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 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 (WFA1 PNNs) in several brain regions involved in emotional memory processing. Sleep deprivation prevented the daytime decrease of WFA1 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, WFA1 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.

Sleep and Memory Consolidation Dysfunction in Psychiatric Disorders: Evidence for the Involvement of Extracellular Matrix Molecules

“…PNNs labeled by chondroitin sulfate synthase 1 antibodies display rhythms in densities of CHSY-immunoreactive PNNs in the hippocampus that mirror WFA labeled PNN densities in the same animals (Figures 2, 3), suggesting that some components of PNNs are produced in a circadian manner. Furthermore, 5 h of sleep deprivation prevented decreases in PNN densities (Pantazopoulos et al, 2020a). Recent work shows similar increases in PNN composition in the prelimbic cortex of sleep deprived rats and provides further support that PNN composition is decreased during sleep (Harkness et al, 2019).…”

“…We recently reported circadian rhythms in PNN composition in the mouse brain, with lower densities of wisteria floribunda agglutinin (WFA) labeled PNNs during sleep and higher densities during wakefulness (Pantazopoulos et al, 2020a). PNNs labeled by chondroitin sulfate synthase 1 antibodies display rhythms in densities of CHSY-immunoreactive PNNs in the hippocampus that mirror WFA labeled PNN densities in the same animals (Figures 2, 3), suggesting that some components of PNNs are produced in a circadian manner.…”

An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity

“…Interestingly, very rapid changes in PNN CS-GAGs (i.e., within 24 h) occur in mouse brain regions involved in emotional memory, where these changes follow circadian rhythms. The number of WFA+ neurons generally peaks at zeitgeber time 20, when mice are awake, and is lowest at zeitgeber time 6–8, when mice sleep [ 174 ]. These data suggest that PNN composition may be regulated in a circadian manner to allow for circadian synaptic plasticity, with PNN decrease during sleep favoring the synaptic refinement that is required for memory consolidation [ 175 ].…”

“…For instance, during rapid eye movement (REM) sleep, memory representations are transferred from short-term storage sites, such as the hippocampus, into long-term storage sites in neocortical areas, where they are strengthened, and short-term storage memories are removed via synaptic pruning [ 176 ]. Indeed, sleep deprivation, which is known to prevent synaptic modification that occurs in the hippocampus during sleep [ 177 , 178 ], precludes a WFA-PNN decrease and leads to an increase in fear memory extinction [ 174 ]. Alternatively, since PNNs are known to be important for protecting neurons from oxidative stress [ 179 , 180 , 181 ], rhythms in PNN composition may reflect periods of reduced oxidative stress during sleep.…”

“…For example, cathepsis-S is a microglia-derived matrix protease, which shows a diurnal rhythmic expression that is opposite to PNN rhythms. The application of cathepsin-S to mouse brain sections eliminates WFA-PNN labeling, suggesting that it may be crucially involved in circadian PNN modulation [ 174 ]. Microglia cells are also able to engulf ECM molecules and clear them from the perisynaptic milieu, where this process may contribute to experience-dependent synapse remodeling and memory consolidation [ 215 ].…”