Although peripheral nerve injury in neonatal rats is associated with poor axonal regeneration and extensive motoneurone death, the same injury in rats aged 5 days and older results in minimal motoneurone loss. It is known that mature nerves are better conduits for axon regeneration than immature ones. Here, we tested the possibility that the mature nerve microenvironment is also more conducive to motoneurone survival after injury. We also examined the role that integrins (extracellular matrix receptors) play in axon regeneration and motoneurone survival.

Three-day-old (P3) ‘host’ rats were anaesthetised with a 2 % halothane-oxygen mixture. The right sciatic nerve was transected and 5 mm nerve grafts from syngeneic ‘donor’ rats aged P3-P21 were attached to the cut ends of the host nerves. Seven days later, the host rats were terminally anaesthetised with phenobarbitone (100-200 mg, I.P.) and motoneurone survival was assessed. Nerve grafts were sectioned and stained for neurofilament and β-1 integrin immunoreactivity. In separate experiments, β-1 integrin function was blocked in P5 nerve grafts using a function-blocking antibody (monoclonal hamster anti-rat CD29, Pharmingen) and integrin expression was increased in P3 nerve grafts using a phorbol ester. Motoneurone survival and axonal regeneration was determined in both groups. All animal experiments conformed to Home Office Guidelines. Results were analysed using the Mann-Whitney U test for comparison of independent samples. Two-tailed tests were used in all instances, and the significance was set as P < 0.05.

Only 38.3 % (± 1.0 S.E.M., n = 4) of sciatic motoneurones in animals receiving P3 grafts survived. When nerve grafts from rats aged P5 were implanted, motoneurone survival rose to 71.4 % (± 3.2 S.E.M., n = 5). No further increase in motoneurone survival was observed with grafts from older donors. Blockade of β-1 integrins in P5 grafts reduced their survival-promoting effect and only 38.4 % (± 1.8 S.E.M., n = 7) of motoneurones survived. Increasing integrin expression in P3 grafts improves their ability to support motoneurone survival and 70.2 % (± 2.3 S.E.M.) of motoneurones survive. These changes in motoneurone survival were reflected in the extent of axon regeneration.

Thus the microenvironment that growth cones encounter during regeneration influences motoneurone survival. Implantation of mature nerve grafts can rescue motoneurones from cell death by a mechanism involving β-1 integrins.

The microenvironment that presents to regenerating growth cones greatly affects motoneurone survival and axon regeneration, with the ‘mature’ nerve microenvironment being more favourable. β-1 integrins appear to play a crucial role in this process.

This work was supported by the Brain Research Trust and The Wellcome Trust.