James N Weiss

University of California, Los Angeles, Los Angeles, California, United States

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Publications (205)1050.86 Total impact

  • Circulation Heart Failure 03/2014; 7(2):359-366. · 6.68 Impact Factor
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    Zhilin Qu, Gang Hu, Alan Garfinkel, James N. Weiss
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    ABSTRACT: In a normal human life span, the heart beats about 2 to 3 billion times. Under diseased conditions, a heart may lose its normal rhythm and degenerate suddenly into much faster and irregular rhythms, called arrhythmias, which may lead to sudden death. The transition from a normal rhythm to an arrhythmia is a transition from regular electrical wave conduction to irregular or turbulent wave conduction in the heart, and thus this medical problem is also a problem of physics and mathematics. In the last century, clinical, experimental, and theoretical studies have shown that dynamical theories play fundamental roles in understanding the mechanisms of the genesis of the normal heart rhythm as well as lethal arrhythmias. In this article, we summarize in detail the nonlinear and stochastic dynamics occurring in the heart and their links to normal cardiac functions and arrhythmias, providing a holistic view through integrating dynamics from the molecular (microscopic) scale, to the organelle (mesoscopic) scale, to the cellular, tissue, and organ (macroscopic) scales. We discuss what existing problems and challenges are waiting to be solved and how multi-scale mathematical modeling and nonlinear dynamics may be helpful for solving these problems.
    Physics Reports. 01/2014;
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    ABSTRACT: Apamin is commonly used as a small-conductance Ca2+-activated K+ (SK) current inhibitor. However, the specificity of apamin in cardiac tissues remains unclear. To test the hypothesis that apamin does not inhibit any major cardiac ion currents. We studied human embryonic kidney (HEK) 293 cells that expressed human voltage-gated Na+, K+ and Ca2+ currents and isolated rabbit ventricular myocytes. Whole-cell patch clamp techniques were used to determine ionic current densities before and after apamin administration. Ca2+ currents (CACNA1c+CACNB2b) were not affected by apamin (500 nM) (data are presented as median [25th percentile;75th percentile] (from -16 [-20;-10] to -17 [-19;-13] pA/pF, P = NS), but were reduced by nifedipine to -1.6 [-3.2;-1.3] pA/pF (p = 0.008). Na+ currents (SCN5A) were not affected by apamin (from -261 [-282;-145] to -268 [-379;-132] pA/pF, P = NS), but were reduced by flecainide to -57 [-70;-47] pA/pF (p = 0.018). None of the major K+ currents (IKs, IKr, IK1 and Ito) were inhibited by 500 nM of apamin (KCNQ1+KCNE1, from 28 [20]; [37] to 23 [18]; [32] pA/pF; KCNH2+KCNE2, from 28 [24]; [30] to 27 [24]; [29] pA/pF; KCNJ2, from -46 [-48;-40] to -46 [-51;-35] pA/pF; KCND3, from 608 [505;748] to 606 [454;684]). Apamin did not inhibit the INa or ICaL in isolated rabbit ventricular myocytes (INa, from -67 [-75;-59] to -68 [-71;-59] pA/pF; ICaL, from -16 [-17;-14] to -14 [-15;-13] pA/pF, P = NS for both). Apamin does not inhibit human cardiac Na+ currents, L-type Ca2+ currents or other major K+ currents. These findings indicate that apamin is a specific SK current inhibitor in hearts as well as in other organs.
