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 m ild stretch; the beading is seen as varicosities, a series of alternat ing 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 mye lin sheath or its Schwann cell initiates beading. We now report, howev er, that a similar beading is seen in the axons of unmyelinated fibres . In electron micrographs, longitudinal sections of axons show the ser ies of constrictions and expansions typical of beading. In cross-secti ons, axons with unusually small diameter, corresponding to the constri ctions, are seen to contain closely packed microtubules and neurofilam ents while neighbouring swollen axons with widely dispersed microtubul es correspond to the beading expansions. Another possibility for the f orm change is that the cytoskeleton is responsible for beading. We dis covered that direct exposure of nerves to beta,beta'-iminodipropionitr ile in vitro for 1-6 h causes both axonal microtubules and neurofilame nts to become degraded and replaced by an amorphous residue. Neverthel ess, beta,beta'-iminodipropionitrile-treated nerves show constrictions in myelinated fibres when stretched. An even greater degree of beadin g with narrower and longer constrictions appears in some fibres, with the expanded regions having oblate ends giving the appearance of a str ing 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 degre e of constriction. With longer exposure to beta,beta'-iminodipropionit rile more fibres show complete degeneration of the cytoskeleton and fo rm ovoids typical of Wallerian degeneration. Unmyelinated axons of bet a,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 cyt oskeleton within the axon is necessary for beading, by exclusion, we c onsider beading constrictions to be initiated at the level of the axol emma. In our hypothesis the membrane skeleton is responsible; namely, the spectrin, actin and other molecular species lining the inside of t he axolemma and binding to transmembrane proteins. The membrane skelet on may be activated by stretch via transmembrane proteins (e.g. beta 1 -integrins). The membrane skeleton mechanism may also be directly enga ged in the production of Wallerian degeneration or be induced by neuro toxic agents.