Fetal iron deficiency alters the proteome of adult rat hippocampal synaptosomes

Abstract

Fetal and neonatal iron deficiency results in cognitive impairments in adulthood despite prompt postnatal iron replenishment. To systematically determine whether abnormal expression and localization of proteins that regulate adult synaptic efficacy are involved, we used a quantitative proteomic approach (isobaric tags for relative and absolute quantitation, iTRAQ) and pathway analysis to identify dysregulated proteins in hippocampal synapses of fetal iron deficiency model. Rat pups were made iron deficient (ID) from gestational day 2 through postnatal day (P) 7 by providing pregnant and nursing dams an ID diet (4 ppm Fe) after which they were rescued with an iron-sufficient diet (200 ppm Fe). This paradigm resulted in a 40% loss of brain iron at P15 with complete recovery by P56. Synaptosomes were prepared from hippocampi of the formerly iron-deficient (FID) and always iron-sufficient controls rats at P65 using a sucrose gradient method. Six replicates per group that underwent iTRAQ labeling and LC-MS/MS analysis for protein identification and comparison elucidated 331 differentially expressed proteins. Western analysis was used to confirm findings for selected proteins in the glutamate receptor signaling pathway, which regulates hippocampal synaptic plasticity, a cellular process critical for learning and memory. Bioinformatics were performed using knowledge-based Interactive Pathway Analysis. FID synaptosomes show altered expression of synaptic proteins-mediated cellular signalings, supporting persistent impacts of fetal iron deficiency on synaptic efficacy, which likely cause the cognitive dysfunction and neurobehavioral abnormalities. Importantly, the findings uncover previously unsuspected pathways, including neuronal nitric oxide synthase signaling, identifying additional mechanisms that may contribute to the long-term biobehavioral deficits.

Keywords: glutamate signaling; hippocampal function; iTRAQ; iron; micronutrient deficiency; neuroplasticity; synaptosome.

Fig. 1.

Validation analysis of isobaric tags for relative and absolute quantitation (iTRAQ) datasets. A: summary of identified proteins that were functionally mapped by Interactive Pathway Anaysis (IPA). B: Venn diagrams showing protein overlapped between two iTRAQ datasets with IPA-mapped proteins (right) and formerly iron deficient (FID)/iron sufficent (IS) comparison (left). C: Western blot quantified protein expressed in ratio of FID/IS with P value (in parentheses), unpaired t-test, n = 4–7/group. ^aNot computable due to negligible phosphoprotein level.

Fig. 2.

Altered molecular and cellular functions mapped by IPA comparison analysis. Relative protein levels of FID/IS identified by iTRAQ were analyzed by knowledge-based IPA software. Six replicates are indicated (S1–S6). Y-axis is [−log (P value)] with threshold (line) set at P = 0.05.

Fig. 3.

Canonical pathways with reduced activity implicated for synaptic long-term potentiation (LTP) in FID synapses: glutamate receptor signaling (A) and nitric oxide synthase (NOS) signaling (B). Data show proteins with decreased level (green) and increased level (red).

Fig. 4.

Canonical pathways with increased activity include p70S6K signaling (A) and 14-3-3 signaling (B). Data show proteins with decreased level (green) and increased level (red).