**1. Molecular composition of the basal lamina**

The basal lamina constitutes a thin extracellular matrix, which is located between the connec‐ tive tissue and the basolateral side of a cell layer. This cellular layer can consist of either endothelial or epithelial cells, and those cell types secrete the different molecular components of the basal lamina. The main components of the basal lamina are laminin, collagen IV, the perlecan complex, and entactin, which are also known as nidogen [1, 2]. Together, those macromolecules form a complex network as illustrated in **Figure 1**. In addition, the basal lamina may contain several proteases such as matrix metalloproteinase-2 (MMP-2), MMP-9, and growth factors such as transforming growth factor beta (TGF-β), insulin-like growth factor (IGF) and fibroblast growth factor (FGF) [3].

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**Figure 1.** Schematic illustration of the basal lamina network. Both laminin and collagen IV assemble into a sheet-like network. Those two networks are cross-linked by entactin as well as the perlecan complex.

Laminin is a glycoprotein mainly found in basement membranes such as the basal lamina and is composed of three polypeptide chains: α-chain, β-chain, and γ-chain [4]. These three chains assemble into a cross-like structure, where the short arms of the cross are formed by the Ntermini of the three subunits. The long arm of this cross-like structure is formed by all three subunits which assemble into an α-helical coiled-coil structure with a globular end [4, 5]. Laminin self-assembles into a sheet-like structure by binding the short arms of different laminins to each other [6]. The polymerized laminin network is anchored to the underlying cell layer via integrin interactions mediated by the globular end of the cross-like structure [4, 7]. Collagen IV is a collagen variant mostly found in the basal lamina and forms a helix similar to collagen I [8]. Type IV collagen self-assembles via covalent bonding, disulfide cross-linking, and non-covalent side-by-side interactions into a sheet-like structure [2]. Both sheet-like structures, the laminin and collagen IV network, do not interact with each other; however, both laminin and collagen IV can bind to perlecan as well as entactin. As a consequence, the latter two molecules act as cross-linkers between the two sheet-like structures, thus maintain‐ ing the complex architecture of the basal lamina [9]. The perlecan complex is a basal laminaspecific proteoglycan. In general, proteoglycans consist of a protein core with glycosaminoglycans covalently attached to the protein. Thus, the perlecan complex consists of perlecan as core protein and 2–15 heparan sulfate (HS) side chains [10]. Also entactin is a glycoprotein and consists of three globular units connected by rod-like structures [11]. Two of the three globular units (G1 and G2) are situated at the N-terminus of entactin. The third globular unit (G3) is found at the C-terminus of the protein. G3 strongly binds to the γ-laminin short arm but can also bind to collagen IV. In contrast, G2 only binds to collagen type IV [11], thus connecting the networks built by laminin and collagen type IV.

In addition to building a complex network and serving as an anchoring matrix for a neigh‐ boring cell layer, all of these basal lamina components can directly influence the cell fate: laminin, in combination with collagen IV supports cell attachment, differentiation, migration, and growth [12]. It was suggested that in addition to fibronectin, type IV collagen and laminin are involved in the formation of tight junctions [13]. Laminin and collagen IV are also key players in establishing the mechanical stability of the basal lamina [10]. As mentioned above, the proteoglycan perlecan consists of a core protein to which HS, a heavily charged glycosa‐ minoglycan, is attached. In addition to acting as a cross-linker between laminin and collagen IV, perlecan and, in particular, the highly charged HS chains are responsible for the hydration of the matrix and contribute to the selective filtering properties of the basal lamina [14–16].

Although this highly specific structure–function relationship suggests that the microarchitec‐ ture of the basal lamina might be rather static, proteolysis of extracellular matrix (ECM) components and thus matrix remodeling is a process which continuously takes place *in vivo*. Remodeling of the ECM is, for example, a crucial part of wound healing and cell differentiation [17]. In addition, the degradation of ECM components can be responsible for cell apoptosis but, depending on the ECM component degraded, can also enhance cell viability [18]. In particular, the degradation of laminin is thought to be harmful for cells: In a study conducted in mice, it was suggested that the breakdown of laminin by the MMP-9 induces neuronal apoptosis but can be prevented by the addition of MMP-9 inhibitors [19].

