Christopher T Pappas

Interests

Research

My research is focused on understanding the mechanisms underlying the regulation of actin filament assembly in striated muscle. Precise regulation of actin filament polarity, spacing and length is essential for proper muscle function and its dysregulation results in human myopathies. I'm specifically interested in how actin-binding proteins that reside at the ends the filament cooperate to regulate its architecture.

Courses

Scholarly Contributions

Journals/Publications

• Yuen, M., Worgan, L., Iwanski, J., Pappas, C. T., Joshi, H., Churko, J. M., Arbuckle, S., Kirk, E. P., Zhu, Y., Roscioli, T., Gregorio, C. C., & Cooper, S. T. (2022). Neonatal-lethal dilated cardiomyopathy due to a homozygous LMOD2 donor splice-site variant. European journal of human genetics : EJHG.
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  Dilated cardiomyopathy (DCM) is characterized by cardiac enlargement and impaired ventricular contractility leading to heart failure. A single report identified variants in leiomodin-2 (LMOD2) as a cause of neonatally-lethal DCM. Here, we describe two siblings with DCM who died shortly after birth due to heart failure. Exome sequencing identified a homozygous LMOD2 variant in both siblings, (GRCh38)chr7:g.123656237G > A; NM_207163.2:c.273 + 1G > A, ablating the donor 5' splice-site of intron-1. Pre-mRNA splicing studies and western blot analysis on cDNA derived from proband cardiac tissue, MyoD-transduced proband skin fibroblasts and HEK293 cells transfected with LMOD2 gene constructs established variant-associated absence of canonically spliced LMOD2 mRNA and full-length LMOD2 protein. Immunostaining of proband heart tissue unveiled abnormally short actin-thin filaments. Our data are consistent with LMOD2 c.273 + 1G > A abolishing/reducing LMOD2 transcript expression by: (1) variant-associated perturbation in initiation of transcription due to ablation of the intron-1 donor; and/or (2) degradation of aberrant LMOD2 transcripts (resulting from use of alternative transcription start-sites or cryptic splice-sites) by nonsense-mediated decay. LMOD2 expression is critical for life and the absence of LMOD2 is associated with thin filament shortening and severe cardiac contractile dysfunction. This study describes the first splice-site variant in LMOD2 and confirms the role of LMOD2 variants in DCM.
• Mi-Mi, L., Farman, G. P., Mayfield, R. M., Strom, J., Chu, M., Pappas, C. T., & Gregorio, C. C. (2020). In vivo elongation of thin filaments results in heart failure. PloS one, 15(1), e0226138.
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  A novel cardiac-specific transgenic mouse model was generated to identify the physiological consequences of elongated thin filaments during post-natal development in the heart. Remarkably, increasing the expression levels in vivo of just one sarcomeric protein, Lmod2, results in ~10% longer thin filaments (up to 26% longer in some individual sarcomeres) that produce up to 50% less contractile force. Increasing the levels of Lmod2 in vivo (Lmod2-TG) also allows us to probe the contribution of Lmod2 in the progression of cardiac myopathy because Lmod2-TG mice present with a unique cardiomyopathy involving enlarged atrial and ventricular lumens, increased heart mass, disorganized myofibrils and eventually, heart failure. Turning off of Lmod2 transgene expression at postnatal day 3 successfully prevents thin filament elongation, as well as gross morphological and functional disease progression. We show here that Lmod2 has an essential role in regulating cardiac contractile force and function.
• Ahrens-Nicklas, R. C., Pappas, C. T., Farman, G. P., Mayfield, R. M., Larrinaga, T. M., Medne, L., Ritter, A., Krantz, I. D., Murali, C., Lin, K. Y., Berger, J. H., Yum, S. W., Carreon, C. K., & Gregorio, C. C. (2019). Disruption of cardiac thin filament assembly arising from a mutation in : A novel mechanism of neonatal dilated cardiomyopathy. Science advances, 5(9), eaax2066.
