Circadian Rhythms of Perineuronal Net Composition

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 vary during sleep, a period of active synaptic modification. Notably, PNN components are cleaved by matrix proteases such as the protease cathepsin-S. This protease is diurnally expressed in the mouse cortex, coinciding with dendritic spine density rhythms. Thus, cathepsin-S may contribute to 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, as well as in a circadian manner in the rodent brain, and that these rhythms are disrupted by sleep deprivation. 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 and enhances fear memory extinction. 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 and this is disrupted by sleep deprivation. We suggest that rhythmic modification of PNNs may contribute to memory consolidation during sleep.Significance Statement The mechanisms underlying memory consolidation are not completely understood. Perineuronal nets are extracellular matrix structures enveloping subsets of neurons and are involved in regulating synaptic plasticity. Recent studies indicate that perineuronal nets are modified during learning to allow for formation of new synapses. During sleep, synapses are proposed to undergo modification as memory consolidation processes occur. Furthermore, microglia are involved in synaptic regulation and produce several proteases that cleave perineuronal net components. We demonstrate that perineuronal nets are modified in a circadian manner and coincide with expression rhythms of the protease cathepsin-S. These rhythms may contribute to altered synaptic plasticity reported during sleep, suggesting a key process through which proteases modify PNNs to allow for to memory consolidation.

Keywords: circadian rhythms; extracellular matrix; memory consolidation; perineuronal nets; psychiatric disorders; sleep.