Radial Glial Development Research Articles

High Angular Resolution Diffusion MRI Reveals Conserved and Deviant Programs in the Paths that Guide Human Cortical Circuitry.

Diffusion magnetic resonance (MR) tractography represents a novel opportunity to investigate conserved and deviant developmental programs between humans and other species such as mice. To that end, we acquired high angular resolution diffusion MR scans of mice [embryonic day (E) 10.5 to postnatal week 4] and human brains [gestational week (GW) 17-30] at successive stages of fetal development to investigate potential evolutionary changes in radial organization and emerging pathways between humans and mice. We compare radial glial development as well as commissural development (e.g., corpus callosum), primarily because our findings can be integrated with previous work. We also compare corpus callosal growth trajectories across primates (i.e., humans and rhesus macaques) and rodents (i.e., mice). One major finding is that the developing cortex of humans is predominated by pathways likely associated with a radial glial organization at GW 17-20, which is not as evident in age-matched mice (E 16.5, 17.5). Another finding is that, early in development, the corpus callosum follows a similar developmental timetable in primates (i.e., macaques and humans) as in mice. However, the corpus callosum grows for an extended period of time in primates compared with rodents. Taken together, these findings highlight deviant developmental programs underlying the emergence of cortical pathways in the human brain.

Pax6 Mediates ß-Catenin Signaling for Self-Renewal and Neurogenesis by Neocortical Radial Glial Stem Cells

The Wnt/ß-catenin pathway is a critical stem cell regulator and plays important roles in neuroepithelial cells during early gestation. However, the role of Wnt/ß-catenin signaling in radial glia, a major neural stem cell population expanded by midgestation, remains poorly understood. This study shows that genetic ablation of ß-catenin with hGFAP-Cre mice inhibits neocortical formation by disrupting radial glial development. Reduced radial glia and intermediate progenitors are found in the ß-catenin-deficient neocortex during late gestation. Increased apoptosis and divergent localization of radial glia in the subventricular zone are also observed in the mutant neocortex. In vivo and in vitro proliferation and neurogenesis as well as oligodendrogenesis by cortical radial glia or by dissociated neural stem cells are significantly defective in the mutants. Neocortical layer patterning is not apparently altered, while astrogliogenesis is ectopically increased in the mutants. At the molecular level, the expression of the transcription factor Pax6 is dramatically diminished in the cortical radial glia and the sphere-forming neural stem cells of ß-catenin-deficient mutants. Chromatin immunoprecipitation and luciferase assays demonstrate that ß-catenin/Tcf complex binds to Pax6 promoter and induces its transcriptional activities. The forced expression of Pax6 through lentiviral transduction partially rescues the defective proliferation and neurogenesis by ß-catenin-deficient neural stem cells. Thus, Pax6 is a novel downstream target of the Wnt/ß-catenin pathway, and ß-catenin/Pax6 signaling plays critical roles in self-renewal and neurogenesis of radial glia/neural stem cells during neocortical development.

MARCKS modulates radial progenitor placement, proliferation and organization in the developing cerebral cortex

The radial glial cells serve as neural progenitors and as a migratory guide for newborn neurons in the developing cerebral cortex. These functions require appropriate organization and proliferation of the polarized radial glial scaffold. Here, we demonstrate in mice that the myristoylated alanine-rich C-kinase substrate protein (MARCKS), a prominent cellular substrate for PKC, modulates radial glial placement and expansion. Loss of MARCKS results in ectopic collection of mitotically active radial progenitors away from the ventricular zone (VZ) in the upper cerebral wall. Apical restriction of key polarity complexes [CDC42, beta-catenin (CTNNB1), N-cadherin (CDH2), myosin IIB (MYOIIB), aPKCzeta, LGL, PAR3, pericentrin, PROM1] is lost. Furthermore, the radial glial scaffold in Marcks null cortex is compromised, with discontinuous, non-radial processes apparent throughout the cerebral wall and deformed, bulbous, unbranched end-feet at the basal ends. Further, the density of radial processes within the cerebral cortex is reduced. These deficits in radial glial development culminate in aberrant positioning of neurons and disrupted cortical lamination. Genetic rescue experiments demonstrate, surprisingly, that phosphorylation of MARCKS by PKC is not essential for the role of MARCKS in radial glial cell development. By contrast, the myristoylation domain of MARCKS needed for membrane association is essential for MARCKS function in radial glia. The membrane-associated targeting of MARCKS and the resultant polarized distribution of signaling complexes essential for apicobasal polarity may constitute a critical event in the appropriate placement, proliferation and organization of polarized radial glial scaffold in the developing cerebral cortex.

Genetic lineage tracing defines distinct neurogenic and gliogenic stages of ventral telencephalic radial glial development

BackgroundRadial glia comprise a molecularly defined neural progenitor population but their role in neurogenesis has remained contested due to the lack of a single universally accepted genetic tool for tracing their progeny and the inability to distinguish functionally distinct developmental stages.ResultsBy direct comparisons of Cre/loxP lineage tracing results obtained using three different radial glial promoters (Blbp, Glast, and hGFAP), we show that most neurons in the brain are derived from radial glia. Further, we show that hGFAP promoter induction occurs in ventral telencephalic radial glia only after they have largely completed neurogenesis.ConclusionThese data establish the major neurogenic role of radial glia in the developing central nervous system and genetically distinguish an early neurogenic Blbp+Glast+hGFAP- stage from a later gliogenic Blbp+Glast+hGFAP+ stage in the ventral telencephalon.

