Domain IV of mouse laminin beta1 and beta2 chains

Dual transgene amelioration of Lama2-null muscular dystrophy

Null mutations of the Lama2-gene cause a severe congenital muscular dystrophy and associated neuropathy. In the absence of laminin-α2 (Lmα2) there is a compensatory replacement by Lmα4, a subunit that lacks the polymerization and α-dystroglycan (αDG)-binding properties of Lmα2. The dystrophic phenotype in the dy3K/dy3K Lama2-/- mouse were evaluated with transgenes driving expression of two synthetic laminin-binding linker proteins. Transgenic muscle-specific expression of αLNNd, a chimeric protein that enables α4-laminin polymerization, and miniagrin (mag), a protein that increases laminin binding to the receptor αDG, separately improved median mouse survival two-fold. The double transgenes (DT) improved mean survival three-fold with increases in overall body weight, muscle size, and grip strength, but, given absence of neuronal expression, did not prevent hindlimb paresis. Muscle improvements included increased myofiber size and number and reduced fibrosis. Myofiber hypertrophy with increased mTOR and Akt phosphorylation were characteristics of mag-dy3K/dy3K and DT-dy3K/dy3K muscle. Elevations of matrix-bound α4-, β1 and γ1 laminin subunits were detected in muscle extracts and immunostained sections in response to DT expression. Collectively, these findings reveal a complimentary polymerization and αDG-binding benefit to Lama2-/- mouse muscle largely mediated through modified laminin-411.

Synaptopodin deficiency exacerbates kidney disease in a mouse model of Alport syndrome

Synaptopodin (Synpo) is an actin-associated protein in podocyte foot processes. By generating mice that completely lack Synpo, we previously showed that Synpo is dispensable for normal kidney function. However, lack of Synpo worsened adriamycin-induced nephropathy, indicating a protective role for Synpo in injured podocytes. Here, we investigated whether lack of Synpo directly impacts a genetic disease, Alport syndrome (AS), because Synpo is reduced in podocytes of affected humans and mice; whether this is merely an association or pathogenic is unknown. We used collagen type IV-α₅ (Col4a5) mutant mice, which model X-linked AS, showing glomerular basement membrane (GBM) abnormalities, eventual foot process effacement, and progression to end-stage kidney disease. We intercrossed mice carrying mutations in Synpo and Col4a5 to produce double-mutant mice. Urine and tissue were taken at select time points to evaluate albuminuria, histopathology, and glomerular capillary wall composition and ultrastructure. Lack of Synpo in Col4a5^-/Y, Col4a5^-/-, or Col4a5^+/- Alport mice led to the acceleration of disease progression, including more severe proteinuria and glomerulosclerosis. Absence of Synpo attenuated the shift of myosin IIA from the podocyte cell body and major processes to actin cables near the GBM in the areas of effacement. We speculate that this is mechanistically associated with enhanced loss of podocytes due to easier detachment from the GBM. We conclude that Synpo deletion exacerbates the disease phenotype in Alport mice, revealing the podocyte actin cytoskeleton as a target for therapy in patients with AS.NEW & NOTEWORTHY Alport syndrome (AS) is a hereditary disease of the glomerular basement with hematuria and proteinuria. Podocytes eventually exhibit foot process effacement, indicating actin cytoskeletal changes. To investigate how cytoskeletal changes impact podocytes, we generated Alport mice lacking synaptopodin, an actin-binding protein in foot processes. Analysis showed a more rapid disease progression, demonstrating that synaptopodin is protective. This suggests that the actin cytoskeleton is a target for therapy in AS and perhaps other glomerular diseases.

EPB41L5 controls podocyte extracellular matrix assembly by adhesome-dependent force transmission

The integrity of the kidney filtration barrier essentially relies on the balanced interplay of podocytes and the glomerular basement membrane (GBM). Here, we show by analysis of in vitro and in vivo models that a loss of the podocyte-specific FERM-domain protein EPB41L5 results in impaired extracellular matrix (ECM) assembly. By using quantitative proteomics analysis of the secretome and matrisome, we demonstrate a shift in ECM composition characterized by diminished deposition of core GBM components, such as LAMA5. Integrin adhesome proteomics reveals that EPB41L5 recruits PDLIM5 and ACTN4 to integrin adhesion complexes (IACs). Consecutively, EPB41L5 knockout podocytes show insufficient maturation of integrin adhesion sites, which translates into impaired force transmission and ECM assembly. These observations build the framework for a model in which EPB41L5 functions as a cell-type-specific regulator of the podocyte adhesome and controls a localized adaptive module in order to prevent podocyte detachment and thereby ensures GBM integrity.

Characterization of Laminins in Healthy Human Aortic Valves and a Modified Decellularized Rat Scaffold

Aortic valve stenosis is one of the most common cardiovascular diseases in western countries and can only be treated by replacement with a prosthetic valve. Tissue engineering is an emerging and promising treatment option, but in-depth knowledge about the microstructure of native heart valves is lacking, making the development of tissue-engineered heart valves challenging. Specifically, the basement membrane (BM) of heart valves remains incompletely characterized, and decellularization protocols that preserve BM components are necessary to advance the field. This study aims to characterize laminin isoforms expressed in healthy human aortic valves and establish a small animal decellularized aortic valve scaffold for future studies of the BM in tissue engineering. Laminin isoforms were assessed by immunohistochemistry with antibodies specific for individual α, β, and γ chains. The results indicated that LN-411, LN-421, LN-511, and LN-521 are expressed in human aortic valves (n = 3), forming a continuous monolayer in the endothelial BM, whereas sparsely found in the interstitium. Similar results were seen in rat aortic valves (n = 3). Retention of laminin and other BM components, concomitantly with effective removal of cells and residual DNA, was achieved through 3 h exposure to 1% sodium dodecyl sulfate and 30 min exposure to 1% Triton X-100, followed by nuclease processing in rat aortic valves (n = 3). Our results provide crucial data on the microenvironment of valvular cells relevant for research in both tissue engineering and heart valve biology. We also describe a decellularized rat aortic valve scaffold useful for mechanistic studies on the role of the BM in heart valve regeneration.

