Steady-state levels of mRNAs coding for the type IV collagen and laminin polypeptide chains of basement membranes exhibit marked tissue-specific stoichiometric variations in the rat

PI3K block restores age-dependent neurovascular coupling defects associated with cerebral small vessel disease

Neurovascular coupling (NVC), a vital physiological process that rapidly and precisely directs localized blood flow to the most active regions of the brain, is accomplished in part by the vast network of cerebral capillaries acting as a sensory web capable of detecting increases in neuronal activity and orchestrating the dilation of upstream parenchymal arterioles. Here, we report a Col4a1 mutant mouse model of cerebral small vessel disease (cSVD) with age-dependent defects in capillary-to-arteriole dilation, functional hyperemia in the brain, and memory. The fundamental defect in aged mutant animals was the depletion of the minor membrane phospholipid phosphatidylinositol 4,5 bisphosphate (PIP 2 ) in brain capillary endothelial cells, leading to the loss of inwardly rectifier K + (Kir2.1) channel activity. Blocking phosphatidylinositol-3-kinase (PI3K), an enzyme that diminishes the bioavailability of PIP 2 by converting it to phosphatidylinositol (3,4,5)-trisphosphate (PIP 3 ), restored Kir2.1 channel activity, capillary-to-arteriole dilation, and functional hyperemia. In longitudinal studies, chronic PI3K inhibition also improved the memory function of aged Col4a1 mutant mice. Our data suggest that PI3K inhibition is a viable therapeutic strategy for treating defective NVC and cognitive impairment associated with cSVD.

One-sentence summary

PI3K inhibition rescues neurovascular coupling defects in cerebral small vessel disease.

Gene cloning to clinical trials-the trials and tribulations of a life with collagen

This review, based on the BSMB Fell-Muir Lecture I presented in July 2018 at the Matrix Biology Europe Conference in Manchester, gives a personal perspective of my own laboratory's contributions to research into type X collagen, metaphyseal chondrodysplasia type Schmid and potential treatments for this disorder that are currently entering clinical trial. I have tried to set the advances made in the context of the scientific technologies available at the time and how these have changed over the more than three decades of this research.

Increased Sp1-dependent transactivation of the LAMgamma 1 promoter in hepatic stellate cells co-cultured with HepG2 cells overexpressing cytochrome P450 2E1

Laminin is a basement-membrane protein that increases in liver fibrosis. To study the role of oxidative stress on laminin expression, hepatic stellate cells (HSC) were co-cultured with HepG2 cells that do or do not express (E47 or C34 cells, respectively) CYP2E1, a potent generator of oxygen radicals. Co-incubation of HSC with E47 cells increased laminin beta1 and gamma1 proteins compared with co-incubation with C34 cells; this increase was prevented by antioxidants and CYP2E1 inhibitors. Similar results were observed in co-culture with primary hepatocytes from saline- or pyrazole-treated (with high levels of CYP2E1) rats. Laminin alpha1 chain was not detectable in the HSC in any of the systems; however, laminin alpha2 chain increased in HSC co-cultured with E47 cells. Synthesis but not turnover of laminin beta1 and gamma1 proteins was increased in HSC in the E47 co-culture. Laminin beta1 and gamma1 mRNAs were up-regulated in HSC in the E47 co-culture because of transcriptional activation of both genes. Transfection experiments in HSC with reporter constructs driven by the laminin gamma1 promoter showed maximal responsiveness with the -230/+106 and the -1400/+106 constructs in the E47 system. Gel-shift assays demonstrated an increase in Sp1 binding to the laminin gamma1 promoter in HSC when co-incubated with E47 cells, which was blocked by an anti-Sp1 antibody. Co-transfection of a Sp1 expression vector further increased the responsiveness of the -330LAMgamma1-CAT reporter vector in HSC in the HSC/E47 system. These results show that diffusable CYP2E1-derived oxidative-stress mediators induce synthesis of laminins by a transcriptional mechanism in HSC. Such interactions between hepatocytes and HSC may be important during liver fibrosis.