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  Neonatal heart failure is a rare, poorly-understood presentation of familial dilated cardiomyopathy (DCM). Exome sequencing in a neonate with severe DCM revealed a homozygous nonsense variant in leiomodin 2 (, p.Trp398*). Leiomodins (Lmods) are actin-binding proteins that regulate actin filament assembly. While disease-causing mutations in smooth () and skeletal () muscle isoforms have been described, the cardiac () isoform has not been previously associated with human disease. Like our patient, -null mice have severe early-onset DCM and die before weaning. The infant's explanted heart showed extraordinarily short thin filaments with isolated cardiomyocytes displaying a large reduction in maximum calcium-activated force production. The lack of extracardiac symptoms in -null mice, and remarkable morphological and functional similarities between the patient and mouse model informed the decision to pursue cardiac transplantation in the patient. To our knowledge, this is the first report of aberrant cardiac thin filament assembly associated with human cardiomyopathy.
• Ly, T., Pappas, C. T., Johnson, D., Schlecht, W., Colpan, M., Galkin, V. E., Gregorio, C. C., Dong, W. J., & Kostyukova, A. S. (2019). Effects of cardiomyopathy-linked mutations K15N and R21H in tropomyosin on thin-filament regulation and pointed-end dynamics. Molecular biology of the cell, 30(2), 268-281.
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  Missense mutations K15N and R21H in striated muscle tropomyosin are linked to dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Tropomyosin, together with the troponin complex, regulates muscle contraction and, along with tropomodulin and leiomodin, controls the uniform thin-filament lengths crucial for normal sarcomere structure and function. We used Förster resonance energy transfer to study effects of the tropomyosin mutations on the structure and kinetics of the cardiac troponin core domain associated with the Ca-dependent regulation of cardiac thin filaments. We found that the K15N mutation desensitizes thin filaments to Ca and slows the kinetics of structural changes in troponin induced by Ca dissociation from troponin, while the R21H mutation has almost no effect on these parameters. Expression of the K15N mutant in cardiomyocytes decreases leiomodin's thin-filament pointed-end assembly but does not affect tropomodulin's assembly at the pointed end. Our in vitro assays show that the R21H mutation causes a twofold decrease in tropomyosin's affinity for F-actin and affects leiomodin's function. We suggest that the K15N mutation causes DCM by altering Ca-dependent thin-filament regulation and that one of the possible HCM-causing mechanisms by the R21H mutation is through alteration of leiomodin's function.
• Pappas, C. T., Farman, G. P., Mayfield, R. M., Konhilas, J. P., & Gregorio, C. C. (2018). Cardiac-specific knockout of Lmod2 results in a severe reduction in myofilament force production and rapid cardiac failure. Journal of molecular and cellular cardiology, 122, 88-97.
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  Leiomodin-2 (Lmod2) is a striated muscle-specific actin binding protein that is implicated in assembly of thin filaments. The necessity of Lmod2 in the adult mouse and role it plays in the mechanics of contraction are unknown. To answer these questions, we generated cardiac-specific conditional Lmod2 knockout mice (cKO). These mice die within a week of induction of the knockout with severe left ventricular systolic dysfunction and little change in cardiac morphology. Cardiac trabeculae isolated from cKO mice have a significant decrease in maximum force production and a blunting of myofilament length-dependent activation. Thin filaments are non-uniform and substantially reduced in length in cKO hearts, affecting the functional overlap of the thick and thin filaments. Remarkably, we also found that Lmod2 levels are directly linked to thin filament length and cardiac function in vivo, with a low amount (
• Wu, T., Mu, Y., Bogomolovas, J., Fang, X., Veevers, J., Nowak, R. B., Pappas, C. T., Gregorio, C. C., Evans, S. M., Fowler, V. M., & Chen, J. (2017). HSPB7 is indispensable for heart development by modulating actin filament assembly. Proceedings of the National Academy of Sciences of the United States of America, 114(45), 11956-11961.
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  Small heat shock protein HSPB7 is highly expressed in the heart. Several mutations within HSPB7 are associated with dilated cardiomyopathy and heart failure in human patients. However, the precise role of HSPB7 in the heart is still unclear. In this study, we generated global as well as cardiac-specific HSPB7 KO mouse models and found that loss of HSPB7 globally or specifically in cardiomyocytes resulted in embryonic lethality before embryonic day 12.5. Using biochemical and cell culture assays, we identified HSPB7 as an actin filament length regulator that repressed actin polymerization by binding to monomeric actin. Consistent with HSPB7's inhibitory effects on actin polymerization, HSPB7 KO mice had longer actin/thin filaments and developed abnormal actin filament bundles within sarcomeres that interconnected Z lines and were cross-linked by α-actinin. In addition, loss of HSPB7 resulted in up-regulation of Lmod2 expression and mislocalization of Tmod1. Furthermore, crossing HSPB7 null mice into an Lmod2 null background rescued the elongated thin filament phenotype of HSPB7 KOs, but double KO mice still exhibited formation of abnormal actin bundles and early embryonic lethality. These in vivo findings indicated that abnormal actin bundles, not elongated thin filament length, were the cause of embryonic lethality in HSPB7 KOs. Our findings showed an unsuspected and critical role for a specific small heat shock protein in directly modulating actin thin filament length in cardiac muscle by binding monomeric actin and limiting its availability for polymerization.
