Matrix metalloproteinase inhibition counteracts impairment of cortical experience-dependent plasticity after photothrombotic stroke

Impact of somatostatin interneurons on interactions between barrels in plasticity induced by whisker deprivation

The activity of inhibitory interneurons has a profound role in shaping cortical plasticity. Somatostatin-expressing interneurons (SOM-INs) are involved in several aspects of experience-dependent cortical rewiring. We addressed the question of the barrel cortex SOM-IN engagement in plasticity formation induced by sensory deprivation in adult mice (2-3 months old). We used a spared vibrissa paradigm, resulting in a massive sensory map reorganization. Using chemogenetic manipulation, the activity of barrel cortex SOM-INs was blocked or activated by continuous clozapine N-oxide (CNO) administration during one-week-long deprivation. To visualize the deprivation-induced plasticity, [¹⁴C]-2-deoxyglucose mapping of cortical functional representation of the spared whisker was performed at the end of the deprivation. The plasticity was manifested as an extension of cortical activation in response to spared vibrissae stimulation. We found that SOM-IN inhibition in the cortical column of the spared whisker did not influence the areal extent of the cortex activated by the spared whisker. However, blocking the activity of SOM-INs in the deprived column, adjacent to the spared one, decreased the plasticity of the spared whisker representation. SOM-IN activation did not affect plasticity. These data show that SOM-IN activity is part of cortical circuitry that affects interbarrel interactions underlying deprivation-induced plasticity in adult mice.

Spreading Depressions and Periinfarct Spreading Depolarizations in the Context of Cortical Plasticity

Studies of cortical function-recovery require a comparison between normal and post-stroke conditions that lead to changes in cortical metaplasticity. Focal cortical stroke impairs experience-dependent plasticity in the neighboring somatosensory cortex and usually evokes periinfarct depolarizations (PiDs) - spreading depression-like waves. Experimentally induced spreading depressions (SDs) affect gene expression and some of these changes persist for at least 30 days. In this study we compare the effects of non-stroke depolarizations that impair cortical experience-dependent plasticity to the effects of stroke, by inducing experience-dependent plasticity in rats with SDs or PiDs by a month of contralateral partial whiskers deprivation. We found that whiskers' deprivation after SDs resulted in normal cortical representation enlargement suggesting that SDs and PiDs depolarization have no influence on experience-dependent plasticity cortical map reorganization. PiDs and the MMP-9, -3, -2 or COX-2 proteins, which are assumed to influence metaplasticity in rats after stroke were compared between SDs induced by high osmolarity KCl solution and the PiDs that followed cortical photothrombotic stroke (PtS). We found that none of these factors directly caused cortical post-stroke metaplasticity changes. The only significant difference between stoke and induced SD was a greater imbalance in interhemispheric activity equilibrium after stroke. The interhemispheric interactions that were modified by stroke may therefore be promising targets for future studies of post-stroke experience-dependent plasticity and of recuperation studies.

Spike Timing-Dependent Plasticity in the Mouse Barrel Cortex Is Strongly Modulated by Sensory Learning and Depends on Activity of Matrix Metalloproteinase 9

Experience and learning in adult primary somatosensory cortex are known to affect neuronal circuits by modifying both excitatory and inhibitory transmission. Synaptic plasticity phenomena provide a key substrate for cognitive processes, but precise description of the cellular and molecular correlates of learning is hampered by multiplicity of these mechanisms in various projections and in different types of neurons. Herein, we investigated the impact of associative learning on neuronal plasticity in distinct types of postsynaptic neurons by checking the impact of classical conditioning (pairing whisker stroking with tail shock) on the spike timing-dependent plasticity (t-LTP and t-LTD) in the layer IV to II/III vertical pathway of the mouse barrel cortex. Learning in this paradigm practically prevented t-LTP measured in pyramidal neurons but had no effect on t-LTD. Since classical conditioning is known to affect inhibition in the barrel cortex, we examined its effect on tonic GABAergic currents and found a strong downregulation of these currents in the layer II/III interneurons but not in pyramidal cells. Matrix metalloproteinases emerged as crucial players in synaptic plasticity and learning. We report that the blockade of MMP-9 (but not MMP-3) abolished t-LTP having no effect on t-LTD. Moreover, associative learning resulted in an upregulation of gelatinolytic activity within the "trained" barrel. We conclude that LTP induced by spike timing-dependent plasticity (STDP) paradigm is strongly correlated with associative learning and critically depends on the activity of MMP-9.

