Perisynaptic chondroitin sulfate proteoglycans restrict structural plasticity in an integrin-dependent manner

Neuroregeneration and plasticity: a review of the physiological mechanisms for achieving functional recovery postinjury

Neuronal networks, especially those in the central nervous system (CNS), evolved to support extensive functional capabilities while ensuring stability. Several physiological "brakes" that maintain the stability of the neuronal networks in a healthy state quickly become a hinderance postinjury. These "brakes" include inhibition from the extracellular environment, intrinsic factors of neurons and the control of neuronal plasticity. There are distinct differences between the neuronal networks in the peripheral nervous system (PNS) and the CNS. Underpinning these differences is the trade-off between reduced functional capabilities with increased adaptability through the formation of new connections and new neurons. The PNS has "facilitators" that stimulate neuroregeneration and plasticity, while the CNS has "brakes" that limit them. By studying how these "facilitators" and "brakes" work and identifying the key processes and molecules involved, we can attempt to apply these theories to the neuronal networks of the CNS to increase its adaptability. The difference in adaptability between the CNS and PNS leads to a difference in neuroregenerative properties and plasticity. Plasticity ensures quick functional recovery of abilities in the short and medium term. Neuroregeneration involves synthesizing new neurons and connections, providing extra resources in the long term to replace those damaged by the injury, and achieving a lasting functional recovery. Therefore, by understanding the factors that affect neuroregeneration and plasticity, we can combine their advantages and develop rehabilitation techniques. Rehabilitation training methods, coordinated with pharmacological interventions and/or electrical stimulation, contributes to a precise, holistic treatment plan that achieves functional recovery from nervous system injuries. Furthermore, these techniques are not limited to limb movement, as other functions lost as a result of brain injury, such as speech, can also be recovered with an appropriate training program.

Lateralized Expression of Cortical Perineuronal Nets during Maternal Experience is Dependent on MECP2

Cortical neuronal circuits along the sensorimotor pathways are shaped by experience during critical periods of heightened plasticity in early postnatal development. After closure of critical periods, measured histologically by the formation and maintenance of extracellular matrix structures called perineuronal nets (PNNs), the adult mouse brain exhibits restricted plasticity and maturity. Mature PNNs are typically considered to be stable structures that restrict synaptic plasticity on cortical parvalbumin+ (PV+) GABAergic neurons. Changes in environment (i.e., novel behavioral training) or social contexts (i.e., motherhood) are known to elicit synaptic plasticity in relevant neural circuitry. However, little is known about concomitant changes in the PNNs surrounding the cortical PV+ GABAergic neurons. Here, we show novel changes in PNN density in the primary somatosensory cortex (SS1) of adult female mice after maternal experience [called surrogate (Sur)], using systematic microscopy analysis of a whole brain region. On average, PNNs were increased in the right barrel field and decreased in the left forelimb regions. Individual mice had left hemisphere dominance in PNN density. Using adult female mice deficient in methyl-CpG-binding protein 2 (MECP2), an epigenetic regulator involved in regulating experience-dependent plasticity, we found that MECP2 is critical for this precise and dynamic expression of PNN. Adult naive Mecp2-heterozygous (Het) females had increased PNN density in specific subregions in both hemispheres before maternal experience, compared with wild-type (WT) littermate controls. The laterality in PNN expression seen in naive Het (NH) was lost after maternal experience in Sur Het (SH) mice, suggesting possible intact mechanisms for plasticity. Together, our results identify subregion and hemisphere-specific alterations in PNN expression in adult females, suggesting extracellular matrix plasticity as a possible neurobiological mechanism for adult behaviors in rodents.

Targeting chondroitinase ABC to axons enhances the ability of chondroitinase to promote neurite outgrowth and sprouting

BACKGROUND

There is currently no effective treatment for promoting regeneration of injured nerves in patients who have sustained injury to the central nervous system such as spinal cord injury. Chondroitinase ABC is an enzyme, which promotes neurite outgrowth and regeneration. It has shown considerable promise as a therapy for these conditions. The aim of the study is to determine if targeting chondroitinase ABC expression to the neuronal axon can further enhance its ability to promote axon outgrowth. Long-distance axon regeneration has not yet been achieved, and would be a significant step in attaining functional recovery following spinal cord injury.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate this, neuronal cultures were transfected with constructs encoding axon-targeted chondroitinase, non-targeted chondroitinase or GFP, and the effects on neuron outgrowth and sprouting determined on substrates either permissive or inhibitory to neuron regeneration. The mechanisms underlying the observed effects were also explored. Targeting chondroitinase to the neuronal axon markedly enhances its ability to promote neurite outgrowth. The increase in neurite length is associated with an upregulation of β-integrin staining at the axonal cell surface. Staining for phosphofocal adhesion kinase, is also increased, indicating that the β-integrins are in an activated state. Expression of chondroitinase within the neurons also resulted in a decrease in expression of PTEN and RhoA, molecules which present a block to neurite outgrowth, thus identifying two of the pathways by which ChABC promotes neurite outgrowth.

CONCLUSIONS / SIGNIFICANCE

The novel finding that targeting ChABC to the axon significantly enhances its ability to promote neurite extension, suggests that this may be an effective way of promoting long-distance axon regeneration following spinal cord injury. It could also potentially improve its efficacy in the treatment of other pathologies, where it has been shown to promote recovery, such as myocardial infarction, stroke and Parkinson's disease.

Mechanotransduction in neuronal cell development and functioning

Although many details remain still elusive, it became increasingly evident in recent years that mechanosensing of microenvironmental biophysical cues and subsequent mechanotransduction are strongly involved in the regulation of neuronal cell development and functioning. This review gives an overview about the current understanding of brain and neuronal cell mechanobiology and how it impacts on neurogenesis, neuronal migration, differentiation, and maturation. We will focus particularly on the events in the cell/microenvironment interface and the decisive extracellular matrix (ECM) parameters (i.e. rigidity and nanometric spatial organisation of adhesion sites) that modulate integrin adhesion complex-based mechanosensing and mechanotransductive signalling. It will also be outlined how biomaterial approaches mimicking essential ECM features help to understand these processes and how they can be used to control and guide neuronal cell behaviour by providing appropriate biophysical cues. In addition, principal biophysical methods will be highlighted that have been crucial for the study of neuronal mechanobiology.

Versatile control of synaptic circuits by astrocytes: where, when and how?

Close structural and functional interactions of astrocytes with synapses play an important role in brain function. The repertoire of ways in which astrocytes can regulate synaptic transmission is complex so that they can both promote and dampen synaptic efficacy. Such contrasting effects raise questions regarding the determinants of these divergent astroglial functions. Recent findings provide insights into where, when and how astroglial regulation of synapses takes place by revealing major molecular and functional intrinsic heterogeneity as well as switches in astrocytes occurring during development or specific patterns of neuronal activity. Astrocytes may therefore be seen as boosters or gatekeepers of synaptic circuits depending on their intrinsic and transformative properties throughout life.

The roles of perineuronal nets and the perinodal extracellular matrix in neuronal function

Perineuronal nets (PNNs) are extracellular matrix (ECM) chondroitin sulfate proteoglycan (CSPG)-containing structures that surround the soma and dendrites of various mammalian neuronal cell types. PNNs appear during development around the time that the critical periods for developmental plasticity end and are important for both their onset and closure. A similar structure - the perinodal ECM - surrounds the axonal nodes of Ranvier and appears as myelination is completed, acting as an ion-diffusion barrier that affects axonal conduction speed. Recent work has revealed the importance of PNNs in controlling plasticity in the CNS. Digestion, blocking or removal of PNNs influences functional recovery after a variety of CNS lesions. PNNs have further been shown to be involved in the regulation of memory and have been implicated in a number of psychiatric disorders.

