The structure and mechanism of the nematocyst – the organelle responsible for jellyfish and sea anemone stings, has been elucidated and caught in action in the starlet sea anemone for the first time.
During jellyfish season, people are often reminded to avoid swimming in the sea or to wear proper protective gear when snorkeling, lest one learns a painful lesson. But what goes behind the stinging process?
Researchers from the Stowers Institute for Medical Research have created a realistic working model of the nematocyst – the stinging organelle – of Nematostella vectensis, commonly known as the starlet sea anemone. The team of researchers published their findings in Nature Communications in June, under the lead of Ahmet Karabulut, a predoctoral researcher supervised by Matt Gibson, Ph.D.
Their efforts to understand the underlying mechanism of stinging were aided by the latest advanced microscopic imaging techniques, together with the creation of a realistic model for a more holistic picture of the complex mechanisms that underlie the stinging function of the nematocyst. This knowledge may potentially be applied to other fields, for example, in microscopic drug delivery devices.
The researchers’ model for the nematocysts, specialised stinging organelles, allows for a deep understanding of the complicated structure and the biomechanisms behind their firing actions. 3D electron microscopic techniques and the targeted reduction of chosen gene functions (gene knockdown) were used to examine them to locate the source of the kinetic energy that renders it sufficient to puncture and inject venom into its prey.
“We utilised fluorescence microscopy, advanced imaging techniques, and 3D electron microscopy combined with genetic perturbations to understand the structure and operating mechanism of nematocysts,” said Karabulut.
Three distinguishable phases of the nematocysts were observed during the biomechanical transformation and instantaneous release of the toxin when firing. The first phase occurs when a capsule is ejected from the nematocyst, further releasing a tightly-coiled structure from it, penetrating the target. The release of the structure phase is powered by sudden changes in the osmotic pressure, due to the entry of water and the elastic movements of the capsule.
The second phase occurs when the part of the ejected thread, called the shaft, is further extended by the conversion of elastic energy to kinetic energy in a process named eversion. Eversion is the process in which the shaft of the capsule turns inside out, forming a structure with three helixes that encapsulate a delicate inner capsule covered in barbs, containing a mixture of neurotoxins.
The third phase culminates in the ejection of the neurotoxins from the inner capsule through its eversion, moving deeper into the soft tissues as it stretches and releases the toxins.
All three phases are completed within several thousandths of a second, and as a result, the complete stinging process of a nematocyte ranks among one of the quickest, naturally-occurring biological processes. “The earliest phase of the firing of the nematocyst is extremely fast and hard to capture in detail,” said Karabulut.
Karabulut managed to observe the phases of the live firing of the fluorescent dye-treated nematocysts after successfully formulating a chemical treatment that was able to both trigger the process and maintain the integrity of the different internal structures of the nematocysts, serendipitously getting to document the whole process unfolding.
According to Karabulut, the entire process viewed under the microscope resembled a burst of fireworks due to the ejection of the threads from the nematocysts. The nematocysts ejected a portion of their threads out while the reagent used fixed them.
He was also able to record the geometric transformations of the nematocyst thread throughout the entire firing process, allowing the geometric transformations throughout the whole process to be studied closely for further insights into the transformations during firing.
A better understanding of nematocyst firing in sea anemones allows for potential developments in the engineering of miniature devices, which would help advance technologies in the field of drug delivery at the nanoscale.
Source: Karabulut et al. (2022). The architecture and operating mechanism of a cnidarian stinging organelle. Nature Communications, 13(1), 1-12.