Formation of the Left-Right Axis;
15 Insights from the Xenopus Model
Axel Schweickert and Tim Ott
CONTENTS
15.1. Historical Background ............................................................................................................................................. 225
15.2. Present State of the Field ......................................................................................................................................... 225
15.2.1. The Nodal Cascade Dictates Laterality ..................................................................................................... 225
15.2.2. The Left-Right Organizer .......................................................................................................................... 226
15.2.3. The Leftward Flow Target: Dand5 ............................................................................................................ 226
15.2.4. Post-Transcriptional Regulation of the dand5 mRNA .............................................................................. 228
15.2.5. Evolution of Dand5 as Flow Target .......................................................................................................... 229
15.3. Future Directions and Important Questions ............................................................................................................. 229
References ............................................................................................................................................................................ 230
15.1. HISTORICAL BACKGROUND
Organ asymmetries are found throughout the animal kingdom, referred to as asymmetric positioning, asymmetric
morphology, or both, as exemplifed by the vertebrate heart
(Blum et al., 2014). The evolutionary origin of organ asymmetries may have arisen with the necessity for a longer-thanbody length gut that allows effcient retrieval of nutrients and
the need to stow this long gut in the body cavity in an orderly
manner ( Blum et al., 2014; Blum and Ott, 2018a). Vertebrate
organ asymmetries (situs solitus) are quite sophisticated. In
humans, the apex of the asymmetrically built heart, with two
atria and ventricles each that connect to lung and body circulation, points to the left. The lung in turn, due to space
restrictions, has fewer lobes on the left than on the right side
(in humans two and three, respectively). Stomach and spleen
are found on the left and the liver on the right. The small
and large intestines coil in a chiral manner. In very rare
cases (1:10.000), the organ situs is inverted (situs inversus).
Heterotaxia describes another rare situation (about 1:10.000),
in which subsets of organs show normal or aberrant positioning and/or morphology, which inevitably are associated
with severe disease syndromes (Duncan and Khokha, 2016;
Grimes and Burdine, 2017; Hamada et al., 2002)
The knowledge of human organ asymmetry date back to
the antiquity, as mentioned in Aristotle’s history of animals.
Interestingly, scientifc approaches to understand left-right
(LR) development were performed in the 19th century using
chick embryos (Blum et al., 1999). The frst systematic experimental analysis of the LR axis was conducted in the early
1920s by Hans Spemann and co-workers before their focus
shifted towards the gastrula organizer phenomenon. They
used two main approaches: (1) Regional ablations and transplantations at gastrula and neurula stages showed that the
left side contains a specifc information which is required for
proper LR development. (2) Experimental induction of double
axes, twinned embryos, to address the pathological outcomes
in human conjoined twins, in which specifcally the right, but
not the left, twin exhibits heterotaxia in 50% of cases (see
Blum et al., 2009, 1999; Tisler et al., 2017a, for details and
original references). Based on the work of many laboratories,
including ours, we now understand the molecular basis of
both observations by Spemann and co-workers. Basically, two
spatially and temporally distinct processes were interfered
with at the time: propagation of Nodal signaling in the left
lateral plate mesoderm (LPM) and ciliary-based symmetry
breakage at the left-right organizer (LRO), respectively (see
the following).
15.2. PRESENT STATE OF THE FIELD
15.2.1. THE NODAL CASCADE DICTATES LATERALITY
Modern research on the molecular basis of LR asymmetry
principally started with the identifcation of nodal, which
encodes a Tgfß growth factor transiently expressed exclusively in the left LPM. nodal transcription is regulated by
a positive feedback loop. In addition, Nodal induces the
expression of the Tgfß feedback inhibitor lefty. Lefty in turn
terminates Nodal signaling by binding to Nodal and to its
cognitive receptor complex ( Shiratori and Hamada, 2014 ).
An additional Nodal target, the homeobox transcription factor pitx2, which we accidentally identifed in a PCR screen
for goosecoid-related homeobox genes in mouse ( Campione
et al., 1999 ), became an important tool to study the LR axis.
