The genetics and molecular structure of the Drosophila pair-rule gene odd Oz (odz)

Drosophila Ten-m and filamin affect motor neuron growth cone guidance

The Drosophila Ten-m (also called Tenascin-major, or odd Oz (odz)) gene has been associated with a pair-rule phenotype. We identified and characterized new alleles of Drosophila Ten-m to establish that this gene is not responsible for segmentation defects but rather causes defects in motor neuron axon routing. In Ten-m mutants the inter-segmental nerve (ISN) often crosses segment boundaries and fasciculates with the ISN in the adjacent segment. Ten-m is expressed in the central nervous system and epidermal stripes during the stages when the growth cones of the neurons that form the ISN navigate to their targets. Over-expression of Ten-m in epidermal cells also leads to ISN misrouting. We also found that Filamin, an actin binding protein, physically interacts with the Ten-m protein. Mutations in cheerio, which encodes Filamin, cause defects in motor neuron axon routing like those of Ten-m. During embryonic development, the expression of Filamin and Ten-m partially overlap in ectodermal cells. These results suggest that Ten-m and Filamin in epidermal cells might together influence growth cone progression.

Expression of Ten-m/Odz3 in the fibrous layer of mandibular condylar cartilage during postnatal growth in mice

It has been speculated that the mandibular condyle develops via the differentiation of the fibroblast-like cells covering the condyle into chondrocytes; however, the developmental mechanisms behind this process have not been revealed. We used laser-capture microdissection and cDNA microarray analysis to elucidate the genes that are highly expressed in these fibroblast-like cells. Among these genes, the transcription of Ten-m/Odz3 was significantly increased in the fibroblast-like cells compared with other cartilage tissues. For the first time, we describe the temporal and spatial expression of Ten-m/Odz3 mRNA in relation to the expression of type I, II, and X collagen mRNA, as determined by in-situ hybridization in mouse mandibular condylar cartilage and mouse femoral cartilage during the early stages of development. Ten-m/Odz3 was expressed in the fibrous layer and the proliferating and mature chondrocyte layers, which expressed type I and II collagen, respectively, but was not detected in the hypertrophic chondrocyte layer. Furthermore, we evaluated the in-vitro expression of Ten-m/Odz3 using ATDC5 cells, a mouse chondrogenic cell line. Ten-m/Odz3 was expressed during the early stage of the differentiation of mesenchymal cells into chondrocytes. These findings suggest that Ten-m/Odz3 is involved in the differentiation of chondrocytes and that it acts as a regulatory factor in the early stages of the development of mandibular condylar cartilage.

C. elegans ten-1 is synthetic lethal with mutations in cytoskeleton regulators, and enhances many axon guidance defective mutants

BACKGROUND

Teneurins are transmembrane proteins that assist morphogenetic processes in many organisms. ten-1 is the C. elegans teneurin homolog with two transcripts, ten-1a and ten-1b, that respectively encode a long (TEN-1L) and short (TEN-1S) form of the protein. We previously isolated a C. elegans mutant where one pharyngeal neuron was frequently misplaced, and now show that it corresponds to a novel allele of ten-1.

RESULTS

The novel ten-1(et5) allele is a hypomorph since its post-embryonic phenotype is weaker than the null alleles ten-1(ok641) and ten-1(tm651). ten-1 mutants have defects in all pharyngeal neurons that we examined, and in vivo reporters show that only the long form of the ten-1 gene is expressed in the pharynx, specifically in six marginal cells and the M2 neurons. Defects in the pharyngeal M2 neurons were enhanced when the ten-1(ok641) mutation was combined with mutations in the following genes: mig-14, unc-5, unc-51, unc-52 and unc-129. None of the body neurons examined show any defects in the ten-1(ok641) mutant, but genetic interaction studies reveal that ten-1(ok641) is synthetic lethal with sax-3, unc-34 and unc-73, and examination of the hypodermal cells in embryos of the ten-1(ok641) mutant point to a role of ten-1 during hypodermal cell morphogenesis.

CONCLUSIONS

Our results are consistent with ten-1 normally providing a function complementary to the cytoskeletal remodeling processes that occur in migrating cells or cells undergoing morphogenesis. It is possible that ten-1 influences the composition/distribution of extracellular matrix.

