The Role of Tenascin-C on the Structural Plasticity of Perineuronal Nets and Synaptic Expression in the Hippocampus of Male Mice

Neuronal plasticity is a crucial mechanism for an adapting nervous system to change. It is shown to be regulated by perineuronal nets (PNNs), the condensed forms of the extracellular matrix (ECM) around neuronal bodies. By assessing the changes in the number, intensity, and structure of PNNs, the ultrastructure of the PNN mesh, and the expression of inhibitory and excitatory synaptic inputs on these neurons, we aimed to clarify the role of an ECM glycoprotein, tenascin-C (TnC), in the dorsal hippocampus. To enhance neuronal plasticity, TnC-deficient (TnC-/-) and wild-type (TnC+/+) young adult male mice were reared in an enriched environment (EE) for 8 weeks. Deletion of TnC in TnC-/- mice showed an ultrastructural reduction of the PNN mesh and an increased inhibitory input in the dentate gyrus (DG), and an increase in the number of PNNs with a rise in the inhibitory input in the CA2 region. EE induced an increased inhibitory input in the CA2, CA3, and DG regions; in DG, the change was also followed by an increased intensity of PNNs. No changes in PNNs or synaptic expression were found in the CA1 region. We conclude that the DG and CA2 regions emerged as focal points of alterations in PNNs and synaptogenesis with EE as mediated by TnC.

Chondroitin sulfate proteoglycans mRNA expression and degradation in the zebra finch following traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of fatality and disability worldwide. From minutes to months following damage, injury can result in a complex pathophysiology that can lead to temporary or permanent deficits including an array of neurodegenerative symptoms. These changes can include behavioral dysregulation, memory dysfunctions, and mood changes including depression. The nature and severity of impairments resulting from TBIs vary widely given the range of injury type, location, and extent of brain tissue involved. In response to the injury, the brain induces structural and functional changes to promote repair and minimize injury size. Despite its high prevalence, effective treatment strategies for TBI are limited. PNNs are part of the neuronal extracellular matrix (ECM) that mediate synaptic stabilization in the adult brain and thus neuroplasticity. They are associated mostly with inhibitory GABAergic interneurons and are thought to be responsible for maintaining the excitatory/inhibitory balance of the brain. The major structural components of PNNs include multiple chondroitin sulfate proteoglycans (CSPGs) as well as other structural proteins. Here we examine the effects of injury on CSPG expression, specifically around the changes in the side change moieties. To investigate CSPG expression following injury, adult male and female zebra finches received either a bilateral penetrating, or no injury and qPCR analysis and immunohistochemistry for components of the CSPGs were examined at 1- or 7-days post-injury. Next, to determine if CSPGs and thus PNNs should be a target for therapeutic intervention, CSPG side chains were degraded at the time of injury with chondroitinase ABC (ChABC) CSPGs moieties were examined. Additionally, GABA receptor mRNA and aromatase mRNA expression was quantified following CSPG degradation as they have been implicated in neuronal survival and neurogenesis. Our data indicate the CSPG moieties change following injury, potentially allowing for a brief period of synaptic reorganization, and that treatments that target CSPG side chains are successful in further targeting this brief critical period by decreasing GABA mRNA receptor expression, but also decreasing aromatase expression.

Link Protein 1 Is Involved in the Activity-Dependent Modulation of Perineuronal Nets in the Spinal Cord

One of the challenges of the mature nervous system is to maintain the stability of neural networks while providing a degree of plasticity to generate experience-dependent modifications. This plasticity-stability dynamism is regulated by perineuronal nets (PNNs) and is crucial for the proper functioning of the system. Previously, we found a relation between spinal PNNs reduction and maladaptive plasticity after spinal cord injury (SCI), which was attenuated by maintaining PNNs with activity-dependent therapies. Moreover, transgenic mice lacking the cartilage link protein 1 (Crtl1 KO mice) showed aberrant spinal PNNs and increased spinal plasticity. Therefore, the aim of this study is to evaluate the role of link protein 1 in the activity-dependent modulation of spinal PNNs surrounding motoneurons and its impact on the maladaptive plasticity observed following SCI. We first studied the activity-dependent modulation of spinal PNNs using a voluntary wheel-running protocol. This training protocol increased spinal PNNs in WT mice but did not modify PNN components in Crtl1 KO mice, suggesting that link protein 1 mediates the activity-dependent modulation of PNNs. Secondly, a thoracic SCI was performed, and functional outcomes were evaluated for 35 days. Interestingly, hyperreflexia and hyperalgesia found at the end of the experiment in WT-injured mice were already present at basal levels in Crtl1 KO mice and remained unchanged after the injury. These findings demonstrated that link protein 1 plays a dual role in the correct formation and in activity-dependent modulation of PNNs, turning it into an essential element for the proper function of PNN in spinal circuits.

Astrocyte-secreted neurocan controls inhibitory synapse formation and function

Astrocytes strongly promote the formation and maturation of synapses by secreted proteins. Several astrocyte-secreted synaptogenic proteins controlling excitatory synapse development were identified; however, those that induce inhibitory synaptogenesis remain elusive. Here, we identify neurocan as an astrocyte-secreted inhibitory synaptogenic protein. After secretion from astrocytes, neurocan is cleaved into N- and C-terminal fragments. We found that these fragments have distinct localizations in the extracellular matrix. The neurocan C-terminal fragment localizes to synapses and controls cortical inhibitory synapse formation and function. Neurocan knockout mice lacking the whole protein or only its C-terminal synaptogenic domain have reduced inhibitory synapse numbers and function. Through super-resolution microscopy, in vivo proximity labeling by secreted TurboID, and astrocyte-specific rescue approaches, we discovered that the synaptogenic domain of neurocan localizes to somatostatin-positive inhibitory synapses and strongly regulates their formation. Together, our results unveil a mechanism through which astrocytes control circuit-specific inhibitory synapse development in the mammalian brain.

Alterations of Perineuronal Net Expression and Abnormal Social Behavior and Whisker-dependent Texture Discrimination in Mice Lacking the Autism Candidate Gene Engrailed 2

GABAergic interneurons and perineuronal nets (PNNs) are important regulators of plasticity throughout life and their dysfunction has been implicated in the pathogenesis of several neuropsychiatric conditions, including autism spectrum disorders (ASD). PNNs are condensed portions of the extracellular matrix (ECM) that are crucial for neural development and proper formation of synaptic connections. We previously showed a reduced expression of GABAergic interneuron markers in the hippocampus and somatosensory cortex of adult mice lacking the Engrailed2 gene (En2-/- mice), a mouse model of ASD. Since alterations in PNNs have been proposed as a possible pathogenic mechanism in ASD, we hypothesized that the PNN dysfunction may contribute to the neural and behavioral abnormalities of En2-/- mice. Here, we show an increase in the PNN fluorescence intensity, evaluated by Wisteria floribunda agglutinin, in brain regions involved in social behavior and somatosensory processing. In addition, we found that En2-/- mice exhibit altered texture discrimination through whiskers and display a marked decrease in the preference for social novelty. Our results raise the possibility that altered expression of PNNs, together with defects of GABAergic interneurons, might contribute to the pathogenesis of social and sensory behavioral abnormalities.

The Role of Cerebellar Intrinsic Neuronal Excitability, Synaptic Plasticity, and Perineuronal Nets in Eyeblink Conditioning

Evidence is strong that, in addition to fine motor control, there is an important role for the cerebellum in cognition and emotion. The deep nuclei of the mammalian cerebellum also contain the highest density of perineural nets-mesh-like structures that surround neurons-in the brain, and it appears there may be a connection between these nets and cognitive processes, particularly learning and memory. Here, we review how the cerebellum is involved in eyeblink conditioning-a particularly well-understood form of learning and memory-and focus on the role of perineuronal nets in intrinsic membrane excitability and synaptic plasticity that underlie eyeblink conditioning. We explore the development and role of perineuronal nets and the in vivo and in vitro evidence that manipulations of the perineuronal net in the deep cerebellar nuclei affect eyeblink conditioning. Together, these findings provide evidence of an important role for perineuronal net in learning and memory.

