Ion exchanger in the brain: Quantitative analysis of perineuronally fixed anionic binding sites suggests diffusion barriers with ion sorting properties.

Perineuronal nets (PNs) are a specialized form of brain extracellular matrix, consisting of negatively charged glycosaminoglycans, glycoproteins and proteoglycans in the direct microenvironment of neurons. Still, locally immobilized charges in the tissue have not been accessible so far to direct observations and quantifications. Here, we present a new approach to visualize and quantify fixed charge-densities on brain slices using a focused proton-beam microprobe in combination with ionic metallic probes. For the first time, we can provide quantitative data on the distribution and net amount of pericellularly fixed charge-densities, which, determined at 0.4-0.5 M, is much higher than previously assumed. PNs, thus, represent an immobilized ion exchanger with ion sorting properties high enough to partition mobile ions in accord with Donnan-equilibrium. We propose that fixed charge-densities in the brain are involved in regulating ion mobility, the volume fraction of extracellular space and the viscosity of matrix components.

Tenascin-R promotes assembly of the extracellular matrix of perineuronal nets via clustering of aggrecan.

Perineuronal nets (PNs) in the brains of tenascin-R-deficient (tn-r(-/-)) mice develop in temporal concordance with those of wild-type (tn-r(+/+)) mice. However, the histological appearance of PNs is abnormal in adult tn-r(-/-) mice. Here, we investigated whether similar defects are also seen in dissociated and organotypic cultures from hippocampus and forebrain of tn-r(-/-) mice and whether the structure of PNs could be normalized. In tn-r(-/-) cultures, accumulations of several extracellular matrix molecules were mostly associated with somata, whereas dendrites were sparsely covered, compared with tn-r(+/+) mice. Experiments to normalize the structure of PNs in tn-r(-/-) organotypic slice cultures by depolarization of neurons, or by co-culturing tn-r(+/+) and tn-r(-/-) brain slices failed to restore a normal PN phenotype. However, formation of dendritic PNs in cultures was improved by the application of tenascin-R protein and rescued by polyclonal antibodies to aggrecan and a bivalent, but not monovalent form of the lectin Wisteria floribunda agglutinin. These results show that tenascin-R and aggrecan are decisive contributors to formation and stabilization of PNs and that tenascin-R may implement these functions by clustering of aggrecan. Proposed approaches for restoration of normal PN structure are noteworthy in the context of PN abnormalities in neurological disorders, such as epilepsy, schizophrenia and addiction.

Involvement of perineuronal and perisynaptic extracellular matrix in Alzheimer's disease neuropathology.

Brain extracellular matrix (ECM) is organized in specific patterns assumed to mirror local features of neuronal activity and synaptic plasticity. Aggrecan-based perineuronal nets (PNs) and brevican-based perisynaptic axonal coats (ACs) form major structural phenotypes of ECM contributing to the laminar characteristics of cortical areas. In Alzheimer's disease (AD), the deposition of amyloid proteins and processes related to neurofibrillary degeneration may affect the integrity of the ECM scaffold. In this study we investigate ECM organization in primary sensory, secondary and associative areas of the temporal and occipital lobe. By detecting all major PN components we show that the distribution, structure and molecular properties of PNs remain unchanged in AD. Intact PNs occurred in close proximity to amyloid plaques and were even located within their territory. Counting of PNs revealed no significant alteration in AD. Moreover, neurofibrillary tangles never occurred in neurons associated with PNs. ACs were only lost in the core of amyloid plaques in parallel with the loss of synaptic profiles. In contrast, hyaluronan was enriched in the majority of plaques. We conclude that the loss of brevican is associated with the loss of synapses, whereas PNs and related matrix components resist disintegration and may protect neurons from degeneration.

Perineuronal nets are largely unaffected in Alzheimer model Tg2576 mice.

