Activin/inhibin beta B subunit gene disruption leads to defects in eyelid development and female reproduction

Reconstitution and analysis of soluble inhibin and activin receptor complexes in a cell-free system

Activins and inhibins compose a heterogeneous subfamily within the transforming growth factor-beta (TGF-beta) superfamily of growth and differentiation factors with critical biological activities in embryos and adults. They signal through a heteromeric complex of type II, type I, and for inhibin, type III receptors. To characterize the affinity, specificity, and activity of these receptors (alone and in combination) for the inhibin/activin subfamily, we developed a cell-free assay system using soluble receptor-Fc fusion proteins. The soluble activin type II receptor (sActRII)-Fc fusion protein had a 7-fold higher affinity for activin A compared with sActRIIB-Fc, whereas both receptors had a marked preference for activin A over activin B. Although inhibin A and B binding was 20-fold lower compared with activin binding to either type II receptor alone, the mixture of either type II receptor with soluble TGF-beta type III receptor (TbetaRIII; betaglycan)-Fc reconstituted a soluble high affinity inhibin receptor. In contrast, mixing either soluble activin type II receptor with soluble activin type I receptors did not substantially enhance activin binding. Our results support a cooperative model of binding for the inhibin receptor (ActRII.sTbetaRIII complex) but not for activin receptors (type II + type I) and demonstrate that a complex composed of activin type II receptors and TbetaRIII is both necessary and sufficient for high affinity inhibin binding. This study also illustrates the utility of this cell-free system for investigating hypotheses of receptor complex mechanisms resulting from crystal structure analyses.

The control of cell motility and epithelial morphogenesis by Jun kinases

Originally identified as stress-activated protein kinases that control cell survival and proliferation through transcription factor c-Jun, the Jun N-terminal kinase (JNK) subgroup of MAP kinases (MAPKs) have recently emerged as crucial regulators of cell migration and the morphogenetic movement of epithelial sheets. In Drosophila, a well-orchestrated JNK signaling pathway controls formation of actin stress fibers and cell shape changes, which are required for the sealing of embryonic epidermis in a process known as dorsal closure. The JNK pathway is also involved in morphogenetic processes in mice including closure of the eyelid, neural tube and optic fissure. This article focuses on recent advances in understanding the role of JNK pathway in the regulation of cell migration, cytoskeleton rearrangement and the morphogenesis of epithelial sheets.

The signaling pathways in tissue morphogenesis: a lesson from mice with eye-open at birth phenotype

Tissue morphogenesis during development is regulated by growth factors and cytokines, and is characterized by constant remodeling of extracellular matrix in response to signaling molecules. MEK kinase 1 (MEKK1) is a mitogen-activated protein kinase (MAPK) kinase kinase originally identified as an upstream activator for several MAPK pathways. During mouse embryogenesis, MEKK1 controls cell shape changes and formation of actin stress fibers that are required for sealing epidermis in the embryos in a process known as eyelid closure. MEKK1-null mice display eye-open at birth (EOB), a phenotype found also in mice impaired in activin, a subgroup of the transforming growth factor beta (TGFbeta) family, or in epidermal growth factor receptor (EGFR) or its ligand TGFalpha, or in transcription factor c-Jun. Molecular analyses have revealed at least two signaling mechanisms in the control of eyelid closure. One is originated from the activins and is transduced through MEKK1, leading to transcription-independent actin stress fiber formation and transcription-dependent keratinocyte migration. Another is the TGFalpha/EGFR signal that is transduced through a MEKK1-independent pathway to the activation of the ERK MAPK, which also leads to keratinocyte migration. c-Jun might serve as a connection between the two pathways. As embryonic eyelid closure is a specific morphogenetic process that is easily detectable, genetic mutant mice with EOB will be ideal models to understand the signaling mechanisms in the control of epithelial cell migration and the morphogenetic process of epithelial sheet movement.

Ovarian Follicle Development Requires Smad3

Smad3 is an important mediator of the TGFβ signaling pathway. Interestingly, Smad3-deficient (Smad3−/−) mice have reduced fertility compared with wild-type (WT) mice. To better understand the molecular mechanisms underlying the reduced fertility in Smad3−/− animals, this work tested the hypothesis that Smad3 deficiency interferes with three critical aspects of folliculogenesis: growth, atresia, and differentiation. Growth was assessed by comparing the size of follicles, expression of proliferating cell nuclear antigen, and expression of cell cycle genes in Smad3−/− and WT mice. Atresia was assessed by comparing the incidence of atresia and expression of bcl-2 genes involved in cell death and cell survival in Smad3−/− and WT mice. Differentiation was assessed by comparing the expression of FSH receptor (FSHR), estrogen receptor (ER) α, ERβ, and inhibin α-, βA-, and βB-subunits in Smad3−/− and WT mice. Because growth, atresia, and differentiation are regulated by hormones, estradiol, FSH, and LH levels were compared in Smad3−/− and WT mice. Moreover, because alterations in folliculogenesis can affect the ability of mice to ovulate, the number of corpora lutea and ovulated eggs in response to gonadotropin treatments were compared in Smad3−/− and WT animals. The results indicate that Smad3 deficiency slows follicle growth, which is characterized by small follicle diameters, low levels of proliferating cell nuclear antigen, and low expression of cell cycle genes (cyclin-dependent kinase 4 and cyclin D2). Smad3 deficiency also causes atretic follicles, degenerated oocytes, and low expression of bcl-2. Furthermore, Smad3 deficiency affects follicular differentiation as evidenced by decreased expression of ERβ, increased expression of ERα, and decreased expression of inhibin α-subunits. Smad3 deficiency causes low estradiol and high FSH levels. Finally, Smad3−/− ovaries have no corpora lutea, and they do not ovulate after ovulatory induction with exogenous gonadotropins. Collectively, these data provide the first evidence that reduced fertility in Smad3−/− mice is due to impaired folliculogenesis, associated with altered expression of genes that control cell cycle progression, cell survival, and cell differentiation. The findings that Smad3−/− follicles have impaired growth, increased atresia, and altered differentiation in the presence of high FSH levels, normal expression of FSHR, and lower expression of cyclin D2, suggest a possible interaction between Smad3 and FSH signaling downstream of FSHR in the mouse ovary.

