Dystrophin and utrophin: genetic analyses of their role in skeletal muscle

Molecular Therapies for Muscular Dystrophies

PURPOSE OF REVIEW

To construct a framework to understand the different molecular interventions for muscular dystrophy.

RECENT FINDINGS

The recent approval of antisense oligonucleotides treatment for Duchenne muscular dystrophy and spinal muscular atrophy and current clinical trials using recombinant adeno-associated virus for the treatment of those diseases suggests that we are at a tipping point where we are able to treat and potentially cure muscular dystrophies. Understanding the basic molecular pathogenesis of muscular dystrophies and the molecular biology of the treatment allows for critical evaluation of the proposed therapies.

Metabolic dysfunction and altered mitochondrial dynamics in the utrophin-dystrophin deficient mouse model of duchenne muscular dystrophy

The utrophin-dystrophin deficient (DKO) mouse model has been widely used to understand the progression of Duchenne muscular dystrophy (DMD). However, it is unclear as to what extent muscle pathology affects metabolism. Therefore, the present study was focused on understanding energy expenditure in the whole animal and in isolated extensor digitorum longus (EDL) muscle and to determine changes in metabolic enzymes. Our results show that the 8 week-old DKO mice consume higher oxygen relative to activity levels. Interestingly the EDL muscle from DKO mouse consumes higher oxygen per unit integral force, generates less force and performs better in the presence of pyruvate thus mimicking a slow twitch muscle. We also found that the expression of hexokinase 1 and pyruvate kinase M2 was upregulated several fold suggesting increased glycolytic flux. Additionally, there is a dramatic increase in dynamin-related protein 1 (Drp 1) and mitofusin 2 protein levels suggesting increased mitochondrial fission and fusion, a feature associated with increased energy demand and altered mitochondrial dynamics. Collectively our studies point out that the dystrophic disease has caused significant changes in muscle metabolism. To meet the increased energetic demand, upregulation of metabolic enzymes and regulators of mitochondrial fusion and fission is observed in the dystrophic muscle. A better understanding of the metabolic demands and the accompanied alterations in the dystrophic muscle can help us design improved intervention therapies along with existing drug treatments for the DMD patients.

Impacts of dystrophin and utrophin domains on actin structural dynamics: implications for therapeutic design

We have used time-resolved phosphorescence anisotropy (TPA) of actin to evaluate domains of dystrophin and utrophin, with implications for gene therapy in muscular dystrophy. Dystrophin and its homolog utrophin bind to cytoskeletal actin to form mechanical linkages that prevent muscular damage. Because these proteins are too large for most gene therapy vectors, much effort is currently devoted to smaller constructs. We previously used TPA to show that both dystrophin and utrophin have a paradoxical effect on actin rotational dynamics-restricting amplitude while increasing rate, thus increasing resilience, with utrophin more effective than dystrophin. Here, we have evaluated individual domains of these proteins. We found that a "mini-dystrophin," lacking one of the two actin-binding domains, is less effective than dystrophin in regulating actin dynamics, correlating with its moderate effectiveness in rescuing the dystrophic phenotype in mice. In contrast, we found that a "micro-utrophin," with more extensive internal deletions, is as effective as full-length dystrophin in the regulation of actin dynamics. Each of utrophin's actin-binding domains promotes resilience in actin, while dystrophin constructs require the presence of both actin-binding domains and the C-terminal domain for full function. This work supports the use of a utrophin template for gene or protein therapy designs. Resilience of the actin-protein complex, measured by TPA, correlates remarkably well with previous reports of functional rescue by dystrophin and utrophin constructs in mdx mice. We propose the use of TPA as an in vitro method to aid in the design and testing of emerging gene therapy constructs.

Introduction to gene therapy: a clinical aftermath

Despite three decades of huge progress in molecular genetics, in cloning of disease causative gene as well as technology breakthroughs in viral biotechnology, out of thousands of gene therapy clinical trials that have been initiated, only very few are now reaching regulatory approval. We shall review some of the major hurdles, and based on the current either positive or negative examples, we try to initiate drawing a learning curve from experience and possibly identify the major drivers for future successful achievement of human gene therapy trials.