• Chu, M., Gregorio, C. C., & Pappas, C. T. (2016). Nebulin, a multi-functional giant. The Journal of experimental biology, 219(Pt 2), 146-52.
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  Efficient muscle contraction in skeletal muscle is predicated on the regulation of actin filament lengths. In one long-standing model that was prominent for decades, the giant protein nebulin was proposed to function as a 'molecular ruler' to specify the lengths of the thin filaments. This theory was questioned by many observations, including experiments in which the length of nebulin was manipulated in skeletal myocytes; this approach revealed that nebulin functions to stabilize filamentous actin, allowing thin filaments to reach mature lengths. In addition, more recent data, mostly from in vivo models and identification of new interacting partners, have provided evidence that nebulin is not merely a structural protein. Nebulin plays a role in numerous cellular processes including regulation of muscle contraction, Z-disc formation, and myofibril organization and assembly.
• Kolb, J., Li, F., Methawasin, M., Adler, M., Escobar, Y. N., Nedrud, J., Pappas, C. T., Harris, S. P., & Granzier, H. (2016). Thin filament length in the cardiac sarcomere varies with sarcomere length but is independent of titin and nebulin. Journal of molecular and cellular cardiology, 97, 286-94.
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  Thin filament length (TFL) is an important determinant of the force-sarcomere length (SL) relation of cardiac muscle. However, the various mechanisms that control TFL are not well understood. Here we tested the previously proposed hypothesis that the actin-binding protein nebulin contributes to TFL regulation in the heart by using a cardiac-specific nebulin cKO mouse model (αMHC Cre Neb cKO). Atrial myocytes were studied because nebulin expression has been reported to be most prominent in this cell type. TFL was measured in right and left atrial myocytes using deconvolution optical microscopy and staining for filamentous actin with phalloidin and for the thin filament pointed-end with an antibody to the capping protein Tropomodulin-1 (Tmod1). Results showed that TFLs in Neb cKO and littermate control mice were not different. Thus, deletion of nebulin in the heart does not alter TFL. However, TFL was found to be ~0.05μm longer in the right than in the left atrium and Tmod1 expression was increased in the right atrium. We also tested the hypothesis that the length of titin's spring region is a factor controlling TFL by studying the Rbm20(ΔRRM) mouse which expresses titins that are ~500kDa (heterozygous mice) and ~1000kDa (homozygous mice) longer than in control mice. Results revealed that TFL was not different in Rbm20(ΔRRM) mice. An unexpected finding in all genotypes studied was that TFL increased as sarcomeres were stretched (~0.1μm per 0.35μm of SL increase). This apparent increase in TFL reached a maximum at a SL of ~3.0μm where TFL was ~1.05μm. The SL dependence of TFL was independent of chemical fixation or the presence of cardiac myosin-binding protein C (cMyBP-C). In summary, we found that in cardiac myocytes TFL varies with SL in a manner that is independent of the size of titin or the presence of nebulin.
• Ly, T., Moroz, N., Pappas, C. T., Novak, S. M., Tolkatchev, D., Wooldridge, D., Mayfield, R. M., Helms, G., Gregorio, C. C., & Kostyukova, A. S. (2016). The N-terminal tropomyosin- and actin-binding sites are important for leiomodin 2's function. Molecular biology of the cell, 27(16), 2565-75.
