Exon Skipping Using Antisense Oligonucleotides for Laminin-Alpha2-Deficient Muscular Dystrophy

Phosphorodiamidate morpholino oligomer (PMO)-mediated exon skipping is among the more promising approaches available for the treatment of several neuromuscular disorders, including Duchenne muscular dystrophy. The main weakness of this treatment arises from the low efficiency and sporadic nature of delivery of the neutrally charged PMO into muscle fibers, the mechanism of which is unknown.Recently, using wild-type and dystrophic mdx52 mice, we showed that muscle fibers took up PMO more efficiently during myotube formation. Interestingly, through in situ hybridization, we detected PMO mainly in embryonic myosin heavy chain-positive regenerating fibers. Next, we tested the therapeutic potential of PMO in laminin-alpha2 (laminin-α2) chain-null dy ^3K/dy ^3K mice, a model of merosin-deficient congenital muscular dystrophy 1A (MDC1A) with active muscle regeneration. We confirmed the recovery of the laminin-α2 chain following skipping of the mutated exon 4 in dy ^3K/dy ^3K mice, which prolonged the life span of the animals slightly. These findings support the theory that PMO entry into fibers is dependent on the developmental stage in myogenesis rather than on dystrophinless muscle membranes, and provide a platform for the future development of PMO-mediated therapies for a variety of muscular disorders, such as MDC1A, that involve active muscle regeneration. Herein, we describe the methods for PMO transfection/injection and evaluation of the efficacy of exon skipping in the laminin-α2-deficient dy ^3K/dy ^3K mouse model both in vitro and in vivo.

Natural disease history of the dy2J mouse model of laminin α2 (merosin)-deficient congenital muscular dystrophy

Merosin deficient congenital muscular dystrophy 1A (MDC1A) is a very rare autosomal recessive disorder caused by mutations in the LAMA2 gene leading to severe and progressive muscle weakness and atrophy. Although over 350 causative mutations have been identified for MDC1A, no treatment is yet available. There are many therapeutic approaches in development, but the lack of natural history data of the mouse model and standardized outcome measures makes it difficult to transit these pre-clinical findings to clinical trials. Therefore, in the present study, we collected natural history data and assessed pre-clinical outcome measures for the dy^2J/dy^2J mouse model using standardized operating procedures available from the TREAT-NMD Alliance. Wild type and dy^2J/dy^2J mice were subjected to five different functional tests from the age of four to 32 weeks. Non-tested control groups were taken along to assess whether the functional test regime interfered with muscle pathology. Respiratory function, body weights and creatine kinase levels were recorded. Lastly, skeletal muscles were collected for further histopathological and gene expression analyses. Muscle function of dy^2J/dy^2J mice was severely impaired at four weeks of age and all mice lost the ability to use their hind limbs. Moreover, respiratory function was altered in dy^2J/dy^2J mice. Interestingly, the respiration rate was decreased and declined with age, whereas the respiration amplitude was increased in dy^2J/dy^2J mice when compared to wild type mice. Creatine kinase levels were comparable to wild type mice. Muscle histopathology and gene expression analysis revealed that there was a specific regional distribution pattern of muscle damage in dy^2J/dy^2J mice. Gastrocnemius appeared to be the most severely affected muscle with a high proportion of atrophic fibers, increased fibrosis and inflammation. By contrast, triceps was affected moderately and diaphragm only mildly. Our study presents a complete natural history dataset which can be used in setting up standardized studies in dy^2J/dy^2J mice.

Absence of microRNA-21 does not reduce muscular dystrophy in mouse models of LAMA2-CMD

MicroRNAs (miRNAs) are short non-coding RNAs that modulate gene expression post-transcriptionally. Current evidence suggests that miR-21 plays a significant role in the progression of fibrosis in muscle diseases. Laminin-deficient congenital muscular dystrophy (LAMA2-CMD) is a severe form of congenital muscular dystrophy caused by mutations in the gene encoding laminin α2 chain. Mouse models dy^3K/dy^3K and dy^2J/dy^2J, respectively, adequately mirror severe and milder forms of LAMA2-CMD. Both human and mouse LAMA2-CMD muscles are characterized by extensive fibrosis and considering that fibrosis is the final step that destroys muscle during the disease course, anti-fibrotic therapies may be effective strategies for prevention of LAMA2-CMD. We have previously demonstrated a significant up-regulation of the pro-fibrotic miR-21 in dy^3K/dy^3K and dy^2J/dy^2J skeletal muscle. Hence, the objective of this study was to explore if absence of miR-21 reduces fibrogenesis and improves the phenotype of LAMA2-CMD mice. Thus, we generated dy^3K/dy^3K and dy^2J/dy^2J mice devoid of miR-21 (dy^3K/miR-21 and dy^2J/miR-21 mice, respectively). However, the muscular dystrophy phenotype of dy^3K/miR-21 and dy^2J/miR-21 double knock-out mice was not improved compared to dy^3K/dy^3K or dy^2J/dy^2J mice, respectively. Mice displayed the same body weight, dystrophic muscles (with fibrosis) and impaired muscle function. These data indicate that miR-21 may not be involved in the development of fibrosis in LAMA2-CMD.

Congenital myasthenic syndromes: pathogenesis, diagnosis, and treatment

The congenital myasthenic syndromes (CMS) are a diverse group of genetic disorders caused by abnormal signal transmission at the motor endplate, a special synaptic contact between motor axons and each skeletal muscle fibre. Most CMS stem from molecular defects in the muscle nicotinic acetylcholine receptor, but they can also be caused by mutations in presynaptic proteins, mutations in proteins associated with the synaptic basal lamina, defects in endplate development and maintenance, or defects in protein glycosylation. The specific diagnosis of some CMS can sometimes be reached by phenotypic clues pointing to the mutated gene. In the absence of such clues, exome sequencing is a useful technique for finding the disease gene. Greater understanding of the mechanisms of CMS have been obtained from structural and electrophysiological studies of the endplate, and from biochemical studies. Present therapies for the CMS include cholinergic agonists, long-lived open-channel blockers of the acetylcholine receptor ion channel, and adrenergic agonists. Although most CMS are treatable, caution should be exercised as some drugs that are beneficial in one syndrome can be detrimental in another.

Laminin-α2 Chain-Deficient Congenital Muscular Dystrophy: Pathophysiology and Development of Treatment

Laminin-211 is a major constituent of the skeletal muscle basement membrane. It stabilizes skeletal muscle and influences signal transduction events from the myomatrix to the muscle cell. Mutations in the gene encoding the α2 chain of laminin-211 lead to congenital muscular dystrophy type 1A (MDC1A), a life-threatening disease characterized by severe hypotonia, progressive muscle weakness, and joint contractures. Common complications include severely impaired motor ability, respiratory failure, and feeding difficulties. Several adequate animal models for laminin-α2 chain deficiency exist and analyses of different MDC1A mouse models have led to a significant improvement in our understanding of MDC1A pathogenesis. Importantly, the animal models have been indispensable tools for the preclinical development of new therapeutic approaches for laminin-α2 chain deficiency, highlighting a number of important disease driving mechanisms that can be targeted by pharmacological approaches. In this chapter, I will describe laminin-211 and discuss the cellular and molecular pathophysiology of MDC1A as well as progression toward development of treatment.

