Chemical Constituents from Hericium erinaceus Promote Neuronal Survival and Potentiate Neurite Outgrowth via the TrkA/Erk1/2 Pathway

Hericium erinaceus is a culinary-medicinal mushroom used traditionally in Eastern Asia to improve memory. In this work, we investigated the neuroprotective and neuritogenic effects of the secondary metabolites isolated from the MeOH extract of cultured mycelium of H. erinaceus and the primary mechanisms involved. One new dihydropyridine compound (6) and one new natural product (2) together with five known compounds (1,3–5,7) were obtained and their structures were elucidated by spectroscopic analysis, including 2D NMR and HRMS. The cell-based screening for bioactivity showed that 4-chloro-3,5-dimethoxybenzoic methyl ester (1) and a cyathane diterpenoid, erincine A (3), not only potentiated NGF-induced neurite outgrowth but also protected neuronally-differentiated cells against deprivation of NGF in PC12 pheochromocytoma cells. Additionally, compound 3 induced neuritogenesis in primary rat cortex neurons. Furthermore, our results revealed that TrkA-mediated and Erk1/2-dependant pathways could be involved in 1 and 3-promoted NGF-induced neurite outgrowth in PC12 cells.


Introduction
Neurotrophins (NTs), such as nerve growth factor (NGF), have been extensively explored for their essential contribution for the survival and differentiation in neuronal cells [1,2]. There is growing evidence that the edecline of NTs is an important factor in the pathogenesis of neurodegenerative diseases (ND) [3]. In fact, genetic or direct induction of NGF has already been elucidated as a promising treatment for ND [4,5]. However, NGF's high molecular weight restricts its delivery across the blood-brain barrier (BBB). Hence, NGF-mimic or NGF-potentiated small molecules show significant therapeutic potential for their pharmacological advantages [6,7].
Hericium erinaceus (Bull.: Fr) Pers is an edible and medicinal mushroom widely consumed in Asian countries and has been paid more and more attention recently for its prominent neurotrophic and neuroprotective properties [8][9][10][11][12][13]. In in vitro assays, extracts from H. erinaceus were reported to stimulate NGF synthesis and/or promote NGF-induced neurite outgrowth in various cell types [14,15]. In in vivo assays, H. erinaceus's fruiting bodies exhibited significant anti-dementia activity [16,17]. The 2 of 13 aqueous extract from fruiting bodies also promoted the injured nerve regeneration in a Sprague-Dawley rat model [18].
In fact, the active components obtained from H. erinaceus, as well as other mushrooms, can be divided into two groups based on their molecular weight [10]. Erinacines and hericenones are representative small molecules with significant neurotrophic and neuroprotective effects and high convenience to cross the BBB [19,20]. Erinacines A-I showed a stronger ability to increase mRNA expression in NGF synthesis than epinephrine as a positive control in murine astroglial cells [21]. Erinacine P, Q, and their biosynthetically-related compounds were also reported to induce NGF synthesis [21]. In vivo, erinacine A, a cyathane diterpenoid, obviously upregulated the level of NGF in the rat's locus caeruleus and hippocampus other than the cerebral cortex [22]. Additionally, erinacine A effectively prevented PC12 apoptosis induced by glutamate insult [23]. Erinacine A also protected neurons from death by inhibiting nitrotyrosine-containing proteins, phosphorylation of p38 mitogen-activated protein kinases (MAPK), CCAAT/enhancer-binding protein (C/EBP), and homologous protein (CHOP) in a rat model of global ischemic stroke [24]. Above all, these findings revealed erinacine A as one of the definite elements responsible for the NGF-synthesis stimulating and neuroprotective activities of H. Erinaceus, which can be further proved by the fact that erinacine A was able to obviously ameliorate pathologies in Alzheimer's disease (AD) and Parkinson's disease (PD) mouse models [25,26]. However, little is known about the direct neuritogenic activity and its detailed mechanism of the functional erinacines, such as erinacine A.
The objective of this research was to explore the potential neuroprotective and neuritogenic effects of 4-chloro-3,5-dimethoxybenzoic methyl ester (1) and erinacine A (3), components of H. erinaceus cultured mycelium extracts, on PC 12 cells as well as primary cultured rat cortex neurons. Furthermore, the involved signaling pathways were revealed by monitoring phosphorylation level as well as cell morphological changes due to selective inhibitors. Our hypothesis is that compounds 1 and 3 can strengthen the activation of tyrosine kinase A (TrkA) triggered by NGF to promote neurite outgrowth by upregulating related downstream signaling, such as extracellular signal-regulated protein kinase 1/2 (Erk1/2) which is essential for neurite outgrowth in PC12 cells.

