N-Propionylmannosamine-induced over-expression and secretion of thioredoxin leads to neurite outgrowth of PC12 cells

Redox Protein Thioredoxin Mediates Neurite Outgrowth in Primary Cultured Mouse Cerebral Cortical Neurons

Thioredoxin system plays an important role in maintaining the cellular redox balance. Recent evidence suggests that thioredoxin (Trx) system may promote cell survival and neuroprotection. In this study, we explored the role of thioredoxin system in neuronal differentiation using a primary mouse cortical neuronal cell culture. First, Trx and Trx reductase (TrxR) protein levels were analyzed in cultured neurons from 1 to 32 days in vitro (DIV). The result showed that Trx and TrxR protein levels time-dependently increased in the neuron cell culture from 1 to 18 DIV. To establish the role of Trx in neuronal differentiation, Trx gene expression was knockdown in cultured neurons using Trx sgRNA CRISPR/Cas9 technology. Treatment with CRISPR/Cas9/Trx sgRNA decreased Trx protein levels and caused a reduction in dendritic outgrowth and branching of cultured neurons. Then, primary cortical neurons were treated with the Trx inhibitor PX12 to block Trx reducing activity. Treatment with PX12 also reduced dendritic outgrowth and branching. Furthermore, PX12 treatment reduced the ratio of phosphorylated cyclic AMP response element-binding protein (CREB)/total CREB protein levels. To investigate whether CREB phosphorylation is redox regulated, SH-SY5Y cells were treated with H2O2, which reduced phosphorylated CREB protein levels and increased CREB thiol oxidation. However, treatment with CB3, a Trx-mimetic tripeptide, rescued H2O2-decreased CREB phosphorylation. Our results suggest that Trx regulates neuronal differentiation and maturation of primary mouse cortical neurons by targeting CREB neurotrophic pathway. Trx may regulate CREB activation by maintaining the cellular redox balance.

The Applications of Metabolic Glycoengineering

Mammalian cell membranes are decorated by the glycocalyx, which offer versatile means of generating biochemical signals. By manipulating the set of glycans displayed on cell surface, it is vital for gaining insight into the cellular behavior modulation and medical and biotechnological adhibition. Although genetic engineering is proven to be an effective approach for cell surface modification, the technique is only suitable for natural and genetically encoded molecules. To circumvent these limitations, non-genetic approaches are developed for modifying cell surfaces with unnatural but functional groups. Here, we review latest development of metabolic glycoengineering (MGE), which enriches the chemical functions of the cell surface and is becoming an intriguing new tool for regenerative medicine and tissue engineering. Particular emphasis of this review is placed on discussing current applications and perspectives of MGE.

Y-27632 Induces Neurite Outgrowth by Activating the NOX1-Mediated AKT and PAK1 Phosphorylation Cascades in PC12 Cells

Y-27632 is known as a selective Rho-associated coiled coil-forming kinase (ROCK) inhibitor. Y-27632 has been shown to induce neurite outgrowth in several neuronal cells. However, the precise molecular mechanisms linking neurite outgrowth to Y-27632 are not completely understood. In this study, we examined the ability of Y-27632 to induce neurite outgrowth in PC12 cells and evaluated the signaling cascade. The effect of Y-27632 on the neurite outgrowth was inhibited by reactive oxygen species (ROS) scavengers such as N-acetyl cysteine (NAC) and trolox. Furthermore, Y-27632-induced neurite outgrowth was not triggered by NADPH oxidase 1 (NOX1) knockdown or diphenyleneiodonium (DPI), a NOX inhibitor. Suppression of the Rho-family GTPase Rac1, which is under the negative control of ROCK, with expression of the dominant negative Rac1 mutant (Rac1N17) prevented Y-27632-induced neurite outgrowth. Moreover, the Rac1 inhibitor NSC23766 prevented Y-27632-induced AKT and p21-activated kinase 1 (PAK1) activation. AKT inhibition with MK2206 suppressed Y-27632-induced PAK1 phosphorylation and neurite outgrowth. In conclusion, our results suggest that Rac1/NOX1-dependent ROS generation and subsequent activation of the AKT/PAK1 cascade contribute to Y-27632-induced neurite outgrowth in PC12 cells.

