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From the Departments of Laboratory Medicine and
Pathobiochemistry *
and
Dermatology,
Charité, Humboldt
University, Berlin; the Growth Factor and Regeneration
Group,
Max Delbrück Centrum (MDC) for
Molecular Medicine, Berlin; and the Department of Anatomy and Cell
Biology,§
Justus Liebig University,
Giessen, Germany; and the Department of
Dermatology,¶
University Hospital, Eppendorf
University, Hamburg, Germany
| Abstract |
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| Introduction |
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Though there is good evidence for the fundamental role of target-derived BDNF for the development of visceral innervation,13,14 the role of target-derived BDNF for adult visceral neurons is rather unknown. Recently, it has been observed that inflammatory diseases of the adult viscera are associated with a strong increase in local BDNF mRNA and protein production.15-17 These observations raised the possibility that BDNF might mediate changes in neuronal function in pathological conditions, in that there is growing evidence for a functional role for BDNF in the normal adult peripheral nervous system.18-21 The involvement of target-derived mechanisms has been suggested, because there is recent evidence for retrograde transport of BDNF in adult visceroafferent and visceroefferent neurons.22 This is supported by the finding that there are many more neurons in the adult nodose and petrosal ganglion (NPG) and (DRG) that contain BDNF protein than produce BDNF mRNA.23,24 Though target-derived actions of BDNF in the adult viscera have been discussed, a systematic study of BDNF expression in the viscera is still lacking. Moreover, most reports do not identify the cellular sources of BDNF. There is some evidence for the presence of BDNF mRNA in extracts from the lung, heart, and spleen25-27 and of BDNF protein in extracts of the rat liver and thymus.28 As possible physiological sources of BDNF, only fibroblasts,29-31 vascular smooth muscle cells,32,33 and thymic stroma cells have been identified so far.34
It was the aim of this study, therefore, to investigate systematically the expression and potential role of BDNF in the targets of adult visceral sensory and motor neurons. Using a nonradioactive in situ hybridization technique, which gives very good cellular resolution, we identified the cells synthesizing BDNF mRNA in all gastrointestinal regions and in tissues of the cardiorespiratory and urogenital systems. In addition, we quantified the amounts of BDNF protein present in these internal organs. Furthermore, we have examined the distribution of BDNF receptors and the morphology of viscera in mice lacking BDNF. We found that BDNF is expressed in certain viscera in even higher amounts than in the brain. The distribution of BDNF receptors and the phenotype of BDNF-/- mice suggest a primarily neurotrophic role for BDNF made by visceral epithelia. We conclude that visceral BDNF could indeed regulate functional properties of adult PNS neurons.
| Materials and Methods |
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Female Balb/c mice were obtained from Harlan-Winkelmann (Borchen, Germany). FVB/N transgenics were genotyped by polymerase chain reaction (PCR) analysis as described before.35,36 Wild-type and BDNF-/- mice were obtained from the mating of BDNF+/- mice maintained at the Max Delbrück Centrum, Berlin. The production and maintenance of these mice have been described elsewhere.21 Paraffin sections (2 µm) of internal organs from 2-week-old wild-type (+/+) and BDNF-/- mice were stained with hematoxylin-eosin (HE) following standard laboratory procedures.
In Situ Hybridization (ISH)
The riboprobe for BDNF was prepared as described by Schaeren-Wiemers and Gerfin-Moser.37 Briefly, for in vitro transcription 1 µg of linearized plasmid containing 510 bp of the BDNF coding sequence (nucleotides 224734) was used as a template.38 The reaction was performed in a 50-µl volume using the DIG-RNA-labeling mix from Boehringer Mannheim (Mannheim, Germany) and a T7 (anti-sense) or T3 (sense control) polymerase (Promega, Madison, WI). After a 3-hour incubation the reaction was stopped by adding DNase. The probe was hydrolyzed by adding two volumes of carbonate buffer (60 mmol/L Na2CO3, 40 mmol/L NaHCO3, pH 10.2) followed by 45 minutes' incubation at 60°C. After neutralization with an equal volume of neutralization buffer (200 mmol/L Na-acetate, 1% acetic acid, pH 6.0), the probe was purified by ethanol precipitation. To estimate the concentration of the probe a dot blot was performed as recommended by the manufacturer (Boehringer Mannheim) for nonradioactive ISH. Probes were stored at -80°C.
