Axonal growth inhibitors and their receptors in spinal cord injury:from biology to clinical translation

Axonal growth inhibitors are released during traumatic injuries to the adult mammalian central nervous system, including after spinal cord injury. These molecules accumulate at the injury site and form a highly inhibitory environment for axonal regeneration. Among these inhibitory molecules, myelinassociated inhibitors, including neurite outgrowth inhibitor A, oligodendrocyte myelin glycoprotein, myelin-associated glycoprotein, chondroitin sulfate proteoglycans and repulsive guidance molecule A are of particular importance. Due to their inhibitory nature, they represent exciting molecular targets to study axonal inhibition and regeneration after central injuries. These molecules are mainly produced by neurons, oligodendrocytes, and astrocytes within the scar and in its immediate vicinity. They exert their effects by binding to specific receptors, localized in the membranes of neurons. Receptors for these inhibitory cues include Nogo receptor 1, leucine-rich repeat, and Ig domain containing 1 and p75 neurotrophin receptor/tumor necrosis factor receptor superfamily member 19(that form a receptor complex that binds all myelin-associated inhibitors), and also paired immunoglobulin-like receptor B. Chondroitin sulfate proteoglycans and repulsive guidance molecule A bind to Nogo receptor 1, Nogo receptor 3, receptor protein tyrosine phosphatase σ and leucocyte common antigen related phosphatase, and neogenin, respectively. Once activated, these receptors initiate downstream signaling pathways, the most common amongst them being the Rho A/ROCK signaling pathway. These signaling cascades result in actin depolymerization, neurite outgrowth inhibition, and failure to regenerate after spinal cord injury. Currently, there are no approved pharmacological treatments to overcome spinal cord injuries other than physical rehabilitation and management of the array of symptoms brought on by spinal cord injuries. However, several novel therapies aiming to modulate these inhibitory proteins and/or their receptors are under investigation in ongoing clinical trials. Investigation has also been demonstrating that combinatorial therapies of growth inhibitors with other therapies, such as growth factors or stem-cell therapies, produce stronger results and their potential application in the clinics opens new venues in spinal cord injury treatment.

Upregulation of Nogo receptor expression induces apoptosis of retinal ganglion cells in diabetic rats

The Nogo receptor is an essential factor for neuronal apoptosis, but the changes in Nogo receptor expression in the retina and the effects of the Nogo receptor on retinal ganglion cell apoptosis in diabetes mellitus remain unclear. We found that Nogo receptor expression was mainly visible in retinal ganglion cells of a rat model of diabetes mellitus induced by streptozotocin. At 12 weeks after onset of diabetes mellitus, Nogo receptor and Rho kinase expression signiifcantly increased in the retina, and retinal ganglion cell apoptosis was apparent. When RNA interference was used to suppress Nogo receptor expression in rat retina, Rho kinase expression was obviously inhibit-ed, and retinal ganglion cell apoptosis was evidently reduced in rats with diabetes mellitus. These results indicate that upregulation of Nogo receptor expression is an important mechanism of retinal ganglion cell apoptosis in rats with diabetes mellitus.

Nogo receptor expression in microglia/macrophages during experimental autoimmune encephalomyelitis progression

