Cerebrovascular degradation of TRKB by MMP9 in the diabetic brain
Deepti Navaratna, … , Xiaoying Wang, Eng H. Lo
Deepti Navaratna, … , Xiaoying Wang, Eng H. Lo
Published July 15, 2013
Citation Information: J Clin Invest. 2013;123(8):3373-3377. https://doi.org/10.1172/JCI65767.
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Brief Report Neuroscience

Cerebrovascular degradation of TRKB by MMP9 in the diabetic brain

Abstract

Diabetes elevates the risk for neurological diseases, but little is known about the underlying mechanisms. Brain-derived neurotrophic factor (BDNF) is secreted by microvascular endothelial cells (ECs) in the brain, functioning as a neuroprotectant through the activation of the neurotrophic tyrosine kinase receptor TRKB. In a rat model of streptozotocin-induced hyperglycemia, we found that endothelial activation of MMP9 altered TRKB-dependent trophic pathways by degrading TRKB in neurons. Treatment of brain microvascular ECs with advanced glycation endproducts (AGE), a metabolite commonly elevated in diabetic patients, increased MMP9 activation, similar to in vivo findings. Recombinant human MMP9 degraded the TRKB ectodomain in primary neuronal cultures, suggesting that TRKB could be a substrate for MMP9 proteolysis. Consequently, AGE-conditioned endothelial media with elevated MMP9 activity degraded the TRKB ectodomain and simultaneously disrupted the ability of endothelium to protect neurons against hypoxic injury. Our findings demonstrate that neuronal TRKB trophic function is ablated by MMP9-mediated degradation in the diabetic brain, disrupting cerebrovascular trophic coupling and leaving the brain vulnerable to injury.

Authors

Deepti Navaratna, Xiang Fan, Wendy Leung, Josephine Lok, Shuzhen Guo, Changhong Xing, Xiaoying Wang, Eng H. Lo

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Figure 1

Cerebrovascular MMP9 is upregulated in diabetes.

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Cerebrovascular MMP9 is upregulated in diabetes. (A) Representative in s...
(A) Representative in situ zymograms of brain sections from 6-week diabetic and age-matched nondiabetic rats. Scale bars: 100 μm. (B) Representative zymogram showing increased MMP9 in microvessel-enriched fractions in 6-week diabetic rat cortex. (C and D) Quantification of MMP9 levels after in situ zymography and gelatin zymography of microvessel-enriched fractions, respectively. *P < 0.05 between control and diabetic rat sections; n = 4 per group. Error bars indicate SEM. (E) Representative zymogram showing a dose-dependent increase in MMP9 after AGE treatment (0–200 μg/ml). (F) Representative zymogram showing that exposure to 50 μg/ml AGE-BSA increased MMP9 secretion, while nonglycated BSA did not. (G) Densitometric analysis of zymogram (zym.) showing that AGE-BSA significantly increased pro- and active MMP9 compared with untreated cells or cells treated with normal BSA. *P < 0.05 between untreated and AGE-BSA treatment; n = 3 per group. Error bars indicate SEM. (H) Representative zymogram showing that the ability of AGE-BSA to upregulate endothelial MMP9 is blocked by anti-RAGE antibodies. (I) Representative Western blot shows increased pERK in cells exposed to AGE-BSA for 48 hours. (J) Representative zymogram shows that U0126 (5 μM) prevents the AGE-BSA–induced increase in MMP9. Std, standard.