Maternal folate deficiency results in selective upregulation of folate receptors and heterogeneous nuclear ribonucleoprotein-E1 associated with multiple subtle aberrations in fetal tissues

BACKGROUND: Homocysteine, which increases in folate deficiency, can upregulate folate receptors (FR) at the translational level by increasing the interaction between a short cis-element in the 5'-untranslated region of FR-alpha mRNA and heterogeneous nuclear ribonucleoprotein-E1 (hnRNP-E1). Perturbation of this RNA-protein interaction on GD8.5 induces neural tube defects and neurocristopathies in mice. FR upregulation can also reduce cell proliferation independently of folate deficiency in some human cells. Accordingly, we tested the hypothesis that sustained murine maternal folate deficiency would negatively impact pregnancy outcomes, upregulate FR, and selectively reduce fetal cell proliferation. METHODS: Dams were fed chow with various levels of folic acid added for eight weeks before and throughout pregnancy. Following sacrifice on GD17, dams were compared for folate and homocysteine status as well as pregnancy outcomes. Fetuses from some groups were evaluated by specific biochemical, molecular, and immunohistochemical studies for FR, hnRNP-E1, and apoptosis. RESULTS: When compared to dams fed a folate-replete diet, those dams on a folate-depleted diet developed reduced red cell folates and hyperhomocysteinemia and an inverse dose-dependent upregulation of FR and hnRNP-E1 on GD17 without alterations in cell number in the majority of tissues. However, FR overexpression was accompanied by a significant reduction in the net number of cells in the midgut, lung, pons, tongue, and olfactory epithelium, and with premature differentiation in dorsal root ganglion cells and dysplasia of taste buds. By contrast, in the brain, spinal cord, diaphragm, and primordium of follicles of vibrissae, there was less FR expression, which accompanied a net reduction in number of cells and architectural anomalies. Subtle "immunohistochemical footprints" of apoptosis on GD17 fetuses corresponded with net cell loss in the lung and olfactory epithelium. Upregulation of FR could be explained by a homocysteine-induced RNA-protein interaction in folate-depleted fetuses that led to a proportionate increase in murine FR biosynthesis. CONCLUSIONS: Maternal folate deficiency results in selective upregulation of FR and hnRNP-E1 associated with multiple aberrations in fetal tissues that include increased cell loss, architectural anomalies, and premature differentiation. The potential significance of these findings to explain the wide spectrum of folate-responsive birth defects in humans is discussed.