Biology subjects

Loo, T. Y. J.

Publications and source records attributed to Loo, T. Y. J..

5 recordsLinked to original sources

bioRxiv2024

Receptor binding and tissue architecture explain the morphogen local-to-global mobility transition

Morphogens are intercellular signaling molecules providing spatial information to cells in developing tissues to coordinate cell fate decisions. The spatial information is encoded within long-ranged concentration gradients of the morphogen. Direct measurement of morphogen dynamics in a range of systems suggests that local and global diffusion coefficients can differ by orders of magnitude. Further, local diffusivity can be large, which would potentially abolish any concentration gradient rapidly. Such observations have led to alternative transport models being proposed, including transcytosis and cytonemes. Here, we show that accounting for tissue architecture combined with receptor binding is sufficient to hinder the diffusive dynamics of morphogens, leading to an order of magnitude decrease in the effective diffusion coefficient from local to global scales. In particular, we built a realistic in silico architecture of the extracellular spaces of the zebrafish brain using light and electron microscopy data. Simulations on realistic architectures demonstrate that tortuosity and receptor binding within these spaces are sufficient to reproduce experimentally measured morphogen dynamics. Importantly, this work demonstrates that hindered diffusion is a viable mechanism for gradient formation, without requiring additional regulatory control. SIGNIFICANCEMeasurements of morphogen diffusivity vary significantly depending on experimental approach. Such differences have been used to argue against diffusion as a viable mechanism of morphogen gradient formation. Here, we demonstrate that accounting for the local tissue architecture in concert with including receptor binding is sufficient to explain a range of biological observations. This demonstrates that (hindered) diffusion-driven transport is a viable mechanism of gradient formation.

biophysics↗

bioRxiv2023

Midkine and Ptprz1b act upstream of Wnt planar cell polarity to establish a midline in the developing zebrafish hindbrain

A midline in the developing central nervous system (CNS) is essential for the symmetric distribution of neural progenitors that later establish functional, bilaterally symmetric neural circuits. In the zebrafish hindbrain, a midline forms early during neurulation and requires a coordinated interplay of cell convergence and midline-crossing cell divisions (C-divisions). These two processes are controlled by the Wnt/planar cell polarity (PCP) pathway. However, upstream cues that control the timely production of PCP components remain unknown. Midkine (Mdk) and pleiotrophin (Ptn) are structurally related heparin-binding growth factors that are dynamically expressed in the developing zebrafish hindbrain. We used proximity ligation assays (PLAs) and fluorescence cross correlation spectroscopy (FCCS) in vivo to show that two zebrafish Mdks, Mdka and Mdkb, as well as Ptn interact with protein tyrosine phosphatase receptors type Z1, Ptprz1a and Ptprz1b, with distinct affinities. Ligand binding triggered Ptprz1b internalization and thereby determined the availability of signaling receptor on cell membranes. In zebrafish mdka, ptn and ptprz1b mutants, cell migration and convergence were significantly impaired during hindbrain neurulation. Impaired convergence led to misplaced C-divisions, defective cell polarity and consequently duplicated midlines. These duplications were rescued by overexpression of Drosophila Prickle, a key component of the Wnt/PCP pathway. Here, we provide evidence that zygotic Mdka controls the distribution of maternally provided Ptprz1b, which in turn is needed for transcription of zebrafish prickle1b. Our findings thus reveal a role for Mdka and Ptprz1b upstream of Wnt/PCP to coordinate neural plate convergence, neural progenitor positioning and midline formation.

developmental biology↗

bioRxiv2022

Echolocation-like model of directed cell migrationwithin growing tissues

During development and regeneration, cells migrate to specific locations within growing tissues. These cells can respond to both biochemical signals and mechanical cues, resulting in directed migration. Such migration is often highly stereotypic. Yet, how cells respond to migratory signals in a robust manner within a growing domain remains an open problem. Here, we propose a model of directed migration in growing tissues motivated by echolocation. The migrating cells generate a signaling gradient that induces a response signal from the moving system boundary. This response signal mediates cellular adhesion to the surrounding matrix and hence modulates the cell migration. We find that such a mechanism can align a series of cells at stable positions within growing systems and can effectively scale to system size. Finally, we discuss the relevance of such a model to fibroblast migration and location within the developing zebrafish caudal fin, which may be regulated by opposing signaling gradients of Slit-Robo pathway components. Significance StatementHow do cells reliably migrate within growing environments? Here, we show that cells can take advantage of an echolocation-like process, whereby they induce a response from the tissue boundary. As they approach the boundary, the response signal strengthens and brings the cell to a fixed position from the boundary. This simple system may be applicable to fibroblast migration in the fin.

biophysics↗

bioRxiv2021

Slit-Robo Signalling Establishes a Sphingosine-1-Phosphate Gradient to Polarise Fin Mesenchyme and Establish Fin Morphology

Immigration of mesenchymal cells into the growing fin and limb buds drives distal outgrowth, with subsequent tensile forces between these cells essential for fin and limb morphogenesis. Morphogens derived from the apical domain of the fin, orientate limb mesenchyme cell polarity, migration, division and adhesion. The zebrafish mutant stomp displays defects in fin morphogenesis including blister formation and associated loss of orientation and adhesion of immigrating fin mesenchyme cells. Positional cloning of stomp identified a mutation in the gene encoding the axon guidance ligand, Slit3. We provide evidence that Slit ligands derived from immigrating mesenchyme act via Robo receptors at the Apical Ectodermal Ridge (AER) to promote release of sphingosine-1-phosphate (S1P). S1P subsequently diffuses back to the mesenchyme to promote their polarisation, orientation, positioning and adhesion to the interstitial matrix of the fin fold. We thus demonstrate coordination of the Slit-Robo and S1P signalling pathways in fin fold morphogenesis. Our work introduces a mechanism regulating the orientation, positioning and adhesion of its constituent cells.

cell biology↗

bioRxiv2017

Disrupting different Distal-less exons leads to ectopic and missing eyespots accurately modeled by reaction-diffusion mechanisms

Eyespots on the wings of nymphalid butterflies represent colorful examples of the process of pattern formation, yet the developmental origins and the mechanisms behind eyespot differentiation are still not fully understood. Here we re-examine the function of Distal-less (Dll) in eyespot development, which is still unclear. We show that CRISPR-Cas9 induced exon 2 mutations in Bicyclus anynana leads to exon skipping and ectopic eyespots on the wing. Exon 3 mutations, however, lead to null/missense transcripts, missing eyespots, lighter wing coloration, loss of scales, and a variety of other phenotypes implicating Dll in the process of eyespot differentiation. Reaction-diffusion modeling enabled exploration of the function of Dll in eyespot formation, and accurately replicated a wide-range of mutant phenotypes. These results confirm that Dll is a required activator of eyespot development, scale growth and melanization and point to a new mechanism of alternative splicing to achieve Dll over-expression phenotypes.

developmental biology↗