Activity-dependent regulation of a chondroitin sulfate proteoglycan 6B4 phosphacan/RPTPbeta in the hypothalamic supraoptic nucleus

An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity

During restricted time windows of postnatal life, called critical periods, neural circuits are highly plastic and are shaped by environmental stimuli. In several mammalian brain areas, from the cerebral cortex to the hippocampus and amygdala, the closure of the critical period is dependent on the formation of perineuronal nets. Perineuronal nets are a condensed form of an extracellular matrix, which surrounds the soma and proximal dendrites of subsets of neurons, enwrapping synaptic terminals. Experimentally disrupting perineuronal nets in adult animals induces the reactivation of critical period plasticity, pointing to a role of the perineuronal net as a molecular brake on plasticity as the critical period closes. Interestingly, in the adult brain, the expression of perineuronal nets is remarkably dynamic, changing its plasticity-associated conditions, including memory processes. In this review, we aimed to address how perineuronal nets contribute to the maturation of brain circuits and the regulation of adult brain plasticity and memory processes in physiological and pathological conditions.

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.

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.

New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

Activity-dependent Notch signalling in the hypothalamic-neurohypophysial system of adult mouse brains

Notch signalling has a key role in cell fate specification in developing brains; however, recent studies have shown that Notch signalling also participates in the regulation of synaptic plasticity in adult brains. In the present study, we examined the expression of Notch3 and Delta-like ligand 4 (DLL4) in the hypothalamic-neurohypophysial system (HNS) of the adult mouse. The expression of DLL4 was higher in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) compared to adjacent hypothalamic regions. Double-labelling immunohistochemistry using vesicular GABA transporter and glutamate transporter revealed that DLL4 was localised at a subpopulation of excitatory and inhibitory axonal boutons against somatodendrites of arginine vasopressin (AVP)- and oxytocin (OXT)-containing magnocellular neurones. In the neurohypophysis (NH), the expression of DLL4 was seen at OXT- but not AVP-containing axonal terminals. The expression of Notch3 was seen at somatodendrites of AVP- and OXT-containing magnocellular neurones in the SON and PVN and at pituicytes in the NH. Chronic physiological stimulation by salt loading, which remarkably enhances the release of AVP and OXT, decreased the number of DLL4-immunoreactive axonal boutons in the SON and PVN. Moreover, chronic and acute osmotic stimulation promoted proteolytic cleavage of Notch3 to yield the intracellular fragments of Notch3 in the HNS. Thus, the present study demonstrates activity-dependent reduction of DLL4 expression and proteolytic cleavage of Notch3 in the HNS, suggesting that Notch signalling possibly participates in synaptic interaction in the hypothalamic nuclei and neuroglial interaction in the NH.

Enhanced cognitive flexibility in reversal learning induced by removal of the extracellular matrix in auditory cortex

During brain maturation, the occurrence of the extracellular matrix (ECM) terminates juvenile plasticity by mediating structural stability. Interestingly, enzymatic removal of the ECM restores juvenile forms of plasticity, as for instance demonstrated by topographical reconnectivity in sensory pathways. However, to which degree the mature ECM is a compromise between stability and flexibility in the adult brain impacting synaptic plasticity as a fundamental basis for learning, lifelong memory formation, and higher cognitive functions is largely unknown. In this study, we removed the ECM in the auditory cortex of adult Mongolian gerbils during specific phases of cortex-dependent auditory relearning, which was induced by the contingency reversal of a frequency-modulated tone discrimination, a task requiring high behavioral flexibility. We found that ECM removal promoted a significant increase in relearning performance, without erasing already established-that is, learned-capacities when continuing discrimination training. The cognitive flexibility required for reversal learning of previously acquired behavioral habits, commonly understood to mainly rely on frontostriatal circuits, was enhanced by promoting synaptic plasticity via ECM removal within the sensory cortex. Our findings further suggest experimental modulation of the cortical ECM as a tool to open short-term windows of enhanced activity-dependent reorganization allowing for guided neuroplasticity.

Alterations in sulfated chondroitin glycosaminoglycans following controlled cortical impact injury in mice

Chondroitin sulfate proteoglycans (CSPGs) play a pivotal role in many neuronal growth mechanisms including axon guidance and the modulation of repair processes following injury to the spinal cord or brain. Many actions of CSPGs in the central nervous system (CNS) are governed by the specific sulfation pattern on the glycosaminoglycan (GAG) chains attached to CSPG core proteins. To elucidate the role of CSPGs and sulfated GAG chains following traumatic brain injury (TBI), controlled cortical impact injury of mild to moderate severity was performed over the left sensory motor cortex in mice. Using immunoblotting and immunostaining, we found that TBI resulted in an increase in the CSPGs neurocan and NG2 expression in a tight band surrounding the injury core, which overlapped with the presence of 4-sulfated CS GAGs but not with 6-sulfated GAGs. This increase was observed as early as 7 days post injury (dpi), and persisted for up to 28 dpi. Labeling with markers against microglia/macrophages, NG2+ cells, fibroblasts, and astrocytes showed that these cells were all localized in the area, suggesting multiple origins of chondroitin-4-sulfate increase. TBI also caused a decrease in the expression of aggrecan and phosphacan in the pericontusional cortex with a concomitant reduction in the number of perineuronal nets. In summary, we describe a dual response in CSPGs whereby they may be actively involved in complex repair processes following TBI.