Perineuronal net deglycosylation associates with tauopathy-induced gliosis and neurodegeneration

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by clinical symptoms of memory and cognitive deficiencies. Postmortem evaluation of AD brain tissue shows proteinopathy that closely associate with the progression of this dementing disorder, including the accumulation of extracellular beta amyloid (Aβ) and intracellular hyperphosphorylated tau (pTau) with neurofibrillary tangles (NFTs). Current therapies targeting Aβ have limited clinical efficacy and life-threatening side effects and highlight the need for alternative treatments targeting pTau and other pathophysiologic mechanisms driving AD pathogenesis. The brain's extracellular matrices (ECM), particularly perineuronal nets (PNNs), play a crucial role in brain functioning and neurocircuit stability, and reorganization of these unique PNN matrices has been associated with the progression of AD and accumulation of pTau in humans. We hypothesize that AD-associated changes in PNNs may in part be driven by the accumulation of pTau within the brain. In this work, we investigated whether the presence of pTau influenced PNN structural integrity and PNN chondroitin sulfate-glycosaminoglycan (CS-GAG) compositional changes in two transgenic mouse models expressing tauopathy-related AD pathology, PS19 (P301S) and Tau4RTg2652 mice. We show that PS19 mice exhibit an age-dependent loss of hippocampal PNN CS-GAGs, but not the underlying aggrecan core protein structures, in association with pTau accumulation, gliosis, and neurodegeneration. The loss of PNN CS-GAGs were linked to shifts in CS-GAG sulfation patterns to favor the neuroregenerative isomer, 2S6S-CS. Conversely, Tau4RTg2652 mice exhibit stable PNN structures and normal CS-GAG isomer composition despite robust pTau accumulation, suggesting a critical interaction between neuronal PNN glycan integrity and neighboring glial cell activation. Overall, our findings provide insights into the complex relationship between PNN CS-GAGs, pTau pathology, gliosis, and neurodegeneration in mouse models of tauopathy, and offer new therapeutic insights and targets for AD treatment.

The last stage of development: The restructuring and plasticity of the cortex during adolescence especially at puberty

There is considerable evidence of reorganization in the prefrontal cortex during adolescence in humans, as well as in rodents, where the cellular basis can be explored. Studies from my laboratory in the rat medial prefrontal cortex are reviewed here. In general, growth predominates before puberty. Pruning mainly occurs at puberty and after with decreases in the number of synapses, dendrites, and neurons. Perineuronal nets, extracellular structures that control plasticity, are pruned peripubertally only in female rats, which may further open the adolescent prefrontal cortex to environmental influences. This is supported by our recent evidence that exposure to mild stress early, but not late, in adolescence decreases prepulse inhibition. Additionally, exposure to methamphetamine in females early in adolescence increases the number of a major class of inhibitory interneurons, parvalbumin neurons, while the opposite occurs late in adolescence. In females, even estrogen receptor beta mRNA decreases at puberty in the prefrontal cortex. Interestingly, rats of both sexes perform better after puberty on a test of cognitive flexibility in the water maze. Thus, evidence is accruing that adolescence is not a single entity but rather an ongoing set of processes, and environmental effects will differ depending on timing and sex.

Developmental and adult stress: effects of steroids and neurosteroids

In humans, exposure to early life adversity has profound implications for susceptibility to developing neuropsychiatric disorders later in life. Studies in rodents have shown that stress experienced during early postnatal life can have lasting effects on brain development. Glucocorticoids and sex steroids are produced in endocrine glands and the brain from cholesterol; these molecules bind to nuclear and membrane-associated steroid receptors. Unlike other steroids that can also be made in the brain, neurosteroids bind specifically to neurotransmitter receptors, not steroid receptors. The relationships among steroids, neurosteroids, and stress are multifaceted and not yet fully understood. However, studies demonstrating altered levels of progestogens, androgens, estrogens, glucocorticoids, and their neuroactive metabolites in both developmental and adult stress paradigms strongly suggest that these molecules may be important players in stress effects on brain circuits and behavior. In this review, we discuss the influence of developmental and adult stress on various components of the brain, including neurons, glia, and perineuronal nets, with a focus on sex steroids and neurosteroids. Gaining an enhanced understanding of how early adversity impacts the intricate systems of brain steroid and neurosteroid regulation could prove instrumental in identifying novel therapeutic targets for stress-related conditions.

From molecules to behavior: Implications for perineuronal net remodeling in learning and memory

Perineuronal nets (PNNs) are condensed extracellular matrix (ECM) structures found throughout the central nervous system that regulate plasticity. They consist of a heterogeneous mix of ECM components that form lattice-like structures enwrapping the cell body and proximal dendrites of particular neurons. During development, accumulating research has shown that the closure of various critical periods of plasticity is strongly linked to experience-driven PNN formation and maturation. PNNs provide an interface for synaptic contacts within the holes of the structure, generally promoting synaptic stabilization and restricting the formation of new synaptic connections in the adult brain. In this way, they impact both synaptic structure and function, ultimately influencing higher cognitive processes. PNNs are highly plastic structures, changing their composition and distribution throughout life and in response to various experiences and memory disorders, thus serving as a substrate for experience- and disease-dependent cognitive function. In this review, we delve into the proposed mechanisms by which PNNs shape plasticity and memory function, highlighting the potential impact of their structural components, overall architecture, and dynamic remodeling on functional outcomes in health and disease.

Age-related upregulation of perineuronal nets on inferior collicular cells that project to the cochlear nucleus

Introduction

Disruptions to the balance of excitation and inhibition in the inferior colliculus (IC) occur during aging and underlie various aspects of hearing loss. Specifically, the age-related alteration to GABAergic neurotransmission in the IC likely contributes to the poorer temporal precision characteristic of presbycusis. Perineuronal nets (PNs), a specialized form of the extracellular matrix, maintain excitatory/inhibitory synaptic environments and reduce structural plasticity. We sought to determine whether PNs increasingly surround cell populations in the aged IC that comprise excitatory descending projections to the cochlear nucleus.

Method

We combined Wisteria floribunda agglutinin (WFA) staining for PNs with retrograde tract-tracing in three age groups of Fischer Brown Norway (FBN) rats.

Results

The data demonstrate that the percentage of IC-CN cells with a PN doubles from ~10% at young age to ~20% at old age. This was true in both lemniscal and non-lemniscal IC.

Discussion

Furthermore, the increase of PNs occurred on IC cells that make both ipsilateral and contralateral descending projections to the CN. These results indicate that reduced structural plasticity in the elderly IC-CN pathway, affecting excitatory/inhibitory balance and, potentially, may lead to reduced temporal precision associated with presbycusis.

Enhanced expression of parvalbumin and perineuronal nets in the medial prefrontal cortex after extended-access cocaine self-administration in rats

The medial prefrontal cortex (mPFC) drives cocaine-seeking behaviour in rodent models of cocaine use disorder. Parvalbumin (PV)-containing GABAergic interneurons powerfully control the output of the mPFC, yet few studies have focused on how these neurons modulate cocaine-seeking behaviour. Most PV neurons are surrounded by perineuronal nets (PNNs), which regulate the firing of PV neurons. We examined staining intensity and number of PV and PNNs after long-access (6 h/day) cocaine self-administration in rats followed by either 8-10 days extinction ± cue-induced reinstatement or short-term (1-2 days) or long-term (30-31 days) abstinence ± cue-induced reinstatement. The intensity of PNNs was increased in the prelimbic and infralimbic PFC after long-term abstinence in the absence of cue reinstatement and after cue reinstatement following both daily extinction sessions and after a 30-day abstinence period. PV intensity was increased after 30 days of abstinence in the prelimbic but not infralimbic PFC. Enzymatic removal of PNNs with chondroitinase ABC (ABC) in the prelimbic PFC did not prevent incubation of cue-induced reinstatement but decreased cocaine-seeking behaviour at both 2 and 31 days of abstinence, and this decrease at 31 days was accompanied by reduced c-Fos levels in the prelimbic PFC. Increases in PNN intensity have generally been associated with the loss of plasticity, suggesting that the persistent and chronic nature of cocaine use disorder may in part be attributed to long-lasting increases in PNN intensity that reduce the ability of stimuli to alter synaptic input to underlying PV neurons.

Impact of hyaluronan size on localization and solubility of the extracellular matrix in the mouse brain

Hyaluronan (HA) is a central component of the extracellular matrix (ECM) in the brain and plays a pivotal role in neural development and plasticity. Brain HA exists in 2 distinct forms of the ECM: the diffuse ECM, which is soluble in saline and detergents, and the condensed ECM, which forms aggregates, such as perineuronal nets (PNNs). Although the physiological functions of HA significantly differ depending on its size, size differences in HA have not yet been examined in the 2 ECM types, which is partly because of the lack of methods to rapidly and accurately measure the molecular weight (MW) of HA. In this study, we established a simple method to simultaneously assess the MW of HA in multiple crude biological samples. HA was purified through single-step precipitation from tissue extracts using biotinylated HA-binding protein and streptavidin-coupled magnetic beads, followed by separation on gel electrophoresis. By applying this method to HA in the mouse brain, we revealed that the condensed ECM contained higher MW HA than the diffuse ECM. Higher MW HA and lower MW HA exhibited different spatial distributions: the former was confined to PNNs, whereas the latter was widely present throughout the brain. Furthermore, the limited degradation of HA showed that only higher MW HA was required to form an insoluble HA-aggrecan complex. The present study demonstrated that the MW of HA in the brain strongly correlates with the localization and solubility of the ECM it forms.

