Embryonic sympathoblasts transiently express TrkB in vivo and proliferate in response to brain-derived neurotrophic factor in vitro
© Straub et al; licensee BioMed Central Ltd. 2007
Received: 03 October 2006
Accepted: 19 February 2007
Published: 19 February 2007
Nerve growth factor and neurotrophin-3 are involved in the development of sympathetic neurons; however, whether brain derived neurotrophic factor also plays a role is not known. The purpose of this study was to determine whether BDNF and its receptor, TrkB, are expressed during the development of paravertebral sympathetic ganglia in vivo and to determine the effect of BDNF in vitro.
As neural crest cells coalesce to form sympathetic ganglia, TrkB-positive cells are seen in both chicken and mouse embryos. In chicken embryos, TrkB-expressing cells first appear at Hamburger-Hamilton Stage (St) 27 and they co-express HNK-1, confirming that they are migrating neural crest cells. The TrkB-positive cells lack neural markers at this stage; however, they migrate with other neurally differentiating cells that are TrkA and TrkC-positive. By St. 29/30, TrkB-positive cells begin to express the neural specific markers Hu C/D and Islet-1; eventually, all TrkB positive cells commence neural differentiation. By St. 34, TrkB and TrkC staining are lost. BDNF transcript expression parallels that of TrkB. In the mouse, TrkB-positive cells surround newly formed sympathetic ganglia and a small number of TrkB positive cells that co-express tyrosine hydroxylase are seen within ganglia between E13.5-15. In cell culture, many cells from St. 29–30 chicken lumbar sympathetic ganglia express neural markers and are dividing, indicating that they are sympathoblasts. Sympathoblasts and neurons require both nerve growth factor and neurotrophin-3 for survival. BDNF increases the number of cells expressing neural markers in culture by increasing number of cells that incorporate bromodeoxyuridine. In contrast, most TrkB-positive sympathetic cells in vivo are not actively proliferating between E6–E8.
Developing paravertebral sympathetic ganglia in avian and murine embryos contain a subpopulation of sympathoblasts that transiently express TrkB and ultimately commence neuronal differentiation. These TrkB expressing sympathoblasts are not actively dividing in vivo; yet, when placed in vitro, will divide in response to BDNF. This suggests that the availability of BDNF in vivo fails to reach a threshold necessary to induce proliferation. We suggest that excess TrkB stimulation of sympathoblasts in vivo may lead to the genesis of neuroblastoma.
Neural crest cells destined to become paravertebral sympathetic neurons proliferate and differentiate during migration and gangliogenesis. In chicken embryos, migrating neural crest cells express catecholamines at Hamburger/Hamilton Stage (St.) 19, and these cells form the primary sympathetic chain dorsolateral to the aorta at St. 22 (E3.5) . Between St. 23 (E4) and St. 28 (E6), these cells disperse and undergo a secondary migration to form the paravertebral sympathetic chain that resides ventral to the spinal cord and dorsal root ganglion . After ganglia coalesce, sympathoblasts express markers of neuronal differentiation, such as Q211 and tyrosine hydroxylase (TH), at a time when they also incorporate [3H]-thymidine . Time lapse photography has shown that cultured E15.5–E16.5 sympathetic neurons from rat embryos extend axons while they divide [3–5]. Although proliferation appears to be an important process to expand the sympathetic neuron population during differentiation, the mechanisms that guide sympathoblast proliferation have not been identified.
The development of sympathetic neurons is guided by neurotrophins. Neurotrophin-3 (NT-3) binds to its receptor, TrkC, to promote the survival of cultured sympathoblasts from early lumbar paravertebral ganglia . Nerve growth factor (NGF) signals through its receptor, TrkA, to promote the survival of sympathetic neurons upon target innervation . There are severe sympathetic defects in the superior cervical ganglion of individual NT-3 and NGF knockout mice [8–10]. Furthermore, there is no additional cell death in the superior cervical ganglion of NT-3 and NGF double knockout mouse embryos, suggesting that all of the neurons are dependent on both neurotrophins for survival . There is also an increase in sympathetic neuron cell death in TrkA knockout mice . However, in TrkB and BDNF knockout mice, there is no apparent phenotype in the superior cervical ganglion, and there is little evidence that TrkB or BDNF is expressed in sympathetic ganglia. Thus, it is generally thought that TrkB and BDNF have little or no roles in guiding the development of sympathetic neurons.
