mGCMa is a murine transcription factor that overrides cell fate decisions in Drosophila

The Repo Homeodomain Transcription Factor Suppresses Hematopoiesis in Drosophila and Preserves the Glial Fate

Despite their different origins, Drosophila glia and hemocytes are related cell populations that provide an immune function. Drosophila hemocytes patrol the body cavity and act as macrophages outside the nervous system, whereas glia originate from the neuroepithelium and provide the scavenger population of the nervous system. Drosophila glia are hence the functional orthologs of vertebrate microglia, even though the latter are cells of immune origin that subsequently move into the brain during development. Interestingly, the Drosophila immune cells within (glia) and outside (hemocytes) the nervous system require the same transcription factor glial cells deficient/glial cells missing (Glide/Gcm) for their development. This raises the issue of how do glia specifically differentiate in the nervous system, and hemocytes in the procephalic mesoderm. The Repo homeodomain transcription factor and panglial direct target of Glide/Gcm is known to ensure glial terminal differentiation. Here we show that Repo also takes center stage in the process that discriminates between glia and hemocytes. First, Repo expression is repressed in the hemocyte anlagen by mesoderm-specific factors. Second, Repo ectopic activation in the procephalic mesoderm is sufficient to repress the expression of hemocyte-specific genes. Third, the lack of Repo triggers the expression of hemocyte markers in glia. Thus, a complex network of tissue-specific cues biases the potential of Glide/Gcm. These data allow us to revise the concept of fate determinants and help us to understand the bases of cell specification. Both sexes were analyzed.SIGNIFICANCE STATEMENT Distinct cell types often require the same pioneer transcription factor, raising the issue of how one factor triggers different fates. In Drosophila, glia and hemocytes provide a scavenger activity within and outside the nervous system, respectively. While they both require the glial cells deficient/glial cells missing (Glide/Gcm) transcription factor, glia originate from the ectoderm, and hemocytes from the mesoderm. Here we show that tissue-specific factors inhibit the gliogenic potential of Glide/Gcm in the mesoderm by repressing the expression of the homeodomain protein Repo, a major glial-specific target of Glide/Gcm. Repo expression in turn inhibits the expression of hemocyte-specific genes in the nervous system. These cell-specific networks secure the establishment of the glial fate only in the nervous system and allow cell diversification.

The effect of neurospecific knockdown of candidate genes for locomotor behavior and sound production in Drosophila melanogaster

Molecular mechanisms underlying the functioning of central pattern generators (CPGs) are poorly understood. Investigations using genetic approaches in the model organism Drosophila may help to identify unknown molecular players participating in the formation or control of motor patterns. Here we report Drosophila genes as candidates for involvement in the neural mechanisms responsible for motor functions, such as locomotion and courtship song. Twenty-two Drosophila lines, used for gene identification, were isolated from a previously created collection of 1064 lines, each carrying a P element insertion in one of the autosomes. The lines displayed extreme deviations in locomotor and/or courtship song parameters compared with the whole collection. The behavioral consequences of CNS-specific RNAi-mediated knockdowns for 10 identified genes were estimated. The most prominent changes in the courtship song interpulse interval (IPI) were seen in flies with Sps2 or CG15630 knockdown. Glia-specific knockdown of these genes produced no effect on the IPI. Estrogen-induced knockdown of CG15630 in adults reduced the IPI. The product of the CNS-specific gene, CG15630 (a predicted cell surface receptor), is likely to be directly involved in the functioning of the CPG generating the pulse song pattern. Future studies should ascertain its functional role in the neurons that constitute the song CPG. Other genes (Sps2, CG34460), whose CNS-specific knockdown resulted in IPI reduction, are also worthy of detailed examination.

Glial cell progenitors in the Drosophila embryo

Development and general organization of the nervous system is comparable between insects and vertebrates. Our current knowledge on the formation of neurogenic anlagen and the generation of neural stem cells is deeply influenced by work done in invertebrate model organisms such as Drosophila and Caenorhabditis elegans. It is the aim of this review to summarize the most important steps in neurogenesis in the Drosophila embryo with a special emphasis on glial cell progenitors and the specification of glial cells. Induction of neurogenic regions during early embryogenesis and determination of neural stem cells are briefly described. Special attention is given to the formation of neural precursors called neuroblasts (NB) and their lineages. NBs divide in a stem cell mode to generate a cell clone of either neurons and/or glial cells. The latter require the activation of the transcription factor glial cells missing (gcm), thus providing a binary switch between neuronal and glial cell fates. Further aspects of glial cell specification and the resulting heterogeneity of the glial population in Drosophila are discussed.

