Search PubMed⌕ Search

Biomedical subjects

C Desplan

Publications and source records attributed to C Desplan.

At least 19 recordsLinked to original sources
PubMed2025

RNA-RNA interactions coordinate GABA metabolism and signaling.

Production, signaling, and degradation of the inhibitory neurotransmitter GABA requires tight regulation for proper nervous system function, yet the gene regulatory mechanisms underlying this control remain poorly understood. Here, we identify a post-transcriptional mechanism that coordinates expression of the GABA synthetase Gad1 with its vesicular transporter VGAT in Drosophila. While Gad1 is transcribed specifically in GABAergic neurons, VGAT mRNA is transcribed broadly across the nervous system, but is selectively translated in GABAergic neurons. We show that this specificity depends on CG14989, a putative long non-coding RNA located adjacent to the Gad1 gene locus that shares its GABAergic expression pattern. CG14989 RNA contains sequences complementary to three predicted miR-7 sites in the VGAT 3'UTR, and ectopic expression experiments revealed that it is sufficient for VGAT translation in non-GABAergic neurons, consistent with antagonism of miR-7-mediated repression. Furthermore, complementary regions to CG14989 are present in multiple other genes related to metabolism and signaling of GABA, suggesting a broader role for this mechanism. Together, our findings suggest that CG14989 functions as a regulatory hub that coordinates the molecular identity of GABAergic neurons.

Journal Article↗
PubMed2001

Two-step process for photoreceptor formation in Drosophila.

The formation of photoreceptor cells (PRCs) in Drosophila serves as a paradigm for understanding neuronal determination and differentiation. During larval stages, a precise series of sequential inductive processes leads to the recruitment of eight distinct PRCs (R1-R8). But, final photoreceptor differentiation, including rhabdomere morphogenesis and opsin expression, is completed four days later, during pupal development. It is thought that photoreceptor cell fate is irreversibly established during larval development, when each photoreceptor expresses a particular set of transcriptional regulators and sends its projection to different layers of the optic lobes. Here, we show that the spalt (sal) gene complex encodes two transcription factors that are required late in pupation for photoreceptor differentiation. In the absence of the sal complex, rhabdomere morphology and expression of opsin genes in the inner PRCs R7 and R8 are changed to become identical to those of outer R1-R6 PRCs. However, these cells maintain their normal projections to the medulla part of the optic lobe, and not to the lamina where outer PRCs project. These data indicate that photoreceptor differentiation occurs as a two-step process. First, during larval development, the photoreceptor neurons become committed and send their axonal projections to their targets in the brain. Second, terminal differentiation is executed during pupal development and the photoreceptors adopt their final cellular properties.

Animals↗
PubMed2001

Photoreceptor subtype specification: from flies to humans.

Multiple cell types often differentiate from a pluripotent cell. These cells may then further diversify as distinct subtypes. The visual system provides an ideal model for studying subtype specification as various photoreceptors acquire different functions based on the type of opsin they express. Opsin expression is mostly controlled through transcriptional mechanisms that are evolutionary conserved from Drosophila to humans. In addition, it appears that, from a "default" developmental state, distinct "acquired" photoreceptor states develop upon receiving intrinsic or extrinsic signals. This review discusses factors involved in opsin gene regulation and how their integration may explain how subtype specificity is achieved.

Animals↗
PubMed2001

A conserved regulatory element present in all Drosophila rhodopsin genes mediates Pax6 functions and participates in the fine-tuning of cell-specific expression.

