Lefty Proteins Research Articles

A functional role of LEFTY during progesterone therapy for endometrial carcinoma

BackgroundThe left-right determination factor (LEFTY) is a novel member of the TGF-β/Smad2 pathway and belongs to the premenstrual/menstrual repertoire in human endometrium, but little is known about its functional role in endometrial carcinomas (Em Cas). Herein, we focused on LEFTY expression and its association with progesterone therapy in Em Cas.MethodsRegulation and function of LEFTY, as well as its associated molecules including Smad2, ovarian hormone receptors, GSK-3β, and cell cycle-related factors, were assessed using clinical samples and cell lines of Em Cas.ResultsIn clinical samples, LEFTY expression was positively correlated with estrogen receptor-α, but not progesterone receptor (PR), status, and was inversely related to phosphorylated (p) Smad2, cyclin A2, and Ki-67 levels. During progesterone therapy, expression of LEFTY, pSmad2, and pGSK-3β showed stepwise increases, with significant correlations to morphological changes toward secretory features and decreased Ki-67 values. In Ishikawa cells, an Em Ca cell line that expresses PR, progesterone treatment reduced proliferation and induced increased expression of LEFTY and pGSK-3β, although LEFTY promoter regions were inhibited by transfection of PR. Moreover, inhibition of GSK-3β resulted in increased LEFTY expression through a decrease in its ubiquitinated form, suggesting posttranslational regulation of LEFTY protein via GSK-3β suppression in response to progesterone. In addition, overexpression or knockdown of LEFTY led to suppression or enhancement of Smad2-dependent cyclin A2 expression, respectively.ConclusionUpregulation of LEFTY may serve as a useful clinical marker for the therapeutic effects of progesterone for Em Cas, leading to inhibition of tumor cell proliferation through alteration in Smad2-dependent transcription of cyclin A2.

Lefty Glycoproteins in Human Embryonic Stem Cells: Extracellular Delivery Route and Posttranslational Modification in Differentiation.

Lefty is a member of transforming growth factor-beta (TGF-β) superfamily and a potent antagonist of the TGF-β/Nodal/Activin signaling pathway. Lefty is critical in sustaining self-renewal/pluripotency status, and implicated in the differentiation of embryonic stem cells (ESCs). However, emerging studies depict Lefty as a multifaceted protein involved in myriad cellular events. Lefty proteins (human Lefty A and B) are secreted glycoproteins, but their mode of secretion and the significance of their "glycan" moiety remain mostly unexplored. By employing an in vitro system of human ESCs (hESCs), we observed that Lefty protein(s) are encased in exosomes for extracellular release. The exosomal- and cell-associated Lefty diverge in their proteolytic processing, and possess N-glycan structures of high mannose and complex nature. Differentiation of hESCs to mesenchymal cells (MSCs) or neuronal progenitor cells (NPCs) entails distinct changes in the Lefty A/Lefty B gene(s), and protein expression. Specifically, the proteolytic cleavage and N-glycan composition of the cell-associated and exosomal Lefty differ in the differentiated progenies. These modifications affected Lefty's inhibitory effect on Nodal signaling in aggressive melanoma cells. The microheterogeneity in the processing and glycosylation of Lefty protein(s) between hESCs, MSCs, and NPCs could present efficient means of diversifying the endogenous functions of Lefty. Whether Lefty's diverse functions in embryonic patterning, as well as its diffusion range in the extracellular environment, are similarly affected remains to be determined. Our studies underscore the potential relevance of Lefty-packaged exosomes for combating debilitating diseases such as cancer.

Lefty inhibits glioma growth by suppressing Nodal-activated Smad and ERK1/2 pathways

As a common malignant tumor, glioma has long been a refractory disease in the field of neurosurgery. Exploration of its etiology, pathogenesis, biological characteristics, and new treatment measures is a hot topic as well as a problem most difficult to solve in the neurosurgical division. In recent years, the role of Nodal and its feedback inhibitor Lefty in tumors has drawn more and more attention and may form a new target for cancer therapy. Western blot detection indicated that there was almost no expression of Lefty protein in glioma cells. Transfection of Lefty-overexpressing vector into GBM8401 and GBM glioma cells significantly decreased the expression of Nodal. Nodal can significantly increase the phosphorylation levels of Smad2 and Smad3 and activate the ERK1/2 pathway; meanwhile, Nodal promotes the proliferation and invasion of glioma cells and inhibits their apoptosis. However, when cells were co-transfected with both Lefty- and Nodal-overexpressing vectors, Lefty inhibited the above effects of Nodal in glioma cells, hence significantly reduced the levels of phosphorylated Smad2, Smad3, and ERK1/2, inhibited the proliferation and invasion of glioma cells, and increased their apoptosis. These results indicate that in glioma cells, Lefty inhibits Nodal-mediated activation of Smad and ERK1/2 signaling pathways, thereby suppressing the promoting effect of Nodal on tumor growth.

