Difference in the maternal and zygotic contributions of tumorhead on embryogenesis

NMDA Receptor Signaling Is Important for Neural Tube Formation and for Preventing Antiepileptic Drug-Induced Neural Tube Defects

Failure of neural tube closure leads to neural tube defects (NTDs), which can have serious neurological consequences or be lethal. Use of antiepileptic drugs (AEDs) during pregnancy increases the incidence of NTDs in offspring by unknown mechanisms. Here we show that during Xenopus laevis neural tube formation, neural plate cells exhibit spontaneous calcium dynamics that are partially mediated by glutamate signaling. We demonstrate that NMDA receptors are important for the formation of the neural tube and that the loss of their function induces an increase in neural plate cell proliferation and impairs neural cell migration, which result in NTDs. We present evidence that the AED valproic acid perturbs glutamate signaling, leading to NTDs that are rescued with varied efficacy by preventing DNA synthesis, activating NMDA receptors, or recruiting the NMDA receptor target ERK1/2. These findings may prompt mechanistic identification of AEDs that do not interfere with neural tube formation.SIGNIFICANCE STATEMENT Neural tube defects are one of the most common birth defects. Clinical investigations have determined that the use of antiepileptic drugs during pregnancy increases the incidence of these defects in the offspring by unknown mechanisms. This study discovers that glutamate signaling regulates neural plate cell proliferation and oriented migration and is necessary for neural tube formation. We demonstrate that the widely used antiepileptic drug valproic acid interferes with glutamate signaling and consequently induces neural tube defects, challenging the current hypotheses arguing that they are side effects of this antiepileptic drug that cause the increased incidence of these defects. Understanding the mechanisms of neurotransmitter signaling during neural tube formation may contribute to the identification and development of antiepileptic drugs that are safer during pregnancy.

Tumorhead distribution to cytoplasmic membrane of neural plate cells is positively regulated by Xenopus p21-activated kinase 1 (X-PAK1)

Tumorhead (TH) regulates neural plate cell proliferation during Xenopus early development, and gain or loss of function prevents neural differentiation. TH shuttles between the nuclear and cytoplasmic/cortical cell compartments in embryonic cells. In this study, we show that subcellular distribution of TH is important for its functions. Targeting TH to the cell cortex/membrane potentiates a TH gain of function phenotype and results in neural plate expansion and inhibition of neuronal differentiation. We have found that TH subcellular localization is regulated, and that its shuttling between the nucleus and the cell cortex/cytoplasm is controlled by the catalytic activity of p21-activated kinase, X-PAK1. The phenotypes of embryos that lack, or have excess, X-PAK1 activity mimic the phenotypes induced by loss or gain of TH functions, respectively. We provide evidence that X-PAK1 is an upstream regulator of TH and discuss potential functions of TH at the cell cortex/cytoplasmic membrane and in the nucleus.

Disruption of the dynamic sub-cellular localization of the Xenopus tumorhead protein causes embryonic lethality at the early gastrula transition

The Xenopus laevis tumorhead (TH) protein, a positive regulator of cell proliferation during embryogenesis, shuttles from the cell periphery into the nucleus during embryogenesis. In these studies, we performed a detailed analysis of TH's subcellular localization pattern to characterize its dynamic behavior. We found that TH exhibits distinct patterns of localization in different germ layers. At the blastula stage, TH is present in the apical cell periphery of prospective mesodermal and ectodermal cells. At the gastrula stage, TH is distributed throughout the entire cytoplasm of prospective mesodermal and ectodermal cells, whereas it shows nuclear localization in presumptive endodermal cells. TH moves into the nucleus of mesodermal and ectodermal cells during the neurula and early tailbud stages. To understand if TH is regulated by changes in its subcellular localization, we used a TH mutant containing signals for farnesylation and palmitoylation to tether the protein to the plasma membrane. Ubiquitous overexpression of this mutant causes embryonic lethality at the early gastrula transition. Further examination using TUNEL assays indicated that wild-type TH overexpression induces apoptosis during gastrulation, and that this effect is exacerbated by the overexpression of the membrane-bound TH mutant. Taken together, our results suggest that changes in the sub-cellular localization of the TH protein are important for its function because blocking the nuclear translocation of overexpressed TH increases apoptosis and causes embryos to die. Our data also suggest that TH plays a role outside the nucleus when it is present at the cell periphery.

The Xenopus laevis morphogenetic factor, tumorhead, causes defects in polarized growth and cytokinesis in the fission yeast, Schizosaccharomyces pombe

Tumorhead (TH) is a maternally expressed gene product that regulates neural tube morphogenesis in the amphibian, Xenopus laevis. Here we describe the effects of TH expression in the rod-shaped fission yeast, Schizosaccharomyces pombe. Expression of TH in S. pombe resulted in severe morphological defects, including ovoid, bottle-shaped, and enlarged cells. Multi-septated cells were also observed in TH expressing cultures, indicating that TH is inhibitory to a process required for the completion of cytokinesis. TH expression caused significant actin and microtubule cytoskeletal defects, including depolarization of the cortical F-actin cytoskeleton and increased microtubule formation. Immunostaining experiments showed that TH is localized to the cell cortex, cell tips, and septum in S. pombe cells. Localization of TH to the cell cortex was dependent on the S. pombe PAK homolog, Shk1. Moreover, TH expression was inhibitory to the growth of a mutant defective in Shk1 function, suggesting that TH may interact with a component(s) of a PAK-mediated morphogenetic regulatory pathway in S. pombe. Taken together, our findings demonstrate that S. pombe may be a useful model organism for identifying potential TH interacting factors.