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Address correspondence to J. Troy Littleton, Picower Center for Learning and Memory, MIT, 50 Ames St., Bldg. E18-672, Cambridge, MA 02139. Tel.: (617) 452-2605. Fax: (617) 452-2249. email: troy{at}mit.edu
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Abstract |
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Key Words: myosin; neuromuscular; dystrophy; cardiomyopathy; Drosophila
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Introduction |
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The genetic tractability of Drosophila has made it an ideal system to characterize mutations affecting neuromuscular function. Many of these mutants have been identified by screening for temperature-sensitive (TS) behavioral defects, allowing the identification of gene products important for neuromuscular function (Ganetzky and Wu, 1983; Littleton et al., 1998). Additionally, the Drosophila flight system has provided an efficient genetic model for hypertrophic cardiomyopathies. Mutated genes leading to flightless behavior in Drosophila are also disrupted in many forms of primary hypertrophic cardiomyopathies, including Myosin heavy chain (Mhc), Tropomyosin2 (Tm2), wings up A (wupA [troponin I]), actin88F (act88F), and upheld (troponin T). Mutations in these genes also show extensive genetic interactions, providing key insights into the regulatory pathways underlying muscle function (Ferrus et al., 2000; Vigoreaux, 2001).
Flightless behavior in Drosophila can arise from mutations that lead to muscle hypercontraction. Hypercontraction of the Drosophila indirect flight muscles (IFM) occurs due to specific mutations of the contractile machinery that lead to either decreased structural integrity of the sarcomere, or dysregulation of the contractile process in vivo. These mutations result in a degeneration of the IFM of adult flies after muscle differentiation and development (Fekete and Szidonya, 1979; Deak et al., 1982; Homyk and Emerson, 1988; Beall and Fyrberg, 1991). Hypercontraction mutants can be genetically suppressed by specific mutant alleles of Mhc. The mechanism of suppression has been suggested to be a potential MhcTroponin I direct interaction (Kronert et al., 1999). However, additional data suggests that an overall decrease in actomyosin force is sufficient to explain suppression of hypercontraction by mutant Mhc (Nongthomba et al., 2003). The identification of Mhc alleles that directly cause hypercontraction and enhance the hypercontraction defects of other mutants may facilitate defining the role of myosin in the regulation of contraction.
To further understand the molecular and cellular processes underlying neuromuscular function, we performed a screen for Drosophila TS behavioral mutants. One complementation group isolated in our screens, Samba, disrupts the Mhc locus, leading to hypercontraction and muscle degeneration. Characterization of the Samba mutants has revealed potential molecular mechanisms that lead to muscle degeneration through hypercontraction via distinct mechanisms from hypercontraction mutants characterized previously. In addition, these mutants give insight into the role of Mhc in the regulation of the contractile process in addition to its role in ATP-dependent motor function.
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Results |
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Segregation analysis of Samba revealed an autosomal-dominant mutation on the second chromosome, refined to 252 cM by recombination mapping. Deficiency mapping by lethality narrowed the cytological interval between 36A8 and 36C4 on the left arm of chromosome 2. To help identify the Samba locus, we screened for revertants of TS seizure behavior in a -irradiation reversion screen in order to isolate potential loss of function mutations in the Samba locus. Three revertants were identified by loss of TS behavioral defects. These revertants were embryonic lethal with normal morphological development, but showed complete loss of muscle wave propagation in late stage embryos (unpublished results). Noncomplementation to Mhc1 by both the TS mutants and the three revertants identified the Samba mutations as new alleles of the Mhc locus (Mogami and Hotta, 1981). We designated the Samba1 and Samba2 alleles MhcS1 and MhcS2, respectively, and the revertants Mhcrv1, Mhcrv2, and Mhcrv3.
The Mhc locus is complex, encoding all muscle-specific isoforms through the use of extensive alternative splice patterns (Rozek and Davidson, 1983; Bernstein et al., 1983). The locus contains 19 coding exons, 5 of which are alternatively spliced, and one that is either included or excluded (Wassenberg et al., 1987; George et al., 1989; Collier et al., 1990; Hess and Bernstein, 1991; Zhang and Bernstein, 2001). An allele isolated previously, Mhc5 (G200D), causes similar hypercontraction defects to the Samba mutants. Mhc5 introduces a point mutation in exon 4 of the Mhc locus, disrupting the ATPase domain of Mhc (Homyk and Emerson, 1988). Due to the phenotypic similarities with Mhc5, exon 4 of these new alleles was sequenced. Both MhcS1 and MhcS2 were found to be point mutations (V235E and E187K, respectively) mapping to the ATP binding and hydrolysis site of the protein (Fig. 2, AE).
