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Biophys J. Feb 2004; 86(2): 991–1001.
PMCID: PMC1303946

Disruption of Caenorhabditis elegans Muscle Structure and Function Caused by Mutation of Troponin I

Abstract

Caenorhabditis elegans strains mutant for the unc-27 gene show abnormal locomotion and muscle structure. Experiments revealed that unc-27 is one of four C. elegans troponin I genes and that three mutant alleles truncate the protein: recessive and presumed null allele e155 terminates after nine codons; semidominant su142sd eliminates the inhibitory and C-terminal regions; and semidominant su195sd abbreviates the extreme C-terminus. Assays of in vivo muscular performance at high and low loads indicated that su142sd is most deleterious, with e155 least and su195sd intermediate. Microscopy revealed in mutant muscle a prevalent disorder of dense body positioning and a less well defined sarcomeric structure, with small islands of thin filaments interspersed within the overlap region of A bands and even within the H zone. The mutants' rigid paralysis and sarcomeric disarray are consistent with unregulated contraction of the sarcomeres, in which small portions of each myofibril shorten irregularly and independently of one another, thereby distorting the disposition of filaments. The exacerbated deficits of su142sd worms are compatible with involvement in vivo of the N-terminal portion of troponin I in enhancing force production, and the severe impairment associated with su195sd highlights importance of the extreme C-terminus in the protein's inhibitory function.

INTRODUCTION

Nearly a century ago, A. V. Hill (1927) drew attention to a problem that remains incompletely solved, namely how muscular performance is varied from one striated muscle to the next despite considerable similarity in structure. One solution likely lies in the rich diversity of muscle protein isoforms. In the case of troponin, which confers calcium sensitivity on the interaction of actin with myosin and hence on tension development, it has been difficult to understand how sequence differences among the isoforms of each of the three subunits might alter calcium regulation and muscular function, given a lack of information concerning the in vivo role of domains of each subunit. Shedding light on these roles, experiments reported here examine with Caenorhabditis elegans the functional and structural impairment caused by three truncation mutations of troponin I (TnI), the subunit responsible for inhibiting actin-myosin interaction.

Recent structural analysis of crystalline human cardiac troponin (Takeda et al., 2003) complements extensive biochemical work (comprehensively reviewed by Perry, 1999), and both provide the framework of current understanding of the protein switch. The core of crystalline troponin, bound with calcium, appears to comprise an arm flexibly linked to a regulatory head. The so-called IT-arm is formed from the first two of four α-helices of TnI, the C-terminal half of troponin C (TnC), and a portion of the C-terminal half of troponin T (TnT). The head is built from the third α-helix of TnI and the N-terminal half of TnC. Toggling of the troponin switch from on, as represented by the crystalline structure, to off is envisaged to involve dissociation of TnI's third helix from the N-lobe of TnC and the subsequent association of the C-terminal half of TnI with actin-tropomyosin (Takeda et al., 2003).

Experiments with model genetic organisms expressing troponin mutations allow testing of the proposed function of troponin within intact muscle in vivo, and the extraordinary conservation of muscle composition, organization, and function suggests that such studies can yield information broadly applicable. The nematode C. elegans moves through the use of four strips, or quadrants, of obliquely striated muscle (termed body-wall muscle) running the length of the worm and operating in conjunction with a hydrostatic skeleton (C. elegans muscle development, organization, and function are comprehensively reviewed by Moerman and Fire, 1997). Body-wall muscle contracts at 0.1–2 Hz during the worm's locomotion, and tension development is regulated by troponin, although some evidence (Harris et al., 1977) exists for myosin-linked regulation or modulation, as well. Among existing C. elegans mutants, the unc-27 class drew our attention, because its phenotype (rigidity, sluggishness, and sarcomeric disorganization; Brenner, 1974; Zengel and Epstein, 1980) suggested defects in the troponin switch. Moreover, the position of unc-27 on the mutation-based genetic map correlates well with the position of DNA potentially encoding TnI on the clone-based physical map. Allowing a rare opportunity to examine the role of particular TnI domains in vivo, the experiments reported here identify unc-27 as a TnI gene and examine the functional and structural impairment caused to body-wall muscle by the three available mutant alleles.

