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Flavonoids target a variety of pathophysiological mechanisms and are therefore increasingly considered as compounds encompassed with therapeutic potentials in diseases such as cancer, diabetes, arteriosclerosis, and neurodegenerative diseases and mood disorders. Hops (Humulus lupulus L.) is rich in flavonoids such as the flavanone 8-prenylnaringenin, which is the most potent phytoestrogen identified so far, and the prenylchalcone xanthohumol, which has potent tumor-preventive, anti-inflammatory and antiviral activities. In the present study, we questioned whether hops-derived prenylflavonoids and synthetic derivatives thereof act on neuronal precursor cells and neuronal cell lines to induce neuronal differentiation, neurite outgrowth and neuroprotection. Therefore, mouse embryonic forebrain-derived neural precursors and Neuro2a neuroblastoma-derived cells were stimulated with the prenylflavonoids of interest, and their potential to activate the promoter of the neuronal fate-specific doublecortin gene and to stimulate neuronal differentiation and neurite outgrowth was analyzed. In this screening, we identified highly "neuroactive" compounds, which we termed "enhancement of neuronal differentiation factors" (ENDFs). The most potent molecule, ENDF1, was demonstrated to promote neuronal differentiation of neural stem cells and neurite outgrowth of cultured dorsal root ganglion neurons and protected neuronal PC12 cells from cobalt chloride-induced as well as cholinergic neurons of the nucleus basalis of Meynert from deafferentation-induced cell death. The results indicate that hops-derived prenylflavonoids such as ENDFs might be powerful molecules to promote neurogenesis, neuroregeneration and neuroprotection in cases of chronic neurodegenerative diseases, acute brain and spinal cord lesion and age-associated cognitive impairments.

After injury to the central nervous system intrinsic factors such as myelin associated inhibitory factors inhibit cellular and axonal regeneration resulting in permanent disability. Three of these factors (Nogo-A, oligodendrocyte myelin glycoprotein, myelin-associated glycoprotein) bind to a common receptor: the Nogo-66 receptor (NgR1). NgR1 is expressed mainly on neurons and is usually associated in a trimolecular complex. The second member of the complex, LINGO-1, is often connected to NgR1 function and is further found to function independently as a negative regulator of oligodendrocyte proliferation and differentiation. The third member of the NgR complex is either the p75 neurotrophin receptor, TROY, or an as yet unidentified co-receptor. Targeting of factors contained in this complex has been described to lead to the promotion of neurite outgrowth, oligodendrocyte proliferation and differentiation and inhibition of cell death. In the current review, we aim to describe the mechanisms of action of the chemical and biological compounds used in targeting NgR1 and LINGO-1. This will be achieved using three examples: blocking of ligand binding to NgR1 in treatment of spinal cord injury, antibody-mediated inhibition of LINGO-1 to promote oligodendrocyte differentiation in multiple sclerosis, and the use of soluble NgR1 to sequester Abeta peptide in the periphery in Alzheimer's disease.

Nogo-A is a potent neurite growth inhibitorin vitroand plays a role both in the restriction of axonal regeneration after injury and in structural plasticity in the CNS of higher vertebrates. The regions that mediate inhibition and the topology of the molecule in the plasma membrane have to be defined. Here we demonstrate the presence of three different active sites: (1) an N-terminal region involved in the inhibition of fibroblast spreading, (2) a stretch encoded by the Nogo-A-specific exon that restricts neurite outgrowth and cell spreading and induces growth cone collapse, and (3) a C-terminal region (Nogo-66) with growth cone collapsing function. We show that Nogo-A-specific active fragments bind to the cell surface of responsive cells and to rat brain cortical membranes, suggesting the existence of specific binding partners or receptors. Several antibodies against different epitopes on the Nogo-A-specific part of the protein as well as antisera against the 66 aa loop in the C-terminus stain the cell surface of living cultured oligodendrocytes. Nogo-A is also labeled by nonmembrane-permeable biotin derivatives applied to living oligodendrocyte cultures. Immunofluorescent staining of intracellular, endoplasmic reticulum-associated Nogo-A in cells after selective permeabilization of the plasma membrane reveals that the epitopes of Nogo-A, shown to be accessible at the cell surface, are exposed to the cytoplasm. This suggests that Nogo-A could have a second membrane topology. The two proposed topological variants may have different intracellular as well as extracellular functions.

