Although the molecular, cellular, and systems mechanisms required for initial memory processing have been intensively investigated, those underlying permanent memory storage remain elusive. We present neuroanatomical, pharmacological, and genetic results demonstrating that the anterior cingulate cortex plays a critical role in remote memory for contextual fear conditioning. Imaging of activity-dependent genes shows that the anterior cingulate is activated by remote memory and that this activation is impaired by a null alpha-CaMKII mutation that blocks remote memory. Accordingly, reversible inactivation of this structure in normal mice disrupts remote memory without affecting recent memory.

Matrix metalloproteinases (MMPs) are extracellular proteases that have well recognized roles in cell signaling and remodeling in many tissues. In the brain, their activation and function are customarily associated with injury or pathology. Here, we demonstrate a novel role for MMP-9 in hippocampal synaptic physiology, plasticity, and memory. MMP-9 protein levels and proteolytic activity are rapidly increased by stimuli that induce late-phase long-term potentiation (L-LTP) in area CA1. Such regulation requires NMDA receptors and protein synthesis. Blockade of MMP-9 pharmacologically prevents induction of L-LTP selectively; MMP-9 plays no role in, nor is regulated during, other forms of short-term synaptic potentiation or long-lasting synaptic depression. Similarly, in slices from MMP-9 null-mutant mice, hippocampal LTP, but not long-term depression, is impaired in magnitude and duration; adding recombinant active MMP-9 to null-mutant slices restores the magnitude and duration of LTP to wild-type levels. Activated MMP-9 localizes in part to synapses and modulates hippocampal synaptic physiology through integrin receptors, because integrin function-blocking reagents prevent an MMP-9-mediated potentiation of synaptic signal strength. The fundamental importance of MMP-9 function in modulating hippocampal synaptic physiology and plasticity is underscored by behavioral impairments in hippocampal-dependent memory displayed by MMP-9 null-mutant mice. Together, these data reveal new functions for MMPs in synaptic and behavioral plasticity.

Neurons of adult brain are able to remodel their synaptic connections in response to various stimuli. Modifications of the peridendritic environment, including the extracellular matrix, are likely to play a role during synapse remodeling. Proteolytic disassembly of ECM is a complex process using the regulated actions of specific extracellular proteinases. One of bestcharacterized families of matrix-modifying enzymes is the matrix metalloproteinase (MMP) family. Here, we describe changes in the expression and function of two well known MMPs, MMP-9 and MMP-2, in adult rat brain before and after systemic administration of the glutamate receptor agonist kainate. Kainate application results in enhanced synaptic transmission and seizures followed by selective tissue remodeling, primarily in hippocampal dentate gyrus. MMP-9 but not MMP-2 was highly expressed by neurons in normal adult rat brain. MMP-9 protein was localized in neuronal cell bodies and dendrites. Kainate upregulated the level of MMP-9 mRNA and protein within hours after drug administration. This was followed several hours later by MMP-9 enzymatic activation. Within hippocampus, MMP-9 mRNA and activity were increased selectively in dentate gyrus, including its dendritic layer. In addition, MMP-9 mRNA levels decreased in areas undergoing neuronal cell loss. This unique spatiotemporal pattern of MMP-9 expression suggests its involvement in activity-dependent remodeling of dendritic architecture with possible effects on synaptic physiology.

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Temporal lobe epilepsy (TLE) is a devastating disease in which aberrant synaptic plasticity plays a major role. We identify matrix metalloproteinase (MMP) 9 as a novel synaptic enzyme and a key pathogenic factor in two animal models of TLE: kainate-evoked epilepsy and pentylenetetrazole (PTZ) kindling–induced epilepsy. Notably, we show that the sensitivity to PTZ epileptogenesis is decreased in MMP-9 knockout mice but is increased in a novel line of transgenic rats overexpressing MMP-9. Immunoelectron microscopy reveals that MMP-9 associates with hippocampal dendritic spines bearing asymmetrical (excitatory) synapses, where both the MMP-9 protein levels and enzymatic activity become strongly increased upon seizures. Further, we find that MMP-9 deficiency diminishes seizure-evoked pruning of dendritic spines and decreases aberrant synaptogenesis after mossy fiber sprouting. The latter observation provides a possible mechanistic basis for the effect of MMP-9 on epileptogenesis. Our work suggests that a synaptic pool of MMP-9 is critical for the sequence of events that underlie the development of seizures in animal models of TLE.