Physical Plasma Membrane Perturbation Using Subcellular Optogenetics Drives Integrin-Activated Cell Migration

Cells experience physical deformations to the plasma membrane that can modulate cell behaviors like migration. Understanding the molecular basis for how physical cues affect dynamic cellular responses requires new approaches that can physically perturb the plasma membrane with rapid, reversible, subcellular control. Here we present an optogenetic approach based on light-inducible dimerization that alters plasma membrane properties by recruiting cytosolic proteins at high concentrations to a target site. Surprisingly, this polarized accumulation of proteins in a cell induces directional amoeboid migration in the opposite direction. Consistent with known effects of constraining high concentrations of proteins to a membrane in vitro, there is localized curvature and tension decrease in the plasma membrane. Integrin activity, sensitive to mechanical forces, is activated in this region. Localized mechanical activation of integrin with optogenetics allowed simultaneous imaging of the molecular and cellular response, helping uncover a positive feedback loop comprising SFK- and ERK-dependent RhoA activation, actomyosin contractility, rearward membrane flow, and membrane tension decrease underlying this mode of cell migration.

"Teaching old dogs new tricks": targeting neural extracellular matrix for normal and pathological aging-related cognitive decline

Cognitive decline is a feature of normal and pathological aging. As the proportion of the global aged population continues to grow, it is imperative to understand the molecular and cellular substrates of cognitive aging for therapeutic discovery. This review focuses on the critical role of neural extracellular matrix in the regulation of neuroplasticity underlying learning and memory in another under-investigated “critical period”: the aging process. The fascinating ideas of neural extracellular matrix forming a synaptic cradle in the tetrapartite synapse and possibly serving as a substrate for storage of very long-term memories will be introduced. We emphasize the distinct functional roles of diffusive neural extracellular matrix and perineuronal nets and the advantage of the coexistence of two structures for the adaptation to the ever-changing external and internal environments. Our study of striatal neural extracellular matrix supports the idea that chondroitin sulfate proteoglycan-associated extracellular matrix is restrictive on synaptic neuroplasticity, which plays important functional and pathogenic roles in early postnatal synaptic consolidation and aging-related cognitive decline. Therefore, the chondroitin sulfate proteoglycan-associated neural extracellular matrix can be targeted for normal and pathological aging. Future studies should focus on the cell-type specificity of neural extracellular matrix to identify the endogenous, druggable targets to restore juvenile neuroplasticity and confer a therapeutic benefit to neural circuits affected by aging.

Regulatory roles of perineuronal nets and semaphorin 3A in the postnatal maturation of the central vestibular circuitry for graviceptive reflex

Perineuronal nets (PN) restrict neuronal plasticity in the adult brain. We hypothesize that activity-dependent consolidation of PN is required for functional maturation of behavioral circuits. Using the postnatal maturation of brainstem vestibular nucleus (VN) circuits as a model system, we report a neonatal period in which consolidation of central vestibular circuitry for graviception is accompanied by activity-dependent consolidation of chondroitin sulfate (CS)-rich PN around GABAergic neurons in the VN. Postnatal onset of negative geotaxis was used as an indicator for functional maturation of vestibular circuits. Rats display negative geotaxis from postnatal day (P) 9, coinciding with the condensation of CS-rich PN around GABAergic interneurons in the VN. Delaying PN formation, by removal of primordial CS moieties on VN with chondroitinase ABC (ChABC) treatment at P6, postponed emergence of negative geotaxis to P13. Similar postponement was observed following inhibition of GABAergic transmission with bicuculline, in line with the reported role of PN in increasing excitability of parvalbumin neurons. We further reasoned that PN-CS restricts bioavailability of plasticity-inducing factors such as semaphorin 3A (Sema3A) to bring about circuit maturation. Treatment of VN explants with ChABC to liberate PN-bound Sema3A resulted in dendritic growth and arborization, implicating structural plasticity that delays synapse formation. Evidence is thus provided for the role of PN-CS-Sema3A in regulating structural and circuit plasticity at VN interneurons with impacts on the development of graviceptive postural control.

Digestion of the glycosaminoglycan extracellular matrix by chondroitinase ABC supports retinal ganglion cell dendritic preservation in a rodent model of experimental glaucoma

Retinal ganglion cell dendritic atrophy is an early feature of glaucoma, and the recovery of retinal ganglion cell dendrites is a viable option for vision improvement in glaucoma. Retinal ganglion cell neurites are surrounded by a specialised glycosaminoglycan extracellular matrix which inhibits dendritic plasticity. Since digestion of the extracellular matrix by chondroitinase ABC has been reported to have neuro-regenerative and neuro-plastic effects within the central nervous system, we explored its potential for dendritic recovery in a rat model of ocular hypertension. Chondroitinase ABC was administrated intravitreally 1 week after ocular hypertension (a time point where dendritic atrophy has already occurred). Retinal ganglion cell dendritic morphology was unaffected by chondroitinase ABC in normal retina. In ocular hypertensive eyes retinal ganglion cells showed significantly decreased dendritic length and area under the Sholl curve with atrophy confined to higher order dendrites. These changes were not observed in chondroitinase ABC injected eyes despite similar total retinal ganglion cell loss (i.e. dendritic protection of surviving retinal ganglion cells). These data suggest that glycosaminoglycan digestion could have a therapeutic role in mitigating the effects of elevated pressure on retinal ganglion cell dendritic structure in glaucoma.

The Biology of Regeneration Failure and Success After Spinal Cord Injury

Since no approved therapies to restore mobility and sensation following spinal cord injury (SCI) currently exist, a better understanding of the cellular and molecular mechanisms following SCI that compromise regeneration or neuroplasticity is needed to develop new strategies to promote axonal regrowth and restore function. Physical trauma to the spinal cord results in vascular disruption that, in turn, causes blood-spinal cord barrier rupture leading to hemorrhage and ischemia, followed by rampant local cell death. As subsequent edema and inflammation occur, neuronal and glial necrosis and apoptosis spread well beyond the initial site of impact, ultimately resolving into a cavity surrounded by glial/fibrotic scarring. The glial scar, which stabilizes the spread of secondary injury, also acts as a chronic, physical, and chemo-entrapping barrier that prevents axonal regeneration. Understanding the formative events in glial scarring helps guide strategies towards the development of potential therapies to enhance axon regeneration and functional recovery at both acute and chronic stages following SCI. This review will also discuss the perineuronal net and how chondroitin sulfate proteoglycans (CSPGs) deposited in both the glial scar and net impede axonal outgrowth at the level of the growth cone. We will end the review with a summary of current CSPG-targeting strategies that help to foster axonal regeneration, neuroplasticity/sprouting, and functional recovery following SCI.

Impaired Cognitive Function after Perineuronal Net Degradation in the Medial Prefrontal Cortex

Perineuronal nets (PNNs) are highly organized components of the extracellular matrix that surround a subset of mature neurons in the CNS. These structures play a critical role in regulating neuronal plasticity, particularly during neurodevelopment. Consistent with this role, their presence is associated with functional and structural stability of the neurons they ensheath. A loss of PNNs in the prefrontal cortex (PFC) has been suggested to contribute to cognitive impairment in disorders such as schizophrenia. However, the direct consequences of PNN loss in medial PFC (mPFC) on cognition has not been demonstrated. Here, we examined behavior after disruption of PNNs in mPFC of Long-Evans rats following injection of the enzyme chondroitinase ABC (ChABC). Our data show that ChABC-treated animals were impaired on tests of object oddity perception. Performance in the cross-modal object recognition (CMOR) task was not significantly different for ChABC-treated rats, although ChABC-treated rats were not able to perform above chance levels whereas control rats were. ChABC-treated animals were not significantly different from controls on tests of prepulse inhibition (PPI), set-shifting (SS), reversal learning, or tactile and visual object recognition memory. Posthumous immunohistochemistry confirmed significantly reduced PNNs in mPFC due to ChABC treatment. Moreover, PNN density in the mPFC predicted performance on the oddity task, where higher PNN density was associated with better performance. These findings suggest that PNN loss within the mPFC impairs some aspects of object oddity perception and recognition and that PNNs contribute to cognitive function in young adulthood.

