3. Neurodegeneration and Sphingolipid Metabolism
Alzheimer’s disease (AD) is the most prevalent neurodegenerative disease. It is characterized by extracellular deposits of β-amyloid (previously cleaved by secretases), called senile plaques, and intracellular build-up of hyperphosphorylated Tau protein in neurofibrillary tangles
[74][75]. High levels of different species of Cer have been found in human samples from AD patients
[76][77][78]. In addition, senile plaques were enriched in C
18:1/18:0 and C
18:1/20:0-Cer
[79]. Likewise, aSMase and nSMase2 were found overexpressed in AD brain samples, correlated with increased Cer levels in blood
[77]. Moreover, treatment with a cell-permeable analog of Cer (C
6-Cer) or stimulation of endogenous Cer by nSMase activation stabilized β-site amyloid precursor protein cleaving enzyme 1 (BACE1) increasing β-amyloid accumulation
[80]. Interestingly, β-amyloid has been reported to stimulate SMase activity in neurons
[43][81][82], oligodendrocytes
[83], dendritic
[82] and endothelial cells
[84], stimulating Cer accumulation and, thereby cell death. Additionally, overexpression of S1P lyase has been described to reduce β-amyloid production in N2a neuroblastoma cells
[85]. Furthermore, it was observed a significant reduction of SphK1 and an increase of S1P lyase in AD human brain samples
[86].
Different genetic diseases disrupt the metabolism of several molecules in the lysosomes, knowns as Lysosomal storage diseases (LSD). One of the main causes is lipid metabolism dysfunction, due to the alteration of enzymes such as aSMase or β-GCase
[23][87][88][89]. LSD include different diseases, such as Niemann-Pick’s disease, Gaucher’s disease, Farber’s disease, Krabbe’s disease, Fabry’s disease, Tay-Sach’s disease, Sandhoff’s disease and ganglioside synthase deficiency. Lysosomal lipid storage occurs in all types of the disease, again highlighting the link between altered sphingolipid metabolism and neurodegeneration.
Niemann-Pick’s disease is a genetic disease that can be caused by two different types of mutations. Mutations in the
SMPD1 gene lead to build-up of SM and the develop of Niemann-Pick’s disease type A and B
[45][90]. Meanwhile, mutations in NPC Intracellular Cholesterol Transporter 1 or 2 (
NPC1 or
NPC2) alter cellular cholesterol trafficking and lipid metabolism disruption, leading to Niemann-Pick’s disease type C1 and C2
[91]. Recently, Torres et al. have shown that ASAH1 is downregulated in patients with Niemann-Pick’s disease type C1
[92]. They have also observed that the overexpression of ASAH1 improves mitochondrial function and reduces oxidative stress by decreasing STARD1.
Gaucher’s disease is due to a mutation in the gene encoding β-GCase (
GBA), resulting in a deficit of the lysosomal enzyme, leading to an accumulation of GlcCer mainly in macrophages
[53]. Elevated levels of glucosylsphingosine (GlcSph) were also found in the brain and were correlated with the phenotype of the disease
[93]. Gaucher disease is associated with an increased risk of PD and dementia, since
GBA deficiency increase α-syn aggregates
[5].
Faber’s disease is caused by mutations in the
ASAH1 gene, leading to an accumulation of Cer and cerebral atrophy. Interestingly, a rare epileptic disorder known as spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME) is also associated with ASAH1 deficit
[94]. Recently, C
26-Cer was proposed as a biomarker for Faber’s disease diagnosis
[95].
Krabbe’s disease is a genetic disease characterized by extensive demyelination, apoptosis of oligodendrocytes and Schwann cells and neurodegeneration due to mutations in the
GALC gene that encode for galactocerebrosidase
[96]. Recently, the possible link of greater cognitive impairment in PD patients with mutations in
GALC gene was evidenced
[97].
