Myotonic dystrophy protein kinase phosphorylates phospholamban and regulates calcium uptake in cardiomyocyte sarcoplasmic reticulum

Identification of phosphorylated small ORF-encoded peptides in Hep3B cells by LC/MS/MS

Small ORF-encoded peptides (SEPs) are a class of low molecular weight proteins and peptides comprising <100 amino acids with important functions in various life activities. Although the sequence length is short, SEPs might also have post-translational modification (PTM). Phosphorylation is one of the most essential PTMs of proteins. In this work, we enriched phosphopeptides with IMAC and TiO2 materials and analyzed the phosphorylated SEPs in Hep3B cells. A total of 24 phosphorylated SEPs were identified, and 11 SEPs were coded by ncRNA. For the sequence analysis, we found that the general characteristics of phosphorylated SEPs are roughly the same as canonical proteins. Besides, two phosphorylation SEPs have the Stathmin family signature 2 motif, which can regulate the microtubule cytoskeleton. Some SEPs have domains or signal peptides, indicating their specific functions and subcellular locations. Kinase network analysis found a small number of kinases that may be a clue to the specific functions of some SEPs. However, only one-fifth of the predicted phosphorylation sites were identified by LC/MS/MS, indicating that many SEP PTMs are hidden in the dark, waiting to be uncovered and verified. This study helps expand our understanding of SEP and provides information for further SEP function investigation. SIGNIFICANCE: Small ORF-encoded peptides (SEPs) are important in various life activities. Although the sequence length is short (<100AA), SEPs might also have post-translational modification (PTM). Phosphorylation is one of the most essential PTMs of proteins. We enriched phosphopeptides and analyzed the phosphorylated SEPs in Hep3B cells. That is the first time to explore the PTM of SPEs systematically. Kinase network analysis found a small number of kinases that may be a clue to the specific functions of SEPs. More SEP PTMs are hidden in the dark and waiting to be uncovered and verified. This study helps expand our understanding of SEP and provides information for further SEP function investigation.

Sex-specific sequels of early life stress on serine/threonine kinase activity in visceral adipose tissue from obese mice

Adverse childhood experiences (ACEs) are an established independent risk factor for chronic disease including obesity and hypertension; however, only women exposed to multiple ACEs show a positive relationship with BMI. Our lab has reported that maternal separation and early weaning (MSEW), a mouse model of early life stress, induces sex-specific mechanisms underlying greater blood pressure response to a chronic high fat diet (HF). Specifically, female MSEW mice fed a HF display exacerbated perigonadal white adipose tissue (pgWAT) expansion and a metabolic syndrome-like phenotype compared to control counterparts, whereas hypertension is caused by sympathoactivation in male MSEW mice. Thus, this study aimed to determine whether there is a sex-specific serine/threonine kinase (STKA) activity in pgWAT adipose tissue associated with early life stress. Frozen pgWAT was collected from MSEW and control, male and female mice fed a HF to assess STKA activity using the Pamstation12 instrument. Overall, MSEW induces significant reduction of 7 phosphokinases (|Z| >=1.5) in females (QIK, MLK, PKCH, MST, STE7, PEK, FRAY) and 5 in males (AKT, SGK, P38, MARK, CDK), while 15 were downregulated in both sexes (DMPK, PKA, PKG, RSK, PLK, DYRK, NMO, CAMK1, JNK, PAKA, RAD53, ERK, PAKB, PKD, PIM, AMPK). This data provides new insights into the sex-specific dysregulation of the molecular network controlling cellular phosphorylation signals in visceral adipose tissue and identifies possible target phosphokinases implicated in adipocyte hypertrophy as a result of exposure to early life stress. Identifying functional metabolic signatures is critical to elucidate the underlying molecular mechanisms behind the sex-specific obesity risk associated with early life stress.

Mechanical Responses of a Single Myelin Layer: A Molecular Simulation Study

The myelin sheath provides insulation to the brain's neuron cells, which aids in signal transmission and communication with the body. Degenerated myelin hampers the connection between the glial cells, which are the front row responders during traumatic brain injury mitigation. Thus, the structural integrity of the myelin layer is critical for protecting the brain tissue from traumatic injury. At the molecular level, myelin consists of a lipid bilayer, myelin basic proteins (MBP), proteolipid proteins (PLP), water and ions. Structurally, the myelin sheath is formed by repeatedly wrapping forty or more myelin layers around an axon. Here, we have used molecular dynamic simulations to model and capture the tensile response of a single myelin layer. An openly available molecular dynamic solver, LAMMPS, was used to conduct the simulations. The interatomic potentials for the interacting atoms and molecules were defined using CHARMM force fields. Following a standard equilibration process, the molecular model was stretched uniaxially at a deformation rate of 5 Å/ps. We observed that, at around 10% applied strain, the myelin started to cohesively fail via flaw formation inside the bilayers. Further stretching led to a continued expansion of the defect inside the bilayer, both radially and transversely. This study provides the cellular-level mechanisms of myelin damage due to mechanical load.

