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Research

1. Regulation of ion channel function and structure-function relation of ion channels

My early studies on the regulation of the CFTR chloride channel and the L-type Ca channel revealed a novel pathway via Gi-like protein, cAMP, and PKA by endothelin-1 and Angiotensin-II receptor stimulation, which exerts an antagonizing effect against β1 adrenergic stimulation by isoproterenol.  It has been reported that phosphatidylinositol-4, 5-diphosphate (PIP2) activates the inwardly rectifying K (Kir) channels. Further study using cloned channels clearly demonstrated that G-protein coupled receptor stimulation and consequent PIP2 metabolism can modulate KATP channel activities. Although the channel-PIP2 interaction may serve as the end effector regulating Kir channel activity, my studies make it clear that a variety of signaling partners also influence Kir channel activity by modulating the interaction of Kir channels with PIP2. I have also studied the molecular mechanisms of the inward rectification and gating mechanism of Kir channels. The mechanism underlying inward rectification is attributed to the voltage-dependent block of the channels by intracellular Mg2+ and polyamines. Polyamine block of the Kir2.1 channel shows a very complex dependence on both voltage and concentration. My studies suggested a model in which positively-charged polyamines can bind to the cytoplasmic vestibule region. The shallow voltage-dependent current reduction at more negative voltages is caused by the screening or reduction of negative surface charges. Polyamines most likely occlude outward ion permeation at the selectivity filter.

  1. James F., Xie L.-H., Fujitani Y., Hayashi S., Horie M. Inhibition of the cardiac protein kinase A-dependent chloride conductance by endothelin-1. Nature. 1994; 370:297-300.
  2. Xie L.-H., Horie M., Takano M. Phospholipase C-linked receptors regulate the ATP-sensitive potassium channel via phosphatidylinositol-4,5-bisphosphate metabolism. Proc Natl Acad Sci U S A. 1999; 96:15292-15297. PMC24813
  3. Xie L.H., John S., Ribalet B., Weiss J. N. Long polyamines act as cofactors in PIP2 activation of inward rectifier potassium (Kir2.1) channels. J General Physiol. 2005; 126:541-549 (Featured in the cover figure). PMC2266595
  4. Xie H. (correspondence author), S.A. John, B. Ribalet and J.N. Weiss. Phosphatidylinositol-4,5-bisphosphate (PIP2) Regulation of Strong Inward Rectifier Kir2.1 Channels: Multilevel Positive Cooperativity. J Physiol. (London). 2008; 2008, 586; 1833-48. PMC2375719

2. Ionic and Cellular Mechanisms for Afterdepolarizations and Triggered Arrhythmias-Involvement of ROS

Severe arrhythmias, particularly self-sustained arrhythmias such as ventricular tachycardia or fibrillation (VT/VF), are a major cause of sudden cardiac death.  Afterdepolarizations and triggered activities play essential roles in the genesis of the clinical arrhythmias. One of my current projects is to explore the ionic and cellular mechanism(s) underlying afterdepolarizations and triggered activities in cardiac cells isolated from various animals.  For example, reactive oxygen species (ROS) are generated as natural byproducts of normal oxygen metabolism and play important roles in cell signaling. However, under pathological conditions, such as heart failure and ischemia-reperfusion, ROS levels can become elevated and predispose the heart to arrhythmias via activation of Calcium/Calmodulin-Dependent Protein Kinase II (CaMKII). Furthermore, the transient outward current (Ito) plays an important role of in the generation of early afterdepolarization (EAD). In addition, sarcoplasmic reticulum (SR) Ca spontaneous release and intracellular Ca2+ waves have been implicated in cardiac arrhythmogenesis under Ca2+ overload condition.

