miR-31-mediated local translation at the mitotic spindle is important for early development.

miR-31 is a highly conserved microRNA that plays critical roles in cell proliferation, migration, and differentiation. We discovered miR-31 and some of its validated targets are enriched on the mitotic spindle of the dividing sea urchin embryo and mammalian cells. Using the sea urchin embryo, we found that miR-31 inhibition led to developmental delay correlated with increased cytoskeleton and chromosomal defects. We identified miR-31 to directly suppress several actin remodeling transcripts, ß-actin, Gelsolin, Rab35 and Fascin, which were localized to the mitotic spindle. miR-31 inhibition leads to increased newly translated Fascin at the spindles. Forced ectopic localization of Fascin transcripts to the cell membrane and translation led to significant developmental and chromosomal segregation defects, leading to our hypothesis that miR-31 regulates local translation at the mitotic spindle to ensure proper cell division. Furthermore, miR-31-mediated post-transcriptional regulation at the mitotic spindle may be an evolutionarily conserved regulatory paradigm of mitosis.

microRNA-124 directly suppresses Nodal and Notch to regulate mesodermal development.

MicroRNAs regulate gene expression post-transcriptionally by destabilizing and/or inhibiting translation of target mRNAs in animal cells. MicroRNA-124 (miR-124) has been examined mostly in the context of neurogenesis. This study discovers a novel role of miR-124 in regulating mesodermal cell differentiation in the sea urchin embryo. The expression of miR-124 is first detectable at 12hours post fertilization at the early blastula stage, during endomesodermal specification. Mesodermally-derived immune cells come from the same progenitor cells that give rise to blastocoelar cells (BCs) and pigment cells (PCs) that must make a binary fate decision. We determined that miR-124 directly represses Nodal and Notch to regulate BC and PC differentiation. miR-124 inhibition does not impact the dorsal-ventral axis formation, but result in a significant increase in number of cells expressing BC-specific transcription factors (TFs) and a concurrent reduction of differentiated PCs. In general, removing miR-124's suppression of Nodal phenocopies miR124 inhibition. Interestingly, removing miR-124's suppression of Notch leads to an increased number of both BCs and PCs, with a subset of hybrid cells that express both BC- and PC-specific TFs in the larvae. Removal of miR-124's suppression of Notch not only affects differentiation of both BCs and PCs, but also induces cell proliferation of these cells during the first wave of Notch signaling. This study demonstrates that post-transcriptional regulation by miR-124 impacts differentiation of BCs and PCs by regulating the Nodal and Notch signaling pathways.

The actin bundling protein Fascin is important for proper early development in Strongylocentrotus purpuratus embryos.

Fascin is a conserved protein that has been shown to modulate the cytoskeleton. Its role in early development remains unclear. After fertilization, embryos undergo rapid cell divisions, requiring the precise regulation of cytoskeleton to segregate chromosomes. Results indicate that Fascin is in the cell cortex, enriched in the perinuclear region of non-dividing blastomeres and on the mitotic spindle of dividing blastomeres of the early embryo. Loss-of-function of Fascin leads to a significant developmental delay or arrest, indicating that Fascin is important for proper early embryonic development.

Rab35 regulates skeletogenesis and gastrulation by facilitating actin remodeling and vesicular trafficking.

Rab35 is a small GTPase that regulates plasma membrane to early endosome vesicular trafficking and mediates actin remodeling to form actin-rich cellular structures. While the function of Rab35 in the cellular context has been examined, its role during development has not been well studied. In this study, we take advantage of the sea urchin's high fecundity, external fertilization, and transparent embryos to determine the function of Rab35 during development. We found that loss of function of Rab35 results in defects in skeletogenesis and gastrulation, which were rescued by co-injection of sea urchin Rab35. The loss of Rab35's function results in decreased endocytosis and impaired exocytosis, which may be important for skeletogenesis and gastrulation. Skeletal spicules of Rab35 knockdown embryos have reduced organized actin compared to the control, supporting the notion that Rab35 regulates actin dynamics. In addition, the skeletal and gastrulation defects induced by Rab35 knockdown were rescued by co-injection with Fascin, an actin-bundling protein, indicating that proper actin dynamics play a critical role for both skeletogenesis and gastrulation. Overall, results indicate that through its role in mediating vesicular trafficking and actin remodeling, Rab35 is an important regulator of embryonic structure formation in early development.