Large diameter primary afferent input is required for expression of the Cat-301 proteoglycan on the surface of motor neurons

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.

Reorganization of Synaptic Connections and Perineuronal Nets in the Deep Cerebellar Nuclei of Purkinje Cell Degeneration Mutant Mice

The perineuronal net (PN) is a subtype of extracellular matrix appearing as a net-like structure around distinct neurons throughout the whole CNS. PNs surround the soma, proximal dendrites, and the axonal initial segment embedding synaptic terminals on the neuronal surface. Different functions of the PNs are suggested which include support of synaptic stabilization, inhibition of axonal sprouting, and control of neuronal plasticity. A number of studies provide evidence that removing PNs or PN-components results in renewed neurite growth and synaptogenesis. In a mouse model for Purkinje cell degeneration, we examined the effect of deafferentation on synaptic remodeling and modulation of PNs in the deep cerebellar nuclei. We found reduced GABAergic, enhanced glutamatergic innervations at PN-associated neurons, and altered expression of the PN-components brevican and hapln4. These data refer to a direct interaction between ECM and synapses. The altered brevican expression induced by activated astrocytes could be required for an adequate regeneration by promoting neurite growth and synaptogenesis.

Models of vocal learning in the songbird: Historical frameworks and the stabilizing critic

Birdsong is a form of sensorimotor learning that involves a mirror-like system that activates with both song hearing and production. Early models of song learning, based on behavioral measures, identified key features of vocal plasticity, such as the requirements for memorization of a tutor song and auditory feedback during song practice. The concept of a comparator, which compares the memory of the tutor song to auditory feedback, featured prominently. Later models focused on linking anatomically-defined neural modules to behavioral concepts, such as the comparator. Exploiting the anatomical modularity of the songbird brain, localized lesions illuminated mechanisms of the neural song system. More recent models have integrated neuronal mechanisms identified in other systems with observations in songbirds. While these models explain multiple aspects of song learning, they must incorporate computational elements based on unknown biological mechanisms to bridge the motor-to-sensory delay and/or transform motor signals into the sensory domain. Here, I introduce the stabilizing critic hypothesis, which enables sensorimotor learning by (1) placing a purely sensory comparator afferent of the song system and (2) endowing song system disinhibitory interneuron networks with the capacity both to bridge the motor-sensory delay through prolonged bursting and to stabilize song segments selectively based on the comparator signal. These proposed networks stabilize an otherwise variable signal generated by both putative mirror neurons and a cortical-basal ganglia-thalamic loop. This stabilized signal then temporally converges with a matched premotor signal in the efferent song motor cortex, promoting spike-timing-dependent plasticity in the premotor circuitry and behavioral song learning.

Perineuronal nets and schizophrenia: the importance of neuronal coatings

Schizophrenia is a complex brain disorder associated with deficits in synaptic connectivity. The insidious onset of this illness during late adolescence and early adulthood has been reported to be dependent on several key processes of brain development including synaptic refinement, myelination and the physiological maturation of inhibitory neural networks. Interestingly, these events coincide with the appearance of perineuronal nets (PNNs), reticular structures composed of components of the extracellular matrix that coat a variety of cells in the mammalian brain. Until recently, the functions of the PNN had remained enigmatic, but are now considered to be important in development of the central nervous system, neuronal protection and synaptic plasticity, all elements which have been associated with schizophrenia. Here, we review the emerging evidence linking PNNs to schizophrenia. Future studies aimed at further elucidating the functions of PNNs will provide new insights into the pathophysiology of schizophrenia leading to the identification of novel therapeutic targets with the potential to restore normal synaptic integrity in the brain of patients afflicted by this illness.

The perineuronal net component of the extracellular matrix in plasticity and epilepsy

During development the extracellular matrix (ECM) of the central nervous system (CNS) facilitates proliferation, migration, and synaptogenesis. In the mature nervous system due to changes in the ECM it provides structural stability and impedes proliferation, migration, and synaptogensis. The perineuronal net (PN) is a specialized ECM structure found primarily surrounding inhibitory interneurons where it forms a mesh-like structure around points of synaptic contact. The PN organizes the extracellular space by binding multiple components of the ECM and bringing them into close proximity to the cell membrane, forming dense aggregates surrounding synapses. The PN is expressed late in postnatal development when the nervous system is in the final stages of maturation and the critical periods are closing. Once fully expressed the PN envelopes synapses and leads to decreased plasticity and increases synaptic stability in the CNS. Disruptions in the PN have been studied in a number of disease states including epilepsy. Epilepsy is one of the most common neurologic disorders characterized by excessive neuronal activity which results in recurrent spontaneous seizures. A shift in the delicate balance between excitation and inhibition is believed to be one of the underlying mechanisms in the development of epilepsy. During epileptogenesis, the brain undergoes numerous changes including synaptic rearrangement and axonal sprouting, which require structural plasticity. Because of the PNs location around inhibitory cells and its role in limiting plasticity, the PN is an important candidate for altering the progression of epilepsy. In this review, an overview of the ECM and PN in the CNS will be presented with special emphasis on potential roles in epileptogenesis.

Activity-dependent plasticity of spinal circuits in the developing and mature spinal cord

Part of the development and maturation of the central nervous system (CNS) occurs through interactions with the environment. Through physical activities and interactions with the world, an animal receives considerable sensory information from various sources. These sources can be internally (proprioceptive) or externally (such as touch and pressure) generated senses. Ample evidence exists to demonstrate that the sensory information originating from large diameter afferents (Ia fibers) have an important role in inducing essential functional and morphological changes for the maturation of both the brain and the spinal cord. The Ia fibers transmit sensory information generated by muscle activity and movement. Such use or activity-dependent plastic changes occur throughout life and are one reason for the ability to acquire new skills and learn new movements. However, the extent and particularly the mechanisms of activity-dependent changes are markedly different between a developing nervous system and a mature nervous system. Understanding these mechanisms is an important step to develop strategies for regaining motor function after different injuries to the CNS. Plastic changes induced by activity occur both in the brain and spinal cord. This paper reviews the activity-dependent changes in the spinal cord neural circuits during both the developmental stages of the CNS and in adulthood.

Treadmill training induces plasticity in spinal motoneurons and sciatic nerve after sensorimotor restriction during early postnatal period: new insights into the clinical approach for children with cerebral palsy

The aim of the present study was to investigate whether locomotor stimulation training could have beneficial effects on the morphometric alterations of spinal cord and sciatic nerve consequent to sensorimotor restriction (SR). Male Wistar rats were exposed to SR from postnatal day 2 (P2) to P28. Control and experimental rats underwent locomotor stimulation training in a treadmill for three weeks (from P31 to P52). The cross-sectional area (CSA) of spinal motoneurons innervating hind limb muscles was determined. Both fiber and axonal CSA of myelinated fibers were also assessed. The growth-related increase in CSA of motoneurons in the SR group was less than controls. After SR, the mean motoneuron soma size was reduced with an increase in the proportion of motoneurons with a soma size of between 0 and 800 μm(2). The changes in soma size of motoneurons were accompanied by a reduction in the mean fiber and axon CSA of sciatic nerve. The soma size of motoneurons was reestablished at the end of the training period reaching controls level. Our results suggest that SR during early postnatal life retards the growth-related increase in the cell body size of motoneurons in spinal cord and the development of sciatic nerve. Additionally, three weeks of locomotor stimulation using a treadmill seems to have a beneficial effect on motoneurons' soma size.

