We have demonstrated that CK2α−/−
mice die in mid-embryogenesis, while CK2α+/−
mice appear to be completely normal. The CK2α−/−
embryos exhibited extensive cardiac and neural tube defects, and the embryonic lethality is likely attributable to abnormalities in the structure and function of the heart. Embryonic hearts were defective in CK2α−/−
embryos isolated from E8.5 to E10.5, although they were able to beat. Sections of the CK2α−/−
hearts revealed defective formation of the chambers and poor trabeculation. The CK2α−/−
embryos died at ~E11, an age consistent with lethality due to defects in cardiac looping and chamber formation (6
). Our RT-PCR results showed that transcript levels of the axial mesodermal marker brachyury and the striated muscle transcription factor myogenin were present but somewhat diminished in CK2α−/−
embryos compared to wild-type embryos. In contrast, transcripts for cardiac markers such as MLC-2A, MLC-2V, and ANF were unchanged. Thus, the defect in the CK2α−/−
embryos is not due to a global defect in mRNA transcription.
The heart is the first organ to develop, and it is essential for embryonic development. The process of cardiogenesis is complex but begins with the specification of the cardiac mesoderm. The primitive heart tube develops from this specialized tissue and progresses through a tightly regulated series of morphological changes including looping of the heart tube, emergence of the endocardial cushion, and formation of the four chambers found in the adult. This process is controlled by the spatiotemporal expression of a number of developmental pathways including the heregulin, bone morphogenetic protein, fibroblast growth factor, and Wnt pathways and transcription factors that include targets of these pathways as well as GATA, T-box, and Nkx proteins (33
). Targeting of genes in these pathways typically results in abnormal cardiac development.
Similarly, the coordinate expression of multiple developmental pathways is required for normal neural tube closure (reviewed in reference 8
), and pathways involved in this and subsequent brain development include sonic hedgehog signaling, Notch, and, again, the Wnt pathway. Neural tube defects are a common developmental abnormality in humans, including spina bifida, when the posterior neural tube fails to close; anencephaly, when the defect is anterior; or craniorachischisis, when the entire neural tube is open. All of these neural tube defects were observed in the CK2α−/−
embryos. Closure of the neural tube allows the rapid expansion of brain volume due to fluid pressure exerted on the lumen of the closed neural tube (9
); when the neural tube fails to close, the neural tube collapses, as was seen in the CK2α−/−
embryos. Neural tube defects, while severe, generally do not lead to embryonic lethality (7
Thus, one of the pathways that is common to heart and brain development is the Wnt pathway (24
). The Wnt transcriptional cofactor β-catenin is required for normal heart formation (21
), and the Wnt target cripto is required for the differentiation of cardiomyocytes, cardiogenesis, and neural tube formation (11
). Targeted deletion of Wnt1 or Wnt3a causes defects in the midbrain and hindbrain regions and ectopic secondary neural tubes, respectively (24
). Dvl knockouts have defects of closure of the neural folds (48
). We have shown that CK2 is a critical regulator of Wnt signaling in cells and in Xenopus laevis
). Thus, the neural tube and heart defects in CK2α−/−
embryos could be due, in part, to the dysregulation of the Wnt pathway. Because of the complex developmental regulation of the neural tube and heart, and the many cellular processes regulated by CK2, a precise determination of the mechanisms behind the developmental defects in the CK2α−/−
embryos will require a thorough investigation of transcriptional and posttranslational regulation of components of Wnt and other signaling pathways using a variety of genomic and proteomic techniques.
In contrast to the essential role of CK2α in embryogenesis, CK2α′ plays a required role in male germ cell development only (51
). Thus, CK2α and CK2α′ are not redundant. This may be due to the fact that CK2α is the more abundant catalytic subunit in the developing embryo, accounting for more than three-fourths of the CK2 catalytic activity. Furthermore, the loss of CK2α leads to diminished CK2β levels in the embryo, similar to what we observed previously with a reduction of CK2α levels in cells using small interfering RNA oligonucleotides (39
). Alternatively, the CK2α and CK2α′ subunits may have functional differences; functional specialization of CK2 subunits has been seen in biochemical studies using dominant negative catalytic subunits (47
) and through studies identifying unique partners (3
). In the future, the issue of functional specialization could be resolved through knock-in experiments by substituting one catalytic subunit for another.