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

Microglia enable cross-modal plasticity by removing inhibitory synapses

Cross-modal plasticity is the repurposing of brain regions associated with deprived sensory inputs to improve the capacity of other sensory modalities. The functional mechanisms of cross-modal plasticity can indicate how the brain recovers from various forms of injury and how different sensory modalities are integrated. Here, we demonstrate that rewiring of the microglia-mediated local circuit synapse is crucial for cross-modal plasticity induced by visual deprivation (monocular deprivation [MD]). MD relieves the usual inhibition of functional connectivity between the somatosensory cortex and secondary lateral visual cortex (V2L). This results in enhanced excitatory responses in V2L neurons during whisker stimulation and a greater capacity for vibrissae sensory discrimination. The enhanced cross-modal response is mediated by selective removal of inhibitory synapse terminals on pyramidal neurons by the microglia in the V2L via matrix metalloproteinase 9 signaling. Our results provide insights into how cortical circuits integrate different inputs to functionally compensate for neuronal damage.

Transcriptomic profiling and differential analysis revealed the neurodevelopmental toxicity mechanisms of zebrafish (Danio rerio) larvae in response to tetrabromobisphenol A bis(2-hydroxyethyl) ether (TBBPA-DHEE) exposure

Tetrabromobisphenol A bis(2-hydroxyetyl) ether (TBBPA-DHEE) is among the main derivatives of Tetrabromobisphenol A (TBBPA). Result from previous study showed that TBBPA-DHEE can cause neurotoxicity in rat. In this study, zebrafish larvae were used for evaluation of TBBPA-DHEE-induced developmental toxicity, apoptosis, oxidative stress and the potential molecular mechanisms of action. Our result showed that TBBPA-DHEE exposure caused a significant concentration-dependent developmental toxicity endpoints like death rate, malformation rate, growth rate. TBBPA-DHEE altered locomotor and enzymes activities of larvae and caused apoptosis within the brain indicating the potential TBBPA-DHEE-induced cardiac, brain impairment in the zebrafish larvae. Our transcriptomic analysis shows that 691 genes were differentially expressed (DEGs) (539 upregulated, 152 downregulated). The KEGG and GO enrichment pathway analysis shows that the DEGs were involved in development, immunity, enzyme activity. Our study provides novel evidence on the neurodevelopmental toxicity and toxicity mechanism of TBBPA-DHEE which are vital for assessment of the environmental toxicity and risk assessment of the chemical.

Parvalbumin-Positive Interneurons Regulate Cortical Sensory Plasticity in Adulthood and Development Through Shared Mechanisms

Parvalbumin-positive neurons are the largest class of GABAergic, inhibitory neurons in the central nervous system. In the cortex, these fast-spiking cells provide feedforward and feedback synaptic inhibition onto a diverse set of cell types, including pyramidal cells, other inhibitory interneurons, and themselves. Cortical inhibitory networks broadly, and cortical parvalbumin-expressing interneurons (cPVins) specifically, are crucial for regulating sensory plasticity during both development and adulthood. Here we review the functional properties of cPVins that enable plasticity in the cortex of adult mammals and the influence of cPVins on sensory activity at four spatiotemporal scales. First, cPVins regulate developmental critical periods and adult plasticity through molecular and structural interactions with the extracellular matrix. Second, they activate in precise sequence following feedforward excitation to enforce strict temporal limits in response to the presentation of sensory stimuli. Third, they implement gain control to normalize sensory inputs and compress the dynamic range of output. Fourth, they synchronize broad network activity patterns in response to behavioral events and state changes. Much of the evidence for the contribution of cPVins to plasticity comes from classic models that rely on sensory deprivation methods to probe experience-dependent changes in the brain. We support investigating naturally occurring, adaptive cortical plasticity to study cPVin circuits in an ethologically relevant framework, and discuss recent insights from our work on maternal experience-induced auditory cortical plasticity.

