022 - Postnatal environmental enrichment improves hippocampal dentate gyrus neuron morphology in IUGR mouse offspring

Publication Number: 22.6335

Ashley Brown, University of Utah School of Medicine, salt lake, UT, United States; Matthew Wieben, University of Utah, Cottonwood Heights, UT, United States; Frank Strnad, University of Utah School of Medicine, South Salt Lake, UT, United States; Megan Williams, University of Utah, salt lake city, UT, United States; Camille Fung, University of Utah, Dept. of Pediatrics, Salt Lake City, UT, United States

Background: Offspring born with intrauterine growth restriction (IUGR) have a 5-fold higher risk for learning and memory impairment compared to being appropriately-grown (AG). In developed countries, hypertensive diseases of pregnancy (HDP) cause uteroplacental insufficiency (UPI) leading to IUGR. We have previously shown in a translational mouse model of HDP/UPI that IUGR hippocampal dentate gyrus (DG) has decreased neuron number and reduced dendritic branching one month after birth, which culminate in adult recognition memory deficits. Whether non-invasive postnatal environmental enrichment can improve neuronal changes is unexplored.

Objective: To institute postnatal environmental enrichment (EE) at weaning to improve DG neuron number and morphology.

Design/Methods: We produced HDP/UPI/IUGR mice by a continuous micro-osmotic pump infusion of a potent vasoconstrictor, TXA2-analog, in timed-pregnant C57BL/6 mouse dams beginning on embryonic day (E)12.5 until term (~20 days). Sham-operated dams with vehicle infusion produced AG mice. After delivery, all mice were cross-fostered, raised to postnatal day (P) 21, and received a retro-orbital injection of a recombinant adeno-associated virus conjugated to green fluorescent protein (AAV-GFP) to label DG neurons. We placed half of the sham and IUGR offspring into postnatal EE for one hour each day for 7 days, while the other half remained in their home cages. Postnatal EE consists of a large box that mimics a playground with colorful toys to enhance sensory, motor, social, and cognitive stimuli. At P28, we harvested all brains for immunofluorescent staining of GFP-labeled neurons. We measured average DG number, dendritic volume, and spine width and spine density on dendrites.

Results: Similar to previous, P28 IUGR DG neurons had decreased number and reduced dendritic branching without EE. IUGR DG dendrites had increased average spine width but had no change in average spine density per dendrite length. One week after postnatal EE, P28 IUGR DG have increased number of neurons and more dendritic branching compared to IUGR without EE. The principal dendrite, defined as the longest dendrite from soma to entorhinal cortex, was less defined (Figure 1).

Conclusion(s): We have uncovered a non-invasive yet fun therapy to improve DG neuron development after IUGR. Whether these neuronal changes are sustained over time and translate to improved memory function will be examined. We posit that improved neuron number is a function of increased neural stem proliferation and neuronal differentiation and improved dendritic branching is a function of decreased synaptic pruning by microglia.

Figure 1. P28 Sham and IUGR mouse hippocampal DG neurons at baseline and after postnatal EE in IUGR.
After one week of EE, P28 IUGR DG neurons (last panel) are more numerous and have more dendritic branching compared to IUGR without EE (middle panel). The dendrites however appear shorter without a principal dendrite, suggesting a developing dendritic tree after EE.