Loss of SPNS1, a lysosomal transporter, in the nervous system causes dysmyelination and white matter dysplasia

Reviewer #1 (Public Review):

Summary:

In this study, the authors studied the roles of SPNS1 which is a lysolipid transporter from the lysosomes in the nervous system using cell and mouse models. The authors tried to show that reduced sphingosine release from the lysosomes via SPNS1 affects myelination.

Strengths:

The authors used knockout models for cells and animals so the results are solid. They also used electron microscopic analysis of the phenotypes of the cells and mouse tissues.

Weaknesses:

The biochemical methods are not fully described at the moment. There is a lack of solid evidence to support the major claim.

If the authors could provide solid evidence that lipids that are released from the lysosomes via SPNS1 are used for myelination, this would be a major finding for the sources of lipids for the formation of axons.

Reviewer #2 (Public Review):

Summary:

Spns1 is a recently identified lysosomal transporter of lysophospholipids and sphingosine and its mutations in humans lead to neurodegeneration with white matter dysplasia. Since global Spns1 deficiency is embryonic lethal, the role of this particular lipid transporter in the nervous system remained unclear. In this study, Ichimura et al generated and analyzed nervous system-specific Spns1 knockout mice. The mutant mice showed epilepsy, growth retardation, demyelination, and early death, with accumulation of various LPC, LPE, and LPI species as well as sphingosine in specific areas of the brain. Probably due to impaired lysosomal efflux of sphingosine, brain levels of sphingolipids (ceramides, sulfatides, and glycolipids), which are main myelin components, were markedly reduced in the KO brain.

Strengths:

This study has provided convincing evidence for the first time that nervous system-specific deletion of Spns1 in mice leads to neurodegeneration, with disturbed lysophospholipid and sphingolipid metabolism in the brain. The results support the idea that the defective transport of lysosomal sphingosine by loss of Spns1 leads to a marked reduction of sphingolipid species required for myelin formation. This study significantly contributes to the research fields of neurodegeneration, lysosomal biology, and lipid biology.

Weaknesses:

It remains unclear why oligodendrocytes but not neurons are specifically damaged and how astroglia are affected by Spns1 deficiency. Lysosomal efflux of lysophospholipids and sphingosine by Spns1 relied solely on the knowledge from published studies and was not addressed in this study. The expression of key lipid-metabolizing genes and molecular markers should be examined more deeply. Several images lack quantification.

Reviewer #3 (Public Review):

Summary:

The authors attempted to understand the effect of Spns1 deficiency in the brain using a brain-specific knockout mouse model. Basic phenotyping of the brain KO line was performed that included mass spectroscopy for lipids, metabolomics, mass spec imaging of brain tissue, and some histology. Similar methods were used for characterising the liver KO model. The main findings supported by the data are that brain KO results in hypomyelinated brains, brain KO mice presented with symptoms akin to epilepsy, and postnatal lethality at 5 weeks of age. In addition, biochemical studies showed that brain KO mice had significant accumulation in whole brain lysates of the lysolipids LPC and LPE and sphingosine with reduced levels of ceramide, sphingomyelin, and sulfatide. Some of the substantial claims made by the authors in an attempt to provide a mechanistic understanding of the data are not strongly supported by experimental data. Some of the major concerns are that the authors claim hypomyelination is not caused by changes in oligodendrocyte differentiation, but experimental evidence to support this was not provided. The authors also claim that hypomyelination and other neurological phenotypes are caused by reduced sphingosine transport by Spns1 leading to reduced sphingolipid synthesis. However, this conclusion is not supported by experimental data and the authors do not address other equally plausible hypotheses.