Nerve cells communicate by conducting electrical signals along slender cytoplasmic extensions known as axons. Animals have evolved two basic mechanisms for increasing axonal conduction velocity. One is to increase axonal diameter and the other is to insulate axons by a process called myelination, which is a tight spiral wrapping of the axons that is formed by myelinating cells. In vertebrates the growth of axon diameter is caused principally by the accumulation of space-filling cytoskeletal polymers called neurofilaments inside the axons, and this is regulated locally by chemical signals from the myelinating cells. It is known that neurofilaments are transported along axons and that they alternate between rapid movements and prolonged pauses. The proportion of the time that the neurofilaments spend pausing is likely to be a principal determinant of their residence time in axons. This is a collaborative experimental and modeling project involving a biologist at Ohio State University and a physicist at Ohio University. The central hypothesis to be tested is that myelinating cells control axonal caliber by regulating neurofilament pausing. A computational model will be developed that relates the moving and pausing behavior of neurofilaments to their distribution along axons. The model will be based on detailed kinetic parameters of neurofilament movement derived experimentally in cultured neurons and will be verified experimentally by fluorescence microscopy of neurofilament movement in myelinated axons in tissue culture. The proposed research will generate a rigorous and quantitative framework that relates the size and shape of axons, which is a key influence on their electrical properties, to the moving and pausing behavior of their internal constituents. The research will involve graduate and undergraduate students in both the physical and biological sciences, providing an integrated and cross-disciplinary training experience at the interface between computational and experimental biology.
An important aspect of the nervous system is how it regulates the size of the axons. Proper brain function relies on nerve signals arriving at their destination at the right time. Hence, the conduction speed of a nerve fiber must be precisely tuned to its physiologic function. Conduction speed in turn is determined by axon diameter that ranges from less than 1 micrometer (for unmyelinated axons) to more than 20 micrometer for myelinated axons. The main thrust of the project was to understand the mechanisms by which axons acquire caliber and how it is regulated. Insights into these mechanisms are the base on which the formation of axonal structures (such as nodes of Ranvier or changes in axonal caliber at branch points) and possibly also neurodegenerative diseases such as amyotrophic lateral sclerosis and giant neuronal neuropathy, associated with axonal swelling, can be understood. While this field of research has been driven so far predominantly by methods of cellular and molecular biology, this project, directed by an interdisciplinary team of a neurobiologist and a physicist, has added a new set of tools - mathematical and computational modeling. The approach of using computational modeling in conjunction with experimentation has let to important and significant insights. These include that neurofilaments, the main structural elements in mature axons, do not form a stationary skeleton as it has been suggested in some previous literature, but move relentlessly on their journey from the cell body to the nerve terminal - resolving a 20-year controversy on this issue. Strong evidence has been provided that the local size of an axon is determined by the local velocity of neurofilaments. In simple terms, if the cell body generates a certain flux of neurofilaments into the axon, and the average velocity is slower in some segments of the axon than others, than the neurofilaments will accumulate there, leading to an enlarged axonal caliber. Specifically, we have demonstrated that neurofilament transport is slower in myelinated segments of axons (internodes) than in non-myelinated segments (nodes of Ranvier) and that this velocity profile is accompanied by a sausage-like shape of myelinated axons with significant constrictions at the nodes of Ranvier. Furthermore it has been demonstrated through mathematical modeling how the nervous system can take advantage of these constrictions at the nodes of Ranvier to reduce fiber diameter. While textbook knowledge states that the caliber of the axon determines conduction speed our finding is that the constrictions at the nodes of Ranvier allow a reduction of the fiber diameter of up to 50% at the same target conduction speed. This project was directed by an interdisciplinary team of a neurobiologist and a physicist, offering highly interdisciplinary opportunities in education and research for graduate students and undergraduates in physics as well as in biology. Physics graduate students at Ohio University participating in the project, interacted directly with students from the collaborating neurobiology lab at Ohio State University. They will leave Ohio University with a unique set of skills which enable them to take advantage of opportunities not only in a physics environment but also in a life-science environment. They will be able to offer different perspectives in whatever fields of biological research they are going to engage in and help innovate in their unique ways.