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.

Immunohistochemical parcellation of the ferret (Mustela putorius) visual cortex reveals substantial homology with the cat (Felis catus)

Electrophysiological mapping of the adult ferret visual cortex has until now determined the existence of 12 retinotopically distinct areas; however, in the cat, another member of the Carnivora, 20 distinct visual areas have been identified by using retinotopic mapping and immunolabeling. In the present study, the immunohistochemical approach to demarcate the areal boundaries of the adult ferret visual cortex was applied in order to overcome the difficulties in accessing the sulcal surfaces of a small, gyrencephalic brain. Nonphosphorylated neurofilament (NNF) expression profiles were compared with another classical immunostain of cortical nuclei, Cat-301 chondroitin sulfate proteoglycan (CSPG). Together, these two markers reliably demarcated the borders of the 12 previously defined areas and revealed further arealization beyond those borders to a total of 19 areas: 21a and 21b; the anterolateral, posterolateral, dorsal, and ventral lateral suprasylvian areas (ALLS, PLLS, DLS, and VLS, respectively); and the splenial and cingulate visual areas (SVA and CVA). NNF expression profile and location of the newly defined areas correlate with previously defined areas in the cat. Moreover, NNF and Cat-301 together revealed discrete expression domains in the posteroparietal (PP) cortex, demarcating four subdivisions in the caudal lateral and medial domains (PPcL and PPcM) and rostral lateral and medial domains (PPrL and PPrM), where only two retinotopic maps have been previously identified (PPc and PPr). Taken together, these studies suggest that NNF and Cat-301 can illustrate the homology between cortical areas in different species and draw out the principles that have driven evolution of the visual cortex.

Development-based compartmentalization of the Drosophila central brain

The neuropile of the Drosophila brain is subdivided into anatomically discrete compartments. Compartments are rich in terminal neurite branching and synapses; they are the neuropile domains in which signal processing takes place. Compartment boundaries are defined by more or less dense layers of glial cells as well as long neurite fascicles. These fascicles are formed during the larval period, when the approximately 100 neuronal lineages that constitute the Drosophila central brain differentiate. Each lineage forms an axon tract with a characteristic trajectory in the neuropile; groups of spatially related tracts congregate into the brain fascicles that can be followed from the larva throughout metamorphosis into the adult stage. Here we provide a map of the adult brain compartments and the relevant fascicles defining compartmental boundaries. We have identified the neuronal lineages contributing to each fascicle, which allowed us to compare compartments of the larval and adult brain directly. Most adult compartments can be recognized already in the early larval brain, where they form a "protomap" of the later adult compartments. Our analysis highlights the morphogenetic changes shaping the Drosophila brain; the data will be important for studies that link early-acting genetic mechanisms to the adult neuronal structures and circuits controlled by these mechanisms.

Influence of aging on chondroitin sulfate proteoglycan expression and neural stem/progenitor cells in rat brain and improving effects of a herbal medicine, yokukansan

There is evidence of structural and functional deterioration in the brain, including the prefrontal cortex (PFC) and hippocampus, during the normal aging process in animals and humans. Extracellular matrix-associated glycoproteins, such as chondroitin sulfate proteoglycans (CSPGs), are involved in not only maintaining the structures and functions of adult neurons, but also regulating the proliferation, migration, and neurite outgrowth of neural stem cells in the brain. On the other hand, a herbal medicine, yokukansan (YKS), is used in a variety of clinical situations for treating symptoms associated with age-related neurodegenerative disorders such as Alzheimer's disease, but its pharmacological properties have not been fully understood. The present study was designed to clarify the influence of aging and the improving effects of YKS on the expression of aggrecan, a major molecule of CSPGs, and on the proliferation and migration of neural stem/progenitor cells identified by bromodeoxyuridine (BrdU) incorporation in the PFC and hippocampus including the dentate gyrus. Aged rats (24 months old) showed a significant increase in aggrecan expression throughout the PFC and in the hippocampus particularly in the CA3 subfield, but not the dentate gyrus compared to young rats (5 months old), evaluated by the immunohistochemical method. YKS treatment decreased the age-related increase in aggrecan expression as well as normal expression in young rats. Aged rats also showed a decreased number of BrdU-labeled cells in the PFC and hippocampus, and these decreases were improved by YKS treatment, which also increased the numbers in young rats. These results suggest that aging influences the microenvironment for adult and immature neurons in the brain, which may affect the proliferation and migration of neural stem/progenitor cells, and YKS has pharmacological potency for these age-related events. These findings help to understand the physiology and pathology of the aged brain and provide an anti-aging strategy for the brain.

Extracellular matrix of the central nervous system: from neglect to challenge

The basic concept, that specialized extracellular matrices rich in hyaluronan, chondroitin sulfate proteoglycans (aggrecan, versican, neurocan, brevican, phosphacan), link proteins and tenascins (Tn-R, Tn-C) can regulate cellular migration and axonal growth and thus, actively participate in the development and maturation of the nervous system, has in recent years gained rapidly expanding experimental support. The swift assembly and remodeling of these matrices have been associated with axonal guidance functions in the periphery and with the structural stabilization of myelinated fiber tracts and synaptic contacts in the maturating central nervous system. Particular interest has been focused on the putative role of chondroitin sulfate proteoglycans in suppressing central nervous system regeneration after lesions. The axon growth inhibitory properties of several of these chondroitin sulfate proteoglycans in vitro, and the partial recovery of structural plasticity in lesioned animals treated with chondroitin sulfate degrading enzymes in vivo have significantly contributed to the increased awareness of this long time neglected structure.

Total number, distribution, and phenotype of cells expressing chondroitin sulfate proteoglycans in the normal human amygdala

Chondroitin sulfate proteoglycans (CSPGs) are a key structural component of the brain extracellular matrix. They are involved in critical neurodevelopmental functions and are one of the main components of pericellular aggregates known as perineuronal nets. As a step toward investigating their functional and pathophysiological roles in the human amygdala, we assessed the pattern of CSPG expression in the normal human amygdala using wisteria floribunda agglutinin (WFA) lectin histochemistry. Total numbers of WFA-labeled elements were measured in the lateral (LN), basal (BN), accessory basal (ABN) and cortical (CO) nuclei of the amygdala from 15 normal adult human subjects. For interspecies qualitative comparison, we also investigated the pattern of WFA labeling in the amygdala of naïve rats (n=32) and rhesus monkeys (Macaca mulatta; n=6). In human amygdala, WFA lectin histochemistry resulted in labeling of perineuronal nets and cells with clear glial morphology, while neurons did not show WFA labeling. Total numbers of WFA-labeled glial cells showed high interindividual variability. These cells aggregated in clusters with a consistent between-subjects spatial distribution. In a subset of human subjects (n=5), dual color fluorescence using an antibody raised against glial fibrillary acidic protein (GFAP) and WFA showed that the majority (93.7%) of WFA-labeled glial cells correspond to astrocytes. In rat and monkey amygdala, WFA histochemistry labeled perineuronal nets, but not glial cells. These results suggest that astrocytes are the main cell type expressing CSPGs in the adult human amygdala. Their highly segregated distribution pattern suggests that these cells serve specialized functions within human amygdalar nuclei.

