FGF /FGFR Signal Induces Trachea Extension in the Drosophila Visual System

Citation: Chu W-C, Lee Y-M, Henry Sun Y ( FGF /FGFR Signal Induces Trachea Extension in the Drosophila Visual System Wei-Chen Chu 0 Yuan-Ming Lee 0 Yi Henry Sun 0 Christos Samakovlis, Stockholm University, Sweden 0 1 Graduate Institute of Life Sciences, National Defense Medical Center , Taipei, Taiwan , 2 Institute of Molecular Biology, Academia Sinica , Taipei, Taiwan , 3 Department of Life Sciences and Institute of Genome Sciences, National Yang-Ming University , Taipei , Taiwan The Drosophila compound eye is a large sensory organ that places a high demand on oxygen supplied by the tracheal system. Although the development and function of the Drosophila visual system has been extensively studied, the development and contribution of its tracheal system has not been systematically examined. To address this issue, we studied the tracheal patterns and developmental process in the Drosophila visual system. We found that the retinal tracheae are derived from air sacs in the head, and the ingrowth of retinal trachea begin at mid-pupal stage. The tracheal development has three stages. First, the air sacs form near the optic lobe in 42-47% of pupal development (pd). Second, in 47-52% pd, air sacs extend branches along the base of the retina following a posteriorto-anterior direction and further form the tracheal network under the fenestrated membrane (TNUFM). Third, the TNUFM extend fine branches into the retina following a proximal-to-distal direction after 60% pd. Furthermore, we found that the trachea extension in both retina and TNUFM are dependent on the FGF(Bnl)/FGFR(Btl) signaling. Our results also provided strong evidence that the photoreceptors are the source of the Bnl ligand to guide the trachea ingrowth. Our work is the first systematic study of the tracheal development in the visual system, and also the first study demonstrating the interactions of two well-studied systems: the eye and trachea. - Funding: This study was supported by grants to Y.H.S. [NSC 96-2321-B-001-002, 97-2321-B-001-002, 98-2321-B-001-034, 99-2321-B-001-016, 100-2321-B-001-012] from the National Science Council of the Republic of China (http://web1.nsc.gov.tw/mp.aspx?mp=7). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. As an organ grows in size, its surface to volume ratio decreases, and simple diffusion through the surface is not sufficient to support the exchange of nutrients, wastes and gases. In vertebrates, the vascular systems form highly branched networks to fulfill these transport needs. In insects, the tracheal system formed by a network of hollow tubes takes care of the gas exchanges by passive diffusion or by active transport during flight [1]. The tracheal system in the Drosophila embryo has been extensively studied [27]. The embryonic tracheal development begins from the specification of distinct tracheal placodes in the posterior thoracic and abdominal segments by spatial patterning genes. The placode cells express two transcription factors, Trachealess (Trh) and Ventral veinless (Vvl), that together specify the tracheal fate. The tracheal placodes invaginate to form tracheal sacs and these cells undergo one round of mitosis to generate the final number of about 80 cells per tracheal metamere. Further morphogenesis does not involve cell division. Subsets of tracheal cells then migrate along stereotypical directions to form distinct tracheal branches. The migration is dependent on the fibroblast growth factor (FGF) and FGF receptor (FGFR) signaling. All tracheal cells express the FGFR Breathless (Btl), induced by Trh and Vvl. The tracheal cells then migrate toward the source of FGF ligand Branchless (Bnl) expressed from target cells. The migration along distinct pathways also depends on integrin, EGF and Slit/Robo signalings. Adjacent and contralateral tracheal metameres are then connected by specialized fusion cells to form the interconnected tracheal network. The terminal cells can extend long subcellular tubes for close contact with cells in the target tissue. The patterns of primary and secondary branches are controlled by a hard-wired developmental program. In contrast, terminal branches are variable and regulated by the tissue oxygen requirement. Bnl expression is regulated by hypoxia to ensure tracheal structure matches the cellular oxygen requirement [8]. In addition to the target tissue, tracheal cells themselves can also sense hypoxia and regulate Btl expression for the tracheal branch remodeling [9]. Bnl/Btl signaling also regulates cell proliferation and migration of the trachea that associated with the larval wing disc (dorsal air sac primordium or tracheoblast) [1012]. The larval tracheal system is largely remodeled during metamorphosis [13]. The Drosophila compound eye contains 750~800 ommatidia (unit eyes), each composed of eight photoreceptor neurons (R1 ~ R8), four cone cells, two primary pigment cells, in addition to sharing secondary and tertiary pigment cells and the interommatidial bristles with adjacent ommatidia. The axons of photoreceptors project basally through the fenestrated membrane (FM) and terminate at different layers of the optic lobe. The energy metabolism of insect photoreceptor is predominantly aerobic [14], therefore it places a high demand on oxygen supply. It has been shown that function of retina is sensitive to hypoxia in many organisms including human, mice and honeybee [1517]. Oxygen transport to the visual system is therefore important to support its neuronal activities. The compound eye develops from the larval eye-antenna imaginal disc, which is composed of two epithelial sheets and does not contain tracheal cells. The photoreceptors begin to differentiate at third instar larval stage and the retina begins to thicken in the mid to late pupal stage. The thickening and increase in volume suggest a requirement for tracheal ingrowth to provide oxygen. Although it has been shown that many insects have tracheae in the retina with different distribution patterns [1823], the pattern and development of trachea in the compound eye of Drosophila is largely unknown. In this study, we examined the tracheal patterns in the Drosophila visual system and studied the molecular mechanism for its development. We generated a 3D reconstruction of the tracheal system in the adult compound eye and optic lobe. We found that the retinal tracheae are derived from air sacs in the head, and the ingrowth of retina trachea begin at mid-pupal stage. There are three major steps for the development of retinal trachea. First, air sacs become apparent near the optic lobe in 42-47% of pupal development (pd). Second, in 47-52% pd, air sacs extend branches along the fenestrated membrane following a posterior-to-anterior direction and further form the tracheal network under the fenestrated membrane (TN (...truncated)