Quantitative demonstration of the retrograde axonal transport of endogenous nerve growth factor

[Search for Neuroprotective Compounds -From 4-Methycatechol to Citrus Compounds]

In the 1980s, the authors developed the enzyme immunoassay (EIA) system for mouse nerve growth factor (NGF) to clarify its important physiological roles. Our EIA system was a new and powerful tool for measurement of extremely low levels of NGF in vitro and in vivo, and it contributed to investigation into the regulatory mechanism of NGF synthesis. After that, we demonstrated that the compounds with a low molecular weight, such as 4-methylcatechol, which elicit stimulatory activity toward NGF synthesis, were useful and practical for therapeutic purposes; as NGF has potent activity on neuronal degeneration in both the central nervous system (CNS) and the peripheral nervous system. Since 2008, we have been searching for and isolating neuroprotective component(s) from citrus peels. As a result, our study revealed that 1) 3,5,6,7,8,3',4'-heptamethoxyflavone (HMF) has neuroprotective ability in the CNS by inducing brain-derived neurotrophic factor (BDNF) and by suppressing inflammation; 2) auraptene (AUR) also has neuroprotective ability in the CNS by suppressing inflammation and by probably inducing neurotrophic factor(s). As the content of AUR in the peels of Kawachi Bankan is exceptionally high, 1) we found this peel powder to exert neuroprotective effects in the brain of various pathological model mice; 2) some of the AUR transited from the peel to the juice during the squeezing process to obtain the juice. Therefore, K. Bankan juice, which is enriched in AUR by adding peel paste to the raw juice, was shown to be practical for suppression of cognitive dysfunction of aged healthy volunteers.

The evolution of nerve growth factor inhibition in clinical medicine

Nerve growth factor (NGF) is a neurotrophin that activates nociceptive neurons to transmit pain signals from the peripheral to the central nervous system and that exerts its effects on neurons by signalling through tyrosine kinase receptors. Antibodies that inhibit the function of NGF and small molecule inhibitors of NGF receptors have been developed and tested in clinical studies to evaluate the efficacy of NGF inhibition as a form of analgesia in chronic pain states including osteoarthritis and chronic low back pain. Clinical studies in individuals with painful knee and hip osteoarthritis have revealed that NGF inhibitors substantially reduce joint pain and improve function compared with NSAIDs for a duration of up to 8 weeks. However, the higher tested doses of NGF inhibitors also increased the risk of rapidly progressive osteoarthritis in a small percentage of those treated. This Review recaps the biology of NGF and the studies that have been performed to evaluate the efficacy of NGF inhibition for chronic musculoskeletal pain states. The adverse events associated with NGF inhibition and the current state of knowledge about the mechanisms involved in rapidly progressive osteoarthritis are also discussed and future studies proposed to improve understanding of this rare but serious adverse event.

Effect of Guizhi Decoction ([symbols; see text]) on heart rate variability and regulation of cardiac autonomic nervous imbalance in diabetes mellitus rats

OBJECTIVE

To observe abnormalities in heart rate variability (HRV) in diabetic rats and to explore the effects of treatment with Guizhi Decoction ([symbols; see text]) on cardiac autonomic nervous (CAN) imbalance.

METHODS

A radio-telemetry system for monitoring physiological parameters was implanted into rats to record electrocardiac signals and all indictors of HRV [time domain measures: standard deviation of all RR intervals in 24 h (SDNN), root mean square of successive differences (RMSSD), percentage of differences between adjacent RR intervals greater than 50 ms (PNN50), and standard deviation of the averages of RR intervals (SDANN); frequency domain measures: low frequency (LF), high frequency (HF), total power (TP), and LF/HF ratio]. The normal group was randomly selected, and the remaining rats were used to establish streptozocin (STZ)-induced diabetic model. After 4 weeks, the model rats were divided into the model group, the methycobal group, and the Guizhi Decoction group, 9 rats in each group. Four weeks after intragastric administration of the corresponding drugs, the right atria of the rats were collected for immunohistochemical staining of tyrosine hydroxylase (TH) and choline acetyltransferase (CHAT) to observe the distribution of the sympathetic and vagus nerves in the right atrium. The myocardial homogenate from the interventricular septum and the left ventricle was used for determination of TH, CHAT, growth-associated protein 43 (GAP-43), nerve growth factor (NGF), and ciliary neurotrophic factor (CNTF) levels using an enzyme-linked immunosorbent assay.

RESULTS

(1) STZ rats had elevated blood glucose levels, reduced body weight, and decreased heart rate; there was no difference between the model group and the drug treated groups. (2) Compared with the model group, only RMSSD and TP increased in the methycobal group significantly (P<0.05); SDNN, RMSSD, PNN50, LF, HF, and TP increased, LF/HF decreased (P<0.05), and SDANN just showed a decreasing trend in the Guizhi Decoction group (P>0.05). TH increased, CHAT decreased, and TH/CHAT increased in the myocardial homogenate of the model group (P<0.05). Compared with the model group, left ventricular TH reduced in the methycobal group; and in the Guizhi Decoction group CHAT increased, while TH and TH/CHAT decreased (P<0.05). Compared with the model group, CNTF in the interventricular septum increased in the methycobal group (P<0.05); GAP-43 increased, NGF decreased, and CNTF increased (P<0.05) in the Guizhi Decoction group. There were significant differences in the reduction of NGF and elevation of CNTF between the Guizhi Decoction group and the methycobal group (P<0.05). (3) Immunohistochemical results showed that TH expression significantly increased and CHAT expression significantly decreased in the myocardia of the model group, whereas TH expression decreased and CHAT expression increased in the Guizhi Decoction group (P<0.05).

CONCLUSION

Guizhi Decoction was effective in improving the function of the vagus nerve, and it could alleviate autonomic nerve damage.