Perineuronal nets in brain physiology and disease

Perineuronal nets (PNNs) in the brain are condensed glycosaminoglycan-rich extracellular matrix structures with heterogeneous composition yet specific organization. They typically assemble around a subset of fast-spiking interneurons that are implicated in learning and memory. Owing to their unique structural organization, PNNs have neuroprotective capacities but also participate in signal transduction and in controlling neuronal activity and plasticity. In this review, we define PNN structure in detail and describe its various biochemical and physiological functions. We further discuss the role of PNNs in brain disorders such as schizophrenia, bipolar disorder, Alzheimer disease and addictions. Lastly, we describe therapeutic approaches that target PNNs to alter brain physiology and counter brain dysfunction.

Neurocan Inhibits Semaphorin 3F Induced Dendritic Spine Remodeling Through NrCAM in Cortical Neurons

Neurocan is a chondroitin sulfate proteoglycan present in perineuronal nets, which are associated with closure of the critical period of synaptic plasticity. During postnatal development of the neocortex dendritic spines on pyramidal neurons are initially overproduced; later they are pruned to achieve an appropriate balance of excitatory to inhibitory synapses. Little is understood about how spine pruning is terminated upon maturation. NrCAM (Neuron-glial related cell adhesion molecule) was found to mediate spine pruning as a subunit of the receptor complex for the repellent ligand Semaphorin 3F (Sema3F). As shown here in the postnatal mouse frontal and visual neocortex, Neurocan was localized at both light and electron microscopic level to the cell surface of cortical pyramidal neurons and was adjacent to neuronal processes and dendritic spines. Sema3F-induced spine elimination was inhibited by Neurocan in cortical neuron cultures. Neurocan also blocked Sema3F-induced morphological retraction in COS-7 cells, which was mediated through NrCAM and other subunits of the Sema3F holoreceptor, Neuropilin-2, and PlexinA3. Cell binding and ELISA assays demonstrated an association of Neurocan with NrCAM. Glycosaminoglycan chain interactions of Neurocan were required for inhibition of Sema3F-induced spine elimination, but the C-terminal sushi domain was dispensable. These results describe a novel mechanism wherein Neurocan inhibits NrCAM/Sema3F-induced spine elimination.

Erasure of striatal chondroitin sulfate proteoglycan-associated extracellular matrix rescues aging-dependent decline of motor learning

Cognitive decline is a feature of aging. Accumulating evidence suggests that the brain extracellular matrix (ECM) is involved in the process of aging-dependent cognitive impairment and neurodegeneration by regulating synaptic neurotransmission and affecting neuroplasticity. Age-related changes in brain structure and cognition are not uniform across the whole brain. Being one of the most vulnerable brain regions to aging-dependent alterations, striatum is integral to several central nervous system functions, such as motor, cognition, and affective control. However, the striatal ECM is largely understudied. We first describe 2 major types of chondroitin sulfate proteoglycan (CSPG)-associated ECM in striatum: perineuronal nets and diffusive ECM. Both types of ECM accumulate in an aging-dependent manner. The accumulation of CSPG-associated ECM correlates with aging-dependent decline in striatum-related cognitive functions, including motor learning and working memory. Enzymatic depletion of CSPG-associated ECM in aged mice via chondroitinase ABC significantly improves motor learning, suggesting that changes in neural ECM CSPGs regulate striatal plasticity. Our study provides a greater understanding of the role of neural ECM underlying striatal plasticity, which is an important precursor to design appropriate therapeutic strategies for normal and pathologic aging.

The Impact of Perineuronal Net Digestion Using Chondroitinase ABC on the Intrinsic Physiology of Cortical Neurons

Perineuronal nets (PNNs) are a form of aggregate Extracellular Matrix (ECM) in the brain. Recent evidence suggests that the postnatal deposition of PNNs may play an active role in regulating neuroplasticity and, potentially, neurological disorders. Observations of high levels of PNN expression around somas, proximal dendrites, and axon initial segments of a subtype of neurons have also led to proposals that PNNs may modulate the intrinsic properties of the neurons they ensheathe. While high levels of PNNs are postnatally expressed throughout the neocortex, it is still unclear how they impact the neuronal physiology of the many classes and subtypes of neurons that exist. In this study, we demonstrate that Chondroitinase ABC digestion of PNNs from acute cortical slices from juvenile mice (P28-35) resulted in neuron-specific impacts on intrinsic physiology. Fast spiking (FS) interneurons showed decreased input resistance, resting membrane potential (RMP), reduced action potential (AP) peaks and altered spontaneous synaptic inputs. Low-Threshold Spiking interneurons showed altered rebound depolarizations and decreased frequency of spontaneous synaptic inputs. Putative excitatory neurons; regular spiking, bursting, and doublet phenotypes did not demonstrate any alterations. Our data indicate that chABC-sensitive PNNs may specifically regulate the intrinsic and synaptic physiology of inhibitory interneurons.

Integrin activity in neuronal connectivity

The formation of correct synaptic structures and neuronal connections is paramount for normal brain development and a functioning adult brain. The integrin family of cell adhesion receptors and their ligands play essential roles in the control of several processes regulating neuronal connectivity - including neurite outgrowth, the formation and maintenance of synapses, and synaptic plasticity - that are affected in neurodevelopmental disorders, such as autism spectrum disorders (ASDs) and schizophrenia. Many ASD- and schizophrenia-associated genes are linked to alterations in the genetic code of integrins and associated signalling pathways. In non-neuronal cells, crosstalk between integrin-mediated adhesions and the actin cytoskeleton, and the regulation of integrin activity (affinity for extracellular ligands) are widely studied in healthy and pathological settings. In contrast, the roles of integrin-linked pathways in the central nervous system remains less well defined. In this Review, we will provide an overview of the known pathways that are regulated by integrin-ECM interaction in developing neurons and in adult brain. We will also describe recent advances in the identification of mechanisms that regulate integrin activity in neurons, and highlight the interesting emerging links between integrins and neurodevelopment.

Increased Excitability and Reduced Excitatory Synaptic Input Into Fast-Spiking CA2 Interneurons After Enzymatic Attenuation of Extracellular Matrix

The neural extracellular matrix (ECM) is enriched with hyaluronic acid, chondroitin sulfate proteoglycans (CSPGs) and the glycoprotein tenascin-R, which play important roles in synaptic plasticity, as shown by studies of the CA1 region of the hippocampus. However, ECM molecules are strongly expressed in the CA2 region, which harbors a high number of fast-spiking interneurons (FSIs) surrounded by a particularly condensed form of ECM, perineuronal nets. Despite this intriguing peculiarity, the functional role of ECM in the CA2 region is mostly unknown. Here, we investigate the acute and delayed effects of chondroitinase ABC (ChABC), an enzyme that digests chondroitin sulfate side chains of CSPGs and greatly attenuates neural ECM, on neuronal excitability and excitatory transmission in the CA2 region. Whole-cell patch clamp recordings of CA2 pyramidal cells (PCs) and FSIs in hippocampal slices revealed that 7 days after injection of ChABC into the CA2 region in vivo, there are alterations in excitability of FSIs and PCs. FSIs generated action potentials with larger amplitudes and longer durations in response to less depolarizing currents compared to controls. PCs were excited at less depolarized membrane potentials, resulted in lower latency of spike generation. The frequency of excitatory postsynaptic currents in FSIs was selectively reduced, while the frequency of inhibitory postsynaptic currents was selectively increased. Acute treatment of hippocampal slices with ChABC did not result in any of these effects. This increase in excitability and changes in synaptic inputs to FSIs after attenuation of ECM suggests a crucial role for perineuronal nets associated with FSIs in regulation of synaptic and electrical properties of these cells.

