Reply to: "On the robustness of germ cell migration and microRNA-mediated regulation of chemokine signaling"

Genome wide analysis of 3' UTR sequence elements and proteins regulating mRNA stability during maternal-to-zygotic transition in zebrafish

Posttranscriptional regulation plays a crucial role in shaping gene expression. During the maternal-to-zygotic transition (MZT), thousands of maternal transcripts are regulated. However, how different cis-elements and trans-factors are integrated to determine mRNA stability remains poorly understood. Here, we show that most transcripts are under combinatorial regulation by multiple decay pathways during zebrafish MZT. By using a massively parallel reporter assay, we identified cis-regulatory sequences in the 3' UTR, including U-rich motifs that are associated with increased mRNA stability. In contrast, miR-430 target sequences, UAUUUAUU AU-rich elements (ARE), CCUC, and CUGC elements emerged as destabilizing motifs, with miR-430 and AREs causing mRNA deadenylation upon genome activation. We identified trans-factors by profiling RNA-protein interactions and found that poly(U)-binding proteins are preferentially associated with 3' UTR sequences and stabilizing motifs. We show that this activity is antagonized by C-rich motifs and correlated with protein binding. Finally, we integrated these regulatory motifs into a machine learning model that predicts reporter mRNA stability in vivo.

Mutation in cpsf6/CFIm68 (Cleavage and Polyadenylation Specificity Factor Subunit 6) causes short 3'UTRs and disturbs gene expression in developing embryos, as revealed by an analysis of primordial germ cell migration using the medaka mutant naruto

Our previous studies analyzing medaka mutants defective in primordial germ cell (PGC) migration identified cxcr4b and cxcr7, which are both receptors of the chemokine sdf1/cxcl12, as key regulators of PGC migration. Among PGC migration mutants, naruto (nar) is unique in that the mutant phenotype includes gross morphological abnormalities of embryos, suggesting that the mutation affects a broader range of processes. A fine genetic linkage mapping and genome sequencing showed the nar gene encodes Cleavage and Polyadenylation Specificity Factor subunit 6 (CPSF6/CFIm68). CPSF6 is a component of the Cleavage Factor Im complex (CFIm) which plays a key role in pre-mRNA 3'-cleavage and polyadenylation. 3'RACE of sdf1a/b and cxcr7 transcripts in the mutant embryos indicated shorter 3'UTRs with poly A additions occurring at more upstream positions than wild-type embryos, suggesting CPSF6 functions to prevent premature 3'UTR cleavage. In addition, expression of the coding region sequences of sdf1a/b in nar mutants was more anteriorly extended in somites than wild-type embryos, accounting for the abnormally extended distribution of PGCs in nar mutants. An expected consequence of shortening 3'UTR is the escape from the degradation mechanism mediated by microRNAs interacting with distal 3'UTR sequence. The abnormal expression pattern of sdf1a coding sequence may be at least partially accounted for by this mechanism. Given the pleiotropic effects of nar mutation, further analysis using the nar mutant will reveal processes in which CPSF6 plays essential regulatory roles in poly A site selection and involvement of 3'UTRs in posttranscriptional gene regulation in various genes in vivo.

RESA identifies mRNA-regulatory sequences at high resolution

Gene expression is regulated extensively at the level of mRNA stability, localization, and translation. However, decoding functional RNA regulatory features remains a limitation to understanding post-transcriptional regulation in vivo. Here, we developed RNA Element Selection Assay (RESA), a method that selects RNA elements based on their activity in vivo and uses high-throughput sequencing to provide quantitative measurement of their regulatory function with near nucleotide resolution. We implemented RESA to identify sequence elements modulating mRNA stability during zebrafish embryogenesis. RESA provides a sensitive and quantitative measure of microRNA activity in vivo and also identifies novel regulatory sequences. To uncover specific sequence requirements within regulatory elements, we developed a bisulfite-mediated nucleotide conversion strategy for large-scale mutational analysis (RESA-bisulfite). Finally, we used the versatile RESA platform to map candidate protein-RNA interactions in vivo (RESA-CLIP). The RESA platform can be broadly applicable to uncover the regulatory features shaping gene expression and cellular function.

Finding their way: themes in germ cell migration

Embryonic germ cell migration is a vital component of the germline lifecycle. The translocation of germ cells from the place of origin to the developing somatic gonad involves several processes including passive movements with underlying tissues, transepithelial migration, cell adhesion dynamics, the establishment of environmental guidance cues and the ability to sustain directed migration. How germ cells accomplish these feats in established model organisms will be discussed in this review, with a focus on recent discoveries and themes conserved across species.

Chemokine signaling in development and disease

Chemokines are a group of small, secreted molecules that signal through G protein-coupled receptors to promote cell survival and proliferation and to provide directional guidance to migrating cells. CXCL12 is one of the most evolutionary conserved chemokines and signals through the chemokine receptor CXCR4 to guide cell migration during embryogenesis, immune cell trafficking and cancer metastasis. Here and in the accompanying poster, we provide an overview of chemokine signaling, focusing on CXCL12, and we highlight some of the different chemokine-dependent strategies used to guide migrating cells.