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To confirm that the Samba phenotype results from hypercontraction, we analyzed the structure of the IFM in MhcS1/+ flies. The ordered array of filaments in muscles leads to birefringent properties, allowing muscle visualization under polarized light microscopy. In hypercontraction mutants characterized previously, the IFM exhibit one of two defects. Some show loss of birefringence in the middle of the muscles due to breakage or degradation, with the bulk of the muscle fiber at either one or both of the attachment sites. Others show separation from the attachment sites, with birefringence found only in the middle of the fiber (Nongthomba et al., 2003). MhcS1/+ flies exhibited the former defect, showing birefringence at the attachment sites, with loss of birefringence in the middle of the IFM. This defect was partially suppressed in the background of Tm2D53, as the IFM of double mutants displayed less degradation despite the presence of indented thoraces compared with similarly aged Samba flies (Fig. 1, EG). These data confirm that the Samba mutants lead to hypercontraction defects in the muscle.
Samba mutant muscles do not alter synaptic function but move independently of neuronal input
Hypercontraction in Drosophila muscles induced by the Samba mutants leads to progressive degradation of fibers, similar to degeneration observed in muscular dystrophies. In some animal models of muscular dystrophy, loss of acetylcholine receptor clustering results in the functional denervation of diseased fibers (Rafael et al., 2000). Because MhcS1 and MhcS2 were isolated by TS behavioral defects and abnormal extracellular DLM activity, we hypothesized that these mutations may cause functional or structural changes at the neuromuscular junction (NMJ).
Bouton number at the NMJ is tightly regulated and is sensitive to disruptions in both presynaptic and postsynaptic function. Though poorly understood, postsynaptic defects can alter presynaptic structural and functional properties through homeostatic regulatory pathways (Petersen et al., 1997; Davis et al., 1998). To analyze the morphology of the NMJ in Samba mutants, we stained third instar larvae with -synaptotagmin I antisera, a marker for presynaptic terminals, and analyzed muscle fibers 6 and 7 (Canton-S n = 18 larvae, 97 muscles; MhcS1/+ n = 27, 157). Type I innervation from glutamatergic motor neurons was not altered in MhcS1/+ animals, suggesting little effect of dysfunctional muscles on excitatory innervation (Fig. 4 J). The number of muscles showing ectopic innervation, however, was found to be more frequent than wild type, increasing from 4.1% in control animals to 15.9% in MhcS1/+ animals (Fig. 4, AF and K). These were determined to be type II synapses due to their morphology and the absence of postsynaptic DLG staining (Fig. 4, GI). Type II synapses are neuromodulatory, influencing the state of excitation in body wall muscles through release of octopamine (Gramates and Budnik, 1999). Increases in type II innervation have also been reported in mutants such as tipE and nap, which reduce nerve excitability (Jarecki and Keshishian, 1995). The increase in type II innervation suggests that alterations in muscle function can lead to altered neuromodulation. Similar data was obtained with MhcS2/+ larvae (unpublished data).
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Discussion |
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Hypercontraction and Mhc
Genetic perturbations can lead to hypercontraction in the Drosophila IFM by increasing actomyosin force through either a decrease in structural integrity of the sarcomere or an alteration in thin filament regulation of the cross-bridge cycle (Kronert et al., 1995; Reedy et al., 2000). Hypercontraction by the latter mechanism can be suppressed by mutations in Mhc, though it remains unclear whether suppression is obtained through an overall decrease in actomyosin force or by a direct role of Mhc on regulating thin filament dynamics (Kronert et al., 1999; Nongthomba et al., 2003). Here, we characterized two new alleles of Mhc. These alleles enhance the defects of up101 and wupAhdp2 and are partially suppressed by Tm2D53. These genetic interactions suggest that although MhcS1and MhcS2 lead to hypercontraction defects that are similar to up101 and wupAhdp2, they do so through a different molecular mechanism, as up101 and wupAhdp2 are fully suppressed by Tm2D53. Using a simplified five state model of contraction based upon the allosteric/cooperative model described previously, we can hypothesize the molecular mechanisms behind hypercontraction (Fig. 6; Lehrer and Geeves, 1998).