    PLoS ONE 01/2014; 9(5):e96691. · 3.53 Impact Factor
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    Thao P Nguyen, Zhilin Qu, James N Weiss
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    ABSTRACT: In the healthy heart, cardiac myocytes form an electrical syncytium, embedded in a supportive fibroblast-rich extracellular matrix designed to optimize electromechanical coupling for maximal contractile efficiency of the heart pump. In the injured heart, however, fibroblasts are activated and differentiate into myofibroblasts that proliferate and generate fibrosis as a component of the wound-healing response. This review discusses how fibroblasts and fibrosis, while essential for maintaining the structural integrity of the heart wall after injury, have undesirable electrophysiological effects by disrupting the normal electrical connectivity of cardiac tissue to increase the vulnerability to arrhythmias. We emphasize the dual contribution of fibrosis in altering source-sink relationships to create a vulnerable substrate while simultaneously facilitating the emergence of triggers such as afterdepolarization-induced premature ventricular complexes- both factors combining synergistically to promote initiation of reentry. We also discuss the potential role of fibroblasts and myofibroblasts in directly altering myocyte electrophysiology in a pro-arrhythmic fashion. Insight into these processes may open up novel therapeutic strategies for preventing and treating arrhythmias in the setting of heart disease as well as avoiding potential arrhythmogenic consequences of cell-based cardiac regeneration therapy. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signaling in Myocardium."
    Journal of Molecular and Cellular Cardiology 10/2013; · 5.15 Impact Factor
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    ABSTRACT: In mammalian tumor cell lines, localization of hexokinase (HK) isoforms to the cytoplasm or mitochondria has been shown to control their anabolic (glycogen synthesis) and catabolic (glycolysis) activities. In this study, we examined whether HK isoform differences could explain the markedly different metabolic profiles between normal adult and neonatal cardiac tissue. We used a set of novel genetically encoded optical imaging tools to track, in real-time in isolated adult (ARVM) and neonatal (NRVM) rat ventricular myocytes, the subcellular distributions of HKI and HKII, and the functional consequences on glucose utilization. We show that HKII, the predominant isoform in ARVM, dynamically translocates from mitochondria and cytoplasm in response to removal of extracellular glucose or addition of iodoacetate (IAA). In contrast, HKI, the predominant isoform in NRVM, is only bound to mitochondria and is not displaced by the above interventions. In ARVM, overexpression of HKI, but not HKII, increased glycolytic activity. In neonatal rat ventricular myocytes (NVRM), knockdown of HKI, but not HKII, decreased glycolytic activity. In conclusion, differential interactions of HKI and HKII with mitochondria underlie the different metabolic profiles of ARVM and NRVM, accounting for the markedly increased glycolytic activity of NRVM.
    The Journal of General Physiology 10/2013; 142(4):425-436. · 4.73 Impact Factor
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    ABSTRACT: Mitochondria are a common energy source for organs and organisms; their diverse functions are specialized according to the unique phenotypes of their hosting environment. Perturbation of mitochondrial homeostasis accompanies significant pathological phenotypes. However, the connections between mitochondrial proteome properties and function remain to be experimentally established on a systematic level. This uncertainty impedes the contextualization and translation of proteomic data to the molecular derivations of mitochondrial diseases. We present a collection of mitochondrial features and functions from four model systems, including two cardiac mitochondrial proteomes from distinct genomes (human and mouse), two unique organ mitochondrial proteomes from identical genetic codons (mouse heart and mouse liver), as well as a relevant metazoan out-group (drosophila). The data, comprised of mitochondrial protein abundance and their biochemical activities, capture the core functionalities of these mitochondria. This investigation allowed us to redefine the core mitochondrial proteome from organs and organisms, as well as the relevant contributions from genetic information and hosting milieu. Our study has identified significant enrichment of disease-associated genes and their products. Furthermore, correlational analyses suggest that mitochondrial proteome design is primarily driven by cellular environment. Taken together, these results connect proteome feature with mitochondrial function, providing a prospective resource for mitochondrial pathophysiology and developing novel therapeutic targets in medicine.
