Destruction of sympathetic and sensory neurons in the developing rat by a monoclonal antibody against the nerve growth factor (NGF) receptor

Evaluation of the Toxicity and Neurological Effects of Fulranumab in Adult Cynomolgus Monkeys

Fulranumab, an anti-human nerve growth factor antibody, was evaluated in a series of nonclinical toxicology studies. No treatment effects were observed in adolescent cynomolgus monkeys in standard design, repeat-dose toxicology studies of up to 6 months. Adverse effects on the developing nervous system were observed in offspring of pregnant cynomolgus monkeys treated with fulranumab. Subsequent studies including detailed morphologic investigations of the nervous system did reveal fulranumab-related changes in adult cynomolgus monkeys; this article is focused on those findings. A single dose of ≥1 mg/kg fulranumab administered subcutaneously (SC) caused a decrease in neuron and sympathetic ganglion size (superior cervical ganglion), observed morphologically and stereologically, with a resulting appearance of increased glial cell density. Similar results were observed in repeat-dose (15 to 52 weeks) toxicity studies at ≤50 mg/kg/wk fulranumab SC. These effects recovered after a 3-month treatment-free period. Fulranumab did not cause any neuronal death, necrosis, apoptosis, or any apparent decrease in function of sympathetic neurons/ganglia at any time point examined. A no observed effect level (NOEL) was established at 0.25 mg/kg fulranumab SC every 4 weeks for 28 weeks.

Nicotine is neuroprotective to neonatal neurons of sympathetic ganglion in rat

Sympathetic neurons of SCG are dependent on availability of nerve growth factor (NGF) for their survival. SCG neurons express nicotinic receptors (nAChR) whose expression levels are modulated by nicotine. Nicotine exerts multiple effects on neurons, including neuroprotection, through nAChR binding. Although sympathetic neurons express robust levels of nAChR, a possible neuroprotective role for nicotine in these neurons is not well-understood. Therefore we determined the effect of nicotine exposure on survival of SCG neurons during NGF withdrawal in a well-established cell culture system. NGF was withdrawn in rat neonatal SCG neuron cultures which were then treated with either 10 μM nicotine alone or with nAChR antagonists 0.1 μM α-bungarotoxin (antagonist for α7 subunit bearing nAChR) and 10 μM mecamylamine (non-specific antagonist for ganglionic nAChR) for 48 h. Apoptotic death was determined by TUNEL staining. Cell survival was also determined by MTS assay. Western blot analysis of ERK1/2 was also performed. Our results showed that exposure to 10 μM nicotine significantly reduced apoptotic cell death in SCG neurons resulting from NGF withdrawal as shown by fewer TUNEL positive cells. The MTS assay results also revealed that 10 μM nicotine concentration significantly increased cell survival thus indicating neuroprotective effect of nicotine against cell death resulting from NGF withdrawal. Nicotinic receptor antagonists (bungarotoxin & mecamylamine) attenuated the effect of nicotine's action of neuroprotection. Western blot analysis showed an increased expression of ERK1/2 in nicotine treated cultures suggesting nicotine provided neuroprotection in SCG neurons by increasing the expression of ERK1/2 through nicotinic receptor dependent mechanisms.

Morphologic, stereologic, and morphometric evaluation of the nervous system in young cynomolgus monkeys (Macaca fascicularis) following maternal administration of tanezumab, a monoclonal antibody to nerve growth factor

Tanezumab, an antibody to nerve growth factor, was administered to pregnant cynomolgus monkeys at 0, 0.5, 4, and 30 mg/kg weekly, beginning gestation day (GD) 20 through parturition (∼GD165). Maternal tanezumab administration appeared to increase stillbirths and infant mortality, but no consistent pattern of gross and/or microscopic change was detected to explain the mortality. Offspring exposed in utero were evaluated at 12 months of age using light microscopy (all tissues), stereology (basal forebrain cholinergic and dorsal root ganglia neurons), and morphometry (sural nerve). Light microscopy revealed decreased number of neurons in sympathetic ganglia (superior mesenteric, cervicothoracic, and ganglia in the thoracic sympathetic trunk). Stereologic assessment indicated an overall decrease in dorsal root ganglion (thoracic) volume and number of neurons in animals exposed to tanezumab 4 mg/kg (n = 9) and 30 mg/kg (n = 1). At all tanezumab doses, the sural nerve was small due to decreases in myelinated and unmyelinated axons. Existing axons/myelin sheaths appeared normal when viewed with light and transmission electron microscopy. There was no indication of tanezumab-related, active neuron/nerve fiber degeneration/necrosis in any tissue, indicating decreased sensory/sympathetic neurons and axonal changes were due to hypoplasia or atrophy. These changes in the sensory and sympathetic portions of the peripheral nervous system suggest some degree of developmental neurotoxicity, although what effect, if any, the changes had on normal function and survival was not apparent. Overall, these changes were consistent with published data from rodent studies.

