Regulation of Embryonic Cell Adhesion by the Prion Protein

Prion proteins (PrPs) are key players in fatal neurodegenerative disorders, yet their physiological functions remain unclear, as PrP knockout mice develop rather normally. We report a strong PrP loss-of-function phenotype in zebrafish embryos, characterized by the loss of embryonic cell adhesion and arrested gastrulation. Zebrafish and mouse PrP mRNAs can partially rescue this knockdown phenotype, indicating conserved PrP functions. Using zebrafish, mouse, and Drosophila cells, we show that PrP: (1) mediates Ca+2-independent homophilic cell adhesion and signaling; and (2) modulates Ca+2-dependent cell adhesion by regulating the delivery of E-cadherin to the plasma membrane. In vivo time-lapse analyses reveal that the arrested gastrulation in PrP knockdown embryos is due to deficient morphogenetic cell movements, which rely on E-cadherin–based adhesion. Cell-transplantation experiments indicate that the regulation of embryonic cell adhesion by PrP is cell-autonomous. Moreover, we find that the local accumulation of PrP at cell contact sites is concomitant with the activation of Src-related kinases, the recruitment of reggie/flotillin microdomains, and the reorganization of the actin cytoskeleton, consistent with a role of PrP in the modulation of cell adhesion via signaling. Altogether, our data uncover evolutionarily conserved roles of PrP in cell communication, which ultimately impinge on the stability of adherens cell junctions during embryonic development

Genome duplications and accelerated evolution of Hox genes and cluster architecture in teleost fishes

The early origin of four vertebrate Hox gene clusters during the evolution of gnathostomes was likely caused by two consecutive duplications of the entire genome and the subsequent loss of individual genes. The presumed conserved and important roles of these genes in tetrapods during development led to the general assumption that Hox cluster architecture had remained unchanged since the last common ancestor of all jawed vertebrates. But recent data from teleost fishes reveals that this is not the case. Here, we present an analysis of the evolution of vertebrate Hox genes and clusters, with emphasis on the differences between the Hox A clusters of fish (actinopterygian) and tetrapod (sarcopterygian) lineages. In contrast to the general conservation of genomic architecture and gene sequence observed in sarcopterygians, the evolutionary history of actinopterygian Hox clusters likely includes an additional (third) genome duplication that initially increased the number of clusters from four to eight. We document, for the first time, higher rates of gene loss and gene sequence evolution in the Hox genes of fishes compared to those of land vertebrates. These two observations might suggest that two different molecular evolutionary strategies exist in the two major vertebrate lineages. Preliminary data from the African cichlid fish Oreochromis niloticus compared to those of the pufferfish and zebrafish reveal important differences in Hox cluster architecture among fishes and, together with genetic mapping data from Medaka, indicate that the third genome duplication was not zebrafish-specific, but probably occurred early in the history of fishes. Each descending fish lineage that has been characterized so far, distinctively modified its Hox cluster architecture through independent secondary losses. This variation is related to the large body plan differences observed among fishes, such as the loss of entire sets of appendages and ribs in some lineages

Vertebrate genomics : More fishy tales about Hox genes

Zebrafish Hox genes are arranged in at least seven clusters, rather than the four clusters typical of vertebrates. This suggests that an additional genome duplication occurred on the fish lineage and explains why many gene families are typically about half the size in land vertebrates than they are in fish

PrPs: Proteins with a purpose: Lessons from the zebrafish

The best-known attribute of the prion protein (PrP) is its tendency to misfold into a rogue isoform. Much less understood is how this misfolded isoform causes deadly brain illnesses. Neurodegeneration in prion disease is often seen as a consequence of abnormal PrP function yet, amazingly little is known about the normal, physiological role of PrP. In particular, the absence of obvious phenotypes in PrP knockout mice has prevented scientists from answering this important question. Using knockdown approaches, we previously produced clear PrP loss-of-function phenotypes in zebrafish embryos. Analysis of these phenotypes revealed that PrP can modulate E-cadherin-based cell-cell adhesion, thereby controlling essential morphogenetic cell movements in the early gastrula. Our data also showed that PrP itself can elicit homophilic cell-cell adhesion and trigger intracellular signaling via Src-related kinases. Importantly, these molecular functions of PrP are conserved from fish to mammals. Here we discuss the use of the zebrafish in prion biology and how it may advance our understanding of the roles of PrP in health and disease

