CMYA5 establishes cardiac dyad architecture and positioning

Cardiac excitation-contraction coupling requires dyads, the nanoscopic microdomains formed adjacent to Z-lines by apposition of transverse tubules and junctional sarcoplasmic reticulum. Disruption of dyad architecture and function are common features of diseased cardiomyocytes. However, little is known about the mechanisms that modulate dyad organization during cardiac development, homeostasis, and disease. Here, we use proximity proteomics in intact, living hearts to identify proteins enriched near dyads. Among these proteins is CMYA5, an under-studied striated muscle protein that co-localizes with Z-lines, junctional sarcoplasmic reticulum proteins, and transverse tubules in mature cardiomyocytes. During cardiac development, CMYA5 positioning adjacent to Z-lines precedes junctional sarcoplasmic reticulum positioning or transverse tubule formation. CMYA5 ablation disrupts dyad architecture, dyad positioning at Z-lines, and junctional sarcoplasmic reticulum Ca2+ release, leading to cardiac dysfunction and inability to tolerate pressure overload. These data provide mechanistic insights into cardiomyopathy pathogenesis by demonstrating that CMYA5 anchors junctional sarcoplasmic reticulum to Z-lines, establishes dyad architecture, and regulates dyad Ca2+ release.

The long noncoding RNA Meg3 regulates myoblast plasticity and muscle regeneration through epithelial-mesenchymal transition

Formation of skeletal muscle is among the most striking examples of cellular plasticity in animal tissue development, and while muscle progenitor cells are reprogrammed by epithelial-mesenchymal transition (EMT) to migrate during embryonic development, the regulation of EMT in post-natal myogenesis remains poorly understood. Here, we demonstrate that the long noncoding RNA (lncRNA) Meg3 regulates EMT in myoblast differentiation and skeletal muscle regeneration. Chronic inhibition of Meg3 in C2C12 myoblasts induced EMT, and suppressed cell state transitions required for differentiation. Furthermore, adenoviral Meg3 knockdown compromised muscle regeneration, which was accompanied by abnormal mesenchymal gene expression and interstitial cell proliferation. Transcriptomic and pathway analyses of Meg3-depleted C2C12 myoblasts and injured skeletal muscle revealed a significant dysregulation of EMT-related genes, and identified TGFβ as a key upstream regulator. Importantly, inhibition of TGFβR1 and its downstream effectors, and the EMT transcription factor Snai2, restored many aspects of myogenic differentiation in Meg3-depleted myoblasts in vitro We further demonstrate that reduction of Meg3-dependent Ezh2 activity results in epigenetic alterations associated with TGFβ activation. Thus, Meg3 regulates myoblast identity to facilitate progression into differentiation.

ERK Regulates NeuroD1-mediated Neurite Outgrowth via Proteasomal Degradation

Neurogenic differentiation 1 (NeuroD1) is a class B basic helix-loop-helix (bHLH) transcription factor and regulates differentiation and survival of neuronal and endocrine cells by means of several protein kinases, including extracellular signal-regulated kinase (ERK). However, the effect of phosphorylation on the functions of NeuroD1 by ERK has sparked controversy based on context-dependent differences across diverse species and cell types. Here, we evidenced that ERK-dependent phosphorylation controlled the stability of NeuroD1 and consequently, regulated proneural activity in neuronal cells. A null mutation at the ERK-dependent phosphorylation site, S274A, increased the half-life of NeuroD1 by blocking its ubiquitin-dependent proteasomal degradation. The S274A mutation did not interfere with either the nuclear translocation of NeuroD1 or its heterodimerization with E47, its ubiquitous partner and class A bHLH transcription factor. However, the S274A mutant increased transactivation of the E-box-mediated gene and neurite outgrowth in F11 neuroblastoma cells, compared to the wild-type NeuroD1. Transcriptome and Gene Ontology enrichment analyses indicated that genes involved in axonogenesis and dendrite development were downregulated in NeuroD1 knockout (KO) mice. Overexpression of the S274A mutant salvaged neurite outgrowth in NeuroD1-deficient mice, whereas neurite outgrowth was minimal with S274D, a phosphomimicking mutant. Our data indicated that a longer protein half-life enhanced the overall activity of NeuroD1 in stimulating downstream genes and neuronal differentiation. We propose that blocking ubiquitin-dependent proteasomal degradation may serve as a strategy to promote neuronal activity by stimulating the expression of neuron-specific genes in differentiating neurons.

A schizophrenia associated CMYA5 allele displays differential binding with desmin

CMYA5 is a candidate gene for schizophrenia because of the genetic association of variant rs10043986 (C > T) to this severe mental disorder. Studies of CMYA5 and its gene product, myospryn, in the brain and neuronal cells have not been previously reported. The SNP rs10043986 changes the 4,063rd amino acid from Pro to Leu, which is likely to alter protein function. To understand its potential role in the brain, we examined the neuronal expression of myospryn and its binding partner, desmin, an intermediate filament (IF) protein, and investigated how the two alleles of myospryn affect its binding to desmin. Myospryn and desmin are shown to be expressed in the brain and myospryn is shown to localize to the cytoplasm and nucleus of myoblast, neuroblastoma, and glioblastoma cell lines. Peripherin and vimentin, known brain IF proteins, have high protein similarity to desmin but were found not to interact with myospryn using yeast two-hybrid (Y2H). Using a quantitative Y2H assay and surface plasmon resonance, the T allele (Leu) of rs10043986 was found to have stronger binding to desmin than the C allele (Pro). Based on findings described in this report, we hypothesize that the interaction between myospryn to IF provides structural support and efficient rearrangement of the cytoskeleton network during early neuritogenesis.

Transcriptome Analysis of Cardiac Hypertrophic Growth in MYBPC3-Null Mice Suggests Early Responders in Hypertrophic Remodeling

Rationale: With a prevalence of 1 in 200 individuals, hypertrophic cardiomyopathy (HCM) is thought to be the most common genetic cardiac disease, with potential outcomes that include severe hypertrophy, heart failure, and sudden cardiac death (SCD). Though much research has furthered our understanding of how HCM-causing mutations in genes such as cardiac myosin-binding protein C (MYBPC3) impair contractile function, it remains unclear how such dysfunction leads to hypertrophy and/or arrhythmias, which comprise the HCM phenotype. Identification of early response mediators could provide rational therapeutic targets to reduce disease severity. Our goal was to differentiate physiologic and pathophysiologic hypertrophic growth responses and identify early genetic mediators in the development of cardiomegaly in the cardiac myosin-binding protein C-null (cMyBP-C^-/-) mouse model of HCM.

Methods and Results: We performed microarray analysis on left ventricles of wild-type (WT) and cMyBPC^-/- mice (n = 7 each) at postnatal day (PND) 1 and PND 9, before and after the appearance of an overt HCM phenotype. Applying the criteria of ≥2-fold change, we identified genes whose change was exclusive to pathophysiologic growth (n = 61), physiologic growth (n = 30), and genes whose expression changed ≥2-fold in both WT and cMyBP-C^-/- hearts (n = 130). Furthermore, we identified genes that were dysregulated in PND1 cMyBP-C^-/- hearts prior to hypertrophy, including genes in mechanosensing pathways and potassium channels linked to arrhythmias. One gene of interest, Xirp2, and its protein product, are regulated during growth but also show early, robust prehypertrophic upregulation in cMyBP-C^-/- hearts. Additionally, the transcription factor Zbtb16 also shows prehypertrophic upregulation at both gene and protein levels.

