Retinoic acid (RA) is a pleiotropic derivative of vitamin A, or retinol, which is responsible for all of the bioactivity associated with this vitamin. The teratogenic influences of vitamin A deficiency and excess RA in rodents were first observed more than 50 years ago. Efforts over the last 15-20 years have refined these observations by defining the molecular mechanisms that control RA availability and signaling during murine embryonic development. This review will discuss our current understanding of the role of RA in teratogenesis, with specific emphasis on the essential function of the RA catabolic CYP26 enzymes in preventing teratogenic consequences caused by uncontrolled distribution of RA. Particular focus will be paid to the RA-sensitive tissues of the caudal and cranial regions, the limb, and the testis, and how genetic mutation of factors controlling RA distribution have revealed important roles for RA during embryogenesis.
In mammals, germ cells within the developing gonad follow a sexually dimorphic pathway. Germ cells in the murine ovary enter meiotic prophase during embryogenesis, whereas germ cells in the embryonic testis arrest in G0 of mitotic cell cycle and do not enter meiosis until after birth. In mice, retinoic acid (RA) signaling has been implicated in controlling entry into meiosis in germ cells, as meiosis in male embryonic germ cells is blocked by the activity of a RA-catabolizing enzyme, CYP26B1. However, the mechanisms regulating mitotic arrest in male germ cells are not well understood. Cyp26b1 expression in the testes begins in somatic cells at embryonic day (E) 11.5, prior to mitotic arrest, and persists throughout fetal development. Here, we show that Sertoli cell-specific loss of CYP26B1 activity between E15.5 and E16.5, several days after germ cell sex determination, causes male germ cells to exit from G0, re-enter the mitotic cell cycle and initiate meiotic prophase. These results suggest that male germ cells retain the developmental potential to differentiate in meiosis until at least at E15.5. CYP26B1 in Sertoli cells acts as a masculinizing factor to arrest male germ cells in the G0 phase of the cell cycle and prevents them from entering meiosis, and thus is essential for the maintenance of the undifferentiated state of male germ cells during embryonic development.
Cyp26b1 encodes a cytochrome-P450 enzyme that catabolizes retinoic acid (RA), a vitamin A derived signaling molecule. We have examined Cyp26b1(-/-) mice and report that mutants exhibit numerous abnormalities in cranial neural crest cell derived tissues. At embryonic day (E) 18.5 Cyp26b1(-/-) animals exhibit a truncated mandible, abnormal tooth buds, reduced ossification of calvaria, and are missing structures of the maxilla and nasal process. Some of these abnormalities may be due to defects in formation of Meckels cartilage, which is truncated with an unfused distal region at E14.5 in mutant animals. Despite the severe malformations, we did not detect any abnormalities in rhombomere segmentation, or in patterning and migration of anterior hindbrain derived neural crest cells. Abnormal migration of neural crest cells toward the posterior branchial arches was observed, which may underlie defects in larynx and hyoid development. These data suggest different periods of sensitivity of anterior and posterior hindbrain neural crest derivatives to elevated levels of RA in the absence of CYP26B1.
Human cellular models of Alzheimers disease (AD) pathogenesis would enable the investigation of candidate pathogenic mechanisms in AD and the testing and developing of new therapeutic strategies. We report the development of AD pathologies in cortical neurons generated from human induced pluripotent stem (iPS) cells derived from patients with Down syndrome. Adults with Down syndrome (caused by trisomy of chromosome 21) develop early-onset AD, probably due to increased expression of a gene on chromosome 21 that encodes the amyloid precursor protein (APP). We found that cortical neurons generated from iPS cells and embryonic stem cells from Down syndrome patients developed AD pathologies over months in culture, rather than years in vivo. These cortical neurons processed the transmembrane APP protein, resulting in secretion of the pathogenic peptide fragment amyloid-?42 (A?42), which formed insoluble intracellular and extracellular amyloid aggregates. Production of A? peptides was blocked by a ?-secretase inhibitor. Finally, hyperphosphorylated tau protein, a pathological hallmark of AD, was found to be localized to cell bodies and dendrites in iPS cell-derived cortical neurons from Down syndrome patients, recapitulating later stages of the AD pathogenic process.
Trisomy 21 is associated with hematopoietic abnormalities in the fetal liver, a preleukemic condition termed transient myeloproliferative disorder, and increased incidence of acute megakaryoblastic leukemia. Human trisomy 21 pluripotent cells of various origins, human embryonic stem (hES), and induced pluripotent stem (iPS) cells, were differentiated in vitro as a model to recapitulate the effects of trisomy on hematopoiesis. To mitigate clonal variation, we isolated disomic and trisomic subclones from the same parental iPS line, thereby generating subclones isogenic except for chromosome 21. Under differentiation conditions favoring development of fetal liver-like, ?-globin expressing, definitive hematopoiesis, we found that trisomic cells of hES, iPS, or isogenic origins exhibited a two- to fivefold increase in a population of CD43(+)(Leukosialin)/CD235(+)(Glycophorin A) hematopoietic cells, accompanied by increased multilineage colony-forming potential in colony-forming assays. These findings establish an intrinsic disturbance of multilineage myeloid hematopoiesis in trisomy 21 at the fetal liver stage.
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