Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

TIMPs and MMPs expression in CSF from patients with TSP/HAM

The tropical spastic paraparesis or human T-cell lymphotropic virus associated myelopathy (TSP/HAM), has been related with an overexpression of matrix metalloproteinases (MMPs), especially MMP-9. Initial studies of reverse zymography with cerebrospinal fluid (CSF) from TSP/HAM patients, and controls showed the presence of TIMPs, endogenous MMP inhibitors. We determined in CSF the levels of TIMPs by immunoanalysis in 25 patients with TSP/HAM, and compared with two groups: controls and patients with acute and subacute inflammatory neurological diseases. We found that TIMP-2, TIMP-3 and TIMP-4 levels were significantly higher than in controls in both TSP/HAM and inflammatory patients, while TIMP-1 was increased only in the inflammatory group. Levels of MMP-3 and MMP-9 from the two groups of patients showed a significant upregulation in CSF. In the CSF of around the 70% of TSP-HAM and inflammatory patients the presence MMP-9 was detected by zymography, but not in controls. MMP-2 was only overexpressed in the acute inflammatory group. The active form of MMP-2 was observed in both groups of patients with a similar high frequency (60%). MMPs overexpressions are independent of the evolution time of the disease in TSP/HAM. The chronic overexpression of these extracelullar matrix proteins detected in CSF of TSP/HAM should be indirectly produced by secreted viral proteins being responsible for the progression of this disease, accounting for the observed differences with acute inflammatory patients. Our results support the existence of an imbalance between MMPs and their endogenous tissue inhibitors, which could be a pathogenic factor in the chronicity of TSP/HAM.

Age‐Related Decline in Spatial Learning and Memory: Attenuation by Captopril

Spontaneously hypertensive rats (SHR) compared to normotensive rats display an accelerated decline in spatial learning and memory. However, few studies have systematically examined the independent contribution of hypertension vs. other age-related mechanisms to this decline. The present study uses a repeated acquisition water maze task to test the hypothesis that hypertension and/or the presence of angiotensin II can accelerate the age-related decrease of spatial learning and memory in rats. We have previously shown that both SHR and Wistar-Kyoto rats (WKY) display age-related decreases in spatial learning and memory; however, the rate of decline differs between the strains. The present results demonstrate that compared to young rats of the same strain, learning and memory in SHR declines significantly already at 12 months of age, and at 24 months of age both SHR and WKY rats are severely impaired in the water maze task. Lifetime treatment of either SHR or WKY with the antihypertensive drug captopril [an angiotensin converting enzyme (ACE) inhibitor] significantly attenuates the age-related impairment in learning and memory. In contrast, chronic treatment of SHR with captopril from 6 months of age only modestly decreases the decline in learning and memory. Whereas lifetime treatment with the vasodilator drug hydralazine also reduces arterial pressure in SHR, this treatment does not significantly preserve learning in 24-month-old SHR. Together, the data suggest captopril can delay the decline in spatial learning and memory in both aging SHR and WKY. Further, the results indicate that the memory enhancing effects of captopril are not primarily the due to the ability of captopril to lower blood pressure.

Ethanol-induced impairment of spatial memory and brain matrix metalloproteinases

The formation of spatial memory appears to be dependent upon an intact hippocampus capable of the specific biochemical changes associated with synaptic remodeling. Hippocampal damage results in the disruption of synaptic remodeling and the acquisition of spatial memory tasks. Ethanol also disrupts normal hippocampal functioning and spatial memory. The present investigation established a dose-response relationship between ethanol treatment and impairment of spatial memory as measured using the circular water maze task. Intraperitoneal ethanol doses of 1.5 and 2 g/kg significantly increased the latency and distance swam to find the submerged pedestal as compared with a 1 g/kg dose, and 0.15 M NaCl vehicle control treatments. On days 2, 4, and 6 of acquisition animals were sacrificed and brain tissues were retained from the hippocampus, prefrontal neocortex, and cerebellum for measurement of matrix metalloproteinases (MMPs). The results indicated that ethanol treatment interfered with MMP-9, but not MMP-2, activity in the hippocampus, and to a lesser degree in the prefrontal cortex. No changes in the cerebellum were measured. Elevations in MMP activity appear to be a prerequisite to reconfiguration of extracellular matrix cell adhesion molecules thought to be important in the process of synaptic plasticity, which in turn appears to be necessary for memory consolidation. Thus, ethanol-induced impairment in the acquisition of spatial memory tasks may, in part, be due to disruption of brain MMP activity.