Cortical and subcortical plasticity in the brains of humans, primates, and rats after damage to sensory afferents in the dorsal columns of the spinal cord

The failure of injured axons to regenerate following spinal cord injury deprives brain neurons of their normal sources of activation. These injuries also result in the reorganization of affected areas of the central nervous system that is thought to drive both the ensuing recovery of function and the formation of maladaptive neuronal circuitry. Better understanding of the physiological consequences of novel synaptic connections produced by injury and the mechanisms that control their formation are important to the development of new successful strategies for the treatment of patients with spinal cord injuries. Here we discuss the anatomical, physiological and behavioral changes that take place in response to injury-induced plasticity after damage to the dorsal column pathway in rats and monkeys. Complete section of the dorsal columns of the spinal cord at a high cervical level in monkeys and rats interrupts the ascending axon branches of low threshold mechanoreceptor afferents subserving the forelimb and the rest of the lower body. Such lesions render the corresponding part of the somatotopic representation of primary somatosensory cortex totally unresponsive to tactile stimuli. There are also behavioral consequences of the sensory loss, including an impaired use of the hand/forelimb in manipulating small objects. In monkeys, if some of the afferents from the hand remain intact after dorsal column lesions, these remaining afferents extensively reactivate portions of somatosensory cortex formerly representing the hand. This functional reorganization develops over a postoperative period of 1 month, during which hand use rapidly improves. These recoveries appear to be mediated, at least in part, by the sprouting of preserved afferents within the cuneate nucleus of the dorsal column-trigeminal complex. In rats, such functional collateral sprouting has been promoted by the post-lesion digestion of the perineuronal net in the cuneate nucleus. Thus, this and other therapeutic strategies have the potential of enhancing sensorimotor recoveries after spinal cord injuries in humans.

Purkinje cell axon collaterals terminate on Cat-301+ neurons in Macaca monkey cerebellum

The monoclonal antibody Cat-301 identifies perineuronal nets around specific neuronal types, including those in the cerebellum. This report finds in adult Macaca monkey that basket cells in the deep molecular layer; granule cell layer (GCL) interneurons including Lugaro cells; large neurons in the foliar white matter (WM); and deep cerebellar nuclei (DCN) neurons contain subsets of Cat-301 positive (+) cells. Most Cat-301+ GCL interneurons are glycine+ and all are densely innervated by a meshwork of calbindin+/glutamic acid decarboxylase+ Purkinje cell collaterals and their synapses. DCN and WM Cat-301+ neurons also receive a similar but less dense innervation. Due to the heavy labeling of adjacent Purkinje cell dendrites, the innervation of Cat-301+ basket cells was less certain. These findings suggest that several complex feedback circuits from Purkinje cell to cerebellar interneurons exist in primate cerebellum whose function needs to be investigated. Cat-301 labeling begins postnatally in WM and DCN, but remains sparse until at least 3 months of age. Because the appearance of perineuronal nets is associated with maturation of synaptic circuits, this suggests that the Purkinje cell feedback circuits develop for some time after birth.

Mapping of aggrecan, hyaluronic acid, heparan sulphate proteoglycans and aquaporin 4 in the central nervous system of the mouse

The extracellular matrix (ECM) of the central nervous system (CNS) is found dispersed in the neuropil or forming aggregates around the neurons called perineuronal nets (PNNs). The ECM mainly contains chondroitin sulphate proteoglycans (CSPG), hyaluronic acid (HA) and tenascin-R. Heparan sulphate proteoglycans (HSPG) can also be secreted in the ECM or be part of the cell membrane. The ECM has a heterogeneous distribution which has been linked to several functions, such as specific regional maintenance of hydrodynamic properties in the CNS, in which aquaporins (AQP) play an important role. AQP are a family of membrane proteins which acts as a water channel and AQP4 is the most abundant isoform in the brain. Nevertheless the importance of these proteins, their distribution and correlation in the whole CNS of mice is only partially known. In the present study, the histochemical and immunohistochemical distribution of PNNs, using Wisteria floribunda agglutinin (WFA), aggrecan, HA, HSPGs and AQP4 is described, and their perineuronal and neuropil staining has been semi-quantitatively evaluated in the whole CNS of mice. The results showed that the aggrecan, HA and HSPGs perineuronal distribution coincided partially and this could be related to ECM functional properties. AQP4 showed a heterogeneous distribution throughout the CNS. In some areas, an inverse correlation between AQP4 and ECM components has been observed, suggesting a complementary role for both in the maintenance of water homeostasis. A common location for AQP4 and HSPGs has also been observed in CNS neuropil.

Increased chondroitin sulfate proteoglycan expression in denervated brainstem targets following spinal cord injury creates a barrier to axonal regeneration overcome by chondroitinase ABC and neurotrophin-3

Increased chondroitin sulfate proteoglycan (CSPG) expression in the vicinity of a spinal cord injury (SCI) is a primary participant in axonal regeneration failure. However, the presence of similar increases of CSPG expression in denervated synaptic targets well away from the primary lesion and the subsequent impact on regenerating axons attempting to approach deafferented neurons have not been studied. Constitutively expressed CSPGs within the extracellular matrix and perineuronal nets of the adult rat dorsal column nuclei (DCN) were characterized using real-time PCR, Western blot analysis and immunohistochemistry. We show for the first time that by 2 days and through 3 weeks following SCI, the levels of NG2, neurocan and brevican associated with reactive glia throughout the DCN were dramatically increased throughout the DCN despite being well beyond areas of trauma-induced blood brain barrier breakdown. Importantly, regenerating axons from adult sensory neurons microtransplanted 2 weeks following SCI between the injury site and the DCN were able to regenerate rapidly within white matter (as shown previously by Davies et al. [Davies, S.J., Goucher, D.R., Doller, C., Silver, J., 1999. Robust regeneration of adult sensory axons in degenerating white matter of the adult rat spinal cord. J. Neurosci. 19, 5810-5822]) but were unable to enter the denervated DCN. Application of chondroitinase ABC or neurotrophin-3-expressing lentivirus in the DCN partially overcame this inhibition. When the treatments were combined, entrance by regenerating axons into the DCN was significantly augmented. These results demonstrate both an additional challenge and potential treatment strategy for successful functional pathway reconstruction after SCI.

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.

Upregulation of aggrecan, link protein 1, and hyaluronan synthases during formation of perineuronal nets in the rat cerebellum

Extracellular matrix molecules accumulate around central nervous system neurons during postnatal development, forming so-called perineuronal nets (PNNs). PNNs play a role in restricting plasticity at the end of critical periods. In the adult rat cerebellum, PNNs are found around large, deep cerebellar nuclei (DCN) neurons and Golgi neurons and are composed of chondroitin sulfate proteoglycans (CSPGs), tenascin-R (TN-R), hyaluronan (HA), and link proteins, such as cartilage link protein 1 (Crtll). Granule cells and Purkinje cells are surrounded by a partially organized matrix. Both glial cells and neurons surrounded by PNNs are the site of synthesis of some CSPGs and of TN-R, but only neurons produce HA synthetic enzymes (HASs), thus HA, and link proteins, which are scaffolding molecules for an organized matrix. To elucidate the mechanisms of formation of PNNs, we analyzed by immunohistochemistry and in situ hybridization which PNN components are upregulated during PNN formation in rat cerebellar postnatal development and what cell types express them. We observed that Wisteria floribunda agglutinin-binding PNNs develop around DCN neurons from postnatal day (P)7 and around Golgi neurons from P14. At the same time as their PNNs start to form, these neurons upregulate aggrecan, Crtll, and HASs mRNAs. However, Crtll is the only PNN component to be expressed exclusively in neurons surrounded by PNNs. The other link protein that shows a perineuronal net pattern in the DCN, Bral2, is upregulated later during development. These data suggest that aggrecan, HA, and, particularly, Crtll might be crucial elements for the initial assembly of PNNs.

