Biochemical composition and dynamics of the axonal cytoskeleton in the corticospinal system of the adult hamster.

The corticospinal system is an important central nervous system (CNS) pathway that is implicated in debilitating diseases such as amyotrophic lateral sclerosis and in traumatic injuries to the spinal cord. This study characterizes some of the fundamental biochemical and kinetic properties of normal corticospinal axons, establishing an important reference for studies that aim to elucidate the cellular modifications that result during pathological conditions of these axons. Slow axonal transport which conveys the axonal cytoskeleton as well as cytomatrix constituents, such as many of the metabolic enzymes and regulatory proteins, has been examined. For these studies, [35S]methionine was injected into the sensorimotor cortex of adult male Golden hamsters, and labeled, transported proteins present in corticospinal axons at 1-42 days after injection were assessed using one- and two-dimensional gel electrophoresis/fluorography. The complex group of slow component b (SCb) proteins (including clathrin, actin, enolase, creatine phosphokinase, and many others) was observed to move at a rate of approximately 2 mm/day in adult corticospinal axons. The slow component a (SCa) proteins (tubulins, neurofilament proteins, and actin) were transported at a substantially slower rate of approximately 0.4 mm/day. The biochemical and kinetic properties of slow transport in corticospinal axons were very similar to those previously described in another CNS pathway, axons of retinal ganglion cells, and substantially different from those documented in large, peripheral sensory or motor axons. These findings suggest that some of the basic properties of axonal transport which determine many of the structural and functional properties of axons may be different in the CNS compared to the peripheral nervous system.