neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and Huntington's disease, illustrating the dual-character of NSCs in cell differentiation: following exposure to ionizing radiation, the normal differentiation of NSCs was compromised, and the upregulated oxidized proteins implied a degenerative differentiation trajectory. These findings could be integrated into research on neurodegenerative diseases and future preventive strategies.

Amelioration of Radiation-Induced Cell Death in Neuro2a Cells by Neutralizing Oxidative Stress and Reducing Mitochondrial Dysfunction Using N-Acetyl-L-Tryptophan

The deleterious effects of ionizing radiation on the central nervous system (CNS) are poorly understood. Radiation exposure during an accidental nuclear explosion, nuclear war, or radiotherapy causes severe brain damage. As a result, the current work is carried out to assess the radioprotective potential of N-acetyl-L-tryptophan (L-NAT) in neuronal cells. Radiation-induced cell death and its amelioration by L-NAT pretreatment were investigated using MTT, SRB, CFU, and comet assays. Flow cytometric and microscopic fluorescence assays were used to investigate radiation-induced oxidative stress, alteration in mitochondrial redox, Ca2+ homeostasis, depolarization of mitochondrial membrane potential, and its prevention with L-NAT pretreatment. Western blot analysis of Caspase-3, γ-H2aX, p53, ERK-1/2, and p-ERK-1/2 expression was carried out to identify the effects of L-NAT pretreatment on radiation-induced apoptosis and its regulatory proteins expression. The study demonstrated (MTT, SRB, and CFU assay) significant (~80%; p <0.001%) radioprotection in irradiated (LD50 IR dose) Neuro2a cells that were pretreated with L-NAT. In comparison to irradiated cells, L-NAT pretreatment resulted in significant (p <0.001%) DNA protection. A subsequent study revealed that L-NAT pretreatment of irradiated Neuro2a cells establishes oxidative stress by increasing antioxidant enzymes and mitochondrial redox homeostasis by inhibiting Ca2+ migration from the cytoplasm to the mitochondrial matrix and thus protects the mitochondrial membrane hyperpolarization. Caspase-3 and γ-H2aX protein expression decreased, while p-ERK1/2 and p53 expression increased in L-NAT pretreated irradiated cells compared to irradiated cells. Hence, L-NAT could be a potential radioprotective that may inhibit oxidative stress and DNA damage and maintain mitochondrial health and Ca2+ levels by activating p-ERK1/2 and p53 expression in Neuronal cells.

Microglial TREM2 Mitigates Inflammatory Responses and Neuronal Apoptosis in Angiotensin II-Induced Hypertension in Middle-Aged Mice

Growing evidence suggests that hypertension and aging are prominent risk factors for the development of late-onset Alzheimer's disease (LOAD) by inducement of neuroinflammation. Recent study showed that neuroinflammation via activated microglia induces reactive astrocytes, termed A1 astrocytes, that highly upregulate numerous classical complement cascade genes that are destructive to neurons in neurodegeneration diseases. Moreover, triggering receptor expressed on myeloid cells 2 (TREM2) is considered as one of the strongest single-allele genetic risk factors and plays important roles in neuroinflammation for LOAD. However, the mechanisms of microglia in the regulation of A1 astrocytic activation are still not clear. We introduced angiotensin II-induced hypertension in middle-aged mice and found that hypertension-upregulated TREM2 expression and A1 astrocytic activation were involved in neuroinflammation in the animal models used in this study. The in vitro results revealed that overexpression of microglial TREM2 not only mitigated microglial inflammatory response but also had salutary effects on reverse A1 astrocytic activation and neuronal toxicity.