Changes in the molecular organization of the extracellular matrix are key factors in neuropathology. We investigated aggrecan-based perineuronal nets (PNs) in relation to neurodegeneration and activation of glial cells in a transgenic mouse (Tg2576) model of Alzheimer's disease. The formation of amyloid plaques in the cerebral cortex occurred independently of the area-specific distribution of PNs. Matrix components were only affected in the core of plaques in advanced stages of pathology. PNs remained unchanged in the large marginal zone occupied by reactive astrocytic processes. We conclude that the aggrecan-based extracellular matrix of PNs is not enzymatically altered in peripheral plaque territories and is only removed after neuronal death.

Chondroitin sulfate proteoglycan-based extracellular matrix in chicken (Gallus domesticus) brain.

A specialised form of extracellular matrix consisting of large aggregating chondroitin sulphate proteoglycans connected to hyaluronan and tenascins, as main components, is termed perineuronal nets. These perineuronal nets surround subpopulations of neurons in many vertebrates including man. In this study we investigated the distribution and the postnatal development of perineuronal nets in the brain of the domestic chicken using immunohistochemical, lectin-histochemical and biochemical methods. Perineuronal nets could be identified very early, already on the first postnatal day throughout various regions and nuclei in chicken fore- and midbrains, most expressively in nidopallium, hyperpallium, lateral striatum, globus pallidus and mesopallium. These mostly delicate, scanty structures around the cell bodies of neurons thicken and complete during the first 2 weeks, however, differ in shape and clearness of contours from the mature form of perineuronal nets found in the adult, 3 year old animals. Perineuronal nets frequently co-localized with the potassium channel subunit Kv3.1b characteristic for fast spiking neurons but remained unrevealed around cholinergic or monoaminergic neurons. The early appearance of perineuronal nets in the precocial birds' brain is probably due to the rapid establishment of neuronal morphology and function which is required for the immediate functional and behavioural performance of chicken.

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.

Activity-dependent formation and functions of chondroitin sulfate-rich extracellular matrix of perineuronal nets.

Extracellular matrix molecules--including chondroitin sulfate proteoglycans, hyaluronan, and tenascin-R--are enriched in perineuronal nets (PNs) associated with subsets of neurons in the brain and spinal cord. In the present study, we show that similar cell type-dependent extracellular matrix aggregates are formed in dissociated cell cultures prepared from early postnatal mouse hippocampus. Starting from the 5th day in culture, accumulations of lattice-like extracellular structures labeled with Wisteria floribunda agglutinin were detected at the cell surface of parvalbumin-expressing interneurons, which developed after 2-3 weeks into conspicuous PNs localized around synaptic contacts at somata and proximal dendrites, as well as around axon initial segments. Physiological recording and intracellular labeling of PN-expressing neurons revealed that these are large fast-spiking interneurons with morphological characteristics of basket cells. To study mechanisms of activity-dependent formation of PNs, we performed pharmacological analysis and found that blockade of action potentials, transmitter release, Ca2+ permeable AMPA subtype of glutamate receptors or L-type Ca2+ voltage-gated channels strongly decreased the extracellular accumulation of PN components in cultured neurons. Thus, we suggest that Ca2+ influx via AMPA receptors and L-type channels is necessary for activity-dependent formation of PNs. To study functions of chondroitin sulfate-rich PNs, we treated cultures with chondroitinase ABC that resulted in a prominent reduction of several major PN components. Removal of PNs did not affect the number and distribution of perisomatic GABAergic contacts but increased the excitability of interneurons in cultures, implicating the extracellular matrix of PNs in regulation of interneuronal activity.

Distribution of pyramidal cells associated with perineuronal nets in the neocortex of rat.

