Analysis of Drosophila cardiac hypertrophy by microcomputerized tomography for genetic dissection of heart growth mechanisms

Septins regulate heart contractility through modulation of cardiomyocyte store-operated calcium entry

Highly regulated cardiomyocyte Ca 2+ fluxes drive heart contractions. Recent findings from multiple organisms demonstrate that the specific Ca 2+ transport mechanism known as store-operated Ca 2+ entry (SOCE) is essential in cardiomyocytes for proper heart function, and SOCE dysregulation results in cardiomyopathy. Mechanisms that regulate SOCE in cardiomyocytes are poorly understood. Here we tested the role of cytoskeletal septin proteins in cardiomyocyte SOCE regulation. Septins are essential SOCE modulators in other cell types, but septin functions in cardiomyocytes are nearly completely unexplored. We show using targeted genetics and intravital imaging of heart contractility in Drosophila that cardiomyocyte-specific depletion of septins 1, 2, and 4 results in heart dilation that phenocopies the effects of SOCE suppression. Heart dilation caused by septin 2 depletion was suppressed by SOCE upregulation, supporting the hypothesis that septin 2 is required in cardiomyocytes for sufficient SOCE function. A major function of SOCE is to support SERCA-dependent sarco/endoplasmic reticulum (S/ER) Ca 2+ stores, and augmenting S/ER store filling by SERCA overexpression also suppressed the septin 2 phenotype. We also ruled out several potential SOCE-independent septin functions, as septin 2 phenotypes were not due to septin function during development and septin 2 was not required for z-disk organization as defined by α-actinin labeling. These results demonstrate, for the first time, an essential role of septins in cardiomyocyte physiology and heart function that is due, at least in part, to septin regulation of SOCE function.

Attention LSTM U-Net model for Drosophila melanogaster heart tube segmentation in optical coherence microscopy images

Optical coherence microscopy (OCM) imaging of the Drosophila melanogaster (fruit fly) heart tube has enabled the non-invasive characterization of fly heart physiology in vivo. OCM generates large volumes of data, making it necessary to automate image analysis. Deep-learning-based neural network models have been developed to improve the efficiency of fly heart image segmentation. However, image artifacts caused by sample motion or reflections reduce the accuracy of the analysis. To improve the precision and efficiency of image data analysis, we developed an Attention LSTM U-Net model (FlyNet3.0), which incorporates an attention learning mechanism to track the beating fly heart in OCM images. The new model has improved the intersection over union (IOU) compared to FlyNet2.0 + with reflection artifacts from 86% to 89% and with movement from 81% to 89%. We also extended the capabilities of OCM analysis through the introduction of an automated, in vivo heart wall thickness measurement method, which has been validated on a Drosophila model of cardiac hypertrophy. This work will enable the comprehensive, non-invasive characterization of fly heart physiology in a high-throughput manner.

The opportunities and challenges of using Drosophila to model human cardiac diseases

The Drosophila heart tube seems simple, yet it has notable anatomic complexity and contains highly specialized structures. In fact, the development of the fly heart tube much resembles that of the earliest stages of mammalian heart development, and the molecular-genetic mechanisms driving these processes are highly conserved between flies and humans. Combined with the fly's unmatched genetic tools and a wide variety of techniques to assay both structure and function in the living fly heart, these attributes have made Drosophila a valuable model system for studying human heart development and disease. This perspective focuses on the functional and physiological similarities between fly and human hearts. Further, it discusses current limitations in using the fly, as well as promising prospects to expand the capabilities of Drosophila as a research model for studying human cardiac diseases.

