Origin of beading constrictions at the axolemma: presence in unmyelinated axons and after beta,beta'-iminodipropionitrile degradation of the cytoskeleton.

Myelinated nerve fibres become beaded when nerves are subjected to a mild stretch; the beading is seen as varicosities, a series of alternating constrictions and enlargements, when using freeze-substitution or cold-fixation to hold this labile form change in place during fixation. One possibility for how this form change comes about is that the myelin sheath or its Schwann cell initiates beading. We now report, however, that a similar beading is seen in the axons of unmyelinated fibres. In electron micrographs, longitudinal sections of axons show the series of constrictions and expansions typical of beading. In cross-sections, axons with unusually small diameter, corresponding to the constrictions, are seen to contain closely packed microtubules and neurofilaments while neighbouring swollen axons with widely dispersed microtubules correspond to the beading expansions. Another possibility for the form change is that the cytoskeleton is responsible for beading. We discovered that direct exposure of nerves to beta, beta'-iminodipropionitrile in vitro for 1-6 h causes both axonal microtubules and neurofilaments to become degraded and replaced by an amorphous residue. Nevertheless, beta,beta'-iminodipropionitrile-treated nerves show constrictions in myelinated fibres when stretched. An even greater degree of beading with narrower and longer constrictions appears in some fibres, with the expanded regions having oblate ends giving the appearance of a string of sausages. In cross-sections taken through the constrictions, a greater than usual reduction of axonal area was seen, this was due to the loss of cytoskeletal organelles which would act to limit the degree of constriction. With longer exposure to beta, beta'-iminodipropinitrile more fibres show complete degeneration of the cytoskeleton and form ovoids typical of Wallerian degeneration. Unmyelinated axons of beta, beta'-iminodipropionitrile-treated nerves which showed degeneration of their cytoskeleton with its replacement by amorphous material still demonstrated beading. As neither the myelin sheath nor the intact cytoskeleton within the axon is necessary for beading, by exclusion, we consider beading constrictions to be initiated at the level of the axolemma. In our hypothesis the membrane skeleton is responsible; namely, the spectrin, actin and other molecular species lining the inside of the axolemma and binding to transmembrane proteins. The membrane skeleton may be activated by stretch via transmembrane proteins (e.g. beta 1-integrins). The membrane skeleton mechanism may also be directly engaged in the production of Wallerian degeneration or be induced by neurotoxic agents.