Topology changes of the regenerating Hydra define actin nematic defects as mechanical organizers of morphogenesis

Background

Morphogenesis, describing how tissues are shaped during development, is tightly coupled to regeneration, sharing common processes and principles. Regenerative systems can therefore be an accessible way to investigate development.

One organism famous for its regeneration is the small cnidarian Hydra, its name derived from the mythical head-regenerating beast¹. If you haven’t come across one before, it has a very simple body plan along a single axis, essentially a two-cell thick tentacled tube with a head at one end and a foot at the other. The outer layer has a network of contractile actin fibres spanning multiple cells (supracellular) aligned with the body axis, while the inner layer has actin in rings encircling the body. Hydra became a favourite of biologists for its simplicity but also its regenerative powers, but, unlike its namesake, Hydra only recover one head when one is lost. Well, usually…

Previous studies of the actin networks considered them as liquid crystal-like nematic orders – the fibres are parallel, but not arranged in defined planes. Such systems can have ‘defects’ in their topology, focal points of disruption in the ordered actin arrangement. Mature Hydra has several types of these defined by their ‘topological charge’ – how many times the director orientation rotates when going once around the defect centre. The head and foot have +1 ‘aster’ defects with actin converging on/diverging from a central point, while the tentacle bases have -1/2 ‘comet’ defects with actin ‘curving’ around the defect centre (see Figure 1). These are powerful regulators of tissue stress, controlling how tissues move and deform. In particular, the sites of topological defects in regenerating Hydra coincide with the future mouth and foot².

Figure 1: Hydra body plan and actin nematic defects.

In this work, the authors set out to manipulate actin organisation in regenerating Hydra to probe its role further, particularly in defining new heads. Through an elegant interplay of experiment and theory, they reveal that topological defects act as organisers of the regenerating body plan. Taking a broader outlook, they speculate that topology could be a recurring theme in the evolution of common structures in body plans.

Results

To alter actin nematic order while regeneration was underway, Hydra were chopped in half and the tissue piece with the foot preserved (head-regenerating) then sandwiched between agarose and glass. By tuning the stiffness of agarose, they altered the degree of compression. They were confined for four days and released on the fifth, later assayed for regeneration on the twelfth day.

When compressed, the tissue could lie ‘flat’ (lateral), with perpendicular compression, or ‘upright’, with the regenerating head (aboral) or preserved tail (oral) against the glass and compression parallel to the axis. On the softest compression conditions, all tissues were lateral, but with stiffer agarose more were unable to reorientate and hence were trapped in aboral or oral conformations.

In the soft gel conditions, a significant number of embryos regenerated with an extra fully-functional head (so-called biaxial animals). Stiffer gels resulted in more dying, particularly the aboral/oral tissues. This hinted that there might be a ‘sweet spot’ of compression that promoted biaxial regeneration, while not restricting the bodily contractions that are essential for regeneration.

 

Figure 2: Phenotypes generated by regenerating Hydra under compression/release

They checked out what the actin looked like in the two-headed animals. While the organisation was largely preserved, additional defects were found at the mouth and between the heads. To see what actin was up to as the regeneration process was underway, they switched to live imaging using Hydra with GFP-tagged actin. In cases where the tissue was orientated orally and regenerated biaxially, they could see two +1 asters where the heads emerged and speculated that the orchestration of forces by this defect would sculpt the tissue into a projecting head.

To dive into this further, they turned to computational modelling. Assuming the Hydra tissue to be an elastic force-generating material, they encoded the actin field observed experimentally for biaxial animals. When simulated, a spherical surface split along the centre to give a biaxial form, each head with a +1 aster defect.

Figure 3: Uniaxial and biaxial regeneration predicted computationally based on actin defects.

Despite being released from compression, some stubborn tissues simply refused to regenerate. Instead, they formed a toroid (doughnut-shape), particularly when aboral/oral, by essentially tearing themselves from the centre and re-healing. Toroids are pretty special topologies as they are the only form in which actin superstructures can be perfectly ordered; with rotational symmetry in their actin orientation, they are devoid of defects. Is this why they don’t regenerate?

To test this, they took spheroids with disordered actin (spheres arising from an excised Hydra body column) and compressed them to form toroids. However, these toroidal tissues did have aster topological defects that emerged as the actin re-organised. These became the later sites of heads and feet, showing that if a toroid does gain a defect, it gains regeneration too. What’s more, when left to fully regenerate, they emerged as toroidal ring-like Hydra, complete with a head and foot. Some even sported some new phenotypes, like two-footed animals!

Figure 4: Toroidal models with and without defects and toroidal Hydra

What I liked about this preprint

The role of physical cues in development and regeneration has been greatly underappreciated, with biochemical models dominating the field for decades. This study places mechanics in the spotlight and uses, in my opinion, a really intriguing system to experimentally pin down its role. It’s also refreshing to read a paper that uses a model organism I’m less familiar with – I doubt there are many systems where you can prod and poke so intensively and still successfully generate such outcomes as animals with a hole in the middle! I’ll be on the lookout for more exciting insights from Hydra.

References

1. Vogg, M. C., Galliot, B. & Tsiairis, C. D. Model systems for regeneration: Hydra. Development 146, dev177212 (2019).
2. Maroudas-Sacks, Y. et al. Topological defects in the nematic order of actin fibres as organization centres of Hydra morphogenesis. Nature Physics 17, 251–259 (2021).

Questions for the authors

• Could a system be developed to compress the Hydra biaxially, or stretch them uniaxially? What would you expect to see?
• Are there any other developmental systems where actin is similarly organised to generate a protrusion?

 

doi: https://doi.org/10.1242/prelights.37285

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