Identification and expression of mouse netrin-4

Netrins are secreted proteins that serve as potent axon guidance molecules in vertebrates and invertebrates. We report the identification of a novel mammalian member of this family. Netrin-4 is similar in predicted size and secondary structure to the other three netrins; all contain, in order, an amino-terminal signal sequence, a laminin-type globular domain of the 'VI' type, three laminin-type epidermal growth factor (EGF) repeats, and a carboxyl-terminal 'netrin module'. In terms of primary sequence, however, netrin-4 is a distant relative of netrins-1-3, and its globular domain is more closely related to those of laminins than to those of other netrins. Netrin-4 is broadly expressed in both neural and non-neural tissues of embryonic and adult mice. In embryonic spinal cord, it is selectively expressed by cells at the lateral margins of the floor plate. In postnatal brain, it is selectively expressed in subsets of neurons, including cerebellar granule and hippocampal pyramidal cells.

Gelatinase B (MMP-9) is not essential in the normal kidney and does not influence progression of renal disease in a mouse model of Alport syndrome

Matrix metalloproteinases are matrix degrading enzymes implicated in many biological processes, including development and inflammation. Gelatinase B (gelB; also known as MMP-9) is expressed in the kidney and is hypothesized to be involved in basement membrane remodeling and in preventing pathogenic accumulation of extracellular matrix in the kidney. Inhibition of gelB activity in metanephric organ culture disrupts branching morphogenesis of the ureteric bud, suggesting that gelB plays a role in kidney development in vivo. We studied kidneys of gelB-deficient mice to search for developmental, histological, molecular, ultrastructural, and functional defects. Surprisingly, no differences between gelB-/- and control kidneys were detected, and renal function was normal in gelB mutants. In addition, gelB-/- embryonic kidneys developed normally in organ culture. Gelatinase B-deficient mice were bred with Col4a3-/- mice, a model for Alport syndrome, to determine whether gelB influences the progression of glomerulonephritis. This is an important question, as it has been hypothesized that proteases are involved in damaging Alport glomerular basement membrane. However, the presence or absence of gelB did not affect the rate of progression of renal disease. Thus, gelB does not have a discernible role in the normal kidney and gelB is not involved in the progression of glomerulonephritis in a mouse model of Alport syndrome.

The presynaptic calcium channel is part of a transmembrane complex linking a synaptic laminin (alpha4beta2gamma1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate neurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminins (LNs) containing alpha4, alpha5, or beta2 chains. Results from a beta2 laminin knockout mouse have suggested a linkage of this extracellular laminin to cytosolic synaptic vesicle clusters. Here we report such a transmembrane link at the electric organ synapse, which is homologous to the NMJ. We immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alpha4beta2gamma1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha4 and beta2 chains are concentrated at the electric organ synapse. Using detergent-solubilized synaptosomes, we immunoprecipitated a complex containing alpha4beta2gamma1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha4beta2gamma1 laminin in the synaptic basal lamina attaches to calcium channel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.

Defective glomerulogenesis in the absence of laminin alpha5 demonstrates a developmental role for the kidney glomerular basement membrane

Laminins are major components of all basement membranes. They are a diverse group of alpha/beta/gamma heterotrimers formed from five alpha, three beta, and three gamma chains. Laminin alpha5 is a widely expressed chain found in many embryonic and adult basement membranes. During embryogenesis, alpha5 has a role in disparate developmental processes, including neural tube closure, digit septation, and placentation. Here, we analyzed kidney development in Lama5 mutant embryos and found a striking defect in glomerulogenesis associated with an abnormal glomerular basement membrane (GBM). This correlates with failure of the developmental switch in laminin alpha chain deposition in which alpha5 replaces alpha1 in the GBM at the capillary loop stage of glomerulogenesis. In the absence of a normal GBM, glomerular epithelial cells were in disarray, and endothelial and mesangial cells were extruded from within the constricting glomerulus, leading to a complete absence of vascularized glomeruli. In addition, a minority of Lama5 mutant mice lacked one or both kidneys, indicating that laminin alpha5 is also important in earlier kidney development. Our results demonstrate a dual role for laminin alpha5 in kidney development, illustrate a novel defect in glomerulogenesis, and indicate a heretofore unappreciated developmental role for the GBM in influencing the behavior of epithelial and endothelial cells.

The synaptic vesicle protein SV2 is complexed with an alpha5-containing laminin on the nerve terminal surface

Interactions between growing axons and synaptic basal lamina components direct the formation of neuromuscular junctions during nerve regeneration. Isoforms of laminin containing alpha5 or beta2 chains are potential basal lamina ligands for these interactions. The nerve terminal receptors are unknown. Here we show that SV2, a synaptic vesicle transmembrane proteoglycan, is complexed with a 900-kDa laminin on synaptosomes from the electric organ synapse that is similar to the neuromuscular junctions. Although two laminins are present on synaptosomes, only the 900-kDa laminin is associated with SV2. Other nerve terminal components are absent from this complex. The 900-kDa laminin contains an alpha5, a beta1, and a novel gamma chain. To test whether SV2 directly binds the 900-kDa laminin, we looked for interaction between purified SV2 and laminin-1, a laminin isoform with a similar structure. We find SV2 binds with high affinity to purified laminin-1. Our results suggest that a synaptic vesicle component may act as a laminin receptor on the presynaptic plasma membrane; they also suggest a mechanism for activity-dependent adhesion at the synapse.

Cell elongation induces laminin alpha2 chain expression in mouse embryonic mesenchymal cells: role in visceral myogenesis

Bronchial smooth muscle (SM) mesenchymal cell precursors change their shape from round to spread/elongated while undergoing differentiation. Here we show that this change in cell shape induces the expression of laminin (LM) alpha2 chain not present in round mesenchymal cells. LM alpha2 expression is reversible and switched on and off by altering the cell's shape in culture. In comparison, the expression of LM beta1 and gamma1 remains unchanged. Functional studies showed that mesenchymal cell spreading and further differentiation into SM are inhibited by an antibody against LM alpha2. Dy/dy mice express very low levels of LM alpha2 and exhibit congenital muscular dystrophy. Lung SM cells isolated from adult dy/dy mice spread defectively and synthesized less SM alpha-actin, desmin, and SM-myosin than controls. These deficiencies were completely corrected by exogenous LM-2. On histological examination, dy/dy mouse airways and gastrointestinal tract had shorter SM cells, and lungs from dy/dy mice contained less SM-specific protein. The intestine, however, showed compensatory hyperplasia, perhaps related to its higher contractile activity. This study therefore demonstrated a novel role for the LM alpha2 chain in SM myogenesis and showed that its decrease in dy/dy mice results in abnormal SM.