Antagonism of nerve growth factor-TrkA signaling and the relief of pain

Nerve growth factor (NGF) was originally discovered as a neurotrophic factor essential for the survival of sensory and sympathetic neurons during development. However, in the adult NGF has been found to play an important role in nociceptor sensitization after tissue injury. The authors outline mechanisms by which NGF activation of its cognate receptor, tropomyosin-related kinase A receptor, regulates a host of ion channels, receptors, and signaling molecules to enhance acute and chronic pain. The authors also document that peripherally restricted antagonism of NGF-tropomyosin-related kinase A receptor signaling is effective for controlling human pain while appearing to maintain normal nociceptor function. Understanding whether there are any unexpected adverse events and how humans may change their behavior and use of the injured/degenerating tissue after significant pain relief without sedation will be required to fully appreciate the patient populations that may benefit from these therapies targeting NGF.

Proteomic assessment of sympathetic ganglia from adult mice that possess null mutations of ExonIII or ExonIV in the p75 neurotrophin receptor gene

Neurotrophins, such as nerve growth factor (NGF), are capable of binding to the transmembrane p75 neurotrophin receptor (p75NTR), which regulates a variety of cellular responses including apoptosis and axonal elongation. While the development of mutant mouse strains that lack functional p75NTR expression has provided further insight into the importance of this neurotrophin receptor, there remains a paucity of information concerning how the loss of p75NTR expression may alter neural phenotypes. To address this issue, we assessed the proteome of the cervical sympathetic ganglia from two mutant lines of mice, which were compared to the ganglionic proteome of age-matched wild type mice. The ganglionic proteome of mice possessing two mutant alleles of either exonIII or exonIV for the p75NTR gene displayed detectable alterations in levels of Lamin A, tyrosine hydroxylase, and Annexin V, as compared to ganglionic proteome of wild type mice. Decreased expression of the basic isoform of tyrosine hydroxylase may be linked to perturbed NGF signaling in the absence of p75NTR in mutant mice. Stereological measurement showed significant increases in the number of sympathetic neurons in both lines of p75NTR-deficient mice, relative to wild type mice. This enhanced survival of sympathetic neurons coincides with shifts toward the more basic isoforms of Annexin V in mutant mice. This study, in addition to providing the first comparative proteomic assessment of sympathetic ganglia, sheds new light onto the phenotypic changes that occur as a consequence of a loss of p75NTR expression in adult mice.

Nerve growth factor survival signaling in cultured hippocampal neurons is mediated through TrkA and requires the common neurotrophin receptor P75

The role of the common neurotrophin receptor p75 (p75NTR) in neuronal survival and cell death remains controversial. On the one hand, p75NTR provides a positive modulatory influence on nerve growth factor (NGF) signaling through the high affinity neurotrophin receptor TrkA, and hence increases NGF survival signaling. However, p75NTR may also signal independently of TrkA, causing cell death or cell survival, depending on the cell type and stage of development. Here we demonstrate that TrkA is expressed in primary cultures of hippocampal neurons and is activated by NGF within 10 min of exposure. In primary hippocampal cultures neuroprotection by NGF against glutamate toxicity was mediated by NF-kappaB and accompanied by an increased expression of neuroprotective NF-kappaB target genes Bcl-2 and Bcl-xl. In mouse hippocampal cells lacking p75NTR (p75NTR-/-) activation of TrkA by NGF was not detectable. Moreover, neuroprotection by NGF against glutamate toxicity was abolished in p75NTR-/- neurons, and the expression of bcl-2 and bcl-xl was markedly reduced as compared to wildtype cells. NGF increased TrkA phosphorylation in hippocampal neurons and provided protection that required phosphoinositol-3-phosphate (PI3)-kinase activity and Akt phosphorylation, whereas the mitogen-activated protein kinases (MAPK), extracellular-regulated kinases (Erk) 1/2, were not involved. P75NTR signaling independent of TrkA, such as increased neutral sphingomyelinase (NSMase) activity causing enhanced levels of ceramide, were not detected after exposure of hippocampal neurons to NGF. Interestingly, inhibition of sphingosine-kinase blocked the neuroprotective effect of NGF, suggesting that sphingosine-1-phosphate was also involved in NGF-mediated survival in our cultured hippocampal neurons. Overall, our results indicate an essential role for p75NTR in supporting NGF-triggered TrkA signaling pathways mediating neuronal survival in hippocampal neurons.

Neurotrophin-induced rapid enhancement of membrane potential oscillations in mesencephalic trigeminal neurons

We have proposed that neurotrophins, in addition to their trophic actions, act as neuromodulators in the adult central nervous system. As a first step to test this hypothesis, we examined in the adult rat slice preparation whether nerve growth factor and neurotrophin-3 are capable of altering the excitability of neurons of the mesencencephalic trigeminal nucleus. In contrast to vehicle pressure microapplication, which did not evoke changes in the electrophysiological properties of these neurons, neurotrophin application produced a significant increase in amplitude of the membrane potential oscillatory activity that is observed in these cells and a significant decrease in their threshold current. The latency of these effects ranged from 2 to 80 s and the duration ranged from 2 to 11 min. Neurotrophin-3 induced a decrease in input resistance and resting membrane potential in 58% of the cells; nerve growth factor induced a decrease in input resistance and resting membrane potential in 35% of the neurons. The spike configuration and action potential afterhyperpolarization potential remained unchanged following neurotrophin application. Tetrodotoxin blocked the membrane potential oscillatory activity of trigeminal mesencephalic neurons. Neurotrophin-induced effects were not blocked by the tyrosine kinase inhibitor K-252a, whereas IgG-192, an antibody directed to the neurotrophin low-affinity receptor, enhanced excitability, as did neurotrophins. These results demonstrate that neurotrophins are capable of producing a rapid increase in the excitability of trigeminal mesencephalic neurons and suggest that their effects may be mediated by low-affinity neurotrophin receptors.