Molecular characterization of a de novo 6q24.2q25.3 duplication interrupting UTRN in a patient with arthrogryposis

Chromosome 6q duplications have been documented repeatedly, allowing the delineation of a "6q duplication syndrome," characterized by hypertelorism, downslanting palpebral fissures, tented upper lip, short neck, severe mental and growth retardation, and joint contractures. Most reported cases result from malsegregation of a reciprocal translocation leading to a terminal 6q duplication and partial monosomy of another chromosome. Only 11 cases of de novo pure duplication have been reported so far. The breakpoints do not appear to be recurrent, but in most cases they have not been characterized molecularly, precluding genotype-phenotype correlation. We report on an 8-year-old girl with a phenotype consistent with mild 6q duplication syndrome, including characteristic physical findings, mild mental retardation, and joint contractures. She carries a 13 Mb de novo 6q24.2q25.3 duplication, diagnosed by high-resolution karyotype and confirmed by array-CGH. Molecular characterization of the duplicated segment with quantitative PCR showed that the proximal breakpoint is localized within the UTRN gene, encoding utrophin, the autosomal homologue of dystrophin. We discuss the possible implication of UTRN in arthrogryposis associated with duplications spanning the 6q23q26 region.

Formation of complex AChR aggregates in vitro requires alpha-dystrobrevin

Efficient function at the neuromuscular junction requires high-density aggregates of acetylcholine receptors (AChRs) to be precisely aligned with the motor nerve terminal. A collaborative effort between the motor neuron and muscle intrinsic factors drives the formation and maintenance of these AChR aggregates. alpha-Dystrobrevin (alpha DB), a cytoplasmic protein found at the postsynaptic membrane, has been implicated in the regulation of AChR aggregate density and patterning. To investigate the contribution of alpha DB to the muscle intrinsic program regulating AChR aggregate development, we analyzed the formation of complex, pretzel-like AChR aggregates on primary muscle cell cultures derived from alpha DB knockout (alpha DB-KO) mice in the absence of nerve or agrin. In myotubes lacking alpha DB, complex AChR aggregates failed to form, whereas aggregates formed readily in wildtype myotubes. Five major isoforms of alpha DB are expressed in skeletal muscle: alpha DB1, alpha DB1(-), alpha DB2, alpha DB2(-), and alpha DB3. Expression of alpha DB1 or alpha DB1(-) in alpha DB-KO myotubes restored formation of complex AChR aggregates similar to those in wildtype myotubes. In contrast, individual expression of alpha DB2, alpha DB2(-), alpha DB3, or an alpha DB1 phosphorylation mutant resulted in the formation of few, if any, complex AChR aggregates. Collectively, these data suggest that alpha DB is a significant component of the muscle intrinsic program that mediates the formation of complex AChR aggregates and that alpha DB's tyrosine phosphorylation sites are of particular functional importance to this program. Although the muscle intrinsic program appears to influence synaptogenesis, the formation of complex mature AChR aggregates in alpha DB-KO mice (with the motor neuron present) suggests the motor neuron, not the muscle intrinsic program, is the major stimulus driving the maturation of AChRs from plaque to pretzel in vivo.

Glutamate receptors localize postsynaptically at neuromuscular junctions in mice

Dlg (Discs Large) is a multidomain protein that interacts with glutamate receptors and potassium channels at Drosophila neuromuscular junctions (NMJs) and at mammalian central nervous system synapses. Dlg also localizes postsynaptically at cholinergic mammalian NMJs. We show here that alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) receptor subunits, together with glutamate, are present at the mammalian NMJ. Both AMPA and NMDA (N-methyl-D-aspartate) glutamate receptor subunits display overlapping postsynaptic localization patterns with Dlg at all NMJs examined in normal mice. Kir2 potassium channels also localize with Dlg and glutamate receptors at this synapse. Localization of the components of a glutamatergic system suggests novel mechanisms at mammalian neuromuscular synapses.

Evolution of insect proteomes: insights into synapse organization and synaptic vesicle life cycle

BACKGROUND

The molecular components in synapses that are essential to the life cycle of synaptic vesicles are well characterized. Nonetheless, many aspects of synaptic processes, in particular how they relate to complex behaviour, remain elusive. The genomes of flies, mosquitoes, the honeybee and the beetle are now fully sequenced and span an evolutionary breadth of about 350 million years; this provides a unique opportunity to conduct a comparative genomics study of the synapse.

