Andrew Chojnacki

White Matter Plasticity and Enhanced Remyelination in the Maternal CNS

Myelination, the process in which oligodendrocytes coat CNS axons with a myelin sheath, represents an important but poorly understood form of neural plasticity that may be sexually dimorphic in the adult CNS. Remission of multiple sclerosis during pregnancy led us to hypothesize that remyelination is enhanced in the maternal brain. Here we report an increase in the generation of myelin-forming oligodendrocytes and in the number of myelinated axons in the maternal murine CNS. Remarkably, pregnant mice have an enhanced ability to remyelinate white matter lesions. The hormone prolactin regulates oligodendrocyte precursor proliferation and mimics the regenerative effects of pregnancy. This suggests that maternal white matter plasticity imparts a striking ability to repair demyelination and identifies prolactin as a potential therapeutic agent.

Identity crisis for adult periventricular neural stem cells: subventricular zone astrocytes, ependymal cells or both?

A population of neural stem cells (NSCs) resides adjacent to the lateral ventricles in the adult mammalian brain. Despite knowledge of their existence since the early 1990s, their identity remains controversial, with evidence suggesting that they may be ependymal cells, glial fibrillary acidic protein (GFAP)-expressing subventricular zone (SVZ) cells or several distinct NSC populations. This issue has major implications for the therapeutic use of NSCs as well as for the study and treatment of brain cancers. Recent studies have both shed light on the issue and added to the controversy.

Production of neurons, astrocytes and oligodendrocytes from mammalian CNS stem cells

The isolation and expansion of precursor cells in a serum-free culture system allows for the systematic characterization of their properties and the intrinsic and extrinsic signals that regulate their function. The discovery of neural stem cells in the adult mouse brain was made possible by the creation of a novel culture system subsequently termed the neurosphere assay. Therein, the dissociated adult mouse periventricular area was plated in the presence of epidermal growth factor, but in the absence of adhesive substrates, which resulted in the generation of spheres of proliferating cells that detached from the plate bottom and remained suspended in the media. Since its inception, the neurosphere culture system has been widely used in the neural precursor cell field and has been extensively adapted for the isolation and expansion of corneal, cardiac, skin, prostate, mammary and brain tumor stem cells. The original neurosphere culture protocol, which takes approximately 10 d to complete, is described here in detail.

Promoting oligodendrogenesis and myelin repair using the multiple sclerosis medication glatiramer acetate

The formation of oligodendrocytes (oligodendrogenesis) and myelin is regulated by several neurotrophic factors. Strategies to increase the level of these trophic molecules may facilitate repair in demyelinating conditions, such as multiple sclerosis (MS). Because leukocytes are a source of neurotrophic factors, and as glatiramer acetate (GA) generates T helper 2 (Th2) lymphocytes that are not known to be harmful, we tested the hypothesis that GA regulates oligodendrogenesis and myelin formation. First, we generated GA-reactive Th2 cells and determined that they produced transcripts for neurotrophic factors, including insulin-like growth factor-1 (IGF-1). The conditioned medium from GA-reactive T cells elevated IGF-1 protein and promoted the formation of oligodendrocyte precursor cells (OPCs) from embryonic brainderived forebrain cells in culture. We next subjected mice to lysolecithin-induced demyelination of the spinal cord. At 7 days after the insult, the number of OPCs in the demyelinated dorsal column was higher than that in uninjured controls, and was further increased by the daily s.c. injection with GA. Increased OPC generation by GA was associated temporally with the elevation of IGF-1 and brain-derived neurotrophic factor (BDNF) in the spinal cord. Finally, the resultant remyelination at 28 days was higher in mice treated with GA during the first 7 days of injury compared with vehicle controls. These results indicate that GA promotes oligodendrogenesis and remyelination through mechanisms that involve the elevation of growth factors conducive for repair.

Isolation of a Novel Platelet-Derived Growth Factor-Responsive Precursor from the Embryonic Ventral Forebrain

Oligodendrocyte progenitor cells express platelet-derived growth factor (PDGF) receptor-α and, when expanded in PDGF only, have been shown to generate oligodendrocytes and astrocytes but never neurons. Recent evidence suggests that oligodendrocytes are generated by a common progenitor that also generates neurons but not astrocytes. We used the neurosphere culture system to isolate embryonic ventral forebrain, PDGF-responsive precursors (PRPs). We report that the medial ganglionic eminence is the source of PRP-generated neurospheres and that the progeny can differentiate into parvalbumin-positive interneurons, oligodendrocytes, and astrocytes. Thyroid hormone and bone morphogenetic protein-2 (BMP-2) promote the mutually exclusive differentiation of oligodendrocytes and neurons, respectively, whereas ciliary neurotrophic factor acts with BMP-2 to suppress OLIG2 expression and promote astroglial differentiation. PRPs require fibroblast growth factor-2 together with PDGF to maintain self-renewal, which is dependent on sonic hedgehog signaling. We present evidence for forebrain oligodendrocytes and parvalbumin-positive interneurons being generated by a common precursor and elucidate signals regulating the multiple differentiation routes of the progeny of this precursor.