  1. Xie L.H., Chen F., Karagueuzian H.S., Weiss J.N. Oxidative Stress-Induced Afterdepolarizations and Calmodulin Kinase II Signaling. Circ Res. 2009; 104(1):79-86. PMC2747806
  2. Zhao Z., Y. Xie, H. Wen, D. Xiao, C. Allen, N. Fefelova, W. Dun, P.A. Boyden, Z. Qu, L.H. Xie. Role of the transient outward potassium current in the genesis of early afterdepolarizations in cardiac cells. Cardiovascular Research 2012; 95:308-16. PMC3400356
  3. Zhao Z., Fefelova N., Mayilvahanan S., Bishara P., J. Babu G.J., L.H. Xie. Angiotensin II Induces afterdepolarizations via reactive oxygen species and calmodulin kinase II signaling. Journal of Molecular and Cellular Cardiology 2011; 50:128-36. PMC3019274
  4. Zhao Z., Kudej RK., Wen H., Fefelova N., Yan L., Vatner DE., Vatner SF., Xie L.H. Antioxidant Defense and Protection to Cardiac Arrhythmias: Lessons from a Mammalian Hibernator (Woodchuck). FASEB Journal 2018;32:4229-4240. PMC6044062

3. Regulation of Ca handling behaviors and Ca waves

We have also extensively studied the modulation of Ca handling properties including Ca alternans, which is a precursor for severe arrhythmias.  We have shown that Ca dynamics plays an important role in APD alternans and arrhythmogenesis. In addition, we have elucidated how the Ca2+ waves are regulated by other cellular factors.

  1. Goldhaber J.I., Xie L.H., Duong T., Motter C., Khuu K., and Weiss J. N. Action potential duration restitution and alternans in rabbit ventricular myocytes: The key role of intracellular calcium cycling. Circulation Research. 96:459-66, 2005. PMID15662034
  2. Xie, L.H., Sato D., Garfinkel A., Qu Z., Weiss J.N.. Intracellular Ca Alternans: Coordinated Regulation by Sarcoplasmic Reticulum Release, Uptake and Leak. Biophys J. 2008; 95:3100-10. PMC2527258
  3. Sato D.*, Xie L.H. *, Sovari A.A., Tran D.X., Morita N., Xie F., Karagueuzian H., Garfinkel A., Weiss J.N., Qu Z. Synchronization of Chaotic Early Afterdepolarizations in the Genesis of Cardiac Arrhythmias. Proc Natl Acad Sci U S A. 2009; 106:2983-8 (* co-first author). PMC2651322
  4. Zhao Z., Wen H., Fefelova N., Allen C., Baba, A., Matsuda, T., Xie L.H. Revisiting the Ionic Mechanisms of Early Afterdepolarizations in Cardiomyocytes: Predominant by Ca Waves or Ca Currents. American Journal of Physiology- Heart Circ Physiology 2012; 302:H1636-44. PMC3330805

4. Mitochondrial Ca Regulation and arrhythmogenesis

Recent studies have shown that mitochondria play important roles in Ca2+ homeostasis of cardiac myocytes. However, it is still unclear if mitochondrial Ca2+ flux can regulate the generation of Ca2+ waves and triggered activities in cardiomyocytes.  My lab is currently studying whether mitochondrial Ca2+ release and uptake can control the local Ca2+ level in the micro-domain near SR ryanodine receptors and plays an important role in regulation of intracellular Ca2+ waves and arrhythmogenesis by using mPTP (CypD) and mCU deficient mouse models.

  1. Zhao Z., Gordan R., Wen H., Fefelova N., Zang W.J., Xie L.H.. Modulation of intracellular Calcium waves and triggered activities by mitochondrial Ca flux in mouse cardiomyocytes. PLoS One 2013 8(11): e80574. PMC3857829
  2. Gordan R., Fefelova N., Gwathmey J.K., Xie L.H. Involvement of Mitochondrial Permeability Transition Pore (mPTP) in Cardiac Arrhythmias: Evidence from Cyclophilin D Knockout Mice. Cell Calcium 2016;60:363-37. PMC5127715
  3. Xie A., Song Z, Liu H., Zhou A, Shi G., Wang Q., Gu L, Liu M., Xie L.H., Qu, Z., and Dudley S.C. Mitochondrial Ca2+ Influx Contributes to Arrhythmic Risk in Nonischemic Cardiomyopathy. JAHA-Journal of the American Heart Association 2018, 7: e007805. PMC6015427
  4. Song Z., Xie L.H., Weiss J.N., Qu Z. A spatiotemporal ventricular myocyte model incorporating mitochondrial calcium cycling. Biophysical Journal 2019, 117 (12), 2349-2360

