Role of Folbp1 in the regional regulation of apoptosis and cell proliferation in the developing neural tube and craniofacies.
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Folic acid is essential for many cellular reactions, including synthesis of nucleotides and regulation of cell cycle. Folic acid-binding protein one (Folbp1), a membrane-bounded protein, is the primary mediator of folic acid transport. Mice deficient in Folbp1 gene die in utero with multiple malformations, including severe exencephaly and craniofacial defects. Fusion of the neural tube and craniofacies require precisely regulated interactions of apoptosis, cell proliferation, and differentiation. To understand the role of Folbp1 in regulating the fusions of these primordia, levels of dead and proliferating precursor cells from Folbp1 embryos were quantified before the fusion processes. Massive apoptosis was detected in the Folbp1-/- defective tissues, with Bax and activated caspase-3 distributed evenly across the apico-basal axis of the lateral neural plate. 5-Bromodeoxyuridine (BrdU) and PCNA labeling assays revealed a reduced cell proliferation as well. However, telomerase activity was unaltered, arguing against telomere shortening and consequently, chromosomal instability, as the cause of the apoptosis. Notably, Islet-1 and 2H3 immunohistochemistry demonstrated the presence of differentiating neuronal cells, albeit in decreased numbers. Interestingly, Folbp1-/- embryos also elaborated novel neural structures that sprouted orthogonally from the embryonic neuraxis. Assays on the defective craniofacies exhibited similar phenomena, suggesting the neural crest precursor population that gives rise to both these structures is selectively vulnerable to Folbp1 inactivation. The results demonstrate a prominent role of Folbp1 in the regional regulation of apoptosis and cell proliferation that underlies the aberrant neural tube and craniofacial defects.