Transcription regulation and animal diversity

Whole-genome sequence assemblies are now available for seven different animals, including nematode worms, mice and humans. Comparative genome analyses reveal a surprising constancy in genetic content: vertebrate genomes have only about twice the number of genes that invertebrate genomes have, and the increase is primarily due to the duplication of existing genes rather than the invention of new ones. How, then, has evolutionary diversity arisen? Emerging evidence suggests that organismal complexity arises from progressively more elaborate regulation of gene expression.     

Paramyosin and actin in schistosomal teguments

Schistosomes are blood-dwelling trematode parasites that infect 200 million people in developing countries. The critical role served by the tegument in immune evasion and parasite homeostasis suggests that a detailed knowledge of tegumental components would be helpful in the design of new drugs and the production of vaccines. We demonstrate here, by immunoelectron microscopy, that the cytoskeletal proteins actin and paramyosin are organized into major tegumental structures of Schistosoma mansoni. The surface spines are composed of paracrystalline arrays of actin filaments. Actin is also present in areas recovering from damage, implying an important role for this structural protein in tegumental repair. Paramyosin exists predominantly in the tegument in a non-filamentous form, the membrane-bounded elongate bodies. The localization of this protein to the tegument of the parasite is the likely basis for resistance to S. mansoni observed in mice immunized with paramyosin (refs 1, 2 and T. P. Flanigen et al., in preparation).     

Soluble factors and the development of rod photoreceptors

Photoreceptors are the most abundant cell type in the vertebrate neural retina. Like the other retinal neurons and the Müller glia, they arise from a population of precursor cells that are multipotent and intrinsic to the retina. Approximately 10 years ago, several studies demonstrated that retinal precursor cells (RPCs) are competent to respond to environmental factors that promote cell type determination and differentiation. Since those studies, significant effort has been directed at identifying the molecular nature of these environmental signals and understanding the precise mechanisms they employ to drive RPCs towards the different retinal fates. In this review, we describe the recent progress toward understanding how environmental factors influence the development of vertebrate rod photoreceptors.     

Conservation of the sequence and temporal expression of let-7 heterochronic regulatory RNA

Two small RNAs regulate the timing of Caenorhabditis elegans development. Transition from the first to the second larval stage fates requires the 22-nucleotide lin-4 RNA, and transition from late larval to adult cell fates requires the 21-nucleotide let-7 RNA. The lin-4 and let-7 RNA genes are not homologous to each other, but are each complementary to sequences in the 3' untranslated regions of a set of protein-coding target genes that are normally negatively regulated by the RNAs. Here we have detected let-7 RNAs of approximately 21 nucleotides in samples from a wide range of animal species, including vertebrate, ascidian, hemichordate, mollusc, annelid and arthropod, but not in RNAs from several cnidarian and poriferan species, Saccharomyces cerevisiae, Escherichia coli or Arabidopsis. We did not detect lin-4 RNA in these species. We found that let-7 temporal regulation is also conserved: let-7 RNA expression is first detected at late larval stages in C. elegans and Drosophila, at 48 hours after fertilization in zebrafish, and in adult stages of annelids and molluscs. The let-7 regulatory RNA may control late temporal transitions during development across animal phylogeny.     

Image-based genome-wide siRNA screen identifies selective autophagy factors

Selective autophagy involves the recognition and targeting of specific cargo, such as damaged organelles, misfolded proteins, or invading pathogens for lysosomal destruction. Yeast genetic screens have identified proteins required for different forms of selective autophagy, including cytoplasm-to-vacuole targeting, pexophagy and mitophagy, and mammalian genetic screens have identified proteins required for autophagy regulation. However, there have been no systematic approaches to identify molecular determinants of selective autophagy in mammalian cells. Here, to identify mammalian genes required for selective autophagy, we performed a high-content, image-based, genome-wide small interfering RNA screen to detect genes required for the colocalization of Sindbis virus capsid protein with autophagolysosomes. We identified 141 candidate genes required for viral autophagy, which were enriched for cellular pathways related to messenger RNA processing, interferon signalling, vesicle trafficking, cytoskeletal motor function and metabolism. Ninety-six of these genes were also required for Parkin-mediated mitophagy, indicating that common molecular determinants may be involved in autophagic targeting of viral nucleocapsids and autophagic targeting of damaged mitochondria. Murine embryonic fibroblasts lacking one of these gene products, the C2-domain containing protein, SMURF1, are deficient in the autophagosomal targeting of Sindbis and herpes simplex viruses and in the clearance of damaged mitochondria. Moreover, SMURF1-deficient mice accumulate damaged mitochondria in the heart, brain and liver. Thus, our study identifies candidate determinants of selective autophagy, and defines SMURF1 as a newly recognized mediator of both viral autophagy and mitophagy.     

Structural basis for the function and inhibition of an influenza virus proton channel

The M2 protein from influenza A virus is a pH-activated proton channel that mediates acidification of the interior of viral particles entrapped in endosomes. M2 is the target of the anti-influenza drugs amantadine and rimantadine; recently, resistance to these drugs in humans, birds and pigs has reached more than 90% (ref. 1). Here we describe the crystal structure of the transmembrane-spanning region of the homotetrameric protein in the presence and absence of the channel-blocking drug amantadine. pH-dependent structural changes occur near a set of conserved His and Trp residues that are involved in proton gating. The drug-binding site is lined by residues that are mutated in amantadine-resistant viruses. Binding of amantadine physically occludes the pore, and might also perturb the pK(a) of the critical His residue. The structure provides a starting point for solving the problem of resistance to M2-channel blockers.