Spindle assembly checkpoint genes reveal distinct as well as overlapping expression that implicates MDF-2/Mad2 in postembryonic seam cell proliferation in Caenorhabditis elegans

An RNAi screen for conserved kinases that enhance microRNA activity after dauer in Caenorhabditis elegans

Gene regulation in changing environments is critical for maintaining homeostasis. Some animals undergo a stress-resistant diapause stage to withstand harsh environmental conditions encountered during development. MicroRNAs are one mechanism for regulating gene expression during and after diapause. MicroRNAs downregulate target genes posttranscriptionally through the activity of the microRNA-induced silencing complex. Argonaute is the core microRNA-induced silencing complex protein that binds to both the microRNA and to other microRNA-induced silencing complex proteins. The 2 major microRNA Argonautes in the Caenorhabditis elegans soma are ALG-1 and ALG-2, which function partially redundantly. Loss of alg-1 [alg-1(0)] causes penetrant developmental phenotypes including vulval defects and the reiteration of larval cell programs in hypodermal cells. However, these phenotypes are essentially absent if alg-1(0) animals undergo a diapause stage called dauer. Levels of the relevant microRNAs are not higher during or after dauer, suggesting that activity of the microRNA-induced silencing complex may be enhanced in this context. To identify genes that are required for alg-1(0) mutants to develop without vulval defects after dauer, we performed an RNAi screen of genes encoding conserved kinases. We focused on kinases because of their known role in modulating microRNA-induced silencing complex activity. We found RNAi knockdown of 4 kinase-encoding genes, air-2, bub-1, chk-1, and nekl-3, caused vulval defects and reiterative phenotypes in alg-1(0) mutants after dauer, and that these defects were more penetrant in an alg-1(0) background than in wild type. Our results implicate these kinases as potential regulators of microRNA-induced silencing complex activity during postdauer development in C. elegans.

The C. elegans gene gvd-1 promotes late larval development and germ cell proliferation

Limiting maternal resources necessitates deferring the development of adult-specific structures, notably the reproductive structures, to the postembryonic phase. These structures form postembryonically from blast cells generated during embryogenesis. A close coordination of developmental timing and pattern among the various postembryonic cell lineages is essential to form a functional adult. Here, we show that the C. elegans gene gvd-1 is essential for the development of several structures that form during the late larval stages. In gvd-1 mutant animals, blast cells that normally divide during the late larval stages (L3 and L4) fail to divide. In addition, germ cell proliferation is also severely reduced in these animals. Expression patterns of relevant reporter transgenes revealed a delay in G1/S transition in the vulval precursor cell P6.p and cytokinesis failure in seam cells in gvd-1 larvae. Our analyses of GVD-1::GFP transgenes indicate that GVD-1 is expressed in both soma and germ line, and functions in both. Sequence comparisons revealed that the sequence of gvd-1 is conserved only among nematodes, which does not support a broadly conserved housekeeping function for gvd-1. Instead, our results indicate a crucial role for gvd-1 that is specific to the larval development of nematodes.

Spindle assembly checkpoint strength is linked to cell fate in the Caenorhabditis elegans embryo

The spindle assembly checkpoint (SAC) is a conserved mitotic regulator that preserves genome stability by monitoring kinetochore-microtubule attachments and blocking anaphase onset until chromosome biorientation is achieved. Despite its central role in maintaining mitotic fidelity, the ability of the SAC to delay mitotic exit in the presence of kinetochore-microtubule attachment defects (SAC "strength") appears to vary widely. How different cellular aspects drive this variation remains largely unknown. Here we show that SAC strength is correlated with cell fate during development of Caenorhabditis elegans embryos, with germline-fated cells experiencing longer mitotic delays upon spindle perturbation than somatic cells. These differences are entirely dependent on an intact checkpoint and only partially attributable to differences in cell size. In two-cell embryos, cell size accounts for half of the difference in SAC strength between the larger somatic AB and the smaller germline P₁ blastomeres. The remaining difference requires asymmetric cytoplasmic partitioning downstream of PAR polarity proteins, suggesting that checkpoint-regulating factors are distributed asymmetrically during early germ cell divisions. Our results indicate that SAC activity is linked to cell fate and reveal a hitherto unknown interaction between asymmetric cell division and the SAC.