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Hypercontraction in Mhc mutants can be caused by two different mechanisms. One mechanism leads to hypercontraction by decreasing structural integrity of the sarcomere (Mhc6, Mhc13). The other mechanism involves the mutations MhcS1, MhcS2, and Mhc5. Hypercontraction by MhcS1, MhcS2, and Mhc5 may be caused by stabilizing actinmyosin interactions during the state Estate A transition, probably by preventing proper ADPATP exchange. This is consistent with the localization of the amino acid substitutions in the ATPase domain of the protein. Further stabilization during this transition could lead to contraction oscillations long after the nerve-stimulated Ca2+ transient, increasing actomyosin force during relaxation periods. The spontaneous contraction oscillations observed in third instar larvae support this model. However, mutants such as up101, wupAhdp2, and Tm2D53 have defects which affect the state Astate B equilibrium. Therefore, in double mutants of wupAhdp2 or up101 and MhcS1, MhcS2, or Mhc5, lethality may reflect the additive effects of two distinct molecular mechanisms of hypercontraction. The differential suppressive effects of Tm2D53 on these hypercontraction mutants support distinct molecular mechanisms as well. Alternatively, hypercontraction by MhcS1, MhcS2, and Mhc5 could occur through direct MhcTroponin I interactions, as proposed previously (Kronert et al., 1999). However, this seems unlikely as these mutants are single amino acid substitutions which map to the ATPase domain as opposed to surfaces more accessible to proteinprotein interactions.
Excitability and hypercontraction mutants
The TS seizure activity in Mhc5, MhcS1, and MhcS2 as well as up101 and wupAhdp2 is likely to reflect a temperature-dependent defect caused by an alteration in the cellular state of a hypercontractive muscle, rather than direct temperature-dependent defects of mutant proteins. The model proposed for hypercontraction may account for the activity through a dysregulation of calcium homeostasis. In normal muscles, calcium levels dramatically increase in the sarcomere in order to increase the fraction of troponin complexes in state B during regulated contraction. However, in mutants such as up101 and wupAhdp2, rather than calcium returning to intracellular stores, the calcium remains buffered in the sarcomere. This may be due to two possibilities. One possibility is that these mutations respond to lower calcium concentrations, where calcium ions are continually binding a mutant complex, transitioning to state B, released upon return to state A, and then repeating this binding cycle long after the large calcium transient has passed for regulated contraction. Although the [Ca2+]free remains relatively low, there is an overall buffering of a significant amounts of calcium in the sarcomere by the troponin complex. The other possibility, though not mutually exclusive, is that these mutations lead to a lower activation energy for the AB transition in the absence of calcium. State B, having a higher affinity for calcium, allows binding of calcium away from endogenous muscle calcium buffers. This can also lead to an overall aberrant buffering of calcium. Likewise, Mhc5, MhcS1, and MhcS2 lead to buffering because the sarcomere is continually cycling through states C
D
E
(C). After a single cycle of unregulated contraction, calcium may unbind the troponin complexes. However, a significant number of myosins remain bound in MhcS1, MhcS2, Mhc5, stimulating a second cycle through cooperative mechanisms, allowing state B troponin complexes to bind calcium. Buffered calcium in both mutant groups is thus continually binding and unbinding troponin complexes. During an increase in temperature, diffusion rates increase, allowing Ca2+ to diffuse farther from the contractile machinery when unbound. At sufficiently high temperatures, calcium may reach the membrane, effectively depolarizing the membrane and leading to a hyperexcitable state. This state would allow for multiple muscle action potentials once threshold is reached, but neuronal input would be required to stimulate the spike train. The suppression of seizure activity by parats1 occurs by preventing threshold through loss of presynaptic release.
Alternatively, though not mutually exclusive, excitability defects may be caused by further increases in ectopic innervation at the adult IFM. Although the innervation defects at the third instar neuromuscular junction are modest, defects may be exacerbated at adult muscles. However, at the adult flight muscles, type II innervation seems to represent a more molecularly diverse set of synapses, and it is unknown whether muscle hypercontraction would induce increased innervation of any, a subset, or all type IIlike synapses in the adult (Rivlin et al., 2004). Moreover, it is unclear how increases in type II innervation may alter excitability of muscles in a temperature-dependent fashion.
