Proteoglycans in the developing brain: new conceptual insights for old proteins

A microglial hypothesis of globoid cell leukodystrophy pathology

Globoid cell leukodystrophy (GLD), also known as Krabbe's disease, is a fatal demyelinating disease accompanied by the formation of giant, multinucleated cells called globoid cells. Previously believed to be a byproduct of inflammation, these cells can be found early in disease before evidence of any damage. The precise mechanism by which these globoid cells cause oligodendrocyte dysfunction is not completely understood, nor is their cell type defined. This Review outlines the idea that microglial cells are transformed into an unknown and undefined novel M3 phenotype in GLD, which is cytotoxic to oligodendrocytes, leading to disease progression. © 2016 Wiley Periodicals, Inc.

Age-dependent and region-specific alteration of parvalbumin neurons and perineuronal nets in the mouse cerebral cortex

Cognitive function declines with age. Such function depends on γ-oscillation in the frontal cortex. Pyramidal neurons, and the parvalbumin-expressing interneurons (PV neurons) that control them, are important for the generation of γ-oscillation. The mechanism by which cognitive function declines is unclear. Perineuronal nets (PNNs) mainly surround the soma and proximal dendrites and axon segments of PV neurons in the cerebral cortex. Previous evidence indicates that PNNs inhibit neural plasticity. If this is true, an increase in the number of neurons surrounded by PNNs or in the thickness or density of the PNNs around neurons could decrease plasticity in the cortex. To determine if an aging-related change in cortical PNNs occurs, we examined the influence of aging on PV neurons and whether Wisteria floribunda agglutinin-positive PNNs differ depending on the cortical area. The results showed that the number of PV neurons/mm² did not change in many areas of the cortex as mice aged. In contrast, the number of neurons in the sensory cortex surrounded by PNNs increased as mice aged. Thus, with age, PNN density increases in some cortical areas but not in others. In addition, the expression level of PV protein in PV neurons decreased with aging in the whole cortex. We suggest that decreased expression of PV protein impairs fast spiking in PV neurons. We propose that PNNs surround more neurons as age increases. This aging-related increase in PNNs decreases plasticity in the cerebral cortex and reduces cognitive function. The first step in investigating this proposal would be to determine if PNN density increases with age.

Laminin-derived Ile-Lys-Val-ala-Val: a promising bioactive peptide in neural tissue engineering in traumatic brain injury

The adult brain has a very limited regeneration capacity and there is no effective treatment currently available for brain injury. Neuroprotective drugs aim to reduce the intensity of cell degeneration but do not trigger tissue regeneration. Cell replacement therapy is a novel strategy to overcome brain injury-induced disability. To enhance cell viability and neuronal differentiation, developing bioactive scaffolds combined with stem cells for transplantation is a crucial approach in brain tissue engineering. Cell interactions with the extracellular matrix (ECM) play a vital role in neuronal cell survival, neurite outgrowth, attachment, migration, differentiation, and proliferation. Thus, appropriate cell-ECM interactions are essential when designing and modifying scaffolds for application in neural tissue engineering. To improve cell-ECM interactions, scaffolds can be modified with bioactive peptides. Here, we discuss the characteristic features of laminin-derived Ile-Lys-Val-Ala-Val (IKVAV) sequence as a bio-functional motif in scaffolds and the behavior of stem cells in scaffolds conjugated with the IKVAV peptide. The incorporation of this bioactive peptide in nanofiber scaffolds markedly improves stem cell behavior and may be a potential method for cell replacement therapy in traumatic brain injury.

Regulation of glycosaminoglycan biogenesis is critical for sensitive-period-dependent vocal ontogeny

In the brain, the extracellular matrix (ECM) plays a central role during neural development and thus modulates critical-period regulated behavioral ontogeny. The major components of the ECM are glycosaminoglycans (GAGs) including chondroitin sulfate (CS). However, the specific roles of GAGs in behavioral development are largely unknown. It has been shown that xylosides affect the biological functions of GAGs through modulating GAG biosynthesis. Particularly, xylosides affect GAG biosynthesis through priming of GAG chains (priming activity), competing with endogenous core proteins that carry GAG initiation sites (decoy activity), or both. Using birdsong as our model, we investigated, for the first time, how xyloside-mediated modulation of GAG biogenesis affects song development. Xylosides infused into motor cortex of juvenile birds alter song development by specifically affecting ontogeny of the stereotyped sequence rather than the acoustic structure of syllables. Further analyses reveal that observed changes can be attributed to the priming activity rather than the decoy activity of xylosides. Collectively, these results suggest that regulation of GAG biogenesis through chemical biology approaches may allow promising therapeutic interventions of critical-period-dependent central nervous system plasticity. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1401-1412, 2017.

ADAMTS-4 in oligodendrocytes contributes to myelination with an impact on motor function

Myelination is a late developmental process regulated by a set of inhibitory and stimulatory factors, including extracellular matrix components. Accordingly, chondroitin sulfate proteoglycans (CSPGs) act as negative regulators of myelination processes. A disintegrin and metalloproteinase with thrombospondin motifs type 4 (ADAMTS-4) is an extracellular protease capable of degrading CSPGs. Although exogenous ADAMTS-4 has been proven to be beneficial in several models of central nervous system (CNS) injuries, the physiological functions of endogenous ADAMTS-4 remain poorly understood. We first used Adamts4/LacZ reporter mice to reveal that ADAMTS-4 is strongly expressed in the CNS, especially in the white matter, with a cellular profile restricted to mature oligodendrocytes. Interestingly, we evidenced an abnormal myelination in Adamts4^-/- mice, characterized by a higher diameter of myelinated axons with a shifting g-ratio. Accordingly, lack of ADAMTS-4 is accompanied by motor deficits and disturbed nervous electrical activity. In conclusion, we demonstrate that ADAMTS-4 is a new marker of mature oligodendrocytes contributing to the myelination processes and thus to the control of motor capacities.