Neuroanatomical Alterations in the CNTNAP2 Mouse Model of Autism Spectrum Disorder

Autism spectrum disorder (ASD) is associated with neurodevelopmental alterations, including atypical forebrain cellular organization. Mutations in several ASD-related genes often result in cerebral cortical anomalies, such as the abnormal developmental migration of excitatory pyramidal cells and the malformation of inhibitory neuronal circuitry. Notably here, mutations in the CNTNAP2 gene result in ectopic superficial cortical neurons stalled in lower cortical layers and alterations to the balance of cortical excitation and inhibition. However, the broader circuit-level implications of these findings have not been previously investigated. Therefore, we assessed whether ectopic cortical neurons in CNTNAP2 mutant mice form aberrant connections with higher-order thalamic nuclei, potentially accounting for some autistic behaviors, such as repetitive and hyperactive behaviors. Furthermore, we assessed whether the development of parvalbumin-positive (PV) cortical interneurons and their specialized matrix support structures, called perineuronal nets (PNNs), were altered in these mutant mice. We found alterations in both ectopic neuronal connectivity and in the development of PNNs, PV neurons and PNNs enwrapping PV neurons in various sensory cortical regions and at different postnatal ages in the CNTNAP2 mutant mice, which likely lead to some of the cortical excitation/inhibition (E/I) imbalance associated with ASD. These findings suggest neuroanatomical alterations in cortical regions that underlie the emergence of ASD-related behaviors in this mouse model of the disorder.

Defining hippocampal area CA2 in the fox (Vulpes vulpes) brain

Since 1959, the Russian Farm-Fox study has bred foxes to be either tame or, more recently, aggressive, and scientists have used them to gain insight into the brain structures associated with these behavioral features. In mice, hippocampal area CA2 has emerged as one of the essential regulators of social aggression, and so to eventually determine whether we could identify differences in CA2 between tame and aggressive foxes, we first sought to identify CA2 in foxes (Vulpes vulpes). As no clearly defined area of CA2 has been described in species such as cats, dogs, or pigs, it was not at all clear whether CA2 could be identified in foxes. In this study, we cut sections of temporal lobes from male and female red foxes, perpendicular to the long axis of the hippocampus, and stained them with markers of CA2 pyramidal cells commonly used in tissue from rats and mice. We observed that antibodies against Purkinje cell protein 4 best stained the pyramidal cells in the area spanning the end of the mossy fibers and the beginning of the pyramidal cells lacking mossy fibers, resembling the pattern seen in rats and mice. Our findings indicate that foxes do have a "molecularly defined" CA2, and further, they suggest that other carnivores like dogs and cats might as well. With this being the case, these foxes could be useful in future studies looking at CA2 as it relates to aggression.

Extracellular Matrix Regulation in Physiology and in Brain Disease

The extracellular matrix (ECM) surrounds cells in the brain, providing structural and functional support. Emerging studies demonstrate that the ECM plays important roles during development, in the healthy adult brain, and in brain diseases. The aim of this review is to briefly discuss the physiological roles of the ECM and its contribution to the pathogenesis of brain disease, highlighting the gene expression changes, transcriptional factors involved, and a role for microglia in ECM regulation. Much of the research conducted thus far on disease states has focused on "omic" approaches that reveal differences in gene expression related to the ECM. Here, we review recent findings on alterations in the expression of ECM-associated genes in seizure, neuropathic pain, cerebellar ataxia, and age-related neurodegenerative disorders. Next, we discuss evidence implicating the transcription factor hypoxia-inducible factor 1 (HIF-1) in regulating the expression of ECM genes. HIF-1 is induced in response to hypoxia, and also targets genes involved in ECM remodeling, suggesting that hypoxia could contribute to ECM remodeling in disease conditions. We conclude by discussing the role microglia play in the regulation of the perineuronal nets (PNNs), a specialized form of ECM in the central nervous system. We show evidence that microglia can modulate PNNs in healthy and diseased brain states. Altogether, these findings suggest that ECM regulation is altered in brain disease, and highlight the role of HIF-1 and microglia in ECM remodeling.

Wild-type MeCp2 expression coincides with age-dependent sensory phenotypes in a female mouse model for Rett syndrome

Rett syndrome is characterized by an early period of typical development and then, regression of learned motor and speech skills in girls. Loss of MECP2 protein is thought to cause Rett syndrome phenotypes. The specific underlying mechanisms from typical developmental trajectory to regression features throughout life are unclear. Lack of established timelines to study the molecular, cellular, and behavioral features of regression in female mouse models is a major contributing factor. Due to random X-chromosome inactivation, female patients with Rett syndrome and female mouse models for Rett syndrome (Mecp2^Heterozygous , Het) express a functional copy of wild-type MeCp-2 protein in approximately half of all cells. As MeCp2 expression is regulated during early postnatal development and experience, we characterized the expression of wild-type MeCp2 in the primary somatosensory cortex of female Het mice. Here, we report increased MeCp2 levels in non-parvalbumin-positive neurons of 6-week-old adolescent Het relative to age-matched wild-type controls, while also displaying typical levels of perineuronal net expression in the barrel field subregion of the primary somatosensory cortex, mild tactile sensory perception deficits, and efficient pup retrieval behavior. In contrast, 12-week-old adult Het express MeCp2 at levels similar to age-matched wild-type mice, show increased perineuronal net expression in the cortex, and display significant tactile sensory perception deficits. Thus, we have identified a set of behavioral metrics and the cellular substrates to study regression during a specific time in the female Het mouse model, which coincides with changes in wild-type MeCp2 expression. We speculate that the precocious increase in MeCp2 expression within specific cell types of adolescent Het may provide compensatory benefits at the behavioral level, while the inability to further increase MeCp2 levels leads to regressive behavioral phenotypes over time.

Ketamine and its metabolite 2R,6R-hydroxynorketamine promote ocular dominance plasticity and release tropomyosin-related kinase B from inhibitory control without reducing perineuronal nets enwrapping parvalbumin interneurons

Ketamine has been described as a fast-acting antidepressant, exerting effects in depressed patients and in preclinical models with a rapid onset of action. The typical antidepressant fluoxetine is known to induce plasticity in the adult rodent visual cortex, as assessed by a shift in ocular dominance, a classical model of brain plasticity, and a similar effect has been described for ketamine and its metabolite 2R,6R-hydroxynorketamine (R,R-HNK). Here, we demonstrate that ketamine (at 3 or 20 mg/kg) and R,R-HNK facilitated the shift in ocular dominance in monocularly deprived mice, after three injections, throughout the 7-day monocular deprivation regimen. Notably, the comparison between the treatments indicates a higher effect size of R,R-HNK compared with ketamine. Treatment with ketamine or R,R-HNK failed to influence the levels of perineuronal nets (PNNs) surrounding parvalbumin-positive interneurons. However, we observed in vitro that both ketamine and R,R-HNK are able to disrupt the tropomyosin-related kinase B (TRKB) interaction with the protein tyrosine phosphatase sigma (PTPσ), which upon binding to PNNs dephosphorylates TRKB. These results support a model where diverse drugs promote the reinstatement of juvenile-like plasticity by directly binding TRKB and releasing it from PTPσ regulation, without necessarily reducing PNNs deposits.

Degradation of Perineuronal Nets in the Cerebellar Interpositus Nucleus Ameliorated Social Deficits in Shank3-deficient Mice

Perineuronal nets (PNNs) are structures that contain extracellular matrix chondroitin sulfate proteoglycan and surround the soma and dendrites of various neuronal cell types. They are involved in synaptic plasticity and undertake important physiological functions. Altered expression of PNNs has been demonstrated in the brains of autism-related animal models. However, the underlying mechanism is still unknown. In this study, we demonstrated that the PNNs in the cerebellum are involved in modulating social and repetitive/inflexible behaviors in Shank3B^-/- mice, an established animal model of autism spectrum disorder. First, we performed wisteria floribunda agglutinin staining of the whole brain of Shank3B^-/- mice, and found wisteria floribunda agglutinin-positive PNNs are significantly increased in the cerebellar interpositus nucleus (IntP) in Shank3B^-/- mice compared to control littermates. After degradation of PNNs in the IntP by chondroitinase ABC, the repetitive behaviors of Shank3B^-/- mice were decreased, while their social behaviors were ameliorated. These results suggested that PNNs homeostasis is involved in the regulation of social behavior, revealing a potential therapeutic strategy targeting PNNs in the IntP for the treatment of autism spectrum disorder.

