The extracellular matrix in the mouse brain: its reactions to endo-alpha-N-acetylgalactosaminidase and certain other enzymes

Blocking Pannexin-1 Channels Alleviates Thalamic Hemorrhage-Induced Pain and Inflammatory Depolarization of Microglia in Mice

Central post-stroke pain (CPSP) is a neuropathic pain syndrome that frequently occurs following cerebral stroke. The pathogenesis of CPSP is mainly due to thalamic injury caused by ischemia and hemorrhage. However, its underlying mechanism is far from clear. In the present study, a thalamic hemorrhage (TH) model was established in young male mice by microinjection of 0.075 U of type IV collagenase into the unilateral ventral posterior lateral nucleus and ventral posterior medial nucleus of the thalamus. We found that TH led to microglial pannexin (Panx)-1, a large-pore ion channel, opening within the thalamus accompanied with thalamic tissue injury, pain sensitivities, and neurological deficit, which were significantly prevented by either intraperitoneal injection of the Panx1 blocker carbenoxolone or intracerebroventricular perfusion of the inhibitory mimetic peptide 10Panx. However, inhibition of Panx1 has no additive effect on pain sensitivities upon pharmacological depletion of microglia. Mechanistically, we found that carbenoxolone alleviated TH-induced proinflammatory factors transcription, neuronal apoptosis, and neurite disassembly within the thalamus. In summary, we conclude that blocking of microglial Panx1 channels alleviates CPSP and neurological deficit through, at least in part, reducing neural damage mediated by the inflammatory response of thalamic microglia after TH. Targeting Panx1 might be a potential strategy in the treatment of CPSP.

Morphological, histological and immunohistochemical study of the area postrema in the dog

Circumventricular organs are specialized brain structures that are located mainly at the midsagittal line, around the third and fourth ventricles, often protruding into the lumen. They are positioned at the interface between the neuroparenchyma and the ventricular system of the brain. These highly vascularized nervous tissue structures differ from the brain parenchyma, as they lack a blood-brain barrier. Circumventricular organs have specialized sensory and secretory functions. It is essential for any pathologist who evaluates brain sections to have a solid knowledge of microscopic neuroanatomy and to recognize these numerous specialized structures within the nervous system as normal and not mistake them for pathological changes. The purpose of this study was to provide, for the first time, a detailed and complete histological description of the healthy canine area postrema and to determine its resemblance to that of other mammalian species. Anatomical dissections with routine histological and immunohistochemical techniques were carried out on ten canine brains. The cellular composition of area postrema proved to be largely comparable to that of other mammal species.

Perineuronal and perisynaptic extracellular matrix in the human spinal cord

Extracellular matrix (ECM) forms an active interface around neurons of the central nervous system (CNS). Whilst the components, chemical heterogeneity and cellular recruitment of this intercellular assembly in various parts of the brain have been discussed in detail, the spinal cord received limited attention in this context. This is in sharp contrast to its clinical relevance since the overall role of ECM especially that of its chondroitin sulphate-based proteoglycan components (CSPGs) was repeatedly addressed in neuropathology, regeneration, CNS repair and therapy models. Based on two post-mortem human specimen, this study gives the first and detailed description of major ECM components of the human spinal cord. Immunohistochemical investigations were restricted to the systematic mapping of aggrecan, brevican, proteoglycan link-protein as well as tenascin-R and hyaluronan containing matrices in the whole cranio-caudal dimension of the human spinal cord. Other proteoglycans like versican, neurocan and NG2 were exemplarily investigated in restricted areas. We show the overall presence of tenascin-R and hyaluronan in both white and grey matters whereas aggrecan, proteoglycan link-protein and brevican were restricted to the grey matter. In the grey matter, the ECM formed aggrecan-based perineuronal nets in the ventral and lateral horns but established single perisynaptic assemblies, axonal coats (ACs), containing link-protein and brevican in all regions except of the Lissauer's zone. Intersegmental differences were reflected in the appearance of segment-specific nuclei but not in overall matrix distribution pattern or chemical heterogeneity. Perineuronal nets were typically associated with long-range projection neurons including cholinergic ventral horn motorneurons or dorsal spinocerebellar tract neurons of the Clarke-Stilling nuclei. Multiple immunolabelling revealed that nociceptive afferents were devoid of individual matrix assemblies unlike glycinergic or GABAergic synapses. The detailed description of ECM distribution in the human spinal cord shall support clinical approaches in injury and regenerative therapy.