Factors Released from Endothelial Cells Exposed to Flow Impact Adhesion, Proliferation, and Fate Choice in the Adult Neural Stem Cell Lineage

The microvasculature within the neural stem cell (NSC) niche promotes self-renewal and regulates lineage progression. Previous work identified endothelial-produced soluble factors as key regulators of neural progenitor cell (NPC) fate and proliferation; however, endothelial cells (ECs) are sensitive to local hemodynamics, and the effect of this key physiological process has not been defined. In this study, we evaluated adult mouse NPC response to soluble factors isolated from static or dynamic (flow) EC cultures. Endothelial factors generated under dynamic conditions significantly increased neuronal differentiation, while those released under static conditions stimulated oligodendrocyte differentiation. Flow increases EC release of neurogenic factors and of heparin sulfate glycosaminoglycans that increase their bioactivity, likely underlying the enhanced neuronal differentiation. Additionally, endothelial factors, especially from static conditions, promoted adherent growth. Together, our data suggest that blood flow may impact proliferation, adhesion, and the neuron-glial fate choice of adult NPCs, with implications for diseases and aging that reduce flow.

Casting a Wide Net: Role of Perineuronal Nets in Neural Plasticity

Perineuronal nets (PNNs) are unique extracellular matrix structures that wrap around certain neurons in the CNS during development and control plasticity in the adult CNS. They appear to contribute to a wide range of diseases/disorders of the brain, are involved in recovery from spinal cord injury, and are altered during aging, learning and memory, and after exposure to drugs of abuse. Here the focus is on how a major component of PNNs, chondroitin sulfate proteoglycans, control plasticity, and on the role of PNNs in memory in normal aging, in a tauopathy model of Alzheimer's disease, and in drug addiction. Also discussed is how altered extracellular matrix/PNN formation during development may produce synaptic pathology associated with schizophrenia, bipolar disorder, major depression, and autism spectrum disorders. Understanding the molecular underpinnings of how PNNs are altered in normal physiology and disease will offer insights into new treatment approaches for these diseases.

Region-specific impairments in parvalbumin interneurons in social isolation-reared mice

Many neuropsychiatric disorders show localized dysfunction in specific cortical regions. The mechanisms underlying such region-specific vulnerabilities are unknown. Post-mortem analyses have demonstrated a selective reduction in the expression of parvalbumin (PV) in GABAergic interneurons in the frontal rather than the sensory cortex of patients with neuropsychiatric disorders such as schizophrenia, autism spectrum disorders, and bipolar disorders. PV neurons are surrounded by perineuronal nets (PNNs), and are protected from oxidative stress. Previous studies have shown that the characteristics of PNNs are brain region-specific. Therefore, we hypothesized that PV neurons and PNNs may be targeted in region-specific lesions in the brain. Oxidative stress was induced in mice by rearing them in socially isolated conditions. We systemically examined the distribution of PV neurons and PNNs in the brains of these mice as well as a control group. Our results show that the regions frequently affected in neuropsychiatric disorders show significantly lower PV expression and a lower percentage of PV neurons surrounded by PNNs in the brains of socially isolated mice. These results indicate that PV neurons and PNNs exhibit region-specific vulnerabilities. Our findings may be useful for elucidating the mechanisms underlying region-specific disruption of the brain in neuropsychiatric disorders.

Perineuronal Nets in the Adult Sensory Cortex Are Necessary for Fear Learning

Lattice-like structures known as perineuronal nets (PNNs) are key components of the extracellular matrix (ECM). Once fully crystallized by adulthood, they are largely stable throughout life. Contrary to previous reports that PNNs inhibit processes involving plasticity, here we report that the dynamic regulation of PNN expression in the adult auditory cortex is vital for fear learning and consolidation in response to pure tones. Specifically, after first confirming the necessity of auditory cortical activity for fear learning and consolidation, we observed that mRNA levels of key proteoglycan components of PNNs were enhanced 4 hr after fear conditioning but were no longer different from the control groups 24 hr later. A similar pattern of regulation was observed in numbers of cells surrounded by PNNs and area occupied by them in the auditory cortex. Finally, the removal of auditory cortex PNNs resulted in a deficit in fear learning and consolidation.

Scaffolds for 3D in vitro culture of neural lineage cells

Understanding how neurodegenerative disorders develop is not only a key challenge for researchers but also for the wider society, given the rapidly aging populations in developed countries. Advances in this field require new tools with which to recreate neural tissue in vitro and produce realistic disease models. This in turn requires robust and reliable systems for performing 3D in vitro culture of neural lineage cells. This review provides a state of the art update on three-dimensional culture systems for in vitro development of neural tissue, employing a wide range of scaffold types including hydrogels, solid porous polymers, fibrous materials and decellularised tissues as well as microfluidic devices and lab-on-a-chip systems. To provide some context with in vivo development of the central nervous system (CNS), we also provide a brief overview of the neural stem cell niche, neural development and neural differentiation in vitro. We conclude with a discussion of future directions for this exciting and important field of biomaterials research.

STATEMENT OF SIGNIFICANCE

Neurodegenerative diseases, including dementia, Parkinson's and Alzheimer's diseases and motor neuron diseases, are a major societal challenge for aging populations. Understanding these conditions and developing therapies against them will require the development of new physical models of healthy and diseased neural tissue. Cellular models resembling neural tissue can be cultured in the laboratory with the help of 3D scaffolds - materials that allow the organization of neural cells into tissue-like structures. This review presents recent work on the development of different types of scaffolds for the 3D culture of neural lineage cells and the generation of functioning neural-like tissue. These in vitro culture systems are enabling the development of new approaches for modelling and tackling diseases of the brain and CNS.

