Exocyst-mediated apical Wg secretion activates signaling in the Drosophila wing epithelium

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

The logistics of Wnt production and delivery

Wnts are secreted proteins that control stem cell maintenance, cell fate decisions, and growth during development and adult homeostasis. Wnts carry a post-translational modification not seen in any other secreted protein: during biosynthesis, they are appended with a palmitoleoyl moiety that is required for signaling but also impairs solubility and hence diffusion in the extracellular space. In some contexts, Wnts act only in a juxtacrine manner but there are also instances of long range action. Several proteins and processes ensure that active Wnts reach the appropriate target cells. Some, like Porcupine, Wntless, and Notum are dedicated to Wnt function; we describe their activities in molecular detail. We also outline how the cell infrastructure (secretory, endocytic, and retromer pathways) contribute to the progression of Wnts from production to delivery. We then address how Wnts spread in the extracellular space and form a signaling gradient despite carrying a hydrophobic moiety. We highlight particularly the role of lipid-binding Wnt interactors and heparan sulfate proteoglycans. Finally, we briefly discuss how evolution might have led to the emergence of this unusual signaling pathway.

Butterfly Wing Color Pattern Modification Inducers May Act on Chitin in the Apical Extracellular Site: Implications in Morphogenic Signals for Color Pattern Determination

Butterfly wing color patterns are modified by various treatments, such as temperature shock, injection of chemical inducers, and covering materials on pupal wing tissue. Their mechanisms of action have been enigmatic. Here, we investigated the mechanisms of color pattern modifications usingthe blue pansy butterfly Junoniaorithya. We hypothesized that these modification-inducing treatments act on the pupal cuticle or extracellular matrix (ECM). Mechanical load tests revealed that pupae treated with cold shock or chemical inducers were significantly less rigid, suggesting that these treatments made cuticle formation less efficient. A known chitin inhibitor, FB28 (fluorescent brightener 28), was discovered to efficiently induce modifications. Taking advantage of its fluorescent character, fluorescent signals from FB28 were observed in live pupae in vivo from the apical extracellular side and were concentrated at the pupal cuticle focal spots immediately above the eyespot organizing centers. It was shown that chemical modification inducers and covering materials worked additively. Taken together, various modification-inducing treatments likely act extracellularly on chitin or other polysaccharides to inhibit pupal cuticle formation or ECM function, which probably causes retardation of morphogenic signals. It is likely that an interactive ECM is required for morphogenic signals for color pattern determination to travel long distances.

Microscopic and biochemical monitoring of endosomal trafficking and extracellular vesicle secretion in an endogenous in vivo model

Extracellular vesicle (EV) secretion enables cell-cell communication in multicellular organisms. During development, EV secretion and the specific loading of signalling factors in EVs contributes to organ development and tissue differentiation. Here, we present an in vivo model to study EV secretion using the fat body and the haemolymph of the fruit fly, Drosophila melanogaster. The system makes use of tissue-specific EV labelling and is amenable to genetic modification by RNAi. This allows the unique combination of microscopic visualisation of EVs in different organs and quantitative biochemical purification to study how EVs are generated within the cells and which factors regulate their secretion in vivo. Characterisation of the system revealed that secretion of EVs from the fat body is mainly regulated by Rab11 and Rab35, highlighting the importance of recycling Rab GTPase family members for EV secretion. We furthermore discovered a so far unknown function of Rab14 along with the kinesin Klp98A in EV biogenesis and secretion.

Regulation of Wnt distribution and function by Drosophila glypicans

The extracellular distribution of secreted Wnt proteins is crucial for their ability to induce a response in target cells at short and long ranges to ensure proper development. Wnt proteins are evolutionarily conserved ligands that are lipid-modified, and their hydrophobic nature interferes with their solubility in the hydrophilic extracellular environment. This raises the question of how Wnt proteins spread extracellularly despite their lipid modifications, which are essential for both their secretion and function. Seminal studies on Drosophila Wingless (Wg), a prototypical Wnt, have discovered multiple mechanisms by which Wnt proteins spread. A central theme emerges from these studies: the Wnt lipid moiety is shielded from the aqueous environment, allowing the ligands to spread and remain viable for signaling. Wnt distribution in vivo is primarily facilitated by glypicans, which are cell-surface heparan sulfate proteoglycans, and recent studies have further provided mechanistic insight into how glypicans facilitate Wnt distribution. In this Review, we discuss the many diverse mechanisms of Wnt distribution, with a particular focus on glypican-mediated mechanisms.