Characterization of hydra type IV collagen. Type IV collagen is essential for head regeneration and its expression is up-regulated upon exposure to glucose

Hydra vulgaris mesoglea is a primitive basement membrane that also exhibits some features of an interstitial matrix. We have characterized cDNAs that encode the full-length hydra alpha1(IV) chain. The 5169-base pair transcript encodes a protein of 1723 amino acids, including an interrupted 1455-residue collagenous domain and a 228-residue C-terminal noncollagenous domain. N-terminal sequence analyses of collagen IV peptides suggest the molecule is homotrimeric. Denatured hydra type IV collagen protein occurs as dimers and higher order aggregates held together by nonreducible cross-links. Hydra collagen IV exhibits no functional evidence for the presence of a 7 S domain. Type IV collagen is expressed by the ectoderm along the entire longitudinal axis of the animal but is most intense at the base of the tentacles at the site of battery cell transdifferentiation. Antisense studies show that inhibition of collagen IV translation causes a blockage in head regeneration, indicating its importance in normal hydra development. Exposure of adult hydra to 15 mm glucose resulted in up-regulation of type IV collagen mRNA levels within 48 h and significant thickening of the mesoglea within 14 days, suggesting that basement membrane thickening seen in diabetes may be, in evolutionary terms, an ancient glucose-mediated response.

Low protein diet blunts the rise in glomerular gene expression in focal glomerulosclerosis

The present study was designed to assess whether expression of mRNA for extracellular matrix (ECM) components, metalloproteinases (MMP) and tissue inhibitor of metalloproteinases (TIMP) in glomeruli is affected by a low protein diet during the course of focal glomerulosclerosis (FGS). Puromycin aminonucleoside (PAN) was injected intraperitoneally in rats and the right kidney was removed on day 22. Nephrotic rats received successive intraperitoneal injections of PAN on days 27, 34, and 41. Control rats were subjected to a nephrectomy or a sham operation on day 22. Animals were divided into six groups. In group 1, the PAN-injected rats were fed a standard diet containing 22% protein. In group 2, the PAN-injected rats were fed a low protein diet containing 6% protein, starting on the same day as the first PAN injection. In group 3, the nephrectomized rats without PAN were fed a standard diet. In group 4, the nephrectomized rats without PAN were fed a low protein diet for the same period. In group 5, the sham operated rats were fed a standard diet. In group 6, the sham operated rats were fed a low protein diet for the same period. Rats were sacrificed on days 0, 60 or 80 after the initial PAN or saline injection. The percentage of sclerotic glomeruli in group 1 rats increased markedly with time, reaching 77% on day 80. The mRNA levels encoding for alpha 1(I), alpha 1(III), alpha 1(IV) collagen chains, laminin B1 and B2 chains, heparan sulfate proteoglycan (HSPG), MMP-2, TIMP-1 and TIMP-2 increased significantly as glomerulosclerosis progressed, whereas MMP-1 and MMP-3 mRNA levels were unchanged, and no MMP-9 mRNA was detected throughout the experiments. In group 2, the low protein diet reduced the prevalence of glomerulosclerosis and attenuated the increased mRNA expression for ECM components, MMP-2, TIMP-1 and TIMP-2 in FGS glomeruli. In groups 3 through 6, mRNA levels for ECM components decreased with age, whereas those for MMPs and TIMPs changed little throughout the experiments. Immunofluorescence studies revealed the accumulation of types I, III and IV collagens, laminin, and HSPG in the sclerotic area and low protein diet attenuated the accumulation of these proteins. These data suggest that glomerulosclerosis may result from an imbalance among ECM components, MMPs and TIMPs and that a low protein diet attenuates the otherwise increased levels of mRNA for ECM components, MMP-2, TIMP-1 and TIMP-2 in glomerulosclerosis.

