Attenuation of the extracellular matrix increases the number of synapses but suppresses synaptic plasticity through upregulation of SK channels

Highlights

• ECM attenuation triggers the appearance of new synapses in the hippocampus.

• GABAergic signalling remains unaffected.

• Excitability of pyramidal neurons and LTP are reduced by SK channels.

• Blockade of SK channels reveals enhanced LTP.

• LTP enhancement is mediated by the ROCK pathway.

Abstract

The effect of brain extracellular matrix (ECM) on synaptic plasticity remains controversial. Here, we show that targeted enzymatic attenuation with chondroitinase ABC (ChABC) of ECM triggers the appearance of new glutamatergic synapses on hippocampal pyramidal neurons, thereby increasing the amplitude of field EPSPs while decreasing both the mean miniature EPSC amplitude and AMPA/NMDA ratio. Although the increased proportion of ‘unpotentiated’ synapses caused by ECM attenuation should promote long-term potentiation (LTP), surprisingly, LTP was suppressed. The upregulation of small conductance Ca²⁺-activated K⁺ (SK) channels decreased the excitability of pyramidal neurons, thereby suppressing LTP. A blockade of SK channels restored cell excitability and enhanced LTP; this enhancement was abolished by a blockade of Rho-associated protein kinase (ROCK), which is involved in the maturation of dendritic spines. Thus, targeting ECM elicits the appearance of new synapses, which can have potential applications in regenerative medicine. However, this process is compensated for by a reduction in postsynaptic neuron excitability, preventing network overexcitation at the expense of synaptic plasticity.

Introduction

The components of the extracellular matrix (ECM) form a molecular meshwork in the extracellular space of the brain. The major components of the ECM are hyaluronic acid, chondroitin sulfate proteoglycans (CSPGs), link proteins such as hyaluronan, and proteoglycan link protein 1 (HAPLN1/CRTL1), and glycoproteins such as tenascinR, which cross-link CSPGs and stabilize ECM structure [1]. The ubiquitous localization of ECM allows it to broadly affect neuronal function, both through mechanical and electrical regulation of diffusion in the extracellular space, as well as by interacting with several transmembrane receptors and channels [2].

During prenatal and early postnatal brain development, the ECM provides either adhesive or repellent cues that control cell migration and navigation of growing axons. These processes involve signaling through a diverse group of transmembrane receptors, such as integrins [3], ApoER2, VLDL, EphB [4,5], and RPTPσ [6,7]. In the mature brain, a number of ECM molecules and ECM receptors trigger various biochemical cascades that regulate neuronal function [8]. Attenuation of the ECM in the adult brain can potentially restore the brain to an immature state and promote (re-)wiring [9]. Hence, manipulation of the ECM has been suggested as a way to stimulate brain repair after injury and boost brain plasticity [10,11]. Nevertheless, the role of ECM in synaptic plasticity remains controversial [12,13]. Initial studies suggested that acute enzymatic digestion of CSPGs with chondroitinase ABC (ChABC) impaired both long-term potentiation (LTP) and depression (LTD) in the CA1 region of the hippocampus [14]. Similarly, LTP in hippocampal CA1 neurons was reduced by a deficiency in the ECM components brevican, or tenascin-R reduced [[14], [15], [16], [17]], as well as after acute enzymatic digestion of hyaluronic acid with hyaluronidase [18]. Moreover, intra-hippocampal injection of hyaluronidase impaired contextual fear conditioning [18]. These studies demonstrate that ECM attenuation may disrupt synaptic plasticity and some forms of learning and memory. On the other hand, mice deficient in CSPG phosphacan/RPTPbeta exhibit increased LTP [19]. HAPLN1 knockout mice retain high juvenile-type levels of ocular dominance plasticity in adulthood [20] and exhibit enhanced long-term object recognition memory and LTD in the perirhinal cortex [21]. Similarly, recognition memory is enhanced following ChABC injection [21]. These findings suggest that genetic or enzymatic attenuation of the ECM may promote some forms of synaptic plasticity and learning.

Here, we resolve this controversy by showing that ECM attenuation with ChABC increases the number of ‘plastic’ glutamatergic synapses onto CA1 pyramidal neurons while at the same time reducing neuronal excitability through the upregulation of small conductance Ca²⁺-activated K⁺ (SK) channels that suppress LTP.