Activity-dependent plasticity and gene expression modifications in the adult CNS

Information processing, memory formation, or functional recovery after nervous system damage depend on the ability of neurons to modify their functional properties or their connections. At the cellular/molecular level, structural modifications of neural circuits are finely regulated by intrinsic neuronal properties and growth-regulatory cues in the extracellular milieu. Recently, it has become clear that stimuli coming from the external world, which comprise sensory inflow, motor activity, cognitive elaboration, or social interaction, not only provide the involved neurons with instructive information needed to shape connection patterns to sustain adaptive function, but also exert a powerful influence on intrinsic and extrinsic growth-related mechanisms, so to create permissive conditions for neuritic remodeling. Here, we present an overview of recent findings concerning the effects of experience on molecular mechanisms underlying CNS structural plasticity, both in physiological conditions and after damage, with particular focus on activity-dependent modulation of growth-regulatory genes and epigenetic modifications.

Oncomodulin affords limited regeneration to injured sensory axons in vitro and in vivo

Oncomodulin, an ~12 kDa Ca(2+)-binding protein secreted from activated macrophages, has been shown to promote axonal regeneration from retinal ganglion cells (RGCs) following optic nerve injury. However, to date, the axonal growth-promoting capacity of oncomodulin in other models of 'regenerative failure' has not been evaluated. We assessed the capability of preconditioning treatment with oncomodulin to promote sensory axonal regeneration in an in vitro spot model of regenerative failure, and across the dorsal root zone (DREZ) after root crush injury. Neither the direct exposure of adult rat DRGs to oncomodulin, nor preconditioning of DRGs by intraganglionic injection of oncomodulin, stimulated axonal outgrowth in the in vitro proteoglycan spot gradient assay. However, direct exposure of unconditioned DRGs to both oncomodulin and db-cAMP in vitro, as well as preconditioning of DRGs with the combined treatment in vivo, resulted in significant, albeit modest, neurite extension across the inhibitory proteoglycan barrier. We next quantified axon regeneration through the C8 DREZ in adult rats after oncomodulin and/or db-cAMP preconditioning and chondroitinase (ChABC) injection into the DREZ immediately following a root crush injury. Axonal regeneration across the DREZ was not observed in control animals, or after injection of ChABC-alone. Treatment with oncomodulin- or db-cAMP-alone resulted in extremely sparse regeneration. However, significant, but meager, sensory axon regeneration across the DREZ was observed using the oncomodulin/ db-cAMP combination (p<0.001), supporting findings from previous studies suggesting that cAMP is necessary for the growth-promoting effects of oncomodulin. Although our results support a role for oncomodulin in macrophage-induced axonal regeneration, the effects of oncomodulin/db-cAMP on sensory regeneration were extremely limited in comparison to previous studies in the same injury model using zymosan.

Chondroitin sulphate proteoglycans: key modulators of spinal cord and brain plasticity

Chondroitin sulphate proteoglycans (CSPGs) are a family of inhibitory extracellular matrix molecules that are highly expressed during development, where they are involved in processes of pathfinding and guidance. CSPGs are present at lower levels in the mature CNS, but are highly concentrated in perineuronal nets where they play an important role in maintaining stability and restricting plasticity. Whilst important for maintaining stable connections, this can have an adverse effect following insult to the CNS, restricting the capacity for repair, where enhanced synapse formation leading to new connections could be functionally beneficial. CSPGs are also highly expressed at CNS injury sites, where they can restrict anatomical plasticity by inhibiting sprouting and reorganisation, curbing the extent to which spared systems may compensate for the loss function of injured pathways. Modification of CSPGs, usually involving enzymatic degradation of glycosaminoglycan chains from the CSPG molecule, has received much attention as a potential strategy for promoting repair following spinal cord and brain injury. Pre-clinical studies in animal models have demonstrated a number of reparative effects of CSPG modification, which are often associated with functional recovery. Here we discuss the potential of CSPG modification to stimulate restorative plasticity after injury, reviewing evidence from studies in the brain, the spinal cord and the periphery.

