Glass plates to catch the model organism Trichoplax in its natural habitat, warm coastal waters. Scientists at Kiel University use the tiny placozoan for evolutionary research.
Photo Credit: © Harald Gruber-Vodicka, Kiel University
Scientific Frontline: "At a Glance" Summary
- Main Discovery: The simple marine animal Trichoplax utilizes an ancient, bacteria-derived lysozyme for acidic extracellular digestion, proving that essential animal immune mechanisms evolved from early digestive processes.
- Methodology: Scientists characterized the enzyme in Trichoplax sp. H2 using proteomics and Western blotting, monitored in situ pH levels with fluorescence reporters, and reconstructed the enzyme's evolutionary history via structure-based phylogenetics.
- Key Data: The research identified a glycoside hydrolase family 23 (GH23) lysozyme that exhibits peak activity at pH 5.0, precisely matching the acidic environment generated within the animal's temporary feeding grooves during nutrient uptake.
- Significance: This study provides the first evidence that metazoan GH23 lysozymes originated from a horizontal gene transfer event from bacteria to a pre-bilaterian ancestor, functioning simultaneously in nutrition and pathogen defense.
- Future Application: Elucidating these ancient dual-use mechanisms clarifies the evolutionary trajectory of the innate immune system and may inform the development of bio-inspired antimicrobial agents.
- Branch of Science: Evolutionary Biology, Immunology, and Marine Biology
- Additional Detail: The lysozyme features a unique N-terminal cysteine-rich domain that stabilizes the protein during transport but is cleaved off to maximize enzymatic potency at the site of action.
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| Trichoplax under a high-resolution dissecting microscope. The tiny sea creature has no organs, muscles, or nerves. CAU researchers discovered that the animals possess a highly effective enzyme that plays an important role in our defence mechanism against bacteria. Photo Credit: © Harald Gruber-Vodicka, Kiel University |
Marine animals, such as the extremely simple flatworm Trichoplax, are ideal model organisms for studying the early evolutionary origins of animal life processes. Despite measuring only a few millimeters and lacking true organs or nervous system, this animal interacts effectively with bacteria. A highly efficient enzyme, goose-type lysozyme (PLys, GH23), plays a key role in this process. Trichoplax uses this enzyme specifically during digestion to degrade bacterial cell walls and neutralize ingested bacteria.
Researchers at the Zoological Institute of Kiel University (CAU) have now shown for the first time that this lysozyme is highly active and widely used by animals. Additionally, they traced the evolutionary origin of this enzyme. These important G-type lysozymes, which humans also process in two copies, originally came from bacteria. They were transferred to early animal ancestors through horizontal gene transfer, the inheritance of genes between unrelated organisms and further evolved. This indicates that this antibacterial mechanism appeared much earlier in the animal family tree than previously assumed. The study offering a new perspective on the early development of animal defense mechanisms was published in the journal Communications Biology.
Enzymes as central players in bacterial defense and digestion
Animals constantly interact with bacteria. These interactions range from beneficial symbiosis between host and microorganism to one-sided exploitation, as in the case of organisms that consume bacteria. Animals have a variety of molecular tools to defend themselves against bacteria and facilitate digestion. Lysozymes, which are enzymes that destroy bacterial cell walls, play a central role in this process. Lysozymes are found in humans, insects, and mollusks, and they are classified into different types. One of the most important groups is the G-type lysozymes (GH23), named after the first species in which they were detected: a grey goose protein. However, as previous research results show, these lysozymes appear to be unevenly distributed among animals. Their evolutionary origin was unclear for a long time. Researchers from the CAU working groups “Marine Symbiosis” and “Comparative Immunobiology,” in collaboration with colleagues from microbiology and experimental medicine, have now clarified this in their current study.
Simplest marine organisms as model organisms
The Kiel researchers investigated the ancient placozoan lysozyme PLys, which occurs in the extremely simple, disc-shaped animal Trichoplax. These tiny, flat marine organisms are free-living invertebrates belonging to one of the most primitive animal groups on Earth: the placozoans. Trichoplax has no intestines, muscles, or nervous system, and its extremely simple structure makes it an important model organism for evolutionary research.
Advanced methods from CAU working groups help to reveal complex mechanism
To characterize the role of PLys in Trichoplax, the researchers used in vitro methods to determine its natural protein forms and cellular distribution. They employed state-of-the-art mass spectrometry and a variety of biochemical, immunobiological, and microscopic methods in collaboration with several CAU working groups. "We were able to prove that PLys is a highly active GH23 lysozyme. Trichoplax produces it specifically in cells on its underside, where it takes in food by grazing on biofilms of algae and bacteria. The enzyme plays a key role in efficiently breaking down this food,” explains biochemist Henry Berndt, first author of the study and doctoral researcher at Kiel University in the Marine Symbiosis working group. "The animal actively adjusts the pH value in the seawater beneath its body, in the area where it is digesting, so that the lysozyme can work optimally. Here, we have encountered complex defense and digestive mechanisms in a very simple animal," Berndt continues. Prof. Matthias Leippe, whose supervised Berndt’s early work, is delighted with the successful handover to his successor, Prof. Harald Gruber-Vodicka. Leippe adds, "We had suspected that such an important and efficient enzyme must have originated earlier in the family tree than previously assumed."
Horizontal gene transfer introduces key functions into early animals
The team led by Professor Gruber-Vodicka systematically investigated these clues and analysed the evolutionary origin of this enzyme using modern protein structure-based phylogenetic analyses. In doing so, the researchers constructed a deep family tree of GH23 lysozymes for the first time. Their findings revealed that animal lysozymes of this type do not have an animal origin but rather trace back to an ancient horizontal gene transfer. In this process, genetic material is transferred from one organism to another without a family relationship, such as between parents and offspring.
"Our results show that this central antibacterial lysozyme was transferred directly from bacteria to an early animal ancestor, presumably in a lineage predating the emergence of bilaterally symmetrical animals, including humans, by more than 600 million years," says Professor Harald Gruber-Vodicka, head of the Marine Symbiosis working group at the Zoological Institute of the CAU. Professor Gruber-Vodicka has researched this group of animals and Trichoplax for a long time and has already identified two bacterial symbioses in them. "After this gene transfer, the enzyme was retained in many animal lineages, including humans. It was partially duplicated and functionally developed. Humans, for example, have two copies, one of which plays an important role in defending against bacteria through our skin."
The study provides crucial insights into the origins of animal defense mechanisms and digestion and into deep branches of the animal family tree in which horizontal gene transfer played a greater role than researchers had previously assumed.
Funding: Collaborative Research Center (CRC) 1182, "Origin and Function of Metaorganisms," and was developed at the intersection of the university's priority research areas Kiel Marine Science and Kiel Life Science.
Published in journal: Communications Biology
Title: An ancient lysozyme in placozoans participates in acidic extracellular digestion
Authors: Henry Berndt, Igor Duarte, Urska Repnik, Michel A. Struwe, Mohammad Abukhalaf, Axel J. Scheidig, Andreas Tholey, Harald R. Gruber-Vodicka, and Matthias Leippe
Source/Credit: Kiel University
Reference Number: ebio021126_01
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