Chloride Dysregulation, Seizures, and Cerebral Edema: A Relationship with Therapeutic Potential

Pharmacoresistant seizures and cytotoxic cerebral edema are serious complications of ischemic and traumatic brain injury. Intraneuronal Cl^- concentration ([Cl^-]_i) regulation impacts on both cell volume homeostasis and Cl^--permeable GABA_A receptor-dependent membrane excitability. Understanding the pleiotropic molecular determinants of neuronal [Cl^-]_i - cytoplasmic impermeant anions, polyanionic extracellular matrix (ECM) glycoproteins, and plasmalemmal Cl^- transporters - could help the identification of novel anticonvulsive and neuroprotective targets. The cation/Cl^- cotransporters and ECM metalloproteinases may be particularly druggable targets for intervention. We establish here a paradigm that accounts for recent data regarding the complex regulatory mechanisms of neuronal [Cl^-]_i and how these mechanisms impact on neuronal volume and excitability. We propose approaches to modulate [Cl^-]_i that are relevant for two common clinical sequela of brain injury: edema and seizures.

Alterations in expression of Cat-315 epitope of perineuronal nets during normal ageing, and its modulation by an open-channel NMDA receptor blocker, memantine

The perineuronal net (PNN), a specialized aggregate of the extracellular matrix, is involved in neuroprotection against oxidative stress, which is now recognized as a major contributor to age-related decline in brain functions. In this study, we investigated the age-related molecular changes of PNNs using monoclonal antibody Cat-315, which recognizes human natural killer-1 (HNK-1) glycan on aggrecan-based PNNs. Western blot analysis showed that the expression levels of Cat-315 epitope in the hippocampus were higher in middle-aged (MA, 12-month-old) mice than in young adult (YA, 2-month-old) mice. Although there were no differences in the expression levels of Cat-315 epitope between old age (OA, 20-month-old) and MA mice, Cat-315 immunoreactivity was also detected in astrocytes of OA mice. To focus on Cat-315 epitope in PNNs, we used YA and MA mice in the following experiments. Optical disector analysis showed that there were no differences in the numbers of Cat-315-positive (Cat-315⁺ ) PNNs between YA and MA mice. Fluorescence intensity analysis indicated that Cat-315 immunoreactivity in PNNs increased with age in the dorsal hippocampus, which is mainly involved in cognitive functions. Administration of an open-channel blocker of NMDA receptor, memantine, reduced the expression levels of Cat-315 epitope in the hippocampus. Furthermore, the numbers of glutamatergic and GABAergic terminals colocalized with Cat-315 epitope around parvalbumin-positive neurons were decreased by memantine. These findings provide novel insight into the involvement of PNNs in normal brain ageing, and suggest that memantine may counteract the age-related alterations in expression levels of Cat-315 epitope via regulation of its subcellular localization.

Sulfation of Glycosaminoglycans and Its Implications in Human Health and Disorders

Sulfation is a dynamic and complex posttranslational modification process. It can occur at various positions within the glycosaminoglycan (GAG) backbone and modulates extracellular signals such as cell-cell and cell-matrix interactions; different sulfation patterns have been identified for the same organs and cells during their development. Because of their high specificity in relation to function, GAG sulfation patterns are referred to as the sulfation code. This review explores the role of GAG sulfation in different biological processes at the cell, tissue, and organism levels. We address the connection between the sulfation patterns of GAGs and several physiological processes and discuss the misregulation of GAG sulfation and its involvement in several genetic and metabolic disorders. Finally, we present the therapeutic potential of GAGs and their synthetic mimics in the biomedical field.

Combined chondroitinase and KLF7 expression reduce net retraction of sensory and CST axons from sites of spinal injury

Axon regeneration in the central nervous system is limited both by inhibitory extracellular cues and by an intrinsically low capacity for axon growth in some CNS populations. Chondroitin sulfate proteoglycans (CSPGs) are well-studied inhibitors of axon growth in the CNS, and degradation of CSPGs by chondroitinase has been shown to improve the extension of injured axons. Alternatively, axon growth can be improved by targeting the neuron-intrinsic growth capacity through forced expression of regeneration-associated transcription factors. For example, a transcriptionally active chimera of Krüppel-like Factor 7 (KLF7) and a VP16 domain improves axon growth when expressed in corticospinal tract neurons. Here we tested the hypothesis that combined expression of chondroitinase and VP16-KLF7 would lead to further improvements in axon growth after spinal injury. Chondroitinase was expressed by viral transduction of cells in the spinal cord, while VP16-KLF7 was virally expressed in sensory neurons of the dorsal root ganglia or corticospinal tract (CST) neurons. After transection of the dorsal columns, both chondroitinase and VP16-KLF7 increased the proximity of severed sensory axons to the injury site. Similarly, after complete crush injuries, VP16-KLF7 expression increased the approach of CST axons to the injury site. In neither paradigm however, did single or combined treatment with chondroitinase or VP16-KLF7 enable regenerative growth distal to the injury. These results substantiate a role for CSPG inhibition and low KLF7 activity in determining the net retraction of axons from sites of spinal injury, while suggesting that additional factors act to limit a full regenerative response.

Parvalbumin neurons and perineuronal nets in the mouse prefrontal cortex

The prefrontal cortex (PFC) plays a key role in cognitive functions, memory, and attention. Alterations in parvalbumin interneurons (PV neurons) and perineuronal nets (PNNs) within the PFC have been implicated in schizophrenia and autism spectrum disorder pathology. However, it remains unclear why PV neurons and PNNs in the PFC are selectively impaired. Here we aimed to clarify if PV neurons and PNNs in the PFC have region-specific features. We found that PV neurons and PNNs were increased in a region-specific manner in the PFC during postnatal development. In the mature PFC, the expression of PV protein is lower than in other parts of the cortex. Furthermore, PNNs in the mature PFC are not typical lattice-like structures and do not have the major components of PNNs and tenascin-R. The present study indicates that PV neurons and PNNs have region-specific features, and our results suggest that PV neurons and PNNs have structural vulnerability within the PFC.

