A convenient in vitro assay for the inhibition of neurite outgrowth by adult mammalian CNS myelin using immortalized neuronal cells

Erythrocarpines IN, new limonoids from the barks of Chisocheton erythrocarpus and their neuroprotective effects against hydrogen peroxide in NG108-15 cells

A phytochemical study on the bark of Chisocheton erythrocarpus Hiern (Meliaceae) has led to the isolation of six new phragmalin-type limonoids named erythrocarpines I - N (1-6) along with one known limonoid, erythrocarpine F (7). Their structures were fully characterized by spectroscopic methods. The pre-treatment of NG108-15 cells with 1-5, 7 (2 h) demonstrated low to good protective effects against H2O2 exposure; 1 (83.77% ± 1.84 at 12.5 μM), 2 (69.07 ± 2.01 at 12.5 μM), 3 (80.38 ± 2.1 at 12.5 μM), 4 (62.33 ± 1.95 at 25 μM),5 (58.67 ± 1.85 at 50 μM) and 7 (66.07 ± 2.03 at 12.5 μM). Interestingly, 1 and 3 demonstrated comparable protective effects to positive control epigallocatechin gallate (EGCG) with similar cell viability capacity (≈ 80%), having achieved that at lower concentration (12.5 μM) than EGCG (50 μM). Collectively, the results suggested the promising use of 1 and 3 as potential neuroprotective agents against hydrogen peroxide-induced cytotoxicity in neuronal model.

Impact of Dual Cell Co-culture and Cell-conditioned Media on Yield and Function of a Human Olfactory Cell Line for Regenerative Medicine

Olfactory ensheathing cells (OECs) are a promising candidate therapy for neuronal tissue repair. However, appropriate priming conditions to drive a regenerative phenotype are yet to be determined. We first assessed the effect of using a human fibroblast feeder layer and fibroblast conditioned media on primary rat olfactory mucosal cells (OMCs). We found that OMCs cultured on fibroblast feeders had greater expression of the key OEC marker p75NTR (25.1 ± 10.7 cells/mm²) compared with OMCs cultured on laminin (4.0 ± 0.8 cells/mm², p = 0.001). However, the addition of fibroblast-conditioned media (CM) resulted in a significant increase in Thy1.1 (45.9 ± 9.0 cells/mm² versus 12.5 ± 2.5 cells/mm² on laminin, p = 0.006), an undesirable cell marker as it is regarded to be a marker of contaminating fibroblasts. A direct comparison between human feeders and GMP cell line Ms3T3 was then undertaken. Ms3T3 cells supported similar p75NTR levels (10.7 ± 5.3 cells/mm²) with significantly reduced Thy1.1 expression (4.8 ± 2.1 cells/mm²). Ms3T3 cells were used as feeder layers for human OECs to determine whether observations made in the rat model were conserved. Examination of the OEC phenotype (S100β expression and neurite outgrowth from NG108-15 cells) revealed that co-culture with fibroblast feeders had a negative effect on human OECs, contrary to observations of rat OECs. CM negatively affected rat and human OECs equally. When the best and worst conditions in terms of supporting S100β expression were used in NG108-15 neuron co-cultures, those with the highest S100β expression resulted in longer and more numerous neurites (22.8 ± 2.4 μm neurite length/neuron for laminin) compared with the lowest S100β expression (17.9 ± 1.1 μm for Ms3T3 feeders with CM). In conclusion, this work revealed that neither dual co-culture nor fibroblast-conditioned media support the regenerative OEC phenotype. In our case, a preliminary rat model was not predictive of human cell responses.

Myelin and Modeling: Bootstrapping Cortical Microcircuits

Histological studies of myelin-stained sectioned cadaver brain and in vivo myelin-weighted magnetic resonance imaging (MRI) show that the cerebral cortex is organized into cortical areas with generally well-defined boundaries, which have consistent internal patterns of myelination. The process of myelination is largely driven by neural experience, in which the axonal passage of action potentials stimulates neighboring oligodendrocytes to perform their task. This bootstrapping process, such that the traffic of action potentials facilitates increased traffic, suggests the hypothesis that the specific pattern of myelination (myeloarchitecture) in each cortical area reveals the principal cortical microcircuits required for the function of that area. If this idea is correct, the observable sequential maturation of specific brain areas can provide evidence for models of the stages of cognitive development.

