Fluorescent microscopy showing the mechanism for the sea anemone's stinging organelle in three distinct phases. Credit: Gibson Lab, Stowers Institute for Medical Research
Jellyfish, sea anemones and coral, members of the phylum Cnidaria, use specialized organelles known as nematocysts to sting other organisms for food or as a defense mechanism. With the entire stinging operation occuring in the course of just a few thousandths of a second, the firing of a nematocyst is one of the fastest biological processes occurring in nature, and therefore difficult to observe and study to fully understand how the stinging process works. Now, researchers from the Stowers Institute for Medical Research have used a combination of techniques, including live and super resolution microscopy, 3D electron microscopy and genetic alterations, to capture the dynamics of a sea anemone’s sting in unprecedented detail and develop a precise operational model for nematocysts.
Predoctoral researcher Ahmet Karabulut, who led the study, was inspired after serendipitously capturing an image of multiple nematocysts at different stages of firing. This occurred after he incorporated a fluorescent dye into a sea anemone and then applied a combination of solutions that both activated nematocyst discharge and simultaneously fixed the sample. To study the structure and firing of starlet sea anemone (Nematostella vectensis) nematocysts in more detail, the team utilized a spinning disk microscopy system for live imaging of a discharging nematocyst at 5 millisecond intervals, as well as super resolution confocal microscopy to unveil the precise structure of barbs and tubules, labeled in different colors, at certain phases in the firing process. The team further imaged serial sections of the organelles using scanning electron microscopy to create detailed 3D reconstructions of nematocysts’ microarchitecture. Gene knockdown studies revealed the importance of certain genes and proteins, such as lectins, in the development of nematocysts, including their tubule and barb structures and their ability to discharge effectively.
The researchers found that the kinetic energy for piercing and poisoning a target involves both osmotic pressure and elastic energy stored within multiple nematocyst sub-structures. The discharge process of N. vectensis nematocysts was divided into three distinct phases. In the first phase is the initial, projectile-like discharge and target penetration of a densely coiled thread from the nematocyst capsule. This process is driven by osmotic pressure from the sudden influx of water and elastic stretching of the capsule. The second phase marks the discharge and elongation of the thread’s shaft substructure, which is further propelled by the release of elastic energy through a process called eversion. During eversion, the shaft turns inside out and forms a triple helical structure surrounding a fragile inner tubule covered with barbs containing a cocktail of toxins. In the third phase, the tubule begins its own eversion process to elongate into the soft tissue of the target, releasing neurotoxins along the way. This study was published in Nature Communications.
“Understanding this complex stinging mechanism can have potential future applications for humans,” said corresponding author Matt Gibson. “This could lead to the development of new therapeutic or targeted delivery methods of medicines as well as the design of microscopic devices.”