METHODS

Nomenclature and maintenance of strains

In addition to the N2 wild-type strain of C. elegans variant Bristol, the following alleles of unc-27, on chromosome X, were used: su142sd, e155, and su195sd. The naming is descriptive. The mutant alleles define the 27th family of mutant worms having uncoordinated movement as the originally described and most easily detected phenotype. The letter prefix in the allele name refers to the laboratory of isolation, and the letter suffix, when present, indicates characteristics of the mutation, e.g., semidominance in the case of su142sd and su195sd. Strains were handled according to published methods (Brenner, 1974).

Transformation rescue

Cosmid ZK721, which spans a TnI gene predicted to lie in the vicinity of unc-27 on the genetic map, was injected into a strain carrying the temperature-sensitive genetic marker pha-1(e2123). The injected DNA included a cloned copy of the wild-type pha-1 gene to allow selection of transformants. For both of the stably transformed lines recovered, PCR with primers matching sequences unique to ZK721 was used to confirm the presence of ZK721 in the transformed lines. Standard genetic crossing enabled introduction of the transformed copy of the TnI gene into a strain carrying unc-27(e155). The chromosomal genotype of the resulting strains was pha-1(e2123)III; unc-27(e155)X. In addition, the resulting strains carried one of the two extrachromosomal elements containing transformed copies of the pha-1 gene and of ZK721. As assayed by motility and muscle organization, the transformed copies of ZK721 rescued the unc-27 mutant phenotype. Further, as expected, the unc-27 mutant phenotype reappeared when the extrachromosomal element was allowed to segregate away.

Analysis of TnI sequences

Four genomic regions potentially encoding TnI were identified by a search with BLAST (Altschul et al., 1990), using default settings, of GenBank (url: www.ncbi.nlm.nih.gov) for sequences similar to rabbit fast twitch TnI. The activity of the four loci as bona fide genes was confirmed by amplification of wild-type cDNA for each, and the exon-intron boundaries of the genes followed from comparison of the genomic sequences deposited in Genbank with cDNA sequences. The cDNA was prepared from total RNA isolated from ~0.1 g N2 worms with Trizol per its accompanying protocol (Invitrogen, Carlsbad, CA), and sequencing was done with the Silver Sequencing Kit (Promega, Madison, WI). Sites of mutation were identified through sequencing of genomic DNA isolated from single mutant worms (Williams et al., 1992) and amplified under conditions promoting high fidelity (either with ~25 μM of each nucleotide or with the proofreading polymerase pfu (Stratagene, La Jolla, CA)) using primers specific to locus ZK721.2. Mutations were verified by sequencing multiple independent genomic DNA clones, as well as ZK721.2 cDNA that had been prepared from mutant strains.

Accession numbers of TnI sequences used to determine the evolutionary relationships among TnI isoforms follow: C. elegans F42E11.4 (Z66562), C. elegans unc-27/ZK721.2 (U40951), C. elegans T20B3.2 (Z81593), C. elegans W03F8.1 (AF039041), Chlamys nipponensis (Japanese scallop) (JE0233), Drosophila melanogaster heldup (P36188), Astacus astacus (broad-fingered crayfish) (A31484), Homo sapiens cardiac (P19429), H. sapiens slow (P19237), H. sapiens fast (AJ245761), Oryctolagus cuniculus (rabbit) cardiac (P02646), O. cuniculus slow (P02645), O. cuniculus fast (P02643), Coturnix coturnix (quail) cardiac (A41030), C. coturnix slow (U37118), C. coturnix fast (A23569), and Salmo salar (salmon) fast (U83878). Sequences were aligned using ClustalX at default settings (Thompson et al., 1997), followed by minor adjustment by eye to ensure proper alignment of characteristic TnI protein motifs. Phylogenetic analysis to identify evolutionary relationships was performed in PAUP v4.0b8 (Swofford, 2000) using the branch and bound search analysis followed by a full heuristic bootstrap analysis using 1000 replicates. The Gibbs and Motif algorithms (url: www.blocks.fhcrc.edu) were used to define conserved blocks within aligned sequences (Henikoff et al., 1995).

Localization of mRNA

TnI RNA was localized in larvae and adults with digoxigenin-labeled single-stranded DNA probes complementary to 3′UTR by the procedure of Kolmerer et al. (2000). The signal generated in this procedure is a stable, dark precipitate at sites with probes, and the anatomical location of this signal is readily identified via differential interference microscopy. Probes were reacted with embryos in the manner of Greenstein et al. (1994) and detected by the procedure of Kolmerer et al. (2000).

