How molecular clocks can help chickens get wings (05/2007)
A fundamental biological question is on how the body plan is organized during embryonic development. Embryos start as a group of non-differentiated cells that divide and eventually specialise into the different tissues and organs of the body. But how do cells know when to go through their different destinies? Research to be published on the 27th of April edition of the Journal of Molecular Biology1 by a group of Portuguese scientists provides a piece of the puzzle by showing that formation of the cartilage during chicken wing embryonic development is linked to a gene called hairy2, which seems to act as molecular clock helping cells to know when is time to stop dividing and differentiate. Their research also suggests that these molecular clocks can be a widespread time measurement mechanism among multicellular organisms. The work has important implications by contributing to a better understanding of embryonic development and consequently also of the diseases resulting from problems in it.
Cyclic or oscillatory genes are genes characterised by going from inactivated (not expressed) to activated (expressed) and back to an inactivated state in continuous cycles. They were discovered almost ten years ago involved in the embryo early segmentation steps although their function in this process is unclear and for a long time they were not found anywhere else. Recent research, however, have report an oscillatory gene in several cells of mouse not connected to segmentation, raising the prospect that their presence could be much more widespread than previously thought.
In order to test this hypothesis Susana Pascoal and Isabel Palmeirim (which was associated with the discovery of the first cyclic genes) at Minho University, Portugal together with colleagues from Gulbenkian Institute of Science and Lisbon University in Portugal and Pierre and Marie Currie University in France decided to study the embryonic development of the chicken wing (one of the most common model of embryonic studies) looking at the expression of the gene hairy2, which is the chicken equivalent of the cyclic gene recently reported in mouse cells.
Limbs in vertebrates develop from small buds on the side of the body and as these buds grow, the undifferentiated cells in them first multiply and then differentiate into the various tissues of the limb, including the cartilage and later the bone that make up the limb skeleton. Pascoal, Palmeirim and colleagues started their study by following, during the different stages of embryonic development, the cells constituting the wing bud to find that hairy2 was expressed in cartilage precursor cells. In fact, it was found that hairy2 oscillated between inactivated and activated states while these cells divided, but, as soon as cells started differentiating hairy2 oscillations stopped and a cartilaginous element of the wing is formed. Furthermore, as cartilage precursor cells divide and the limb bud grows it is even possible to see a wavelike expression of the hairy2 gene through adjacent cells, starting from cells with a low hairy2 expression through to moderate and then onto high hairy2 levels of expression.
Pascoal, Palmeirim and colleagues measured each hairy2 cycle – corresponding to the time necessary for the gene to go from an inactivated state, into activation and back into inactivation – to find that these lasted exactly 6 hours.
The next step was to see if the hairy2 cycle could be related with the formation of cartilage/bone elements in chicken wing since it has been proposed that cyclic genes can work as molecular clocks helping cells decide when to change behaviour. In order to test this supposition the team of researcher followed the formation of the 2nd phalanx of the wing and found that the average time for this process was 12 hours which corresponded to two hairy2 cycles.
This result led Pascoal, Palmeirim and colleagues to propose that the 2nd phalanx precursor cells were able to count hairy2 cycles and use the gene as a molecular clock to measure time and allow an accurate control of the cells growth and differentiation. In the case of the 2nd phalanx the cells “counted” two 6-hours cycles for the formation of one bone element (12 hours). Furthermore, the fact that time control is crucial for development and that cyclic genes seem to present in several embryonic tissues led the researchers to also suggest that oscillators (or cyclic genes) playing the role of molecular clocks are probably quite widespread among the different embryonic tissues of multicellular organisms.
The researchers also suggest that the reason why it has been so difficult to find prove of these cyclic genes comes from the fact that only when adjacent cells are in synchrony (activated-inactivated-activated-etc) is possible to detect a pattern big enough to identify the presence of a molecular clock.
"The discovery of the first cyclic gene by Palmeirim and colleagues was an amazing discovery and our work now supports the idea they are used as molecular clocks by embryonic cells in several tissues” says Susana Pascoal, the first author of the article “we know this is only part of the story, but it is crucial information to understand how embryonic development is regulated”
Pascoal, Palmeirim and colleagues’ work has in fact many and important implications as it contributes to the understanding of how embryos develop, and consequently also the bases of human congenital development defects, but also helps paving the way for one day future technology aiming at biological tissue engineering and repair be developed.
1 Journal of Molecular Biology Volume 368, Issue 2 (April 27, 2007) “A Molecular Clock Operates During Chick Autopod Proximal-distal Outgrowth”
Authors of the original paper
Susana Pascoal- firstname.lastname@example.org