Clinical and Molecular Insights into Gastrointestinal Dysfunction in Myotonic Dystrophy Types 1 & 2

Myotonic dystrophy (DM) is a highly variable, multisystemic disorder that clinically affects one in 8000 individuals. While research has predominantly focused on the symptoms and pathological mechanisms affecting striated muscle and brain, DM patient surveys have identified a high prevalence for gastrointestinal (GI) symptoms amongst affected individuals. Clinical studies have identified chronic and progressive dysfunction of the esophagus, stomach, liver and gallbladder, small and large intestine, and rectum and anal sphincters. Despite the high incidence of GI dysmotility in DM, little is known regarding the pathological mechanisms leading to GI dysfunction. In this review, we summarize results from clinical and molecular analyses of GI dysfunction in both genetic forms of DM, DM type 1 (DM1) and DM type 2 (DM2). Based on current knowledge of DM primary pathological mechanisms in other affected tissues and GI tissue studies, we suggest that misregulation of alternative splicing in smooth muscle resulting from the dysregulation of RNA binding proteins muscleblind-like and CUGBP-elav-like is likely to contribute to GI dysfunction in DM. We propose that a combinatorial approach using clinical and molecular analysis of DM GI tissues and model organisms that recapitulate DM GI manifestations will provide important insight into defects impacting DM GI motility.

Recent Progress and Challenges in the Development of Antisense Therapies for Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a dominant genetic disease in which the expansion of long CTG trinucleotides in the 3' UTR of the myotonic dystrophy protein kinase (DMPK) gene results in toxic RNA gain-of-function and gene mis-splicing affecting mainly the muscles, the heart, and the brain. The CUG-expanded transcripts are a suitable target for the development of antisense oligonucleotide (ASO) therapies. Various chemical modifications of the sugar-phosphate backbone have been reported to significantly enhance the affinity of ASOs for RNA and their resistance to nucleases, making it possible to reverse DM1-like symptoms following systemic administration in different transgenic mouse models. However, specific tissue delivery remains to be improved to achieve significant clinical outcomes in humans. Several strategies, including ASO conjugation to cell-penetrating peptides, fatty acids, or monoclonal antibodies, have recently been shown to improve potency in muscle and cardiac tissues in mice. Moreover, intrathecal administration of ASOs may be an advantageous complementary administration route to bypass the blood-brain barrier and correct defects of the central nervous system in DM1. This review describes the evolution of the chemical design of antisense oligonucleotides targeting CUG-expanded mRNAs and how recent advances in the field may be game-changing by forwarding laboratory findings into clinical research and treatments for DM1 and other microsatellite diseases.

An overview of heart rhythm disorders and management in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is the most common adult form of muscular dystrophy, presenting with a constellation of systemic findings secondary to a CTG triplet expansion of the noncoding region of the DMPK gene. Cardiac involvement is frequent, with conduction disease and supraventricular and ventricular arrhythmias being the most prevalent cardiac manifestations, often developing from a young age. The development of cardiac arrhythmias has been linked to increased morbidity and mortality, with sudden cardiac death well described. Strategies to mitigate risk of arrhythmic death have been developed. In this review, we outline the current knowledge on the pathophysiology of rhythm abnormalities in patients with myotonic dystrophy and summarize available knowledge on arrhythmic risk stratification. We also review management strategies from an electrophysiological perspective, attempting to underline the substantial unmet need to address residual arrhythmic risks for this population.

Comparative genome-wide DNA methylation analysis in myocardial tissue from donors with and without Down syndrome

Down syndrome (DS, trisomy 21) is the most common major chromosomal aneuploidy compatible with life. The additional whole or partial copy of chromosome 21 results in genome-wide imbalances that drive the complex pathobiology of DS. Differential DNA methylation in the context of trisomy 21 may contribute to the variable architecture of the DS phenotype. The goal of this study was to examine the genomic DNA methylation landscape in myocardial tissue from non-fetal individuals with DS. >480,000 unique CpG sites were interrogated in myocardial DNA samples from individuals with (n = 12) and without DS (n = 12) using DNA methylation arrays. A total of 93 highly differentially methylated CpG sites and 16 differentially methylated regions were identified in myocardial DNA from subjects with DS. There were 18 differentially methylated CpG sites in chromosome 21, including 5 highly differentially methylated sites. A CpG site in the RUNX1 locus was differentially methylated in DS myocardium, and linear regression suggests that donors' age, gender, DS status, and RUNX1 methylation may contribute up to ~51% of the variability in RUNX1 mRNA expression. In DS myocardium, only 58% of the genes overlapping with differentially methylated regions codify for proteins with known functions and 24% are non-coding RNAs. This study provides an initial snapshot on the extent of genome-wide differential methylation in myocardial tissue from persons with DS.