    Journal of Proteome Research 09/2013; · 5.06 Impact Factor
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    ABSTRACT: Calcium (Ca) waves generating oscillatory Ca signals are widely observed in biological cells. Experimental studies have shown that under certain conditions, initiation of Ca waves is random in space and time, while under other conditions, waves occur repetitively from preferred locations (pacemaker sites) from which they entrain the whole cell. In this study, we use computer simulations to investigate the self-organization of Ca sparks into pacemaker sites generating Ca oscillations. In both ventricular myocyte experiments and computer simulations of a heterogeneous Ca release unit (CRU) network model, we show that Ca waves occur randomly in space and time when the Ca level is low, but as the Ca level increases, waves occur repetitively from the same sites. Our analysis indicates that this transition to entrainment can be attributed to the fact that random Ca sparks self-organize into Ca oscillations differently at low and high Ca levels. At low Ca, the whole cell Ca oscillation frequency of the coupled CRU system is much slower than that of an isolated single CRU. Compared to a single CRU, the distribution of inter spike intervals (ISIs) of the coupled CRU network exhibits a greater variation, and its ISI distribution is asymmetric with respect to the peak, exhibiting a fat-tail. At high Ca, however, the coupled CRU network has a faster frequency and lesser ISI variation compared to an individual CRU. The ISI distribution of the coupled network no longer exhibits a fat-tail and is well-approximated by a Gaussian distribution. This same Ca oscillation behavior can also be achieved by varying the number of ryanodine receptors per CRU or the distance between CRUs. Using these results, we develop a theory for the entrainment of random oscillators which provides a unified explanation for the experimental observations underlying the emergence of pacemaker sites and Ca oscillations.
    The Journal of Physiology 09/2013; · 4.38 Impact Factor
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    ABSTRACT: Rationale: Omics sciences enable a systems-level perspective in characterizing cardiovascular biology. Integration of diverse proteomics data via a computational strategy will catalyze the assembly of contextualized knowledge, foster discoveries through multidisciplinary investigations, and minimize unnecessary redundancy in research efforts. Objective: The goal of this project is to develop a consolidated cardiac proteome knowledgebase with novel bioinformatics pipeline and web portals, thereby serving as a new resource to advance cardiovascular biology and medicine. Methods and Results: We created Cardiac Organellar Protein Atlas Knowledgebase (COPaKB), a centralized platform of high quality cardiac proteomic data, bioinformatics tools and relevant cardiovascular phenotypes. Currently, COPaKB features eight organellar modules, comprising 4,203 LC-MS/MS experiments from human, mouse, drosophila and C. elegans as well as expression images of 10,924 proteins in human myocardium. In addition, the Java-coded bioinformatics tools provided by COPaKB enable cardiovascular investigators in all disciplines to retrieve and analyze pertinent organellar protein properties of interest. Conclusions: COPaKB (www.HeartProteome.org) provides an innovative and interactive resource, which connects research interests with the new biological discoveries in protein sciences. With an array of intuitive tools in this unified web server, non-proteomics investigators can conveniently collaborate with proteomics specialists to dissect the molecular signatures of cardiovascular phenotypes.
    Circulation Research 08/2013; · 11.86 Impact Factor
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    ABSTRACT: A secondary rise of intracellular Ca(2+) (Cai) and an upregulation of IKAS are characteristic findings of failing ventricular myocytes. We hypothesize that apamin, a specific IKAS blocker, may induce torsades de pointes (TdP) ventricular arrhythmia from failing ventricles exhibiting secondary rises of Cai. To test the hypothesis that small conductance Ca(2+) activated apamin sensitive K(+) current (IKAS) maintains repolarization reserve and prevents ventricular arrhythmia in a rabbit model of heart failure (HF). We performed Langendorff perfusion and optical mapping studies in 7 hearts with pacing-induced HF and in 5 normal control rabbit hearts. Atrioventricular (AV) block was created by cryoablation to allow pacing at slow rates. The left ventricular ejection fraction reduced from 69.1 [95% confidence interval 62.3-76.0]% pre-pacing to 30.4 [26.8-34.0]% (N=7, p<0.001) post-pacing. The QTc in failing ventricles was 337 [313-360] ms at baseline and 410 [381-439] ms after applying 100 nmol/L of apamin (p=0.01). Apamin induced early afterdepolarizations (EADs) in 6 ventricles, premature ventricular beats (PVBs) in 7 ventricles and polymorphic ventricular tachycardia consistent with TdP in 4 ventricles. The earliest activation site of the EADs and PVBs always occurred at the site with long APD and large amplitude of the secondary rises of Cai. Apamin induced secondary rises of Cai in 1 non-failing ventricles, but no EAD or TdP were observed. In HF ventricles, apamin induces EADs, PVBs and TdP from areas with secondary rises of Cai. IKAS is important in maintaining repolarization reserve and preventing TdP in HF ventricles.
