271
Digestive and Respiratory System
indeed, SATB2 expression has proven to be a useful early patterning marker to assess successful directed differentiation
of human PSCs into colonic organoids (Múnera et al. 2017).
The developmental effects resulting from simultaneous
functional depletion of the three cdx genes (cdx1/2/4) typically present in vertebrate genomes was f rst determined in
Xenopus; this work provided evidence that Cdx and Wnt3a
function in a positive feedback loop operating in the hindgut
(Faas and Isaacs 2009; Keenan et al. 2006). More recent
RNA-seq studies of individual and triple Cdx loss-of-function embryos revealed further cross-talk between Cdx and
FGF, identifying unappreciated negative feedback loops
wherein Cdx restrains fgf4/8 levels as well as a unique
restraint on HH signaling by Cdx4 (Marlétaz et al. 2015).
Mutations in human CDX2 are associated with the congenital defect persistent cloaca (Hsu et al. 2018), and imbalances
in posterior WNT, FGF, and HH signaling result in defective hindgut development in mice and humans (Runck et al.
2014). As the GRNs downstream of these signaling pathways and Cdx activity are still only partially understood, the
rich resource of Cdx-regulated genes in Xenopus, combined
with the robust ability to test epistatic relationships among
signaling pathways and screen the effect of mutations in
potentially causative factors, has the potential for Xenopus
to provide needed insight into congenital syndromes affecting the intestine.
Extensive work in Xenopus has also identif ed conserved
Wnt/PCP-dependent mechanisms regulating intestinal morphogenesis. During NF32-NF46, dynamic changes in intestinal endoderm cell properties, including cell-cell adhesion
and microtubule architecture, are regulated by Wnt/PCP signaling mediators (Rho, ROCK, non-muscle Myosin, JNK);
loss of function of these factors disrupts endoderm cell intercalation, epithelial morphogenesis, and gut tube elongation
(Reed et al. 2009; Dush and Nascone-Yoder 2019). Such cell
biological insights into intestinal development are also relevant to cancer, as many parallels exist between tumor pathogenesis and early embryo development. Indeed, Xenopus
has been a useful model for studying cancer (Hardwick and
Philpott 2018; see Chapter 21 of this book).
18.6. CONCLUDING REMARKS
Xenopus continues to be a powerful system to study fundamental mechanisms of digestive and respiratory system
organogenesis and disease. As high-throughput genomics,
genome editing, proteomics, and pharmacological screening are now routine in Xenopus, functional genomics and
human disease modeling at a systems level is accelerating.
We feel confdent that future Xenopus research will continue
to provide additional signifcant insights into both normal
human organogenesis and congenital malformations.
ACKNOWLEDGMENTS
This work is supported in part by grants P41HD054556,
P01HD093363, and R01 DK123092 to AMZ, and we
acknowledge Xenbase, the Xenopus model organism knowledge base (RRID:SCR_003280), and the National Xenopus
Resource (RRID:SCR_013731) for their support. Drawings
of Xenopus embryos are reproduced with permission from
Xenbase and Natalya Zahn, licensed under CC BY-NC 4.0.
To view a copy of this license, visit http://creativecommons.
org/licenses/by-nc/4.0/ .
REFERENCES
Afelik, S., Chen, Y., & Pieler, T. (2006). Combined ectopic expression of Pdx1 and Ptf1a/p48 results in the stable conversion of
posterior endoderm into endocrine and exocrine pancreatic
tissue. Genes & Development, 20 (11), 1441–1446.
Afouda, B. A., Nakamura, Y., Shaw, S., Charney, R. M., Paraiso,
K. D., Blitz, I. L., Cho, K., & Hoppler, S. (2020). Foxh1/
Nodal def nes context-specifc direct maternal Wnt/β-Catenin
target gene regulation in early development. iScience, 23(7),
101314.
Agius, E., Oelgeschläger, M., Wessely, O., Kemp, C., & De
Robertis, E. M. (2000). Endodermal Nodal-related signals
and mesoderm induction in Xenopus. Development, 127(6),
1173–1183.
Arima, K., Shiotsugu, J., Niu, R., Khandpur, R., Martinez, M.,
Shin, Y., Koide, T., Cho, K. W., Kitayama, A., Ueno, N.,
Chandraratna, R. A., & Blumberg, B. (2005). Global analysis
of RAR-responsive genes in the Xenopus neurula using cDNA
microarrays. Developmental Dynamics, 232 (2), 414–431.
Asashima, M., Nakano, H., Shimada, K., Kinoshita, K., & Ishii, K.