Two PTP receptors mediate CSPG inhibition by convergent and divergent signaling pathways in neurons

Receptor protein tyrosine phosphatase σ (PTPσ) and its subfamily member LAR act as transmembrane receptors that mediate growth inhibition of chondroitin sulfate proteoglycans (CSPGs). Inhibition of either receptor increases axon growth into and beyond scar tissues after CNS injury. However, it is unclear why neurons express two similar CSPG receptors, nor whether they use the same or different intracellular pathways. We have now studied the signaling pathways of these two receptors using N2A cells and primary neurons derived from knockout mice. We demonstrate that both receptors share certain signaling pathways (RhoA, Akt and Erk), but also use distinct signals to mediate CSPG actions. Activation of PTPσ by CSPGs selectively inactivated CRMP2, APC, S6 kinase and CREB. By contrast LAR activation inactivated PKCζ, cofilin and LKB1. For the first time, we propose a model of the signaling pathways downstream of these two CSPG receptors. We also demonstrate that deleting both receptors exhibits additive enhancement of axon growth in adult neuronal cultures in vitro. Our findings elucidate the novel downstream pathways of CSPGs and suggest potential synergy of blocking their two PTP receptors.

Common pitfalls of stem cell differentiation: a guide to improving protocols for neurodegenerative disease models and research

Induced pluripotent stem cells and embryonic stem cells have revolutionized cellular neuroscience, providing the opportunity to model neurological diseases and test potential therapeutics in a pre-clinical setting. The power of these models has been widely discussed, but the potential pitfalls of stem cell differentiation in this research are less well described. We have analyzed the literature that describes differentiation of human pluripotent stem cells into three neural cell types that are commonly used to study diseases, including forebrain cholinergic neurons for Alzheimer's disease, midbrain dopaminergic neurons for Parkinson's disease and cortical astrocytes for neurodegenerative and psychiatric disorders. Published protocols for differentiation vary widely in the reported efficiency of target cell generation. Additionally, characterization of the cells by expression profile and functionality differs between studies and is often insufficient, leading to highly variable protocol outcomes. We have synthesized this information into a simple methodology that can be followed when performing or assessing differentiation techniques. Finally we propose three considerations for future research, including the use of physiological O2 conditions, three-dimensional co-culture systems and microfluidics to control feeding cycles and growth factor gradients. Following these guidelines will help researchers to ensure that robust and meaningful data is generated, enabling the full potential of stem cell differentiation for disease modeling and regenerative medicine.

Accumulation of Extracellular Matrix in Advanced Lesions of Canine Distemper Demyelinating Encephalitis

In demyelinating diseases, changes in the quality and quantity of the extracellular matrix (ECM) may contribute to demyelination and failure of myelin repair and axonal sprouting, especially in chronic lesions. To characterize changes in the ECM in canine distemper demyelinating leukoencephalitis (DL), histochemical and immunohistochemical investigations of formalin-fixed paraffin-embedded cerebella using azan, picrosirius red and Gomori`s silver stain as well as antibodies directed against aggrecan, type I and IV collagen, fibronectin, laminin and phosphacan showed alterations of the ECM in CDV-infected dogs. A significantly increased amount of aggrecan was detected in early and late white matter lesions. In addition, the positive signal for collagens I and IV as well as fibronectin was significantly increased in late lesions. Conversely, the expression of phosphacan was significantly decreased in early and more pronounced in late lesions compared to controls. Furthermore, a set of genes involved in ECM was extracted from a publically available microarray data set and was analyzed for differential gene expression. Gene expression of ECM molecules, their biosynthesis pathways, and pro-fibrotic factors was mildly up-regulated whereas expression of matrix remodeling enzymes was up-regulated to a relatively higher extent. Summarized, the observed findings indicate that changes in the quality and content of ECM molecules represent important, mainly post-transcriptional features in advanced canine distemper lesions. Considering the insufficiency of morphological regeneration in chronic distemper lesions, the accumulated ECM seems to play a crucial role upon regenerative processes and may explain the relatively small regenerative potential in late stages of this disease.

The role of heparan sulphate in development: the ectodermal story

Heparan sulphate (HS) is ubiquitously expressed and is formed of repeating glucosamine and glucuronic/iduronic acid units which are generally highly sulphated. HS is found in tissues bound to proteins forming HS proteoglycans (HSPGs) which are present on the cell membrane or in the extracellular matrix. HSPGs influence a variety of biological processes by interacting with physiologically important proteins, such as morphogens, creating storage pools, generating morphogen gradients and directly mediating signalling pathways, thereby playing vital roles during development. This review discusses the vital role HS plays in the development of tissues from the ectodermal lineage. The ectodermal layer differentiates to form the nervous system (including the spine, peripheral nerves and brain), eye, epidermis, skin appendages and tooth enamel.

Expression of ADAMTS-1, -4, -5 and TIMP-3 in normal and multiple sclerosis CNS white matter

ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) -1, -4 and -5 proteases have been identified in the CNS at the mRNA level. These glutamyl endopeptidases, inhibited by tissue inhibitor of metalloproteinases (TIMP)-3, are key enzymes in the degradation of the aggregating chondroitin sulphate proteoglycans (CSPGs), and may therefore play a role in CNS extracellular matrix (ECM) changes in multiple sclerosis (MS). We have investigated ADAMTS and TIMP-3 expression in normal and MS CNS white matter by real-time RT-PCR, western blotting and immunohistochemistry. We report for the first time the presence of ADAMTS-1, -4 and -5 in normal and MS white matter. Levels of ADAMTS-1 and -5 mRNA were decreased in MS compared to normal tissue, with no significant change in ADAMTS-4 mRNA levels. Protein levels of ADAMTS-4 were significantly higher in MS tissue compared to normal tissue. Immunohistochemical studies demonstrated that ADAMTS-4 was associated predominantly with astrocytes with increased expression within MS lesions. TIMP-3 mRNA was significantly decreased in MS compared to controls. These studies suggest a role for ADAMTS-4 in the pathogenesis of MS. Further studies on the activity of ADAMTS-4 will enable a better understanding of its role in the turnover of the ECM of white matter in MS.

