In mammals, the unique midkine (mdk) gene encodes a secreted heparin-binding growth factor with neurotrophic activity. Here, we show the presence of two functional mdk genes named mdka and mdkb in zebrafish and rainbow trout. Both midkine proteins are clearly different from the related pleiotrophin, which was also identified in zebrafish and other fishes. Zebrafish mdka and mdkb genes map to linkage groups LG7 and LG25, respectively, both presenting synteny to human chromosome 11, in which the unique human ortholog mdk is located. At least four other genes unique in mammals are also present as duplicates on LG7 and LG25. Phylogenetic and divergence analyses suggested that LG7/LG25 paralogs including mdka and mdkb have been formed at approximately the same time, early during the evolution of the fish lineage. Hence, zebrafish and rainbow trout mdka and mdkb might have been generated by an ancient block duplication, and might be remnants of the proposed fish-specific whole-genome duplication. In contrast to the ubiquitous expression of their mammalian counterpart, zebrafish mdka and mdkb are expressed in spatially restricted, mostly nonoverlapping patterns during embryonic development and strongly in distinct domains in the adult brain. Ectopic ubiquitous expression of both mdk genes in early zebrafish embryos caused completely distinct effects on neural crest and floorplate development. These data indicate that mdka and mdkb underwent functional divergence after duplication. This provides an outstanding model to analyze the molecular mechanisms that lead to differences in pathways regulating the formation of homologous embryonic structures in different vertebrates.

In recent years it turned out that there is not only extensive communication between the nucleus and mitochondria but also between mitochondria and lipid droplets (LDs) as well. We were able to demonstrate that a number of proteins shuttle between LDs and mitochondria and it depends on the metabolic state of the cell on which organelle these proteins are predominantly localized. Responsible for the localization of the particular proteins is a protein domain consisting of two α-helices, which we termed V-domain according to the predicted structure. So far we have detected this domain in the following proteins: mammalian BAX, BCL-XL, TCTP and yeast Mmi1p and Erg6p. According to our experiments there are two functions of this domain: (1) shuttling of proteins to mitochondria in times of stress and apoptosis; (2) clearing the outer mitochondrial membrane from pro- as well as anti-apoptotic proteins by moving them to LDs after the stress ceases. In this way the LDs are used by the cell to modulate stress response.

The main function of the epidermis is to protect us against a multitude of hostile attacks from the environment. Its main cell type, the keratinocytes have a sophisticated system of different proteins and lipids available to form the cornified envelope, which is responsible for the barrier function of the skin. During ageing, dramatic changes are taking place. Some proteins of the SPRR-, S100- and LCE3-family are massively up-regulated, whereas others like loricrin, filaggrin and the LCE1&2 protein families are significantly down-regulated. The latter ones are known to be under control of calcium and/or 'calcium response elements'. We were able to show that the calcium peak specific for the stratum granulosum, which is the site where loricrin and the LCE1&2 families are synthesized, is reduced during ageing. The resulting cornified envelope in old skin has an extensively changed composition on the molecular level compared to young skin. This knowledge is of critical importance to understand chronic wound formation and ulcers in old age.

The dmrtgene family of vertebrates comprises several transcription factors that share a highly conserved DNA-binding domain, the DM domain. Like some of their invertebrate counterparts, e.g. Drosophila doublesex (dsx) and the Caenorhabditis elegans Mab3, several are implicated in sex determination and differentiation. Thus far, dmrt genes represent the only factors involved in sexual development that are conserved across phyla. In the teleost Medaka (Oryzias latipes), a duplicated copy of dmrt1, designated dmrt1bY or dmy, has recently been postulated to be the master regulator of male development in this species. Here, we have analyzed the expression of four additional Medaka dmrt genes during embryonic and larval development. In contrast to other vertebrates, the autosomally located dmrt1a gene of Medaka is not expressed at detectable levels during embryogenesis. On the other hand, dmrt2, dmrt3 and dmrt4 show highly restricted and non-overlapping expression patterns during embryogenesis. While dmrt2 is expressed in early somites, dmrt3 transcripts are found in dorsal interneurons and dmrt4 is expressed in the developing olfactory system. Other than in mouse, they do not show any sex specific expression and no transcription could be detected in the early developing gonads. However, all four analyzed dmrt genes share expression in the differentiating gonad of larvae and in adult testis.

No abstract

Xmrk is a receptor tyrosine kinase closely related to the human EGF receptor. In the teleost ®sh Xiphophorus two versions of the Xmrk gene exist, an oncogene (ONC) and a proto-oncogene (INV). While ONC-Xmrk is the melanoma-inducing gene, INV-Xmrk appears not to be involved in transformation of pigment cells. To elucidate the mechanism that converts the proto-oncogene into a transforming oncogene a comparative analysis of the structure, expression and function of both versions of the gene was performed. In contrast to ONC-Xmrk which is expressed at high levels in melanoma cells, the protooncogene INV-Xmrk is ubiquitously expressed at very low levels indicating overexpression as one possible reason for tumorigenicity by ONC-Xmrk. As sequence comparison of the proto-oncogene and the oncogene revealed a number of amino acid changes, a possible eect of these mutations on the activation of the ONCXmrk receptor was determined. A constitutive activation of the oncogenic receptor was found and ectopic expression of INV-Xmrk after microinjection into medaka®sh embryos did not lead to the high tumour rate in transgenic ®sh as observed for the oncogene. Our data therefore suggest that overexpression of the receptor alone is not sucient for melanoma induction, but that in addition activating mutations in ONC-Xmrk are responsible for its full tumorigenic potential.

The genes coding for bacterioopsin, haloopsin, and sensory opsin I

Jule is the second complete long-terminal-repeat (LTR) Ty3/Gypsy retrotransposon identified to date in vertebrates. Jule, first isolated from the poeciliid fish Xiphophorus maculatus, is 4.8 kb in length, is flanked by two 202-bp LTRs, and encodes Gag (structural core protein) and Pol (protease, reverse transcriptase, RNase H, and integrase, in that order) but no envelope. There are three to four copies of Jule per haploid genome in X. maculatus. Two of them are located in a subtelomeric region of the sex chromosomes, where they are associated with the Xmrk receptor tyrosine kinase genes, of which oncogenic versions are responsible for the formation of hereditary melanoma in Xiphophorus. One almost intact copy of Jule was found in the first intron of the X-chromosomal allele of the Xmrk proto-oncogene, and a second, more corrupted copy is present only 56 nt downstream of the polyadenylation signal of the Xmrk oncogene. Jule-related elements were detected by Southern blot hybridization with less than 10 copies per haploid genome in numerous other poeciliids, as well as in more divergent fishes, including the medakafish Oryzias latipes and the tilapia Oreochromis niloticus. Database searches also identified Jule-related sequences in the zebrafish Danio rerio and in both genome project pufferfishes, Fugu rubripes and Tetraodon nigroviridis. Phylogenetic analysis revealed that Jule is the first member of the Mag family of Ty3/Gypsy retrotransposons described to date in vertebrates. This family includes the silkworm Mag and sea urchin SURL retrotransposons, as well as sequences from the nematode Caenorhabditis elegans. Additional related elements were identified in the genomes of the malaria mosquito Anopheles gambiae and the nematode Ascaris lumbricoides. Phylogeny of Mag-related elements suggested that the Mag family of retrotransposons is polyphyletic and is constituted of several ancient lineages that diverged before their host genomes more than 600 MYA.