Neurotrophin signaling via long-distance axonal transport

Neurotrophins are a family of target-derived growth factors that support survival, development, and maintenance of innervating neurons. Owing to the unique architecture of neurons, neurotrophins that act locally on the axonal terminals must convey their signals across the entire axon for subsequent regulation of gene transcription in the cell nucleus. This long-distance retrograde signaling, a motor-driven process that can take hours or days, has been a subject of intense interest. In the last decade, live-cell imaging with high sensitivity has significantly increased our capability to track the transport of neurotrophins, their receptors, and subsequent signals in real time. This review summarizes recent research progress in understanding neurotrophin-receptor interactions at the axonal terminal and their transport dynamics along the axon. We emphasize high-resolution studies at the single-molecule level and also discuss recent technical advances in the field.

Nerve growth factor promoter activity revealed in mice expressing enhanced green fluorescent protein

Nerve growth factor (NGF) and its precursor proNGF are perhaps the best described growth factors of the mammalian nervous system. There remains, however, a paucity of information regarding the precise cellular sites of proNGF/NGF synthesis. Here we report the generation of transgenic mice in which the NGF promoter controls the ectopic synthesis of enhanced green fluorescent protein (EGFP). These transgenic mice provide an unprecedented resolution of both neural cells (e.g., neocortical and hippocampal neurons) and non-neural cells (e.g., renal interstitial cells and thymic reticular cells) that display NGF promoter activity from postnatal development to adulthood. Moreover, the transgene is inducible by injury. At 2 days after sciatic nerve ligation, a robust population of EGFP-positive cells is seen in the proximal nerve stump. These transgenic mice offer novel insights into the cellular sites of NGF promoter activity and can be used as models for investigating the regulation of proNGF/NGF expression after injury.

Motor neuron trophic factors: therapeutic use in ALS?

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Real-time visualization of axonal transport in neurons

The normal function of neurons depends on the integrity of microtubule-dependent transport of cellular materials and organelles to/from their cell bodies or axon terminus. In this chapter, we describe the design and implementation of a fluorescence imaging method to visualize axonal transport in neurons directly. We combine a pseudo total internal reflection microscopy, quantum dot fluorescence labeling, microfluidic neuronal culture chamber, and single molecule detection methods to achieve a high spatial and temporal resolution in tracking nerve growth factor transport in dorsal root ganglia neurons.

Impaired activation of celiac ganglion neurons in vivo after damage to their sympathetic nerve terminals

Because damage to sympathetic nerve terminals occurs in a variety of diseases, we tested the hypothesis that nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo. First, we measured the effect of nerve terminal damage produced by the sympathetic nerve terminal toxin 6-hydroxydopamine (6-OHDA) on ganglionic levels of several neurotrophins thought to promote neurotransmission. 6-OHDA-induced nerve terminal damage did not decrease the expression of neurotrophin-4 or brain-derived neurotrophic factor mRNA in the celiac ganglia but did decrease the ganglionic content of both nerve growth factor protein (nadir = -63%) and the mRNA of the alpha-3 subunit of the nicotinic cholinergic receptor (nadir = -49%), a subunit required for neurotransmission. Next, we tested whether this degree of receptor deficiency was sufficient to impair activation of celiac ganglia neurons. Impaired fos mRNA responses to nicotine administration in the celiac ganglia of 6-OHDA-pretreated rats correlated temporally with suppressed expression of functional nicotinic receptors. We verified by Fos protein immunohistochemistry that this ganglionic impairment was specific to principal ganglionic neurons. Last, we tested whether centrally initiated ganglionic neurotransmission is also impaired following nerve terminal damage. The principal neurons in rat celiac ganglia were reflexively activated by 2-deoxy-glucose-induced glucopenia, and the Fos response in the celiac ganglia was markedly inhibited by pretreatment with 6-OHDA. We conclude that sympathetic nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo and that decreased nicotinic receptor production is a likely mediator.

Nerve growth factor

In contrast to all other molecules which are labelled 'growth factor', NGF is not a mitogen. It is a neurotrophic molecule essential for the development and maintenance of function of specific populations of peripheral and possibly also central neurons. The availability of NGF in large quantities from exocrine glands (e.g. male mouse submandibular gland), where NGF does not play a neurotrophic role, has allowed the purification of NGF, the production of specific antibodies, the determination of its amino acid sequence and finally the molecular cloning of NGF leading to the elucidation of its precursor structure and its genomic organization. Comparison of the biological activities and the immunological properties of NGF isolated from different sources demonstrated that the active centre of the molecule has been highly conserved during evolution, whereas other parts of the molecule determining immunological properties have undergone considerable changes. After a survey of the essential biological actions of NGF, this paper concentrates on two actual questions of NGF research, namely the regulation of NGF synthesis in the target tissues of NGF-responsive neurons, and the molecular mechanism(s) of action of NGF on these neurons.

One at a time, live tracking of NGF axonal transport using quantum dots

Retrograde axonal transport of nerve growth factor (NGF) signals is critical for the survival, differentiation, and maintenance of peripheral sympathetic and sensory neurons and basal forebrain cholinergic neurons. However, the mechanisms by which the NGF signal is propagated from the axon terminal to the cell body are yet to be fully elucidated. To gain insight into the mechanisms, we used quantum dot-labeled NGF (QD-NGF) to track the movement of NGF in real time in compartmentalized culture of rat dorsal root ganglion (DRG) neurons. Our studies showed that active transport of NGF within the axons was characterized by rapid, unidirectional movements interrupted by frequent pauses. Almost all movements were retrograde, but short-distance anterograde movements were occasionally observed. Surprisingly, quantitative analysis at the single molecule level demonstrated that the majority of NGF-containing endosomes contained only a single NGF dimer. Electron microscopic analysis of axonal vesicles carrying QD-NGF confirmed this finding. The majority of QD-NGF was found to localize in vesicles 50-150 nm in diameter with a single lumen and no visible intralumenal membranous components. Our findings point to the possibility that a single NGF dimer is sufficient to sustain signaling during retrograde axonal transport to the cell body.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Increased nerve growth factor after rat plantar incision contributes to guarding behavior and heat hyperalgesia