Myelin-associated inhibitory factors within the central nervous system（CNS） are considered to be one of the main obstacles for axonal regeneration following disease or injury. The nogo receptor 1（NgR1） has been well documented to play a key role in limiting axonal regrowth in the injured and diseased mammalian CNS. However, the role of nogo receptor in immune cell activation during CNS inflammation is yet to be mechanistically elucidated. Microglia/macrophages are immune cells that are regarded as pathogenic contributors to inflammatory demyelinating lesions in multiple sclerosis（MS）. In this study, the animal model of MS, experimental autoimmune encephalomyelitis（EAE） was induced in ngr1^＋/＋ and ngr1^–/– female mice following injection with the myelin oligodendrocyte glycoprotein（MOG_（35–55）） peptide. A fatemap analysis of microglia/macrophages was performed throughout spinal cord sections of EAE-induced mice at clinical scores of 0, 1, 2 and 3, respectively（increasing locomotor disability） from both genotypes, using the CD11 b and Iba1 cell markers. Western immunoblotting using lysates from isolated spinal cord microglia/macrophages, along with immunohistochemistry and flow cytometric analysis, was performed to demonstrate the expression of nogo receptor and its two homologs during EAE progression. Myelin protein engulfment during EAE progression in ngr1^＋/＋ and ngr1^–/– mice was demonstrated by western immunblotting of lysates from isolated spinal cord microglia/macrophages, detecting levels of Nogo-A and MOG. The numbers of M1 and M2 microglia/macrophage phenotypes present in the spinal cords of EAE-induced ngr1^＋/＋ and ngr1^–/– mice, were assessed by flow cytometric analysis using CD38 and Erg-2 markers. A significant difference in microglia/macrophage numbers between ngr1^＋/＋ and ngr1^–/– mice was identified during the progression of the clinical symptoms of EAE, in the white versus gray matter regions of the spinal cord. This difference was unrelated to the expression of Ng R on these macrophage/microglial cells. We have identified that as EAE progresses, the phagocytic activity of microglia/macrophages with myelin debris, in ngr1^–/– mice, was enhanced. Moreover, we show a modulation from a predominant M1-pathogenic to the M2-neurotrophic cell phenotype in the ngr1^–/– mice during EAE progression. These findings suggest that CNS-specific macrophages and microglia of ngr1^–/– mice may exhibit an enhanced capacity to clear inhibitory molecules that are sequestered in inflammatory lesions.

Experience-dependent expression of Nogo-A and Nogo receptor in the developing rat visual cortex

Nogo-A and Nogo receptor （NgR） expression in the visual cortex following a critical developmental period （postnatal days 20-60） has been previously shown. However, little is known regarding Nogo-A and NgR expression between postnatal day 0 and initiation of the critical period. The present study analyzed Nogo-A and NgR expression at four different time points： postnatal day 0 （P0）, before critical period （P14）, during critical period （P28）, and after critical period （P60）. Results showed significantly increased Nogo-A mRNA and protein expression levels in the visual cortex following birth, and expression levels remained steady between P28 and P60. NgR mRNA or protein expression was dramatically upregulated with age and peaked at P14 or P28, respectively, and maintained high expression to P60. In addition, Nogo-A and NgR expression was analyzed in each visual cortex layer in normal developing rats and rats with monocular deprivation. Monocular deprivation decreased Nogo-A and NgR mRNA and protein expression in the rat visual cortex, in particular in layers Ⅱ-Ⅲ and Ⅳ in the visual cortex contralateral to the deprived eye. These findings suggested that Nogo-A and NgR regulated termination of the critical period in experience- dependent visual cortical plasticity.

Ultrasound microbubbles combined with liposome-mediated pNogo-R shRNA delivery into neural stem cells

In the present study, ultrasound-mediated microbubble destruction （UMMD） alone and combined with liposome technology was used as a novel nonviral technique to transfect a Nogo receptor （Nogo-R） shRNA plasmid （pNogo-R shRNA） into neural stem cells （NSCs）. Using green fluorescent protein as a reporter gene, transfection efficiency of NSCs was significantly higher in the group transfected with UMMD combined with liposomes compared with that of the group transfected with UMMD or liposomes alone, and did not affect cell vitality. In addition, Nogo-R mRNA and protein expression was dramatically decreased in the UMMD combined with liposome-mediated group compared with that of other groups after 24 hours of transfection. The UMMD technique combined with liposomes is a noninvasive gene transfer method, which showed minimal effects on cell viability and effectively increased transfer of Nogo-R shRNA into NSCs.

Lateral olfactory tract usher substance(LOTUS) protein,an endogenous Nogo receptor antagonist,converts a non-permissive to permissive brain environment for axonal regrowth

It is well known that primates,including humans,hardly recover motor function after spinal cord injury（SCI）when compared with non-primate mammals such as rodents.This limited functional recovery is in part due to a non-permissive environment of the central nervous system（CNS）inhibiting axonal regrowth.