The Drosophila rhodopsin genes (rh's) represent a unique family of highly regulated cell-specific genes, where each member has its own expression pattern in the visual system. Extensive analysis of the rh's has revealed several functional elements that are involved in cell-specificity. We have investigated the functional role of the RCSI/P3 site that is found in the proximal promoter of all Drosophila rh genes. This sequence is remarkably conserved in evolution and is located 15-30 bp upstream of the TATA box. We have previously shown that, in the context of the rh1 promoter, this element is recognized in vivo by a Pax6 protein, the master regulator of eye development. Thus, rh regulation might represent the ancestral function of Pax6. Here, we investigated the role of the RCSI/P3 sequence in the other rh genes and show that they also mediate Pax6 function. We also tested the potential impact of the various RCSI/P3 sequences on the precise cell-specific expression of rh genes. Our results demonstrate that, even though all RCSI/P3 sequences bind Pax6, they are clearly distinct in various rh promoters and these differences are conserved throughout evolution: RCSI/P3 appears to participate in the fine-tuning of cell-specificity. We also show that Pax6 or a related Pax protein may be involved in the regulation of olfactory genes. Therefore, in addition to performing a global photoreceptor-specific function, RCSI also appears to mediate the combined action of Pax6 and other factors and to contribute to rh regulation in subsets of photoreceptors.

Amino Acid Sequence↗
PubMed2001

A new visualization approach for identifying mutations that affect differentiation and organization of the Drosophila ommatidia.

The Drosophila eye is widely used as a model system to study neuronal differentiation, survival and axon projection. Photoreceptor differentiation starts with the specification of a founder cell R8, which sequentially recruits other photoreceptor neurons to the ommatidium. The eight photoreceptors that compose each ommatidium exist in two chiral forms organized along two axes of symmetry and this pattern represents a paradigm to study tissue polarity. We have developed a method of fluoroscopy to visualize the different types of photoreceptors and the organization of the ommatidia in living animals. This allowed us to perform an F(1) genetic screen to isolate mutants affecting photoreceptor differentiation, survival or planar polarity. We illustrate the power of this detection system using known genetic backgrounds and new mutations that affect ommatidial differentiation, morphology or chirality.

Animals↗
PubMed2000

Bicoid-independent formation of thoracic segments in Drosophila.

The maternal determinant Bicoid (Bcd) represents the paradigm of a morphogen that provides positional information for pattern formation. However, as bicoid seems to be a recently acquired gene in flies, the question was raised as to how embryonic patterning is achieved in organisms with more ancestral modes of development. Because the phylogenetically conserved Hunchback (Hb) protein had previously been shown to act as a morphogen in abdominal patterning, we asked which functions of Bcd could be performed by Hb. By reestablishing a proposed ancient regulatory circuitry in which maternal Hb controls zygotic hunchback expression, we show that Hb is able to form thoracic segments in the absence of Bcd.

Animals↗
PubMed2000

A green fluorescent protein enhancer trap screen in Drosophila photoreceptor cells.

The Drosophila ommatidia contain two classes of photoreceptor cells (PR's), the outer and the inner PR's. We performed an enhancer trap screen in order to target genes specifically expressed in PR's. Using the UAS/GAL4 method with enhanced green fluorescent protein (eGFP) as a vital marker, we screened 180000 flies. Out of 2730 lines exhibiting new eGFP patterns, we focused on 16 lines expressing eGFP in particular subsets of PR's. In particular, we describe three lines inserted near the spalt major, m-spondin and furrowed genes, whose respective expression patterns resemble those genes. These genes had not been reported to be expressed in the adult eye. These examples clearly show the ability of our screen to target genes expressed in the adult Drosophila eye.

Animals↗
PubMed2000

High bicoid levels render the terminal system dispensable for Drosophila head development.

In Drosophila, the gradient of the Bicoid (Bcd) morphogen organizes the anteroposterior axis while the ends of the embryo are patterned by the maternal terminal system. At the posterior pole, expression of terminal gap genes is mediated by the local activation of the Torso receptor tyrosine kinase (Tor). At the anterior, terminal gap genes are also activated by the Tor pathway but Bcd contributes to their activation. Here we present evidence that Tor and Bcd act independently on common target genes in an additive manner. Furthermore, we show that the terminal maternal system is not required for proper head development, since high levels of Bcd activity can functionally rescue the lack of terminal system activity at the anterior pole. This observation is consistent with a recent evolution of an anterior morphogenetic center consisting of Bcd and anterior Tor function.

Animals↗
PubMed2000

Phosphorylation of bicoid on MAP-kinase sites: contribution to its interaction with the torso pathway.