New insights in the clockwork mechanism regulating lineage specification: Lessons from the Drosophila nervous system

BACKGROUND

Powerful transcription factors called fate determinants induce robust differentiation programs in multipotent cells and trigger lineage specification. These factors guarantee the differentiation of specific tissues/organs/cells at the right place and the right moment to form a fully functional organism. Fate determinants are activated by temporal, positional, epigenetic, and post-transcriptional cues, hence integrating complex and dynamic developmental networks. In turn, they activate specific transcriptional/epigenetic programs that secure novel molecular landscapes.

RESULTS

In this review, we use the Drosophila Gcm glial determinant as a model to discuss the mechanisms that allow lineage specification in the nervous system. The dynamic regulation of Gcm via interlocked loops has recently emerged as a key event in the establishment of stable identity. Gcm induces gliogenesis while triggering its own extinction, thus preventing the appearance of metastable states and neoplastic processes.

CONCLUSIONS

Using simple animal models that allow in vivo manipulations provides a key tool to disentangle the complex regulation of cell fate determinants.

Characterization of missense alleles of the glial cells missing gene of Drosophila

Glial cells missing (Gcm) is the primary regulator of glial cell fate in Drosophila. Gcm belongs to a small family of transcriptional regulators involved in fundamental developmental processes found in diverse animal phyla including vertebrates. Gcm proteins contain the highly conserved DNA-binding GCM domain, which recognizes an octamer DNA sequence. To date, studies in Drosophila have primarily relied on gcm alleles caused by P-element induced DNA deletions at the gcm locus, as well as a null allele caused by a single base pair substitution in the GCM domain that completely abolishes DNA binding. Here I characterize two hypomorphic missense alleles of gcm with intermediate glial cells missing phenotypes. In embryos homozygous for either of these gcm alleles the number of glial cells in the central nervous cystem (CNS) is reduced approximately in half. Both alleles have single amino acid changes in the GCM domain. These results suggest that Gcm protein activities in these mutant alleles have been attenuated such that they are operating at threshold levels, and trigger glial cell differentiation in neural precursors in the CNS in a stochastic fashion. These hypomorphic alleles provide additional genetic resources for understanding Gcm functions and structure in Drosophila and other species.

Gcm proteins function in the developing nervous system

A fundamental issue during nervous system development is how individual cells are formed from the undefined precursors. Differentiated neurons and glia, two major cell types mediating neuronal function, are acquired from immature precursors via a series of explicit controls exerted by transcription factors such as proteins in the family of Glial cells missing (Gcm). In mammals, Gcm proteins are involved in placenta and parathyroid gland development, whereas in the invertebrate organism Drosophila, Gcm proteins act as fate determinants for glial cell fate, regulate neural stem cell (NSC) induction and conversion, and promote glial proliferation. In particular, Gcm protein levels are carefully tuned for Drosophila gliogenesis and their stability is under precise control via the ubiquitin-proteasome system (UPS). Here we summarize recent advances on Gcm proteins function. In addition to describe various features of Gcm protein family, the significance of their functions in the developing nervous system is also discussed.

Dominant-negative GCMB mutations cause an autosomal dominant form of hypoparathyroidism

CONTEXT

Hypoparathyroidism (HP) is characterized by low PTH levels, hypocalcemia, and hyperphosphatemia. Heterozygous mutations in pre-pro-PTH or the calcium-sensing receptor (CaSR) cause some forms of autosomal dominant HP (AD-HP). Furthermore, homozygous mutations in glial cells missing B (GCMB) have been implicated in autosomal recessive HP (AR-HP). In most other HP patients, however, the molecular defect remains undefined.

OBJECTIVE

Our objectives were to determine the genetic defect in the affected members of two unrelated families with AD-HP and define the underlying disease mechanism.

SUBJECTS

Several family members affected by AD-HP were investigated. The proband in family A had low calcium detected on routine blood testing, whereas the proband in family B had symptomatic hypocalcemia.

METHODS

Mutational analysis of the genes encoding pre-pro-PTH, CaSR, and GCMB was performed using PCR-amplified genomic DNA of the probands and other available members of each family. The identified GCMB mutants were characterized by Western blot analysis and luciferase reporter assay using DF-1 fibroblasts.