Unlike neonatal axons, mammalian adult axons of the CNS do not regenerate after injury. This developmental loss of regenerative capacity, is correlated with the onset of myelination. Likewise, myelin, or myelin-associated components such as Nogo-A and myelin-associated glycoprotein (MAG) inhibit regeneration from older but not younger neurons. Identification of the molecular events responsible for this developmental loss of regenerative capacity is central to devise strategies to encourage regeneration in adults after injury. Endogenous levels of the cyclic nucleotides cAMP and cGMP have been suggested to determine the neuronal responsiveness to various axonal guidance factors. Elevating cAMP concentrations block Nogo-A or MAG induced inhibition of neurite outgrowth in older neurons, whereas suppressing cAMP levels in young neurons renders them susceptible to Nogo-A and MAG. Interestingly, elevated cAMP levels abrogated the Nogo-A and MAG mediated activation of RhoA and down regulation of Rac1 in adult neurons. In contrast, elevation of cAMP leads to the inactivation of RhoA and prevents activation of downstream effector proteins, while Rac is activated. We therefore conclude that the endogenous neuronal cAMP levels determine the neuronal responsiveness to myelin-associated neurite growth inhibitors by regulating rho GTPase activities.

The adult mammalian CNS has a limited capacity for nerve regeneration and structural plasticity. The presence of glia-derived inhibitory factors myelin-associated glycoprotein (MAG) and Nogo-A have been suggested to provide a nonpermissive environment for elongating nerve fibers. In particular, Nogo-A, an integral membrane protein predominantly expressed by oligodendrocytes, has been demonstrated to impair neurite growth in vitro and in vivo . Structure function analysis revealed that Nogo-A protein contains at least two active domains, NiG and Nogo-66, with diverse effects on neurite outgrowth and cell spreading. We now provide evidence that these inhibitory domains mediate their effects via an antagonistic regulation of the small GTPases RhoA and Rac1, resulting in activation of RhoA and suppression of Rac1. By inactivating RhoA with C3 transferase or the downstream effector Rho-kinase ROCK with[Y27632][1], the inhibitory effects of both Nogo-A fragments and MAG on neurite outgrowth and oligodendrocyte-mediated growth cone collapse were abolished. Furthermore, we show that the recently cloned receptor for Nogo-66 and MAG, NgR, is not necessary for either NiG- or MAG-induced RhoA activation. [1]: /lookup/external-ref?link_type=GEN&access_num=Y27632&atom=%2Fjneuro%2F22%2F23%2F10368.atom

After nerve injury, adult sensory neurons can regenerate peripheral axons and reconnect with their target tissue. Initiation of outgrowth, as well as elongation of neurites over long distances, depends on the signaling of receptors for neurotrophic growth factors. Here, we investigated the importance of gp130, the signaling subunit of neuropoietic cytokine receptors in peripheral nerve regeneration.After sciatic nerve crush, functional recoveryin vivowas retarded inSNS-gp130−/−mice, which specifically lackgp130in sensory neurons. Correspondingly, a significantly reduced number of free nerve endings was detected in glabrous skin fromSNS-gp130−/−compared with control mice after nerve crush. Neurite outgrowth and STAT3 activationin vitrowere severely reduced in cultures in gp130-deficient cultured neurons. Surprisingly, in neurons obtained fromSNS-gp130−/−mice the increase in neurite length was reduced not only in response to neuropoietic cytokine ligands of gp130 but also to nerve growth factor (NGF), which does not bind to gp130-containing receptors. Neurite outgrowth in the absence of neurotrophic factors was partially rescued in gp130-deficient neurons by leptin, which activates STAT3 downstream of leptic receptor and independent of gp130. The neurite outgrowth response of gp130-deficient neurons to NGF was fully restored in the presence of leptin.Based on these findings, gp130 signaling via STAT3 activation is suggested not only to be an important regulator of peripheral nerve regenerationin vitroandin vivo, but as determining factor for the growth promoting action of NGF in adult sensory neurons.