Perineuronal nets are lattice-like accumulations of extracellular matrix components around the cell body and perisomatic portion of certain neurons. Whereas interneurons associated to this specific neuron-associated sheath have been elaborately classified, less effort has been undertaken to describe the occurrence of perineuronal nets around pyramidal neurons. Our aim was to give a detailed and comparative description of the occurrence of net-associated pyramidal cells throughout the rat neocortex as well as to systematically and comparatively analyze the relation of main projection types of principal neurons to the presence of perineuronal nets. The present study revealed that perineuronal nets stained with WFA were associated rather rarely to pyramidal cells compared to interneurons in layers II/III and V/VI of rat neocortex. However, their frequency was considerably different between various cortical areas with a maximum in visual cortex and with a minimum in secondary motor cortices. Further analysis revealed that neuron-associated matrix sheaths around principal cells were more common in the primary than in the secondary fields of corresponding areas and they were more numerous in infra-than in supragranular layers in most regions. Subfields of cortical areas also differed regarding the occurrence of net-associated principal cells, and the subtlety of cortical representation seemed to correlate with the frequency of perineuronal nets around pyramidal neurons in the primary somatosensory cortex. It appears that net-associated pyramidal cells do not have a projection pattern restricted to distinct target regions. Rather a functional heterogeneity of the pyramidal cell population contributing to specific intra-or subcortical projections is suggested.

Organization of brain extracellular matrix in the Chilean fat-tailed mouse opossum Thylamys elegans (Waterhouse, 1839).

We investigated the structural and molecular organization of the extracellular matrix in Thylamys elegans, a marsupial representative of the mammalian order Didelphimorphia. Perineuronal nets (PNs) associated with distinct types of neurons were visualized by detection of chondroitin sulfate proteoglycans and hyaluronan, and by labeling with Wisteria floribunda agglutinin (WFA), a marker for PNs in the mammalian brain. In the neocortex of Thylamys, these methods revealed PNs on pyramidal cells. In contrast, parvalbumin-immunoreactive interneurons in the neocortex and hippocampal formation (displaying robust, WFA-labeled PNs in placental mammals) were ensheathed only with a delicate rim of hyaluronan and proteoglycans not detectable with WFA. The absence of WFA staining was characteristic also of some subcortical regions which contained PNs intensely labeled for chondroitin sulfate proteoglycan and hyaluronan. However, corresponding to placental mammals, numerous subcortical nuclei showed clearly WFA-stained PNs. Similar as in placental mammals, cholinergic basal forebrain neurons and tyrosine hydroxylase-immunoreactive neurons of the substantia nigra and locus coeruleus were devoid of PNs. Together with our earlier study on Monodelphis, the present results reveal that South American opossums show either a particular "marsupial" or "Didelphid" type of extracellular matrix chemoarchitecture, supporting the view that these components may vary phylogenetically as integral parts of neuronal physiology at the systems and single cell level.

Decomposition and long-lasting downregulation of extracellular matrix in perineuronal nets induced by focal cerebral ischemia in rats.

The upregulation of extracellular matrix components, especially chondroitin sulfate proteoglycans, after brain injury and stroke is known to accompany the glial reaction, forming repellent scars that hinder axonal growth and the reorganization of the injured neuronal networks. The extracellular matrix associated with perineuronal nets (PNs) in the primarily injured and remote regions has not yet been systematically analyzed. We use the model of permanent middle cerebral artery occlusion (MCAO) to investigate the acute and long-lasting consequences of ischemia for PNs, related to the damage of neurons and reactions of glial cells, in spontaneously hypertensive rats. Extracellular matrix components associated with PNs around cortical interneurons and neurons in thalamic nuclei were characterized 1, 7, 14, and 35 days after MCAO, using Wisteria floribunda agglutinin (WFA) staining and immunocytochemistry. The degradation of PNs in the infarct core was initiated by loss of WFA-binding matrix components, indicating the cleavage of glycosaminoglycan chains of chondroitin sulfate proteoglycans. Immunostaining showed the subsequent removal of proteoglycan core proteins within the extending microglia/macrophage invasion zone lasting for 2 weeks after MCAO. In the cortical periinfarct region, delineated by an astrocytic scar against the infarct core, the number of WFA-stained and proteoglycan core protein-immunoreactive PNs was permanently reduced. In the homolateral ventroposterior thalamus, the delayed decrease in perineuronal matrix was related to the distribution pattern of activated microglia and massive neuronal degeneration. It can be concluded from these results that complementary to the known upregulation of matrix components in the glial scar, deficits in the expression of the neuron-associated extracellular matrix develop in the periinfarct and remote regions. These deficits may contribute to the long-lasting functional impairments after stroke.