Analysis of the Drosophila gene for the laminin B1 chain

We have isolated and sequenced a Drosophila genomic DNA that encodes the entire coding region of the laminin B1 chain. The genomic DNA sequenced spans 11,787 bp, including a 1.1-kb 5'-flanking region, 5 exons, 4 introns, and a 1.4-kb 3'-flanking region. The open reading frame is within the two largest exons, the exons 3 and 4, while the first two and the last exons are much smaller and are untranslated. The structure of the Drosophila laminin B1 gene is similar to the Drosophila laminin B2 gene. Their exon-intron lengths and Eco RI, Pst I restriction maps are quite conserved. Both of their open reading frames are very compact, and their first introns are much larger than all of the rest of the introns. These results are consistent with the suggestion that the B1 and B2 genes could be derived from an ancestral gene. The similarity of the proximal 5'-flanking regions of the Drosophila B1 and B2 genes is 46.6%. Also, similar sequences of transcriptional regulatory elements, even though not site conserved, are found in both proximal 5'-flanking regions of the B1 and B2 genes. When transfected into Drosophila SL-2 cells, pCAT plasmid containing 1,048 bp of 5'-flanking region shows a strong expression of chloramphenicol acetyltransferase (CAT) activity. The deletion clones that contain sequences between nucleotides -462 to +150, and -282 to +150 all show strong CAT activity. These results suggest that this 5'-flanking promoter region may contain DNA sequences that can promote the expression of the laminin B1 gene.

Renal basement membranes by ultrahigh resolution scanning electron microscopy

Three-dimensional ultrastructures of basement membranes of the rat kidney were investigated with an ultrahigh resolution scanning electron microscope (HSEM) equipped with a resolving power of 0.5 nm. All cellular components were extracted from renal cortical tissues by sequential-detergent treatment. Four types of acellular basement membranes were observed after tannin-osmium conductive staining: the glomerular basement membrane (GBM) associated with the mesangial matrix, the tubular basement membrane (TBM), the Bowman's capsule basement membrane (BCBM), and the peritubular capillary basement membrane (PTCBM). We could demonstrate the polygonal meshwork structures composed of strands in the respective basement membranes. The strands averaged 6 to 7 nm wide, whereas the pore sizes within the meshworks were variable and differed according to the basement membrane type. Moreover, we confirmed the presence of the heterogeneity of the GBM suggested by several approaches. Present data support the proposition that a polygonal meshwork structure may represent the basic structure of basement membrane. Some of the observed architectural dissimilarities in basement membrane types may reflect their different functional properties, which in turn may reflect the heterogeneous distribution of major basement membrane components as demonstrated by immunohistochemical and biochemical studies.

The laminins: a family of basement membrane glycoproteins important in cell differentiation and tumor metastases

Laminins are a family of basement membrane-derived glycoproteins that are very biologically active with a number of diverse cell types. The response of the cells is dependent on the cell type and various cell-specific intracellular events are activated. Multiple active sites on laminin and cellular receptors have been described. Both laminin and the synthetic peptides that define the active sites may have important clinical uses. For example, the neurite-promoting peptides may be useful in vivo in regeneration studies because of their potent activity with neural cells and their lack of antigenicity. Also, peptides, such as YIGSR, that inhibit angiogenesis are potentially useful for treating the vascularization of the eye that occurs in conditions such as diabetes mellitus. Likewise, the angiogenic peptide SIKVAV, because of its role in endothelial cell block vessel formation, may be useful for treating ischemia. The recent progress that has been made in characterizing basic mechanisms of action of laminin has laid the groundwork for more direct studies of its clinical relevance.

Laminin variants: why, where and when?

