The InteractiveFly: Drosophila as a Model for Human Diseases


Cardiomyopathy
Reviews | PubMed | Wikipedia | More in IF

Drosophila genes associated with Cardiomyopathy

Calcineurin
CG3226
Dystrophin
Galactokinase
Mitofusin (Marf)
Opa1
Pygopus
scox
Talin
Zwischenferment (G6PD)
Related terms


Heart (dorsal vessel)
Mitochondria
Muscle
RAS pathway
Overview of the disease

Cardiomyopathies are generally associated with a myocyte growth program that leads to an increase in the size of individual muscle cells. Individuals who have cardiac hypertrophy and cardiomyopathies are predisposed to the development of heart failure (Yu, 2013 and references therein).

Drosophila circulatory system. Drosophila have an open circulatory system, in which the contractile heart and aorta comprise a tube with an anterior opening. The larval and adult hearts have a single layer of cardiomyocytes that enwrap the posterior heart chamber and the anterior aorta, with myocytes in close apposition to their contralateral partners at the dorsal and ventral midlines (Bogatan, 2015 and references therein).

Go to top


Relevant studies of Cardiomyopathy

Bogatan, S., Cevik, D., Demidov, V., Vanderploeg, J., Panchbhaya, A., Vitkin, A. and Jacobs, J.R. (2015). Talin is required continuously for cardiomyocyte remodeling during heart growth in Drosophila. PLoS One 10: e0131238. PubMed ID: 26110760

Abstract
Mechanotransduction of tension can govern the remodeling of cardiomyocytes during growth or cardiomyopathy. Tension is signaled through the integrin adhesion complexes found at muscle insertions and costameres but the relative importance of signalling during cardiomyocyte growth versus remodelling has not been assessed. Employing the Drosophila cardiomyocyte as a genetically amenable model, this study depleted the levels of Talin, a central component of the integrin adhesion complex, at different stages of heart growth and remodeling. A continuous requirement for Talin was demonstrated during heart growth to maintain the one-to-one apposition of myofibril ends between cardiomyocytes. Retracted myofibrils cannot regenerate appositions to adjacent cells after restoration of normal Talin expression, and the resulting deficit reduces heart contraction and lifespan. Reduction of Talin during heart remodeling after hatching or during metamorphosis results in pervasive degeneration of cell contacts, myofibril length and number, for which restored Talin expression is insufficient for regeneration. Resultant dilated cardiomyopathy results in a fibrillating heart with poor rhythmicity. Cardiomyocytes have poor capacity to regenerate deficits in myofibril orientation and insertion, despite an ongoing capacity to remodel integrin based adhesions (Bogatan, 2015).

Highlights

  • Talin is required to maintain cardiomyocyte insertions.
  • Reduced larval Talin expression results in heart dilation.
  • Heart morphogenesis is more vulnerable than growth to reduced Talin function.

Discussion
In late embryogenesis, the Drosophila heart is a 4 micron diameter tube enclosed by 2 cardiomyocytes, attached at the dorsal and ventral midline with cadherin based cell junctions, and an integrin rich lumen. The larval cardiomyocytes are dominated by myofibrils that terminate in integrin rich insertions at the dorsal and ventral midline, without a cadherin rich domain. Therefore early heart development is marked by dramatic reorganisation of cell adhesion and polarity. For the remainder of a fly’s life, cardiomyocyte differentiation is remarkable for increase in cell size but not cell number, and for the pupal remodelling of posterior aorta myocytes into heart myocytes (Bogatan, 2015).

This study examined the role of Talin production in the differentiation, growth and remodelling of cardiomyocytes. The requirement of integrin function for cardiomyocyte adhesion was verified, and it was noted that, like body wall muscle, the insertions are integrin-rich, and that the muscle costameres coincide with myocyte surface integrin adhesions. Normally, myofibrils of each cell are aligned end-to-end with myofibrils of the contralateral cardiomyocyte, suggesting that extracellular matrix (ECM) linkages at the end of myofibrils are different from the rest of the cell surface, reminiscent of the mammalian intercalated disc. If levels of Talin production are reduced, cardiomyocyte insertion, particularly at points of myofibril termination are vulnerable to degeneration (Bogatan, 2015).

During first instar heart differentiation and pupal remodelling, cardiomyocytes are most susceptible to depletion of Talin, resulting in significant cell shrinkage. At less susceptible stages of heart growth, less cell shrinkage, but loss of myofibril apposition between cells results. The resulting degeneration of heart structure is likely due to the loss of adhesion caused by the depletion of Talin. This reflects the ongoing turnover of Talin and Integrin at adhesions, shown to be modulated by tension in Drosophila muscle. Remarkably, restoration of normal Talin expression does not enable regeneration of myofibril length, inter-cardiomyocyte cell junctions or apposition of myofibril ends between myocytes at any larval or adult stage. Instead, the cardiomyocyte perimeter is marked by a broader band of integrin, suggestive of expanded adhesion to the heart ECM, and hence less direct transmission of tension between cardiomyocytes. Nevertheless, affected cardiomyocytes continue to grow as the larva grows, without restoring cell to cell apposition and alignment of myofibrils (Bogatan, 2015).

Heart contraction is reduced subsequent to Talin reduction at each larval stage, including during the second instar, when myocyte degeneration is minimal, but midline apposition of myofibrils is disrupted. Nevertheless, this disruption does not reduce the rhythmicity of second instar treated hearts. Heart dilation, rhythmicity and contraction are most affected by transient depletion of Talin during cardiomyocyte remodelling in the first instar, suggesting that synchronicity of cardiomyocyte contraction requires cell to cell contact, possibly along the ipsilateral domains of cardiomyocytes. This cell surface domain contains the costameres, where components of the IAC are implicated in tension signalling (Bogatan, 2015).

Myofibril stability may depend upon linkage to integrin adhesion at insertions, or at the costameres, as Talin depleted cardiomyocytes have fewer myofibrils. However muscle insertion structure is far more sensitive to the level of Talin than the structure of the costamere. Weakened costameres, observed in Drosophila mutants of muscle Trim32, are depleted of Integrin Adhesion complex (IAC) proteins, including Talin, resulting in unbundling of myofibrils and muscle “wasting”. Similarly, increased or decreased Integrin function in vertebrate heart muscle alters intercalated disc structure and cardiomyocyte contractility. In Drosophila and vertebrates, integrin adhesion signalling is required for homeostasis of the contractile apparatus (Bogatan, 2015).

ECM is visible on the luminal and abluminal surfaces of cardiomyocytes. As heart diameter grows normally, new matrix must be deposited on both surfaces. Similarly, when cardiomyocytes retract, the remaining ECM likely stretches and expands as the heart vessel becomes dilated. In the Drosophila model, this dilation results in the deposition of a more elaborate network of Pericardin containing ECM fibrils. This process is analogous to mammalian Dilated Cardiomyopathy (DCM). DCM can be triggered by mutations in proteins that link the sarcomere to the ECM, such as IAC proteins vinculin and tintin. Expression of IAC proteins is elevated in cardiac hypertrophy. Analysis of IAC gene function in genetic models such as Drosophila reveals the temporal dimension of the stability and remodeling of myofibrils. This study indicates myofibril stability requires ongoing Talin renewal, and that regeneration after perturbation is very limited. Further study of IAC function subsequent to changes in cardiac load in Drosophila cardiomyocytes should be instructive in revealing the signalling pathways activated in DCM (Bogatan, 2015).

Go to top

Martínez-Morentin, L., Martínez, L., Piloto, S., Yang, H., Schon, E.A., Garesse, R., Bodmer, R., Ocorr, K., Cervera, M. and Arredondo, J.J. (2015). Cardiac deficiency of single cytochrome oxidase assembly factor scox induces p53-dependent apoptosis in a Drosophila cardiomyopathy model. Hum Mol Genet 24: 3608-3622. PubMed ID: 25792727

Abstract
The heart is a muscle with high energy demands. Hence, most patients with mitochondrial disease produced by defects in the oxidative phosphorylation (OXPHOS) system are susceptible to cardiac involvement. The presentation of mitochondrial cardiomyopathy includes hypertrophic, dilated and left ventricular noncompaction, but the molecular mechanisms involved in cardiac impairment are unknown. One of the most frequent OXPHOS defects in humans frequently associated with cardiomyopathy is cytochrome c oxidase (COX) deficiency caused by mutations in COX assembly factors such as Sco1 and Sco2. To investigate the molecular mechanisms that underlie the cardiomyopathy associated with Sco deficiency, this study interfered with scox (the single Drosophila Sco orthologue) expression in the heart. Cardiac-specific knockdown of scox reduces fly lifespan, and it severely compromises heart function and structure, producing dilated cardiomyopathy. Cardiomyocytes with low levels of scox have a significant reduction in COX activity and they undergo a metabolic switch from OXPHOS to glycolysis, mimicking the clinical features found in patients harbouring Sco mutations. The major cardiac defects observed are produced by a significant increase in apoptosis, which is dp53-dependent. Genetic and molecular evidence strongly suggest that dp53 is directly involved in the development of the cardiomyopathy induced by scox deficiency. Remarkably, apoptosis is enhanced in the muscle and liver of Sco2 knock-out mice, clearly suggesting that cell death is a key feature of the COX deficiencies produced by mutations in Sco genes in humans (Martínez-Morentin, 2015).

Highlights

  • Cardiac-specific interference of scox causes mitochondrial impairment.
  • scox RNAi knockdown causes cardiomyopathy in Drosophila melanogaster.
  • scox knockdown alters myofibril structure.
  • COX deficiency enhances the production of reactive oxygen species.
  • scox cardiomyopathy is p53-dependent.
  • Cardiac-specific knockdown of scox induces apoptosis.
  • Disruption of dp53 activity rescues scox cardiomyopathy.
  • Sco2KI/KO mice undergo apoptosis.