Molecular mechanisms which stabilize dendrites and dendritic spines are essential for regulation of neuronal plasticity in development and adulthood. The class of Nogo receptor proteins, which are critical for restricting neurite outgrowth inhibition signaling, have been shown to have roles in developmental, experience and activity induced plasticity. Here we investigated the role of the Nogo receptor homolog NgR2 in structural plasticity in a transgenic null mutant for NgR2. Using Golgi-Cox staining to analyze morphology, we show that loss of NgR2 alters spine morphology in adult CA1 pyramidal neurons of the hippocampus, significantly increasing mushroom-type spines, without altering dendritic tree complexity. Furthermore, this shift is specific to apical dendrites in distal CA1 stratum radiatum (SR). Behavioral alterations in NgR2(-/-) mice were investigated using a battery of standardized tests and showed that whilst there were no alterations in learning and memory in NgR2(-/-) mice compared to littermate controls, NgR2(-/-) displayed reduced fear expression in the contextual conditioned fear test, and exhibited reduced anxiety- and depression-related behaviors. This suggests that the loss of NgR2 results in a specific phenotype of reduced emotionality. We conclude that NgR2 has role in maintenance of mature spines and may also regulate fear and anxiety-like behaviors.

In this paper, we identify the chondroitin sulfate proteoglycan versican V2 as a major inhibitor of axonal growth in the extracellular matrix of the mature central nervous system. In immunohistochemical and in situ hybridization experiments we show that this tissue-specific splice variant of versican is predominantly present in myelinated fiber tracts of the brain and in the optic nerve, most likely being expressed by oligodendrocytes. We demonstrate that isolated versican V2 strongly inhibits neurite outgrowth of central and peripheral neurons in stripe-choice assays using laminin-1 as permissive substrate. The inhibitory character of versican V2 is maintained after removal of chondroitin sulfate and N- and O-linked oligosaccharide side chains, but it is abolished after core protein digestion with proteinase-K. Our data support the notion, that intact versican V2 prevents excessive axonal growth during late phases of development and hereby participates in the structural stabilization of the mature central nervous system.

Retinal ganglion cell (RGC) axons in lizards (reptiles) were found to regenerate after optic nerve injury. To determine whether regeneration occurs because the visual pathway has growth-supporting glia cells or whether RGC axons regrow despite the presence of neurite growth-inhibitory components, the substrate properties of lizard optic nerve myelin and of oligodendrocytes were analyzed in vitro, using rat dorsal root ganglion (DRG) neurons. In addition, the response of lizard RGC axons upon contact with rat and reptilian oligodendrocytes or with myelin proteins from the mammalian central nervous system (CNS) was monitored. Lizard optic nerve myelin inhibited extension of rat DRG neurites, and lizard oligodendrocytes elicited DRG growth cone collapse. Both effects were partially reversed by antibody IN-1 against mammalian 35/250 kD neurite growth inhibitors, and IN-1 stained myelinated fiber tracts in the lizard CNS. However, lizard RGC growth cones grew freely across oligodendrocytes from the rat and the reptilian CNS. Mammalian CNS myelin proteins reconstituted into liposomes and added to elongating lizard RGC axons caused at most a transient collapse reaction. Growth cones always recovered within an hour and regrew. Thus, lizard CNS myelin and oligodendrocytes possess nonpermissive substrate properties for DRG neurons--like corresponding structures and cells in the mammalian CNS, including mammalian-like neurite growth inhibitors. Lizard RGC axons, however, appear to be far less sensitive to these inhibitory substrate components and therefore may be able to regenerate through the visual pathway despite the presence of myelin and oligodendrocytes that block growth of DRG neurites.

Numerous studies have shown that the developing tip of a neurite, the growth cone, can respond to environmental cues with behaviors such as guidance or collapse. To assess whether a given cell type can use more than one second-messenger pathway for a single behavior, we compared the influence of two well-characterized guidance cues on growth cones of chick temporal retinal ganglion cells. The first cue was the repulsive activity derived from the posterior optic tectum (p-membranes), and the second was the collapse-inducing activity derived from oligodendrocytes known as NI35/NI250. p-Membranes caused permanent growth cone collapse with no recovery after several hours, while NI35 caused transient collapse followed by recovery after about 10 min. The p-membrane-induced collapse was found to be Ca2+ independent, as shown using the Ca2+-sensitive dye Fura-2 and by the persistence of collapse in Ca2+-free medium. Dantrolene, a blocker of the ryanodine receptor, had only a minor effect on the collapse frequency caused by p-membranes. In contrast, the NI35-induced collapse was clearly Ca2+ dependent. [Ca2+]i increased sevenfold preceding collapse, and both dantrolene and antibodies against NI35 significantly reduced both the Ca2+ increase and the collapse frequency. Thus, even in a single cell type, growth cone collapse induced by two different signals can be mediated by two different second-messenger systems.

