Neurotrophins: neurotrophic modulation of neurite growth

Bioinspired Nanoplatforms Based on Graphene Oxide and Neurotrophin-Mimicking Peptides

Neurotrophins (NTs), which are crucial for the functioning of the nervous system, are also known to regulate vascularization. Graphene-based materials may drive neural growth and differentiation, and, thus, have great potential in regenerative medicine. In this work, we scrutinized the nano-biointerface between the cell membrane and hybrids made of neurotrophin-mimicking peptides and graphene oxide (GO) assemblies (pep-GO), to exploit their potential in theranostics (i.e., therapy and imaging/diagnostics) for targeting neurodegenerative diseases (ND) as well as angiogenesis. The pep-GO systems were assembled via spontaneous physisorption onto GO nanosheets of the peptide sequences BDNF(1-12), NT3(1-13), and NGF(1-14), mimicking the brain-derived neurotrophic factor (BDNF), the neurotrophin 3 (NT3), and the nerve growth factor (NGF), respectively. The interaction of pep-GO nanoplatforms at the biointerface with artificial cell membranes was scrutinized both in 3D and 2D by utilizing model phospholipids self-assembled as small unilamellar vesicles (SUVs) or planar-supported lipid bilayers (SLBs), respectively. The experimental studies were paralleled via molecular dynamics (MD) computational analyses. Proof-of-work in vitro cellular experiments with undifferentiated neuroblastoma (SH-SY5Y), neuron-like, differentiated neuroblastoma (dSH-SY5Y), and human umbilical vein endothelial cells (HUVECs) were carried out to shed light on the capability of the pep-GO nanoplatforms to stimulate the neurite outgrowth as well as tubulogenesis and cell migration.

Peripheral nerve-derived fibroblasts promote neurite outgrowth in adult dorsal root ganglion neurons more effectively than skin-derived fibroblasts

NEW FINDINGS

What is the central question of this study? Although fibroblasts are involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. What is the main finding and its importance? Nerve-derived fibroblasts have a greater neurite-promoting effect than skin-derived fibroblasts, and epineurium-derived fibroblasts can promote neurite outgrowth more effectively than parenchyma-derived fibroblasts. The epineurium-derived fibroblasts and parenchyma-derived fibroblasts have distinctly different molecular profiles, including genes of soluble factors to promote axonal growth. Fibroblasts are molecularly and functionally different depending on their localization in nerve tissue, and epineurium-derived fibroblasts might be involved in axon regeneration after peripheral nerve injury more than previously thought.

ABSTRACT

Although fibroblasts (Fb) are components of a peripheral nerve involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. The objective of the present study was to investigate the capacity of Fb derived from peripheral nerves to stimulate the outgrowth of neurites from adult dorsal root ganglion neurons and to clarify their molecular characteristics. Fibroblasts were prepared from the epineurium and parenchyma of rat sciatic nerves and skin. The Fb derived from epineurium showed the greatest effect on neurite outgrowth, followed by the Fb derived from parenchyma, indicating that Fb derived from nerves promote neurite outgrowth more effectively than skin-derived Fb. Although both soluble and cell-surface factors contributed evenly to the neurite-promoting effect of nerve-derived Fb, in crush and transection injury models, Fb were not closely associated with regenerating axons, indicating that only soluble factors from Fb are available to regenerating axons. A transcriptome analysis revealed that the molecular profiles of these Fb were distinctly different and that the gene expression profiles of soluble factors that promote axonal growth are unique to each Fb. These findings indicate that Fb are molecularly and functionally different depending on their localization in nerve tissue and that Fb derived from epineurium might be involved more than was previously thought in axon regeneration after peripheral nerve injury.

OUP accepted manuscript

Hereditary spastic paraplegia type 15 (HSP15) is a neurodegenerative condition caused by the inability to produce SPG15 protein, which leads to lysosomal swelling. However, the link between lysosomal aberrations and neuronal death is poorly explored. To uncover the functional consequences of lysosomal aberrations in disease pathogenesis, we analyze human dermal fibroblasts from HSP15 patients as well as primary cortical neurons derived from an SPG15 knockout (KO) mouse model. We find that SPG15 protein loss induces defective anterograde transport, impaired neurite outgrowth, axonal swelling and reduced autophagic flux in association with the onset of lysosomal abnormalities. Additionally, we observe lipid accumulation within the lysosomal compartment, suggesting that distortions in cellular lipid homeostasis are intertwined with lysosomal alterations. We further demonstrate that SPG15 KO neurons exhibit synaptic dysfunction, accompanied by augmented vulnerability to glutamate-induced excitotoxicity. Overall, our study establishes an intimate link between lysosomal aberrations, lipid metabolism and electrophysiological impairments, suggesting that lysosomal defects are at the core of multiple neurodegenerative disease processes in HSP15.

Inulin ameliorates schizophrenia via modulation of the gut microbiota and anti-inflammation in mice

The microbiome-gut-brain (MGB) axis, which regulates neurological and cognitive functions, plays an essential role in schizophrenia (SCZ) progression. Dietary inulin could be a novel strategy for the treatment of SCZ due to its modulating effects on the gut microbiota. In this study, the effects of inulin on mice with SCZ were studied. As indicated by the behavioural tests, expression of neurotransmitters, inflammatory indicators, and brain morphology, inulin administration ameliorated aberrant behaviours (locomotor hypoactivity, anxiety disorders and depressive behaviours, and impaired learning and spatial recognition memory) and effectively reduced neuroinflammation and neuronal damage. In addition, inulin improved intestinal integrity and permeability, as indicated by the elevated expression of tight junction proteins (p < 0.05). The results of 16S rRNA sequencing and analysis showed that inulin increased the abundance of Lactobacillus and Bifidobacterium, which were negatively correlated with 5-hydroxytryptamine and inflammatory cytokines and positively correlated with brain-derived neurotrophic factor (BDNF). Inulin caused a reduction in Akkermansia that was positively correlated with inflammatory cytokines and negatively correlated with BDNF. These results suggested that dietary inulin modulated the gut microbiota and exerted anti-inflammatory effects in mice though the MGB axis, which further ameliorated SCZ. Therefore, the results of this study provide a potential explanation for inulin intervention in the treatment of SCZ.

Porcine liver decomposition product-derived lysophospholipids promote microglial activation in vitro

Cognitive impairments such as dementia are common in later life, and have been suggested to occur via a range of mechanisms, including oxidative stress, age-related changes to cellular metabolism, and a loss of phospholipids (PLs) from neuronal membranes. PLs are a class of amphipathic lipids that form plasma membrane lipid bilayers, and that occur at high concentrations in neuronal membranes. Our previous study suggested that a porcine liver decomposition product (PLDP) produced via protease treatment may improve cognitive function at older ages, by acting as a rich source of PLs and lysophospholipids (LPLs); however, its specific composition remains unclear. Thus, the present study used a novel liquid chromatography electrospray ionization tandem mass spectrometric (LC-MS/MS) protocol to identify the major PLs and LPLs in PLDP. Furthermore, it assessed the effect of identified LPLs on microglial activation in vitro, including cell shape, proliferation, and cell morphology. The results of the conducted analyses showed that PLDP and PLDP-derived LPLs concentration-dependently modulate microglial activation in vitro. In particular, lysophosphatidylcholine (LPC) concentration-dependently promotes cell morphology, likely via effects mediated by the enzyme autotaxin (ATX), since inhibiting ATX also promoted cell morphology, while conversely, increasing ATX production (via treatment with high levels of LPC) abolished this effect. These findings suggest that LPC is likely neuroprotective, and thus, support the importance of further research to assess its use as a therapeutic target to treat age-related cognitive impairments, including dementia.

