Cell death in normal and abnormal development

Malformations and mortality in zebrafish early stages associated with elevated caspase activity after 24 h exposure to MS-222

Tricaine methanesulfonate (MS-222) is a commonly used anaesthetic agent for immobilization of aquatic species. However, delayed development and malformations have been observed in 24 hpf (hours post-fertilization) zebrafish embryos after long-term immobilization. Still, no comprehensive study has been described regarding zebrafish exposure to MS-222 during the first hours of development, which are one of the most sensitive life stages to toxicants. Therefore, this research aimed to assess the toxicity of a 24 h exposure to MS-222 on zebrafish embryonic development. Based on the MS-222 LC₅₀, early blastula stage embryos (~2 hpf) were exposed to 0, 12.5, 25 and 50 mg L^-1 for 24 h and then allowed to develop up to 144 hpf. The chromatographic analysis showed that this anaesthetic agent bioaccumulates in 26 hpf zebrafish larvae in a concentration-dependent manner. In addition, increased mortalities and skeletal abnormalities were observed at 144 hpf, namely in the highest tested concentration. Yet, no craniofacial anomalies were observed either by alcian blue or calcein staining methods. Independently of the tested concentration, decreased speed and distance travelled were perceived in 144 hpf larvae. At the biochemical level, decreased in vivo reactive oxygen species (ROS) generation and apoptosis was observed. Additionally, catalase activity was increased at 26 hpf while results of mRNA expression showed a decreased gclc transcript content at the same time-point. Overall, data obtained highlight the toxicological risk of MS-222 and support ROS-mediated cell death signalling changes through the elevation of catalase activity as an adaptative or protective response.

The role of Trp53 in the mouse embryonic response to DNA damage

Apoptosis occurs primarily in the blastocyst inner cell mass, cells of which go on to form the foetus. Apoptosis is likely to play a role in ensuring the genetic integrity of the foetus, yet little is known about its regulation. In this study, the role of the mouse gene, transformation-related protein 53 (Trp53) in the response of embryos to in vitro culture and environmentally induced DNA damage was investigated using embryos from a Trp53 knockout mouse model. In vivo-derived blastocysts were compared to control embryos X-irradiated at the two-cell stage and cultured to Day 5. An analysis of DNA by comet assay demonstrated that 1.5 Gy X-irradiation directly induced damage in cultured two-cell mouse embryos; this was correlated with retarded development to blastocyst stage and increased apoptosis at the blastocyst stage but not prior to this. Trp53 null embryos developed to blastocysts at a higher frequency and with higher cell numbers than wild-type embryos. Trp53 also mediates apoptosis in conditions of low levels of DNA damage, in vivo or in vitro in the absence of irradiation. However, following DNA damage induced by X-irradiation, apoptosis is induced by Trp53 independent as well as dependent mechanisms. These data suggest that Trp53 and apoptosis play important roles in normal mouse embryonic development both in vitro and in vivo and in response to DNA damage. Therefore, clinical ART practices that alter apoptosis in human embryos and/or select embryos for transfer, which potentially lack a functional Trp53 gene, need to be carefully considered.

MicroRNA-34a is dispensable for p53 function as teratogenesis inducer

The tumor suppressor protein p53 is a powerful regulator of the embryo's susceptibility to diverse teratogenic stimuli, functioning both as a teratogenesis inducer and suppressor. However, the targets that p53 engages to fulfill its functions remain largely undefined. We asked whether the microRNA (miRNA) miR-34 family, identified as one of the main targets of p53, mediates its function as a teratogenesis inducer. For this, pregnant ICR-, p53- and miR-34a-deficient mice, as well as rats, were exposed to 5-aza-2'-deoxycytidine (5-aza), a teratogen inducing limb reduction anomalies (LRA) of the hindlimbs in mice and either the hindlimbs or forelimbs in rats. Using hind- and forelimb buds of 5-aza-exposed embryos, we identified that the miR-34 family members are the most upregulated miRNAs in mouse and rat limb buds, with their increase level being significantly higher in limb buds destined for LRA. We showed that p53 mediates the 5-aza-induced miR-34 transcription followed by met proto-oncogene and growth-arrest-specific 1 target suppression in embryonic limb buds. We demonstrated that p53 regulates the teratogenic response to 5-aza acting as a teratogenesis inducer albeit miR-34a deletion does not affect the susceptibility of mice to 5-aza. Overall, our study thoroughly characterizes the expression and regulation of miR-34 family in teratogen-resistant and teratogen-sensitive embryonic structures and discusses the involvement of epigenetic miRNA-mediated pathway(s) in induced teratogenesis.