Renal basement membrane components

Renal basement membrane components. Basement membranes are specialized extracellular matrices found throughout the body. They surround all epithelia, endothelia, peripheral nerves, muscle cells, and fat cells. They play particularly important roles in the kidney, as demonstrated by the fact that defects in renal basement membranes are associated with kidney malfunction. The major components of all basement membranes are laminin, collagen IV, entactin/nidogen, and sulfated proteoglycans. Each of these describes a family of related proteins that assemble with each other in the extracellular space to form the basement membrane. Over the last few years, new basement membrane components that are expressed in the kidney have been discovered. Here, the major components and their localization in mature and developing renal basement membranes are described. In addition, the phenotypes of basement membrane component gene mutations, both naturally occurring and experimental, are discussed, as is the aberrant deposition of basement membrane proteins in the extracellular matrix in several renal diseases.

Insertional mutation of the collagen genes Col4a3 and Col4a4 in a mouse model of Alport syndrome

Mice homozygous for the transgenic insertion in line OVE250 exhibit severe progressive glomerulonephritis. Ultrastructural changes in the glomerular basement membrane (GBM) at 2 weeks of age resemble those in Alport syndrome. The transgenic insertion site was mapped by FISH to mouse chromosome 1 close to Pax3. Genetic and molecular analyses identified a deletion of genomic DNA at the transgene insertion site. Exons 1 through 12 of the collagen IV gene Col4a4, exons 1 and 2 of the adjacent Col4a3 gene, and the intergenic promoter region are deleted. Transcripts of Col4a3 and Col4a4 are undetectable in mutant kidney, and both proteins are missing from the GBM. Persistent cellular proliferation in mutant kidneys suggests that interaction with the extracellular matrix may be important for cell maturation. Evolutionarily conserved sequence elements in the promoter regions of human and mouse Col4a3 and Col4a4 include a 19-bp element that was tandemly duplicated in the human lineage and a CTC box element common to several genes encoding extracellular matrix proteins. This new animal model of Alport syndrome, Col4Delta3-4, lacks both alpha3 and alpha4 chains of collagen IV and exhibits an earlier disease onset than mice lacking alpha3 only.

Congenital nephrotic syndrome in mice lacking CD2-associated protein

CD2-associated protein (CD2AP) is an 80-kilodalton protein that is critical for stabilizing contacts between T cells and antigen-presenting cells. In CD2AP-deficient mice, immune function was compromised, but the mice died at 6 to 7 weeks of age from renal failure. In the kidney, CD2AP was expressed primarily in glomerular epithelial cells. Knockout mice exhibited defects in epithelial cell foot processes, accompanied by mesangial cell hyperplasia and extracellular matrix deposition. Supporting a role for CD2AP in the specialized cell junction known as the slit diaphragm, CD2AP associated with nephrin, the primary component of the slit diaphragm.

Distribution of ten laminin chains in dystrophic and regenerating muscles

Using immunohistochemical methods, we assessed the distribution of all 10 known laminin chains (alpha1-5, beta1-3, gamma1 and gamma2) in skeletal muscles from patients with Duchenne, congenital, limb girdle, or Emery-Dreifuss muscular dystrophies. The alpha2, beta1 and gamma1 chains were abundant in the basal lamina surrounding muscle fibers in normal controls; alpha1, alpha3-alpha5, beta3, and gamma2 were undetectable; and beta2 was present at a low level. Compared to controls, levels of the alpha5 chain were increased in muscles from many dystrophic patients; levels of beta1 were reduced and/or levels of beta2 were increased in a minority. However, these changes were neither specific for, nor consistent within, diagnostic categories. In contrast, levels of alpha4 were increased in muscles from all patients with alpha2 laminin (merosin)-deficient congenital muscular dystrophy. Loss of alpha2 laminin in congenital dystrophy is disease-specific but some other changes in laminin isoform expression in dystrophic muscles could be secondary consequences of myopathy, denervation, regeneration or immaturity. To distinguish among these possibilities, we compared the laminins of embryonic, denervated, regenerating, and mutant mouse muscles with those in normal adult muscle. Embryonic muscle basal lamina contained alpha4 and alpha5 along with alpha2, and regenerating muscle re-expressed alpha5 but not alpha4. Levels of alpha5 but not alpha4 were increased in dystrophin (mdx) mutants and in dystrophin/utrophin double mutants (mdx:utrn -/-), models for Duchenne dystrophy. In contrast, laminin alpha4 was upregulated more than alpha5 in muscles of laminin alpha2 mutant mice (dy/dy; a model for alpha2-deficient congenital dystrophy). Based on these results, we hypothesize that the expression of alpha5 in many dystrophies reflects the regenerative process, whereas the selective expression of alpha4 in alpha2-deficient muscle is a specific compensatory response to loss of alpha2.

Regulation of renal laminin in mice with type II diabetes

This study examines the regulation of renal laminin in the db/db mouse, a model of type II diabetes characterized by extensive remodeling of extracellular matrix. Immunohistochemistry demonstrated an increase in the contents of laminin chains including beta1 chain in the mesangium and tubular basement membranes at 1, 2, 3, and 4 mo of diabetes. Immunofluorescence with an antibody against the recently discovered laminin alpha5 chain showed that in the normal mouse, the protein had a restricted distribution to the glomerular and tubular basement membranes with scant expression in the mesangium of older mice. In the diabetic mouse, the laminin alpha5 chain content of the glomerular and tubular basement membranes was increased, with marked expression in the mesangium. Northern analysis revealed a significant decrease in the renal cortical contents of alpha5, beta1, and gamma1 chain mRNA in the diabetic mice compared to control, at each of the time points. In situ hybridization showed decreased abundance of alpha5 transcripts in the glomeruli of diabetic mice compared to nondiabetic controls. Analysis of mRNA changes by Northern and in situ hybridization studies demonstrated that the reduction in laminin transcripts involved both glomerular and tubular elements. These observations demonstrate that laminin accumulation in the db/db mice with type II diabetes is due to nontranscriptional mechanisms. Because previous investigations in rodents with type I diabetes have shown that the increase in renal laminin content was associated with a corresponding increment in laminin chain transcript levels, it appears that the mechanisms underlying augmentation in renal matrix laminin content may be distinct in the two types of diabetes.