Chronic Monocular Deprivation Reveals MMP9-Dependent and -Independent Aspects of Murine Visual System Plasticity

The deletion of matrix metalloproteinase MMP9 is combined here with chronic monocular deprivation (cMD) to identify the contributions of this proteinase to plasticity in the visual system. Calcium imaging of supragranular neurons of the binocular region of primary visual cortex (V1b) of wild-type mice revealed that cMD initiated at eye opening significantly decreased the strength of deprived-eye visual responses to all stimulus contrasts and spatial frequencies. cMD did not change the selectivity of V1b neurons for the spatial frequency, but orientation selectivity was higher in low spatial frequency-tuned neurons, and orientation and direction selectivity were lower in high spatial frequency-tuned neurons. Constitutive deletion of MMP9 did not impact the stimulus selectivity of V1b neurons, including ocular preference and tuning for spatial frequency, orientation, and direction. However, MMP9^-/- mice were completely insensitive to plasticity engaged by cMD, such that the strength of the visual responses evoked by deprived-eye stimulation was maintained across all stimulus contrasts, orientations, directions, and spatial frequencies. Other forms of experience-dependent plasticity, including stimulus selective response potentiation, were normal in MMP9^-/- mice. Thus, MMP9 activity is dispensable for many forms of activity-dependent plasticity in the mouse visual system, but is obligatory for the plasticity engaged by cMD.

Role of matrix metalloproteinase-9 in neurodevelopmental deficits and experience-dependent plasticity in Xenopus laevis

Matrix metalloproteinase-9 (MMP-9) is a secreted endopeptidase targeting extracellular matrix proteins, creating permissive environments for neuronal development and plasticity. Developmental dysregulation of MMP-9 may also lead to neurodevelopmental disorders (ND). Here, we test the hypothesis that chronically elevated MMP-9 activity during early neurodevelopment is responsible for neural circuit hyperconnectivity observed in Xenopus tadpoles after early exposure to valproic acid (VPA), a known teratogen associated with ND in humans. In Xenopus tadpoles, VPA exposure results in excess local synaptic connectivity, disrupted social behavior and increased seizure susceptibility. We found that overexpressing MMP-9 in the brain copies effects of VPA on synaptic connectivity, and blocking MMP-9 activity pharmacologically or genetically reverses effects of VPA on physiology and behavior. We further show that during normal neurodevelopment MMP-9 levels are tightly regulated by neuronal activity and required for structural plasticity. These studies show a critical role for MMP-9 in both normal and abnormal development.

Microglia enable mature perineuronal nets disassembly upon anesthetic ketamine exposure or 60-Hz light entrainment in the healthy brain

Perineuronal nets (PNNs), components of the extracellular matrix, preferentially coat parvalbumin-positive interneurons and constrain critical-period plasticity in the adult cerebral cortex. Current strategies to remove PNN are long-lasting, invasive, and trigger neuropsychiatric symptoms. Here, we apply repeated anesthetic ketamine as a method with minimal behavioral effect. We find that this paradigm strongly reduces PNN coating in the healthy adult brain and promotes juvenile-like plasticity. Microglia are critically involved in PNN loss because they engage with parvalbumin-positive neurons in their defined cortical layer. We identify external 60-Hz light-flickering entrainment to recapitulate microglia-mediated PNN removal. Importantly, 40-Hz frequency, which is known to remove amyloid plaques, does not induce PNN loss, suggesting microglia might functionally tune to distinct brain frequencies. Thus, our 60-Hz light-entrainment strategy provides an alternative form of PNN intervention in the healthy adult brain.

Safety and Efficacy of Otaplimastat in Patients with Acute Ischemic Stroke Requiring tPA (SAFE-TPA): A Multicenter, Randomized, Double-Blind, Placebo-Controlled Phase 2 Study

Objective

Otaplimastat is a neuroprotectant that inhibits matrix metalloprotease pathway, and reduces edema and intracerebral hemorrhage induced by recombinant tissue plasminogen activator (rtPA) in animal stroke models. We aimed to assess the safety and efficacy of otaplimastat in patients receiving rtPA.

Methods

This was a phase 2, 2‐part, multicenter trial in stroke patients (19–80 years old) receiving rtPA. Intravenous otaplimastat was administered <30 minutes after rtPA. Stage 1 was a single‐arm, open‐label safety study in 11 patients. Otaplimastat 80 mg was administered twice daily for 3 days. Stage 2 was a randomized, double‐blind, placebo‐controlled study involving 69 patients, assigned (1:1:1) to otaplimastat 40 mg, otaplimastat 80 mg, or a placebo. The primary endpoint was the occurrence of parenchymal hematoma (PH) on day 1. Secondary endpoints included serious adverse events (SAEs), mortality, and modified Rankin scale (mRS) distribution at 90 days (http://clinicaltrials.gov identifier: NCT02787278).

