Luo C et al / Acta Pharmacol Sin 2003 Feb; 24 (2): 152-156

Ginkgo biloba leaf extract enhances levels of caspase-3 and amyloid precursor protein in normal rat hippocampus¹

LUO Can, WU Qin, HUANG Xie-Nan, SUN An-Sheng, SHI Jing-Shan²

Department of Pharmacology, Zunyi Medical College, Zunyi 563003, China

¹ Project supported by Natural Science Fundation of Guizhou Province, No (2000)3044. ² Correspondence to Prof SHI Jing-Shan, MD, PhD. Phn 86-852-820-5416. E-mail jsshi@zmc.gz.cn

Received 2002-02-27 Accepted 2002-12-11

KEY WORDS Ginkgo biloba; caspases; amyloid beta-protein precursor; immunohistochemistry

ABSTRACT

AIM: To explore the effect of Ginkgo biloba extract (GbE) on the levels of caspase-3 and amyloid precursor protein (APP) in normal rats' hippocampus. METHODS: Immunohistochemistry method was used for qualitative analysis of the expressions of caspase-3 and APP, and an image analysis method was used for the quantification of the levels of caspase-3 and APP after GbE was administered to rats of different ages for 14 d. RESULTS: The mean absorbance of staining of caspase-3 and APP was markedly higher in GbE group than that in control groups. The expressions of caspase-3 and APP were intensified in the hippocampus of rats after GbE administration. CONCLUSION: GbE can raise the levels of caspase-3 and APP in the hippocampus of normal rats.

INTRODUCTION

The pathogenesis of Alzheimer's disease (AD) is still uncertain and we have not effective therapy yet. In the cascade hypothesis of -amyloid protein (A-)^[1], Hardy et al indicated that A played an important role in pathophysiologic and pathologic process of AD. AD is accompanied by the deposition of A- in senile plaques and cerebral blood vessels. The abnormal secretion and /or deposit of A- are involved in the pathogenesis. Being the main constituent of senile plaque, A- and its precursor-amyloid precursor protein (APP) were studied by many scientists^[2-4]. The latest studies showed that caspase-3 could convert APP to produce A-^[5,6]. GbE is used to combat a variety of neurological disturbances in Alzheimer's disease including depression, lack of attention, and short-term memory loss^[7-9]. In Wirth and colleagues' research, their results provided us evidence with a short-term memory enhancement effect of GbE (60 mg/kg and 120 mg/kg) in both young and aged rats^[10]. Winter also found that GbE (50 mg/kg and 200 mg/kg) had beneficial effects on cognitive performance and longevity in rats^[11]. Yang's and other's studies suggested that GbE could protect neurons against apoptosis induced by free radicals^[12-14]. However, the possible effect of GbE on caspases and APP remained unclear. In the present study, the effect of GbE, at an effective medium dose (100 mg/kg) employed in previous experiments^[10-14], on expressions of caspase-3 and APP, was observed in rat hippocampus.

MATERIALS AND METHODS

Reagents GbE (containing 26 % flavone glycosides and 6 % diterpen lactone, Xinbang Pharmaceutical Factory, Guizhou province) was prepared as 0.1 % solution with normal saline. The antibody of APP-(rabbit-anti-rat) was purchased from Sigma Co in USA. The antibody of caspase-3 (rabbit-anti-rat), SABC (streptavidin-biotin complex) kit (rabbit) and diaminobenzidine (DAB) staining kit were purchased from BOSTER Biological Engineering Co in Wuhan, China.

Apparatus BI2000 image analysis system (Taimeng Co in Chengdu ) and HL-2 transfusion pump.

Treatment of animals Forty-two male Wistar rats (grade II) were supplied by the Experimental Animal Center of the Third Military Medical University. Qualification number is 2001015. All rats were divided into three age groups: One-month-old, four-month-old, and over twelve-month-old. The rats were fed with ordinary forage with free access to tap water. Each age group was randomly divided into two sub-groups: control group and GbE group. GbE 100 mg/kg was administered through intra-gastric method in GbE group for 14 d.