Discussion
Cardiomyopathies are a collection of myocardial disorders in which the heart muscle is structurally and functionally abnormal. In the past decade, it has become clear that an important proportion of cases of hypertrophic and dilated cardiomyopathies are caused by mutations in genes encoding sarcomeric or desmosomal proteins. In addition, cardiomyopathies (both hypertrophic and dilated) are frequently associated to syndromic and non-syndromic mitochondrial diseases. The importance of oxidative metabolism for cardiac function is supported by the fact that 25–35% of the myocardial volume is taken by mitochondria. The current view of mitochondrial involvement in cardiomyopathy assumes that ETC malfunction results in an increased ROS production, triggering a “ROS-induced ROS release” vicious circle which in turn perpetuates ETC dysfunction via damage in mtDNA and proteins involved in electron transport. Under this view, accumulated mitochondrial damage would eventually trigger apoptosis through mitochondrial permeability transition pore (mPTP) opening other mechanisms. Under normal circumstances, damaged mitochondria would be eliminated through mitophagy. Excessive oxidative damage is supposed to overcome the mitophagic pathway resulting in apoptosis. Nevertheless, although several potential mechanisms have been suggested, including apoptosis deregulation, oxidative stress, disturbed calcium homeostasis or impaired iron metabolism, the molecular basis of the pathogenesis of mitochondrial cardiomyopathy is virtually unknown (Martínez-Morentin, 2015).

Pathogenic mutations in human SCO1 and SCO2 have been reported to cause hypertrophic cardiomyopathy, among other clinical symptoms. However, the molecular mechanisms underlying this cardiac dysfunction have yet to be elucidated. This study reports the first cardiac-specific animal model to study human SCO1/2-mediated cardiomyopathy. Cardiac-specific scox KD in Drosophila provokes a severe dilated cardiomyopathy, as reflected by a significant increase in the conical chamber size, due to mitochondrial dysfunction. It presents a concomitant metabolic switch from glucose oxidation to glycolysis and an increase in ROS levels, leading to p53-dependent cell death. Interestingly, previous studies on patients and rat models have shown that mitochondrial dysfunction is associated with abnormalities in cardiac function and changes in energy metabolism, resulting in glycolysis optimization and lactic acidosis. Furthermore, in the Sco2KI/KO mouse model, where no evidence of cardiomyopathy has been described, partial loss of Sco2 function induces apoptosis in liver and skeletal muscle. In flies scox KD causes a significant reduction in FS and in the DI, as well as cardiac myofibril disorganization. This degenerative process is most likely due to mitochondrial dysfunction rather than to a developmental defect and moreover, the dilated cardiomyopathy developed by flies resembles that caused by mitochondrial fusion defects in flies (Martínez-Morentin, 2015).

The ETC is the major site of ROS production in cells, and aging and many neurodegenerative diseases have been linked to mitochondrial dysfunction that results in excessive oxidative stress. Interestingly, there is an increase in ROS formation associated with oxidative DNA damage in human Sco2−/− cells. Accordingly, it was found that cardiac-specific knockdown of scox increases oxidative stress, although it could not be distinguished whether this increase in free radical accumulation arises from the mitochondria or whether it comes from non-mitochondrial sources due to a loss of cellular homeostasis, as reported in yeast and in a neuro-specific COX-deficient Alzheimer disease mouse model (Martínez-Morentin, 2015).

Sco2 expression is known to be modulated by p53, a transcription factor that participates in many different processes, including cancer development, apoptosis and necrosis. p53 regulates homeostatic cell metabolism by modulating Sco2 expression and contributes to cardiovascular disorders. In addition, p53 activation in response to stress signals, such as increased oxidative stress or high lactic acid production, is well documented. Data from this study, showing that p53 is upregulated in response to scox KD, but not in response to KD of another Complex IV assembly factor, Surf1, suggest a specific genetic interaction between dp53 and scox. This is corroborated by the dramatic effects observed in the heart structure and function when dp53 is overexpressed in scox KD hearts. Furthermore, the functional and structural defects seen in scox KD hearts can be rescued in dp53-DN OE or dp53 null backgrounds, indicating that the scox-induced defects are mediated by increased p53 expression. Interestingly, opposed to scox KD, the heart structure defects induced by dp53 OE can be fully rescued by heart-specific Surf1 KD, further confirming the specificity of the genetic interaction between dp53 and scox (Martínez-Morentin, 2015).

It has recently been shown that SCO2 OE induces p53-mediated apoptosis in tumour xenografts and cancer cells. Furthermore, SCO2 KD sensitizes glioma cells to hypoxia-induced apoptosis in a p53-dependent manner and induces necrosis in tumours expressing WT p53, further linking the SCO2/p53 axis to cell death. In Drosophila, there is a dp53-mediated upregulation of Reaper, Hid and Grim in response to scox KD. This, coupled with the observation that Reaper overexpression in the adult heart enhances the structural defects caused by cardiac-specific scox KD, suggests that scox normally prevents the triggering of dp53-mediated cell death in cardiomyocytes in stress response. Indeed, it was found that there is massive cell death in the skeletal muscle and liver of Sco2KI/KO mice, supporting the hypothesis that Sco proteins might play this role also in mammals (Martínez-Morentin, 2015).

The study provides evidence that scox KD hearts exhibit partial loss of COX activity, with cardiomyocytes undergoing apoptosis. There is evidence from vertebrate and invertebrate models that partial inhibition of mitochondrial respiration promotes longevity and metabolic health due to hormesis. In fact, it has recently been shown that mild interference of the OXPHOS system in Drosophila IFMs preserves mitochondrial function, improves muscle performance and increases lifespan through the activation of the mitochondrial unfolded pathway response and IGF/like signalling pathways. This study speculates that cell death, rather than mitochondrial dysfunction itself, is likely to be the main reason for the profound heart degeneration observed in TinCΔ4-Gal4>scoxi flies. Expression of dominant negative dp53 in scox KD hearts rescues dysfunction and cardiac degeneration, and, most importantly, scox KD in dp53−/− animals causes no apparent heart defects, which could attribute the rescue observed to blockade of the p53 pathway. Indeed, inhibiting apoptosis by p35 or Diap1 OE almost completely rescues the morphological scox KD phenotype. As scox KD in the absence of dp53 causes no symptoms of heart disease, coupled with the inability of p35 and Diap1 to completely rescue the morphological phenotype, suggests that, in addition to inducing apoptosis, dp53 plays a key role in the development of cardiomyopathy (Martínez-Morentin, 2015).

The fact that heart-specific Surf1 KD neither upregulates p53 nor induces apoptosis supports the idea that the partial loss of scox function itself triggers dp53 upregulation and apoptosis, rather than it being a side effect of COX dysfunction and the loss of cellular homeostasis. In this context, it is noteworthy that SCO2 interference in mammalian cells induces p53 re-localization from mitochondria to the nucleus. It is therefore tempting to hypothesize that scox might play another role independent of its function as a COX assembly factor, perhaps in redox regulation as suggested previously and that it may act in conjunction with dp53 to fulfil this role. Another issue deserves further attention, the possibility of this interaction being a tissue-specific response. It may be possible that the threshold of COX deficiency tolerated by the heart might be lower than in other tissues, thus the scox/dp53 genetic interaction may be a tissue-dependent phenomenon or the consequence of a tissue-specific role of scox. In fact, mitochondrial dysfunction in mice is sensed independently from respiratory chain deficiency, leading to tissue-specific activation of cellular stress responses. Thus, more work is necessary to test these hypotheses and try to understand how the partial lack of scox induces cell death through dp53 (Martínez-Morentin, 2015).

Although the role of mitochondria in Drosophila apoptosis remains unclear, there is strong evidence that, as in mammals, mitochondria play an important role in cell death in flies. The localization of Rpr, Hid and Grim in the mitochondria is essential to promote cell death, and fly mitochondria undergo Rpr-, Hid- and Drp1-dependent morphological changes and disruption following apoptotic stimulus. Moreover, the participation of the mitochondrial fission protein Drp1 in cell death is conserved in worms and mammals. It has been proposed that p53 plays a role in the opening of the mPTP that induces necrotic cell death. According to this model, p53 translocates to the mitochondrial matrix upon ROS stimulation, where it binds cyclophilin D (CypD) to induce mPTP opening independent of proapoptotic Bcl-2 family members Bax and Bak, and in contrast to traditional concepts, independent of Ca2+ (Martínez-Morentin, 2015).

Apoptotic and necrotic pathways have a number of common steps and regulatory factors, including mPTP opening that is thought to provoke mitochondrial swelling and posterior delivery of necrotic factors, although Drosophila mPTP activation is not accompanied by mitochondrial swelling. Interestingly, although the p53 protein triggers mitochondrial outer membrane permeabilization (MOMP) in response to cellular stress in mammals, releasing mitochondrial death factors, MOMP in Drosophila is more likely a consequence rather than cause of caspase activation and the release of mitochondrial factors does not appear to play a role in apoptosis. Thus, in cardiac-specific scox KD flies, dp53 might induce mPTP opening to trigger cell death, which in the absence of mitochondrial swelling would result in apoptosis instead of necrosis, as occurs in mammals. Drosophila mPTP has been shown to be cyclosporine A (CsA)-insensitive in vitro, although CsA administration ameliorates the mitochondrial dysfunction with a severely attenuated ATP and enhanced ROS production displayed by collagen XV/XVIII mutants. Interestingly, mice lacking collagen VI display altered mitochondrial structure and spontaneous apoptosis, defects that are caused by mPTP opening and that are normalized in vivo by CsA treatment (Martínez-Morentin, 2015).

