Research in my lab has recently become involved in tissue morphogenesis, from the molecular/cellular to the physiological level, confronting questions concerning the positioning of organs in the body as well as how they attain their characteristic shapes.
By using Drosophila, the fruit fly, we are able to use a combination of molecular, genetic, cellular, microscopical/live-imaging and physiological techniques to analyse the development of a simple tubular epithelium, the renal or Malpighian tubules. Our overall aim has been to understand how an epithelial tissue develops during embryogenesis. More recently our focus has been on the post-mitotic morphogenetic movements that achieve tubule elongation, stereotypic positioning relative to other organs in the body cavity and the 3-dimensional looped shape that is characteristic of excretory tubules in a wide variety of organisms including man. By tracking living cells we have established that these morphogenetic movements result from the circumferential convergence of neighbouring cells to produce tubule narrowing and extension. This depends on the establishment of the proximo-distal axis of polarity in the tissue through graded EGF signalling from the distally-placed tubule tip cell lineage, which acts as a localised source. The establishment of tissue polarity results in cell movements that rely on highly dynamic and asymmetric activity of the actin-myosin cytoskeleton.
We have uncovered interactions between specific subsets of tubule cells and other organs, which regulate tubule positioning and their final looped structure. Firstly haemocytes lay down ECM components, sensitising a subset of tubule cells to TGFb signals from other tissues, which act as guidepost cues as the tubules extend through the body. In a second study we have shown that serial interactions between tubule tip cells and alary muscles oppose these guided movements to establish tubule position within the body and to preserve the characteristic tubule looping. Physiological analysis reveals the functional importance of tubule elongation, looping and positioning in the body cavity.
An additional area of interest is in the differentiation of specific excretory cell types both in the developing tubules and around the heart and gut, the nephrocytes . An ongoing project is to unravel the pathways required for the normal specification and differentiation of tubule tip cells. More recently we have also focused on the pathways that regulate the specification and differentiation of the two secretory cell types that together orchestrate the regulated secretion of primary urine, the Principal and Stellate cells. In both projects we have found that it is the interaction of activating and inhibitory signalling pathways that lead to the reliable and patterned differentiation of the specialised cell types that ultimately produce a physiologically responsive tissue.
The nephrocytes filter, endoctyose, metabolise and sequester haemolymph constituents. We have shown that the filtration diaphragm of nephrocytes bears striking morphological, genetic, protein and functional similarities to the slit diaphragm of mammalian podocytes. In ongoing projects we are analysing the development of these structures and the role of specific genes, that are conserved between flies and mammals, in diaphragm stabilisation and in the endocytic pathways that are active in nephrocytes.