H. Robert Horvitz

Many families of C. elegans microRNAs are not essential for development or viability.

MicroRNAs (miRNAs) are approximately 23 nt regulatory RNAs that posttranscriptionally inhibit the functions of protein-coding mRNAs. We previously found that most C. elegans miRNAs are individually not essential for development or viability and proposed that paralogous miRNAs might often function redundantly. To test this hypothesis, we generated mutant C. elegans strains that each lack multiple or all members of one of 15 miRNA families. Mutants for 12 of these families did not display strong synthetic abnormalities, suggesting that these miRNA families have subtle roles during development. By contrast, mutants deleted for all members of the mir-35 or mir-51 families died as embryos or early larvae, and mutants deleted for four members of the mir-58 family showed defects in locomotion, body size, and egg laying and an inability to form dauer larvae. Our findings indicate that the regulatory functions of most individual miRNAs and most individual families of miRNAs related in sequence are not critical for development or viability. Conversely, because in some cases miRNA family members act redundantly, our findings emphasize the importance of determining miRNA function in the absence of miRNAs related in sequence.

Caenorhabditis elegans early embryogenesis and vulval morphogenesis require chondroitin biosynthesis

Defects in glycosaminoglycan biosynthesis disrupt animal development and can cause human disease. So far much of the focus on glycosaminoglycans has been on heparan sulphate. Mutations in eight squashed vulva (sqv) genes in Caenorhabditis elegans cause defects in cytokinesis during embryogenesis and in vulval morphogenesis during postembryonic development. Seven of the eight sqv genes have been shown to control the biosynthesis of the glycosaminoglycans chondroitin and heparan sulphate. Here we present the molecular identification and characterization of the eighth gene, sqv-5. This gene encodes a bifunctional glycosyltransferase that is probably localized to the Golgi apparatus and is responsible for the biosynthesis of chondroitin but not heparan sulphate. Our findings show that chondroitin is crucial for both cytokinesis and morphogenesis during C. elegans development.

The SQV-1 UDP-glucuronic acid decarboxylase and the SQV-7 nucleotide-sugar transporter may act in the golgi apparatus to affect Caenorhabditis elegans vulval morphogenesis and embryonic development

Recent findings indicate that glycosaminoglycans can play important roles in animal development. The genes sqv-3, -7, and -8, which are necessary for vulval morphogenesis in Caenorhabditis elegans, affect the biosynthesis of chondroitin and heparan sulfate glycosaminoglycans. We cloned sqv-1 and showed that the SQV-1 protein is a type II transmembrane protein that functions as a UDP-glucuronic acid decarboxylase. SQV-1 localizes to punctate cytoplasmic compartments and colocalizes with the SQV-7 nucleotide-sugar transporter, which probably acts in the Golgi apparatus. SQV-1 and SQV-7 are both expressed in the vulva and in oocytes, where they likely act in vulval morphogenesis and embryonic development, respectively. Progeny of sqv-7 and sqv-1 null mutants fail to initiate cytokinesis, possibly because they are unable to separate the plasma membrane from the eggshell, a defect analogous to that of incomplete vulval invagination.

C. elegans ISWI and NURF301 antagonize an Rb-like pathway in the determination of multiple cell fates

The class A, B and C synthetic multivulva (synMuv) genes act redundantly to negatively regulate the expression of vulval cell fates in Caenorhabditis elegans. The class B and C synMuv proteins include homologs of proteins that modulate chromatin and influence transcription in other organisms similar to members of the Myb-MuvB/dREAM, NuRD and Tip60/NuA4 complexes. To determine how these chromatin-remodeling activities negatively regulate the vulval cell-fate decision, we isolated a suppressor of the synMuv phenotype and found that the suppressor gene encodes the C. elegans homolog of Drosophila melanogaster ISWI. The C. elegans ISW-1 protein likely acts as part of a Nucleosome Remodeling Factor (NURF) complex with NURF-1, a nematode ortholog of NURF301, to promote the synMuv phenotype. isw-1 and nurf-1 mutations suppress both the synMuv phenotype and the multivulva phenotype caused by overactivation of the Ras pathway. Our data suggest that a NURF-like complex promotes the expression of vulval cell fates by antagonizing the transcriptional and chromatin-remodeling activities of complexes similar to Myb-MuvB/dREAM, NuRD and Tip60/NuA4. Because the phenotypes caused by a null mutation in the tumor-suppressor and class B synMuv gene lin-35 Rb and a gain-of-function mutation in let-60 Ras are suppressed by reduction of isw-1 function, NURF complex proteins might be effective targets for cancer therapy.