An FGF-responsive astrocyte precursor isolated from the neonatal forebrain

No Longer Underappreciated: The Emerging Concept of Astrocyte Heterogeneity in Neuroscience

Astrocytes are ubiquitous in the central nervous system (CNS). These cells possess thousands of individual processes, which extend out into the neuropil, interacting with neurons, other glia and blood vessels. Paralleling the wide diversity of their interactions, astrocytes have been reported to play key roles in supporting CNS structure, metabolism, blood-brain-barrier formation and control of vascular blood flow, axon guidance, synapse formation and modulation of synaptic transmission. Traditionally, astrocytes have been studied as a homogenous group of cells. However, recent studies have uncovered a surprising degree of heterogeneity in their development and function, in both the healthy and diseased brain. A better understanding of astrocyte heterogeneity is urgently needed to understand normal brain function, as well as the role of astrocytes in response to injury and disease.

CYP1B1: A key regulator of redox homeostasis

CYP1B1 is a member of the CYP1 subfamily of CYP superfamily of enzymes, which contains three members, CYP1A1, CYP1A2, and CYP1B1. CYP1B1 is expressed in both adult and fetal human extrahepatic tissues, including the parenchymal and stromal cells of most organs. Mutations in the CYP1B1 gene are linked to the development of primary congenital glaucoma in humans. However, the underlying mechanisms remain unknown. Using Cyp1b1-deficient mice, we showed that CYP1B1 is constitutively expressed in retinal vascular cells with a significant role in retinal neovascularization during oxygen-induced ischemic retinopathy. We also showed CYP1B1 is constitutively expressed in trabecular meshwork (TM) cells and its expression plays a significant role in the normal development and function of the TM tissue. We have observed that germline deletion of Cyp1b1 is associated with increased oxidative stress in the retinal vascular and TM cells in culture, and retinal and TM tissue in vivo. We showed increased oxidative stress was responsible for altered production of the extracellular matrix proteins and had a significant impact on cellular integrity and function of these tissues. Collectively, our studies have established an important role for CYP1B1 expression in modulation of tissue integrity and function through the regulation of cellular redox homeostasis and extracellular microenvironment.

Resolving Astrocyte Heterogeneity in the CNS

Astrocytes play essential roles in nearly all aspects of brain function from modulating synapses and neurovasculature to preserving appropriate extracellular solute concentrations. To meet the complex needs of the central nervous system (CNS), astrocytes possess highly specialized properties that are optimized for their surrounding neural circuitry. Precisely how these diverse astrocytes types are generated in vivo, however, remains poorly understood. Key to this process is a critical balance of intrinsic developmental patterning and context-dependent environmental signaling events that configures astrocyte phenotype. Indeed, emerging lines of evidence indicate that persistent cues from neighboring cells in the mature CNS cooperate with early patterning events to promote astrocyte diversity. Consistent with this, manipulating Sonic hedgehog (Shh), Notch and fibroblast growth factor (FGF) signaling in the adult brain, have profound effects on the structural, morphological and physiological state of mature astrocytes. These pathways may become disrupted in various neurological diseases and contribute to CNS pathology. This mini-review article focuses on how context-dependent environmental cues cooperate with intrinsic developmental patterning events to control astrocyte diversity in vivo in order to promote healthy brain function.

Reactive astrocytes as neural stem or progenitor cells: In vivo lineage, In vitro potential, and Genome-wide expression analysis

