Long-lived extracellular matrix molecules called perineuronal nets are essential for distant memories, researchers from the Centre for Integrative Neuroplasticity (CINPLA) at the University of Oslo have shown. The mechanism by which the brain is able to store memories over long periods of time has been a persistent mystery to neuroscientists.

The study shows that removal of the perineuronal nets disrupts distant but not recent memories.

Previously, researchers have mainly focused on molecules inside the nerve cells. The team of investigators, led by Drs. Marianne Fyhn and Torkel Hafting, studied perineuronal nets that tightly cover the outside of neurons. The nets are made up of sugar-coated proteins, forming a rigid structure that contains holes where connections to other neurons are kept in place.

Surprisingly Strong Effects

When new memories are formed, the connections between neurons change. The authors hypothesized that perineuronal nets might stabilize the new, memory-related connections to support long-term memories.

To test memory function, the team performed a classical conditioning experiment, where rats learn to associate a light blink with an unpleasant event. This type of learning creates a robust and long-lasting memory.

After learning, the rats were divided into two groups, one where the perineuronal nets (PNNs) were left intact and one where they were removed in a small area of the cortex, termed secondary visual cortex, an area known to be involved in memory storage. When the rats were asked to recall the memory a month later, the results were astonishing — the group without the nets did not remember anything.

[caption id=“attachment_93969” align=“aligncenter” width=“680”]Removal of PNNs in lateral secondary visual cortex Removal of PNNs in lateral secondary visual cortex disrupts recall of a remote fear memory.
(A, Left) Coronal section of a rat brain, PNNs detected by Wisteria floribunda agglutinin (WFA; green) and neuronal cell bodies detected by Nissl staining (red).
Au1, primary auditory cortex; V1, primary visual cortex; V2L, lateral secondary visual cortex; V2M, medial secondary visual cortex.
(A, Right) Neuron expressing parvalbumin (Pvalb; red) enwrapped in a PNN (WFA; green).
(B) Coronal section from a rat injected with chABC in V2L 1 wk before perfusion. PNNs detected by WFA staining (green).
Activity by chABC causes reduced WFA staining (green) restricted to V2L. RSp, retrosplenial cortex; V1b, binocular V1; V1m, monocular V1.
(C) Experimental timeline for remote fear conditioning (FC).
(D) The extent of chABC digestion in gray superimposed on illustrations of brain sections (40) five different distances (mm) from bregma, red squares indicates V2L (Left), or V1 and V2L (Right).
(E) Bilateral chABC injection in V2L (n = 7 rats) reduced freezing to light CS compared with sham controls (n = 12 rats).
Each dot represents one animal, bars indicate population mean. Two-way ANOVA, treatment (aCSF or chABC) × condition (baseline or light CS) followed by post hoc Sidak test revealed that chABC treatment disrupted CS memory retrieval (***P < 0.0001).
(F) Bilateral chABC injection in V1 (n = 9 rats) did not influence freezing to light CS compared with sham controls (n = 13 rats).
(G) The extent of chABC activity confined to V2L (mean from both hemispheres) was correlated with the amount of freezing during light cues; r = −0.67, P = 0.003, n = 17 rats.
(H) Experimental timeline for recent FC. (I) Recent memory testing 1 wk after FC. Bilateral ChABC injections in V2L (n = 8 rats) did not influence freezing to light CS compared with sham controls (n = 8 rats).
(J) Recent memory testing 1 wk after FC. Bilateral ChABC injections in V1 (n = 4 rats) did not influence freezing to light CS compared with sham controls (n = 8 rats).
Credit: Elise Holter Thompson et al CC-BY[/caption]

The experiments show that perineuronal nets are essential for long-term memories, because without them, the memory is lost.

“We were quite surprised by how strong the effect was in those first experiments, since we only manipulated molecules outside the neurons and not inside. While we expected to see some effect of the intervention, previous studies on the nets had focused on their role in development and learning, not memory storage. It was very exciting to see that the memory was in fact gone,"

said Elise H. Thompson, one of the leading authors of the paper.

Stable Perineuronal Nets

In a follow-up experiment where the memory was tested only a few days after learning, the team found that the memory was intact, and that the disappearing effect was specific to old memories.

“Because the net is a very stable structure it may stabilize memories as they age, but when a memory is new, it survives without extra stabilizing factors,"

says Dr. Kristian K. Lensjø, another leading author of the paper.

While scientists understanding of the processes that govern the transition from short-term to long-term memory has expanded greatly in recent years, those needed for a memory to persist across years remain unresolved. This research is an important step toward understanding what components are needed to store memories for a lifetime.

“If we can increase our understanding of how memories are processed over months and years in the healthy brain, we can start to untangle what goes wrong when they are eventually lost in detrimental diseases like Alzheimer’s and dementia. The surprising finding that extracellular molecules are involved in these processes also suggests potential novel drug targets,"

explains Marianne Fyhn, leader of the CINPLA project.

The research was supported by the Research Council of Norway.

Elise Holter Thompson, Kristian Kinden Lensjø, Mattis Brænne Wigestrand, Anders Malthe-Sørenssen, Torkel Hafting, and Marianne Fyhn Removal of perineuronal nets disrupts recall of a remote fear memory PNAS 2017 ; published ahead of print December 26, 2017, doi:10.1073/pnas.1713530115

Top Image: perineuronal net (green) surrounding a neuron Credit: Kristian K. Lensjø

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