Role of Phosphocholine Cytidylyltransferase α in Lung Development

Abstract

Lung development depends upon the differentiation and expansion of a variety of specialized epithelial cell types, including distal type I and type II pneumocytes in the late term. Previous studies have shown a strict dependence on the choline cytidylyltransferase ␣ isoform (CCT␣) to mediate membrane phospholipid formation in cultured cells and during preimplantation embryogenesis. CCT␣ expression is highest in lung, and there has long been speculation about its precise role, due to the dual requirement for phospholipid in proliferating cell membranes and for lung surfactant production from alveolar type II cells. We investigated the function of CCT␣ in lung development, using an inducible, epithelial cell-specific CCT␣ knockout mouse line. Deletion of CCT␣ beginning at embryonic day 7.5 did not restrict lung development but resulted in severe respiratory failure at birth. Alveolar lavage and lung lipid analyses showed significant decreases in the major surfactant phospholipid, dipalmitoyl-phosphatidylcholine. The fatty acids destined for the surfactant phospholipid were redirected to an expanded triglyceride pool. Transcripts encoding type II cell-specific markers were expressed in the knockout mice, indicating the expected progression of differentiation in lung epithelia. However, surfactant protein levels were reduced, with the exception of that for surfactant protein B, which was elevated. Ultrastructural analysis of the type II cells showed Golgi complex abnormalities and aberrant lamellar bodies, which deliver surfactant lipid and protein to the alveolar lumen. Thus, CCT␣ was not required for the proliferation or differentiation of lung epithelia but was essential for the secretory component of phospholipid synthesis and critical for the proper formation of lamellar bodies and surfactant protein homeostasis.

Physiological consequences of disruption of mammalian phospholipid biosynthetic genes

“…Dipalmitoyl-PC in lungs of the knockout mice, as well as in the alveolar lavage, was significantly reduced. Thus, CTa is not required for proliferation or differentiation of lung epithelia but is essential for normal PC secretion by the lung (35).…”

“…The Pcyt1a gene was selectively inactivated in mouse lung epithelial cells (35). Lack of CTa did not impair lung development, but at birth, the mice suffered respiratory failure.…”

CTP:Phosphocholine Cytidylyltransferase α Is Required for B-cell Proliferation and Class Switch Recombination

“…However, CCT␣ Ϫ/Ϫ primary animal cells survive and differentiate, as exemplified by macrophages (13), hepatocytes (19), lung alveolar cells (18), and B-cells (this study). These results strongly suggest that another source of PtdCho is available that circumvents a requirement for CCT␣.…”

“…The functions of CCT␣ in adult animals have been studied by tissue-specific deletion of CCT␣ gene expression using the Cre-loxP system. These studies have revealed distinct roles for CCT␣ in professional secretory cells, including the formation and secretion of surfactant by alveolar epithelial cells (18), the assembly and secretion of lipoproteins by hepatocytes (19), and cytokine secretion by stimulated macrophages (20). In this study, the role of CCT␣ in B lymphocyte function was examined by selectively deleting the Pcyt1a gene encoding CCT␣ in murine B-cells.…”

A mechanism for suppression of the CDP-choline pathway during apoptosis

“…For example, knockout of the ubiquitous CCT ␣ isoform in mice is embryonic lethal, indicating an essential developmental role ( 5 ). However, conditional CCT ␣ deletion in tissues that express suffi cient CCT ␤ or phosphatidylethanolamine Nmethyltransferase is not lethal, resulting in phenotypes related to partial deduction in biosynthetic capacity (6)(7)(8).…”