Neuroplastic Changes in Addiction Memory-How Music Therapy and Music-Based Intervention May Reduce Craving: A Narrative Review

Recent findings indicate that Music Therapy (MT) and Music-Based Interventions (MBIs) may reduce craving symptoms in people with Substance Use Disorders (SUD). However, MT/MBIs can lead SUD clients to recall memories associated with their drug history and the corresponding strong emotions (addiction memories). Craving is a central component of SUD, possibly linked to relapse and triggered by several factors such as the recall of memories associated with the drug experience. Therefore, to address the topic of what elements can account for an improvement in craving symptoms after MT/MBIs, we conducted a narrative review that (1) describes the brain correlates of emotionally salient autobiographical memories evoked by music, (2) outlines neuroimaging and neurophysiological studies suggesting how the experience of craving may encompass the recall of emotionally filled moments, and (3) points out the role of perineuronal nets (PNNs) in addiction memory neuroplasticity. We highlight how autobiographical memory retrieval, music-evoked autobiographical memories, and craving share similar neural activations with PNNs which represent a causal element in the processing of addiction memory. We finally conclude by considering how the neuroplastic characteristics of addiction memory might represent the ground to update and/or recalibrate, within the therapy, the emotional content related to the recall.

Elimination of perineuronal nets in CA1 disrupts GABA release and long-term contextual fear memory retention

Perineuronal nets (PNNs) which mostly surround the parvalbumin (PV) neurons, have been shown to play critical roles in neural plasticity. Recently, PNNs have been shown to regulate fear-associated memory, but the molecular mechanism is still unclear. In this study, we found that removal of PNNs in vivo using chondroitinase ABC (ChABC) injection resulted in reduced firing rate of PV neurons and decreased inhibitory synaptic transmission in both PV neurons and excitatory neurons in the CA1 hippocampus. Interestingly, altered synaptic transmission appears to be mediated by presynaptic changes. Furthermore, ChABC treatment disrupts long-term contextual fear memory retention. These results suggest PNNs might alter fear memory by reducing the presynaptic GABA release.

Low frequency repetitive transcranial magnetic stimulation promotes plasticity of the visual cortex in adult amblyopic rats

The decline of visual plasticity restricts the recovery of visual functions in adult amblyopia. Repetitive transcranial magnetic stimulation (rTMS) has been shown to be effective in treating adult amblyopia. However, the underlying mechanisms of rTMS on visual cortex plasticity remain unclear. In this study, we found that low-frequency rTMS reinstated the amplitude of visual evoked potentials, but did not influence the impaired depth perception of amblyopic rats. Furthermore, the expression of synaptic plasticity genes and the number of dendritic spines were significantly higher in amblyopic rats which received rTMS when compared with amblyopic rats which received sham stimulation, with reduced level of inhibition and perineuronal nets in visual cortex, as observed via molecular and histological investigations. The results provide further evidence that rTMS enhances functional recovery and visual plasticity in an adult amblyopic animal model.

Regulation of the E/I-balance by the neural matrisome

In the mammalian cortex a proper excitatory/inhibitory (E/I) balance is fundamental for cognitive functions. Especially γ-aminobutyric acid (GABA)-releasing interneurons regulate the activity of excitatory projection neurons which form the second main class of neurons in the cortex. During development, the maturation of fast-spiking parvalbumin-expressing interneurons goes along with the formation of net-like structures covering their soma and proximal dendrites. These so-called perineuronal nets (PNNs) represent a specialized form of the extracellular matrix (ECM, also designated as matrisome) that stabilize structural synapses but prevent the formation of new connections. Consequently, PNNs are highly involved in the regulation of the synaptic balance. Previous studies revealed that the formation of perineuronal nets is accompanied by an establishment of mature neuronal circuits and by a closure of critical windows of synaptic plasticity. Furthermore, it has been shown that PNNs differentially impinge the integrity of excitatory and inhibitory synapses. In various neurological and neuropsychiatric disorders alterations of PNNs were described and aroused more attention in the last years. The following review gives an update about the role of PNNs for the maturation of parvalbumin-expressing interneurons and summarizes recent findings about the impact of PNNs in different neurological and neuropsychiatric disorders like schizophrenia or epilepsy. A targeted manipulation of PNNs might provide an interesting new possibility to indirectly modulate the synaptic balance and the E/I ratio in pathological conditions.

Longitudinal in vivo imaging of perineuronal nets

SIGNIFICANCE

Perineuronal nets (PNNs) are extracellular matrix structures implicated in learning, memory, information processing, synaptic plasticity, and neuroprotection. However, our understanding of mechanisms governing the evidently important contribution of PNNs to central nervous system function is lacking. A primary cause for this gap of knowledge is the absence of direct experimental tools to study their role in vivo.

AIM

We introduce a robust approach for quantitative longitudinal imaging of PNNs in brains of awake mice at subcellular resolution.

APPROACH

We label PNNs in vivo with commercially available compounds and monitor their dynamics with two-photon imaging.

RESULTS

Using our approach, we show that it is possible to longitudinally follow the same PNNs in vivo while monitoring degradation and reconstitution of PNNs. We demonstrate the compatibility of our method to simultaneously monitor neuronal calcium dynamics in vivo and compare the activity of neurons with and without PNNs.

CONCLUSION

Our approach is tailored for studying the intricate role of PNNs in vivo, while paving the road for elucidating their role in different neuropathological conditions.

The fate of interneurons, GABAA receptor sub-types and perineuronal nets in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurological disease, which is associated with gradual memory loss and correlated with synaptic hyperactivity and abnormal oscillatory rhythmic brain activity that precedes phenotypic alterations and is partly responsible for the spread of the disease pathology. Synaptic hyperactivity is thought to be because of alteration in the homeostasis of phasic and tonic synaptic inhibition, which is orchestrated by the GABA_A inhibitory system, encompassing subclasses of interneurons and GABA_A receptors, which play a vital role in cognitive functions, including learning and memory. Furthermore, the extracellular matrix, the perineuronal nets (PNNs) which often go unnoticed in considerations of AD pathology, encapsulate the inhibitory cells and neurites in critical brain regions and have recently come under the light for their crucial role in synaptic stabilisation and excitatory-inhibitory balance and when disrupted, serve as a potential trigger for AD-associated synaptic imbalance. Therefore, in this review, we summarise the current understanding of the selective vulnerability of distinct interneuron subtypes, their synaptic and extrasynaptic GABA_A R subtypes as well as the changes in PNNs in AD, detailing their contribution to the mechanisms of disease development. We aim to highlight how seemingly unique malfunction in each component of the interneuronal GABA inhibitory system can be tied together to result in critical circuit dysfunction, leading to the irreversible symptomatic damage observed in AD.

Transcriptional profiling reveals roles of intercellular Fgf9 signaling in astrocyte maturation and synaptic refinement during brainstem development

Neural tissue maturation is a coordinated process under tight transcriptional control. We previously analyzed the kinetics of gene expression in the medial nucleus of the trapezoid body (MNTB) in the brainstem during the critical postnatal phase of its development. While this work revealed timed execution of transcriptional programs, it was blind to the specific cells where gene expression changes occurred. Here, we utilized single-cell RNA-Seq to determine transcriptional profiles of each major MNTB cell type. We discerned directional signaling patterns between neuronal, glial, and vascular-associated cells for VEGF, TGFβ, and Delta-Notch pathways during a robust period of vascular remodeling in the MNTB. Furthermore, we describe functional outcomes of the disruption of neuron-astrocyte fibroblast growth factor 9 (Fgf9) signaling. We used a conditional KO (cKO) approach to genetically delete Fgf9 from principal neurons in the MNTB, which led to an early onset of glial fibrillary acidic protein (Gfap) expression in astrocytes. In turn, Fgf9 cKO mice show increased levels of astrocyte-enriched brevican (Bcan), a component of the perineuronal net matrix that ensheaths principal neurons in the MNTB and the large calyx of Held terminal, while levels of the neuron-enriched hyaluronan and proteoglycan link protein 1 (Hapln1) were unchanged. Finally, volumetric analysis of vesicular glutamate transporters 1 and 2 (Vglut1/2), which serves as a proxy for terminal size, revealed an increase in calyx of Held volume in the Fgf9 cKO. Overall, we demonstrate a coordinated neuron-astrocyte Fgf9 signaling network that functions to regulate astrocyte maturation, perineuronal net structure, and synaptic refinement.

