By Haley Dion, VI Form

Unique Advances in Transplant Research with Hydractinia

Transplantation is the future of medicine. It is an ever-evolving field of research. For three weeks this summer, I was given the opportunity to take part in the research by interning at the Thomas E. Starzl Transplant Institute. At the institute, I worked in the Nicotra Lab under the mentorship of Dr. Matthew Nicotra. The Nicotra Lab is one of the Stuart K. Patrick Research Laboratories at the Institute named after St. Mark’s alumnus, Stuart K. Patrick ’57. The lab I worked in is unique because it works with an organism that is very rarely used in research: Hydractinia

Hydractinia are invertebrates that live on hermit crab shells. These organisms are part of the cnidarian species, and they grow as colonies. Hydractinia grow mat tissue, which is the base of their colony. Within the mat, there are gastrovascular canals that allow cells to flow throughout the colony. Some Hydractinia have stolons, branched stem-like structures, that extend from their mat. Hydractinia also have polyps that protrude from the top of their mat. These polyps are tubes surrounded by tentacles that are used to consume food. In addition to the polyps that help the Hydractinia eat, there are reproductive polyps that can be used to tell whether the colony is male or female. This image illustrates the development of a Hydractinia embryo to a colony. The image shows what an adult polyp looks like, in addition to both the male and female sexual polyps.

In their natural habitat, Hydractinia grow in attempt to cover the entire shell, but there are more than one colony on the shells. The colonies that come into contact with each other can perform a fusion, rejection, or transitory fusion. These three phenotypes of Hydractinia are determined by their genotypes. A genotype is the set of genes an organism carries, and a phenotype is the physical characteristics of an organism that are influenced by their genotypes. Hydractinia have allorecognition genes that help them detect other colonies. Allorecognition is an organism’s ability to distinguish self from non-self. Research has proven that Hydractinia have at least two allorecognition genes, alr1 and alr2. If the colonies touch and are recognized as self, they fuse together and become one larger colony. Here is an image of two Hydractinia colonies fusing together. If the colonies’ do not have the same genotypes, then they reject each other and one colony overpowers the other until there is only one colony left standing. The other possible interaction between two colonies is a transitory fusion. A transitory fusion consists of the colonies first fusing together but rejecting after they realize they are not the same.

Hydractinia research can help advance transplant research because just as Hydractinia determine whether colonies are self or non-self, the human body does the same with transplanted tissue. This is why there are many instances when patients reject their transplant. Although both humans and Hydractinia have allorecognition systems, they are not the same. In the human body there are MHC molecules that act as self-tags on cells. Although these tags help the body distinguish self from non-self, they are not allorecognition genes. Despite the differences between Hydractinia and humans, Hydractinia research is still valuable.

While in the lab, I conducted many experiments with Hydractinia. I extracted DNA and genotyped one of the colonies. I looked at triple fusion colonies, which consisted of three different Hydractinia colonies on a slide. These colonies had been previously genotyped and were expected to fuse together and become one large colony. To examine the slides, I used a fluorescent microscope because the two end colonies were injected with fluorescent proteins. Under the microscope, I could see cells from one colony moving through the gastrovascular canals into the adjacent colony. In addition to this experiment, I conducted research with two colonies per slide. These colonies’ genotypes were not the same, so they were meant to reject. For the experiment, I observed the colonies and placed them in 2% DMSO when they were about to touch. 2% DMSO is an important solvent that can dissolve polar and nonpolar compounds. This experiment had not yet been conducted before, but it was predicted that the 2% DMSO would alter the phenotype and the colonies would fuse together. This prediction was put forward because a similar experiment with single polyps had been conducted by a postdoc in the lab, and the polyps fused when placed in 2% DMSO. This research is an example of how working with Hydractinia can improve transplantation in humans. If there is something that can cause two non-self colonies of Hydractinia to fuse, then who says it cannot be used to ensure that the human body accepts transplanted tissue?

Transplantation research continues to evolve, and I imagine that there will be many discoveries and advances in the near future. I am incredibly grateful for the opportunity to observe and contribute to the amazing research being done at the Starzl Institute!

Haley Dion is a VI Form boarding student from Medway, MA. Her passions include rowing, running, and helping those in need.