Acutely, nerve growth factor (NGF) exerts profound effects on nociceptive transmission and produces pain and hyperalgesia. In the present study, we sought to determine the tissue levels and role of NGF after a plantar incision. A substantial increase in NGF protein expression occurred in skin 4-h, 1-day and 2-days and 5-days after incision comparing contralateral uninjured skin. Plantar incision did not change NGF levels in the tibial nerve and L4-L6 dorsal root ganglia. The therapeutic effect of a monoclonal antibody against endogenous NGF was evaluated by intraperitoneal administration of a single preoperative dose of anti-NGF. Of three different doses of anti-NGF used, the highest dose 2.8 mg/kg anti-NGF attenuated or almost abolished guarding pain scores at 4-h, 1-day (>80% decrease) and 2-days after incision. This effect is dose dependent in that lower doses (1, 0.1 mg/kg) were effective only at 1-day after incision. Anti-NGF also attenuated heat hyperalgesia at 1-day and 2-days after incision when the highest dose was used. However, treatment by anti-NGF did not affect C-fibers sensitized 1-day after incision in a glabrous skin-tibial nerve in vitro preparation. In conclusion, increased NGF was present in skin after plantar incision. NGF contributes to some incision-induced pain behaviors, guarding and heat hyperalgesia. Anti-NGF did not affect the extent of sensitization of C-fibers observed in vitro.

A hind limb tourniquet induces interleukin-6 expression in a rat dorsal root ganglion

We investigated the mRNA levels of interleukin-6-related genes in a rat dorsal root ganglion after application of a tourniquet to a hind limb in order to identify the molecules that are induced immediately after peripheral nerve injury at the early stage. Induction of interleukin-6 and upregulation of glycoprotein 130 mRNA expressions were observed in the ipsilateral dorsal root ganglion at 4 h after tourniquet application. Interleukin-6 protein was detected in small-sized and medium-sized dorsal root ganglion cells by immunohistochemical analysis. The induction of interleukin-6 expression is likely to play a role in the protection of injured neurons perhaps related to growth of their axons. Glycoprotein 130 might also account for the inhibitory effects following nerve injury.

Long-distance retrograde neurotrophic signaling

The retrograde communication of neurotrophic signals from axon terminals to neuron cell bodies is crucial for neuron survival and plasticity. Several mechanisms have been proposed in the past, but recent evidence strongly supports the hypothesis that the retrograde propagation of self-regenerating signaling organelles, derived from the endocytosis of activated neurotrophin-bound receptor tyrosine kinases, is the primary mechanism responsible for this long-distance communication.

Dynein motors transport activated Trks to promote survival of target-dependent neurons

Mutations that alter dynein function are associated with neurodegenerative diseases, but it is not known why defects in dynein-dependent transport impair neuronal survival. Here we show that dynein function in axons is selectively required for the survival of neurons that depend on target-derived neurotrophins. Stimulation of axon terminals with neurotrophins causes internalization of neurotrophin receptors (Trks). Using real-time imaging of fluorescently tagged Trks, we show that dynein is required for rapid transport of internalized, activated receptors from axon terminals to remote cell bodies. When dynein-based transport is inhibited, neurotrophin stimulation of axon terminals does not support survival. These studies indicate that defects in dynein-based transport reduce trafficking of activated Trks and thereby obstruct the prosurvival effect of target-derived trophic factors, leading to degeneration of target-dependent neurons.

Signaling endosome hypothesis: A cellular mechanism for long distance communication

The kinetics of signaling endosome retrograde transport along axons is analyzed and offered as evidence that such transport is more efficient than diffusion or calcium wave-based signaling systems over even relatively small distances. Evidence is provided to support the signaling endosome hypothesis and to expand the hypothesis to include signaling in many cell types and many cellular dimensions. Finally, a saltatory, regenerating inositol 1,4,5-trisphosphate wave model is offered to reconcile current discrepancies in the literature regarding endosomal-based retrograde signaling.

Beneficial effect of nerve growth factor-7S on peripheral nerve regeneration through inside-out vein grafts: An experimental study

This study investigated the effect of local administration of nerve growth factor-7S (NGF-7S) on the axonal regrowth of mixed peripheral nerves through inside-out vein grafts. Sixty male Wistar rats were randomized into two groups (n = 30). A defect 12 mm long in the right sciatic nerve was created and repaired with an inside-out vein graft from the right jugular vein. NGF-7S (group A) or phosphate-buffered saline (group B; control) was locally administered daily during the first 3 weeks. Walking-track analysis and electrophysiological and histological-morphometric studies were carried out 4, 6, 8, 10, and 12 weeks postoperatively (subgroups a, b, c, d, and e, respectively, n = 6 each). Data analysis showed that 1) the recovery of motor function, as measured by walk pattern analysis and evoked muscle action potential, and 2) the orientation, number, myelin thickness, and diameter of myelinated fibers were better in the NGF-7S than in the control group. These findings present strong evidence of the beneficial effect of NGF-7S on peripheral nerve regeneration through inside-out vein grafts.

Ten years on: mediation of cell death by the common neurotrophin receptor p75(NTR)

The common neurotrophin receptor p75(NTR) remains one of the most enigmatic of the tumor necrosis factor receptor (TNFR) superfamily: on the one hand, it displays a death domain and has been shown to be capable of mediating programmed cell death (PCD) upon ligand binding; on the other hand, its death domain is of type II (unlike that of Fas or TNFR I), and it has also been shown to be capable of mediating cell death in response to the withdrawal of ligand. Thus, p75(NTR) may function as a death receptor-similar to Fas or TNFR I-or a dependence receptor-similar to deleted in colorectal cancer (DCC) or uncoordinated gene-5 homologues 1-3 (UNC5H1-3). Here, we review the data relating to the mediation of PCD by p75(NTR), and suggest that one reasonable model for the apparently paradoxical effects of p75(NTR) is that this receptor functions as a "quality control" in that it is capable of mediating PCD in at least four situations: (1). withdrawal of neurotrophins; (2). exposure to mismatched neurotrophins; (3). exposure to unprocessed neurotrophins; and (4). exposure of inappropriately immature cells to neurotrophins. Results to date suggest that these functions are mediated through different underlying mechanisms, and that their respective signaling pathways are cell type and co-receptor dependent.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

