TotM: Human PLP1 and the importance of a good cable


On July 27th of 1866 the world suddenly became smaller. After ten years of hardships and invested fortunes, The Atlantic Telegraphic Company finished the construction of the first telegraphic wire connecting two continents, allowing instantaneous communication between the United Kingdom and The United States of America. Many other projects followed, and by the end of the XIXth century any citizen of the world was able to know what was going on in any part of the world virtually in real time. Such wonder was possible thanks to the discovery of the gutta-percha, a potent electrical insulator that allowed the telegraphic signals to travel thousands of kilometers without diffusing into the salty ocean.




Living beings also face similar problems. Microscopic organisms, such as bacteria, can easily communicate within their colonies by means of chemical signals that spread by simple diffusion. However, the speed of such processes allow for fast communication only at very small distances. In bigger organisms, composed by aggregates of cells that must work coordinately, diffusible signals are useful for slow responses, such as hormonal changes. Plants can function well with such limitations, but animals require speed to be able to fight and flee. In order to solve that. metazoans invented a cell type specialized in conducting membrane depolarizing signals across a long celular body, the neurons. But neurons are not unlike that submarine cable, in the sense that the signal can easily lose strength because the cable is submerged in a saline solution.



Having bigger cables is a solution to this problem, as in many invertebrates. The higher diameter allows for better electrical conductivity, but the size of such cells (the giant axon of the squid, which was used by Alan Lloyd Hodgkin and Andrew Fielding Huxley to describe the electrical properties of neurons) makes this strategy prohibitive. The other option is the use of an electrical insulator. In vertebrates (except lampreys), and some crustaceans and annelids, neurons are covered by a multilayered coat of lipidic material, called myelin. Myelin allow for high conduction speed, even through small neurons. This in turn allow for great compaction, making physically possible the wonder that are our brains.


The myelin proteolipid protein (PLP1) is one of the main structural components in the central nervous system of tetrapods. It is an adhesive protein involved in the compaction of the multilayered myelin sheats. As we can observe in our tree of the month, the protein is highly conserved across terrestrial vertebrates. In bony fishes, where its role is not as capital, the gene has duplicated. The gene itself appears as part of a gene expansion at the base of vertebrates, signaling the origin of myelin sheats.





origin of myelin sheatsAll pictures are published under p.