Go to top

Bhandari, P., Song, M. and Dorn, G.W. (2015). Dissociation of mitochondrial from sarcoplasmic reticular stress in Drosophila cardiomyopathy induced by molecularly distinct mitochondrial fusion defects. J Mol Cell Cardiol 80: 71-80. PubMed ID: 25555803

Abstract
Mitochondrial dynamism (fusion and fission) is responsible for remodeling interconnected mitochondrial networks in some cell types. Adult cardiac myocytes lack mitochondrial networks, and their mitochondria are inherently “fragmented”. Mitochondrial fusion/fission is so infrequent in cardiomyocytes as to not be observable under normal conditions, suggesting that mitochondrial dynamism may be dispensable in this cell type. However, it has been previously reported that cardiomyocyte-specific genetic suppression of mitochondrial fusion factors optic atrophy 1 (Opa1) and mitofusin/MARF evokes cardiomyopathy in Drosophila hearts. Fusion-mediated remodeling of mitochondria may be critical for cardiac homeostasis, although never directly observed. Alternately, inner membrane Opa1 and outer membrane mitofusin/MARF might have other as-yet poorly described roles that affect mitochondrial and cardiac function. This study compared heart tube function in three models of mitochondrial fragmentation in Drosophila cardiomyocytes: Drp1 expression, Opa1 RNAi, and mitofusin MARF RNA1. Mitochondrial fragmentation evoked by enhanced Drp1-mediated fission did not adversely impact heart tube function. In contrast, RNAi-mediated suppression of either Opa1 or mitofusin/MARF induced cardiac dysfunction associated with mitochondrial depolarization and ROS production. Inhibiting ROS by overexpressing superoxide dismutase (SOD) or suppressing ROMO1 prevented mitochondrial and heart tube dysfunction provoked by Opa1 RNAi, but not by mitofusin/MARF RNAi. In contrast, enhancing the ability of endoplasmic/sarcoplasmic reticulum to handle stress by expressing Xbp1 rescued the cardiomyopathy of mitofusin/MARF insufficiency without improving that caused by Opa1 deficiency. The study concludes that decreased mitochondrial size is not inherently detrimental to cardiomyocytes. Rather, preservation of mitochondrial function by Opa1 located on the inner mitochondrial membrane, and prevention of ER stress by mitofusin/MARF located on the outer mitochondrial membrane, are central functions of these “mitochondrial fusion proteins” (Bhandari, 2015).

Highlights

  • Mitochondrial fragmentation provoked by cardiomyocyte Drp1 overexpression does not compromise mitochondrial fitness or heart tube function.
  • Structural cardiac and mitochondrial abnormalities in Opa1-deficient hearts, and benefits of ROS suppression.
  • Structural cardiac and mitochondrial abnormalities in mitofusin/MARF-deficient hearts, and lack of sustained improvement with ROS suppression.
  • SOD rescues mitochondrial depolarization in fragmented mitochondria of Opa1-deficient, but not mitofusin/MARFideficient heart tubes.
  • Mitochondria are the source of damaging ROS in Opa1-deficient, but not mitofusin/MARF-deficient cardiomyocytes.
  • Cardiomyocyte mitofusin/MARF deficiency, but not Opa1 suppression, induces ER-stress that causes cardiac dysfunction.

Discussion
By performing a side-by-side detailed comparison of cardiomyopathies provoked by interrupting fusion of either the outer or inner mitochondrial membranes, this study identified distinct cellular mechanisms for the different molecular lesions. RNAi-mediated suppression of outer mitochondrial membrane mitofusin/MARF and inner mitochondrial membrane Opa1 provokes similar overt phenotypes: in both models mitochondrial size is approximately halved, the proportion of depolarized (sick) mitochondria increases to ~ 40%, mitochondrial ROS production is comparably greater (~ 50%), and the heart tubes exhibit similar reductions in fractional shortening. However, the cardiac defect caused by Opa1 deficiency is readily corrected by attacking the disease at the level of mitochondrial ROS production, through SOD expression or ROMO1 suppression. Indeed, mitochondrial structural and functional abnormalities are also improved by these genetic maneuvers, suggesting that they interrupt a vicious cycle of ROS-induced mitochondrial degeneration provoked by Opa1 insufficiency. Thus, interrupting endogenous mitochondrial ROS production greatly abrogates both the mitotoxicity and the cytotoxicity evoked by Opa1 suppression. This suggests a central role for mitochondrial degeneration in the Opa1-deficient fly heart model and, by extension, other heart diseases caused by defective inner mitochondrial membrane fusion proteins (Bhandari, 2015).

Whereas mitochondrial size and polarization status are similarly impaired in mitofusin/MARF insufficient heart tubes, neither of the mitochondrial-targeted interventions directed at reducing ROS, both of which rescue Opa1 deficient hearts, improve the cardiomyopathy induced by mitofusin/MARF deficiency. Indeed, whereas transgenic expression of SOD1 or SOD2 normalizes both ROS and heart tube function in Parkin-deficient heart tubes and Opa1-deficient heart tubes, it is remarkable that SOD fails to improve ROS levels in the mitofusin/MARF deficient heart tubes. Together with the original observation of transient heart tube functional improvement with SOD1, these observations suggest that there is “ROS escape” in the mitochondrial fusion impaired model that confers resistance to SOD expression or ROMO1 suppression. Instead, heart tube function is normalized without improving either mitochondrial structural or functional abnormalities by genetically enhancing the cardiomyocytes' ability to handle ER stress through XBP1 expression. These results, although surprising, are consistent with an essential role for mitofusin-mediated mitochondrial-ER/SR cross-talk in managing the ER stress response as proposed earlier in heart and in other tissues (Bhandari, 2015).

Previously, the consequences of Opa1 deficiency on Drosophila eye phenotypes have been found to be rescuable with SOD1, which is in accordance with findings of this study. Another study in Drosophila neurons and skeletal muscle also supports an important role for mitofusins, but not Opa1, as modulators of ER stress. However, only human Mfn2, and not human Mfn1, can correct abnormalities induced by mitofusin/MARF suppression in flies. This contrasts with findings in this study that cardiac-specific expression of either human Mfn1 or Mfn2 will fully correct cardiomyopathy induced by cardiomyocyte-specific MARF RNAi. Furthermore, ER dysmorphology has been found in MARF-deficient fly tissues, which was not detected in MARF-deficient heart tubes. It is likely either that the heart has different requirements for mitofusins and ER/SR morphology, or that the powerful tinman gene promoter used for cardiomyocyte-specific gene manipulation confers different expression characteristics to the heart models. Either way, the overall conclusions regarding a role of ER stress in mitofusin deficiency are in agreement (Bhandari, 2015).

Cardiomyocyte mitochondria are the Oompa-Loompas of the heart (with apologies to Roald Dahl): They are diminutive, structurally homogenous, and frequently overlooked despite toiling endlessly behind the scenes to keep the place running. Research has tended to focus on the mitochondrial work product (cardiac metabolism) and the means by which the general mitochondrial population is sustained (through biogenesis), rather than the fate of individual organelles. Indeed, individual cardiomyocyte mitochondria seem hardly worthy of observation, being monotonously similar in appearance and lacking the morphometric remodeling or intra-cellular mobility that has sparked detailed investigations (and visually engaging movie clips) in other cell types. The data presented in this study emphasize that (for mitochondria as well as Oompa-Loompas) size is not the critical determinant of function; it is literally what is inside that counts. Accordingly, one should eschew generalizations and extrapolations of mitochondrial status and dysfunction based strictly on morphometry (Bhandari, 2015).

Go to top

Taghli-Lamallem, O., Jagla, K., Chamberlain, J.S. and Bodmer, R. (2014). Mechanical and non-mechanical functions of Dystrophin can prevent cardiac abnormalities in Drosophila. Exp Gerontol 49: 26-34. PubMed ID: 24231130

Abstract
Dystrophin-deficiency causes cardiomyopathies and shortens the life expectancy of Duchenne and Becker muscular dystrophy (D/BMD) patients. Restoring Dystrophin expression in the heart by gene transfer is a promising avenue to explore as a therapy. Truncated Dystrophin gene constructs have been engineered and shown to alleviate dystrophic skeletal muscle disease, but their potential in preventing the development of cardiomyopathy is not fully understood. This study shows that either the mechanical or the signaling functions of Dystrophin are able to reduce the dilated heart phenotype of Dystrophin mutants in a Drosophila model. Data suggest that Dystrophin retains some function in fly cardiomyocytes in the absence of a predicted mechanical link to the cytoskeleton. Interestingly, cardiac-specific manipulation of nitric oxide synthase expression also modulates cardiac function, which can in part be reversed by loss of Dystrophin function, further implying a signaling role of Dystrophin in the heart. These findings suggest that the signaling functions of Dystrophin protein are able to ameliorate the dilated cardiomyopathy, and thus might help to improve heart muscle function in micro-Dystrophin-based gene therapy approaches (Taghli-Lamallem, 2014).

Highlights

  • Dystrophin proteins carrying either the ‘mechanical’ or ‘signaling’ domains ameliorate Dystrophin-deficient heart dysfunction.
  • Dystroglycan does not play a critical role in Drosophila heart function.
  • Dp116 Dystrophin increases the heart period in Dystrophin mutant flies.
  • Nitric oxide synthase (Nos) maintains youthful heart function of Dys−/− flies during aging.

Discussion
Cardiomyopathy is a major health threat to DMD and BMD patients. Focusing on the gene therapy strategy to redress cardiac pump failure is of great importance to alleviate the severity of this heart disease. This study evaluated truncated Dys genes for their potential to rescue the heart phenotypes in Drosophila Dys−/− mutants. It was found that both micro-Dys constructs with a predicted mechanical role and Dp116-Dys with a predicted signaling role can markedly reverse morphological and functional features of dystrophic hearts. Specifically, the beneficial effect of Dp116 in rescuing the Dys−/− heart abnormalities even in the absence of a predicted actin cytoskeleton link was highlighted (Taghli-Lamallem, 2014).

The expression of micro-Dys in mdx mice results in cardiac histopathology correction and partial normalization of heart function, consistent with data of this study, which also demonstrate partially normalized heart contractility of young and aged dystrophic flies. Both micro-Dys proteins fail to correct the increased heart rate of old Dys−/− flies, again consistent with ΔH2-R19 also having no effect on tachycardia in mdx mice. This suggests that micro-Dys functions in the fly heart mirror in many aspects their functional capacities in the mdx mouse. Interestingly, in Drosophila, the micro-Dys rescue of heart function also includes restoration of cardiomyocyte myofibril integrity. Whether a residual myofibrillar mis-orientation is a similar phenomenon to that of micro-Dys ΔR4-23/ΔCT generating ringed fibers in skeletal muscle of mdx mice remains to be established (Taghli-Lamallem, 2014). 

