However, in addition specific direct interactions of neurotransmitter receptors with components of the ECM may influence the lateral diffusion of neurotransmitter receptors in particular and cell surface receptors in general. One example of such interaction is the clustering of AMPA receptors by the pentraxin family
member Narp (O’Brien et al., 1999). This immediate early gene forms clusters on the neuronal surface and co-aggregates AMPA receptors on spinal cord neurons. The size of surface compartments and the density of the ECM meshwork may play Adriamycin in vivo an important role in controlling the access of AMPA receptors to the synapse und thus influence their synaptic properties. For example, ECM structures could regulate the accessibility of exocytic and endocytic sites for a distinct receptor population. Both size of compartments and density of the ECM may be actively regulated by neurons and astrocytes contributing to the synthesis of ECM material.
On the one hand this may be achieved by altered expression of components of the ECM. In particular, the synthesis of hyaluronic acid seems to X-396 nmr be crucial for the formation of the dense ECM (Carulli et al., 2006). Moreover, it has been reported that formation of PNN-like structures in vitro is regulated by activity and that the removal of the ECM altered interneuron activity (Dityatev et al., 2007). However, these effects are rather slow and may account for long-term changes in neuronal properties rather than fast changes. On the other hand, local removal of ECM structures can contribute to plasticity regulation. Degradation of pre-existing ECM may be achieved on a much shorter time scale and could regulate both compartment size and the density of the ECM. A number of proteases, especially from the family of the matrix metalloproteases (MMPs) have been reported to cleave components cAMP of the ECM (Nakamura et al., 2000; Ethell & Ethell, 2007). In particular, ADAMTS4 (a
disintegrin and metalloproteinase with thrombospondin motifs 4) has the potential to modify ECM structure as it degrades two of the major components of the hyaluronan–CSPG-based ECM, i.e. aggrecan and brevican. However, its impact on neuronal function and its regulation during neuronal activity remain to be clarified. One of the best-studied MMPs in the nervous system is MMP9. Its activity has been associated with enhanced neuronal activity (Michaluk et al., 2007) and depletion of MMP9 results in impairment of long-term potentiation at hippocampal synapses (Nagy et al., 2006). Application of MMP9 to neuronal primary cultures affects lateral diffusion of NMDA receptors without changing the mobility of AMPA receptors or the structure of the hyaluronic acid-based ECM (Michaluk et al., 2009). Rather, extracellular MMP9 proteolysis induced integrin beta1-dependent signaling, which then led to the mobilization of NMDA receptors.
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