(Alpha)3(beta)1 integrin regulates epithelial cytoskeletal organization

Epithelial cell morphology and cytoskeletal organization are determined by interactions, with both adjacent cells and the extracellular matrix, which are mediated by integrins and cadherins. Little is known, however, of the relative contributions of integrins and cadherins to maintaining the sub-cortical cytoskeleton characteristic of epithelial cells. Since most studies that utilize integrin-blocking antibodies result in a loss of both cell-cell adhesion and sub-cortical cytoskeletal organization, it has been difficult to distinguish whether integrins and cadherins both mediate cytoskeletal assembly in epithelial cells. Therefore, cells derived from kidney collecting ducts of (alpha)3(beta)1 integrin-deficient mice were used to examine the role of integrins in epithelial cell morphology and cytoskeletal organization. In primary cell culture, (alpha)3(beta)1 integrin-deficient kidney collecting duct cells maintain cadherin-mediated cell-cell adhesions but fail to form the sub-cortical cytoskeleton that is characteristic of epithelial cells, and instead assemble actin stress fibers. Moreover, the cell-cell junctions in mutant cells were irregular, rather than being uniformly oriented perpendicular to the culture substrate. These results demonstrated that integrins have an primary and essential function in establishing and maintaining the sub-cortical cytoskeleton that is characteristic of epithelial cells. To further study the role of (alpha)3(beta)1 integrin in establishing and maintaining cytoskeletal organization in tubular epithelial cells, we derived immortalized cell lines from wild-type and (alpha)3(beta)1 integrin-deficient kidney collecting ducts that duplicated the cytoskeletal and cadherin organization observed in primary cells. E-cadherin and (alpha)- and (beta)-catenin were complexed together in equal amounts in membranes of wild-type and (alpha)3(beta)1 integrin-deficient cells. However, association of the cadherin:catenin complex with (alpha)-actinin was greatly decreased in mutant cells, indicating that integrin-mediated assembly of the sub-cortical cytoskeleton is essential for subsequent association of the cytoskeleton with the cadherin:catenin complex. These results present direct evidence for integrin:cadherin cross-regulation in which cadherin function is dependent on the presence of an integrin.

Laminin-11

Laminins are a family of glycoproteins which are ubiquitous components of basement membranes and play key structural and functional roles. Eleven isoforms have been identified to date; each is an alpha beta gamma heterotrimer assembled from a repertoire of five alpha, three beta and two gamma chains. Studies of laminin-11 (alpha 5 beta 2 gamma 1) illustrate the unique expression patterns and distinct functions that can be displayed by laminin isoforms. Laminin-11 is found in the glomerular basement membrane in kidney, in the neuromuscular synaptic cleft in skeletal muscle and in other tissues such as placenta and lung. Mice lacking laminin-11 exhibit defective glomerular filtration and developmental defects in neuromuscular synapse formation, with Schwann cells invading the synaptic cleft. Consistent with these observations, both motoneurons and Schwann cells distinguish laminin-11 from other isoforms in vitro. These results suggest that laminin-11 is a structural component of the basement membrane which influences cell behavior in physiologically relevant ways. A greater understanding of laminin-11 assembly and basement membrane incorporation could provide a logical basis for therapy in merosin-deficient congenital muscular dystrophy.

Adhesion of cultured bovine aortic endothelial cells to laminin-1 mediated by dystroglycan

Expression of dystroglycan (DG) by cultured bovine aortic endothelial (BAE) cells was confirmed by cDNA cloning from a BAE cDNA library, Northern blotting of mRNA, Western blotting of membrane proteins, and double immunostaining with antibodies against betaDG and platelet endothelial cell adhesion molecule-1. Immunocytochemical analysis revealed localization of DG in multiple plaques on the basal side of resting cells. This patchy distribution was obscured in migrating cells, in which the most prominent staining was observed in the trailing edge anchoring the cells to the substratum. Biotin-labeled laminin-1 overlay assay of dissociated BAE membrane proteins indicated the interaction of laminin-1 with alphaDG. The laminin alpha5 globular domain fragment expressed in bacteria and labeled with biotin could also bind alphaDG on the membrane blot, and the unlabeled fragment disrupted the binding of biotin-laminin-1 to alphaDG. The interaction of biotin-laminin-1 with alphaDG was inhibited by soluble alphaDG contained in the conditioned medium from DG cDNA-transfected BAE cells and by a series of glycosaminoglycans (heparin, dextran sulfate, and fucoidan). Soluble alphaDG in the conditioned medium inhibited the adhesion of BAE cells to laminin-1-coated dishes, whereas it had no effect on their adhesion to fibronectin. All three glycosaminoglycans that disrupted the biotin-laminin-1 binding to alphaDG inhibited BAE cell adhesion to laminin-1, whereas they failed to inhibit the adhesion to fibronectin. These results indicate a role of DG as a non-integrin laminin receptor involved in vascular endothelial cell adhesion to the extracellular matrix.

Sex-specific localization of laminin alpha 5 chain in the differentiating rat testis and ovary

The localization of laminin (Ln) alpha 5, beta 1 and beta 2 chains in the differentiating rat testis and ovary was studied by immunolabeling light and electron microscopy. The initial formation of the male and female gonadal blastemas included an emergence of Ln alpha 5 and beta 1 chains, but not of Ln beta 2 chain. The sexual differentiation of the embryonic male gonadal cords included rapid sex-specific disappearance of the incipient Ln alpha 5 chain. The rete testis cords, in contrast, remained positive for Ln alpha 5 chain. In the postnatal testis, the Ln alpha 5 chain reappeared in Ln beta 1 chain-positive cord basement membranes, which also became positive for Ln beta 2 chain. The differentiating myoid cells also gradually became positive for both Ln alpha 5 and Ln beta 1 chains. In the ovary Ln alpha 5 chain persisted in BMs of the cords throughout the fetal phase. Small and newly formed follicles in the early postnatal rat ovary were also positive for Ln alpha 5 chain, whereas growing and large follicles were negative. During the early postnatal phase, Ln beta 1-chain positive follicular BMs became also positive for the Ln beta 2 chain. Basement membranes of testicular and ovarian surface epithelia contained the Ln alpha 5 chain throughout the study. The blood vessels of the male and female gonad showed differentiation-dependent variation in their reactivity for the Ln alpha 5 and beta 2 chains. The present results show that the Ln alpha 5 chain is an early molecular marker for sexual differentiation, which therefore may be regulated by the testis-determining factors. The results also show that in the early postnatal rat ovary, the follicular basement membranes are heterogeneous in their Ln content, which may offer a means to distinguish different follicular populations from each other and to identify the different stages of follicular growth.