The expression of Dp116 does not seem to ameliorate the dystrophic phenotypes of mdx mice or the extensive muscle degeneration in mdx; utrn−/− double knockout mice, but it improves their mobility and lifespan, suggesting that predicted non-mechanical functions of Dys are indeed important. Also, Dys isoform Dp71 (predicted to have a non-mechanical function) is not sufficient to prevent the mdx;utrn−/− cardiomyopathy, probably because Dp71, but not Dp116, lacks the entire rod domain as well as a functional WW domain, and only weakly associates with the sarcolemma. It was shown that the Dp116 ameliorates the Dys−/− flies’ cardiomyopathy and is effective in restoring the cytoarchitectural myofibrils, also suggesting an important signaling role of Dys. It will be interesting to examine the rescue abilities further in the future with other assays, for example the atomic force microscopy-based analysis to measure the passive mechanical stiffness of the cardiomyocytes (Taghli-Lamallem, 2014). 

To further explore the non-mechanical role of Dys, Nos, known to be involved in Dys-complex signaling and the progression of myopathy, was analyzed. In vertebrate heart muscle, Nos modulates excitation–contraction coupling and thus myocardial function, but its role in regulating heart rhythm remains controversial. Unexpectedly, genetic disruption of all Nos genes [neuronal (nNos), inducible (iNos) and endothelial (eNos)] results in viable mice with left ventricular hypertrophy and diastolic dysfunction, consistent with previous studies indicating that Nos affects cardiac remodeling. It was found that in flies, cardiac-specific RNAi knockdown of Nos results in lower heart rates, due to enlarged relaxation periods. Recent studies reveal that myocardial nNos promotes the [Ca2+]i decay and relaxation by stimulating sarcoplasmic reticulum (SR) Ca2 + reuptake, possibly in close proximity to SR and Z-lines as suggested by data from this study. One of the Nos targets could be the sarcoplasmic/endoplasmic reticulum Ca2 ± ATPase (SERCA). In intact arteries, NO induced post-translational modifications such as S-glutathiolation of SERCA, thus increasing SERCA activity and enabling Ca2 ± uptake by the SR. Alternatively, Nos may also directly affect transcriptional activation of effector genes. Human and mice ventricular myocardial sections show a similar SR-related pattern of nNos. Interestingly, the slower heart rate of NosRNAi flies is reversed in Dys−/− mutant background, so that Dys−/−; Hand > NosRNAi flies exhibit a significant decrease in both diastolic and systolic intervals. These results allow the hypothesis that Dys and Nos differentially regulate a common but still unknown target involved in Ca2+ homeostasis and in modulating heart periods (Taghli-Lamallem, 2014). 

In conclusion, findings from this study reveal an age-related cardioprotective effect of truncated Dys proteins, including Dp116 lacking the actin cytoskeleton link, raising intriguing possibilities of Dys signaling functions that warrant further investigation in a less complex cardiac model such as Drosophila. This study also supports the importance of micro-Dys based gene therapy approaches in improving cardiac performance in DMD patients (Taghli-Lamallem, 2014).

Go to top

Viswanathan, M.C., Kaushik, G., Engler, A.J., Lehman, W. and Cammarato, A. (2014). A Drosophila melanogaster model of diastolic dysfunction and cardiomyopathy based on impaired troponin-T function. Circ Res 114: e6-e17. PubMed ID: 24221941

Abstract
Regulation of striated muscle contraction is achieved by Ca2+-dependent steric modulation of myosin cross-bridge cycling on actin by the thin filament troponin-tropomyosin complex. Alterations in the complex can induce contractile dysregulation and disease. For example, mutations between or near residues 112–136 of cardiac troponin-T, the crucial N-terminal TnT1 tropomyosin-binding region, cause cardiomyopathy. The Drosophila up101 Glu/Lys amino acid substitution lies C-terminally adjacent to this phylogenetically conserved sequence. Using a highly integrative approach, this study sought to determine the molecular trigger of up101 myofibrillar degeneration, to evaluate contractile performance in the mutant cardiomyocytes, and to examine the effects of the mutation on the entire Drosophila heart to elucidate regulatory roles for conserved TnT1 regions and provide possible mechanistic insight into cardiac dysfunction. Live video imaging of Drosophila cardiac tubes revealed the troponin-T mutation prolongs systole and restricts diastolic dimensions of the heart, due to increased numbers of actively cycling myosin cross-bridges. Elevated resting myocardial stiffness, consistent with up101 diastolic dysfunction, was confirmed by an atomic force microscopy-based nanoindentation approach. Direct visualization of mutant thin filaments via electron microscopy and three-dimensional reconstruction resolved destabilized tropomyosin positioning and aberrantly exposed myosin binding sites under low Ca2+ conditions. As a result of troponin-tropomyosin dysinhibition, up101 hearts exhibit cardiac dysfunction and remodeling comparable to that observed during human restrictive cardiomyopathy. Thus, reversal of charged residues about the conserved tropomyosin-binding region of TnT1 may perturb critical intermolecular associations required for proper steric regulation, which likely elicits myopathy in the Drosophila model used in this study (Viswanathan, 2014).

Highlights

  • The homologous Tm binding region of TnT1 is associated with multiple disease mutations.
  • The Drosophila up101 TnT mutation alters cardiac morphology.
  • The Drosophila up101 TnT mutation induces restrictive physiology and diastolic dysfunction.
  • The up101 TnT1 mutation reduces cardiac tube diameters during diastole by increasing actively cycling cross-bridges.
  • The Drosophila up101 TnT mutation enhances myocardial stiffness under low Ca2+.
  • The Drosophila up101 TnT mutation promotes the C-state even in the absence of Ca2+.

Discussion
TnT makes extensive associations with multiple thin filament components including the TnI-TnC binary complex, Tm and actin and therefore is central to thin filament-mediated regulation of striated muscle contraction. TnT mutations, which predominately localize to the N-terminal TnT1 domain, are frequently associated with a host of myopathic responses. A number of investigative efforts have shown cardiomyopathic cTnT lesions induce a range of complex effects including altered myofilament in vitro sliding velocity, disturbed Ca2+ sensitivity of force generation, decreased cTnT-Tm affinity, impaired ability to stabilize the Tm overlap, perturbed efficacy of promoting Tm binding to actin, disrupted folding stability and secondary structure of cTnT1 and mutation-specific changes in peptide flexibility, all of which likely contribute to disease pathogenesis (Viswanathan, 2014).

Drosophila muscles are also sensitive to TnT alterations. Previously, constitutive TnT1 mutations have been shown to drastically disrupt both the structure and function of IFMs within days of adulthood. This study speculates that since the up101 mutation resides in the N-terminal region of TnT and it induces myosin-dependent IFM degeneration, the lesion likely causes cardiomyopathy in flies due to contractile dysinhibition and perturbed steric regulation (Viswanathan, 2014).

TnT-binding Tms from vertebrates and invertebrates share a well-conserved amino acid segment believed to be a critical TnT1 recognition site. Similarly, multiple sequence alignments illustrate a high degree of evolutionary conservation among stretches of TnT1 postulated to be critical for Tm associations. Residues 112–136 of human cTnT were found to be 70% homologous across analyzed sequences, which suggests an essential Tm binding role for this element throughout the animal kingdom. This region is highly charged and intermolecular electrostatic associations likely dictate proper function. Importantly, engineered Tn constructs bind tightly to the thin filament only if they contain the entire 112–136 TnT1 domain. The Drosophila up101 mutation lies just downstream of this region. Introduction of basic charges could disturb conserved interactions immediately at the Tm-TnT1 interface that are required for proper steric regulation. Similarly cTnT cardiomyopathy mutations located in and adjacent to this region are also expected to influence Tm-TnT1 associations. Interestingly, it was found that the up101 charge reversal mutation results in a cardiac phenotype reminiscent of human RCM (Viswanathan, 2014).

The up101 amino acid substitution may also promote molecular pathogenesis by potentially altering overall TnT performance, including mutation-driven propagated effects that could influence TnT function at a distance. These effects may involve 1) changes in helical stability of TnT1 and subsequently the flexibility of this region, which could compromise effective interactions with Tm; 2) alterations in local helical electrostatic compaction and consequently distal helical expansion that drives unwinding and flexibility changes at remote distances along TnT; and 3) disruptions that possibly propagate through a complex structural pathway to perturb the affinity of cardiac TnC for Ca2+. The latter could potentially contribute to changes in myoflament Ca2+ sensitivity, as frequently observed in other models of TnT-based cardiomyopathies (Viswanathan, 2014).

Regardless of gender, the up101 Drosophila mutation markedly reduces diastolic volumes and extends systole in mutant relative to control hearts. These changes are consistent with diastolic dysfunction, a hallmark of HCM and RCM. Diastolic dysfunction is characterized by impaired relaxation, decreased distensibility and increased myocardial stiffness, which can result from excessive acto-myosin interactions. Thus, up101 Drosophila serve as a unique model to investigate the root of these pathological alterations in myocyte properties (Viswanathan, 2014).

The diameter changes across the Drosophila heart, in response to the myosin-specific inhibitor blebbistatin, suggest that diastole in flies is accompanied by a small but significant population of residual, force generating cross-bridges that actively shorten the cardiomyocytes and impact diastolic tone. Detection of cycling myosin cross-bridges at submaximal, diastolic levels of Ca2+, which affect cardiomyocyte mechanical properties, is not unprecedented. Although beyond the scope of this study, these basal acto-myosin associations could bolster the prominent stretch activation response common to insect muscle and may have important implications in myofilament length dependent activation of cardiomyocytes. Interestingly, specific TnI-inducing RCM and Tm-causing HCM mutations are associated with excessive cross-bridge cycling during diastole. These thin filament mutations were shown to reduce basal sarcomere length and elevate diastolic tension of cardiomyocytes. Likewise, the reduced diastolic diameter of up101 hearts is due, in part, to excessive, less inhibited force generating acto-myosin interactions that promote enhanced cell shortening relative to control hearts. Application of the myosin inhibitor however did not restore mutant heart diameter to that of blebbistatin-treated control flies. Thus, additional remodeling events must transpire over the three week pre-analysis period that influence up101 myocyte dimensions. Changes in cellular dimensions are consistent with pathological responses and tissue disorganization that accompany cardiac disease. For example, pathological stimuli associated with several cardiomyopathies induce changes in myocyte geometry and shape that help determine contractile function and whole heart morphology (Viswanathan, 2014).