Growth Differentiation Factor 9 Regulates Expression of the Bone Morphogenetic Protein Antagonist Gremlin

Growth differentiation factor 9 (GDF9) is an oocyte-expressed member of the transforming growth factor beta (TGF-beta) superfamily and is required for normal ovarian follicle development and female fertility. GDF9 acts as a paracrine factor and affects granulosa cell physiology. Only a few genes regulated by GDF9 are known. Our microarray analysis has identified gremlin as one of the genes up-regulated by GDF9 in cultures of granulosa cells. Gremlin is a known member of the DAN family of bone morphogenetic protein (BMP) antagonists, but its expression and function in the ovary are unknown. We have investigated the regulation of gremlin in mouse granulosa cells by GDF9 as well as other members of the TGF-beta superfamily. GDF9 and BMP4 induce gremlin, but TGF-beta does not. In addition, in cultures of granulosa cells, gremlin negatively regulates BMP4 signaling but not GDF9 activity. The expression of gremlin in the ovary was also examined by in situ hybridization. A distinct change in gremlin mRNA compartmentalization occurs during follicle development and ovulation, indicating a highly regulated expression pattern during folliculogenesis. We propose that gremlin modulates the cross-talk between GDF9 and BMP signaling that is necessary during follicle development because both ligands use components of the same signaling pathway.

Changes in circulating and testicular levels of inhibin A and B and activin A during postnatal development in the rat

This study describes the testicular levels of inhibin/activin subunits by Northern analysis and in situ hybridization and serum and testicular levels of inhibins A and B and activin A by enzyme linked immunosorbent assays (ELISA) during postnatal development in the rat. We show that serum inhibin A levels are less than 4 pg/ml throughout postnatal life. Serum inhibin B levels peak at 572 +/- 119 pg/ml (mean +/- se) at d 40 post partum (pp) before falling to 182 +/- 35 pg/ml in mature males. Serum activin A decreases from 294 +/- 29 pg/ml at d 6 to 132 +/- 27 pg/ml at maturity. Within the testis, inhibin A levels fall from 0.330 +/- 0.108 ng/g at d 15 to less than 0.004 ng/g at maturity. Inhibin B levels peak at 43.9 +/- 4.2 ng/g at d 6 before falling to 1.6 +/- 0.13 ng/g at maturity. Testicular activin A levels fall from 18.6 +/- 2.2 ng/g at d 6 to 0.094 +/- 0.013 ng/g at maturity. Northern profiles of testicular inhibin/activin subunits correlate with immunoreactive levels demonstrated by ELISA. In situ hybridization suggests that beta(A) and beta(B) subunit expression is largely restricted to the seminiferous tubule, particularly Sertoli cells, spermatogonia, and primary spermatocytes. These data support the view that inhibin B is the major inhibin in the male rat and that levels relate to Sertoli cell number and activity. Furthermore, the demonstration of high local concentrations of activin A during the period of Sertoli cell proliferation and the onset of spermatogenesis support its proposed role because a modulator of testicular development and function.

Activin isoforms signal through type I receptor serine/threonine kinase ALK7

Activins play a fundamental role in cell differentiation and development. Activin A signaling is mediated through a combination of activin type II receptors (ActRIIs) and the activin type IB receptor, ALK4. Signaling receptors of other activin isoforms remain to be elucidated. Here, we found that activin AB and activin B are ligands for ALK7. ALK7 is an orphan receptor serine/threonine kinase expressed in neuroendocrine tissues including pancreatic islets. The combination of ActRIIA and ALK7, preferred by activin AB and activin B but not by activin A, is responsible for activin-mediated secretion of insulin from pancreatic beta cell line, MIN6. In contrast, all activins activate a combination of ActRIIA and ALK4 with various levels of potency. Thus, variation in activin signaling through type I receptors is dependent upon homo- and heterodimeric assembly of activin isoforms. Thus, the differential combination of receptor heterodimers mediates variation in activin isoform signaling.

Nodal signaling and vertebrate germ layer formation

The understanding of germ layer formation in vertebrates began with classical experimental embryology. Early in the 20th century, Spemann and Mangold (1924) identified a region of the early embryo capable of inducing an entire embryonic axis. Termed the dorsal organizer, the tissue and the activity have been shown to exist in all vertebrates examined. In mice, for example, the activity resides in a region of the gastrula embryo known as the node. Experiments by the Dutch embryologist Nieuwkoop (1967a, 1967b, 1973, 1977) showed that a signal derived from the vegetal half of the amphibian embryo is responsible for the formation of mesoderm. Nieuwkoop's results allowed the development of in vitro assays that led, in the late 1980s and early 1990s, to the identification of growth factors essential for germ layer formation. Through more recent genetic investigations in mice and zebrafish, we now know that one class of secreted growth factor, called Nodal because of its localized expression in the mouse node, is essential for formation of mesoderm and endoderm and for the morphological rearrangements that occur during gastrulation.

Expression of activin subunits and receptors in the developing human ovary: activin A promotes germ cell survival and proliferation before primordial follicle formation

The formation of the essential functional unit of the ovary, the primordial follicle, occurs during fetal life in humans. Factors regulating oogonial proliferation and interaction with somatic cells before primordial follicle formation are largely unknown. We have investigated the expression, localisation and functional effects of activin and its receptors in the human fetal ovary at 14-21 weeks gestation. Expression of mRNA for the activin betaA and betaB subunits and the activin receptors ActRIIA and ActRIIB was demonstrated by RT-PCR. Expression of betaA mRNA increased 2-fold across the gestational range examined. Activin subunits and receptors were localised by immunohistochemistry. The betaA subunit was expressed by oogonia, and the betaB subunit and activin receptors were expressed by both oogonia and somatic cells. BetaA expression was increased in larger oogonia at later gestations, but was low in oocytes within newly formed primordial follicles. Treatment of ovary fragments with activin A in vitro increased both the number of oogonia present and oogonial proliferation, as detected by bromodeoxyuridine (BrdU) incorporation. These data indicate that activin may be involved in the autocrine and paracrine regulation of germ cell proliferation in the human ovary during the crucial period of development leading up to primordial follicle formation.

Development of definitive endoderm from embryonic stem cells in culture

The cellular and molecular events regulating the induction and tissue-specific differentiation of endoderm are central to our understanding of the development and function of many organ systems. To define and characterize key components in this process, we have investigated the potential of embryonic stem (ES) cells to generate endoderm following their differentiation to embryoid bodies (EBs) in culture. We found that endoderm can be induced in EBs, either by limited exposure to serum or by culturing in the presence of activin A (activin) under serum-free conditions. By using an ES cell line with the green fluorescent protein (GFP) cDNA targeted to the brachyury locus, we demonstrate that endoderm develops from a brachyury(+) population that also displays mesoderm potential. Transplantation of cells generated from activin-induced brachyury(+) cells to the kidney capsule of recipient mice resulted in the development of endoderm-derived structures. These findings demonstrate that ES cells can generate endoderm in culture and, as such, establish this differentiation system as a unique murine model for studying the development and specification of this germ layer.