Although more experimentation will be required to discern between these possibilities as well as other potential mechanisms, it is clear that hypercontraction creates a distinct muscle state that is different from hypocontracted and normal muscles. In support of this, hypercontraction mutants have TS behavioral defects that are not evident in hypocontraction mutants or in wild-type flies. In addition, TS behavioral defects are not likely due to mixtures of differentially active myosins being expressed in the IFM, as MhcD45/+ heterozygotes do not display TS behavioral defects such as those found in MhcS1/+, MhcS2/+, and Mhc5/+. Future studies in determining the components that contribute to this altered state will be critical in understanding the underlying causes of excitability defects in hypercontraction mutants. Characterization of genetic interactors of MhcS1 and MhcS2 mutants such as the Swing and Breakdance loci described here may also provide insights into the molecular pathways underlying hypercontraction myopathies, as well as contribute to understanding of the mechanisms underlying human muscle diseases such as hypertrophic cardiomyopathy.
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Materials and methods |
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Adult behavior analysis
10 flies were placed into a preheated glass vial at 38°C. Flies showing TS behavioral defects were scored in 15-s intervals. The analysis was done with 10 repetitions for each genotype and each repetition contained an independent set of 10 flies.
Polarized light miocrographs
Polarized light micrographs of the adult flight muscles were analyzed as described previously with the modification of using Xylenes as a clearing agent (Fyrberg et al., 1994). Thoraces were mounted using Permount (Fisher Scientific) and analyzed under Nomarski optics.
Mutation and crystal structure analysis
Mutations were determined by PCR and sequencing. Genomic DNA was isolated from Canton-S and MhcS1/Df(2L)H20 flies (Simpson, 1983). Exons 46 were amplified by PCR and the product was sequenced at the MIT Cancer Center sequencing facility. Genomic DNA from homozygous MhcS2 embryos was isolated and similarly processed. Amino acids are numbered according to the Mhc-P11 sequence. Crystal structure analysis of the mutations was done using the Swiss PDB Viewer software available at http://us.expasy.org/spdbv/. Crystal structures 2MYS and 1MMG were downloaded from the National Center for Biotechnology Information (NCBI) and mutations mapped according to BLAST alignments done through the NCBI BLAST website (Fisher et al., 1995; Rayment et al., 1993, 1995).
Antibodies and immunohistochemistry of third instar larvae
Wandering third instar larvae were raised at 25°C, and then dissected and fixed by standard procedures. Affinity-purified rabbit -sytI antibodies (Littleton et al., 1993) were used at 1:1,000 and Cy2-conjugated goat
rabbit secondary antibodies at 1:200 (Jackson ImmunoResearch Laboratories). Texas redconjugated phalloidin was incubated simultaneously with the secondary antibody at 1:500 (Molecular Probes). Visualization and quantification was performed under light microscopy using a 40x/1.3NA oil-immersion lens. Images were taken using confocal microscopy under similar conditions and processed with Zeiss PASCAL software.
Electrophysiology
Extracellular DLM recordings.
Extracellular DLM recordings were done in male flies raised at 25°C. 15-M electrodes were filled with 3 M KCl. The recording electrode was inserted into the lateral thorax with the ground electrode inserted into the eye. Basal activity was recorded for 2 min at 22°C. Temperature was then shifted to 38°C for 1 min, and returned to 22°C. Recordings were done using an Axoclamp-2B amplifier (Axon Instruments, Inc.) and digitized with an digitizer (model 100; Instrunet) at 10 kHz and analyzed with Superscope 3.0 software (GW Instruments). To attenuate extracellular signals from the eye, experiments were done in constant light.
Intracellular recordings.
Intracellular recordings were done at room temperature in wandering third instar larve raised at 25°C. Dissections and recordings were done in 0.4 mM Ca2+ HL3 (Stewart et al., 1994) with 4 mM MgCl2. Recordings were done from muscle 6 at segments A3A5. 50100-M electrodes were filled with 3 M KCl. Muscles were analyzed if the resting membrane potential was below -50 mV. Data was digitized with a Digidata 1322, filtered at 10 kHz online, and analyzed using pCLAMP v8.0 software (Axon Instruments, Inc.). mEJP amplitude and frequency was determined by manual analysis, analyzing representative samples from each muscle recording. EJP amplitude was similarly analyzed, using the maximal response to suprathreshold stimulation (determined for each individual muscle). Ca2+-free recordings were done in a similar manner. Failure to evoke release was used to verify that Ca2+ was minimal in the external solution. Recordings with an intact central nervous system were done in 0.2 mM Ca2+ to prevent substantial depolarization during central pattern activity in the presence or absence of 3 µM TTX.
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Acknowledgments |
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This work was supported by grants from the National Institutes of Health, the Human Frontiers Science Program, the Searle Scholars Program, and the Packard Foundation. J. Troy Littleton is an Alfred P. Sloan Research Fellow.
Submitted: 31 August 2003
Accepted: 18 February 2004
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