Single-cell profiling of the developing embryonic heart in Drosophila

AlkTango reveals a role for Jeb/Alk signaling in the Drosophila heart

Anaplastic lymphoma kinase (Alk) signaling is important in a variety of biological contexts such as cell type specification, regulation of metabolic and endocrine programs, behavior, and cancer. In this work, we generated a Tango GPCR assay-based, dimerization-sensitive Alk activity reporter (AlkTango) and followed receptor activation throughout Drosophila development. AlkTango reports Alk activation in embryonic and larval tissues previously linked to Alk signaling. Remarkably, AlkTango was active in the heart of Drosophila larvae and adult flies. We show that cardiomyocytes express Alk from late embryonic stages to adulthood, while jeb expression in pericardial cells coincided with AlkTango activity. Perturbation of cardiac Alk signaling leads to decreased adult survival as well as lower fitness and increased lethality in response to heat stress. In keeping with a role for Alk, heart measurements reveal arrythmia and irregular muscle contraction upon ligand stimulation. Finally, activation of cardiac Alk signaling induces hyperplasia in the accessory wing hearts of adult flies.

Mutual Regulation of Cardiovascular and Hematopoietic Development

PURPOSE OF REVIEW

The cardiovascular and hematopoietic systems share molecular mechanisms and regulatory interactions across species. Endocardial hematopoiesis, a debated topic in mice, is actually an evolutionarily conserved process from Drosophila. This review explores the origins and significance of endocardial hematopoiesis, highlighting its role in cardiac development and macrophage formation.

RECENT FINDINGS

Despite extensive lineage-tracing and transcriptome studies, it remained unclear until single-cell RNA sequencing (scRNA-seq) identified that endocardial cells possess an intrinsic hematopoietic program independent of known hematopoietic organs. These endocardial-derived macrophages contribute uniquely to cardiac morphogenesis, supporting valve maturation and tissue remodeling. Endocardial hematopoiesis is an evolutionarily conserved phenomenon that is essential for developmental process. The heterogeneity of tissue-resident macrophages and their specialized functions in cardiac development have been further unraveled by single-cell analysis. This review provides an evolutionary perspective on endocardial hematopoiesis and highlights its critical contributions of hematopoietic cells to heart formation and homeostasis.

Single-cell RNA sequencing reveals the gene expression profile and cellular communication in human fetal heart development

The heart is the central organ of the circulatory system, and its proper development is vital to maintain human life. As fetal heart development is complex and poorly understood, we use single-cell RNA sequencing to profile the gene expression landscapes of human fetal hearts from the four-time points: 8, 10, 11, 17 gestational weeks (GW8, GW10, GW11, GW17), and identified 11 major types of cells: erythroid cells, fibroblasts, heart endothelial cells, ventricular cardiomyocytes, atrial cardiomyocytes, macrophage, DCs, smooth muscle, pericytes, neural cells, schwann cells. In addition, we identified a series of differentially expressed genes and signaling pathways in each cell type between different gestational weeks. Notably, we found that ANNEXIN, MIF, PTN, GRN signalling pathways were simple and fewer intercellular connections in GW8, however, they were significantly more complex and had more intercellular communication in GW10, GW11, and GW17. Notably, the interaction strength of OSM signalling pathways was gradually decreased during this period of time (from GW8 to GW17). Together, in this study, we presented a comprehensive and clear description of the differentiation processes of all the main cell types in the human fetal hearts, which may provide information and reference data for heart regeneration and heart disease treatment.

Actin Cytoskeleton and Integrin Components Are Interdependent for Slit Diaphragm Maintenance in Drosophila Nephrocytes

In nephrotic syndrome, the podocyte filtration structures are damaged in a process called foot process effacement. This is mediated by the actin cytoskeleton; however, which actins are involved and how they interact with other filtration components, like the basement membrane, remains poorly understood. Here, we used the well-established Drosophila pericardial nephrocyte-the equivalent of podocytes in flies-knockdown models (RNAi) to study the interplay of the actin cytoskeleton (Act5C, Act57B, Act42A, and Act87E), alpha- and beta-integrin (basement membrane), and the slit diaphragm (Sns and Pyd). Knockdown of an actin gene led to variations of formation of actin stress fibers, the internalization of Sns, and a disrupted slit diaphragm cortical pattern. Notably, deficiency of Act5C, which resulted in complete absence of nephrocytes, could be partially mitigated by overexpressing Act42A or Act87E, suggesting at least partial functional redundancy. Integrin localized near the actin cytoskeleton as well as slit diaphragm components, but when the nephrocyte cytoskeleton or slit diaphragm was disrupted, this switched to colocalization, both at the surface and internalized in aggregates. Altogether, the data show that the interdependence of the slit diaphragm, actin cytoskeleton, and integrins is key to the structure and function of the Drosophila nephrocyte.