Drosophila odz gene is required for multiple cell types in the compound retina

The Drosophila melanogaster pair-rule gene odz (odd Oz, or Ten-m) is expressed in distinct patterns in the larval eye imaginal disc. Its earliest eye expression occurs in ommatidial precursors starting from the posterior edge of the morphogenetic furrow. Loss of function of odz activity leads to visible light photoreceptor loss; R7 photoreceptor loss; ommatidial size, shape, and rotation defects; ommatidial disorder and fusions; interommatidial bristle defects; and ommatidial lens defects. The same effects are seen in odz eye mitotic clones, in odz-Ten-a transheterozygous combinations, and in eyes expressing an Odz-Dominant Negative transgene (Odz-DN). Effects of the same strength are also seen when the Odz-DN transgene is driven only in regions of scabrous expression, which overlaps the four columns of Odz expression clusters behind the furrow. Small odz mitotic clones suggest an odz role in cell proliferation or survival. Senseless is expressed in odz mutant clones, in a fairly ordered manner, indicating that Odz acts downstream of R8 specification. Disorder within each ommatidium in odz clones is accompanied by some loss of R7 precursors and visible photoreceptor precursor order.

Drosophila Ten-a is a maternal pair-rule and patterning gene

The Ten-a gene of Drosophila melanogaster encodes several alternative variants of a full length member of the Odz/Tenm protein family. A number of Ten-a mutants created by inexact excisions of a resident P-element insertion are embryonic lethal, but show no pair-rule phenotype. In contrast, these mutants, and deficiencies removing Ten-a, do enhance the segmentation phenotype of a weak allele of the paralog gene odz (or Ten-m) to the odz amorphic phenotype. Germ line clone derived Ten-a(-) embryos display a pair-rule phenotype which phenocopies that of odz. Post segmentation eye patterning phenotypes of Ten-a mutants establish it as a pleiotropic patterning co-partner of odz.

Teneurins: a conserved family of transmembrane proteins involved in intercellular signaling during development

Teneurins, which were initially described as ten-a and the pair-rule gene ten-m/odz in Drosophila, are a family of highly conserved proteins that have recently been characterized in Caenorhabditis elegans and a number of vertebrates. We have proposed the nomenclature teneurin 1-4 for the four members of this gene family found in vertebrates. Recent evidence shows that teneurins belong to a novel class of signaling molecules that function both at the cell surface as type II transmembrane receptors and, after the release of the intracellular domain, as transcriptional regulators. Nuclear localization of the intracellular domain has been observed in vitro in mammalian cells and confirmed in vivo in C. elegans. RNAi studies and mutational analysis has revealed that Ten-1 in C. elegans is an important regulator of many aspects of morphogenesis, including germ cell development and neuronal pathfinding. In vertebrates, teneurins are concentrated in the developing and adult central nervous system and at sites of pattern formation, including the developing limb. Teneurins also possess a carboxy terminal sequence that may be processed to generate a neuromodulatory peptide. Teneurin function appears to be required for a fundamentally important signaling mechanism conserved between invertebrates and vertebrates having an impact on many processes relying on cell-cell contact throughout development.

Teneurin proteins possess a carboxy terminal sequence with neuromodulatory activity

We have previously shown that a bioactive neuropeptide-like sequence is present at the carboxy-terminus of the teneurin transmembrane proteins. We have subsequently called this peptide 'teneurin C-terminal associated peptide' (TCAP). The sequence encodes a peptide 40 or 41 amino acids long flanked by a cleavage motif on the amino terminus and an amidation motif on the carboxy terminus, characteristic of bioactive peptides. This sequence is highly conserved in all vertebrates. A TCAP-like sequence is encoded by each of the four teneurin genes. We have therefore examined the neurological role TCAP-1 may play in mice and rats. In situ hybridization studies showed that the teneurin-1 mRNA containing the TCAP-1 sequence is expressed in regions of the forebrain and limbic system regulating stress and anxiety. A synthetic version of amidated mouse/rat TCAP-1 was prepared by solid-phase synthesis and used to investigate the in vitro and in vivo activity. TCAP-1 induces a dose-dependent change in cAMP accumulation and MTT activity in immortalized mouse neurons. Administration of synthetic TCAP-1 into the basolateral amygdala significantly increases the acoustic startle response in low-anxiety rats and decreases the response in high-anxiety animals in a dose-dependent manner. When 30 pmol TCAP-1 is administered into the lateral ventricles each day for 5 days, the sensitization of the rats to the acoustic startle response is abolished. These data indicate that TCAP may possess functions that are independent of the teneurin proprotein and together, the teneurins and TCAP, may represent a novel system to regulate neuronal function and emotionality.

