To elucidate how tissue morphogenesis and signaling molecules are regulated during development, precise spatial and temporal control of cellular signal transduction is an essential feature of animal development.

Since developmental signaling is often accompanied by dynamic morphogenesis, it has been postulated that signaling and dynamic morphogenesis are mutually coordinated. In order to address these issues, we work on the topics in the field of cell and developmental biology.

Coupling between 3D tissue architecture and morphogen signaling

At the level of organogenesis, tissue morphogenesis drives developmental processes in animals often involving the rearrangement of simple 2D structures into more complex 3D tissues. These processes can be directed by growth factor signaling pathways. Although how the cellular mechanisms of developmental signaling affect cell and tissue shapes have been actively studied, it remains to be addressed how the dynamic 3D architecture of the entire tissue affects developmental signaling in a precise spatiotemporal manner.

By using the Drosophilapupal wing, we addressed how Dpp/BMP signaling and 3D wing morphogenesis are coupled. We re-evaluated the function of Dpp signaling in pupal wing development by employing a conditional knockout approach and found that Dpp signaling is needed not only in vein differentiation and patterning, but also has a key role in tissue proliferation. Dpp expressed in the LVs diffuses laterally to regulate tissue size during the inflation stage. Dpp is then tightly retained in LVs for vein differentiation within a planar epithelium. Intriguingly, Dpp is actively trafficked between dorsal and ventral epithelia, which is critical for refining signaling range. This represents a change in signaling directionality from lateral planar to interplanar. Our data suggest that 3D tissue architecture directs spatial distribution of BMP signaling (Gui et al).


Coordination between epithelial morphogenesis and signaling

By studying Drosophila PCV development as a model, we addressed how extracellular signal and morphogenesis are coordinated. Our data indicate that BMP preferentially accumulates at the basal side of the PCV region, raising the possibility that BMP transport is associated with wing vein morphogenesis. Our results show that RhoGAP Crossveinless-C (Cv-C) is the key molecule coupling BMP transport and wing vein morphogenesis. Cv-C is induced at the PCV primordial cells by BMP signaling and mediates PCV morphogenesis cell-autonomously by inactivating members of the Rho-type small GTPases. Cv-C is also required non-cell-autonomously for BMP transport into the PCV region. Our data suggest that lack of β-integrin regulated by the Rho GTPase at the basal side of PCV epithelial cells provides an optimal extracellular environment for guiding BMP transport. These data suggest that Cv-C mediates a feed-forward loop coupling BMP transport and PCV morphogenesis (Matsuda et al). 

We further found that the apical-basal cell polarity gene Scribbled (Scrib) is required for PCV formation by optimizing BMP signaling in the PCV region. Scrib regulates BMP type-I receptor Tkv localization at the basolateral region of PCV cells and subsequently facilitates Tkv internalization to the Rab5 endosomes to optimize signal transduction after receptor-ligand binding. BMP signaling also up-regulates scribtranscription in the pupal wing to form a positive feedback loop. Therefore, the Scrib-Rab5 system is a flexible module for receptor trafficking in a context-dependent manner and can be used for signal amplification (Gui et al).

Spatiotemporal regulation of BMP signal and morphogenesis

The development of the Drosophila wing is a classical model for studying the genetic control of tissue size, shape and patterning. The Drosophila wing venation pattern is relatively simple when compared to other insects and consists of four main longitudinal veins (LVs) and two crossveins (anterior crossvein (ACV) and posterior crossvein (PCV)). Systematic studies of wing phenotypes led to a model for vein patterning involving the sequential action of determination of the position in which the vein will differentiate, and differentiation of cells to form vein tissues. Since the same signaling and transcriptional factors are repeatedly used in tissue growth, differentiation and morphogenesis in a wide range of organisms, wing patterning provides a unique paradigm for investigating molecular details of such interactions.

During the pupal stages, BMP functions as a wing vein determinant. By establishing a system in which functional GFP-tagged Dpp, a BMP-type ligand in Drosophila, can be visualized in the pupal wing, we have demonstrated that GFP-Dpp is moved from LVs into PCV by BMP binding proteins Sog and Crossveinless. Incontrast, majority of ligands appear to be immobilized in the LVs of the pupal wing to maintain a short-range signaling. Thus, the spatial distribution of BMP type ligands is achieved by a blend of short- and long-range signaling (Matsuda and Shimmi). 

Dpp/BMP transport system might be a key mechanism underlying diversified wing venation among insects

Insect wings are great resources for studying morphological diversities in nature as well as in fossil records. Among them, variation in wing venation is one of the most characteristic features of insect species. Venation is a key factor of species-specific functional traits of the wings. However, little is known about how wing venation is established and diversified among insects. Recent works on the sawfly Athalia rosae, of the order Hymenoptera, suggested that the Dpp/BMP transport system is required to specify fore- and hindwing vein patterns. Given that Dpp redistribution via transport is likely to be a key mechanism for establishing wing vein patterns, this raises the interesting possibility that distinct wing vein patterns are generated, based on where Dpp is transported. These data suggest that the BMP transport system is widely used to redistribute Dpp to specify wing venation and may be a basal mechanism underlying diversified wing vein patterns among insects (Matsuda et alHuang et alShimmi et al).

Post-translational regulation of BMP ligands

The expression and activity of TGF-beta type ligands can be regulated at both transcriptional and post-transcriptional levels. One of the important post-transcriptional regulations is the cleavage of proproteins by furin-type proprotein convertases. Our previous findings indicate that cleavage sites of the BMP2/4/Dpp family have been evolutionary diversified and can be categorized into four different types. We studied how the three furin sites of DrosophilaDpp coordiate the maturation of ligands and contribute to signaling in vivo, suggesting that the Dpp precursor is cleaved at three furin sites, and the first cleavage at an upstream furin site is critical and enough for long-range Dpp signaling. These indicate that the furin cleavage sites in BMP2/4/Dpp precursors have adjusted to different systems in diversified species (künnapuu et al). 

A mutant allele of scw contains a mutation at the furin cleavage site of its prodomain that shows strong interaction with dpp. Our data reveal that the Scw precursor needs to be cleaved at the prodomain in addition to the furin recognition site adjacent to the ligand domain and prodomain cleavage of Scw is critical for forming BMP morphogen gradient in the early embryo (Künnapuu et al).

We investigated the signaling capacity of BMP5/6/7/8 type ligands Scw and Gbb in embryonic dorsal-ventral patterning and PCV formation. We found that Scw ligand contains two N-glycosylation motifs, both of which play roles in boosting BMP signaling in cell culture and in the embryo, whilst Gbb was unable to participate in embryonic DV patterning. On the other hand, both Gbb and Scw could contribute to PCV development. Interestingly, N-glycosylation motifs of Gbb or Scw did not cause advantage to PCV formation, rather decreased the developmental fitness during PCV development. These observations suggest that structural changes of BMP ligands rely on developmental and evolutionary constraints (Tauscher et al).