The Innovative Role of Nuclear Receptor Interaction Protein in Orchestrating Invadosome Formation for Myoblast Fusion

BACKGROUND

Nuclear receptor interaction protein (NRIP) is versatile and engages with various proteins to execute its diverse biological function. NRIP deficiency was reported to cause small myofibre size in adult muscle regeneration, indicating a crucial role of NRIP in myoblast fusion.

METHODS

The colocalization and interaction of NRIP with actin were investigated by immunofluorescence and immunoprecipitation assay, respectively. The participation of NRIP in myoblast fusion was demonstrated by cell fusion assay and time-lapse microscopy. The NRIP mutants were generated for mechanism study in NRIP-null C2C12 (termed KO19) cells and muscle-specific NRIP knockout (NRIP cKO) mice. A GEO profile database was used to analyse NRIP expression in Duchenne muscular dystrophy (DMD) patients.

RESULTS

In this study, we found that NRIP directly and reciprocally interacted with actin both in vitro and in cells. Immunofluorescence microscopy showed that the endogenous NRIP colocalized with components of invadosome, such as actin, Tks5, and cortactin, at the tips of cells during C2C12 differentiation. The KO19 cells were generated and exhibited a significant deficit in myoblast fusion compared with wild-type C2C12 cells (3.16% vs. 33.67%, p < 0.005). Overexpressed NRIP in KO19 cells could rescue myotube formation compared with control (3.37% vs. 1.00%, p < 0.01). We further confirmed that NRIP directly participated in cell fusion by using a cell-cell fusion assay. We investigated the mechanism of invadosome formation for myoblast fusion, which depends on NRIP-actin interaction, by analysing NRIP mutants in NRIP-null cells. Loss of actin-binding of NRIP reduced invadosome (enrichment ratio, 1.00 vs. 2.54, p < 0.01) and myotube formation (21.82% vs. 35.71%, p < 0.05) in KO19 cells and forced NRIP expression in KO19 cells and muscle-specific NRIP knockout (NRIP cKO) mice increased myofibre size compared with controls (over 1500 μm2, 61.01% vs. 20.57%, p < 0.001). We also found that the NRIP mRNA level was decreased in DMD patients compared with healthy controls (18 072 vs. 28 289, p < 0.001, N = 10 for both groups).

CONCLUSIONS

NRIP is a novel actin-binding protein for invadosome formation to induce myoblast fusion.

Autoantibody of NRIP, a novel AChR-interacting protein, plays a detrimental role in myasthenia gravis

BACKGROUND

Nuclear receptor interaction protein (NRIP) co-localizes with acetylcholine receptor (AChR) at the neuromuscular junction (NMJ), and NRIP deficiency causes aberrant NMJ architecture. However, the normal physiological and pathophysiological roles of NRIP in NMJ are still unclear.

METHODS

We investigated the co-localization and interaction of NRIP with AChR-associated proteins using immunofluorescence and immunoprecipitation assay, respectively. The binding affinity of AChR-associated proteins was analysed in muscle-restricted NRIP knockout mice and NRIP knockout muscle cells (C2C12). We further collected the sera from 43 patients with myasthenia gravis (MG), an NMJ disorder. The existence and features of anti-NRIP autoantibody in sera were studied using Western blot and epitope mapping.

RESULTS

NRIP co-localized with AChR, rapsyn and α-actinin 2 (ACTN2) in gastrocnemius muscles of mice; and α-bungarotoxin (BTX) pull-down assay revealed NRIP with rapsyn and ACTN2 in complexes from muscle tissues and cells. NRIP directly binds with α subunit of AChR (AChRα) in vitro and in vivo to affect the binding affinity of AChR with rapsyn and rapsyn with ACTN2. In 43 patients with MG (age, 58.4 ± 14.5 years; female, 55.8%), we detected six of them (14.0%) having anti-NRIP autoantibody. The presence of anti-NRIP autoantibody correlated with a more severe type of MG when AChR autoantibody existed (P = 0.011). The higher the titre of anti-NRIP autoantibody, the more severe MG severity (P = 0.032). The main immunogenic region is likely on the IQ motif of NRIP. We also showed the IgG subclass of anti-NRIP autoantibody mainly to be IgG1.

CONCLUSIONS

NRIP is a novel AChRα binding protein and involves structural NMJ formation, which acts as a scaffold to stabilize AChR-rapsyn-ACTN2 complexes. Anti-NRIP autoantibody is a novel autoantibody in MG and plays a detrimental role in MG with the coexistence of anti-AChR autoantibody.