Electroacupuncture treatment improves motor function and neurological outcomes after cerebral ischemia/reperfusion injury

Electroacupuncture(EA)has been widely used for functional restoration after stroke.However,its role in post-stroke rehabilitation and the associated regulatory mechanisms remain poorly understood.In this study,we applied EA to the Zusanli(ST36)and Quchi(LI11)acupoints in rats with middle cerebral artery occlusion and reperfusion.We found that EA effectively increased the expression of brain-derived neurotrophic factor and its receptor tyrosine kinase B,synapsin-1,postsynaptic dense protein 95,and microtubule-associated protein 2 in the ischemic penumbra of rats with middle cerebral artery occlusion and reperfusion.Moreover,EA greatly reduced the expression of myelin-related inhibitors Nogo-A and NgR in the ischemic penumbra.Tyrosine kinase B inhibitor ANA-12 weakened the therapeutic effects of EA.These findings suggest that EA can improve neurological function after middle cerebral artery occlusion and reperfusion,possibly through regulating the activity of the brain-derived neurotrophic factor/tyrosine kinase B signal pathway.All procedures and experiments were approved by the Animal Research Committee of Shanghai University of Traditional Chinese Medicine,China(approval No.PZSHUTCM200110002)on January 10,2020.

Cholesterol synthesis inhibition or depletion in axon regeneration

Cholesterol is biosynthesized by all animal cells. Beyond its metabolic role in steroidogenesis, it is enriched in the plasma membrane where it has key structural and regulatory functions. Cholesterol is thus presumably important for post-injury axon regrowth, and this notion is supported by studies showing that impairment of local cholesterol reutilization impeded regeneration. However, several studies have also shown that statins, inhibitors of 3-hydroxy-3-methylglutaryl-Co A reductase, are enhancers of axon regeneration, presumably acting through an attenuation of the mevalonate isoprenoid pathway and consequent reduction in protein prenylation. Several recent reports have now shown that cholesterol depletion, as well as inhibition of cholesterol synthesis per se, enhances axon regeneration. Here, I discussed these findings and propose some possible underlying mechanisms. The latter would include possible disruptions to axon growth inhibitor signaling by lipid raft-localized receptors, as well as other yet unclear neuronal survival signaling process enhanced by cholesterol lowering or depletion.

Anti-apoptotic efficacy of Qingnao Yizhi formula(清脑益智方)in hypoxia/reoxygenation primary cortical neurons through the promotion of synaptic plasticity by modulating the NogoA-Nogo receptor/Rho-Rho kinase signaling pathway

OBJECTIVE:To evaluate the anti-apoptotic efficacy of Qingnao Yizhi formula(清脑益智方,QNYZ)in cultured cerebral cortical neuronal cells(CNCs)and the regulation of the NogoA-Nogo receptor(NgR)/Rho-Rho kinase(ROCK)signaling pathway.METHODS:Primary cultured CNCs were randomly divided into the following groups:normal control group(N-C),hypoxia-reoxygenation group(H/R),high-dose QNYZ group(Q-H),low-dose QNYZ group(Q-L)butylphthalide(NBP)group,and Y-27632(a selective ROCK transduction pathway inhibiter)group.Except those in the N-C group,CNCs were placed in hypoxic conditions for 24 h and then in reoxygenation conditions for 24 h.Cell media was changed every 48 h,and various assays were performed on the 7 th day.Cell viability was evaluated by measuring mitochondrial dehydrogenase activity,using a CCK-8 assay,in triplicate.Synapsin(SYN)protein concentrations were evaluated by enzyme-linked immunosorbent assay.NogoA and RhoA protein expression were evaluated through Western blotting.The gene expression of NogoA,NgR,RhoA,and ROCK was evaluated by reverse transcription-polymerase chain reaction.Cell apoptosis was measured using a terminal deoxynucleotidyl transferase biotin-d UTP nick end labeling assay.RESULTS:Compared with the N-C group,the cell viability of the H/R group decreased significantly(P<0.05).The cell viability values for the Q-H and Q-L groups increased compared with that for the H/R group,and the difference was significant for the Q-H group(P<0.05).The NogoA and RhoA protein levels and the NogoA,NgR,RhoA,and ROCK m RNA expression levels increased in the H/R group,compared with the N-C group,and decreased significantly in the Q-H and Q-L groups(P<0.05)and in the Y-27632 group(P<0.05)compared with the H/R group.The SYN levels in the Q-H,Q-L,and NBP groups significantly increased compared with that in the H/R group(P<0.05).Compared with the H/R group,the numbers of apoptotic cells in the Q-H,Q-L,and NBP groups significantly decreased(P<0.05).CONCLUSION:The presented study demonstrated that QNYZ exerted anti-apoptotic effects on H/R-induced CNCs,possibly through the modulation of the NogoA-NgR/Rho-ROCK signaling pathway and the promotion of synaptic plasticity in H/R CNCs.