Overexpression of neurotrophin receptor p75 contributes to the excitotoxin-induced cholinergic neuronal death in rat basal forebrain

Both excitotoxicity and altered trophic factor support have been implicated in the pathogenesis of Alzheimer's disease. To determine whether stimulation of p75, the low-affinity receptor for nerve growth factor, contributes to the excitotoxin-induced apoptotic death of cholinergic neurons, we examined the effect of unilateral kainic acid (KA; PBS vehicle, 1.25, 2.5 and 5.0 nmol) administration into rat basal forebrain on neuronal loss and p75 expression. KA (2. 5 nmol) destroyed 43% of Nissl-stained neurons and 70% of choline acetyltransferase (ChAT)-positive neurons 5 days after injection. Agarose gel electrophoresis revealed that KA (2.5 nmol) induced local internucleosomal DNA fragmentation after 6-48 h. Immunohistochemical analysis further showed that KA (2.5 nmol) augmented p75 immunoreactivity at a time when terminal transferase-mediated deoxyuridine trophosphate (d-UTP)-digoxigenin nick end labeling (TUNEL)-positive nuclei were increased. Many fragmented nuclei were co-labeled with ChAT antibody. The chronic administration of anti-rat p75 or the protein synthesis inhibitor, cycloheximide, but not anti-human p75, substantially reduced the KA-induced destruction of cholinergic neurons and the induction of internucleosomal DNA fragmentation. Anti-rat p75, but not cycloheximide, also reversed the spatial memory impairment produced by KA. These findings suggest that overexpression of p75 contributes to the excitotoxin-induced death of rat basal forebrain cholinergic neurons by an apoptotic-like mechanism.

Regulation of NGFI-A (Egr-1) gene expression by the POU domain transcription factor Brn-3a

NGFI-A is an immediate early gene (IEG) that is transcriptionally induced by nerve growth factor (NGF) in PC12 cells and has been implicated in a number of cellular responses. Studies have shown that elements within the first 106 base pairs of the NGFI-A promoter contribute to its induction by NGF in PC12 cells. One element, within the serum response element (SRE) bridge region, bears strong homology to a motif previously identified in promoters regulated by the Brn-3a POU domain transcription factor. We report here that Brn-3a activates the NGFI-A promoter in neurons (both primary and cell lines). Analysis revealed that this response requires sequences between positions -49 and -106. Whilst DNA-protein interaction studies failed to identify a site bound directly by Brn-3a, the data presented here suggest that Brn-3a may cooperate in the regulation of NGFI-A gene expression in neurons, possibly during the developmental switch between neurotrophin dependency that occurs during neurogenesis.

p75 neurotrophin receptor as a modulator of survival and death decisions

The p75 receptor is the founding member of the TNF receptor superfamily. Members in this receptor family share a common cysteine motif repeated two to six times that serves as the ligand binding domain. In addition, several members contain a cytoplasmic region designated the death domain. The neurotrophins NGF, BDNF, NT-3, and NT-4 each bind to the p75 receptor and also more selectively to members of the Trk family of receptor tyrosine kinases. Although the biological functions of p75 have been elusive, recent experimental evidence supports an involvement of this receptor in apoptosis. This presents a counter-intuitive function for neurotrophins, which are normally required for the survival of neurons during development. The life-and-death decisions by neurotrophins appear to be governed by the level of expression and signaling activities of the p75 and Trk tyrosine kinase receptors and their downstream effector molecules. The generation of the correct number of cells in the nervous system is a highly controlled and coordinated process that is the consequence of cell proliferation and cell death decisions. The appropriate number of neuronal and glial cells formed during development guarantees the establishment of proper innervation and functional synaptic connections. One common mechanism to account for the number of viable cells is the ability to form ligand-receptor complexes that promote cell survival under conditions of limiting concentrations of trophic factors. Another diametrically opposed mechanism is to produce ligand-receptor interactions that can activate programmed cell death directly.