The extracellular matrix niche microenvironment of neural and cancer stem cells in the brain

Numerous studies demonstrated that neural stem cells and cancer stem cells (NSCs/CSCs) share several overlapping characteristics such as self-renewal, multipotency and a comparable molecular repertoire. In addition to the intrinsic cellular properties, NSCs/CSCs favor a similar environment to acquire and maintain their characteristics. In the present review, we highlight the shared properties of NSCs and CSCs in regard to their extracellular microenvironment called the NSC/CSC niche. Moreover, we point out that extracellular matrix (ECM) molecules and their complementary receptors influence the behavior of NSCs/CSCs as well as brain tumor progression. Here, we focus on the expression profile and functional importance of the ECM glycoprotein tenascin-C, the chondroitin sulfate proteoglycan DSD-1-PG/phosphacan but also on other important glycoprotein/proteoglycan constituents. Within this review, we specifically concentrate on glioblastoma multiforme (GBM). GBM is the most common malignant brain tumor in adults and is associated with poor prognosis despite intense and aggressive surgical and therapeutic treatment. Recent studies indicate that GBM onset is driven by a subpopulation of CSCs that display self-renewal and recapitulate tumor heterogeneity. Based on the CSC hypothesis the cancer arises just from a small subpopulation of self-sustaining cancer cells with the exclusive ability to self-renew and maintain the tumor. Besides the fundamental stem cell properties of self-renewal and multipotency, GBM stem cells share further molecular characteristics with NSCs, which we would like to review in this article.

Emerging tools to study proteoglycan function during skeletal development

In the past 20years, appreciation for the varied roles of proteoglycans (PGs), which are specific types of sugar-coated proteins, has increased dramatically. PGs in the extracellular matrix were long known to impart structural functions to many tissues, especially articular cartilage, which cushions bones and allows mobility at skeletal joints. Indeed, osteoarthritis is a debilitating disease associated with loss of PGs in articular cartilage. Today, however, PGs have a demonstrated role in cell biological processes, such as growth factor signalling, prompting new perspectives on the etiology of PG-associated diseases. Here, we review diseases associated with defects in PG synthesis and sulfation, also highlighting current understanding of the underlying genetics, biochemistry, and cell biology. Since most research has analyzed a class of PGs called heparan sulfate PGs, more attention is paid here to studies of chondroitin sulfate PGs (CSPGs), which are abundant in cartilage. Interestingly, CSPG synthesis is tightly linked to the cell biological processes of secretion and lysosomal degradation, suggesting that these systems may be linked genetically. Animal models of loss of CSPG function have revealed CSPGs to impact skeletal development. Specifically, our work from a mutagenesis screen in zebrafish led to the hypothesis that cartilage PGs normally delay the timing of endochondral ossification. Finally, we outline emerging approaches in zebrafish that may revolutionize the study of cartilage PG function, including transgenic methods and novel imaging techniques. Our recent work with X-ray fluorescent imaging, for example, enables direct correlation of PG function with PG-dependent biological processes.

Self-assembling peptides for stem cell and tissue engineering

Regenerative medicine holds great potential to address many shortcomings in current medical therapies. An emerging avenue of regenerative medicine is the use of self-assembling peptides (SAP) in conjunction with stem cells to improve the repair of damaged tissues. The specific peptide sequence, mechanical properties, and nanotopographical cues vary widely between different SAPs, many of which have been used for the regeneration of similar tissues. To evaluate the potential of SAPs to guide stem cell fate, we extensively reviewed the literature for reports of SAPs and stem cell differentiation. To portray the most accurate summary of these studies, we deliberately discuss both the successes and pitfalls, allowing us to make conclusions that span the breadth of this exciting field. We also expand on these conclusions by relating these findings to the fields of nanotopography, mechanotransduction, and the native composition of the extracellular matrix in specific tissues to identify potential directions for future research.

Extracellular matrix hyaluronan signals via its CD44 receptor in the increased responsiveness to mechanical stimulation

We propose that the extracellular matrix (ECM) signals CD44, a hyaluronan receptor, to increase the responsiveness to mechanical stimulation in the rat hind paw. We report that intradermal injection of hyaluronidase induces mechanical hyperalgesia, that is inhibited by co-administration of a CD44 receptor antagonist, A5G27. The intradermal injection of low (LMWH) but not high (HMWH) molecular weight hyaluronan also induces mechanical hyperalgesia, an effect that was attenuated by pretreatment with HMWH or A5G27. Pretreatment with HMWH also attenuated the hyperalgesia induced by hyaluronidase. Similarly, intradermal injection of A6, a CD44 receptor agonist, produced hyperalgesia that was inhibited by HMWH and A5G27. Inhibitors of protein kinase A (PKA) and Src, but not protein kinase C (PKC), significantly attenuated the hyperalgesia induced by both A6 and LMWH. Finally, to determine if CD44 receptor signaling is involved in a preclinical model of inflammatory pain, we evaluated the effect of A5G27 and HMWH on the mechanical hyperalgesia associated with the inflammation induced by carrageenan. Both A5G27 and HMWH attenuated carrageenan-induced mechanical hyperalgesia. Thus, while LMWH acts at its cognate receptor, CD44, to induce mechanical hyperalgesia, HMWH acts at the same receptor as an antagonist. That the local administration of HMWH or A5G27 inhibits carrageenan-induced hyperalgesia supports the suggestion that carrageenan produces changes in the ECM that contributes to inflammatory pain. These studies define a clinically relevant role for signaling by the hyaluronan receptor, CD44, in increased responsiveness to mechanical stimulation.

