The extracellular matrix of cerebral gray matter: Golgi's pericellular net and Nissl's nervösen grau revisited

Derivatization of Hyaluronan to Target Neuroblastoma and Neuroglioma Expressing CD44

Therapeutics for actively targeting over-expressed receptors are of great interest because the majority of diseased tissues originate from normal cells and do not possess a unique receptor from which they can be differentiated. One such receptor is CD44, which has been shown to be highly overexpressed in many breast cancers and other types of cancer cells. While CD44 has been documented to express low levels in normal adult neurons, astrocytes, and microglia, this receptor may be overexpressed by neuroblastoma and neuroglioma. If differential expression exists between normal and cancerous cells, hyaluronan (HA) could be a useful carrier that targets carcinomas. Thus, HA was conjugated with resveratrol (HA-R), and its efficacy was tested on cortical-neuroblastoma hybrid, neuroblastoma, and neuroglioma cells. Confocal and flow cytometry showed these cells express CD44 and are able to bind and uptake HA-R. The toxicity of HA-R correlated well with CD44 expression in this study. Therefore, conjugating resveratrol and other chemotherapeutics to HA could minimize the side effects for normal cells within the brain and nervous system and could be a viable strategy for developing targeted therapies.

Damaged Neocortical Perineuronal Nets Due to Experimental Focal Cerebral Ischemia in Mice, Rats and Sheep

As part of the extracellular matrix (ECM), perineuronal nets (PNs) are polyanionic, chondroitin sulfate proteoglycan (CSPG)-rich coatings of certain neurons, known to be affected in various neural diseases. Although these structures are considered as important parts of the neurovascular unit (NVU), their role during evolution of acute ischemic stroke and subsequent tissue damage is poorly understood and only a few preclinical studies analyzed PNs after acute ischemic stroke. By employing three models of experimental focal cerebral ischemia, this study was focused on histopathological alterations of PNs and concomitant vascular, glial and neuronal changes according to the NVU concept. We analyzed brain tissues obtained 1 day after ischemia onset from: (a) mice after filament-based permanent middle cerebral artery occlusion (pMCAO); (b) rats subjected to thromboembolic MACO; and (c) sheep at 14 days after electrosurgically induced focal cerebral ischemia. Multiple fluorescence labeling was applied to explore simultaneous alterations of NVU and ECM. Serial mouse sections labeled with the net marker Wisteria floribunda agglutinin (WFA) displayed largely decomposed and nearly erased PNs in infarcted neocortical areas that were demarcated by up-regulated immunoreactivity for vascular collagen IV (Coll IV). Subsequent semi-quantitative analyses in mice confirmed significantly decreased WFA-staining along the ischemic border zone and a relative decrease in the directly ischemia-affected neocortex. Triple fluorescence labeling throughout the three animal models revealed up-regulated Coll IV and decomposed PNs accompanied by activated astroglia and altered immunoreactivity for parvalbumin, a calcium-binding protein in fast-firing GABAergic neurons which are predominantly surrounded by neocortical PNs. Furthermore, ischemic neocortical areas in rodents simultaneously displayed less intense staining of WFA, aggrecan, the net components neurocan, versican and the cartilage link protein (CRTL) as well as markers in net-bearing neurons such as the potassium channel subunit Kv3.1b and neuronal nuclei (NeuN). In summary, theconsistent observations based on three different stroke models confirmed that PNs are highly sensitive constituents of the NVU along with impaired associated GABAergic neurons. These results suggest that PNs could be promising targets of future stroke treatment, and further studies should address their reorganization and plasticity in both stabilizing the acute stroke as well as supportive effects during the chronic phase of stroke.