Distinct roles of COMPASS subunits to Drosophila heart development

The multiprotein complexes known as the complex of proteins associated with Set1 (COMPASS) play a crucial role in the methylation of histone 3 lysine 4 (H3K4). In Drosophila, the COMPASS series complexes comprise core subunits Set1, Trx, and Trr, which share several common subunits such as ash2, Dpy30-L1, Rbbp5, and wds, alongside their unique subunits: Wdr82 for Set1/COMPASS, Mnn1 for Trx/COMPASS-like, and Ptip for Trr/COMPASS-like. Our research has shown that flies deficient in any of these common or unique subunits exhibited high lethality at eclosion (the emergence of adult flies from their pupal cases) and significantly shortened lifespans of the few adults that do emerge. Silencing these common or unique subunits led to severe heart morphological and functional defects. Moreover, specifically silencing the unique subunits of the COMPASS series complexes, Wdr82, Mnn1, and Ptip, in the heart results in decreased levels of H3K4 monomethylation and dimethylation, consistent with effects observed from silencing the core subunits Set1, Trx, and Trr. These findings underscore the critical roles of each subunit of the COMPASS series complexes in regulating histone methylation during heart development and provide valuable insights into their potential involvement in congenital heart diseases, thereby informing ongoing research in heart disease.

Distinct Roles for COMPASS Core Subunits Set1, Trx, and Trr in the Epigenetic Regulation of Drosophila Heart Development

Highly evolutionarily conserved multiprotein complexes termed Complex of Proteins Associated with Set1 (COMPASS) are required for histone 3 lysine 4 (H3K4) methylation. Drosophila Set1, Trx, and Trr form the core subunits of these complexes. We show that flies deficient in any of these three subunits demonstrated high lethality at eclosion (emergence of adult flies from their pupal cases) and significantly shortened lifespans for the adults that did emerge. Silencing Set1, trx, or trr in the heart led to a reduction in H3K4 monomethylation (H3K4me1) and dimethylation (H3K4me2), reflecting their distinct roles in H3K4 methylation. Furthermore, we studied the gene expression patterns regulated by Set1, Trx, and Trr. Each of the COMPASS core subunits controls the methylation of different sets of genes, with many metabolic pathways active early in development and throughout, while muscle and heart differentiation processes were methylated during later stages of development. Taken together, our findings demonstrate the roles of COMPASS series complex core subunits Set1, Trx, and Trr in regulating histone methylation during heart development and, given their implication in congenital heart diseases, inform research on heart disease.

APOL1-G2 accelerates nephrocyte cell death by inhibiting the autophagy pathway

People of African ancestry who carry the APOL1 risk alleles G1 or G2 are at high risk of developing kidney diseases through not fully understood mechanisms that impair the function of podocytes. It is also not clear whether the APOL1-G1 and APOL1-G2 risk alleles affect these cells through similar mechanisms. Previously, we have developed transgenic Drosophila melanogaster lines expressing either the human APOL1 reference allele (G0) or APOL1-G1 specifically in nephrocytes, the cells homologous to mammalian podocytes. We have found that nephrocytes that expressed the APOL1-G1 risk allele display accelerated cell death, in a manner similar to that of cultured human podocytes and APOL1 transgenic mouse models. Here, to compare how the APOL1-G1 and APOL1-G2 risk alleles affect the structure and function of nephrocytes in vivo, we generated nephrocyte-specific transgenic flies that either expressed the APOL1-G2 or both G1 and G2 (G1G2) risk alleles on the same allele. We found that APOL1-G2- and APOL1-G1G2-expressing nephrocytes developed more severe changes in autophagic pathways, acidification of organelles and the structure of the slit diaphragm, compared to G1-expressing nephrocytes, leading to their premature death. We conclude that both risk alleles affect similar key cell trafficking pathways, leading to reduced autophagy and suggesting new therapeutic targets to prevent APOL1 kidney diseases.