Characterization of nodal/TGF-lefty signaling pathway gene variants for possible roles in congenital heart diseases.

BackgroundNodal/TGF-Lefty signaling pathway has important effects at early stages of differentiation of human embryonic stem cells in directing them to differentiate into different embryonic lineages. LEFTY, one of transforming growth factors in the Nodal/TGF-Lefty signaling pathway, plays an important role in the development of heart. The aim of this work was to find evidence on whether Lefty variations are associated with congenital heart diseases (CHD).MethodsWe sequenced the Lefty gene for 230 Chinese Han CHD patients and evaluated SNPs rs2295418, rs360057 and g.G169A, which are located within the translated regions of the genes. The statistical analyses were conducted using Chi-Square Tests as implemented in SPSS (version 13.0). The Hardy-Weinberg equilibrium test of the population was carried out using online software OEGE, and multiple-sequence alignments of LEFTY proteins were carried out using the Vector NTI software.ResultsTwo heterozygous variants in Lefty1 gene, g.G169A and g.A1035C, and one heterozygous variant in Lefty2 gene, g.C925A, were identified. Statistical analyses showed that the rs2295418 (g.C925A) variant in Lefty2 gene was obviously associated with the risk of CHD (P value = 0.016<0.05). The genotype frequency of rs360057 (g.A1035C) variant in Lefty1 gene was associated with the risk of CHD (P value = 0.007<0.05), but the allele frequency was not (P value = 0.317>0.05).ConclusionsThe SNP rs2295418 in the Lefty2 gene is associated with CHD in Chinese Han populations.

Left-right determination factor is down-regulated in fibrotic renal tissue of human hydronephrosis

• To compare the expressions of common fibrosis-relevant genes in hydronephrosis-induced fibrotic renal tissues and normal human renal tissues, thereby providing insights into the cellular and molecular mechanisms of renal fibrosis resulting from hydronephrosis. • A total of 12 extensively fibrotic renal tissue samples from patients with hydronephrosis (H-group) and six normal renal tissue samples from patients who underwent nephrectomy for renal cell carcinoma (N-group), along with their clinical data, were collected at Renmin Hospital of Wuhan University in China between October 2005 and August 2007. • These tissue samples were compared for their transforming growth factor-β (TGF-β)/bone morphogenetic protein (BMP) pathway-related gene profiles using a real-time polymerase chain reaction (PCR) microarray. • Subsequently, reverse transcriptase-PCR assays were used to validate the expression changes of left-right determination factor (LEFTY), a gene of interest, at the mRNA level. • The different expression of LEFTY at the protein level was confirmed by western blotting and immunohistochemistry assays. • The results showed that 49 genes were differently expressed in fibrotic renal tissues relative to normal control tissues. Among these genes, 25 were up-regulated and 24 were down-regulated. • LEFTY-B, one of the most markedly altered genes, was down-regulated 13.55-fold compared with N-group tissues. • RT-PCR showed that the LEFTY-A (6.05-fold down-regulated, P < 0.001) and LEFTY-B (12.5-fold down-regulated, P < 0.001) genes, two members of the LEFTY family in human tissues, were both significantly down-regulated in H-group tissues. • Similarly, down-regulations of LEFTY-A (0.25-fold vs N-group, P < 0.001) and LEFTY-B (0.20-fold vs N-group, P < 0.001) proteins were detected by western blotting (P < 0.001). • Immunohistochemical staining showed different distributions of LEFTY in the two tissue samples, and quantitative image analyses confirmed that LEFTY protein expression was lower in H-group tissues than in N-group tissues (P < 0.001). • The gene and protein expressions of LEFTY were found to be down-regulated in extensively fibrotic renal tissues induced by hydronephrosis. • LEFTY may represent an ideal candidate for a therapeutic target for renal fibrosis.