Perineuronal nets characterized by vital labelling, confocal and electron microscopy in organotypic slice cultures of rat parietal cortex and hippocampus.

Perineuronal nets (PNs) of the extracellular matrix have been shown to develop in organotypic slice cultures largely corresponding with regional patterns known from in vivo experiments. In the present study, we use vital labelling to investigate aspects of the cell type-dependent development of PNs associated with nonpyramidal neurons and pyramidal cells in the parietal cortex and hippocampus. Frontal sections were cut from brains of 3-5-day-old rats and were cultured for 3-5 weeks. PNs were sequentially labelled using biotinylated Wisteria floribunda agglutinin and chromogen-tagged streptavidin either in living slice cultures, examined by confocal microscopy in vitro, or in cultures examined by confocal and electron microscopy after fixation. Nonpyramidal and pyramidal cells were characterized by immunoreaction for parvalbumin and the ionotropic glutamate receptor subunits 2/3. Vital labelling and examination of fixed slices correspondingly revealed that large numbers of PNs developed around cortical and hippocampal interneurons under depolarizing conditions induced by elevated external potassium concentration. After culture in standard medium, PNs were mainly found in association with subpopulations of pyramidal cells in the parietal cortex. PNs showed ultrastructural characteristics resembling those known from perfusion-fixed brain. A zone of labelled extracellular matrix aggregates was found in close proximity to the neuronal cell surface, surrounding presynaptic boutons and preterminal axons. The results show that characteristic features of PNs are retained after vital labelling in slice cultures. Moreover, our findings suggest that the cell type-specific development of PNs is regulated by patterns of intrinsic activity mediated by intra-cortical and -hippocampal synaptic contacts on potentially net-associated neurons.

Perineuronal nets potentially protect against oxidative stress.

A specialized form of extracellular matrix (ECM) termed perineuronal nets (PNs) consisting of large aggregating chondroitin sulfate proteoglycans (CSPGs), with hyaluronan and tenascin as main components, surrounds subpopulations of neurons. The glycosaminoglycan components of perineuronal nets form highly charged structures in the direct microenvironment of neurons and thus might be involved in local ion homeostasis. The polyanionic character suggests that perineuronal nets also potentially contribute to reduce the local oxidative potential in the neuronal microenvironment by scavenging and binding redox-active iron, thus providing some neuroprotection to net-associated neurons. Here, we show that neurons ensheathed by a perineuronal net in the human cerebral cortex are less frequently affected by lipofuscin accumulation than neurons without a net both in normal-aged brain and Alzheimer's disease (AD). As lipofuscin is an intralysosomal pigment composed of cross-linked proteins and lipids generated by iron-catalyzed oxidative processes, the present results suggest a neuroprotective function of perineuronal nets against oxidative stress, potentially involved in neurodegeneration.

Region and lamina-specific distribution of extracellular matrix proteoglycans, hyaluronan and tenascin-R in the mouse hippocampal formation.

