Ultrastructural Adaptation of the Cardiomyocyte to Chronic Mitral Regurgitation

Abstract

Introduction: Mitral regurgitation (MR) imposes volume overload on the left ventricle (LV) and elevates wall stress, triggering its adverse remodeling. Pronounced LV dilation, minimal wall thinning, and a gradual decline in cardiac ejection fraction (EF) are observed. The structural changes in the myocardium that define these gross, organ level remodeling are not known. Cardiomyocyte elongation and slippage have both been hypothesized, but neither are confirmed, nor are the changes to the cardiomyocyte structure known. Using a rodent model of MR, we used immunohistochemistry and transmission electron microscopy (TEM) to describe the ultrastructural remodeling of the cardiomyocyte.Methods: Twenty-four male Sprague-Dawley rats (350–400 g) were assigned to two groups: group (1) rats induced with severe MR (n = 18) and group (2) control rats that were healthy and age and weight matched (n = 6). MR was induced in the beating heart using a 23-G ultrasound-guided, transapical needle to perforate the anterior mitral leaflet, and the rats were followed to 2, 10, and 20 weeks (n = 6/time-point). Echocardiography was performed to quantify MR severity and to measure LV volume and function at each time-point. Explanted myocardial tissue were examined with TEM and immunohistochemistry to investigate the ultrastructural changes.Results: MR induced rapid and significant increase in end-diastolic volume (EDV), with a 50% increase by 2 weeks, compared with control. Rise in end-systolic volume (ESV) was more gradual; however, by 20 weeks, both EDV and ESV in MR rats were increased by 126% compared with control. A significant decline in EF was measured at 10 weeks of MR. At the ultrastructural level, as early as 2 weeks after MR, cardiomyocyte elongation and increase in cross-sectional area were observed. TEM depicted sarcomere shortening, with loss of Z-line and I-band. Desmin, a cytoskeletal protein that is uniformly distributed along the length of the cardiomyocyte, was disorganized and localized to the intercalated disc, in the rats induced with MR and not in the controls. In the rats with MR, the linear registry of the mitochondrial arrangement along the sarcomeres was lost, with mitochondrial fragmentation, aggregation around the nucleus, and irregularities in the cristae.Discussion: In the setting of chronic mitral regurgitation, LV dilatation occured by cardiomyocyte elongation, which manifests at the subcellular level as distinct ultrastructural alterations of the sarcomere, cytoskeleton, and mitochondria. Since the cytoskeleton not only provides tensegrity but has functional consequences on myocyte function, further investigation into the impact of cytoskeletal remodeling on progressive heart failure or recovery of function upon correcting the valve lesion are needed.

Pimobendan prevents cardiac dysfunction, mitigates cardiac mitochondrial dysfunction, and preserves myocyte ultrastructure in a rat model of mitral regurgitation

“…In addition, MR causes a loss of the linearity of the mitochondria along the sarcomeres, clustering around the nucleus and sarcomeres, and abnormalities in the cristae. Studies have reported that these structural changes are associated with mitochondrial dysfunction [ 31 ] and may contribute to the development of oxidative stress [ 38 , 40 , 41 ], which supports our findings of mitochondrial quality degradation in MR. Furthermore, in the current manuscript, we demonstrated the presence of vacuoles in the heart of MR rats more than in the heart of treatment groups or sham rats.…”

“…In this study, we also explored ultrastructural changes and measured sarcomeric length in the myocytes of a rodent model of MR. In MR rats, a reduction in sarcomeric length was observed in this study similar to previous reports [ 38 , 39 ], which could be due to the breakdown of structural organization, especially the thin filament [ 38 ]. In addition, MR causes a loss of the linearity of the mitochondria along the sarcomeres, clustering around the nucleus and sarcomeres, and abnormalities in the cristae.…”

Large and Small Animal Models of Heart Failure With Reduced Ejection Fraction

“…Chronic mitral regurgitation (after 2, 10, and 20 weeks), confirmed by Doppler measurements, resulted in LV dilatation and maladaptive cardiomyocyte changes but did not result in HFrEF. 105 A closed-chest approach was presented recently via inserting a needle through the xiphoidal cartilage and the apex to perforate the mitral valve leaflet under echocardiography guidance. 106 Besides the advantages of less invasive techniques, use of these approaches may increase with improved imaging devices.…”

Rodent Models of Dilated Cardiomyopathy and Heart Failure for Translational Investigations and Therapeutic Discovery

“…A drop in ejection fraction is noticed at 14 weeks with myocyte and extracellular matrix remodeling and a transcriptomic shift illustrating upregulation of oxidative stress pathways [ 92 ]. Cardiomyocyte elongation, cytoskeletal disorganization, and loss of mitochondrial networking represent the ultrastructural bases of this remodeling [ 93 ]. The low-pressure volume overloaded mitral regurgitation model can be combined with the LAD ligation-based ischemic model to better reproduce the chronic ischemic DCM profile [ 94 ].…”