Mitochondria are secreted in extracellular vesicles when lysosomal function is impaired

Mitochondrial quality control is critical for cardiac homeostasis as these organelles are responsible for generating most of the energy needed to sustain contraction. Dysfunctional mitochondria are normally degraded via intracellular degradation pathways that converge on the lysosome. Here, we identified an alternative mechanism to eliminate mitochondria when lysosomal function is compromised. We show that lysosomal inhibition leads to increased secretion of mitochondria in large extracellular vesicles (EVs). The EVs are produced in multivesicular bodies, and their release is independent of autophagy. Deletion of the small GTPase Rab7 in cells or adult mouse heart leads to increased secretion of EVs containing ubiquitinated cargos, including intact mitochondria. The secreted EVs are captured by macrophages without activating inflammation. Hearts from aged mice or Danon disease patients have increased levels of secreted EVs containing mitochondria indicating activation of vesicular release during cardiac pathophysiology. Overall, these findings establish that mitochondria are eliminated in large EVs through the endosomal pathway when lysosomal degradation is inhibited.

The Small GTPase Rab7 Regulates Release of Mitochondria in Extracellular Vesicles in Response to Lysosomal Dysfunction

Mitochondrial quality control is critical for cardiac homeostasis as these organelles are responsible for generating most of the energy needed to sustain contraction. Dysfunctional mitochondria are normally degraded via intracellular degradation pathways that converge on the lysosome. Here, we identified an alternative mechanism to eliminate mitochondria when lysosomal function is compromised. We show that lysosomal inhibition leads to increased secretion of mitochondria in large extracellular vesicles (EVs). The EVs are produced in multivesicular bodies, and their release is independent of autophagy. Deletion of the small GTPase Rab7 in cells or adult mouse heart leads to increased secretion of EVs containing ubiquitinated cargos, including intact mitochondria. The secreted EVs are captured by macrophages without activating inflammation. Hearts from aged mice or Danon disease patients have increased levels of secreted EVs containing mitochondria indicating activation of vesicular release during cardiac pathophysiology. Overall, these findings establish that mitochondria are eliminated in large EVs through the endosomal pathway when lysosomal degradation is inhibited.

Cardiolipin Remodeling Defects Impair Mitochondrial Architecture and Function in a Murine Model of Barth Syndrome Cardiomyopathy

BACKGROUND

Cardiomyopathy is a major clinical feature in Barth syndrome (BTHS), an X-linked mitochondrial lipid disorder caused by mutations in Tafazzin (TAZ), encoding a mitochondrial acyltransferase required for cardiolipin remodeling. Despite recent description of a mouse model of BTHS cardiomyopathy, an in-depth analysis of specific lipid abnormalities and mitochondrial form and function in an in vivo BTHS cardiomyopathy model is lacking.

METHODS

We performed in-depth assessment of cardiac function, cardiolipin species profiles, and mitochondrial structure and function in our newly generated Taz cardiomyocyte-specific knockout mice and Cre-negative control mice (n≥3 per group).

RESULTS

Taz cardiomyocyte-specific knockout mice recapitulate typical features of BTHS and mitochondrial cardiomyopathy. Fewer than 5% of cardiomyocyte-specific knockout mice exhibited lethality before 2 months of age, with significantly enlarged hearts. More than 80% of cardiomyocyte-specific knockout displayed ventricular dilation at 16 weeks of age and survived until 50 weeks of age. Full parameter analysis of cardiac cardiolipin profiles demonstrated lower total cardiolipin concentration, abnormal cardiolipin fatty acyl composition, and elevated monolysocardiolipin to cardiolipin ratios in Taz cardiomyocyte-specific knockout, relative to controls. Mitochondrial contact site and cristae organizing system and F1F0-ATP synthase complexes, required for cristae morphogenesis, were abnormal, resulting in onion-shaped mitochondria. Organization of high molecular weight respiratory chain supercomplexes was also impaired. In keeping with observed mitochondrial abnormalities, seahorse experiments demonstrated impaired mitochondrial respiration capacity.

CONCLUSIONS

Our mouse model mirrors multiple physiological and biochemical aspects of BTHS cardiomyopathy. Our results give important insights into the underlying cause of BTHS cardiomyopathy and provide a framework for testing therapeutic approaches to BTHS cardiomyopathy, or other mitochondrial-related cardiomyopathies.

Mcl-1-mediated mitochondrial fission protects against stress but impairs cardiac adaptation to exercise

Myeloid cell leukemia-1 (Mcl-1) is a structurally and functionally unique anti-apoptotic Bcl-2 protein. While elevated levels of Mcl-1 contribute to tumor cell survival and drug resistance, loss of Mcl-1 in cardiac myocytes leads to rapid mitochondrial dysfunction and heart failure development. Although Mcl-1 is an anti-apoptotic protein, previous studies indicate that its functions extend beyond regulating apoptosis. Mcl-1 is localized to both the mitochondrial outer membrane and matrix. Here, we have identified that Mcl-1 in the outer mitochondrial membrane mediates mitochondrial fission, which is independent of its anti-apoptotic function. We demonstrate that Mcl-1 interacts with Drp1 to promote mitochondrial fission in response to various challenges known to perturb mitochondria morphology. Induction of fission by Mcl-1 reduces nutrient deprivation-induced cell death and the protection is independent of its BH3 domain. Finally, cardiac-specific overexpression of Mcl-1_OM, but not Mcl-1_Matrix, contributes to a shift in the balance towards fission and leads to reduced exercise capacity, suggesting that a pre-existing fragmented mitochondrial network leads to decreased ability to adapt to an acute increase in workload and energy demand. Overall, these findings highlight the importance of Mcl-1 in maintaining mitochondrial health in cells.

Nuclear Parkin Activates the ERRα Transcriptional Program and Drives Widespread Changes in Gene Expression Following Hypoxia

Parkin is an E3 ubiquitin ligase well-known for facilitating clearance of damaged mitochondria by ubiquitinating proteins on the outer mitochondrial membrane. However, knowledge of Parkin's functions beyond mitophagy is still limited. Here, we demonstrate that Parkin has functions in the nucleus and that Parkinson's disease-associated Parkin mutants, ParkinR42P and ParkinG430D, are selectively excluded from the nucleus. Further, Parkin translocates to the nucleus in response to hypoxia which correlates with increased ubiquitination of nuclear proteins. The serine-threonine kinase PINK1 is responsible for recruiting Parkin to mitochondria, but translocation of Parkin to the nucleus occurs independently of PINK1. Transcriptomic analyses of HeLa cells overexpressing wild type or a nuclear-targeted Parkin revealed that during hypoxia, Parkin contributes to both increased and decreased transcription of genes involved in regulating multiple metabolic pathways. Furthermore, a proteomics screen comparing ubiquitinated proteins in hearts from Parkin-/- and Parkin transgenic mice identified the transcription factor estrogen-related receptor α (ERRα) as a potential Parkin target. Co-immunoprecipitation confirmed that nuclear-targeted Parkin interacts with and ubiquitinates ERRα. Further analysis uncovered that nuclear Parkin increases the transcriptional activity of ERRα. Overall, our study supports diverse roles for Parkin and demonstrates that nuclear Parkin regulates transcription of genes involved in multiple metabolic pathways.

