Living Force

The number of “species” in the “genus” Scientist is diverse – ranging from biologists to geneticists, chemists to physicists, and everything in between – each adapted to his or her own particular niche. What would happen if two such species were to “crossbreed”? Once inconceivable, a viable and growing population of physicists engaging in biology research has arisen in the past few years. Says Dr. Nir Gov of the Institute’s Chemical Physics Department in the Faculty of Chemistry: “With scientific equipment becoming ever more powerful, biologists are beginning to face gridlock in the vast amount of data they’ve accrued. Some physicists have seen this as an invitation to help biologists understand phenomena on a more fundamental level and, in the process, discover new physical principles that are unique to active, living matter.”

 

One area that Gov and his research team have been actively studying is how cells get their variety of shapes. Hair cells in the ear, for example, grow finger-like protrusions on their outer surface, and these are organized into rows of graded length. These fingers (stereocilia) convert sound vibrations to electrical signals that are then relayed to the brain. Biologists can now describe in detail the different stages of ear cell formation in a developing embryo, from the initial “deaf” stages, when the nascent fingers start growing in a disorderly fashion, to the final, highly ordered structure. Peeling away the outer membrane of these fingers reveals a scaffold-type protein structure – the cytoskeleton – within the stereocilia, and this scaffolding is the driving force behind their formation. Yet biologists still do not know exactly how stereocilia development takes place. For this, another layer needs to be peeled away to reveal the “invisible forces” that are at play. Gov: “By ‘forces,’ we mean such things as tension, compression, friction, and kinetic and chemical energies – physical mechanisms that act on the objects in a given system. Putting these basic ingredients together in equations gives us mathematical models. We can then use our models to make quantitative predictions about how cellular formations arise and how they behave under various conditions – predictions which can then be tested experimentally.”

 

The model that Gov and his team have been building starts with ATP – the energy currency of the cell. ATP causes elements in the protein scaffold structure – a tight bundle of protein filaments – to dissociate from and re-associate with each other. When balanced by forces in the membrane, the cell’s outer shape remains roughly constant; but the forces exerted by the scaffold proteins also deform the membrane, giving rise to the growth of the finger-like protrusions. The researchers have calculated how these forces exerted on the membranes initiate the process, and a group of experimentalists has recently validated some of their predictions.

 

The same mathematical model has helped the biophysicists gain insights into other, similar systems. For example, another member of Gov’s team is working on brain cells, which grow branched spines for communicating with their neighbors. The basic process underlying their growth appears to be the same as that which initiates the formation of the ear cell fingers, and it  may apply, as well, to the finger-like protrusions of both immune cells and cancer cells that are essential to motility.