Analysis of low affinity neurotrophin receptor (p75) expression in glia of the CNS-PNS transition zone following dorsal root transection

Peripheral nerves exit from the brain through the transition zone where oligodendroyctes and astrocytes of the central nervous system (CNS) and Schwann cells of the peripheral nervous system (PNS) are in close proximity. In this zone, the same axons are ensheathed by oligo-dendrocytes and Schwann cells. We examined, in adult rats, the expression of the low affinity neurotrophin receptor (p75) in central glia and Schwann cells in response to lesion of lumbar dorsal roots. In normal rats, scattered p75-immunoreactive glial cells were present in the CNS-PNS transition zone. A marked increase of p75 immunoreactivity occurred in Schwann cells near the transition zone from 4 days to at least 3 weeks after dorsal root transection. In contrast, the p75 immunoreactivity remained unchanged in central glia. The differential expression of p75 in the two types of glial cells was sharply demarcated at the CNS-PNS border. Our results are consistent with earlier observations that axon damage is less potent in its ability to induce central glial expression of p75, and further, suggests a possible mechanism for the failure of regenerating dorsal root axons growing into the spinal cord.

Endogenous nerve growth factor is required for regulation of the low affinity neurotrophin receptor (p75) in sympathetic but not sensory ganglia

During development, many sympathetic and sensory neurons are dependent on nerve growth factor (NGF) for survival. The low affinity neurotrophin receptor (p75), expressed in these neurons, is regulated by exogenous NGF in vitro and in vivo. However, whether p75 expression in vivo is under the control of endogenous NGF has not been determined. The role of NGF in regulating the expression of p75 in sympathetic and sensory nerves was investigated in Sprague-Dawley rats treated with an antiserum specific for NGF. P75 was differentially regulated. P75 immunoreactivity (-ir) within sympathetic neurons in the superior cervical ganglia (SCG) was reduced after 2 days, and disappeared after 5 days, of treatment with the NGF antiserum. In contrast, a significant increase in p75-ir was detected in nerve bundles within and close to the SCG from 3 to 14 days after treatment. A similar pattern of p75 expression was observed in the stellate and coeliac ganglia. In contrast, p75 expression in nerve terminals of the mesenteric arteries and irides was reduced. However, in the same animals the expression of p75 was not significantly affected by the treatment in dorsal root, trigeminal or nodose ganglia, salivary gland or small intestine. In contrast to p75, the NGF high affinity receptor trkA was little affected in sympathetic neurons by depletion of endogenous NGF for 2 weeks. These results indicate that endogenous NGF is required in sympathetic ganglia for the expression of p75 but not trkA in neurons, but for the down-regulation of p75 in glia. In contrast, endogenous NGF is not essential for the regulation of p75 in neurons or glia within sensory ganglia.

The concept of uptake and retrograde transport of neurotrophic molecules during development: history and present status

In the present review honoring Hans Thoenen's contributions to the concept of uptake and retrograde transport of trophic molecules, I have attempted to identify the major historical pathways that had to converge before this concept could be accepted as a fundamental principle in neurobiology. Some of the critical events in this history which are discussed here include: neuron-target interactions, bidirectional trophic signals, axoplasmic transport, receptor-mediated endocytosis, transneuronal trophic signals, the discovery of NGF, the retrograde transport of NGF, and the production of NGF by target tissues. Only when all of these diverse pieces of the puzzle were in place was the concept finally confirmed as being the mechanism that mediates the many phenomena attributed to the regulation and maintenance of neurons by their targets.

Retrograde transport of neurotrophins from the eye to the brain in chick embryos: roles of the p75NTR and trkB receptors

The receptors involved in retrograde transport of neurotrophins from the retina to the isthmo-optic nucleus (ION) of chick embryos were characterized using antibodies to the p75 neurotrophin receptor and trkB receptors. Survival of neurons in the ION has been shown previously to be regulated by target-derived trophic factors with survival promoted or inhibited by ocular injection of brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF), respectively. In the present paper, we show that during the period of target dependence, these neurons express trkB and p75 neurotrophin receptor but not trkA or trkC mRNAs. We also show that BDNF and NT-3 were transported efficiently at low doses, whereas NGF was transported significantly only at higher doses. The transport of BDNF and NT-3 was reduced by high concentrations of NGF or by antibodies to either trkB or the p75 neurotrophin receptor. Thus both receptors help mediate retrograde transport of these neurotrophins. Ocular injection of the comparatively specific trk inhibitor K252a did not reduce transport of exogenous BDNF, but did induce significant neuronal death in the ION, which could not be prevented by co-injection of BDNF. Thus, transport of BDNF alone does not generate a trophic signal at the cell body when axonal trkB is inactivated. In summary, our results indicate that both p75 neurotrophin and trkB receptors can mediate internalization and retrograde transport of BDNF, but activation of trkB seems to be essential for the survival-promoting actions of this neurotrophin.

Autocrine regulation of proliferation of cerebellar granule neurons by nerve growth factor

Premigratory cerebellar granule neurons, which highly express nerve growth factor (NGF), low (gp75NGFR) and high (gp140trkA) affinity NGF receptors, were used as a physiological model to investigate the effects of NGF on neuronal replication. Studies in vivo and on cultures showed that NGF stimulates DNA synthesis, mitotic activity and related cell acquisition by initiating the entry of cells into the S phase and regulating their time in the G1 and S phases. The NGF-induced effects were blocked in vivo and in vitro by both monoclonal anti-blocked in vivo and in vitro by both monoclonal anti-NGF and anti-gp75NGFR antibodies. These results clearly demonstrate that NGF is essential for the crucial first step of cerebellar ontogenesis and support the idea that low affinity receptors are involved in the biological response, possibly by interacting with gp140trkA. By comparison with a number of well known mitogens, the high affinity form could be the main transducer of the mitogenic signal pathway. The early developing cerebellum appears therefore to be the first autocrine (and/or paracrine) model of NGF action on neurogenesis in the CNS.

