Gag proteins encoded by endogenous retroviruses are required for zebrafish development.

Transposable elements (TEs) make up the bulk of eukaryotic genomes and examples abound of TE-derived sequences repurposed for organismal function. The process by which TEs become coopted remains obscure because most cases involve ancient, transpositionally inactive elements. Reports of active TEs serving beneficial functions are scarce and often contentious due to difficulties in manipulating repetitive sequences. Here we show that recently active TEs in zebrafish encode products critical for embryonic development. Knockdown and rescue experiments demonstrate that the endogenous retrovirus family BHIKHARI-1 (Bik-1) encodes a Gag protein essential for mesoderm development. Mechanistically, Bik-1 Gag associates with the cell membrane and its ectopic expression in chicken embryos alters cell migration. Similarly, depletion of BHIKHARI-2 Gag, a relative of Bik-1, causes defects in neural crest development in zebrafish. We propose an "addiction" model to explain how active TEs can be integrated into conserved developmental processes.

Role of K+ and amino acids in osmoregulation by the free-living microaerophilic protozoon Hexamita inflata.

The primitive free-living protozoon Hexamita inflata was found to maintain a cell volume of approximately 260 fI under standard culture conditions. On increasing the extracellular osmolality the volume decreased and the cells remained shrunken for >30 min. By contrast, a decrease in the external osmolality resulted in a transient increase in cell volume which was followed by an efficient 'regulatory volume decrease' (RVD). H. inflata contains high concentrations of amino acids, with alanine constituting over 70% of the total amino acid pool. Exposure to hypo-osmotic medium resulted in the loss from the cell of both amino acids and K+, via one or more swelling-activated pathways. The efflux of amino acids and K+, together with a charge-balancing counter-anion, accounted almost fully for the observed RVD. The pharmacological properties of the swelling-activated pathways differ from those of volume-sensitive transporters and channels described previously in other cell types.