Introduction
Interleukin-1 beta (IL-1β) is a pro-inflammatory cytokine, which is produced as an immune response to both injury and infection [
1]. Increased production of IL-1β is involved in a variety of cellular activities, including cell proliferation, differentiation, and apoptosis [
2]. IL-1β induces an intracellular signaling pathway, leading to altered expression of its target genes and the induction of a pro-inflammatory response [
3]. It is considered that inflammatory processes stimulated by IL-1β are associated with an increased risk of neurodevelopmental disorders. Previous studies showed that IL-1β can aggravate the primary damage caused by infection of the central nervous system (CNS), and IL-1β deficient mice display reduced neuronal loss and infarct volumes after ischemic brain damage in in vivo studies [
4,
5]. On the contrary, IL-1β showed beneficial effects on neuronal survival in cultures; it is highly expressed in the developing brain and can directly influence neural precursor cells (NPCs) [
6]. In other in vitro models, IL-1β has been shown to stimulate the migration of cultured cortical neurons and promote neurite outgrowth [
1,
7]. The findings in various neuronal culture models show that long-term exposure (3–5 days) to high concentrations of IL-1β (500 ng/ml) had neurotoxic effects; however, short-term exposure (1 day) to lower concentrations (10 ng/ml) did not [
8]. Thus, it is important to elucidate whether IL-1β mediates neuronal differentiation to understand neural development and the pathogenesis of various neurodevelopmental disorders.
Wnt signaling plays an important role in embryogenesis and in the late stages of development, by regulating cell survival, growth, and, differentiation via various signaling pathways [
9,
10]. Wnt signaling involves a group of pathways, such as the canonical Wnt/β-catenin pathway, the noncanonical Wnt/Ca
2+ pathway, and the Wnt/planar cell polarity (PCP) pathway [
11,
12]. Several Wnt isoforms (Wnt3b, Wnt5a, Wnt7a, and Wnt11) activate various signaling cascades during the maintenance of many organs and tissues [
12‐
14]. A recent report suggested that Wnt5a plays a role in cell differentiation and specialization [
15]. Wnt5a promotes the differentiation of mesenchymal stem cells (MSCs) into type II alveolar epithelial cells (AT II cells) [
16] and acts as a repulsive guidance cue for cortical axons extending through the corpus callosum in vivo [
17]. Wnt5a is also involved in IL-1β-mediated cell migration and differentiation. IL-1β-induced Wnt5a protein enhanced human corneal endothelial cell migration through the regulation of Cdc42 and RhoA [
18], and silencing of Wnt5a prevents IL-1β-induced collagen type II degradation in rat chondrocytes [
19]. Furthermore, Wnt5a signaling is involved in IL-1β-induced matrix metalloproteinase (MMP)-3-regulated proliferation of ES cell-derived odontoblast-like cells [
20]. Although Wnt5a has been associated with the regulation of proliferation and differentiation in different cell types, little is known about the role of Wnt5a signaling in neuronal differentiation of NPCs.
Rho-family GTPases play an important role in regulating intracellular cytoskeletal and signaling pathways that facilitate axonal morphological changes [
21,
22]. The most commonly studied Rho-GTPases are RhoA, Cdc42, and Rac1 [
23]. Rho family GTPases serve as a molecular switch by converting from an inactive GDP-bound state to an active GTP-bound state and, once activated, they can interact with their specific effectors [
24]. The GTP-bound form of RhoA causes Rho-associate kinase (ROCK) activation [
25]. The RhoA/ROCK pathway is associated with various neuronal functions, such as migration, dendrite development, and axonal extension [
25,
26]. The role of RhoA is complicated in axonal branching and growth cone formation [
27]. In some cases, RhoA negatively regulates axon branch that inactivation of intracellular Rho to stimulate axon growth and regeneration [
28]. In contrast, other studies indicated that RhoA is involved in the promotion of axon branching [
29,
30]. It is, therefore, necessary to determine the distinct physiological functions of RhoA in neuronal differentiation of NPCs.
In the present study, we first investigated the role of IL-1β in neuronal differentiation of NPCs and found that IL-1β activated noncanonical Wnt5a signaling (Wnt5a/RhoA/ROCK/JNK pathway). We also demonstrated that IL-1β-induced noncanonical Wnt5a signaling is required for neuronal differentiation, especially for neurite outgrowth, suggesting that IL-1β can promote the neuronal differentiation of NPCs.
