Why do animals develop

Stepping stone to evolution

What coeled animals reveal about the development of humans
by Thomas Holstein

They are mainly made of water, and inside they are hollow. At first glance, coeled animals do not appear to be very promising objects of molecular research. And yet science can learn a lot from them, for example, on what the extraordinary regenerative ability of the simply built beings is based on, which can regenerate almost at will - even if they are divided into 100 pieces. This amazing regenerative power is by no means the only remarkable ability of these fascinating animals. You can also learn amazing things from them about the workings of evolution, which has used important gene groups as a springboard - right up to humans.

They have no blood, no brains and no heart and yet they are superlative organisms: the cnidarians, scientifically called cnidaria, probably better known as sea anemones, jellyfish and corals. They have conquered every underwater habitat, from the Antarctic to the tropics, from the deep sea to freshwater, their ability to regenerate is legendary, and the latest findings show that the complexity of their genome is amazingly close to that of vertebrates. What else do we know about the supposedly simple beings and their remarkable abilities?

Cnidarians consist of 99 percent water and are among the oldest animals still alive today. They are living fossils and are at the base of the evolution of all higher animals, near the transition from the unicellular to the multicellular level of organization. Cnidarians were found in the fossils of the Ediacaran fauna 600 million years ago, long before the majority of all animal phyla known today were created in the so-called Cambrian explosion.

In their development, cnidarians stop at the so-called gastrula stage: They have only one body axis that leads to a sac, into which food flows and from which indigestible food is excreted in the opposite direction. More highly developed animals only go through this stage of development as a brief intermediate stage, from which an organism with a mouth and intestinal opening develops. Cnidarians have a primitive nervous system that is organized as a simple neural network. There is no central nervous system, but some already have complex eyes and other sensory organs.

Cnidarians often appear in two forms: as fixed polyps and as free-swimming jellyfish (medusa), whose graceful beauty was impressively documented by the famous zoologist Ernst Haeckel in his book "Kunstformen der Natur". Many of the coral polyps have shaped our geological history as forms that form rocks and reefs in the truest sense of the word. The almost unlimited regenerative capacity of animals, which has been known since ancient times, is also famous: Similar to the many-headed "Hydra of Lerna" known from Greek mythology, many polyps can regenerate their heads, which are covered with poisonous stinging cells.
Some cnidarians are among the most poisonous animals in the world. One example is the tropical sea wasp, a box jellyfish that feeds on fish - contact with it can be fatal for humans too. But even the encounter of a swimmer with far less dangerous jellyfish or polyps can leave very painful marks.

Discharge in nanoseconds

The toxic effect of the cnidarians is due to the cells from which they owe their name: the nettle cells. These are highly specialized sensory cells, each of which houses a complex small organ, the nettle capsule, scientifically correct called "nematocyst" or "cnide". A long tube is rolled up inside the cylindrically shaped, approximately ten micrometer small nettle capsule. This is the basic construction plan of the nettle capsule - based on it, nature has formed a large number of, in some cases very complex, nettle capsules, all of which are used for prey and defense.

The functioning of the nettle cells is extremely remarkable. If a nettle cell is mechanically stimulated from the outside, for example by a prey, it discharges within a very short period of time: The tube rolled up in the capsule shoots out like a harpoon, penetrates the victim's outer skin or wraps its body. Our high-speed analysis has shown that even with the most complex capsule types, the entire discharge is completed in less than three milliseconds; the critical phase of the discharge even takes place in the nanosecond range. Accelerations are achieved that are more than 5,000,000 times the acceleration due to gravity - the stinging capsule discharge is one of the fastest processes in biology.

At the molecular level, the discharge can be explained as an interplay between high pressure and the elastically tensioned capsule wall. The high pressure results from the high concentration of poly-gamma-glutamate (two moles) and associated cations (internal pressure more than 150 bar). In recent years we have characterized a family of unusually small collagens (“mini collagens”) and a new protein family (NOWA) as the essential structural proteins of the elastic capsule wall. When the capsule is formed within a "giant" vesicle that is part of the cellular protein synthesis machinery, these proteins are in soluble form and initially combine to form a preliminary structure, which then becomes the final capsule through a polymerization reaction. The proteome of a nettle cell, i.e. the entirety of all proteins present in the cell, comprises around 200 proteins, the structure and function of which are currently being identified in a separate proteome project.

At the center of our work at the Heidelberg Institute for Zoology is the question of how cnidarians develop. An unexpected result of comparative developmental biology and genome research is that animal organisms apparently already had an astonishingly large repertoire of genes at a very early stage of evolution, with which they control the development of body structure. In our search for such genes, we selected the cnidarians as the most important representatives of simple multicellular organisms, examined their shape-forming genes and compared them with those of more highly developed animals.

The so-called Wnt genes play a key role. These genes are a group of developmental genes that are responsible in all animals for the development of a body axis and the maturation of the respective organs and the nervous system. The genes supply signal molecules (glycoproteins) containing sugar. These molecules instruct their target cells to develop in a certain direction.