Phosphacan and receptor protein tyrosine phosphatase β expression mediates deafferentation-induced synaptogenesis

This study documents the spatial and temporal expression of three structurally related chondroitin sulfated proteoglycans (CSPGs) during synaptic regeneration induced by brain injury. Using the unilateral entorhinal cortex (EC) lesion model of adaptive synaptogenesis, we documented mRNA and protein profiles of phosphacan and its two splice variants, full length receptor protein tyrosine phosphatase β (RPTPβ) and the short transmembrane receptor form (sRPTPβ), at 2, 7, and 15 days postlesion. We report that whole hippocampal sRPTPβ protein and mRNA are persistently elevated over the first two weeks after UEC. As predicted, this transmembrane family member was localized adjacent to synaptic sites in the deafferented neuropil and showed increased distribution over that zone following lesion. By contrast, whole hippocampal phosphacan protein was not elevated with deafferentation; however, its mRNA was increased during the period of sprouting and synapse formation (7d). When the zone of synaptic reorganization was sampled using molecular layer/granule cell (ML/GCL) enriched dissections, we observed an increase in phosphacan protein at 7d, concurrent with the observed hippocampal mRNA elevation. Immunohistochemistry also showed a shift in phosphacan distribution from granule cell bodies to the deafferented ML at 2 and 7d postlesion. Phosphacan and sRPTPβ were not colocalized with glial fibrillary acid protein (GFAP), suggesting that reactive astrocytes were not a major source of either proteoglycan. While transcript for the developmentally prominent full length RPTPβ was also increased at 2 and 15d, its protein was not detected in our adult samples. These results indicate that phosphacan and RPTPβ splice variants participate in both the acute degenerative and long-term regenerative phases of reactive synaptogenesis. These results suggest that increase in the transmembrane sRPTPβ tyrosine phosphatase activity is critical to this plasticity, and that local elevation of extracellular phosphacan influences dendritic organization during synaptogenesis.

Experience-dependent plasticity and modulation of growth regulatory molecules at central synapses

Structural remodeling or repair of neural circuits depends on the balance between intrinsic neuronal properties and regulatory cues present in the surrounding microenvironment. These processes are also influenced by experience, but it is still unclear how external stimuli modulate growth-regulatory mechanisms in the central nervous system. We asked whether environmental stimulation promotes neuronal plasticity by modifying the expression of growth-inhibitory molecules, specifically those of the extracellular matrix. We examined the effects of an enriched environment on neuritic remodeling and modulation of perineuronal nets in the deep cerebellar nuclei of adult mice. Perineuronal nets are meshworks of extracellular matrix that enwrap the neuronal perikaryon and restrict plasticity in the adult CNS. We found that exposure to an enriched environment induces significant morphological changes of Purkinje and precerebellar axon terminals in the cerebellar nuclei, accompanied by a conspicuous reduction of perineuronal nets. In the animals reared in an enriched environment, cerebellar nuclear neurons show decreased expression of mRNAs coding for key matrix components (as shown by real time PCR experiments), and enhanced activity of matrix degrading enzymes (matrix metalloproteinases 2 and 9), which was assessed by in situ zymography. Accordingly, we found that in mutant mice lacking a crucial perineuronal net component, cartilage link protein 1, perineuronal nets around cerebellar neurons are disrupted and plasticity of Purkinje cell terminal is enhanced. Moreover, all the effects of environmental stimulation are amplified if the afferent Purkinje axons are endowed with enhanced intrinsic growth capabilities, induced by overexpression of GAP-43. Our observations show that the maintenance and growth-inhibitory function of perineuronal nets are regulated by a dynamic interplay between pre- and postsynaptic neurons. External stimuli act on this interaction and shift the balance between synthesis and removal of matrix components in order to facilitate neuritic growth by locally dampening the activity of inhibitory cues.

Animals lacking link protein have attenuated perineuronal nets and persistent plasticity

Chondroitin sulphate proteoglycans in the extracellular matrix restrict plasticity in the adult central nervous system and their digestion with chondroitinase reactivates plasticity. However the structures in the extracellular matrix that restrict plasticity are unknown. There are many changes in the extracellular matrix as critical periods for plasticity close, including changes in chondroitin sulphate proteoglycan core protein levels, changes in glycosaminoglycan sulphation and the appearance of dense chondroitin sulphate proteoglycan-containing perineuronal nets around many neurons. We show that formation of perineuronal nets is triggered by neuronal production of cartilage link protein Crtl1 (Hapln1), which is up-regulated in the visual cortex as perineuronal nets form during development and after dark rearing. Mice lacking Crtl1 have attenuated perineuronal nets, but the overall levels of chondroitin sulphate proteoglycans and their pattern of glycan sulphation are unchanged. Crtl1 knockout animals retain juvenile levels of ocular dominance plasticity and their visual acuity remains sensitive to visual deprivation. In the sensory pathway, axons in knockout animals but not controls sprout into the party denervated cuneate nucleus. The organization of chondroitin sulphate proteoglycan into perineuronal nets is therefore the key event in the control of central nervous system plasticity by the extracellular matrix.

