Sialic acid mediates the initial binding of positively charged inorganic particles to alveolar macrophage membranes

Our plain-English summary of Sialic acid mediates the initial binding of positively charged inorganic particles to alveolar macrophage membranes, by J.E. Gallagher, G. George, A.R. Brody (The American Review of Respiratory Disease, 1987).

The study

• Authors: J.E. Gallagher, G. George, A.R. Brody
• Journal: The American Review of Respiratory Disease
• Published: 1987;135(6):1345-52
• Cited by: 45 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Pulmonary macrophages phagocytize inhaled particles and are postulated to play a role in the development of pulmonary interstitial fibrogenesis. The basic biologic mechanisms through which inhaled particles bind to macrophage membranes and subsequently are phagocytized remain unclear. We hypothesize that positively charged particles bind to negatively charged sialic acid (SA) residues on macrophage membranes. Alveolar Macrophages (AM) were collected by saline lavage from normal rat lungs. The cells adhered to plastic coverslips in serum-free phosphate buffered saline at 37 degrees C for 45 min and then were maintained at 4 degrees C for the binding experiments. Even distribution of SA groups on AM surfaces was demonstrated by scanning electron microscopy of wheat germ agglutinin (WGA) conjugated to 50 nm gold spheres. The WGA is a lectin that binds specifically to sialic acid, and pretreatment of AM with this lectin prevented the binding of positively charged carbonyl iron (C-Fe) spheres, aluminum (Al) spheres, and chrysotile asbestos fibers to AM surfaces. Limulus protein, another lectin with binding specificity for SA, similarly blocked the binding of positively charged spheres and chrysotile asbestos fibers but not negatively charged glass spheres or crocidolite asbestos fibers. Con A and ricin, lectins that bind to mannose and galactose residues, respectively, did not block particle binding. When both positively charged iron spheres and negatively charged glass spheres were prebound to AM membranes, subsequent treatment with WGA displaced only the positively charged spheres from macrophage surfaces. Con A and ricin had no effect on prebound positively charged C-Fe and Al spheres.(ABSTRACT TRUNCATED AT 250 WORDS)

In plain English

This was a laboratory study of rat lung cells. The authors washed alveolar macrophages (scavenger cells of the lung's air sacs) out of normal rat lungs. Spheres were counted per cell by scanning electron microscopy; fibers, harder to count, were graded as a percentage of binding on control cells without lectin. The particles were positively charged carbonyl iron spheres, aluminum spheres and chrysotile asbestos fibers, and negatively charged glass spheres and crocidolite asbestos fibers. Negatively charged sialic acid groups were spread evenly over the cell surface. Blocking these groups with a protein called WGA reduced binding of iron and aluminum spheres, more at higher doses, and significantly reduced chrysotile binding. Proteins that bind other sugars did not block binding. Another sialic-acid-binding protein, limulus, blocked iron but not glass spheres. At 1.0 and 3.0 mg/ml, limulus reduced chrysotile binding, relative to fiber-exposed control cells without lectin, more than crocidolite binding. An enzyme removed 30% of the available sialic acid without significantly changing chrysotile or iron binding; the authors thought this probably too little. More aluminum spheres bound as the pH was lowered. The authors concluded that sialic acid mediates initial binding of positively charged particles in vitro.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) experiment on rat alveolar macrophages, using blocking and competition tests with lectins (sugar-binding proteins) and sugars
• Who was studied: Alveolar macrophages washed (lavaged) from the lungs of normal white CD rats aged 8 to 10 weeks. The number of rats is not stated. At least 50 cells per treatment group were counted.
• Exposure: Positively charged carbonyl iron spheres, aluminum spheres and chrysotile asbestos fibers, and negatively charged glass spheres and crocidolite asbestos fibers, added to cells kept at 4 degrees C. Spheres were used at 3.3 mg/ml and asbestos fibers at 0.5 mg/ml.
• Main results:
  • Sialic acid groups were evenly distributed on the macrophage surface. Blocking with WGA, or adding the competing sugar GlcNAc, cut binding of the gold-labelled WGA marker by 80 to 85%.
  • WGA pretreatment inhibited binding of positively charged carbonyl iron and aluminum spheres. Con A and ricin showed no dose-dependent effect on carbonyl iron binding.
  • WGA significantly reduced binding of chrysotile asbestos fibers to macrophages (p less than 0.05).
  • Limulus lectin blocked binding of iron spheres but had no effect on glass spheres. At 1.0 and 3.0 mg/ml limulus, fiber binding expressed as a percentage of binding on fiber-exposed control cells not treated with lectin was reduced more for chrysotile than for crocidolite.
  • WGA added after particles had bound displaced a significant number of carbonyl iron and aluminum spheres, but not glass spheres. Ricin and Con A did not displace carbonyl iron.
  • The competing sugars GlcNAc and NeuNAc reversed WGA's blocking effect. The noncompeting sugars GalNAc and mannose did not.
  • Neuraminidase treatment reduced the available sialic acid residues by 30% but caused no significant change in binding of chrysotile or carbonyl iron.
  • Across pH 2.0 to 9.0, more aluminum spheres bound to the cells as pH was lowered.
• The authors' conclusion: The authors concluded that negatively charged sialic acid groups on macrophage membranes mediate the initial binding of positively charged particles in vitro. They suggested this step could come before the cell swallows the particle and be needed for later toxicity.
• Limitations the authors state:
  • The authors did not know where on the macrophage membrane negatively charged particles bind.
  • Neuraminidase removed only 30% of the total available sialic acid, which the authors said was probably not enough to affect binding.
  • The sugar NeuNAc could not be tested on its own because it made the positively charged spheres clump.
  • Fibers could not easily be counted one by one, so asbestos binding was graded rather than counted.
  • The experiments were in vitro. The authors said particle-membrane interactions still need to be understood in the living body.

Topics

Medical terms in this study, explained in our Lexicon: Alveolar Macrophage, Macrophage, Interstitial, Scanning Electron Microscopy (SEM), Chrysotile, Crocidolite.

Related studies

• Hemolysis by chrysotile asbestos fibers. I. Influence of the sialic acid content in human, rat, and sheep red blood cell membranes — J.P. Pelé, R. Calvert (1983)
• Morphologic characterization of alveolar macrophages from subjects with occupational exposure to inorganic particles — T. Takemura, W.N. Rom, et al. (1989)
• Influence of the sialic acid content of the membrane on its susceptibility to chrysotile — J. Depasse (1982)
• Pulmonary macrophages are attracted to inhaled particles through complement activation — D.B. Warheit, L.H. Overby, et al. (1988)
• Production of arachidonic acid metabolites by macrophages exposed in vitro to asbestos, carbonyl iron particles, or calcium ionophore — S. Kouzan, A.R. Brody, et al. (1985)
• Asbestos fibers and silica particles stimulate rat alveolar macrophages to release tumor necrosis factor. Autoregulatory role of leukotriene B4 — C.M. Dubois, E. Bissonnette, et al. (1989)

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