Reorganization of the Brain Extracellular Matrix in Hippocampal Sclerosis

The extracellular matrix (ECM) is an important regulator of excitability and synaptic plasticity, especially in its highly condensed form, the perineuronal nets (PNN). In patients with drug-resistant mesial temporal lobe epilepsy (MTLE), hippocampal sclerosis type 1 (HS1) is the most common histopathological finding. This study aimed to evaluate the ECM profile of HS1 in surgically treated drug-resistant patients with MTLE in correlation to clinical findings. Hippocampal sections were immunohistochemically stained for aggrecan, neurocan, versican, chondroitin-sulfate (CS56), fibronectin, Wisteria floribunda agglutinin (WFA), a nuclear neuronal marker (NeuN), parvalbumin (PV), and glial-fibrillary-acidic-protein (GFAP). In HS1, besides the reduced number of neurons and astrogliosis, we found a significantly changed expression pattern of versican, neurocan, aggrecan, WFA-specific glycosylation, and a reduced number of PNNs. Patients with a lower number of epileptic episodes had a less intense diffuse WFA staining in Cornu Ammonis (CA) fields. Our findings suggest that PNN reduction, changed ECM protein, and glycosylation expression pattern in HS1 might be involved in the pathogenesis and persistence of drug-resistant MTLE by contributing to the increase of CA pyramidal neurons’ excitability. This research corroborates the validity of ECM molecules and their modulators as a potential target for the development of new therapeutic approaches to drug-resistant epilepsy.

Removal of perineuronal nets leads to altered neuronal excitability and synaptic transmission in the visual cortex with distinct time courses

Parvalbumin-expressing (PV) interneurons fast inhibit excitatory neurons in various brain areas. Perineuronal nets (PNNs), accumulating around PV neurons, have been shown to play critical roles in neuronal function and plasticity. The cellular mechanisms underlying their functions are still in debate, for example, do PNNs contribute significantly to the excitability of inhibitory neurons especially those containing PV? On the other hand, whether PNNs have significant contributions to synaptic transmission of PV neurons is much less unknown. In this study, we designed experiments to address these questions and found that removing PNNs in vivo using chondroitinase ABC (ChABC) led to distinct changes in neuronal excitability and synaptic transmission, depending on the duration of ChABC treatment. The results showed 7 days after ChABC treatment reduced both intrinsic excitability of PV neurons and synaptic transmission to both PV neurons and excitatory neurons in the primary visual cortex. However, 1 day after ChABC treatment digested PNNs effectively but had no effects on intrinsic excitability and synaptic transmission. These results suggest the contribution of PNNs to neuronal excitability and synaptic transmission depends on different time courses of ChABC digestion.

The "Loss" of Perineuronal Nets in Alzheimer's Disease: Missing or Hiding in Plain Sight?

Perineuronal nets (PNNs) are chondroitin-sulfate glycosaminoglycan (CS-GAG) containing extracellular matrix structures that assemble around neurons involved in learning, memory, and cognition. Owing to the unique patterning of negative charges stemming from sulfate modifications to the attached CS-GAGs, these matrices play key roles in mediating glycan-protein binding, signaling interactions, and charged ion buffering of the underlying circuitry. Histochemical loss of PNN matrices has been reported for a range of neurocognitive and neurodegenerative diseases, implying that PNNs might be a key player in the pathogenesis of neurological disorders. In this hypothesis and theory article, we begin by highlighting PNN changes observed in human postmortem brain tissue associated with Alzheimer's disease (AD) and corresponding changes reported in rodent models of AD neuropathology. We then discuss the technical limitations surrounding traditional methods for PNN analyses and propose alternative explanations to these historical findings. Lastly, we embark on a global re-evaluation of the interpretations for PNN changes across brain regions, across species, and in relation to other neurocognitive disorders.

Structural and Functional Deviations of the Hippocampus in Schizophrenia and Schizophrenia Animal Models

Schizophrenia is a grave neuropsychiatric disease which frequently onsets between the end of adolescence and the beginning of adulthood. It is characterized by a variety of neuropsychiatric abnormalities which are categorized into positive, negative and cognitive symptoms. Most therapeutical strategies address the positive symptoms by antagonizing D2-dopamine-receptors (DR). However, negative and cognitive symptoms persist and highly impair the life quality of patients due to their disabling effects. Interestingly, hippocampal deviations are a hallmark of schizophrenia and can be observed in early as well as advanced phases of the disease progression. These alterations are commonly accompanied by a rise in neuronal activity. Therefore, hippocampal formation plays an important role in the manifestation of schizophrenia. Furthermore, studies with animal models revealed a link between environmental risk factors and morphological as well as electrophysiological abnormalities in the hippocampus. Here, we review recent findings on structural and functional hippocampal abnormalities in schizophrenic patients and in schizophrenia animal models, and we give an overview on current experimental approaches that especially target the hippocampus. A better understanding of hippocampal aberrations in schizophrenia might clarify their impact on the manifestation and on the outcome of this severe disease.

Impact of adolescent intermittent ethanol exposure on interneurons and their surrounding perineuronal nets in adulthood

BACKGROUND

Binge alcohol exposure during adolescence results in long-lasting alterations in the brain and behavior. For example, adolescent intermittent ethanol (AIE) exposure in rodents results in long-term loss of functional connectivity among prefrontal cortex (PFC) and striatal regions as well as a variety of neurochemical, molecular, and epigenetic alterations. Interneurons in the PFC and striatum play critical roles in behavioral flexibility and functional connectivity. For example, parvalbumin (PV) interneurons are known to contribute to neural synchrony and cholinergic interneurons contribute to strategy selection. Furthermore, extracellular perineuronal nets (PNNs) that surround some interneurons, particularly PV+ interneurons, further regulate cellular plasticity. The effect of AIE exposure on the expression of these markers within the PFC is not well understood.

METHODS

The present study tested the hypothesis that AIE exposure reduces the expression of PV+ and choline acetyltransferase (ChAT)+ interneurons in the adult PFC and striatum and increases the related expression of PNNs (marked by binding of Wisteria floribunda agglutinin lectin) in adulthood. Male rats were exposed to AIE (5 g/kg/day, 2-days-on/2-days-off, i.e., P25 to P54) or water (CON), and brain tissue was harvested in adulthood (>P80). Immunohistochemistry and co-immunofluorescence were used to assess the expression of ChAT, PV, and PNNs within the adult PFC and striatum following AIE exposure.

RESULTS

ChAT and PV interneuron densities in the striatum and PFC were unchanged after AIE exposure. However, PNN density in the PFC of AIE-exposed rats was greater than in CON rats. Moreover, significantly more PV neurons were surrounded by PNNs in AIE-exposed subjects than controls in both PFC subregions assessed: orbitofrontal cortex (CON = 34%; AIE = 40%) and medial PFC (CON = 10%; AIE = 14%).

CONCLUSIONS

These findings indicate that, following AIE exposure, PV interneuron expression in the adult PFC and striatum is unaltered, while PNNs surrounding these neurons are increased. This increase in PNNs may restrict the plasticity of the ensheathed neurons, thereby contributing to impaired microcircuitry in frontostriatal connectivity and related behavioral impairments.

Single-Nucleus RNA Sequencing Reveals that Decorin Expression in the Amygdala Regulates Perineuronal Nets Expression and Fear Conditioning Response after Traumatic Brain Injury

Traumatic brain injury (TBI) is a risk factor for posttraumatic stress disorder (PTSD). Augmented fear is a defining characteristic of PTSD, and the amygdala is considered the main brain region to process fear. The mechanism by which the amygdala is involved in fear conditioning after TBI is still unclear. Using single-nucleus RNA sequencing (snRNA-seq), transcriptional changes in cells in the amygdala after TBI are investigated. In total, 72 328 nuclei are obtained from the sham and TBI groups. 7 cell types, and analysis of differentially expressed genes (DEGs) reveals widespread transcriptional changes in each cell type after TBI are identified. In in vivo experiments, it is demonstrated that Decorin (Dcn) expression in the excitatory neurons of the amygdala significantly increased after TBI, and Dcn knockout in the amygdala mitigates TBI-associated fear conditioning. Of note, this effect is caused by a Dcn-mediated decrease in the expression of perineuronal nets (PNNs), which affect the glutamate-γ-aminobutyric acid balance in the amygdala. Finally, the results suggest that Dcn functions by interacting with collagen VI α3 (Col6a3). Consequently, the findings reveal transcriptional changes in different cell types of the amygdala after TBI and provide direct evidence that Dcn relieves fear conditioning by regulating PNNs.