Preparations of brain On d 15, rats were fixed in a supine position after 35 % chloral hydrate anesthesia to fully expose thorax and heart. A trocar (No 16) connected with transfusion pump was inserted into aorta through the tip of heart. Ventroaorta was clamped, and atria was cut open, then transfused forward with cold (4 ºC) phosphate buffer solution (PBS, 0.1 mol/L). When the outflow of PBS from left atria was limpid, PBS was replaced by cold formalin (4 ºC) until the carcass turned rigid. Brain tissues were removed and put into formalin (4 ºC) over night. On the next day, the tissues were removed and deposited in saccharose solution (20 %, containing anti-septics). Freezing coronal brain tissue slice was prepared at 30-m thick each.

Immunohistochemistry Free floating staining and SABC (streptavidin-biotin complex) method were used for detection. DAB was used as a staining agent. Rabbit normal immune serum was used to replace primary antibody (antibody-APP, antibody- caspase-3) in negative control group. PBS was used to replace immediate antibody in working control group.

Image analysis Immunohistochemistry analysis module of BI2000 image analysis system was used to quantify the levels of APP and caspase-3 of hippocampus in brain tissue slices. The values of mean absorbance and mean gradation were used to show the levels.

Statistical method Adopt q-test of Microsoft excel. There is discrepancy when P0.05, and significant discrepancy when P0.01.

RESULTS

Effect of GbE 100 mg/kg on level of APP in normal rats' hippocampus In all brain tissue slices, cytoplasm and membrane of cone-shaped cell in hippocampus showed deeper yellow stain in GbE group than in control group. Image analysis showed that, in four-month-old and over twelve-month-old rats, the value of mean absorbance was higher in GbE group than in control group. The value of mean gradation was lower in GbE treated group than control group, correspondingly. The results indicated that the level of APP in normal rats' hippocampus was elevated in GbE group. While in one-month-old rats, no statistical difference was observed between these two groups.

Tab 1. Effect of GbE 100 mg/kg on level of APP in normal rats' hippocampus. n=7. Mean±SD. ^bP<0.05, ^cP<0.01 vs control. The number in the parentheses means the age of rats in months.

Effect of GbE 100 mg/kg on level of caspase-3 in normal rats' hippocampus In all brain tissue slices, cone-shaped cell in hippocampus showed deeper yellow stain in GbE group than in control group. Markedly higher value of mean absorbance in GbE group than in control group was observed through image analysis (P<0.01). The value of mean gradation was significantly lower correspondingly (P<0.01). The results indicated that the level of caspase-3 in normal rats' hippocampus was increased in GbE group.

Tab 2. Effect of GbE 100 mg/kg on level of caspase-3 in normal rats' hippocampus. n=7. Mean±SD. ^cP<0.01 vs control. The number in the parentheses means the age of rats in months.

Fig 1. Photos of immunohistochemistry. A) Staining of APP in rat hippocampus in GbE group (×400). APP is a transmembrane glycoprotein that extends its carboxylic terminal into cytoplasm. A distinct yellow stain could be observed in cytoplasm and membrane of the cone-shaped cell. B) Staining of APP in rat hippocampus in GbE group (×100). C) Staining of APP in rat hippocampus in control group (×100). D) Staining of caspase-3 in rat hippocampus in GbE group (×400). E) Staining of caspase-3 in rat hippocampus in GbE group (×100). F) Staining of caspase-3 in rat hippocampus in control group (×100).