Abolished perineuronal nets and altered parvalbumin-immunoreactivity in the nucleus reticularis thalami of wildtype and 3xTg mice after experimental stroke

Treatment strategies for ischemic stroke are still limited, since numerous attempts were successful only in preclinical research but failed under clinical condition. To overcome this translational roadblock, clinical relevant stroke models should consider co-morbidities, age-related effects and the complex neurovascular unit (NVU) concept. The NVU includes neurons, vessels and glial cells with astrocytic endfeet in close relation to the extracellular matrix (ECM). However, the role of the ECM after stroke-related tissue damage is poorly understood and mostly neglected for treatment strategies. This study is focused on alterations of perineuronal nets (PNs) as ECM constituents and parvalbumin-containing GABAergic neurons in mice with emphasis on the nucleus reticularis thalami (NRT) in close proximity to the ischemic lesion as induced by a filament-based stroke model. One day after ischemia onset, immunofluorescence-based quantitative analyses revealed drastically declined PNs in the ischemia-affected NRT from 3- and 12-month-old wildtype and co-morbid triple-transgenic (3xTg) mice with Alzheimer-like alterations. Parvalbumin-positive cells decreased numerically in the ischemia-affected NRT, while staining intensity did not differ between the affected and non-affected hemisphere. Additional qualitative analyses demonstrated ischemia-induced loss of PNs and allocated neuropil ECM immunoreactive for aggrecan and neurocan, and impaired immunoreactivity for calbindin, the potassium channel subunit Kv3.1b and the glutamate decarboxylase isoforms GAD65 and GAD67 in the NRT. In conclusion, these data confirm PNs as highly sensitive constituents of the ECM along with impaired neuronal integrity of GABAergic neurons. Therefore, specific targeting of ECM components might appear as a promising strategy for future treatment strategies in stroke.

Hyaluronic acid and neural stem cells: implications for biomaterial design

While in the past hyaluronic acid (HA) was considered a passive structural component, research over the past few decades has revealed its diverse and complex biological functions resulting in a major ideological shift. This review describes recent advances in biological interactions of HA with neural stem cells, with a focus on leveraging these interactions to develop advanced biomaterials that aid regeneration of the central nervous system.

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.

Cortical areas abundant in extracellular matrix chondroitin sulphate proteoglycans are less affected by cytoskeletal changes in Alzheimer's disease

In the human brain, the distribution of perineuronal nets occurring as lattice-like neuronal coatings of extracellular matrix proteoglycans ensheathing several types of non-pyramidal neurons and subpopulations of pyramidal cells in the cerebral cortex is largely unknown. Since proteoglycans are presumably involved in the pathogenesis of Alzheimer's disease, we analysed the distribution pattern of extracellular chondroitin sulphate proteoglycans in cortical areas, including primary motor, primary auditory and several prefrontal and temporal association areas, in normal human brains and in those showing neuropathological criteria of Alzheimer's disease. In both groups, neurons with perineuronal nets were most numerous in the primary motor cortex (approximately 10% in Brodmann's area 4) and in the primary auditory cortex as a representative of the primary sensory areas. Their number was lower in secondary and higher order association areas. Net-associated pyramidal cells occurred predominantly in layers III and V in motor areas, as well as throughout lower parts of layer III in the primary auditory cortex and neocortical association areas. In the entorhinal cortex, net-associated pyramidal cells were extremely rare. In brains showing hallmarks of Alzheimer's disease, the characteristic patterns of hyperphosphorylated tau protein, stained with the AT8 antibody, largely excluded the zones abundant in perineuronal nets and neuropil-associated chondroitin sulphate proteoglycans. As shown in double-stained sections, pyramidal and non-pyramidal neurons ensheathed by perineuronal nets were virtually unaffected by the formation of neurofibrillary tangles even in severely damaged regions. The distribution patterns of amyloid B deposits overlapped but showed no congruence with that of the extracellular chondroitin sulphate proteoglycans. It can be concluded that low susceptibility of neurons and cortical areas to neurofibrillary changes corresponds with high proportions of aggregating chondroitin sulphate proteoglycans in the neuronal microenvironment.

Low expression of extracellular matrix components in rat brain stem regions containing modulatory aminergic neurons

