Interplay of cell shape and division orientation promotes robust morphogenesis of developing epithelia.

Epithelial cells acquire functionally important shapes (e.g., squamous, cuboidal, columnar) during development. Here, we combine theory, quantitative imaging, and perturbations to analyze how tissue geometry, cell divisions, and mechanics interact to shape the presumptive enveloping layer (pre-EVL) on the zebrafish embryonic surface. We find that, under geometrical constraints, pre-EVL flattening is regulated by surface cell number changes following differentially oriented cell divisions. The division pattern is, in turn, determined by the cell shape distribution, which forms under geometrical constraints by cell-cell mechanical coupling. An integrated mathematical model of this shape-division feedback loop recapitulates empirical observations. Surprisingly, the model predicts that cell shape is robust to changes of tissue surface area, cell volume, and cell number, which we confirm in vivo. Further simulations and perturbations suggest the parameter linking cell shape and division orientation contributes to epithelial diversity. Together, our work identifies an evolvable design logic that enables robust cell-level regulation of tissue-level development.

Specified neural progenitors sort to form sharp domains after noisy Shh signaling.

Sharply delineated domains of cell types arise in developing tissues under instruction of inductive signal (morphogen) gradients, which specify distinct cell fates at different signal levels. The translation of a morphogen gradient into discrete spatial domains relies on precise signal responses at stable cell positions. However, cells in developing tissues undergoing morphogenesis and proliferation often experience complex movements, which may affect their morphogen exposure, specification, and positioning. How is a clear pattern achieved with cells moving around? Using in toto imaging of the zebrafish neural tube, we analyzed specification patterns and movement trajectories of neural progenitors. We found that specified progenitors of different fates are spatially mixed following heterogeneous Sonic Hedgehog signaling responses. Cell sorting then rearranges them into sharply bordered domains. Ectopically induced motor neuron progenitors also robustly sort to correct locations. Our results reveal that cell sorting acts to correct imprecision of spatial patterning by noisy inductive signals.

Otolith tethering in the zebrafish otic vesicle requires Otogelin and α-Tectorin.

Otoliths are biomineralised structures important for balance and hearing in fish. Their counterparts in the mammalian inner ear, otoconia, have a primarily vestibular function. Otoliths and otoconia form over sensory maculae and are attached to the otolithic membrane, a gelatinous extracellular matrix that provides a physical coupling between the otolith and the underlying sensory epithelium. In this study, we have identified two proteins required for otolith tethering in the zebrafish ear, and propose that there are at least two stages to this process: seeding and maintenance. The initial seeding step, in which otolith precursor particles tether directly to the tips of hair cell kinocilia, fails to occur in the einstein (eis) mutant. The gene disrupted in eis is otogelin (otog); mutations in the human OTOG gene have recently been identified as causative for deafness and vestibular dysfunction (DFNB18B). At later larval stages, maintenance of otolith tethering to the saccular macula is dependent on tectorin alpha (tecta) function, which is disrupted in the rolling stones (rst) mutant. α-Tectorin (Tecta) is a major constituent of the tectorial membrane in the mammalian cochlea. Mutations in the human TECTA gene can cause either dominant (DFNA8/12) or recessive (DFNB21) forms of deafness. Our findings indicate that the composition of extracellular otic membranes is highly conserved between mammals and fish, reinforcing the view that the zebrafish is an excellent model system for the study of deafness and vestibular disease.

Recovery of shape and size in a developing organ pair.

BACKGROUND: Paired organs in animals are largely bilaterally symmetric despite inherent noise in most biological processes. How is precise organ shape and size achieved during development despite this noise? Examining paired organ development is a challenge because it requires repeated quantification of two structures in parallel within living embryos. Here we combine bilateral quantification of morphology through time with asymmetric perturbations to study regulation of organ shape, size, and symmetry in developing organ pairs. RESULTS: We present quantitative live imaging tools to measure the shape and size of the developing inner ears on both the left and right side simultaneously over time. By quantifying variation between the left and right inner ear (intrinsic noise) and between different individuals (extrinsic noise), we find that initial variability decreases over time in normal development to achieve symmetry. Early asymmetry is increased by environmental stress, but symmetry is still recovered over subsequent developmental time. Using multiple unilateral perturbations including Fgf signaling and ultraviolet light, we find that growth can be adjusted to compensate for a range of initial size and shape differences. CONCLUSIONS: We propose that symmetry in developmental systems does not emerge through precise deterministic bilateral development, but rather through feedback mechanisms that adjust morphogenesis rates to account for variation. Developmental Dynamics 246:451-465, 2016. © 2017 Wiley Periodicals, Inc.