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  Leiomodin is a potent actin nucleator related to tropomodulin, a capping protein localized at the pointed end of the thin filaments. Mutations in leiomodin-3 are associated with lethal nemaline myopathy in humans, and leiomodin-2-knockout mice present with dilated cardiomyopathy. The arrangement of the N-terminal actin- and tropomyosin-binding sites in leiomodin is contradictory and functionally not well understood. Using one-dimensional nuclear magnetic resonance and the pointed-end actin polymerization assay, we find that leiomodin-2, a major cardiac isoform, has an N-terminal actin-binding site located within residues 43-90. Moreover, for the first time, we obtain evidence that there are additional interactions with actin within residues 124-201. Here we establish that leiomodin interacts with only one tropomyosin molecule, and this is the only site of interaction between leiomodin and tropomyosin. Introduction of mutations in both actin- and tropomyosin-binding sites of leiomodin affected its localization at the pointed ends of the thin filaments in cardiomyocytes. On the basis of our new findings, we propose a model in which leiomodin regulates actin poly-merization dynamics in myocytes by acting as a leaky cap at thin filament pointed ends.
• Winter, J. M., Joureau, B., Lee, E. J., Kiss, B., Yuen, M., Gupta, V. A., Pappas, C. T., Gregorio, C. C., Stienen, G. J., Edvardson, S., Wallgren-Pettersson, C., Lehtokari, V. L., Pelin, K., Malfatti, E., Romero, N. B., Engelen, B. G., Voermans, N. C., Donkervoort, S., Bönnemann, C. G., , Clarke, N. F., et al. (2016). Mutation-specific effects on thin filament length in thin filament myopathy. Annals of neurology, 79(6), 959-69.
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  Thin filament myopathies are among the most common nondystrophic congenital muscular disorders, and are caused by mutations in genes encoding proteins that are associated with the skeletal muscle thin filament. Mechanisms underlying muscle weakness are poorly understood, but might involve the length of the thin filament, an important determinant of force generation.
• Novak, S. M., Ottenheijm, C. A., Yuen, M., Wallgren-pettersson, C., Waddell, L. B., Todd, E. J., Thomas, B. P., Telfer, W. R., Swanson, L. C., Sloboda, D. D., Shannon, P., Sandaradura, S. A., Romero, N. B., Ravenscroft, G., Quinlan, K. G., Pelin, K., Pappas, C. T., Ottenheijm, C. A., Novak, S. M., , North, K. N., et al. (2015). Leiomodin-3 dysfunction results in thin filament disorganization and nemaline myopathy.. The Journal of clinical investigation, 125(1), 456-7. doi:10.1172/jci80057
• Pappas, C. T., Mayfield, R. M., Henderson, C., Jamilpour, N., Cover, C., Hernandez, Z., Hutchinson, K. R., Chu, M., Nam, K., Valdez, J. M., Wong, P. K., Granzier, H. L., & Gregorio, C. C. (2015). Knockout of Lmod2 results in shorter thin filaments followed by dilated cardiomyopathy and juvenile lethality. Proceedings of the National Academy of Sciences of the United States of America, 112(44), 13573-8.
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  Leiomodin 2 (Lmod2) is an actin-binding protein that has been implicated in the regulation of striated muscle thin filament assembly; its physiological function has yet to be studied. We found that knockout of Lmod2 in mice results in abnormally short thin filaments in the heart. We also discovered that Lmod2 functions to elongate thin filaments by promoting actin assembly and dynamics at thin filament pointed ends. Lmod2-KO mice die as juveniles with hearts displaying contractile dysfunction and ventricular chamber enlargement consistent with dilated cardiomyopathy. Lmod2-null cardiomyocytes produce less contractile force than wild type when plated on micropillar arrays. Introduction of GFP-Lmod2 via adeno-associated viral transduction elongates thin filaments and rescues structural and functional defects observed in Lmod2-KO mice, extending their lifespan to adulthood. Thus, to our knowledge, Lmod2 is the first identified mammalian protein that functions to elongate actin filaments in the heart; it is essential for cardiac thin filaments to reach a mature length and is required for efficient contractile force and proper heart function during development.