RNA interference

T7 RNA polymerase promoter sites were incorporated at the 5′ end of primers specific to the TnI isoforms, and PCR was used to generate sufficient double-stranded DNA template, purified on agarose gels, for transcription with the AmpliScribe T7 Kit (Epicentre, Madison, WI). The transcription product was denatured and reannealed to ensure a high percentage of double-stranded RNA by heating for 10 min at 75°C and cooling to 37°C over 30 min. Elimination of F42E11.4 transcripts from embryos, by RNA interference (RNAi) with fourth-stage larvae (L4) and recovery of progeny, caused significant embryonic lethality (of 266 progeny produced within 24 h by 6 treated wild-type hermaphrodites, 160 arrested as early embryos). Thus, RNAi was performed by soaking first-stage larvae (L1) for 24 h in solution containing 1–2 μg/μl RNA in the manner of Maeda et al. (2001), and worms were examined ~72 h later, by which time they had attained adulthood.

Electron microscopy

Adult worms were fixed in 3% glutaraldehyde in 0.1 M Na cacodylate buffer, pH 7.2–7.4, on ice for 3 h, cut transversely in half in glutaraldehyde, and left in the fixative at room temperature overnight. Next day, the worms were washed in buffer, postfixed in 2% OsO4 in cacodylate buffer for 1 h, washed in water and en-block stained in saturated aqueous uranyl acetate for 1 h, all at room temperature. All material was dehydrated in graded concentrations of ethyl alcohol (EtOH; 15 min each in 70% and 90%) followed by three changes each of absolute EtOH and acetone; material was then embedded in Epon. Ultrathin sections were stained with 3% uranyl acetate in 50% EtOH and with lead salts (Sato, 1968), and they were photographed in a Philips 410 EM.

Immunofluorescence and polarized light microscopy

Worms were labeled with mouse antivinculin (MH24, a kind gift of M. Coutu Hresko, Washington University, St. Louis, MO) at a dilution of 1:200 by the procedure described by Finney and Ruvkin (1990), as modified by Miller and Shakes (1995). The fixative contained 2% formaldehyde and 25% v/v methanol. The secondary antibody was goat antimouse conjugated with fluorescein (AP181F, Chemicon International, Temecula, CA). Polarized light microscopy with live worms was achieved by inserting between the ocular and plan apochromat objective a sheet polarizer crossed in orientation with a polarizer between the condenser and light source of an Eclipse TE300 (Nikon,Tokyo, Japan).

Tests of locomotion and measurements of sizes

For dimensional analysis, worms were synchronized in growth and photographed at defined ages via a camera attached to a stereomicroscope (MZFlIII, Leica, Bannockburn, IL). Negatives were digitized, magnified, and printed, allowing measurement of the length and diameter. For swimming assays, worms were transferred from agar plates to liquid (unless specified otherwise in text, solution was 0.1 M NaCl, 50 mM potassium-phosphate, pH 6) and were given 30 s to recover from the transfer before commencement of the measurement, which involved counting the contractile waves propagating along the length of worms during 60 s. The creeping behavior of worms was assessed in the manner of Epstein et al. (1976), who used a chemotaxis assay to resolve differences in crawling capability among strains. C. elegans move toward and feed on bacteria. In the assay, a drop of 10–100 age-synchronized adults in a minimal volume of liquid (<2 μl) was placed at the center of a ring of bacteria, and the chemotaxis of worms from the center toward the ring was monitored. Statistical significance was evaluated with Student's t-test and the Wilcoxon-Mann-Whitney test for comparison of two populations; for multiple comparisons, ANOVA was used and followed when appropriate with a post hoc test (Tukey's honestly significant difference test and Games-Howell test, both within SPSS version 11.5).

RESULTS

Linkage of unc-27 with TnI and identification of multiple TnI isoforms

Two lines of evidence showed that the C. elegans unc-27 gene encodes TnI. First, a transformed wild-type copy of the TnI gene on cosmid ZK721 completely rescued unc-27 worms from the mutant phenotype (see Materials and Methods). Second, mutations in this TnI gene were found for the three unc-27 mutant alleles: Gln10stop for e155, Gln122stop for su142sd, and Glu207stop for su195sd. Fig. 1 aligns the inferred amino acid sequence of the TnI encoded by ZK721 with representative vertebrate TnIs, as well as with three other C. elegans TnI isoforms uncovered via similarity searches of genetic data deposited in GenBank by the C. elegans mapping/sequencing consortium. These other isoforms are F42E11.4 (on the X chromosome), T20B3.2 (on chromosome V), and W03F8.1 (on chromosome IV).