Proteoglycans and axon guidance: a new relationship between old partners

Neural circuits are formed with great precision during development. Accumulated evidence over the past three decades has demonstrated that growing axons are navigated toward their targets by the combined actions of attractants and repellents together with their receptors. It has long been known that proteoglycans, glycosylated proteins possessing covalently attached glycosaminoglycans, play a critical role in axon guidance; however, the molecular mechanisms by which proteoglycans regulate axon behaviors remain largely unknown. Glycosaminoglycans such as heparan sulfate and chondroitin sulfate are large linear polysaccharides composed of repeating disaccharide units that are highly modified by specific sulfation and epimerization. Recent biochemical and molecular biological studies have identified the enzymes that are involved in the biosynthesis of glycosaminoglycans. Interestingly, many mutants lacking glycosaminoglycan-synthesizing enzymes or proteoglycans in several model organisms show defects in specific nerve tract formation. In parallel, detailed biochemical studies have identified the molecular interactions between axon guidance molecules and glycosaminoglycans that have specific modification in their sugar chains. This review summarizes the structure and function of axon guidance molecules and glycosaminoglycans, and then tries to combine the knowledge from these studies to understand the role of proteoglycans from a new vantage point. Deciphering the sugar code is important for understanding the complicated nature of proteoglycans in axon guidance. Neural circuits are formed by the combined actions of axon guidance molecules. Proteoglycans play critical roles in regulating axon guidance through the interaction between signaling molecules and glycosaminoglycan chains attached to the core protein. This paper summarizes the structure and functions of axon guidance molecules and glycosaminoglycans and reviews the molecular mechanisms by which proteoglycans regulate axon guidance from a new vantage point. This article is part of the 60th Anniversary special issue.

Implications of irradiating the subventricular zone stem cell niche

Radiation therapy is a standard treatment for brain tumor patients. However, it comes with side effects, such as neurological deficits. While likely multi-factorial, the effect may in part be associated with the impact of radiation on the neurogenic niches. In the adult mammalian brain, the neurogenic niches are localized in the subventricular zone (SVZ) of the lateral ventricles and the dentate gyrus of the hippocampus, where the neural stem cells (NSCs) reside. Several reports showed that radiation produces a drastic decrease in the proliferative capacity of these regions, which is related to functional decline. In particular, radiation to the SVZ led to a reduced long-term olfactory memory and a reduced capacity to respond to brain damage in animal models, as well as compromised tumor outcomes in patients. By contrast, other studies in humans suggested that increased radiation dose to the SVZ may be associated with longer progression-free survival in patients with high-grade glioma. In this review, we summarize the cellular and functional effects of irradiating the SVZ niche. In particular, we review the pros and cons of using radiation during brain tumor treatment, discussing the complex relationship between radiation dose to the SVZ and both tumor control and toxicity.

Canadian Association of Neuroscience Review: Axonal Regeneration in the Peripheral and Central Nervous Systems – Current Issues and Advances

Injured nerves regenerate their axons in the peripheral (PNS) but not the central nervous system (CNS). The contrasting capacities have been attributed to the growth permissive Schwann cells in the PNS and the growth inhibitory environment of the oligodendrocytes in the CNS. In the current review, we first contrast the robust regenerative response of injured PNS neurons with the weak response of the CNS neurons, and the capacity of Schwann cells and not the oligodendrocytes to support axonal regeneration. We then consider the factors that limit axonal regeneration in both the PNS and CNS. Limiting factors in the PNS include slow regeneration of axons across the injury site, progressive decline in the regenerative capacity of axotomized neurons (chronic axotomy) and progressive failure of denervated Schwann cells to support axonal regeneration (chronic denervation). In the CNS on the other hand, it is the poor regenerative response of neurons, the inhibitory proteins that are expressed by oligodendrocytes and act via a common receptor on CNS neurons, and the formation of the glial scar that prevent axonal regeneration in the CNS. Strategies to overcome these limitations in the PNS are considered in detail and contrasted with strategies in the CNS.

Dysregulation of astrocyte extracellular signaling in Costello syndrome

Astrocytes produce an assortment of signals that promote neuronal maturation according to a precise developmental timeline. Is this orchestrated timing and signaling altered in human neurodevelopmental disorders? To address this question, the astroglial lineage was investigated in two model systems of a developmental disorder with intellectual disability caused by mutant Harvey rat sarcoma viral oncogene homolog (HRAS) termed Costello syndrome: mutant HRAS human induced pluripotent stem cells (iPSCs) and transgenic mice. Human iPSCs derived from patients with Costello syndrome differentiated to astroglia more rapidly in vitro than those derived from wild-type cell lines with normal HRAS, exhibited hyperplasia, and also generated an abundance of extracellular matrix remodeling factors and proteoglycans. Acute treatment with a farnesyl transferase inhibitor and knockdown of the transcription factor SNAI2 reduced expression of several proteoglycans in Costello syndrome iPSC-derived astrocytes. Similarly, mice in which mutant HRAS was expressed selectively in astrocytes exhibited experience-independent increased accumulation of perineuronal net proteoglycans in cortex, as well as increased parvalbumin expression in interneurons, when compared to wild-type mice. Our data indicate that astrocytes expressing mutant HRAS dysregulate cortical maturation during development as shown by abnormal extracellular matrix remodeling and implicate excessive astrocyte-to-neuron signaling as a possible drug target for treating mental impairment and enhancing neuroplasticity.

Signaling pathways regulating neuron-glia interaction and their implications in Alzheimer's disease

Astrocytes are the most abundant cells in the central nervous system. They play critical roles in neuronal homeostasis through their physical properties and neuron-glia signaling pathways. Astrocytes become reactive in response to neuronal injury and this process, referred to as reactive astrogliosis, is a common feature accompanying neurodegenerative conditions, particularly Alzheimer's disease. Reactive astrogliosis represents a continuum of pathobiological processes and is associated with morphological, functional, and gene expression changes of varying degrees. There has been a substantial growth of knowledge regarding the signaling pathways regulating glial biology and pathophysiology in recent years. Here, we attempt to provide an unbiased review of some of the well-known players, namely calcium, proteoglycan, transforming growth factor β, NFκB, and complement, in mediating neuron-glia interaction under physiological conditions as well as in Alzheimer's disease. This review discusses the role of astrocytic NFκB and calcium as well as astroglial secreted factors, including proteoglycans, TGFβ, and complement in mediating neuronal function and AD pathogenesis through direct interaction with neurons and through cooperation with microglia.