Conclusion: Our transcriptome analysis generated a comprehensive data set comparing physiologic vs. hypertrophic growth in mice lacking cMyBP-C. It highlights the importance of extracellular matrix pathways in hypertrophic growth and early dysregulation of potassium channels. Prehypertrophic upregulation of Xirp2 in cMyBP-C^-/- hearts supports a growing body of evidence suggesting Xirp2 has the capacity to elicit both hypertrophy and arrhythmias in HCM. Dysregulation of Xirp2, as well as Zbtb16, along with other genes associated with mechanosensing regions of the cardiomyocyte implicate stress-sensing in these regions as a potentially important early response in HCM.

The transcription factor MEF2A fine-tunes gene expression in the atrial and ventricular chambers of the adult heart

The distinct morphological and functional properties of the cardiac chambers arise from an elaborate developmental program involving cell lineage determination, morphogenesis, and dynamic spatiotemporal gene expression patterns. Although a number of transcription factors have been identified for proper gene regulation in the chambers, the complete transcriptional network that controls these patterns remains poorly defined. Previous studies have implicated the MEF2C transcription factor in the regulation of chamber-restricted enhancers. To better understand the mechanisms of MEF2-mediated regional gene regulation in the heart, we took advantage of MEF2A knock-out (KO) mice, a model that displays a predominantly ventricular chamber phenotype. Transcriptomic analysis of atrial and ventricular tissue from adult MEF2A KO hearts revealed a striking difference in chamber gene expression, with a larger proportion of dysregulated genes in the atrial chambers. Canonical pathway analysis of genes preferentially dysregulated in the atria and ventricles revealed distinct MEF2A-dependent cellular processes in each cardiac chamber. In the atria, MEF2A regulated genes involved in fibrosis and adhesion, whereas in the ventricles, it controlled inflammation and endocytosis. Finally, analysis of transcription factor-binding site motifs of differentially dysregulated genes uncovered distinct MEF2A co-regulators for the atrial and ventricular gene sets, and a subset of these was found to cooperate with MEF2A. In conclusion, our results suggest a mechanism in which MEF2 transcriptional activity is differentially recruited to fine-tune gene expression levels in each cardiac chamber. This regulatory mechanism ensures optimal output of these gene products for proper physiological function of the atrial and ventricular chambers.

Antagonistic regulation of cell-cycle and differentiation gene programs in neonatal cardiomyocytes by homologous MEF2 transcription factors

Cardiomyocytes acquire their primary specialized function (contraction) before exiting the cell cycle. In this regard, proliferation and differentiation must be precisely coordinated for proper cardiac morphogenesis. Here, we have investigated the complex transcriptional mechanisms employed by cardiomyocytes to coordinate antagonistic cell-cycle and differentiation gene programs through the molecular dissection of the core cardiac transcription factor, MEF2. Knockdown of individual MEF2 proteins, MEF2A, -C, and -D, in primary neonatal cardiomyocytes resulted in radically distinct and opposite effects on cellular homeostasis and gene regulation. MEF2A and MEF2D were absolutely required for cardiomyocyte survival, whereas MEF2C, despite its major role in cardiac morphogenesis and direct reprogramming, was dispensable for this process. Inhibition of MEF2A or -D also resulted in the activation of cell-cycle genes and down-regulation of markers of terminal differentiation. In striking contrast, the regulation of cell-cycle and differentiation gene programs by MEF2C was antagonistic to that of MEF2A and -D. Computational analysis of regulatory regions from MEF2 isoform-dependent gene sets identified the Notch and Hedgehog signaling pathways as key determinants in coordinating MEF2 isoform-specific control of antagonistic gene programs. These results reveal that mammalian MEF2 family members have distinct transcriptional functions in cardiomyocytes and suggest that these differences are critical for proper development and maturation of the heart. Analysis of MEF2 isoform-specific function in neonatal cardiomyocytes has yielded insight into an unexpected transcriptional regulatory mechanism by which these specialized cells utilize homologous members of a core cardiac transcription factor to coordinate cell-cycle and differentiation gene programs.

The Function of the MEF2 Family of Transcription Factors in Cardiac Development, Cardiogenomics, and Direct Reprogramming

Proper formation of the mammalian heart requires precise spatiotemporal transcriptional regulation of gene programs in cardiomyocytes. Sophisticated regulatory networks have evolved to not only integrate the activities of distinct transcription factors to control tissue-specific gene programs but also, in many instances, to incorporate multiple members within these transcription factor families to ensure accuracy and specificity in the system. Unsurprisingly, perturbations in this elaborate transcriptional circuitry can lead to severe cardiac abnormalities. Myocyte enhancer factor–2 (MEF2) transcription factor belongs to the evolutionarily conserved cardiac gene regulatory network. Given its central role in muscle gene regulation and its evolutionary conservation, MEF2 is considered one of only a few core cardiac transcription factors. In addition to its firmly established role as a differentiation factor, MEF2 regulates wide variety of, sometimes antagonistic, cellular processes such as cell survival and death. Vertebrate genomes encode multiple MEF2 family members thereby expanding the transcriptional potential of this core transcription factor in the heart. This review highlights the requirement of the MEF2 family and their orthologs in cardiac development in diverse animal model systems. Furthermore, we describe the recently characterized role of MEF2 in direct reprogramming and genome-wide cardiomyocyte gene regulation. A thorough understanding of the regulatory functions of the MEF2 family in cardiac development and cardiogenomics is required in order to develop effective therapeutic strategies to repair the diseased heart.

miR-410 and miR-495 Are Dynamically Regulated in Diverse Cardiomyopathies and Their Inhibition Attenuates Pathological Hypertrophy

Noncoding RNAs have emerged as important modulators in cardiac development and pathological remodeling. Recently, we demonstrated that regulation of the Gtl2-Dio3 noncoding RNA locus is dependent on the MEF2 transcription factor in cardiac muscle, and that two of its encoded miRNAs, miR-410 and miR-495, induce robust cardiomyocyte proliferation. Given the possibility of manipulating the expression of these miRNAs to repair the damaged heart by stimulating cardiomyocyte proliferation, it is important to determine whether the Gtl2-Dio3 noncoding RNAs are regulated in cardiac disease and whether they function downstream of pathological cardiac stress signaling. Therefore, we examined expression of the above miRNAs processed from the Gtl2-Dio3 locus in various cardiomyopathies. These noncoding RNAs were upregulated in all cardiac disease models examined including myocardial infarction (MI) and chronic angiotensin II (Ang II) stimulation, and in the cardiomyopathies associated with muscular dystrophies. Consistent with these observations, we show that the Gtl2-Dio3 proximal promoter is activated by stress stimuli in cardiomyocytes and requires MEF2 for its induction. Furthermore, inhibiting miR-410 or miR-495 in stressed cardiomyocytes attenuated the hypertrophic response. Thus, the Gtl2-Dio3 noncoding RNA locus is a novel marker of cardiac disease and modulating the activity of its encoded miRNAs may mitigate pathological cardiac remodeling in these diseases.