In addition to their developmental functions, neurotrophin receptors regulate cell behavior in neuroblastoma, a tumor found in sympathetic ganglia and adrenal medulla. Tumors that express TrkA often spontaneously regress, while those that express TrkB and its ligand, brain-derived neurotrophic factor (BDNF), grow aggressively, are invasive, and fail to respond to chemotherapeutic agents . The presence of TrkA in neuroblastoma tumors is consistent with its expression in developing sympathetic neurons, and suggests that regressive neuroblastoma tumors arise from early sympathetic neurons that express TrkA. The function of TrkB in early sympathetic development is unknown, which makes understanding the etiology of aggressive neuroblastoma tumors difficult. Based on its function in neuroblastoma tumors, we hypothesize that BDNF and TrkB expression in differentiating sympathoblasts is responsible for expanding the neuronal population through proliferation.
We sought to determine whether BDNF and TrkB are involved in sympathetic development. We report that during early embryonic development, TrkB is expressed in a subset of differentiating sympathoblasts in both avian and murine embryos. We also find that BDNF promotes the proliferation of TrkB-positive sympathoblasts in cell culture. However, the majority of TrkB positive cells in vivo fail to take up bromodeoxyuridine (BrdU) over a 24 hr period, suggesting that endogenous BDNF concentrations do not reach a threshold necessary to stimulate proliferation of sympathoblasts. Shortly after all of the TrkB positive cells commence neuronal differentiation, TrkB immunoreactivity is lost. These results suggest that prolonged expression and/or activation of TrkB signaling at these early stages may be an early event triggering the formation of neuroblastoma.
TrkB is expressed during migration of neural crest cells to sympathetic ganglia
We first determined whether TrkB is expressed in neural crest-derived cells in the region ventral to the spinal cord and dorsal root ganglia where sympathetic ganglia coalesce between Hamburger/Hamilton Stages (St.) 25–28/29. To identify cells that have commenced neuronal differentiation, transverse sections of the lumbar spinal column region were stained with antibodies against Hu C/D , a neuronal-specific RNA-binding protein, or Islet-1, a transcription factor found in sympathetic neurons . We found that Hu C/D and Islet-1 are expressed in the same cells both in vivo and in vitro throughout sympathetic development. In experiments done between St. 25 and 28, Islet-1 staining appeared weaker than Hu C/D staining, and thus we used Hu C/D to identify differentiating neurons at these stages. At later stages, Islet-1 was used to facilitate the identification of neurons because of the nuclear location of its immunoreactivity.
Developmental regulation of TrkA, TrkB, TrkC, and BDNF expression
NT3 and NGF promote survival of differentiating sympathetic neurons in culture
p75 NTR and Hu C/D Expression in Acutely Isolated Cells from St. 29/30 (E6.5) Sympathetic Ganglia
Experiment 1 (mean values; n = 3)
Experiment 2 (mean values; n = 3)
p75NTR (% of total)
1974.3 +/- 368 (91%)
2128.7 +/- 490 (80%)
Hu C/D (% of total)
719.7 +/- 137 (33%)
751 +/- 47 (28%)
2177.7 +/- 381
2637 +/- 701
We then determined how many of the acutely isolated cells were proliferating by incubating them for 12 hrs in BrdU-containing medium. For these experiments, we identified differentiating neurons with the transcription factor Islet-1 because this marker labels nuclei, thus it co localizes with any BrdU that has been incorporated into the DNA, allowing us to determine whether the cell had undergone S-phase of the cell cycle. After 12 hrs in BrdU, 59% of Islet-1-positive nuclei stain for BrdU immunoreactivity. Thus, cultures of St. 29/30 sympathetic ganglia contain many cells that proliferate while exhibiting markers of neuronal differentiation, confirming previous observations . We call these dividing neuronal precursors sympathoblasts. The remaining non-BrdU incorporating, Islet-1 positive cells are likely to be post mitotic neurons.
BDNF promotes proliferation of TrkB-positive sympathetic neurons in culture
Effects of BDNF on BrdU Incorporation in TrkB-Expressing Cells from St. 29/30 (E6.5) Sympathetic Cultures
219 ± 34
845* ± 95
438 ± 55
1111* ± 153
258 ± 36
187 ± 14
962 ± 42
900 ± 109
258 ± 54
673* ± 64
508 ± 77
947* ± 126
211 ± 14
180 ± 48
892 ± 75
834 ± 105
We report that the neurotrophin receptor TrkB is expressed in a subset of embryonic sympathoblasts during the early development of lumbar paravertebral sympathetic ganglia in chicken and mouse embryos. In the chicken, TrkB expression is transient, and completely lost by St 34 (E8). Since BDNF induces the proliferation of sympathoblasts in cell culture, yet in vivo there is little proliferation observed in TrkB-positive cells in nascent ganglia, we propose that if TrkB activation becomes unregulated by excess BDNF or constitutive phosphorylation of TrkB , this transient population of TrkB-positive sympathoblasts may trigger the genesis of neuroblastoma, a childhood tumor found in the paravertebral chain and adrenal medulla.
