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 α-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 best-characterized 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.

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 ofMMP‐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 ofMMP‐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. NeuronalMMP‐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 ofMMP‐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 blood–brain barrier disruption, facilitating the extravasation of leukocytes into brain parenchyma. However, another emerging possibility (i.e., the control ofMMP‐9 over synaptic plasticity) should not be neglected. The translational potential ofMMP‐9 has already been recognized in both the diagnosis and treatment domains. The most striking translational aspect may be the discovery ofMMP‐9 up‐regulation in a mouse model of Fragile X syndrome, quickly followed by human studies and promising clinical trials that have sought to inhibitMMP‐9. With regard to diagnosis, suggestions have been made to useMMP‐9 alone or combined with tissue inhibitor of matrix metalloproteinase‐1 or brain‐derived neurotrophic factor as disease biomarkers. image MMP‐9, through cleavage of specific target proteins, plays a major role in synaptic plasticity and neuroinflammation, and by those virtues contributes to brain physiology and a host of neurological and psychiatric disorders.This article is part of the60th Anniversary special issue.

While a number of oncogenes are expressed in a cell cycle-dependent manner, their role in the control of cell proliferation can only be established by a direct functional assay. The c- myc protein, upon microinjection into nuclei of quiescent Swiss 3T3 cells, cooperated with platelet-poor plasma in the stimulation of cellular DNA synthesis. This suggests that c- myc protein, like platelet-derived growth factor (PDGF), may act as a competence factor in the cell cycle to promote the progression of cells to S phase. The presence in the medium of an antibody against PDGF abolished DNA synthesis induced by microinjected PDGF; however, the microinjected c- myc protein stimulated DNA synthesis even when its own antibody was present in the medium. The c- myc protein may act as an intracellular competence factor, while PDGF expresses its biological activity only from outside the cells.