Cloning and characterization of teneurin C-terminus associated peptide (TCAP)-3 from the hypothalamus of an adult rainbow trout (Oncorhynchus mykiss)

Neuropeptides that evolved early in metazoan evolution may possess much larger networks of paralogous genes than later evolving peptides due to the increased exposure to gene and genomic duplication events. The corticotropin-releasing factor family of peptides, which also include invertebrate CRF-like peptides, are a candidate group that appear to have an early origin. We have attempted to find additional paralogous genes to the CRF family by doing a low-stringency screen of a rainbow trout hypothalamic cDNA library using a hamster urocortin probe. A clone was identified that represented the rainbow trout ortholog of teneurin-3. The C-terminal region of the last exon teneurin transmembrane protein gene possesses a neuropeptide-like sequence with a primary structure similarity to the corticotropin-releasing factor family of peptides. We have called this sequence teneurin C-terminal associated peptide (TCAP). The predicted peptide is 40 residues long and possesses an expected pyroglutamyl residue in the first position and an amidated carboxy terminus. A synthetic version of the rainbow trout (rt) TCAP-3 is potent at increasing the concentration of cAMP and stimulating proliferation in a neuronal cell line. The synthetic peptide can also either increase or decrease the expression of the teneurin-1 gene, depending upon its concentration. The teneurin/TCAP system may represent a novel and highly conserved regulatory signalling system in the vertebrate brain.

The intracellular domain of teneurin-2 has a nuclear function and represses zic-1-mediated transcription

Teneurin-2, a vertebrate homologue of the Drosophila pair-rule gene ten-m/odz, is revealed to be a membrane-bound transcription regulator. In the nucleus, the intracellular domain of teneurin-2 colocalizes with promyelocytic leukemia (PML) protein in nuclear bodies implicated in transcription control. Since Drosophila ten-m acts epistatically to another pair-rule gene opa, we investigated whether gene regulation by the mammalian opa homologue zic-1 was influenced by the intracellular domain of teneurin-2. We found that zic-mediated transcription from the apolipoprotein E promoter was inhibited. Release of the intracellular domain of teneurin-2 could be stimulated by homophilic interaction of the extracellular domain, and the intracellular domain was stabilized by proteasome inhibitors. We have previously shown that teneurin-2 is expressed by neurons belonging to the same functional circuit. Therefore, we hypothesize that homophilic interaction enables neurons to identify their targets and that the release of the intracellular domain of teneurin-2 provides them with a signal to switch their gene expression program from growth towards differentiation once the proper contact has been made.

The Drosophila odz/ten-m gene encodes a type I, multiply cleaved heterodimeric transmembrane protein

The product of the Drosophila melanogaster odd Oz (odz)/Tenascin-major (ten-m) pair-rule gene consists of eight epidermal growth factor (EGF)-like repeats followed by a novel 1800 amino acid polypeptide stretch unique to proteins of the Odz/Ten-m family. The structure and membrane orientation of this large enigmatic protein was characterized by raising and employing antibodies directed against discrete Odz polypeptide regions. Protein-modifying reagents impermeable to the plasma membrane were used in concert with the battery of antibodies to demonstrate that Odz is a type I transmembrane protein with the vast C-terminal portion in the intracellular space, and with the EGF repeats deployed extracellularly. The polypeptide was shown to undergo multiple cleavages at discrete intracellular and extracellular sites, and its extreme C-terminus was shown to undergo either processing at a very large number of sites or programmed degradation. The polypeptide is presented at the cell surface with additional post-translational modifications, and as two subunits of previously cleaved Odz joined by cysteine disulphide bridges maintaining their association. The model derived for the Odz protein is discussed in light of other models proposed for proteins of the Odz/Ten-m family, and in terms of functional implications.

