Oligodendrocytic but not neuronal Nogo restricts corticospinal axon sprouting after CNS injury

Nogo-B mediates endothelial oxidative stress and inflammation to promote coronary atherosclerosis in pressure-overloaded mouse hearts

AIMS

Endothelial dysfunction plays a pivotal role in atherosclerosis, but the detailed mechanism remains incomplete understood. Nogo-B is an endoplasmic reticulum (ER)-localized protein mediating ER-mitochondrial morphology. We previously showed endothelial Nogo-B as a key regulator of endothelial function in the setting of hypertension. Here, we aim to further assess the role of Nogo-B in coronary atherosclerosis in ApoE-/- mice with pressure overload.

METHODS AND RESULTS

We generated double knockout (DKO) mouse models of systemically or endothelium-specifically excising Nogo-A/B gene on an ApoE-/- background. After 7 weeks of transverse aortic constriction (TAC) surgery, compared to ApoE-/- mice DKO mice were resistant to the development of coronary atherosclerotic lesions and plaque rapture. Sustained elevation of Nogo-B and adhesion molecules (VCAM-1/ICAM-1), early markers of atherosclerosis, was identified in heart tissues and endothelial cells (ECs) isolated from TAC ApoE-/- mice, changes that were significantly repressed by Nogo-B deficiency. In cultured human umbilical vein endothelial cells (HUVECs) exposure to inflammatory cytokines (TNF-α, IL-1β), Nogo-B was upregulated and activated reactive oxide species (ROS)-p38-p65 signaling axis. Mitofusin 2 (Mfn2) is a key protein tethering ER to mitochondria in ECs, and we showed that Nogo-B expression positively correlated with Mfn2 protein level. And Nogo-B deletion in ECs or in ApoE-/- mice reduced Mfn2 protein content and increased ER-mitochondria distance, reduced ER-mitochondrial Ca2+ transport and mitochondrial ROS generation, and prevented VCAM-1/ICAM-1 upregulation and EC dysfunction, eventually restrained atherosclerotic lesions development.

CONCLUSION

Our study revealed that Nogo-B is a critical modulator in promoting endothelial dysfunction and consequent pathogenesis of coronary atherosclerosis in pressure overloaded hearts of ApoE-/- mice. Nogo-B may hold the promise to be a common therapeutic target in the setting of hypertension.

Microglial Nogo delays recovery following traumatic brain injury in mice

Nogo-A, B, and C are well described members of the reticulon family of proteins, most well known for their negative regulatory effects on central nervous system (CNS) neurite outgrowth and repair following injury. Recent research indicates a relationship between Nogo-proteins and inflammation. Microglia, the brain's immune cells and inflammation-competent compartment, express Nogo protein, although specific roles of the Nogo in these cells is understudied. To examine inflammation-related effects of Nogo, we generated a microglial-specific inducible Nogo KO (MinoKO) mouse and challenged the mouse with a controlled cortical impact (CCI) traumatic brain injury (TBI). Histological analysis shows no difference in brain lesion sizes between MinoKO-CCI and Control-CCI mice, although MinoKO-CCI mice do not exhibit the levels of ipsilateral lateral ventricle enlargement as injury matched controls. Microglial Nogo-KO results in decreased lateral ventricle enlargement, microglial and astrocyte immunoreactivity, and increased microglial morphological complexity compared to injury matched controls, suggesting decreased tissue inflammation. Behaviorally, healthy MinoKO mice do not differ from control mice, but automated tracking of movement around the home cage and stereotypic behavior, such as grooming and eating (termed cage "activation"), following CCI is significantly elevated. Asymmetrical motor function, a deficit typical of unilaterally brain lesioned rodents, was not detected in CCI injured MinoKO mice, while the phenomenon was present in CCI injured controls 1-week post-injury. Overall, our studies show microglial Nogo as a negative regulator of recovery following brain injury. To date, this is the first evaluation of the roles microglial specific Nogo in a rodent injury model.

