Smad5 determines murine amnion fate through the control of bone morphogenetic protein expression and signalling levels

Short telomeres impede germ cell specification by upregulating MAPK and TGFβ signaling

Functional telomeres protect chromosome ends and play important roles in stem cell maintenance and differentiation. Short telomeres negatively impact germ cell development and can contribute to age-associated infertility. Moreover, telomere syndrome resulting from mutations of telomerase or telomere-associated genes exhibits short telomeres and reduced fertility. It remains elusive whether and how telomere lengths affect germ cell specification. We report that functional telomere is required for the coordinated germ cell and somatic cell fate decisions. Using telomerase gene Terc deficient mice as a model, we show that short telomeres restrain germ cell specification of epiblast cells but promote differentiation towards somatic lineage. Short telomeres increase chromatin accessibility to elevate TGFβ and MAPK/ERK signaling for somatic cell differentiation. Notably, elevated Fst expression in TGFβ pathway represses the BMP4-pSmad signaling pathway, thus reducing germ cell formation. Re-elongation of telomeres by targeted knock-in of Terc restores normal chromatin accessibility to suppress TGFβ and MAPK signaling, thereby facilitating germ cell formation. Taken together, our data reveal that functional telomeres are required for germ cell specification by repressing TGFβ and MAPK signaling.

The extended analogy of extraembryonic development in insects and amniotes

It is fascinating that the amnion and serosa/chorion, two extraembryonic (EE) tissues that are characteristic of the amniote vertebrates (mammals, birds and reptiles), have also independently evolved in insects. In this review, we offer the first detailed, macroevolutionary comparison of EE development and tissue biology across these animal groups. Some commonalities represent independent solutions to shared challenges for protecting the embryo (environmental assaults, risk of pathogens) and supporting its development, including clear links between cellular properties (e.g. polyploidy) and physiological function. Further parallels encompass developmental features such as the early segregation of the serosa/chorion compared to later, progressive differentiation of the amnion and formation of the amniotic cavity from serosal-amniotic folds as a widespread morphogenetic mode across species. We also discuss common developmental roles for orthologous transcription factors and BMP signalling in EE tissues of amniotes and insects, and between EE and cardiac tissues, supported by our exploration of new resources for global and tissue-specific gene expression. This highlights the degree to which general developmental principles and protective tissue features can be deduced from each of these animal groups, emphasizing the value of broad comparative studies to reveal subtle developmental strategies and answer questions that are common across species. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Growth Factor Roles in Soft Tissue Physiology and Pathophysiology

Repair and healing of injured and diseased tendons has been traditionally fraught with apprehension and difficulties, and often led to rather unsatisfactory results. The burgeoning research field of growth factors has opened new venues for treatment of tendon disorders and injuries, and possibly for treatment of disorders of the aorta and major arteries as well. Several chapters in this volume elucidate the role of transforming growth factor β (TGFß) in pathogenesis of several heritable disorders affecting soft tissues, such as aorta, cardiac valves, and tendons and ligaments. Several members of the bone morphogenetic group either have been approved by the FDA for treatment of non-healing fractures or have been undergoing intensive clinical and experimental testing for use of healing bone fractures and tendon injuries. Because fibroblast growth factors (FGFs) are involved in embryonic development of tendons and muscles among other tissues and organs, the hope is that applied research on FGF biological effects will lead to the development of some new treatment strategies providing that we can control angiogenicity of these growth factors. The problem, or rather question, regarding practical use of imsulin-like growth factor I (IGF-I) in tendon repair is whether IGF-I acts independently or under the guidance of growth hormone. FGF2 or platelet-derived growth factor (PDGF) alone or in combination with IGF-I stimulates regeneration of periodontal ligament: a matter of importance in Marfan patients with periodontitis. In contrast, vascular endothelial growth factor (VEGF) appears to have rather deleterious effects on experimental tendon healing, perhaps because of its angiogenic activity and stimulation of matrix metalloproteinases-proteases whose increased expression has been documented in a variety of ruptured tendons. Other modalities, such as local administration of platelet-rich plasma (PRP) and/or of mesenchymal stem cells have been explored extensively in tendon healing. Though treatment with PRP and mesenchymal stem cells has met with some success in horses (who experience a lot of tendon injuries and other tendon problems), the use of PRP and mesenchymal stem cells in people has been more problematic and requires more studies before PRP and mesenchymal stem cells can become reliable tools in management of soft tissue injuries and disorders.