Abstract 435: SUMOylation Site in MCL-1 Regulates its Anti-Apoptotic Activity

Myeloid Cell Leukemia-1 (MCL-1) is an anti-apoptotic BCL-2 protein upregulated in various types of cancer. It is also highly expressed in the myocardium. We previously found that MCL-1 plays a key role in maintaining mitochondrial homeostasis and myocyte survival. Cardiac specific deletion of MCL-1 in myocytes leads to mitochondrial dysfunction, loss of myocytes, and rapid development of heart failure. However, MCL-1’s function in the heart is unclear and the exact mechanism by which MCL-1 protects against apoptotic stimuli is not fully understood. Studies have reported that MCL-1 is subjected to phosphorylation and ubiquitination at several different residues which leads to changes in its stability. MCL-1 has a short half-life and proteasomal-mediated degradation of MCL-1 allows for apoptosis to proceed. Here, we screened the amino acid sequence of MCL-1 to identify novel sites of post-translational modifications involved in regulating MCL-1 function. We discovered the presence of a sumoylation site at lysine 219 that is conserved between species. To examine the function of the site, we generated a mutant of MCL-1 where the lysine residue was mutated to an arginine, MCL-1K219R. First, we investigated whether the sumoylation site was involved in regulating protein stability of MCL-1, but we found no differences in the degradation rates between MCL-1 and MCL-1K219R at baseline and after staurosporine treatment. Interestingly, MCL-1K219R failed to protect against staurosporine-induced apoptosis, while both MCL-1 and MCL-1K219R protected against doxorubicin-mediated cell death. This suggest that the mechanism of doxorubicin-mediated cell death is different from staurosporine. Moreover, MCL-1 has been reported to prevent activation of apoptosis via the mitochondrial pathway by sequestering pro-apoptotic BCL-2 proteins such as PUMA and NOXA. Hence, we examined whether MCL-1 sumoylation is involved in regulating its interaction with pro-apoptotic proteins. A better understanding of how MCL-1 regulates cell survival will not only aid the development of drugs that combat harmful cells, but will also help the production of drugs that effectively protect limited and valuable cells like myocytes.

Abstract 105: MCL-1 Promotes Drp1-Mediated Mitochondrial Fission as an Adaptive Response to Stress

The anti-apoptotic BCL-2 family member, Myeloid Cell Leukemia-1 (MCl-1), is highly expressed in myocardium and plays a critical role in maintaining mitochondrial homeostasis. We have previously found that cardiomyocyte-specific MCL-1 knockout mice develop rapid cardiac dysfunction and cardiomyopathy but show little activation of apoptotic cell death. Instead, loss of MCL-1 resulted in atypical mitochondrial morphology and function. This suggests that besides its anti-apoptotic role, MCL-1 may have broader functions in regulating mitochondrial dynamics and function. MCL-1 localizes to two different mitochondrial locations in myocytes. One form exists on the outer mitochondrial membrane (MCL-1 OM ) and a shorter cleaved form resides in the mitochondrial matrix (MCL-1 Matrix ). We found that overexpression of MCL-1 WT or MCL-1 OM , but not MCL-1 Matrix , induces fragmentation and perinuclear aggregation of the mitochondria in a Drp1-dependent manner. Mutating MCL-1’s BH3 domain (G198E D199A), which is required for MCL-1’s anti-apoptotic function, completely abrogates its ability to induce perinuclear aggregation. Interestingly, a MCL-1-BCL-2 chimera, in which MCL-1’s BH domains are replaced with those of BCL-2, is still able to induce perinuclear aggregation. This suggests that the presence of a functional BH3 domain is sufficient for induction of perinuclear aggregation. We confirmed that there is increased interaction between endogenous MCL-1 and Drp1 in response to a variety of fission-promoting stimuli, including glucose deprivation, hypoxia, and treatment with rotenone or FCCP. MCL-1 overexpression also protects against cell death in response to these stimuli, but this protection is abrogated when Drp1 is knocked down using siRNA. Additionally, Drp1 levels are significantly increased at the mitochondria in the hearts of MCL-1 OM transgenic mice, and this increase corresponds to an increased interaction between MCL-1 and Drp1 in these transgenic mice. Consistent with these findings, many of the mitochondria in these transgenic mice appear to be smaller than those from the WT mice, indicative of enhanced mitochondrial fission. Thus, our data suggests that MCL-1 functions as a positive regulator of Drp1-mediated mitochondrial fission.

Abstract 865: The E3 Ubiquitin Ligase Parkin Regulates Metabolism From the Nucleus

Parkin facilitates mitophagy by ubiquitinating depolarized mitochondria to label them for degradation. We have previously shown that Parkin is important for adaptation to myocardial infarction (MI) and that loss of Parkin leads to accumulation of dysfunctional mitochondria. Although Parkin is mainly studied for its role at the mitochondria, it is unclear whether Parkin also functions in other subcellular compartments. Here, we used immunofluorescence and subcellular fractionation to find that a portion of Parkin localizes to the nucleus under basal conditions in vitro and in vivo . Pathogenic Parkin mutants, however, are selectively excluded from the nucleus. To further examine conditions that induce Parkin nuclear enrichment, we subjected HeLa cells stably expressing YFP-Parkin to starvation or hypoxia. During nutrient-deprivation, Parkin rapidly exits the nucleus and accumulates in the cytosol. Conversely, exposure to hypoxia causes Parkin enrichment in the nucleus and depletion from the cytosol. To separate the roles of nuclear versus mitochondrial Parkin, we generated nuclear- and mitochondrial- targeted constructs: NLS-Parkin and Mito-Parkin. RNA-sequencing analysis revealed widespread transcriptional changes in response to targeting Parkin to the nucleus, particularly in metabolic processes. Through a non-biased proteomics screen, we identified the transcription factor estrogen related receptor α (ERRα) as a potential Parkin target. ERRα is highly expressed in the heart and promotes transcription of genes involved in mitochondrial biogenesis and metabolism. We therefore used co-immunoprecipitation and qPCR to determine whether Parkin regulates cellular energetics through ERRα. Indeed, we found that NLS-Parkin interacts with and ubiquitinates ERRα. Additionally, expressing NLS-Parkin increases protein levels of both endogenous and overexpressed ERRα and promotes transcription of ERRα-targets. These data indicate that Parkin-mediated ubiquitination of ERRα leads to its increased stability and activity resulting in increased transcription of genes involved in mitochondrial biogenesis and metabolism. This study provides insight into new ways Parkin may be targeted to enhance energetic function in the heart.

Abstract 166: MCL-1 Facilitates the Removal of Damaged Mitochondria via the Mitophagy Receptor BNIP3

Myeloid Cell Leukemia-1 (MCL-1) is an anti-apoptotic BCL-2 family protein that is necessary to maintain cardiac homeostasis in the adult heart. MCL-1 localizes to two distinct mitochondrial locations in myocytes, both on the outer mitochondrial membrane (MCL-1 OM ) and in the mitochondrial matrix (MCL-1 Matrix ). Our lab previously showed that cardiac-specific ablation of MCL-1 at both of these mitochondrial locations in mice led to severe contractile dysfunction and compromised mitochondrial function. Intriguingly, these defects were accompanied by signs of necrotic, rather than apoptotic, cell death. This indicates that MCL-1 has an alternate role in maintaining mitochondrial homeostasis in cardiac myocytes. Unexpectedly, we found that MCL-1 induces mitochondrial clearance in response to treatment with the chemical uncoupler FCCP in a Parkin-independent manner. Hypoxia is also known to induce mitochondrial clearance, and overexpression of MCL-1 further enhances hypoxia-mediated mitophagy. Fluorescence imaging identified MCL-1-positive mitochondria sequestered inside autophagosomes. MCL-1-mediated clearance is abrogated in autophagy deficient Atg5-/- cells, confirming that clearance is occurring via the autophagy pathway. Mutation of MCL-1’s BH3 domain (G198E D199A) does not affect its ability to induce clearance, suggesting that this role may be independent of its anti-apoptotic function. Also, replacing MCL-1’s BH domains with those of BCL-2, does not affect its ability to induce mitophagy. Next, we investigated whether MCL-1 functions as a mitophagy receptor and promotes removal of damaged mitochondria by binding to directly to LC3 through one or more of its three putative LC3-Interacting Region (LIR) motifs. Endogenous MCL-1 and LC3 co-immunoprecipitate in response to stress induced by FCCP. However, mutating each of MCL-1’s individual LIR motifs, as well as generating combined mutations in all three, does not affect MCL-1-mediated mitophagy. Instead, we found that MCL-1 interacts with the known mitophagy receptor BNIP3 both in vitro and in vivo . Thus, our data suggest that MCL-1 promotes elimination of dysfunctional mitochondria by positively regulating the mitophagy receptor BNIP3.

