Nanobodies as novel tools to study morphogen function "in vivo"

Nanobodies are small, monomeric antibody mimetic proteins produced by members of the camelid family (camels and llamas), that can be engineered by fusion to proteins carrying a specific function. These “functionalized” protein binders emerge as novel tools for protein manipulation in vivo. During my PhD studies I have generated scaffold-bound nanobodies (SBNs) specific to EGFP in order to interfere with gradient formation of a EGFP-tagged version of the Decapentaplegic (EGFP::Dpp) morphogen. Morphogens are secreted signaling molecules forming concentration gradients and controlling organ patterning and growth during animal development. Drosophila Decapentaplegic (Dpp) is one of the best studied morphogens, but it remains unclear how its concentration gradient is established and how it and controls patterning and growth of the Drosophila wing imaginal disc. In this PhD Thesis I summarize the development and characterization of SBNs and their applications in studying the formation and function of the Decapentaplegic morphogen gradient in the Drosophila melanogaster wing imaginal disc. In the first part of this Thesis, I will discuss how SBNs allowed us to investigate the importance of the Dpp gradient on proliferation and growth control of the wing imaginal disc. Using morphotrap, a SBN that localizes to the outer cell surface, we could completely block gradient formation and study the effect of a loss of the Dpp gradient on patterning and growth. We find that induction of Dpp target genes, and hence patterning, directly depends on the spreading of Dpp. Furthermore, we show that the Dpp gradient is crucial for growth and size control of the medial wing disc region. Moreover, we find that the Dpp gradient is not necessary for proliferation and size control of the lateral region of the wing disc. This data challenges previously published growth models, in which growth control solely depends on the signaling dynamics of Dpp. In the second part of this Thesis I investigate the mechanism of Dpp gradient formation in the wing disc. The wing disc is a complex three-dimensional structure, consisting of two contiguous epithelial layers. How the long-range Dpp gradient is established in the wing disc remains controversial. I have created different SBNs that localize to specific subcellular regions along the apicobasal axis. These SBNs allow us to reduce or block the dispersal of specific gradient subfractions and assess their contribution to wing development. We find that EGFP::Dpp disperses along three main routes: within the epithelial plane of the wing disc, in the luminal cavity between the two epithelial layers and along the basal lamina. Preliminary results suggest that these subfractions encode for different functions of Dpp. While we find that the patterning function of Dpp is encoded by the basolateral subfractions, the growth function of Dpp seems to be influenced by all three subfraction. Further experiments will investigate how target cells perceive and integrate Dpp input from these different subfractions

Myosin II is not required for Drosophila tracheal branch elongation and cell intercalation

The Drosophila tracheal system consists of an interconnected network of monolayered epithelial tubes that ensures oxygen transport in the larval and adult body. During tracheal dorsal branch (DB) development, individual DBs elongate as a cluster of cells, led by tip cells at the front and trailing cells in the rear. Branch elongation is accompanied by extensive cell intercalation and cell lengthening of the trailing stalk cells. Although cell intercalation is governed by Myosin II (MyoII)-dependent forces during tissue elongation in the Drosophila embryo that lead to germ-band extension, it remained unclear whether MyoII plays a similar active role during tracheal branch elongation and intercalation. Here, we have used a nanobody-based approach to selectively knock down MyoII in tracheal cells. Our data show that, despite the depletion of MyoII function, tip cell migration and stalk cell intercalation (SCI) proceed at a normal rate. This confirms a model in which DB elongation and SCI in the trachea occur as a consequence of tip cell migration, which produces the necessary forces for the branching process

Formation of polarized contractile interfaces by self-organized Toll-8/Cirl GPCR asymmetry

International audienc

Forward and feedback control mechanisms of developmental tissue growth

International audienc

Protein binders and their applications in developmental biology

Developmental biology research would benefit greatly from tools that enable protein function to be regulated, both systematically and in a precise spatial and temporal manner, in vivo In recent years, functionalized protein binders have emerged as versatile tools that can be used to target and manipulate proteins. Such protein binders can be based on various scaffolds, such as nanobodies, designed ankyrin repeat proteins (DARPins) and monobodies, and can be used to block or perturb protein function in living cells. In this Primer, we provide an overview of the protein binders that are currently available and highlight recent progress made in applying protein binder-based tools in developmental and synthetic biology

A nanobody-based toolset to investigate the role of protein localization and dispersal in Drosophila

