Selective disruption of perineuronal nets in mice lacking Crtl1 is sufficient to make fear memories susceptible to erasure

Chondroitin sulphate proteoglycans (CSPGs) in the extracellular matrix (ECM) restrict plasticity in the adult central nervous system and their digestion with chondroitinase ABC promotes plasticity in the adult visual cortex (Pizzorusso et al., 2002; Pizzorusso et al., 2006) and allows juvenile-like erasure of fear memories (Gogolla et al., 2009). However the structures in the ECM that restrict plasticity are unknown. There are many changes in the ECM as critical periods for plasticity close, including changes in CSPG core protein levels, changes in glycosaminoglycan sulphation and the appearance of dense CSPG-containing perineuronal nets (PNNs) around many neurons. We showed that formation of PNNs is triggered by neuronal production of cartilage link protein Crtl1 (Hapln1), which is up-regulated in the visual cortex as PNNs form during development and after dark rearing (Carulli et al., 2010). Mice lacking Crtl1 have attenuated PNNs, but the overall levels of CSPGs and their pattern of glycan sulphation are unchanged. We exploited these mice to assess whether preventing the aggregation of CSPGs into PNNs is sufficient to induce fear memories erasure. We find that Crtl1 KO animals retain the juvenile feature of erasing a specific fear memory associated with an initially neutral auditory tone (conditioned stimulus, CS) following a protocol of extinction. Indeed, they show neither spontaneous recovery nor renewal of fear 7 and 42 days after conditioned fear extinction, while wild type mice do. Acceleration of extinction and persistent reduction of fear in Crtl1 KO mice do not depend on passive loss of memory (oblivion), since fear memory assessed 10 days after learning without intervening extinction protocol is comparable in Crtl1 KO and WT mice. The organization of CSPGs into PNNs during development is therefore the key event in the control of fear memory erasure by ECM.