RESULTS

Two novel heterozygous mutations located in the last GCMB exon (c.1389delT and c.1399delC in families A and B, respectively) were identified that both lead to frame-shifts and replacement of the putative second transactivation domain within carboxyl-terminal region by unrelated amino acid sequence. The mutant GCMB proteins were well expressed, and both showed dose-dependent inhibition of the transactivation capacity of wild-type protein in luciferase reporter assays.

CONCLUSIONS

The dominant-negative effect observed in vitro for both GCMB mutations provides a plausible explanation for the impaired PTH secretion observed in the two unrelated families with AD-HP.

R-spondin3 is required for mouse placental development

Mouse R-spondin3 (Rspo3) is a member of the R-spondin protein family, which is characterized by furin-like cysteine-rich domains and a thrombospondin type 1 repeat. Rspo3 has been proposed to function as a secretory molecule that promotes the Wnt/beta-catenin signaling pathway. We generated mice bearing a mutant Rspo3 allele in which a lacZ-coding region replaced the coding region of the first exon. The homozygous mutant mice died at about embryonic day 10, due to impaired formation of the labyrinthine layer of the placenta. Rspo3 was expressed in the allantoic component of the labyrinth. In the homozygous mutant placentas, fetal blood vessels did not penetrate into the chorion, and expression of Gcm1, encoding the transcription factor glial cells missing-1 (Gcm1), was dramatically reduced in the chorionic trophoblast cells. These findings suggest a critical role for Rspo3 in the interaction between chorion and allantois in labyrinthine development.

Sloppy paired 1/2 regulate glial cell fates by inhibiting Gcm Function

Organization of the central nervous system during embryonic development is an intricate process involving a host of molecular players. The Drosophila segmentation genes, sloppy paired (slp) 1/2 have been shown to be necessary for development of a neuronal precursor cell subtype, the NB4-2 cells. Here, we show that slp1/2 also have roles in regulating glial cell fates. Using slp1/2 loss-of-function mutants, we show an increase in glial cell markers, glial cells missing (gcm) and reversed polarity. In contrast, misexpression of either slp1 or slp2 causes downregulation of glial cell-specific genes and alters the fate of glial and neuronal cells. Furthermore, we demonstrate that Slp1 and its mammalian ortholog, Foxg1, inhibit Gcm transcriptional activity as well as bind Gcm. Taken together, these data show that Slp1/Foxg1 regulate glial cell fates by inhibiting Gcm function.

Signaling in glial development: differentiation migration and axon guidance

Glial cells have diverse functions that are necessary for the proper development and function of complex nervous systems. During development, a variety of reciprocal signaling interactions between glia and neurons dictate all parts of nervous system development. Glia may provide attractive, repulsive, or contact-mediated cues to steer neuronal growth cones and ensure that neurons find their appropriate synaptic targets. In fact, both neurons and glia may act as migrational substrates for one another at different times during development. Also, the exchange of trophic signals between glia and neurons is essential for the proper bundling, fasciculation, and ensheathement of axons as well as the differentiation and survival of both cell types. The growing number of links between glial malfunction and human disease has generated great interest in glial biology. Because of its relative simplicity and the many molecular genetic tools available, Drosophila is an excellent model organism for studying glial development. This review will outline the roles of glia and their interactions with neurons in the embryonic nervous system of the fly.Key words: glia, axon guidance, migration, EGF receptor.

Interaction, cooperative promoter modulation, and renal colocalization of GCMa and Pitx2

The transcription factor GCMa is a member of a new small family of transcription factors with a conserved zinc-containing DNA-binding domain. All members of this transcription factor family play crucial roles as master regulators during development. GCMa is restricted to placenta during development and to kidney and thymus at postnatal stages. It is essential for the formation of the placental labyrinth and as a consequence for survival of the embryo from mid-embryogenesis onwards. Here, we identify Pitx transcription factors as GCMa-interacting proteins. We show that Pitx proteins interact via their conserved homeodomain with the DNA-binding domain of GCMa. As a consequence, Pitx proteins and GCMa exhibit cooperative DNA binding. Furthermore, Pitx proteins influence GCMa-dependent promoter activation in a cell-specific manner. One of the three Pitx paralogues in mice, Pitx2, is the predominant Pitx member present in the placenta and colocalizes on the cellular level with GCMa in the kidney. This is the first description of a regulatory cross-talk between a transcription factor of the GCM family and a homeodomain protein.

