''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of highly aquatic
frogs
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...
native to
sub-Saharan Africa
Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the List of sov ...
. Twenty species are currently described within it. The two best-known species of this genus are ''
Xenopus laevis
The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
'' and ''
Xenopus tropicalis
The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it a ...
'', which are commonly studied as
model organism
A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
s for developmental biology, cell biology, toxicology, neuroscience and for modelling human disease and birth defects.
The genus is also known for its
polyploid
Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei ( eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...
y, with some species having up to 12 sets of
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s.
Characteristics
''Xenopus laevis'' is a rather inactive creature. It is incredibly hardy and can live up to 15 years. At times the ponds that ''Xenopus laevis'' is found in dry up, compelling it, in the dry season, to burrow into the mud, leaving a tunnel for air. It may lie dormant for up to a year. If the pond dries up in the rainy season, ''Xenopus laevis'' may migrate long distances to another pond, maintaining hydration by the rains. It is an adept swimmer, swimming in all directions with ease. It is barely able to hop, but it is able to crawl. It spends most of its time underwater and comes to surface to breathe. Respiration is predominantly through its well developed lungs; there is little cutaneous respiration.
Description
All species of ''Xenopus'' have flattened, somewhat egg-shaped and streamlined bodies, and very slippery skin (because of a protective mucus covering).
The frog's skin is smooth, but with a
lateral line
The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
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
claw
A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tarsus ...
s.
The frog's eyes are on top of the head, looking upwards. The
pupils
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
are circular. They have no moveable
eyelids
An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyel ...
,
tongue
The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
s (rather it is completely attached to the floor of the mouth
) or
eardrum
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear
The outer ear, external ear, or auris externa is the extern ...
s (similarly to
''Pipa pipa'', the common Suriname toad).
Unlike most amphibians, they have no
haptoglobin
Haptoglobin (abbreviated as Hp) is the protein that in humans is encoded by the ''HP'' gene. In blood plasma, haptoglobin binds with high affinity to ''free'' hemoglobin released from erythrocytes, and thereby inhibits its deleterious oxidative ...
in their
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
.
Behaviour
''Xenopus'' species are entirely
aquatic, though they have been observed migrating on land to nearby bodies of water during times of
drought
A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...
or in heavy rain. They are usually found in
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s,
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
s,
swamps
A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...
, potholes in streams, and man-made reservoirs.
Adult frogs are usually both
predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s and
scavengers
Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding be ...
, and since their tongues are unusable, the frogs use their small fore limbs to aid in the feeding process. Since they also lack
vocal sac
The vocal sac is the flexible membrane of skin possessed by most male frogs and toads. The purpose of the vocal sac is usually as an amplification of their mating or advertisement call. The presence or development of the vocal sac is one way o ...
s, they make clicks (brief pulses of sound) underwater (again similar to ''
Pipa pipa
The pipa, pípá, or p'i-p'a () is a traditional China, Chinese List of traditional Chinese musical instruments, musical instrument, belonging to the Plucked string instrument, plucked category of instruments. Sometimes called the "Chinese lute" ...
'').
Males establish a hierarchy of social dominance in which primarily one male has the right to make the advertisement call.
The females of many species produce a release call, and ''
Xenopus laevis
The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
'' females produce an additional call when sexually receptive and soon to lay eggs.
The ''Xenopus'' species are also active during the twilight (or
crepuscular
In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal, vespertine, or both. This is distinguished from diurnal and nocturnal behavior, where an animal is active during the hours of daylig ...
) hours.
During breeding season, the males develop ridge-like nuptial pads (black in color) on their fingers to aid in grasping the female. The frogs' mating embrace is inguinal, meaning the male grasps the female around her waist.
Species
Extant species
*''
Xenopus allofraseri''
*''
Xenopus amieti'' (
volcano clawed frog)
*''
Xenopus andrei'' (
Andre's clawed frog)
*''
Xenopus borealis'' (
Marsabit clawed frog)
*''
Xenopus boumbaensis
''Xenopus boumbaensis'', the Mawa clawed frog, is a species of frog in the family Pipidae. It is known from a few localities in central and south-eastern Cameroon, and from north-western Republic of Congo and extreme south-western Central African ...
'' (
Mawa clawed frog
''Xenopus boumbaensis'', the Mawa clawed frog, is a species of frog in the family Pipidae. It is known from a few localities in central and south-eastern Cameroon, and from north-western Republic of Congo and extreme south-western Central Afric ...
)
*''
Xenopus calcaratus
''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...