The Drosophila CG1674 gene encodes a synaptopodin 2-like related protein that localizes to the Z-disc and is required for normal flight muscle development and function

BACKGROUND

To identify novel myofibrillar components of the Drosophila flight muscles, we carried out a proteomic analysis of chemically demembranated flight muscle myofibrils, and characterized the knockdown phenotype of a novel gene identified in the screen, CG1674.

RESULTS

The CG1674 protein has some similarity to vertebrate synaptopodin 2-like, and when expressed as a FLAG-tagged fusion protein, it was localized during development to the Z-disc and cytoplasm. Knockdown of CG1674 expression affected the function of multiple muscle types, and defective flight in adults was accompanied by large actin-rich structures in the flight muscles that resembled overgrown Z-discs. Localization of CG1674 to the Z-disc depended predominantly upon presence of the Z-disc component alpha-actinin, but also depended upon other Z-disc components, including Mask, Zasp52, and Sals. We also observed re-localization of FLAG-CG1674 to the nucleus in Alpha-actinin and sals knockdown animals.

CONCLUSIONS

These studies identify and characterize a previously unreported myofibrillar component of Drosophila muscle that is necessary for proper myofibril assembly during development.

The Role of Cullin-RING Ligases in Striated Muscle Development, Function, and Disease

The well-orchestrated turnover of proteins in cross-striated muscles is one of the fundamental processes required for muscle cell function and survival. Dysfunction of the intricate protein degradation machinery is often associated with development of cardiac and skeletal muscle myopathies. Most muscle proteins are degraded by the ubiquitin-proteasome system (UPS). The UPS involves a number of enzymes, including E3-ligases, which tightly control which protein substrates are marked for degradation by the proteasome. Recent data reveal that E3-ligases of the cullin family play more diverse and crucial roles in cross striated muscles than previously anticipated. This review highlights some of the findings on the multifaceted functions of cullin-RING E3-ligases, their substrate adapters, muscle protein substrates, and regulatory proteins, such as the Cop9 signalosome, for the development of cross striated muscles, and their roles in the etiology of myopathies.

GSK3 inhibition with low dose lithium supplementation augments murine muscle fatigue resistance and specific force production

Calcineurin is a Ca2+ -dependent serine/threonine phosphatase that dephosphorylates nuclear factor of activated T cells (NFAT), allowing for NFAT entry into the nucleus. In skeletal muscle, calcineurin signaling and NFAT activation increases the expression of proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α) and slow myosin heavy chain (MHC) I ultimately promoting fatigue resistance. Glycogen synthase kinase 3 (GSK3) is a serine/threonine kinase that antagonizes calcineurin by re-phosphorylating NFAT preventing its entry into the nucleus. Here, we tested whether GSK3 inhibition in vivo with low dose lithium chloride (LiCl) supplementation (10 mg kg-1  day-1 for 6 weeks) in male C57BL/6J mice would enhance muscle fatigue resistance in soleus and extensor digitorum longus (EDL) muscles by activating NFAT and augmenting PGC-1α and MHC I expression. LiCl treatment inhibited GSK3 by elevating Ser9 phosphorylation in soleus (+1.8-fold, p = .007) and EDL (+1.3-fold p = .04) muscles. This was associated with a significant reduction in NFAT phosphorylation (-50%, p = .04) and a significant increase in PGC-1α (+1.5-fold, p = .05) in the soleus but not the EDL. MHC isoform analyses in the soleus also revealed a 1.2-fold increase in MHC I (p = .04) with no change in MHC IIa. In turn, a significant enhancement in soleus muscle fatigue (p = .04), but not EDL (p = .26) was found with LiCl supplementation. Lastly, LiCl enhanced specific force production in both soleus (p < .0001) and EDL (p = .002) muscles. Altogether, our findings show the skleletal muscle contractile benefits of LiCl-mediated GSK3 inhibition in mice.

