Neural crest-derived spinal and cranial sensory neurones are equally sensitive to NGF but differ in their response to tissue extracts

Expression of recombinant rat Neurotrophin-3 in Chinese hamster ovary cells

The CHO cell line stably producing recombinant rat NT-3 was established. The insertion of rNT-3 cDNA into transferred cell gonome was analyzed with Southern blot. The expressed protein was identified by Dot ELISA (enzyme-linked immunosorbent assay) and Western blot. Western blot showed a clear specific band of about 14 ku for NT-3. The mean level of rNT-3 in four NT-3cDNA/CHO cell lines was about 2 100 ng/10(6) cells/48 h determined by EIA. The conditioned-medium (CM) of NT-3cDNA/CHO cells could promote the fiber outgrowth of the dissociated dorsal root ganglion of 8-day-old chick embryos, which shows a dose-response relationship. A half-maximal concentration of the biological activity (EC50) of the recombinant protein was approximately 16.7 ng/mL. The MoAb 3W3 of NT-3 could neutralize the biological activity of the rNT-3.

The changing sensitivity in the life of the nociceptor

Plasticity of the central nervous system has been shown to be an important correlate in the generation of chronic pain. However, there is now also increasing evidence for profound changes of the primary sensory neurons including nociceptors throughout the life of an organism and these changes account for clinically relevant alterations of pain perception. During development sensory neurons require one or more growth factors that rescue neurons during critical periods of programmed cell death and growth factors also play an important role for the development of the appropriate phenotype. Neurotrophin-3 may initially have an effect on proliferation of many subtypes of sensory neurons including cells destined to become nociceptors during early development. During a critical period of late prenatal development nerve growth factor (NGF) signalling through its cognate high affinity receptor trkA has been shown to be the main survival factor during a critical period of prenatal development. Humans deficient of trkA suffer from the rare disorder of congenital analgesia. Postnatally, the subpopulation of non-peptidergic nociceptors lose their ability to respond to NGF, start to express receptor element for and begin to respond to glial cell line-derived neurotrophic factor (GDNF). Both NGF and GDNF have also been shown to regulate the sensitivity of nociceptors to heat and capsaicin in the adult. Changes in the levels of endogenous trophic factors have also been implicated for the generation of ongoing activity and sensitisation to heat that are the hallmark of nociceptors innervating inflamed tissue. Whereas the development of ongoing activity correlates with the intensity of ongoing pain, sensitisation of nociceptors to heat can explain the hyperalgesia to heat that typically accompanies inflammatory lesions in the skin. Dramatic changes of nociceptor phenotype occur following nerve injury. Sensory neurons, including nociceptors, start to express adrenoceptors and become responsive for catecholamines and these changes appear to be responsible for the development of sympathetically maintained pain in some patients.

Spatiotemporal distribution of dying neurons during early mouse development

Apoptosis is a critical cellular event during several stages of neuronal development. Recently, we have shown that biotinylated annexin V detects apoptosis in vivo in various cell lineages of a wide range of species by binding to phosphatidylserines that are exposed at the outer leaflet of the plasma membrane. In the present study, we tested the specificity by which annexin V binds apoptotic neurons, and subsequently investigated developmental cell death in the central and peripheral nervous system of early mouse embryos at both the cellular and histological level, and compared the phagocytic clearance of apoptotic neurons with that of apoptotic mesodermal cells. Our data indicate: (i) that biotinylated annexin V can be used as a sensitive marker that detects apoptotic neurons, including their extensions at an early stage during development; (ii) that apoptosis plays an important part during early morphogenesis of the central nervous system, and during early quantitative matching of brain-derived neurotrophic factor and neurotrophic factor 3 responsive postmitotic large clear neurons in the peripheral ganglia with their projection areas; and (iii) that apoptotic neurons are removed by a process that differs from classical phagocytosis of non-neuronal tissues.