''
*''
Xenopus clivii'' (
Eritrea clawed frog
The Eritrea clawed frog or Peracca's clawed frog (''Xenopus clivii'') is a species of frog in the family Pipidae found in Eritrea, Ethiopia, Kenya, and Sudan. Its natural habitats are subtropical or tropical moist lowland forests, subtropical ...
)
*''
Xenopus epitropicalis'' (
Cameroon clawed frog)
*''
Xenopus eysoole''
*''
Xenopus fischbergi
''Xenopus fischbergi'', the Fischberg's clawed frog, is a species of frog native to Central and Western Africa. It occurs in habitats such as savannas and wetlands. It can be found in elevations between . It is listed as Least Concern on the IUCN ...
''
*''
Xenopus fraseri
Fraser's platanna (''Xenopus fraseri'') is a species of frogs in the family Pipidae found in Angola, Cameroon, Central African Republic, Republic of the Congo, Democratic Republic of the Congo, Equatorial Guinea, Gabon, and possibly Rwanda.
Its ...
'' (
Fraser's platanna)
*''
Xenopus gilli'' (
Cape platanna
The Cape clawed frog, Cape platanna or Gill's platanna (''Xenopus gilli'') is a species of frogs in the family Pipidae endemic to South Africa. Its natural habitats are Mediterranean-type shrubby vegetation, freshwater marshes, intermittent fres ...
)
*''
Xenopus itombwensis
''Xenopus itombwensis'' is a species of frogs in the family Pipidae endemic to the Itombwe Massif of the South Kivu Province of the Democratic Republic of the Congo.
References
Xenopus, itombwensis
Amphibians described in 2008
Frogs of ...
''
*''
Xenopus kobeli
''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...
''
*''
Xenopus laevis
The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
'' (
African clawed frog
The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
or common platanna)
*''
Xenopus largeni'' (
Largen's clawed frog)
*''
Xenopus lenduensis'' (
Lendu Plateau clawed frog)
*''
Xenopus longipes'' (
Lake Oku clawed frog)
*''
Xenopus mellotropicalis''
*''
Xenopus muelleri'' (
Müller's platanna)
*''
Xenopus parafraseri''
*''
Xenopus petersii
Peters' platanna (''Xenopus petersii'') is a species of frog in the family Pipidae found in Angola, Botswana, the Republic of the Congo, the Democratic Republic of the Congo, Gabon, Namibia, Zambia, Zimbabwe, and possibly Tanzania. Its natural h ...
'' (
Peters' platanna)
*''
Xenopus poweri''
*''
Xenopus pygmaeus'' (
Bouchia clawed frog)
*''
Xenopus ruwenzoriensis
The Uganda clawed frog (''Xenopus ruwenzoriensis'') is a species of frog in the family Pipidae found in Uganda and possibly the Democratic Republic of the Congo. Its natural habitats are subtropical or tropical moist lowland forest, freshwater ma ...
'' (
Uganda clawed frog)
*''
Xenopus tropicalis
The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it a ...
'' (
western clawed frog
The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it ...
)
*''
Xenopus vestitus
The Kivu clawed frog (''Xenopus vestitus'') is a species of frog in the family Pipidae found in the Democratic Republic of the Congo, Rwanda, and Uganda. Its natural habitats are subtropical or tropical moist montane forests, subtropical or tro ...
'' (
Kivu clawed frog)
*''
Xenopus victorianus'' (
Lake Victoria clawed frog)
*''
Xenopus wittei
De Witte's clawed frog, ''Xenopus wittei'', is a species of frog in the family Pipidae found in the Democratic Republic of the Congo, Rwanda, Uganda, and possibly Burundi. Its natural habitats are subtropical or tropical moist montane forests ...
'' (
De Witte's clawed frog
De Witte's clawed frog, ''Xenopus wittei'', is a species of frog in the family Pipidae found in the Democratic Republic of the Congo, Rwanda, Uganda, and possibly Burundi. Its natural habitats are subtropical or tropical moist montane forests ...
)
Fossil species
The following fossil species have been described:
[ ''Xenopus''at ]Fossilworks
Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database
The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...
.org
* †''
Xenopus arabiensis
''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...
'' -
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
Yemen Volcanic Group, Yemen
* †''
Xenopus hasaunus''
* †''
Xenopus romeri'' -
Itaboraian
The Itaboraian ( pt, Itaboraiense) age is a period within the Early Eocene geologic time (53.0–50.0 Mya (unit), Ma) epoch of the Paleogene, used more specifically with South American land mammal ages (SALMA). It follows the Riochican and precedes ...
