Xenopus (Gk., xeno=strange, pous=foot) is a genus of highly aquatic frogs native to Sub-Saharan Africa. There are 20 species in the Xenopus genus. They are known collectively as African Clawed Frogs or Platanna. The best-known species belonging to this genus is Xenopus laevis, which is commonly studied as a model organism for developmental biology, cell biology, toxicology, and neuroscience.
All species of Xenopus have flattened, somewhat egg-shaped and streamlined bodies, and very slippery skin (because of a protective mucous covering). The frog's skin is smooth but with a lateral line sensory organ that has a stitch-like appearance. The frogs are all excellent swimmers and have powerful fully webbed toes, though the fingers lack webbing. Three of the toes on each foot have conspicuous black claws.
The frog's eyes are on top of the head, looking upwards. The pupils are circular. They have no moveable eyelids, tongues (rather it is completely attached to the floor of the mouth) or eardrums (similarly to the Surinam toad).
Xenopus species are entirely aquatic, though they have been observed migrating on land to nearby bodies of water during times of drought. They are usually found in lakes, rivers, swamps and man-made reservoirs.
Adult frogs are usually both predators and scavengers, and since their tongue is unusable, the frogs use their small forelimbs to aid in the feeding process. Since they also lack a vocal sac, they make clicking sounds underwater (again similar to the Surinam toad). The Xenopus species are also active during the twilight (or crepuscular) hours.
During breeding season, the males develop ridge-like nuptial pads (black in color) on the fingers to aid in grasping the female. The frogs' mating embrace is inguinal, meaning that the male grasps the female around her waist.
- Xenopus amieti (Volcano Clawed Frog)
- Xenopus andrei (Andre's Clawed Frog)
- Xenopus borealis (Marsabit Clawed Frog)
- Xenopus boumbaensis (Mawa Clawed Frog)
- Xenopus clivii (Eritrea Clawed Frog)
- Xenopus fraseri (Fraser's Platanna)
- Xenopus gilli (Cape Platanna)
- Xenopus itombwensis (Itombwe Massif Clawed Frog)
- Xenopus laevis (Common Platanna)
- Xenopus largeni (Largen's Clawed Frog)
- Xenopus lenduensis (Lendu Plateau Clawed Frog)
- Xenopus longipes (Lake Oku Clawed Frog)
- Xenopus muelleri (Muller's Platanna)
- Xenopus petersii (Peters' Platanna)
- Xenopus pygmaeus (Bouchia Clawed Frog)
- Xenopus ruwenzoriensis (Uganda Clawed Frog)
- Xenopus tropicalis (Western clawed frog)
- Xenopus vestitus (Kivu Clawed Frog)
- Xenopus victorianus (Lake Victoria Clawed Frog)
- Xenopus wittei (De Witte's Clawed Frog)
Xenopus as a model organism for biomedical research
Xenopus embryos and eggs are a popular model system for a wide variety of biological studies. This animal is widely used because of its powerful combination of experimental tractability and close evolutionary relationship with humans, at least compared to many model organisms.
Xenopus has long been an important tool for in vivo studies in molecular, cell, and developmental biology of vertebrate animals. However, the wide breadth of Xenopus research stems from the additional fact that cell-free extracts made from Xenopus are a premier in vitro system for studies of fundamental aspects of cell and molecular biology. Thus, Xenopus is the only vertebrate model system that allows for high-throughput in vivo analyses of gene function and high-throughput biochemistry. Finally, Xenopus oocytes are a leading system for studies of ion transport and channel physiology.
The community-maintained model organism database for Xenopus is Xenbase 
Investigations of human disease genes using Xenopus
All modes of Xenopus research (embryos, cell-free, extracts, and oocytes) are commonly used in direct studies of human disease genes.
