Astrocytes in culture express fibrillar collagen

Glial scar survives until the chronic phase by recruiting scar-forming astrocytes after spinal cord injury

Spinal cord injury (SCI) causes reactive astrogliosis, the sequential phenotypic change of astrocytes in which naïve astrocytes (NAs) transform into reactive astrocytes (RAs) and subsequently become scar-forming astrocytes (SAs), resulting in glial scar formation around the lesion site and thereby limiting axonal regeneration and motor/sensory functional recovery. Inhibiting the transformation of RAs into SAs in the acute phase attenuates the reactive astrogliosis and promotes regeneration. However, whether or not SAs once formed can revert to RAs or SAs is unclear. We performed selective isolation of astrocytes from glial scars at different time points for a gene expression analysis and found that the expression of Sox9, an important transcriptional factor for glial cell differentiation, was significantly increased in chronic phase astrocytes (CAs) compared to SAs in the sub-acute phase. Furthermore, CAs showed a significantly lower expression of chondroitin sulfate proteoglycan (CSPG)-related genes than SAs. These results indicated that SAs changed their phenotypes according to the surrounding environment of the injured spinal cord over time. Even though the integrin-N-cadherin pathway is critical for glial scar formation, collagen-I-grown scar-forming astrocytes (Col-I-SAs) did not change their phenotype after depleting the effect of integrin or N-cadherin. In addition, we found that Col-I-SAs transplanted into a naïve spinal cord formed glial scar again by maintaining a high expression of genes involved in the integrin-N-cadherin pathway and a low expression of CSPG-related genes. Interestingly, the transplanted Col-I-SAs changed NAs into SAs, and anti-β₁-integrin antibody blocked the recruitment of SAs while reducing the volume of glial scar in the chronic phase. Our findings indicate that while the characteristics of glial scars change over time after SCI, SAs have a cell-autonomous function to form and maintain a glial scar, highlighting the basic mechanism underlying the persistence of glial scars after central nervous system injury until the chronic phase, which may be a therapeutic target.

Astrocyte depletion alters extracellular matrix composition in the demyelinating phase of Theiler's murine encephalomyelitis

Astrocytes produce extracellular matrix (ECM) glycoproteins contributing to the blood-brain barrier and regulating the immune response in the central nervous system (CNS). The aim of this study was to investigate the impact of astrocyte depletion upon the clinical outcome and the composition of ECM glycoproteins in a virus-induced animal model of demyelination. Glial fibrillary acidic protein (GFAP)-thymidine-kinase transgenic SJL (GFAP-knockout) and wildtype mice were infected with Theiler’s murine encephalomyelitis virus (TMEV). Astrocyte depletion was induced during the progressive, demyelinating disease phase by ganciclovir administration once daily between 56 and 77 days post infection (dpi). At 77 dpi GFAP-knockout mice showed a significant deterioration of clinical signs associated with a reduction of azan and picrosirius red stained ECM-molecules in the thoracic spinal cord. Basement-membrane-associated ECM-molecules including laminin, entactin/nidogen-1 and Kir4.1 as well as non-basement membrane-associated ECM-molecules like collagen I, decorin, tenascin-R and CD44 were significantly reduced in the spinal cord of GFAP-knockout mice. The reduction of the investigated ECM-molecules demonstrates that astrocytes play a key role in the production of ECM-molecules. The present findings indicate that the detected loss of Kir4.1 and CD44 as well as the disruption of the integrity of perineuronal nets led to the deterioration of clinical signs in GFAP-knockout mice.

Generation of Human iPSC-Derived Astrocytes with a mature star-shaped phenotype for CNS modeling