Laminin is a member of a family of proteins that are composed of three subunits, one heavy chain and two light chains. Five subunits in the laminin family have been cloned and sequenced so far. These include two heavy chains, the laminin A chain and the merosin M chain, and three light chains, B1, B2, and S. These five subunits can form four different laminin variants: A-B1-B2, A-S-B2, M-B1-B2, and M-S-B2, all having the B2 chain in common. Major basement membranes in tissues contain at least one of the four laminin variants. For example, the adult muscle and nerve basement membranes contain M-B1-B2, smooth muscle contains A-B1-B2, the myotendinous junction and the trophoblast basement membrane in the placenta contain M-S-B2, and blood vessels contain A-B1-B2 and/or A-S-B2. In the brain, the merosin M chain is present in association with neuronal fibers. The four members of the laminin family interact with cells in a similar manner. Thus, they promote outgrowth of neurites from neuronal cells and promote attachment and spreading of non-neuronal cells. The interaction of cells with laminins is mediated largely by integrin type receptors, including integrins alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, and alpha 6 beta 1. The expression of the different laminin-like proteins is developmentally regulated. The laminin A chain is the first heavy chain expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of human collagen type IV genes is regulated by transcriptional and post-transcriptional mechanisms

The molecules of the basement membrane specific collagen type IV are heterotrimers consisting of two alpha 1(IV) and one alpha 2(IV) polypeptide chains. Comparison of the ratios of transcription by nuclear run-on analysis and mRNAs by RNAse protection assay indicates the involvement of transcriptional as well as post-transcriptional events in the control of overall collagen type IV expression. The relative ratios of transcription of the respective genes COL4A1 and COL4A2 remained near 2:1 in most cells, whereas the ratio of mRNA steady-state levels alpha 1(IV)/alpha 2(IV) varied from 0.3:1 to 1:1 and did not parallel the subunit structure of the protein. Nevertheless, secreted protein shows a 2:1 ratio of the subunit polypeptides. This indicates that post-translational processes during chain selection, aggregation and secretion finally determine the amount of secreted protein.

Basal lamina formation by epithelial cell lines correlates with laminin A chain synthesis and secretion

Laminin is a major component of the basal lamina upon which all epithelial cells reside in vivo. The synthesis of basal lamina components and their subsequent assembly into a morphologically distinct basal lamina is a differentiated function of epithelial cells in vivo. Ultrastructural studies in our laboratory show that some epithelial cell lines (P-MDCK) form a basal lamina when cultured on membrane-permeable substrate (Millipore Millicells or type I collagen gels). Under the same conditions other epithelial cell lines (MDCK-AA7, M-mTAL-1P, and T84) do not form a basal lamina. When metabolically labeled with [35S]methionine, laminin A and B chains can be immunoprecipitated from the culture medium and culture lysates of P-MDCK cells. In contrast, labeled laminin chains cannot be immunoprecipitated from the culture medium of MDCK-AA7, M-mTAL-1P, and T84 cells. Immunoprecipitates of MDCK-AA7, M-mTAL-1P, and T84 culture lysates demonstrate the presence of one or both B chains but not A chains. These results suggest that laminin B chain synthesis is constitutive in MDCK-AA7, M-mTAL-1P, and T84 cells and that B chains, in the absence of A chains, are not secreted. Furthermore, laminin secretion and basal lamina formation are not required to maintain structural and functional polarity in these cell lines.

Hepatic fibrosis in rats produced by carbon tetrachloride and dimethylnitrosamine: observations suggesting immunoassays of serum for the 7S fragment of type IV collagen are a more sensitive index of liver damage than immunoassays for the NH2-terminal propeptide of type III procollagen

Liver fibrosis was induced in rats both with carbon tetrachloride and dimethylnitrosamine. Assays were performed on steady-state levels of messenger RNAs in the liver for several collagens and basement membrane components. The results indicated marked increases in the steady-state levels of messenger RNA for type I collagen, type III collagen, type IV collagen and the B2 component of laminin. In the same animals, immunoassays were performed for serum levels of the N-terminal propeptide of type III procollagen and the 7S fragment of type IV collagen. The results demonstrated an increase in the serum levels of 7S fragment that occurred early and closely paralleled the increase in the steady-state levels of messenger RNA for the alpha 1(IV) chain of type IV collagen. In contrast, no significant increase was seen in the serum levels of the N-propeptide of type III procollagen. The results suggest that immunoassays for 7S fragment of type IV collagen in serum are a more sensitive index for liver cell damage and fibrosis than assays for the N-propeptide of type III procollagen. The results suggest that greater attention should be paid to assays of 7S fragments in assessing hepatic fibrosis in man.

