Some of the most evolutionarily conserved ECM molecules are those that comprise basal laminae. While our understanding of the molecular organization and function of basal laminae is largely derived from vertebrates, studies of invertebrate basal lamina indicate a high degree of structural and functional conservation between vertebrates and invertebrates. Basal lamina was originally referred to as “basement membranes” based on low resolution light microscopic images of tissue sections stained with hematological dyes. More sophisticated higher resolution transmission electron microscopy (TEM) studies revealed that what was originally referred to as basement membranes, included interstitial matrix as well as what is now defined as basal laminae. A basal lamina is a 50-100 nm zone visible as an electron dense domain (lamina densa) sandwiched between two almost electron transparent zones. Despite this many researchers continue to use the terms interchangeably. Throughout this thesis, the term basal lamina will be used exclusively.
Basal lamina is multifunctional, ECM sheets that underlie epithelial and endothelial cells and surrounding muscle fibers, nerves, and adipose tissue. In addition to tissue compartmentalization, basal lamina have diverse morphoregulatory activities such as acting as substrates for cell migration, axon guidance, growth factor localization and activation, maintenance of epithelial cell polarity, and differentiation, growth and apoptosis. In some tissues, like the kidney, the basal lamina has been modified to play a key role in filtration.
Fig. 1 Basal laminae are specialized extracellular matrix networks.
Studies have shown that Laminin, Type IV Collagen, Perlecan, and Nidogen are major components of invertebrate basal lamina. The Drosophila genome contains four Laminin genes, which encode two α, one β, one γ subunits, two Type IV Collagen genes, which encode the α1(IV) and α2(IV) subunits, one Nidogen and one Perlecan gene.
Molecular and genetics studies in Drosophila have shown that mutations that prevent the assembly of Laminin-1 (α3, α5, β1, γ1) molecules lead to the disruption or absence of basal laminae during embryogenesis resulting in embryonic lethality. Furthermore, mutations in Perlecan and Nidogen do not affect the assembly and stability of basal laminae during embryogenesis, raising the possibility that other components contribute to the assembly and stability of basal laminae.
There are two major sources of basal laminae components in Drosophila: the hemocytes and the fat body. With the exception of the hemocytes, all Drosophila tissues are associated with a basal lamina.
Biochemical and genetic studies conducted have provided the following model of basal lamina assembly. Laminin is the first component of the basal lamina to be secreted and assembled into a network and in its absence no basal lamina formation occurs. Secreted Laminin binds to the ECM cell surface receptors β1-integrin and α-dystroglycan via its C-terminal LG domain. Binding to cell surface receptors facilitates its polymerization through the N-terminal short arms. Following Laminin polymerization into a network, Collagen IV is secreted and polymerized into a network that is tethered to the Laminin network. Linkage of the Collagen IV network to the Laminin network is believed to be mediated through the small ECM glycoprotein Nidogen that can bind to both Type IV Collagen and Laminin. Perlecan does not polymerize like Collagen IV and Laminin but can bind to both of these network forming molecules and Nidogen.