The role of protein localization along the apical-basal axis of polarized cells is difficult to investigate in vivo, partially due to lack of suitable tools. Here, we present the GrabFP system, a collection of four nanobody-based GFP-traps that localize to defined positions along the apical-basal axis. We show that the localization preference of the GrabFP traps can impose a novel localization on GFP-tagged target proteins and results in their controlled mislocalization. These new tools were used to mislocalize transmembrane and cytoplasmic GFP fusion proteins in the Drosophila wing disc epithelium and to investigate the effect of protein mislocalization. Furthermore, we used the GrabFP system as a tool to study the extracellular dispersal of the Decapentaplegic (Dpp) protein and show that the Dpp gradient forming in the lateral plane of the Drosophila wing disc epithelium is essential for patterning of the wing imaginal disc

BMP morphogen gradients in flies

Bone morphogenetic proteins (BMPs) act as morphogens to control patterning and growth in a variety of developing tissues in different species. How BMP morphogen gradients are established and interpreted in the target tissues has been extensively studied in Drosophila melanogaster. In Drosophila, Decapentaplegic (Dpp), a homologue of vertebrate BMP2/4, acts as a morphogen to control dorsal-ventral patterning of the early embryo and anterior-posterior patterning and growth of the wing imaginal disc. Despite intensive efforts over the last twenty years, how the Dpp morphogen gradient in the wing imaginal disc forms remains controversial, while gradient formation in the early embryo is well understood. In this review, we first focus on the current models of Dpp morphogen gradient formation in these two tissues, and then discuss new strategies using genome engineering and nanobodies to tackle open questions

Myosin II is not required for Drosophila tracheal branch elongation and cell intercalation

The Drosophila tracheal system consists of an interconnected network of monolayered epithelial tubes that ensures oxygen transport in the larval and adult body. During tracheal dorsal branch (DB) development, individual DBs elongate as a cluster of cells, led by tip cells at the front and trailing cells in the rear. Branch elongation is accompanied by extensive cell intercalation and cell lengthening of the trailing stalk cells. Although cell intercalation is governed by Myosin II (MyoII)-dependent forces during tissue elongation in the Drosophila embryo that lead to germ-band extension, it remained unclear whether MyoII plays a similar active role during tracheal branch elongation and intercalation. Here, we have used a nanobody-based approach to selectively knock down MyoII in tracheal cells. Our data show that, despite the depletion of MyoII function, tip cell migration and stalk cell intercalation (SCI) proceed at a normal rate. This confirms a model in which DB elongation and SCI in the trachea occur as a consequence of tip cell migration, which produces the necessary forces for the branching process

Dpp spreading is required for medial but not for lateral wing disc growth

Drosophila Decapentaplegic (Dpp) has served as a paradigm to study morphogen-dependent growth control. However, the role of a Dpp gradient in tissue growth remains highly controversial. Two fundamentally different models have been proposed: the ‘temporal rule’ model suggests that all cells of the wing imaginal disc divide upon a 50% increase in Dpp signalling, whereas the ‘growth equalization model’ suggests that Dpp is only essential for proliferation control of the central cells. Here, to discriminate between these two models, we generated and used morphotrap, a membrane-tethered anti-green fluorescent protein (GFP) nanobody, which enables immobilization of enhanced (e)GFP::Dpp on the cell surface, thereby abolishing Dpp gradient formation. We find that in the absence of Dpp spreading, wing disc patterning is lost; however, lateral cells still divide at normal rates. These data are consistent with the growth equalization model, but do not fit a global temporal rule model in the wing imaginal disc

Growth anisotropy of the extracellular matrix shapes a developing organ

Final organ size and shape result from volume expansion by growth and shape changes by contractility. Complex morphologies can also arise from differences in growth rate between tissues. We address here how differential growth guides the morphogenesis of the growing Drosophila wing imaginal disc. We report that 3D morphology results from elastic deformation due to differential growth anisotropy between the epithelial cell layer and its enveloping extracellular matrix (ECM). While the tissue layer grows in plane, growth of the bottom ECM occurs in 3D and is reduced in magnitude, thereby causing geometric frustration and tissue bending. The elasticity, growth anisotropy and morphogenesis of the organ are fully captured by a mechanical bilayer model. Moreover, differential expression of the Matrix metalloproteinase MMP2 controls growth anisotropy of the ECM envelope. This study shows that the ECM is a controllable mechanical constraint whose intrinsic growth anisotropy directs tissue morphogenesis in a developing organ