Molecular and functional characterization of a novel cardiac-specific human tropomyosin isoform.

BACKGROUND: Tropomyosin (TM), an essential actin-binding protein, is central to the control of calcium-regulated striated muscle contraction. Although TPM1alpha (also called alpha-TM) is the predominant TM isoform in human hearts, the precise TM isoform composition remains unclear. METHODS AND RESULTS: In this study, we quantified for the first time the levels of striated muscle TM isoforms in human heart, including a novel isoform called TPM1kappa. By developing a TPM1kappa-specific antibody, we found that the TPM1kappa protein is expressed and incorporated into organized myofibrils in hearts and that its level is increased in human dilated cardiomyopathy and heart failure. To investigate the role of TPM1kappa in sarcomeric function, we generated transgenic mice overexpressing cardiac-specific TPM1kappa. Incorporation of increased levels of TPM1kappa protein in myofilaments leads to dilated cardiomyopathy. Physiological alterations include decreased fractional shortening, systolic and diastolic dysfunction, and decreased myofilament calcium sensitivity with no change in maximum developed tension. Additional biophysical studies demonstrate less structural stability and weaker actin-binding affinity of TPM1kappa compared with TPM1alpha. CONCLUSIONS: This functional analysis of TPM1kappa provides a possible mechanism for the consequences of the TM isoform switch observed in dilated cardiomyopathy and heart failure patients.

An internal domain of beta-tropomyosin increases myofilament Ca(2+) sensitivity.

Tropomyosin (TM) is involved in Ca(2+)-mediated muscle contraction and relaxation in the heart. Striated muscle alpha-TM is the major isoform expressed in the heart. The expression of striated muscle beta-TM in the murine myocardium results in a decreased rate of relaxation and increased myofilament Ca(2+) sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of alpha-TM with beta-TM (a troponin T-binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament Ca(2+) sensitivity. We hypothesized that the putative internal troponin T-binding domain (amino acids 175-190) of beta-TM may be responsible for the increased myofilament Ca(2+) sensitivity observed when the entire beta-TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that expressed chimeric TM containing beta-TM amino acids 175-190 in the backbone of alpha-TM (amino acids 1-174 and 191-284). These mice expressed 16-57% chimeric TM and did not develop cardiac hypertrophy or any other morphological changes. Physiological analysis showed that these hearts exhibited decreased rates of contraction and relaxation and a positive response to isoproterenol. Skinned fiber bundle analyses showed a significant increase in myofilament Ca(2+) sensitivity. Biophysical experiments demonstrated that the exchanged amino acids did not influence the flexibility of the TM. This is the first study to demonstrate that a specific domain within TM can increase the Ca(2+) sensitivity of the thin filament and affect sarcomeric performance. Furthermore, these results enhance the understanding of why TM mutations associated with familial hypertrophic cardiomyopathy demonstrate increased myofilament sensitivity to Ca(2+).

Atonal homolog 1 is a tumor suppressor gene.

Colon cancer accounts for more than 10% of all cancer deaths annually. Our genetic evidence from Drosophila and previous in vitro studies of mammalian Atonal homolog 1 (Atoh1, also called Math1 or Hath1) suggest an anti-oncogenic function for the Atonal group of proneural basic helix-loop-helix transcription factors. We asked whether mouse Atoh1 and human ATOH1 act as tumor suppressor genes in vivo. Genetic knockouts in mouse and molecular analyses in the mouse and in human cancer cell lines support a tumor suppressor function for ATOH1. ATOH1 antagonizes tumor formation and growth by regulating proliferation and apoptosis, likely via activation of the Jun N-terminal kinase signaling pathway. Furthermore, colorectal cancer and Merkel cell carcinoma patients show genetic and epigenetic ATOH1 loss-of-function mutations. Our data indicate that ATOH1 may be an early target for oncogenic mutations in tissues where it instructs cellular differentiation.

Dilated cardiomyopathy mutant tropomyosin mice develop cardiac dysfunction with significantly decreased fractional shortening and myofilament calcium sensitivity.

Mutations in striated muscle alpha-tropomyosin (alpha-TM), an essential thin filament protein, cause both dilated cardiomyopathy (DCM) and familial hypertrophic cardiomyopathy. Two distinct point mutations within alpha-tropomyosin are associated with the development of DCM in humans: Glu40Lys and Glu54Lys. To investigate the functional consequences of alpha-TM mutations associated with DCM, we generated transgenic mice that express mutant alpha-TM (Glu54Lys) in the adult heart. Results showed that an increase in transgenic protein expression led to a reciprocal decrease in endogenous alpha-TM levels, with total myofilament TM protein levels remaining unaltered. Histological and morphological analyses revealed development of DCM with progression to heart failure and frequently death by 6 months. Echocardiographic analyses confirmed the dilated phenotype of the heart with a significant decrease in the left ventricular fractional shortening. Work-performing heart analyses showed significantly impaired systolic, and diastolic functions and the force measurements of cardiac myofibers revealed that the myofilaments had significantly decreased Ca(2+) sensitivity and tension generation. Real-time RT-PCR quantification demonstrated an increased expression of beta-myosin heavy chain, brain natriuretic peptide, and skeletal actin and a decreased expression of the Ca(2+) handling proteins sarcoplasmic reticulum Ca(2+)-ATPase and ryanodine receptor. Furthermore, our study also indicates that the alpha-TM54 mutation decreases tropomyosin flexibility, which may influence actin binding and myofilament Ca(2+) sensitivity. The pathological and physiological phenotypes exhibited by these mice are consistent with those seen in human DCM and heart failure. As such, this is the first mouse model in which a mutation in a sarcomeric thin filament protein, specifically TM, leads to DCM.

