Self-Organization of Embryonic Genetic Oscillators into Spatiotemporal Wave Patterns.

In vertebrate embryos, somites, the precursor of vertebrae, form from the presomitic mesoderm (PSM), which is composed of cells displaying signaling oscillations. Cellular oscillatory activity leads to periodic wave patterns in the PSM. Here, we address the origin of such complex wave patterns. We employed an in vitro randomization and real-time imaging strategy to probe for the ability of cells to generate order from disorder. We found that, after randomization, PSM cells self-organized into several miniature emergent PSM structures (ePSM). Our results show an ordered macroscopic spatial arrangement of ePSM with evidence of an intrinsic length scale. Furthermore, cells actively synchronize oscillations in a Notch-signaling-dependent manner, re-establishing wave-like patterns of gene activity. We demonstrate that PSM cells self-organize by tuning oscillation dynamics in response to surrounding cells, leading to collective synchronization with an average frequency. These findings reveal emergent properties within an ensemble of coupled genetic oscillators.

Model systems for regeneration: Hydra.

The freshwater polyp Hydra provides a potent model system for investigating the conditions that promote wound healing, reactivation of a developmental process and, ultimately, regeneration of an amputated body part. Hydra polyps can also be dissociated to the single cell level and can regenerate a complete body axis from aggregates, behaving as natural organoids. In recent years, the ability to exploit Hydra has been expanded with the advent of new live-imaging approaches, genetic manipulations that include stable transgenesis, gene silencing and genome editing, and the accumulation of high-throughput omics data. In this Primer, we provide an overview of Hydra as a model system for studying regeneration, highlighting recent results that question the classical self-enhancement and long-range inhibition model supposed to drive Hydra regeneration. We underscore the need for integrative explanations incorporating biochemical as well as mechanical signalling.

Scaling of embryonic patterning based on phase-gradient encoding.

A fundamental feature of embryonic patterning is the ability to scale and maintain stable proportions despite changes in overall size, for instance during growth. A notable example occurs during vertebrate segment formation: after experimental reduction of embryo size, segments form proportionally smaller, and consequently, a normal number of segments is formed. Despite decades of experimental and theoretical work, the underlying mechanism remains unknown. More recently, ultradian oscillations in gene activity have been linked to the temporal control of segmentation; however, their implication in scaling remains elusive. Here we show that scaling of gene oscillation dynamics underlies segment scaling. To this end, we develop a new experimental model, an ex vivo primary cell culture assay that recapitulates mouse mesoderm patterning and segment scaling, in a quasi-monolayer of presomitic mesoderm cells (hereafter termed monolayer PSM or mPSM). Combined with real-time imaging of gene activity, this enabled us to quantify the gradual shift in the oscillation phase and thus determine the resulting phase gradient across the mPSM. Crucially, we show that this phase gradient scales by maintaining a fixed amplitude across mPSM of different lengths. We identify the slope of this phase gradient as a single predictive parameter for segment size, which functions in a size- and temperature-independent manner, revealing a hitherto unrecognized mechanism for scaling. Notably, in contrast to molecular gradients, a phase gradient describes the distribution of a dynamical cellular state. Thus, our phase-gradient scaling findings reveal a new level of dynamic information-processing, and provide evidence for the concept of phase-gradient encoding during embryonic patterning and scaling.

Autoregulatory and repressive inputs localize Hydra Wnt3 to the head organizer.

Polarized Wnt signaling along the primary body axis is a conserved property of axial patterning in bilaterians and prebilaterians, and depends on localized sources of Wnt ligands. However, the mechanisms governing the localized Wnt expression that emerged early in evolution are poorly understood. Here we find in the cnidarian Hydra that two functionally distinct cis-regulatory elements control the head organizer-associated Hydra Wnt3 (HyWnt3). An autoregulatory element, which mediates direct inputs of Wnt/ß-catenin signaling, highly activates HyWnt3 transcription in the head region. In contrast, a repressor element is necessary and sufficient to restrict the activity of the autoregulatory element, thereby allowing the organizer-specific expression. Our results reveal that a combination of autoregulation and repression is crucial for establishing a Wnt-expressing organizing center in a basal metazoan. We suggest that this transcriptional control is an evolutionarily old strategy in the formation of Wnt signaling centers and metazoan axial patterning.