Amniotic ectoderm expansion in mouse occurs via distinct modes and requires SMAD5-mediated signalling

Upon gastrulation, the mammalian conceptus transforms rapidly from a simple bilayer into a multilayered embryo enveloped by its extra-embryonic membranes. Impaired development of the amnion, the innermost membrane, causes major malformations. To clarify the origin of the mouse amnion, we used single-cell labelling and clonal analysis. We identified four clone types with distinct clonal growth patterns in amniotic ectoderm. Two main types have progenitors in extreme proximal-anterior epiblast. Early descendants initiate and expand amniotic ectoderm posteriorly, while descendants of cells remaining anteriorly later expand amniotic ectoderm from its anterior side. Amniogenesis is abnormal in embryos deficient in the bone morphogenetic protein (BMP) signalling effector SMAD5, with delayed closure of the proamniotic canal, and aberrant amnion and folding morphogenesis. Transcriptomics of individual Smad5 mutant amnions isolated before visible malformations and tetraploid chimera analysis revealed two amnion defect sets. We attribute them to impairment of progenitors of the two main cell populations in amniotic ectoderm and to compromised cuboidal-to-squamous transition of anterior amniotic ectoderm. In both cases, SMAD5 is crucial for expanding amniotic ectoderm rapidly into a stretchable squamous sheet to accommodate exocoelom expansion, axial growth and folding morphogenesis.

Hematopoietic activity in putative mouse primordial germ cell populations

In the present paper, starting from the observation of heterogeneous expression of the GOF-18ΔPE-GFP Pou5f1 (Oct3/4) transgene in putative mouse PGC populations settled in the aorta-gonad-mesonephros (AGM) region, we identified various OCT3/4 positive populations showing distinct expression of PGC markers (BLIMP-1, AP, TG-1, STELLA) and co-expressing several proteins (CD-34, CD-41, FLK-1) and genes (Brachyury, Hox-B4, Scl/Tal-1 and Gata-2) of hematopoietic precursors. Moreover, we found that Oct3/4-GFP(weak) CD-34(weak/high) cells possess robust hematopoietic colony forming activity (CFU) in vitro. These data indicate that the cell population usually considered PGCs moving toward the gonadal ridges encompasses a subset of cells co-expressing several germ cell and hematopoietic markers and possessing hematopoietic activity. These results are discussed within of the current model of germline segregation.

Mouse primordial germ cells: a reappraisal

Current dogma is that mouse primordial germ cells (PGCs) segregate within the allantois, or source of the umbilical cord, and translocate to the gonads, differentiating there into sperm and eggs. In light of emerging data on the posterior embryonic-extraembryonic interface, and the poorly studied but vital fetal-umbilical connection, we have reviewed the past century of experiments on mammalian PGCs and their relation to the allantois. We demonstrate that, despite best efforts and valuable data on the pluripotent state, what is and is not a PGC in vivo is obscure. Furthermore, sufficient experimental evidence has yet to be provided either for an extragonadal origin of mammalian PGCs or for their segregation within the posterior region. Rather, most evidence points to an alternative hypothesis that PGCs in the mouse allantois are part of a stem/progenitor cell pool that exhibits all known PGC "markers" and that builds/reinforces the fetal-umbilical interface, common to amniotes. We conclude by suggesting experiments to distinguish the mammalian germ line from the soma.

Loss of function of mouse Pax-Interacting Protein 1-associated glutamate rich protein 1a (Pagr1a) leads to reduced Bmp2 expression and defects in chorion and amnion development

BACKGROUND

Human PAX-Interacting Protein 1 (PAXIP1)-associated glutamate rich protein 1 (PAGR1, also known as PA1) originally was discovered as part of a complex containing PAXIP1 and histone H3K4 methyltransferases MLL3 and MLL4, suggesting a role in epigenetic gene regulation. Further in vitro studies suggested additional functions in DNA damage repair and transcription. However, in vivo analysis of PAGR1 function has been lacking.