The cellular origin of laminin determines its role in blood pressure regulation

Laminin of different cellular sources has distinct functions. In addition to vascular smooth muscle cells (SMCs), aorta also contains a small population of nestin(+) cells, whose function remains unknown. This study investigates the role of SMC- and nestin(+) cell-derived laminin in blood pressure (BP) regulation and SMC contractibility. Using mice with laminin deficiency in SMCs (SKO) or nestin(+) cells (NKO), we examined laminin-dependent changes in BP. Contractile protein expression was reduced in SKO but not NKO mice, consistent with their, respectively, low and normal baseline BP measurements. At the ultrastructural level, SKO SMCs maintained the contractile phenotype with reduced elasticity, whereas NKO SMCs switched to the synthetic phenotype and showed degeneration. Additionally, angiotensin II (Ang II) significantly increased BP in SKO but not NKO mice. It also enhanced contractile proteins to the same levels and induced SMC degeneration in both knockout mice. These data suggest that SMC laminin regulates BP via modulating contractile protein expression, whereas nestin(+) cell-derived laminin contributes to SMC phenotypic switch.

[Congenital muscular dystrophies in children]

From the clinical and genetic point of view, congenital muscular dystrophies (CMD) are a heterogenic group of diseases within neuromuscular pathologies. The best known forms are: merosin deficiency CMD, collagen VI deficiency CMD, LMNA-related CMD, selenoprotein-related CMD (SEPN1) and alpha-dystroglycan-related CMD. They present with a broad spectrum of clinical phenotypes. Most of them are transmitted by recessive autosomal inheritance. The initial manifestations very often begin in infancy or in the neonatal period. There are clinical suspicions of the existence of hypotonia and paresis, and they are characterised by a dystrophic pattern in the muscular biopsy (muscle replaced by fibroadipose tissue, with necrosis and cell regeneration). Advances in the understanding of the molecular pathogenesis of CMD have made it possible to make further progress in the classification of the different subtypes. The aim of this review is to comment on the advances made in recent years as regards the classification of CMD in terms of genetics, the proteins involved and their clinical presentation.

Omigapil treatment decreases fibrosis and improves respiratory rate in dy(2J) mouse model of congenital muscular dystrophy

Introduction

Congenital muscular dystrophy is a distinct group of diseases presenting with weakness in infancy or childhood and no current therapy. One form, MDC1A, is the result of laminin alpha-2 deficiency and results in significant weakness, respiratory insufficiency and early death. Modification of apoptosis is one potential pathway for therapy in these patients.

Methods

dy^2J mice were treated with vehicle, 0.1 mg/kg or 1 mg/kg of omigapil daily via oral gavage over 17.5 weeks. Untreated age matched BL6 mice were used as controls. Functional, behavioral and histological measurements were collected.

Results

dy^2J mice treated with omigapil showed improved respiratory rates compared to vehicle treated dy^2J mice (396 to 402 vs. 371 breaths per minute, p<0.03) and similar to control mice. There were no statistical differences in normalized forelimb grip strength between dy^2J and controls at baseline or after 17.5 weeks and no significant differences seen among the dy^2J treatment groups. At 30–33 weeks of age, dy^2J mice treated with 0.1 mg/kg omigapil showed significantly more movement time and less rest time compared to vehicle treated. dy^2J mice showed normal cardiac systolic function throughout the trial. dy^2J mice had significantly lower hindlimb maximal (p<0.001) and specific force (p<0.002) compared to the control group at the end of the trial. There were no statistically significant differences in maximal or specific force among treatments. dy^2J mice treated with 0.1 mg/kg/day omigapil showed decreased percent fibrosis in both gastrocnemius (p<0.03) and diaphragm (p<0.001) compared to vehicle, and in diaphragm (p<0.013) when compared to 1 mg/kg/day omigapil treated mice. Omigapil treated dy^2J mice demonstrated decreased apoptosis.

Conclusion

Omigapil therapy (0.1 mg/kg) improved respiratory rate and decreased skeletal and respiratory muscle fibrosis in dy^2J mice. These results support a putative role for the use of omigapil in laminin deficient congenital muscular dystrophy patients.

Functional consequences of cell type-restricted expression of laminin α5 in mouse placental labyrinth and kidney glomerular capillaries

The labyrinth is the highly vascularized part of the rodent placenta that allows efficient transfer of gases, nutrients, wastes, and other molecules between the maternal and embryonic circulations. These two blood compartments are separated by blastocyst-derived trophoblasts and endothelial cells with an intervening basement membrane that contains laminin and other typical basement membrane components. Previously we reported that the labyrinth of laminin α5 knockout (LMα5−/−) embryos exhibits reduced vascularization and detachment of endothelial cells from the basement membrane, which normally contains LMα5. As very little is known about the origin of this vascular basement membrane, we investigated the cellular requirements for LMα5 expression in the mouse placental labyrinth. By fluorescence-activated cell sorting and RT-PCR we confirmed that both endothelial cells and trophoblasts normally express LMα5. Using Cre-loxP technology and doxycycline-mediated gene expression, we generated genetically mosaic placentas in which either the trophoblasts or the endothelial cells, but not both, expressed LMα5. We found that the overall architecture of the labyrinth was normal as long as one of these two cell types expressed LMα5, even if it was transgene-derived human laminin α5. These results suggest that laminin trimers containing α5 that are synthesized and secreted by endothelium or by trophoblasts are capable of integrating into the basement membrane and promoting normal vascularization of the placenta. Additional studies showed that endothelium-expressed human LMα5 can support vascularization of the kidney glomerulus, consistent with previous studies using a tissue grafting approach.

Ex vivo stretch reveals altered mechanical properties of isolated dystrophin-deficient hearts

Duchenne muscular dystrophy (DMD) is a progressive and fatal disease of muscle wasting caused by loss of the cytoskeletal protein dystrophin. In the heart, DMD results in progressive cardiomyopathy and dilation of the left ventricle through mechanisms that are not fully understood. Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes. To expand upon these findings, here we have subjected the left ventricles of dystrophin-deficient mdx hearts to mechanical stretch. Unexpectedly, isolated mdx hearts showed increased left ventricular (LV) compliance compared to controls during stretch as LV volume was increased above normal end diastolic volume. During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts. This suggests that the mechanical properties of the intact heart cannot be modeled as a simple extrapolation of findings in single cardiac myocytes. To explain these findings, a model is proposed in which disruption of the dystrophin-glycoprotein complex perturbs cell-extracellular matrix contacts and promotes the apparent slippage of myocytes past each other during LV distension. In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan-null and laminin-α₂ mutant mice, but not in dysferlin-null mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes. Collectively, these findings suggest a novel and cell-death independent mechanism for the progressive pathological LV dilation that occurs in DMD.

