Loss of flrt2 gene leads to microphthalmia in zebrafish.

As a member of the fibronectin leucine-rich transmembrane (flrt) gene family, fibronectin leucine-rich transmembrane 2 (flrt2) is strongly expressed in a subset of sclerotome cells, and the resultant protein interacts with FGFR1 in the FGF signaling pathway during development. Studies on flrt2 have focused mainly on its roles in the brain, heart and chondrogenesis. However, reports on its expression and function in the zebrafish retina are lacking. Here, we detected the high expression of flrt2 in zebrafish retina using in situ hybridization technique and developed an flrt2-knockout (KO) zebrafish line using CRISPR/Cas9 genome editing. Quantitative real-time PCR was used to measure the expression levels of flrt2, which results in an approximately 60% mRNA reduction. The flrt2-KO zebrafish eyes' altered morphological, cellular, and molecular events were identified using BrdU labeling, TUNEL assay, immunofluorescent staining, fluorescent dye injection and RNA sequencing. Abnormal eye development, known as microphthalmia, was found in flrt2-KO larvae, and the retinal progenitor cells exhibited increased apoptosis, perhaps owing to the combined effects of crx, neurod4, atoh7, and pcdh8 downregulation and Casp3a and Caspbl upregulation. In contrast, the retinal neural development, as well as retinal progenitor cell differentiation and proliferation, were not affected by the flrt2 deletion. Thus, flrt2 appears to play important roles in retinal development and function, which may provide the basis for further investigations into the molecular mechanisms of retinal development and evolution.

m6A writer complex promotes timely differentiation and survival of retinal progenitor cells in zebrafish.

N6-Methyladenosine (m6A) is the most prevalent internal modification in eukaryotic mRNAs that modulates mRNA metabolism and function. Most m6A modifications on mRNAs are catalyzed by a core writer complex consisting of a methyltransferase, Mettl3, and two ancillary components, Mettl14 and Wtap. Recent studies have demonstrated important roles of m6A in various physiological and pathological processes, such as stem cell multipotency, cell differentiation, and cancer progression. However, our knowledge about m6A in the retina is still lacking. In this study, we used zebrafish as a model vertebrate to study the function of the m6A modification during retinal development. We show that the three main components of the m6A writer complex, mettl3, mettl14 and wtap, are abundantly expressed in the developing zebrafish eyes, and that knocking down m6A writer complex in zebrafish embryos caused microphthalmia formation, delayed retinal progenitor cells differentiation and increased cell death. By examining the retinal developmental processes in m6A writer complex-deficient fish, we show that m6A modification regulates zebrafish retinal development through ensuring the timely differentiation and survival of the retinal progenitor cells.