FIGURE 1
Alignment of the inferred amino acid sequence of the TnI encoded by ZK721.2/unc-27 with the three other C. elegans isoforms (F42E11.4, T20B3.2, and W03F8.1), as well as with human cardiac TnI (accession No. ...

Conservation of sequence among TnIs, although particularly striking over the region termed the inhibitory peptide (underlined in Fig. 1 and corresponding to residues Asn96-Met116 of rabbit fast TnI, accession No. P02643, with glycine of the second codon taken as residue 1), can be categorized into several blocks, detected in a search of seventeen invertebrate and vertebrate TnI sequences with the MOTIF and GIBBS algorithms. Referenced to the sequence of rabbit fast TnI, these blocks follow: MOTIF block 1, Lys5 to His47; MOTIF block 2, Tyr79 to Val114; MOTIF block 3, Lys129 to Glu151; Gibbs block 1, Leu63 to Val114; and Gibbs block 2, Asp135 to Glu151. The similarity of size of N-terminal extensions in vertebrate cardiac and worm TnIs hinted of a stronger evolutionary relationship between these two classes than between vertebrate skeletal and cardiac isoforms, but evolutionary analysis (Fig. 2) did not support this idea. The clustering of chordate troponins by muscle-specific isoform (e.g., cardiac) rather than by species suggests that diversification into multiple isoforms occurred in a common ancestor to these chordates. However, the C. elegans isoforms do not cluster with any vertebrate equivalent, indicating that the diversification of invertebrate isoforms occurred independently.

FIGURE 2
A phylogenetic array comparing troponin I isoforms of vertebrates and invertebrates suggests that diversification of troponin I isoforms occurred independently in the two lineages. Bootstrap probabilities are noted at each node.

In wild-type worms, ZK721.2/unc-27 is transcriptionally active in the body-wall musculature, with weak expression in late-stage embryos (Fig. 3 A) and strong expression during the larval and adult stages (Fig. 4, A and B), when it appears to be the major body-wall isoform. Transcripts were also detected in the diagonal sex muscles of the male (Fig. 4 B). F42E11.4 is the major body-wall isoform during embryogenesis (Fig. 3 B), but its body-wall expression wanes during the larval stages. In late-stage larvae and adults, expression of F42E11.4 is evident in the proximal gonad of both hermaphrodites (Fig. 4, C and D) and males (Fig. 4 E). T20B3.2 (Figs. 3 C and 4, F and G) transcripts were detected in the body-wall musculature, but expression in hermaphrodites appeared limited to the anteriormost body-wall muscle cells. Expression in the body-wall muscle of males was less restricted, typically including posterior body-wall muscle cells, in addition to those at the head (Fig. 4 G). T20B3.2 transcripts were found also in the vulval muscles of hermaphrodites and the sex muscles of males (Fig. 4, F and G). From embryos to adults, expression of W03F8.1 was detected only in pharyngeal muscle cells (Figs. 3 D and 4 H).

FIGURE 3
Localization of TnI mRNA in embryos. (A) ZK721.2/unc-27 transcripts in body-wall muscle of threefold embryo. (B) F42E11.4 in body-wall muscle of 1.25-fold embryo. (C) T20B3.2 in anteriormost body-wall muscle cells of threefold embryo. (D) W03F8.1 in pharyngeal ...
FIGURE 4
Localization of TnI mRNA in wild-type larvae and adults (AH), TnI mRNA in unc-27 mutants (I and J), and TnT and TnC mRNA in mutants (KM). Transcripts of ZK721.2/unc-27 were detected in the body-wall musculature of larvae and adults, ...

When compared, the mutants evidenced similar expression patterns that differed in two respects from the wild-type patterns: in each of the mutant strains, transcripts of F42E11.4 (Fig. 4 I) and T20B3.2 (Fig. 4 J) were detected in body-wall muscle of a fraction of adult hermaphrodites (mutant males were not examined; in mutant larvae, expression of T20B3.2 matched that of wild-types). This fraction, for animals showing staining, was ~0.3 for F42E11.4; it was ~0.1 for T20B3.2 with su142sd and e155 worms, but ~0.5 with su195sd worms. Expression of body-wall isoforms of TnT (Fig. 4, K and L) and TnC (Fig. 4 M) in the mutants resembled the wild-type patterns (Allen et al., 1997; Terami et al., 1999), with the exception that transcripts of the major embryonic body-wall TnT (mup-2/tnt-1/T22E5.5) remained detectable in body-wall muscle of some mutant adults.