• Winter, J. M., Stienen, G. J., Romero, N. B., Pappas, C. T., Ottenheijm, C. C., Malfatti, E., Joureau, B., Gregorio, C. C., Granzier, H., Clarke, N. F., & Beggs, A. H. (2015). Force-Sarcomere Length Relations in Patients with Thin Filament Myopathy Caused by Mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 and KLHL41. Biophysical Journal, 108(2), 338a-339a. doi:10.1016/j.bpj.2014.11.1847
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  Background: Mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 and −41 lead to thin filament myopathies, such as nemaline myopathy, congenital fiber type disproportion and cap disease. A hallmark feature of these myopathies is muscle weakness. Here, we aimed to elucidate whether mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 and KLHL41 affect the maximal force generating capacity of muscle fibers. Subsequently, by determining the sarcomere length-dependence of force, we investigated whether changes in thin filament length contributed to muscle weakness.Methods: Biopsies from NM, CFTD, and CAP patients (n=39) with mutations in NEB, ACTA1, TPM2, TPM3, KBTBD13, KLHL40 or KLHL41 were compared to biopsies from healthy controls (n=9). Using permeabilized muscle fibers, maximal active tension was determined at incremental sarcomere lengths (range 2.0-3.5 µm) to obtain the force-sarcomere length relationship.Results: The maximal active tension (Fmax (in mN/mm2, mean±SEM)) was significantly lower in biopsies from NEB (46±5), ACTA1 (48±4), TPM2 (71±8), TPM3 (85±10), KBTBD13 (78±3), KLHL40 (2.8±0.2) and KLHL41 (63±4) patients compared to biopsies of controls (129±7).No shift in the force-sarcomere length relationship was observed in TPM3, TPM2, KBTBD 13, KLHL 40 and KLHL41 patients. In contrast, several patients with ACTA1 and NEB mutations showed a leftward shift of the force-sarcomere length relationship indicating shorter thin filaments. Furthermore, the slope of the descending limb of the force-sarcomere length relationship in these patients is less steep than the slope of the CTRL curve, suggesting heterogeneity of thin filaments.Conclusion: Our data suggest that mutations in NEB and ACTA1 result in changes in thin filament length. Insights in the mechanisms underlying weakness in patients with thin filament mutations are necessary to improve specific treatment strategies.
• Granzier, H. L., Hutchinson, K. R., Tonino, P., Methawasin, M., Li, F. W., Slater, R. E., Bull, M. M., Saripalli, C., Pappas, C. T., Gregorio, C. C., & Smith, J. E. (2014). Deleting titin's I-band/A-band junction reveals critical roles for titin in biomechanical sensing and cardiac function. Proceedings of the National Academy of Sciences of the United States of America, 111(40), 14589-94.
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  Titin, the largest protein known, forms a giant filament in muscle where it spans the half sarcomere from Z disk to M band. Here we genetically targeted a stretch of 14 immunoglobulin-like and fibronectin type 3 domains that comprises the I-band/A-band (IA) junction and obtained a viable mouse model. Super-resolution optical microscopy (structured illumination microscopy, SIM) and electron microscopy were used to study the thick filament length and titin's molecular elasticity. SIM showed that the IA junction functionally belongs to the relatively stiff A-band region of titin. The stiffness of A-band titin was found to be high, relative to that of I-band titin (∼ 40-fold higher) but low, relative to that of the myosin-based thick filament (∼ 70-fold lower). Sarcomere stretch therefore results in movement of A-band titin with respect to the thick filament backbone, and this might constitute a novel length-sensing mechanism. Findings disproved that titin at the IA junction is crucial for thick filament length control, settling a long-standing hypothesis. SIM also showed that deleting the IA junction moves the attachment point of titin's spring region away from the Z disk, increasing the strain on titin's molecular spring elements. Functional studies from the cellular to ex vivo and in vivo left ventricular chamber levels showed that this causes diastolic dysfunction and other symptoms of heart failure with preserved ejection fraction (HFpEF). Thus, our work supports titin's important roles in diastolic function and disease of the heart.
• Yuen, M., Sandaradura, S. A., Dowling, J. J., Kostyukova, A. S., Moroz, N., Quinlan, K. G., Lehtokari, V., Ravenscroft, G., Todd, E. J., Ceyhan-Birsoy, O., Gokhin, D. S., Maluenda, J., Lek, M., Nolent, F., Pappas, C. T., Novak, S. M., D'Amico, A., Malfatti, E., Thomas, B. P., , Gabriel, S. B., et al. (2014). Leiomodin-3 dysfunction results in thin filament disorganization and nemaline myopathy. The Journal of clinical investigation, 124(11), 4693-708.