Evaluation of mRNA steady-state and protein levels for basement membrane proteins in cultured murine cells

We have determined the mRNA steady-state levels for the six constituent polypeptide chains of the basement membrane proteins collagen IV, laminin and nidogen in murine cell lines derived from a teratocarcinoma, and in some other cell lines of different origin in stationary cultures and during different growth phases. The mRNA and protein levels change in response to growth phase. The amounts of the mRNAs for the single chains do not agree with the ratios needed for the different peptide chains of collagen IV and laminin. While the mRNA and protein levels for laminin are in a similar range for the teratocarcinoma-derived cell lines, the mRNA and protein levels vary by at least a factor of 10 for collagen IV. These results point to complex posttranscriptional regulatory mechanisms for the biosynthesis of basement membrane proteins.

Growth and function of primary rabbit kidney proximal tubule cells in glucose-free serum-free medium

The properties of primary rabbit kidney proximal tubule cells in glucose-free serum-free medium have been examined. Primary rabbit kidney proximal tubule cells were observed to grow at the same rate, 1.0 doublings/day, both in glucose-free and in glucose-supplemented medium. Growth in glucose-free medium was dependent upon the presence of an additional nutritional supplement, such as glutamine, pyruvate, palmitate, lactate, or beta hydroxybutyrate. Lactate, pyruvate, and glutamate are utilized for renal gluconeogenesis in vivo. The growth of the primary rabbit kidney proximal tubule cells in glucose-free medium was also dependent upon the presence of the three growth supplements insulin, transferrin, and hydrocortisone. Insulin was growth stimulatory to the primary proximal tubule cells in glucose-free medium, although insulin causes a reduction in the phosphoenolpyruvate carboxykinase (PEPCK) activity in these cells. PEPCK is a key regulatory enzyme in the gluconeogenic pathway. In order to evaluate whether or not the primary cells have gluconeogenic capacity, their glucose content was determined. The cells contained 5 pmoles D-glucose/mg protein. However, no significant glucose was detected in the medium. Presumably, the primary cells were either utilizing or storing the glucose made by the gluconeogenic pathway. Consistent with this latter possibility, cellular glycogen levels were observed to increase with time in culture. The effect of glucose on the expression of the alpha I(IV) collagen and laminin B1 chain genes was examined. Northern analysis indicated that the level of alpha I(IV) collagen mRNA was significantly elevated in glucose containing, as compared with glucose deficient, medium. In contrast, laminin B1 chain mRNA levels were not significantly affected by the glucose content of the medium.

Basement membrane proteins: structure, assembly, and cellular interactions

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.

Effects of methylprednisolone on glomerular and medullary mRNA levels for extracellular matrices in puromycin aminonucleoside nephrosis

We examined the effects of methylprednisolone (MPSL) on type IV collagen, laminin and heparan sulfate proteoglycan (HSPG) mRNA levels in the renal glomeruli and medulla of puromycin aminonucleoside (PAN) nephrosis. mRNA levels encoding for type IV collagen and laminin increased markedly, whereas those for HSPG decreased significantly in glomeruli of PAN nephrosis. Administration of MPSL partially ameliorated the abnormal gene expression for basement membrane components. Furthermore, we showed that medullary mRNA levels for all these basement membrane components decreased with age in PAN nephrosis with or without MPSL treatment, suggesting that neither PAN nor MPSL has any effect on basement membrane component mRNA levels in the renal medulla. In contrast, mRNA levels for the interstitial collagens including alpha 1 (I) and alpha 1 (III) chains in glomeruli showed little change with or without MPSL treatment, whereas those in medulla increased significantly in PAN nephrosis when compared with the control. MPSL ameliorated the abnormal gene expression of alpha 1 (I) and alpha 1 (III) collagen in renal medulla. These results indicate that PAN affects both glomerular mRNA encoding for basement membrane components and medullary mRNA encoding for interstitial collagens, and that MPSL has marked effects on the amelioration of abnormal gene expression in both glomeruli and medulla of PAN nephrosis.

