The roles of sphingosine kinase 2 in behaviour, myelination and protection against Alzheimer’s disease

Abstract

Sphingosine 1-phos¬phate (S1P) is a po¬tent vasculo- and neuro-pro¬tective signalling lipid that pro¬motes neuro¬trophic growth factor ex¬pression and pre-synaptic acetylcholine and glu¬tam¬ate release. It signals through its own family of five G-protein coupled receptors. S1P is synthesised by sphingosine kinase 1 and 2 (SK1/2). Our research group demonstrated pro-nounced loss of S1P and SK2 activity early in Alz¬heimer’s dis¬ease (AD) patho¬genesis, leading us to speculate that loss of S1P sensitises to AD de¬velop¬ment. Paradoxically, SK2 was reported to mediate amyloid β (Aβ) formation from amyloid precursor protein (APP) in vitro, which implies a role for this enzyme in promoting AD pathology. In this study we investigated the consequences of global SK2 knockout (SK2Δ) on neurological functions, particularly in the context of myelination and Alzheimer’s disease. We first examined behavioural effects of SK2 deletion in the absence of pathology. We then crossed SK2Δ mice to the J20 mouse model of familial AD amyloidosis to test whether SK2 deficiency synergises with amyloid beta (Aβ) in pro¬moting AD. S1P levels were reduced 85-90% in brains of SK2Δ mice, indicating that SK2 is responsible for the majority of S1P synthesis in the brain. SK2Δ mice displayed notably higher freezing response when placed in a novel context in the fear conditioning paradigm, independently of the fear stimulus, indicating a general anxiety phenotype in this strain. In agreement with a role in amyloid formation, SK2 deficiency re¬duced Aβ content, plaque burden, and reactive astrogliosis in J20 mice. This was as¬so¬ci¬ated with sig¬nifi¬cant impro-vements in hyper¬synchronous activity and cross-frequency coupling measured by hippo¬campal electro¬encephalo¬graphy. However, SK2-deficient J20 mice exhibited severe hypo¬mye¬lina¬tion in the hippo¬campus and cortex, hippo¬campal atrophy, and sig¬nifi¬cant deficits in the Y-maze and social novelty memory tests, when com¬pared to the J20 or SK2Δ strains. We also uncovered a significant hypomyelination phenotype in SK2Δ mice, leading us to investigate the role of S1P receptors in myelination, and particularly the oligodendrocyte-specific S1PR5 receptor. Female SK2Δ mice were administered the S1PR5-selective agonist A-971432 or S1PR1/5 dual-agonist Siponimod, to determine if activation of these receptors would preserve myelination. Despite observing pronounced myelin depletion in male SK2Δ mice, no such impairments were identified in female SK2Δ mice. Nonetheless, a significant increase of understudied 18:2 base sphingadiene lipids was observed in both male and female SK2Δ mice. As myelin is comprised 80% of lipids, further investigation on the potential impact of these 18:2 lipids on myelin structural integrity is required. In summary, SK2-generated S1P was required for oligodendrocyte survival in an AD mouse model, yet was paradoxically also an important endogenous regulator of Aβ formation. These results urge consideration of the importance of myelin loss as a driver of neurodegeneration in AD. Future work should further investigate the different hypomyelination phenotypes in male and female SK2Δ mice, including any potential interaction between sex hormones and SK2-generated S1P in myelination.