BNIP3L/NIX and FUNDC1-mediated mitophagy is required for mitochondrial network remodeling during cardiac progenitor cell differentiation

Cell-based therapies represent a very promising strategy to repair and regenerate the injured heart to prevent progression to heart failure. To date, these therapies have had limited success due to a lack of survival and retention of the infused cells. Therefore, it is important to increase our understanding of the biology of these cells and utilize this information to enhance their survival and function in the injured heart. Mitochondria are critical for progenitor cell function and survival. Here, we demonstrate the importance of mitochondrial autophagy, or mitophagy, in the differentiation process in adult cardiac progenitor cells (CPCs). We found that mitophagy was rapidly induced upon initiation of differentiation in CPCs. We also found that mitophagy was mediated by mitophagy receptors, rather than the PINK1-PRKN/PARKIN pathway. Mitophagy mediated by BNIP3L/NIX and FUNDC1 was not involved in regulating progenitor cell fate determination, mitochondrial biogenesis, or reprogramming. Instead, mitophagy facilitated the CPCs to undergo proper mitochondrial network reorganization during differentiation. Abrogating BNIP3L- and FUNDC1-mediated mitophagy during differentiation led to sustained mitochondrial fission and formation of donut-shaped impaired mitochondria. It also resulted in increased susceptibility to cell death and failure to survive the infarcted heart. Finally, aging is associated with accumulation of mitochondrial DNA (mtDNA) damage in cells and we found that acquiring mtDNA mutations selectively disrupted the differentiation-activated mitophagy program in CPCs. These findings demonstrate the importance of BNIP3L- and FUNDC1-mediated mitophagy as a critical regulator of mitochondrial network formation during differentiation, as well as the consequences of accumulating mtDNA mutations. Abbreviations: Baf: bafilomycin A₁; BCL2L13: BCL2 like 13; BNIP3: BCL2 interacting protein 3; BNIP3L: BCL2 interacting protein 3 like; CPCs: cardiac progenitor cells; DM: differentiation media; DNM1L: dynamin 1 like; EPCs: endothelial progenitor cells; FCCP: carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone; FUNDC1: FUN14 domain containing 1; HSCs: hematopoietic stem cells; MAP1LC3B/LC3: microtubule-associated protein 1 light chain 3 beta; MFN1/2: mitofusin 1/2; MSCs: mesenchymal stem cells; mtDNA: mitochondrial DNA; OXPHOS: oxidative phosphorylation; PPARGC1A: PPARG coactivator 1 alpha; PHB2: prohibitin 2; POLG: DNA polymerase gamma, catalytic subunit; SQSTM1: sequestosome 1; TEM: transmission electron microscopy; TMRM: tetramethylrhodamine methyl ester.

Abstract 280: MCL-1 Matrix Promotes Mitochondrial Fusion and Protects Against TAC Induced Hypertrophy

The anti-apoptotic BCL-2 protein MCL-1 is highly expressed in the myocardium, and we have previously found that deletion in cardiomyocytes leads to mitochondrial dysfunction and rapid development of heart failure. MCL-1 exists both in the outer mitochondrial membrane (MCL-1 OM ) and in the mitochondrial matrix (MCL-1 Matrix ). Because Mcl-1 -/- mice lack both MCL-1 Matrix and MCL-1 OM , the functional role of each form remains unknown. While studies have implicated MCL-1 OM in regulating apoptosis, very little is known about the functional role of MCL-1 Matrix . Here, we show that mitochondria are responsive to alterations in cellular metabolism, and culturing mouse embryonic fibroblasts (MEFs) in “oxidative” media containing galactose or acetoacetate led to a significant increase in the MCL-1 Matrix /MCL-1 OM ratio and elongation of mitochondria. We confirmed that MCL-1 Matrix overexpression promoted mitochondrial fusion and reduced maximal mitochondrial respiration under baseline conditions. Interestingly, the elongated mitochondria were initially protected against FCCP-induced fission and mitophagy. However, prolonged exposure to FCCP resulted in abrogation of MCL-1 Matrix import. Accumulation of MCL-1 Matrix on the outer membrane led to its degradation and subsequent induction of mitophagy. Furthermore, to investigate its functional role in the heart, we generated cardiac specific MCL-1 Matrix (αMHC-MCL-1 Matrix ) transgenic mice. The mice show no differences in cardiac structure and function compared to wild-type litter mates at 3 months. However, ultrastructural analysis revealed the presence of enlarged mitochondria in myocytes from αMHC-MCL-1 Matrix mice. Finally, we subjected WT and αMHC-MCL-1 Matrix mice to transverse aortic constriction (TAC) and we found that αMHC-MCL-1 Matrix mice had reduced heart weight to body weight (HW/BW) ratio and decreased expression of hypertrophy markers ANF, BNP, and MHC-β at two weeks post-TAC compared to WT. Thus, our studies indicate that cardiac overexpression of MCL-1 Matrix promotes mitochondrial enlargement in myocytes and alleviates stress placed on the heart during pressure overload.

Isolation of Rab5-positive endosomes reveals a new mitochondrial degradation pathway utilized by BNIP3 and Parkin

Degradation of mitochondria is an important cellular quality control mechanism mediated by two distinct pathways: one involving Parkin-mediated ubiquitination and the other dependent on mitophagy receptors. It is known that mitochondria are degraded by the autophagy pathway; however, we recently reported that the small GTPase Rab5 and early endosomes also participate in Parkin-mediated mitochondrial clearance. Here, we have developed a protocol to isolate Rab5-positive vesicles from cells for proteomics analysis and provide additional data confirming that mitophagy regulators and mitochondrial proteins are present in these vesicles. We also demonstrate that the mitophagy receptor BNIP3 utilizes the Rab5-endosomal pathway to clear mitochondria in cells. These findings indicate that a redundancy exists in the downstream degradation pathways to ensure efficient mitochondrial clearance.

