Induction of Excess Centrosomes in Neural Progenitor Cells during the Development of Radiation-Induced Microcephaly

Dimethyl Sulfoxide Attenuates Ionizing Radiation-induced Centrosome Overduplication and Multipolar Cell Division in Human Induced Pluripotent Stem Cells

Centrosomes are important organelles for cell division and genome stability. Ionizing radiation exposure efficiently induces centrosome overduplication via the disconnection of the cell and centrosome duplication cycles. Over duplicated centrosomes cause mitotic catastrophe or chromosome aberrations, leading to cell death or tumorigenesis. Pluripotent stem cells, including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), can differentiate into all organs. To maintain pluripotency, PSCs show specific cellular dynamics, such as a short G1 phase and silenced cell-cycle checkpoints for high cellular proliferation. However, how exogenous DNA damage affects cell cycle-dependent centrosome number regulation in PSCs remains unknown. This study used human iPSCs (hiPSCs) derived from primary skin fibroblasts as a PSC model to address this question. hiPSCs derived from somatic cells could be a useful tool for addressing the radiation response in cell lineage differentiation. After radiation exposure, the hiPSCs showed a higher frequency of centrosome overduplication and multipolar cell division than the differentiated cells. To suppress the indirect effect of radiation exposure, we used the radical scavenger dimethyl sulfoxide (DMSO). Combined treatment with radiation and DMSO efficiently suppressed DNA damage and centrosome overduplication in hiPSCs. Our results will contribute to the understanding of the dynamics of stem cells and the assessment of the risk of genome instability for regenerative medicine.

DNA damage and repair: underlying mechanisms leading to microcephaly

DNA-damaging agents and endogenous DNA damage constantly harm genome integrity. Under genotoxic stress conditions, the DNA damage response (DDR) machinery is crucial in repairing lesions and preventing mutations in the basic structure of the DNA. Different repair pathways are implicated in the resolution of such lesions. For instance, the non-homologous DNA end joining and homologous recombination pathways are central cellular mechanisms by which eukaryotic cells maintain genome integrity. However, defects in these pathways are often associated with neurological disorders, indicating the pivotal role of DDR in normal brain development. Moreover, the brain is the most sensitive organ affected by DNA-damaging agents compared to other tissues during the prenatal period. The accumulation of lesions is believed to induce cell death, reduce proliferation and premature differentiation of neural stem and progenitor cells, and reduce brain size (microcephaly). Microcephaly is mainly caused by genetic mutations, especially genes encoding proteins involved in centrosomes and DNA repair pathways. However, it can also be induced by exposure to ionizing radiation and intrauterine infections such as the Zika virus. This review explains mammalian cortical development and the major DNA repair pathways that may lead to microcephaly when impaired. Next, we discuss the mechanisms and possible exposures leading to DNA damage and p53 hyperactivation culminating in microcephaly.

Development of an Adverse Outcome Pathway for radiation-induced microcephaly via expert consultation and machine learning

BACKGROUND

Brain development during embryogenesis and in early postnatal life is particularly complex and involves the interplay of many cellular processes and molecular mechanisms, making it extremely vulnerable to exogenous insults, including ionizing radiation (IR). Microcephaly is one of the most frequent neurodevelopmental abnormalities that is characterized by small brain size, and is often associated with intellectual deficiency. Decades of research span from epidemiological data on in utero exposure of the A-bomb survivors, to studies on animal and cellular models that allowed deciphering the most prominent molecular mechanisms leading to microcephaly. The Adverse Outcome Pathway (AOP) framework is used to organize, evaluate and portray the scientific knowledge of toxicological effects spanning different biological levels of organizations, from the initial interaction with molecular targets to the occurrence of a disease or adversity. In the present study, the framework was used in an attempt to organize the current scientific knowledge on microcephaly progression in the context of ionizing radiation (IR) exposure. This work was performed by a group of experts formed during a recent workshop organized jointly by the Multidisciplinary European Low Dose Initiative (MELODI) and the European Radioecology Alliance (ALLIANCE) associations to present the AOP approach and tools. Here we report on the development of a putative AOP for congenital microcephaly resulting from IR exposure based on discussions of the working group and we emphasize the use of a novel machine-learning approach to assist in the screening of the available literature to develop AOPs.