The generation of astrocytes from human induced pluripotent stem cells has been hampered by either prolonged differentiation—spanning over two months—or by shorter protocols that generate immature astrocytes, devoid of salient mature astrocytic traits pivotal for central nervous system (CNS) modeling. We directed stable hiPSC-derived neuroepithelial stem cells to human iPSC-derived Astrocytes (hiAstrocytes) with a high percentage of star-shaped cells by orchestrating an astrocytic-tuned culturing environment in 28 days. We employed RT-qPCR and ICC to validate the astrocytic commitment of the neuroepithelial stem cells. To evaluate the inflammatory phenotype, we challenged the hiAstrocytes with the pro-inflammatory cytokine IL-1β (interleukin 1 beta) and quantitatively assessed the secretion profile of astrocyte-associated cytokines and the expression of intercellular adhesion molecule 1 (ICAM-1). Finally, we quantitatively assessed the capacity of hiAstrocytes to synthesize and export the antioxidant glutathione. In under 28 days, the generated cells express canonical and mature astrocytic markers, denoted by the expression of GFAP, AQP4 and ALDH1L1. In addition, the notion of a mature phenotype is reinforced by the expression of both astrocytic glutamate transporters EAAT1 and EAAT2. Thus, hiAstrocytes have a mature phenotype that encompasses traits critical in CNS modeling, including glutathione synthesis and secretion, upregulation of ICAM-1 and a cytokine secretion profile on a par with human fetal astrocytes. This protocol generates a multifaceted astrocytic model suitable for in vitro CNS disease modeling and personalized medicine.

Glial Interfaces: Advanced Materials and Devices to Uncover the Role of Astroglial Cells in Brain Function and Dysfunction

Research over the past four decades has highlighted the importance of certain brain cells, called glial cells, and has moved the neurocentric vision of structure, function, and pathology of the nervous system toward a more holistic perspective. In this view, the demand for technologies that are able to target and both selectively monitor and control glial cells is emerging as a challenge across neuroscience, engineering, chemistry, and material science. Frequently neglected or marginally considered as a barrier to be overcome between neural implants and neuronal targets, glial cells, and in particular astrocytes, are increasingly considered as active players in determining the outcomes of device implantation. This review provides a concise overview not only of the previously established but also of the emerging physiological and pathological roles of astrocytes. It also critically discusses the most recent advances in biomaterial interfaces and devices that interact with glial cells and thus have enabled scientists to reach unprecedented insights into the role of astroglial cells in brain function and dysfunction. This work proposes glial interfaces and glial engineering as multidisciplinary fields that have the potential to enable significant advancement of knowledge surrounding cognitive function and acute and chronic neuropathologies.

Second Harmonic Generation Imaging of Collagen in Chronically Implantable Electrodes in Brain Tissue

Advances in neural engineering have brought about a number of implantable devices for improved brain stimulation and recording. Unfortunately, many of these micro-implants have not been adopted due to issues of signal loss, deterioration, and host response to the device. While glial scar characterization is critical to better understand the mechanisms that affect device functionality or tissue viability, analysis is frequently hindered by immunohistochemical tissue processing methods that result in device shattering and tissue tearing artifacts. Devices are commonly removed prior to sectioning, which can itself disturb the quality of the study. In this methods implementation study, we use the label free, optical sectioning method of second harmonic generation (SHG) to examine brain slices of various implanted intracortical electrodes and demonstrate collagen fiber distribution not found in normal brain tissue. SHG can easily be used in conjunction with multiphoton microscopy to allow direct intrinsic visualization of collagen-containing glial scars on the surface of cortically implanted electrode probes without imposing the physical strain of tissue sectioning methods required for other high resolution light microscopy modalities. Identification and future measurements of these collagen fibers may be useful in predicting host immune response and device signal fidelity.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

Extracellular matrix in the CNS induced by neuropathogenic viral infection

During the early phase of infection with an extremely neurovirulent murine coronavirus, cl-2, the ER-TR7 antigen (ERag)-positive fibers (ERfibs) associated with laminin and collagen III show a rapid increase in expression levels in the meninges, followed by an appearance of the antigens in the ventricle and brain parenchyma. Then, cl-2 invades the ventricle and ventricular wall along the newly assembled ERfibs after infection, using them as a pathway from the meninges, the initial site of infection. In the lymph nodes and spleen, ERag is mainly produced by fibroblastic reticular cells (FRCs), which play a key role in nursing the ERfibs to form a fibroblastic reticular network (FRN). The FRN functions as a conduit system to transfer antigens, cytokines or leukocytes in the lymphoid organs. In the brain parenchyma, astrocytes were found to produce the main components of mature ERfibs, such as collagen, laminin and ERag, which have been identified in the lymphoid organs. The producibility of these extracellular matrices (ECMs) by astrocytes was further confirmed by primary brain cultures, which disclosed the dissociation of laminin and ERag production, and the close association of ERag production with that of collagen, forming a fibrous structure. The pattern of ECM production in vitro indicated the process of forming mature ERfibs in the brain, that is, fibers made of collagen fibers and ERag are wrapped by laminin prepared as a sheet structure. In addition, the brain parenchymal cells that produce interferon β after infection in spite of their residence away from the sites of viral invasion were surrounded by ERfibs, which were closely associated with astrocytic fibers. These findings indicate that astrocytes play a central role in forming the astrocytic reticular network (ARN) in the brain parenchyma, as FRCs do to form FRN in the lymphoid organs.