Itaboraí Formation
The Itaboraí Formation ( pt, Formação Itaboraí) is a highly list of fossiliferous stratigraphic units in Brazil, fossiliferous geologic Formation (geology), formation and LagerstätteKellner & Campos, 1999, p.399 of the Itaboraí Basin in Rio ...
, Brazil
* †''
Xenopus stromeri''
* ''cf. Xenopus sp.'' -
Campanian
The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...
-
Los Alamitos Formation
The Los Alamitos Formation is a geological formation of the North Patagonian Massif in Rio Negro Province, northwestern Patagonia, Argentina, whose strata date back to the Late Cretaceous (Late Campanian to Maastrichtian). Dinosaur remains are amo ...
, Argentina
* ''Xenopus (Xenopus) sp.'' -
Late Oligocene
The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stage ...
Nsungwe Formation
The Nsungwe Formation is a formation in the Rukwa Rift Basin of the East African Rift System, it is Oligocene in age based on U-Pb dating of a tuff horizon within the formation. It is part of the Red Sandstone Group along with the uncomfortably ...
, Tanzania
* ''Xenopus sp.'' -
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
Morocco
* ''Xenopus sp.'' -
Early Pleistocene
The Early Pleistocene is an unofficial sub-epoch in the international geologic timescale in chronostratigraphy, being the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently estimated to span the time ...
Olduvai Formation, Tanzania
Model organism for biological research
Like many other
frogs
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...
, they are often used in laboratory as research subjects.
''Xenopus'' embryos and eggs are a popular model system for a wide variety of biological studies.
This animal is 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 a vertebrate model system that allows for high-throughput ''in vivo'' analyses of gene function and high-throughput biochemistry. Furthermore, ''Xenopus'' oocytes are a leading system for studies of ion transport and channel physiology.
''Xenopus'' is also a unique system for analyses of genome evolution and whole genome duplication in vertebrates, as different ''Xenopus'' species form a
ploidy
Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
series formed by
interspecific hybridization
In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Hybrids are not always intermediates between their parents (such as in ...
.
In 1931,
Lancelot Hogben
Lancelot Thomas Hogben FRS FRSE (9 December 1895 – 22 August 1975) was a British experimental zoologist and medical statistician. He developed the African clawed frog ''(Xenopus laevis)'' as a model organism for biological research in his ear ...
noted that ''Xenopus laevis'' females ovulated when injected with the urine of pregnant women. This led to a pregnancy test that was later refined by South African researchers
Hillel Abbe Shapiro
Hillel Abbe Shapiro (2 February 1909 – 31 October 1984) was a South African forensic pathologist with a range of specialisms in experimental physiology and forensic medicine. He was editor of medical journals, medical text books and a universi ...
and Harry Zwarenstein.
A female Xenopus frog injected with a woman's urine was put in a jar with a little water. If eggs were in the water a day later it meant the woman was pregnant. Four years after the first ''Xenopus'' test, Zwarenstein's colleague, Dr Louis Bosman, reported that the test was accurate in more than 99% of cases. From the 1930s to the 1950s, thousands of frogs were exported across the world for use in these pregnancy tests.
The of the
Marine Biological Laboratory
The Marine Biological Laboratory (MBL) is an international center for research and education in biological and environmental science. Founded in Woods Hole, Massachusetts, in 1888, the MBL is a private, nonprofit institution that was independent ...
is an ''in vivo'' repository for transgenic and mutant strains and a training center.
Online Model Organism Database
Xenbase
Xenbase is a Model Organism Database (MOD), providing informatics resources, as well as genomic and biological data on Xenopus frogs.K. Karimi et al. (2017Xenbase: a genomic, epigenomic and transcriptomic model organism database Nucleic Acids Re ...
is the
Model Organism Database (MOD) for both ''
Xenopus laevis
The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...
'' and ''
Xenopus tropicalis
The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it a ...
''.
Investigation of human disease genes
All modes of ''Xenopus'' research (embryos, cell-free extracts, and oocytes) are commonly used in direct studies of human disease genes and to study the basic science underlying initiation and progression of cancer. ''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 to allow 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 vertebrate embryos, 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
Nephronophthisis is a genetic disorder of the kidneys which affects children. It is classified as a medullary cystic kidney disease. The disorder is inherited in an autosomal recessive fashion and, although rare, is the most common genetic cause o ...