Activity-dependent remodeling of chondroitin sulfate proteoglycans extracellular matrix in the hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) consisting of arginine vasopressin (AVP) and oxytocin (OXT) magnocellular neurons shows the structural plasticity including the rearrangement of synapses, dendrites, and neurovascular contacts during chronic physiological stimulation. In this study, we examined the remodeling of chondroitin sulfate proteoglycans (CSPGs), main extracellular matrix (ECM), in the HNS after salt loading known as a chronic stimulation to cause the structural plasticity. In the supraoptic nucleus (SON), confocal microscopic observation revealed that the immunoreactivity of 6B4 proteoglycans (PG) was observed mainly at AVP-positive magnocellular neurons but that of neurocan was seen chiefly at OXT-positive magnocellular neurons. The immunoreactivity of phosphacan and aggrecan was seen at both AVP- and OXT-positive magnocellular neurons. Electron microscopic observation further showed that the immunoreactivity of phosphacan and neurocan was observed at astrocytic processes to surround somata, dendrites, and terminals, but not synaptic junctions. In the neurohypophysis (NH), the immunoreactivity of phosphacan, 6B4 PGs, and neurocan was observed at AVP-positive magnocellular terminals, but the reactivity of Wisteria floribunda agglutinin lectin was seen at OXT-positive ones. The immunoreactivity of versican was found at microvessel and that of aggrecan was not detected in the NH. Quantitative morphometrical analysis showed that the chronic physiological stimulation by 7-day salt loading decreased the level of 6B4 PGs in the SON and the level of phosphacan, 6B4 PGs, and neurocan in the NH. These results suggest that the extracellular microenvironment of CSPGs is different between AVP and OXT magnocellular neurons and activity-dependent remodeling of CSPGs could be involved in the structural plasticity of the HNS.

Traumatic brain injury results in disparate regions of chondroitin sulfate proteoglycan expression that are temporally limited

Axonal injury is a major hallmark of traumatic brain injury (TBI), and it seems likely that therapies directed toward enhancing axon repair could potentially improve functional outcomes. One potential target is chondroitin sulfate proteoglycans (CSPGs), which are major axon growth inhibitory molecules that are generally, but not always, up-regulated after central nervous system injury. The current study was designed to determine temporal changes in cerebral cortical mRNA or protein expression levels of CSPGs and to determine their regional localization and cellular association by using immunohistochemistry in a controlled cortical impact model of TBI. The results showed significant increases in versican mRNA at 4 and 14 days after TBI but no change in neurocan, aggrecan, or phosphacan. Semiquantitative Western blot (WB) analysis of cortical CSPG protein expression revealed a significant ipsilateral decrease of all CSPGs at 1 day after TBI. Lower CSPG protein levels were sustained until at least 14 days, after which the levels began to normalize. Immunohistochemistry data confirm previous reports of regional increases in CSPG proteins after CNS injury, seen primarily within the developing glial scar after TBI, but also corroborate the WB data by revealing wide areas of pericontusional tissue that are deficient in both extracellular and perineuronal net-associated CSPGs. Given the evidence that CSPGs are largely inhibitory to axonal growth, we interpret these data to indicate a potential for regional spontaneous plasticity after TBI. If this were the case, the gradual normalization of CSPG proteins over time postinjury would suggest that this may be temporally as well as regionally limited.

Synaptic Mechanisms of Activity-Dependent Remodeling in Visual Cortex during Monocular Deprivation

It has long been appreciated that in the visual cortex, particularly within a postnatal critical period for experience-dependent plasticity, the closure of one eye results in a shift in the responsiveness of cortical cells toward the experienced eye. While the functional aspects of this ocular dominance shift have been studied for many decades, their cortical substrates and synaptic mechanisms remain elusive. Nonetheless, it is becoming increasingly clear that ocular dominance plasticity is a complex phenomenon that appears to have an early and a late component. Early during monocular deprivation, deprived eye cortical synapses depress, while later during the deprivation open eye synapses potentiate. Here we review current literature on the cortical mechanisms of activity-dependent plasticity in the visual system during the critical period. These studies shed light on the role of activity in shaping neuronal structure and function in general and can lead to insights regarding how learning is acquired and maintained at the neuronal level during normal and pathological brain development.