Genomic and non-genomic pathways are both crucial for peak induction of neurite outgrowth by retinoids

Retinoic acid (RA) is the active metabolite of vitamin A and essential for many physiological processes, particularly the induction of cell differentiation. In addition to regulating genomic transcriptional activity via RA receptors (RARs) and retinoid X receptors (RXRs), non-genomic mechanisms of RA have been described, including the regulation of ERK1/2 kinase phosphorylation, but are poorly characterised. In this study, we test the hypothesis that genomic and non-genomic mechanisms of RA are regulated independently with respect to the involvement of ligand-dependent RA receptors. A panel of 28 retinoids (compounds with vitamin A-like activity) showed a marked disparity in genomic (gene expression) versus non-genomic (ERK1/2 phosphorylation) assays. These results demonstrate that the capacity of a compound to activate gene transcription does not necessarily correlate with its ability to regulate a non-genomic activity such as ERK 1/2 phosphorylation. Furthermore, a neurite outgrowth assay indicated that retinoids that could only induce either genomic, or non-genomic activities, were not strong promoters of neurite outgrowth, and that activities with respect to both transcriptional regulation and ERK1/2 phosphorylation produced maximum neurite outgrowth. These results suggest that the development of effective retinoids for clinical use will depend on the selection of compounds which have maximal activity in non-genomic as well as genomic assays.

Preconditioning of Human Dental Pulp Stem Cells with Leukocyte- and Platelet-Rich Fibrin-Derived Factors Does Not Enhance Their Neuroregenerative Effect

Pathologies of the central nervous system are characterized by loss of brain tissue and neuronal function which cannot be adequately restored by endogenous repair processes. This stresses the need for novel treatment options such as cell-based therapies that are able to restore damaged tissue or stimulate repair. This study investigated the neuroregenerative potential of the conditioned medium of human dental pulp stem cells (CM-hDPSCs) on neural stem cell (NSC) proliferation and migration as well as on neurite outgrowth of primary cortical neurons (pCNs). Additionally, the effect of leukocyte- and platelet-rich fibrin (L-PRF) priming on the neuroregenerative potential of the hDPSC secretome on NSCs and pCNs was evaluated. L-PRF contains factors that enhance stem cell-induced regeneration, but its effect on hDPSC-mediated neuroregeneration is unknown. This study demonstrated that CM-hDPSCs enhanced neuritogenesis. Moreover, CM-hDPSCs had a chemoattractant effect on NSCs. Although priming hDPSCs with L-PRF increased brain-derived neurotrophic factor secretion, no additional effects on the paracrine-mediated repair mechanisms were observed. These data support the neuroregenerative potential of hDPSCs, and although priming had no additional effect, the potential of L-PRF-primed hDPSCs on distinct regenerative mechanisms remains to be clarified.

Nerves and Proliferation of Progenitor Cells in Limb Regeneration

Nerves, in conjunction with the apical epidermal cap (AEC), play an important role in the proliferation of the mesenchymal progenitor cells comprising the blastema of regenerating urodele amphibian limbs. Reinnervation after amputation requires factors supplied by the forming blastema, and neurotrophic factors must be present at or above a quantitative threshold for mitosis of the blastema cells. The AEC forms independently of nerves, but requires nerves to be maintained. Urodele limb buds are independent of nerves for regeneration, but innervation imposes a regenerative requirement for nerve factors on their cells as they differentiate. There are three main ideas on the functional relationship between nerves, AEC, and blastema cells: (1) nerves and AEC produce factors with different roles in maintaining progenitor status and mitosis; (2) the AEC produces the factors that promote blastema cell mitosis, but requires nerves to express them; (3) blastema cells, nerves, and AEC all produce the same factor(s) that additively attain the required threshold for mitosis.

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Venenum Bufonis induces rat neuroinflammation by activiating NF-κB pathway and attenuation of BDNF

ETHNOPHARMACOLOGICAL RELEVANCE

Venenum Bufonis (VB), also called toad venom, has been widely used in clinic as a cardiotonic, anohyne and antineoplastic agents both in China and other Asian countries. However, its neurotoxicity and cardiotoxicity limit its wide clinical application. Compared with extensive attention attracted with cardiotoxicity, the toxic effect of VB on Central Nervous System (CNS) is much less studied.

AIM OF THE RESEARCH

This study was performed to examine the neurotoxicity caused by VB on Sprague Dawley (SD) rats, then to clarify the mechanism in vivo by investigating its action on the neuroinflammation which possibly attributed to the activation of nuclear factor κB (NF-κB) pathway and the attenuation of brain-derived neurotrophic factor (BDNF).

MATERIALS AND METHODS

Rats administrated with 0.5% carboxymethyl cellulose sodium salt (CMC-Na) aqueous solution and VB (100mg/kg, 200mg/kg and 400mg/kg) were sacrificed at 2h, 4h, 6h, 8h, 24h and 48h. The brain level of neurotransmitters and their corresponding receptors, pro-inflammatory cytokines, BDNF/TrkB and NF-κB pathway-related proteins were examined, respectively.

RESULTS

VB administration induced severe neurologic damage and neuroinflammation, as indicated by the disordered 5-hydroxytryptamine (5-HT), dopamine (DA) and their corresponding receptors, together with the over production of inflammatory cytokines including interleukin-6 (IL-6), interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). VB also notably promoted the expression of p-NF-κBp65, p-IκBα, p-IKKα and p-IKKβ and down-regulated the expression of BDNF and TrkB.

CONCLUSION

This study demonstrates that VB triggers neurotoxicity which probably is induced by neuroinflammation via activating of NF-κB pathway and attenuating the expression of BDNF.

Novel p75 neurotrophin receptor ligand stabilizes neuronal calcium, preserves mitochondrial movement and protects against HIV associated neuropathogenesis

Human immunodeficiency virus (HIV) rapidly penetrates into the brain and establishes a persistent infection of macrophages/microglia. Activation of these cells by HIV results in the secretion of soluble factors that destabilize neuronal calcium homeostasis, encourage oxidative stress and result in neural damage. This damage is thought to underlie the cognitive-motor dysfunction that develops in many HIV-infected patients. Studies have suggested that neurotrophins may protect neurons from the toxic effects of HIV-associated proteins. To better understand the pathogenic mechanisms and the neuroprotective potential of neurotrophin ligands, we evaluated neuronal damage, calcium homeostasis and mitochondrial functions after exposure of cultured rat neurons directly to HIV gp120 or to conditioned medium from human monocyte-derived macrophages treated with gp120. We then assessed the ability of a new non-peptide p75 neurotrophin receptor ligand, LM11A-31, to stabilize calcium homeostasis and prevent the development of pathology. Each toxic challenge resulted in a delayed accumulation of intracellular calcium coupled to a decrease in the rate of calcium clearance from the cell. The delayed calcium accumulation correlated with the development of focal dendritic swellings (beading), cytoskeletal damage and impaired movement of mitochondria. Addition of LM11A-31 to the cultures at nanomolar concentrations eliminated cell death, significantly reduced the pathology, suppressed the delayed accumulation of calcium and restored mitochondrial movements. The potent neuroprotection and the stabilization of calcium homeostasis indicate that LM11A-31 may have excellent potential for the treatment of HIV-associated neurodegeneration.