Growth of trigeminal neurites and interactions with corneal cells in embryonic chick organ cultures

The investigation of interactions between growing neurites and target cells during the development of the sensible corneal innervation is of crucial importance for understanding certain corneal diseases which are related to abnormal patterns of innervation. The purpose of the present work was to establish a culture system of cornea and trigeminal neurons and to examine interactions between these tissues. The responses of neurons derived from explanted embryonic chick trigeminal ganglia to co-explanted slices prepared from embryonic cornea were monitored over several days in culture. The growth of trigeminal fibers, but not of neurites derived from control tissues such as trigeminal mesencephalic nucleus or ciliary ganglion, was preferentially directed towards the co-cultured corneal slices. The ingrowth of trigeminal axons into the cornea was followed by formation of elaborate axonal terminal branches. Individual dissociated trigeminal neurons of pseudo-unipolar or bipolar classes developed their typical morphologies in culture. In co-cultures with corneal slices, they reacted to the corneal co-explant by frequently retracting some branches and forming or elongating other ones, which were predominantly directed towards the target tissue. In addition, the presence of a co-explanted trigeminal ganglion increased the rate of growth in the dissociated trigeminal neurons. The effect was not additive when cornea was present. Antibodies against nerve growth factor (NGF) and the low-affinity p75-NGF receptor (LANGFR) revealed that trigeminal ganglion cells support neuritic growth by secreting NGF, whereas corneal cells secrete additional factor(s) which act via the LANGFR.

Purification and characterization of biologically active recombinant human neurotrophin-3 produced by expression of a chimera gene in Chinese hamster ovary cells

In order to obtain high-level expression of recombinant human neurotrophin-3 (NT-3), we constructed several types of expression plasmids and examined several cell lines for expression of the human NT-3 gene. The highest level production of the recombinant protein was attained in Chinese hamster ovary cells transfected with an expression plasmid that contains a chimera gene encoding the human nerve growth factor (NGF) prepro-region and human NT-3 mature-region under control of a murine leukemia virus-derived long terminal repeat (MuLV-LTR). This cell line can produce more than 1 mg recombinant human NT-3/1 conditioned medium. The recombinant protein was purified to apparent homogeneity with a cation exchange column, a gel filtration column and a reversed-phase HPLC column with a recovery of about 30%. The purified NT-3, at a concentration as low as 0.2 ng/ml, induced neurite out-growth in neurons prepared from 8-day-old chick embryonic dorsal root ganglia; however, it showed little neurotrophic effect on rat PC12 pheochromocytoma cells, which are known to be NGF-responding cells. In addition, this protein promoted colony formation by human peripheral blood lymphocytes in soft agar culture.

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.

Development of trophic interactions in the vertebrate peripheral nervous system

During embryogenesis, the neurons of vertebrate sympathetic and sensory ganglia become dependent on neurotrophic factors, derived from their targets, for survival and maintenance of differentiated functions. Many of these interactions are mediated by the neurotrophins NGF, BDNF, and NT3 and the receptor tyrosine kinases encoded by genes of the trk family. Both sympathetic and sensory neurons undergo developmental changes in their responsiveness to NGF, the first neurotrophin to be identified and characterized. Subpopulations of sensory neurons do not require NGF for survival, but respond instead to BDNF or NT3 with enhanced survival. In addition to their classic effects on neuron survival, neurotrophins influence the differentiation and proliferation of neural crest-derived neuronal precursors. In both sympathetic and sensory systems, production of neurotrophins by target cells and expression of neurotrophin receptors by neurons are correlated temporally and spatially with innervation patterns. In vitro, embryonic sympathetic neurons require exposure to environmental cues, such as basic FGF and retinoic acid to acquire neurotrophin-responsiveness; in contrast, embryonic sensory neurons acquire neurotrophin-responsiveness on schedule in the absence of these molecules.

Retinoic acid responsive gene product, midkine, has neurotrophic functions for mouse spinal cord and dorsal root ganglion neurons in culture

Midkine (MK) is the product of a retinoic acid responsive gene and is a member of a new family of heparin-binding growth factors. Neurotrophic effects of MK were examined using cultured spinal cord and dorsal root ganglion (DRG) neurons derived from fetal mouse. MK, which was added to the culture medium at concentrations of 1-100 ng/ml, promoted survival of both types of neurons approximately 5-fold after 7 days in culture. For spinal cord neurons, the increased survival was reflected in an increase of choline acetyltransferase activity. MK also promoted neurite extension in spinal cord (2-fold) and DRG (1.7-fold) neurons. The survival-promoting activity of MK to these neurons was comparable to that of basic fibroblast growth factor (bFGF) and leukemia inhibitory factor (LIF). In spite of its significant effects on fetal neurons, MK was ineffective in sustaining survival of DRG neurons derived from postnatal mice. From these results, we conclude that MK is a neurotrophic factor to embryonic spinal cord and DRG neurons, and we propose that MK plays a significant role in embryogenesis of the nervous system.

