Perineuronal nets (PNNs) are chondroitin sulfate proteoglycan-containing structures on the surface of some neurons which surround synapses and are known to be involved in memory. PNNs become more inhibitory with aging due to the changes in sulfation of chondroitin sulfate glycosaminoglycan chains (CS-GAGs) of chondroitin sulfate proteoglycans (CSPGs), and the resulting inability of neuronal processes to penetrate PNNs may block the synaptogenesis underlying memory formation. CS composition is dominated by C6S and C4S, and a team of researchers from the University of Cambridge and collaborating institutions hypothesized that the age-related increase in the ratio of C4S/C6S may make PNNs more inhibitory, leading in turn to memory loss associated with diminished inhibitory synapse formation onto GABAergic parvalbumin+ (PV+) interneurons.
In testing the hypothesis, aged 20-month-old mice were first shown to have memory deficits in spontaneous object recognition (SOR), spontaneous alternation (SA) and marble burying (MB) tasks. PNNs were implicated in the loss of SOR memory in aged mice, as shown in experiments in which PNNs were attenuated by transgenic deletion of hapln1. Studies of chondroitinase (ChABC) digestion of CS-GAGs in the perirhinal cortex (PRh) showed restoration of memory and that CS-GAGs in the PRh are responsible for SOR memory loss in the aged mice. Increased PV expression in PV+ interneurons in aged mice was also returned to the levels of young animals by ChABC treatment. A reduction of C6S was observed in the PNNs of mice at 20 months of age, leading to an increase in the ratio of C4S to C6S. C6S loss was also associated with early memory deficits on SOR, SA and MB tasks in young C6 sulfotransferase-1 (chst3) knockout animals. In mice injected into the PRh with an AAV1 vector expressing mouse chst3 at 4 months of age, SOR memory was restored. When elderly animals with memory deficits were injected with AAV1-chst3 into PRh, overexpression of chst3 led to a restoration of SOR memory. In addition, transgenic chst3-overexpressing mice with increased C6 levels had no memory deficits at 20 months on SOR and SA tests. Restoring C6S levels in aged animals using AAV-chst3 also restored cortical long-term potentiation. Memory restoration by AAV-chst3 led to a restoration in the number of inhibitory synapses contacting PV+ interneurons.
The studies showed that aged PNNs lose C6S, become more inhibitory, leading to a decrease in inhibitory inputs to GABAergic PV+ neurons, causing increased PV expression and increased cortical inhibition; increased GABAergic inhibition impairs memory acquisition. Removing PNNs or restoring C6S levels increases inhibitory synapses on PV+ interneurons and restores normal memory. The authors concluded that treatments targeting PNNs could ameliorate memory deficits associated with aging