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  Nemaline myopathy (NM) is a genetic muscle disorder characterized by muscle dysfunction and electron-dense protein accumulations (nemaline bodies) in myofibers. Pathogenic mutations have been described in 9 genes to date, but the genetic basis remains unknown in many cases. Here, using an approach that combined whole-exome sequencing (WES) and Sanger sequencing, we identified homozygous or compound heterozygous variants in LMOD3 in 21 patients from 14 families with severe, usually lethal, NM. LMOD3 encodes leiomodin-3 (LMOD3), a 65-kDa protein expressed in skeletal and cardiac muscle. LMOD3 was expressed from early stages of muscle differentiation; localized to actin thin filaments, with enrichment near the pointed ends; and had strong actin filament-nucleating activity. Loss of LMOD3 in patient muscle resulted in shortening and disorganization of thin filaments. Knockdown of lmod3 in zebrafish replicated NM-associated functional and pathological phenotypes. Together, these findings indicate that mutations in the gene encoding LMOD3 underlie congenital myopathy and demonstrate that LMOD3 is essential for the organization of sarcomeric thin filaments in skeletal muscle.
• Donlin, L. T., Andresen, C., Just, S., Rudensky, E., Pappas, C. T., Kruger, M., Jacobs, E. Y., Unger, A., Zieseniss, A., Dobenecker, M., Voelkel, T., Chait, B. T., Gregorio, C. C., Rottbauer, W., Tarakhovsky, A., & Linke, W. A. (2012). Smyd2 controls cytoplasmic lysine methylation of Hsp90 and myofilament organization. Genes & development, 26(2), 114-9.
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  Protein lysine methylation is one of the most widespread post-translational modifications in the nuclei of eukaryotic cells. Methylated lysines on histones and nonhistone proteins promote the formation of protein complexes that control gene expression and DNA replication and repair. In the cytoplasm, however, the role of lysine methylation in protein complex formation is not well established. Here we report that the cytoplasmic protein chaperone Hsp90 is methylated by the lysine methyltransferase Smyd2 in various cell types. In muscle, Hsp90 methylation contributes to the formation of a protein complex containing Smyd2, Hsp90, and the sarcomeric protein titin. Deficiency in Smyd2 results in the loss of Hsp90 methylation, impaired titin stability, and altered muscle function. Collectively, our data reveal a cytoplasmic protein network that employs lysine methylation for the maintenance and function of skeletal muscle.
• Pappas, C. T., Bliss, K. T., Zieseniss, A., & Gregorio, C. C. (2011). The Nebulin family: an actin support group. Trends in cell biology, 21(1), 29-37.
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  Nebulin, a giant, actin-binding protein, is the largest member of a family of proteins (including N-RAP, nebulette, lasp-1 and lasp-2) that are assembled in a variety of cytoskeletal structures, and expressed in different tissues. For decades, nebulin has been thought to act as a molecular ruler, specifying the precise length of actin filaments in skeletal muscle. However, emerging evidence suggests that nebulin should not be viewed as a ruler but as an actin filament stabilizer required for length maintenance. Nebulin has also been implicated recently in an array of regulatory functions independent of its role in actin filament length regulation. In this review, we discuss the current evolutionary, biochemical, and functional data for the nebulin family of proteins - a family whose members, both large and small, function as cytoskeletal scaffolds and stabilizers.
• Pappas, C. T., Krieg, P. A., & Gregorio, C. C. (2010). Nebulin regulates actin filament lengths by a stabilization mechanism. The Journal of cell biology, 189(5), 859-70.
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  Efficient muscle contraction requires regulation of actin filament lengths. In one highly cited model, the giant protein nebulin has been proposed to function as a molecular ruler specifying filament lengths. We directly challenged this hypothesis by constructing a unique, small version of nebulin (mini-nebulin). When endogenous nebulin was replaced with mini-nebulin in skeletal myocytes, thin filaments extended beyond the end of mini-nebulin, an observation which is inconsistent with a strict ruler function. However, under conditions that promote actin filament depolymerization, filaments associated with mini-nebulin were remarkably maintained at lengths either matching or longer than mini-nebulin. This indicates that mini-nebulin is able to stabilize portions of the filament it has no contact with. Knockdown of nebulin also resulted in more dynamic populations of thin filament components, whereas expression of mini-nebulin decreased the dynamics at both filament ends (i.e., recovered loss of endogenous nebulin). Thus, nebulin regulates thin filament architecture by a mechanism that includes stabilizing the filaments and preventing actin depolymerization.