The Torso signal transduction pathway exhibits two opposite effects on the activity of the Bicoid (Bcd) morphogen: (i) Bcd function is repressed by Torso (Tor) at the anterior pole of the embryo leading to a retraction of the expression of many Bcd targets from the most anterior region of the embryo, where the Tor tyrosine kinase receptor is activated, and (ii) Bcd function is strengthened by Tor in a broader anterior region, as indicated by a shift of the posterior border of Bcd targets towards the anterior pole in embryos deprived from Tor activity. Anterior repression of Bcd targets was not observed in embryos lacking maternal contribution of D-sor, which acts downstream of Tor and encodes a MAP-kinase kinase. This indicates that the Ras signalling cascade is directly involved in this process, although the known transcriptional effectors of the Tor pathway, tll and hkb, are not (Ronchi, E., Treisman, J., Dostatni, N., Struhl, G. and Desplan, C. (1993) Cell 74, 347-355). Bcd is a good in vitro substrate for phosphorylation by MAP-kinase and phosphorylation of the protein occur in vivo on MAP-kinase sites. In the presence of a Bcd mutant that could no longer be phosphorylated by MAP-kinase, expression of Bcd targets remained repressed by Tor at the pole while strengthening of Bcd activity was reduced. These experiments indicate that phosphorylation of Bcd by MAP-kinase is likely to be required for the Tor pathway to induce its full positive effect on Bcd. This suggests that Tor signalling acts at a distance from the anterior pole by direct modification of the diffusing Bcd morphogen.

Amino Acid Sequence↗
PubMed1999

Bicoid functions without its TATA-binding protein-associated factor interaction domains.

Four maternal systems are known to pattern the early Drosophila embryo. The key component of the anterior system is the homeodomain protein Bicoid (Bcd). Bcd needs the contribution of another anterior morphogen, Hunchback (Hb), to function properly: Bcd and Hb synergize to organize anterior development. A molecular mechanism for this synergy has been proposed to involve specific interactions of Bcd and Hb with TATA-binding protein-associated factors (TAFIIs) that are components of the general transcription machinery. Bcd contains three putative activation domains: a glutamine-rich region, which interacts in vitro with TAFII110; an alanine-rich domain, which targets TAFII60; and a C-terminal acidic region, which has an unknown role. We have generated flies carrying bcd transgenes lacking one or several of these domains to test their function in vivo. Surprisingly, a bcd transgene that lacks all three putative activation domains is able to rescue the bcdE1 null phenotype to viability. Moreover, the development of these embryos is not affected by the presence of dominant negative mutations in TAFII110 or TAFII60. This means that the interactions observed in vitro between Bcd and TAFII60 or TAFII110 aid transcriptional activation but are dispensable for normal development.

Animals↗
PubMed1999

Munster, a novel paired-class homeobox gene specifically expressed in the Drosophila larval eye.

Munster (Mu) is a homeobox-containing gene of the Paired-class which is specifically expressed in the developing Bolwig organs, the Drosophila larval eyes. This expression is first detected during early germ band retraction stage (stage 12 from 7 h 20 at 25 degrees C) and persists until the end of embryogenesis. Mu homeodomain is most similar to that of Aristaless and D-Goosecoid. Strikingly, the Munster gene maps within 6 kb of D-goosecoid, in the same genomic region as aristaless, suggesting that these genes are part of a homeobox gene cluster.

Amino Acid Sequence↗
PubMed1999

Evolution of color vision.

Color vision is achieved by comparing the inputs from retinal photoreceptor neurons that differ in their wavelength sensitivity. Recent studies have elucidated the distribution and phylogeny of opsins, the family of light-sensitive molecules involved in this process. Interesting new findings suggest that animals have evolved a strategy to achieve specific sensitivity through the mutually exclusive expression of different opsin genes in photoreceptors.

Animals↗
PubMed1998

Lune/eye gone, a Pax-like protein, uses a partial paired domain and a homeodomain for DNA recognition.