Methods
Materials
Coon’s modified Ham’s F-12 medium and human insulin were purchased from Sigma Chemical Co (St Louis, MO, USA). Penicillin/streptomycin solution, neurobasal medium, and B27 were purchased from Gibco (Grand Island, NY, USA), and bFGF, recombinant rat IL-1β, and recombinant Wnt5a were purchased from R&D Systems (Minneapolis, MN, USA). The following antibodies were purchased: anti NT3 (#SC-547) and anti Ngn1 (#SC-19231) from Santa Cruz Biotechnology (Santa Cruz, CA, USA); anti p-JNK (#9251S), anti JNK (#9252S), anti p-NF-κB (#3031S) and anti NF-κB (#6956S) from Cell Signaling Technology (Beverly, MA, USA); anti Wnt5a (#MABD136) from Merck (Darmstadt, Germany); anti β-tubulin type III (TUJ1) (#801202) from BioLegend (San Diego, CA, USA); anti calnexin (#ADI-SPA-860-F) from Enzo Life Sciences (Farmingdale, NY, USA). Fluorescein-(DTAF)-conjugated streptavidin (#016–010-084) was purchased from Jackson Immuno Research (Westgrove, PA, USA). Y27632 and SP600125 were purchased from Abcam (Cambridge, UK).
Primary culture of neural precursor cells
Embryonic brain cortices from E14 rat embryos were mechanically triturated in Ca2+/Mg2+-free Hank’s balanced salt solution (Gibco), seeded at 2 × 105 cells in 10-cm culture dishes (Corning Life Sciences, Acton, MA, USA), which were precoated with 15 μg/ml poly-L-ornithine (Sigma) and 1 μg/ml fibronectin (Invitrogen, Carlsbad, CA, USA). The cells were then cultured for 5–6 days in serum-free N2 medium supplemented with 20 ng/ml bFGF (R&D Systems Inc.). Cell clusters generated by precursor cell proliferation were dissociated in Hank’s balanced salt solution and plated at 2 × 104 cells per well on coated 24-well plates, 2 × 105 cells per well on coated 6-well plates, and 8 × 105 cells per dish on coated 6-cm culture dishes. All experiments were performed using passage 1 (P1) neural precursor cells.
Transient transfection of NSCs
RhoA (Dharmacon, Lafayette, CO, USA, Catalog No. L-095222-02), Wnt5a (Dharmacon, Catalog No. L-088203-02), Control (Dharmacon, Catalog No. D-001810-10) or NF-κB p65 siRNA (Bioneer, Daejeon, KOREA, Catalog No. 1783704) were introduced into cells in knockdown experiments using a Nucleofector™ kit (Lonza, Basel, Switzerland, Catalog No. VPG-1003, Program A-031).
Real time PCR and RT (reverse transcription)-PCR
cDNA was prepared using the total mRNA extracted from cells with TRIzol® reagent (Thermo Fisher Scientific); 2 μg samples of RNA were reverse-transcribed using random hexamer mixed primers. The cDNA thus formed was amplified by PCR using the following primers:
Wnt3a (5′-TGCAAATGCCACGGACTATC-3′ and 5′-AGACTCTCGGTGTTTCTCTACC-3′),
Wnt5a (5′-TGCCACTTGTATCAGGACCA-3′ and 5′-GGCTCATGGCATTTACCACT-3′),
Wnt5b (5′-CAAGCTGGAACTGACCAACA-3′ and 5′-AAAGCAACACCAGTGGAACC-3′),
Wnt7a (5′-CACAATTCCGAGAGCTAGGC-3′ and 5′-TAGCCTGAGGGGCTGTCTTA-3′),
Wnt7b (5′-GCTATCAGAAGCCGATGGAG-3′ and 5′-ACGTGTTGCACTTGACGAAG-3′),
Nt3 (5′-GGCACACACACAGGAAGTGTC-3′ and 5′-CTGGACGTCAGGCACGGCCTGT-3′),
Ngn1 (5′-ATGCCTGCCCCTTTGGAGAC-3′ and 5′-TGCATACGGTTGCGCTCGC-3′),
p65 NF-κB (5′-CTAGGAGGACTCGGGCTCTT-3′ and 5′-AGGAGCTCCACAGGACAGAA-3′),
Gapdh (5′-CTCGTCTCATAGACAAGATG-3′ and 5′-AGACTCCACGACATACTCAGCAC-3′).
