In Vivo incorporation of choline‐³H, leucine‐³H and galactose‐³H in alveolar type II pneumocytes in relation to surfactant synthesis. A quantitative radioautographic study in mouse by electron microscopy

Abstract

A quantitative time study of the incorporation of choline-SH, le~cine-~H and gala~tose-~H in lung epithelial cells, was undertaken in vivo with electron microscopic autoradiography. Type I1 pneumocytes were selectively labeled with ~holine-~H, a specific precursor of phosphatidylcholine, which is the main component of pulmonary surfactant. Choline was initially localized in endoplasmic reticulum, then was rapidly transferred through the Golgi complex and stored in lamellar bodies. Previously undescribed small lamellar bodies are suggested as phospholipid carriers between Golgi complex and lamellar bodies.After initial incorporation in the endoplasmic reticulum, the leucine label migrated through the Golgi complex into lamellar bodies by fusion of multivesicular bodies, which are the carriers between the two structures. Galactose was modestly incorporated into lamellar bodies via the Golgi complex. Intra-alveolar myelin figures, recognized as excreted surfactant, were labeled 120 minutes after injection with the three precursors.These findings indicate that the synthesis of a secretory product by type I1 pneumocytes involves phospholipid, protein and polysaccharide precursors; the secretory product is segregated as lamellar bodies which are destined to be excreted into the alveolar cavity to become part of the lining layer.

Phosphatidylcholine-Lysophosphatidylcholine Cycle Pathway Enzymes in Rabbit Lung. I. Subcellular Localization and Properties

“…as also indicatcrl by other investigators (10,12,16,19,25,32). These bioche~nical studies also confirmed tlie electron microscopic autoradiography findings that I'C \vas initially synthesi/ed in cndoplasmic ~e t i c u l u m (8). T h e rnitochondri;~ may hiivc enzymes required for the synthesis of membrane pliosl~liolipids (25).…”

“…12, 17, 18, 20, 36) have suygcstccl that the PC-IysoPC cyclc p:rth\v;~y may he import:rnt in the rcmocleling of tlic lung surfactant, the mcc1i;rnism is not cle;~r. Chcvalicr and Collctt (8) obscrvcd that PC w:ls initially syntllcsizcd in crldoplasrnic rcticulunl ;111d then transferretl to Ii~rncllar bodies through Golgi complex. Rooncy c.1 nl.…”

Surfactant Phospholipid Metabolism in 3-Day and 3-Day Postmature Rabbits In Vivo

“…Recent studies in rats (19), lambs (I), and rabbits (30) suggest that surfactant replacement therapy may be applicable to the human; however, the basic difference between lung surfactant metabolism between perinatal and adult mammals have been used to study subcellular synthesis, storage, and secretion of phosphatidylcholine in fetal (26), premature (23), term newborn (14,24), and adult rabbits (21,25). These studies are consistent with the anatomic descriptions of the subcellular metabolism of surfactant (7,8), and they have shown striking differences between perinatal and adult rabbits in the kinetics of labeled phosphatidylcholine secretion to the alveolar space.…”

Biogenesis of Multilamellar Bodies via Autophagy

“…Autoradiographic studies of murine type II alveolar cells of mouse lungs showed that although phospholipids labeled with [ 3 H]choline are delivered directly from the Golgi to the MLB, proteins metabolically labeled with [ 3 H]leucine are visualized within multivesicular bodies before delivery to MLBs (Chevalier and Collet, 1972). Surfactant proteins A, B, and C are delivered via multivesicular bodies to MLBs, and multivesicular bodies are proposed as the site of processing of surfactant precursor to mature forms; both multivesicular bodies and MLBs express the lysosomal marker CD63 and are therefore part of the lysosomal pathway (Voorhut et al, 1992(Voorhut et al, , 1993.…”