Gene groups that are millions of years old

In the sea anemone Nematostella vectensis, we found twelve Wnt gene families, which is astonishing in several respects: The simple cnidarians thus have more Wnt development genes than some more highly developed animals, such as insects or nematodes, which only have seven groups of these shaping genetic make-up feature. Contrary to what was previously assumed, there does not seem to be any direct connection between the number of genes and the morphological complexity of animal organisms. Mammals, including humans, have twelve Wnt gene groups like the cnidarians, whereby in mammals at least one of the developmental genes has been lost during evolution and has been replaced by a new one. In protozoa, which only consist of a single cell, and in organisms which, like slime molds, form cell colonies but do not develop into true multicellular cells, no Wnt genes have so far been detected. The appearance of these genes around 650 million years ago is likely to have been the prerequisite for the development of multicellular cells.

The genetic comparisons currently taking place make it more and more clear that cnidarians and more highly developed vertebrates (vertebrates) are much more similar than previously believed. The variety of signaling pathways known from animal organisms is already established in the genome of the cnidarians. The new genetic data also underscores our discovery that many of these old genes have been lost in some animal groups; conversely, the data show how important these genes must have been for the rapid expansion of the genetic repertoire during evolution and for the emergence of multicellularity.

The example of the “mesodermal” genes shows how genetic complexity has progressed during the evolution of morphological complexity. These are hereditary factors which, during the development of the embryo, ensure that a middle cotyledon, the so-called mesoderm, is formed. Cnidarians have only two cotyledons: an external protective ectoderm and an internal digestive endoderm. Cnidarians do not have a mesoderm, from which the vascular system and muscles emerge in higher living beings. Nevertheless, cnidarians have the complete catalog of mesodermal genes that are responsible for the maturation of the so-called epithelial muscle cells - these are cells that are found in the outer ectoderm and that are equipped with muscle-cell-like contractile fibers.

The genes for embryonic development and the maturation of cells into certain cell types with special tasks, such as muscle cells, go back to the early days of evolution. How it came about that nature repeatedly used the same signal chains in new cell types, structures and organs in the course of evolution is still completely incomprehensible and is currently being intensively researched.

The phenomenon of "regeneration" is a vivid example of how deeply basic development processes are anchored in the family tree of life. Certain representatives of the cnidarians, the freshwater polyps (hydrozoans), are the "champions of regeneration" in the animal kingdom, which can be impressively shown when a freshwater polyp is cut into 100 parts: after a few days, 100 new, well-formed polyps have emerged. It seems like a miracle that complete bodies can emerge from just a few cells. But not only polyps, but also flatworms, starfish and salamanders are capable of this miracle and regenerate limbs and internal organs immediately after the original has been lost.

In recent years it has been possible to identify genes, proteins and signaling pathways in freshwater polyps and other regenerating animals that contribute to this amazing regenerative power. We humans also basically still have the genes that simple animals use to regenerate themselves - the gap between these organisms and humans is therefore smaller than expected.

Regenerating organisms replace lost or damaged body parts and organs with the help of stem cells: freshwater polyps, for example, have a population of stem cells throughout their life that they can mobilize and use when necessary to create a wide variety of parts of the body. Other organisms, such as newts and fish, convert mature (“differentiated”) cells, which have already specialized in skin, muscle or nerve cells, back into stem cells, a process known as “dedifferentiation”. Humans also have stem cells in many tissues. However, the ability of these “adult” stem cells to regenerate certain cell types is limited. In all cases it is important to understand where the regenerating cells get their instructions from and which genes, proteins and signaling pathways are responsible for the regenerative capacity.

In the case of freshwater polyps, we were able to show that the products (proteins) of the Wnt genes do not only arise during embryonic development or budding. They also arise when a freshwater polyp, which has lost its upper body part, its "head", begins to regenerate. We wanted to know: How many cells are required for a new head to develop? To answer this question, we dismantled the regenerating tip of the polyp into individual cells and initially let these cells grow into groups of different sizes. If you add these cell nests to accumulations of body cells, you can find out how many cells are required for head formation. The result: only about ten cells are required for this.

In addition to the Wnt molecules, other signaling molecules and regulatory proteins are involved in the regeneration of freshwater polyps - all genes that are also active during the development of higher animals, including that of mammals. We therefore assume that there is a minimum common set of genes required for the pattern formation and growth of limbs and organs of complex animals.

We are only at the beginning of our work - but fascinating application possibilities are already conceivable today. Research on freshwater polyps and other simple developmental systems could reveal how genes and proteins that control development can be switched on and off during regeneration. This knowledge could perhaps be used to specifically induce the regeneration of injured or diseased tissue - including that of humans.

Prof. Dr. Thomas W. Holstein has headed the Department of Molecular Evolution and Genomics since 2004 and is Managing Director of the Institute for Zoology at Heidelberg University. Before that he worked at the Universities of Darmstadt and Frankfurt and was a visiting professor for molecular developmental biology at the University of Vienna.

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