MEF2D deficiency in neonatal cardiomyocytes triggers cell cycle re-entry and programmed cell death in vitro

The cardiomyocyte cell cycle is a poorly understood process. Mammalian cardiomyocytes permanently withdraw from the cell cycle shortly after birth but can re-enter the cell cycle and proliferate when subjected to injury within a brief temporal window in the neonatal period. Thus, investigating the mechanisms of cell cycle regulation in neonatal cardiomyocytes may provide critical insight into the molecular events that prevent adult myocytes from proliferating in response to injury or stress. MEF2D is a key transcriptional mediator of pathological remodeling in the adult heart downstream of various stress-promoting insults. However, the specific gene programs regulated by MEF2D in cardiomyocytes are unknown. By performing genome-wide transcriptome analysis using MEF2D-depleted neonatal cardiomyocytes, we found a significant impairment in the cell cycle, characterized by the up-regulation of numerous positive cell cycle regulators. Expression of Pten, the primary negative regulator of PI3K/Akt, was significantly reduced in MEF2D-deficient cardiomyocytes and found to be a direct target gene of MEF2D. Consistent with these findings mutant cardiomyocytes showed activation of the PI3K/Akt survival pathway. Paradoxically, prolonged deficiency of MEF2D in neonatal cardiomyocytes did not trigger proliferation but instead resulted in programmed cell death, which is likely mediated by the E2F transcription factor. These results demonstrate a critical role for MEF2D in cell cycle regulation of post-mitotic, neonatal cardiomyocytes in vitro.

MicroRNAs in the Myocyte Enhancer Factor 2 (MEF2)-regulated Gtl2-Dio3 Noncoding RNA Locus Promote Cardiomyocyte Proliferation by Targeting the Transcriptional Coactivator Cited2

Understanding cell cycle regulation in postmitotic cardiomyocytes may lead to new therapeutic approaches to regenerate damaged cardiac tissue. We have demonstrated previously that microRNAs encoded by the Gtl2-Dio3 noncoding RNA locus function downstream of the MEF2A transcription factor in skeletal muscle regeneration. We have also reported expression of these miRNAs in the heart. Here we investigated the role of two Gtl2-Dio3 miRNAs, miR-410 and miR-495, in cardiac muscle. Overexpression of miR-410 and miR-495 robustly stimulated cardiomyocyte DNA synthesis and proliferation. Interestingly, unlike our findings in skeletal muscle, these miRNAs did not modulate the activity of the WNT signaling pathway. Instead, these miRNAs targeted Cited2, a coactivator required for proper cardiac development. Consistent with miR-410 and miR-495 overexpression, siRNA knockdown of Cited2 in neonatal cardiomyocytes resulted in robust proliferation. This phenotype was associated with reduced expression of Cdkn1c/p57/Kip2, a cell cycle inhibitor, and increased expression of VEGFA, a growth factor with proliferation-promoting effects. Therefore, miR-410 and miR-495 are among a growing number of miRNAs that have the ability to potently stimulate neonatal cardiomyocyte proliferation.

MEF2 transcription factors regulate distinct gene programs in mammalian skeletal muscle differentiation

Skeletal muscle differentiation requires precisely coordinated transcriptional regulation of diverse gene programs that ultimately give rise to the specialized properties of this cell type. In Drosophila, this process is controlled, in part, by MEF2, the sole member of an evolutionarily conserved transcription factor family. By contrast, vertebrate MEF2 is encoded by four distinct genes, Mef2a, -b, -c, and -d, making it far more challenging to link this transcription factor to the regulation of specific muscle gene programs. Here, we have taken the first step in molecularly dissecting vertebrate MEF2 transcriptional function in skeletal muscle differentiation by depleting individual MEF2 proteins in myoblasts. Whereas MEF2A is absolutely required for proper myoblast differentiation, MEF2B, -C, and -D were found to be dispensable for this process. Furthermore, despite the extensive redundancy, we show that mammalian MEF2 proteins regulate a significant subset of nonoverlapping gene programs. These results suggest that individual MEF2 family members are able to recognize specific targets among the entire cohort of MEF2-regulated genes in the muscle genome. These findings provide opportunities to modulate the activity of MEF2 isoforms and their respective gene programs in skeletal muscle homeostasis and disease.

MEF2A regulates the Gtl2-Dio3 microRNA mega-cluster to modulate WNT signaling in skeletal muscle regeneration

Understanding the molecular mechanisms of skeletal muscle regeneration is crucial to exploiting this pathway for use in tissue repair. Our data demonstrate that the MEF2A transcription factor plays an essential role in skeletal muscle regeneration in adult mice. Injured Mef2a knockout mice display widespread necrosis and impaired myofiber formation. MEF2A controls this process through its direct regulation of the largest known mammalian microRNA (miRNA) cluster, the Gtl2-Dio3 locus. A subset of the Gtl2-Dio3 miRNAs represses secreted Frizzled-related proteins (sFRPs), inhibitors of WNT signaling. Consistent with these data, Gtl2-Dio3-encoded miRNAs are downregulated in regenerating Mef2a knockout muscle, resulting in upregulated sFRP expression and attenuated WNT activity. Furthermore, myogenic differentiation in Mef2a-deficient myoblasts is rescued by overexpression of miR-410 and miR-433, two miRNAs in the Gtl2-Dio3 locus that repress sFRP2, or by treatment with recombinant WNT3A and WNT5A. Thus, miRNA-mediated modulation of WNT signaling by MEF2A is a requisite step for proper muscle regeneration, and represents an attractive pathway for enhancing regeneration of diseased muscle.

The Mef2A transcription factor coordinately regulates a costamere gene program in cardiac muscle

The Mef2 family of transcription factors regulates muscle differentiation, but the specific gene programs controlled by each member remain unknown. Characterization of Mef2A knock-out mice has revealed severe myofibrillar defects in cardiac muscle indicating a requirement for Mef2A in cytoarchitectural integrity. Through comprehensive expression analysis of Mef2A-deficient hearts, we identified a cohort of dysregulated genes whose products localize to the peripheral Z-disc/costamere region. Many of these genes are essential for costamere integrity and function. Here we demonstrate that these genes are directly regulated by Mef2A, establishing a mechanism by which Mef2A controls the costamere. In an independent model system, acute knockdown of Mef2A in primary neonatal cardiomyocytes resulted in profound malformations of myofibrils and focal adhesions accompanied by adhesion-dependent programmed cell death. These findings indicate a role for Mef2A in cardiomyocyte survival through regulation of costamere integrity. Finally, bioinformatics analysis identified over-represented transcription factor-binding sites in this network of costamere promoters that may provide insight into the mechanism by which costamere genes are regulated by Mef2A. The global control of costamere gene expression adds another dimension by which this essential macromolecular complex may be regulated in health and disease.