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 net formation and structure in aggrecan knockout mice

Perineuronal nets (PNNs) are specialized substructures of the neural extracellular matrix (ECM) which envelop the cell soma and proximal neurites of particular sets of neurons with apertures at sites of synaptic contact. Previous studies have shown that PNNs are enriched with chondroitin sulfate proteoglycans (CSPGs) and hyaluronan, however, a complete understanding of their precise molecular composition has been elusive. In addition, identifying which specific PNN components are critical to the formation of this structure has not been demonstrated. Previous work in our laboratory has demonstrated that the CSPG, aggrecan, is a key activity-dependent component of PNNs in vivo. In order to assess the contribution of aggrecan to PNN formation, we utilized cartilage matrix deficiency (cmd) mice, which lack aggrecan. Herein, we utilized an in vitro model, dissociated cortical culture, and an ex vivo model, organotypic slice culture, to specifically investigate the role aggrecan plays in PNN formation. Our work demonstrates that staining with the lectin, Wisteria floribunda agglutinin (WFA), considered a broad PNN marker, is eliminated in the absence of aggrecan, suggesting the loss of PNNs. However, in contrast, we found that the expression patterns of other PNN markers, including hyaluronan and proteoglycan link protein 1 (HAPLN1), tenascin-R, brevican, and hyaluronan are unaffected by the absence of aggrecan. Lastly, we determined that while all PNN components are bound to the surface in a hyaluronan-dependent manner, only HAPLN1 remains attached to the cell surface when neurons are treated with chondroitinase. These results suggest a different model for the molecular association of PNNs to the cell surface. Together our work has served to assess the contribution of aggrecan to PNN formation while providing key evidence concerning the molecular composition of PNNs in addition to determining how these components ultimately form PNNs.

Rapid reversal of chondroitin sulfate proteoglycan associated staining in subcompartments of mouse neostriatum during the emergence of behaviour

Background

The neostriatum, the mouse homologue of the primate caudate/putamen, is the input nucleus for the basal ganglia, receiving both cortical and dopaminergic input to each of its sub-compartments, the striosomes and matrix. The coordinated activation of corticostriatal pathways is considered vital for motor and cognitive abilities, yet the mechanisms which underlie the generation of these circuits are unknown. The early and specific targeting of striatal subcompartments by both corticostriatal and nigrostriatal terminals suggests activity-independent mechanisms, such as axon guidance cues, may play a role in this process. Candidates include the chondroitin sulfate proteoglycan (CSPG) family of glycoproteins which have roles not only in axon guidance, but also in the maturation and stability of neural circuits where they are expressed in lattice-like perineuronal nets (PNNs).

Methodology/Principal Findings

The expression of CSPG-associated structures and PNNs with respect to neostriatal subcompartments has been examined qualitatively and quantitatively using double-labelling for Wisteria floribunda agglutinin (WFA), and the μ-opioid receptor (μOR), a marker for striosomes, at six postnatal ages in mice. We find that at the earliest ages (postnatal day (P)4 and P10), WFA-positive clusters overlap preferentially with the striosome compartment. By P14, these clusters disappear. In contrast, PNNs were first seen at P10 and continued to increase in density and spread throughout the caudate/putamen with maturation. Remarkably, the PNNs overlap almost exclusively with the neostriatal matrix.

Conclusions/Significance

This is the first description of a reversal in the distribution of CSPG associated structures, as well as the emergence and maintenance of PNNs in specific subcompartments of the neostriatum. These results suggest diverse roles for CSPGs in the formation of functional corticostriatal and nigrostriatal connectivity within the striosome and matrix compartments of the developing caudate/putamen.

Discordant localization of WFA reactivity and brevican/ADAMTS-derived fragment in rodent brain

Background

Proteoglycan (PG) in the extracellular matrix (ECM) of the central nervous system (CNS) may act as a barrier for neurite elongation in a growth tract, and regulate other characteristics collectively defined as structural neural plasticity. Proteolytic cleavage of PGs appears to alter the environment to one favoring plasticity and growth. Brevican belongs to the lectican family of aggregating, chondroitin sulfate (CS)-bearing PGs, and it modulates neurite outgrowth and synaptogenesis. Several ADAMTSs (a disintegrin and metalloproteinase with thrombospondin motifs) are glutamyl-endopeptidases that proteolytically cleave brevican. The purpose of this study was to localize regions of adult CNS that contain a proteolytic-derived fragment of brevican which bears the ADAMTS-cleaved neoepitope sequence. These regions were compared to areas of Wisteria floribunda agglutin (WFA) reactivity, a common reagent used to detect "perineuronal nets" (PNNs) of intact matrix and a marker which is thought to label regions of relative neural stability.