**Figure 1.** Schematic illustration of the basal lamina network. Both laminin and collagen IV assemble into a sheet-like

Laminin is a glycoprotein mainly found in basement membranes such as the basal lamina and is composed of three polypeptide chains: α-chain, β-chain, and γ-chain [4]. These three chains assemble into a cross-like structure, where the short arms of the cross are formed by the Ntermini of the three subunits. The long arm of this cross-like structure is formed by all three subunits which assemble into an α-helical coiled-coil structure with a globular end [4, 5]. Laminin self-assembles into a sheet-like structure by binding the short arms of different laminins to each other [6]. The polymerized laminin network is anchored to the underlying cell layer via integrin interactions mediated by the globular end of the cross-like structure [4, 7]. Collagen IV is a collagen variant mostly found in the basal lamina and forms a helix similar to collagen I [8]. Type IV collagen self-assembles via covalent bonding, disulfide cross-linking, and non-covalent side-by-side interactions into a sheet-like structure [2]. Both sheet-like structures, the laminin and collagen IV network, do not interact with each other; however, both laminin and collagen IV can bind to perlecan as well as entactin. As a consequence, the latter two molecules act as cross-linkers between the two sheet-like structures, thus maintain‐ ing the complex architecture of the basal lamina [9]. The perlecan complex is a basal laminaspecific proteoglycan. In general, proteoglycans consist of a protein core with glycosaminoglycans covalently attached to the protein. Thus, the perlecan complex consists of perlecan as core protein and 2–15 heparan sulfate (HS) side chains [10]. Also entactin is a glycoprotein and consists of three globular units connected by rod-like structures [11]. Two of

network. Those two networks are cross-linked by entactin as well as the perlecan complex.

204 Composition and Function of the Extracellular Matrix in the Human Body

Moreover, the degradation of laminin does not only result in cell apoptosis but also impacts the stability of the basal lamina [20]. Since laminin interacts with the integrins on the cell surface and anchors the cells onto the basal lamina, a breakdown of laminin results in a separation of the basal lamina from the endothelial/epithelial cell layer which in turn induces a loss of cell– matrix communication [21, 22]. It was shown in an *in vivo* study in a mouse model that when the second structural main component of the basal lamina, collagen IV, is knocked out, embryos develop normal during the first few days, but after 10 days of development lethality occurs [23]. It was suggested that collagen IV is essential for the function and integrity of the basal lamina when mechanical stress increases. However, collagen IV seems to be unimportant in the assembly of the basal lamina at early embryonic states [23]. Similar results were obtained when an enzyme, which catalyzes the assembly of collagen IV, was modified and thus nonfunctional. In these mice, collagen IV was present but did not assemble properly and the mouse embryos died after 10 days [24].

In contrast to those structural main components, loss of the small cross-linking molecule entactin seems to have a weaker influence on basal lamina structure and function. In mice, the inactivation or mutation of the gene encoding entactin results in a normal basal lamina phenotype, and the viability of the mutant mice seems not be strongly impaired by a loss of entactin [25–27]. Exceptions are the lung and the kidney, organs which fulfill important filtering tasks and thus contain a huge amount of basal lamina. Here, a loss of entactin crosslinking function entailed strong alteration of those tissues during embryonal development and ultimately led to death immediately after birth [28]. Of course, alterations in basal lamina properties can also have less severe consequences. For instance, long-term diabetes patients not only often suffer from retinopathies but also show an increased thickness and stiffness in the ocular basal lamina. Here, however, the higher amount of basal lamina proteins is due to the expression of diabetes-specific proteins whereas the production of the normal basal lamina components is not increased [29].
