Physiological consequences of disruption of mammalian phospholipid biosynthetic genes

Abstract

By 1959, Eugene Kennedy and coworkers had outlined most pathways of phospholipid biosynthesis. In the next four decades, the emphasis was on enzymology and regulation of these pathways. In the last 12 years, several lines of mice with disrupted genes of phospholipid biosynthesis were generated. From this research, we have learned that embryonic lethality occurs in mice that lack choline kinase (CK) a, CTP:phosphocholine cytidylyltransferase a, CTP:phosphoethanolamine cytidylyltransferase, or phosphatidylserine decarboxylase. Whereas mice that lack CK b are viable but develop hindlimb muscular dystrophy and neonatal bone deformity. Mice that lack CTP:phosphocholine cytidylytransferase b have gonadal dysfunction and defective axon branching. Mice that lack phosphatidylethanolamine N-methyltransferase exhibit no phenotype until fed a choline-deficient diet, which leads to rapid liver failure. Future research should extend our knowledge about the function of these and other enzymes of phospholipid biosynthesis.-Vance, D. E., and J. E. Vance. Physiological consequences of disruption of mammalian phospholipid biosynthetic genes. J. Lipid Res. 2009. 50: S132-S137. Supplementary key words phosphatidylcholineIn 1959, the general outline of biosynthesis of major phospholipids had been established in studies pioneered by Eugene Kennedy (1-3). A key finding was that CTP, not ATP, was the ribonucleoside triphosphate required for phospholipid biosynthesis (1). The next major focus was to purify enzymes involved in phospholipid biosynthesis, many of which were integral membrane proteins. Thus, success was limited. Choline kinase (CK) was purified in 1984 (4), CTP:phosphocholine cytidylyltransferase (CT) was purified in 1987 (5), and the integral membrane protein, phosphatidylethanolamine (PE) N-methyltransferase (PEMT) that converts PE to PC was purified in 1987 (6). Several phospholipid biosynthetic enzymes have never been purified.In the 1970s, investigations into the regulation of phospholipid biosynthesis were initiated. In 1975, primary rat hepatocytes were used to demonstrate that the second enzyme in the Kennedy pathway for PC biosynthesis, CT, catalyzed the rate-limiting step (7). These findings were confirmed under most metabolic conditions (8). In addition, the rate-limiting enzyme of PE synthesis via CDP-ethanolamine was identified as CTP:phosphoethanolamine cytidylyltransferase (ET) (7). A striking regulatory feature of PC biosynthesis is that CT is inactive in the cytosolic fraction of cells but is activated by translocation to membranes (9). Insights into the mechanism of translocation of CT to membranes have been largely provided by Cornell and Northwood (10) and Taneva et al. (11). Studies on regulation of expression of mRNAs encoding CTa have concluded that CTa expression is linked to the cell cycle, cell growth, and differentiation rather than energy metabolism (12).Clearly, significant progress has been made during the last 50 years in understanding the enzymology and regulation of PC biosynthesis. ...

The Kennedy pathway—De novo synthesis of phosphatidylethanolamine and phosphatidylcholine

“…Deletion of both ECT alleles in mice is embryonic lethal, whereas heterozygous animals are outwardly normal and maintain normal PE levels in tissues by downregulating its degradation, rather than stimulating PS decarboxylation (54,65,66). Hepatocyte-specific deletion of the ECT gene in mice did not give rise to liver injury or inflammation.…”

“…It has been calculated that every day the mouse liver secretes into the bile the equivalent of its entire pool of PC, and with simultaneous lack of PEMT and dietary choline, PC synthesis is not able to sustain its utilization. This causes a decrease in the PC/PE ratio and an increase in membrane permeability that in turn is responsible for the development of steatohepatitis and liver failure (54,92). In addition, PEMT activity seems to be necessary for very low-density lipoprotein secretion and the regulation of the plasma supply of homocysteine, generated by the hydrolysis of the PEMT reaction byproduct S-adenosylhomocysteine.…”

“…The physiological importance of C/EKs was studied in mice by gene disruption (54). Deletion of CKa caused embryonic lethality and highlighted its role in development, whereas loss of function of CKb activity, as present in mice with a spontaneous recessive mutation in CKb gene, led instead to progressive Figure 4.…”

Mechanism of hypertriglyceridemia in CTP:phosphoethanolamine cytidylyltransferase-deficient mice

“…In contrast, our understanding of how membrane phospholipids contribute in the development of chronic disease is considerably less studied and it is generally poorly understood. There have been select lines of evidence that have highlighted the relationship between phospholipids, mainly phosphatidylcholine (PC) and triglyceride (TAG) metabolism (1)(2)(3)(4)(5). FAs released from phospholipid degradation can be utilized for TAG synthesis ( 2 ) and changes in membrane PC content are suffi cient to cause changes in whole body TAG homeostasis ( 2,3 ).…”

Daily rhythms of glycerophospholipid synthesis in fibroblast cultures involve differential enzyme contributions

“…In most mammals, there are two genes encoding for ChoK: Chka codes for ChoK ␣ 1/2 and Chkb codes for ChoK ␤ ( 34,35 ). Mice lacking ChoK ␣ die early in embryogenesis, whereas mice lacking ChoK ␤ survive to adulthood ( 30 ). ChoK overexpression has been implicated in the development of human carcinogenic processes ( 36,37 ).…”