The bone morphogenetic protein system in mammalian reproduction

Using molecular, cellular, and genetic approaches, recent studies examining the role of the bone morphogenetic protein (BMP) family of growth factors in the reproductive system have led to significant breakthroughs in our understanding of mammalian reproduction and fertility. Gene expression studies have revealed that key components of the BMP system (ligands, receptors, signaling molecules, and binding proteins) exhibit coordinated spatial and temporal expression patterns in fundamental cell types throughout the reproductive system. Availability of recombinant BMPs has enabled functional studies that have demonstrated important biological activities of BMPs in controlling cellular proliferation, differentiation, and apoptosis in reproductive tissues. The physiological importance of the BMP system for mammalian reproduction has been further highlighted by the elucidation of the aberrant reproductive phenotypes of animals with naturally occurring mutations or targeted deletions of certain BMP family genes. Collectively, these studies have established the concept that the BMP system plays a crucial role in fertility in female and male mammals. The purpose of this article is to review the evidence underpinning the importance of the BMP system in mammalian reproduction.

Activins are critical modulators of growth and survival

Activins betaA and betaB (encoded by Inhba and Inhbb genes, respectively) are related members of the TGF-beta superfamily. Previously, we generated mice with an Inhba knock-in allele (InhbaBK) that directs the expression of activin betaB protein in the spatiotemporal pattern of activin betaA. These mice were small and had shortened life spans, both influenced by the dose of the hypomorphic InhbaBK allele. To understand the mechanism(s) underlying these abnormalities, we now examine growth plates, liver, and kidney and analyze IGF-I, GH, and major urinary proteins. Our studies show that activins modulate the biological effects of IGF-I without substantial effects on GH, and that activin signaling deficiency also has modest effects on hepatic and renal function. To assess the relative influences of activin betaA and activin betaB, we produced mice that express activin betaB from the InhbaBK allele, and not from its endogenous Inhbb locus. InhbaBK/BK, Inhbb-/- mice have failure of eyelid fusion at birth and demonstrate more severe effects on somatic growth and survival than either of the corresponding single homozygous mutants, showing that somatic growth and life span are supported by both activins betaA and betaB, although activin betaA plays a more substantial role.

A role for MEK kinase 1 in TGF-beta/activin-induced epithelium movement and embryonic eyelid closure

MEKK1-deficient mice show an eye open at birth phenotype caused by impairment in embryonic eyelid closure. MEK kinase 1 (MEKK1) is highly expressed in the growing tip of the eyelid epithelium, which displays loose cell-cell contacts and prominent F-actin fibers in wild-type mice, but compact cell contacts, lack of polymerized actin and a concomitant impairment in c-Jun N-terminal phosphorylation in MEKK1-deficient mice. In cultured keratinocytes, MEKK1 is essential for JNK activation by TGF-beta and activin, but not by TGF-alpha. MEKK1-driven JNK activation is required for actin stress fiber formation, c-Jun phosphorylation and cell migration. However, MEKK1 ablation does not impair other TGF-beta/activin functions, such as nuclear translocation of Smad4. These results establish a specific role for the MEKK1-JNK cascade in transmission of TGF-beta and activin signals that control epithelial cell movement, providing the mechanistic basis for the regulation of eyelid closure by MEKK1. This study also suggests that the signaling mechanisms that control eyelid closure in mammals and dorsal closure in Drosophila are evolutionarily conserved.

The gonadotropin releasing hormone (GnRH) receptor activating sequence (GRAS) is a composite regulatory element that interacts with multiple classes of transcription factors including Smads, AP-1 and a forkhead DNA binding protein

Activin responsiveness of the murine GnRH receptor gene promoter is mediated at a regulatory element we termed the GnRH receptor activating sequence (GRAS). Here, we have sought to define the complex of transcription factors that interact at this element. Consistent with activin regulation at GRAS, gel shift analyses and yeast one-hybrid assays reveal Smad4 interaction at the 5' end of GRAS. While overexpression of Smad3 activates a GRAS reporter, Smad3 binding at GRAS was not detectable. A functional interaction of Smad3 at GRAS was, however, detectable in yeast expressing Smad4. Thus, Smad3 interaction at GRAS appears to be dependent on the presence of Smad4. Mutations located at the 3' end of GRAS do not affect Smad binding but eliminate functional activity. Thus, Smad binding alone cannot account for the functional attributes of GRAS. Consistent with this notion, we find that AP-1 binding is immediately juxtaposed to and, in fact, partially overlaps the Smad binding site. Finally, a recently identified member of the forkhead family of transcription factors, FoxL2, is also capable of interacting at GRAS. Furthermore, FoxL2 activation at GRAS is lost with mutation of either the 5' Smad binding site or a putative forkhead binding site located at the 3' end of the element. We suggest that GRAS is a composite regulatory element whose functional activity is dependent on the organization of a multi-protein complex consisting of Smads, AP-1 and a member of the forkhead family of DNA binding proteins.

Identification of a phylogenetically conserved activin-responsive enhancer in the Zic3 gene

Multiple signaling pathways are involved in the induction of the organizer, a major center controlling vertebrate body plan formation. To study these signals, we have focused on the regulation of the Zic3 gene, which codes for a zinc finger transcription factor expressed in the organizer region at the beginning of gastrulation. We searched for DNA regulatory elements in the Zic3 promoter by testing their ability to drive reporter gene expression in early embryos. By this approach, we identified an activin responsive enhancer (Zic3-ARE), which was located in the Zic3 first intron and was essential for dorsal activation of the reporter. The Zic3-ARE was stimulated by activin and Nodal ligands, but not by a dominant negative bone morphogenetic protein (BMP) receptor. The Zic3-ARE contains a repeating consensus homeodomain binding sequence, CTAATTAAA, suggesting involvement of a homeodomain transcription factor(s). Mutations in this motif abolished enhancer activity in dorsal marginal zone and its response to activin in animal pole explants. Inhibition of either Wnt/beta-catenin or activin/Nodal signaling suppressed Zic3-ARE activity in dorsal blastomeres, further illustrating the importance of these pathways in activation of organizer genes.

Etiology of ovarian failure in blepharophimosis ptosis epicanthus inversus syndrome: FOXL2 is a conserved, early-acting gene in vertebrate ovarian development

Blepharophimosis ptosis epicanthus inversus syndrome (BPES) is a human disorder caused by mutations in the forkhead transcription factor gene FOXL2 and is characterized by facial dysmorphology combined in some cases with ovarian failure. To better understand the role of FOXL2 in the etiology of ovarian failure in BPES, we examined its expression in embryonic ovaries of mice, chickens, and red-eared slider turtles, representatives of three phylogenetically distant vertebrate groups that have different mechanisms of sex determination. Expression of Foxl2 was detected in early ovaries of all three species around the time of sex determination and was associated with both somatic and germ cell populations in mice. Expression was sexually dimorphic in all cases. Sequence analysis of turtle and chicken FoxL2 orthologues indicated an unusually high degree of structural conservation during evolution. FoxL2 was found to be autosomal in chickens, and therefore unlikely to represent the dominant ovarian-determining gene that has been postulated to exist as a possible explanation for female heterogamety in birds. Our observations suggest that BPES may result from early abnormalities in regulating the development of the fetal ovary, rather than premature degeneration of the postnatal or adult ovary. Further, our results suggest that FOXL2 is a highly conserved early regulator of vertebrate ovarian development.