• Tonino, P., Pappas, C. T., Hudson, B. D., Labeit, S., Gregorio, C. C., & Granzier, H. (2010). Reduced myofibrillar connectivity and increased Z-disk width in nebulin-deficient skeletal muscle. Journal of cell science, 123(Pt 3), 384-91.
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  A prominent feature of striated muscle is the regular lateral alignment of adjacent sarcomeres. An important intermyofibrillar linking protein is the intermediate filament protein desmin, and based on biochemical and structural studies in primary cultures of myocytes it has been proposed that desmin interacts with the sarcomeric protein nebulin. Here we tested whether nebulin is part of a novel biomechanical linker complex, by using a recently developed nebulin knockout (KO) mouse model and measuring Z-disk displacement in adjacent myofibrils of both extensor digitorum longus (EDL) and soleus muscle. Z-disk displacement increased as sarcomere length (SL) was increased and the increase was significantly larger in KO fibers than in wild-type (WT) fibers; results in 3-day-old and 10-day-old mice were similar. Immunoelectron microscopy revealed reduced levels of desmin in intermyofibrillar spaces adjacent to Z-disks in KO fibers compared with WT fibers. We also performed siRNA knockdown of nebulin and expressed modules within the Z-disk portion of nebulin (M160-M170) in quail myotubes and found that this prevented the mature Z-disk localization of desmin filaments. Combined, these data suggest a model in which desmin attaches to the Z-disk through an interaction with nebulin. Finally, because nebulin has been proposed to play a role in specifying Z-disk width, we also measured Z-disk width in nebulin KO mice. Results show that most Z-disks of KO mice were modestly increased in width (approximately 80 nm in soleus and approximately 40 nm in EDL fibers) whereas a small subset had severely increased widths (up to approximately 1 microm) and resembled nemaline rod bodies. In summary, structural studies on a nebulin KO mouse show that in the absence of nebulin, Z-disks are significantly wider and that myofibrils are misaligned. Thus the functional roles of nebulin extend beyond thin filament length regulation and include roles in maintaining physiological Z-disk widths and myofibrillar connectivity.
• Tsukada, T., Pappas, C. T., Moroz, N., Antin, P. B., Kostyukova, A. S., & Gregorio, C. C. (2010). Leiomodin-2 is an antagonist of tropomodulin-1 at the pointed end of the thin filaments in cardiac muscle. Journal of cell science, 123(Pt 18), 3136-45.
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  Regulation of actin filament assembly is essential for efficient contractile activity in striated muscle. Leiomodin is an actin-binding protein and homolog of the pointed-end capping protein, tropomodulin. These proteins are structurally similar, sharing a common domain organization that includes two actin-binding sites. Leiomodin also contains a unique C-terminal extension that has a third actin-binding WH2 domain. Recently, the striated-muscle-specific isoform of leiomodin (Lmod2) was reported to be an actin nucleator in cardiomyocytes. Here, we have identified a function of Lmod2 in the regulation of thin filament lengths. We show that Lmod2 localizes to the pointed ends of thin filaments, where it competes for binding with tropomodulin-1 (Tmod1). Overexpression of Lmod2 results in loss of Tmod1 assembly and elongation of the thin filaments from their pointed ends. The Lmod2 WH2 domain is required for lengthening because its removal results in a molecule that caps the pointed ends similarly to Tmod1. Furthermore, Lmod2 transcripts are first detected in the heart after it has begun to beat, suggesting that the primary function of Lmod2 is to maintain thin filament lengths in the mature heart. Thus, Lmod2 antagonizes the function of Tmod1, and together, these molecules might fine-tune thin filament lengths.
• Pappas, C. T., Bhattacharya, N., Cooper, J. A., & Gregorio, C. C. (2008). Nebulin interacts with CapZ and regulates thin filament architecture within the Z-disc. Molecular biology of the cell, 19(5), 1837-47.
