The Sculpting Gene

The breakthrough work of Prof. Beni Podbilewicz and doctoral student Meital Oren-Suissa of the Faculty of Biology exposes a surprising new facet in the functioning of the nervous system

Meital Oren-Suissa and Prof. Beni Podbilewicz

By Noam Bercovitz

A well known fact, which at times people tend to forget, is that most research conducted in the institutions of higher education is not carried out solely by the researchers themselves - in actual fact, research is done together with Masters and doctoral students. The students are the ones who conduct most experiments. While the majority of researchers are careful to note the important contribution of their research assistants to their studies, it seems that Prof. Podbilewicz of the Faculty of Biology has set a new standard by proposing, when asked to appear in this column, that his doctoral student, Meital Oren-Suissa, also join the discussion.

The two scientists are particularly excited nowadays-their paper has been published by Science, one of the top two most prestigious science journals in the world, and their work appears to be a breakthrough with broad significance. The joint interview, obviously, dealt with the work itself but also allowed a glimpse into the complex and interesting relations between a supervisor and his students.

Cells do not only divide

Prof. Podbilewicz's lab deals with the field of cellular fusion. Previously this field was considered to be somewhat esoteric, but over the past few years it has been garnering more and more interest. In opposition to cellular division, in cellular fusion two cells unite and form one cell. In the human body, cellular fusion processes occur in various mechanisms:  fertilization is the union of an ovum and a sperm cell, muscle cells are composed of rows of fused cells, placenta is made up of powerful multinucleated cells that are actually numerous individual cells that have fused, the eyes' lenses are formed of rows of fused cells, and in bones too cellular fusion occurs. The fusion processes are also involved in cancer, viral infections and stem cells.

The exact way fusion takes place is still not completely clear and its clarification is the focus of work in Prof. Podbilewicz's lab and in the small community of scientists that has developed in this area around the world. The working supposition is that proteins in the membranes of two cells meant to fuse know how to curve the membranes so that they merge, generating holes that grow larger to the point that the cell wall disappears.

How is a young woman finishing her undergraduate degree drawn to this particular field? Meital Oren-Suissa explains that Prof. Podbilewicz's lab suited her as a graduate research student because, among other things, the subject was very interesting and the lab was not overly large (between five to ten researchers). As a Masters student, according to Oren-Suissa, the focus is on learning the practical work and research methodologies, and Prof. Podbilewicz's lab has sophisticated and advanced work methods without being so large that a student disappears in the crowd.

Celeb  worm

The research in Beni Podbilewicz's lab studies a small worm about a millimeter in length called C. elegans. Three Nobel Prizes in the past decade were linked to work that used this worm, and it has become a celebrity among lab animals in the same way that the E. coli bacterium and the Drosophila fly have been the subjects of tens of thousands of papers. The worm suits cell fusion research because in its skin and other organs intensive cell-cell fusion processes take place that can be easily followed.

In earlier research in the lab, Prof. Podbilewicz and his doctoral student, Gidi Shemer, successfully identified the protein responsible for the worm's fusion activity - the EFF-1 protein. They created a mutant line of C. elegans worms that did not have this gene and showed that in the mutant worms, skin cells do not fuse and the cells begin to migrate through the body. Giving the EFF-1 protein to the mutants restored, to a great degree, skin cell fusion, and giving an extra high amount of the protein to normal worms raised the fusion rate abnormally.

Oren-Suissa's work for her M.Sc. proved that EFF-1 initiates the fusion process and is not an intermediate product of the process started by another protein. She also showed that EFF-1 must be present in the two cells undergoing fusion; it is not enough for it to be present in only one. She began her doctoral work on another subject in the field of fusion but then the hand of fate intervened and she switched her research topic completely as a result of research done on  C. elegans in a field very different from the field of cell fusion.