Positive and negative effects of neurotrophins on the isthmo-optic nucleus in chick embryos

The survival of neurons in the developing isthmo-optic nucleus (ION) is believed to depend on the retrograde transport of trophic molecules from the target, the contralateral retina. We now show that ION neurons transport nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) retrogradely and that BDNF and NT-3 support the survival of ION neurons in vivo and promote neurite outgrowth in vitro. Surprisingly, NGF enhanced normal developmental cell death in vivo in a dose-dependent way. These findings show that increased levels of NGF can have adverse effects on differentiated neurons. The negative effect of NGF could be mimicked by intraocular injection of antibodies that block binding of neurotrophins to the 75 kd neurotrophin receptor (p75). These data implicate a role for the p75 receptor in NGF's neurotoxicity and indicate that this receptor is involved in the mechanism by which ION neurons respond to BDNF and NT-3 in the target.

Age-related effects of nerve growth factor on the morphology of embryonic sensory neurons in vitro

Studies of neonatal and adult mammals have shown that neuronal morphology is regulated in part by the availability of target-derived neurotrophic factor. To test whether the same is true for embryonic neurons, which are dependent on target-derived neurotrophic factors for survival, we grew neural crest-derived sensory neurons from the trigeminal ganglion of avian embryos of different ages in vitro in different concentrations of nerve growth factor (NGF) and measured the number of branch points and total length of the resulting arborizations. Although the size and complexity of arborizations increased with embryonic age up to embryonic day (E)14, neuronal morphology for embryos younger than E14 was unaffected by the concentration of NGF in the culture medium. However, beginning at E14, the stage at which trigeminal neurons start to lose their absolute requirement for NGF for survival, the neurons had significantly more branch points and larger arborizations in higher concentrations of NGF. Thus, it appears that the extent of neurite outgrowth in young embryos is independent of neurotrophic factor concentration; each neuron that receives enough neurotrophic factor to survive elaborates approximately the same size arbor. As trigeminal neurons mature and become less dependent on neurotrophic factor for survival, they acquire the ability to respond to neurotrophic factor with increased neurite growth and branching, as in neonates and adults.

Lesion-induced expression of low-affinity nerve growth factor receptor-immunoreactive protein in Purkinje cells of the adult rat

Normal adult cerebellar Purkinje cells in the rat rarely express low-affinity nerve growth factor receptor immunoreactivity. However, intense anti-low-affinity nerve growth factor receptor immunostaining was observed as early as one day after a lesion of the cerebellar cortex. Low-affinity nerve growth factor receptor immunoreactivity was confined to a selected group of Purkinje cells, the number of which reached a maximum at three days postlesion, and, in some neurons, persisted up to 10 days after damage. The intensity of Purkinje cell immunolabeling decayed abruptly with distance from the lesion site. Reactive Purkinje cells exhibited deposition of immunoreaction product in the cell soma, dendrites and axons. Characteristically, most Purkinje cell axons exhibiting intense low-affinity nerve growth factor receptor immunoreactivity had beaded, varicose morphology. Varicose fibres with the appearance of recurrent collaterals of Purkinje cell axons were also low-affinity nerve growth factor receptor-positive. Our results indicate that adult rat Purkinje cells increase low-affinity nerve growth factor receptor-immunoreactive protein in response to injury, suggesting that, in the cerebellum, low-affinity nerve growth factor receptor or low-affinity nerve growth factor receptor-like molecules may be involved in regulating neuronal plasticity during adulthood.

Dorsal root ganglion neurons expressing trk are selectively sensitive to NGF deprivation in utero

In utero immune deprivation of the neurotrophic molecule nerve growth factor (NGF) results in the death of most, but not all, mammalian dorsal root ganglion (DRG) neurons. The recent identification of trk, trkB, and trkC as the putative high affinity receptors for NGF, brain-derived neurotrophic factor, and neurotrophin-3, respectively, has allowed an examination of whether their expression by DRG neurons correlates with differential sensitivity to immune deprivation of NGF. In situ hybridization demonstrates that virtually all neurons expressing trk are lost during in utero NGF deprivation. Most, if not all, neurons expressing trkB and trkC survive this treatment. In contrast, the low affinity NGF receptor, p75NGFR, is expressed in both NGF deprivation-resistant and -sensitive neurons. These experiments show that DRG neurons expressing trk require NGF for survival. Furthermore, at least some of the DRG neurons that do not require NGF express the high affinity receptor for another neurotrophin. Finally, these experiments provide evidence that trk, and not p75NGFR, is the primary effector of NGF action in vivo.

