Glomerular laminin isoform transitions: errors in metanephric culture are corrected by grafting

Tissue-specific parameters for the design of ECM-mimetic biomaterials

The extracellular matrix (ECM) is a complex network of biomolecules that mechanically and biochemically directs cell behavior and is crucial for maintaining tissue function and health. The heterogeneous organization and composition of the ECM varies within and between tissue types, directing mechanics, aiding in cell-cell communication, and facilitating tissue assembly and reassembly during development, injury and disease. As technologies like 3D printing rapidly advance, researchers are better able to recapitulate in vivo tissue properties in vitro; however, tissue-specific variations in ECM composition and organization are not given enough consideration. This is in part due to a lack of information regarding how the ECM of many tissues varies in both homeostatic and diseased states. To address this gap, we describe the components and organization of the ECM, and provide examples for different tissues at various states of disease. While many aspects of ECM biology remain unknown, our goal is to highlight the complexity of various tissues and inspire engineers to incorporate unique components of the native ECM into in vitro platform design and fabrication. Ultimately, we anticipate that the use of biomaterials that incorporate key tissue-specific ECM will lead to in vitro models that better emulate human pathologies. STATEMENT OF SIGNIFICANCE: Biomaterial development primarily emphasizes the engineering of new materials and therapies at the expense of identifying key parameters of the tissue that is being emulated. This can be partially attributed to the difficulty in defining the 3D composition, organization, and mechanics of the ECM within different tissues and how these material properties vary as a function of homeostasis and disease. In this review, we highlight a range of tissues throughout the body and describe how ECM content, cell diversity, and mechanical properties change in diseased tissues and influence cellular behavior. Accurately mimicking the tissue of interest in vitro by using ECM specific to the appropriate state of homeostasis or pathology in vivo will yield results more translatable to humans.

Generation of Functioning Nephrons by Implanting Human Pluripotent Stem Cell-Derived Kidney Progenitors

Human pluripotent stem cells (hPSCs) hold great promise for understanding kidney development and disease. We reproducibly differentiated three genetically distinct wild-type hPSC lines to kidney precursors that underwent rudimentary morphogenesis in vitro. They expressed nephron and collecting duct lineage marker genes, several of which are mutated in human kidney disease. Lentiviral-transduced hPSCs expressing reporter genes differentiated similarly to controls in vitro. Kidney progenitors were subcutaneously implanted into immunodeficient mice. By 12 weeks, they formed organ-like masses detectable by bioluminescence imaging. Implants included perfused glomeruli containing human capillaries, podocytes with regions of mature basement membrane, and mesangial cells. After intravenous injection of fluorescent low-molecular-weight dextran, signal was detected in tubules, demonstrating uptake from glomerular filtrate. Thus, we have developed methods to trace hPSC-derived kidney precursors that formed functioning nephrons in vivo. These advances beyond in vitro culture are critical steps toward using hPSCs to model and treat kidney diseases.

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Reproducible differentiation to kidney progenitors in 3 hESC lines

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After subcutaneous implantation, kidney-like tissues detectable by bioluminescence

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Implant nephrons contain glomeruli, proximal and distal tubules, and collecting ducts

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Vascularized glomeruli filter intravenously injected low-molecular-weight dextran

Glomerular development--shaping the multi-cellular filtration unit

The glomerulus represents a highly structured filtration unit, composed of glomerular endothelial cells, mesangial cells, podocytes and parietal epithelial cells. During glomerulogenesis an intricate network of signaling pathways involving transcription factors, secreted factors and cell-cell communication is required to guarantee accurate evolvement of a functional, complex 3-dimensional glomerular architecture. Here, we want to provide an overview on the critical steps and relevant signaling cascades of glomerular development.

