The Drosophila Hox gene Ultrabithorax controls appendage shape by regulating extracellular matrix dynamics

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.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Micromanagement of Drosophila Post-Embryonic Development by Hox Genes

Hox genes function early in development to determine regional identity in animals. Consequently, the loss or gain of Hox gene expression can change this identity and cause homeotic transformations. Over 20 years ago, it was observed that the role of Hox genes in patterning animal body plans involves the fine-scale regulation of cell fate and identity during development, playing the role of 'micromanagers' as proposed by Michael Akam in key perspective papers. Therefore, as well as specifying where structures develop on animal bodies, Hox genes can help to precisely sculpt their morphology. Here, we review work that has provided important insights about the roles of Hox genes in influencing cell fate during post-embryonic development in Drosophila to regulate fine-scale patterning and morphology. We also explore how this is achieved through the regulation of Hox genes, specific co-factors and their complex regulation of hundreds of target genes. We argue that further investigating the regulation and roles of Hox genes in Drosophila post-embryonic development has great potential for understanding gene regulation, cell fate and phenotypic differentiation more generally.

Atypical laminin spots and pull-generated microtubule-actin projections mediate Drosophila wing adhesion

During Drosophila metamorphosis, dorsal and ventral wing surfaces adhere, separate, and reappose in a paradoxical process involving cell-matrix adhesion, matrix production and degradation, and long cellular projections. The identity of the intervening matrix, the logic behind the adhesion-reapposition cycle, and the role of projections are unknown. We find that laminin matrix spots devoid of other main basement membrane components mediate wing adhesion. Through live imaging, we show that long microtubule-actin cables grow from those adhesion spots because of hydrostatic pressure that pushes wing surfaces apart. Formation of cables resistant to pressure requires spectraplakin, Patronin, septins, and Sdb, a SAXO1/2 microtubule stabilizer expressed under control of wing intervein-selector SRF. Silkworms and dead-leaf butterflies display similar dorso-ventral projections and expression of Sdb in intervein SRF-like patterns. Our study supports the morphogenetic importance of atypical basement-membrane-related matrices and dissects matrix-cytoskeleton coordination in a process of great evolutionary significance.

To form and function: on the role of basement membrane mechanics in tissue development, homeostasis and disease

The basement membrane (BM) is a special type of extracellular matrix that lines the basal side of epithelial and endothelial tissues. Functionally, the BM is important for providing physical and biochemical cues to the overlying cells, sculpting the tissue into its correct size and shape. In this review, we focus on recent studies that have unveiled the complex mechanical properties of the BM. We discuss how these properties can change during development, homeostasis and disease via different molecular mechanisms, and the subsequent impact on tissue form and function in a variety of organisms. We also explore how better characterization of BM mechanics can contribute to disease diagnosis and treatment, as well as development of better in silico and in vitro models that not only impact the fields of tissue engineering and regenerative medicine, but can also reduce the use of animals in research.

A corset function of exoskeletal ECM promotes body elongation in Drosophila

Body elongation is a general feature of development. Postembryonically, the body needs to be framed and protected by extracellular materials, such as the skeleton, the skin and the shell, which have greater strength than cells. Thus, body elongation after embryogenesis must be reconciled with those rigid extracellular materials. Here we show that the exoskeleton (cuticle) coating the Drosophila larval body has a mechanical property to expand less efficiently along the body circumference than along the anteroposterior axis. This “corset” property of the cuticle directs a change in body shape during body growth from a relatively round shape to an elongated one. Furthermore, the corset property depends on the functions of Cuticular protein 11 A and Tubby, protein components of a sub-surface layer of the larval cuticle. Thus, constructing a stretchable cuticle and supplying it with components that confer circumferential stiffness is the fly’s strategy for executing postembryonic body elongation.

Tajiri et al. describe how the cuticle coating the Drosophila larval body expands less efficiently along the body circumference than along the anteroposterior axis to drive body elongation. This “corset” property depends on cuticular proteins Cpr11A and Tubby, which may work together to change larval body shape.

