THE L2/HNK-1 CARBOHYDRATE EPITOPE IS INVOLVED IN THE PREFERENTIAL OUTGROWTH OF MOTOR NEURONS ON VENTRAL ROOTS AND MOTOR NERVES

Based on the observation that in adult mice the carbohydrate epitope L2/HNK-1 is detectable on Schwann cells in ventral spinal roots, but only scarcely in dorsal roots (Martini et al., Dev. Biol., 129, 330 - 338, 1988), the possibility was investigated that the carbohydrate is involved in the outgrowth of regenerating motor neuron axons on peripheral nerve substrates expressing the epitope. To monitor whether the L2 carbohydrate remains present during the time periods in which regenerating axons penetrate the denervated distal nerve stumps, the expression of L2 in motor and sensory branches of the femoral nerve was investigated in normal animals and after a crush lesion. During the first two postoperative weeks, L2 immunoreactivity remained high in the myelinating Schwann cells of the motor branch, whereas L2 immunoreactivity was virtually absent in the sensory branch. In a first experimental approach, cryosections of ventral and dorsal spinal roots and of motor and sensory nerves of adult rats and mice were used as substrates for neurite outgrowth. Neurites of motor neurons from chicken embryos were approximately 35% longer after 30 h of maintenance on ventral roots than on dorsal roots. Neurites from sensory neurons had the same length on dorsal as on ventral motors and were as long as neurites from motor neurons grown on dorsal roots. L2 antibodies reduced neurite outgrowth of motor neurons on ventral roots but not on dorsal roots. Neurite outgrowth of sensory neurons on both roots was not altered by the antibodies. Neurite outgrowth of motor neurons on a mixture of the extracellular matrix glycoprotein laminin and the L2 carbohydrate-carrying glycolipid was significantly higher than on the laminin substrate mixture with GD1b ganglioside or sulphatide. L2 antibodies reduced neurite outgrowth of motor neurons by 50% on the L2 glycolipid, but not on GD1b or sulphatide. These observations indicate that the L2 carbohydrate promotes neurite outgrowth of motor neurons in vitro and may thus contribute to the preferential reinnervation of motor nerves by regenerating motor axons in vivo.