By David N. Leff
As students of high school French learn, with a snicker, the word derrihre refers to the lower rear portion of the human anatomy.
In English, it now has a new embryological meaning: the derrihre gene and its Derrihre protein dictate the formation of a frog¿s future body from the neck down. This genomic nomenclature was jointly adopted by scientists at the Whitehead Institute for Biomedical Research and Genetics Institute Inc. (GI), both in Cambridge, Mass.
¿The derrihre gene,¿ recalled Whitehead molecular embryologist Hazel Sive, ¿was discovered as a joint venture between ourselves and GI about two years ago. The word derrihre in French,¿ she continued, ¿means posterior,¿ or behind.¿ We thought it was one of the less risqui names for our new gene. My laboratory, which focuses on how an embryo knows where to put its various body parts, came up with that name. GI liked it, too, so we went with it.¿
Sive¿s lab researches embryogenesis in the claw-toed frog (Xenopus laevis). ¿Its gestation is very quick,¿ she explained, ¿which makes our assays kind of easy. In two days following fertilization, the embryo has a very highly developed nervous system, functional muscles and eyes that are starting to develop. Its digestive tract is not quite mature, but it¿s getting there. In human terms, this is a stage when the mother may not even know she¿s pregnant.
¿Things get to the point were we can actually see what the frog¿s basic body plan looks like, in just a couple of days. Then, about a week later, it¿s a free-swimming tadpole feeding for itself.¿ However, she pointed out, after this fast-forward breakaway, ¿for X. laevis to get to sexual maturity so it can breed takes a lot longer, a couple of years.¿
Sive¿s research goal is ¿trying to understand how a vertebrate embryo ¿ like ourselves ¿ decides where to put its different body parts and tissues very, very early during embryonic life. The posterior of the embryo,¿ she explained, ¿includes the whole spinal cord, but not the brain. And it includes all the good stuff that¿s down there in the part of the body from the neck down.¿
Sive continued: ¿We were really looking to try and figure out how this particular posterior part of the body forms. And we isolated this new molecule we called Derrihre. We think, but don¿t know for sure, that it¿s somehow involved in telling cells that they are going to be part of the body from the neck down, and not the part from the neck up.¿
She and her collaborators assume that this protein, and the gene that encodes it, occurs all the way up the vertebrate family tree to Homo sapiens. ¿We know,¿ she told BioWorld Today, ¿that the class of proteins to which Derrihre belongs, the transforming growth factor-betas (TGF-b), is very well represented in the human genome. It is our opinion that there is likely to be a gene in it with a very similar function to derrihre.¿
¿Whether it looks very much like derrihre we don¿t know,¿ she went on,¿ and we¿re currently trying to figure that out by going incrementally into different organisms that are more closely related to frogs than humans ¿ from zebra fish to chicks to mice ¿ and eventually working our way up to people. That process,¿ she observed, ¿might take a year if we¿re lucky.¿
Sive is senior author of a progress report in the current issue of the twice-monthly British journal Development, published April 7, 1999. It¿s titled: ¿derrihre: a TGF-b family member required for posterior development in Xenopus.¿
Of the article¿s eight co-authors, three are from the Whitehead, five from GI.
¿It was a very good formal collaboration,¿ Sive recalled. ¿We physically went over to GI to help isolate this protein. They did all of the DNA sequencing for us, put a considerable effort into characterizing the gene, and actually isolated the purified protein. The technique to do this was entirely theirs. And we applied that to the question we were interested in, all the biological studies to try to figure out what this molecule actually did.¿
The two partners have filed a joint patent application covering their derrihre¿s intellectual property.
To find out what the gene and its protein did in the embryonic frog, the co-authors conducted three key experiments:
¿The first thing we did,¿ Sive recounted, ¿was to look and see where the gene was active, and when. We could see that the RNA for making its protein was present at the right time and in the right place for a role in posterior development of the embryo.
¿Then,¿ she went on, ¿we did gain-of-function experiments, in which we put the active protein in the wrong place in the body, and asked what it does. So one thing we did was put the protein in the developing head, at a time when the embryo doesn¿t have a head, which is just beginning to form.
¿We found that the embryo developed with no head. There were no eyes, no chin region; the brain was very tiny. That suggested that this protein¿s action was not compatible with formation of the head. This was consistent with its being involved in formation of the other parts of the body, the posterior.
¿Next, we put the active protein into the belly region of the embryo. What we saw there was that another little embryo, a kind of Siamese twin, was growing out of that embryonic belly. And this Siamese-twinned embryo didn¿t have a head, just a posterior region. It had a lot of nervous tissue, and a lot of muscle.¿
Putative Therapeutic Target: Muscle Wasting
¿So this Siamese twin growing out of the belly of the embryo,¿ Sive narrated, ¿under the direction of Derrihre, suggested that this protein was able tell the embryo to make a little posterior part but not a head. Armed with this data, we then did the more difficult loss-of-function experiment ¿ trying to get rid of the protein¿s posterior-inducing activity in the embryo.
¿We took out a four-residue chunk of Derrihre¿s amino-acid sequence. That made it act like the normal protein, but not quite. When we did that, we found that the embryos developed with a perfectly normal head. Everything looked fine, all the way down to the base of the neck. But from the shoulders on down, the tissue was clearly very abnormal. Just how abnormal is something that we¿re looking into now. There was clearly no muscle tissue, for example.
¿We know,¿ Sive pointed out, ¿that Derrihre is very good at telling muscles to form. One potential therapeutic application of our findings is in the regeneration of muscle, or in the generation of new muscle. That would be something for other labs, which are interested, for example, in trying to get muscle regeneration in muscular dystrophy. It¿s possible that Derrihre could encourage certain cells in the human body to go and form muscle. We know that it¿s very good at doing it in the embryo.¿