Extracellular matrix proteoglycans, particularly those accumulated in perineuronal nets (PNs), have been shown to form characteristic distribution patterns in cortical and subcortical regions of adult mammals. Their involvement in sustaining mechanisms that are especially related to fast activities of neurons has been discussed as one of the possible functions. The present study deals with the spatial organization of extracellular matrix proteoglycans in brain stem regions that contain aminergic neurons, such as substantia nigra, ventral tegmental area (VTA), raphe nuclei and locus coeruleus (LC). As these nuclei are known to influence brain activity by modulatory functions exerting patterns of slow electric activity, it could be expected that PNs would be absent around aminergic cells. The staining of PNs with Wisteria floribunda agglutinin (WFA) was combined with the detection of catecholaminergic neurons by tyrosine hydroxylase immunoreactivity and of serotonergic neurons by tryptophan hydroxylase (TH) immunoreactivity using double fluorescence microscopy. It was found that the catecholaminergic and serotonergic neurons in the nuclear accumulations, as well as those scattered in adjacent regions, were not ensheathed by PNs. In contrast, several non-aminergic neurons intermingled with aminergic neurons in the raphe nuclei, in the substantia nigra pars compacta (SNC) and in the VTA, as well as many cells in the reticular part of the substantia nigra, were found to be surrounded by PNs. It can be concluded from these results that the absence of PNs around aminergic brain stem neurons, also previously shown for cholinergic basal forebrain neurons, appears as a characteristic feature common to cells that exert slow modulatory functions.

One hundred years of Golgi's "perineuronal net": history of a denied structure

Perineuronal nets are reticular structures enwrapping cell bodies and the largest dendrites of several neuronal populations. Discovered by Camillo Golgi, who described them in detail in 1898, they were intensely studied by the most famous contemporary neurohistologists for about twenty years. The opinion of Ramon y Cajal that perineuronal nets were a fixation artifact ended the first period of studies. Only a few researchers, among whom the Italian neurologists Besta and Belloni, went on with their studies up to the 1930s documenting the morphology of perineuronal nets of different mammals and of man both in normal and in pathological conditions. Only after about fifty years, the advances in the field of cytochemistry allowed the elucidation of not only the actual existence of perineuronal nets, but also their chemical nature, showing conclusively that they are complex organisations of extracellular matrix molecules, namely glycoproteins and proteoglycans. The research on perineuronal nets today involves several groups engaged to elucidate their biological properties and functional role.

Neurons produce a neuronal cell surface-associated chondroitin sulfate proteoglycan

Monoclonal antibody Cat-315 recognizes a chondroitin sulfate proteoglycan (CSPG) expressed on the surface of subsets of neurons in many areas of the mammalian CNS (). The cell type-specific expression exhibited by the Cat-315 CSPG and other perineuronal net CSPGs imparts a distinct molecular surface identity to a neuron (Celio and Blumcke, 1994; Lander et al., 1997). The cell type(s) producing these surface-associated proteins and yielding this cellular diversity has remained in question. The expression of the Cat-315 CSPG in primary rat cortical cultures has permitted an examination of the cellular source of the Cat-315 antigen, as well as a determination of its spatial relationship to the neuronal surface. Live-cell labeling of primary neuronal cultures demonstrates that the Cat-315 CSPG is on the extracellular surface of neurons. Furthermore, extraction experiments demonstrate that the Cat-315 CSPG lacks a transmembrane domain and that the entire molecule is extracellular and, therefore, can be considered a constituent of brain extracellular matrix. Several lines of evidence indicate that neurons with cell surface staining produce the Cat-315 CSPG. First, neurons with cell surface staining also show intracellular Cat-315 immunoreactivity. Second, beta-xyloside or monensin, reagents that inhibit the synthesis and transport of CSPGs, increase intracellular Cat-315 immunoreactivity within neurons that express cell surface Cat-315 immunoreactivity. Third, double labeling with Cat-315 and a polyclonal antibody for the Golgi complex demonstrates a precise colocalization of the intracellular Cat-315 immunoreactivity with the Golgi. Together, these observations demonstrate that neurons contribute to the extracellular matrix of brain and that the Cat-315 CSPG is produced by the neurons that carry Cat-315 cell surface immunoreactivity.