RESULTS

Here we show that expression of the cognate mouse gene Pagr1a is found predominately in the extraembryonic and chorionic ectoderm from pregastrulation stages and is up-regulated within the embryo proper after gastrulation. Embryos with a germ line deletion of Pagr1a establish the anterior-posterior axis, and show normal neuroectodermal, mesodermal, and endodermal patterning, but fail to develop beyond the four- to five-somite stage or to undergo axial rotation. Pagr1a(-) (/) (-) embryos also show abnormal development of extraembryonic tissues with defects seen in the amnion, chorion and visceral yolk sac. At the molecular level, Pagr1a(-) (/) (-) embryos have reduced expression of BMP2, a known regulator of extraembryonic development.

CONCLUSIONS

Loss of mouse Pagr1a function leads to defective extraembryonic development, likely due at least in part to altered BMP signaling, contributing to developmental arrest.

Differentiation of trophoblast cells from human embryonic stem cells: to be or not to be?

It is imperative to unveil the full range of differentiated cell types into which human pluripotent stem cells (hPSCs) can develop. The need is twofold: it will delimit the therapeutic utility of these stem cells and is necessary to place their position accurately in the developmental hierarchy of lineage potential. Accumulated evidence suggested that hPSC could develop in vitro into an extraembryonic lineage (trophoblast (TB)) that is typically inaccessible to pluripotent embryonic cells during embryogenesis. However, whether these differentiated cells are truly authentic TB has been challenged. In this debate, we present a case for and a case against TB differentiation from hPSCs. By analogy to other differentiation systems, our debate is broadly applicable, as it articulates higher and more challenging standards for judging whether a given cell type has been genuinely produced from hPSC differentiation.

Dullard/Ctdnep1 modulates WNT signalling activity for the formation of primordial germ cells in the mouse embryo

Dullard/Ctdnep1 is a member of the serine/threonine phosphatase family of the C-terminal domain of eukaryotic RNA polymerase II. Embryos lacking Dullard activity fail to form primordial germ cells (PGCs). In the mouse, the formation of PGCs is influenced by BMP4 and WNT3 activity. Although Dullard is reputed to negatively regulate BMP receptor function, in this study we found mutations in Dullard had no detectable effect on BMP4 and p-Smad activity. Furthermore Dullard mutations did not influence the dosage-dependent inductive effect of Bmp4 in PGC formation. However, Dullard may function as a positive regulator of WNT signalling. Combined loss of one copy each of Dullard and Wnt3 had a synergistic effect on the reduction of PGC numbers in the compound heterozygous embryo. In addition, loss of Dullard function was accompanied by down-regulation of WNT/β-catenin signalling activity and a reduction in the level of Dishevelled 2 (Dvl2). Therefore, Dullard may play a role in the fine-tuning of WNT signalling activity by modulating the expression of ligands/antagonists and the availability of Dvl2 protein during specification of the germ cell lineage.

Antagonism of Nodal signaling by BMP/Smad5 prevents ectopic primitive streak formation in the mouse amnion

The strength and spatiotemporal activity of Nodal signaling is tightly controlled in early implantation mouse embryos, including by autoregulation and feedback loops, and involves secreted and intracellular antagonists. These control mechanisms, which are established at the extra-embryonic/embryonic interfaces, are essential for anterior-posterior patterning of the epiblast and correct positioning of the primitive streak. Formation of an ectopic primitive streak, or streak expansion, has previously been reported in mutants lacking antagonists that target Nodal signaling. Here, we demonstrate that loss-of-function of a major bone morphogenetic protein (BMP) effector, Smad5, results in formation of an ectopic primitive streak-like structure in mutant amnion accompanied by ectopic Nodal expression. This suggests that BMP/Smad5 signaling contributes to negative regulation of Nodal. In cultured cells, we find that BMP-activated Smad5 antagonizes Nodal signaling by interfering with the Nodal-Smad2/4-Foxh1 autoregulatory pathway through the formation of an unusual BMP4-induced Smad complex containing Smad2 and Smad5. Quantitative expression analysis supports that ectopic Nodal expression in the Smad5 mutant amnion is induced by the Nodal autoregulatory loop and a slow positive-feedback loop. The latter involves BMP4 signaling and also induction of ectopic Wnt3. Ectopic activation of these Nodal feedback loops in the Smad5 mutant amnion results in the eventual formation of an ectopic primitive streak-like structure. We conclude that antagonism of Nodal signaling by BMP/Smad5 signaling prevents primitive streak formation in the amnion of normal mouse embryos.