In a study done on the nervous system of C. elegans, researchers, and among them Dr. Millet Treinin of the Hadassah School of Medicine in Jerusalem, detected that some of the worm's neurons have a more complex structure than was originally thought. Consequently, Dr. Treinin turned to Beni Podbilewicz and asked him to examine the structure using an advanced microscope in the Faculty of Biology at the Technion. Oren-Suissa began investigating the subject, studying a pair of neurons running through the whole body (right and left). To the surprise of Podbilewicz and Oren-Suissa, they found that at the ends of these neurons there are branched dendrite systems, which are tiny nerve branches, shaped in structures that resemble menorahs.

Change direction

The subject looked promising and Oren-Suissa, temporarily giving up on her original doctoral topic, under Podbilewicz's supervision began studying the dendritic tree structures. This time they were again in for a surprise - they found that the protein that was identified as responsible for cellular fusion also affects the dendritic tree. Worm mutants whose protein-producing gene was damaged grew very dense and chaotic dendritic trees, whereas giving a surplus amount of the protein to the mutants fixed the problem to a certain degree and created a more normal looking tree. Giving normal worms extra amounts of the protein produced a stunted tree having almost no branches.

The dendrites form almost immediately under the worm's skin, where the fusion processes occur, hence the first hypothesis of Podbilewicz, Oren-Suissa and Dr. Gidi Shemer was that a disruption in the fusion activity was harming the dendrite creation, and in the absence of healthy skin, a normal nervous system could not develop. They summarized their results in a paper and submitted it for publication. The paper was not accepted and was returned with a few comments and questions from the reviewers. One question was: Why are you sure that EFF-1 affects only the skin fusion mechanism? Would it not be worthwhile to check if it does not directly affect the worm's nervous system?

And special thanks to the anonymous reviewer

 "It was frustrating not to have the paper accepted but today I can only thank the anonymous reviewer," says Oren-Suissa. It should be noted that paper reviewers in scientific journals are professionals in the field and they respond anonymously in order to prevent conflict of interest or personal unpleasantness.  Podbilewicz and Oren-Suissa began investigating the protein's effect using a variety of methods that separated the protein's effect on the skin and on the neuron, and found that the protein directly affects the worm's nervous system.

This was the first time that anyone identified a protein involved in shaping the branched dendritic structure of the C. elegans' nervous system, and the implications of the discovery are liable to be far-reaching. According to Prof. Podbilewicz, if discovering EFF-1 was akin to finding the Holy Grail of the field of fusion, linking it to the control of the nervous system is a Holy Grail of a completely different and surprising kind.

Tracking the protein's behavior over the long term showed that it operates in two ways: retraction of superfluous branches growing on the tree, and fusion of its errant branches. The protein sculpts the dendritic trees - pruning what is unnecessary and preserving the unique structure needed for neuron functioning - pain sensing. In collaboration with Prof. David Hall of the Albert Einstein School of Medicine in New York, researchers were able, for the first time, to prove that neurons can fuse their dendrites (auto-fusion).

Can two walk together?

The questions at this point and for the future are many and intriguing. For instance, the eff-1 gene produces four different proteins (isoforms) and so does the same variant affect both the skin and the nervous system or are we talking about two different forms? In our joint discussion, it seems that the route for future research is open for discussion - Prof. Podbilewicz remarks that it is important to gather quantitative data, and a small smile of despair on Oren-Suissa's face reveals what she perhaps feels about repeating the same experiment over and over again in order to verify that its results are reliable. But if it appears that Prof. Podbilewicz is the level-headed and responsible factor here, taking care not to run too far ahead, Oren-Suissa begs to note that sometimes it is the supervisor who is the one who gets excited and wants to rush ahead and explore new issues, and the student who is the one who must conduct the experiments and also submit the report tries to keep the research focused.

And speaking of work - Oren-Suissa must still submit her dissertation. On this subject Prof. Podbilewicz can reassure us, if anyone is anxious, and expects the work to come out just fine.

Links

http://www.elegansfusion.org - The group's site with links to articles and more