Novel Approaches to the Prevention and Treatment of Rabies

Rabies is a highly lethal disease caused by the neurotropic rabies virus (RABV), and it remains an important public health problem globally. Effective vaccines have been developed for pre- and post-exposure prophylaxis (PEP). PEP is only effective if it is initiated promptly after recognizing exposure. Once neurological symptoms develop, however, it is widely accepted that there is no effective treatment available. Recent studies indicate that the presence of RABV-specific immunity (i.e. Virus neutralizing antibodies, VNA) and the transient enhancement of the BBB permeability are absolutely required for effective virus clearance from the CNS. In principle, it has been shown in mice using various live-attenuated RABVs or recombinant RABVs expressing three copies of the G or expressing chemokine/cytokines, which can induce high levels of VNA in the serum and also capable of transiently enhancing the BBB permeability that it is possible to clear the virus from CNS. Also, it has been demonstrated that, intravenous administration of VNA together with MCP-1 (shown to transiently open up BBB) can clear RABV from the CNS in both immunocompetent and immunocompromised mice, as late as 5 days after lethal challenge. Novel therapeutic approaches aimed at allowing the peripheral VNA to cross the BBB by administration of the VNA in combination with biological or chemical agents that can transiently open up the BBB would be useful to establish an effective therapy for rabies in humans. In this review, we focus on the some of the approaches that can be used to meet the challenges in the field of rabies treatment.

Neurotrophins regulate ApoER2 proteolysis through activation of the Trk signaling pathway

Background

ApoER2 and the neurotrophin receptors Trk and p75^NTR are expressed in the CNS and regulate key functional aspects of neurons, including development, survival, and neuronal function. It is known that both ApoER2 and p75^NTR are processed by metalloproteinases, followed by regulated intramembrane proteolysis. TrkA activation by nerve growth factor (NGF) increases the proteolytic processing of p75^NTR mediated by ADAM17. Reelin induces the sheeding of ApoER2 ectodomain depending on metalloproteinase activity. However, it is not known if there is a common regulation mechanism for processing these receptors.

Results

We found that TrkA activation by NGF in PC12 cells induced ApoER2 processing, which was dependent on TrkA activation and metalloproteinases. NGF-induced ApoER2 proteolysis was independent of mitogen activated protein kinase activity and of phosphatidylinositol-3 kinase activity. In contrast, the basal proteolysis of ApoER2 increased when both kinases were pharmacologically inhibited. The ApoER2 ligand reelin regulated the proteolytic processing of its own receptor but not of p75^NTR. Finally, in primary cortical neurons, which express both ApoER2 and TrkB, we found that the proteolysis of ApoER2 was also regulated by brain-derived growth factor (BDNF).

Conclusions

Our results highlight a novel relationship between neurotrophins and the reelin-ApoER2 system, suggesting that these two pathways might be linked to regulate brain development, neuronal survival, and some pathological conditions.

Cell cycle regulation and regeneration

Regeneration of ear punch holes in the MRL mouse and amputated limbs of the axolotl show a number of similarities. A large proportion of the fibroblasts of the uninjured MRL mouse ear are arrested in G2 of the cell cycle, and enter nerve-dependent mitosis after injury to form a ring-shaped blastema that regenerates the ear tissue. Multiple cell types contribute to the establishment of the regeneration blastema of the urodele limb by dedifferentiation, and there is substantial reason to believe that the cells of this early blastema are also arrested in G2, and enter mitosis under the influence of nerve-dependent factors supplied by the apical epidermal cap. Molecular analysis reveals other parallels, such as; (1) the upregulation of Evi5, a centrosomal protein that prevents mitosis by stabilizing Emi1, a protein that inhibits the degradation of cyclins by the anaphase promoting complex and (2) the expression of sodium channels by the epidermis. A central feature in the entry into the cell cycle by MRL ear fibroblasts is a natural downregulation of p21, and knockout of p21 in wild-type mice confers regenerative capacity on non-regenerating ear tissue. Whether the same is true for entry into the cell cycle in regenerating urodele limbs is presently unknown.

The roles of glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor and nerve growth factor during the final stage of folliculogenesis: a focus on oocyte maturation

Neurotrophic factors were first identified to promote the growth, survival or differentiation of neurons and have also been associated with the early stages of ovarian folliculogenesis. More recently, their effects on the final stage of follicular development, including oocyte maturation and early embryonic development, have been reported. Glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), which are expressed in numerous peripheral tissues outside of the CNS, most notably the ovary, are now known to stimulate oocyte maturation in various species, also enhancing developmental competence. The mechanisms that underlie their actions in antral follicles, as well as the targets ultimately controlled by these factors, are beginning to emerge. GDNF, BDNF and NGF, alone or in combination, could be added to the media currently utilized for in vitro oocyte maturation, thereby potentially increasing the production and/or quality of early embryos.

Brain-derived neurotrophic factor genotype is associated with brain gray and white matter tissue volumes recovery in abstinent alcohol-dependent individuals

Neuroimaging studies have linked the methionine (Met) allele of the brain-derived neurotrophic factor (BDNF) gene to abnormal regional brain volumes in several psychiatric and neurodegenerative diseases. However, no neuroimaging studies assessed the effects of this allele on brain morphology in alcohol use disorders and its demonstrated change during abstinence from alcohol. Here we assessed the effects of the BDNF Val66Met (rs6265) polymorphism on regional brain tissue volumes and their recovery during short-term abstinence in treatment-seeking alcohol-dependent individuals. 3D T1 weighted magnetic resonance images from 62 individuals were acquired at 1.5 T at one week of abstinence from alcohol; 41 of the participants were rescanned at 5 weeks of abstinence. The images were segmented into gray matter (GM), white matter (WM) and cerebrospinal fluid and parcellated into regional volumes. The BDNF genotype was determined from blood samples using the TaqMan technique. Alcohol-dependent Val (Valine)/Met heterozygotes and Val homozygotes had similar regional brain volumes at either time point. However, Val homozygotes had significant GM volume increases, while Val/Met heterozygotes increased predominantly in WM volumes over the scan interval. Longitudinal increases in GM but not WM volumes were related to improvements in neurocognitive measures during abstinence. The findings suggest that functionally significant brain tissue volume recovery during abstinence from alcohol is influenced by BDNF genotype.

Daidzein induces neuritogenesis in DRG neuronal cultures

Absract

Propofol neurotoxicity is mediated by p75 neurotrophin receptor activation

BACKGROUND

Propofol exposure to neurons during synaptogenesis results in apoptosis, leading to cognitive dysfunction in adulthood. Previous work from our laboratory showed that isoflurane neurotoxicity occurs through p75 neurotrophin receptor (p75(NTR)) and subsequent cytoskeleton depolymerization. Given that isoflurane and propofol both suppress neuronal activity, we hypothesized that propofol also induces apoptosis in developing neurons through p75(NTR).

METHODS

Days in vitro 5-7 neurons were exposed to propofol (3 μM) for 6 h and apoptosis was assessed by cleaved caspase-3 (Cl-Csp3) immunoblot and immunofluorescence microscopy. Primary neurons from p75(NTR-/-) mice or wild-type neurons were treated with propofol, with or without pretreatment with TAT-Pep5 (10 μM, 15 min), a specific p75(NTR) inhibitor. P75(NTR-/-) neurons were transfected for 72 h with a lentiviral vector containing the synapsin-driven p75(NTR) gene (Syn-p75(NTR)) or control vector (Syn-green fluorescent protein) before propofol. To confirm our in vitro findings, wild-type mice and p75(NTR-/-) mice (PND5) were pretreated with either TAT-Pep5 or TAT-ctrl followed by propofol for 6 h.