Laminin α1 regulates age-related mesangial cell proliferation and mesangial matrix accumulation through the TGF-β pathway

Laminin α1 (LAMA1), a subunit of the laminin-111 basement membrane component, has been implicated in various biological functions in vivo and in vitro. Although LAMA1 is present in kidney, its roles in the kidney are unknown because of early embryonic lethality. Herein, we used a viable conditional knockout mouse model with a deletion of Lama1 in the epiblast lineage (Lama1(CKO)) to study the role of LAMA1 in kidney development and function. Adult Lama1(CKO) mice developed focal glomerulosclerosis and proteinuria with age. In addition, mesangial cell proliferation was increased, and the mesangial matrix, which normally contains laminin-111, was greatly expanded. In vitro, mesangial cells from Lama1(CKO) mice exhibited significantly increased proliferation compared with those from controls. This increased proliferation was inhibited by the addition of exogenous LAMA1-containing laminin-111, but not by laminin-211 or laminin-511, suggesting a specific role for LAMA1 in regulating mesangial cell behavior. Moreover, the absence of LAMA1 increased transforming growth factor (TGF)-β1-induced Smad2 phosphorylation, and inhibitors of TGF-β1 receptor I kinase blocked Smad2 phosphorylation in both control and Lama1(CKO) mesangial cells, indicating that the increased Smad2 phosphorylation occurred in the absence of LAMA1 via the TGF-β1 receptor. These findings suggest that LAMA1 plays a critical role in kidney function and kidney aging by regulating the mesangial cell population and mesangial matrix deposition through TGF-β/Smad signaling.

Laminin β2 gene missense mutation produces endoplasmic reticulum stress in podocytes

Mutations in the laminin β2 gene (LAMB2) cause Pierson syndrome, a severe congenital nephrotic syndrome with ocular and neurologic defects. LAMB2 is a component of the laminin-521 (α5β2γ1) trimer, an important constituent of the glomerular basement membrane (GBM). The C321R-LAMB2 missense mutation leads to congenital nephrotic syndrome but only mild extrarenal symptoms; the mechanisms underlying the development of proteinuria with this mutation are unclear. We generated three transgenic mouse lines, in which rat C321R-LAMB2 replaced mouse LAMB2 in the GBM. During the first postnatal month, expression of C321R-LAMB2 attenuated the severe proteinuria exhibited by Lamb2(-/-) mice in a dose-dependent fashion; proteinuria eventually increased, however, leading to renal failure. The C321R mutation caused defective secretion of laminin-521 from podocytes to the GBM accompanied by podocyte endoplasmic reticulum (ER) stress, likely resulting from protein misfolding. Moreover, ER stress preceded the onset of significant proteinuria and was manifested by induction of the ER-initiated apoptotic signal C/EBP homologous protein (CHOP), ER distention, and podocyte injury. Treatment of cells expressing C321R-LAMB2 with the chemical chaperone taurodeoxycholic acid (TUDCA), which can facilitate protein folding and trafficking, greatly increased the secretion of the mutant LAMB2. Taken together, these results suggest that the mild variant of Pierson syndrome caused by the C321R-LAMB2 mutation may be a prototypical ER storage disease, which may benefit from treatment approaches that target the handling of misfolded proteins.

Role of the podocyte (and glomerular endothelium) in building the GBM

This article summarizes the basic cellular and extracellular events in the development of the glomerulus and assembly of the glomerular basement membrane (GBM), paying special attention to laminin (LM) and type IV collagen. Cellular receptors for GBM proteins, including the integrins, dystroglycan, and discoidin domain receptor 1 also are discussed. Evidence is reviewed showing that the laminin isoform present in the earliest GBM, LM-111, and final isoform found in the mature GBM, LM-521, are each derived from both endothelial cells and podocytes. Although the early collagen α1α2α1(IV) similarly derives from endothelial cells and podocytes, collagen α3α4α5(IV) found in fully mature GBM is a product solely of podocytes. Genetic diseases affecting laminin and type IV collagen synthesis also are presented, with an emphasis on mutations to LAMB2 (Pierson syndrome) and COL4A3, COL4A4, and COL4A5 (Alport syndrome), and their experimental mouse models. Stress is placed on the assembly of a compositionally correct GBM for the acquisition and maintenance of glomerular barrier properties.

Mouse stem cells seeded into decellularized rat kidney scaffolds endothelialize and remodel basement membranes

INTRODUCTION

To address transplant organ shortage, a promising strategy is to decellularize kidneys in a manner that the scaffold retains signals for seeded pluripotent precursor cells to differentiate and recapitulate native structures: matrix-to-cell signaling followed by cell-cell and cell-matrix interactions, thereby remodeling and replacing the original matrix. This would reduce scaffold antigenicity and enable xeno-allografts.