PDGFRα Expression Distinguishes GFAP-Expressing Neural Stem Cells from PDGF-Responsive Neural Precursors in the Adult Periventricular Area

Jackson et al. (2006) have reported that adult glial fibrillary acid protein (GFAP)-expressing neural stem cells (NSCs) also express platelet-derived growth factor (PDGF) receptor-α (PDGFRα), and that their stimulation by PDGF induced the formation of a glioma-like mass. Here, we reexamined the relationship between PDGFRα and GFAP expression within the three-dimensional organization of the adult periventricular area. Using four independent PDGFRα antibodies, we found that adult mouse GFAP-expressing NSCs and PDGFRα-expressing cells represent two distinct populations of neural precursors. Examination of the adult periventricular area in a mouse line that expresses nuclear-localized enhanced green fluorescent protein under the control of the PDGFRα promoter confirmed that GFAP-expressing NSCs do not express PDGFRα. Furthermore, PDGF-responsive neural precursors were found at least one cell layer subjacent to the ependymal layer, and were evenly distributed across the lateral ventricular wall, which contrasts with the reported patchy and often ependymal localization of adult GFAP-expressing NSCs. Adult human PDGFRα-expressing neural precursors were also found not to express GFAP. PDGF-responsive neural precursors, but not GFAP-expressing NSCs, responded to infusions of PDGF by generating glioma-like masses. Our results do not support the view that GFAP-expressing NSCs are the origin of glioma-like masses that form after intraventricular PDGF infusion.

Radial glial cells as neuronal precursors: The next generation?

One of the major challenges in precursor cell biology of the central nervous system (CNS) has been to unambiguously identify different precursor cells so that we may better study them. Precursors have been largely characterized by the cell types they produce, compelling us to study them retrospectively without knowledge of which particular cell gave rise to specific progeny. Radial glial cells, easily identifiable cell types in the embryonic germinal zone, have been suggested recently to comprise a significant proportion of the neuronal precursor cell population of the developing brain (Malatesta et al., 2000; Miyata et al., 2001; Noctor et al., 2001, 2002). Radial glia can be identified by their characteristic bipolar morphology in which the cell soma resides in the ventricular zone (VZ) or subventricular zone (SVZ), bearing a long basal process that extends outwards toward the pial surface and a second, short apical process that contacts the ventricular wall. The existence of such cells has long been recognized and they have borne many names over the course of the field’s history (Bentivoglio and Mazzarello, 1999). Pasko Rakic recognized that these cells express the astrocytic marker glial fibrillary acidic protein (GFAP) in the developing primate CNS and termed them “radial glia” (Rakic, 1972; Levitt and Rakic, 1980). In the mouse these cells do not begin to express GFAP until late in embryonic development, but can be identified with several other specific markers, including RC2 (Misson et al., 1988), brain lipidbinding protein (BLBP; Feng et al., 1994), vimentin (Dahl et al., 1981), nestin (Hockfield and McKay, 1985) and GLAST (Shibata et al., 1997). Rakic went on to show that radial glial cells act as a scaffold to support the migration of newly generated neurons from the embryonic germinal zone into the developing layers of the cortex, and until recently this was considered to be the major role for radial glial cells, before their postnatal transdifferentiation into astrocytes. The new role for radial glial cells as neuronal precursor cells in the embryo may dramatically change our understanding of CNS development, yet, at the same time, it raises several important and possibly controversial issues. In this mini-review, we critically examine our present understanding of radial glial cell regulation, specifically regarding the processes of induction, maintenance, and transdifferentiation. We also discuss our knowledge of cell lineage in the developing forebrain as it pertains to a putative role for radial glia as the major precursor population in vivo.

Platelet-derived growth factor-responsive neural precursors give rise to myelinating oligodendrocytes after transplantation into the spinal cords of contused rats and dysmyelinated mice

Spinal cord injury (SCI) results in substantial oligodendrocyte death and subsequent demyelination leading to white‐matter defects. Cell replacement strategies to promote remyelination are under intense investigation; however, the optimal cell for transplantation remains to be determined. We previously isolated a platelet‐derived growth factor (PDGF)‐responsive neural precursor (PRP) from the ventral forebrain of fetal mice that primarily generates oligodendrocytes, but also astrocytes and neurons. Importantly, human PRPs were found to possess a greater capacity for oligodendrogenesis than human epidermal growth factor‐ and/or fibroblast growth factor‐responsive neural stem cells. Therefore, we tested the potential of PRPs isolated from green fluorescent protein (GFP)‐expressing transgenic mice to remyelinate axons in the injured rat spinal cord. PRPs were transplanted 1 week after a moderate thoracic (T9) spinal cord contusion in adult male rats. After initial losses, PRP numbers remained stable from 2 weeks posttransplantation onward and those surviving cells integrated into host tissue. Approximately one‐third of the surviving cells developed the typical branched phenotype of mature oligodendrocytes, expressing the marker APC‐CC1. The close association of GFP cells with myelin basic protein as well as with Kv1.2 and Caspr in the paranodal and juxtaparanodal regions of nodes of Ranvier indicated that the transplanted cells successfully formed mature myelin sheaths. Transplantation of PRPs into dysmyelinated Shiverer mice confirmed the ability of PRP‐derived cells to produce compact myelin sheaths with normal periodicity. These findings indicate that PRPs are a novel candidate for CNS myelin repair, although PRP‐derived myelinating oligodendrocytes were insufficient to produce behavioral improvements in our model of SCI.

Distinctions between fetal and adult human platelet-derived growth factor–responsive neural precursors

Platelet‐derived growth factor (PDGF)–responsive neural precursors (PRPs; also known as oligodendrocyte progenitor cells) are one of the best characterized precursor cell populations of the rodent central nervous system. Yet little is known about the biology of human PRPs because of an apparent inability to culture and expand them in large numbers. This study was designed to establish an approach that allows direct comparisons between the biology of fetal and adult human PRPs, as a means to address potential differences in intrinsic myelin‐production capabilities.

Pigment epithelium-derived growth factor: modulating adult neural stem cell self-renewal

The vascular niche-derived factor PEDF enhances Notch signaling in adult neural stem cells via an unexpected mechanism involving nuclear export of a transcriptional repressor, to promote both proliferation and multipotentiality.