Calmodulin-Binding Proteins in Muscle: A Minireview on Nuclear Receptor Interacting Protein, Neurogranin, and Growth-Associated Protein 43

Calmodulin (CaM) is an important Ca2+-sensing protein with numerous downstream targets that are either CaM-dependant or CaM-regulated. In muscle, CaM-dependent proteins, which are critical regulators of dynamic Ca2+ handling and contractility, include calcineurin (CaN), CaM-dependant kinase II (CaMKII), ryanodine receptor (RyR), and dihydropyridine receptor (DHPR). CaM-regulated targets include genes associated with oxidative metabolism, muscle plasticity, and repair. Despite its importance in muscle, the regulation of CaM-particularly its availability to bind to and activate downstream targets-is an emerging area of research. In this minireview, we discuss recent studies revealing the importance of small IQ motif proteins that bind to CaM to either facilitate (nuclear receptor interacting protein; NRIP) its activation of downstream targets, or sequester (neurogranin, Ng; and growth-associated protein 43, GAP43) CaM away from their downstream targets. Specifically, we discuss recent studies that have begun uncovering the physiological roles of NRIP, Ng, and GAP43 in skeletal and cardiac muscle, thereby highlighting the importance of endogenously expressed CaM-binding proteins and their regulation of CaM in muscle.

Skeletal muscle: A review of molecular structure and function, in health and disease

Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The "omics" revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross-talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems-level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Developmental Biology > Developmental Processes in Health and Disease Models of Systems Properties and Processes > Cellular Models.

Deficiency of nuclear receptor interaction protein leads to cardiomyopathy by disrupting sarcomere structure and mitochondrial respiration

BACKGROUND

Cardiomyopathy is a common and lethal complication in patients with limb-girdle muscular dystrophy (LGMD), one of the most prevalent forms of muscular dystrophy. The pathogenesis underlying LGMD-related cardiomyopathy remains unclear. NRIP (gene name DCAF6), a Ca²⁺-dependent calmodulin binding protein, was reduced in dystrophic muscles from LGMD patients. Mice lacking NRIP exhibit a myopathic phenotype resembling that in LGMD patients, making NRIP deficiency a potential culprit leading to cardiomyopathy. This study aimed to determine if NRIP deficiency leads to cardiomyopathy and to explore the underlying molecular mechanisms.

METHODS AND RESULTS

NRIP expression was reduced in both human and mouse failing hearts. Muscle-specific NRIP knockout (MCK-Cre::Dcaf6^flox/flox) mouse heart and isolated cardiomyocytes exhibited markedly reduced contractility. Transmission electron microscopy revealed abnormal sarcomere structures and mitochondrial morphology in MCK-Cre::Dcaf6^flox/flox hearts. Protein co-immunoprecipitation and confocal imaging revealed that NRIP interacts with α-actinin 2 (ACTN2) at the Z-disc. We found that NRIP facilitated ACTN2-mediated F-actin bundling, and that NRIP deficiency resulted in reduced binding between Z-disc proteins ACTN2 and Cap-Z. In addition, NRIP-deficiency led to increased mitochondrial ROS and impaired mitochondrial respiration/ATP production owing to elevated cellular NADH/NAD⁺ ratios. Treatment with mitochondria-directed antioxidant mitoTEMPO or NAD⁺ precursor nicotinic acid restored mitochondrial function and cardiac contractility in MCK-Cre::Dcaf6^flox/flox mice.

CONCLUSIONS

NRIP is essential to maintain sarcomere structure and mitochondrial/contractile function in cardiomyocytes. Our results revealed a novel role for NRIP deficiency in the pathogenesis of LGMD and heart failure. Targeting NRIP, therefore, could be a powerful new approach to treat myocardial dysfunction in LGMD and heart failure patients.

Neurogranin is expressed in mammalian skeletal muscle and inhibits calcineurin signaling and myoblast fusion

Calcineurin is a Ca²⁺/calmodulin (CaM)-dependent phosphatase that plays a critical role in promoting the slow fiber phenotype and myoblast fusion in skeletal muscle, thereby making calcineurin an attractive cellular target for enhancing fatigue resistance, muscle metabolism, and muscle repair. Neurogranin (Ng) is a CaM-binding protein thought to be expressed solely in brain and neurons, where it inhibits calcineurin signaling by sequestering CaM, thus lowering its cellular availability. Here, we demonstrate for the first time the expression of Ng protein and mRNA in mammalian skeletal muscle. Both protein and mRNA levels are greater in slow-oxidative compared with fast-glycolytic muscles. Coimmunoprecipitation of CaM with Ng in homogenates of C2C12 myotubes, mouse soleus, and human vastus lateralis suggests that these proteins physically interact. To determine whether Ng inhibits calcineurin signaling in muscle, we used Ng siRNA with C2C12 myotubes to reduce Ng protein levels by 60%. As a result of reduced Ng expression, C2C12 myotubes had enhanced CaM-calcineurin binding and calcineurin signaling as indicated by reduced phosphorylation of nuclear factor of activated T cells and increased utrophin mRNA. In addition, calcineurin signaling affects the expression of myogenin and stabilin-2, which are involved in myogenic differentiation and myoblast fusion, respectively. Here, we found that both myogenin and stabilin-2 were significantly elevated by Ng siRNA in C2C12 cells, concomitantly with an increased fusion index. Taken together, these results demonstrate the expression of Ng in mammalian skeletal muscle where it appears to be a novel regulator of calcineurin signaling.