Myotonic Dystrophy and Developmental Regulation of RNA Processing

Myotonic dystrophy (DM) is a multisystemic disorder caused by microsatellite expansion mutations in two unrelated genes leading to similar, yet distinct, diseases. DM disease presentation is highly variable and distinguished by differences in age-of-onset and symptom severity. In the most severe form, DM presents with congenital onset and profound developmental defects. At the molecular level, DM pathogenesis is characterized by a toxic RNA gain-of-function mechanism that involves the transcription of noncoding microsatellite expansions. These mutant RNAs disrupt key cellular pathways, including RNA processing, localization, and translation. In DM, these toxic RNA effects are predominantly mediated through the modulation of the muscleblind-like and CUGBP and ETR-3-like factor families of RNA binding proteins (RBPs). Dysfunction of these RBPs results in widespread RNA processing defects culminating in the expression of developmentally inappropriate protein isoforms in adult tissues. The tissue that is the focus of this review, skeletal muscle, is particularly sensitive to mutant RNA-responsive perturbations, as patients display a variety of developmental, structural, and functional defects in muscle. Here, we provide a comprehensive overview of DM1 and DM2 clinical presentation and pathology as well as the underlying cellular and molecular defects associated with DM disease onset and progression. Additionally, fundamental aspects of skeletal muscle development altered in DM are highlighted together with ongoing and potential therapeutic avenues to treat this muscular dystrophy. © 2018 American Physiological Society. Compr Physiol 8:509-553, 2018.

Can Human Pluripotent Stem Cell-Derived Cardiomyocytes Advance Understanding of Muscular Dystrophies?

Muscular dystrophies (MDs) are clinically and molecularly a highly heterogeneous group of single-gene disorders that primarily affect striated muscles. Cardiac disease is present in several MDs where it is an important contributor to morbidity and mortality. Careful monitoring of cardiac issues is necessary but current management of cardiac involvement does not effectively protect from disease progression and cardiac failure. There is a critical need to gain new knowledge on the diverse molecular underpinnings of cardiac disease in MDs in order to guide cardiac treatment development and assist in reaching a clearer consensus on cardiac disease management in the clinic. Animal models are available for the majority of MDs and have been invaluable tools in probing disease mechanisms and in pre-clinical screens. However, there are recognized genetic, physiological, and structural differences between human and animal hearts that impact disease progression, manifestation, and response to pharmacological interventions. Therefore, there is a need to develop parallel human systems to model cardiac disease in MDs. This review discusses the current status of cardiomyocytes (CMs) derived from human induced pluripotent stem cells (hiPSC) to model cardiac disease, with a focus on Duchenne muscular dystrophy (DMD) and myotonic dystrophy (DM1). We seek to provide a balanced view of opportunities and limitations offered by this system in elucidating disease mechanisms pertinent to human cardiac physiology and as a platform for treatment development or refinement.

Mast cells regulate CD4+ T-cell differentiation in the absence of antigen presentation

BACKGROUND

Given their unique capacity for antigen uptake, processing, and presentation, antigen-presenting cells (APCs) are critical for initiating and regulating innate and adaptive immune responses. We have previously shown the role of nicotinamide adenine dinucleotide (NAD⁺) in T-cell differentiation independently of the cytokine milieu, whereas the precise mechanisms remained unknown.

OBJECTIVE

The objective of this study is to further dissect the mechanism of actions of NAD⁺ and determine the effect of APCs on NAD⁺-mediated T-cell activation.

METHODS

Isolated dendritic cells and bone marrow-derived mast cells (MCs) were used to characterize the mechanisms of action of NAD⁺ on CD4⁺ T-cell fate in vitro. Furthermore, NAD⁺-mediated CD4⁺ T-cell differentiation was investigated in vivo by using wild-type C57BL/6, MC^-/-, MHC class II^-/-, Wiskott-Aldrich syndrome protein (WASP)^-/-, 5C.C7 recombination-activating gene 2 (Rag2)^-/-, and CD11b-DTR transgenic mice. Finally, we tested the physiologic effect of NAD⁺ on the systemic immune response in the context of Listeria monocytogenes infection.