Impacts of a new transcription factor family: mammalian GCM proteins in health and disease

GCM proteins constitute a small transcription factor family with a DNA-binding domain exhibiting a novel fold composed of two subdomains rigidly held together by coordination of one of two structural zinc cations. In all known cases, GCM proteins exert the role of master regulators: the prototypical family member determines gliogenesis in Drosophila melanogaster, whereas mammalian GCM proteins orchestrate divergent aspects of development and physiology in placenta, kidney, thymus, and parathyroid gland. Recent data point to an involvement of GCM proteins in different pathological contexts, such as preeclampsia, hyper- or hypoparathyroidism, and parathyroid gland tumors.

Conservation and variation of structure and function in a newly identified GCM homolog from chicken

Glial cell missing (GCM) proteins constitute a small family of transcription factors with two members each described in Drosophila and several mammalian species. Here, we report the identification of a GCM homolog from chicken. Although the exon-intron structure is well conserved between chicken GCM and other family members, sequence similarity is largely restricted to the DNA-binding GCM-domain (residues 24-176). In accord with the high degree of sequence conservation within the GCM-domain, the chicken GCM protein has a DNA-binding specificity similar to that of other GCM proteins. Like other GCM proteins, it is located to the nucleus and can act as a transcriptional activator despite the strong divergence in sequences outside the GCM-domain. The chicken GCM protein contains two transactivation domains with cell-specific function, one immediately following the DNA-binding domain, the other at its extreme carboxy terminus. Intriguingly, chicken GCM is expressed only transiently during embryogenesis and is restricted exclusively to extraembryonic tissues where it was detected in close vicinity to embryonic blood vessels. Taking the extraembryonic expression of chicken GCM and mammalian GCMa into account, it is tempting to speculate that a conserved extraembryonic function exists for GCM proteins in birds and mammals.

Structural requirements for nuclear localization of GCMa/Gcm-1

GCM proteins constitute a small transcription factor family. Nuclear localization of Drosophila GCM is mediated by a typical bipartite nuclear localization sequence (NLS) close to the DNA-binding GCM domain. Here, we have analyzed nuclear localization of the mammalian GCM proteins. Whereas GCMb/Gcm-2 contained a classical bipartite NLS, nuclear localization of GCMa/Gcm-1 was mediated by two regions without resemblance to known NLS, one corresponding to the amino-terminal part of the GCM domain, the second defined as a tyrosine-and-proline-rich carboxy-terminal region. Nuclear import was counteracted by an amino-terminal nuclear export activity. This complex regulation of subcellular localization has important implications for GCMa/Gcm-1 function.

The potential to induce glial differentiation is conserved between Drosophila and mammalian glial cells missing genes

Drosophila glial cells missing (gcm) is a key gene that determines the fate of stem cells within the nervous system. Two mouse gcm homologs have been identified, but their function in the nervous system remains to be elucidated. To investigate their function, we constructed retroviral vectors harboring Drosophila gcm and two mouse Gcm genes. Expression of these genes appeared to influence fibroblast features. In particular, mouse Gcm1 induced the expression of astrocyte-specific Ca(2+)-binding protein, S100beta, in those cells. Introduction of the mouse Gcm1 gene in cultured cells from embryonic brains resulted in the induction of an astrocyte lineage. This effect was also observed by in utero injection of retrovirus harboring mouse Gcm1 into the embryonic brain. However, cultures from mouse Gcm1-deficient mouse brains did not exhibit significant reductions in the number of astrocytes. Furthermore, in situ hybridization analysis of mouse Gcm1 mRNA revealed distinct patterns of expression in comparison with other well-known glial markers. The mammalian homolog of Drosophila gcm, mouse Gcm1, exhibits the potential to induce gliogenesis, but may function in the generation of a minor subpopulation of glial cells.