Extent of paralysis of unc-27 mutants

The locomotion of mutants was clearly compromised, indicative of deficits in the activation and relaxation of muscle. In the swimming assay, a comparatively low-force/high-velocity test, impairment was apparent as early as the first larval stage and grew worse as the worms developed (Table 1). Additionally, the severity of the mutations (“allelic strength”) differed: adults homozygous for e155 operated their body-wall muscle in this assay at one-third of the wild-type frequency, whereas mutants homozygous for su142sd or for su195sd operated at about one-ninth. The somewhat compromised swimming behavior of su142sd and su195sd heterozygotes, with one wild-type and one mutant copy of unc-27, confirms the designation of these mutations as semidominant. Moreover, given the absence of haploinsufficiency (i.e., given that heterozygotes with one wild-type allele and one null allele, e155, are indistinguishable from wild-type homozygotes), the semidominance of these mutations offers strong evidence that they are manifested at the protein level. Performance of the mutants in the chemotaxis assay, a comparatively low-velocity/high-force test, also differed from that of wild-types. Fig. 5 displays the results, showing a rapid migration of wild-types to food (t1/2 = 3.29 ± 0.48 min, n =7 trials) and slower movement of mutants (t1/2 = 24.35 ± 4.56 min for e155, 25.07 ± 5.62 min for su195sd, and 59.72 ± 5.17 min for su142sd; n = 4 trials with each).

FIGURE 5
Chemotaxis assay with wild-type (solid squares), su142sd (solid diamonds), e155 (open circles), and su195sd (crosses) adults. The percentage of worms remaining within the annular ring of bacteria is plotted as a function of time, in minutes. Data are ...
TABLE 1
Frequency of contractile waves passing along body wall of swimming worms

Whether variations in compensation by wild-type TnI (possibly F42E11.4 and T20B3.2, as suggested by the data on expression patterns) for mutant ZK721.2/UNC-27 protein contributed to differences in muscular performance among mutants was addressed by examining worms in which expression of wild-type body-wall isoforms was blocked by RNA interference (RNAi; Fire et al., 1998). Combined interference of F42E11.4 and T20B3.2 did not prove useful. A fraction, varying from one trial to another, of doubly treated mutants died prior to adulthood, and those that attained adulthood frequently displayed detachment of the hypodermis from the cuticle, as well as a very sickly, scrawny appearance suggestive of starvation, likely arising from inability of the worms to move their heads to obtain food.

Following interference of F42E11.4, a high percentage of su142sd and e155 animals became completely immobile (e.g., 100% immobility in two of four trials; these worms still could move their heads to forage, presumably because of expression of T20B3.2 in the anteriormost body-wall muscle cells). Wild types and, to a lesser extent, su195sd worms subjected to RNAi of F42E11.4 remained mobile. Wave frequency was 1.48 ± 0.27 Hz (n = 12) for wild-type (compare with 1.56 ± 0.19 Hz, n = 12, for “negative control” wild-types soaked in buffer lacking dsRNA; P > 0.40 by nondirectional t-test) and 0.28 ± 0.14 Hz (n = 10) for the mutant. The degree of rigid paralysis within the mutants was consequently gauged from dimensions of the worms, because hypercontracted worms have previously been shown to take on a “dumpy” appearance, due to diminution of both the body length and the length/width ratio (Reiner et al., 1995). Results following interference of F42E11.4 are given in Table 2, with column two (labeled immobile subset) giving mean length of worms from the two trials in which complete immobility was displayed by su142sd and e155 worms, and with column three summarizing results pooled from all four trials. In both the subset and the pooled data, treated su142sd worms were shorter than the e155 ones (for the subset data P = 0.011 with Tukey's honestly significant difference test and 0.004 with the Games-Howell test; for the pooled data, P [double less-than sign] 0.001 with both tests). Although exhibiting some ability to move, the su195sd worms also were short, with a length indistinguishable from that of su142sd worms (P > 0.15 in both cases).

TABLE 2
Adult body length of worms subjected to RNAi of F42E11.4 TnI

The relative extents of compensation by F42E11.4 and T20B3.2 for mutant ZK721.2 was addressed with e155 worms, because working with these avoided complications of semidominance. Length of adults following RNAi of F42E11.4 was 0.62 ± 0.15 mm (mean ± SD, n = 47 worms), but for those subjected to RNAi of T20B3.2 length was 0.77 ± 0.10 mm (n = 18) (comparison of the two means gives P < 0.001). Note that e155 worms subjected to a control RNAi procedure, having an incubation in 0 μg/μl RNA, had a mean adult length of 0.88 ± 0.11 mm (n = 23). Thus, the impact on mean adult length following depletion of T23B3.2 is about one-third of that following depletion of F42E11.4.

