Circadian Rhythms of Perineuronal Net Composition

Abstract

Perineuronal Nets (PNNs) are extracellular matrix (ECM) structures that envelop neurons and regulate synaptic functions. Long thought to be stable structures, PNNs have been recently shown to respond dynamically during learning, potentially regulating the formation of new synapses. We postulated that PNNs may vary during sleep, a period of active synaptic modification. Notably, PNN components are cleaved by matrix proteases such as the protease cathepsin-S, synthesized by microglia. Cathepsin-S is expressed in a diurnal manner in the mouse cortex, coinciding with dendritic spine density rhythms. Thus, cathepsin-S may induce PNN remodeling during sleep, mediating synaptic reorganization. These studies were designed to test the hypothesis that PNN numbers vary in a diurnal manner in the rodent and human brain, and in a circadian manner in the rodent brain, in association with cathepsin-S expression rhythms coinciding with reported rhythms in synaptic plasticity. In mice, we observed diurnal and circadian rhythms of PNNs labeled with the lectin wisteria floribunda agglutinin (WFA+PNNs) in several brain regions involved in emotional memory processing. Sleep deprivation prevented the daytime decrease of WFA+ PNNs. Diurnal rhythms of cathepsin-S expression in microglia were observed in the same brain regions, opposite to PNN rhythms. Finally, incubation of mouse sections with cathepsin-S eliminated PNN labeling. In humans, WFA+PNNs showed a diurnal rhythm in the amygdala and thalamic reticular nucleus (TRN). Our results demonstrate that PNNs vary in a circadian manner that coincides with expression rhythms of cathepsin-S in the mouse hippocampus. We suggest that rhythmic modification of PNNs may contribute to memory consolidation during sleep.

Diurnal changes in perineuronal nets and parvalbumin neurons in the rat medial prefrontal cortex

“…A circadian contribution (vs. sleep contribution) to the diurnal rhythm is consistent with the minimal changes in PNN intensity we previously observed at ZT6 after 6 hr sleep disruption [48]. In contrast to the Pantazopoulos study in mice [49], we found no diurnal rhythm in the number of WFA-labeled PNNs but instead a diurnal rhythm in the intensity of PNNs, suggesting that perhaps very dim PNNs were not visible in the mouse study or that there are species differences in PNN expression. Importantly, the intensity of PNNs influences the extent of plasticity of their underlying neurons, based on studies in knockout mice missing a critical proteoglycan for PNN formation (link protein 1); these mice have much less intense PNNs yet exhibit delayed critical period plasticity in the visual cortex [2].…”

“…The study in mice revealed that the number of PNNs in several brain regions, including in the mPFC, followed a diurnal pattern, with higher numbers of PNNs in the dark, active phase. PNN-surrounded neurons in mPFC also demonstrated a similar diurnal pattern when mice were held under conditions of constant darkness, indicating the presence of a true circadian rhythm [49]. A circadian contribution (vs. sleep contribution) to the diurnal rhythm is consistent with the minimal changes in PNN intensity we previously observed at ZT6 after 6 hr sleep disruption [48].…”

“…The intensity of PNNs was higher in the dark phase (ZT12, ZT18) compared with the light phase (ZT0, ZT6). Diurnal fluctuation in the number of WFA-labeled PNNs has recently been reported for mice and humans [49]. The study in mice revealed that the number of PNNs in several brain regions, including in the mPFC, followed a diurnal pattern, with higher numbers of PNNs in the dark, active phase.…”

“…The particular component(s) of PNNs that are diurnally regulated is unknown. The daily decrease in PNNs appears to be regulated at least in part by cathepsin-S in the mPFC [49].…”

“…It is possible that these changes in marker intensity were masked by time-of-day changes created by diurnal fluctuations. Pantazopolous et al [49] recently reported that the number of PNNs in mice fluctuated diurnally in several brain regions, including the prelimbic mPFC, and that this same pattern persisted when mice were maintained under constant darkness, indicating a circadian influence. Here we tested the hypotheses that both PNN and PV intensity fluctuate diurnally.…”