Activin a produced by ureteric bud is a differentiation factor for metanephric mesenchyme

The present study was conducted to investigate the role of the activin-follistatin system in the development of metanephros. Organ culture system and cultured metanephric mesenchymal cells were used to address this issue. Activin A was localized in ureteric bud. Activin type II receptor was localized in ureteric bud as well as metanephric mesenchyme. In an organ culture system, exogenous activin A reduced the size of cultured metanephroi, delayed ureteric bud branching, and enlarged the tips of ureteric bud. Follistatin, an antagonist of activin A was used to clarify the role of endogenous activin A. Exogenous follistatin enlarged the size of cultured metanephroi, increased ureteric bud branching, and promoted cell growth in ureteric bud. Blockade of activin signaling by adenoviral transfection of dominantly negative activin mutant receptor mimics the effect of follistatin. In cultured metanephric mesenchymal cells, activin A promoted cell growth; conversely, follistatin induced apoptosis. Furthermore, activin A induced the expressions of epithelial differentiation markers in these cells. These results suggest that activin A produced by ureteric bud is not only an important regulator of ureteric bud branching, but also a differentiation factor for metanephric mesenchyme during kidney development.

c-Jun regulates eyelid closure and skin tumor development through EGFR signaling

To investigate the function of c-Jun during skin development and skin tumor formation, we conditionally inactivated c-jun in the epidermis. Mice lacking c-jun in keratinocytes (c-jun(Deltaep)) develop normal skin but express reduced levels of EGFR in the eyelids, leading to open eyes at birth, as observed in EGFR null mice. Primary keratinocytes from c-jun(Deltaep) mice proliferate poorly, show increased differentiation, and form prominent cortical actin bundles, most likely because of decreased expression of EGFR and its ligand HB-EGF. In the absence of c-Jun, tumor-prone K5-SOS-F transgenic mice develop smaller papillomas, with reduced expression of EGFR in basal keratinocytes. Thus, using three experimental systems, we show that EGFR and HB-EGF are regulated by c-Jun, which controls eyelid development, keratinocyte proliferation, and skin tumor formation.

c-Jun is essential for organization of the epidermal leading edge

The migration of epithelial layers requires specific and coordinated organization of the cells at the leading edge of the sheet. Mice that are conditionally deleted for the c-jun protooncogene in epidermis are born at expected frequencies, but with open eyes and with defects in epidermal wound healing. Keratinocytes lacking c-Jun are unable to migrate or elongate properly in culture at the border of scratch assays. Histological analyses in vitro and in vivo demonstrate an inability to activate EGF receptor at the leading edge of wounds, and we demonstrate that this can be rescued by supplementation with conditioned medium or the EGF receptor ligand HB-EGF. Lack of c-Jun prevents EGF-induced expression of HB-EGF, indicating that c-jun controls formation of the epidermal leading edge through its control of an EGF receptor autocrine loop.

Proteotyping of mammary tissue from transgenic and gene knockout mice with immunohistochemical markers: a tool to define developmental lesions

Through the use of transgenic and gene knockout mice, several studies have identified specific genes required for the functional development of mammary epithelium. Although histological and milk protein gene analyses can provide useful information regarding functional differentiation, they are limited in their ability to precisely define the molecular lesions. For example, mice that carry a mutation in one of the subunits of the IkappaB kinase, IKKalpha, cannot lactate despite the presence of histologically normal alveolar compartment and the expression of milk protein genes. To further define and understand such lesions on a molecular level, we sought evidence for proteins that are differentially expressed during mammary gland development with a view to generating a tissue proteotype. Using database screens and immunohistochemical analyses, we have identified three proteins that exhibit distinct profiles. Here, using mouse models as test biological systems, we demonstrate the development and application of mammary tissue proteotyping and its use in the elucidation of specific developmental lesions. We propose that the technique of proteotyping will have wide applications in the analyses of defects in other mouse models.

Consequences of knocking out BMP signaling in the mouse

During the past two decades, a significant amount of data has been accumulated revealing the intriguing functions of bone morphogenetic proteins (BMPs) in all aspects of embryonic development and organogenesis. Numerous genes encoding BMPs, BMP receptors, and their downstream signal transducers have been mutated in the mouse through targeted mutagenesis. This review focuses on what is known about the role of BMP signaling in gastrulation, mesoderm formation, left-right asymmetry, neural patterning, skeletal and limb development, organogenesis, and gametogenesis as revealed by BMP-signaling mutants.

The mammalian SIR2alpha protein has a role in embryogenesis and gametogenesis

The yeast Sir2p protein has an essential role in maintaining telomeric and mating type genes in their transcriptionally inactive state. Mammalian cells have a very large proportion of their genome inactive and also contain seven genes that have regions of homology with the yeast sir2 gene. One of these mammalian genes, sir2alpha, is the presumptive mammalian homologue of the yeast sir2 gene. We set out to determine if sir2alpha plays a role in mammalian gene silencing by creating a strain of mice carrying a null allele of sir2alpha. Animals carrying two null alleles of sir2alpha were smaller than normal at birth, and most died during the early postnatal period. In an outbred background, the sir2alpha null animals often survived to adulthood, but both sexes were sterile. We found no evidence for failure of gene silencing in sir2alpha null animals, suggesting that either SIR2alpha has a different role in mammals than it does in Saccharomyces cerevisiae or that its role in gene silencing in confined to a small subset of mammalian genes. The phenotype of the sir2alpha null animals suggests that the SIR2alpha protein is essential for normal embryogenesis and for normal reproduction in both sexes.

Genetic analysis of the mammalian transforming growth factor-beta superfamily

Members of the TGF-beta superfamily, which includes TGF-betas, growth differentiation factors, bone morphogenetic proteins, activins, inhibins, and glial cell line-derived neurotrophic factor, are synthesized as prepropeptide precursors and then processed and secreted as homodimers or heterodimers. Most ligands of the family signal through transmembrane serine/threonine kinase receptors and SMAD proteins to regulate cellular functions. Many studies have reported the characterization of knockout and knock-in transgenic mice as well as humans or other mammals with naturally occurring genetic mutations in superfamily members or their regulatory proteins. These investigations have revealed that TGF-beta superfamily ligands, receptors, SMADs, and upstream and downstream regulators function in diverse developmental and physiological pathways. This review attempts to collate and integrate the extensive body of in vivo mammalian studies produced over the last decade.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.