A deletion in the N-terminal polymerizing domain of laminin β2 is a new mouse model of chronic nephrotic syndrome

The importance of the glomerular basement membrane (GBM) in glomerular filtration is underscored by the manifestations of Alport and Pierson syndromes, caused by defects in type IV collagen α3α4α5 and the laminin β2 chain, respectively. Lamb2 null mice, which model the most severe form of Pierson syndrome, exhibit proteinuria prior to podocyte foot process effacement and are therefore useful for studying GBM permselectivity. We hypothesize that some LAMB2 missense mutations that cause mild forms of Pierson syndrome induce GBM destabilization with delayed effects on podocytes. While generating a CRISPR/Cas9-mediated analogue of a human LAMB2 missense mutation in mice, we identified a 44-amino acid deletion (LAMB2-Del44) within the laminin N-terminal domain, a domain mediating laminin polymerization. Laminin heterotrimers containing LAMB2-Del44 exhibited a 90% reduction in polymerization in vitro that was partially rescued by type IV collagen and nidogen. Del44 mice showed albuminuria at 1.8-6.0 g/g creatinine (ACR) at one to two months, plateauing at an average 200 g/g ACR at 3.7 months, when GBM thickening and hallmarks of nephrotic syndrome were first observed. Despite the massive albuminuria, some Del44 mice survived for up to 15 months. Blood urea nitrogen was modestly elevated at seven-nine months. Eight to nine-month-old Del44 mice exhibited glomerulosclerosis and interstitial fibrosis. Similar to Lamb2^-/- mice, proteinuria preceded foot process effacement. Foot processes were widened but not effaced at one-two months despite the high ACRs. At three months some individual foot processes were still observed amid widespread effacement. Thus, our chronic model of nephrotic syndrome may prove useful to study filtration mechanisms, long-term proteinuria with preserved kidney function, and to test therapeutics.

Differential expression of parietal epithelial cell and podocyte extracellular matrix proteins in focal segmental glomerulosclerosis and diabetic nephropathy

In healthy glomeruli, parietal epithelial cell (PEC)-derived extracellular matrix (ECM) proteins include laminin-β1, perlecan, and collagen type IV-α2 and podocyte-specific ECM proteins include laminin-β2, agrin, and collagen type IV-α4. This study aimed to define individual ECM protein isoform expression by PECs in both experimental and human focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy (DN) and to determine if changes were CD44 dependent. In experimental FSGS induced with a cytotoxic podocyte antibody and in the BTBR ob/ob mouse model of DN, PEC-derived protein staining was significantly increased in PECs. Dual staining also showed de novo expression of the podocyte-specific ECM proteins laminin-β2 and agrin in PECs. Similar findings were observed in biopsies from patients with FSGS and DN. Increases in individual ECM proteins colocalized with CD44 in PECs in disease. To determine the role of CD44, FSGS was induced in CD44-/- and CD44+/+ mice. PEC staining for perlecan, collagen type IV-α2, laminin-β2, and agrin were significantly lower in diseased CD44-/- mice compared with diseased CD44+/+ mice. These results show that in experimental and human FSGS and DN, PECs typically in an activated state, produce both PEC-derived and podocyte-specific ECM protein isoforms, and that the majority of these changes were dependent on CD44.

Switch in Laminin β2 to Laminin β1 Isoforms During Aging Controls Endothelial Cell Functions-Brief Report

OBJECTIVE

Endothelial cells play important roles in tissue homeostasis and vascularization, a function that is impaired by aging. Here, we aim to decipher the role of the microenvironment underlying the impairment of endothelial cell functions by aging.

APPROACH AND RESULTS

RNA sequencing of isolated cardiac endothelial cells derived from young and 18-month-old mouse hearts revealed that aging affects the endothelial expression of genes encoding extracellular matrix proteins, specifically the laminin β1 (Lamb1) and laminin β2 (Lamb2) chains. Whereas Lamb1 was upregulated, Lamb2 was decreased in endothelial cells in old mice compared with young controls. A similar change in expression patterns was observed after induction of acute myocardial infarction. Mimicking aging and injury conditions by plating endothelial cells on laminin β1-containing laminin 411 matrix impaired endothelial cell adhesion, migration, and tube formation and augmented endothelial-to-mesenchymal transition and endothelial detachment compared with laminin 421, which contains the laminin β2 chain. Because laminins can signal via integrin receptors, we determined the activation of ITGB1 (integrin β1). Laminin 421 coating induced a higher activation of ITGB1 compared with laminin 411. siRNA-mediated silencing of ITGB1 reduced laminin β2-dependent adhesion, suggesting that laminin β2 more efficiently activates ITGB1.

CONCLUSIONS

Mimicking age-related modulation of laminin β1 versus β2 chain expression changes the functional properties and phenotype of endothelial cells. The dysregulation of the extracellular matrix during vascular aging may contribute to age-associated impairment of organ function and fibrosis.

Bone marrow laminins influence hematopoietic stem and progenitor cell cycling and homing to the bone marrow

Hematopoietic stem and progenitor cell (HSPC) functions are regulated by a specialized microenvironment in the bone marrow - the hematopoietic stem cell niche - of which the extracellular matrix (ECM) is an integral component. We describe here the localization of ECM molecules, in particular the laminin α4, α3 and α5 containing isoforms in the bone marrow. Laminin 421 (composed of laminin α4, β2, γ1 chains) is identified as a major component of the bone marrow ECM, occurring abundantly surrounding venous sinuses and in a specialized reticular fiber network of the intersinusoidal spaces of murine bone marrow (BM) in close association with HSPC. Bone marrow from Lama4^-/- mice is significantly less efficient in reconstituting the hematopoietic system of irradiated wildtype (WT) recipients in competitive bone marrow transplantation assays and shows reduced colony formation in vitro. This is partially due to retention of Lin^-c-kit⁺Sca-1⁺CD48^- long-term and short-term hematopoietic stem cells (LT-HSC/ST-HSC) in the G0 phase of the cell cycle in Lama4^-/- bone marrow and hence a more quiescent phenotype. In addition, the extravasation of WT BM cells into Lama4^-/- bone marrow is impaired, influencing the recirculation of HSPC. Our data suggest that these effects are mediated by a compensatory expression of laminin α5 containing isoforms (laminin 521/522) in Lama4^-/- bone marrow. Collectively, these intrinsic and extrinsic effects lead to reduced HSPC numbers in Lama4^-/- bone marrow and reduced hematopoietic potential.

Skeletal impairment in Pierson syndrome: Is there a role for lamininβ2 in bone physiology?

INTRODUCTION

Pierson syndrome is caused by a mutation of LAMB2, encoding for laminin β2. Clinical phenotype is variable but usually associates congenital nephrotic syndrome (CNS) and ocular abnormalities. Neuromuscular impairment has also been described.

METHODS

We report on a 15-year old girl, suffering from Pierson Syndrome, who developed severe bone deformations during puberty. This patient initially displayed CNS and microcoria, leading to the clinical diagnosis of Pierson syndrome. Genetic analysis revealed a truncating mutation and a splice site mutation of LAMB2. The patient received a renal transplantation (R-Tx) at the age of 3. After R-Tx, renal evolution was simple, the patient receiving low-dose corticosteroids, tacrolimus and mycophenolate mofetil. At the age of 12, bone deformations progressively appeared. At the time of bone impairment, renal function was subnormal (glomerular filtration rate using iohexol clearance 50mL/min per 1.73m²), and parameters of calcium/phosphate metabolism were normal (calcium 2.45mmol/L, phosphorus 1.30mmol/L, PTH 81ng/L, ALP 334U/L, 25OH-D 73nmol/L). Radiographs showed major deformations such as scoliosis, genu varum and diffuse epiphyseal abnormalities. A high resolution scanner (HR-pQCT) was performed, demonstrating a bone of "normal low" quantity and quality; major radial and cubital deformations were observed. Stainings of laminin β2 were performed on bone and renal samples from the patient and healthy controls: as expected, laminin β2 was expressed in the control kidney but not in the patient's renal tissue, and a similar pattern was observed in bone.

