In the search for new and better materials, many a scientist has turned to the study of living organisms. Generally these studies have focused on such tissues as bone or shell – complex mixtures of minerals, proteins and sugars that result in hard, strong, fracture-resistant materials. Recent research in the Institute's Structural Biology Department has shown that bones and shells share similar formation processes.

Bone and shell are composite materials in which the mineral component forms within a framework of soft macromolecules such as proteins. The framework is essentially a "mold" that directs the mineral formation. Bone grows continuously, layer by layer; but, unlike shell, it also undergoes constant remodeling. This makes it hard for scientists studying bone to distinguish new material from old, much less to trace the process by which bone is formed. Because of this difficulty, conflicting theories have arisen as to the mechanisms of bone formation. Many scientists thought that the minerals precipitate directly from a solution in the way that stalactites and stalagmites grow from elements dissolved in water. But findings published in the 1960s already hinted at the possibility that a different process was at work.

Ten years ago, research in a group headed by Profs. Lia Addadi and Stephen Weiner of the Institute's Structural Biology Department confirmed these earlier observations for shells. They showed that organisms first produce packets of amorphous material (unorganized material, as opposed to the ordered organization of crystal). These packets are transferred from the inside of cells to the building site – the spot where the mineralized tissue eventually forms. There, the packets undergo structural changes that turn them into a hard crystal. This observation triggered widespread interest in amorphous precursor mineral phases, and many different invertebrate mineralization processes were investigated.

Addadi and Weiner revealed this process in sea urchin spines, and their findings have been joined by a body of global research confirming that this method of construction is common to many different invertebrates that have shells, spines or other hard body structures. Addadi: "The original idea of precipitation from solution would require huge quantities of liquid to flow from inside the mollusk to the outside of its body. Transferring the materials in solid form – 'bricks and cement' – is a much more energy-efficient way of doing things."

While the issue has been more or less settled for the shells of invertebrates, the question of how vertebrate bones are formed remained unresolved. This is partly because of the difficulty inherent in attempting to observe a substance with no fixed location that exists for only a fleeting stage of growth. Recently, however, research student Julia Mahamid, together with Addadi and Weiner, found a biological system that enabled them to follow bone formation step by step and identify the processes taking place at each stage. This system is the fin of a small aquarium denizen called a zebrafish. The zebrafish fin bones, which continue to grow throughout the fish's lifetime, form a sort of fan. Each bony rib of the fan is composed of segments, and the segments nearest the fin's edge are always the newest. Thus the segments can be studied as a sort of timeline of bone formation. In addition, zebrafish, which live in relatively cold water, grow slowly, and the leisurely pace of their bone development enabled the scientists to get a good look at each stage.

Using both light microscopy and scanning electron microscopy, the scientists succeeded in observing abundant spherical parcels of amorphous mineral material in the newly formed fin bone. Their findings, which appeared in the Proceedings of the National Academy of Sciences (PNAS), USA, showed that about half the mineral makeup of the newer bone segments is amorphous – a fraction that dwindles in the segments farther away from the fin edge. Their observations suggest that the amorphous material does, indeed, turn to crystal over time.

These findings are shedding light on a number of scientific mysteries, giving scientists a unique perspective on how the hard substances in our body – bones and teeth – are formed. The scientists hope that a deeper understanding of these biological processes may, in the future, help researchers find cures for diseases involving faulty bone development or repair.

Prof. Lia Addadi's research is supported by the Clore Center for Biological Physics; the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; the Helen and Martin Kimmel Center for Nanoscale Science; the Helen and Milton A. Kimmelman Center for Biomolecular Structure and Assembly; and the Carolito Stiftung. Prof. Lia Addadi is the incumbent of the Dorothy and Patrick Gorman Professorial Chair.

Prof. Stephen Weiner's research is supported by the Kekst Family Center for Medical Genetics; the Helen and Martin Kimmel Center for Archaeological Science; the Helen and Milton A. Kimmelman Center for Biomolecular Structure and Assembly; and George Schwartzman, Sarasota, FL. Prof. Weiner is the incumbent of the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.

More than 80 years ago, the German scientist Hans Spemann conducted a famous experiment that laid the foundations for the field of embryonic development – research that would later earn him a Nobel Prize. After dividing a salamander embryo in half, Spemann noticed that one half – the ventral half that gives rise to the salamander’s “belly” – starts to wither away. However, the “back” (dorsal) half, from which the head, brain and spinal cord arise, continues to grow, regenerating the missing belly and developing into a complete – though small – embryo. Spemann then conducted another experiment. This time, he removed a few cells from the back half of one embryo and transplanted them into the belly half of a different embryo. To his surprise, the result was a Siamese twin embryo with an extra head generated from the transplanted cells. Although the resulting embryo was smaller than normal, all its tissues and organs developed in the right proportions, and they functioned properly.

 

Sensations like pain or heat travel in a fraction of a second along the extensions – the axons – of nerve cells, an impressive feat considering that axons such as those that run from the base of the spine all the way down the leg can reach over a meter in length – 40,000 times longer than their width. But when a nerve cell is injured somewhere along its length, a different kind of signal must be sent – one that more closely resembles a mechanical railway car than an electrical impulse. Thus getting the call for help back to the remote cell nucleus is something like sending a message by boxcar to headquarters halfway across the state.

Prof. Michael Fainzilber of the Weizmann Institute’s Biological Chemistry Department has been investigating emergency transportation systems in nerve cells. His previous research had shown that this information is packaged in a molecular “railcar” complex: A motor protein called dynein works together with importins to move the complex along microtubule “tracks.” The importins are nuclear import proteins that ferry various molecules in and out of the nucleus, thus delivering the message straight to the command center of the cell.

Fainzilber and research students Dmitry Yudin and Shlomit Hanz, working in collaboration with the group of Dr. Jeffrey Twiss (Nemours Institute, Wilmington, DE) wanted to know what controls the triggering of this system right after injury, as it gears up for the long journey. Using rat sciatic nerves as their model, the scientists looked for changes in the molecular makeup up of the nerve cell material around the injury site. Their findings recently appeared in Neuron.

To their surprise, they found molecules that until now were believed to exist only around the cell nucleus. These molecules, known collectively as the Ran system, are found in two main configurations, one inside the nucleus and one outside, and they help to control the importins’ molecule-ferrying activities through gates in the walls surrounding the nucleus. The research team found that the nuclear form of Ran sits on the axonal complex, preventing importins from coupling with the rest of the transport machinery. When damage occurs, Ran is switched to the second configuration, which causes it to disengage. The waiting importin can then plug into the machinery, and the expedition gets under way.

The researchers believe the Ran system works like a safety catch – preventing unwanted activation of the alarm machinery until an actual emergency occurs, and then switching quickly to allow efficient triggering of the system. Understanding this mechanism may help devise ways by which nerve cells can be induced to repair damage and may aid in developing treatments for nerve injury in the future.

Prof. Michael Fainzilber’s research is supported by the M.D. Moross Institute for Cancer Research; the Nella and Leon Benoziyo Center for Neurological Diseases; the Dr. Miriam and Sheldon G. Adelson Medical Research Foundation; the PW-Bes Foundation; the European Union FP6 NEST Axon Support project; the J&R Center for Scientific Research; the Minerva Foundation; and the USA-Israel Binational Science Foundation. Prof. Fainzilber is the incumbent of the Chaya Professorial Chair for Molecular Neuroscience.