Myospryn is a calcineurin-interacting protein that negatively modulates slow-fiber-type transformation and skeletal muscle regeneration

The calcium-calmodulin-regulated protein phosphatase calcineurin plays an important regulatory role in muscle differentiation, fiber-type determination, hypertrophy, and muscle regeneration. Because calcineurin functions in numerous processes in muscle, multiple mechanisms are likely necessary to ensure that the activity of this phosphatase is appropriately regulated. Here we demonstrate that the muscle-specific scaffolding protein myospryn modulates calcineurin signaling by inhibiting calcineurin-dependent transcriptional activity in C2C12 myoblasts through direct interaction with the enzyme via its noncanonical tripartite motif (TRIM-like). Consistent with these data, transgenic mice overexpressing both the TRIM-like domain of myospryn and constitutively active calcineurin displayed a severe attenuation in the ability of calcineurin to induce a slow-fiber phenotype. Furthermore, transgenic mice overexpressing the TRIM-like domain of myospryn displayed attenuated muscle regeneration after cardiotoxin-induced muscle injury. These results indicate that myospryn functions as a novel inhibitor of the calcineurin signaling pathway in skeletal muscle.

Modulation of angiotensin II-mediated cardiac remodeling by the MEF2A target gene Xirp2

RATIONALE

The vasoactive peptide angiotensin II (Ang II) is a potent cardiotoxic hormone whose actions have been well studied, yet questions remain pertaining to the downstream factors that mediate its effects in cardiomyocytes.

OBJECTIVE

The in vivo role of the myocyte enhancer factor (MEF)2A target gene Xirp2 in Ang II-mediated cardiac remodeling was investigated.

METHODS AND RESULTS

Here we demonstrate that the MEF2A target gene Xirp2 (also known as cardiomyopathy associated gene 3 [CMYA3]) is an important effector of the Ang II signaling pathway in the heart. Xirp2 belongs to the evolutionarily conserved, muscle-specific, actin-binding Xin gene family and is significantly induced in the heart in response to systemic administration of Ang II. Initially, we characterized the Xirp2 promoter and demonstrate that Ang II activates Xirp2 expression by stimulating MEF2A transcriptional activity. To further characterize the role of Xirp2 downstream of Ang II signaling we generated mice harboring a hypomorphic allele of the Xirp2 gene that resulted in a marked reduction in its expression in the heart. In the absence of Ang II, adult Xirp2 hypomorphic mice displayed cardiac hypertrophy and increased beta myosin heavy chain expression. Strikingly, Xirp2 hypomorphic mice chronically infused with Ang II exhibited altered pathological cardiac remodeling including an attenuated hypertrophic response, as well as diminished fibrosis and apoptosis.

CONCLUSIONS

These findings reveal a novel MEF2A-Xirp2 pathway that functions downstream of Ang II signaling to modulate its pathological effects in the heart.

Complete loss of murine Xin results in a mild cardiac phenotype with altered distribution of intercalated discs

AIMS

Xin is a striated muscle-specific F-actin binding protein that has been implicated in cardiomyopathies. In cardiomyocytes, Xin is localized at intercalated discs (IDs). Mice lacking only two of the three Xin isoforms (XinAB(-/-) mice) develop severe cardiac hypertrophy. To further investigate the function of Xin variants in the mammalian heart, we generated XinABC(-/-) mice deficient in all Xin isoforms.

METHODS AND RESULTS

XinABC(-/-) mice showed a very mild phenotype: heart weight, heart weight to tibia length ratios, and cardiac dimensions were not altered. Increased perivascular fibrosis was only observed in hearts of young XinABC(-/-) mice. Striking differences were revealed in isolated cardiomyocytes: XinABC(-/-) cells demonstrated a significantly increased number of non-terminally localized ID-like structures. Furthermore, resting sarcomere length was increased, sarcomere shortening, peak shortening at 0.5-1 Hz, and the duration of shortening were decreased, and shortening and relengthening velocities were accelerated at frequencies above 4 Hz in XinABC(-/-) cardiomyocytes. ECG showed a significantly shorter HV interval and a trend towards shorter QRS interval in XinABC(-/-) mice, suggesting a faster conduction velocity of the ventricular-specific conduction system. In human cardiac tissue, expression of XinC protein was detected solely in samples from patients with cardiac hypertrophy.

CONCLUSION

Total Xin deficiency leads to topographical ID alterations, premature fibrosis and subtle changes in contractile behaviour; this is a milder cardiac phenotype than that observed in XinAB(-/-) mice, which still can express XinC. Together with the finding that XinC is detected solely in cardiomyopathic human tissues, this suggests that its expression is responsible for the stronger dominant phenotype in XinAB(-/-) mice. Furthermore, it indicates that XinC may be involved in the development of human cardiac hypertrophy.

Deregulated protein kinase A signaling and myospryn expression in muscular dystrophy

Alterations in signaling pathway activity have been implicated in the pathogenesis of Duchenne muscular dystrophy, a degenerative muscle disease caused by a deficiency in the costameric protein dystrophin. Accordingly, the notion of the dystrophin-glycoprotein complex, and by extension the costamere, as harboring signaling components has received increased attention in recent years. The localization of most, if not all, signaling enzymes to this subcellular region relies on interactions with scaffolding proteins directly or indirectly associated with the dystrophin-glycoprotein complex. One of these scaffolds is myospryn, a large, muscle-specific protein kinase A (PKA) anchoring protein or AKAP. Previous studies have demonstrated a dysregulation of myospryn expression in human Duchenne muscular dystrophy, suggesting a connection to the pathophysiology of the disorder. Here we report that dystrophic muscle exhibits reduced PKA activity resulting, in part, from severely mislocalized myospryn and the type II regulatory subunit (RIIalpha) of PKA. Furthermore, we show that myospryn and dystrophin coimmunoprecipitate in native muscle extracts and directly interact in vitro. Our findings reveal for the first time abnormalities in the PKA signal transduction pathway and myospryn regulation in dystrophin deficiency.

Identification and mapping of protein kinase A binding sites in the costameric protein myospryn

Recently we identified a novel target gene of MEF2A named myospryn that encodes a large, muscle-specific, costamere-restricted alpha-actinin binding protein. Myospryn belongs to the tripartite motif (TRIM) superfamily of proteins and was independently identified as a dysbindin-interacting protein. Dysbindin is associated with alpha-dystrobrevin, a component of the dystrophin-glycoprotein complex (DGC) in muscle. Apart from these initial findings little else is known regarding the potential function of myospryn in striated muscle. Here we reveal that myospryn is an anchoring protein for protein kinase A (PKA) (or AKAP) whose closest homolog is AKAP12, also known as gravin/AKAP250/SSeCKS. We demonstrate that myospryn co-localizes with RII alpha, a type II regulatory subunit of PKA, at the peripheral Z-disc/costameric region in striated muscle. Myospryn interacts with RII alpha and this scaffolding function has been evolutionarily conserved as the zebrafish ortholog also interacts with PKA. Moreover, myospryn serves as a substrate for PKA. These findings point to localized PKA signaling at the muscle costamere.