RESULTS

DAPI-labeled cells in arterial vessels and glomeruli were positive for both endothelial lineage markers, BsLB4 and VEGFR2. Rat scaffold's basement membrane demonstrated immunolabeling with anti-mouse laminin β1. Labeling intensified over time with 14 day incubations.

CONCLUSION

We provide new evidence for matrix-to-cell signaling in acellular whole organ scaffolds that induces differentiation of pluripotent precursor cells to endothelial lineage. Production of mouse basement membrane supports remodeling of host (rat)-derived scaffolds and thereby warrants further investigation as a promising approach for xenotransplantation.

METHODS

We previously showed that murine embryonic stem cells arterially seeded into acellular rat whole kidney scaffolds multiply and demonstrate morphologic, immunohistochemical and gene expression evidence for differentiation. Vascular cell endothelialization was now further tested by endothelial specific BsLB4 lectin and anti-VEGFR2 (Flk1) antibodies. Remodeling of the matrix basement membranes from rat to mouse ("murinization") was assessed by a monoclonal antibody specific for mouse laminin β1 chain.

The role of cell-extracellular matrix interactions in glomerular injury

Glomerulosclerosis is characterized by excessive deposition of extracellular matrix within the glomeruli of the kidney, glomerular cell death, and subsequent loss of functional glomeruli. While in physiological situations the levels of extracellular matrix components are kept constant by a tight balance between formation and degradation, in the case of injury that results in fibrosis there is increased matrix deposition relative to its breakdown. Multiple factors control matrix synthesis and degradation, thus contributing to the development of glomerulosclerosis. This review focuses primarily on the role of cell-matrix interactions, which play a critical role in governing glomerular cell cues in both healthy and diseased kidneys. Cell-extracellular matrix interactions are made possible by various cellular receptors including integrins, discoidin domain receptors, and dystroglycan. Upon binding to a selective extracellular matrix protein, these receptors activate intracellular signaling pathways that can either downregulate or upregulate matrix synthesis and deposition. This, together with the observation that changes in the expression levels of matrix receptors have been documented in glomerular disease, clearly emphasizes the contribution of cell-matrix interactions in glomerular injury. Understanding the molecular mechanisms whereby extracellular matrix receptors regulate matrix homeostasis in the course of glomerular injury is therefore critical for devising more effective therapies to treat and ideally prevent glomerulosclerosis.

A missense LAMB2 mutation causes congenital nephrotic syndrome by impairing laminin secretion

Laminin β2 is a component of laminin-521, which is an important constituent of the glomerular basement membrane (GBM). Null mutations in laminin β2 (LAMB2) cause Pierson syndrome, a severe congenital nephrotic syndrome with ocular and neurologic defects. In contrast, patients with LAMB2 missense mutations, such as R246Q, can have less severe extrarenal defects but still exhibit congenital nephrotic syndrome. To investigate how such missense mutations in LAMB2 cause proteinuria, we generated three transgenic lines of mice in which R246Q-mutant rat laminin β2 replaced the wild-type mouse laminin β2 in the GBM. These transgenic mice developed much less severe proteinuria than their nontransgenic Lamb2-deficient littermates; the level of proteinuria correlated inversely with R246Q-LAMB2 expression. At the onset of proteinuria, expression and localization of proteins associated with the slit diaphragm and foot processes were normal, and there were no obvious ultrastructural abnormalities. Low transgene expressors developed heavy proteinuria, foot process effacement, GBM thickening, and renal failure by 3 months, but high expressors developed only mild proteinuria by 9 months. In vitro studies demonstrated that the R246Q mutation results in impaired secretion of laminin. Taken together, these results suggest that the R246Q mutation causes nephrotic syndrome by impairing secretion of laminin-521 from podocytes into the GBM; however, increased expression of the mutant protein is able to overcome this secretion defect and improve glomerular permselectivity.