Organs from 8-week-old Balb/c mice were immediately frozen in Tissue Tek (Miles, Elkhart, IN). Slides were coated with 2% APES (3-Aminopropyltriethoxysilane, Sigma, Deisenhofen, Germany) in Aceton under RNase-free conditions. Ten-micron cryosections were dried for 30 minutes, fixed in 4% cold paraformaldehyde for 10 minutes, and then washed in RNase-free PBS. Two sequential sections of each organ were used for anti-sense and sense staining. After acetylation, sections were washed in PBS. For prehybridization, 500 µl of hybridization buffer (50% formamide, 4x SSC, 2x Denhardt's solution, 50 µg/ml RNA-core from baker's yeast) were added to each slide. The slides were placed in a humid chamber containing a 50% formamide/4x SSC mix at the bottom. The hybridization mixture was prepared by adding 150 ng/ml digoxigenin (DIG)-labeled cRNA (anti-sense or sense) to the hybridization buffer. To denature the probe the mixture was incubated at 85°C for 5 minutes. The prehybridization solution was allowed to drip off the slides, and 200 µl of hybridization mixture were added to each slide. Hybridization was performed overnight at 56°C. Posthybridization washes were carried out in the following sequence: 4x 10 minutes in 2x SSC at 67°C, 45 minutes in 2x SSC at 67°C, 60 minutes in 0.1x SSC at 67°C, 10 minutes in 0.2x SSC at room temperature. Detection of the DIG-labeled probe was performed as described in the manufacturer's instructions, with antibody incubation overnight at 4°C. Color development was allowed to proceed in the dark for 2 hours. The reaction was terminated by immersing the slides in PBS, pH 7.5.
Preparation of Tissue Lysates
Organs from 8-week-old Balb/c mice were prepared and pulverized in liquid nitrogen. The lysing buffer contained 50 mmol/L Tris, pH 8.0, 150 mmol/L NaCl, 1 mmol/L EDTA, 1 mmol/L phenylmethylsulfonylfluoride, 5 mmol/L Iodacetamid, 10 mg/ml aprotinin, and (as detergents) 0.2% SDS, 1% Triton X-100, and 1% Igepal (Sigma). Protein extraction was performed as described before.39 The lysing solutions were centrifuged at 2000 x g, 4°C, for 30 minutes and supernatants stored at -80°C. Total protein content was measured using the detergent-compatible BCA protein assay (Pierce, Rockford, IL).
Determination of BDNF Protein by Enzyme-Linked Immunosorbent Assay (ELISA)
BDNF in lysates was measured using commercial ELISA kits according to the manufacturer's instructions (Promega) as described.17 The detection limit was 4 pg/ml. Measurements were performed in duplicate. Concentrations of BDNF were calculated as nanograms of BDNF per gram of total protein.
trkB and p75NTR Immunoreactivity
Immunoreactivity against the full-length form of mouse trkB (gp 145) was studied on acetone-fixed cryosections (10 µm) of 8-week-old Balb/c mice internal organs using rabbit antisera (Transduction Laboratories, Lexington, KY) and TRITC-conjugated goat anti-rabbit IgG as the secondary antibody, as described before.40,41 Sections were counterstained by Hoechst 33342 (Sigma, St. Louis, MO; 10 mg/ml in TBS, 30 minutes) for identification of cell nuclei as described.42 For detection of p75NTR immunoreactivity, APAAP-staining was performed as described elsewhere,43 using a monoclonal rat anti-mouse p75NTR primary antibody (Chemicon, Temecula, CA). Sections were counterstained with haemalaun. Incubation of internal organ cryosections without primary antibody was used as a negative control in both experiments. Slides were studied using a fluorescence Zeiss Axioscope microscope and photodocumented using a digital image analysis system (ISIS Metasystem, Altlussheim, Germany).