Genetic disruption of calcineurin improves skeletal muscle pathology and cardiac disease in a mouse model of limb-girdle muscular dystrophy

Calcineurin (Cn) is a Ca(2+)/calmodulin-dependent serine/threonine phosphatase that regulates differentiation-specific gene expression in diverse tissues, including the control of fiber-type switching in skeletal muscle. Recent studies have implicated Cn signaling in diminishing skeletal muscle pathogenesis associated with muscle injury or disease-related muscle degeneration. For example, use of the Cn inhibitor cyclosporine A has been shown to delay muscle regeneration following toxin-induced injury and inhibit regeneration in the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy. In contrast, transgenic expression of an activated mutant of Cn in skeletal muscle was shown to increase utrophin expression and reduce overall disease pathology in mdx mice. Here we examine the effect of altered Cn activation in the context of the delta-sarcoglycan-null (scgd(-/-)) mouse model of limb-girdle muscular dystrophy. In contrast to results discussed in mdx mice, genetic deletion of a loxP-targeted calcineurin B1 (CnB1) gene using a skeletal muscle-specific cre allele in the scgd(-/-) background substantially reduced skeletal muscle degeneration and histopathology compared with the scgd(-/-) genotype alone. A similar regression in scgd-dependent disease manifestation was also observed in calcineurin Abeta (CnAbeta) gene-targeted mice in both skeletal muscle and heart. Conversely, increased Cn expression using a muscle-specific transgene increased cardiac fibrosis, decreased cardiac ventricular shortening, and increased muscle fiber loss in the quadriceps. Our results suggest that inhibition of Cn may benefit select types of muscular dystrophy.

Myomaxin is a novel transcriptional target of MEF2A that encodes a Xin-related alpha-actinin-interacting protein

The physiological targets regulated by MEF2 in striated muscle are not completely known. Several recent studies have identified novel downstream target genes and shed light on the global transcriptional network regulated by MEF2 in muscle. In our continuing effort to identify novel, downstream pathways controlled by MEF2, we have used mef2a knock-out mice to find those genes dependent on MEF2A transcriptional activity. Here, we describe the characterization of a direct, downstream target gene for the MEF2A transcription factor encoding a large, muscle-specific protein that localizes to the Z-disc/costameric region in striated muscle. This gene, called myomaxin, was identified as a gene markedly down-regulated in MEF2A knock-out hearts. Myomaxin is the mouse ortholog of a partial human cDNA of unknown function named cardiomyopathy associated gene 3 (CMYA3). Myomaxin is expressed as a single, large transcript of approximately 11 kilobases in adult heart and skeletal muscle with an open reading frame of 3,283 amino acids. The protein encoded by the myomaxin gene is related to the actin-binding protein Xin and interacts with the sarcomeric Z-disc protein, alpha-actinin-2. Our findings demonstrate that Myomaxin functions directly downstream of MEF2A at the peripheral Z-disc complex in striated muscle potentially playing a role in regulating cytoarchitectural integrity.

Myospryn Is a Direct Transcriptional Target for MEF2A That Encodes a Striated Muscle, α-Actinin-interacting, Costamere-localized Protein

The full repertoire of proteins that comprise the striated muscle Z-disc and peripheral structures, such as the costamere, have yet to be discovered. Recent studies suggest that this elaborate protein network, which acts as a structural and signaling center for striated muscle, harbors factors that function as mechanosensors to ensure coordinated contractile activity. Mutations in genes whose products reside in this region often result in skeletal and cardio myopathies, demonstrating the importance of this macromolecular complex in muscle structure and function. Here, we describe the characterization of a direct, downstream target gene for the MEF2A transcription factor encoding a large, muscle-specific protein that localizes to the costamere in striated muscle. This gene, called myospryn, was identified by microarray analysis as a transcript down-regulated in MEF2A knock-out mice. MEF2A knock-out mice develop cardiac failure during the perinatal period with mutant hearts exhibiting several cardiac abnormalities including myofibrillar disarray. Myospryn is the mouse ortholog of a partial human cDNA of unknown function named cardiomyopathy-associated gene 5 (CMYA5). Myospryn is expressed as a single, large transcript of approximately 12 kilobases in adult heart and skeletal muscle with an open reading frame of 3739 amino acids. This protein, belonging to the tripartite motif superfamily of proteins, contains a B-box coiled-coil (BBC), two fibronectin type III (FN3) repeats, and SPRY domains and interacts with the sarcomeric Z-disc protein, alpha-actinin-2. Our findings demonstrate that myospryn functions directly downstream of MEF2A at the costamere in striated muscle potentially playing a role in myofibrillogenesis.

Calcineurin activation influences muscle phenotype in a muscle-specific fashion

Background

The calcium activated protein phosphatase 2B, also known as calcineurin, has been implicated as a cell signaling molecule involved with transduction of physiological signals (free cytosolic Ca²⁺) into molecular signals that influence the expression of phenotype-specific genes in skeletal muscle. In the present study we address the role of calcineurin in mediating adaptations in myosin heavy chain (MHC) isoform expression and muscle mass using 3-month old wild-type (WT) and transgenic mice displaying high-level expression of a constitutively active form of calcineurin (MCK-CN* mice).

Results

Slow muscles, e.g., soleus, were significantly larger (by ~24%), whereas fast muscles, e.g., medial gastrocnemius (MG) and tibialis anterior were significantly smaller (by ~26 and ~16%, respectively) in MCK-CN* mice compared to WT. The masses of mixed phenotype muscles, such as the plantaris and the extensor digitorum longus, were not significantly changed from WT. The soleus, plantaris, MG and diaphragm displayed shifts toward slower MHC isoforms, e.g., soleus from WT mice contained ~52% MHC-I, ~39% MHC-IIa, and ~9% MHC-IIx, whereas MCK-CN* mice had ~67% MHC-I, ~26% MHC-IIa, and ~7% MHC-IIx. The specific isoforms that were either up or down-regulated were muscle-specific. For instance, the proportion of MHC-IIa was decreased in the soleus and diaphragm, but increased in the plantaris and MG of MCK-CN* mice. Also, the proportion of MHC-IIx was unchanged in the soleus, decreased in the diaphragm and increased in the plantaris and MG of MCK-CN* relative to WT mice. Fast to slow shifts in fiber type proportions were evident for the plantaris, but not the soleus. Fast, but not slow, plantaris fibers of MCK-CN* mice had higher oxidative and lower glycolytic properties than WT.

Conclusion

These data suggest that calcineurin activation can influence muscle phenotype and that the specific influence of calcineurin activation on the phenotypic and mass characteristics of a muscle is dependent upon the original phenotypic state of the muscle.

Mitochondrial deficiency and cardiac sudden death in mice lacking the MEF2A transcription factor

The four MEF2 transcription factors (MEF2A, -B, -C, and -D) regulate differentiation and calcium-dependent gene expression in muscle cells. We generated mice deficient in MEF2A, the predominant Mef2 gene product expressed in post-natal cardiac muscle. Most mice lacking Mef2a died suddenly within the first week of life and exhibited pronounced dilation of the right ventricle, myofibrillar fragmentation, mitochondrial disorganization and activation of a fetal cardiac gene program. The few Mef2a(-/-) mice that survived to adulthood also showed a deficiency of cardiac mitochondria and susceptibility to sudden death. Paradoxically, MEF2 transcriptional activity, revealed by the expression of a MEF2-dependent transgene, was enhanced in the hearts of Mef2a-mutant mice, reflecting the transcriptional activation of residual MEF2D. These findings reveal specific roles for MEF2A in maintaining appropriate mitochondrial content and cyto-architectural integrity in the post-natal heart and show that other MEF2 isoforms cannot support these activities.