The Broad Transcription Factor Links Hormonal Signaling, Gene Expression, and Cellular Morphogenesis Events During Drosophila Imaginal Disc Development

Imaginal disc morphogenesis during metamorphosis in Drosophila melanogaster provides an excellent model to uncover molecular mechanisms by which hormonal signals effect physical changes during development. The broad (br) Z2 isoform encodes a transcription factor required for disc morphogenesis in response to 20-hydroxyecdysone, yet how it accomplishes this remains largely unknown. Here, we use functional studies of amorphic br5 mutants and a transcriptional target approach to identify processes driven by br and its regulatory targets in leg imaginal discs. br5 mutants fail to properly remodel their basal extracellular matrix (ECM) between 4 and 7 hr after puparium formation. Additionally, br5 mutant discs do not undergo the cell shape changes necessary for leg elongation and fail to elongate normally when exposed to the protease trypsin. RNA-sequencing of wild-type and br5 mutant leg discs identified 717 genes differentially regulated by br, including a large number of genes involved in glycolysis, and genes that encode proteins that interact with the ECM. RNA interference-based functional studies reveal that several of these genes are required for adult leg formation, particularly those involved in remodeling the ECM. Additionally, brZ2 expression is abruptly shut down at the onset of metamorphosis, and expressing it beyond this time results in failure of leg development during the late prepupal and pupal stages. Taken together, our results suggest that brZ2 is required to drive ECM remodeling, change cell shape, and maintain metabolic activity through the midprepupal stage, but must be switched off to allow expression of pupation genes.

Atypical basement membranes and basement membrane diversity - what is normal anyway?

The evolution of basement membranes (BMs) played an essential role in the organization of animal cells into tissues and diversification of body plans. The archetypal BM is a compact extracellular matrix polymer containing laminin, nidogen, collagen IV and perlecan (LNCP matrix) tightly packed into a homogenously thin planar layer. Contrasting this clear-cut morphological and compositional definition, there are numerous examples of LNCP matrices with unusual characteristics that deviate from this planar organization. Furthermore, BM components are found in non-planar matrices that are difficult to categorize as BMs at all. In this Review, I discuss examples of atypical BM organization. First, I highlight atypical BM structures in human tissues before describing the functional dissection of a plethora of BMs and BM-related structures in their tissue contexts in the fruit fly Drosophila melanogaster To conclude, I summarize our incipient understanding of the mechanisms that provide morphological, compositional and functional diversity to BMs. It is becoming increasingly clear that atypical BMs are quite prevalent, and that even typical planar BMs harbor a lot of diversity that we do not yet comprehend.

Control of tissue morphogenesis by the HOX gene Ultrabithorax

Mutations in the Ultrabithorax (Ubx) gene cause homeotic transformation of the normally two-winged Drosophila into a four-winged mutant fly. Ubx encodes a HOX family transcription factor that specifies segment identity, including transformation of the second set of wings into rudimentary halteres. Ubx is known to control the expression of many genes that regulate tissue growth and patterning, but how it regulates tissue morphogenesis to reshape the wing into a haltere is still unclear. Here, we show that Ubx acts by repressing the expression of two genes in the haltere, Stubble and Notopleural, both of which encode transmembrane proteases that remodel the apical extracellular matrix to promote wing morphogenesis. In addition, Ubx induces expression of the Tissue inhibitor of metalloproteases in the haltere, which prevents the basal extracellular matrix remodelling necessary for wing morphogenesis. Our results provide a long-awaited explanation for how Ubx controls morphogenetic transformation.

On growth and force: mechanical forces in development

The EMBO/EMBL Symposium 'Mechanical Forces in Development' was held in Heidelberg, Germany, on 3-6 July 2019. This interdisciplinary symposium brought together an impressive and diverse line-up of speakers seeking to address the origin and role of mechanical forces in development. Emphasising the importance of integrative approaches and theoretical simulations to obtain comprehensive mechanistic insights into complex morphogenetic processes, the meeting provided an ideal platform to discuss the concepts and methods of developmental mechanobiology in an era of fast technical and conceptual progress. Here, we summarise the concepts and findings discussed during the meeting, as well as the agenda it sets for the future of developmental mechanobiology.