RNA-seq-based mapping and candidate identification of mutations from forward genetic screens.

Forward genetic screens have elucidated molecular pathways required for innumerable aspects of life; however, identifying the causal mutations from such screens has long been the bottleneck in the process, particularly in vertebrates. We have developed an RNA-seq-based approach that identifies both the region of the genome linked to a mutation and candidate lesions that may be causal for the phenotype of interest. We show that our method successfully identifies zebrafish mutations that cause nonsense or missense changes to codons, alter transcript splicing, or alter gene expression levels. Furthermore, we develop an easily accessible bioinformatics pipeline allowing for implementation of all steps of the method. Overall, we show that RNA-seq is a fast, reliable, and cost-effective method to map and identify mutations that will greatly facilitate the power of forward genetics in vertebrate models.

Size control of the inner ear via hydraulic feedback.

Animals make organs of precise size, shape, and symmetry but how developing embryos do this is largely unknown. Here, we combine quantitative imaging, physical theory, and physiological measurement of hydrostatic pressure and fluid transport in zebrafish to study size control of the developing inner ear. We find that fluid accumulation creates hydrostatic pressure in the lumen leading to stress in the epithelium and expansion of the otic vesicle. Pressure, in turn, inhibits fluid transport into the lumen. This negative feedback loop between pressure and transport allows the otic vesicle to change growth rate to control natural or experimentally-induced size variation. Spatiotemporal patterning of contractility modulates pressure-driven strain for regional tissue thinning. Our work connects molecular-driven mechanisms, such as osmotic pressure driven strain and actomyosin tension, to the regulation of tissue morphogenesis via hydraulic feedback to ensure robust control of organ size. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Myc and Fgf Are Required for Zebrafish Neuromast Hair Cell Regeneration.

Unlike mammals, the non-mammalian vertebrate inner ear can regenerate the sensory cells, hair cells, either spontaneously or through induction after hair cell loss, leading to hearing recovery. The mechanisms underlying the regeneration are poorly understood. By microarray analysis on a chick model, we show that chick hair cell regeneration involves the activation of proliferation genes and downregulation of differentiation genes. Both MYC and FGF are activated in chick hair cell regeneration. Using a zebrafish lateral line neuromast hair cell regeneration model, we show that the specific inhibition of Myc or Fgf suppresses hair cell regeneration, demonstrating that both pathways are essential to the process. Rapid upregulation of Myc and delayed Fgf activation during regeneration suggest a role of Myc in proliferation and Fgf in differentiation. The dorsal-ventral pattern of fgfr1a in the neuromasts overlaps with the distribution of hair cell precursors. By laser ablation, we show that the fgfr1a-positive supporting cells are likely the hair cell precursors that directly give rise to new hair cells; whereas the anterior-posterior fgfr1a-negative supporting cells have heightened proliferation capacity, likely to serve as more primitive progenitor cells to replenish lost precursors after hair cell loss. Thus fgfr1a is likely to mark compartmentalized supporting cell subtypes with different capacities in renewal proliferation and hair cell regeneration. Manipulation of c-MYC and FGF pathways could be explored for mammalian hair cell regeneration.

Multibow: digital spectral barcodes for cell tracing.

We introduce a multicolor labeling strategy (Multibow) for cell tracing experiments in developmental and regenerative processes. Building on Brainbow-based approaches that produce colors by differential expression levels of different fluorescent proteins, Multibow adds a layer of label diversity by introducing a binary code in which reporters are initially OFF and then probabilistically ON or OFF following Cre recombination. We have developed a library of constructs that contains seven different colors and three different subcellular localizations. Combining constructs from this library in the presence of Cre generates cells labeled with multiple independently expressed colors based on if each construct is ON or OFF following recombination. These labels form a unique "barcode" that allows the tracking of the cell and its clonal progenies in addition to expression level differences of each color. We tested Multibow in zebrafish which validates its design concept and suggests its utility for cell tracing applications in development and regeneration.