The myosin-dependent changes in up101 cardiac dimensions are accompanied by differences in resting myocardial stiffness. This study employed a novel AFM-based technique to resolve Drosophila myocardial tension disparities, in vivo with single cell resolution. The geometric nature of the cardiac tube allows indentation and transverse stiffness determination from discrete cellular loci in the intact organ with no mechanical artifacts due to myocyte isolation, seeding or plating. By determining the transverse stiffness at cell junctions, the degree of active longitudinal tension generated and transmitted to the connections between the ends of coupled conical chamber myocytes can be directly assessed. Thus, the relative extent of contractile dysinhibition can be quantified by comparing the transverse stiffness at the midventral seam from different genotypes before and after incubation with blebbistatin. Control cardiomyocytes show roughly a 25% drop in transverse stiffness at the midline upon incubation with the myosin inhibitor, consistent with a small number of residual force-generating acto-myosin associations under low Ca2+, diastole-like conditions. The transverse stiffness of the up101 cardiomyocyte junction is over 60% greater than that of control and shows an ~50% decrease following blebbistatin treatment. The large discrepancies in resting transverse stiffness and in response to the myosin-specific inhibitor indicate the TnT1 mutation promotes a greater number of actively cycling, force generating cross-bridges in up101 myocytes relative to Canton-S myocytes and these contribute to the diastolic dysfunction observed under relaxing conditions (Viswanathan, 2014).

TnT is pivotal in modulating the average position of Tm between the B-, C- and M-states along thin filaments. Disease-causing mutations in Tn subunits may lead to changes in the distribution of these states and therefore disrupt regulation of contractile force. To directly assess a mutation-specific redistribution of regulatory states up101 IFM thin filaments were purified and imaged. In the complete absence of Ca2+, surface views of three-dimensional reconstructions of up101 thin filaments reveal Tm stands, on average, making contact with the inner domains of successive actin monomers along the long pitch helices of the filament. However, a small amount of density could be seen extending from the extreme inner edge of the outer domains of actin to the Tm strands on these filaments. This extra density is the result of a small population of up101 thin filaments, which in the absence of Ca2+ exhibit Tm in the B-state, as seen with wildtype filaments lacking Ca2. Nonetheless, the vast majority of the up101 thin filaments in the absence of Ca2+ were shown to be in the C-state. Thus, TnT products of the up101 allele may alter the equilibrium of the Tm position at rest such that at any given time the majority of thin filament regulatory units are in the C-state and not the B-state. This is consistent with the mutation disrupting the inherent, and possibly vital, B-state promoting property of the TnT1 tail, a property that appears dependent upon a TnT1 subdomain that includes the up101 locus. When the up101 mutation is expressed in vivo and in the presence of thick filaments, excessive cross-bridge cycling would occur in a dysregulated fashion. Over time this could lead to destruction of the IFM and drive restrictive remodeling in the heart (Viswanathan, 2014).

Analysis of myocytes from several TnT-based cardiomyopathy models has revealed significant alterations in cellular Ca2+ handling. Moreover, a Drosophila TnI mutant, which is characterized by an enlarged cardiac chamber and cardiac contractile dysfunction, displays abnormalities in the cytosolic Ca2+ transient as well as changes in transcription of proteins associated with Ca2+ handling. Therefore, the up101 TnT mutant may similarly exhibit downstream alterations in Ca2+ kinetics and homeostasis that potentially contribute to the observed cardiac phenotype. Quantitative polymerase chain reaction analysis was performed to assay possible changes in the Ca2+-handling biosignature of up101 relative to Canton-S hearts. No significant differences were identified between the lines in transcript levels of L-type Ca2+ channels, ryanodine receptors, SERCA, Na/Ca exchangers, or in inositol-3-phosphate receptors. Although this assay does not preclude possible post-transcriptional or post-translational modifications that could influence the encoded proteins, nor does it completely rule out all potential adaptive Ca2+ responses, the results suggest the cardiac phenotype that was observed is primarily due to the direct, Ca2+-independent, C-state promoting effect the up101 TnT mutation exerts on thin filaments (Viswanathan, 2014). 

These integrative data are consistent with a mechanism of diastolic dysfunction and restrictive cardiac pathology based on a fundamental inability of the homozygous, up101 TnT1 mutant thin filaments to properly block myosin cross-bridge cycling at rest. Here, a disproportionally large number of up101 regulatory units adopt the C-state under low Ca2+ conditions. This permits an exceedingly high number of strong stereospecific acto-myosin associations and excessive formation of the M-state that would promote decreased diastolic heart chamber volumes and elevated diastolic myocardial stiffness. Moreover, inordinate myosin binding increases the affinity of Tn for Ca2+. Thus, fewer mutant regulatory units required to undergo Ca2+-dependent unblocking combined with enhanced Ca2+ sensitivity “primes” the system for systole. As a result, for a given Ca2+ transient, systole would commence earlier and terminate later, which is consistent with the highly prolonged systolic intervals observed in up101 hearts relative to control (Viswanathan, 2014).

The study anticipates similar, however potentially less severe responses in up101 heterozygotes. Due to cooperativity of contractile activation and the continuous nature of regulatory units, the effects of the mutation could potentially be transmitted to neighboring, non-mutant regulatory units and thus influence the regulatory status of regions up- and down-stream of the lesion. These propagated effects along the thin filament may also be sufficient to promote myosin cross-bridge cycling, decreased diastolic chamber volumes and elevated diastolic myocardial stiffness, but potentially to a lesser extent than that found in homozygotes due to the presence of some normally functioning, wildtype regulatory units. Thus, as with other models of cardiomyopathy, a relationship could be expected between the number of mutant up101 alleles and phenotype severity (Viswanathan, 2014).

Go to top

Lee, T.E., Yu, L., Wolf, M.J. and Rockman, H.A. (2014). Galactokinase is a novel modifier of calcineurin-induced cardiomyopathy in Drosophila. Genetics 198: 591-603. PubMed ID: 25081566

Abstract
Activated/uninhibited calcineurin is both necessary and sufficient to induce cardiac hypertrophy, a condition that often leads to dilated cardiomyopathy, heart failure, and sudden cardiac death. This study expressed constitutively active calcineurin in the adult heart of Drosophila melanogaster and identified enlarged cardiac chamber dimensions and reduced cardiac contractility. In addition, expressing constitutively active calcineurin in the fly heart using the Gal4/UAS system induces an increase in heart wall thickness. By performing a targeted genetic screen for modifiers of calcineurin-induced cardiac enlargement based on previous calcineurin studies in the fly, galactokinase was identified as a novel modifier of calcineurin-induced cardiomyopathy. Genomic deficiencies spanning the galactokinase locus, transposable elements that disrupt galactokinase, and cardiac-specific RNAi knockdown of galactokinase suppress constitutively active calcineurin-induced cardiomyopathy. In addition, in flies expressing constitutively active calcineurin using the Gal4/UAS system, a transposable element in galactokinase suppresses the increase in heart wall thickness. Finally, genetic disruption of galactokinase suppresses calcineurin-induced wing vein abnormalities. Collectively, this study generated a model for discovering novel modifiers of calcineurin-induced cardiac enlargement in the fly and identified galactokinase as a previously unknown regulator of calcineurin-induced cardiomyopathy in adult Drosophila (Lee, 2014).

Highlights

  • Expression of CanAact in the adult fly heart causes cardiac enlargement.
  • A region on chromosome 3L harbors novel modifiers of tinC-CanAact-mediated cardiac enlargement.
  • Identification of galactokinase as a novel modifier of calcineurin-induced cardiac enlargement.
  • Tissue-specific suppression of CanAact-induced phenotypes in noncardiac tissues with genetic deficiency of Galk.

Discussion
Calcineurin is a known mediator of cardiac hypertrophy in mammalian hearts and understanding the signals that regulate calcineurin has the potential to alter cardiac pathology. This study shows that (1) CanAact in the Drosophila heart induces cardiac enlargement and reduces cardiac contractility; (2) Galk disruption suppresses the CanAact-mediated cardiac enlargement, increase in wall thickness, and posterior wing vein phenotypes; and (3) Galk regulation of CanAact-induced phenotypes is tissue specific. Using the resources that are available in fly genetics, the ability to phenotype in vivo cardiac chamber sizes, and the unique fact that flies lack calcineurin-regulated NFAT, a potential new regulator of calcineurin was identified in the Drosophila heart. It was also observed that CanAact induces sustained cardiac enlargement in Drosophila during aging and reduces life span. Furthermore, known regulators of calcineurin signaling, including Mef2, rescue the observed cardiac abnormalities, supporting the hypothesis that these approaches can identify modifiers of calcineurin-mediated cardiac abnormalities (Lee, 2014).

Two prior screens of the fly have scored changes in eye morphology or lethality caused by activated calcineurin and have identified several genomic regions that harbor potential modifiers. Four enhancer regions and five suppressor regions have been identified in a mutagenesis screen of the fly eye, whereas seven suppressor regions have been discovered in a deficiency screen for lethality. These studies identify cytological regions in the fly genome and, based on these important findings, this study focused on a region that is common to both studies but lacks identification of the candidate modifier of calcineurin. Using molecularly defined genomic deficiencies, transposable element insertions, precise excisions, and transgenic RNAi, it was identified that galactokinase is a candidate modifier of calcineurin-mediated cardiac abnormalities in the fly. Galactokinase may function either as an enhancer of calcineurin signaling or in a pathway downstream of calcineurin activation. Interestingly, previous studies have not implicated an interaction between calcineurin and galactokinase (Lee, 2014).

