Homozygous mutation of foxh1 arrests oogenesis causing infertility in female Nile tilapia†

Foxh1, a member of fox gene family, was first characterized as a transcriptional partner in the formation of the Smad protein complex. Recent studies have shown foxh1 is highly expressed in the cytoplasm of oocytes in both tilapia and mouse. However, its function in oogenesis remains unexplored. In the present study, foxh1-/- tilapia was created by CRISPR/Cas9. At 180 dah (days after hatching), the foxh1-/- XX fish showed oogenesis arrest and a significantly lower GSI. The transition of oocytes from phase II to phase III and follicle cells from one to two layers was blocked, resulting in infertility of the mutant. Transcriptomic analysis revealed that expression of genes involved in estrogen synthesis and oocyte growth were altered in the foxh1-/- ovaries. Loss of foxh1 resulted in significantly decreased Cyp19a1a and increased Cyp11b2 expression, consistent with significantly lower concentrations of serum estradiol-17β (E2) and higher concentrations of 11-ketotestosterone (11-KT). Moreover, administration of E2 rescued the phenotypes of foxh1-/- XX fish, as indicated by the appearance of phase III and IV oocytes and absence of Cyp11b2 expression. Taken together, these results suggest that foxh1 functions in the oocytes to regulate oogenesis by promoting cyp19a1a expression, and therefore estrogen production. Disruption of foxh1 may block the estrogen synthesis and oocyte growth, leading to the arrest of oogenesis and thus infertility in tilapia.

Zebrafish let-7b acts downstream of hypoxia-inducible factor-1α to assist in hypoxia-mediated cell proliferation and cell cycle regulation

AIMS

Hypoxia-inducible factor-1α (HIF-1α) is a transcriptional regulator of cellular responses to hypoxic stress. MicroRNAs (miRNAs) play an essential role in hypoxia-mediated cellular responses. Previous studies have identified some let-7 family members as hypoxia-regulated miRNAs (HRMs). In the present study, we aimed to investigate whether zebrafish let-7b/7f contribute cellular hypoxic response in a Hif-1α-dependent manner.

MAIN METHODS

Stable suppression of zebrafish hif-1α was achieved by microinjection of an optimized short-hairpin RNA (shRNA) expression vector. Next-generation sequencing was conducted to characterize miRNA and mRNA expression profiles. MiRNA promoter analysis and target detection was performed by dual-luciferase assay. Quantitative real-time PCR (qRT-PCR) and western blot were used to determine the expression of let-7b/7f, Hif-1α and Foxh1. Proliferation of ZF4 cells was examined using Cell Counting Kit-8 (CCK-8) and cell cycle progression was analyzed by flow cytometry assay.

KEY FINDINGS

Correlation between 7 miRNAs and 76 putative targets was identified based on integrated analysis of miRNA-mRNA profiles. Let-7b and let-7f were further considered as potential HRMs, with let-7b further validated as Hif-1α up-regulated. In addition, Forkhead-box H1 (Foxh1) was confirmed as a bona fide downstream target of let-7b. Furthermore, overexpression of both let-7b and let-7f repressed cell proliferation through blocking cell cycle progression of the G1-S transition.

SIGNIFICANCE

Our findings for the first time suggest zebrafish let-7b acts downstream of Hif-1α to assist in hypoxia-mediated cell proliferation and cell cycle regulation at least in part through the downregulation of foxh1. We also identified 4 novel potential HIF-1α-regulated miRNAs in zebrafish.

The role of a Williams-Beuren syndrome-associated helix-loop-helix domain-containing transcription factor in activin/nodal signaling

We investigated the regulation of the activin/nodal-inducible distal element (DE) of the Xenopus goosecoid (gsc) promoter. On the basis of its interaction with the DE, we isolated a Xenopus homolog of the human Williams-Beuren syndrome critical region 11 (XWBSCR11), and further, show that it interacts with pathway-specific Smad2 and Smad3 in a ligand-dependent manner. Interestingly, we also find that XWBSCR11 functions cooperatively with FoxH1 (Fast-1) to stimulate DE-dependent transcription. We propose a mechanism in which FoxH1 functions together with Smads as a cofactor for the recruitment of transcription factors like XWBSCR11 in the process of activin/nodal-mediated gsc-specific induction. This mechanism provides considerable opportunities for modulation of transcription across a variety of activin/nodal-inducible genes, increasing diversity in promoter selection, thus leading to the differential induction of activin/nodal target genes.

The transcription factor FoxH1 (FAST) mediates Nodal signaling during anterior-posterior patterning and node formation in the mouse

FoxH1 (FAST) is a transcription factor that mediates signaling by transforming growth factor-beta, Activin, and Nodal. The role of FoxH1 in development has now been investigated by the generation and analysis of FoxH1-deficient (FoxH1(-/-)) mice. The FoxH1(-/-) embryos showed various patterning defects that recapitulate most of the defects induced by the loss of Nodal signaling. A substantial proportion of FoxH1(-/-) embryos failed to orient the anterior-posterior (A-P) axis correctly, as do mice lacking Cripto, a coreceptor for Nodal. In less severely affected FoxH1(-/-) embryos, A-P polarity was established, but the primitive streak failed to elongate, resulting in the lack of a definitive node and its derivatives. Heterozygosity for nodal renders the FoxH1(-/-) phenotype more severe, indicative of a genetic interaction between FoxH1 and nodal. The expression of FoxH1 in the primitive endoderm rescued the A-P patterning defects, but not the midline defects, of FoxH1(-/-) mice. These results indicate that a Nodal-FoxH1 signaling pathway plays a central role in A-P patterning and node formation in the mouse.