gcm2 promotes glial cell differentiation and is required with glial cells missing for macrophage development in Drosophila

glial cells missing (gcm) is the primary regulator of glial cell fate in Drosophila. In addition, gcm has a role in the differentiation of the plasmatocyte/macrophage lineage of hemocytes. Since mutation of gcm causes only a decrease in plasmatocyte numbers without changing their ability to convert into macrophages, gcm cannot be the sole determinant of plasmatocyte/macrophage differentiation. We have characterized a gcm homolog, gcm2. gcm2 is expressed at low levels in glial cells and hemocyte precursors. We show that gcm2 has redundant functions with gcm and has a minor role promoting glial cell differentiation. More significant, like gcm, mutation of gcm2 leads to reduced plasmatocyte numbers. A deletion removing both genes has allowed us to clarify the role of these redundant genes in plasmatocyte development. Animals deficient for both gcm and gcm2 fail to express the macrophage receptor Croquemort. Plasmatocytes are reduced in number, but still express the early marker Peroxidasin. These Peroxidasin-expressing hemocytes fail to migrate to their normal locations and do not complete their conversion into macrophages. Our results suggest that both gcm and gcm2 are required together for the proliferation of plasmatocyte precursors, the expression of Croquemort protein, and the ability of plasmatocytes to convert into macrophages.

Ectopic expression ofGcm1induces congenital spinal cord abnormalities

Brief ectopic expression of Gcm1 in mouse embryonic tail bud profoundly affects the development of the nervous system. All mice from 5 independently derived transgenic lines exhibited either one or both of two types of congenital spinal cord pathologies: failure of the neural tube to close (spina bifida) and multiple neural tubes (diastematomyelia). Because the transgene is expressed only in a restricted caudal region and only for a brief interval (E8.5 to E13.5), there was no evidence of embryonic lethality. The dysraphisms develop during the period and within the zone of transgene expression. We present evidence that these dysraphisms result from an inhibition of neuropore closure and a stimulation of secondary neurulation. After transgene expression ceases, the spinal dysraphisms are progressively resolved and the neonatal animals, while showing signs of scarring and tissue resorption, have a closed vertebral column. The multiple spinal cords remain but are enclosed in a single spinal column as in the human diastematomyelia. The animals live a normal life time, are fertile and do not exhibit any obvious weakness or motor disabilities.

glide/gcm: at the crossroads between neurons and glia

Neurons and glia are generated by multipotent precursors. Recent studies indicate that the choice between the two fates depends on the combined activity of extracellular influences and factors that respond to precise spatial and temporal cues. Drosophila provides a simple genetic model to study the cellular and molecular mechanisms controlling fate choice, mode of precursor division and generation of cell diversity. Moreover, all glial precursors and glial-promoting activities have been identified in Drosophila, which provides us with a unique opportunity to dissect regulatory pathways controlling glial differentiation and specification.

Glial cell development in the Drosophila embryo

Glial cells play a central role in the development and function of complex nervous systems. Drosophila is an excellent model organism for the study of mechanisms underlying neural development, and recent attention has been focused on the differentiation and function of glial cells. We now have a nearly complete description of glial cell organization in the embryo, which enables a systematic genetic analysis of glial cell development. Most glia arise from neural stem cells that originate in the neurogenic ectoderm. The bifurcation of glial and neuronal fates is under the control of the glial promoting factor glial cells missing. Differentiation is propagated through the regulation of several transcription factors. Genes have been discovered affecting the terminal differentiation of glia, including the promotion glial-neuronal interactions and the formation of the blood-nerve barrier. Other roles of glia are being explored, including their requirement for axon guidance, neuronal survival, and signaling.

Peripheral glia direct axon guidance across the CNS/PNS transition zone

CNS glia have integral roles in directing axon migration of both vertebrates and insects. In contrast, very little is known about the roles of PNS glia in axonal pathfinding. In vertebrates and Drosophila, anatomical evidence shows that peripheral glia prefigure the transition zones through which axons migrate into and out of the CNS. Therefore, peripheral glia could guide axons at the transition zone. We used the Drosophila model system to test this hypothesis by ablating peripheral glia early in embryonic neurodevelopment via targeted overexpression of cell death genes grim and ced-3. The effects of peripheral glial loss on sensory and motor neuron development were analyzed. Motor axons initially exit the CNS in abnormal patterns in the absence of peripheral glia. However, they must use other cues within the periphery to find their correct target muscles since early pathfinding errors are largely overcome. When peripheral glia are lost, sensory axons show disrupted migration as they travel centrally. This is not a result of motor neuron defects, as determined by motor/sensory double-labeling experiments. We conclude that peripheral glia prefigure the CNS/PNS transition zone and guide axons as they traverse this region.

