Precise SDF1-mediated cell guidance is achieved through ligand clearance and microRNA-mediated decay.

During animal development, SDF1 simultaneously guides various cell types to different targets. As many targets are in close proximity to one another, it is unclear how the system avoids mistargeting. Zebrafish trigeminal sensory neurons express the SDF1 receptor Cxcr4b and encounter multiple SDF1 sources during migration, but ignore all but the correct one. We show that miR-430 and Cxcr7b regulation of SDF1a are required for precise guidance. In the absence of miR-430 or Cxcr7b, neurons responded to ectopic SDF1a sources along their route and did not reach their target. This was due to a failure to clear SDF1a transcript and protein from sites of expression that the migrating neurons had already passed. Our findings suggest an "attractive path" model in which migrating cells closely follow a dynamic SDF1a source that is refined on a transcript and protein level by miR-430 and Cxcr7b, respectively.

Use of target protector morpholinos to analyze the physiological roles of specific miRNA-mRNA pairs in vivo.

MicroRNAs (miRNAs) regulate gene expression by pairing with complementary sequences in the 3' untranslated regions (UTRs) of transcripts. Although the molecular mechanism underlying miRNA biogenesis and activity is becoming better understood, determining the physiological role of the interaction of an miRNA with its target remains a challenge. A number of methods have been developed to inhibit individual miRNAs, but it can be difficult to determine which specific targets are responsible for any observed phenotypes. To address this problem, we use target protector (TP) morpholinos that interfere with a single miRNA-mRNA pair by binding specifically to the miRNA target sequence in the 3' UTR. In this protocol, we detail the steps for identifying miRNA targets, validating their regulation and using TPs to interrogate their function in zebrafish. Depending on the biological context, this set of experiments can be completed in 6-8 weeks.

miRNA regulation of Sdf1 chemokine signaling provides genetic robustness to germ cell migration.

microRNAs (miRNAs) function as genetic rheostats to control gene output. Based on their role as modulators, it has been postulated that miRNAs canalize development and provide genetic robustness. Here, we uncover a previously unidentified regulatory layer of chemokine signaling by miRNAs that confers genetic robustness on primordial germ cell (PGC) migration. In zebrafish, PGCs are guided to the gonad by the ligand Sdf1a, which is regulated by the sequestration receptor Cxcr7b. We find that miR-430 regulates sdf1a and cxcr7 mRNAs. Using target protectors, we demonstrate that miR-430-mediated regulation of endogenous sdf1a (also known as cxcl12a) and cxcr7b (i) facilitates dynamic expression of sdf1a by clearing its mRNA from previous expression domains, (ii) modulates the levels of the decoy receptor Cxcr7b to avoid excessive depletion of Sdf1a and (iii) buffers against variation in gene dosage of chemokine signaling components to ensure accurate PGC migration. Our results indicate that losing miRNA-mediated regulation can expose otherwise buffered genetic lesions leading to developmental defects.

Zebrafish miR-1 and miR-133 shape muscle gene expression and regulate sarcomeric actin organization.

microRNAs (miRNAs) represent approximately 4% of the genes in vertebrates, where they regulate deadenylation, translation, and decay of the target messenger RNAs (mRNAs). The integrated role of miRNAs to regulate gene expression and cell function remains largely unknown. Therefore, to identify the targets coordinately regulated by muscle miRNAs in vivo, we performed gene expression arrays on muscle cells sorted from wild type, dicer mutants, and single miRNA knockdown embryos. Our analysis reveals that two particular miRNAs, miR-1 and miR-133, influence gene expression patterns in the zebrafish embryo where they account for >54% of the miRNA-mediated regulation in the muscle. We also found that muscle miRNA targets (1) tend to be expressed at low levels in wild-type muscle but are more highly expressed in dicer mutant muscle, and (2) are enriched for actin-related and actin-binding proteins. Loss of dicer function or down-regulation of miR-1 and miR-133 alters muscle gene expression and disrupts actin organization during sarcomere assembly. These results suggest that miR-1 and miR-133 actively shape gene expression patterns in muscle tissue, where they regulate sarcomeric actin organization.