Results

WFA reactivity was found primarily as PNNs, whereas brevican and the ADAMTS-cleaved fragment of brevican were more broadly distributed in neuropil, and in particular regions localized to PNNs. One example is hippocampus where the ADAMTS-cleaved brevican fragment is found surrounding pyramidal neurons, in neuropil of stratum oriens/radiatum and the lacunosum moleculare. The fragment was less abundant in the molecular layer of the dentate gyrus. Mostly PNNs of scattered interneurons along the pyramidal layer were identified by WFA. In lateral thalamus, the reticular thalamic nucleus stained abundantly with WFA whereas ventral posterior nuclei were markedly immunopositive for ADAMTS-cleaved brevican. Using Western blotting techniques, no common species were reactive for brevican and WFA.

Conclusion

In general, a marked discordance was observed in the regional localization between WFA and brevican or the ADAMTS-derived N-terminal fragment of brevican. Functionally, this difference may correspond to regions with varied prevalence for neural stability/plasticity.

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.

Distribution of a brain-specific extracellular matrix protein in developing and adult zebrafish

A monoclonal antibody (IgG) that recognizes a 53-kDa zebrafish brain protein was isolated and used to characterize the distribution of this protein in zebrafish. (1) The antigen was found only in the brain and not in any other tissues such as muscle, dermis and cartilage. Within the brain, the antibody recognized extracellular matrix (ECM) outside neuronal cells. (2) Digestion by hyaluronidase released the antigen from brain tissue, and the monoclonal antibody staining was also decreased by the digestion by hyaluronidase. (3) The pattern of antigen distribution is not perineuronal, as the density of the antigen at the periphery of the cells was practically identical to that of the empty intercellular spaces. Therefore, this monoclonal antibody does not recognize the perineuronal glycocortex. (4) The antigen is distributed only in limited areas of the brain, namely in the periphery of the forebrain, the hypothalamus, the optic tectum, the interpeduncular nucleus, the cerebellum and the ventricular rim of the medulla. In the optic tectum, the antibody strongly stained the most superficial layer, and in the cerebellum, it stained the molecular but not the granular layer. These patterns of distribution are very different from those of other typical brain ECM proteins and suggest that this protein may play quite distinct roles in brain development and maintenance.

Perisynaptic barrier of proteoglycans in the mature brain and spinal cord

Cell bodies and their dendrites of motor neurons, motor-related neurons, and certain other subsets of neurons such as GABAergic interneurons in the mature brain and spinal cord possess intensely negatively charged perineuronal or perisynaptic nets of proteoglycans which are linked to the nerve cell surface glycoproteins. These perineuronal nets of proteoglycans are digested by chondroitinase ABC, hyaluronidase, or collagenase, but not by endo-alpha-N-acetylgalactosaminidase, which is reactive to the nerve cell surface glycoproteins. Aggrecan, versican, neurocan, and brevican are members of a family of chondroitin sulfate proteoglycans that bind to hyaluronan. Neurocan- or brevican-deficient mice showed a regionally heterogeneous composition of proteoglycans in perineuronal nets. Aggrecan glycoforms contribute to the molecular heterogeneity of the perineuronal nets. Proteoglycans such as phosphacan are included in matrix-associated proteoglycans. The extracellular matrix glycoprotein tenascin-R is accumulated in the perineuronal nets. The perineuronal proteoglycans are produced by associated satellite astrocytes just before weaning, while the nerve cell surface glycoproteins are produced by the associated nerve cells at earlier stages after birth. The perineuronal proteoglycans may entrap the tissue fluid and form a perineuronal gel layer which protects the synapses as a "perisynaptic barrier". Degradation of the perineuronal proteoglycans or perisynaptic barrier by treatment with chondroitinase ABC or hyaluronidase reactivates the neuronal plasticity or promotes the functional recovery of a severed nervous system. Another set of perineuronal nets occurs, which are intensely positively charged and contain guanidino compounds. It is considered that these intensely positively charged nets are intermingled with the intensely negatively charged ones of proteoglycans.