Selective dependence of mammalian dorsal root ganglion neurons on nerve growth factor during embryonic development

We have investigated the NGF dependence of dorsal root ganglion (DRG) neurons in mammals using a paradigm of multiple in utero injections of a high titer anti-NGF antiserum. We have determined the specificity of our antiserum in relation to other members of the NGF neurotrophin family and found no cross-reactivity with brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). To identify various classes of DRG neurons, we have stained their characteristic central projections with Dil. We show here that the NGF dependence of DRG neurons is strikingly selective. Although a majority of DRG neurons are lost after NGF deprivation during embryonic life, these are almost exclusively small diameter neurons that project to laminae I and II of the dorsal horn and presumably subserve nociception and thermoreception. Larger neurons that project to more ventral spinal laminae and subserve other sensory modalities do not require NGF for survival. These NGF-independent DRG neurons likely require one of the more recently identified neurotrophins, BDNF or NT-3.

Changes in nerve growth factor receptor-like immunoreactivity in the spinal cord after ventral funiculus lesion in adult cats

Spinal motoneurons have a capability to regenerate CNS-type axons after intramedullary lesions in the adult cat. Regrowing axons have been traced through CNS-type scar tissue in the ventral funiculus of the spinal cord and into adjacent ventral root fascicles. This scar tissue, which appears to support and sustain regenerating axons, has been shown to have a persistent defect in the blood-brain barrier. It has been suggested that the blood-brain barrier may play a vital role in CNS regeneration by regulating the access of blood-borne trophic factors to the lesion area. In the present study, the binding of antibodies to the human nerve growth factor receptor in the cat spinal cord was examined with immunohistochemical methods 2 days to 8 weeks after a ventral funiculus lesion. The results show that, while no neurons in the ventral horn of the control material contained nerve growth factor receptor-like immunoreactivity as revealed by fluorescence microscopy, affected motoneurons expressed nerve growth factor receptor after ventral funiculus lesion. Nerve growth factor receptor-like immunoreactivity associated to both capillaries and interstitium was present in the scar tissue. Electron microscopic examination of sections labelled with the immunogold-silver method showed that perivascular nerve growth factor receptor-like immunoreactivity was located exclusively to non-pericytic perivascular cells. These cells were abundant in the expanded capillary perivascular spaces adjacent to the traumatic lesion. Similar cells, with or without relation to blood vessels, were observed in the scar tissue and in the pia mater. In a separate set of specimens it was observed that a ventral funiculus lesion combined with ventral root avulsion, which removes denervated PNS tissue, resulted in an expression of nerve growth factor receptor-like immunoreactivity which was similar to the one observed after ventral funiculus lesion only. The results of the present study show that affected motoneurons and cells in the scar tissue express nerve growth factor receptor after ventral funiculus lesion which implies that neurotrophic factors related to nerve growth factor may be of importance for the regenerative response.

Carnosine, nerve growth factor receptor and tyrosine hydroxylase expression during the ontogeny of the rat olfactory system

The localizations of carnosine, nerve growth factor (NGF) receptor and tyrosine hydroxylase (TH) were studied in the embryonic and postnatal rat olfactory bulb and epithelium by means of single- and double-immunostaining methods. Tyrosine hydroxylase ontogeny was also evaluated at the mRNA level by in situ hybridization. All these molecules were expressed in the olfactory bulb but with different developmental patterns and cellular localization: carnosine immunoreactivity is seen from embryonic day 17 in primary olfactory neurons scattered in the nasal cavity and in fibres projecting from them to the olfactory bulb. Nerve growth factor-receptor immunoreactivity associated with small glial-like cells is visible in some glomeruli starting from the second day of postnatal life. At postnatal day 10 NGF-receptor immunoreactivity is extended to all glomeruli. Periglomerular neurons expressing TH mRNA and protein are present prenatally and their number sharply increases during the early postnatal development. Double-staining methods show that TH and NGF-receptor immunoreactivity do not overlap in cell bodies and processes. In addition, NGF-receptor immunoreactivity is not colocalized with carnosine. These findings definitely exclude NGF-receptor expression in periglomerular and primary olfactory neurons, suggesting that at least part of NGF-receptor expression in the olfactory bulb is associated with glial cells. In addition, they provide the first immunohistochemical data on carnosine ontogeny and confirm at the mRNA level previous studies on the ontogeny of TH protein.

Nerve growth factor stimulates neurite regeneration but not survival of adult auditory neurons in vitro

Injury to either the peripheral or central nervous system results in the accumulation of growth factors at the wound site. Some of these growth factors have been shown to participate in the neural repair process. Adult auditory neurons grown in dissociated spiral ganglion cell cultures are injured (i.e. bilateral axotomy) as a result of the initial preparation of these cultures. Therefore, cell cultures of dissociated spiral ganglia provide a model for the study of repair processes of adult auditory neurons (e.g. effects of exogenous growth factors on the process of neuritogenesis by injured neurons). Auditory neurons do not survive in these dissociated ganglion cell cultures when only exogenous NGF is added to the defined culture medium. Previous work has identified substrate bound basic fibroblast growth factor (bFGF) as a survival factor for adult auditory neurons in vitro. Auditory neurons cultured on substrate bound bFGF also do not show increased survival in response to the addition of increasing concentrations of nerve growth factor (NGF) to the defined medium. This is in sharp contrast to the pronounced neurite outgrowth-promoting effects (concentration dependent) observed when exogenous NGF is added to adult auditory neurons cultured on substrate bound bFGF. We propose that several neuronotrophic factors (e.g. TGFB1, bFGF, NGF and other neurotrophins) are active in the spiral ganglions' response to injury. Several of these growth factors (i.e. bFGF, NGF) act in cooperation to promote the regeneration or repair of severed or traumatized neuritic processes.