Results

No safety issues were encountered in stage 1. The incidence of PH during stage 2 was comparable: 0 of 22 with the placebo, 0 of 22 with otaplimastat 40 mg, and 1 of 21 with the 80 mg dose. No differences in SAEs (13%, 17%, 14%) or death (8.3%, 4.2%, 4.8%) were observed among the 3 groups. Three adverse events (chills, muscle rigidity, hepatotoxicity) were judged to be related to otaplimastat.

Interpretation

Intravenous otaplimastat adjunctive therapy in patients receiving rtPA is feasible and generally safe. The functional efficacy of otaplimastat needs to be investigated with further large trials. ANN NEUROL 2020;87:233–245

MMPs in learning and memory and neuropsychiatric disorders

Matrix metalloproteinases (MMPs) are a group of over twenty proteases, operating chiefly extracellularly to cleave components of the extracellular matrix, cell adhesion molecules as well as cytokines and growth factors. By virtue of their expression and activity patterns in animal models and clinical investigations, as well as functional studies with gene knockouts and enzyme inhibitors, MMPs have been demonstrated to play a paramount role in many physiological and pathological processes in the brain. In particular, they have been shown to influence learning and memory processes, as well as major neuropsychiatric disorders such as schizophrenia, various kinds of addiction, epilepsy, fragile X syndrome, and depression. A possible link connecting all those conditions is either physiological or aberrant synaptic plasticity where some MMPs, e.g., MMP-9, have been demonstrated to contribute to the structural and functional reorganization of excitatory synapses that are located on dendritic spines. Another common theme linking the aforementioned pathological conditions is neuroinflammation and MMPs have also been shown to be important mediators of immune responses.

Light reintroduction after dark exposure reactivates plasticity in adults via perisynaptic activation of MMP-9

The sensitivity of ocular dominance to regulation by monocular deprivation is the canonical model of plasticity confined to a critical period. However, we have previously shown that visual deprivation through dark exposure (DE) reactivates critical period plasticity in adults. Previous work assumed that the elimination of visual input was sufficient to enhance plasticity in the adult mouse visual cortex. In contrast, here we show that light reintroduction (LRx) after DE is responsible for the reactivation of plasticity. LRx triggers degradation of the ECM, which is blocked by pharmacological inhibition or genetic ablation of matrix metalloproteinase-9 (MMP-9). LRx induces an increase in MMP-9 activity that is perisynaptic and enriched at thalamo-cortical synapses. The reactivation of plasticity by LRx is absent in Mmp9^−/− mice, and is rescued by hyaluronidase, an enzyme that degrades core ECM components. Thus, the LRx-induced increase in MMP-9 removes constraints on structural and functional plasticity in the mature cortex.

Differential Expression and Cell-Type Specificity of Perineuronal Nets in Hippocampus, Medial Entorhinal Cortex, and Visual Cortex Examined in the Rat and Mouse

Perineuronal nets (PNNs) are specialized extracellular matrix (ECM) structures that condense around the soma and proximal dendrites of subpopulations of neurons. Emerging evidence suggests that they are involved in regulating brain plasticity. However, the expression of PNNs varies between and within brain areas. A lack of quantitative studies describing the distribution and cell-specificity of PNNs makes it difficult to reveal the functional roles of PNNs. In the current study, we examine the distribution of PNNs and the identity of PNN-enwrapped neurons in three brain areas with different cognitive functions: the dorsal hippocampus, medial entorhinal cortex (mEC) and primary visual cortex (V1). We compared rats and mice as knowledge from these species are often intermingled. The most abundant expression of PNNs was found in the mEC and V1, while dorsal hippocampus showed strikingly low levels of PNNs, apart from dense expression in the CA2 region. In hippocampus we also found apparent species differences in expression of PNNs. While we confirm that the PNNs enwrap parvalbumin-expressing (PV+) neurons in V1, we found that they mainly colocalize with excitatory CamKII-expressing neurons in CA2. In mEC, we demonstrate that in addition to PV+ cells, the PNNs colocalize with reelin-expressing stellate cells. We also show that the maturation of PNNs in mEC coincides with the formation of grid cell pattern, while PV+ cells, unlike in other cortical areas, are present from early postnatal development. Finally, we demonstrate considerable effects on the number of PSD-95-gephyrin puncta after enzymatic removal of PNNs.

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.