Redox Signaling Mechanisms in Nervous System Development

SIGNIFICANCE

Numerous studies have demonstrated the actions of reactive oxygen species (ROS) as regulators of several physiological processes. In this study, we discuss how redox signaling mechanisms operate to control different processes such as neuronal differentiation, oligodendrocyte differentiation, dendritic growth, and axonal growth. Recent Advances: Redox homeostasis regulates the physiology of neural stem cells (NSCs). Notably, the neuronal differentiation process of NSCs is determined by a change toward oxidative metabolism, increased levels of mitochondrial ROS, increased activity of NADPH oxidase (NOX) enzymes, decreased levels of Nrf2, and differential regulation of different redoxins. Furthermore, during the neuronal maturation processes, NOX and MICAL produce ROS to regulate cytoskeletal dynamics, which control the dendritic and axonal growth, as well as the axonal guidance.

CRITICAL ISSUES

The redox homeostasis changes are, in part, attributed to cell metabolism and compartmentalized production of ROS, which is regulated, sensed, and transduced by different molecules such as thioredoxins, glutaredoxins, peroxiredoxins, and nucleoredoxin to control different signaling pathways in different subcellular regions. The study of how these elements cooperatively act is essential for the understanding of nervous system development, as well as the application of regenerative therapies that recapitulate these processes.

FUTURE DIRECTIONS

The information about these topics in the last two decades leads us to the conclusion that the role of ROS signaling in development of the nervous system is more important than it was previously believed and makes clear the importance of exploring in more detail the mechanisms of redox signaling. Antioxid. Redox Signal. 28, 1603-1625.

Axonal protection by thioredoxin-1 with inhibition of interleukin-1β in TNF-induced optic nerve degeneration

Interleukin (IL)-1β, a proinflammatory cytokine, is a key mediator in several acute and chronic neurological diseases. Thioredoxin-1 (TRX1) acts as an antioxidant and plays a protective role in certain neurons. We examined whether exogenous TRX1 exerts axonal protection and affects IL-1β levels in tumor necrosis factor (TNF)-induced optic nerve degeneration in rats. Immunoblot analysis showed that IL-1β was upregulated in the optic nerve after intravitreal injection of TNF. Treatment with recombinant human (rh) TRX1 exerted substantial protective effects against TNF-induced axonal loss. The increase in the IL-1β level in the optic nerve was abolished by rhTRX1. Treatment with rhTRX1 also significantly inhibited increased glial fibrillary acidic protein (GFAP) levels induced by TNF. Immunohistochemical analysis showed substantial colocalization of IL-1β and GFAP in the optic nerve after TNF injection. These results suggest that IL-1β is upregulated in astrocytes in the optic nerve after TNF injection and that exogenous rhTRX1 exerts axonal protection with an inhibitory effect on IL-1β.

Thioredoxin-2 Modulates Neuronal Programmed Cell Death in the Embryonic Chick Spinal Cord in Basal and Target-Deprived Conditions

Thioredoxin-2 (Trx2) is a mitochondrial protein using a dithiol active site to reduce protein disulfides. In addition to the cytoprotective function of this enzyme, several studies have highlighted the implication of Trx2 in cellular signaling events. In particular, growing evidence points to such roles of redox enzymes in developmental processes taking place in the central nervous system. Here, we investigate the potential implication of Trx2 in embryonic development of chick spinal cord. To this end, we first studied the distribution of the enzyme in this tissue and report strong expression of Trx2 in chick embryo post-mitotic neurons at E4.5 and in motor neurons at E6.5. Using in ovo electroporation, we go on to highlight a cytoprotective effect of Trx2 on the programmed cell death (PCD) of neurons during spinal cord development and in a novel cultured spinal cord explant model. These findings suggest an implication of Trx2 in the modulation of developmental PCD of neurons during embryonic development of the spinal cord, possibly through redox regulation mechanisms.

N-Propionylmannosamine stimulates axonal elongation in a murine model of sciatic nerve injury

Increasing evidence indicates that sialic acid plays an important role during nerve regeneration. Sialic acids can be modified in vitro as well as in vivo using metabolic oligosaccharide engineering of the N-acyl side chain. N-Propionylmannosamine (ManNProp) increases neurite outgrowth and accelerates the reestablishment of functional synapses in vitro. We investigated the influence of systemic ManNProp application using a specific in vivo mouse model. Using mice expressing axonal fluorescent proteins, we quantified the extension of regenerating axons, the number of regenerating axons, the number of arborising axons and the number of branches per axon 5 days after injury. Sciatic nerves from non-expressing mice were grafted into those expressing yellow fluorescent protein. We began a twice-daily intraperitoneal application of either peracetylated ManNProp (200 mg/kg) or saline solution 5 days before injury, and continued it until nerve harvest (5 days after transection). ManNProp significantly increased the mean distance of axonal regeneration (2.49 mm vs. 1.53 mm; P < 0.005) and the number of arborizing axons (21% vs. 16%; P = 0.008) 5 days after sciatic nerve grafting. ManNProp did not affect the number of regenerating axons or the number of branches per arborizing axon. The biochemical glycoengineering of the N-acyl side chain of sialic acid might be a promising approach for improving peripheral nerve regeneration.