Human medulloblastoma cell line DEV is a potent tool to screen for factors influencing differentiation of neural stem cells

The aim of our study was to investigate whether a human neural cell line could be used as a reliable screening tool to examine the functional conservation, in humans, of transcription factors involved in neuronal or glial specification in other species. Gain-of-function experiments were performed on DEV cells, a cell line derived from a human medulloblastoma. Genes encoding nine different transcription factors were tested for their influence on the process of specification of human DEV cells towards a neuronal or glial fate. In a first series of experiments, DEV cells were transfected with murine genes encoding transcription factors known to be involved in the neuronal differentiation cascade. Neurogenins-1, -2, and -3; Mash-1; and NeuroD increased the differentiation of DEV cells towards a neuronal phenotype by a factor of 2-3.5. In a second series of experiments, we tested transcription factors involved in invertebrate glial specification. In the embryonic Drosophila CNS, the development of most glial cells depends on the master regulatory gene glial cell missing (gcm). Expression of gcm in DEV cells induced a twofold increase of astrocytic and a sixfold increase of oligodendroglial cell types. Interestingly, expression of tramtrack69, which is required in all Drosophila glial cells, resulted in a fourfold increase of only the oligodendrocyte phenotype. Expression of the related tramtrack88 protein, which is not expressed in the fly glia, or the C. elegans lin26 protein showed no effect. These results show that the Drosophila transcription factor genes tested can conserve their function upon transfection into the human DEV cells, qualifying this cell line as a screening tool to analyze the mechanisms of neuronal and glial specification.

Chronicles of a switch hunt: gcm genes in development

Co-conservation of sequence and function is an important principle during evolution. As a consequence, sequence-related genes often have similar functions in evolutionarily distant species. Enter the 'glial cells missing' (gcm) genes. They code for a small family of novel transcription factors that share DNA-binding properties and domain structure. However, no evolutionarily conserved function is apparent as yet. The prototypical gcm from Drosophila dominates nervous system development as a fate switch and master regulator of gliogenesis, whereas mammalian gcm genes have roles in placental morphogenesis and development of the parathyroid gland. Apparently, structure and function sometimes can go separate ways.

The midline glia of Drosophila: a molecular genetic model for the developmental functions of glia

The Midline Glia of Drosophila are required for nervous system morphogenesis and midline axon guidance during embryogenesis. In origin, gene expression and function, this lineage is analogous to the floorplate of the vertebrate neural tube. The expression or function of over 50 genes, summarised here, has been linked to the Midline Glia. Like the floorplate, the cells which generate the Midline Glia lineage, the mesectoderm, are determined by the interaction of ectoderm and mesoderm during gastrulation. Determination and differentiation of the Midline Glia involves the Drosophila EGF, Notch and segment polarity signaling pathways, as well as twelve identified transcription factors. The Midline Glia lineage has two phases of cell proliferation and of programmed cell death. During embryogenesis, the EGF receptor pathway signaling and Wrapper protein both function to suppress apoptosis only in those MG which are appropriately positioned to separate and ensheath midline axonal commissures. Apoptosis during metamorphosis is regulated by the insect steroid, Ecdysone. The Midline Glia participate in both the attraction of axonal growth cones towards the midline, as well as repulsion of growth cones from the midline. Midline axon guidance requires the Drosophila orthologs of vertebrate genes expressed in the floorplate, which perform the same function. Genetic and molecular evidence of the interaction of attractive (Netrin) and repellent (Slit) signaling is reviewed and summarised in a model. The Midline Glia participate also in the generation of extracellular matrix and in trophic interactions with axons. Genetic evidence for these functions is reviewed.

Genomic organization, chromosomal localization, and the complete 22 kb DNA sequence of the human GCMa/GCM1, a placenta-specific transcription factor gene

The genomic sequence of the human GCMa/GCM1 gene, a mammalian homologue of Drosophila melanogaster GCM, was determined. Drosophila GCM is a neural transcription factor that regulates glial cell fate. The mammalian homolog however, is a placenta-specific transcription factor that is necessary for placental development. The 22 kb DNA sequence spanning the GCMa gene contains six exons and five introns, encoding a 2.8 kb cDNA. Overall genomic organization is similar for the human and mouse. Several potential binding sites for transcription factors like GATA, Oct-1, and bHLH proteins were found in the 5'-flanking region of the human gene. A DNA motif for GCM protein binding exists in the 5'-flanking region that is highly homologous with that of the mouse gene. The location of this gene was mapped to chromosome 6 using fluorescence in situ hybridization.

