Molecular cloning and expression of mouse and human cDNA encoding AES and ESG proteins with strong similarity to Drosophila enhancer of split groucho protein

Overexpression of a zebrafish homologue of the Drosophila neurogenic gene Delta perturbs differentiation of primary neurons and somite development

We describe here the isolation and characterization of a zebrafish Delta homologue (delta D). A PCR fragment was used to obtain overlapping cDNA clones encoding a protein of 717 amino acids with all characteristic features of proteins of this family, a signal peptide, a transmembrane domain, and an extracellular region comprising the DSL domain and eight EGF-like repeats. The gene is transcribed in a complex pattern in the developing nervous system as well as in the hypoblast. Overexpression of this gene following mRNA injections leads to a reduction in the number of islet-I positive cells, which are assumed to be primary neurons, and to various defects in the adaxial mesoderm, as well as in the somites and myotomes. This suggests that delta D, and the Notch signalling pathway are involved in the differentiation of primary neurons within the neural plate, as well as in somite development.

Grg3, a murine Groucho-related gene, is expressed in the developing nervous system and in mesenchyme-induced epithelial structures

We have isolated cDNAs representing multiple members of murine groucho homologues, designated Grg for groucho-related genes. Among them, Grg3 appears to produce two transcripts. One of the Grg3 transcripts contains coding sequence for a complete Groucho protein homologue. The second transcript contains coding sequence for only the two amino-terminal domains of the Groucho protein, followed by a hydrophobic tail and a stop codon. We analyzed the expression of both transcripts in mouse embryos using RNase protection and in situ hybridization. Expression was detected during cell determination in the nervous system and in somitic mesoderm, overlapping Notch1 expression and adjacent to Mash1, MyoD and Myf5 expression. Thus, the expression pattern of Grg3 suggests a conserved role in the Notch signalling pathway to regulate expression of basic helix-loop-helix proteins and cell determination. Grg3 expression was also consistently detected in epithelial structures undergoing mesenchyme induction.

Products of the grg (Groucho-related gene) family can dimerize through the amino-terminal Q domain

The murine grg (Groucho-related gene) products are believed to interact with transcription factors and repress transcription, thereby regulating cell proliferation and differentiation. Most proteins in the grg family contain all of the domains found in the Drosophila Groucho protein, including the S/P (Ser-Pro-rich) domain required for interaction with transcription factors and the WD40 domain, which is thought to interact with other proteins. However, at least two Grg proteins contain only the amino-terminal Q (glutamine-rich) domain. We examined whether the Q domain is used for dimerization between Grg proteins, using the yeast two-hybrid system and binding assays with glutathione S-transferase fusion proteins. We found that Grg proteins are able to dimerize through the Q domain and that dimerization requires a core of 50 amino acids. Surprisingly, the dimerization does not require the leucine zipper located within the Q domain.

Transcripts of Grg4, a murine groucho-related gene, are detected in adjacent tissues to other murine neurogenic gene homologues during embryonic development

The groucho-related genes (Grg) of the mouse comprise at least four family members. In Drosophila, groucho is one of the neurogenic genes that participates in the Notch signalling pathway. The Groucho protein interacts with Hairy-related transcription factors to regulate segmentation, neurogenesis and sex determination. Thus, by analogy to the Drosophila proteins, murine Grg proteins may interact with mammalian Hairy and E(spl) homologues (Hes proteins) and take part in a signalling pathway downstream of murine Notch. We have isolated murine Grg4 cDNAs and examined Grg4 expression during embryogenesis. Transcripts of Grg4 were detected in proliferating epithelial tissues undergoing mesenchymal induction, overlapping with Grg3, Notch1 and Hes1 expression. Grg4 was also expressed in the central nervous system and somites, but in cells adjacent to Grg3-, Notch1-, and Hes1-expressing cells. This distinct pattern of expression suggests a role for Grg4 in later stages of cell differentiation than for the other mouse neurogenic gene homologues.

