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

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.

Designing Patient-Driven, Tissue-Engineered Models of Primary and Metastatic Breast Cancer

The rising survival rate for early-stage breast cancer in the United States has created an expanding population of women in remission at risk for distant recurrence, with metastatic spread to the brain demonstrating an especially poor prognosis. The current standard of care for breast cancer brain metastases is not well defined or differentiated from the treatment of brain metastases from other primary sites. Here, we present tissue-engineered models of the primary and brain metastatic breast cancer microenvironments informed by analysis of patient tumor resections. We find that metastatic resections demonstrate distinct cellular and matrix components compared with primary resections or non-cancerous controls. Using our model systems, we find that the observed deposition of collagen I after metastasis to the brain may enhance breast cancer invasion. Future optimization of these models will present a novel platform to examine tumor-stroma interactions and screen therapeutics for the management of metastatic breast cancer.

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.

Formation of fibroblastic reticular network in the brain after infection with neurovirulent murine coronavirus

cl‐2 virus is an extremely neurovirulent murine coronavirus. However, during the initial phase of infection between 12 and 24 h post‐inoculation (hpi), the viral antigens are detected only in the meninges, followed by viral spread into the ventricular wall before invasion into the brain parenchyma, indicating that the viruses employ a passage between the meninges and ventricular wall as an entry route into the brain parenchyma. At 48 hpi, the passage was found to be constructed by ER‐TR7 antigen (ERag)‐positive fibers (ERfibs) associated with laminin and collagen III between the fourth ventricle and meninges at the cerebellopontine angle. The construct of the fibers mimics the reticular fibers of the fibroblastic reticular network, which comprises a conduit system in the lymphoid organs. In the meninges, ERfibs together with collagen fibers, lining in a striped pattern, made up a pile of thin sheets. In the brain parenchyma, mature ERfibs associated with laminin were found around blood vessels. Besides mature ERfibs, immature Erfibs without associations with other extracellular matrix components like laminin and collagen appeared after infection, suggesting that the CNS creates a unique conduit system for immune communication triggered by viral invasion.

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.

Influence of the extracellular matrix on endogenous and transplanted stem cells after brain damage

The limited regeneration capacity of the adult central nervous system (CNS) requires strategies to improve recovery of patients. In this context, the interaction of endogenous as well as transplanted stem cells with their environment is crucial. An understanding of the molecular mechanisms could help to improve regeneration by targeted manipulation. In the course of reactive gliosis, astrocytes upregulate Glial fibrillary acidic protein (GFAP) and start, in many cases, to proliferate. Beside GFAP, subpopulations of these astroglial cells coexpress neural progenitor markers like Nestin. Although cells express these markers, the proportion of cells that eventually give rise to neurons is limited in many cases in vivo compared to the situation in vitro. In the first section, we present the characteristics of endogenous progenitor-like cells and discuss the differences in their neurogenic potential in vitro and in vivo. As the environment plays an important role for survival, proliferation, migration, and other processes, the second section of the review describes changes in the extracellular matrix (ECM), a complex network that contains numerous signaling molecules. It appears that signals in the damaged CNS lead to an activation and de-differentiation of astrocytes, but do not effectively promote neuronal differentiation of these cells. Factors that influence stem cells during development are upregulated in the damaged brain as part of an environment resembling a stem cell niche. We give a general description of the ECM composition, with focus on stem cell-associated factors like the glycoprotein Tenascin-C (TN-C). Stem cell transplantation is considered as potential treatment strategy. Interaction of transplanted stem cells with the host environment is critical for the outcome of stem cell-based therapies. Possible mechanisms involving the ECM by which transplanted stem cells might improve recovery are discussed in the last section.

Meningeal and choroid plexus cells--novel drug targets for CNS disorders

The meninges and choroid plexus perform many functions in the developing and adult human central nervous system (CNS) and are composed of a number of different cell types. In this article I focus on meningeal and choroid plexus cells as targets for the development of drugs to treat a range of traumatic, ischemic and chronic brain disorders. Meningeal cells are involved in cortical development (and their dysfunction may be involved in cortical dysplasia), fibrotic scar formation after traumatic brain injuries (TBI), brain inflammation following infections, and neurodegenerative disorders such as Multiple Sclerosis (MS) and Alzheimer's disease (AD) and other brain disorders. The choroid plexus regulates the composition of the cerebrospinal fluid (CSF) as well as brain entry of inflammatory cells under basal conditions and after injuries. The meninges and choroid plexus also link peripheral inflammation (occurring in the metabolic syndrome and after infections) to CNS inflammation which may contribute to the development and progression of a range of CNS neurological and psychiatric disorders. They respond to cytokines generated systemically and secrete cytokines and chemokines that have powerful effects on the brain. The meninges may also provide a stem cell niche in the adult brain which could be harnessed for brain repair. Targeting meningeal and choroid plexus cells with therapeutic agents may provide novel therapies for a range of human brain disorders.

