Unit 15b Left right Vertebrates are left right Situs solitus normal Lungs Heart Liver Spleen Stomach The position of the organs in the body reflects laterality Normal positioning is known as situs solitus Patel A Honor E 2010 Nat Rev Nephrol doi L R asymmetry is established during early Gilbert 10e Fig 9 33 Humans can exhibit left right Situs solitus normal Situs inversus totalis Lungs Heart Liver Spleen Stomach Situs inversus totalis total organ reversal often healthy Heterotaxia randomization of organ position many defects Patel A Honor E 2010 Nat Rev Nephrol doi Situs inversus totalis results in dextrocardia heart Dextrocardia the heart is on the right instead of the left http pictures doccheck com de photos 10204 4888 situs Vertebrate L R axis specification uses Early events are poorly understood Some other components e g Shh are not obviously identical L R Organizer typically has cilia Nodal Pitx2 Pitx2 is a transcript ion factor Lefty1 Lefty1 is a Nodal inhibitor expressed along the midline midline Right Left adapted from Gilbert 10e Fig 9 15 Mammals have cilia in the Gilbert 10e Fig 9 33 Clues that humans require nodal cilia Kartagener s triad 50 situs inversus totalis Males have immotile heart sperm All patients have respiratory problems due to mucus accumulation in the bronchi and lungs http pictures doccheck com de photos 10204 4888 situs Karatagener s Triad results from loss of normal PCD Kartagener s triad is often called primary ciliary dyskinesia It results from loss of outer dynein arms leading to immotile cilia and flagella http upload wikimedia org wikipedia commons thumb 7 79 Kartagener svg 1000px L R dynein mutants misexpress Pitx2 No reversal Full reversal Both sides None L R dynein is required for intraflagellar transport IFT IFT transports material to and from the tips of cilia and is required for their maintenance iv mutant mice are mutant for this dynein and have laterality defects Blum et al 1999 Development 126 1225 1234 KIF3B mutants have abnormal cilia KIF3B is a kinesin required for intraflagellar transport IFT IFT transports material to the tip of cilia and is required for their maintenance Hirokawa 2000 Traffic1 29 34 KIF3B mutants express lefty2 bilaterally wildtype kif3b knockout lefty2 is a marker for the left side Conclusion cilia are needed for development of left right asymmetry Nonaka et al 1998 Cell 95 829 837 KIF3B mutants have abnormal flow WT kif3b KO Text kif3b knockouts have no flow Conclusion cilia are necessary for nodal flow Hirokawa 2000 Traffic 1 29 34 Overriding normal flow can reverse the L R axis Axis often reversed Node override flow Nonaka et al 2002 Nature 418 96 99 Conclusion Overriding flow can reverse the L R axis as often as 50 of the time This is similar to primary ciliary dyskinesia This also suggests nodal flow may be crucial in mammals Model normal flow in the node Model nodal vesicular parcels NVPs transport materials to the left side of the Node This triggers signaling there adapted from Gilbert that leads to L R asymmetries 9e Fig 8 35 Calcium is elevated on the left side of the node Gilbert 9e Fig 8 35 Cilia are in posterior axial structures in all vertebrates frog gastrocoel roof plate GRP mammals posterior notochord PNC mouse rabbit Blum et al 2009 Dev Dyn 238 1215 1225 Nodal cilia in fish are in Kupffer s vesicle Kupffer s vesicle comes from dorsal forerunner cells special cells that do not involute during gastrulation and come to lie at the posterior of the embryo They are ciliated Gilbert 10e Fig 8 47 Removing cells that make Kupffer s vesicle frequently results in L R reversals Normal L KV precursor cells removed heart mesoderm notochord Conclusion Kupffer s vesicle is necessary for L R specification Essner et al 2004 Development 132 1247 1260 R Vertebrate L R axis specification uses conserved components L R Organizer typically has cilia Nodal and Ptx2 are implicated in L R axis specification as a response to initial signals Nodal Pitx2 Pitx2 is a transcription factor Left Lefty1 Lefty1 is a Nodal inhibitor expressed along the midline midline Right adapted from Gilbert 10e Fig 9 15 Pitx2 is on the left side in frog embryos Left Right M I Chung et al 2012 Dev Biol 363 155 165 Pitx2 expression influences L R axis specification in frogs Wildtype Loss of pitx2 pitx2 mRNA injected injected expression or expression on the right side in fish frogs chick and mice results in laterality defects frog shown Rieger syndrome in humans laterality defects due to pitx2 mutation Gilbert 10e Fig 8 35 Gilbert 9e Fig 7 36 normal gut coiling reversed gut coiling Chick L R axis specification involves Nodal and Pitx2 Nodal mRNA Pitx2 mRNA L Gilbert 10e Fig 9 15 R Note flipped relative to book since the book image is a view from below from ventral side Mouse L R Axis involves Nodal and Pitx2 Nodal mRNA Pitx2 mRNA left view R L Oki et al 2007 Development 134 3893 3904 Blum et al 1999 Development 126 1225 1234 Chick Nodal regulates the location of Pitx2 Implanting cells that express Nodal induceds cells nearby to express Pitx2 Piedra et al 1998 Cell 94 319 324 Nodal mRNA Pitx2 mRNA Conclusion Nodal induces Pitx2 expression later