Laminin deposition in the extracellular matrix: a complex picture emerges

Abstract

Laminins are structural components of basement membranes. In addition, they are key extracellular-matrix regulators of cell adhesion, migration, differentiation and proliferation. This Commentary focuses on a relatively understudied aspect of laminin biology: how is laminin deposited into the extracellular matrix? This topic has fascinated researchers for some time, particularly considering the diversity of patterns of laminin that can be visualized in the matrix of cultured cells. We discuss current ideas of how laminin matrices are assembled, the role of matrix receptors in this process and how laminin-associated proteins modulate matrix deposition. We speculate on the role of signaling pathways that are involved in laminin-matrix deposition and on how laminin patterns might play an important role in specifying cell behaviors, especially directed migration. We conclude with a description of new developments in the way that laminin deposition is being studied, including the use of tagged laminin subunits that should allow the visualization of laminin-matrix deposition and assembly by living cells.

Fig. 1.

Laminin subunits and examples of three heterotrimers. (A) The major laminin-subunit splice isoforms, showing some of the prominent, functionally important domains within each of the subunits (see key). (B) Organization of laminin heterotrimers. Laminins 111, 332 and 331 are depicted. Binding sites for those matrix molecules and receptors that are mentioned in the text are indicated on each diagram (red boxes).

Fig. 2.

Laminin subunits are deposited in a variety of complex patterns in the matrix of cultured cells in vitro. (A) In the matrix of non-migrating keratinocytes, the α3 laminin subunit appears in a rosette-like pattern. (B) In alveolar epithelial cells of the lung, the α3 laminin subunit appears in fibrillar arrays. (C) In immortalized endothelial cells, the α4 laminin subunit localizes to focal-adhesion-like structures. (D) Cells shown in C are stained by an antibody against αv integrin. C and D have been adapted from Gonzales et al. (Gonzales et al., 2001), with permission. In A-D, cells were prepared for immunofluorescence and then viewed by confocal microscopy with the focal plane of the image being as close as possible to the substratum-attached surface of the cells. Scale bars: 20 μm.

Fig. 3.

Imaging of laminin matrix in live motile and non-migrating keratinocytes. (A) Keratinocytes expressing YFP-tagged laminin-332 (green) were plated onto a glass coverslip and visualized by confocal microscopy 8 hours later. Phase-fluorescence overlays at the indicated time points are shown. This set of images was taken from Sehgal et al. (Sehgal et al., 2006), with permission. Scale bar: 50 μm. (B-D) A group of live, stationary keratinocytes expressing mCherry-tagged laminin-332 was viewed by confocal microscopy. The fluorescence and phase contrast images in B and C are overlaid in D. Scale bar: 20 μm.