Promoting plasticity in the spinal cord with chondroitinase improves functional recovery after peripheral nerve repair

Functional recovery after peripheral nerve repair in humans is often disappointing. A major reason for this is the inaccuracy of re-innervation of muscles and sensory structures. We hypothesized that promoting plasticity in the spinal cord, through digestion of chondroitin sulphate proteoglycans (CSPGs) with chondroitinase ABC (ChABC), might allow the CNS to compensate for inaccurate peripheral re-innervation and improve functional recovery. The median and ulnar nerves were injured and repaired to produce three grades of inaccuracy of peripheral re-innervation by (i) crush of both nerves; (ii) correct repair of median to median and ulnar to ulnar; and (iii) crossover of the median and ulnar nerves. Mapping of the motor neuron pool of the flexor carpi radialis muscle showed precise re-innervation after nerve crush, inaccurate regeneration after correct repair, more inaccurate after crossover repair. Recovery of forelimb function, assessed by skilled paw reaching, grip strength and sensory testing varied with accuracy of re-innervation. This was not due to differences in the number of regenerated axons. Single injections of ChABC into the spinal cord led to long-term changes in the extracellular matrix, with hyaluronan and neurocan being removed and not fully replaced after 8 weeks. ChABC treatment produce increased sprouting visualized by MAP1BP staining and improved functional recovery in skilled paw reaching after correct repair and in grip strength after crossover repair. There was no hyperalgesia. Enhanced plasticity in the spinal cord, therefore, allows the CNS to compensate for inaccurate motor and sensory re-innervation of the periphery, and may be a useful adjunct therapy to peripheral nerve repair.

Differential expression of several molecules of the extracellular matrix in functionally and developmentally distinct regions of rat spinal cord

We have examined the regional distribution of several chondroitin sulfate proteoglycans (neurocan, brevican, versican, aggrecan, phosphacan), of their glycosaminoglycan moieties, and of tenascin-R in the spinal cord of adult rat. The relationships of these molecules with glial and neuronal populations, identified with appropriate markers, were investigated by using multiple fluorescence labeling combined with confocal microscopy. The results showed that the distribution of the examined molecules was similar at all spinal cord levels but displayed area-specific differences along the dorso-ventral axis, delimiting functionally and developmentally distinct areas. In the gray matter, laminae I and II lacked perineuronal nets (PNNs) of extracellular matrix and contained low levels of chondroitin sulfate glycosaminoglycans (CS-GAGs), brevican, and tenascin-R, possibly favoring the maintenance of local neuroplastic properties. Conversely, CS-GAGs, brevican, and phosphacan were abundant, with numerous thick PNNs, in laminae III-VIII and X. Motor neurons (lamina IX) were surrounded by PNNs that contained all molecules investigated but displayed various amounts of CS-GAGs. Double-labeling experiments showed that the presence of PNNs could not be unequivocally related to specific classes of neurons, such as motor neurons or interneurons identified by their expression of calcium-binding proteins (parvalbumin, calbindin, calretinin). However, a good correlation was found between PNNs rich in CS-GAGs and the neuronal expression of the Kv3.1b subunit of the potassium channel, a marker of fast-firing neurons. This observation confirms the correlation between the electrophysiological properties of these neurons and the specific composition of their microenvironment.

Chondroitinase ABC digestion of the perineuronal net promotes functional collateral sprouting in the cuneate nucleus after cervical spinal cord injury

Upregulation of extracellular chondroitin sulfate proteoglycans (CSPGs) after CNS injuries contributes to the impediment of functional recovery by restricting both axonal regeneration and synaptic plasticity. In the present study, the effect of degrading CSPGs with the application of the bacterial enzyme chondroitinase ABC (chABC) into the cuneate nucleus of rats partially denervated of forepaw dorsal column axons was examined. A dorsal column transection between the C6-C7 dorsal root entry zones was followed immediately by an ipsilateral brainstem injection of either chABC or a bacterial-derived control enzyme [penicillinase (P-ase)] and then subsequently (1 week later) followed with a second brainstem enzyme injection and cholera toxin B subunit (CTB) tracer injection into the ipsilateral forepaw digits and pads. After 1 additional week, the rats underwent electrophysiological receptive field mapping of the cuneate nucleus and/or anatomical evaluation. Examination of the brainstems of rats from each group revealed that CSPGs had been reduced after chABC treatment. Importantly, in the chABC-treated rats (but not in the P-ase controls), a significantly greater area of the cuneate nucleus was occupied by physiologically active CTB traced forepaw afferents that had been spared by the initial cord lesion. These results demonstrate, for the first time, a functional change directly linked to anatomical evidence of sprouting by spinal cord afferents after chABC treatment.

Composition of perineuronal nets in the adult rat cerebellum and the cellular origin of their components

The decrease in plasticity that occurs in the central nervous system during postnatal development is accompanied by the appearance of perineuronal nets (PNNs) around the cell body and dendrites of many classes of neuron. These structures are composed of extracellular matrix molecules, such as chondroitin sulfate proteoglycans (CSPGs), hyaluronan (HA), tenascin-R, and link proteins. To elucidate the role played by neurons and glial cells in constructing PNNs, we studied the expression of PNN components in the adult rat cerebellum by immunohistochemistry and in situ hybridization. In the deep cerebellar nuclei, only large excitatory neurons were surrounded by nets, which contained the CSPGs aggrecan, neurocan, brevican, versican, and phosphacan, along with tenascin-R and HA. Whereas both net-bearing neurons and glial cells were the sources of CSPGs and tenascin-R, only the neurons expressed the mRNA for HA synthases (HASs), cartilage link protein, and link protein Bral2. In the cerebellar cortex, Golgi neurons possessed PNNs and also synthesized HASs, cartilage link protein, and Bral2 mRNAs. To see whether HA might link PNNs to the neuronal cell surface by binding to a receptor, we investigated the expression of the HA receptors CD44, RHAMM, and LYVE-1. No immunolabelling for HA receptors on the membrane of net-bearing neurons was found. We therefore propose that HASs, which can retain HA on the cell surface, may act as a link between PNNs and neurons. Thus, HAS and link proteins might be key molecules for PNN formation and stability.

Immature developmental pattern of the monosynaptic reflex in isolated spinal cord of glial mutant taiep rats

There is increasing evidence suggesting that glial cells play a crucial role in the formation and maturation of neural circuits. However, little is known about the effects of glial alterations on the establishment of functional circuitry in vivo during the development. The taiep rat, a long-lived neurological mutant characterized by early astrogliosis and demyelination affecting selectively the CNS, provides an interesting model to study the glia-neuron interaction in situ. In the present study, we evaluated the functional development of segmental neural circuits recording the monosynaptic reflex responses (MSR) in the isolated spinal cord of neonatal taiep rats. To evaluate the developmental changes during the first two postnatal weeks, we measured the latency of MSR, the magnitude of depression to paired pulses and the time course of post-tetanic recovery. During the early postnatal period, the MSR of control rats reduced their latency and decreased their sensitivity to depression, as a function of age. By contrast, the MSR of taiep rats failed to develop further from neonatal stage. Near the end of the second postnatal week, the MSR latencies were still prolonged, and the MSR showed a significantly stronger paired pulse depression, and higher post-tetanic recovery times than the age-matched controls. The lack of MSR maturation in taiep rats suggests an early alteration of functional mechanisms underlying the maturation of the spinal reflexes, probably due to the characteristic glial dysfunction(s) of this mutant.