Differential expression of laminin chains in hepatic lipocytes

The lipocyte is an important source of laminin in the normal liver. We have investigated the expression of the 3 chains of laminin in isolated rat lipocytes. Both B1 and B2 chains, but not A, were found in medium from 5-day-old lipocyte primary cultures by immunoblotting and immunoprecipitation of 35S-labeled proteins after reducing SDS-polyacrylamide gel electrophoresis. An additional polypeptide of Mr = 380,000 was identified by immunoprecipitation. Under non-reducing conditions only one Mr = 900,000 band was revealed. High levels of B1 and B2 mRNAs were also demonstrated in 5-day-old cultured lipocytes while at the time of seeding, only B2 chain mRNAs were clearly detectable. A chain mRNA was constantly absent. These results suggest that lipocytes produce a variant form of laminin in primary culture and that the Mr = 380,000 polypeptide could be unrelated to the A chain of laminin.

High glucose concentration suppresses mesangial laminin B2 gene expression

To study the role of laminin in the rat mesangial cell attachment, cell adhesion to laminin-coated dishes was assayed by adding various amounts of a pentapeptide (YIGSR). YIGSR (20-100 micrograms/mL) inhibited the cell attachment to laminin-coated dishes. Mesangial laminin B2 messenger RNA (mRNA) levels were attenuated during exposure to 30 mM glucose or high osmolarity achieved by the addition of 25 mM choline chloride to 5 mM glucose (310 mOsm/kg H2O) for 3 hours. These results suggest that laminin, which may have a physiologic role in mesangial cell attachment, is produced in mesangial cells, and that the decrease of laminin B2 mRNA during hyperglycemia or exposure to high osmolarity may have some pathophysiologic significance in the development of diabetic nephropathy.

Cyclosporine induces elevated procollagen alpha 1 (I) mRNA levels in the rat renal cortex

Chronic cyclosporine nephrotoxicity is a poorly understood drug side-effect characterized by renal cortical interstitial scarring. To evaluate procollagen mRNA levels as an early factor in the development of this form of renal fibrosis, we measured renal procollagen alpha 1 (I), alpha 1 (III), alpha 1 (IV) and beta-actin mRNA levels in rats treated with cyclosporine (CsA) or the olive oil vehicle (OO) for one or four weeks. Renal morphology was similar without atrophy or fibrosis in one week CsA and OO and four week OO rats. Four week CsA rats had focal cortical interstitial fibrosis and tubular atrophy. Cortical procollagen alpha 1 (I) mRNA levels were increased in CsA versus OO rats at one week (P less than 0.02) and four weeks (P less than 0.02). One week medullary procollagen alpha 1 (I) and all other one week medullary, and one and four week cortical procollagen and beta-actin mRNA levels were no different in CsA versus OO rats. The early increase in renal cortical procollagen alpha 1 (I) mRNA levels precedes renal morphologic abnormalities, and may represent an important step in the pathogenesis of cyclosporine-induced renal cortical fibrosis.