Pharmacological Suppression of CNS Scarring by Deferoxamine Reduces Lesion Volume and Increases Regeneration in an In Vitro Model for Astroglial-Fibrotic Scarring and in Rat Spinal Cord Injury In Vivo

Lesion-induced scarring is a major impediment for regeneration of injured axons in the central nervous system (CNS). The collagen-rich glial-fibrous scar contains numerous axon growth inhibitory factors forming a regeneration-barrier for axons. We demonstrated previously that the combination of the iron chelator 2,2’-bipyridine-5,5’-decarboxylic acid (BPY-DCA) and 8-Br-cyclic AMP (cAMP) inhibits scar formation and collagen deposition, leading to enhanced axon regeneration and partial functional recovery after spinal cord injury. While BPY-DCA is not a clinical drug, the clinically approved iron chelator deferoxamine mesylate (DFO) may be a suitable alternative for anti-scarring treatment (AST). In order to prove the scar-suppressing efficacy of DFO we modified a recently published in vitro model for CNS scarring. The model comprises a co-culture system of cerebral astrocytes and meningeal fibroblasts, which form scar-like clusters when stimulated with transforming growth factor-β (TGF-β). We studied the mechanisms of TGF-β-induced CNS scarring and compared the efficiency of different putative pharmacological scar-reducing treatments, including BPY-DCA, DFO and cAMP as well as combinations thereof. We observed modulation of TGF-β-induced scarring at the level of fibroblast proliferation and contraction as well as specific changes in the expression of extracellular matrix molecules and axon growth inhibitory proteins. The individual and combinatorial pharmacological treatments had distinct effects on the cellular and molecular aspects of in vitro scarring. DFO could be identified as a putative anti-scarring treatment for CNS trauma. We subsequently validated this by local application of DFO to a dorsal hemisection in the rat thoracic spinal cord. DFO treatment led to significant reduction of scarring, slightly increased regeneration of corticospinal tract as well as ascending CGRP-positive axons and moderately improved locomotion. We conclude that the in vitro model for CNS scarring is suitable for efficient pre-screening and identification of putative scar-suppressing agents prior to in vivo application and validation, thus saving costs, time and laboratory animals.

Astrocytes alignment and reactivity on collagen hydrogels patterned with ECM proteins

To modulate the surface properties of collagen and subsequent cell-surface interactions, a method was developed to transfer protein patterns from glass coverslips to collagen type I hydrogel surfaces. Two proteins and one proteoglycan found in central nervous system extracellular matrix as well as fibrinogen were patterned in stripes onto collagen hydrogel and astrocytes were cultured on these surfaces. The addition of the stripe protein patterns to hydrogels created astrocyte layers in which cells were aligned with underlying patterns and had reduced chondroitin sulfate expression compared to the cells grown on collagen alone. Protein patterns were covalently cross-linked to the collagen and stable over four days in culture with no visible cellular modifications. The present method can be adapted to transfer other types of protein patterns from glass coverslips to collagen hydrogels.

Kallikrein 6 is a novel molecular trigger of reactive astrogliosis

Kallikrein-related peptidase 6 (KLK6) is a trypsin-like serine protease upregulated at sites of central nervous system (CNS) injury, including de novo expression by reactive astrocytes, yet its physiological actions are largely undefined. Taken with recent evidence that KLK6 activates G-protein-coupled protease-activated receptors (PARs), we hypothesized that injury-induced elevations in KLK6 contribute to the development of astrogliosis and that this occurs in a PAR-dependent fashion. Using primary murine astrocytes and the Neu7 astrocyte cell line, we show that KLK6 causes astrocytes to transform from an epitheliod to a stellate morphology and to secrete interleukin 6 (IL-6). By contrast, KLK6 reduced expression of glial fibrillary acidic protein (GFAP). The stellation-promoting activities of KLK6 were shown to be dependent on activation of the thrombin receptor, PAR1, as a PAR1-specific inhibitor, SCH79797, blocked KLK6-induced morphological changes. The ability of KLK6 to promote astrocyte stellation was also shown to be linked to activation of protein kinase C (PKC). These studies indicate that KLK6 is positioned to serve as a molecular trigger of select physiological processes involved in the development of astrogliosis and that this is likely to occur at least in part by activation of the G-protein-coupled receptor, PAR1.