The two populations of sympathoblasts that we observe support previous findings of heterogeneity among developing sympathetic neurons and neural crest cells. Early sympathetic ganglia contain at least two subpopulations: early differentiating neurons that lack TrkB expression and express TrkA and TrkC, and late differentiating sympathoblasts that express TrkB. Explant cultures of sympathetic ganglia from E16 chick embryos give rise to two neuronal populations: one that remains close to the explant, and one that migrates away from the explant . In addition, early neuronal subpopulations have been observed in cultures of neural crest cells from St. 13/14 quail embryos as evidenced by the expression of neuronal cell type-specific gangliosides . Perhaps these different subpopulations will ultimately give rise to the two neurochemically distinct populations found in lumbar sympathetic ganglia: the noradrenergic, NPY-containing neurons that innervate internal organs and enteric ganglia and the cholinergic, VIP-containing neurons that innervate vasculature in the hind limbs.
The effects of BDNF and TrkB deletion and over expression have been studied on superior cervical ganglion and preganglionic neurons in thoracic segments of the spinal column, but not on paravertebral sympathetic neurons. In the superior cervical ganglion, an increase in the number of neurons of BDNF null mice is likely due to apoptosis induced by BDNF via p75NTR . In contrast, the responses of paravertebral sympathetic neurons to BDNF are complex and subtype dependent. Over expression of BDNF leads to an increase in the number of noradrenergic fibers innervating the erector pilli muscles of hair follicles, while noradrenergic fibers innervating blood vessels were unaffected . If our results indicating that BDNF promotes proliferation of TrkB-positive sympathoblasts in the chicken embryo can be extrapolated to the subset of TrkB-positive sympathoblasts in murine ganglia, then these TrkB-positive cells may be neurons destined to innervate the erector pilli. In other studies, TrkB null mice showed no changes in morphology or cell number in superior cervical ganglia  or in the intermediolateral column ; but this may not be predictive of a phenotype in the lumbar paravertebral chain. It is thus possible that BDNF/TrkB signaling could play a specific role in other regions of the paravertebral sympathetic chain, such as the lumbar region. However, if TrkB-positive cells are not normally actively proliferating in vivo, then it would not be surprising that the development of the paravertebral sympathetic chain is not disrupted in TrkB or BDNF null mice. It may be more informative to examine mice that over express BDNF on a promoter that targets expression to embryonic lumbar ganglia. Unfortunately, such mice do not exist.
Our findings that the St. 29/30 (E6.5) sympathoblasts are dependent on both NT-3 and NGF for survival in culture are consistent with previous work on mouse sympathoblasts from the superior cervical ganglion . In these studies, NT-3 and NGF deletion separately led to a decrease in the number of sympathetic neurons at E17.5 compared to control. Deletion of both NT-3 and NGF together did not enhance cell death. In contrast, cultured rat superior cervical ganglion sympathetic neurons respond to NT-3 at E14.5 and then to NGF at E19.5, although time points in between were not analyzed .
In addition to promoting survival, NT-3, NGF, and BDNF also induce proliferation of various neuronal precursors at different stages of development. NT-3 can promote the incorporation of [3H]-thymidine into cultured quail neural crest cells from the trunk region [21, 22], Later in rat sympathetic development, NT-3 supports survival of neurons, but does not promote proliferation , which is consistent with our results. NGF promotes an increase in BrdU incorporation from 25% to 35% in the DRG cervical segment 2 in the chick embryo . In chicken embryos that are treated with NGF in ovo at St. 18 and 21, there is an increase in BrdU uptake after formation of the primary sympathetic chain at St. 23 . Since NGF does not appear to affect proliferation of St. 29/30 (E6.5) chick sympathoblasts, NGF may only promote proliferation in primary, but not secondary chain sympathoblasts. Motor neuron progenitors in the ventral neural tube from the chick embryo express TrkB and when ventral neural tube explants are treated with BDNF, there is an increase in the number of motor neurons produced and BrdU incorporation . BDNF also promotes the proliferation of cultured neuroblastoma cells . Taken together, these results are consistent with our findings that NT-3 and NGF do not promote proliferation of St. 29/30 (E6.5) sympathoblasts, and support the assertion that BDNF promotes proliferation of TrkB-positive sympathoblasts in culture.