) have been extensively studied in ''Xenopus'' embryos, shedding new light on the link between these disorders,
ciliogenesis
Ciliogenesis is defined as the building of the cell's antenna (primary cilia) or extracellular fluid mediation mechanism (motile cilium). It includes the assembly and disassembly of the cilia during the cell cycle. Cilia are important organelles ...
and
Wnt signaling
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...
. ''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 of ''PYCR1'' in
cutis laxa
Cutis laxa or pachydermatocele is a group of rare connective tissue disorders in which the skin becomes inelastic and hangs loosely in folds.
Signs and symptoms
It is characterised by skin that is loose, hanging, wrinkled, and lacking in elasti ...
with progeroid features.
Transgenic ''Xenopus'' for studying transcriptional regulation of human disease genes: ''Xenopus'' embryos develop rapidly, 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
Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). Signs and symptoms may include blood in the stool, a change in bowel m ...
. The mutations lay in conserved, but noncoding sequences, suggesting these mutations impacted the patterns of ''SMAD7'' transcription. To test this hypothesis, the authors used ''Xenopus'' transgenesis, and revealed this genomic region drove expression of
GFP GFP may refer to:
Organisations
* Gaelic Football Provence, a French Gaelic Athletic Association club
* Geheime Feldpolizei, the German secret military police during the Second World War
* French Group for the Study of Polymers and their Applicat ...
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 ataxia telangiectasia, ''
BRCA1
Breast cancer type 1 susceptibility protein is a protein that in humans is encoded by the ''BRCA1'' () gene. Orthologs are common in other vertebrate species, whereas invertebrate genomes may encode a more distantly related gene. ''BRCA1'' is a h ...
'' inherited breast and ovarian cancer, ''Nbs1'' Nijmegen breakage syndrome, ''RecQL4'' Rothmund-Thomson syndrome, ''
c-Myc
''Myc'' is a family of regulator genes and proto-oncogenes that code for transcription factors. The ''Myc'' family consists of three related human genes: ''c-myc'' (MYC), ''l-myc'' (MYCL), and ''n-myc'' (MYCN). ''c-myc'' (also sometimes referre ...
'' oncogene and FANC proteins (
Fanconi anemia
Fanconi anaemia (FA) is a rare genetic disease resulting in impaired response to DNA damage. Although it is a very rare disorder, study of this and other bone marrow failure syndromes has improved scientific understanding of the mechanisms of nor ...
).
''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
Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...
with
ataxia
Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of ...
and
sensorineural deafness
Sensorineural hearing loss (SNHL) is a type of hearing loss in which the root cause lies in the inner ear or sensory organ (cochlea and associated structures) or the vestibulocochlear nerve (cranial nerve VIII). SNHL accounts for about 90% of rep ...
Catastrophic
cardiac arrhythmia
Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adults ...
(
Long-QT syndrome
Long QT syndrome (LQTS) is a condition affecting repolarization (relaxing) of the heart after a heartbeat, giving rise to an abnormally lengthy QT interval. It results in an increased risk of an irregular heartbeat which can result in fainting, ...
) and Megalencephalic leukoencephalopathy.
Gene editing by the CRISPR/CAS system has recently been demonstrated in ''Xenopus'' ''tropicalis'' and ''Xenopus laevis''. This technique is being used to screen the effects of human disease genes in ''Xenopus'' and the system is sufficiently efficient to study the effects within the same embryos that have been manipulated.
Investigation of fundamental biological processes
Signal transduction
Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...
: ''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 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
Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
: ''Xenopus'' egg extracts have allowed the study of many complicated cellular events ''in vitro''. 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.
Cell-free system A cell-free system is an ''in vitro'' tool widely used to study biological reactions that happen within cells apart from a full cell system, thus reducing the complex interactions typically found when working in a whole cell. Subcellular fractions ...
s have recently become practical investigatory tools, and ''Xenopus'' oocytes are often the source of the extracts used. This has produced significant results in understanding
mitotic
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
oscillation and
microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
s.
Embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
: ''Xenopus'' embryos are widely used in developmental biology. A summary of recent advances made by ''Xenopus'' research in recent years would include:
#
Epigenetics
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
of cell fate specification and epigenome reference maps
#
microRNA
MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRN ...
in germ layer patterning and eye development
#Link between
Wnt signaling
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...
and
telomerase
Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most euka ...
#Development of the
vasculature
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
#Gut morphogenesis
#Contact inhibition and
neural crest
Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...
cell migration and the generation of neural crest from pluripotent blastula cells
# - Role of ''
Notch'': Dorsky et al 1995 elucidated a pattern of expression followed by downregulation
DNA replication
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritanc ...
