Brain matrix: structure, turnover and necessity

Perisynaptic aggrecan-based extracellular matrix coats in the human lateral geniculate body devoid of perineuronal nets

The extracellular matrix surrounds different neuronal compartments in the mature nervous system. In a variety of vertebrates, most brain regions are loaded with a distinct type of extracellular matrix around the somatodendritic part of neurons, termed perineuronal nets. The present study reports that chondrotin sulfate proteoglycan-based matrix is structured differently in the human lateral geniculate body. Using various chondrotin sulfate proteoglycan-based extracellular matrix antibodies, we show that perisomatic matrix labeling is rather weak or absent, whereas dendrites are contacted by axonal coats appearing as small, oval structures. Confocal laser scanning microscopy and electron microscopy demonstrated that these typical structures are associated with synaptic loci on dendrites. Using multiple labelings, we show that different chondrotin sulfate proteoglycan components of the extracellular matrix do not associate exclusively with neuronal structures but possibly associate with glial structures as well. Finally, we confirm and extend previous findings in primates that intensity differences of various extracellular matrix markers between magno- and parvocellular layers reflect functional segregation between these layers in the human lateral geniculate body.

Chondroitin sulphate proteoglycans: extracellular matrix proteins that regulate immunity of the central nervous system

The extracellular matrix (ECM) is a complex network of scaffolding molecules that also plays an important role in cell signalling, migration and tissue structure. In the central nervous system (CNS), the ECM is integral to the efficient development/guidance and survival of neurons and axons. However, changes in distribution of the ECM in the CNS may significantly enhance pathology in CNS disease or following injury. One group of ECM proteins that is important for CNS homeostasis is the chondroitin sulphate proteoglycans (CSPGs). Up-regulation of these molecules has been demonstrated to be both desirable and detrimental following CNS injury. Taking cues from arthritis, where there is a strong anti-CSPG immune response, there is evidence that suggests that CSPGs may influence immunity during CNS pathological conditions. This review focuses on the role of CSPGs in CNS pathologies as well as in immunity, both from a viewpoint of how they may inhibit repair and exacerbate damage in the CNS, and how they are involved in activation and function of peripheral immune cells, particularly in multiple sclerosis. Lastly, we address how CSPGs may be manipulated to improve disease outcomes.

Distribution and classification of aggrecan-based extracellular matrix in the thalamus of the rat

Extracellular matrix molecules take part in functional isolation and stabilization of neuronal compartments but form a vivid interface between neuronal elements at the same time. Previous studies have shown that the accumulation of extracellular matrix, especially its typical phenotypic form, termed perineuronal nets, correlates not only with the functional properties of the single neuron but also with the functional properties of the whole brain area. In contrast to recent advances in investigating neocortex, the present study mapped the occurrence and phenotypic appearance of aggrecan-based matrix accumulation throughout the rat thalamus. Results showed that divisions of thalamus that relay information to cortical fields known rather for their plastic properties exibit a poor matrix immunoreactivity, whereas matrix accumulation is more enhanced in nuclei connected to primary cortical regions. In addition to perineuronal nets, extracellular matrix condensed in another peculiar form, in 2-5-μm, large, round or oval structures, as described by Brückner et al. ([ 2008] Neuroscience 151:489-504) as axonal coats (ACs). Multiple labelling experiments showed that specific excitatory afferents were not ensheathed with these structures. At the same time, inhibitory endings were occasionally enwrapped in ACs. Electron microscopic analysis showed that aggrecan-immunoreactive profiles were present mostly around inhibitory terminals but also in all neuronal compartments. We suggest that aggrecan-based extracellular matrix is formed by both pre- and postsynaptic elements and is preferably associated with inhibitory terminals in the extracellular space.

Nanofibers in a hyaluronan-based pericellular matrix

The extracellular matrix of the brain is a highly organized hyaluronan-based supramolecular assembly that is involved in neuronal pathfinding, cell migration, synaptogenesis and neuronal plasticity. Here, we analyze the structure of the hyaluronan-rich pericellular matrix of an oligodendroglial precursor cell line using helium ion beam scanning microscopy at a subnanometer resolution. We find that thin nanofibers are the ultimate building elements of this oligodendroglial pericellular matrix. These structures may participate in the regulation of oligodendroglial maturation and motility.

Proteoglycans: from structural compounds to signaling molecules

Our knowledge of proteoglycan biology has significantly expanded over the past decade with the discovery of a host of new members of this multifunctional family leading to their present classification into three major categories: (1) small leucine-rich proteoglycans, 2) modular proteoglycans, and 3) cell-surface proteoglycans. In addition to being structural proteins, proteoglycans play a major role in signal transduction with regulatory functions in various cellular processes. Being mostly extracellular, they are upstream of many signaling cascades and are capable of affecting intracellular phosphorylation events and modulating distinct pathways, including those driven by bone morphogenetic protein/transforming growth factor superfamily members, receptor tyrosine kinases, the insulin-like growth factor-I receptor, and Toll-like receptors. Mechanistic insights into the molecular and cellular functions of proteoglycans have revealed both the sophistication of these regulatory proteins and the challenges that remain in uncovering the entirety of their biological functions. This review aims to summarize the multiple functions of proteoglycans with special emphasis on their intricate composition and the newly described signaling events in which these molecules play a key role.

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.

Effect of hydrocephalus on rat brain extracellular compartment

Background

The cerebral cortex may be compressed in hydrocephalus and some experiments suggest that movement of extracellular substances through the cortex is impaired. We hypothesized that the extracellular compartment is reduced in size and that the composition of the extracellular compartment changes in rat brains with kaolin-induced hydrocephalus.