Results from this study demonstrate that deficiency of Mef2 suppresses cardiac-CanAact-induced cardiac enlargement. Since Drosophila do not express calcineurin-regulated NFAT, this implies that Mef2 functions independent of NFAT. In agreement with these results, studies in skeletal muscle and C2C12 myogenic cells have shown that calcineurin binds to Mef2, leading to hypophosphorylation and enhanced transcription activity. However, studies in Jurkat T-lymphocytes suggest that calcineurin-regulated Mef2 activation requires recruitment of NFATc2. In the mouse heart, the major pathway appears to be NFAT since expressing dominant negative Mef2 rescues only cardiac dilation and not hypertrophy. In the same mouse study, overexpressing Mef2 induces a cardiac dilation phenotype without hypertrophy, suggesting that Mef2 mainly contributes to the dilation phenotype induced by calcineurin. Interestingly, flies expressing tinC-Gal4>UAS-Mef2 have significantly increased end-diastolic dimensions, similar to the tinC-CanAact phenotype, although the flies overexpressing Mef2 do not display reduced fractional shortening, suggesting that Mef2 is necessary but not sufficient for calcineurin-induced reduction in contractility. These results suggest that an NFAT-independent pathway through Mef2 in Drosophila may regulate cardiac enlargement and that additional factors outside of Mef2 may be involved in controlling cardiac function (Lee, 2014).

This study excluded several potential genes as modifiers from the original candidate region. The Doc genes are known to regulate Drosophila cardiac development, and it was shown that knocking down Doc expression with RNAi causes cardiac enlargement and does not rescue tinC-CanAact. This implies that Doc genes are important for cardiac development but evidence for them regulating calcineurin was not found. Argk has high expression in the adult Drosophila heart. Argk is an enzyme that transfers the phosphate on ATP to arginine, creating an energy-rich buffer for maintaining ATP concentration. Although it is conceivable that Argk functions to provide sufficient energy for the fly myocardium, it was shown that disrupting Argk with a transposable element does not produce a phenotype and does not rescue calcineurin-induced cardiac enlargement. These results suggest that Argk does not regulate calcineurin-induced cardiac enlargement. However, it should be noted that the gene expression decreased by only 30% in the heterozygous PBac{PB}Argkf05255 line and it is possible that the lack of rescue is due to incomplete knockdown of gene expression (Lee, 2014).

A deficiency encompassing Galk was found to suppress the CanAact-induced phenotypes in heart and posterior wing. However, this deficiency did not suppress phenotypes driven by ubiquitous Act5C-Gal4, ectodermal dpp-Gal4, or mesodermal Mef2-Gal4 drivers. These observations could be explained by the tissue-specific context in which calcineurin is expressed. The expression of each driver occurs in different tissues. Although e16E-Gal4 and dpp-Gal4 expression of CanAact induces wing phenotypes, e16E-Gal4 drives expression in the posterior wing while dpp-Gal4 drives expression anterior to the anterior–posterior boundary during imaginal disc development at the third instar larva stage. Many signaling factors are differentially expressed during wing development in these two separate compartments to guide correct patterning. For example, engrailed and hedgehog are expressed posteriorly, guiding formation of the posterior wing veins, while dpp and the EGFR inhibitor knot are expressed specifically in the anterior wing imaginal disc. It is possible that Galk modification of calcineurin signaling is regulated by these differentially expressed factors. In addition, Act5C-Gal4 and Mef2-Gal4 driving CanAact induce lethality that is not suppressed by deficiency of Galk. These drivers induce transcript expression in multiple tissues during early embryonic development. One explanation for the findings is that the signals at an early stage of development involve pathways that are not modulated by Galk (Lee, 2014).

The survival of tinC-CanAact and in the context of the insertion Mi{ET1}GalkMB10638 was examined. Of note, in mice, cardiac-specific expression of CanAact also induces premature sudden death. Flies heterozygously expressing tinC-CanAact have a significantly reduced life span compared to the control group. This reduction is suppressed in the context of one copy of Mi{ET1}GalkMB10638, further suggesting that deficiency in Galk suppresses multiple cardiac calcineurin-induced phenotypes. However, a caveat to the interpretation of this experiment is the possibility of genetic background confounding the beneficial effects seen by disruption of Galk (Lee, 2014).

Galactokinase belongs to the GHMP ATP-dependent kinase family (named after the four representative kinases in this family: galactokinase, homoserine kinase, mevalonate kinase, and phosphomevalonate kinase). In the fly, galactokinase phosphorylates galactose and N-acetyl-galactosamine (GalNAc), allowing further utilization in either metabolism (energy production) or glycosylation (protein modification). These pathways potentially lead to cardiac enlargement: either galactokinase promotes a higher level of phosphorylated galactose, galactose-1-p, leading to a diseased state, or downstream reactions involving UDP-galactose incorporation into glycosylated proteins promote cardiac enlargement, or both mechanisms may be required. A previous screen examining Drosophila cardiac development discovered that mutations in HMG-CoA reductase (the rate-limiting enzyme in the mevalonate pathway) induce a cardiac phenotype by geranylgeranylation of the G-protein Gγ1, suggesting a pathway by which post-translational modifications can alter the fly heart (Lee, 2014). 

In additional experiments, it was shown that a transposable element insertion in galectin (a galactoside-binding lectin) suppresses the tinC-CanAact cardiac phenotype. Mammalian Galectin3 has been shown to bind to Galk-regulated N-acetyllactosamine side chains on EGFR, preventing endocytosis and enhancing signaling of isolated mouse mammary cells. Driving activated EGFR in the Drosophila heart has been shown to induce a cardiac hypertrophy phenotype and sprouty, a regulator of EGFR signaling, has been shown to modify calcineurin-induced rough eye. Although speculation, one potential mechanism by which galactokinase could function to modulate calcineurin-induced cardiac enlargement is by influencing cotranslational glycosylation of EGFR or another yet unidentified cell surface protein that is bound by galectin (Lee, 2014).

A yeast homolog of Galk, Gal3p, has been found to act as a transcriptional activator by interacting with Gal80p; transcriptional regulation is activated with the binding of galactose and ATP to Gal3p. This suggests the possibility that Galk may act as a modifier of transcription for known pathways including Mef2. Whether Galk functions as part of the transcriptional machinery remains to be determined (Lee, 2014).

Go to top

Tang, M., Yuan, W., Fan, X., Liu, M., Bodmer, R., Ocorr, K. and Wu, X. (2013). Pygopus maintains heart function in aging Drosophila independently of canonical Wnt signaling. Circ Cardiovasc Genet 6: 472-480. PubMed ID: 24046329

Abstract
Heart function declines with age, but the genetic factors underlying such deterioration are largely unknown. Wnt signaling is known to play a role in heart development, but it has not been shown to be important in adult heart function. This study investigated the nuclear adapter protein encoded by pygopus (pygo), which mediates canonical Wnt signaling, for roles in aging-related cardiac dysfunction. Using the Drosophila heart model, it was shown that cardiac-specific pygo knockdown in adult flies causes a significant (4- to 5-fold) increase in cardiac arrhythmias (P<0.001) that worsene with age and cause a significant decrease in contractility (−54%; P<0.001) with systolic dysfunction. Immunohistochemistry revealed structural abnormalities that worsen with age, and both functional and morphological alterations are ameliorated by pygo overexpression. Unexpectedly, knockdown of 2 other Wnt signaling components, β-cat/armadillo or TCF/pangolin, have relatively milder effects on cardiac function. Double-heterozygous combinations of mutants for pygo and canonical Wnt signaling components have no additional effect on heart function over pygo heterozygotes alone. However, double knockdown of pygo and Ca2+/calmodulin-dependent protein kinase II causes additional arrhythmia compared with pygo knockdown alone, suggesting that some of the effects of pygo are mediated by Ca2+ signaling. In the isoproterenol-induced hypertrophic mouse model, it was shown that Pygo1 protein levels are increased. These data indicate that Pygo plays a critical role in adult heart function that is Wnt signaling independent and is likely conserved in mammals (Tang, 2013).

Highlights

  • pygo is expressed in the adult heart.
  • pygo is required to maintain normal heart physiology.
  • pygo is required for normal heart morphology.
  • Cardiac pygo KD causes an accelerated age-dependent deterioration of heart function.
  • pygo is required in the adult heart to maintain its function.
  • pygo functions independently of canonical Wnt signaling in the adult heart.
  • pygo does not interact genetically with β-Cat/TCF complex genes to regulate adult heart function.
  • pygo interacts genetically with CaMKII to regulate heart function.

Go to top

Chen, Y., Sparks, M., Bhandari, P., Matkovich, S.J. and Dorn, G.W. 2nd. (2014). Mitochondrial genome linearization is a causative factor for cardiomyopathy in mice and Drosophila. Antioxid Redox Signal 21: 1949-1959. PubMed ID: 23909626

Abstract
Mitofusin (Mfn)2 redundantly promotes mitochondrial outer membrane tethering and organelle fusion with Mfn1, and uniquely functions as the mitochondrial receptor for Parkin during PTEN-induced putative kinase 1 (PINK1)-Parkin-mediated mitophagy. Selective deletion of Mfn2 with retention of Mfn1 preserves mitochondrial fusion while rendering damaged mitochondria resistant to normal quality control culling mechanisms. Consequently, neuron and cardiomyocyte-specific Mfn2 gene ablation is associated with accumulation of damaged mitochondria and organ dysfunction. This study determined how mitochondrial DNA (mtDNA) damage contributes to cardiomyopathy in Mfn2-deficient hearts. RNA sequencing of Mfn2-deficient hearts reveals increased expression of some nuclear-encoded mitochondrial genes, but mitochondrial-encoded transcripts are not upregulated in parallel and mtDNA content is decreased. Ultra-deep sequencing of mtDNA shows no increase in single nucleotide mutations, but copy number variations representing insertion–deletion (in–del) mutations are induced over time by cardiomyocyte-specific Mfn2 deficiency. Double-strand mtDNA breaks in the form of in–dels were confirmed by polymerase chain reaction, and in the form of linear mitochondrial genomes were identified by southern blot analysis. Linearization of Drosophila cardiomyocyte mtDNA using conditional cardiomyocyte-specific expression of mitochondrial targeted XhoI recapitulated the cardiomyopathy of Mfn2-deficient mouse hearts. This is the first description of mitochondrial genome linearization as a causative factor in cardiomyopathy. One of the consequences of interrupting mitochondrial culling by the PINK1-Mfn2-Parkin mechanism is an increase in mtDNA double-stranded breaks, which adversely impact mitochondrial function and DNA replication (Chen, 2014).