HIV-I induced destruction of neocortical extracellular matrix components in AIDS victims

Neurological dysfunction is not uncommon in patients suffering from acquired immunodeficiency syndrome (AIDS) and, when manifested, intimates involvement of the central nervous system. Here, the human immunodeficiency virus (HIV) infects preferentially microglial cells, which thereby release substances known to interfere with neuronal function. One class of agents set free in this manner are proteases; these degrade certain components within, and thereby undermine the integrity of, the extracellular matrix (ECM) compartment, which plays a vital role in cell-to-cell communication. We wished to ascertain whether the ECM compartment is indeed disrupted in the brains of AIDS victims. We examined the neocortical areas of 27 AIDS autopsy cases, including 9 with diagnosed HIV-encephalopathy (HIVE); 8 HIV-seronegative cases with various types of brain lesion, including viral infections, were also included in this study. HIV-antigens and DNA were identified by use of immunohistochemistry and in situ hybridization, and ECM components by lectin staining and immunohistochemistry. Of the 27 AIDS cases examined, each of the 9 with HIVE was completely devoid of labeled ECM components; 8 of the 18 without HIVE had incurred substantial losses, and only 2 manifested a normal complement of constituents within this compartment. With respect to stratal and topographic variations, layers II and III were less affected than layers V to VII, as was the frontal cortex relative to other areas. These findings confirmed our expectations of the brain's ECM undergoing degradation following HIV infection, and these changes may well underlie the neurological disturbances manifested in AIDS patients.

CD44: structure, function, and association with the malignant process

CD44 is a ubiquitous multistructural and multifunctional cells surface adhesion molecule involved in cell-cell and cell-matrix interactions. Twenty exons are involved in the genomic organization of this molecule. The first five and the last 5 exons are constant, whereas the 10 exons located between these regions are subjected to alternative splicing, resulting in the generation of a variable region. Differential utilization of the 10 variable region exons, as well as variations in N-glycosylation, O-glycosylation, and glycosaminoglycanation (by heparan sulfate or chondroitin sulfate), generate multiple isoforms (at least 20 are known) of different molecular sizes (85-230 kDa). The smallest CD44 molecule (85-95 kDa), which lacks the entire variable region, is standard CD44 (CD44s). As it is expressed mainly on cells of lymphohematopoietic origin, CD44s is also known as hematopoietic CD44 (CD44H). CD44s is a single-chain molecule composed of a distal extracellular domain (containing, the ligand-binding sites), a membrane-proximal region, a transmembrane-spanning domain, and a cytoplasmic tail. The molecular sequence (with the exception of the membrane-proximal region) displays high interspecies homology. After immunological activation, T lymphocytes and other leukocytes transiently upregulate CD44 isoforms expressing variant exons (designated CD44v). A CD44 isform containing the last 3 exon products of the variable region (CD44V8-10, also known as epithelial CD44 or CD44E), is preferentially expressed on epithelial cells. The longest CD44 isoform expressing in tandem eight exons of the variable region (CD44V3-10) was detected in keratinocytes. Hyaluronic acid (HA), an important component of the extracellular matrix (ECM), is the principal, but by no means the only, ligand of CD44. Other CD44 ligands include the ECM components collagen, fibronectin, laminin, and chondroitin sulfate. Mucosal addressin, serglycin, osteopontin, and the class II invariant chain (Ii) are additional, ECM-unrelated, ligands of the molecule. In many, but not in all cases, CD44 does not bind HA unless it is stimulated by phorbol esters, activated by agonistic anti-CD44 antibody, or deglycosylated (e.g., by tunicamycin). CD44 is a multifunctional receptor involved in cell-cell and cell-ECM interactions, cell traffic, lymph node homing, presentation of chemokines and growth factors to traveling cells, and transmission of growth signals. CD44 also participates in the uptake and intracellular degradation of HA, as well as in transmission of signals mediating hematopoiesis and apoptosis. Many cancer cell types as well as their metastases express high levels of CD44. Whereas some tumors, such as gliomas, exclusively express standard CD44, other neoplasms, including gastrointestinal cancer, bladder cancer, uterine cervical cancer, breast cancer and non-Hodgkin's lymphomas, also express CD44 variants. Hence CD44, particularly its variants, may be used as diagnostic or prognostic markers of at least some human malignant diseases. Furthermore, it has been shown in animal models that injection of reagents interfering with CD44-ligand interaction (e.g., CD44s- or CD44v-specific antibodies) inhibit local tumor growth and metastatic spread. These findings suggest that CD44 may confer a growth advantage on some neoplastic cells and, therefore, could be used as a target for cancer therapy. It is hoped that identification of CD44 variants expressed on cancer but not on normal cells will lead to the development of anti-CD44 reagents restricted to the neoplastic growth.