Structural Elucidation of Compounds 1-7
One new compound (6), one new natural product 3-(hydroxymethyl)-2-furaldehyde (2) and five known compounds, namely, 4-chloro-3,5-dimethoxybenzoic methyl ester (1) [27], erinacine A (3) [28], herierin III (4) [29], herierin IV (5) [29], and erinacerin G (7) [30], were isolated from the MeOH extract of the mycelium of H. erinaceus (Figure 1). The known compounds were identified by comparing their spectral data with those reported in the literature.  [16,17]. The aqueous extract from fruiting bodies also promoted the injured nerve regeneration in a Sprague-Dawley rat model [18]. In fact, the active components obtained from H. erinaceus, as well as other mushrooms, can be divided into two groups based on their molecular weight [10]. Erinacines and hericenones are representative small molecules with significant neurotrophic and neuroprotective effects and high convenience to cross the BBB [19,20]. Erinacines A-I showed a stronger ability to increase mRNA expression in NGF synthesis than epinephrine as a positive control in murine astroglial cells [21]. Erinacine P, Q, and their biosynthetically-related compounds were also reported to induce NGF synthesis [21]. In vivo, erinacine A, a cyathane diterpenoid, obviously upregulated the level of NGF in the rat's locus caeruleus and hippocampus other than the cerebral cortex [22]. Additionally, erinacine A effectively prevented PC12 apoptosis induced by glutamate insult [23]. Erinacine A also protected neurons from death by inhibiting nitrotyrosine-containing proteins, phosphorylation of p38 mitogen-activated protein kinases (MAPK), CCAAT/enhancer-binding protein (C/EBP), and homologous protein (CHOP) in a rat model of global ischemic stroke [24]. Above all, these findings revealed erinacine A as one of the definite elements responsible for the NGF-synthesis stimulating and neuroprotective activities of H. Erinaceus, which can be further proved by the fact that erinacine A was able to obviously ameliorate pathologies in Alzheimer's disease (AD) and Parkinson's disease (PD) mouse models [25,26]. However, little is known about the direct neuritogenic activity and its detailed mechanism of the functional erinacines, such as erinacine A.
The objective of this research was to explore the potential neuroprotective and neuritogenic effects of 4-chloro-3,5-dimethoxybenzoic methyl ester (1) and erinacine A (3), components of H. erinaceus cultured mycelium extracts, on PC 12 cells as well as primary cultured rat cortex neurons. Furthermore, the involved signaling pathways were revealed by monitoring phosphorylation level as well as cell morphological changes due to selective inhibitors. Our hypothesis is that compounds 1 and 3 can strengthen the activation of tyrosine kinase A (TrkA) triggered by NGF to promote neurite outgrowth by upregulating related downstream signaling, such as extracellular signal-regulated protein kinase 1/2 (Erk1/2) which is essential for neurite outgrowth in PC12 cells.