RESULTS

Our in vivo and in vitro findings indicate that after NAD⁺ administration, MCs exclusively promote CD4⁺ T-cell differentiation, both in the absence of antigen and independently of major APCs. Moreover, we found that MCs mediated CD4⁺ T-cell differentiation independently of MHC II and T-cell receptor signaling machinery. More importantly, although treatment with NAD⁺ resulted in decreased MHC II expression on CD11c⁺ cells, MC-mediated CD4⁺ T-cell differentiation rendered mice resistant to administration of lethal doses of L monocytogenes.

CONCLUSIONS

Collectively, our study unravels a novel cellular and molecular pathway that regulates innate and adaptive immunity through MCs exclusively and underscores the therapeutic potential of NAD⁺ in the context of primary immunodeficiencies and antimicrobial resistance.

The pyrazolyl-urea GeGe3 inhibits tumor angiogenesis and reveals dystrophia myotonica protein kinase (DMPK)1 as a novel angiogenesis target

The limitation of targeting VEGF/VEGFR2 signalling to stop angiogenesis in cancer therapy has been blamed on re-activation of alternative receptor tyrosine kinases by compensatory angiogenic factors. Targeting MAPK and PI3K signaling pathways in endothelial cells may be an alternative or complementary approach. Herein we aimed to evaluate the antitumor and antiangiogenic potential of a novel pyrazolyl-urea kinase inhibitor, GeGe3, and to identify its kinase targets.

We found GeGe3 to inhibit the proliferation of HUVEC and endothelial tube formation. GeGe3 impaired inter-segmental angiogenesis during development of zebrafish embryos. In mice, GeGe3 blocked angiogenesis and tumor growth in transplanted subcutaneous Lewis Lung Carcinomas. Screening for GeGe3-targeted kinases revealed Aurora B, Aurora C, NEK10, polo-like kinase (PLK)2, PLK3, DMPK1 and CAMK1 as candidate targets. Biochemical analysis of these kinases showed DMPK1 regulation upon VEGF challenge. Investigation of the role of DMPK1 in endothelial cells revealed DMPK1 as a novel mediator of angiogenesis that controls the activation of MAPK signaling, proliferation and migration. GeGe3 alters angiogenesis by targeting DMPK in tumor endothelial cells and pericytes.

The pyrazolyl-urea GeGe3, a novel blocker of MAPK and PI3K pathways, strongly inhibits physiological and tumor angiogenesis. We also report GeGe3-targeted kinase DMPK as a novel mediator of angiogenesis.

Serum response factor regulates smooth muscle contractility via myotonic dystrophy protein kinases and L-type calcium channels

Serum response factor (SRF) transcriptionally regulates expression of contractile genes in smooth muscle cells (SMC). Lack or decrease of SRF is directly linked to a phenotypic change of SMC, leading to hypomotility of smooth muscle in the gastrointestinal (GI) tract. However, the molecular mechanism behind SRF-induced hypomotility in GI smooth muscle is largely unknown. We describe here how SRF plays a functional role in the regulation of the SMC contractility via myotonic dystrophy protein kinase (DMPK) and L-type calcium channel CACNA1C. GI SMC expressed Dmpk and Cacna1c genes into multiple alternative transcriptional isoforms. Deficiency of SRF in SMC of Srf knockout (KO) mice led to reduction of SRF-dependent DMPK, which down-regulated the expression of CACNA1C. Reduction of CACNA1C in KO SMC not only decreased intracellular Ca2+ spikes but also disrupted their coupling between cells resulting in decreased contractility. The role of SRF in the regulation of SMC phenotype and function provides new insight into how SMC lose their contractility leading to hypomotility in pathophysiological conditions within the GI tract.

