Structural requirements for DNA binding of GCM proteins

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.

DNA repair: models for damage and mismatch recognition

Maintaining the integrity of the genome is critical for the survival of any organism. To achieve this, many families of enzymatic repair systems which recognize and repair DNA damage have evolved. Perhaps most intriguing about the workings of these repair systems is the actual damage recognition process. What are the chemical characteristics which are common to sites of nucleic acid damage that DNA repair proteins may exploit in targeting sites? Importantly, thermodynamic and kinetic principles, as much as structural factors, make damage sites distinct from the native DNA bases, and indeed, in many cases, these are the features which are believed to be exploited by repair enzymes. Current proposals for damage recognition may not fulfill all of the demands required of enzymatic repair systems given the sheer size of many genomes, and the efficiency with which the genome is screened for damage. Here we discuss current models for how DNA damage recognition may occur and the chemical characteristics, shared by damaged DNA sites, of which repair proteins may take advantage. These include recognition based upon the thermodynamic and kinetic instabilities associated with aberrant sites. Additionally, we describe how small changes in base pair structure can alter also the unique electronic properties of the DNA base pair pi-stack. Further, we describe photophysical, electrochemical, and biochemical experiments in which mismatches and other local perturbations in structure are detected using DNA-mediated charge transport. Finally, we speculate as to how this DNA electron transfer chemistry might be exploited by repair enzymes in order to scan the genome for sites of damage.

Glia development in the embryonic CNS of Drosophila

Glial cells are pivotal players during the development and function of complex nervous systems. In Drosophila, recent genetic analyses have revealed several genes that control differentiation and function of CNS glial cells and their interactions with neurons can be studied in detail at the CNS midline, where it is essential for the correct establishment of the commissural axon pattern.

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

During neural development of Drosophila melanogaster, Glial Cells Missing (GCM), functions as a binary switch that promotes glial cell fate while simultaneously inhibiting the neuronal fate. Sequence similarities between GCM and the recently identified mouse protein mGCMa are strictly limited to the aminoterminal DNA-binding domain. Here we show that mGCMa efficiently activates transcription in Drosophila cells just as Drosophila GCM activates transcription in mammalian cells. Transactivation potential was present in two separate regions of mGCMa outside the DNA-binding domain. One of them mapped to the carboxyterminal 88 amino acids, a location corresponding exactly to the transactivation domain of GCM. Similarities between GCM and mGCMa were also observed in vivo. Overexpression of mGCMa in the developing nervous system of Drosophila embryos led to an increase in glial-like cells at the expense of neurons. Outside the neurogenic region, mGCMa interfered with epidermal development, as evident from changes in cell morphology and marker expression. Thus, mGCMa function is at least partially independent of a cell's predisposition to a neural fate. The potent activity of mGCMa in Drosophila and its extensive functional similarities to GCM make mGCMa a candidate for a regulator of mouse glial development.

Murine Gcm1 gene is expressed in a subset of placental trophoblast cells

The gcm gene of Drosophila melanogaster encodes a transcription factor that is an important component in cell fate specification within the nervous system. In the absence of a functional gcm gene, progenitor cells differentiate into neurons, whereas when the gene is ectopically expressed the cells produce excess glial cells at the expense of neuronal differentiation. Recent searches of databases have uncovered high sequence similarity between the Drosophila gem gene and an anonymous human placental cDNA clone (Altschuller et al., 1996; this communication). Here we report the molecular organization of the murine Gcm1, its spatio-temporal pattern of expression in developing placenta, and its map position at E1-E3 on murine chromosome 9. The murine gene is composed of at least 6 exons. The promoter region contains an "initiation sequence" and is GC rich, characteristics of the promoters of several transcription factors. The mRNA has a modest 5'UTR (ca. 200 bases) but an extensive 3' UTR (ca. 2 kb). Northern blot and mRNA in situ hybridization studies showed that Gcm1 expression was readily detectable only in the placenta. It began at embryonic day 7.5 within trophoblast cells of the chorion and continued to about embryonic day 17.5 within a subset of labyrinthine trophoblast cells. Comparison with other transcription factors revealed that Gcm1 expression defines a unique subset of trophoblast cells.

Isolation and expression analysis of a novel human homologue of the Drosophila glial cells missing (gcm) gene

A novel human homologue (GCMB) of the Drosophila glial cells missing gene (dGCM) was isolated using RACE. GCMB contained a gcm motif sequence and a nuclear targeting sequence similar to that of dGCM and mouse GCMb. Homology searches indicated that GCMB was located within chromosome 6p24.2. Transcripts of GCMB were detected by means of RT-PCR in fetal brain, normal adult kidney, 3/3 medulloblastomas, 1/3 gliomas and 4/8 non-neuroepithelial tumor cell lines. Our data suggest that humans have two homologues of gcm like mice and that human gcm genes form a novel family which may function not only during fetal development but also in the postnatal or pathological stage.

Positive autoregulation of the glial promoting factor glide/gcm

Fly gliogenesis depends on the glial-cell-deficient/glial-cell-missing (glide/gcm) transcription factor. glide/gcm expression is necessary and sufficient to induce the glial fate within and outside the nervous system, indicating that the activity of this gene must be tightly regulated. The current model is that glide/gcm activates the glial fate by inducing the expression of glial-specific genes that are required to maintain such a fate. Previous observations on the null glide/gcmN7-4 allele evoked the possibility that another role of glide/gcm might be to maintain and/or amplify its own expression. Here we show that glide/gcm does positively autoregulate in vitro and in vivo, and that the glide/gcmN7-4 protein is not able to do so. We thereby provide the first direct evidence of both a target and a regulator of glide/gcm. Our data also demonstrate that glide/gcm transcription is regulated at two distinct steps: initiation, which is glide/gcm-independent, and maintenance, which requires glide/gcm. Interestingly, we have found that autoregulation requires the activity of additional cell-specific cofactors. The present results suggest transcriptional autoregulation is a mechanism for glial fate induction.

Isolation and characterization of mammalian homologs of the Drosophila gene glial cells missing

The glial cells missing (gcm) gene in Drosophila encodes a transcription factor that determines the choice between glial and neuronal fates. We report here the isolation of two mammalian gcm homologs, Gcm1 and Gcm2, and the characterization of their expression patterns during embryonic development. Although Gcm2 is expressed in neural tissues at a low level, the major sites of expression for both of the mammalian genes are nonneural, suggesting that the functions of the mammalian homologs have diverged and diversified. However, when expressed ectopically, Gcm1 can substitute functionally for Drosophila gcm by transforming presumptive neurons into glia. Thus, certain biochemical properties, although not the specificity of the tissue in which the gene is expressed, have been conserved through the evolution of the Gcm gene family.