Cell Polarity: Keeping Worms LeGaL

SummaryThe protein Lethal giant larvae (LGL) regulates cell polarity in diverse animal models. Now, an LGL orthologue has been identified in the worm Caenorhabditis elegans and is shown to function redundantly with a worm-specific polarity protein, PAR-2

Myosin and the PAR proteins polarize microfilament-dependent forces that shape and position mitotic spindles in Caenorhabditis elegans

In Caenorhabditis elegans, the partitioning proteins (PARs), microfilaments (MFs), dynein, dynactin, and a nonmuscle myosin II all localize to the cortex of early embryonic cells. Both the PARs and the actomyosin cytoskeleton are required to polarize the anterior-posterior (a-p) body axis in one-cell zygotes, but it remains unknown how MFs influence embryonic polarity. Here we show that MFs are required for the cortical localization of PAR-2 and PAR-3. Furthermore, we show that PAR polarity regulates MF-dependent cortical forces applied to astral microtubules (MTs). These forces, which appear to be mediated by dynein and dynactin, produce changes in the shape and orientation of mitotic spindles. Unlike MFs, dynein, and dynactin, myosin II is not required for the production of these forces. Instead, myosin influences embryonic polarity by limiting PAR-3 to the anterior cortex. This in turn produces asymmetry in the forces applied to MTs at each pole and allows PAR-2 to accumulate in the posterior cortex of a one-cell zygote and maintain asymmetry

The evolution of protostome GATA factors: Molecular phylogenetics, synteny, and intron/exon structure reveal orthologous relationships

<p>Abstract</p> <p>Background</p> <p>Invertebrate and vertebrate GATA transcription factors play important roles in ectoderm and mesendoderm development, as well as in cardiovascular and blood cell fate specification. However, the assignment of evolutionarily conserved roles to GATA homologs requires a detailed framework of orthologous relationships. Although two distinct classes, GATA123 and GATA456, have been unambiguously recognized among deuterostome GATA genes, it has been difficult to resolve exact orthologous relationships among protostome homologs. Protostome GATA genes are often present in multiple copies within any one genome, and rapidly evolving gene sequences have obscured orthology among arthropod and nematode GATA homologs. In addition, a lack of taxonomic sampling has prevented a stepwise reconstruction of protostome GATA gene family evolution.</p> <p>Results</p> <p>We have identified the complete GATA complement (53 genes) from a diverse sampling of protostome genomes, including six arthropods, three lophotrochozoans, and two nematodes. Reciprocal best hit BLAST analysis suggested orthology of these GATA genes to either the ancestral bilaterian GATA123 or the GATA456 class. Using molecular phylogenetic analyses of gene sequences, together with conserved synteny and comparisons of intron/exon structure, we inferred the evolutionary relationships among these 53 protostome GATA homologs. In particular, we resolved the orthology and evolutionary birth order of all arthropod GATA homologs including the highly divergent <it>Drosophila </it>GATA genes.</p> <p>Conclusion</p> <p>Our combined analyses confirm that all protostome GATA transcription factor genes are members of either the GATA123 or GATA456 class, and indicate that there have been multiple protostome-specific duplications of GATA456 homologs. Three GATA456 genes exhibit linkage in multiple protostome species, suggesting that this gene cluster arose by tandem duplications from an ancestral GATA456 gene. Within arthropods this GATA456 cluster appears orthologous and widely conserved. Furthermore, the intron/exon structures of the arthropod GATA456 orthologs suggest a distinct order of gene duplication events. At present, however, the evolutionary relationship to similarly linked GATA456 paralogs in lophotrochozoans remains unclear. Our study shows how sampling of additional genomic data, especially from less derived and interspersed protostome taxa, can be used to resolve the orthologous relationships within more divergent gene families.</p

Whole genome duplications and expansion of the vertebrate GATA transcription factor gene family

<p>Abstract</p> <p>Background</p> <p>GATA transcription factors influence many developmental processes, including the specification of embryonic germ layers. The GATA gene family has significantly expanded in many animal lineages: whereas diverse cnidarians have only one GATA transcription factor, six GATA genes have been identified in many vertebrates, five in many insects, and eleven to thirteen in <it>Caenorhabditis </it>nematodes. All bilaterian animal genomes have at least one member each of two classes, GATA123 and GATA456.</p> <p>Results</p> <p>We have identified one GATA123 gene and one GATA456 gene from the genomic sequence of two invertebrate deuterostomes, a cephalochordate (<it>Branchiostoma floridae</it>) and a hemichordate (<it>Saccoglossus kowalevskii</it>). We also have confirmed the presence of six GATA genes in all vertebrate genomes, as well as additional GATA genes in teleost fish. Analyses of conserved sequence motifs and of changes to the exon-intron structure, and molecular phylogenetic analyses of these deuterostome GATA genes support their origin from two ancestral deuterostome genes, one GATA 123 and one GATA456. Comparison of the conserved genomic organization across vertebrates identified eighteen paralogous gene families linked to multiple vertebrate GATA genes (GATA paralogons), providing the strongest evidence yet for expansion of vertebrate GATA gene families via genome duplication events.</p> <p>Conclusion</p> <p>From our analysis, we infer the evolutionary birth order and relationships among vertebrate GATA transcription factors, and define their expansion via multiple rounds of whole genome duplication events. As the genomes of four independent invertebrate deuterostome lineages contain single copy GATA123 and GATA456 genes, we infer that the 0R (pre-genome duplication) invertebrate deuterostome ancestor also had two GATA genes, one of each class. Synteny analyses identify duplications of paralogous chromosomal regions (paralogons), from single ancestral vertebrate GATA123 and GATA456 chromosomes to four paralogons after the first round of vertebrate genome duplication, to seven paralogons after the second round of vertebrate genome duplication, and to fourteen paralogons after the fish-specific 3R genome duplication. The evolutionary analysis of GATA gene origins and relationships may inform understanding vertebrate GATA factor redundancies and specializations.</p

