Cellular mechanisms that might affect the degradation of neurofilament proteins (NFPs) were examined in the distal segments
of severed goldfish Mauthner axons (M-axons), which do
not degenerate for more than 2 months after severance. Calpain
levels, as determined by reactivity to a polyclonal antibody,
remained constant for 80 d postseverance in distal segments
of M-axons and then declined from 80 to 85 d
postseverance. Calpain activity in rat brain, as determined by a
spectrophotometric assay, was much higher than calpain activity
in control and severed goldfish brain, spinal cord, muscle,
or M-axons. Calpain activity was extremely low in M-axons
compared with that in all other tissues and remained low for up
to 80 d postseverance in distal segments of M-axons. Phosphorylated
NFPs, as determined by Stains-All treatment of SDS
gels, were maintained for up to 72 d postseverance and then decreased noticeably at 75 d postseverance when NFP breakdown
products appeared on silver-stained gels. By 85 d postseverance,
phosphorylated NFPs no longer were detected, and
NFP breakdown products were the most prominent bands on
silver-stained gels. These results suggest that the distal segments
of M-axons survive for months after severance, because
NFPs are maintained in a phosphorylated state that stabilizes
and protects NFPs from degradation by low levels of calpain
activity in the M-axon; the distal segments of severed M-axons
degenerate eventually when NFPs no longer are maintained in
a phosphorylated state and become susceptible to degradation,
possibly by low levels of calpain activity in the M-axon.This work was supported by an Advanced Technology Project Grant to G.D.B.Neuroscienc