A new role for laminins as modulators of protein toxicity in Caenorhabditis elegans

Protein misfolding is a common theme in aging and several age-related diseases such as Alzheimer's and Parkinson's disease. The processes involved in the development of these diseases are many and complex. Here, we show that components of the basement membrane (BM), particularly laminin, affect protein integrity of the muscle cells they support. We knocked down gene expression of epi-1, a laminin α-chain, and found that this resulted in increased proteotoxicity in different Caenorhabditis elegans transgenic models, expressing aggregating proteins in the body wall muscle. The effect could partially be rescued by decreased insulin-like signaling, known to slow the aging process and the onset of various age-related diseases. Our data points to an underlying molecular mechanism involving proteasomal degradation and HSP-16 chaperone activity. Furthermore, epi-1-depleted animals had altered synaptic function and displayed hypersensitivity to both levamisole and aldicarb, an acetylcholine receptor agonist and an acetylcholinesterase inhibitor, respectively. Our results implicate the BM as an extracellular modulator of protein homeostasis in the adjacent muscle cells. This is in agreement with previous research showing that imbalance in neuromuscular signaling disturbs protein homeostasis in the postsynaptic cell. In our study, proteotoxicity may indeed be mediated by the neuromuscular junction which is part of the BM, where laminins are present in high concentration, ensuring the proper microenvironment for neuromuscular signaling. Laminins are evolutionarily conserved, and thus the BM may play a much more causal role in protein misfolding diseases than currently recognized.

Peripheral nerve pathology, including aberrant Schwann cell differentiation, is ameliorated by doxycycline in a laminin-α2-deficient mouse model of congenital muscular dystrophy

The most common form of childhood congenital muscular dystrophy, Type 1A (MDC1A), is caused by mutations in the human LAMA2 gene that encodes the laminin-α2 subunit. In addition to skeletal muscle deficits, MDC1A patients typically show a loss of peripheral nerve function. To identify the mechanisms underlying this loss of nerve function, we have examined pathology and cell differentiation in sciatic nerves and ventral roots of the laminin-α2-deficient (Lama2(-/-)) mice, which are models for MDC1A. We found that, compared with wild-type, sciatic nerves of Lama2(-/-) mice had a significant increase in both proliferating (Ki67+) cells and premyelinating (Oct6+) Schwann cells, but also had a significant decrease in both immature/non-myelinating [glial fibrillary acidic protein (GFAP)(+)] and myelinating (Krox20+) Schwann cells. To extend our previous work in which we found that doxycycline, which has multiple effects on mammalian cells, improves motor behavior and more than doubles the median life-span of Lama2(-/-) mice, we also determined how nerve pathology was affected by doxycycline treatment. We found that myelinating (Krox20+) Schwann cells were significantly increased in doxycycline-treated compared with untreated sciatic nerves. In addition, doxycycline-treated peripheral nerves had significantly less pathology as measured by assays such as amount of unmyelinated or disorganized axons. This study thus identified aberrant proliferation and differentiation of Schwann cells as key components of pathogenesis in peripheral nerves and provided proof-of-concept that pharmaceutical therapy can be of potential benefit for peripheral nerve dysfunction in MDC1A.

Lack of laminin gamma1 in embryonic stem cell-derived cardiomyocytes causes inhomogeneous electrical spreading despite intact differentiation and function

Laminins form a large family of extracellular matrix (ECM) proteins, and their expression is a prerequisite for normal embryonic development. Herein we investigated the role of the laminin gamma1 chain for cardiac muscle differentiation and function using cardiomyocytes derived from embryonic stem cells deficient in the LAMC1 gene. Laminin gamma1 (-/-) cardiomyocytes lacked basement membranes (BM), whereas their sarcomeric organization was unaffected. Accordingly, electrical activity and hormonal regulation were found to be intact. However, the inadequate BM formation led to an increase of ECM deposits between adjacent cardiomyocytes, and this resulted in defects of the electrical signal propagation. Furthermore, we also found an increase in the number of pacemaker areas. Thus, although laminin and intact BM are not essential for cardiomyocyte development and differentiation per se, they are required for the normal deposition of matrix molecules and critical for intact electrical signal propagation.

Genetic and pharmacologic inhibition of mitochondrial-dependent necrosis attenuates muscular dystrophy

Muscular dystrophies comprise a diverse group of genetic disorders that lead to muscle wasting and, in many instances, premature death. Many mutations that cause muscular dystrophy compromise the support network that connects myofilament proteins within the cell to the basal lamina outside the cell, rendering the sarcolemma more permeable or leaky. Here we show that deletion of the gene encoding cyclophilin D (Ppif) rendered mitochondria largely insensitive to the calcium overload-induced swelling associated with a defective sarcolemma, thus reducing myofiber necrosis in two distinct models of muscular dystrophy. Mice lacking delta-sarcoglycan (Scgd(-/-) mice) showed markedly less dystrophic disease in both skeletal muscle and heart in the absence of Ppif. Moreover, the premature lethality associated with deletion of Lama2, encoding the alpha-2 chain of laminin-2, was rescued, as were other indices of dystrophic disease. Treatment with the cyclophilin inhibitor Debio-025 similarly reduced mitochondrial swelling and necrotic disease manifestations in mdx mice, a model of Duchenne muscular dystrophy, and in Scgd(-/-) mice. Thus, mitochondrial-dependent necrosis represents a prominent disease mechanism in muscular dystrophy, suggesting that inhibition of cyclophilin D could provide a new pharmacologic treatment strategy for these diseases.

Long-term follow-up of laminin alpha2 (merosin)-deficient muscular dystrophy in a cat

Progressive muscle weakness beginning at 6 months of age was observed in a male Persian-mix cat. Muscle atrophy and joint contracture progressed over the next 3 years. The cat had developed gait difficulty at 8 months of age. The cat died at age of 5 years and 3 months due to an acute respiratory disorder. The clinical, laboratory, necropsy and histopathological findings of the cat were consistent with those of muscular dystrophy. The cat was diagnosed as having laminin alpha2 (merosin)-deficient muscular dystrophy on the basis of immunohistochemical findings. The cat was born in an inbred colony, and another related cat exhibited similar clinical signs. Few cases of laminin alpha2-deficient muscular dystrophy have been reported in cats, and this report provides additional information about the disease.

Isotonic fatigue in laminin alpha2-deficient dy/dy dystrophic mouse diaphragm

Laminin alpha2 deficiency causes approximately 50% of human congenital muscular dystrophies. Muscle in the corresponding dy/dy mouse model has reduced force but increased fatigue resistance during isometric contractions. To determine whether a similar pattern of alterations is present during isotonic contractions, dy/dy diaphragm was studied in vitro. During 20% load, dystrophic diaphragm had significantly reduced shortening, shortening velocity, work and power deficits, which persisted during the fatigue-inducing stimulation. In contrast, during 40% load, isotonic contractile performance of diseased muscle was impaired only mildly and only for some contractile parameters. At both loads, rate of isotonic fatigue when expressed relative to initial contractile values was similar for dystrophic and normal muscle, or in some instances slightly higher for dystrophic muscle. Therefore, fatigue resistance is considerably impaired during isotonic contractions relative to that reported previously for isometric contractions. This has important implications for increased susceptibility to respiratory failure in laminin alpha2-deficient muscular dystrophy.

