Effect of polygonatum polysaccharide on the hypoxia-induced apoptosis and necrosis in in vitro cultured cerebral cortical neurons from neonatal rats

BACKGROUND: Cardiocerebrovascular diseases induced cerebral circulation insufficiency and senile vascular dementia can result in ischemic/hypoxic apoptosis of central neurons, which we should pay more attention to and prevent and treat as early as possible. Traditional Chinese medicine possesses the unique advantage in this field. Polygonatum, a Chinese herb for invigorating qi, may play a role against the hypoxic apoptosis of brain neurons. OBJECTIVE: To observe the protective effect of polygonatum polysaccharide on hypoxia-induced apoptosis and necrosis in cerebral cortical neurons cultured in vitro. DESIGN: A comparative experiment. SETTING: Laboratory of Cell Biology, Institute of Basic Medical Sciences, Jiangxi Provincial Academy of Traditional Chinese Medicine. MATERIALS: The experiment was carried out in the Laboratory of Cell Biology, Institute of Basic Medical Sciences, Jiangxi Provincial Academy of Traditional Chinese Medicine from November 2003 to April 2005. Totally 218 Wistar rats (male or female) of clean degree within 24 hours after birth were purchased from the animal center of Jiangxi Medical College (certification number was 021-97-03). METHODS: ① Preparation of cerebral cortical neurons of rats: The cerebral cortical tissues were isolated from the Wistar rats within 24 hours after birth, and prepared to single cell suspension, and the cerebral cortical neurons of neonatal rats were in vitro cultured in serum free medium with Neurobasal plus B27 Supplement. ② Observation on the non-toxic dosage of polygonatum polysaccharide on neurons: After the neurons were cultured for 4 days, polygonatum polysaccharide of different dosages (1-20 g/L) was added for continuous culture for 48 hours, the toxicity and non-toxic dosage of polygonatum polysaccharide on neurons were observed and detected with trypan blue staining. ③ Grouping: After hypoxia/reoxygenation, the cultured neurons were divided into normal control group, positive apoptotic group and polygonatum polysaccharide group. In the normal control group, the neurons were cultured at 37 ℃ in CO2 with the volume fraction of 0.05 under saturated humidity for 6 days. In the apoptotic positive group, the neurons were cultured with hypoxia for 12 hours after 4-day culture, and followed by reoxygenation for 48 hours. In the polygonatum polysaccharide group, polygonatum polysaccharide with the terminal concentration of 0.5, 1 and 1.5 g/L was added to some neurons at 10 hours before the hypoxia culture, and then the neurons were cultured with hypoxia for 12 hours, followed by reoxygenation for 48 hours; polygonatum polysaccharide with the terminal concentration of 0.5, 1 and 1.5 g/L was added to the other neurons at 12 hours after hypoxia followed by reoxygenation for 48 hours. ④ The Hoechst33342 fluorescence staining, Annexin V/PI flow cytometer, appearance of DNA agarose gel electrophoresis gradient strap and immunohistochemical staining were used to observe the expressions of Bcl-2, Bax and Caspase-3 apoptotic and anti-apoptotic proteins and the ratio of Bcl-2/Bax, and observe the effect of polygonatum polysaccharide against the hypoxic apoptosis of cerebral cortical neurons of neonatal rats. MAIN OUTCOME MEASURES: ① Toxicity and non-toxic dosage of polygonatum polysaccharide on neurons; ② Apoptotic rate of neurons detected with Hoechst33342 fluorescence staining; ③ Early apoptotic rate and necrotic rate of neurons detected with Annexin V/PI flow cytometer; ④ DNA agarose gel electrophoresis ladder-like strap appeared or not; ⑤ Expressions of Bcl-2, Bax and Caspase-3 apoptotic and anti-apoptotic proteins and the ratio of Bcl-2/Bax. RESULTS: ① Polygonatum polysaccharide within 6 g/L had no cytotoxicity on the normal cultured cerebral cortical neurons (P > 0.05). ② The apoptotic rates of neurons detected with Hoechst33342 fluorescence staining had significant differences between the polygonatum polysaccharide groups and positive apoptosis group added to neurons at 10 hours before the hypoxia culture [(13.00±4.52)%,(12.72±2.15)%,(11.80±1.18)%,(38.03±1.05)%, P < 0.01], and had no significant differences between the polygonatum polysaccharide groups and positive apoptosis group added to neurons at 12 hours after the hypoxia culture (36.77±1.45)%, (36.60±1.61)%, (36.37±2.02)%, (38.03±1.05)%, P > 0.05].③ Annexin V/PI flow cytometer detected that the anti-necrotic effect was enhanced with the increased concentration of polygonatum polysaccharide within 0.5-1.5 g/L (P < 0.01). Polygonatum polysaccharide of 0.5-1.5 g/L added before hypoxia could significantly decrease the apoptotic rate of neurons induced by hypoxia/reoxygenation (P < 0.01). ④ No DNA agarose gel electrophoresis ladder-like strap appeared in the groups with polygonatum polysaccharide of 0.5-1.5 g/L added at 10 hours before hypoxia; ⑤ After Polygonatum polysaccharide of 0.5-1.5 g/L was added before hypoxia, the expression of Bcl-2 protein of hypoxic neurons was increased (P < 0.01), and those of Bax protein and Caspase-3 protein were reduced (P < 0.01), and the ratio of Bcl-2/Bax was increased (P < 0.01). CONCLUSION: Polygonatum polysaccharide within 6 g/L has no cytotoxicity on the normal cultured cerebral cortical neurons. Polygonatum polysaccharide of 0.5-1.5 g/L added before hypoxia plays a role agains necrosis of neurons induced by hypoxia. Polygonatum polysaccharide of 0.5-1.5 g/L can significantly reduce the apoptosis of neurons induced by hypoxia through up-regulating the expression of Bcl-2 protein, down-regulating the expressions of Bax protein and Caspase-3 protein, and increasing the ratio of Bcl-2/Bax.