Neurohormonal activation in cardiac hypertrophy and heart failure 

Our early research combined my cardiologist’s perspective of heart disease and my investigative interest in G-protein coupled receptor signaling pathways. In the mid-1990s it was appreciated that the sympathetic adrenergic and renin-angiotensin-aldosterone systems played roles in cardiac hypertrophy and heart failure, but it was unclear if these were direct effects on the heart vs indirect effects on vascular resistance and hemodynamic loading. Because signaling pathways of the agonists that provoke in vitro hypertrophy of neonatal rat cardiac myocytes (norepinephrine, angiotensin II, endothelin) all converged on the Gq G-protein, we reasoned that intrinsic activation of cardiomyocyte Gq signaling by overexpressing Gαq in living mouse hearts would recapitulate direct cardiomyocyte activation by these neuroendocrine signaling pathways. We discovered that modest Gαq overexpression alone was sufficient to provoke cardiac hypertrophy resembling pressure overload hypertrophy at the whole heart, cardiomyocyte, and transcriptional levels. It also evoked characteristic hypertrophy decompensation by inducing cardiomyocyte apoptosis. Our conclusions that Gq-coupled agonists directly stimulate pathological cardiomyocyte hypertrophy and programmed death in vivo were validated by loss-of-function studies, and are now universally accepted. We distributed the Gq mouse model worldwide; it has been used in ~300 published studies and continues to be widely investigated as a cardiomyocyte-autonomous genetic model of pathological cardiac hypertrophy.

  • D’ Angelo DD, Sakata Y, Lorenz JN, Boivin GP, Walsh RA, Liggett SB, Dorn GW II. Transgenic Galphaq overexpression induces cardiac contractile failure in mice. Proc Natl Acad Sci U S A. 1997 Jul 22;94(15):8121-6. PubMed PMID: 9223325; PubMed Central PMCID: PMC21567
  • Sakata Y, Hoit BD, Liggett SB, Walsh RA, Dorn GW II. Decompensation of pressure-overload hypertrophy in G alpha q-overexpressing mice. Circulation. 1998 Apr 21;97(15):1488-95. PubMed PMID: 9576430.
  • Adams JW, Sakata Y, Davis MG, Sah VP, Wang Y, Liggett SB, Chien KR, Brown JH, Dorn GW II. Enhanced Galphaq signaling: a common pathway mediates cardiac hypertrophy and apoptotic heart failure. Proc Natl Acad Sci U S A. 1998 Aug
  • Matkovich SJ, Zhang Y, Van Booven DJ, Dorn GW II. Deep mRNA sequencing for in vivo functional analysis of cardiac transcriptional regulators: application to Galphaq. Circ Res. 2010 May 14;106(9):1459-67. PubMed PMID: 20360248; PubMed Central PMCID: PMC2891025.

Cardiomyocyte apoptosis in heart failure

Because Gq-coupled receptor agonists stimulate growth of vascular smooth muscle and other cells, the hypertrophic effects of Gq signaling at the in vivo heart were consistent with known cell biology; stimulation of cardiomyocyte apoptosis was not. Thus, the next evolution of our work was to discover the molecular mechanisms behind Gq-mediated cardiomyocyte apoptosis. Using early mRNA arrays we identified Nix/Bnip3L as a transcriptionally upregulated pro-apoptotic mitochondrial death protein in Gq and pressure overloaded mouse hearts and in human hypertensive hypertrophy. Again employing in vivo genetic manipulation we demonstrated that cardiomyocyte-directed overexpression of Nix was sufficient to induce apoptotic dilated cardiomyopathy, and that cardiomyocyte-specific ablation of Nix prevented cardiomyopathic decompensation or murine pressure overload hypertrophy. We identified and validated a parallel role for Bnip3 after ischemic heart injury. These studies and others in a large body of related work revealed that the molecular seeds of cardiomyopathic decompensation are planted early in cardiac hypertrophy in the form of a transcriptional program for programmed cardiomyocyte death evoked by Gq-coupled signaling pathways.