Maintenance of glomerular filtration barrier integrity requires laminin alpha5

Mutation of the mouse laminin alpha5 gene results in a variety of developmental defects, including defects in kidney structure and function. Whereas the total absence of laminin alpha5 results in breakdown of the glomerular basement membrane (GBM) and failed glomerular vascularization, a hypomorphic Lama5 mutation (the Lama5(neo) allele) results in proteinuria, hematuria, polycystic kidney disease (PKD), and death 3 to 4 weeks after birth. Here, we examined the role of podocyte-derived laminin alpha5 via podocyte-specific inactivation of Lama5 and podocyte-specific rescue of the Lama5(neo) mutation. Podocyte-specific inactivation of Lama5 resulted in varying degrees of proteinuria and rates of progression to nephrotic syndrome. The GBM of proteinuric mice appeared thickened and "moth-eaten," and podocyte foot processes became effaced. Podocyte-specific restoration of laminin alpha5 production using two distinct strategies in Lama5(neo/neo) mice resulted in the resolution of proteinuria, hematuria, and PKD. These results suggest that the development of normal GBM structure and function requires podocyte-derived laminin alpha5 during and after glomerulogenesis and present a unique mechanism for the pathogenesis of PKD in these mice.

Laminin alpha4-null mutant mice develop chronic kidney disease with persistent overexpression of platelet-derived growth factor

Each extracellular matrix compartment in the kidney has a unique composition, with regional specificity in the expression of various laminin isoforms. Although null mutations in the majority of laminin chains lead to specific developmental abnormalities in the kidney, Lama4-/- mice have progressive glomerular and tubulointerstitial fibrosis. These mice have a significant increase in expression of platelet-derived growth factor (PDGF)-BB, PDGF-DD, and PDGF receptor beta in association with immature glomerular and peritubular capillaries. In addition, mesangial cell exposure to alpha4-containing laminins, but not other isoforms, results in down-regulation of PDGF receptor mRNA and protein, suggesting a direct effect of LN411/LN421 on vessel maturation. Given the known role of overexpression of PDGF-BB and PDGF-DD on glomerular and tubulointerstitial fibrosis, these data suggest that failure of laminin alpha4-mediated down-regulation of PDGF activity contributes to the progressive renal lesions in this animal model. Given the recent demonstration that individuals with laminin alpha4 mutations develop cardiomyopathy, these findings may be relevant to kidney disease in humans.

Laminin alpha 5 influences the architecture of the mouse small intestine mucosa

The mammalian intestine displays two distinct patterns of mucosal organization. The small intestine contains mucosal epithelial invaginations (the crypts of Lieberkühn) that are continuous with evaginations (villi) into the lumen. The colon also contains crypts of Lieberkühn, but its epithelial surface is lined by flat surface cuffs. The epithelial cells of both organs communicate with the underlying mesenchyme through a basement membrane that is composed of a variety of extracellular matrix proteins, including members of the laminin family. The basement membranes of the small intestine and colon contain distinct laminin subtypes; notably, the villus basement membrane is rich in laminin alpha 5. Here, we show that the diminution of laminin alpha 5 in a mouse model led to a compensatory deposition of colonic laminins, which resulted in a transformation from a small intestinal to a colonic mucosal architecture. The alteration in mucosal architecture was associated with reduced levels of nuclear p27Kip1 - a cell-cycle regulator - and altered intestinal epithelial cell proliferation, migration and differentiation. Our results suggest that laminin alpha 5 has a crucial role in establishing and maintaining the specific mucosal pattern of the mouse small intestine.

Beta1 integrin expression by podocytes is required to maintain glomerular structural integrity

Integrins are transmembrane heteromeric receptors that mediate interactions between cells and extracellular matrix (ECM). beta1, the most abundantly expressed integrin subunit, binds at least 12 alpha subunits. beta1 containing integrins are highly expressed in the glomerulus of the kidney; however their role in glomerular morphogenesis and maintenance of glomerular filtration barrier integrity is poorly understood. To study these questions we selectively deleted beta1 integrin in the podocyte by crossing beta1(flox/flox) mice with podocyte specific podocin-cre mice (pod-Cre), which express cre at the time of glomerular capillary formation. We demonstrate that podocyte abnormalities are visualized during glomerulogenesis of the pod-Cre;beta1(flox/flox) mice and proteinuria is present at birth, despite a grossly normal glomerular basement membrane. Following the advent of glomerular filtration there is progressive podocyte loss and the mice develop capillary loop and mesangium degeneration with little evidence of glomerulosclerosis. By 3 weeks of age the mice develop severe end stage renal failure characterized by both tubulointerstitial and glomerular pathology. Thus, expression of beta1 containing integrins by the podocyte is critical for maintaining the structural integrity of the glomerulus.

