Developmental genetics in Xenopus tropicalis

Immunomodulatory role of Xenopus tropicalis immature Sertoli cells in tadpole muscle regeneration via macrophage response modulation

BACKGROUND

Regenerative medicine and transplantation science continuously seek methods to circumvent immune-mediated rejection and promote tissue regeneration. Sertoli cells, with their inherent immunoprotective properties, emerge as pivotal players in this quest. However, whether Sertoli cells can play immunomodulatory role in tadpole tail regeneration and can thus benefit the regeneration process are needed to be discovered.

METHODS

Immature Sertoli cells from Xenopus tropicalis (XtiSCs) were transplanted into X. tropicalis tadpoles, followed by the amputation of the final third of their tails. We assessed the migration of XtiSCs, tail regeneration length, muscle degradation and growth, and macrophage counts across various regions including the entire tail, tail trunk, injection site, and regeneration site. The interactions between XtiSCs and macrophages were examined using a confocal microscope. To deplete macrophages, clodronate liposomes were administered prior to the transplantation of XtiSCs, while the administration of control liposomes acted as a negative control. Student's t-test was used to compare the effects of XtiSCs injection to those of a 2/3PBS injection across groups with no liposomes, control liposomes, and clodronate liposomes.

RESULTS

XtiSCs have excellent viability after transplantation to tadpole tail and remarkable homing capabilities to the regeneration site after tail amputation. XtiSCs injection increased macrophage numbers at 3 days post-amputation and 5 days post-amputation in the tail trunk, specifically at the injection site and at the regenerated tail, in a macrophage depleted environment (clodronate-liposome injection). What's more, XtiSCs injection decreased muscle fibers degradation significantly at 1 day post-amputation and facilitated new muscle growth significantly at 3 days post-amputation. In addition, whole-mount immunostaining showed that some XtiSCs co-localized with macrophages. And we observed potential mitochondria transport from XtiSCs to macrophages using MitoTracker staining in tadpole tail.

CONCLUSIONS

Our study delineates the novel role of XtiSCs in facilitating muscle regeneration post tadpole tail amputation, underscoring a unique interaction with macrophages that is crucial for regenerative success. This study not only highlights the therapeutic potential of Sertoli cells in regenerative medicine but also opens avenues for clinical translation, offering insights into immunoregulatory strategies that could enhance tissue regeneration and transplant acceptance.

Interaction between sex-determining genes from two species: clues from Xenopus hybrids

Hybrids provide an interesting model to study the evolution of sex-determining genes and sex chromosome systems as they offer the opportunity to see how independently evolving sex-determining pathways interact in vivo. In this context, the genus Xenopus represents a stimulating model, since species with non-homologous sex chromosomes and different sex-determining genes have been identified. In addition, the possibility of interspecies breeding is favoured in this group, which arose by alloploidization events, with species ploidy ranging from 2n = 2x = 20 in X. tropicalis (the only diploid representative of the genus) to 2n = 12x = 108 in X. ruwenzoriensis. To study how two sex-determining genes interact in vivo, X. laevis × X. tropicalis hybrids were produced. Gonadal differentiation in these hybrids revealed that the dm-w gene is dominant over X. tropicalis male-determining sex chromosomes (Y or Z), even though the Y chromosome is dominant in X. tropicalis (Y > W>Z). In the absence of the dm-w gene (the Z chromosome from X. laevis is present), the W chromosome from X. tropicalis is able to trigger ovarian development. Testicular differentiation will take place in the absence of W chromosomes from any of the parental species. The dominance/recessivity relationships between these sex-determining loci in the context of either parental genome remains unknown. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

The interconnection between cytokeratin and cell membrane-bound β-catenin in Sertoli cells derived from juvenile Xenopus tropicalis testes

Sertoli cells (SCs) play a central role in the determination of male sex during embryogenesis and spermatogenesis in adulthood. Failure in SC development is responsible for male sterility and testicular cancer. Before the onset of puberty, SCs are immature and differ considerably from mature cells in post-pubertal individuals regarding their morphology and biochemical activity. The major intermediate filament (IF) in mature SCs is vimentin, anchoring germ cells to the seminiferous epithelium. The collapse of vimentin has resulted in the disintegration of seminiferous epithelium and subsequent germ cell apoptosis. However, another IF, cytokeratin (CK) is observed only transiently in immature SCs in many species. Nevertheless, its function in SC differentiation is poorly understood. We examined the interconnection between CK and cell junctions using membrane β-catenin as a marker during testicular development in the Xenopus tropicalis model. Immunohistochemistry on juvenile (5 months old) testes revealed co-expression of CK, membrane β-catenin and E-cadherin. Adult (3-year-old males) samples confirmed only E-cadherin expression; CK and β-catenin were lost. To study the interconnection between CK and β-catenin-based cell junctions, the culture of immature SCs (here called XtiSCs) was employed. Suppression of CK by acrylamide in XtiSCs led to breakdown of membrane-bound β-catenin but not F-actin and β-tubulin or cell-adhesion proteins (focal adhesion kinase and integrin β1). In contrast to the obvious dependence of membrane β-catenin on CK stability, the detachment of β-catenin from the plasma membrane via uncoupling of cadherins by Ca2+ chelator EGTA had no effect on CK integrity. Interestingly, CHIR99021, a GSK3 inhibitor, also suppressed the CK network, resulting in the inhibition of XtiSCs cell-to-cell contacts and testicular development in juvenile frogs. This study suggests a novel role of CK in the retention of β-catenin-based junctions in immature SCs, and thus provides structural support for seminiferous tubule formation and germ cell development.