Bimodal expression of Wnt5a in the tooth germ: A comparative study using in situ hybridization and immunohistochemistry

BACKGROUND

During tooth development, Wnt5a, a member of the noncanonical Wnt ligand, is expressed prominently in the dental mesenchyme. However, the spatiotemporal profiles of Wnt5a protein production and distribution in tooth germs are largely unknown, which impairs elucidation of the Wnt5a-mediated regulatory mechanism of tooth development.

METHODS

We performed analyzes of the spatiotemporal expression of Wnt5a in embryonic tooth germs (E11.5-E18.5) by using in situ hybridization and immunohistochemistry in parallel. The developmental stages of the embryonic tooth germs were determined by HE staining. In order to compare the spatiotemporal distribution patterns of Wnt5a mRNA-expressing cells and those of Wnt5a protein-expressing cells, serial frontal sections of paraffinized mouse embryo heads were used for the analyzes. When needed, the immunohistochemistry images were subjected to digital detection analysis of Wnt5a immunostaining signal using the WinROOF 2018 Ver. 4.19.0 image processing software program.

RESULTS

Throughout the developmental process, cells expressing Wnt5a mRNA were found in various tissues including the dental follicle, dental papilla, inner and outer enamel epithelium, stratum intermediate, and stellate reticulum. However, odontoblasts differentiating and polarizing at E18.5 were the only cells representing an accumulation of Wnt5a protein in the apical region of the odontoblast process. The Wnt5a protein was undetectable in undifferentiated mesenchymal cells as well as any other cells positive for Wnt5a mRNA.

CONCLUSION

Differentiating odontoblasts execute Wnt5a expression, the mode of which is distinct from that executed by the other cells constituting tooth germ. Change of the mode of Wnt5a expression begins to take place in the mesenchymal cells by E18.5, starting the elongation of the cytoplasmic process.

Extracellular WNTs: Trafficking, Exosomes, and Ligand-Receptor Interaction

WNT signaling is a key developmental pathway in tissue organization. A recent focus of research is the secretion of WNT proteins from source cells. Research over the past decade on how WNTs are produced and released into the extracellular space has unravelled very specific control mechanisms in the early secretory pathway, specialized trafficking routes, and redundant forms of packaging for delivery to target cells. In this review I discuss the findings that WNT proteins have been found on extracellular vesicles (EVs) such as exosomes and possible functional implications. There is an ongoing debate in the WNT signaling field whether EV are relevant in vivo and can fulfill specific functions, also fueled by the general preconception of EV secretion as cellular garbage disposal. As part of the EV research community, I want to give an overview of what we know and don't know about WNT secretion on EVs and offer a more unifying model that can explain current discrepancies in observations regarding WNT secretion.

The Emerging Mechanisms of Wnt Secretion and Signaling in Development

Wnts are highly-conserved lipid-modified secreted proteins that activate multiple signaling pathways. These pathways regulate crucial processes during various stages of development and maintain tissue homeostasis in adults. One of the most fascinating aspects of Wnt protein is that despite being hydrophobic, they are known to travel several cell distances in the extracellular space. Research on Wnts in the past four decades has identified several factors and uncovered mechanisms regulating their expression, secretion, and mode of extracellular travel. More recently, analyses on the importance of Wnt protein gradients in the growth and patterning of developing tissues have recognized the complex interplay of signaling mechanisms that help in maintaining tissue homeostasis. This review aims to present an overview of the evidence for the various modes of Wnt protein secretion and signaling and discuss mechanisms providing precision and robustness to the developing tissues.

Visualization and Quantitation of Wg trafficking in the Drosophila Wing Imaginal Epithelium