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  The barbed ends of actin filaments in striated muscle are anchored within the Z-disc and capped by CapZ; this protein blocks actin polymerization and depolymerization in vitro. The mature lengths of the thin filaments are likely specified by the giant "molecular ruler" nebulin, which spans the length of the thin filament. Here, we report that CapZ specifically interacts with the C terminus of nebulin (modules 160-164) in blot overlay, solid-phase binding, tryptophan fluorescence, and SPOTs membrane assays. Binding of nebulin modules 160-164 to CapZ does not affect the ability of CapZ to cap actin filaments in vitro, consistent with our observation that neither of the two C-terminal actin binding regions of CapZ is necessary for its interaction with nebulin. Knockdown of nebulin in chick skeletal myotubes using small interfering RNA results in a reduction of assembled CapZ, and, strikingly, a loss of the uniform alignment of the barbed ends of the actin filaments. These data suggest that nebulin restricts the position of thin filament barbed ends to the Z-disc via a direct interaction with CapZ. We propose a novel molecular model of Z-disc architecture in which nebulin interacts with CapZ from a thin filament of an adjacent sarcomere, thus providing a structural link between sarcomeres.
• Cutter, A. D., Good, J. M., Pappas, C. T., Saunders, M. A., Starrett, D. M., & Wheeler, T. J. (2005). Transposable element orientation bias in the Drosophila melanogaster genome. Journal of molecular evolution, 61(6), 733-41.
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  Nonrandom distributions of transposable elements can be generated by a variety of genomic features. Using the full D. melanogaster genome as a model, we characterize the orientations of different classes of transposable elements in relation to the directionality of genes. DNA-mediated transposable elements are more likely to be in the same orientation as neighboring genes when they occur in the nontranscribed region's that flank genes. However, RNA-mediated transposable elements located in an intron are more often oriented in the direction opposite to that of the host gene. These orientation biases are strongest for genes with highly biased codon usage, probably reflecting the ability of such loci to respond to weak positive or negative selection. The leading hypothesis for selection against transposable elements in the coding orientation proposes that transcription termination poly(A) signal motifs within retroelements interfere with normal gene transcription. However, after accounting for differences in base composition between the strands, we find no evidence for global selection against spurious transcription termination signals in introns. We therefore conclude that premature termination of host gene transcription due to the presence of poly(A) signal motifs in retroelements might only partially explain strand-specific detrimental effects in the D. melanogaster genome.
• Pappas, C. T., Sram, J., Moskvin, O. V., Ivanov, P. S., Mackenzie, R. C., Choudhary, M., Land, M. L., Larimer, F. W., Kaplan, S., & Gomelsky, M. (2004). Construction and validation of the Rhodobacter sphaeroides 2.4.1 DNA microarray: transcriptome flexibility at diverse growth modes. Journal of bacteriology, 186(14), 4748-58.
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  A high-density oligonucleotide DNA microarray, a genechip, representing the 4.6-Mb genome of the facultative phototrophic proteobacterium, Rhodobacter sphaeroides 2.4.1, was custom-designed and manufactured by Affymetrix, Santa Clara, Calif. The genechip contains probe sets for 4,292 open reading frames (ORFs), 47 rRNA and tRNA genes, and 394 intergenic regions. The probe set sequences were derived from the genome annotation generated by Oak Ridge National Laboratory after extensive revision, which was based primarily upon codon usage characteristic of this GC-rich bacterium. As a result of the revision, numerous missing ORFs were uncovered, nonexistent ORFs were deleted, and misidentified start codons were corrected. To evaluate R. sphaeroides transcriptome flexibility, expression profiles for three diverse growth modes--aerobic respiration, anaerobic respiration in the dark, and anaerobic photosynthesis--were generated. Expression levels of one-fifth to one-third of the R. sphaeroides ORFs were significantly different in cells under any two growth modes. Pathways involved in energy generation and redox balance maintenance under three growth modes were reconstructed. Expression patterns of genes involved in these pathways mirrored known functional changes, suggesting that massive changes in gene expression are the major means used by R. sphaeroides in adaptation to diverse conditions. Differential expression was observed for genes encoding putative new participants in these pathways (additional photosystem genes, duplicate NADH dehydrogenase, ATP synthases), whose functionality has yet to be investigated. The DNA microarray data correlated well with data derived from quantitative reverse transcription-PCR, as well as with data from the literature, thus validating the R. sphaeroides genechip as a powerful and reliable tool for studying unprecedented metabolic versatility of this bacterium.

Poster Presentations

• Pappas, C. T., Farman, G. P., Mayfield, R. M., & Gregorio, C. C. (2017, December). Leiomodin-2 regulates thin filament assembly and is necessary for proper contractile force production in the hearts of adult mice. American Society of Cell Biology/European Molecular Biology Organization Annual Meeting.