5. Iron overload, ferroptosis and cardiomyopathy

One of my recent studies is focused on the effects of iron overload on oxidative stress, intracellular calcium handling, mitochondrial function, and ferroptosis-related cardiomyopathy. The importance of ferroptosis is well known in the cancer field but has only recently begun to be studied in regard to the heart. We have demonstrated that mitochondrial iron uptake is dependent on MCU, which plays an essential role in causing mitochondrial dysfunction and ferroptosis under iron overload conditions in the heart. Cardiac-specific deficiency of MCU prevents the development of ferroptosis and iron overload induced cardiac dysfunction.

  1. Gordan , Wongjaikam S., Gwathmey J.K., Chattipakorn N., Chattipakorn S.C., and Xie L.H. Involvement of Cytosolic and Mitochondrial Iron in Iron Overload Cardiomyopathy. Heart Failure Review 2018;23:801-816. PMC6093778
  2. Gordan R., Fefelova N., Gwathmey J. K., and Xie L.H. Iron Overload, Oxidative Stress and Calcium Mishandling in Cardiomyocytes: Role of The Mitochondrial Permeability Transition Pore. Antioxidants 2020; 9:758. PMC7465659
  3. Siri-Angkul N., Song Z., Fefelova N., Gwathmey J.K., Chattipakorn S.C., Qu Z., Chattipakorn N., and Xie L.H. Activation of TRPC Channel Currents in Iron Overloaded Cardiac Myocytes. Circ Arrhythm Electrophysiol. 2021;14(2):e009291. PMID: 33417472
  4. Fefelova N., Wongjaikam S., Sri Harika Pamarthi, Siri-Angkul N., Comollo T., Ivessa A. Chattipakorn N., Chattipakorn S.C., Gwathmey J.K., and Xie L.H. Deficiency of Mitochondrial Calcium Uniporter Abrogates Iron Overload-Induced Cardiac Ferroptosis and Cardiomyopathy in Mice. Basic Research in Cardiology 2023;118:21.

6. Involvement of Cx43 and Hemichannels in Cell Death and Arrhythmogenesis

Another research topic of mine has been on the involvement of Cx43 and hemichannels in cell death and arrhythmogenesis. For example, we have shown that oxidative stress induced by reactive oxygen species or extracts from cigarette smoke depolarize the cell membrane and open connexin hemichannels, that consequently induces cell injury. In addition, we have also demonstrated that the dephosphorylation of Cx43 is linked to IR-induced arrhythmias in cultured neonatal rat cardiomyocyte monolayer. A most recent collaborative research of mine has revealed that Cx43 lateralization contributes significantly to arrhythmogenesis in mice with muscular dystrophy and that selective inhibition of the Cx43 hemichannel may provide substantial benefit.

  1. de Diego C., Pai R.K., Chen F., Xie L.H, De Leeuw J., Weiss J.N., Valderrábano M. Electrophysiological Consequences of Acute Regional Ischemia/Reperfusion in Neonatal Rat Ventricular Myocyte Monolayers. Circulation. 2008; 118:2330-7. PMC2730415
  2. Gonzalez JP, Ramachandran J, Xie L.H, Contreras JE, Fraidenraich D. Selective Connexin43 Inhibition Prevents Isoproterenol-induced Arrhythmias and Lethality in Muscular Dystrophy Mice. Scientific Reports. 2015; 5:13490. PMC4550874
  3. Himelman E., Lillo M.A., Nouet J., Gonzalez J.P., Zhao Q., Xie L.H., Li H., Liu T., Wehrens X.H.T., Lampe P.D., Fishman G.I., Shirokova N., Contreras J.E., Fraidenraich D. Prevention of Connexin43 remodeling protects against Duchenne muscular dystrophy cardiomyopathy. J Clin Invest 2020; 130: 1713–1727. PMC7108916
  4. Lillo M., Himelman E., Shirokova N., Xie L.H., Fraidenrach D., Contreras J.E. S-nitrosylation of Cx43 hemichannels elicits cardiac stress-induced arrhythmias in Duchenne muscular dystrophy mice. JCI Insight 2019; 4:e130091. PMC6975272