Unlike the transient nodal and lefty asymmetry, left-sided
pitx2 expression persists to later developmental stages and
mediates left positional identity throughout organogenesis
( Blum et al., 1999 ; Campione et al., 1999 ). Because
transcriptional regulation is direct in all cases, expression of
DOI: 10.1201/9781003050230-18
225
15 Insights from the Xenopus Model
Axel Schweickert and Tim Ott
CONTENTS
15.1. Historical Background ............................................................................................................................................. 225
15.2. Present State of the Field ......................................................................................................................................... 225
15.2.1. The Nodal Cascade Dictates Laterality ..................................................................................................... 225
15.2.2. The Left-Right Organizer .......................................................................................................................... 226
15.2.3. The Leftward Flow Target: Dand5 ............................................................................................................ 226
15.2.4. Post-Transcriptional Regulation of the dand5 mRNA .............................................................................. 228
15.2.5. Evolution of Dand5 as Flow Target .......................................................................................................... 229
15.3. Future Directions and Important Questions ............................................................................................................. 229
References ............................................................................................................................................................................ 230
15.1. HISTORICAL BACKGROUND
Organ asymmetries are found throughout the animal kingdom, referred to as asymmetric positioning, asymmetric
morphology, or both, as exemplifed by the vertebrate heart
(Blum et al., 2014). The evolutionary origin of organ asymmetries may have arisen with the necessity for a longer-thanbody length gut that allows effcient retrieval of nutrients and
the need to stow this long gut in the body cavity in an orderly
manner ( Blum et al., 2014; Blum and Ott, 2018a). Vertebrate
organ asymmetries (situs solitus) are quite sophisticated. In
humans, the apex of the asymmetrically built heart, with two
atria and ventricles each that connect to lung and body circulation, points to the left. The lung in turn, due to space
restrictions, has fewer lobes on the left than on the right side
(in humans two and three, respectively). Stomach and spleen
are found on the left and the liver on the right. The small
and large intestines coil in a chiral manner. In very rare
cases (1:10.000), the organ situs is inverted (situs inversus).
Heterotaxia describes another rare situation (about 1:10.000),
in which subsets of organs show normal or aberrant positioning and/or morphology, which inevitably are associated
with severe disease syndromes (Duncan and Khokha, 2016;
Grimes and Burdine, 2017; Hamada et al., 2002)
The knowledge of human organ asymmetry date back to
the antiquity, as mentioned in Aristotle’s history of animals.
Interestingly, scientifc approaches to understand left-right
(LR) development were performed in the 19th century using
chick embryos (Blum et al., 1999). The frst systematic experimental analysis of the LR axis was conducted in the early
1920s by Hans Spemann and co-workers before their focus
shifted towards the gastrula organizer phenomenon. They
used two main approaches: (1) Regional ablations and transplantations at gastrula and neurula stages showed that the
left side contains a specifc information which is required for
proper LR development. (2) Experimental induction of double
axes, twinned embryos, to address the pathological outcomes
in human conjoined twins, in which specifcally the right, but
not the left, twin exhibits heterotaxia in 50% of cases (see
Blum et al., 2009, 1999; Tisler et al., 2017a, for details and
original references). Based on the work of many laboratories,
including ours, we now understand the molecular basis of
both observations by Spemann and co-workers. Basically, two
spatially and temporally distinct processes were interfered
with at the time: propagation of Nodal signaling in the left
lateral plate mesoderm (LPM) and ciliary-based symmetry
breakage at the left-right organizer (LRO), respectively (see
the following).
15.2. PRESENT STATE OF THE FIELD
15.2.1. THE NODAL CASCADE DICTATES LATERALITY
Modern research on the molecular basis of LR asymmetry
principally started with the identifcation of nodal, which
encodes a Tgfß growth factor transiently expressed exclusively in the left LPM. nodal transcription is regulated by
a positive feedback loop. In addition, Nodal induces the
expression of the Tgfß feedback inhibitor lefty. Lefty in turn
terminates Nodal signaling by binding to Nodal and to its
cognitive receptor complex ( Shiratori and Hamada, 2014 ).
An additional Nodal target, the homeobox transcription factor pitx2, which we accidentally identifed in a PCR screen
for goosecoid-related homeobox genes in mouse ( Campione
et al., 1999 ), became an important tool to study the LR axis.
Unlike the transient nodal and lefty asymmetry, left-sided
pitx2 expression persists to later developmental stages and
mediates left positional identity throughout organogenesis
( Blum et al., 1999 ; Campione et al., 1999 ). Because
transcriptional regulation is direct in all cases, expression of
DOI: 10.1201/9781003050230-18
225