(1990). Mesodermal induction in early amphibian embryos
by activin A (erythroid differentiation factor). Roux’s archives
of developmental biology: the offcial organ of the EDBO,
198(6), 330–335. https://doi.org/10.1007/BF00383771
Blitz, I. L., Paraiso, K. D., Patrushev, I., Chiu, W., Cho, K., &
Gilchrist, M. J. (2017). A catalog of Xenopus tropicalis transcription factors and their regional expression in the early
gastrula stage embryo. Developmental Biology, 426(2),
409–417.
Blythe, S. A., Cha, S. W., Tadjuidje, E., Heasman, J., & Klein, P.
S. (2010). Beta-Catenin primes organizer gene expression by
recruiting a histone H3 arginine 8 methyltransferase, Prmt2.
Developmental Cell, 19 (2), 220–231.
Briggs, J. A., Weinreb, C., Wagner, D. E., Megason, S., Peshkin,
L., Kirschner, M. W., & Klein, A. M. (2018). The dynamics
of gene expression in vertebrate embryogenesis at single-cell
resolution. Science, 360 (6392), eaar5780.
Brosens, E., Ploeg, M., van Bever, Y., Koopmans, A. E., IJsselstijn,
H., Rottier, R. J., Wijnen, R., Tibboel, D., & de Klein, A.
(2014). Clinical and etiological heterogeneity in patients with
tracheo-esophageal malformations and associated anomalies.
European Journal of Medical Genetics, 57 (8), 440–452.
Brown, D. D., & Cai, L. (2007). Amphibian metamorphosis.
Developmental Biology, 306 (1), 20–33.
Buchholz, D. R. (2015). More similar than you think: Frog metamorphosis as a model of human perinatal endocrinology.
Developmental Biology, 408 (2), 188–195.
Cebola, I., Rodríguez-Seguí, S. A., Cho, C. H., Bessa, J., Rovira,
M., Luengo, M., Chhatriwala, M., Berry, A., Ponsa-Cobas,
J., Maestro, M. A., Jennings, R. E., Pasquali, L., Morán, I.,
Castro, N., Hanley, N. A., Gomez-Skarmeta, J. L., Vallier,
L., & Ferrer, J. (2015). TEAD and YAP regulate the enhancer
network of human embryonic pancreatic progenitors. Nature
Cell Biology, 17 (5), 615–626.
Digestive and Respiratory System
indeed, SATB2 expression has proven to be a useful early patterning marker to assess successful directed differentiation
of human PSCs into colonic organoids (Múnera et al. 2017).
The developmental effects resulting from simultaneous
functional depletion of the three cdx genes (cdx1/2/4) typically present in vertebrate genomes was f rst determined in
Xenopus; this work provided evidence that Cdx and Wnt3a
function in a positive feedback loop operating in the hindgut
(Faas and Isaacs 2009; Keenan et al. 2006). More recent
RNA-seq studies of individual and triple Cdx loss-of-function embryos revealed further cross-talk between Cdx and
FGF, identifying unappreciated negative feedback loops
wherein Cdx restrains fgf4/8 levels as well as a unique
restraint on HH signaling by Cdx4 (Marlétaz et al. 2015).
Mutations in human CDX2 are associated with the congenital defect persistent cloaca (Hsu et al. 2018), and imbalances
in posterior WNT, FGF, and HH signaling result in defective hindgut development in mice and humans (Runck et al.
2014). As the GRNs downstream of these signaling pathways and Cdx activity are still only partially understood, the
rich resource of Cdx-regulated genes in Xenopus, combined
with the robust ability to test epistatic relationships among
signaling pathways and screen the effect of mutations in
potentially causative factors, has the potential for Xenopus
to provide needed insight into congenital syndromes affecting the intestine.
Extensive work in Xenopus has also identif ed conserved
Wnt/PCP-dependent mechanisms regulating intestinal morphogenesis. During NF32-NF46, dynamic changes in intestinal endoderm cell properties, including cell-cell adhesion
and microtubule architecture, are regulated by Wnt/PCP signaling mediators (Rho, ROCK, non-muscle Myosin, JNK);
loss of function of these factors disrupts endoderm cell intercalation, epithelial morphogenesis, and gut tube elongation
(Reed et al. 2009; Dush and Nascone-Yoder 2019). Such cell
biological insights into intestinal development are also relevant to cancer, as many parallels exist between tumor pathogenesis and early embryo development. Indeed, Xenopus
has been a useful model for studying cancer (Hardwick and
Philpott 2018; see Chapter 21 of this book).