Overexpression of Smad7 results in severe pathological alterations in multiple epithelial tissues

Biochemical studies have shown that Smad7 blocks signal transduction of transforming growth factor beta (TGFbeta); however, its in vivo functions are largely unknown. To determine the functions of Smad7, we have expressed Smad7 in transgenic mice, utilizing a keratin K5 promoter (K5.Smad7). K5.Smad7 mice exhibited pathological changes in multiple tissues and died within 10 days after birth. These mice were born with open eyelids and corneal defects, significantly delayed and aberrant hair follicle morphogenesis, and hyperproliferation in the epidermis and other stratified epithelia. Furthermore, K5.Smad7 mice developed severe thymic atrophy and massive thymocyte death, suggesting that Smad signaling in thymic epithelia is essential for thymocyte survival. Interestingly, in addition to a reduction in Smad phosphorylation, the protein levels of the receptors for TGFbeta, activin and bone morphogenetic protein were significantly decreased in the affected tissues of K5.Smad7 mice. Our study provides evidence that Smad7 is a potent in vivo inhibitor for signal transduction of the TGFbeta superfamily during development and maintenance of homeostasis of multiple epithelial tissues.

Follistatin inhibits the function of the oocyte-derived factor BMP-15

Recent studies have highlighted the importance of a novel oocyte-derived growth factor, bone morphogenetic protein-15 (BMP-15) in the regulation of proliferation and differentiation of granulosa cells in the ovary. Namely, BMP-15 stimulates granulosa cell mitosis and inhibits follicle-stimulating hormone (FSH) receptor mRNA expression in granulosa cell, thereby playing a critical role in the elaborate mechanism controlling ovarian folliculogenesis. At present, however, nothing is known about molecules which may regulate the biological activity of BMP-15. Here we demonstrate evidence that follistatin can form an inactive complex with BMP-15, through which follistatin inhibits BMP-15 bioactivities. The binding of follistatin to BMP-15 was directly demonstrated by a surface plasmon resonance biosensor, and the ability of follistatin to inhibit BMP-15 functions was determined by established BMP-15 bioassays using primary rat granulosa cells. Specifically, follistatin attenuated BMP-15 stimulation of granulosa cell proliferation and reversed BMP-15 inhibition of FSH receptor mRNA expression leading to the suppression of FSH-induced progesterone synthesis. This is the first demonstration of the biochemical interaction and biological antagonism of follistatin and BMP-15, which may be involved in the complex yet well-controlled mechanism of the regulation of follicle growth and development.

The role of the activin-follistatin system in the developmental and regeneration processes of the kidney

Regeneration processes in many tissues are modulated by various factors, which are involved in their organogenesis. Activin A, a member of the TGF-beta superfamily, inhibits branching tubulogenesis of the kidney in organ culture system as well as in in vitro tubulogenesis model. On the other hand, follistatin, an antagonist activin A, reverses the effect of activin A on kidney development, induces branching tubulogenesis, and also promotes tubular regeneration after ischemia/reperfusion injury by blocking the action of endogenous activin A. The activin-follistatin system is one of the important regulatory systems modulating developmental and regeneration processes of the kidneys.

Reproductive deficiencies in transgenic mice expressing the rat inhibin alpha-subunit gene

Inhibin is an important modulator of reproductive function at both the endocrine level, through its regulation of pituitary FSH biosynthesis, and at the paracrine and autocrine levels, as an intragonadal regulatory factor. To investigate the in vivo actions of inhibin in FSH regulation and gonadal function, transgenic mice that overexpress the rat inhibin alpha-subunit gene were generated. A transgene that includes the mouse metallothionein-I gene promoter (MT-alpha) fused to the rat inhibin alpha-subunit precursor coding sequences was used to produce three lines of transgenic mice. Transgene mRNA is expressed in numerous tissues, including the pituitary, liver, testis, ovary, and kidney. Inhibin alpha-subunit protein was also increased in transgenic pituitary and ovary. Serum inhibin alpha-subunit levels are highly increased compared with control mice. Inhibin beta(A)- and beta(B)-subunit protein amounts are lower in transgenic ovaries compared with wild type, although serum levels of activin A are not significantly reduced in transgenic female mice. FSH levels are reduced in both male and female transgenic mice, whereas LH levels are increased in MT-alpha female mice. MT-alpha transgenic females are subfertile and exhibit a 52% reduction in litter size compared with wild-type females. The smaller litter size of MT-alpha female mice was correlated with a reduction in the number of oocytes ovulated during a normal cycle. Treatment of the transgenic females with exogenous gonadotropins resulted in an ovulation rate similar to that of stimulated wild-type animals, suggesting that altered gonadotropin levels may be responsible for the decreased ovulation rates. MT-alpha transgenic male mice are fertile and sire litters of equivalent size to those sired by wild-type males, despite an approximately 50% reduction in sperm numbers. These results indicate that overexpression of the rat inhibin alpha-subunit gene in mice leads to a disruption of the normal inhibin-to-activin ratio and to reproductive deficiencies, and they support the hypothesis that inhibin and activin act to regulate FSH secretion in vivo and are essential for normal gonadal function.

Fibroblast growth factor receptor 2 (Fgfr2) plays an important role in eyelid and skin formation and patterning

Initiating as protruding ridges above and below the optic vesicle, the eyelids of mice grow across the eye and temporarily fuse in fetal life. Mutations of a number of genes disrupt this developmental process and result in a birth defect, "open-eyelids at birth." Here we show that a critical event for eyelid induction occurs at embryonic day 11.5 (E11.5) when the single cell-layered ectoderm in the presumptive eyelid territory increases proliferation and undergoes morphologic transition to form cube-shaped epithelial cells. Using embryos lacking the Fgfr2 Ig domain III (Fgfr2(DeltaIII/DeltaIII)) generated by tetraploid rescue and chimeric embryo formation approaches, we demonstrate that this event is controlled by Fgfr2 signals as the Fgfr2(DeltaIII/DeltaIII) mutation blocks these changes and results in embryos without eyelids. Fgfr2 and its ligands are differentially expressed in the ectoderm and underlying mesenchyme and function in a reciprocal interacting loop that specifies eyelid development. We also demonstrate that similar defects account for failure of skin formation at early stages. Interestingly, Fgfr2-independent skin formation occurs at E14.5 mutant embryos, resulting in much thinner, yet well-differentiated epidermis. Notably, mutant skin remains thin with decreased hair density after transplantation to wild-type recipients. These data demonstrate an essential role of Fgfr2 in eyelid and skin formation and patterning.

Vertebrate development: the subtle art of germ-layer specification

Nodal signalling is essential for vertebrate germ-layer formation. How this single signal can generate such a diverse array of tissues remains a mystery and is an area of intense research. Three recent reports reveal unanticipated subtleties to the process and provide new mechanisms for generating distinct responses.