A Combined In Vitro Assay for Evaluation of Neurotrophic Activity and Cytotoxicity

Neurotrophic assays are phenotypic methods to identify molecules that stimulate differentiation of neuronal cells. Bioactive small molecules with neurotrophic actions hold great promise as therapeutic agents for the treatment of neurodegenerative diseases and neuronal injuries by virtue of their ability to stimulate neuritic outgrowth. A combined in vitro method, which measures neurotrophic activity and cytotoxicity in a single assay, has been described. This assay, performed in 96-well microplates with PC12 and Neuroscreen-1 (NS-1; a subclone of PC12) cells, is a simple tool for identification of new neurotrophic agents. Stimulation of neurite outgrowth was measured with NIS software by analysis of digital cell images as multiple parameters, namely, mean neurite length, neurite length/cell, nodes/cell, and number of neurites/cell. The assay has been standardized and validated with dose-response analysis for nerve growth factor (NGF) and mechanism-based inhibitors of NGF-induced neurite outgrowth, namely, SU6656 (an Src family kinase inhibitor) and PD98059 (a MEK inhibitor). The assay has been successfully applied for screening natural and synthetic compound libraries for cytotoxicity and neurotrophic activity. Screening of a set of harmala alkaloids identified harmine as a potential neurotrophic molecule that significantly stimulated NGF-induced neurite outgrowth in the NS-1 cells. Important advantages of this method are its simplicity and determination of cytotoxicity and neurotrophic activity in a single assay. This assay may be suitable for primary and cultured neuronal cells.

Deficiency of GDNF Receptor GFRα1 in Alzheimer's Neurons Results in Neuronal Death

We have recently developed aged cortical neuron cultures from autopsied human brains with Alzheimer's disease (AD). During the culturing process, we found that glutamatergic cortical neurons from the AD brain lacked a response to glial cell line-derived neurotrophic factor (GDNF), including no axonal regrowth, and were starting to undergo apoptosis. Here we showed that, in cortical neurons from age- and gender-matched cognitively normal control (NC) subjects (NC neurons), GDNF enhanced the expression of GDNF family receptor subtype α1 (GFRα1), but not the other three subtypes (GFRα2, GFRα3, and GFRα4), whereas GDNF failed to induce GFRα1 expression in cortical neurons from the AD brain (AD neurons). The exogenous introduction of GFRα1, but not of its binding partner α1-neural cell adhesion molecule, or RET into AD neurons restored the effect of GDNF on neuronal survival. Moreover, between NC and AD neurons, the AMPA receptor blocker CNQX and the NMDA receptor blocker AP-5 had opposite effects on the GFRα1 expression induced by GDNF. In NC neurons, the presence of glutamate receptors was necessary for GDNF-linked GFRα1 expression, while in AD neurons the absence of glutamate receptors was required for GFRα1 expression by GDNF stimulation. These results suggest that, in AD neurons, specific impairments of GFRα1, which may be linked to glutamatergic neurotransmission, shed light on developing potential therapeutic strategies for AD by upregulation of GFRα1 expression.

A quantitative method for analysis of in vitro neurite outgrowth

The adult mammalian CNS is extremely limited in its ability to regenerate axons following injury. Glial scar, neuroinflammatory processes and molecules released from myelin impair axonal regrowth and contribute to the lack of neural regeneration. An in vitro assay that quantitates neurite outgrowth from cultured neurons as a model of neuronal regenerative potential is described. Specifically, the neurite outgrowth from primary neurons (rat cerebellar granule neurons; CGNs) and a neuronal cell line (NG108-15) were quantitatively measured after optimization of culture conditions. After cultures were fixed and immunostained to label neurons and nuclei, microscope images were captured and an image analysis algorithm was developed using Image-Pro Plus software to allow quantitative analysis. The algorithm allowed the determination of total neurite length, number of neurons, and number of neurons without neurites. The algorithm also allows for end-user control of thresholds for staining intensity and cell/nuclei size. This assay represents a useful tool for quantification of neurite outgrowth from a variety of neuronal sources with applications that include: (1) assessment of neurite outgrowth potential; (2) identification of molecules that can block or stimulate neurite outgrowth in conventional culture media; and (3) identification of agents that can overcome neurite outgrowth inhibition by inhibitory substrates.

Neurite growth capability of rat fetal neuronal cells against matured CNS myelin in vitro

Reconstruction of neurocircuits by transplanted cells is expected to become an effective therapy for brain damage. In order to establish the transplantation therapy, it is necessary to find transplantable cells capable of reconstructing the lesioned neurocircuitry. We have reported that the younger neuronal cells such as neural stem cells are useful transplant materials because of their vigorous capacity for forming abundant neurites. On the other hand, it was reported that myelin-associated neurite growth inhibitor prevents neurite regeneration. In this study, we used rat fetal neuronal cells to examine the neurite growth capacity in the presence of mature CNS myelin. Crude CNS myelin was prepared from the brains of adult Wistar rats using previously described procedures. Testing wells were precoated with poly-L-lysine and additionally by over-night drying of a suspension containing 0, 5, 10, 15, or 20 microg/cm2 of the crude myelin protein. On embryonic days 10, 12, 15, and 17 (E10, E12, E15, and E17) embryos were surgically removed, mesencephalic neural plates were dissected out from the E10 embryos, and midbrain cells were taken from the E12, E15, and E17 embryos. The neural plates and midbrain cells were placed on the myelin-coated wells. After 24 h of culture (72 h in the case of neural plates), the number of surviving cells and the length of the neurites were examined immunocytochemically using anti-neurofilament (NF) antibody. Neurite length was measured by image analyzer Luzex-F. The mesencephalic neural plate was able to grow neurites even on 20 microg/cm2 central myelin. Almost the same number of midbrain cells attached themselves to the wells without myelin in every culture obtained from various stages of embryos. The number of cells attached on the myelin-coated wells decreased with the concentration of myelin. The number of NF-positive cells was higher in cultures of materials obtained from older embryos than in cultures obtained from younger embryos. The younger cells grew longer neurites than the older cells in the myelin noncoated wells. Neurite growth was inhibited strongly when the concentration of the central myelin was 10 microg/cm2 or greater, but on the 5 microg/cm2 myelin, the younger the cells were, the longer neurites they had. When the length of the longest neurites in one field of the image analyzer was further examined in the same way, the younger the cells were, the longer their axons grew on 0 and 5 microg/cm2 myelin. Thus, CNS myelin was seen to be a significant inhibitor of the recovery of injured neural tissue of the adult CNS. Younger cells grew longer neurites than older cells on CNS myelin, and so it was suggested that neural stem cells or younger neurons may serve as tissue for transplantation therapy.