Neurotrophin-4 is a target-derived neurotrophic factor for neurons of the trigeminal ganglion

The cellular localization of mRNA for neurotrophin-4 (NT-4), a novel neurotrophic factor, in the developing whisker follicles and skin of the embryonic rat is demonstrated by in situ hybridization. Levels of NT-4 mRNA in the whisker pad decrease between embryonic day 13 (E13) and E20, correlating in time with the onset of naturally occurring neuronal death in the innervating trigeminal ganglion. In addition to NT-4, brain-derived neuotrophic factor (BDNF) mRNA is also shown to be expressed in the rat embryonic whisker follicles although in a different cellular localization, which combined with previous data on the expression of NGF and NT-3 mRNAs, shows that all four neurotrophins are expressed during development of this structure. NT-4 protein is shown to elicit neurite outgrowth from explanted embryonic trigeminal ganglia and to promote neuronal survival of dissociated trigeminal ganglion neurons when cultured during the phase of cell death. NT-4 and NT-3 mainly support different neuronal subpopulations, whereas some NT-4-responsive cells appear to respond also to NGF and BDNF. Analysis of mRNAs for members of the Trk family of neurotrophin receptors in neurons rescued by different neurotrophins demonstrates the presence of distinct neuronal subpopulations that respond to specific combinations of these factors. Based on these results we propose that NT-4, together with the other three neurotrophins, orchestrate the innervation of the different structures of the developing whisker pad by the trigeminal ganglion, acting as target-derived neurotrophic factors for different subpopulations of trigeminal ganglion neurons.

Cellular localization of brain-derived neurotrophic factor and neurotrophin-3 mRNA expression in the early chicken embryo

Degenerate primers from conserved regions in nerve growth factor, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) were used in the polymerase chain reaction to isolate DNA fragments from the chicken BDNF and NT-3 genes. A genomic clone coding for chicken NT-3 was isolated and the structure of the chicken NT-3 mature protein was subsequently deduced from nucleotide sequence analysis of the isolated chicken NT-3 gene. Comparison of the chicken BDNF and NT-3 with the corresponding rat molecules showed that the avian molecules are very similar to their mammalian homologues. Northern blot analyses of messenger RNA (mRNA) from chicken embryos from embryonic day 3.5 (E3.5), E4.5, E8, E12 and E18 showed that expression of both BDNF and NT-3 mRNA peaked at E4.5 and decreased at later stages of development. Both probes revealed two transcripts; larger mRNAs of 4.5 kilobases (kb) for BDNF and 4.0 kb for NT-3 predominated over the smaller transcripts of 1.4 and 1.3 kb, respectively. The cellular localization of BDNF and NT-3 mRNA in the E4 and E6 embryos was studied by in situ hybridization. In the E4 embryo, labelling for BDNF was seen over cells in restricted parts of the epithelium of the otic vesicle. Analysis of adjacent sections for the low-affinity nerve growth factor receptor mRNA showed that regions in the otic vesicle epithelium which labelled for BDNF mRNA also labelled for low-affinity nerve growth factor receptor mRNA. No labelling for NT-3 was detected in the otic vesicle. Labelling for BDNF mRNA was also found over mesenchyme dorsal to the wing bud, in the wing bud and in the splanchnopleural lining of the stomach. Labelling for NT-3 mRNA was found at E4 over the epidermis on the ventral side in the region of the branchial arches. The labelling extended up the maxillary processes to Rathke's pouch. The closely located infundibulum was weakly labelled for NT-3 mRNA. NT-3 mRNA was also detected in the mesenchyme surrounding the oesophagus and lung buds. The regional expression pattern is in agreement with the established role for BDNF and NT-3 as target-derived neurotrophic factors, but the results also suggest that BDNF may be an intrinsic factor important for the development of the inner ear. The results support the emerging view that neurotrophic factors can play a role in early differentiation of both neuronal and non-neuronal tissues.

