Activation of sphingosine 1-phosphate receptor-1 by SEW2871 improves cognitive function in Alzheimer's disease model rats

Abstract

Sphingosine-1 phosphate (S1P) is involved in a variety of cellular processes via activation of S1P receptors (S1PRs; S1PR1 to S1PR5) that are highly expressed in the brain. It has been shown that the level of S1P is reduced in the brain of Alzheimer's disease (AD) patients. However, there is no study designed to evaluate the expression of S1PRs in AD brains. The objectives of the present work are (1) to examine the expression of S1PR1-3 in the hippocampus of beta amyloid (Aβ) 1-42 injected rats and (2) to clarify the effects of chronic S1PR1 activation on S1PR1-3 levels, spatial memory deficit and hippocampal damage in AD rats. SEW2871, the S1PR1 selective agonist, repeatedly was injected intraperitoneally during a period of two weeks. Upon Western Blot data bilateral intrahippocampal injection of Aβ1-42 decreased the expression of S1PR1 while increased S1PR2 level and did not affect that of S1PR3. We found that chronic administration of SEW2871 inhibited the reduction of S1PR1 expression and ameliorated spatial memory impairment in the Morris water maze task in rats. In addition, SEW2871 attenuated the Aβ1-42-induced hippocampal neuronal loss according to Nissl staining findings. Data in the current study highlights the importance of S1PR1 signaling pathway deregulation in AD development and suggests that activation of S1PR1 may represent a potential approach for developing new therapeutics to manage memory deficit and apoptosis associated with neurodegenerative disorders such as AD.

Keywords: Alzheimer's disease; SEW2871; cognitive function; sphingosine-1 phosphate receptors.

Table 1. Effect of chronically treatment of SEW2871 on Aβ1-42-induced neuronal loss in CA1 region of hippocampus

Figure 1. Diagrammatic representation of the study protocol. One day after the stereotaxic surgery, daily treatment with SEW2871 or vehicle started, which continued for 14 days. Each small black square represents one day and the “gray mark” shows the time of the Morris water maze test performance. Scarification of animals was done 14 days after the surgery.

Figure 2. Effect of SEW2871 on S1PR1-3 expression in Aβ1-42 injected rats evaluated by Western blotting method. 15 days after the stereotaxic procedure, the expression of S1PR1-3 in hippocampus was detected. Results showed decreasing of S1PR1 (A) and increasing of S1PR2 (B) in AD model rats. Treatment of AD model rats with SEW2871 resulted in elevated levels of S1PR1 (A) in hippocampus and had no effect on S1P2 (B). The expression of S1P3 did not show any change between groups (C). β-actin protein was used here as an internal control. One representative Western blot is shown; n = 6. The band density values were calculated and the values from the control were used as 1. Values are the mean ± S.E.M., **P<0.01 and ***P<0.001 vs. control group; ### P<0.001 vs. Aβ group.

Figure 3. Effect of SEW2871 on spatial learning and memory performance in Aβ1-42 injected rats in Morris water maze test. Intrahippocampal injection of PBS or Aβ1-42 (2 μg/2 μL/ rat hippocampus) was performed and from the next day, rats were treated with a daily intraperitoneal injection of SEW2871 (0.5 mg/kg) or DMSO (vehicle). Hidden platform test was carried out on days 11-14 after the injection of Aβ1-42 and a probe trial was conducted in the day after the final training trial. Significant deficit in learning and memory performance was observed in Aβ injected group and chronic treatment with SEW2871 improved learning and memory task. Distance moved to reach platform (cm) (A) Escape latency in target quadrant (sec) (B) and Time spent in target quadrant (sec) (C) are shown and compared between the experimental groups. Data are expressed as mean ± S.E.M., n=10. *P<0.05, ***P<0.001 vs. control; #P<0.05, ###P<0.001 vs. Aβ group

Figure 4. Effect of SEW2871 on Aβ-induced damage in CA1 region of rat hippocampus. Investigation of Nissl stained photomicrographs from CA1 region of hippocampus showed that the pyramidal cells layer in CA1 area of Aβ1-42-injected rats are markedly thinner than that in the control group. While in AD rats which chronically received SEW2871 it was as thick as that in control group. From left to right: control, Aβ, Aβ + SEW groups. Photos represent ×400 magnification

Figure 5. Effect of SEW2871 on level of cleaved caspase-3 in the hippocampus of Aβ1-42 injected rats. The expression of cleaved caspase-3 was determined by Western blotting method. The level of cleaved caspase-3 in control, Aβ and Aβ + SEW groups (One representative Western blot is shown; n=6) (A). The relative densities of corresponding bands were measured and compared (B). Each point shows the mean ± S.E.M.,***P < 0.001 vs. control group and ## P< 0.01 vs. Aβ group