Matrix metalloproteinase-9 (MMP-9) is a member of the metzincin family of mostly extracellularly operating proteases. Despite the fact that all of these enzymes might be target promiscuous, with largely overlapping catalogs of potential substrates, MMP-9 has recently emerged as a major and apparently unique player in brain physiology and pathology. The specificity of MMP-9 may arise from its very local and time-restricted actions, even when released in the brain from cells of various types, including neurons, glia, and leukocytes. In fact, the quantity of MMP-9 is very low in the naive brain, but it is markedly activated at the levels of enzymatic activity, protein abundance, and gene expression following various physiological stimuli and pathological insults. Neuronal MMP-9 participates in synaptic plasticity by controlling the shape of dendritic spines and function of excitatory synapses, thus playing a pivotal role in learning, memory, and cortical plasticity. When improperly unleashed, MMP-9 contributes to a large variety of brain disorders, including epilepsy, schizophrenia, autism spectrum disorder, brain injury, stroke, neurodegeneration, pain, brain tumors, etc. The foremost mechanism of action of MMP-9 in brain disorders appears to be its involvement in immune/inflammation responses that are related to the enzyme's ability to process and activate various cytokines and chemokines, as well as its contribution to bloodbrain barrier disruption, facilitating the extravasation of leukocytes into brain parenchyma. However, another emerging possibility (i.e., the control of MMP-9 over synaptic plasticity) should not be neglected. The translational potential of MMP-9 has already been recognized in both the diagnosis and treatment domains. The most striking translational aspect may be the discovery of MMP-9 up-regulation in a mouse model of Fragile X syndrome, quickly followed by human studies and promising clinical trials that have sought to inhibit MMP-9. With regard to diagnosis, suggestions have been made to use MMP-9 alone or combined with tissue inhibitor of matrix metalloproteinase-1 or brain-derived neurotrophic factor as disease biomarkers.

Dicer-dependent noncoding RNAs, including microRNAs (miRNAs), play an important role in a modulation of translation of mRNA transcripts necessary for differentiation in many cell types. In vivo experiments using cell type-specific Dicer1 gene inactivation in neurons showed its essential role for neuronal development and survival. However, little is known about the consequences of a loss of miRNAs in adult, fully differentiated neurons. To address this question, we used an inducible variant of the Cre recombinase (tamoxifeninducible CreERT2) under control of Camk2a gene regulatory elements. After induction of Dicer1 gene deletion in adult mouse forebrain, we observed a progressive loss of a whole set of brain-specific miRNAs. Animals were tested in a battery of both aversively and appetitively motivated cognitive tasks, such as Morris water maze, IntelliCage system, or trace fear conditioning. Compatible with rather long half-life of miRNAs in hippocampal neurons, we observed an enhancement of memory strength of mutant mice 12 weeks after the Dicer1 gene mutation, before the onset of neurodegenerative process. In acute brain slices, immediately after high-frequency stimulation of the Schaffer collaterals, the efficacy at CA3-to-CA1 synapses was higher in mutant than in control mice, whereas long-term potentiation was comparable between genotypes. This phenotype was reflected at the subcellular and molecular level by the elongated filopodia-like shaped dendritic spines and an increased translation of synaptic plasticity-related proteins, such as BDNF and MMP-9 in mutant animals. The presented work shows miRNAs as key players in the learning and memory process of mammals.

Members of the metzincin family of metalloproteinases have long been considered merely degradative enzymes for extracellular matrix molecules. Recently, however, there has been growing appreciation for these proteinases and their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs), as fine modulators of nervous system physiology and pathology. Present all along the phylogenetic tree, in all neural cell types, from the nucleus to the synapse and in the extracellular space, metalloproteinases exhibit a complex spatiotemporal profile of expression in the nervous parenchyma and at the neurovascular interface. The irreversibility of their proteolytic activity on numerous biofactors (e.g., growth factors, cytokines, receptors, DNA repair enzymes, matrix proteins) is ideally suited to sustain structural changes that are involved in physiological or postlesion remodeling of neural networks, learning consolidation or impairment, neurodegenerative and neuroinflammatory processes, or progression of malignant gliomas. The present review provides a state of the art overview of the involvement of the metzincin/TIMP system in these processes and the prospects of new therapeutic strategies based on the control of metalloproteinase activity.The importance of proteolysis in tissue structure/function is reflected not only in the evolutionary conservation of protease genes in all kingdoms (e.g., from archaea and eubacteria to plants and animals) but also the genomic complexity of this protein class. The "degradome," the repertoire of proteases produced by cells, consists of at least 569 human, 629 rat, and 644 mouse proteases or protease-like proteins and homologs, whereas 156 human protease inhibitor genes have been identified (Puente et al., 2003). The proteases are classified into five major catalytic classes, including metalloproteinases and serine, cysteine, threonine, and aspartic proteinases, with the metalloproteinases representing the largest class (Fig. 1 A). The metzincin family of metalloproteinases is so named for the conserved Met residue at the active site and the use of a zinc ion in the enzymatic reaction. This family comprises matrix metalloproteinases (MMPs), a disintegrin and metalloproteinases (ADAMs), and ADAM proteases with thrombospondin motifs (ADAMTSs). Interest in MMPs began with the identification of an enzyme that contributes to tail resorption during tadpole metamorphosis (collagenase-1, MMP-1) (Gross and Lapiere, 1962) and increased on the discovery that these enzymes not only play a role in normal tissue remodeling but were upregulated in diverse human diseases, including chronic inflammatory disorders and cancer. MMPsMMPs, encoded by 24 human and 23 mouse genes, include secreted and membrane-associated members divided into four main subgroups according to their domain structure, including collagenases, stromelysins, gelatinases, and membrane-type MMPs (MT-MMPs) (Fig.