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Fhod3 is a cardiac member of the formin family proteins that play pivotal roles in actin filament assembly in various cellular contexts. The targeted deletion of mouse Fhod3 gene leads to defects in cardiogenesis, particularly during myofibrillogenesis, followed by lethality at embryonic day (E) 11.5. However, it remains largely unknown how Fhod3 functions during myofibrillogenesis. In this study, to assess the mechanism whereby Fhod3 regulates myofibrillogenesis during embryonic cardiogenesis, we generated transgenic mice expressing Fhod3 selectively in embryonic cardiomyocytes under the control of the β-myosin heavy chain (MHC) promoter. Mice expressing wild-type Fhod3 in embryonic cardiomyocytes survive to adulthood and are fertile, whereas those expressing Fhod3 (I1127A) defective in binding to actin die by E11.5 with cardiac defects. This cardiac phenotype of the Fhod3 mutant embryos is almost identical to that observed in Fhod3 null embryos, suggesting that the actin-binding activity of Fhod3 is crucial for embryonic cardiogenesis. On the other hand, the β-MHC promoter-driven expression of wild-type Fhod3 sufficiently rescues cardiac defects of Fhod3-null embryos, indicating that the Fhod3 protein expressed in a transgenic manner can function properly to achieve myofibril maturation in embryonic cardiomyocytes. Using the transgenic mice, we further examined detailed localization of Fhod3 during myofibrillogenesis in situ and found that Fhod3 localizes to the specific central region of nascent sarcomeres prior to massive rearrangement of actin filaments and remains there throughout myofibrillogenesis. Taken together, the present findings suggest that, during embryonic cardiogenesis, Fhod3 functions as the essential reorganizer of actin filaments at the central region of maturating sarcomeres via the actin-binding activity of the FH2 domain.

Summary Heart development requires organized integration of actin filaments into the sarcomere, the contractile unit of myofibrils, although it remains largely unknown how actin filaments are assembled during myofibrillogenesis. Here we show that Fhod3, a member of the formin family of proteins that play pivotal roles in actin filament assembly, is essential for myofibrillogenesis at an early stage of heart development. Fhod3−/− mice appear normal up to embryonic day (E) 8.5, when the developing heart, composed of premyofibrils, initiates spontaneous contraction. However, these premyofibrils fail to mature and myocardial development does not continue, leading to embryonic lethality by E11.5. Transgenic expression of wild-type Fhod3 in the heart restores myofibril maturation and cardiomyogenesis, which allow Fhod3−/− embryos to develop further. Moreover, cardiomyopathic changes with immature myofibrils are caused in mice overexpressing a mutant Fhod3, defective in binding to actin. These findings indicate that actin dynamics, regulated by Fhod3, participate in sarcomere organization during myofibrillogenesis and thus play a crucial role in heart development.

The formin family proteins play pivotal roles in actin filament assembly via the FH2 domain. The mammalian formin Fhod3 is highly expressed in the heart, and its mRNA in the adult heart contains exons 11, 12, and 25, which are absent from non-muscle Fhod3 isoforms. In cultured neonatal cardiomyocytes, Fhod3 localizes to the middle of the sarcomere and appears to function in its organization, although it is suggested that Fhod3 localizes differently in the adult heart. Here we show, using immunohistochemical analysis with three different antibodies, each recognizing distinct regions of Fhod3, that Fhod3 localizes as two closely spaced bands in middle of the sarcomere in both embryonic and adult hearts. The bands are adjacent to the M-line that crosslinks thick myosin filaments at the center of a sarcomere but distant from the Z-line that forms the boundary of the sarcomere, which localization is the same as that observed in cultured cardiomyocytes. Detailed immunohistochemical and immuno-electron microscopic analyses reveal that Fhod3 localizes not at the pointed ends of thin actin filaments but to a more peripheral zone, where thin filaments overlap with thick myosin filaments. We also demonstrate that the embryonic heart of mice specifically expresses the Fhod3 mRNA isoform harboring the three alternative exons, and that the characteristic localization of Fhod3 in the sarcomere does not require a region encoded by exon 25, in contrast to an essential role of exons 11 and 12. Furthermore, the exon 25-encoded region appears to be dispensable for actin-organizing activities both in vivo and in vitro, albeit it is inserted in the catalytic FH2 domain.