Here, we review the stem cell hallmarks of endogenous neural stem cells (NSCs) during development and in some niches of the adult mammalian brain to then compare these with reactive astrocytes acquiring stem cell hallmarks after traumatic and ischemic brain injury. Notably, even endogenous NSCs including the earliest NSCs, the neuroepithelial cells, generate in most cases only a single type of progeny and self‐renew only for a rather short time in vivo. In vitro, however, especially cells cultured under neurosphere conditions reveal a larger potential and long‐term self‐renewal under the influence of growth factors. This is rather well comparable to reactive astrocytes in the traumatic or ischemic brain some of which acquire neurosphere‐forming capacity including multipotency and long‐term self‐renewal in vitro, while they remain within their astrocyte lineage in vivo. Both reactive astrocytes and endogenous NSCs exhibit stem cell hallmarks largely in vitro, but their lineage differs in vivo. Both populations generate largely a single cell type in vivo, but endogenous NSCs generate neurons and reactive astrocytes remain in the astrocyte lineage. However, at some early postnatal stages or in some brain regions reactive astrocytes can be released from this fate restriction, demonstrating that they can also enact neurogenesis. Thus, reactive astrocytes and NSCs share many characteristic hallmarks, but also exhibit key differences. This conclusion is further substantiated by genome‐wide expression analysis comparing NSCs at different stages with astrocytes from the intact and injured brain parenchyma. GLIA 2015;63:1452–1468

A DNA hybridization system for labeling of neural stem cells with SPIO nanoparticles for MRI monitoring post-transplantation

Neural stem cells (NSCs) demonstrate encouraging results in cell replacement therapy for neurodegenerative disorders and traumatic injury in the central nervous system. Monitor the survival and migration of transplanted cells would provide us important information concerning the performance and integration of the graft during the therapy time course. Magnetic resonance imaging (MRI) allow us to monitor the transplanted cells in a non-invasive way. The only requirement is to use an appropriate contrast agent to label the transplanted cells. Superparamagnetic iron oxide (SPIO) nanoparticles are one of the most commonly used contrast agent for MRI detection of transplanted cells. SPIO nanoparticles demonstrated to be suitable for labeling several types of cells including NSCs. However, the current methods for SPIO labeling are non-specific, depending mostly on electrostatic interactions, demanding relatively high SPIO concentration, and long incubation time, which can affect the viability of cells. In this study, we propose a specific and relatively fast method to label NSCs with SPIO nanoparticles via DNA hybridization. Two short single stranded DNAs (ssDNAs), oligo[dT]20 and oligo[dA]20 were conjugated with a lipid molecule and SPIO nanoparticle respectively. The labeling process comprises two simple steps; first the cells are modified to present oligo[dT]20 ssDNA on the cell surface, then the oligo[dA]20 ssDNA conjugated with SPIO nanoparticles are presented to the modified cells to allow the oligo[dT]20-oligo[dA]20 hybridization. The method showed to be non-toxic at concentrations up to 50 μg/mL oligo[dA]20-SPIO nanoparticles. Presence of SPIO nanoparticles at cell surface and cell cytoplasm was verified by transmission electron microscopy (TEM). SPIO labeling via DNA hybridization demonstrated to not interfere on NSCs proliferation, aggregates formation, and differentiation. NSCs labeled with SPIO nanoparticles via DNA hybridization system were successfully detected by MRI in vitro as well in vivo. Cells transplanted into the rat brain striatum could be detected by MRI scanning up to 1 month post-transplantation.

Human stem cell-derived spinal cord astrocytes with defined mature or reactive phenotypes

Differentiation of astrocytes from human stem cells has significant potential for analysis of their role in normal brain function and disease, but existing protocols generate only immature astrocytes. Using early neuralization, we generated spinal cord astrocytes from mouse or human embryonic or induced pluripotent stem cells with high efficiency. Remarkably, short exposure to fibroblast growth factor 1 (FGF1) or FGF2 was sufficient to direct these astrocytes selectively toward a mature quiescent phenotype, as judged by both marker expression and functional analysis. In contrast, tumor necrosis factor alpha and interleukin-1β, but not FGFs, induced multiple elements of a reactive inflammatory phenotype but did not affect maturation. These phenotypically defined, scalable populations of spinal cord astrocytes will be important both for studying normal astrocyte function and for modeling human pathological processes in vitro.

Retroviral delivery of platelet-derived growth factor to spinal cord progenitor cells drives the formation of intramedullary gliomas

BACKGROUND

High-grade gliomas of the spinal cord are poorly understood tumors that are very commonly associated with bad outcomes. The transforming effects of platelet-derived growth factor (PDGF) on spinal cord glial progenitor cells may play an important role in the development of these tumors.