Laminin and s-laminin are produced and released by astrocytes, Schwann cells, and schwannomas in culture

Components of the extracellular matrix (ECM) have been implicated in the regulation of neuronal migration, axonal growth, and synaptogenesis. We have examined cultures of glial cells, Schwann cells, and schwannomas for the expression of two components of the ECM, laminin and s-laminin, using immunohistochemical and Western blot techniques. Laminin is a potent promotor of neurite outgrowth in cultures of both central and peripheral neurons, and is present in all ECMs. In contrast, s-laminin (for synaptic laminin), a recently described homolog of laminin, is highly localized at the neuromuscular synaptic cleft (Sanes and Chiu, Cold Spring Harbor Symp. Quant. Biol. 1983;48:667-678; Chiu and Sanes, Dev. Biol. 1984;103:456-467) and shows selective adhesivity for motor neurons (Hunter et al. Cell 1989;59:905-913). While the distribution of these ECM components have been well documented in situ, the sources of these extracellular molecules are unclear. We report that astrocytes cultured in serum-free medium maintain an organized ECM that only bears laminin immunoreactivity; s-laminin appears to be sequestered intracellularly. However, both molecules are found in the astrocyte conditioned medium. Thus, under these growth conditions, astrocytes produce and release laminin and s-laminin, but only incorporate the former into an ECM. In contrast, neither molecule is present in comparable cultures of oligodendrocytes. Although no established ECM is seen in cultures of Schwann cells or schwannomas, laminin and s-laminin immunoreactivity are present within cells and in the conditioned media. These results indicate that certain populations of non-neuronal support cells and cell lines can produce and release both synaptic and extrasynaptic components of the ECM. The assembly of these different molecules into an organized basal lamina may require the presence of additional factors or interaction with neurons.

Effects of long-term diabetes and galactosaemia upon lens and retinal mRNA levels in the rat

The levels of mRNAs encoding the alpha 1 chain of collagen IV and the B1 chain of laminin were assayed in the lenses and retinas of long-term (28-week) diabetic and galactosaemic rats in order to gain some insight into the effects on basement membrane (BM) synthesis in these tissues. mRNAs coding for beta-actin, glucose transporter protein and the alpha 2 catalytic subunit of Na+,K(+)-ATPase were also assayed to determine whether any effects on BM-coding mRNA levels were specific. Long-term diabetes had no significant effect on the levels of alpha 1 (IV) collagen mRNA but caused a significant reduction in the laminin B1 message in the lens. In the same samples, the level of the glucose transporter protein mRNA was found to be elevated significantly in the diabetic tissue, whereas the mRNAsen coding beta-actin and alpha 2 Na+,K(+)-ATPase were unaffected in comparison with age-matched controls. Long-term galactosaemia resulted in significant increases in the levels of all mRNAs assayed when expressed per micrograms total RNA used for each analysis. However, this effect appeared to be due to a specific loss of ribosomal RNA from these severely cataractous lenses. When related to the beta-actin mRNA internal control, the levels of mRNA in the galactosaemic lenses were very similar to that found in the diabetics. Laminin B1 mRNA levels were decreased significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a specific mechanism of laminin assembly

The specificity of laminin chain assembly was investigated using fragments E8 and C8-9, derived from the long arm of the molecule, whose rod-like domain consists of the alpha-helical regions of the A, B1 and B2 chains. Urea-induced chain separation and unfolding were monitored by transverse urea/polyacrylamide gel electrophoresis (PAGE) and circular dichroism. Separation of the A and disulphide-linked B1-B2 chains occurred at 3.5-4.0 M urea and by 7.0 M urea all residual alpha-helicity was lost. Removal of urea by dialysis resulted in high recoveries (87-100%) of renatured protein which in its apparent molecular mass, alpha-helix content, chain composition, degree of association and ultrastructural appearance was indistinguishable from native E8. Reduction or reduction and alkylation of the chains did not lead to a decrease in their ability to reassemble specifically. Reformation of the single interchain disulphide, linking the B1 and B2 chains, clearly demonstrates that these chains are correctly aligned in parallel and in register in E8 renatured from its reduced chains. Renaturation of E8 from its reduced and alkylated chains precludes a role for disulphide formation in determining chain alignment but suggests rather than it is involved in the stabilisation of the correctly assembled molecule. These results, together with recent sequence data, provide evidence for the interaction of the alpha-helical regions of the A, B1 and B2 chains in the formation of a triple coiled-coil within the long arm of the molecule. The highly specific nature of this interaction suggests that it is the mechanism by which laminin is assembled in vivo.

