| Popis: |
Neurofilaments (NFs), the most abundant cytoskeletal structure in large myelinated axons, are considered to provide mechanical support to neurons including modulation of the axonal caliber. They are made by the copolymerization of three intermediate filament proteins, NF-L, NF-M, and NF-H that are expressed in most neurons, particularly in the PNS. Abnormal accumulation of NF in proximal axons has been associated with neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), and overt neuronopathy in transgenic animals hyperexpressing human NF-H. Recent evidence suggests that NF proteins can play a causative role in motor neuron disease. In order to examine consequences of NF expresion we have studied electrophysiological properties of myelinated axons with mutation deletions of NF (NF-H and NH-M-/-). Steady state memebrane properties, action potentials and single axon refractory period were measured on isolated sciatic nerve in vitro using intracellular current clamp technique (intra-axonal microelectdrode recording from single large myelinated axons). Axon from control and transgenic animnals showed similar inward and outward rectification, suggesting qualitatively similar properties of voltage dependent K+ conductances. However, in NF-H-/-conduction velocity (CV) was significantly reduced (26ą5 m/s c vs. 43ą10 m/s; n=24; 22 C) in spite of axonal diameters similar to controls. Action potential refractory period was significantly prolonged in NF-H-/-. Similar results were obtained by extracellular recording or sucrose gap recording on whole nerves indicating uniform population defect. These results suggest that Na+ channels in axons lacking NF-H have slower kinetics. Since phosphorylation od Na+ channels is recognized to control its one is compelled to search for possible links between NF expression and changes inprtoein kinases in axons. |