Glial cell fate specification modulated by the bHLH gene Hes5 in mouse retina

Neurons and glial cells differentiate from common precursors. Whereas the gene glial cells missing (gcm) determines the glial fate in Drosophila, current data about the expression patterns suggest that, in mammals, gcm homologues are unlikely to regulate gliogenesis. Here, we found that, in mouse retina, the bHLH gene Hes5 was specifically expressed by differentiating Muller glial cells and that misexpression of Hes5 with recombinant retrovirus significantly increased the population of glial cells at the expense of neurons. Conversely, Hes5-deficient retina showed 30–40% decrease of Muller glial cell number without affecting cell survival. These results indicate that Hes5 modulates glial cell fate specification in mouse retina.

Placental failure in mice lacking the mammalian homolog of glial cells missing, GCMa

The GCM family of transcription factors consists of Drosophila melanogaster GCM, an important regulator of gliogenesis in the fly, and its two mammalian homologs, GCMa and GCMb. To clarify the function of these mammalian homologs, we deleted GCMa in mice. Genetic ablation of murine GCMa (mGCMa) is embryonic lethal, with mice dying between 9.5 and 10 days postcoitum. At the time of death, no abnormalities were apparent in the embryo proper. Nervous system development, in particular, was not impaired, as might have been expected in analogy to Drosophila GCM. Instead, placental failure was the cause of death. In agreement with the selective expression of mGCMa in labyrinthine trophoblasts, mutant placentas did not develop a functional labyrinth layer, which is necessary for nutrient and gas exchange between maternal and fetal blood. Only a few fetal blood vessels entered the placenta, and these failed to thrive and branch normally. Labyrinthine trophoblasts did not differentiate. All other layers of the placenta, including spongiotrophoblast and giant cell layer, formed normally. Our results indicate that mGCMa plays a critical role in trophoblast differentiation and the signal transduction processes required for normal vascularization of the placenta.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.

Protein stability and domain topology determine the transcriptional activity of the mammalian glial cells missing homolog, GCMb

The glial cells missing (GCM) family of transcription factors consists of Drosophila GCM and the mammalian proteins GCMa and GCMb. They are expressed in a highly restricted manner during development and are known or assumed to be important regulators of developmental fate decisions. As the biochemical properties of GCMb have not been studied so far, we have undertaken a detailed structure-function analysis of the mouse GCMb (mGCMb) protein. DNA-binding specificity was very similar to that of other GCM proteins. Nevertheless, mGCMb was only a weak transcriptional activator in a number of different tissue culture systems. Interestingly, this was not due to an intrinsic absence of transactivation potential. In effect, we were able to identify two separate transactivation domains within mGCMb, one carboxyl-terminally adjacent to the DNA-binding domain and the second within the extreme carboxyl terminus. Activity of both transactivation domains was, however, modulated by an inhibitory region unique to mGCMb and located between the two transactivation domains. Furthermore, pulse-chase experiments proved that the mGCMb protein has a half-life approximately four times shorter than mGCMa. Introduction of the above mentioned inhibitory domain of mGCMb into mGCMa shortened the half-life of mGCMa to a value typical of mGCMb with a concomitant reduction in transactivation potential. Given the strong correlation between protein stability and transactivation potential, functional differences between the two mammalian GCM homologs are likely due to differences in stability with a single inhibitory region in mGCMb being involved in the reduction of both.

A GCM motif protein is involved in placenta-specific expression of human aromatase gene

A new cis-element, trophoblast-specific element 2 (TSE2) is located in the placenta-specific enhancer of the human aromatase gene that dictates its tissue-specific expression. In the minimum enhancer region, an element similar to the trophoblast-specific element (TSE), originally described for the human chorionic gonadotropin alpha-subunit gene, also exists (Yamada, K., Harada, N., Honda, S., and Takagi, Y. (1995) J. Biol. Chem. 270, 25064-25069). The co-presence of TSE and TSE2 is required to direct trophoblast-specific expression driven by a heterologous thymidine kinase promoter. A 2562-base pair cDNA clone encoding a 436-amino acid protein that binds to TSE2 was isolated from a human placental cDNA library using a yeast one-hybrid system with the TSE2 as a reporter sequence. The protein was revealed to be identical to hGCMa, a mammalian homologue of the Drosophila GCM (glia cells missing) protein. Expression of hGCMa is restricted to the placenta. The protein also binds to PLE1 in the leptin promoter among other cis-elements reported to confer placenta-specific expression, suggesting that hGCMa is a placenta-specific transcription regulator, possibly involved in the expression of multiple placenta-specific genes.