Brain ageing changes proteoglycan sulfation, rendering perineuronal nets more inhibitory

Chondroitin sulfate (CS) proteoglycans in perineuronal nets (PNNs) from the central nervous system (CNS) are involved in the control of plasticity and memory. Removing PNNs reactivates plasticity and restores memory in models of Alzheimer’s disease and ageing. Their actions depend on the glycosaminoglycan (GAG) chains of CS proteoglycans, which are mainly sulfated in the 4 (C4S) or 6 (C6S) positions. While C4S is inhibitory, C6S is more permissive to axon growth, regeneration and plasticity. C6S decreases during critical period closure. We asked whether there is a late change in CS-GAG sulfation associated with memory loss in aged rats. Immunohistochemistry revealed a progressive increase in C4S and decrease in C6S from 3 to 18 months. GAGs extracted from brain PNNs showed a large reduction in C6S at 12 and 18 months, increasing the C4S/C6S ratio. There was no significant change in mRNA levels of the chondroitin sulfotransferases. PNN GAGs were more inhibitory to axon growth than those from the diffuse extracellular matrix. The 18-month PNN GAGs were more inhibitory than 3-month PNN GAGs. We suggest that the change in PNN GAG sulfation in aged brains renders the PNNs more inhibitory, which lead to a decrease in plasticity and adversely affect memory.

The tetrapartite synapse: a key concept in the pathophysiology of schizophrenia

Growing evidence points to synaptic pathology as a core component of the pathophysiology of schizophrenia (SZ). Significant reductions of dendritic spine density and altered expression of their structural and molecular components have been reported in several brain regions, suggesting a deficit of synaptic plasticity. Regulation of synaptic plasticity is a complex process, one that requires not only interactions between pre- and post-synaptic terminals, but also glial cells and the extracellular matrix (ECM). Together, these elements are referred to as the ‘tetrapartite synapse’, an emerging concept supported by accumulating evidence for a role of glial cells and the extracellular matrix in regulating structural and functional aspects of synaptic plasticity. In particular, chondroitin sulfate proteoglycans (CSPGs), one of the main components of the ECM, have been shown to be synthesized predominantly by glial cells, to form organized perisynaptic aggregates known as perineuronal nets (PNNs), and to modulate synaptic signaling and plasticity during postnatal development and adulthood. Notably, recent findings from our group and others have shown marked CSPG abnormalities in several brain regions of people with SZ. These abnormalities were found to affect specialized ECM structures, including PNNs, as well as glial cells expressing the corresponding CSPGs. The purpose of this review is to bring forth the hypothesis that synaptic pathology in SZ arises from a disruption of the interactions between elements of the tetrapartite synapse.

A Sweet Talk: The Molecular Systems of Perineuronal Nets in Controlling Neuronal Communication

Perineuronal nets (PNNs) are mesh-like structures, composed of a hierarchical assembly of extracellular matrix molecules in the central nervous system (CNS), ensheathing neurons and regulating plasticity. The mechanism of interactions between PNNs and neurons remain uncharacterized. In this review, we pose the question: how do PNNs regulate communication to and from neurons? We provide an overview of the current knowledge on PNNs with a focus on the cellular interactions. PNNs ensheath a subset of the neuronal population with distinct molecular aspects in different areas of the CNS. PNNs control neuronal communication through molecular interactions involving specific components of the PNNs. This review proposes that the PNNs are an integral part of neurons, crucial for the regulation of plasticity in the CNS.

Hyaluronic acid based extracellular matrix regulates surface expression of GluN2B containing NMDA receptors

Cortical areas of the juvenile rodent brain display a high degree of structural and functional plasticity, which disappears later in development. Coincident with the decline of plasticity 1) the hyaluronic acid-based extracellular matrix (ECM) of the brain, which stabilizes synapses and neuronal circuit is formed and 2) N-methyl-D-aspartate subtype of ionotropic glutamate receptors (NMDARs) implied in synaptic plasticity switch from mainly GluN2B to GluN2A subunit-containing receptors. Here we tested the hypothesis that ECM influences the NMDAR subunit composition in dissociated neuronal cultures. Experimental removal of ECM using hyaluronidase induced an increase in surface expression of GluN2B. This was due to decreased endocytosis of surface GluNB-containing receptors. We further found a reduction in phosphorylation at Tyr1472, which negatively regulates their binding to the endocytotic AP2 complex. We propose that maturation of ECM could induce switch in NMDAR composition necessary for normal adult synaptic plasticity and that increased expression of GluN2B contributes to rejuvenation of plasticity after ECM removal in vivo.

Maturation, Refinement, and Serotonergic Modulation of Cerebellar Cortical Circuits in Normal Development and in Murine Models of Autism

The formation of the complex cerebellar cortical circuits follows different phases, with initial synaptogenesis and subsequent processes of refinement guided by a variety of mechanisms. The regularity of the cellular and synaptic organization of the cerebellar cortex allowed detailed studies of the structural plasticity mechanisms underlying the formation of new synapses and retraction of redundant ones. For the attainment of the monoinnervation of the Purkinje cell by a single climbing fiber, several signals are involved, including electrical activity, contact signals, homosynaptic and heterosynaptic interaction, calcium transients, postsynaptic receptors, and transduction pathways. An important role in this developmental program is played by serotonergic projections that, acting on temporally and spatially regulated postsynaptic receptors, induce and modulate the phases of synaptic formation and maturation. In the adult cerebellar cortex, many developmental mechanisms persist but play different roles, such as supporting synaptic plasticity during learning and formation of cerebellar memory traces. A dysfunction at any stage of this process can lead to disorders of cerebellar origin, which include autism spectrum disorders but are not limited to motor deficits. Recent evidence in animal models links impairment of Purkinje cell function with autism-like symptoms including sociability deficits, stereotyped movements, and interspecific communication by vocalization.

Antibody recognizing 4-sulfated chondroitin sulfate proteoglycans restores memory in tauopathy-induced neurodegeneration

Chondroitin sulfate proteoglycans (CSPGs) are the main active component of perineuronal nets (PNNs). Digestion of the glycosaminoglycan chains of CSPGs with chondroitinase ABC or transgenic attenuation of PNNs leads to prolongation of object recognition memory and activation of various forms of plasticity in the adult central nervous system. The inhibitory properties of the CSPGs depend on the pattern of sulfation of their glycosaminoglycans, with chondroitin 4-sulfate (C4S) being the most inhibitory form. In this study, we tested a number of candidates for functional blocking of C4S, leading to selection of an antibody, Cat316, which specifically recognizes C4S and blocks its inhibitory effects on axon growth. It also partly blocks binding of semaphorin 3A to PNNs and attenuates PNN formation. We asked whether injection of Cat316 into the perirhinal cortex would have the same effects on memory as chondroitinase ABC treatment. We found that masking C4S with the Cat316 antibody extended long-term object recognition memory in normal wild-type mice to 24 hours, similarly to chondroitinase or transgenic PNN attenuation. We then tested Cat316 for restoration of memory in a neurodegeneration model. Mice expressing tau with the P301S mutation showed profound loss of object recognition memory at 4 months of age. Injection of Cat316 into the perirhinal cortex normalized object recognition at 3 hours in P301S mice. These data indicate that Cat316 binding to C4S in the extracellular matrix can restore plasticity and memory in the same way as chondroitinase ABC digestion. Our results suggest that antibodies to C4S could be a useful therapeutic to restore memory function in neurodegenerative disorders.