  • Yussman MG, Toyokawa T, Odley A, Lynch RA, Wu G, Colbert MC, Aronow BJ, Lorenz JN, Dorn GW II. Mitochondrial death protein Nix is induced in cardiac hypertrophy and triggers apoptotic cardiomyopathy. Nat Med. 2002 Jul;8(7):725-30. PubMed PMID: 12053174.
  • Diwan A, Krenz M, Syed FM, Wansapura J, Ren X, Koesters AG, Li H, Kirshenbaum LA, Hahn HS, Robbins J, Jones WK, Dorn GW II. Inhibition of ischemic cardiomyocyte apoptosis through targeted ablation of Bnip3 restrains postinfarction remodeling in mice. J Clin Invest. 2007 Oct;117(10):2825-33. PubMed PMID: 17909626; PubMed Central PMCID: PMC1994631.
  • Diwan A, Matkovich SJ, Yuan Q, Zhao W, Yatani A, Brown JH, Molkentin JD, Kranias EG, Dorn GW II. Endoplasmic reticulum-mitochondria crosstalk in NIX-mediated murine cell death. J Clin Invest. 2009 Jan;119(1):203-12. PubMed PMID: 19065046; PubMed Central PMCID: PMC2613462.
  • Chen Y, Lewis W, Diwan A, Cheng EH, Matkovich SJ, Dorn GW II. Dual autonomous mitochondrial cell death pathways are activated by Nix/BNip3L and induce cardiomyopathy. Proc Natl Acad Sci U S A. 2010 May 18;107(20):9035-42. PubMed PMID: 20418503; PMCID: PMC2889094.

Mitochondrial dynamism and heart development

Our studies of Nix and Bnip3 exposed the role for mitochondrial- reticular calcium signaling in programmed cardiomyocyte death observed in functional decompensation of reactive cardiac hypertrophy. Building upon Scorrano’s observation (Nature 2008) that the mitochondrial fusion protein Mfn2 facilitates trans-organelle calcium crosstalk by tethering mitochondria to endoplasmic reticulum, we used cardiac-specific ablation to demonstrate that Mfn2, but not Mfn1, tethers in vivo cardiomyocyte mitochondria to SR, thus demonstrating the importance of calcium microdomains formed by Mfn2 in the rapid metabolic response to increased cardiomyocyte work.

  • Chen Y, Liu Y, Dorn GW II. Mitochondrial fusion is essential for organelle function and cardiac homeostasis. Circ Res. 2011 Dec 9;109(12):1327-31. PubMed PMID: 22052916; PubMed Central PMCID: PMC3237902.
  • Kasahara A, Cipolat S, Chen Y, Dorn GW II, Scorrano L. Mitochondrial fusion directs cardiomyocyte differentiation via calcineurin and Notch signaling. Science. 2013 Nov 8;342(6159):734-7. PubMed PMID: 24091702.

The PINK1-Mfn2-Parkin mitophagy pathway and metabolic remodeling in hearts

During our in vivo studies of cardiac mitochondrial fusion we were intrigued by the apparent proliferation of abnormally large (not small as expected from fusion defects) cardiomyocyte mitochondria provoked by Mfn2 (but not Mfn1) deficiency. We hypothesized that absence of Mfn2 created dysfunction within pathways that would normally eliminate abnormal or dysfunctional mitochondria, i.e. in mitophagy. In exploring this notion we observed that Parkin translocation to cardiomyocyte mitochondria was interrupted by Mfn2 deficiency, which lead us to the discovery that phosphorylation of Mfn2 by the mitochondrial-localized and damage-stabilized kinase PINK1 transformed Mfn2 into a mitochondrial receptor for the mitophagy factor, Parkin. These seminal studies made fertile ground for our ongoing efforts to understand the inter-relationships between mitochondrial dynamism, mitochondrial biogenesis, and mitophagic mitochondrial quality control in hearts. Our studies have revealed how these processes, far from being functionally distinct, are connected through molecular cross-talk, duplication and multiplicity of function, and counter-regulation. This work evolved into a detailed mechanistic evaluation of the role of PINK1-Mfn2-Parkin mitophagy signaling in the perinatal cardiac metabolic transition and in cardiac metabolic remodeling after hemodynamic or ischemic stress.

  • Chen Y, Dorn GW II. PINK1-phosphorylated mitofusin 2 is a Parkin receptor for culling damaged mitochondria. Science. Apr 26;340(6131):471-5, 2013. PubMed PMID: 23620051; PubMed Central PMCID: PMC3774525.
  • Song M, Mihara K, Chen Y, Scorrano L, Dorn GW II. Mitochondrial fission and fusion factors reciprocally orchestrate mitophagic culling in mouse hearts and cultured fibroblasts. Cell Metab. Feb 3;21(2):273-85, 2015. PubMed PMID: 25600785; PubMed Central PMCID: PMC4318753.
  • Gong G, Song M, Csordas G, Kelly DP, Matkovich SJ, Dorn GW II. Parkin-mediated mitophagy directs perinatal cardiac metabolic maturation in mice. Science, 350:aad2459, 2015. PubMed PMID : 26785495; PubMed Central PMCID: PMC4747105.
  • Song M, Franco A, Fleischer JA, Zhang L, Dorn GW II. Abrogating mitochondrial dynamics in mouse hearts accelerates mitochondrial senescence.  Cell Metab. 26:872-885, 2017. PubMed PMID: 29107503; PubMed Central PMCID: PMC5718956.