SGPL1 (sphingosine phosphate lyase 1) modulates neuronal autophagy via phosphatidylethanolamine production

Abstract

Macroautophagy/autophagy defects have been identified as critical factors underlying the pathogenesis of neurodegenerative diseases. The roles of the bioactive signaling lipid sphingosine-1-phosphate (S1P) and its catabolic enzyme SGPL1/SPL (sphingosine phosphate lyase 1) in autophagy are increasingly recognized. Here we provide in vitro and in vivo evidence for a previously unidentified route through which SGPL1 modulates autophagy in neurons. SGPL1 cleaves S1P into ethanolamine phosphate, which is directed toward the synthesis of phosphatidylethanolamine (PE) that anchors LC3-I to phagophore membranes in the form of LC3-II. In the brains of SGPL1fl/fl/Nes mice with developmental neural specific SGPL1 ablation, we observed significantly reduced PE levels. Accordingly, alterations in basal and stimulated autophagy involving decreased conversion of LC3-I to LC3-II and increased BECN1/Beclin-1 and SQSTM1/p62 levels were apparent. Alterations were also noticed in downstream events of the autophagic-lysosomal pathway such as increased levels of lysosomal markers and aggregate-prone proteins such as APP (amyloid β [A4] precursor protein) and SNCA/α-synuclein. In vivo profound deficits in cognitive skills were observed. Genetic and pharmacological inhibition of SGPL1 in cultured neurons promoted these alterations, whereas addition of PE was sufficient to restore LC3-I to LC3-II conversion, and control levels of SQSTM1, APP and SNCA. Electron and immunofluorescence microscopy showed accumulation of unclosed phagophore-like structures, reduction of autolysosomes and altered distribution of LC3 in SGPL1fl/fl/Nes brains. Experiments using EGFP-mRFP-LC3 provided further support for blockage of the autophagic flux at initiation stages upon SGPL1 deficiency due to PE paucity. These results emphasize a formerly overlooked direct role of SGPL1 in neuronal autophagy and assume significance in the context that autophagy modulators hold an enormous therapeutic potential in the treatment of neurodegenerative diseases.

Neural sphingosine 1‐phosphate accumulation activates microglia and links impaired autophagy and inflammation

“…Sphingolipids are pleiotropic signaling molecules and we have earlier shown that S1P is neurotoxic (Hagen et al, ) and that systemic knock out of SGPL1 that catalyzes the breakdown of S1P can cause neurodegeneration (Hagen et al, ). Further, with SGPL1 fl/fl/Nes mice, we have demonstrated autophagy defects that could, in turn, lead to accumulation of aggregate‐prone proteins and behavioral changes in mice (Mitroi et al, ).…”

“…In contrast, in neurons SGPL1 deficiency causes accumulation of S1P on the one hand and a drop in phosphatidylethanolamine on the other hand. The latter proved to block neuronal autophagy flux via an mTOR independent mechanism (Mitroi et al, ), whereas the former was shown to stimulate neuronal autophagy (Moruno Manchon et al, )…”

“…Interestingly, the neurotoxic effect of S1P was receptor independent (Hagen, Hans, Hartmann, Swandulla, & van Echten‐Deckert, ). Further, we have recently shown that the deficiency of SGPL1 in the brain not only increases the amount of sphingoid bases but also reduces the amount of phosphatidylethanolamine, synthesized from ethanolamine phosphate, one of the degradation products of S1P (Mitroi et al, ). This causes a block of the autophagic flux in neurons leading to neurodegeneration (Mitroi et al, ).…”

“…This causes a block of the autophagic flux in neurons leading to neurodegeneration (Mitroi et al, 2017).…”

Inhibition of acid sphingomyelinase disrupts LYNUS signaling and triggers autophagy

“…Alternatively, subsequent reductions in S1P levels may be implicated, although increased rather than decreased intracellular S1P have been associated with triggering autophagy (22). The pro-autophagic effect of S1P production and the importance of the subcellular localization of S1P accumulation have been recently described in neuronal cells (23)(24)(25). This body of work suggests that the autophagy triggered by ASM inhibition, which was associated with decreased S1P levels occurs via a different mechanism than by S1P overproduction.…”