Abstract 212: Dilated Cardiomyopathy and Sarcomeric Disarray following Cardiac Myocyte-Specific Deletion of Smad5
Bone morphogenetic protein (BMP) signaling is critical during early heart development, but its potential role in the adult heart is not investigated. BMP signaling is mediated by the Smad transcription factors (1/5/8). Germline deletion of Smad1 and -5 resulted in non-overlapping developmental defects that suggest a unique role for Smad5 in the heart. To test the role of Smad5 specifically in cardiac myocytes in vivo, and to circumvent embryonic lethality and extrinsic defects seen in Smad5-null mice, we generated a cardiac-specific deletion of Smad5 using the Cre/lox system. KO mice (αMHC-Cre+, Smad5null/flox) followed a Mendelian distribution at weaning but died prematurely between 10 and 12 months of age (54% mortality), compared to control littermates (αMHC-Cre+, Smad5wt/flox) (15% mortality). Echocardiographic measurements indicated a dilated cardiomyopathy with preserved function at 3 months of age (LVEDD: Ctrl, 3.88 ± 0.05 mm; KO, 4.36 ± 0.09 mm; LVESD: Ctrl, 2.58 ± 0.04 mm; KO: 2.98 ± 0.08 mm; EF: Ctrl, 71 ± 1%; KO, 68 ± 1%; p<0.05 for all), but overt heart failure at 9 months (EF: Ctrl, 63 ± 3%; KO, 48 ± 5%; p<0.05) with liver and lung congestion, intra-atrial thrombosis, and significant increase in heart weight/tibial length (Ctrl: 8.8 ± 2.2 g/mm; KO: 18.7 ± 2.4 g/mm, p<0.05). Histological analysis of 3–9 months old KO revealed disarray of the sarcomeres, accompanied by disruption of the plasma membrane. At 3 months of age, electron microscopy showed lower density of myofibrils and defect in M line demarcation. Immuno-staining with phalloidin, which binds to polymerized F-actin, was decreased in KO mice. The KO showed a 1.5 fold increase in the expression of coronin 1A mRNA (p<0.05 vs Ctrl), an actin-binding protein which promotes assembly of actin filaments but does not stabilize them. The immunostaining patterns of α-actinin, a constituent of the Z-disc, and of vinculin, a component of intercalated disks, were unchanged. In conclusion, mice with cardiac-specific deletion of Smad5 develop dilated cardiomyopathy and heart failure as a result of sarcomeric disarray and myocyte disruption. These data demonstrate a unique role for Smad5 in cardiac myocyte integrity and therefore for BMP signaling in adult heart.