Formation and remodeling of the brain extracellular matrix in neural plasticity: Roles of chondroitin sulfate and hyaluronan

BACKGROUND

The extracellular matrix (ECM) of the brain is rich in glycosaminoglycans such as chondroitin sulfate (CS) and hyaluronan. These glycosaminoglycans are organized into either diffuse or condensed ECM. Diffuse ECM is distributed throughout the brain and fills perisynaptic spaces, whereas condensed ECM selectively surrounds parvalbumin-expressing inhibitory neurons (PV cells) in mesh-like structures called perineuronal nets (PNNs). The brain ECM acts as a non-specific physical barrier that modulates neural plasticity and axon regeneration.

SCOPE OF REVIEW

Here, we review recent progress in understanding of the molecular basis of organization and remodeling of the brain ECM, and the involvement of several types of experience-dependent neural plasticity, with a particular focus on the mechanism that regulates PV cell function through specific interactions between CS chains and their binding partners. We also discuss how the barrier function of the brain ECM restricts dendritic spine dynamics and limits axon regeneration after injury.

MAJOR CONCLUSIONS

The brain ECM not only forms physical barriers that modulate neural plasticity and axon regeneration, but also forms molecular brakes that actively controls maturation of PV cells and synapse plasticity in which sulfation patterns of CS chains play a key role. Structural remodeling of the brain ECM modulates neural function during development and pathogenesis.

GENERAL SIGNIFICANCE

Genetic or enzymatic manipulation of the brain ECM may restore neural plasticity and enhance recovery from nerve injury. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Mechanisms of axon regeneration: The significance of proteoglycans

BACKGROUND

Therapeutics specific to neural injury have long been anticipated but remain unavailable. Axons in the central nervous system do not readily regenerate after injury, leading to dysfunction of the nervous system. This failure of regeneration is due to both the low intrinsic capacity of axons for regeneration and the various inhibitors emerging upon injury. After many years of concerted efforts, however, these hurdles to axon regeneration have been partially overcome.

SCOPE OF REVIEW

This review summarizes the mechanisms regulating axon regeneration. We highlight proteoglycans, particularly because it has become increasingly clear that these proteins serve as critical regulators for axon regeneration.

MAJOR CONCLUSIONS

Studies on proteoglycans have revealed that glycans not only assist in the modulation of protein functions but also act as main players-e.g., as functional ligands mediating intracellular signaling through specific receptors on the cell surface. By regulating clustering of the receptors, glycans in the proteoglycan moiety, i.e., glycosaminoglycans, promote or inhibit axon regeneration. In addition, proteoglycans are involved in various types of neural plasticity, ranging from synaptic plasticity to experience-dependent plasticity.

GENERAL SIGNIFICANCE

Although studies on proteins have progressively facilitated our understanding of the nervous system, glycans constitute a new frontier for further research and development in this field. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Juvenile Hippocampal CA2 Region Expresses Aggrecan

Perineuronal nets (PNNs) are distributed primarily around inhibitory interneurons in the hippocampus, such as parvalbumin-positive interneurons. PNNs are also present around excitatory neurons in some brain regions and prevent plasticity in these neurons. A recent study demonstrated that PNNs also exist around mouse hippocampal pyramidal cells, which are the principle type of excitatory neurons, in the CA2 subregion and modulate the excitability and plasticity of these neurons. However, the development of PNNs in the CA2 region during postnatal maturation was not fully investigated. This study found that a main component of PNNs, aggrecan, existed in the pyramidal cell layer of the putative CA2 subarea prior to the appearance of the CA2 region, which was defined by the CA2 marker protein regulator of G protein signaling 14 (RGS14). We also found that aggrecan immunoreactivity was more evident in the anterior sections of the CA2 area than the posterior sections, which suggests that the function of CA2 PNNs varies along the anterior-posterior axis.

New Applications of Heparin and Other Glycosaminoglycans

Heparin, the widely used pharmaceutical anticoagulant, has been in clinical use for well over half a century. Its introduction reduced clotting risks substantially and subsequent developments, including the introduction of low-molecular-weight heparin, made possible many major surgical interventions that today make heparin an indispensable drug. There has been a recent burgeoning of interest in heparin and related glycosaminoglycan (GAG) polysaccharides, such as chondroitin sulfates, heparan sulfate, and hyaluronate, as potential agents in various applications. This ability arises mainly from the ability of GAGs to interact with, and alter the activity of, a wide range of proteins. Here, we review new developments (since 2010) in the application of heparin and related GAGs across diverse fields ranging from thrombosis and neurodegenerative disorders to microbiology and biotechnology.

Differential expression of cytoskeletal regulatory factors in the adolescent prefrontal cortex: Implications for cortical development

The prevalence of depression, anxiety, schizophrenia, and drug and alcohol use disorders peaks during adolescence. Further, up to 50% of "adult" mental health disorders emerge in adolescence. During adolescence, the prefrontal cortex (PFC) undergoes dramatic structural reorganization, in which dendritic spines and synapses are refined, pruned, and stabilized. Understanding the molecular mechanisms that underlie these processes should help to identify factors that influence the development of psychiatric illness. Here we briefly discuss the anatomical connections of the medial and orbital prefrontal cortex (mPFC and OFC, respectively). We then present original findings suggesting that dendritic spines on deep-layer excitatory neurons in the mouse mPFC and OFC prune at different adolescent ages, with later pruning in the OFC. In parallel, we used Western blotting to define levels of several cytoskeletal regulatory proteins during early, mid-, and late adolescence, focusing on tropomyosin-related kinase receptor B (TrkB) and β1-integrin-containing receptors and select signaling partners. We identified regional differences in the levels of several proteins in early and midadolescence that then converged in early adulthood. We also observed age-related differences in TrkB levels, both full-length and truncated isoforms, Rho-kinase 2, and synaptophysin in both PFC subregions. Finally, we identified changes in protein levels in the dorsal and ventral hippocampus that were distinct from those in the PFC. We conclude with a general review of the manner in which TrkB- and β1-integrin-mediated signaling influences neuronal structure in the postnatal brain. Elucidating the role of cytoskeletal regulatory factors throughout adolescence may identify critical mechanisms of PFC development. © 2016 Wiley Periodicals, Inc.

Crosstalk between glia, extracellular matrix and neurons

Extracellular matrix (ECM) molecules in the central nervous system form highly organized ECM structures around cell somata, axon initial segments, and synapses and play prominent roles in early development by guiding cell migration, neurite outgrowth and synaptogenesis, and by regulating closure of the critical period of development, synaptic plasticity and stability, cognitive flexibility, and axonal regeneration in adults. Major components of neural ECM, including chondroitin sulfate proteoglycans (CSPGs), tenascin-R and hyaluronic acid, are synthesized by both neurons and glial cells. The expression of these molecules is dynamically regulated during brain development in physiological conditions, shaping both neuronal and glial functions through multitude of molecular mechanisms. Upregulation of particular CSPGs and other ECM molecules, in particular by reactive astrocytes, after CNS injuries, during aging, neuroinflammation, and neurodegeneration on the one hand results in formation of growth-impermissive environment and impaired synaptic plasticity. On the other hand, ECM appeared to have a neuroprotective effect, at least in the form of perineuronal nets. CSPGs-degrading matrix metalloproteinases (MMPs) and several members of the disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family of proteases are secreted by neurons and glia and may drive neural ECM remodeling in physiological conditions as well as after brain injury and other brain disorders. Thus, targeting expression of specific ECM molecules, associated glycans and degrading enzymes may lead to development of new therapeutic strategies promoting regeneration and synaptic plasticity.

