Isolation and characterization of a cDNA that encodes mouse fibroblast tropomyosin isoform 2

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Distinct localizations of tropomyosin isoforms in LLC-PK1 epithelial cells suggests specialized function at cell-cell adhesions

At least eight nonmuscle, nonbrain tropomyosin isoforms have been described. We used antibodies, microinjection, and transfection to characterize their expression and localization in LLC-PK1 kidney epithelial cells and compared them with other cells. Similar to primary enterocytes, LLC-PK1 cells exhibited predominantly TM-1 and TM-3 of the high-molecular-weight (HMW) isoforms; TM-5 and TM-5b of the low-molecular-weight (LMW) isoforms. Neither TM-4 nor TM-5a was detectable in the LLC-PKI cells. Immunofluorescence studies revealed that HMW isoforms were localized only on stress fibers, not adhesion belts, whereas the adhesion belts were stained by LMW isoform antibodies. When exogenous proteins are introduced either by transfection or microinjection, the HMW isoforms do not incorporate into the adhesion belt, whereas the LMW isoforms can incorporate into the stress fibers, thus indicating there are different mechanisms at work for the selective localization. Temporal changes in the microfilament system of the LLC-PK1 cells were studied during differentiation in culture as defined by spectrin expression and F-actin architecture. Western blot analysis indicated that TM-5b is only expressed in the LLC-PK1 cells after a certain degree of maturation in culture, which suggests isoform switching after the cell-cell contacts are developed. Collectively these results demonstrate that epithelial cells express a complex pattern of TM isoforms, which exhibit differential localizations within the cells and different patterns of expression depending on their origin and stage of differentiation. The implication of differential localization of TM isoforms on their specific functions is discussed.

Clinical implications of hypertrophic cardiomyopathy associated with mutations in the alpha-tropomyosin gene

OBJECTIVE

The disease-bearing genes for hypertrophic cardiomyopathy (HCM) in HCM families have been identified as the beta-myosin heavy chain, alpha-tropomyosin, and cardiac troponin T genes. Three HCM kindreds with three distinct point mutations in the alpha-tropomyosin gene had extensive clinical evaluations.

DESIGN AND RESULTS

Single-strand conformation polymorphism gel analysis of polymerase chain reaction amplified products was used to capture each of the nine exons from the alpha-tropomyosin gene to identify mutations in 60 familial HCM patients. Two missense mutations in exon 2 (Ala63Val and Lys70Thr) and one missense mutation in exon 5 (Asp175Asn) were found in three unrelated HCM kindreds. These kindreds were the subject of clinical, electrocardiographic and echocardiographic studies. The morphological appearance of HCM was similar in the three kindreds. All the patients had severe hypertrophy of the left ventricle with asymmetrical septal hypertrophy during the early stage of the disease, which gradually progressed to dilatation of the left ventricle. Moreover, these kindreds showed similar disease penetrance, age of onset, and incidence of premature sudden death. The disease in these kindreds was severe and resulted in frequent sudden deaths.

CONCLUSIONS

Among Japanese patients with familial HCM mutations in the alpha-tropomyosin gene are not as rare as reported, accounting for about 5% of all cases. These mutations are characterised by hypertrophy of the left ventricle which then progresses to dilatation and a high incidence of sudden or disease-related death.

Chromosome mapping of nine tropomyosin-related sequences in mice

Tropomyosins are a group of actin-binding proteins expressed as different isoforms in muscle and non-muscle cells. Two tropomyosin loci have already been mapped in the mouse genome, on Chromosomes (Chrs) 6 and 9. By using a human cDNA fragment of tropomyosin non-muscle isoform (TPM3) gene that maps on human Chr 1q, and a mapping panel from a murine interspecific cross, we mapped nine distinct tropomyosin-related loci in the mouse genome, on seven different chromosomes: Chrs 3, 4, 6, 7, 14, 17, and X.

Suppression of syntheses of high molecular weight nonmuscle tropomyosins in macrophages

