Specific transfer of Otx2 homeoprotein into GABAergic interneurons expressing parvalbumin (PV) is necessary and sufficient to open, then close, a critical period (CP) of plasticity in the developing mouse visual cortex. The accumulation of endogenous Otx2 in PV-cells suggests the presence of specific Otx2 binding sites. Here, we find that perineuronal nets (PNNs) on the surface of PV-cells permit the specific, constitutive capture of Otx2. We identify a 15 amino-acid domain containing an arginine-lysine doublet (RK-peptide) within Otx2, bearing prototypic traits of a glycosaminoglycan (GAG) binding sequence that mediates Otx2 binding to PNNs and specifically Chondroitin sulfate D and E with high affinity. Accordingly, PNN hydrolysis by Chondroitinase ABC reduces the amount of endogenous Otx2 in PV-cells. Direct infusion of RK-peptide similarly disrupts endogenous Otx2 localization to PV-cells, reduces PV and PNN expression and reopens plasticity in adult mice. The closure of one eye during this transient window reduces cortical acuity and is specific to the RK motif, as an AA variant or scrambled peptide fail to reactivate plasticity. Conversely, this transient reopening of plasticity in the adult restores binocular vision in amblyopic mice. Thus, one function of PNNs is to facilitate the persistent internalization of Otx2 by PV-cells to maintain CP closure. The pharmacological use of the Otx2 GAG-binding domain offers a novel, potent therapeutic tool with which to restore cortical plasticity in the mature brain.

Accumulation of non-cell autonomous Otx2 homeoprotein in postnatal mouse visual cortex (V1) has been implicated in both the onset and closure of critical period plasticity. Here, we show that a genetic point mutation in the glycosaminoglycan-recognition motif of Otx2 broadly delays the maturation of pivotal parvalbumin-positive (PV+) interneurons not only in V1 but also in the primary auditory (A1) and medial prefrontal cortex (mPFC). Consequently, not only visual, but also auditory plasticity is delayed, including the experience-dependent expansion of tonotopic maps in A1 and the acquisition of acoustic preferences in mPFC which mitigates anxious behavior. In addition, Otx2 mis-localization leads to dynamic turnover of selected perineuronal net (PNN) components well beyond the normal critical period in V1 and mPFC. These findings reveal widespread actions of Otx2 signaling in the postnatal cortex controlling the maturational trajectory across modalities. Disrupted PV+ network function and deficits in PNN integrity are implicated in a variety of psychiatric illnesses, suggesting a potential global role for Otx2 function in establishing mental health.