Our observations suggest a transient function of TrkB during early sympathetic development in supporting proliferation of this early subpopulation of sympathoblasts. However, the in vivo labeling suggests that only a minority (10–20%) of this population is dividing during the window that TrkB is expressed. In light of the very strong proliferative effect produced in cell culture, these TrkB expressing cells could respond more strongly if endogenous BDNF rises to higher levels, or if the mechanism that down regulates TrkB expression becomes nonfunctional. Such events could trigger an early proliferative event that leads to a cascade of changes that initiates transformation of cells to neuroblastoma. Thus, these early TrkB expressing cells help solve the puzzle as to why TrkB is expressed in aggressive and invasive forms of neuroblastoma, particularly because BDNF induces cultured neuroblastoma cells to become more proliferative, invasive, angiogenic, and resistant to chemotherapeutic reagents than untreated cultures . Future studies will determine whether constitutive expression of BDNF and TrkB in the chick embryo sustains proliferation of differentiating sympathoblasts.
We have identified a time point during development when differentiating lumbar sympathetic neurons transiently express TrkB and proliferate in response to high concentrations of BDNF in culture. These studies suggest that elevated BDNF expression above basal levels and signaling through TrkB may be a mechanism that contributes to the onset of neuroblastoma. A further understanding of the two populations of sympathetic neurons and the fate of the TrkB-positive cells will provide additional insight into the development of paravertebral sympathetic ganglia and the genesis of neuroblastoma.
Preparation of tissue for immunohistochemistry
The lumbar spinal column and surrounding tissues were dissected from chicken embryos at the indicated stages and placed in Zamboni's fixative (4% (w/v) paraformaldehyde, 15% (v/v) picric acid in 0.1 M sodium phosphate buffer, pH 7.4) for two hours at room temperature. Mouse embryos at 13–15 days post-coitus were collected according to an IACUC-approved protocol to Dr. L. Sherman at the Oregon Health and Science University. The mouse embryos were immersion-fixed in Zamboni's fixative overnight at 4 degrees C then washed with phosphate buffered saline (PBS; 130 mM NaCl, 20 mM sodium phosphate buffer, pH 7.4). Fixed tissues were equilibrated in 30% sucrose in 1× phosphate-buffered saline (PBS). Fixed mouse embryos were shipped to Vermont in sucrose. Transverse 30 μM sections of the spinal columns were cut at on a Microm HM cryostat (knife temperature: 16°C; object temperature: 23°C) and collected on Superfrost Plus slides (Fisher). Sections were dried at room temperature, washed in 1× PBS and incubated overnight in blocking buffer (1× PBS consisting of 10% (v/v) heat-inactivated horse serum (Invitrogen/Gibco), 0.5% Triton X-100 (Sigma), and 0.1% sodium azide (Fisher)).
Sections were incubated with primary antibodies overnight at 4°C, followed by incubation with secondary antibodies for 2 hours at room temperature. Primary antibodies used were: rabbit anti-p75 (1:1500, generous gift from Louis Reichardt, UCSF ), mouse IgG2b anti-Hu C/D, (1:250, Molecular Probes); mouse IgG1 anti-Islet-1, (1:10, Developmental Studies Hybridoma Bank); rabbit anti-chicken TrkA (1:500); rabbit anti-chicken TrkB (1:500); rabbit anti-chicken TrkC (1:500) (all Trk antibodies were generous gifts of Dr. Louis Reichardt, UCSF [26–28]); mouse anti-HNK-1 (1:50, Developmental Studies Hybridoma Bank); mouse IgG2a anti-tyrosine hydroxylase (1:10, Developmental Studies Hybridoma Bank), sheep anti-BrdU (1:100, Biodesign International), rabbit anti-tyrosine hydroxylase (1:100, Chemicon), and goat anti-TrkB (1:1000, R&D Systems). Immunofluorescence was imaged using a Nikon C1 confocal mounted on a Nikon Eclipse E800 microscope with a 10× Plan Apo (NA 0.785) air objective or a 60× Plan Apo (NA 1.4) oil objective lens, E7-C1 software, and UV, Argon, and He/Ne lasers exciting at 408, 488, and 543 nm and emitting at 404 500–530, and 555–615 nm, respectively. A Nikon Eclipse E800 microscope in the nearby COBRE Molecular/Cellular Core Facility was used for counting immunofluorescent cells at 200× using epifluorescence optics.
RNA Extraction/cDNA synthesis
Sympathetic ganglia were removed from chick embryos and RNA was isolated using TriReagent (Molecular Research Center), an acidified guanidinium with phenol extraction method . RNA was transcribed to cDNA using oligo-dT with Superscript II Reverse Transcriptase (Invitrogen) at 42°C for 1 hour.