Analysis of gene-chip and expression of left-fight determination factor 1 in human extensive flbrotic renal tissues of patients with hydronephrosis

Objective To investigate the fibrosis-associated genes expressed in the tissue of extensive fibrotic kidney of patients with severe hydronephrosis.Methods Twelve extensive fibmtic kidneys in patients with severe hydronephrosis(hydrnephrosis group)were compared with 6 normal renal tissues (control group)by using real-time polymerase chain reaction(PCR)assay.All samples were collected from our hospital from Oct.2005 to Aug.2007.The results of real time PCR assay were processed with ΔΔCt method.When 2~(ΔΔc1) wag≤0.5 or≥2.the difference Was considered significant.Then the expression of left-right determination factor A(LEFTY-A)in both groups were detected by immunohistochemistry.Results A total of 49 differential genes expressed in the fibrotic renal tissues were screened out.of which,25 were up-regulated and 24 were down-regulated.The differential genes included members and their receptors of transforming growth factor(TGF)-β super family or bone morphogenetic protein(BMP) family,target molecular of TGF-β or BMP signal pathway and several relative regulators.And the gene of LEFTY-1.one of the most markedly-changed genes,in hydrnephrosis group Was 13.55-fold down-regulated as compared with control group.Immunohistochemistry analysis reveled that LEGTU-A protein in the sample of hydronephrosis group was significantly decreased as compared with that in control group(P＜0.05).Conclusion The LEFTY mRNA and protein expression Was decreased in human severely hydronephrotic renal tissue.indicating that LEFTY may play an important role in the inhibition of renal fibrosis resulting from long-term hydronephrosis. Key words: Hydronephrosis; Reflal fibrosis; Gene chip; Left-right determination factor

Lefty promotes trophoblast lineage specification in human placenta

VISWANATHAN RAVISHANKAR, KRISTIN PATZKOWSKI, MEIYI TANG, CECILIA AVILA, PAUL OGBURN, J. QUIRK, SIAMAK TABIBZADEH, State University of New York at Stony Brook, Obstetrics, Gynecology and Reproductive Medicine, Stony Brook, New York, State University of New York at Stony Brook, Department of Obstetrics, Gynecology and Reproductive Medicine, Stony Brook, New York, Columbia University, Obstetrics and Gynecology, Port Jefferson, New York, SUNY Stony Brook, Stony Brook, New York OBJECTIVE: Microarray analysis has shown a member of the TGF-beta super family, Lefty A, to be the single most abundant inhibitor in stem cells and in the maternal decidua that supports embryo implantation. Based on the foundation that Lefty is induced by the transcriptional factor Oct3/4 that regulates trophoectodermal differentiation, we hypothesized that Lefty is involved in placental development. We sought to determine the presence and distribution of Lefty in human Placenta and examine the biologic impact of lefty A in specification of trophoblasts. STUDY DESIGN: Placental samples were obtained from normal term pregnancies (cesarean or vaginal deliveries). Lefty mRNA was detected by reverse transcription followed by polymerase chain reaction (RT-PCR) and Lefty proteins by Western blot analysis. Immunoreactive Lefty was localized in tissue sections by immunohistochemical staining. Decidual cells and BeWo trophoblast cells were subjected to transduction with empty control adenovirus particles and with particles driving Lefty expression. Cells were harvested and analyzed by RT-PCR to determine trophoblast lineage specification and assessed for fusion. RESULTS: 1) Lefty was expressed in villous and extra-villous trophoblasts including binucleated cells. 2) Lefty transduction of maternal decidual cells derived from third trimester placenta led to the development of binucleation, expression of cytokeratin and CGbeta. 3) Transduction of BeWo cells by Lefty increase expression of trophoblast markers including, Imfa, and Stral3. 4) Lefty also led to increased number of multinucleated cells with a large size within 48 hr of treatment. CONCLUSION: The findings suggest a role for Lefty in cell-cell fusion and trophoblast fate specification.

