We have long extrapolated from animal models to better understand our own biology
and health. Amongst such models, amphibia, and in particular Xenopus, have emerged
as a powerhouse of biological discovery, providing startling insights into fundamental
processes in embryology, cell biology, genetics, physiology, toxicology, evolution,
ecology, and disease. Indeed, research in amphibians has consistently thrown open
new fields of discovery, a fact reflected in contributions to numerous Nobel prizes
in Physiology or Medicine, beginning with August (Lindstedt, 2014) prize for discovery
of capillary motor-regulating mechanism and most recently with John Gurdon's 2012
award for reprogramming mature cells to pluripotency (Krogh, 1919; Gurdon et al.,
1958; Gurdon and Hopwood, 2000; Burggren and Warburton, 2007; Blum and Ott, 2018).
Over the last 70 years, Xenopus has emerged as the predominant amphibian model and
one of most widely used model systems globally, making a tremendous impact on biological
research.
Native to south and central Africa, Xenopus laevis initially expanded into European
and North American laboratories in the 1930's and 40's as the leading pregnancy test
of the time; one injection of human urine containing gonadotrophic hormone is sufficient
to induce egg laying within hours (Gurdon and Hopwood, 2000). However, this ability
to produce thousands of eggs and externally developing embryos on demand year-round
by simple hormone injection gave Xenopus a distinct advantage over other available
experimental models. This, combined with its large oocytes and embryos that are well-suited
to biochemical, cell biological and embryological manipulations, its ease of genomic
manipulation, its relative evolutionary proximity to humans, low maintenance, short
life cycle, and low cost, continue to make Xenopus an exceptionally valuable model.
In the past two decades, the establishment of X. tropicalis, a diploid species, as
a laboratory model has added additional powerful genetic tools (Grainger, 2012; Tandon
et al., 2017). Together, X. laevis and X. tropicalis allow us to rapidly investigate
fundamental biological processes both in vivo and ex vivo. This makes Xenopus an ideal
system in the genomic era, where we are in need of efficient models suitable for testing
human disease gene function.
The purpose of this Research Topic is to highlight the outstanding versatility and
utility of Xenopus as a model system in which to investigate human development, disease,
and pathology. It comprises 18 primary research and review articles exploring a diverse
array of topics, including development, regeneration, cancer, biological scaling,
and human disease modeling, as well as providing an overview of the extensive resources
available to support Xenopus research. It is our hope that it will be a resource both
for established Xenopus researchers, and Xenopus newcomers looking to identify the
appropriate model system and approach for their research.
Several articles in this Research Topic illustrate the power of Xenopus in modeling
and investigating a broad variety of inherited human diseases. For example, Mills
et al. investigate the genetic and developmental causes of Wolf-Hirschhorn Syndrome
(WHS), a multigenic disorder that results in characteristic facial abnormalities.
In particular, they determine requirements for four distinct WHS candidate genes during
cranial neural crest migration and facial morphogenesis. Depletion of these genes
in frog can disrupt facial morphogenesis, recapitulating much of the patient phenotype.
Importantly, this work demonstrates the relative ease with which complex multigenic
syndromes can be dissected in Xenopus. Expanding upon this point, Lasser et al. contribute
a complementary review of WHS and discuss how Xenopus might be exploited to further
investigate the ontogeny of this and other multigenic conditions. Ott et al. identify
and functionally analyze novel compound heterozygous variants of PIBF1 that were identified
in a Joubert syndrome patient. Importantly, they discover that these disease variants
affect cilia function and discuss their likely contribution to the disease. In other
examples, Sempou et al. report an unexpected role for the heterotaxy candidate gene,
Fgfr4, in gastrulation and development of the left—right body axis, providing insight
into the origin of the patient phenotype, while Popov et al. use Xenopus laevis to
determine the functional consequence of a candidate disease variant in YWHAZ, and
investigate the molecular mechanisms underlying its contribution to the RASopathy,
Cardiofaciocutaneous syndrome. Lichtig et al. develop a Xenopus model of Bainbridge-Ropers
syndrome and reveal that depletion of the asxl3 disease gene perturbs early neural
development. In doing so they produce a powerful tool for further investigation of
the condition. Finally, Hwang et al. review recent methodological advances that allow
organ specific phenotypic investigations in Xenopus and discuss their utility in modeling
genetic disease. Together, these articles add a wealth of knowledge to our understanding
of congenital disease.