A Rab5 endosomal pathway mediates Parkin-dependent mitochondrial clearance

Damaged mitochondria pose a lethal threat to cells that necessitates their prompt removal. The currently recognized mechanism for disposal of mitochondria is autophagy, where damaged organelles are marked for disposal via ubiquitylation by Parkin. Here we report a novel pathway for mitochondrial elimination, in which these organelles undergo Parkin-dependent sequestration into Rab5-positive early endosomes via the ESCRT machinery. Following maturation, these endosomes deliver mitochondria to lysosomes for degradation. Although this endosomal pathway is activated by stressors that also activate mitochondrial autophagy, endosomal-mediated mitochondrial clearance is initiated before autophagy. The autophagy protein Beclin1 regulates activation of Rab5 and endosomal-mediated degradation of mitochondria, suggesting cross-talk between these two pathways. Abrogation of Rab5 function and the endosomal pathway results in the accumulation of stressed mitochondria and increases susceptibility to cell death in embryonic fibroblasts and cardiac myocytes. These data reveal a new mechanism for mitochondrial quality control mediated by Rab5 and early endosomes.

Rapid Discovery of Functional Small Molecule Ligands against Proteomic Targets through Library-Against-Library Screening

Identifying “druggable” targets and their corresponding therapeutic agents are two fundamental challenges in drug discovery research. The one-bead-one-compound (OBOC) combinatorial library method has been developed to discover peptides or small molecules that bind to a specific target protein or elicit a specific cellular response. The phage display cDNA expression proteome library method has been employed to identify target proteins that interact with specific compounds. Here, we combined these two high-throughput approaches, efficiently interrogated approximately 10¹³ possible molecular interactions, and identified 91 small molecule compound beads that interacted strongly with the phage library. Of 19 compounds resynthesized, 4 were cytotoxic against cancer cells; one of these compounds was found to interact with EIF5B and inhibit protein translation. As more binding pairs are confirmed and evaluated, the “library-against-library” screening approach and the resulting small molecule–protein domain interaction database may serve as a valuable tool for basic research and drug development.

Hexamethylene amiloride engages a novel reactive oxygen species- and lysosome-dependent programmed necrotic mechanism to selectively target breast cancer cells

Anticancer chemotherapeutics often rely on induction of apoptosis in rapidly dividing cells. While these treatment strategies are generally effective in debulking the primary tumor, post-therapeutic recurrence and metastasis are pervasive concerns with potentially devastating consequences. We demonstrate that the amiloride derivative 5-(N,N-hexamethylene) amiloride (HMA) harbors cytotoxic properties particularly attractive for a novel class of therapeutic agent. HMA is potently and specifically cytotoxic toward breast cancer cells, with remarkable selectivity for transformed cells relative to non-transformed or primary cells. Nonetheless, HMA is similarly cytotoxic to breast cancer cells irrespective of their molecular profile, proliferative status, or species of origin, suggesting that it engages a cell death mechanism common to all breast tumor subtypes. We observed that HMA induces a novel form of caspase- and autophagy-independent programmed necrosis relying on the orchestration of mitochondrial and lysosomal pro-death mechanisms, where its cytotoxicity was attenuated with ROS-scavengers or lysosomal cathepsin inhibition. Overall, our findings suggest HMA may efficiently target the heterogeneous populations of cancer cells known to reside within a single breast tumor by induction of a ROS- and lysosome-mediated form of programmed necrosis.

Staying young at heart: autophagy and adaptation to cardiac aging

Aging is a predominant risk factor for developing cardiovascular disease. Therefore, the cellular processes that contribute to aging are attractive targets for therapeutic interventions that can delay or prevent the development of age-related diseases. Our understanding of the underlying mechanisms that contribute to the decline in cell and tissue functions with age has greatly advanced over the past decade. Classical hallmarks of aging cells include increased levels of reactive oxygen species, DNA damage, accumulation of dysfunctional organelles, oxidized proteins and lipids. These all contribute to a progressive decline in the normal physiological function of the cell and to the onset of age-related conditions. A major cause of the aging process is progressive loss of cellular quality control. Autophagy is an important quality control pathway and is necessary to maintain cardiac homeostasis and to adapt to stress. A reduction in autophagy has been observed in a number of aging models and there is compelling evidence that enhanced autophagy delays aging and extends life span. Enhancing autophagy counteracts age-associated accumulation of protein aggregates and damaged organelles in cells. In this review, we discuss the functional role of autophagy in maintaining homeostasis in the heart, and how a decline is associated with accelerated cardiac aging. We also evaluate therapeutic approaches being researched in an effort to maintain a healthy young heart.

Novel sorafenib-based structural analogues: in-vitro anticancer evaluation of t-MTUCB and t-AUCMB

In the current work, we carried out a mechanistic study on the cytotoxicity of two compounds, trans-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-N-methyl-benzamide (t-AUCMB) and trans-N-methyl-4-{4-[3-(4-trifluoromethoxy-phenyl)-ureido]-cyclohexyloxy}-benzamide (t-MTUCB), that are structurally similar to sorafenib. These compounds show strong cytotoxic responses in various cancer cell lines, despite significant differences in the induction of apoptotic events such as caspase activation and lactate dehydrogenase release in hepatoma cells. Both compounds induce autophagosome formation and LC3I cleavage, but there was little observable effect on mTORC1 or the downstream targets, S6K1 and 4E-binding protein. In addition, there was an increase in the activity of upstream signaling through the IRS1/PI3K/Akt-signaling pathway, suggesting that, unlike sorafenib, both compounds induce mammalian target of rapamycin (mTOR)-independent autophagy. The autophagy observed correlates with mitochondrial membrane depolarization, apoptosis-inducing factor release, and oxidative stress-induced glutathione depletion. However, there were no observable changes in the endoplasmic reticulum-stress markers such as binding immunoglobulin protein, inositol-requiring enzyme-α, phosphorylated eukaryotic initiation factor 2, and the lipid peroxidation marker, 4-hydroxynonenal, suggesting endoplasmic reticulum-independent oxidative stress. Finally, these compounds do not have the multikinase inhibitory activity of sorafenib, which may be reflected in their difference in the ability to halt cell cycle progression compared with sorafenib. Our findings indicate that both compounds have anticancer effects comparable with sorafenib in multiple cell lines, but they induce significant differences in apoptotic responses and appear to induce mTOR-independent autophagy. t-AUCMB and t-MTUCB represent novel chemical probes that are capable of inducing mTOR-independent autophagy and apoptosis to differing degrees, and may thus be potential tools for further understanding the link between these two cellular stress responses.

A cell-permeant amiloride derivative induces caspase-independent, AIF-mediated programmed necrotic death of breast cancer cells

Amiloride is a potassium-sparing diuretic that has been used as an anti-kaliuretic for the chronic management of hypertension and heart failure. Several studies have identified a potential anti-cancer role for amiloride, however the mechanisms underlying its anti-tumor effects remain to be fully delineated. Our group previously demonstrated that amiloride triggers caspase-independent cytotoxic cell death in human glioblastoma cell lines but not in primary astrocytes. To delineate the cellular mechanisms underlying amiloride’s anti-cancer cytotoxicity, cell permeant and cell impermeant derivatives of amiloride were synthesized that exhibit markedly different potencies in cancer cell death assays. Here we compare the cytotoxicities of 5-benzylglycinyl amiloride (UCD38B) and its free acid 5-glycinyl amiloride (UCD74A) toward human breast cancer cells. UCD74A exhibits poor cell permeability and has very little cytotoxic activity, while UCD38B is cell permeant and induces the caspase-independent death of proliferating and non-proliferating breast cancer cells. UCD38B treatment of human breast cancer cells promotes autophagy reflected in LC3 conversion, and induces the dramatic swelling of the endoplasmic reticulum, however these events do not appear to be the cause of cell death. Surprisingly, UCD38B but not UCD74A induces efficient AIF translocation from the mitochondria to the nucleus, and AIF function is necessary for the efficient induction of cancer cell death. Our observations indicate that UCD38B induces programmed necrosis through AIF translocation, and suggest that its cytosolic accessibility may facilitate drug action.