Molecular Analysis of LEFTY-Expressing Cells in Early Human Embryoid Bodies

Human ESCs (HESCs) are self-renewing pluripotent cell lines that are derived from the inner cell mass of blastocyst-stage embryos. These cells can produce terminally differentiated cells representing the three embryonic germ layers. We thus hypothesized that during the course of in vitro differentiation of HESCs, progenitor-like cells are transiently formed. We demonstrated that LEFTY proteins, which are known to play a major role during mouse gastrulation, are transiently expressed during HESC differentiation. Moreover, LEFTY proteins seemed to be exclusively expressed by a certain population of cells in the early human embryoid bodies that does not overlap with the population expressing the ESC marker OCT4. We also showed that LEFTY expression is regulated at the cellular transcription level by molecular labeling of LEFTY-positive cells. A DNA microarray analysis of LEFTY-overexpressing cells revealed a signature of cell surface markers such as CADHERIN 2 and 11. Expression of LEFTY controlled by NODAL appears to have a substantial role in mesodermal origin cell population establishment, since inhibition of NODAL activity downregulated expression not only of LEFTY A and LEFTY B but also of BRACHYURY, an early mesodermal marker. In addition, other mesodermal lineage-related genes were downregulated, and this was accompanied by an upregulation in ectoderm-related genes. We propose that during the initial step of HESC differentiation, mesoderm progenitor-like cells appear via activation of the NODAL pathway. Our analysis suggests that in vitro differentiation of HESCs can model early events in human development.

Lefty is expressed in mouse endometrium in estrous cycle and peri-implantation period

Reproductive tissues are unique structures that exhibit cyclic stromal remodelling during menstrual cycles in humans. Ebaf/lefty participates in tissue remodelling of human endometrium by induction of matrix metalloproteases (MMP). We describe the temporal expression and spatial distribution of lefty and tissue remodelling events in mouse endometrium. RT-PCR and real-time PCR were used to identify mRNA expression and western blots to analyse Lefty protein. Immunolocalization was performed with specific antibodies and horseradish peroxidase staining. Lefty was expressed in endometrium throughout the estrous cycle. Expression of MMP (MMP-2, -3, -7 and -14) was higher at estrus, metestrus and/or diestrus while collagen content of endometrium decreased in these phases. During pregnancy, lefty levels were higher on days 3-5 and were minimal by day 9. Similarly, expression of endometrial MMP was higher on days 3 and 5 of pregnancy and was low on day 9. During pregnancy, loss of collagen was initiated on day 3, persisted to day 5, and led to a significantly reduced collagen on day 9. Immunoreactive lefty decorated basal laminae, and was associated with extracellular matrix in stroma. Regulated expression and spatial distribution of lefty in mouse endometrium confines its biological impact on tissues that undergo remodelling during estrous cycle and pregnancy.

Two Modes by which Lefty Proteins Inhibit Nodal Signaling

During vertebrate embryogenesis, members of the Lefty subclass of Transforming Growth Factor-β (TGFβ) proteins act as extracellular antagonists of the signaling pathway for Nodal, a TGFβ-related ligand essential for mesendoderm formation and left-right patterning [1–3]. Genetic and biochemical analyses have shown that Nodal signaling is mediated by activin receptors but also requires EGF-CFC coreceptors, such as mammalian Cripto or Cryptic [4–8]. Misexpression experiments in zebrafish and frogs have suggested that Lefty proteins can act as long-range inhibitors for Nodal, possibly through competition for binding to activin receptors [9–13]. Here we demonstrate two distinct and unexpected mechanisms by which Lefty proteins can antagonize Nodal activity. In particular, using a novel assay for Lefty activity in mammalian cell culture, we find that Lefty can inhibit signaling by Nodal but not by Activin or TGFβ1, which are EGF-CFC independent. We show that Lefty can interact with Nodal in solution and thereby block Nodal from binding to activin receptors. Furthermore, Lefty can also interact with EGF-CFC proteins and prevent their ability to form part of a Nodal receptor complex. Our results provide mechanistic insights into how Lefty proteins can achieve efficient and stringent regulation of a potent signaling factor.

Lefty Proteins Are Long-Range Inhibitors of Squint-Mediated Nodal Signaling

The regulation of signaling pathways by feedback inhibitors has become an emerging theme in the control of pattern formation during development [1]. Nodal and Lefty proteins belong to divergent subfamilies of the TGF-β family [2–5]. Nodal signals promote mesendoderm induction in vertebrates, and Lefty proteins antagonize it [2–5]. In zebrafish, Squint functions as a long-range Nodal signal during mesoderm induction [6]. We report that the range over which Squint induces mesoderm is reduced by Lefty proteins. In contrast, the activity range of the short-range Nodal signal Cyclops is not regulated by Lefty activity. We present three lines of evidence that Lefty proteins diminish the range of Squint signaling by acting not only as antagonists of Squint autoregulation but also as long-range inhibitors of Squint activity. First, Lefty can block Nodal signaling at a distance. Second, Lefty regulates the range of Squint signaling before regulating squint expression. Third, Lefty restricts the range of Squint activity in squint mutant embryos, in which the endogenous gene is not subject to autoregulation. We also find that Lefty restricts the response to both high and low levels of Nodal signaling. These results indicate that Lefty proteins restrict the activity range of Nodal signals by dampening Nodal signaling in surrounding cells.