Regulation of prostate branching morphogenesis by activin A and follistatin

Ventral prostate development occurs by branching morphogenesis and is an androgen-dependent process modulated by growth factors. Many growth factors have been implicated in branching morphogenesis including activins (dimers of beta(A) and beta(B) subunits); activin A inhibited branching of lung and kidney in vitro. Our aim was to examine the role of activins on prostatic development in vitro and their localization in vivo. Organ culture of day 0 rat ventral prostates for 6 days with activin A (+/- testosterone) inhibited prostatic branching and growth without increasing apoptosis. The activin-binding protein follistatin increased branching in vitro in the absence (but not presence) of testosterone, suggesting endogenous activins may reduce prostatic branching morphogenesis. In vivo, inhibin alpha subunit was not expressed until puberty, therefore inhibins (dimers of alpha and beta subunits) are not involved in prostatic development. Activin beta(A) was immunolocalized to developing prostatic epithelium and mesenchymal aggregates at ductal tips. Activin beta(B) immunoreactivity was weak during development, but was upregulated in prostatic epithelium during puberty. Activin receptors were expressed throughout the prostatic epithelium. Follistatin mRNA and protein were expressed throughout the prostatic epithelium. The in vitro evidence that activin and follistatin have opposing effects on ductal branching suggests a role for activin as a negative regulator of prostatic ductal branching morphogenesis.

Purification of activins from androgen-independent Shionogi carcinoma cells demonstrates enhanced expression of activin betaB-subunit under androgen-depleted cell conditions in vitro and in vivo

Here, we report characterization of growth factors secreted from androgen-independent mouse mammary Shionogi carcinoma cells. Previous isolation of fibroblast growth factor 8 (FGF8) from androgen-dependent Shionogi carcinoma SC-3 cells prompted us to characterize growth factors secreted from the androgen-independent cells. After several purification procedures, mitogens for NIH3T3 cells from the androgen-independent cells were identified as activins on the grounds that activin betaA- and betaB-subunits are detected in the active fractions by Western blotting and that the growth-promoting effects by the active fractions are specifically inhibited in the presence of follistatin. In addition, exogenous activins, but not inhibin, stimulated the growth of NIH3T3 cells in a dose-dependent manner. Interestingly, transcripts of activin betaB-subunit were predominantly found in the androgen-independent cells while its betaA-subunit was universally expressed in both androgen-dependent and -independent Shionogi carcinoma cells. In concordant with this in vitro finding, transcripts of activin betaB-subunit were enhanced in murine prostates after castration. Therefore, expression of activin betaB-subunit, but not its betaA-subunit, is likely to be related with androgen-depleted cell conditions in prostates, and possibly in androgen-related cancers.

Male reproductive phenotypes in double mutant mice lacking both FSHbeta and activin receptor IIA

Activins are known to signal through two serine/threonine kinase type II receptors. Activin receptor IIA is widely expressed in the male reproductive axis, including the pituitary and testis. Our previous studies using gene knockout mice have confirmed the essential in vivo role of activin receptor IIA in FSH homeostasis. Activin receptor IIA-null male mice are fertile, have suppressed pituitary and serum FSH levels, and demonstrate a decrease in testis size as a result of reduced Sertoli cells and germ cells. Similarly, FSHbeta null male mice are fertile despite reduced testis size and Sertoli cell number. To define the direct roles of activin receptor IIA signaling locally in the testis, independent of its effects on FSH homeostasis, we generated double mutant mice lacking both activin receptor IIA and FSH by a genetic intercross and analyzed the male reproductive phenotypes. The double mutant male mice lacking both FSH and activin receptor IIA are fertile, demonstrate no significant reduction in testis size, and produce small litters compared with mice lacking either FSH or activin receptor IIA alone. Histological analyses of the testes from double mutant mice revealed the presence of normal stages of spermatogenesis. However, there was a significant reduction in the epididymal sperm number compared with that of the individual mutants. Northern blot analyses of total RNA from testes of double mutants did not reveal transcriptional up-regulation of activin receptor IIB, the other activin type II receptor. Although RNA expression profiles of many testis cell-specific markers are unaltered, stereological analysis of the testes from double mutants indicates that there was a reduction in type A and I spermatogonial number compared with that observed in individual mutants. Our results provide in vivo genetic evidence to demonstrate that activin receptor IIA signaling plays an important local role within the testis, independent of its actions via FSH homeostasis in the pituitary.

Role of the activin-follistatin system in the morphogenesis and regeneration of the renal tubules

Activin A inhibits branching tubulogenesis of the kidney during development. Activin A also inhibits branching tubulogenesis in MDCK cells, an in vitro tubulogenesis model. On the other hand, follistatin, an antagonist of activin A, reverses the effect of activin A and induces branching tubulogenesis. Follistatin also promotes tubular regeneration after ischemia/reperfusion injury. The activin/follistatin system is one of the important regulatory systems modulating developmental and regeneration processes of the kidney.

Inhibins, activins and follistatin: actions on the testis

While the early studies of the inhibins, activins and follistatins concentrated on their role as endocrine regulators of FSH secretion, recent data has emphasized the local actions of the activins and follistatin. Inhibin, through its capacity to suppress FSH secretion can modulate numerous processes within the testis. However, to date, evidence to support a local role for inhibin is limited. In contrast, activin and its binding protein follistatin are produced by a large number of cell-types within the testis raising the possibility of a range of paracrine and autocrine actions. These include the modulation of androgen production, influence on the proliferation of Sertoli cells and germ cells as well as the capacity to influence the structural and functional features of mitochondria within germ cells. Some of these actions are carefully controlled in a temporal relationship during the development of testicular function in the rat in which there is no separation in time between birth and the onset of spermatogenesis. Given the range of actions of activin in different cell-types, recognition of systems that are designed to modulate its actions are crucial in enhancing our understanding of how these many roles can be compartmentalized.

Studying TGF-beta superfamily signaling by knockouts and knockins

The transforming growth factor beta (TGF-beta) superfamily has profound effects on many aspects of animal development. In the last decade, our laboratory and others have performed in vivo functional studies on multiple components of the TGF-beta superfamily signal transduction pathway, including upstream ligands, transmembrane receptors, receptor-associated proteins and downstream Smad proteins. We have taken gene knockout approaches to generate null alleles of the genes of interest, as well as a gene knockin approach to replace the mature region of one TGF-beta superfamily ligand with another. We found that activin betaB, expressed in the spatiotemporal pattern of activin betaA, can function as a hypomorphic allele of activin betaA and rescue the craniofacial defects and neonatal lethal phenotype of activin betaA-deficient mice. With the knockout approach, we have shown that the expression pattern of a component in the TGF-beta superfamily signal transduction cascade does not necessarily predict its in vivo function. Two liver-specific activins, activin betaC and activin betaE are dispensable for liver development, regeneration and function, whereas ubiquitously expressed Smad5 has specific roles in the development of multiple embryonic and extraembryonic tissues.