Abstract 595: A cell-permeant amiloride derivative induces caspase-independent, AIF-mediated programmed necrotic death of breast cancer cells

Amiloride is a potassium-sparing diuretic that has been used as an anti-kaliuretic for the chronic management of hypertension and heart failure. Several studies have identified a potential anti-cancer role for amiloride, however the mechanisms underlying its anti-tumor effects remain to be fully delineated. Our group previously demonstrated that amiloride triggers caspase-independent cytotoxic cell death in human glioblastoma cell lines but not in primary astrocytes. To delineate the cellular mechanisms underlying amiloride's anti-cancer cytotoxicity, cell permeant and cell impermeant derivatives of amiloride were synthesized that exhibit markedly different potencies in cancer cell death assays. Here we compare the cytotoxicities of 5-benzylglycinyl amiloride (UCD38B), and its free acid 5-glycinyl amiloride (UCD74A) toward human breast cancer cells. UCD74A exhibits poor cell permeability and has very little cytotoxic activity, while UCD38B is cell permeant and induces the caspase-independent death of proliferating and non-proliferating breast cancer cells. UCD38B treatment of human breast cancer cells induces autophagy, reflected in LC3 conversion, however this does not appear to be the cause of cell death. Electron microscopy reveals that UCD38B induces a dramatic swelling of intracellular organelles such as the endoplasmic reticulum and mitochondria. Surprisingly UCD38B, but not UCD74A, induces efficient AIF translocation from the mitochondria to the nucleus, and AIF function is necessary for the efficient induction of cancer cell death. Our observations indicate that UCD38B induces programmed necrosis through AIF translocation, and suggest that cytosolic accessibility may facilitate drug action.

Citation Format: Leonardo J. Leon, Nagarekha Pasupuleti, Fredric Gorin, Kermit Carraway. A cell-permeant amiloride derivative induces caspase-independent, AIF-mediated programmed necrotic death of breast cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 595. doi:10.1158/1538-7445.AM2013-595

Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.

Abstract 5489: An amiloride derivative capable of inducing ER stress and cell death in highly invasive breast cancer

Metastasis, which is the spread of tumors from their place of origin to other locations in the body via the lymph or blood, is associated with a significant increase in morbidity and mortality. Studies on amiloride, an FDA-approved diuretic, have shown it to suppress metastatic characteristics such as cell invasion and suppression, and impair their proliferation and viability. Here we show that 38B, a cell-permeant amiloride derivative, is more efficacious at causing cell death of cancer cells than amiloride. We also show that the cell death produced by 38B does not seem to be caused by apoptosis, autophagy, or calcium-dependent necrosis. However, bright field and immunofluorescence microscopy reveal vacuole formation and ER stress in cells, characteristics typically associated with a recently discovered type of cell death known as paraptosis. Since cancer cells have shown to be efficient at evading common forms of cell death such as apoptosis, the discovery of alternative pathways of cell death and a good understanding of the drugs that activate them can be can be key in the improvement of cancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5489. doi:10.1158/1538-7445.AM2011-5489

Atrial cell action potential parameter fitting using genetic algorithms

Understanding of the considerable variation in action potential (AP) shape throughout the heart is necessary to explain normal and pathological cardiac function. Existing mathematical models reproduce typical APs, but not all measured APs, as fitting the sets of non-linear equations is a tedious process. The study describes the integration of a pre-existing mathematical model of an atrial cell AP with a genetic algorithm to provide an automated tool to generate APs for arbitrary cells by fitting ionic channel conductances. Using the Nygren model as the base, the technique was first verified by starting with random values and fitting the Nygren model to itself with an error of only 0.03%. The Courtemanche model, which has a different morphology from that of the Nygren model, was successfully fitted. The AP duration restitution curve generated by the fit matched that of the target model very well. Finally, experimentally recorded APs were reproduced. To match AP duration restitution behaviour properly, it was necessary simultaneously to fit over several stimulation frequencies. Also, fitting of the upstroke was better if the stimulating current pulse replicated that found in situ as opposed to a rectangular pulse. In conclusion, the modelled parameters were successfully able to reproduce any given atrial AP. This tool can be useful for determining parameters in new AP models, reproducing specific APs, as well as determining the locus of drug action by examining changes in conductance values.

Apolipophorin III: lipopolysaccharide binding requires helix bundle opening

Apolipophorin III (apoLp-III) is a prototypical apolipoprotein used for structure-function studies. Besides its crucial role in lipid transport, apoLp-III is able to associate with fungal and bacterial membranes and stimulate cellular immune responses. We recently demonstrated binding interaction of apoLp-III of the greater wax moth, Galleria mellonella, with lipopolysaccharides (LPS). In the present study, the requirement of helix bundle opening for LPS binding interaction was investigated. Using site-directed mutagenesis, two cysteine residues were introduced in close spatial proximity (P5C/A135C). When the helix bundle was locked by disulfide bond formation, the tethered helix bundle failed to associate with LPS. In contrast, the mutant protein regained its ability to bind upon reduction with dithiothreitol. Thus, helix bundle opening is a critical event in apoLp-III binding interaction with LPS. This mechanism implies that the hydrophobic interior of the protein interacts directly with LPS, analogous to that observed for lipid interaction.

Tyrosine fluorescence analysis of apolipophorin III–lipopolysaccharide interaction

Apolipophorin III (apoLp-III) is an exchangeable apolipoprotein that binds to lipopolysaccharides (LPS). Polyacrylamide gel electrophoresis analysis demonstrated that apoLp-III from Galleria mellonella associated with various truncated LPS variants, including lipid A. Subsequent binding studies were performed employing the intrinsic tyrosine fluorescence properties of apoLp-III, which is highly quenched in the unbound state. A marked increase in tyrosine fluorescence intensity was observed upon binding to LPS or detoxified LPS, indicating a new microenvironment for Tyr-142. This also implies that the LPS carbohydrate region is involved in LPS binding. Dissociation constants (Kd) measured by apoLp-III titration were estimated at approximately 1 microM. Increasing the ionic strength did not decrease the Kd, neither did LPS phosphate removal. In addition, truncation apoLp-III mutants, lacking two complete helices, were still able to associate with LPS. This indicates that the association of apoLp-III with LPS may not be governed by charge but by hydrophobic interactions.

Cardiac near-field morphology during conduction around a microscopic obstacle--a computer simulation study

In a recent paper, we described the behavior of the cardiac electric near-field, E, parallel to the tissue surface during continuous conduction. We found that the tip of E describes a vector-loop during depolarization with the peak field, E, pointing opposite to the direction of propagation, phiI(m). Experimentally recorded loop morphologies of E, however, frequently showed significant deviations from the theoretically predicted behavior. We hypothesized that this variety of morphologies might be caused by conduction obstacles at a microscopic size scale. This study examines the influence of obstacles on the morphology of vector loops of E and whether the peak of distorted loops remains a reliable indicator for the direction of propagation. We used a computer model of a sheet of cardiac tissue with a central conduction obstacle immersed in an unbounded volume conductor. We studied the loop morphologies of E and the differences between the intracellularly determined direction of propagation, phiI(m), and the direction of E, phiE. Distortions of the vector loop were morphologically similar to those observed experimentally. Differences between phiI(m) and phiE were less than 18 degrees at all observation sites. The obstacle led to deformations of the loop morphology, particularly during the initial and terminal phases, and to a lesser degree near the instant of E. We concluded that E is a reliable indicator of phiI(m).

