Temporal manipulation of transferrin‐receptor‐1‐dependent iron uptake identifies a sensitive period in mouse hippocampal neurodevelopment

Abstract

Iron is a necessary substrate for neuronal function throughout the lifespan, but particularly during development. Early life iron deficiency (ID) in humans (late gestation through 2–3 years) results in persistent cognitive and behavioral abnormalities despite iron repletion. Animal models of early life ID generated using maternal dietary iron restriction also demonstrate persistent learning and memory deficits, suggesting a critical requirement for iron during hippocampal development. Precise definition of the temporal window for this requirement has been elusive due to anemia and total body and brain ID inherent to previous dietary restriction models. To circumvent these confounds, we developed transgenic mice that express tetracycline transactivator regulated, dominant negative transferrin receptor (DNTfR1) in hippocampal neurons, disrupting TfR1 mediated iron uptake specifically in CA1 pyramidal neurons. Normal iron status was restored by doxycycline administration. We manipulated the duration of ID using this inducible model to examine long-term effects of early ID on Morris water maze learning, CA1 apical dendrite structure, and defining factors of critical periods including parvalbmin (PV) expression, perineuronal nets (PNN), and brain derived neurotrophic factor (BDNF) expression. Ongoing ID impaired spatial memory and resulted in disorganized apical dendrite structure accompanied by altered PV and PNN expression and reduced BDNF levels. Iron repletion at P21, near the end of hippocampal dendritogenesis, restored spatial memory, dendrite structure, and critical period markers in adult mice. However, mice that remained hippocampally iron deficient until P42 continued to have spatial memory deficits, impaired CA1 apical dendrite structure, and persistent alterations in PV and PNN expression and reduced BDNF despite iron repletion. Together, these findings demonstrate that hippocampal iron availability is necessary between P21 and P42 for development of normal spatial learning and memory, and that these effects may reflect disruption of critical period closure by early life ID.

Disrupted iron homeostasis causes dopaminergic neurodegeneration in mice

“…In a previous study, murine hippocampal neurons expressing dominant negative Tfr1 had markedly decreased iron associated with altered dendrite structure and delayed GABAergic maturation, but the authors did not report an increase in cell death (10). Mice lacking IRP2 developed lower motor neuron degeneration, raising the possibility that a lack of utilizable iron can cause the death of some types of neurons (47).…”

“…It has been widely assumed that neuronal iron uptake depends on TFR1-mediated endocytosis of Fe 2 -TF (9), but in vivo evidence is scant. Expression of a dominant negative form of Tfr1 was shown to decrease iron uptake by hippocampal CA1 pyramidal neurons (10). However, others noted that iron distribution does not correlate with Tfr1 expression in the brain and suggested that Tfr1 may have roles unrelated to iron uptake (11).…”

Early Life Iron Deficiency Impairs Spatial Cognition in Neonatal Piglets ,2

“…TFR is important for iron uptake (53). The expression of TfR mRNA was increased in the prefrontal cortex of MID and SID piglets, which may indicate the mechanism by which this brain region was able to maintain higher iron concentrations despite the piglets being fed deficient diets.…”

Neuronal-Specific Iron Deficiency Dysregulates Mammalian Target of Rapamycin Signaling during Hippocampal Development in Nonanemic Genetic Mouse Models

“…Mitochondrial enzymes, including cytochromes, NADPH, and flavoproteins, require iron in the form of heme and iron-sulfur clusters (9), which are integral for oxidative phosphorylation and ATP production. Iron status also regulates levels of BDNF and insulin-like growth factors and their receptors, which are primary regulators of neuronal growth and plasticity (6,(10)(11)(12).…”

“…In the current study, we examined the specific role of iron in developmental regulation of mTOR signaling using 2 unique genetic mouse models of in vivo neuronal ID that are not confounded by the hypoxia or increased uptake of other divalent metals that occur in dietary IDA (11,27). One model is generated by permanent, conditional Cre-lox P knockout of Slc11a2, the gene that codes for divalent metal transporter-1 (DMT-1) conditional knockout (DMT-1 CKO) in hippocampal neurons and results in a 40% decrease in the hippocampal iron concentration without affecting the remainder of the brain (27).…”