Muscle wasting in myotonic dystrophies: a model of premature aging

Myotonic dystrophy type 1 (DM1 or Steinert’s disease) and type 2 (DM2) are multisystem disorders of genetic origin. Progressive muscular weakness, atrophy and myotonia are the most prominent neuromuscular features of these diseases, while other clinical manifestations such as cardiomyopathy, insulin resistance and cataracts are also common. From a clinical perspective, most DM symptoms are interpreted as a result of an accelerated aging (cataracts, muscular weakness and atrophy, cognitive decline, metabolic dysfunction, etc.), including an increased risk of developing tumors. From this point of view, DM1 could be described as a progeroid syndrome since a notable age-dependent dysfunction of all systems occurs. The underlying molecular disorder in DM1 consists of the existence of a pathological (CTG) triplet expansion in the 3′ untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene, whereas (CCTG)n repeats in the first intron of the Cellular Nucleic acid Binding Protein/Zinc Finger Protein 9(CNBP/ZNF9) gene cause DM2. The expansions are transcribed into (CUG)n and (CCUG)n-containing RNA, respectively, which form secondary structures and sequester RNA-binding proteins, such as the splicing factor muscleblind-like protein (MBNL), forming nuclear aggregates known as foci. Other splicing factors, such as CUGBP, are also disrupted, leading to a spliceopathy of a large number of downstream genes linked to the clinical features of these diseases. Skeletal muscle regeneration relies on muscle progenitor cells, known as satellite cells, which are activated after muscle damage, and which proliferate and differentiate to muscle cells, thus regenerating the damaged tissue. Satellite cell dysfunction seems to be a common feature of both age-dependent muscle degeneration (sarcopenia) and muscle wasting in DM and other muscle degenerative diseases. This review aims to describe the cellular, molecular and macrostructural processes involved in the muscular degeneration seen in DM patients, highlighting the similarities found with muscle aging.

Lack of myotubularin (MTM1) leads to muscle hypotrophy through unbalanced regulation of the autophagy and ubiquitin-proteasome pathways

Mutations in the phosphoinositide phosphatase myotubularin (MTM1) results in X-linked myotubular/centronuclear myopathy (XLMTM), characterized by a severe decrease in muscle mass and strength in patients and murine models. However, the molecular mechanism involved in the muscle hypotrophy is unclear. Here we show that the IGF1R/Akt pathway is affected in Mtm1-deficient murine muscles, characterized by an increase in IGF1 receptor and Akt levels in both the presymptomatic and symptomatic phases. Moreover, up-regulation of atrogenes was observed in the presymptomatic phase of the myopathy, supporting overactivation of the ubiquitin-proteasome pathway. In parallel, the autophagy machinery was affected as indicated by the increase in the number of autophagosomes and of autophagy markers, such as LC3 and P62. However, phosphorylation of FOXO3a and mTOR were abnormal at late but not at early stages of the disease, suggesting that myotubularin acts both upstream in the IGF1R/Akt pathway and downstream on the balance between the autophagy and ubiquitin-proteasome pathways in vivo. Adeno-associated virus-mediated delivery of Mtm1 into Mtm1-null muscles rescued muscle mass and normalized the expression levels of IGF1 receptor, the ubiquitin-proteasome pathway, and autophagy markers. These data support the hypothesis that the unbalanced regulation of the ubiquitin proteasome pathway and the autophagy machinery is a primary cause of the XLMTM pathogenesis.

Phosphorylation target site specificity for AGC kinases DMPK E and Lats2

Serine/threonine kinases of the AGC group are important regulators of cell growth and motility. To examine the candidate substrate profile for two members of this group, DMPK E and Lats2, we performed in vitro kinase assays on peptide arrays. Substrate peptides for both kinases exhibited a predominance of basic residues surrounding the phosphorylation target site. 3D homology modeling of the kinase domains of DMPK E and Lats2 indicated that presence of two negative pockets in the peptide binding groove provides an explanation for the substrate preference. These findings will aid future research toward signaling functions of Lats2 and DMPK E within cells.

Myotonic dystrophy protein kinase is critical for nuclear envelope integrity

Myotonic dystrophy 1 (DM1) is a multisystemic disease caused by a triplet nucleotide repeat expansion in the 3' untranslated region of the gene coding for myotonic dystrophy protein kinase (DMPK). DMPK is a nuclear envelope (NE) protein that promotes myogenic gene expression in skeletal myoblasts. Muscular dystrophy research has revealed the NE to be a key determinant of nuclear structure, gene regulation, and muscle function. To investigate the role of DMPK in NE stability, we analyzed DMPK expression in epithelial and myoblast cells. We found that DMPK localizes to the NE and coimmunoprecipitates with Lamin-A/C. Overexpression of DMPK in HeLa cells or C2C12 myoblasts disrupts Lamin-A/C and Lamin-B1 localization and causes nuclear fragmentation. Depletion of DMPK also disrupts NE lamina, showing that DMPK is required for NE stability. Our data demonstrate for the first time that DMPK is a critical component of the NE. These novel findings suggest that reduced DMPK may contribute to NE instability, a common mechanism of skeletal muscle wasting in muscular dystrophies.

[Heart and Steinert's disease]

Myotonic dystrophy type 1 (Steinert disease) is an autosomal dominant disease characterized by myotonia and multiorgan damage. This latter is the most frequent of the adult-onset muscular dystrophies. Heart involvement is often associated, including cardiomyopathies, atrioventricular block, atrial and ventricular arrhythmias.