Structure of unc-27 mutant muscle

Structural analyses were undertaken to gain insight on the mechanism by which the mutations compromise muscular activity. Both polarized light microscopy with living worms and antivinculin labeling of fixed worms revealed a distortion of sarcomeric organization in the body-wall muscle of the mutants (Fig. 6). The wild-type pattern of obliquely arranged, alternating isotropic and anisotropic bands that is seen in wild-type worms (Fig. 6 B) was absent in mutants (exemplified with su142sd, shown in Fig. 6 D). Dense bodies, marking the ends of the sarcomeres and equivalent to Z-lines, were evenly spaced along parallel rows in wild-type worms, as indicated by the position of vinculin foci (Fig. 6 A), but their disposition was greatly distorted in the mutants (Fig. 6 C).

FIGURE 6
Structure of wild-type and unc-27 mutant body-wall muscle revealed by reaction of adults with antivinculin antibody (A and C) and by visualization of living worms between crossed polarizers (B and D). The wild-type muscle (A and B) shows obliquely arranged, ...

Electron microscopy of body-wall muscle confirmed a prevalent disorder of dense body positioning and a less well-defined sarcomeric structure. In longitudinal sections (Fig. 7), it is clear that the oblique striation is disarrayed, so that the positioning of the dense bodies (equivalent to Z-lines) is not as regular. The reason for this appearance was identified from transverse sections. In wild-type worms (Fig. 8, A and B), well-oriented cross sections showed sharp profiles of thin and thick filaments and a regular alternation of the sarcomere bands. Within half-sarcomeres, dense bodies were followed by I bands, composed of thin filaments, then by overlap zones of A bands, with regularly packed thick filaments surrounded by rings of thin filaments, and finally by the H zone of A bands, containing only thick filaments. The boundaries between the various bands were fairly abrupt and regular. With the mutants, the same bands were seen, but their positioning was irregular. The defects appeared strongest in the su142sd mutants (Fig. 8, CF), but they were also clearly evident in the other two (Fig. 9). Small islands of thin filaments were interspersed within the overlap region of A bands and even within the H zone. In the overlap region, the content of thin filaments was quite variable, and the H zone was fragmented into several small areas of the cross section. The Z-lines had irregular boundaries. The images do not suggest any basic defect in the formation of filaments or in their assembly into the basic components of a sarcomere. The structural disruption evidenced by the mutants' muscle could arise from unregulated contraction of the sarcomere, in which small portions of each myofibril shorten irregularly and independently from one another, thereby distorting the filament disposition (Fig. 7 D).

FIGURE 7
Longitudinal sections through wild-type (A) and su142sd (B and C) muscle fibers. In A, the vinculin containing dense bodies are located at periodic intervals and are well aligned. Note short, well delineated vinculin-containing dense bodies (equivalent ...
FIGURE 8
Comparison of cross sections through muscle fibers of wild-type (A and B) and su142sd mutant (C–F) worms. In the wild-type (A and B), orderly arrangement of thin and thick filaments within “obliquely striated” sarcomeres is indicated ...
FIGURE 9
Cross sections through muscle fibers of the su195sd mutant (A and B) and the e155 mutant (C and D). The sharp, round profiles of thin and thick filaments indicate that the sections are well oriented in a direction perpendicular to the long axis of the ...

DISCUSSION

When expressed in vivo, all three truncation mutants of TnI lead to disarray of the sarcomere and rigid paralysis. Three observations suggest that this phenotype is a primary consequence of the mutations, as opposed to a secondary complication resulting, for example, from alterations of relative protein levels. First, the semidominance of su142sd and su195sd, given the absence of haploinsufficiency, indicates that mutant TnI protein is indeed synthesized and able to compete with wild-type TnI. Second, although associated with the mutations, changes in the expression pattern of troponin subunits appeared limited, affecting a minority of examined worms and involving persistence of embryonic-larval isoforms in adult muscle, as well as in the case of TnI T20B3.2 an increase in number of cells showing expression. Third, the structure of mutant muscle is entirely consistent with disarrangement of the sarcomeres, rather than with defects typically associated with altered protein levels such as improperly formed filaments and failed assembly of basic components of the sarcomere. Thus, in the subsequent discussion, the phenotype is interpreted in light of both the structure of crystalline human cardiac troponin (Takeda et al., 2003) and the substantial body of biochemical work with TnI in vitro (comprehensively reviewed by Perry, 1999).

