Although there are many other less abundant proteins in the extracellular matrix, laminin is the final ECM molecule to be discussed in this chapter. Laminins are a family of secreted glycoproteins that are found in many ECM formations, and like fibronectin, bind to cells via integrin receptors. The laminin protein is composed of three subunits (α, β, γ) arranged in a cruciform shape. There are multiple isoforms of each subunit yielding the variety (15) of laminin proteins catalogued to date. Although laminin contains an RGD sequence like fibronectin, its role in cell adhesion is not universal. Although some cell types have been demonstrated to bind to the RGD site, others clearly bind to other domains of laminin, primarily located on the opposite end of the protein.
Figure 7. The cruciform structure of laminin is composed of three subunits. The arms bind collagens and sulfolipids, while the foot contains LG domains that bind integrins and carbohydrates.
At present, there are 5 α-chain genes, 4 β-chains, and 3 γ-chains known. The α2/β1/γ1 combination is known as laminin-2 or merosin, and is found primarily in the basal lamina of striated muscle. Mutations that affect the function of this laminin cause a form of congenital muscular dystrophy.
Laminin plays a crucial role in neural development, where it acts as a guiding path along which certain axons extend to find their eventual synaptic targets. One prominent example is the retinotectal pathway that leads retinal ganglion cell axons from the eye to the brain. Another example of the role of laminin in development is the guidance of primordial germ cells (PGC). These are cells that eventually become the gametes, but need to migrate from the yolk sac, which is outside the embryo proper, to the site of gonad formation. Laminin is found along this pathway. Interestingly, as the PGCs reached a stretch of laminin very close to the final destination, the adhesion to laminin increased, and this adhesion was found to involve not integrin receptors, but an interaction with a cell surface heparan-sulfate proteoglycan. This and other evidence suggests that migration on laminin may be mediated by integrin receptors, whose adhesivity can be regulated intracellularly, while more static interactions with laminin may be mediated by other types of binding proteins. Finally, as already discussed, laminin is an important component of basal lamina, able to form fibrils and networks itself, as well as with collagen IV.