CONCLUSION

This is the first case of skeletal impairment ever described in Pierson syndrome. Integrin α3β1, receptor for laminin β2, are found in podocytes and osteoblasts, and the observation of both the presence of laminin β2 staining in healthy bone and its absence in the patient's bone raises the question of a potential role of laminin β2 in bone physiology.

Pathogenicity of a Human Laminin β2 Mutation Revealed in Models of Alport Syndrome

Pierson syndrome is a congenital nephrotic syndrome with eye and neurologic defects caused by mutations in laminin β2 (LAMB2), a major component of the glomerular basement membrane (GBM). Pathogenic missense mutations in human LAMB2 cluster in or near the laminin amino-terminal (LN) domain, a domain required for extracellular polymerization of laminin trimers and basement membrane scaffolding. Here, we investigated an LN domain missense mutation, LAMB2-S80R, which was discovered in a patient with Pierson syndrome and unusually late onset of proteinuria. Biochemical data indicated that this mutation impairs laminin polymerization, which we hypothesized to be the cause of the patient's nephrotic syndrome. Testing this hypothesis in genetically altered mice showed that the corresponding amino acid change (LAMB2-S83R) alone is not pathogenic. However, expression of LAMB2-S83R significantly increased the rate of progression to kidney failure in a Col4a3^-/- mouse model of autosomal recessive Alport syndrome and increased proteinuria in Col4a5^+/- females that exhibit a mild form of X-linked Alport syndrome due to mosaic deposition of collagen α3α4α5(IV) in the GBM. Collectively, these data show the pathogenicity of LAMB2-S80R and provide the first evidence of genetic modification of Alport phenotypes by variation in another GBM component. This finding could help explain the wide range of Alport syndrome onset and severity observed in patients with Alport syndrome, even for family members who share the same COL4 mutation. Our results also show the complexities of using model organisms to investigate genetic variants suspected of being pathogenic in humans.

Endothelial basement membrane laminin 511 is essential for shear stress response

Shear detection and mechanotransduction by arterial endothelium requires junctional complexes containing PECAM-1 and VE-cadherin, as well as firm anchorage to the underlying basement membrane. While considerable information is available for junctional complexes in these processes, gained largely from in vitro studies, little is known about the contribution of the endothelial basement membrane. Using resistance artery explants, we show that the integral endothelial basement membrane component, laminin 511 (laminin α5), is central to shear detection and mechanotransduction and its elimination at this site results in ablation of dilation in response to increased shear stress. Loss of endothelial laminin 511 correlates with reduced cortical stiffness of arterial endothelium in vivo, smaller integrin β1-positive/vinculin-positive focal adhesions, and reduced junctional association of actin-myosin II In vitro assays reveal that β1 integrin-mediated interaction with laminin 511 results in high strengths of adhesion, which promotes p120 catenin association with VE-cadherin, stabilizing it at cell junctions and increasing cell-cell adhesion strength. This highlights the importance of endothelial laminin 511 in shear response in the physiologically relevant context of resistance arteries.

Crystallographic analysis of the laminin β2 short arm reveals how the LF domain is inserted into a regular array of LE domains

Laminins are a major constituent of all basement membranes. The polymerisation of laminins at the cell surface is mediated by the three short arms of the cross-shaped laminin heterotrimer. The short arms contain repeats of laminin-type epidermal growth factor-like (LE) domains, interspersed with globular domains of unknown function. A single LF domain is inserted between LE5 and LE6 of the laminin β1 and β2 chains. We report the crystal structure at 1.85 Å resolution of the laminin β2 LE5-LF-LE6 region. The LF domain consists of a β-sandwich related to bacterial family 35 carbohydrate binding modules, and more distantly to the L4 domains present in the short arms of laminin α and γ chains. An α-helical region mediates the extensive interaction of the LF domain with LE5. The relative arrangement of LE5 and LE6 is very similar to that of consecutive LE domains in uninterrupted LE tandems. Fitting atomic models to a low-resolution structure of the first eight domains of the laminin β1 chain determined by small-angle X-ray scattering suggests a deviation from the regular LE array at the LE4–LE5 junction. These results advance our understanding of laminin structure.

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The crystal structure of the laminin β2 LE5-LF-L6 region has been determined.

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The LF domain is a β-sandwich with distant homology to the L4 domains in laminin α and γ chains.

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A unique α-helical region in the LF domain interacts extensively with LE5.

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LE5 and LE6 are arranged in a manner typical of tandem LE domains, despite the insertion of the LF domain.

Laminin α5 in the keratinocyte basement membrane is required for epidermal-dermal intercommunication

Laminin α5 is broadly expressed in the epidermal basement membrane (BM) of mature mice and its elimination at this site (Lama5^Ker5 mouse) results in hyperproliferation of basal keratinocytes and a delay in hair follicle development, which correlated with upregulation of the dermally-derived laminin α2 and laminin α4 chains in the epidermal BM and of tenascin-C subjacent to the BM. In vitro studies revealed laminin 511 to be strongly adhesive for primary keratinocytes and that loss of laminin α5 does not result in cell autonomous defects in proliferation. Flow cytometry reveals that the loss of laminin α5 resulted in increased numbers of CD45⁺, CD4⁺ and CD11b⁺ immune cells in the skin, which temporo-spatial analyses revealed were detectable only subsequent to the loss of laminin α5 and the appearance of the hyperproliferative keratinocyte phenotype. These findings indicate that immune cell changes are the consequence and not the cause of keratinocyte hyperproliferation. Loss of laminin α5 in the epidermal BM was also associated with changes in the expression of several dermally-derived growth factors involved in keratinocyte proliferation and hair follicle development in adult but not new born Lama5^Ker5 skin, including KGF, EGF and KGF-2. In situ binding of FGF-receptor-2α (IIIb)-Fc chimera (FGFR2IIIb) to mouse skin sections revealed decoration of several BMs, including the epidermal BM, which was absent in Lama5^Ker5 skin. This indicates reduced levels of FGFR2IIIb ligands, which include KGF and KGF-2, in the epidermal BM of adult Lama5^Ker5 skin. Our data suggest an initial inhibitory effect of laminin α5 on basal keratinocyte proliferation and migration, which is exacerbated by subsequent changes in growth factor expression by epidermal and dermal cells, implicating laminin α5 in epidermal-dermal intercommunication.