OBJECTIVE

To investigate the possible tumor-initiating effects of PDGF overexpression in the spinal cord, we delivered a PDGF retrovirus directly into the substance of the spinal cord.

METHODS

The spinal cords of wild-type adult rats were surgically exposed and injected with 10⁶ colony-forming units of a green fluorescent protein-tagged, PDGF-expressing retrovirus. A control virus was injected to assess the cell types that become infected during retroviral delivery to the spinal cord.

RESULTS

It was observed that PDGF overexpression in the spinal cord causes morbidity from high-grade intramedullary glioma formation between 27 and 49 days after PDGF retrovirus injection. Retroviral transduction was highly efficient with 100% of injected animals displaying the tumor phenotype. The tumors produced were highly proliferative, were locally invasive, and displayed the immunophenotype of virus-targeted glial progenitor cells (Olig2+PDGFR+NG2+GFAP-).

CONCLUSION

PDGF is capable of driving glial progenitor cells within the adult spinal cord to form high-grade gliomas. Further investigation of PDGF signaling in the spinal cord is needed to better understand and treat these devastating tumors.

A critical cell-intrinsic role for serum response factor in glial specification in the CNS

Astrocytes and oligodendrocytes play crucial roles in nearly every facet of nervous system development and function, including neuronal migration, synaptogenesis, synaptic plasticity, and myelination. Previous studies have widely characterized the signaling pathways important for astrocyte differentiation and unveiled a number of transcription factors that guide oligodendrocyte differentiation in the CNS. However, the identities of the transcription factors critical for astrocyte specification in the brain remain unknown. Here we show that deletion of the stimulus-dependent transcription factor, serum response factor (SRF), in neural precursor cells (NPCs) (Srf-Nestin-cKO) results in nearly 60% loss in astrocytes and 50% loss in oligodendrocyte precursors at birth. Cultured SRF-deficient NPCs exhibited normal growth rate and capacity to self-renew. However, SRF-deficient NPCs generated fewer astrocytes and oligodendrocytes in response to several lineage-specific differentiation factors. These deficits in glial differentiation were rescued by ectopic expression of wild-type SRF in SRF-deficient NPCs. Interestingly, ectopic expression of a constitutively active SRF (SRF-VP16) in NPCs augmented astrocyte differentiation in the presence of pro-astrocytic factors. However, SRF-VP16 expression in NPCs had an inhibitory effect on oligodendrocyte differentiation. In contrast, mice carrying conditional deletion of SRF in developing forebrain neurons (Srf-NEX-cKO) did not exhibit any deficits in astrocytes in the brain. Together, our observations suggest that SRF plays a critical cell-autonomous role in NPCs to regulate astrocyte and oligodendrocyte specification in vivo and in vitro.

Regulated temporal-spatial astrocyte precursor cell proliferation involves BRAF signalling in mammalian spinal cord

Expansion of astrocyte populations in the central nervous system is characteristic of evolutionarily more complex organisms. However, regulation of mammalian astrocyte precursor proliferation during development remains poorly understood. Here, we used Aldh1L1-GFP to identify two morphologically distinct types of proliferative astrocyte precursors: radial glia (RG) in the ventricular zone and a second cell type we call an 'intermediate astrocyte precursor' (IAP) located in the mantle region of the spinal cord. Astrogenic RG and IAP cells proliferated in a progressive ventral-to-dorsal fashion in a tight window from embryonic day 13.5 until postnatal day 3, which correlated precisely with the pattern of active ERK signalling. Conditional loss of BRAF function using BLBP-cre resulted in a 20% decrease in astrocyte production, whereas expression of activated BRAFV600E resulted in astrocyte hyperproliferation. Interestingly, BRAFV600E mitogenic effects in astrocytes were restricted, in part, by the function of p16INK4A-p19(ARF), which limited the temporal epoch for proliferation. Together, these findings suggest that astrocyte precursor proliferation involves distinct RG and IAP cells; is subjected to temporal and spatial control; and depends in part on BRAF signalling at early stages of mammalian spinal cord development.