Matrix metalloproteinases in the adult brain physiology: a link between c-Fos, AP-1 and remodeling of neuronal connections?

Matrix metalloproteinases (MMPs), together with their endogenous inhibitors (TIMPs) form an enzymatic system that plays an important role in a variety of physiological and pathological conditions. These proteins are also expressed in the brain, especially under pathological conditions, in which glia as well as invading inflammatory cells provide the major source of the MMP activity. Surprisingly little is known about the MMP function(s) in adult neuronal physiology. This review describes available data on this topic, which is presented in a context of knowledge about the MMP/TIMP system in other organs as well as in brain disorders. An analysis of the MMP and TIMP expression patterns in the brain, along with a consideration of their regulatory mechanisms and substrates, leads to the proposal of possible roles of the MMP system in the brain. This analysis suggests that MMPs may play an important role in the neuronal physiology, especially in neuronal plasticity, including their direct participation in the remodeling of synaptic connections-a mechanism pivotal for learning and memory.

Memory-specific temporal profiles of gene expression in the hippocampus

Many experiments in the past have demonstrated the requirement of de novo gene expression during the long-term retention of learning and memory. Although previous studies implicated individual genes or genetic pathways in learning and memory, they did not uncover the collective behaviors or patterns of the genes. We have used genome-scale screening to analyze gene expression during spatial learning of rats in the Morris water maze. Our results show distinct temporal gene expression profiles associated with learning and memory. Exogenous administration of one peptide whose sustained increase during memory retention was implicated by microarray analysis, fibroblast growth factor (FGF)-18, improved spatial learning behavior, suggesting that pharmacological modulation of pathways and targets identified may allow new therapeutic approaches for improving learning and memory. Results of this study also suggest that while learning and physical activity involve common groups of genes, the behavior of learning and memory emerges from unique patterns of gene expression across time.

Brevican-deficient mice display impaired hippocampal CA1 long-term potentiation but show no obvious deficits in learning and memory

Brevican is a brain-specific proteoglycan which is found in specialized extracellular matrix structures called perineuronal nets. Brevican increases the invasiveness of glioma cells in vivo and has been suggested to play a role in central nervous system fiber tract development. To study the role of brevican in the development and function of the brain, we generated mice lacking a functional brevican gene. These mice are viable and fertile and have a normal life span. Brain anatomy was normal, although alterations in the expression of neurocan were detected. Perineuronal nets formed but appeared to be less prominent in mutant than in wild-type mice. Brevican-deficient mice showed significant deficits in the maintenance of hippocampal long-term potentiation (LTP). However, no obvious impairment of excitatory and inhibitory synaptic transmission was found, suggesting a complex cause for the LTP defect. Detailed behavioral analysis revealed no statistically significant deficits in learning and memory. These data indicate that brevican is not crucial for brain development but has restricted structural and functional roles.

Neural plasticity and the brain renin-angiotensin system

The brain renin-angiotensin system mediates several classic physiologies including body water balance, maintenance of blood pressure, cyclicity of reproductive hormones and sexual behaviors, and regulation of pituitary gland hormones. In addition, angiotensin peptides have been implicated in neural plasticity and memory. The present review initially describes the extracellular matrix (ECM) and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases, and tissue inhibitors of metalloproteinases in the maintenance and degradation of the ECM. It is the ECM that appears to permit synaptic remodeling and thus is critical to the plasticity that is presumed to underlie mechanisms of memory consolidation and retrieval. The interrelationship among long-term potentiation (LTP), CAMs, and synaptic strengthening is described, followed by the influence of angiotensins on LTP. There is strong support for an inhibitory influence by angiotensin II (AngII) and a facilitory role by angiotensin IV (AngIV), on LTP. Next, the influences of AngII and IV on associative and spatial memories are summarized. Finally, the impact of sleep deprivation on matrix metalloproteinases and memory function is described. Recent findings indicate that sleep deprivation-induced memory impairment is accompanied by a lack of appropriate changes in matrix metalloproteinases within the hippocampus and neocortex as compared with non-sleep deprived animals. These findings generally support an important contribution by angiotensin peptides to neural plasticity and memory consolidation.