Sphingosine-1-Phosphate (S1P) Impacts Presynaptic Functions by Regulating Synapsin I Localization in the Presynaptic Compartment.
Growing evidence indicates that sphingosine-1-P (S1P) upregulates glutamate secretion in hippocampal neurons. However, the molecular mechanisms through which S1P enhances excitatory activity remain largely undefined. The aim of this study was to identify presynaptic targets of S1P action controlling exocytosis. Confocal analysis of rat hippocampal neurons showed that S1P applied at nanomolar concentration alters the distribution of Synapsin I (SynI), a presynaptic phosphoprotein that controls the availability of synaptic vesicles for exocytosis. S1P induced SynI relocation to extrasynaptic regions of mature neurons, as well as SynI dispersion from synaptic vesicle clusters present at axonal growth cones of developing neurons. S1P-induced SynI relocation occurred in a Ca(2+)-independent but ERK-dependent manner, likely through the activation of S1P3 receptors, as it was prevented by the S1P3 receptor selective antagonist CAY1044 and in neurons in which S1P3 receptor was silenced. Our recent evidence indicates that microvesicles (MVs) released by microglia enhance the metabolism of endogenous sphingolipids in neurons and stimulate excitatory transmission. We therefore investigated whether MVs affect SynI distribution and whether endogenous S1P could be involved in the process. Analysis of SynI immunoreactivity showed that exposure to microglial MVs induces SynI mobilization at presynaptic sites and growth cones, whereas the use of inhibitors of sphingolipid cascade identified S1P as the sphingolipid mediating SynI redistribution. Our data represent the first demonstration that S1P induces SynI mobilization from synapses, thereby indicating the phosphoprotein as a novel target through which S1P controls exocytosis.


CONCLUSIONS
Growing evidence indicates that the bioactive lipid sphingosine and its metabolite sphingosine-1-P (S1P) stimulate excitatory transmission. While it has been recently clarified that sphingosine influences directly the exocytotic machinery by activating the synaptic vesicle protein VAMP2 to form SNARE fusion complexes, the molecular mechanism by which S1P promotes neurotransmission remained largely undefined. In this study, we identify Synapsin I, a presynaptic phosphoprotein involved in the control of availability of synaptic vesicles for exocytosis, as the key target of S1P action. In addition, we provide evidence that S1P can be produced at mature axon terminals as well as at immature growth cones in response to microglia-derived signals, which may be important to stabilize nascent synapses and to restore or potentiate transmission.
Consiglio Nazionale delle Ricerche Institute of Neuroscience, Milano 20129, Italy, Department of Biotechnology and Translational Medicine, University of Milano, Milano 20129, Italy.
Loredana
L
Riganti
Consiglio Nazionale delle Ricerche Institute of Neuroscience, Milano 20129, Italy, Department of Biotechnology and Translational Medicine, University of Milano, Milano 20129, Italy.
Flavia
F
Antonucci
Department of Biotechnology and Translational Medicine, University of Milano, Milano 20129, Italy.
Martina
http://orcid.org/0000-0003-4958-541X
M
Gabrielli
Consiglio Nazionale delle Ricerche Institute of Neuroscience, Milano 20129, Italy.
Ilaria
I
Prada
Department of Biotechnology and Translational Medicine, University of Milano, Segrate 20090, Italy.
Paola
P
Giussani
Department of Biotechnology and Translational Medicine, University of Milano, Segrate 20090, Italy.
Paola
http://orcid.org/0000-0002-5728-6631
P
Viani
San Raffaele Scientific Institute/Vita e Salute University, Milan 20132, Italy, and.
Flavia
http://orcid.org/0000-0001-9556-4746
F
Valtorta
Consiglio Nazionale delle Ricerche Institute of Neuroscience, Milano 20129, Italy, Istituto Di Ricovero e Cura a Carattere Scientifico Humanitas, Rozzano 20089, Italy.
Elisabetta
E
Menna
Consiglio Nazionale delle Ricerche Institute of Neuroscience, Milano 20129, Italy, Istituto Di Ricovero e Cura a Carattere Scientifico Humanitas, Rozzano 20089, Italy.
Michela
M
Matteoli
Consiglio Nazionale delle Ricerche Institute of Neuroscience, Milano 20129, Italy, Istituto Di Ricovero e Cura a Carattere Scientifico Humanitas, Rozzano 20089, Italy c.verderio@in.cnr.it.
Claudia
http://orcid.org/0000-0001-7216-5873
C
Verderio
D000818:Animals; D002478:Cells, Cultured; D005260:Female; D006624:Hippocampus / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D008246:Lysophospholipids / Q000032:analysis / Q000502:physiology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017729:Presynaptic Terminals / Q000737:chemistry / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000032:analysis / Q000502:physiology; D013569:Synapses / Q000737:chemistry / Q000378:metabolism*; D016704:Synapsins / Q000032:analysis / Q000096:biosynthesis*
D008246:Lysophospholipids; D016704:Synapsins; C060506:sphingosine 1-phosphate; D013110:Sphingosine
S-FTY720-vinylphosphonate; S1P3 receptor; microglia; neurotransmitter release; sphingosine-1-phosphate; synapsin I