Background-In patients with left ventricular infarction or dilatation, leaflet tethering by displaced papillary muscles frequently induces mitral regurgitation, which doubles mortality. Little is known about the biological potential of the mitral valve (MV) to compensate for ventricular remodeling. We tested the hypothesis that MV leaflet surface area increases over time with mechanical stretch created by papillary muscle displacement through cell activation, not passive stretching. Methods and Results-Under cardiopulmonary bypass, the papillary muscle tips in 6 adult sheep were retracted apically short of producing mitral regurgitation to replicate tethering without confounding myocardial infarction or turbulence. Diastolic leaflet area was quantified by 3-dimensional echocardiography over 61Ϯ6 days compared with 6 unstretched sheep MVs. Total diastolic leaflet area increased by 2.4Ϯ1.3 cm 2 (17Ϯ10%) from 14.3Ϯ1.9 to 16.7Ϯ1.9 cm 2 (Pϭ0.006) with stretch with no change in the unstretched valves despite sham open heart surgery. Stretched MVs were 2.8 times thicker than normal (1.18Ϯ0.14 versus 0.42Ϯ0.14 mm; PϽ0.0001) at 60 days with an increased spongiosa layer. Endothelial cells (CD31 ϩ ) coexpressing ␣-smooth muscle actin were significantly more common by fluorescent cell sorting in tethered versus normal leaflets (41Ϯ19% versus 9Ϯ5%; Pϭ0.02), indicating endothelial-mesenchymal transdifferentiation. ␣-Smooth muscle actin-positive cells appeared in the atrial endothelium, penetrating into the interstitium, with increased collagen deposition. Thickened chordae showed endothelial and subendothelial ␣-smooth muscle actin. Endothelial-mesenchymal transdifferentiation capacity also was demonstrated in cultured MV endothelial cells. Conclusions-Mechanical stresses imposed by papillary muscle tethering increase MV leaflet area and thickness, with cellular changes suggesting reactivated embryonic developmental pathways. Understanding such actively adaptive mechanisms can potentially provide therapeutic opportunities to augment MV area and reduce ischemic mitral regurgitation. (Circulation. 2009;120:334-342.)Key Words: echocardiography Ⅲ mitral valve Ⅲ valves I n population studies, valvular heart disease is common, with mitral regurgitation (MR) most prevalent. 1 Although degeneration is the leading cause of MR surgical repair, 2 coronary artery disease with myocardial infarction and left ventricular (LV) dysfunction frequently causes functional MR as a result of global LV remodeling and sphericity 3-5 or localized inferoposterior wall remodeling 6 -12 ; both cause apical, posterior, and outward displacement of the papillary muscles (PMs) 6 -12 and mitral valve (MV) leaflet tethering 13 that prevents effective closure ( Figure 1A and 1B). 12,14 Editorial see p 275 Clinical Perspective on p 342Patients who develop MR after myocardial infarction or with congestive heart failure, even after surgical or catheter revascularization, have doubled mortality and in- To observe leaflet adaptation to tethering over time, we asse...

BACKGROUND In patients with myocardial infarction (MI), leaflet tethering by displaced papillary muscles induces mitral regurgitation (MR), which doubles mortality. Mitral valves (MVs) are larger in such patients but fibrosis sets in counterproductively. The investigators previously reported that experimental tethering alone increases mitral valve area in association with endothelial-to-mesenchymal transition. OBJECTIVES This study explored the clinically relevant situation of tethering and MI, testing the hypothesis that ischemic milieu modifies MV adaptation. METHODS Twenty-three adult sheep were examined. Under cardiopulmonary bypass, the PM tips in 6 sheep were retracted apically to replicate tethering, short of producing MR (tethered-alone). PM retraction was combined with apical MI created by coronary ligation in another 6 sheep (tethered + MI), and left ventricular (LV) remodeling was limited by external constraint in 5 additional sheep (LV constraint). Six sham-operated sheep were controls. Diastolic MV surface area was quantified by 3-dimensional echocardiography at baseline and after 58 ± 5 days, followed by histopathology and flow cytometry of excised leaflets. RESULTS Tethered + MI leaflets were markedly thicker than tethered-alone valves and sham controls. Leaflet area also increased significantly. EMT, detected as α-smooth muscle actin-positive endothelial cells, significantly exceeded that in tethered-alone and control valves. Transforming growth factor-β, matrix metalloproteinase expression, and cellular proliferation were markedly increased. Uniquely, tethering + MI showed endothelial activation with vascular adhesion molecule expression, neovascularization, and cells positive for CD45, considered a hematopoietic cell marker. Tethered + MI findings were comparable with external ventricular constraint. CONCLUSIONS MI altered leaflet adaptation, including a profibrotic increase in valvular cell activation, CD45-positive cells, and matrix turnover. Understanding cellular and molecular mechanisms underlying leaflet adaptation and fibrosis could yield new therapeutic opportunities for reducing ischemic MR.

Background After myocardial infarction (MI), mitral valve (MV) tethering stimulates adaptive leaflet growth, but counterproductive leaflet thickening and fibrosis augment mitral regurgitation (MR), doubling heart failure and mortality. MV fibrosis post-MI is associated with excessive endothelial-to-mesenchymal transition (EMT), driven by transforming growth factor (TGF)-β overexpression. In vitro, losartan-mediated TGF-β inhibition reduces EMT of MV endothelial cells. Objectives The authors tested the hypothesis that profibrotic MV changes post-MI are therapeutically accessible, specifically by losartan-mediated TGF-β inhibition. Methods We studied 17 sheep, including 6 sham-operated controls and 11 with apical MI and papillary muscle retraction short of producing MR: 6 treated with daily losartan, and 5 untreated, with flexible epicardial mesh comparably limiting left ventricular (LV) remodeling. LV volumes, tethering, and MV area were quantified by 3-dimensional echocardiography at baseline and at 60 ± 6 days, and excised leaflets were analyzed by histopathology and flow cytometry.’ Results Post-MI LV dilation and tethering were comparable in losartan-treated and untreated LV-constraint sheep. Telemetered sensors (n = 6) showed no significant losartan-induced arterial pressure changes. Losartan strongly reduced leaflet thickness (0.9 ± 0.2 mm vs. 1.6 ± 0.2 mm; p < 0.05; 0.4 ± 0.1 mm shams), TGF-β and downstream phosphorylated extracellular-signal–regulated kinase and EMT (27.2% ± 12.0% vs. 51.6% ± 11.7% α-smooth-muscle-actin-positive endothelial cells, p < 0.05; 7.2% ± 3.5% shams), cellular proliferation, collagen deposition, endothelial cell activation (vascular cell adhesion molecule-1 expression), neovascularization, and cells positive for cluster of differentiation (CD)45, a hematopoietic marker associated with post-MI valve fibrosis. Leaflet area increased comparably (17%) in constrained and Losartan-treated sheep. Conclusions Profibrotic changes of tethered MV leaflets post-MI can be modulated by losartan without eliminating adaptive growth. Understanding the cellular and molecular mechanisms could provide new opportunities to reduce ischemic MR.