In mouse fibroblasts, at least five TM isoforms are identified and they can be grouped into the high (TM1, TM2, and TM3) and low molecular weight TM isoforms (TM4 and TM5). Suppression of one of the high molecular weight tropomyosin (TM) isoforms in nonmuscle cells is implicated to be one of the causes for disorganization of actin microfilament bundles and subsequent changes in cell motility and cell shape. In this study, we studied the expression of tropomyosin isoforms in macrophages that exhibit high motility and ability to change cell shape. Two-dimensional gel electrophoresis followed by Western blot analysis using polyclonal anti-TM antiserum revealed that the high molecular weight TM isoforms were lacking in both resident and activated mouse peritoneal macrophages. Analyses of newly synthesized TM isoforms, Northern blot analyses using isoform-specific cDNA probes, and immunostaining with monoclonal anti-TM antibody that recognizes only the high molecular weight TM isoforms also demonstrated that the syntheses of the high molecular weight TM isoforms (TM1, TM2, and TM3) were completely suppressed, whereas the low molecular weight TM isoforms (TM4 and TM5) were expressed in macrophages. These results indicate that macrophages intrinsically lack the high molecular weight TM isoforms. In order to obtain information about cellular localization of the low molecular weight TM isoforms in macrophages, they were immunostained with polyclonal anti-TM antiserum that recognizes both the high and low molecular weight TM isoforms. The results showed that the low molecular weight TM isoforms were co-localized with F-actin in punctate and short fibrous structures. In addition, we performed in situ hybridization analysis to examine localizations of the TM mRNAs in fibroblasts and macrophages. The results showed that TM mRNAs were localized throughout the cytoplasm.

Restoration of microfilament bundle organization in v-raf-transformed NRK cells after transduction with tropomyosin 2 cDNA

The syntheses of tropomyosin (TM) isoforms, especially those of TM1 and TM2, were suppressed in v-raf-transformed NRK cells. To test whether restoration of one of the suppressed TM expressions affects cellular phenotypes of v-raf-transformed NRK cells, the cells were transduced with mouse fibroblast TM2 cDNA by retrovirally mediated DNA transfer method. Clones expressing the inserted TM2 cDNA and accordingly higher amounts of TM2 than the parental and control clones displayed a flatter morphology which was accompanied by partial restoration of microfilament organization, indicating that restoration of one of the diminished TM isoforms results in reorganization of microfilament bundles. However, no significant decrease in cell growth rate and the ability to grow in soft agar was observed in the TM2 cDNA-transduced cells.

Expression, cytoskeletal utilization and dimer formation of tropomyosin derived from retroviral-mediated cDNA transfer. Metabolism of tropomyosin from transduced cDNA

Expression of the tropomyosin-1 isoform was enhanced by cDNA transfer in non-transformed murine 3T3 fibroblasts and also in v-Ki-ras transformed fibroblasts in which native tropomyosin-1 expression had been reduced and tropomyosin-2 synthesis virtually eliminated by action of the oncogene. The level of synthesis of insert-derived tropomyosin-1 was similar in normal and transformed transductants (3-5 times normal levels). The high level of insert-derived tropomyosin-1 expression resulted in a considerable increase in tropomyosin-1 utilization in the cytoskeleton of transformed cells, but this expression still did not reach normal levels, suggesting an oncogene-related inhibition of tropomyosin utilization. A large proportion of newly synthesized native tropomyosin-1 in normal, unmodified fibroblasts appeared in homodimers which, upon prolonged incubation, were largely converted to the heterodimers. Excess tropomyosin-1 derived from the inserted cDNA also appeared largely as the homodimer in both normal and transformed cells. This homodimer was utilized effectively in the formation of cytoskeletal structures but was partially converted to heterodimer by chain exchange. Under steady-state conditions, approximately 33% of the cytoskeletal tropomyosin-1-containing dimers were homodimers, compared to approximately 10% in normal fibroblasts. The results show that the increased amount of tropomyosin-1 homodimer entering the cytoskeleton under conditions of tropomyosin-1 excess, results in an atypical microfilament composition. The effect of this excess of tropomyosin-1 homodimers on stability or function of microfilament fibers remains to be determined. The results also confirm that the mechanisms of rapid homodimer formation with conversion to heterodimers by chain exchange, known from in vitro studies, also occur in vivo.

Alpha-tropomyosin and cardiac troponin T mutations cause familial hypertrophic cardiomyopathy: a disease of the sarcomere

We demonstrate that missense mutations (Asp175Asn; Glu180Gly) in the alpha-tropomyosin gene cause familial hypertrophic cardiomyopathy (FHC) linked to chromosome 15q2. These findings implicated components of the troponin complex as candidate genes at other FHC loci, particularly cardiac troponin T, which was mapped in this study to chromosome 1q. Missense mutations (Ile79Asn; Arg92Gln) and a mutation in the splice donor sequence of intron 15 of the cardiac troponin T gene are also shown to cause FHC. Because alpha-tropomyosin and cardiac troponin T as well as beta myosin heavy chain mutations cause the same phenotype, we conclude that FHC is a disease of the sarcomere. Further, because the splice site mutation is predicted to function as a null allele, we suggest that abnormal stoichiometry of sarcomeric proteins can cause cardiac hypertrophy.