Requirement for basement membrane laminin α5 during urethral and external genital development

Hypospadias, a congenital malformation of the penis characteristic of an abnormal urethral orifice, affects 1 in every 125 boys, and its incidence is rising. Herein we test the hypothesis that the basement membrane protein laminin α5 (LAMA5) plays a key role in the development of the mouse genital tubercle, the embryonic anlage of the external genitalia. Using standard histological analyses and electron microscopy, we characterized the morphology of the external genitalia in Lama5 knockout (LAMA5-KO) mouse embryos during both androgen-independent genital tubercle development and androgen-mediated sexual differentiation. We compared regulatory gene expression between control and LAMA5-KO by in situ hybridization. We also examined the epithelial structure of the mutant genital tubercle using immunofluorescence staining and histological analyses of semi-thin sections. We found that Lama5 was expressed in both ectodermal and endodermal epithelia of the cloaca. The LAMA5-KO displayed a profound external genital malformation in which the genital tubercle was underdeveloped with a large ectopic orifice at the proximal end. In older embryos, the urethra failed to form a tubular structure and was left completely exposed. These defects were not associated with a significant alteration in regulatory gene expression, but rather with a defective ectodermal epithelium and an abnormal disintegration of the cloacal membrane. We conclude that LAMA5 is required in the basement membrane to maintain normal architecture of the ventral ectoderm during genital tubercle development, which is essential for the formation of a tubular urethra. Perturbation of LAMA5, and possibly other basement membrane components, may cause hypospadias in humans.

Neuregulin1 displayed on motor axons regulates terminal Schwann cell-mediated synapse elimination at developing neuromuscular junctions

Synaptic connections in the nervous system are rearranged during development and in adulthood as a feature of growth, plasticity, aging, and disease. Glia are implicated as active participants in these changes. Here we investigated a signal that controls the participation of peripheral glia, the terminal Schwann cells (SCs), at the neuromuscular junction (NMJ) in mice. Transgenic manipulation of the levels of membrane-tethered neuregulin1 (NRG1-III), a potent activator of SCs normally presented on motor axons, alters the rate of loss of motor inputs at NMJs during developmental synapse elimination. In addition, NMJs of adult transgenic mice that expressed excess axonal NRG1-III exhibited continued remodeling, in contrast to the more stable morphologies of controls. In fact, synaptic SCs of these adult mice with NRG1-III overexpression exhibited behaviors evident in wild type neonates during synapse elimination, including an affinity for the postsynaptic myofiber surface and phagocytosis of nerve terminals. Given that levels of NRG1-III expression normally peak during the period of synapse elimination, our findings identify axon-tethered NRG1 as a molecular determinant for SC-driven neuromuscular synaptic plasticity.

Molecular Basis of Laminin-Integrin Interactions

Laminins are composed of three polypeptide chains, designated as α, β, and γ. The C-terminal region of laminin heterotrimers, containing coiled-coil regions, short tails, and laminin globular (LG) domains, is necessary and sufficient for binding to integrins, which are the major laminin receptor class. Laminin recognition by integrins critically requires the α chain LG domains and a glutamic acid residue of the γ chain at the third position from the C-terminus. Furthermore, the C-terminal region of the β chain contains a short amino acid sequence that modulates laminin affinity for integrins. Thus, all three of the laminin chains act cooperatively to facilitate integrin binding. Mammals possess 5 α (α1-5), 3 β (β1-3), and 3 γ (γ1-3) chains, combinations of which give rise to 16 distinct laminin isoforms. Each isoform is expressed in a tissue-specific and developmental stage-specific manner, exerting its functions through binding of integrins. In this review, we detail the current knowledge surrounding the molecular basis and physiological relevance of specific interactions between laminins and integrins, and describe the mechanisms underlying laminin action through integrins.

Laminins 411 and 421 differentially promote tumor cell migration via α6β1 integrin and MCAM (CD146)

α4-laminins, such as laminins 411 and 421, are mesenchymal laminins expressed by blood and lymphatic vessels and some tumor cells. Laminin-411 promotes migration of leukocytes and endothelial cells, but the effect of this laminin and laminin-421 on tumor cells is poorly understood. In the present study, we demonstrate that laminin-411 and, to a greater extent, laminin-421 significantly promote migration of tumor cells originated from melanomas, gliomas and different carcinomas via α6β1 integrin. In solid-phase binding assays, both laminins similarly bound α6β1 integrin but only laminin-421, among several laminin isoforms, readily bound MCAM (CD146), a cell-surface adhesion molecule strongly associated with tumor progression. Accordingly, a function-blocking mAb to MCAM inhibited tumor cell migration on laminin-421 but not on laminins 411 or 521. In tumor tissues, melanoma cells co-expressed MCAM, laminin α4, β1, β2 and γ1 chains, and integrin α6 and β1 chains. The present data highlight the novel role of α4-laminins in tumor cell migration and identify laminin-421 as a primary ligand for MCAM and a putative mediator of tumor invasion and metastasis.

Rac1 activation in podocytes induces rapid foot process effacement and proteinuria

The kidney's vital filtration function depends on the structural integrity of the glomerulus, the proximal portion of the nephron. Within the glomerulus, the architecturally complex podocyte forms the final cellular barrier to filtration. Injury to the podocyte results in a morphological change called foot process effacement, which is a ubiquitous feature of proteinuric diseases. The exact mechanism underlying foot process effacement is not known, but recently it has been proposed that this change might reflect activation of the Rac1 GTPase. To test this hypothesis, we generated a podocyte-specific, inducible transgenic mouse line that expressed constitutively active Rac1. When the Rac1 transgene was induced, we observed a rapid onset of proteinuria with focal foot process effacement. Using superresolution imaging, we verified that the induced transgene was expressed in damaged podocytes with altered foot process morphology. This work sheds new light on the complex balance of Rho GTPase signaling that is required for proper regulation of the podocyte cytoskeleton.

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.

Muscles in a mouse model of spinal muscular atrophy show profound defects in neuromuscular development even in the absence of failure in neuromuscular transmission or loss of motor neurons

A mouse model of the devastating human disease "spinal muscular atrophy" (SMA) was used to investigate the severe muscle weakness and spasticity that precede the death of these animals near the end of the 2nd postnatal week. Counts of motor units to the soleus muscle as well as of axons in the soleus muscle nerve showed no loss of motor neurons. Similarly, neither immunostaining of neuromuscular junctions nor the measurement of the tension generated by nerve stimulation gave evidence of any significant impairment in neuromuscular transmission, even when animals were maintained up to 5days longer via a supplementary diet. However, the muscles were clearly weaker, generating less than half their normal tension. Weakness in 3 muscles examined in the study appears due to a severe but uniform reduction in muscle fiber size. The size reduction results from a failure of muscle fibers to grow during early postnatal development and, in soleus, to a reduction in number of fibers generated. Neuromuscular development is severely delayed in these mutant animals: expression of myosin heavy chain isoforms, the elimination of polyneuronal innervation, the maturation in the shape of the AChR plaque, the arrival of SCs at the junctions and their coverage of the nerve terminal, the development of junctional folds. Thus, if SMA in this particular mouse is a disease of motor neurons, it can act in a manner that does not result in their death or disconnection from their targets but nonetheless alters many aspects of neuromuscular development.