Compounds 1 and 3 Promoted NGF-Induced Neurite Outgrowth in PC12 Cells
At first, we evaluated the neuritogenic effects of the MeOH extract from the mycelium of H. erinaceus by using PC12 cells since it was a widely-employed in vitro model for neuronal differentiation and neurite outgrowth studies. The extract (12.5-200 µg/mL) promoted NGF-induced neurite outgrowth in PC12 cells ( Figure S8). Then, PC12 cells were treated with NGF (2 ng/mL) alone or in combination with the seven isolated compounds at the concentrations of 0.3, 3, and 30 µM to identify the specific components responsible for the neurotrophic activity. Consequently, 1 and 3 enhanced the growth of neurites in a dose-dependent manner and significantly increased the maximal neurite length in the presence of NGF (Figure 3a,b). In parallel, the NGF-induced expression of neuronal differentiation marker proteins (i.e., MAP2 and β3-tubulin), were also obviously upregulated by the combination of 1 or 3 with NGF ( Figure 3c). Taken together, these data indicated that compounds 1 and 3 could effectively induce neuritogenesis when the PC12 cells were triggered by a low concentration of NGF. This finding agrees well with other research. Hericenones C-E from H. Erinaceus markedly potentiated the neurite outgrowth of PC12 cells in the presence of NGF (5 ng/mL) [32]. Treatment of PC12 cells with corallocins A-or C-conditioned media produced by 1321N1 astrocytes resulted in significant increase of neurite outgrowth [33]. Furthermore, it was demonstrated that increasing neurite outgrowth activity in PC12 cells by the above compounds exhibited a strong correlation with the stimulation of NGF synthesis [32,33]. However, erinacine A was unable to stimulate NGF synthesis with or without NGF in PC12 cells ( Figure S9). This indicated that erinacine A induced neuritogenesis in PC12 acting as a stimuli for NGF potency instead of NGF level.

Compounds 1 and 3 Promoted NGF-Induced Neurite Outgrowth by Activating TrkA/Erk1/2 Pathway in PC12 Cells
It was reported that stimulation of PC12 cells with NGF induces TrkA phosphorylation. Tyr-490-phophorylated form TrkA is required for Shc association and activation of the Ras-MAP kinase cascade, which is one of vital pathways involved into NGF-induced neurite outgrowth [34][35][36]. To explore the possible mechanism underlying the neurotrophic activity, the influences of compound 1 and 3 on TrkA and Erk1/2 were examined.

Compounds 1 and 3 Promoted NGF-Induced Neurite Outgrowth by Activating TrkA/Erk1/2 Pathway in PC12 Cells
It was reported that stimulation of PC12 cells with NGF induces TrkA phosphorylation. Tyr-490-phophorylated form TrkA is required for Shc association and activation of the Ras-MAP kinase cascade, which is one of vital pathways involved into NGF-induced neurite outgrowth  (Figure 4a). Specifically, k252a abolished the neuritogenesis induced by NGF and 1 or 3, alone or in combination at a level comparable to the DMSO control (around 5%). Similarly, the neuritogenic effect of iridoidcatalpol from Rehmannia glutinosa (Shengdihuang) was also completely abolished by k252a in a primary culture of forebrain neurons [37]. In contrast, the activity of potentiating NGF-mediated PC12 neurite outgrowth by hericenone E was found to be partially blocked by k252a [32]. Furthermore, U0126 reduced compounds 1 and 3-potentiated neurite outgrowth by 50%. It was consistent with the fact that Erk1/2 was not the sole pathway involved into the NGF-induced neurite outgrowth in PC12 cells [36]. Taken together, it indicated that NGF-induced neurite outgrowth promoted by compounds 1 and 3 may be entirely TrkA-dependent and partially Erk1/2-mediated. . Normalized intensity of phospho-protein versus protein is expressed as the mean ± SD from three independent experiments. ** p < 0.01, *** p < 0.001 represent significant differences compared with NGF-treated PC12 cells (positive control) (ANOVA followed by Dunnett's test).