Merosin-deficient congenital muscular dystrophy type 1A

Merosin-deficient congenital muscular dystrophy type 1A (MDC1A) is the most common form of congenital muscular dystrophy. MDC1A is caused by mutation of the laminin alpha-2 gene (LAMA2), localized to chromosome 6q22-23. The diagnosis of merosin-deficient CMD is based on the clinical findings of severe congenital hypotonia, weakness, with high blood levels of creatine kinase, WM abnormalities, and dystrophy associated with negative immunostaining of biopsied muscle for merosin. We investigated clinical and laboratory a patient: a girl with merosin-deficient congenital muscular dystrophy type 1A. Clinically the particularity of the case is the association of merosin-negative congenital muscular dystrophy (MN-CMD) with congenital feet deformity. The level of serum creatine kinase is elevated 1045 U/L. Immunohistochemistry show presence of dystrophin, lack of merosin, also the utrophin is normally expressed. Nerve conduction studies are normally, while electromyography suggested a myopathic process with early recruitment and decreased amplitude and duration of response. Magnetic resonance imaging: MRI T1 and MRI T2 show hypointensity and diffuse hyperintensity respectively in the white matter. Supratentorial MRI images showed hypotrophy of the corpus callosum and almost absent cingulate gyrus. In addition, hypophysis is reduced size.

LAMA2 mRNA processing alterations generate a complete deficiency of laminin-alpha2 protein and a severe congenital muscular dystrophy

An increasing number of genomic variations are no more regarded as harmless changes in protein coding sequences or as genetic polymorphisms. Studying the impact of these variations on mRNA metabolism became a central issue to better understand the biological significance of disease. We describe here a severe congenital muscular dystrophy (CMD) with lumbar scoliosis and respiratory complications in a patient, who died at the age of 10. Despite a poor linkage to any form of CMD, total deficiency of laminin-alpha2 rather suggested the occurrence of an MDC1A form. Extensive analysis of LAMA2 gene revealed two novel mutations: a (8007delT) frameshift deletion in exon 57, and a de novo 7nt deletion in intron 17. Using an ex vivo approach, we provided strong evidence that the intron mutation is responsible for complete exon 17 skipping. The mutations are in trans and they each generate a nonsense mRNA potentially elicited to degradation by NMD. We further discuss the impact of mRNA alterations on the subtle phenotypic discrepancies.

Severe congenital muscular dystrophy in a LAMA2-mutated case

Clinical features and molecular data are described for a patient with undetectable expression of laminin alpha2 chain (merosin) and severe congenital muscular dystrophy. Molecular analysis of the LAMA2 gene revealed two previously un-described mutations. The patient achieved independent sitting at age 2, but lost head balance at age 7; he was never able to stand unsupported. Cerebral magnetic resonance imaging revealed diffuse hypomyelination in both cerebral hemispheres; electrophysiological assessment revealed progressive sensorimotor axonal polyneuropathy. Investigation of the primary molecular defect in congenital muscular dystrophy patients is important for genetic counseling, because the clinical features of the various forms overlap, and because significant laminin alpha2 chain reduction may occur in patients with primary defects in other genes.

Laminin deficits induce alterations in the development of dopaminergic neurons in the mouse retina

Genetically modified mice lacking the beta2 laminin chain (beta2null), the gamma3 laminin chain (gamma3 null), or both beta2/gamma3 chains (compound null) were produced. The development of tyrosine hydroxylase (TH) immunoreactive neurons in these mouse lines was studied between birth and postnatal day (P) 20. Compared to wild type mice, no alterations were seen in gamma3 null mice. In beta2 null mice, however, the large, type I TH neurons appeared later in development, were at a lower density and had reduced TH immunoreactivity, although TH process number and size were not altered. In the compound null mouse, the same changes were observed together with reduced TH process outgrowth. Surprisingly, in the smaller, type II TH neurons, TH immunoreactivity was increased in laminin-deficient compared to wild type mice. Other retinal defects we observed were a patchy disruption of the inner limiting retinal basement membrane and a disoriented growth of Müller glial cells. Starburst and AII type amacrine cells were not apparently altered in laminin-deficient relative to wild type mice. We postulate that laminin-dependent developmental signals are conveyed to TH amacrine neurons through intermediate cell types, perhaps the Müller glial cell and/or the retinal ganglion cell.

Quantitative proton MRS of cerebral metabolites in laminin alpha2 chain deficiency

Congenital muscular dystrophy (CMD) due to merosin (laminin alpha2 chain) deficiency is an autosomal recessively inherited disorder characterized by severe muscular weakness and hypotonia from birth on. Brain involvement is the rule and characterized by variable T2 hyperintensities of white matter which appears swollen on cranial MRI. The pathophysiology of these white matter changes is not clear. In five patients with laminin alpha2 deficient CMD we performed short-echo time localized proton MRS with determination of absolute metabolite concentrations in grey and white matter. In affected white matter, a consistent pattern of metabolites was detected comprising reduced concentrations of N-acetylaspartate and N-acetylaspartylglutamate, creatine, and phosphocreatine, and to a milder degree of choline-containing compounds. In contrast, concentrations of myo-inositol were in the normal range. Spectra of cortical and subcortical grey matter were normal. The observed metabolite profile is consistent with white matter edema, that is reduced cellular density, and relative astrocytosis. This interpretation is in line with the hypothesis that laminin alpha2 deficiency results in leakage of fluids across the blood-brain barrier and a histopathological report of astrocytic proliferation in CMD.

MR spectroscopy and diffusion tensor imaging of the brain in congenital muscular dystrophy with merosin deficiency: metabolite level decreases, fractional anisotropy decreases, and apparent diffusion coefficient increases in the white matter

Brain magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) in one patient with merosin-deficient congenital muscular dystrophy (MDCMD) revealed significant metabolite (choline, creatine, N-acetyl aspartate) level reductions, fractional anisotropy (FA) reduction and increased apparent diffusion coefficient (ADC) in the white matter (p<0.01, all). In the gray matter, the MRS properties did not differ significantly from those in controls. The ADC and FA, however, differed significantly as in the white matter, although the differences were less pronounced. This is the first quantitative MR study of the brain in a patient with MDCMD, which revealed that the concentrations of all MRS measured metabolites were decreased only in the white matter. This observation, combined with the DTI observed ADC increases and FA decrease, indicated a presence of vasogenic edema in the white matter.

