Background
HSCR is caused by failures of the proliferation, differentiation or migration of enteric neural crest cells (ENCCs), resulting in aganglionosis of the distal part of the gastrointestinal tract [
1]. According to the length of the aganglionic segment, it can be further classified as short segment HSCR, long segment HSCR, total colonic aganglionosis (TCA) and total intestinal aganglionosis [
2]. The aganglionosis then leads to severe intestinal obstruction, which requires surgical removal of the aganglionic segment to treat [
3].
HSCR is a highly heritable disorder (OMIM 142623). Familial and syndromic HSCR show a Mendelian pattern of inheritance. However, the etiology of the sporadic HSCR seems to be intricate, presenting a non-Mendelian type inheritance and involving many genetic and environmental factors [
4]. So far, more than 20 HSCR susceptibility genes have been found to be associated with the development of ENS and most of them were involved in the RET (encoding a tyrosine kinase) and EDNRB signaling pathways [
5,
6]. The RET/GDNF/GFRA1 signaling pathway and the Endothelin 3-Endothelin Receptor B signaling pathway are the most common known pathways involved in HSCR development. Besides, transcription factors like SOX10, PHOX2B, ZEB2 etc. are also playing important roles in the development of ENS [
4]. Recent studies have also demonstrated the association between SEMA3 and HSCR, which was applicable to specific ethnic groups [
7‐
9]. However, only approximately 30% of all HSCR patients carry mutations in these genes, suggesting that there must be many other genes participating in the etiology of HSCR.
In the last few years, lots of novel susceptibility genes and variants for HSCR were identify by genome-wide association studies (GWAS) [
10,
11]. In a recent GWAS of HSCR,
NOX5 was identified as a susceptibility gene [
12]. Furthermore, a follow-up association analysis between
NOX5 polymorphisms and risk of HSCR indicated that several hereditary variants in
NOX5 were significantly associated with HSCR susceptibility [
13].
NOX5, which belongs to the NADPH oxidase family, is one of the major producers of reactive oxygen species (ROS) in mammalian cells [
14]. NADPH oxidases are membrane proteins that generate superoxides, particularly ROS, which have been shown to be participated in various signaling cascades and cellular processes including proliferation, apoptosis and migration. Dysfunction of the NOX enzyme could lead to abnormal levels of ROS that may cause diseases [
15]. Previous studies have demonstrated that
NOX5 is involved in various pathological conditions, including cancer, cardiovascular and atherosclerotic diseases [
16‐
18]. However, no in vivo or in vitro study has provided immediate evidence that
NOX5 is required for the development or function of the ENS. In this study, we aimed to test whether loss of
NOX5 would lead to the disruption of the biological processes of enteric neurons using zebrafish models.
Methods
Tissue collection
This study complied with the Declaration of Helsinki and was approved by the Review Board of Ethics Committee of Tongji Hospital. Consent forms were sent to patients at the age between 0 and 5 years and signed by their legal custodians. Full-length resected bowel specimens obtained during pull-through operations for HSCR were collected from ten patients. Three of these patients had a history of preoperative HAEC. Resected tissues included aganglionic and ganglionic segments. Ganglionic segments were taken from the most proximal margin of the resected pull-through specimen while aganglionic segments were taken from the most distal margin of the resected specimen. Control group of specimens were obtained from imperforate anus patients after colostomy (n = 10). Tissue specimens were stored in three ways following collection. One portion of each specimen was fixed in formalin at room temperature, for paraffin embedding and immunochemistry. A second portion was snap frozen in a mold containing optimal cutting temperature medium and stored at − 80 °C for immunofluorescence. The remaining specimen was stored at − 80 °C for protein or total RNA extraction.
Protein extraction and Western blot
Each protein sample of zebrafish was extracted from 30 embryos. The western blot was then performed as previously described [
19]. The rabbit anti-
NOX5 antibody (Abcam, Cambridge, UK, ab191010) was used at a concentration of 1:1000. The rabbit anti-beta III Tubulin (Tuj1) antibody (Abcam, Cambridge, UK, ab18207) was used at a concentration of 1:1000. The HRP linked goat anti-rabbit secondary antibody (Abcam, Cambridge, UK) was used at 1:10,000 dilution. Beta actin (dilution 1:1000, Abcam, Cambridge, UK) was used as the loading control. The relative level of protein was determined by the normalized density of each band in the western blot using the ImageJ software.
Immunohistochemistry
Sections (4 μm) on silane-coated slides (Muto Pure Chemicals Co., Ltd., Tokyo, Japan) were deparaffinized in xylene and dehydrated in solutions with decreasing concentrations of ethanol. After rehydration and blocking of endogenous peroxidase activity with 3% of hydrogen peroxide for 10 min, heat-induced epitope retrieval was performed for 20 min in 0.01 M citrate buffer (pH 6.0) in a pressure cooker. Primary antibody for NOX5 was used at 1:100 and incubated for 30 min. After washing and incubation with EnVision™ for 30 min, color products were developed using the Liquid DAB+ as chromogen. The sections were counterstained with hematoxylin before dehydration and coverslipping. Slides processed without primary antibody were prepared as negative controls.