Extracellular matrix in plasticity and epileptogenesis

Extracellular matrix (ECM) in the brain is composed of molecules synthesized and secreted by neurons and glial cells in a cell-type-specific and activity-dependent manner. During development, ECM plays crucial roles in proliferation, migration and differentiation of neural cells. In the mature brain, ECM undergoes a slow turnover and supports multiple physiological processes, while restraining structural plasticity. In the first part of this review, we discuss the contribution of ECM molecules to different forms of plasticity, including developmental plasticity in the cortex, long-term potentiation and depression in the hippocampus, homeostatic scaling of synaptic transmission and metaplasticity. In the second part, we focus on pathological changes associated with epileptogenic mutations in ECM-related molecules or caused by seizure-induced remodeling of ECM. The available data suggest that ECM components regulating physiological plasticity are also engaged in different aspects of epileptogenesis, such as dysregulation of excitatory and inhibitory neurotransmission, sprouting of mossy fibers, granule cell dispersion and gliosis. At the end, we discuss combinatorial approaches that might be used to counteract seizure-induced dysregulation of both ECM molecules and extracellular proteases. By restraining ECM modification and preserving the status quo in the brain, these treatments might prove to be valid therapeutic interventions to antagonize the progression of epileptogenesis.

Egr-1 regulates expression of the glial scar component phosphacan in astrocytes after experimental stroke

Ischemic brain injury causes tissue damage and neuronal death. The deficits can often be permanent because adult neurons fail to regenerate. One barrier to neuronal regeneration is the formation of the glial scar, a repair mechanism that is otherwise necessary to seal off necrotic areas. The process of gliosis has been well described, but the mechanisms regulating the robust production of scar components after injury remain poorly understood. Here we show that the early growth response 1 transcriptional factor (Egr-1, also called Krox24, Zif268, and NGFI-A) is expressed in astrocytes in the ventricular wall, corpus callosum, and striatum of normal mouse brain. After experimental stroke caused by permanent occlusion of the middle cerebral artery, Egr-1 was expressed long term in reactive astrocytes that accumulate around the injury site. Gain- and loss-of-function studies in primary astrocytes indicated that Egr-1 regulates the transcription of chondroitin sulfate proteoglycans genes, the main extracellular matrix proteins of the glial scar. Egr-1 bound to a site within the phosphacan promoter and transactivated its expression. Egr-1-deficient mice accumulated lower levels of phosphacan RNA and protein than wild-type mice after stroke, but there were no measurable differences in neurite outgrowth toward the infarct area between the two groups. Our findings suggest that Egr-1 is an important component of the transcriptional network regulating genes involved in gliosis after ischemic injury.

Distribution and synthesis of extracellular matrix proteoglycans, hyaluronan, link proteins and tenascin-R in the rat spinal cord

Perineuronal nets (PNNs) are dense extracellular matrix (ECM) structures that form around many neuronal cell bodies and dendrites late in development. They contain several chondroitin sulphate proteoglycans (CSPGs), hyaluronan, link proteins and tenascin-R. Their time of appearance correlates with the ending of the critical period for plasticity, and they have been implicated in this process. The distribution of PNNs in the spinal cord was examined using Wisteria floribunda agglutinin lectin and staining for chondroitin sulphate stubs after chondroitinase digestion. Double labelling with the neuronal marker, NeuN, showed that PNNs were present surrounding approximately 30% of motoneurons in the ventral horn, 50% of large interneurons in the intermediate grey and 20% of neurons in the dorsal horn. These PNNs formed in the second week of postnatal development. Immunohistochemical staining demonstrated that the PNNs contain a mixture of CSPGs, hyaluronan, link proteins and tenascin-R. Of the CSPGs, aggrecan was present in all PNNs while neurocan, versican and phosphacan/RPTPbeta were present in some but not all PNNs. In situ hybridization showed that aggrecan and cartilage link protein (CRTL 1) and brain link protein-2 (BRAL 2) are produced by neurons. PNN-bearing neurons express hyaluronan synthase, and this enzyme and phosphacan/RPTPbeta may attach PNNs to the cell surface. During postnatal development the expression of link protein and aggrecan mRNA is up-regulated at the time of PNN formation, and these molecules may therefore trigger their formation.

Formation of perineuronal nets in organotypic mouse brain slice cultures is independent of neuronal glutamatergic activity

Perineuronal nets (PNs) are a specialized form of the extracellular matrix and cover specific sets of neurons in distinct brain areas. Animal experiments on sensory visual deprivation have demonstrated that the generation of PNs around neurons of the visual cortex is dependent on neuronal activity during the critical period of visual experience. The importance of the activity of specific neurotransmitter systems for PN formation has, however, not yet been demonstrated. Based on the predominantly glutamatergic innervation of the visual cortex we hypothesized that reduced glutamatergic activity impairs the development of PNs. To address this question, genetic mouse models with compromised glutamate release [Munc13-1-knockout (KO) and Munc13-1/2 double-KO (DKO)] and chronic pharmacological treatments interfering with specific steps of glutamatergic transmission were used. Under experimental conditions of glutamatergic hypofunction PN formation was studied in organotypic brain slice cultures with Wisteria floribunda lectin binding and with aggrecan immunohistochemistry. After cultivation for 21 days a regular PN formation was observed in brain slices (i) derived from Munc13-1-KO and Munc13-1/2-DKO mice, (ii) after blockade of metabotropic and ionotropic glutamate receptors with MCPG and kynurenate, and (iii) after suppression of glutamate release by blockade of presynaptic Ca++ channels with riluzole. Nonselective suppression of neuronal activity by blockade of voltage-gated sodium channels with tetrodotoxin clearly inhibited PN formation. These results indicate that neuronal activity is required but that the glutamatergic system is not essential for PN development.

