Diverse anatomical and metabolic effects of amyloid deposits in transthyretin cardiac amyloidosis revealed by 3D reconstruction of serial block face-scanning electron microscopy images

Background The interactions and interplay between cells and the extracellular matrix are crucial in both the steady state and pathological conditions, including wild-type transthyretin cardiac amyloidosis (wtATTR-CA). However, it has not been well analyzed how amyloid deposits alter cardiomyocytes and cardiac niches anatomically and metabolically in wtATTR-CA. Normal histological staining is limited in resolution, and 2D electron microscopy lacks 3-dimensional (3D) information. Hence, novel approaches are being attempted. Methods In this study, we utilized serial block-face scanning electron microscopy and a 3D structural reconstruction system to investigate the microstructure of myocardial biopsy samples. Endomyocardial biopsy was conducted for the diagnosis of clinically suspected cardiac amyloidosis. After the diagnosis was confirmed by detecting the presence of amyloid transthyretin deposits, biopsy samples obtained at the same time were analyzed. Results In wtATTR-CA samples, the capillary density reduced with its abnormal kinking, which could cause microenvironmental ischemia. Although the fibroblasts were severely deformed to have extended morphology, of note, they showed no evidence of cell death, including necrosis and apoptosis, indicating their strong resistance against microenvironmental changes. The arrangement of myocardial cells was disrupted, and some myocardial cells were compressed from the lateral side to form a dumbbell-like shape, sometimes with abnormal intercalated disc-like structures. These could explain intramyocardial conduction heterogeneity and impaired contractility. While mitochondria of cardiomyocytes in wtATTR-CA sample were more abundant than those in the control cardiomyocytes (32.2% and 23.0%, p < 0.01), ATTR-CA mitochondria had more spherical shapes and higher sphericity (0.73 and 0.65, p < 0.001). Some ATTR-CA mitochondria also showed abnormal morphology characterized by few cristae. These were characteristics of mitochondria swelling and cristae disruption, suggesting mitochondrial dysfunction, subsequent impaired ATP production and decreased myocardial contractility, which were potentially associated with chronic microenvironmental ischemia/hypoxia and oxidative stress due to amyloid deposition. Conclusion Our novel technologies uncovered multifaceted pathological consequences of amyloid deposits in ATTR-CA, suggesting microenvironmental ischemia due to capillary narrowing, intramyocardial conduction heterogeneity, reduced energy production and impaired myocardial contractility caused by anatomical and metabolic limitations. Our findings have the potential to advance our understanding of the disease progression mechanism in ATTR-CA.