Secretory Wnt trafficking can be studied in the polarized epithelial monolayer of Drosophila wing imaginal discs (WID). In this tissue, Wg (Drosophila Wnt-I) is presented on the apical surface of its source cells before being internalized into the endosomal pathway. Long-range Wg secretion and spread depend on secondary secretion from endosomal compartments, but the exact post-endocytic fate of Wg is poorly understood. Here, we summarize and present three protocols for the immunofluorescence-based visualization and quantitation of different pools of intracellular and extracellular Wg in WID: (1) steady-state extracellular Wg; (2) dynamic Wg trafficking inside endosomal compartments; and (3) dynamic Wg release to the cell surface. Using a genetic driver system for gene manipulation specifically at the posterior part of the WID (EnGal4) provides a robust internal control that allows for direct comparison of signal intensities of control and manipulated compartments of the same WID. Therefore, it also circumvents the high degree of staining variability usually associated with whole-tissue samples. In combination with the genetic manipulation of Wg pathway components that is easily feasible in Drosophila, these methods provide a tool-set for the dissection of secretory Wg trafficking and can help us to understand how Wnt proteins travel along endosomal compartments for short- and long-range signal secretion. Graphic abstract: Figure 1. Visualization of extracellular and intracellular Wg trafficking in Drosophila wing imaginal discs. While staining of extracellular Wg without permeabilization exclusively visualizes Wg bound to the extracellular surface (left), Wg uptake and endosomal trafficking can be visualized using an antibody uptake assay (middle). Dynamic Wg release can be visualized by performing a non-permeabilizing staining at a permissive temperature that sustains secretory Wg transport (right).

Transcytosis in the development and morphogenesis of epithelial tissues

Transcytosis is a form of specialized transport through which an extracellular cargo is endocytosed, shuttled across the cytoplasm in membrane-bound vesicles, and secreted at a different plasma membrane surface. This important process allows membrane-impermeable macromolecules to pass through a cell and become accessible to adjacent cells and tissue compartments. Transcytosis also promotes redistribution of plasma membrane proteins and lipids to different regions of the cell surface. Here we review transcytosis and highlight in vivo studies showing how developing epithelial cells use it to change shape, to migrate, and to relocalize signaling molecules.

HumanaFly: high-throughput transgenesis and expression of breast cancer transcripts in Drosophila eye discovers the RPS12-Wingless signaling axis

Drosophila melanogaster has been a model for multiple human disease conditions, including cancer. Among Drosophila tissues, the eye development is particularly sensitive to perturbations of the embryonic signaling pathways, whose improper activation in humans underlies various forms of cancer. We have launched the HumanaFly project, whereas human genes expressed in breast cancer patients are screened for their ability to aberrate development of the Drosophila eye, hoping to thus identify novel oncogenes. Here we report identification of a breast cancer transgene, which upon expression in Drosophila produces eye malformation similar to the famous Glazed phenotype discovered by Thomas Morgan and decades later dissected to originate from mis-expression of Wingless (Wg). Wg is the ortholog of human Wnt proteins serving as ligands to initiate the developmental/oncogenic Wnt signaling pathway. Through genetic experiments we identified that this transgene interacted with the Wg production machinery, rather than with Wg signal transduction. In Drosophila imaginal discs, we directly show that the transgene promoted long-range diffusion of Wg, affecting expression of the Wg target genes. The transgene emerged to encode RPS12—a protein of the small ribosomal subunit overexpressed in several cancer types and known to also possess extra-ribosomal functions. Our work identifies RPS12 as an unexpected regulator of secretion and activity of Wnts. As Wnt signaling is particularly important in the context of breast cancer initiation and progression, RPS12 might be implicated in tumorigenesis in this and other Wnt-dependent cancers. Continuation of our HumanaFly project may bring further discoveries on oncogenic mechanisms.

The kinesin motor Klp98A mediates apical to basal Wg transport

Development and tissue homeostasis rely on the tight regulation of morphogen secretion. In the Drosophila wing imaginal disc epithelium, Wg secretion for long-range signal transduction occurs after apical Wg entry into the endosomal system, followed by secretory endosomal transport. Although Wg release appears to occur from the apical and basal cell sides, its exact post-endocytic fate and the functional relevance of polarized endosomal Wg trafficking are poorly understood. Here, we identify the kinesin-3 family member Klp98A as the master regulator of intracellular Wg transport after apical endocytosis. In the absence of Klp98A, functional mature endosomes accumulate in the apical cytosol, and endosome transport to the basal cytosol is perturbed. Despite the resulting Wg mislocalization, Wg signal transduction occurs normally. We conclude that transcytosis-independent routes for Wg trafficking exist and demonstrate that Wg can be recycled apically via Rab4-recycling endosomes in the absence of Klp98A.

Summary: In the polarized wing disc epithelium of Drosophila, Kinesin-like protein 98A mediates transcytosis of multivesicular endosomes.