Regulation of axonal regeneration after mammalian spinal cord injury

One hundred years ago, Ramón y Cajal, considered by many as the founder of modern neuroscience, stated that neurons of the adult central nervous system (CNS) are incapable of regenerating. Yet, recent years have seen a tremendous expansion of knowledge in the molecular control of axon regeneration after CNS injury. We now understand that regeneration in the adult CNS is limited by (1) a failure to form cellular or molecular substrates for axon attachment and elongation through the lesion site; (2) environmental factors, including inhibitors of axon growth associated with myelin and the extracellular matrix; (3) astrocyte responses, which can both limit and support axon growth; and (4) intraneuronal mechanisms controlling the establishment of an active cellular growth programme. We discuss these topics together with newly emerging hypotheses, including the surprising finding from transcriptomic analyses of the corticospinal system in mice that neurons revert to an embryonic state after spinal cord injury, which can be sustained to promote regeneration with neural stem cell transplantation. These gains in knowledge are steadily advancing efforts to develop effective treatment strategies for spinal cord injury in humans.

Dynamic induction of the myelin-associated growth inhibitor Nogo-A in perilesional plasticity regions after human spinal cord injury

The myelin-associated inhibitor Nogo-A (Reticulon 4, RTN4) restricts axonal outgrowth, plasticity, and neural circuitry formation in experimental models of spinal cord injury (SCI) and is targeted in clinical interventions starting treatment within 4 weeks post-SCI. Specifically, Nogo-A expressed by oligodendroglia restricts compensatory neurite sprouting. To interrogate the hypothesis of an inducible, lesion reactive Nogo-A expression over time, we analyzed the spatiotemporal Nogo-A expression at the spinal lesion core (region of tissue necrosis and axonal damage/pruning) and perilesional rim (region of plasticity formation). Spinal cord specimens of SCI subjects (n = 22) were compared to neuropathologically unaltered controls (n = 9). Nogo-A expression was investigated ranging from acute (0-3 days), early subacute (4-21 days), late subacute (22-90 days) to early chronic-chronic (91 days to 1.5 years after SCI) stages after SCI. Nogo-A expression in controls is confined to motoneurons in the anterior horn and to oligodendrocytes in gray and white matter. After SCI, the number of Nogo-A+ and TPPP/p25+ oligodendrocytes (i) inclined at the organizing perilesional rim specifically, (ii) increased further over time, and (iii) peaked at chronic stages after SCI. By contrast, at the lesion core, the number of Nogo-A+ and TPPP/p25+ oligodendrocytes did not increase. Increasing numbers of Nogo-A+ oligodendrocytes coincided with oligodendrogenesis corroborated by Nogo-A coexpression of Ki67+ , TPPP/p25+ proliferating oligodendrocytes. Nogo-A oligodendrocyte expression emerges at perilesional (plasticity) regions over time and suggests an extended therapeutical window for anti-Nogo-A pathway targeting interventions beyond 4 weeks in patients after SCI.

Targeting PTEN but not SOCS3 resists an age-dependent decline in promoting axon sprouting

Axonal repair is critical for functional recovery after injury of the CNS. We previously reported that neuronal PTEN deletion exhibits an age-dependent decline in promoting axon regeneration from the corticospinal tract (CST). How sprouting of uninjured axons, a naturally occurring form of axonal repair, is impacted by age is unknown. We assessed CST sprouting after unilateral pyramidotomy in PTEN and/or SOCS3-deleted mice at different ages. While PTEN deletion enhances sprouting independently of age, SOCS3 deletion loses its sprouting-promoting effect with age. The synergistic effect of PTEN/SOCS3 co-deletion on CST sprouting is rapidly lost with increased age. Overall, promoting sprouting appears more robust across age than regeneration, yet distinct molecular pathways are differentially impacted by age. Importantly, six-week delayed PTEN deletion promotes CST sprouting across age groups, supporting a clinically relevant time frame for this neural repair strategy independently of age.

In vivo peptide-based delivery of a gene-modifying enzyme into cells of the central nervous system

We report on the successful delivery of the Cre recombinase enzyme in the neural cells of mice in vivo by simple coinjection with peptides derived from HIV-TAT. Cre delivery activates the expression of a reporter gene in both neurons and astrocytes of the cortex without tissue damage and with a transduction efficiency that parallels or exceeds that of a commonly used adeno-associated virus. Our data indicate that the delivery peptides mediate efficient endosomal leakage and cytosolic escape in cells that have endocytosed Cre. The peptides, therefore, act in trans and do not require conjugation to the payload, greatly simplifying sample preparation. Moreover, the delivery peptides are exclusively composed of natural amino acids and are consequently readily degradable and processed by cells. We envision that this approach will be beneficial to applications that require the transient introduction of proteins into cells in vivo.