Physical Exercise Preserves Adult Visual Plasticity in Mice and Restores it after a Stroke in the Somatosensory Cortex

The primary visual cortex (V1) is widely used to study brain plasticity, which is not only crucial for normal brain function, such as learning and memory, but also for recovery after brain injuries such as stroke. In standard cage (SC) raised mice, experience-dependent ocular dominance (OD) plasticity in V1 declines with age and is compromised by a lesion in adjacent and distant cortical regions. In contrast, mice raised in an enriched environment (EE), exhibit lifelong OD plasticity and are protected from losing OD plasticity after a stroke-lesion in the somatosensory cortex. Since SC mice with an access to a running wheel (RW) displayed preserved OD plasticity during aging, we investigated whether physical exercise might also provide a plasticity promoting effect after a cortical stroke. To this end, we tested if adult RW-raised mice preserved OD plasticity after stroke and also if short-term running after stroke restored OD plasticity to SC mice. Indeed, unlike mice without a RW, adult RW mice continued to show OD plasticity even after stroke, and a 2 weeks RW experience after stroke already restored lost OD plasticity. Additionally, the experience-enabled increase of the spatial frequency and contrast threshold of the optomotor reflex of the open eye, normally lost after a stroke, was restored in both groups of RW mice. Our data suggest that physical exercise alone can not only preserve visual plasticity into old age, but also restore it after a cortical stroke.

Optimal level activity of matrix metalloproteinases is critical for adult visual plasticity in the healthy and stroke-affected brain

The ability of the adult brain to undergo plastic changes is of particular interest in medicine, especially regarding recovery from injuries or improving learning and cognition. Matrix metalloproteinases (MMPs) have been associated with juvenile experience-dependent primary visual cortex (V1) plasticity, yet little is known about their role in this process in the adult V1. Activation of MMPs is a crucial step facilitating structural changes in a healthy brain; however, upon brain injury, upregulated MMPs promote the spread of a lesion and impair recovery. To clarify these seemingly opposing outcomes of MMP-activation, we examined the effects of MMP-inhibition on experience-induced plasticity in healthy and stoke-affected adult mice. In healthy animals, 7-day application of MMP-inhibitor prevented visual plasticity. Additionally, treatment with MMP-inhibitor once but not twice following stroke rescued plasticity, normally lost under these conditions. Our data imply that an optimal level of MMP-activity is crucial for adult visual plasticity to occur.

DOI:http://dx.doi.org/10.7554/eLife.11290.001

When part of the brain becomes damaged as a result of injury or disease – for example, a stroke – other brain regions can sometimes take over from the damaged part. This is one example of a phenomenon called brain plasticity. The strengthening and weakening of connections between neurons that underlies learning and memory is another, less extreme, example of plasticity. While the brain is most plastic during childhood, it remains malleable to some degree throughout life.

The brain’s visual system in particular shows robust and predictable plasticity, and so is often used by neuroscientists to study mechanisms behind plasticity. In young rodents, taping one eye shut for a few days causes inputs from that eye to visual areas of the brain to become weaker. Inputs from the open eye meanwhile become stronger, leading to improved vision in the open eye. Such plasticity also occurs in adult rodents, but the eye must be closed for longer to produce an effect.

In young animals, this plasticity depends, in part, on enzymes called matrix metalloproteinases (MMPs). These help to regulate a network of proteins called the extracellular matrix, which provides structural support for cells. Pielecka-Fortuna et al. now provide the first evidence that MMP enzymes also contribute to visual plasticity in adult animals. Blocking the activity of MMPs prevented reorganisation of visual areas of the brains of adult mice in response to eye closure, and prevented vision improvements in the open eye.

However, blocking MMP in adult mice whose brains had been damaged by a stroke had the opposite effect. Whereas stroke normally prevents visual system plasticity in response to eye closure, treatment with a single dose of MMP blocker rescued this plasticity. Strikingly, these benefits were lost if the mice were given two doses of MMP blocker, rather than one.

These experiments show that MMP levels must be within a narrow range to support plasticity. In the healthy adult brain, blocking MMPs impairs plasticity. After stroke, MMP levels are increased and reducing them rescues plasticity. The next challenge is to identify the specific MMP enzymes responsible, and to determine whether these changes can be exploited to improve recovery from stroke.

DOI:http://dx.doi.org/10.7554/eLife.11290.002

Inhibition of Tnf-α R1 signaling can rescue functional cortical plasticity impaired in early post-stroke period

Tumor necrosis factor-α (TNF-α) is one of the key players in stroke progression and can interfere with brain functioning. We previously documented an impairment of experience-dependent plasticity in the cortex neighboring the stroke-induced lesion, which was accompanied with an upregulation of Tnf-α level in the brain of ischemic mice 1 week after the stroke. Because TNF receptor 1 (TnfR1) signaling is believed to be a major mediator of the cytotoxicity of Tnf-α through activation of caspases, we used an anti-inflammatory intervention aimed at Tnf-α R1 pathway, in order to try to attenuate the detrimental effect of post-stroke inflammation, and investigated if this will be effective in protecting plasticity in the infarct proximity. Aged mice (12-14 months) were subjected to the photothrombotic stroke localized near somatosensory cortex, and immediately after ischemia sensory deprivation was introduced to induce plasticity. Soluble TNF-α R1 (sTNF-α R1), which competed for TNF-α with receptors localized in the brain, was delivered chronically directly into the brain tissue for the whole period of deprivation using ALZET Micro-Osmotic pumps. We have shown that such approach undertaken simultaneously with the stroke reduced the level of TNF-α in the peri-ischemic tissue and was successful in preserving the post-stroke deprivation-induced brain plasticity.