The proteoglycan DSD-1-PG occurs in perineuronal nets around parvalbumin-immunoreactive interneurons of the rat cerebral cortex

Proteoglycans involved in the shaping of the developing brain are often preserved in the adult brain in more restricted locations. We have studied the fate of DSD-1-PG, a chondroitin sulfate proteoglycan containing the hybrid epitope DSD-1. DSD-1-PG exerts neurite outgrowth promoting activity and has been shown to occur in the developing brain during late brain development and into adulthood. In the adult rat brain, monoclonal and polyclonal antibodies against DSD-1-PG labelled only the circumference of a selected subpopulation of neurons. These nerve cells invariably expressed the calcium-binding protein parvalbumin. The label occupied the extracellular space in close vicinity to the cell body, surrounding axon terminals and glial end feet, but was absent from synaptic clefts. DSD-1-PG is thus shown to be an additional representative of the growing list of substances found in perineuronal locations in the adult mammalian brain.

In vivo and in vitro labelling of perineuronal nets in rat brain

Previous lectin-histochemical and immunocytochemical investigations using fixed tissue revealed perineuronal nets as lattice-like accumulations of extracellular matrix proteoglycans at the surface of several types of neurons. In the present study, perineuronal nets in the rat brain were labelled for the first time in vivo by stereotaxic injections of biotinylated Wisteria floribunda agglutinin (Bio-WFA), as well as in vitro, by incubation of unfixed brain slices with the same lectin. Six days after Bio-WFA injections into the parietal cortex, medial septum, reticular thalamic nucleus and red nucleus, the lectin remaining bound to perineuronal nets was detected by streptavidin/biotinylated peroxidase complexes or red fluorescent Cy3-streptavidin, respectively. Double-fluorescence labelling showed that Bio-WFA applied in vivo reacted with the chondroitin sulphate proteoglycan immunoreactive perineuronal nets in the injection zone. Labelling of perineuronal nets in unfixed slices was obtained with either Cy3-tagged WFA or Bio-WFA and subsequent visualization by Cy3-streptavidin which confirmed the region-dependent distribution patterns and the structural characteristics of perineuronal nets known from histochemical studies. These results provide support for the role of extracellular matrix proteoglycans to maintain a considerable chemical and, probably, spatial heterogeneity of the extracellular space in vivo. The ability of in vivo and in vitro labelling may promote the functional characterization of the extracellular matrix in various brain structures including its species-dependent neuronal association patterns.

Extracellular matrix organization in various regions of rat brain grey matter

Previous studies revealed the concentration of extracellular matrix proteoglycans in the so-called perineuronal nets on the one hand and in certain zones of the neuropil on the other. This nonhomogeneous distribution suggested a non-random chemical and spatial heterogeneity of the extracellular space. In the present investigation, regions dominated by one of both distribution patterns, i.e. piriform and parietal cortex, reticular thalamic nucleus, medial septum/diagonal band complex and cerebellar nuclei, were selected for correlative light and electron microscopic analysis. The labelling was performed by the use of the N-acetylgalactosamine-binding plant lectin Wisteria floribunda agglutinin visualized by peroxidase staining and additionally by photoconversion of red carbocyanine fluorescence labelling for electron microscopy. The intense labelling of the neuropil of a superficial piriform region, presumably identical with sublayer Ia, was confined to a fine meshwork spreading over the extracellular space between non-myelinated axons, dendrites and glial profiles. In the reticular thalamic nucleus the neuronal cell bodies were embedded in zones of labelled neuropil. In contrast to these patterns, the labelled extracellular matrix in different cortical layers and in the other subcortical regions was concentrated in perineuronal nets as large accumulations at surface areas of the neuronal perikarya and dendrites and the attached presynaptic boutons. Astrocytic processes usually were separated from the neuronal surface by the interposed extracellular material. Despite a great variability, the width of the extracellular space containing the labelled matrix components in all perineuronal nets appeared to be considerably larger than that in the labelled zones of neuropil and the non-labelled microenvironment of other neurons. Our results support the view that differences expressed in topographical and spatial peculiarities of the extracellular matrix constituents are related to neuron-type and system-specific functional properties.