Specificity Control
The heart of BDNF-overexpressing mice (FVB/N, alpha-myosin
heavy-chain promoter36
) and BDNF-/- knockout mice served
as a positive and negative control for ELISA and ISH. Transgenic heart
lysates showed 20-fold higher concentrations of BDNF (583.3 ±
151.8 ng BDNF/g total protein) than FVB/N wild-type heart lysates
(30.5 ± 3.7 ng/g), whereas BDNF was not detectable in heart
lysates of BDNF knockouts. ISH revealed a strong and ubiquitous BDNF
mRNA staining in cardiomyocytes of BDNF overexpressors. Wild-type
hearts displayed distinct BDNF mRNA-positive cardiomyocytes, BDNF-/-
hearts were completely BDNF mRNA-negative (not shown). BDNF levels in
back skin lysates (telogen) of Balb/c mice ranged at 8.5 ± 2.1
ng/g (Figure 1A)
, concentrations of BDNF
in total brain lysates at 5.9 ± 1.5 ng/g, according to reported
data.38
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| Results |
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In the gastrointestinal tract, the BDNF protein content differed
markedly between organs (Figure 1B)
. High concentrations of BDNF were
detected in the colon and duodenum, low concentrations in the ileum. In
the liver and pancreas, BDNF levels were comparable to those measured
in the total brain (Figure 1A)
. In contrast, the submandibular gland
contained very low concentrations of BDNF. In order to identify the
cellular sources of BDNF, mRNA expression was examined with
nonradioactive ISH (Table 1)
. The
internal circular muscle layer of the tunica muscularis was BDNF mRNA
positive throughout the intestine. In contrast, the outer longitudinal
muscle layer remained negative (Figure 2
,
colon). An exception was the upper
esophagus, where distinct muscle fibers of the outer layer appeared
positive (Figure 3
, esophagus). BDNF
expression was detected in epithelia throughout the intestinal tract
(Table 1)
. Intense signals were observed on the bottom of the crypts or
of gastric foveolae, respectively. Expression levels appeared to be
lower toward the apical cell layers (Figure 2
, colon and stomach).
Prominent BDNF expression was observed in epithelia of the colon
(Figure 2
, colon), in contrast, only weak expression was seen in the
ileum. Ganglia of the myenteric plexus were BDNF mRNA positive
throughout the intestine (arrow in Figure 2
, colon). BDNF mRNA was
detectable in the peritoneal cell layer of the intestine as on other
internal organs (Figure 3
, peritoneum). In the liver, hepatocytes were
identified as the main sources of BDNF mRNA (Figure 2
, liver).
Epithelia of exocrine ducts remained unstained in all examined glands
(Table 1)
.
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A striking pattern of BDNF mRNA-expressing cells was observed in
the lung. Respiratory epithelium was strongly BDNF mRNA positive from
trachea up to the bronchioli. Airway smooth muscle cells as well as
smooth muscle cells of pulmonary vessels (Table 1)
were moderately
positive (Figure 3
, lung). This expression pattern was mirrored in the
very high BDNF content measured in lysates taken from lung (Figure 1A)
.
The BDNF protein contents found in the heart were significantly lower
than those found in the lung. ISH showed that only a few cardiomyocytes
were BDNF mRNA-positive (Table 1)
.
Urogenital Tract
BDNF protein levels in the kidney were comparable to those found
in the brain. In contrast, lysates taken from the urinary bladder
revealed a much higher concentration of BDNF (Figure 1C)
. BDNF protein
was also detectable in the urine (24.98 ± 14.78 pg/ml), though
serum levels were below the detection limit (<4 pg/ml). BDNF protein
levels in the oviduct were significantly higher than in the uterus
(Figure 1C)
. For the identification of cellular BDNF expression, BDNF
mRNA was detected by ISH. Proximal and distal tubules of the kidney
displayed strong BDNF expression. No expression was observed in
glomerula (Figure 2
, kidney). Renal vascular smooth muscle cells
appeared to be BDNF mRNA negative (not shown). Urothelia of the urinary
bladder revealed very strong BDNF expression. In contrast, the tunica
muscularis was BDNF mRNA-negative (Figure 3
, bladder). BDNF mRNA was
found in epithelia of the uterus and oviduct. There was also a light
expression in adjacent cells of the lamina propria (Figure 3
, tuba
uterina). Interestingly, BDNF mRNA was not detectable in the squamous
epithelium of the portio vaginalis uteri (Figure 3
, cervix uteri).
