Self-assembly of laminin induced by acidic pH.
The supramolecular architecture of the basement membrane is provided by two enmeshed networks of collagen IV and laminin. The laminin network is maintained exclusively by interactions among individual laminin molecules and does not depend on the presence of other extracellular matrix components. Laminin polymers can be obtained in vitro either in solution or in association with the surface of bilayers containing acidic lipids. In this work, we have tested the hypothesis that the negative charges present on acidic lipids establish an acid microenvironment that is directly responsible for inducing laminin aggregation. Using light-scattering measurements, we show that laminin does not aggregate on vesicles of neutral lipids, whereas instantaneous aggregation occurs to progressively greater extents as the proportion of acidic phospholipids in the vesicles is increased. Aggregation of laminin induced by vesicles containing acidic phospholipids occurs very rapidly, so that maximal aggregation for each condition is reached within 1 min after laminin dilution. Aggregation depends on the presence of Ca(2+) ions, is reversed by increasing ionic strength, and can be detected at laminin concentrations as low as 6 nM. In addition, we show that, in the absence of vesicles, acidification of the bulk solution can also induce laminin self-polymerization through a process that exhibits the same properties as lipid-induced polymerization. The fact that there is a correspondence between the processes of self-polymerization of laminin in acidic medium and in neutral medium but in the presence of vesicles containing negatively charged lipids leads us to propose that the microenvironment of an acidic surface may trigger the assembly of laminin networks. In vivo, such an acidic microenvironment would be provided by negatively charged sialic acid and sulfate groups present in the glycocalyx surrounding the cells.
Freire
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Departamento de Bioquímica Médica, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro, RJ 21941-590, Brazil.
Coelho-Sampaio
T
T
D000818:Animals; D004492:Edetic Acid / Q000494:pharmacology; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D007797:Laminin / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism*; D008027:Light; D008051:Lipid Bilayers; D046911:Macromolecular Substances; D051379:Mice; D009282:Naphthalenesulfonates; D017830:Octoxynol / Q000494:pharmacology; D010718:Phosphatidylserines / Q000494:pharmacology; D011487:Protein Conformation / Q000187:drug effects; D012542:Scattering, Radiation; D012965:Sodium Chloride / Q000494:pharmacology; D012996:Solutions
D005456:Fluorescent Dyes; D007797:Laminin; D008051:Lipid Bilayers; D046911:Macromolecular Substances; D009282:Naphthalenesulfonates; D010718:Phosphatidylserines; D012996:Solutions; D012965:Sodium Chloride; C009497:2-(4-toluidino)-6-naphthalenesulfonic acid; D017830:Octoxynol; D004492:Edetic Acid