A soluble Nogo receptor differentially affects plasticity of spinally projecting axons

Abstract

In the central nervous system, regeneration of injured axons and sprouting of intact axons are suppressed by myelin-derived molecules that bind to the Nogo receptor (NgR). We used a soluble form of the NgR (sNgR), constructed as an IgG of the human NgR extracellular domain, to manipulate plasticity of uninjured primary afferent and descending monoaminergic projections to the rat spinal cord following dorsal rhizotomy. Rats with quadruple dorsal rhizotomies were treated with intrathecal sNgR or saline, or were left untreated for 2 weeks. Rhizotomy alone resulted in sprouting of serotonergic axons and to a lesser extent, tyrosine-hydroxylase (TH)-expressing axons, while axons expressing dopamine-beta-hydroxylase (DbetaH) were unaffected. Human IgG immunohistochemistry revealed that sNgR infused into the intrathecal space penetrated approximately 300 microm into spinal white and grey matter. Separate axonal populations differed in their responses to intrathecal sNgR: TH-expressing and DbetaH-expressing axons responded most and least vigorously, respectively. Serotonergic axons were identified by serotonin (5-HT) or serotonin transporter (SERT) immunohistochemistry. Interestingly, a large increase in 5-HT compared to SERT-positive axons density in both saline and sNgR-treated rats indicated that serotonergic axons both sprouted and increased their transmitter content in response to rhizotomy and sNgR treatment. Calcitonin gene-related peptide-positive axons were largely depleted ipsilaterally by rhizotomy, and sNgR increased axon density only in deeper contralateral laminae (III-V). GAP-43 immunohistochemistry revealed a small increase in axon density following dorsal rhizotomy that was further augmented by sNgR treatment. These results reveal a differential effect of myelin antagonism on distinct populations of spinally projecting axons.

Calcitonin gene-related peptide immunoreactivity in chronic human spinal cord injury

“…31,32 In order to prevent observer bias, all sections were coded in order to blind the analysis. Each photomicrograph was captured using Northern Eclipse software (Empix Neuroimaging, version 6.0) and was thresholded using an edge-detecting algorithm.…”

“…A computerized program provided average density measurements for each 20-mm segment of the 720-mm long rectangular strip. To quantify CGRP-positive axon density in the spinal cord, we carried an automated thresholding procedure, 31,33,34 which first defines the thickest and brightest regions, and then, following application of a Laplacian omnidirectional edge-detection filter, the finer processes ( Figure 1, bottom). The edge-detection filter effectively normalizes the signal-to-noise ratio such that small variations in immunoreactivity across sections are eliminated.…”

Endogenous TrkB Ligands Suppress Functional Mechanosensory Plasticity in the Deafferented Spinal Cord

“…C7/8 DRI produced a consistent deafferentation gap in laminas III-IV of the C7 dorsal horn. Densitometric analyses of VGLUT1-and CTB-positive terminals were performed within the deafferentation gap, by measuring terminal density as a function of depth in the dorsal horn (Ramer et al, 2001MacDermid et al, 2004;Scott et al, 2005). After C7/8 DRI, the density of both VGLUT1-and CTB-labeled terminals was decreased but remained unchanged for at least 20 d postoperatively.…”

“…Quantitative analyses of terminal density in the dorsal horn were performed by measuring terminal density as a function of depth in the dorsal horn, as we have done previously (Ramer et al, 2001MacDermid et al, 2004;Scott et al, 2005). Densitometric analyses of vesicular glutamate transporter 1 (VGLUT1)-and CTB-positive terminals were performed within the consistent deafferentation gap apparent in C7 after C7/8 DRI.…”

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

“…51,52 Another antagonist, NgREcto, is a soluble, truncated form of NgR, which has been shown to alleviate inhibition of both Nogo and myelin in vitro (Fournier et al, 2002). Recent work from our group examines the effects of NgR activity using a different form of soluble NgR in vivo; 53 in these experiments, we found that NgR antagonism augmented rhizotomy-induced sprouting of both descending monoaminergic axons and peptidgeric primary afferents in the cervical dorsal horn. The discovery of an anti-NgR monoclonal antibody, capable of inhibiting binding of Nogo, MAG, and OMgp, and of blocking inhibition of myelin in vitro, has now been reported.…”