Histochemical study of perineuronal nets in the retrosplenial cortex of adult rats

The retrosplenic cortex of rats, similar to many cortical or subcortical regions, is provided with special subsets of neurons that exhibited a fenestrated or reticular coat of condensed extracellular matrix on their soma, initial dendrites and proximal axon segment. This pericellular coating, currently termed "Perineuronal Nets", was detected on the surfaces of some neurons distributing throughout the cortical layers II-V. They presented direct interconnections with each other, and appeared in close association to the astroglial processes. In addition to their collagenous ligands, the perineuronal nets (PNs) were enriched with proteoglycans (PGs, sulfated glycoconjugates) and/or glycoproteins (GPs, unsulfated glycoconjugates with terminal N-acetylgalactosamine). Accordingly, the PNs were differentially identified as belonging to three categories, depending upon their organic nature or chemical composition. First, coats exclusively formed of PGs (stained with iron colloid); second, coats formed of GPs (labeled with plant lectins binding to terminal N-acetylgalactosamine); and third, complex coats formed of PG networks intermingled with glycoprotein molecules (double stained with iron colloid and lectin). Since differential distribution of protein containing substances (GPs and/or PGs) in the extracellular matrix contributes to functional terms, we suggest that these biochemical or morphological differences in the microenvironment of some retrosplenial neurons might reflect certain functional aspects concerned with processing of navigation or episodic memory.

The intensely positively charged perineuronal net in the adult rat brain, with special reference to its reactions to oxine, chondroitinase ABC, hyaluronidase and collagenase

Light microscopic observations of healthy adult rat brain sections stained with anionic iron colloid indicated that 5-10% of neurons in the hippocampal subiculum and all neurons in the medial cerebellar nucleus possessed an intensely positively charged perineuronal net. This net was demonstrated to react to oxine, and therefore suggested to consist of guanidino compounds. It was further shown that the intensely positively charged perineuronal net, in accordance with the intensely negatively charged perineuronal net of proteoglycans, was digested by chondroitinase ABC, hyaluronidase, and collagenase, but not by endo-alphaN-acetylgalactosaminidase. This finding suggested that the former positively charged net might be linked to the latter negatively charged one.

Purkinje cells in the adult cat cerebellar cortex possess a perineuronal net of proteoglycans

The Purkinje cells in the adult cat cerebellar cortex were found to possess perineuronal proteoglycans which could be stained with our fine cationic iron colloid and Fujita's highly concentrated aldehyde fuchsin, and digested by chondroitinase ABC/keratanase/ heparitinase and hyaluronidase. The Purkinje cells are surrounded by some collagenous elements which are stained with Gömöri's ammoniacal silver and digested by collagenase. The Purkinje cells also express nerve cell surface glycoproteins which are labeled with lectin Vicia villosa agglutinin and digested by a double treatment with collagenase and endo-alpha-N-acetylgalactosaminidase. Sole digestion by endo-alpha-N-acetylgalactosaminidase never erased the lectin labeling of the nerve cell surface glycoproteins. These findings suggest that the collagenous elements mediate the linkage of the perineuronal proteoglycans to the nerve cell surface glycoproteins. It is presumed that in mice and rats, the perineuronal nets of proteoglycans and nerve cell surface glycoproteins of the Purkinje cells are so thin or coarse that they can not be sufficiently visualized under the light microscope.

Enhanced visualization of weak colloidal iron signals with Bodian's protein silver for demonstration of perineuronal nets of proteoglycans in the central nervous system

The present study aimed for a clear visualization of faintly deposited colloidal iron in tissue sections for light microscopy. Paraffin blocks containing paraformaldehyde-fixed brain tissue from healthy adult mice were cut into sections 10-15 microm thick. After deparaffinization, the sections were stained with fine cationic iron colloid at a pH value of 1.0-1.5, and treated with a mixture of potassium ferrocyanide and hydrochloride for Prussian blue reaction. Some sections were further treated with Bodian's protein silver after the Prussian blue reaction. This sensitized development of Prussian blue reaction with Bodian's protein silver more clearly visualized the faintly deposited cationic colloidal irons than the demonstration by Prussian blue reaction alone, and allowed an enhanced visualization of the perineuronal nets of sulfated proteoglycans in the brain. Thus, such fine perineuronal sulfated proteoglycans as those in the CA3 field of the hippocampus, which are weakly stained with cationic iron colloid and usually overlooked by a demonstration with only a Prussian blue reaction, could be clearly visualized with striking contrast by the sensitized development with Bodian's protein silver after the Prussian blue reaction. Preliminary hyaluronidase digestion erased Bodian's protein silver development of perineuronal sulfated proteoglycans. Though some axonal fibers were also additionally stained with Bodian's protein silver itself, this sensitized development is useful to enhance such weak colloidal iron signals as are hardly detectable by only Prussian blue reaction.