Glial-neurotrophic mechanisms in Down syndrome

Complex interactions and interconnectivity between neurons are hallmarks of normal neuronal differentiation and development. Neurons also interact with other cell types, notably glia, and rely on substances released by glia for their normal function. A deficit in glial response may disturb this critical neuronal-glial-neuronal interaction in Down syndrome (DS), leading to loss of neurons and other defects of development, and contribute to cognitive limitation and early onset of Alzheimer disease. The hypothesis this paper will discuss is that normal neural development involves an activity-dependent release of substances from neurons, and that these substances act upon glia cells which in turn release substances that influence neurons to promote their survival and development. This glial influence affects cortical neurons and also the subcortical cholinergic neurons that project to the cerebral and hippocampal cortices to maintain cortical neuronal excitability and activity. The neuronal activity stimulates glial secretion of sustaining substances, in a reciprocally interactive cycle. Some aspect of this "virtuous cycle" is deficient in Down syndrome. The result is a small but slowly increasing deficit in activity-dependent support by glia cells which produces a gradually increasing abnormality of cortical and subcortical, perhaps especially cholinergic, function.

Retrograde support of neuronal survival without retrograde transport of nerve growth factor

Application of nerve growth factor (NGF) covalently cross-linked to beads increased the phosphorylation of TrkA and Akt, but not of mitogen-activated protein kinase, in cultured rat sympathetic neurons. NGF beads or iodine-125-labeled NGF beads supplied to distal axons resulted in the survival of over 80% of the neurons for 30 hours, with little or no retrograde transport of iodine-125-labeled NGF; whereas application of free iodine-125-labeled NGF (0.5 nanograms per milliliter) produced 20-fold more retrograde transport, but only 29% of the neurons survived. Thus, in contrast to widely accepted theory, a neuronal survival signal can reach the cell bodies unaccompanied by the NGF that initiated it.

Neurotropins and their receptors are expressed in the human fetal ovary

Mammalian ovarian development is characterized by a sequential pattern of mitotic proliferation of oogonia, initiation then arrest of meiosis, and primordial follicle formation. The factors regulating these processes are poorly understood. The neurotropins are survival and differentiation factors in the nervous system, acting via high affinity receptors of the trk protooncogene family and the low affinity p75 nerve growth factor receptor, and have also been described in the rodent ovary, where changes in NT4/TrkB gene expression have been detected at the time of primordial follicle formation. There are no data on neurotropin expression in the normal human ovary. We have investigated the expression and localization of neurotropins and their receptors in the midtrimester human fetal ovary (13-21 wk gestation). Expression of mRNA for neurotropins and their receptors was detected by RT-PCR. Clusters of oogonia were found to be the predominant site of NT4 mRNA expression using in situ hybridization. However, at later gestations granulosa cells of primordial follicles showed increased expression, with lesser expression in the enclosed oocytes. NT4 protein was also localized to the granulosa cells by immunohistochemistry and at earlier developmental stages to epithelioid cells, which were mingled with clusters of oogonia not expressing NT4. TrkB receptor protein was localized by immunohistochemistry to germ cells at all gestations examined. The p75 nerve growth factor receptor protein was exclusively expressed in the ovarian stroma. These data demonstrate the expression of neurotropins and their receptors within the human fetal ovary. Developmental changes in the pattern of expression of NT4 around the time of primordial follicle formation suggest that neurotropins may be involved in signaling between somatic cells and germ cells at this crucial stage of ovarian development.

Short-term estrogen replacement increases beta-preprotachykinin mRNA levels in uninjured dorsal root ganglion neurons, but not in axotomized neurons

Dorsal root ganglion (DRG) neurons that mediate nociception express the high affinity NGF receptor (trkA) gene and the preprotachykinin (PPT) gene. NGF has been shown to regulate both of these DRG neuronal genes. Our laboratory has shown that these genes are also regulated by estrogen. Long-term daily estrogen replacement, in adult ovariectomized (OVX) rats, causes a coordinate decline in trkA and beta-PPT mRNA levels in lumbar DRG neurons, while short-term estrogen replacement increases trkA mRNA levels in uninjured as well as in axotomized lumbar DRG neurons. The purpose of the current study was to test the hypothesis that short-term estrogen replacement increases DRG beta-PPT mRNA levels in lumbar DRG neurons of OVX rats and that the increase is dependent on target-derived NGF. Sciatic nerve transection (SNT) was used to eliminate target-derived NGF in L4 and L5 DRGs in adult OVX rats. Seven days later, one-half of the SNT and one-half of the animals that had received sham sciatic nerve transactions (SHAM) received two daily injections of estradiol benzoate (EB). The remaining rats received two daily injections of vehicle alone. Quantitative in situ hybridization analyses of sections from L4 and L5 DRGs showed that two daily injections of EB significantly increased beta-PPT mRNA levels in DRGs of SHAM animals, but had no effect on beta-PPT mRNA levels in DRGs from SNT animals. These data coupled with our earlier observations of the effect of short-term estrogen replacement on DRG trkA mRNA levels, indicate that the regulation of DRG beta-PPT mRNA levels by estrogen requires target-derived NGF.

Target-derived BDNF (brain-derived neurotrophic factor) is essential for the survival of developing neurons in the isthmo-optic nucleus

Neurons in the peripheral nervous system depend on single neurotrophic factors, whereas those in the brain are thought to utilize many different trophic factors. This study examined whether some neurons in the brain critically depend on a single trophic factor during development. Neurons in the isthmo-optic nucleus (ION) of chick embryos respond to exogenous brain-derived neurotrophic factor (BDNF). Relatively high concentrations of endogenous BDNF were present in the ION of 14-18-day-old chick embryos. ION target cells in the retina were immunolabeled for BDNF but showed surprisingly low levels of BDNF mRNA. These data suggest that ION target cells derive some BDNF from other retinal sources. No BDNF mRNA was detected in the ION itself. ION neurons had a very efficient retrograde transport system for BDNF and exogenous BDNF arrived in the ION intact. When the ION was deprived of endogenous trkB ligands by injection of trkB fusion proteins in the eye, cell death of ION neurons was enhanced, and this effect was mimicked by BDNF-specific blocking antibodies in the eye. TrkB fusion proteins in the retina induced cell death of ION neurons prior to visible effects on ION target cells in the retina. Immunolabel for endogenous BDNF was sparse in pyknotic ION neurons, suggesting that ION neurons with low BDNF content were eliminated by apoptosis. These data show that BDNF is an essential target-derived trophic factor for developing ION neurons and thereby validate the neurotrophic hypothesis for at least one neuronal population in the brain.

