Serine metabolism in the human brain: serine racemase regulation and pathological variants of phosphoserine aminotransferase

Abstract

L-serine and D-serine play crucial roles in the central nervous system (CNS). Indeed, L-serine is a neurotrophic factor and represents the precursor of the neurotransmitters glycine and D-serine, the co-agonists of N-methyl-D-aspartate (NMDA) glutamate receptors. Serine racemase (SR) is the enzyme responsible for converting L-serine into D-serine and eliminating both enantiomers to pyruvate and ammonia. NMDA receptors are involved in the brain's development, plasticity and function. In addition, they are related to Alzheimer's and Parkinson's diseases, ischemia and amyotrophic lateral sclerosis. In the CNS, the primary route of L-serine biosynthesis is the phosphorylated pathway (PP), which starts from the glycolytic precursor 3-phosphoglycerate. The PP takes place in astrocytes thanks to three sequential enzymatic reactions catalyzed by 3-phosphoglycerate dehydrogenase (3-PHGDH), phosphoserine aminotransferase (PSAT) and phosphoserine phosphatase (PSP). Defects in the genes that encode the PP’s enzymes lead to severe CNS diseases, known as serine deficiency disorders (SDDs), characterized by a low concentration of L-serine in the cerebrospinal fluid and plasma. These syndromes exhibit highly variable neurological phenotypes, with retardation, microcephaly, and seizures typical in newborns and progressive polyneuropathy in adults. Early recognition is crucial for their successful treatment with L-serine repletion. In addition, since SDDs can affect all three enzymes of the PP, with overlapping phenotypes, the molecular understanding of their pathogenetic variants is fundamental for diagnosis, prognosis, and therapy. Chapter 1 of this PhD dissertation offers a general introduction to the state of the art of human brain serine metabolism. In particular, the role and characteristics of the pyridoxal-5'-phosphate-dependent enzymes human phosphoserine aminotransferase (hPSAT) and human serine racemase (hSR) are described in detail. The following chapters report the experimental activity. Chapter 2 focuses on the functional and biochemical characterization of hPSAT, especially its pathogenic variants S43R, G79W, A99V, D100A, S179L, C245R, and R342W associated in the literature with SDDs. In addition, the non-pathogenic P87A variant was studied for comparison. All PSAT variants were investigated to understand the loss-of-function mechanisms that can be responsible for SDDs pathogenesis. Chapters 3 and 4 focus on hSR regulation, especially evaluating the possible biochemical link between serine metabolism and the glycolytic flux in humans. Chapter 3 explores the interaction of hSR with the glycolytic human enzyme glyceraldehyde-3-phosphate dehydrogenase (hGAPDH). Chapter 4 reports the investigation of hSR inhibition by glyceraldehyde 3-phosphate (G3P), the hGAPDH substrate.