The PCR conditions for amplification of Wnt3a, Wnt5a, Wnt5b, Wnt7a, Wnt7b, Nt3, Ngn1, p65 NF-κB, and Gapdh were as follows: denaturation at 94 °C for 30 s, annealing at 58 °C for 1 min, and extension at 72 °C for 1 min. 30 cycles were used for amplification of all cDNAs. The PCR products were analyzed on a 1.5% agarose gel. For real-time PCR, 5 μl of RT reaction product was amplified in duplicates at a final volume of 20 μl iQ™ SYBR®Green Super mix. Thermocycling conditions were 95 °C for 10 min, followed by 40 cycles of 95 °C for 15 s and 60 °C for 1 min. The primer sequences for real-time PCR were same as those used for RT-PCR, and all gene expression values were normalized to those of Gapdh.
Western blot analysis
Cells were lysed in 20 mM Tris–HCl (pH 7.5) containing 150 mM NaCl, 1 mM Ethylenediaminetetraacetic acid (EDTA), 1 mM Ethylene glycol-bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA), 2.5 mM sodium pyrophosphate, 1% Triton X-100, 1 mM phenylmethylsulfonyl fluoride (PMSF), and 1 mM Na3VO4. Protein samples (20–30 μg) were loaded on 10% SDS–polyacrylamide gels, electrophoresed, and transferred to nitrocellulose membranes (Amersham Pharmacia Biotech, Amersharm, UK) after electrophoresis. After blocking with 5% non-fat dried milk for 1 h, membranes were incubated with primary antibodies followed by incubation with HRP-conjugated secondary antibody (Anti-mouse IgG (#7076) and Anti-rabbit IgG (#7074)) (1:2000) (New England Biolabs, Beverly, MA, USA), and specific bands were detected by ECL (Amersham Pharmacia Biotech).
RhoA activation assay
RhoA activity in cells was measured using Rho Activation assay kit (Merck, Catalog No. 17–294), according to the manufacturer’s instructions. Briefly, cells were washed with ice-cold PBS, and lysed with Mg2+ Lysis/Wash Buffer (MLB). Lysates were collected by centrifugation at 14,000×g for 5 min at 4 °C. Equal amounts of protein from cell lysates were incubated for 45 min at 4 °C with 20 μg of Rhotekin-RBD protein agarose beads. Pellets were washed with MLB and subjected to western blotting using the anti-RhoA antibody.
Immunofluorescence staining
Cells were washed with PBS and fixed with 4% paraformaldehyde in PBS, followed by three washes with PBS at room temperature. They were then permeabilized with 0.1% Triton X-100 in PBS for 10 min, followed by three washes with PBS, and then blocked with 10% normal goat serum in PBS containing 0.5% Tween 20 for 1 h at room temperature. Next, the cells were incubated with the mouse monoclonal anti-β-tubulin type III (TUJ1) antibody (1:2000 dilution) primary antibody at 4 °C. Cells were then stained with streptavidin-conjugated secondary antibody (1:400) for 1 h before mounting with Vectashield (Vector Laboratories, Burlingame, CA, USA) containing 4, 6-diamidino-2-phenylindole (DAPI). Immunoreactive cells were detected using a TCS SP5 confocal imaging system (Leica Microsystems, Wetzlar, Germany) at magnification between 40× and 60 ×.
Measurement of neurite outgrowth
Cells were cultured on coverslips coated with fibronectin in 24 well plates, fixed with 0.1% (w/v) picric acid/PBS containing 4% (w/v) paraformaldehyde, and incubated overnight at 4 °C with β-tubulin type III (TUJ1) antibody 1:2000. After incubation with Cy3-conjugated secondary antibody (Jackson Immuno Research Laboratories, PA, USA), cells were mounted on slides with Vectashield. TUJ1-positive cells were photographed using the TCS SP5 confocal imaging system (Leica Microsystems) and morphological characteristics were quantitated using Image J software (NIH-http//rsb.Info.nih.gov/ij/). The length of primary neurite was defined as the distance from the soma to the tip of the longest branch. For each graph, data on neurite length were generated from randomly selected areas of at least five independent cultures from three independent experiments and more than 100 cells were counted for each condition in each experiment.
Statistical analysis
Cells were counted in randomly chosen (fractionator) microscope fields. Data are expressed as means ± SDs of three independent experiments. Differences were analyzed using the Student’s t test and were considered significant at p < 0.05.