RESULTS

We compiled a list of 120 gene prototypes that comprise the core of presynaptic structures in insects. Insects lack several scaffolding proteins in the active zone, such as bassoon and piccollo, and the most abundant protein in the mammalian synaptic vesicle, namely synaptophysin. The pattern of evolution of synaptic protein complexes is analyzed. According to this analysis, the components of presynaptic complexes as well as proteins that take part in organelle biogenesis are tightly coordinated. Most synaptic proteins are involved in rich protein interaction networks. Overall, the number of interacting proteins and the degrees of sequence conservation between human and insects are closely correlated. Such a correlation holds for exocytotic but not for endocytotic proteins.

CONCLUSION

This comparative study of human with insects sheds light on the composition and assembly of protein complexes in the synapse. Specifically, the nature of the protein interaction graphs differentiate exocytotic from endocytotic proteins and suggest unique evolutionary constraints for each set. General principles in the design of proteins of the presynaptic site can be inferred from a comparative study of human and insect genomes.

Dp71, utrophin and beta-dystroglycan expression and distribution in PC12/L6 cell cocultures

Function of dystrophin Dp71 isoforms is unknown but seems related to neurite outgrowth and synapse formation. To evaluate Dp71 role in myoneural synapses, we established a coculture model using PC12 cells and L6 myotubes and analyzed expression and localization of Dp71 and related proteins, utrophin and beta-dystroglycan, in PC12 cells. Confocal microscopy showed Dp71d isoform in PC12 nuclei, golgi-complex-like and endoplasmic reticulum-like structures, whereas Dp71ab concentrates at neurite tips and cytoplasm, colocalizing with beta-dystroglycan, utrophin, synaptophysin and acetylcholine receptors. Evidences suggest that Dp71ab isoform, unlike Dp71d, may take part in neurite-related processes. This is the first work on Dp and members of Dp-associated protein complex roles in a cell-line based coculturing system, which may be useful in determining Dp71 isoforms associations.

Identification of CAP as a costameric protein that interacts with filamin C

Cbl-associated protein (CAP) is an adaptor protein that interacts with both signaling and cytoskeletal proteins. Here, we characterize the expression, localization and potential function of CAP in striated muscle. CAP is markedly induced during myoblast differentiation, and colocalizes with vinculin during costamerogenesis. In adult mice, CAP is enriched in oxidative muscle fibers, and it is found in membrane anchorage complexes, including intercalated discs, costameres, and myotendinous junctions. Using both yeast two-hybrid and proteomic approaches, we identified the sarcomeric protein filamin C (FLNc) as a binding partner for CAP. When overexpressed, CAP recruits FLNc to cell-extracellular matrix adhesions, where the two proteins cooperatively regulate actin reorganization. Moreover, overexpression of CAP inhibits FLNc-induced cell spreading on fibronectin. In dystrophin-deficient mdx mice, the expression and membrane localization of CAP is increased, concomitant with the elevated plasma membrane content of FLNc, suggesting that CAP may compensate for the reduced membrane linkage of the myofibrils due to the loss of the dystroglycan-sarcoglycan complex in these mice. Thus, through its interaction with FLNc, CAP provides another link between the myofibril cytoskeleton and the plasma membrane of muscle cells, and it may play a dynamic role in the regulation and maintenance of muscle structural integrity.

Haploinsufficiency of utrophin gene worsens skeletal muscle inflammation and fibrosis in mdx mice

To address whether mdx mice with haploinsufficiency of utrophin (mdx/utrn+/-) develop more severe skeletal muscle inflammation and fibrosis than mdx mice, to represent a better model for Duchenne muscular dystrophy (DMD), we performed qualitative and quantitative analysis of skeletal muscle inflammation and fibrosis in mdx and mdx/utrn+/- littermates. Inflammation was significantly worse in mdx/utrn+/- quadriceps at age 3 and 6 months and in mdx/utrn+/- diaphragm at age 3 but not 6 months. Fibrosis was more severe in mdx/utrn+/- diaphragm at 6 months, and at this age, mild fibrosis was noted in quadriceps of mdx/utrn+/- but not mdx mice. The findings indicate that utrophin compensates, although insufficiently, for the effects of dystrophin loss with regard to inflammation and fibrosis of both quadriceps and diaphragm muscles in mdx mice. With more severe muscle dystrophy than mdx mice and a longer life span than utrophin-dystrophin-deficient (dko) mice, mdx/utrn+/- mice provide a better mouse model for testing potential therapies for muscle inflammation and fibrosis associated with DMD.