Spatial considerations for stimulus-dependent transcription in neurons

Most neurons have elaborate dendrites as well as an axon emanating from the cell body that form synaptic connections with one or many target cells, which may be located a considerable distance from the cell body. Such complex and impressive morphologies allow some types of neurons to integrate inputs from one to many thousands of pre-synaptic partners and to rapidly propagate electrical signals, often over long distances, to post-synaptic target cells. Much slower, non-electrical signals also propagate from dendrites and distal axons to neuronal nuclei that influence survival, growth, and plasticity. The distances between distal dendrites and/or distal axons and cell bodies of neurons can be hundreds of microns to more than one meter. This long-range biochemical signal propagation from distal dendrites and distal axons to neuronal nuclei is entirely unique to neurons. This review is focused on excitatory neurotransmitter signaling from dendritic synapses to neuronal nuclei as well as on retrograde growth factor signaling from distal axons to neuronal nuclei.

Spatially and functionally distinct roles of the PI3-K effector pathway during NGF signaling in sympathetic neurons

NGF is a target-derived growth factor for developing sympathetic neurons. Here, we show that application of NGF exclusively to distal axons of sympathetic neurons leads to an increase in PI3-K signaling in both distal axons and cell bodies. In addition, there is a more critical dependence on PI3-K for survival of neurons supported by NGF acting exclusively on distal axons as compared to neurons supported by NGF acting directly on cell bodies. Interestingly, PI3-K signaling within both cell bodies and distal axons contributes to survival of neurons. The requirement for PI3-K signaling in distal axons for survival may be explained by the finding that inhibition of PI3-K in the distal axons attenuates retrograde signaling. Therefore, a single TrkA effector, PI3-K, has multiple roles within spatially distinct cellular locales during retrograde NGF signaling.

Molecular mechanisms regulating the retrograde axonal transport of neurotrophins

Neurotrophins are released from target tissues following neural innervation and bind to specific receptors situated on the nerve terminal plasma membrane. The neurotrophin-receptor complex undergoes retrograde axonal transport towards the cell soma, where it signals to the nucleus. This process allows neurotrophins to perform their numerous functions, which include the promotion of neuronal survival and the outgrowth of axons towards certain target tissues. The molecular events controlling each of the components of retrograde axonal transport are beginning to become defined. There is good evidence for the participation of phosphatidylinositol 3-kinase, phosphatidylinositol 4-kinase and the actin cytoskeleton in neurotrophin retrograde axonal transport in vivo. It also appears that the retrograde motor protein dynein mediates the retrograde axonal transport in vivo of neurotrophins such as nerve growth factor. This review discusses the role of the neurotrophin receptors in binding and axonal transport, the endocytic processes required for neurotrophin internalization, the targeting and trafficking of neurotrophins, and the propagation of neurotrophin-induced signals along the axon.

Levels of nerve growth factor and neurotrophin-3 are affected differentially by the presence of p75 in sympathetic neurons in vivo

The development and survival of sympathetic neurons is critically dependent on the related neurotrophic factors nerve growth factor (NGF) and neurotrophin-3 (NT3), the actions of which must be executed appropriately despite spatial and temporal overlaps in their activities. The tyrosine receptor kinases, trkA and trkC, are the cognate receptors for NGF and NT3, respectively. The p75 neurotrophin receptor has been implicated in neurotrophin binding and signaling for both NGF and NT3. In this study, the authors used mice that overexpressed NGF (NGF-OE) or NT3 (NT3-OE) in skin and mice that lacked p75 (p75(-/-)) to understand the dynamics of sympathetic neuron response to each neurotrophin and to address the role of p75. NGF and NT3 were measured in sympathetic ganglia and skin (a major target of sympathetic neurons) by using the enzyme-linked immunosorbent assay (ELISA) technique. A three- to four-fold increase in skin NT3 was seen in both NT3-OE and p75(-/-) mice. Moreover, both mouse lines exhibited a three-fold increase in ganglionic NT3. However, the increase in ganglionic NT3 was accompanied by a decrease in ganglionic NGF in p75(-/-) mice but not in NT3-OE mice. This indicated that p75 plays an important role in determining the level of NGF within sympathetic neurons. In NGF-OE mice, the overexpression of NGF was correlated with increased ganglionic NGF and increased ganglionic expression of p75 mRNA. In addition, in NGF-OE mice, ganglionic trkC expression was decreased, as was the amount of NT3 present within sympathetic ganglia. These results indicate that the level of p75 is integral in determining the level of sympathetic NGF and that NGF competes with NT3 by increasing the expression of p75 and decreasing the expression of trkC.

Characterization of an NGF-P-TrkA retrograde-signaling complex and age-dependent regulation of TrkA phosphorylation in sympathetic neurons

Nerve growth factor (NGF) is a target-derived trophic factor for developing sympathetic and cutaneous sensory neurons. NGF promotes growth and survival of neurons via activation of the receptor tyrosine kinase TrkA. We used compartmentalized cultures of sympathetic neurons to address the mechanism of NGF signaling from distal axons and terminals to proximal axons and cell bodies. Our results demonstrate that an NGF-phospho-TrkA (NGF-P-TrkA)-signaling complex forms in distal axons and is retrogradely transported as a complex to cell bodies of sympathetic neurons. Although a minor fraction of both NGF and TrkA is retrogradely transported, a large fraction of the NGF that is retrogradely transported is found complexed with retrogradely transported TrkA. Interestingly, the metabolism of the P-TrkA complex is dramatically different in young, NGF-dependent sympathetic neurons as compared to older, NGF-independent sympathetic neurons. After withdrawal of NGF from distal axons of young neurons, P-TrkA within distal axons, as well as within proximal axons and cell bodies, dephosphorylates rapidly. In contrast, after withdrawal of NGF from distal axons of older neurons, P-TrkA within distal axons dephosphorylates completely, although more slowly than that in young neurons, whereas dephosphorylation of P-TrkA within proximal axons and cell bodies occurs markedly more slowly, with at least one-half of the level of P-TrkA remaining 2 d after NGF withdrawal. Thus, P-TrkA within the cell bodies of young, NGF-dependent sympathetic neurons is derived from distal axons. A more stable P-TrkA complex within cell bodies of mature sympathetic neurons may contribute to the acquisition of NGF independence for survival of mature sympathetic neurons.