Activation of MEF2 by muscle activity is mediated through a calcineurin-dependent pathway

Gene expression in skeletal muscles of adult vertebrates is altered profoundly by changing patterns of contractile work. Here we observed that the functional activity of MEF2 transcription factors is stimulated by sustained periods of endurance exercise or motor nerve pacing, as assessed by expression in trans genic mice of a MEF2-dependent reporter gene (desMEF2-lacZ). This response is accompanied by transformation of specialized myofiber subtypes, and is blocked either by cyclosporin A, a specific chemical inhibitor of calcineurin, or by forced expression of the endogenous calcineurin inhibitory protein, myocyte-enriched calcineurin interacting protein 1. Calcineurin removes phosphate groups from MEF2, and augments the potency of the transcriptional activation domain of MEF2 fused to a heterologous DNA binding domain. Across a broad range, the enzymatic activity of calcineurin correlates directly with expression of endogenous genes that are transcriptionally activated by muscle contractions. These results delineate a molecular pathway in which calcineurin and MEF2 participate in the adaptive mechanisms by which skeletal myofibers acquire specialized contractile and metabolic properties as a function of changing patterns of muscle contraction.

Neuronal basic helix-loop-helix proteins (NEX and BETA2/Neuro D) regulate terminal granule cell differentiation in the hippocampus

The transcription factors neuronal helix-loop-helix protein (NEX)/mammalian atonal homolog 2 (Math-2), BETA2/neuronal determination factor (NeuroD), and NeuroD-related factor (NDRF)/NeuroD2 comprise a family of Drosophila atonal-related basic helix-loop-helix (bHLH) proteins with highly overlapping expression in the developing forebrain. The ability of BETA2/NeuroD and NDRF to convert ectodermal cells into neurons after mRNA injection into Xenopus oocytes suggested a role in specifying neuronal cell fate. However, neuronal bHLH genes are largely transcribed in CNS neurons, which are fully committed. Here we analyze a defect in mice lacking BETA2/NeuroD, and in NEX*BETA2/NeuroD double mutants, demonstrating that bHLH proteins are required in vivo for terminal neuronal differentiation. Most strikingly, presumptive granule cells of the dentate gyrus are generated but fail to mature, lack normal sodium currents, and show little dendritic arborization. Long-term hippocampal slice cultures demonstrate secondary alterations of entorhinal and commissural/associational projections. The primary developmental arrest appears to be restricted to granule cells in which an autoregulatory system involving all three neuronal bHLH genes has failed.

CaM kinase signaling induces cardiac hypertrophy and activates the MEF2 transcription factor in vivo

Hypertrophic growth is an adaptive response of the heart to diverse pathological stimuli and is characterized by cardiomyocyte enlargement, sarcomere assembly, and activation of a fetal program of cardiac gene expression. A variety of Ca(2+)-dependent signal transduction pathways have been implicated in cardiac hypertrophy, but whether these pathways are independent or interdependent and whether there is specificity among them are unclear. Previously, we showed that activation of the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin or its target transcription factor NFAT3 was sufficient to evoke myocardial hypertrophy in vivo. Here, we show that activated Ca(2+)/calmodulin-dependent protein kinases-I and -IV (CaMKI and CaMKIV) also induce hypertrophic responses in cardiomyocytes in vitro and that CaMKIV overexpressing mice develop cardiac hypertrophy with increased left ventricular end-diastolic diameter and decreased fractional shortening. Crossing this transgenic line with mice expressing a constitutively activated form of NFAT3 revealed synergy between these signaling pathways. We further show that CaMKIV activates the transcription factor MEF2 through a posttranslational mechanism in the hypertrophic heart in vivo. Activated calcineurin is a less efficient activator of MEF2-dependent transcription, suggesting that the calcineurin/NFAT and CaMK/MEF2 pathways act in parallel. These findings identify MEF2 as a downstream target for CaMK signaling in the hypertrophic heart and suggest that the CaMK and calcineurin pathways preferentially target different transcription factors to induce cardiac hypertrophy.

MEF2 responds to multiple calcium-regulated signals in the control of skeletal muscle fiber type

Different patterns of motor nerve activity drive distinctive programs of gene transcription in skeletal muscles, thereby establishing a high degree of metabolic and physiological specialization among myofiber subtypes. Recently, we proposed that the influence of motor nerve activity on skeletal muscle fiber type is transduced to the relevant genes by calcineurin, which controls the functional activity of NFAT (nuclear family of activated T cell) proteins. Here we demonstrate that calcineurin-dependent gene regulation in skeletal myocytes is mediated also by MEF2 transcription factors, and is integrated with additional calcium-regulated signaling inputs, specifically calmodulin-dependent protein kinase activity. In skeletal muscles of transgenic mice, both NFAT and MEF2 binding sites are necessary for properly regulated function of a slow fiber-specific enhancer, and either forced expression of activated calcineurin or motor nerve stimulation up-regulates a MEF2-dependent reporter gene. These results provide new insights into the molecular mechanisms by which specialized characteristics of skeletal myofiber subtypes are established and maintained.

Stimulation of slow skeletal muscle fiber gene expression by calcineurin in vivo

Adult skeletal muscle fibers can be categorized into fast and slow twitch subtypes based on specialized contractile and metabolic properties and on distinctive patterns of muscle gene expression. Muscle fiber-type characteristics are dependent on the frequency of motor nerve stimulation and are thought to be controlled by calcium-dependent signaling. The calcium, calmodulin-dependent protein phosphatase, calcineurin, stimulates slow fiber-specific gene promoters in cultured skeletal muscle cells, and the calcineurin inhibitor, cyclosporin A, inhibits slow fiber gene expression in vivo, suggesting a key role of calcineurin in activation of the slow muscle fiber phenotype. Calcineurin has also been shown to induce hypertrophy of cardiac muscle and to mediate the hypertrophic effects of insulin-like growth factor-1 on skeletal myocytes in vitro. To determine whether activated calcineurin was sufficient to induce slow fiber gene expression and hypertrophy in adult skeletal muscle in vivo, we created transgenic mice that expressed activated calcineurin under control of the muscle creatine kinase enhancer. These mice exhibited an increase in slow muscle fibers, but no evidence for skeletal muscle hypertrophy. These results demonstrate that calcineurin activation is sufficient to induce the slow fiber gene regulatory program in vivo and suggest that additional signals are required for skeletal muscle hypertrophy.