Effect of fibre rotation on the initiation of re-entry in cardiac tissue

Transmural rotation of cardiac fibres can have a big influence on the initiation of re-entry in the heart. However, owing to computational demands, this has not been fully explored in a three-dimensional model of cardiac tissue that has a microscopic description of membrane currents, such as the Luo-Rudy model. Using a previously described model that is computationally fast, re-entry in three-dimensional blocks of cardiac tissue is induced by a cross-shock protocol, and the activity is examined. In the study, the effect of the transmural fibre rotation is ascertained by examining differences between a tissue block with no rotation and ones with 1, 2 and 3 degrees of rotation per fibre layer. The direction of the re-entry is significant in establishing whether or not re-entry can be induced, with clockwise re-entry being easier to initiate. Owing to the rotating anisotropy that results in preferential propagation along the fibre axis, the timing of the second stimulus in the cross-shock protocol has to be changed for different rates of fibre rotation. The fibre rotation either increases or decreases the window of opportunity for re-entry, depending on whether the activation front is perpendicular or parallel to the fibre direction. By varying the transmural extent of the S2, it is found that a deeper stimulus has to be applied to the blocks with fibre rotation to create re-entry. Increasing the transmural resistance also tends to reduce the extent of the S2 required to induce re-entry. Results suggest that increasing fibre rotation reduces the susceptibility of the tissue to re-entry, but that more complex spatiotemporal patterns are possible, e.g. stable figure-of-eight re-entries and transient rotors. Three mechanisms of re-entry annihilation are identified: front catchup, filling of the excitable gap and core wander.

Electrophysiological basis of mono-phasic action potential recordings

Monophasic action potentials (MAPs) have been recorded for over a century, however, the exact mechanism responsible for their genesis has yet to be elucidated fully. The goal of the paper is to examine the physical basis of MAP recordings. MAP recordings are simulated by modelling a three-dimensional block of cardiac tissue. The effect of the MAP electrode is modelled by introducing a large, non-specific leakage conductance to the small region under the electrode. From the spread of the electrical activity, the equivalent extracellular current flow can be efficiently determined. These computed current sources are then input into a boundary element model of the tissue to determine the surface potentials. Finally, differences in surface potentials are used to compute waveforms that closely resemble MAP recordings. By varying model parameters, the mechanisms responsible for the MAP are determined, and a theory is put forward that can account for all observations. It is hypothesised that the leakage current causes the formation of a double-layer potential with a strength equal to the difference in transmembrane voltage between the regions under the electrode and those outside the electrode, leading to a recorded potential that mimics the transmembrane voltage outside the electrode region, although offset. Based on experimental MAP recordings, an equivalent leakage channel with a conductance of 0.1 mS cm-2 and a reversal potential of -43 mV is introduced by the electrode.

Computationally efficient model for simulating electrical activity in cardiac tissue with fiber rotation

Transmural rotation of cardiac fibers may have a large influence on the initiation, stabilization, and termination of several life threatening cardiac arrhythmias. However, three-dimensional modeling of reentry in cardiac tissue is computationally demanding, as a tissue on the order of centimeters in size must be used to sustain reentry and several seconds must be simulated. Numerical accuracy requires time steps on the order of microseconds and spatial discretization on the order of microns. Consequently, the resultant numerical systems are extremely large. In this article, a computationally efficient model of a three-dimensional block of cardiac tissue with fiber rotation is presented. Computational speedup is achieved by using a discrete cable model which allowed for system order reduction, and also by using a scheme for tracking the activation wave front which identified regions requiring integration with a small time step. Simulating 1.2 s of activity of the approximately 2 x 10(6) cells constituting a block measuring 2.0 x 4.0 x 0.29 cm was performed in 26 h. Effects of model parameters on performance are discussed. The effect of fiber rotation on the spread of electrical activity after point source stimulation and a cross shock protocol is clearly demonstrated.

Spatiotemporal evolution of ventricular fibrillation

Sudden cardiac death is the leading cause of death in the industrialized world, with the majority of such tragedies being due to ventricular fibrillation. Ventricular fibrillation is a frenzied and irregular disturbance of the heart rhythm that quickly renders the heart incapable of sustaining life. Rotors, electrophysiological structures that emit rotating spiral waves, occur in several systems that all share with the heart the functional properties of excitability and refractoriness. These re-entrant waves, seen in numerical solutions of simplified models of cardiac tissue, may occur during ventricular tachycardias. It has been difficult to detect such forms of re-entry in fibrillating mammalian ventricles. Here we show that, in isolated perfused dog hearts, high spatial and temporal resolution mapping of optical transmembrane potentials can easily detect transiently erupting rotors during the early phase of ventricular fibrillation. This activity is characterized by a relatively high spatiotemporal cross-correlation. During this early fibrillatory interval, frequent wavefront collisions and wavebreak generation are also dominant features. Interestingly, this spatiotemporal pattern undergoes an evolution to a less highly spatially correlated mechanism that lacks the epicardial manifestations of rotors despite continued myocardial perfusion.

Interactions between adjacent fibers in a cardiac muscle bundle

A strand of cardiac muscle was modeled as a small bundle of individual fibers surrounded by a large volume conductor. The bundle is a uniform assembly of small identical cylindrical fibers, arranged as a series of concentric layers, and its behavior is examined in the presence (coupled bundle) or absence (uncoupled bundle) of transverse resistive coupling between adjacent fibers. Individual fibers are continuous cables of excitable membrane, with circumferential segmentation into 12 equal patches to make the membrane potential changes dependent upon the local interstitial potential. The minimum spacing (d) between adjacent fibers is used to modify the interstitial microstructural organization and the intracellular volume fraction (fi). When d is small enough (d < 0.01 micron), fi remains unchanged at its maximum of about 90%, the interstitial potential is large, the transverse interstitial resistance is high, and the proximity effect arising from the close juxtaposition of adjacent fibers is important. A surface fiber of the uncoupled bundle exhibits little sensitivity to changes in the interstitial microstructure, owing to the dominant influence of the external volume conductor, whereas the central fiber shows a large decrease in velocity, substantial waveshape modifications, and a large increase in interstitial potential as d is reduced. In the coupled bundle, all fibers adopt the same velocity during uniform propagation, owing to the strong transverse resistive coupling; when d is reduced in the range of d < 0.01 micron, the velocity and interstitial potential changes are less pronounced than in the uncoupled bundle. When d is large enough (d > 0.01 micron), the bundle behavior (coupled and uncoupled) approaches that obtained with a bidomain formulation.

Propagation on a central fiber surrounded by inactive fibers in a multifibered bundle model

We studied uniform propagation on a central active fiber surrounded by inactive fibers in a multifibered bundle model lying in a large volume conductor. The behavior of a fully active bundle is considered in a companion paper. The bundle is formed by concentric layers of small cylindrical fibers (radius 5 microns), with a uniform minimum distance (d) between any two adjacent fibers, to yield a bundle radius of about 72 microns. Individual fibers are identical continuous cables of excitable membrane based on a modified Beeler-Reuter model. The intracellular volume fraction (fi) increases to a maximum of about 90% as d is reduced and remains unchanged for d < 0.01 micron. In the range of d < 0.01 micron, the central fiber is effectively shielded from external effects by the first concentric layer of inactive fibers, and a large capacitive load current flows across the surrounding inactive membranes. In addition, the fiber proximity produces a circumferentially nonuniform current density (proximity effect) that is equivalent to an increased average longitudinal interstitial resistance. The conduction velocity is reduced as d becomes smaller in the range of d < 0.1 micron, the interstitial potential becomes larger, and both the maximum rate of rise and time constant of the foot of the upstroke are increased. On the other hand, for d > 0.1 micron, there are negligible changes in the shape of the upstroke, and the behavior of the central fiber is close to that of a uniform cable in a restricted volume conductor. For d larger than about 1.2 microns, the active fiber environment is close to an unbounded isotropic volume conductor.