Neurotrophic factors promote the maturation of developing sensory neurons before they become dependent on these factors for survival

We have studied the early development of chicken embryo sensory neurons in culture before they become dependent on neurotrophic factors for survival. During this period, they undergo a distinct change in morphology:initially they have small, spindle-shaped, phase-dark cell bodies, which become spherical and phase bright and extend long neurites. Although this maturational change occurs in isolated cells grown in chemically defined medium, it is accelerated by brain-derived neurotrophic factor (BDNF) or neurotrophin-3 and is retarded by antisense oligonucleotides that inhibit expression of the common, low affinity neurotrophic factor receptor (gp75NGFR) and by antisense BDNF oligonucleotides. We conclude that neurotrophic factors play a role in the earliest stages of sensory neuron development and suggest that they operate by an autocrine mechanism at this time.

The duration of neurotrophic factor independence in early sensory neurons is matched to the time course of target field innervation

To investigate how the onset of neurotrophic factor dependence in neurons is coordinated with the arrival of their axons in the target field, we have studied the survival of four populations of cranial sensory neurons whose axons reach their common central target field, the hindbrain, at different times. We show that neurons whose axons reach the hindbrain first survive for a short time in culture before responding to brain-derived neurotrophic factor (BDNF). Neurons whose axons reach the hindbrain later survive longer before responding to BDNF. These differences in survival, which arise prior to gangliogenesis, may play a role in coordinating trophic interactions for cranial sensory neurons.

Extension of sympathetic neurites in vitro towards explants of embryonic and neonatal mouse heart and stomach: ontogeny of neuronotrophic factors

In order to establish when target organs first produce neuronotrophic factors, extension of neurites in vitro from sympathetic ganglia (superior cervical and coeliac) of 1-day neonatal mice towards explants of 10-, 11-, 14- and 17-day embryonic and 1-day neonatal atrium and stomach was examined in co-cultures. Longer neurites extended from ganglia towards, than away from, atrial targets at all stages examined, and was most marked towards 17-day embryonic and neonatal explants. Treatment of atrial co-cultures with antiserum to nerve growth factor (NGF) almost totally blocked preferential neurite outgrowth. Directional growth of neurites towards stomach explants in co-cultures was not as pronounced as that towards atrium; extension of neurites was most marked when stomach was provided by 11-, 14- and 17-day embryos. Such outgrowth was only partially blocked by antiserum to NGF, significant preferential extension of neurites towards stomach persisting in the presence of the antiserum. These results indicate that atrium and stomach produce neuronotrophic factors from the earliest ages studied; the evidence indicates that in the case of atrium, NGF predominates but that stomach produces NGF as well as another factor immunologically distinct from NGF. It is of interest that both types of target explanted before they receive sympathetic innervation show evidence of producing NGF in culture.

The response of cultured trigeminal and spinal cord motoneurons to nerve growth factor

Dissociated neurons from the trigeminal (V) region of the metencephalic basal plate or the ventral spinal cord from chick embryos of Day 4 (V basal plate) or Day 5 (spinal cord) were cultured on a laminin substratum either in the presence of nerve growth factor (NGF) or in control medium. Assessment was made of neuronal survival, the amount of neurite elaborated, and the percentage of neurons initiating neurites. The presence of motoneurons was verified by retrograde labeling with the fluorescent dye diI. NGF was found to significantly increase the quantity of neuritic processes produced by the spinal cord dissociates at both 24 and 48 hr in vitro. The percentage of neurons initiating neuritic processes was significantly increased by NGF in the trigeminal population at 48 hr in vitro. Neuronal survival was not enhanced by NGF in either group. Both trigeminal and spinal cord neurons were also found to specifically bind 125I-NGF in culture. These results provide direct evidence for an influence of NGF on process formation of early embryonic motoneurons in culture.

Fetal brain grafts rescue adult retinal ganglion cells from axotomy-induced cell death

After intraorbital transection of the optic nerve of adult rats, 90% of the retinal ganglion cells die within 30 days. Since fetal brain extracts and cocultured fetal target regions support the survival of retinal ganglion cells in vitro (Nurcombe and Bennett: Exp. Brain Res. 44: 249-258, '81; McCaffery et al.: Exp. Brain Res. 48: 377-386, '82; Armson and Bennett: Neurosci. Lett. 38: 181-186, '83) we investigated whether cell death in the adult retina could be prevented by transplanting fetal (E16) thalamus and tectum to the proximal stump of the optic nerve of adult rats that was completely transected 2-3 mm behind the optic disc. Unoperated eyes contained 119,973 (+/- 939, SEM) retinal ganglion cells, estimated from axon counts of the intact optic nerve. Of these, 11,601 (+/- 1,857) remained in control operated eyes at 30 days postoperation while in the eyes of grafted rats, 35,086 (+/- 2,278) retinal ganglion cells were counted. Thus, 23,485 (= 22% of those normally dying after transection of the optic nerve) ganglion cells were rescued by the fetal grafts from cell death normally following axotomy. These results indicate that fetal target regions of retinal ganglion cells contain and/or produce neurotrophic molecules that promote the survival of adult axotomized retinal ganglion cells.

