Manfred Frasch

Research

Genetic and Molecular Pathways Controlling Drosophila Mesoderm Differentiation

Our lab is studying the processes of mesoderm patterning and mesodermal tissue development in Drosophila. Genetic and molecular studies aim to define the identities and functions of regulatory molecules that spatially subdivide the mesodermal cell layer and determine the primordial cells of the heart, skeletal, and visceral musculatures at defined locations in the early embryo.

Our recent results have shown that several of these patterning processes involve the synergistic activities of mesoderm-specific transcription factors and signaling molecules that are secreted from the ectoderm. For example, the mesodermally-expressed homeobox genetinman functionally cooperates with the ectodermally-secreted factor Dpp, a TGF-b molecule of the BMP family, to subdivide the mesoderm into a ventral and a dorsal portion and to determine dorsal mesodermal derivatives. Molecularly, this process involves the binding of Dpp-activated signal transducers (the Smad proteins Mad and Medea), together with the Tinman protein itself, to an enhancer element of the tinman gene, which results in the spatially-restricted activation of tinman mRNA expression in the dorsal mesoderm. Downstream events that further subdivide the dorsal mesoderm into heart and gut muscle primordia also require the combined functions of Tinman and Dpp, together with additional spatially restricted cues such as Wingless, Hedgehog, and the forkhead domain protein Sloppy Paired. We are interested in finding out how these combinatorial activities cooperate at the molecular level to activate or repress developmental control genes in defined groups of cells and how some of these control genes (e.g., the homeobox genes bagpipe and S59/slouch, the forkhead domain encoding gene biniou, and additional genes which we are trying to identify) ultimately specify cell fates of the cardiac, visceral, and skeletal musculatures.

Interestingly, most of these molecules and regulatory pathways are evolutionarily conserved and it appears that mesoderm patterning and muscle/heart development in insect and vertebrate embryos involves many closely related mechanisms. Therefore, Drosophila can serve as an excellent model system for developmental processes of the heart, skeletal and gut muscles, as well as organogenesis of other internal organs, in vertebrate embryos.

Figure 1 Development of body wall muscles (top), heart (middle) and midgut muscles (bottom) during Drosophila embryogenesis. The embryos on the left are ca. 6 hrs. old and were stained to visualize primordial cells of the somatic muscles, heart, and midgut muscles, respectively. 10 hrs. later, these cells have differentiated to form the mature muscle tissues seen in the embryos on the right. (Markers, from top left to bottom right: rP298 enhancer trap/bGal antibodies; myosin heavy chain antibodies; Even-skipped Antibodies; Even-skipped + cardioblast-specific enhancer trap/bGal antibodies; bagpipe riboprobe; visceral mesoderm-specific enhancer trap/bGal antibodies)

Figure 2 Reciprocal arrangement of Bagpipe expressing cells (green) and Sloppy Paired expressing cells (red) in the early embryonic mesoderm. (Probes: Green: anti-Bagpipe; red: anti Sloppy Paired; done by Hsiu-Hsiang Lee)

Figure 3 Development of the midgut musculature during embryogenesis. The bagpipe expressing cells have merged into a band of trunk visceral mesoderm (red) which will form circular midgut muscles. The cells of the caudal visceral mesoderm (green) migrate along the trunk visceral mesoderm and will form an outer layer of longitudinal midgut muscles. (Probes: Red: anti-fasciclin III; green: tinman-lacZ)

Figure 4 The homeobox gene slouch (S59) specifies the identities of a small subset of body wall muscles. This confocal image shows that three of the thirty muscle fibers in each hemisegment maintain slouch expression. (Probes: Blue: slouch-lacZ/anti bGal; Green: anti myosin heavy chain; done by Stefan Knirr)