RESULTS

Neurons exposed to propofol showed a significant increase in Cl-Csp3, an effect attenuated by TAT-Pep5 and hydroxyfasudil. Apoptosis was significantly attenuated in p75(NTR-/-) neurons. In p75(NTR-/-) neurons transfected with Syn-p75(NTR), propofol significantly increased Cl-Csp3 in comparison with Syn-green fluorescent protein-transfected p75(NTR-/-) neurons. Wild-type mice exposed to propofol exhibited increased Cl-Csp3 in the hippocampus, an effect attenuated by TAT-Pep5. By contrast, propofol did not induce apoptosis in p75(NTR-/-) mice.

CONCLUSION

These results demonstrate that propofol induces apoptosis in developing neurons in vivo and in vitro and implicate a role for p75(NTR) and the downstream effector RhoA kinase.

The role of peripheral nerves in urodele limb regeneration

Nerve axons and the apical epidermal cap (AEC) are both essential for the formation of an accumulation blastema by amputated limbs of urodele salamanders. The AEC forms in the absence of axons, but is not maintained, and blastema formation fails. Growth stages of the blastema become nerve-independent for morphogenesis, but remain dependent on the nerve for blastema growth. Denervated growth stage blastemas form smaller than normal skeletal parts, owing to diminished mitosis, but form the full proximodistal array of skeletal elements. This difference in nerve dependency of morphogenesis and proliferation is hypothesized to be the result of a dependence of the AEC on nerves for blastema cell proliferation but not for blastema morphogenesis. Regenerating axons induce the synthesis and secretion of the anterior gradient protein (AGP) by distal Schwann cells during dedifferentiation and by the gland cells of the AEC during blastema growth stages. AGP promotes the regeneration of a denervated limb to digit stages when electroporated into the limb during dedifferentiation. Once a critical mass of blastema cells has been attained, the blastema can undergo morphogenesis in the absence of the nerve, but the regenerate will be a miniature, because the nerve is no longer inducing the AEC to carry out its AGP-mediated proliferative function. AGP expression by both Schwann cells and the AEC is induced by axons, but the nature of the inductive agent is unclear.

Neurotrophins acting via TRKB receptors activate the JAGGED1-NOTCH2 cell-cell communication pathway to facilitate early ovarian development

Tropomyosin-related kinase (TRK) receptor B (TRKB) mediates the supportive actions of neurotrophin 4/5 and brain-derived neurotrophic factor on early ovarian follicle development. Absence of TRKB receptors reduces granulosa cell (GC) proliferation and delays follicle growth. In the present study, we offer mechanistic insights into this phenomenon. DNA array and quantitative PCR analysis of ovaries from TrkB-null mice revealed that by the end of the first week of postnatal life, Jagged1, Hes1, and Hey2 mRNA abundance is reduced in the absence of TRKB receptors. Although Jagged1 encodes a NOTCH receptor ligand, Hes1 and Hey2 are downstream targets of the JAGGED1-NOTCH2 signaling system. Jagged1 is predominantly expressed in oocytes, and the abundance of JAGGED1 is decreased in TrkB(-/-) oocytes. Lack of TRKB receptors also resulted in reduced expression of c-Myc, a NOTCH target gene that promotes entry into the cell cycle, but did not alter the expression of genes encoding core regulators of cell-cycle progression. Selective restoration of JAGGED1 synthesis in oocytes of TrkB(-/-) ovaries via lentiviral-mediated transfer of the Jagged1 gene under the control of the growth differentiation factor 9 (Gdf9) promoter rescued c-Myc expression, GC proliferation, and follicle growth. These results suggest that neurotrophins acting via TRKB receptors facilitate early follicle growth by supporting a JAGGED1-NOTCH2 oocyte-to-GC communication pathway, which promotes GC proliferation via a c-MYC-dependent mechanism.

Serum inducible kinase is a positive regulator of cortical dendrite development and is required for BDNF-promoted dendritic arborization

Serum inducible kinase (SNK), also known as polo-like kinase 2 (PLK2), is a known regulator of mitosis, synaptogenesis and synaptic homeostasis. However, its role in early cortical development is unknown. Herein, we show that snk is expressed in the cortical plate from embryonic day 14, but not in the ventricular/subventricular zones (VZ/SVZ), and SNK protein localizes to the soma and dendrites of cultured immature cortical neurons. Loss of SNK impaired dendritic but not axonal arborization in a dose-dependent manner and overexpression had opposite effects, both in vitro and in vivo. Overexpression of SNK also caused abnormal branching of the leading process of migrating cortical neurons in electroporated cortices. The kinase activity was necessary for these effects. Extracellular signal-regulated kinase (ERK) pathway activity downstream of brain-derived neurotrophic factor (BDNF) stimulation led to increases in SNK protein expression via transcriptional regulation, and this upregulation was necessary for the growth-promoting effect of BDNF on dendritic arborization. Taken together, our results indicate that SNK is essential for dendrite morphogenesis in cortical neurons.

Anticonvulsant Activity of Extracts of Plectranthus barbatus Leaves in Mice

Plectranthus barbatus is a medicinal plant used to treat a wide range of disorders including seizure. However, the anticonvulsant activity of this plant has not been studied in depth. We therefore sought to evaluate the anticonvulsant activity of a hydroalcoholic extract of P. barbatus leaves on seizures induced by strychnine sulphate (2.0 mg/kg) and pilocarpine (600 mg/kg) in mice. The extract was administered orally at 1, 10, 30, and 100 mg/kg. We report that the P. barbatus extract had marked anticonvulsant activity against strychnine-induced convulsions, but was quite ineffective against pilocarpine-induced convulsions. Further experiments will be required to identify the active molecules(s) and their mechanism(s) of action.

Ret-dependent and Ret-independent mechanisms of Gfl-induced sensitization

BACKGROUND

The GDNF family ligands (GFLs) are regulators of neurogenic inflammation and pain. We have previously shown that GFLs increase the release of the sensory neuron neuropeptide, calcitonin gene-related peptide (CGRP) from isolated mouse DRG.

RESULTS

Inhibitors of the mitogen-activated protein kinase (MAPK) pathway abolished the enhancement of CGRP release by GDNF. Neurturin-induced enhancement in the stimulated release of CGRP, used as an indication of sensory neuronal sensitization, was abolished by inhibition of the phosphatidylinositol-3 kinase (PI-3K) pathway. Reduction in Ret expression abolished the GDNF-induced sensitization, but did not fully inhibit the increase in stimulus-evoked release of CGRP caused by neurturin or artemin, indicating the presence of Ret-independent GFL-induced signaling in sensory neurons. Integrin β-1 and NCAM are involved in a component of Ret-independent GFL signaling in sensory neurons.

CONCLUSIONS

These data demonstrate the distinct and variable Ret-dependent and Ret-independent signaling mechanisms by which GFLs induce sensitization of sensory neurons. Additionally, there is a clear disconnect between intracellular signaling pathway activation and changes in sensory neuronal function.