Pax3 inhibits Neuro-2a cells proliferation and neurite outgrowth

Pax3 and Pax7 are closely related transcription factors that are widely expressed in the developing nervous system and somites. During the normal development in the central nervous system (CNS), Pax3 and Pax7 are mainly expressed in the dorsal part of the neural tube. Further analysis revealed that Pax3 and Pax7 shared redundant functions in the spinal cord development. However, it is still unknown whether Pax3 and Pax7 play a role in neuronal differentiation. In this study, Pax3 and Pax7 genes were overexpressed in Neuro‐2a, the mouse neuroblastoma cell line. CCK‐8 and EdU assay results showed that overexpression of Pax3 inhibited cell viability and proliferation of Neuro‐2a cells, whereas the overexpression of Pax7 had no significant difference on their cell viability and proliferation. Overexpression of Pax3 not only increased the percentage of cells in the S phase and G0/G1 phase, but also decreased that in the G2 phase. Moreover, the total neurite lengths of Neuro‐2a cells were significantly shorter in Pax3 overexpressed group than those in negative control group and showed no significant difference between Pax7 overexpressed group and negative control group. These results suggested that Pax3 not only inhibited the cell viability and proliferation but also affected the cell cycle and the neurite outgrowth of Neuro‐2a cells. RNA sequencing analysis showed up‐regulated genes in Pax3 overexpressed group were involved in cell cycle machinery, which may reveal the potential mechanism of Neuro‐2a cells proliferation.

p53 regulates katanin-p60 promoter in HCT 116 cells

Tumor suppressor protein p53, which functions in the cell cycle, apoptosis and neuronal differentiation via transcriptional regulations of target genes or interactions with several proteins, has been associated with neurite outgrowth through microtubule re-organization. We previously demonstrated in neurons that upon p53 induction, the level of microtubule severing protein Katanin-p60 increases, indicating that p53 might be a transcriptional regulator of the KATNA1 gene encoding Katanin-p60. In this context, we firstly elucidated the activity of KATNA1 regulatory regions and endogenous KATNA1 mRNA levels in the presence or absence of p53 using HCT 116 WT and HCT 116 p53 (-/-) cells. Next, we demonstrated the binding of p53 to the KATNA1 promoter and then investigated the role of p53 on KATNA1 gene expression by ascertaining KATNA1 mRNA and Katanin-p60 protein levels upon p53 overexpression and activation in both cells. Moreover, we showed changes in microtubule network upon increased Katanin-p60 level due to p53 overexpression. Also, the changes in KATNA1 mRNA and Katanin-p60 protein levels upon p53 knockdown were investigated. Our results indicate that p53 is an activator of KATNA1 gene expression and we show that both p53 and Katanin-p60 expression have strict regulations and are maintained at balanced levels as they are vital proteins to orchestrate either survival and apoptosis or differentiation.

Inhibition of neurite outgrowth using commercial myelin associated glycoprotein-Fc in neuro-2a cells

Myelin-associated glycoprotein (MAG) inhibits the growth of neurites from nerve cells. Extraction and purification of MAG require complex operations; therefore, we attempted to determine whether commercially available MAG-Fc can replace endogenous MAG for research purposes. Immunofluorescence using specific antibodies against MAG, Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB) was used to determine whether MAG-Fc can be endocytosed by neuro-2a cells. In addition, neurite outgrowth of neuro-2a cells treated with different doses of MAG-Fc was evaluated. Enzyme linked immunosorbent assays were used to measure RhoA activity. Western blot assays were conducted to assess Rho-associated protein kinase (ROCK) phosphorylation. Neuro-2a cells expressed NgR and PirB, and MAG-Fc could be endocytosed by binding to NgR and PirB. This activated intracellular signaling pathways to increase RhoA activity and ROCK phosphorylation, ultimately inhibiting neurite outgrowth. These findings not only verify that MAG-Fc can inhibit the growth of neural neurites by activating RhoA signaling pathways, similarly to endogenous MAG, but also clearly demonstrate that commercial MAG-Fc is suitable for experimental studies of neurite outgrowth.