Perineuronal Nets in the Prefrontal Cortex of a Schizophrenia Mouse Model: Assessment of Neuroanatomical, Electrophysiological, and Behavioral Contributions

Schizophrenia is a neurodevelopmental disorder whose etiopathogenesis includes changes in cellular as well as extracellular structures. Perineuronal nets (PNNs) associated with parvalbumin-positive interneurons (PVs) in the prefrontal cortex (PFC) are dysregulated in schizophrenia. However, the postnatal development of these structures along with their associated neurons in the PFC is unexplored, as is their effects on behavior and neural activity. Therefore, in this study, we employed a DISC1 (Disruption in Schizophrenia) mutation mouse model of schizophrenia to assess these developmental changes and tested whether enzymatic digestion of PNNs in the PFC affected schizophrenia-like behaviors and neural activity. Developmentally, we found that the normal formation of PNNs, PVs, and colocalization of these two in the PFC, peaked around PND 22 (postnatal day 22). However, in DISC1, mutation animals from PND 0 to PND 60, both PNNs and PVs were significantly reduced. After enzymatic digestion of PNNs with chondroitinase in adult animals, the behavioral pattern of control animals mimicked that of DISC1 mutation animals, exhibiting reduced sociability, novelty and increased ultrasonic vocalizations, while there was very little change in other behaviors, such as working memory (Y-maze task involving medial temporal lobe) or depression-like behavior (tail-suspension test involving processing via the hypothalamic pituitary adrenal (HPA) axis). Moreover, following chondroitinase treatment, electrophysiological recordings from the PFC exhibited a reduced proportion of spontaneous, high-frequency firing neurons, and an increased proportion of irregularly firing neurons, with increased spike count and reduced inter-spike intervals in control animals. These results support the proposition that the aberrant development of PNNs and PVs affects normal neural operations in the PFC and contributes to the emergence of some of the behavioral phenotypes observed in the DISC1 mutation model of schizophrenia.

Decoding perineuronal net glycan sulfation patterns in the Alzheimer's disease brain

The extracellular matrix (ECM) of the brain comprises unique glycan "sulfation codes" that influence neurological function. Perineuronal nets (PNNs) are chondroitin sulfate-glycosaminoglycan (CS-GAG) containing matrices that enmesh neural networks involved in memory and cognition, and loss of PNN matrices is reported in patients with neurocognitive and neuropsychiatric disorders including Alzheimer's disease (AD). Using liquid chromatography tandem mass spectrometry (LC-MS/MS), we show that patients with a clinical diagnosis of AD-related dementia undergo a re-coding of their PNN-associated CS-GAGs that correlates to Braak stage progression, hyperphosphorylated tau (p-tau) accumulation, and cognitive impairment. As these CS-GAG sulfation changes are detectable prior to the regional onset of classical AD pathology, they may contribute to the initiation and/or progression of the underlying degenerative processes and implicate the brain matrix sulfation code as a key player in the development of AD clinicopathology.

Effects of Early Life Stress on the Developing Basolateral Amygdala-Prefrontal Cortex Circuit: The Emerging Role of Local Inhibition and Perineuronal Nets

The links between early life stress (ELS) and the emergence of psychopathology such as increased anxiety and depression are now well established, although the specific neurobiological and developmental mechanisms that translate ELS into poor health outcomes are still unclear. The consequences of ELS are complex because they depend on the form and severity of early stress, duration, and age of exposure as well as co-occurrence with other forms of physical or psychological trauma. The long term effects of ELS on the corticolimbic circuit underlying emotional and social behavior are particularly salient because ELS occurs during critical developmental periods in the establishment of this circuit, its local balance of inhibition:excitation and its connections with other neuronal pathways. Using examples drawn from the human and rodent literature, we review some of the consequences of ELS on the development of the corticolimbic circuit and how it might impact fear regulation in a sex- and hemispheric-dependent manner in both humans and rodents. We explore the effects of ELS on local inhibitory neurons and the formation of perineuronal nets (PNNs) that terminate critical periods of plasticity and promote the formation of stable local networks. Overall, the bulk of ELS studies report transient and/or long lasting alterations in both glutamatergic circuits and local inhibitory interneurons (INs) and their associated PNNs. Since the activity of INs plays a key role in the maturation of cortical regions and the formation of local field potentials, alterations in these INs triggered by ELS might critically participate in the development of psychiatric disorders in adulthood, including impaired fear extinction and anxiety behavior.

Extracellular Metalloproteinases in the Plasticity of Excitatory and Inhibitory Synapses

Long-term synaptic plasticity is shaped by the controlled reorganization of the synaptic proteome. A key component of this process is local proteolysis performed by the family of extracellular matrix metalloproteinases (MMPs). In recent years, considerable progress was achieved in identifying extracellular proteases involved in neuroplasticity phenomena and their protein substrates. Perisynaptic metalloproteinases regulate plastic changes at synapses through the processing of extracellular and membrane proteins. MMP9 was found to play a crucial role in excitatory synapses by controlling the NMDA-dependent LTP component. In addition, MMP3 regulates the L-type calcium channel-dependent form of LTP as well as the plasticity of neuronal excitability. Both MMP9 and MMP3 were implicated in memory and learning. Moreover, altered expression or mutations of different MMPs are associated with learning deficits and psychiatric disorders, including schizophrenia, addiction, or stress response. Contrary to excitatory drive, the investigation into the role of extracellular proteolysis in inhibitory synapses is only just beginning. Herein, we review the principal mechanisms of MMP involvement in the plasticity of excitatory transmission and the recently discovered role of proteolysis in inhibitory synapses. We discuss how different matrix metalloproteinases shape dynamics and turnover of synaptic adhesome and signal transduction pathways in neurons. Finally, we discuss future challenges in exploring synapse- and plasticity-specific functions of different metalloproteinases.

Perineuronal Net Receptor PTPσ Regulates Retention of Memories

Perineuronal nets (PNNs) have an important physiological role in the retention of learning by restricting cognitive flexibility. Their deposition peaks after developmental periods of intensive learning, usually in late childhood, and they help in long-term preservation of newly acquired skills and information. Modulation of PNN function by various techniques enhances plasticity and regulates the retention of memories, which may be beneficial when memory persistence entails negative symptoms such as post-traumatic stress disorder (PTSD). In this study, we investigated the role of PTPσ [receptor-type tyrosine-protein phosphatase S, a phosphatase that is activated by binding of chondroitin sulfate proteoglycans (CSPGs) from PNNs] in retention of memories using Novel Object Recognition and Fear Conditioning models. We observed that mice haploinsufficient for PTPRS gene (PTPσ^+/–), although having improved short-term object recognition memory, display impaired long-term memory in both Novel Object Recognition and Fear Conditioning paradigm, as compared to WT littermates. However, PTPσ^+/– mice did not show any differences in behavioral tests that do not heavily rely on cognitive flexibility, such as Elevated Plus Maze, Open Field, Marble Burying, and Forced Swimming Test. Since PTPσ has been shown to interact with and dephosphorylate TRKB, we investigated activation of this receptor and its downstream pathways in limbic areas known to be associated with memory. We found that phosphorylation of TRKB and PLCγ are increased in the hippocampus, prefrontal cortex, and amygdaloid complex of PTPσ^+/– mice, but other TRKB-mediated signaling pathways are not affected. Our data suggest that PTPσ downregulation promotes TRKB phosphorylation in different brain areas, improves short-term memory performance but disrupts long-term memory retention in the tested animal models. Inhibition of PTPσ or disruption of PNN-PTPσ-TRKB complex might be a potential target for disorders where negative modulation of the acquired memories can be beneficial.