Intrinsic and therapeutic factors determining the recovery of motor function after peripheral nerve transection

Insufficient recovery after peripheral nerve injury has been attributed to (i) poor pathfinding of regrowing axons, (ii) excessive collateral axonal branching at the lesion site and (iii) polyneuronal innervation of the neuromuscular junctions (NMJ). The facial nerve transection model has been used initially to measure restoration of function after varying therapies and to examine the mechanisms underlying their effects. Since it is very difficult to control the navigation of several thousand axons, efforts concentrated on collateral branching and NMJ-polyinnervation. Treatment with antibodies against trophic factors to combat branching improved the precision of reinnervation, but had no positive effects on functional recovery. This suggested that polyneuronal reinnervation--rather than collateral branching--may be the critical limiting factor. The former could be reduced by pharmacological agents known to perturb microtubule assembly and was followed by recovery of function. Because muscle polyinnervation is activity-dependent and can be manipulated, attempts to design a clinically feasible therapy were performed by electrical stimulation or by soft tissue massage. Electrical stimulation applied to the transected facial nerve or to paralysed facial muscles did not improve vibrissal motor performance and failed to diminish polyinnervation. In contrast, gentle stroking of the paralysed muscles (vibrissal, orbicularis oculi, tongue musculature) resulted in full recovery of function. This manual stimulation was also effective after hypoglossal-facial nerve suture and after interpositional nerve grafting, but not after surgical reconstruction of the median nerve. All these findings raise hopes that clinically feasible and effective therapies could be soon designed and tested.

Isoflurane neurotoxicity is mediated by p75NTR-RhoA activation and actin depolymerization

BACKGROUND

The mechanisms by which isoflurane injured the developing brain are not clear. Recent work has demonstrated that it is mediated in part by activation of p75 neurotrophin receptor. This receptor activates RhoA, a small guanosine triphosphatase that can depolymerize actin. It is therefore conceivable that inhibition of RhoA or prevention of cytoskeletal depolymerization might attenuate isoflurane neurotoxicity. This study was conducted to test these hypotheses using primary cultured neurons and hippocampal slice cultures from neonatal mouse pups.

METHODS

Primary neuron cultures (days in vitro, 4-7) and hippocampal slice cultures from postnatal day 4-7 mice were exposed to 1.4% isoflurane (4 h). Neurons were pretreated with TAT-Pep5, an intracellular inhibitor of p75 neurotrophin receptor, the cytoskeletal stabilizer jasplakinolide, or their corresponding vehicles. Hippocampal slice cultures were pretreated with TAT-Pep5 before isoflurane exposure. RhoA activation was evaluated by immunoblot. Cytoskeletal depolymerization and apoptosis were evaluated with immunofluorescence microscopy using drebrin and cleaved caspase-3 staining, respectively.

RESULTS

RhoA activation was increased after 30 and 120 min of isoflurane exposure in neurons; TAT-Pep5 (10 μm) decreased isoflurane-mediated RhoA activation at both time intervals. Isoflurane decreased drebrin immunofluorescence and enhanced cleaved caspase-3 in neurons, effects that were attenuated by pretreatment with either jasplakinolide (1 μm) or TAT-Pep5. TAT-Pep5 attenuated the isoflurane-mediated decrease in phalloidin immunofluorescence. TAT-Pep5 significantly attenuated isoflurane-mediated loss of drebrin immunofluorescence in hippocampal slices.

CONCLUSIONS

Isoflurane results in RhoA activation, cytoskeletal depolymerization, and apoptosis. Inhibition of RhoA activation or prevention of downstream actin depolymerization significantly attenuated isoflurane-mediated neurotoxicity in developing neurons.

Differential expression and regulation by activin of the neurotrophins BDNF and NT4 during human and mouse ovarian development

The tropomyosin-related kinase (Trk) B neurotrophin receptor is essential for ovarian germ cell survival and primordial follicle formation, but the contributions of its ligands, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4), are unknown. We have investigated their expression and regulation in developing human and mouse ovaries. BDNF expression increased with increasing gestation, expression of human NTF4 and of both Ntf5 and Bdnf in the mouse was unchanged. Bdnf expression was dramatically lower than Ntf5 in the mouse, but levels were comparable in the human. Human fetal ovarian somatic cells expressed BDNF. Activin A selectively regulated BDNF and Ntf5 expression in human and mouse, respectively, identifying an oocyte/somatic signaling pathway which might mediate the pro-survival effects of activin. These data reveal that expression and regulation of the TrkB ligands are differentially controlled in the developing ovaries of humans and mice, and identify BDNF as a potential regulator of germ cell fate in the human fetal ovary.

Transgenic BDNF induces nerve fiber regrowth into the auditory epithelium in deaf cochleae

Sensory organs typically use receptor cells and afferent neurons to transduce environmental signals and transmit them to the CNS. When sensory cells are lost, nerves often regress from the sensory area. Therapeutic and regenerative approaches would benefit from the presence of nerve fibers in the tissue. In the hearing system, retraction of afferent innervation may accompany the degeneration of auditory hair cells that is associated with permanent hearing loss. The only therapy currently available for cases with severe or complete loss of hair cells is the cochlear implant auditory prosthesis. To enhance the therapeutic benefits of a cochlear implant, it is necessary to attract nerve fibers back into the cochlear epithelium. Here we show that forced expression of the neurotrophin gene BDNF in epithelial or mesothelial cells that remain in the deaf ear induces robust regrowth of nerve fibers towards the cells that secrete the neurotrophin, and results in re-innervation of the sensory area. The process of neurotrophin-induced neuronal regeneration is accompanied by significant preservation of the spiral ganglion cells. The ability to regrow nerve fibers into the basilar membrane area and protect the auditory nerve will enhance performance of cochlear implants and augment future cell replacement therapies such as stem cell implantation or induced transdifferentiation. This model also provides a general experimental stage for drawing nerve fibers into a tissue devoid of neurons, and studying the interaction between the nerve fibers and the tissue.

High-resolution fractionation of signaling endosomes containing different receptors

Receptor endocytosis is regulated by ligand binding, and receptors may signal after endocytosis in signaling endosomes. We hypothesized that signaling endosomes containing different types of receptors may be distinct from one another and have different physical characteristics. To test this hypothesis, we developed a high-resolution organelle fractionation method based on mass and density, optimized to resolve endosomes from other organelles. Three different types of receptors undergoing ligand-induced endocytosis were localized predominately in endosomes that were resolved from one another using this method. Endosomes containing activated receptor tyrosine kinases (RTKs), TrkA and EGFR, were similar to one another. Endosomes containing p75(NTR) (in the tumor necrosis receptor superfamily) and PAC1 (a G-protein-coupled receptor) were distinct from each other and from RTK endosomes. Receptor-specific endosomes may direct the intracellular location and duration of signal transduction pathways to dictate response to signals and determine cell fate.

Role of neurotrophic factors in early ovarian development

Much is known about the endocrine hormonal mechanisms controlling ovarian development. More recently, attention has focused on identifying regulatory pathways that, operating within the ovarian microenvironment, contribute to the acquisition of ovarian reproductive competence. Within this framework, the concept has developed that neurotrophins (NTs) and their Trk tyrosine kinase receptors, long thought to be exclusively required for the development of the nervous system, are also involved in the control of ovarian maturation. The ovary of several species, including rodents, sheep, cows, nonhuman primates, and humans, produce NTs and express both the high-affinity receptors and the common p75 (NTR) receptor required for signaling. Studies in humans and rodents have shown that this expression is initiated during fetal life, before the formation of primordial follicles. Gene targeting approaches have identified TrkB, the high-affinity receptor for neurotrophin-4/5 and brain-derived neurotrophic factor, as a signaling module required for follicular assembly, early follicular growth, and oocyte survival. A similar approach has shown that nerve growth factor contributes independently to the growth of primordial follicles into gonadotropin-responsive structures. Altogether, these observations indicate that NTs are important contributors to the gonadotropin-independent process underlying the formation and initiation of ovarian follicular growth.