Caught in the Net: Perineuronal Nets and Addiction

Exposure to drugs of abuse induces plasticity in the brain and creates persistent drug-related memories. These changes in plasticity and persistent drug memories are believed to produce aberrant motivation and reinforcement contributing to addiction. Most studies have explored the effect drugs of abuse have on pre- and postsynaptic cells and astrocytes; however, more recently, attention has shifted to explore the effect these drugs have on the extracellular matrix (ECM). Within the ECM are unique structures arranged in a net-like manner, surrounding a subset of neurons called perineuronal nets (PNNs). This review focuses on drug-induced changes in PNNs, the molecules that regulate PNNs, and the expression of PNNs within brain circuitry mediating motivation, reward, and reinforcement as it pertains to addiction.

Neuron-Glia Interactions in Neural Plasticity: Contributions of Neural Extracellular Matrix and Perineuronal Nets

Synapses are specialized structures that mediate rapid and efficient signal transmission between neurons and are surrounded by glial cells. Astrocytes develop an intimate association with synapses in the central nervous system (CNS) and contribute to the regulation of ion and neurotransmitter concentrations. Together with neurons, they shape intercellular space to provide a stable milieu for neuronal activity. Extracellular matrix (ECM) components are synthesized by both neurons and astrocytes and play an important role in the formation, maintenance, and function of synapses in the CNS. The components of the ECM have been detected near glial processes, which abut onto the CNS synaptic unit, where they are part of the specialized macromolecular assemblies, termed perineuronal nets (PNNs). PNNs have originally been discovered by Golgi and represent a molecular scaffold deposited in the interface between the astrocyte and subsets of neurons in the vicinity of the synapse. Recent reports strongly suggest that PNNs are tightly involved in the regulation of synaptic plasticity. Moreover, several studies have implicated PNNs and the neural ECM in neuropsychiatric diseases. Here, we highlight current concepts relating to neural ECM and PNNs and describe an in vitro approach that allows for the investigation of ECM functions for synaptogenesis.

In Sickness and in Health: Perineuronal Nets and Synaptic Plasticity in Psychiatric Disorders

Rapidly emerging evidence implicates perineuronal nets (PNNs) and extracellular matrix (ECM) molecules that compose or interact with PNNs, in the pathophysiology of several psychiatric disorders. Studies on schizophrenia, autism spectrum disorders, mood disorders, Alzheimer's disease, and epilepsy point to the involvement of ECM molecules such as chondroitin sulfate proteoglycans, Reelin, and matrix metalloproteases, as well as their cell surface receptors. In many of these disorders, PNN abnormalities have also been reported. In the context of the “quadripartite” synapse concept, that is, the functional unit composed of the pre- and postsynaptic terminals, glial processes, and ECM, and of the role that PNNs and ECM molecules play in regulating synaptic functions and plasticity, these findings resonate with one of the most well-replicated aspects of the pathology of psychiatric disorders, that is, synaptic abnormalities. Here we review the evidence for PNN/ECM-related pathology in these disorders, with particular emphasis on schizophrenia, and discuss the hypothesis that such pathology may significantly contribute to synaptic dysfunction.

Fetal brain extracellular matrix boosts neuronal network formation in 3D bioengineered model of cortical brain tissue

The extracellular matrix (ECM) constituting up to 20% of the organ volume is a significant component of the brain due to its instructive role in the compartmentalization of functional microdomains in every brain structure. The composition, quantity and structure of ECM changes dramatically during the development of an organism greatly contributing to the remarkably sophisticated architecture and function of the brain. Since fetal brain is highly plastic, we hypothesize that the fetal brain ECM may contain cues promoting neural growth and differentiation, highly desired in regenerative medicine. Thus, we studied the effect of brain-derived fetal and adult ECM complemented with matricellular proteins on cortical neurons using in vitro 3D bioengineered model of cortical brain tissue. The tested parameters included neuronal network density, cell viability, calcium signaling and electrophysiology. Both, adult and fetal brain ECM as well as matricellular proteins significantly improved neural network formation as compared to single component, collagen I matrix. Additionally, the brain ECM improved cell viability and lowered glutamate release. The fetal brain ECM induced superior neural network formation, calcium signaling and spontaneous spiking activity over adult brain ECM. This study highlights the difference in the neuroinductive properties of fetal and adult brain ECM and suggests that delineating the basis for this divergence may have implications for regenerative medicine.

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.

Development and Structural Variety of the Chondroitin Sulfate Proteoglycans-Contained Extracellular Matrix in the Mouse Brain

Chondroitin sulfate proteoglycans (CSPGs) are major components of the extracellular matrix (ECM) in the brain. In adult mammals, CSPGs form the specialized ECM structure perineuronal nets (PNNs) that surround somata and dendrites of certain types of neurons. PNNs restrict synaptic plasticity and regulate the closure of critical periods. Although previous studies have examined the starting period of PNN formation, focusing on primary sensory cortices, there are no systematic studies at the whole brain level. Here, we examined the starting period of PNN formation in male mice ranging in age from postnatal day 3 to week 11, mainly focusing on several cortical areas, limbic structures, hypothalamus, and brain stem, using lectin histochemistry with Wisteria floribunda agglutinin (WFA). Results showed that early PNN formation was observed in several reticular formations of the brain stem related to the cranial nerves and primary somatosensory cortices. In the limbic system, PNN formation in the hippocampus started earlier than that of the amygdala. Furthermore, in the medial amygdaloid nucleus and some hypothalamic regions, WFA labeling did not show typical PNN-like forms. The present study suggests spatiotemporal differences at the beginning of PNN formation and a structural variety of CSPG-contained ECM in the brain.