Phylogenetic analysis of teneurin genes and comparison to the rearrangement hot spot elements of E. coli

Teneurins are a novel family of transmembrane proteins conserved between invertebrates and vertebrates. There are two members in Drosophila, one in C. elegans and four members in mouse. Here, we describe the analysis of the genomic structure of the human teneurin-1 gene. The entire human teneurin-1 (TEN1) gene is contained in eight PAC clones representing part of the chromosomal locus Xq25. Interestingly, many X-linked mental retardation syndromes (XLMR) and non-specific mental retardation (MRX) are mapped to this region. The location of the human TEN1 together with the neuronal expression makes TEN1 a candidate gene for XLMR and MRX. We also identified large parts of the human teneurin-2 sequence on chromosome 5 and sections of human teneurin-4 at chromosomal position 11q14. Database searches resulted in the identification of ESTs encoding parts of all four human members of the teneurin family. Analysis of the genomic organization of the Drosophila ten-a gene revealed the presence of exons encoding a long form of ten-a, which can be aligned with all other teneurins known. Sequence comparison and phylogenetic trees of teneurins show that insects and vertebrates diverged before the teneurin ancestor was duplicated independently in the two phyla. This is supported by the presence of conserved intron positions between teneurin genes of man, Drosophila and C. elegans. It is therefore not possible to class any of the vertebrate teneurins with either Drosophila Ten-a or Ten-m. The C-terminal part of all teneurins harbours 26 repetitive sequence motifs termed YD-repeats. YD-repeats are most similar to the repeats encoded by the core of the rearrangement hot spot (rhs) elements of Escherichia coli. This makes the teneurin ancestor a candidate gene for the source of the rhs core acquired by horizontal gene transfer.

RINX(VSX1), a novel homeobox gene expressed in the inner nuclear layer of the adult retina

The locus control region (LCR) of the human red and green visual pigment genes is critical for the formation of functional red and green cones in the retina. A 37-bp core of the LCR is perfectly conserved among mammals and binds specific retinal nuclear proteins. Here, we employed a yeast one-hybrid screen of an adult retinal cDNA library to clone and characterize these proteins. We identified clones encoding homeodomain (HD) transcription factors Pax6, Rx, and Chx10 and a novel paired-like HD protein, RINX. In the adult retina, RINX is exclusively expressed in a subset of cells (likely to be bipolar cells) of the retinal inner nuclear layer (INL). RINX is closely related to Chx10, which is also exclusively expressed in the INL of the adult retina and is critical for retinal development. The RINX gene is expressed in two classes of mRNA. One class encodes proteins that lack either part of or all of the HD, but retain the transcriptional activation domain. The RINX gene maps to chromosome 20p11.2 to which no retinal disease has been assigned. In conclusion, the LCR contains two adjacent motifs that are targets for binding of HD proteins that may specify the development and differentiation of cone photoreceptors and a subset of INL bipolar cells. Mutations in the related human CHX10 gene cause microphthalmia in a subset of families, and, therefore, the RINX gene is a candidate for this phenotype in another subset of patients. Since the RINX gene is likely an ortholog of the goldfish Vsx1 gene, it has been named VSX1 by the Human Gene Nomenclature Committee.

The mammalian Odz gene family: homologs of a Drosophila pair-rule gene with expression implying distinct yet overlapping developmental roles

The Drosophila pair-rule gene odz (Tenm) has many patterning roles throughout development. We have identified four mammalian homologs of this gene, including one previously described as a mouse ER stress response gene, Doc4 (Wang et al., 1998). The Odz genes encode large polypeptides displaying the hallmarks of Drosophila Odz: a putative signal peptide; eight EGF-like repeats; and a putative transmembrane domain followed by a 1800-amino-acid stretch without homology to any proteins outside of this family. The mouse genes Odz3 and Doc4/Odz4 exhibit partially overlapping, but clearly distinct, embryonic expression patterns. The major embryonic sites of expression are in the nervous system, including the tectum, optic recess, optic stalk, and developing eye. Additional sites of expression include trachea and mesodermally derived tissues, such as mesentery, and forming limb and bone. Expression of the Odz2 gene is restricted to the nervous system. The expression patterns suggest that each of the genes has its own distinct developmental role. Comparisons of Drosophila and vertebrate Odz expression patterns suggest evolutionarily conserved functions.