Anti-cancer drug induced neurotoxicity and identification of Rho pathway signaling modulators as potential neuroprotectants

Many chemotherapy drugs are known to cause significant clinical neurotoxicity, which can result in the early cessation of treatment. To identify and develop more effective means of neuroprotection it is important to understand the toxicity of these drugs at the molecular and cellular levels. In the present study, we examine the effects of paclitaxel (taxol), cisplatin, and methotrexate on primary rat neurons including hippocampal, cortical, and dorsal horn/dorsal root ganglion neuronal cultures. We found that all of these anti-cancer drugs induce substantial neurotoxicity evidenced by neurite degeneration. The neurons are capable of recovering after treatment withdrawal, but taxol exerts a biphasic effect that results in the collapse of processes days after treatment is withdrawn. After cisplatin and methotrexate treatment, we observed the degeneration of neuronal processes including the reduction of dendritic branching, length, and altered growth cone formation, indicating an abnormal arrangement of the actin cytoskeleton consistent with the involvement of Rho family small GTPases. Inhibiting RhoA downstream effector p160 ROCK/Rho kinase using Y-27632, or activating p75 neurotrophin receptor (p75 NTR) using non-peptide mimetic LM11A-31, were able to reverse the degeneration caused by cisplatin and methotrexate. Therefore, the neurotoxicity resulting from exposure to the anti-cancer drugs cisplatin and methotrexate can be alleviated by inhibiting Rho signaling pathway.

Bone morphogenetic protein down-regulation of neuronal pituitary adenylate cyclase-activating polypeptide and reciprocal effects on vasoactive intestinal peptide expression

Among bone morphogenetic proteins (BMPs), the decapentaplegic (Dpp; BMP2, BMP4) and glass bottom boat (Gbb/60A; BMP5, BMP6, BMP7) subgroups have well-described functions guiding autonomic and sensory neuronal development, fiber formation and neurophenotypic identities. Evaluation of rat superior cervical ganglia (SCG) post-ganglionic sympathetic neuron developmental regulators identified that selected BMPs of the transforming growth factor beta superfamily have reciprocal effects on neuronal pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) expression. Dpp and Gbb/60A BMPs rapidly down-regulated PACAP expression, while up-regulating other sympathetic neuropeptides, including PACAP-related VIP. The suppressive effects of BMP on PACAP mRNA and peptide expression were potent, efficacious and phosphorylated mothers against decapentaplegic homolog (Smad) signaling-dependent. Axotomy of SCG dramatically increases PACAP expression, and the possibility that abrogation of inhibitory retrograde target tissue BMP signaling may contribute to this up-regulation of sympathetic neuron PACAP was investigated. Replacement of BMP6 to SCG explant preparations significantly blunted the injury-induced elevated PACAP expression, with a concomitant decrease in sympathetic PACAP-immunoreactive neuron numbers. These studies suggested that BMPs modulate neuropeptide identity and diversity by stimulating or restricting the expression of specific peptidergic systems. Furthermore, the liberation of SCG neurons from target-derived BMP inhibition following axotomy may be one participating mechanism associated with injury-induced neuropeptidergic plasticity.

In Vivo Induction of Glial Cell Proliferation and Axonal Outgrowth and Myelination by Brain-Derived Neurotrophic Factor

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family of neuronal cell survival and differentiation factors but is thought to be involved in neuronal cell proliferation and myelination as well. To explore the role of BDNF in vivo, we employed the intermediate pituitary melanotrope cells of the amphibian Xenopus laevis as a model system. These cells mediate background adaptation of the animal by producing high levels of the prohormone proopiomelanocortin (POMC) when the animal is black adapted. We used stable X. transgenesis in combination with the POMC gene promoter to generate transgenic frogs overexpressing BDNF specifically and physiologically inducible in the melanotrope cells. Intriguingly, an approximately 25-fold overexpression of BDNF resulted in hyperplastic glial cells and myelinated axons infiltrating the pituitary, whereby the transgenic melanotrope cells became located dispersed among the induced tissue. The infiltrating glial cells and axons originated from both peripheral and central nervous system sources. The formation of the phenotype started around tadpole stage 50 and was induced by placing white-adapted transgenics on a black background, i.e. after activation of transgene expression. The severity of the phenotype depended on the level of transgene expression, because the intermediate pituitaries from transgenic animals raised on a white background or from transgenics with only an approximately 5-fold BDNF overexpression were essentially not affected. In conclusion, we show in a physiological context that, besides its classical role as neuronal cell survival and differentiation factor, in vivo BDNF can also induce glial cell proliferation as well as axonal outgrowth and myelination.

Gene delivery to the spinal cord: comparison between lentiviral, adenoviral, and retroviral vector delivery systems

Viral gene delivery for spinal cord injury (SCI) is a promising approach for enhancing axonal regeneration and neuroprotection. An understanding of spatio-temporal transgene expression in the spinal cord is essential for future studies of SCI therapies. Commonly, intracellular marker proteins (e.g., EGFP) were used as indicators of transgene levels after viral delivery, which may not accurately reflect levels of secreted transgene. This study examined transgene expression using ELISA after viral delivery of D15A, a neurotrophin with BDNF and NT-3 activities, at 1, 2, and 4weeks after in vivo and ex vivo delivery using lentiviral, adenoviral, and retroviral vectors. Further, the inflammatory responses and viral infection patterns after in vivo delivery were examined. Lentiviral vectors had the most stable pattern of gene expression, with D15A levels of 536 +/- 38 and 363 +/- 47 pg/mg protein seen at 4 weeks after the in vivo and ex vivo delivery, respectively. Our results show that protein levels downregulate disproportionately to levels of EGFP after adenoviral vectors both in vivo and ex vivo. D15A dropped from initial levels of 422 +/- 87 to 153 +/- 18 pg/mg protein at 4 weeks after in vivo administration. Similarly, ex vivo retrovirus-mediated transgene expression exhibited rapid downregulation by 2 weeks post-grafting. Compared to adenoviral infection, macrophage activation was attenuated after lentiviral infection. These results suggest that lentiviral vectors are most suitable in situations where stable long-term transgene expression is needed. Retroviral ex vivo delivery is optional when transient expression within targeted spinal tissue is desired, with adenoviral vectors in between.

Role of all-trans retinoic acid in neurite outgrowth and axonal elongation

The vitamin A metabolite, all-trans retinoic acid (atRA) plays essential roles in nervous system development, including neuronal patterning, survival, and neurite outgrowth. Our understanding of how the vitamin A acid functions in neurite outgrowth comes largely from cultured embryonic neurons and model neuronal cell systems including human neuroblastoma cells. Specifically, atRA has been shown to increase neurite outgrowth from embryonic DRG, sympathetic, spinal cord, and olfactory receptor neurons, as well as dissociated cerebra and retina explants. A role for atRA in axonal elongation is also supported by a limited number of studies in vivo, in which a deficiency in retinoid signaling produced either by dietary or genetic means has been shown to alter neurite outgrowth from the spinal cord and hindbrain regions. Human neuroblastoma cells also show enhanced numbers of neurites and longer processes in response to atRA. The mechanism whereby retinoids regulate neurite outgrowth includes, but is not limited to, the regulation of the transcription of neurotrophin receptors. More recent evidence supports a role for atRA in regulating components of other signaling pathways or candidate neurite-regulating factors. Some of these effects, such as that on neuron navigator 2 (NAV2), may be direct, whereas others may be secondary to other atRA-induced changes in the cell. This review focuses on what is currently known about neurite initiation and growth, with emphasis on the manner in which atRA may influence these events.