Proteínas de prión : de la patogénesis a la función

The prion protein (PrP) is a membrane-anchored glycoprotein normally present in all vertebrates. Under unusual circumstances, it can undergo structural transformation and become the sole constituent of prions, a unique class of infectious agent devoid of nucleic acid. Prions are the cause of a group of lethal neurodegenerative disorders that include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE) or “mad cow disease”. Much is known about PrP’s pathogenic properties; however, its natural role remains elusive despite extensive characterization of its biochemical and cellular properties. Here we review prion diseases and their molecular basis, as well as the evolution and function of prion proteins. We include work carried out in our laboratory, particularly our analysis of PrP loss-of-function phenotypes in the zebrafish and the heterologous expression of various vertebrate PrPs in zebrafish, mouse and Drosophila cells. Our data show that an evolutionarily conserved function of all vertebtrate PrPs is the establishment of homotypic trans-interactions between neighboring cells, thus mediating cell contact formation and signaling via lipid rafts

Ancient origin of reggie (flotillin), reggie-like, and other lipid-raft proteins : convergent evolution of the SPFH domain

Reggies (flotillins) are detergent-resistant microdomains involved in the scaffolding of large heteromeric complexes that signal across the plasma membrane. Based on the presence of an evolutionarily widespread motif, reggies/flotillins have been included within the SPFH (stomatin-prohibitin-flotillin-HflC/K) protein superfamily. To better understand the origin and evolution of reggie/flotillin structure and function, we searched databases for reggie/flotillin and SPFH-like proteins in organisms at the base and beyond the animal kingdom, and used the resulting dataset to compare their structural and functional domains. Our analysis shows that the SPFH grouping has little phylogenetic support, probably due to convergent evolution of its members. We also find that reggie/flotillin homologues are highly conserved among metazoans but are absent in plants, fungi and bacteria, where only proteins with "reggie-like" domains can be found. However, despite their low sequence similarities, reggie/flotillin and "reggie-like" domains appear to subserve related functions, suggesting that their basic biological role was acquired independently during evolution

An evolutionary basis for scrapie disease : identification of a fish prion mRNA

Infectious prion proteins cause neurodegenerative disease in mammals owing to the acquisition of an aberrant conformation. We cloned a Fugu rubripes gene that encodes a structurally conserved prion protein, and found rapid rates of molecular divergence among prions from different vertebrate classes, along with molecular stasis within each class. We propose that a directional trend in the evolution of prion sequence motifs associated with pathogenesis and infectivity could account for the origin of scrapie in mammals

Complete nucleotide sequence of the mitochondrial genome of a salamander, Mertensiella luschani

The complete nucleotide sequence (16,650 bp) of the mitochondrial genome of the salamander Mertensiella luschani (Caudata, Amphibia) was determined. This molecule conforms to the consensus vertebrate mitochondrial gene order. However, it is characterized by a long non-coding intervening sequence with two 124-bp repeats between the tRNAThr and tRNAPro genes. The new sequence data were used to reconstruct a phylogeny of jawed vertebrates. Phylogenetic analyses of all mitochondrial protein-coding genes at the amino acid level recovered a robust vertebrate tree in which lungfishes are the closest living relatives of tetrapods, salamanders and frogs are grouped together to the exclusion of caecilians (the Batrachia hypothesis) in a monophyletic amphibian clade, turtles show diapsid affinities and are placed as sister group of crocodiles+birds, and the marsupials are grouped together with monotremes and basal to placental mammals. The deduced phylogeny was used to characterize the molecular evolution of vertebrate mitochondrial proteins. Amino acid frequencies were analyzed across the main lineages of jawed vertebrates, and leucine and cysteine were found to be the most and least abundant amino acids in mitochondrial proteins, respectively. Patterns of amino acid replacements were conserved among vertebrates. Overall, cartilaginous fishes showed the least variation in amino acid frequencies and replacements. Constancy of rates of evolution among the main lineages of jawed vertebrates was rejected