The Aurora-related kinase AIR-2 recruits ZEN-4/CeMKLP1 to the mitotic spindle at metaphase and is required for cytokinesis

AbstractBackground: The Aurora/Ipl1p-related kinase AIR-2 is required for mitotic chromosome segregation and cytokinesis in early Caenorhabditis elegans embryos. Previous studies have relied on non-conditional mutations or RNA-mediated interference (RNAi) to inactivate AIR-2. It has therefore not been possible to determine whether AIR-2 functions directly in cytokinesis or if the cleavage defect results indirectly from the failure to segregate DNA. One intriguing hypothesis is that AIR-2 acts to localize the mitotic kinesin-like protein ZEN-4 (also known as CeMKLP1), which later functions in cytokinesis.Results: Using conditional alleles, we established that AIR-2 is required at metaphase or early anaphase for normal segregation of chromosomes, localization of ZEN-4, and cytokinesis. ZEN-4 is first required late in cytokinesis, and also functions to maintain cell separation through much of the subsequent interphase. DNA segregation defects alone were not sufficient to disrupt cytokinesis in other mutants, suggesting that AIR-2 acts specifically during cytokinesis through ZEN-4. AIR-2 and ZEN-4 shared similar genetic interactions with the formin homology (FH) protein CYK-1, suggesting that AIR-2 and ZEN-4 function in a single pathway, in parallel to a contractile ring pathway that includes CYK-1. Using in vitro co-immunoprecipitation experiments, we found that AIR-2 and ZEN-4 interact directly.Conclusions: AIR-2 has two functions during mitosis: one in chromosome segregation, and a second, independent function in cytokinesis through ZEN-4. AIR-2 and ZEN-4 may act in parallel to a second pathway that includes CYK-1

The convergence of Cesaro averages for certain nonstationary Markov chains

AbstractIf P is a stochastic matrix corresponding to a stationary, irreducible, positive persistent Markov chain of period d>1, the powers Pn will not converge as n → ∞. However, the subsequences Pnd+k for k=0,1,...d-1, and hence Cesaro averages Σnk-1 Pk/n, will converge. In this paper we determine classes of nonstationary Markov chains for which the analogous subsequences and/or Cesaro averages converge and consider the rates of convergence. The results obtained are then applied to the analysis of expected average cost

Heterotrimeric G protein signaling functions with dynein to promote spindle positioning in C. elegans

Proper orientation and positioning of the mitotic spindle is essential for the correct segregation of fate determinants during asymmetric cell division. Although heterotrimeric G proteins and their regulators are essential for spindle positioning in many cell types, their mechanism of action remains unclear. In this study, we show that dyrb-1, which encodes a dynein light chain, provides a functional link between heterotrimeric G protein signaling and dynein activity during spindle positioning in Caenorhabditis elegans. Embryos depleted of dyrb-1 display phenotypes similar to a weak loss of function of dynein activity, indicating that DYRB-1 is a positive regulator of dynein. We find that the depletion of dyrb-1 enhances the spindle positioning defect of weak loss of function alleles of two regulators of G protein signaling, LIN-5 and GPR-1/2, and that DYRB-1 physically associates with these two proteins. These results indicate that dynein activity functions with regulators of G protein signaling to regulate common downstream effectors during spindle positioning in the early C. elegans embryo

Dynein Modifiers in C. elegans: Light Chains Suppress Conditional Heavy Chain Mutants

Cytoplasmic dynein is a microtubule-dependent motor protein that functions in mitotic cells during centrosome separation, metaphase chromosome congression, anaphase spindle elongation, and chromosome segregation. Dynein is also utilized during interphase for vesicle transport and organelle positioning. While numerous cellular processes require cytoplasmic dynein, the mechanisms that target and regulate this microtubule motor remain largely unknown. By screening a conditional Caenorhabditis elegans cytoplasmic dynein heavy chain mutant at a semipermissive temperature with a genome-wide RNA interference library to reduce gene functions, we have isolated and characterized twenty dynein-specific suppressor genes. When reduced in function, these genes suppress dynein mutants but not other conditionally mutant loci, and twelve of the 20 specific suppressors do not exhibit sterile or lethal phenotypes when their function is reduced in wild-type worms. Many of the suppressor proteins, including two dynein light chains, localize to subcellular sites that overlap with those reported by others for the dynein heavy chain. Furthermore, knocking down any one of four putative dynein accessory chains suppresses the conditional heavy chain mutants, suggesting that some accessory chains negatively regulate heavy chain function. We also identified 29 additional genes that, when reduced in function, suppress conditional mutations not only in dynein but also in loci required for unrelated essential processes. In conclusion, we have identified twenty genes that in many cases are not essential themselves but are conserved and when reduced in function can suppress conditionally lethal C. elegans cytoplasmic dynein heavy chain mutants. We conclude that conserved but nonessential genes contribute to dynein function during the essential process of mitosis