Roles for laminin in embryogenesis: exencephaly, syndactyly, and placentopathy in mice lacking the laminin alpha5 chain

Laminins are the major noncollagenous glycoproteins of all basal laminae (BLs). They are alpha/beta/gamma heterotrimers assembled from 10 known chains, and they subserve both structural and signaling roles. Previously described mutations in laminin chain genes result in diverse disorders that are manifested postnatally and therefore provide little insight into laminin's roles in embryonic development. Here, we show that the laminin alpha5 chain is required during embryogenesis. The alpha5 chain is present in virtually all BLs of early somite stage embryos and then becomes restricted to specific BLs as development proceeds, including those of the surface ectoderm and placental vasculature. BLs that lose alpha5 retain or acquire other alpha chains. Embryos lacking laminin alpha5 die late in embryogenesis. They exhibit multiple developmental defects, including failure of anterior neural tube closure (exencephaly), failure of digit septation (syndactyly), and dysmorphogenesis of the placental labyrinth. These defects are all attributable to defects in BLs that are alpha5 positive in controls and that appear ultrastructurally abnormal in its absence. Other laminin alpha chains accumulate in these BLs, but this compensation is apparently functionally inadequate. Our results identify new roles for laminins and BLs in diverse developmental processes.

Human corneal epithelial basement membrane and integrin alterations in diabetes and diabetic retinopathy

Corneas of diabetic patients have abnormal healing and epithelial adhesion, which may be due to alterations of the corneal extracellular matrix (ECM) and basement membrane (BM). To identify such alterations, various ECM and BM components and integrin receptors were studied by immunofluorescence on sections of normal and diabetic human corneas. Age-matched corneas from 15 normal subjects, 10 diabetics without diabetic retinopathy (DR), and 12 diabetics with DR were used. In DR corneas, the composition of the central epithelial BM was markedly altered, compared to normal or non-DR diabetic corneas. In most cases the staining for entactin/nidogen and for chains of laminin-1 (alpha1beta1gamma1) and laminin-10 (alpha5beta1gamma1 was very weak, discontinuous, or absent over large areas. Other BM components displayed less frequent changes. The staining for alpha3beta1 (VLA-3) laminin binding integrin was also weak and discontinuous in DR corneal epithelium. Components of stromal ECM remained unchanged even in DR corneas. Therefore, distinct changes were identified in the composition of the epithelial BM in DR corneas. They may be due to increased degradation or decreased synthesis of BM components and related integrins. These alterations may directly contribute to the epithelial adhesion and wound healing abnormalities found in diabetic corneas.

Expression of laminin alpha3, alpha4, and alpha5 chains by alveolar epithelial cells and fibroblasts

Laminins are principal components of basement membranes. Eleven laminin isoforms are known, each a heterotrimer composed of polypeptide chains designated alpha, beta, and gamma. Five alpha chains have been identified to date: alpha1, alpha2, alpha3, alpha4, and alpha5. Recent studies of fetal and adult mouse lung show prominence of alpha3, alpha4, and alpha5 in alveolar tissue, and point to differences in the cellular expression of these alpha chains in the developing alveolus. We examined isolated rat alveolar type II cells and lung fibroblasts for expression of laminins alpha3, alpha4, and alpha5. We found that laminin alpha3 was expressed only by alveolar epithelial cells, that laminin alpha4 was expressed only by lung fibroblasts, and that laminin alpha5 was expressed primarily by alveolar epithelial cells. Metabolic labeling and immunoprecipitation confirmed the production of laminin alpha4 by fibroblasts and laminin alpha5 by alveolar epithelial cells in culture. These studies indicate that different alveolar cell types contribute different laminin alpha chains to the laminin isoforms in alveolar basement membranes. Immunohistochemistry showed colocalization of these laminin alpha chains with the laminin beta1, beta2, and gamma1 chains, indicating the likelihood that laminins 6 to 11 are present in alveolar basement membranes.

Development of the neuromuscular junction: genetic analysis in mice

Formation of the skeletal neuromuscular junction is a multi-step process that requires communication between the nerve and muscle. Studies in many laboratories have led to identification of factors that seem likely to mediate these interactions. 'Knock-out' mice have now been generated with mutations in several genes that encode candidate transsynaptic messengers and components of their effector mechanisms. Using these mice, it is possible to test hypotheses about the control of synaptogenesis. Here, we review our studies on neuromuscular development in mutant mice lacking agrin alpha CGRP, rapsyn, MuSK, dystrophin, dystrobrevin, utrophin, laminin alpha 5, laminin beta 2, collagen alpha 3 (IV), the acetylcholine receptor epsilon subunit, the collagenous tail of acetylcholinesterase, fibroblast growth factor-5, the neural cell adhesion molecule, and tenascin-C.

Developmental biology of glomerular basement membrane components

The main components of basement membranes are collagen IV, laminin, entactin/nidogen, and proteoglycans. In the past few years, new basement membrane components have been discovered, including new collagen i.v. alpha chains, new laminin alpha, beta, and gamma chains, and new proteoglycans. Most of these are expressed in the kidney, a virtual treasure chest of molecularly and functionally distinct basement membranes. One well characterized renal basement membrane is the glomerular basement membrane, the primary filtration barrier of the kidney. Immunohistochemical studies have shown not only that the glomerular basement membrane is molecularly distinct from other renal basement membranes, but also that as it develops, there is a complex series of defined transitions in the basement membrane components that are deposited there. This review summarizes these transitions and discusses their relevance in our understanding of the pathogenesis of renal disease.