Pax proteins, characterized by the presence of a paired domain, play key regulatory roles during development. The paired domain is a bipartite DNA-binding domain that contains two helix-turn-helix domains joined by a linker region. Each of the subdomains, the PAI and RED domains, has been shown to be a distinct DNA-binding domain. The PAI domain is the most critical, but in specific circumstances, the RED domain is involved in DNA recognition. We describe a Pax protein, originally called Lune, that is the product of the Drosophila eye gone gene (eyg). It is unique among Pax proteins, because it contains only the RED domain. eyg seems to play a role both in the organogenesis of the salivary gland during embryogenesis and in the development of the eye. A high-affinity binding site for the Eyg RED domain was identified by using systematic evolution of ligands by exponential enrichment techniques. This binding site is related to a binding site previously identified for the RED domain of the Pax-6 5a isoform. Eyg also contains another DNA-binding domain, a Prd-class homeodomain (HD), whose palindromic binding site is similar to other Prd-class HDs. The ability of Pax proteins to use the PAI, RED, and HD, or combinations thereof, may be one mechanism that allows them to be used at different stages of development to regulate various developmental processes through the activation of specific target genes.

Amino Acid Sequence↗
PubMed1998

Drosophila Goosecoid requires a conserved heptapeptide for repression of paired-class homeoprotein activators.

Goosecoid (Gsc) is a homeodomain protein expressed in the organizer region of vertebrate embryos. Its Drosophila homologue, D-Gsc, has been implicated in the formation of the Stomatogastric Nervous System. Although there are no apparent similarities between the phenotypes of mutations in the gsc gene in flies and mice, all known Gsc proteins can rescue dorsoanterior structures in ventralized Xenopus embryos. We describe how D-Gsc behaves as a transcriptional repressor in Drosophila cells, acting through specific palindromic HD binding sites (P3K). D-Gsc is a 'passive repressor' of activator homeoproteins binding to the same sites and an 'active repressor' of activators binding to distinct sites. In addition, D-Gsc is able to strongly repress transcription activated by Paired-class homeoproteins through P3K, via specific protein-protein interactions in what we define as 'interactive repression'. This form of repression requires the short conserved GEH/eh-1 domain, also present in the Engrailed repressor. Although the GEH/eh-1 domain is necessary for rescue of UV-ventralized Xenopus embryos, it is dispensable for ectopic induction of Xlim-1 expression, demonstrating that this domain is not required for all Gsc functions in vivo. Interactive repression may represent specific interactions among Prd-class homeoproteins, several of which act early during development of invertebrate and vertebrate embryos.

Amino Acid Sequence↗
PubMed1997

Direct regulation of rhodopsin 1 by Pax-6/eyeless in Drosophila: evidence for a conserved function in photoreceptors.

Pax-6 is a transcription factor containing both a homeodomain (HD) and a Paired domain (PD). It functions as an essential regulator of eye development in both Drosophila and vertebrates, suggesting an evolutionarily conserved origin for different types of metazoan eyes. Classical morphological and phylogenetic studies, however, have concluded that metazoan eyes have evolved many times independently. These apparently contradictory findings may be reconciled if the evolutionarily ancient role of Pax-6 was to regulate structural genes (e.g., rhodopsin) in primitive photoreceptors, and only later did it expand its function to regulate the morphogenesis of divergent and complex eye structures. In support of this, we present evidence that eyeless (ey), which encodes the Drosophila homolog of Pax-6, directly regulates rhodopsin 1 (rh1) expression in the photoreceptor cells. We detect ey expression in both larval and adult terminally differentiated photoreceptor cells. We show that the HD of Ey binds to a palindromic HD binding site P3/RCS1 in the rh1 promoter, which is essential for rh1 expression. We further demonstrate that, in vivo, P3/RCS1 can be replaced by binding sites specific for the PD of Ey. P3/RCS1 is conserved in the promoters of all Drosophila rhodopsin genes as well as in many opsin genes in vertebrates. Mutimerized P3 sites in front of a basal promoter are able to drive the expression of a reporter gene in all photoreceptors. These results suggest that Pax-6/Ey directly regulates rhodopsin 1 gene expression by binding to the conserved P3/RCS1 element in the promoter.

Animals↗