    Heart rhythm: the official journal of the Heart Rhythm Society 07/2013; · 4.56 Impact Factor
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    ABSTRACT: INTRODUCTION: The apamin-sensitive small-conductance calcium-activated potassium current (IKAS ) is increased in heart failure. It is unknown if myocardial infarction (MI) is also associated with an increase of IKAS . METHODS AND RESULTS: We performed Langendorff perfusion and optical mapping in 6 normal hearts and 10 hearts with chronic (5 weeks) MI. An additional 6 normal and 10 MI hearts were used for patch clamp studies. The infarct size was 25% (95% confidence interval, 20-31) and the left ventricular ejection fraction was 50 (46-54). The rabbits did not have symptoms of heart failure. The action potential duration measured to 80% repolarization (APD80 ) in the peri-infarct zone (PZ) was 150 (142-159) milliseconds, significantly (P = 0.01) shorter than that in the normal ventricles (167 [158-177] milliseconds. The intracellular Ca transient duration was also shorter in the PZ (148 [139-157] milliseconds) than that in normal ventricles (168 [157-180] milliseconds; P = 0.017). Apamin prolonged the APD80 in PZ by 9.8 (5.5-14.1)%, which is greater than that in normal ventricles (2.8 [1.3-4.3]%, P = 0.006). Significant shortening of APD80 was observed at the cessation of rapid pacing in MI but not in normal ventricles. Apamin prevented postpacing APD80 shortening. Patch clamp studies showed that IKAS was significantly higher in the PZ cells (2.51 [1.55-3.47] pA/pF, N = 17) than in the normal cells (1.08 [0.36-1.80] pA/pF, N = 15, P = 0.019). CONCLUSION: We conclude that IKAS is increased in MI ventricles and contributes significantly to ventricular repolarization especially during tachycardia.
    Journal of Cardiovascular Electrophysiology 05/2013; · 3.48 Impact Factor
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    ABSTRACT: Atrial fibrillation (AF) is the most common cardiac arrhythmia. Patients with AF have up to seven-fold higher risk of suffering from ischemic stroke. Better understanding of etiologies of AF and its thromboembolic complications are required for improved patient care, as current anti-arrhythmic therapies have limited efficacy and off target effects. Accumulating evidence has implicated a potential role of oxidative stress in the pathogenesis of AF. Excessive production of reactive oxygen species (ROS) is likely involved in the structural and electrical remodeling of the heart, contributing to fibrosis and thrombosis. In particular, NADPH oxidase (NOX) has emerged as a potential enzymatic source for ROS production in AF based on growing evidence from clinical and animal studies. Indeed, NOX can be activated by known upstream triggers of AF such as angiotensin II and atrial stretch. In addition, treatments such as Statins, antioxidants, ACEI or AT1RB have been shown to prevent post-operative AF; among which ACEI/AT1RB and Statins can attenuate NOX activity. On the other hand, detailed molecular mechanisms by which specific NOX isoform(s) are involved in the pathogenesis of AF and the extent to which activation of NOX plays a causal role in AF development remains to be determined. The current review discusses causes and consequences of oxidative stress in AF with a special focus on the emerging role of NOX pathways.