Compounds 1 and 3 Prevented the Death of Neuronally-Differentiated PC12 Cells against the Removal of NGF
Differentiated PC12 cells need NGF to maintain survival, proliferation and the phenotypic properties of sympathetic neurons. It was reported that several natural products such as senegenin can protect neurites of differentiated PC12 cells from the removal of NGF [40]. In order to investigate the effect of compounds 1 and 3 on the neuronally-differentiated PC12 without NGF, the cell viability was measured by using MTT assay. PC12 cells were neuronally-differentiated treated with NGF (30 ng/mL) for 7 d. The cells were treated with 1 and 3 at the concentrations of 0.03, 0.3, and 3 μM after NGF withdrawal and cell viability was measured 48 h later. After NGF withdrawal, 50-60% of neuronally-differentiated PC12 cells were dead 48 h later (Figure 5a). At the concentration h pretreatment with or without inhibitors followed by 96 h exposure to 2 ng/mL NGF or indicated agents (with 2 ng/mL NGF). Data are expressed as the mean ± SD from three independent experiments. ** p < 0.01, *** p < 0.001 represent significant differences compared with the inhibitor-free groups (ANOVA followed by Dunnett's test); (b) Western blotting analysis of 1 and 3 on the NGF-induced TrkA and Erk1/2 phosphorylation in PC12 cells. The cells were treated with NGF (2 ng/mL) and compound 1 or 3 (3 and 30 µM), alone or in combination for 15 min (TrkA) or 30 min (Erk1/2). β-actin was detected as the control of protein loading; (c) Densitometric analysis of protein phosphorylation levels on Western blotting (b). Normalized intensity of phospho-protein versus protein is expressed as the mean ± SD from three independent experiments. ** p < 0.01, *** p < 0.001 represent significant differences compared with NGF-treated PC12 cells (positive control) (ANOVA followed by Dunnett's test). 1 and 3 Enhanced NGF-Induced TrkA and Erk1/2 Phosphorylation in PC12 Cells PC12 cells were starved and exposed to NGF and 1 or 3, alone or in combination for 15 min (TrkA) or 30 min (Erk1/2). The phosphorylation of TrkA (Y490) and a downstream protein, Erk1/2, were analyzed by Western blotting. Both 1 and 3 promoted NGF-induced phosphorylation of TrkA (Y490) and Erk1/2 in a concentration-dependent manner (Figure 4b,c). Previous reports have revealed that upregulation of TrkA and/or Erk1/2 phosphorylation by some natural products agreed well with their potency to promote neurite outgrowth [32,38,39]. It is, thus, predicted that compounds 1 and 3 may mediate the interaction between NGF and its receptor TrkA, enhance the activation of TrkA and its downstream signaling Erk1/2 and finally potentiate NGF-induced neurite outgrowth in PC12 cells. 1 and 3 Prevented the Death of Neuronally-Differentiated PC12 Cells against the Removal of NGF Differentiated PC12 cells need NGF to maintain survival, proliferation and the phenotypic properties of sympathetic neurons. It was reported that several natural products such as senegenin can protect neurites of differentiated PC12 cells from the removal of NGF [40]. In order to investigate the effect of compounds 1 and 3 on the neuronally-differentiated PC12 without NGF, the cell viability was measured by using MTT assay. PC12 cells were neuronally-differentiated treated with NGF (30 ng/mL) for 7 d. The cells were treated with 1 and 3 at the concentrations of 0.03, 0.3, and 3 µM after NGF withdrawal and cell viability was measured 48 h later. After NGF withdrawal, 50-60% of neuronally-differentiated PC12 cells were dead 48 h later (Figure 5a). At the concentration of 3 µM, the cells treated with 1 and 3 increased NGF-deprived PC12 cell viability up to 131% and 138%, respectively, versus untreated cells, which is comparable with the cells treated with 20 ng/mL NGF (Figure 5b). These results indicated that compounds 1 and 3 were able to protect differentiated PC 12 cells from the removal of NGF. Since compound 1 was reported to protect mouse neuroblastoma N2a (Neuro-2a) cells against endoplasmic reticulum stress [27], our findings confirmed the neuroprtective function of 4-chloro-3,5-dimethoxybenzoic methyl ester. 138%, respectively, versus untreated cells, which is comparable with the cells treated with 20 ng/ml NGF (Figure 5b). These results indicated that compounds 1 and 3 were able to protect differentiated PC 12 cells from the removal of NGF. Since compound 1 was reported to protect mouse neuroblastoma N2a (Neuro-2a) cells against endoplasmic reticulum stress [27], our findings confirmed the neuroprtective function of 4-chloro-3,5-dimethoxybenzoic methyl ester.