DISCUSSION

GbE, at doses of 50-200 mg/kg, is beneficial in a broad range of physiological dysfunctions and pathological conditions, including those involved with circulation, blood conditions, metabolism, and immune function^[15-17]. Pharmacological effects of the extract include antioxidant, free radical scavenging, nerve protection, and platelet inhibition properties^[18]. GbE also helps modulate cerebral energy metabolism and inhibit acetylcholinesterase^[19,20]. Numerous pharmacological and clinical studies of GbE have demonstrated a positive effect in enhancing short-term memory^[21]. Generally, GbE is primarily an important remedy for aging and the elderly for restoring proper function and preventing degenerative changes. However, other antioxidants have no equivalent therapeutic properties compared with GbE. As to the ability of inhibiting acetylcholinesterase, GbE is not as good as tacrine^[20]. The mechanism by which it may be used to treat AD needs further research. Although the use of only one dosage of GbE was a drawback in the present study, we argue that an effective dosage could also provide sufficient pharmacological evidence of GbE, especially when the effect was observed by positive response, such as in the immunohistochemistry determination. The GbE 100 mg/kg, which was used in present study, is considered appropriate according to results of our and other laboratory^[10-14]. Results had shown that the levels of APP and caspase-3 in rats' hippocampus were increased after administration of GbE for14 successive days. The results indicated that GbE might affect the caspase-3-mediated proteolysis process of APP.

The proteolysis process of APP has two pathways. One is secretive pathway. The other is endocytotic pathway. In secretive pathway, APP is cleaved by -secretase to generate A segment. While in endocytotic pathway, APP is first cleaved by -secretase where abutting against the amino-terminal of A segment. The process generates a surface APP (APPs) and a P₁₁ segment which contains an integrated A segment. Then P₁₁ is cleaved by -secretase at the carboxyl end of A segment to generate integrated A^[22,23]. A number of laboratories have recently demonstrated that APP can be cleaved in the cytoplasmic domain by caspases after the aspartate-664 residue, Val-Glu-Val-Asp⁶⁶⁴ Ala^[5,23]. This cleavage would generate a C-terminal-truncated APP molecule (APPC31) ^[5,24]. Previous observations show that the APP cytoplasmic region contains a tetrapeptide motif (⁶⁸³YENP⁶⁸⁶) that functions as a signal for endocytosis^[25]. Accordingly, loss of this endocytotic signal abrogates processing of cell surface APP in the endocytotic pathway and severely impairs A production and secretion. Because caspase-cleaved APP lacks the endocytotic signal, the truncated APPC31 should in theory release less, rather than more. This process is indicated in Fig 2.

Fig 2. Alleged interaction between caspase-3 and APP as proposed by Gervais FG^[2,3].

In this experiments, it shows that the levels of APP and caspases were improved in GbE group compared with controls. The result indicates, in agreement with the endocytosis-hypothesis, that GbE might affect the generation of A through caspase-3-mediated proteolytic process of APP.