The extracellular matrix is known to show region-specific characteristics in the adult brain. Our comparative cytochemical study is focused on the laminar organisation of major extracellular matrix constituents in the murine hippocampal formation, including the regions CA1, CA2 and CA3 of the hippocampus proper, the dentate gyrus, the subiculum and the presubiculum. Components related to chondroitin sulphate proteoglycans were detected by N-acetylgalactosamine-binding Wisteria floribunda agglutinin, colloidal iron staining, and antibodies to different proteoglycan domains, including the Cat-301 and Cat-315 epitopes of aggrecan, as well as neurocan, brevican and phosphacan. The distribution patterns of these components were correlated with the patterns revealed for hyaluronan and the brain-specific extracellular matrix glycoprotein, tenascin-R, known to be ligands of extracellular matrix proteoglycans. Lectin binding clearly labelled perineuronal nets of the extracellular matrix around interneurons, which were preferentially located within or near the principal cell layers in all regions. In the hippocampus proper, the CA2 subfield showed an intense labelling of the neuropil around pyramidal cell bodies and the neuropil zones in the strata oriens and radiatum. These patterns were also seen after immunoreaction for chondroitin proteoglycan domains, brevican and phosphacan, as well as after detection of hyaluronan and tenascin-R. Characteristic laminar and intralaminar patterns were additionally expressed in the neuropil in all regions. In the dentate gyrus, the staining intensity for brevican, phosphacan and tenascin-R was predominant in the middle molecular layer, and for Cat-315 in the inner molecular layer, whereas immunoreactivity for neurocan increased within the outer molecular layer towards the hippocampal fissure. Our findings indicate that proteoglycans, hyaluronan and tenascin-R show differential patterns of co-expression in the individual regions and laminae of the hippocampal formation. The inhomogeneous composition of these major components suggests that the extracellular matrix is specifically adapted to the functional domains of intrahippocampal connections and afferent fibre systems.

Tenascin-R-deficient mice show structural alterations of symmetric perisomatic synapses in the CA1 region of the hippocampus.

Accumulating evidence suggests that extracellular matrix (ECM) molecules play important roles in formation of synapses. Our previous electrophysiologic study of mice deficient in the extracellular matrix glycoprotein tenascin-R (TN-R) showed an impaired gamma-aminobutyric acid release at perisomatic inhibitory synapses in the CA1 pyramidal cell layer of the hippocampus. The present study investigated possible ultrastructural correlates of abnormal perisomatic inhibition. Topographic, morphometric, and stereologic methods were applied at the light and electron microscopic levels to quantify the density and spatial arrangement of cell bodies of CA1 pyramidal neurons and density and architecture of symmetric synapses formed on them in TN-R(-/-) and wild-type mice of different ages. The spatial arrangement of neuronal cell bodies in the CA1 pyramidal cell layer was found more diffuse and disordered in TN-R(-/-) mice than in wild-type animals. The coverage of the plasma membrane of pyramidal cell bodies by active zones of symmetric synapses was reduced by at least 40% in TN-R(-/-) animals compared with control animals. Further, the length of active zone profiles of perisomatic inhibitory synapses in the CA1 pyramidal cell layer was 8-14% smaller, whereas the number of active zones calculated per length unit of cell body profile was 30-40% smaller in TN-R mutants than in wild-type animals. The density and spatial arrangement of synaptic vesicles in the synaptic terminals provided ultrastructural evidence for reduced synaptic activity in TN-R mutants. Thus, TN-R appears to play an important role in the regulation of the number and architecture of perisomatic inhibitory synapses, which play crucial roles in the synchronization of neuronal activity and modulation of synaptic plasticity in the hippocampus.

Diffuse perineuronal nets and modified pyramidal cells immunoreactive for glutamate and the GABA(A) receptor alpha1 subunit form a unique entity in rat cerebral cortex.