Linker molecules between laminins and dystroglycan ameliorate laminin-alpha2-deficient muscular dystrophy at all disease stages

Mutations in laminin-α2 cause a severe congenital muscular dystrophy, called MDC1A. The two main receptors that interact with laminin-α2 are dystroglycan and α7β1 integrin. We have previously shown in mouse models for MDC1A that muscle-specific overexpression of a miniaturized form of agrin (mini-agrin), which binds to dystroglycan but not to α7β1 integrin, substantially ameliorates the disease (Moll, J., P. Barzaghi, S. Lin, G. Bezakova, H. Lochmuller, E. Engvall, U. Muller, and M.A. Ruegg. 2001. Nature. 413:302–307; Bentzinger, C.F., P. Barzaghi, S. Lin, and M.A. Ruegg. 2005. Matrix Biol. 24:326–332.). Now we show that late-onset expression of mini-agrin still prolongs life span and improves overall health, although not to the same extent as early expression. Furthermore, a chimeric protein containing the dystroglycan-binding domain of perlecan has the same activities as mini-agrin in ameliorating the disease. Finally, expression of full-length agrin also slows down the disease. These experiments are conceptual proof that linking the basement membrane to dystroglycan by specifically designed molecules or by endogenous ligands, could be a means to counteract MDC1A at a progressed stage of the disease, and thus opens new possibilities for the development of treatment options for this muscular dystrophy.

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.

The zebrafish candyfloss mutant implicates extracellular matrix adhesion failure in laminin alpha2-deficient congenital muscular dystrophy

Mutations in the human laminin alpha2 (LAMA2) gene result in the most common form of congenital muscular dystrophy (MDC1A). There are currently three models for the molecular basis of cellular pathology in MDC1A: (i) lack of LAMA2 leads to sarcolemmal weakness and failure, followed by cellular necrosis, as is the case in Duchenne muscular dystrophy (DMD); (ii) loss of LAMA2-mediated signaling during the development and maintenance of muscle tissue results in myoblast proliferation and fusion defects; (iii) loss of LAMA2 from the basement membrane of the Schwann cells surrounding the peripheral nerves results in a lack of motor stimulation, leading to effective denervation atrophy. Here we show that the degenerative muscle phenotype in the zebrafish dystrophic mutant, candyfloss (caf) results from mutations in the laminin alpha2 (lama2) gene. In vivo time-lapse analysis of mechanically loaded fibers and membrane permeability assays suggest that, unlike DMD, fiber detachment is not initially associated with sarcolemmal rupture. Early muscle formation and myoblast fusion are normal, indicating that any deficiency in early Lama2 signaling does not lead to muscle pathology. In addition, innervation by the primary motor neurons is unaffected, and fiber detachment stems from muscle contraction, demonstrating that muscle atrophy through lack of motor neuron activity does not contribute to pathology in this system. Using these and other analyses, we present a model of lama2 function where fiber detachment external to the sarcolemma is mechanically induced, and retracted fibers with uncompromised membranes undergo subsequent apoptosis.

[Bart syndrome associated to lethal junctional epidermolysis bullosa (Herlitz form)]

We present the case of a newborn with congenital absence of skin in the anterior part of the left leg that shortly after developed bulla and erosions in hands, feet, ears, buttocks and mouth. The cutaneous biopsy and ultrastructural and immunohistochemical studies showed a subepidermal bulla in the lamina lucida, absence of hemidesmosomes and marked decrease of laminin 5, thus establishing the diagnosis of Bart syndrome associated to the Herlitz form of lethal junctional epidermolysis bullosa. Bart syndrome consists of congenital and localized absence of skin, nail abnormalities and mucoc-cutaneous bullae. It is usually associated to dystrophic epidermolysis bullosa. The Herlitz form of junctional epidermolysis bullosa is a rare variant, usually lethal that is produced by mutations in the genes coding for the anchor protein laminin 5. To our knowledge this is the second case that reports an association between Bart syndrome and lethal junctional epidermolysis bullosa and the first in which the results of immunofluorescence mapping are published.

Thymus acetylcholinesterase activity is reduced in mice with congenital muscular dystrophy

Lama2dy mice constitute an animal model for congenital muscular dystrophy (CMD) by merosin (laminin alpha2-chain) deficiency. This pathology affects the properties of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) of mouse skeletal muscle and nerves (Moral-Naranjo et al., 1999, 2002). AChE and BChE are involved through catalytic and noncatalytic actions in multiple processes, such as hydrolysis of acetylcholine (ACh), morphogenesis, hematopoiesis, and tumorigenesis (Soreq and Seidman, 2001). AChE and BChE molecules can be globular (G1, G2, and G4) or asymmetric forms (A4, A8, and A12) (Massoulié, 2002), and G molecules can show amphiphilic (detergent-interacting, GA) or hydrophilic (GH) behavior. AChE catalytic subunits are encoded by three mRNAs (T, H, or R) generated by alternative splicing. The presence of AChE in lymphoid tissues (Rossi et al., 1991; Nieto-Cerón et al., 2004), the role of immune responses in muscular dystrophy (Spencer and Tidball, 2001), the abnormalities of Lama2dy thymus (Magner et al., 2000), and the role of ACh in thymocyte function (Kawashima and Fujii, 2000) prompted us to investigate thymus AChE and the possible effect of merosin deficiency on it.

Laminins containing the beta2 chain modulate the precise organization of CNS synapses

Synapses are formed and stabilized by concerted interactions of pre-, intra-, and post-synaptic components; however, the precise nature of the intrasynaptic components in the CNS remains obscure. Potential intrasynaptic components include extracellular matrix molecules such as laminins; here, we isolate beta2-containing laminins, including perhaps laminins 13 (alpha3beta2gamma3) and 14 (alpha4beta2gamma3), from CNS synaptosomes suggesting a role for these molecules in synaptic organization. Indeed, hippocampal synapses that form in vivo in the absence of these laminins are malformed at the ultrastructural level and this malformation is replicated in synapses formed in vitro, where laminins are provided largely by the post-synaptic neuron. This recapitulation of the in vivo function of laminins in vitro suggests that the malformations are a direct consequence of the removal of laminins from the synapse. Together, these results support a role for neuronal laminins in the structural integrity of central synapses.

[Diagnosis of congenital muscular dystrophy and clinical significance of merosin expression]

OBJECTIVE

The congenital muscular dystrophies (CMD) are a clinically and genetically heterogeneous group of neuromuscular disorders with progressive muscle wasting and weakness that begin during neonatal or early infantile period. To study the clinical diagnosis, immunohistochemical feature and follow-up information of CMD, data of 8 cases with CMD were analyzed.

METHODS

Immunohistochemical features of biopsied muscle specimens were summarized and analyzed by using anti-laminin alpha2 (merosin), anti alpha-dystroglycan (alpha-DG) and anti beta-dystroglycan (beta-DG) antibodies.

RESULTS

These patients mostly presented at birth or during the first six months of life with muscle weakness, hypotonia, contractures, and feeding difficulty or respiratory dysfunction. Hematoxylin-eosin staining of skeletal muscle specimens from these patients showed typical characteristics of CMD. Differences in fiber size, with predominantly small and round fibers, and dense connective tissue infiltration were seen. Four of the 8 patients were merosin-stain negative, which might be due to primary merosin deficiency. T2-weighted magnetic resonance imaging of the brain shows abnormalities of the white matter. Four cases were merosin-stain positive, and two of them also had hypoglycosylation of alpha-dystroglycan. Two patients had mental retardation. One of them had optic nerve atrophy and abnormal brain structure.