REFERENCES

• 1 Hardy JA, Higgins GA. Alzheimer's disease: The amyloid cascade hypothesis. Science 1992; 256: 184-5.
• 2 Elkoe DJ. Translating cell biology into therapeutic advances in Alzheimer's disease. Nature 1999; 399: A23-31.
• 3 Games D, Adams D, Alessandrini R, Barbour R, Berthelette P, Blackwell C, et al. Alzheimer-type neuropathology in transgenic mice overexpressing V717F beta-amyloid precursor protein. Nature 1995; 373: 523-7.
• 4 Hsiao K, Chapman P, Nilsen S, Eckman C, Harigaya Y, Younkin S, et al. Correlative memory deficits, A elevation, and amyloid plaques in transgenic mice. Science 1996; 274: 99-102.
• 5 Gervais FG, Xu D, Robertson GS, Vaillancourt JP, Zhu Y, Huang J, et al. Involvement of caspases in proteolytic cleavage of Alzheimer's amyloid-beta precursor protein and amyloidogenic A beta peptide formation. Cell 1999; 97: 395-406.
• 6 Soriano S, Lu DC, Chandra S, Pietrzik CU, Koo EH. The amyloidogenic pathway of amyloid precursor protein (APP) is independent of its cleavage by caspases. J Biol Chem 2001; 276: 29045-50.
• 7 Kanowski S, Herrmann WM, Stephan K, Wierich W, Horr R. Proof of efficacy of the Ginkgo biloba special extract (GbE 761) in outpatients suffering from mild to moderate primary degenerative dementia of the Alzheimer type or multi-infarct dementia. Pharmacopsychiatry 1996; 29: 47-56.
• 8 Le Bars PL, Katz MM, Berman N, Itil TM, Freedman AM, Schatzberg AF, et al. A placebo-controlled, double-blind, randomized trial of an extract of Ginkgo biloba for dementis. North American EGb study group. JAMA 1997; 278: 1327-32.
• 9 Hofferberth B. The efficacy of GbE 761 in patients with senile dementia of Alzheimer type, A double-blind, placebo-controlled study on different levels of investigation. Human Psychopharmacol 1994; 9: 215-22.
• 10 Wirth S, Stemmelin J, Will B, Christen YVES, Di Scala G. Facilitative effects of EGb 761 on olfactory recognition in young and aged rats. Pharmacol Biochem Behav 2000; 65: 321-6.
• 11 Winter JC. The effects of an extract of Ginkgo biloba, EGb 761, on cognitive behavior and longevity in the rat. Physiol Behav 1998; 63: 425-33.
• 12 Yang SF, Yang ZQ, Wu Q, Sun AS, Huang XN, Shi JS. Protective effect and mechanism of Ginkgo biloba leaf extracts for Parkinson disease induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Acta Pharmacol Sin 2001; 22: 1089-93.
• 13 Kiegstein J. Neuroprotective properties of Ginkgo biloba constituents demonstrated in vivo and vitro. J Rev Bras Neurol Agosto 1994; 30 (Suppl 1): 7.
• 14 Zhang Y, Ming L, Li JP, Li WP, Fang M. Protective effects of Egb on apoptosis of neurons. Chin J Pharmacol Ther 2001; 6: 25-7.
• 15 Pietri S, Maurelli E, Drieu K, Culcasi M. Cardioprotective and anti-oxidant effects of the terpenoid constituents of Ginkgo biloba extract (EGb 761). J Mol Cell Cardiol 1997; 29: 733-42.
• 16 Janssens D, Michiels C, Delaive E, Eliaers F, Drieu K, et al. Protection of hypoxia-induced ATP decrease in endothelial cells by Ginkgo biloba extract and bilobalide. Biochem Pharmacol 1995; 50: 991-9.
• 17 Turcan M, Cozmei C, Carasevici E, Haulica I. Effect of the EGb 761 (Ginkgo bilboa extract) on primary immune response in experimental chronic stress. Rev Med Chir Soc Med Nat Iasi 1995; 99: 154-9.
• 18 Kim YS, Pyo MK, Park KM, Park PH, Hahn BS, Wu SJ, et al. Antiplatelet and antithrombotic effects of a combination of ticlopidine and Ginkgo biloba ext (EGb 761). Thromb Res 1998; 91: 33-8.
• 19 Sastre J, Millan A, Garcia de la Asuncion J, Pla R, Juan G, Pallardo, et al. A Ginkgo biloba extract (EGb 761) prevents mitochondrial aging by protecting against oxidative stress. Free Radic Biol Med 1998; 24: 298-304.
• 20 Wettstein A. Cholinesterase inhibitor and ginkgo extracts in therapy of dementia. A comparison of effectiveness based on controlled studies. Fortschr Med 1999;117: 48-9.
• 21 Winter JC. The effects of an extract of Ginkgo biloba, EGb 761, on cognitive behavior and longevity in the rat. Physiol Behav 1998; 63: 425-33.
• 22 Selkoe DJ. Physiological production of the ss-amyloid protein and the mechanism of Alzheimer's disease. Trends Neurol Sci 1993; 16: 403-9.
• 23 Selkoe DJ. Alzheimer's disease: genotypes, phenotype, and treatments. Science 1997; 275: 630-1.
• 24 Lu DC, Rabizadeh S, Chandra S, Shayya RF, Ellerby LM, Ye X, et al. A second cytotoxic proteolytic peptide derived from amyloid beta-protein precursor. Nat Med 2000; 6: 397-404.
• 25 Perez RG, Soriano S, Hayes JD, Ostaszewski B, Xia W, Selkoe DJ, et al. Mutagenesis identifies new signals for -amyloid precursor protein endocytosis, turnover, and the generation of secreted fragments, including A42. J Biol Chem 1999; 274: 18851-6.