Bmp indicator mice reveal dynamic regulation of transcriptional response

Cellular responses to Bmp ligands are regulated at multiple levels, both extracellularly and intracellularly. Therefore, the presence of these growth factors is not an accurate indicator of Bmp signaling activity. While a common approach to detect Bmp signaling activity is to determine the presence of phosphorylated forms of Smad1, 5 and 8 by immunostaining, this approach is time consuming and not quantitative. In order to provide a simpler readout system to examine the presence of Bmp signaling in developing animals, we developed BRE-gal mouse embryonic stem cells and a transgenic mouse line that specifically respond to Bmp ligand stimulation. Our reporter identifies specific transcriptional responses that are mediated by Smad1 and Smad4 with the Schnurri transcription factor complex binding to a conserved Bmp-Responsive Element (BRE), originally identified among Drosophila, Xenopus and human Bmp targets. Our BRE-gal mES cells specifically respond to Bmp ligands at concentrations as low as 5 ng/ml; and BRE-gal reporter mice, derived from the BRE-gal mES cells, show dynamic activity in many cellular sites, including extraembryonic structures and mammary glands, thereby making this a useful scientific tool.

Periostin as a biomarker of the amniotic membrane

Tracing the precise developmental origin of amnion and amnion-derived stem cells is still challenging and depends chiefly on analyzing powerful genetic model amniotes like mouse. Profound understanding of the fundamental differences in amnion development in both the disc-shaped primate and human embryo and the cup-shaped mouse embryo is pivotal in particular when sampling amniotic membrane from nonprimate species for isolating candidate amniotic stem cells. The availability of molecular marker genes that are specifically expressed in the amniotic membrane and not in other extraembryonic membranes would be instrumental to validate unequivocally the starting material under investigation. So far such amniotic markers have not been reported. We postulated that bone morphogenetic protein (BMP) target genes are putative amniotic membrane markers mainly because deficiency in one of several components of the BMP signaling cascade in mice has been documented to result in defective development of the early amnion. Comparative gene expression analysis of acknowledged target genes for BMP in different extraembryonic tissues, combined with in situ hybridization, identified Periostin (Postn) mRNA enrichment in amnion throughout gestation. In addition, we identify and propose a combination of markers as transcriptional signature for the different extraembryonic tissues in mouse.

Few Smad proteins and many Smad-interacting proteins yield multiple functions and action modes in TGFβ/BMP signaling in vivo

Signaling by the many ligands of the TGFβ family strongly converges towards only five receptor-activated, intracellular Smad proteins, which fall into two classes i.e. Smad2/3 and Smad1/5/8, respectively. These Smads bind to a surprisingly high number of Smad-interacting proteins (SIPs), many of which are transcription factors (TFs) that co-operate in Smad-controlled target gene transcription in a cell type and context specific manner. A combination of functional analyses in vivo as well as in cell cultures and biochemical studies has revealed the enormous versatility of the Smad proteins. Smads and their SIPs regulate diverse molecular and cellular processes and are also directly relevant to development and disease. In this survey, we selected appropriate examples on the BMP-Smads, with emphasis on Smad1 and Smad5, and on a number of SIPs, i.e. the CPSF subunit Smicl, Ttrap (Tdp2) and Sip1 (Zeb2, Zfhx1b) from our own research carried out in three different vertebrate models.