Microglia Depletion-Induced Remodeling of Extracellular Matrix and Excitatory Synapses in the Hippocampus of Adult Mice

The extracellular matrix (ECM) plays a key role in synaptogenesis and the regulation of synaptic functions in the central nervous system. Recent studies revealed that in addition to dopaminergic and serotoninergic neuromodulatory systems, microglia also contribute to the regulation of ECM remodeling. In the present work, we investigated the physiological role of microglia in the remodeling of perineuronal nets (PNNs), predominantly associated with parvalbumin-immunopositive (PV+) interneurons, and the perisynaptic ECM around pyramidal neurons in the hippocampus. Adult mice were treated with PLX3397 (pexidartinib), as the inhibitor of colony-stimulating factor 1 receptor (CSF1-R), to deplete microglia. Then, confocal analysis of the ECM and synapses was performed. Although the elimination of microglia did not alter the overall number or intensity of PNNs in the CA1 region of the hippocampus, it decreased the size of PNN holes and elevated the expression of the surrounding ECM. In the neuropil area in the CA1 str. radiatum, the depletion of microglia increased the expression of perisynaptic ECM proteoglycan brevican, which was accompanied by the elevated expression of presynaptic marker vGluT1 and the increased density of dendritic spines. Thus, microglia regulate the homeostasis of pre- and postsynaptic excitatory terminals and the surrounding perisynaptic ECM as well as the fine structure of PNNs enveloping perisomatic-predominantly GABAergic-synapses.

Nutritional regulation of oligodendrocyte differentiation regulates perineuronal net remodeling in the median eminence

The mediobasal hypothalamus (MBH; arcuate nucleus of the hypothalamus [ARH] and median eminence [ME]) is a key nutrient sensing site for the production of the complex homeostatic feedback responses required for the maintenance of energy balance. Here, we show that refeeding after an overnight fast rapidly triggers proliferation and differentiation of oligodendrocyte progenitors, leading to the production of new oligodendrocytes in the ME specifically. During this nutritional paradigm, ME perineuronal nets (PNNs), emerging regulators of ARH metabolic functions, are rapidly remodeled, and this process requires myelin regulatory factor (Myrf) in oligodendrocyte progenitors. In genetically obese ob/ob mice, nutritional regulations of ME oligodendrocyte differentiation and PNN remodeling are blunted, and enzymatic digestion of local PNN increases food intake and weight gain. We conclude that MBH PNNs are required for the maintenance of energy balance in lean mice and are remodeled in the adult ME by the nutritional control of oligodendrocyte differentiation.

Hidden talents: Poly (I:C)-induced maternal immune activation improves mouse visual discrimination performance and reversal learning in a sex-dependent manner

While there is a strong focus on the negative consequences of maternal immune activation (MIA) on developing brains, very little attention is directed towards potential advantages of early life challenges. In this study, we utilized a polyinosine-polycytidylic acid (poly(I:C)) MIA model to test visual pairwise discrimination (PD) and reversal learning (RL) in mice using touchscreen technology. Significant sex differences emerged in that MIA reduced the latency for males to make a correct choice in the PD task while females reached criterion sooner, made fewer errors, and utilized fewer correction trials in RL compared to saline controls. These surprising improvements were accompanied by the sex-specific upregulation of several genes critical to cognitive functioning, indicative of compensatory plasticity in response to MIA. In contrast, when exposed to a 'two-hit' stress model (MIA + loss of the social component of environmental enrichment [EE]), mice did not display anhedonia but required an increased number of PD and RL correction trials. These animals also had significant reductions of CamK2a mRNA in the prefrontal cortex. Appropriate functioning of synaptic plasticity, via mediators such as this protein kinase and others, are critical for behavioral flexibility. Although EE has been implicated in, delaying the appearance of symptoms associated with certain brain disorders, these findings are in line with evidence that it also makes individuals more vulnerable to its loss. Overall, with the right 'dose', early life stress exposure can confer at least some functional advantages, which are lost when the number or magnitude of these exposures become too great.

Deletion of Fmr1 from Forebrain Excitatory Neurons Triggers Abnormal Cellular, EEG, and Behavioral Phenotypes in the Auditory Cortex of a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is a leading genetic cause of autism with symptoms that include sensory processing deficits. In both humans with FXS and a mouse model [Fmr1 knockout (KO) mouse], electroencephalographic (EEG) recordings show enhanced resting state gamma power and reduced sound-evoked gamma synchrony. We previously showed that elevated levels of matrix metalloproteinase-9 (MMP-9) may contribute to these phenotypes by affecting perineuronal nets (PNNs) around parvalbumin (PV) interneurons in the auditory cortex of Fmr1 KO mice. However, how different cell types within local cortical circuits contribute to these deficits is not known. Here, we examined whether Fmr1 deletion in forebrain excitatory neurons affects neural oscillations, MMP-9 activity, and PV/PNN expression in the auditory cortex. We found that cortical MMP-9 gelatinase activity, mTOR/Akt phosphorylation, and resting EEG gamma power were enhanced in CreNex1/Fmr1Flox/y conditional KO (cKO) mice, whereas the density of PV/PNN cells was reduced. The CreNex1/Fmr1Flox/y cKO mice also show increased locomotor activity, but not the anxiety-like behaviors. These results indicate that fragile X mental retardation protein changes in excitatory neurons in the cortex are sufficient to elicit cellular, electrophysiological, and behavioral phenotypes in Fmr1 KO mice. More broadly, these results indicate that local cortical circuit abnormalities contribute to sensory processing deficits in autism spectrum disorders.

Structural and Functional Modulation of Perineuronal Nets: In Search of Important Players with Highlight on Tenascins

The extracellular matrix (ECM) of the brain plays a crucial role in providing optimal conditions for neuronal function. Interactions between neurons and a specialized form of ECM, perineuronal nets (PNN), are considered a key mechanism for the regulation of brain plasticity. Such an assembly of interconnected structural and regulatory molecules has a prominent role in the control of synaptic plasticity. In this review, we discuss novel ways of studying the interplay between PNN and its regulatory components, particularly tenascins, in the processes of synaptic plasticity, mechanotransduction, and neurogenesis. Since enhanced neuronal activity promotes PNN degradation, it is possible to study PNN remodeling as a dynamical change in the expression and organization of its constituents that is reflected in its ultrastructure. The discovery of these subtle modifications is enabled by the development of super-resolution microscopy and advanced methods of image analysis.

Sexually dimorphic perineuronal nets in the rodent and primate reproductive circuit

Perineuronal nets are extracellular glycoprotein structures that have been found on some neurons in the central nervous system and that have been shown to regulate their structural plasticity. Until now work on perineuronal nets has been focused on their role in cortical structures where they are selectively expressed on parvalbumin-positive neurons and are reported to restrict the experience-dependent plasticity of inhibitory afferents. Here, we examined the expression of perineuronal nets subcortically, showing that they are expressed in several discrete structures, including nuclei that comprise the brain network controlling reproductive behaviors (e.g., mounting, lordosis, aggression, and social defense). In particular, perineuronal nets were found in the posterior dorsal division of the medial amygdala, the medial preoptic nucleus, the posterior medial bed nucleus of the stria terminalis, the ventrolateral ventromedial hypothalamus and adjacent tuberal nucleus, and the ventral premammillary nucleus in both the mouse and primate brain. Comparison of perineuronal nets in male and female mice revealed a significant sexually dimorphic expression, with expression found prominently on estrogen receptor expressing neurons in the medial amygdala. These findings suggest that perineuronal nets may be involved in regulating neural plasticity in the mammalian reproductive system.

Cocaine memory reactivation induces functional adaptations within parvalbumin interneurons in the rat medial prefrontal cortex

Substance use disorder is a complex disease created in part by maladaptive learning and memory mechanisms following repeated drug use. Exposure to drug-associated stimuli engages prefrontal cortex circuits, and dysfunction of the medial prefrontal cortex (mPFC) is thought to underlie drug-seeking behaviors. Growing evidence supports a role for parvalbumin containing fast-spiking interneurons (FSI) in modulating prefrontal cortical microcircuit activity by influencing the balance of excitation and inhibition, which can influence learning and memory processes. Most parvalbumin FSIs within layer V of the prelimbic mPFC are surrounded by specialized extracellular matrix structures called perineuronal nets (PNN). Previous work by our group found that cocaine exposure altered PNN-surrounded FSI function, and pharmacological removal of PNNs reduced cocaine-seeking behavior. However, the role of FSIs and associated constituents (parvalbumin and PNNs) in cocaine-related memories was not previously explored and is still unknown. Here, we found that reactivation of a cocaine conditioned place preference memory produced changes in cortical PNN-surrounded parvalbumin FSIs, including decreased parvalbumin intensity, increased parvalbumin cell axis diameter, decreased intrinsic excitability, and increased excitatory synaptic input. Further investigation of intrinsic properties revealed changes in the interspike interval, membrane capacitance, and afterhyperpolarization recovery time. Changes in these specific properties suggest an increase in potassium-mediated currents, which was validated with additional electrophysiological analysis. Collectively, our results indicate that cocaine memory reactivation induces functional adaptations in PNN-surrounded parvalbumin neurons, which likely alters cortical output to promote cocaine-seeking behavior.