Discussion
IL-1β is known to be a key modulator of stress and inflammation in the CNS. Neuroinflammation is implicated in the pathophysiology of many psychiatric and neurodegenerative disorders where cognitive dysfunction and reductions in neurogenesis are evident, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and depression [
2]. IL-1β has been shown to negatively influence the proliferation and differentiation of hippocampal NPCs [
37], and that it impairs neurotrophin-induced neuronal cell survival [
38,
39]. In the spinal cord, IL-1β has been implicated in extensive inflammation and progressive neuro-degeneration after ischemic and traumatic injury [
40,
41]. These results indicate that IL-1β might be involved in the pathogenesis of some neurodevelopmental disorders. However, there is increasing evidence that inflammation-associated cytokines can play a key role in stimulating neurite outgrowth and regeneration [
42,
43]. Recent studies have provided an indication that IL-1β is able to stimulate the migration of cultured cortical neurons [
7], and the potent neurotropic action of IL-1β leads to rapid neurite growth [
44]. In this study, we found that stimulation with IL-1β facilitates neurite outgrowth, as well as increases the expression levels of neuronal factors, such as NT3 and Ngn1. Moreover, IL-1β-induced Wnt5a expression has a critical role in neuronal differentiation of NPCs. These results demonstrate a novel physiological function of IL-1β in neuronal differentiation of cortical NPCs.
Recently, Wnt5a, identified as an axon guidance cue, was shown to activate a non-canonical pathway essential for cortical axonal morphogenesis. In this study, we found that the Wnt5a/RhoA/ROCK/JNK pathway is required for IL-1β-mediated neurite outgrowth of NPCs. The effect of Wnt5a signaling on axon outgrowth of cortical neurons is consistent with a recent study showing that Wnt5a increases axon length and promotes axon differentiation of dissociated hippocampal neurons [
45]. In addition, a recent study suggests that Wnt5a promotes axonal growth through the regulation of calcium signaling during neuronal polarization [
46]. In contrast, a previous study has shown that Wnt5a acts via Ryk receptors to inhibit the neurite outgrowth of sensorimotor cortex [
47]. The different roles of Wnt5a in neurons are probably related to their activity dependent mechanisms. Therefore, it is necessary to elucidate a more detailed mechanism of Wnt5a signaling in developing neurite outgrowth and axon guidance.
Axon outgrowth and navigation during synapse formation are regulated by extracellular signals. The cytoskeletal mechanisms of axonal branching are regulated by Rho GTPases [
23]. Recent reports suggest that the Rho GTPases, including RhoA, Rac1, and Cdc42 play a central role in dendritic development, and that differential activation of Rho-related GTPases contributes to the generation of morphological diversity in the developing cortex [
24,
48]. Cdc42 and Rac1 facilitate axonal branching and growth cone formation [
21]. However, RhoA activity in axonal branch regulation is complex. It has been shown that RhoA negatively regulated axon formation [
26]. RhoA/ROCK suppresses axonal protrusive activity by negatively regulating the filopodia from actin patches [
27]. In contrast, our present study showed that RhoA/ROCK signaling induced by IL-1β or Wnt5a promotes neurite outgrowth, as well as increases the expression levels of neuronal factors, such as NT3 and Ngn1. In addition, recent studies reported that RhoA/ROCK pathway increased neurite outgrowth in hippocampal immortalized cells [
49], and inhibition of RhoA activity reduces axonal branching in hippocampal neurons of embryonic mice [
29]. These findings are consistent with our study showing that RhoA/ROCK signaling plays a positive role in neurite outgrowth. Considering that RhoA plays a different role in the regulation of neurite outgrowth depending on the cell environment (culture conditions and type of stimulation) or cell types, it will be necessary to determine the specific contribution of RhoA activity to all components of cytoskeletal mechanics and neurite outgrowth.
Here, we demonstrate a novel function of IL-1β in neuronal differentiation of NPCs. The findings of the present study are summarized in Fig.
5k. First, we found that stimulation with IL-1β promotes neuronal differentiation. Second, IL-1β induces Wnt5a expression through NF-κB activity. Third, IL-1β-induced Wnt5a is required for neuronal differentiation through a RhoA/ROCK/JNK pathway. Taken together, we conclude that IL-1β promotes neuronal differentiation through a Wnt5a/RhoA/ROCK/JNK pathway in cortical NPCs.