Sarcolipin deletion in mdx mice impairs calcineurin signalling and worsens dystrophic pathology

Duchenne muscular dystrophy (DMD) is the most severe form of muscular dystrophy affecting 1 in 3500 live male births. Although there is no cure for DMD, therapeutic strategies aimed at enhancing calcineurin signalling and promoting the slow fibre phenotype have shown promise in mdx mice, which is the classical mouse model for DMD. Sarcolipin (SLN) is a small protein that regulates the sarco(endo)plasmic reticulum Ca2+-ATPase pump and its expression is highly upregulated in dystrophic skeletal muscle. We have recently shown that SLN in skeletal muscle amplifies calcineurin signalling thereby increasing myofibre size and the slow fibre phenotype. Therefore, in the present study we sought to determine the physiological impact of genetic Sln deletion in mdx mice, particularly on calcineurin signalling, fibre-type distribution and size and dystrophic pathology. We generated an mdx/Sln-null (mdx/SlnKO) mouse colony and hypothesized that the soleus and diaphragm muscles from these mice would display blunted calcineurin signalling, smaller myofibre sizes, an increased proportion of fast fibres and worsened dystrophic pathology compared with mdx mice. Our results show that calcineurin signalling was impaired in mdx/SlnKO mice as indicated by reductions in utrophin, stabilin-2 and calcineurin expression. In addition, mdx/SlnKO muscles contained smaller myofibres, exhibited a slow-to-fast fibre-type switch that corresponded with reduced expression of mitochondrial proteins and displayed a worsened dystrophic pathology compared with mdx muscles. Altogether, our findings demonstrate a critical role for SLN upregulation in dystrophic muscles and suggest that SLN can be viewed as a potential therapeutic target.

Muscle-restricted nuclear receptor interaction protein knockout causes motor neuron degeneration through down-regulation of myogenin at the neuromuscular junction

BACKGROUND

Nuclear receptor interaction protein (NRIP) is a calcium/calmodulin (CaM) binding protein. Nuclear receptor interaction protein interacts with CaM to activate calcineurin and CaMKII signalling. The conventional NRIP knockout mice (global knockout) showed muscular abnormality with reduction of muscle oxidative functions and motor function defects.

METHODS

To investigate the role of NRIP on neuromuscular system, we generated muscle-restricted NRIP knockout mice [conditional knockout (cKO)]. The muscle functions (including oxidative muscle markers and muscle strength) and lumbar motor neuron functions [motor neuron number, axon denervation, neuromuscular junction (NMJ)] were tested. The laser-captured microdissection at NMJ of skeletal muscles and adenovirus gene therapy for rescued effects were performed.

RESULTS

The cKO mice showed muscular abnormality with reduction of muscle oxidative functions and impaired motor performances as global knockout mice. To our surprise, cKO mice also displayed motor neuron degeneration with abnormal architecture of NMJ. Specifically, the cKO mice revealed reduced motor neuron number with small neuronal size in lumbar spinal cord as well as denervating change, small motor endplates, and decreased myonuclei number at NMJ in skeletal muscles. To explore the mechanisms, we screened various muscle-derived factors and found that myogenin is a potential candidate that myogenin expression was lower in skeletal muscles of cKO mice than wild-type mice. Because NRIP and myogenin were colocalized around acetylcholine receptors at NMJ, we extracted RNA from synaptic and extrasynaptic regions of muscles using laser capture microdissection and showed that myogenin expression was especially lower at synaptic region in cKO than wild-type mice. Notably, overexpression of myogenin using intramuscular adenovirus encoding myogenin treatment rescued abnormal NMJ architecture and preserved motor neuron death in cKO mice.

