Ultrastructural localization and characterization of proteoglycans in human lung alveoli.

In order to localize and characterize proteoglycans in human lung alveoli, we have used the cationic dye Cuprolinic Blue according to the critical electrolyte concentration method. After staining, five types of Cuprolinic Blue-positive filaments become apparent: two types in the basement membranes of type I and type II epithelial cells respectively and lying in one or two layers; one type, more scattered, localized in the basement membrane of the endothelial cells and another kind associated with collagen fibrils and separated from each other according to the main banding period (+/- 60 nm) of these fibrils. Finally, there was a type of filament which was only locally present at a variety of places. The basement membrane filaments were sensitive to heparinase, heparitinase, pronase (without prefixation) and nitrous acid treatment, but not to Streptomyces hyaluronidase, neuraminidase, chondroitinase ABC, chondroitinase AC, pronase (after prefixation) and 2.0 M MgCl2 treatment. The basement membrane filaments, therefore, represent heparan sulphate-containing proteoglycans. On the other hand, the collagen fibril associated filaments were sensitive to treatment with heparinase, chondroitinase ABC and pronase (without prefixation), but insensitive to Streptomyces hyaluronidase, neuraminidase, nitrous acid, heparitinase, chondroitinase AC, pronase (after prefixation) and 2.0 M MgCl2 (after prefixation) treatment. These filaments thus represent iduronic acid-rich dermatan sulphate-containing proteoglycans. Several physiological functions for these proteoglycans are discussed.

Staining of proteoglycans in mouse lung alveoli. I. Ultrastructural localization of anionic sites.

In order to contrast anionic sites in mouse lung alveoli, two staining procedures were applied: (a) staining with Ruthenium Red and Alcian Blue and (b) staining with Cuprolinic Blue in a critical electrolyte concentration method. The Ruthenium Red-Alcian Blue staining procedure revealed electron-dense granules in the alveolar basement membrane. The granules were closely associated with the epithelial cell membrane and continued to stain even when the procedure was carried out at a low pH, indicating the presence of sulphate groups in the granules. After staining with Cuprolinic Blue, electron-dense filaments, also closely associated with the cell membrane, became visible in the basement membrane of type I epithelial cells. Their length depended on the MgCl2 concentration used during staining. At 0.4 M MgCl2, the length was mostly within the range 100-180 nm. Using a modified Cuprolinic Blue method, the appearance of the filaments closely resembled that of spread proteoglycan monomers with their side-chains condensed. The basement membrane of type II epithelial cells also contained filaments positive towards Cuprolinic Blue; their length, however, was smaller in comparison with those of type I epithelial cells. The filaments lay in one plane and provided the whole alveolus with an almost continuous sheet of anionic sites. Cuprolinic Blue staining also revealed filaments in the basement membrane of the capillary endothelial cells. Furthermore, Cuprolinic Blue-positive filaments (average length about 40 nm) became apparent in close contact with collagen fibrils and separated from each other according to the main banding period of the collagen fibrils (about 60 nm), indicating a specific ultrastructural interaction between these two components. Filaments connecting collagen fibrils with each other were also detected.

Staining of proteoglycans in mouse lung alveoli. II. Characterization of the Cuprolinic blue-positive, anionic sites.

The nature of Cuprolinic Blue-positive anionic filaments in mouse lung alveoli has been characterized. The contrast of filaments in the alveolar basement membrane of type I epithelial cells was lost on treatment with nitrous acid and pronase (without prefixation). In contrast, neither neuraminidase, chondroitinase ABC or AC, nor Streptomyces hyaluronidase had any effect. Treatment with pronase (after prefixation) and 2.0 M MgCl2 (after prefixation) also had no effect, indicating that the filaments are heparan sulphate proteoglycans. The filaments in the alveolar basement membrane of type II epithelial cells and in the capillary basement membrane of the endothelial cells were also nitrous acid sensitive, but chondroitinase ABC-insensitive. A model in which the whole alveolus contains a single layer of heparan sulphate-containing proteoglycan monomers is proposed. Furthermore, the collagen fibril associated filaments remained unaffected after treatment with nitrous acid, neuraminidase or Streptomyces hyaluronidase, or after digestion with pronase (after prefixation) and treatment with 2.0 M MgCl2 (after prefixation). These filaments, however, could no longer be detected when digestion with chondroitinase ABC or pronase (without prefixation) was applied; chondroitinase AC treatment clearly affected the filaments, although they still were visible. These results indicate that the filaments are dermatan sulphate-containing proteoglycans. Some functional aspects of the proteoglycans are discussed.