Electroacupuncture Restores Locomotor Functions After Mouse Spinal Cord Injury in Correlation With Reduction of PTEN and p53 Expression

Background: We previously showed that electroacupuncture (EA) at Jiaji points promotes expression of adhesion molecule L1 in spinal cord tissue after mouse spinal cord injury (SCI) and contributes to recovery of neural functions. Objective: We investigated the effects of EA on downstream signaling molecules of L1 and molecules relevant to apoptosis with the aim to understand the underlying molecular mechanisms. Methods: Female C57BL/6 mice were divided into a sham group, injury group, injury+acupuncture (AP) group and injury+EA group. We investigated the changes in cognate L1-triggered signaling molecules after SCI by immunofluorescence staining and immunoblot analysis. Results: Protein levels of phosphatase and tensin homolog (PTEN) and p53 were decreased by EA at different time points after injury, whereas the levels of phosphorylated mammalian target of rapamycin (pmTOR), p-Akt and phosphorylated extracellular signal-regulatedkinase (p-Erk) were increased. Also, levels of myelin basic protein (MBP) were increased by EA. AP alone showed less pronounced changes in expression of the investigated molecules, when compared to EA. Conclusion: We propose that EA contributes to neuroprotection by inhibiting PTEN and p53 expression and by increasing the levels of pmTOR/Akt/Erk and of MBP after SCI. These observations allow novel insights into the beneficial effects of EA via L1-triggered signaling molecules after injury.

Red (660 nm) or near-infrared (810 nm) photobiomodulation stimulates, while blue (415 nm), green (540 nm) light inhibits proliferation in human adipose-derived stem cells

We previously showed that blue (415 nm) and green (540 nm) wavelengths were more effective in stimulating osteoblast differentiation of human adipose-derived stem cells (hASC), compared to red (660 nm) and near-infrared (NIR, 810 nm). Intracellular calcium was higher after blue/green, and could be inhibited by the ion channel blocker, capsazepine. In the present study we asked what was the effect of these four wavelengths on proliferation of the hASC? When cultured in proliferation medium there was a clear difference between blue/green which inhibited proliferation and red/NIR which stimulated proliferation, all at 3 J/cm². Blue/green reduced cellular ATP, while red/NIR increased ATP in a biphasic manner. Blue/green produced a bigger increase in intracellular calcium and reactive oxygen species (ROS). Blue/green reduced mitochondrial membrane potential (MMP) and lowered intracellular pH, while red/NIR had the opposite effect. Transient receptor potential vanilloid 1 (TRPV1) ion channel was expressed in hADSC, and the TRPV1 ligand capsaicin (5uM) stimulated proliferation, which could be abrogated by capsazepine. The inhibition of proliferation caused by blue/green could also be abrogated by capsazepine, and by the antioxidant, N-acetylcysteine. The data suggest that blue/green light inhibits proliferation by activating TRPV1, and increasing calcium and ROS.

Rab GTPase signaling in neurite outgrowth and axon specification

Neurons are highly polarized cells that contain specialized subcellular domains involved in information transmission in the nervous system. Specifically, the somatodendritic compartment receives neuronal inputs while the axons convey information through the synapse. The establishment of asymmetric domains requires a specific delivery of components, including organelles, proteins, and membrane. The Rab family of small GTPases plays an essential role in membrane trafficking. Signaling cascades triggered by extrinsic and intrinsic factors tightly regulate Rab functions in cells, with Rab protein activation depending on GDP/GTP binding to establish a binary mode of action. This review summarizes the contributions of several Rab family members involved in trans-Golgi, early/late endosomes, and recycling endosomes during neurite development and axonal outgrowth. The regulation of some Rabs by guanine exchanging factors and GTPase activating proteins will also be addressed. Finally, discussion will be provided on how specific effector-mediated Rab activation modifies several molecules essential to neuronal differentiation. © 2016 Wiley Periodicals, Inc.

Regulation of Cancer Cell Behavior by the Small GTPase Rab13

The members of the Rab family of GTPases are master regulators of cellular membrane trafficking. With ∼70 members in humans, Rabs have been implicated in all steps of membrane trafficking ranging from vesicle formation and transport to vesicle docking/tethering and fusion. Vesicle trafficking controls the localization and levels of a myriad of proteins, thus regulating cellular functions including proliferation, metabolism, cell-cell adhesion, and cell migration. It is therefore not surprising that impairment of Rab pathways is associated with diseases including cancer. In this review, we highlight evidence supporting the role of Rab13 as a potent driver of cancer progression.

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

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.