CONCLUSIONS

Two types of CMD were present in our group. Merosin-deficient congenital muscular dystrophy (congenital muscular dystrophy 1A, MDC1A) was more common, accompanied by abnormalities of the white matter. "Alpha-dystroglycanopathy" could be seen in merosin-positive cases.

The glomerular basement membrane: not gone, just forgotten

The glomerular capillaries function as the filtration barrier that retains albumin and other plasma proteins in the circulation. The unresolved question that has been asked for more than 50 years is, Which structural component of these capillaries constitutes the main molecular sieve that normally retains albumin and allows its passage in diseases associated with proteinuria? There is considerable evidence implicating both the glomerular basement membrane (GBM) and the epithelial filtration slits as the barrier. However, the prevailing point of view at present is that the slit diaphragms bridging the filtration slits are responsible for this important function, and evidence implicating the GBM is largely ignored or forgotten. In this issue of the JCI, Jarad et al. show that in laminin beta2-deficient (Lamb2-/-) mice, proteinuria can be directly attributed to the altered composition of the GBM (see the related article beginning on page 2272). Changes in the permeability of the GBM and its organization were primary to changes in the epithelium, as they preceded foot process effacement and loss of slit diaphragms.

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.

Laminin alpha1 chain improves laminin alpha2 chain deficient peripheral neuropathy

Absence of laminin alpha2 chain leads to a severe form of congenital muscular dystrophy (MDC1A) associated with peripheral neuropathy. Hence, future therapies should be aimed at alleviating both muscle and neurological dysfunctions. Pre-clinical studies in animal models have mainly focused on ameliorating the muscle phenotype. Here we show that transgenic expression of laminin alpha1 chain in muscles and the peripheral nervous system of laminin alpha2 chain deficient mice reduced muscular dystrophy and largely corrected the peripheral nerve defects. The presence of laminin alpha1 chain in the peripheral nervous system resulted in near-normal myelination, restored Schwann cell basement membranes and improved rotarod performance. In summary, we postulate that laminin alpha1 chain is an excellent substitute for laminin alpha2 chain in multiple tissues and suggest that treatment with laminin alpha1 chain may be beneficial for MDC1A in humans.

Regulated synthesis and functions of laminin 5 in polarized madin-darby canine kidney epithelial cells

Renal tubular epithelial cells synthesize laminin (LN)5 during regeneration of the epithelium after ischemic injury. LN5 is a truncated laminin isoform of particular importance in the epidermis, but it is also constitutively expressed in a number of other epithelia. To investigate the role of LN5 in morphogenesis of a simple renal epithelium, we examined the synthesis and function of LN5 in the spreading, proliferation, wound-edge migration, and apical-basal polarization of Madin-Darby canine kidney (MDCK) cells. MDCK cells synthesize LN5 only when subconfluent, and they degrade the existing LN5 matrix when confluent. Through the use of small-interfering RNA to knockdown the LN5 alpha3 subunit, we were able to demonstrate that LN5 is necessary for cell proliferation and efficient wound-edge migration, but not apical-basal polarization. Surprisingly, suppression of LN5 production caused cells to spread much more extensively than normal on uncoated surfaces, and exogenous keratinocyte LN5 was unable to rescue this phenotype. MDCK cells also synthesized laminin alpha5, a component of LN10, that independent studies suggest may form an assembled basal lamina important for polarization. Overall, our findings indicate that LN5 is likely to play an important role in regulating cell spreading, migration, and proliferation during reconstitution of a continuous epithelium.

Pierson syndrome: a novel cause of congenital nephrotic syndrome

In this report, we describe a newborn infant who presented with congenital nephrotic syndrome and renal insufficiency, as well as bilateral microcoria. This constellation of findings is a hallmark of Pierson syndrome, a newly recognized genetic disorder that is caused by a deficiency of beta2 laminin in the basement membrane. Our patient demonstrated classic histopathologic findings of Pierson syndrome on renal biopsy, including absence of beta2 laminin on immunofluorescent staining, and genetic testing confirmed the diagnosis. We conclude that Pierson syndrome should be included in the differential diagnosis for congenital nephrotic syndrome, especially in patients with ocular abnormalities.

Mutations in laminin alpha 1 result in complex, lens-independent ocular phenotypes in zebrafish

We report phenotypic and genetic analyses of a recessive, larval lethal zebrafish mutant, bal(a69), characterized by severe eye defects and shortened body axis. The bal(a69) mutation was mapped to chromosome 24 near the laminin alpha 1 (lama1) gene. We analyzed the lama1 gene sequence within bal(a69) embryos and two allelic mutants, bal(arl) and bal(uw1). Missense (bal(a69)), nonsense (bal(arl)), and frameshift (bal(uw1)) alterations in lama1 were found to underlie the phenotypes. Extended analysis of bal(a69) ocular features revealed disrupted lens development with subsequent lens degeneration, focal cornea dysplasia, and hyaloid vasculature defects. Within the neural retina, the ganglion cells showed axonal projection defects and ectopic photoreceptor cells were noted at inner retinal locations. To address whether ocular anomalies were secondary to defects in lens differentiation, bal(a69) mutants were compared to embryos in which the lens vesicle was surgically removed. Our analysis suggests that many of the anterior and posterior ocular defects in bal(a69) are independent of the lens degeneration. Analysis of components of focal adhesion signaling complexes suggests that reduced focal adhesion kinase activation underlies the anterior segment dysgenesis in lama1 mutants. To assess adult ocular phenotypes associated with lama1 mutations, genetic mosaics were generated by transplanting labeled bal cells into ocular-fated regions of wild-type blastulas. Adult chimeric eyes displayed a range of defects including anterior segment dysgenesis and cataracts. Our analysis provides mechanistic insights into the developmental defects and ocular pathogenesis caused by mutations in laminin subunits.

Molecular dissection of laminin alpha 5 in vivo reveals separable domain-specific roles in embryonic development and kidney function

Laminins are a family of basement membrane proteins with diverse roles in fundamental developmental processes such as epiblast polarization and gastrulation, as well as in organ development and function. We have focused on the laminin alpha1 and alpha5 chains, the ancestral laminin alpha chains required for development. To elucidate the unique functions of laminin alpha1 and alpha5 and their COOH-terminal LG domains, we have produced a collection of laminin knockout and transgenic mice expressing full length and chimeric laminin alpha5/alpha1 chains. Crossing the transgenes onto the Lama5-/- background generates "pseudo-knockins", so called because endogenous laminin alpha5 is replaced by transgene-encoded proteins. Expression of a chimera with the entire alpha5LG domain replaced by alpha1LG had minimal ameliorative effects on the defects observed in Lama5-/- embryos. In contrast, high level expression of a chimera with only the alpha5LG3-5 tandem replaced by alpha1LG3-5 completely rescued defects in digit septation, neural tube closure, placental labyrinth morphology, lung lobe septation, hair growth, and vascularization of kidney glomeruli. These mice were viable for several months, but they developed a lethal nephrotic syndrome. Our results show that: (1) the laminin alpha5LG1-2 tandem plays an essential role during development and harbors the great majority of the functionality of the alpha5LG domain; and (2) the alpha5LG3-5 tandem serves as a novel determinant required for the kidney's glomerular filtration barrier to plasma protein.