Amnion formation in the mouse embryo: the single amniochorionic fold model

BACKGROUND

Despite the detailed knowledge obtained over the last decade on the molecular regulation of gastrulation in amniotes, the process of amnion development has been poorly described and illustrated in mice, and conflicting descriptions exist. Understanding the morphogenesis and development not only of the early mouse embryo, but also of its extraembryonic tissues, is crucial for correctly interpreting fate-mapping data and mouse mutants with gastrulation defects. Moreover, the recent isolation from amnion of cells with stem cell features further argues for a better understanding of the process of amnion formation. Here, we revisit the highly dynamic process of amnion formation in the mouse. Amnion development starts early during gastrulation and is intimately related to the formation of the exocoelom and the expansion of the amniotic fold. The authoritative description involves the fusion of two amniotic folds, a big posterior and a smaller anterior fold. We challenged this 'two amniotic folds' model by performing detailed histomorphological analyses of dissected, staged embryos and 3D reconstructions using historical sections.

RESULTS

A posterior fold of extraembryonic ectoderm and associated epiblast is formed early during gastrulation by accumulation of extraembryonic mesoderm posterior to the primitive streak. Previously called the "posterior amniotic fold", we rename it the "amniochorionic fold" (ACF) because it forms both amnion and chorion. Exocoelom formation within the ACF seems not to involve apoptosis within the mesoderm. The ACF and exocoelom expand without disrupting the anterior junction of epiblast, extraembryonic ectoderm and visceral endoderm. No separate anterior fold is formed; its absence was confirmed in 3D reconstructions. Amnion and chorion closure is eccentric, close to the anterior margin of the egg cylinder: we name it the "anterior separation point".

CONCLUSIONS

Here, we reconcile previous descriptions of amnion formation and provide new nomenclature, as well as an animation, that clarify and emphasize the arrangement of the tissues that contribute to amnion development and the dynamics of the process. According to our data, the amnion and the chorion are formed by a single amniochorionic fold initiated posteriorly. Finally, we give an overview on mutant mouse models with impaired amnion development.

Mesothelium of the murine allantois exhibits distinct regional properties

The rodent allantois is thought to be unique amongst mammals in not having an endodermal component. Here, we have investigated the mesothelium, or outer surface, of murine umbilical precursor tissue, the allantois (∼7.25-8.5 days postcoitum, dpc) to discover whether it exhibits the properties of an epithelium. A combination of morphology, challenge with biotinylated dextran amines (BDAs), and immunohistochemistry revealed that the mesothelium of the mouse allantois exhibits distinct regional properties. By headfold stages (∼7.75-8.0 dpc), distal mesothelium was generally squamous in shape, and highly permeable to BDA challenge, whereas ventral proximal mesothelium, referred to as "ventral cuboidal mesothelium" (VCM) for the characteristic cuboidal shape of its cells, was relatively impermeable. Although "dorsal cuboidal mesothelium" (DCM) resembled the VCM in cell shape, its permeability to BDA was intermediate between the other two regions. Results of immunostaining for Zonula Occludens-1 (ZO-1) and Epithelial-cadherin (E-cadherin), together with transmission electron microscopy (TEM), suggested that impermeability in the VCM may be due to greater cellular contact area between cells and close packing rather than to maturity of tight junctions, the latter of which, by comparison with the visceral yolk sac, appeared to be rare or absent from the allantoic surface. Both VCM and DCM exhibited an ultrastructure more favorable for protein synthesis than did the distal squamous mesothelium; however, at most stages, VCM exhibited robust afadin (AF-6), whereas the DCM uniquely contained alpha-4-integrin. These observations demonstrate that the allantoic mesothelium is not a conventional epithelium but possesses regional ultrastructural, functional and molecular differences that may play important roles in the correct deployment of the umbilical cord and its associated vascular, hematopoietic, and other cell types.