Calculation of transmembrane current from extracellular potential recordings: a model study

INTRODUCTION

A mathematical/computer model of cardiac tissue was used to study the estimation of transmembrane current (EIm) from extracellular potential recordings.

METHODS AND RESULTS

The simulated EIm of transmembrane current was compared with the simulated transmembrane current (Im), and both simulated values were compared with experimentally derived EIm obtained during sinus rhythm and ventricular fibrillation in dogs. We found that although EIm measurements slightly overestimate the duration of the Im waveform, they provide a reasonable approximation of Im during normal conduction and during decremental conduction and conduction block.

CONCLUSIONS

There is a very clear linear correlation between the time spent at or below 25% of the peak inward transmembrane current (Im25), its corresponding estimate (EIm25), the peak inward Im and EIm, and the peak ionic current, providing some evidence that EIm25 may be a suitable in vivo measure of peak ionic current.

Simulation of two-dimensional anisotropic cardiac reentry: effects of the wavelength on the reentry characteristics

A two-dimensional sheet model was used to study the dynamics of reentry around a zone of functional block. The sheet is a set of parallel, continuous, and uniform cables, transversely interconnected by a brick-wall arrangement of fixed resistors. In accord with experimental observations on cardiac tissue, longitudinal propagation is continuous, whereas transverse propagation exhibits discontinuous features. The width and length of the sheet are 1.5 and 5 cm, respectively, and the anisotropy ratio is fixed at approximately 4:1. The membrane model is a modified Beeler-Reuter formulation incorporating faster sodium current dynamics. We fixed the basic wavelength and action potential duration of the propagating impulse by dividing the time constants of the secondary inward current by an integer K. Reentry was initiated by a standard cross-shock protocol, and the rotating activity appeared as curling patterns around the point of junction (the q-point) of the activation (A) and recovery (R) fronts. The curling R front always precedes the A front and is separated from it by the excitable gap. In addition, the R front is occasionally shifted abruptly through a merging with a slow-moving triggered secondary recovery front that is dissociated from the A front and q-point. Sustained irregular reentry associated with substantial excitable gap variations was simulated with short wavelengths (K = 8 and K = 4). Unsustained reentry was obtained with a longer wavelength (K = 2), leading to a breakup of the q-point locus and the triggering of new activation fronts.

A model study of extracellular stimulation of cardiac cells

Point source extracellular stimulation of a myocyte model was used to study the efficacy of excitation of cardiac cells, taking into account the shape of the pulse stimulus and its time of application in the cardiac cycle. The myocyte was modeled as a small cylinder of membrane (10 microns in diameter and 100 microns in length) capped at both ends and placed in an unbounded volume conductor. A Beeler-Reuter model modified for the Na+ dynamics served to simulate the membrane ionic current. The stimulus source was located on the cylinder axis, close to the myocyte (50 microns) in order to generate a nonlinear extracellular field (phi e). The low membrane impedance associated with the high frequency component of the make and break of the rectangular current pulse leads to a current flow across the membrane and an abrupt change in intracellular potential (phi i). Because the intracellular space is very small, phi i is nearly uniform over the length of the myocyte and the membrane potential (V = phi i-phi e) is governed by the applied field phi e. There is then a longitudinal gradient of membrane polarization which is the inverse of the gradient of extracellular potential. With an anodal (positive) pulse, for instance, the proximal portion of the myocyte is hyperpolarized and the distal portion is depolarized. Based on this principle and considering the voltage-dependent activation/inactivation dynamics of the membrane, it is shown that a cathodal (negative) pulse is the most efficacious stimulus at diastolic potentials, an anodal current is preferable during the plateau phase of the action potential, and a biphasic pulse is optimal during the relative refractory phase. Thus a biphasic pulse would constitute the best choice for maximum efficacy at all phases of the action potential.

On the interpretation of voltage-clamp data using the Hodgkin-Huxley model

This paper describes a method to extract membrane model parameters from experimental voltage-clamp records. The underlying theory is based on two premises: (1) the membrane dynamics can be described by a Hodgkin-Huxley (HH) model, and (2) the most reliable data provided by voltage clamp experiments are peak current (Ip) measurements. First, the steady-state characteristics of activation (x infinity) and inactivation (z infinity) must be estimated, and it is shown that Ip data provided by standard voltage-clamp stimulation protocols are sufficient for this purpose for the case of well-separated activation (tau x) and inactivation (tau z) time constants, tau x << tau z. Next, we propose a test (R test) to establish the suitability of the HH model to represent the data. When the HH model is applicable (successful R test), the procedure yields the degree of the gating variables, a range of maximum membrane conductance (g) values, and a tau x/tau z ratio that relates x infinity and z infinity to the Ip data. When additional information is available, such as the time of occurrence of Ip or an estimate of tau z from the late portion of the ionic current response, one can narrow down the value of g and estimate all the HH parameters and functions. Otherwise, when the R test is not successful, one can conclude that x infinity and z infinity have been incorrectly estimated because tau x and tau z are not sufficiently separated or that the HH model is not applicable to the data.

A model study of electric field interactions between cardiac myocytes

The transmission of excitation via electric field coupling was studied in a model comprising two myocytes abutted end-to-end and placed in an unbounded volume conductor. Each myocyte was modeled as a small cylinder of membrane (10 microns in diameter and 100 microns in length) capped at both ends. A Beeler-Reuter model modified for the Na+ current dynamics served to simulate the membrane ionic current. There was no resistive coupling between the myocytes and the intercellular junction consisted of closely apposed pre- and post-junctional membranes, separated by a uniform cleft distance. The membrane current crossing the prejunctional membrane during the action potential upstroke tends to flow out of the cleft, but it is partly prevented from doing so by the shunt resistance constituted by the cleft volume conductor. The prejunctional upstroke gives rise to a pulse of positive potential within the cleft which induces a small capacitive current across the post-junctional membrane to yield a small positive change in the intracellular potential in the post-junctional cell. The net result is an hyperpolarization of the post-junctional cleft membrane and a slight depolarization of the rest of the cell membrane since the extracellular potential outside of the cell is zero. The magnitude of this depolarization is quite small for a flat junctional membrane and it can be increased by membrane folding and interdigitation, so as to increase the junctional membrane area by a factor of 10 or more. Even then the post-junctional depolarization does not reach threshold when the extracellular potential around the post-junctional cell is effectively zero. Threshold depolarization occurs in the presence of a large decrease of post-junctional load, by increasing the junctional membrane capacitance and/or decreasing the volume of the post-junctional cell. Assuming that the normal resistive coupling between two cardiac myocytes is 1-4 M omega, our model study indicates that electric field coupling would then be about two orders of magnitude smaller. However, substantial enhancement of the efficacy of electric field transmission was observed in the case of cells with substantial junctional membrane folding.