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.

Dystroglycan does not contribute significantly to kidney development or function, in health or after injury

Dystroglycan (DG or DAG1) is considered a critical link between the basement membrane and the cytoskeleton in multiple tissues. DG consists of two subunits, an extracellular α-subunit that binds laminin and other basement membrane components, and a transmembrane β-subunit. DG-null mouse embryos die during early embryogenesis because DG is required for Reichert's membrane formation. DG also forms an integral part of the dystrophin-glycoprotein complex in muscle. Although no human DG mutations have been reported, multiple forms of muscular dystrophy have been linked to DG glycosylation defects, and targeted deletion of muscle DG causes muscular dystrophy in mice. Moreover, DG is widely distributed in endothelial and epithelial cells, including those in the kidney. There has therefore been significant interest in DG's role in the kidney, especially in podocytes. Previous reports suggested that DG's disturbance in podocytes might cause glomerular filtration barrier abnormalities. To fully understand DG's contribution to nephrogenesis and kidney function, we used a conditional DG allele and a variety of Cre mice to systematically delete DG from podocytes, ureteric bud, metanephric mesenchyme, and then from the whole kidney. Surprisingly, none of these conditional deletions resulted in significant morphological or functional abnormalities in the kidney. Furthermore, DG-deficient podocytes did not show increased susceptibility to injury, and DG-deficient kidneys did not show delayed recovery. Integrins are therefore likely the primary extracellular matrix receptors in renal epithelia.

Basement membrane components are key players in specialized extracellular matrices

More than three decades ago, basement membranes (BMs) were described as membrane-like structures capable of isolating a cell from and connecting a cell to its environment. Since this time, it has been revealed that BMs are specialized extracellular matrices (sECMs) with unique components that support important functions including differentiation, proliferation, migration, and chemotaxis of cells during development. The composition of these sECM is as unique as the tissues to which they are localized, opening the possibility that such matrices can fulfill distinct functions. Changes in BM composition play significant roles in facilitating the development of various diseases. Furthermore, tissues have to provide sECM for their stem cells during development and for their adult life. Here, we briefly review the latest research on these unique sECM and their components with a special emphasis on embryonic and adult stem cells and their niches.

Tissue distribution of the laminin beta1 and beta2 chain during embryonic and fetal human development

Laminins are the major glycoproteins present in all basement membranes. Previously, we showed that perlecan is present during human development. Although an overview of mRNA-expression of the laminin β1 and β2 chains in various developing fetal organs is already available, a systematic localization of the laminin β1 and β2 chains on the protein level during embryonic and fetal human development is missing. Therefore, we studied the immunohistochemical expression and tissue distribution of the laminin β1 and β2 chains in various developing embryonic and fetal human organs between gestational weeks 8 and 12. The laminin β1 chain was ubiquitously expressed in the basement membrane zones of the brain, ganglia, blood vessels, liver, kidney, skin, pancreas, intestine, heart and skeletal system. Furthermore, the laminin β2 chain was present in the basement membrane zones of the brain, ganglia, skin, heart and skeletal system. The findings of this study support and expand upon the theory that these two laminin chains are important during human development.

Laminin-121--recombinant expression and interactions with integrins

Laminin-121, previously referred as to laminin-3, was expressed recombinantly in human embryonic kidney (HEK) 293 cells by triple transfection of full-length cDNAs encoding mouse laminin α1, β2 and γ1 chains. The recombinant laminin-121 was purified using Heparin-Sepharose followed by molecular sieve chromatography and shown to be correctly folded by electron microscopy and circular dichroism (CD). The CD spectra of recombinant laminin-121 were very similar to those of laminin-111 isolated from Engelbreth-Holm-Swarm tumor (EHS-laminin) but its T(m) value was smaller than EHS-laminin and recombinant lamnin-111 suggesting that the replacement of the β chain reduced the stability of the coiled-coil structure of laminin-121. Its binding to integrins was compared with EHS-laminin, laminin-3A32 purified from murine epidermal cell line and recombinantly expressed laminins-111, -211 and -221. Laminin-121 showed the highest affinity to α6β1 and α7β1 integrins and furthermore, laminin-121 most effectively supported neurite outgrowth. Together, this suggests that the β2 laminins have higher affinity for integrins than the β1 laminins.

The C-terminal region of laminin beta chains modulates the integrin binding affinities of laminins

Laminins are major cell-adhesive proteins in basement membranes that are capable of binding to integrins. Laminins consist of three chains (alpha, beta, and gamma), in which three laminin globular modules in the alpha chain and the Glu residue in the C-terminal tail of the gamma chain have been shown to be prerequisites for binding to integrins. However, it remains unknown whether any part of the beta chain is involved in laminin-integrin interactions. We compared the binding affinities of pairs of laminin isoforms containing the beta1 or beta2 chain toward a panel of laminin-binding integrins, and we found that beta2 chain-containing laminins (beta2-laminins) bound more avidly to alpha3beta1 and alpha7X2beta1 integrins than beta1 chain-containing laminins (beta1-laminins), whereas alpha6beta1, alpha6beta4, and alpha7X1beta1 integrins did not show any preference toward beta2-laminins. Because alpha3beta1 contains the "X2-type" variable region in the alpha3 subunit and alpha6beta1 and alpha6beta4 contain the "X1-type" region in the alpha6 subunit, we hypothesized that only integrins containing the X2-type region were capable of discriminating between beta1-laminins and beta2-laminins. In support of this possibility, a putative X2-type variant of alpha6beta1 was produced and found to bind preferentially to beta2-laminins. Production of a series of swap mutants between the beta1 and beta2 chains revealed that the C-terminal 20 amino acids in the coiled-coil domain were responsible for the enhanced integrin binding by beta2-laminins. Taken together, the results provide evidence that the C-terminal region of beta chains is involved in laminin recognition by integrins and modulates the binding affinities of laminins toward X2-type integrins.

A synaptic nidogen: developmental regulation and role of nidogen-2 at the neuromuscular junction

Background

The skeletal neuromuscular junction is a useful model for elucidating mechanisms that regulate synaptogenesis. Developmentally important intercellular interactions at the neuromuscular junction are mediated by the synaptic portion of a basal lamina that completely ensheaths each muscle fiber. Basal laminas in general are composed of four main types of glycosylated proteins: laminins, collagens IV, heparan sulfate proteoglycans and nidogens (entactins). The portion of the muscle fiber basal lamina that passes between the motor nerve terminal and postsynaptic membrane has been shown to bear distinct isoforms of the first three of these. For laminins and collagens IV, the proteins are deposited by the muscle; a synaptic proteoglycan, z-agrin, is deposited by the nerve. In each case, the synaptic isoform plays key roles in organizing the neuromuscular junction. Here, we analyze the fourth family, composed of nidogen-1 and -2.