Compound 3 Induced Neurite Outgrowth in Primary Rat Cortex Neurons
Primary cultured neurons possess majority of the in vivo neuronal properties. Thus, rat cortical neurons were chosen to screen neurotrophic molecules from H. erinaceum. As a result, compound 3 was investigated to induce the neurite extension (Figure 6a). The maximal neurite length for the neurons treated with 3 (3 and 30 μM) were significantly larger than untreated neurons, but smaller than the basic fibroblast growth factor (bFGF, 10 ng/mL) control (Figure 6b). These data indicated that compound 3 can mimic NTs finitely in primary rat cortex neurons. (a) Representive cell morphology images and (b) cell viability was assessed by MTT assay. Data are expressed as the mean ± SD from three independent experiments. ** p < 0.01 represent significant differences compared with vehicle (ANOVA followed by Dunnett's test).

Compound 3 Induced Neurite Outgrowth in Primary Rat Cortex Neurons
Primary cultured neurons possess majority of the in vivo neuronal properties. Thus, rat cortical neurons were chosen to screen neurotrophic molecules from H. erinaceum. As a result, compound 3 was investigated to induce the neurite extension (Figure 6a). The maximal neurite length for the neurons treated with 3 (3 and 30 µM) were significantly larger than untreated neurons, but smaller than the basic fibroblast growth factor (bFGF, 10 ng/mL) control (Figure 6b). These data indicated that compound 3 can mimic NTs finitely in primary rat cortex neurons. cells after removal of NGF. PC12 cells were seeded in collagen-coated 96-well plates in normal serum medium for 24 h, then changed to low serum medium (2% HS and 1% FBS) containing NGF (30 ng/mL). After seven days, PC12 cells were exposure to vehicle (0.1% DMSO) as a negative control, NGF (20 ng/mL) as a positive control, or 1, 3 (0.03, 0.3, 3 μM) for an additional 48 h. Scale bar: 100 μm. (a) Representive cell morphology images and (b) cell viability was assessed by MTT assay. Data are expressed as the mean ± SD from three independent experiments. ** p < 0.01 represent significant differences compared with vehicle (ANOVA followed by Dunnett's test).

Compound 3 Induced Neurite Outgrowth in Primary Rat Cortex Neurons
Primary cultured neurons possess majority of the in vivo neuronal properties. Thus, rat cortical neurons were chosen to screen neurotrophic molecules from H. erinaceum. As a result, compound 3 was investigated to induce the neurite extension (Figure 6a). The maximal neurite length for the neurons treated with 3 (3 and 30 μM) were significantly larger than untreated neurons, but smaller than the basic fibroblast growth factor (bFGF, 10 ng/mL) control (Figure 6b). These data indicated that compound 3 can mimic NTs finitely in primary rat cortex neurons. Figure 6. The effect of compound 3 on promoting neurite outgrowth in primary cultured rat cortex neurons. Primary rat cortex neurons were seeded in poly-L-lysine-coated 24-well plates in DMEM for 24 h, then changed to neurobasal medium (2% B27 and 0.5 mM L-glutamine) and exposure to vehicle (0.1% DMSO) as a negative control, bFGF (10 ng/mL) as a positive control, or 3 (0.3, 3, 30 µM) for an additional six days. Scale bar: 100 µm. (a) Cell morphology was observed and photographed as described in Section 3.8; (b) Neurite length was measured as described in Section 3.8. Data are expressed as the mean ± SD from three independent experiments. ** p < 0.01 represent significant differences compared with vehicle (ANOVA followed by Dunnett's test).

