Like mammalian neurons, C. elegans neurons lose axon regeneration ability as they age, but it is not known why. Here, we report that let-7 contributes to a developmental decline in AVM axon regeneration. In older AVM axons, let-7 inhibits regeneration by down regulating LIN-41, an important AVM axon regeneration-promoting factor. While let-7 inhibits lin-41 expression in older neurons through the lin-41 3'UTR, lin-41 inhibits let-7 expression in younger neurons through Argonaute ALG-1. This reciprocal inhibition ensures axon regeneration is only inhibited in older neurons. These findings show that a let-7-lin-41 regulatory circuit, which was previously shown to control timing of events in mitotic stem cell lineages, is re-utilized in postmitotic neurons to control post-differentiation events.

Members of the Wnt family of secreted glycoproteins regulate many developmental processes, including cell migration. We and others have previously shown that the Wnts egl-20, cwn-1, and cwn-2 are required for cell migration and axon guidance. However, the roles in cell migration of all of the Caenorhabditis elegans Wnt genes and their candidate receptors have not been explored fully. We have extended our analysis to include all C. elegans Wnts and six candidate Wnt receptors: four Frizzleds, the sole Ryk family receptor LIN-18, and the Ror receptor tyrosine kinase CAM-1. We show that three of the Wnts, CWN-1, CWN-2, and EGL-20, play major roles in directing cell migrations and that all five Wnts direct specific cell migrations either by acting redundantly or by antagonizing each other's function. We report that all four Frizzleds function to direct Q-descendant cell migrations, but only a subset of the putative Wnt receptors function in directing migrations of other cells. Finally, we find striking differences between the phenotypes of the Wnt quintuple and Frizzled quadruple mutants.

microRNAs (miRNAs) play a central role in the regulation of gene expression by targeting specific mRNAs for degradation or translational repression. Each miRNA is post‐transcriptionally processed into a duplex comprising two strands. One of the two miRNA strands is selectively loaded into an Argonaute protein to form the miRNA‐Induced Silencing Complex (miRISC) in a process referred to as miRNA strand selection. The other strand is ejected from the complex and is subject to degradation. The target gene specificity of miRISC is determined by sequence complementarity between the Argonaute‐loaded miRNA strand and target mRNA. Each strand of the miRNA duplex has the capacity to be loaded into miRISC and possesses a unique seed sequence. Therefore, miRNA strand selection plays a defining role in dictating the specificity of miRISC toward its targets and provides a mechanism to alter gene expression in a switch‐like fashion. Aberrant strand selection can lead to altered gene regulation by miRISC and is observed in several human diseases including cancer. Previous and emerging data shape the rules governing miRNA strand selection and shed light on how these rules can be circumvented in various physiological and pathological contexts. This article is categorized under: RNA Processing > Processing of Small RNAs Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs Regulatory RNAs/RNAi/Riboswitches > Biogenesis of Effector Small RNAs

Members of the Frizzled family of integral membrane proteins are implicated in many developmental events, including specifying cell fate, orienting cell and planar polarity, and directing cell migration. Frizzleds function as cell surface receptors for secreted Wnt proteins. We report here the isolation of a mutation in cfz-2, a Caenorhabditis elegans Frizzled gene. Mutation of cfz-2 causes defective cell migration, disorganization of head neurons, and can cause ectopic axon outgrowth. Analysis of mosaic animals shows that CFZ-2 functions cell nonautonomously, but does not rule out an autonomous role. CFZ-2 is expressed primarily in the anterior of embryos and in several cells in the head of adults. Our analysis of interactions between CFZ-2 and other Wnt pathways reveals that three Wnts, CWN-1, CWN-2 and EGL-20, and a Frizzled, MOM-5, function redundantly with one another and with CFZ-2 for specific cell migrations. In contrast, CWN-1, CWN-2, EGL-20, CFZ-2, and MOM-5 antagonize one another for other migrations. Therefore, CFZ-2 functions by collaborating with and/or antagonizing other Wnt signaling pathways to regulate specific cell migrations.

microRNAs function in diverse developmental and physiological processes by regulating target gene expression at the post-transcriptional level. ALG-1 is one of two Caenorhabditis elegans Argonautes (ALG-1 and ALG-2) that together are essential for microRNA biogenesis and function. Here, we report the identification of novel antimorphic (anti) alleles of ALG-1 as suppressors of lin-28(lf) precocious developmental phenotypes. The alg-1(anti) mutations broadly impair the function of many microRNAs and cause dosage-dependent phenotypes that are more severe than the complete loss of ALG-1. ALG-1(anti) mutant proteins are competent for promoting Dicer cleavage of microRNA precursors and for associating with and stabilizing microRNAs. However, our results suggest that ALG-1(anti) proteins may sequester microRNAs in immature and functionally deficient microRNA Induced Silencing Complexes (miRISCs), and hence compete with ALG-2 for access to functional microRNAs. Immunoprecipitation experiments show that ALG-1(anti) proteins display an increased association with Dicer and a decreased association with AIN-1/GW182. These findings suggest that alg-1(anti) mutations impair the ability of ALG-1 miRISC to execute a transition from Dicer-associated microRNA processing to AIN-1/GW182 associated effector function, and indicate an active role for ALG/Argonaute in mediating this transition.