Adult-born granule cell mossy fibers preferentially target parvalbumin-positive interneurons surrounded by perineuronal nets

Adult-born granule cells (abGCs) integrate into the hippocampus and form connections with dentate gyrus parvalbumin-positive (PV+) interneurons, a circuit important for modulating plasticity. Many of these interneurons are surrounded by perineuronal nets (PNNs), extracellular matrix structures known to participate in plasticity. We compared abGC projections to PV+ interneurons with negative-to-low intensity PNNs to those with high intensity PNNs using retroviral and 3R-Tau labeling in adult mice, and found that abGC mossy fibers and boutons are more frequently located near PV+ interneurons with high intensity PNNs. These results suggest that axons of new neurons preferentially stabilize near target cells with intense PNNs. Next, we asked whether the number of abGCs influences PNN formation around PV+ interneurons, and found that near complete ablation of abGCs produced a decrease in the intensity and number of PV+ neurons with PNNs, suggesting that new neuron innervation may enhance PNN formation. Experience-driven changes in adult neurogenesis did not produce consistent effects, perhaps due to widespread effects on plasticity. Our study identifies abGC projections to PV+ interneurons with PNNs, with more presumed abGC mossy fiber boutons found near the cell body of PV+ interneurons with strong PNNs.

Accelerated Hyper-Maturation of Parvalbumin Circuits in the Absence of MeCP2

Methyl-CpG-binding protein 2 (MeCP2) mutations are the primary cause of Rett syndrome, a severe neurodevelopmental disorder. Cortical parvalbumin GABAergic interneurons (PV) make exuberant somatic connections onto pyramidal cells in the visual cortex of Mecp2-deficient mice, which contributes to silencing neuronal cortical circuits. This phenotype can be rescued independently of Mecp2 by environmental, pharmacological, and genetic manipulation. It remains unknown how Mecp2 mutation can result in abnormal inhibitory circuit refinement. In the present manuscript, we examined the development of GABAergic circuits in the primary visual cortex of Mecp2-deficient mice. We identified that PV circuits were the only GABAergic interneurons to be upregulated, while other interneurons were downregulated. Acceleration of PV cell maturation was accompanied by increased PV cells engulfment by perineuronal nets (PNNs) and by an increase of PV cellular and PNN structural complexity. Interestingly, selective deletion of Mecp2 from PV cells was sufficient to drive increased structure complexity of PNN. Moreover, the accelerated PV and PNN maturation was recapitulated in organotypic cultures. Our results identify a specific timeline of disruption of GABAergic circuits in the absence of Mecp2, indicating a possible cell-autonomous role of MeCP2 in the formation of PV cellular arbors and PNN structures in the visual cortex.

Poly I:C Activated Microglia Disrupt Perineuronal Nets and Modulate Synaptic Balance in Primary Hippocampal Neurons in vitro

Perineuronal nets (PNNs) are specialized, reticular structures of the extracellular matrix (ECM) that can be found covering the soma and proximal dendrites of a neuronal subpopulation. Recent studies have shown that PNNs can highly influence synaptic plasticity and are disrupted in different neuropsychiatric disorders like schizophrenia. Interestingly, there is a growing evidence that microglia can promote the loss of PNNs and contribute to neuropsychiatric disorders. Based on this knowledge, we analyzed the impact of activated microglia on hippocampal neuronal networks in vitro. Therefore, primary cortical microglia were cultured and stimulated via polyinosinic-polycytidylic acid (Poly I:C; 50 μg/ml) administration. The Poly I:C treatment induced the expression and secretion of different cytokines belonging to the CCL- and CXCL-motif chemokine family as well as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α). In addition, the expression of matrix metalloproteinases (MMPs) could be verified via RT-PCR analysis. Embryonic hippocampal neurons were then cultured for 12 days in vitro (DIV) and treated for 24 h with microglial conditioned medium. Interestingly, immunocytochemical staining of the PNN component Aggrecan revealed a clear disruption of PNNs accompanied by a significant increase of glutamatergic and a decrease of γ-aminobutyric acid-(GABA)ergic synapse numbers on PNN wearing neurons. In contrast, PNN negative neurons showed a significant reduction in both, glutamatergic and GABAergic synapses. Electrophysiological recordings were performed via multielectrode array (MEA) technology and unraveled a significantly increased spontaneous network activity that sustained also 24 and 48 h after the administration of microglia conditioned medium. Taken together, we could observe a strong impact of microglial secreted factors on PNN integrity, synaptic plasticity and electrophysiological properties of cultured neurons. Our observations might enhance the understanding of neuron-microglia interactions considering the ECM.

Erythropoietin Stimulates GABAergic Maturation in the Mouse Hippocampus

Several neurodevelopmental disabilities are strongly associated with alterations in GABAergic transmission, and therapies to stimulate its normal development are lacking. Erythropoietin (EPO) is clinically used in neonatology to mitigate acute brain injury, and to stimulate neuronal maturation. Yet it remains unclear whether EPO can stimulate maturation of the GABAergic system. Here, with the use of a transgenic mouse line that constitutively overexpresses neuronal EPO (Tg21), we show that EPO stimulates postnatal GABAergic maturation in the hippocampus. We show an increase in hippocampal GABA-immunoreactive neurons, and postnatal elevation of interneurons expressing parvalbumin (PV), somatostatin (SST), and neuropeptide Y (NPY). Analysis of perineuronal net (PNN) formation and innervation of glutamatergic terminals onto PV+ cells, shows to be enhanced early in postnatal development. Additionally, an increase in GABA_Aergic synapse density and IPSCs in CA1 pyramidal cells from Tg21 mice is observed. Detection of EPO receptor (EPOR) mRNA was observed to be restricted to glutamatergic pyramidal cells and increased in Tg21 mice at postnatal day (P)7, along with reduced apoptosis. Our findings show that EPO can stimulate postnatal GABAergic maturation in the hippocampus, by increasing neuronal survival, modulating critical plasticity periods, and increasing synaptic transmission. Our data supports EPO’s clinical use to balance GABAergic dysfunction.

Chondroitinase and Antidepressants Promote Plasticity by Releasing TRKB from Dephosphorylating Control of PTPσ in Parvalbumin Neurons

Perineuronal nets (PNNs) are an extracellular matrix structure rich in chondroitin sulfate proteoglycans (CSPGs), which preferentially encase parvalbumin-containing (PV+) interneurons. PNNs restrict cortical network plasticity but the molecular mechanisms involved are unclear. We found that reactivation of ocular dominance plasticity in the adult visual cortex induced by chondroitinase ABC (chABC)-mediated PNN removal requires intact signaling by the neurotrophin receptor TRKB in PV+ neurons. Additionally, we demonstrate that chABC increases TRKB phosphorylation (pTRKB), while PNN component aggrecan attenuates brain-derived neurotrophic factor (BDNF)-induced pTRKB in cortical neurons in culture. We further found that protein tyrosine phosphatase σ (PTPσ, PTPRS), receptor for CSPGs, interacts with TRKB and restricts TRKB phosphorylation. PTPσ deletion increases phosphorylation of TRKB in vitro and in vivo in male and female mice, and juvenile-like plasticity is retained in the visual cortex of adult PTPσ-deficient mice (PTPσ+/-). The antidepressant drug fluoxetine, which is known to promote TRKB phosphorylation and reopen critical period-like plasticity in the adult brain, disrupts the interaction between TRKB and PTPσ by binding to the transmembrane domain of TRKB. We propose that both chABC and fluoxetine reopen critical period-like plasticity in the adult visual cortex by promoting TRKB signaling in PV+ neurons through inhibition of TRKB dephosphorylation by the PTPσ-CSPG complex.SIGNIFICANCE STATEMENT Critical period-like plasticity can be reactivated in the adult visual cortex through disruption of perineuronal nets (PNNs) by chondroitinase treatment, or by chronic antidepressant treatment. We now show that the effects of both chondroitinase and fluoxetine are mediated by the neurotrophin receptor TRKB in parvalbumin-containing (PV+) interneurons. We found that chondroitinase-induced visual cortical plasticity is dependent on TRKB in PV+ neurons. Protein tyrosine phosphatase σ (PTPσ, PTPRS), a receptor for PNNs, interacts with TRKB and inhibits its phosphorylation, and chondroitinase treatment or deletion of PTPσ increases TRKB phosphorylation. Antidepressant fluoxetine disrupts the interaction between TRKB and PTPσ, thereby increasing TRKB phosphorylation. Thus, juvenile-like plasticity induced by both chondroitinase and antidepressant treatment is mediated by TRKB activation in PV+ interneurons.