Spinal cord transplants permit the growth of serotonergic axons across the site of neonatal spinal cord transection

These experiments were designed to determine whether transplants of fetal spinal cord tissue into lesioned spinal cord in newborn rats provide a terrain that supports the growth of serotonergic (5-HT) axons across the site of the lesion. Although descending serotonergic axons can regenerate after chemical lesions in adult animals, they show little regrowth after surgical lesions. In newborn animals, 5-HT axons do not regrow after either chemical or mechanical lesions since the axotomized raphe-spinal neurons die. After partial spinal cord lesions made in developing animals, immature axons can take an aberrant route around the site of the lesion to reach normal target areas. Even these robust, late-growing, uninjured axons, however, are unable to grow through the site of the spinal cord lesion. Immunocytochemical labeling was used to determine if descending serotonergic axons grow into fetal spinal cord transplants, and whether these axons cross the transplant to reach spinal cord levels caudal to the lesion. Spinal cord transection at a mid-thoracic spinal cord level on the day of birth resulted in a dramatic decrease in 5-HT immunoreactivity caudal to the lesion by one day postoperative. 5-HT immunoreactivity caudal to the lesion was abolished by 5 days postoperative and did not return after acute or chronic (6 months) survival periods. When a transplant of fetal spinal cord tissue was placed into the lesion site, 5-HT axons were identified throughout the transplant. At spinal cord levels caudal to the transection and transplant, the serotonergic axons were identified in the host spinal cord in both the white and gray matter. This 5-HT innervation was not confined to spinal cord segments adjacent to the lesion site but extended to spinal cord segments as far as lower lumbar levels. The reinnervation of the host spinal cord caudal to the transection was far less than that seen in unlesioned adult rat spinal cord. Horseradish peroxidase (HRP) injected caudal to the transection and transplant, retrogradely labeled neurons within the medullary raphe nuclei. The HRP and 5-HT results both depended on apposition of the transplant with the rostral and caudal stumps of the host spinal cord; without such apposition, labeling was abolished. These results indicate that the presence of a transplant at the site of the neonatal lesion modifies the environment at the lesion site in such a manner as to support the elongation of identified axons across the site of the lesion and into the host cord caudal to the lesion.

Effects of nerve growth factor on sensory neurons in the chick embryo: a stereological study

Chicken embryos on days 6-13 of incubation received injections of purified beta NGF (80 micrograms/day) for 3 or 4 days and were then killed. Sensory ganglia were fixed and taken for embedding and sectioning. A stereological method based on unfolding of cell-diameter frequencies was used to determine the number of neurons of different size in the spinal, trigeminal and nodose ganglia. The total volume of the ganglia was also determined. NGF induced increases in diameter of the neural crest-derived dorsomedial (DM) neurons in spinal and trigeminal ganglia. Injected NGF did not influence ventrolateral (VL) neurons of neural crest origin in the spinal ganglia nor the ventrolateral neurons of placodal origin in the trigeminal ganglion. The volumes of spinal and trigeminal ganglia increased by 50 and 100%, respectively. The volume of the nodose ganglion and the total number and size of the placodal nodose neurons were unaffected by NGF. The results demonstrate a clear difference in the response to NGF in vivo between smaller and larger sensory neurons.