Zebrafish housing/breeding
The zebrafish protocols were approved by the Institutional Animal Care and Use Committee at Tongji Hospital. The AB strain zebrafish were maintained according to standard procedures [
20]. Embryos were raised in E3 medium at 28.5 °C and staged as previously described [
21]. 0.003% N-phenylthiourea was added to E3 medium to inhibit melanization.
Real-time quantitative PCR
Total RNA was isolated from 0.2, 6, 12, 24, 48, and 72hpf embryos and colon tissues with TRIzol reagent (Life Technologies, Carlsbad, CA). qPCR and data analysis were performed using LightCycler96 (Roche Diagnostics). Relative expression levels were calculated using β-actin as internal reference. The experiments were repeated three times with biological replicates. Zebrafish primers (NOX5 primer: Forward, 5′- ATT CACGGCACT GAAACGGA-3′, Reverse, 5′-GGAGCTCCGCATGATT TACCT A-3′; β-actin primer: Forward, 5′-CGAGCTGTCTTCCCATCCA-3′, Reverse, 5′-TCACC AACGTAGCTG TCTTTCTG-3′). Human primers (Tuj1 primer: Forward, 5′- GGA AGAGGGCGAGATGTACG-3′, Reverse, 5′- GGGTTTAGACACTGCTGGCT-3′; β-actin primer: Forward, 5′- CCTTCCTGGGCATGGAGTC-3′, Reverse, 5′- TGA TCTTCATTGTGCTGGGTG-3′. NOX5 primer: Forward, 5′-CCAGAAAGTGGCTG CTGAGA-3′, Reverse, 5′-AGCTTGGAGAGGTGAGGCTA-3).
Whole-mount in situ hybridization
The 0.2, 6, 12, 48hpf Embryos (
n = 15 for each phase) were collected and processed for whole-mount ISH as previously described [
22].
A 735-bp fragment of the NOX5 cDNA was amplified using the following primers: Forward: 5′-CGGAGGTCTCTGGATCATGC-3′, Reverse: 5′-ATGTGCAGCCACAA CGTTTC-3′. A T7 promoter was added to the reverse primer. The ISH probe was then generated by in vitro transcription using T7 RNA polymerase.
Microinjection of morpholino antisense oligonucleotides
NOX5 knockdown experiments using MO were carried out as previously described [
23]. ATG morpholino antisense oligonucleotides targeting
NOX5 were designed and synthesized as follows:
NOX5-MO 5′-CGGGTGTCATCATCCAGACTCAT-3′, a 5-nucleotide-mismatch morpholino was used as control: 5′- CGGcTGaCtTgATCCAcAC TCAT − 3′. Zebrafish embryos were injected with 5 ng of the MO at the one cell stage. The knockdown efficiency was validated using western blot.
Whole-mount Immunofluorescent staining
Whole-mount Immunofluorescent staining with the anti-HuC/D antibody (A-21271, Life Technologies) was performed to examine the enteric neurons along the GI tract. The 5dpf embryos (n = 20) were collected and fixed with 4% PFA overnight. The embryos were washed with PBS. After incubation in blocking solution (2% goat serum, 2 mg/ml BSA in 1 x PBS) for 1 h at room temperature, embryos were incubated with the anti-HuC/D antibody (1500) in blocking solution overnight at 4 °C. After two washes in PBS for 10 min each time, embryos were incubated in the secondary antibody solution, 1: 1000 Alexa Fluor rabbit anti-mouse IgG (A11001, Life Technologies) in PBS, for 1 h at room temperature. Finally, the images were acquired using LSM 800 confocal microscope (Zeiss, Germany). The number of HuC/D-positive cells in the gut was then quantified using ImageJ. All of the experiments were repeated for three times.
Zebrafish intestinal transit assay
The tracer was prepared by mixing 100 mg of egg yolk, 150 μL of yellow-green fluorescent 2.0-um polystyrene microspheres (Invitrogen, Carlsbad, CA, USA) and 50 μL of deionized water as previously described [
24]. For 7dpf zebrafish larvae (
n = 65 for Control,
n = 75 for
NOX5-Mo), approximately 2 mg of tracer powder was administered per Petri dish in the morning. After 3, 6, and 9 h, the larvae were anaesthetized by 0.2%. tricaine (Sigma, St Louis, MO, USA) and imaged using a fluorescent dissecting microscope (Axio Zoom.V16, Zeiss, Germany). For scoring the transit efficiency, the zebrafish intestine was artificially divided into four zones according to anatomical landmarks and the larvae was grouped based on the anterior extent of the tracer.
Statistical analysis
The embryos were selected by Simple random sampling. Data were analyzed using the GraphPad Prism software package (version 5; GraphPad Software Inc., La Jolla, CA, USA) and are presented as the mean ± standard error of the mean. Differences between two groups were analyzed using an unpaired t-test with Welch’s correction. Analysis of variance (ANOVA) was used to compare data of more than two groups. Pearson’s chi-square tests were used to assess the difference between 7 dpf wild-type and NOX5-MO group transit profiles at different time points. The experiments and data analyzing were finished by different researchers. The analyst didn’t know the grouping scheme in advance.