Paralysis caused by e155

Adult e155 animals showed a surprising degree of swimming and crawling capability, contrary to predictions based on previous work with vertebrate troponins. Biochemical studies show that in the absence of TnI vertebrate thin filaments are disinhibited (reviewed by Perry, 1999). Extending this finding, extraction of TnI from permeabilized ventricular muscle leads to calcium-independent tension (Strauss et al., 1992), and myocardium from mice in which the cardiac TnI gene had been deleted exhibited abnormally shortened sarcomeres and elevated resting tension (Huang et al., 1999). The e155 mutation eliminates all but 9 of the 242 residues of ZK721.2, including all of the hallmarks of TnI, and therefore it is unlikely that the truncated protein could fulfill any roles of wild-type TnI. One possible explanation for the residual locomotion of e155 animals is redundancy within the activation process due to potential presence of thick- and thin-filament regulation, whereas another is overlapping expression of TnI isoforms.

Dual-regulation is prevalent throughout the animal kingdom (Lehman and Szent-Gyorgyi, 1975), and early evidence suggested existence in C. elegans of thick-filament-based regulation or modulation (Harris et al., 1977). Little is known about the specific roles of thick- and thin-filament regulatory proteins in dually regulated muscle, but available data, from studies with Limulus muscle, suggest that both regulatory systems need to be switched on before activation of actomyosin MgATPase and tension development can occur (Wang et al., 1993). If dually regulated in the same manner as Limulus muscle, C. elegans body-wall muscle depleted of TnI should be capable of contracting and relaxing: thin filaments would be persistently disinhibited, and the thick-filament-based system would toggle on and off. The complete immobility of e155 worms depleted of the embryonic-larval TnI (F42E11.4) conflicts with this prediction and suggests that body-wall muscle is regulated solely by troponin, a possibility which does not exclude thick-filament-based modulation by a protein such as twitchin/UNC-22 (Benian et al., 1989). Additionally, the loss of mobility, as well as diminution of body length, of e155 worms subjected to RNAi offers clear evidence that the embryonic-larval isoform can compensate for loss of the larval-adult isoform (ZK721.2/unc-27) and account for regulated contraction in body-wall muscle of e155 animals.

Paralysis caused by su142sd

An interesting question arising from the data is why su142sd, which eliminates the inhibitory region of TnI, is more deleterious than e155, a presumed null. The disparity likely reflects differing extents of compensation for the two alleles by other TnI isoforms, as well as true difference in effects of the mutant alleles. At a qualitative level, both mutations are associated with comparable alterations of the expression pattern of troponin isoforms, e.g., persistence of embryonic-larval TnI (F42E11.4) and TnT (mup-2/tnt-1/T22E5.5) transcripts in body-wall muscle in a minority of adults. Yet, the semidominance of su142sd, in the absence of haploinsufficiency, indicates that this mutant TnI can compete with wild-type TnI and hence that compensation by wild-type TnI should be less in su142sd animals than in e155 ones.

Although fully supportive of differing extents of compensation for su142sd and e155, the data are also compatible with a true difference between effects of the mutations. When depleted of the embryonic-larval body-wall TnI (F42E11.4), the su142sd worms were shorter than the e155 ones, an observation compatible with a greater degree of hypercontraction in the former. It remains possible that RNAi did not remove all traces of compensation by F42E11.4; however, the complete immobility of both e155 and su142sd worms following RNAi of F42E11.4 suggests at the functional level absence of significant compensation. This immobility likewise argues against meaningful compensation afforded by T20B3.2, transcripts of which were detected in the middle and posterior body-wall muscle of a minority of adult mutants.

Thus, the fragment encoded by su142sd, terminating at a position equivalent to Lys87 of rabbit fast TnI and Lys120 of human cardiac TnI, both competes with wild-type TnI and likely wreaks more damage than does the absence of TnI. The fragment's ability to compete in vivo agrees well with the structure of crystalline human cardiac troponin, in which TnI residues 42–136 associate with TnT residues 203–271 and TnC residues 93–161 to form the IT-arm (Takeda et al., 2003). Biochemical work substantiates an interaction in vitro between vertebrate TnT and the N-terminal half of TnI (Farah et al., 1994; Van Eyk et al., 1997).