Weaving a Net of Neurobiological Mechanisms in Schizophrenia and Unraveling the Underlying Pathophysiology

Perineuronal nets (PNNs) are enigmatic structures composed of extracellular matrix molecules that encapsulate the soma, dendrites, and axon segments of neurons in a lattice-like fashion. Although most PNNs condense around parvalbumin-expressing gamma-aminobutyric acidergic interneurons, some glutamatergic pyramidal cells in the brain are also surrounded by PNNs. Experimental findings suggest pivotal roles of PNNs in the regulation of synaptic formation and function. Also, an increasing body of evidence links PNN abnormalities to schizophrenia. The number of PNNs progressively increases during postnatal development until plateauing around the period of late adolescence and early adulthood, which temporally coincides with the age of onset of schizophrenia. Given the established role of PNNs in modulating developmental plasticity, the PNN represents a possible candidate for altering the onset and progression of schizophrenia. Similarly, the reported function of PNNs in regulating the trafficking of glutamate receptors places them in a critical position to modulate synaptic pathology, considered a cardinal feature of schizophrenia. We discuss the physiologic role of PNNs in neural function, synaptic assembly, and plasticity as well as how they interface with circuit/system mechanisms of cognition. An integrated understanding of these neurobiological processes should provide a better basis to elucidate how PNN abnormalities influence brain function and contribute to the pathogenesis of neurodevelopmental disorders such as schizophrenia.

Regional Regulation of Purkinje Cell Dendritic Spines by Integrins and Eph/Ephrins

Climbing fibres and parallel fibres compete for dendritic space on Purkinje cells in the cerebellum. Normally, climbing fibres populate the proximal dendrites, where they suppress the multiple small spines typical of parallel fibres, leading to their replacement by the few large spines that contact climbing fibres. Previous work has shown that ephrins acting via EphA4 are a signal for this change in spine type and density. We have used an in vitro culture model in which to investigate the ephrin effect on Purkinje cell dendritic spines and the role of integrins in these changes. We found that integrins α3, α5 and β4 are present in many of the dendritic spines of cultured Purkinje cells. pFAK, the main downstream signalling molecule from integrins, has a similar distribution, although the intenstity of pFAK staining and the percentage of pFAK+ spines was consistently higher in the proximal dendrites. Activating integrins with Mg2+ led to an increase in the intensity of pFAK staining and an increase in the proportion of pFAK+ spines in both the proximal and distal dendrites, but no change in spine length, density or morphology. Blocking integrin binding with an RGD-containing peptide led to a reduction in spine length, with more stubby spines on both proximal and distal dendrites. Treatment of the cultures with ephrinA3-Fc chimera suppressed dendritic spines specifically on the proximal dendrites and there was also a decrease of pFAK in spines on this domain. This effect was blocked by simultaneous activation of integrins with Mn2+. We conclude that Eph/ephrin signaling regulates proximal dendritic spines in Purkinje cells by inactivating integrin downstream signalling.

In Sickness and in Health: Perineuronal Nets and Synaptic Plasticity in Psychiatric Disorders

Rapidly emerging evidence implicates perineuronal nets (PNNs) and extracellular matrix (ECM) molecules that compose or interact with PNNs, in the pathophysiology of several psychiatric disorders. Studies on schizophrenia, autism spectrum disorders, mood disorders, Alzheimer's disease, and epilepsy point to the involvement of ECM molecules such as chondroitin sulfate proteoglycans, Reelin, and matrix metalloproteases, as well as their cell surface receptors. In many of these disorders, PNN abnormalities have also been reported. In the context of the “quadripartite” synapse concept, that is, the functional unit composed of the pre- and postsynaptic terminals, glial processes, and ECM, and of the role that PNNs and ECM molecules play in regulating synaptic functions and plasticity, these findings resonate with one of the most well-replicated aspects of the pathology of psychiatric disorders, that is, synaptic abnormalities. Here we review the evidence for PNN/ECM-related pathology in these disorders, with particular emphasis on schizophrenia, and discuss the hypothesis that such pathology may significantly contribute to synaptic dysfunction.

Development and Structural Variety of the Chondroitin Sulfate Proteoglycans-Contained Extracellular Matrix in the Mouse Brain

Chondroitin sulfate proteoglycans (CSPGs) are major components of the extracellular matrix (ECM) in the brain. In adult mammals, CSPGs form the specialized ECM structure perineuronal nets (PNNs) that surround somata and dendrites of certain types of neurons. PNNs restrict synaptic plasticity and regulate the closure of critical periods. Although previous studies have examined the starting period of PNN formation, focusing on primary sensory cortices, there are no systematic studies at the whole brain level. Here, we examined the starting period of PNN formation in male mice ranging in age from postnatal day 3 to week 11, mainly focusing on several cortical areas, limbic structures, hypothalamus, and brain stem, using lectin histochemistry with Wisteria floribunda agglutinin (WFA). Results showed that early PNN formation was observed in several reticular formations of the brain stem related to the cranial nerves and primary somatosensory cortices. In the limbic system, PNN formation in the hippocampus started earlier than that of the amygdala. Furthermore, in the medial amygdaloid nucleus and some hypothalamic regions, WFA labeling did not show typical PNN-like forms. The present study suggests spatiotemporal differences at the beginning of PNN formation and a structural variety of CSPG-contained ECM in the brain.

The hyaluronan and proteoglycan link proteins: Organizers of the brain extracellular matrix and key molecules for neuronal function and plasticity

The hyaluronan and proteoglycanbinding link protein (Hapln) is a key molecule in the formation and control of hyaluronan-based condensed perineuronal matrix in the adult brain. This review summarizes the recent advances in understanding the role of Haplns in the formation and control of two distinct types of perineuronal matrices, one for "classical" PNN and the other for the specialized extracellular matrix (ECM) at the node of Ranvier in the central nervous system (CNS). We introduce the structural components of each ECM organization including the basic concept of supramolecular structure named "HLT model". We furthermore summarize the developmental and physiological role of perineuronal ECMs from the studies of Haplns and related molecules. Finally, we also discuss the potential mechanism modulating PNNs in the adult CNS. This layer of organized matrices may exert a direct effect via core protein or sugar moiety from the structure or by acting as a binding site for biologically active molecules, which are important for neuronal plasticity and saltatory conduction.

Sugar-dependent modulation of neuronal development, regeneration, and plasticity by chondroitin sulfate proteoglycans

Chondroitin sulfate proteoglycans (CSPGs) play important roles in the developing and mature nervous system, where they guide axons, maintain stable connections, restrict synaptic plasticity, and prevent axon regeneration following CNS injury. The chondroitin sulfate glycosaminoglycan (CS GAG) chains that decorate CSPGs are essential for their functions. Through these sugar chains, CSPGs are able to bind and regulate the activity of a diverse range of proteins. CSPGs have been found both to promote and inhibit neuronal growth. They can promote neurite outgrowth by binding to various growth factors such as midkine (MK), pleiotrophin (PTN), brain-derived neurotrophic factor (BDNF) and other neurotrophin family members. CSPGs can also inhibit neuronal growth and limit plasticity by interacting with transmembrane receptors such as protein tyrosine phosphatase σ (PTPσ), leukocyte common antigen-related (LAR) receptor protein tyrosine phosphatase, and the Nogo receptors 1 and 3 (NgR1 and NgR3). These CS-protein interactions depend on specific sulfation patterns within the CS GAG chains, and accordingly, particular CS sulfation motifs are upregulated during development, in the mature nervous system, and in response to CNS injury. Thus, spatiotemporal regulation of CS GAG biosynthesis may provide an important mechanism to control the functions of CSPGs and to modulate intracellular signaling pathways. Here, we will discuss these sulfation-dependent processes and highlight how the CS sugars on CSPGs contribute to neuronal growth, axon guidance, and plasticity in the nervous system.