Morphological plasticity of rodent astroglia

In the past two decades, there has been an explosion of research on the role of neuroglial interactions in the control of brain homeostasis in both physiological and pathological conditions. Astrocytes, a subtype of glia in the central nervous system, are dynamic signaling elements that regulate neurogenesis and development of brain circuits, displaying intimate dynamic relationships with neurons, especially at synaptic sites where they functionally integrate the tripartite synapse. When astrocytes are isolated from the brain and maintained in culture, they exhibit a polygonal shape unlike their precursors in vivo. However, cultured astrocytes can be induced to undergo morphological plasticity leading to process formation, either by interaction with neurons or by the influence of pharmacological agents. This review highlights studies on the molecular mechanisms underlying morphological plasticity in astrocyte cultures and intact brain tissue, both in situ and in vivo.

Astrogliosis during acute and chronic cuprizone demyelination and implications for remyelination

In multiple sclerosis, microglia/macrophage activation and astrocyte reactivity are important components of the lesion environment that can impact remyelination. The current study characterizes these glial populations relative to expression of candidate regulatory molecules in cuprizone demyelinated corpus callosum. Importantly, periods of recovery after acute or chronic cuprizone demyelination are examined to compare conditions of efficient versus limited remyelination, respectively. Microglial activation attenuates after early demyelination. In contrast, astrocyte reactivity persists throughout demyelination and a 6-week recovery period following either acute or chronic demyelination. This astrocyte reaction is characterized by (a) early proliferation, (b) increased expression of GFAP (glial fibrillary acidic protein), Vim (vimentin), Fn1 (fibronectin) and CSPGs (chondroitin sulphate proteoglycans) and (c) elaboration of a dense network of processes. Glial processes elongated in the axonal plane persist throughout lesion areas during both the robust remyelination that follows acute demyelination and the partial remyelination that follows chronic demyelination. However, prolonged astrocyte reactivity with chronic cuprizone treatment does not progress to barrier formation, i.e. dense compaction of astrocyte processes to wall off the lesion area. Multiple candidate growth factors and inflammatory signals in the lesion environment show strong correlations with GFAP across the acute cuprizone demyelination and recovery time course, yet there is more divergence across the progression of chronic cuprizone demyelination and recovery. However, differential glial scar formation does not appear to be responsible for differential remyelination during recovery in the cuprizone model. The astrocyte phenotype and lesion characteristics in this demyelination model inform studies to identify triggers of non-remyelinating sclerosis in chronic multiple sclerosis lesions.

Trafficking of immune cells in the central nervous system

The CNS is an immune-privileged environment, yet the local control of multiple pathogens is dependent on the ability of immune cells to access and operate within this site. However, inflammation of the distinct anatomical sites (i.e., meninges, cerebrospinal fluid, and parenchyma) associated with the CNS can also be deleterious. Therefore, control of lymphocyte entry and migration within the brain is vital to regulate protective and pathological responses. In this review, several recent advances are highlighted that provide new insights into the processes that regulate leukocyte access to, and movement within, the brain.

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.

Behavior of parasite-specific effector CD8+ T cells in the brain and visualization of a kinesis-associated system of reticular fibers

To understand lymphocyte behavior in the brain, we used two-photon microscopy to visualize effector CD8(+) T cells during toxoplasmic encephalitis. These cells displayed multiple behaviors with two distinct populations of cells apparent: one with a constrained pattern of migration and one with a highly migratory subset. The proportion of these populations varied over time associated with changes in antigen availability as well as T cell expression of the inhibitory receptor PD1. Unexpectedly, the movement of infiltrating cells was closely associated with an infection-induced reticular system of fibers. This observation suggests that, whereas in other tissues pre-existing scaffolds exist that guide lymphocyte migration, in the brain specialized structures are induced by inflammation that guide migration of T cells in this immune-privileged environment.