Neuronal age influences the response to neurite outgrowth inhibitory activity in the central and peripheral nervous systems

Axonal regeneration is abortive in the central nervous system (CNS) of adult mammals, but readily occurs in the injured peripheral nervous system (PNS). Recent experiments indicate an important role for both intrinsic neuronal features and extrinsic substrate properties in determining the propensity for axonal regrowth. In particular, certain components of adult mammalian CNS myelin have been shown to exert a strong inhibitory influence on neurite outgrowth. To determine whether the potent neurite outgrowth inhibitory activity found in CNS myelin may also be present in PNS myelin and to study the influence of neuronal age on neurite outgrowth, we used a cryoculture assay in which dissociated rat dorsal root ganglion (DRG) neurons of different ages were challenged to extend neurites on fractionated myelin and cryostat sections from the PNS (sciatic nerve and myelin-free degenerated sciatic nerve) and CNS (optic nerve) of adult rats. The CNS environment of the optic nerve did not support E17 to P8 DRG neurite adhesion or outgrowth. E17 DRG neurons, unlike their older counterparts, however, were able to attach and extend neurites onto normal sciatic nerve and onto purified PNS myelin. In contrast, a vigorous neurite outgrowth response from all the ages tested was observed on the myelin-free degenerated sciatic nerve. These results indicate that PNS myelin is a potent inhibitor of neurite outgrowth and that DRG neuronal age plays an important role in determining the propensity for neurite outgrowth and regenerative response on inhibitory PNS and CNS substrata.

Alpha 1-tubulin mRNA level is increased during neurite outgrowth of NG 108-15 cells but not during neurite outgrowth inhibition by CNS myelin

alpha 1-tubulin is an isotype of alpha-tubulin, and its mRNA is expressed in the rodent nervous system. A high level of alpha 1-tubulin mRNA in neurones is associated with axonal outgrowth during development as well as with axonal regeneration after axotomy in adult animals. We quantitated alpha 1-tubulin mRNA levels in motor neurone-like NG 108-15 cells using Northern blots in order to determine whether the expression of this neurite outgrowth-associated gene is regulated in NG 108-15 cells during neurite extension and during inhibition of this process by CNS myelin. Here we report that during the acute phase of neurite outgrowth, alpha 1-tubulin mRNA level increases in NG 108-15, a maximal induction of 1.7-fold over the initial level occurring 24 h after neurite outgrowth onset. By contrast, when these cells are plated on CNS myelin alpha 1-tubulin mRNA levels show no such increase. These findings indicate that an increase of the alpha 1-tubulin mRNA level is associated with neurite outgrowth of NG 108-15 cells. More interestingly, this study also demonstrates that the inhibition of neurite outgrowth by CNS myelin may affect the expression of a gene encoding a protein involved in neurite extension.

Human central nervous system myelin inhibits neurite outgrowth

In vitro and animal studies have identified molecules in mammalian CNS myelin which inhibit neuritic extension and which may be responsible, at least in part, for the lack of axonal regeneration after injury in the injured brain, optic nerve and spinal cord. To determine whether such inhibitory activity may be present in human CNS myelin, we used a bioassay to characterize neurite outgrowth on this substrate. Human CNS myelin strongly inhibited neuritic outgrowth from newborn rat dorsal root ganglion neurons and NG-108-15 cells, a neuroblastoma-glioma hybrid cell line. Similar but less potent inhibitory activity was identified in human gray matter. The CNS myelin inhibition of neuritic outgrowth appeared to be dependent on direct contact between the myelin substrate and neurites. The inhibitory activity in human CNS myelin closely resembled that described in adult rodents. Inhibition of neurite growth by human CNS myelin in this in vitro bioassay mirrors the lack of regeneration in vivo and can be used as a model to develop strategies designed to enhance axonal regeneration and neural recovery.