Rapid nuclear responses to target-derived neurotrophins require retrograde transport of ligand-receptor complex

Target-derived neurotrophins initiate signals that begin at nerve terminals and cross long distances to reach the cell bodies and regulate gene expression. Neurotrophin receptors, Trks, themselves serve as retrograde signal carriers. However, it is not yet known whether the retrograde propagation of Trk activation reflects movement of Trk receptors from neurites to cell bodies or reflects serial activation of stationary Trk molecules. Here, we show that neurotrophins selectively applied to distal neurites of sensory neurons rapidly induce phosphorylation of the transcription factor cAMP response element-binding protein (CREB) and also cause a slower increase in Fos protein expression. Both nuclear responses require activation of neurotrophin receptors (Trks) at distal nerve endings and retrograde propagation of Trk activation to the nerve cell bodies. Using photobleach and recovery techniques to follow biologically active, green fluorescent protein (GFP)-tagged BDNF receptors (TrkB-GFP) in live cells during retrograde signaling, we show that TrkB-GFP moves rapidly from neurites to the cell bodies. This rapid movement requires ligand binding, Trk kinase activity, and intact axonal microtubules. When they reach the cell bodies, the activated TrkB receptors are in a complex with ligand. Thus, the retrograde propagation of activated TrkB from neurites to cell bodies, although rapid, reflects microtubule-dependent transport of phosphorylated Trk-ligand complexes. Moreover, the relocation of activated Trk receptors from nerve endings to cell bodies is required for nuclear signaling responses. Together, these data support a model of retrograde signaling whereby rapid vesicular transport of ligand-receptor complex from the neurites to the cell bodies mediates the nuclear responses.

Endogenous neurotrophin-3 is retrogradely transported in the rat sciatic nerve

To address the active transport of neurotrophins, nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 in the peripheral nerves, we examined the levels of proteins and messenger RNAs in the sciatic nerve of adult rats following transection, using enzyme immunoassays and reverse transcription polymerase chain reaction method, respectively. Neurotrophin-3 protein increased one day after transection only in the distal segment next to the transection site and returned to the original level two days later. This was considered to reflect accumulation of neurotrophin-3 transported from the periphery toward the neuronal cell bodies, because the neurotrophin-3 messenger RNA level was not changed in any sciatic segments during this experimental period. An increase in brain-derived neurotrophic factor protein was observed simultaneously in both the distal and proximal stumps three days after transection. Brain-derived neurotrophic factor messenger RNA was elevated in the same stumps two days after transection, suggesting that brain-derived neurotrophic factor was produced within the transected stumps. These observations demonstrate that neurotrophin-3, like nerve growth factor, is retrogradely transported in the sciatic nerve but that brain-derived neurotrophic factor is not. This suggests that neurotrophin-3 plays a role in the conveyance of trophic signals from target organs to neurons.

Pituitary adenylate cyclase-activating polypeptide and islet amyloid polypeptide in primary sensory neurons: functional implications from plasticity in expression on nerve injury and inflammation

Primary sensory neurons serve a dual role as afferent neurons, conveying sensory information from the periphery to the central nervous system, and as efferent effectors mediating, e.g., neurogenic inflammation. Neuropeptides are crucial for both these mechanisms in primary sensory neurons. In afferent functions, they act as messengers and modulators in addition to a principal transmitter; by release from peripheral terminals, they induce an efferent response, "neurogenic inflammation," which comprises vasodilatation, plasma extravasation, and recruitment of immune cells. In this article, we introduce two novel members of the sensory neuropeptide family: pituitary adenylate cyclase-activating polypeptide (PACAP) and islet amyloid polypeptide (IAPP). Whereas PACAP, a vasoactive intestinal polypeptide-resembling peptide, predominantly occurs in neuronal elements, IAPP, which is structurally related to calcitonin gene-related peptide, is most widely known as a pancreatic beta-cell peptide; as such, it has been recognized as a constituent of amyloid deposits in type 2 diabetes. In primary sensory neurons, under normal conditions, both peptides are predominantly expressed in small-sized nerve cell bodies, suggesting a role in nociception. On axotomy, the expression of PACAP is rapidly induced, whereas that of IAPP is reduced. Such a regulation of PACAP suggests that it serves a protective role during nerve injury, but that of IAPP may indicate that it is an excitatory messenger under normal conditions. In contrast, in localized adjuvant-induced inflammation, expression of both peptides is rapidly induced. For IAPP, studies in IAPP-deficient mice support the notion that IAPP is a pronociceptive peptide, because these mutant mice display a reduced nociceptive response when challenged with formalin.

