Tbx5 and evolution of ventricular septation
Human heart is responsible for pumping blood through the blood vessels by repeated, rhythmic contractions and similar thing holds true for majority of animals on this planet. In spite of looking so different in various animals ,the molecular pathways controlling heart formation and differentiation are evolutionarily conserved.
Similar rules are followed for specification of heart precursors , be it in Drosophila or frog or mouse or annelids or humans. Just to cite an example if one takes an example of a NK gene(NK4 or tinman).Tinman is broadly expressed in mesoderm of flies and later restricted to cardiac mesoderm where it plays vital role is Heart Development. Tinaman/NK4 or NKx 2.5, in which ever species it is known, performs a similar role of heart or dorsal tube patterning.Tinman homologs in Humans ,Mouse,Amphioxus, Ciona and Chicken are known to pattern heart, which is an fantastic exhibition of functional conservation.Recently, the expression of the homologue of NK4/tinman in the pulsatile dorsal vessel of annelid Platynereis has been shown.
Like humans ,birds,crocodiles and other mammals exhibit four chambered heart with two atria and two lower ventricles. Amphibians have just one ventricle, but when it comes to most reptiles, the situation is rather unclear.During evolution when animals started moving to land ,there was a need for more sophisticated circulatory systems , in order to adopt to high-energy terrestrial life.This is the reason why we see a complete four chambered heart with complete division in ventricle also(which is not the case in amphibians) in birds,crocodiles and mammals. Four chambered heart has allowed a separate pulmonary and systemic circulatory systems ,which is vital for warm blooded animals.
Things become rather fuzzy when it comes to other reptiles : do they possess a single ventricular chamber or two incompletely septated ventricles? To address this question Kazuko Koshiba-Takeuchi et al; undertook new embryological study of a lizard (the green anole) and a turtle (the slider turtle),focusing on expression of a vital gene in Heart development ,a T-box transcription factor gene Tbx5.
Members of the T-box family of proteins play a fundamental role in patterning the developing vertebrate heart.The T-box encodes a 180 amino acid domain that binds to DNA. Genes encoding T-box proteins are found in a wide range of animals, but not in other kingdoms such as plants.
Authors saw that in both reptiles, red-eared slider turtle, Trachemys scripta elegans (a chelonian), and the
green anole, Anolis carolinensis (a squamate),Tbx5 is expressed homogeneously in the ventricular chamber. Interesting when one check for the expression of Tbx5 in embryonic tissues of birds and mammals , it is only restricted to precursor cells of left ventricle.Later when they checked the expression of Tbx5 in development, in turtles but not in lizards,(remember in lizards there is just one ventricle and Turtle has a primitive septum that partially separates the ventricles into left and right sides) it is later restricted to the left side of the chamber. This gives us an idea that altered expression of Tbx5 could be a possible evolutionary force responsible for embryonic heart patterning required in order to get used to tough terrestrial life.This remarkable study clearly demonstrated that the expression of the T-box transcription factor Tbx5 is responsible for the division of an ancestral single ventricle into two chambers in land animals.
The above hypothesis was further supported by the experiments performed in mouse.The complete loss of Tbx5 results in single ventricle and in gain of function studies ,where in when you misexpress Tbx5 throughout the developing myocardium, mimicing the reptilian expression pattern also results in a single mispatterned ventricular chamber lacking septation. These experiments show that for septation in ventricles is achieved by proper positioning of Tbx5 gradient during development.
Reference:
Reptilian heart development and the molecular basis of cardiac chamber evolution
Kazuko Koshiba-Takeuchi, Alessandro D. Mori, Bogac L. Kaynak, Judith Cebra-Thomas, Tatyana Sukonnik, Romain O. Georges, Stephany Latham, Laural Beck, R. Mark Henkelman, Brian L. Black, Eric N. Olson, Juli Wade, Jun K. Takeuchi, Mona Nemer, Scott F. Gilbert & Benoit G. Bruneau
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