BMPs: from bone to body morphogenetic proteins

The family of bone morphogenetic proteins (BMPs) comprises approximately 30 secreted cytokines that signal through transmembrane serine/threonine kinase receptors. The BMP signaling pathways are fine-tuned on multiple levels: Extracellular antagonists modify ligand activity; several co-receptors enhance or inhibit downstream signaling events through multiple mechanisms; and intracellular molecules further regulate the signaling outcome and mediate crosstalk with other pathways. BMPs affect structures and processes throughout the entire body, ranging from embryonic patterning and development through stem cells and their niches, to tissue homeostasis and regeneration. This comprehensive involvement in various tissues had not been expected by Marshall Urist, who initially discovered the ability of an unknown factor in bone to induce bone growth in muscle and subsequently suggested the name "bone morphogenetic protein." Today, recombinant BMPs are used in clinical practice for the treatment of bone and kidney disorders, and new genetically modified BMPs are emerging as promising tools in regenerative medicine and tissue engineering. Clearly, the functions of BMPs within the body are more versatile than initially suspected. To discuss modern trends in BMP signaling, leaders in the field met for the First International BMP Workshop in Berlin in September 2009. Here, we summarize new insights on the roles of BMPs in various tissues and highlight recent findings in cell, structural, and developmental biology as well as the therapeutic potential of BMPs. Finally, we conclude that BMPs today deserve to be called body morphogenetic proteins.

Transforming Growth Factor type beta and Smad family signaling in stem cell function

Ligands of the Transforming Growth Factor type beta (TGFbeta) family exert multiple and sometimes opposite effects on most cell types in vivo depending on cellular context, which mainly includes the stage of the target cell, the local environment of this cell or niche, and the identity and the dosage of the ligand. Significant progress has been made in the molecular dissection of the regulation of the activity of the ligands and their intracellular signal transduction pathways, including via the canonical Smad pathway where Smads interact with many transcription factors. This knowledge together with results from functional studies within the embryology and stem cell research fields is giving us insight in the role of individual ligands and other components of this signaling system and where and how it regulates many properties of embryonic and adult stem/progenitor cells, which is anticipated to contribute to successful cell-based therapy in the future. We review and discuss recent progress on the effects of Nodal/Activin and Bone Morphogenetic Proteins (BMPs) and their canonical signaling in cells with stem cell properties. We focus on embryonic stem cells and their maintenance and pluripotency, and conversion into selected cell types of neuroectoderm, mesoderm and endoderm, on induced pluripotent cells and on neurogenic cells in the adult brain.

Stem cell potential of the mammalian gonad

Stem cells have enormous potential for therapeutic application because of their ability to self-renew and differentiate into different cell types. Gonads, which consist of somatic cells and germ cells, are the only organs capable of transmitting genetic materials to the offspring. Germ-line stem cells and somatic stem cells have been found in the testis; however, the presence of stem cells in the ovary remains controversial. In this review, we discuss studies focusing on whether stem cell properties are present in the different cell types of male and female gonads and their implications on stem cell research.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Inactivation of Smad5 in endothelial cells and smooth muscle cells demonstrates that Smad5 is required for cardiac homeostasis

Smads are intracellular signaling proteins that transduce signals elicited by members of the transforming growth factor (TGF)-beta superfamily. Smad5 and Smad1 are highly homologous, and they mediate primarily bone morphogenetic protein (Bmp) signals. We used the Cre-loxP system and Sm22-Cre and Tie-1-Cre mice to study the function of Smad5 in the developing blood vessel wall. Analysis of embryos demonstrated that deletion of Smad5 in endothelial or smooth muscle cells resulted in a normal organization of embryonic and extra-embryonic vasculature. Angiogenic assays performed in adult mice revealed that mutant mice display a comparable angiogenic and vascular remodeling response to control mice. In Sm22-Cre; Smad5(fl/-) mice, Smad5 is also deleted in cardiomyocytes. Echocardiographic analysis on those 9-month-old female mice demonstrated larger left ventricle internal diameters and decreased fractional shortening compared with control littermates without signs of cardiac hypertrophy. The decreased cardiac contractility was associated with a decreased performance in a treadmill experiment. In isolated cardiomyocytes, fractional shortening was significantly reduced compared with control cells. These data demonstrate that restricted deletion of Smad5 in the blood vessel wall results in viable mice. However, loss of Smad5 in cardiomyocytes leads to a mild heart defect.