The greater severity of the su142sd mutation in comparison with the null following RNAi may reflect participation of the N-terminal half of TnI in activation, i.e., the enhancement of actomyosin MgATPase in the presence of activating calcium to an activity exceeding that observed with actin-tropomyosin-myosin in vitro (Malnic et al., 1998). In this sense, a parallel may be drawn between the severity of su142sd and the muscle degeneration caused by the Drosophila TnI heldup2 mutation, which replaces with valine an invariant alanine falling within the IT-arm and aligning with rabbit fast residue 25 and human cardiac residue 57 (Beall and Fyrberg, 1991). The degeneration can be ameliorated by elimination of myosin filaments, as well as by other myosin mutations, suggesting that heldup2 promotes excessive actin-myosin interaction and force production (Beall and Fyrberg, 1991; Kronert et al., 1999).

Paralysis caused by su195sd

The most striking aspect of sarcomeric disarray and paralysis caused by su195sd is the extent to which the phenotype resembles that of the presumed null, e155, even though the su195sd mutation leaves intact all but the extreme C-terminus. Loss of TnI function caused by su195sd could suggest that the mutant protein simply fails to associate with the thin filament, but both the semidominance of the mutation and the mobility of su195sd worms subjected to RNAi of F42E11.4 argue against this view. Another possibility is that the extreme C-terminus is needed for the full inhibitory potential of TnI to be manifested. This view is supported by work with vertebrate TnI, even though the extreme C-termini of vertebrate and invertebrate isoforms differ in length and composition. For example, removal of the C-terminal 17 residues from cardiac TnI leads to diminished calcium sensitivity of force, and removal of 23 residues leads to increased sensitivity, as well as impaired ability of the truncated TnI to inhibit (Rarick et al., 1997). Elimination of as few as 18 residues from the C-terminus of chicken fast skeletal TnI compromises inhibition of actomyosin MgATPase (Ramos, 1999).

Biological relevance of the extreme C-terminus of TnI is reflected in several observations. Four missense mutations and an eight-residue deletion near the C-terminus of human cardiac TnI have been linked to cardiomyopathy (Kimura et al., 1997; Morner et al., 2000; Morgensen et al., 2003). Cleavage of the C-terminal 17 residues from cardiac TnI ensues ischemia and reperfusion injury (McDonough et al., 1999), and the steady-state force-calcium relation obtained with myocardium regulated by the truncated TnI exhibits both diminished peak force and depressed calcium sensitivity (Murphy et al., 2000). The ability of the C-terminus to modulate TnI's inhibitory strength potentially is tapped in nature. Encoding the C-terminus of Drosophila TnI are two exons (exons 9 and 10) that are spliced in a mutually exclusive manner (Barbas et al., 1991). The sequence encoded by exon 9 is detected only in adult isoforms and differs in two obvious respects from that of exon 10. The terminus generated by exon 9 is shorter by nine residues, and it lacks a motif (P/G–D/E–W–R/S–K) found in almost all other TnI isoforms of invertebrates and vertebrates, with notable exceptions being C. elegans isoform W03F8.1, expressed exclusively in the pharynx, and TnI of crayfish and scallop.

In summary, the rigid paralysis and sarcomeric disarray of the three mutants are consistent with unregulated contraction of the mutant sarcomeres, in which small portions of each myofibril shorten irregularly and independently from one another, thereby distorting the filament disposition. Semidominance of two of the alleles, in the absence of haploinsufficiency, highlights that mutant protein is synthesized and competes with wild-type isoforms. The exacerbated deficits exhibited by su142sd (Gln122stop) mutants strengthen the view that in vivo the N-terminal portion of TnI enhances force production. The deleterious effects of the su195sd (Glu207stop) allele point strongly to a role of the extreme C-terminus in TnI's inhibitory function.

Acknowledgments

We thank Drs. S. Takeda and Y. Maéda for kindly providing a preprint of their article on crystalline troponin.

We gratefully acknowledge funding from National Institutes of Health grant 5-PO1-HL15835 to the Pennsylvania Muscle Institute (C.F.-A.), as well as from the Templeton Medical Research Foundation and the National Science Foundation (IBN 9985315) (T.A.). Nematode strains used in this work were provided by the Caenorhabditis Genetics Center, which is funded by the National Center for Research Resources of the National Institutes of Health.

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