18.6. CONCLUDING REMARKS
Xenopus continues to be a powerful system to study fundamental mechanisms of digestive and respiratory system
organogenesis and disease. As high-throughput genomics,
genome editing, proteomics, and pharmacological screening are now routine in Xenopus, functional genomics and
human disease modeling at a systems level is accelerating.
We feel confdent that future Xenopus research will continue
to provide additional signifcant insights into both normal
human organogenesis and congenital malformations.
ACKNOWLEDGMENTS
This work is supported in part by grants P41HD054556,
P01HD093363, and R01 DK123092 to AMZ, and we
acknowledge Xenbase, the Xenopus model organism knowledge base (RRID:SCR_003280), and the National Xenopus
Resource (RRID:SCR_013731) for their support. Drawings
of Xenopus embryos are reproduced with permission from
Xenbase and Natalya Zahn, licensed under CC BY-NC 4.0.
To view a copy of this license, visit http://creativecommons.
org/licenses/by-nc/4.0/ .
REFERENCES
Afelik, S., Chen, Y., & Pieler, T. (2006). Combined ectopic expression of Pdx1 and Ptf1a/p48 results in the stable conversion of
posterior endoderm into endocrine and exocrine pancreatic
tissue. Genes & Development, 20 (11), 1441–1446.
Afouda, B. A., Nakamura, Y., Shaw, S., Charney, R. M., Paraiso,
K. D., Blitz, I. L., Cho, K., & Hoppler, S. (2020). Foxh1/
Nodal def nes context-specifc direct maternal Wnt/β-Catenin
target gene regulation in early development. iScience, 23(7),
101314.
Agius, E., Oelgeschläger, M., Wessely, O., Kemp, C., & De
Robertis, E. M. (2000). Endodermal Nodal-related signals
and mesoderm induction in Xenopus. Development, 127(6),
1173–1183.
Arima, K., Shiotsugu, J., Niu, R., Khandpur, R., Martinez, M.,
Shin, Y., Koide, T., Cho, K. W., Kitayama, A., Ueno, N.,
Chandraratna, R. A., & Blumberg, B. (2005). Global analysis
of RAR-responsive genes in the Xenopus neurula using cDNA
microarrays. Developmental Dynamics, 232 (2), 414–431.
Asashima, M., Nakano, H., Shimada, K., Kinoshita, K., & Ishii, K.
(1990). Mesodermal induction in early amphibian embryos
by activin A (erythroid differentiation factor). Roux’s archives
of developmental biology: the offcial organ of the EDBO,
198(6), 330–335. https://doi.org/10.1007/BF00383771
Blitz, I. L., Paraiso, K. D., Patrushev, I., Chiu, W., Cho, K., &
Gilchrist, M. J. (2017). A catalog of Xenopus tropicalis transcription factors and their regional expression in the early
gastrula stage embryo. Developmental Biology, 426(2),
409–417.
Blythe, S. A., Cha, S. W., Tadjuidje, E., Heasman, J., & Klein, P.
S. (2010). Beta-Catenin primes organizer gene expression by
recruiting a histone H3 arginine 8 methyltransferase, Prmt2.
Developmental Cell, 19 (2), 220–231.
Briggs, J. A., Weinreb, C., Wagner, D. E., Megason, S., Peshkin,
L., Kirschner, M. W., & Klein, A. M. (2018). The dynamics
of gene expression in vertebrate embryogenesis at single-cell
resolution. Science, 360 (6392), eaar5780.
Brosens, E., Ploeg, M., van Bever, Y., Koopmans, A. E., IJsselstijn,
H., Rottier, R. J., Wijnen, R., Tibboel, D., & de Klein, A.
(2014). Clinical and etiological heterogeneity in patients with
tracheo-esophageal malformations and associated anomalies.
European Journal of Medical Genetics, 57 (8), 440–452.
Brown, D. D., & Cai, L. (2007). Amphibian metamorphosis.
Developmental Biology, 306 (1), 20–33.
Buchholz, D. R. (2015). More similar than you think: Frog metamorphosis as a model of human perinatal endocrinology.
Developmental Biology, 408 (2), 188–195.
Cebola, I., Rodríguez-Seguí, S. A., Cho, C. H., Bessa, J., Rovira,
M., Luengo, M., Chhatriwala, M., Berry, A., Ponsa-Cobas,
J., Maestro, M. A., Jennings, R. E., Pasquali, L., Morán, I.,
Castro, N., Hanley, N. A., Gomez-Skarmeta, J. L., Vallier,
L., & Ferrer, J. (2015). TEAD and YAP regulate the enhancer
network of human embryonic pancreatic progenitors. Nature
Cell Biology, 17 (5), 615–626.