Doenjang, A Korean Traditional Fermented Soybean Paste, Ameliorates Neuroinflammation and Neurodegeneration in Mice Fed a High-Fat Diet

Abstract

Obesity is considered a risk factor for neurodegeneration. Because fermentation of soybean increases contents of various bioactive compounds with anti-obesity and anti-diabetic activities, we investigated the protective effect of doenjang, a Korean traditional fermented soybean paste, against neuroinflammation and neurodegeneration in the cortex and hippocampus of mice fed a high-fat (HF) diet. C57BL/6J mice were fed a low-fat diet, an HF diet, an HF-containing steamed soybean diet, or an HF-containing doenjang (DJ) diet for 11 weeks. Doenjang consumption alleviated hippocampal neuronal loss, which was increased by the HF diet. Accordingly, we observed higher cell proliferation and neurotrophic factor mRNA levels in the DJ group. Contents of oxidative metabolites and mRNA levels of oxidative stress- and neuroinflammation-related genes were lower in the DJ group compared to the HF group. Dietary doenjang reduced β-amyloid peptide (Aβ) levels by regulating gene expressions involved in Aβ production and degradation. Furthermore, doenjang consumption reduced tau hyperphosphorylation induced by HF feeding. Overall, doenjang was more effective than steamed soybean in suppressing neuroinflammation and neurodegeneration in mice fed an HF diet. These results suggest that bioactive compounds produced during the fermentation and aging of soybean may be involved in the enhanced neuroprotective effects of doenjang.

Keywords: fermentation; high-fat diet; isoflavone; neurodegenerative disease; neuroinflammation; soybean.

Figure 1

Effects of doenjang on neuronal death and neurogenesis in the cortex and hippocampus of mice fed a high-fat diet. (a) Brain weights (n = 11–12/group). (b) Representative hippocampal CA1 subfield sections stained with cresyl violet (n = 4/group). Relative protein levels of apoptotic markers: (c) SBDP, (d) cleaved caspase-3, and (e) CHOP were determined using immunoblotting (n = 3/group). (f) Relative protein levels of a neurogenesis marker, p-CREB, were determined using immunoblotting (n = 3/group). (g) Relative mRNA expression of a gene involved in synaptic plasticity, BDNF, were determined using qPCR (n = 4–5/group). (h) Relative protein levels of a proliferation marker, PCNA, were determined using immunoblotting (n = 3/group). Each bar represents the mean ± SEM. Data with a different alphabetical superscript were significantly different from one another (p < 0.05). LF, a low-fat diet; HF, a high-fat diet; SS, an HF diet containing freeze-dried steamed soybean; DJ, an HF diet containing freeze-dried doenjang; BDNF, brain-derived neurotrophic factor; CHOP, C/EBP homologous protein; CREB, cAMP-response element binding protein; SDBP, spectrin α breakdown products.

Figure 2

Effects of doenjang on oxidative stress and neuroinflammation in the cortex and hippocampus of mice fed a high-fat diet. (a) Relative mRNA expression of genes involved in oxidative stress, HO-1, was determined using qPCR (n = 4–5/group). Levels of (b) TBARS and (c) carbonylated proteins were determined using enzymatic assays (n = 5/group). (d) Relative protein levels of catalase were determined using immunoblotting (n = 3/group). Relative mRNA expression of genes involved in (e) pro-inflammation and in (f) astrocyte activation were determined using qPCR (n = 4–5/group). Each bar represents the mean ± SEM. Data with a different alphabetical superscript were significantly different from one another (p < 0.05). LF, a low-fat diet; HF, a high-fat diet; SS, an HF diet containing freeze-dried steamed soybean; DJ, an HF diet containing freeze-dried doenjang; GFAP, glial fibrillary acidic protein

Figure 3

Effects of doenjang on Aβ deposition in the cortex and hippocampus of mice fed a high-fat diet. (a) Relative protein levels of Aβ were determined using immunoblotting (n = 3/group). (b) Relative mRNA expression of genes involved in Aβ production, BACE1 and PS1, were determined using qPCR (n = 4–5/group). (c) Relative protein levels of APP kinase, JNK, were determined using immunoblotting (n = 3/group). (d) Relative mRNA expression of a gene involved in Aβ degradation, IDE, were determined using qPCR (n = 4–5/group). (e) Relative ubiquitinated protein levels were determined using immunoblotting (n = 3/group). Each bar represents the mean ± SEM. Data with a different alphabetical superscript were significantly different from one another (p < 0.05). LF, a low-fat diet; HF, a high-fat diet; SS, an HF diet containing freeze-dried steamed soybean; DJ, an HF diet containing freeze-dried doenjang; IDE, insulin degrading enzyme.

Figure 3

Effects of doenjang on Aβ deposition in the cortex and hippocampus of mice fed a high-fat diet. (a) Relative protein levels of Aβ were determined using immunoblotting (n = 3/group). (b) Relative mRNA expression of genes involved in Aβ production, BACE1 and PS1, were determined using qPCR (n = 4–5/group). (c) Relative protein levels of APP kinase, JNK, were determined using immunoblotting (n = 3/group). (d) Relative mRNA expression of a gene involved in Aβ degradation, IDE, were determined using qPCR (n = 4–5/group). (e) Relative ubiquitinated protein levels were determined using immunoblotting (n = 3/group). Each bar represents the mean ± SEM. Data with a different alphabetical superscript were significantly different from one another (p < 0.05). LF, a low-fat diet; HF, a high-fat diet; SS, an HF diet containing freeze-dried steamed soybean; DJ, an HF diet containing freeze-dried doenjang; IDE, insulin degrading enzyme.

Figure 4

Effects of doenjang on tau hyperphosphorylation in the cortex and hippocampus of mice fed on a high-fat diet. Relative protein levels of (a) dephosphorylated tau, (b) phosphorylated tau (ser422), (c) phosphorylated tau kinase, GSK3β, and (d) methylated tau phosphatase, PP2A, were determined using immunoblotting (n = 3/group). Each bar represents the mean ± SEM. Data with a different alphabetical superscript were significantly different from one another (p < 0.05). LF, a low-fat diet; HF, a high-fat diet; SS, an HF diet containing freeze-dried steamed soybean; DJ, an HF diet containing freeze-dried doenjang.