Discussion
HSCR is a highly heritable disorder [
25]. Previously, a genome-wide association study (GWAS) with 123 sporadic HSCR patients and 432 unaffected controls identified
NOX5 as a new susceptibility gene for HSCR [
12]. Furthermore, an association analysis between
NOX5 polymorphisms and risk of HSCR in 187 patients and 283 unaffected controls showed that the genetic variants in
NOX5 were significantly associated with HSCR susceptibility, particularly for the L-HSCR and TCA subtypes [
13]. Encouraged by these findings, we embarked on elucidating
NOX5 function in ENS.
NADPH oxidases (
NOX), comprising seven family members (
NOX1-
NOX5 and dual oxidase 1 and 2), are the major producers of reactive oxygen species in mammalian cells.
NOX5 was first reported in 2001 based on a blast search using the C-terminus of gp91
phox as bait to identify novel transcripts [
26]. The exact pathophysiological significance of
NOX5 remains unclear, but it seems to be important in the physiological regulation of sperm motility, vascular contraction and lymphocyte differentiation, and
NOX5 hyper activation has been implicated in cardiovascular disease, kidney injury and cancer. One of the distinguishing features of
NOX5 is the dependence on Ca
2+ for its regulation [
27]. Activation of
NOX5 in response to elevated Ca
2+ is a multi-phased process [
28]. The amount of Ca
2+ required to activate
NOX5 fully is relatively high, and accordingly, additional systems involving regulatory proteins are operational that increase sensitivity to Ca
2+, thereby facilitating ROS generation. Hence,
NOX5 can be activated directly by Ca
2+ or indirectly by interacting with other proteins and kinases, such as Ca
2+-bound calmodulin or PKC [
29]. Intriguingly,
NOX5 is the first and only NADPH oxidase to be crystallized, providing opportunities to design specific
NOX5 inhibitors and activators, which is crucial for biomedical research and potentially for therapeutic utility [
30].
We used zebrafish as the animal model to explore the function of
NOX5. The result of spatiotemporal expression spectrum of
NOX5 in zebrafish embryo indicated that
NOX5 might play a role in the early development of zebrafish, which is similar to the previous study about the expression of NOX family in zebrafish [
31]. However, after significant knockdown of the
NOX5 expression, there were no difference of HuC/D positive neuron numbers of the GI tract between the normal group and the
NOX5 knockdown group, indicated that the development of ENS in zebrafish do not require the
NOX5. Besides, the results of GI transit assay suggested that the GI motility were not affected by the absence of
NOX5 protein either. Generally, GI motility is controlled by enteric neural crest cells that form the ENS and undergo extensive migration from the caudal hindbrain to colonize the total GI tract [
32,
33]. The results of the in vivo study in zebrafish showed that the loss function of
NOX5 did not cause the absence of enteric neuron in zebrafish, which is the most important characteristic of HSCR.
Interestingly, our data of IHC showed that
NOX5 is indeed located on the enteric neuron membrane. qPCR, western blot showed that
NOX5 is strongly expressed in the myenteric ganglionic cell in the proximal and normal segment of HSCR colon and hardly expressed in the distal segment of colon with the absence of ganglionic cell. Recent researches have revealed that several aberrant gene expressions were involved in the pathological processes of HSCR, including UBR4 [
34]. UBR4, a ubiquitin ligase protein, has been showed to be a novel HSCR gene [
6]. It was required for neurogenesis and played an important role in myofiber hypertrophy [
35,
36]. Therefore, NOX5 might be the downstream factor of UBR4. Downregulation of UBR4 expression might impact the development of ENS which probably reduce the expression of NOX5. On the one hand, abnormal distribution of ICCs had been observed in the aganglionic segment colon of HSCR patients [
1]. What’ more, cell migration, contraction and proliferation cannot complete without Ca2
+-dependent processes, and this is particularly pertinent to
NOX5, because
NOX5 itself is regulated by Ca2
+ [
37]. Moreover, in bowel motility, Ca2
+ is regulated by ICCs, which are essential to generate and propagate the electrical cyclical activity (slow waves) in the intestines. Therefore, we hypothesize that the aberrant expression of
NOX5 in aganglionic segment may occur due to the abnormal release of Ca2
+. More importantly, our study demonstrated that
NOX5 expresses in the ganglionic cells specifically in colon tissue.
NOX5 may serve as a typical neuron marker for enteric ganglionic cell to determine the occurrence and development of HSCR. It is noteworthy that the disruption of the balance between Ca2
+ and
NOX5 may lead to further deterioration of spasm in distal segment of HSCR patients. Potentially, abnormal release of Ca2
+ may result in the decreased expression of
NOX5 through an unknown mechanism, which may further cause dysregulation of Ca2
+ the concentration.
However, the association between NOX5 and Ca2+ remain unclear and further studies are required to explain the decreased level of NOX5 in the aganglionic segment colon of HSCR patients.
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