Unidirectional and phase-gated signaling synchronizes murine presomitic mesoderm cells.

Oscillator systems achieve synchronization when oscillators are coupled. The presomitic mesoderm is a system of cellular oscillators, where coordinated genetic activity is necessary for proper periodic generation of somites. While Notch signaling is required for the synchronization of these cells, it is unclear what information the cells exchange and how they react to this information to align their oscillatory pace with that of their neighbors. Combining mathematical modeling and experimental data, we found that interaction between murine presomitic mesoderm cells is controlled by a phase-gated and unidirectional coupling mechanism and results in deceleration of their oscillation pace upon Notch signaling. This mechanism predicts that isolated populations of well-mixed cells synchronize, revealing a stereotypical synchronization in the mouse PSM and contradicting expectations from previously applied theoretical approaches. Collectively, our theoretical and experimental findings reveal the underlying coupling mechanisms of the presomitic mesoderm cells and provide a framework to quantitatively characterize their synchronization.

Studying Mechanical Oscillations During Whole-Body Regeneration in Hydra.

Cells of the freshwater cnidarian Hydra possess an exceptional regeneration ability. In small groups of these cells, organizer centers emerge spontaneously and instruct the patterning of the surrounding population into a new animal. This property makes them an excellent model system to study the general rules of self-organization. A small tissue fragment or a clump of randomly aggregated cells can form a hollow spheroid that is able to establish a body axis de novo. Interestingly, mechanical oscillations (inflation/deflation cycles of the spheroid) driven by osmosis accompany the successful establishment of axial polarity. Here we describe different approaches for generating Hydra tissue spheroids, along with imaging and image analysis techniques to investigate their mechanical behavior.

The transcription factor Zic4 promotes tentacle formation and prevents epithelial transdifferentiation in Hydra.

The molecular mechanisms that maintain cellular identities and prevent dedifferentiation or transdifferentiation remain mysterious. However, both processes are transiently used during animal regeneration. Therefore, organisms that regenerate their organs, appendages, or even their whole body offer a fruitful paradigm to investigate the regulation of cell fate stability. Here, we used Hydra as a model system and show that Zic4, whose expression is controlled by Wnt3/ß-catenin signaling and the Sp5 transcription factor, plays a key role in tentacle formation and tentacle maintenance. Reducing Zic4 expression suffices to induce transdifferentiation of tentacle epithelial cells into foot epithelial cells. This switch requires the reentry of tentacle battery cells into the cell cycle without cell division and is accompanied by degeneration of nematocytes embedded in these cells. These results indicate that maintenance of cell fate by a Wnt-controlled mechanism is a key process both during homeostasis and during regeneration.

Three homologous subunits form a high affinity peptide-gated ion channel in Hydra.

Recently, three ion channel subunits of the degenerin (DEG)/epithelial Na(+) channel (ENaC) gene family have been cloned from the freshwater polyp Hydra magnipapillata, the Hydra Na(+) channels (HyNaCs) 2-4. Two of them, HyNaC2 and HyNaC3, co-assemble to form an ion channel that is gated by the neuropeptides Hydra-RFamides I and II. The HyNaC2/3 channel is so far the only cloned ionotropic receptor from cnidarians and, together with the related ionotropic receptor FMRFamide-activated Na(+) channel (FaNaC) from snails, the only known peptide-gated ionotropic receptor. The HyNaC2/3 channel has pore properties, like a low Na(+) selectivity and a low amiloride affinity, that are different from other channels of the DEG/ENaC gene family, suggesting that a component of the native Hydra channel might still be lacking. Here, we report the cloning of a new ion channel subunit from Hydra, HyNaC5. The new subunit is closely related to HyNaC2 and -3 and co-localizes with HyNaC2 and -3 to the base of the tentacles. Coexpression in Xenopus oocytes of HyNaC5 with HyNaC2 and -3 largely increases current amplitude after peptide stimulation and affinity of the channel to Hydra-RFamides I and II. Moreover, the HyNaC2/3/5 channel has altered pore properties and amiloride affinity, more similarly to other DEG/ENaC channels. Collectively, our results suggest that the three homologous subunits HyNaC2, -3, and -5 form a peptide-gated ion channel in Hydra that could contribute to fast synaptic transmission.