Distribution and function of laminins in the neuromuscular system of developing, adult, and mutant mice

Laminins, heterotrimers of alpha, beta, and gamma chains, are prominent constituents of basal laminae (BLs) throughout the body. Previous studies have shown that laminins affect both myogenesis and synaptogenesis in skeletal muscle. Here we have studied the distribution of the 10 known laminin chains in muscle and peripheral nerve, and assayed the ability of several heterotrimers to affect the outgrowth of motor axons. We show that cultured muscle cells express four different alpha chains (alpha1, alpha2, alpha4, and alpha5), and that developing muscles incorporate all four into BLs. The portion of the muscle's BL that occupies the synaptic cleft contains at least three alpha chains and two beta chains, but each is regulated differently. Initially, the alpha2, alpha4, alpha5, and beta1 chains are present both extrasynaptically and synaptically, whereas beta2 is restricted to synaptic BL from its first appearance. As development proceeds, alpha2 remains broadly distributed, whereas alpha4 and alpha5 are lost from extrasynaptic BL and beta1 from synaptic BL. In adults, alpha4 is restricted to primary synaptic clefts whereas alpha5 is present in both primary and secondary clefts. Thus, adult extrasynaptic BL is rich in laminin 2 (alpha2beta1gamma1), and synaptic BL contains laminins 4 (alpha2beta2gamma1), 9 (alpha4beta2gamma1), and 11 (alpha5beta2gamma1). Likewise, in cultured muscle cells, alpha2 and beta1 are broadly distributed but alpha5 and beta2 are concentrated at acetylcholine receptor-rich "hot spots," even in the absence of nerves. The endoneurial and perineurial BLs of peripheral nerve also contain distinct laminin chains: alpha2, beta1, gamma1, and alpha4, alpha5, beta2, gamma1, respectively. Mutation of the laminin alpha2 or beta2 genes in mice not only leads to loss of the respective chains in both nerve and muscle, but also to coordinate loss and compensatory upregulation of other chains. Notably, loss of beta2 from synaptic BL in beta2(-/-) "knockout" mice is accompanied by loss of alpha5, and decreased levels of alpha2 in dystrophic alpha2(dy/dy) mice are accompanied by compensatory retention of alpha4. Finally, we show that motor axons respond in distinct ways to different laminin heterotrimers: they grow freely between laminin 1 (alpha1beta1gamma1) and laminin 2, fail to cross from laminin 4 to laminin 1, and stop upon contacting laminin 11. The ability of laminin 11 to serve as a stop signal for growing axons explains, in part, axonal behaviors observed at developing and regenerating synapses in vivo.

The immunohistochemical composition of corneal basement membrane in keratoconus

PURPOSE

Keratoconus is a gradually progressing disease of unknown cause, characterized by central thinning, increased curvature, and finally scarring of the cornea. This causes myopia and astigmatism and the ultimate treatment is keratoplasty. We studied the composition of basement membranes (BMs) in normal, scarred and keratoconus corneas to find out possible changes specific for keratoconus.

METHODS

Frozen sections of normal, scarred and keratoconus corneas were immunostained with various antibodies against basement membrane (BM) proteins and integrin beta 4.

RESULTS

In the keratoconus corneas, we found discontinuities or defects in Bowman's layer, sometimes distorted stroma beneath the defects, and also thinning of the stroma. The results show that within the defects in keratoconus corneas, there is an expression of proteins that are not normally present in the corneal BM, i.e. collagen alpha 1/2 (IV) chains, and on the contrary, absence of the expression of some proteins, i.e. collagen alpha 5-6 (IV) chains that normally are continuously expressed in the corneal epithelial BM. In addition, either increased or decreased expression of laminin-1 (alpha 1 beta 1 gamma 1), laminin-5 (alpha 3 beta 3 gamma 2) and collagen type VII, depending on the keratoconus defect, was seen and the expression of integrin beta 4 was decreased. These findings seem to be specific for keratoconus, as they were not found in scarred corneas.

CONCLUSIONS

The results show that the defects in BM and changes in the BM composition are involved in the pathogenesis of keratoconus. Furthermore, it seems that scarring alone does not explain the breaks in Bowman's layer and immunohistochemical changes seen in keratoconus. Therefore, we suggest that a process similar to wound healing, which is initiated by breaks in Bowman's layer, would largely contribute to the differences seen in keratoconus corneas.

Expression of type IV collagen alpha1(IV)-alpha6(IV) polypeptides in normal and developing human kidney and in renal cell carcinomas and oncocytomas

Type IV collagen trimer is a major component of basement membranes (BMs). It is composed of polypeptides named alpha1(IV)-alpha6(IV) chains. Chains alpha1,2(IV) are widely expressed in BMs while alpha3(IV)-alpha6(IV) are more restricted in human tissues. We have now studied by immunohistochemical means the distribution of collagen IV chains in fetal and adult human kidney, in oncocytomas, in renal cell carcinomas (RCCs) and their metastases and in experimental xenografts of human tumors. alpha1,2(IV) chains were found in all BMs of fetal and adult kidney as well as of renal tumors, while alpha3(IV)-alpha6(IV) chains were found in BMs of distal segments of developing and mature tubules. alpha3(IV)-alpha5(IV) chains were seen also in BMs of developing fetal glomeruli after the capillary loop stage. Most of the RCCs and their metastases showed occasional expression of alpha3(IV)-alpha6(IV) with papillary variants showing only expression of alpha5(IV) chain. There was a distinct expression of alpha3(IV)-alpha5(IV) chains in BMs of 3 oncocytomas. In 2 of them a variable expression of the alpha6(IV) chain was seen. In 3 of 4 xenografts, immunoreactivity for human-specific monoclonal antibody (MAb) for alpha1,2(IV) was seen in the BM-like structures. No alpha3-alpha6(IV) was seen in any of the xenografts, while polyclonal antiserum for type IV collagen presented immunoreactivity in BMs of all xenografts. Our results show that oncocytomas and most of the RCCs express scarce variants of type IV collagen containing alpha3(IV)-alpha6(IV) chains. In experimental xenograft tumors, both implanted RCC cells and host stromal cells have a capacity to produce type IV collagen.

