Biology subjects

Megason, S.

Publications and source records attributed to Megason, S..

7 recordsLinked to original sources

bioRxiv2025

A single-cell transcriptomic atlas of inner ear morphogenesis in zebrafish

The inner ear constitutes different cell types next to one another: the sensory patches whose hair cells synapse with neurons, the thin channels of three semicircular canals whose perpendicular organization enables detection of directional head rotation, and the endolymphatic duct and sac whose conditional epithelial barrier relieves excess pressure and promotes fluid pressure homeostasis. How the ears component cell states are established during development has remained unknown. We use single-cell RNA sequencing to distinguish cell states within the developing ear with wild-type zebrafish embryos and lmx1bb mutants that exhibit defects in canal and sac morphogenesis. We identify the earliest marker for the semicircular canal-genesis zone (ccn1l1), unexpected genes in the endolymphatic sac that suggest a role for tissue contraction in its function (smtnb), parallel gene sets for sensory patches in the neuromast and ear, and a conserved role for cell-cycle pausing (cdkn1bb expression in the canals and sac as previously observed in the developing mouse ear). This atlas provides the most comprehensive transcriptional profiling of the developing inner ear, identifying new molecular leads to understand ear morphogenesis. Summary statementSingle-cell transcriptomic analysis of developing wild-type and lmx1bb mutant zebrafish reveals cell-states and effectors that distinguish the inner ears sensory patches, semicircular canals, endolymphatic duct and sac, and periotic mesenchyme.

developmental biology↗

bioRxiv2025

Microtubule-dependent cell polarity regulates skin-resident macrophage phagocytosis and directed cell migration

Immune cells rapidly respond to tissue damage through dynamic properties of the cytoskeleton. How microtubules control immune cell functions during injury responses remains poorly understood. Within skin, tissue-resident macrophages known as Langerhans cells use dynamic dendrites to surveil the epidermis for damage and migrate through a densely packed epithelium to wounds. Here, we use Langerhans cells within the adult zebrafish epidermis as a model to investigate roles for microtubules in immune cell tissue surveillance, phagocytosis, and directed migration. We describe microtubule organization within Langerhans cells, and show that depolymerizing the microtubule cytoskeleton alters dendrite morphology, debris engulfment, and migration efficiency. We find that the microtubule organizing center positions adjacent to engulfed debris and that its position correlates with navigational pathfinding during directed cell migration. Stabilizing microtubules prevents Langerhans cell motility during directed cell migration by impairing navigation around cellular obstacles. Collectively, our work demonstrates requirements for microtubules in the dynamic actions of tissue-resident macrophages during epithelial surveillance and wound repair.

cell biology↗

bioRxiv2023

IQGAP2 regulates blood-brain barrier immune dynamics

Brain endothelial cells (BECs) play an important role in maintaining central nervous system (CNS) homeostasis through blood-brain barrier (BBB) functions. BECs express low baseline levels of adhesion receptors, which limits entry of leukocytes. However, the molecular mediators governing this phenotype remain mostly unclear. Here, we explored how infiltration of immune cells across the BBB is influenced by the scaffold protein IQ motif containing GTPase activating protein 2 (IQGAP2). In mice and zebrafish, we demonstrate that loss of Iqgap2 increases infiltration of peripheral leukocytes into the CNS under homeostatic and inflammatory conditions. Using single-cell RNA sequencing and immunohistology, we further show that BECs from mice lacking Iqgap2 exhibit a profound inflammatory signature, including extensive upregulation of adhesion receptors and antigen-processing machinery. Human tissue analyses also reveal that Alzheimers disease is associated with reduced hippocampal IQGAP2. Overall, our results implicate IQGAP2 as an essential regulator of BBB immune privilege and immune cell entry into the CNS.