    Journal of Molecular and Cellular Cardiology 05/2013; · 5.15 Impact Factor
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    ABSTRACT: Early afterdepolarizations (EADs) are secondary voltage depolarizations during the repolarizing phase of the action potential, which can cause lethal cardiac arrhythmias. The occurrence of EADs requires a reduction in outward current and/or an increase in inward current, a condition called reduced repolarization reserve. However, this generalized condition is not sufficient for EAD genesis and does not explain the voltage oscillations manifesting as EADs. Here we summarize recent progress that uses dynamical theory to build on and advance our understanding of EADs beyond the concept of repolarization reserve, towards the goal of developing a holistic and integrative view of EADs and their role in arrhythmogenesis. We first introduce concepts from nonlinear dynamics that are relevant to EADs, namely, Hopf bifurcation leading to oscillations and basin of attraction of an equilibrium or oscillatory state. We then present a theory of phase-2 EADs in nonlinear dynamics, which includes the formation of quasi-equilibrium states at the plateau voltage, their stabilities, and the bifurcations leading to and terminating the oscillations. This theory shows that the L-type calcium channel plays a unique role in causing the nonlinear dynamical behaviors necessary for EADs. We also summarize different mechanisms of phase-3 EADs. Based on the dynamical theory, we discuss the roles of each of the major ionic currents in the genesis of EADs, and potential therapeutic targets.
    Cardiovascular research 04/2013; · 5.81 Impact Factor
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    ABSTRACT: Background Under conditions promoting early afterdepolarizations (EADs), ventricular tissue can become bi-excitable, that is, capable of wave propagation mediated by either the Na current (INa) or the L-type calcium current (ICa,L), raising the possibility that ICa,L-mediated reentry may contribute to polymorphic ventricular tachycardia (PVT) and torsades de pointes. ATP-sensitive K current (IKATP) activation suppresses EADs, but the effects on ICa,L-mediated reentry are unknown. Objective To investigate the effects of IKATP activation on ICa,L-mediated reentry. Methods We performed optical voltage mapping in cultured neonatal rat ventricular myocyte monolayers exposed to BayK8644 and isoproterenol. The effects of pharmacologically activating IKATP with pinacidil were analyzed. Results In 13 monolayers with anatomic ICa,L-mediated reentry around a central obstacle, pinacidil (50 μM) converted ICa,L-mediated reentry to INa-mediated reentry. In 33 monolayers with functional ICa,L-mediated reentry (spiral waves), pinacidil terminated reentry in 17, converted reentry into more complex INa-mediated reentry resembling fibrillation in 12, and had no effect in 4. In simulated 2-dimensional bi-excitable tissue in which ICa,L- and INa-mediated wave fronts coexisted, slow IKATP activation (over minutes) reliably terminated rotors but rapid IKATP activation (over seconds) often converted ICa,L-mediated reentry to INa-mediated reentry resembling fibrillation. Conclusions IKATP activation can have proarrhythmic effects on EAD-mediated arrhythmias if ICa,L-mediated reentry is present.
    Heart rhythm: the official journal of the Heart Rhythm Society 04/2013; 10(4):575–582. · 4.56 Impact Factor
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    ABSTRACT: BACKGROUND: -Apamin-sensitive K currents (I(KAS)) are upregulated in heart failure (HF). We hypothesize that apamin can flatten action potential duration restitution (APDR) curve and reduce ventricular fibrillation (VF) duration in failing ventricles. METHODS AND RESULTS: -We simultaneously mapped membrane potential and intracellular Ca (Ca(i)) in 7 rabbits hearts with pacing-induced HF and in 7 normal hearts. A dynamic pacing protocol was used to determine APDR at baseline and after apamin (100 nM) infusion. Apamin did not change APD(80) in normal ventricles, but prolonged APD(80) in failing ventricles at either long (≥300 ms) or short (≤170 ms) pacing cycle length (PCL), but not at intermediate PCL. The maximal slope of APDR curve was 2.03 [95% CI, 1.73 to 2.32] in failing ventricles and 1.26 [95% CI, 1.13 to 1.40] in normal ventricles at baseline (p=0.002). After apamin administration, the maximal slope of APDR in failing ventricles decreased to 1.43 [95% CI, 1.01 to 1.84] (p=0.018) whereas no significant changes were observed in normal ventricles. During VF in failing ventricles, the number of phase singularities (baseline vs apamin, 4.0 vs 2.5), dominant frequency (13.0 Hz vs 10.0 Hz), and VF duration (160 s vs 80 s) were all significantly (p<0.05) decreased by apamin. CONCLUSIONS: -Apamin prolongs APD at long and short, but not at intermediate PCL in failing ventricles. I(KAS) upregulation may be antiarrhythmic by preserving the repolarization reserve at slow heart rate, but is proarrhythmic by steepening the slope of APDR curve which promotes the generation and maintenance of VF.