Propagation of activation in cardiac muscle

The hypothesis of local circuit current flow underlying propagation of activation in cardiac muscle has been extensively documented by one-dimensional and two-dimensional simulation studies. The assumptions of spatially uniform membrane capacitance and membrane ionic properties yield simulation results that are in good agreement with experimental observations in healthy cardiac muscle, thereby indicating that differences in propagation velocity and action potential upstroke between longitudinal and transverse directions can be explained solely on the basis of anisotropic intercellular coupling. Two-dimensional model studies of anisotropic propagation have also stressed the more efficient charging of the membrane capacitance and higher safety factor of propagation in the transverse direction. These conditions favor the occurrence of longitudinal unidirectional block and the initiation of reentry via transverse propagation. The authors simulated rotating waves initiated by properly phased transverse and longitudinal plane waves in a two-dimensional sheet model. Sustained propagation requires a minimum anisotropy ratio, corresponding to a velocity ratio of about 4:1. It was found, for uniform anisotropy, that the central focus wandered slightly. A higher anisotropy ratio favors a more stable rotating pattern and a more restricted movement of the central focus.

Structural complexity effects on transverse propagation in a two-dimensional model of myocardium

A thin sheet of cardiac tissue was modeled as a set of resistively coupled excitable cables with membrane dynamics described by the modified Beeler Reuter model. Transverse connections have a resistance Rn and are regularly distributed with a spacing delta on any given cable, to provide alternating input and output junctions. Flat wave longitudinal propagation corresponds to propagation along a single continuous cable since all units of the network are functionally isolated due to the absence of transverse current flow. Events on a given cable during flat transverse propagation include electrotonic spread of potential from input to output junctions, action potential initiation at input junctions, and collision at output junctions. The propagating two-dimensional transverse wavefront is an undulating transmembrane potential surface with highs at the input junctions and lows at the output junctions. The action potential upstroke is also modulated in a periodic manner with minimum and maximum Vmax at the input and output junctions respectively. Thus, the network is capable of a diversity of dynamic behavior spatially distributed in relation to the specific pattern of transverse connections chosen. Overall, the behavior of the network model is in good agreement with available structural and electrophysiological data on myocardium. In addition, this network topology allows to handle more easily parameters governing propagation and to avoid very large matrices which are costly in computational effort and overall computer time.

Directional characteristics of action potential propagation in cardiac muscle. A model study

Propagation of an elliptic excitation wave front was studied in a two-dimensional model of a thin sheet of cardiac muscle. The sheet model of 2.5 x 10 mm consisted of a set of 100 parallel cables coupled through a regular array of identical transverse resistors. The membrane dynamics was represented by a modified Beeler-Reuter model. We defined the charging factor (CF) to represent by a single number the proportion of input current used to charge the membrane locally below threshold and showed that CF is inversely correlated with the time constant of the foot of the action potential (tau foot) during propagation on a cable. A safety factor of propagation (SF) was also defined for the upstroke of the action potential, with SF directly correlated with the maximum rate of depolarization (Vmax) and, for cablelike propagation, with propagation velocity. Propagation along the principal longitudinal axis of the elliptic wave front is cablelike but, in comparison with a flat wave front, transverse current flow provides a drag effect that somewhat reduces the propagation velocity, Vmax, SF, and CF. With a longitudinal-to-transverse velocity ratio of 3:1 or more, the wave front propagating along the principal transverse axis is essentially flat and is characterized by multiple collisions between successive pairs of input junctions on a given cable; Vmax, SF, and CF are larger than for longitudinal propagation, but CF is no longer correlated with tau foot. There are transient increases in propagation velocity and Vmax with distance from the stimulation site along both principal axes until stablized values are achieved, and a similar transient decrease in tau foot. Away from the principal axes, the action potential characteristics change progressively along the elliptic wave front.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer model of excitation and recovery in the anisotropic myocardium. I. Rectangular and cubic arrays of excitable elements

A computer model of propagated excitation and recovery in anisotropic cardiac tissue is presented that consists of a large number of excitable elements whose subthreshold interactions are governed by the anisotropic bidomain theory but whose suprathreshold behavior (action potential) is largely preassigned. The model's performance was first tested in a two-dimensional configuration with uniform anisotropy; this method allowed comparison of simulated isochrones of excitation and extracellular electrograms with the results of experimental in vitro studies of cardiac tissue. Next the model was used to study propagated excitation in a three-dimensional region representing the anisotropic properties of the ventricular wall, with attention to the effects produced by variable fiber direction from "endocardium" to "epicardium."

Computer model of excitation and recovery in the anisotropic myocardium. II. Excitation in the simplified left ventricle

A computer model of propagated excitation and recovery in anisotropic cardiac tissue was described in the first report of this series. The model consists of a large number of excitable elements whose subthreshold interactions are governed by the anisotropic bidomain theory but whose suprathreshold behavior (action potential) is largely preassigned. As described in the first report, the model's performance was tested in rectangular and cubic arrays of excitable elements. This second report deals with three-dimensional simulations in a simplified left ventricle with anisotropy; specifically, the activation process in the "normal" ventricle is described (exemplified by the activation sequences started from various endocardial, intramural, and epicardial sites). To further substantiate our model's validity, we compare simulated epicardial and body-surface potential distributions with experimental findings in isolated canine hearts and with clinical evidence provided by electrocardiographic body-surface mapping.

Computer model of excitation and recovery in the anisotropic myocardium. III. Arrhythmogenic conditions in the simplified left ventricle

A computer model of propagated excitation and recovery in anisotropic cardiac tissue has been described in the first two reports of this series. The model consists of a large number of excitable elements whose subthreshold interactions are governed by the anisotropic bidomain theory but whose suprathreshold behavior (action potential) is largely preassigned. As described in the previous two reports, the model's performance was tested in rectangular and cubic arrays of excitable elements and in the "normal" three-dimensional simplified left ventricle with anisotropy. The present report deals with arrhythmogenic conditions in the simplified left ventricle with anisotropy and ventricular-gradient properties; specifically, we studied activation and recovery in the presence of an ischemic region and under various stimulation protocols. The aim of these simulations was to elucidate the role of reentry in the genesis of ventricular tachycardia. Our simulations produced reentrant activation as a result of appropriate endocardial stimulation.

A new cable model formulation based on Green's theorem

We describe an alternative formulation of the cable equation to model excitation in a cylinder of cardiac fiber. The formulation uses Green's theorem to develop equations for the extracellular and intracellular potential on either side of the excitable membrane, the dynamics of which are described by a Hodgkin-Huxley type model, without assuming that the radial current is zero. These equations are discretized to yield a system of linear equations which are solved at each instant in time. We found no qualitative differences between this approach and the standard cable model for parameters within accepted physiological limits. When the cable diameter is of the same order as the length constant the new formulation takes into account the intracellular potential change in the radial direction and gives an accurate expression of the conduction velocity.

The effect of torso geometry on magnetocardiographic isofield maps

Using a computer model of a realistically shaped human torso with lungs and intraventricular blood masses, we have assessed how torso geometry and composition affect the extracorporal magnetic field produced by a current dipole in the centre of the ventricular mass. The magnetic induction vector B arising from the dipole has been calculated at points of a precordial measuring grid and the influence of boundaries has been assessed qualitatively, by comparing contour maps of the B component normal to the torso's frontal plane. We found that the maps reflected relatively faithfully the underlying dipolar source for the homogeneous torso and even for the torso with lungs. However, the intraventricular blood masses caused a noticeable rotation of the maps' extrema. Both lungs and blood masses tended to swing the distribution towards the distribution that would have been caused by a dipole oriented along the anatomical axis of the heart.