Hereditary sensory neuropathy type 1-associated deoxysphingolipids cause neurotoxicity, acute calcium handling abnormalities and mitochondrial dysfunction in vitro

Hereditary sensory neuropathy type 1 (HSN-1) is a peripheral neuropathy most frequently caused by mutations in the SPTLC1 or SPTLC2 genes, which code for two subunits of the enzyme serine palmitoyltransferase (SPT). SPT catalyzes the first step of de novo sphingolipid synthesis. Mutations in SPT result in a change in enzyme substrate specificity, which causes the production of atypical deoxysphinganine and deoxymethylsphinganine, rather than the normal enzyme product, sphinganine. Levels of these abnormal compounds are elevated in blood of HSN-1 patients and this is thought to cause the peripheral motor and sensory nerve damage that is characteristic of the disease, by a largely unresolved mechanism. In this study, we show that exogenous application of these deoxysphingoid bases causes dose- and time-dependent neurotoxicity in primary mammalian neurons, as determined by analysis of cell survival and neurite length. Acutely, deoxysphingoid base neurotoxicity manifests in abnormal Ca2+ handling by the endoplasmic reticulum (ER) and mitochondria as well as dysregulation of cell membrane store-operated Ca2+ channels. The changes in intracellular Ca2+ handling are accompanied by an early loss of mitochondrial membrane potential in deoxysphingoid base-treated motor and sensory neurons. Thus, these results suggest that exogenous deoxysphingoid base application causes neuronal mitochondrial dysfunction and Ca2+ handling deficits, which may play a critical role in the pathogenesis of HSN-1. Graphical illustration demonstrating the proposed pathomechanism of the toxic, atypical deoxysphinganine and deoxymethylsphinganine, produced by mutant SPT in HSN-1. The toxic sphingoid bases DSp and DMSp (or downstream metabolites) are released into the extracellular space where they destabilize neuronal cell membranes, resulting in dysfunction of neuronal ion channels such as store-operated Ca2+ (SOC) channels, upon stimulation. This results in elevated Ca2+ entry when SOC channels open, which is buffered by mitochondria. This increase in mitochondrial Ca2+ uptake in turn results in mitochondrial dysfunction, leading to loss of mitochondrial membrane potential and ultimately dysfunction of cellular processes and neuronal degeneration. (DSp: deoxysphinganine; DMSp: deoxymethylsphinganine). [Display omitted] •Mutation in SPTLC1 or SPTLC2 are the most common cause of HSN-1.•Abnormal sphingolipids produced by the mutant SPTLC1/2 enzyme cause neuronal death.•Early targets of abnorm