Affinity for the nuclear compartment and expression during cell differentiation implicate phosphorylated Groucho/TLE1 forms of higher molecular mass in nuclear functions

The Drosophila protein Groucho is involved in embryonic segmentation and neural development, and is implicated in the Notch signal transduction pathway. We are investigating the molecular mechanisms underlying the function of Groucho and of its mammalian homologues, the TLE ('transducin-like Enhancer of split') proteins. We show that Groucho/TLE1 proteins are phosphorylated. We also show that two populations of phosphorylated Groucho proteins can be identified based on their interaction with the nuclear compartment. More slowly migrating proteins with an apparent molecular mass of roughly 110 kDa interact strongly with the nuclei, while faster migrating proteins displaying molecular masses roughly 84-85 kDa show lower affinity for the nuclear compartment. Similarly, TLE1 proteins with an apparent molecular mass of roughly 118 kDa exhibit higher affinity for the nuclear compartment than do faster migrating forms with apparent molecular masses of 90-93 kDa. Moreover, we show that the nuclear, more slowly migrating, TLE1 proteins are induced during neural determination of P19 embryonic carcinoma cells. These results implicate phosphorylation in the activity of Groucho/TLE1 proteins and suggest that phosphorylated forms of higher molecular mass are involved in nuclear functions. Finally, we show that different TLE proteins respond in different ways to the neural commitment of P19 cells, suggesting that individual members of this protein family may have non-redundant functions.

The WRPW motif of the hairy-related basic helix-loop-helix repressor proteins acts as a 4-amino-acid transcription repression and protein-protein interaction domain

Hairy-related proteins include the Drosophila Hairy and Enhancer of Split proteins and mammalian Hes proteins. These proteins are basic helix-loop-helix (bHLH) transcriptional repressors that control cell fate decisions such as neurogenesis or myogenesis in both Drosophila melanogaster and mammals. Hairy-related proteins are site-specific DNA-binding proteins defined by the presence of both a repressor-specific bHLH DNA binding domain and a carboxyl-terminal WRPW (Trp-Arg-Pro-Trp) motif. These proteins act as repressors by binding to DNA sites in target gene promoters and not by interfering with activator proteins, indicating that these proteins are active repressors which should therefore have specific repression domains. Here we show the WRPW motif to be a functional transcriptional repression domain sufficient to confer active repression to Hairy-related proteins or a heterologous DNA-binding protein, Ga14. This motif was previously shown to be necessary for interactions with Groucho, a genetically defined corepressor for Drosophila Hairy-related proteins. Here we show that the WRPW motif is sufficient to recruit Groucho or the TLE mammalian homologs to target gene promoters. We also show that Groucho and TLE proteins actively repress transcription when directly bound to a target gene promoter and identify a novel, highly conserved transcriptional repression domain in these proteins. These results directly demonstrate that Groucho family proteins are active transcriptional corepressors for Hairy-related proteins and are recruited by the 4-amino acid protein-protein interaction domain, WRPW.

Protein characterization and targeted disruption of Grg, a mouse gene related to the groucho transcript of the Drosophila Enhancer of split complex

The Grg gene encodes a 197 amino acid protein homologous to the amino-terminal domain of the product of the groucho gene of the Drosophila Enhancer of split complex. Analysis with a polyclonal antisera specific for the Grg protein revealed that Grg is a 25 kd nuclear protein that can participate in specific protein-protein interactions. A null mutation of the Grg gene was constructed by gene targeting. Mice homozygous for this mutation completed embryogenesis and were born, but exhibited varying degrees of post-natal growth deficiency. No dosage-sensitive genetic interaction was detected between the Notch1 and Grg genes in mice heterozygous for a Notch1 mutant allele and homozygous for the Grg null mutation.

TLE expression correlates with mouse embryonic segmentation, neurogenesis, and epithelial determination

The TLE proteins are the mammalian homologues of Groucho, a member of the Drosophila Notch signaling pathway. Notch signaling controls the differentiation of a variety of tissues in invertebrates and vertebrates. We are investigating the role of the TLE genes during mammalian development. We show that TLE 1 and TLE 3 are expressed during a number of cell-determination events, including embryonic segmentation, central and peripheral neurogenesis, and epithelial differentiation. This expression pattern is in agreement with the involvement of Groucho in similar fate choices in Drosophila and suggests that Groucho and TLE proteins perform similar developmental roles. Our results also show that TLE genes are co-expressed during a variety of cell-fate choices with several vertebrate homologues of genes implicated in the Drosophila Notch cascade, suggesting a role for the TLE proteins in mammalian Notch signaling.