The role of chondroitin sulfate proteoglycans in regeneration and plasticity in the central nervous system

Chondroitin sulfate proteoglycans (CSPGs) consist of a core protein and glycosaminoglycan (GAG) chains. There is enormous structural diversity among CSPGs due to variation in the core protein, the number of GAG chains and the extent and position of sulfation. Most CSPGs are secreted from cells and participate in the formation of the extracellular matrix (ECM). CSPGs are able to interact with various growth-active molecules and this may be important in their mechanism of action. In the normal central nervous system (CNS), CSPGs have a role in development and plasticity during postnatal development and in the adult. Plasticity is greatest in the young, especially during critical periods. CSPGs are crucial components of perineuronal nets (PNNs). PNNs have a role in closure of the critical period and digestion of PNNs allows their re-opening. In the adult, CSPGs play a part in learning and memory and the hypothalamo-neurohypophysial system. CSPGs have an important role in CNS injuries and diseases. After CNS injury, CSPGs are the major inhibitory component of the glial scar. Removal of CSPGs improves axonal regeneration and functional recovery. CSPGs may also be involved in the pathological processes in diseases such as epilepsy, stroke and Alzheimer's disease. Several possible methods of manipulating CSPGs in the CNS have recently been identified. The development of methods to remove CSPGs has considerable therapeutic potential in a number of CNS disorders.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Extracellular matrix and synaptic functions

Comprehensive analysis of neuromuscular junction formation and recent data on synaptogenesis and long-term potentiation in the central nervous system revealed a number of extracellular matrix (ECM) molecules regulating different aspects of synaptic differentiation and function. The emerging mechanisms comprise interactions of ECM components with their cell surface receptors coupled to tyrosine kinase activities (agrin, integrin ligands, and reelin) and interactions with ion channels and transmitter receptors (Narp, tenascin-R and tenascin-C). These interactions may shape synaptic transmission and plasticity of excitatory synapses either via regulation of Ca2+ entry and postsynaptic expression of transmitter receptors or via control of GABAergic inhibition. The ECM molecules, derived from both neurons and glial cells and secreted into the extracellular space in an activity-dependent manner, may also shape synaptic plasticity through setting diffusion constraints for neurotransmitters, trophic factors and ions.

Chronic enhancement of the intrinsic growth capacity of sensory neurons combined with the degradation of inhibitory proteoglycans allows functional regeneration of sensory axons through the dorsal root entry zone in the mammalian spinal cord

Peripherally conditioned sensory neurons have an increased capacity to regenerate their central processes. However, even conditioned axons struggle in the presence of a hostile CNS environment. We hypothesized that combining an aggressive conditioning strategy with modification of inhibitory reactive astroglial-associated extracellular matrix could enhance regeneration. We screened potential treatments using a model of the dorsal root entry zone (DREZ). In this assay, a gradient of inhibitory chondroitin sulfate proteoglycans (CSPGs) stimulates formation of dystrophic end bulbs on adult sensory axons, which mimics regeneration failure in vivo. Combining inflammation-induced preconditioning of dorsal root ganglia in vivo before harvest, with chondroitinase ABC (ChABC) digestion of proteoglycans in vitro allows for significant regeneration across a once potently inhibitory substrate. We then assessed regeneration through the DREZ after root crush in adult rats receiving the combination treatment, ChABC, or zymosan pretreatment alone or no treatment. Regeneration was never observed in untreated animals, and only minimal regeneration occurred in the ChABC- and zymosan-alone groups. However, remarkable regeneration was observed in a majority of animals that received the combination treatment. Regenerated fibers established functional synapses, as demonstrated electrophysiologically by the presence of an H-reflex. Two different postlesion treatment paradigms in which the timing of both zymosan and ChABC administration were varied after injury were ineffective in promoting regeneration. Therefore, zymosan pretreatment, but not posttreatment, of the sensory ganglia, combined with ChABC modification of CSPGs, resulted in robust and functional regeneration of sensory axons through the DREZ after root injury.