CONCLUSIONS

In summary, we demonstrated that deprivation of NRIP decreases myogenin expression at NMJ, possibly leading to abnormal NMJ formation, denervation of acetylcholine receptor, and subsequent loss of spinal motor neuron. Overexpression of myogenin in cKO mice can partially rescue abnormal NMJ architecture and motor neuron death. Therefore, muscular NRIP is a novel trophic factor supporting spinal motor neuron via stabilization of NMJ by myogenin expression.

NRIP/DCAF6 stabilizes the androgen receptor protein by displacing DDB2 from the CUL4A-DDB1 E3 ligase complex in prostate cancer

Both nuclear receptor interaction protein (NRIP) and DNA damage binding protein 2 (DDB2) belong to the Cullin 4 (CUL4)-DDB1 binding protein family and are androgen receptor (AR)-interacting proteins. Here, we investigated the expression patterns of the NRIP, DDB2 and AR proteins in human prostate cancer tissues and found that the expression levels of NRIP and AR were higher, but the DDB2 level was lower, in prostate cancer tissues than in non-neoplastic controls, suggesting NRIP as a candidate tumor promoter and DDB2 as a tumor suppressor in prostate cancer. Furthermore, both NRIP and DDB2 shared the same AR binding domain; they were competitors for the AR, but not for DDB1 binding, in the AR-DDB2-DDB1-CUL4A complex. Conclusively, NRIP stabilizes the AR protein by displacing DDB2 from the AR-DDB2 complex. Consistent with our hypothesis, a specific expression pattern with high levels of NRIP and AR, together with a low level of DDB2, was found more frequently in the human prostate cancer tissues with a cribriform pattern than in non-cribriform tumors, suggesting that disruption of the balance between NRIP and DDB2 may change AR protein homeostasis and contribute to pathogenesis in certain aggressive types of prostate cancer.

Distinct roles of NFATc1 and NFATc4 in human primary myoblast differentiation and in the maintenance of reserve cells

Ca²⁺ signaling plays a key role during human myoblast differentiation. Among Ca²⁺-sensitive pathways, calcineurin is essential for myoblast differentiation and muscle regeneration. Nuclear factor of activated T-cell (NFAT) transcription factors are the major calcineurin targets. We investigated the expression and the role of each NFAT gene during human primary myoblast differentiation. We found that three NFAT isoforms are present, NFATc1, NFATc3 and NFATc4. Importantly, while their mRNA expression increases during differentiation, NFATc1 is more highly expressed in myotubes, whilst NFATc4 is specifically maintained in reserve cells. NFATc3 is present in both cell types, although no specific role during myoblast differentiation was observed. Knockdown of either NFATc1 or NFATc4 affects the differentiation process similarly, by decreasing the expression of late differentiation markers, but impairs myotube formation differently. Whereas NFATc1 knockdown strongly reduced the number and the surface area of myotubes, NFATc4 knockdown increased the surface area of myotubes and reduced the pool of reserve cells. We conclude that NFAT genes have specific roles in myotube formation and in the maintenance of the reserve cell pool during human postnatal myogenesis.

Conditional Knockout of Breast Carcinoma Amplified Sequence 2 (BCAS2) in Mouse Forebrain Causes Dendritic Malformation via β-catenin

Breast carcinoma amplified sequence 2 (BCAS2) is a core component of the hPrP19 complex that controls RNA splicing. Here, we performed an exon array assay and showed that β-catenin is a target of BCAS2 splicing regulation. The regulation of dendrite growth and morphology by β-catenin is well documented. Therefore, we generated conditional knockout (cKO) mice to eliminate the BCAS2 expression in the forebrain to investigate the role of BCAS2 in dendrite growth. BCAS2 cKO mice showed a microcephaly-like phenotype with a reduced volume in the dentate gyrus (DG) and low levels of learning and memory, as evaluated using Morris water maze analysis and passive avoidance, respectively. Golgi staining revealed shorter dendrites, less dendritic complexity and decreased spine density in the DG of BCAS2 cKO mice. Moreover, the cKO mice displayed a short dendrite length in newborn neurons labeled by DCX, a marker of immature neurons, and BrdU incorporation. To further examine the mechanism underlying BCAS2-mediated dendritic malformation, we overexpressed β-catenin in BCAS2-depleted primary neurons and found that the dendritic growth was restored. In summary, BCAS2 is an upstream regulator of β-catenin gene expression and plays a role in dendrite growth at least partly through β-catenin.