Proteinuria precedes podocyte abnormalities inLamb2-/- mice, implicating the glomerular basement membrane as an albumin barrier

Primary defects in either podocytes or the glomerular basement membrane (GBM) cause proteinuria, a fact that complicates defining the barrier to albumin. Laminin beta2 (LAMB2) is a GBM component required for proper functioning of the glomerular filtration barrier. To investigate the GBM's role in glomerular filtration, we characterized GBM and overlying podocyte architecture in relation to development and progression of proteinuria in Lamb2-/- mice, which model Pierson syndrome, a rare congenital nephrotic syndrome. We found ectopic deposition of several laminins and mislocalization of anionic sites in the GBM, which together suggest that the Lamb2-/- GBM is severely disorganized, although it is ultrastructurally intact. Importantly, albuminuria was detectable shortly after birth and preceded podocyte foot process effacement and loss of slit diaphragms by at least 7 days. Expression and localization of slit diaphragm and foot process-associated proteins appeared normal at early stages. GBM permeability to the electron-dense tracer ferritin was dramatically elevated in Lamb2-/- mice, even before widespread foot process effacement. Increased ferritin permeability was not observed in nephrotic CD2-associated protein-null (Cd2ap-/-) mice, which have a primary podocyte defect. Together these data show that the GBM serves as a barrier to protein in vivo and that the glomerular slit diaphragm alone is not sufficient to prevent the passage of albumin into the urinary space.

Abnormal development of glomerular endothelial and mesangial cells in mice with targeted disruption of the lama3 gene

Mice with targeted disruption of the lama3 gene, which encodes the alpha3 chain of laminin-5 (alpha3beta3gamma2, 332), develop a blistering skin disease similar to junctional epidermolysis bullosa in humans. These animals also develop abnormalities in glomerulogenesis. In both wild-type and mutant animals (lama3(-/-)), podocytes secrete glomerular basement membrane and develop foot processes. Endothelial cells migrate into this scaffolding and secrete a layer of basement membrane that fuses with the one formed by the podocyte. In lama3(-/-) animals, glomerular maturation arrests at this stage. Endothelial cells do not attenuate, develop fenestrae, or form typical lumens, and mesangial cells (MCs) were not identified. LN alpha3 subunit (LAMA3) protein was identified in the basement membrane adjacent to glomerular endothelial cells (GEnCs) in normal rats and mice. In developing rat glomeruli, the LAMA3 subunit was first detectable in the early capillary loop stage, which corresponds to the stage at which maturation arrest was observed in the mutant mice. Lama3 mRNA and protein were identified in isolated rat and mouse glomeruli and cultured rat GEnCs, but not MC. These data document expression of LAMA3 in glomeruli and support a critical role for it in GEnC differentiation. Furthermore, LAMA3 chain expression and/or another product of endothelial cells are required for MC migration into the developing glomerulus.

Transgenic isolation of skeletal muscle and kidney defects in laminin beta2 mutant mice: implications for Pierson syndrome

Pierson syndrome is a recently defined disease usually lethal within the first postnatal months and caused by mutations in the gene encoding laminin beta2 (LAMB2). The hallmarks of Pierson syndrome are congenital nephrotic syndrome accompanied by ocular abnormalities, including microcoria (small pupils), with muscular and neurological developmental defects also present. Lamb2(-/-) mice are a model for Pierson syndrome; they exhibit defects in the kidney glomerular barrier, in the development and organization of the neuromuscular junction, and in the retina. Lamb2(-/-) mice fail to thrive and die very small at 3 weeks of age, but to what extent the kidney and neuromuscular defects each contribute to this severe phenotype has been obscure, though highly relevant to understanding Pierson syndrome. To investigate this, we generated transgenic mouse lines expressing rat laminin beta2 either in muscle or in glomerular epithelial cells (podocytes) and crossed them onto the Lamb2(-/-) background. Rat beta2 was confined in skeletal muscle to synapses and myotendinous junctions, and in kidney to the glomerular basement membrane. In transgenic Lamb2(-/-) mice, beta2 deposition in only glomeruli prevented proteinuria but did not ameliorate the severe phenotype. By contrast, beta2 expression in only muscle restored synaptic architecture and led to greatly improved health, but the mice died from kidney disease at 1 month. Rescue of both glomeruli and synapses was associated with normal weight gain, fertility and lifespan. We conclude that muscle defects in Lamb2(-/-) mice are responsible for the severe failure to thrive phenotype, and that renal replacement therapy alone will be an inadequate treatment for Pierson syndrome.