Developmental expression of activin/inhibin alpha- and beta(A)-subunit genes in the gonads of male and female chick embryos

The expression of inhibin alpha- and beta(A)-subunits was investigated in gonads of male and female chick embryos during the last week of their 21-day incubation period. Fertilized Hisex brown laying hen eggs were incubated at 37.8 +/- 0.2 degrees and 60% relative humidity in an automatic forced-draft incubator with constant lighting. Embryos were killed after 14, 18, and 21 days of incubation, sexed by macroscopical inspection of the gonadal phenotype, and further dissected to obtain the gonads. Total RNA was isolated using the ultraspec RNA method. The expression of alpha- and beta(A)-subunits was evaluated by competitive RT-PCR. Significant differences were found within and between sexes in the expression of the alpha- and beta(A)-subunits. The level of the alpha-subunit in the testis was about 23-fold higher than that in the ovary at all ages. Testicular content of inhibin alpha mRNA levels was similar at days 14 and 18 but declined significantly at day 21 of incubation, whereas no significant differences were observed between the three age groups in the ovary. Testicular and ovarian inhibin beta(A)-subunit increased significantly from day 14 to day 18 followed by a significant decline before hatch. However, inhibin beta(A) level at day 14 was significantly higher in the ovary than in the testis. At days 18 and 21, there were no differences in the levels of the inhibin beta(A) in the sexes. The expression of inhibin beta(A)-subunit in the ovary was significantly higher than that of the alpha-subunit at all ages. In the testis, however, the expression of the beta(A)-subunit was higher at days 18 and 21 than at day 14. The sex difference in gonadal inhibin subunits expression suggests differential roles of inhibin/activin in the development of the chicken gonads. The changing level of expression during incubation also suggests changing biological roles within sexes.

A novel role for bone morphogenetic proteins in the synthesis of follicle-stimulating hormone

FSH is produced in pituitary gonadotropes as an alpha/beta heterodimer, and synthesis of the beta-subunit is the rate-limiting step in overall FSH production. Synthesis of FSHbeta can be regulated by activin and inhibin, both of which are members of the transforming growth factor-beta superfamily. Bone morphogenetic proteins (BMPs) also belong to the transforming growth factor-beta family and are multifunctional growth factors involved in many aspects of tissue development and morphogenesis, including regulation of FSH action in the ovary. Here we report a novel function for BMP-7 and BMP-6 in regulating FSH synthesis in the pituitary. Using primary pituitary cell cultures derived from transgenic mice that carry the ovine FSHbeta promoter linked to a luciferase reporter gene (oFSHbetaLuc), BMP-7 or BMP-6 was found to stimulate oFSHbetaLuc expression by 6-fold. Transient expression of the oFSHbetaLuc in a transformed gonadotrope cell line, LbetaT2, was induced 4-fold by BMP-7 or BMP-6 treatment. More importantly, BMP-7 and BMP-6 increased endogenous FSH secretion by 10- and 14-fold, respectively, from LbetaT2 cells, demonstrating for the first time that a functional signaling BMP system is present in gonadotropes. Two bioneutralizing antibodies to BMP-7, which cross-react with BMP-6, but not with activin A, decreased basal oFSHbetaLuc expression and FSH secretion from transgenic mouse pituitary cultures by 83-88% and 47-48%, respectively, suggesting an autocrine or paracrine role for BMP-7 or BMP-6 in FSH synthesis. Neither bioneutralizing antibody to activin A or activin B decreased basal oFSHbetaLuc expression or mouse FSH secretion significantly. Dose-dependent inhibition of FSH synthesis by anti-BMP7 was also observed in rat and sheep pituitary cultures. These results combined with the fact that the messenger RNAs for BMP-7 and BMP-6 were detected in mouse pituitaries and LbetaT2 cells indicate that BMP-7 and/or BMP-6 can function as FSH stimulators and may be significant physiological factors maintaining basal FSH expression in vivo.

TGF beta 2, LIF and FGF2 cooperate to induce nephrogenesis

The metanephric kidney develops from interactions between the epithelial ureteric bud and adjacent metanephric mesenchyme, which is induced by the bud to form the epithelia of the nephron. We have found that leukemia inhibitory factor (LIF) and transforming growth factor beta 2 (TGF beta 2) are secreted by inductive rat bud cells and cooperate to enhance and accelerate renal tubule formation in uninduced rat metanephric mesenchymal explants. LIF alone or TGF beta 2 with fibroblast growth factor 2 induced numerous tubules in isolated mesenchymes over an 8 day period, while (in combination) all three caused abundant tubule formation in 72 hours. Furthermore, neutralization of Wnt ligands with antagonist-secreted Frizzled-related protein 1 abrogated these responses and combinatorial cytokine/growth factor stimulation of explants augmented nuclear activation of Tcf1/Lef1, suggesting that LIF and TGF beta 2/FGF2 cooperate to regulate nephrogenesis through a common Wnt-dependent mechanism.

Gonadal development in mammals at the cellular and molecular levels

In mammals, although sex is determined chromosomally, gonads in both sexes begin development as similar structures. Until recently it was widely held that female development constituted a "default" pathway of development, which would occur in the absence of a testis-determining gene. This master gene on the Y chromosome, SRY in the human and Sry in the mouse, is thought to act in a cell-autonomous fashion to determine that cells in the gonadal somatic population develop as pre-Sertoli cells. Triggering of somatic cell differentiation along the Sertoli cell pathway is therefore a key event; it was thought that further steps in gonadal differentiation would follow in a developmental cascade. In the absence of Sertoli cells, the lack of anti-Mullerian hormone would allow development of the female Mullerian duct and absence of Leydig cells would prevent maintenance of the Wolffian duct. Recent findings that female signals not only maintain the Mullerian duct and repress the Wolffian duct but also suppress the development of Leydig cells and maintain meiotic germ cells, together with the finding that an X-linked gene is required for ovarian development and must be silenced in the male, have shown that the female default pathway model is an oversimplification. Morphological steps in gonadal differentiation can be correlated with emerging evidence of molecular mechanisms; growth factors, cell adhesion, and signaling molecules interact together, often acting within short time windows via reciprocal control relationships.

The putative forkhead transcription factor FOXL2 is mutated in blepharophimosis/ptosis/epicanthus inversus syndrome

In type I blepharophimosis/ptosis/epicanthus inversus syndrome (BPES), eyelid abnormalities are associated with ovarian failure. Type II BPES shows only the eyelid defects, but both types map to chromosome 3q23. We have positionally cloned a novel, putative winged helix/forkhead transcription factor gene, FOXL2, that is mutated to produce truncated proteins in type I families and larger proteins in type II. Consistent with an involvement in those tissues, FOXL2 is selectively expressed in the mesenchyme of developing mouse eyelids and in adult ovarian follicles; in adult humans, it appears predominantly in the ovary. FOXL2 represents a candidate gene for the polled/intersex syndrome XX sex-reversal goat.

A fork in the road to fertility

Haploinsufficiency of FOXL2, a new forkhead transcription factor, causes blepharophimosis/ptosis/epicanthus inversus syndrome (BPES), a rare developmental disorder affecting the eyelid and sometimes the ovary. A new study implicates FOXL2 as the first human gene required for the maintenance of ovarian follicles. The discovery of FOXL2 may provide insight into the causes of idiopathic premature ovarian failure, a disease that burdens many infertile couples.

