Matrix metalloproteinase-9 (MMP-9) at the synapse : functions and targets

Abstract

Matrix metalloproteinases-9 (MMP-9) is a zinc-dependent endopeptidase which acts outside the cell and can cleave various extracellular matrix (ECM) proteins as well as adhesion molecules. MMP-9 has recently emerged as one of the important molecules involved in synaptic plasticity, however the exact role of this protease at the neuronal junctions is still to be described. The objective of this work was to reveal in more details the functions and targets of MMP-9 at the synapse. In the Chapter 2 we show that beta-dystroglycan (beta-DG), a transmembrane protein, is a synaptic target for MMP-9 which is cleaved upon neuronal stimulation. We observed that stimulation of neuronal cultures caused an increase in MMP-9 activity which coincides with cleavage of beta-DG and that this cleavage can be blocked by over-expression of tissue inhibitor of metalloproteinases-1 (TIMP-1). We also showed the involvement of MMP-9 activity in cleavage of beta-DG in vivo , as well as the co-localization of both proteins at the same asymmetric synapses in the hippo-campus. Next, in the Chapter 3 we examined the influence of MMP-9 on mobility of one of the most important receptors for the synaptic plasticity – NMDA receptor (NMDAR). Using single Quantum Dot tracking we demonstrated that enzymatic activity of MMP-9 increased lateral trafficking of NR1 subunit-containing NMDA receptors and not AMPA receptors. More interestingly, we showed that the mechanism of MMP-9 action on NMDARs mobility was neither mediated by a general change in the structure of ECM, as this was unchanged after incubation with recombinant MMP-9, nor by direct cleavage of receptor subunits, but rather through integrin beta1-dependent pathway. Finally, in the Chapter 4 we focused on the influence of MMP-9 on structural plasticity of dendritic spines which are known to bear excitatory synapses. In particular, using three independent experimental models: dissociated and organotypic cultures, as well as transgenic rats over-expressing auto-activating MMP-9, we showed that enzymatic activity of MMP-9 caused transition of dendritic spines into more filopodia-like structures. Moreover, we demonstrated again the involvement of integrin beta1 subunit in this process. Our results described and discussed in this thesis show MMP-9 is released shortly after neuronal stimulation, is potent to cleave synaptic proteins without causing large changes in ECM structure, to modify behaviour of important synaptic receptors and to change morphology of synapses. Thus, as discussed in chapter 5, our results together with several recent ones, strongly indicate MMP-9 function in synaptic remodelling.