Neurons from radial glia: the consequences of asymmetric inheritance

The problem of astrocyte identity

Astrocytes were the original neuroglia of Ramón y Cajal but after 100 years there is no satisfactory definition of what should comprise this class of cells. This essay takes a historical and philosophical approach to the question of astrocytic identity. The classic approach of identification by morphology and location are too limited to determine new members of the astrocyte population. I also critically evaluate the use of protein markers measured by immunoreactivity, as well as the newer technique of marking living cells by using promoters for these same proteins to drive reporter genes. These two latter approaches have yielded an expanded population of astrocytes with diverse functions, but also mark cells that traditionally would not be defined as astrocytes. Thus we need a combination of measures to define an astrocyte but it is not clear what this combination should be. The molecular approach, especially promoter driven fluorescent reporter genes, does have the advantage of pre marking living astrocytes for electrophysiological or imaging recordings. However, lack of sufficient understanding of the behavior of the inserted constructs has led to unclear results. This approach will no doubt be perfected with time but at present an acceptable, practical definition of what constitutes the class of astrocytes remains elusive.

Asymmetric production of surface-dividing and non-surface-dividing cortical progenitor cells

Mature neocortical layers all derive from the cortical plate (CP), a transient zone in the dorsal telencephalon into which young neurons are continuously delivered. To understand cytogenetic and histogenetic events that trigger the emergence of the CP, we have used a slice culture technique. Most divisions at the ventricular surface generated paired cycling daughters (P/P divisions) and the majority of the P/P divisions were asymmetric in daughter cell behavior; they frequently sent one daughter cell to a non-surface (NS) position, the subventricular zone (SVZ), within a single cell-cycle length while keeping the other mitotic daughter for division at the surface. The NS-dividing cells were mostly Hu+ and their daughters were also Hu+, suggesting their commitment to the neuronal lineage and supply of early neurons at a position much closer to their destiny than from the ventricular surface. The release of a cycling daughter cell to SVZ was achieved by collapse of the ventricular process of the cell, followed by its NS division. Neurogenin2 (Ngn2) was immunohistochemically detected in a certain cycling population during G1 phase and was further restricted during G2-M phases to the SVZ-directed population. Its retroviral introduction converted surface divisions to NS divisions. The asymmetric P/P division may therefore contribute to efficient neuron/progenitor segregation required for CP initiation through cell cycle-dependent and lineage-restricted expression of Ngn2.

An inhibition of cyclin-dependent kinases that lengthens, but does not arrest, neuroepithelial cell cycle induces premature neurogenesis

The G1 phase of the cell cycle of neuroepithelial cells, the progenitors of all neurons of the mammalian central nervous system, has been known to lengthen concomitantly with the onset and progression of neurogenesis. We have investigated whether lengthening of the G1 phase of the neuroepithelial cell cycle is a cause, rather than a consequence, of neurogenesis. As an experimental system, we used whole mouse embryo culture, which was found to exactly reproduce the temporal and spatial gradients of the onset of neurogenesis occurring in utero. Olomoucine, a cell-permeable, highly specific inhibitor of cyclin-dependent kinases and G1 progression, was found to significantly lengthen, but not arrest, the cell cycle of neuroepithelial cells when used at 80 microM. This olomoucine treatment induced, in the telencephalic neuroepithelium of embryonic day 9.5 to 10.5 mouse embryos developing in whole embryo culture to embryonic day 10.5, (i) the premature up-regulation of TIS21, a marker identifying neuroepithelial cells that have switched from proliferative to neuron-generating divisions, and (ii) the premature generation of neurons. Our data indicate that lengthening G1 can alone be sufficient to induce neuroepithelial cell differentiation. We propose a model that links the effects of cell fate determinants and asymmetric cell division to the length of the cell cycle.

Asymmetric distribution of the apical plasma membrane during neurogenic divisions of mammalian neuroepithelial cells

At the onset of neurogenesis in the mammalian central nervous system, neuroepithelial cells switch from symmetric, proliferative to asymmetric, neurogenic divisions. In analogy to the asymmetric division of Drosophila neuroblasts, this switch of mammalian neuroepithelial cells is thought to involve a change in cleavage plane orientation from perpendicular (vertical cleavage) to parallel (horizontal cleavage) relative to the apical surface of the neuroepithelium. Here, we report, using TIS21-GFP knock-in mouse embryos to identify neurogenic neuroepithelial cells, that at the onset as well as advanced stages of neurogenesis the vast majority of neurogenic divisions, like proliferative divisions, show vertical cleavage planes. Remarkably, however, neurogenic divisions of neuroepithelial cells, but not proliferative ones, involve an asymmetric distribution to the daughter cells of the apical plasma membrane, which constitutes only a minute fraction (1-2%) of the entire neuroepithelial cell plasma membrane. Our results support a novel concept for the cell biological basis of asymmetric, neurogenic divisions of neuroepithelial cells in the mammalian central nervous system.

