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1 Lung Biology Program, Hospital for Sick Children, Toronto, Ontario, Canada
2 Institute of Medical Sciences, University of Toronto, Toronto, Ontario, Canada
* To whom correspondence should be addressed. E-mail: martin.post{at}sickkids.ca.
Pulmonary surfactant is a lipoprotein complex that functions to reduce surface tension at the air liquid interface in the alveolus of the mature lung. In late gestation glycogen-laden type II cells shift their metabolic program towards the synthesis of surfactant, of which phosphatidylcholine (PtdCho) is by far the most abundant lipid. To investigate the cellular site of surfactant PtdCho synthesis in these cells we determined the subcellular localization of two key enzymes for PtdCho biosynthesis, fatty acid synthase (FAS) and
CTP:phosphocholine cytidylyltransferase (CCT
), and compared their localization with that of surfactant storage organelles, the lamellar bodies (LBs), and surfactant
protein(SP)s in fetal mouse lung. Ultrastructural analysis showed that immature and mature LBs were present within the glycogen pools of fetal type II cells. Multivesicular
bodies were noted only in the cytoplasm. Immunogold electron microscopy (EM) revealed that the glycogen pools were the prominent cellular sites for FAS and CCT.
Energy filtering EM demonstrated that CCT bound to phosphorus-rich (phospholipid) structures in the glycogen. SP-B and SP-C, but not SP-A, localized predominantly to the
glycogen stores. Collectively, these data suggest that the glycogen stores in fetal type II cells are a cellular site for surfactant PtdCho synthesis and LB formation/maturation
consistent with the idea that the glycogen is a unique substrate for surfactant lipids.
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