Allocation of perineuronal nets and parvalbumin-, calbindin-D28k- and glutamic acid decarboxylase-immunoreactivity in the amygdala of the rhesus monkey

Lattice-like coatings, known as perineuronal nets, were lectin-cytochemically stained with the Wisteria floribunda agglutinin in the lateral nucleus and the intermediate division of the basal nucleus of the amygdala in rhesus monkeys. Perineuronal nets were demonstrated around neurons with parvalbumin- or calbindin-D28k-immunoreactivity, but not around calretinin-containing cells. In parallel dual-peroxidase staining experiments, it was demonstrated that lattice-like coatings exclusively surround GABAergic neurons in this brain region. The novel finding of calbindin-D28k-immunoreactivity in neurons ensheathed by perineuronal nets amplifies the panel of revealed markers in such nerve cells and indicates their cytochemical heterogeneity.

Relationship between astrocytic processes and "perineuronal nets" in rat neocortex

"Perineuronal nets" (PNs) ensheath a subtype of inhibitory neurons in the mammalian neocortex. In the light of the proposal that PNs consist of glial processes, we have analyzed the relationship between intracellularly injected glial cells and PNs in the rat neocortex. Glial cells were injected iontophoretically with Lucifer Yellow in lightly fixed tissue slices and PNs were visualized with the lectin from Vicia villosa. Using confocal laser scanning microscopy, glial processes and PNs were identified as distinct structures. Lectin labeling was consistently associated with the extracellular space interposed between LY-labeling was consistently associated with the extracellular space interposed between LY-labeled astrocyte processes and neurons. Of the different types of glial cells injected, only the densely-ramifying protoplasmic astrocytes extended processes which could be traced to contact PNs. These protoplasmic astrocytes also sent out processes to adjacent neurons not ensheathed by PNs, and to capillaries. The present data strongly suggests that PNs do not consist of glial processes but rather support the idea that PNs represent specialized extracellular material interposed between the surface of some inhibitory interneurons and astrocytic processes.

Tenascin-C expression by neurons and glial cells in the rat spinal cord: changes during postnatal development and after dorsal root or sciatic nerve injury

We have used in situ hybridization with a digoxigenin-labelled probe for tenascin-C mRNA and immunocytochemistry with antibodies against tenascin-C, glial fibrillary acidic protein, OX-42 and the 200 kDa neurofilament protein to study the expression, distribution and cellular relationships of tenascin-C mRNA and protein in the developing (postnatal) and adult spinal cord of rat, and the effects thereon of dorsal root, ventral root and sciatic nerve injuries. The most interesting finding was that on postnatal day 7 (P7), P14 and in the adult, but not on P0 or P3, a group of neurons in the lumbar ventral horn expressed the tenascin-C mRNA gene. They represented about 5% of ventral horn neurons in the adult and were among the smaller such neurons. Since 40-60% of such cells were lost at P13 following sciatic nerve crush on P0, some were almost certainly motor neurons. In addition, we found that at P0 and P3, mRNA-containing glial cells were widespread in grey and white matter but sparse in the developing dorsal columns; tenascin-C immunofluorescence showed a similar distribution. By P7 there were fewer mRNA-containing cells in the ventral horns and in the area of the dorsal columns containing the developing corticospinal tract where immunofluorescence was also weak. At P14 there were no glial-like mRNA-containing cells in the grey matter; such cells were confined to the periphery of the lateral and ventral white columns but were present throughout the dorsal columns where tenascin-C immunofluorescence was also strong. No glial-like mRNA-containing cells were present in the adult lumbar spinal cord and tenascin-C immunofluorescence was confirmed to irregular patches in the ventral horn, especially around immunonegative cell bodies of small neurons, a zone around the central canal, and a thin zone adjacent to the glia limitans. Thus the expression of tenascin-C is differentially developmentally regulated in the grey matter and in different parts of the white matter. Three days after injury of dorsal roots L4-6, many cells containing tenascin-C mRNA, some identified as glial fibrillary acidic protein-positive astrocytes, were present in the ipsilateral dorsal column, but were rare after longer survivals. Immunoreactivity, however, was elevated in the ipsilateral dorsal column at 3 days, remained high for several months and disappeared at 6.5 months. Dorsal root injury had no effect on tenascin-C mRNA or protein in the grey matter. Sciatic nerve or ventral root injury had no effect on these molecules in any part of the spinal cord.