Mass spectrometry analysis of complexes formed by myotonic dystrophy protein kinase (DMPK)

Myotonic dystrophy type 1 (DM1) is caused by an expansion of CTG repeats at the 3'-UTR of the serine/threonine protein kinase DMPK. Expanded CTG repeats are toxic since they are transcribed into an RNA molecule which is then sequestered within the nucleus in the form of foci. RNA cytotoxicity is linked to the aberrant splicing of several developmentally regulated genes. DMPK transcripts undergo alternative splicing giving rise to many isoforms but do not seem to be involved in the splicing dysregulation of DM1. However, decreased levels of DMPK in DM1 patients and DMPK involvement in muscle weakness and cardiac dysfunction in animal models have been reported. The variability in phenotypic expression of DMPK together with its differential subcellular targeting, suggests that different splicing isoforms may be involved in different signalling pathways, possibly through DMPK-interacting proteins. To gain better insight into the DMPK function, we used mass spectrometry to identify proteins co-segregating with DMPK in soluble complexes isolated from high-speed supernatant of rat muscles. We carried out experiments with native DMPK to preserve the physiological stoichiometry with potential partners. DMPK-containing complexes were isolated and immuno-detected by non-denaturing electrophoresis, gel filtration, ionic-exchange chromatography and immunoprecipitation. DMPK peptides were identified by high-resolution mass spectrometry together with several putative DMPK-binding proteins, including several heat shock proteins such as HSP20/HSPB6, HSP60/CPN60, HSP70 and HSP90. We also obtained evidence of a direct interaction of DMPK with alphaB-crystallin/HSPB5 and HSP25/HSPB1.

DMPK protein isoforms are differentially expressed in myogenic and neural cell lineages

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by an unstable (CTG . CAG)n segment in the 3' untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. It is commonly accepted that DMPK mRNA-based toxicity is the main contributor to DM1 manifestations; however, not much is known about the significance of the DMPK protein. To appreciate its normal and possible pathobiological role, we analyzed the patterns of DMPK splice isoform expression in mouse tissues. Long membrane-anchored DMPK dominated in heart, diaphragm, and skeletal muscle, whereas short cytosolic isoforms were highly expressed in bladder and stomach. Both isoform types were present in diverse brain regions. DMPK protein was also detectable in cultured myoblasts, myotubes, cortical astrocytes, and related cell lines of neural or muscle origin, but not in hippocampal neurons. This work identifies DMPK as a kinase with pronounced expression in diverse muscle and neural tissues that are affected in DM1.

Rational design of ligands targeting triplet repeating transcripts that cause RNA dominant disease: application to myotonic muscular dystrophy type 1 and spinocerebellar ataxia type 3

Herein, we describe the design of high affinity ligands that bind expanded rCUG and rCAG repeat RNAs expressed in myotonic dystrophy type 1 (DM1) and spinocerebellar ataxia type 3. These ligands also inhibit, with nanomolar IC(50) values, the formation of RNA-protein complexes that are implicated in both disorders. The expanded rCUG and rCAG repeats form stable RNA hairpins with regularly repeating internal loops in the stem and have deleterious effects on cell function. The ligands that bind the repeats display a derivative of the bisbenzimidazole Hoechst 33258, which was identified by searching known RNA-ligand interactions for ligands that bind the internal loop displayed in these hairpins. A series of 13 modularly assembled ligands with defined valencies and distances between ligand modules was synthesized to target multiple motifs in these RNAs simultaneously. The most avid binder, a pentamer, binds the rCUG repeat hairpin with a K(d) of 13 nM. When compared to a series of related RNAs, the pentamer binds to rCUG repeats with 4.4- to >200-fold specificity. Furthermore, the affinity of binding to rCUG repeats shows incremental gains with increasing valency, while the background binding to genomic DNA is correspondingly reduced. Then, it was determined whether the modularly assembled ligands inhibit the recognition of RNA repeats by Muscleblind-like 1 (MBNL1) protein, the expanded-rCUG binding protein whose sequestration leads to splicing defects in DM1. Among several compounds with nanomolar IC(50) values, the most potent inhibitor is the pentamer, which also inhibits the formation of rCAG repeat-MBNL1 complexes. Comparison of the binding data for the designed synthetic ligands and MBNL1 to repeating RNAs shows that the synthetic ligand is 23-fold higher affinity and more specific to DM1 RNAs than MBNL1. Further studies show that the designed ligands are cell permeable to mouse myoblasts. Thus, cell permeable ligands that bind repetitive RNAs have been designed that exhibit higher affinity and specificity for binding RNA than natural proteins. These studies suggest a general approach to targeting RNA, including those that cause RNA dominant disease.

