Posts in Category: Cardiac Myocyte Biology

Mending a Faltering Heart

Mending a Faltering Heart

Mo Li, Juan Carlos Izpisua Belmonte

Recent strategies for cardiomyocyte regeneration. Cardiomyocytes can be induced to re-enter the cell cycle through modulation of endogenous cardiomyocyte proliferation programs. Reprogramming to induced pluripotent stem cell (iPSCs) followed by directed differentiation could provide a large number of immature cardiomyocytes. Other noncardiomyocyte cells, such as fibroblasts, can be directly converted to induced cardiomyocytes (iCMs). These in vitro derived cardiomyocytes may be further matured using tissue engineering technologies. In situ conversion of cardiac fibroblasts also represents a promising strategy for heart repair. MI indicates myocardial infarction. [Powerpoint File]

Cardiac Nonmyocytes in the Hub of Cardiac Hypertrophy

Cardiac Nonmyocytes in the Hub of Cardiac Hypertrophy

Takehiro Kamo, Hiroshi Akazawa, Issei Komuro

Intercellular communications between cardiomyocytes and nonmyocytes in the development of cardiac hypertrophy. Myocardial cell responses are regulated by intercellular communications between cardiomyocytes and nonmyocytes, such as fibroblasts, endothelial cells, and inflammatory cells. Cardiomyocytes secrete angiogenic factors to induce angiogenesis. An increase in capillary mass leads to increase in supply of nutrients and O2 necessary for hypertrophic growth. In addition, endothelial cells produce a variety of paracrine factors that exert prohypertrophic and antihypertrophic responses. During the development of cardiac hypertrophy, localized proliferation of both epicardium-derived and endocardium-derived fibroblast lineages contributes to extracellular matrix protein deposition and cardiac fibrosis. A wide variety of paracrine and autocrine factors mediate the intercellular communications between cardiac fibroblasts and cardiomyocytes in cardiac hypertrophy. During the development of cardiac hypertrophy, macrophages accumulate in ventricle both through localized proliferation and recruitment of circulating monocytes. Cardiac macrophages and mast cells are involved in induction of cardiac hypertrophy, although mechanistic link between T cells and the development of cardiac hypertrophy remains unclear. [Powerpoint File]

Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes: Insights Into Molecular, Cellular, and Functional Phenotypes

Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes: Insights Into Molecular, Cellular, and Functional Phenotypes

Ioannis Karakikes, Mohamed Ameen, Vittavat Termglinchan, Joseph C. Wu

Expression of key structural and functional genes in induced pluripotent stem cell–derived cardiomyocytes (iPSC-CMs). A, Schematic of the major structural and functional features of iPSC-CMs. In adult CMs, on membrane depolarization a small amount of Ca2+ influx induced by activation of voltage-dependent L-type Ca2+ channels (CACNAC1) triggers the release of Ca2+ from the sarcoplasmic reticulum (SR) through the ryanodine receptors (ryanodine receptor 2 [RYR2]), termed Ca2+-induced Ca2+-release mechanism. The released Ca2+ ions diffuse through the cytosolic space and bind to troponin C (TNNC1), resulting in the release of inhibition induced by troponin I (TNNI3), which activates the sliding of thin and thick filaments, and lead to cardiac contraction. Recovery occurs as Ca2+ is extruded by the Na2+/Ca2+ exchanger (NCX1) and returned to the SR by the sarco(endo)plasmic Ca2+-ATPase pumps on the nonjunctional region of the SR that are regulated by phospholamban (PLN). This process is conserved in iPSC-CMs, but major differences exist, such as a nascent SR, the presence of an inositol 1,4,5-trisphosphate–releasable Ca2+ pool, and the complete absence of T-tubules. The genes encoding the major transmembrane ion channels involved in the generation of action potential are also shown. B, Immunofluorescence staining of cardiac troponin T and α-sarcomeric actinin in iPSC-CMs. C, Line-scan images and spontaneous Ca2+ transients in iPSC-CMs. ACTC1 indicates actin, α, cardiac muscle 1; CACNA1C, calcium channel, voltage-dependent, L type, α1C subunit; IPTR3, inositol 1,4,5-trisphosphate receptor, type 3; KCNH2, potassium voltage-gated channel, subfamily H (eag-related), member 2; KCNIP2, potassium channel–interacting protein 2; KCNJ2, potassium inwardly rectifying channel, subfamily J, member 2; KCNQ1, potassium voltage-gated channel, KQT-like subfamily, member 1; MYBPC3, myosin binding protein C; MYH6, myosin, heavy chain 6, α; MYH7, myosin, heavy chain 7, β; MYL2, myosin, light chain 2; MYL3, myosin, light chain 3; SCN5A, sodium channel, voltage-gated, type V, α-subunit; SERCA2, SR Ca2+-ATPase 2; TNNI3, troponin I type 3; TNNT2, troponin T type 2; TTN, titin; and TPM1, tropomyosin 1 (α). [Powerpoint File]

Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes: Insights Into Molecular, Cellular, and Functional Phenotypes

Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes: Insights Into Molecular, Cellular, and Functional Phenotypes

Ioannis Karakikes, Mohamed Ameen, Vittavat Termglinchan, Joseph C. Wu

Current applications of patient-specific induced pluripotent stem cell–derived cardiomyocyte (iPSC-CM) technology. iPSC-CMs have been used for disease modeling of inherited cardiomyopathies and channelopathies, regenerative therapies, drug discovery, and cardiotoxicity testing, as well as for studying metabolic abnormalities and cardiac development. [Powerpoint File]

Proteotoxicity and Cardiac Dysfunction

Proteotoxicity and Cardiac Dysfunction

Patrick M. McLendon, Jeffrey Robbins

Schematic diagram of selected protection strategies or proteotoxic pathways in the cardiomyocyte. Strategy 1: Rescue. The cell’s chaperones, which can be constitutively expressed or inducible, act alone or in concert to maintain or restore the peptide’s functional conformation. Strategy 2: Degradation. The misfolded protein is recognized by the protein quality control machinery, ubiquitinated, and trafficked to the proteasome for degradation and amino acid recycling. Strategy 3: Sequestration. The misfolded protein forms larger aggregates, which are unable to be processed by the proteasome. These aggregates can be attached to dynein motors and transported on microtubules to a perinuclear location where the potentially proteotoxic peptides are sequestered. These aggregates can be cleared from the cytoplasm by autophagy (see text for details).37,38,39 Strategy 4: ERAD. The endoplasmic reticulum associated degradation (ERAD) pathway is complex and a simplified schematic diagram simply outlines the process. Mutated or terminally misfolded proteins are recognzied by ER-associated proteins that may have either enzymatic or chaperone activities. The protein is then shepherded to the retrotranslocation machinery whose exact identity(ies) remain obscure. Membrane-associated E3 ligases and associated proteins are able to mediate ubiquitination of the protein, which may partially drive its retrotranslocation. There are multiple checkpoints and components underlying these processes as well (see text for details). The ubiquitinated proteins are then targeted for proteasomal degradation. [Powerpoint File]

Proteotoxicity and Cardiac Dysfunction

Proteotoxicity and Cardiac Dysfunction

Patrick M. McLendon, Jeffrey Robbins

Selective cargo recognition and degradation by autophagy. Step 1, Ubiquitin binding is necessary for most modes of selective autophagy. Polyubiquitin conjugation onto a substrate protein is accomplished via an E3 ubiquitin ligase. Many of these proteins are ubiquitinated via a K63 linkage, which signals lysosomal degradation rather than the K48 linkage that is typical for proteasomal targeting. Step 2, Ubiquitinated proteins can be recognized by cargo receptors, such as p62, NBR1, or histone deacetylase (HDAC)6 for transport towards the lysosome or aggresome. p62 (icon) binds ubiquitinated proteins for interaction with LC3 in a growing autophagosome, thus selectively targeting the protein for autophagic degradation after fusion with lysosomes. In addition, HDAC6 can bind ubiquitinated proteins through its ubiquitin-binding domain (icon) and transport them in a retrograde fashion along microtubules by binding to the motor protein dynein through the dynein motor–binding domain (icon). These can be single misfolded species or soluble proto-aggregates. This event can happen many times, causing the proteins to coalesce and form aggresomes in the perinuclear region of the cell. Aggresomal proteins can also be degraded by autophagy, and these cargo transport proteins may be involved in this process as well, particularly HDAC6 with known roles in mediating the autophagosome–lysosome fusion event. [Powerpoint File]