Neurotrophin effects on intracellular Ca2+ and force in airway smooth muscle

Neurotrophins [e.g., brain-derived neurotrophic factor (BDNF), neurotrophin 4 (NT4)], known to affect neuronal structure and function, are expressed in nonneuronal tissues including the airway. However, their function is unclear. We examined the effect of acute vs. prolonged neurotrophin exposure on regulation of airway smooth muscle (ASM) intracellular Ca(2+) concentration ([Ca(2+)](i)): sarcoplasmic reticulum (SR) Ca(2+) release and Ca(2+) influx (specifically store-operated Ca(2+) entry, SOCE). Human ASM cells were incubated for 30 min in medium (control) or 1 or 10 nM BDNF, NT3, or NT4 (acute exposure) or overnight in 1 nM BDNF, NT3, or NT4 (prolonged exposure) and imaged after loading with the Ca(2+) indicator fura-2 AM. [Ca(2+)](i) responses to ACh, histamine, bradykinin, and caffeine and SOCE following SR Ca(2+) depletion were compared across cell groups. Force measurements were performed in human bronchial strips exposed to neurotrophins. Basal [Ca(2+)](i), peak responses to all agonists, SOCE, and force responses to ACh and histamine were all significantly enhanced by both acute and prolonged BDNF exposure (smaller effect of NT4) but decreased by NT3. Inhibition of the BDNF/NT4 receptor trkB by K252a prevented enhancement of [Ca(2+)](i) responses. ASM cells showed positive immunostaining for BDNF, NT3, NT4, trkB, and trkC (NT3 receptor). These novel data demonstrate that neurotrophins influence ASM [Ca(2+)](i) and force regulation and suggest a potential role for neurotrophins in airway diseases.

A Simple, Flexible, Nonfluorescent System for the Automated Screening of Neurite Outgrowth

Measurement of neurite outgrowth is a common assay of neurotrophic activity. However, currently available techniques for measuring neurite outgrowth are either time or resource intensive. The authors established a system in which chronic treatment of a subcloned SH-SY5Y cell line with aphidicolin and various concentrations of nerve growth factor (NGF) induced discernable alterations in proliferation and differentiation. Cells were fixed, labeled with a nonfluorescent dye, and evaluated both manually and with an automated analysis system. NGF increased multiple parameters of differentiation, including neurite length, the proportion of cells extending neurites, and branching, as well as promoting cellular survival/proliferation. Interestingly, although NGF treatment increased the total number of branches, it actually decreased the proportion of branches per neurite length. The authors observed no differences in results obtained using the manual and automated systems, but the automated system was orders of magnitude faster. To demonstrate the flexibility of the system, the authors also show that they could measure changes in differentiation induced by a small-molecule Rho kinase inhibitor, as well as by retinoic acid cotreatment with brain-derived neurotrophic factor. In addition to this flexibility, this system does not require specialized equipment or fluorescent antibodies for analysis and therefore provides a less resource-intensive alternative to fluorescence-based systems.

Effects of glossopharyngeal nerve section on the expression of neurotrophins and their receptors in lingual taste buds of adult mice

The expression of neurotrophins and neurotrophin receptors is essential for the proper establishment and function of many sensory systems. To determine which neurotrophins and neurotrophin receptors are expressed in taste buds, and in taste buds of mice following denervation, antibodies directed against the neurotrophins and their receptors were applied to adult mouse gustatory tissue. Immunohistochemistry reveals that nerve growth factor (NGF)-like immunoreactive (LIR), tyrosine kinase (trk) A-LIR, trkB-LIR, and p75-LIR elongated, differentiated taste cells are present within all lingual taste buds, whereas neither neurotrophin (NT)-3- nor trkC-LIR was detected in taste cells. Double-label immunohistochemistry using markers of different taste cell types in brain-derived neurotrophic factor (BDNF)LacZ mice reveals that BDNF (beta-gal) and trkB colocalize, mainly in type III taste cells. NGF, pro-NGF, and trkA coexist in type II taste cells, i.e., those expressing phospholipase Cbeta2 (PLCbeta2). p75-LIR also is present in both BDNF and NGF taste cell populations. To determine the neural dependence of neurotrophin expression in adult taste buds, glossopharyngeal nerves were cut unilaterally. During the period of denervation (10 days to 3 weeks), taste buds largely disappear, and few neurotrophin-expressing cells are present. Three weeks after nerve transection, nerve fascicles on the operated side of the tongue exhibit BDNF-LIR, NGF-LIR, and ubiquitin carboxyl terminal hydrolase (PGP 9.5)-LIR. However, BDNF-LIR staining intensity but not NGF-LIR or PGP 9.5-LIR is increased in nerve fascicles on the operated compared with the unoperated side. Five weeks following nerve transection, NT and NT receptor expression resumes and appears normal in taste buds and nerves. These results indicate that neurotrophin expression in taste buds is dependent on gustatory innervation, but expression in nerves is not dependent on contact with taste buds.

IGF-1 and BDNF promote chick bulbospinal neurite outgrowth in vitro

Injured neurons in the CNS do not experience significant functional regeneration and so spinal cord insult often results in permanently compromised locomotor ability. The capability of a severed axon to re-grow is thought to depend on numerous factors, one of which is the decreased availability of neurotrophic factors. Application of trophic factors to axotomized neurons has been shown to enhance survival and neurite outgrowth. Although brainstem-spinal connections play a pivotal role in motor dysfunction after spinal cord injury, relatively little is known about the trophic sensitivity of these populations. This study explores the response of bulbospinal populations to various trophic factors. Several growth factors were initially examined for potential trophic effects on the projection neurons of the brainstem. Brain derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1) significantly enhance mean process length in both the vestibulospinal neurons and spinal projection neurons from the raphe nuclei. Nerve growth factor (NGF), neurotrophin-4 (NT-4) and glial derived neurotrophic factor (GDNF) did not effect process outgrowth in vestibulospinal neurons. At the developmental stages used in this study, it was determined that receptors for BDNF and IGF-1 were present both on bulbospinal neurons and on surrounding cells with a non-neuronal morphology.

Neurite extension in central neurons: a novel role for the receptor tyrosine kinases Ror1 and Ror2

Neurite elongation and branching are key cellular events during brain development as they underlie the formation of a properly wired neuronal network. Here we report that the receptor tyrosine kinases Ror1 and Ror2 modulate the growth of neurites as well as their branching pattern in hippocampal neurons. Upon Ror1 or Ror2 suppression using antisense oligonucleotides or RNA interference (RNAi), neurons extended shorter and less branched minor processes when compared to those in control cells. In addition, Ror-depleted cells elongated longer, albeit less branched, axons than seen in control cells. Conversely, Ror overexpression both in non-neuronal cells and in hippocampal neurons resulted in the enhanced extension of short and highly branched processes. These phenotypes were accompanied by changes in the microtubule-associated proteins MAP1B and MAP2. Taken together, these results support a novel role for Ror receptors as modulators of neurite extension in central neurons.

