Gene Regulation And Function Of Basement Membranes

Dental &Craniofacial Research

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Abstract

Basement membranes are thin extracellular matrices that separate cells, such as endothelial, epithelial, muscular, and neural cells, from their adjacent stroma. Basement membranes are the first extracellular matrix to appear in development and are critical for organ development and tissue repair. They not only provide the scaffold for cells and cell layers, but they also have an essential role in morphogenesis that affects cell adhesion, migration, proliferation, and differentiation. Additionally, basement membranes provide major barriers in blood vessels to the passage of proteins and invasion by metastatic tumor cells. The thickness and components of basement membranes are different in various tissues, suggesting tissue-specific functions. The major molecules in basement membranes are collagen IV, laminin, perlecan, and nidogen/entactin, which interact with each other and other molecules to form the supramolecular structure. Recently, genetic diversity in the subunits of laminin and type IV collagen has been found and the existence of a large family of these molecules has been demonstrated. Our primary objectives have been to identify the specific functions of basement membrane components, to study the structure and function relationships, to elucidate the mechanisms by which they are regulated, and to describe related protein interactions in development and diseases. Our effort has also focused on establishing animal models to study functions of basement membrane components in development and disease and on creating diagnostic and therapeutic reagents for diseases associated with basement membranes. We have also identified bioactive sites on laminin and perlecan that have a number of biological activities, such as promoting cell adhesion, migration, and neurite outgrowth, and affecting metastatic activity of tumor cells. These studies are aimed towards developing reagents useful for diagnostic and therapeutic applications. We previously created perlecan knockout mice, which developed a severe chondrodysplasia with micromelia and died as embryos or perinatally. We subsequently identified functional-null mutations of perlecan, which cause a lethal chondrodysplasia in humans, dyssegmental dysplasia, Silverman-Handmaker type (DDSH). Partially functional mutations of perlecan also cause a milder human genetic disorder Schwartz-Jampel syndrome (SJS), characterized by myotonia and chondrodysplasia. The myotonia phenotype of SJS suggests the involvement of perlecan in neuromuscular junction (NMJ) activity, since perlecan is enriched at the NMJ and co-localizes with acetylcholinesterase (AChE). In perlecan-null mice AChE is absent at the NMJ, while other molecules enriched at the normal NMJ, such as acetylcholine receptor (AChR), were present at the perlecan-null NMJ. In order to define the mechanism of myotonia caused by the defect in perlecan, we have rescued the perinatal lethality of perlecan-null mice by mating heterozygote perlecan-null mice with transgenic mice expressing recombinant perlecan specifically in cartilage. The mice survived and developed myotonia, showing a continuous discharge on the EMG (electromyography), and degeneration of muscle. The discharge was blocked by treatment with curare. Thus, the rescued mice are useful to study the mechanism of myotonia and to develop therapeutic agents for the disease. Perlecan is implicated in atherosclerosis, because lipoproteins associate with proteoglycans. In collaboration with Dr. Ira Goldberg, we showed that heterozygous perlecan mice developed less atherosclerosis compared to wild-type mice.