Subtractive cDNA cloning as a tool to analyse secondary effects of a muscle disease. Characterization of affected genes in the myotonic ADR mouse

In myotonic ADR mice that are homozygous for a defect in the muscular chloride channel gene adr/Clc-1, the hyperexcitability of fast muscles is associated with secondary changes in gene expression and fibre type composition. cDNA clones derived from a set of genes down regulated in fast muscles of the myotonic ADR mouse were isolated by a subtractive cloning procedure. A total of 1200 clones were analysed for high expression in fast muscle of wild type and low expression in mutant mouse. Differential transcript levels were verified by northern blot hybridizations. The identities of the corresponding transcripts were determined by sequencing as myosin heavy chain IIB, alpha-tropomyosin, troponin C, a Ca2+ ATPase and parvalbumin mRNAs. Of these, mRNAs for parvalbumin and myosin heavy chain IIB were drastically downregulated in myotonic muscle (to < 10% of control). A full length cDNA clone for skeletal muscle alpha-tropomyosin was homologous to the mouse fibroblast tropomyosin isoform 2, except for the portion encoding the alpha-tropomyosin specific amino acids 258-284. A cDNA derived from the 1100 nucleotide parvalbumin transcript was cloned and the sequence for the as yet unknown 3' extended trailer, generated by alternative polyadenylation, was determined.

Isolation of genes from complex sources of mammalian genomic DNA using exon amplification

Modifications to exon amplification have been instituted that increase its speed, efficiency and reliability. Exons were isolated from target human or mouse genomic DNA sources ranging from 30 kilobases (kb) to 3 megabases (Mb) in complexity. The efficiency was dependent upon the amount of input DNA, and ranged from isolation of an exon for every 20 kb to an exon for every 80 kb of target genomic DNA. In these studies, several novel genes and a smaller number of genes isolated previously that reside on human chromosome 9 have been identified. These results indicate that exon amplification is presently adaptable to large scale isolation of exons from complex sources of genomic DNA.

Human fibroblast tropomyosin isoforms: characterization of cDNA clones and analysis of tropomyosin isoform expression in human tissues and in normal and transformed cells

A tropomyosin-specific oligonucleotide probe (REN29) designed to hybridize to all known human tropomyosin isoforms was used to study tropomyosin mRNA levels in normal and transformed human cells. At least four different sizes of RNAs were detected in normal human fibroblast KD cells by Northern blot analysis. The major bands of 1.1 kb RNA for hTM1 and 3.0 kb RNA for hTM4 were decreased substantially in various transformed cell lines. One of the minor RNA bands (2.0 kb for hTM2 and hTM3) appeared to be absent in a human pancreatic carcinoma cell line. The level of the other minor RNA band (2.5 kb for hTM5) was found to be unchanged or slightly decreased in transformed cells. This differential expression of tropomyosin isoforms at the RNA level was not totally in agreement with the difference in the protein amounts found in normal and transformed cells, suggesting that translational control may also play an important role in the expression of some tropomyosin isoforms. The REN29 probe was further used to screen lambda gt10 and lambda gt11 cDNA libraries, which were constructed from poly(A)+ RNAs of human fibroblast cell lines HuT-14 and WI-38, respectively. In addition to cDNA clones encoding known isoforms, we obtained three classes of new cDNA clones that encode two low M(r) isoforms (hTM5a and hTM5b), and a high M(r) isoform (hTMsm alpha). Sequence comparison revealed that hTM5a and hTM5b are alternatively spliced products derived from the same gene that encodes hTM2 and hTM3. Northern blot analysis and amino acid sequence comparison suggested that the hTMsm alpha represents a smooth muscle tropomyosin which is also expressed in human fibroblasts. The exon specific for, and common to, hTM5a and hTM5b was found to be highly expressed in small intestine. However, there was no detectable expression of this exon in stomach and skeletal muscle. The difference in tissue-specific expression suggests that different isoforms may perform distinct functions in different tissues.

Avian cardiac tropomyosin gene produces tissue-specific isoforms through alternative RNA splicing

We have isolated a quail cardiac tropomyosin gene which encodes three distinct isoforms through the use of alternative exon splicing. Characterization of cDNA clones produced by this gene indicate that the gene encodes a unique 284 amino acid cardiac tropomyosin isoform, along with a 248 amino acid cytoskeletal and 284 amino acid smooth muscle isoforms. Northern analyses indicate that the gene is primarily expressed in cardiac muscle, with only minor expression of the cytoskeletal and smooth muscle transcripts.