Projection of non-cholinergic basal forebrain neurons ensheathed with perineuronal nets to rat mesocortex

The existence of non-cholinergic (GABAergic) components in the septo-hippocampal system but also in basal forebrain projections terminating in the olfactory bulb and certain cortical areas has been documented by several authors using retrograde and anterograde tracing techniques. On the other hand, the basal forebrain also contains a high number of mainly parvalbumin-positive neurons ensheathed by a lattice-like matrix of polyanionic proteoglycans forming so-called perineuronal nets of as yet unknown function. By a combination of retrograde tracing using Fluoro-Gold injection into mesocortical areas of rats and staining of perineuronal nets by Wisteria floribunda agglutinin (WFA) the present study describes the projection pattern and distribution of non-cholinergic projection neurons characterized by perineuronal nets in the anterior parts of the basal forebrain complex (medial septal nucleus, nucleus of the diagonal band of Broca, magnocellular preoptic nucleus). After tracer injection into the cingulate cortex labelled net-associated neurons were distributed within the rostrocaudal extension of the basal forebrain complex but were predominantly found in the horizontal limb of the diagonal band of Broca. Retrograde labelling of neurons with perineuronal nets after tracer injection into the retrosplenial cortex was more pronounced in the medial septum. Choline acetyltransferase-immunoreactive (ChAT-ir) projection neurons were in no case associated with perineuronal nets. The results demonstrate that a large portion of the non-cholinergic projection neurons of the basal forebrain are endowed with a specialized microenvironment of proteoglycans and form a strong input system of mesocortical components of the limbic system.

Cortical areas are revealed by distribution patterns of proteoglycan components and parvalbumin in the Mongolian gerbil and rat

Cortical areas in rodents have been basically characterized by its cytoarchitecture, connectivity or by physiological parameters. In this study we show that they are revealed by distribution patterns of proteoglycans and parvalbumin-immunoreactivity. Brains of young adult Mongolian gerbils (Meriones unguiculatus) and Wistar rats were cut into series of transversal sections. Proteoglycan components were detected using the N-acetylgalactosamine binding Wisteria floribunda agglutinin (WFA) and antibodies against chondroitin sulphate proteoglycan (CSPG). Differences between cortical areas were found to exist with regard to the occurrence and the density of perineuronal nets, but were also expressed in varying staining intensities for WFA and CSPG of the neuropil. Primary neocortical areas (somatosensory, auditory, visual cortex) were characterized by an intense neuropil staining in layer IV and the upper part of layer VI. Using the same methods strong labelling was also typical of the neuropil in the retrosplenial cortex, of layer Ia in the prepiriform cortex and the hippocampal CA3 field. In tangential sections cut from gerbil cortical hemispheres, some of the heavily lectin-stained cortical areas were sharply delineated from adjacent faintly labelled regions, others showed more diffuse borders. In the rat, the area-specific staining for WFA was less clearly expressed than in the gerbil. Immunocytochemistry of the calcium-binding protein parvalbumin in alternate sections showed labelling patterns of neuropil which resembled those of WFA-binding and CSPG-immunoreactivity in the entire neocortex and hippocampus. From these results it can be concluded that functional peculiarities of cortical fields may not only be determined by neuronal network parameters but also by the spatial arrangement of extracellular matrix proteoglycans.

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.

'Perineuronal nets' around cortical interneurons expressing parvalbumin are rich in tenascin

'Perineuronal nets' are ill-known structures enwrapping the cell bodies and proximal dendrites of certain neurons in the brain. It is as yet unclear if they represent a cytological entity or extracellular material. Using immunohistochemical methods we have detected the presence of the extracellular matrix-protein, tenascin, in the 'perineuronal nets' surrounding certain cortical interneurons. We have also shown that tenascin antibodies label the circumference of parvalbumin-immunoreactive neurons preferentially. We conclude that this classical matrix protein is a major component of 'perineuronal nets'. Therefore, 'perineuronal nets' may represent sites of privileged adhesion between nerve and glial cells.