Relative RNA levels were determined using quantitative real-time PCR with an ABI 7500 Fast Real Time PCR System. TaqMan probes were used to quantify the progression of the PCR reaction and reactions were normalized using the constitutively expressed gene chick ribosomal binding protein s17 (CHRPS). The sequences were used for primer/probes sets: for BDNF: forward: 5'-AGCCCAGTGAGGAAAACAAG-3', reverse: 5'-ACTCCTCGAGCAGAAAGAGC-3', probe: 5'-[6-FAM]-TACACATCCCGAGTCATGCTGAGCA-[BHQ]-3'; for CHRPS (chick ribosomal binding protein S-17): 5'AACGACTTCCACACCAACAA3', reverse: 5'CTTCATCAGGTGGGTGACAT3', probe: 5'-[6-FAM]-CGCCATCATCCCCAGCAAGA [BHQ]-3'. Primers and probes were synthesized by Operon Technologies, Inc (Alameda, CA). The primers for BDNF were validated against primers for CHRPS according to an Applied BioSystems protocol by serially diluting the target cDNA 1:10, determining the cycle threshold (Ct) for each reaction, and plotting the Ct versus log concentration. Slopes of the resulting lines were calculated and primers were accepted if their Ct slopes were between -3.2 and -3.4 (a perfect efficiency of 1.0 yields a slope of -3.3). To analyze the data, the delta Ct method of relative quantification was used, where the Ct of Chrps was subtracted from the Ct of the gene of interest (Delta Ct) and the arbitrary units of mRNA were expressed as 10000/2^(Delta Ct).
Sympathetic neurons were cultured as previously described  with a few modifications. Sympathetic ganglia were removed from the lumbar region of the paravertebral chain of St. 29/30 (E6.5) chick embryos and placed in Modified Puck's solution with glucose (MPG). The cells were dissociated by incubation of sympathetic ganglia with 0.1% trypsin in MPG at 37°C for 10 minutes followed by triturating with a fire polished 9" Pasteur pipette. Cells were then resuspended in Dulbecco's Modified Eagle Medium (DMEM) consisting of 10% horse serum, 2% fetal calf serum, and 10 mg/ml penicillin/streptomycin. For neurotrophin studies, the culture medium was supplemented with 25 ng/ml NT-3 (R & D Systems) and 1 μg/ml 7S NGF (Alomone Labs) upon plating, and 50 ng/ml, 100 ng/ml, or 200 ng/ml BDNF (R & D Systems) once the cells adhered to the wells. Cells were plated on poly-D-lysine/laminin coated wells or cover slips (Fisher) as previously described .
Quantification of neurons and sympathoblasts using phase microscopy
Embryonic sympathoblasts and neurons are small, phase bright cells with neurites. The total number of cells with neurites the length of two cell bodies were counted in 10 non-overlapping fields of view evenly spaced in a grid-like pattern across the bottom of a well from a 24 well plate at 200× using a Nikon Eclipse TE200 microscope.
For in vitro studies, approximately 2 hours after plating cells from St. 29/30 (E6.5) sympathetic ganglia, cells were labeled with 10 μM bromodeoxyuridine (BrdU, Sigma) for 12 hours at 37°C. Following this labeling period, cells were incubated in complete medium without BrdU for an additional 10 hrs. Cells were then fixed in Zamboni's fixative for 30 min at room temperature and rinsed with 1× PBS. For in vivo studies, 25 μg BrdU was injected into the amnion of chick embryos at St. 27. The cells and sections were denatured with 2 N HCl at 37°C for 1 hr, and were then neutralized with 0.1 M borate buffer, pH 8.5, for 10 min at room temperature. Immunochemistry was performed as described above.
brain-derived neurotrophic factor
Dulbecco's Modified Eagle's Medium
dorsal root ganglion
Modified Puck's solution with glucose
nerve growth factor
superior cervical ganglion
standard error of the mean
Rabbit anti-chicken TrkA, rabbit anti-chicken TrkB, rabbit anti-chicken TrkC, and rabbit anti-chicken p75 were generous gifts of Dr. Louis Reichardt, University of California at San Francisco. Mouse embryos were processed by Dr. Steven Matsumoto, Oregon Health and Science University. We thank Drs. Felix Eckenstein and Victor May for their critical review of this manuscript. We also thank the Center for Biomedical Research Excellence (COBRE) in Neuroscience Molecular Cellular Core Facility for providing access to the ABI 7500 and the epifluorescence microscope. Financial support was from NIH RO1 NS 25767 (RN), the Vermont Cancer Center (GLSS, JAS), the Lake Champlain Cancer Research Organization (JAS), and COBRE grant NIH NCRR P20 RR16435.
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