Phosphatidylcholine and the CDP-choline cycle

Abstract

The CDP-choline pathway of phosphatidylcholine (PtdCho) biosynthesis was first described more than 50 years ago. Investigation of the CDP-choline pathway in yeast provides a basis for understanding the CDP-choline pathway in mammals. PtdCho is considered as an intermediate in a cycle of synthesis and degradation, and the activity of a CDP-choline cycle is linked to subcellular membrane lipid movement. The components of the mammalian CDP-choline pathway include choline transport, choline kinase, phosphocholine cytidylyltransferase, and choline phosphotransferase activities. The protein isoforms and biochemical mechanisms of regulation of the pathway enzymes are related to their cell- and tissue-specific functions. Regulated PtdCho turnover mediated by phospholipases or neuropathy target esterase participates in the mammalian CDP-choline cycle. Knockout mouse models define the biological functions of the CDP-choline cycle in mammalian cells and tissues. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Fig. 1

Timeline of publications that advanced our ability to interrogate the regulation, the cell biology and the physiological role of the mammalian CDP-choline pathway. The biochemical characterization of the CDP-choline pathway was accomplished in a short period of time, from 1949 to 1956 [1,19,80,81]. Between 1956 and 1986, the unpurified enzymes in the pathway were investigated, with a major focus on CCT. During this time it was recognized that CCT activity regulated the flux through the pathway [–147], that CCT activity was stimulated by selected lipids [–150], that CCT was regulated by reversible phosphorylation [151] and that increased CCT activity correlated with greater CCT association with microsomal membranes [145,146,152]. A fibroblast cell line containing temperature-sensitive CCT activity was isolated in 1982 [153]. The rat CCT was purified to homogeneity in 1986 [154]. The genes in the pathway were first cloned and sequenced from yeast between 1987 and 1991 [,–157] which enabled the identification and cloning of the mammalian homologs thereafter. The rat CK was purified in 1990 [158]. The CCTα gene was cloned in 1990 [159]. The CKα gene was cloned in 1992 [160]. The nuclear localization of CCTα was discovered in 1993 [161]. The expression of recombinant CCTα was achieved using a baculovirus vector in 1993 [162,163] and the phosphorylation sites of CCT were identified in 1994 [164]. The first CPT clone from higher eukaryotes was obtained from soybean in 1994 [83]. The structure of the CCT membrane-binding domain was described in 1996 [165]. The CKβ gene [166] and the CCTβ gene [167] were cloned in 1998. The gene encoding CEPT was cloned in 1999 [86]. The neuropathy target esterase was cloned in 1999 [168]. The participation of the group VIA calcium-independent phospholipase A₂ in PtdCho turnover was recognized in 1999 [123]. The major membrane property that governs CCT association was discovered in 2000 [66]. The first CCT tissue-specific knockout mouse model was derived in 2000 [131]. A CK protein structure from C. elegans was published in 2003 [26]. The neuropathy target esterase [120] and the group VIA calcium-independent phospholipase A2 [118] were identified as regulating PtdCho turnover in 2004. CCTα was found to be phosphorylated by the ERK kinase [169]. A structure of the CCTα catalytic domain was described in 2009 [61].

Fig. 2

Biochemical relationships between the CDP-choline pathway of PtdCho synthesis, the degradation of PtdCho, and general phospholipid metabolism in mammals. Choline uptake is mediated by the organic cation transporters (OCTs) which rely on facilitated diffusion governed by the choline concentration gradient and the electrical potential across the membrane. Active transport of choline is mediated by the choline transporter-like proteins (CTLs) that are energized by adenosine triphosphate (ATP) hydrolysis. The high-affinity choline transporters (CHTs) are Na+-dependent and require ATP hydrolysis. Intracellular choline can be acted upon by three different enzymes: the choline dehydrogenase (CHDH), choline acetyltransferase (CAT) and choline kinase (CK). CHDH is localized inside the mitochondria and oxidizes choline to betaine aldehyde, with oxygen being the final electron acceptor. The betaine aldehyde is oxidized to betaine by the betaine aldehyde dehydrogenase (BADH) in concert with the conversion of oxidized nicotinamide adenine nucleotide (NAD⁺) into the reduced nicotinamide adenine nucleotide (NADH). Choline acetylation is catalyzed by the choline acetyltransferase (CAT) that transfers the acetyl group from acetyl-coenzyme A (AcCoA) to choline, yielding free coenzyme A (CoA) and acetylcholine. CK catalyzes the esterification of the choline hydroxyl group with the γ-phosphate of the ATP to produce phosphocholine (P-choline) plus adenosine diphosphate (ADP). The phosphocholine cytidylyltransferase (CCT) uses cytidine triphosphate (CTP) to convert P-choline into CDP-choline, with the release of pyrophosphate (PPi). The CDP-choline is esterified with diacylglycerol (DAG) by the cholinephosphotransferase (CPT) or the choline/ethanolaminephosphotransferase (CEPT) to produce 1,2-diacyl-glycerophosphocholine (PtdCho) and cytidine monophosphate (CMP). The Legionella pneumophila-encoded AnkX(Lp) can hijack CDP-choline to esterify a serine hydroxyl group of Rab1 protein. PtdCho can be hydrolyzed into choline and phosphatidic acid (PtdOH) by phospholipase D (PLD); in turn, PtdOH can be hydrolyzed into DAG and Pi. PtdCho can also be hydrolyzed into DAG and P-choline by phospholipase C (PLC). Neuropathy target esterase (NTE) hydrolyzes the two acyl chains (FA) of PtdCho to yield glycerophosphocholine (GroPCho), which can then be further hydrolyzed into glycerophosphate (GroP) and choline by the glycerophosphodiesterase (GPD). Phosphatidylserine synthase 1 (PSS1) catalyzes a base-exchange reaction with PtdCho substituting the choline group with serine (Ser) to produce phosphatidylserine (PtdSer). The sphingomyeline synthase (SMS) substitutes DAG for ceramide (Cer) to yield sphingomyelin (Sph). Sph can be hydrolyzed into P-choline and Cer by sphingomyelinase (SMase). Phospholipases A (PLA) can degrade PtdCho to yield lysophosphatidylcholine (lyso-PtdCho) and further hydrolysis of lyso-PtdCho yields GroPCho. The free hydroxyl group in lyso-PtdCho can be esterified by the acylglycerophosphate acyltransferase (AGPAT) using acylCoA as the donor of the acyl-group to yield PtdCho.