The Guardians of the Sea Urchins

Sannidhi Krovvidi

In any good game, defense is the secret weapon wielded behind the offense. Whether it is soccer, football, or basketball, no amount of offense can beat a truly strong defense. They are the guardians of the game on either side, working towards warding away the offensive threats. These guardians are systems built foundationally in most organisms – also known as immunity!

Lytechinus pictus, otherwise known as the painted urchin, contributes a special quality to research aimed towards discovering the role of innate immunity as it is fully genetically enabled, meaning that genes of interest are easy to amplify and study. Innate immunity is the organism’s front line defenders, naturally ingrained in the organism. In contrast, adaptive immunity grows and evolves as the organism is exposed to varied and diverse environments. 

Gloria Renaudin, undergraduate researcher at UCSD, sought to explore this picture within sea urchins further. In sea urchins, coelomocytes, a type of immune cell, play an instrumental role in this defense, transcribing what are known as Scavenger Receptor Cysteine-Rich (SRCR) genes; these are key in the defense line of the urchins. SRCR receptors, similar to defensive guards blocking the opposing offense, help with pathogen recognition and clearance. A well known example of the influence of SRCR expression can be found in the model of Strongylocentrotus Purpuratus, the Pacific purple sea urchin; Abundant along the California coast, is easily identifiable by its vibrant dark purple color. Identified in the early pluteus stage, an early larval stage for sea urchins, it possesses over one thousand SRCR domains. However the location and function of these SRCR genes are unknown in the L. Pictus. In invertebrates specifically, SRCRs are more diverse than in mammals. While humans have 81 SRCR genes, S. Purpuratus has 1095 SRCR genes. Considering that sea urchins lack adaptive immunity, or the ability to shift their defense strategies based on the opponents, it is interesting to delve deeper into the fundamentals of their immune system.

Blastocoelar cells pertain to the development of the embryo, as well as pigment cells, which in turn develop into coelomocytes, immune cells that attack or digest invading organisms in the immune system. Blastocoelar cells similarly aid in identifying and phagocytizing, or ingesting, foreign particles. Blastocoelar cells & pigment cells are types of immune cells present before metamorphosis, or the transformation into adult sea urchins. After metamorphosis, they are known as coelomocytes since they’re in the coelomic cavity, a fluid filled body between the outer wall and digestive tract, of the sea urchin. These coelomocytes act as the active defense, and are able to function in a timely manner against imminent threats. The bright red color of pigment cells is derived from polyketide synthase, or PKS1, which synthesizes echinochrome A, a common pigment in sea urchins utilized in pharmaceutical drugs. All immune cells express SRCRs, but for the purposes of this research, SRCR142 was chosen to be of interest due to previous studies. SRCR142 was found along with PKS1 in the early developmental stage, as seen in S. purpuratus, whose model was used to base this further study within the L. pictus. Additionally, L. pictus was chosen as it has larger identifiable transparent eggs, with faster development compared to S. purpuratus. Preliminary data shows complex interaction between SRCR142 and PKS1, which can be interpreted as any sort of interaction within close proximity of the two molecules. Additionally, there was increased expression of SRCR142 in both coelomic pouches, which are areas responsible for the growth and development of the sea urchin. PKS1 localized in the gut of the sea urchin when an immune threat was observed. Proceeding experimentally, the expression of SRCR142 and PKS1 in terms of the development of sea urchins could be of interest, Renaudin states.

Hypothesizing that SRCR142 would be expressed in pigment cells throughout development of the urchins, and that its expression would increase as the larvae approach metamorphosis, the goal is to locate SRCR142 during development to find its specific function within the innate immune system of the sea urchin, i.e., to pinpoint its role on the defense team as the sea urchin grows.

Initially, Renaudin raised the sea urchin larvae in seawater with an airpick to oxygenate the water, and a rotating paddle to keep them in the water column without settling at the bottom. As they grew, they were fed the phytoplankton Rhodomonas Lens. To begin metamorphosis, the competent larvae were added to a biofilm plate to keep them at the same metamorphosis stages.

As the larvae developed, they were checked under the microscope to confirm that the respective developmental stages were present, and were then collected. Consequently, the stages were visualized. This visualization was done with the help of a ‘fix’ buffer, which helped essentially freeze the stages as they were in order to collect them. Following the fixing of the stages, Hybridized Chain Reactions, or HCRs are performed with the help of probes, specific DNA sequences, designed considering the L. pictus transcriptome. In this case, they were designed in the lab. HCRs consisted of adding the probes to the fixed larvae samples then adding hairpins to amplify the signal. They were then imaged, edited, and compiled for further analysis to visualize SRCR142 expression.

The fundamental hypothesis while investigating the defensive roles within these sea urchins was that SRCR142 would be expressed in the pigment cells. These pigment cells are what we know as the active defense, moving towards sites of infection in order to protect the urchin; Our offensive players moving towards the opponents goal. However, it could be seen that SRCR142 was not only closely found with the pigment cells, but also specifically had a higher expression in the left coelomic pouch. It is interesting to note that this pouch is what gives rise to the rudiment, which leads to the development of the adult sea urchin. In a game of basketball, this pouch is the basket and it is of the utmost importance to keep the opposing offense away from it. This shows that SRCR142 intriguingly has an unknown part to play in protecting the growing sea urchin and its development instead of overall larvae immune response. It protects cells from the germline, which contains the genetic material passed down through future generations, playing the role of perhaps the most important defense of all.

The data demonstrates that SRCR142 is being expressed in the left coelomic pouch where the adult rudiment develops from the larval stages. The foundation of an unmovable defensive team comes from its ability to spot and respond to imminent offensive threats from the other team. An efficient defense is present where the largest opposing offense is. Thus, like any solid defense, the expression of SRCR142 moves towards the gut, where there is growing waste and unknown harm from ingested bacteria and foreign particles. The gut is also right next to the left coelomic pouch so the space in between these two structures creates the main court for the defense and offense. Imagine a baseball field, the left coelomic pouch is our homebase to defend. The space between the left coelomic pouch and the gut is the field, while the gut itself is another important base to defend: We are the team on the field, and the offense is batting! Keeping the larger picture in mind, to keep the opposing forces away from home ground, SRCR142 can be seen protecting the most important area of the urchin in terms of its future growth and development. In the sea urchin, we can see how nature seems to be a natural skilled athlete by creating the perfect defensive team, or immune system.