The Nogo–Nogo Receptor Pathway Limits a Spectrum of Adult CNS Axonal Growth

PyX-induced sprouting of intact CST fibers in ngr1 ^−/− mice. Photomicrographs A–C illustrate C7 transverse sections of spinal cord from ngr1 ^−/− sham, ngr1 ^+/+ PyX, and ngr1 ^−/− PyX-lesioned animals. Fasciculated BDA⁺ axons can be seen in the left ventrodorsal column ( A ) from which CST collaterals project unilaterally into both dorsal and ventral horns in sham-lesioned ngr1 ^−/− ( A ) and PyX ngr1 ^+/+ mice ( B ). PyX ngr1 ^−/− mice illustrate robust sprouting of intact BDA⁺ CST axons into the deafferented side of the spinal cord ( C ). Scale bar, 100 μm. The optical density of BDA⁺ axons from the pial surface into gray matter was assessed in zones I–VI ( A ). Quantification of zones IV–VI is shown in D–F . The integrated CST density (area under the curve) of ngr1 ^−/− animals is significantly greater in zones IV and V than in sham-lesioned or PyX ngr1 ^+/+ mice (*p < 0.05, ANOVA). G , H , As another measure of CST innervation, fiber length per cross-sectional area was measured throughout the spinal cord gray matter. Fiber length was not significantly different on the intact side between ngr1 ^+/+ and ngr1 ^−/− mice after sham or PyX lesion ( G ); however, CST fiber length was significantly greater on the injured side of ngr1 ^−/− mice ( H ) compared with ngr1 ^+/+ PyX, ngr1 ^−/− sham, and ngr1 ^+/+ sham-lesioned mice (*p < 0.001, ANOVA). For G and H , data are the mean ± SEM for n = 7–10 mice.

CST fibers crossing the midline of the spinal cord. A–C , Photomicrographs illustrate significant numbers of BDA-immunoreactive CST axons sprouting across the midline in ngr1 ^−/− mice after PyX compared with ngr1 ^−/− sham and ngr1 ^+/+ PyX-lesioned mice. Scale bar, 100 μm. D , The average number of CST fibers crossing the midline of the cervical spinal per transverse section is significantly greater in ngr1 ^−/− mice compared with ngr1 ^+/+ PyX, ngr1 ^−/− sham, and ngr1 ^+/+ sham-lesioned mice (*p < 0.001, ANOVA). Data are the mean ± SEM for n = 7–10 mice.

PyX-induced sprouting of intact CST fibers in nogo-ab^atg/atg mice. Photomicrographs A–C illustrate C7 transverse section of spinal cord from nogo-ab^atg/atg sham, nogo-ab ^+/+ PyX, and nogo-ab^atg/atg PyX-lesioned animals, respectively. Fasciculated BDA⁺ axons can be seen in the left ventrodorsal column ( A ) from which CST collaterals project unilaterally into both dorsal and ventral horns in sham-lesioned nogo-ab^atg/atg ( A ) and PyX nogo-ab ^+/+ mice ( B ). PyX nogo-ab^atg/atg mice illustrated increased sprouting of intact BDA⁺ CST axons into the deafferented side of the spinal cord ( C ). Scale bar, 200 μm. The optical density of BDA reactivity from the pial surface into gray matter was assessed in zones I–VI ( A ). Quantification of zones IV–VI is shown in D–F . Assessment of the integrated axonal density (area under the curve) revealed that nogo-ab^atg/atg animals have significantly more BDA reactivity in zones IV and V in comparison to sham-lesioned and PyX nogo-ab ^+/+ mice (*p < 0.05, ANOVA). G , H , The absolute length of CST axon per cross-sectional area was measured throughout the spinal cord gray matter for each condition. Fiber length was not significantly different on the intact side between nogo-ab ^+/+ sham and nogo-ab ^+/+ PyX-lesioned mice ( G ). However, PyX-lesioned ngr1 ^−/− mice had significantly greater CST length compared with nogo-ab^atg/atg sham-lesioned mice (*p < 0.05, ANOVA). CST fiber length was also significantly greater on the injured side of nogo-ab^atg/atg mice ( H ) compared with nogo-ab ^+/+ PyX, nogo-ab^atg/atg sham, and nogo-ab ^+/+ sham-lesioned mice (*p < 0.001, ANOVA). For G , H , data are the mean ± SEM for n = 7–10 mice.

Increased numbers of nogo-ab^atg/atg CST fibers crossing the midline of the spinal cord after PyX. A–C , Photomicrographs illustrate significant numbers of BDA-immunoreactive CST axons sprouting across the midline in nogo-ab^atg/atg mice after PyX compared with nogo-ab^atg/atg sham and nogo-ab ^+/+ PyX-lesioned mice. Scale bar, 100 μm. CC, Central canal. D , The average number of CST fibers crossing the midline of the cervical spinal per transverse section was significantly greater in nogo-ab^atg/atg mice compared with nogo-ab ^+/+ PyX, nogo-ab^atg/atg sham, and nogo-ab ^+/+ sham-lesioned mice (*p < 0.001, ANOVA). Data are the mean ± SEM for n = 7–10 mice.

Pyridotomized ngr1 ^−/− and nogo-ab^atg/atg mice recover fine forelimb function. A , ngr1 ^+/+ and ngr1 ^−/− mice were trained to retrieve food pellets through an aperture displaced to one side of a transparent plastic box. Ipsilateral paw usage was identical between genotypes during presurgical training. At 4 weeks after PyX, the ngr1 ^+/+ illustrated a significant inability to retrieve food with their ipsilateral paws (*p < 0.001, ANOVA). ngr1 ^−/− mice recovered the ability to use their ipsilateral paws and were not significantly different from sham-lesioned controls. B , Ipsilateral paw usage was identical between nogo-ab ^+/+ and nogo-ab^atg/atg mice during presurgical training. At 28 d after PyX, the nogo-ab ^+/+ showed a significant inability to retrieve food with the ipsilateral forepaw (*p < 0.001, ANOVA). In contrast, nogo-ab^atg/atg mice fully recovered the ability to use their ipsilateral paws and were not significantly different from sham-lesioned controls.