Highlights

  • Altered gene expression in Mfn2-deficient mouse hearts.
  • Mitochondrial gene expression and mtDNA abnormalities in Mfn2-deficient mouse hearts.
  • Mitochondrial genomic rearrangements are caused by cardiac Mfn2 ablation.
  • Linear mitochondrial genomes accumulate in cardiac Mfn2 deficiency.
  • mtDNA linearization is sufficient to cause mitochondrial cardiomyopathy.
  • mtDNA linearization with MitoXhoI induces cardiomyopathy in Drosophila.

Go to top

Xie, H.B., Cammarato, A., Rajasekaran, N.S., Zhang, H., Suggs, J.A., Lin, H.C., Bernstein, S.I., Benjamin, I.J. and Golic, K.G. (2013). The NADPH metabolic network regulates human αB-crystallin cardiomyopathy and reductive stress in Drosophila melanogaster. PLoS Genet 9: e1003544. PubMed ID: 23818860

Abstract
Dominant mutations in the alpha-B crystallin (CryAB) gene are responsible for a number of inherited human disorders, including cardiomyopathy, skeletal muscle myopathy, and cataracts. The cellular mechanisms of disease pathology for these disorders are not well understood. Among recent advances is that the disease state can be linked to a disturbance in the oxidation/reduction environment of the cell. In a mouse model, cardiomyopathy caused by the dominant CryABR120G missense mutation has been found to be suppressed by mutation of the gene that encodes glucose 6-phosphate dehydrogenase (G6PD), one of the cell's primary sources of reducing equivalents in the form of NADPH. This study reports the development of a Drosophila model for cellular dysfunction caused by this CryAB mutation. With this model, the link between G6PD and mutant CryAB pathology was confirmed by finding that reduction of G6PD expression suppresses the phenotype while overexpression enhances it. Moreover, it was found that expression of mutant CryAB in the Drosophila heart impairs cardiac function and increases heart tube dimensions, similar to the effects produced in mice and humans, and that reduction of G6PD ameliorates these effects. Finally, to determine whether CryAB pathology responds generally to NADPH levels mutants or RNAi-mediated knockdowns of phosphogluconate dehydrogenase (PGD), isocitrate dehydrogenase (IDH), and malic enzyme (MEN), the other major enzymatic sources of NADPH,  were tested and it was found that all are capable of suppressing CryABR120G pathology, confirming the link between NADP/H metabolism and CryAB (Xie, 2013).

Highlights

  • Drosophila heart dysfunction caused by human CryABR120G.
  • CryABR120G-induced defects in non-heart tissues.

Discussion
This study shows that Drosophila provides a suitable model in which to study the pathology of the human CryABR120G mutation. Expression of the mutant allele, but not the wildtype, in fly hearts, causes heart dilation and dysfunction very reminiscent of the cardiomyopathy produced in humans carrying this dominant allele. It was also found that a reduced level of G6PD ameliorates many of the perturbed cardiac functional parameters in CryABR120G flies, just as it does in the CryABR120G mouse. Despite a lack of reduction in CryABR120G diastolic diameters in response to Zw (G6PD) RNAi co-expression, systolic diameters in the double mutants were rescued and did not significantly differ from those found in fly hearts expressing wildtype human CryAB. Thus, fractional shortening was indistinguishable between wildtype CryAB-expressing and CryABR120G+Zw RNAi-expressing hearts. Furthermore, cardiac restricted expression of Zw RNAi, either with CryABR120G or alone, significantly increased heart rates relative to control hearts. This suggests G6PD deficiency can improve cardiac output in either mutant or non-mutant backgrounds and may be a potent modifier of cardiac function (Xie, 2013).

In Drosophila, overexpression of G6PD can extend lifespan and protect against oxidative stress. In mammalian cells, overexpression of wildtype small heat shock proteins leads to increased G6PD expression and protection against oxidative stress. The finding in this study, that reduction of G6PD can be beneficial in some circumstances, is also not without precedent. Some studies have suggested a link between G6PD deficiency and protection against cardiovascular disease in humans, although such findings have not been replicated in larger patient studies. Previous work has shown that G6PD reduction is highly beneficial in one specific case — when human CryABR120G is expressed in the mouse heart. Although these are differing outcomes, instead this study concludes that the effect of modifying G6PD levels may range from beneficial to deleterious, with the outcome determined by the constellation of genetic variation present in individuals' genomes and the environmental stressors that they experience (Xie, 2013).

To generate an experimental paradigm suited to rapid genetic exploration CryABR120G was expressed in the fly eye and it was found that it strongly disturbs normal development and pattern formation. The eye phenotype is also responsive to altered G6PD levels, validating it as a model for investigation of the underlying mechanism of CryABR120G pathology. Unlike an earlier report, abnormal eye phenotype that could be attributed to expression of the wild-type human CryAB gene was not found. In the one case where an eye phenotype was observed, it most likely resulted from induced expression of the esg gene that lay adjacent to that particular insertion of the CryAB transgene. These findings indicate that CryABR120G induces cellular dysfunction in both the heart and the eye, or lethality if expressed ubiquitously, while wild-type CryAB is relatively benign (Xie, 2013).

The human disease produced by the CryABR120G allele is sometimes called Desmin-Related Myopathy (DRM), owing to the presence of desmin in the characteristic cytoplasmic protein aggregates, and to similarities with diseases caused by mutations in the gene encoding the intermediate filament desmin. Although Drosophila do not have a desmin homolog, several lines of evidence argue that the cellular dysfunction in Drosophila resulting from CryABR120G expression is, nonetheless, a legitimate model for this disease. First, a number of other proteins have been identified within the aggregates, including (at least) another small heat shock protein and G6PD, both of which have homologs in Drosophila. Second, CryABR120G causes the formation of cytoplasmic aggregates even when expressed in human cell types that do not express desmin, and it was shown that a portion of the CryABR120G is also found in aggregates in Drosophila. Third, the identical response of mouse and Drosophila CryABR120G pathologies to G6PD reduction strongly suggests an identical mechanism of action (Xie, 2013).

The pyridine nucleotides NADH and NADPH are essential co-factors of oxidative and reductive enzymatic processes involved in energetics, oxidative metabolism, redox homeostasis, calcium homeostasis, macromolecular biosynthesis, mitochondrial functions, gene expression, aging and cell death. In this study, the effect of altered levels of the four enzymes that are primarily responsible for reducing NADP to NADPH were examined: two enzymes of the pentose phosphate pathway, G6PD and PGD which together account for ∼40% of NADPH levels in the adult; MEN which generates pyruvate for import into mitochondria and accounts for another ∼30%; and, IDH, which accounts for ∼20% of NADPH. These enzymes constitute a metabolic network linked by a common substrate (NADP) and interacting regulation. A major finding was that, even though G6PD, PGD, MEN and IDH carry out varied metabolic reactions, alterations to any of their activities have significant consequences for the phenotypes produced by CryABR120G expression, implying a common mechanism of action through NADP/H (Xie, 2013).

It was found that reduction of IDH is less effective at CryABR120G suppression than reductions of G6PD, PGD or MEN, a result that is not entirely surprising. Alteration of either G6PD or PGD activity is likely to affect both activities coordinately since they constitute sequential steps in the tightly regulated pentose phosphate pathway, and MEN by itself produces more NADPH than any of the other three enzymes of this network. IDH produces less NADPH than either MEN or the G6PD/PGD couple. Additionally, RNAi-mediated knockdowns of IDH were relatively ineffective. What is surprising was that knockdown of the mitochondrial NAD-dependent IDH results in significant suppression of the CryABR120G phenotype. The study surmises that mitochondrial metabolism affects the cytoplasmic NADP/H network (Xie, 2013).

The NADP∶NADPH redox couple, and the linked glutathione redox couple (GSSG∶GSH), participate in a diverse array of biological processes. Therefore, a number of possible mechanisms through which CryABR120G could alter the cellular redox potential and thereby contribute to toxicity can be envisioned. Most obviously, redox-sensitive sequestration of both existing and newly synthesized proteins could seriously disturb cellular regulation. The function of many proteins depends on the reduced or oxidized state of thiol-containing cysteine residues. It is conceivable that structurally flexible hydrophobic protein surfaces, which are normally buried within a folded protein, could engage in non-productive protein-protein interactions, in part, due to alterations of intra-chain disulfide links. Partially folded proteins might, in this way, become soluble toxic intermediates. Alternatively, misfolding might occur as a result of alterations in other redox-sensitive post-translational modifications, for example glutathionylation, nitrosylation, and (de)acetylation. Co-aggregation of several distinct polypeptides might cripple multiple disparate functions within the cell. Results from this study also suggest that alterations in mitochondrial homeostasis and energy metabolism could affect the levels of oxidized or reduced NADP/H. The reciprocal is most certainly true as well, with normal mitochondrial function dependent upon the function of the NADP-reducing enzyme MEN. Additionally there are scores of enzymes that use NADP/H as a cofactor, and the activity of one or more of these enzymes could be affected to generate the phenotypes were observed. It will require significant further work to identify the critical determinants of NADP/H involvement in CryABR120G pathology. Drosophila provides powerful tools for genetic screening to identify such factors and the model for CryABR120G pathology that is described in this study provides a context for carrying out such screens (Xie, 2013).

Go to top

Yu, L., Daniels, J., Glaser, A.E. and Wolf, M.J. (2013). Raf-mediated cardiac hypertrophy in adult Drosophila. Dis Model Mech 6: 964-976. PubMed ID: 23580199

Abstract
In response to stress and extracellular signals, the heart undergoes a process called cardiac hypertrophy during which cardiomyocytes increase in size. If untreated, cardiac hypertrophy can progress to overt heart failure that causes significant morbidity and mortality. The identification of molecular signals that cause or modify cardiomyopathies is necessary to understand how the normal heart progresses to cardiac hypertrophy and heart failure. Receptor tyrosine kinase (RTK) signaling is essential for normal human cardiac function, and the inhibition of RTKs can cause dilated cardiomyopathies. However, neither investigations of activated RTK signaling pathways nor the characterization of hypertrophic cardiomyopathy in the adult fly heart has been previously described. This study developed strategies using Drosophila as a model to circumvent some of the complexities associated with mammalian models of cardiovascular disease. Transgenes encoding activated EGFRA887T, Ras85DV12 and Ras85DV12S35, which preferentially signal to Raf, or constitutively active human or fly Raf caused hypertrophic cardiomyopathy as determined by decreased end diastolic lumen dimensions, abnormal cardiomyocyte fiber morphology and increased heart wall thicknesses. There were no changes in cardiomyocyte cell numbers. Additionally, activated Raf also induced an increase in cardiomyocyte ploidy compared with control hearts. However, preventing increases in cardiomyocyte ploidy using fizzy-related (Fzr) RNAi did not rescue Raf-mediated cardiac hypertrophy, suggesting that Raf-mediated polyploidization is not required for cardiac hypertrophy. Similar to mammals, the cardiac-specific expression of RNAi directed against MEK or ERK rescued Raf-mediated cardiac hypertrophy. However, the cardiac-specific expression of activated ERKD334N, which promotes hyperplasia in non-cardiac tissues, did not cause myocyte hypertrophy. These results suggest that ERK is necessary, but not sufficient, for Raf-mediated cardiac hypertrophy (Yu, 2013).