Expression of peroxiredoxins and thioredoxins in the mouse spinal cord during embryonic development

Reactive oxygen and nitrogen species (ROS/RNS) are natural byproducts of cellular metabolism. Although these molecules are deleterious at high concentrations, moderate levels of ROS/RNS are essential for normal cell function and take part in numerous cellular processes. The regulation of ROS/RNS is largely attended by peroxiredoxins (Prdxs) and their main reductants, thioredoxins (Trxs). Through their oxidoreductase activities, the members of the Trx/Prdx system can also affect certain cellular processes, notably many implicated in central nervous system (CNS) development. Although several studies have investigated the expression of Prdxs and Trxs in mouse, rat, and human adult CNS, few data are available concerning embryonic stages. In this work, we use immunofluorescence analyses to study the distribution of these enzymes during prenatal mouse spinal cord development. Our results highlight several patterns that contrast with available data for the adult. Indeed, Prdx1, Prdx4, and Prdx6, which are expressed in glial cells in the adult CNS, present clear neuronal localization in mouse spinal cord during embryonic development. Additionally, Prdx1, Prdx2, and to a lesser extent Prdx4, Prdx6, and Trx1 are localized mainly in the nucleus of neural cells. Finally, we identified a consistent, intense expression of all Prdxs and Trxs in groups of cells located in ventral regions of the spinal cord that express motor neuronal markers. These striking expression patterns suggest novel functions of these enzymes at these stages and offer clues to the role of the Trx/Prdx system during embryonic development of the spinal cord.

In vivo stimulation of early peripheral axon regeneration by N-propionylmannosamine in the presence of polysialyltransferase ST8SIA2

The key enzyme of sialic acid (Sia) biosynthesis is the bifunctional UDP-N-acetylglucosamine 2-epimerase/ManNAc kinase (GNE/MNK). It metabolizes the physiological precursor ManNAc and N-acyl modified analogues such as N-propionylmannosamine (ManNProp) to the respective modified sialic acid. Polysialic acid (polySia) is a crucial compound for several functions in the nervous system and is synthesized by the polysialyltransferases ST8SIA2 and ST8SIA4. PolySia can be modified in vitro and in vivo by metabolic glycoengineering of the N-acyl side chain of Sia. In vitro studies show that the application of ManNProp increases neurite outgrowth and accelerates the re-establishment of functional synapses. In this study, we investigate in vivo how ManNProp application might benefit peripheral nerve regeneration. In mice expressing axonal fluorescent proteins (thy-1-YFP), we transected the sciatic nerve and then replaced part of it with a sciatic nerve graft from non-expressing mice (wild-type mice or St8sia2(-/-) mice). Analyses conducted 5 days after grafting showed that systemic application of ManNProp (200 mg/kg, twice a day, i.p.), but not of physiological ManNAc (1 g/kg, twice a day, i.p.), significantly increased the extent of axonal elongation, the number of arborizing axons and the number of branches per regenerating axon within the grafts from wild-type mice, but not in those from St8sia2(-/-) mice. The results demonstrate that the application of ManNProp has beneficial effects on early peripheral nerve regeneration and indicate that the stimulation of axon growth depends on ST8SIA2 activity in the nerve graft.

Tissue-based metabolic labeling of polysialic acids in living primary hippocampal neurons

The posttranslational modification of neural cell-adhesion molecule (NCAM) with polysialic acid (PSA) and the spatiotemporal distribution of PSA-NCAM play an important role in the neuronal development. In this work, we developed a tissue-based strategy for metabolically incorporating an unnatural monosaccharide, peracetylated N-azidoacetyl-D-mannosamine, in the sialic acid biochemical pathway to present N-azidoacetyl sialic acid to PSA-NCAM. Although significant neurotoxicity was observed in the conventional metabolic labeling that used the dissociated neuron cells, neurotoxicity disappeared in this modified strategy, allowing for investigation of the temporal and spatial distributions of PSA in the primary hippocampal neurons. PSA-NCAM was synthesized and recycled continuously during neuronal development, and the two-color labeling showed that newly synthesized PSA-NCAMs were transported and inserted mainly to the growing neurites and not significantly to the cell body. This report suggests a reliable and cytocompatible method for in vitro analysis of glycans complementary to the conventional cell-based metabolic labeling for chemical glycobiology.

