Self-assembly of Laminin Induced by Acidic pH

Abstract

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.

Extraction and Assembly of Tissue-Derived Gels for Cell Culture and Tissue Engineering

“…Assembly of laminin-111 triggered by a local acidic pH at the cell surface 38 has been suggested as a mechanism to explain observations that in vivo protein levels in BMs may be lower than gelation thresholds observed in vitro. 24 Our results potentially agree with this mechanism, as the resultant gels have structure similar to BMs in vivo. 39 It is also possible that decreasing pH interferes with the solubility of the protein in urea.…”

“…Although the ECM mechanism of assembly in vivo is not clear, it has been suggested that BM assembly in vivo is induced by a reduction in pH at cell surfaces. 24,25 These extracts may provide a tool for studying this process. All tissue extracts could be assembled into gels by acid mechanisms of gelation and maintained this structure at pH 7.4 and 378C.…”

Sialic acid residues on astrocytes regulate neuritogenesis by controlling the assembly of laminin matrices

“…First, our previous work had suggested that negative lipids could modulate laminin self-assembly (Freire and Coelho-Sampaio, 2000;Freire et al, 2002). Second, we knew that surface charges had been proposed to fine-tune the cellular response to ECM molecules, such as chondroitin and heparan sulfate proteoglycans (Powell et al, 1997;Wettreich et al, 1999).…”

Structure of laminin substrate modulates cellular signaling for neuritogenesis

“…More recently, we showed that solution polymerization could alternatively be triggered at low laminin concentrations by acidification of bulk pH (Freire and Coelho-Sampaio, 2000). We hypothesized that acidic polymerization may occur in vivo in regions of the plasma membrane where circumstantially the negatively charged groups provided by glycoproteins, glycolipids and proteoglycans reduce the local pH (Wettreich et al, 1999;Freire and Coelho-Sampaio, 2000).…”