Although neuronal stress circuits have been identified, little is known about the mechanisms that underlie the stress-induced neuronal plasticity leading to fear and anxiety. Here we found that the serine protease tissue-plasminogen activator (tPA) was upregulated in the central and medial amygdala by acute restraint stress, where it promoted stress-related neuronal remodeling and was subsequently inhibited by plasminogen activator inhibitor-1 (PAI-1). These events preceded stress-induced increases in anxiety-like behavior of mice. Mice in which the tPA gene has been disrupted did not show anxiety after up to three weeks of daily restraint and showed attenuated neuronal remodeling as well as a maladaptive hormonal response. These studies support the idea that tPA is critical for the development of anxiety-like behavior after stress.

Accumulation of the amyloid-beta (Abeta) peptide depends on both its generation and clearance. To better define clearance pathways, we have evaluated the role of the tissue plasminogen activator (tPA)-plasmin system in Abeta degradation in vivo. In two different mouse models of Alzheimer's disease, chronically elevated Abeta peptide in the brain correlates with the upregulation of plasminogen activator inhibitor-1 (PAI-1) and inhibition of the tPA-plasmin system. In addition, Abeta injected into the hippocampus of mice lacking either tPA or plasminogen persists, inducing PAI-1 expression and causing activation of microglial cells and neuronal damage. Conversely, Abeta injected into wild-type mice is rapidly cleared and does not cause neuronal degeneration. Thus, the tPA-plasmin proteolytic cascade aids in the clearance of Abeta, and reduced activity of this system may contribute to the progression of Alzheimer's disease.

SummaryA minority of individuals experiencing traumatic events develop anxiety disorders. The reason for the lack of correspondence between the prevalence of exposure to psychological trauma and the development of anxiety is unknown. Extracellular proteolysis contributes to fear-associated responses by facilitating neuronal plasticity at the neuron-matrix interface1-4. Here we show that the serine protease neuropsin is critical for stress-related plasticity in the amygdala by regulating the dynamics of EphB2/NMDA receptor interaction, the expression of Fkbp5 and anxiety-like behaviour. Stress results in neuropsin-dependent cleavage of EphB2 in the amygdala causing dissociation of EphB2 from the NR1-subunit of NMDA receptor and promoting membrane turnover of EphB2 receptors. Dynamic EphB2/NR1 interaction enhances NMDA receptor current, induces the Fkbp5 gene expression and enhances behavioural signatures of anxiety. Upon stress, neuropsin-deficient mice do not show EphB2 cleavage and its dissociation from NR1 resulting in a static EphB2/NR1 interaction, attenuated induction of the Fkbp5 gene and low anxiety. The behavioural response to stress can be restored by intra-amygdala injection of neuropsin into neuropsin-deficient mice and disrupted by the injection of either anti-EphB2 antibodies or silencing the Fkbp5 gene in the amygdala of wild-type animals. Our findings establish a novel neuronal pathway linking stress-induced proteolysis of EphB2 in the amygdala to anxiety.

Repeated stress can impair function in the hippocampus, a brain structure essential for learning and memory. Although behavioral evidence suggests that severe stress triggers cognitive impairment, as seen in major depression or posttraumatic stress disorder, little is known about the molecular mediators of these functional deficits in the hippocampus. We report here both pre-and postsynaptic effects of chronic stress, manifested as a reduction in the number of NMDA receptors, dendritic spines, and expression of growth-associated protein-43 in the cornu ammonis 1 region. Strikingly, the stress-induced decrease in NMDA receptors coincides spatially with sites of plasminogen activation, thereby predicting a role for tissue plasminogen activator (tPA) in this form of stress-induced plasticity. Consistent with this possibility, tPA؊͞؊ and plasminogen؊͞؊ mice are protected from stress-induced decrease in NMDA receptors and reduction in dendritic spines. At the behavioral level, these synaptic and molecular signatures of stressinduced plasticity are accompanied by impaired acquisition, but not retrieval, of hippocampal-dependent spatial learning, a deficit that is not exhibited by the tPA؊͞؊ and plasminogen؊͞؊ mice. These findings establish the tPA͞plasmin system as an important mediator of the debilitating effects of prolonged stress on hippocampal function at multiple levels of neural organization.dendritic spines ͉ learning ͉ NMDA receptor P sychological stress induces neuronal responses that can be either adaptive and directed toward maintaining homeostasis or maladaptive, leading to severe behavioral abnormalities (1). Posttraumatic stress disorder (PTSD) is a devastating disease triggered by a severe traumatic event(s) and characterized by cognitive impairment, depression, fear, and anxiety (2). Although little is known about the cellular mechanisms of PTSD, its different aspects are mediated by different brain structures (3). Animal and human studies suggest that stress-induced fear and anxiety are mediated by the amygdala (4, 5), and cognitive decline is a result of hippocampal dysfunction (6, 7). It has been hypothesized that the decrease in complexity of the hippocampal dendritic tree contributes to learning deficits (8), but molecular mechanisms underlying this dendritic plasticity are poorly understood.One molecule strategically positioned to control neuronal activity, dendritic remodeling, and learning is the NMDA receptor. It is located on dendritic spines and is critically involved in spine motility (9) and experience-induced neuronal plasticity (10). Although the decrease in the number of NMDA receptors leads to memory deficits (10), overexpression of some of its subunits results in more efficient learning (11).There is evidence that NMDA receptor function is linked to stress-induced neuronal and cognitive changes, because stressinduced remodeling is blocked by NMDA-receptor antagonists (12). The NMDA receptor has numerous ligands and modulators, and it is likely that the above processes may involve a n...