Aggrecan glycoforms contribute to the molecular heterogeneity of perineuronal nets

The perineuronal net forms the extracellular matrix of many neurons in the CNS, surrounding neuron cell bodies and proximal dendrites in a mesh-like structure with open "holes" at the sites of synaptic contacts. The perineuronal net is first detected late in development, approximately coincident with the transformation of the CNS from an environment conducive to neuronal growth and motility to one that is restrictive, suggesting a role for the perineuronal net in this developmental transition. Perineuronal nets show a great degree of molecular heterogeneity. Using monoclonal antibodies Cat-301, Cat-315, and Cat-316, we have shown previously that although all antibodies recognize chondroitin sulfate proteoglycans of similar sizes, each antibody recognizes perineuronal nets on distinct but overlapping sets of neurons in the adult cat CNS. An understanding of the heterogeneity demonstrated by these antibodies is critical to understanding the organization and function of perineuronal nets. Using aggrecan knock-out mice (cmd), we have now determined that all three antibodies recognize aggrecan. Chemical and enzymatic deglycosylation show that the differences revealed by the three antibodies arise from differential glycosylation of aggrecan. We further demonstrate that aggrecan mRNA is expressed relatively late in development and that neurons themselves are likely the predominant cellular sites of aggrecan expression. This work indicates that neurons can directly regulate the composition of their extracellular matrix by regulated synthesis and differential glycosylation of aggrecan in a cell type-specific manner. These results have important implications for the role of regulated microheterogeneity of glycosylation in the CNS.

Intact aggrecan and fragments generated by both aggrecanse and metalloproteinase-like activities are present in the developing and adult rat spinal cord and their relative abundance is altered by injury

Aggrecan is a large proteoglycan (PG) that has been grouped with different PG families on the basis of its physical characteristics. These families include the chondroitin sulfate PGs, which appear to inhibit the migration of cells and axons during development. Although aggrecan has been studied primarily in cartilage, in the present study, tissue samples from developing, mature, and injured-adult rat spinal cords were used to determine whether aggrecan is present in the mammalian spinal cord. By the use of Western blot analysis, tissues were probed with aggrecan-specific antibodies (ATEGQV, TYKHRL, and LEC-7) and aggrecan-specific neoepitope antibodies (NITEGE, FVDIPEN, and TFKEEE) to identify full-length aggrecan and several fragments. Unlike many other aggrecan gene family members, aggrecan species were similar in embryonic day 14, postnatal day 1, and adult spinal cords. Spinal cord injury caused significant decreases in aggrecan. Partial recovery in some aggrecan species was evident by 2 weeks after injury. The presence of specific aggrecan neoepitopes suggested that aggrecan is cleaved in the spinal cord by both a disintegrin and metalloproteinase thrombospondin (also known as aggrecanase) and metalloproteinase-like activities. Many aggrecan species found in the spinal cord were similar to species in cartilage. Additional antibodies were used to identify two other aggrecan gene family members, neurocan and brevican, in the adult spinal cord. These studies present novel information on the aggrecan core protein species and enzymes involved in aggrecan cleavage in vivo in the rat spinal cord throughout development and after injury. They also provide the basis for investigating the function of aggrecan in the spinal cord.

Development of inhibitory synaptic transmission to motoneurons

The inhibitory effects of the neurotransmitters glycine and gamma-aminobutyric acid (GABA) on motoneurons and their role in mediating the timing of motor output have been understood for some years. Recent work, however, has revealed that these neurotransmitters function very differently in developing motor circuits. Most strikingly, both GABA and glycine depolarize neonatal motoneurons, and, in many instances, provide excitatory drive to developing motor networks. Additionally, the relative contributions of GABA and glycine to inhibitory synaptic transmission in a circuit or, indeed, within the same synapse, change with postnatal development. Here, we review three fundamental properties of inhibitory neurotransmission that are altered postnatally and may be important in shaping the unique behaviors of these synapses early in development.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Development of Cat-301 immunoreactivity in auditory brainstem nuclei of the gerbil

The developing brainstem auditory system has been studied in detail by using anatomical and physiological techniques. However, it is not known whether immature auditory neurons exhibit different molecular characteristics than those of physiologically mature neurons. To address this issue, we examined the distribution of Cat-301 immunoreactivity in the developing auditory brainstem of gerbils. Cat-301 is a monoclonal antibody that recognizes a 680-kD chondroitin sulfate proteoglycan similar to aggrecan, a high-molecular-weight chondroitin sulfate proteoglycan found in cartilage. In the central nervous system, Cat-301 immunoreactivity is localized to the extrasynaptic surface of neurons. It has been hypothesized by Hockfield and co-workers (Hockfield et al. [1990a]Cold Spring Harbor Symp. Quart. Biol. 55:504-514) that the Cat-301 proteoglycan is a molecular marker indicating that a neuron has acquired mature neuronal properties. In the current study, Cat-301 staining is first seen at 7 days after birth in the anterior ventral cochlear nucleus (AVCN), the posterior VCN (PVCN), and the medial nucleus of the trapezoid body (MNTB) shortly before the onset of sound-evoked activity. By 21 days after birth, neurons in the AVCN, the PVCN, and the lateral and medial superior olive have attained adult-like distributions of Cat-301 staining concomitant with the physiological maturation of these neurons. Neurons in MNTB attain adult-like distributions of Cat-301 immunoreactivity at 1 year. The maturation of Cat-301 immunoreactivity parallels the physiological maturation of gerbil auditory neurons, and the Cat-301 proteoglycan may play a role in the formation and/or stabilization of auditory synapses.

A family of activity-dependent neuronal cell-surface chondroitin sulfate proteoglycans in cat visual cortex

Monoclonal antibody Cat-301 recognizes a chondroitin sulfate proteoglycan (CSPG) expressed on the extracellular surface of cell bodies and proximal dendrites of specific subsets of neurons in many areas of the mammalian CNS, including the cat visual cortex. The Cat-301 CSPG is first detected at the close of the critical period in development, a period during which the pattern of neuronal activity determines the mature synaptic circuitry and neuronal phenotype. In the cat visual cortex, dark-rearing from birth prolongs the duration of the critical period and attenuates the expression of the Cat-301 antigen, implicating the Cat-301 CSPG in the cellular mechanisms that terminate the period of synaptic plasticity. Because the Cat-301 antigen is expressed on only a limited subset of neurons, we have further examined the molecular heterogeneity among neuronal cell-surface CSPGs and have asked (1) whether other neuronal subsets carry distinct CSPGs and (2) whether the activity-dependent expression of the Cat-301 CSPG is a property generalizable to related cell-surface CSPGs. Here, we report two new monoclonal antibodies, Cat-315 and Cat-316, which together with Cat-301 define a family of at least seven related yet distinct CSPGs. These three antibodies define nonidentical subsets of neurons in the cat visual cortex. The expression of normal levels of these CSPGs is reduced by dark-rearing. Together, these data show that the family of cell-surface CSPGs is molecularly diverse, that different sets of neurons express distinct complements of cell-surface antigens, and that the regulation of CSPG expression by activity may be a general feature of neuronal cell-surface CSPGs.

