Comprehensive corticospinal labeling with mu-crystallin transgene reveals axon regeneration after spinal cord trauma in ngr1−/− mice

Abstract

Spinal cord injury interrupts descending motor tracts and creates persistent functional deficits due to the absence of spontaneous axon regeneration. Of descending pathways, the corticospinal tract (CST) is thought to be the most critical for voluntary function in primates. Even with multiple tracer injections and genetic tools, the CST is visualized to only a minor degree in experimental studies. Here, we identify and validate the mu-crystallin (crym) gene as a high-fidelity marker of the CST. In transgenic mice expressing green fluorescent protein (GFP) under crym regulatory elements (crym-GFP), comprehensive and near complete CST labeling is achieved throughout the spinal cord. Bilateral pyramidotomy eliminated the 17,000 GFP-positive CST axons that were reproducibly labeled in brainstem from the spinal cord.Weshow that CST tracing with crym-GFP is 10-fold more efficient than tracing with biotinylated dextran amine (BDA). Using crym-GFP, we reevaluated the CST in mice lacking nogo receptor 1 (NgR1), a protein implicated in limiting neural repair. The number and trajectory of CST axons in ngr1−/− mice without injury was indistinguishable from ngr1+/+ mice. After dorsal hemisection in the midthoracic cord, CST axons did not significantly regenerate in ngr1+/+ mice, but an average of 162 of the 6000 labeled thoracic CST axons (2.68%) regenerated >100μ mpast the lesion site in crym-GFP ngr1−/− mice. Although traditional BDA tracing cannot reliably visualize regenerating ngr1−/− CST axons, their regenerative course is clear with crym-GFP. Therefore the crym-GFP transgenic mouse is a useful tool for studies of CST anatomy in experimental studies of motor pathways.