Xenopus embryos for in vivo studies of human disease gene function: Xenopus embryos are large and easily manipulated, and moreover, thousands of embryos can be obtained in a single day. Indeed, Xenopus was the first vertebrate animal for which methods were developed that allowed rapid analysis of gene function using misexpression (by mRNA injection ). Injection of mRNA in Xenopus that led to the cloning of interferon. Moreover, the use of morpholino-antisense oligonucleotides for gene knockdowns in vertebrates, which is now widely used, was first developed by Janet Heasman using Xenopus.
In recent years these approaches have played in important role in studies of human disease genes. The mechanism of action for several genes mutated in human cystic kidney disorders (e.g. nephronophthisis) have been extensively studied in Xenopus embryos, shedding new light on the link between these disorders, ciliogenesis and Wnt signaling (). Xenopus embryos have also provided a rapid test bed for validating newly-discovered disease genes. For example, studies in Xenopus confirmed and elucidated the role PYCR1 in cutis laxa with progeroid features ().
Transgenic Xenopus for studying transcriptional regulation of human disease genes: Xenopus embryos develop rapidly, and so transgenesis in Xenopus is a rapid and effective method for analyzing genomic regulatory sequences. In a recent study, mutations in the SMAD7 locus were revealed to associate with human colorectal cancer. Interestingly, the mutations lay in conserved, but non-coding sequences, suggesting that these mutations impacted the patterns of SMAD7 transcription. To test this hypothesis, the authors used Xenopus transgenesis, and revealed that this genomic region drove expression of GFP in the hindgut. Moreover, transgenics made with the mutant version of this region displayed substantially less expression in the hindgut ().
Xenopus cell-free extracts for biochemical studies of proteins encoded by human disease genes: A unique advantage of the Xenopus system is that cytosolic extracts contain both soluble cytoplasmic and nuclear proteins (including chromatin proteins). This is in contrast to cellular extracts prepared from somatic cells with already distinct cellular compartments. Xenopus egg extracts have provided numerous insights into the basic biology of cells with particular impact on cell division and the DNA transactions associated with it (see below).
Studies in Xenopus egg extracts have also yielded critical insights into the mechanism of action of human disease genes associated with genetic instability and elevated cancer risk, such as ATM (Ataxia telangiectasia), BRCA1 (Inherited Breast and Ovarian cancer), Nbs1 (Nijmegen Breakage Syndrome), RecQL4 (Rothmund-Thomson Syndrome), c-Myc oncogene and FANC proteins (Fanconi anemia) ().
Xenopus oocytes for studies of gene expression and channel activity related to human disease: Yet another strength of Xenopus is the ability to rapidly and easily assay the activity of channel and transporter proteins using expression in oocytes. This application has also led to important insights into human disease, including studies related to trypanosome transmission (), Epilepsy with ataxia and sensorineural deafness, Catastrophic cardiac arrhythmia (Long-QT syndrome;) and Megalencephalic leukoencephalopathy ().
Investigation of fundamental biological processes using Xenopus
Signal transduction: Xenopus embryos and cell-free extracts are widely used for basic research in signal transduction. In just the last few years, Xenopus embryos have provided crucial insights into the mechanisms of TGF-beta and Wnt signal transduction. For example, Xenopus embryos were used to identify the enzymes that control ubiquitination of Smad4 (), and also to demonstrate direct links between TGF-beta superfamily signaling pathways and other important networks, such as the MAP kinase pathway () and the Wnt pathway (). Moreover, new methods using egg extracts revealed novel, important targets of the Wnt/GSK3 destruction complex ().
Cell division: Xenopus egg extracts have allowed the study of many complicated cellular events in a test tube. Because egg cytosol can support successive cycling between mitosis and interphase in vitro, it has been critical to diverse studies of cell division. For example, the small GTPase Ran was first found to regulate interphase nuclear transport, but Xenopus egg extracts revealed the critical role of Ran GTPase in mitosis independent of its role in interphase nuclear transport (). Similarly, the cell-free extracts were used to model nuclear envelope assembly from chromatin, revealing the function of RanGTPase in regulating nuclear envelope reassembly after mitosis (). More recently, using Xenopus egg extracts, it was possible to demonstrate the mitosis-specific function of the nuclear lamin B in regulating spindle morphogenesis () and to identify new proteins that mediate kinetochore attachment to microtubules.