SC1, a brain extracellular matrix glycoprotein related to SPARC and follistatin, is expressed by rat cerebellar astrocytes following injury and during development

In the nervous system, extracellular matrix components are believed to influence cell shape, proliferation and migration during development and following injury. SC1 is a secreted glycoprotein expressed during neural development and in the adult brain. The molecule shows partial sequence homology to the anti-adhesive extracellular matrix molecule SPARC/osteonectin and to follistatin. We have made a surgical lesion in the adult rat cerebellum and examined changes in SC1 expression at 1 to 14 days after injury. Dual in situ hybridization/immunohistochemistry demonstrated that SC1 mRNA was induced in astrocytes surrounding the wound, reaching maximal levels at 10 days post-lesion. Immunohistochemistry revealed changes in the deposition of SC1 protein in radial fibres of Bergmann glia. SC1 protein was also detected at the border of the lesion, suggesting an association with the glial scar. Double immunohistochemistry with the astrocytic marker GFAP demonstrated that astrocytes also express SC1 during postnatal development.

Early postnatal muscle contractile activity regulates the carbonic anhydrase phenotype of proprioceptive neurons in young and mature mice: evidence for a critical period in development

Carbonic anhydrase activity, a marker of mouse proprioceptive neurons in adult dorsal root ganglia, is first detectable in the perinatal period, increases until postnatal day 60 and remains stable in adulthood. The onset of carbonic anhydrase staining begins after the neurons have made connections with their targets suggesting that neuron-target interactions regulate carbonic anhydrase phenotype development. To examine this possibility, we first analysed carbonic anhydrase expression in mdx mice which are characterized by a massive but reversible degeneration of skeletal muscle concomitant with the carbonic anhydrase ontogenesis. Neuronal carbonic anhydrase expression in mdx mice stopped developing when the period of muscular degeneration-regeneration began. Furthermore this alteration persisted during adulthood. We then analysed carbonic anhydrase expression in fifth lumbar dorsal root ganglion of developing control mice before and after surgical procedures that might interfere with central and peripheral target influences on dorsal root ganglion neurons. Central disconnection (dorsal rhizotomy) did not affect the development of carbonic anhydrase activity. Disrupting neuron-peripheral target interactions by sciatic nerve transection or blocking muscle contraction by tenotomy stopped the development of neuronal carbonic anhydrase content. Finally, recovery was monitored following sciatic nerve crush. In adults, recovery of carbonic anhydrase activity was obtained after functional recuperation; similar manipulations during the first month of life induced irreversible alteration of the carbonic anhydrase phenotype. These results show that the development of carbonic anhydrase activity in proprioceptive neurons is regulated by neuron-muscle interactions (i.e. muscle contraction). They also provide evidence for a critical period in the development of the carbonic anhydrase phenotype. We suggest that these two mechanisms are responsible for the altered carbonic anhydrase phenotype of the dorsal root ganglion neurons in mdx mice, a model of human muscular dystrophy.

Effects of early periods of monocular deprivation and reverse lid suture on the development of Cat-301 immunoreactivity in the dorsal lateral geniculate nucleus (dLGN) of the cat

During certain sensitive periods early in postnatal life, the anatomical and physiological development of the central visual pathways of cats and monkeys can be affected by the nature of an animal's early visual experience. In the last few years, studies have been started on some of the molecular and biochemical events that underlie the many functional changes induced by early selected visual deprivation in the visual cortex of kittens. In this respect, the monoclonal antibody Cat-301 provides a potentially powerful tool, because it recognizes in the cat dorsal lateral geniculate nucleus (dLGN) a proteoglycan associated with the surface of a particular class of cells, namely Y cells. In the dLGN, the Cat-301 proteoglycan appears late in postnatal development, and it expression has been shown to be experience dependent in both the dLGN and visual cortex (M. Sur, D. Frost, and S. Hockfield, 1988, J. Neurosci. 8:874-882; A. Guimaraes, S. Zaremba, and S. Hockfield, 1990, J. Neurosci. 10:3014-3024). We have explored further the experience-dependent nature of Cat-301 expression in the dLGN with a view to establishing a biochemical correlate of the many functional changes induced by early monocular deprivation and its reversal in the kitten visual system. In addition to demonstrating differences in Cat-301 expression between deprived and nondeprived laminae of the dLGNs of kittens monocularly deprived to only 4 or 5 weeks of age, the magnitude of the laminar difference was found to increase as the period of deprivation was extended. Moreover, monocularly deprived kittens that subsequently received long periods of reverse lid suture exhibited a reversal of the pattern of immunoreactivity, so that the greatest immunoreactivity occurred in laminae innervated by the initially deprived eye. However, possibly the most surprising and important finding was the extremely low levels of immunoreactivity observed in both A laminae of monocularly deprived animals that had received relatively short periods of reverse lid suture. These data suggest that Y cell development can be drastically altered depending on the time of initiation of the period of reverse lid suture and its duration.

In situ hybridization analysis of AMPA receptor subunit gene expression in the developing rat spinal cord

In early postnatal life the acquisition of mature morphological and molecular features of motor neurons is influenced by synaptic activity within the spinal cord. Glutamatergic synaptic neurotransmission is believed to play a central role in this process. We hypothesize that the repertoire of glutamate receptors expressed by neurons in the young spinal cord differ from those expressed in adults and such receptors support activity-dependent developmental plasticity. To explore this idea, we used in situ hybridization histochemistry to determine the distribution, temporal expression, and potential subunit composition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the developing rat spinal cord and compared these findings with those in adult rats. We find qualitative and quantitative changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit gene expression over the first month of postnatal life. alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit genes GluR1, 2 and 4 are expressed at greater levels throughout the spinal cord of the neonate versus the adult animals. The developmental down-regulation is most pronounced for GluR1 transcripts, less for GluR2 and GluR4 transcripts, and minimal for GluR3 transcripts. Analysis of flip and flop splice variants of each subunit show that receptors expressed by adult motor neurons are potentially composed of the subunits GluR1 flop, GluR2 flip, GluR3 flip and flop, and GluR4 flip. In neonatal motor neuron all subunits are potentially expressed (except GluR2 flop) with quantitatively the dominent subunits being the flip splice variants of GluR1, 2 and 4. Receptors in the substantia gelatinosa undergo equally dramatic, developmentally independent changes. Changes in the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit composition are likely to have an important effect on the electrophysiological properties of motor neurons and may form part of the molecular identity of neurons capable of undergoing activity-dependent developmental plasticity.