The laminin alpha chains: expression, developmental transitions, and chromosomal locations of alpha1-5, identification of heterotrimeric laminins 8-11, and cloning of a novel alpha3 isoform

Laminin trimers composed of alpha, beta, and gamma chains are major components of basal laminae (BLs) throughout the body. To date, three alpha chains (alpha1-3) have been shown to assemble into at least seven heterotrimers (called laminins 1-7). Genes encoding two additional alpha chains (alpha4 and alpha5) have been cloned, but little is known about their expression, and their protein products have not been identified. Here we generated antisera to recombinant alpha4 and alpha5 and used them to identify authentic proteins in tissue extracts. Immunoprecipitation and immunoblotting showed that alpha4 and alpha5 assemble into four novel laminin heterotrimers (laminins 8-11: alpha4beta1gamma1, alpha4beta2gamma1, alpha5beta1gamma1, and alpha5beta2gamma1, respectively). Using a panel of nucleotide and antibody probes, we surveyed the expression of alpha1-5 in murine tissues. All five chains were expressed in both embryos and adults, but each was distributed in a distinct pattern at both RNA and protein levels. Overall, alpha4 and alpha5 exhibited the broadest patterns of expression, while expression of alpha1 was the most restricted. Immunohistochemical analysis of kidney, lung, and heart showed that the alpha chains were confined to extracellular matrix and, with few exceptions, to BLs. All developing and adult BLs examined contained at least one alpha chain, all alpha chains were present in multiple BLs, and some BLs contained two or three alpha chains. Detailed analysis of developing kidney revealed that some individual BLs, including those of the tubule and glomerulus, changed in laminin chain composition as they matured, expressing up to three different alpha chains and two different beta chains in an elaborate and dynamic progression. Interspecific backcross mapping of the five alpha chain genes revealed that they are distributed on four mouse chromosomes. Finally, we identified a novel full-length alpha3 isoform encoded by the Lama3 gene, which was previously believed to encode only truncated chains. Together, these results reveal remarkable diversity in BL composition and complexity in BL development.

Distribution of the ten known laminin chains in the pathways and targets of developing sensory axons

Laminins are heterotrimers of alpha, beta, and gamma chains. At present, five alpha, three beta, and two gamma chains have been described. The best characterized laminin (laminin 1 = alpha 1, beta 1, gamma 1) promotes neurite outgrowth from virtually all classes of developing neurons, implying that laminins may serve as axon guidance molecules in vivo. Moreover, different laminin trimers exert distinct effects on subsets of laminin-1-responsive cells, suggesting that isoform diversity may underlie some axonal choices in vivo. As a first step toward evaluating these hypotheses, we have documented the expression patterns of all 10-known laminin chains in the peripheral nervous system and spinal cord of the murine embryo. The alpha 2, alpha 4, beta 1, and gamma 1 chains are expressed in peripheral axonal pathways by embryonic day (E) 11.5, when sensory and motor axonal outgrowth is underway. Thus, laminins (but not laminin 1) may promote peripheral axonal outgrowth. By E 13.5, laminin chains are differentially expressed in the limb-bud, with prominent expression of alpha 2 and alpha 4 in muscle and of alpha 3 and alpha 5 in skin. This pattern raises the possibility that laminin isoform diversity contributes to the ability of cutaneous and muscle sensory axons to distinguish their targets. Later in development, some chains (e.g., alpha 2, alpha 4, and beta 1) are downregulated in peripheral nerve while others (e.g., gamma 1), continue to be expressed by Schwann cells into adulthood. In contrast to peripheral nerves and ganglia, laminin chains are expressed at low levels, if at all, in the developing spinal cord gray matter.

Molecular and functional defects in kidneys of mice lacking collagen alpha 3(IV): implications for Alport syndrome

Collagen IV is a major structural component of all basal laminae (BLs). Six collagen IV alpha chains are present in mammals; alpha 1 and alpha 2(IV) are broadly expressed in embryos and adults, whereas alpha 3-6(IV) are restricted to a defined subset of BLs. In the glomerular BL of the kidney, the alpha 1 and alpha 2(IV) chains are replaced by the alpha 3-5(IV) chains as development proceeds. In humans, mutation of the collagen alpha 3, alpha 4, or alpha 5(IV) chain genes results in a delayed onset renal disease called Alport syndrome. We show here that mice lacking collagen alpha 3(IV) display a renal phenotype strikingly similar to Alport syndrome: decreased glomerular filtration (leading to uremia), compromised glomerular integrity (leading to proteinuria), structural changes in glomerular BL, and glomerulonephritis. Interestingly, numerous changes in the molecular composition of glomerular BL precede the onset of renal dysfunction; these include loss of collagens alpha 4 and alpha 5(IV), retention of collagen alpha 1/2(IV), appearance of fibronectin and collagen VI, and increased levels of perlecan. We suggest that these alterations contribute, along with loss of collagen IV isoforms per se, to renal pathology.

The functions of laminins: lessons from in vivo studies

This series of three short reviews is an attempt to summarize our current knowledge of the in vivo tests of hypotheses of laminin functions. The structures of the laminins have been thoroughly reviewed recently (P. Ekblom and R. Timpl, in press), and I will not attempt to repeat this information here. Instead, I will focus on the recent evidence gathered from gene knock out experiments in mice and from naturally occurring human and mouse gene mutations. The most obvious lesson from the above studies--other than demonstrating the importance of laminins in general--is that the structural diversity of the laminin family members makes highly specialized functions possible. While all laminins may share many functional properties, the individual chains are involved in interactions which cannot be substituted for by other laminins or by other basement membrane components. While this concept is not new, it is very satisfying to see its validity so dramatically confirmed. It is therefore predictable that additional gene ablation experiments using other known and yet undescribed laminin genes will be equally interesting and informative. To me, one of the most striking lessons from these studies is how strongly the induced mouse mutations mimic human disease. With all the concerns with genetic background differences and species specific effects, manipulation of the laminin genes appears to be a particularly good first approach to identifying the causes of human disease. There is an abundant literature accumulated from biochemical and, more recently, molecular structural analyses, and from in vitro systems, suggesting a role of laminins contributing directly to the stability of the basement membrane. There is an equally vast literature supporting an indirect role in mediating cellular behavior, through interactions with various receptors. It is interesting that the in vivo studies summarized above support both activities. In the case of laminin 5 mutations, the phenotypic consequence appears to be due primarily to the loss of an important structural link between the epithelial cytokeratins and the dermal anchoring fibrils. The ultrastructure of the epithelium appears normal, as does the architecture of the papillary dermis. Only the anchoring complex itself is aberrant. The absence of laminin 5 appears not to compromise the development or viability of the epidermis. The basement membrane appears normal-other than the anchoring complex itself. The pathology observed in the newborn is believed to be due to the frictional trauma of birth, with the expectation that the function of the fetal skin is normal in utero. The Herlitz epidermolysis bullosa phenotype is obvious immediately at birth, and it does not progress postnatally beyond the extent to which the affected individual experiences additional frictional trauma or secondary consequences such as infection or fluid loss. Since laminin 5 is only one of a series of structural links within the anchoring complex, one would predict that a loss of any of these links would result in the same phenotype. Current evidence supports this view, as the absence of integrin alpha 6 beta 4 (Vidal et al., 1995; Dowling et al., 1996; Georges-Labouesse et al., 1996; van der Neut et al., 1996) or of collagen VII (A. M. Christiano and J. Uitto, in press) also results in dramatic neonatal dermal-epidermal fragility. The differences in phenotype, such as the pyloric atresia in the case of loss of integrin alpha 6 beta 4, are presumably due to additional functions of the integrin in other tissues or in other developmental processes. Therefore, the laminin 5 mutations may be unique, in that the in vivo studies suggest that the primary role of the molecule is in the elaboration and stability of the anchoring complex, but not in the basement membrane itself. Of course, since the in vivo phenotype reflects only losses that cannot be compensated, this interpretation may be much too narrow. (ABSTRACT TRUNCATED)