Mechanical oscillations orchestrate axial patterning through Wnt activation in Hydra.

Mechanical input shapes cell fate decisions during development and regeneration in many systems, yet the mechanisms of this cross-talk are often unclear. In regenerating Hydra tissue spheroids, periodic osmotically driven inflation and deflation cycles generate mechanical stimuli in the form of tissue stretching. Here, we demonstrate that tissue stretching during inflation is important for the appearance of the head organizer­a group of cells that secrete the Wnt3 ligand. Exploiting time series RNA expression profiles, we identify the up-regulation of Wnt signaling as a key readout of the mechanical input. In this system, the levels of Wnt3 expression correspond to the levels of stretching, and Wnt3 overexpression alone enables successful regeneration in the absence of mechanical stimulation. Our findings enable the incorporation of mechanical signals in the framework of Hydra patterning and highlight the broad significance of mechanochemical feedback loops for patterning epithelial lumens.

Disp1 regulates growth of mammalian long bones through the control of Ihh distribution.

Dispatched1 (Disp1) is required for the release of cholesterol modified hedgehog (Hh) proteins from producing cells. We investigated the role of Disp1 in Indian hedgehog (Ihh) signaling in the developing bone bypassing the lethality of the Disp1(C829F) allele at early somite stages through the supply of non-cholesterol modified Sonic hedgehog (N-Shh). The long bones that develop in the absence of wild-type Disp1, while clearly shorter, have a juxtaposition of proliferating and non-proliferating hypertrophic chondrocytes that is markedly more normal in organization than those of ihh null mutants. Direct analysis of Ihh trafficking in the target field demonstrates that Ihh is distributed well beyond Ihh expressing cells though the range of movement and signaling action is more restricted than in wild-type long bones. Consequently, a PTHrP-Ihh feedback loop is established, but over a shorter distance, reflecting the reduced range of Ihh movement. These analyses of the Disp1(C829F) mutation demonstrate that Disp1 is not absolutely required for the paracrine signaling role of Ihh in the skeleton. However, Disp1 is critical for the full extent of signaling within the chondrocyte target field and consequently the establishment of a normal skeletal growth plate.

An Hh-dependent pathway in lateral plate mesoderm enables the generation of left/right asymmetry.

Breaking bilateral symmetry is critical for vertebrate morphogenesis. In the mouse, directional looping of the heart and rotation of the embryo, the first overt evidence of left/right asymmetry (L/R), are observed at early somite stages ( approximately E8.5) [1, 2]. Activation of a Nodal-Pitx2 regulatory pathway specifically within the left lateral plate mesoderm (LPM) is critical for these events [3-10]. Asymmetric expression of Nodal is thought to be triggered by left-oriented, cilia-generated flow within the ventral, midline node [11, 12]. Genetic removal of Hedgehog (Hh) signaling in the mouse demonstrates a requirement for Hedgehog signals in the symmetry-breaking process [13], and analysis of node trafficking has suggested a mechanism of directional transport in the node that might relate to symmetry breaking in the LPM [14]. Here we provide evidence that Hedgehog signaling in the node is not essential for breaking bilateral symmetry. In contrast, direct Hh signaling in the LPM is critical. Evidence is presented that Sonic and Indian hedgehog signals act together, through a Foxf1/Bmp4 pathway, to enable the initiation and propagation of Nodal signaling within the LPM, regulating the competence of that tissue to respond to the Nodal pathway.