Perineuronal nets (PNs) consisting of polyanionic chondroitin sulfate proteoglycans (CSPG) and other extracellular matrix components create an exceptional microenvironment around certain types of neurons. In rat neocortex, three types of PNs can be distinguished after staining with Wisteria floribunda agglutinin (WFA) by their different morphological structure: lattice-like PNs associated with subpopulations of nonpyramidal neurons, weakly labeled PNs showing a pyramidal morphology, and diffuse PNs that possess a thick, strongly labeled matrix sheath located mainly in layer VIb above the white matter. The type of neuron surrounded by diffuse nets has not been described so far. This study is focused on the cytochemical and morphological characteristics of neurons associated with diffusely contoured PNs in rat parietal cortex using immunocytochemical staining, intracellular injection, and retrograde tracing methods. Cells surrounded by diffuse PNs were glutamate-immunoreactive in contrast to nonpyramidal, net-associated neurons that showed immunoreactivity for GABA, the calcium-binding protein parvalbumin and the potassium channel subunit Kv3.1b. Both groups of PN-ensheathed cells were mostly immunoreactive for the GABA(A) receptor alpha1 subunit. Lucifer Yellow-injected neurons surrounded by diffuse PNs displayed the morphological properties of modified pyramidal cells with intracortical main axons. Many neurons with diffuse PNs were retrogradely labeled over a long distance after Fluoro-Gold tracer injection in the parietal cortex, but remained unlabeled after intrathalamic injection. We conclude that neurons associated with diffuse PNs are a subpopulation of glutamatergic modified pyramidal cells that could act as excitatory long-range intracortically projecting neurons.

Saccadic omnipause and burst neurons in monkey and human are ensheathed by perineuronal nets but differ in their expression of calcium-binding proteins.

The extracellular matrix of the brain contains large aggregates of chondroitin sulfate proteoglycans (CSPG), which form lattice-like cell coatings around distinct neuron populations and are termed perineuronal nets. The function of perineuronal nets is not fully understood, but they are often found around neurons containing the calcium-binding protein parvalbumin, suggesting a function in primarily highly active neurons. In the present paper the distribution of perineuronal nets was studied in two functional cell groups of the primate oculomotor system with well-known firing properties: 1) the saccadic omnipause neurons in the nucleus raphe interpositus (RIP) exhibit a high tonic firing rate, which is only interrupted during saccades; they are inhibitory and use glycine as a transmitter; and 2) premotor burst neurons for vertical saccades in the rostral interstitial nucleus of the medial longitudinal fascicle (RiMLF) fire with high-frequency bursts during saccades; they are excitatory and use glutamate and/or aspartate as a transmitter. In the macaque monkey, both cell populations were identified by their parvalbumin immunoreactivity and were studied for the presence of perineuronal nets using CSPG antibodies or lectin binding with Wisteria floribunda agglutinin. In addition, the expression of another calcium-binding protein, calretinin, was studied in both cell groups. Double- and triple-immunofluorescence methods revealed that both omnipause and burst neurons are selectively ensheathed with strongly labeled perineuronal nets. Calretinin was coexpressed in at least 70% of the saccadic burst neurons, but not in the omnipause neurons. Parallel staining of human tissue revealed strongly labeled perineuronal nets around the saccadic omnipause and burst neurons, in corresponding brainstem regions, which specifically highlighted these neurons within the poorly structured reticular formation. These findings support the hypothesis that perineuronal nets may provide a specialized microenvironment for highly active neurons to maintain their fast-spiking activity and are not related to the transmitter or the postsynaptic action of the ensheathed neurons.

Perineuronal nets of extracellular matrix around hippocampal interneurons resist destruction by activated microglia in trimethyltin-treated rats.

The destruction of the extracellular matrix by inflammatory processes may induce neuronal dysfunction and accelerate neurodegeneration. We describe that chondroitin sulphate proteoglycan-immunoreactive perineuronal nets and the enwrapped interneurons persisted 2 weeks after trimethyltin intoxication of rats (TMT, 8 mg/kg, i.p.) in all regions of the severely affected hippocampus and dentate gyrus, whereas the diffuse immunoreactivity around the CA2 pyramidal cells was reduced. Fluoro-Jade staining of degenerating neurons and staining of microglia by Griffonia simplicifolia agglutinin showed that net-associated neurons survived in the vicinity of damaged pyramidal cells and that perineuronal nets were not removed by activated microglia. We conclude that the extracellular matrix of perineuronal nets resists destruction after TMT treatment in the inflamed neural tissue. A permanent reconstitution of matrix components may be one of the factors that may support the viability of distinct types of neurons during neurodegenerative diseases.