Molecular cloning of a novel laminin chain, alpha 5, and widespread expression in adult mouse tissues

We have identified a fifth member of the alpha subfamily of vertebrate laminin chains. Sequence analysis revealed a close relationship of alpha 5 to the only known Drosophila alpha chain, suggesting that the ancestral alpha gene was more similar to alpha 5 than to alpha 1-4. Analysis of RNA expression showed that alpha 5 is widely expressed in adult tissues, with highest levels in lung, heart, and kidney. Our results suggest that alpha 5 may be a major laminin chain of adult basal laminae.

Disruption of the mouse MRF4 gene identifies multiple waves of myogenesis in the myotome

MRF4 (herculin/Myf-6) is one of the four member MyoD family of transcription factors identified by their ability to enforce skeletal muscle differentiation upon a wide variety of nonmuscle cell types. In this study the mouse germline MRF4 gene was disrupted by targeted recombination. Animals homozygous for the MRF4bh1 allele, a deletion of the functionally essential bHLH domain, displayed defective axial myogenesis and rib pattern formation, and they died at birth. Differences in somitogenesis between homozygous MRF4bh1 embryos and their wild-type littermates provided evidence for three distinct myogenic regulatory programs (My1-My3) in the somite, which correlate temporally and spatially with three waves of cellular recruitment to the expanding myotome. The first program (My1), marked initially by Myf-5 expression and followed by myogenin, began on schedule in the MRF4bh1/bh1 embryos at day 8 post coitum (E8). A second program (My2) was highly deficient in homozygous mutant MRF4 embryos, and normal expansion of the myotome failed. Moreover, expression of downstream muscle-specific genes, including FGF-6, which is a candidate regulator of inductive interactions, did not occur normally. The onset of MyoD expression around E10.5 in wild-type embryos marks a third myotomal program (My3), the execution of which was somewhat delayed in MRF4 mutant embryos but ultimately led to extensive myogenesis in the trunk. By E15 it appeared to have largely compensated for the defective My2 program in MRF4 mutants. Homozygous MRF4bh1 animals also showed improper rib pattern formation perhaps due to the absence of signals from cells expressing the My2 program. Finally, a later and relatively mild phenotype was detected in intercostal muscles of newborn animals.

The renal glomerulus of mice lacking s-laminin/laminin beta 2: nephrosis despite molecular compensation by laminin beta 1

S-laminin/laminin beta 2, a homologue of the widely distributed laminin B1/beta 1 chain, is a major component of adult renal glomerular basement membrane (GBM). Immature GBM bears beta 1, which is replaced by beta 2 as development proceeds. In mutant mice that lack beta 2, the GBM remains rich in beta 1, suggesting that a feedback mechanism normally regulates GBM maturation. The beta 2-deficient GBM is structurally intact and contains normal complements of several collagenous and noncollagenous glycoproteins. However, mutant mice develop massive proteinuria due to failure of the glomerular filtration barrier. These results support the idea that laminin beta chains are functionally distinct although they assemble to form similar structures. Laminin beta 2-deficient mice may provide a model for human congenital or idiopathic nephrotic syndromes.

Collagen IV alpha 3, alpha 4, and alpha 5 chains in rodent basal laminae: sequence, distribution, association with laminins, and developmental switches

Collagen IV is a major component of vertebrate basal laminae (BLs). Studies in humans have revealed a family of genes encoding alpha 1-alpha 6 collagen IV chains and implicated alpha 3-alpha 6 in disease processes (Goodpasture and Alport syndromes and diffuse leiomyomatosis). To extend studies of these components to an experimentally accessible animal, we cloned cDNAs encoding partial collagen alpha 3, alpha 4, and alpha 5(IV) chains from the mouse. Ribonuclease protection assays showed that all three genes were expressed at highest levels in kidney and lung; alpha 5(IV) was also expressed at high levels in heart. We then made antibodies specific for each collagen IV chain. Immunohistochemical studies of several tissues revealed many combinations of collagen IV chains; however, alpha 3 and alpha 4 (IV) were always coexpressed, and only appeared in BLs that were alpha 5(IV) positive. The alpha 3-alpha 5(IV) chains were frequently but not exclusively associated with the S (beta 2) chain of laminin, as were the alpha 1, 2 (IV) collagen chains with laminin B1 (beta 1). An analysis of developing rat kidney BLs showed that newly formed (S-shaped) nephrons harbored collagen alpha 1 and alpha 2(IV) and laminin B1; maturing (capillary loop stage) BLs contained collagen alpha 1-alpha 5(IV) and laminin B1 and S-laminin; and mature glomerular BLs contained mainly collagen alpha 3-alpha 5(IV) and S-laminin. Thus, collagen alpha 1 and alpha 2(IV) and laminin B1 appear to be fetal components of the glomerular BL, and there is a developmental switch to collagen alpha 3-alpha 5(IV) and S-laminin expression.