A recombinant Fab fragment was prepared from the monoclonal IgM/kappa antibody IN-1, which neutralizes central nervous system myelin-associated neurite growth inhibitors both in vitro and in vivo. The variable domain gene sequences were amplified and cloned after cDNA synthesis from the hybridoma RNA. After insertion into the tet promoter vector pASK85, which provided the constant domains of class IgG1/kappa, equipped with a His6 tag, large amounts of the Fab fragment were produced in Escherichia coli by medium cell density fermentation. The Fab fragment was purified to homogeneity by immobilized metal-affinity chromatography and its biochemical activity was compared with the original IN-1 antibody. In an assay for neurite outgrowth and fibroblast spreading, the Fab fragment showed a similar neutralizing effect on inhibitory substrate properties of central nervous system myelin as the unpurified IgM, although an approximately tenfold higher concentration was necessary. Immunoprecipitation experiments revealed a more selective antigen-binding behaviour for the Fab fragment. The Fab fragment was also successfully applied for antigen detection in immunohistochemical analyses. Therefore, the recombinant Fab fragment of IN-1 shows full functionality in vitro and appears to be well suited for replacing the monoclonal IgM in investigations on fiber tract regeneration in vivo.

Adhesion and neurite outgrowth of PC12 cells, as well as the spreading of 3T3 fibroblasts, were inhibited in a dose dependent manner by detergent solubilized mouse central nervous system myelin proteins as a tissue culture substrate. These inhibitory effects could be neutralized by the monoclonal antibody IN-1 directed against the neurite growth inhibiting proteins NI-35 and NI-250. Separation of the detergent soluble proteins of bovine spinal cord by an anion exchange column showed that the peaks of inhibitory activity for the two cell lines overlapped, such that the PC12 cells were inhibited by a larger number of fractions comprising those inhibitory for 3T3 cells. Neurite outgrowth of PC12 cells was not influenced by the myelin associated glycoprotein, MAG.

Peripheral nerve injury triggers the activation of RhoA in spinal motor and peripheral sensory neurons. RhoA activates a number of effector proteins including the Rho-associated kinase, ROCK, which targets the cytoskeleton and leads to inhibition of neurite outgrowth. Blockade of the Rho/ROCK pathway by pharmacological means improves axon regeneration after experimental injury. C3(bot) transferase, an exoenzyme produced by Clostridium botulinum, inactivates RhoA by ADP-ribosylation. It has been successfully applied in experimental CNS lesions to facilitate axon regeneration. Up to now it was not investigated thoroughly whether C3(bot) exerts positive effects on peripheral axon regeneration as well. In the present study, recombinant membrane permeable C3(bot) produced a small, but significant, axon outgrowth effect on peripheral sensory neurons dissociated from adult dorsal root ganglia (DRG) of the rat. Neuronal overexpression of C3, however, did not enhance axonal growth. Moreover, transfection of plasmids encoding dominant negative RhoA or RhoA specific shRNAs failed to increase axonal growth. Furthermore, we show that the C3(bot) mutant, C3(E174Q), which lacks RhoA inhibitory activity, still stimulates axonal growth. When analyzing possible signaling mechanisms we found that extracellular signal-regulated kinase (ERK) and Akt are activated by C3(bot) and ERK is induced by the C3(E174Q) mutant. Upregulation of kinase activities by C3(bot) occurs significantly faster than inactivation of RhoA indicating a RhoA-independent pathway of action by C3(bot). The induction of ERK signaling by C3(bot) was detected in embryonic hippocampal neurons, too. Taken together, although RhoA plays a central role for inhibition of axon outgrowth by myelin-derived inhibitors, it does not interfere with axonal growth of sensory neurons on a permissive substrate in vitro. C3(bot) blocks neuronal RhoA activity, but its positive effects on axon elongation and branching appear to be mediated by Rho independent mechanisms involving activation of axon growth promoting ERK and Akt kinases.