Chondroitin sulfate proteoglycan staining in astrocyte-Schwann cell co-cultures

Transplantation of Schwann cells (SCs) in the central nervous system (CNS) for remyelination in pathological situations has been considered a promising approach. However, numerous studies have indicated that astrocytes have a restrictive effect on SC migration within the CNS. We have previously established an in vitro model which demonstrates the restrictive effect of astrocytes on SCs (Ghirnikar and Eng, Glia 4:367-377, 1994). Using this culture model, in the present study, we have characterized the molecular basis underlying astrocyte-SC interaction and demonstrated chondroitin sulfate proteoglycan (CSP) staining in the co-cultures. Following 1-2 weeks of incubation, CSP staining was specifically associated with SCs co-cultured with astrocytes. Staining with antibodies specific for the different chondroitin sulfate isomers revealed the presence of both, chondroitin-4- and 6-sulfates in SCs. In contrast, SCs when cultured alone, or in the presence of astrocytes conditioned medium did not show CSP staining. These data suggest that CSP staining is associated with SCs following co-culture with astrocytes and mediated by cell to cell contact. We hypothesize that the CSP, alone or in combination with other molecules expressed by astrocytes and/or SCs, may be involved in the restrictive effects of astrocytes on SCs. Identification of molecules involved in the unfavorable interaction between astrocytes and SCs will have an important bearing on efforts to remyelinate demyelinated axons by SC transplantation within the damaged CNS.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Perineuronal nets--a specialized form of extracellular matrix in the adult nervous system

One century ago, Camillo Golgi described 'perineuronal nets' enwrapping the cell bodies and proximal dendrites of certain neurons in the adult mammalian central nervous system and suggested that they represent a supportive and protective scaffolding. Although other neuroanatomists validated the existence of these nets on selected neurons in the adult brain, there was a lack of agreement on their origins, composition and function. The application of modern molecular and ultrastructural methods has brought new insights and a renewed interest in these classic observations. Recent data suggest that perineuronal nets result from the visualization of extracellular matrix molecules that are confined to the space interposed between glial processes and the nerve cells that they outline. The material confined to these spaces can be visualized selectively by antibodies directed to glycoproteins (e.g., tenascin and restrictin/janusin), proteoglycans (e.g., chondroitin sulfates), markers for hyaluronan as well as by lectins recognizing N-acetylgalactosamine and by monoclonal antibodies directed to epitopes on unknown molecules (e.g., HNK-1, VC1.1 and Cat 301). This review examines the emerging clarification of classical observations of perineuronal nets and the functional implications suggested by their molecular composition. Also discussed are studies that further extend observations on the time of development and of the specificity in the occurrence of perineuronal nets. In the adult brain the molecules constituting the 'perineuronal nets of matrix' could serve as recognition molecules between certain neurons and their surrounding cells and participate in the selection and consolidation of their relationship.

Molecular evidence for nitric oxide-mediated motor neuron development

The complex morphological, electrophysiological and molecular properties of the adult vertebrate nervous system emerge over an extended period in prenatal and early postnatal life. Numerous studies have shown that synaptic activity plays a key role in the postnatal acquisition of mature neuronal phenotype. The cellular and molecular mechanisms subserving activity-dependent development are largely unknown. Several lines of evidence suggest that a rise in intracellular Ca2+ as a consequence of synaptic activity may regulate neuronal differentiation through its interactions with calcium-activated signal transduction molecules such as calcium/calmodulin kinase type II, protein kinase C or nitric oxide synthase (NOS). The aim of the present study is to identify potential signal transduction events subserving postnatal motor neuron development. Here we show that NOS antagonists block the molecular maturation of motor neurons and this effect is likely to be mediated by a subpopulation of ventral horn cells that express NOS transiently during early postnatal life. These results suggest that the local production of nitric oxide within the ventral horn may contribute to a late phase in motor neuron differentiation.

Perineuronal nets provide a polyanionic, glia-associated form of microenvironment around certain neurons in many parts of the rat brain

The nature and function of previously described perineuronal nets are still obscure. In the present study their polyanionic components were demonstrated in the rat brain using colloidal iron hydroxide (CIH) staining. In subcortical regions, such as the red nucleus, cerebellar, and vestibular nuclei, most neurons were ensheathed by CIH-binding material. In the cerebral cortex perineuronal nets were seen around numerous nonpyramidal neurons. Biotinylated hyaluronectin revealed that hyaluronan occurs in perineuronal nets. Two plant lectins [Wisteria floribunda agglutinin (WFA) and Vicia villosa agglutinin (VVA)] with affinity for N-acetylgalactosamine visualized perineuronal nets similar to those rich in anionic components. Glutamic acid decarboxylase (GAD)-immunoreactive synaptic boutons were shown to occupy numerous meshes of perineuronal VVA-positive nets. Electron microscopically, VVA binding sites were scattered throughout perisynaptic profiles, but accumulated at membranes and in the extracellular space except not in synaptic clefts. To investigate the spatial relationship between glial cell processes and perineuronal nets, two astrocytic markers (S100-protein and glutamine synthetase) were visualized at the light and electron microscopic level. Two methods to detect microglia by the use of Griffonia simplicifolia agglutinin (GSA I-B4) and the monoclonal antibody, OX-42, were also applied. Labelled structures forming perineuronal nets were observed with both astrocytic, but not with microglial, markers. It is concluded that perineuronal nets are composed of a specialized type of glia-associated extracellular matrix rich in polyanionic groups and N-acetylgalactosamine. The net-like appearance is due to perisynaptic arrangement of the astrocytic processes and these extracellular components. Similar to the ensheathment of nodes of Ranvier, perineuronal nets may provide a special ion buffering capacity required around various, perhaps highly active, types of neurons.

Large chondroitin sulfate proteoglycans of developing chick CNS are expressed in cerebral hemisphere neuronal cultures

Chondroitin sulfate proteoglycans (CSPG) of the extracellular matrix may play regulatory roles in central nervous system (CNS) development. We have examined the expression of two large CSPGs of the embryonic chick brain, which can be differentiated using the monoclonal antibodies HNK-1 and S103L, in cultures of embryonic day 6 chick cerebral hemisphere neurons. Western blot analysis following immunoprecipitation and endoglycosidase treatment revealed that these cultures produce S103L- and HNK-1-reactive proteoglycans which are biochemically indistinguishable from the CSPGs (previously) identified in homogenized chick embryo brain extracts. The HNK-1-reactive CSPG accumulated in the medium throughout the course of cultures. In contrast, the S103L-reactive CSPG was found in a neuron-associated form during the period of aggregate establishment in culture, as well as in a soluble form secreted into the medium. Immunocytochemical staining of cultures with the S103L antibody localized reactivity to most neurons during the period of aggregate formation, while neuronal processes and the few flat cells present (presumably neuroblasts and early glia) were negative. Cell selection experiments confirmed that neurofilament-positive cells were the source of the S103L-reactive CSPG. The use of differential fixation techniques suggested that the cell-associated S103L reactivity may be intracellular. Because of this pattern of expression and localization, we propose that the developmentally regulated S103L-reactive CSPG may play a role in neuronal migration arrest and organization of neurons into functional aggregates.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Murine retrovirus-induced spongiform encephalopathy: disease expression is dependent on postnatal development of the central nervous system

In this report, we have examined the role of central nervous system (CNS) development in the pathogenesis of neurodegenerative disease induced by murine retroviruses. This was accomplished by comparing the effect of delivering viruses, with either severe or marginal neurovirulence (J. L. Portis, S. Czub, C. F. Garon, and F. J. McAtee, J. Virol. 64:1648-1656, 1990), during the midgestational development of the mouse (gestation days 9 to 10). Midgestation inoculation of the marginally neurovirulent virus, 15-1, resulted in high level CNS infection, as determined by viral DNA and protein analysis. The high-level infection resulted in rapid, severe disease with 100% incidence and an average clinical onset on postnatal day 17 (P17). The disease onset was comparable to that observed for the highly neurovirulent virus, FrCasE, when inoculated neonatally (onset ca. P16). To evaluate whether disease could be induced even earlier in CNS development, FrCasE was inoculated during midgestation. Surprisingly, neither clinical nor histological manifestations of CNS disease were accelerated but rather appeared at the same developmental time as seen for neonatally inoculated animals (onset of neuropathology at ca. P10; onset of clinical disease at ca. P15). CNS infection, on the other hand, occurred at earlier times (< P0), at higher levels, and with a broader distribution than in neonatally inoculated animals. No infection of the neurons which ultimately degenerate was observed in any regimen of virus inoculation. It was observed, however, that the gp70 viral envelope protein from the CNS showed an increase mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis compared with the envelope protein from infected spleens or purified virions. These results indicate that a postnatal developmental event must occur to allow the presence of a neurovirulent virus to precipitate spongiform degeneration and that an altered envelope protein may be participating in the process.