Fungal Materials
The fungal strain H. erinaceum H6 (accession no. DQ185914) was purchased from the Applied Research Institute of Microbiology, Academy of Agricultural Sciences, Xinjiang, China. It was identified based on 16S rDNA sequence analysis with 99% similarity by Beijing Sunbiotech Co. and has been deposited at Shaanxi Key Labotory of Natural Products and Chemical Biology, Northwest A and F University. H. erinaceus was cultured on slants of potato dextrose agar at 25 • C for 10 days. Agar plugs were inoculated in a 500 mL Erlenmeyer flask containing 120 mL of the culture medium and incubated at 25 • C on a rotary shaker at 100 rpm for 10 days. The components of the culture medium were listed as follows: glucose 0.4%, malt extract 1%, yeast extract 0.4% in distilled water. The culture medium was adjusted to pH 6.5 before sterilization. The scale-up fermentation was carried out in 40 × 500 mL Erlenmeyer flasks each containing 80 g of rice and 120 mL of distilled water. Each flask was inoculated with 5.0 mL of the culture medium and incubated at 25 • C for 40 days.
Primary rat cortex neurons were purchased from Gibco BRL. The cells were seeded into poly-L-lysine (Sigma-Aldrich Inc.: Saint Louis, MI, USA) coated 24-well plates at a density of 2 × 10 4 cells/mL. After 24 h, the medium was replaced with Neurobasal medium supplement with B27 (2%), L-glutamine (0.5 mM) and 1% penicillin/streptomycin. The cells were maintained in a humidified 5% CO 2 incubator.

MTT Assay for Cell Viability
PC12 cells were seeded in collagen coated 96-well plates at a density of 8 × 10 3 cells/mL. After 24 h, the culture medium was replaced with the differential medium containing 30 ng/mL of NGF for seven days. After NGF withdrawal, the cells were treated with compounds for 48 h. Then the cells were incubated with of MTT (0.5 mg/mL) for 2 h at 37 • C. The medium was carefully removed and 100 µL of 50% DMSO/50% EtOH was added into each well. The absorbance was measured at 560 nm by a microplate reader. The results were expressed as the percentage of MTT reduction, assuming the absorbance of blank control was 100%.

Assay of Neurite Outgrowth of PC12 Cells
The neurite outgrowth was measured as previously reported [42,43]. Briefly PC12 cells were seeded in poly-L-lysine-coated 24-well plates at a density of 8 × 10 3 cells/mL with normal serum medium for 24 h, and then treated with the drugs and NGF, alone or in combination. PC12 cells were photographed by an inverted microscope and examined for the percentage of neurite-bearing cells relative to the total cells in the field or the average of maximal neurite length in individual cells.

Statistical Analysis
Data were expressed as mean ± standard error (SD). All experiments were carried out at least three times. The results were analyzed by ANOVA followed by Dunnett's test, with p < 0.01 and p < 0.001 taken to indicate significant difference.

Conclusions
In the present study, one new compound 6 and one new natural product 2 from the cultured mycelium of H. erinaceus were elucidated. Two known compounds, 1 and 3, have been explored with new neurotrophic and neuroprotective activities. Specifically, compounds 1 and 3 can effectively protect the neuronally-differentiated PC12 cells against deprival of NGF. Compound 3 can mimic finitely neuritogenic activity of NTs in primary rat cortex neurons. Compounds 1 and 3 can also obviously potentiate NGF-induced neurite outgrowth independent of the stimulation of NGF synthesis in PC12 cells. Furthermore, analysis of cellular signaling pathways revealed that NGF-induced neurite outgrowth potentiated by compounds 1 and 3 is completely TrkA-mediated and partially Erk1/2-dependent. Collectively, these facts strengthen the perception that Hericium erinaceus as a potential agent to reduce the risk of various neurodegenerative diseases, which may offer useful benefits to help the development of new drugs against these diseases.