Peripheral nerve injury fails to induce growth of lesioned ascending dorsal column axons into spinal cord scar tissue expressing the axon repellent Semaphorin3A

We have investigated the hypothesis that the chemorepellent Semaphorin3A may be involved in the failure of axonal regeneration after injury to the ascending dorsal columns of adult rats. Following transection of the thoracic dorsal columns, fibroblasts in the dorsolateral parts of the lesion site showed robust expression of Semaphorin3A mRNA. In addition, dorsal root ganglion (DRG) neurons with projections through the dorsal columns to the injury site persistently expressed both Semaphorin3A receptor components, neuropilin-1 and plexin-A1. These ascending DRG collaterals failed to invade scar regions occupied by Semaphorin3A-positive fibroblasts, even in animals which had received conditioning lesions of the sciatic nerve to enhance regeneration. Other axon populations in the dorsal spinal cord were similarly unable to penetrate Semaphorin3A-positive scar tissue. These data suggest that Semaphorin3A may create an exclusion zone for regenerating dorsal column fibres and that enhancing the intrinsic regenerative response of DRG neurons has only limited effects on axonal regrowth. Tenascin-C and chondroitin sulphate proteoglycans were also detected at the injury site, which was largely devoid of central nervous system (CNS) myelin, showing that several classes of inhibitory factors, including semaphorins, with only partially overlapping spatial and temporal patterns of expression are in a position to participate in preventing regenerative axonal growth in the injured dorsal columns. Interestingly, conditioning nerve injuries enabled numerous ascending DRG axons to regrow across areas of strong tenascin-C and chondroitin sulphate proteoglycan expression, while areas containing Semaphorin3A and CNS myelin were selectively avoided by (pre)primed axonal sprouts.