Mammalian neurons require a constant supply of oxygen to maintain adequate cellular functions and survival. Following sustained hypoxia during ischemic events in brain, the energy status of neurons and glia is compromised, which may subsequently lead to cell death by apoptosis and necrosis. Concomitant with energy depletion is the formation of the purine nucleoside adenosine, a powerful endogenous neuroprotectant. In this paper the effect of chemical hypoxia on cell survival and neurite outgrowth of primary cerebellar granule cells was investigated. Rotenone, a mitochondrial complex I inhibitor, induced a 30.4 +/- 3.6% loss of viable cells and a 35.0 +/- 4.4% loss of neurite formation of cerebellar granule cells, which was partially restored by the addition of purine nucleosides adenosine, inosine and guanosine. Inosine had the most striking effect of 37.7 +/- 2.9% rescue of viability and 71.2 +/- 18.4% rescue of neurite outgrowth. Data confirm the suggested role of purine nucleosides for the neuronal regeneration of primary brain cells following hypoxic insult.

The absence of fiber regrowth in the injured mammalian CNS is influenced by several different factors and mechanisms. Besides the nonconducive properties of the glial scar tissue that forms around the lesion site, individual molecules present in CNS myelin and expressed by oligodendrocytes, such as NI-35/NI-250, bNI-220, and myelin-associated glycoprotein (MAG), have been isolated and shown to inhibit axonal growth. Here, we report an additional neurite growth-inhibitory activity purified from bovine spinal cord myelin that is not related to bNI-220 or MAG. This activity can be ascribed to the presence of two chondroitin sulfate proteoglycans (CSPGs), brevican and the brain-specific versican V2 splice variant. Neurite outgrowth of neonatal cerebellar granule cells and of dorsal root ganglion neuronsin vitrowas strongly inhibited by this myelin fraction enriched in CSPGs. Immunohistochemical staining revealed that brevican and versican V2 are present on the surfaces of differentiated oligodendrocytes. We provide evidence that treatment of oligodendrocytes with the proteoglycan synthesis inhibitors β-xylosides can strongly influence the growth permissiveness of oligodendrocytes. β-Xylosides abolished cell surface presentation of brevican and versican V2 and reversed growth cone collapse in encounters with oligodendrocytes as demonstrated by time-lapse video microscopy. Instead, growth cones were able to grow along or even into the processes of oligodendrocytes. Our results strongly suggest that brevican and versican V2 are additional components of CNS myelin that contribute to its nonpermissive substrate properties for axonal growth. Expression of these CSPGs on oligodendrocytes may indicate that they participate in the restriction of structural plasticity and regeneration in the adult CNS.

The poor axonal regeneration that follows lesions of the central nervous system (CNS) is crucially influenced by the local CNS tissue environment through which neurites have to grow. In addition to an inhibitory role of the glial scar, inhibitory substrate effects of CNS myelin and oligodendrocytes have been demonstrated. Several proteins including NI-35/250, myelin-associated glycoprotein, tenascin-R, and NG-2 have been described to have neurite outgrowth inhibitory or repulsive properties in vitro. Antibodies raised against NI-35/250 (monoclonal antibody IN-1) were shown to partially neutralize the growth inhibitory effect of CNS myelin and oligodendrocytes, and to result in long distance fiber regeneration in the lesioned adult mammalian CNS in vivo. We report here the purification of a myelin protein to apparent homogeneity from bovine spinal cord which exerts a potent neurite outgrowth inhibitory effect on PC12 cells and chick dorsal root ganglion cells, induces collapse of growth cones of chick dorsal root ganglion cells, and also inhibits the spreading of 3T3 fibroblasts. These activities could be neutralized by the monoclonal antibody IN-1. The purification procedure includes detergent solubilization, anion exchange chromatography, gel filtration, and elution from high resolution SDS-polyacrylamide gel electrophoresis. The active protein has a molecular mass of 220 kDa and an isoelectric point between 5.9 and 6.2. Its inhibitory activity is sensitive to protease treatment and resists harsh treatments like 9 M urea or short heating. Glycosylation is, if present at all, not detectable. Microsequencing resulted in six peptides and strongly suggests that this proteins is novel.

