Interaction of 42Sp50 with the vegetal RNA localization machinery in Xenopus laevis oocytes

Germ plasm dynamics during oogenesis and early embryonic development in Xenopus and zebrafish

Specification of the germline and its segregation from the soma mark one of the most crucial events in the lifetime of an organism. In different organisms, this specification can occur through either inheritance or inductive mechanisms. In species such as Xenopus and zebrafish, the specification of primordial germ cells relies on the inheritance of maternal germline determinants that are synthesized and sequestered in the germ plasm during oogenesis. In this review, we discuss the formation of the germ plasm, how germline determinants are recruited into the germ plasm during oogenesis, and the dynamics of the germ plasm during oogenesis and early embryonic development.

Remnants of the Balbiani body are required for formation of RNA transport granules in Xenopus oocytes

The Balbiani body (Bb), an organelle comprised of mitochondria, ER, and RNA, is found in the oocytes of most organisms. In Xenopus, the structure is initially positioned immediately adjacent to the nucleus, extends toward the vegetal pole, and eventually disperses, leaving behind a region highly enriched in mitochondria. This area is later transversed by RNP complexes that are being localized to the vegetal cortex. Inhibition of mitochondrial ATP synthesis prevents perinuclear formation of the transport complexes that can be reversed by a nonhydrolyzable ATP analog, indicating the nucleotide is acting as a hydrotrope. The protein composition, sensitivity to hexanediol, and coalescence in the absence of transport provide evidence that the transport RNP complexes are biocondensates. The breakdown of the Bb engenders regions of clustered mitochondria that are used not to meet extraordinary energy demands, but rather to promote a liquid-liquid phase separation.

Heterozygous mutation of eEF1A1b resulted in spermatogenesis arrest and infertility in male tilapia, Oreochromis niloticus

Eukaryotic elongation factor 1 alpha (eEF1A) is an essential component of the translational apparatus. In the present study, eEF1A1b was isolated from the Nile tilapia. Real-time PCR and Western blot revealed that eEF1A1b was expressed highly in the testis from 90 dah (days after hatching) onwards. In situ hybridization and immunohistochemistry analyses showed that eEF1A1b was highly expressed in the spermatogonia of the testis. CRISPR/Cas9 mediated mutation of eEF1A1b resulted in spermatogenesis arrest and infertility in the F0 XY fish. Consistently, heterozygous mutation of eEF1A1b (eEF1A1b+/-) resulted in an absence of spermatocytes at 90 dah, very few spermatocytes, spermatids and spermatozoa at 180 dah, and decreased Cyp11b2 and serum 11-ketotestosterone level at both stages. Further examination of the fertilization capacity of the sperm indicated that the eEF1A1b+/- XY fish were infertile due to abnormal spermiogenesis. Transcriptomic analyses of the eEF1A1b+/- testis from 180 dah XY fish revealed that key elements involved in spermatogenesis, steroidogenesis and sperm motility were significantly down-regulated compared with the control XY. Transgenic overexpression of eEF1A1b rescued the spermatogenesis arrest phenotype of the eEF1A1b+/- testis. Taken together, our data suggested that eEF1A1b is crucial for spermatogenesis and male fertility in the Nile tilapia.

A novel role for Celf1 in vegetal RNA localization during Xenopus oogenesis

The localization of certain mRNAs to the vegetal cortex of Xenopus oocytes is of crucial importance for germ cell development and early embryonic patterning. Vegetal RNA localization is mediated by cis-acting RNA localization elements (LE). Several proteins assemble on the RNA LE and direct transport to the vegetal cortex. Although a number of localization RNP components have been identified, their full composition is unknown. In an RNA affinity purification approach, using the dead end 1 (dnd1) RNA LE, we identified Xenopus Celf1 as a novel component of vegetal localization RNP complexes. Celf1 is part of an RNP complex together with known vegetal localization factors and shows specific interactions with LEs from several but not all vegetally localizing RNAs. Immunostaining experiments reveal co-localization of Celf1 with vegetally localizing RNA and with known localization factors. Inhibition of Celf1 protein binding by localization element mutagenesis as well as Celf1 overexpression interfere with vegetal RNA localization. These results argue for a role of Celf1 in vegetal RNA localization during Xenopus oogenesis.

Global gene expression during early differentiation of Xenopus (Silurana) tropicalis gonad tissues

African clawed frog Xenopus sp. is used extensively for developmental biology and toxicology research. Amid concerns of environmental pollutants disrupting endocrine systems and causing altered reproductive development in wildlife, eco-toxicology research has led to a focus on linking molecular initiating events to population-level effects. As such, efforts to better understand reproductive development at the molecular level in these model species are warranted. To that end, transcriptomes were characterized in differentiating Xenopus tropicalis gonad tissues at Nieuwkoop and Faber (NF) stage 58 (pro-metamorphosis), NF66 (completion of metamorphosis), 1week post-metamorphosis (1WPM), and 2weeks post-metamorphosis (2WPM). Differential expression analysis between tissue types at each developmental stage revealed a substantial divergence of ovary and testis transcriptomes starting between NF58 and NF66; transcriptomes continued to diverge through 2WPM. Generally, testis-enriched transcripts were expressed at relatively constant levels, while ovary-enriched transcripts were up-regulated within this developmental period. Functional analyses of differentially expressed transcripts allowed linkages to be made between their putative human orthologues and specific cellular processes associated with differentiating gonad tissues. In ovary tissue, genetic programs direct germ cells through meiosis to the diplotene stage when maternal mRNAs are transcribed and trafficked to oocytes for translation following fertilization. In the testis, gene expression is consistent with connective tissue development, tubule formation, and germ cell support (Leydig and Sertoli cells). This dataset exhibited remarkable consistency with transcript profiles previously described in gonad tissues across species, and emphasizes the universal importance of certain transcripts for germ cell development and preparation of these tissues for reproduction.

RNA localization in Xenopus oocytes uses a core group of trans-acting factors irrespective of destination

The 3' untranslated region of mRNA encoding PHAX, a phosphoprotein required for nuclear export of U-type snRNAs, contains cis-acting sequence motifs E2 and VM1 that are required for localization of RNAs to the vegetal hemisphere of Xenopus oocytes. However, we have found that PHAX mRNA is transported to the opposite, animal, hemisphere. A set of proteins that cross-link to the localization elements of vegetally localized RNAs are also cross-linked to PHAX and An1 mRNAs, demonstrating that the composition of RNP complexes that form on these localization elements is highly conserved irrespective of the final destination of the RNA. The ability of RNAs to bind this core group of proteins is correlated with localization activity. Staufen1, which binds to Vg1 and VegT mRNAs, is not associated with RNAs localized to the animal hemisphere and may determine, at least in part, the direction of RNA movement in Xenopus oocytes.