Introduction:
Looks might be really deceiving and this statement holds true in case of heart formation in organisms. Even though Drosophila heart which looks very different from its vertebrates counter parts, but The molecular pathways controlling heart formation and differentiation are evolutionarily conserved. The similar rules in forming heart in flies and higher vertebrates and availability for various tools in Drosophila , helped it in emerging as an excellent model system to study cardiac development, function and aging.
Heart Development:
In Drosophila melanogaster, heart precursor cells originate from the dorsal-most region of the mesoderm in response to Decapentaplegic (dpp) and wingless (wg) signalling. These cardiac precursor cells later form heart-like organ called the dorsal vessel. The Drosophila heart is a simple organ composed of two major cell types: cardioblasts, which form the simple contractile tube of the heart, and pericardial cells, which flank the cardioblasts.
The development of embryonic Drosophila heart can be viewed under two parts:
1) Cardiac specification : Results from combined action of various transcription factors and signaling pathways ensuring proper specification and position of cardiac mesoderm.
2) Cardiac morphogenesis: Deals with cell migration cell polarization, assembly into a linear tube, and terminal differentiation of already specified cells.
Cardiac Specification:
dpp activates its target tinman and dpp is also required for maintainence of tinman ,these two play vital roles in establishing dorsal mesoderm fates.During gastrulation and mesoderm migration, dpp is expressed in a broadband of cells within the dorsal ectoderm and signals across germ layers to pattern the underlying dorsal mesoderm similar to another signalling molecule wingless.Genetic studies prove beyond doubt that cardiac progenitors cells are formed in mesoderm where both wg and dpp intersect.
Apart from these signaling events mentioned above, interactions among various transcription factors in mesoderm ensure proper specification of heart in flies. tinman/NKx 2.5 , a NK gene leads the pack of transcription factors, which along with GATA protein pannier and set of T-BOX proteins Dorso cross (doc 1,doc2 and doc3) promote cardiac specification.
The two signaling proteins dpp, wg and three transcription factors tinman, pannier, dorso cross are involved in complex interactions among themselves which is required for either activating or maintaining expression of all five involved. Similar to the heart in vertebrates at the linear tube stage, the Drosophila ‘dorsal vessel’ is also built with an A-P polarity. Hedgehog and Ras signaling along with Nk gene lady bird ,homeobox gene even skipped and Seven up transcription factors Takes care of AP aspect of progenitor cardiac cells
Hox Genes:
Its very difficult to keep Hox genes away for long in any developmental pathway. Apart from segmental A-P of heart tube, further broad division of Heart tube into three parts also occurs and thus brings Hox genes into picture. Heart tube can be divided into Aorta and Heart. Aorta again into anterior(formed in segments T3-A1 by Antp hox gene) and posterior aorta ( A2-A4) results from action of Ultrabithorax Hox gene. Heart proper is formed in segments A5-A7 and Abdominal-A Hox gene plays premier role for its formation. Heart contains inlet-outlet valves for hemolymph, called as Ostia. There is also one valve present between Aorta and Heart. In flies mutant for Abdominal-A gene heart is transformed into aorta according to posterior prevalence rule Hox proteins.
2) Cardiac morphogenesis:
Proteins involved in morphogenesis are mostly cell adhesion and extracellular matrix (ECM) molecules, membrane-associated glycoproteins,and intracellular cell membrane-associated proteins.These proteins make sure that heart tube is properly patterned once they are specified in mesoderm.
Five different types of proteins essential for cardiac morphogenesis are listed below:
Transcription factors : Nmr / T-Box 20
Cell adhesion molecules : faint sausage, laminin A, E-cadherin
ECM molecules: Pericardin, Dystroglycan
Guidance molecules : Slit, Robo
Cell polarity genes : PDZ protein discs large, a-Spectrin, Armadillo/b-catenin, Toll.
These proteins along with those involved in cardiac specification process ensure proper functioning of heart in Drosophila.
Here i tried to summarize the important events in embryonic heart development in Drosophila , for detailed study including remodeling of heart in adult fly refer the following articles:
1) Heart Development in Drosophila
Li Qian, Jiandong Liu, Rolf Bodmer
Pages 1-29, Volume 18,Cardiovascular Development,Advances in Developmental Biology.(Book Series).
2) Drosophila cardiac tube organogenesis requires multiple phases of Hox activity.
Perrin L, Monier B, Ponzielli R, Astier M, Semeriva M.
Dev Biol. 2004 Aug 15;272(2):419-31.
3) Steroid-dependent modification of Hox function drives myocyte reprogramming in the Drosophila heart.
Monier B, Astier M, Semeriva M, Perrin L.
Development. 2005 Dec;132(23):5283-93.