Inhibition of Nodal signalling by Lefty mediated through interaction with common receptors and efficient diffusion.

Two TGFbeta-related proteins, Nodal and Lefty, are implicated in early embryonic patterning of vertebrates. Genetic data suggest that Nodal is a signalling molecule, while Lefty is an antagonist of Nodal, but their precise function remains unknown. The signalling pathway of Nodal was investigated with the use of a Nodal-responsive assay system based on frog animal caps. Expression of dominant negative mutants of various receptors indicated that ALK4, and either ActRIIA or ActRIIB, function as type I and type II receptors for Nodal, respectively. A soluble form of Cripto lacking the COOH-terminal region interacted with Nodal but failed to mediate Nodal signalling, indicating that the native Cripto protein functions as a membrane-bound co-receptor for Nodal. Processed forms of Lefty proteins, both smaller and larger forms, inhibited Nodal signalling. Such Lefty-induced inhibition was rescued by excess ActRIIA or ActRIIB, suggesting that Lefty antagonizes Nodal signalling through competitive binding to the common receptor ActRIIA or ActRIIB. This idea was supported by the demonstration of a genetic interaction between lefty2 and ActRIIB in mouse. Behaviours of GFP-Nodal and GFP-Lefty2 proteins were also investigated in chick embryos. Both proteins could diffuse over a long distance, but the latter diffused faster than the former. Efficient inhibition of Nodal signals by Lefty may involve competitive binding of Lefty to the common receptors and faster diffusion of Lefty.

Establishment of vertebrate left-right asymmetry.

The generation of morphological, such as left-right, asymmetry during development is an integral part of the establishment of a body plan. Until recently, the molecular basis of left-right asymmetry was a mystery, but studies indicate that Nodal and the Lefty proteins, transforming growth factor-beta-related molecules, have a central role in generating asymmetric signals. Although the initial mechanism of symmetry breaking remains unknown, developmental biologists are beginning to analyse the pathway that leads to left-right asymmetry establishment and maintenance.

Roles of nodal-lefty regulatory loops in embryonic patterning of vertebrates.

Nodal is a signalling molecule that belongs to the transforming growth factor-beta superfamily of proteins, and Lefty proteins are antagonists of Nodal signalling. The nodal and lefty genes form positive and negative regulatory loops that resemble the reaction-diffusion system. As a pair, these genes control various events of vertebrate embryonic patterning, including left-right specification and mesoderm formation. In this review, we will focus on recent studies that have addressed the roles of nodal and lefty in mouse development.

Lefty Proteins Exhibit Unique Processing and Activate the MAPK Pathway

Lefty polypeptides, novel members of the transforming growth factor-beta (TGF-beta) superfamily, are involved in the formation of embryonic lateral patterning. Members of the TGF-beta superfamily require processing for their activation, suggesting cleavage to be an essential step for lefty activation. Transfection of different cell lines with lefty resulted in expression of a 42-kDa protein, which was proteolytically processed to release two polypeptides of 34 and 28 kDa. Since members of the proprotein convertase (PC) family cleave different TGF-beta factors and are involved in the establishment of embryonic laterality, we studied their role in lefty processing. Cotransfection analysis showed that PC5A processed the lefty precursor to the 34-kDa form in vivo, whereas furin, PACE4, PC5B, and PC7 had a limited activity. None of these PCs showed activity in the processing of the lefty polypeptide to the 28-kDa lefty form. The mutation of the consensus sequences for PC cleavage in the lefty protein allowed the lefty cleavage sites to be identified. Mutations of the sequence RGKR to GGKG (amino acids 74-77) and of RHGR to GHGR (amino acids 132-135) prevented the proteolytic processing of the lefty precursor to the 34- and 28-kDa forms, respectively. To identify the biologically active form of lefty, we studied the effect of lefty treatment on pluripotent P19 cells. Lefty did not induce Smad2 or Smad5 phosphorylation, Smad2/Smad4 heterodimerization, or nuclear translocation of Smad2 or Smad4, but activated the MAPK pathway in a time- and dose-dependent fashion. Further analysis showed the 28-kDa (but not the 34-kDa) polypeptide to induce MAPK activity. Surprisingly, the 42-kDa lefty protein was also capable of inducing MAPK activity, indicating that the lefty precursor is biologically active. The data support a molecular model of processing as a mechanism for regulation of lefty signaling.