Glia unglued: how signals from the extracellular matrix regulate the development of myelinating glia

The health and function of the nervous system relies on glial cells that ensheath neuronal axons with a specialized plasma membrane termed myelin. The molecular mechanisms by which glial cells target and enwrap axons with myelin are only beginning to be elucidated, yet several studies have implicated extracellular matrix proteins and their receptors as being important extrinsic regulators. This review provides an overview of the extracellular matrix proteins and their receptors that regulate multiple steps in the cellular development of Schwann cells and oligodendrocytes, the myelinating glia of the PNS and CNS, respectively, as well as in the construction and maintenance of the myelin sheath itself. The first part describes the relevant cellular events that are influenced by particular extracellular matrix proteins and receptors, including laminins, collagens, integrins, and dystroglycan. The second part describes the signaling pathways and effector molecules that have been demonstrated to be downstream of Schwann cell and oligodendroglial extracellular matrix receptors, including FAK, small Rho GTPases, ILK, and the PI3K/Akt pathway, and the roles that have been ascribed to these signaling mediators. Throughout, we emphasize the concept of extracellular matrix proteins as environmental sensors that act to integrate, or match, cellular responses, in particular to those downstream of growth factors, to appropriate matrix attachment.

Extracellular matrix in multiple sclerosis lesions: Fibrillar collagens, biglycan and decorin are upregulated and associated with infiltrating immune cells

Extracellular matrix (ECM) proteins can modify immune reactions, e.g. by sequestering or displaying growth factors and by interacting with immune and glial cells. Here we quantified by quantitative polymerase chain reaction (qPCR) expression of 50 ECM components and 34 ECM degrading enzymes in multiple sclerosis (MS) active and inactive white matter lesions. COL1A1, COL3A1, COL5A1 and COL5A2 chains were induced strongly in active lesions and even more in inactive lesions. These chains interact to form collagen types I, III and V, which are fibrillar collagens. Biglycan and decorin, which can decorate fibrillar collagens, were also induced strongly. The fibrillar collagens, biglycan and decorin were largely found between the endothelium and astrocytic glia limitans in the perivascular space where they formed a meshwork which was closely associated with infiltrating immune cells. In active lesions collagen V was also seen in the heavily infiltrated parenchyma. Fibrillar collagens I and III inhibited in vitro human monocyte production of CCL2 (MCP-1), an inflammatory chemokine involved in recruitment of immune cells. Together, ECM changes in lesions with different activities were quantified and proteins forming a perivascular fibrosis were identified. Induced fibrillar collagens may contribute to limiting enlargement of MS lesions by inhibiting the production of CCL2 by monocytes.

Brain-specific deletion of extracellular signal-regulated kinase 2 mitogen-activated protein kinase leads to aberrant cortical collagen deposition

The mitogen-activated protein kinases extracellular signal-regulated kinase (ERK)1 and 2 are essential intracellular mediators of numerous transmembrane signals. To investigate neural-specific functions of ERK2 in the brain, we used a Cre/lox strategy using Nestin:Cre to drive recombination in neural precursor cells. Nestin:Cre;ERK2(fl/fl) conditional knockout (cKO) mice have architecturally normal brains and no gross behavioral deficits. However, all cKO mice developed early-onset (postnatal day 35 to 40) frontal cortical astrogliosis, without evidence of neuronal degeneration. Frontoparietal cortical gray matter, but not underlying white matter, was found to contain abundant pericapillary and parenchymal reticulin fibrils, which were shown by immunohistochemistry to contain fibrillar collagens, including type I collagen. ERK1 general KO mice showed neither fibrils nor astrogliosis, indicating a specific role for ERK2 in the regulation of brain collagen. Collagen fibrils were also observed to a lesser extent in GFAP:Cre;ERK2(fl/fl) mice but not in CamKII-Cre;ERK2(fl/fl) mice (pyramidal neuron specific), consistent with a possible astroglial origin. Primary astroglial cultures from cKO mice expressed elevated fibrillar collagen levels, providing further evidence that the phenotype may be cell autonomous for astroglia. Unlike most other tissues, brain and spinal cord parenchyma do not normally contain fibrillar collagens, except in disease states. Determining mechanisms of ERK2-mediated collagen regulation may enable targeted suppression of glial scar formation in diverse neurological disorders.

