Alk3/Bmpr1a receptor is required for development of the atrioventricular canal into valves and annulus fibrosus.

Endocardial cushions are precursors of mature atrioventricular (AV) valves. Their formation is induced by signaling molecules originating from the AV myocardium, including bone morphogenetic proteins (BMPs). Here, we hypothesized that BMP signaling plays an important role in the AV myocardium during the maturation of AV valves from the cushions. To test our hypothesis, we used a unique Cre/lox system to target the deletion of a floxed Alk3 allele, the type IA receptor for BMPs, to cardiac myocytes of the AV canal (AVC). Lineage analysis indicated that cardiac myocytes of the AVC contributed to the tricuspid mural and posterior leaflets, the mitral septal leaflet, and the atrial border of the annulus fibrosus. When Alk3 was deleted in these cells, defects were seen in the same leaflets, ie, the tricuspid mural leaflet and mitral septal leaflet were longer, the tricuspid posterior leaflet was displaced and adherent to the ventricular wall, and the annulus fibrosus was disrupted resulting in ventricular preexcitation. The defects seen in mice with AVC-targeted deletion of Alk3 provide strong support for a role of Alk3 in human congenital heart diseases, such as Ebstein's anomaly. In conclusion, our mouse model demonstrated critical roles for Alk3 signaling in the AV myocardium during the development of AV valves and the annulus fibrosus.

Bone morphogenetic protein type IA receptor signaling regulates postnatal osteoblast function and bone remodeling.

Bone morphogenetic proteins (BMPs) function during various aspects of embryonic development including skeletogenesis. However, their biological functions after birth are less understood. To investigate the role of BMPs during bone remodeling, we generated a postnatal osteoblast-specific disruption of Bmpr1a that encodes the type IA receptor for BMPs in mice. Mutant mice were smaller than controls up to 6 months after birth. Irregular calcification and low bone mass were observed, but there were normal numbers of osteoblasts. The ability of the mutant osteoblasts to form mineralized nodules in culture was severely reduced. Interestingly, bone mass was increased in aged mutant mice due to reduced bone resorption evidenced by reduced bone turnover. The mutant mice lost more bone after ovariectomy likely resulting from decreased osteoblast function which could not overcome ovariectomy-induced bone resorption. In organ culture of bones from aged mice, ablation of the Bmpr1a gene by adenoviral Cre recombinase abolished the stimulatory effects of BMP4 on the expression of lysosomal enzymes essential for osteoclastic bone resorption. These results demonstrate essential and age-dependent roles for BMP signaling mediated by BMPRIA (a type IA receptor for BMP) in osteoblasts for bone remodeling.

Genetic studies of the AMH/MIS signaling pathway for Müllerian duct regression.

Anti-Müllerian hormone (AMH)/Müllerian-inhibiting substance (MIS) is a member of the transforming growth factor-beta (TGF-beta) superfamily. Like other TGF-beta family members, AMH is likely to signal through two transmembrane serine/threonine kinase receptors. Whereas the AMH type II receptor has been clearly defined, only recently has there been evidence about the identity of the AMH type I receptor for Müllerian duct regression in vivo. We generated a new cre mouse line expressing the recombinase in AMH target cells. This line was then used to conditionally inactivate the Bmpr1a gene in the Müllerian duct, resulting in males with a uterus. Thus, Bmpr1a plays an essential role in the process of Müllerian duct regression. To investigate the role of Bmpr1a in granulosa cells, we took advantage of transgenic mice overexpressing human AMH. Surprisingly, these transgenic females that were also conditionally mutant for Bmpr1a in the Müllerian duct had no uterus. These results suggest that when AMH is overexpressed, other TGF-beta family type I receptors can potentially transduce AMH signals.

Reengineering inducible cardiac-specific transgenesis with an attenuated myosin heavy chain promoter.

Despite the advantages of reversibly altering cardiac transgene expression, the number of successful studies with inducible cardiac-specific transgene expression remains limited. The utility of the current system is hampered by the large number of lines needed before a nonleaky inducible line is isolated and by the use of a heterologous virus-based minimal promoter in the responder line. We developed an efficient, experimentally flexible system that enables us to reversibly affect both abundant and nonabundant cardiomyocyte proteins. The use of bacterial-codon-based transactivators led to aberrant splicing, whereas other more efficient transactivators, by themselves, caused disease when expressed in the heart. The redesign of the system focused on developing stable transactivator-expressing lines in which expression was driven by the mouse alpha-myosin heavy chain promoter. A minimal responder locus was derived from the same promoter, in which the GATA sites and thyroid responsive elements responsible for robust cardiac specific expression were ablated, leading to an attenuated promoter that could be inducibly controlled. In all cases, whether activated or not, expression mimicked that of the parental promoter. By use of this system, an inducible expression of an abundant contractile protein, the atrial isoform of essential myosin light chain 1, and a powerful biological effector, glycogen synthase kinase-3beta (GSK-3beta), were obtained. Subsequently, we tested the hypothesis that GSK-3beta expression could reverse a preexisting hypertrophy. Inducible expression of GSK-3beta could both attenuate a hypertrophic response and partially reverse a pressure-overload-induced hypertrophy. The system appears to be robust and can be used to temporally control high levels of cardiac-specific transgene expression.

Requirement of Bmpr1a for Müllerian duct regression during male sexual development.

Elimination of the developing female reproductive tract in male fetuses is an essential step in mammalian sexual differentiation. In males, the fetal testis produces the transforming growth factor beta (TGF-beta) family member anti-Müllerian hormone (Amh, also known as Müllerian-inhibiting substance (Mis)), which causes regression of the Müllerian ducts, the primordia of the oviducts, uterus and upper vagina. Amh induces regression by binding to a specific type II receptor (Amhr2) expressed in the mesenchyme surrounding the ductal epithelium. Mutations in AMH or AMHR2 in humans and mice disrupt signaling, producing male pseudohermaphrodites that possess oviducts and uteri. The type I receptor and Smad proteins that are required in vivo for Müllerian duct regression have not yet been identified. Here we show that targeted disruption of the widely expressed type I bone morphogenetic protein (BMP) receptor Bmpr1a (also known as Alk3) in the mesenchymal cells of the Müllerian ducts leads to retention of oviducts and uteri in males. These results identify Bmpr1a as a type I receptor for Amh-induced regression of Müllerian ducts. Because Bmpr1a is evolutionarily conserved, these findings indicate that a component of the BMP signaling pathway has been co-opted during evolution for male sexual development in amniotes.

Endocardial cushion and myocardial defects after cardiac myocyte-specific conditional deletion of the bone morphogenetic protein receptor ALK3.

Receptors for bone morphogenetic proteins (BMPs), members of the transforming growth factor-beta (TGFbeta) superfamily, are persistently expressed during cardiac development, yet mice lacking type II or type IA BMP receptors die at gastrulation and cannot be used to assess potential later roles in creation of the heart. Here, we used a Cre/lox system for cardiac myocyte-specific deletion of the type IA BMP receptor, ALK3. ALK3 was specifically required at mid-gestation for normal development of the trabeculae, compact myocardium, interventricular septum, and endocardial cushion. Cardiac muscle lacking ALK3 was specifically deficient in expressing TGFbeta2, an established paracrine mediator of cushion morphogenesis. Hence, ALK3 is essential, beyond just the egg cylinder stage, for myocyte-dependent functions and signals in cardiac organogenesis.