Role of the cellular prion protein in oligodendrocyte precursor cell proliferation and differentiation in the developing and adult mouse CNS

There are numerous studies describing the signaling mechanisms that mediate oligodendrocyte precursor cell (OPC) proliferation and differentiation, although the contribution of the cellular prion protein (PrP^c) to this process remains unclear. PrP^c is a glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein involved in diverse cellular processes during the development and maturation of the mammalian central nervous system (CNS). Here we describe how PrP^c influences oligodendrocyte proliferation in the developing and adult CNS. OPCs that lack PrP^c proliferate more vigorously at the expense of a delay in differentiation, which correlates with changes in the expression of oligodendrocyte lineage markers. In addition, numerous NG2-positive cells were observed in cortical regions of adult PrP^c knockout mice, although no significant changes in myelination can be seen, probably due to the death of surplus cells.

Genetically modified human umbilical cord blood cells expressing vascular endothelial growth factor and fibroblast growth factor 2 differentiate into glial cells after transplantation into amyotrophic lateral sclerosis transgenic mice

Current therapy of a number of neuropsychiatric maladies has only symptomatic modality. Effective treatment of these neuro-degenerative diseases, including amyotrophic lateral sclerosis (ALS), may benefit from combined gene/stem-cell approaches. In this report, mononuclear fraction of human umbilical cord blood cells (hUCBCs) were transfected by electroporation with dual plasmid constructs, simultaneously expressing vascular endothelial growth factor 165 (VEGF(165)) and human fibroblast growth factor 2 (FGF(2)) (pBud-VEGF-FGF(2)). These genetically modified hUCBCs were injected retro-orbitally into presymptomatic ALS transgenic animal models ((G)93(A) mice). Lumbar spinal cords of rodents were processed for immunofluoresent staining with antibodies against human nuclear antigen (HNA), oligodendrocyte-specific protein, S100, iba1, neuronal β(3)-tubulin and CD34. Co-localization of HNA and S100 was found in the spinal cord of mice after transplantation of genetically modified hUCBCs over-expressing VEGF-FGF(2). Double staining in control animals treated with unmodified hUCBCs, however, revealed HNA+ cells expressing iba1 and CD34. Neuron-specific β(3)-tubulin or oligodendrocyte-specific protein were not expressed in hUCBCs in either control or experimental mice. These results demonstrate that genetically naïve hUCBCs may differentiate into endothelial (CD34+) and microglial (iba1+) cells; however when over-expressing VEGF-FGF(2), hUCBCs transform into astrocytes (S100+). Autocrine regulation of VEGF and FGF(2) on hUCBCs, signal molecules from dying motor neurons in spinal cord, as well as self-differentiating potential may provide a unique microenvironment for the transformation of hUCBCs into astrocytes that eventually serve as a source of growth factors to enhance the survive potential of surrounding cells in the diseased regions.

The interface between glial progenitors and gliomas

The mammalian brain and spinal cord contain heterogeneous populations of cycling, immature cells. These include cells with stem cell-like properties as well as progenitors in various stages of early glial differentiation. This latter population is distributed widely throughout gray and white matter and numerically represents an extremely large cell pool. In this review, we discuss the possibility that the glial progenitors that populate the adult CNS are one source of gliomas. Indeed, the marker phenotypes, morphologies, and migratory properties of cells in gliomas strongly resemble glial progenitors in many ways. We review briefly some salient features of normal glial development and then examine the similarities and differences between normal progenitors and cells in gliomas, focusing on the phenotypic plasticity of glial progenitors and the responses to growth factors in promoting proliferation and migration of normal and glioma cells, and discussing known mutational changes in gliomas in the context of how these might affect the proliferative and migratory behaviors of progenitors. Finally, we will discuss the "cancer stem cell" hypothesis in light of the possibility that glial progenitors can generate gliomas.