Maintaining Ancient Organelles: Mitochondrial Biogenesis and Maturation

Maintaining Ancient Organelles: Mitochondrial Biogenesis and Maturation

Rick B. Vega, Julie L. Horton, Daniel P. Kelly

Peroxisome-proliferator activated receptor γ (PPARγ) coactivator-1α (PGC-1α) mediates physiological control of mitochondrial biogenesis and function. The transcriptional coactivator PGC-1α interacts directly with multiple transcription factors to integrate upstream signaling events with mitochondrial biogenesis and functional capacity. The downstream transcription factors control virtually every aspect of mitochondrial function and energy production, including biogenesis, dynamics, and maintenance of protein levels. The control of PGC-1α expression and activity is dynamic, responding to multiple intracellular second messengers and signaling molecules transmitting inputs from various physiological and metabolic stimuli (top). AMPK indicates AMP-activated protein kinase; CaMK, calmodulin-dependent kinase; CN, calcineurin; CREB, cAMP-response element binding protein; ERR, estrogen-related receptor; ETC/OXPHOS, electron transport chain/oxidative phosphorylation; NAD+, nicotinamide adenine dinucleotide; and NRF, nuclear respiratory factor. [Powerpoint FIle]

Maintaining Ancient Organelles: Mitochondrial Biogenesis and Maturation

Maintaining Ancient Organelles: Mitochondrial Biogenesis and Maturation

Rick B. Vega, Julie L. Horton, Daniel P. Kelly

Two predominant models of mtDNA replication are shown here. Both models concur the replisome consists of at least a helicase, T7 gp4-like protein with intramitochondrial nucleoid localization (TWINKLE; orange) and polymerase γ (POLG; yellow). Leading strand synthesis begins at OH and lagging strand synthesis at OL (red arrow). A, Strand-displacement model proposes single-stranded binding proteins (green spheres) bind the displaced H-strand during leading strand replication. B, Alternatively, the ribonucleotide incorporation throughout the lagging strand (RITOLS) model proposes portions of transcribed mitochondrial DNA bind the H-strand (green dotted line). [Powerpoint File]

Getting to the Heart of the Matter: New Insights Into Cardiac Fibrosis

Getting to the Heart of the Matter: New Insights Into Cardiac Fibrosis

Andrew Leask

Cellular interactions in cardiac fibrosis. The cells involved with the interactions described in Figure 1 are shown. Note that profibrotic proteins can originate from several different sources; however, the final effector cell of scarring is the myofibroblast. Ang-II indicates angiotensin II; ECM, extracellular matrix; ET, endothelin-1; IL, interleukin; PDGFD, platelet-derived growth factor D; and TGF-β, transforming growth factor-β (illustration credit: Ben Smith). [Powerpoint File]

Getting to the Heart of the Matter: New Insights Into Cardiac Fibrosis

Getting to the Heart of the Matter: New Insights Into Cardiac Fibrosis

Andrew Leask

Transforming growth factor-β (TGF-β) signaling in fibroblasts. TGF-β is a major fibrogenic cytokine. Latent TGF-β is released by integrin-mediated contraction, binds to its type I and II receptors, and activates the canonical Smad3/4 pathway and the noncanonical TGF-β–activated kinase-1 (TAK1)/p38/c-Jun N-terminal kinase (JNK) and NADPH oxidase 4 (NOX4)/reactive oxygen species (ROS) pathway resulting in induction of fibrogenic genes, such as α-smooth muscle actin (α-SMA), collagen, and CCN2. Myofibroblasts may perpetuate their fibrotic phenotype at least in part via this pathway (illustration credit: Ben Smith). [Powerpoint File]

“String Theory” of c-kitpos Cardiac Cells: A New Paradigm Regarding the Nature of These Cells That May Reconcile Apparently Discrepant Results

“String Theory” of c-kitpos Cardiac Cells: A New Paradigm Regarding the Nature of These Cells That May Reconcile Apparently Discrepant Results

