Estrogenic regulation of perineuronal nets in the mouse insular cortex and hippocampus

Estrogen has profound effects on the brain, affecting neuronal plasticity and behavior. Perineuronal nets (PNNs) are perforated extracellular matrix structures that mostly surround parvalbumin (PV)-expressing inhibitory interneurons and regulate neuronal activity, synaptic plasticity and behavior. PNNs have sex-specific effects on behavior, suggesting that hormones like estrogen may be involved in these sex differences. In this study, we investigated the effects of estrous cycle and estrogen on PNNs in the hippocampus and insular cortex of female mice. PNNs and PV protein were detected by fluorescence labeling with the plant lectin WFA and PV immunostaining, respectively, throughout the estrous cycle, following treatment with estradiol, and after chronic administration of the aromatase inhibitor letrozole to block estrogen synthesis. PNN labeling was highest during estrus and bidirectionally modulated by estradiol levels, with estradiol decreasing, and letrozole increasing, PNN labeling intensity. To interrogate potential mechanisms of estrogenic regulation of PNNs, we used RNAScope to probe the estrogen receptors Esr1 and Esr2, and Cyp19a1, encoding aromatase, in PV neurons during estrus and diestrus. In insular PV neurons, Esr2 was elevated during estrus, and in hippocampal PV neurons, Cyp19a1 was elevated during diestrus. Finally, expression of genes encoding matrix metalloproteinases (MMPs/ADAMTSs) and their endogenous inhibitors (TIMPs) were examined by qPCR following letrozole treatment, with expression changes observed in both the insula and hippocampus. Together, these results indicate that PNN accumulation and degradation in females are regulated by estrogen with distinct region-specific mechanisms contributing to PNN structural changes by estrogen.