DAAM1 and DAAM2 are co-required for myocardial maturation and sarcomere assembly

Abstract

Wnt ligands regulate heart morphogenesis but the underlying mechanisms remain unclear. Two Formin-related proteins, DAAM1 and 2, were previously found to bind the Wnt effector Disheveled. Here, since DAAM1 and 2 nucleate actin and mediate Wnt-induced cytoskeletal changes, a floxed-allele of Daam1 was used to disrupt its function specifically in the myocardium and investigate Wnt-associated pathways. Homozygous Daam1 conditional knockout (CKO) mice were viable but had misshapen hearts and poor cardiac function. The defects in Daam1 CKO mice were observed by mid-gestation and were associated with a loss of protrusions from cardiomyocytes invading the outflow tract. Further, these mice exhibited noncompaction cardiomyopathy (NCM) and deranged cardiomyocyte polarity. Interestingly, Daam1 CKO mice that were also homozygous for an insertion disrupting Daam2 (DKO) had stronger NCM, severely reduced cardiac function, disrupted sarcomere structure, and increased myocardial proliferation, suggesting that DAAM1 and DAAM2 have redundant functions. While RhoA was unaffected in the hearts of Daam1/2 DKO mice, AKT activity was lower than in controls, raising the issue of whether DAAM1/2 are only mediating Wnt signaling. Daam1-floxed mice were thus bred to Wnt5a null mice to identify genetic interactions. The hearts of Daam1 CKO mice that were also heterozygous for the null allele of Wnt5a had stronger NCM and more severe loss of cardiac function than Daam1 CKO mice, consistent with DAAM1 and Wnt5a acting in a common pathway. However, deleting Daam1 further disrupted Wnt5a homozygousnull hearts, suggesting that DAAM1 also has Wnt5a-independent roles in cardiac development.

The actin-organizing formin protein Fhod3 is required for postnatal development and functional maintenance of the adult heart in mice

“…including sarcomeric organization and cell size control (42), probably via cooperation with formins. Daam1, another major cardiac formin (41), has been reported to be required for myocardial maturation and sarcomere assembly (7,43). In contrast to the localization of Fhod3 at the central region of sarcomeres, mouse Daam1 in the cardiomyocytes mainly localizes to the cell membrane not in a sarcomeric pattern (7), suggesting that Daam1 and Fhod3 regulate cardiac actin dynamics in a different manner.…”

FHOD-1 is the only formin inCaenorhabditis elegansthat promotes striated muscle growth and Z-line organization in a cell autonomous manner

“…Loss of DAAM-family formins significantly perturbs sarcomere organization in mammalian and insect muscle (Molnar et al, 2014;Ajima et al, 2015), while worm DAAM-1 appears dispensable (Fig. S2).…”

“…Similar, but lesser, defects were also observed in heart and larval body wall muscles partially deficient for DAAM (Molnar et al, 2014). In mice, conditional DAAM1 knock out led to non-compaction cardiomyopathy with misshapen hearts and poor cardiac function, and simultaneous knock out of DAAM1 and DAAM2 led to even stronger myopathy, with severely reduced cardiac function and disrupted sarcomere structure (Ajima et al, 2015). An RNA interference (RNAi)-based study in neonatal mouse cardiomyocytes confirmed the importance of DAAM1 for myofibril organization, and also implicated FMNL-family formins FMNL1 and FMNL2, as well as the DIAPH-family formin DIAPH3 (Rosado et al, 2014).…”

Assembly and Maintenance of Sarcomere Thin Filaments and Associated Diseases

“…Factors DAAM 1 and 2 appear at the Z-disc before actin filament assembly and recruit the critical factor, profilin, for actin polymerization [45][46][47]. DAAM1/2 mouse knockout models display disrupted cardiac sarcomere assembly and intercalated disc structures [48] and DAAM Drosophila mutants demonstrate disorganized sarcomeres with reduced thin filament numbers [49]. Since DAAM1/2 are members of the formin family of proteins, DAAM1/2 possess formin homology domains 1 and 2 (FH1 and FH2, respectively).…”

“…A second model for actin recycling relies on reusing factors that were involved in sarcomere assembly. DAAM1/2, a formin protein, is localized to the Z-disc, where it recruits profilin and synthesizes F-actin during sarcomere assembly [48,51,55,143]. FHOD3, the debated formin discussed in thin filament assembly, is required for sarcomere maintenance by regulating actin dynamics and a lack of formin proteins increases the cytoplasmic concentration of G-actin [51,143,144].…”