Results

In adult muscle, nidogen-1 is present throughout muscle fiber basal lamina, while nidogen-2 is concentrated at synapses. Nidogen-2 is initially present throughout muscle basal lamina, but is lost from extrasynaptic regions during the first three postnatal weeks. Neuromuscular junctions in mutant mice lacking nidogen-2 appear normal at birth, but become topologically abnormal as they mature. Synaptic laminins, collagens IV and heparan sulfate proteoglycans persist in the absence of nidogen-2, suggesting the phenotype is not secondary to a general defect in the integrity of synaptic basal lamina. Further genetic studies suggest that synaptic localization of each of the four families of synaptic basal lamina components is independent of the other three.

Conclusion

All four core components of the basal lamina have synaptically enriched isoforms. Together, they form a highly specialized synaptic cleft material. Individually, they play distinct roles in the formation, maturation and maintenance of the neuromuscular junction.

Laminins promote postsynaptic maturation by an autocrine mechanism at the neuromuscular junction

A prominent feature of synaptic maturation at the neuromuscular junction (NMJ) is the topological transformation of the acetylcholine receptor (AChR)-rich postsynaptic membrane from an ovoid plaque into a complex array of branches. We show here that laminins play an autocrine role in promoting this transformation. Laminins containing the alpha4, alpha5, and beta2 subunits are synthesized by muscle fibers and concentrated in the small portion of the basal lamina that passes through the synaptic cleft at the NMJ. Topological maturation of AChR clusters was delayed in targeted mutant mice lacking laminin alpha5 and arrested in mutants lacking both alpha4 and alpha5. Analysis of chimeric laminins in vivo and of mutant myotubes cultured aneurally demonstrated that the laminins act directly on muscle cells to promote postsynaptic maturation. Immunohistochemical studies in vivo and in vitro along with analysis of targeted mutants provide evidence that laminin-dependent aggregation of dystroglycan in the postsynaptic membrane is a key step in synaptic maturation. Another synaptically concentrated laminin receptor, Bcam, is dispensable. Together with previous studies implicating laminins as organizers of presynaptic differentiation, these results show that laminins coordinate post- with presynaptic maturation.

Deletion of CD151 results in a strain-dependent glomerular disease due to severe alterations of the glomerular basement membrane

Alterations in CD151 have been associated with primary glomerular disease in both humans and mice, implicating CD151 as a key component of the glomerular filtration barrier. CD151 belongs to the tetraspanin family and associates with cell-matrix adhesion complexes such as alpha3beta1-integrin. Here we show that Cd151-deficient mice develop severe kidney disease on an FVB background but are healthy on a B6 background, providing a new and unique tool for the identification of genes that modulate the onset of proteinuria. To better understand the function of CD151 in the kidney, we studied its expression pattern and characterized early ultrastructural defects in Cd151-null kidneys. CD151 is expressed in podocytes of the mouse kidney and co-localizes with alpha3-integrin at the base of podocyte foot processes, at the site of anchorage to the glomerular basement membrane (GBM). Interestingly, the first ultrastructural lesions seen at the onset of proteinuria in Cd151-null kidneys were severe alterations of the GBM, reminiscent of Alport syndrome and consisting of massive thickening and splitting of the GBM. These lesions are associated with increased expression of GBM components. Podocyte abnormalities, effacement of foot processes, and podocyte loss appear to occur consequently to the GBM damage. In conclusion, CD151 appears to be involved in the establishment, maturation, and/or maintenance of the GBM structure in addition to its role in integrin-mediated adhesion strengthening.

Crystal structure of thioflavin T bound to the peripheral site of Torpedo californica acetylcholinesterase reveals how thioflavin T acts as a sensitive fluorescent reporter of ligand binding to the acylation site

Acetylcholinesterase plays a key role in cholinergic synaptic transmission by hydrolyzing the neurotransmitter acetylcholine with one of the highest known catalytic rate constants. Hydrolysis occurs in a narrow and deep gorge that contains two sites of ligand binding: A peripheral site, or P-site, near the gorge entrance that contributes to catalytic efficiency both by transiently trapping substrate molecules as they enter the gorge and by allosterically accelerating the transfer of the substrate acyl group to a serine hydroxyl in an acylation site or A-site at the base of the gorge. Thioflavin T is a useful reporter of ligand interactions with the A-site. It binds specifically to the P-site with fluorescence that is enhanced approximately 1000-fold over that of unbound thioflavin T, and the enhanced fluorescence is quenched 1.5- to 4-fold when another ligand binds to the A-site in a ternary complex. To clarify the structural basis of this advantageous signal change, we here report the X-ray structure of the complex of thioflavin T with Torpedo californica acetylcholinesterase. The two aromatic rings in thioflavin T are coplanar and are packed snugly parallel to the aromatic side chains of Trp279, Tyr334, and Phe330. Overlays of this structure with the crystal structures of Torpedo californica acetylcholinesterase complexes with either edrophonium or m-( N, N, N-trimethylammonio)-2,2,2-trifluoroacetophenone, two small aromatic ligands that bind specifically to the A-site, indicate that the phenyl side chain of Phe330 must rotate to sterically accommodate both thioflavin T and the A-site ligand in the ternary complex. This rotation may allow some relaxation of the strict coplanarity of the aromatic rings in the bound thioflavin T and result in partial quenching of its fluorescence.

Laminin alpha5 is necessary for submandibular gland epithelial morphogenesis and influences FGFR expression through beta1 integrin signaling

Laminin alpha chains have unique spatiotemporal expression patterns during development and defining their function is necessary to understand the regulation of epithelial morphogenesis. We investigated the function of laminin alpha5 in mouse submandibular glands (SMGs). Lama5(-/-) SMGs have a striking phenotype: epithelial clefting is delayed, although proliferation occurs; there is decreased FGFR1b and FGFR2b, but no difference in Lama1 expression; later in development, epithelial cell organization and lumen formation are disrupted. In wild-type SMGs alpha5 and alpha1 are present in epithelial clefts but as branching begins alpha5 expression increases while alpha1 decreases. Lama5 siRNA decreased branching, p42 MAPK phosphorylation, and FGFR expression, and branching was rescued by FGF10. FGFR siRNA decreased Lama5 suggesting that FGFR signaling provides positive feedback for Lama5 expression. Anti-beta1 integrin antibodies decreased FGFR and Lama5 expression, suggesting that beta1 integrin signaling provides positive feedback for Lama5 and FGFR expression. Interestingly, the Itga3(-/-):Itga6(-/-) SMGs have a similar phenotype to Lama5(-/-). Our findings suggest that laminin alpha5 controls SMG epithelial morphogenesis through beta1 integrin signaling by regulating FGFR expression, which also reciprocally regulates the expression of Lama5. These data link changes in basement membrane composition during branching morphogenesis with FGFR expression and signaling.

