Abstract
BackgroundThere has been ongoing research impetus to uncover novel blood-based diagnostic and prognostic biomarkers for Alzheimer’s disease (AD), vascular dementia (VaD), and related cerebrovascular disease (CEVD)-associated conditions within the spectrum of vascular cognitive impairment (VCI). Sphingosine-1-phosphates (S1Ps) are signaling lipids which act on the S1PR family of cognate G-protein-coupled receptors and have been shown to modulate neuroinflammation, a process known to be involved in both neurodegenerative and cerebrovascular diseases. However, the status of peripheral S1P in AD and VCI is at present unclear.MethodsWe obtained baseline bloods from individuals recruited into an ongoing longitudinal cohort study who had normal cognition (N = 80); cognitive impairment, no dementia (N = 160); AD (N = 113); or VaD (N = 31), along with neuroimaging assessments of cerebrovascular diseases. Plasma samples were processed for the measurements of major S1P species: d16:1, d17:1, d18:0, and d18:1, along with pro-inflammatory cytokines interleukin (IL)-6, IL-8, and tumor necrosis factor (TNF). Furthermore, in vitro effects of S1Ps on cytokine expression were also studied in an astrocytoma cell line and in rodent primary astrocytes.ResultsOf the S1Ps species measured, only d16:1 S1P was significantly reduced in the plasma of VaD, but not AD, patients, while the d18:1 to d16:1 ratios were increased in all cognitive subgroups (CIND, AD, and VaD). Furthermore, d18:1 to d16:1 ratios correlated with levels of IL-6, IL-8, and TNF. In both primary astrocytes and an astroglial cell line, treatment with d16:1 or d18:1 S1P resulted in the upregulation of mRNA transcripts of pro-inflammatory cytokines, with d18:1 showing a stronger effect than d16:1. Interestingly, co-treatment assays showed that the addition of d16:1 reduced the extent of d18:1-mediated gene expression, indicating that d16:1 may function to “fine-tune” the pro-inflammatory effects of d18:1.ConclusionTaken together, our data suggest that plasma d16:1 S1P may be useful as a diagnostic marker for VCI, while the d18:1 to d16:1 S1P ratio is an index of dysregulated S1P-mediated immunomodulation leading to chronic inflammation-associated neurodegeneration and cerebrovascular damage.
Highlights
Alzheimer’s disease (AD) and vascular dementia (VaD) are the two major causes of cognitive impairment in the elderly and represent a major healthcare burden on both developed and developing countries [1, 2]
Altered peripheral inflammatory and S1P markers in cognitive subgroups With respect to peripheral cytokines and S1P levels, Table 2 shows that IL-8 levels were significantly increased in all cognitive subgroups, while tumor necrosis factor (TNF) levels were raised only in AD and VaD
D16:1 S1P levels were significantly lower in all cognitive subgroups, while ratios of d18:1 to d16:1 S1P were significantly higher in all cognitive subgroups, with the highest values found in the VaD group
Summary
Alzheimer’s disease (AD) and vascular dementia (VaD) are the two major causes of cognitive impairment in the elderly and represent a major healthcare burden on both developed and developing countries [1, 2]. Given the extended prodromal stages where potential disease-modifying therapies may more likely be efficacious [16], especially for certain forms of small vessel CEVD [14, 17], the availability of reliable, measurable diagnostic biomarkers is essential for optimal clinical management and will help in the advancement and assessment of new therapeutic strategies for AD and VCI To this end, the development of bloodbased biomarkers has been ongoing, with novel markers for a wide range of processes being evaluated [3, 18,19,20,21]. The status of peripheral S1P in AD and VCI is at present unclear
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