Expression profiling of muscles from Fukuyama-type congenital muscular dystrophy and laminin-α2 deficient congenital muscular dystrophy; is congenital muscular dystrophy a primary fibrotic disease?

Fukuyama-type congenital muscular dystrophy (FCMD) and laminin-alpha2 deficient congenital muscular dystrophy (MDC1A) are congenital muscular dystrophies (CMDs) and they both are categorized into the same clinical entity of muscular dystrophy as Duchenne muscular dystrophy (DMD). All three disorders share a common etiologic defect in the dystrophin-glycoprotein complex, which connects muscle structural proteins with the extracellular basement membrane. To investigate the pathophysiology of these CMDs, we generated microarray gene expression profiles of skeletal muscle from patients in various clinical stages. Despite diverse pathological changes, the correlation coefficient of overall gene expression among these samples was considerably high. We performed a multi-dimensional statistical analysis, the Distillation, to extract determinant genes that distinguish CMD muscle from normal controls. Up-regulated genes were primarily extracellular matrix (ECM) components, whereas down-regulated genes included structural components of mature muscle. These observations reflect active interstitial fibrosis with less active regeneration of muscle cell components in the CMDs, characteristics that are clearly distinct from those of DMD. Although the severity of fibrosis varied among the specimens tested, ECM gene expression was consistently high without substantial changes through the clinical course. Further, in situ hybridization showed more prominent ECM gene expression on muscle cells than on interstitial tissue cells, suggesting that ECM components are induced by regeneration process rather than by 'dystrophy.' These data imply that the etiology of FCMD and MDC1A differs from that of the chronic phase of classical muscular dystrophy, and the major pathophysiologic change in CMDs might instead result from primary active fibrosis.

Suspected malignant hyperthermia in a child with laminin alpha2 (merosin) deficiency in the absence of a triggering agent

Malignant hyperthermia (MH) is an inherited disorder of the skeletal muscles that can be triggered by many anesthetic agents. MH has different presentations and manifestations that makes it difficult to diagnose. Patients with laminin alpha2 deficiency have never been reported to be susceptible to MH. We present a suspected MH episode in the absence of classic triggering agents in a 7-year-old boy with laminin alpha2 (merosin) deficiency and congenital muscular dystrophy. The episode was diagnosed using the MH clinical grading scale and responded well to prompt management with dantrolene. We conclude that patients with laminin alpha2 deficiency may be susceptible to MH, and early suspicion and rapid treatment is vital in the management of MH. Anesthesiologists should be prepared to treat MH in susceptible patients even in the absence of a classical triggering agent.

Schwann cell-specific ablation of laminin gamma1 causes apoptosis and prevents proliferation

To investigate the function of laminin in peripheral nerve development, we specifically disrupted the laminin gamma1 gene in Schwann cells. Disruption of laminin gamma1 gene expression resulted in depletion of all other laminin chains known to be expressed in Schwann cells. Schwann cells lacking laminin do not extend processes required for initiating axonal sorting and mediating axon-Schwann cell interaction. They fail to downregulate Oct-6 and arrest at the premyelinating stage. The impaired axon-Schwann cell interaction prevents phosphorylation of beta-neuregulin-1 receptors and results in decreased cell proliferation. Postnatally, laminin-null Schwann cells exhibit reduced phosphatidylinositol 3 (PI3)-kinase activity and activation of caspase cascades, leading to apoptosis. Injection of a laminin peptide into mutant sciatic nerves partially restores PI3-kinase activity and reduces apoptotic signals. These results demonstrate the following: (1) that laminin initiates axonal sorting and mediates axon-Schwann cell interactions required for Schwann cell proliferation and differentiation, and (2) that laminin provides a PI3-kinase/Akt-mediated Schwann cell survival signal.

Both laminin and Schwann cell dystroglycan are necessary for proper clustering of sodium channels at nodes of Ranvier

Nodes of Ranvier are specialized axonal domains, at which voltage-gated sodium channels cluster. How axons cluster molecules in discrete domains is mostly unknown. Both axons and glia probably provide constraining mechanisms that contribute to domain formation. Proper sodium channel clustering in peripheral nerves depends on contact from Schwann cell microvilli, where at least one molecule, gliomedin, binds the sodium channel complex and induces its clustering. Furthermore, mice lacking Schwann cell dystroglycan have aberrant microvilli and poorly clustered sodium channels. Dystroglycan could interact at the basal lamina or at the axonglial surface. Because dystroglycan is a laminin receptor, and laminin 2 mutations [merosin-deficient congenital muscular dystrophy (MDC1A)] cause reduced nerve conduction velocity, we asked whether laminins are involved. Here, we show that the composition of both laminins and the dystroglycan complex at nodes differs from that of internodes. Mice defective in laminin 2 have poorly formed microvilli and abnormal sodium clusters. These abnormalities are similar, albeit less severe, than those of mice lacking dystroglycan. However, mice lacking all Schwann cell laminins show severe nodal abnormalities, suggesting that other laminins compensate for the lack of laminin 2. Thus, although laminins are located at a distance from the axoglial junction, they are required for proper clustering of sodium channels. Laminins, through their specific nodal receptors and cytoskeletal linkages, may participate in the formation of mechanisms that constrain clusters at nodes. Finally, abnormal sodium channel clusters are present in a patient with MDC1A, providing a molecular basis for the reduced nerve conduction velocity in this disorder.

Impeded interaction between Schwann cells and axons in the absence of laminin alpha4

The Schwann cell basal lamina (BL) is required for normal myelination. Loss or mutations of BL constituents, such as laminin-2 (alpha2beta1gamma1), lead to severe neuropathic diseases affecting peripheral nerves. The function of the second known laminin present in Schwann cell BL, laminin-8 (alpha4beta1gamma1), is so far unknown. Here we show that absence of the laminin alpha4 chain, which distinguishes laminin-8 from laminin-2, leads to a disturbance in radial sorting, impaired myelination, and signs of ataxia and proprioceptive disturbances, whereas the axonal regenerative capacity is not influenced. In vitro studies show poor axon growth of spinal motoneurons on laminin-8, whereas it is extensive on laminin-2. Schwann cells, however, extend longer processes on laminin-8 than on laminin-2, and, in contrast to the interaction with laminin-2, solely use the integrin receptor alpha6beta1 in their interaction with laminin-8. Thus, laminin-2 and laminin-8 have different critical functions in peripheral nerves, mediated by different integrin receptors.

