**Background:** Brain-derived neurotrophic factor (BDNF) and kisspeptin-1 (KISS-1) are promising effect biomarkers for neurodevelopmental and reproductive outcomes, respectively, within the European Human Biomonitoring Initiative (HBM4EU). BDNF is essential for fetal brain development, synaptic plasticity, and placental function, while KISS-1 regulates puberty onset, fertility, and trophoblast invasion during pregnancy. Although both are expressed in the placenta, the predictive value of maternal serum concentrations for placental and fetal levels had not been explored. This study aimed to determine whether maternal serum BDNF and KISS-1 levels correlate with umbilical cord serum levels and placental gene expression, whether they can serve as biomarkers of effect for lead (Pb) and cadmium (Cd) exposure, and whether maternal iron status influences these biomarkers.
**Methods:** Sixty-five healthy pregnant women with single term pregnancies (gestational weeks 37-42, median age 36.0 years) were recruited at General Hospital Vienna between July 2019 and October 2021. Maternal venous blood was collected before delivery, umbilical cord blood after birth, and full-thickness placental pieces from four quadrants were homogenized. Pro-BDNF, mature BDNF, and KISS-1 were quantified by ELISA in serum and cell culture supernatants. Placental BDNF and KISS-1 gene expression was measured by qPCR normalized to Ubiquitin C. Pb and Cd were measured in erythrocytes and placenta by graphite-furnace atomic absorbance spectroscopy. In vitro experiments used BeWo choriocarcinoma cells and primary human trophoblast cells (hTCs) isolated from placentas, treated with PbCl₂ (0.5-50 µM), CdCl₂ (0.5-5 µM), deferoxamine (DFO, 25-500 µM for iron deficiency), or ferric ammonium citrate (FAC, 5-100 µM for iron overload) for 72 hours. Statistical analyses included Spearman correlations, Mann-Whitney U tests, and categorical regression (CATREG) analyses.
**Key Results:** Both pro-BDNF and mature BDNF levels were slightly higher in maternal serum (mean pro-BDNF: 5365 pg/ml; mean mature BDNF: 17,644 pg/ml) than in cord serum (mean pro-BDNF: 4639 pg/ml; mean mature BDNF: 15,932 pg/ml). KISS-1 levels were lower in maternal serum (mean 1265 pg/ml) than in cord serum (mean 1962 pg/ml). The cord-to-maternal serum ratio was approximately 1 for both BDNF isoforms and 1.65 for KISS-1. Maternal and cord serum levels were moderately to strongly correlated (pro-BDNF: rS=0.60; mature BDNF: rS=0.54; KISS-1: rS=0.71, all P<0.05). Moderate to strong correlations were also found between maternal serum levels and placental gene expression for all three markers. Maternal erythrocyte Pb levels were inversely correlated with maternal serum KISS-1 concentrations (rS=-0.322, P<0.05) and placental KISS-1 gene expression (rS=-0.274, P<0.05). In CATREG analysis, maternal Ery-Pb remained the only significant factor for maternal KISS-1 levels (β=-0.371, P<0.001, R²=0.138). Placental Cd content correlated positively with placental BDNF gene expression (rS=0.328, P<0.05). Maternal iron therapy (β=0.403, P=0.008), maternal thrombocyte count (β=0.414, P<0.001), and transferrin saturation (β=0.291, P=0.027) were significant modulators of maternal serum pro-BDNF levels (final model R²=0.373). In vitro, Pb-treated BeWo cells showed decreased BDNF and KISS-1 expression and reduced secretion of pro-BDNF, mature BDNF, and KISS-1. Cd-treated cells showed increased BDNF expression and specifically upregulated pro-BDNF secretion. Iron deficiency (DFO) increased pro-BDNF secretion but decreased mature BDNF and KISS-1 secretion from BeWo cells in a dose-dependent manner. Cd accumulated 34 times more efficiently than Pb in BeWo cells (0.17 pg/cell vs. 0.005 pg/cell at 5 µM), and strongly induced metallothionein 2A (MT2A) expression.
**Clinical Implications:** The strong correlations between maternal serum BDNF and KISS-1 levels with both cord serum levels and placental gene expression suggest that maternal levels could serve as non-invasive biomarkers for placental health and fetal neurodevelopment. The inverse association between maternal lead exposure and KISS-1 levels, confirmed by in vitro experiments, identifies KISS-1 as a potential effect biomarker for Pb-induced reproductive toxicity. The influence of maternal iron status on BDNF levels highlights the importance of considering iron supplementation in biomonitoring studies. However, the small sample size (n=65) necessitates validation in larger cohorts, and the clinical utility of these biomarkers for predicting neurodevelopmental outcomes requires further investigation. The hybrid epidemiological-in vitro approach strengthens the causal interpretation of the observed associations.