Role of asymmetric signals in left-right patterning in the mouse

Left-right asymmetric signaling molecules in mammals include three transforming growth factor beta (TGFbeta)-related factors, Nodal, Lefty1 and Lefty2. They are all expressed on the left half of developing mouse embryos. Nodal acts as a left-side determinant by transducing signals through Smad and FAST and by inducing Pitx2 expression on the left side. Lefty proteins are antagonists that inhibit Nodal signaling. There are positive and negative transcriptional regulatory loops between nodal and lefty2 genes. Thus, Nodal activates its own gene and lefty2. Lefty2 protein produced then inhibits Nodal signaling and terminates expression of both genes. This feedback mechanism can restrict the range and duration of Nodal signaling in developing embryos.

Regulation of Gut and Heart Left–Right Asymmetry by Context-Dependent Interactions between Xenopus Lefty and BMP4 Signaling

The Lefty subfamily of TGFβ signaling molecules has been implicated in early development in mouse, zebrafish, and chick. Here, we show that Xenopus lefty (Xlefty) is expressed both bilaterally in symmetric midline domains and unilaterally in left lateral plate mesoderm and anterior dorsal endoderm. To examine the roles of Xlefty in left–right development, we created a system for scoring gut asymmetry and examined the effects of unilateral Xlefty misexpression on gut development, heart development, and Xnr-1 and XPitx2 expression. In contrast to the unilateral effects of Vg1, Activin, Nodal, or BMPs, targeted expression of Xlefty in either the left or the right side of Xenopus embryos randomized the direction of heart looping, gut coiling, and left–right positioning of the gut and downregulated the asymmetric expression of Xnr-1 and XPitx2. It is currently thought that Lefty proteins act as feedback inhibitors of Nodal signaling. However, this would not explain the effects of right-sided Xlefty misexpression. Here, we show that Xlefty interacts with the signaling pathways of other members of the TGFβ family during left–right development. Results from coexpression of Xlefty and Vg1 indicate that Xlefty can nullify the effects of Vg1 ectopic expression and that Xlefty is downstream of left-sided Vg1 signaling. Results from coexpression of Xlefty and XBMP4 indicate that XLefty and XBMP4 interact both synergistically and antagonistically in a context-dependent manner. We propose a model in which interactions of Xlefty with multiple members of the TGFβ family enhance the differences between the right-sided BMP/ALK2/Smad pathway and the left-sided Vg1/anti-BMP/Nodal pathway, leading to left–right morphogenesis of the gut and heart.

Pitx2, a Bicoid-Type Homeobox Gene, Is Involved in a Lefty-Signaling Pathway in Determination of Left-Right Asymmetry

Signaling molecules such as Activin, Sonic hedgehog, Nodal, Lefty, and Vg1 have been found to be involved in determination of left-right (L-R) asymmetry in the chick, mouse, or frog. However, a common signaling pathway has not yet been identified in vertebrates. We report that Pitx2, a bicoid-type homeobox gene expressed asymmetrically in the left lateral plate mesoderm, may be involved in determination of L-R asymmetry in both mouse and chick. Since Pitx2 appears to be downstream of lefty-1 in the mouse pathway, we examined whether mouse Lefty proteins could affect the expression of Pitx2 in the chick. Our results indicate that a common pathway from lefty-1 to Pitx2 likely exists for determination of L-R asymmetry in vertebrates.

Left-right asymmetric expression of the TGF beta-family member lefty in mouse embryos.

Examples of lateral asymmetry are often found in vertebrates, such as the heart being on the left side, but the molecular mechanism governing the establishment of this left-right (L-R) handedness is unknown. A diffusible morphogen may determine L-R polarity, but a likely molecule has not so far been identified. Here we report on the gene lefty, a member of the transforming growth factor-beta family, which may encode a morphogen for L-R determination. Lefty protein contains the cysteine-knot motif characteristic of this superfamily and is secreted as a processed form of relative molecular mass 25K-32K. Surprisingly, lefty is expressed in the left half of gastrulating mouse embryos. This asymmetric expression is very transient and occurs just before the first sign of lateral asymmetry appears. In the mouse mutants iv and inv, which cause situs inversus, the sites of lefty expression are inverted, indicating that lefty is downstream of iv and inv. These results suggest that lefty may be involved in setting up L-R asymmetry in the organ systems of mammals.