Evidence for distinct leptomeningeal cell-dependent paracrine and EGF-linked autocrine regulatory pathways for suppression of fibrillar collagens in astrocytes

A unique and unresolved property of the central nervous system is that its extracellular matrix lacks fibrillar elements. In the present report, we show that astrocytes secrete triple helices of fibrillar collagens type I, III and V in culture, while no astroglial collagen expression could be detected in vivo. We discovered two inhibitory mechanisms that could underlie this apparent discrepancy. Thus, we uncover a strong inhibitory effect of meningeal cells on astrocytic collagen expression in coculture assays. Furthermore, we present evidence that EGF-receptor activation downregulates collagen expression in astrocytes via an autocrine loop. These investigations provide a rational framework to explain why the brain is devoid of collagen fibers, which is a unique feature that characterizes the structure of the neural extracellular matrix. Moreover, fibrillar collagens were found transiently upregulated in a laser-induced cortical lesion, suggesting that these could contribute to the glial scar that inhibits axonal regeneration.

The extracellular matrix glycoprotein Tenascin-C is expressed by oligodendrocyte precursor cells and required for the regulation of maturation rate, survival and responsiveness to platelet-derived growth factor

Analysis of Tenascin-C (TN-C) knockout mice revealed novel roles for this extracellular matrix (ECM) protein in regulation of the developmental programme of oligodendrocyte precursor cells (OPCs), their maturation into myelinating oligodendrocytes and sensitivity to growth factors. A major component of the ECM of developing nervous tissue, TN-C was expressed in zones of proliferation, migration and morphogenesis. Examination of TN-C knockout mice showed roles for TN-C in control of OPC proliferation and migration towards zones of myelination [E. Garcion et al. (2001) Development, 128, 2485-2496]. Extending our studies of TN-C effects on OPC development we found that OPCs can endogenously express TN-C protein. This expression covered the whole range of possible TN-C isoforms and could be strongly up-regulated by leukaemia inhibitory factor and ciliary neurotrophic factor, cytokines known to modulate OPC proliferation and survival. Comparative analysis of TN-C knockout OPCs with wild-type OPCs reveals an accelerated rate of maturation in the absence of TN-C, with earlier morphological differentiation and precocious expression of myelin basic protein. TN-C knockout OPCs plated on poly-lysine displayed higher levels of apoptosis than wild-type OPCs and there was also an earlier loss of responsiveness to the protective effects of platelet-derived growth factor (PDGF), indicating that TN-C has anti-apoptotic effects that may be associated with PDGF signalling. The existence of mechanisms to compensate for the absence of TN-C in the knockout is indicated by the development of oligodendrocytes derived from TN-C knockout neurospheres. These were present in equivalent proportions to those found in wild-type neurospheres but displayed enhanced myelin membrane formation.

BDNF and NT-3 promote thalamocortical axon growth with distinct substrate and temporal dependency

The role of neurotrophins in thalamic axon growth was studied by culturing embryonic rat thalamus on collagen-coated substrate or fixed cortical slices in the presence of either brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). Both BDNF and NT-3 promoted axonal growth, but the axonal growth-promoting activity depended on culture substrates. Axonal growth on collagen-coated membrane was accelerated by BDNF, but not by NT-3. In contrast, axonal outgrowth on fixed cortex was significantly enhanced by NT-3, but not by BDNF. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis of cultured thalamic cells demonstrated that culture substrates did not alter the expression of their receptors, trkB and trkC. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) staining further demonstrated that axonal growth promoted by neurotrophins was not due to reduction of cell death. Measurement of the developmental changes in BDNF and NT-3 levels revealed that, in contrast to the rapid elevation of BDNF after the arrival of thalamocortical axons to their target layer, the regulation of NT-3 protein accompanies the phase of their outgrowth in neocortex. These findings suggest that BDNF and NT-3 promote thalamic axon growth in different manners in terms of substrate dependency and developmental stage.