neuroscience↗

bioRxiv2021

Single cell response landscape of graded Nodal signaling in zebrafish explants

Nodal, as a morphogen, plays important roles in cell fate decision, pattern formation and organizer function. But because of the complex context in vivo and technology limitations, systematic studying of genes, cell types and patterns induced by Nodal alone is still missing. Here, by using a relatively simplified model, the zebrafish blastula animal pole explant avoiding additional instructive signals and prepatterns, we constructed a single cell response landscape of graded Nodal signaling, identified 105 Nodal immediate targets and depicted their expression patterns. Our results show that Nodal signaling is sufficient to induce anterior-posterior patterned axial mesoderm and head structure. Surprisingly, the endoderm induced by Nodal alone is mainly the anterior endoderm which gives rise to the pharyngeal pouch only, but not internal organs. Among the 105 Nodal targets, we identified 14 genes carrying varying levels of axis induction capability. Overall, our work provides new insights for understanding of the Nodal function and a valuable resource for future studies of patterning and morphogenesis induced by it.

developmental biology↗

bioRxiv2020

Extracellular hyaluronate pressure shaped by cellular tethers drives tissue morphogenesis

How tissues acquire complex shapes is a fundamental question in biology and regenerative medicine. Zebrafish semicircular canals form from invaginations in the otic epithelium (buds) that extend and fuse to form the hubs of each canal. We find that conventional actomyosin-driven behaviors are not required. Instead, local secretion of hyaluronan, made by the enzymes ugdh and has3, drives canal morphogenesis. Charged hyaluronate polymers osmotically swell with water and generate isotropic extracellular pressure to deform the overlying epithelium into buds. The mechanical anisotropy needed to shape buds into tubes is conferred by a polarized distribution of cellular protrusions, linked between cells, that we term cytocinches. Most work on tissue morphogenesis ascribes actomyosin contractility as the driving force, while the extracellular matrix shapes tissues through differential stiffness. Our work inverts this expectation. Hyaluronate-pressure shaped by anisotropic tissue stiffness may be a widespread mechanism for powering morphological change in organogenesis and tissue engineering.

developmental biology↗

bioRxiv2018

A Scube2-Shh feedback loop links morphogen release to morphogen signaling to enable scale invariant patterning of the ventral neural tube

To enable robust patterning, morphogen systems should be resistant to variations in gene expression and tissue size. Here we explore how the Sonic Hedgehog (Shh) morphogen gradient in the ventral neural tube enables scaled patterning in embryos of varying sizes. Using zebrafish eggs that have been surgically reduced in size, we find that ventral neural tube patterning remains proportional in smaller embryos. Intriguingly, a secreted protein implicated in Shh release, Scube2, is expressed in the dorsal-intermediate neural tube far from Shh producing cells. Overexpression of scube2 expands the Shh gradient whereas loss of scube2 causes gradient contraction. Conversely, upregulation of Shh represses scube2 expression while Shh downregulation increases scube2 expression thus establishing a negative feedback loop. This regulatory feedback is necessary for scaling, as demonstrated by its loss in scube2 overexpressing embryos. Using mathematical modeling, we show that feedback control on diffusion and release rates of Shh allows the morphogen gradient to be robust to differences in field length and Shh gene dosage. We conclude that Scube2 promotes release and diffusion of Shh to allow gradient scaling in an extension to the expander-repressor model. Summary StatementThe Shh morphogen gradient can scale to different size tissues by feedback between Scube2 mediated release and diffusion of Shh and Shh based inhibition of Scube2 expression

developmental biology↗

bioRxiv2018

Heterogeneity of Sonic Hedgehog Response Dynamics and Fate Specification in Single Neural Progenitors

During neural tube patterning, a gradient of Sonic hedgehog (Shh) signaling specifies ventral progenitor fates. The cellular response to Shh is processed through a genetic regulatory network (GRN) to specify distinct fate decisions. This process integrates Shh response level, duration and other inputs and is affected by noise in signaling and cell position. How reliably Shh response profile predicts the fate choice of a single cell is unclear. Here we use live imaging to track neural progenitors in developing zebrafish and quantify both Shh and fate reporters in single cells over time. We found that there is significant heterogeneity between Shh response and fate choice in single cells. We quantitatively modeled reporter intensities to obtain single cell response levels over time and systematically evaluated their correlation with cell fate specification. Motor neuron progenitors (pMN) exhibit a high degree of variability in their Shh responses, which is particularly prominent in the posterior neural tube where the Shh response dynamics are similar to those of the more ventrally fated lateral floor plate cells (LFPs). Our results highlight the precision limit of morphogen-interpretation GRNs in small and dynamic target cell fields.

developmental biology↗