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.

Chondroitin sulfate perlecan enhances collagen fibril formation. Implications for perlecan chondrodysplasias

Inactivation of the perlecan gene leads to perinatal lethal chondrodysplasia. The similarity to the phenotypes of the Col2A1 knock-out and the disproportionate micromelia mutation suggests perlecan involvement in cartilage collagen matrix assembly. We now present a mechanism for the defect in collagen type II fibril assembly by perlecan-null chondrocytes. Cartilage perlecan is a heparin sulfate or a mixed heparan sulfate/chondroitin sulfate proteoglycan. The latter form binds collagen and accelerates fibril formation in vitro, with more defined fibril morphology and increased fibril diameters produced in the presence of perlecan. Interestingly, the enhancement of collagen fibril formation is independent on the core protein and is mimicked by chondroitin sulfate E but neither by chondroitin sulfate D nor dextran sulfate. Furthermore, perlecan chondroitin sulfate contains the 4,6-disulfated disaccharides typical for chondroitin sulfate E. Indeed, purified glycosaminoglycans from perlecan-enriched fractions of cartilage extracts contain elevated levels of 4,6-disulfated chondroitin sulfate disaccharides and enhance collagen fibril formation. The effect on collagen assembly is proportional to the content of the 4,6-disulfated disaccharide in the different cartilage extracts, with growth plate cartilage glycosaminoglycan being the most efficient enhancer. These findings demonstrate a role for perlecan chondroitin sulfate side chains in cartilage extracellular matrix assembly and provide an explanation for the perlecan-null chondrodysplasia.

Podocyte-Specific Deletion of Integrin-Linked Kinase Results in Severe Glomerular Basement Membrane Alterations and Progressive Glomerulosclerosis

Alterations in glomerular podocyte cell-cell and cell-matrix contacts are key events in progressive glomerular failure. Integrin-linked kinase (ILK) has been implicated in podocyte cell-matrix interaction and is induced in proteinuria. For evaluation of ILK function in vivo, mice with a Cre-mediated podocyte-specific ILK inactivation were generated. These mice seemed normal at birth but developed progressive focal segmental glomerulosclerosis and died in terminal renal failure. The first ultrastructural lesions that are seen at onset of albuminuria are glomerular basement membrane (GBM) alterations with a significant increase in true harmonic mean GBM thickness. Podocyte foot process effacement and loss of slit diaphragm followed with progression to unselective proteinuria. No significant reduction of slit membrane molecules (podocin and nephrin), key GBM components (fibronectin, laminins, and collagen IV isoforms), or podocyte integrins could be observed at onset of proteinuria. However, alpha3-integrins were relocalized into a granular pattern along the GBM, consistent with altered integrin-mediated matrix assembly in ILK-deficient podocytes. As the increased GBM thickness precedes structural podocyte lesions and key components of the GBM were expressed at comparable levels to controls, these data suggest an essential role of ILK for the close interconnection of GBM structure and podocyte function.

TGF-beta1 induces aberrant laminin chain and collagen type IV isotype expression in the glomerular basement membrane

Transforming growth factor-beta1 (TGF-beta1) contributes to the thickening of the glomerular basement membrane (GBM), abnormal deposition of extracellular matrix (ECM) therein and expansion of the mesangial matrix (MM) in several glomerular kidney diseases. However, the influence of TGF-beta1 on the expression of collagen IV isotypes and laminin chains in the GBM and the MM in vivo is not known in detail. By using transgenic mice with TGF-beta1 expression targeted to the juxtaglomerular apparatus and a combination of immunohistochemistry, Western blotting, immunoelectron microscopy and in situ hybridization, we investigated the contribution of different laminin chains and collagen type IV isotypes to the basement membrane thickening and mesangial expansion. We report that exposure of the glomerulus to TGF-beta1 in vivo induces aberrant deposition of fetal laminin alpha1, alpha2 and beta1 chains and collagen type IValpha1/alpha2 in the GBM. On the other hand, the TGF-beta1-mediated expansion of the mesangial ECM is dominated by the normal components. We found that the cellular origin of at least laminin alpha1 and alpha2 chains may be the glomerular endothelial cells. We speculate that the endothelial cells could contribute to TGF-beta1-induced glomerulopathy and should be considered as target cells for early intervention in glomerular diseases associated with TGF-beta1 in man.

Strength at the extracellular matrix-muscle interface

Mechanical force is generated within skeletal muscle cells by contraction of specialized myofibrillar proteins. This paper explores how the contractile force generated at the sarcomeres within an individual muscle fiber is transferred through the connective tissue to move the bones. The initial key point for transfer of the contractile force is the muscle cell membrane (sarcolemma) where force is transferred laterally to the basement membrane (specialized extracellular matrix rich in laminins) to be integrated within the connective tissue (rich in collagens) before transmission to the tendons. Connections between (1) key molecules outside the myofiber in the basement membrane to (2) molecules within the sarcolemma of the myofiber and (3) the internal cytoplasmic structures of the cytoskeleton and sarcomeres are evaluated. Disturbances to many components of this complex interactive system adversely affect skeletal muscle strength and integrity, and can result in severe muscle diseases. The mechanical aspects of these crucial linkages are discussed, with particular reference to defects in laminin-alpha2 and integrin-alpha7. Novel interventions to potentially increase muscle strength and reduce myofiber damage are mentioned, and these are also highly relevant to muscle diseases and aging muscle.

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.

Laminin {alpha}1 chain corrects male infertility caused by absence of laminin {alpha}2 chain

Laminins are important for basement membrane structure and function. The laminin alpha2 chain is a major component of muscle basement membranes, and mutations in the laminin alpha2 gene lead to congenital muscular dystrophy in humans and mice. Although the laminin alpha2 chain is prominently expressed in testicular basement membranes, its role in testis has remained unclear. Here, we show that laminin alpha1, alpha2, beta1, beta2, gamma 1, and gamma 3 chains are the major laminin chains in basement membranes of seminiferous tubules. In laminin alpha2 chain-deficient dy(3 K)/dy(3 ASK) mice, lack of laminin alpha2 chain led to concurrent reduction of laminin gamma 3 chain and abnormal testicular basement membranes. Seminiferous tubules of laminin alpha2 chain-deficient dy(3 K)/dy(3 K) mice displayed a defect in the timing of lumen formation, resulting in production of fewer spermatides. We also demonstrate that overexpression of laminin alpha1 chain in testis of dy(3 K)/dy(3 K) mice compensated for laminin alpha2 chain deficiency and significantly reversed the appearance of the histopathological features. We thus provide genetic data that laminin alpha chains are essential for normal testicular function in vivo.