Degradation of chondroitin sulfate proteoglycans induces sprouting of intact purkinje axons in the cerebellum of the adult rat

Chondroitin sulfate proteoglycans are major constituents of the extracellular matrix and form perineuronal nets. Information regarding the growth-inhibitory activity of these molecules after injury is rapidly expanding. However, less is known about their physiological role in the adult undamaged CNS. Here, we investigated the function of chondroitin sulfate proteoglycans in maintaining the proper structure of Purkinje axons in the cerebellum of adult rats. To this end, we examined the morphology and distribution of intracortical Purkinje neurites after intraparenchymal injection of chondroitinase ABC. Staining with the lectin Wisteria floribunda agglutinin or 2B6 antibodies showed that this treatment efficiently removed chondroitin sulfate proteoglycans from wide areas of the cerebellar cortex. In the same sites, there was a profuse outgrowth of terminal branches from the Purkinje infraganglionic plexus, which invaded the deeper regions of the granular layer. In contrast, myelinated axon segments were not affected and maintained their normal relationship with oligodendroglial sheaths. Purkinje axon sprouting was first evident at 4 d and increased further at 7 d after enzyme application. Within 42 d, the expression pattern of chondroitin sulfate proteoglycans gradually recovered, whereas axonal modifications progressively regressed. Our results show that, in the absence of injury or novel external stimuli, degradation of chondroitin sulfate proteoglycans is sufficient to induce Purkinje axon sprouting but not the formation of long-lasting synaptic contacts. Together with other growth-inhibitory molecules, such as myelin-associated proteins, chondroitin sulfate proteoglycans restrict structural plasticity of intact Purkinje axons to maintain normal wiring patterns in the adult cerebellar cortex.

Synaptic localization of receptor-type protein tyrosine phosphatase zeta/beta in the cerebral and hippocampal neurons of adult rats

Receptor-type protein tyrosine phosphatase (RPTP) zeta/beta is a nervous tissue-specific chondroitin sulfate proteoglycan. In this study, we investigated the immunohistochemical localization of RPTPzeta/beta in adult rat cerebral cortex and hippocampus at light and electron microscopic levels. Double labeling immunofluorescence microscopy revealed that the immunoreactivity of RPTPzeta/beta was observed at MAP2-positive dendrites and PSD-95-positive spines of pyramidal neurons in the cerebral cortex and hippocampus. Electron microscopic observation demonstrated a strong immunoreactivity of RPTPzeta/beta at the postsynaptic membrane of dendritic spines and shafts, and its moderate immunoreactivity at the dendritic membrane. In cultured cortical neurons, the immunoreactivity of RPTPzeta/beta was observed at some of PSD-95-positive spines. These results demonstrate that RPTPzeta/beta is localized mainly at the postsynaptic membrane of pyramidal neurons in adult cerebral cortex and hippocampus.

Matrix-degrading enzymes tissue plasminogen activator and matrix metalloprotease-3 in the hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS), synthesizing arginine vasopressin (AVP) and oxytocin (OXT), is well known to show structural plasticity during chronic physiological stimulation such as salt loading and lactation. In the present study, we undertook in the HNS to study localization and activity-dependent changes in the expression of matrix-degrading enzymes such as tissue plasminogen activator (tPA) and matrix metalloprotease-3 (MMP-3). Double labeling confocal microscopy demonstrated that the immunoreactivity of tPA was localized at AVP-positive dendrites in the supraoptic nucleus (SON) and AVP-positive terminals in the neurohypophysis (NH). The immunoreactivity of tPA was also seen at astrocytic processes in the HNS. Likewise, the immunoreactivity of MMP-3 was observed at AVP-positive dendrites and terminals. High magnification observation further revealed punctate distribution of tPA and MMP-3 immunoreactivity at dendrites and terminals, suggesting that they are localized at neurosecretory granules. Salt loading, known as the chronic stimulation to cause the structural plasticity, increased protein and mRNA levels of tPA in the SON but reduced protein levels of it in the NH. The chronic stimulation also increased protein levels of urokinase plasminogen activator in the SON, but the stimulation did not change protein levels of MMP-3 in the SON and NH. Depolarizing agent KCl released tPA from isolated neurosecretosomes, and this depolarization-dependent release was abolished by verapamil, a Ca(2+) channel blocker. These results demonstrate that tPA and MMP-3 are localized mainly at dendrites and terminals of AVP-expressing magnocellular neurons and tPA is released in an activity-dependent manner, suggesting that matrix-degrading proteases are candidate molecules to be concerned with the structural plasticity in the HNS.