A newly identified mouse hypothalamic area having bidirectional neural connections with the lateral septum: the perifornical area of the anterior hypothalamus rich in chondroitin sulfate proteoglycans

While previous studies and brain atlases divide the hypothalamus into many nuclei and areas, uncharacterised regions remain. Here, we report a new region in the mouse anterior hypothalamus (AH), a triangular-shaped perifornical area of the anterior hypothalamus (PeFAH) between the paraventricular hypothalamic nucleus and fornix, that abundantly expresses chondroitin sulfate proteoglycans (CSPGs). The PeFAH strongly stained with markers for chondroitin sulfate/CSPGs such as Wisteria floribunda agglutinin and antibodies against aggrecan and chondroitin 6 sulfate. Nissl-stained sections of the PeFAH clearly distinguished it as a region of comparatively low density compared to neighboring regions, the paraventricular nucleus and central division of the anterior hypothalamic area. Immunohistochemical and DNA microarray analyses suggested that PeFAH contains sparsely distributed calretinin-positive neurons and a compact cluster of enkephalinergic neurons. Neuronal tract tracing revealed that both enkephalin- and calretinin-positive neurons project to the lateral septum (LS), while the PeFAH receives input from calbindin-positive LS neurons. These results suggest bidirectional connections between the PeFAH and LS. Considering neuronal subtype and projection, part of PeFAH that includes a cluster of enkephalinergic neurons is similar to the rat perifornical nucleus and guinea pig magnocellular dorsal nucleus. Finally, we examined c-Fos expression after several types of stimuli and found that PeFAH neuronal activity was increased by psychological but not homeostatic stressors. These findings suggest that the PeFAH is a source of enkephalin peptides in the LS and indicate that bidirectional neural connections between these regions may participate in controlling responses to psychological stressors.

"GAG-ing with the neuron": The role of glycosaminoglycan patterning in the central nervous system

Proteoglycans (PGs) are a diverse family of proteins that consist of one or more glycosaminoglycan (GAG) chains, covalently linked to a core protein. PGs are major components of the extracellular matrix (ECM) and play critical roles in development, normal function and damage-response of the central nervous system (CNS). GAGs are classified based on their disaccharide subunits, into the following major groups: chondroitin sulfate (CS), heparan sulfate (HS), heparin (HEP), dermatan sulfate (DS), keratan sulfate (KS) and hyaluronic acid (HA). All except HA are modified by sulfation, giving GAG chains specific charged structures and binding properties. While significant neuroscience research has focused on the role of one PG family member, chondroitin sulfate proteoglycan (CSPG), there is ample evidence in support of a role for the other PGs in regulating CNS function in normal and pathological conditions. This review discusses the role of all the identified PG family members (CS, HS, HEP, DS, KS and HA) in normal CNS function and in the context of pathology. Understanding the pleiotropic roles of these molecules in the CNS may open the door to novel therapeutic strategies for a number of neurological conditions.

Insights into the physiological role of CNS regeneration inhibitors

The growth inhibitory nature of injured adult mammalian central nervous system (CNS) tissue constitutes a major barrier to robust axonal outgrowth and functional recovery following trauma or disease. Prototypic CNS regeneration inhibitors are broadly expressed in the healthy and injured brain and spinal cord and include myelin-associated glycoprotein (MAG), the reticulon family member NogoA, oligodendrocyte myelin glycoprotein (OMgp), and chondroitin sulfate proteoglycans (CSPGs). These structurally diverse molecules strongly inhibit neurite outgrowth in vitro, and have been most extensively studied in the context of nervous system injury in vivo. The physiological role of CNS regeneration inhibitors in the naïve, or uninjured, CNS remains less well understood, but has received growing attention in recent years and is the focus of this review. CNS regeneration inhibitors regulate myelin development and axon stability, consolidate neuronal structure shaped by experience, and limit activity-dependent modification of synaptic strength. Altered function of CNS regeneration inhibitors is associated with neuropsychiatric disorders, suggesting crucial roles in brain development and health.

Chondroitin Sulfate Induces Depression of Synaptic Transmission and Modulation of Neuronal Plasticity in Rat Hippocampal Slices

It is currently known that in CNS the extracellular matrix is involved in synaptic stabilization and limitation of synaptic plasticity. However, it has been reported that the treatment with chondroitinase following injury allows the formation of new synapses and increased plasticity and functional recovery. So, we hypothesize that some components of extracellular matrix may modulate synaptic transmission. To test this hypothesis we evaluated the effects of chondroitin sulphate (CS) on excitatory synaptic transmission, cellular excitability, and neuronal plasticity using extracellular recordings in the CA1 area of rat hippocampal slices. CS caused a reversible depression of evoked field excitatory postsynaptic potentials in a concentration-dependent manner. CS also reduced the population spike amplitude evoked after orthodromic stimulation but not when the population spikes were antidromically evoked; in this last case a potentiation was observed. CS also enhanced paired-pulse facilitation and long-term potentiation. Our study provides evidence that CS, a major component of the brain perineuronal net and extracellular matrix, has a function beyond the structural one, namely, the modulation of synaptic transmission and neuronal plasticity in the hippocampus.

Chondroitin Sulfate Proteoglycans Negatively Modulate Spinal Cord Neural Precursor Cells by Signaling Through LAR and RPTPσ and Modulation of the Rho/ROCK Pathway

Multipotent adult neural precursor cells (NPCs) have tremendous intrinsic potential to repair the damaged spinal cord. However, evidence shows that the regenerative capabilities of endogenous and transplanted NPCs are limited in the microenvironment of spinal cord injury (SCI). We previously demonstrated that injury-induced upregulation of matrix chondroitin sulfate proteoglycans (CSPGs) restricts the survival, migration, integration, and differentiation of NPCs following SCI. CSPGs are long-lasting components of the astroglial scar that are formed around the lesion. Our recent in vivo studies demonstrated that removing CSPGs from the SCI environment enhances the potential of transplanted and endogenous adult NPCs for spinal cord repair; however, the mechanisms by which CSPGs regulate NPCs remain unclear. In this study, using in vitro models recapitulating the extracellular matrix of SCI, we investigated the direct role of CSPGs in modulating the properties of adult spinal cord NPCs. We show that CSPGs significantly decrease NPCs growth, attachment, survival, proliferation, and oligodendrocytes differentiation. Moreover, using genetic models, we show that CSPGs regulate NPCs by signaling on receptor protein tyrosine phosphate sigma (RPTPσ) and leukocyte common antigen-related phosphatase (LAR). Intracellularly, CSPGs inhibitory effects are mediated through Rho/ROCK pathway and inhibition of Akt and Erk1/2 phosphorylation. Downregulation of RPTPσ and LAR and blockade of ROCK in NPCs attenuates the inhibitory effects of CSPGS. Our work provide novel evidence uncovering how upregulation of CSPGs challenges the response of NPCs in their post-SCI niche and identifies new therapeutic targets for enhancing NPC-based therapies for SCI repair.