trkB and p75NTR Immunoreactivity (IR)
The strong BDNF expression in visceral epithelia raised the
question whether BDNF could also play an autocrine role for
non-neuronal structures in the adult viscera. However, almost all
epithelia in the examined internal organs were both trkB-IR and
p75NTR-IR-negative (Table 1)
. Arrowheads in Figure 4
(colon) show epithelial structures of
the transverse colon, which are negative for trkB-IR and
p75NTR-IR. An exception was the squamous epithelium of the
portio vaginalis uteri, which was trkB-IR-positive (red fluorescence),
but p75NTR-IR-negative (arrowheads in Figure 4
, cervix
uteri). Most smooth muscle layers appeared to possess neither trkB nor
p75NTR receptors (Figure 4
, colon). However, prominent
p75NTR-IR was detected in the tunica muscularis of the
oviduct and in the myometrium (Table 1)
. Strongest
p75NTR-IR was revealed in the lamina propria underneath the
epithelia of the uterus and oviduct, probably representing connective
tissue cells. Figure 4
, cervix uteri, shows p75NTR-IR (red
staining) underneath the squamous epithelium of the portio vaginalis
uteri. Scattered trkB-IR was detectable in muscle fibers of the upper
esophagus (Table 1)
. Neurons and nerve fibers of the myenteric plexus
showed trkB-IR as well as p75NTR-IR (Table 1)
. White arrows
in Figure 4
, Colon show trkB-IR (red fluorescence), black arrows
p75NTR-IR (red staining) on myenteric neurons and nerve
fibers of the colon.
|
To further analyze the role of BDNF for non-neuronal structures of
the viscera, we examined the viscera of mice lacking BDNF. The
morphology of internal organs of 2-week-old wild-type
(n = 4) and BDNF-/- mice
(n = 4) was examined using HE-stained 2-µm
paraffin sections. Throughout the gastrointestinal tract, the
intestinal mucosa was present and displayed no gross morphological
changes. However, the whole intestine appeared markedly hypotrophic.
The ileum and duodenum showed no reduction in mucosal thickness and a
regular relation of crypts and villi. The colon displayed a mucosal
atrophy. Goblet cells of the colon appeared enlarged, probably due to a
retention of mucus. Figure 5
, colon shows
the wall of a wild-type and a BDNF-/- transverse colon. The
morphology of pancreatic and hepatic epithelia was unaltered; these
organs appeared normal (not shown). Furthermore, the BDNF-/- lung and
heart were indistinguishable from wild-type organs. Figure 5
, lung,
shows the respiratory epithelium and airway smooth muscle of a main
bronchus. Epithelial and smooth muscle structures in organs of the
urogenital tract (kidney, uterus, oviduct, urinary bladder) appeared
unaltered in thickness and morphology as well (not shown).
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| Discussion |
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In the last few years, BDNF mRNA expression has been reported in various gustatory and olfactory sensory epithelia46-48 and in epithelial cells of the cochlea and vestibulum.49-52 These sensory epithelia represent innervation targets for neuronal populations, which were reported to be dependent on BDNF. Visceroafferent neurons of internal organs, which are mainly located in the nodose/petrosal ganglion (NPG) and dorsal root ganglia (DRG), were shown to require BDNF during development.9,10 Recently, it has been shown that during normal development BDNF is transiently expressed at high levels in the targets of arterial baroreceptive and chemoreceptive sensory neurons that have their cell bodies in the NPG.13 This expression is coordinated with the arrival of sensory axons in these targets.13 In addition, the onset of trkB expression in neurons of the NPG has been shown to correlate with the onset of BDNF expression in peripheral targets of NPG neurons.14,27 In the adult animal, retrograde transport of BDNF was recently demonstrated by neurons with their axons in the vagus nerve.22 A small number of these neurons had their cell bodies in the NPG, but many motoneurons with their cell bodies in the brainstem also retrogradely transported radiolabeled BDNF. Thus, although the presence of trkB-expressing NPG neurons in the adult animal has been disputed,23,53 motoneurons innervating the viscera appear to be able to utilize endogenous target-derived BDNF.22 It is also not in dispute that many adult sensory neurons innervating the viscera with their cell bodies in the DRG possess trkB receptors.23,45 The fact that more sensory neurons in the adult NPG and DRG contain BDNF protein than produce BDNF mRNA further support a role of retrogradely transported BDNF.23,24,54 The possible function of target-derived BDNF in the adult animal is still relatively obscure, although evidence does exist that this factor can influence the functional properties of mature sensory and motoneurons.21,55 It may still be the case that BDNF is required for the survival of adult neurons, but, as no conditional knockouts have yet been described, this issue is still open.