Activity-dependent structural changes during neuronal development

Activity during the early postnatal period can have a pronounced effect on the structure of neurons in the central nervous system. Recent studies in the cat visual system and in the vertebrate and invertebrate neuromuscular system, have provided new insights into the cellular and molecular features of this process.

N-methyl-D-aspartate receptors are transiently expressed in the developing spinal cord ventral horn

Quantitative receptor autoradiography was used to map the distribution of N-methyl-D-aspartate (NMDA) receptors in the developing rat spinal cord. Three different specific ligands, which label partially overlapping subpopulations of NMDA receptors, were used: an agonist (L-[3H]glutamate), a noncompetitive antagonist ([3H]MK-801), and a competitive antagonist ([3H]CGP-39653). In the adult, NMDA receptors labeled with all three ligands are restricted to the substantia gelatinosa in the spinal dorsal horn. In marked distinction, at postnatal day 7 NMDA receptors labeled with L-[3H]glutamate and [3H]MK-801 are present throughout the spinal gray matter. NMDA receptors in the neonatal spinal ventral horn have a higher affinity for L-[3H]glutamate than those in the adult substantia gelatinosa. Over the second and third postnatal weeks, NMDA receptors are lost from all areas of the spinal gray matter except for the substantia gelatinosa. Neonatal NMDA receptors identified with [3H]CGP-39653 are restricted to the substantia gelatinosa. These results show that the immature ventral horn contains a subpopulation of NMDA receptors and raise the possibility that motor neurons transiently express NMDA receptors in early postnatal life. Ventral horn NMDA receptors may be a component of the mechanisms by which the mature phenotype of motor neurons is acquired through activity-dependent processes. The loss of NMDA receptors over the course of development may play a role in limiting the period of motor neuron plasticity.

Activity-dependent development of spinal cord motor neurons

Patterned neuronal activity in early postnatal life can regulate the acquisition of the mature morphological and electrophysiological properties of neurons. Many properties of motor neurons are developmentally regulated and may be influenced by epigenetic factors. The pattern of activation of motor neurons can regulate axon terminal morphology and synaptic efficacy at the neuromuscular junction. Motor neuron morphology and synaptic connections can also be modified by exposure to specific hormones in the early postnatal period. The acquisition of mature physiological and anatomical properties is paralleled by the acquisition of specific molecular properties. Recent experiments using molecular markers for motor neuron differentiation indicate that motor neurons undergo activity-dependent development during a circumscribed period in early postnatal life. Normal motor neuron differentiation requires a normal pattern of neuronal activity in early postnatal life. Differentiation also requires activation of the NMDA receptor over the same time period. The activity-dependent development of morphological, electrophysiological and molecular properties of motor neurons is similar to activity-dependent development in the vertebrate visual system. The neuromuscular system may provide an accessible system for characterizing the molecules subserving the translation of patterned neuronal activity into mature neuronal phenotype.

Distribution of the novel developmentally-regulated protein EAP-300 in the embryonic chick nervous system

In a previous study we described a 300 kDa, developmentally regulated protein identified in embryonic chick neural retina with a monoclonal antibody. Because this protein has been shown to be undetectable in the adult nervous system, and the monoclonal antibody is species-specific, the protein has been named embryonal avian polypeptide of 300 kDa (EAP-300). In the present study we have analyzed the histological expression of EAP-300 during chick embryogenesis. In the developing nervous system, EAP-300 expression was detected as early as Stage 5 (19 h), and was subsequently down-regulated to undetectable levels in the adult. Of particular interest was the association of EAP-300 with putative barriers of neuronal growth, such as the telencephalon/diencephalon glial knot, the dorsal midline in the mesencephalon and the midline in myelencephalon, and the spinal cord roof plate. EAP-300 was also shown to be expressed by Bergmann glia during the period of granule cell migration in the cerebellum. The expression of EAP-300 by radial astrocytes was confirmed in culture by immunofluorescent co-labeling with a MAb to EAP-300 and the R5 MAb, which is a radial astrocyte-specific marker. It has also been shown that EAP-300, when immunopurified from embryonic brain under non-dissociating conditions, co-purifies with a neural keratan sulfate proteoglycan that is also associated with CNS barrier structures during brain development. The restricted expression of EAP-300 in nervous tissue, particularly in CNS barrier structures, suggests that EAP-300 may play an important, but transient, role in the development of the chick nervous system.

Characterization of a cell surface adhesion molecule expressed by a subset of developing chick neurons

We have isolated a 105-kDa membrane glycoprotein expressed by subsets of developing chick neurons. This glycoprotein, identified by the JC7 monoclonal antibody, is present on the surface of axons and cell bodies of developing spinal motor neurons, dorsal root ganglion sensory neurons, sympathetic and parasympathetic neurons, and a small subset of brain neurons. Late in development the JC7 antigen is expressed at high levels on CNS nonneuronal glial-like cells. When attached to latex beads this glycoprotein can mediate homophilic adhesion and when used as a culture substrate stimulates a highly branched pattern of neurite outgrowth from dorsal root ganglion explants. The JC7 antigen appears to be identical to the SC1, BEN, and DM antigens. Its limited distribution, adhesive qualities, and ability to stimulate neurite outgrowth suggest it may play a role in the selective growth of neural processes during development.

Chondroitin sulfate proteoglycan surrounds a subset of human and rat CNS neurons

Chondroitin sulfate proteoglycan (CS-PG) bearing glycosaminoglycan (GAG) chains containing unsulfate (COS) and 6-sulfate (C6S) disaccharides was immunolocalized in rat and human CNS by using monoclonal antibodies (MAb) specific for the two disaccharides. The immunostaining with both MAb was restricted to the periphery of a neuronal subset in rat and human CNS. Double immunofluorescence showed codistribution of the antigens around the same neuronal population. The staining with anti-COS MAb was stronger than with anti-C6S MAb, suggesting that the proteoglycan (PG) contains mainly COS disaccharides. In different rat cortical areas, 40-60/mm2 positive interneurons were found, the visual cortex showing the highest value. In human cortex, positivity was also observed around the soma of some pyramidal cells. In the rat, positive neurons were also localized in deep cerebellar nuclei, reticular nucleus of the thalamus, and other structures of the midbrain and hindbrain. CA3 region of hippocampus and the external layer of pyriform cortex were characterized by positivity of the neuropil. Immunoelectronmicroscopy showed the antigens in the extracellular space around the neuronal soma, the synaptic elements and the cell processes of the neuropil. The neuronal surface of the soma and of the proximal dendrites were positive, but the pre- and postsynaptic membranes and clefts were negative.