Basement membrane composition in the early mouse embryo day 7

Basement membranes (BM) are specialized structures of the extracellular matrix known to be involved in various early developmental processes. Despite numerous investigations on the localization of BM components, it remains unknown which molecules are expressed in early developmental stages and by which germ layers these proteins are produced. Therefore, we tested for all known laminin chains, nidogens, collagen type IV, and perlecan by means of light microscopic immunostaining and performed in situ reverse transcriptase-polymerase chain reaction to detect the mRNAs specific for laminin alpha1, laminin beta1, the alpha1 chain of collagen type IV, nidogen-2, and perlecan in the early mouse embryo, day 7, in vivo. Only the laminin chains alpha1, beta1, and gamma1 were detected immunohistochemically throughout the entire endodermal and ectodermal BM zones of the embryo proper. The mRNA of laminin alpha1, laminin beta1, collagen type IV, nidogen-2 and perlecan were expressed in the ectoderm-derived mesoderm, in the endoderm as well as in the ectoderm. In contrast, Reichert's membrane was positive for all laminin chains except for the alpha4, alpha5, beta3, and gamma3 chains. Moreover, maternal epithelial as well as mesenchymal cells expressed laminins, nidogen-1 and nidogen-2, collagen type IV, and perlecan. In conclusion, laminin-1 might be the only laminin isoform in the early mouse embryo that, together with the other main BM components, nidogens, collagen type IV, and perlecan, is synthesized by all three germ layers.

Laminin-induced autoimmune myositis in rats

The present study aimed to examine if immunization with laminin causes myositis in rats and whether the pathologic findings mirror human polymyositis and dermatomyositis. Rats were immunized with an emulsion of laminin and complete Freund's adjuvant. As a result, muscle fiber necrosis with infiltrating macrophages was frequently observed and mononuclear cells were observed in the endomysium. These mononuclear cells were composed of CD4+ cells, CD8+ T cells, and macrophages. CD4+ cells and CD8+ T cells were mainly located in the endomysium, whereas a large number of macrophages were located in the endomysium and infiltrating muscle fibers. A small number of B cells, detected by immunohistochemical staining, were mainly located in the perimysium. The nonnecrotic muscle fiber to which CD4+ T cells, CD8+ T cells, and perforin+ cells adhered was negative for antimerosin and antidystrophin antibodies. Muscle fiber necrosis in rats immunized with laminin may occur after denaturation of basement membrane proteins. In conclusion, the immunization with laminin induces moderate to severe myositis. We suggest that laminin may be an important antigen for connective tissue diseases such as polymyositis and dermatomyositis.

Transient expression of laminin ?1 chain in regenerating murine liver: Restricted localization of laminin chains and nidogen-1

Most interstitia between epithelial and endothelial cells contain basal laminae (BLs), as defined by electron microscopy. However, in liver, the sinusoidal interstitium (called space of Disse) between hepatocytes and sinusoidal endothelial cells (SECs) lacks BLs. Because laminins are major components of BLs throughout the body, whether laminins exist in sinusoids has been a controversial issue. Despite recent advances, the distribution and expression of laminin chains have not been well defined in mammalian liver. Here, using a panel of antibodies, we examined laminins in normal and regenerating mouse livers. Of alpha chains, alpha5 was widely observed in all BLs except for sinusoids, while the other alpha chains were variously expressed in Glisson's sheath and central veins. Laminin gamma1 was also distributed to all BLs except for sinusoids. Although the beta2 chain was observed in all BLs and sinusoids, the expression of beta1 chain was restricted to Glisson's sheath. Detailed analysis of regenerating liver revealed that alpha1 and gamma1 chains appeared in sinusoids and were produced by stellate cells. The staining of alpha1 and gamma1 chains reached its maximum intensity at 6 days after two-thirds partial hepatectomy (PHx). Moreover, in vitro studies showed that alpha1-containing laminin promoted spreading of sinusoidal endothelial cells (SECs) isolated from normal liver, but not other hepatic cells. In addition, SECs isolated from regenerating liver elongated pseudopodia on alpha1-containing laminin more so than did cells from normal liver. The transient expression of laminin alpha1 may promote formation of sinusoids after PHx.

Laminin functions in tissue morphogenesis

Significant advances have been made in the application of genetics to probe the functions of basement membrane laminins. These studies have shown that different laminin subunits profoundly affect tissue morphogenesis, starting around the time of embryonic implantation and extending through organogenesis and into the postnatal period. Collectively they have revealed common functions that include the induction and maintenance of cell polarity, the establishment of barriers between tissue compartments, the organization of cells into tissues, and the protection of adherent cells from detachment-induced cell death, anoikis. Interpreted in light of what is known about laminin structure and self-assembly and binding activities, these advances have begun to provide insights into mechanisms of action. In this review we focus on the contributions of the laminins in invertebrate and vertebrate tissue morphogenesis.

Compositional and structural requirements for laminin and basement membranes during mouse embryo implantation and gastrulation

Laminins are components of all basement membranes and have well demonstrated roles in diverse developmental processes, from the peri-implantation period onwards. Laminin 1 (alpha1beta1gamma1) is a major laminin found at early stages of embryogenesis in both embryonic and extraembryonic basement membranes. The laminin gamma1 chain has been shown by targeted mutation to be required for endodermal differentiation and formation of basement membranes; Lamc1(-/-) embryos die within a day of implantation. We report the generation of mice lacking laminin alpha1 and laminin beta1, the remaining two laminin 1 chains. Mutagenic insertions in both Lama1 and Lamb1 were obtained in a secretory gene trap screen. Lamb1(-/-) embryos are similar to Lamc1(-/-) embryos in that they lack basement membranes and do not survive beyond embryonic day (E) 5.5. However, in Lama1(-/-) embryos, the embryonic basement membrane forms, the embryonic ectoderm cavitates and the parietal endoderm differentiates, apparently because laminin 10 (alpha5beta1gamma1) partially compensates for the absent laminin 1. However, such compensation did not occur for Reichert's membrane, which was absent, and the embryos died by E7. Overexpression of laminin alpha5 from a transgene improved the phenotype of Lama1(-/-) embryos to the point that they initiated gastrulation, but this overexpression did not rescue Reichert's membrane, and trophoblast cells did not form blood sinuses. These data suggest that both the molecular composition and the integrity of basement membranes are crucial for early developmental events.

Laminins alpha2 and alpha4 in pancreatic acinar basement membranes are required for basal receptor localization

Basement membranes (BMs) are thin layers of extracellular matrix (ECM) found at the basal surface of many cell types, including epithelial cells. BMs present growth, differentiation, and anti-apoptotic signals and provide structural support to cells, compartmentalize tissues, and serve as filters. The structure and function of BMs depend on their complement of laminins, a family of alpha beta gamma heterotrimeric glycoproteins. We found that laminins containing the alpha2 and alpha4 chains are the major laminins in pancreatic acinar BMs. Importantly, these laminins were required for proper basal localization on acinar cells of two laminin receptors, dystroglycan and integrin alpha6beta4 .

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.