Information on BDNF's role in the ENS is very limited. Furthermore, there are conflicting data about trkB receptor expression in human and rat enteric plexuses.56,57 The finding that BDNF receptors were identified only on neuronal structures of the gut suggest that the observed BDNF expression in epithelial and smooth muscle cells could influence predominantly innervating neurons. Because there are no data available about the role of BDNF in adult myenteric plexus neurons, both survival and nonsurvival functions are conceivable. The examined BDNF-/- mice did not feed properly, as described elsewhere.11 Though there was some food in the stomach, the intestinal tract appeared nearly empty. Hence, the hypotrophy of the whole intestine in BDNF-/- mice is most likely due to malnutrition. The marked reduction of food intake is probably also the reason that the (completely empty) colon displayed a mucosal atrophy and significant mucus retention in goblet cells.
BDNF levels in the lung and urinary bladder were 5- to 15-fold higher than in total brain lysates and even higher than BDNF levels previously described in the hippocampus.58 The levels of BDNF in the urinary bladder are, to some extent, consistent with the finding that nearly all adult afferents projecting through the pelvic nerve are trkB-positive.45 However, the viscera is a relatively sparsely innervated region in comparison to other somatic tissues.59 It is, therefore, surprising that the amount of BDNF message and protein expressed by certain viscera is so large compared to somatic tissues, eg, the densely innervated skin. BDNF has been described as playing a role in skin innervation.21,60-62 BDNF protein levels in the skin, however, were significantly lower than in certain inner organs (8 ng/g in the back skin versus, e.g., 80 ng/g in the urinary bladder). It thus appears that, at least in adults, visceral BDNF may also play a role in the functional regulation of visceral motor and sensory as well as possibly enteric neurons.18,63 There is recent evidence demonstrating that BDNF can regulate the capsaicin sensitivity of adult visceral sensory neurons20 as well as several functional properties of adult motoneurons.18,19 Furthermore, it has been well established that BDNF plays a nonsurvival role in CNS neurons.11 Hippocampal neurons especially require BDNF for the expression of synaptic changes associated with long-term potentiation (LTP).64,65 It is, therefore, conceivable that BDNF, in contrast to developmental stages, could act primarily on functional properties of PNS neurons.
Recent studies demonstrate that inflammatory diseases of the adult viscera are associated with a local up-regulation of BDNF mRNA and protein production. Interestingly, these observations focus on inner organs we found to be the predominant physiological sources of BDNF in adult viscera (lung and urinary bladder). Allergic asthmatic patients respond with a marked increase of BDNF levels during inflammation in the lung after allergen provocation.17 In a mouse model of allergic bronchial asthma, we demonstrated that this local increase is due at least in part to an up-regulation of BDNF mRNA production in infiltrating immune cells, including macrophages and T cells.15 The production of BDNF in activated human immune cells has been demonstrated recently.66 In addition, a strong local up-regulation of BDNF mRNA was demonstrated in the inflamed urinary bladder.16 It is well established that the closely related nerve growth factor (NGF) contributes to the characteristic neuronal changes in allergic bronchial asthma and in cystitis. In animal models of these diseases, blocking of NGF partly prevented neuronal changes which follow inflammation.43,67 Therefore, a similar functional role of locally produced BDNF has been suggested in inflammatory conditions. These observations indicate that BDNF could mediate functional neuronal changes in pathological conditions of the viscera, especially of the lung and urinary bladder.
An additional novel finding is the p75NTR expression on smooth muscle cells of the myometrium. A major role of p75NTR has been postulated recently in myogenic differentiation.68,69 These studies showed that NGF is capable of stimulating myoblast proliferation and differentiation via p75NTR. In addition, NGF and p75NTR down-regulation was shown to be essential for the terminal myogenic differentiation. The observed p75NTR-IR on the myometrium could, therefore, indicate the plasticity and growth potency of the uterine smooth muscle. Autocrine processes of BDNF seem not to be involved, because the myometrium was completely negative for BDNF mRNA. Paracrine actions of BDNF, however, have to be considered not only on uterine smooth muscle cells, but also on the squamous epithelium of the portio, because BDNF has been demonstrated recently to promote keratinocyte proliferation via trkB.70
In summary, we have shown the extensive cellular BDNF expression in non-neuronal innervation targets of adult murine viscera. Non-neuronal tissues expressing BDNF did not display BDNF receptors and revealed no architectural changes in BDNF-/- mice. The surprisingly high concentrations of BDNF protein in certain internal organs suggest that this protein probably also regulates functional properties of adult PNS neurons innervating the viscera.
| Acknowledgements |
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| Footnotes |
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Supported by Volkswagen Stiftung.
This work is dedicated to the 60th birthday of Prof. Eckart Köttgen.
Accepted for publication June 30, 1999.
| References |
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