Methods

We studied neonatal (newborn) onset hydrocephalus for 1 or 3 weeks, juvenile (3 weeks) onset hydrocephalus for 3–4 weeks or 9 months, and young adult (10 weeks) onset hydrocephalus for 2 weeks, after kaolin injection. Freeze substitution electron microscopy was used to measure the size of the extracellular compartment. Western blotting and immunohistochemistry with quantitative image densitometry was used to study the extracellular matrix constituents, phosphacan, neurocan, NG2, decorin, biglycan, and laminin.

Results

The extracellular space in cortical layer 1 was reduced significantly from 16.5 to 9.6% in adult rats with 2 weeks duration hydrocephalus. Western blot and immunohistochemistry showed that neurocan increased only in the periventricular white matter following neonatal induction and 3 weeks duration hydrocephalus. The same rats showed mild decorin increases in white matter and around cortical neurons. Juvenile and adult onset hydrocephalus was associated with no significant changes.

Conclusion

We conclude that compositional changes in the extracellular compartment are negligible in cerebral cortex of hydrocephalic rats at various ages. Therefore, the functional change related to extracellular fluid flow should be reversible.

Gamma-secretase and metalloproteinase activity regulate the distribution of endoplasmic reticulum to hippocampal neuron dendritic spines

The neuronal endoplasmic reticulum (ER) contributes to many physiological and pathological processes in the brain. A subset of dendritic spines on hippocampal neurons contains ER that may contribute to synapse-specific intracellular signaling. Distribution of ER to spines is dynamic, but knowledge of the regulatory mechanisms is lacking. In live cell imaging experiments we now show that cultured hippocampal neurons rapidly lost ER from spines after phorbol ester treatment. ER loss was reduced by inhibiting gamma-secretase (DAPT at 2 microM) and metalloproteinase (TAPI-0 and GM6001 at 4 microM) activity. Inhibition of protein kinase C also diminished loss of ER by preventing exit of ER from spines. Furthermore, gamma-secretase and metalloproteinase inhibition, in the absence of phorbol ester, triggered a dramatic increase in spine ER content. Metalloproteinases and gamma-secretase cleave several transmembrane proteins. Many of these substrates are known to localize to adherens junctions, a structural specialization with which spine ER interacts. One interesting possibility is thus that ER content within spines may be regulated by proteolytic activity affecting adherens junctions. Our data demonstrate a hitherto unknown role for these two proteolytic activities in regulating dynamic aspects of cellular ultrastructure, which is potentially important for cellular calcium homeostasis and several intracellular signaling pathways.

A 'GAG' reflex prevents repair of the damaged CNS

The extracellular matrix of the central nervous system (CNS) serves as both a supporting structure for cells and a rich source of signaling molecules that can influence cell proliferation, survival, migration and differentiation. A large proportion of this matrix is composed of proteoglycans--proteins with long chains of polysaccharides, called glycosaminoglycans (GAGs), covalently attached. Although many of the activities of proteoglycans depend on their core proteins, GAGs themselves can influence cell signaling. Here we review accumulating evidence that two GAGs, chondroitin sulfate and hyaluronan, play essential roles during nervous system development but also accumulate in chronic CNS lesions and inhibit axonal regeneration and remyelination, making them significant hindrances to CNS repair. We propose that the balance between the synthesis and degradation of these molecules dictates, in part, how regeneration and recovery from CNS damage occurs.

Factors controlling matrix turnover in health and disease

The impact of changes in matrix turnover on disease processes is gradually becoming more widely understood and appreciated. Similarly, the importance of interactions between the cellular and acellular components of any given tissue is finally being realized. An unhealthy cell does not make a healthy matrix; likewise an unhealthy matrix often leads to the demise of the cells within it, or at the very least to major changes in cell phenotype. We can therefore no longer investigate these two components in isolation, because the matrix so often contributes to cellular signalling pathways, and these in turn can lead to changes in matrix turnover. This is a long way from the traditional view of the role of the extracellular matrix, or 'ground substance', in filling the spaces between the cells and providing physical support for them. Just over 100 delegates assembled at Sheffield Hallam University for the Joint BSMB (British Society for Matrix Biology)/Biochemical Society Focused Meeting on Matrix Turnover: Mechanisms and Common Denominators on 2-3 April 2007. The stated aim of the meeting was to aid and encourage interactions between scientists working in various areas of matrix biology, and to this end there were sessions on intervertebral disc, turnover in the CNS (central nervous system), fibroses and tumour-stroma interactions, as well as a session covering general topics. The involvement of both the BSMB and Biochemical Society membership increased the potential for interactions between scientists and hopefully increased the value of the meeting for all the delegates. This issue of Biochemical Society Transactions contains papers written by those who gave oral presentations at the meeting. I think it is fair to conclude from their talks and the papers presented here that the 'common denominators' involved in matrix turnover include cytokine and growth factor signalling pathways that control the rates of matrix synthesis and breakdown, and which, in many disease processes, lead to an uncoupling of synthesis and breakdown and thereby the loss of homoeostasis. Evidently, the make-up of the matrix surrounding cells profoundly affects cell phenotype and behaviour through various signalling pathways. Numerous environmental stimuli may trigger these events, and a host of genes are undoubtedly involved in generating predisposing genotypes. Such factors appear to be common to many diseases involving matrix turnover.