Embryonic development: Xenopus embryos are widely used in developmental biology. A summary of recent advances made by Xenopus research in recent years would include:
- Epigenetics of cell fate specification 
- microRNA in germ layer patterning and eye development,
- Link between Wnt signaling and telomerase 
- Development of the vasculature 
- Gut morphogenesis 
- Contact inhibition and neural crest cell migration 
DNA replication: Xenopus cell-free extracts also support the synchronous assembly and the activation of origins of DNA replication. They have been instrumental in characterizing the biochemical function of the pre-replicative complex, including MCM proteins.
DNA damage response: Cell-free extracts have been instrumental to unravel the signaling pathways that are activated in response to DNA double-strand breaks (ATM), replication fork stalling (ATR) or DNA interstrand crosslinks (FA proteins and ATR). Notably, several mechanisms and components of these signal transduction pathways were first identified in Xenopus.
Apoptosis: Xenopus oocytes provide a tractable model for biochemical studies of apoptosis. Recently, oocytes were used recently to study the biochemical mechanisms of caspase-2 activation; importantly, this mechanism turns out to be conserved in mammals.
Regenerative medicine: In recent years, there has been tremendous interest in developmental biology stoked by the promise of regenerative medicine. Xenopus has played a role here as well. For example, it has been found that expression of seven transcription factors in pluripotent Xenopus cells rendered those cells able to develop into functional eyes when implanted into Xenopus embryos, providing potential insights into the repair of retinal degeneration or damage. In a vastly different study, Xenopus embryos was used to study the effects of tissue tension on morphogenesis, an issue that will be critical for in vitro tissue engineering.
Physiology: The directional beating of multi-ciliated cells is essential to development and homeostasis in the central nervous system, the airway, and the oviduct. Interestingly, the multi-ciliated cells of the Xenopus epidermis have recently been developed as the first in vivo test-bed for live-cell studies of such ciliated tissues, and these studies have provided important insights into the biomechanical and molecular control of directional beating.
Use of Xenopus for small molecule screens to develop novel therapeutics
Because huge amounts of material are easily obtained, all modalities of Xenopus research are now being used for small-molecule based screens.
Chemical genetics of vascular growth in Xenopus tadpoles: Given the important role of neovascularization in cancer progression, Xenopus embryos were recently used to identify new small molecules inhibitors of blood vessel growth. Notably, compounds identified in Xenopus were effective in mice. Notably, frog embryos figured prominently in a study that used evolutionary principles to identify a novel vascular disrupting agent that may have chemotherapeutic potential. That work was featured in the New York Times Science Times 
In vivo testing of potential endocrine disruptors in transgenic Xenopus embryos: Endocrine disrupting chemicals released into the environment pose a potential public health risk, but our ability to identify such compounds in vitro vastly outstrips our ability to monitor the in vivo effects of such chemicals. A high-throughput assay for thyroid disruption has recently been developed using transgenic Xenopus embryos.
Small molecule screens in Xenopus egg extracts: Egg extracts provide ready analysis of molecular biological processes and can rapidly screened. This approach was used to identify novel inhibitors of proteasome-mediated protein degradation and DNA repair enzymes.
Use of Xenopus tropicalis for genetic studies
While Xenopus laevis is the most commonly used species for developmental biology studies, genetic studies, especially forward genetic studies, can be complicated by their pseudotetraploid genome. Xenopus tropicalis provides a simpler model for genetic studies, having a diploid genome.
Use of Morpholino knockdown techniques for gene knockdown in Xenopus
Morpholino-oligos or MOs are also used in both X. laevis and X. tropicalis to probe the function of a protein by observing the results of eliminating the protein's activity, for example, as was done to screen a set of X. tropicalis genes, published in 2006.
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