CAPON expression in skeletal muscle is regulated by position, repair, NOS activity, and dystrophy

In skeletal muscle, the localization of nNOS is destabilized in the absence of dystrophin, which impacts muscle function and satellite cell activation. In neurons, the adaptor protein, carboxy-terminal PDZ ligand of nNOS (CAPON), regulates the distribution of neuronal nitric oxide synthase (nNOS), which produces the key signaling molecule nitric oxide (NO). While a CAPON-like gene is known to compensate functionally for a dystrophic phenotype in muscle of Caenorhabditis elegans, CAPON expression has not been reported for mammalian muscle. Here, CAPON expression was identified in mouse muscle using Northern and Western blotting and in situ hybridization in combination with immunostaining for laminin. CAPON RNA was expressed in developing normal and dystrophic muscles near fiber junctions with tendons, and levels increased from 1 to 3 weeks. In regenerating normal muscle and also in dystrophic muscles in the mdx mouse, CAPON transcripts were prominent in satellite cells and new myotubes. Expression of CAPON RNA increased in diaphragm muscle of normal and mdx mice after treatment with L-arginine, the NOS substrate. Both CAPON and utrophin protein levels increased in dystrophic quadriceps muscle after treatment with the steroid deflazacort plus L-arginine, known to reduce the dystrophic phenotype. The identification of CAPON transcripts and protein in mammalian muscle and responses to L-arginine suggest CAPON may have a functional role in stabilizing neuronal NOS in skeletal muscle in the cytoskeletal complex associated with dystrophin/utrophin, with possible applications to therapy for human muscular dystrophy.

Stretch-induced nitric oxide modulates mechanical properties of skeletal muscle cells

In this study, we examined the hypothesis that stretch-induced (nitric oxide) NO modulates the mechanical properties of skeletal muscles by increasing accumulation of protein levels of talin and vinculin and by inhibiting calpain-induced proteolysis, thereby stabilizing the focal contacts and the cytoskeleton. Differentiating C2C12myotubes were subjected to a single 10% step stretch for 0–4 days. The apparent elastic modulus of the cells, Eapp, was subsequently determined by atomic force microscopy. Static stretch led to significant increases ( P < 0.01) in Eappbeginning at 2 days. These increases were correlated with increases in NO activity and neuronal NO synthase (nNOS) protein expression. Expression of talin was upregulated throughout, whereas expression of vinculin was significantly increased only on days 3 and 4. Addition of the NO donor l-arginine onto stretched cells further enhanced Eapp, NOS activity, and nNOS expression, whereas the presence of the NO inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) reversed the effects of mechanical stimulation and of l-arginine. Overall, viscous dissipation, as determined by the value of hysteresis, was not significantly altered. For assessment of the role of vinculin and talin stability, cells treated with l-NAME showed a significant decrease in Eapp, whereas addition of a calpain inhibitor abolished the effect. Thus our results show that NO inhibition of calpain-initiated cleavage of cytoskeleton proteins was correlated with the changes in Eapp. Together, our data suggest that NO modulates the mechanical behavior of skeletal muscle cells through the combined action of increased talin and vinculin levels and a decrease in calpain-mediated talin proteolysis.

Genetic compensation for sarcoglycan loss by integrin α7β1 in muscle

Disruption of the sarcoglycan complex leads to muscle membrane instability and muscular dystrophy in humans and mice. Through the dystrophin glycoprotein complex, sarcoglycan participates in connecting the internal cytoskeleton to the membrane and the extracellular matrix. Integrin α7β1 is also a transmembrane protein of skeletal and cardiac muscle that similarly links the cytoskeleton to the extracellular matrix. Mice lacking integrin α7 develop mild muscle degeneration, while sarcoglycan mutant mice display overt muscle degeneration and muscular dystrophy. In sarcoglycan-deficient muscle, integrin α7 protein was upregulated at the plasma membrane. To ascertain whether integrin α7 upregulation compensates for the loss of the transmembrane sarcoglycan linkage in sarcoglycan-deficient muscle, we generated mice lacking both integrin α7 and γ-sarcoglycan (gxi). These double-mutant gxi mice exhibit profound, rapid muscle degeneration leading to death before one month of age consistent with a weakened cellular attachment to the extracellular matrix. The regenerative capacity of gxi muscle was intact with increased embryonic myosin heavy chain expression, myofiber central nucleation and normal in vivo myoblast differentiation. Therefore, upregulation of integrin α7β1 compensates as a transmembrane muscle cell attachment for sarcoglycan consistent with overlapping roles for sarcoglycan and integrins in mediating cytoskeletal-membrane-extracellular matrix interaction.