Structure of dystrophia myotonica protein kinase

Dystrophia myotonica protein kinase (DMPK) is a serine/threonine kinase composed of a kinase domain and a coiled-coil domain involved in the multimerization. The crystal structure of the kinase domain of DMPK bound to the inhibitor bisindolylmaleimide VIII (BIM-8) revealed a dimeric enzyme associated by a conserved dimerization domain. The affinity of dimerisation suggested that the kinase domain alone is insufficient for dimerisation in vivo and that the coiled-coil domains are required for stable dimer formation. The kinase domain is in an active conformation, with a fully-ordered and correctly positioned alphaC helix, and catalytic residues in a conformation competent for catalysis. The conserved hydrophobic motif at the C-terminal extension of the kinase domain is bound to the N-terminal lobe of the kinase domain, despite being unphosphorylated. Differences in the arrangement of the C-terminal extension compared to the closely related Rho-associated kinases include an altered PXXP motif, a different conformation and binding arrangement for the turn motif, and a different location for the conserved NFD motif. The BIM-8 inhibitor occupies the ATP site and has similar binding mode as observed in PDK1.

Adenylyl cyclase type VI increases Akt activity and phospholamban phosphorylation in cardiac myocytes

Increased expression of adenylyl cyclase VI has beneficial effects on the heart, but strategies that increase cAMP production in cardiac myocytes usually are harmful. Might adenylyl cyclase VI have beneficial effects unrelated to increased beta-adrenergic receptor-mediated signaling? We previously reported that adenylyl cyclase VI reduces cardiac phospholamban expression. Our focus in the current studies is how adenylyl cyclase VI influences phospholamban phosphorylation. In cultured cardiac myocytes, increased expression of adenylyl cyclase VI activates Akt by phosphorylation at serine 473 and threonine 308 and is associated with increased nuclear phospho-Akt. Activated Akt phosphorylates phospholamban, a process that does not require beta-adrenergic receptor stimulation or protein kinase A activation. These previously unrecognized signaling events would be predicted to promote calcium handling and increase contractile function of the intact heart independently of beta-adrenergic receptor activation. We speculate that phospholamban phosphorylation, through activation of Akt, may be an important mechanism by which adenylyl cyclase VI increases the function of the failing heart.

Myotonic dystrophy protein kinase (DMPK) and its role in the pathogenesis of myotonic dystrophy 1

Myotonic dystrophy 1 (DM1) is an autosomal, dominant inherited, neuromuscular disorder. The DM1 mutation consists in the expansion of an unstable CTG-repeat in the 3'-untranslated region of a gene encoding DMPK (myotonic dystrophy protein kinase). Clinical expression of DM1 is variable, presenting a progressive muscular dystrophy that affects distal muscles more than proximal and is associated with the inability to relax muscles appropriately (myotonia), cataracts, cardiac arrhythmia, testicular atrophy and insulin resistance. DMPK is a Ser/Thr protein kinase homologous to the p21-activated kinases MRCK and ROCK/rho-kinase/ROK. The most abundant isoform of DMPK is an 80 kDa protein mainly expressed in smooth, skeletal and cardiac muscles. Decreased DMPK protein levels may contribute to the pathology of DM1, as revealed by gene target studies. Here we review current understanding of the structural, functional and pathophysiological characteristics of DMPK.

Toward biologically targeted therapy of calcium cycling defects in heart failure

A growing body of evidence indicates that heart failure progression is tightly associated with dysregulation of phosphorylation of Ca2+ regulators localized in the sub-cellular microdomain of the sarcoplasmic reticulum. Chemical or genetic correction of abnormalities in cardiac phosphorylation cascades is emerging as a potential target in the treatment of heart failure. Here, we review how specific kinases and phosphatases finely tune Ca2+ cycling and regulate excitation-contraction (E-C) coupling in cardiomyocytes.