Overexpressing eukaryotic elongation factor 1 alpha (eEF1A) proteins to promote corticospinal axon repair after injury

Although protein synthesis is hypothesized to have a pivotal role in axonal repair after central nervous system (CNS) injury, the role of core components of the protein synthesis machinery has not been examined. Notably, some elongation factors possess non-canonical functions that may further impact axonal repair. Here, we examined whether overexpressing eukaryotic elongation factor 1 alpha (eEF1A) proteins enhances the collateral sprouting of corticospinal tract (CST) neurons after unilateral pyramidotomy, along with the underlying molecular mechanisms. We found that overexpressing eEF1A proteins in CST neurons increased the levels of pS6, an indicator for mTOR activity, but not pSTAT3 and pAKT levels, in neuronal somas. Strikingly, overexpressing eEF1A2 alone, but neither eEF1A1 alone nor both factors simultaneously, increased protein synthesis and actin rearrangement in CST neurons. While eEF1A1 overexpression only slightly enhanced CST sprouting after pyramidotomy, eEF1A2 overexpression substantially enhanced this sprouting. Surprisingly, co-overexpression of both eEF1A1 and eEF1A2 led to a sprouting phenotype similar to wild-type controls, suggesting an antagonistic effect of overexpressing both proteins. These data provide the first evidence that overexpressing a core component of the translation machinery, eEF1A2, enhances CST sprouting, likely by a combination of increased protein synthesis, mTOR signaling and actin cytoskeleton rearrangement.

The Role of Tissue Geometry in Spinal Cord Regeneration

Unlike peripheral nerves, axonal regeneration is limited following injury to the spinal cord. While there may be reduced regenerative potential of injured neurons, the central nervous system (CNS) white matter environment appears to be more significant in limiting regrowth. Several factors may inhibit regeneration, and their neutralization can modestly enhance regrowth. However, most investigations have not considered the cytoarchitecture of spinal cord white matter. Several lines of investigation demonstrate that axonal regeneration is enhanced by maintaining, repairing, or reconstituting the parallel geometry of the spinal cord white matter. In this review, we focus on environmental factors that have been implicated as putative inhibitors of axonal regeneration and the evidence that their organization may be an important determinant in whether they inhibit or promote regeneration. Consideration of tissue geometry may be important for developing successful strategies to promote spinal cord regeneration.

Promotion of corticospinal tract growth by KLF6 requires an injury stimulus and occurs within four weeks of treatment

Axons in the corticospinal tract (CST) display a limited capacity for compensatory sprouting after partial spinal injuries, potentially limiting functional recovery. Forced expression of a developmentally expressed transcription factor, Krüppel-like factor 6 (KLF6), enhances axon sprouting by adult CST neurons. Here, using a pyramidotomy model of injury in adult mice, we confirm KLF6's pro-sprouting properties in spared corticospinal tract neurons and show that this effect depends on an injury stimulus. In addition, we probed the time course of KLF6-triggered sprouting of CST axons and demonstrate a significant enhancement of growth within four weeks of treatment. Finally, we tested whether KLF6-induced sprouting was accompanied by improvements in forelimb function, either singly or when combined with intensive rehabilitation. We found that regardless of rehabilitative training, and despite robust cross-midline sprouting by corticospinal tract axons, treatment with KLF6 produced no significant improvement in forelimb function on either a modified ladder-crossing task or a pellet-retrieval task. These data clarify important details of KLF6's pro-growth properties and indicate that additional interventions or further optimization will be needed to translate this improvement in axon growth into functional gains.

