Sphingosine 1-phosphate lyase ablation disrupts presynaptic architecture and function via an ubiquitin- proteasome mediated mechanism.

Daniel N Mitroi,André U Deutschmann,Konstantine Glebov,Michael Hans,Dieter Swandulla,Maren Raucamp,Gerhild Van Echten-Deckert,Indulekha Karunakaran,Julie Saba,Oleg Shupliakov,Jochen Walter,Markus Gräler,Dan Ehninger,Elena Sopova

doi:10.1038/srep37064

Abstract

The bioactive lipid sphingosine 1-phosphate (S1P) is a degradation product of sphingolipids that are particularly abundant in neurons. We have shown previously that neuronal S1P accumulation is toxic leading to ER-stress and an increase in intracellular calcium. To clarify the neuronal function of S1P, we generated brain-specific knockout mouse models in which S1P-lyase (SPL), the enzyme responsible for irreversible S1P cleavage was inactivated. Constitutive ablation of SPL in the brain (SPLfl/fl/Nes) but not postnatal neuronal forebrain-restricted SPL deletion (SPLfl/fl/CaMK) caused marked accumulation of S1P. Hence, altered presynaptic architecture including a significant decrease in number and density of synaptic vesicles, decreased expression of several presynaptic proteins, and impaired synaptic short term plasticity were observed in hippocampal neurons from SPLfl/fl/Nes mice. Accordingly, these mice displayed cognitive deficits. At the molecular level, an activation of the ubiquitin-proteasome system (UPS) was detected which resulted in a decreased expression of the deubiquitinating enzyme USP14 and several presynaptic proteins. Upon inhibition of proteasomal activity, USP14 levels, expression of presynaptic proteins and synaptic function were restored. These findings identify S1P metabolism as a novel player in modulating synaptic architecture and plasticity.

Highlights

The major regulator of intracellular Sphingosine 1-phosphate (S1P) levels is S1P-lyase (SPL)
We further show that S1P accumulation is essential for the assessed elevation of ubiquitin-proteasome system (UPS) which is responsible for the decreased expression of several presynaptic proteins and of the deubiquitinating protease USP14
In contrast to systemic SPL deletion, mice lacking SPL only in neural tissue (SPLfl/fl/Nes) exhibit a rather unremarkable phenotype and their lifespan is comparable to that of their wild type littermates, representing a promising model to analyze the role of SPL in brain physiology

Summary

Introduction

The major regulator of intracellular S1P levels is S1P-lyase (SPL). It catalyses the irreversible cleavage of S1P to hexadecenal and ethanolamine phosphate, the final step of sphingolipid catabolism[11]. S1P accumulation in SPL-deficient neurons was associated with increased cytosolic calcium levels[4] and ER-stress[13], which mediated apoptotic neuronal death[4] Based on these findings and due to early postnatal death of systemic SPL knockouts[13], we assumed that brain-specific ablation of SPL might serve as a tool to clarify the role of neuronal S1P. We further show that S1P accumulation is essential for the assessed elevation of ubiquitin-proteasome system (UPS) which is responsible for the decreased expression of several presynaptic proteins and of the deubiquitinating protease USP14. The latter was shown to play a critical role in synaptic plasticity and its loss is associated with several physiological impairments in the central nervous system[14]

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