Fetal circulation occurs through the umbilical vein that carries maternal blood and nutrients filtered through the placenta to the fetus, and 2 umbilical arteries that transport blood to and from the placenta. The placenta forms a barrier to macromolecules, including most proteins, to cross from maternal to fetal circulation [30]. However, fluoride crosses the placenta [23, 31], and is also increased in the placenta relative to cord blood fluoride concentrations [32, 33]. Previous studies have shown that fetal cord blood fluoride concentrations reflect maternal blood fluoride [34], and therefore in this study we used maternal serum fluoride concentrations as a biomarker for fetal fluoride exposure [23].
Cord blood serum was collected between 2014 and 2016 from second trimester pregnant women residing in Northern California [23]. All women were healthy without known underlying medical conditions, and their community water fluoride concentrations were at or below optimal water fluoride concentrations, as recommended by the US Center for Disease Control (CDC) at the time of sample collection (1.0 ppm fluoride). The 9 cord blood proteins found to be significantly associated with the maternal serum fluoride concentration were not correlated with risk factors for pregnancy outcomes including maternal age [35, 36], smoking status [37, 38], BMI [39], and race/ethnicity.
These proteins included protein delta-like homolog 1 (DLK1), a transmembrane protein highly expressed by stromal cells of the placental villi [28] and by the fetal liver hepatocytes [40], which was negatively associated with maternal serum fluoride. DLK1 regulates glucose metabolism [41] and placental growth hormone [42], and is reduced in small for gestational age fetuses [28]. The placenta is a primary hematopoietic stem cell (HSC) niche during pregnancy, which is believed to seed the fetal liver [43, 44], and depletion of DLK1 from of human fetal liver hepatoblasts in culture reduces the percentage of mature hematopoietic cells [40].
DLK1 also regulates notch signaling and affects neurodevelopment [45]. In mice, DLK1 was shown to regulate hippocampal neurogenesis and cognition. Reduced DLK1 levels triggers cognitive abnormalities [46], suggesting the possibility that fluoride-related effects on fetal neurodevelopment may be associated with reductions in DLK1.
The fetal liver is the primary organ for erythropoiesis during development, functioning as a vascular connection between the developing placental vessels connecting the heart [47]. It does not perform the traditional digestive and filtration functions because nutrients are normally carried to the fetus from the mother via the placenta. Other proteins significantly downregulated relative to maternal serum fluoride are also associated with hematopoiesis, and include apolipoprotein B-100 (APOB), coagulation factor X, plasma kallikrein, and vasorin. APOB, which is synthesized by the placenta, is a likely pathway for lipid transfer required for fetal growth [24]. Vasorin is a transmembrane glycoprotein[48] with a possible role in the regulation of the glycogen-mediated mTOR-ULK1 signaling pathway in the liver[49], and mimecan, also known as osteoglycin which is a small leucine-rich proteoglycan (SLRPs) present in the extracellular matrix of multiple tissues, including adipose tissue.
The possibility that fluoride may affect the liver in humans is supported by NHANES survey data from 2013–2016 that shows higher water fluoride and plasma fluoride concentrations are associated with lower blood urea nitrogen among US adolescents. However, while the fetal hematopoietic system is separate from that of the maternal circulatory system, the placenta is of dual origin, comprised of both fetally and maternally derived cells and maternal inflammation can alter maternal–fetal interactions. Maternal immune activation during pregnancy, can affect the development of the fetal hematopoietic system [50], and therefore it possible that fluoride related effects to increase maternal inflammation may also have a role in fetal development. Analysis of available NHANES survey data (2013–2014 and 2015–2016) of children aged 6–19, shows plasma fluoride concentrations positively correlated with total white blood cell count, segmented neutrophils and monocytes when adjusted for age, gender, and BMI as predictors. While there is no available data linking second trimester maternal white blood cell counts to fluoride exposure, further studies to explore this possibility are warrented.
In summary, this is the first study to assess possible fluoride related changes associated with human fetal development at low level fluoride exposure. Our finding in this pilot study, that cord blood proteins associated with the development of the fetal hematopoietic system were significantly downregulated relative to maternal fluoride exposure, supports the possibility that fluoride related mechansims affecting fetal development and may be associated with the placenta and fetal hematopoesis during second trimester fetal development. The second trimester of fetal development is also critical for development of the prefrontal cortex and integration of control of attention, thought, emotions and actions [51]. Therefore, the identification of cord blood proteins altered by relative levels of maternal fluoride exposure may direct future studies of fluoride related mechanism that may affect fetal development, including neurodevelopment.