Summary: Cytokeratin (CK) and β-catenin are expressed in juvenile testicles and cultivated Xenopus tropicalis immature Sertoli cells (SC). Acrylamide and CHIR99021 disrupted the CK network, immature SC connections and testes development.

Epithelial-Mesenchymal Transition Promotes the Differentiation Potential of Xenopus tropicalis Immature Sertoli Cells

Epithelial-mesenchymal transition (EMT) is a fundamental process in embryonic development by which sessile epithelial cells are converted into migratory mesenchymal cells. Our laboratory has been successful in the establishment of Xenopus tropicalis immature Sertoli cells (XtiSCs) with the restricted differentiation potential. The aim of this study is the determination of factors responsible for EMT activation in XtiSCs and stemness window acquisition where cells possess the broadest differentiation potential. For this purpose, we tested three potent EMT inducers—GSK-3 inhibitor (CHIR99021), FGF2, and/or TGF-β1 ligand. XtiSCs underwent full EMT after 3-day treatment with CHIR99021 and partial EMT with FGF2 but not with TGF-β1. The morphological change of CHIR-treated XtiSCs to the typical spindle-like cell shape was associated with the upregulation of mesenchymal markers and the downregulation of epithelial markers. Moreover, only CHIR-treated XtiSCs were able to differentiate into chondrocytes in vitro and cardiomyocytes in vivo. Interestingly, EMT-shifted cells could migrate towards cancer cells (HeLa) in vitro and to the injury site in vivo. The results provide a better understanding of signaling pathways underlying the generation of testis-derived stem cells.

Identification and characterization of Xenopus tropicalis common progenitors of Sertoli and peritubular myoid cell lineages

The origin of somatic cell lineages during testicular development is controversial in mammals. Employing basal amphibian tetrapod Xenopus tropicalis we established a cell culture derived from testes of juvenile male. Expression analysis showed transcription of some pluripotency genes and Sertoli cell, peritubular myoid cell and mesenchymal cell markers. Transcription of germline-specific genes was downregulated. Immunocytochemistry revealed that a majority of cells express vimentin and co-express Sox9 and smooth muscle α-actin (Sma), indicating the existence of a common progenitor of Sertoli and peritubular myoid cell lineages. Microinjection of transgenic, red fluorescent protein (RFP)-positive somatic testicular cells into the peritoneal cavity of X. tropicalis tadpoles resulted in cell deposits in heart, pronephros and intestine, and later in a strong proliferation and formation of cell-to-cell net growing through the tadpole body. Immunohistochemistry analysis of transplanted tadpoles showed a strong expression of vimentin in RFP-positive cells. No co-localization of Sox9 and Sma signals was observed during the first three weeks indicating their dedifferentiation to migratory-active mesenchymal cells recently described in human testicular biopsies.

Summary: We identified cells co-expressing differentiation markers of Sertoli and peritubular myoid cell lineages in X. tropicalis through the establishment and characterization of cell culture derived from juvenile testis.

Coexistence of Y, W, and Z sex chromosomes in Xenopus tropicalis

Homomorphic sex chromosomes and rapid turnover of sex-determining genes can complicate establishing the sex chromosome system operating in a given species. This difficulty exists in Xenopus tropicalis, an anuran quickly becoming a relevant model for genetic, genomic, biochemical, and ecotoxicological research. Despite the recent interest attracted by this species, little is known about its sex chromosome system. Direct evidence that females are the heterogametic sex, as in the related species Xenopus laevis, has yet to be presented. Furthermore, X. laevis' sex-determining gene, DM-W, does not exist in X. tropicalis, and the sex chromosomes in the two species are not homologous. Here we identify X. tropicalis' sex chromosome system by integrating data from (i) breeding sex-reversed individuals, (ii) gynogenesis, (iii) triploids, and (iv) crosses among several strains. Our results indicate that at least three different types of sex chromosomes exist: Y, W, and Z, observed in YZ, YW, and ZZ males and in ZW and WW females. Because some combinations of parental sex chromosomes produce unisex offspring and other distorted sex ratios, understanding the sex-determination systems in X. tropicalis is critical for developing this flexible animal model for genetics and ecotoxicology.

Development of Xenopus resource centers: the National Xenopus Resource and the European Xenopus Resource Center

Xenopus is an essential vertebrate model system for biomedical research that has contributed to important discoveries in many disciplines, including cell biology, molecular biology, physiology, developmental biology, and neurobiology. However, unlike other model systems no central repository/stock center for Xenopus had been established until recently. Similar to mouse, zebrafish, and fly communities, which have established stock centers, Xenopus researchers need to maintain and distribute rapidly growing numbers of inbred, mutant, and transgenic frog strains, along with DNA and protein resources, and individual laboratories struggle to accomplish this efficiently. In the last 5 years, two resource centers were founded to address this need: the European Xenopus Resource Center (EXRC) at the University of Portsmouth in England, and the National Xenopus Resource (NXR) at the Marine Biological Laboratory in Woods Hole, MA. These two centers work together to provide resources and support to the Xenopus research community. The EXRC and NXR serve as stock centers and acquire, produce, maintain and distribute mutant, inbred and transgenic Xenopus laevis and Xenopus tropicalis lines. Independently, the EXRC is a repository for Xenopus cDNAs, fosmids, and antibodies; it also provides oocytes and wild-type frogs within the United Kingdom. The NXR will complement these services by providing research training and promoting intellectual interchange through hosting mini-courses and workshops and offering space for researchers to perform short-term projects at the Marine Biological Laboratory. Together the EXRC and NXR will enable researchers to improve productivity by providing resources and expertise to all levels, from graduate students to experienced PIs. These two centers will also enable investigators that use other animal systems to take advantage of Xenopus' unique experimental features to complement their studies.