Loss of cell polarity causes severe brain dysplasia in Lgl1 knockout mice

Disruption of cell polarity is seen in many cancers; however, it is generally considered a late event in tumor progression. Lethal giant larvae (Lgl) has been implicated in maintenance of cell polarity in Drosophila and cultured mammalian cells. We now show that loss of Lgl1 in mice results in formation of neuroepithelial rosette-like structures, similar to the neuroblastic rosettes in human primitive neuroectodermal tumors. The newborn Lgl1(-/-) pups develop severe hydrocephalus and die neonatally. A large proportion of Lgl1(-/-) neural progenitor cells fail to exit the cell cycle and differentiate, and, instead, continue to proliferate and die by apoptosis. Dividing Lgl1(-/-) cells are unable to asymmetrically localize the Notch inhibitor Numb, and the resulting failure of asymmetric cell divisions may be responsible for the hyperproliferation and the lack of differentiation. These results reveal a critical role for mammalian Lgl1 in regulating of proliferation, differentiation, and tissue organization and demonstrate a potential causative role of disruption of cell polarity in neoplastic transformation of neuroepithelial cells.

Neurons arise in the basal neuroepithelium of the early mammalian telencephalon: a major site of neurogenesis

Neurons of the mammalian CNS are thought to originate from progenitors dividing at the apical surface of the neuroepithelium. Here we use mouse embryos expressing GFP from the Tis21 locus, a gene expressed throughout the neural tube in most, if not all, neuron-generating progenitors, to specifically reveal the cell divisions that produce CNS neurons. In addition to the apical, asymmetric divisions of neuroepithelial (NE) cells that generate another NE cell and a neuron, we find, from the onset of neurogenesis, a second population of progenitors that divide in the basal region of the neuroepithelium and generate two neurons. Basal progenitors are most frequent in the telencephalon, where they outnumber the apically dividing neuron-generating NE cells. Our observations reconcile previous data on the origin and lineage of CNS neurons and show that basal, rather than apical, progenitors are the major source of the neurons of the mammalian neocortex.

NOVOcan: a molecular link among selected glial cells

The nervous system is generated from cells lining the ventricular system. Our understanding of the fate potentials and lineage relationships of these cells is being re-evaluated, both because of recent demonstrations that radial glia can generate neurons and because of the identification of fate-determining genes. A variety of intrinsic and extrinsic molecules, including proteoglycans, regulate embryonic and postnatal brain development. Using probes modeled after species conserved domains of heparan sulfate proteoglycans, we cloned a novel gene called novocan, raised monoclonal antibodies against a segment of the predicted amino acid sequence of the expressed protein (NOVOcan) and used the antibodies to establish the cell and tissue localization of NOVOcan in postnatal rat brains by immunohistochemistry. NOVOcan was expressed in cells lining the ventricles, including a variety of radial glia during early postnatal development. Later, as radial glia disappeared and ependymal cells appeared, NOVOcan was detected in ependymal cells and in tanycytes, a specialized form of ependymal cell resembling radial glia. NOVOcan was absent in two known progeny of radial glia, mature astrocytes and neurons. Whereas NOVOcan was also absent in mature oligodendrocytes (OLGs), it was present in OLG precursors in developing white matter. These studies set the stage for determining the roles of NOVOcan in brain cell lineage patterns as well as in other aspects of development.

Radial glia defines boundaries and subdivisions in the embryonic midbrain of the lizardGallotia galloti

We have studied the organization of the midbrain radial glia in embryos of Gallotia galloti using the fluorescent lipophilic dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) and the antibodies H5 and RC2. Our goal was to verify if the radial glia takes part in the midbrain boundaries formation and if it defines different zones. Our exam reveals two clear limits, anterior or mesencephalic-diencephalic (m/d) and posterior or mesencephalic-rhombencephalic (m/r), that can be defined as the borders where the midbrain radial glia processes end. Moreover, fasciculate radial glia processes characterize these limits totally or partially. They coincide with gene expression limits and with cytoarchitectonic limits defined by other criteria. Six different subdivisions, five alar and one basal, can be defined according to radial glia distribution, fasciculation, and immunohistochemical features. The ventral part of the alar region is defined by an RC2-positive bundle of radial glial cells. This bundle supposes a trustworthy landmark to point out the tectal/tegmental boundary. We hypothesize that this pattern of midbrain radial glia represents a basic model in amniota.