Secondary coronary artery vasospasm promotes cardiomyopathy progression

Genetic defects in the plasma membrane-associated sarcoglycan complex produce cardiomyopathy characterized by focal degeneration. The infarct-like pattern of cardiac degeneration has led to the hypothesis that coronary artery vasospasm underlies cardiomyopathy in this disorder. We evaluated the coronary vasculature of gamma-sarcoglycan mutant mice and found microvascular filling defects consistent with arterial vasospasm. However, the vascular smooth muscle sarcoglycan complex was intact in the coronary arteries of gamma-sarcoglycan hearts with perturbation of the sarcoglycan complex only within the adjacent myocytes. Thus, in this model, coronary artery vasospasm derives from a vascular smooth muscle-cell extrinsic process. To reduce this secondary vasospasm, we treated gamma-sarcoglycan-deficient mice with the calcium channel antagonist verapamil. Verapamil treatment eliminated evidence of vasospasm and ameliorated histological and functional evidence of cardiomyopathic progression. Echocardiography of verapamil-treated, gamma-sarcoglycan-null mice showed an improvement in left ventricular fractional shortening (44.3 +/- 13.3% treated versus 37.4 +/- 15.3% untreated), maximal velocity at the aortic outflow tract (114.9 +/- 27.9 cm/second versus 92.8 +/- 22.7 cm/second), and cardiac index (1.06 +/- 0.30 ml/minute/g versus 0.67 +/- 0.16 ml/minute/g, P < 0.05). These data indicate that secondary vasospasm contributes to the development of cardiomyopathy and is an important therapeutic target to limit cardiomyopathy progression.

Constitutive properties, not molecular adaptations, mediate extraocular muscle sparing in dystrophic mdx mice

Extraocular muscle (EOM) is spared in Duchenne muscular dystrophy. Here, we tested putative EOM sparing mechanisms predicted from existing dystrophinopathy models. Data show that mdx mouse EOM contains dystrophin-glycoprotein complex (DGC)-competent and DGC-deficient myofibers distributed in a fiber type-specific pattern. Up-regulation of a dystrophin homologue, utrophin, mediates selective DGC retention. Counter to the DGC mechanical hypothesis, an intact DGC is not a precondition for EOM sarcolemmal integrity, and active adaptation at the level of calcium homeostasis is not mechanistic in protection. A partial, fiber type-specific retention of antiischemic nitric oxide to vascular smooth muscle signaling is not a factor in EOM sparing, because mice deficient in dystrophin and alpha-syntrophin, which localizes neuronal nitric oxide synthase to the sarcolemma, have normal EOMs. Moreover, an alternative transmembrane protein, alpha7beta1 integrin, does not appear to substitute for the DGC in EOM. Finally, genomewide expression profiling showed that EOM does not actively adapt to dystrophinopathy but identified candidate genes for the constitutive protection of mdx EOM. Taken together, data emphasize the conditional nature of dystrophinopathy and the potential importance of nonmechanical DGC roles and support the hypothesis that broad, constitutive structural cell signaling, and/or biochemical differences between EOM and other skeletal muscles are determinants of differential disease responsiveness.

Localizing synaptic mRNAs at the neuromuscular junction: it takes more than transcription

The neuromuscular junction has been used for several decades as an excellent model system to examine the cellular and molecular events involved in the formation and maintenance of a differentiated chemical synapse. In this context, several laboratories have focused their efforts over the last 15 years on the important contribution of transcriptional mechanisms to the regulation of the development and plasticity of the postsynaptic apparatus in muscle fibers. Converging lines of evidence now indicate that post-transcriptional events, operating at the level of mRNA stability and targeting, are likely to also play key roles at the neuromuscular junction. Here, we present the recent findings highlighting the role of these additional molecular events and extend our review to include data showing that post-transcriptional events are also important in the control of the expression of genes encoding synaptic proteins in muscle cells placed under different conditions. Finally, we discuss the possibility that mis-regulation of post-transcriptional events can occur in certain neuromuscular diseases and cause abnormalities of the neuromuscular junction.