Role of presenilin-1 in cortical lamination and survival of Cajal-Retzius neurons

Presenilin-1 ( PS1), the major causative gene of familial Alzheimer disease, regulates neuronal differentiation and Notch signaling during early neural development. To investigate the role of PS1 in neuronal migration and cortical lamination of the postnatal brain, we circumvented the perinatal lethality of PS1-null mice by generating a conditional knockout (cKO) mouse in which PS1 inactivation is restricted to neural progenitor cells (NPCs) and NPC-derived neurons and glia. BrdU birthdating analysis revealed that many late-born neurons fail to migrate beyond the early-born neurons to arrive at their appropriate positions in the superficial layer, while the migration of the early-born neurons is largely normal. The migration defect of late-born neurons coincides with the progressive reduction of radial glia in PS1 cKO mice. In contrast to the premature loss of Cajal-Retzius (CR) neurons in PS1-null mice, generation and survival of CR neurons are unaffected in PS1 cKO mice. Furthermore, the number of proliferating meningeal cells, which have been shown to be important for the survival of CR neurons, is increased in PS1-null mice but not in PS1 cKO mice. These findings show a cell-autonomous role for PS1 in cortical lamination and radial glial development, and a non-cell-autonomous role for PS1 in CR neuron survival.

A Notch Above ErbB

Radial glia are present early in development and help guide neuronal migration in the developing brain, which is essential for normal brain function. In the cerebellum, the formation of radial glia depends on contact with neurons and involves Notch1 and ErbB signaling. The mechanisms whereby neuronal contact initiates radial glial development, however, and the relationship between these two pathways are unclear. Patten et al. used cocultures to show that cerebellar granule cells stimulated Notch-dependent activation of a gene reporter expressed in cerebellar glia, as well as development of radial glial morphology. Glial expression of a constitutively active form of Notch1 also promoted radial morphology, as did coculture with fibroblasts expressing the Notch ligand Jagged1 or exposure to the ErbB ligand neuregulin (NRG). Expression in glia of dominant-negative forms of Notch1 or the ErbB receptor blocked the ability of granule cells to stimulate radial morphology, whereas a dominant-negative form of the Notch signal transducer Suppressor of Hairless (SuH) did not. Experiments involving various dominant-negatives and gene reporters indicated that Notch1 signaling was upstream of ErbB signaling and that granule cell (and Notch1)-dependent transcriptional activation of brain lipid-binding protein (BLBP, a protein expressed by radial glia)--but not of Erb2--depended on SuH. This suggests that Notch1 promotes radial glial differentiation through at least two distinct signaling pathways. The authors propose a model in which sequential activation of Notch1 and ErbB signaling pathways are required for radial glial development and suggest that they may also act in tandem in other developmental events.B. A. Patten, J. M. Peyrin, G. Weinmaster, G. Corfas, Sequential signaling through Notch1 and ErbB receptors mediates radial glia differentiation. J. Neurosci. 23, 6132-6140 (2003). [Abstract] [Full Text]

A Notch Above ErbB

Radial glia are present early in development and help guide neuronal migration in the developing brain, which is essential for normal brain function. In the cerebellum, the formation of radial glia depends on contact with neurons and involves Notch1 and ErbB signaling. The mechanisms whereby neuronal contact initiates radial glial development, however, and the relationship between these two pathways are unclear. Patten et al . used cocultures to show that cerebellar granule cells stimulated Notch-dependent activation of a gene reporter expressed in cerebellar glia, as well as development of radial glial morphology. Glial expression of a constitutively active form of Notch1 also promoted radial morphology, as did coculture with fibroblasts expressing the Notch ligand Jagged1 or exposure to the ErbB ligand neuregulin (NRG). Expression in glia of dominant-negative forms of Notch1 or the ErbB receptor blocked the ability of granule cells to stimulate radial morphology, whereas a dominant-negative form of the Notch signal transducer Suppressor of Hairless (SuH) did not. Experiments involving various dominant-negatives and gene reporters indicated that Notch1 signaling was upstream of ErbB signaling and that granule cell (and Notch1)-dependent transcriptional activation of brain lipid-binding protein (BLBP, a protein expressed by radial glia)--but not of Erb2--depended on SuH. This suggests that Notch1 promotes radial glial differentiation through at least two distinct signaling pathways. The authors propose a model in which sequential activation of Notch1 and ErbB signaling pathways are required for radial glial development and suggest that they may also act in tandem in other developmental events. B. A. Patten, J. M. Peyrin, G. Weinmaster, G. Corfas, Sequential signaling through Notch1 and ErbB receptors mediates radial glia differentiation. J. Neurosci . 23 , 6132-6140 (2003). [Abstract] [Full Text]

Role of GGF/neuregulin signaling in interactions between migrating neurons and radial glia in the developing cerebral cortex.

During neuronal migration to the developing cerebral cortex, neurons regulate radial glial cell function and radial glial cells, in turn, support neuronal cell migration and differentiation. To study how migrating neurons and radial glial cells influence each others' function in the developing cerebral cortex, we examined the role of glial growth factor (a soluble form of neuregulin), in neuron-radial glial interactions. Here, we show that GGF is expressed by migrating cortical neurons and promotes their migration along radial glial fibers. Concurrently, GGF also promotes the maintenance and elongation of radial glial cells, which are essential for guiding neuronal migration to the cortex. In the absence of GGF signaling via erbB2 receptors, radial glial development is abnormal. Furthermore, GGF's regulation of radial glial development is mediated in part by brain lipid-binding protein (BLBP), a neuronally induced, radial glial molecule, previously shown to be essential for the establishment and maintenance of radial glial fiber system. The ability of GGF to influence both neuronal migration and radial glial development in a mutually dependent manner suggests that it functions as a mediator of interactions between migrating neurons and radial glial cells in the developing cerebral cortex.