Immunohistochemical localization of nerve growth factor (NGF) and NGF receptor (NGF-R) in the developing first molar tooth of the rat

Nerve growth factor (NGF) is a well established target-derived trophic factor supporting sympathetic and sensory innervation in the peripheral tissues as well as cholinergic innervation in the brain. Despite its name, NGF may have broader biological functions early in development in a wide range of non-neuronal differentiating cells. The many effects of NGF are directly dependent on initial binding of NGF to specific plasma membrane receptors on target cells. Here we use immunohistochemical methods to show that NGF and its receptor (NGF-R) are localized in a variety of embryonic epithelial and mesenchymal cells in the rat developing molar tooth. Dental cells known to play important roles in morphogenesis and inductive tissue interactions show NGF-like reactivity. Thus, labelling is seen in epithelial preameloblasts and mesenchymal odontoblasts. We also show a transient expression of NGF-R in restricted parts of the dental epithelium (inner dental epithelium) and dental mesenchyme differentiating cells (post-mitotic, polarizing odontoblasts). The expression patterns of NGF are different to those of NGF-R during embryogenesis and this is illustrated in detail in the developing tooth. The histochemical findings reported here support the notion that NGF may have multiple roles during morphogenetic and cytodifferentiation events in the tooth.

Nerve growth factor synthesis in vascular smooth muscle

Details of the interdependent, trophic relation between smooth muscle and its neural innervation are not well known despite suggestions that neural influences may contribute significantly to hypertensive and other cardiovascular disease. Vascular smooth muscle is a major target of innervation by neurons of the sympathetic nervous system. Sympathetic neurons depend on a constant supply of the potent neurotrophic peptide nerve growth factor. Nerve growth factor regulates an impressive list of neuronal and perhaps muscle properties, yet its source in vessels and the determinants of its synthesis are not known. We have taken advantage of the cytoarchitecture of the aorta to demonstrate that vascular smooth muscle cells synthesize nerve growth factor. The survival of cultured sympathetic neurons is supported in a nerve growth factor-dependent manner by co-culture with pure rat aortic vascular smooth muscle cells. Furthermore, pure smooth muscle cell cultures contain nerve growth factor-specific messenger RNA. Levels of messenger nucleic acid coding for nerve growth factor in smooth muscle are regulated by contractile agonists (angiotensin II, arginine vasopressin) and the adrenergic agonist phenylephrine. This suggests a link between muscle activity and growth factor production. Secretion of nerve growth factor protein by vascular smooth muscle was measured using a sensitive two-site immunoassay. Secretion is highest during muscle growth. Secretion is elevated by angiotensin II and arginine vasopressin but slightly inhibited by phenylephrine. These results suggest that cultured vascular smooth muscle can serve as a useful model in which to study the cellular regulation of trophic factor synthesis in health and disease.

Role of the neurotrophic factors BDNF and NGF in the commitment of pluripotent neural crest cells

Since trophic factors are increasingly recognized as playing a role in some decision-making steps during development, the influence of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) on the commitment of pluripotent neural crest cells was investigated by in vitro clonal analysis. BDNF caused an increase of up to 21-fold in the number of sensory neuron precursors per colony without a corresponding increase in the total number of cells. By contrast, BDNF treatment caused an equivalent decrease in the number of undifferentiated cells per colony. The data suggest that BDNF, but not NGF, directs pluripotent neural crest cells to differentiate along the primary sensory neuron lineage.

Nerve growth factor enhances neurite arborization of adult sensory neurons; a study in single-cell culture

Nerve growth factor (NGF) is a well-established trophic factor of sympathetic and sensory neurons during development. NGF is, however, little known to be required for the maintenance or regulation of differentiated phenotypes of matured peripheral neurons. Since trophic factors, including NGF, are currently known to be secreted by non-neuronal cells, like Schwann cells and fibroblasts, a highly pure-neuron culture is required to assess the direct action of trophic factors on neurons. We have developed a single-neuron culture from neonatal and adult rat dorsal root ganglia in serum-free conditions, and estimated the primary effect of NGF on the morphological geometry of sensory neurons. We found that NGF promoted the neurite length of neonatal sensory neurons, rather than promoting arborization (branching of neurites), while in adult matured neurons NGF significantly enhanced neurite arborizations, rather than the maximal neurite extension, distance from the cell soma to the maximum margin of the territory of neurite extension. Total neurite length, the summed length of all neurites per neuron was significantly increased by NGF in both neonatal and adult neurons. NGF also increased the size of neuronal soma independent of neuronal maturation. Neonatal sensory neurons tended to die in 1 week despite the presence of NGF. In contrast, some adult sensory neurons were alive for more than 2 weeks in the absence of NGF. These results indicate that NGF more than simply accelerates a pre-existing developmental program in the matured stage, and that the promotion of neurite arborization by NGF in adult sensory neurons suggests that NGF may have some role in peripheral nerve regeneration via promotion of axonal sprouting.