The molecular basis for tropomyosin isoform diversity

The tropomyosins are a family of actin filament binding proteins. In multicellular animals, they exhibit extensive cell type specific isoform diversity. In this essay we discuss the genetic mechanisms by which this diversity is generated and its possible significance to cellular function.

Nucleotide sequence of cDNA for nonmuscle tropomyosin 5 of mouse fibroblast

A cDNA clone for mouse fibroblast tropomyosin (TM) 5 was obtained from a cDNA library using human TM pseudogene as a probe. Sequence analysis of the clone revealed that the deduced amino acid sequence is different only at the 4th position from the human counterpart and that both the 5' and 3' untranslated regions are highly conserved.

Expression of tropomyosin genes during the development of the rat cerebellum

The expression of the tropomyosin genes in the rat nervous system was examined during the postnatal development of the cerebellum, using human-specific alpha-, beta-, gamma-, and delta-tropomyosin cDNA probes and rat-specific alpha-, beta-, and delta-tropomyosin oligonucleotide probes. The beta- and gamma-genes do not seem to be expressed in the rat brain. The delta-tropomyosin gene produces two mRNAs: a major one of 2.4 kb, which is highly concentrated during the first postnatal week and then decreases fourfold in level until the age of 35 days, and a minor one of 2 kb, with the same developmental profile as the 2.4-kb mRNA. A 3-kb mRNA is expressed by the alpha-tropomyosin gene and is characteristic of the mature rat. The expression of the tropomyosin genes during the development of the rat cerebellum does not seem to be regulated through alternative splicing but rather implies the differential expression of two different isogenes. The multiple isoforms of tropomyosin produced during neuronal differentiation may be intimately involved in the regulation of the organization and function of actin microfilaments.

Structure and complete nucleotide sequence of the gene encoding rat fibroblast tropomyosin 4

We have isolated and determined the complete nucleotide sequence of the gene that encodes the 248 amino acid residue fibroblast tropomyosin, TM-4. The TM-4 sequence is encoded by eight exons, which span approximately 16,000 bases. The position of the intron-exon splice junctions relative to the final transcript are identical to those present in other vertebrate tropomyosin genes and the Drosophila melanogaster TMII gene. We have found no evidence that the rat TM-4 gene is alternatively spliced, unlike all the other tropomyosin genes from multicellular organisms that have been described. Typical vertebrate tropomyosin genes contain some, or all, of alternatively spliced exons 1a and 1b, 2a and 2b, 6a and 6b, and 9a, 9b, 9c and 9d in addition to common exons 3, 4, 5, 7 and 8. The rat fibroblast TM-4 mRNA is encoded by sequences most similar to exons 1b, 3, 4, 5, 6b, 7, 8 and 9d. Two exon-like sequences that are highly similar to alternatively spliced exons 2b and 9a of the rat beta-tropomyosin gene and the human TMnm gene have been located in the appropriate region of the gene encoding rat fibroblast TM-4. However, several mutations in these sequences render them non-functional as tropomyosin coding exons. We have termed these exon-like sequences, vestigial exons. The evolutionary relationship of the rat TM-4 gene relative to other vertebrate tropomyosin genes is discussed.

Differential control of tropomyosin mRNA levels during myogenesis suggests the existence of an isoform competition-autoregulatory compensation control mechanism

We have isolated tropomyosin cDNAs from human skeletal muscle and nonmuscle cDNA libraries and constructed gene-specific DNA probes for each of the four functional tropomyosin genes. These DNA probes were used to define the regulation of the corresponding mRNAs during the process of myogenesis. Tropomyosin regulation was compared with that of beta- and gamma-actin. No two striated muscle-specific tropomyosin mRNAs are coordinately accumulated during myogenesis nor in adult striated muscles. Similarly, no two nonmuscle tropomyosins are coordinately repressed during myogenesis. However, mRNAs encoding the 248 amino acid nonmuscle tropomyosins and beta- and gamma-actin are more persistent in adult skeletal muscle than those encoding the 284 amino acid nonmuscle tropomyosins. In particular, the nonmuscle tropomyosin Tm4 is expressed at similar levels in adult rat nonmuscle and striated muscle tissues. We conclude that each tropomyosin mRNA has its own unique determinants of accumulation and that the 248 amino acid nonmuscle tropomyosins may have a role in the architecture of the adult myofiber. The variable regulation of nonmuscle isoforms during myogenesis suggests that the different isoforms compete for inclusion into cellular structures and that compensating autoregulation of mRNA levels bring gene expression into alignment with the competitiveness of each individual gene product. Such an isoform competition-autoregulatory compensation mechanism would readily explain the unique regulation of each gene.