Integration of embryonic stem cells in metanephric kidney organ culture

Many stages of nephrogenesis can be studied using cultured embryonic kidneys, but there is no efficient technique available to readily knockdown or overexpress transgenes for rapid evaluation of resulting phenotypes. Embryonic stem (ES) cells have unlimited developmental potential and can be manipulated at the molecular genetic level by a variety of methods. The aim of this study was to determine if ES cells could respond to developmental signals within the mouse embryonic day 12 to embryonic day 13 (E12 to E13) kidney microenvironment and incorporate into kidney structures. ROSA26 ES cells were shown to express beta-galactosidase ubiquitously when cultured in the presence of leukemia inhibitory factor to suppress differentiation. When these cells were microinjected into E12 to E13 metanephroi and then placed in transwell organ culture, ES cell-derived, beta-galactosidase-positive cells were identified in epithelial structures resembling tubules. On rare occasions, individual ES cells were observed in structures resembling glomerular tufts. Electron microscopy showed that the ES cell-derived tubules were surrounded by basement membrane and had apical microvilli and junctional complexes. Marker analysis revealed that a subset of these epithelial tubules bound Lotus tetragonolobus and expressed alpha(1) Na(+)/K(+) ATPase. ES cells were infected before injection with a cytomegalovirus promoter-green fluorescence protein (GFP) adenovirus and GFP expression was found as early as 18 h, persisting for up to 48 h in cultured kidneys. This ES cell technology may achieve the objective of obtaining a versatile cell culture system in which molecular interventions can be used in vitro and consequences of these perturbations on the normal kidney development program in vivo can be studied.

Overexpression of beta1-chain-containing laminins in capillary basement membranes of human breast cancer and its metastases

INTRODUCTION

Laminins are the major components of vascular and parenchymal basement membranes. We previously documented a switch in the expression of vascular laminins containing the alpha4 chain from predominantly laminin-9 (alpha4beta2gamma1) to predominantly laminin-8 (alpha4beta1gamma1) during progression of human brain gliomas to high-grade glioblastoma multiforme. Here, differential expression of laminins was studied in blood vessels and ductal epithelium of the breast.

METHOD

In the present study the expressions of laminin isoforms alpha1-alpha5, beta1-beta3, gamma1, and gamma2 were examined during progression of breast cancer. Forty-five clinical samples of breast tissues including normal breast, ductal carcinomas in situ, invasive ductal carcinomas, and their metastases to the brain were compared using Western blot analysis and immunohistochemistry for various chains of laminin, in particular laminin-8 and laminin-9.

RESULTS

Laminin alpha4 chain was observed in vascular basement membranes of most studied tissues, with the highest expression in metastases. At the same time, the expression of laminin beta2 chain (a constituent of laminin-9) was mostly seen in normal breast and carcinomas in situ but not in invasive carcinomas or metastases. In contrast, laminin beta1 chain (a constituent of laminin-8) was typically found in vessel walls of carcinomas and their metastases but not in those of normal breast. The expression of laminin-8 increased in a progression-dependent manner. A similar change was observed from laminin-11 (alpha5beta2gamma1) to laminin-10 (alpha5beta1gamma1) during breast tumor progression. Additionally, laminin-2 (alpha2beta1gamma1) appeared in vascular basement membranes of invasive carcinomas and metastases. Chains of laminin-5 (alpha3beta3gamma2) were expressed in the ductal epithelium basement membranes of the breast and diminished with tumor progression.

CONCLUSION

These results suggest that laminin-2, laminin-8, and laminin-10 are important components of tumor microvessels and may associate with breast tumor progression. Angiogenic switch from laminin-9 and laminin-11 to laminin-8 and laminin-10 first occurs in carcinomas in situ and becomes more pronounced with progression of carcinomas to the invasive stage. Similar to high-grade brain gliomas, the expression of laminin-8 (and laminin-10) in breast cancer tissue may be a predictive factor for tumor neovascularization and invasion.