Short-form Ron receptor is required for normal IFN-gamma production in concanavalin A-induced acute liver injury.

Abrogation of Ron receptor tyrosine kinase function results in defects in macrophage activation and dysregulated acute inflammatory responses in vivo. Several naturally occurring constitutively active alternative forms of Ron have been identified, including from primary human tumors and tumor cell lines. One of these alternative forms, short-form (SF) Ron, is generated from an alternative start site in intron 10 of the Ron gene that eliminates most of the extracellular portion of the receptor and is overexpressed in several human cancers. To test the physiological significance of SF-Ron in vivo, mice were generated that solely express the full-length form of Ron (FL-Ron). Our results show that elimination of the capacity to express SF-Ron in vivo leads to augmented production of IFN-gamma from splenocytes following stimulation ex vivo with either concanavalin A or anti-CD3/T cell receptor monoclonal antibody. Moreover, in a concanavalin A-induced murine model of acute liver injury, FL-Ron mice have increased production of serum INF-gamma and serum alanine aminotransferase levels and worsened liver histology and overall survival compared with wild-type control mice. Taken together, these results suggest for the first time that SF-Ron impacts the progression of inflammatory immune responses in vivo and further support a role for the Ron receptor and its various forms in liver pathophysiology.

A mouse model of familial hypertrophic cardiomyopathy caused by a alpha-tropomyosin mutation.

Familial hypertrophic cardiomyopathy, a disease caused by mutations in cardiac contractile proteins, is characterized by left and/or right ventricular hypertrophy, myocyte disarray, fibrosis, and cardiac arrhythmias that may lead to premature sudden death. Five distinct point mutations within alpha-tropomyosin are associated with the development of familial hypertrophic cardiomyopathy. Two of these mutations are found within a troponin T binding site, located at amino acids 175 and 180. In this study, we analyze a transgenic mouse model for one of the mutations that occur at codon 180: a substitution of a glutamic acid for a glycine. These mice develop severe cardiac hypertrophy, substantial interstitial fibrosis, and have an increased heart weight/ body weight ratio. Results show that calcium-handling proteins associated with the sarcoplasmic reticulum exhibit decreased expression. These alterations in gene expression, coupled with the structurally-altered tropomyosin, may contribute to the demonstrated decreased physiological performance exhibited by these transgenic mice. A DNA hybridization microarray analysis of the transgenic vs. control ventricular RNAs shows that 50 transcripts are differentially expressed by more than 100% during the onset of the hypertrophic process, many of which are associated with the extracellular matrix. This study demonstrates that mutations within tropomyosin can be severely disruptive of sarcomeric function, triggering a hypertrophic response coupled with a cascade of alterations in gene expression.

Tropomyosin 3 expression leads to hypercontractility and attenuates myofilament length-dependent Ca(2+) activation.

Tropomyosin (TM), an integral component of the thin filament, is encoded by three striated muscle isoforms: alpha-TM, beta-TM, and TPM 3. Although the alpha-TM and beta-TM isoforms are well characterized, less is known about the function of the TPM 3 isoform, which is predominantly found in the slow-twitch musculature of mammals. To determine its functional significance, we ectopically expressed this isoform in the hearts of transgenic mice. We generated six transgenic mouse lines that produce varying levels of TPM 3 message with ectopic TPM 3 protein accounting for 40-60% of the total striated muscle tropomyosin. The transgenic mice have normal life spans and exhibit no morphological abnormalities in their sarcomeres or hearts. However, there are significant functional alterations in cardiac performance. Physiological assessment of these mice by using closed-chest analyses and a work-performing model reveals a hyperdynamic effect on systolic and diastolic function. Analysis of detergent-extracted fiber bundles demonstrates a decreased sensitivity to Ca(2+) in force generation and a decrease in length-dependent Ca(2+) activation with no detectable change in interfilament spacing as determined by using X-ray diffraction. Our data are the first to demonstrate that TM isoforms can affect sarcomeric performance by decreasing sensitivity to Ca(2+) and influencing the length-dependent Ca(2+) activation.