Technological advances are rapidly augmenting the Xenopus experimental repertoire
and opening innovative new avenues of investigation. This is strongly evident in the
field of oncology, where the marriage of classical Xenopus attributes and modern gene
editing tools is creating efficacious new experimental platforms. In this Research
Topic, Hardwick and Philpott review Xenopus' many contributions to our knowledge of
tumor biology and discuss how genome editing technologies are revolutionizing its
utility in oncology research. In addition, Dimitrakopoulou et al. highlight the untapped
potential of Xenopus as an emerging system in which to study hematologic malignancies,
and outline their experimental pipeline for generating leukemia models in Xenopus
using CRISPR/Cas9. Deniz et al. provide another example of technological application,
by demonstrating the power of hemoglobin contrast subtraction angiography as a non-destructive
and efficient method to quantify cardiac function, a technique that greatly facilitates
high throughput investigation of candidate congenital heart disease genes.
Several articles showcase Xenopus' unrivaled power for studying fundamental processes
in early vertebrate development and organogenesis. For example, despite being an integral
biological process, we have little understanding of how size and scaling are controlled
at the cell and organism levels. Gibeaux et al. exploit the size difference between
the Xenopus species, and the ability to generate viable intermediately sized hybrids,
to investigate this mystery. Based on their findings, they propose a model whereby
cell and organism size are regulated through a combination of genome size and transcriptional
regulation in Xenopus. Haworth et al. use the ease with which embryonic tissue can
be isolated and manipulated in Xenopus to create ex vivo models of cardiac and liver
induction, and use these systems to explore the differential requirements for Wnt,
FGF, and BMP signaling in liver formation, information critical to the refinement
of protocols for liver cell differentiation from pluripotent stem cells. DeLay et
al. advance our understanding of kidney development by demonstrating that the CDC42-GEF,
dynamin binding protein (Dnmbp/Tuba), is essential for pronephric patterning and nephrogenesis
in Xenopus, while Kho et al. reveal that CEP3 regulates the coordinated cell shape
changes and movements required in somite segmentation.
The regenerative abilities of amphibians have long captivated biologists and inspired
hope that these healing mechanisms could be applied to human injuries. While Xenopus
tadpoles can readily regenerate damaged tissues, this ability is lost during metamorphosis,
making them an ideal system for studying both the mechanisms that promote and prevent
regeneration. Furthermore, as developmental processes have been extensively studied
in Xenopus, it is an ideal model in which to examine regeneration. In this collection,
Kha et al., take advantage of Xenopus tadpoles' ability to regrow a functional and
morphologically normal eye, to investigate how similarly developmental processes are
employed during embryogenesis and regrowth following injury, while Kakebeen and Wills
review the biophysical, biochemical, and epigenetic processes that underlie regeneration.
Xenopus research is supported by powerful resources, including Xenbase, an extensive
online bioinformatics and research database. Three Xenopus resource centers also exist
to support and encourage research in Xenopus. In this collection, Horb et al. provide
an overview of these centralized resources and the support available to both specialist
and non-specialist researchers, including the availability of transgenic, inbread,
and mutant animals, molecular resources, training, and experimental support. Nenni
et al. add a complimentary description of advancements in Xenbase, highlighting its
application to the study of disease. They also report a very fitting meta-analysis
of Xenopus research which provides a fascinating snapshot of the breath of human diseases
being investigated using Xenopus and the diverse experimental approaches taken by
the community to understand them.
We hope that this collection of articles will be of interest to the storied Xenopus
community as well as to clinicians and investigators working in the broader field
of developmental biology and disease research.
Author Contributions
All authors listed have made a substantial, direct and intellectual contribution to
the work, and approved it for publication.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.