Functional assessment of sensory functions after photothrombotic stroke in the barrel field of mice

Motor, sensory and cognitive deficits are common impairments observed in human stroke as well as in animal stroke models. Using a battery of behavioural tests we assessed sensorimotor deficits after photothrombotic stroke localized within or beyond cortical representation of mouse sensory vibrissae. We found restricted, modality specific behavioural consequences in the acute post-stroke period. Among incorporated tests, adhesive removal test, novelty exploration test and sensory labyrinth task were sensitive to the somatosensory cortical deficits. Injured animals explored new objects significantly longer, they also needed distinctly more time to contact and to remove the adhesive tape placed on whiskers contralateral to the infarct. Moreover, we observed that after stroke animals were unable to solve the sensory labyrinth depending only upon tactile sensation from whiskers with injured cortical representation. Spontaneous recovery could be observed within the first post-stroke week for adhesive tape removal and within 14 days for labyrinth performance. However, for the novel object exploration we did not observed the recovery for the period of 18 days after stroke. Moreover, new object exploration test performance differed between the somatosensory and visual cortical impairments. We suggest that those three tests might be valuable in assessing the usefulness of therapies designed to support brain repair after experimental stroke.

Influence of inflammation on poststroke plasticity

Age-related brain injuries including stroke are a leading cause of morbidity and mental disability worldwide. Most patients who survive stroke experience some degree of recovery. The restoration of lost functions can be explained by neuronal plasticity, understood as brain ability to reorganize and remodel itself in response to changed environmental requirements. However, stroke triggers a cascade of events which may prevent the normal development of the plastic changes. One of them may be inflammatory response initiated immediately after stroke, which has been found to contribute to neuronal injury. Some recent evidence though has suggested that inflammatory reaction can be also neuroprotective. This paper attempts to discuss the influence of poststroke inflammatory response on brain plasticity and stroke outcome. We also describe the recent anti-inflammatory strategies that have been effective for recovery in experimental stroke.

Synaptic circuit remodelling by matrix metalloproteinases in health and disease

Matrix metalloproteinases (MMPs) are extracellularly acting enzymes that have long been known to have deleterious roles in brain injury and disease. In particular, widespread and protracted MMP activity can contribute to neuronal loss and synaptic dysfunction. However, recent studies show that rapid and focal MMP-mediated proteolysis proactively drives synaptic structural and functional remodelling that is crucial for ongoing cognitive processes. Deficits in synaptic remodelling are associated with psychiatric and neurological disorders, and aberrant MMP expression or function may contribute to the molecular mechanisms underlying these deficits. This Review explores the paradigm shift in our understanding of the contribution of MMPs to normal and abnormal synaptic plasticity and function.

Experience-dependent brain plasticity after stroke: effect of ibuprofen and poststroke delay

Despite indications that brain plasticity may be enhanced after stroke, we have described impairment of experience-dependent plasticity in rat cerebral cortex neighboring the stroke-induced lesion. Photothrombotic stroke was centered behind the barrel cortex in one cerebral hemisphere of rats. Plasticity of cortical representation of one row of vibrissae was induced by sensory deprivation of all surrounding whiskers for 1 month, and visualized with [(14)C]-2-deoxyglucose autoradiography. In control rats deprivation resulted in an enlargement of functional cortical representation of the spared row of vibrissae. After a focal stroke neighbouring the barrel cortex, no plasticity of the spared row representation was found. Investigation of plastic changes with deprivation initiated 1 week and 1 month after stroke have shown that later poststroke onset of deprivation resulted in a partial recovery of cortical plasticity in the barrel field. Western blot analysis of proinflammatory enzyme cyclooxygenase-2 (COX-2) expression revealed its strong upregulation in the barrel cortex 24 h after stroke. When chronic treatment with the anti-inflammatory drug ibuprofen (10 mg/kg or 20 mg/kg) accompanied deprivation, plasticity was restored. Ibuprofen applied before the ischemia also prevented the poststroke upregulation of COX-2. The results strongly suggest that poststroke impairment of experience-dependent cortical plasticity is caused by stroke-induced inflammatory reactions that subside with poststroke delay and can be at least partially ameliorated by pharmacological treatment.