Metabolic glycoengineering of mesenchymal stromal cells with N-propanoylmannosamine

There is an increasing interest in the modification of cell surface glycosylation to improve the properties of therapeutic cells. For example, glycosylation affects the biodistribution of mesenchymal stromal cells (MSCs). Metabolic glycoengineering is an efficient way to modify the cell surface. The mammalian biosynthetic machinery tolerates the unnatural sialic acid precursor, N-propanoylmannosamine (ManNProp), and incorporates it into cell surface glycoconjugates. We show here by mass spectrometric analysis of cell surface N-glycans that about half of N-acetylneuraminic acid was replaced by N-propanoylneuraminic acid in the N-glycans of human umbilical cord blood-derived MSCs supplemented with ManNProp. In addition, the N-glycan profile was altered. ManNProp-supplemented cells had more multiply fucosylated N-glycan species than control cells. The fucosylated epitopes were shown in tandem mass spectrometric analysis to be Lewis x or blood group H epitopes, but not sialyl Lewis x (sLex). The amounts of tri- and tetra-antennary and polylactosamine-containing N-glycans also increased in ManNProp supplementation. In accordance with previous studies of other cell types, increased expression of the sLex epitope in ManNProp-supplemented MSCs was demonstrated by flow cytometry. In light of the N-glycan analysis, the sLex epitope in these cells is likely to be carried by O-glycans or glycolipids. sLex has been shown to target MSCs to bone marrow, which may be desirable in therapeutic applications. The present results represent the first structural analysis of an N-glycome of ManNProp-supplemented cells and demonstrate the feasibility of modifying cell surface glycosylation of therapeutic cells by this type of metabolic glycoengineering.

Activation of Rac1-dependent redox signaling is critically involved in staurosporine-induced neurite outgrowth in PC12 cells

Staurosporine, a non-specific protein kinase inhibitor, has been shown to induce neurite outgrowth in PC12 cells, but the mechanism by which staurosporine induces neurite outgrowth is still obscure. In the present study, we investigated whether the activation of Rac1 was responsible for the neurite outgrowth triggered by staurosporine. Staurosporine caused rapid neurite outgrowth independent of the ERK signaling pathways. In contrast, neurite outgrowth in response to staurosporine was accompanied by activation of Rac1, and the Rac1 inhibitor NSC23766 attenuated the staurosporine-induced neurite outgrowth in a concentration-dependent manner. In addition, suppression of Rac1 activity by expression of the dominant negative mutant Rac1N17 also blocked the staurosporine-induced morphological differentiation of PC12 cells. Staurosporine caused an activation of NADPH oxidase and increased the production of reactive oxygen species (ROS), which was prevented by NSC23766 and diphenyleneiodonium (DPI), an NADPH oxidase inhibitor. Staurosporine-induced neurite outgrowth was attenuated by pretreatment with DPI and exogenous addition of sublethal concentration of H2O2 accelerated neurite outgrowth triggered by staurosporine. These results indicate that activation of Rac1, which leads to ROS generation, is required for neurite outgrowth induced by staurosporine in PC12 cells.

The role of redox environment in neurogenic development

The dynamic changes of cellular redox elements during neurogenesis allow the control of specific programs for selective lineage progression. There are many redox couples that influence the cellular redox state. The shift from a reduced to an oxidized state and vice versa may act as a cellular switch mechanism of stem cell mode of action from proliferation to differentiation. The redox homeostasis ensures proper functioning of redox-sensitive signaling pathways through oxidation/reduction of critical cysteine residues on proteins involved in signal transduction. This review presents the current knowledge on the relation between changes in the cellular redox environment and stem cell programming in the course of commitment to a restricted neural lineage, focusing on in vivo neurogenesis and in vitro neuronal differentiation. The first two sections outline the main systems that control the intracellular redox environment and make it more oxidative or reductive. The last section provides the background on redox-sensitive signaling pathways that regulate neurogenesis.