RhoA, RhoB, RhoC, Rac1, Cdc42, and Tc10 mRNA levels in spinal cord, sensory ganglia, and corticospinal tract neurons and long-lasting specific changes following spinal cord injury

Inhibition of RhoA has been shown to enhance axonal regeneration following spinal cord injury. Here we mapped mRNA expression patterns of RhoA, B, and C, Rac1, Cdc42, and Tc10 in spinal cord, sensory ganglia, and sensorimotor cortex in uninjured rats, and following spinal cord injury or sham laminectomy. In the intact spinal cord, neurons displayed high levels of Rac1, Cdc42, and Tc10 mRNA hybridization signal. GFAP-immunoreactive astrocytes expressed primarily RhoB and Rac1, while oligodendrocyte-like cells expressed RhoA, Rac1, and Cdc42. Injury caused profound, long-lasting upregulation of RhoA, Rac1, Cdc42, and Tc10 mRNA in the spinal cord, while RhoB was modestly increased and RhoC did not change. GFAP-immunoreactive reactive astrocytes exhibited a dramatic increase of RhoA mRNA expression along with increases of Rac1 and Cdc42. Injury also led to elevation of RhoA, Cdc42, and Tc10 in neurons and modest increases of RhoA, Rac1, and Tc10 in oligodendrocyte-like cells. Laminectomy caused similar, but less pronounced alterations of investigated mRNA species. In dorsal root ganglia neuronal RhoA, Rac1, Cdc42, and Tc10 mRNA levels were increased similarly by spinal cord injury and sham surgery. The CST pyramidal cells expressed Tc10 mRNA and the CST itself was Tc10-immunoreactive. Tc10-immunoreactivity disappeared distal to injury. We conclude that there are gene-specific patterns of expression of the six different Rho-GTPases in normal spinal cord and dorsal root ganglia, and that specific changes of temporal and spatial expression patterns occur in response to spinal cord injury, suggesting different roles of these GTPases in the cellular sequelae of CNS injury.

Biological activity of neurotrophins is dependent on recruitment of Rac1 to lipid rafts

The Rho family of small GTPases, key regulators of the actin cytoskeleton in eukaryotic cells, is implicated in the control of neuronal morphology. Here, we report that neurotrophin dependent cytoskeletal changes, characteristic of the phenotype of Rac1, in the hippocampal neurons or PC12 cells are inhibited by the disruption of lipid raft integrity. Activation of Rac1 induced by NGF is impaired in cholesterol-depleted PC12 cells. Pretreatment with gammaGTP shifted significant amount of Rac1, presumably in a GTP-bound form, from non-raft to raft fractions. Proper recruitment of activated Rac1 to lipid rafts, structures that represent specialized signaling organelles, is of fundamental importance in determining neurotrophins' bioactivity.

Promotion of axon regeneration by myelin-associated glycoprotein and Nogo through divergent signals downstream of Gi/G

Several myelin-derived proteins have been identified as components of the CNS myelin that prevents axonal regeneration in the adult vertebrate CNS. Activation of RhoA has been shown to be an essential part of the signaling mechanism of these proteins. Here we report an additional signal, which determines whether these proteins promote or inhibit axon outgrowth. Myelin-associated glycoprotein (MAG) and Nogo trigger the intracellular elevation of Ca2+ as well as the activation of PKC, presumably mediated by G(i)/G. Neurite outgrowth inhibition and growth cone collapse by MAG or Nogo can be converted to neurite extension and growth cone spreading by inhibiting conventional PKC, but not by inhibiting inositol 1,4,5-triphosphate (IP3). Conversely, neurite growth of immature neurons promoted by MAG is abolished by inhibiting IP3. Activation of RhoA is independent of PKC. Thus, a balance between PKC and IP3 is important for bidirectional regulation of axon regeneration by the myelin-derived proteins.

Activation of Rho in the injured axons following spinal cord injury

Axons of the adult central nervous system have very limited ability to regenerate after injury. This inability may be, at least partly, attributable to myelin-derived proteins, such as myelin-associated glycoprotein, Nogo and oligodendrocyte myelin glycoprotein. Recent evidence suggests that these proteins inhibit neurite outgrowth by activation of Rho through the neurotrophin receptor p75(NTR)/Nogo receptor complex. Despite rapidly growing knowledge on these signals at the molecular level, it remained to be determined whether Rho is activated after injury to the central nervous system. To assess this question, we establish a new method to visualize endogenous Rho activity in situ. After treatment of cerebellar granular neurons with the Nogo peptide in vitro, Rho is spatially activated and colocalizes with p75(NTR). Following spinal cord injury in vivo, massive activation of Rho is observed in the injured neurites. Spatial regulation of Rho activity may be necessary for axonal regulation by the inhibitory cues.

The death receptor antagonist FAIM promotes neurite outgrowth by a mechanism that depends on ERK and NF-kapp B signaling

Fas apoptosis inhibitory molecule (FAIM) is a protein identified as an antagonist of Fas-induced cell death. We show that FAIM overexpression fails to rescue neurons from trophic factor deprivation, but exerts a marked neurite growth–promoting action in different neuronal systems. Whereas FAIM overexpression greatly enhanced neurite outgrowth from PC12 cells and sympathetic neurons grown with nerve growth factor (NGF), reduction of endogenous FAIM levels by RNAi decreased neurite outgrowth in these cells. FAIM overexpression promoted NF-κB activation, and blocking this activation by using a super-repressor IκBα or by carrying out experiments using cortical neurons from mice that lack the p65 NF-κB subunit prevented FAIM-induced neurite outgrowth. The effect of FAIM on neurite outgrowth was also blocked by inhibition of the Ras–ERK pathway. Finally, we show that FAIM interacts with both Trk and p75 neurotrophin receptor NGF receptors in a ligand-dependent manner. These results reveal a new function of FAIM in promoting neurite outgrowth by a mechanism involving activation of the Ras–ERK pathway and NF-κB.

The effects of brain-derived neurotrophic factor (BDNF) administration on kindling induction, Trk expression and seizure-related morphological changes

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family that mediates synaptic plasticity and excitability in the CNS. Recent evidence has shown that increased BDNF levels can lead to hyperexcitability and epileptiform activities, while suppression of BDNF function in transgenic mice or by antagonist administration retards the development of seizures. However, several groups, including our own, have reported that increasing BDNF levels by continuous intrahippocampal infusion inhibits epileptogenesis. It is possible that the continuous administration of BDNF produces a down-regulation of its high-affinity TrkB receptor, leading to a decrease of neuronal responsiveness to BDNF. If so, then animals should respond differently to bolus injections of BDNF, which presumably do not alter Trk expression, compared with continuous infusion. To test this hypothesis, we compared the effects of intrahippocampal BDNF continuous infusion and bolus injections on kindling induction. We showed that continuous infusion of BDNF inhibited the development of behavioral seizures and decreased the level of phosphorylated Trks or TrkB receptors. In contrast, multiple bolus microinjections of BDNF accelerated kindling development and did not affect the level of phosphorylated Trks or TrkB receptors. Our results indicate that different administration protocols yield opposite effects of BDNF on neuronal excitability, epileptogenesis and Trk expression. Unlike nerve growth factor and neurotrophin-3, which affect mossy fiber sprouting, we found that BDNF administration had no effect on the mossy fiber system in naive or kindled rats. Such results suggest that the effects of BDNF on epileptogenesis are not modulated by its effect on sprouting, but rather by its effects on excitability.