Isolated sequences from the linked Myf-5 and MRF4 genes drive distinct patterns of muscle-specific expression in transgenic mice

In developing mouse embryos, MyoD family regulatory genes are expressed specifically in muscle precursors and mature myofibers. This pattern, taken together with the well-established ability of MyoD family members to convert a variety of cell types to skeletal muscle, suggests a significant role for these genes in regulating skeletal myogenesis. The possibility that expression of these genes may be causally associated with segregation of the myogenic lineage from other mesodermal derivatives, or with the subsequent maintenance of muscle phenotypes at later times, raises the issue of how MyoD family genes are themselves regulated during development. In this work, we have initiated studies to identify DNA sequences that govern Myf-5 and MRF4 (herculin, myf-6) transcription. Myf-5 is the first of the MyoD family to be expressed in the developing mouse embryo, while MRF4 is the most abundantly expressed myogenic factor in postnatal animals. In spite of their strikingly divergent patterns of expression, Myf-5 and MRF4 are tightly linked in the mouse genome; their translational start codons are only 8.5 kilobases apart. Here, the 5′ flanking regions of the mouse Myf-5 and MRF4 genes were separately linked to a bacterial beta-galactosidase (lacZ) gene, and these constructs were each used to produce several lines of transgenic mice. Transgene expression was monitored by X-gal staining of whole embryos and by in situ hybridization of embryo sections. For the Myf-5/lacZ lines, the most intense transgene expression was in the visceral arches and their craniofacial muscle derivatives, beginning at day 8.75 post coitum (p.c.). This correlates with endogenous Myf-5 expression in visceral arches.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle phenotypes initiated by ectopic MyoD in transgenic mouse heart

Forced expression of the myogenic regulatory gene MyoD in many types of cultured cells initiates their conversion into skeletal muscle. It is not known, however, if MyoD expression serves to activate all or part of the skeletal muscle program in vivo during animal development, nor is it known how limiting the influences of cellular environment may be on the regulatory effects of MyoD. To begin to address these issues, we have produced transgenic mice which express MyoD in developing heart, where neither MyoD nor its three close relatives—myogenin, Myf-5, and MRF4/herculin/Myf-6—are normally expressed. The resulting gross phenotype in offspring from multiple, independent transgenic founders includes abnormal heart morphology and ultimately leads to death. At the molecular level, affected hearts exhibit activation of skeletal muscle-specific regulatory as well as structural genes. We conclude that MyoD is able to initiate the program that leads to skeletal muscle differentiation during mouse development, even in the presence of the ongoing cardiac differentiation program. Thus, targeted misexpression of this tissue-specific regulator during mammalian embryogenesis can activate, either directly or indirectly, a diverse set of genes normally restricted to a different cell lineage and a different cellular environment.

c-myc inhibition of MyoD and myogenin-initiated myogenic differentiation

In vertebrate development, a prominent feature of several cell lineages is the coupling of cell cycle regulation with terminal differentiation. We have investigated the basis of this relationship in the skeletal muscle lineage by studying the effects of the proliferation-associated regulator, c-myc, on the differentiation of MyoD-initiated myoblasts. Transient cotransfection assays in NIH 3T3 cells using MyoD and c-myc expression vectors demonstrated c-myc suppression of MyoD-initiated differentiation. A stable cell system was also developed in which MyoD expression was constitutive, while myc levels could be elevated conditionally. Induction of this conditional c-myc suppressed myogenesis effectively, even in the presence of MyoD. c-myc suppression also prevented up-regulation of a relative of MyoD, myogenin, which is normally expressed at the onset of differentiation in all muscle cell lines examined and may be essential for differentiation. Additional experiments tested whether failure to differentiate in the presence of myc could be overcome by providing myogenin ectopically. Cotransfection of c-myc with myogenin, MyoD, or a mixture of myogenin and MyoD showed that neither myogenin alone nor myogenin plus MyoD together could bypass the c-myc block. The effects of c-myc were further dissected by showing that c-myc can inhibit differentiation independently of Id, a negative regulator of muscle differentiation. These results lead us to propose that c-myc and Id constitute independent negative regulators of muscle differentiation, while myogenin and any of the other three related myogenic factors (MyoD, Myf-5, and MRF4/herculin/Myf-6) act as positive regulators.

Herculin, a fourth member of the MyoD family of myogenic regulatory genes

We have identified and cloned herculin, a fourth mouse muscle regulatory gene. Comparison of its DNA and deduced amino acid sequences with those of the three known myogenic genes (MyoD, myogenin, and Myf-5) reveals scattered short spans with similarity to one or more of these genes and a long span with strong similarity to all three. This long span includes a sequence motif that is also present in proteins of the myc, achaete-scute, and immunoglobulin enhancer-binding families. The herculin gene is physically linked to the Myf-5 gene on the chromosome; only 8.5 kilobases separate their translational start sites. A putative 27-kDa protein is encoded by three exons contained within a 1.7-kilobase fragment of the herculin gene. When expressed under the control of the simian virus 40 early promoter, transfected herculin renders murine NIH 3T3 and C3H/10T1/2 fibroblasts myogenic. In doing so, it also activates expression of myogenin, MyoD, and endogenous herculin in NIH 3T3 recipients. In adult mice, herculin is expressed in skeletal muscle but is absent from smooth muscle, cardiac muscle, and all nonmuscle tissues assayed. Direct comparison of the four known myogenic regulators in adult muscle showed that herculin is expressed at a significantly higher level than is any of the others. This quantitative dominance suggests an important role in the establishment or maintenance of adult skeletal muscle.

Cell type-specific expression of JC virus T antigen in primary and established cell lines from transgenic mice

The highly restricted host range of JC virus (JCV) has made it difficult to study the biology of this common human papovavirus. To increase our understanding of the tissue specificity of this virus, we have examined the expression of the T antigen (T-Ag) in primary and established cell lines from various tissues of transgenic mice containing the JCV early region. In contrast to earlier results from a simian virus 40-containing transgenic mouse, there was no T-Ag expression in mesenchymal fibroblasts derived from two lines of JCV-transgenic mice. Instead, we isolated T-Ag-positive (T-Ag+) cells that had characteristics consistent with a neural crest origin. Furthermore, primary brain cultures contained many T-Ag+ astrocytes, but no expression was detected in macrophages, epithelial cells, neuronal cells nor, surprisingly, in oligodendrocytes. Continued passage of these cultures resulted in vigorously growing glial fibrillary acidic protein-positive, T-Ag+ astrocytes. Thus, the strict tissue specificity of JCV expression was maintained, despite the fact that the viral genome pre-existed in every tissue of these transgenic mice and these constraints on expression were preserved even when cells were explanted in vitro.