Loss of BETA2/NeuroD leads to malformation of the dentate gyrus and epilepsy

BETA2/NeuroD is a homologue of the Drosophila atonal gene that is widely expressed during development in the mammalian brain and pancreas. Although studies in Xenopus suggest that BETA2/NeuroD is involved in cellular differentiation, its function in the mammalian nervous system is unclear. Here we show that mutant mice homozygous for a deletion at the BETA2/NeuroD locus fail to develop a granule cell layer within the dentate gyrus, one of the principal structures of the hippocampal formation. To understand the basis of this abnormality, we analyzed dentate gyrus development by using immunocytochemical markers in BETA2/NeuroD-deficient mice. The early cell populations in the dentate gyrus, including Cajal-Retzius cells and radial glia, are present and appear normally organized. The migration of dentate precursor cells and newly born granule cells from the neuroepithelium to the dentate gyrus remains intact. However, there is a dramatic defect in the proliferation of precursor cells once they reach the dentate and a significant delay in the differentiation of granule cells. This leads to malformation of the dentate granule cell layer and excess cell death. BETA2/NeuroD null mice also exhibit spontaneous limbic seizures associated with electrophysiological evidence of seizure activity in the hippocampus and cortex. These findings thus establish a critical role of BETA2/NeuroD in the development of a specific class of neurons. Furthermore, failure to express BETA2/NeuroD leads to a stereotyped pattern of pathological excitability of the adult central nervous system.

MEF2: a transcriptional target for signaling pathways controlling skeletal muscle growth and differentiation

Skeletal muscle development involves a multistep pathway in which mesodermal precursor cells are selected, in response to inductive cues, to form myoblasts that later withdraw from the cell cycle and differentiate. The transcriptional circuitry controlling muscle differentiation is intimately linked to the cell cycle machinery, such that muscle differentiation genes do not become transcribed until myoblasts have exited the cell cycle. Members of the MyoD and MEF2 families of transcription factors associate combinatorially to control myoblast specification, differentiation and proliferation. Recent studies have revealed multiple signaling systems that stimulate and inhibit myogenesis by altering MEF2 phosphorylation and its association with other transcriptional cofactors.

IGF-1 induces skeletal myocyte hypertrophy through calcineurin in association with GATA-2 and NF-ATc1

Localized synthesis of insulin-like growth factors (IGFs) has been broadly implicated in skeletal muscle growth, hypertrophy and regeneration. Virally delivered IGF-1 genes induce local skeletal muscle hypertrophy and attenuate age-related skeletal muscle atrophy, restoring and improving muscle mass and strength in mice. Here we show that the molecular pathways underlying the hypertrophic action of IGF-1 in skeletal muscle are similar to those responsible for cardiac hypertrophy. Transfected IGF-1 gene expression in postmitotic skeletal myocytes activates calcineurin-mediated calcium signalling by inducing calcineurin transcripts and nuclear localization of calcineurin protein. Expression of activated calcineurin mimics the effects of IGF-1, whereas expression of a dominant-negative calcineurin mutant or addition of cyclosporin, a calcineurin inhibitor, represses myocyte differentiation and hypertrophy. Either IGF-1 or activated calcineurin induces expression of the transcription factor GATA-2, which accumulates in a subset of myocyte nuclei, where it associates with calcineurin and a specific dephosphorylated isoform of the transcription factor NF-ATc1. Thus, IGF-1 induces calcineurin-mediated signalling and activation of GATA-2, a marker of skeletal muscle hypertrophy, which cooperates with selected NF-ATc isoforms to activate gene expression programs.

Transcriptional activity of MEF2 during mouse embryogenesis monitored with a MEF2-dependent transgene

The four members of the MEF2 family of MADS-box transcription factors, MEF2-A, MEF2-B, MEF2-C and MEF2-D, are expressed in overlapping patterns in developing muscle and neural cell lineages during embryogenesis. However, during late fetal development and postnatally, MEF2 transcripts are also expressed in a wide range of cell types. Because MEF2 expression is controlled by translational and post-translational mechanisms, it has been unclear whether the presence of MEF2 transcripts in the embryo reflects transcriptionally active MEF2 proteins. To define the temporospatial expression pattern of transcriptionally active MEF2 proteins during mouse embryogenesis, we generated transgenic mice harboring a lacZ reporter gene controlled by three tandem copies of the MEF2 site and flanking sequences from the desmin enhancer, which is active in cardiac, skeletal and smooth muscle cells. Expression of this MEF2-dependent transgene paralleled expression of MEF2 mRNAs in developing myogenic lineages and regions of the adult brain. However, it was not expressed in other cell types that express MEF2 transcripts. Tandem copies of the MEF2 site from the c-jun promoter directed expression in a similar pattern to the desmin MEF2 site, suggesting that transgene expression reflects the presence of transcriptionally active MEF2 proteins, rather than other factors specific for DNA sequences flanking the MEF2 site. These results demonstrate the presence of transcriptionally active MEF2 proteins in the early muscle and neural cell lineages during embryogenesis and argue against the existence of lineage-restricted MEF2 cofactors that discriminate between MEF2 sites with different immediate flanking sequences. The discordance between MEF2 mRNA expression and MEF2 transcriptional activity in nonmuscle cell types of embryos and adults also supports the notion that post-transcriptional mechanisms regulate the expression of MEF2 proteins.

The basic helix-loop-helix protein BETA2 interacts with p300 to coordinate differentiation of secretin-expressing enteroendocrine cells

The major epithelial cell types lining the intestine comprise a perpetually self-renewing population of cells that differentiate continuously from a stem cell in the intestinal crypts. Secretin-producing enteroendocrine cells represent a nondividing subpopulation of intestinal epithelial cells, suggesting that expression of the hormone is coordinated with cell cycle arrest during the differentiation of this cell lineage. Here we report that the basic helix-loop-helix protein BETA2 associates functionally with the coactivator, p300 to activate transcription of the secretin gene as well as the gene encoding the cyclin-dependent kinase inhibitor p21. Overexpression of BETA2 in cell lines induces both cell cycle arrest and apoptosis suggesting that BETA2 may regulate proliferation of secretin cells. Consistent with this role, we observed both reentry of normally quiescent cells into the cell cycle and disrupted cell number regulation in the small intestine of BETA2 null mice. Thus, BETA2 may function to coordinate transcriptional activation of the secretin gene, cell cycle arrest, and cell number regulation, providing one of the first examples of a transcription factor that controls terminal differentiation of cells in the intestinal epithelium.

Diabetes, defective pancreatic morphogenesis, and abnormal enteroendocrine differentiation in BETA2/neuroD-deficient mice

Candidate transcription factors involved in pancreatic endocrine development have been isolated using insulin gene regulation as a paradigm. The cell-type restricted basic helix-loop-helix (bHLH) gene, BETA2/NeuroD, expressed in pancreatic endocrine cells, the intestine, and the brain, activates insulin gene transcription and can induce neurons to differentiate. To understand the importance of BETA2 in pancreatic endocrine cell differentiation, mice lacking a functional BETA2 gene were generated by gene targeting experiments. Mice carrying a targeted disruption of the BETA2 gene developed severe diabetes and died perinatally. Homozygous BETA2 null mice had a striking reduction in the number of insulin-producing beta cells and failed to develop mature islets. Islet morphogenesis appeared to be arrested between E14.5 and E17.5, a period characterized by major expansion of the beta cell population. The presence of severe diabetes in these mice suggests that proper islet structure plays an important role in blood glucose homeostasis. In addition, secretin- and cholecystokinin-producing enteroendocrine cells failed to develop in the absence of BETA2. The absence of these two pancreatic secretagogs may explain the abnormal cellular polarity and inability to secrete zymogen granules in pancreatic acinar exocrine cells. The nervous system appeared to develop normally, despite abundant expression of BETA2 in differentiating neurons. Thus, BETA2 is critical for the normal development of several specialized cell types arising from the gut endoderm.