Nogo-A aggravates oxidative damage in oligodendrocytes

Nogo-A is considered one of the most important inhibitors of myelin-associated axonal regeneration in the central nervous system. It is mainly expressed by oligodendrocytes. Although previous studies have found regulatory roles for Nogo-A in neurite outgrowth inhibition, neuronal homeostasis, precursor migration, plasticity, and neurodegeneration, its functions in the process of oxidative injury are largely uncharacterized. In this study, oligodendrocytes were extracted from the cerebral cortex of newborn Sprague-Dawley rats. We used hydrogen peroxide (H₂O₂) to induce an in vitro oligodendrocyte oxidative damage model and found that endogenously expressed Nogo-A is significantly upregulated in oligodendrocytes. After recombinant virus Ad-ZsGreen-rat Nogo-A infection of oligodendrocytes, Nogo-A expression was increased, and the infected oligodendrocytes were more susceptible to acute oxidative insults and exhibited a markedly elevated rate of cell death. Furthermore, knockdown of Nogo-A expression in oligodendrocytes by Ad-ZsGreen-shRNA-Nogo-A almost completely protected against oxidative stress induced by exogenous H₂O₂. Intervention with a Nogo-66 antibody, a LINGO1 blocker, or Y27632, an inhibitor in the Nogo-66-NgR/p75/LINGO-1-RhoA-ROCK pathway, did not affect the death of oligodendrocytes. Ad-ZsGreen-shRNA-Nogo-A also increased the levels of phosphorylated extracellular signal-regulated kinase 1/2 and inhibited BCL2 expression in oligodendrocytes. In conclusion, Nogo-A aggravated reactive oxygen species damage in oligodendrocytes, and phosphorylated extracellular signal-regulated kinase 1/2 and BCL2 might be involved in this process. This study was approved by the Ethics Committee of Peking University People’s Hospital, China (approval No. 2018PHC081) on December 18, 2018.

Application of Antibodies to Neuronally Expressed Nogo-A Increases Neuronal Survival and Neurite Outgrowth

Nogo-A, a glycoprotein expressed in oligodendrocytes and central nervous system myelin, inhibits regeneration after injury. Antibodies against Nogo-A neutralize this inhibitory activity, improve locomotor recovery in spinal cord-injured adult mammals, and promote regrowth/sprouting/saving of damaged axons beyond the lesion site. Nogo-A is also expressed by neurons. Complete ablation of Nogo-A in all cell types expressing it has been found to lead to recovery in some studies but not in others. Neuronal ablation of Nogo-A reduces axonal regrowth after injury. In view of these findings, we hypothesized that, in addition to neutralizing Nogo-A in oligodendrocytes and myelin, Nogo-A antibodies may act directly on neuronal Nogo-A to trigger neurite outgrowth and neuronal survival. Here, we show that polyclonal and monoclonal antibodies against Nogo-A enhance neurite growth and survival of cultured cerebellar granule neurons and increase expression of the neurite outgrowth-promoting L1 cell adhesion molecule and polysialic acid. Application of inhibitors of signal transducing molecules, such as c-src, c-fyn, protein kinase A, and casein kinase II reduce antibody-triggered neurite outgrowth. These observations indicate that the recovery-promoting functions of antibodies against Nogo-A may not only be due to neutralizing Nogo-A in oligodendrocytes and myelin, but also to their interactions with Nogo-A on neurons.

A Mini-Review of the Role of Glutamate Transporter in Drug Addiction

Goals: The development of new treatment for drug abuse requires identification of targetable molecular mechanisms. The pathology of glutamate neurotransmission system in the brain reward circuit is related to the relapse of multiple drugs. Glutamate transporter regulates glutamate signaling by removing excess glutamate from the synapse. And the mechanisms between glutamate transporter and drug addiction are still unclear.

Methods: A systematic review of the literature searched in Pubmed and reporting drug addiction in relation to glutamate transporter. Studies were screened by title, abstract, and full text.

Results: This review is to highlight the effects of drug addiction on glutamate transporter and glutamate uptake, and targeting glutamate transporter as an addictive drug addiction treatment. We focus on the roles of glutamate transporter in different brain regions in drug addiction. More importantly, we suggest the functional roles of glutamate transporter may prove beneficial in the treatment of drug addiction.

Conclusion: Overall, understanding how glutamate transporter impacts central nervous system may provide a new insight for treatment of drug addiction.

The Application of Omics Technologies to Study Axon Regeneration and CNS Repair

Traumatic brain and spinal cord injuries cause permanent disability. Although progress has been made in understanding the cellular and molecular mechanisms underlying the pathophysiological changes that affect both structure and function after injury to the brain or spinal cord, there are currently no cures for either condition. This may change with the development and application of multi-layer omics, new sophisticated bioinformatics tools, and cutting-edge imaging techniques. Already, these technical advances, when combined, are revealing an unprecedented number of novel cellular and molecular targets that could be manipulated alone or in combination to repair the injured central nervous system with precision. In this review, we highlight recent advances in applying these new technologies to the study of axon regeneration and rebuilding of injured neural circuitry. We then discuss the challenges ahead to translate results produced by these technologies into clinical application to help improve the lives of individuals who have a brain or spinal cord injury.