Neural crest-derived proprioceptive neurons express nerve growth factor receptors but are not supported by nerve growth factor in culture

The neural crest-derived, first-order, sensory neurons of the embryonic chick trigeminal mesencephalic nucleus were grown in dissociated, glia-free culture. Whereas brain-derived neurotrophic factor promoted the survival and growth of the majority of these neurons (over 70% after 48 h incubation), nerve growth factor had no effect on their survival. The percentage survival in cultures supplemented with nerve growth factor at concentrations ranging from 0.2 to 625 ng/ml was only 2%, the same percentage survival as in control cultures. Furthermore, nerve growth factor did not change the dose-response of these neurons to brain-derived neurotrophic factor. Although nerve growth factor did not influence the survival of trigeminal mesencephalic neurons in culture, nerve growth factor specifically bound to the great majority of neurons growing in the presence of brain-derived neurotrophic factor. Autoradiographs of cultures incubated with iodinated nerve growth factor showed that the perikarya and processes of neurons were heavily labelled with silver grains. These findings demonstrate the existence of a population of neural crest-derived sensory neurons which express nerve growth factor receptors but are not supported by nerve growth factor in culture.

The survival and growth of embryonic proprioceptive neurons is promoted by a factor present in skeletal muscle

To date, the neurotrophic factor requirements of developing sensory neurons have been studied using heterogeneous populations of neurons that innervate a wide variety of different sensory structures. To ascertain the particular neurotrophic factor requirements of different kinds of sensory neurons and to determine whether these requirements are related to the type of sensory receptors innervated, it is necessary to study homogeneous preparations of functionally distinct sensory neurons. For this reason I have studied the influence of a soluble extract of skeletal muscle on the survival and growth of proprioceptive neurons isolated from the trigeminal mesencephalic nucleus (TMN) of the embryonic chick. Explants of the TMN and dissociated glia-free cultures of TMN neurons were established from chick embryos of 10 to 18 days incubation (E10 to E18). Skeletal muscle extract prepared from E18 chick pectoral muscle and enriched for neurotrophic activity by ammonium sulfate fractionation promoted marked neurite outgrowth from explants and substantial survival in dissociated cultures established during the period of natural neuronal death in the TMN. In these latter cultures 70 to 80% of the neurons survived and grew in the presence of the extract compared with less than 2% in control cultures. At later ages, following the period of natural neuronal death, these effects were less marked. The neurotrophic activity of extracts prepared from muscle of different ages increased steadily from E10 to E20 (the oldest muscle studied). The active factor is heat labile, trypsin sensitive, and non-dialyzable, it is neither functionally nor immunochemically related to NGF and it has negligible neurotrophic effect on the predominantly cutaneous sensory neuron population of the trigeminal ganglion. These findings demonstrate that skeletal muscle contains a neurotrophic factor which supports the survival and growth of proprioceptive neurons and suggest that this factor has some specificity among functionally distinct kinds of sensory neurons.

Quantitative effects of NGF on the growth of embryonic frog axons

Sparse, dissociated cultures of embryonic Xenopus CNS neurons were grown with and without NGF. Under both conditions the same number of neurons survived and extended neurites, and under both conditions the neurites moved at approximately the same overall rates and with the same degree of straightness. On the other hand, neurons in the NGF-supplemented cultures had more neurites and these neurites branched 64% more often. Detailed measurements showed that the axons elongated 44% faster in NGF and that this increase could be ascribed to a selective increase in the stepping rate of axonal elongation. These observations raise the possibility that NGF may selectively modulate the rate of movement of the core cytoskeleton of the axon.

Neural tissue transplants rescue axotomized rubrospinal cells from retrograde death

Rubrospinal tract cells undergo massive retrograde degeneration following spinal cord damage in newborn rats (Prendergast and Stelzner, J. Comp. Neurol. 166:163-172, '76b). In the current study, fetal spinal cord tissue (E12-14) was grafted into midthoracic spinal cord lesions in newborn rats (less than 72 hours old) in order to determine whether such transplants could modify the response of the immature host central nervous system (CNS) to axotomy. These transplants grew, differentiated, and formed extensive areas of apposition with the recipient spinal cords. Counts of red nucleus (RN) neurons indicated a significant loss of RN neurons in animals with lesion alone, but a rescuing of most of these cells if a transplant was placed into the lesion site. In fact, the number of neurons in animals with lesions and transplants was not significantly different from control animals. Horseradish peroxidase injected 10-15 mm caudal to the transplant (at 1-12 months post-transplantation) labeled neurons within the transplant and RN neurons contralateral to the spinal cord lesions and transplant. In animals with spinal cord lesion but no transplant, only the unaxotomized RN was labeled. Thus, spinal cord transplants prevented the massive retrograde cell death of immature axotomized rubrospinal neurons. Some of these rescued neurons projected to the host spinal cord caudal to the transplant.