A comparison was made of neurite growth across spinal cord lesions in the isolated central nervous system (CNS) of newborn opossums (Monodelphis domestica) at various stages of development. The aim was to define the critical period at which growth after injury ceases to occur, with emphasis on growth-inhibitory proteins, myelin and glial cells. In postnatal opossums 3-6 days old (P3-6), repair was observed 5 days after lesions were made in culture at the cervical level (C7) by crushing with forceps. Through-conduction of action potentials was re-established and axons stained by Dil grew into and beyond the crush. In a series of 66 animals 29 showed repair. In 28 animals at P11-12 with comparable lesions repair was observed in five preparations. At P13-14, the CNS was still viable in culture, but none of the 25 preparations examined showed any axonal growth into the crush or conduction through it. The rostro-caudal gradient of development permitted lesions to be made in mature cervical and immature lumbar regions of P11-12 spinal cord. Growth across crushes occurred in lumbar but not in cervical segments of the same preparation. The development of glial cells and myelin was assessed by electron microscopy and by staining with specific antibodies (Rip-1 and myelin-associated glycoprotein) in cervical segments of neonatal P6-14 opossums. At P8, oligodendrocytes and thin myelin sheaths started to appear followed at P9 by astrocytes stained with antibody against glial fibrillary acidic protein. By P14, astrocytes, oligodendrocytes and well-developed myelin sheaths were abundant. The cervical crush sites of P12 cords contained occasional astrocytes but no oligodendrocytes. Specific antibodies (IN-1) to neurite growth-inhibiting proteins (NI-35/250) associated with oligodendrocytes and myelin in the rat CNS cross-reacted with opossum proteins. Assays using the spreading of 3T3 fibroblasts and IN-1 showed that by P7 inhibitory proteins became apparent, particularly in the hindbrain and cervical spinal cord. The concentrations of NI-35/250 thereafter increased and became abundant in the adult opossum. Our finding of a well-defined critical period, encompassing only 5 days, in CNS preparations that can be maintained in culture offers advantages for analysing mechanisms that promote or prevent CNS repair.

In the central nervous system (CNS) the ability of nerve fibres to regenerate upon injury is very limited. In contrast, peripheral nerve fibres have been observed to regrow over long distances. Transplantation experiments with CNS and PNS nerve explants have shown that the local microenvironment is crucial for regenerating nerve fibers: CNS neurons can extend their lesioned neurites into the peripheral nerve environment, but cease growth at the transition between PNS and CNS (Aguayo et al 1990; David and Aguayo 1981; Ramon y Cajal 1928; Tello 1911). To test if a possible lack of trophic factors in the adult CNS might be responsible for this observation, perinatal rat sensory, sympathetic, or retinal neurons were cocultured with explants of adult rat sciatic (PNS) or optic nerves (CNS) in the presence of nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) (Schwab & Thoenen 1985). The pattern of fiber growth corresponded exactly to that found in vivo: many axons grew into the sciatic nerve explants, but none grew into the optic nerves. These results suggested that there is a neurite growth inhibitory activity present in adult CNS tissue that cannot be overcome by the stimulatory effects of neurotrophic factors (Schwab & Thoenen, 1985).

A myelin-associated protein from the central nervous system, the neurite growth inhibitor NI-35, inhibits regeneration of lesioned neuronal fiber tracts in vivo and growth of neurites in vitro. Growth cones of cultured rat dorsal root ganglion neurons arrested their growth and collapsed when exposed to liposomes containing NI-35. Before morphological changes, the concentration of free intracellular calcium ([Ca2+]i) showed a rapid and large increase in growth cones exposed to liposomes containing NI-35. Neither an increase in [Ca2+]i nor collapse of growth cones was detected in the presence of antibodies to NI-35. Dantrolene, an inhibitor of calcium release from caffeine-sensitive intracellular calcium stores, protected growth cones from collapse evoked by NI-35. Depletion of these caffeine-sensitive intracellular calcium stores prevented the increase in [Ca2+]i evoked by NI-35. The NI-35-evoked cascade of intracellular messengers that mediates collapse of growth cones includes the crucial step of calcium release from intracellular stores.