Axonal degeneration is a major feature in many degenerative, heritable, and metabolic disorders including Alzheimer disease, Charcot-Marie-Tooth, and most sporadic neuropathies. A more thorough understanding of axonal degeneration, which initiates the sequence of morphologic changes collectively termed 'Wallerian degeneration,' can give new insight into the biology of axons and the treatment of axonal disorders. The goal of this research proposal is to better understand the structural and functional biology of mammalian axons that have been separated from their cell bodies. Until now, these basic questions have been unapproachable because of the rapid degeneration (hours to days) that occurs in transected axons. This proposal will take advantage of the C57BL/6/Ola mouse, a unique strain that shows prolonged axonal survival of an axotomized distal stump. Isolated axons in this strain survive for several weeks, allowing analysis of the structural changes and biologic behavior of axons separated from their cell bodies. Pilot studies from this laboratory have shown that the transport of neurofilaments, which normally occurs in a proximoi distal direction, may become bidirectional in surviving, transected axons in C57BL/6/Ola mice. Thus, Specific Aim 1 will test the hypothesis that the normal polarized organization of the axon is disrupted when it is separated from its cell body. Three polarized features of the axon will be examined: 1) slow transport of cytoskeletal proteins; 2) orientation of microtubules; 3) capacity for regeneration. Whether axons require newly synthesized proteins (GAPs and tubulins) for sprouting will be studied by examining 'naive' axons and axons 'primed' by earlier axotomy for new growth.
Aim 2 will examine the pathophysiology of axonal degeneration. Nerve xenografts will be used to ask whether axon or Schwann cell is responsible for the phenotype of prolonged axonal survival in C57BL/6/Ola mice. The evolution of axonal pathology in isolated axons will be analyzed, including the temporal and spatial sequence of axonal degeneration and the development of neurofilamentous changes. Both of these issues have a direct bearing on human neurologic disorders that demonstrate neurofibrillary pathology and/or axonal degeneration. Computer assisted image analysis and quantitative EM techniques will be used to measure the structural relationship of cytoskeletal structures in swollen and non-swollen regions of axons, and to determine whether dissolution of the axon occurs in a pattern of 'dying forward' or """"""""dying back.' The proposed series of studies presents an unprecedented opportunity to examine some fundamental issues in cellular neuropathology. First, they will further elucidate the relationship between the perikaryon and the axon. Second, they will provide data about the pathogenesis of a type of axonal pathology that is common to a variety of neurological disorders. An understanding of the mechanisms that underlie the axonal changes in this unique animal model will stimulate new research into the pathogenesis of these human diseases.