Highlights

  • Activated EGFR or Ras causes cardiac hypertrophy in adult flies.
  • Ras-mediated cardiac hypertrophy occurs via signals directed to Raf.
  • Activated fly or human Raf causes cardiac hypertrophy and an increase in cardiomyocyte ploidy in adult flies.
  • ERK is necessary, but not sufficient, for the development of cardiac hypertrophy in adult Drosophila.

Discussion
The inhibition of EGF ligand or EGFR by dominant-negative EGFR molecules results in enlarged cardiac chambers in adult flies. This study shows that the cardiac-specific expression of activated EGFR, Ras or Raf (human or fly) causes decreased heart chamber lumens. These results raise the question: how does the lumen of a heart composed of pairs of single myocytes undergo reduction or enlargement (i.e. dilation)? One proposed explanation is that molecular signals that drive the addition of sarcomeres added in series produce an eccentric hypertrophy and resultant enlarged heart lumen. Conversely, signals that promote either the addition of sarcomeres in parallel or myofiber disarray produce enlarged myocytes and resultant concentric hypertrophy in the fly. This explanation is consistent with a model proposed earlier that shows that ERKs regulate the balance between eccentric and concentric cardiac growth in mammals. Transgenic mice that have genetically ablated ERK1 and ERK2 in the heart (Erk1−/−, Erk2fl/flαMHC-Cre mice) develop an eccentric hypertrophy, and isolated cardiomyocytes from these mice display elongation. Results from this study show that the inhibition of ERK or MEK causes a thinner cardiomyocyte and in some cases an enlargement in EDDs, consistent with findings in mammalian models (Yu, 2013).

The expression of activated EGFR, Ras or Raf promotes cell proliferation in the eye or wing. The activation of these pathway components in the fly heart causes cardiac hypertrophy. Myocyte polyploidy was observed in control hearts and increase in myocyte ploidy in the RafAct hearts suggests incomplete endocycling, repetitive rounds of genome replication. An increase in cell ploidy is achieved through endoreplication, in which genomic DNA content increases without cellular division. Endoreplication is a common occurrence among species and, in the fly, some cells can have up to 2048 copies of the euchromatic genome. This process has been described as an effective strategy for cell growth, often found in differentiated cells that are large or have high metabolic activity (Yu, 2013).

Prior studies have shown changes in cardiomyocyte DNA content, ploidy level and nuclear number in mammalian hearts across multiple species, including humans. Post-mortem examinations of human hearts has demonstrated that the degree of polyploidy closely correlates with myocardial hypertrophy. Endoreduplication has been observed in mouse cardiomyocytes after cardiac injury. Augmented endoreplication, and an increase in ploidy, might represent a mechanism by which cells can adapt to high metabolic demands. Therefore, growth conditions or stimuli that promote cardiac hypertrophy might cause the myocyte to adapt by increasing ploidy, thereby providing more copies of essential genes required to respond to these cues (Yu, 2013).

It was found that Fzr RNAi preventa Raf-mediated increases in cardiomyocyte ploidy. Interestingly, the cardiac-specific expression of Fzr RNAi does not significantly change the degree of Raf-mediated cardiac hypertrophy. Therefore, the cardiac hypertrophy observed in the context of activated Raf does not require increased polyploidization. One interpretation of these findings is that the signals emanating from Raf bifurcate towards two distinct pathways: one that drives DNA replication and one that promotes cardiac hypertrophy (Yu, 2013).

The results further support the concept that the cellular context in which signaling molecules are expressed defines the cell growth response, namely hyperplasia or hypertrophy. During fly development, signals from EGFR are required for the specification and diversification of embryonic muscle progenitors, including cardiac cells. Somatic muscles and the cardiac cells develop from specialized progenitors. Each progenitor cell divides asymmetrically and produces two founder cells that specify individual muscle cell fates and give rise to multinucleate myofibers. It has been shown earlier that hyperactive EGFR, using early mesodermal GAL4 drivers including twist-GAL4 or 24B-GAL4, generates supernumerary mesodermal founder cells and causes a duplication of dorsal acute muscle 1 (DA1), a larval body wall muscle. Moreover, Wingless and the TGFβ family member Decapentaplegic, two members of the signals from the Wnt family, prepattern the mesoderm and render cells competent to respond to Ras-MAPK activation. Thus, the timing of EGFR and Ras signals during development and the integration of cellular cues dictates the response of myocytes, including the cardiomyocytes (Yu, 2013).

This study used the cardiac driver tinC-Gal4, which is expressed later in mesodermal development: this might explain why the increase in cardiomyocytes was not observed that has been observed with drivers that are expressed earlier in mesodermal development. These findings suggest that adult Drosophila cardiomyocytes lose the ability to proliferate when EGFR signals are activated after cardiomyocyte differentiation, similar to observations in mammals. Whether this lack of myocyte proliferation is specific to EGFR, Ras and Raf or is more generalizable to other stimuli remains to be investigated (Yu, 2013).

In mice, the indirect activation of ERKs via upstream signaling molecules has been shown to cause cardiac hypertrophy. Transgenic knock-in mice expressing RafL613V, a mutation that is associated with Noonan syndrome, develop cardiac hypertrophy through the activation of the MEK-ERK pathway. The cardiac-specific expression of MEK produces concentric hypertrophy attributed to the activation of ERK. The homozygous deletion of ERK1 prevents valvular abnormalities caused by selective overexpression of an activated mutant of protein tyrosine phosphatase, non-receptor type 11 (Shp2Q79R) in the developing endocardial cushions. However, studies of the cardiac-specific expression of activated transgenic ERKs on cardiac hypertrophy are lacking. Thus, these prior studies cannot rule out ERK-independent signals that contribute to Raf-mediated cardiac hypertrophy (Yu, 2013).

Findings from this study suggest that the activation of ERK is necessary, but not sufficient, for the development of Raf-mediated cardiac hypertrophy. This suggests that ERK-independent signals emanating from Raf function in concert with ERK to produce cardiac hypertrophy. Alternatively, the mutation in ERKD334N might differentially influence downstream effector molecules that promote cell proliferation in tissues receptive to cell cycle progression (i.e. the eye and wing) but do not produce a growth response in the fly heart. Alternatively, the inability of ERKD334N to produce cardiac hypertrophy might be explained by the degree to which this allele is ‘activated’ such that it might suffice to cause visible effects in the eyes and wings but perhaps be insufficient for producing effects in the heart. In either case, the fly model could provide a means to identify ERK-independent signals that promote hypertrophy or are differentially regulated to control cell proliferation and growth in different tissues (Yu, 2013).

Go to top

Casad, M.E., Yu, L., Daniels, J.P., Wolf, M.J. and Rockman, H.A. (2012). Deletion of Siah-interacting protein gene in Drosophila causes cardiomyopathy. Mol Genet Genomics 287: 351-360. PubMed ID: 22398840

Abstract
Drosophila is a useful model organism in which to study the genetics of human diseases, including recent advances in identification of the genetics of heart development and disease in the fly. To identify novel genes that cause cardiomyopathy, this study performed a deficiency screen in adult Drosophila. Using optical coherence tomography to phenotype cardiac function in awake adult Drosophila, the Df(1)Exel6240 was identified as having cardiomyopathy. Using a number of strategies including customized smaller deletions, screening of mutant alleles, and transgenic rescue, CG3226 was identified as the causative gene for this deficiency. CG3226 is an uncharacterized gene in Drosophila possessing homology to the mammalian Siah-interacting-protein (SIP) gene. Mammalian SIP functions as an adaptor protein involved in one of the β-catenin degradation complexes. To investigate the effects of altering β-catenin/Armadillo signaling in the adult fly, heart function was measured in flies expressing either constitutively active Armadillo or transgenic constructs that block Armadillo signaling, specifically in the heart. While increasing Armadillo signaling in the heart does not have an effect on adult heart function, decreasing Armadillo signaling in the fly heart causes significant reduction in heart chamber size. In summary, it was shown that deletion of CG3226, which has homology to mammalian SIP, causes cardiomyopathy in adult Drosophila. Alterations in Armadillo signaling during development lead to important changes in the size and function of the adult heart (Casad, 2012).

Highlights

  • Deficiency Df(1)Exel6240 causes cardiomyopathy.
  • Identification of the causative gene within Df(1)Exel6240.
  • CG3226 is the candidate gene for the abnormal cardiac function of Df(1)Exel6240.
  • Reduction of Armadillo signaling results in a small heart.

Go to top


Reviews

Qian, L. and Bodmer, R. (2012). Probing the polygenic basis of cardiomyopathies in Drosophila. J Cell Mol Med 16: 972-977. PubMed ID: 22268758

Go to top


More in IF

ER stress might contribute to the dilated cardiomyopathy in cases of lamin associated muscular dystrophy

SMAD signaling drives heart and muscle dysfunction in a Drosophila model of muscular dystrophy

Huntington's disease induced cardiac amyloidosis is reversed by modulating protein folding and oxidative stress pathways in the Drosophila heart

Role of eyes absent (eya) in cadiomyopathy


Go to top

Back to Drosophila as a Model for Human Diseases


Date revised: 20 August 2015

Home page: The Interactive Fly © 2015 Thomas Brody, Ph.D.

The Interactive Fly resides on the web server of the Society for Developmental Biology