Abstract Purpose  We evaluated the clinical results of commissure plication annuloplasty for mitral regurgitation (MR) in children. Methods  Twenty-eight patients underwent a valve repair with commissure plication annuloplasty for MR from 1988 to 2005. The mean age was 2.7 ± 3.3 years. Several appropriate techniques were combined (cleft closure in 5 patients, chordal shortening in 2 patients, artificial chordal replacement in 4 patients, leaflet fixation in 2 patients, and so on). The mean follow-up period was 6.2 years. Results  There was one operative death (3.6%) and no late deaths. Two patients underwent a second repair 19 and 23 months after their initial repairs. The actuarial freedom from the reoperation rate was 90.4% ± 0.6% at 10 years. The freedom from moderate MR or more was shown to decrease over time, 87.8% ± 0.7% at 5 years and 78.0% ± 11.0% at 10 years. Furthermore, the 10-year freedom from mild MR or more was 56.5% ± 11.9%. A progression of MR was seen. Most of the residual or recurrent MR cases weighed less than 10 kg at operation. Conclusions  The combination of commissure plication annuloplasty and several appropriate techniques provided adequate results for MR in children. Since a progression of MR was observed, a careful follow-up is therefore needed in such cases. [ABSTRACT FROM AUTHOR]

A 54-year old man underwent redo mitral valve (MV) plasty because of recurrent mitral regurgitation (MR). Intraoperative transoesophageal echocardiography revealed severe MR and turbulent flow at the left ventricular (LV) outflow tract associated with systolic anterior motion of the MV. Various medical treatments, additional surgical correction, and atrial and right ventricular pacing had failed to resolve the MR associated with systolic anterior motion. LV pacing, however, markedly attenuated MR. Temporary LV pacing was discontinued on postoperative day 2, and subsequently MR associated with systolic anterior motion has not recurred. LV dyssynchrony resulting from conduction disturbances might cause systolic anterior motion immediately after MV plasty. We speculate that LV pacing eliminated LV dyssynchrony and improved the MR associated with systolic anterior motion. Temporary LV pacing can be performed easily and safely at the time of MV plasty. LV pacing can be a complementary treatment for systolic anterior motion and resultant MR. [ABSTRACT FROM AUTHOR]

Cardiac development and function require actin-myosin interactions in the sarcomere, a highly organized contractile structure. Sarcomere assembly mediated by formin homology 2 domain-containing 3 (Fhod3), a member of formins that directs formation of straight actin filaments, is essential for embryonic cardiogenesis. However, the role of Fhod3 in the neonatal and adult stages has remained unknown. Here, we generated floxed Fhod3 mice to bypass the embryonic lethality of an Fhod3 knockout (KO). Perinatal KO of Fhod3 in the heart caused juvenile lethality at around day 10 after birth with enlarged hearts composed of severely impaired myofibrils, indicating that Fhod3 is crucial for postnatal heart development. Tamoxifen-induced conditional KO of Fhod3 in the adult heart neither led to lethal effects nor did it affect sarcomere structure and localization of sarcomere components. However, adult Fhod3-deleted mice exhibited a slight cardiomegaly and mild impairment of cardiac function, conditions that were sustained over 1 year without compensation during aging. In addition to these age-related changes, systemic stimulation with the α1-adrenergic receptor agonist phenylephrine, which induces sustained hypertension and hypertrophy development, induced expression of fetal cardiac genes that was more pronounced in adult Fhod3-deleted mice than in the control mice, suggesting that Fhod3 modulates hypertrophic changes in the adult heart. We conclude that Fhod3 plays a crucial role in both postnatal cardiac development and functional maintenance of the adult heart. [ABSTRACT FROM AUTHOR]