Opposing effects of genetic variation in MTCH2 for obesity versus heart failure

Genetic variation in genes regulating metabolism may be advantageous in some settings but not others. The non-failing adult heart relies heavily on fatty acids as a fuel substrate and source of ATP. In contrast, the failing heart favors glucose as a fuel source. A bootstrap analysis for genes with deviant allele frequencies in cardiomyopathy cases versus controls identified the MTCH2 gene as having unusual variation. MTCH2 encodes an outer mitochondrial membrane protein, and prior genome-wide studies associated MTCH2 variants with body mass index, consistent with its role in metabolism. We identified the referent allele of rs1064608 (p.Pro290) as being overrepresented in cardiomyopathy cases compared to controls, and linkage disequilibrium analysis associated this variant with the MTCH2 cis eQTL rs10838738 and lower MTCH2 expression. To evaluate MTCH2, we knocked down Mtch in Drosophila heart tubes which produced a dilated and poorly functioning heart tube, reduced adiposity and shortened life span. Cardiac Mtch mutants generated more lactate at baseline, and they displayed impaired oxygen consumption in the presence of glucose but not palmitate. Treatment of cardiac Mtch mutants with dichloroacetate, a pyruvate dehydrogenase kinase inhibitor, reduced lactate and rescued lifespan. Deletion of MTCH2 in human cells similarly impaired oxygen consumption in the presence of glucose but not fatty acids. These data support a model in which MTCH2 reduction may be favorable when fatty acids are the major fuel source, favoring lean body mass. However, in settings like heart failure, where the heart shifts toward using more glucose, reduction of MTCH2 is maladaptive.

The SOCE Machinery: An Unbalanced Knowledge between Left and Right Ventricular Pathophysiology

Right ventricular failure (RVF) is the most important prognostic factor for morbidity and mortality in pulmonary arterial hypertension (PAH) or pulmonary hypertension (PH) caused by left heart diseases. However, right ventricle (RV) remodeling is understudied and not targeted by specific therapies. This can be partly explained by the lack of basic knowledge of RV remodeling. Since the physiology and hemodynamic function of the RV differ from those of the left ventricle (LV), the mechanisms of LV dysfunction cannot be generalized to that of the RV, albeit a knowledge of these being helpful to understanding RV remodeling and dysfunction. Store-operated Ca²⁺ entry (SOCE) has recently emerged to participate in the LV cardiomyocyte Ca²⁺ homeostasis and as a critical player in Ca²⁺ mishandling in a pathological context. In this paper, we highlight the current knowledge on the SOCE contribution to the LV and RV dysfunctions, as SOCE molecules are present in both compartments. he relative lack of studies on RV dysfunction indicates the necessity of further investigations, a significant challenge over the coming years.

Reduction of Drosophila Mitochondrial RNase P in Skeletal and Heart Muscle Causes Muscle Degeneration, Cardiomyopathy, and Heart Arrhythmia

In this study, we examine the cause and progression of mitochondrial diseases linked to the loss of mtRNase P, a three-protein complex responsible for processing and cleaving mitochondrial transfer RNAs (tRNA) from their nascent transcripts. When mtRNase P function is missing, mature mitochondrial tRNA levels are decreased, resulting in mitochondrial dysfunction. mtRNase P is composed of Mitochondrial RNase P Protein (MRPP) 1, 2, and 3. MRPP1 and 2 have their own enzymatic activity separate from MRPP3, which is the endonuclease responsible for cleaving tRNA. Human mutations in all subunits cause mitochondrial disease. The loss of mitochondrial function can cause devastating, often multisystemic failures. When mitochondria do not provide enough energy and metabolites, the result can be skeletal muscle weakness, cardiomyopathy, and heart arrhythmias. These symptoms are complex and often difficult to interpret, making disease models useful for diagnosing disease onset and progression. Previously, we identified Drosophila orthologs of each mtRNase P subunit (Roswell/MRPP1, Scully/MRPP2, Mulder/MRPP3) and found that the loss of each subunit causes lethality and decreased mitochondrial tRNA processing in vivo. Here, we use Drosophila to model mtRNase P mitochondrial diseases by reducing the level of each subunit in skeletal and heart muscle using tissue-specific RNAi knockdown. We find that mtRNase P reduction in skeletal muscle decreases adult eclosion and causes reduced muscle mass and function. Adult flies exhibit significant age-progressive locomotor defects. Cardiac-specific mtRNase P knockdowns reduce fly lifespan for Roswell and Scully, but not Mulder. Using intravital imaging, we find that adult hearts have impaired contractility and exhibit substantial arrhythmia. This occurs for roswell and mulder knockdowns, but with little effect for scully. The phenotypes shown here are similar to those exhibited by patients with mitochondrial disease, including disease caused by mutations in MRPP1 and 2. These findings also suggest that skeletal and cardiac deficiencies induced by mtRNase P loss are differentially affected by the three subunits. These differences could have implications for disease progression in skeletal and heart muscle and shed light on how the enzyme complex functions in different tissues.