Extraocular mesenchyme patterns the optic vesicle during early eye development in the embryonic chick

The vertebrate eye develops from the neuroepithelium of the ventral forebrain by the evagination and formation of the optic vesicle. Classical embryological studies have shown that the surrounding extraocular tissues - the surface ectoderm and extraocular mesenchyme - are necessary for normal eye growth and differentiation. We have used explant cultures of chick optic vesicles to study the regulation of retinal pigmented epithelium (RPE) patterning and differentiation during early eye development. Our results show that extraocular mesenchyme is required for the induction and maintenance of expression of the RPE-specific genes Mitf and Wnt13 and the melanosomal matrix protein MMP115. In the absence of extraocular tissues, RPE development did not occur. Replacement of the extraocular mesenchyme with cranial mesenchyme, but not lateral plate mesoderm, could rescue expression of the RPE-marker Mitf. In addition to activating expression of RPE-specific genes, the extraocular mesenchyme inhibits the expression of the neural retina-specific transcription factor Chx10 and downregulates the eye-specific transcription factors Pax6 and Optx2. The TGF(β) family member activin can substitute for the extraocular mesenchyme by promoting expression of the RPE-specific genes and downregulating expression of the neural retina-specific markers. These data indicate that extraocular mesenchyme, and possibly an activin-like signal, pattern the domains of the optic vesicle into RPE and neural retina.

Hepatocyte growth factor induces branching tubulogenesis in MDCK cells by modulating the activin-follistatin system

BACKGROUND

The activin-follistatin system is expressed in tubular cells of the kidney. The present study was conducted to examine the role of the activin-follistatin system in tubulogenesis using Madin-Darby canine kidney (MDCK) cells as a model system.

METHODS

Tubulogenesis was assessed using MDCK cells cultured in collagen gel. The effect of recombinant human activin A on tubulogenesis was examined. Blockade of the action of endogenous activin was achieved by either adding follistatin or transfection of dominant-negative mutant of the type II activin receptor. The production of activin A was examined by Northern blotting, in situ hybridization, and Western blotting.

RESULTS

MDCK cells expressed mRNA for the betaA subunit of activin. These cells formed spherical cysts when cultured in collagen gel. Hepatocyte growth factor (HGF) added to the spherical cysts induced branching tubulogenesis. When activin A was added together with HGF, activin A blocked the branching tubulogenesis induced by HGF, and the activin-treated cells were scattered. Conversely, follistatin, an antagonist of activin A, induced branching tubulogenesis qualitatively similar to that induced by HGF. Adenovirus vector-mediated transfer of the gene encoding truncated type II activin receptor, which acts as a dominant negative mutant, also induced branching tubulogenesis. Finally, HGF markedly inhibited the production of activin A in MDCK cells cultured in collagen gel.

CONCLUSION

Activin A produced in MDCK cells tonically inhibits branching tubulogenesis, and HGF induced branching tubulogenesis mainly by blocking the production of activin A.

Insertion of Inhbb into the Inhba locus rescues the Inhba-null phenotype and reveals new activin functions

The activins (dimers of betaA or betaB subunits, encoded by the genes Inhba and Inhbb, respectively) are TGF-beta superfamily members that have roles in reproduction and development. Whereas mice homozygous for the Inhba-null allele demonstrate disruption of whisker, palate and tooth development, leading to neonatal lethality, homozygous Inhbb-null mice are viable, fertile and have eye defects. To determine if these phenotypes were due to spatiotemporal expression differences of the ligands or disruption of specific ligand-receptor interactions, we replaced the region of Inhba encoding the mature protein with Inhbb, creating the allele Inhbatm2Zuk (hereafter designated InhbaBK). Although the craniofacial phenotypes of the Inhba-null mutation were rescued by the InhbaBK allele, somatic, testicular, genital and hair growth were grossly affected and influenced by the dosage and bioactivity of the allele. Thus, functional compensation within the TGF-beta superfamily can occur if the replacement gene is expressed appropriately. The novel phenotypes in these mice further illustrate the usefulness of insertion strategies for defining protein function.

Activin betaC and betaE genes are not essential for mouse liver growth, differentiation, and regeneration

The liver is an essential organ that produces several serum proteins, stores vital nutrients, and detoxifies many carcinogenic and xenobiotic compounds. Various growth factors positively regulate liver growth, but only a few negative regulators are known. Among the latter are the transforming growth factor beta (TGF-beta) superfamily members TGF-beta1 and activin A. To study the function of novel activin family members, we have cloned and generated mice deficient in the activin betaC and betaE genes. Expression analyses demonstrated that these novel genes are liver specific in adult mice. Here, we show by RNase protection that activin betaC transcripts are present in the liver beginning at embryonic day 11.5 (E11.5) whereas activin betaE expression is detected starting from E17.5. Gene targeting in embryonic stem cells was used to generate mice with null mutations in either the individual activin betaC and betaE genes or both genes. In contrast to the structurally related activin betaA and betaB subunits, which are necessary for embryonic development and pituitary follicle-stimulating hormone homeostasis, mice deficient in activin betaC and betaE were viable, survived to adulthood, and demonstrated no reproductive abnormalities. Although activin betaC and betaE mRNAs are abundantly expressed in the liver of wild-type mice, the single and double mutants did not show any defects in liver development and function. Furthermore, in the homozygous mutant mice, liver regeneration after >70% partial hepatectomy was comparable to that in wild-type mice. Our results suggest that activin betaC and betaE are not essential for either embryonic development or liver function.

Revelations of ovarian follicle biology from gene knockout mice

Autocrine, paracrine and endocrine factors are necessary for normal ovarian folliculogenesis. A number of studies have demonstrated that the pituitary hormones follicle stimulating hormone (FSH) and luteinizing hormone (LH) as well as granulosa cell-derived growth factors such as activins, inhibins, and kit ligand are necessary for follicle development. Recent knockout studies from our laboratory have demonstrated that mice lacking the pituitary proteins FSHbeta and activin receptor type II are infertile due to blocks at the pre-antral and antral follicle stages, respectively. Although the somatic cells of the ovary (the granulosa and theca cells) have long been implicated in ovarian function, only recently have we shown that the oocyte plays an essential role in controlling its own fate by influencing somatic cell functions. Mice lacking the oocyte-secreted growth factor, growth differentiation factor-9 (GDF-9), are infertile due to a block at the one-layer primary follicle stage, leading to secondary defects in thecal cell layer formation, oocyte growth and meiotic competence, and granulosa cell differentiation. Furthermore, using recombinant GDF-9, we demonstrate that GDF-9 also regulates cumulus expansion and expression of several key granulosa cell-specific genes. Thus, GDF-9 functions as an oocyte-secreted growth and differentiation factor during early and late folliculogenesis and at ovulation to regulate several key somatic cell functions essential for female reproduction.