CONCLUSION

The expert consultation led to the identification of crucial biological events for the progression of microcephaly upon exposure to IR, and highlighted current knowledge gaps. The machine learning approach was successfully used to screen the existing knowledge and helped to rapidly screen the body of evidence and in particular the epidemiological data. This systematic review approach also ensured that the analysis was sufficiently comprehensive to identify the most relevant data and facilitate rapid and consistent AOP development. We anticipate that as machine learning approaches become more user-friendly through easy-to-use web interface, this would allow AOP development to become more efficient and less time consuming.

Puma, noxa, p53, and p63 differentially mediate stress pathway induced apoptosis

Cellular stress can lead to several human disease pathologies due to aberrant cell death. The p53 family (tp53, tp63, and tp73) and downstream transcriptional apoptotic target genes (PUMA/BBC3 and NOXA/PMAIP1) have been implicated as mediators of stress signals. To evaluate the importance of key stress response components in vivo, we have generated zebrafish null alleles in puma, noxa, p53, p63, and p73. Utilizing these genetic mutants, we have deciphered that the apoptotic response to genotoxic stress requires p53 and puma, but not p63, p73, or noxa. We also identified a delayed secondary wave of genotoxic stress-induced apoptosis that is p53/puma independent. Contrary to genotoxic stress, ER stress-induced apoptosis requires p63 and puma, but not p53, p73, or noxa. Lastly, the oxidative stress-induced apoptotic response requires p63, and both noxa and puma. Our data also indicate that while the neural tube is poised for apoptosis due to genotoxic stress, the epidermis is poised for apoptosis due to ER and oxidative stress. These data indicate there are convergent as well as unique molecular pathways involved in the different stress responses. The commonality of puma in these stress pathways, and the lack of gross or tumorigenic phenotypes with puma loss suggest that a inhibitor of Puma may have therapeutic application. In addition, we have also generated a knockout of the negative regulator of p53, mdm2 to further evaluate the p53-induced apoptosis. Our data indicate that the p53 null allele completely rescues the mdm2 null lethality, while the puma null completely rescues the mdm2 null apoptosis but only partially rescues the phenotype. Indicating Puma is the key mediator of p53-dependent apoptosis. Interestingly the p53 homozygous null zebrafish develop tumors faster than the previously described p53 homozygous missense mutant zebrafish, suggesting the missense allele may be hypomorphic allele.

Collimated Microbeam Reveals that the Proportion of Non-Damaged Cells in Irradiated Blastoderm Determines the Success of Development in Medaka (Oryzias latipes) Embryos

It has been widely accepted that prenatal exposure to ionizing radiation (IR) can affect embryonic and fetal development in mammals, depending on dose and gestational age of the exposure, however, the precise machinery underlying the IR-induced disturbance of embryonic development is still remained elusive. In this study, we examined the effects of gamma-ray irradiation on blastula embryos of medaka and found transient delay of brain development even when they hatched normally with low dose irradiation (2 and 5 Gy). In contrast, irradiation of higher dose of gamma-rays (10 Gy) killed the embryos with malformations before hatching. We then conducted targeted irradiation of blastoderm with a collimated carbon-ion microbeam. When a part (about 4, 10 and 25%) of blastoderm cells were injured by lethal dose (50 Gy) of carbon-ion microbeam irradiation, loss of about 10% or less of blastoderm cells induced only the transient delay of brain development and the embryos hatched normally, whereas embryos with about 25% of their blastoderm cells were irradiated stopped development at neurula stage and died. These findings strongly suggest that the developmental disturbance in the IR irradiated embryos is determined by the proportion of severely injured cells in the blastoderm.

Ionizing radiation reduces larval brain size by inducing premature differentiation of Drosophila neural stem cells

DNA damaging agents, such as ionizing radiation (IR), induce cell cycle arrest, senescence, differentiation, or cell death of stem cells, which may affect tissue homeostasis. The specific response of stem cells upon irradiation seems to vary depending on the cell type and their developmental stages. Drosophila larval brain contains neural stem cells called neuroblasts (NBs) and maintaining an appropriate number of NBs is critical to maintain brain size. Irradiation of larvae at early larval stage results in microcephaly, whereas the DNA damage response of NBs that could explain this small brain size is not clearly understood. We observed that the irradiation of larvae in the second instar retarded brain growth, accompanied by fewer NBs. The IR-induced microcephaly does not seem to result from apoptosis since the irradiated larval brain was not stained with activated Caspase nor was the microcephaly affected by the ectopic expression of the apoptosis inhibitor. When analyzed for the percentage of mitotic cells, irradiated NBs recovered their proliferative potential within 6 h post-irradiation after transient cell cycle arrest. However, IR eventually reduced the proliferation of NBs at later time points and induced the premature differentiation of NBs. In summary, IR-induced microcephaly occurs by NB loss due to premature differentiation, rather than apoptotic cell death.