Neuronal and glial differentiation within expanded glial cultures derived from the lateral and medial ganglionic eminences

Attached glial-like cell cultures were established from the lateral and medial ganglionic eminences (LGE and MGE) and from the neocortex (Cx) of E13.5 mouse embryos, and expanded over four to five passages under epidermal growth factor (EGF) stimulation. Following removal of EGF and serum, we analysed the generation of neurons and glial cells within the cultures. Significant numbers of betaIII-tubulin-positive neurons were generated in both the LGE (about 7% of total cell numbers) and the MGE (around 2%). However, only few betaIII-tubulin-positive cells with neuronal morphologies were detected in the differentiated Cx cultures. The newly formed neurons were to a large extent GABAergic, and many of the MGE-derived, but not the LGE-derived, cells expressed the MGE-marker NKX2.1. Most cells in all cultures still appeared astroglial-like, expressing glial fibrillary acidic protein (GFAP), but in addition, CNPase-positive cells with oligodendroglial morphologies were present in the MGE (0.68%), and, to a lesser extent (0.2%), in the LGE cultures. The present results demonstrate that cells of expanded glial cultures from both the LGE and MGE can give rise to significant and, to a certain extent, region-specific neuronal and glial cell types under differentiating conditions.

Asymmetric cell division during neurogenesis in Drosophila and vertebrates

The majority of cells that build the nervous system of animals are generated early in embryonic development in a process called neurogenesis. Although the vertebrate nervous system is much more complex than that of insects, the underlying principles of neurogenesis are intriguingly similar. In both cases, neuronal cells are derived from polarized progenitor cells that divide asymmetrically. One daughter cell will continue to divide and the other daughter cell leaves the cell cycle and starts to differentiate as a neuron or a glia cell. In Drosophila, this process has been analyzed in great detail and several of the key players that control asymmetric cell division in the developing nervous system have been identified over the past years. Asymmetric cell division in vertebrate neurogenesis has been studied mostly at a descriptive level and so far little is known about the molecular mechanisms that control this process. In this review we will focus on recent findings dealing with asymmetric cell division during neurogenesis in Drosophila and vertebrates and will discuss common principles and apparent differences between both systems.

Environment temperature affects cell proliferation in the spinal cord and brain of juvenile turtles

The spinal cords and brains--comprising dorsal cortex (DC), medial cortex (MC) and diencephalon (Dien)--of juvenile turtles acclimated to warm temperature [27-30 degrees C; warm-acclimated turtles (WATs)] revealed higher density values of bromodeoxyuridine-labeled cells (BrdU-LCs) than those acclimated to a cooler environment [5-14 degrees C; cold-acclimated turtles (CATs)]. Both populations were under the influence of the seasonal daily light-dark rhythms. Pronounced differences between WATs and CATs (independent t-test; confidence level, P<0.01) were found in the central area of the spinal gray matter and in the ependymal epithelium lining the brain ventricles. Forebrain regions (DC, MC and Dien) also revealed significant differences between WATs and CATs (independent t-test; confidence level, P<0.01-0.05). Unexplored biological clocks that may be affecting cell proliferation were equalized by performing paired experiments involving one WAT and one CAT. Both animals were injected on the same day at the same time and both were sacrificed 24 h later. These experiments confirmed that a warm environment increased cell proliferation in the CNS of turtles. Double- and triple-labeling experiments involving anti-BrdU antibody together with anti-glial protein antibodies revealed that temperature modulates not only cell populations expressing glial markers but also other cells that do not express them. As expected, in the case of short post-injection (BrdU) surviving time points, no cells were found colabeling for BrdU and NeuN (neuronal marker). The probable direct effect of temperature on the cell division rate should be analyzed together with potential indirect effects involving increased motor activity and increased food intake. The fate of the increased BrdU-LCs (death, permanence as progenitor cells or differentiation following neuronal or glial lines) remains a matter for further investigation. Results are discussed in the light of current opinions concerned with post-natal neurogenesis in vertebrates.

Morphological asymmetry in dividing retinal progenitor cells

For the understanding of histogenetic events in the 3-D retinal neuroepithelium, direct observation of the progenitor cells and their morphological changes is required. A slice culture method has been developed by which the behavior of single progenitor cells can be monitored. Although it has been believed that each retinal progenitor cell loses its basal process while it is in M phase, it is reported here that the process is retained throughout M phase and is inherited by one daughter cell, which can be a neuron or a progenitor cell. Daughter neurons used an inherited process for neuronal translocation and positioning. In divisions that produced two mitotic daughters, both of which subsequently divided to form four granddaughter cells, only one daughter cell inherited the original basal process while the other extended a new process. Interestingly, behavioral differences were often noted between such mitotic sisters in the trajectory of interkinetic nuclear movement, cell cycle length, and the composition of the granddaughter pair. Therefore, "symmetric" (progenitor --> progenitor + progenitor) divisions are in fact morphologically asymmetric, and the behavior of the mitotic daughters can often be asymmetric, indicating the necessity for studying possible associations between the process inheritance and the cell fate choice.