Chondroitin sulfate proteoglycan phosphacan associates with parallel fibers and modulates axonal extension and fasciculation of cerebellar granule cells

Phosphacan is a nervous system-specific chondroitin sulfate proteoglycan and one of the major components of extracellular matrix in the brain. In the present study, we examined its spatiotemporal expression, ultrastructural localization, binding manner, and in vitro analysis on cell adhesion, axonal extension, and fasciculation in rat cerebellum. The present light microscopic immunohistochemistry showed that phosphacan immunoreactivity was localized mainly at the molecular layer in the cerebellum, but not at the external granular layer. Further double labeling immunohistochemical and immunoelectron microscopic studies revealed that phosphacan was localized around parallel fibers, but not at synapses. The binding of phosphacan to membrane and/or extracellular matrix partly required Ca2+ and was mediated through its core glycoprotein. Phosphacan inhibited adhesion and axonal extension of cerebellar granule cells in dissociated culture, while it promoted axonal fasciculation of their aggregated culture. These results indicate that phosphacan around parallel fibers may be the repulsive substratum for adhesion and extension of granule cells and promote the fasciculation of parallel fibers.

Neuronal expression of the chondroitin sulfate proteoglycans receptor-type protein-tyrosine phosphatase beta and phosphacan

Receptor-type protein-tyrosine phosphatase beta (RPTPbeta) and its spliced variant phosphacan are major components of chondroitin sulfate proteoglycans in the CNS. In this study, expression and localization of RPTPbeta and phosphacan were examined in developing neurons by immunological analyses using 6B4, 3F8, and anti-PTP antibodies and reverse transcription-polymerase chain reaction (RT-PCR). Light microscopic immunohistochemistry showed that 6B4 RPTPbeta/phosphacan immunoreactivity was observed around neurons in the cortical plate. Further ultrastructural observation showed that 6B4 RPTPbeta/phosphacan immunoreactivity was observed mainly at the membrane of migrating neurons and radial glia. Immunocytochemical analysis revealed that RPTPbeta immunoreactivity was observed in cultured cerebral, hippocampal, and cerebellar neurons in addition to type-1 and type-2 astrocytes. Western analysis further demonstrated that the shorter receptor form of RPTPbeta (sRPTPbeta) was detected from cell lysate of cortical and hippocampal neurons using 6B4 and anti-PTP antibodies, while sRPTPbeta of cerebellar neurons and type-1 astrocytes was recognized only by anti-PTP antibody. Phosphacan was detected from neuronal culture supernatants of cortical, hippocampal, and cerebellar neurons, but not from type-1 astrocytes using 6B4 and 3F8 antibodies. RT-PCR analysis demonstrated the prominent expression of sRPTPbeta and phosphacan mRNAs in cortical neurons, and that of sRPTPbeta mRNA in type-1 astrocytes. During culture development of cortical neurons, the immunoreactivity of 6B4 sRPTPbeta was observed entirely on the neuronal surface including somata, dendrites, axons, and growth cones at earlier stages of cortical neuronal culture such as stages 2 and 3, while, after longer culture, 6B4 sRPTPbeta immunoreactivity in stages 4 and 5 neurons was detected at dendrites and somata and disappeared from axons, and was not observed over axonal terminals and postsynaptic spines. These results demonstrate that neurons are able to express sRPTPbeta on their cellular surface and to secrete phosphacan, and neuronal expression of sRPTPbeta may modulate neuronal differentiation including neuritogenesis and synaptogenesis.

Construction of perineuronal net-like structure by cortical neurons in culture

Perineuronal nets consisting of chondroitin sulfate proteoglycans and hyaluronic acid are associated with distinct neuronal populations in mammalian brain. Whether neurons or glia cells produce these surface-associated chondroitin sulfate proteoglycan perineuronal nets has remained in question. In the present study, we observed perineuronal net-like structure by rat cortical neurons in dissociated culture using Wisteria floribunda aggulutinin, hyaluronic acid binding protein, and the antibodies recognizing chondroitin sulfate proteoglycans. The double labeling experiments showed that perineuronal net-like structure labeled with Wisteria floribunda aggulutinin was observed often at parvalbumin-positive neurons in dissociated cortical culture without glia. Perineuronal net-like structure was not seen at the early stage of culture, but they became visible concomitantly with neuronal maturation after longer culture. High magnification observation further demonstrated that Wisteria floribunda aggulutinin labeling on cortical neurons was seen as numerous puncta along surface of somata and proximal dendrites, but not axons and synapses. Perineuronal net-like structure on cultured neurons was also visualized using chondroitin sulfate proteoglycan-specific antibodies and hyaluronic acid binding protein. Double labeling study demonstrated that perineuronal net-like structure in cultured cortical neurons was composed of chondroitin sulfate proteoglycans such as neurocan and phosphacan. The hyaluronidase treatment of live neurons abolished cellular labeling of hyaluronic acid binding protein and concomitantly diminished that of Wisteria floribunda aggulutinin. These results indicate that cultured cortical neurons are able to construct perineuronal net-like structure without glial cells.