We have studied the reconstitution of basement membrane molecules from extracts prepared from the basement membrane of the EHS tumor. Under physiological conditions and in the presence of added type IV collagen and heparan sulfate proteoglycan, gellike structures form whose ultrastructure appears as interconnected thin sheets resembling the lamina dense zone of basement membrane. The major components of the reconstituted structures include laminin, type IV collagen, heparan sulfate proteoglycan, entactin, and nidogen. These components polymerize in constant proportions on reconstitution, suggesting that they interact in defined proportions. Molecular sieve studies on the soluble extract demonstrate that laminin, entactin, and nidogen are associated in large but dissociable complexes which may be a necessary intermediate in the deposition of basement membrane. The reconstituted matrix was biologically active and stimulated the growth and differentiation of certain cells.

Perlecan, a large, multi-domain, heparan sulfate proteoglycan originally identified in basement membrane, interacts with extracellular matrix proteins, growth factors and receptors, and influences cellular signalling. Perlecan is present in a variety of basement membranes and in other extracellular matrix structures. We have disrupted the gene encoding perlecan (Hspg2) in mice. Approximately 40% of Hspg2-/- mice died at embryonic day (E) 10.5 with defective cephalic development. The remaining Hspg2-/- mice died just after birth with skeletal dysplasia characterized by micromelia with broad and bowed long bones, narrow thorax and craniofacial abnormalities. Only 6% of Hspg2-/- mice developed both exencephaly and chondrodysplasia. Hspg2-/- cartilage showed severe disorganization of the columnar structures of chondrocytes and defective endochondral ossification. Hspg2-/- cartilage matrix contained reduced and disorganized collagen fibrils and glycosaminoglycans, suggesting that perlecan has an important role in matrix structure. In Hspg2-/- cartilage, proliferation of chondrocytes was reduced and the prehypertrophic zone was diminished. The abnormal phenotypes of the Hspg2-/- skeleton are similar to those of thanatophoric dysplasia (TD) type I, which is caused by activating mutations in FGFR3 (refs 7, 8, 9), and to those of Fgfr3 gain-of-function mice. Our findings suggest that these molecules affect similar signalling pathways.

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When primary mouse mammary epithelial cells are cultured on plastic, they rapidly lose their ability to synthesize and secrete most milk proteins even in the presence of lactogenic hormones, whereas cells cultured on released type I collagen gels show greatly enhanced mRNA levels and secretion rates of ,B-casein and of some other milk proteins. We show here that culture on a reconstituted basement membrane from Engelbreth-Holm-Swarm tumor (EHS) allows >90% of cells to produce high levels of 13-casein. By 1-3). Until a decade ago, most investigators cultured cells on plastic surfaces, which led to drastic alterations of morphology and function from the parent tissue (4). Following the example of Emerman and Pitelka (5), we and others have shown that several aspects of functional epithelium can be maintained when primary mouse mammary epithelial cells (PMME) are cultured on "released" (RG or "floating") collagen type I gels instead of plastic. These include polarization of organelles, appearance of apical microvilli, formation of a basal lamina (5, 6), changes in glucose metabolite pattern (7), lumina formation (8), altered synthesis and compartmentalization of extracellular matrix (ECM) components (9, 10), enhancement ofmost milk protein synthesis and secretion (11), and increases in p-casein (12) and transferrin (13,14) mRNA levels. These results signify the importance of cell-substratum interactions in regulating tissue-specific functions and point to a possible regulatory role for ECM in vivo.Cells on released gels (and not on plastic or flat gels) were shown to synthesize an intact basement membrane (5) containing high levels of heparan sulfate and other sulfated glycosaminoglycans (9), type IV collagen, and laminin (10). We reasoned that the released gel may act through the influence of newly synthesized basement membrane components. To test the mechanism of cell-ECM interaction more directly, we have analyzed the consequences of culturing PMME cells on a reconstituted basal lamina derived from Engelbreth-Holm-Swarm (EHS) tumor (15) and on some of its individual components.MATERIALS AND METHODS PMME from 14-to 17-day pregnant BALB/c mice and collagen from rat tail tendon were prepared as described (5, 10, 11). Tumors (EHS) from normal or lathyritic mice were extracted with high salt and 2 M urea as described by Kleinman et al. (15). Dialyzed EHS extract (100-200 1.d) was spread either directly on 35-mm dishes or on top of rat-tail collagen gels. Collagen gels were released 3-4 days after seeding. Matrigel, an EHS preparation from Collaborative Research (Waltham, MA), was used for some experiments. Laminin and type IV collagen (Bethesda Research Laboratories) were spread at 2.5-10 pug/cm2. Heparan sulfate proteoglycan (HSPG; low density form) was prepared from EHS tumors as described by Hassell et al. (16) and was spread at 1-5 ,ug/cm2 or was added to the medium at 5 ,ug/ml every other day. The latter was less toxic but also less effective. EHS (10 ,ug/ml) and individual substrata other than HSPG we...

We have isolated a unique, basement membrane proteoglycan from the Engelbreth-Holm-Swarm (EHS) sarcoma. This proteoglycan, estimated to be 0.75 X 10w daltons, was found to contain about equal amounts of protein and covalently linked heparan sulfate. Antibody prepared against this proteoglycan reacts with the basement membrane matrix in the tumor and with the basement membranes in skin, kidney, and cornea. These studies indicate that the heparan sulfate proteoglycan is a normal constituent of basement membranes that presumably plays an important role in the organization of basement membrane components and that also may determine the permeability of basement membranes to acidic molecules.