Reduced sphingosine kinase-1 and enhanced sphingosine 1-phosphate lyase expression demonstrate deregulated sphingosine 1-phosphate signaling in Alzheimer's disease

Abstract

Background: The accumulation of beta amyloid (Aβ) peptides, a hallmark of Alzheimer's disease (AD) is related to mechanisms leading to neurodegeneration. Among its pleiotropic cellular effects, Aβ accumulation has been associated with a deregulation of sphingolipid metabolism. Sphingosine 1-phosphate (S1P) derived from sphingosine is emerging as a critical lipid mediator regulating various biological activities including cell proliferation, survival, migration, inflammation, or angiogenesis. S1P tissue level is low and kept under control through equilibrium between its synthesis mostly governed by sphingosine kinase-1 (SphK1) and its degradation by sphingosine 1-phosphate lyase (SPL). We have previously reported that Aβ peptides were able to decrease the activity of SphK1 in cell culture models, an effect that could be blocked by the prosurvival IGF-1/IGF-1R signaling.

Results: Herein, we report for the first time the expression of both SphK1 and SPL by immunohistochemistry in frontal and entorhinal cortices from 56 human AD brains. Immunohistochemical analysis revealed a decreased expression of SphK1 and an increased expression of SPL both correlated to amyloid deposits in the entorhinal cortex. Otherwise, analysis of brain tissue extracts showed a decrease of SphK1 expression in AD brains whereas SPL expression was increased. The content of IGF-1R, an activator of SphK1, was found decreased in AD brains as well as S1P1, the major receptor for S1P.

Conclusions: Collectively, these results highlight the importance of S1P in AD suggesting the existence of a global deregulation of S1P signaling in this disease from its synthesis by SphK1 and degradation by SPL to its signaling by the S1P1 receptor.

Figure 1

Schematic representation of the sphingolipid rheostat in AD context. Ceramide can be generated de novo or by hydrolysis of sphingomyelin by sphingomyelinase. Ceramide is subsequently metabolized by ceramidase to generate sphingosine which in turn produces sphingosine 1-phosphate through phosphorylation by sphingosine kinase-1 and sphingosine kinase-2. All these reactions are reversible. Sphingosine 1-phosphate can be catabolized into hexadecenal + phospho-ethanolamine by the action of sphingosine 1-phosphate lyase. Sphingosine 1-phosphate exerts intracellular signaling which can promote, calcium mobilization, epigenetic modulation and survival effects. S1P has also extracellular effects by signaling through 5 G protein-coupled receptors. The agonist FTY720 can bind S1P_1,3,4,5. IGF-1/IGF-1R signaling pathway is able to activate SphK1, inducing its translocation to the plasma membrane. In AD, Aβ peptide is able to upregulate sphingomyelinase activity which consequently increases the level of ceramide. In turn, ceramide stabilizes BACE-1 which increases APP cleavage leading to generation of Aβ peptide [22]. S1P level as IGF-1R level are decreased in AD [23]. SMSyn, sphingomyelin synthase; SMase, sphingomyelinase; Cerase, ceramidase; CerSyn, ceramide synthase; Sph, sphingosine; SphK1/2, sphingosine kinase 1/2; S1Pase, sphingosine 1-phosphate phosphatase.

Figure 2

Immunohistochemical study of SphK1/SPL expression and Aβ deposits. (a) Double labeling of SphK1 and Aβ in AD brain section from entorhinal cortex. Diffuse and focal deposits are seen in different areas. X200. Inset: Strongly stained neurons (SphK1++, black arrow) and mild stained neurons (SphK1+, white arrow) are clearly distinguishable. X400 (b) Double labeling of SPL and Aβ in AD brain section from entorhinal cortex. A senile plaque is mainly surrounded by strong labeled neurons (SLP++). X400.

Figure 3

Correlation between neurons with strong expression of SphK1 and Aβ deposits. (a) Packing density of SphK1++ neurons and density of Aβ deposits are not correlated in frontal cortex. (b) A negative correlation between the packing density of SphK1++ neurons and the density of Aβ deposits is observed in entorhinal cortex. Data are represented as scatter plot with regression line.

Figure 4

Correlation between neurons with strong expression of SPL and Aβ deposits. (a) Packing density of SPL++ neurons and density of Aβ deposits are not correlated in frontal cortex. (b) A positive correlation between the packing density of SPL++ neurons and the density of Aβ deposits is observed in entorhinal cortex. Data are represented as scatter plot with regression line.

Figure 5

Immunoblot analysis of SphK1, SPL, S1P₁ and IGF-1R. Tissue lysates from frontal cortex (a) and temporal cortex in the hippocampal area (b) of AD and control brains were assayed for SphK1, SPL, S1P₁ and IGF-1R expression. Equal loading of protein was monitored using antibody to β-actin. The intensity of each band was quantified using ImageJ software and is provided as representative under corresponding blot. Values are expressed at 1/1000.