The regeneration of peripheral noradrenergic nerves after chemical sympathectomy in diabetic rats: effects of nerve growth factor

The study investigated the role of nerve growth factor (NGF) in the regeneration of noradrenergic nerves of the right atria from control and 8-week diabetic rats, after lesion caused by a single injection of 6-hydroxydopamine (6-OHDA, 100 mg/kg ip). This treatment caused a profound depletion of tissue noradrenaline (NA) of the right atria from both control and diabetic groups, followed by a progressive repletion that was not complete at 49 days. Immunoreactivity for the NGF receptors trkA and p75(NTR) was decreased and increased, respectively, between days 3 and 28 in right atria from diabetic rats and returned to pretreatment levels at day 49. Receptor levels were not significantly altered in controls. In contrast to tissue NA, at day 14 functional responses to electrical nerve stimulation of the right atria had completely returned to the pretreatment state in diabetic rats and were very close to normal in nondiabetic rats. NGF treatment (1 mg/kg, three times/week, for 2 weeks) increased tissue NA only in control rats; the pattern was similar after 6-OHDA. These findings are consistent with the hypothesis that NGF normally plays a role in the regulation of autonomic sympathetic nerves in the adult rat atrium and that mature and uninjured sympathetic neurons remain responsive to NGF. In injured noradrenergic neurons, NGF promotes regeneration in nondiabetic rats. The ability of NGF to promote regeneration of noradrenergic nerves is lost in diabetes and this may relate to the loss of trkA receptor on prejunctional nerve terminals after denervation.

Expression of neurotrophin mRNAs in the dorsal root ganglion after spinal nerve injury

Neurotrophins have specificity toward distinct subpopulations of dorsal root ganglion (DRG) neurons with different neurotrophin receptors. It has been suggested that neurotrophins also play important roles in mature DRG neurons after injury. In the present study, we examined the expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) mRNAs in the DRG after a peripheral nerve injury. The data showed that following a spinal nerve ligation, the level of NGF mRNA increased 4 times over the normal level and was maintained at a high level for a period of 3 weeks. The induction of BDNF mRNA was brief (lasting less than 3 days) and lesser in quantity ( approximately 1. 7 times increase) compared to NGF expression. The expression of NT-3 mRNA was not detected either in normal or nerve injured rats. Results suggest that different neurotrophins play different functional roles in the DRG after spinal nerve injury.

Expression of gp145trkB in the early stage of Schwann cell tube formation

We investigated the expression of tyrosine kinase receptors (trkBs) and their ligand, brain-derived neurotrophic factor (BDNF), in isolated sciatic nerve segments of rat in a silicone chamber model devoid of axonal contact, using an immunohistochemical technique. The receptor gp 95trkB was not detected during the first 3 weeks but was strongly expressed in the nerve stumps on day 21. In contrast, gp 145trkB immunoreactivity was readily detected in the Schwann cells in the nerve stumps between days 1 and 7 after isolation. Immunohistochemical analysis using anti-S-100 antibody demonstrated that Schwann cell tube formation within the silicone chamber had been completed by the third week, which suggested that gp 95trkB began to be produced by Schwann cells after completion of Schwann cell tube formation. Considering the low level of BDNF production during the first week, molecules other than BDNF may be ligands for gp 145trkB in the silicone chamber model devoid of axonal contact.

Identification of macrophage migration inhibitory factor (MIF) in rat peripheral nerves: its possible involvement in nerve regeneration

Macrophage migration inhibitory factor (MIF) is known as a pluripotent immunoregulatory cytokine involved in T-cell activation and inflammatory responses; however, no study on this protein in the peripheral nervous systems has been carried out. We here demonstrated for the first time expression of MIF mRNA and MIF protein in rat sciatic nerves by reverse transcription-polymerase chain reaction, Western blotting, and immunohistochemistry. Immunohistochemical analysis revealed positive staining of MIF, which was largely observed in Schwann cells. Furthermore, we examined MIF mRNA expression in the sciatic nerves by Northern blot analysis in the case of nerve transection. In both proximal and distal segments, the level of MIF mRNA started to increase 12 h after the nerve transection. The level remained high from 24 h up to day 7 after the injury. During the period from days 14 to 21, MIF mRNA sharply decreased to the pre-transection level. In immunohistochemistry, positive staining of MIF was largely observed in axons as well as non-neuronal cells in proximal segments at day 4 after transection. In the distal segments, contrastingly, endoneurial fibroblasts or Schwann cells migrating into neuronal fibers showed positive staining with Wallerian degeneration. Although the precise functions of MIF in the peripheral nerves remain to be elucidated, the present results could represent a major departure from the current state of knowledge, revealing a novel function in the degenerative-regenerative process.

Clenbuterol stimulates neurotrophic support in streptozotocin-diabetic rats

BACKGROUND

The diabetic neuropathies are characterized by axonal degeneration of peripheral nerves. The commonest variant of this condition is distal symmetrical polyneuropathy, in which the principal presenting features can be explained by impaired neurotrophic support of sensory neurones.

AIMS

The aims of these experiments were to determine whether deficits in nerve growth factor (NGF) content and the products of its target genes (substance P and calcitonin-gene related peptide; CGRP) in the sciatic nerve of streptozotocin-induced rats could be prevented using clenbuterol, which has been demonstrated to induce NGF synthesis in vitro.

RESULTS

Diabetes of 8 weeks duration produced a reduction in measurable sciatic nerve NGF-like immunoreactivity (NGF-LI) content of 53% (p < 0.01). This was accompanied by depletion of substance P (of 43%; p < 0.01) and CGRP (of 45%; p < 0.01). Treatment with clenbuterol (30 microg/kg, 100 microg/kg and 300 microg/kg; daily subcutaneous injection) significantly increased the NGF-LI and neuropeptide content of the sciatic nerve, with 30 microg/kg of clenbuterol normalising the levels in diabetic animals. The alterations in steady state sciatic nerve NGF-LI were proportionally related to the amount of NGF-LI delivered to the primary afferent neurones via retrograde transport. Thus, there was a significant reduction in accumulation of NGF-LI distal to a 12-h ligature in diabetic untreated rats (p < 0.01), which was normalized by clenbuterol (30 microg/kg; p < 0.05). Treatment of control or diabetic rats with clenbuterol did not affect the local production of NGF-LI in the sciatic nerve adjacent to and triggered by an occluding ligature. The change in NGF-LI content of neuronal tissue in the clenbuterol treated rats was a reflection of the capture of NGF derived from the peripheral targets of the sciatic nerve. The soleus and EDL muscles showed reductions in NGF-LI in untreated diabetic animals. Clenbuterol significantly (p < 0.05) increased NGF-LI in the soleus muscle.

