Abstract
Dysregulation of sphingolipid (SL) metabolism has been associated with the development of various monogenic and metabolic diseases. SL synthesis starts by conjugation of L-Serine and Palmitoyl-CoA catalyzed by the enzyme serine palmitoyltransferase (SPT). In this way, a long-chain base is formed, which serves as a substrate for the synthesis of higher SL classes such as Ceramides, Sphingomyelins and Glycosphingolipids. Catabolism of SL is mediated via the degradation of Sphingosine-1-phosphate by sphingosine-1-phosphate lyase (SPL). Under certain conditions, SPT utilizes L-Alanine instead of L-Serine, forming atypical toxic 1-deoxysphingolipids (1-deoxySL). This thesis explores the connections between disturbances in SL profiles and the development of associated diseases, shedding light on their pathomechanisms and potential therapies, respectively. In the first part of this work, we explore the metabolic consequences of defective SL catabolism. Sphingosine-1-phosphate lyase insufficiency syndrome (SPLIS), also known as Nephrotic syndrome type 14, is a rare monogenic disease caused by defective SPL. Clinically, SPLIS patients develop renal, endocrinological, and neurological symptoms with up to 50% mortality within the first two years of life. We report a new SPLIS patient with yet undescribed mutation p.Ser362Thr. Using isotopic labelling, we show fine-tuned metabolic regulatory mechanisms maintaining homeostasis in SPLIS. Moreover, our data suggest that absorption of S1P mediates the disease development and that the clinically approved drug Fasudil effectively alleviates SPLIS-induced cellular phenotypes. In the second chapter, we explore the metabolism of atypical neurotoxic 1-deoxySL and their relation to the development of diabetic polyneuropathy. Increased plasma 1-deoxySL levels were repeatedly reported in type 2 diabetic (T2D) patients and are associated with the development of diabetic polyneuropathy and Hereditary Sensory and Autonomic Neuropathy type 1 (HSAN1). HSAN1-associated SPT mutations are causing a permanent shift in substrate specificity from L-Serine to L-Alanine. However, the reason for the increased 1-deoxySL in T2D is not known. We show that due to the diabetic sarcopenia and decreased L-Serine levels, increased plasma Alanine to Serine ratio is the driver of 1-deoxySL formation in T2D. Our lipidomic profiling of plasma and skin biopsies from 38 T2D and 39 healthy controls reveals an accumulation of 1-deoxySL in T2D patients. Moreover, our data suggest that L-Serine supplementation might be a safe and easy way of preventing diabetic polyneuropathy. In the third chapter, we explored 1-deoxySL-mediated toxicity. Using a genome-wide CRISPRi screen, we identified Ceramide synthase 2 (CerS2), and Elongation of very-long-chain fatty acids protein 1 (ELOVL1) as enzymes mediating 1-deoxySL toxicity. Furthermore, we show that a preclinically tested ELOVL1 inhibitor (compound 22) completely mitigates 1-deoxySL mediated mitochondrial, cellular, and neuronal toxicity. Altogether, this thesis provides three new pathomechanisms and three potential therapies for associated diseases.
Majcher, Adam