Distribution of nerve growth factor receptor-like immunoreactivity in the adult rat central nervous system. Effect of colchicine and correlation with the cholinergic system--I. Forebrain

Nerve growth factor receptor, as recognized by the monoclonal antibody 192-IgG, was localized to multiple regions of the adult rat forebrain. Immunoreactive cell bodies and fibers were seen in both sensory and motor regions which are known to contain cholinergic and non-cholinergic neurons. Specifically, nerve growth factor receptor immunoreactivity was present in cells lining the olfactory ventricle, rostral portion of the lateral ventricle, in basal forebrain nuclei, caudate putamen, globus pallidus, zona incerta and hypothalamus. Immunoreactive cells which were situated subpially along the olfactory ventricle and anterior portions of the lateral ventricle, and in the arcuate nucleus resembled neuroglia but could not definitively identified at the light microscopic level. Animals pretreated with intracerebroventricular colchicine displayed significantly increased nerve growth factor receptor immunoreactivity in all previously positive neurons and particularly in the medial preoptic area and ventral premammillary nucleus of the hypothalamus. In such animals, receptor immunoreactivity also appeared in previously non-immunoreactive cells of the hippocampal CA3 region and polymorph layer of the dentate gyrus as well as in the mitral cell layer of the olfactory bulb. Nerve growth factor receptor-immunoreactive fibers and varicosities were seen in the olfactory bulb, piriform cortex, neocortex, amygdala, hippocampus, thalamus, olivary pretectal nucleus and hypothalamus. In most regions, such fiber-like immunoreactive structures likely represented axon terminals, although in some areas, neuroglial or extracellular localizations could not be excluded. In this context, diffuse, non-fibrillar receptor immunoreactivity occurred in the lateral habenular nucleus and medial terminal nucleus of the accessory optic tract. Furthermore, intense nerve growth factor receptor immunoreactivity occurred along certain regions of the pial surface on the ventral surface of the brain. The distribution of nerve growth factor receptor-immunoreactive cell bodies and fibers in multiple sensory and motor nuclei suggests wide-spread influences of nerve growth factor throughout the adult rat forebrain. There is a high degree of overlap with regions containing choline acetyltransferase immunoreactivity. However, significant disparities exist suggesting that certain nerve growth factor receptor-containing non-cholinergic neurons of the rat forebrain may also be affected by nerve growth factor.

An interaction between thyroid hormone and nerve growth factor promotes the development of hippocampus, olfactory bulbs and cerebellum: a comparative biochemical study of normal and hypothyroid rats

The effects of treatment with L-thyroxine (T4;20 ng/g body weight, given subcutaneously on days 1, 3, 5, 7 and 9), 2.5 S nerve growth factor (NGF; 2 ng/mg brain weight, given intracerebroventricularly on days 1, 3, 5, 7 and 9), monoclonal anti-NGF (2 ng/mg wet weight, given intracerebroventricularly on days 1, 3, 5, 7 and 9), and monoclonal anti-NGF receptor (192 IgG; 2 ng/mg wet weight, injected daily from day 1 to day 9) antibodies, separately or together, were studied on the biochemical development of hippocampal formation, olfactory bulbs and cerebellum in 10-day-old and 15-day-old normal and hypothyroid rats. The results provide the following information: (1) CNS structures other than the basal forebrain are sensitive to NGF during early development. (2) Both normal and hypothyroid rats are more sensitive to NGF deprivation than NGF supplementation. (3) The effects of anti-NGF antibodies in normal rats are similar to those induced by anti-NGFr antibodies. (4) NGF alone had little or no effect, but interacts with T4 in promoting cell maturation, especially in hypothyroid rats. (5) Hypothyroid rats are more sensitive to T4 and to T4 plus NGF than are normal ones. (6) The synergistic action of both trophic factors, but not that of T4, tend to disappear at long term in hypothyroid rats. (7) The differential sensitivity of the brain areas to T4, NGF, or both trophic factors correlates with their cell acquisition rate, especially in hypothyroid rats. (8) T4 and NGF together act more markedly (but not exclusively) on the cholinergic structures in both normal and hypothyroid rats. (9) RNA appears to be very sensitive to NGF, especially in hypothyroid rats. In close correlation with preliminary morphological observations, the results clearly demonstrate that an interaction between T4 and NGF regulates the ontogeny of a number of neuronal structures in CNS independently of their neurotransmitter phenotype, but with a regional specificity. The possibilities of accounting for this interaction, in particular the major role of thyroxine, are discussed.