Role of myotonic dystrophy protein kinase (DMPK) in glucose homeostasis and muscle insulin action

Myotonic dystrophy 1 (DM1) is caused by a CTG expansion in the 3′-unstranslated region of the DMPK gene, which encodes a serine/threonine protein kinase. One of the common clinical features of DM1 patients is insulin resistance, which has been associated with a pathogenic effect of the repeat expansions. Here we show that DMPK itself is a positive modulator of insulin action. DMPK-deficient (dmpk^−/−) mice exhibit impaired insulin signaling in muscle tissues but not in adipocytes and liver, tissues in which DMPK is not expressed. Dmpk^−/− mice display metabolic derangements such as abnormal glucose tolerance, reduced glucose uptake and impaired insulin-dependent GLUT4 trafficking in muscle. Using DMPK mutants, we show that DMPK is required for a correct intracellular trafficking of insulin and IGF-1 receptors, providing a mechanism to explain the molecular and metabolic phenotype of dmpk^−/− mice. Taken together, these findings indicate that reduced DMPK expression may directly influence the onset of insulin-resistance in DM1 patients and point to dmpk as a new candidate gene for susceptibility to type 2-diabetes.

Phospholamban Interacts with HAX-1, a Mitochondrial Protein with Anti-apoptotic Function

Phospholamban (PLN) is a key regulator of Ca(2+) homeostasis and contractility in the heart. Its regulatory effects are mediated through its interaction with the sarcoplasmic reticulum Ca(2+)-ATPase, (SERCA2a), resulting in alterations of its Ca(2+)-affinity. To identify additional proteins that may interact with PLN, we used the yeast-two-hybrid system to screen an adult human cardiac cDNA library. HS-1 associated protein X-1 (HAX-1) was identified as a PLN-binding partner. The minimal binding regions were mapped to amino acid residues 203-245 for HAX-1 and residues 16-22 for PLN. The interaction between the two proteins was confirmed using GST-HAX-1, bound to the glutathione-matrix, which specifically adsorbed native PLN from human or mouse cardiac homogenates, while in reciprocal binding studies, recombinant His-HAX-1 bound GST-PLN. Kinetic studies using surface plasmon resonance yielded a K(D) of approximately 1 muM as the binding affinity for the PLN/HAX-1 complex. Phosphorylation of PLN by cAMP-dependent protein kinase reduced binding to HAX-1, while increasing concentrations of Ca(2+) diminished the PLN/HAX-1 interaction in a dose-dependent manner. HAX-1 concentrated to mitochondria, but upon transient co-transfection of HEK 293 cells with PLN, HAX-1 redistributed and co-localized with PLN at the endoplasmic reticulum. Analysis of the anti-apoptotic function of HAX-1 revealed that the presence of PLN enhanced the HAX-1 protective effects from hypoxia/reoxygenation-induced cell death. These findings suggest a possible link between the Ca(2+) handling by the sarcoplasmic reticulum and cell survival mediated by the PLN/HAX-1 interaction.

Downregulation of two novel genes in Sl/Sld and W(LacZ)/Wv mouse jejunum

Interstitial cells of Cajal (ICC) are the so-called pacemaker cells of the gut. W(LacZ)/Wv and Sl/Sld mice lack ICC surrounding the myenteric plexus (MP) in the jejunum. We compared the gene expression profile of wild type (WT) and W(LacZ)/Wv and Sl/Sld mice using suppression subtractive hybridization (SSH), generating a cDNA library of 1303 clones from which 48 unique sequences were differentially expressed with Southern blot. Among them, we identified heme oxygenase2, TROY, and phospholamban in ICC using immunohistochemistry. Using RT-qPCR, c-Kit and two new transcripts Dithp and prenylcysteine oxidase1 were significantly lower expressed in Sl/Sld and W(LacZ)/Wv versus WT. Prenylcysteine oxidase1 appeared cytotoxic for COS-7 cells and was highly expressed in liver while Dithp was mainly expressed in small intestine. The combination of SSH, Southern blot, RT-qPCR, and immunohistochemistry turned out to be a useful approach to identify rarely expressed genes and genes with small differences in expression.

Misregulation of alternative splicing causes pathogenesis in myotonic dystrophy

Myotonic dystrophy (DM), the most common form of adult onset muscular dystrophy, affects skeletal muscle, heart, and the central nervous system (CNS). Mortality results primarily from muscle wasting and cardiac arrhythmias. There are two forms of the disease: DM1 and DM2. DM1, which constitutes 98% of cases, is caused by a CTG expansion in the 3' untranslated region (UTR) of the DMPK gene. DM2 is caused by a CCTG expansion in the first intron of the ZNF9 gene. RNA containing CUG- or CCUG-expanded repeats are transcribed but are retained in the nucleus in foci. Disease pathogenesis results primarily from a gain of function of the expanded RNAs, which alter developmentally regulated alternative splicing as well as pathways of muscle differentiation. The toxic RNA has been implicated in sequestration of splicing regulators and transcription factors thereby causing specific symptoms of the disease. Here we review the proposed mechanisms for the toxic effects of the expanded repeats and discuss the molecular mechanisms of splicing misregulation and disease pathogenesis.