Time-dependent localization of high- and low-sulfated keratan sulfates in the song nuclei of developing zebra finches

Keratan sulfate proteoglycans (KSPGs) and chondroitin sulfate proteoglycans (CSPGs) consist of a protein core with covalently attached glycosaminoglycan side chain. Although CSPGs are known to regulate the end of the critical period, the role of KSPGs in brain development remains unclear. Young male zebra finches memorise song templates during development. The brain regions that are responsible for song learning, known as song nuclei, are recognized as a suitable model for the study of brain development. To understand the potential role of KSPGs, here we examined the localization of KSs with different degrees of sulfation in the brain of developing male zebra finches. Exclusively in the song nuclei, an increase in expression of 5-D-4-positive (5-D-4(+)) high-sulfated KS started after hatching, and reached a plateau at the end of the sensory period, during which the young bird listens to and memorises the song of an adult tutor. By contrast, weak and ubiquitous expression of BCD-4(+) low-sulfated KS remained unchanged until the end of the sensory period, and first increased in the song nuclei at the end of the sensorimotor period, during which the young bird produces plastic songs. Immunoblot analysis showed that phosphacan was a common core protein of 5-D-4(+) KS and BCD-4(+) KS. Finally, we confirmed that the sulfotransferase responsible for the synthesis of high-sulfated KS was exclusively localised in the song nuclei. Our observations suggest that time-dependent localization of KSPGs with different sulfation patterns in the song nuclei may underlie song learning in developing male zebra finches.

The hyaluronan and proteoglycan link proteins: Organizers of the brain extracellular matrix and key molecules for neuronal function and plasticity

The hyaluronan and proteoglycanbinding link protein (Hapln) is a key molecule in the formation and control of hyaluronan-based condensed perineuronal matrix in the adult brain. This review summarizes the recent advances in understanding the role of Haplns in the formation and control of two distinct types of perineuronal matrices, one for "classical" PNN and the other for the specialized extracellular matrix (ECM) at the node of Ranvier in the central nervous system (CNS). We introduce the structural components of each ECM organization including the basic concept of supramolecular structure named "HLT model". We furthermore summarize the developmental and physiological role of perineuronal ECMs from the studies of Haplns and related molecules. Finally, we also discuss the potential mechanism modulating PNNs in the adult CNS. This layer of organized matrices may exert a direct effect via core protein or sugar moiety from the structure or by acting as a binding site for biologically active molecules, which are important for neuronal plasticity and saltatory conduction.

Proteolytic regulation of synaptic plasticity in the mouse primary visual cortex: analysis of matrix metalloproteinase 9 deficient mice

The extracellular matrix (ECM) is known to play important roles in regulating neuronal recovery from injury. The ECM can also impact physiological synaptic plasticity, although this process is less well understood. To understand the impact of the ECM on synaptic function and remodeling in vivo, we examined ECM composition and proteolysis in a well-established model of experience-dependent plasticity in the visual cortex. We describe a rapid change in ECM protein composition during Ocular Dominance Plasticity (ODP) in adolescent mice, and a loss of ECM remodeling in mice that lack the extracellular protease, matrix metalloproteinase-9 (MMP9). Loss of MMP9 also attenuated functional ODP following monocular deprivation (MD) and reduced excitatory synapse density and spine density in sensory cortex. While we observed no change in the morphology of existing dendritic spines, spine dynamics were altered, and MMP9 knock-out (KO) mice showed increased turnover of dendritic spines over a period of 2 days. We also analyzed the effects of MMP9 loss on microglia, as these cells are involved in extracellular remodeling and have been recently shown to be important for synaptic plasticity. MMP9 KO mice exhibited very limited changes in microglial morphology. Ultrastructural analysis, however, showed that the extracellular space surrounding microglia was increased, with concomitant increases in microglial inclusions, suggesting possible changes in microglial function in the absence of MMP9. Taken together, our results show that MMP9 contributes to ECM degradation, synaptic dynamics and sensory-evoked plasticity in the mouse visual cortex.

Local Delivery of High-Dose Chondroitinase ABC in the Sub-Acute Stage Promotes Axonal Outgrowth and Functional Recovery after Complete Spinal Cord Transection

Chondroitin sulfate proteoglycans (CSPGs) are glial scar-associated molecules considered axonal regeneration inhibitors and can be digested by chondroitinase ABC (ChABC) to promote axonal regeneration after spinal cord injury (SCI). We previously demonstrated that intrathecal delivery of low-dose ChABC (1 U) in the acute stage of SCI promoted axonal regrowth and functional recovery. In this study, high-dose ChABC (50 U) introduced via intrathecal delivery induced subarachnoid hemorrhage and death within 48 h. However, most SCI patients are treated in the sub-acute or chronic stages, when the dense glial scar has formed and is minimally digested by intrathecal delivery of ChABC at the injury site. The present study investigated whether intraparenchymal delivery of ChABC in the sub-acute stage of complete spinal cord transection would promote axonal outgrowth and improve functional recovery. We observed no functional recovery following the low-dose ChABC (1 U or 5 U) treatments. Furthermore, animals treated with high-dose ChABC (50 U or 100 U) showed decreased CSPGs levels. The extent and area of the lesion were also dramatically decreased after ChABC treatment. The outgrowth of the regenerating axons was significantly increased, and some partially crossed the lesion site in the ChABC-treated groups. In addition, retrograde Fluoro-Gold (FG) labeling showed that the outgrowing axons could cross the lesion site and reach several brain stem nuclei involved in sensory and motor functions. The Basso, Beattie and Bresnahan (BBB) open field locomotor scores revealed that the ChABC treatment significantly improved functional recovery compared to the control group at eight weeks after treatment. Our study demonstrates that high-dose ChABC treatment in the sub-acute stage of SCI effectively improves glial scar digestion by reducing the lesion size and increasing axonal regrowth to the related functional nuclei, which promotes locomotor recovery. Thus, our results will aid in the treatment of spinal cord injury.