Huntington’s disease is a neurodegenerative disease strongly correlated with the expansion of CAG trinucleotide repeat within the
huntingtin gene (
HTT). It is characterized by progressive neurodegeneration and cognitive, motor and behavioral disturbances. Different studies carried out in in vivo models of Huntington’s disease have discovered a dysregulation in ganglioside metabolism
[98][99]. Furthermore, a recent work has described a downregulation of SPT and CerS in mouse models, with a decrease in dihydroSphingosine, dihydroSphingosine-1-phosphate and dihydroCeramide (C
18)
[100].
Multiple sclerosis, also known as encephalomyelitis disseminate, is a chronic inflammatory disorder of the central nervous system characterized by demyelination and subsequent degeneration leading to neuronal damage and axonal loss. Its underlying etiology is unknown; however, genetic and environmental risks related to its development have been described
[101]. CerS2 was found upregulated in monocytes and neutrophils isolated from mouse models
[102]; meanwhile, CerS6 was increased in monocytes/macrophages
[103][104]. Their overexpression has been associated with an increase in granulocyte stimulating factor (G-CSF)-induced C-X-C Motif Chemokine Receptor 2 (CXCR2) expression
[103]. Additionally, downregulation of CerS2 and CerS6 were shown to inhibit the migration capacity of macrophages and neutrophils
[103][104]. Therefore, CerS2 and CerS6 may represent a promising target for multiple sclerosis treatment. Moreover, plasma levels of C
16-Cer, C
24:1-Cer, C
16-GlcCer and C
24:1-GlcCer were increased and C
16-LacCer was decreased in multiple sclerosis patients compared to healthy controls
[105]. Furthermore, increased levels of C
16:0- and C
24:0-Cer were found in the cerebrospinal fluid samples from patients with multiple sclerosis
[106].
Vascular dysfunction has been associated with the risk of neurodegeneration
[107]. Notably, cerebral ischemia has been linked to pro-inflammatory processes in endothelial cells and loss of the integrity of the blood–brain barrier
[108][109]. Sphingolipid metabolism has been described as a key factor in the progression and prognosis of brain ischemia. SMS1 was expressed in a time-dependent manner with a decrease in the first 24 h and recuperation at 72 h after transient middle cerebral artery occlusion (tMCAO) in rats
[110]. Additionally, mice lacking aSMase exhibited a reduction in the infarct size in tMCAO, related to a decrease in Cer levels
[111]. Moreover, a recent study demonstrated that aSMase protects against mild focal cerebral ischemia
[112]. In preclinical studies, the levels of ceramides were increased 24 h after tMCAO in the ipsilateral hemisphere, especially in long-chain Cers, and decreased in SM
[113]. Furthermore, recent studies in stroke patients showed elevated levels of long-chain Cers, while S1P and very long-chain Cers were decreased. Interestingly, high levels of long-chain Cers were associated with poor outcome at 48–72 h
[114][115].
Glioblastoma is the most common and aggressive malignant brain tumor diagnosed in adults. The sphingolipids metabolism has emerged as a potential target for tumor cancer
[116]. SPT inhibition by myriocin or specific siRNA inhibited the proliferation of human U87MG glioblastoma cells
[117]. Des1 inhibitors such as γ-tocotrienol, phenoxodiol, or celecoxib have been described to induce autophagy in T98G and U87MG glioblastoma cell lines by dhCer accumulation
[118]. Furthermore,
N-[(1R,2S)-2-hydroxy-1-hydroxymethyl-2-(2-tridecyl-1-cyclopropenyl)ethyl]octanamide (GT11), another specific inhibitor of Des1, has been found to activate autophagy and apoptosis of the human U87MG glioma cell line
[119]. Additionally, treatment with tetrahydrocannabinol (THC) produced an alteration of the lipid composition in the endoplasmic reticulum and reduction of Des1 expression, promoting autophagy and apoptosis in human U87MG glioma cells
[119]. Interestingly, a correlation between SphK1 and poor survival has been observed in a clinical study with patients with glioblastoma
[120]. Moreover, specific inhibition of SphK1 or SphK2 resulted in a cell-cycle arrest in U-1242 and U-87MG glioblastoma
[120]. In addition, chemical or transcriptional down-regulation of SphK1 induces apoptosis and suppresses the growth of human glioblastoma cells and xenografts
[121].