Reprogramming and differentiation-dependent transcriptional alteration of DNA damage response and apoptosis genes in human induced pluripotent stem cells

Pluripotent stem cells (PSCs), such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), have a dual capability to self-renew and differentiate into all cell types necessary to develop an entire organism. Differentiation is associated with dynamic epigenetic alteration and transcriptional change, while self-renewal depends on maintaining the genome DNA accurately. Genome stability of PSCs is strictly regulated to maintain pluripotency. However, the DNA damage response (DDR) mechanism in PSCs is still unclear. There is accumulating evidence that genome stability and pluripotency are regulated by a transcriptional change in undifferentiated and differentiated states. iPSCs are ideal for analyzing transcriptional regulation during reprogramming and differentiation. This study aimed to elucidate the transcriptional alteration surrounding genome stability maintenance, including DNA repair, cell cycle checkpoints and apoptosis in fibroblasts, iPSCs and neural progenitor cells (NPCs) derived from iPSCs as differentiated cells. After ionizing radiation exposure, foci for the DNA double-stranded break marker γ-H2AX increased, peaking at 0.5 h in all cells (>90%), decreasing after 4 h in fibroblasts (32.3%) and NPCs (22.3%), but still remaining at 52.5% (NB1RGB C2 clone) and 54.7% (201B7 cells) in iPSCs. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells were detected, indicating that iPSCs' apoptosis increases. In addition, RNA sequencing (RNA-Seq) analysis showed high expression of apoptosis genes (TP53, CASP3 and BID) in iPSCs. Results suggested that increased apoptosis activity maintains accurate, undifferentiated genome DNA in the cell population.

Counter-Balance Between Gli3 and miR-7 Is Required for Proper Morphogenesis and Size Control of the Mouse Brain

Brain morphogenesis requires precise regulation of multiple genes to control specification of distinct neural progenitors (NPs) and neuronal production. Dysregulation of these genes results in severe brain malformation such as macrocephaly and microcephaly. Despite studies of the effect of individual pathogenic genes, the counter-balance between multiple factors in controlling brain size remains unclear. Here we show that cortical deletion of Gli3 results in enlarged brain and folding structures in the cortical midline at the postnatal stage, which is mainly caused by the increased percentage of intermediate progenitors (IPs) and newborn neurons. In addition, dysregulation of neuronal migration also contributes to the folding defects in the cortical midline region. Knockdown of microRNA (miRNA) miR-7 can rescue abnormal brain morphology in Gli3 knockout mice by recovering progenitor specification, neuronal production and migration through a counter-balance of the Gli3 activity. Moreover, miR-7 likely exerts its function through silencing target gene Pax6. Our results indicate that proper brain morphogenesis is an outcome of interactive regulations of multiple molecules such as Gli3 and miR-7. Because miRNAs are easy to synthesize and deliver, miR-7 could be a potential therapeutic means to macrocephaly caused by Gli3-deficiency.

Microcephaly

Microcephaly is defined as a head circumference more than two standard deviations below the mean for gender and age. Congenital microcephaly is present at birth, whereas postnatal microcephaly occurs later in life. Genetic abnormalities, syndromes, metabolic disorders, teratogens, infections, prenatal, perinatal, and postnatal injuries can cause both congenital and postnatal microcephaly. Evaluation of patients with microcephaly begins with a thorough history and physical examination. In cases of worsening microcephaly or neurological signs or symptoms, neuroimaging, metabolic, or genetic testing should be strongly considered. Any further studies and workup should be directed by the presence of signs or symptoms pointing to an underlying diagnosis and are usually used as confirmatory testing for certain conditions. Neuroimaging with magnetic resonance imaging (MRI) is often the first diagnostic test in evaluating children with microcephaly. Genetic testing is becoming more common and is often the next step following neuroimaging when there is no specific evidence in the history or physical examination suggesting a diagnosis. Microcephaly is a lifelong condition with no known cure. The prognosis is usually worse for children who experienced an intrauterine infection or have a chromosomal or metabolic abnormality. Zika virus has rapidly spread since 2015, and maternal infection with this virus is associated with microcephaly and other serious brain abnormalities. Microcephaly has become much more prevalent in the news and scientific community with the recent emergence of Zika virus as a cause of congenital microcephaly.