**4. The hyaluronic acid-based ECM regulates adult synaptic plasticity**

The functional mechanisms by which the HA-based ECM implements adult forms of brain plasticity including classical (Hebbian) and homeostatic plasticity are still elusive (for an overview see [3]). Research investigating knockout models lacking particular components of the ECM have provided major insights into the impact of the ECM on adult synaptic plasticity. For instance, different mouse models deficient in specific matrix components, as tenascin-R, brevican, or neurocan, all showed impaired forms of hippocampal long-term potentiation (LTP). However, during the brain development of k.o.-models compensatory mechanisms might mimic the deficit of a particular matrix component and hence limit the significance of the findings. Therefore, another experimental strategy is to use acute enzymatic weakening of the ECM based on the local application of different matrix-degrading enzymes (**Figure 1C**). For instance, it has been shown that treatment with chABC impaired theta-burst-induced LTP in CA1–CA3 pyramidal synapses in hippocampal slices [19]. It has been suggested that this is due to an increased excitability of GABAergic perisomatic interneurons [3]. Similar findings have been found with Injection of the hyaluronidase (HYase) from *Streptomyces hyalurolyti‐ cus* [20]. In contrast to other hyaluronidases, this enzyme is highly specific to HA and does not digest CS. The phenotype in these experiments could be rescued by perfusion with HA. It has been suggested that this is due to the fact that HA directly regulates L-type voltage-gated calcium channels (L-VDCC; Cav1.2) of CA1 neurons and thus postsynaptic Ca2+ entry and hippocampal-dependent forms of learning [3,20]. In a recent study, we have described a molecular mechanism by which acute ECM removal was altering short-term-dependent forms of synaptic plasticity [21]. We measured paired pulse ratios in dissociated hippocampal cultures by cell-attached recordings. We found the typical robust paired pulse depression (PPD) under control conditions. Interestingly, digestion of the ECM by infusion of HYase prevented cells from expressing PPD. We further found increased lateral diffusion of extra‐ synaptic AMPA receptors after ECM digestion as key correlate of this effect [21]. This results in a higher exchange between synaptic desensitized receptors with extrasynaptic naïve ones, which quickly replenishes the pool of excitable receptors in the active zone of a synapse (**Figure 1C**). In such conditions, synapses are able to follow higher firing frequencies. Blockade of lateral diffusion of AMPA receptors by cross-linking with antibodies, on the other hand, led to stronger PPD caused by accumulated desensitized synaptic AMPA receptors [22]. Similarly, mobility of other receptor types can be modulated by specific proteolytic enzymes. For instance, MMP9 increases the mobility of NMDA-type, but not AMPA receptors. These results have shown that the perisynaptic ECM forms surface compartments that act as diffusion barrier for membrane proteins such as AMPA receptors. Modulation of lateral receptor diffusion provides a novel mechanism of short-term plasticity due to a changed extracellular micromillieu at individual synapses. Interestingly, lack of the major hyaluronan synthetase HAS3 in the hippocampus did not lead to a striking morphological change in the ECM. However, the extracellular space was reduced, which resulted in a more dense packing of cells and lower diffusion of soluble molecules in the CA1 *stratum pyramidale* [23]. Further, these animals were prone to epileptic seizures which underlines the importance of the regulatory function of the HA-based ECM that is required for instance for volume transmission and ion homeostasis.

for rapid experience-based behavioral adaptations of higher vertebrates [4]. Nonetheless, the adult, healthy brain retains a remarkable capability of plastic reorganization that is essential

**4. The hyaluronic acid-based ECM regulates adult synaptic plasticity**

The functional mechanisms by which the HA-based ECM implements adult forms of brain plasticity including classical (Hebbian) and homeostatic plasticity are still elusive (for an overview see [3]). Research investigating knockout models lacking particular components of the ECM have provided major insights into the impact of the ECM on adult synaptic plasticity. For instance, different mouse models deficient in specific matrix components, as tenascin-R, brevican, or neurocan, all showed impaired forms of hippocampal long-term potentiation (LTP). However, during the brain development of k.o.-models compensatory mechanisms might mimic the deficit of a particular matrix component and hence limit the significance of the findings. Therefore, another experimental strategy is to use acute enzymatic weakening of the ECM based on the local application of different matrix-degrading enzymes (**Figure 1C**). For instance, it has been shown that treatment with chABC impaired theta-burst-induced LTP in CA1–CA3 pyramidal synapses in hippocampal slices [19]. It has been suggested that this is due to an increased excitability of GABAergic perisomatic interneurons [3]. Similar findings have been found with Injection of the hyaluronidase (HYase) from *Streptomyces hyalurolyti‐ cus* [20]. In contrast to other hyaluronidases, this enzyme is highly specific to HA and does not digest CS. The phenotype in these experiments could be rescued by perfusion with HA. It has been suggested that this is due to the fact that HA directly regulates L-type voltage-gated calcium channels (L-VDCC; Cav1.2) of CA1 neurons and thus postsynaptic Ca2+ entry and hippocampal-dependent forms of learning [3,20]. In a recent study, we have described a molecular mechanism by which acute ECM removal was altering short-term-dependent forms of synaptic plasticity [21]. We measured paired pulse ratios in dissociated hippocampal cultures by cell-attached recordings. We found the typical robust paired pulse depression (PPD) under control conditions. Interestingly, digestion of the ECM by infusion of HYase prevented cells from expressing PPD. We further found increased lateral diffusion of extra‐ synaptic AMPA receptors after ECM digestion as key correlate of this effect [21]. This results in a higher exchange between synaptic desensitized receptors with extrasynaptic naïve ones, which quickly replenishes the pool of excitable receptors in the active zone of a synapse (**Figure 1C**). In such conditions, synapses are able to follow higher firing frequencies. Blockade of lateral diffusion of AMPA receptors by cross-linking with antibodies, on the other hand, led to stronger PPD caused by accumulated desensitized synaptic AMPA receptors [22]. Similarly, mobility of other receptor types can be modulated by specific proteolytic enzymes. For instance, MMP9 increases the mobility of NMDA-type, but not AMPA receptors. These results have shown that the perisynaptic ECM forms surface compartments that act as diffusion barrier for membrane proteins such as AMPA receptors. Modulation of lateral receptor diffusion provides a novel mechanism of short-term plasticity due to a changed extracellular micromillieu at individual synapses. Interestingly, lack of the major hyaluronan synthetase

for constantly adapting to our ever-changing environment.

148 Composition and Function of the Extracellular Matrix in the Human Body

Weakening of the ECM and the corresponding synaptic networks might further promote architectural remodeling and even increased spine motility. In *in vitro* hippocampal cultures, microinjections of chABC treatment increased the motility of local spines and induced spine remodeling in a β1-integrin-dependent manner [24].