Matthew C.L. Keith, Roberto Bolli

Proposed origin of c-kitpos cells in the adult heart. The figure illustrates the concept that, in the adult heart, c-kitpos cells are intermediates derived from c-kitneg epicardial progenitors that undergo epithelial-to-mesenchymal transition (EMT). (EMT is known to be associated with expression of c-kit.) These c-kitpos intermediates have a mesenchymal phenotype and give rise to c-kitneg epicardium-derived cells (EPDCs) with differentiation potential limited to noncardiomyocytic lineages. A, Wilm’s tumor protein (WT)-1+/T-box transcription factor (Tbx) 18+ epicardial progenitors contribute predominantly to smooth muscle and cardiac adventitial fibroblasts, with minimal endothelial cell formation. B, Sema3D+/Scx+ epicardial progenitors (distinct from WT-1+/Tbx18+ cells) form vascular endothelium and cardiac fibroblasts and contribute minimally to smooth muscle cells. Both of these epicardium-derived c-kitpos intermediates express CD105 and possess a mesenchymal phenotype with canonical mesenchymal stromal/stem cell (MSC) markers; neither of these two populations has shown any significant ability to form myocytes. The expression of c-kit in these epicardium-derived intermediates is postulated to be high relative to that of c-kitpos intermediates from the FHF (shown in Figure 1), which have bipotent cardiomyocyte and smooth muscle differentiation capacity. This differential c-kit expression among cardiac progenitors is currently a conjecture and has not been demonstrated experimentally; nevertheless, it is inferred from the results of the van Berlo study, which did not detect significant myogenesis from first heart field c-kitpos progenitors but detected robust evidence of a c-kitpos progenitor of fibroblasts, smooth muscle cells, and endothelial cells (all of which are of epicardial origin), and from the proposed insensitivity of the van Berlo model to low levels of c-kit expression, which could theoretically result in underestimation of low expressers of c-kit. Shown below the figure is the relative contribution of mesenchymally transitioned EPDCs to cardiac lineages of the adult heart; this pattern is consistent with studies of adult c-kitpos cells, which have shown preferential adventitial and vasculogenic differentiation and a paucity of direct cardiomyogenic potential. [Powerpoint File]

Investigating the Transcriptional Control of Cardiovascular Development

Investigating the Transcriptional Control of Cardiovascular Development

Irfan S. Kathiriya*, Elphège P. Nora*, Benoit G. Bruneau

Studying cardiac regulatory elements. A, Left, Evolutionary constraints on some regulatory elements render them identifiable by comparative genomics, as exemplified by enhancers upstream of mouse Nkx2-5.86 Right, Combinations of specific chromatin features can reveal potential regulatory elements active in a given cell type. B, Left, Enhancer activity is classically tested by an ability of candidate elements to drive tissue- or stage-specific activity of a reporter. Middle, New technologies, such as SIF-seq,87 allow screening of large genomic neighborhoods for tissue-specific enhancers. Right, Genome Regulatory Organization Mapping with Integrated Transposons92 is a technology that can reveal integrated regulatory inputs exerted at a locus. [Powerpoint FIle]

Investigating the Transcriptional Control of Cardiovascular Development

Investigating the Transcriptional Control of Cardiovascular Development

Irfan S. Kathiriya*, Elphège P. Nora*, Benoit G. Bruneau

Molecular players for transcriptional regulation. A, Cis-regulatory elements containing DNA-binding sites are bound by transcription factors and (B) modulate the assembly of the preinitiation complex at promoters through (C) physical contacts driven by a 3-dimensional arrangement of chromatin, thereby acting as (D) a molecular platform between cellular signaling and gene activity. [Powerpoint File]

Molecular Mechanisms of Autophagy in the Cardiovascular System

Molecular Mechanisms of Autophagy in the Cardiovascular System

Damián Gatica, Mario Chiong, Sergio Lavandero, Daniel J. Klionsky

Autophagy regulation. Through serine/threonine kinase 11 (STK11/LKB1), AMP-activated protein kinase (AMPK) senses decreases in the ATP/AMP ratio and phosphorylates tuberous sclerosis (TSC) 1-TSC2, which then targets Ras homolog enriched in brain (RHEB), leading to mechanistic target of rapamycin complex 1 (MTORC1) inhibition and autophagy activation. insulin receptor/insulin-like growth factor 1 receptor (INSR/IGF1R) triggers the activation of the class I phosphoinositide 3-kinase (PI3K), inducing the formation of phosphatidylinositol(3,4,5)trisphosphate (PIP3) and AKT/PKB activation; AKT can inhibit TSC1-TSC2, blocking autophagy. Phosphatase and tensin homolog (PTEN) works as a PIP3 phosphatase generating phosphatidylinositol(4,5)bisphosphate (PIP2) and inducing autophagy. [Powerpoint File]

Molecular Mechanisms of Autophagy in the Cardiovascular System

Molecular Mechanisms of Autophagy in the Cardiovascular System

Damián Gatica, Mario Chiong, Sergio Lavandero, Daniel J. Klionsky

The 3 main types of autophagy. A, Yeast cells carry out both macroautophagy and microautophagy. Although macroautophagy consists of the bulk degradation of cytoplasmic material that is sequestered inside double-membrane autophagosomes that then fuse with the vacuole, microautophagy works by directly taking up the substrates through invagination of the vacuole. B, In mammals along with microautophagy and macroautophagy, chaperone-mediated autophagy enables the degradation of specific protein substrates that contain a KFERQ motif that is recognized by chaperones that mediate the translocation of the unfolded protein into the lysosome through LAMP2A. [Powerpoint File]