Laminin alpha1 chain mediated reduction of laminin alpha2 chain deficient muscular dystrophy involves integrin alpha7beta1 and dystroglycan

Transgenically introduced laminin (LN) alpha1 chain prevents muscular dystrophy in LNalpha2 chain deficient mice. We now report increased integrin alpha7Bbeta1D synthesis in dystrophic LNalpha2 chain deficient muscle. Yet, immunofluorescence demonstrated a reduced expression of integrin alpha7B subunit at the sarcolemma. Transgenic expression of LNalpha1 chain reconstituted integrin alpha7B at the sarcolemma. Expression of alpha- and beta-dystroglycan is enhanced in LNalpha2 chain deficient muscle and normalized by transgenic expression of LNalpha1 chain. We suggest that LNalpha1 chain in part ameliorates the development of LNalpha2 chain deficient muscular dystrophy by retaining the binding sites for integrin alpha7Bbeta1D and alpha-dystroglycan, respectively.

Lung development in laminin gamma2 deficiency: abnormal tracheal hemidesmosomes with normal branching morphogenesis and epithelial differentiation

Background

Laminin γ2 (Lamc2), one of the polypeptides in laminin-332 (laminin-5), is prominent in the basement membrane of alveolar walls and airways of developing and adult lung. Laminins are important for lung morphogenesis and based on its localization, a function for laminin γ2 in lung development has been hypothesized. Targeted deletion of the laminin γ2 gene in mice results in skin blistering and neonatal death at 3–5 days after birth due to failure to thrive.

Methods

Examination of lung development in Lamc2-/- mice through 1–2 days postnatal was accomplished by morphometric analysis, lung bud culture, electron microscopy, immunohistochemical and immunofluorescence staining.

Results

Compared to littermate controls, Lamc2-/- lungs were similar in morphology during embryonic life. At post-natal day 1–2, distal saccules were mildly dilated by chord length measurements. Epithelial differentiation as evaluated by immunohistochemical staining for markers of ciliated cells, Clara cells, alveolar type I cells and alveolar type II cells did not reveal a difference between Lamc2-/- and littermate control lungs. Likewise, vascular development, smooth muscle cell differentiation, and elastic fiber formation looked similar, as did airway basement membrane ultrastructure. Branching morphogenesis by lung bud culture was similar in Lamc2-/- and littermate control lungs. Since laminin-332 is important for hemidesmosome formation, we examined the structure of tracheal hemidesmosomes by transmission electron microscopy. Compared to littermate controls, Lamc2-/- tracheal hemidesmosomes were less organized and lacked the increased electron density associated with the basement membrane abutting the hemidesmosome.

Conclusion

These findings indicate that laminin γ2 and laminin-332, despite their prominence in the lung, have a minimal role in lung development through the saccular stage.

Novel mutations in LAMA2 gene responsible for a severe phenotype of congenital muscular dystrophy in two Tunisian families

Congenital muscular dystrophies are a group of common genetically determined disorders often transmitted with a recessive mode of inheritance. In recent years, several deficiencies of proteins from the muscle membrane, extra cellular matrix, sarcomere, muscle cytosol and the nucleus have been described to cause CMD. The occidental type of CMD (MDC1A) in which the primary defect is a deficiency in laminin alpha2 chain (merosin) encoded by LAMA2 gene, accounts for 30-40% of cases. The clinical course of CMD with complete laminin alpha2 chain deficiency may be variable but most often; severe forms characterized by hypotonia at birth, profound muscle weakness, marked delay in motor milestones are observed. Since the identification of the first LAMA2 gene mutations leading to merosin deficiency in 1995, several mutations have subsequently been reported in many exons of this gene without any "hotspot" region. In this work, we report two novel homozygous mutations c.8005delT and c.8244+1G>A in the LAMA2 gene in four Tunisian patients with a severe MDC1A phenotype belonging to two unrelated consanguineous families.

The increased ecto-5′-nucleotidase activity in muscle, heart and liver of laminin α2-deficient mice is not caused by an elevation in the mRNA content

We have previously shown that mouse muscle and liver contain catalytically active and inactive ecto-5'-nucleotidase (eNT) variants and that eNT activity in these tissues increases in laminin alpha2 (merosin)-deficient Lama2dy mice. These results prompted us to study whether: (1) the increase of eNT activity depends on the change in the content of merosin between healthy and dystrophic organs; (2) the active and inactive eNT variants arise from the same or distinct mRNAs; (3) the enhancement of the activity is caused by an increase in the eNT mRNA content. Compared to healthy organs, the activity in dystrophic organs increased four-fold in muscle, 1.7-fold in liver, 1.4-fold in heart and not at all in kidney and lung. The level of immunolabelled eNT protein per unit of activity suggested a similar ratio of inactive to active eNT in healthy liver, kidney, heart and muscle, which increased greatly in lung. The size of the eNT subunit in liver, kidney, heart and muscle (72 kDa) decreased to 66 kDa in lung. The identification of a single RT-PCR product suggested that active and inactive eNT arise from the same mRNA and are generated by a differential post-translational processing. Compared to the content in muscle, the amount of eNT mRNA was 12-fold higher in liver and kidney, eight-fold in heart and five-fold in lung. The relative content of eNT mRNA was unaffected by the deficiency of merosin.

Gene expression profiling of diaphragm muscle in alpha2-laminin (merosin)-deficient dy/dy dystrophic mice

Deficiency of alpha2-laminin (merosin) underlies classical congenital muscular dystrophy in humans and dy/dy muscular dystrophy in mice and causes severe muscle dysfunction in both species. To gain greater insight into the biochemical and molecular events that link alpha2-laminin deficiency with muscle fiber necrosis, and the associated compensatory responses, gene expression profiles were characterized in diaphragm muscle from 8-wk-old dy/dy mice using oligonucleotide microarrays. Compared with age-matched normal muscle, dystrophic diaphragm was characterized by predominantly augmented gene expression, irrespective of the fold-change threshold. Among the 69 genes with at least plus or minus twofold significantly altered expression, 30 belonged to statistically overrepresented Gene Ontology (GO) biological process groups. These covered four specific themes: development including muscle development, cell motility with an emphasis on muscle contraction, defense/immune response, and cell adhesion. An additional 11 gene transcripts were assigned to more general overrepresented GO biological process groups (e.g., cellular process, organismal physiological process); the remaining 28 did not belong to any overrepresented groups. GO cellular constituent assignment resulted in the highest degree of overrepresentation in extracellular and muscle fiber locations, whereas GO molecular function assignment was most notable for various types of binding. RT-PCR was performed on 38 of 41 genes with at least plus or minus twofold significantly altered expression that were assigned to overrepresented GO biological process groups, with expression changes verified for 36 of 38 genes. These results indicate that several specific groups of genes have altered expression in response to genetic alpha2-laminin deficiency, with both similarities and differences compared with data reported for dystrophin-deficient muscular dystrophies.

Muscular dystrophy with laminin deficiency decreases acetylcholinesterase activity in thymus of dystrophic Lama2dy mice

We have studied the effect of muscular dystrophy by merosin deficiency on mouse thymus acetyl- (AChE) and butyrylcholinesterase (BuChE). The organ contains AChE and BuChE activities. Merosin deficiency causes an important decrease (46%) in AChE specific activity. Thymus produces dimers, monomers and tetramers of AChE, and the three kinds of AChE mRNAs. The drop in AChE activity in dystrophic animals could affect the amount of ACh reaching cholinergic receptors in cells of lymphoid organs.