Placode and neural crest-derived sensory neurons are responsive at early developmental stages to brain-derived neurotrophic factor

The response of embryonic chick nodose ganglion (neural placode-derived) and dorsal root ganglion (neural crest-derived) sensory neurons to the survival and neurite-promoting activity of brain-derived neurotrophic factor (BDNF) was studied in culture. In dissociated, neuron-enriched cultures established from chick embryos between Day 6 (E6) and Day 12 (E12) of development, both nodose ganglion (NG) and dorsal root ganglion (DRG) neurons were responsive on laminin-coated culture dishes to BDNF. In the case of NG, BDNF elicited neurite outgrowth from 40 to 50% of the neurons plated at three embryonic ages; E6, E9, and E12. At the same ages, nerve growth factor (NGF) alone or in combination with BDNF, had little or no effect upon neurite outgrowth from NG neurons. The response of NG neurons to BDNF was dose dependent and was sustainable for at least 7 days in culture. Surprisingly, in view of a previous study carried out using polyornithine as a substrate for neuronal cell attachment, on laminin-coated dishes BDNF also sustained survival and neurite outgrowth from a high percentage (60-70%) of DRG neurons taken from E6 embryos. In marked contrast to NG neurons, the combined effect of saturating levels of BDNF and NGF activity on DRG neurons was greater than the effect of either agent alone at all embryonic ages studied. Under similar culture conditions, BDNF did not elicit survival and neurite outgrowth from paravertebral chain sympathetic neurons or parasympathetic ciliary ganglion neurons. We propose that primary sensory neurons, regardless of their embryological origin, are responsive to a "central-target" (CNS) derived neurotrophic factor--BDNF, while they are differentially responsive to "peripheral-target"-derived growth factors, such as NGF, depending on whether the neurons are of neural crest or placodal origin.

Placodal sensory neurons in culture: nodose ganglion neurons are unresponsive to NGF, lack NGF receptors but are supported by a liver-derived neurotrophic factor

Explant and dissociated neuron-enriched cultures of nodose ganglia (inferior or distal sensory ganglion of the Xth cranial nerve) were established from chick embryos taken between embryonic Day 4 (E4) and Day 16 (E16). The response of each type of culture to nerve growth factor (NGF) was examined over this developmental range. At the earliest ages taken (E4-E6), NGF elicited modest neurite outgrowth from ganglion explants cultured in collagen gel for 24 hr, although the effect of NGF on ganglia taken from E4 chicks was only marginally greater than spontaneous neurite extension from control ganglia of the same developmental age. The response of nodose explants to NGF was maximal at E6-E7, but declined to a negligible level in ganglia taken from E9-E10 or older chick embryos. In dissociated neuron-enriched cultures, nodose ganglion neurons were unresponsive to NGF throughtout the entire developmental age range between E5 and E12. In contrast to the lack of effect of NGF, up to 50% of nodose ganglion neurons survived and produced extensive neurites in dissociated cultures, on either collagen- or polylysine-coated substrates, in the presence of extracts of late embryonic or early posthatched chick liver (E18-P7). Antiserum to mouse NGF did not block the neurotrophic activity of chick (or rat or bovine) liver extracts. Whether cultured with chick liver extract alone or with chick liver extract plus NGF, nodose ganglion neurons taken from E6-E12 chick embryos and maintained in culture for 2 days were devoid of NGF receptors, as assessed by autoradiography of cultures incubated with 125I-NGF. Under similar conditions 70-95% of spinal sensory neurons (dorsal root ganglion--DRG) were heavily labeled. 2+

The effect of nerve growth factor on developing primary sensory neurons of the trigeminal nerve in chick embryos

The response to nerve growth factor (NGF) of two sensory neuron populations of the trigeminal nerve was studied in chick embryos. NGF promoted neuronal survival and cellular hypertrophy in the Gasserian ganglia with minimal effect on the neuron population of the mesencephalic trigeminal nucleus. NGF induced prolific neurite outgrowth from cultured Gasserian ganglia, in contrast, cultured mesencephalic trigeminal neurons remained refractory to NGF treatment. The apparent lack of response of mesencephalic trigeminal neurons to NGF may be explained either by their derivation from placodal material rather than from the neural crest, or their lost sensitivity to NGF due to interaction with the local environment in the central nervous system.