Chondroitin sulfate proteoglycans: Key modulators in the developing and pathologic central nervous system

Chondroitin Sulfate Proteoglycans (CSPGs) are a major component of the extracellular matrix in the central nervous system (CNS) and play critical role in the development and pathophysiology of the brain and spinal cord. Developmentally, CSPGs provide guidance cues for growth cones and contribute to the formation of neuronal boundaries in the developing CNS. Their presence in perineuronal nets plays a crucial role in the maturation of synapses and closure of critical periods by limiting synaptic plasticity. Following injury to the CNS, CSPGs are dramatically upregulated by reactive glia which form a glial scar around the lesion site. Increased level of CSPGs is a hallmark of all CNS injuries and has been shown to limit axonal plasticity, regeneration, remyelination, and conduction after injury. Additionally, CSPGs create a non-permissive milieu for cell replacement activities by limiting cell migration, survival and differentiation. Mounting evidence is currently shedding light on the potential benefits of manipulating CSPGs in combination with other therapeutic strategies to promote spinal cord repair and regeneration. Moreover, the recent discovery of multiple receptors for CSPGs provides new therapeutic targets for targeted interventions in blocking the inhibitory properties of CSPGs following injury. Here, we will provide an in depth discussion on the impact of CSPGs in normal and pathological CNS. We will also review the recent preclinical therapies that have been developed to target CSPGs in the injured CNS.

The extracellular matrix in plasticity and regeneration after CNS injury and neurodegenerative disease

Chondroitin sulfate proteoglycans (CSPGs) are involved in several processes relevant to recovery of function after CNS damage. They restrict axon regeneration through their presence in glial scar tissue and plasticity through their presence in perineuronal nets (PNNs), affect memory through their effect on dendritic spines, and influence the inflammatory reaction. Much of our knowledge of these CSPG effects comes from digestion of their glycosaminoglycan chains by the enzyme chondroitinase ABC (ChABC). ChABC after spinal cord injury permits some axon regeneration and greatly increases plasticity through increased sprouting and through digestion of PNNs. When combined with appropriate rehabilitation, ChABC treatment can lead to considerable restoration of function. ChABC treatment of the perirhinal cortex greatly increases retention of object recognition memory. When applied to tauopathy animals that model Alzheimer's disease, ChABC digestion can restore normal object recognition memory. CSPGs in the adult CNS are found throughout the ECM, but 2% is concentrated in PNNs that surround GABAergic parvalbumin interneurons and other neurons. Knockout of the PNN-organizing protein Crtl1 link protein attenuates PNNs and leads to continued plasticity into adulthood, demonstrating that the CSPGs in PNNs are the key components in the control of plasticity. CSPGs act mainly through their sulfated glycosaminoglycan chains. A disulfated CS-E motif in these chains is responsible for binding of Semaphorin 3A to PNNs where it affects ocular dominance plasticity and probably other forms of plasticity. In addition OTX2 binds to CS-E motifs, where it can act on parvalbumin interneurons to maintain the PNNs.

Early postnatal expression and localization of matrix metalloproteinases-2 and -9 during establishment of rat hippocampal synaptic circuitry

Matrix metalloproteinases (MMPs) are extracellular proteolytic enzymes that contribute to pericellular remodeling in a variety of tissues, including brain, where they function in adult hippocampal synaptic structural and functional plasticity. Synaptic plasticity and remodeling are also important for development of connectivity, but it is unclear whether MMPs--particularly MMP-2 and -9, the major MMPs operative in brain--contribute at these stages. Here, we use a combination of biochemical and anatomical methods to characterize expression and localization of MMP-2 and MMP-9 in early postnatal and adult rat hippocampus. Gene and protein expression of these MMPs were evident throughout hippocampus at all ages examined, but expression levels were highest during the first postnatal week. MMP-2 and MMP-9 immunolocalized to punctate structures within the neuropil that codistributed with foci of proteolytic activity, as well as with markers of growing axons and synapses. Taken together, discrete foci of MMP proteolysis are likely important for actively shaping and remodeling cellular and connectional architecture as hippocampal circuitry is becoming established during early postnatal life.

Extracellular matrix control of dendritic spine and synapse structure and plasticity in adulthood

Dendritic spines are the receptive contacts at most excitatory synapses in the central nervous system. Spines are dynamic in the developing brain, changing shape as they mature as well as appearing and disappearing as they make and break connections. Spines become much more stable in adulthood, and spine structure must be actively maintained to support established circuit function. At the same time, adult spines must retain some plasticity so their structure can be modified by activity and experience. As such, the regulation of spine stability and remodeling in the adult animal is critical for normal function, and disruption of these processes is associated with a variety of late onset diseases including schizophrenia and Alzheimer's disease. The extracellular matrix (ECM), composed of a meshwork of proteins and proteoglycans, is a critical regulator of spine and synapse stability and plasticity. While the role of ECM receptors in spine regulation has been extensively studied, considerably less research has focused directly on the role of specific ECM ligands. Here, we review the evidence for a role of several brain ECM ligands and remodeling proteases in the regulation of dendritic spine and synapse formation, plasticity, and stability in adults.

Plasticity of binocularity and visual acuity are differentially limited by nogo receptor

The closure of developmental critical periods consolidates neural circuitry but also limits recovery from early abnormal sensory experience. Degrading vision by one eye throughout a critical period both perturbs ocular dominance (OD) in primary visual cortex and impairs visual acuity permanently. Yet understanding how binocularity and visual acuity interrelate has proven elusive. Here we demonstrate the plasticity of binocularity and acuity are separable and differentially regulated by the neuronal nogo receptor 1 (NgR1). Mice lacking NgR1 display developmental OD plasticity as adults and their visual acuity spontaneously improves after prolonged monocular deprivation. Restricting deletion of NgR1 to either cortical interneurons or a subclass of parvalbumin (PV)-positive interneurons alters intralaminar synaptic connectivity in visual cortex and prevents closure of the critical period for OD plasticity. However, loss of NgR1 in PV neurons does not rescue deficits in acuity induced by chronic visual deprivation. Thus, NgR1 functions with PV interneurons to limit plasticity of binocularity, but its expression is required more extensively within brain circuitry to limit improvement of visual acuity following chronic deprivation.

Semaphorin 5A inhibits synaptogenesis in early postnatal- and adult-born hippocampal dentate granule cells

Human SEMAPHORIN 5A (SEMA5A) is an autism susceptibility gene; however, its function in brain development is unknown. In this study, we show that mouse Sema5A negatively regulates synaptogenesis in early, developmentally born, hippocampal dentate granule cells (GCs). Sema5A is strongly expressed by GCs and regulates dendritic spine density in a cell-autonomous manner. In the adult mouse brain, newly born Sema5A-/- GCs show an increase in dendritic spine density and increased AMPA-type synaptic responses. Sema5A signals through PlexinA2 co-expressed by GCs, and the PlexinA2-RasGAP activity is necessary to suppress spinogenesis. Like Sema5A-/- mutants, PlexinA2-/- mice show an increase in GC glutamatergic synapses, and we show that Sema5A and PlexinA2 genetically interact with respect to GC spine phenotypes. Sema5A-/- mice display deficits in social interaction, a hallmark of autism-spectrum-disorders. These experiments identify novel intra-dendritic Sema5A/PlexinA2 interactions that inhibit excitatory synapse formation in developmentally born and adult-born GCs, and they provide support for SEMA5A contributions to autism-spectrum-disorders.

Transformation of cortical and hippocampal neural circuit by environmental enrichment

It has been half a century since brain volume enlargement was first reported in animals reared in an enriched environment (EE). As EE animals show improved memory task performance, exposure to EE has been a useful model system for studying the effects of experience on brain plasticity. We review EE-induced neural changes in the cerebral cortex and hippocampus focusing mainly on works published in the recent decade. The review is organized in three large domains of changes: anatomical, electrophysiological, and molecular changes. Finally, we discuss open issues and future outlook toward better understanding of EE-induced neural changes.