Regulation of divergent transcription of the genes coding for basement membrane type IV collagen

The genes coding for the two polypeptide chains, alpha 1(IV) and alpha 2(IV), which form together the molecule of the basement membrane type IV collagen, were found to have a special and unusual genomic arrangement. The two genes are very closely linked, they are transcribed in opposite directions, and they apparently use a common and bidirectional promoter with a length of 127 bp. This region is characterized by a symmetrical arrangement of typical elements and by the palindromic structure of the sequence. In accordance with the symmetry of the promoter itself, a symmetrical organization of sequence motifs (SP1, CCAAT) was also observed in flanking regions. For the promoter and the flanking regions we could detect specific binding of nuclear factors that indicates their involvement in transcriptional activation. This suggests that the intrinsic symmetry of the type IV collagen promoter and its flanking regions may be a structural prerequisite for its bidirectional function. In transient gene expression systems no significant activity of the type IV collagen promoter was observed in either direction. This implies that additional enhancing elements are essential for the efficient and tissue-specific transcription of both type IV collagen genes. The screening for such controlling elements within the alpha 1(IV) and the alpha 2(IV) gene led to the observation that the transcription in direction of the alpha 2(IV) gene is activated by an element located in the first intron of the alpha 2 gene. Its enhancing effect is strictly dependent on the intact genomic structure of this region. Alternation of orientation and distance to the promoter destroys its activity completely. This element, located about 100-600 bp downstream from the start site of alpha 2(IV) transcription, seems to form a synergistically acting unit with the common promoter, essential for transcriptional activity in alpha 2 direction. We have not found additional enhancing elements in other regions of both genes. Explanations for the discrepancy with previous data, which define an enhancing element within the first intron of the alpha 1(IV) gene of mouse, are only speculative at present.

Differential expression of type IV procollagen and laminin genes by fetal vs adult skin fibroblasts in culture: determination of subunit mRNA steady-state levels

Basement membrane zone gene expression by fibroblast cultures, established from individuals varying in age from 14 fetal weeks to 61 years, was examined by molecular hybridizations with human sequence specific cDNAs corresponding to type IV procollagen and laminin subunit polypeptides. Northern transfer analysis with poly(A)+RNA revealed the presence of specific mRNA transcripts for alpha 1(IV) and alpha 2(IV) chains of type IV procollagen as well as B1 and B2 chains of laminin. Laminin A chain mRNAs were not detected using the same RNA preparations. Quantitative estimates of the steady-state levels of type IV procollagen and laminin mRNAs indicated that they were of relatively low abundance, as compared with mRNA to type I procollagen. The expression of alpha 1(IV) and alpha 2(IV) collagen genes was high in fetal fibroblasts but was reduced to low, yet detectable, levels in cultures established from 3-d to 61-year-old individuals. In contrast, the laminin B1 and B2 chain mRNA levels showed little age-associated variation within the cultures examined. These results provide evidence for differential regulation of the expression of different basement membrane zone macromolecules during chronologic aging.

Structure and biological activity of basement membrane proteins

Collagen type IV, laminin, heparan sulfate proteoglycans, nidogen (entactin) and BM-40 (osteonectin, SPARC) represent major structural proteins of basement membranes. They are well-characterized in their domain structures, amino acid sequences and potentials for molecular interactions. Such interactions include self-assembly processes and heterotypic binding between individual constituents, as well as binding of calcium (laminin, BM-40) and are likely to be used for basement membrane assembly. Laminin, collagen IV and nidogen also possess several cell-binding sites which interact with distinct cellular receptors. Some evidence exists that those interactions are involved in the control of cell behaviour. These observations have provided a more defined understanding of basement membrane function and the definition of new research goals in the future.