: ''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 prereplicative complex, including MCM proteins.
DNA damage
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
response: Cell-free extracts have been instrumental to unravel the signaling pathways 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
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
: ''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
Regenerative medicine deals with the "process of replacing, engineering or regenerating human or animal cells, tissues or organs to restore or establish normal function". This field holds the promise of engineering damaged tissues and organs by st ...
: In recent years, tremendous interest in developmental biology has been stoked by the promise of regenerative medicine. ''Xenopus'' has played a role here, as well. For example, 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. ''Xenopus'' species are important model organisms for the study of spinal cord regeneration, because while capable of regeneration in their larval stages, ''Xenopus'' lose this capacity in early metamorphosis.
Physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
: The directional beating of multiciliated cells is essential to development and homeostasis in the central nervous system, the airway, and the oviduct. The multiciliated 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.
Actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
: Another result from cell-free ''Xenopus'' oocyte extracts has been improved understanding of actin.
Small molecule screens to develop novel therapies
Because huge amounts of material are easily obtained, all modalities of ''Xenopus'' research are now being used for small-molecule based screens.
Chemical genetics Chemical genetics is the investigation of the function of proteins and signal transduction pathways in cells by the screening of chemical libraries of small molecules. Chemical genetics is analogous to classical genetic screen where random mutations ...
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
Endocrine disruptors, sometimes also referred to as hormonally active agents, endocrine disrupting chemicals, or endocrine disrupting compounds are chemicals that can interfere with endocrine (or hormonal) systems. These disruptions can cause c ...
in transgenic ''Xenopus'' embryos; 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.
Genetic studies
While ''Xenopus laevis'' is the most commonly used species for
developmental biology
Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of Regeneration (biology), regeneration, asexual reproduction, metamorphosis, and the growth and di ...
studies, genetic studies, especially forward genetic studies, can be complicated by their
pseudotetraploid genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
. ''
Xenopus tropicalis
The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it a ...
'' provides a simpler model for genetic studies, having a
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
genome.
Gene expression knockdown techniques
The expression of genes can be reduced by a variety of means, for example by using antisense oligonucleotides targeting specific mRNA molecules. DNA oligonucleotides complementary to specific mRNA molecules are often chemically modified to improve their stability ''in vivo''. The chemical modifications used for this purpose include phosphorothioate, 2'-O-methyl, morpholino, MEA phosphoramidate and DEED phosphoramidate.
Morpholino oligonucleotides
Morpholino oligos are 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, a set of ''X. tropicalis'' genes has been screened in this fashion.
Morpholino oligos (MOs) are short, antisense oligos made of modified nucleotides. MOs can knock down gene expression by inhibiting mRNA translation, blocking RNA splicing, or inhibiting miRNA activity and maturation. MOs have proven to be effective knockdown tools in developmental biology experiments and RNA-blocking reagents for cells in culture. MOs do not degrade their RNA targets, but instead act via a steric blocking mechanism RNAseH-independent manner. They remain stable in cells and do not induce immune responses. Microinjection of MOs in early ''Xenopus'' embryos can suppress gene expression in a targeted manner.
Like all antisense approaches, different MOs can have different efficacy, and may cause off-target, non-specific effects. Often, several MOs need to be tested to find an effective target sequence. Rigorous controls are used to demonstrate specificity,
including:
* Phenocopy of genetic mutation
* Verification of reduced protein by western or immunostaining
* mRNA rescue by adding back a mRNA immune to the MO
* use of 2 different MOs (translation blocking and splice blocking)
* injection of control MOs
Xenbase
Xenbase is a Model Organism Database (MOD), providing informatics resources, as well as genomic and biological data on Xenopus frogs.K. Karimi et al. (2017Xenbase: a genomic, epigenomic and transcriptomic model organism database Nucleic Acids Re ...
provides a searchable catalog of over 2000 MOs that have been specifically used in Xenopus'' research.'' The data is searchable via sequence, gene symbol and various synonyms (as used in different publications).
Xenbase
/ref> Xenbase maps the MOs to the latest ''Xenopus'' genomes in GBrowse, predicts 'off-target' hits, and lists all ''Xenopus'' literature in which the morpholino has been published.
References
External links
Xenbase
~ A ''Xenopus laevis'' and ''tropicalis'' Web Resource
{{Taxonbar, from=Q1342298
Pipidae
Amphibian genera
Amphibians of Sub-Saharan Africa
Vertebrate developmental biology
Animal models
Taxa named by Johann Andreas Wagner