Our findings suggest that trehalose administration in NP type A and B patients is effective in the breakdown of sphingomyelins and also can improve the overall metabolism of the cells via protective activity on mitochondria. In addition, it seems that trehalose can change the metabolism of glucose in favor of reducing fatty acid synthesis and is also able to facilitate the breakdown of lipids (Fig. 4).
In this study, we observed a clear increase (about 2-fold) in the concentration of phosphate after treatment with trehalose. Increased plasma level of phosphate is mainly attributed to chronic kidney failure 37, while in our study no adverse effects on kidney function have been reported. We previously showed that trehalose administration can increase the breakdown of lyso-SM509 and lyso-SM 34. Therefore, we hypothesize that the increase in phosphate level may be related to sphingomyelin degradation after the treatment. The breakdown of sphingomyelins typically occurs through the hydrolysis of the phosphocholine headgroups resulting in the production of ceramide and free phosphocholine 38. In the next step, phosphocholine can be metabolized by the enzymes such as phosphoethanolamine/phosphocholine phosphatase to choline and phosphate 39. Interestingly, in our previous study, we found that one of the patients
(patient 5) did not benefit from the treatment at all in terms of lyso-SM509 and lyso-SM breakdown and even a slight increase in the level of these two sphingomyelins was observed 34. Accordingly, in the current study, the phosphate level of this patient was reduced after treatment indicating that the source of phosphate can be sphingomyelin degradation. In addition, only in three patients marked decrease in sphingomyelin was observed, which is in accordance with our findings for phosphate level (Fig. 2).
Another metabolite that was significantly (p-value: 0.0001) altered following trehalose treatment was lactic acid. Lactic acid is a carboxylic acid that plays a crucial role in human cellular metabolism, particularly in glycolysis and the Cori cycle. It is produced as a result of the enzymatic conversion of pyruvate by the enzyme lactate dehydrogenase (LDH). In aerobic conditions, pyruvate generated during glycolysis is further oxidized in the mitochondria through the tricarboxylic acid cycle and oxidative phosphorylation, yielding a substantial amount of ATP. However, under anaerobic conditions or during instances of oxygen deprivation, such as intense exercise, hypoxia, or the malfunction of mitochondria due to mitochondrial damage, pyruvate is converted into lactic acid to regenerate the NAD + required for glycolysis 40. Mitochondrial dysfunction due to oxidative damage is a common pathological condition that has been reported in several lysosomal storage diseases such as Gaucher and NP diseases 41,42. We previously showed that trehalose administration has antioxidant activity and increases glutathione peroxidase (GPX) activity in the human body 34. In this study, we observed a marked decrease in the serum level of lactic acid that can be attributed to the protective activity of trehalose against mitochondrial damage. Moreover, facilitating sphingomyelin breakdown can partly reduce the oxidative damage to mitochondria since sphingomyelin deposition is linked to cellular stress responses, such as endoplasmic reticulum stress, oxidative stress, and reactive oxygen species generation in mitochondria 43. Interestingly, in one patient, lactic acid did not reduce which is in accordance with our previous findings that only four out of five patients could benefit from the treatment. Moreover, this pattern was observed for almost all other biomarkers. However, since lactic acid is not a direct biomarker related to mitochondria function, more evidence from the measurement of biomarkers related to mitochondria function such as mtDNA is needed to assess the real protective impact of trehalose on mitochondria.
Among the 12 assessed biomarkers, mannose and 1-monopalmitin were among the metabolites for which a significant decrease was observed after the treatment. 1-Monopalamitin is a monoacylglycerol with an essential role in the production of ceramides in the human body and ceramides are also the important building blocks of sphingolipids and sphingomyelins. Therefore, a reduction in 1-monopalmitin shows that the treatment could shift the overall metabolism towards less fatty acid production via the acetyl-CoA pathway. These findings suggest that trehalose shifts glucose metabolism from glycolysis, which can produce lactic acid and acetyl-CoA, towards other pathways that use glucose as a source to produce other sugars such as mannose, D-ribose, ribitol, and threitol. This was evidenced by the increase in the serum levels of these four sugars.
A rise in the serum concentration of myo-inositol may explain another mechanism of trehalose for the reduction in sphingomyelin accumulation regarding the lipotropic activity of myo-inositol. Myo-inositol facilitates removal of lipids and cholesterol from liver and myocardial cells 44.
Several changes in the serum level of amino acids were recorded, while only the change in threonine level was statistically significant. Animal studies have exhibited that threonine and lysine have an impact on the lipid composition of the liver and the lower level of threonine and lysine is in favor of lecithin, sphingomyelin, and free fatty acids accumulation 45. Hence, an increase in the serum concentration of threonine may play a role in the sphingomyelin decrement that was observed in our study. However, more studies and particularly clinical trials are needed to prove the relationship between the plasma concentration of threonine and lipid composition of the liver.
Our study has several limitations including small sample size and the relatively short duration of intervention. Such limitations are inherent to every first-in-human and proof-of-concept study involving an intravenous intervention in the pediatric setting. The short length of treatment may affect the effectiveness and impact of treatment on serum metabolome. Although we checked several databases for the identification of biomarkers, a higher level of confidence could be achieved using chemical reference standards.