Analysis of candidate genes for susceptibility to type I diabetes: a case-control and family-association study of genes on chromosome 2q31-35

Recent genome searches suggest a putative linkage of many loci to susceptibility to type I diabetes. The chromosome 2q31-35 region is reported to be linked to susceptibility to type I diabetes and is thought to contain several diabetes susceptibility loci. These candidate genes include the HOXD gene cluster, BETA2, CTLA4, CD28, IGFBP2, and IGFBP5. Association studies in populations and families are required to confirm and/or identify the actual susceptibility loci. We hereby report several previously unknown DNA polymorphisms for HOXD8, BETA2, and IGFBP5, which we have used along with previously known polymorphisms of HOXD8 and CTLA4 to test whether these candidate loci are the susceptibility genes on chromosome 2q31-35. Using a case-control design with a subsequent family-association approach to confirm associations, we find no evidence that these candidate genes are associated with susceptibility to type I diabetes.

The basic helix-loop-helix transcription factor BETA2/NeuroD is expressed in mammalian enteroendocrine cells and activates secretin gene expression

The gene encoding the hormone secretin is expressed only in enteroendocrine S cells and insulin-producing pancreatic beta cells during development. A 120-bp enhancer directs cell-specific expression of the rat secretin gene in secretin-expressing cells. The enhancer includes an E-box sequence, CAGCTG, which is important for transcriptional activity. To further characterize the role of the E box, a consensus binding site for basic helix-loop-helix (bHLH) proteins, we have examined factors that interact with this element in the secretin gene. The results suggest that transcription is activated by a recently isolated tissue-specific bHLH protein, BETA2, heterodimerized to the ubiquitously expressed bHLH proteins, Pan 1 and Pan 2, the rodent homologues of E47 and E12. The importance of BETA2 for transcriptional activation of secretin is further illustrated by antisense experiments inhibiting BETA2 expression in secretin-producing cell lines, which resulted in the inhibition of most E box-dependent transcription. Expression of BETA2 in a nonendocrine cell line conferred the ability to express secretin-reporter genes that are transcribed at minimal levels in the absence of BETA2. Secretin-producing enteroendocrine cells in the murine small intestine showed specific immunostaining with BETA2 antibodies, corroborating observations in cell lines. Thus BETA2 is to our knowledge the first transcription factor identified that specifically activates cell type-specific expression of an intestinal hormone gene.

BETA3, a novel helix-loop-helix protein, can act as a negative regulator of BETA2 and MyoD-responsive genes

Using degenerate PCR cloning we have identified a novel basic helix-loop-helix (bHLH) transcription factor, BETA3, from a hamster insulin tumor (HIT) cell cDNA library. Sequence analysis revealed that this factor belongs to the class B bHLH family and has the highest degree of homology with another bHLH transcription factor recently isolated in our laboratory, BETA2 (neuroD) (J. E. Lee, S. M. Hollenberg, L. Snider, D. L. Turner, N. Lipnick, and H. Weintraub, Science 268:836-844, 1995; F. J. Naya, C. M. M. Stellrecht, and M.-J. Tsai, Genes Dev. 8:1009-1019, 1995). BETA2 is a brain- and pancreatic-islet-specific bHLH transcription factor and is largely responsible for the tissue-specific expression of the insulin gene. BETA3 was found to be tissue restricted, with the highest levels of expression in HIT, lung, kidney, and brain cells. Surprisingly, despite the homology between BETA2 and BETA3 and its intact basic region, BETA3 is unable to bind the insulin E box in bandshift analysis as a homodimer or as a heterodimer with the class A bHLH factors E12, E47, or BETA1. Instead, BETA3 inhibited both the E47 homodimer and the E47/BETA2 heterodimer binding to the insulin E box. In addition, BETA3 greatly repressed the BETA2/E47 induction of the insulin enhancer in HIT cells as well as the MyoD/E47 induction of a muscle-specific E box in the myoblast cell line C2C12. In contrast, expression of BETA3 had no significant effect on the GAL4-VP16 transcriptional activity. Immunoprecipitation analysis demonstrates that the mechanism of repression is via direct protein-protein interaction, presumably by heterodimerization between BETA3 and class A bHLH factors.

Tissue-specific regulation of the insulin gene by a novel basic helix-loop-helix transcription factor

The insulin gene is one of the best paradigms of tissue-specific gene expression. It is developmentally regulated and is expressed exclusively in the pancreatic beta-cell. This restricted expression is directed by a tissue-specific enhancer, within the promoter, which contains an E-box sequence. The insulin E-box binds an islet-specific protein complex, termed 3a1. E-boxes bind proteins belonging to the basic helix-loop-helix (bHLH) family of transcription factors. The bHLH proteins function as potent transcriptional activators of tissue-specific genes by forming heterodimers between ubiquitous and cell-restricted family members. In addition, the cell-restricted bHLH members play an important role in specifying cell fate. To isolate the tissue-specific bHLH factor controlling insulin gene expression and study its role in islet cell differentiation, a modified yeast two-hybrid system was utilized to clone a novel bHLH factor, BETA2 (beta-cell E-box trans-activator 2), from a hamster insulin tumor (HIT) cell cDNA library. Northern analysis demonstrates that high-level expression of the BETA2 gene is restricted to pancreatic alpha- and beta-cell lines. As expected of tissue-specific bHLH members, BETA2 binds to the insulin E-box sequence with high affinity as a heterodimer with the ubiquitous bHLH factor E47. More importantly, antibody supershift experiments clearly show that BETA2 is a component of the native insulin E-box-binding complex. Transient transfection assays demonstrate that the BETA2/E47 heterodimer synergistically interacts with a neighboring beta-cell-specific complex to activate an insulin enhancer. In contrast, other bHLH factors such as MyoD and E47, which can bind to the insulin E-box with high affinity, fail to do so. Thus, a unique, cooperative interaction is the basis by which the insulin E-box enhancer discriminates between various bHLH factors to achieve tissue-specific activation of the insulin gene.

Distribution of R-cognin and choline acetyltransferase in the ganglion cell layer of developing chick neural retina

Retina cognin, a cell membrane glycoprotein which mediates cell-cell recognition and adhesion in vitro, is initially present throughout the retina and becomes confined to the ganglion cell layer at 14-15 days of embryogenesis. Within this layer it is found on membranes of virtually all ganglion and displaced amacrine cells, but not on membranes of retinal glial cells (Müller fibers) which traverse this layer. The distribution of cognin as determined by immunocytochemistry is described and compared with that of choline acetyltransferase. The significance of cognin as a possible address marker during development of neural retina is discussed.