Developmentally regulated expression and localization of dystrophin and utrophin in the human fetal brain

Expression of dystrophin and the dystrophin-related protein utrophin has been studied in the human fetal brain both in vivo and in vitro. Results showed that both these proteins were developmentally regulated, even if their expression followed a different pattern. Utrophin was found since very early stages of development, reached a peak between week 15-20 of gestation, declining then, so that at week 32 was barely detectable. The protein was mainly found in neuronal cell bodies, partially associated to the plasma membrane, and in astrocytes cytoplasm. On the contrary, the brain form of dystrophin was first detectable at week 12, increased up to week 15 and then remained stable. Dystrophin localization was similar but not identical to utrophin. In neurons, it was also partially associated with the plasma membrane of cell body and axon hillock. However, the most was concentrated in the cytoplasm and in the processes, where it appeared associated to neurofilaments. Astrocytes were negative for brain dystrophin, but positive for the muscle isoform. Results suggest that utrophin and dystrophin are likely to play a key, though different, role in the immature brain. They help in understanding the basic mechanism(s) underlying cognition defects frequently observed in Duchenne and Becker dystrophic patients.

zipper Nonmuscle myosin-II functions downstream of PS2 integrin in Drosophila myogenesis and is necessary for myofibril formation

Nonmuscle myosin-II is a key motor protein that drives cell shape change and cell movement. Here, we analyze the function of nonmuscle myosin-II during Drosophila embryonic myogenesis. We find that nonmuscle myosin-II and the adhesion molecule, PS2 integrin, colocalize at the developing muscle termini. In the paradigm emerging from cultured fibroblasts, nonmuscle actomyosin-II contractility, mediated by the small GTPase Rho, is required to cluster integrins at focal adhesions. In direct opposition to this model, we find that neither nonmuscle myosin-II nor RhoA appear to function in PS2 clustering. Instead, PS2 integrin is required for the maintenance of nonmuscle myosin-II localization and we show that the cytoplasmic tail of the beta(PS) integrin subunit is capable of mediating this PS2 integrin function. We show that embryos that lack zygotic expression of nonmuscle myosin-II fail to form striated myofibrils. In keeping with this, we demonstrate that a PS2 mutant that specifically disrupts myofibril formation is unable to mediate proper localization of nonmuscle myosin-II at the muscle termini. In contrast, embryos that lack RhoA function do generate striated muscles. Finally, we find that nonmuscle myosin-II localizes to the Z-line in mature larval muscle. We suggest that nonmuscle myosin-II functions at the muscle termini and the Z-line as an actin crosslinker and acts to maintain the structural integrity of the sarcomere.

Just in time and place: NOS/NO system assembly in neuromuscular junction formation

Recent advances in the molecular, biochemical, and anatomical aspects of postsynaptic membrane components at the neuromuscular junction (NMJ) are briefly reviewed focussing on assembly, architecture, and function of the multi-subunit dystrophin-protein complex (DPC) and its associated nitric oxide (NO)-signaling complex. Elucidation of unique structural binding motifs of NO-synthases (NOS), and microscopical codistribution of neuronal NOS (nNOS), the major isoform of NOS expressed at the NMJ, with known synaptic proteins, i.e., family members of the DPC, nicotinic acetylcholine receptor (AChR), NMDA-receptor, type-1 sodium and Shaker K(+)-channel proteins, and linker proteins (e.g., PSD-95, 43K-rapsyn), suggests targeting and assembly of the NO-signaling pathway at postsynaptic membrane components. NO mediates agrin-induced AChR-aggregation and downstream signal transduction in C2 skeletal myotubes while administration of L-arginine, the limiting substrate for NO-biosynthesis, enhances aggregation of synapse-specific components such as utrophin. At the NMJ, NO appears to be a mediator of (1) early synaptic protein clustering, (2) synaptic receptor activity and transmitter release, or (3) downstream signaling for transcriptional control. Multidisciplinary data obtained from cellular and molecular studies and from immunolocalization investigations have led us to propose a working model for step-by-step binding of nNOS, e.g., to subunit domains of targeted and/or preexisting membrane components. Formation of NOS-membrane complexes appears to be governed by agrin-signaling as well as by NO-signaling, supporting the idea that parallel signaling pathways may account for the spatiotemporally defined postsynaptic assembly thereby linking the NOS/NO-signaling cascade to early membrane aggregations and at the right places nearby preexisting targets (e.g., juxtaposition of NO source and target) in synapse formation.