Normal location of thumb/big toe may be related to programmed cell death in the preaxial area of embryonic limb

The forelimbs and hindlimbs of newborn Polydactyly Nagoya (Pdn) mice were examined to analyze the roles of programmed cell death (PCD) in the preaxial region of the limb. Special attention was paid to the relationship between the PCD in the preaxial area and the location and shape of the first digit (thumb/big toe). Although a large, bifurcated or duplicated thumb/big toe appeared in Pdn/+ mice, digit I (thumb/big toe) in Pdn/+ mice, as in +/+ ones, was located more ventro-proximally than the other four digits. On the other hand, abnormal preaxial digits of the fore/hindlimb in Pdn/Pdn mice lay distally and were aligned at the radial/tibial end of a serial curved plane formed by digits II-V; that is, a thumb and big toe of normal shape and location were not detectable in any preaxial digits of Pdn/Pdn mice. In the limb development of Pdn mouse embryos on Day 11-12, PCD did not occur in the preaxial mesoderm of fore/hindlimb only in one-fourths of all embryos obtained by Pdn/+ x Pdn/+ mating. In addition to digital rays II-V, extra preaxial digital rays appeared in the prominent preaxial expansion of fore/hindlimbs in these embryos on early Day 12. These abnormal limb configurations in embryos were closely similar to those in Pdn/Pdn newborn mice. The present findings suggest that PCD in the preaxial region not only prevents the formation of extra digits but also determines the location of the thumb/big toe for the normal limb morphogenesis.

Mechanisms of the embryo's response to embryopathic stressors: a focus on p53

Whether the embryo develops normally or not depends not only on the mechanisms regulating embryonic development, but also on the mechanisms acting to resist and repair injures in the embryo due to harmful maternal stimuli or exposure to developmental toxicants. The key role of p53 in the regulation of the embryo's response to embryopathic stress inducing DNA damage is beyond doubt. Yet, the question why p53 in some cases acts as a suppressor of teratogenesis, whereas in other cases it induces teratogenesis, remains unanswered. In this minireview we analyze studies in which organogenesis-stage embryos were exposed to various developmental toxicants and suggest a model unifying the teratogenesis-suppressing and teratogenesis-promoting role of p53. This model predicts that p53 protects embryos from developmental toxicant inducing oxidative stress and promotes the process of maldevelopment induced by developmental toxicants activating apoptotic machinery. Certainly, many questions must be answered before concluding the extent to which this model is correct. Yet, it does allow us to explain some discrepancies obtained in studies performed to date. Also, the model might be useful in choosing molecular targets for further studies addressing p53-controlled and p53-independent mechanisms, which determine the embryo's resistance to embryopathic stress.

Teratogen-induced alterations in microRNA-34, microRNA-125b and microRNA-155 expression: correlation with embryonic p53 genotype and limb phenotype

Background

In a large number of studies, members of the microRNA (miRNA)-34 family such as miRNA-34a, miRNA-34b, miRNA-34c, as well as miRNA-125b and miRNA-155, have been shown to be regulators of apoptosis. The ability of these miRNAs to perform this function is mainly attributed to their ability to interact with the p53 tumor suppressor, which is a powerful regulator of the teratologic susceptibility of embryos. We chose to explore whether miRNA-34a/b/c, miRNA-125b and miRNA-155 may play a role in teratogenesis by using p53^+/- pregnant mice treated with cyclophosphamide (CP) as a model. We evaluated how CP-induced alterations in the expression of these miRNAs in the embryonic limbs correlate with embryonic p53 genotype and CP-induced limb phenotypes.

Results

The limbs of p53 positive embryos were more sensitive to CP-induced teratogenic insult than the limbs of p53 negative embryos. The hindlimbs were more severely affected than the forelimbs. Robust miRNA-34a expression was observed in the fore- and hindlimbs of p53^+/+ embryos exposed to 12.5 mg/kg CP. The dose of 20 mg/kg CP induced almost a two-fold increase in the level of miRNA-34a expression as compared to that exhibited by p53^+/+ embryos exposed to a lower dose. Increased miRNA-34b and miRNA-34c expression was also observed. Of note, this dose activated miRNA-34a and miRNA-34c in the forelimbs of p53^-/- embryos. When embryos were exposed to 40 mg/kg CP, the expression pattern of the miRNA-34a/b/c was identical to that registered in the limbs of embryos exposed to 20 mg/kg CP. However, this dose suppressed miRNA-125b and miRNA-155 expression in the fore- and hindlimbs of p53^+/+ embryos.

Conclusion

This study demonstrates that teratogen-induced limb dysmorphogenesis may be associated with alterations in miRNA-34, miRNA-125b and miRNA-155 expression. It also suggests for the first time that p53-independent mechanisms exist contributing to teratogen-induced activation of miRNA-34a and miRNA-34c. At the same time, teratogen-induced suppression of miRNA-125b and miRNA-155 expression may be p53 dependent. The analysis of correlations between the expression pattern of the tested miRNAs and CP induced limb phenotypes implies that miRNAs regulating apoptosis may differ from each other with respect to their functional role in teratogenesis: some miRNAs act to protect embryos, whereas other miRNAs boost a teratogen-induced process of maldevelopment to induce embryonic death.