Establishment and characterization of a parietal endoderm-like cell line derived from Engelbreth–Holm–Swarm tumor (EHSPEL), a possible resource for an engineered basement membrane matrix

Engelbreth-Holm-Swarm (EHS) tumor produces large amounts of basement membrane (BM) components, which are widely used as cell culture substrates mimicking BM functions. EHS tumor arose spontaneously in an ST/Eh strain mouse and has been propagated by transplantation. In the present study, we established a cell line, EHSPEL (EHS Parietal Endoderm-Like), which can be cultured ex vivo and preserves the capacity to form tumors in vivo. EHSPEL cells secreted large amounts of laminin-1 into the medium and deposited BM components onto dishes. To further characterize EHSPEL cells, their gene expression profile was compared to those of parietal endoderm cells from Reichert's membrane at embryonic day 13.5, differentiated F9 embryonal carcinoma cells, and PYS-2 parietal endoderm cells. These analyses outlined not only common features of parietal endoderm-like cells that underlie the efficient production of BM components, but also germline cell-like features of EHSPEL cells, at least some of which may play crucial roles in their capacity to form tumors that accumulate abundant BM components in vivo. Karyotyping of EHSPEL cells using chromosome painting probes showed a large number of interchromosomal rearrangements and partial chromosome hyperploidy. Exogenous introduction of a human laminin-alpha(4)-EGFP fusion protein into EHSPEL cells resulted in the production and deposition of human-mouse-hybrid laminin-8. This strategy should be applicable for creating efficient systems to produce chimeric laminins as well as BM-like gels with modified biological activity.

Laminin alpha5 chain is required for intestinal smooth muscle development

Laminins (comprised of alpha, beta, and gamma chains) are heterotrimeric glycoproteins integral to all basement membranes. The function of the laminin alpha5 chain in the developing intestine was defined by analysing laminin alpha5(-/-) mutants and by grafting experiments. We show that laminin alpha5 plays a major role in smooth muscle organisation and differentiation, as excessive folding of intestinal loops and delay in the expression of specific markers are observed in laminin alpha5(-/-) mice. In the subepithelial basement membrane, loss of alpha5 expression was paralleled by ectopic or accelerated deposition of laminin alpha2 and alpha4 chains; this may explain why no obvious defects were observed in the villous form and enterocytic differentiation. This compensation process is attributable to mesenchyme-derived molecules as assessed by chick/mouse alpha5(-/-) grafted associations. Lack of the laminin alpha5 chain was accompanied by a decrease in epithelial alpha3beta1 integrin receptor expression adjacent to the epithelial basement membrane and of Lutheran blood group glycoprotein in the smooth muscle cells, indicating that these receptors are likely mediating interactions with laminin alpha5-containing molecules. Taken together, the data indicate that the laminin alpha5 chain is essential for normal development of the intestinal smooth muscle and point to possible mesenchyme-derived compensation to promote normal intestinal morphogenesis when laminin alpha5 is absent.

Expression of laminin chains by central neurons: analysis with gene and protein trapping techniques

Laminins exert numerous effects on neurons in vitro, but expression of laminin subunit genes by neurons in vivo remains controversial. To reexamine this issue, we generated mice from ES cells in which the laminin alpha1, alpha5, beta1, and gamma1 genes had been "trapped" by insertion of a histochemically detectable selectable marker, betageo (beta-galactosidase fused to neomycin phosphotransferase). The presence of laminin-betageo fusion proteins was assayed histochemically and immunochemically, revealing expression of laminin beta1 and gamma1 genes, but not alpha chain genes, by defined subsets of neurons in brain and retina. We also used the gene traps in a novel way to assay expression of endogenous laminin subunits, which were barely detectable by ordinary immunohistochemical methods. The trapping vector included a transmembrane domain that anchors proteins otherwise destined for secretion. Laminin alpha/beta/gamma heterotrimers are assembled intracellularly, and we show that the trapped laminin gamma1 fusion protein "co-trapped" endogenous beta1 intracellularly. The laminin gamma1 fusion was also able to co-trap transgene-derived alpha chains, but we detected no co-trapped endogenous alpha chains. The co-trapping method may be generally useful for identifying proteins or isolating protein complexes associated with trapped gene products.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.