CONCLUSION

Clenbuterol could be a useful tool in alleviating the deficits in neurotrophin support in diabetes.

A proposed role for nerve growth factor in the etiology of multiple sclerosis

It is proposed that, in addition to genetic factors involved in immune attack on myelin, higher concentrations of nerve growth factor in certain tissues during development determine susceptibility to multiple sclerosis. High early nerve growth factor in some vasculature of spontaneously hypertensive rats increases sympathetic innervation and catecholamine production in these vessels. They become more sensitive than controls to noradrenaline after chemical sympathectomy. Continued exposure to high noradrenaline can result in sympathectomy-like effects, heightening sensitivity to constricting neurotransmitters. Vasoresponses of spontaneously hypertensive rats are impaired with submaximal but not maximal hypoxia. Such a situation in multiple sclerosis patients could result in insufficient blood flow by vasoconstriction until it becomes maximal. Glutamate increase by ischemia and hyperemic release of free radicals could injure neurons, prompting an immune response to myelin proteins in susceptible people. Developmental adaptation to situations requiring lower sympathetic activity might help counteract these effects.

Motoneurotrophins derived from limb buds protect the motoneurons in anterior spinal cord after nerve injury and promote nerve regeneration

The effects of limb bud-derived motoneurotrophins (LBMNTs) as seen in the motoneurons in the anterior spinal cord and sciatic nerve regeneration of adult rats, were evaluated in the present study. A nerve regeneration chamber with a nerve gap of 9 mm was created by suturing the proximal and distal ends of a random sciatic nerve into a silicone tube after removal of a 5 mm piece of nerve in the distal end, The chamber of the experimental group was filled with 34.34 microg LBMNTs and PBS (0.01 mol/ml, pH 7.0),and the control group with PBS only. At 1 day, 4 days, 1 week, 2 weeks, 4 weeks and 6 weeks post surgery, the content of acetylcholine esterase (AchE) and acid phosphatase (ACP) of the anterior spinal cord (injured side) was quantified, and the corresponding motoneuron's ultrastructure and the existant ratio were also examined. Meanwhile, the regenerated nerve from within the silicone tube was examined at 2, 4 and 6 weeks post surgery for histological studies at both the light microscopic and ultrastructural levels. The experimental group showed a smaller decrease of AchE and an increase of ACP, a larger existant ratio of motoneurons, better ultrastructure and a more mature regenerated nerve based on a larger diameter of the regenerated nerve trunk, a greater number of axons and thicker myelin sheaths than the control group. So it was concluded that LBMNTs had a high activity of protecting motoneurons in the anterior spinal cord after nerve injury and promoting nerve regeneration, and it may be a new source of neurotrophic factors (NTFs).

Expression of nerve growth factor in the dorsal root ganglion after peripheral nerve injury

Nerve growth factor (NGF) is believed to play a critical role in altering the phenotypic and functional properties of dorsal root ganglion (DRG) cells after a pathological insult. The present study examined NGF protein levels and NGF immunoreactivity (NGF-IR) in the DRG at multiple time points following peripheral nerve injury. The NGF protein level in the ipsilateral DRG decreased dramatically at 6 h after the injury, but recovered to an almost normal level at 2 days. In accordance with the NGF level, the proportion of NGF-IR neurons also showed a significant decrease at 6 h after the injury, but recovered to the normal level at 3 days. In addition, NGF-IR can also be found in satellite cells at a time point of 3 days after the injury. These data suggest that there is an increase in synthesis of NGF within the DRG after peripheral nerve injury, which contributes to the recovery of NGF levels. The newly synthesized NGF may play important roles in the reactions of DRG neurons to peripheral nerve injury.

Sympathetic axons surround nerve growth factor-immunoreactive trigeminal neurons: observations in mice overexpressing nerve growth factor

It has been postulated that the aberrant projection of sympathetic axons to individual primary sensory neurons may provide the morphological basis for pain-related behaviors in rat models of chronic pain syndrome. Since nerve growth factor (NGF) can elicit the collateral sprouting of noradrenergic sympathetic terminals, it might be predicted that NGF plays a role in mediating the sprouting of sympathetic axons into sensory ganglia. Using a line of transgenic mice overexpressing NGF among glial cells, it was first found that trigeminal ganglia from adult transgenic mice possessed significantly higher levels of NGF protein in comparison to age-matched wild-type mice; as well, detectable levels of NGF mRNA transgene expression were present in both the ganglia and brain stem. Within the trigeminal ganglia, a small proportion of the sensory neuronal population stained immunohistochemically for NGF; a higher percentage of NGF-positive neurons was evident in transgenic mice. New sympathetic axons extended into the trigeminal ganglia of transgenic mice only and formed perineuronal plexuses surrounding only those neurons immunostained for NGF. In addition, such plexuses were accompanied by glial processes from nonmyelinating Schwann cells. From these data, we propose that accumulation of glial-derived NGF by adult sensory neurons and its putative release into the ganglionic environment induce the directional growth of sympathetic axons to the source of NGF, namely, the cell bodies of primary sensory neurons.

An NGF-TrkA-mediated retrograde signal to transcription factor CREB in sympathetic neurons

Nerve growth factor (NGF) is a neurotrophic factor secreted by cells that are the targets of innervation of sympathetic and some sensory neurons. However, the mechanism by which the NGF signal is propagated from the axon terminal to the cell body, which can be more than 1 meter away, to influence biochemical events critical for growth and survival of neurons has remained unclear. An NGF-mediated signal transmitted from the terminals and distal axons of cultured rat sympathetic neurons to their nuclei regulated phosphorylation of the transcription factor CREB (cyclic adenosine monophosphate response element-binding protein). Internalization of NGF and its receptor tyrosine kinase TrkA, and their transport to the cell body, were required for transmission of this signal. The tyrosine kinase activity of TrkA was required to maintain it in an autophosphorylated state upon its arrival in the cell body and for propagation of the signal to CREB within neuronal nuclei. Thus, an NGF-TrkA complex is a messenger that delivers the NGF signal from axon terminals to cell bodies of sympathetic neurons.