    Circulation Arrhythmia and Electrophysiology 02/2013; · 5.95 Impact Factor
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    ABSTRACT: Animal and clinical studies have demonstrated that oxidative stress, a common pathophysiological factor in cardiac disease, reduces repolarization reserve by enhancing the L-type calcium current, the late Na, and the Na-Ca exchanger, promoting early afterdepolarizations (EADs) that can initiate ventricular tachycardia and ventricular fibrillation (VT/VF) in structurally remodeled hearts. Increased ventricular fibrosis plays a key facilitatory role in allowing oxidative-stress induced EADs to manifest as triggered activity and VT/VF, since normal non-fibrotic hearts are resistant to arrhythmias when challenged with similar or higher levels of oxidative stress. The findings imply that antifibrotic therapy, in addition to therapies designed to suppress EAD formation at the cellular level, may be synergistic in reducing the risk of sudden cardiac death.
    Frontiers in Physiology 01/2013; 4:19.
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    Biophysical Journal 01/2013; 104(2):295-. · 3.67 Impact Factor
  • Biophysical Journal 01/2013; 104(2):435-. · 3.67 Impact Factor
  • Michael Nivala, James N. Weiss, Zhilin Qu
    Biophysical Journal 01/2013; 104(2):435-. · 3.67 Impact Factor
  • Biophysical Journal 01/2013; 104(2):314-. · 3.67 Impact Factor
  • Biophysical Journal 01/2013; 104(2):435-. · 3.67 Impact Factor

Publication Stats

5k Citations
1,050.86 Total Impact Points


  • 1997–2014
    • University of California, Los Angeles
      • • Department of Medicine
      • • Division of Cardiology
      Los Angeles, California, United States
  • 2013
    • Chang Gung Memorial Hospital
      • Department of Internal Medicine
      T’ai-pei, Taipei, Taiwan
    • Catholic University of Daegu
      • Department of Internal Medicine
      Hayang, North Gyeongsang, South Korea
  • 2009–2013
    • CSU Mentor
      Long Beach, California, United States
  • 2005–2013
    • University of Southern California
      Los Angeles, California, United States
  • 2011
    • University of Leeds
      Leeds, England, United Kingdom
    • Indiana University-Purdue University School of Medicine
      • Department of Medicine
      Indianapolis, IN, United States
    • Toho University
      • Department of Cardiovascular Medicine
      Edo, Tōkyō, Japan
  • 2009–2011
    • Indiana University-Purdue University Indianapolis
      • Krannert Institute of Cardiology
      Indianapolis, IN, United States
  • 2000–2011
    • UCLA Cardiovascular Research Laboratory
      Los Angeles, California, United States
  • 2010
    • Loyola Marymount University
      • Department of Mathematics
      Los Angeles, California, United States
  • 1998–2009
    • Cedars-Sinai Medical Center
      • Division of Cardiology
      Los Angeles, CA, United States
  • 2008
    • Shiga University of Medical Science
      • Department of Medicine
      Ōtsu-shi, Shiga-ken, Japan
    • Rutgers New Jersey Medical School
      • Department of Cell Biology and Molecular Medicine (NJ Medical School)
      Newark, NJ, United States
  • 2006–2008
    • Northeastern University
      • Center for Interdisciplinary Research on Complex Systems (CIRCS)
      Boston, MA, United States
  • 2006–2007
    • Kaiser Permanente
      Oakland, California, United States
  • 2001–2006
    • Children's Hospital Los Angeles
      • • Division of Hospital Medicine
      • • Division of Cardiology
      Los Angeles, California, United States
  • 2002–2004
    • Taichung Veterans General Hospital
      • Department of Internal Medicine
      Taichung, Taiwan, Taiwan